Burn marks


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.

FIGURE 1. Burn marks

Causes of burn marks


Entrapped air
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.


Material degradation
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:

High melt temperature  
Excessive melt temperature can be caused by improper barrel temperature setting, a broken thermocouple, or a malfunctioning temperature controller.

High screw rotation speed  
If the screw speed is too high during the plasticization period, it will create too much frictional heat, which could degrade the material.

Restrictive flow path  
When the melt flows through restrictive nozzle, runner, gate, or part sections, it creates a lot of shear (frictional) heat, which could degrade the material.


Other defects resulting from the same causes

Remedies


Alter the mold design


Place an adequate venting system throughout the mold to help vent out the entrapped air.
Vents are especially important near the end of the flow path and in the blind area. The recommended venting size is 0.025 mm (0.001 inches) for crystalline polymers, and 0.038 mm (0.0015 inches) for amorphous polymers.


Enlarge the sprue, runner, and/or gate.
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.


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.