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

The how behavior of a low-density-polyethylene melt in a 14:1 planar contra ction 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. Tw o 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 e ntry plane. Under stable flow conditions there is no material exchange betw een the vortices and the primary flow. It is shown that very small velociti es within the vortices can be measured accurately. In the case of the obliq ue entry die no vortices are found. By the normalization of the velocity pr ofiles with the average velocity in the die, calculated independently from the mass flow rate, an output-invariant presentation is found for the appar ent 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 ve locity field within the die is about 15 times the height of the slit. Due t o 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 de monstrated by the comparison of the strain rate tensor components that the elongational deformation in the center plane of the flow channel is planar. (C) 1999 The Society of Rheology. [S0148-6055(99)02106-9].