Injection moulding

Process:   Plastic pellets are fed into a heated cylinder and driven forward by a turning screw.  The compacted and melted material is then injected into a metal mould often with branching sprues to enable the production of multiple pieces at the same time.

Introduced:   First used successfully with cellulose acetate after 1928; since 1946 it has been the most widely used method of processing thermoplastics; since 1960 it has also been used for processing some thermosets.

Plastics:   Commonly all thermoplastics such as polyethylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, and polyamide.

Marks:   The plastic enters the mould through what is known as a gate which leaves a 'sprue' which is then broken off but leaves a slightly rough, often circular area; there are sometimes also smooth circular marks left by the ejector pins used to help release the warm moulding from the mould; mould lines are sometimes also visible.

Tooling cost:   High.

Production volume:  High.

Uses:   Precision technique capable of complicated shapes: e.g. medical components; Airfix kits; cheap products produced in very large numbers: Lego; plastic cutlery; machine housings; washing-up bowls.