This paper presents a mathematical-physical model for tightly intermeshing co-rotating twin screw extruders which describes the throughput and pressure characteristics of these extruders in the sections filled with molten plastic. With the equations that have been compiled, it is possible to calculate pressure profiles and back-pressure lengths for individual sections of isothermal and two-dimensional flow with non-Newtonian melts, making allowance for the screw geometry and for the parameters of relevance to the process. A comparison of the results supplied by the formulated theory and those obtained from the experiment shows that good computed results are obtained for various materials and different points of operation, although no allowance is made for the leakage flow that occurs in reality.