Die Casting

Module-I of Manufacturing Science-I

1.6.1 DIE CASTING (PRESSURE DIE CASTING) It involves the preparation of components by injecting molten metal at high pressure (2 to 200 N/ mm2) into a metallic die. Because of high pressure involved in die casting, any narrow sections, complex shapes and fine surface details can easily be produced. The metallic die consists of two parts. One part called as stationary or cover die is fixed to the die casting machine while the other part called ejector die is moved out for the extraction of the casting. The casting cycle begins when the twp parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system. Then the two halves are closed and clamped. The required amount of metal is injected into the die. After the casting is solidified under pressure the die is opened and the casting is ejected.

Die Casting Machine Die casting machine performs the following functions: • Holding two die halves firmly together. • Spraying lubricant on the die cavity. • Closing the die. • Injecting molten metal into the die. • Opening the die. • Ejecting the casting out of the die. Die casting machines are of two types: • Hot chamber die casting machine in which the holding furnace for the liquid metal is integral with it. • Cold chamber die casting machine in which the molten metal, which is melted in a separate furnace, is poured into the die casting machine with a ladle for each casting cycle.

Hot Chamber Process In this, a gooseneck is used for pumping the liquid metal into the die cavity. The gooseneck, made of grey, alloy or ductile iron or cast steel, is submerged in the holding furnace containing the molten metal. A plunger made of alloy cast iron and which is hydraulically operated, moves up in the gooseneck to uncover the entry port for the intake of liquid metal into the gooseneck. The plunger can then develop the necessary pressure for forcing the metal into the die cavity through the nozzle.

Figure 1.6.1: Hot chamber die casting

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Module-I of Manufacturing Science-I In another arrangement, the gooseneck container is operated by a lifting mechanism. Initially it is submerged in the molten metal and is filled by the gravity. Then it is raised so as to bring the nozzle in contact with the die opening and is locked in that position. Compressed air then forces the metal into the die and pressure is maintained till solidification. When solidification is complete, the gooseneck is lowered down and casting is removed by ejector pins after opening the dies. Operating Sequence: The cycle starts with the closing of the die, when the plunger is in the highest position in the gooseneck, thus facilitating the filling of the gooseneck by the liquid metal. The plunger then starts moving down to force the metal in the gooseneck to be injected into the cavity. The metal is then held at the same pressure till it is solidified. The die is opened, any cores, if present are also retracted. The plunger then moves back returning the unused liquid metal to the gooseneck. The casting which is in the ejector die is now ejected and at the same time the plunger uncovers the filling hole, letting the liquid metal from the furnace to enter the gooseneck.

Figure 1.6.2: Hot chamber die casting –operation sequences

Cold Chamber Process The hot chamber process is used for most of the low melting temperature alloys such as zinc, lead and tin. For materials such as aluminium and brass, their high melting temperatures make it difficult to cast them by hot chamber process, because gooseneck of the hot chamber machine is continuously in contact with the molten metal. Also liquid aluminium would attack the gooseneck material and thus hot chamber process is not used with aluminium alloys. In cold chamber process, the molten metal is poured with a ladle into the shot chamber for every shot. This process reduces the contact time between the liquid metal and the shot chamber. Operating Sequence: The operation starts with the spraying of die lubricants through out the die cavity and closing of the die when molten metal is ladled into the shot chamber of the machine either manually by a hand ladle or by means of an automatic robotic ladle. The metal volume and pouring temperature can be precisely controlled with a robotic ladle and hence the desired casting quality can be held. Then plunger forces the metal into the die cavity and maintains the pressure till it solidifies. After that the die opens and the casting is ejected. At the same time the plunger returns to its original position completing the operation. The main disadvantage of this process is the longer cycle time needed compared to the hot chamber process. Since the metal is ladled into the machine from the furnace, it may lose the superheat and sometimes may cause defects such cold shuts.

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Module-I of Manufacturing Science-I

Figure 1.6.3: Cold chamber die casting

Figure 1.6.4: Cold chamber die casting –operation sequences

The Dies of Die Casting Process The stationary die consists of sprue (biscuit), runner and gates, and is also in contact with the nozzle of the gooseneck in case of hot chamber and with the shot chamber in case of cold chamber process. The ejector pins move through the moving die to free the casting from the ejector die. The number of ejector pins must be sufficient so as to remove the hot casting without distortion. The placement of ejector pin positions should be so that the pin marks left on the casting are not objectionable. The cores used are all metallic and are of two types. The fixed cores are the ones which are fixed to the die halves. These are parallel to the die movement. The others called moving cores, are not parallel

