The elementary processes during heat transfer in screw extruders are described by a simulation model. The balance equations are solved numerically with a personal computer. By contrast with known computational approaches, consideration is given to the non-stationary processes in the melt layer between the screw flight and the barrel wall. This is guaranteed by incorporating the ＇＇pulsating shear＇＇ in this layer into the model as a dissipative energy source. The computational results are used to demonstrate, in an exemplary fashion for one extruder series, the influence of all the relevant machine and process parameters on the internal heat transfer.