Friction of Polymer Pellets at Screw Surfaces: A new apparatus for measuring the frictional behaviour of polymer pellets at screw surfaces

The external coefficients of friction of the polymer pellets at the inner barrel surface, as well as at the screw surface are decisive factors for solids conveying in single screw plasticizing units. Besides 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, such as plasma-nitriding, CrN, TiN and TiAlN coatings, additionally affect the frictional conditions between the screw surface and the polymer pellets. A new apparatus for analyzing the frictional behaviour of polymer pellets at the screw surface is presented in this paper. In this testing method the pellets are pressed onto a heated, rotating shaft at a defined pressure. The induced frictional force is precisely measured with a load cell. Due to the apparatus’ construction, measurement errors caused by friction in the bearings, thermal strains, and forces caused by the contact device can be avoided. This allows for the accurate measurement and regulation of the pressure applied to the pellets, the frictional speed and the temperature at the screw surface. The shaft and the pellets can be changed easily, which allows various plastics and shafts with different surfaces and coatings to be used. Furthermore, the temperature of the heated shaft, the frictional speed and the pressure applied to the pellets and the shaft can be widely varied to simulate real processing conditions. First results of the external coefficient of friction, dependent upon pressure, temperature and velocity, at different treated and coated surfaces are presented in this paper.