Parting across Class A face
- Flash control and maintenance sensitivity
- Texture match across the seam
- Customer acceptance of witness line
DFM
Parting Line Placement: Injection Molding Design Rules
Parting line placement sets flash risk, cosmetics, and whether side actions are required.
DFM
Parting line placement drives flash, cosmetics, and side actions
Every molded part splits somewhere. Make that split a design choice.
The parting line is where cavity and core meet. Flash and witness follow that seam.
Hiding the parting on a sharp external corner is common when cosmetics allow a faint line.
Side actions often start because the parting cannot follow the ideal cosmetic path.
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.
Cosmetics
Parting on visible edge versus hidden flange
Moving parting after steel cut is expensive.
Parting on hidden step or flange
- Easier flash control on shutoff
- May require assembly features to hide step
- Better fit for high gloss programs
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 articleDraft angle
Release along pull direction.
Draft angleSide actions
When parting cannot follow shape.
Side actionsCorner design
External corners as parting anchors.
Corner designVisual 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
Parting line questions
Can parting line move after T1?
Major moves need steel rework. Plan on CAD before cut.
Does texture affect parting?
Texture depth needs draft and shutoff planning at the seam.
Should parting show on the drawing?
A parting line view or note reduces DFM back and forth.
Next step
Show parting intent on CAD
Upload a parting line sketch or marked view with cosmetic faces.