DescriptionThe surface properties of the extrusion screw are vital for the process, as they influence the mass flow rate and the pressure build-up of the extruder as well as the lifetime of the screw itself. Especially in the solids conveying zone, those properties are of importance as the coefficients of friction on the surfaces of screw and barrel determine the conveying efficiency of the system. In the course of this research, the influence of different screw-surface/polymer pairings on the processing conditions of a smooth barrel single-screw extruder with 20 mm screw diameter is evaluated. For this reason, screws of equal geometry but with diverging surface properties (polished screw steel, TiN-, TiAlN- and CrN coated) are used for processing polymethylmethacrylate (PMMA) and polypropylene (PP). By recording the resulting melt temperature and the pressure at three different positions along the barrel wall as well as the mass flow rate of the extruder, a quantitative comparison can be achieved. Moreover, screw pull tests after the experiments allow surface evaluations and can reveal material depositions. The different screw surfaces are characterized by confocal microscopy to reveal possible differences in the surface roughness and texture. Furthermore, the surface energies are determined using the Owens Wendt Rabel Kaelble method for sessile drops of standard test liquids. In addition, measurements of the external coefficient of friction between the polymeric materials and steel shafts with different surface properties to exhibit possible differences in solids conveying are performed. The tests are conducted on a screw tribometer utilizing various sets of parameters. It can be shown, that the external coefficient of friction depends on the combination of polymeric material and the surface properties of the frictional partner. Not only the surface roughness, but also the choice of the coating plays a role in the development of the frictional coefficient. Those divergences are also reflected in the extrusion experiments, where the mass flow rate as well as the processing pressure depend on the polymer/surface pairing.
|Period||24 Sep 2015|
|Event title||Polymer Processing Society Conference 2015: null|