116860058 Handbook on Investment Casting Gold Jewellery

WORLD GOLD COUNCIL

HANDBOOK ON INVESTMENT CASTING THE LOST WAX CASTING PROCESS FOR CARAT GOLD JEWELLERY MANUFACTURE

WORLD GOLD COUNCIL

HANDBOOK ON INVESTMENT CASTING THE LOST WAX CASTING PROCESS FOR CARAT GOLD JEWELLERY MANUFACTURE

By Valerio Faccenda Consultant to World Gold Council with Chapter 3 written by Dieter Ott Formerly at FEM, Schwäbisch Gmünd, Germany

Copyright © 2003 by World Gold Council, London Publication Date: May 2003 Published by World Gold Council, International Technology, 45 Pall Mall, London SW1Y 5JG, United Kingdom Telephone: +44 20 7930 5171. Fax: +44 20 7839 6561 E-mail: [email protected] www.gold.org

Produced by Dr Valerio Faccenda, Aosta, Italy Editor: Dr Christopher W. Corti Translated by Professor Giovanni Baralis, Turin, Italy Originated and printed by Trait Design Limited

Note: Whilst every care has been taken in the preparation of this publication, World Gold Council cannot be responsible for the accuracy of any statement or representation made or the consequences arising from the use of information contained in it. The Handbook is intended solely as a general resource for practising professionals in the field and specialist advice should be obtained wherever necessary. It is always important to use appropriate and approved health and safety procedures. All rights reserved. No part of this publication may be reproduced, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission in writing of the copyright holder.

CONTENTS Preface

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Glossary

7

1 Introduction 1.1 1.2 1.3 1.4

Development of the modern process The modern process and product quality Choice of equipment and consumables Health and safety

2 The process of investment casting 2.1 Design 2.2 Making the master model 2.2.1 Alloy of manufacture 2.2.2 Feed sprue 2.3 Making the rubber mould 2.3.1 Types of mould rubber 2.3.2 Making the mould 2.3.3 Cutting the mould 2.3.4 Storing and using the mould 2.3.5 Common problems 2.4 Production of the wax patterns 2.4.1 Types of waxes 2.4.2 Wax injection 2.4.3 Common problems 2.5 Assembling the tree 2.5.1 Bases and sprues 2.5.2 Tree design 2.6 Investing the mould 2.6.1 Flasks 2.6.2 Investment powders 2.6.3 Safety and storage of investment powders 2.6.4 Checking the condition of the investment: the ‘gloss-off’ test 2.6.5 Mixing the investment 2.7 Dewaxing the flask 2.8 Burnout 2.8.1 The burnout cycle 2.8.2 Behaviour of calcium sulphate-bonded investment during burnout 2.9 Melting 2.10 Casting 2.10.1 Test for system temperature 2.10.2 Inspection criteria 2.10.3 Test for best feed sprue design 2.10.4 Casting with stones in place 2.11 Cooling and recovery of cast items 2.12 Summary of the basic guidelines for each step of the process 2.13 Schematic list of possible defects

13 13 14 15 16 19 20 21 21 21 23 24 25 26 28 29 29 29 30 33 33 33 35 36 36 36 37 38 39 41 42 42 44 45 46 48 49 50 51 52 53 56

3 Alloys for Investment Casting 3.1 Yellow and red gold alloys 3.1.1. Metallurgy and its effects on physical properties 3.1.2 High carat golds with enhanced properties 3.2 White gold alloys 3.3 Influence of small alloying additions 3.3.1 Improving properties 3.3.2 Effect of individual additions

4 Equipment 4.1 4.2 4.3 4.4 4.5 4.6

Vulcanisers Wax injectors Investing machines Dewaxers Burnout ovens Melting/casting machines 4.6.1 Comparison between centrifugal and static machines 4.6.2 Centrifugal machines 4.6.3 Static machines

59 59 55 64 64 67 67 68 75 76 77 78 79 79 81 81 82 83

5 Sources of equipment and consumables

6 Further reading 7 Acknowledgements 8 World Gold Council technical publications

89 97 102

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PREFACE Investment (or Lost Wax) Casting is one of the earliest processes developed by man, dating back 6,000 years or more. Today, it is the most widely used process in jewellery manufacture but probably the least understood by practitioners of the art. It comprises a series of process steps, each of which must be performed properly, if good castings are to result. It never ceases to surprise me just how many casters do not realize what quality of casting it is possible to achieve consistently, if each process step is done carefully in a controlled manner. There are comparatively few good manuals on investment casting. Many date back some years and focus on centrifugal casting. Our first WGC technical manual, the Investment Casting Manual, was published in 1995 and has proved popular. Since then, there have been substantial developments in the technology and our understanding of the process. Thus, we considered it timely to update it, particularly as stocks of the original are running out. This Handbook is the result. It has given me great pleasure to work with Valerio Faccenda and Dieter Ott (Chapter 3) in the production of this Handbook. Both Valerio and Dieter are well known to many of you as experts in jewellery technology, especially in investment casting, with each contributing several articles to Gold Technology magazine over the years and presenting at several WGC International Technology Symposia at Vicenza, Italy. Valerio, as a technical consultant to World Gold Council, has also presented at many WGC technical seminars in countries around the world. He is, of course, author of the Finishing Handbook. Dieter has made major contributions to our understanding of the Investment Casting process and to the metallurgy of the carat gold alloys and is author of the Handbook on Casting and Other Defects which complements this Handbook. Both have presented at the prestigious Santa Fe Symposia, Dieter on many occasions. I know that this Handbook will become a classic in the jewellery field and meets a demand for a good comprehensive and authoritative book on the subject. I know you will find it useful and enjoyable. I must also mention Giovanni Baralis who translated this Handbook from Italian into English. Whilst known to relatively few of you, Giovanni has been responsible for translating Gold Technology into Italian over many years. He certainly makes my job easier. This Handbook is the seventh in the range of technical publications published by World Gold Council. These are designed to assist the manufacturing jeweller and goldsmith to use the optimum technology and best practice in the production of jewellery, thereby improving quality of the product, reducing defects and process time which, in turn, results in lower costs of manufacture. We believe that it is important for the practising jeweller to understand the technology underpinning his or her materials and processes if he or she is to achieve consistent good quality. That is one aim of these Manuals and Handbooks – not only to provide good basic guidelines and procedures but to explain, in simple terms, why they are important and how they impact quality. Armed with such knowledge, a jeweller should be better able to solve problems that will inevitably arise from time to time. Christopher W. Corti, London, April 2003

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Handbook on Investment Casting

GLOSSARY Note: Certain technical terms exist in two spellings (e.g. carat or karat, mould or mold), reflecting English and American common usage. In this Handbook, the English versions have been used, although both are given in this Glossary. Accelerator: Compound which speeds up setting of investment, mainly to increase the productivity. In general it is based on crystalline substances such as sodium chloride, sodium citrate, Rochelle salt. Alloy: A combination of two or more metals, usually prepared by melting them together. They are designed to have certain desired properties, e.g. strength, hardness, ductility, colour, etc. Annealing: Restoration of softness and ductility to metals and alloys after cold working by heating to a temperature that promotes recrystallisation. Assay: The testing of items to determine their precious metal content, e.g. by fire assay or other analytical technique. Base metal: The non precious metals in a jewellery alloy. For instance in a gold-silver-copper-zinc alloy, copper and zinc are the base metals. Binder: A substance used to hold together the investment powder, e.g. for casting jewellery, this can be the Plaster of Paris (q.v.) or acid phosphate. Burnout: The firing of the invested flask at temperature in an oven after dewaxing (q.v.), to condition the mould for casting and to completely eliminate any residual wax or other model materials. CAD/Computer Assisted Design: A sophisticated software system for bi-dimensional or threedimensional designing. CAM/Computer Assisted Machining: A software system for automated machining of a component, driven by computer software, typically from a CAD system. Carat/Karat: A unit for designating the fineness of gold alloys, based on an arbitrary division into twenty-four carats. Pure gold is 24 carat or 100% pure. A 75% gold alloy is 18 carat and so on. (The carat is also a unit of weight for gemstones, equal to 0.2 grams). Carat/Karat gold: A gold alloy which conforms to national or international standards of fineness and can be legally marked or hallmarked. Castability: The ability of a molten alloy to be poured into a mould, retaining sufficient fluidity to fill the mould completely and to take up an accurate impression of the details of the mould cavity. Casting: This word can have two different meanings: 1) the process of pouring a molten metal in a mould; 2) the metallic object taken out from the mould, after solidification of the cast metal. Casting grain: Metals or, more usually, alloys prepared for melting and subsequent casting by dividing the charge material into small particles (like gravel) by pouring a melt into water to form shot or grains. Casting temperature: Temperature at which power is switched off and the molten alloy is poured into the mould. Centrifugal casting: A method for casting metals in which the molten metal is driven by centrifugal and tangential forces from the crucible into a heated mould whilst both are rapidly rotated. Chilling factor: Cooling capacity of a mould calculated from volume specific heat of the mould material and the mould/melt temperature difference. Value for gypsum binder - low; for silica medium; for cold copper - very high. Cold work/working: Deformation of a piece of metal or alloy to effect a change in shape at temperatures sufficiently below the annealing temperature to cause work (strain)-hardening, usually with a loss in residual ductility. The amount of cold work imparted is often measured in terms of reduction in cross-sectional area (e.g. wire drawing) or in thickness (e.g. rolling of strip). Cristobalite: The highest temperature phase of silica, stable and with high strength retention from 1470°C (2678°F) to the melting point, 1700°C (3092°F). De-airing: Removal of air bubbles from an investment slurry, to avoid bubble defects on the final casting. Assisted by vibration and/or vacuum. Devesting: Separation of the cast tree from the refractory mould. This can be done by quenching the flask in water or by hammering or with high pressure water jets, depending on the refractory type. Dewaxing: The removal of the largest part of the wax from the invested mould. It can be done dry, in an oven, or wet, with steam.

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Dross: The scum that forms on the surface of molten metals, due largely to oxidation, but sometimes also to the rising of impurities and inclusions to the surface. Feeding: The necessary process of introducing molten metal through suitable channels (the sprues) and into the cavities of the mould to fill them and to compensate for contraction (shrinkage) as castings solidify. Can be gravity assisted, or otherwise pressurised. Feed sprue: A system of wax rods connecting the central sprue to the pattern to be cast. It forms the channel for the melt to fill the mould cavity. It should be kept as short as possible and must not freeze prematurely. Its junction with the pattern is called the “gate”. Fineness: Precious metal content expressed in parts per thousands (‰). 18 carat is 750 fineness. Flask: The outer metal container of an investment casting mould, used from the investment process through to extraction of the cast tree. It is available in standard sizes and reusable. It may be a solid cylinder or a cylinder perforated with holes to allow escape of air from the mould under vacuum. Fluidity: Complex property describing the ability of a molten alloy to run into a mould and take up an accurate impression of the mould cavity. It generally increases with superheat, freedom from oxidation and some alloying additions (such as zinc & silicon). Flux: Inorganic mixture applied to melt surface to protect the melt from oxidation. It should melt at a temperature lower than melting temperature of the alloy. Form filling: The ability of a molten metal to fill the mould cavity completely. Fuel gas to oxygen ratio: The volumetric flow ratio matching the molecular ratio for complete combustion. With a hydrogen/oxygen flame a ratio of 2 gives a neutral flame with a sharp inner cone. A lower ratio gives an oxidising flame; a higher, a reducing flame. Furnace: See Oven Gate: The part of the feed system that controls the flow of metal from the feed sprue to the pattern. When it freezes, it is closed and no more metal can pass that point into the pattern. Gloss time /Gloss-off time: The time between the addition of the investment powder to the water and the moment where the slurry surface loses its “gloss”. It denotes the start of setting of the investment. Gloss-off test: A test for determining the gloss-off time of a batch of investment powder. Useful in defining or eliminating possible causes of casting problems and defects. Grain: See “casting grain”. It can also refer to the tiny crystals - or “grains” - forming the bulk or microstructure of a metal or alloy. Graining: The process of preparing casting grain, normally by pouring the molten alloy into water. Grain control/ grain size control: The metallurgical procedure to keep the grain (crystal) size of an alloy under control, by addition of particular metals or compounds (grain refiners, q.v.). Grain refiner: An addition of suitable metals or compounds to control the grain size of an alloy during solidification or annealing (recrystallisation). Grain size: Dimension of the crystalline grain of metals and alloys. In jewellery alloys, a fine grain size is usually preferred. Gypsum: Calcium sulphate, used as a binder in investment. Gypsum-bonded (investment): The traditional refractory investment based on silica powder bonded with Plaster of Paris (selected hemihydrated calcium sulphate) mainly used by jewellery industry for investment casting of gold alloys. Hallmarking: The stamping of precious metal articles by an independent assay office to show the fineness of that article. Term derives from the U.K. for marks applied by Goldsmiths Hall -’marked by the Hall’. Term is often used loosely to describe a mark applied by a manufacturer to show fineness in non-hallmarking countries. Heat treatment: A treatment given to metals and alloys, involving a combination of temperature, time, heating and cooling, to effect a change in microstructure and other properties. Hot shortness: Brittleness at high temperature, often intergranular, caused by either low melting point segregates or other non-ductile grain boundary constituents. Hygroscopic: A material possessing a marked ability to absorb water vapour from the atmosphere. Some compounds can react with atmospheric water vapour to form new compounds (e.g. calcium sulphate hemihydrate forms calcium sulphate dihydrate). Gypsum-bonded investment is hygroscopic and should not be left exposed to the atmosphere.

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Handbook on Investment Casting

Inclusion: Non-metallic particle that is found in a metallic body. It can be generated from fragments of extraneous materials (e.g. refractory from furnace crucible or mould) or from the reaction of the metal with foreign materials (e.g. atmospheric oxygen, sulphur compounds generated from investment reaction, etc.). Induction melting: Heating to above the melting point by generating eddy currents within a conducting material surrounded by a water-cooled copper coil carrying an alternating current at low (500 Hz) or high (>100 kHz) frequency. Also creates a stirring effect in the melt by induced electromagnetic forces. Investment/Investment powder/Investment mould: The investment is a mixture of fine silica powder and a binder, formulated to withstand the high temperatures of burnout and casting. For the investment casting of gold jewellery, the commonest binder is gypsum, in its hemihydrated form (Plaster of Paris). Besides these main ingredients, commercial investment contains small amounts of other chemicals (modifiers), designed to impart to the investment the required characteristics for optimum performance. When mixed with water to form a slurry, the binder undergoes a hydration reaction (like cement) to set the investment into a solid mould. Liquidus temperature: The temperature above which an alloy is completely liquid, i.e. no more solid metal is present. Below liquidus temperature there is an increasing proportion of solid phase until at the solidus temperature no liquid remains in equilibrium. Lost wax: Original name for investment casting. A wax model (or pattern) forms the cavity in the investment. Then the wax is melted out before firing of the mould. So the wax is "lost", from which the name of the process derives. Master alloy: A premixed metal alloy (q.v.) that is added to fine gold to produce the final carat gold alloy. Generally contains silver and copper with other additions, e.g. zinc, nickel, palladium, deoxidisers and grain refiners. Master model: The master model is the reference model for the design and can be made of wax or plastic or metal. Nickel silver or silver alloys are frequently used. Metal models can be rhodium plated to improve wear and corrosion resistance. CAD/CAM systems can also be used to produce master models. Melting range: The temperature interval between the solidus and liquidus temperatures (see Solidus temperature and Liquidus temperature). Mould/Mold: A hollow object, containing a cavity that is the outer form of the piece(s) to be reproduced by wax injection or by metal casting. In the case of investment casting, the mould can be made of various materials, e.g.: metal, rubber (for wax patterns) or refractory investment (for casting). Mould clamp: A pneumatic device for maintaining a constant clamping pressure to a rubber mould during wax injection. Mould/Mold Frame: A metal frame, usually rectangular (but can be circular), used to contain the rubber layers and master model during production of the rubber mould in the vulcanising press. Negative tolerance: Used in the context of standards of fineness. It implies a small allowance in precious metal content below the specified minimum that is acceptable in some countries. Oven: A furnace where a controlled and relatively uniform temperature can be held for the required length of time. It can be heated by combustion of a suitable fuel (e.g. natural gas, propane, etc.) or by electrical resistance elements. The temperature is controlled through suitable regulators. For burnout, the oven should be of the muffle type with a large volume to contain several flasks. It may be fan-assisted and/or have a rotary hearth to aid temperature uniformity. Overheating: When the temperature of the material becomes too high. Not to be confused with superheat (q.v.). Overheating is an unwanted and potentially detrimental occurrence. The overheated material can begin to decompose or react with other materials into which it comes in contact. Overheated melts can oxidise more readily. Pasty zone: The pasty zone corresponds to the temperature range between the liquidus and the solidus (q.v.). In this temperature range the metal is not fully liquid nor fully solid. It is in a "pasty" state. Compensating shrinkage by feeding liquid alloy under these conditions may be difficult. Pure metals and eutectic alloys do not show a pasty zone. Pattern: A master (usually metal) or consumable (lost wax process) model of a component that is to be reproduced by casting. Pattern dimensions may need to allow for net shrinkage or expansion over the whole casting process. Phosphate-bonded (investment): Investment with acid-phosphate and magnesia, which first gels silica flour and then bonds it by subsequent dehydration. It is used preferably for high melting point alloys, e.g. palladium white gold and platinum. Pickling: The process of dissolving away surface oxides and flux by immersion in a suitable dilute acid bath (‘pickle’). Normally used for cleaning cast trees, soldered or welded parts or scrap (before melting). Handbook on Investment Casting

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Plaster of Paris: White powder of calcium sulphate hemihydrate (2CaSO4.H2O). It is obtained by suitable heat treatment at relatively low temperature of the mineral gypsum (calcium sulphate dihydrate - CaSO4.2H2O). It reacts with water to form the more stable dihydrate. This reaction is used for investment setting. Porosity: Network of holes in castings, often at the surface, caused by entrapped or dissolved gas or by shrinkage on solidification. Protective atmosphere: An oxygen-free or low oxygen gas atmosphere used to protect a material from oxidation during melting, casting, soldering, welding or heat treatment. Quenching: Fast cooling of a hot material by rapid immersion in a suitable fluid, such as water, oil, or even air or other fluid mixture. The quenchant is usually water for carat gold alloys. Rapid prototyping: Modern technique for producing prototypes with automated machines driven by CAD/CAM systems. Many quite different techniques of rapid prototyping have been developed. A modern method for producing a master model. Reducing flame: A torch flame with excess fuel gas in comparison with available oxygen. Often used for shielding molten metal from oxidation. Refractory: High melting point inorganic (ceramic) material used for furnace linings, crucibles or moulds, usually based on graphite, oxides, nitrides or silicates. Often needs a suitable binder to hold the refractory particles together. Preferably, it should also be resistant to thermal shock and chemical attack. Retarder: Many organic compounds and colloids retard the start of setting of gypsum-bonded investment. This increases the available working time (q.v.). Scrap: Any redundant or reject metal/alloy from a manufacturing operation, that may be suitable for recycling as feedstock to the primary operation. Segregation: The non-uniform distribution or localized concentration of alloying elements, impurities or precipitates within the alloy microstructure, originating from solidification or heat treatment. Setting time: The length of time the investment slurry requires to set, harden or cure. Shot: See Casting grain. Shrinkage: Volume contraction of a molten metal during solidification, typically about 5% for carat golds. Can give rise to porosity in investment castings. Silica: Silicon dioxide selectively processed for producing refractory and abrasive materials. Exists in the vitreous state or as quartz, tridymite or cristobalite phases in equilibrium at increasing temperatures. Silicone rubber: A heat stable, flexible material containing organic radicals and silicon. Can be used in the place of natural rubber for making rubber moulds. Can also be used for heat resistant sealing gaskets. Silicosis: A serious lung disease caused by the inhalation of very fine silica (SiO2) particles. Precautions must be taken when handling investment powders. Soaking: Holding the material in an oven at a constant temperature for the purpose of obtaining a uniform temperature throughout the mass. Solidus temperature: The temperature below which an alloy is completely solid, i.e. finishes freezing on cooling or begins to melt on heating. Above the solidus temperature there is an increasing proportion of liquid phase until at the liquidus temperature no solid remains in equilibrium. Spalling: The breakaway of the surface of the mould due to internal or external stresses, mechanical and/or thermal. Can be a sign of a weak investment. Sprue: Main, central pouring channel in the mould. It forms the stem of the tree and is connected to the castings through the feed sprues (q.v.). It is obtained by melting the wax sprue used to build the wax tree (q.v.). Sprue base: A pad, often of rubber, that makes a bottom for the flask during mould making. The cone (or hemisphere) on a sprue base makes the recess that will be the pouring basin for the molten metal. Superheat: Difference between the melting point / liquidus of an alloy and the casting temperature, required to allow the molten metal to fill the mould without premature freezing. Experience shows that it should be as low as possible to avoid overheating (q.v.). Third hand: Mechanical device, usually fixed to the workbench, to assist in rubber mould cutting.

