November 28, 2026
A sink mark forms when the outer skin of a molded part cools and solidifies before the thicker interior section finishes shrinking, pulling the surface inward slightly as the core continues to contract. On a textured or matte surface, a shallow sink mark can be nearly invisible. On a high-gloss cosmetic jar, the same depth of sink mark reflects light differently than the surrounding surface and reads as an obvious defect, which is why gloss finishes have a much lower tolerance for wall thickness inconsistency than textured parts do.
The most common sink mark location on cosmetic jars is directly opposite a rib, boss, or wall thickness transition on the inside of the part, anywhere the effective wall thickness increases locally creates a slower-cooling zone that pulls the outer surface as it shrinks. Uniform wall thickness throughout the part, generally the single best sink mark prevention strategy, isn't always achievable given structural or functional requirements, which is where gate placement and cooling channel design become the actual mitigation tools.
Draft angle, the slight taper (typically a minimum of 1 to 2 degrees) built into vertical walls to allow a part to release from the mold without dragging or scuffing, doesn't directly prevent sink marks, but insufficient draft can cause a part to stick momentarily during ejection, which can distort a still-warm high-gloss surface enough to create a visible flaw that looks similar to a sink mark. On deep-draw jars with tall vertical walls, draft angle below the 1 to 2 degree minimum is one of the more common causes of surface defects that get misdiagnosed as a cooling problem when the real issue is ejection.
Uniform wall thickness where structurally possible, adequate draft angle on every vertical surface, and cooling channel layout designed specifically around the part's thick-zone geometry are the three levers that address sink marks together, addressing only one of the three often isn't enough on a high-gloss finish.
We run mold flow simulation specifically checking for sink risk on gloss-finish parts before cutting steel, not after a first-article sample comes out flawed. For the underlying principles, see Wall Thickness, Draft Angles, and Gate Placement.
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