Injection molding defects
An air trap forms when the advancing melt front closes off a pocket of air with no vent path. Compressed gas can burn the resin, stall fill, or create weak knit regions at ribs, bosses, and last-to-fill corners.
Defect troubleshooting
Air traps are not voids. They are regions where air could not escape before plastic sealed the cavity. The trapped gas may compress, heat, and burn the surrounding melt or prevent complete packing in
Air traps are not voids. They are regions where air could not escape before plastic sealed the cavity. The trapped gas may compress, heat, and burn the surrounding melt or prevent complete packing in that pocket.
Simulation and bench trials often show traps at rib intersections, deep bosses, and cosmetic pockets with inadequate vent lands. Map trap locations to CAD features before adding arbitrary vent count without direction.
Vent strategy should be planned while part geometry is still editable. NetProto DFM with instant quotes flags deep pocket and rib patterns that commonly trap air.
Root causes
Use this table to decide whether the next change belongs in material prep, process, mold steel, or part design.
| Likely driver | What you see | First lever to try |
|---|---|---|
| No vent path at last-to-fill pocket | Burn or incomplete fill at rib tip | Add vent to parting line or insert vent steel at trap point |
| Fill too fast into closed pocket | Burn marks with trap location fixed in CAD | Slow fill segment into pocket; verify vent depth and land width |
| Parting line vent blocked by flash or grease | Sudden burn after maintenance | Clean vents; inspect for closed-off parting line gaps |
| Deep pocket with parallel walls | Short fill in pocket only | Add vent pin, sintered insert, or geometry relief per mold vendor |
Diagnosis
Splitting ownership tells you whether material, process, or tooling is the honest fix path.
Troubleshooting
Document what you see, prove a fair process window, then escalate to steel or design when the limit is proven.
Map
Overlay defect photos on the model rib or boss name.
Housekeeping
Verify parting line vents at last fill are open and not polished shut.
Process
Reduce speed into the trapping region while monitoring short fill elsewhere.
Fill study
Use fill-only shots to see melt front sequence relative to the trap pocket.
Permanent
When trap persists at fair speed on clean vents, add vent steel or relieve geometry with mold vendor.
Prevention
Most recurring defects trace to setup discipline and design margin rather than a single out-of-spec shot.
Review rib and boss intersections in CAD for last-to-fill pockets.
Specify vent locations on mold drawings before CNC starts.
Include vent cleaning in mold maintenance when burn marks return.
Document fill speed segments tied to trap-prone features.
Run mold-flow or DFM review on new programs with deep pockets.
Upload CAD for instant quote DFM on vent-risk geometry.
Program context
Air traps are cheaper to fix in CAD and mold design than on the floor with repeated burn scrap.
FAQ
Voids are shrink or gas pockets inside thick sections after pack. Air traps are gas compressed during fill before the cavity is fully packed.
Vacuum assists venting on some tools but requires seal discipline and mold design support. It does not replace adequate parting line vent lands.
Compressed air heats adiabatically and can degrade local melt. That is why traps and burn marks often appear together.
Pack cannot displace gas sealed before the pocket filled. Fix vent path and fill sequence first.
Upload CAD for an instant quote and DFM feedback. Engineers can review gates, vents, wall transitions, and ejection risk while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Use instant quote DFM to review ribs, bosses, and last-fill regions while CAD changes are still low cost.
