Simulation of 3D flow field of RPVC in twin-screw extrusion under wall slip conditions

Three-dimensional isothermal flow fields of rigid polyvinyl chloride are calculated under different wall slip conditions in the metering section of the twin-screw extruder by using the evolution technique in POLYFLOW. The relationship between the shear stress at the screw wall and the slip velocity of the flowing melt obeys Navier's linear law. Calculations show that the differences in velocity, pressure, shear rate, and viscosity between partial-slip condition and full-slip condition are so small. Then, only the results of full-slip and no-slip conditions were presented in the paper. The results show, when the slip coefficient is smaller than the critical value, the pressure difference between the entrance and exit of the melting section of the twin-screw extruder and the volumetric flow rate are constant, corresponding to the full slip condition. When the slip coefficient exceeds the critical value, with the slip coefficient increasing, the pressure difference and the gradients of velocity and pressure increase. The residual stress of the product is thus reduced. Therefore, increasing wall slip is good for the stability of polymer extrusion and the product quality. By using the particle tracking analysis method, the dispersive and the distributive mixing of the twin-screw extruder under full slip and no slip conditions are also studied. Results show that the dispersive mixing performance of the extruder is better under no-slip conditions than under full-slip conditions, but the distributive mixing performance is better under slip conditions than no-slip conditions. The slip at the wall is good for the extrusion of heat-sensitive materials.