Color Discoloration in Injection Molding: Causes and Solutions

During injection molding, plastic parts may sometimes show unwanted color changes — such as yellowing, browning, or dark streaks. This defect is often referred to as discoloration. Understanding the causes is the first step to preventing it.

1. Main Causes of Discoloration

① Unstable colorants or additives

Some colorants or additives do not have sufficient thermal stability. They break down easily when exposed to high temperatures inside the barrel, causing discoloration, as shown in the image below. This problem is more likely to occur in areas with high shear flow, such as weld lines or rib structures.

② Thermal degradation of the resin

When the barrel temperature is too high, the molten plastic may overheat and decompose. High temperature at the nozzle can also cause material to stagnate and burn, leading to surface discoloration on the molded part.

③ Excessive injection speed

If the injection speed is too fast, the air inside the cavity cannot escape in time. The trapped air is compressed and heats up rapidly, which can cause the material to overheat and decompose. In addition, high‑speed injection creates strong shear heat as the melt passes through the runner and gate, further raising the melt temperature and potentially causing thermal degradation and discoloration.

2. How to Prevent Discoloration

① Choose stable colorants and additives

Select colorants and additives with good thermal stability that can withstand the processing temperatures of the base resin.

② Lower barrel or nozzle temperature

If discoloration occurs throughout the entire part, the molding temperature is likely too high. Reduce the barrel temperature or nozzle temperature to stay within the recommended range for the material.

③ Reduce injection speed and ensure proper venting

Discoloration caused by high injection speed and poor venting typically appears at the end of the flow path. Lowering the injection speed allows trapped air to escape more easily. Also, make sure the mold has adequate venting to prevent air compression and overheating.