The design and construction of the mould is essential for a high quality moulded component or product. A mould may consist of two or more parts in which the shape of the moulding has been cut.
Where large quantities of a product are required ( long production run) a hard wearing, durable mould material is required (steel).

Aluminium can be used but only if the production run is in hundreds instead of thousands of mouldings.

The cost of a mould that may make a moulding the size of a lunchbox may cost £10000 to £20000. The capital cost is of course expensive but consider that a single mould may produce tens of thousands of mouldings. Therefore the unit cost of each moulding can be very low because they are manufactured in such large quantities.

The cost of a particular mould is expensive. To solve this problem quick- change tooling systems are used. These kits offer inserts to modify existing moulds rather than making new ones every time a different type of moulding is needed. This has also reduced the setting up times of the injection moulding machine. This is called (Quick change injection moulding techniques).

In order to guarantee a quality moulding;

1. Sharp corners and sudden changes in section must be avoided. This may weaken the plastic moulding wall.
2. Large, flat surfaces should be avoided on a mould as the plastic will not come out completely flat.
3. All surfaces on the mould should be smooth and highly polished.
4. The Sprue (the hole in which the plastic is injected) needs to be carefully designed. It needs to round or half round in section. This allows ease of flow. The sprue also needs to be large enough to allow the mould to be filled very quickly.