Design Tip

How to Choose Core and Cavity Placement for Moulded Parts

Determining how the part will be placed in the mould is critical in injection moulding
Mould core and cavity example
An A-side (cavity) and B-side (core) of a mould are represented with the ejector plate and pins placed on the B-side.

One of the goals of rapid injection moulding is to produce parts quickly. Proper design helps ensure that good parts will be produced on the first run. Determining how the part will be placed in the mould is critical. The overriding consideration is that the part must stay in the mould half that contains the ejector system.

Cavity and Core

In a typical injection moulding machine, one half of the mould (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 clamp (or B) side contains the ejection actuator, which controls the ejector pins. The clamp forces the A and B-sides 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 the part releases from the mould.

Let's use a mould for a plastic drinking glass as an example. To ensure that the part stays in the mould half with the ejector system, we would design the moud so that the outside of the glass is formed in the cavity of the mould (A-side) and the inside would be formed by the core of the mould (B-Side). As the plastic cools, the part would shrink away from the A-side of the mould and shrink onto the core in the B-side. As the mould opens, the glass will release from the A-Side, and stay in the B-side, where it can be pushed off from the core by the ejector system.

If the mould design were reversed, the outside of the glass would shrink away from the cavity in the B-side and onto the core in the A-Side. The glass would release from the B-side and stick to the A-side where there are no ejector pins. At this point, we have a serious problem.

At Proto Labs, our design staff uses software tools and extensive experience to make the correct A-side vs. B-side choice. On some parts, it is difficult to predict in advance which side of the mould the part will stick to. Well thought-out part design ensures that the part will naturally stick to the correct side of the mould.

Moulded part with four holes
A part with four through holes and a tab drafted to the B-side.
A Rectangular Example

Let's consider a rectangular enclosure with four through holes. The outside of the enclosure will be a cavity in the A-side of the mould and the inside will be a core on the B-side. 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 the A-side of the mould. A better approach would be to draft the cores to the B-side, ensuring that the part would stick to the B-side of the mould. Similarly, any tab or strip sticking from the part or spanning an internal hole should be drafted to the B-side to prevent sticking in the A-side and bending or tearing off when the mould opens. And, of course, the design should also avoid heavy texture on the outside of a part without adequate draft, as this could cause the part to stick in the A-Side.

As always, you can contact a Proto Labs applications engineer at

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