Entry flow of a low-density-polyethylene melt into a slit die: An experimental study by laser-Doppler velocimetry

Abstract

The flow behavior of a low-density-polyethylene melt in a 14:1 planar contraction was investigated by laser-Doppler velocimetry. The velocity field in the central plane of the flow channel is composed of the measurement of the velocity components in and perpendicular to the direction of extrusion. Two dies differing in their entrance angles are compared. In the case of the flat entry die large recirculating vortices are found in front of the die entry plane. Under stable flow conditions there is no material exchange between the vortices and the primary flow. It is shown that very small velocities within the vortices can be measured accurately. In the case of the oblique entry die no vortices are found. By the normalization of the velocity profiles with the average velocity in the die, calculated independently from the mass flow rate, an output-invariant presentation is found for the apparent shear rate range from 53 to 182 s−1. The velocity distribution along the centerline of the flow channel exhibits a pronounced velocity overshoot shortly after the die entry plane. The distance up to a fully developed velocity field within the die is about 15 times the height of the slit. Due to the acceleration of the melt in front of the die entry large elongational deformations occur. The resulting maximum elongation rates (20 s−1) are very high compared to those achievable by elongational rheometers. It is demonstrated by the comparison of the strain rate tensor components that the elongational deformation in the center plane of the flow channel is planar.