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5. THERMOFORMING

5. THERMOFORMING. CORPORATE TRAINING AND PLANNING. INTRODUCTION. It is the combination of two words Thermo & Forming. The plastic sheet retains the moulds shape and details. The process involves heating a thermoplastic sheet to its softening temp (pliable State).

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5. THERMOFORMING

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  1. 5. THERMOFORMING CORPORATE TRAINING AND PLANNING

  2. INTRODUCTION • It is the combination of two words Thermo & Forming. • The plastic sheet retains the moulds shape and details. • The process involves heating a thermoplastic sheet to its softening temp (pliable State). • Processing or forcing the hot & flexible sheet against the contours of mould by applying vacuum or air pressure. • The sheet is held there for cooling and then removed. • Thermoforming is secondary processing technique. CORPORATE TRAINING AND PLANNING 2

  3. The sheet is heated to the point only enough to soften it. • Cooling step is usually short due to low wall thickness of the part as compared to other parts. • The essential characteristics of thermoplastic sheet material should be such that when they are heated to just below melting point they should become rubbery or plastic in nature to an extent which enables them to be stretched out rather like a balloon. 3 CORPORATE TRAINING AND PLANNING

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  5. APPLICATION • Refrigerator door liners • Cheese containers • Soft drink cups • Signs • Packaging of Tablets and capsules 5 CORPORATE TRAINING AND PLANNING

  6. Ice cream cups • Plastic tray • Helmets • Telecommunication Joints • Luggage • Light and instrument panels. 6 CORPORATE TRAINING AND PLANNING

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  11. ADVANTAGES • Tooling cost is inexpensive. • Suitable for large parts • Thin walled components can be made by this method only. • Suitable for small number of parts, samples, prototypes etc. • Low capital cost. • Moulds can readily modified and quickly changed. 11 CORPORATE TRAINING AND PLANNING

  12. LIMITATION • The process is confined to the use of sheet material only. • All the parts to be made by this process must have uniform well thickness. • Ribs or mounting bosses cannot be made. 12 CORPORATE TRAINING AND PLANNING

  13. MATERIALS CHARACTERISTIC FOR THERMOFORMING • Ability of the materials to be deep drawn without tearing. • Plastic Memory. • Good hot melt strength. • Hot Elongation. • Forming temperature range a. Wide range is preferred b. No sharp melting point should be there. 13 CORPORATE TRAINING AND PLANNING

  14. TYPES OF MATERIAL USED • Basically thermoplastic materials used for thermoforming process. • Such types of material when heated will exhibit a reduction in their modulus of elasticity, their stiffness and load bearing capacity. 14 CORPORATE TRAINING AND PLANNING

  15. A wide softening range, i.e a broad temperature span in which plastic is soft, pliable and elastic is desirable since, duringthermoforming process the temperature of material drops rapidly. • High molecular weight thermoplastics mostly preferred for thermoforming. • The material to be thermoformed should have higher thermal expansion. • The thermal stability of the material must be good. Thermal diffusivity is ideal for establishing cooling time for thermoformed parts. 15 CORPORATE TRAINING AND PLANNING

  16. Thermal Diffusivity = Thermal Conductivity Density X specific heat • The water absorption capacity of the plastic material should be low for thermoforming, because slow rate of water absorption also causes difficulties in thermoforming 16 CORPORATE TRAINING AND PLANNING

  17. PLASTIC MATERIALS FOR FORMING PS, ABS, PVC, PMMA, CAB, PC, HDPE, PP 17 CORPORATE TRAINING AND PLANNING

  18. MATERIAL PREPARATION • Most available sheet materials are prepared by sheet extrusion process which employs medium to high molecular weight polymers that are subjected to minimal heat stress. • Sheets with excellent optical properties are obtained either by casting or by laminating and / or press polishing of otherwise manufactured sheets. 18 CORPORATE TRAINING AND PLANNING

  19. SHEET FOR THERMOFORMING The sheets are manufactured from : • Extrusion Process • Calendaring Process • Casting Process The sheet thickness ranges from 0.25mm to 12.5mm 19 CORPORATE TRAINING AND PLANNING

  20. THERMO FORMING MOULD Mould Materials (i) Plaster of Paris : • Most commercial moulding, plasters are not strong enough to be used in prototyping. • Plasters are inorganic calcious materials that hydrolytically react and harden when mixed with water. 20 CORPORATE TRAINING AND PLANNING

