Chapter 15 Thermoforming

1. Chapter 15Thermoforming ETPL 1100

2. Introdution • Thermoforming is the process where plastic materials are shaped from softened sheet • The process involves • Heating the plastic sheet to a temperature range where it softens • Then stretching the softened plastic against a cold surface mold • When the sheet has cooled, it is removed from the mold and excess plastic is trimmed

3. Introduction • Thermoforming is a gemeric term for a group of processes • Vacuum forming • Drape forming • Billow forming • Mechanical bending • Match-Mold forming • Pressure forming

4. Introduction • Thermoforming is one of the oldest methods of forming useful articles

6. Introduction • Markets • Thermoformed products are categorized as • Permanent( Industrial) • Disposable

7. Introduction • Typical Industrial Products • Equipment cabinets • Tote bins • Pallets • Trays • Head liners • Shelves

8. Introduction • Typical disposable products • Point of purchase containers • Hand and power tool cases • Meat and poultry trays • Egg cartons

9. Introduction • Major growth area for thermoformed products • Multi layer containers • Returnable containers • Modified atmospheric packages • Medical device equipment

10. Introduction • Terminology • Thermoforming processes are divided by thickness or gauge of the sheet used • Thin gauge for sheet less than .06in (1.5mm) • Heavy gauge for sheet greater than 0.12in. (3mm) • “gray area” between the thickness, depends on whether the material can be rolled or comes in sheets

12. Introduction • Competition to thermoforming • Blow Molding • Rotational Molding • Injection molding

14. Introduction • Advantages of thermoforming • Low temperature, low pressure required • Only a single surface mold is required • Molds are easy to fabricate and use inexpensive materials • No need for the plastic to flow • Can make very large surface area to thickness ratios

15. Introduction • Disadvantages to thermoforming • Plastic material is more expensive because the pellets have to be made into sheets • Generally more waste to reprocess • One sided • Can get a great deal of wall thickness variation

16. Plastics and Polymers

17. Plastics and Polymers • Thermoforming uses plastic sheet, which is heated, stretched, cooled and mechanically cut • The plastic sheet is manipulated as a rubbery solid or elastic liquid • The solid or elastic liquid properties are more important than the viscous properties when thermoforming

18. Plastics and Polymers • You can thermoform both amorphous and crystalline polymers • Amorphous • No organization, glass transition • PS, ABS, PVC, PMMA, PC • Crystalline • Organized region called crystals, glass transition and melting • PE, PP, Nylon, Acetal

19. Plastics and Polymers • Thermoforming Window • Temperature range over which the polymer is sufficiently subtle or deformable for stretching and shaping • Typically amorphous have broader window than crystalline • PS – 260 to 360 F • PP – few degrees below melting point of 330F

21. Plastics and Polymers • Important thermal properties • Enthalpy or heat capacity • Thermal conductivity • Temperature dependent density

23. Plastics and Polymers • Heat Capacity or specific heat • Measure of the amount of energy required to elevate the polymer temperature • One measure of energy uptake is enthalpy • It increase with increasing temperature • When a material goes through a phase change, such as melting, the enthalpy-temperature curve changes dramatically • When a material goes the glass transition, the enthalpy-temperature curve changes very little

24. Plastics and Polymers • Heat Capacity or specific heat • It takes twice as much energy to heat heat a crystalline polymer from room temperature to above its melt temperature than to heat an amorphous polymer the same

26. Plastics and Polymers • Heat Capacity or specific heat • In general enthalpy changes can be calculated as: • M is mass • Cp is heat capacity • T is temperature

27. Plastics and Polymers • Heat Capacity or specific heat • If going through a melting transition • ∆H gives the energy required

28. Plastics and Polymers • Thermal conductivity • The measure of energy transmission through a material • Plastics have substantially lower rates than metals • Aluminum has 1000 times greater than PS • Th rate of heat transfer to the sheet is very important because it determines the time the process takes

29. Plastics and Polymers • Density • Mass per volume • It decreases with increasing temperature • Small change at glass transition, large change at melting temperature • At forming temperature amorphous has 10-15% less density than at room temperature • At forming temperature crystalline has 25% less density than at room temperature • Shrinkage must be accounted for to meet final specifications

30. Plastics and Polymers • Thermal diffusivity • Combination of polymer heat properties • Thermal conductivity/density/heat capacity • Fundamental polymer property in time dependent heat transfer to materials

31. Plastics and Polymers • Infrared energy absorption • Most commercial thermoforming heaters emit energy in far infrared wavelength • Thermoforming is concerned most with the wavelength range 2.5 to 15 microns • Very important when heating sheet

33. Plastics and Polymers • Thermoformable polymers • Polystyrene is by far the most widely used • Other PS family • HIPS • ABS • SAN • OPS – orientated PS • PVC • PMMA

34. Plastics and Polymers • Thermoformable polymers • Cellulosics • PC • PET • PE • PP • If a polymer can be made into a sheet it can be thermoformed

35. General Forming Concepts

36. General Forming Concepts • The simplest thermoforming process consists of simply heating the sheet and forcing it against a solid shaped mold • There are many variations of this simple method

37. General Forming Concepts • Simple heating and stretching • The basic thermoforming process is of differential stretching • Only the sheet that is not touching the mold stretches • As stretching continues, the sheet becomes thinner and thinner • Area last formed are the thinnest, most orientated and weakest • Final part has very non-uniform wall thickness

39. General Forming Concepts • Drape forming • Earliest method • The sheet is heated and then manually shaped over the mold • Uses a male or positive mold • Yields a part that is thinner along its side walls, rim and corners than at the bottom • Used to make heavy gauge products such as signs and refrigerator liners

41. General Forming Concepts • Vacuum forming • The sheet is heated and stretched into a female mold • Requires a vacuum system • The part is thinner in the bottom and corners than at the top • Used to make heavy gauge products such as signs and thin gauge product such as picnic plates

42. General Forming Concepts • Free forming • Billow or free bubble forming • Does not use a mold • The sheet is heated to its forming temperature, then air pressure is applied against the sheet, and the sheet expands • As the bubble expands it touches a shut off device for the air, controlling the final size of the bubble

44. General Forming Concepts • Free forming • Because the bubble never touches a solid surface, it remains mar free • The bubble is uniform in thickness • Transparent polymers are most used • Heavy gauge, free fromed shapes are used as skylights and aircraft windows • Thin gauge, free fromed shapes are used in blister packaging

45. General Forming Concepts • Assisted forming • Used to improve the wall thickness uniformity for deep draw parts • Three types • Non uniform heating • Pneumatic preforming • Plug assist

46. General Forming Concepts • Non uniform heating • For heavy gauge froming its produces a sheet that is hotter in certain ares than others • Hotter sheet stretches more that cooler sheet • Regions of the sheet that would normally be over thinned are not heated as much as regions that would normally be over thick

47. General Forming Concepts • Pneumatic preforming • The heated sheet is first inflated with air • A mold is plunged into the inflated sheet • Vacuum is applied to the mold to ensure the sheet replicates the mold surface

50. General Forming Concepts • Plug assist • Plugs are mechanically driven, shaped solid structures that are pressed into the softened sheet prior to forming • Used to locally stretch a sheet • Used for heavy gauge products such as tote bins and equipment cabinets • Used for thin gauge products such as drink cups