Moulds for plastics (as well as rubber and other softer materials) are similar to dies in that they usually consist of an upper and lower portion. Plastic or other material is heated to a workable temperature and is compressed or injected into the mould cavity. The material quickly hardens as the halves of the mold are opened and the parts may be extracted. Moulds often have heating and cooling piping built into the tool itself to allow for uniform temperatures within the cavity in which the material is formed. Virtually all of the plastic things in our everyday life have been produced in moulds. Mouldmakers use the same high levels of machining and assembly skills as die makers.

Moulded parts
Moulded plastics have been used as replacements for many materials which are more costly or more difficult to work with. Moulds with textured beryllium-copper cavity surfaces can produce "wood grained" parts that not only look like wood, but also feel like wood.
A single cavity mould produced for a complex electronics component's housing required 1,756 critical dimensions for the housing itself, and 5,541 dimensions for the tool drawings. The mould contains approximately 300 bypass shutoff valves to insure uniform temperature and pressure within the mould.