Internal versus external
Inside corners concentrate stress. Outside breaks are a different choice.
Sharp corner stress
Sharp Corner Stress in Molded Parts: Why Zero Radius Cracks
Zero radius inside a loaded pocket or rib root is a stress riser. Leave an edge sharp only when function requires it. DFM review comments on CAD.
Design guideline
Zero radius is a stress riser, not a default
A sharp internal corner concentrates stress and can stall melt as it turns the wall. That is a common crack start. This page owns the mechanism, not the radius rule.
Mold steel at a sharp internal feature also vents poorly and wears faster. When function demands a sharp edge, document the exception. Fillets and radii own how to radius the corner. DFM review comments before steel is cut.
This spoke owns the mechanism. Corner design owns corner types. Ribs and bosses own those roots. The design guide hub lists every topic.
Roles
Which page answers which corner question
Stay here for why zero radius cracks. Open a sibling for the radius rule or corner type.
| Page | Use it for |
|---|---|
| Sharp corner stress (this page) | Why a zero-radius inside corner cracks |
| Fillets and radii | How to radius the corner |
| Corner design | Corner types and parting-line choices |
| Rib design | Root blend on a stiffener |
| Boss design | Root blend on a post |
| Gussets | Brace roots need a blend too |
| Cantilever snap fit | Beams crack at a sharp inside corner |
| Wall thickness | A large root can hide a thick island |
| DFM review | CAD check before steel |
Checks
What DFM review looks for on sharp corners
Qualitative checks. No figure on this page is a NetProto specification.
Loaded roots
Ribs, bosses, and snap beams fail at a sharp inside corner first.
Flow stall
Melt hesitates at a knife edge. That can leave a knit or a short.
Keep-sharp exceptions
Shutoffs, seals, and snap catches may stay sharp. Flag them on the RFQ.
Decision
Insist on zero radius versus radius the loaded inside corner
Wrong: CNC-style knife edges inside every pocket. Right: radius loaded insides; keep sharp only where function needs it.
What goes wrong
- Zero radius on a loaded rib or pocket
- Assume a tiny radius will also fix sink
- Leave keep-sharp edges unmarked
What to try first
- Radius loaded internal corners
- Open fillets for the radius rule
- Mark shutoff and snap edges that must stay sharp
RFQ
Send these notes with CAD for a sharp-corner review
The quote can start from CAD alone. These notes make the DFM pass specific.
Loaded internal corners identified
Keep-sharp edges: shutoff, seal, or snap catch
Cosmetic faces that cannot show sink from a large root
Resin family
Whether EDM-sharp internals were requested on purpose
Section views through stressed corners
Related
Related design guidelines
Stay on this spoke for its question. Open a sibling for the neighbor topic or the CAD check.
DFM review
How NetProto checks stress risers before steel.
DFM reviewFAQ
Sharp corner stress FAQ
Why are sharp internal corners bad?
They concentrate stress and can stall flow. Loaded ribs, bosses, and snaps often crack there first.
Will a small radius fix sink?
Not by itself. A large root can thicken the junction and sink the opposite face. Fillets own that balance.
Are sharp external corners OK?
Often they are a break or a chamfer, not a stress riser like an inside knife edge. Corner design owns that choice.
Can EDM create sharp internal corners?
Steel can be cut sharp. The plastic and the steel both suffer. Document the exception if function requires it.
When should a corner stay sharp?
Shutoffs, seals, and some snap catches. Flag those keep-sharp edges on the RFQ.
How is this page different from fillets or corner design?
This spoke owns why zero radius cracks. Fillets own the radius rule. Corner design owns types. DFM review checks CAD.
Next step
Upload CAD for DFM review of sharp corners
Share loaded corners and keep-sharp edges so review can check stress risers before quoting.