Influence of raw material shape on Single Screw Extruder Performance

In this paper, the influence of the raw material shape considering shredded film, cylindrical and spheroidal polypropylene pellets on the extrusion process is investigated. It is shown that the mass flow rate and the pressure build up behavior of a single screw extruder (Measuring extruder type Dr. Collin E20M) 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 strongly influenced by the external coefficient of friction and by the bulk density in the screw channel. A previous developed tribometer is used to study these parameters at pressures and velocities which are comparable to the extrusion process. The results of this model experiment show clear differences in the frictional behavior and in the density of the different raw material geometries. The density is investigated as a function of pressure and temperature. A higher density usually leads to a higher mass flow rate and a better pressure buildup behavior in a single screw extruder. The external coefficient of friction is depending on pressure, velocity, the temperature and on the frictional distance. To achieve a good pressure buildup behavior, the frictional coefficient between the polymeric material and the barrel surface should be high, and that between the polymeric material and the screw surface should be low.