When making round parts—gears, pistons, plugs, and anything else that needs to fit, seal, or spin—start by asking just how round does it need to be? If the answer is “as round as possible,” you probably need a center gate. In injection molding, resin fans out into the mold from the gate. With a center gate, that means an expanding flow front that stops at the outside edge of the mold where it meets the parting line once the mold is full. Because filling and cooling occur at roughly the same time around the radius, the part stays as round as possible.
If you’re designing a Frisbee®, which has featureless material at its center point, gate location can be easy, but what if there is a hole or feature at the center of your part that keeps you from placing a gate there?
If roundness is important, you may need to consider adding a dome that can be machined off, or a plug in the hole that can be drilled or machined out.
The alternative to adding material at a challenging center point is to place your gate off center, resulting in an off-center fill pattern and inviting potential problems. If your part has a center core and you use an off-center gate, a knit line will form where the material flows to meet around the core. That knit line will almost certainly cool and shrink differently from the rest of the material. Even without a core, an off-center 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.
A center gate on a round part is particularly important with glass-reinforced resins. The radial fill allows the fiber 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 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.