Modelling the Solids Throughput of Single Screw Smooth Barrel Extruders as a Function of the Feed Section Parameters

Abstract

In a smooth barrel extruder, the throughput is generally dominated by the metering sections pumping ability. Within the feed zone, more granules can always be conveyed than can melt in the compression zone and be discharged in the metering zone. At high screw speeds, this effect will only occur if the granules flow quickly enough out of the hopper into the screw channel and this is fully filled. In this case, the solids throughput up to maximum screw speed shows an approximately linear relationship to the speed. These linear solids conveying characteristics necessitate constructional changes to the hopper opening. In this article, solids conveying processes with approximately linear conveying characteristics up to speeds of 2,000 min−1 (peripheral velocities up to 3 m/s) are simulated with various feed hopper geometries. The simulations are carried out by means of DEM (Discrete Element Method) in which the granules are approximated by single spherical particles. From the simulation results, via dimensional analysis a model is derived for describing the throughput as a function of the geometry and process parameters of the solids feed zone.

Experimental analysis of the influence of pellet shape on single screw extrusion

“…Although the apparent bulk density of the long cylinders is the lowest, the results of the extrusion experiments show that the long cylindrical pellets exhibit a higher mass flow rate than the other two pellet geometries in all the experiments. There is a decline of the specific output with rising screw speed that was also reported by Wortberg and Rahal and by Lessmann et al . This can be seen for all pellet geometries in this experiment.…”

“…The models conclude that there is a higher possible output with a higher ratio between the external coefficients of friction of the pellets on the barrel and the screw surface. More recent works deal with discrete polymer pellets using the discrete element method …”

Fundamentals of Global Modeling for Polymer Extrusion

“…In addition, several studies were performed in which the material data for the solid conveying section were no longer assumed to be constant, e.g., by Spalding and Hyun [42]. The research on solid conveying was recently reviewed in detail by Schöppner et al [43,44].…”

“…A further description of the solid conveying section based on the discrete element method (DEM) DEM was performed by Schöppner et al [43,44] who developed the model, which fully comprehends the effects occurring in the solid conveying section and enables the calculation of a solid conveying zone with consideration of the pressure build-up and filling degree.…”

Experimental study of the influences of the pellet shape on the bulk density and the frictional behavior of polypropylene

“…There is a clear drop in pressure at the end of the solids conveying zone when accelerating the screw while processing the long cylinders. This might be due to the fact that the feeding behavior works better at low screw speeds, which was also found by Wortberg and Rahal and Lessmann et al 14,25,32 This would also explain the higher mass flow rate increase at lower screw speeds in comparison with the virgin pellets. Another reason for this could be that the faster movement of the screw leads to a higher compression of the virgin pellets (as shown in Figure 6) that could not be achieved with the long cylindrical pellets.…”