How to design plastic shutoffs
Shutoffs are areas with no plastic. Where metal from the two sides of the mold meet each other. If those two areas meet parallel to the parting line, that is easy, but if they are perpendicular to the parting line, they must be angled slightly to prevent inaccuracy in the alignment of the two mold halves from causing them to overlap each other and shear. We prevent that by angling the shutoff.
In the picture at right, you see a slide by area with a small gap. That gap comes from the angle. The angle should be chosen so that the gap will be at least 0.010 and the shutoff angle should be 3 degrees or more.
The more shutoffs a part has, and the more complicated the shutoffs, the more expensive the mold will be to make.