A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

Abstract

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (∼1.9) and low (∼0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.

Rheological properties and melt strength of LDPE during coextrusion process

“…On exiting the die (Point B), the velocities of the melt across the die diameter had to equalize to form a plug flow 1. During the transition of velocity pattern the melt velocity near the wall, usually has relatively high shear rate, had to accelerate to compete with the melt at the center and this could generate a considerably high shear heating and thus affected the bulk viscosity of the melt 20. On the contrary, if the $ {{\dot \gamma }}_{{\rm{core}}} $ was increased, this effect would expect to be very small as the shear rate of the melt at the centre was relatively low.…”

“…1 During the transition of velocity pattern the melt velocity near the wall, usually has relatively high shear rate, had to accelerate to compete with the melt at the center and this could generate a considerably high shear heating and thus affected the bulk viscosity of the melt. 20 On the contrary, if the _ c core was Figure 5 Variations in swelling ratio for coextruded virgin LDPE as a function of _ c core = _ c total at various die temperatures. Figure 6 Flow curves for coextruded virgin LDPE at different shear rate of skin and core layers.…”

Rheological properties, flow visualization and extrudate swell of NR compound by rotating‐die rheometer

“…In single channel die, the extrudate swell ratio of both PS and LLDPE melts in circular flow channel die was greater than that of the slit flow channel, whereas in the dual channel die the slit channel exhibited a higher extrudate swell ratio. The discrepancies in the extrudate-swell results from single and dual dies were caused by differences in the flow patterns in the barrel and die, and a sudden change in the melt velocities flowing from the barrel and into the die to the die exit [12].…”

Magnetic Effects on Extrudate Swell of a Polystyrene Melt in Capillary Extrusion Dies

“…In general, the extrudate swell ratio increases with decrease in the die length and the temperature, and it increases with increase in the shear rate and molecular weight of polymers. In the actual polymer processing, the extrudate swell ratio is controlled by adjusting the extruder screw speed and the barrel and die temperature.A number of theoretical, computational, and experimental studies [1][2][3][4][5][6][7][8][9][10] have been reported on extrudate swell. It was found that the swell was slightly affected by the change in the melt temperature and flow rate but it was strongly affected by the molecular weight distribution.…”

“…3 The use of magnetic fields to the polymer processing has been paid attention recently. 4,8,10 The magnetic fields were found to act on feeble magnetic materials including polymers through the magnetic force and the magnetic torque. 11 It is well known that the liquid crystalline polymers undergo magnetic alignment.…”