Injection moulding

Injection Moulding is a manufacturing technique for making parts from plastic.

It operates on the principle of injecting molten plastic at high pressure into a mould, which is the "negative" of the wanted shape. The mould is made from metal, and is precision machined to form the wanted part. Injection moulding is very widely used for manufacturing almost anything, from the smallest component to entire body panels of cars.

Considerable thought must be put into the design of moulds, to ensure that the formed part can be removed. Moulds separate into at least two halves to extract the part, and in general a part must be shaped so that it doesn't "grip" the mould. For example, sides of a hollow object cannot be parallel - they must diverge slightly. Examination of most household objects made from plastic will show this aspect of design. More complex parts will call for more complex moulds, which may require moveable sections which are insetted into the mould to form particular features of the part, but are then withdrawn to allow the part to be released.

Raw material for injection moulding is usually in pellet form, and is melted electrically just before being injected into the mould. The channels along which the plastic flows into the actual chamber will also solidify, forming an attached frame called the sprue". This sprue can be cut off and recycled.

The quality of the moulded part depends both on the quality of the mould and the care taken during the moulding process. It is essential that the plastic be at just the right temperature so that it flows easily to all parts of the mould. The parts of the mould must also come together extremely precisely, otherwise small leakages of molten plastic can form, a phenomenon known as "flash".