Tensile stress-strain behavior coupled with fractography was used to investigate the weld-line strength of an injection molded 40？w% talc-filled polypropylene. The relationship between processing conditions, microstructure, and tensile strength was established. Fracture surface of the weld line exhibited skin-core morphology with different degrees of talc particle orientations in the core and in the skin. Experimental results also showed that the thickness of the core decreased and the thickness of the skins increased with increasing melt temperature and increasing hold pressure, which resulted in an increase of yield strength and yield strain with increasing melt temperature and increasing hold pressure. Finally, a three-parameter nonlinear constitutive model was developed to describe the strain softening behavior of the weld-line strength of talc-filled polypropylene. The parameters in this model are the modulus E, the strain exponent m, and the compliance factor β. The simulated stress-strain curves from the model are in good agreement with the test data, and both m and β are functions of skin-core thickness ratio. 1. Introduction Injection molding is one of the most common manufacturing processes in the polymer industry owing to its versatility, high production rate, short cycle time, and low percentage of scrap. In addition, this process can be used for a large variety of thermoplastic polymers. One of the defects observed in injection molded parts is weld line, which is formed when two or more separate melt fronts traveling from different directions meet and join as the mold cavity is filled [1]. This happens in multigated molds but can also occur when a melt front is divided by inserts used in the mold to create holes or other openings in injection-molded parts. Since weld lines cause reduction in mechanical properties and visual defects, many studies have been conducted to explain the weakness at the weld line. For example, Mielewski et al. [2] have investigated the weld-line morphology of injection molded polypropylene. Tomari et al. [3] have reported V-notch at weld lines in polystyrene injection moldings. Fellahi et al. [4] investigated the morphology of the weld region in injection-molded high-density polyethylene/polyamide-6 blends. In the homopolymer system, the weakness of weld line can be explained by incomplete bonding due to inefficient molecular entanglement at the interface, disturbance of molecular orientation parallel to the flow direction, inefficient diffusion time, existence of voids, and V-shape notch owing to entrapped air or
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