Core and Cavity Placement

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Insight: Core and Cavity Placement


Hello and welcome to this week’s Insight.

This time around we’re going to be delving into an important area of injection moulding, which is working out the best core and cavity placement for your parts.

Now, one of the goals of rapid injection moulding is to produce your parts cleanly and quickly. It’s easy to just throw things together and plan on iterating as you go along, but spending a little bit of time on a proper design helps ensure that you’re going to be getting good parts right from your very first run.

Determining how the part will be placed in the mould is a small but really very critical part of this. Our overriding consideration in this is that the part must stay in the mould half that contains the ejector system.

Let me explain. In a typical injection moulding machine, one half of the mould – which we’ll call the A-side – is attached to the fixed side of the press, and the other half of the mould – the B-side – is attached to the moving clamp side of the press.

The B-side contains the ejection actuator, which controls the ejector pins. The clamp forces A and B together, molten plastic is injected into the mould and allowed to cool, the clamp pulls the B-side of the mould away, the ejection pins are actuated, and if all goes well the part releases from the mould. Simple, right?

Well, it depends.

Let's use a mould for a plastic drinking cup as an example. To ensure that the part stays in the mould half with the ejector system, we’d like to design the mould so that the outside of the cup is formed in the cavity and the inside would be formed by the core. As the plastic cools, the part begins to shrink away from the A-side of the mould and shrink onto the core in the B-side.

When the mould opens, the cup should release and stay on the B-side. There, it can be pushed off from the core by the ejector system.

However, if the mould design were reversed – if we didn’t plan it right – the outside of the cup would shrink away from the cavity in the B-side. Instead of staying on the side with the ejector pins to easily release it, it would stick to the A-side, where there are no ejector pins.

And if this happens… we have a problem.

Of course, in the example of the cup, it isn’t hard to think through the process and work out where you want things to end up. However, not every design is quite that simple. On some parts, it is difficult to predict in advance which side of the mould the part will stick to, but well thought-out part design ensures that the part will naturally stick to the correct side of the mould.

Let's consider a rectangular enclosure, like a little box, with four through holes, for example.

The outside of the enclosure will be a cavity in the A-side of the mould. The inside will be a core on the B-side, and that’s where we want it to end up. Okay?

Design for the holes, however, could be handled in two different ways. They could be drafted toward the A-side, requiring cores in the A-side of the mould, but this might cause the part to stick to that side, which is something we want to avoid. Rather than that, we should probably draft the cores to the other direction, which would help with ensuring that the part sticks to the B-side of the mould.

On top of this, we have to think about any tabs or strips sticking from the part or spanning an internal hole. These should also be drafted to the B-side, to prevent sticking in the A-side and bending or tearing off when the mould opens.

Finally, we need to take into account any texturing on the surfaces. If we have a rough pattern or texture on the outside of a part without adequate draft we need to think carefully about its location as it could cause the part to stick in the A-Side.

If you’re concerned about these factors when you’re designing a part, your manufacturer should have both the software tools and the experience on hand to ensure you make the correct choice.

That’s it for this week. I look forward to seeing you again next Friday.


With special thanks to Natalie Constable

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