Surface defects in the injection molding of acrylonitrile-styrene-acrylate (ASA) weatherable polymers, such as ＇＇chevrons＇＇, ＇＇non-uniform haze＇＇ and ＇＇step defect＇＇, are examined. A criterion is proposed based on a flow instability mechanism which suggests that recoverable shear strain, shear stress, and coefficient of friction are the key factors controlling the formation of surface defects. The criterion is supported by various experiments.