Specimens with weld lines were produced via conventional and film insert molding techniques using two types of materials as the substrate resin, i.e. a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blend and glass fiber-filled polycarbonate (PC-gf). The formation and morphology of the weld line region was assessed with and without the presence of 0.5-mm-thick PC film inserts. The weld line formation and characteristics were found to be dependent on the extent of interaction between the injected resin and the mold surface or the film insert. Better interfacial interaction between the substrate and film led to the distortion of the weld line orientation, which significantly enhanced the mechanical properties of the weld line. The incorporation of glass fibers into the substrate resin would usually reduce the resistance of the weld line towards tensile, flexural and impact loadings. However, with the attachment of film inserts, the mechanical properties of the weld line region have significantly improved, even with the presence of rigid fibers. Upon examination of tensile and impact fracture surfaces of film insert specimens, a unique orientation of fibers across the weld line (parallel to the flow direction and perpendicular to the weld line) could be observed at regions directly under the film. The combination of favorable properties from the unique fiber orientation and distortion of the weld line, as well as the ability of the film to effectively dissipate forces towards a larger area, have synergistically contributed towards the mechanical property enhancement of the weld line region in film insert moldings. POLYM. ENG. SCI., 48:21472158, 2008. (C) 2008 Society of Plastics Engineers