What is The Plastic Injection Moulding Process

Feb 28, 2024

The complexities of plastic injection moulding can, at first glance, appear overwhelming, but with many years in the business, we’ve got the process down to a fine art here at Pentagon. Just as we guide our customers through the injection moulding process, here we will walk you through it, so that you know what to expect from injection moulded component production.

We feel it is important to involve an injection moulder at the early stage of a project, to ensure the components design is both viable and designed in a way that suits the moulding process.

So, once the product design is well on its way and your Non-Disclosure Agreement (NDA) is in place, what is the plastic injection moulding process?

Step 1 – Design for manufacture review & choosing materials

The first part of the process is always to understand the product that is required, before carrying out a design for manufacture (DFM) review to evaluate ease of production from a manufacturing perspective. The Pentagon DFM will identify challenges, note complexities, and consider those design changes that may be necessary.

The success of the product and its suitability for purpose depends as much on the thermoplastic material used as the mould used to make it with. Mechanical characteristics, the moulding process, cost per kilo and the material’s availability should all be considered. Underpinning it all should be the understanding that different materials will have different properties, and these will affect the strength and functionality of the final part. Material choice will also have an impact on mould tool design due to the differing behaviours of materials within the moulding process.

Step 2 - Designing the mould

The mould tool is drafted by our design team to optimal configuration to deliver the best tooling solution, be it to meet a particular budget, to achieve the expected life expectancy or to provide optimum design techniques for complex movements. The key is to achieve repeatability at an acceptable cycle time.

It is critical that the mould tool that is manufactured suits the plastic injection moulding process, and, crucially, is fit for the purpose of the product.  

Our designers will create a 3D Model of the mould tool creating each element to exacting standards. From the tool design a Bill of Materials (BOM) will be generated for ordering materials and bought in items, a General Assembly drawing for customer approval will be produced and finally Detailed drawings will be created to support the manufacturing process.

Step 3 - Producing the mould

Moulds are often made from pre-toughened of fully hardened tool steels but can be made from softer aluminium, or have beryllium-copper features.

Individual elements of the mould tools are manufactured in our toolroom working closely with the tool designer at every stage of construction. Our highly skilled CNC technicians will use various 3,4 & 5 axis machining centres and lathes supported by our extensive spark and wire EDM erosion centres. Traditional engineering activities such as milling, turning and grinding are also required to deliver all of the component parts ready for final assembly by our highly skilled Tool Assembly Technicians.

Step 4 - The production process

Regardless of the specifics of the product, the production process itself follows a predictable four stages, which include:

Melting and clamping

Granular thermoplastic is added to a hopper and heated until melted whilst the two halves of the mould are clamped shut (and stay this way until the ejection stage). The machine is then set and cycle times are optimised to control the process.


The now-liquid thermoplastic is pushed through the barrel from the hopper into the injection system and injected into the mould. This system is powered by hydraulics, injecting plastic at a pressure ranging from 35 MPa (megapascal) to 140MPa.


Before the mould is removed, the plastic form needs to cool and harden. How long this takes will depend on the size and material behaviours of the components. Only when the plastic is cool can the ejection process take place; this can be in the form of hand removal from the mould or automated drop depending on the product complexity.

It is essential to gain first off approval from inspection before moving to full production runs.


Once the plastic inside them is cool, the moulds can be removed, and the product ejected forcibly, to unstick it from the mould.

Step 5 - Quality assurance

Just like first off approval, last off approval needs to be sought to ensure a consistent process, intermittent visual inspection throughout the production run is beneficial to identify any quality issues that may occur throughout the run and avoid production of products that are not fit for purpose (waste product).

Step 6 - Post moulding

Some products can require post moulding operations, such as inserting, waxing, light assembly, coating and marking. This should all be done once the product is properly cooled.

Why choose Pentagon Plastics for your plastic injection moulding?

As plastic injection specialists with over 50 years of experience and a commitment to innovation in our field, your plastic products are in safe hands with Pentagon. With dedicated departments who can offer expert advice and skills in all the areas we’ve mentioned to bring your plastic products to life, we are confident in saying that our processes, tooling and output are world leading.

Whether you are at the initial concept stage or are seeking a plastic injection moulder to run production from your existing tooling we are well placed to cater for all of your project requirements.

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