What Causes Sink Marks?

Sink marks are the small surface depressions that show up on molded parts, most often where the plastic is thick — behind a boss, a rib, or a heavy section. They happen because thick areas cool and shrink more than thin ones, pulling the surface inward. The encouraging part is that they're largely a design issue, which means they're largely preventable.

Why They Happen

As a molded part cools, thicker sections shrink more than thinner ones. That uneven shrinkage tugs the outer surface inward, leaving a visible dimple — a sink mark. Thick features like ribs and bosses meeting a wall are classic spots for them to appear.

What Tends to Cause Them

How Design Prevents Them

Keeping walls uniform, thinning and coring out thick features, and avoiding abrupt transitions all help the part cool evenly and shrink predictably. Most sink-mark problems are solved on the design side, before the mold is ever cut — which is exactly why designing for molding matters.

Common Mistakes We See

A Good Next Step

A short conversation can help you spot the parts of your design most likely to sink — and how to adjust them before the mold is made.

Please keep your question general — no confidential product details are needed for the introductory consultation. If confidential review is needed later, an NDA can be arranged before a longer engagement.