Undercut Fundamentals

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Insight: Undercut Fundamentals


Hi and welcome to another insight video.  This week I’m going to be talking about a common problem that we come across when designing parts for injection moulding.  I’m going to cover what you need to think about for undercuts.

Now undercuts can be a bit of a nightmare to deal with, especially when it comes to ejecting parts from their moulds. This means that the most common piece of advice you’re going to hear around them is to just avoid them altogether. Which is great if you can manage it, but sometimes the nature of the part you’re making means that you need an undercut somewhere.

With this in mind, we’re going to run through a handful of different ways that you can reduce the annoyance that stems from using an undercut.

Number one: Make use of your parting lines. These lines are where the two halves of your mould intersect, and if you place them with a bit of thought it can eliminate the issue entirely.

Think of moulding a capital H. Normally you’d have two horrible undercuts if placed on its side, but if you place it upright there would be none.

Of course, not every part is going to be able to accommodate this kind of jiggery-pokery, in which case we need to get a little bit more creative.

For example, if the undercut isn’t too severe you can use a bumpoff. This is where you set your part up so that the plastic can gently bend its way around the lip of the undercut - if you think of something that snaps into place, like a camera lens cover or the lid of a Tupperware container, that was probably made using a bumpoff.

This is a great way to mould these kinds of parts, but it does have some pretty strict requirements. The bumpoff must be smooth and have a decent radius, and the material you’re working with needs to be on the flexible side of things. If it’s completely rigid, it’s not going to get round that corner.

If that doesn’t work with your part, you could always look at using a side-action. These are basically inserts that slide into the mould as it closes, and are slid out as it opens, allowing you to create undercuts in places that would normally make the part difficult – if not impossible to eject.

These are great tools that can help make designs actually workable, but they do add some extra complexity into the equation. Also, you’re fairly limited on how big the side-action can be and where it can be placed if you want to automate things.

Of course, if you’re willing to have the inserts loaded by hand, you get a lot more flexibility. You can make some really impressive shapes by inserting machined pieces of metal into the mould cavity, and because the inserts get ejected along with the rest of the part you don’t need to worry so much about the issues you get from conventional undercuts.

If you’ve been watching and paying attention to the pattern of these videos, you can probably guess that this is the point where I have to point out the limitations, though. The big one is the fact that you need someone to manually load and extract the inserts, and if you’ve ever visited a fancy gift-shop you’ll know that making something “hand-crafted” can drive up the costs. On top of this, the hands that are doing the work are usually covered with pretty thick gloves, what with the hot plastic. This means your inserts can’t be too small or too fiddly, or the operator isn’t going to be able to get them out.

The final technique you can use to make undercuts a little more manageable is a telescoping shutoff, which is also known as a sliding shutoff. This is essentially a long shutoff machined into the mould so that it extends – or telescopes – into the other side of the mould. These are great for making hooks and clips without having to faff around with inserts or the like.

You just have to be a bit careful about having some draft where the shutoff meets the other side of the mould. You really don’t want metal rubbing on metal. That’s very bad for the tools, and can cause flash.

Right, that’s a lot of information to take in, but that’s also all that we have time for! Have a great weekend and make sure to come back next week when I’ll have another Insight video waiting for you.



With special thanks to Natalie Constable.

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