The effects of processing parameters on the tensile properties of weld lines in injection molded thermoplastics

Abstract Weld or knit lines result wherever two or more polymer flow fronts unite. This results in a region of a different level of molecular entanglements than the bulk material. Consequently, weld regions have been observed to have inferior mechanical properties compared to the bulk. Although this phenomenon occurs in almost all the commercially important polymer processes, there has been little systematic investigation. The effects of melt temperature, mold temperature, injection speed and injection pressure on the tensile properties of commercial grades of polystyrene (GPS), high impact polystyrene (HIPS) and polypropylene (PP) are examined. The most important processing parameters seemed to be melt and mold temperature; injection speed and pressure had little effect on the tensile properties of any of the samples. A higher melt temperature increased both the strain and stress at break considerably in GPS. In HIPS increased melt temperature increased only the elongation to break substantially. Increased mold temperature improved the stress and elongation to break in GPS but not as much as melt temperature. Polypropylene showed improved weld yield strength with increased mold temperature. Under the conditions examined, injection pressure and injection speed showed no effect on the tensile properties of any of the materials investigated.