What Are the Design Rules for Ribs in Plastic Parts?

Last updated: 2026-09-01 · Reading time: about 10 min

Ribs add stiffness where walls cannot — and they add it with rules, not habits.
The governing rule is the thickness ratio: a rib’s root runs a share of the nominal wall (commonly half to two-thirds, depending on the resin’s shrinkage) precisely so the junction does not cool fat and sink; height, draft, root radius and spacing follow from the same logic. Break the ratio and the part keeps its stiffness but gains a visible defect on every cosmetic face. This article collects the rules and the reasons behind them.

Key facts

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  • Ribs exist to raise moment of inertia without raising wall thickness — the stiffening a part needs, delivered without the sink and cycle cost of thicker walls.
  • The thickness ratio is the master rule: root thickness as a share of wall, chosen by resin shrinkage, exists to keep the junction cooling in step with the walls around it.
  • Sink above a rib is not a molding error to be fixed at the press; it is geometry already decided at the desk — the sink mechanism starts at the junction.
  • Height, draft and radius interact: taller ribs need draft to eject, draft eats thickness, and the root radius ties the junction into the flow and stress picture.
  • Every rib rule is verifiable at DFM — measured ratios, checked junctions — and the drawing that arrives checked builds faster and cuts cleaner.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

    Why ribs instead of thicker walls?

    Rib
    A thin vertical wall, joined to a main wall or another rib, that raises a part’s bending stiffness by adding section depth where loads run — at a fraction of the material, sink and cycle cost that thickening the whole wall would cost.

    Stiffness scales with section depth, which is why the instinctive fix for a flexible panel is “make it thicker.” The instinct is expensive: thicker walls sink where they meet other walls, lengthen cooling in proportion to the square of the thickness change, add material to every part forever, and invite the uniform-wall discipline to break down everywhere at once.
    Ribs deliver the same section depth as a thin beam standing on edge — stiffness per kilo is dramatically better — and their cost is localized: a junction to cool, a height to eject, a root to fill. The trade is almost always favorable, which is why nearly every stiff plastic part you handle is a shell with ribs, not a slab. But the trade has terms, and the terms are geometry.
    A rib is a promise that its junction cools like the walls around it; break the promise and the defect appears exactly where the rib was supposed to be invisible. The DFM review is where the promise gets checked, ratio by ratio.

    What are the rules, parameter by parameter?

    Parameter Typical practice When violated
    Root thickness ratio About half to two-thirds of the nominal wall, lower for high-shrink resins. The junction cools fat; sink shadows the rib on the opposite face — visible on every cosmetic surface.
    Height As deep as stiffness needs, within fill and ejection limits; very tall ribs become thin walls with all their problems. Fill hesitation and ejection stress; tall thin ribs also buckle in handling before they stiffen in use.
    Draft Per-side draft from the standard draft practice; ribs, being deep, need it more than bosses do. Galling and drag marks on ejection; in worse cases, parts hanging on the rib.
    Root radius A radius that eases the junction into the wall — generous enough for flow and stress, mindful that radius adds local thickness. Sharp roots concentrate stress and resist fill; oversized roots quietly recreate the sink the ratio was avoiding.
    Spacing Enough clearance between parallel ribs for cooling to work between them. Overlapping sink fields and starved steel between close ribs; the tool cools neither rib well.
    Junctions and crossings Crossing ribs staggered or cored so no node accumulates mass. The crossing node becomes the thickest point in the part — sink, voids and cycle time concentrate there.
    Ends and attachments Ribs run into walls or supports, not into air; free ends taper out. Unattached ends flash, warp and read as flaws; stiffness they carry never reaches the load path.

    Two rows carry most programs: the ratio row and the junction row.
    The ratio is a material conversation — a resin that shrinks heavily needs a thinner root for the same defect-free result — which is why the honest check asks for the ratio for your resin, not a universal number. The junction row is the quiet one: two ribs crossing looks tidy on a screen and is the single thickest point of the part in section.
    Cored junctions, staggered crossings andradius transitions are ordinary answers; what is not ordinary is leaving the node unexamined and discovering it as a dimple that polishing cannot . Both rows share one property: they are quick to check on a drawing and expensive to fix in steel.

    How do you check the ribs on your drawing?

    1Measure the ratio, rib by rib type. Root thickness against nominal wall, for each rib family on the part; the number is a five-second caliper check on the CAD and the first thing a sink will grade you on.
    2Walk the junctions. Every place ribs meet walls or each other: is the section locally thicker than the rule allows?
    Crossings cored or staggered? This is where the sink predictions are won or lost.
    3Check draft on the deep ribs. Draft read at the root and at the tip; deep ribs lose proportionally more thickness to draft than shallow ones, and the ratio must survive the taper.
    4Confirm the load path. Each rib should run load into a wall or support; a rib to nowhere carries stiffness the load never sees.
    Stiffness is delivered, not declared.
    5Simulate or sample the questionable ones. Ratios near the limit, cosmetic faces above deep ribs, crossing nodes — the cases worth a flow study or a deliberate first-article look before the pattern is set across the whole part.

    Which drawing checks catch the classic rib errors?

