Complex Undercuts
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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: Complex Undercuts
Transcript
Good afternoon, and welcome to another one of our Insight videos.
Today, we’re going to look at two concepts that are closely intertwined - undercuts and bumpoffs.
And, no, a bumpoff isn’t a polite term for an assassination, but rather a technique you can use to introduce a small undercut into your part design without messing about with fancy moulds or side actions.
You see, under normal circumstances, undercuts are something we want to avoid at all costs when it comes to moulded parts, as they make it incredibly difficult to extract the finished product. Think of trying to pull a plastic cup with a sticking out handle from an aluminium mould – it just wouldn’t work, as the handle would catch and get stuck.
However, sometimes it can be very useful to have a little undercut, and a bumpoff is what we call it when the undercut is small enough to bend around the mould and pop out anyway. Think of the way that a snap on your coat or a camera’s lens cap works – they slightly deform around the lip of their matching part as they click into place, but this doesn’t cause any damage or permanent warping.
The choice of material and design of the snap’s components is what allows this deformation to take place without damage or significant wear to the mating parts, and this is exactly what happens during the ejection of a part with a bumpoff feature.
Now, there are several different factors that go into making sure that a bumpoff works effectively and that it doesn’t cause any damage. These include the shape of the undercut itself, the resin used to form the part, the geometry of the area surrounding the undercut, and the design of the mould.
First of all, when you’re designing a bumpoff you need to ensure that the leading edge of an undercut provides a ramp-like structure that can gently guide the material up the bumpoff, rather than a sharp edge or – even worse – a hook. If the mould can catch on the undercut, it can interfere with the ejection process or simply break.
Think of it like a speedbump – show me one design with a gentle little hump and another that’s just a square-edged concrete slab laying across the road, and I think I know which one I’d rather drive over.
Next up, resin choice. This is critical to the success of a bumpoff, as the material itself will need to stretch and/or bend during the ejection process and then reliably return to its original shape and size. A resin like TPE or unfilled polyethylene is flexible enough to bump off. Glass-filled nylon, on the other hand, is very rigid and isn’t likely to behave itself during ejection – it just doesn’t have enough give.
Finally, we need to consider the shape of the part and the moulding technique.
Think of, for example, a snap-on cap for a drinks bottle – basically a shallow cylinder with a lip running around the inside.
The simplest way to handle this would be a straightforward design with straight sides and a small undercut to make the lip. However, if you do that, you’re going to be squeezing that lip down to the thickness of the wall beneath it, and that means you’re going to have to use a really very compressible material. If not, the lip is going to get stuck on its journey out of the mould. Not good.
One way to help it along, however, is to bend the walls a little to form a more gradual lip, rather than keeping them straight and having a dramatic undercut right at the end. Instead of being compressed, this means the wall of the part can be snaked out during ejection, requiring only that it bend and stretch. This is something that most resins are much better at than simply compressing.
Another, even fancier, technique, is to take advantage of core-cavity moulding. This, essentially, forms the mould in two parts – a core, and a cavity. The important part of this, at least as far as bumpoffs are concerned, is that once the mould is opened there’s nothing pressing against the outside of the part, allowing the part wall to stretch much more freely. This makes it much, much easier for the material to deform around the bumpoff.
In any case, that’s about it for today. I know there’s a lot to think about here, but I hope you can check back in for another insight video next Friday.
Have a great weekend.
With special thanks to Natalie Constable.
