Troubleshooting discoloration problems

Discoloration is a color defect characterized by a molded part's color having changed from the original material color.

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. Autodesk Moldflow Insight 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.