Injection molding burn mark troubleshooting starts by locating the mark against the fill pattern, then separating trapped-gas, material-degradation, process, and tooling causes. A black or brown mark at the end of fill is evidence to investigate, not proof that the injection speed alone is wrong.
This checklist is written for a process engineer, toolmaker, or buyer reviewing a recurring defect. It keeps the investigation controlled so that one adjustment does not create flash, short shots, weld-line weakness, or a new cosmetic defect.
1. Record where and when the mark appears
- Map the mark to the gate, runner, weld line, rib, blind pocket, insert, and end-of-fill location.
- Compare all cavities and record whether the same cavity burns every cycle.
- Photograph the part, mark the orientation, and retain a short-shot sequence if the tool is still being debugged.
- Record resin grade, color, lot, drying history, regrind, barrel residence time, and purge condition.
2. Separate trapped gas from degraded material
| Pattern | Likely direction | First evidence |
|---|---|---|
| Dark mark at a dead end or last-fill region | Air trap, poor venting, or blocked vent | Short shots, vent inspection, and fill pattern |
| Streak follows the flow from the gate | Overheating, shear, contamination, or residence time | Barrel profile, screw recovery, purge, and material condition |
| Only one cavity burns | Cavity-specific vent, gate, or cooling issue | Cavity comparison and tool inspection |
| Burn appears with a short shot | Gas back-pressure or restricted flow | Fill speed profile, venting, and runner restriction |
3. Check the mold before changing the process
- Clean vents, parting-line lands, ejector clearances, and gas-trap areas.
- Confirm vents are placed at the last-fill locations and are not sealed by flash, residue, or incorrect shutoff contact.
- Review gate and runner restrictions, sharp transitions, inserts, ribs, and blind pockets that can trap air.
- Check clamp tonnage and mold alignment; excessive clamp force can reduce the working clearance of some vents.
4. Use controlled process trials
Start inside the resin supplier’s approved processing range. Test a staged injection profile, reducing speed near the burn location rather than slowing the entire shot. Verify melt temperature, mold temperature, back pressure, screw speed, cushion, transfer position, and residence time. Make one change at a time and compare burn severity, fill completeness, pressure, weight, appearance, and cycle stability.
Reducing temperature or speed can reduce degradation or gas compression, but it can also increase viscosity and cause a short shot. The injection molding team should review the process window with the engineering team before a permanent setting is released.
5. Correct the root cause and validate
Permanent corrections may include adding or relocating vents, improving vent cleaning, modifying a gate or runner, changing an insert, reducing residence time, or selecting a better-suited material grade. Do not approve a cosmetic correction until the part still meets dimensions, weld-line requirements, mechanical tests, and repeatability targets. The mold-making team can inspect the steel and vent system; Contact Cavity Mold with the burn location, resin grade, and short-shot photos.
Technical references
The Autodesk Moldflow burn-mark reference separates trapped air, material degradation, and process remedies. Avient’s troubleshooting guide connects burn marks and short shots with gas trapping and poor venting.