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Handbook on Investment Casting

Tree: See Wax Tree Vacuum: A space in which the pressure is much lower than normal atmospheric pressure. Vacuum can be applied to remove air from the mixed investment slurry or to "suck" molten metal in the flask. Vulcanisation/Vulcanization: A chemical reaction of sulphur (or other vulcanising agent) with rubber to cause cross-linking of polymer chains. It increases strength and resiliency of the rubber. Performed as a step in making rubber moulds from the master model. Vulcaniser/Vulcanizer: A piece of equipment used to carry out the vulcanisation, i.e. to produce a rubber mould around a metal master model. Essentially, a press with heated platens. Water blasting: Surface treatment in which high pressure water jets are used to remove the investment from a cast tree. Water quality: The content of ionisable (dissolved) salts and organic matter in the water. Important for mixing of investment slurry, it should be accurately controlled, because it affects the working time and gloss-off time (q.v.). Deionised water is the preferred water quality. Water temperature: The temperature of the water mixed with investment powder. It should be accurately controlled, because it affects the working time and gloss-off time (q.v.). Wax: Any of a group of organic substances resembling beeswax. In general they are formed by esters of fatty acids with higher alcohols. Mixtures of different composition are used to obtain the required properties for making patterns for lost wax casting (melting point, hardness, flexibility, etc.). Usually, the different wax types are differentiated by colour. Wax injector/ Wax pot: Equipment containing molten wax under pressure for injecting into rubber moulds to replicate the desired patterns. Often has a vacuum facility for removing air from the mould prior to injection of wax. Wax pattern: Wax replica of a master model, usually produced by injection of molten wax in a rubber mould. The solidified wax patterns are removed and used in the assembly of a wax tree, which is then invested, to form an investment mould. Wax tree: The assembly of wax patterns on a central sprue, from which the investment mould will be made. Usually shaped like a tree, hence the name. Wettability: The ability of a solid surface to be wetted when in contact with a liquid. Wettability is high when the liquid spreads over the surface. It is related to surface or interfacial tension. Working time (investment): Time available for the preparation of the invested flask. It includes: mixing investment with water, de-airing, pouring the slurry in the flask and de-airing again. In the whole it should be about 1 minute shorter than the gloss time (q.v.).

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The oldest example of a gold investment casting: The Onager or wild ass, cast in electrum (the natural alloy of gold-silver), part of the rein ring from the sledge-chariot of Queen Pu-Abi. From the royal tomb at Ur, Mesopotamia, dating to about 2,600 B.C.

INTRODUCTION

1

1 INTRODUCTION Investment casting is probably the first process used by man for jewellery production, dating back over 6,000 years. This happened long before man used the same process for manufacturing weapons or other objects. Perhaps uniquely, investment casting is the only manufacturing process that has been used first for jewellery production and then subsequently for other production fields, like the mechanical engineering industry. Investment casting is also named lost wax casting: this latter name reminds us that we start from a wax pattern that is invested with a refractory material to form a mould. The wax pattern is then removed by melting (the wax is ‘lost’!) leaving a negative impression in the mould, into which the molten metal is subsequently poured. The word “investment” in the context of investment casting has nothing to do with financial investment. It refers to the fact that the wax patterns are “invested”, i.e. coated, with a refractory material. After setting of the refractory, the wax is melted out and molten metal can be poured in the cavity that accurately reproduces the shape and size of the wax pattern. The cast metal item accurately reproduces also the fine details of the wax model.

1.1

Figure 1.1.1 Greek ring, fourth century B.C. (Schmuckmuseum, Pforzheim)

DEVELOPMENT OF THE MODERN PROCESS

All past civilisations left us wonderful examples of investment cast jewellery. Such jewellery specimens have been found in the treasures of the Pharaohs of Egypt and in Atzec and Inca tombs of Central and South America. Also, in Europe, the ancient Etruscans, the Greeks, Figure 1.1.1, the Romans and the Byzantines, Figure 1.1.2, left us investment cast jewellery, and later, during the Renaissance, the great Masters created wonderful masterpieces. The starting point for the utilization of investment casting in industry has been the application of the refractory investment in the form of a fluid slurry, invented near the end of 1800. But, until the middle of the past century (I refer to the 20th century!), investment casting has been used almost exclusively for the production of one-off pieces for the very few persons who could afford it. Around the middle of the past century, three major breakthroughs made investment casting an industrial process, usable for mass production. The first breakthrough has been connected to automatized chain making. Whilst this process is not related to investment casting, it enabled production of large quantities of jewellery (chain and bracelets) and favoured the access of jewellery to the field of fashion and to an ever wider market. The second breakthrough has been the invention of flexible rubber moulds, for the mass production of wax patterns, by the Canadian, T.G. Jungersen. This invention was rapidly patented in the USA, in 1944, Figures 1.1.3 and 1.1.4, and allowed goldsmiths to reproduce intricate objects, with marked undercuts, without problems or limitations. Finally, the third breakthrough has been the realization that the casting machines developed for use in dentistry, with minor modifications, could be used also for the industrial production of jewellery. These were spring-driven centrifugal casting machines and explain why, even today, centrifugal casting machines are widely used for jewellery production, in spite of the advent of static casting machines, particularly in the last decade. Handbook on Investment Casting

Figure 1.1.2 Byzantine earring, sixth century A.D. (Schmuckmuseum, Pforzheim)

Figure 1.1.3 Patent for elastic rubber moulds, registered in USA in 1944

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INTRODUCTION

Figure 1.1.4 Description of the mould and of the centrifugal wax injector, from the patent of Figure 1.1.3

After flexible rubber moulds and casting machines were made available, a simple optimisation of the consumable materials has been sufficient to allow a profitable industrial utilisation of investment casting. In particular, we refer to the wax and investment powder. The wax types used for dental applications were too brittle and cracked easily during the extraction of the wax pattern from the rubber mould, especially when marked undercuts were present in the pattern. In this case, a correct balance of properties had to be sought, to develop a product that could be used without particular problems. The investment used in dentistry was too expensive for the goldsmith, who didn’t need the high dimensional precision required for dental applications. Therefore, less costly, but in no way inferior quality, investment types have been developed to meet the requirements of the goldsmith. We refer here to silica-based, calcium sulphatebonded investment powder. Since investment casting developed into an industrial process, it has become ever more widely used. Today, we can say that at least 50% of jewellery worldwide is produced by investment casting, as a result of the remarkable technical progress made, whilst the ancient, time honoured basic concepts remain unchanged, Figures 1.1.5-1.1.7. As a result, investment casting has an aura of fascination, still preserving the artistic and craft aspects of jewellery items.

1.2

Figure 1.1.5 Modern investment cast jewellery object: hinged pendant with clasp (Pomellato Spa, Italy)

Figure 1.1.6 Hinged bracelet: the single links have been investment cast (Pomellato Spa, Italy)

Figure 1.1.7 Investment cast pendants for young people. Their weight ranges from 1 to 3 g (Pomellato Spa, Italy)

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THE MODERN PROCESS AND PRODUCT QUALITY

Investment casting is very versatile: both simple and intricate shapes can be produced in small or large numbers. It is not costly: often, when we take into account the cost of a good die, pieces that could be cold forged are more economically produced by casting. However, investment casting is not a simple process. There are many metallurgical principles we must consider and comply with in the many steps of the process, if we are to obtain a good quality product. These steps are made more complicated by the small size of the castings, which makes process control somewhat difficult. Quite often, the goldsmith focuses his attention on the melting and casting stages; these are only the final steps of a multi-stage process but, very commonly, a defective or unsatisfactory product will be obtained, if all process steps preceding the final ones have not been carried out correctly. Some years ago, World Gold Council, with the Santa Fe Symposium, supported research by the German Institute of Precious Metals into the defects occurring in the production of jewellery pieces. This study showed that about 80% of defective jewellery pieces had been produced by investment casting and that more than 50% of the defects were due to porosity, a defect typical of the investment casting process. The most important results of this research were collected together as case studies in the Handbook on Casting and Other Defects, published by World Gold Council, where the most common defect types are described, along with an exhaustive explanation of their origin and useful recommendations for their prevention. This Handbook is a very useful and essential complement to the present Handbook, which is focused on the process. Investment casting is a very ancient process; nevertheless, in its modern form it is not an easy process to control. We mentioned that the small size of the castings we want to produce is a problem. In Figure 1.2.1 we see the progress of solidification in a ring with a large head. From the first to the last picture, only about 10 seconds have elapsed. Solidification is completed in less than 1 minute. This experiment to observe Handbook on Investment Casting

INTRODUCTION

the progress of solidification was conceptually very simple: molten metal has been poured in the mould, and the liquid metal remaining after a pre-established time has been removed by centrifuging. The shortest time has been about 1 second after pouring. After centrifuging, the mould was opened and the amount of solidified metal was evaluated. These pictures show that the solidification process is very fast and, consequently, its control is nearly impossible. Therefore, it is clear that the last steps of the overall casting process should be carried out under the best possible conditions, in addition to the correct execution of all preceding steps. We would be foolish to believe that a completely automatised latest generation melting/casting machine, centrifugal or static, with vacuum and pressure assist, can compensate for carelessness in the preceding steps of the process. The machine will help to achieve a consistent quality of the product, but it will never be able to attain a good quality level, if errors have been made in the preceding steps of the process or simple metallurgical principles have been ignored.

1.3

1

Figure 1.2.1 Development of solidification in a gold alloy ring: a - about 1 second after mould filling

CHOICE OF EQUIPMENT AND CONSUMABLES

The modern goldsmith can choose from a wide range of equipment, from the vulcanisers, wax injectors, investment mixers and burnout furnaces, to melting/casting machines, which represent the largest capital investment. With regard to melting/casting machines, two types of equipment are commercially available that differ in the origin of the force that pushes the melt in the mould: centrifugal machines and static machines. There are no special reasons to prefer one type of machine to the other: both types can produce a high quality product. The main differences between centrifugal machines and static machines will be briefly summarized in Chapter 4, devoted to the equipment, but the final choice should be made by the goldsmith, based on his needs. This choice will depend on the amount of money he is willing to invest, on the type and quantity of product to be produced and, particularly importantly, on the level of technical after-sales service guaranteed by the supplier. A fundamental consideration: a decision taken to purchase new equipment because the current product shows too many defects can be a big mistake! Before considering new equipment, it is absolutely necessary to make a thorough scrutiny of the present production process. When (and only when) we are sure of obtaining the best performance from the existing equipment, can we think to make an investment in new equipment. At this time, when the market offers more and more automatised equipment, there is the danger of committing the full responsibility for product quality to the equipment. The results of such an attitude can be disastrous! Therefore, the most important rule for achieving good results is always to engage your brain and to scrutinize your current process constantly and accurately. Investment casting should never be considered as a routine process. No detail of the process should be neglected, even if, at first sight, it could appear unimportant. In the course of the production process, the goldsmith uses not only equipment, but also various consumable materials: the rubber for making the moulds, the wax for the wax patterns, the investment for filling the flask and, lastly, the alloys. All these materials are the outcome of careful study: they should be selected and used correctly, carefully following the recommendations of the producer on their use. Handbook on Investment Casting

b - after 3 seconds

c - after 7 seconds

d - after 10 seconds

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INTRODUCTION

If the results are unsatisfactory, extemporaneous inventions should be avoided. Please refrain from trying to transform your production workshop in a research laboratory! There is the risk of a further worsening of the problem and of increasing mental confusion! Time can be saved and results improved if we involve the producers of the various materials directly in the problem: generally, the producer is the first to be concerned about the results obtained by use of his product. Usually, he will be able to detect possible errors and recommend suitable corrective action, enabling you to save time and money.

1.4

HEALTH AND SAFETY

We have discussed the complex nature of the investment casting process and the need to ensure the correct procedures are followed at each stage. There are health and safety issues that need to be addressed. It is vital that the interests of the workforce are protected if good quality and productivity are to be ensured. Some of the materials may be hazardous or toxic. Of particular note is that related to handling of investment powder and its removal after casting. This material causes silicosis! Engineered control of investment dust or the use of a respirator, approved for silica dust protection, is essential. Respirators must be properly fitted to each worker, who should be trained in its care and use. Other hazards include hot metal handling, electrical and chemical, etc. Suitable precautions must be taken, including provision of protective clothing and implementation of rigorous safety procedures. These issues will be discussed later in more detail.

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2. THE PROCESS OF INVESTMENT CASTING Investment casting is a typical example of a multistage process. We can list at least 13 separate steps from the initial design to the finishing of the jewellery: 1 – Design 2 – Making the master model 3 – Making the rubber mould 4 – Production of the wax patterns 5 – Assembling the tree 6 – Investing the mould 7 – Dewaxing the flask 8 – Burnout 9 – Melting 10 – Casting 11 – Cooling 12 – Cutting the cast pieces off the tree 13 – Assembly and finishing of the jewellery. With the exception of the last two steps, all other steps directly or indirectly involve metallurgical concepts that should be respected, if a good quality product is to result. The process does not tolerate errors: any careless operation, any apparently innocuous shortcut is a potential source of defects in the finished product. Later, if a defect is observed in the casting, very seldom is the root cause readily found and the proper corrective action identified, because of the complexity of the process. Temperature is an important process parameter in many of the process steps; often the goldsmith tries to improve a situation by changing the temperature, for example of the metal and/or the flask. Usually, a simple temperature change does not solve the problem, but it certainly changes the operating conditions and makes defect diagnosis more difficult. When we have to deal with a defect in our castings, we should first consult the Handbook on Casting and Other Defects to help determine the type and possible causes. The number of defect types is not infinite and many of them, particularly the most common ones, are described in the Handbook. In this way, it is usually possible to identify the defect type and its possible causes correctly. The second step is to scrutinize the process parameters to narrow the possibilities by elimination. Finally, we can try to identify the root cause of the problem. Only at this point can we decide the proper corrective action. Because of process complexity, a defect does not usually originate from a single, simple cause, but from a group of causes that are not necessarily located in a single process step, but over several process steps. Therefore, systematic process data recording is very useful and we never should take anything for granted. The human factor is fundamental for achieving good results. I believe it to be not far from the truth by saying that the contribution of the goldsmith to the achievement of good quality is not less than 80%. The remaining 20% is represented by the equipment, which should be well maintained and reliable. Handbook on Investment Casting

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In this chapter we will discuss each step of the process, with particular attention to the rules or guidelines to follow and to the most common problems that can arise. Later, in separate chapters, we will describe the characteristics of the most commonly used casting alloys and of the different equipment types. We will also give some basic guidelines for making a correct choice.

2.1 Figure 2.1.1 a Design of a ring in 3 parts by means of CAD technique. (Courtesy of Pomellato Spa.)

b

c

d

DESIGN

Design represents the moment of creation, the birth of the idea for a new jewellery product. Although we can cast very complex shapes, thanks to modern technology, the designer should always have a good knowledge of the casting process, so that he/she can design pieces that are easily cast. In the design phase, it is also important that the designer be in regular contact with the caster on the shopfloor who will produce the casting. Today, the design operation can be facilitated by the use of Computer Aided Design (CAD) systems, which enable a dimensioned drawing to be obtained, used for making the master model, Figure 2.1.1 (a – e). Such CAD software is not easy to use by inexperienced persons. Specialised knowledge is required. Small workshops can seldom afford such facilities, but it is possible to access CAD service through a reliable CAD service centre. Considerable advantages can be obtained with the use of CAD systems, e.g. the ready availability of a dimensioned drawing is a great help to the work of the model maker. Moreover, if we use a CAD system, we can also use a Computer Aided Manufacturing (CAM) system and/or one of the many available Rapid Prototyping (RP) methods, Figures 2.1.2 – 2.1.4, for making a first master model, typically in wax or plastic or even metal. With regard to the creative design phase, we should remember that many production problems originate from lack of communication between the designer and the caster. This insular approach is no longer acceptable in a modern jewellery company. A good ‘rule’ says that the relevant production staff should be involved when a new jewellery design is discussed, to scrutinise for potential problems that could arise in the production process. This should be done before the new jewellery design is launched on the market. Good quality starts right from the very beginning! At the Santa Fe Symposium of 1995, in a discussion on the way to shorten the time between the idea and the realization of the product, J. Orrico, Director of Jewellery Manufacturing at Tiffany & Co., said: “Sure a CAD machine will be great. But realize, even though it is an extremely powerful tool, it can only facilitate the process. A round table can do the same thing. If you can justify a CAD machine, great. If not, everyone has a table. The process needs to cut across organisational boundaries to be truly effective. Get started today!” This very simple, easily implemented recommendation should be always present in our mind if we want to achieve a high quality level: it is fundamental to establish a symbiotic relationship among the different departments in the company.

