Coefficient of Friction between Bulk Polymers and Different Metal Surfaces at High Pressures

In single screw plasticising units the external coefficients of friction (ECOF) between polymer pellets and the surfaces of the inner barrel and the screw are decisive factors for conveying solids and wear. In addition to other parameters, such as screw geometry and processing conditions, they affect the pressure build up behaviour and the temperature development in the solid plug. Screw surface treatments, for instance plasma-nitriding or coatings such as CrN, TiN or TiAlN, can further affect the frictional conditions between the screw surface and the polymer pellets. A new apparatus for analysing the frictional behaviour of polymer pellets on the screw surface, in which the bulk polymer is pressed onto a heated, rotating shaft at a defined normal force and temperature, was previously developed by the author. Utilising this method, the newest results of measurements of the ECOF at room temperature are presented in this paper. Two different, stainless, polished shafts, one without surface treatment and the other one with a plasma-nitride surface treatment were investigated in combination with three types of bulk polymers: lenticular-shaped polypropylene (PP) pellets for extrusion and thermoforming; globular-shaped, toughened, medium viscosity polyoxymethylene (POM) pellets and cylindrical-shaped polymethylmethacrylate (PMMA) pellets. The pressure was varied from 4 to 200 bar and the frictional velocity from 0.1 to 1.2 meters per second. The ECOF of PP increases at rising velocities, except at pressures over 100 bar and velocities beyond 0.8 m/s, these values fall slightly. The ECOF ranges between 0.25 and 0.6 at the untreated surface and a bit lower at the nitrided surface. The ECOF of POM is much smaller compared to PP and varies between 0.12 and 0.15 at the untreated surface and between 0.2 and 0.3 at the nitrided surface. In contrast to PP the ECOF is higher at the nitrided surface. The ECOF of PMMA exceeds that of PP and shows an additional peculiarity. At the untreated surface it shows a maximum of the ECOF at medium velocities. At the nitrided surface there is a strictly monotonic decrease at rising velocities.