Review of Existing Models for Boundary Conditions and Instability Mechanisms

In this report a concise review is given of the literature on slip phenomena at the wall of extrusion devices and the role of slip-laws in modeling of extrusion instabilities. In section 2 the main three instabilities are discussed: spurt, in which big pressure oscillations occur and the extrudate leaves the die in an oscillating way, sharkskin, which leads to more or less regular surface defects, and gross melt fracture, which refers to volume distortions of the extrudate. These instabilities have different spatial origins. The spurt defect is due to stick-slip behaviour near the die wall. That is why the development of a slip law describing the interaction with the wall is most important for this defect. The relation between slip at the wall and sharkskin and gross melt fracture is more indirect. In section 3 a review is presented of the literature on the modelling of slip phenomena. In section 4 it is concluded that considerable progress is made in the last years, but the resulting insights still have to be incorporated in models describing the full extrusion system. Then, it can be determined whether the theoretically developed slip-laws indeed give rise to spurt oscillations. If this is the case, these models have to be used as a tool to predict the occurrence of spurt. Models describing sharkskin and gross melt fracture are still lacking and have to be developed ab-initio.