Previously developed equations describing the pressure and discharge rates of Newtonian liquids in the screw-type pump are discussed briefly and are experimentally verified. These equations are then applied to rubberlike materials for the two extreme conditions of extruder operation: Open discharge (with the removed from discharge end of extruder); and zero discharge (with solid plate sealing the discharge end of the extruder). For the condition of closed discharge, calculated pressures agreed with observed pressures. The apparent viscosities of a number of stocks were determined by forcing them through circular orifices at different flow rates and calculating the viscosity from Poiseuille’s equation. The effective shear rate was calculated by using an expression for the average volume weighted rate of shear in circular orifices. The effective shear rate occurring in the extruder was calculated for screws with wide thread troughs from the velocity distribution in the troughs and an expression for the average volume weighted rate of shear in rectangular orifices. An experiment was devised to determine the effective shear rate in the extruder at zero discharge. The result agreed with the calculated value. It is concluded that at zero discharge the mechanism for rubberlike materials is one of viscous shearing and that rubberlike materials behave as viscous but highly thixotropic liquids.