Modeling of Solid Conveying Pressure Throughput Behavior of Single Screw Smooth Barrel Extruders under Consideration of Backpressure and High Screw Speeds

Abstract

The task of the solid conveying section is to collect the entering material and transfer it to the melt dominated sections. Therefore, its upstream interface is the hopper and its downstream interface is the section in which the first melting occurs. Beneath the hopper opening, the screw rotation collects and conveys the material forward. It is hereby assumed that the friction forces among the particles are higher than the transferring forces on the screw and barrel. For that reason commonly known models for describing the solid conveying in single screw extruders assume that the screw channel is always completely filled so that a plug flow occurs. At high screw speeds, however, these assumptions are no longer valid. To analyze and mathematically describe the solid conveying behavior at screw speeds up to 2000 rpm, simulations based on the Discrete Element Method have been executed in previous investigations. Based on this, a model for describing the solid conveying behavior was developed which enables a variation of general screw and hopper geometry parameters but neglects the backpressure that inhibits the solid movement. For that reason, further investigations based on the Discrete Element Method are presented in this paper, which enable a consideration of the backpressure and pressure build-up in the feeding zone. Astonishingly, at high screw speeds a low filling degree in the screw channel was observed which did not fulfill the expectations of an always fully filled channel due to a backpressure forced particle backlog. Based on the investigations, a mathematical model has been developed which now fully comprehends the effects occurring in the solid conveying section. The conducted simulations were subsequently validated by experimental investigations. Therefore, a test bench was designed which enables solid conveying analysis with adjustable backpressures. The results indicate a good agreement with the formulated model which now enables the calculation of solid conveying zones with consideration of the pressure build-up and filling degrees.