COMPUTER-SIMULATION OF CONVENTIONAL AND BARRIER SCREW EXTRUDERS

The single-screw extruder is the single most important processing machine used in the polymer industry. The quality of the final product is very sensitive to screw design and operating conditions, however. The simulation of this process by means of mathematical models has been studied extensively in several research laboratories. Nevertheless, despite the theoretical developments in these models and their complexity, prediction of the overall performance of the extruder and the optimal screw design, with respect to operating conditions, can be improved. As a result, a compreshensive model is developed to be used as a computer-aided design tool for conventional and barrier screws. The model takes into account the three zones of the extruder. The solids conveying zone is analyzed by using a frictional model for pressure calculation and a 2-D thermal analysis to obtain the solid bed temperature profile along this zone. The melting and pumping zones are analyzed by applying the general continuity, mass and energy conservation equations. A finite-difference method is used to solve these equations numerically. A non-Newtonian and non-isothermal polymer melt flow is considered along the screw channel. The leakage flow over the flights is taken into account and the rheological behavior of the polymer melt is described either by a Carreau or by a power-law model. The rheological characterization of the polymers used in the laboratory is obtained by means of Instron and CIL capillary rheometers and by a parallel plate mechanical spectrometer (rheometrics). The solid bed profile, pressure, and polymer temperature profile are calculated along the screw channel. For barrier screw simulation, a new concept of solid bed behavior is introduced to take into account the situation where the melt channel is not completely filled.