• thermoplastics,
• thermosets,
• elastomers.
Thermoplastic articles can be formed at high temperatures, cooled, reheated and reshaped many times without changing their structure and properties irreversibly. The thermoplastic structure is composed of linear chain molecules, sometimes with side branching of the molecules, which have weak, secondary bonding between adjacent chains. When subjected to pressure, thermoplastics can deform in two ways. Elastic deformation occurs in which the initially curled long chains stretch out and this deformation is recovered when the pressure is removed. At higher temperatures or pressures, viscous or plastic deformation occurs in which the secondary bonds weaken and allow the molecular chains to slide over each other. This weakening of the bonds at higher temperatures means that thermoplastics are easy to cast or form. Thermosets
Thermoset polymers are formed by a condensation reaction which forms strong, primary crosslinking bonds between adjacent polymer chains, giving them a rigid threedimensional structure in which the atoms cannot move easily. Heating helps this crosslinking process and so the polymer hardens as it is heated. Common examples of thermosetting polymers are polyester resin (often used with glass fibre reinforcement in boat hulls and furniture), polyurethane (used in upholstery and paint), phenolic resin (Bakelite, used in electrical fittings and pan handles), melamine formaldehyde resin (Formica or Melamine, used in laminated surfaces and moulded products), epoxy resin (Araldite; used as a laminating and bonding agent) and urea formaldehyde resin (used in electrical fittings, adhesives and cavitywall insulation
Different types of procesing plastics
Vacuum forming and blowmoulding
Vacuum forming and blowmoulding are manufacturing processes which are very closely related to the sheet forming .Both these processes rely on the use of pressure forces to bring sheet material into contact with mould. In vacuum forming this is achieved by sucking the sheet material down on to a former; in blowmoulding by raising the pressure to force the material against 1 mould. The tool costs are much lower than for presswork and it is possible to get highquality finish, which is difficult when pressing SNICs, for example. One disadvantage is that the stress levels required to deform the material cannot be too high and this limits the materials which can be used and the shapes that can be generated.
Bending
Plastics can be bent in many ways. By using heat the bonds in the plastics become separated and malleable. This enables the plastic (usually in sheet form) to be bent. Both thermo and thermosetting plastics can be bent however thermosetting plastics can not then be reformed. If the plastic gets too hot however it will begin to deform. The structure and the appearance of the plastic will therefore change.
