INFLUENCE OF UPSTREAM INSTABILITIES AND WALL SLIP ON MELT FRACTURE AND SHARKSKIN PHENOMENA DURING SILICONES EXTRUSION THROUGH ORIFICE DIES

The flow of four different linear or branched silicones through thin-walled orifice dies has been visualized. The observation of the extrudate downstream of the orifice has allowed a thorough examination of the appearance and evolution of the various flow regimes. Thus it has been possible to detect very clearly a surface defect appearing in the form of scratches at the free surface, and whose appearance threshold has been thoroughly measured. Depending on the rheology of the fluids considered, these defects may evolve into more severe forms commonly designated by "loss of gloss" or "sharkskin". Moreover, for all the silicones studied, the well-known melt fracture phenomenon has been observed. This defect first appears in the form of a regular helix, and then evolves into a chaotic regime as flow pressure increases. In other respects, the visualization of the upstream flow shows that this remains perfectly stable as long as the rupture regime has not been reached. Thus, it seems that the surface defect, even in its more severely evolved form, namely the sharkskin defect, is an exit phenomenon related to the relaxation of stretch strains at the orifice outlet. On the other hand, instabilities are generated in the upstream region for the flow regime at which melt fracture appears downstream. Consequently, it seems that this phenomenon should be attributed, at least in part, to the unstable phenomena developing in the elongational flow field upstream of the orifice die. This assumption is supported by the fact that experiments performed with various wall material have shown that wall slip was, in any case, unlikely or minor, so that it could not be responsible for the observed phenomena. © 1990.