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Module-I of Manufacturing Science-I with the die movement and hence are to be removed before the casting is ejected from the die. The cycle time in die casting being very small, the dies would readily get heated. Particularly in sections such as sprues, or heavy sections of the casting, the temperature may be too high. To maintain uniform and requisite temperature for which the casting is designed, water is circulated through the identified hot regions of the die. Sometimes, overflows are provided in the parting plane for the first metal which is normally cold, entering into the die cavity, to solidify. Overflows are primarily provided for small components, to provide enough heat input to the die, so that no cold shuts occur. These can be utilized for positioning the ejector pins so that no objectionable ejector pin marks appear on the casting. Some excess metal may be forced into the parting plane which is called flash. Since molten metal fills the die cavity almost instantly, one of the difficult problems in die casting is to get rid of the air in the cavity without entrapping it in the castings. This produces voids or porosity which affects mechanical properties of the castings. Vents about 0.1 mm deep, cut in the parting surfaces, will help the escape of air. Vents are often provided from the overflow wells to the outside. Hot working tool steels are used for the preparation of the dies, die inserts and cores. For zinc alloys, the normal die material is AISI P20 for low volume and H13 for high volume, whereas for aluminium and magnesium, H13 and H11 are used. For copper allots H21, H20, H22 are the usual die materials. Dies which contain only one die cavity and produce only one casting at a time are known as single impression dies. Dies containing more than one die cavity are called multiple impression dies if all the die cavities are alike or combination dies if the die cavities are not alike.

Advantages • • • • • • • • • • • • •

Because of the use of the movable cores, it is possible to obtain fairly complex castings. Very small thickness (0.4 mm) can be easily filled because the liquid metal is injected at high pressure. Very high production rates (300 to 350 pieces per hour) can be achieved. Because of metallic dies, very good surface finish (1 micron) can be obtained. The surfaces generated by die casting can be directly electroplated without any further processing. Close dimensional tolerances of the order of ± 0.08 mm for small dimensions can be obtained. The die has a long life, which is the order of 3,00,000 pieces for zinc alloys and 1,50,000 for aluminium alloys. Die casting gives better mechanical properties compared to sand casting, because of the fine grained skin formed during solidification. Inserts can be readily cast in place. It is very economical for large scale production. It requires less floor space. The labour cost involved is less. The increased soundness and reduction of defects provide increased yield. Threads and other fine surface details can be easily obtained.

Limitations • • •

• • • •

The maximum size of casting is limited. The normal sizes are less than 4 Kg with a maximum of order of 15 Kg. This is not suitable for all materials because of the limitations on die materials. Normally, zinc, aluminium, magnesium and copper alloys are die cast. The air in the die cavity gets trapped inside the casting and is therefore a problem often with the die castings. Porosity causes reduction of mechanical properties. Heart treatment is usually not possible because at high temperature the metal becomes weaker, and the entrapped air expands, causing blisters to raise on the die casting surfaces. The dies and the machines are very expensive and therefore, economy in production is possible only when large quantities are produced. The life of die decreases rapidly if metal temperature is high. Special skill is required for maintenance and supervision of dies. This technique requires comparatively longer time for going into production (set up time, preparation time, etc.).

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Module-I of Manufacturing Science-I

Applications The typical products made by die casting are carburetor, crank case, magnetos, handle bar housings, parts of scooters, motorcycles, zip fasteners, head lamp bezels, battery parts, light duty bearings, radiation shield, and many decorative parts.

Figure 1.6.5: Die casting products

1.6.2 CENTRIFUGAL CASTING Centrifugal casting is accomplished by rotating a mould rapidly about its central axis as the metal is poured into it. This is done principally to secure higher pressures upon the molten metal before and during its solidification. Denser metal is obtained, since relatively lighter impurities within the metal , such as oxide, sand, slag, and gas, will get separated and float more quickly toward the centre of rotation. There are three main types of centrifugal casting processes. They are • True centrifugal casting • Semi-centrifugal casting • Centrifuging.

True Centrifugal Casting This is used for the making of hollow pipes, tubes, hollow bushes, etc. which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of centrifugal force, no core needs to be used for making the concentric hole. The axis of rotation can be horizontal, vertical or any angle in between. Very long pipes are normally cast with horizontal axis, whereas short pipes are more conveniently cast with a vertical axis.

