Last updated: 2026-09-02 · 5 min read

What Causes Sink Marks in Injection Molded Parts — and How Do You Fix Them?

Sink marks are shallow depressions on the part surface where the material was thicker than the surrounding wall — and they form because the core of a thick section shrinks more than the surface, pulling the skin inward after the gate freezes. The fix is not more packing pressure; it is reducing the local thickness, improving cooling at the thick zone, or redesigning the geometry so the thick section feeds from a rib or boss instead of a flat wall.

Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

Key Facts

  • Sink marks appear opposite ribs, bosses and thick sections — the nominal wall should be uniform; local thickness should not exceed 1.5× the nominal wall.
  • Increasing packing pressure hides sink temporarily but creates other problems: flash, high ejection force, internal stress and warpage — the root cause is geometry, not process.
  • Semi-crystalline resins (PP, PA, POM) shrink more than amorphous resins (PC, ABS, PS) and are more prone to sink — the resin’s volumetric shrinkage rate determines the severity.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.

    Where Sink Marks Appear and Why

    Feature Sink Location Root Cause
    Rib Opposite side of the wall, visible as a depression Rib base thicker than 60% of nominal wall
    Boss Around the boss base on the cosmetic surface Boss diameter too large relative to wall; inadequate core cooling
    Wall transition At the thick-to-thin transition Abrupt thickness change creates a hot spot that shrinks after freeze

    How to Eliminate Sink Marks

  • Coring: remove material from thick sections to create uniform wall thickness — a cored-out boss or rib base shrinks uniformly with the surrounding wall.
  • Rib thickness: keep ribs at 50–60% of the nominal wall; ribs thicker than 60% create a local hot spot that sinks after the surrounding wall freezes.
  • Cooling improvement: add a baffle or bubbler behind the thick section to extract heat faster — the thick zone solidifies closer to the same time as the thin zone.
  • Process adjustment (temporary): increase packing pressure and hold time to push more material into the thick zone — but this is a band-aid, not a fix; it creates flash, stress and warpage.
  • Material selection: amorphous resins (PC, ABS) shrink less than semi-crystalline (PP, PA, POM) — if the application allows, switching to a lower-shrinkage resin reduces sink severity.

    • What This Means for Your Quotation

    The DFM review identifies sink risk zones before steel is cut — mapping thick sections, rib bases and boss locations against the nominal wall. The quotation should specify the wall uniformity requirement, the rib-to-wall ratio and the cooling strategy for thick zones. When the geometry is corrected at the design stage, the mold produces sink-free parts without excessive packing; when the geometry is left unchanged, the process window is narrow and the sink reappears with any material lot change.

    Frequently Asked Questions

    Can texture hide sink marks?

    Texture reduces the visual impact of shallow sink (under 0.1mm depth) by breaking up light reflection — but it does not fix the underlying shrinkage. Deep sink (over 0.1mm) shows through even heavy texture. The fix is geometric: reduce the local thickness, not change the surface finish.

    Does foaming eliminate sink?

    Structural foam molding uses a blowing agent to create a gas core in thick sections — the gas pressure packs out the part from inside, eliminating sink. But the surface has a swirl pattern from the gas bubbles, so foam molding is not suitable for cosmetic surfaces. It is used for thick, non-cosmetic parts like pallets and equipment housings.

    How do you detect sink before production?

    Mold flow analysis predicts sink depth based on wall thickness, packing pressure and material shrinkage. The analysis shows sink risk as a color map — red zones indicate areas where the predicted sink exceeds the acceptable threshold. Running the analysis before steel is cut lets the designer fix the geometry, not fight the process.

    Key Takeaways

    1. Sink marks come from local thickness that exceeds 1.5× the nominal wall — the fix is geometric (coring, rib thickness, cooling), not process-based (more packing).
    2. Semi-crystalline resins shrink more than amorphous resins and are more prone to sink — material selection affects severity.
    3. The DFM review should map sink risk zones and specify rib-to-wall ratios before steel is cut — fixing geometry at design stage costs nothing; fixing it after the mold is built costs weld and re-machine.

    Send aCatMould the part drawing, and the DFM review identifies sink risk zones — with wall uniformity analysis, rib-to-wall ratios and cooling strategy for thick sections documented, so the mold produces sink-free parts without relying on excessive packing pressure.

    Sources
    Plastics Technology — sink marks and wall thickness design
    SPE — injection molding defect analysis
    MoldMaking Technology — DFM for sink prevention

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet