Extrusion of glass melts - Influence of wall friction effects on the die swell phenomenon

Abstract

During the extrusion of glass melts the states "complete wall sticking" and "frictionless wall slipping" are two borderline cases for the flow conditions inside the die channel. Wall sticking leads to an increase of the cross-sectional area of the extruded rod of about 28 %, while for wall slipping the die swell becomes zero. Moreover, there exist special die materials which under certain experimental conditions cause a local shpping depending on the friction of the glass melt along the die wall. In this case Coulomb's law is applied to describe the flow behaviour. Die materials with a high nickel content favour such a wall-gliding behaviour. This is accompanied by a decrease of the die swell phenomenon together with an increase of the precision of shape and dimensional accuracy of the extruded rods. Theoretical considerations as well as experimental results suggest the hypothesis, that the wall slipping of a glass melt only takes place within a specific region of the die channel immediately in front of the die exit. By using die materials with wall-slppping behaviour a significant reduction in the die swell can be obtained, as the flattening of the inhomogeneous velocity profile already occurs inside the die channel. On the other hand, it could also be shown, both by experiments as well as by numerical simulation, that a diminution of the wall friction by shortening the channel length up to the knife-edge die does not effect the complete removal of the die swell phenomenon.