  • The ratio is stated and resin-appropriate — not copied from the previous part’s drawing; shrinkage sets the number, and the datasheet conversation supplies it.
  • No junction is the thickest point of the part — the crossing nodes are cored, staggered or slimmed; check the section, not the rendering.
  • Height survives draft — tip thickness still inside the rule after per-side draft is applied; deep ribs are where drafts eat ratios.
  • Root radii sized deliberately — flow and stress served, junction mass still inside the ratio; radius is a thickness decision wearing a smoothing costume.
  • Cosmetic faces opposite ribs are named — if a class-A surface sits across from rib country, the sink conversation happens at DFM, not at the sorting bench.
  • Spacing leaves cooling room — parallel ribs far enough apart that the steel between them can work; the tool’s cooling has to live there for the mold’s whole life.

    • How does the resin change the rules?

    The ratio table is a material table in disguise.
    High-shrink, semi-crystalline resins pull hard as they cool, so their ribs run lean — a thinner root for the same wall, or the sink shows. Lower-shrink amorphous resins tolerate a deeper root; the same wall can carry more rib before the junction betrays itself. Filled compounds shift the picture again: fillers cut shrinkage but stiffen the melt, and the junction questions move from sink toward fill.
    None of this is folklore; it is shrinkage arithmetic, and it is why the ratio travels with the resin, not with the designer’s habits. The second material effect is cosmetic sensitivity: the same junction that a textured interior hides will announce itself through a glossy exterior face — the surface class decides how honest the ratio has to be.
    Parts that must survive both a high-shrink resin and a glossy face are the parts where the rib rules earn full attention, and where a disciplined DFM conversation pays for itself in the first boxed shipment. Bring the resin and the cosmetic map; the ratios follow from those two facts, and everything else on the rib is negotiation.

    “Every sink I have ever been asked to fix was drawn first. The rib ratio is the most forgiving contract in tooling: half a millimeter at the desk, or a re-cut in the steel later.”
    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    What ratio should we use for our resin?

    The range is set by shrinkage: leaner roots for the high-shrink semi-crystalline family, deeper roots allowed for amorphous resins, filled grades between. The specific number comes from the resin’s data and the shop’s experience with it — which is why the question belongs in the DFM review with the material decision already made. A ratio copied from a different resin’s drawing is a decision you didn’t know you were making.

    Can we hide sink above ribs with texture?

    Partially — texture scatters light and forgives shallow depressions, which is why textured interiors tolerate more than glossy exteriors. It is a mitigation, not a fix: the junction is still cooling fat, the local property loss is still real, and a shadow that survives texture under raking light will still be found. Design the ratio right and the texture is an ally; use it as cover and the part ships with a lottery ticket.

    How tall can a rib be?

    As tall as three things allow: fill (the melt must the tip through a narrowing channel), ejection (deep ribs grip and need draft, ejection geometry, sometimes rib-side release), and stiffness of the rib itself (a thin deep plate can buckle in handling). The limits interact — draft consumes thickness faster on tall ribs — so the honest answer comes from the specific geometry at DFM, not from a table.

    Do ribs always need radii at the root?

    Effectively yes — a sharp internal corner resists fill, concentrates stress and cracks first in service. The design freedom is in the size: generous enough for flow and strength, restrained enough that root mass stays inside the thickness ratio. When the radius and the ratio fight, the ratio wins, and the radius that fits is usually still enough for both jobs.

    What about ribs on transparent parts?

    With more discipline: sinks and voids are optical events in clear resins, and the junction is where they live. Ratios run conservative, junctions get cored rather than stacked, and any rib behind an optical region is a deliberate conversation, not a default. The alternative — moving stiffness into flanges and edges away from the optical window — is often the better answer, and it is a drawing-stage decision.

    Can the press fix a rib that was drawn wrong?

    Process can hide a small sin and cannot reverse geometry. Packing profiles and cycle adjustments shave a shallow sink margin; they do not change the section that caused it. The re-cut — steel removed, weld line moved, texture redone — is the honest price of a ratio ignored. This is why rib checks live at DFM: the desk version of the fix costs minutes, and the steel version costs the schedule.

    Key takeaways

    1. Ribs buy stiffness without wall thickness — the efficient shape, governed by junction geometry.
    2. The thickness ratio is the master rule, set by the resin’s shrinkage and checked rib family by rib family.
    3. Junctions and crossings are the quiet sink factories: cored, staggered or slimmed on purpose.
    4. Height, draft and radius interact — deep ribs lose thickness to draft, and the ratio must survive the taper.
    5. Cosmetic class sets how honest the ratio must be; glossy faces opposite rib country get the conversation at DFM, not at sorting.

    If your drawing carries ribs — most do — send it with the resin callout. You will get the ratio check for your material, the junction walk, and the cosmetic-face risk named before steel is ordered — from a named tooling engineer, in the DFM review, where the fix is still minutes wide.

    Sources
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    DME — mold technologies and components, reference for rib and junction tooling practice
    MatWeb — material property database, reference for resin shrinkage in ratio selection
    Plastics Industry Association — industry resources on part design practice

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