Solving molding burn problems
Burn marks are small, dark or black spots on the part surface. This phenomena is also often referred to as dark streaks or specks.
Adiabatically heated trapped air (see Air Traps)
Air trapped in pockets may compress, heat up and cause burn marks.
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This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.
Eliminate air traps
To prevent burn marks, you should move air traps to places which can be vented, or where ejector pins can be added.
Optimize the runner system design
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.
Modify screw design
Contact material/machine suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.
Select machine with smaller shot size
Optimize melt temperature
Reduce temperature to avoid material degradation from overheating, or increase it to limit residual stress.
Optimize back pressure, screw rotation speed, or injection speed
Balance shear heat against residual stress.
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.