A dark streak or spot on an injection-molded part does not identify one cause, and using a vertical machine does not by itself explain the defect. Autodesk Moldflow identifies compressed trapped air and material degradation as possible causes of burn marks. The investigation should start with where the mark appears and what changed in the actual material, tool, and process.
Separate two investigation paths
- Possible air-trap burn: Compare the mark with the end-of-fill pattern, flow fronts, vents, ejector features, and nearby geometry. Trapped air may compress and heat; venting or flow-path evidence helps assess whether this mechanism is plausible.
- Possible material degradation: Review the resin grade and lot, drying requirements, residence conditions, melt history, contamination, and any restrictive screw, runner, gate, or flow-path features that could contribute to overheating.
Autodesk 的 burn-mark troubleshooting guidance lists trapped-air heating and material degradation as different possible causes and warns that process changes can introduce other effects.
Collect evidence before changing settings
- Photograph and locate the defect by part, cavity, time, and production lot.
- Compare the defect location with the drawing, fill pattern, venting, and any recent tooling change.
- Check the approved resin grade, lot, supplier processing guidance, and handling records.
- Review the machine and process record for the affected lot rather than applying a generic screw-speed or back-pressure value.
- Document the suspected cause and the evidence used to choose the trial change.
Validate a correction on the actual part
A change to venting, gate or runner geometry, injection profile, melt conditions, or screw settings should be reviewed against the tool design and resin supplier guidance. Check the burn mark and other relevant quality results after the change; a correction that removes one visible defect may affect fill, stress, dimensions, or cycle time. Do not use a generic “laminar flow” threshold or universal machine setting as an acceptance rule.
For a related filling defect, see DAYIN’s short-shot troubleshooting guide. For tool and process records, see the mold-processing 檢查清單 和 客製化射出成型服務.