Overmould Design
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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: Overmould Design
Transcript
Hi and welcome to another Insight video.
This week I’m going to be chatting about overmoulding design and how to avoid some common mistakes.
But first a quick recap. Overmoulding involves adding a layer of material over another component using injection moulding. You might do this to add a grip, a handle, a cover or a sleeve.
The process involves placing the component in the mould, so that the overmould material fills the area around the inserted part. This is known as pick and place overmoulding.
There are three things that you need to consider. The bonding, the material you choose and some general design rules.
I’ll touch very briefly on the first two and then concentrate on the third, design rules, in a bit more detail.
Okay bonding. You obviously want the overmould to stay in place. There are a few ways to do this. The first is to choose a material that will chemically bond to the substrate. Another method is to design a mechanical interlock, perhaps by placing an undercut in the original part.
Moving swiftly onto material selection; this really depends on what you want the overmould to do. If it’s to add a non-slip grip then TPU over ABS is a great choice because it is grippy and will bond well.
One fairly obvious point here – make sure that the melting temperature of the overmoulding material is less than that of the substrate that you are covering – it could get messy otherwise!
Another quick point on material selection. Ideally you want something that flows easily into the mould to fill any thin-walled sections. So, pick something with a low viscosity.
Okay let’s get onto some design rules.
The first point is if you know you want an overmould, then you should consider this when you are designing the original part.
My second point is that this is still an injection moulding process, so many of the design rules are the same as for any part using this technology, but there are a few extra things to consider.
First of all, the wall thickness of the overmould should be as uniform as possible. And if it does vary, then make sure that any transitions between different thicknesses are gradual because this will reduce any flow problems such as backfills and gas traps.
How thick should these walls be? Well, we find that a thickness of between 1.5 millimetres to 3 millimetres will ensure good bonding.
If you do need to have thicker sections then design them to be cored out to minimise shrinkage and to reduce weight.
Next if you have a long draw section then try and include a 3-to-5-degree draft to help with the part ejection. Sharp edges are tough to get out.
And talking about sharp edges, a sharp corner will cause localised stress, so use a minimum radius of half a millimetre – again it will help your supplier eject the part cleanly.
Now you can design deep undercuts with the right TPE compounds, but just like before you need to minimise sharp corners. If you use an advancing core when the mould opens, then the elastomer can deflect as it is ejected.
We also need to consider some of the potential issues with the materials that we might use for overmoulding.
So, for example most styrenic TPEs have a fairly high mould shrinkage which can cause it to shrink more than the substrate that you are overmoulding it onto. This can in turn cause warping or cupping of the substrate part. Remember what I said about working collaboratively between the substrate design and the overmoulding? Well, here is an example where it can make a real difference.
There are a few things you can do to avoid this. First of all, try and make the overmould thinner than the substrate. Second you can use a higher modulus substrate, or in other words one that will resist deformation and third you can add ribs to the substrate part to strengthen it.
You can also use a softer TPE and think about the gate location to minimise the flow length to thickness ratio.
Another property that you should be aware of with typical overmoulding compounds is that they are shear responsive. In other words, their viscosity is reduced when processed at high shear rates – which is good because it allows an easy flow into thin-walled sections.
But again, it’s something you need to consider in your mould design. So, think about using small TPE gates so that you get the best fill and minimise any cosmetic gate vestige and try and locate these gates at the thickest wall section.
You also need to think about ejection of the component to minimise marks on the soft elastomer surface. Other considerations to talk to your supplier about are allowing enough cooling to the cavity to minimise cycle time and restricting flow ratios to a maximum flow length to thickness of 150:1.
Okay let me conclude by saying that overmoulding is a great way to improve your product or enhance its appearance. It will save on assembly costs and generally give you a better quality and more durable product.
If you are considering overmoulding then as always talk to your supplier and get their help.
Anyway, that’s enough from me for now so I’ll wish you a good weekend and see you again next Friday for another Insight video.
With special thanks to Natalie Constable.
