All dimensions to tight band
- Inspection cost scales with feature count
- Non mating cosmetics fail without adding function
- Hides which fit actually drives assembly
DFM
Tight Tolerance Strategy: Injection Molding Design Rules
Tight tolerances on molded parts should target mating features, not every dimension.
DFM
Tight tolerances belong on mating features, not every edge
Plastic moves with temperature, moisture, and shrink lot to lot.
Commercial molding tolerances cover many dimensions until you tighten specific mates.
Each tightened band can add tooling, gate tuning, or inspection cost.
NetProto DFM asks which interfaces are critical before quoting inspection heavy programs.
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.
Drawing practice
Blanket tight decimals versus GD&T on mates
Inspectors and molders read drawings differently.
GD&T on mating features only
- Datums show assembly intent
- Looser commercial bands elsewhere
- Easier negotiation on T1 correlation
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 articleStandard tolerances
Commercial baseline.
Standard tolerancesTolerance stack up
Assembly chain math.
Tolerance stack upGD&T for molded parts
Datum frames on plastics.
GD&T for molded partsVisual 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
Tight tolerance questions
Can plastic hold metal like tolerances?
Often no on global dims. Local mates can be tight with plan.
Does tooling steel temperature matter?
Process stability matters for tight mates after shrink.
Should I tighten tolerances to force quality?
Tighten where assembly fails, not as a generic quality signal.
Next step
Mark critical mates on CAD
Upload GD&T or a critical dimension list with your quote.