Thermo Forming

Technical Manual THERMOFORMING

INDEX

Thermoforming principles -History of thermoforming industry -Products manufactured by thermoforming

4

Suitable polymers for thermoforming -Thermal properties -Temperature -Heat measurement -Specific heat -Thermal conductivity

7

Heating plastics -Heat transfer: conductivity, convection and radiation -Thermal properties of plastics -Heat transmission media -Temperatures and forming cycles -Establishing the right temperature for the material

11

Thermoforming equipments -Gas furnaces with pressured air circulation -Infrared heating furnace -Lineal heating electric resistors

17

Complementary equipment: vacuum, pressured air and mechanical forces -Vacuum forming -Pressured air forming -Mechanical forming -Combined techniques -Mechanical support design Thermoforming molds -Choosing thermoforming technique -Criteria to design thermoformed products -Criteria to design thermoforming molds -Considerations in designing thermoforming molds -Materials used to manufacture tthermoform ing molds 2

Thermoforming

25

31

Thermoforming techniques -Bi-dimensional thermoforming -Tri-dimensional thermoforming (with molds) -Molding techniques in infrared heating furnace

46

Cooling thermoformed products -Conventional cooling methods -Non-conventional cooling methods

51

Cutting thermoformed products -Cutting equipment -Cutting techniques

53

Thermoforming variables -Material variables -Mold variables -Pre-stretching variables -Mechanical support variables

58

Problem and solution guide

62

Appendix

68 -Glossary -Glass fiber reinforced plastic -Unit conversion table

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Thermoforming

Thermoforming principles History of thermoforming industry

Since the beginning of the XX century some techniques to form sheets, with materials such as metal, glass and natural fibers, have been known. The true thermoforming principles emerged as thermoplastic materials were developed, which happened during the second world war. The post-war period brought about mass commercialization and rapid development of equipment and machinery capable to adapt to the manufacturing modern methods, to make more useful and income yielding products. In the 50s, the volume of thermoplastic material production and the products made with it reached impressive figures. In the 60s, by developing the thermoforming industry, the foundations for the future were established. Then huge consumers and product competitiveness, in the 70s, required high speed productive machinery. Equipment manufacturers met those needs by making machinery capable to produce about one hundred thousand thermoformed individual containers per hour. Sophisticated controls were also required. Since the 80s up to the present, thermoformers have so much relied on their process that they have gone beyond their expectations and have established production lines that can produce finished thermoformed products, not only from sheets but also from resin pellets; besides, they recycle the scrap with minimum control. Equipments have been computerized and at present, they can perform auto-monitoring and diagnostic functions. Nowadays, very complex equipment does not require more than one worker to handle and control it thanks to electronic advances. Thus, it is believed that the thermoforming industrial labor market will undergo a shortage of technically trained and experienced personnel, since traditional knowledge will no longer be enough. Therefore, lectures, seminars, courses, etc., would be useful to increase thermoformers´ general knowledge, and would further advance this well established industry.

Manufacturing thermoformed products

Many of the thermoformed products in use at present have been manufactured to replace their original use forms. This has taken place so fast that those original ones have been almost forgotten. For example: it is not easy to remember in what hamburgers were packed before the arrival of the one piece polystyrene package or what kind of material lined the interior of refrigerators. The following list begins with the area with the most number of thermoformed pieces and continues in a decreasing order up to the one with the fewer pieces. Packaging industry Since the beginning of the thermoforming process, the packaging industry has been the most benefited due to the high productivity and benefits (cost-profit) that it offers.

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Thermoforming

At present, most of the packaging equipments (blister) are high speed automatically sustained. These equipments are called "form-fill-seal" and are used to pack cosmetics, cold cuts, sodas, candies, stationery, etc. Take away food industry In the growing "take away food" industry, a great deal of thermoformed products are used, ranging from a complete meal container (divided containers), to hamburgers and sandwich packages, sodas, etc. Usually, that industry requires printed thermoformed packages. This printing can be made before or after thermoforming. Some examples of this are trays, cups, sandwich, hamburger, hot-dog packages, etc. Food packaging industry Supermarkets are the great consumers of thermoformed containers. The materials used are low-cost thermoplastics. These are designed to be piled or placed in different forms. Examples: meat, fruit, eggs and vegetables containers. Transport Public and private transport such as bus, train subway, plane, car, etc., has within its equipment many thermoformed plastic parts. Most of these are used for inside finishing or non-structural exterior parts. In others, they are used for seats, backs and arms of seats, fronts of doors, service tables, wind-shields, instrument protectors, guards, spoilers, etc. Signaling and advertisements These are usually made of acrylic and can consist of only one piece and can be very large. Transparent (clear) acrylic is generally used and it is painted on the inside using acrylic based paint. The use of acrylics for exteriors makes advertisements weather resistant and they virtually need no maintenance; furthermore, they can stand extreme cold or hot weather conditions. Exterior lighted bill-boards, interior advertisements, signaling in public places, offices, etc., are some examples. Household products There is a great deal of products that have thermoformed parts; actually, they are produced in great quantities. They can be found in cabinet, washing machines, dish washers, dryers, refrigerators, air conditioning outlets, humidifiers, T.V. and radio cabinets, etc. Food industry One of the oldest and greatest thermoformed product consumers is the food industry. The use of trays and other accessories has a greater potential use, besides the great 5

Thermoforming

users like hospitals, nurseries, schools fairs and others, there are the military sector and international aid organizations. Some examples of products are: trays, cups and plates. Medical industry The medical industry requires a great variety of products and sterilized packaging for hospitals, clinics and doctors´ offices. The specifications for these products are usually very strict and recycling materials is unacceptable. The use of acrylic , since it is physiologically harmless, is growing every day. Some examples are: chirurgical equipment, syringes and needles, chirurgical tables, cabinets, incubators, dentists´ seats and exercise platforms. Agriculture and horticulture Commercialization of decoration plants in supermarkets and specialized shops has generated, for some time, the need to make flower pots and small containers, including with multiple divisions for exhibiting and selling. This kind of containers are made of recycled plastic at low cost. Flower pots, different size and divided containers, small green houses, trays for growing seeds, planting containers, etc., are some examples. Constructión and housing For some years, construction industry has used thermoformed products, which have become quickly popular. Thermoformed parts have easily replaced a lot of products. Actually, there are products that cannot be manufactured any other way, such as skylights or cannon arches. In this sector, acrylic is used a lot because of its weather resistant properties and its thermoforming quality. Examples of these are: skylights, cannon arches, hydro-massage tubs, bath modules, wash basins, bathroom screens and cabinets, tables, chairs, lamp stands, kitchen items, stairs, frontages, partings, windows, aquariums, etc. Luggage Some luggage manufacturers are deciding in favor of using the thermoforming process, since it has advantages over the injection products. Because it is molded effortlessly, the possibility of thermoformed products fracturing is reduced. Examples: all kinds of suitcases, briefcases, etc. Photography equipment One of the oldest thermoformed products is the tray used for developing photos, also flash bulbs (metallic reflector) and the magazine for standing cameras, even though its manufacturing requires a precision thermoforming technique.

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Thermoforming

Suitable polymers for thermoforming Basically, every thermoplastic polymer is suitable for the thermoforming process. Those materials, when exposed to heating, show an elasticity, hardness, and resistance capacity, under load variation in their module. With an increased temperature over the H.D.T., the material will tend to become rubber-like, having as critical value the temperature of annealing of the thermoplastic polymer. This can be seen in the rapid bending of the hot sheet, when the force of gravity is strong enough to cause this deformity. Table 1 shows the suitable and most common polymers for thermoforming, as well as their temperature.

HEATING DEFLECTION TEMPERATURE POLYMERS

Extruded acrylic Cell-cast acrylic Cellulose acetobutyrate High density polyethylene Polypropylene Polystyrene High impact polystyrene SAN ABS Polyvinyl chloride (RV.C.) Polycarbonate

Thermal properties

AT 264 PSI (ºF)

AT 66 PSI (ºF)

201.2 204.8 149-167

208.4 230 167-176 140-176 230-239 158-212 194-203 221 176-248 167 248

131-149 158-203 185-203 212 167-239 158 266

WITHOUT CHARGE (ºF)

THERMOFORMING TEMPERATURE SHEET TEMP. (ºF)

275-347 320-356 248-302 284-320 212 293-374 293-392 284 284-338 212 338-356 248 428-446 248-356 203 275-347 230 356-446 320

MOLD TEMP. (ºF)

AID TEMP (ºF)

149-167 149-167 203

338

113-149 113-149

194 194

158-185 113 203-248

194 176 284

One of the least considered aspects in thermoforming practice, is that of the thermal properties of polymers which is one of the most relevant and critical aspects of the process. Wholly understanding these factors will reduce the risk of long pre-production run or bad adjusting of the product to the outline. When we talk about thermal properties, it is indispensable to establish the concepts related to this topic. First, it must be remembered that energy often dissipates through friction and then it appears as heat or the inner thermal energy of a body. Of course, some times, heat in a substance is increased deliberately to change its temperature or its form.

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Thermoforming

Specific heat and thermal conductivity are two of the physical properties of polymers that are extensively used in thermoforming. Temperature

In the thermal phenomenon debate some terms and concepts must be included. The first thermal property is temperature. Temperature is the measurement of the degree of "heat" or "cold" in an object. A temperature scale must be established, water properties have been taken as a parameter, specially the degree of ice fusion and water boiling. There are three scales to measure the temperature of a substance: the scale in centigrade degrees (°C), Fahrenheit (°F), and Kelvin (°K), the first two are the most commonly used.

Heat measurement

Heat is simply a form of energy, therefore, the suitable physics unit to measure heat is the same as the one for mechanical energy and it is the joule (J). As in the case of temperature, water is used as parameter of substance to define the heat unit. The amount of heat needed to raise the temperature of 2.2 pounds of water by one degree [at present it is taken as 58.1ºF to 59.9 ºF (14.5 °C to 15.5 °C) is defined as 1 calorie (cal)].

Specific heat

When a calorie is added to 2.2 pounds of water, the water temperature increases 33.8 degree, for example: if the same amount of heat is added to the same amount of methylalcohol, the temperature rises about 35.06 degrees, or if 1 cal. is added to 2.2 pounds of aluminum, the temperature of the metal rises about 41 degrees. In fact, each substance will respond differently when exposed to heat. The amount of heat needed to raise 33.8 degree in 2.2 pounds substance is called specific heat of that substance. Water works as a parameter and it has been determined as 1 cal./pounds, and it is taken as a basis to compare every material. Excepting water, most materials have a specific heat, lower than plastics.

Thermal conductivity

Thermal conductivity is one of the three ways by which heat energy can be transferred from one place to another; it results from the molecular movement and therefore, it needs the presence of matter. Heat energy is transferred by collisions where the rapid movement of atoms and molecules of the hotter object transfers part of the energy to the colder object or the one with a slower movement of atoms and molecules. When a substance is heated, it expands, heat increases the volume of a substance and diminishes its density. The thermal conductivity of acrylic is 0.0005 cal./seg. cm2

Thermal expansion

Thermal expansion derives from increasing the temperature of a substance, and as a consequence it expands, actually, almost every substance: solid, liquid or gas has the property to increase its size, as its temperature rises. As for thermoforming, when a polymer is heated the mobility of molecular chains increases, therefore, they tend to separate from each other, increasing the volume and area of the polymer. This property is extremely important especially in thermoformed pieces, which are exposed to sudden changes of temperature or weather conditions. In thermoforming, the plastic sheet is expanded more rapidly than the metal frame, creating some wrinkles near the frame, which disappear when the sheet contracts. The numeric values of the coefficients for heating and cooling are identical; this means that it takes the same time for

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Thermoforming

a piece to get hot as to get cool. It must be taken into consideration that there might be problems when the thermoformed parts have to be within a very close dimensional tolerance. There might be other kinds of problems when there is shrinkage in a male mold, making it difficult to remove the part from the mold. The thermal expansion coefficient of acrylic is 0.00009 cm./cm./°C.

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Thermoforming

Heating plastics Heat transfer: conduction, convection and radiation

In the thermoforming process, the heating operation is one of the longest stages in which there might be present the most difficulties and material and human resources waste. That is why this chapter is devoted to heat transfer, aiming at trying to clarify phenomena that might occur in plastics heating operation. Although scientists have divided heat transfer into three different phenomena: conduction, convection and radiation, in practice, the three phenomena are concurrent. Conduction This is heat transfer from one part of a body to another part of the same body, or from one body to another which is in physical contact with it, without a substantial displacement of the particles of the body. Convection This is heat transfer from one point to another, in a fluid, gas or liquid (by mixing one part of the fluid with another). In natural convection, the movement of the fluid totally derives from the difference in density as a result of different temperatures. In the forced convection, which is the one we are interested in, the movement is produced by mechanical means. When velocity is relatively low, it must be noted that free convection factors, such as different temperature and density, may have an important influence. Radiation This is heat transfer from one body to another that is not in contact with it, by means of a wavy movement through space. For the purposes of thermoforming process, three media for heat transfer are considered: A) Contact with a solid, liquid or hot gas. B) Infrared radiation. C) Internal excitation or by microwaves. The first two ones are very much used in plastic thermoforming and for several of them the scope of temperature is between 120°C and 205°C (250°F and 400°F).

