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Notes on injection moulding: part design, tooling and cost

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How thick and how tall should ribs be on a moulded part?

Make ribs thinner than the wall they stand on, typically 50 to 60 percent of the wall thickness, or less for semi-crystalline resins where sink marks matter. Keep them no taller than about three times the wall, space them at least two wall thicknesses apart, give them draft, and put a modest radius at the base. Several shorter ribs usually beat one tall rib.

Ribs add stiffness without thickening the whole wall, which would lengthen cooling and invite sink and warpage. Getting the proportions wrong moves the problem to the rib itself: sink on the show face, short fills, or parts sticking in the mould.

What three published guides say

RuleEnvalior (formerly DSM)Eurotec design guideAIM Processing
Thicknessabout 50 to 60% of wallby resin, see next table50 to 60% of wall
Heightno more than 3 × wall2.5 to 3 × wallno more than 3 × wall
Spacingat least 2 × wallat least 2 × wallat least 2 × wall
Draft per side1 to 1.5°0.5°at least 1°
Base radius25 to 50% of wall, at least 0.4 mmat least 0.5 mm0.5 to 1.0 × wall at intersections

Thickness, height and spacing agree closely. Draft and base radius vary, so confirm them with the toolmaker and resin supplier. Envalior explains the radius limit: above about half the wall, a mass of material builds up at the base and raises the risk of sink and moulded-in stress.

Thickness by resin type (Eurotec)

Resin typeMinimal sinkSlight sink
Amorphous0.40 × wall0.60 × wall
Semi-crystalline, unfilled0.30 × wall0.40 × wall
Semi-crystalline, filled0.35 × wall0.50 × wall

Eurotec adds that thicker ribs are tolerable on walls under 1.5 mm, where thin ribs become hard to fill.

Worked example (our numbers)

A 2.5 mm wall in polycarbonate, which is amorphous, on a cosmetic face: Eurotec's minimal-sink figure gives a rib of about 1.0 mm, while the 50 to 60 percent rule gives 1.25 to 1.5 mm. Height should stay within about 7.5 mm, spacing at least 5 mm, and the base radius about 0.6 to 1.25 mm on Envalior's rule. With 1° draft per side, a 7.5 mm rib that starts at 1.25 mm at the base narrows by about 0.26 mm to roughly 1.0 mm at the top, so check that the tip still fills.

Placing ribs so they work

  • Run them along the bending direction. Envalior shows that ribs along the length of a plate supported at its ends stiffen it significantly, while ribs across it add little.
  • More ribs, not taller ones. AIM Processing recommends adding ribs rather than height.
  • Core thick ribs. AIM suggests coring a thick rib from behind to reduce sink.
  • Do not add ribs just in case. Eurotec warns that unnecessary ribs can cause warpage and stress.
  • Treat gussets as ribs. Envalior says the same dimensioning rules apply to gussets on corners, walls and bosses.

These are published rules of thumb. Final rib proportions depend on the resin grade, cosmetic requirements and mould, and are confirmed on moulded samples.

Questions buyers ask

Why is there a line on the show face opposite each rib?

That is sink from the rib being too thick or its base radius too large. Thinner ribs, coring or a textured surface are common fixes.

Can ribs have zero draft?

Envalior notes that low ribs can sometimes use less or no draft, but this risks scuff marks and ejection problems.

Should I add ribs later if the part is too flexible?

It is often possible, because a new rib means removing steel from the mould rather than adding it. The toolmaker still needs to check cooling, venting and ejection around the new rib, so it is better to settle stiffness at the design stage.

Do ribs help a part fill?

Eurotec says well-placed ribs can help fill difficult areas, while Envalior warns that over-thick ribs can disturb flow and cause weld lines and voids.

Sources