Screw plasticating of discontinuous fiber filled thermoplastic: Mechanisms and prevention of fiber attrition

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Abstract
Processing of fiber compounds in single‐screw plasticating units can cause 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. Pellets chopped from pultruded strands, containing perfectly wetted fibers of uniform length, eliminate the need for the mixing operation in an extrusion compounder. The attempt to preserve the initial fiber length during processing reised interest in the origin of fiber attrition and in optimized processing parameters. Based upon the knowledge that fiber length breakdown takes place mainly during the process of screw plasticating, extrusion trials have been carried out under varied conditions. Fiber length distrubutions and fiber movement within the cross sections of the screw channel along the screw axis have been investigated. It has been shown that screw speed. Pressure and barrel temperature 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 most suitable hardware and the corresponding adjustment of process parameters to preserve large residual fiber length.