Closely intermeshing, co-rotating, twin-screw extruders are increasingly being used for polymer preparation and reactive extrusion. For a theoretical description of these processes, not only the mechanism of reaction kinetics but also aspects of heat transfer and in particular the conveying behaviour of modular twin-screw extruders must be taken into account. Using practical examples, this work sets out to evaluate the possibility of predicting process parameters such as melt temperature, drive power, degree of filling and pressure build-up as functions of screw geometry. To this end, trials with two twin-screw extruders of different size were carried out.