Spurt in the Extrusion of Polymeric Melts: Discrete Models for Relaxation Oscillations

Abstract

In the extrusion of polymer melts, several types of flow instability can occur. One example of this is spurt. Spurt is manifested by periodic oscillations in the pressure and volumetric flow rate. These oscillations are of relaxation type. An extrusion through a cylindrical die is considered. A discrete model to describe spurt or relaxation oscillations is constructed. This model is based on observations from three-dimensional theory. When spurt occurs, the shear rates very near the wall of the die (i.e., in the spurt layer) are much higher than those in the kernel of the extruded polymeric melt. Therefore, the viscosity in the spurt layer is taken much smaller than in the kernel. In both regions a linear Newtonian fluid model is used. A no-slip boundary condition at the wall is maintained. The model developed here is compared to an analogous model, allowing for slip at the wall of the die. It is shown that corresponding results can be obtained from both models. Application of the model to a piston-driven extrusion flow shows the occurrence of spurt oscillations for a restricted range of prescribèd inlet flow rates. The found oscillations are qualitatively in correspondence with experimental results.