Pins on exterior show face
- Polish witness and vent flash risk
- Customer rejection on Class A
- May require redesign or stripper plate
DFM
Ejector Pin Placement: Injection Molding Design Rules
Ejector pin placement trades push force against visible witness on each face.
DFM
Ejector pin placement is a cosmetic decision as much as a mold decision
Each pin leaves a witness. Place pins where the product team accepts them.
Pins on glossy surfaces show even when flush. Texture helps hide minor witness.
Pin clusters around deep ribs need support without punching through thin walls.
Part stiffness at ejection temperature determines whether pins dent soft regions.
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.
Layout
Pins on show surface versus hidden belt
Assembly covers can hide pins. Visible housings cannot.
Pins on inner belt or rib foot
- Hidden by assembly or label
- Still needs balanced push on flat regions
- Robot pick may supplement pin layout
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 articleEjection design
Overall release strategy.
Ejection designRib design
Rib feet as pin land.
Rib designSurface finish hub
Texture hide strategies.
Surface finish hubVisual 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
Pin placement questions
Can pin size change after T1?
Limited steel safe changes. Large moves rework the core.
Do pins cause flash?
Worn pins or poor fit can flash at pin holes. Maintenance item in production.
Are sleeve ejectors always better?
Sleeves help on cylindrical bosses but add mold cost.
Next step
Upload pin witness map
Mark acceptable pin zones on CAD before T1 samples.