Quantitative Measurement of Fiber Orientation and Fracture, Void and Weld-Lines in Short Fiber Reinforced Thermoplastic Composites

Mechanical properties of short fiber reinforced thermoplastics depend on fiber length and fiber orientation distribution produced during melt flow processing. The incorporation of short fibers into thermoplastic polymer melts increases their viscosity and the presence of fibers, increasing in length, results also in an increasing viscosity. Breakage of glass fibers during flow through a rheometer, also a serious problem in melt processing of short fiber reinforced thermoplastics, is demonstrated and analyzed. Short fibers greatly improve the mechanical properties of polymers, but may severely weaken molded parts at their weld-line. A quantitative determination of the glass fiber ori entation distribution in the weld-line region was done by analyzing photomicrographs of polished sections. Fiber orientation parallel to the weld-line, normal to the flow direction, causes a dramatic loss of tensile strength. Another factor playing an important role in fiber-filled thermoplastics is the associated void content. Unexpectedly high void volume fractions were estimated from density results. Reflected light micrographs of polished extrudates confirmed the existence of voids and their increasing content with increasing shear rate, fiber loading, fiber length and temperature.