Viscoelastic Instabilities in Injection Molding

Abstract

Industrial processing of polymeric materials is often limited by the occurrence of flow instabilities. This can have severe implications on production rates, as well as on final product properties. Some of the most notorious examples include polymer extrusion, where increasing flow rates can lead to the well known short wave sharkskin instability or when production speeds are increased further, gross melt fracture. Another example can be found in the extrusion of multilayer films, where a jump of the normal stresses and viscosity mismatch allows for spatially growing oscillations of the fluid/fluid interface (see for example [1]). An extensive review on the subject of viscoelastic flow instabilities was written by Larson [2]. In spite of their frequent occurrence, very little is known about these flow phenomena, which are found not only in complex flows but also in simple flows (e.g., Taylor-Couette flow). This chapter is focused on the viscoelastic flow instability during the injection molding process. The occurrence of unstable flows in injection molding can result in specific surface defects that are characterized by alternating shiny and dull bands perpendicular to the flow direction (see Fig. 16.1).