Influence of raw material shape on single screw extruder performance

In this paper, the influence of the raw material shape considering shredded film (which often occurs in recycling), cylindrical and spheroidal shaped polypropylene pellets on the extrusion process is investigated. It is shown that the mass flow rate and the pressure buildup behavior of a lab-scale single screw extruder with a diameter of 20 mm is strongly influenced by the shape of the raw material. Detailed analysis revealed that the differences are mainly affected by the solids conveying zone. The behavior in this zone is altered by the external coefficient of friction and by the bulk density in the screw channel. A previously developed tribometer is used to study the external coefficient of friction at pressures temperatures and velocities which are comparable to the extrusion process. The model experiments show clear differences in the frictional behavior of the different raw material geometries. The extrusion experiments reveal that, compared to rather spheroidal virgin material, a specific cylindrical geometry can lead to clearly increased throughput and pressure buildup, while shredded film leads to comparatively low throughput.