Abstract
Long-fiber thermoplastic (LFT) composites consist of an engineering thermoplastic matrix with glass
or carbon reinforcing fibers that are initially 10 to 13 mm long. When an LFT is injection molded, flow
during mold filling orients the fibers and degrades the fiber length. Fiber orientation models for injection
molding are well developed, and special orientation models for LFTs have been developed. Here we
present a detailed quantitative model for fiber length attrition in a flowing fiber suspension. The model
tracks a discrete fiber length distribution (FLD) at each spatial node. Key equations are a conservation
equation for total fiber length, and a breakage rate equation. The breakage rate is based on buckling
of fibers due to hydrodynamic forces, when the fibers are in unfavorable orientations. The FLD model
is combined with a mold filling simulation to predict spatial and temporal variations in fiber length
distribution in a mold cavity during filling. The predictions compare well to experiments on a glassfiber/
PP LFT molding. Fiber length distributions predicted by the model are easily incorporated into
micromechanics models to predict the stress-strain behavior of molded LFT materials.
Author to whom correspondence should be addressed; electronic mail: ctucker@illinois.edu
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