Sink Marks | Definition, Causes & Inspection Criteria

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Sink Marks | Definition, Causes & Inspection Criteria

Last updated on Jul 22, 2026

(1) What are Sink Marks? (Visual Defect)

Sink marks are indentations formed on the surface due to resin shrinkage.

Principle Explanation

Crystalline resins experience significant volume reduction and high shrinkage rate during cooling and solidification. Thick wall sections have greater shrinkage, so sink marks tend to appear on thicker portions of molded parts.

Figure 1: Sink Marks on Visual Evaluation Specimen (Specimen approx. 20 cm) Gate

📄 Definition

Local surface indentation, dullness or loss of gloss.

🎯 Prone Locations

Thick wall zones, backside of ribs, around bosses, and corner areas.

👁 Inspection Focus

Compare surface gloss with acceptable reference; check for obvious indentation; analyze combined with the internal structure on the reverse side.

Inspection Checklist

☑ Observe surface indentations

☑ Pay attention to areas with wall thickness transition

☑ Distinguish sink marks from poor gloss

☑ Take photos for documentation

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Sink Marks | Definition, Causes & Inspection Criteria