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Tolerance Stack Up: Injection Molding Design Rules
Stack up analysis ties each molded part tolerance to real assembly clearance.
DFM
Tolerance stack up shows whether assembly fits before steel is cut
Multiple molded and purchased parts each carry variation.
Stack chains from datum to datum across parts and clearances.
A tight single part tolerance can still fail assembly if the chain is loose elsewhere.
Upload stack sketches with CAD so DFM review catches clearance traps early.
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.
Stack review
Bring this to DFM review
One page prevents expensive T1 surprises.
Worst case and nominal stack for each mating interface
Clearance or interference targets at operating temperature
Which partner parts are molded versus purchased
Adjustment features such as slots or shims if needed
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
DFM article
Long form engineer guide.
DFM articleGD&T for molded parts
Datum reference frames.
GD&T for molded partsStandard tolerances
Default bands on drawings.
Standard tolerancesArticles tolerance walkthrough
Long form example.
Articles tolerance walkthroughVisual 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
Stack up questions
Should molders run stack calculations?
Buyers own assembly intent. Molders validate critical dims on T1.
Does shrink affect stack?
Yes. Model mates at intended service temperature when possible.
Can slots fix a bad stack?
Sometimes. Design adjustment before tooling is cheaper.
Next step
Send stack sketches with CAD
Highlight interfaces that must pass assembly on first T1.