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Thermoforming

Thermal properties of plastics

Plastics are poor heat conductors; therefore, thick sheets need a considerably long time to heat. In table 8, there are some thermal properties of some materials to be compared. In plastic thermoforming the method and size of the heating equipment must be taken into consideration. Heating a sheet on both sides (sandwich-like heating) helps to reduce the time taken in this operation. In some cases, heating time can be reduced if the sheet is pre-heated and kept at a medium temperature; however, this is rarely done with less than 6mm. thick materials. In addition, the amount of heat required to raise the temperature of plastics is high, compared with any other material; except water. To estimate the needed heat for a sheet, the following formula can be used. Required heat = Length X width X thickness X density of material X (specific heat X different temperature + fusion heat) Table 8: Thermal properties of some materials.

MATERIALS

Air Water Ice Soft wood Hard wood Phenol R. Epoxy R. Polyethylene Acrylic Polycarbonate Graphite Glass Quartz Aluminum Steel Copper

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Thermoforming

SPECIFIC GRAVITY g/cm3

SPECIFIC HEAT Btu/ Ib 0F

0.0012 1 0.92 0.5 0.7 1.5 1.6-2.1 0.96 1.19 1.2 1.5 2.5 2.8 2.7 7.8 8.8

0.24 1 0.5 0.4 0.4 0.3 0.3 0.37 0.35 0.30 0.20 0.20 0.20 0.23 0.10 0.092

FUSION HEAT Btu/lb

144 144

55

171 171 88

THERMAL CONDUCTIVITY Btu ft/sq ft hr 0F 0.014 0.343 1.26 0.052 0.094 0.2 0.1-0.8 0.28 0.108 0.112 87 0.59 4y8 90 27 227

THERMAL COEFFICIENT of LINEAL in/in 0F10-5

2.8 1.5 1.5 3-5 1.5-2.8 7 3.5 3.7 0.44 0.5 0.4 y 0.7 1.35 0.84 0.92

Heat transfer media

For practical purposes we will divide the media for heat transfer into 4 types: Heating by contact The fastest heating method is placing a plastic sheet directly in contact with a hot metal sheet. It is specially used in mass production of small and thin items. Heating by immersion With this method, a plastic sheet is immersed in some liquid that transmits heat as evenly and quickly as possible, but its use is restricted to molding parts out of huge or very thick sheets, since handling and cleaning of the piece are very difficult Heating by convection Furnaces with air convection are widely used, because they provide even heating and can, to a certain degree, dry some materials that contain some degree of moisture. These furnaces provide a huge safety margin as for time variations in thermoforming cycles. Important note: All the above mentioned heating media require a considerable amount of time to preheat the equipment. Infrared heating: This method can supply instant heating and therefore, its exposition cycles are very short, and sometimes it takes only a few seconds. The main sources of this kind of energy are: -Quartz lamps that emit in the visible and near infrared. -Ceramic or metal resistors that emit more energy in the far infrared. The surface of these radiation heaters can be between 599 ºF and 1301 (315°C and 705°C). It must be noticed that at the highest temperatures, the mass of radiation occurs at shorter wave lengths. On the other hand, at lower temperatures, radiation expands on longer wave lengths; and this is extremely important, since each plastic absorbs infrared radiation in different areas. Only the radiation absorbed is used to heat plastic directly. Internal heating This method has not had enough application in thermoforming because the equipment used is very expensive. Besides, it is not suitable for every plastic, and cooling time is very long. It is useful in forming processes where localized heating is required on a specific area of the material. For example, when forming edges of material which has a high loss factor, such as P.V.C.

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Thermoforming

In certain applications, thermoformed products show uneven parts, even when a sheet has been uniformly heated. Heterogeneous shrinkage of a sheet is due to the very design of that part. In those special cases, controlling heat by section will give more uniform wall areas. This procedure is called shading or screening and it consists in placing a non-flammable filter to regulate heat (a wire net, asbestos, etc.) between the sheet and the source of heat, this will reduce the flow of heat to certain areas of the material, and will prevent excessive stretching on that area. In more sophisticated equipments, at present, there are electronic controls and ceramic parabolic elements that allow variability when heating different areas of the sheet. Temperatures and forming cycles

Before we start with temperatures and forming cycles, we will establish some terminology: a) Temperature to remove items off a mold b) Operation: bottom limit c) Normal temperature to form d) Operation: top limit Temperature to remove item off a mold It is the temperature at which an item can be removed off the mold without distortion. Some times an item can be removed at higher temperature if cooling devices are used. Operation bottom limit This represents the lowest temperature at which the material can be formed without internal effort. This means that the plastic sheet must touch each corner of the mold before it reaches its bottom limit. The material processed under this limit will show internal effort that later will cause distortions, glow loss, cracking and other physical changes in the finished product. Normal temperature to form This is the temperature at which a sheet must be formed in a normal operation. It must cover the whole sheet. Shallow thermoformed items with the aid of air or vacuum will allow a bit lower temperatures, and this translates into shorter cycles. On the other hand, deep forming requires high temperatures, as well as for pre-stretching operations, details or intricate radiuses. Operation top limit Under this temperature a thermoplastic sheet begins to degrade, and it also turns too fluid and cannot be handled. These temperatures can be exceeded, but only with modified formulations that improve the physical conditions of the sheet. Injection and extrusion molding, actually use much higher temperatures, but only for very short periods of time.

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Thermoforming

General recommendations a) The characteristics of a finished product are determined by the kind of thermoforming technique used. b) The material must be heated evenly at the annealing and forming point, before it cools below its molding temperature. c) Acrylic must cool slowly and evenly while it is in the mold. d) The formed piece must be cool before any finishing is done, like spraying paint or serigraphy. e) In the design of a piece, a 2% shrinkage in both directions and a 4% increase in thickness must be taken into consideration, as well as a 0.6% contraction at 1% when cooling Temperatures and forming cycles As it was previously mentioned, one of the most important steps of the thermoforming process is determining the right temperature of the material. For acrylic, the right selection of annealing or normal temperature will prevent: At a low temperature: Internal effort concentrates in the thermoformed piece which later, under sudden environmental temperature changes, will emerge as fissures or cracking. At high temperature: Bubbles and mold marks, due to extreme heating. Table 9 shows the ranging temperatures for Plastiglas acrylic sheet, for general use, and Sensacryl FP¨, deep molding sheet. Table 9 TEMPERATURE RANGE KIND OF MATERIAL

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BOTTOM LIMIT (OF)

TOP LIMIT (OF)

Plastiglas (general use)

320

356

Sensacryl (deep molding)

356

392

Thermoforming

In Mexico, due to the high cost of electricity, it is more common to use a convection furnace with pressured air re-circulation by means of gas, for which a very practical formula is very useful to determine the permanence time for an acrylic sheet, taking into consideration the annealing temperature range previously adjusted. Formula: 53.3 X E (inches) = T (min.) Where : 53.3 = Factor, E = Thickness of material, T = time. This formula can be used for thin (0.04 to 0.24 inches) Chemcast sheets. For thicker sheets, the factor has to be changed as follows: Formula: 3 X E (inches) = T (min). Ex: 53.3 X 0.118 = 6.30 min. As it has already been mentioned, there are variables that may modify these formulas, such as: environmental temperature of the place where the furnace is located, cure (especially in extreme weather conditions), material thickness fluctuation and the conditions of the equipment among other things. Forming temperature Every thermoplastic material has a process specific temperature. These ranges apply without taking into consideration the way the material will be processed. The most used materials compared with acrylic are mentioned in table 10: Table 10, Ranges of forming temperature

MATERIAL

Acrylic CHEMCAST Sensacryl FP ABS Polycarbonate AD Polyethylene

EstablishIng the right temperature of the material

SHEET TEMP. (0F ) 320- 356 356-392 257-356 392-482 320-428

BOTTOM NORMAL (0F ) LIMIT 0 (F) 320 356 257 392 320

338 374 329 455 374

TOP LIMIT (0F )

REMOVAL TEMP.

(0F )

MOLD TEMP. (0F )

MECHANICAL SUPPORT TEMP . (°F)

356 392 356 482 428

248 266 185 284 185

149-167 158-176 158-185 194-248 194-212

210 248 338

Another important factor in the thermoforming process, is establishing the right temperature for plastic material. You must bear in mind that apart from the heat transmission medium, a sheet must be heated at the recommended range of temperature (annealing range), besides, a sheet has to be heated in an evenly way. In practice, it is not easy to accurately establish the temperature of the sheet, even when using contact thermometers; therefore, this determination is based on the performance of a sheet. The gradual change in which a sheet yields during the heating

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Thermoforming

process (annealing point), is one of the cues to establish the right temperature. Some controls for infrared radiation thermoforming equipment have been developed, where a sheet is fastened horizontally, and the "yielding" or "bending" phenomenon is used, and photo-electric cells control heating time and/or temperature.

Clamp

Frame Vacuum box

Photo-electric cells Solenoid valve controlled by photoelectric cells.

However, this criterion cannot be applied indiscriminately to every plastic, since some materials may over-heat before they begin to yield or bend. Although a range of temperature is established, the expected temperature of a sheet may not be achieved; this may be caused by: a) Fluctuations in the thickness of the material b) Temperature changes in the equipment and/or environment c) Minimum fluctuations in the line voltage (in infrared equipment). d) The regulator of the pressured air circulation gas equipment may not be the right one, there is not enough gas pressure, the burner is not the right one or it may be blocked with soot, etc. There are cone formed pyrometers, infrared radiation or gas (hot air) heating tablets, that can render a more accurate measurement. Although probably, the best way to measure the temperature of a sheet is by means of an infrared pistol, which measures by zones; though the equipment is expensive, it is the only one that measures the temperature of a sheet accurately and reliably.

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Thermoforming

Thermoforming equipments Originally, convection furnaces were the first equipments to heat plastic sheets that were going to be thermoformed, and up to now, that kind of heating is still preferred for sheets of different thickness, and for temperature even distribution. Heat can be applied with gas or electric resistor units. To produce air circulation from 4,500 to 6,100 cm3/min. (150 to 200 feet3/min), pressured air re-circulation and deflectors are crucial to get homogeneous temperatures. The furnace temperature must be adjusted to the plastic forming temperature. Infrared radiation heating, compared with oil immersion or contact heating (the two latter very limited in practice), is extremely rapid. For example, a 3.0 mm sheet heating time by infrared radiation can be achieved in one min. at about 10 watts/inch2. Because infrared radiation heating takes very little time, heat energy absorbed by a sheet may cause over-heating, that may even affect the degrading of the material (bubbles or burning) if it is not controlled. It is important to consider that in long runs, the furnace temperature has to be gradually reduced. In some cases, when the product has intricate or very deep sections, there is the risk of the thickness of the material considerably thinning; in this case screens must be used (they may be made of perforated plate or metallic display) to prevent over-heating. The elements of infrared radiation can be obtained in a very wide range of designs, according to their importance they are: 1.- Tungsten filaments in quartz tubes or lamps, temperature 3992 ºF (2,200 °C). 2.- Spring- like nichrome resistor on refractory ceramic bases. 3.- Nichrome resistors protected by plate or stainless steel tubes. There are manufacturers who make infrared radiation thermoforming machines in a wide variety of sizes, capacity, degree of automation and versatility. The specifications to acquire a thermoforming machine vary depending on the finished product that you want to get and therefore, it is necessary to consider: Voltage, wattage, amperage, useful area of forming, number of heaters (lower and upper), controls to regulate temperatures by zones, degree of automation, capacity to

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Thermoforming

accept mechanical support, type of sheet fastening device, (clamps, mechanical, pneumatic, etc.), ventilators to cool the product, general dimensions, production capacity, cost- profit. Gas furnaces with pressured air circulation

