Influence of Solid Bulk Properties and Frictional Behavior on Solids Conveying in Single Screw Extruders

The properties of solid bulk materials and their frictional behavior predefine the quality of solids conveying and the mass flow rate of single screw extruders. The apparent bulk density is strongly dependent on thermal and mechanical influences. In this work, the apparent bulk density in dependence on temperature and pressure of two different polypropylene pellet shapes of the same grade is examined. The frictional behavior is studied at different velocities and pressures. Differences in the mass flow rate of the pellet geometries are evaluated on a laboratory-scale smooth barrel single screw extruder. The experiments show a strong dependence of the bulk densities on temperature and pressure. In fact, long cylindrical pellets exhibit a low bulk density at standard conditions which increases above the values of spheroidal virgin pellets at elevated temperature and higher pressure. Furthermore the mass flow rate of the cylindrical pellets is significantly higher in the extrusion experiments.