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MAKING THE MASTER MODEL

The quality of the master model is of fundamental importance for the achievement of good quality product: it should be perfect, with a perfect finish. It should not show the slightest defect, because any surface defect will be replicated in the rubber mould and, in turn, on the wax pattern, on the refractory mould and finally on the castings. In most instances, a defect can be removed in the jewellery finishing stage, to obtain the desired quality level, but it requires time and money. However, such a defect limits the use of mechanized finishing. Such finishing is done by hand, with a resulting waste of time and an increased production cost.

2.2.1 Alloy of manufacture The use of an alloy with suitable high hardness is recommended for manufacturing the master model: finishing of the model will be easier, with a better wear resistance. We should remember that, if the jewellery design is a commercial success, the master model will be used for making many rubber moulds. Therefore, good wear and corrosion resistance are important characteristics for a master model. The use of nickel silver (nickel 50%, copper 30%, zinc 20%) is recommended. Many goldsmiths use sterling silver (silver 92.5%) to make the models, because they are accustomed to cast and work this alloy. The only drawbacks to the use of sterling silver are its low hardness and reactivity with the rubber during vulcanising. No matter what alloy is used, rhodium plating of the finished model is strongly recommended. For silver models, it is essential. Rhodium plating is bright and hard, enabling better finishing, increased wear resistance and making it corrosion and oxidation resistant, particularly in the vulcanisation stage, if conventional rubber is used, Figure 2.2.1. Up to now, we have discussed metal models. With the modern techniques of rapid prototyping, it is now possible, with the aid of CAD-CAM systems, to manufacture models in special plastics that can be used directly for making rubber moulds or for casting a metal master model, in the place of a wax pattern, Figures 2.1.2, 2.1.3 and 2.1.4. Some jewellers use their wax or plastic model produced by Rapid Prototyping to cast the master model in carat gold.

Figure 2.1.2 Heads of a rapid prototyping machine: the red head builds the supporting structure, which will be removed later, while the green head builds the actual model

Figure 2.1.3 Operating diagram of the rapid prototyping machine shown in Figure 2.1.2 Vista laterale = Side view Passo della goccia = Spacing of the drops Diametro della goccia = Drop diameter Direzione del movimento dei jets = Advancement direction of the jets Direzione del deposito dei jets = Deposition direction of the jets Altezza di un layer = Thickness of a layer Altezza della parete = Thickness of the whole deposit

2.2.2 Feed sprue Usually the feed sprue is considered as an integral part of the model. It links the pattern to be cast with the central sprue into which the molten metal is poured. Function of the feed sprue The feed sprue is a very important component of investment casting. It should guarantee perfect filling of the pattern cavities in the mould. Even more important, it should act as a liquid metal reservoir to compensate for the unavoidable volume contraction of the gold during solidification of the cast items. If the feed sprue cannot perform this second function, a defect will form - shrinkage porosity, with its typical dendritic appearance, Figures 2.2.2, 2.2.3 and 2.2.4. This defect can be entirely contained inside the casting and, if this is the case, there are no aesthetic problems. However, as is more often the case, if it appears on the surface of the cast piece, it must be repaired or the item scrapped. Repairing is a delicate operation that can be difficult or sometimes impossible, Figure 2.2.5. The criticality of the feed system changes in accordance with the type of casting equipment. Feed sprue design is more critical with the traditional equipment for Handbook on Investment Casting

Figure 2.1.4 Some models manufactured with the rapid prototyping machine

Figure 2.2.1 Master model of a ring made from nickel silver, rhodium plated

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Figure 2.2.2 Shrinkage porosity in a cross section: the dendritic shape is evident

Figure 2.2.3 Shrinkage porosity in a metallographic microsection, observed under the optical microscope

Figure 2.2.4 Dendrites in a shrinkage cavity, observed under the scanning electron microscope

static casting and a little less critical with vacuum assisted static casting. The difficulty of feed sprue design decreases further with pressure and vacuum assisted casting, the more recent evolution of static casting machine technology, and is minimum with centrifugal casting. When we speak of criticality, we usually refer to form filling in metal casting, because the feed system is never critical for wax injection. Therefore, feed sprues should be carefully designed, Figure 2.2.6, as a function of size and shape of the object to be cast. Given that solidification shrinkage, as a physical characteristic, is unavoidable, the feed sprues, in addition to allowing complete form filling, should be able to “drive” shrinkage porosity out of the cast object. Design of the feed sprue Basically, a feed sprue system is a tube or a set of tubes, wherein the metal should flow as smoothly as possible. Turbulence should be reduced as much as possible: so abrupt changes of cross-section, sharp angles, etc. should be avoided. Turbulence in the flowing liquid metal can cause gas entrapment and gas porosity results from entrapped gas in the casting. In all cases, turbulence causes a pressure drop, thus hampering form filling. Therefore, it is always important to think in terms of fluid mechanics and try to imagine the behaviour of liquid metal as it flows towards the cavity to be filled. Patterns with complex geometry or with abrupt changes of cross-sectional area often benefit from multiple feed sprues. However, the best results are not always obtained with a multiple feed sprue on the master model because, although multiple sprueing can be beneficial during casting, it sometimes does not enable high quality wax patterns to be obtained, in contrast to those obtained with a simpler feed sprue. In these instances, many workshops use models with a single feed sprue for wax injection. Later, the single feed sprue is cut off and the wax pattern is fitted with a multiple feed sprue. A set of rubber moulds of multiple feed sprues of different size and shape can be used for this purpose. These multiple wax sprues can be fitted to the wax patterns as required, in accordance with the type of casting to produce, Figure 2.2.7. The “Y” feed sprue design is the simplest and, from the point of view of fluid mechanics, the best type of multiple feed sprue. When the liquid metal gets to the junction, where it splits into two streams, the metal will not favour one side or the other, unless some other force is involved. Therefore a “Y” is a balanced fluid system. The stem of the “Y” becomes the primary feed sprue and must have enough cross-sectional area to supply ample metal to fill the two secondary feed sprues into which it splits.

Figure 2.2.5 Shrinkage defects in a ring with a large head in a vertical section cut through the ring half-way across the band width. Two defective zones are seen: a diffused one in the head and another one in the opposite part of the shank, near the junction with the feed sprue. After pouring, the side parts of the shank solidify first, because they are thinner. Thus, when the thicker head solidifies, feeding of more liquid metal is no longer possible. The defect on the opposite side is known as a “hot spot”, because the sprue junction is heated by the flowing metal, causing a delay in solidification. This zone solidifies after the feed sprue and both sides of the shank are already solid. So it is not possible to feed liquid metal to compensate for the shrinkage.

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If there is the danger of investment erosion at the point of splitting of the secondary feed sprues, or if the shape of the wax pattern requires a large temperature difference between the liquid metal and the investment, the excessive cooling expected where the metal splits off into the two secondary feed sprues of a “Y” can be relieved by using a “V” design. The wax pattern can be produced with a “Y” sprue, with the stem cut off to form a “V”; this junction is attached directly to the main sprue. With all other parameters constant, the “V” feed sprue will deliver molten metal to the pattern with less temperature drop than the “Y”, because the metal path is shorter and less tortuous. Size of the feed sprue Another important point, also based on the principles of fluid mechanics, relates to the constant cross-sectional area in primary and secondary feed sprues. If, for example, the cross-sectional area of the primary sprue is 8mm2, then the crosssectional area of each of the two secondary sprues into which it splits should be 4mm2 and not 8mm2. The total cross-sectional area remains constant. In this way we can reduce turbulence. There are no formulae to calculate the optimum size of a feed sprue for a given casting. As a practical rule, we can say that the cross-sectional area of the feed sprue should range from 50% to 70% of the cross-sectional area of the pattern it will feed.

2.3

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Figure 2.2.6 Examples of split feed sprues (coloured in red) for correct feeding of liquid metal in a ring. They should be connected to the thicker part of the ring with a heavier head; also, to the model with an inclined angle, to reduce turbulence

MAKING THE RUBBER MOULD

The correct design of the rubber mould is another important step in achieving a good quality product. We can say that there are nearly no limits to the shape of jewellery pieces that can be produced by investment casting with the presently available materials. The only limit is the imagination and the creative power of the person who should design and make the mould. ‘Mould engineering’ is an indispensable skill that should be cultivated inside the jewellery company. By mould engineering, we refer to designing the mould, choosing the correct material, deciding how many parts will form the mould and if metal inserts will be necessary, deciding how the mould will be cut to facilitate the extraction of the wax pattern, with minimum interference with the surface of the pattern itself. In a Handbook such as this, we cannot teach mould-making technology, we can only illustrate it through some examples. Mould-making should be learnt with practice and prolonged, assiduous exercise. We recommend practitioners to attend training courses on this particular subject, for example, those given by the producers of mould rubber. In recent years, there has been a steady improvement in the materials, as has occurred also for wax and investment powder. Therefore, regular updating courses meet the need of understanding the new materials and refining the basic technology.

Handbook on Investment Casting

Figure 2.2.7 a Moulds for making complex feed sprues

Figure 2.2.7 b

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2.3.1 Types of mould rubber Many different rubber types are commercially available, both natural and synthetic and also including the silicone rubbers. Each type of rubber has a different balance of properties and should be chosen for use in specific situations, consistent with the objects to be cast. Usually, natural rubber is stronger and more wear resistant. Silicone rubber is less strong, but enables a better replication of fine detail to be obtained. Two component systems, that are not vulcanisable rubber, have been the most recent to become commercially available. Apparently, they are simpler to use, but they show significantly lower wear resistance compared with other rubber types. The advantages and disadvantages offered by the most common rubber types are listed in Table 1. All types of rubber should be used with care and the recommendations of the supplier should be followed accurately. In particular, vulcanisable rubbers have a finite shelf life. Some of their characteristics can gradually deteriorate when this time has elapsed. The producers recommend storage of the rubber (before vulcanisation) away from heat and light sources, at a temperature not higher than 20°C (68°F). If these simple rules are followed, the rubber will keep its favourable properties unchanged for one year at least. This is what producers guarantee. In practice, if correctly stored, a rubber can last much longer, still giving very good results. All batches of vulcanisable rubber are marked with a code number. In the case of

Table 1 Advantages and disadvantages of different rubber types for mould making Type

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Advantages

Disadvantages

Natural rubber (requires vulcanisation)

Excellent tear resistance Ideal for intricate models Requires only few release cuts Very limited shrinkage

More difficult to cut Requires more time for filling the frame It is relatively soft Gives a matt surface Requires the use of spray or talcum Tarnishing of silver models

Silicone rubber (requires vulcanisation)

The frame is filled easily Easy to cut Different hardness levels available Doesn’t require spray or talcum powder Gives a polished finishing

Requires more release cuts Shrinkage slightly higher than natural rubber Good tear resistance but lower than natural rubber

Room temperature silicone rubber (two components)

Very fine surface finishing Short time for preparation Negligible shrinkage Doesn’t require spray or talcum powder

Suitable only for simple wax or metal models, without undercuts Moderate tear resistance Difficult to burn (to enlarge feed sprue)

Liquid silicone rubber (two components)

Very fine surface finishing Doesn’t require spray or talcum powder Very easy to prepare Can be used with wax models Negligible shrinkage

Difficult to burn (to enlarge feed sprue) Moderate tear resistance High cost

Transparent, vulcanisable silicone rubber

Good surface finishing Transparent Soft and flexible Easily vulcanised

Shrinkage not negligible Costly

“No shrink” pink

Very low shrinkage Very good surface finishing

Vulcanising temperature (143°C +–1°C) must be strictly complied with

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complaints, the producer can trace the production date. Therefore, keeping a record of the code number is important. Above all, we should not store large quantities of rubber and we should use the older batches first (‘first purchased, first used’).

2.3.2. Making the mould Before making the mould, the master model should be carefully cleaned with a degreasing solution in ultrasonic cleaning equipment. In the case of vulcanisable rubber, the mould should be prepared by carefully packing the rubber layers inside a suitable metal frame (preferably forged aluminium). The model is placed in the centre of the rubber layers and is then covered with an equal number of rubber layers, Figure 2.3.1 (a and b). The vulcanising press should have temperaturecontrolled platens, preferably with independent thermostatic control. The calibration of the temperature controller should be checked periodically with a reference thermocouple or some other suitable device. Two types of test should be done: with the first one, we verify that both heated platens are at the same temperature. The test can be carried out by putting a small wood block, the same size of the mould and with grooves on the upper and lower surface, between the platens of the vulcaniser. The reference thermocouple is then inserted in the grooves and temperature is measured at different points of the upper and lower surface. The temperature readings should be the same in all positions. The second test aims to verify the correct calibration of the temperature controller. In this case we can use a small aluminium block, of the same thickness as the mould, with a mid height hole for inserting the reference thermocouple. Then we turn the vulcaniser on and we verify that the pilot light of the thermostat turns on and off at the desired temperature of 152-154°C (about 305-309°F). If the light turns on and off at a different temperature, we should adjust the temperature setting knob until the correct temperature is obtained. An incorrect vulcanising temperature is the most common cause of poor quality moulds or of excessive shrinkage. The recommended temperature for vulcanising natural rubber moulds is typically 152-154°C (about 305-309°F). For the silicone rubber moulds, this rises up to 165-177°C (about 329-351°F). Vulcanising time varies with the thickness of the mould: usually a time of 7.5 minutes per rubber layer is recommended (a rubber layer is about 3.2mm/1/8 in. thick). Therefore, a mould 19mm (about 3/4 in.) thick will require vulcanising for about 45 minutes. With particularly complex master models, if good results are not obtained under the conditions cited above, we could lower the vulcanising temperature by about 10°C (18°F) and double the time. In this way the rubber will remain in a putty-like state for a longer time and will have more time to conform to the model perfectly.

Handbook on Investment Casting

Figure 2.3.1 Steps for making a rubber mould a – The model is positioned in the centre of the mould

b – The mould is completed with other rubber layers

Figure 2.3.2 Protective glove made from stainless steel reinforced fibre for mould cutting a – The glove fits either hand b – Cutting with a protected hand

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2.3.3 Cutting the mould

Figure 2.3.3 Bench fixture to facilitate mould cutting (third hand) a – The “third hand” b – The third hand in use

Figure 2.3.4 Convex ring, with a pronounced internal undercut

Figure 2.3.5 a – A single rubber mould is used to produce half of the ring shown in Figure 2.3.4

To cut the moulds after vulcanising (or curing/setting, for non-vulcanising rubbers), we use blades that should be sharpened or replaced frequently, because the cuts must be sharp and perfect, otherwise we will have moulds that will produce defective wax patterns. To make cutting easier, the blade should be wetted frequently with an aqueous solution of surface-active agents. Two important safety recommendations: the blades are very sharp and so we work with the blade moving away from the hand holding the mould. A second recommendation is to protect the hand holding the mould with a cut-resistant glove, knitted with steel wire, Figure 2.3.2 (a and b). As we proceed with cutting, the cut surfaces should be kept well open, by pulling the rubber strongly apart: this is difficult to do with only one hand. For this purpose, it is very helpful to use a simple, but effective device, called a “Third hand”: it will facilitate your work significantly, Figure 2.3.3. The mould should be cut in different ways, depending on the type of injector used for making the wax patterns. This is to avoid the presence of air bubbles in the wax patterns, which will unavoidably lead to the formation of defects. Presently, injectors are frequently used which exhaust the air from the mould before injecting the wax. In this case, the moulds should be vacuum tight. However, traditional injectors are still used in many workshops that do not use the vacuum technique. In this case, the moulds should have suitable vents cut, enabling the air in the mould to escape at the moment of wax injection. In workshops where both vacuum and traditional injectors are used, problems can arise if the moulds are interchanged between the two types, with unfavourable consequences on the quality of the wax patterns. Teaching how to build a perfect mould is quite difficult in a Handbook, but a few examples are given to show what can be obtained from taking the ‘mouldengineering’ approach. The importance of having a good mould maker in the factory is clearly evident from the following example: the model, Figure 2.3.4, is apparently very simple: a ring with a smooth surface, which has a marked undercut on its inner side. At the insistence of the production department, the initial solution has been to produce the wax pattern in two halves, Figure 2.3.5 (a and b). So there was a single mould for each half of the ring. To produce an entire ring, either two wax patterns are joined together or two half rings are cast in carat gold and soldered together. As we can see from the figure, the mould had locating pegs for connecting the two halves, which were removed after soldering. Both solutions showed considerable disadvantages and required a long finishing operation to obtain an acceptable – but never perfect – quality level. A better solution was found later, thanks to a skilled mould maker, and is shown in

Figure 2.3.5 b – Two halves must be joined to make the entire ring

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b

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c Figure 2.3.6 Mould designed to produce the wax pattern of the ring in Figure 2.3.4 as a single piece

d

e

the Figure 2.3.6. It is a complex mould, formed in several parts, where the part corresponding to the undercut has been cut in such a way as to be easily removed without damaging the wax pattern. The wax pattern is obtained as a single piece, the quality of the product is perfect and finishing labour has been reduced to a minimum. We emphasize an important detail that should always be kept in our mind when cutting a mould. The cut between the two halves of the mould has been done to coincide with an edge of the ring: in this way there are no traces of separation lines on the main surfaces of the wax ring and finishing operations of the casting are simplified. So a significant improvement of product quality and a reduction in manufacturing cost have been achieved. Another example, similar to the one described above, is shown in Figure 2.3.7. In this case, a metal insert has been used to prevent mould deformation during wax

a

b

c

d

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f

g

h

Figure 2.3.7 Mould made of two types of room temperature-curing silicone rubber with a metal insert, to produce a ring similar to the ring of Figure 2.3.4 a – The metal master model b to h – The mould. The metal insert prevents mould deformation during wax injection Handbook on Investment Casting

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Figure 2.3.8 a Preparation of a self-parting mould, first half.

Figure 2.3.8 b – The red hatched zones should be dusted with talcum or protected with other means, because they should not bond during vulcanising

injection, because two types of two component silicone rubber have been used for making the mould, instead of natural rubber: one type for the inner part of the ring and another one for the actual mould. If we do not have a skilled mould maker in our factory, we can resort to a solution that should never be considered as optimum, i.e. to use self-parting moulds. In this case, the vulcanised mould will be opened with the simple action of the fingers. Before vulcanising, the mould is assembled in the usual way, by packing the rubber layers in the frame. When nearly half of the layers have been packed, we put small cubes of vulcanised rubber or metal pegs at the outer edge of the mould. These rubber cubes or metal inserts act as locating pegs for the two halves of the mould. Then the free surface is dusted with talcum powder, Figure 2.3.8 (a & b), or is sprayed with a suitable silicone product, or is covered with a thin plastic film. Then a further rubber layer is added, on which the master model is positioned, Figure 2.3.9 (a & b). The previous operation of dusting with talcum or silicone spraying or covering with plastic film is repeated. Then we also repeat all other operations in an inverted order for the second half of the mould, Figure 2.3.9c. The mould is then vulcanised. After vulcanising, the mould will open by the simple pressure of the fingers and will comprise four parts. Two outer parts - the mould shell - and two thin inner parts, formed by the two inner layers, which are the true mould. These two parts will easily separate from the wax pattern, without damaging it, Figure 2.3.10. In this mould type, the separation line is in the centre and will always leave a ‘witness mark’, which must be removed later. Moreover, this mould type is not suitable for vacuum injectors.

