The mathematical model for the shear stress in a polymer melt within a capillary was set up by superimposing a sine vibration of small amplitude in parallel with the extruding direction of the polymer melt. By making use of experimental equipment of the constant velocity type dynamic rheometer of capillary (CVDRC), designed by the authors, the steps required to calculate the shear stress in the polymer melt at the inside wall of the capillary under a vibration force field were also established. By measuring the instantaneous data of capillary entry pressure, capillary volume rate and their phase-difference under a superimposed vibration, the shear stress of the polymer melt at the wall of the capillary can be calculated.
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