Overmolding

Overmolding is a great way to add value and functionality to your product, but it’s not as straightforward as it might seem. Whether you’re dealing in consumer goods, electrical components or medical supplies, it’s worth consulting expert opinion to avoid pitfalls and to find the right approach

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Rosti - John Kidd
John Kidd

Regional Technical Director Asia
E-mail: rosti@rosti.com

Rosti - Tony Austin
Tony Austin

European Technical Director
E-mail: sales@rosti.com

How overmoulding can deliver practical and attractive products

When done correctly overmoulding (also known as insert moulding) can be a reliable way of enhancing, protecting or decorating your product. It’s an injection moulding method that embeds components inside plastic or rubber without the need for adhesives or welding. Rosti have been using this technique to combine components for over 20 years, over which time we have finely tuned our ability to create functional, attractive objects, while reducing the amount of post-moulding assembly processes.

What is overmolding?

Moulds are essential in all forms of injection moulding. In this particular technique, overmolding takes different moulds and uses them all to manufacture one finished product. Each individual part is created using the injection moulding process – melting plastic granules or powders into liquid and injecting this into the mould using a high-pressure device – with the mould taking the form of the finished product. 

Each part is then placed into a further mould, and this is covered with synthetic substance. This may be done manually or automatically. The parts join together as one. Overmolding is typically chosen when the properties required for an item cannot be found in one single material. 

Overmolding is used in practically every industry, from consumer goods to packaging, medical devices to military products. You are likely to encounter numerous overmolded items every day. 

Overmolding is invaluable when you need different functionalities in one item. For example, you want a softer and pliable handle but an unbreakable drill bit on an electrical hand tool. Or you want to add a more tactile feel to an otherwise hard-edged product. The procedure is also commonly used for aesthetic purposes.

Overmolding can make your product more physically robust

Overmolding is a valued technology for producing products that stand the test of time. Rosti helps you manufacture items that feel better and last longer. 

For example, we can partially embed an item to make it easier to grip or to give it a more pleasant feel when your customers handle it. This could be worthwhile in anything from a car key to a toothbrush. You can do it for purely aesthetic reasons, to give your product an interesting look and feel, and you can also improve your products’ sustainability by incorporating post consumer recycled (PCR) resins.

It can transform a delicate item into something durable and solid, with good resistance to vibration and shock. It can form a barrier against dust and water, and if you use a flexible elastomer it can compress and expand to effect a seal. For these reasons, it is a technique we often use for medical and electronic devices – as casings for cables, junction boxes and circuit boards (PCBs).

Once an item is embedded in this way, it can’t be easily separated. This makes your product harder to reverse engineer, so overmolding can also be a tool to protect your intellectual property.

Overmolding has additional benefits. It eliminates the need for costly secondary assembly methods. Products manufactured using overmolding are better insulated, and the process is ideal for sensitive electronic items and cables.

Common overmolding processes

Plastic over plastic overmolding allows one plastic mould to be used with another, producing a seamless finish. This is ideal for producing two-toned or two-coloured items at a competitive manufacturing price. There is also no need for time-consuming assembly. 

Plastic over metal involves, as the name suggests, putting plastic over a metal layer. This is usually seen when plastic handles enhance a metallic tool.

Rubber over plastic overmolding uses an elastomer over the plastic to combine different materials that display markedly differing characteristics. For example, a rubber grip on a lawn mower handle. Rubber over metal also adds a distinct tactile element to a product. 

It is not always as simple as picking the most cost effective or desirable material and combining it with another. There are limitations involved in the compatibility of different materials. And you are also not constrained to just two – overmolding can use multiple different materials in one design. 

It depends on your needs and your budget. Rosti can help you make the final choice.

Is overmolding the same as injection moulding?

Overmolding shares similarities with injection moulding. But there is an additional step to overmolding, where the moulds are transferred to a separate mould and the extra plastic coating is added. Traditional injection moulding uses one single machine. 

Rosti – solving problems before they arise

Overmolding is a useful and fairly straightforward process, but there are pitfalls to be aware of. Without input from experts it’s very easy to choose the wrong materials and techniques for enhancing your product. You need to be especially aware of the heat and pressure tolerances of the component that you want to embed. This should guide your material selection, and determine whether overmolding is the right approach for your product.

With Rosti, you have a partner with extensive experience in overmolding who can help you reach optimal end-results, and guide your from concept to reality. We can spot potential issues early in the production process, and give you an honest assessment of what is feasible. Our expertise extends to all areas of moulding, so we can give you high-level view about the techniques and materials that are most appropriate for your goals and your budget.

We can support you throughout as much of the process as you require, all the way from the design stage through material selection, toolmaking, prototyping, testing, and production. Get in touch to find out how much we have to offer.

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