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Die-flow calculations are quite complex due to the many different shear rates and flow variations that occur in most dies. A similar effect occurs in the screw, as its pumping capability is also related to the shear rate, temperature, and resulting polymer viscosity. The screw characteristic is determined by the difference between the drag flow and pressure flow. The pressure-flow calculation depends on the viscosity in the screw channels. Because the calculations are quite different in the tooling and the screw, there is no parallel path to calculate the net effect, even if the two viscosities were identical. Each one must be dealt with individually.

Since at both ends of the extrusion process the pressure is atmospheric, the pressure buildup through the screw has to equal the pressure drop through the tooling to achieve a balanced system. The screw and tooling therefore interact to determine the actual output. As the pressure drop changes to push polymer through the tooling, the specific output (lb/hr-rev) of the screw is affected in the opposite way by pressure flow in the screw. Pressure flow is a loss of output due to increasing head pressure. What occurs is a sort of induced reverse flow in the screw—or a recirculation of the flow in the screw’s pumping section— much like closing a valve on any fluid pump (and a screw is a spiral pump). The higher the tooling pressure drop, the lower the screw output at a specific screw speed—and conversely for lower die pressure.