Sink Mark (noun)

Meaning A shallow depression on the surface of a moulded part, sitting opposite a thick section, where the plastic pulled inward as it cooled and shrank.

Think of… A dent in the surface over a buried beam — a dip that forms opposite a thick section.

Etymology From sink — the surface visibly sinks below the level of the wall around it as the material contracts.

DFM Meaning A cosmetic defect, and a serious one on visible ("Class A") surfaces. It appears wherever one region cools much more slowly than its neighbours: a thick wall, or the thick junction where a rib or boss meets the wall. It is avoided by keeping walls uniform, keeping a rib to roughly 50–60% of the base wall thickness, and coring out bosses so no section stays thick.

Singular - A single sink mark appeared on the show face, opposite the boss.

Plural - Thick ribs left a row of sink marks along the visible panel.

Industry Example - Thin the rib and add a cooling channel so the thick base stops leaving a sink mark on the styled surface.

---

Part of the Glossary reference.