September 2011 Design Tip



Round Like a Wheel, Not Like an Egg

When making round parts—gears, pistons, plugs, and anything else that needs to fit, seal, or spin—start by asking yourself just how round it needs to be. If the answer is “as round as possible,” you probably need a centre gate. The reason is that, in injection moulding, resin fans out into the mould from the gate. With a centre gate, that means an expanding flow front that stops at the outside edge of the mould where it meets the parting line once the mould is full. In Figure 1, because filling and cooling occur at roughly the same time around the radius, the part stays as round as possible.

Figure 1: ProtoFlow<sup>®</sup> image—gating a part on centre.

Figure 1: ProtoFlow® image—gating a part on centre.


If you’re designing a Frisbee®, which has featureless material at its centre point, gate location can be easy, but what if there is a hole or feature at the centre of your part that keeps you from placing a gate there? (See Figure 2.) If roundness is important, you may need to consider adding a dome (Figure 3) that can be machined off, or a plug in the hole (Figure 4) that can be drilled or machined out.

Figure 2: Cross section of the round part with a hole in the centre.

Figure 2: Cross section of the round part with a hole in the centre.

Figure 3: Adding a round dome to machine off after moulding.

Figure 3: Adding a round dome to machine off after moulding.

Figure 4: Adding a plug to the centre hold that will be drilled/milled out after moulding.

Figure 4: Adding a plug to the centre hold that will be drilled/milled out after moulding.


The alternative to adding material at a challenging centre point is to place your gate off centre, resulting in an off-centre fill pattern and inviting potential problems. If your part has a centre core and you use an off-centre gate, a knit line will form where the material flows to meet around the core (see Figure 5). That knit line will almost certainly cool and shrink differently from the rest of the material. Even without a core, an off-centre gate means unequal flow length as resin moves toward the edges of your part. Uneven cooling can result in a slight egg shape in the resulting part—definitely not what you want in a spinning or precisely fitted part.

Figure 5: ProtoFlow image—gating part off centre.

Figure 5: ProtoFlow image—gating part off centre.


A centre gate on a round part is particularly important with glass-reinforced resins. The radial fill allows the fibre filler to align in the outward direction of flow like the spokes on a wagon wheel. As the resin cools, this will offset the effect of transverse/perpendicular shrink, which can adversely affect flatness.


It is important to consider your ultimate goals and keep your options open when designing round parts. While adding material (like the domes or plugs mentioned earlier) to the part may require additional processes, the benefit of uniform radial fill will help make your part as strong, round, and balanced as possible and reduce the likelihood of problems. If you have questions about this tip, or other items regarding part design for injection moulding, contact our Customer Service Engineers at +44 (0) 1952 683047. We will be glad to discuss the function of your part and help with design considerations.


To learn more about ProtoFlow, visit our ProtoFlow Fill Analysis page.


Check out our video design tip on knit lines.