Burn Marks
Brownish or silvery streaks caused by thermal damage to the melt. Excessive temperature, long residence time or high shear are the main causes.
Burn marks form when the melt suffers thermal damage inside the machine or as it enters the mold. They leave brownish or silvery streaks on the part surface and almost always come down to one of three sources: a melt temperature that is too high, the material waiting in the barrel longer than necessary, or excessive shear inside the mold. Because they look very similar to moisture and air streaks, you need to identify which one you are dealing with before touching any parameter.
How to Recognize It
- Brownish (change in macromolecules) or silvery (chain shortening) discoloration on the surface.
- If the mark appears periodically, it is caused by residence time.
- It is seen behind narrow cross-sections or sharp edges; if lowering the injection speed or melt temperature reduces the defect, the diagnosis is correct.
- VERY similar to moisture and air streaks — the process history is needed to tell them apart.
Physical Causes
- The melt temperature is close to or above the upper limit of the processing range.
- Long residence time in the plasticizing unit (cycle interruptions, small shot volume relative to the machine capacity).
- Excessive screw speed or back pressure (shear heat).
- High shear inside the mold: excessive injection speed, narrow gate, sharp edges.
- Excessively hot/long pre-drying; a high reclaim ratio; residence in the hot runner.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- MEASURE the melt temperature by injecting "into the open" and using a needle thermometer (the panel value is not enough).
- If the temperature is high: lower the barrel temperature, screw speed and back pressure.
- If the residence time is critical: shorten the cycle time, increase the plasticizing delay, reduce the reclaim ratio.
- If the mark is near the gate: lower the injection speed or use a profile (slow→fast).
- Check the pre-drying of the material — excessive drying also causes thermal damage.
Mold, Design and Material Solutions
- Avoid sharp edges and small flow cross-sections in the runner and gate system.
- Check the nozzle cross-section and temperature, and the function of the shut-off nozzle.
- Eliminate dead spots in the unit (areas where the flow stops); check for wear.
- Consider a smaller plasticizing unit or a material/colorant with high thermal stability.
Important Notes
- ⚠️ Thermal degradation reduces the mechanical properties of the part even if no mark is visible on the surface.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Moisture Streaks | Moisture streaks take the form of silvery lines spread over the part surface and disappear when the raw material is dried. A burn mark, on the other hand, tends toward brown and does not change with drying. |
| Diesel Effect | The diesel effect appears at a specific point in the mold, where air is trapped and burns, and it is always in the same position. A burn mark appears scattered along the flow. |
| Black Specks | Black specks are sharply defined, individual particles; their source is degraded material that has built up on the screw. A burn mark is a diffuse discoloration. |
Frequently asked questions
Why do burn marks appear periodically?
Burn marks that recur at certain intervals point to material waiting in the barrel. When the cycle pauses, the waiting charge degrades and comes to the surface in the next few shots. If your shot volume is very small relative to the machine capacity, this pattern becomes continuous.
I lowered the temperature but the mark did not go away — what next?
If temperature alone is not enough, look at the shear heat: lower the injection speed step by step, reduce the screw speed and back pressure. If the mark still appears at the same point, the problem is on the mold side — a narrow gate, a sharp edge or insufficient venting.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice