Wall thickness, ribs and sink marks

Plastic shrinks as it cools. Where a section is thicker than its surroundings (a boss, a rib junction, a heavy wall) the core cools last and pulls the surface inward, leaving a visible dimple called a sink mark, or an internal void. Uneven cooling also warps parts.

Guidance: keep a uniform nominal wall suited to the material and part size, core out thick areas, and transition gradually where thickness must change. Ribs add stiffness without adding thickness; a widely cited rule is to make ribs roughly 50–60% of the adjoining wall thickness, with draft and a small root radius.

Where it bites: sink marks are most visible on glossy, dark cosmetic surfaces, which is often exactly where a consumer product's visible face is. Thick walls also lengthen cycle time and so raise part cost.