Last updated: 2026-09-05 · 5 min read

Sink Marks in Injection Molding — Why Do They Appear on Ribs, Bosses and Thick Sections, and How Are They Eliminated?

Sink marks are shallow depressions caused by localized shrinkage where thick sections cool unevenly — typically at ribs, bosses or wall transitions. The outer skin solidifies first; the thicker core behind continues to shrink, pulling the skin inward. Fix starts at design (rib ≤ 60% of wall, boss cored out, 3:1 transition slope, gate the thick section) and continues through process optimization (packing pressure, cooling time, mold temperature).

Key Facts

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  • Sink marks appear on an estimated 40–60% of parts with ribs or bosses if the design is not optimized. The deeper the thick section relative to its neighbors, the more pronounced the sink.
  • Four design rules: (1) rib thickness ≤60% of nominal wall, (2) boss OD ≤2× wall (cored out), (3) gradual wall transition (3:1 slope), (4) gate the thick section, not the thin section.
  • Semi-crystalline materials (PP, PA, POM) shrink 1.5–3.0% and are 2–3× more prone to sink than amorphous materials (PC, ABS, PS) which shrink 0.4–0.8%. For semi-crystalline, tighten the rib rule to 50%.

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    What Are the Four Design Rules That Prevent Sink Marks?

    Design Feature Rule Why If Violated
    Rib thickness ≤ 60% of nominal wall A rib thicker than 60% creates a local hot spot that shrinks more than the surrounding surface Sink appears on the opposite cosmetic surface above the rib
    Boss diameter OD ≤ 2× wall; core-out center A solid boss creates a thick mass that shrinks unevenly; coring reduces local thickness Sink around boss base; potential void inside
    Wall transition Gradual (3:1 slope) between thick and thin Abrupt transition creates stress concentration and differential cooling Sink at transition line; potential warpage
    Gating location Gate the thick section, not the thin Gate must pack the thick section last — if in thin section, thick cannot be packed adequately Sink in thick section far from gate; voids inside

    The 60% rib rule is the single most important constraint. A rib at 50% of wall produces no visible sink on most materials; at 70%, visible sink appears on semi-crystalline materials (PP, PA, POM) and slight sink on amorphous (PC, ABS, PS); at 100%, pronounced sink on all materials.

    What Process Adjustments Can Reduce Sink Marks?

    1. Increase packing pressure (10–20% increments). Higher packing forces more material into the cavity during hold phase, compensating for shrinkage in thick sections. Limit: flash — if packing is too high, material is forced out at the parting line.
      Maximum packing is typically 80% of injection pressure.
    2. Increase packing time (1–3 second increments). Longer packing keeps pressure applied during more of the cooling phase, allowing more material to flow into the shrinking thick section. Limit: gate freeze-off — once the gate solidifies, additional packing has no effect.
      Optimal packing time is when part weight no longer increases.
    3. Reduce mold temperature on cosmetic surface. A cooler mold solidifies the skin faster, creating a stiffer shell that resists shrinkage pull from the core. Limit: flow — if mold is too cool, material does not fill completely, creating short shots or weld lines.
      For semi-crystalline materials, cooler mold also reduces crystallinity and shrinkage at the surface.
    4. Add cooling channels behind thick sections. Concentrated cooling behind a boss or rib solidifies the core faster, reducing shrinkage time. Limit: thermal shock — if cooling is too aggressive, differential cooling creates internal stresses and warpage. Channel should be 10–15 mm from mold surface, coolant 20–30°C below mold set point.

    Process adjustments can reduce sink but cannot fix a bad design.
    Fix the design first, then optimize the process.

    Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.

    Sink marks appear on 40–60% of parts with ribs or bosses if design is not optimized. The 60% rib thickness rule is the single most important constraint. Semi-crystalline materials (PP, PA, POM) are 2–3× more prone to sink than amorphous (PC, ABS) because of higher shrinkage from crystallization.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Can sink marks be eliminated completely?

    Sink marks can be eliminated completely if the design follows the 60% rib rule, the boss is cored out, and the gating is in the thick section. If the design violates these rules, process adjustments can reduce the sink to below the visible threshold (typically 0.05 mm depth on a Class A surface) but cannot eliminate it entirely.

    The most reliable approach is to fix the design first, then optimize the process. Trying to fix a bad design with process adjustments alone is unreliable and material-dependent.

    Which materials are most prone to sink marks?

    Semi-crystalline materials (PP, PA, POM, PBT) are 2–3× more prone to sink marks than amorphous materials (PC, ABS, PS, PMMA). The reason is shrinkage: semi-crystalline materials shrink 1.5–3.0% as the crystalline structure forms during cooling; amorphous materials shrink only 0.4–0.8% because there is no crystallization.

    Higher shrinkage means more core pull on the skin, and deeper sink marks. For semi-crystalline materials, the 60% rib rule should be tightened to 50% to prevent visible sink.

    How do you detect sink marks not visible to the naked eye?

    Sink marks below 0.05 mm depth are not visible under normal lighting but can be detected with a straight edge and feeler gauge, or with a surface profilometer measuring the depth profile. On Class A surfaces (automotive exterior, consumer electronics), the acceptance criterion is typically 0.03 mm maximum depth over a 50 mm diameter area.

    A sink above 0.05 mm creates a detectable edge when running a fingernail lightly across the surface. For critical programs, specify the sink depth limit on the drawing and verify with a profilometer at T1.

    Key Takeaways

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  • Sink marks: localized shrinkage at thick sections — ribs, bosses, wall transitions — core shrinks more than surrounding surface.
  • Four design rules: rib ≤60% wall, boss OD ≤2× wall (cored), 3:1 transition slope, gate the thick section.
  • Process adjustments (higher packing, longer packing, cooler cosmetic surface, concentrated cooling) reduce sink but cannot fix bad design.
  • Semi-crystalline (PP, PA, POM) 2–3× more prone than amorphous (PC, ABS).
    Class A surface limit: 0.03 mm depth.

    • If your part has ribs, bosses or thick sections showing sink marks at T1, send the drawing and sample photos for a DFM review. The review identifies which design features violate the sink prevention rules and recommends specific changes before T2.

    Sources: Society of Plastics Engineers (SPE) — Sink Marks Technical Papers · ASTM International — Injection Molding Defect Standards · Beaumont, Runner and Gating Design Handbook (2nd Ed.) · ENGEL — Sink Marks Prevention Guide · Arburg — Practical Guide to Sink Mark Elimination

    — Arthur, Tooling Engineer at aCatMould