SCREW PLASTICATING OF DISCONTINUOUS FIBER FILLED THERMOPLASTICS - MECHANISMS AND PREVENTION OF FIBER ATTRITION

Processing of fiber compounds in single-screw plasticating units can c ause fiber fracture. Attempts have been made to obtain large residual fiber length in finished components. Therefore, pultrusion compounded materials for extrusion and injection molding have been developed. Pel lets chopped from pultruded strands, containing perfectly wetted fiber s of uniform length, eliminate the need for the mixing operation in an extrusion compounder. The attempt to preserve the initial fiber lengt h during processing raised interest in the origin of fiber attrition a nd in optimized processing parameters. Based upon the knowledge that f iber length breakdown takes place mainly during the process of screw p lasticating, extrusion trials have been carried out under varied condi tions. Fiber length distributions and fiber movement within the cross sections of the screw channel along the screw axis have been investiga ted. It has been shown that screw speed, pressure and barrel temperatu re profile, in dependence on screw geometry, show a distinct influence on the residual fiber length. This information provides a description of the prevailing fiber attrition mechanisms and helps define the mos t suitable hardware and the corresponding adjustment of process parame ters to preserve large residual fiber length.