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Sharp Corner Stress Concentration: Injection Molding Design Rules
Zero radius internal corners concentrate stress and stall plastic flow.
DFM
Sharp internal corners concentrate stress and stall flow
CAD defaults of zero radius are rarely mold friendly.
Stress risers in plastic parts often trace to sharp internal corners on loaded ribs or pockets.
Mold steel at sharp internal features erodes faster and vents poorly.
When function demands a sharp edge, document the exception and accept DFM pushback on mold cost.
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.
Review
Before you insist on zero radius
Engineering drawings should show intent, not accidental CAD artifacts.
Loaded features identified with FEA or test need if applicable
Minimum radius proposal on internal pockets
Draw direction relative to sharp features
Cosmetic face opposite sharp roots
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
DFM article
Long form engineer guide.
DFM articleFillets and radii
Default radius strategy.
Fillets and radiiCorner design
External versus internal corners.
Corner designGate location
Fill paths near thickness changes.
Gate locationVisual 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
Stress concentration questions
Will a small radius fix sink?
Not alone. Thickness and coring still drive sink.
Are sharp external corners OK?
Often yes with draft and parting plan. Internal sharp corners are harder.
Can EDM create sharp internal corners?
EDM can cut tight radii, but mold life and flow still matter.
Next step
Reduce stress risers in CAD
Upload model with internal radii on loaded ribs and pockets.