Granulated or powdered thermoplastic plastic is fed from a hopper into the Injection Moulding machine.
The Injection Moulding machine consists of a hollow steel barrel, containing a rotating screw (Archemidial Screw). The screw carries the plastic along the barrel to the mould.
Heaters surround the barrel melt the plastic as it travels along the barrel.
The screw is forced back as the melted plastic collects at the end of the barrel.
Once enough plastic has collected a hydraulic ram pushes the screw forward injecting the plastic through a sprue into a mould cavity.
The mould is warmed before injecting and the plastic is injected quickly to prevent it from hardening before the mould is full.
Pressure is maintained for a short time (dwell time) to prevent the material creeping back during setting (hardening). This prevents shrinkage and hollows, therefore giving a better quality product.
The moulding is left to cool before removing (ejected) from the mould. The moulding takes on the shape of the mould cavity.
The image below shows an an Injection Moulding machine.