How does Injection Moulding Work?

Posted on: 6th Jul 2021

Injection moulding is a process used to produce plastic parts, by injecting molten material into a mould.  

As a cost effective, reliable method of manufacturing, it is no surprise that injection moulded parts are produced in their millions, every year. Its versatility allows for both mass volume production of identical items, and smaller quantity runs for testing purposes. Many of the items we utilise in everyday life are a product of injection moulding.

Injection Moulded Everyday Parts

The Injection Moulding Process

Injection moulding can be performed with a wide range of materials, but most commonly thermoplastic polymers. Raw (or virgin) plastic materials usually start in pellet or granule form.  Prior to starting the injection moulding process, plastic pellets are dried to remove excess moisture using hopper dryers. Any excess moisture retained can affect the quality and/or performance of a finished moulded part.

Raw material
Hopper Dryers

The overall process of injection moulding is quite complex, but it can be broken down into a few steps. An injection moulding machine is made up of 3 main parts. The feed hopper, the heated barrel, and the reciprocating screw.

Injection Moulding Machine Diagram

The pre dried plastic pellets are fed into the hopper, then enter the heated barrel where the frictional (turning) action of the reciprocating screw creates heat to melt the pellets. The screw also acts as a ram during the injection phase.

Once in a molten state, the plastic is injected into the mould cavity, through a nozzle.

Once the mould cavity has been filled, it is held at high pressure to compensate for material shrinkage. Whilst in the cavity, the material cools and solidifies to form the desired part, which is then released when the mould opens.

Injection moulded part coming out of a mould

Meanwhile the screw turns again, preparing the next shot of material, ready for the mould cavity to be filled again.

This sequence of events is known as the injection moulding cycle. The new cycle starts when the mould closes, and the polymer is injected into the mould cavity. Moulding cycle times vary depending on the specific characteristics of the material and/or the finished part but tend to range from a few seconds through to around 4 minutes.

The whole process is then repeated until the production run is complete.

After the moulding cycle is complete, the parts are left to cool. On completion of the full production run, parts undergo trimming/finishing as required, before being prepared for dispatch.

Image of manufacturing process

If you would like any further information or advice please visit our Injection Moulding page or email

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