Rib-to-Wall Ratio for Injection Molding — What Is the Correct Ratio, Why Does It Matter, and What Happens When the Rib Is Too Thick or Too Thin?

Last updated: 2026-09-03 · Reading time: about 6 min

The rib-to-wall ratio is the rib base thickness divided by the nominal wall thickness. Hold it in a band — 50–60% for most resins — and both failure modes stay out: below the band the rib will not fill, above it the opposite face sinks. Proportion the whole rib off the same nominal wall so it scales when the wall changes.
See also related article.

Key facts

<>

  • The band is the whole game: hold the rib base at 50–60% of the nominal wall. Below roughly 40–50% the rib is too thin to fill and short-shots at the tip; above about 60% the thick base cools last and sinks the opposite face.
  • Resin family moves the cap — leaner (near 50%) for high-shrinkage PP and PE, 55–60% for amorphous ABS, PS, PC and POM, 60–65% for glass-filled PA66 and PBT, and 65–70% for low-shrinkage PPS and PEEK.
  • Proportion the whole rib off the nominal wall — height ≤3× the wall, base fillet ≥25% of it, draft 0.5–1.0° per side — and keep the ratio constant as the wall changes, so the rib scales instead of sinking.

    • 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. See also related guide.

    What rib-to-wall ratio should you target — and how does the resin family move the number?

    The ratio, not the absolute rib thickness, is the controlling number, because one fraction sets both limits at once: how easily the melt fills the rib, and how hard the thick base pulls the opposite face as it cools. The resin family decides where that ceiling sits.

    Resin family Typical max rib-to-wall ratio Why the cap sits there
    PP, PE — semi-crystalline, high shrinkage ~50% High shrinkage keeps pulling after the wall sets, so a lean base is what keeps the opposite face flat.
    ABS, PS, PC, POM — amorphous, low-to-medium shrinkage 55–60% Moderate shrinkage; 60% is the usual ceiling on a cosmetic face before a sink shows.
    PA66, PBT + glass fiber 60–65% Glass cuts shrinkage and stiffens the base, so a slightly thicker rib is tolerable.
    PPS, PEEK — high-performance, very low shrinkage 65–70% Very low shrinkage lets the base approach the wall with little added sink.

    Read the band as a two-sided window. Below it, the rib is a thin channel — the melt loses heat faster than it advances and freezes before the tip fills, leaving a short shot. Above it, the thick base is the last section to cool, so it keeps shrinking after the wall has set and pulls the opposite face into a sink. The ratio is the single number that holds the rib inside both limits.

    Once the base ratio is set, how do you proportion the rest of the rib off the nominal wall?

    The ratio is really a proportioning discipline. Once the base is a known fraction of the nominal wall, every other rib dimension follows from that same wall — so the whole rib scales together when the thickness changes from one zone of the part to the next.

    1. Set the base as a fraction of the nominal wall. Start at 50–60% and move the cap by resin family from the table above. Reference the part’s main wall, not a locally thickened section at the rib root.
    2. Cap the height near 3× the wall. Stiffness rises with height, roughly with the cube of it, so add height — not base thickness — when you need rigidity. Past about 3–5× the rib gets hard to fill and hard to eject without dragging.
    3. Fillet the base at ≥25% of the wall. A radius at the rib-to-wall junction spreads the stress that would otherwise concentrate at a sharp corner and crack the part there.
    4. Add draft of 0.5–1.0° per side. Draft lets the rib release cleanly so it does not stick, bend or break the tall thin core that forms it during ejection.
    5. Space ribs off the wall too — about 2–4× the rib height. Even spacing supports the wall between ribs without clustering thick sections that sink together.

    Rib-to-wall ratio and nominal wall. The rib-to-wall ratio is the rib base thickness divided by the nominal wall thickness, expressed as a percentage; the nominal wall is the part’s main general wall, not the thickened local section at the rib root. Because it is a ratio, it holds as the part scales: a 3 mm wall takes a 1.5–1.8 mm base, a 1.5 mm wall takes 0.75–0.9 mm — the fraction, not a fixed millimeter value, is what you carry across the drawing.

    “The most common rib mistake we see is a base dimensioned in absolute millimeters instead of as a fraction of the wall. Give us the nominal wall and we set the base at 50–60% of it, cap the height near 3×, and put the stiffness in the height rather than the base. When the wall changes across the part, the ratio holds — the rib thickness follows it.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • 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.

    • When the wall thickness changes across the part, do I hold the rib thickness or the ratio constant?

    Hold the ratio, not the rib thickness.
    The base is re-derived from the local nominal wall in each zone, so a rib over a 2 mm wall gets a different base than one over a 3 mm wall.

    A fixed rib thickness breaks this: over a thinner wall it becomes a larger fraction and sinks, over a thicker wall a smaller fraction and under-reinforces. Keeping the ratio constant is what lets one design logic work across a part whose walls change.

    Does a thicker rib base make the part stiffer?

    Not the way most people expect. Bending stiffness rises with rib height — roughly with the cube of it — so doubling the height multiplies stiffness about eight times, while a thicker base adds comparatively little.

    That is why the base ratio governs fill and sink, and the height governs rigidity. When a part must be stiffer, add height within the ~3× wall cap and with draft, and leave the base lean at 50–60% so the opposite face stays clean.

    What happens below the band — can process rescue a rib that is too thin to fill?

    A rib under roughly 40–50% of the wall is a thin channel: the melt cools against the walls faster than it advances, and the tip freezes before it packs — a short shot at the rib end.

    Higher melt temperature and faster injection buy a little flow length, but they lengthen the cycle and can flash the parting line. The durable fix is geometric — open the base toward the band, or thicken the rib near the gate — rather than chasing a thin rib with process.

    Key takeaways

    1. Treat the rib-to-wall ratio as the master number: base at 50–60% of the nominal wall for most resins, held inside the band so the rib both fills and avoids a sink on the opposite face.
    2. Move the cap by resin family — leaner for high-shrinkage PP and PE, thicker for glass-filled and low-shrinkage high-performance grades.
    3. Proportion everything else off the same nominal wall — height ≤3× (stiffness comes from height, not base), fillet ≥25%, draft 0.5–1.0° per side — and keep the ratio constant as the wall changes.

    The rib-to-wall ratio is a proportioning discipline, not a single dimension. Set the base as a fraction of the nominal wall, move the cap by resin family, and scale the height, fillet, draft and spacing off that same wall. At aCatMould we check rib geometry against these ratios during the DFM review — before the deposit — so a thick base or a thin, short-shot rib is caught on the drawing rather than at T1.

    Sources
    DuPont — injection molding part design guidelines (rib and wall proportioning)
    Autodesk Moldflow — resin shrinkage prediction
    BASF — Ultramid (PA66) glass-filled design data
    Solvay — KetaSpire PEEK design guide
    MatWeb — material property database (mold shrinkage by resin)

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