This kind of furnace supplies uniform heat and constant temperature, with a minimum risk of over-heating an acrylic sheet. Electric ventilators must be used to force hot air circulation on the acrylic sheet at a speed about 4,500 to 6,100 cm3/min., and devices to distribute the air in every zone of the furnace. Gas furnaces need heat inter-changers to prevent accumulation of soot due to the gas flow, as well as controls to interrupt the gas flow, when necessary. Electric furnaces can be heated, using sets of 1000 watts resistors. When using a furnace with a 10 m3 capacity, about 25,000 power watts will be consumed and half of this will be used to compensate heat loss due to leakage, insulating transmission and the use of doors. A minimum 2" thick insulation is advised and the doors of the furnace should be as narrow as possible, to reduce most of the temperature loss. Automatic devices must be used to strictly control temperature between 32 ºF and 482 ºF (0 °C and 250 °C). To get a more uniform sheet heating, it is important to hang it vertically, and this can be done with a system that fastens the material all along with clamps or canals with springs which move on wheels that slide on rails, like the ones used for closets. Basic criteria to construct a gas furnace with pressured air circulation. The best advice in this case, is asking any industrial furnace manufacturer to build one with the mentioned characteristics, since the construction of one, specially the heating and operation systems, is very risky for anybody who has only little knowledge on the subject. This kind of equipment must be approved by specialists in gas installations, it also has to be registered before the corresponding authorities. It is also relevant to point out that the information provided here, is only related to the metallic structure and fastening system for acrylic sheets. A furnace construction can be divided into the following sub-systems: A) Structure B) Fastening acrylic sheet C) Electric system D) Gas installation E) Controls

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Thermoforming

Recommendations to build a furnace Building the structure with commercial iron tubular of 11/2" X 11/2" or 2 X 2". a) Cut it according to the measurements and requirements of design. b) Weld the lateral walls. c) Weld the upper wall, the lower one and the back one; to join them with the lateral ones, and build the whole structure. d) Line the inner part of the structure with a black plate cal. 18 and weld it or rivet it with "pop". e) Cover the holes (thickness of the tubular) with a rigid sheet of glass fiber to get thermal insulation, code RF-4100, or a similar one. f) Line the exterior with a black plate cal. 18 and rivet it with "pop" or weld it. g) Make the doors with a structure of tubular PTR 1" X 1", and follow the same instructions as for the walls, they should be shorter to leave room for the rails. h) Attach the doors to the furnace with hinges. i) Put the closet-type rails, they should be twice as long as the furnace. They are fixed with screws on the upper part of the furnace. Once they are fixed to the furnace and the furnace on its place where it will operate, using bearings fasten the rails to the ceiling or structure of the place.

GAS STRUCTURE WITH AIR RE-CIRCULATION Rectangular tubular profile of 11/2” X 11/2” ó 2” X 2”

The electric ventilator is placed in this section to force the air

Every joint must be welded with electric welding Closet-type rails Plate "U" bearings of 1/4”

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Thermoforming

FASTENING SYSTEM FOR ACRYLIC SHEETS 1/4” iron plate 5/16" Cold rolled bar

Iron hinge Spring

1/4” crossbar handle

Washer

Nut

Type C profile cal.# 18

Acrylic sheet

FURNACE FRONT VIEW AND DOOR DETAIL AND RAILING SYSTEM

Steel cable to fix it to the ceiling of the place.

1

1/2”

x 11/2” iron angle

1 3/4” x 2” (1500 rail) closet- type profile No. 50 wheels Furnace door

Hook formed 1/2" cold-rolled bar. 1/2” cold rolled bar.

Joint of the fastening system for acrylic sheets.

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Thermoforming

LATERAL VIEW AND DETAIL OF THE FURNACE DOOR AND RAILING SYSTEM Steel cable to fix it to the ceiling of the place 1 1/2” x 1 iron angle

1/2”

1 3/4” x 2” (riel 1500) closet -type profile

No. 50 wheel

2 1/2” x 2 1/2” iron angle Furnace door

Infrared heating furnace

21

It is normally used in automatic thermoforming machines, heating a sheet by means of radiation at a speed 3 to 10 times faster than in a pressured air circulation furnace, thus, with very short heating cycles. It should be noted that the ratio temperature/time becomes critical and it is harder to heat the material uniformly.

Thermoforming

Infrared energy is absorbed by the acrylic surface exposed, rapidly reaching temperatures over 356 ºF (180 °C), that later on, is transmitted to the center of the material due to temperature conduction. Infrared radiation heating can be obtained using tubular metal elements, spring electric resistors, or by grouping infrared light lamps. To get a more uniform heating distribution, a net or metallic mesh can be placed among the heating elements and the material which can work to expand the temperature. It is also convenient to place an infrared heating plate, about 12”from the material and 20” from the bottom plate. To regulate energy input into the equipment, we recommend using devices such as different transformers or percentage meters that will help to control temperature. Planning electric energy charges and great capacity equipment is also advisable, an electric sub-station will also be needed.

Lineal heating electric resistors

An electric resistor can only be used to make bends in a straight line; to achieve this, you also need a spring type electric resistor (20) or armored type (about 1KW X 1.2 m.). Lineal resistors are made of wire, inside Pyrex ceramic tubes. The material must not be in contact with the tube to avoid marks on the surface. A distance of 6 mm. from the tube to the material is recommended to get uniform heating on thin material. When more than 3.0 mm thick material is going to be heated with this procedure, the resistors should be placed on both sides of it. In the next picture, it is shown how an asbestos plate bender at the beginning of production will provide a suitable bend, but as production advances, the heating area expands making a bigger radius bend, that is why a resistor with water re-circulation is much better for acrylic bending.

22

Thermoforming

Acrylic Sheet

Heating zone

Asbestos plate

Electric resistor

Acrylic Sheet

Heating zone

Asbesto s plate

Electric resistor

Basic criteria to build a lineal heating electric resistor. Bi-dimensional thermoforming or lineal bending, can be made with a spring type resistor or a tubular one. Building these equipments is conditioned to thickness, kind of bending and volume to be produced. Generally, a 1.32 yd. long resistor is the most common, though a 24” one is also acceptable, the specifications for this resistor are 1Kw for each 1.32 yd., thus, with a rule of three consume can be deduced both for a longer or a shorter resistor. Acrylic benders are more common than the ones built with asbestos plates on the lateral walls, these are suitable as long as you do not have to produce a huge volume, since when asbestos plates are exposed to the same infrared radiation they tend to get hot and therefore, the heating area will expand changing a piece production standard. In other words, at the beginning of production, there will be small radiuses and as production advances, the heating area will be wider creating a bigger radius. An electric resistor bender with water re-circulation will be more effective and produce better quality bent pieces. This equipment needs tubular profiles that allow water recirculation, which will keep the surface cool and will only allow a heating zone. The required materials to build this kind of bender are listed below. It is important to include a rheostat to control temperature intensity on an acrylic sheet, since it will provide the suitable pace of production and, obviously, it will reduce costs of electric energy.

23

Thermoforming

ASBESTOS PLATE FOLDER • Spring-like, tubular or nichrome tape resistor • No. 16 or 18 cable with glass fiber insulator • Terminals. • 2 X 14 Heavy duty cable • Plug • 500, 1000, 2000 or 3000 watts dimmer • 1/8", 3/16" o 1/4" asbestos plate

24

Thermoforming

WATER RE-CIRCULATION FOLDER • Spring-like, tubular or nichrome tape resistor • No. 16 or 18 cable with glass fiber insulator • Terminals • 2 X 14 Heavy duty cable • Plug • 500, 1000, 2000 or 3000 watts dimmer • 3/4" x, 3/4" aluminum tubular profile • 6.6 yd. hose • Clamps • 10 to 20 lt. container • Garden water pump

Complementary equipment: vacuum, pressured air and mechanical forces The thermoforming process consists in heating and softening a sheet of any kind of thermoplastic material and making it adopt the form of the corresponding mold to get an almost finished product with a particular form. Some times, an external force has to be used to turn a flat sheet into a different form and to make it copy the outline and details of the mold. The level of energy or use of this force must be adjusted, so that the plastic sheet can be easily forced to take another form. The most common used forming forces in the thermoforming process are: vacuum or pressured air, mechanical forces and the combination of these three. Choosing a forming force in the forming process generally depends on the size of the product, the volume to be produced and the speed of the forming cycles. In addition, the following factors must be considered, since any of these can make a difference in selecting the forming force: a) Intrinsic limitations of each thermoplastic material b) Construction and material of the mold c) Thermoforming equipment available Vacuum forming

The oldest method to form a plastic sheet into a utilitarian piece is vacuum forming. The original description of the thermoforming process was precisely "vacuum-forming". The basic principle of the vacuum-forming process is having a softened thermoplastic sheet in a mold perfectly sealed and where the air inside is evacuated by the vacuum force or suction. As the air is evacuated from the mold, it creates a negative pressure on the surface of the sheet and therefore, natural atmospheric pressure yields, forcing the hot sheet to take the place of the empty spaces, as it can be seen in the picture. Acrylic sheet

25

Thermoforming

Vacuum equipment There is a great variety of vacuum pumps: reciprocal piston, diaphragm, blades, eccentric rotor, etc. All these provide a good vacuum but cannot evacuate great volumes of air at high speed; that is why a stock tank has to be connected to be used as "vacuum accumulator". On the other hand, there are compressors that can evacuate a great volume of air but are limited for vacuum force. A suitable vacuum system needs a pump that can displace from 28 to 29" Hg or from 0.5 to absolute 1 Psi (710 to 735 mm of Hg.) in the stock tank before the forming cycle. The line, duct or pipe between the stock tank and the mold should be as short as possible with a minimum of angles. It is important to eliminate air leaking due to damaged piping, perforated hoses, loose couples or nipples, as well as unnecessary valves. Rapid action or globe valves should be used. Vacuum pumps are available in one or two steps. A two step vacuum pump can evacuate pressures below 10 Psi; displacement capacity or evacuation for a one step pump is reduced by half. Table 11 shows vacuum pumps typical capacities

Table 11: Vacuum pump typical specifications SPECIFICATIONS No. OF CYLINDERS 1 2 2 2 2 3

DIAMETER (inches) 3.04 3.04 4.08 5.08 5.6 5.6

VACUUM THEORETICAL CAPACITY RUN (inches) 2.8 2.8 2.8 3.2 4.08 4.08

ONE STEP TWO STEPS (yd3/min) (yd3/min) 0.280 0.561 0.996 1.87 3.08 4.64

---0.280 0.498 0.935 1.54 3.08

SPEED

POWER

DIAMETER

(RPM)

NEEDED

OF PIPING

(Kw)

OUTLET

0.56 0.74 1.48 2.2/3.7 3.7 5.6

19 25 32 38 52 52

800 800 800 750 900 900

Vacuum tanks Excepting some vacuum equipments, most have a stock tank. Bearing in mind that work pressure is about 10 Psi (about 21 inches Hg/530 mm. Hg) vacuum, then the volume of the tank should be 2.5 times bigger than the volume between the molds, the vacuum box and the piping. Doubling the volume of the stock tank (along with other similar conditions) pressure can be increased 15% (11.5 Psi), according to what is established, the theoretical limit for the vacuum forming process is only 14.5 Psi.

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Thermoforming

In many cases, a rapid displacement of vacuum is very important. This can only be made by placing the vacuum tank as near the mold as possible and reducing the piping friction as much as possible, which can be done by: a) A bigger piping diameter. b) Piping with wide curves, avoiding 90° angles. c) Changes in the transversal section of the piping (diameter changes). Many equipments in the market do not meet these requirements. In general, the piping must be 1" diameter to displace 1 ft3 of air, for big pieces a 2" or 3" diameter is suitable. There should also be a flexible plastic hose internally reinforced with wire or a similar material that prevents it form collapsing; it should be connected between the mold and the piping, as shown in the picture.

Stock tank (400 lt.) 2” flexible hose

Bearings

globe valve

Solenoid valve

Air deflector

Vacuum forces, applications. In general, pumps work constantly to keep vacuum in the stock tank, there is a variation on the vacuum-meter readings in each cycle. The vacuum generated on the formed part must be kept enough time to cool and stand the internal force of the material which will tend to keep the original form, causing waves and bending. As a general rule, the faster the vacuum is made the better the piece will be formed. Occasionally, slow forming speed for deep forming pieces or intricate sections is recommended. When the matrix is very deep and when the configuration is problematic, slow vacuum can allow plastic more time to contract in the transversal section, this way a deficient configuration can be avoided. 27

Thermoforming

Pressured air forming.

