Influence of pellet shape and size on friction of polymer pellets in single screw plasticizing units

The pressure build-up behaviour, the mass flow rate and the temperature development in the feeding zone of single screw extruders are mainly determined by the prevailing frictional conditions. The internal and the external coefficient of friction have to be distinguished. The internal coefficient of friction describes the friction between the polymer pellets themselves. The external coefficient of friction (ECOF) describes the friction between the polymer pellets and the screw or barrel surface. The ECOF of polymer pellets should be rather low at the screw surface and high at the barrel surface to achieve a high mass flow rate and sufficient pressure in the feeding zone of single screw plasticizing units. Depending on the pelletization after compounding different pellet shapes and sizes occur, which affect solids conveying. The influences of pellet shape and size on the external coefficient of friction of polypropylene (PP) pellets are presented in this paper. The ECOF of spherical shaped PP-pellets and two different cylindrical shaped kinds of PP-pellets was investigated. The tribological measurements were carried out as a function of velocity and normal force, using a previously developed screw tribometer [1]. The applied normal forces were 500, 720, 1000 and 1800 N, and the velocity was varied between 0.1 and 1.2 m/s. The measurements were performed at room temperature. The measured ECOF of the PP-pellets was in the range between 0.25 and 0.6. The results show that an increasing velocity leads to an increase in the ECOF of PP-pellets. On the contrary, an increasing pressure leads to a reduction of the ECOF of PP-pellets in all measured cases. The ECOF of the long cylindrical shaped PP-pellets is always higher in comparison to the short cylindrical shaped and the virgin PP-pellets. This observation can be explained by using a contact mechanics model. The contact area between the pellets and the metal surface can be calculated dependent on the applied normal force. An increase in the ECOF with a higher calculated contact area for constant normal force and velocity can be observed.