Shear rate measures the velocity of a fluid layer passing over another layer of fluid at a different velocity. The frozen plastic material in contact with the cavity wall does not move relative to the wall, which results in a shear rate of zero (0.0 1/sec). The molten plastic material just inside of the frozen layer moves over the frozen material, which yields a positive shear rate (>0.0 1/sec). The shear rate continues to increase until a maximum is reached just inside the wall. Then the shear rate begins to decrease toward the center of flow, reaching a local minimum in the center. This minimum occurs because the polymer chains at the center of the flow move at the same speed and do not move relative to each other, resulting in a zero (0.0 1/sec) shear rate.