In operations where vacuum force is replaced by pressured air, it should be considered that it is harder to seal the mold satisfactorily. The forming force can easily multiply up to 10 times if the pressured air is at 100 Psi. However, the molds can stand such pressure very few times. To form by using pressured air, it is necessary to take as many precautions as possible. A regular size mold requires a closing pressure of some tons, which obviously a common vise (type "C") cannot stand. Then, various clamps or rapid action fasteners, which are very useful in this case, should be used. With the pressure exerted, a badly built mold may explode like a bomb. An aluminum or machine finished metal mold is a good choice; resin or wooden molds must not be used unless they are reinforced with metal. Pressure forming equipment must be stronger than the vacuum forming one. It must have a similar tank for the compressor as well. Piping does not need strict specifications since pressure drop is not considerable. If in a piping pressure drops 5 Psi, pressure loss in the system will be 10 Psi, 50% of the pressure. But if the pressure system is 100 Psi, it will be 5%. A valve to reduce pressure and a manometer should be also installed, as well as a baffle or filter at the entrance of the mold, so that cold air is never in direct contact with a hot sheet. Some times, heaters should be incorporated to the air system, since they will help in great blows, which must be kept hot until a piece is formed on the mold. If possible, there should also be filters to eliminate water that tends to condense in the system and in the long run can make the equipment rusty, in addition, combined with air particles, it can block air ventilation orifices in the molds. Periodical maintenance is a must. Vacuum

Acrylic Mold

Pressured air

Vacuum orifices Acrylic Mold Air exhaust

28

Thermoforming

When needed, the mold should have orifices to eliminate the air caught inside and avoid wrinkles or deficient forming. Pressured air forming has become popular, specially for small pieces. The advantages of this method are: improvement on dimensional tolerance, forming speed can be considerably increased and fine details are better defined.

Mechanical forming

The thermoforming process is not limited to pneumatic techniques. There are several mechanical forces that can be applied. The simplest form of mechanical forming is used for bi-dimensional forming. In this case, a heated sheet is placed on the surface of a curved mold which is usually a smooth surface and gravity is enough to curve the sheet; the edge of the sheet should be fastened to keep it in position until it cools. That is the case for the manufacturing of the cannon arch whose sides are tightly fastened and there is not thickness variation. Mechanical forming, matrix and male mold. Matrix-male molding is used, among other things, to shape complicated pieces. In this molding technique, a heated sheet is shaped between 2 opposing but similarly outlined molds (matrix-male). When the molds are joined, the outlines force the sheet to take the same shape, in the space left between the two molds. Any protuberance on the male mold, mechanically, will force the plastic into the counterpart (matrix). For big or medium production, mechanical equipment is used to close the molds; in other cases, the movement is created by servomotors. If both molds have a controlled temperature, cooling time can be reduced. There are three basic criteria to achieve good thermo-shaping performance when using this technique. The first, is applied force, regardless of its source (pneumatic, hydraulic or mechanical), it must be strong enough to make plastic deform, of course, a huge surface or an intricate mold will need a bigger pressure force. The second refers to suitable elimination of the air caught inside. The pressure exerted between the two molds causes that air gets caught between them and the sheet, and air must be removed to shape the piece well. Boring some holes in one or the two molds in the areas where this anomaly is spotted, can eliminate the air. The third is related to the depth limit of stretching, that derives from the forces used in the process. It can be easily understood that maximum stretching is only successful when the mold has exit angles bigger than 5° and very big and smooth curve radiuses, the angles close to 90° may diminish stretching and even tear the plastic material. This sophisticated thermo-shaping method should not be used on the whole mold, its use is limited to only some parts of the mold.

29

Thermoforming

Combined techniques

Mechanically forming with matrix-male molds does not only depend on the forces used, usually, this kind of forming can be combined with vacuum, pressured air or both at the same time. Therefore, the matrix-male mold does not have to coincide accurately, the male mold may be relatively inferior in dimensions and have a substantially different form from the matrix. When male molds are made like this, they can act as "pushers" of a plastic sheet. This kind of support is called mechanical support, because it presses the softened material into the matrix. The purpose of this support is to stretch the material so that the final form is accomplished in combination of vacuum and/or pressured air. Using mechanical support in the process has the advantage of a better distribution of the thickness of a product, than using any other process. Many variations in the process can be obtained combining these techniques. Those variations can be vacuum pressure changes, vacuum or pressure application time, mold closing speed time or forming cycles.

Mechanical support design

Usually, mechanical supports are made of wood. Hard or tropical wood is the most used to make supports. In some cases, pieces of other plastic material such as: nylon, rigid polyurethane, acrylic, aluminum or steel, which are easily machine finished, can be incorporated. If production volume requires it, a cooling and/or heating system can be incorporated. The decision to heat or cool the support, must be made from the beginning of the design, since later on it will be harder if not impossible to try to adapt a heating element, that is why required machine finishing should be made to incorporate the system. When a support is very cold, a sheet will surely get cold on it. Cooling usually takes place between the points of a support and a sheet and the sheet and the mold. In extreme cases, the sheet may shrink on the support during the forming. The form of a support has a determining influence on the wall or thickness of a finished piece. In the next picture, there are three different kinds of support.

30

Thermoforming

Flat surfaced and blunt edged support This allows a sheet to stretch between the support and the edge of the mold, and meanwhile, the part of the sheet in contact with the edge of the support gets cool. A piece formed this way will have a thick bottom and thin walls Tin-like support In this second alternative, a sheet is in contact with the support and cools fast only on the perimeter of the support. Stretching is similar to that of the flat support, but the central area of the support allows extra stretching. Sphere-like support On the other hand, in this case, only a small area is in contact with the support. There might be a significant stretching as the support moves forward, therefore, the area of the perimeter between the edge and the support decreases.

Flat surfaced and blunt edged support

31

Thermoforming

Tin-like support

Sphere-like support

Thermoforming molds Choosing the type of thermoforming technique

One of the most important aspects to be taken into consideration in thermoforming pieces is the thermoforming technique to be used. Depending on the characteristics of the product if the wrong technique is used, there may be problems before you can get a piece with the specifications initially determined, finished. And many times the operation will fail, with the consequence of a waste of time, money and resources. Thus, before manufacturing a mold, the following should be considered: 1.- Form and dimensions of the piece. 2.- Desired aspect. 3.- Thermoforming technique. Based on these factors, you can plan and anticipate possible defects in the pieces. In this chapter all the variables that emerge when a thermoforming mold has to be manufactured, are analyzed.

Criteria to design thermoformed products

It must be mentioned that: products made using thermoforming technique, though this technique is versatile and flexible, regarding aspect and characteristics, differ from products manufactured using injection molding. In the following comparative table the basic differences can be analyzed. To conclude, to design thermoformed pieces the following criteria must be established: 1. - Thinning of material should be considered, this mostly depends on form, size and technique used (chapter 8). Generally, thinning of material is directly proportional to the height of a piece. 2.- A 3° and 5° exit angle of the mold should be considered. 3. - It must be taken into consideration that a piece will contract 0.6 to 1% when it cools. 4. - In general, the surface of a thermoformed piece will be smooth, though some textures can be obtained. 5.- In designing a piece, big radiuses should be included; there may be edges but they can tear the material.

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Thermoforming

Table 12 Basic differences between Injection and thermo-shaping processes.

VARIABLES Thickness Mold exit angles Molding temperature Dimensional tolerance Inserts Surface finishing Production Mold

May create ribbings, all types of holes, coils, etc. Scrap, material waste Radius

Time to make a piece (design, mold, tests). Subsequent treatment and finishing

33

Thermoforming

PROCESO INJECTIÓN

TERMOFORMING

Constant 0.5° to 1° 392ºF-464ºF (200°C – 240°C) Excellent Possible insertion of elements in other materials. Smooth surfaces or any other texture can be obtained. High production, hundreds or thousands of pieces a day. Steel with alloys or expensive treatment, complex design, matrix-male mold. Yes.

Variable 3° - 5° 320ºF-356ºF (160°C – 180°C) Relatively good, not for accuracy. Mold surface can be prepared for inserts Only smooth surfaces, some shallow textures Medium, some dozens a day.

Very little, recoverable.

From 3 to 6 months.

Depends on the shape, about 25% waste and recoverable. Larger radiuses, 0.4” to 2” needed. Depending on shape and depth. Maximum 1 month.

Any treatment or finishing, painting, hot-stamping, serigraphy, metallization, etc.

Any treatment or finishing, painting, hot-stamping, serigraphy, metallization, etc.

Must blunt edges, about 1.5 thickness of material.

Variety of materials, rather low cost, simple design, may use matrix-male mold. No.

Criteria to design thermoforming molds

The following criteria are key factors to successfully produce thermoformed pieces. They are the core of any development, but it is also vital to thoroughly analyze these concepts and later we will see in detail each consideration in the design of molds. Then, these basic criteria and considerations will be the fundamental parameters to manufacture thermoforming molds, regardless of their complexity. It should be noted that when these molds are manufactured, the following concepts must be assessed. 1. - Form and dimensions of the piece. 2.- Aspect of the piece. 3.- Estimated production volume. Probably the most important of these concepts is the estimated production volume, since it will depend on the definition of the kind of mold, material, finishing, thermoforming technique, etc. Next, the model designs are shown: 1. - A male mold is easier to use, less expensive and more suitable to form deep pieces. In general, a matrix should not be used to form pieces deeper than half the width of the piece. The matrix is used when the concave face of the finished piece must not be in contact with the mold. 2.- The molds must have enough vacuum orifices so that an annealed sheet can conform to the critical parts of the mold, the vacuum orifices have to be made in the deepest parts and areas where air is caught, and must be small enough not to leave marks (1/32" to 1/8" diameter). Vacuum can be more effective if the hole is enlarged from the inside. 3.- There must be ducts that allow water or oil circulation through the mold when temperature control in it is needed.

34

Thermoforming

4. - When the dimensions of a formed piece are critical, molds must be built bigger to compensate for the contraction of the material. Expected contraction from molding temperature to environment temperature is 1% maximum.

5.-A slight curving of the flat big areas of the mold will allow flat areas when the material cools.

6. - Pieces with 90° walls cannot be obtained; the mold must have an exit angle of at least 3°.

7. - Edges should be blunt, since vertex form accumulates internal efforts. A piece will be more resistant designing blunt edges and corners.

8.- The thin or weak parts can be reinforced with reinforcement ribs, which will also reinforce big flat areas.

35

Thermoforming

30

9.- If it is necessary to mold using a permanent incrustation, you should consider: the difference between the expansion coefficient and the various materials, otherwise, there can be a failure due to a forced insert, because of different expansions and contractions of the materials in contact. 10.- The surface of the molds can be lined with cotton flannel, felt, velvet, suede, etc, to diminish mold marks. The most common is cotton flannel.

Considerations in the design of thermoforming molds

One of the advantages of the thermoforming process is the diversity and kinds of molds that can be made at a very low cost and relatively fast, being highly accepted for other applications, over other processes. Usually and unlike injection molds, only half the mold is needed and it depends on the form of the product, desired aspect and chosen technique (may be male mold or matrix). Choosing the right one is much more important when the part to be thermoformed is very deep. When the pieces are shallow, profiles are small or when thinning is irrelevant, choosing will depend on the aspect of the piece. If details of the mold are important, then the side of a plastic sheet in contact with the mold surface should be the front of the piece. Some times, a bigger radius or smooth aspect is desirable if a sheet of material shows a nice surface, then the surface which does not touch the mold will be the front of the piece, besides, a dimensional control closer to the surface of the mold can be obtained. Thinning of the material Under every condition of thermoforming when pieces are formed of a plastic sheet, the area of the surface will get bigger, there will be some stretching and the material will get thinner.

One of the decisive factors of this thinning is the ratio, generally defined as maximum depth or height ratio with a minimum space through the opening. To estimate this thinning, the area of the available sheet to be thermoformed must be determined and divided into the area of the finished piece, including waste. It is always desirable that the

36

Thermoforming

molds and thermoformed pieces have generous curving radiuses. Theoretically, there is a formula to determine the thinning percentage of the material, considering that the material is uniformly annealed and stretched.

Thinning % =

Final thickness of the material Original thickness of material

=

available area of a sheet total area of shaped piece

=

AXB A X B X E (2C + 2D)

A=3 B=4 C=2 D=1 E=1

A=3 B=4 C=2 D=1 E=1

In practice, with a micrometer or calibrator you can determine thickness directly on the thermoformed piece, cutting small pieces on different sections. Other methods use translucent sheets and correlate color intensity vs. thinning of the sheet. Thickness can also be determined making squares with an oil marker on the sheet before thermoforming it and observing stretching of the material. One should consider the possibility of wrinkling on some critical areas or on the bottom of a male mold or matrix. If an annealed sheet cannot contract from the dimension A to E, excess material will create wrinkles.

In a matrix the opposite happens, the sheet will expand to the 4 vertexes of the mold surface, becoming very thin. This can be seen in most of the thermoformed tubs.

