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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Hi, it’s Friday, which means it’s time for another Insight video.
If you’ve watched our Insight videos before you’ll know that they look at some of the many technologies and techniques around digital manufacturing.
Well, this time we’re going deep into a rather useful area of design, which is how to create complex undercuts with bump-offs for injection moulding.
Bump-offs are a clever technique we use when we want to create a part with some kind of undercut, but still want the simplicity of a straight-pull mould. Under normal circumstances these undercuts would catch on the mould when you were ejecting it, but with the right material and the right design we can make parts that work just fine and don’t need the use of any side actions.
They work by allowing the material to deform just enough to make it out of the mould, without suffering any damage or significant wear. Think about snapping a lens cap onto a camera – the plastic bends a little so that the rim can get into place, but the bending isn’t permanent or really a problem at all.
There are several different factors that go into making sure a part can be bumped-off without suffering 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 we will look at the shape of the undercut.
In order for the bump-off to work, the leading edge of an undercut must provide a nice, smooth ramp or radius – it can’t look like a hook or sharp edge, as that’ll mean they catch and get stuck rather than be ejected. If all the other factors are correct, with a bump-off the ramp-shaped lip can ride up over the edge of the groove in the mould that formed it.
It moves up and over, similar to a car riding over a speed bump.
Of course, speed bumps work because our cars’ suspensions have a little bit of give, and making sure that the resin you use has the right kind of properties is also critical to the success of a bump-off. Depending on the shape of the part, the resin will need to stretch and bend during ejection and then return neatly to its original shape and size.
A resin like TPE or unfilled polyethylene is flexible enough to bump-off. It can move around a little and then return to its original shape. However, something rigid like glass-filled nylon, for example, isn’t going to work so well. It just doesn’t have the give needed.
Finally, we need to consider the shape of the part and the moulding technique.
Let’s imagine we’re moulding a snap-on bottle cap – essentially a short cylinder with a lip.
Now, you could potentially just make the most basic mould you can think of, with straight sides and a small undercut, and just hope that you can drag the bump through the gap left for the wall. However, all you’re really doing here is relying on the material to be compressible enough to work, and there are only a handful of resins that are going to be reliable enough in that way.
A smarter solution is to replace the sharp little lip with a more gradual inward bend, with the walls mostly staying at the same thickness rather than having a big bump. 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, much better at than compressing down.
Core-cavity moulding can be used to produce some clever designs that allow for more defined lips, as once one half of the mould is removed the resin can simply stretch its way over a bump, rather than having to compress it through a small gap.
Bump-offs can be a confusing area of design, so if you do run into any questions make sure you speak to your manufacturer’s engineers about it.
That’s it for this week. I look forward to seeing you again next Friday.
With special thanks to Natalie Constable