When identical plastic products are to be manufactured in large quantities industry tends to use Injection Moulding. This system is of course automated in industry but tends to be operated by person on the school workshop environment.

A shot of granulated or powdered thermosetting plastic which has been heated is injected under pressure into a pre-heated mould. The plastic is injected quickly to prevent the plastic hardening before it fills the mould.

Once inside the mould the pressure is maintained (dwell time) to prevent the plastic from creeping back into the screw chamber. The pressure also prevents hollows from occurring inside the plastic moulded shape.

The plastic cools and takes on the shape of the mould cavity. When the plastic component has solidified the mould is opened and the moulding ejected

 As can be seen from the above diagram the plastic (high impact polystyrene- HIPS) or (ABS - acrylonitrile butadienestyrene) is fed into the hopper. It is forced forward under pressure by the Archimedean screw. It is heated by the surrounding heater coils. As the plastic builds up at the front of the chamber the screw moves backward. Eventually the screw is forced forward by the hydraulic ram forcing the plastic through a sprue into the mould.

Such small items as bottles, sink plugs, model kits, dustbins, bowls and milk crates etc. can be made using this system.
Even much larger items for example dinghy hulls and high precision components such as camera parts, razors and gear wheels are injection moulded.

image courtesy of Royal College of Art. Hodder and Stoughton.