37

Thermoforming

Next, some techniques to prevent wrinkling are shown:

When low molding temperature is used, a sheet will keep a greater tenacity and elasticity. For big pieces, molding time and temperature should be increased on difficult zones to be thermoformed, minimizing this kind of defect. For deep molding sheets, because of their partially cross-linked structure, they tend to minimize wrinkling. When there are many molds, there should be enough room to prevent wrinkling, a distance 1.75 times the height of a piece, is suitable. Dimensional shrinking and tolerance. Dimensional shrinkage and tolerance in thermoforming vary for pieces formed on matrix or male mold. On a male mold, shrinkage can be reduced if the piece cools most of the time on the mold. If cooling reaches environmental temperature on the mold, shrinkage will be minimum. Thus, the internal dimension of the piece will be very close to the one of the mold, but then a production cycle will not be productive. However, the fact is that a piece must be removed from the male mold when it is still hot, otherwise removal will be difficult. This is exactly thermal shrinkage, which is the proportional difference between the environmental temperature and the one at the time of removal. Thus, to keep the specified dimension of a piece, the model must be slightly bigger. On the other hand, a piece formed in a matrix will begin shrinking as soon as the temperature of the material is below the one of forming. To keep a close continuous tolerance, the mold dimension must be considerably increased and vacuum pressure kept during the whole operation.

38

Thermoforming

As a guideline it can be assumed that shrinkage on male molds it is .127 mm/mm (0.005 in/in) and in a matrix it is bigger. For acrylic, polycarbonate, thermoplastic polyester and oriented polystyrene .203 mm/mm (0.008 in/in) can be considered. Anyway, one should be cautious about these values, since the following conditions can significantly alter them. 1.-Mold temperature: a difference of 15°F (10°C) can change shrinkage over 0.001 in/in. (0254 mm/mm). 2.- Size and thickness: this refers to the exit angle limited by the mold and the effect of greater thickness regarding temperature profile. 3. - Final use temperature: Due to expansion and contraction proportional to lineal expansion coefficient, a thermoformed piece will keep on varying with environmental temperature changes. 4.- Use extreme conditions: Shrinking can reach top values after the first exposition to the highest temperature of use. 5.- Molecular orientation: There might be bigger shrinkage related to the molecular orientation of the material. Some times, to prevent distortion and shrinkage, cooling templates are needed until a piece reaches the environmental temperature. Further more, the pieces thermoformed at a temperature below the one specified, tend to go back to their original state due to the plastic memory of the material. It is advised to monitor shrinkage and deformation during production. Aspect of the mold. It must be clarified that the surfaces obtained by injection and extrusion processes cannot be reproduced by conventional thermoforming techniques. Even highly brilliant materials may lose their glow during the process. In addition, they tend to emphasize mark and waving when they touch a cold mold and undergo thickness changes. A change of thickness will cause small distortions. Thus, cleaning the working area is a must. All the outlines should be rounded, actually, a mold with big radiuses will benefit the thermoforming operation, since the material will tend to stretch better

NO

39

Thermoforming

YES

If you want a sheet to copy details of a mold, like non-skid textures or similar ones, those detail should be at least three times bigger than the thickness of the material. Actually, it is better to have a not so smooth molding surface, this way, the piece will not copy the mistakes of the mold. It may even be sand-blasted with glass fiber micro spheres or an abrasive material. This way you can eliminate the air caught between the mold and the piece. Some times it is a good idea to sand the surface using rough sandpaper, this helps at the time of removal, to break the vacuum between the mold and the piece.

Superficie lisa, bien pulida

Superficie áspera

Vacuum bores When using thermoforming techniques with vacuum or pressured air, it is very important to eliminate most of the air between a mold and a sheet in a minimum of time. Depending on the kind of mold, 1/2" or 1" orifices can be used, as in the case of thermoformed skylights, up to homogenous distribution in all the vertexes of the mold. Metallic frame

Acrylic

Base 1/2” or 1” piping

These pictures show the distribution of the vacuum pressured air bores, typical for pressure-free forming molds, male mold and matrix In general, the diameter of vacuum bores should be slightly smaller than the thickness of the material. As a starting point, the vacuum bores will have a diameter equivalent to the final thickness of a thermoformed piece. This rule does not apply when the material is very thin or very thick, or when the marks of these orifices are irrelevant. It can be considered that a suitable range is from 1/32" to 1/8" diameter. To eliminate a 40

Thermoforming

great volume of air, 1/8" or _" diameter holes can be drilled. Depending on the manufacture of the mold, the bores can be widened on the inside of the mold, as shown in the picture. To reduce the time to eliminate the volume of air round a softened sheet and a vacuum box, the space can be refilled with polystyrene foam balls or polyurethane pieces.

Widened bores on the inside

Increased diameter bore

Another function of a mold is to contribute along with a frame to stabilize the position of a sheet and provide good sealing all around the mold. In some cases, a canal around the piece is helpful, exactly on the external zone of the cutting line. Mold cooling Some times when production runs are very long, the mold should have a cooling system, generally copper piping is used. It should be placed adequately and have enough capacity to carry a considerable volume of water or refrigerant. A relationship between the temperature of the sheet and the mold should be established so that the material does not get too cold and it does not thermoform below the bottom limit of the molding temperature. There are different methods to cool a mold, for example, when there are critical molding zones, plastic or poly-tetra-fluorine-ethylene inserts can be incorporated. In some cases, a plastic covering can be applied to reduce thermal conductivity, or even after thermoforming, pressured air can be injected through the bores or holes. Three cooling systems are shown in the next picture: First an undulated cooling system, the second is a branch system and the third is an external multiple alternative flow branch system with 2 inputs and 2 outputs. Branch system

Undulated system

41

Thermoforming

External multiple alternative flow branch

Mold supports As it has been mentioned before, when thermoforming a piece the material always gets thinner. Molding supports are used to get a better distribution of material in a thermoformed piece. Their purpose is to stretch a softened sheet, as a pre-forming. This technique is very important, specially with very deep pieces. In general terms, the molding supports can be made of the same material as molds. There are three categories of mold supports: Metallic supports Usually they are made of iron or aluminum, must be very smooth, with radius on the edges. The range of temperature is 10 to 15°C (10°F) below the temperature of the material, if their temperature is too high the sheet will stick to them. Thermal material supports These are made of wood, plastic or metal and they are built under the principle of a good thermal insulator. The surface may be of soft wood, plastics like nylon, or another thermofixed, synthetic foam or any other material including soft flannel. Skeleton type support Skeleton or frame type supports are only rounded bars welded forming intersections, which should be totally rounded to avoid tearing the material. Support dimensions are related to the size of a piece, since they have a great influence on the thickness distribution of the material. It must be noted that in some cases, by only changing the depth penetration of a support (75% depth of the piece), the thickness of the material between the faces and the surface can be controlled. Therefore, the equipment must have the required depth adjustment capacity, penetration power and speed. Materials used to manufacture thermoforming molds

Materials used. Unlike other plastic molding processes, such as injection or compression, thermoforming has the advantage of using relatively low pressure and temperature. That is why a great variety of materials can be used. Usually, wooden molds can be used, they are ideal for low production and as wood has a low thermal conductivity, it helps the annealed sheet not to cool quickly at first contact, but these molds are not good for medium or high production. Manufacturing molds with phenol laminates are better because they are not seriously affected by heat or humidity. There are also molds made of mineral or metallic charges and polyester or epoxy or rigid polyurethane resins. These are easy to remove off a mold and may even have a mold with multiple cavities. The thermal properties of epoxy and polyester resins make them suitable for medium production. Copper piping can be used as cooling system to better control the mold temperature, but even then, it is not enough for high production.

42

Thermoforming

Aluminum molds are the best for high production, but because of the thermal conductivity of aluminum, the mold has to be pre-heated by means of circulating hot water through the cooling/heating system or radiating heat with electric resistors, or even heating the mold with the same material to be thermoformed. For long runs, a thermostat has to be incorporated, to ensure there is the least temperature fluctuation on the surface of the mold, thus, preventing over cooling. Applying poly-tetra-fluorine-ethylene to aluminum can improve its properties. Summarizing, there are 4 groups to manufacture thermoforming molds: 1) 2) 3) 4)

Wood. Minerals. Plastic resins. Metals.

Table 13. Use of materials for thermoforming molds

GROUP Woods

43

Thermoforming

MATERIALS USED

PRODUCTION VOLUME

Pine Mahogany Cedar Maple Triply Agglomerated

Low

ADVANTAGES AND DISADVANTAGES These are low cost molds, their time of manufacturing is short and they have good surface finishing, though in some cases the grain of the wood leaves marks. Wood should be seasoned, for better finishing and preventing dimensional changes due to humidity, molds must be sealed with casein, phenolvarnish or epoxy resin diluted in methylethyl ketone. For better finishing the grain of the wood must be parallel to the length of the mold. Triply or agglomerated molds last longer, which can be prolonged by reinforcing the intersections with metal.

MATERIALS USED

PRODUCTION VOLUME

Cast (Calcium Carbonate) Sodium Fluoric-silicate

Low Medium

Cast Molds are more durable than wooden ones and can be cast of a composite of low shrinking cast, highly resistant and interiorly reinforced with metallic mesh, glass fiber or materials that do not absorb humidity. Cast on the molding is left to cure 5 to 7 days at environmental temperature. If surface is good it does not need finishing. Polyester, epoxy or phenol resin coverings provide more resistant surface. Care must be taken not to chip cast when making vacuum holes, which may be eliminated if pieces of wire are inserted previously and removed after hardening.

Plastic resins

Polyester Epoxy Phenol Plastic laminated Nylon

Medium

Plastic resin molds are more expensive and elaborated than cast or wooden ones but more durable, smoother surfaces and dimensional stability. These resins can be charged with aluminum powder which provides a more homogeneous temperature of the mold or with kaolin, glass fiber etc. A vacuum system can be incorporated to these molds, fitting a cardboard pipe at the back of the mold.

Metallic

Aluminum Berylliumcopper Iron

High

They are ideal for big production runs, high pressure or metallic forming. Aluminum, bronze, or any other low point fusion alloy founding molds can be used, and also machine finished steel, brass or bronze. They are the must expensive, making them takes a long time, have better surface finishing, maintenance low cost and better dimensional stability. Cooling system must be used, and avoid rapid cooling of the piece.

GROUP Minerals

44

Thermoforming

ADVANTAGES AND DISADVANTAGES

Recommendations for thermoforming molds 1. For wooden molds, the best remover is baby powder or flour. 2. For metallic or plastic resin molds, removing waxes are recommended. 3. Soft wood must not be used with very sensitive materials such as polystyrene, foamed or acrylic P.V.C., since they get marked because of the grain of the wood. 4. For long production runs, wood must not be used since slow cooling makes the mold expand, creating separations on the joints. 5. For plastic resin or metallic molds, aerosol removers can also be used. 6. For wash basins, tubs, or bath room modules, a porcelain-like glow can be achieved sand-blasting the surface of the mold, roughness will achieve a finish with these characteristics. .

45

Thermoforming

Thermoforming techniques Thermoforming is the simplest and most used process to form an acrylic sheet. Being a thermoplastic material it softens and it is easy to handle and can take any form when heated at suitable temperature and time. As it cools it recovers its rigidity and keeps the form it was exposed to. The cost of equipment and molds is relatively low and bi or tri-dimensional forms can be obtained by means of a great variety of processes. Bi-dimensional thermoforming

This is a bending process that can be achieved through two methods:

Lineal heating bending. A Chemcast acrylic sheet is heated on a lineal resistor, bending at the desired angle. To bend, remove the protector paper of the bending line (the rest of the paper may be left to protect the areas that are not to be worked on), then place the sheet on the supports with the bending line directly on the heating line, bending on the heated side. Heating time varies according to the thickness of a sheet. To bend an acrylic sheet over 0.16” thick it should be heated on both sides to obtain a suitable bend. Heat the sheet until it gets soft on the bending zone. Do not try to bend the sheet before it is well heated, this may cause irregular or creased corners. Heat carefully, irregular heating may cause arching on the bending line. Some times this is hard to avoid, specially on pieces over 24” long. Arching may be diminished fastening the recently formed material with some clamps or a template until it cools. Templates can be made of wood, fixed or adjustable.

Acrylic

YES

No

With suitable heating, clean shining corners are obtained

46

Thermoforming

Top

Support with adjustable hinge at any angle

Butts

Electric resistor

Place the sheet on the support with the folding line directly on the heating line

Acrylic

Use fixed or adjustable templates to keep the piece at the desired angle

Cold forming Chemcast acrylics sheet can be cold formed on curved frames, as long as the radius of the curve is 180 times bigger than the thickness of the material used. Formula: R (radius) = 180 X T (Thickness of material in inches.)

R=180 X E

Threedimensional thermoforming (with molds).

The procedures for tri-dimensional forming in general, require using vacuum, pressured air, mechanical equipment, or a combination of these to mold Chemcast acrylic sheets to a desired form. These techniques are described next:

Acrylic

Free or gravity shaping This method is the simplest of all, because once the material is softened, the sheet is placed on the mold and the material adopts the form by its own weight. The edges of the material can be fastened to the mold to avoid waves that tend to occur when cooling.

