Burn marks (dieseling): causes and troubleshooting order
Why do brown/black burn marks appear on the part surface — trapped air, or the material itself?
A burn mark (also called dieseling) is a brown-to-black stain usually found near a weld line or vent, in the last area of the part to fill. It differs from a weld line in that the color looks burnt/charred.
Main cause: trapped air heating up
As molten plastic fills the cavity quickly, it compresses the air ahead of it. If there's nowhere for the air to escape (venting), that compression heats the air rapidly, like in a diesel engine (dieseling), burning the plastic in the last-filling area.
Troubleshooting order
- Check venting: Is the escape channel (usually on the parting line) blocked, or too shallow? This is the most common cause — see our venting guide.
- Injection speed: Flow that's too fast in the final filling stage compresses air faster than it can escape. Delaying the speed/pressure switchover point and slowing the last stage often helps.
- Material moisture/degradation: Undried hygroscopic materials (PA, PC, PET) release moisture vapor, which can cause a similar burn/silver-streak mark. Verify drying parameters.
- Melt temperature: A barrel temperature higher than needed can degrade the material before it enters the mold, causing color/odor changes that can be confused with a venting issue.
- Recycled/regrind ratio: Excessively reprocessed material can have a lower degradation temperature; lower the ratio and retest.
How to tell them apart
If it's venting-related, the mark always appears in the same spot and in the same shape (the last-filling corner). If it's material-related, the mark's location and intensity can shift from batch to batch or day to day, and it usually comes with a faint odor.
Practical note
Try the cheapest, fully reversible fix first: slowing the injection speed in the final stage. If that doesn't work, adding extra venting to the mold in that area is usually the permanent fix.
Is something missing or wrong in this article? .