What is a burn mark?
Burn marks are small, dark or black spots that appear near the end of the flow path of a molded part or in the blind area where the air trap forms.
If the injection speed or injection pressure is too high, the air trapped in the runner system and cavity cannot be released to the atmosphere through the venting system properly within a very short filling time. Air traps also occur in improperly vented systems when race-tracking behavior is significant. Consequently, the air will be compressed, resulting in a very high pressure and temperature, and which will cause the polymer to degrade on the surface near the end of the flow path or the blind area.
Burn marks can also result from the degraded (charred) materials being carried downstream and then appearing on the surface of the molded part or near the venting areas. Material degradation is caused by:
Other defects resulting from the same causes
Alter the mold design
Place an adequate venting system throughout the mold to help vent out the entrapped air.
Enlarge the sprue, runner, and/or gate.
Adjust the molding conditions
Reduce the likelihood of burn marks by avoiding excessive melt temperatures during the molding process:
Reduce the injection pressure.
Reduce the injection speed.
Reduce the screw rotation speed.
Decrease the barrel temperature.
Check the band heaters on the barrel and nozzle, and calibrate the thermocouple.