Last updated: 2026-09-05 · 5 min read

Injection Molding Boss and Rib Design — What Are the Thickness, Height and Base Rules to Avoid Sink Marks and Short Shots?

Boss OD: 2–2.5× nominal wall, ID equal to wall, height ≤2.5× OD. Ribs: 40–60% of wall thickness, max height 3× rib thickness, draft ≥0.5° per side. These ratios keep the cross-section uniform so thick areas do not shrink more than thin areas — violate them and defects appear at T1: sink marks, short shots or cracks at the boss base.

Key Facts

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  • Sink marks form when a thick section cools and shrinks more than the surrounding wall. A boss with OD 50% above the wall creates a visible depression on the cosmetic side — the outer skin solidifies first, the molten core shrinks and pulls the surface inward.
  • For a 2.5mm wall: boss OD 5–6.25mm, ID 2.5mm, height ≤15mm. Rib thickness 1.0–1.5mm, height ≤3× thickness (max 4.5mm), draft ≥0.5° per side. These ratios hold for PP, ABS, PC, PA and most engineering resins.
  • Screw bosses: OD = 2× screw diameter + 1mm (M3 → 7mm OD), pilot hole 70–80% of screw major diameter (M3 → 2.1–2.4mm), minimum depth 1.5× screw diameter (M3 → 4.5mm). Add a 0.5mm gusset at the base to prevent hoop-stress cracking.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    What Thickness Ratios Prevent Sink Marks on Bosses and Ribs?

    Feature Rule Why
    Boss OD 2–2.5× nominal wall Above 2.5× the core shrinks visibly on the cosmetic side
    Boss ID Equal to nominal wall Smaller ID creates a thick ring that sinks
    Boss height ≤ 2.5× OD Taller bosses resist filling; melt freezes before reaching the top
    Rib thickness 40–60% of adjacent wall Thicker ribs create a hot spot that sinks opposite
    Rib height ≤ 3× rib thickness Taller ribs are hard to fill and hard to eject without stripping
    Rib draft ≥ 0.5° per side Prevents ejection damage; deep ribs without draft gall and drag

    If the wall is 2.5mm, a boss with OD 7mm and ID 2.5mm holds a flat cosmetic surface. A boss with OD 10mm and the same ID will sink — the 3.75mm effective thickness at the base is 50% thicker than the wall, and that differential is enough to leave a visible mark on the opposite side.

    How Do You Design a Screw Boss That Will Not Crack Under Hoop Stress?

    1. Set the OD to 2× screw diameter + 1mm. For M3: 2 × 3 + 1 = 7mm OD.
      This gives enough material around the pilot hole to resist hoop stress without creating a thick section that sinks.
    2. Set the pilot hole to 70–80% of screw major diameter. For M3 (3.0mm major): 2.1–2.4mm pilot. Smaller pilot → excessive insertion torque → cracks the boss.
      Larger pilot → reduced thread engagement → strips under load.
    3. Add a 0.5mm gusset at the boss base. A fillet where the boss meets the wall distributes hoop stress over a larger area and prevents cracking at the sharp corner. Minimum wall thickness (OD minus ID, divided by 2): 1.5mm for ABS, 1.0mm for PC or glass-filled nylon.
    4. Set boss depth to ≥1.5× screw diameter. For M3: at least 4.5mm deep.
      Ensures enough thread engagement for the screw to full clamp load before threads strip.

    Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost.
    The related guide covers adjacent topics.

    Boss OD: 2–2.5× wall. Boss ID: equal to wall. Height: ≤2.5× OD. Rib: 40–60% wall, height ≤3× thickness, draft ≥0.5°. Screw boss: OD = 2× screw + 1mm, pilot 70–80% major diameter, depth ≥1.5× screw. Every rule exists to keep the cross-section uniform — violate it and the defect appears at T1.

  • 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 I make a rib thicker than 60% of the wall if I add more draft?

    Adding draft helps with ejection, not with sink. The sink mark comes from the thickness differential between the rib base and the wall — more draft does not change that differential.

    If you need a thicker rib for structural reasons, core the base to bring the effective thickness back in line with the wall, or accept the sink and cover it with texture.

    What is the minimum boss wall thickness for a self-tapping screw in ABS?

    For an M3 self-tapping screw in ABS, the minimum boss wall (OD minus ID, divided by 2) should be at least 1.5mm. Below that, hoop stress from screw insertion exceeds the tensile strength of ABS and the boss cracks.

    For tougher resins like PC or glass-filled nylon, the minimum wall can be reduced to 1.0mm because the material has higher tensile strength and better fatigue resistance.

    Should ribs connect to the wall with a fillet or a sharp corner?

    Always use a fillet. A sharp inside corner creates a stress concentration that can crack under impact or thermal cycling. A fillet radius of 0.25–0.5× the rib thickness distributes stress and improves mold filling by giving the melt a smoother path around the corner.

    The fillet also makes the mold easier to polish, which reduces the risk of drag marks during ejection — especially important for deep ribs above 3mm.

    Key Takeaways

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  • Boss OD: 2–2.5× wall; ID: equal to wall; height: ≤2.5× OD.
  • Rib: 40–60% wall, height ≤3× thickness, draft ≥0.5° per side.
  • Screw boss: OD = 2× screw + 1mm, pilot 70–80%, depth ≥1.5× screw, 0.5mm base gusset.
  • Post-T1 fixes: deepen rib for short shots, thin base for sink, add draft for ejection damage.

    • Boss and rib design is one of the most common sources of T1 defects — not because the rules are complicated, but because they are often ignored at the CAD stage.
    The ratios are simple, the consequences are visible at T1, and the fix before the mold is cut costs nothing. Send the part drawing for a DFM review and the geometry will be checked against these rules before any steel is ordered.

    Sources: Beaumont — Runner and Gating Design for Injection Molding · Society of Plastics Engineers (SPE) — Part Design Guidelines · ASTM International — Injection Molding Dimensional Standards · ENGEL — Injection Molding Design Handbook · Brydson, Plastics Materials (8th Ed.)

    — Arthur, Tooling Engineer at aCatMould