In this study, several blends of BPA polycarbonate and acrylonitrile/butadiene/styrene (ABS) resins having different gloss levels and microscopic surface roughness values were injection molded over a range of injection rates. Changes in surface appearance were monitored by visual observation, optical microscopy, and electron microscopy. The onset of unstable flow was marked by the development of regularly spaced surface patches on the molded parts. The surface irregularities were found to be strikingly similar to those produced by dragging crosslinked rubbers across hard surfaces. It was observed that blends having higher surface roughness values had lower coefficients of friction and exhibited unstable flow at lower stresses than compositions with smoother surfaces. The data were used to quantify the rheological parameters and surface characteristics defining an acceptable molding envelope for these materials.