What Are The 4 Stages of Injection Moulding

Mar 7, 2024

Over our 50-year injection moulding career, the team at Pentagon Plastics are proud of the constant innovation that we have been able to pursue to make the plastic products we produce the best they can be, from the compounds to the moulds, and the equipment we use.

However, there are four crucial stages of the injection moulding process that remain the same - in this article, we will outline what exactly those four stages are

What is injection moulding?

Injection moulding is the process used to create certain plastic products; by injecting molten thermoplastic compound into a mould and allowing it to cool, a hard plastic product can be formed.

The materials that we commonly use are thermoplastic polymers. Thermoplastics can be of varying base materials such as Polypropylene, Nylon, ABS and Polycarbonate to name just a few, and the scope of use can be widened with additives such as flame retardancy, UV stabiliser, glass fill or biocompatibility.

What can be produced by injection moulding?

There are a great number of products that can be created using plastic injection moulding; from children’s toys to bottle caps, medical products, and automotive parts, the possibilities are somewhat endless.

How long does the injection moulding process take?

You may be surprised to learn that, compared to some other plastic manufacturing processes, the injection moulding process is relatively short. The length of time involved in the injection moulding process depends on the complexity of the components being produced. For example, a single set of components can take mere seconds, whereas larger, less straightforward components can stretch into minutes.

What are the four stages of injection moulding?

Regardless of the complexity of the components being produced, the following four stages of injection moulding remain the same every time:  


Before injection can begin, the two halves of the mould need to be clamped into the moulding press so that the compound (in this case, plastic) can be injected into it.

Throughout the injection process, the mould remains clamped in position and is made up of two halves. The Fixed Half will be clamped to the fixed platen of the press and connect to the screw & barrel assembly that will feed the thermoplastic moulding material into the mould tool.

The Moving Half is clamped to the opposing moving platen which pulls away from the fixed platen after every cycle splitting the tool on the parting line and allowing the moulded product to be ejected from the mould tool. The force applied between the platens by the moulding machine is the clamping force which also denotes the size of the moulding press.  


Whilst the mould is clamped together, granular thermoplastic is placed inside a hopper, and heated until it’s fully melted.

The molten plastic is then forced through the barrel and injected into the mould; how long this takes will be determined by the size of the mould, the type of plastic and the intricacy of the product. The injection process is powered by an electric or hydraulic system, with the ability to inject at various pressures and speeds.


The compound needs to solidify before the product can be removed so a cooling time is applied. The length of time is determined by the type of thermoplastic, the thickness of the wall sections and the quality of the temperature control applied to the mould tool.

Each component will have an estimated cooling time and this can be the longest part of the process. Only when this time is reached can the mould be removed, and the finished product rejected from the press.


After the part is moulded and the tool is opened, the ejection system within in the mould tool pushes the component out of the cavity or off the core until it clears the mould features. The part is then free to fall clear of the mould tool into a collection box or be physically removed.

On occasions, the moulded part can become stuck to the mould faces, so the ejection system applies force to break the bond. Parts can be removed through automation into a collection barrel at the bottom of the injection mould press, by hand or through robotics.

This completes the injection moulding process, and the mould can then be closed once more to repeat the cycle as required for the number of parts in the production run.

It’s important to note that quality should be monitored throughout the plastic injection moulding process, to ensure that every component is repeatable and meets product specifications.

Why choose Pentagon Plastics for your plastic injection moulding?

As experts in our field, we’ve got a handle on the four stages of plastic injection moulding; they have formed the basis for countless batches of products over our 50 years in the business.

This high level of experience means that we can accurately and efficiently manage the full plastic injection moulding project from start to finish, from the design and development to mould tooling, production, and post moulding support.

There are pre-production and post-production stages to go through to ensure that your components come together seamlessly; we’re here every step of the way to guide you through it.

We will always work with you to establish the best ordering patterns be it make to order, scheduled call offs, or bespoke Kanban agreements. So don't hesitate and get in touch with us today.

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