Mold

Male mold Frame

Matrix

47

Thermoforming

Mechanical forming with matrix and male mold. A Chemcast acrylic sheet can be formed pressing the annealed material between the male mold and the matrix, to produce pieces of very accurate dimensions. This procedure requires excellent finishing of the molds to reduce their marks to a minimum.

Free, pressure or vacuum forming The pieces that require optical clarity like skylights, helicopter cabins, etc., can be formed without mold, Chemcast acrylic can be vacuum or pressured air formed. The form of the finished piece is given by the form and size of the ring that fixes it to the frame and by the given height. However, these forms are limited to spherical outlines or bubbles freely formed. Vacuum is better for this kind of forming, or pressure if it is over 1 atmosphere. Vacuum and pressure forming, matrix. This procedure allows forming pieces, on 1 piece molds whose form requires more accuracy than the ones vacuum formed. However, high pressure leaves marks of the mold on the piece. As high pressure is required, the molds should be of metal, epoxy resins or other materials that can stand high pressure without deforming. Good finishing of the molds is a must to obtain quality pieces. Pressure forming with the help of a piston and matrix The technique of piston help is used to reduce thinning at the bottom of the formed pieces. The piston stretches the material before pressure is applied. Piston speed of 6.6 yd./min., is required, it may damage the material at initial contact. Forming pressure 6.16 pounds/in2

48

Thermoforming

Presión de aire

Vacío

Vacío

Vacío

Vacuum with return and male mold forming. This technique is useful to form pieces that require uniform thickness on the walls and fewer forming marks. An annealed sheet is stretched in a vacuum box until it reaches the necessary depth for the mold; once it is inside it, vacuum is freed gradually so that the acrylic returns to its original form meeting it. More defined forms can be obtained if at the point of returning, vacuum is applied to the male mold

Pressure forming with the help of a piston, matrix and vacuum. This is the most sophisticated of all, since it is a combination of almost all the others, it is generally used for very deep thermoforming which requires more controlled thickness and when breaking is possible because of excessive molding depth. Vacío

49

Thermoforming

Infrared heating furnace molding techniques

50

In this section we will try to expand the techniques mentioned before. Although these examples are designed for infrared heating equipments, it is possible to apply them to the conventional molding systems.

Vacuum forming, matrix and mechanical support

Pressured air pre-stretching, mechanical support and vacuum

Vacuum forming, matrix and mechanical support

Vacuum forming, matrix

Thermoforming

Free pressured air forming

Pressured air pre-stretching, matrix, mechanical support and vacuum

Free pressured air forming

Pressured air stretching, mechanical support and vacuum

Vacuum forming, matrix, mechanical support and pressured air.

51

Thermoforming

Cooling thermoformed pieces Cooling a thermoformed piece is as important as heating it, but in some cases, it takes longer than heating. That is why it is important to choose the right method. Some times, when very thick pieces that can stand less internal effort are formed, normal cooling should be delayed, covering the piece with soft cloth or flannel. If the piece is fastened with clamps, fastening force diminishes as cooling takes place and shrinkage will show the great efforts of this process. Most of the heat absorbed during the heating cycle should dissipate off the plastic before it is removed off the mold, otherwise, the piece might get distorted and warped. If the piece is formed on a male mold, it should be removed before shrinkage, which will make it hard to remove. Conventional cooling methods

Conduction and convection are practically the only methods to dissipate heat, since thermal conductivity is low, pieces over 0.08” thick require long cooling. The most common is using electric ventilators to cool the piece; this method has the advantage of allowing cooling the piece on the mold. The disadvantage is that the air draft is not enough to cool the mold in each cycle, and the mold will be too hot, interfering with the normal heating cycle. Cooling a piece in contact with a mold is very efficient if it is a metallic mold and has cooling ducts with water re-circulation. In these cases, enough volume of refrigerant liquid should be used to keep a constant temperature on the mold. If the cooling water is kept at a certain temperature, marks on the piece (usually known as undulations on its surface) due to a cold mold, can be minimized. Aluminum or epoxy resin and/or polyester molds are very suitable if you want to include a refrigeration system. Wooden molds are not convenient for long runs because they do not dissipate heat quickly.

Non conventional cooling methods

52

There are faster cooling methods that use a spray or a very thin de-ionized water curtain or liquid carbon dioxide, which rapidly cools a thermoformed piece. This method is not common because of its cost, but both methods can be justified, specially if they are applied locally to prevent thermal tearing of very deep pieces. Irregular fast cooling of a formed piece causes great efforts that affect durability.

Thermoforming

Cutting thermoformed pieces Once the forming cycle is finished, pieces have to be cut to eliminate excess material. It is very rarely that a finished piece does not need cutting, as in the case of lighted signs. Most thermoformed products need some kind of cutting. The right equipment and technique must be chosen. Anyway, there are some factors that determine the choice, as sheet measures, size and depth of a piece, acceptable level of roughness of the cutting surface, required dimensional tolerance and cutting speed among others. Cutting equipment.

There are several equipments to cut thermoformed pieces: Electric tools. Circular saw. A circular saw must have straight teeth to help cooling and not to soften the material. Tungsten carbide teeth provide excellent cutting and keep sharp longer. Cutting must be slow to prevent heating or stretching the material. The saw has to be operated at relatively high speed and before starting, make sure that the saw has reached its highest speed. The thicker the material, the bigger the diameter of the saw must be, and have the least number of teeth (minimum 2 teeth per 0.8”.). When a hand circular saw is used, the sheet has to be held and pressed firmly as it cuts at a steady speed to avoid chipping. Table 14. Cutting specifications for circular, radial, or travel saw. DISK

SHEET Thickness inches

DIAMETERS (inches)

Thickness (inches)

No. TEETH (*)

0.06-0.12 0.12-0.16 0.2-0.4 0.48-0.6 0.72-0.84 1-2.08

8 10 10 12 12 12-14

1/16-1/32 3/32-1/8 1/8 1/8 1/8 1/8-5/32

96 82-96 82-96 82-96 48-52 48-52

*Teeth with tungsten carbide bit, teeth with straight surface at the center, combined or alternated

53

Thermoforming

Band saw A band saw is the right one to make curves in flat sheets and rethread formed pieces. A band saw with variable speed up to 5000 feet/min. and minimum 10" deep groove is recommended . It is convenient to use the special bands to cut metal or plastic; the guide must be adjusted as close as possible to the material to avoid chipping on the cutting line and to reduce the vibration of the saw to a minimum. Next, cutting specifications with a band saw are listed: Table 15, cutting specifications with a band saw.

SHEET Thickness (inches) 0.06-0.12 0.16-0.24 0.32-0.48 0.6-1 1-2.08

ENGINE

BAND WIDTH MIN (inches)

TEETH X (inch)

HP

RPM

3/16 3/16 1/4 3/8 3/8

18 14 10 8 8

1 1.5 1.5 1 .5-2 2

DE 2500 A 3500

Table 16 Radial cutting specifications, with band saw SHEET MINIMUM RADIUS TO CUT IN (inches)

WIDTH OF BAND (inches)

TICKNESS OF BAND (inches)

TEETH X INCHES

0.48 0.52-0.76 0.8-1.52 1.56-2.28 2.32-3.04 3.08-4.56 4.6-8.12 8.16-12.2 12.24-20

3/16 1/4 3/8 1/2 5/8 3/4 1 1 1/4 1 1/2

.028 .028 .028 .032 .032 .032 .035 .035 .035

7 7 6 5 5 4 4 3 3

BAND

Router Chemcasts acrylic sheets can be cut with a portable or fixed router (electric or pneumatic). A 1.5 HP and 20,000 to 30,000 RPM electric router is recommended, and bits or cutters with tungsten carbide bits with 1/4 or 3/8" diameter and ideally 1/2" to avoid that vibrations break the bit.

54

Thermoforming

This method provides very uniform cut and is good to form as well as to make big diameter holes. The router can be fixed to a table and a copying guide can be used for intricate designs. The cutting tool of a circular saw or router can be changed for an abrasive normal disk or even a diamond one; this kind of disk should not be used when an acrylic formed piece is reinforced with glass fiber, as in the case of tubs, wash basins, phone booths, etc. Automatic equipment. This kind of cutting equipment is used when a high automatic level is required; generally, this equipment has a computing system and specialized software, like CAD-CAMCAE, which is used to design the cutting pattern, and later send the information to a peripheral one, that in this case may be 1 or 5 head routers, pressured water system or laser. Cutting capacity is not limited to a direction or plane, it can perform any kind of cut or perforation. Pressured water cutting The abrasive system with pressured water eliminates many of the problems related to the machinery and cutting operations of conventional cutting. A very fine jet of pressured water 50.000 Psi, is concentrated, at a speed of about 3.3 yd./min and a pressure of +/- 0.04”. Using a combination of highly pressured water and abrasive materials, such as silica powder, the water jet can cut every material without heating and provide an exceptional finishing on the cutting surface. The advantages of this cutting system on acrylic are: eliminating heating distortions, any cutting angle can be performed because of its multi-directional type integrated to computing systems, it eliminates secondary operations like sanding, and reduces material waste since the cutting area is very reduced. Cutting with laser Cutting with laser is a technique that has already been used in other industrial sectors for several years and its main characteristics are: • High pressure cutting • Manufacturing flexibility • Reduced cost An advantage of the laser cutting is its application versatility, since apart from its direct use to cut acrylic sheets, it offers the possibility of processing many other materials.

55

Thermoforming

With a laser device you can cut, weld and hew surfaces up to 1.2” thick, because laser energy is concentrated on one spot and heat generation can be limited to a minimum zone, which avoids any heat deformation or structural changes in the material. Very fine cuts with accurate edges can be obtained which is good for acrylic pieces with intricate forms. You can make 0.004” diameter bores at a speed up to 150,000 holes per hour. A laser equipment can cut 1/2" of acrylic at a speed of 12”./min. Swaging This technique is not much used because of its limitations; it may be used on thermoformed pieces when they are still hot and are not over 0.08” thick, the blades should be at a temperature between 104ºF and 140ºF (40°C and 60° C). Even then cutting quality is not very good. This kind of cutting is better for plastics-like acetate polystyrene and foamed P.V.C. Cutting techniques

Although there are non conventional cutting techniques and highly automatic ones, their practical application is far from popular, because of their high investment and maintenance cost compared with traditional techniques like router or circular saw cutting. Some cutting alternatives of thermoformed pieces are shown next. As long as it is possible, you should build a cutting template as support for the thermoformed piece, this way you will avoid variations on a piece and production will be standardized.

Cutting with router and bullet bit

56

Thermoforming

Cutting with router, straight bit and copying guide

57

Cutting with bench saw and iron or aluminum angle butt

Cutting with bench saw and wooden butt

Cutting with router and cookie cutter or abrasive disk on the outside

Cutting with router and cookie cutter or abrasive disk on the inside

Cutting with radial saw and template on the inside

Cutting with radial saw and template on the outside

Thermoforming

Thermoforming variables In the thermoforming process there are variables that can affect aspect, quality, dimensions and distribution of the material of a formed piece. Knowing these variables can help to solve difficult production problems in the thermoforming process. Following, the most frequent variables as deviations in the thermoforming process are shown. Material variables

Thickness of a sheet When electric resistors or infrared radiation is used to heat, changes on caliber of the thickness of the material can cause an uneven heating, creating variations in the formed part. In pre-stretching or deep forming, close dimensional tolerance is needed to avoid breaking the material in very thin areas, because of the force exerted by vacuum or pressured air. In very deep pieces there is a variation in the thickness of the material which depends on the thickness used, the area and maximum depth of a piece. When there is a thickness variation between each sheet, the heating temperature must be reduced to prevent material from over softening. If the temperature of a sheet is homogeneous, even a piece with thin areas can be well made. Sheet pigmentation In the case of radiation heating (electric resistors) the different colors of the same material can cause temperature changes and heating cycle changes. In a convection furnace (hot air re-circulation) this variable does not apply. . Size of a sheet. To get a better distribution of the material of a very deep piece, it is more economic to increase the size of a sheet instead of its thickness. Temperature uniformity of a sheet When the temperature of any material is increased, tension force is reduced and therefore the sheet becomes malleable. Simple or deep forming made at a lower range than annealing temperature provides the best results. For high quality pieces, it is important that a sheet heats evenly at annealing point length-wise and width-wise. The sheets that are not evenly heated will be deficiently formed: there will be more stretching in the normal temperature zones than in the ones that were not softened.