  21. Soaps such as Murphy’s Oil soap, found in lather goods stores can also be used as a surface release agent. Vents should be designed in by placing release-agent-coated wires perpendicular to the pattern surfaces before coating. • A very hard surface (Void-free) can be achieved by “Splitting” a thin layer of relatively high water content plaster slurry against the pattern. 21 CORPORATE TRAINING AND PLANNING

  22. Advantages : Cheap, quick, and intimate production of details is possible. Disadvantage : A maximum of only about 50 forming is possible, the surface being very soft and the mould itself is very fragile. 22 CORPORATE TRAINING AND PLANNING

  23. (ii) Wood : • Hardwoods are used for prototype and short production. • The woods must be thoroughly Klin-dried before shaping to minimize stress relief during fabrication. • After thorough drying, the surface can be sealed with temperature–resistant enamel or varnish. • Recently, epoxy enamels and varnishes have been developed that protect wood surface for hundreds of cycles with out refinishing. CORPORATE TRAINING AND PLANNING 23

  24. Advantages : Cheap, longer life span than plaster moulds, higher impact strength. Disadvantages : Limited life say for approximately 500 forming. During repeated forming, wooden mould should not be allowed to become too hot and its dimensions should be checked regularly. 24 CORPORATE TRAINING AND PLANNING

  25. Plastic moulds: • In particular, plastic tooling is economically preferred for thick sheet forming. • Plastic moulds are used where mould surface temperature do not exceed 60oC. where drape or vacuum forming used, epoxy and unsaturated polyester resin (UPE) together with glass fiber are the mould materials of choice. 25 CORPORATE TRAINING AND PLANNING

  26. Advantages: • Fairly cheap, easily manufactured, low thermal conductivity, little or no finish is required, lasts long. Disadvantages: • Some materials are sensitive to high forming temp. • mould surface must be adequately cleaned, waxed and buffed prior to use. 26 CORPORATE TRAINING AND PLANNING

  27. iv) Aluminium moulds: • Aluminium is frequently the material of choice for thermo forming moulds. • Because it can be easily fabricated and it has very high thermal conductivity and so sensible heat from plastic material can easily be removed. • It is light weight, tough metal. • Thermo forming tools can be made from either machined plate or caste material. 27 CORPORATE TRAINING AND PLANNING

  28. The Aluminium mould mostly consists of 1 to 2 % Cu., 0.5 to 1% mg, 0.5% Mn, 4 to 8% Si, 1% (max) Fe, 15 Ni and traces of Ti and Zn. • Typical machined aluminium hardness is 130 Brinell and Aluminium has relatively high thermal expansion co-efficient. Advantage: Dimensionally stable, good surface finish, very good abrasion resistance and indefinite life time. 28 CORPORATE TRAINING AND PLANNING

  29. HEATING SYSTEMS • Convection Ovens: • Convection ovens are originally the most common device used to heat plastic sheets for thermoforming. • The heat can be supplied by gas flames or by electric resistance units. 29 CORPORATE TRAINING AND PLANNING

  30. Forced circulation of air and baffling to equalize the air flow at around 200 feet per minute are crucial to obtain temperature uniformity. • Good thermal insulation of the oven walls and the strategical position and size of entrance and exit doors increase energy efficiency. • Automatic temperature regulators must be provided to keep air temperature fluctuation as low as possible. 30 CORPORATE TRAINING AND PLANNING

  31. (ii) Infrared Radiant Heaters • Besides dialectic heating, oil submersion heating and contact heating, IR radiant heating is the fastest way of heating plastic sheet or films to thermoforming temperature. • Although heater densities may vary with equipment, there are also differences in regard to materials. 31 CORPORATE TRAINING AND PLANNING

  32. High temperature plastics, such as polycarbonates and polyesters are the highest, with about 30 watt/sq. • The cellulosic, styrene and vinyl Polymers are the lowest 15 watts/sq. Thin films can be heated at higher energy densities in a considerably short time. 32 CORPORATE TRAINING AND PLANNING

