Burn marks in injection molding usually indicate that trapped gas was compressed or heated at the end of fill faster than it could escape. The dark brown, black, or scorched area often appears near a rib, blind pocket, weld line, or final-fill location.
Burn is commonly called diesel effect, but not every dark mark is the same. Material degradation, contamination, a hot runner issue, or a damaged mold surface can create similar discoloration. Find the location and timing of the defect before changing the process.
Where burn marks come from
| Area | Possible cause | First check |
|---|---|---|
| End of fill or blind pocket | Air or gas has no effective escape path | Vent location, vent condition, and fill sequence |
| Rib or deep feature | Compression and poor local venting | Vent extension, overflow, or feature geometry |
| Gate or runner | Excessive shear, restriction, degradation, or hot-runner issue | Gate throat, melt residence, nozzle, and runner surface |
| Random dark streak | Contamination or degraded material | Purge, dryer, regrind, colorant, and residence time |
Burn-mark troubleshooting sequence
- Map the mark to the gate, flow front, weld line, rib, insert, vent, and end of fill.
- Run short shots to confirm where the final air pocket forms.
- Inspect and clean vents; confirm they are not blocked by residue or flash.
- Review fill speed, transfer, clamp force, melt temperature, residence time, and material condition.
- Improve the escape path or gate/fill pattern before raising temperatures broadly.
- Validate the fix with appearance, dimensions, strength, and repeatability.
Venting and mold design
Vents must connect the trapped region to an escape path without creating flash or damaging the part. The vent may need to extend through a parting line, ejector, insert, porous steel, overflow, or a redesigned end-of-fill feature. A vent that exists on the drawing but is blocked, too short, or located away from the actual air pocket may not solve the problem.
Check ribs, bosses, blind pockets, inserts, texture, and weld lines during the engineering review. Correct venting can also reduce fill pressure and improve consistency, but the final design must preserve part function and cosmetic requirements.
Process and material controls
- Reduce or profile injection speed at the fill section associated with the mark.
- Confirm melt and mold temperatures are within the grade-specific window.
- Check clamp force and mold alignment; excessive clamp force may restrict parting-line venting.
- Control residence time, screw recovery, purging, color changes, regrind, and contamination.
- Do not keep increasing temperature or pressure when the underlying issue is trapped gas.
How to prove the correction
Keep the trial evidence: before/after photos, short shots, vent condition, process settings, material lot, and inspection results. The injection molding team should verify the stable process window, while the mold-making team should document any vent or steel change. Contact Cavity Mold with the defect location and part geometry for a root-cause review.
Technical reference
The BASF engineering-thermoplastics troubleshooting brochure includes diesel-effect and venting-related molding problems.