Performance of gear pumps in single screw plasticating extrusion process

Gear pumps are essentially positive-displacement devices. Retrofitting gear pumps to plasticating extruders shifted the pressurizing and metering functions from extruders to gear pumps. As a result, gear pumps improved productivity without higher melt temperature, and greatly reduced surges and fluctuations in the extrusion rate. It has been shown that gear pumps equipped with a pump inlet pressure controller delivered stable output most effectively. Melt temperature fluctuations in the machine direction were stabilized because of the high surface to volume ratio of the gear teeth cavities. Overall power economy can be improved by shifting the screen pack to the gear pump discharge side though the hydraulic efficiency of gear pumps were in the range of 0.30-0.43. Gear pumps did not change the residence time distribution function of the single screw extruder, and a plug flow in gear pump is suggested. Unlike conventional extrusion, material residence time can be unaffected by die pressure requirement in gear pump assisted extrusion. Radial mixing by gear pumps was observed but it was not intensive enough to change the melt temperature profiles in the radial direction. No significant molecular size reduction of the polymer by gear pumps was found.