2.3.4 Storing and using the mould

Figure 2.3.9 Preparation of a self-parting mould. a – The model

Figure 2.3.9 b – Positioning of the model in the mould

After making, the mould should be numbered, referenced and stored in a closed container - a drawer or a cupboard - away from sunlight and dust. The mould should always be carefully cleaned after use. It is recommended that a register of the moulds is maintained, where all parameters for the production of wax patterns are recorded for each mould (wax type, wax temperature, injector temperature, vacuum, pressure, cooling time). With some latest generation injectors, it is possible to record these parameters on an electronic chip that is inserted in the mould and is “read” by the injector at the moment of wax injection. When a new mould is made, manufacturing parameters should be recorded with care. If necessary, specific tests should be carried out to obtain a perfect mould. With an optimised manufacturing process, mould shrinkage can be minimized. Recently, some vulcanisable rubber types have become available on the market that are claimed to be “no shrink”. The shrinkage of these rubbers can really be zero or nearly zero, but to achieve this, the recommended vulcanising temperature should be accurately adhered to. If the vulcaniser is not equipped with a very accurate temperature control system, “no shrink” rubber can show some degree of shrinkage, maybe even more conspicuously than with conventional rubber types. This can occur if the temperature is only a few degrees higher or lower than the optimum temperature.

Figure 2.3.9 c – Covering the model

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Table 2 Common problems in the production of rubber moulds Problem

Causes

Remedies

Mould is soft and sticky

Too low temperature or too short time for vulcanising

Check with a suitable instrument the real temperature of the vulcaniser Comply with temperature and time recommended by the producer

The mould is hard and distorted

Pressure is too high, too long vulcanising time and/or too high temperature

Use lower pressure Verify the temperature displayed by the vulcaniser Comply with time and temperature recommended by the producer

The different layers of the mould tend to separate

Pollution of the surface of the rubber layers during mould making (dirty hands, grease, talcum, etc.)

Reject the defective mould and improve cleanliness

Bubbles or depressed areas on the larger surfaces of the mould

Insufficient filling of the frame

Improve filling of the frame

White dust on the rubber surface before vulcanising

Normal occurrence

Do not mind it Do not try to remove it

The rubber is hard and doesn’t vulcanise

The rubber is already partially or completely vulcanised because of an accidental exposure to heat or because of aging

Reject the rubber batch and verify that the rubber is stored correctly

Rubber is hard and stiff

The rubber is “frozen” after a prolonged storage at a too low temperature

Heat very slowly the rubber up to about 38°C (100°F)

Excessive shrinkage

Too high vulcanising temperature

Check with a suitable instrument the real temperature of the vulcaniser Comply with temperature and time recommended by the producer Alternatively, lower vulcanising temperature to 143°C (289°F) and double vulcanising time

The rubber doesn’t fill all The frame has not been correctly packed cavities and undercuts Rubber is too old

Insert small pieces of rubber in cavities and undercuts Check the temperature displayed by the vulcaniser

2.3.5 Common problems Some of the most common problems we can meet when making a mould are listed in Table 2, along with their causes and some simple remedies.

Figure 2.3.10 Details of the self-parting mould of the previous figures, after vulcanisation

2.4 PRODUCTION OF THE WAX PATTERNS 2.4.1 Types of wax The use of a wax with a narrow melting range is recommended. A range of waxes is available to the goldsmith: the wax type should be selected on the basis of the object to be produced. Therefore, it is very important to know the physical characteristics of the different types of wax as thoroughly as possible. Usually, the wax producers give only one quantitative datum: the recommended injection temperature. All other information given is purely qualitative. But this information exists and should be available to the goldsmith if he is to make a correct selection of the wax grade.

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Table 3 Characteristics of some commercial wax types

Wax type

Ring & Ball Softening point

Density g/cm3

Hardness -penetration (100g load) mm

Viscosity Fluidity test

°C (°F)

77°C 170°F

71°C 160°F

66°C 150°F

Volume expansion From 24°C (75.2°F)

CPS

RPM

CPS

RPM

CPS

RPM

48°C 118°F

A

70±3 (158±5)

.954

5,8

>50% 50°C

252

100

311

100

550

50

4%

B

70±3 (158±5)

.960

8,2

>50% 52°C

204

100

282

100

348

100

C

72±3 (162±5)

.940

6,4

>50% 54°C

622

100

759

100

998

D

68±3 (154±5)

.950

8,4

>50% 54°C

764

50

952

20

E

74±3 (165±5)

.955

9,6

>50% 54°C

217

100

267

F

68±3 (154±5)

.960

7,6

>50% 52°C

248

100

307

Cps = Centipoises

60°C 72°C 140°F 162°F 9,1%

Injection temperature °C (°F)

11,6%

68-71 (154-160)

3,5%

10,0% 11,6%

71-74 (160-165)

100

3,0%

7,3%

10,3%

71-74 (160-165)

–

–

3,3%

8,6%

12,8%

73-76 (163-169)

100

400

100

3,5%

9,2%

11,6%

71-74 (160-165)

100

413

100

4,6%

10,6% 11,8%

68±3 (154±5)

Rpm = Revs. per min.

Information on hardness, density, ash content, viscosity, linear and volume thermal expansion is important for making the correct choice. For example, thermal expansion can be used for evaluating the cooling shrinkage of the wax patterns and calculating the real dimensions of the cast pieces. Viscosity will give information on the ability of the wax to fill the mould completely and the density can be used for calculating the precise amount of precious alloy required for casting. The values of some physical parameters for different wax types are shown in Table 3. The values of these parameters should be available from all serious suppliers. We can see that the values of some parameters can change by more than 20% from one wax type to another. Therefore, the usual qualitative information offered, like “high”, “low”, etc., should not be considered sufficient. We should also note that wax grades are often differentiated by wax colour; however, different suppliers use different colours for similar grades. Thus, a blue grade from one will be different from the blue grade from another!

2.4.2 Wax injection Temperature is also a fundamental parameter for the production of wax patterns. Not only wax temperature, but also the injector nozzle temperature and mould temperature are important. Injectors fitted with devices to monitor and control nozzle temperature as well as wax temperature can be most effective in attaining good quality wax patterns. While too low a wax temperature can cause incomplete filling of the mould, too high a wax temperature can give rise to bubbles and excessive pattern shrinkage. It is recommended that the wax patterns of a given type produced throughout the day are weighed systematically, to verify the operation of the wax department. Too large a weight variability of wax patterns says that something is not running as expected. The first thing to consider is the use of mould clamping devices for

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Table 4 Common problems in the production of wax patterns Problem

Causes

Remedies

Bubbles

Insufficient wax quantity in the injector Wax is too hot or too cold Poor fitting between mould and nozzle Too high injection pressure Using vented moulds on vacuum injector

Add wax in the wax pot Calibrate wax temperature Set the mould correctly or adjust the mould mouth Use lower pressure Don’t use vacuum

Incomplete filling of the mould

Injection pressure is too low Wax temperature is too low Cold mould The feed sprue is too thin Insufficient vents (no vacuum injectors) Vents are obstructed or dirty (no vacuum injectors) Obstructed injector

Increase injection pressure Increase wax temperature Heat the mould with repeated use Use a wider feed sprue Increase vents in the mould Clean the vents and keep them open with talcum Clean injector and nozzle

Excessive filling of the mould

Pressure is too high Incorrect clamping pressure on the mould Wax is too hot Too long injection time

Decrease pressure Use correct clamping pressure Make a new mould and cut it with improved tools Lower wax temperature Shorten injection time

Sticky wax pattern that is easily bent

The mould has been opened too early Wax is too hot Mould too hot

Longer cooling time Lower wax temperature Increase cooling time of mould before re-use

Excessive shrinkage

Wax is too hot Insufficient pressure Injection time too short Feed sprue too thin The mould is too cold Wax with excessive shrinkage

Lower wax temperature Increase injection pressure Longer injection time Use wider feed sprue Heat the mould with repeated use Turn to low shrinkage wax

Sinks (depressions in large patterns)

Incorrect selection of wax type Injection time too short Wax is too hot Insufficient injection pressure Feed sprue too thin

Turn to a depression resistant wax type Longer injection time Lower wax temperature Increase injection pressure Enlarge the feed sprue

Poor surface finishing (also wrinkling)

The mould is too cold Wax is too cold

Heat the mould with repeated use Increase wax temperature

Poor surface finishing (rough surface, cavities are present) Too low injection pressure Too much spray for releasing the patterns Too much talcum Spray and talcum have been used at the same time Fins

Too high injection pressure Mould imperfectly cut Insufficient clamping pressure Vents are insufficient or obstructed Wax is too hot

Wax patterns tend to break

Not enough spray for releasing The mould has been incorrectly open or the pattern has been removed badly The mould has not been cut properly to facilitate pattern removal Cooling time too long A brittle wax has been used

Handbook on Investment Casting

Increase injection pressure Clean mould and reduce spray quantity Clean mould and reduce talcum quantity (add a layer of cloth to the linen bag) Clean mould and use spray only Lower injection pressure Make a new mould and improve cutting Increase clamping pressure Clean the mould and the vents accurately Cut additional vents Lower wax temperature Use more spray Improve mould opening and pattern removal methods Make a new mould and improve cutting Shorten cooling time Prefer a more flexible wax

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Figure 2.4.1 Mould clamp, allowing clamping pressure control during wax injection

Figure 2.4.2 Quality control of wax patterns. Generally different colours denote different physical characteristics of the wax

a

b

Figure 2.4.3 Perfect seal between injector nozzle and mould mouth is very important a – Correct geometry b – Incorrect geometry that can favour entrainment of air with the wax and does not ensure a satisfactory vacuum in the mould before wax injection

Figure 2.4.4 Mould frame with screwed-in sprue former ensuring correct geometry of mould mouth

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controlling mould pressure during wax injection, to avoid the variability that occurs when the mould is held by hand, Figure 2.4.1. Weight variation can exceed ±10% for different operators or even for the same operator at different moments during the day. An initial quality control should be done on the wax patterns, Figure 2.4.2. The patterns should not be dirty (talcum powder, for example) and should not show bubbles. When investing the flasks, bubbles can break open and fill up with investment. So they can give rise to more serious defects in the castings. The presence of bubbles can readily be detected by looking at the patterns against a light source. Defective wax patterns should be immediately rejected and should never be used for production. They will unavoidably give rise to defective castings, with considerable loss of time and money. The removal of fins and witness marks of the mould separation line, when very evident, are the only repair operations acceptable for a wax pattern. We should record the number of rejected wax patterns for each model type. High figures suggest that the mould is badly made or has deteriorated. Otherwise, the injection parameters are incorrect and should be modified, or the wrong wax type has been used. Recycling of used wax and defective waxes should be totally avoided. It is a useless and harmful ‘economy’ that will invariably lead to poor products. We should also avoid using too much talcum powder to facilitate removal of the wax patterns from the mould. The aim should be to use as little as possible. During the dewaxing process, it is difficult to remove all the powder remaining on the surface of the patterns or embedded in the wax. Certainly, talcum powder will not disappear during burnout (talc is an inorganic silicate): it will lead to a poor surface or defects! It will also accumulate in the rubber mould. To facilitate easy removal of the wax from the mould, the use of a fine starch powder or a silicone spray is preferred. Excess starch powder will burn in the burnout furnace, leaving no residues. The main parameters involved in wax injection are temperature and pressure. For vacuum injectors, a third one, vacuum, should be included. We can start by discussing the last parameter, vacuum. To get a good effect from vacuuming, the mouth of the mould must be perfectly matched with the nozzle of the injector, Figures 2.4.3 and 2.4.4. If there is a gap between the mould mouth and the nozzle, not only will we not exhaust the mould sufficiently, but in the subsequent step of wax injection some air can be entrained by the wax and enter the mould. This air will be added to the air already present in the mould cavity, with a considerable danger of producing air bubbles in the wax pattern. We should keep in our mind that moulds for vacuum injectors don’t have vents, so the air will find it rather difficult to escape from the mould cavity during wax injection and will never be completely removed. As far as temperature is concerned, usually we should work at the temperature recommended by the supplier of the wax. A higher temperature can lead to wax patterns with air bubbles, while at lower temperatures wax fluidity can be insufficient and the model could be inaccurately replicated, with loss of fine surface detail. Lastly, pressure is the only parameter requiring a real adjustment for each single model. When we have found the correct pressure level, allowing the replication of the model accurately, we should not change it. Changes of injection pressure can cause very significant variation of the weight of the wax patterns and, consequently, of the weight of the gold alloy castings.

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THE PROCESS OF INVESTMENT C ASTING

Each mould and each wax type are different and require a specific pressure level, a specific temperature and an appropriate time for cooling. The best compromise among these different parameters can be achieved only with experience and experimentation on that specific mould with a specific wax type. Moreover, the characteristics of the mould change as we continue with the injection of hot wax. Maybe the combination of parameters giving good results initially (when the mould is cold) will no longer work well when the mould has been heated by a prolonged use. Therefore, we should take into account the time of cooling between subsequent injections. Lastly, we should note that waxes should be stored on flat trays in a cool place and covered to prevent dust settling on the surface by electrostatic attraction. They should not be piled in heaps, as they are liable to distort or to surface damage.

2

Figure 2.5.1 Two traditional rubber base types: with conical or hemispherical sprue button

2.4.3 Common problems Some common problems that can occur in the production of wax patterns are listed in Table 4, along with the possible remedies.

Figure 2.5.2 Possible problems with a hemispherical sprue button

2.5 ASSEMBLING THE TREE 2.5.1 Bases and sprues The rubber base for the tree is the starting point for building the wax tree. It should be selected with care. Usually, the rubber base includes the part that becomes the sprue button of the cast tree. We should check carefully that the base is clean and free from residues of used investment. Residues of used investment can appreciably change the setting time of the new investment, thus impacting on mould quality. Bases with a cone-shaped sprue button are preferable to those rubber bases with a hemispherical sprue button, Figure 2.5.1. A hemispherical sprue button can cause pressure losses and induce turbulence during casting, Figure 2.5.2, with the consequent possibility of gas entrapment in the liquid metal. These problems are more evident when we cast with centrifugal machines rather than with static machines. We should always check that the selected base does not show signs of wear on the tip of the cone of the sprue button, where the main wax sprue is inserted, Figures 2.5.3 and 2.5.4. As before, the presence of a step between the rubber base and the wax sprue can cause turbulence and pressure loss during casting. Each rubber base should be identifiable with a code number and weighed. It is considered better to use main sprues made from a wax with lower melting range than the wax of the patterns. In this way, when dewaxing, the main sprue will melt first and stress generation inside the invested flask will be avoided, when the wax patterns begin to melt. Slightly tapered main sprues are preferred to standard cylindrical ones. Tapering gives a better heat balance: the solidification will progress from the top of the tree (smaller diameter) to the bottom, favouring a directional solidification, Figure 2.5.5. The danger of shrinkage porosity formation in the cast items is reduced.

Figure 2.5.3 Rubber base to reject: the conical sprue button shows wear on the tip

Figure 2.5.4 A worn out rubber base (see Figure 2.5.3) forms an undesirable step in the sprue button

Figure 2.5.5 Variation in temperature distribution in the tree resulting from the use of a cylindrical or tapered main sprue

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Figure 2.5.6 Assembling system for the NeuSprue™ sprue and base

A few years ago, a new patented system was developed, comprising an innovative rubber base, onto which the tapered main sprue is screwed through a special device, Figure 2.5.6. The main wax sprue includes a narrower conical sprue button that is designed to facilitate mould filling with minimum turbulence, Figure 2.5.7. In this way, the rubber base can be removed without stress or torque being applied to the tree and the patterns, Figure 2.5.8, and any danger of cracks in the investment near the sprue button and the main sprue is avoided. Such cracks can cause defects in the castings. In the author’s opinion, this system, tradenamed NeuSprue™, is one of the most interesting new products to appear on the market in recent years, Figure 2.5.9. At first sight, it is a very simple fixture, but its development was based on a rigorous study, using finite element analysis. An optimised dimensioning of the main sprue has been achieved, which enables a reduction in the weight of alloy required for each cast and allows control of the progression of solidification. In all cases, the cross-sectional area of the main sprue should be decided with care, because it depends on the size of the tree and on the items we want to cast (shape, size etc.). Some goldsmiths use a tubular main sprue. It is a tube, with a diameter much larger than a conventional sprue, but it is hollow and its weight is lower. This particular kind of main sprue is used for two reasons: it permits many more pieces to be placed on the tree, because a larger surface area is available on the main sprue, and a smaller amount of precious metal is required for casting, because the main sprue is hollow. Therefore, a higher yield per flask can be obtained and the amount of precious metal reduced. In the author’s opinion, even if the reasons for choosing a hollow sprue are accepted, a hollow central sprue does not allow directional solidification to be obtained in the best way, because of the different distribution of heat release. Therefore, it may be better to stick to the more traditional practice: a solid, slightly tapered, main sprue.

Figure 2.5.7 The NeuSprue™ sprue with its rubber base

Figure 2.5.8 Release system of the rubber base of the NeuSprue™ (right) eliminates stress caused by removal of old style sprue base (left)

34

Figure 2.5.9 a Preparation of a tree with the sprue shown in Figure 2.5.7.

Figure 2.5.9 b The sprue holder can be tilted to facilitate attaching the wax patterns to the main sprue

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2

2.5.2 Tree design As far as possible, we should put wax patterns of similar shape, size and weight together on the same tree. Thin patterns and thick patterns should not be cast on the same tree. When the temperature is high enough to cast the thin patterns beautifully, then the temperature will be too high to get good castings from the thick patterns, if they are tree’d together. In general, where different patterns are included on the same tree, thin or lighter patterns should be put at the top of a tree, because pressure is higher there than near the sprue button. If thin patterns will not fill at the bottom of the tree, then the feed sprue may not be large enough nor attached to the main sprue in the best way (presence of constrictions) or the temperature of the metal and/or of the flask may be too low. Patterns that cast well at the same flask and metal temperature can be mixed on the same tree with more challenging patterns at the top and easy to fill patterns at the bottom. The joints between the main sprue and the feed sprues must be smooth and well filleted. Constrictions at the junction point should be carefully avoided, Figure 2.5.10. When casting, the investment will protrude at this junction and can be eroded or broken off by the flow of liquid metal. Such investment fragments could obstruct the feed sprue and/or form non-metallic inclusions in the castings, Figure 2.5.11. Traditionally, the angle between the feed sprue and the main sprue has been recommended at about 45° – 60°. More recently, a larger angle of 70° – 80° has been recommended for static vacuum casting Figure 2.5.12. Recent research has shown that the best results are obtained when the wax patterns are welded perpendicularly to the main sprue. So we obtain a double advantage: solidification takes place more directionally - and the probability of formation of shrinkage porosity in the casting is lower - and the escape of the gas from the mould cavity is easier, because there is a thinner investment layer to go through to reach the outer surface of the flask. In this way the probability of formation of gas porosity from trapped gas is reduced.

