Rib equals wall thickness
- High sink risk on opposite side
- Longer cooling at rib root
- Harder to hide on glossy parts
DFM
Rib to Wall Thickness Ratio: Injection Molding Design Rules
Rib to wall ratio balances stiffness against sink on the opposite cosmetic face.
DFM
Rib to wall thickness ratio controls sink and fill
Ribs that are too thick pull sink. Ribs that are too thin do not fill.
Most programs size rib thickness well below the primary wall to limit sink on the opposite face.
Tall thin ribs may not fill completely without vent paths and adequate gate pressure.
Rib roots need fillets to reduce stress and improve flow into the rib.
DFM
Quote this rule with your CAD upload
NetProto DFM review flags these geometry patterns before steel is ordered.
- Call out draw direction and cosmetic faces on the model.
- Note resin family even when the grade is not final.
- Tighten tolerances only on mating features that need them.
Sizing
Thick rib on thin wall versus balanced rib
The cosmetic face opposite the rib tells the story on T1.
Rib well below wall thickness
- Stiffness with less sink tendency
- Easier fill when height is reasonable
- Pairs with gussets on large flats
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
Design guide hub
All DFM topics on one route.
Design guide hubDFM article
Long form engineer guide.
DFM articleRib design
Primary rib rules.
Rib designGussets
Triangular support ribs.
GussetsSink marks article
Thick section sink context.
Sink marks articleVisual reference
Molding context for this rule
Images from the Forge media library illustrate typical programs.
Injection molded part design review on CAD
Injection molded part design review on CAD
Plastic housing with ribs and bosses
Plastic housing with ribs and bosses
Engineer measuring molded component
Engineer measuring molded component
FAQ
Rib ratio questions
Is fifty percent of wall always correct?
Starting point only. Geometry and resin adjust the answer.
Do glass filled ribs sink less?
Glass filled resins are stiffer but sink still shows on thin cosmetics.
Can ribs be added after T1?
Steel safe rib adds are limited. Design ribs before tooling.
Next step
Chart rib thickness on CAD
Upload rib section views with primary wall callout.