58

Thermoforming

Mold variables

Vacuum bores or orifices Vacuum speed is directly proportional to the quality of a piece. A slow vacuum makes the part of the sheet where the first contact with the mold takes place to cool faster than the rest. Therefore, there are sections with very thin walls or incomplete pieces. To eliminate air quickly, 1/8" and _" vacuum bores should be used. When possible, there should be vacuum canals or ducts since they display a greater volume of air. Mold surface When a thermoplastic sheet is formed it will take the form of the mold, one with opaque finishing, will give an opaque finishing, a very polished finishing (mirror finishing) of course, will provide a shining piece. Mold temperature. A mold surface temperature influences directly the duration of the forming cycles, the size and a better aspect of a formed piece. A thermoformed piece final shrinkage depends on having a mold temperature similar to the thermal expansion coefficient of the material. Mechanical support temperature To prevent a sheet from getting cold during a pre-stretching operation causing "cooling marks" and deformations, a mechanical support should be heated at a temperature over the distortion point.

Pre-stretching variables

Vacuum box In vacuum with return and free forming it is very effective to use a vacuum box of 3.2” to 4.8” longer than the total depth of the formed bubble to prevent cooling on the perimeter of the sheet in contact with the mold. Before forming the bubble, the sheet must be strongly sealed on the mold. In a vacuum with return operation, maximum thinning will occur at the bottom of the formed bubble. To get thicker walls, there must be a two step edge in the vacuum box which will cool the top area making it thicker. Air temperature Sometimes the air of the system should be pre-heated. When air at room temperature gets into the system, it may cool the sheet, affecting its size and form. With thin materials, the cooling problem is more serious. With pre-heated air, the temperature should be about 10% below the temperature of the sheet. An air deflector or an air diffuser should be used at the intake of the mold since they can prevent a sudden cooling in some areas of the material.

59

Thermoforming

Mechanical support variables

Mechanical support form This must be closely adapted to the form of the cavity of the mold, but must be 10 to 20% smaller length-wise and width-wise (or diameter). When these dimensions are 4.8” or larger, the small supports must allow at least 1/4" margin between the final part and the support, to prevent thickness irregularities of the material as far as possible. When the mold has canals (corrugated tin) with sudden changes from flat to narrow zones it is important that the support is made with detachable parts that fit into the canals of the mold. These parts will help add more material to increase thickness in a particular area. For boxes in the mold, the same projection of the support must be applied. In the case of deep depressions on the walls of the mold, a support mechanism should be incorporated to take material to that zone, all the corners should be softened and have generous radiuses. Support materials. To get good results, the mechanical support must have excellent qualities to transfer heat, and must have constant and prolonged resistance at high temperatures. Aluminum is one of the best materials. For short or prototype runs hard wood is better and to prevent it from getting too dry or cracking because of the heat, the surface has to be greased frequently. Support temperature. The temperature of a support must be kept below that of the forming of a sheet. The support may have low working temperatures, anyway, if the temperature drops, the cooling marks will be more visible. It is not so critical to strictly control the temperature of a mold. In the case of a support, a maximum uniform heating of 50°F must be kept without variation, with suitable regulated temperature the molding marks are generally eliminated. Support surface. A smooth surface with well polished radiuses, dust-free and rubbish-free, will produce good pieces Support height An effective mechanical support is the one that is longer than the depth of the mold, since it can regulate adjusting. Support vacuum speed. Increasing the speed of a support raises air compression capacity in the cavity of the mold. The vacuum system capacity and its duration related to the run of the support affects pressure in the cavity of the mold. Normally, the vacuum cycle must start at the same time as the support touches the material.

60

Thermoforming

Support, depth of action The best results are achieved when a support penetrates 78 or 80% in the cavity of the mold. This creates the best combination between the thickness of the bottom and the walls of a piece. Material variables when forming with support The kind of material used will affect the amount of pressure needed to keep the right contact of the material around the support. High resistance of materials such as acrylic and ABS, need air pressure between 15 and 50 Psi.

61

Thermoforming

Problems and solutions guide DEFECT • Bubble or blister on the sheet

POSIBLE CAUSE • Excessive moisture

• Heating too fast

• Irregular heating.

• Incomplete forms and details

• Insufficient vacuum

• Slow vacuum displacement

• Insufficient heating of a sheet.

• Color change of a sheet

• Excessive heating • Low mold temperature.

62

Thermoforming

SUGGESTED SOLUTION • Pre-dry sheet. • Dry both sides of sheet at 140°F (60°C) • Reduce furnace temperature. • Increase distance between sheet and heater. • Check and fix the furnace. • Check heating elements. • Eliminate obstructions in vacuum system • Increase number of holes • Increase their diameter • More tank and vacuum pump capacity. • Leakage. • Check vacuum system for possible leaks. • Use vacuum canals in possible areas. • Increase temperature or heating time. • Reduce heating time. • Reduce furnace temperature. • Heat mold.

DEFECT • Color change of a sheet.

POSIBLE CAUSE • Low temperature of mechanical support • Too much thinning of a sheet. • Sheet cooling before its formiing is completed.

• Mold wrongly designed.

• Inadequate material • Excessive warping or bending of a sheet

• Cooling marks on a formed piece.

Thermoforming

• Heat mechanical support. • Increase sheet thickness. • Place sheet more quickly on the mold. • Increase vacuum speed. • Heat mold and mechanical support. • Reduce mold depth. • Improve vacuum air flow. • Use more curved radiuses. • Change material.

• Sheet too hot

• Reduce heating time. • Reduce furnace

•Sheet too big.

• If possible, reduce sheet size • Use screens, mainly on center of sheet (only infrared heating furnaces).

• Sheet too hot.

• Reduce mold temperature. . • Reduce heating time.

• Insufficient temperature of support.

• Raise support temperature. . • Use soft flannel filter on sup port surface • Raise mold and/or support temperature, without exceed ing temperature range. • Soften and/or round mold critical areas.

• Mold low temperature (Shrinking stops at contact with mold or cold support).

63

SUGGESTED SOLUTION

DEFECT • Small wrinkles or circular marks..

POSIBLE CAUSE • Sheet too hot

• Reduce mold temperature. • Reduce heating time..

• Too big vacuum bores.

• Refill and bore again smaller diameter.

• Bending variation of sheet.

• Sheet irregular temperature.

• Check there are no drafts in furnace, deflectors must be incorporated.

• Wrinkles while forming.

• Excessive heating of sheet.

• Reduce furnace temperature. • Reduce heating time. • As far as possible, more distance between 2 heaters and sheet (only infrared heating furnaces) • Reduce molding range temperature. • Check vacuum system. • Increase vacuum canals or orifices.

• Excessive bending of sheet. • Insufficient vacuum..

64

SUGGESTED SOLUTION

• Very shiny lines or zones.

• Over heating sheet on shine area.

• Use screens to reduce heat on zone. • As far as possible more distance between 2 heaters and sheet (only infrared heating furnaces). • Reduce heating time.

• Bad surface aspect of piece.

• Defect caused by air caught on flat surface of mold. • Insufficient vacuum.

• Sandblast mold surface.

Thermoforming

• Increase number of vacuum orifices • If marks are isolated, increase number of vacuum orifices in affected area.

DEFECT • Piece surface bad aspect

POSIBLE CAUSE

SUGGESTED SOLUTION

• Excessive mold temperature. • IInsufficient mold temperature.. • Superficie del molde demasiado áspera o rugosa. • Dirty sheet.

• Reduce mold temperature. • Increase mold temperature.

• Excessive distortion or shrinking after removing a piece off a mold.

• Piece removed too fast.

• Prolong cooling cycle. • Move the piece to a cooling template. • Use refrigerant. • Use water spray steam to reduce piece temperature. • use electric ventilators to cool piece inside the mold.

• Excessive thinning of walls of a piece

• Inadequate forming technique.

• Use different forming technique: vacuum with return, pressured air and mechanical support, pressured air and return with vacuum. • Check material meets quality norms and /or complain. • Check furnace operation. • Reduce furnace temperature.

• Material thickness variation. • Uneven sheet heating. • Sheet at excessive temperature. • Cold mold. • Sheet not firmly fastened to frame.

• Soften mold surface. • Make mold of other material. • Clean sheet.

• Reduce heating time. • Heat mold. • IIncrease closing pressured. • Check possible sheet thickness variation.

65

Thermoforming

DEFECT • Pieces twist

POSIBLE CAUSE

SUGGESTED SOLUTION

• Piece cooled wrongly. • Uneven wall thickness distribution.

• Adjust cooling cycle. • Use pre-stretching mechanical or technical support. • Sheet might be unevenly heated. • Increase vacuum orifices. • Modify mold • As far as possible, curve a little flat areas. • Increase mold temperature.

• Wrongly designed mold. • Wrongly designed piece. • Insufficient mold temperature.

• Shrinking marks on corners.

• Mold surface too smooth. • Insufficient vacuum.

• Sandblast mold surface. • Check vacuum system. • Add more orifices.

• Bubble stretches unevenly.

• Insufficient sheet temperature. • Sheet uneven thickness. • Insufficient pressured air.

• Check furnace operation condition • Use cooling screens (Only infrared radiation heating furnace.). • Longer heating time at lower temperature. • Incorporate an air distribution system with deflectors.

• In deep forming, thin corners.

• Wrong forming technique.

• Change forming technique.

• Thin sheet • Sheet unevenly heated

• Increase sheet thickness. • Check furnace operation. • Use screens to change heat distribution. • Change furnace temperature.

• Mold wrongly heated.

66

Thermoforming

DEFECT • Piece sticks to mechanical support.

POSIBLE CAUSE • Mechanical support (wood). • Mechanical support (metal).

• Piece sticks to mold.

• Piece high temperature. • Mold insufficient exit angle. • Wooden mold.

• Corners of formed piece shatter once in use..

67

Thermoforming

• Piece wrongly designed. • Effort concentration on a piece.

SUGGESTED SOLUTION • • • • •

Apply removing agent. Cover with soft felt or flannel. Apply removing agent. Lower support temperature. Cover with felt or flannel.

• • • • •

Longer cooling time. Reduce mold temperature. Give 1° and 3° angle Change matrix. Apply removing agent.

• Redesign piece. • Increase mold curve radius. • Increase thermoforming temperature. • Make sure piece is wholly formed before it cools below forming temperature.

APPENDIX Glossary

ABSORBENCY Fraction of radiant energy taken by a sheet. CAVITY Depression of a vacuum made mold, machine finished or a combination of both,. depending on the number of depressions, it may have one or several cavities. CONDUCTION Energy transferred by directly touching a solid. CONVECTION Energy transferred by the movement of a fluid current. COOLING MARKS Marks caused by using wrong temperature on a plastic sheet, derived from inadequate heating. CO-POLYMER Polymer composed of tow different kinds of monomers. CYCLE Complete repetitive sequence in the thermoforming process, which consist in: heating, forming, cooling and removal. DIMENSIONAL STABILITY Capacity of a piece to keep the accurate shape and dimension of the mold used. ENTHALPY Inner energy of a system. HEAT TRANSFER COEFFICIENT Effectiveness measure of energy transported between a fluid current and a solid surface. HOMO-POLYMER Polymer made of only one monomer INFRARED Part of electro-magnetic spectrum, between the range of visible light and the range of radio waves. Radiant heating is the range at which infrared heaters are used to heat a sheet. Wave length is 0.08” to 0.4”

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Thermoforming

MELTING TEMPERATURE Range of temperature at which a crystalline polymer turns from a solid rubber-like state into a viscous-elastic liquid. MOLDING TEMPERATURE A piece temperature at which it can be removed without deforming. PRESSURED AIR SHAPING Difference of pressure exceeding two atmospheres (30 Psi.). RADIATION It is the transfer or exchange of electromagnetic energy. REFLECTIVITY Fraction of radiant energy reflected on a sheet surface. RESIN Another name to call a polymer or plastic material. SCRAP Material waste that is not part of the final piece. TENSION External charge exerted on a defined area. THERMAL DIFFUSIVENESS Transmission index of calorific energy in a material. TRANSMITTANCE Fraction of energy that is transmitted through a sheet. TRI-POLYMER Polymer composed of three different kinds of monomers. VACUUM TANK Tank between the vacuum pump and the mold, that allows you to apply pressure evenly during forming.

