Plastic Part Design for Injection Molding

Injection molding is the cheapest way to make a million parts — and the most punishing way to design a bad one. The rules come from how plastic flows, shrinks, and cools.

The non-negotiables

• Uniform wall thickness to avoid sink and warp
• Draft on all vertical faces for ejection
• Smooth transitions between thicknesses
• Avoid sharp internal corners

A part that ignores these will mold — once — and then warp, sink, or stick in the mold forever. Fixing it on the drawing is free; fixing the mold is not.