injection molding discoloration

Causes
Material degradation
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.

Remedies
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 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
The typical shot size should be between 20 and 80 percent of machine injection capacity. For temperature-sensitive materials, the range should be narrowed down, depending on the material. Moldflow Plastics Insight (MPI) products can help you select the right size machine for a specific mold. This will help avoid material remaining in the heated barrel for prolonged periods of time.
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.
Vent appropriately
Use the material supplier recommended venting size.

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.

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