In-depth: the injection moulding process

Many of the plastic products that customers take for granted in their cars, homes and workplaces will have been created through a single process: injection moulding

Although injection moulding is often a process of choice for manufacturers due to its cost-effectiveness and high consistency of product, it involves controlling complex systems and parameters.

How do injection moulding machines work?

There are a variety of injection moulding machines used by manufacturers. The main types of machine used are hydraulic, electric and hybrid (a combination of electric and hydraulic) powered, but all use a similar mechanism to create their final product.

Injection moulding machines are designed to force molten materials, usually plastic or metal, into a mould that includes both a cavity, which the material is pushed into, and a clamp, which is used to mould and fix the part into its final shape.

Within one machine, the moulds can be of a single cavity or multiple cavities, meaning that just one or a number of products can be made during a single process. In addition, the multiple cavity moulds can include identical or unique cavities, so the products created can be the same or individual. Each mould is designed to create more than a million parts across their lifetime, meaning they have to be robust and have low tolerances.

What happens during injection moulding?

When thermoplastics are moulded, pelletised raw material is fed through a hopper into a heated barrel with a reciprocating screw. The molten material is then sent down a runner to the gate, where the machine uses a ram or screw-type plunger to shoot it into the cavity.

Sometimes, a two-shot mould process is used to incorporate two separate materials into one part. This is used to add a soft touch to knobs, to provide multiple colours or to produce a part with multiple performance characteristics.

Once the clamp has been pressed down into the cavity to form the part and the material has cooled in the cavity, a pump is used to push the finished part out of the cavity, known as ejection.

Depending on the aesthetic requirements of the product, there may be further processes that help to polish, colour or finish the part, but this is the standard injection moulding process used to make plastic or metal parts.

Are there different types of injection moulding?

Depending on the characteristics of the part that needs to be created, the injection moulding process can be adjusted to create more complex products. Here are a few of the most common processes.

• Overmoulding (two-shot moulding) is when two different types of material are injected into the same cavity so they combine to create a part with mixed qualities, whether this is two colours, for increased durability or a slightly adjusted performance.
• Insert moulding combines two or more different components to create a single part. This may mean putting a finished part into the cavity before refilling it to combine the two together. This can help to make the product stronger or add extra elements to a part without increasing its weight.
• Metal injection moulding uses a slightly altered process to turn metal powders into fully moulded parts. This is a less common process but can be used to create complex pieces that are more robust or have different aesthetic qualities from plastic.

To make injection moulding more efficient, it is now integrating the technologies of Industry 4.0 so the products it creates are a more consistent quality, despite having more complex shapes and characteristics. This means that despite the fact the process has been used by manufacturers for many years, it is highly likely to continue to evolve and be used in the future.