Figure 2.5.10 Joint constriction between the main sprue and the feed sprue: to be avoided!

Handbook on Investment Casting

Figure 2.5.12 Optimum angle between the main sprue and the feed sprue. A 90o angle is now preferred (see text)

Figure 2.5.13 Homogeneous tree with thin patterns

Figure 2.5.11 Possible problems when constrictions are present (see Figure 2.5.10). During pouring, particles can break off from the investment, resulting in non-metallic inclusions in the cast piece

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The length of the feed sprues should be such that the furthest part of the patterns is no more than 10mm (0.4 in) from the wall of the flask, Figures 2.5.13 and 2.5.14. Figure 2.5.15 shows the drawing of a tree, with the names we use for its different parts. Finally, assembled trees should be weighed to determine the weight of wax (subtract the weight of the rubber base), as this allows the amount of carat gold for casting to be calculated. Prior to investing, the trees can be washed in water containing surfactant to remove any electrostatically attracted dust.

2.6 INVESTING THE MOULD 2.6.1 Flasks

Figure 2.5.14 Wax tree with heavy patterns

Steel cylinders or ‘flasks’ are used to contain the investment mould. Stainless steel is preferred. Before use, the flasks should be cleaned with a wire brush to remove all traces of old investment, because residues of used investment can reduce the work time of the new investment, thus influencing mould quality. The flask is placed around the wax tree and sealed at its base. Before filling, the perforated flasks, used in modern static casting machines, should be wrapped or in suitable sleeves, made from rubber or special paper or plastic, to seal the holes until the investment is fully set. In the case of solid flasks, used mainly in centrifugal casting machines, the use of a wax net is recommended, to assist in gas evacuation during casting. The wax net should be positioned near the flask wall, Figure 2.6.1, and will be removed during dewaxing, leaving escape channels for the gases present in mould cavities.

2.6.2 Investment powders

Figure 2.5.15 The tree and its different parts

Figure 2.6.1 Wax webs to facilitate gas evacuation from a solid flask

36

Two basic types of investment are used for jewellery production. These differ in the type of bonding material used, while the true refractory material is always the same: a mixture of quartz and a-cristobalite. The bonding material can be calcium sulphate (gypsum) or a mixture of one or more phosphate-containing materials. Calcium sulphate-bonded (also known as gypsum-bonded) investment is used for casting gold and silver alloys, while phosphate-bonded investment is used for alloys melting at higher temperature, such as palladium white gold and, in particular, platinum alloys. The investment powders contain also a small percentage of proprietary additives to control the rate of setting and the properties of the set investment. There are also special grades with additives that allow for casting with gemstones in place. Alternatively, a standard grade of investment can be used for this purpose which is mixed with water containing about 3.3 grammes (maximum 4 grammes) of boric acid per 100 ml of water. Dissolve the boric acid in the water at 82°C (180°F) and then cool it down before using. These investments must be dry dewaxed only, as will be discussed later. Of the 2 types, the goldsmith prefers calcium sulphate-bonded investment for two main reasons: (1) It is less costly. (2) It is easier to remove. After solidification of the castings, it is sufficient to quench the hot flask in water, which breaks the investment mould and allows recovery of the cast tree.

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THE PROCESS OF INVESTMENT C ASTING

The most common investment type consists of a mixture of 25-30% bonding material (Plaster of Paris (or gypsum), i.e. calcium sulphate hemihydrate: CaSO4.1/2H2O) and 70-75% silica, the true refractory material, in the form of quartz and a-cristobalite. The ratio between quartz and a-cristobalite varies with grade and from producer to producer, Figure 2.6.2. There are several grades of investment powders on the market, Table 5. The quality of an investment powder depends on many factors, such as particle size and purity of minerals. Cheaper grades often contain coarser, less pure powders. These, together with the proprietary additives, affect the performance of an investment. In recent years, research work has led to an improvement of quality and reliability of the product. Investment is now stronger and more reliable and has a wider field of application.

2

Figure 2.6.2 Investment structure. The larger prismatic crystals are calcium sulphate (the binder) and the smaller crystals are silica (the true refractory material)

Table 5 Typical grades of powders for investment casting Ransom & Randolph USA

KerrLab USA

Hoben International UK

SRS, UK Classic (18 carat+) Eurovest (up to 14 carat)

Standard grades for gold

Ultravest (Advantage)

Satin Cast 20 Kerrcast 2000 Supervest 20 Satin Cast regular

Gold Star Ultima Gold Star XL Gold Star 21 Gold Star Plus Investite

White gold/platinum

Platinum Astrovest

Platinite PT

Platincast

Stone-in-place casting

Solitaire

Satin Cast 20

Gemset

Stonecast

Nevertheless, making the investment mould is always the most critical step in the investment casting process. It consists of a sequence of operations, requiring adherence to some strict but simple rules. Unfortunately, these are often neglected, maybe because of their simplicity, with adverse effects on product quality. In the author’s view, there is no argument about using good investment powders, produced by well respected companies, and on the necessity of accurately following the procedure recommended by the producer.

2.6.3 Safety and storage of investment powders Two aspects must be highlighted before discussing the investment process. Firstly: Safety! Fine silica dust, as used for the investment powder, is very dangerous. When inhaled, it remains in pulmonary alveoli and can cause silicosis, a progressive, irreversible lung injury. Silicosis is a serious disease that can result in premature death and the warning labels, which are now a standard part of investment containers, should be taken very seriously. The Materials Safety Data Sheets, supplied by the investment manufacturers should be obtained and heeded. Therefore, it is recommended that investment powder is handled in a separate area, fitted with good exhaust ventilation and regularly cleaned to keep dust to a minimum. When handling investment powder, the operator should always wear special approved dust masks, rated for use with investment. Normal dust masks don’t stop

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the fine silica particles, which are the most dangerous! Protective clothing, including hats, should be worn and regularly laundered. Two operations are the most hazardous: (1) Opening the container of investment powder and taking out the powder. When the container of the investment powder is opened and investment powder is scooped out, the finest particles become suspended and float in the air (2) Quenching the flask. When the flask is quenched after casting, the escaping water vapour (steam) entrains fine silica dust into the surrounding environment. The second point refers to the method of investment storage. We should keep in mind that Plaster of Paris (gypsum) used as the bonding material, is hygroscopic. The Plaster of Paris absorbs moisture when it comes in contact with a humid atmosphere, and becomes unable to play its function. Therefore, investment powder must always be kept in dry conditions. The containers should be closed and sealed after use. Where possible, the containers of the investment should be kept in a room with controlled humidity and temperature, because investment temperature is also an important parameter. Bulk investment powder is a bad heat conductor: if stored in a cold or hot area, it can take a long time to reach the correct process temperature, required for mixing. So the temperature of the investment should also be checked. This can be done with a digital thermometer, now available cheaply. If a room with controlled humidity and temperature is not available, the containers should be preferably kept in a sheltered area, preferably on pallets, not resting on the floor, rather than in open air. Air circulation will prevent the condensation of harmful humidity. Investment powder is the most perishable material used in the process of investment casting. It has a typical shelf life of one year, when correctly stored. Therefore, it is recommended not to store large quantities of investment powder in the factory. The date of manufacturing is normally printed on the containers plainly or in some easily readable code by the manufacturer and should always be checked. In overseas locations, delivery of investment to the local wholesaler or agent by ship can result in investment already well into its storage life.

2.6.4 Checking the condition of the investment: the ‘gloss-off’ test Before using a new batch of investment for production, it is advisable to test it, by measuring the ‘gloss-off’ time. This is a very simple test, requiring no special instrument. Only a plastic coffee cup and a stop watch are required. We weigh a small quantity of investment (30-50 g) and a quantity of water at room temperature (20°C/68°F) in the ratio recommended by the producer. We add the investment powder to the water in the plastic cup and start the stopwatch. We mix with a glass rod for the recommended time and then observe the surface of the slurry. The moment when the mixture starts setting is denoted by a change in appearance of the surface from a bright gloss or shine to a dull matt. This is the gloss-off point. With a good quality investment, with water at 20°C (68°F), the gloss point is reached

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Temperature Effect Pour Time

Time (min)

after 9-10 minutes (all commercial investment types fall in the range 7-10 min.). If a considerably longer time is required to reach the gloss-off point, the investment is not behaving properly (probably due to the hydration of calcium sulphate) and has deteriorated. The ‘working time’ of an investment is the gloss-off time less 1 minute. The ‘glossoff’ test is useful for checking the condition of a batch of investment, if problems (defects) occur in casting, attributable to a poor mould. It is a way of checking if the problem is due to the investment or to the burn-out cycle.

Set Time

22 20 18 16 14 12 10 8 60

65

70 75 80 Temperature (F)

85

90

Figure 2.6.3 Effect of temperature on pour time and set time

2.6.5 Mixing the investment

Handbook on Investment Casting

Water Quality Pour Time

Delta Time (min)

The setting time is very important, because it is the basis for performing all the operations involved in creating the invested flask (mould). If we do not respect the required time, weak or poor moulds will result, leading to various defects such as watermarks, sandy surfaces and fin formation. Setting of the investment slurry is due to hydration of calcium sulphate hemihydrate; this is a chemical reaction, so it is strongly influenced by the temperature of both water and investment powder, Figure 2.6.3. Therefore, it is very important to use water at the recommended temperature, typically about 20°C/68°F, to ensure a consistent behaviour of the investment. Investment made with water that is too hot will set faster. Water that is too cold will slow down the setting time and lead to weak moulds and defects such as watermarks. Recent work shows some tap waters can substantially extend the setting time, Figure 2.6.4. As for water quality, it is preferable to use deionised water, because the setting time can be appreciably changed (lengthened) by the substances dissolved in tap water. The ‘gloss-off’ test will demonstrate this difference if batches of investment are made with both deionised and tap water. Clearly, we can assume that for most locations, the tap water composition will be nearly constant, but we cannot be certain. In some locations, it can change significantly with the seasons. The use of deionised water will remove such uncertainty and variability and thus contribute to the most profitable use of investment powder in quality terms. We should note that the producers of investment powder develop their powders for use with deionised water and their advice on its use is based on deionised water at 20°C/68°F. Should it be difficult to obtain deionised water, the measurement of “gloss-off time” is even more important, because it is the base for determining the time available for all investing operations. The sequence of steps for investing the flask is as follows: 1. Weighing investment powder and water – This must be done accurately. A measuring cylinder should be used for the water, scales for the powder 2. Mixing the powder in the water – Always add the powder to the water to ensure good mixing without ‘lumps’ 3. Vacuuming the mixture – This removes entrapped air

8 7 6 5 4 3 2 1 0 -1 -2 -3 -4

1

2

3

4

5

Set Time

6

7

8

9

10

11

Water Source Number

Figure 2.6.4 Effect of water quality on pour time and set time

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Figure 2.6.5 Watermarks on a cast item as seen under the scanning electron microscope

a

b Figure 2.6.6 Watermarks on a ring, as cast

Figure 2.6.7 Hand mixing in air

4. Filling the flask – To fill the flask around the wax tree 5. Vibrating the flask under vacuum – To remove any remaining air bubbles that may stick to the wax surface and ensure good surface replication 6. The flask is left to stand for investment setting – The investment is weak at the setting point and it strengthens over time. Any movement at this stage risks cracking the investment. Time is a critical parameter: the first five operations must be carried out before the slurry starts setting. This is known as the ‘working time’. A good rule is to vibrate the flask until 1 minute before the slurry starts setting (hence the importance of measuring the “gloss-off time”!). We should keep in our mind that we deal with a liquid-solid mixture, not a solution. If we don’t mix enough or if we let the slurry rest for too long a time between vibrating and final setting, the water will tend to separate at the interface between wax and investment, forming watermarks, Figures 2.6.5 and 2.6.6, a kind of veining that accurately replicates the tiny water streams creeping between the wax surface and the investment. The watermarks will be faithfully reproduced on the castings and will be superimposed on the surface details of the cast item, which will be ruined. The slurry can be prepared by hand, Figure 2.6.7, with very simple equipment, like kitchen mixers, Figure 2.6.8, bell jars for vacuuming the slurry, or rotary vacuum pumps, etc. But, if we want to obtain a consistently good quality, it is advisable to use investment mixing and pouring units, where the whole process, up to filling, vibrating and vacuuming the flask is carried out in an automatic and programmed way. It is important that the recommendations of the invesment manufacturer on powder/water ratio, mixing times, temperatures, etc. are followed. Just as an indication, the data for an investment powder with about 9 min. gloss-off time are: 1. powder to water ratio: 100:38 2. mixing time: about 3 min. 3. vacuuming: about 1.5 min. 4. pouring the slurry in the flask: about 1.5 min. 5. vacuuming and vibrating the flask: 2 min. total working time: 8 min. According to most recent theory, after vacuuming we should let the flasks sit undisturbed from a minimum of 1 hour to a maximum of 2 hours, before dewaxing. The flasks should never become thoroughly dry: if this happens, they should be abundantly sprayed with water before dewaxing. It is not advised to prepare batches of filled flasks and use them in subsequent days. If the flasks become completely dry, there is a high risk of crack formation, rupture or even major blowouts during casting, Figure 2.6.9. The preferred practice is to prepare the flasks, to let them set and to send them directly to dewaxing and burnout. The programmed burnout cycle will be carried out overnight and, on the following day, when equilibrated at the casting temperature, the flasks will be cast.

Figure 2.6.8 Machine mixing in air

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2.7

2

DEWAXING THE FLASK

Recent research carried out by the producers of investment powder suggests that after complete setting of the investment, i.e. 1 to 2 hours after flask filling, the wax of the patterns should be removed, to empty the mould cavities where the liquid metal will be poured. Dewaxing can be carried out in two ways: dry - it is the older method - or by steam. Dry dewaxing is often done in the burnout oven as part of the burnout cycle, but can be done in a separate dewaxing oven, prior to the main burnout cycle. Originally, steam dewaxing was introduced for ecological reasons, to avoid air pollution from the smoke generated by large scale burning of wax, particularly in places where many jewellery factories were operating in close proximity. Later it has been realized that steam dewaxing can lead to better product quality, with reduced gas porosity in the castings. Research performed on this subject, particularly by the German Research Institute for Precious Metals (FEM) of Schwäbisch Gmünd, has shown that there are two types of gas porosity: from trapped gas and from reaction gas. The first type comes from the gas present in mould cavity, in combination with metal turbulence during casting. The second type comes from the decomposition of calcium sulphate (investment binder), which produces gaseous sulphur dioxide, which largely remains in the metal filling the form. Under normal conditions, this decomposition reaction begins around 1140°C (2084°F), Figure 2.7.1, but it is accelerated by silica and, even more, by reducing substances like carbonaceous residues from wax, Figure 2.7.2. In this case, the decomposition temperature of calcium sulphate is lowered to values near to the investment temperature at the moment of casting. The studies have also shown that, with dry dewaxing, the wax impregnates investment surface pores, Figure 2.7.3, and is difficult to remove completely. So, during burnout, carbonaceous residues are formed that favour calcium sulphate decomposition both during burnout, with reduction of investment strength, and during casting, with formation of gas porosity, Figures 2.7.4 (a & b), 2.7.5 and 2.7.6. On the contrary, with steam dewaxing, humidity saturates the porosity of investment and inhibits wax absorption. So the probability of calcium sulphate decomposition is reduced. For this reason steam dewaxing has been preferred or, at least, recommended for some time.

Figure 2.6.9 Burst flask, arising from process errors

Figure 2.7.1 Thermal decomposition curve of calcium sulphate

Figure 2.7.2 Thermal decomposition curve of calcium sulphate when reducing substances are present

400µm

Figure 2.7.4 Gas porosity observed under the optical microscope. a – Surface Figure 2.7.4 b – Cross section. It can be seen that porosity affects not only the surface but also the inner part of the object

Handbook on Investment Casting

Figure 2.7.3 Evidence of wax penetration into investment porosity during dry dewaxing (paler halos)

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Figure 2.7.5 Usually, the sprue button shows a bulge in the centre when calcium sulphate decomposition occurs

More recent research has introduced some doubt: steam dewaxing could modify the morphology of the components of the investment, reducing investment permeability, important in removing air from the mould. Research on this subject is still under way, so at present we cannot clearly recommend one method over the other unless gas porosity is a significant problem. Irrespective of the preferred dewaxing method, the flask should not be allowed to cool down between dewaxing and burnout. The investment will suffer thermal stress and its strength will be decreased. We should note that steam dewaxing should not be used when casting with gemstones. The investment used for this special purpose contains boric acid to protect the stones. Boric acid is dissolved and removed by steam and no longer available to protect the stones. A warning about steam dewaxing! It is important that the steam be vented out, preferably upwards, before removing the flasks from the chamber. Steam burns are nasty and should be avoided!

2.8

Figure 2.7.6 In some cases, the sprue button shows a single inner cavity, produced by strong reaction gas evolution (see also Figure 2.7.5)

BURNOUT

Burnout, as the name implies, is carried out to burn out the last traces of wax and to give the investment mould the refractoriness and characteristics required for casting. The final characteristics of the mould will depend strongly on the burnout cycle selected and particularly on the heating rate and temperature homogenisation in the holding periods. Therefore, it is important to accurately follow the burnout cycle recommended by the producer of the investment. The ratio between quartz and a-cristobalite varies with investment grade and manufacturer and, consequently, the optimum burnout cycle may change.

2.8.1 The burnout cycle

Figure 2.8.1 Fins on cast rings, caused by cracks in the investment

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There are two critical points in the heating cycle. The first one is at about 100-120°C (212-248°F), when absorbed water and part of the gypsum crystallisation water evaporate. This is a slow process, taking place with volume contraction. Therefore, the temperature should be increased slowly, to avoid the creation of stresses that could cause cracks in the mould, with consequent formation of fins on the cast items, Figure 2.8.1. The second critical point is around 250°C (482°F), when a-cristobalite transforms to b-cristobalite. This transformation takes place with a volume increase. In this case temperature should be held constant for sufficient time to ensure that the transformation occurs uniformly in the whole mould. Lastly, with gypsum-bonded investment, we should not exceed the maximum temperature of 750°C (1382°F). Above 750°C (1382°F), because of the presence of silica, calcium sulphate decomposition can start, with consequent degradation of investment strength. This can result in the formation of a sandy surface on the castings, Figure 2.8.2.

