The velocity and pressure field that forms within the gap of a calender is numerically calculated. The numerical calculations are based on a decoupled calculation method for the free surface of the bank. The transport equations for mass and momentum are solved numerically. The position of the free surface of the bank is determined by shifting it until it matches a streamline. The resulting three-dimensional velocity field gives the vortex patterns within the volume of the bank for fluids with non-Newtonian and Newtonian rheological behavior. The shape of the free surface of the bank and its position are determined by the gap height, the circumferential speed of the rolls, the speed ratio, the feed mass flow into the gap and the rheological behavior of the polymer. Additionally, there are special requirements to be considered that determine the product quality, such as the thickness of the produced sheet, its uniformity and surface appearance, air inclusions and the mixing of the polymer. The calendering process itself requires limited operating windows for the variation of the process parameters in order to meet the product quality requirements. These operating windows are discussed and graphically plotted. (C) 2004 Society of Plastics Engineers.