Rheology of Layered Thermoplastic Matrix Composites during Compression Molding

Abstract

Abstract In view of optimizing the industrial compression molding process of thermoplastic composites, the rheological and microstructural behavior of a polypropylene/glass fiber composite is investigated in model squeeze-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 microscopic characterization method. By contrast to pure polypropylene, it is shown that in the high speed range, the macroscopic flow process is controlled by both the viscous extension and the relative sliding of parallel fibrous layers, the latter becoming unstable when the flow undergoes a rapid transition from divergent to convergent. Under high pressure, voids are dissolved in the polymer melt. In the case of non-isothermal compression, the rheology of the composite is not significantly affected by the cooling of the surface layers. The multilayer plug-flow model based on the sliding mechanism of viscous layers is found to reproduce correctly the experimental stress/strain behavior.