Analysis and solution of color change phenomenon of plastic injection molding products

Color change of medical injection molding products, also known as color streaks, refers to the phenomenon that the color of the injection molded product is inconsistent with the standard color. The following is the analysis and troubleshooting methods of color change and color streaks:

1. The quality of the colorant does not meet the requirements

The performance of the colorant directly affects the color quality of the plastic part after molding. If the dispersion, thermal stability and particle morphology of the colorant do not meet the process requirements, it is difficult to produce products with uniform color. For example, some colorants are in the form of aluminum foil or flakes, which will form a directional arrangement after being mixed into the melt, resulting in uneven color on the surface of the plastic part. In addition, if the colorant is mixed with the raw material by dry mixing, it may cause poor dispersion in the barrel, thereby affecting the final color. Colorants with poor thermal stability are easily decomposed by heat in the barrel, which will also cause the plastic part to change color. Therefore, when selecting colorants, the process conditions and color requirements should be carefully compared to ensure that the heat resistance temperature and dispersion characteristics meet the requirements, and it is best to use a wet mixing method. If the injection molding equipment and mold are contaminated by colorants, the hopper, barrel and mold cavity should be thoroughly cleaned.

2. Raw materials do not meet the use requirements

Excessive volatile content in the raw materials, foreign materials or poor drying will lead to uneven color. Uneven fiber distribution or resin loss after fiber-reinforced materials are molded can also cause uneven surface color. In addition, some materials such as high-impact polystyrene and ABS have large internal stress after molding, which may cause stress discoloration. To solve these problems, foreign matter in the raw materials should be removed, pre-dried, and moisture should be reduced; process parameters should be adjusted to improve fiber distribution, reduce the amount of lubricant and release agent; select resins with better crystallization properties or improve the crystallization properties of resins by controlling cooling conditions.

3. Unreasonable molding conditions

The cause of uneven color may be related to molding process conditions. For example, if the color near the feed port or the weld is uneven, it is usually due to uneven distribution of colorants or properties that do not meet the requirements. The discoloration of the entire plastic part may be related to the high barrel temperature, and overheated melts are easy to decompose. In addition, factors such as excessive nozzle temperature, screw speed, injection back pressure, excessive injection pressure, and long injection time may all lead to uneven color. To prevent the molten material from staying at high temperature for too long, the injection volume should be controlled within two-thirds of the capacity of the injection molding machine; if there is accumulation of coked molten material, the barrel and nozzle should be thoroughly cleaned, and the nozzle temperature should be appropriately reduced.

4. Problems with molds for medical injection molding products

The oil, release agent or dirt in the mold mixed with the molten material, poor mold venting and uneven cooling will all cause the surface of the plastic part to change color. Therefore, the mold cavity should be kept clean before injection molding. To reduce the impact of poor venting, the clamping force can be appropriately reduced or the gate can be repositioned, and venting holes can be set at the last filling point. The mold temperature has a significant effect on the crystallinity of the molten material when it solidifies, and the mold should be ensured to be evenly cooled. For example, when molding crystalline plastics such as polyamide, too low a mold temperature will cause the molten material to crystallize slowly and the surface of the plastic part will be transparent; while too high a mold temperature will cause the molten material to crystallize faster and the plastic part will become translucent or milky white. By adjusting the mold and melt temperature, the surface color of the plastic part can be effectively controlled.

Through the above analysis and solutions, the discoloration of injection molded products can be effectively reduced to ensure product quality.