Causes and Solutions for Uneven Color in Injection Molded Products

  Uneven color distribution in injection molded parts is a common quality issue, primarily caused by dispersion problems, thermal instability, and inconsistent processing. The main reasons and their solutions are: (1) Poor Dispersibility of Colorant: This often causes a streaked pattern near the gate. The solution is to use a more compatible color masterbatch, increase the back pressure during plastication, or ensure adequate screw mixing. (2) Poor Thermal Stability: If the plastic or pigment degrades at processing temperatures, it can cause color shifts. Stabilizing the color requires strict control over the material temperature, shot size, and cycle time to ensure consistent thermal history. (3) Inconsistent Cooling in Crystalline Plastics: Variable cooling rates can lead to differential crystallization, which manifests as color differences. For parts with varying wall thickness, colorants can be used to mask the chromatic aberration. For uniform parts, consistent mold and melt temperatures are critical. (4) Part and Mold Design: The geometry of the part and the gate's location and type can cause uneven flow fronts and filler orientation, leading to visible flow marks or color streaks. Modifying the mold design, such as relocating the gate or adjusting runner sizes, can often resolve this.