  33. (iii) Electrically Powered Infrared Heaters • Electrically powered infrared heaters are available in a wide range of designs. In order of decreasing radiant surface temperatures (i.e. increasing wave length of energy emitter). They are: • Tungsten wire filament heaters in quartz tubes and tungsten wire filament glass lamps. 33 CORPORATE TRAINING AND PLANNING

  34. Nichrome wire coil heaters in quartz glass tubes. • Nichrome wire or band in refractory materials embedded or surrounded and protected by stainless steel round. • Heat distribution over entire sheet is more uniform in case of radiant heaters than hot air convection ovens. 34 CORPORATE TRAINING AND PLANNING

  35. STRETCH RATIO A sheet of plastic initially of thickness to and surface area Ao is stretched to provide a part having a surface area A (A > Ao) and an average thickness ta (ta < to) The plastic volume given by :V = toAo = tdA = taA The stretch ratio is given by : Ra = A/Ao or Area ratio Other wise called areal draw ratio 35 CORPORATE TRAINING AND PLANNING

  36. DIFFERENT FORMING PROCESSES • Straight Vacuum Forming Process. • Pressure forming • Plug-Assist Forming • Free forming • Drape forming • Snap-back forming • Matched-die forming • Mechanical forming 36 CORPORATE TRAINING AND PLANNING

  37. STRAIGHT VACUUM FORMING • This techniques is most versatile and widely used. • The plastic sheet is clamped in a frame and heated. • The hot sheet becomes rubbery or elastic. • Then it is placed over a female mould cavity. • The vacuum is now applied. 37 CORPORATE TRAINING AND PLANNING

  38. The atmospheric pressure forces the hot sheet against the walls and contours of the mould. • It is allowed to cool there. • The formed part is removed and final finishing and decoration is done. 38 CORPORATE TRAINING AND PLANNING

  39. This technique is used when the outside of the part (the side against the mould) must have fine details or close tolerances . • This process is limited to draw ratio of 1 ½ . • Draw ratio is the ratio of the draw dept to the part width. 39 CORPORATE TRAINING AND PLANNING

  40. FIGURE 40 CORPORATE TRAINING AND PLANNING

  41. PRESSURE FORMING • It is similar to straight vacuum forming process. • Here also plastic is formed in a female mould. • Here instead of applying vacuum a positive air pressure on the top of the plastic is used to force the material against the female mould. 41 CORPORATE TRAINING AND PLANNING

  42. PROCESS • The sheet is clamped and heated till softened. • The softened sheet is transferred to the moulding area and a seal is made so that the upper chamber, above the plastic is airtight. • The sheet is also sealed against the mould as is done with vacuum farming. • Air pressure is applied into the area above the softened plastic and vacuum is created below it. • The air pressure and the vacuum forces the plastic against the mould. 42 CORPORATE TRAINING AND PLANNING

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  44. ADVANTAGES • Moulding cycle is faster. • The sheet can be formed at lower temp because the forming pressure is higher. • A greater dimensional control and part definition can be achieved. 44 CORPORATE TRAINING AND PLANNING

  45. PLUG ASSIST FORMING • A male plug is used. • Plastic sheet is clamped in the female mould and after the heat-softened sheet is sealed across the mould cavity,the plug pushes the sheet to stretch it. • After completion of penetration stroke vacuum and /or compressed air is introduced to transfer the sheet from the plug surface to the cavity mould surface. 45 CORPORATE TRAINING AND PLANNING

  46. Plugs are made up of metal,wood or thermoset plastic. • Plug is heated to a few degree less than the temp of the plastic in order to prevent premature cooling. • The plug size combined with the rate and depth of penetration affect the amount of stretching that occurs. 46 CORPORATE TRAINING AND PLANNING

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  48. ADVANTAGES • Deeper and more uniform wall thickness is obtained. • It is also responsible for the ultimate material distribution in the finished product. 48 CORPORATE TRAINING AND PLANNING

  49. FREE FORMING • The sheet is expanded with pressure. • The size of bubble is monitored by an electronic eye. • When the bubble reaches the desired size,the air pressure is reduced to a level that maintains the size of the bubble while the part cools. 49 CORPORATE TRAINING AND PLANNING

  50. ADVANTAGES • The products have very high optical clarity. • No mould is used. • No transfer or handling of the sheet. • Simple and Economical • Uniform cooling. 50 CORPORATE TRAINING AND PLANNING

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