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Module-I of Manufacturing Science-I

Figure 1.6.6: True centrifugal casting At first the moulding flask is properly rammed with sand to confirm to the outer contour of the pipe to be made. Any end details like spigot ends, or flanged ends are obtained with the help of dry sand cores located in the ends. Then the flask is dynamically balanced so as to reduce the occurrence of undesirable vibrations during the casting process. The finished flask is mounted in between the rollers and the mould is rotated slowly. Now the molten metal in the requisite quantity is poured into the mould through the movable poring basin. The amount of metal poured determines the thickness of the pipe to be cast. After the pouring is complete, the mould is rotated at its operational speed till it solidifies, to form the requisite tubing. Then the mould is replaced by a new mould and the process is repeated. Metal mould can also be used in true centrifugal casting process for large quantity production. A water jacket is provided around the mould for cooling it. The casting machine is mounted on wheels with the pouring with the pouring ladle which has a long spout extending till the other end of the pipe to be made. Initially the mould is rotated with the metal being delivered at the extreme end of the pipe. The casting machine is slowly moved down the track allowing the metal to be deposited all along the length of the pipe. The machine is continuously rotated till the pipe is completely solidified. Afterwards, the pipe is extracted from the mould and the cycle is repeated. Castings with relatively short lengths are usually more conveniently cast in moulds rotating about a vertical or an inclined axis. The resulting central hole, instead of being cylindrical, will be slightly paraboloidal. However, the high spinning speeds used will produce central holes, which are nearly cylindrical. A centrifugal casting machine, which spins about a vertical or an inclined axis, should be strong and rigid, since the forces encountered when heavy moulds are rapidly rotated may be considerable. When pouring, the metal should be directed against the centre of the mould bottom where the movement is least. The moulds may be sand lined or permanent moulds made of metal, graphite, or other suitable materials. Advantages: • The mechanical properties of centrifugal cast jobs are better compared to other processes, because the inclusions such as slag and oxides get segregated towards the centre and can be easily removed by machining. Also, the pressure acting on the metal throughout the solidification causes the porosity to be eliminated giving rise to dense metal. • Up to a certain thickness of objects, proper directional solidification can be obtained starting from the mould surface to the centre. • No cores are required for making concentric holes in the case of true centrifugal casting. • There is no need for gates and runners, which increases the casting yield, reaching almost 100 %. Limitations: • Only certain shapes which are axisymmetric and having concentric holes are suitable for true centrifugal casting. • The equipment is expensive and thus is suitable only for large quantity production. Applications: • Cylindrical parts ranging from 13 mm to 3 m in diameter and 16 m long can be cast with wall thickness ranging from 6 mm to 125 mm. • In addition to pipes, typical parts made are bushings, engine cylinder liners, and bearing rings with or without flanges.

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Module-I of Manufacturing Science-I Semi-Centrifugal Casting This casting is used for jobs which are more complicated than those possible in true centrifugal casting, but are axisymmetric in nature. It is not necessary that these should have a central hole, which is to be obtained with help of a core. The moulds made of sand or metal are rotated about a vertical axis and the metal enters the mould through the central pouring basin. For larger production rates the moulds can be stacked one over the other, all feeding from the same central pouring basin. The rotating speeds used in this process are not as high as in the case of true centrifugal casting. The general practice is to rotate these moulds at rpm which will give a linear speed at the outside edge of the castings of about 200 m per minute. The typical products made by this process are wheels, gear blanks, sheaves etc.

Figure 1.6.7: Semi centrifugal casting

Centrifuging When casting shapes are not axisymmetric, then centrifuging process is used. This is suitable for small jobs of any shape. A number of small jobs are joined together by means of radial runners with a central sprue on a revolving table. The jobs are uniformly placed on the table around the periphery so that their masses are properly balanced. The process is similar to semi centrifugal casting. Stacked or multiple moulds may be advantageously employed for castings required in large quantities.

Figure 1.6.8: Centrifuging casting

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Module-I of Manufacturing Science-I Continuous Casting Generally the starting point of any structural steel product is the ingot which is subsequently rolled through number of mills before a final product such as slab or bloom is obtained. However, the wide adoption of continuous casting has changed that scenario by directly casting slabs, billets and blooms without going through the rolling process. This process is fast and also economical. In this process, the liquid steel is poured into a double walled, bottomless water cooled mould made up copper where a solid skin is quickly formed and a semi-finished skin emerges from the open mould bottom. The skin formed in the mould is about 10 to 25 mm thick and is further solidified by intensive cooling with water sprays as casting moves downwards. A typical arrangement of continuous casting plant is shown in the figure. The molten steel is collected in a ladle and kept over a refractory lined intermediate pouring vessel named tundish. The steel is then poured into water cooled vertical moulds which are 450 to 750 mm long. Before starting the casting a dummy bar is placed in the mould bottom. After starting the casting process as the metal level rises in the mould to a desirable height, the starter bar is withdrawn at a rate equal to the steel pouring rate. The initial metal freezes onto the starter bar as well as the periphery of the mould. This solidified shell supports the liquid steel as it moves downwards. This steel shell is mechanically supported (rollers) as it moves down through the secondary cooling zone where water is sprayed onto the shell surfaces to complete the solidification process. After the casting is completely solidified, it is cut to the desired lengths by a suitable cutoff apparatus.

Figure 1.6.9: Continuous casting

Reference 1.

Manufacturing Technology by P.N.Rao , TMH page 220to 227

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