Effect of Wall Conditions on PVC-R Co-Rotating Conical Twin-Screw Extrusion

Abstract

The effect of wall conditions on co-rotating conical twin-screw extrusion of rigid polyvinyl chloride (PVC-R) is studied. The relationship between the shear stress at the screw surface and the slip velocity of the flowing melt obeys Navier’s linear law. When the volumetric flow rate is 3.83×10-5 m3/s, the pressure difference between the entrance and exit of the metering section of co-rotating conical twin-screw extruder is calculated under different wall conditions by using the evolution technique in POLYFLOW. The 3D isothermal flow fields of PVC-R are also calculated. The results show that when the slip coefficient is smaller than 104 , the pressure difference is constant, corresponding to the full slip condition. When the slip coefficient is larger than 104 , with the slip coefficient decreasing, the pressure difference, and the gradients of velocity, pressure and shear rate decrease. 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. The dispersive and the distributive mixing of the twin-screw extruder under different slip conditions are also studied. Results show that when the slip coefficient is 107 , it is good for the co-rotating conical twin-screw PVC-R mixing extrusion.