Thick groove only
- Partial flex and early crack initiation
- Sink on the opposite face
- Inconsistent orientation at the hinge
DFM
Living Hinge: Injection Molding Design Rules
Living hinges need a thin uniform flex web and resin that survives repeated flex.
DFM
Living hinges need thin, uniform flex sections and resin discipline
A hinge is a fatigue feature, not a thick wall with a groove.
Living hinges work when the flex section is thin, uniform, and oriented for mold fill.
Polypropylene and some PE grades are common hinge resins. Brittle grades fail in flex.
Gate location should avoid weld lines across the hinge line when possible.
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.
Hinge section
Thick hinge land versus true flex web
Customers feel hinge quality in the first hundred cycles.
Controlled thin flex web
- Uniform thickness along hinge length
- Gradual transition into rigid walls
- Process window documented on T1 samples
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 articleUniform wall
Transitions into hinge web.
Uniform wallGate location
Weld lines across hinge.
Gate locationGlossary cycle time
Flex parts still pay cooling time.
Glossary cycle timeVisual 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
Living hinge questions
Which resin for living hinges?
Often PP family on qualified programs. Confirm in RFQ notes.
Can hinges run on one steel tool?
Yes when DFM accepts hinge geometry and process development.
Do hinges need draft?
Rigid walls need draft. Flex web thickness is its own DFM topic.
Next step
Validate hinge CAD early
Upload hinge section views and resin notes with your quote.