Velocity field for polymer melts extrusion using particle image velocimetry: Stable and unstable flow regimes

Abstract

Extrusion defects and flow instabilities are an important limitation in most polymer processing operations. Observation of the fluid kinematics to deduce the dynamical response of the polymer can be very useful to characterise these instabilities and understand the mechanisms involved in their triggering and enhancement. To do so, the flow of two linear polydimethylsiloxanes (PDMS) through an axisymmetric sudden contraction has been studied using pressure drop measurements as well as particles image velocimetry (PIV). This optical method for measuring flow velocities provides quantitative whole-field information even when flow conditions are non-stationary. It was shown that, for the two polymer melts studied, the flow is symmetric under stable conditions whereas non-symmetric and non-stationary flows are obtained under unstable conditions. These instabilities arise in the upstream part of the contraction. The extruded rod is then excited by these instabilities which generates the phenomenon of melt fracture at the die exit. In addition, the flow is found to be purely elongationnal on the axis during stable and unstable flow regimes. Under the experimental conditions investigated here, the resulting stretch rate on the centreline varies as x −3, x being the distance above the contraction plane. Moreover flow through contractions becomes unstable first around the centreline and close to the orifice plane. The instabilities then develop in the radial direction and invade progressively the entire upstream flow which completely looses its properties of symmetry: the flow is expected to have a spiralling motion and its two-dimensional (2D) projection looks like a knitting system. The frequency of the instabilities remains approximately the same while their amplitude increases with flow regimes. On the flow axis, instantaneous fluctuations of the velocity can reach ±30% of a mean value characteristic of a mean flow. These fluctuations are more important on each side of the axis where they can reach ±105% of a mean value.