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Our Insight video series will help you master digital manufacturing.
Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.
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Insight: Overmoulding Materials
Hello and welcome to this week’s Insight.
Today, we’re going to be taking a look at a process known as overmoulding – and specifically the kinds of materials you might want to use with it.
Now, if you haven’t encountered it before, overmoulding is the process of adding an extra layer of resin to an existing moulded part. Essentially, you add it over the mould.
We do this for a couple of main reasons. Perhaps the most common is to add a nice, hand-friendly grip onto tools and other plastic devices we need to handle a lot. Think of the last handsaw you picked up, or the torch you take on camping trips – there’s a good chance that the grip on both of them was overmoulded onto the hard, slippery plastic substrate below.
Of course, that’s not the only application for the process. Sometimes overmoulding is used simply because it can look good – it’s an easy way to add a colour or texture that can stand out from the substrate. And sometimes it’s used when you want to use two different materials without having to deal with screws, glue and assembly.
Right, hopefully that’s enough background about the process. Now it’s time to get to what we’re here to talk about today – the materials you use.
This may seem like a pretty minor matter, but material selection for overmoulding can be a surprisingly complicated affair. The substrate and overmould resins need to be compatible in order to be effective, both chemically and aesthetically, and what choices you have access to don’t just depend on the application of the part you’re making, but also with the specific method you’re using to produce it. On top of all this, there are quite literally thousands of possible combinations of substrate and overmoulded material out there.
If that sounds overwhelming, don’t worry too much. It’s possible to rule out a wide range of options based on what application you’re looking for.
For example, if the plan is the use the overmoulding as cushioning – a squishy coating to make sure the hard plastic part doesn’t crack if it’s dropped – you’re going to want something nice and soft. The important characteristic you’re after here is the flexural modulus, which is basically just a measurement of its resistance to bending. The lower this number is, the softer and more absorbent it feels. Elastomers such as Versaflex can be ideal here.
Just bear in mind that regardless of the material you’re using, the overmould is going to have to be decently thick for it to have a real impact. Anything below about 10 mil is always going to feel pretty hard, no matter how squishy the resin might be. Sticking some ribs on the substrate where the overmould is going to be applied can help to make things work a lot better and feel more secure.
Next up, grip. If the goal of overmoulding is to act as a secure grip – like on that handsaw we were talking about earlier – you want a material with a decent coefficient of friction. Thermoplastic elastomers – also known as TPEs – are usually pretty good at this kind of thing. They generally have a nice, high coefficient of friction and can be very easy to hold onto in tough situations, even if things get sweaty or otherwise wet.
Also, though it’s not always top of the priority list, aesthetics can also be important. If you’re using the overmoulding on a product that’s going out to customers you probably want it to look good, so it’s worth checking what colour and textures your ideal materials can work with. Sticking things in the same shade as the company logo is very popular.
When it comes to combining overmould resins with substrates, there are also a few things you need to keep an eye on. For example, you really want to make sure that melting temperature of the overmoulding material is less than that of the substrate. It doesn’t matter how grippy the handle might be if the process of applying it turns the rest of the tool into a molten lump of plastic.
Finally, you need to think about whether you need a chemical or a mechanical bond between the substrate and the overmould. Some materials will actually chemically bond during the overmoulding process, while others will be held in place by simple mechanical forces. If, for example, you want the grip to be replaceable, you should probably opt for a pairing that won’t physically stick them together.
That’s it for this week. I look forward to seeing you again next Friday.
With special thanks to Natalie Constable.