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On the other hand, to guarantee complete combustion of carbonaceous residues left by the wax, we should exceed 690°C (1274°F). A nearly universally accepted compromise gives 730°C (1346°F) as maximum burnout temperature. The critical role of temperature comes out clearly from what has been said. Therefore, it is very important to check the temperature control equipment of the burnout oven periodically with a calibrated thermocouple, Figure 2.8.3. The following is a typical burnout cycle. After dewaxing, ramp slowly to 250°C (482°F) in 1 hour, hold at 250°C (482°F) for 2 hours, ramp to 450°C (842°F) in 1 hour, hold at 450°C (842°F) for 2 hours, ramp to 730°C (1346°F) in 11/2 hours, hold at 730°C (1346°F) for 3 hours, then slow cooling to the selected flask casting temperature and equilibrate at the casting temperature for at least 11/2 hours. The casting temperature of the mould is chosen as a function of the pattern being cast and the alloy used. The timing given for the cycle will vary, depending on the size of the flask. Larger flasks require longer cycle times, Table 6.

2

Figure 2.8.2 Sandy surface on a cast item, caused by investment crumbling during casting

Table 6 Flask size and typical burnout cycle time Total cycle time

Times to & at step temperatures* (hrs)

2.5 x 2.5 in. (63 x 63 mm)

5 hours

1 + 1 + 2 +1

3.5 x 4 in. (89 x 100 mm)

8 hours

2+2+3+1

4 x 8 in. (100 x 200 mm)

12 hours

2+2+2+4+1

*300°F/150°C; 700°F/370°C; (900°F/480°C); 1350°F/730°C; Casting temperature Source: KerrLab

It is very important to keep the flask at the holding temperature long enough to equilibrate temperature in the whole volume of the mould. We should remember that investment is a poor conductor of heat. Temperature measurements carried out by inserting thermocouples in different points of the moulds have shown that, independently from temperature level, at least 11/2 hours are required for the centre of the mould to reach oven temperature. The same holds for the heating and the cooling part of the burnout cycle. Flasks should not be allowed to cool down to room temperature during the burnout cycle and then be reheated. They will crack and be of poor quality. If the burnout oven fails or there is a power failure and the temperature of the flask falls below about 250°C (482°F), throw the flasks away! The oven atmosphere must be strongly oxidising, to guarantee complete burning of carbonaceous residues. For the same reason, overfilling the oven with flasks touching each other should be avoided. Sufficient space should be left for air circulation among the flasks. As with investing, the investment manufacturer’s recommended burnout cycle should be followed. Where stones-in-place casting is being done, the burnout cycle must be modified to prevent damage to the stones. Maximum temperature is only 630°C (1166°F) but times may be longer to ensure wax burnout. Follow the recommendation of the investment manufacturer. An example is shown in Figure 2.8.4.

Handbook on Investment Casting

Figure 2.8.3 Reference thermocouple

800

Temperature (°C)

Flask size

600

400

200

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Hours Figure 2.8.4 Example of a burnout cycle for stones-in-place casting. (Courtesy SRS Ltd.)

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2.8.2 Behaviour of calcium sulphate-bonded investment during burnout

Figure 2.8.5 Thermal expansion curve of a typical calcium sulphate bonded investment for jewellery casting

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After dewaxing, when the temperature of the flask rises above 100°C (212°F), free water evaporates and gypsum (CaSO4.2H2O) begins to lose its water of hydration, but the complete transformation of gypsum into the anhydrous form of calcium sulphate (anhydrite) occurs over a wide temperature range, through complex transformations of the crystal lattice. From the point of view of the goldsmith, it is important to note that these transformations take place with a considerable volume contraction, which is particularly severe at 300-450°C (572-842°F). If gypsum alone were used to produce investment for lost wax casting, the moulds would crack in service and would also produce castings a great deal smaller than the original patterns. Silica is used to compensate for this gypsum shrinkage and to regulate the thermal expansion of the mould. Silica exists in several crystalline forms, and two of them are used in the production of investment powders. Quartz is the most readily available form and its conversion from a to b crystal forms is accompanied by an increase in volume at around 570°C (1058°F). Cristobalite is the other major constituent of investment powder and this form of silica also undergoes a significant increase in volume as it transforms from its a to b crystal structure at around 270°C (518°F). Thus, these two allotropic forms of silica are used to override the shrinkage effect of the gypsum binder. A typical thermal expansion curve of a jewellery investment powder, Figure 2.8.5, shows how the cristobalite provides the expansion between 250 and 300°C (482572°F). Then, there is a temperature band up to about 570°C (1058°F) where the gypsum shrinkage dominates. Above 570°C, we see the contribution of quartz transformation. It is important to remember that, when the investment mould cools, it will pass through the silica transformations which, being reversible, will contract an equal amount to the original silica expansion. But the contraction of the plaster is permanent, so there is no more volume compensation. This cooling curve can be used to understand the final casting size and explains why the flasks cannot be cooled too much between burnout and casting. After casting and on cooling the plaster becomes very weak and, coupled with the disruption caused by the cooling contraction of silica, enables the cast investment to be readily removed during quenching.

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2.9

2

MELTING

Not all jewellery workshops buy ready-made carat gold alloys from precious metal alloy producers, so very often melting coincides with the formation of the alloy for casting. Generally fine gold is added to a suitable master alloy. This is generally preferable than producing carat gold alloys in situ, starting from pure metals. The use of reliable master alloys, produced by reputable companies and correctly utilized, can help to avoid many problems and guarantee a consistently good quality of the end product. The alloy to be melted should preferably be used in the form of grains of similar size. This gives an advantage in temperature control. When the alloy is in small pieces, melting is easier and faster and the risk of overheating can be avoided. Many goldsmiths prefer to make the carat gold alloy in a preliminary melt, casting it into water to make grain. Graining is carried out by pouring the molten metal from suitable crucibles, preferably with bottom pouring, into stirred water. Basically, there are three methods for melting: the gas torch, the electric resistance furnace and induction heating. The torch is the most ancient method and finds little use for melting in modern jewellery factories. Propane or natural gas is preferred for heating, supposedly because they give a cleaner flame than acetylene. The flame for melting should be reducing: a reducing flame has an irregular contour, is bright blue and makes little noise. A reducing flame has a low oxygen content, so it captures oxygen from the surrounding atmosphere and shields the melt from oxidation. Nearly all alloy types can be melted with a torch. Electrical resistance heating has been largely used for melting until the more recent introduction of induction heating. Resistance heating allows working in a closed environment, where atmosphere control is possible. Melting can be performed in inert gas (nitrogen or argon) or in slightly reducing atmosphere (forming gas). With resistance heating, it is difficult to obtain the high temperature required for melting some white golds. In all cases melting is rather slow. Induction heating is the most modern method and is used in nearly all latest generation casting machines. Induction melting is very fast and induces stirring of the molten metal, with rapid thermal and chemical homogenisation. The stirring effect is greater, the lower the frequency of the induction heating. Melting is probably the step of investment casting with the highest “metallurgical” content. Therefore, it is very important to follow some basic rules or guidelines. 1. Before melting, the required amount of precious metal alloy should be calculated: the weight of the wax tree multiplied by the density of the alloy gives the minimum weight of alloy required for melting. A further amount of alloy will be added to allow for the sprue button. 2. The amount of recycled scrap in the melt charge should be kept to a mininum but never more than 50% scrap metal should be used for the charge. 3. Any scrap metal to be remelted must be perfectly clean and free from oxides and investment residues.

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4. For preference, grained alloys should be used. When scrap from preceding operations is used, it is recommended to remelt and grain it prior to use for casting. 5. The melt should be stirred after melting, to ensure complete homogenisation. In modern induction heated casting machines, stirring is induced by electromagnetic interaction. In open furnaces, torch or electric resistance heated, stirring should be done manually with a suitable refractory rod, to avoid pollution of the melt. 6. The metal should be kept in the molten state for the shortest possible time, to limit oxidation and the loss of alloying elements by evaporation. 7. Before casting, the molten metal should be heated to a temperature higher than the melting temperature of the alloy (superheat). The required amount of superheat depends on the alloy, on the type of items to cast and also on the casting equipment. In all cases, the degree of superheat should be kept as low as possible: it could range from about 50°C (122°F) with a bottom pouring crucible in a modern casting machine to typically 75100°C (167- 212°F) in an open top pouring crucible.

Figure 2.10.1 Patterns with different shape factor

2.10

Figure 2.10.2 Experimental patterns with different shape factor

CASTING

In modern melting/casting machines, pouring of the molten metal into the mould is carried out automatically. Melting and casting are controlled by the machine through dedicated software. For most current static machines, pouring takes place through the bottom of the crucible, so metal temperature loss is reduced to a minimum. If the machine has a tilting crucible, an additional temperature loss up to 80-100°C (144180°F) should be considered when determining the casting temperature of the molten alloy (i.e. amount of superheat). The liquid metal and flask temperatures should be kept as low as possible to minimise the formation of defects, in particular gas porosity. Therefore, before starting the production of new items, a set of tests should be performed to find the optimum system temperature. The term ‘system temperature’ is used to indicate the set formed by the molten metal and flask temperatures. Solidification starts immediately after the liquid metal has filled the cavity of the mould. The temperature difference between the liquid metal and the flask is always considerable (about 400°C/720°F or higher). Therefore, the liquid metal filling the mould cavity will start freezing from the investment mould surface, Figure 1.8 (a - d), and solidification will progress rapidly to the inner part of the pattern. If the tree has Table 7 Pattern size and relative surface area Pattern Size in mm

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Total Surface Area, mm2

Heat Increase Over 1mm thick pattern

Percent Increased Surface Area increase (over 1 mm Pattern)

15 x 15 x 1

510

0

0%

15 x 15 x 2

570

2x

11 %

15 x 15 x 4

690

4x

27 %

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been assembled correctly, possibly with the patterns making a 90° angle to the main sprue, if the feed sprues have been properly designed and if a main sprue of the right diameter has been used to perform correctly, without exceeding its duty as a heat reservoir, solidification will take place directionally towards the sprue and shrinkage porosity will collect in the main sprue and in the sprue button. If, on the other hand, shrinkage or gas porosity is present in the cast items, the process parameters should be modified in a rational way, after due consideration of the situation. To find the optimum temperature combination for the liquid metal and the flask, it is necessary to clarify some concepts concerning the shape of the items to be cast and, specifically, the shape factor (surface to volume ratio). If we cast three patterns that are 15 x 15 mm x 1, 2 and 4 mm thick respectively, Figure 2.10.1, on the same tree, we could say that the casting conditions were the same for all three patterns because the investment and the metal were at the same temperature when the metal was cast. The surface area on the top and bottom of all the patterns is constant; the only increase in surface area on the larger patterns is on the sides; thus, the volume increases much faster than the surface area, Table 7. All the heat lost to the investment from the metal must go through the mouldmetal interface. Investment is a poor conductor of heat and measurements show that after the metal is cast, only 1 to 1.5mm thickness of investment material next to the metal will experience any temperature change; naturally, as the metal cools, the adjacent investment heats. The temperature of the metal may have been the same when it was cast, but each pattern holds a different amount of metal and, therefore, a corresponding amount of heat energy. The 4mm thick pattern will discharge 4 times the heat to the investment relative to the 1mm pattern. This means the temperature rise of the investment will be much greater around the 4mm pattern than around the 1mm pattern and the 2mm pattern should lie in-between. If the metal temperature and the flask temperature are correct for the 1mm pattern (this is the hardest to fill and requires higher temperature), then the temperature will be too high for the larger patterns and gas porosity is likely. Casters have a practice of classifying their patterns for flask temperature in terms of heavy, medium and light. Most casters would classify two of the patterns on the tree in Figure 2.10.2 as heavy and one each as medium and light. Therefore, the concept of system temperature is also useful for taking into account the effect that surface area and volume (surface area to volume ratio) have on the cooling of the metal and the subsequent increase in the temperature of the investment at the metal interface for a specific pattern, flask and metal temperature and alloy. The pattern with the grooved surface has less volume of metal as the other 4mm thick pattern and the surface area is somewhat larger. Because of that, it might cast better at the ‘medium flask’ temperature. It can be concluded from this that: a) System temperature is pattern specific. When considering which patterns can be on the same tree, the surface to volume ratio should be noted, not just the cross-sectional thickness.

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b) When the pattern has high surface area and low volume (thin patterns), the flask temperature influence is greater than that of the metal temperature. As volume increases in ratio to surface area (thick patterns), the flask temperature influence on the system temperature decreases. c) Flask temperature is controlled by the hardest to fill pattern on the tree. d) When thin and thick patterns are on the same tree, the flask temperature has to be high enough to fill the thin patterns, and would be too high to cast the thick patterns at their best system temperature. e) System temperature is alloy specific. The casting temperature for a metal has to be above the liquidus temperature and since various alloys melt at different temperatures, the casting temperatures will vary as well. For a particular alloy, the casting temperature will generally be lower for thick section patterns and higher for thin section patterns, but in every case the casting temperature of the metal is strongly influenced by size, shape and attachment point of the feed sprue. Better-designed feed sprues will allow casting at a lower system temperature.

2.10.1 Test for system temperature Figure 2.10.3 Experimental tree for system temperature selection

A simple experiment can be used to quickly find the best system temperature for a range of patterns cast with a specific alloy. Build five trees alike with five or six different patterns on each tree, as seen in Figure 2.10.3. The selection of patterns should represent the variety of patterns you cast, for example thin, medium, thick, large and small. Inspect all the wax patterns before using them and attach them the same side up. The patterns are attached in a vertical row at the top, centre and bottom of the main sprue. Do not expect all the different patterns to cast well on any one tree; rather, the purpose is to find out how each pattern casts at a temperature combination. If there are five patterns on the tree, one cast will give a good idea how each of these different patterns will cast at a given temperature set and, therefore, five experiments are performed in one cast. This is called a designed experiment, whereby the normal methodical testing process is shortcut. A set of test trees, as described above, are cast using a grid of flask and metal temperatures. The grid should note the alloy and the patterns being cast. Put the presumed temperature ‘sweet spot’ in the centre of the grid as shown, Table 8. Table 8 System Temperature Test Grid Date: Alloy: 18KY Patterns Tested: A, B, C, D, E Flask Temperature, °C (°F) Metal Temperature, °C (°F)

500 (932)

550 (1022)

600 (1112)

960 (1760) 980 (1796) 1000 (1832) 1020 (1868)

Flask 2 Flask 1

Flask 3

Flask 5

Flask 4

1040 (1904)

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In this case, the flask temperature is 550°C (1022°F) and metal 1000°C (1832°F). Cast one flask at each temperature combination on the grid above, below and at each side of the sweet spot. Make sure all the flasks are well soaked at the casting temperature before casting. Holding the flask for three or four hours at casting temperature is considered prudent to get good experimental results. After casting, inspect the castings in the as-cast condition, record the results and send any promising casting through finishing and normal quality inspection. A simple inspection criterion can be used to grade the castings for evaluating test results.

2.10.2 Inspection criteria All inspected castings are rated as a 1, 2 or 3 where 1 = any casting that can be finished and would pass internal quality control 2 = any casting that can be repaired, finished and would pass internal quality control 3 = any casting that is rejected, not economic to repair In most cases, the castings graded #3 will be sorted out in the as cast condition. Some #2 castings may be identified in the as cast condition, or subsurface defects may show up later. Wax patterns must be free of any powder. By careful inspection of wax patterns before casting, defects attributed to the mould and wax pattern can be eliminated. Care should be given to identify any defect that can be attributed to investment or burnout. Fins from cracked investment, or voids caused by investment inclusions, for example, are not temperature related casting defects and should be excluded from this test grading. A short list of defects that should be attributed to wrong system temperature are incomplete filling, gas porosity, shrinkage porosity, rough surface (where the wax was smooth), and cracks. After the castings are graded, the score (1, 2 or 3) for each pattern number is recorded on a test results chart, Table 9. The test data are easy to understand in this form and trends can quickly be seen. The example in Table 9 clearly shows the best flask and metal temperature for casting pattern A in alloy 18KY (18 carat yellow). Table 9 System Temperature Test Results Chart Date: Alloy: 18KY Pattern A Flask Temperature, °C (°F) Metal Temperature, °C (°F)

500 (932)

980 (1796) 1000 (1832)

550 (1022)

600 (1112)

1/1/1 2/3/3

1020 (1868)

1/2/2

3/3/3

2/2/2 Top / Centre / Bottom

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Table 10 System Temperature Test Results Chart Date: Alloy: 18KY Pattern B Flask Temperature, °C (°F) Metal Temperature, °C (°F)

500 (932)

980 (1796)

550 (1022)

600 (1112)

3/3/3

1000 (1832)

3/3/3

1020 (1868)

2/2/2

1/1/2

1/2/2 Top / Centre / Bottom

a

Pattern A was picked to represent a larger selection of patterns that were judged to have similar surface-to-volume ratios and, therefore, would be expected to cast well at similar flask and metal temperature. So all patterns that are represented by pattern A in the test should be cast at metal 980°C (1796°F) and flask 550°C (1022°F). The goal is to get all grade one castings and it is possible that that is not achieved for a pattern in the temperature grid that was picked for the test. In Table 10, pattern B is used to show how the chart can identify trends. Metal 1000°C (1832°F) and flask 600°C (1112°F) is the best combination, but not good enough. Since metal 1020°C (1868°F) and flask 550°C (1022°F) is much better than metal 980°C (1796°F) and flask 550°C (1022°F), the trend to improve would be to increase metal temperature to 1020°C (1868°F). This could be done as a single cast test, or a new grid could be formed with a new presumed sweet spot.

2.10.3 Test for best feed sprue design b Figure 2.10.4 Selection of best feed sprue design

After the system temperature is found and applied to the range of pattern styles produced, it may become evident that not all the patterns are casting with the desired quality at the system temperature chosen for it. This leaves two options: find a new temperature set for that pattern, or experiment with the feed sprue. If the casting surface is rough and such things as powder in the wax, or a rough wax coming from the mould are eliminated, then the temperature may be too high for that pattern and a lower temperature can be explored. If the surface is very fine but details such as prongs are not filling, the feed sprue may be to blame. Another designed experiment can be

Figure 2.10.5 Often it is necessary to consider the model as an integral part of the feed system, to position feed sprues correctly

a

b

c

Figure 2.10.6 Examples of cast trees

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used to find the feed sprue design that works best for any pattern. This time, only one pattern design will be used on the tree, but it will be attached with five different feed sprue configurations. Using wax wire (or wax feed sprues made in a rubber mould, Figure 2.2.7), attach feed sprues to the patterns in different locations. Build a tree in the same manner as the system temperature and test with three patterns on the tree with each of the five or six feed sprue configurations, Figure 2.10.4. One flask may be all that is required to solve the defect, but if the results are not satisfactory, then make and cast additional flasks on a new temperature grid. In some cases also the pattern should be considered as part of the feed system, Figure 2.10.5. Some examples of successfully cast trees are shown in Figure 2.10.6.

