Surface Discoloration

The color of the part is abnormal or locally yellowed. Discoloration forms include overall color difference, local yellowing, color deepening near the gate, or color streaks appearing along the flow direction. For light-colored and transparent parts, discoloration is particularly obvious. The root cause of discoloration is usually that a component in the material has undergone a chemical change — 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. Material factors: raw material thermal degradation, excessive catalyst causing reaction temperature too high, impurities in raw material, uneven mixing.

1. Process parameter optimization and adjustment: control reaction temperature, reduce catalyst dosage, check raw material purity, improve mixing quality. 2. Systematic troubleshooting method: RIM problem troubleshooting follows the order of "formulation -> mixing -> temperature control -> equipment". First check the two-component ratio and catalyst dosage, then check the mixing head and self-cleaning function, then check the raw material and mold temperature control, and finally check the accuracy of the metering pump and hydraulic system. 3. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluation. For recurring problems, use fishbone diagram or 5Why analysis to trace the root cause, and fix the countermeasures as PFMEA control points.

This problem is a typical manifestation of surface discoloration in the RIM process; the core of the solution lies in ensuring the precise ratio and full mixing of the two components, while controlling the reaction temperature and curing conditions within the optimal window. It is recommended to start troubleshooting from the mixing system and metering accuracy.