Flow Mark / Whitening

White flow marks appear on the surface of the part. Flow marks spread outward from the gate as concentric circles or wavy ring patterns, similar to water surface ripples. The formation of flow marks originates from the "stick-slip" behavior of the melt flow front on the cavity wall — after the front melt contacts the cold mold wall, its viscosity rises and it temporarily stagnates, and the subsequent high-pressure melt pushes it forward again; this repeats to form ripple marks. Although flow marks mainly affect appearance, in severe cases they also imply that the melt front temperature is too low, affecting weld strength.

1. Process parameter factors: unstable flow front of the material in the mold, low mold temperature, injection speed fluctuation, component ratio fluctuation. 2. Mixing quality factors: in the RIM process, the mixing uniformity of the two components is the foundation of all quality; the Reynolds number of the mixing head, the impingement mixing energy, and the residence time distribution are the key engineering parameters determining mixing quality.

1. Process parameter optimization and adjustment: stabilize injection speed, increase mold temperature, calibrate metering pump stability, optimize gate design. 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 flow mark/whitening 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.