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Flow Length to Wall Thickness Ratio: Injection Molding Design Rules
Flow length versus wall thickness predicts how hard it will be to fill the part cleanly.
DFM
Flow length to wall thickness ratio predicts fill difficulty
Long paths through thin walls stress pressure and venting.
High flow length relative to wall thickness makes short fills and hesitation lines more likely.
Gate placement at the geometric start of fill helps pack reach the last thin region.
Resin choice matters. High flow grades help within the same family, but geometry still dominates.
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.
Fill review
Flow length checklist
Flag these on upload for DFM.
Longest flow path marked from gate to last fill
Thinnest section along that path
Resin family or known high flow grade
Cosmetic face along hesitation prone edges
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
DFM article
Long form engineer guide.
DFM articleThin wall design
Thin wall programs.
Thin wall designGate location
Start fill where it helps.
Gate locationMelt flow index glossary
Lab flow comparison.
Melt flow index glossaryVisual 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
Flow length questions
Is there a universal max ratio?
Program dependent. Upload CAD for qualified DFM feedback.
Can multiple gates fix long flow?
Yes when cosmetics and weld lines accept the extra gates.
Does flow length affect warp?
Indirectly through orientation and pack profile.
Next step
Upload gate and flow path notes
Mark longest fill path on CAD when walls are thin or long.