Mould or die are the common terms used to describe the tool used to produce plastic parts in moulding.

Since moulds have been expensive to manufacture, they were usually only used in mass production where thousands of parts were being produced. Typical moulds are constructed from hardened steel, pre-hardened steel, aluminium, and/or beryllium-copper alloy.^[18]:176 The choice of material to build a mould from is primarily one of economics; in general, steel moulds cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel moulds are less wear-resistant and are used for lower volume requirements or larger components; their typical steel hardness is 38–45 on the Rockwell-C scale. Hardened steel moulds are heat treated after machining; these are by far superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminium moulds can cost substantially less, and when designed and machined with modern computerised equipment can be economical for moulding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mould that require fast heat removal or areas that see the most shear heat generated.^[18]:176 The moulds can be manufactured either by CNC machining or by using electrical discharge machining processes.

• Injection moulding die with side pulls
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  "A" side of die for 25% glass-filled acetal with 2 side pulls.

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  Close up of removable insert in "A" side.

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  "B" side of die with side pull actuators.

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  Insert removed from die.