2.10.4 Casting with stones in place The technique of producing jewellery by investment casting with stones in place (stones are set in the wax pattern) is no longer a novelty, but its use has increased considerably in the last 10 years. At the beginning, this technique has been used for large scale industrial setting of synthetic stones, mainly cubic zirconia, where the cost of manual setting was not justifiable, but later its use has rapidly been extended to natural stones, like diamond, ruby, sapphire, etc., Figure 2.10.7. The same steps of the conventional investment casting process are used for stone-in-place casting, but some modifications are required. The master model should be suitably designed for positioning the stones correctly and the stones should have a small groove, just below the girdle, to favour firm clamping by the metal. Wax patterns must be flexible and springy and the stones are set in the wax. This operation is much simpler and faster than setting in the metal. The use of a special vacuum tweezer is advised to facilitate handling of the stones. Invisible setting is the most suitable technique for stone-in-place casting. The use of the special investment grades or classic gypsum-bonded investment, but with a very fine grain, is recommended. In the latter case, boric acid should be added to the investment slurry, to protect the stones during burnout and casting, as described earlier. Dewaxing should be performed dry, to avoid dissolution of the boric acid by steam. The maximum temperature in the burnout cycle should be lower than usual, to avoid spoiling the stones. Consequently, holding time at maximum temperature will be longer than usual, to remove carbonaceous residues left from wax completely. Maximum burnout temperature and recommended holding time should be approximately: • for diamond and emerald: 630°C (1166°F)/6 hours – flask casting temperature 480-530°C (896-986°F), • for zircon, ruby, sapphire and synthetic stones 680°C (1256°F)/5 hours – flask casting temperature 550-600°C (1022-1112°F).

Handbook on Investment Casting

Figure 2.10.7 Example of a cast tree with stones in place

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The cast flasks must not be water quenched immediately, to avoid cracking of the stones by thermal shock. The flasks set with diamonds can be water quenched after at least 20 min. from casting. Flasks with other types of set stones can be quenched after 60-120 minutes.

2.11

COOLING AND RECOVERY OF THE CAST ITEMS

The flasks cast with simple yellow or red gold should be water quenched about 3 minutes after casting but this time will depend on other factors, such as the flask temperature on casting and specific alloy composition. With a longer cooling time, cast items in 18 or lower yellow and red carat gold can harden, because of the precipitation of intermetallic gold-copper phases in the gold matrix. If we want to have the alloy in the condition of maximum softness (e.g. if heavy cold working is required) it is necessary to heat to a high temperature (600-700°C (1112-1292°F)) and then water quench the castings. Flasks cast in low carat golds containing silicon must be cooled longer to avoid quench cracking, preferably to 400°C (750°F) before quenching. Flasks cast with nickel white gold should cool for slightly longer time (5-6 minutes) before water quenching. Nickel white gold can crack if cooled too fast, because of strong internal stresses. The higher is the quenching temperature of the cast flask, the easier is the recovery of the cast tree. The investment crumbles into pieces because of the thermal shock. Safety note: As said earlier, quenching of the flask must be done in a well ventilated area and the operator should wear special protective masks, approved for protection from silica dust. Inhalation of fine silica dust is dangerous and must be avoided. The steam produced by quenching hot flasks entrains very fine silica particles that remain airborne and can be inhaled by an unprotected operator or passer by! The recovered tree should be thoroughly cleaned of investment residues adhering to its surface. Cleaning is performed with high pressure water guns or by wet grit-blasting. The above mentioned process refers only to calcium sulphate-bonded investment. In the case of phosphate-bonded investment, the separation of the cast tree from the investment can be realised only by mechanical means. Subsequently, if the surface of the tree is oxidised (a frequent occurrence), it should be pickled carefully in an acid bath. The most frequently used pickling solution is 20% sulphuric acid in water at a temperature of 50°C (122°F). The cast tree is dipped in the solution for about 2 minutes. Some workshops use “safety pickle” as an alternative to storing and mixing sulphuric acid. This is sodium hydrogen sulphate that, when dissolved in water at a concentration of 220 g/litre, gives what is essentially a dilute solution of sulphuric acid. If phosphate-bonded investment has been used, good results are obtained with a 50% water solution of hydrofluoric acid at 50°C (122°F). The cast tree is dipped in the solution for about 5 minutes. Safety note: Acids can be dangerous: they are strongly corrosive and can cause serious problems, if they come into contact with the skin or the eyes.

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THE PROCESS OF INVESTMENT C ASTING

Hydrofluoric acid is more dangerous than sulphuric acid and must be handled with considerable care under an exhaust system and avoiding contact with the skin. Glass containers or beakers cannot be used; it should be contained in plastic containers and bottles. When diluting a concentrated acid to make a pickle, the acid must be added slowly to water, while stirring, and not the other way round. The exothermic reaction that occurs when adding water to concentrated sulphuric acid produces intense heat and may cause instantaneous boiling and spillage. The operator should always wear protective clothing when handling acids and, most important, suitable eye protection! In the case of contact of an acid with the skin or the eyes, wash immediately with abundant water, then see a doctor. Acids and spent pickle solutions can be polluting and should not be discharged into the drainage system without treatment: all requirements for safety, health and environmental protection should be complied with. After pickling, the cast tree is washed with water and dipped in a sodium carbonate solution, to neutralize acid residues. Then it is carefully washed, to remove all traces of pickle solution, and dried, preferably with a pressure jet of steam. After drying, the cast tree is subjected to visual inspection. Possible defects, like incomplete filling, shrinkage or gas porosity, etc. should be accurately described and the position of the defective castings on the tree should be recorded. The more information collected on the defects, the higher will be the probability of being able to explain what happened and to take corrective action. The subsequent step is cutting the castings off the main sprue. This can be done with hand cutters or pneumatic sprue cutters that largely eliminate physical strain, Figures 2.11.1 and 2.11.2. After a second and deeper quality inspection, the cast items are sent for assembly and finishing. The recommended finishing procedures are described in the Finishing Handbook, published by World Gold Council in 1999.

2.12

2

Figure 2.11.1 Bench sprue cutter

Figure 2.11.2 Hand held sprue cutter

SUMMARY OF THE BASIC RULES FOR THE DIFFERENT STEPS OF INVESTMENT CASTING

In this section, a summary is given of the basic rules and guidelines to be followed in the different steps of investment casting, necessary to obtain good quality cast product. These rules have been distilled from what has been discussed in the preceding sections. Design (2.1): • A good knowledge of the whole process is required. • The designer should be in continual contact with the production staff. • “Castable” objects should be designed. • Sharp changes of cross-section (e.g. thick-thin-thick) should be avoided. Otherwise adequate feed sprues should be provided. • Possible production problems should be discussed before launching a new model. Master model (2.2): • Prefer alloys with suitable hardness.

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THE PROCESS OF INVESTMENT C ASTING

• • • •

Finishing must be perfect. Rhodium plating is recommended. Also rapid prototyping techniques should be considered. The design of the feed system must take into account size and complexity of the model. • In the feed system, bottlenecks (thick-thin-thick patterns) and abrupt changes of direction should be avoided. The principles of fluid mechanics should always be considered. Rubber mould (2.3) • You should cultivate the skill of an expert mould maker. • Knowledge of the characteristics of materials (natural rubber, silicone rubber, etc.) should be the basis for selection of the correct material. • Store the products for mould making as recommended by the producer. • The geometry of the mouth of the mould should be accurately designed (it must fit exactly on the nozzle of the injector). • Vulcanisers with a reliable temperature monitoring and control system should be used. • The temperature in the vulcaniser should be frequently checked with a calibrated instrument. • The moulds should be kept perfectly clean and stored away from heat and light. They should be numbered for identification. Wax patterns (2.4) • Prefer wax types with a narrow melting range. • Understand the properties of different wax types to allow correct selection. • Prefer injectors that apply vacuum in the mould prior to injection. • Use a mould clamp with controlled clamping pressure. • Record production parameters for each model. • Weigh the wax patterns to evaluate weight variability for a single model. • Verify wax quality accurately before use. Reject faulty wax batches. • Don’t use too much talcum powder to facilitate pattern extraction from the mould. • Don’t use recycled wax. Assembling the tree (2.5) • Prefer a main sprue designed for optimum performance. • In any case, prefer a rubber base with a conical sprue button (not hemispherical!). • The rubber base should not show signs of wear. • The rubber base should not contain residues of investment from preceding flasks. If necessary, clean the base thoroughly! • The welds between the main sprue and the feed sprues should be well filleted. Avoid constrictions! • Prefer a 90° angle between main sprue and feed sprue. • The outer end of the wax patterns should be at about 10 mm distance from the flask wall.

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THE PROCESS OF INVESTMENT C ASTING

2

Investing the flask (2.6) • Use investment powders produced by reputable companies. • Store the investment in a well-sealed container and in a dry place. • Check the production date of each new batch. • Before use, clean the flask with a wire brush, to remove all traces of used investment. • Use powder and water at the recommended temperature. • Mix the powder with the water in the ratio recommended by the producer. • Prefer deionised water. • Check the gloss-off time of each new batch of investment. • Let the flasks set for at least 1 hour and no more than 2 hours before dewaxing. Dewaxing (2.7) • There is not clear understanding if dry or steam dewaxing should be preferred. The most important thing is to start the burnout cycle immediately after dewaxing, without letting the flask cool. Burnout (2.8) • In the case of electric resistance heating, prefer ovens with forced ventilation. • Verify that temperature is uniform throughout the oven also during heating. • Avoid loading too many flasks in the oven. Enough space should be left for air circulation. • Follow the burnout cycle recommended by the producer. • Observe the holding times in the heating ramp. • Don’t exceed 750°C (1382°F) maximum temperature (for gypsum bonded investment). • Temperature should be allowed to homogenize in the whole flask before casting. • Preferably, the oven should be equipped with a double control system, with a thermocouple in the work chamber and another one near the heating elements. Melting (2.9) • Calculate the weight of alloy required for casting (from the weight of the wax tree). • Use grained alloy or alloy cut in small pieces. • Use clean metal. • Don’t make a charge with more than 50% scrap metal. • Don’t remelt the alloy more than three times. • Avoid unnecessary overheating. • Stir the molten metal for perfect homogenisation. Casting (2.10) • Keep the alloy molten for the shortest possible time. • Use the minimum degree of superheat consistent with good casting. • Cast in the shortest possible time. Cooling (2.11) • Water quench 3 minutes after casting (yellow and red gold castings) or 6 minutes after casting (nickel white gold). • After recovering the tree, clean it accurately and make a visual inspection. Type and position of defects should be recorded.

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THE PROCESS OF INVESTMENT C ASTING

2.13

SCHEMATIC LIST OF POSSIBLE DEFECTS

As mentioned in the introduction, this Handbook will not deal with defects in detail. The analysis of most common defects, along with a description of their causes and possible remedies to avoid their occurrence, has been dealt with in the Handbook on Casting and Other Defects in gold jewellery manufacture, published by World Gold Council in 1997. The reader should refer to this Handbook. Here it is emphasized that, in most cases, defects don’t have a single cause. Frequently, there are many causes acting together to cause a specific defect. Consequently, corrective action will require compromises, in order to minimize defect formation and improve end product quality. A schematic list of most frequently observed defects is given below, along with most common causes. These causes can act separately or in combination. Shrinkage porosity • Pattern is improperly sprued. Sprues may be too thin, too long or not attached in the proper location. • Not enough liquid metal reservoir after filling mould cavity. Gas porosity: it can consist of trapped or reaction gas. Distinguishing the two causes is very difficult: • Too much turbulence during pouring. • Incorrect assembling of the patterns on the tree. • Too much distance between the extremity of the patterns and the outer surface of the flask. • Too high metal and/or flask temperature. • Metal is contaminated with gas. • Too much moisture in the flux, if used. • Too much recycled scrap has been used. Always use at least 50% new metal. • Poor mould burnout. Incomplete filling • Insufficient feeding system. • Too low metal and/or flask temperature. • Pattern was improperly sprued, creating turbulence when casting in a centrifugal casting machine. • Centrifugal casting machine had too high revolution per minute. Fins on the edges • The investment has absorbed humidity before the preparation of the slurry. • Flask was disturbed while investment was setting. • Rubber base was removed too soon. • The flask has been allowed to partially dry before dewaxing. • Too high burnout temperature. • The flask has been allowed to cool between dewaxing and burnout. • Flask was improperly handled or dropped. • Speed was set too high on centrifugal casting machine. • Flask was placed too close to heat source in burnout oven. • Flasks were not held at low burnout temperature long enough.

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Handbook on Investment Casting

Bubbles or nodules on the surface of the cast items • Air bubbles in the wax patterns because: – Vacuum pump is leaking air. – Vacuum pump has water in the oil. – Vacuum pump is low on oil. – Investment not mixed properly or long enough. – Invested flasks were not vibrated during vacuum cycle. – Vacuum extended past investment working time. Depressions in the surface of the cast items • The defect was already present in the wax patterns (see Table 4). Watermarks • The correct water to investment powder ratio has not been observed. • The flask has been vibrated for too short a time (too long time between end of vibration and investment setting). Inclusions (Foreign particles: oxides, investment, graphite) in castings • Patterns were improperly sprued to wax base or tree or not filleted, causing investment to break at sharp corners during casting. • Flask was not sufficiently cured before placing in the burnout oven. • Improper dewaxing cycle was used. • Flask was not cleaned from prior cast. • Loose investment in sprue hole. • Molten metal contains excess flux or foreign oxides. • Crucible disintegrating or poorly fluxed. • Improperly dried graphite crucible. • Investment was not mixed properly or long enough. • Flask was not held at low burnout temperature long enough. • Flask was placed too close to heat source in burnout oven. • Contaminants in wax patterns. Rough surface • Too much talcum powder has been used to facilitate the extraction of the wax patterns. • Talcum powder and spray have been used at the same time. Sandy surface: often associated with investment particles enclosed in the surface of the metal: • Too high burnout temperature. • The investment has absorbed humidity before the preparation of the slurry. • Flask was not sufficiently cured before placing into burnout oven. • Flask was held in steam dewaxer too long. • Metal, flask or both were too hot. • Patterns were improperly sprued. • Flask was placed too close to heat source in burnout oven. Shiny castings • Carbonaceous residues have been left in the mould, creating a reducing condition on the mould surface.

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Handbook on Investment Casting

Temper

600 500 400

3

A L L O Y S F O R I N V E S T M300E N T C A S T I N G 200

???

???

0 10 20 30 40 50 60 70 80 90 100 Ag Cu

Atomic per cent copper

3 ALLOYS FOR INVESTMENT CASTING

Y

YELLOW AND RED GOLD ALLOYS Metallurgy and its effect on physical properties

0 40

50

60

Temperature °C

779°C

28.1

a

600

L+b

C 92.0

b

Solid a+b

400 G

F

0 10 20 30 40 50 60 70 80 90 100 Ag X Cu

Composition weight, percent copper Figure 3.1.2 Silver-copper phase diagram

L+b

Temperature °C

1100 1064.43°C

30

E

Liquid L

800

Weight percent copper 20

B L+a 8.8

800

200

1000 960° A

casting) behaviour of yellow gold. A relatively small variation in the silver/copper ratio changes the melting range of the alloy considerably. In addition, a separation 400 into two phases, a silver-rich and a copper-rich phase, will occur (especially in 14 carat alloys, see later). 200 F Age hardening can occur below approximately 410°C (770°F) due to the ordering process originating from the gold-copper system.

10

Liquid L

960° A

B L+a The properties of the ternary alloy, gold-silver-copper, are strongly influenced by the8.8 binary systems, especially gold-copper and silver-copper, Figures 3.1.1 and 3.1.2.a The 600 (and low melting eutectic in the silver-copper phase diagram influences the melting

0

1083°

1000

The result of the casting process depends strongly on the properties of the alloys used. The alloy composition should be selected to suit the casting process. That means that alloys tailored for casting should be used. In the past, mainly ‘general purpose’ alloys have been used. This was (and still is, in part) possible for yellow carat gold based on gold-silver-copper due to the beneficial working properties. However, the situation is different for white gold alloys. The steadily increasing requirements for quality and economy have led to the development of alloys modified for casting in recent decades. Such development is difficult because the modifications have to be achieved without any change in fineness and colour. Because of this, the modifications have been restricted to relatively small alloying additions. The development of white gold alloys suitable for casting has been somewhat different. General information on jewellery alloys is Y available in the literature.

