A specific quantity of raw, un-polymerised plastic is placed into a heated mould which is closed under pressure compressing the molten plastic into all areas of the cavity into the desired shape; it is the only process capable of moulding thermosetting plastics.
The molten resin is transfered by pressure from a meltpot into a mould which is at a temperature above the meltpoint of the resin. ( For comparison: in injection moulding, the mould temperature is below the resins meltpoint).
This higher temeprature is important for the filling of moulds with high flow lenght: wall thickness ratios and to allow for shrinkage compensation, otherwisen jeopardized by premature gate freeze-off and cooling in the mould.
Materials such as polystyrene, nylon, polypropylene and polythene can be used in a process called injection moulding. These are thermoplastics - this means when they are heated and then pressured in a mould they can be formed into different shapes.