metal inserts

Demystifying plastic injection moulding – Terminology Part 1

Since the first plastic injection moulding machine was invented 150 years ago, plastic injection moulders have continued to innovate to transform great ideas into functional components. This has improved the design and manufacture of plastic parts, making the process faster and more cost effective while improving the quality and increasing the technical complexity of components.

Much of the innovation comes at the design for injection moulding stage when the OGM Team will help you refine your idea and perfect a design ideally suited to the process.

To understand more you may want to learn some of the terminology that surrounds the injection moulding process and the features of your finished part. Here we aim to demystify three common plastic injection moulding terms.

1. Inserts
Metal inserts may be required for connecting your finished part to other components via screws, bolts or other fasteners. This is often the case if your part will eventually be attached to another component that requires a more robust bond than plastic will allow, for example if the part has a long life where servicing is required or re-assembly.

There are a number of different methods for including inserts in your part:

  • In mould – this is when the insert is placed into the injection mould tool before the molten plastic flows into the mould cavity. On cooling, the insert is securely bonded in place
  • Thermal insertion – the insert is heated and pressed into the plastic part which melts and then cools around the insert
  • Ultrasonic insertion – ultrasonic vibration creates heat that melts the plastic and allows the insert to be fixed in position
  • Press inserts – the inserts are pressed into the part while it is still warm after moulding

Inserts can also be added during assembly. There are many different occasions when this may be the case, particularly if the inserts are designed to be removed from the component as and when required.

Each of these insertion methods offers different benefits such as minimising cycle times, reducing stress on your plastic part, cost efficiency and simplicity or reducing secondary operations. If you need inserts the OGM Team will be able to advise which method will best suit your project.


2. Shut offs
This is a feature of the mould tool that creates a required slot or hole in your finished part. Two pieces of steel on the tool will touch during the moulding process to prevent molten plastic from flowing into the area where you want a hole.

Shut offs can become quite complex where there are angled faces in your mould design. If you need a precision, quality finish, it is important to use an expert moulder who can produce high quality mould tools. This will prevent what is known as flash – a small flap of thin plastic on the edge of the hole – that forms when a gap in the steel has allowed the molten plastic to leak through.

3. Overmoulding
Overmoulding is required when you need to mould two parts together that are designed not to separate. A finished part, which could be made from metal or plastic, is placed in the mould tool and molten resin is injected around it to form a tight bond.

Examples where this process is used include covers for electronics housing devices, such as thermostats and meters, where a window is inserted into the tool and the cover is moulded around it.

Overmoulding can also be used when you want your finished product to have different colours, textures or other qualities. A common example most people will have experienced is a plastic toothbrush that has been overmoulded with a different coloured rubber resin to provide grip and a soft finish. More technical examples include medical injection devices.

OGM will be able to advise you on the best techniques for your product. Overmoulding can reduce the need for assembly which helps cut production costs but you will also need to consider material choice to ensure you get a good bond and a quality finish.

The team at OGM has years of experience supporting customers with their product designs, prototyping and production of plastic injection moulded parts. We have worked on some highly technical and specialist components for general industry such as gas smart metering, fire detection systems and consumer electronics and in the medical industry such as components used in syringes, breathing apparatus, dental equipment and other medical devices.

Get in touch to find out how we can help you with your next project

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