Recognizing a sink mark
A sink mark is a shallow depression on the visible surface of a molded part. It commonly appears above a thick rib, mounting boss or intersection where the internal material stays hot longer than the outer skin. The surface may look acceptable immediately after ejection and become more visible as the part cools.
1. Why thick sections sink
The outer skin freezes against the mold while the core continues to contract. If the cavity cannot receive enough additional melt during packing, the shrinking core pulls the skin inward. A gate that freezes early, low holding pressure or a short holding time can make the effect worse.
2. Troubleshooting steps
- Compare sink location with the position of ribs, bosses and wall intersections.
- Check whether part weight increases when holding pressure or time is raised.
- Confirm that the gate remains open long enough for packing to reach the affected area.
- Inspect cooling near the thick feature and verify that channels are not blocked.
- Check the cushion and transfer point for stable shot-to-shot behavior.
3. The durable design solution
Process changes can reduce a sink mark, but the strongest fix is usually reducing the thickness of the attached feature. A rib is often designed at a fraction of the nominal wall thickness, with draft and a suitable radius. A hollow boss with supporting ribs generally cools more predictably than a solid boss. If appearance is critical, move the feature away from the show surface or use a texture that makes minor variation less visible.
Do not simply increase packing until the machine reaches its limit. Excessive pressure can create flash, stress or difficult ejection without solving a geometry problem.
How to use this guide
This page is designed to explain the relevant material, process, decision point or failure mode in practical terms. It is meant to support understanding, not replace a current supplier datasheet, process validation, regulatory requirement or engineering sign-off.
Typical material behavior, common process logic, defect patterns, material comparisons and practical decision framing.
Exact product specifications, grade-specific values, regulatory opinions, food-contact validation or factory-specific trials.
Before a production or commercial decision is made, check the exact grade, processing window, standards and local rules that apply to your application.
Acceptance criteria for visible surfaces
Define the viewing distance, lighting and allowable depth before production begins. A cosmetic limit that is not written down can lead to repeated process changes without agreement on the result. Confirm the decision on conditioned parts, not only freshly ejected samples.