69

Thermoforming

Plastic reinforced with glass fiber Introduction Reinforced plastics are those thermo-plastic or thermo-fixed materials, in whose shaping process, some reinforcing material is used to improve their mechanic characteristics. This reinforcing material can be continuous or discontinuous. As examples of the former there are fiber materials like: salwort, jute, henequen, rayon, etc., but the most used is glass fiber. Resin, polyester and reinforced plastic

A polyester is made by the reaction of a poly-basic acid and a polyhydric-alcohol, at temperatures over 212°F (100ºC), getting one polyester and water. Depending on the type of acids and alcohol used and modifications performed, the following kinds of products will be obtained. Non saturated polyesters These are lineal polyester resins obtained when dibasic acids and polyvalent alcohols react, and can polymerize in a cross-linking way with vinyl monomers to make thermofixed plastic.. Alkyd polyesters. These are the ones modified with oil, used for decorative and/or protective coverings, for example: paints, varnishes, printing inks, etc. Plasticizing polyesters Polyesters totally saturated that are used to soften other plastics, they are also known as polymeric plasticizers. They are used to make vinyl with or without reinforcement, for example: the one used for car upholstery, wall paper, etc. Fibers and films They are polyesters of heavy molecular weight, molecularly oriented and for which specific acids and alcohol are used. Example: polyethylene, polypropylene, etc. Polyester foam. Polyesters with a great number of hydroxyl groups and that react with interlinked chains with isomeric acid groups, to make foams, elastomers, coverings, etc. According to the previous classification, polyesters are a great variety of chemical composites and products. However, they are generally used to name composites defined as non-saturated polyesters, so unless something else is suggested, this denomination will be adopted. Polyester resins are used in a wide variety of applications, in different industries, for example: forming with reinforcing materials (reinforced plastic), encapsulating, protec-

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tive covering, decorative objects, buttons, etc. Reinforced plastic industry is the one that has the most polyester consumption. Increasing demand and application of plastic reinforced items are basically due to their properties and features, among which, the following can be mentioned: 1) Composites are easy to handle (polyester resin is applied in liquid form). 2) Easy curing and using. 3) Excellent dimensional stability in the final product. 4) Good dielectric properties. 5) Excellent physical and mechanical properties. A reinforced plastic sheet, equivalent to three times steel thickness, has mechanical resistance to tension, weighs about half and is more resilient. 6) Rust resistant and also to a great amount of chemical agents. 7) Easy finishing (coloring, painting, machine finishing, etc.). To obtain optimal reinforced plastic features, the reinforcing material must have the best mechanical and chemical properties. Next, reinforcements most used are mentioned. Reinforcing materials The most important reinforcing material are: 1.- Cellulose fibers. Cellulose alpha. Cotton. Jute. Salwort. Rayon. 2.- Synthetic fibers. Polyamides (nylon). Polyester (Dacron). Polyacrylonitrile. Polyvinyl alcohol fibers. 3.- Asbestos fibers. 4.- Special fibers. Carbonate and graphite fibers. Boron and tungsten fibers. Ceramic fibers.

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5.- Reinforcing charges. 6.- Glass fibers.

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Glass fiber In reinforced plastic industry, the material most used is glass fiber because of its features: 1.- Tension high resistant. 2.- Incombustible. 3.- Biologically inactive. 4.- Excellent weather resistant as well as to a great deal of chemical agents. 5.- Excellent dimensional stability. 6.- Low thermal conductivity. The main uses of glass fiber reinforcements are: Roving. Mat. Woven roving. Surfacing mat. Chopped strand. Following, processes to obtain these and their features are mentioned.

Roving Roving is one of the most used glass fiber products, and it is indispensable when reinforced plastic items are made by sprinkling, directed filament and hot forming (preform manufacturing). Roving comes wound on bobbins, and it usually has 60 threads.

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Mat This is the most popular and known glass fiber product in reinforced plastic industry and it is made of fiber mono-filaments about 2”, long. Woven roving This is roving strings woven at 90° angles as to their longitudinal axes. Combined with mat, it is used as secondary reinforcement, to manufacture boats and big structures. Surfacing mat This material is made of glass fiber sections like the mat, though with less weight/unit area. It is mainly used to improve the finishing of reinforced plastic products and to increase their weather resistance features; since when it is put on the reinforcement material, usually a mat, it does not allow the fiber to crop up and as it absorbs resin, finishing gets smoother. Chopped strand This glass fiber presentation is not much used, it is made by the machine that makes mat. Its size varies from 2/2" to 2" long (1.25 to 5.0 cm). It is mainly used to make items by methods of pre-mixing. According to reinforcing materials classification, there is another type of products used in the manufacture of reinforced plastics, the most important are: Asbestos. Salwort, henequen, jute. Synthetic fibers. Ceramic fibers. Polyvinyl alcohol fibers. Special reinforcements. To improve reinforced plastic efficiency and application, several reinforcing elements have been developed. Their main characteristic is a high elasticity module, which considerably increases mechanical resistance of laminated products. This is specially important in specialized fields like aero-spatial vehicles, submarines, etc. Among these reinforcements are: Boron tungsten filaments. Carbonate and graphite fibers. Metallic filaments. Whiskers Adhesion promoting agents. Hybrid reinforcements. Metalized reinforcements.

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Mechanical resistance of a plastic/reinforcing composite derives from joining, generally mechanically a system composites. This joint, satisfactory in most cases, may reduce composite or product aging as well as moisture, when glass fiber systems are used to reinforce, since the fiber is hydrophilic and tends to absorb water which weakens or destroys plastic joints. To prevent this, chemical composites hydrogen siliceous type are added to the inorganic charge, resin or reinforcing material. They provide a chemical consolidation in the interface of the joint, improving and keeping the mechanical properties of composites, apart from improving dielectric characteristics of the system. Manufacturing reinforced plastic molds. To make a mold, a model or original of the piece to be made is required. When there are only specifications and blue prints, the model can be made of cast, wood, or epoxy paste, depending on how difficult the piece is and how skilled the operators are. Some times, the model can be made by combining polyurethane foam or polystyrene plates covered with a thin coat of cast or epoxy. When the model is finished, roughness can be smoothed with emery cloth and then applying a sealer to eliminate porosity. In most of the cases it can be a nitro-cellulose lacquer, which is spayed, or shellac dissolved in alcohol. To polish the model, a removing agent is applied, its specific function is avoiding adherence of the resin to the mold. Removing agents can be classified in three groups: Solutions. Generally aqueous polyvinyl-alcohol, methyl-cellulose, etc. This kind of removers must be applied in each molding operation. Waxes and wax emulsions This agent is applied with a flannel or felt, and polished manually. Internal removers These agents are mixed with gel-coat. When they mix with the tooling gel-coat, removing characteristics improve, making molding easy. Once the removing agent has been chosen, the mold is coated with a resin preparation known as gel-coat or finishing coat. Gel-coat It is made of a resin that provides a film whose characteristics are: 1.-Uniform surface. 2.-Avoiding that reinforcing material crops up. 3.-Improving weather resistant properties. 75

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Some times, a glass fiber mat should be put to reinforce gel-coat, getting a resin rich coat and avoiding that the reinforcing material crops up. Gel-coat is usually sprayed, but it can also be applied with hair brushes. In that case, accelerator/catalyst quantities should be less than for immersion. Finishing film thickness depends on the use and characteristics of the piece to be made and it can be measured with a calibrator of humid film. Manual finishing process It is often used since it does not require any special equipment. Its process is: A mold prepared with removing agents (wax, removing film or both) is coated with a finishing product using a soft brush or spraying equipment, thickness varies depending on the use of the piece and the supplier’s specifications. Once the gel-coat thickness is determined and it has been cured, the glass fiber mat is placed. Next, using a brush and with vertical movements, a resin of styrene monomers or methyl methacrylate, or both, is applied to the mold, as well as the accelerator, whiskers and/or toxitropic agents, heat concentrator, catalyst, etc. Later, and before the resin jells rolling is done, with a plastic or metal 0.36” to 1” (9.0 to 25mm.) diameter and 2” to 8” (5 to 20 cm.) long roller generally grooved, depending on the case. Rolling the roller in several directions pressing evenly helps to eliminate air caught in the resin and reinforcing material, as well as to obtain good adhesion with the gel-coat. Finishing and rolling should be done by sections no bigger than 1m2 when a piece is big.

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Often, commercial measurements of mat and woven roving (which are always applied with this procedure) are not enough to cover the whole mold; therefore, they have to be joined by sections. Overlapping them 5cm is suggested. The resin to join them should have the least accelerator and catalyst to avoid problems created by material contractions which derived from a bigger amount of resin, that reduces curing time and increases exo-thermal temperature. Some times, one or more woven roving layers have to be used as reinforcement. They must be put between the 2 mat sections, or even better, as a final layer and never directly with the gel-coat, because if the finishing coat is not properly applied, the woven roving will be visible, which will give the product a bad aspect. The brushes and rollers have to be washed intermittently with a solvent like acetone, ethyl acetone, methyl ethyl acetone, etc., since as the resin cures it hardens and they may get damaged. Most of the time it is enough to put them in a container with a solvent or a monomer mixture. Reinforced plastic machine finishing. Reinforced plastic product manufacturing, often includes machine finishing or adjusting, operations that are not highly specialized, but must be done carefully to get good results. Among machine finishing operations are: cutting, perforating, joining, etc. The most important are detailed next Cutting on the mold. It is also known as trimming and it is cutting the material (glass fiber and resin) that surpasses the mold or piece made. This is done with steel blades along the edge of the mold when the resin is jelled and has not been totally cured. In the case of products manufactured with pressure or temperature, cutting must be done immediately after removing the piece; otherwise, it becomes harder to do so. Cutting with equipment. It is performed on totally finished products. Abrasive products are recommended, since metallic disks are not as fast, accurate, and ergonomic as the ones suggested. Water should be used as cutting takes place; since water acts as refrigerant and lubricant helping to eliminate reinforced plastic dust and the cut is cleaner. Reinforced plastic joints Often, 2 or more sections have to be joined to get a final piece; The systems commonly used are

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Joining with adhesives Although this kind of joining can be done in 2 ways, on the edges or overlapping, the latter is the most used. Because contact surface is bigger. The adhesives most used are: polyester resin (modified with flexible resin) or epoxy resin that provides excellent adhesion. Adhesive material can be put directly on the plastic surface, though applying it on a layer of reinforcing material is suggested, placing the layer between the surface to be joined, and pressing next, to obtain uniformity in the joint. Rivets. Rivets are not often used in this industry, but if needed, aluminum or bronze ones are recommended. They must not be bigger than 4.5mm (3/16") diameter. The minimum distance from the edge is three times their diameter. Besides, flat washers should be used to reduce the rivet tendency to penetrate laminated. Using screws Screws are the most commonly used to join reinforced plastic pieces, excepting adhesives. The use of setscrews is not advisable. Using a screw and nut has the advantage that they are easy to place, are adjustable and available. To get the most efficiency, the following rules must be followed: • Distance between the center of the screw and the edge of a laminated must be minimum three times the screw diameter. • Separation between the center of each screw must be 2.5 times perforation diameter. • Flat washers should be used on both sides of laminated, thus, charge and mechanical efforts are uniformly distributed • Perforations must be perpendicular to reinforcing layer, and the screws must adjust perfectly in. (Both screw and perforation diameter must be the same).. • Screws allow using adhesives, which will provide a better quality and more resistant joint.

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Table: unit conversions.

Specific gravit: 1

g lb = 62.4 cm3 cu ft

Specific heat: 1 Btu = 1 cal lb° F g° C

Heat fusion: 1

Btu in Btu ft cal cm =12 = 0.00413 = 0.0173 sq ft hr° F sq hr° F cm2 sec °C

Thermal conductivity: 1

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in = 1.80 cm ln° F cm° C

W cm cm2 °C

Table: temperature scale conversion. ºF 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 250 255 260 265 270

ºC 10 12.8 15.6 18.3 21.1 23.9 26.7 29.4 32.2 35.0 37.8 40.6 43.3 46.1 48.9 51.7 54.4 57.2 60.0 62.8 65.6 68.3 71.1 73.9 76.7 79.4 82.2 85.0 87.8 90.6 93.3 96.1 98.9 101.7 104.4 107.2 110.0 112.8 115.6 118.3 121.1 123.9 126.7 129.4 132.2

Temperature scale conversion formulas. ºF = °C x l.8 + 32 ºC = ºF - 32/1.8 80

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ºF 275 280 285 290 295 300 305 310 315 320 325 330 335 340 345 350 355 360 365 370 375 380 385 390 395 400 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485 490 495 500

ºC 135 137.8 140.6 143.3 146.1 148.9 151.7 154.4 157.2 160.0 162.8 165.6 168.3 171.1 173.9 176.7 179.4 182.2 185.0 187.8 190.6 193.3 196.1 198.9 201.7 204.4 207.2 210.0 212.8 215.6 218.3 221.1 223.9 226.7 229.4 232.2 235.0 237.8 240.6 243.3 246.1 248.9 251.7 254.4 257.2 260.0

IMPORTANT: CHEMCAST is not legally liable for the recommendations or information given in this manual, which are based on information we consider to be true, we offer it bona fide, but we do not guarantee it, since transformation conditions and use of products are beyond our control.

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