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Ejection Design: Injection Molding Design Rules
Ejection design balances release force with pin witness on cosmetic and flat surfaces.
DFM
Ejection design must release the part without cosmetic damage
Pins, sleeves, and stripper plates push on hidden faces when possible.
Ejection happens while the part is still warm and flexible. Uneven push warps flat covers.
Deep cores need sufficient ejector area or air assist to avoid vacuum hold.
Robot grip points can replace some pin marks when automation is in scope.
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.
Ejection
Mark before mold design locks pin layout
These notes reduce pin marks on Class A.
Hidden faces acceptable for pin witness
Flat regions that need uniform ejection support
Deep core pockets that may need vent or air pop
Texture faces where pin polish marks are unacceptable
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
DFM article
Long form engineer guide.
DFM articleEjector pin placement
Pin layout detail.
Ejector pin placementDraft angle
Release before ejection force.
Draft angleEjector pin glossary
Shared term.
Ejector pin glossaryVisual 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
Ejection questions
Can ejection marks be eliminated?
Often reduced by hidden pin placement or texture hide.
Does ejection cause warp?
Uneven ejection on large flats is a common warp driver at demold.
Who approves pin locations?
Mold designer proposes. Customer accepts witness on qualified programs.
Next step
Identify hidden ejection zones on CAD
Upload views showing where pin marks are acceptable.