Product discoloration

The product color is abnormal or locally discolored. Discoloration forms include overall color difference, local yellowing, color darkening near the gate, or color streaks along the flow direction. For light-colored and transparent products, discoloration is particularly obvious. The root cause of discoloration is usually a chemical change in a component of the material——thermal degradation breaks molecular chains to form chromophoric groups, oxidation reactions generate quinone-type yellow substances, or pigments decompose and lose color at high temperatures. Discoloration is not only an appearance problem, but also an external sign of material performance deterioration.

1. Mold design factors: mold temperature too high causing thermal degradation of the material, material residence time too long, shear overheating at the gate, poor thermal stability of the pigment in the material. 2. Feeding factor: The volume of the feeding chamber, the feeding amount and the preheating state of the material directly determine the transfer pressure and filling effect, and are key variables unique to transfer molding.

1. Process parameter optimization and adjustment: lower the mold temperature, reduce material residence, enlarge the gate to reduce shear, select thermally stable pigments. 2. Systematic troubleshooting method: Troubleshooting of transfer molding problems follows the order of 'runner -> feeding -> parameters -> mold'. First check whether the runners and gates are blocked or worn, then verify the feeding amount and material preheating, then adjust the transfer pressure and speed, and finally check the mold venting and temperature uniformity. 3. Preventive management: Establish standardized process parameter cards and quality control limits for each set of mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify the countermeasures as PFMEA control points.

This problem is a typical manifestation of product discoloration in transfer molding; the core of the solution lies in the rational design of the runner and gate system, the effective transmission of transfer pressure, and the uniform control of mold temperature. It is recommended to start troubleshooting from the runner system and transfer parameters.