3.1 3.1.1

D

70 80 90100 Ag X 1084.87°C

10

E

779°C

C 92.0

28.1 Solid a+b

30 40 50 60 70 80 90 20 Composition weight, per cent copper

L

1000

910°C 44

900

Temperature °C

800 700

(Au, Cu)

600 500

AuCuII

400

410°C

AuCu3II390°C 64

385

300 240

200

285

AuCu3I

50

60

70

38.6

Au3Cu

100 0

AuCuI

0

Au

10

20

30

40

Atomic percent copper

80

90

100

Cu

Figure 3.1.1 Gold-copper phase diagram

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A L LOYS F O R I N V E S T M E N T C A S T I NG

Au 1050°

90

10

80 70

10 00 °

30

60

40

50

50

40

0°

60

85 0°

95 0°

950°

80 0°

90

30

1000°

900°

70

80

ld go ent er c tp igh We

We igh tp er c ent silv er 90

20

20 1050°

Ag

Cu 0 10 20 30 40 50 60 70 80 90 Weight per cent copper

Figure 3.1.3 Liquidus surface of gold-coppersilver system

Au

α

20 40 60

α1/AuCu

α1/AuCu3

60 40

α1 + α2

80 100 Ag 0

80

20

40

60

It is not within the scope of this chapter to discuss the ternary alloy system in detail (this is more fully discussed in the literature – see Further Reading at the end of this Handbook). As an example, Figure 3.1.3 shows the influence of the composition on the liquidus temperature. There is a deep ‘valley’ in the liquidus temperature, starting from the eutectic composition on the silver-copper side and continuing in the direction of the gold-copper side (for more practical diagrams see Figures 3.1.7 and 3.1.8). Figure 3.1.4 shows the phase distribution at a temperature of about 300°C (572°F). The formation of a heterogeneous, two phase field and the formation of age hardening intermetallic compounds can be recognised. The diagram gives the situation for an ideal equilibrium state, which is never fully attained under the practical conditions of investment casting. However, it conveys an idea what might happen in yellow gold, with consequences for mechanical properties and tarnishing behaviour. The separation into two phases decreases the tarnishing resistance; the formation of so-called ordered intermetallic compounds increases hardness and strength but reduces the ductility (increases tendency to embrittlement). Table 11 give some examples of the compositions of yellow gold alloys at various finenesses (caratages). Data for melting range, density and standard colour are included as far as available. The data for the melting range are not very reliable in some cases and have to be used with care. Table 11 Examples of yellow gold alloys used in the jewellery industry

20

80

Figure 3.1.4 Two phase region in goldcopper-silver alloys

100

Cu

Solidus Liquidus temperature temperature Density °C °C g/cm3

Carat

Gold ‰

Silver ‰

Copper ‰

Zinc ‰

14 14 14 14 14

585 585 585 585 585

90 100 140 200 260

320 277 270 200 140

5 38 5 15 15

860 835 835 825 830

890 865 865 835 845

13,1 13,1 13,25 13,5 13,7

5N 3N 4N 2N 1N

18 18 18 18 18 18 18 18 18

750 750 750 750 750 750 750 750 750

20 45 90 90 125 140 155 160 210

220 205 160 155 125 90 90 90 40

10 0 0 5 0 20 5 0 0

897 890 880 880 885 865 870 895 960

917 895 885 895 895 903 900 920 990

15,45 15,15 15,3 15,3 15,45 15,36

5N 4N 4N 3N

15,6 15,7

2N 2N 1N

21 21 21

875 875 875

0 17,5 45

125 107,5 80

0 0 0

926 928 940

940 952 964

16,7 16,8 16,8

Red pink yellow-pink

22 22 22 22

916,6 916,6 917 917

21,4 62 32 55

62 21,4 51 28

0 0 0 0

959 1010 964 995

982 1035 982 1020

17,8 18 17,8 17,9

Colour*

* Based on ISO 8654 classifications

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A L LOYS F O R I N V E S T M E N T C A S T I NG

13.8

Copper

%

%

%

J/g

J/cm3

91.7 75.0 58.5

6.2 16.0 30.0 90

2.1 9.0 11.5 10

60 72 76 111 65* 107* 205*

1002 1123 1048

100 100 100

Specific Heat J/(g*K)

Liquidus Temp. °C

Solidus Temp. °C

Superheat 100 K (°C) J/g

0.174 0.212 0.242 0.320 0.157* 0.310* 0.494*

1032.8 933.3 891.4 901.6

1009 902.8 850.9 779.8

17 21 24

Density (g/cm3)

13.5 13.4 13.3 13.3

13.1 60

100

140

180

220

260

300

Silver (‰) Figure 3.1.5 Density of 14 carat yellow gold as a function of silver content 15.7

Density (g/cm3)

15.6

18 ct YG Density as a function of silver content

15.5

Density

15.4 15.3 15.2 15.1 20 40 60 80 100 120 140 160 180

Silver (‰) Figure 3.1.6 Density of 18 carat yellow gold as a function of silver content 960 940

Influence of silver content on the solidification 14 ct yellow 80 120 range 160 of 200 240 gold 280

920 900 880 860 840

SOLIDUS LIQUIDUS

820 800 780

80 90 100 110

260 280 300 320 340 360

Silver (‰) Figure 3.1.7

Temperature (°C)

Heat of solidification

Silver

13.6

13.2

Table 12 Data of thermal analysis for some typical jewellery alloys Gold

Density

13.7

Temperature

All the alloys are based on gold-silver-copper. Most of the 14 ct alloys contain an addition of zinc. About 50% of the 18 ct alloys also have small additions of zinc. Zinc additions are not common in higher carat alloys. The influence of zinc additions on properties are considered separately in section 3.3. Low carat alloys (10, 9 and 8 ct) are, with few exceptions, based on copper- gold-(silver)-zinc (in this order). Zinc can be considered as a main alloying element in this case. In addition to these ‘traditional’ alloying elements, in recent times small additions of other elements are in use, e.g. for grain refining (iridium) and ‘deoxidation’ (silicon, boron). The influence of these additions is also discussed separately in section 3.3. Recently, to improve the mechanical properties of high carat alloys (20 ct up to micro-alloyed ‘pure’ gold) special alloy systems have been developed. They are not in frequent use and shall be mentioned only briefly. The density of 14 and 18 ct yellow gold is strongly influenced by the silver/copper ratio (at constant gold content), Figures 3.1.5 and 3.1.6. Small additions of zinc have a small influence on density, especially with 14 ct alloys where zinc is added more frequently (Note: Figure 3.1.5 also contains alloys with small additions of zinc). The densities of 21 ct alloys (875‰ gold) lie in the range of 16.7-16.8 g/cm3; for 22 ct (917‰ gold), the values lie between 17.8 and 17.9 g/cm3. Variations in the silver/copper ratio are very limited and have no significant influence on density. The solidification range of a yellow gold alloy depends on the composition in a rather complicated way. In principle, it can be read from the ternary phase diagram. However, for practical purposes, diagrams which demonstrate the melting range for the most important 14 and 18 ct alloys as a function of silver content are more useful, Figures 3.1.7 and 3.1.8. For the 18 ct alloys, an increasing silver content mainly influences the liquidus temperature, which also increases, but has less influence on the solidus temperature. For the 14 ct alloys, the liquidus temperature is increased at higher silver concentrations to a limited extent, but the solidus temperature is significantly decreased. Therefore, the solidification range itself is increased for both 14 ct and for 18 ct alloys at higher silver contents. The consequence of a larger solidification range is increased (micro-) segregation and a more pronounced dendritic structure. The solidification behaviour of an alloy during casting is not only influenced by the temperature range of solidification but also by the heat introduced by the melt into the flask. Table 10 shows some examples for the heat of solidification and the specific heat of jewellery alloys and the pure alloying metals. The values are based on mass (as it is common). Gold has the lowest and copper has the highest heat of solidification

945 Solidification range of 18 ct YG 940 as a function of silver content 2N 935 930 925 SOLIDUS 920 LIQUIDUS 915 910 905 900 5N 3N 895 890 4N 885 880 20 40 60 80 100 120 140 160 180

Silver (‰)

* Source: Edelmetall Taschenbuch Figure 3.1.8 Handbook on Investment Casting

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Table 13 Shrinkage at solidification, examples Metal Gold Silver Copper 18ct yellow gold

62

Shrinkage at Solidification, volume,% 4.8 7.3 5.4 6.0

Table 14 Influence of atmosphere on interfacial tension (14 ct yellow gold) Casting atmosphere

Contact angle (deg)

Interfacial tension (N/mm2)

Vacuum 0.1mbar Forming gas (N2+H2) Argon Air

144 148 – 1500 g

High pouring turbulence

Lower pouring turbulence (with a correct feed system)

Risk of investment erosion, because of high metal flow & pressure

Lower risk of investment erosion

Feed system not critical

Critical feed system (it should be suitably designed)

Relatively low productivity (8-10 casts/hour)

Higher productivity (20 casts/hour in the most sophisticated machines, using larger flasks too)

Relatively lower cost

High cost (for the more sophisticated machines)

Handbook on Investment Casting

EQUIPMENT

4

4.6 MELTING/CASTING MACHINES 4.6.1 Comparison between centrifugal and static casting machines The purchase of a casting machine is the largest financial investment made by the goldsmith or caster. Choosing the right machine is no easy decision when there are so many available on the market. Having decided on his requirements in terms of production, the first decision is whether to buy a static casting or a centrifugal casting machine. There are no special reasons to prefer one system or the other. Here the decision is up to the goldsmith and will depend on his needs, experience and preference. A centrifugal casting machine is really needed only in the case where investment casting of platinum is to be done. (Platinum is less fluid than gold and a stronger push, that only a centrifugal machine can give, is required for filling the mould cavity.) For gold and other precious metals, in recent years the preference has shifted strongly towards static casting machines, which are favoured because of their higher level of technological evolution. In the more advanced models, they are nearly fully automated. Automated machines remove, to a large extent, the technical responsibility from the goldsmith or caster in the casting phase of the process and result in a more consistent quality product. Human error is minimised. The main differences between static and centrifugal casting are summarized in Table 20. Centrifugal casting Centrifugal casting has two weak points: more turbulence in the liquid metal during ‘pouring’ and a higher liquid metal pressure. On the other hand, higher pressure facilitates form-filling and makes the feed system less critical, particularly with very thin patterns. As discussed in preceding chapters, high turbulence increases the probability of having gas porosity from trapped gas. Perforated flasks are not used in centrifugal machines, so the escape of gas from mould cavity is more difficult, even with suction from the bottom of the flask. High casting pressure favours complete filling of the mould, but also increases the risk of investment erosion (and mould collapse in the extreme case). Eroded investment particles become entrained in the flowing metal, leading to inclusions in the castings. Such erosion can also lead to sandy surfaces on the castings. This occurrence has been demonstrated by recent studies that have shown that surface defects caused by crumbled investment, formerly ascribed to incorrect burnout, were instead caused by investment erosion produced by the centrifugal force pushing the liquid metal in the mould, Figures 4.6.1 and 4.6.2. Moreover, in centrifugal casting, the pressure exerted on the liquid metal is not constant over the entire length of the tree, but is highest at the top of the tree and lowest at the sprue button. Therefore, the patterns near the sprue button can be incompletely filled, while the patterns near to the treetop can show finning, caused by investment cracks produced by the high pressure.

Handbook on Investment Casting

Figure 4.6.1 Defective surface on centrifuge cast rings, caused by investment erosion

Figure 4.6.2 Another example of defective surface caused by investment erosion during centrifugal casting

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EQUIPMENT

Figure 4.6.3 Sketch of a variable geometry centrifugal casting machine

Figure 4.6.4 Trace of the liquid metal on the crucible wall in a traditional centrifugal casting machine

Figure 4.6.5 Trace of the liquid metal on the crucible wall in a variable geometry centrifugal casting machine. Note the symmetry of the metal flow

Static casting In contrast, in static casting, the pressure is due to gravity and is nearly uniform over the full length of the tree, because the only difference is due to the hydrostatic pressure of the liquid metal from top to bottom. With static machines, maintaining a controlled atmosphere in the crucible and flask chambers is not difficult, whereas only few centrifugal machines can operate in a controlled atmosphere. Productivity Lastly, let us consider productivity. In centrifugal casting machines, the metal high pressure also sets a limit to the weight of the charge that can be safely used; it should not exceed 800 g (1.76lb). Flask height is also limited to 150 mm (6 in). The limit on flask size is not so onerous in static machines, where the weight of the charge can be larger than 1.5 kg (3.36lb) and the flasks can be taller than 250 mm (10 in). Higher charge weight and larger flasks mean better process economy. With a centrifugal machine, the operator will struggle to make more than 8 casts per hour, using 130-150 mm (5-6 in) high flasks. With a vacuum assisted (maybe also pressure assisted) fully automated, latest generation, static casting machine, the operator can carry out up to 20 casts per hour without difficulty, using 250 mm (10 in) high flasks. Obviously, these are extreme cases, but they give an idea of the different potential of these two systems. Even if productivity not always is a critical factor in a workshop or factory, we should consider that the operator working with a static automatic machine has more time available for his other work, i.e. it is less labour intensive.

4.6.2 Centrifugal machines Probably, centrifugal machines are the most widely used for casting jewellery. In recent years, there has been considerable progress in motor technology and programming systems, but the original basic design remains nearly unchanged. In comparison with older centrifuge equipment, the most important innovations include the variable geometry arm, the flask with bottom-applied suction (in some models), the temperature measurement attachment, induction heating and a controlled atmosphere chamber (in some models). Variable geometry In the variable geometry machines, the angle between the flask axis and the centrifuge arm is not longer fixed at 0°, but can change from 90° (in its rest position) to 0° as a function of rotation rate, Figure 4.6.3. In this way, the combination of centrifugal and tangential-inertial forces acting on the molten metal flowing out of the crucible and entering the flask, is taken into account. This device helps to improve the symmetry of metal flow into the mould, Figures 4.6.4 and 4.6.5, and prevents the liquid metal from flowing preferentially along the side of the main sprue cavity opposite to the rotation direction, as occurs in conventional fixed geometry equipment, where this phenomenon can cause incomplete filling of some patterns on the cast tree.

Figure 4.6.6 Sketch of a centrifugal casting machine with suction through the flask bottom

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Applied suction To ease the outflow of the gas contained in the mould cavity, suction systems have been designed that are connected to the flask bottom, Figures 4.6.6 and 4.6.7. These systems form a single unit with the rotating centrifuge assembly and facilitate filling of very thin mould cavities. Temperature measurement As for temperature measurement, the best systems make use of a thermocouple dipping into the molten metal in the melting crucible. The thermocouple is clamped on the rotating system and the electric signal is transmitted through suitable contacts on a commutator, that open when rotation starts, Figures 4.6.8 and 4.6.9. Temperature measurement is less accurate and reliable when a thermocouple contacting the outer crucible surface is used. Crucible and thermocouple have different electrical potential and electric discharges can occur; these oxidise the thermocouple junction and contribute to errors in temperature readings. Optical pyrometers also tend to be less accurate and reliable. Process control Generally, in centrifugal casting machines, the operating parameters must be programmed by the operator and the interaction with the operator is very tight, Figure 4.6.10. The operator chooses the rotation rate and, consequently, the level of the centrifugal force that will push the molten metal into the mould during pouring. There are no fully automated centrifugal casting machines. The best machines are equipped with induction heating and, recently, machines operating under a protective atmosphere have been put on the market. Developing a centrifugal machine to operate under a protective atmosphere is more complicated than for a static machine, not least because of the larger volume involved. Cost The cost of a centrifugal casting machine can range from about 2,000 to about 4,000 Euros/US$ for basic equipment, not programmable, with torch melting. The cost of a machine with some programming and induction heating can reach 10,000 Euros/US$, while machines with more sophisticated programming, induction heating, controlled atmosphere, temperature measurement with a dipping thermocouple and suction from the flask bottom can cost up to 40,000 Euros/US$ or more.

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Figure 4.6.7 Centrifugal casting machine operating in an inert atmosphere, with suction through the flask bottom

Figure 4.6.8 Detail of the crucible zone, without the flask

Figure 4.6.9 Detail of the crucible zone, with the flask and the thermocouple for liquid metal temperature measurement

4.6.3 Static machines All modern, good quality static casting machines are “vacuum assist”, i.e. are equipped with a suction system, acting through the flask, which facilitates mould filling, Figure 4.6.11. The best machines are equipped with separate crucible and flask chambers. In this way, process time can be shortened further. Nearly all static casting machines operate under an inert atmosphere, usually nitrogen or argon although some use a hydrogen/nitrogen reducing atmosphere. Presently, argon is frequently preferred, even if it is more costly than nitrogen. The machines can also be equipped with a pressure system, acting (after pouring) only in the flask chamber on the sprue button to facilitate better mould fill and surface detail. In some very recent machines, pouring is also carried out under pressure.

Figure 4.6.10 Detail of the programming system of a high performance centrifugal casting machine

Figure 4.6.11 Modern basic level static casting machine

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a

d

c

b

Figure 4.6.12 Computer assisted static casting machine: a – General view b – Detail of the crucible chamber c – Detail of the control panel d – Display with operation parameters

In many machines, pouring takes place through the crucible bottom; this minimises heat loss during pouring, allowing a lower degree of superheat and also reduces the risk of oxide entrapment in the casting, since any oxide on the surface of the melt will tend to fill the sprue button. Many casting machines can be equipped with a grain-making accessory, for making casting grain. Heating and temperature measurement Many static machines are induction heated, although more basic small machines can be electrical resistance heated. In general, the better quality machines have medium – low frequency induction. Heating depth and hence melting speed as well as electromagnetic stirring forces increase with decreasing frequency. Temperature measurement can be by optical pyrometer or, better and preferable, by a sheathed thermocouple dipped into the melt, often via the central stopper in bottom pouring crucibles.

d

e a

f

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b

c

g

Figure 4.6.13 Computer controlled static casting machine: a – General view b – Only the essential process parameters are displayed, because the machine is computer operated c – Flask temperature measuring attachment equipped with optical pyrometer d – Operation scheme of the machine e – Graining attachment for making alloy grains f – Connection with the computer for recording process parameters g – Process evolution can be observed on the computer screen

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Trends in process control For all the leading machine manufacturers, the trend is towards an even more complete automation of the machines. In some cases, artificial intelligence software is being utilised. These control systems remove the risk of operator error. He only feeds the metal charge into the crucible and sets the casting temperature. Then the control system takes all other technical decisions on the subsequent steps of the melting and casting process. There are two trends in the technical development of static machines: programmable machines or self-programming machines. We could say “computer assisted” machines, Figure 4.6.12, or “computer controlled” machines, Figure 4.6.13. In the first group, the operating cycle is planned by the operator, who inputs a set of instructions. Generally, with these machines, data collection must be done by the operator, who should write down all data recorded by the machine. In contrast, computer controlled machines are self-programming. They can automatically evaluate the weight of the charged metal and correct the thermocouple temperature readings in real time. This correction is needed, because thermocouples are always enclosed in a refractory sheath and temperature readings always lag slightly behind in comparison with the true metal temperature (they are lower in the heating phase and higher in the cooling phase). Data collection is carried out automatically: the data are recorded in the computer and can be retrieved for subsequent processing. The most recent developments involve the use of pressure assistance in casting, as shown in, Figures 4.6.14, 4.6.15, 4.6.16 and 4.6.17.

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Figure 4.6.14 Operation of a pressure casting machine: 1 – Preparation for melting 2 – Crucible and flask chambers are exhausted (vacuumed) 3 – Inert gas is introduced in the crucible chamber and slowly, also in the flask chamber; induction heating of the crucible is started

4 – The crucible chamber is filled with inert atmosphere with controlled pressure 5 – (Optional) the flask chamber can be put under dynamic vacuum 6 – Pouring is started: the crucible chamber is pressurized, while the flask chamber is vacuumed

a

b

Figure 4.6.15 a – Detail of the crucible of the machine of Figure 4.6.14. The thermocouple is off-centre to facilitate crucible filling b – Detail of the control panel

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7 – Pouring ends. The flask chamber is pressurized, to facilitate mould filling and prevent shrinkage porosity 8 – The cast tree is solidified. The pressure in the flask chamber is lowered 9 – The crucible chamber is filled with inert gas, to protect the heating assembly. In the meanwhile the flask chamber is opened, to recover the cast flask

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Cost The price range is even wider than for centrifugal casting machines. Electric resistance heated, non programmable machines may cost a few thousand Euros/US$, and resistance heated, programmable machines can cost up 7,000-8,000 Euros/US$. But induction heated machines with a good level of programming capability can cost up to 20,000 Euros/US$ and more sophisticated, fully automatic machines that can be interfaced with a computer for data collecting and processing may cost even more than 60,000-70,000 Euros/US$. A typical range of machines produced by one manufacturer is shown in Table 21. Table 21 Range of static casting machines from a US manufacturer

Figure 4.6.16 A machine for static casting under pressure (made in U.S.A.)

Model

Heating

Max. Crucible Temperature capacity*, Grammes

Flask size, Typical maximum, mm cycle time (dia. x height) Mins

Casting rate, flasks/hour

Optional features

J-2

Resistance

1204°C

900

102 x 229

6-8

8-10

J-z

Induction

1513°C

1440

127 x 229

4

12-15

Yes

J-5

Induction, 5kW

1513°C

1440

152 x 254

4

12-15

Yes

J-10

Induction, 10kW

1513°C

1960

152 x 254