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Microstructure of weld-lines in injection-molded polypropylene was characterized using polarized laser Raman spectroscopy, micro-cutting analysis, and fracture testing. A coarse-grained molecular dynamics simulation was conducted to understand the structure–property relationship. The effect of molecular weight and molecular weight distribution on the weld-line structure and mechanical properties were investigated. Property reductions induced by weld-lines were evaluated by the double-edge-notched tensile method as a property ratio for a stress intensity factor of welded specimen to nonwelded one. It was revealed that the significant reduction induced by the weld-line was caused by increasing molecular weight or broadening molecular weight distribution. Using polarized laser Raman spectroscopy, it was observed that molecular orientation in the weld-line was not a dominant cause of the property reduction within these materials. The micro-cutting analysis revealed that the property reduction was closely related to the reduction of the shear strength observed in the area below the surface V-notch. Based on the coarse-grained molecular dynamics simulation, the reduction of the mechanical property in broader molecular weight distribution polypropylene was considered to be responsible for a lack of entanglement owing to the chain segregation of low-molecular-weight molecules in the weld-line interface. POLYM. ENG. SCI., 53:2336–2344, 2013. © 2013 Society of Plastics Engineers