For the implementation of product design by using a virtual die design, it is significant to understand the flow conditions and to be able to predict the flow behaviour accurately. Special rheological flow phenomena in the plastic melt as cross flows, which flow perpendicular to the main flow, should be taken into account. This phenomenon is caused by pressure gradients transverse to the direction of extrusion and both the flow distribution and the pressure consumption are influenced in the die. First, an investigation for the characterization of cross flows was carried out in the slot die. Dead-stop experiments were performed by adding a blowing agent to the extrusion process. Negatives of die with gas-filled bubbles were prepared and evaluated with image analysis software across the half width of die. Cross flows can be deduced in the evaluation of ellipsoid shape according to the flow direction and the alignment according to the flow direction. The cross flow behaviour can´t be described with the simple numerical method a holistic one-dimensional representation (GEB) without cross flows in a spreadsheet program (p.e. Excel). Therefor a linear equation system according to the Gauss algorhythm was developed, which can calculate the cross flows in the die with rectangle cross section. In this network crosslinks are implemented to take into account the cross flows. The equation system is set up from the network, which corresponds to the number of the desired partial volume flows in the number of established equations. Afterwards the die was simulated with the corresponding operating point by Finite Element Method (FEM), calculated by GEB included the equation system, that validated the calculation the cross flow behaviour. Keywords: flat die, extrusion, cross flow. *Corresponding Author: Bianka Henke, Bianka.Henke@ktp.upb.de