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Introductory Guide To Plastics Injection Moulding

If your knowledge of plastic injection moulding is limited and you would like to gain a basic understanding, then this guide is perfect for you.

Having been in the field for over 40 years, we feel well-placed to provide a rudimentary overview detailing the following information:

  • Definition of Plastic Injection Moulding

  • The Plastic Injection Moulding Process

  • The Advantages & Disadvantages

What is Plastic Injection Moulding?

Plastic injection moulding entails melting down small thermoplastic polymers, or plastic pellets, to make them pliable enough to pour into a mould. The now liquid polymer material will then be reformed into the exact shape that is needed to make the component.

How does the Plastic Injection Moulding process work?

Illustration of inside an injection moulding machine

The process can be broken down into four basic steps:

Step 1

  • Granules are dried in ovens or dehumidifiers prior to production.

  • Once dry, granulated thermoplastic plastic is fed from a hopper into the Injection Moulding machine.

Step 2

  • The Injection Moulding machine consists of a hollow steel barrel, containing a rotating screw. The screw carries the plastic along the barrel into the mould through a nozzle. Heaters that surround the barrel melt the plastic as it travels along the barrel.

Step 3

  • Once enough plastic has collected, a hydraulic ram pushes the screw forward injecting the plastic through a sprue into a mould cavity. The nozzle meets the sprue on the tool which is then injected into the mould cavity of the tool.

  • The plastic is injected quickly to prevent it from hardening before the mould is full.

Step 4

  • Pressure is maintained for a short time to prevent the material creeping back during the setting off period. This prevents shrinkage and hollows, therefore giving a better-quality product.

  • The moulding is left to cool before it is ejected from the mould. The moulding takes on the shape of the mould cavity.

Advantages of Plastic Injection Moulding

Fast production.

Injection moulding an extremely efficient process that can produce a high number of parts per hour. Speed is dependent on the complexity and size of the mould and can range anywhere between 15-120 seconds per cycle time.

Low labour costs / Low Cost per Part

Plastic Injection Moulding is very much an automated process, reliant on machines which are usually managed by a sole operator. Automation essentially reduces the manufacturing cost.

Low Waste

Injection moulding produces very little post-production scrap relative to traditional manufacturing processes. Any waste plastic typically comes from the sprue and runners. Any unused or waste plastic, can be reground and recycled for future use.

High-output production.

Thousands of component parts can be manufactured before the tooling needs to be serviced. A well-made mould tool has a very long mould tool life, if treated well by the machine setters.

Design flexibility / Precision.

Injection moulding is perfect for manufacturing highly complex plastic shapes, with intricate and precisely formed details, patterns and geometries. This provides product designers with considerable scope to develop injection moulded parts for a wide range of applications, whilst integrating unique styles and properties.

Choice of Material, Finish & Properties.

An extensive range of plastics are available for injection moulding, including polypropylene, polycarbonate, ABS, and nylon. These offer product designers a broad choice of colours, surface finishes and chemical & mechanical properties.

Good Colour control.

Plastic parts can be manufactured in any required colours with the use of masterbatches or compounding.

Ability to include inserts.

Metal or plastic inserts can be insert moulded.

Product consistency.

Injection Moulding is a repeatable process delivering product consistency, which is an enormous advantage when aiming to produce high tolerances and part reliability in high volumes.

Disadvantages of Plastic Injection Moulding

High Initial Cost

The initial cost of getting a plastic injection moulding tool made is high.

Long Initial lead time

From product conception to final part can take months of design, testing and tool manufacturing.

Careful Design Is Required.

Plastic mouldings need careful design to avoid tooling issues such as undercuts or not enough draft. The material and temperature need to be considered in wall design, otherwise the mould may not fill fully. The placement of ejectors and cooling lines will need to be considered to ensure the product is aesthetically pleasing.

Large Part Size Limitations

Large machines are needed to make plastic injection mouldings. Very large parts need an enormous mould tool and become very expensive to manufacture.

We hope you have found this introductory guide to plastic injection moulding useful.

If you are considering plastic injection moulding for your component part, then why not schedule a free consultation with us.

To arrange your consultation either call us on 01453 833 388 or email us at

PNP's plastics injection moulding facility
PNP's Shop Floor


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