AbstractDuring a routine extrusion compounding of a polymer blend of polyarylate resin and ethylene/acrylate rubber, an unexpected powdery product instead of a cohesive mixture was obtained. Also, the 20% rubber component was not well dispersed in the final product. This process problem was studied by simulation with a laboratory internal mixer following the procedure discussed by Shih. The powder formation was successfully reproduced. From the results of continuous video monitoring and analyses of the intermediate samples collected during the simulation experiments, we found that the progression of mixing from solid pellets to a cohesive mixture in a batch mixer or a twin‐screw extruder needs to “climb over” a prolonged transition state where the plastic polyarylate was dispersed in the 20% rubber. The finding led to two successful solutions to the powdering problem. A “phase‐inversion” mechanism is proposed to explain the observed morphological changes. This mechanism appears to have wider applicability in the processing of polymer blends. © 1992 John Wil