Dynamic shear in the axial direction of a rotor was vertically superposed on the melt flow direction, and its effects on the shear rate and melt strength were investigated theoretically. Polypropylene/high-density polyethylene blends were microcellularly foamed with different vibration parameters. The experimental results were compared with those of a theoretical analysis, and the effects of dynamic shear on the foamability and ultimate cell structure were analyzed in detail. The theoretical results showed that the shear rate and melt strength increased with an increase in the vibration amplitude and frequency. The enhanced melt strength could effectively restrict cell growth, prevent cell rupture, and improve foamability. The experimental results showed that the cell orientation decreased and the cell structure was improved when axial dynamic shear induced by rotor vibrations was superposed on the melt flow direction. Furthermore, the cell diameter decreased and the cell density increased with increases in the vibration amplitude and frequency. The experimental results were very consistent with the theoretical analysis. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1320-1328, 2009