Extrusion defects and flow instabilities are an important limitation in most product processing operations. Investigation of the fluid kinematics to deduce the dynamical response of the fluid can be very useful to characterize these instabilities and understand the mechanisms involved in their triggering and enhancement. To do so, the flow of a Newtonian fluid through a screw press has been studied in order to predict the downstream velocity field in extrusion process. Flow in the screw press is modeled by adapting the classical theory of single screw extrusion. Some publications have addressed this subject matter, especially in the nineties. However, most of these publications develop models that are not so easy to apply. To complement those earlier works, an approximate model base on finite-difference method is developed. The models in this paper may be considered as of pedagogical value because of the ease of their application; nonetheless, results are obtained basing on the real extruding conditions, such as a general screw geometry. In order to achieve this pedagogical purpose, three important properties pertaining to model building are considered in this article. These properties are applicability, simplicity, and level of technique applied. Moreover, the models introduced in this paper are straightforward application by both practitioners and students. The ease of the applicability is confirmed by the suitability to adopt a widely employed software, such as Microsoft Excel, to implement the model developed. (c) 2007 Elsevier Ltd. All rights reserved.