A composite model simulation of solids conveying, melting and melt flow in a modular co-rotating twin screw extruder has been developed. This is based on combining existing melt conveying models with new melting and solids conveying models for the various elements of a modular twin screw extruder. Computations are made for axial fill factor, pressure, temperature, melting profiles and also the power consumption, torque, specific energy consumption and average residence times. The results are compared with experiment.