Burn marks. Air and gasses in front of the flow front cannot escape due to insufficient venting and are compressed via the incoming melt to the point of ignition.
Variable wall thickness can cause the melt to encircle air and gasses at the flow front resulting in a gas trap.
1. Reduced injection speed.
2. Reduce injection pressure.
3. Improve venting in the area of the burn mark.
4. Decrease the melt temperature in case of degradation.
5. Calibrate thermocouples and check band heaters.
6. Consider flow simulation to calculate the flow front behaviour and investigate practical solutions.