After the Second World War, there was exponential growth in the use of plastic in manufacturing. This resinous, malleable and yet incredibly durable material can be moulded into a huge range of shapes and forms, making many of the products we now use on a daily basis. The technique most widely used to shape plastic is plastic injection moulding.

How it works

Although the injection moulding process can be used with a wide range of materials, including metals, glasses, elastomers and even confectionary, it’s used most with thermoplastic. What makes thermoplastic such a popular material is the ease with which it melts and then flows into every crack and crevice of a mould.

The injection moulding process involves feeding the material into a heated barrel, where it’s melted and mixed, and then forced by a screw-type plunger into a mould cavity. Once in the mould, the material slowly cools and hardens.

Lego mould

Image by Arne Hückelheim (CC BY 3.0)

The plastic is usually fed into the heated barrel in the form of small plastic pellets, which melt easily because they’re small. The melted plastic collects at the end of a plunger and is known as a “shot”. There has to be exactly the right volume of shot to fill a mould properly, avoiding any waste but also compensating for shrinkage on cooling. The plunger then forces the shot into the mould, which is also sometimes called a die and is usually made of steel or aluminium.

Industries and applications

Plastic injection moulding is the most widely used modern method of manufacturing parts. This is thanks to its ability to produce the same object at high volumes with little risk of failure, no waste, low labour cost and little to no need for finishing touches. It’s estimated that as many as 70 to 80% of all objects that we use on a daily basis are made by injection moulding.

Plastic injection moulding is used in a huge range of industries, creating everything from bottle caps, musical instruments, hair combs and computer parts to automobile dashboards, furniture, mechanical gears, kitchen equipment and toys.

The plastic moulding industry is by no means a stagnant one. As the dimensions for parts get smaller and more complex, injection moulding machines and processes for creating accurate moulds have to keep up. The world of plastic itself is also one that is evolving. Traditionally plastic has been a comparatively cheap, brittle material, but plastic manufactured now can be as hard as steel and more durable than almost any other substance – while still being relatively inexpensive and easy to mould.