Composite models for the calculation of the filling level profiles, the pressure profiles, the melting profiles, the residence time distributions, the temperature profiles, the shear stress profiles, and the power consumption in modular tightly intermeshing co-rotating twin screw extruders (ZSK) are developed. A complex systematic design procedure was compiled, which is explained in part in this paper. The simulation of the intermeshing co-rotating machine involves both screw and kneading disc elements, including left- and right-handed sections. Kneading blocks were approximated by a screw of ＇＇equivalent pitch＇＇ with making allowance for the leakage flow across the flights from one channel to the adjacent channel. The mathematical treatment of co-rotating twin screw extruders has been based up according to the theory of single screw extruders. There was seen to be a good correlation between calculated and experimental results.