Light knurl on smooth boss
- Easier mold steel
- Risk of pull out on high load joints
- Sensitive to resin shrink and boss ID
DFM
Insert Retention Features: Injection Molding Design Rules
Retention knurls and undercuts keep inserts in plastic after assembly loads.
DFM
Insert retention features stop brass from pulling out in service
Knurls, barbs, and undercuts in plastic hold inserts after cool down.
Retention geometry must survive fill without crushing or shifting the insert.
Opposite cosmetic walls can sink when retention features thicken local sections.
Pull out tests on T1 samples validate retention before production release.
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.
Features
Knurl only versus knurl plus undercut
More retention often means more local thickness.
Knurl with undercut or barb
- Higher pull out resistance
- More DFM scrutiny on boss wall thickness
- May need vent path around insert shutoff
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 articleMolded in inserts
In mold loading context.
Molded in insertsBoss design
Boss wall and coring.
Boss designInsert molding
Program overview.
Insert moldingVisual 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
Retention questions
Can retention be tested on samples?
Yes. Pull or torque tests on T1 are common on insert programs.
Do all inserts need knurls?
Follow vendor guidance for the insert family you select.
Can retention features be added after T1?
Steel safe changes are limited. Design retention early.
Next step
Show retention geometry on CAD
Upload knurl zones and insert vendor sheets for DFM sign off.