Weld-line characteristics in short fiber reinforced thermoplastics

Fibers can greatly improve the mechanical properties of polymers but may also severely weaken molded parts at their weld‐line compared to their bulk strength. The tensile properties and fiber orientation of injection and compression molded fiber reinforced Noryl and polypropylene samples with and without a weld‐zone were studied. Distinct differences in structure and mechanical properties of weld‐containing and weld‐free samples were identified. In unfilled Noryl and unfilled polypropylene, the presence of a weld‐line was found to only have a small effect on the tensile strength and modulus, while in the corresponding fiber reinforced systems, orientation of the fibrous reinforcement parallel to the weld‐line caused a significant reduction of the tensile strength compared to the weld‐free products. The strength ratio of welded and unwelded specimens was found to decrease with increasing fiber concentration. Quantitative determination of the glass fiber orientation distribution within the weld‐line region and in the bulk was carried out by analyzing photomicrographs of polished sections at desired locations.