RHEOLOGY OF LAYERED THERMOPLASTIC MATRIX COMPOSITES DURING COMPRESSION MOLDING

In view of optimizing the industrial compression molding process of th ermoplastic composites, the rheological and microstructural behavior o f a polypropylene/glass fiber composite is investigated in model squee ze-flow geometries. The overall stress/strain behavior of the material at various compression rates is recorded and the induced orientation of the fibers is investigated by means of a special electron microscop ic characterization method. By contrast to pure polypropylene, it is s hown that in the high speed range, the macroscopic flow process is con trolled by both the viscous extension and the relative sliding of para llel fibrous layers, the latter becoming unstable when the flow underg oes a rapid transition from divergent to convergent. Under high pressu re, voids are dissolved in the polymer melt. In the case of non-isothe rmal compression, the rheology of the composite is not significantly a ffected by the cooling of the surface layers. The multilayer plug-flow model based on the sliding mechanism of viscous layers is found to re produce correctly the experimental stress/strain behavior.