Last updated: 2026-09-02 · 5 min read

Injection Mold Rib Design Rules — How Thick, How Tall and How Many Ribs Does Your Part Need?

Ribs reinforce injection molded parts without increasing wall thickness — the three design rules are: rib thickness 50–60% of the nominal wall (to prevent sink marks), rib height no more than 3× the rib thickness (to prevent ejection and fill problems), and rib draft angle 0.5–1.5° per side (to prevent ejection damage). Following these rules produces a part that is stiffer, lighter and free of sink marks — violating them produces sink marks, warp, ejection marks and short shots.

Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. See also related guide.

Key Facts

  • Rib thickness rule: 50–60% of nominal wall thickness. A 2mm wall needs ribs at 1.0–1.2mm. Thicker ribs create a material concentration that causes sink marks on the opposite surface; thinner ribs are difficult to fill and may break during ejection.
  • Rib height rule: maximum 3× rib thickness. A 1mm rib can be up to 3mm tall. Taller ribs are difficult to fill (the melt freezes before reaching the top), difficult to eject (the rib sticks in the cavity) and prone to buckling under load.
  • Rib draft angle: 0.5° minimum for polished cavities, 1.0–1.5° for textured surfaces. Without draft, the rib sticks to the cavity during ejection, causing drag marks, part deformation or mold damage.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

    Rib Design Parameters

    Parameter Rule Violation Consequence
    Rib thickness 50–60% of nominal wall Sink marks (too thick); short shots or breakage (too thin)
    Rib height Max 3× rib thickness Fill problems, ejection sticking, buckling
    Rib draft angle 0.5–1.5° per side Ejection damage, drag marks, part deformation
    Rib base radius 0.25–0.5× wall thickness (fillet) Stress concentration at rib base (no fillet = cracking)
    Rib spacing Min 2× wall thickness between ribs Material concentration, sink marks between ribs

    Common Rib Design Mistakes

  • Rib thickness equal to wall thickness: creates a material concentration at the rib base that causes a visible sink mark on the opposite surface — the most common rib design error. The fix is reducing rib thickness to 50–60% of the wall.
  • No fillet at the rib base: a sharp corner at the rib-wall junction creates a stress concentration that causes cracking under load or impact. The fix is adding a fillet radius of 0.25–0.5× the wall thickness.
  • Ribs too close together: spacing less than 2× wall thickness creates a material concentration between the ribs, causing sink marks. The fix is increasing the spacing or reducing the number of ribs.

    • What This Means for Your Quotation

    The DFM review should check rib thickness, height, draft, fillet and spacing against these rules before the mold is cut — ribs that violate the rules cause sink marks, fill problems and ejection damage that require mold modifications to fix. The DFM report should flag any rib that violates the rules and recommend the correction, so the mold is built right the first time.

    Frequently Asked Questions

    Can ribs be used on cosmetic surfaces?

    Ribs on cosmetic surfaces cause sink marks on the opposite side — if the opposite side is also cosmetic, the rib creates a visible defect. The solution is to use a rib with a textured surface (which hides minor sink marks) or to relocate the rib to a non-cosmetic area. For structural reinforcement on cosmetic parts, consider coring out the back side instead of adding ribs.

    How do ribs affect cycle time?

    Ribs add material thickness at the base, which increases cooling time at the rib-wall junction. The effect is small for properly designed ribs (50–60% thickness) but significant for oversized ribs. A rib at 80% wall thickness adds 5–10% to cycle time; a rib at 100% wall thickness adds 15–25% and causes sink marks.

    What is the difference between a rib and a boss?

    A rib is a linear reinforcement (long and narrow) that adds stiffness along its length; a boss is a cylindrical reinforcement (round tower) that provides a screw attachment point or alignment feature. Both follow the same thickness rule (50–60% of wall), but bosses have additional constraints: they must accommodate the screw diameter and the mold must have a core pin to form the boss hole.

    Key Takeaways

    1. Rib design follows three rules: thickness 50–60% of wall, height max 3× thickness, draft 0.5–1.5° per side — violating any rule causes sink marks, fill problems or ejection damage.
    2. Common mistakes: rib thickness equal to wall (causes sink marks), no fillet at base (causes cracking), ribs too close together (causes material concentration and sink marks between ribs).
    3. The DFM review should check all rib parameters against these rules before the mold is cut — correcting a rib in the DFM review costs nothing; correcting it after T1 costs $500–$3,000 in mold modification.

    Send aCatMould the part drawing, and the DFM review checks every rib against the thickness, height, draft, fillet and spacing rules — with corrections recommended before the mold is cut, so the ribs reinforce the part without causing sink marks or fill problems.

    Sources
    Plastics Technology — rib design guidelines for injection molding
    SPE — part design for injection molding
    MoldMaking Technology — DFM review for rib design

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