Original article Weld line strength factors in a reinforced injection molded part: Relationship with predicted fiber orientation Maria C Quintana and Patricia Frontini Abstract In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line— was evaluated. Injected plates were generated using a double-gated mold under four different process conditions. Quantification of the weld line detrimental effect was made in base on a fracture mechanics experiment. Specimens with and without the weld line—obtained from the same plates—were tested under a clamped single edge notched tension (SENT) configuration. For each set of process conditions, a relative weld line strength factor was defined in terms of the maximum applied stress intensity factor (K Imax ) as: K Imax of specimens with weld line/K Imax of specimens without weld line. In parallel, the fiber distribution pattern was obtained by process simulation software Moldex3D. An orientation factor was determined from simulation in order to quantify the effect of the local fiber orientation around the weld line. Optimal process condition and the most significant variable influencing the weld line strength were calculated via statistical analysis. Results showed a clear correlation between the weld line strenght factors and the orientaion factor. It was demonstrated that the fracture performance of the weld line region is controlled by the fiber orientation arrangement developed on that zone. Keywords Polymer-matrix composites, injection molding, Weld lines performance, process simulation Introduction Weld lines in polymeric injection molded parts occur wherever two or more melt fronts meet in the mold filling stage of the injection process. Its presence causes reduced mechanical properties and visual defects due to the poor intermolecular entanglement, the molecular orientation induced by the fountain flow at this region, and the stress concentration effect of a surface V-notch. 1–4 Such effects may be worsened when considering discontinuous fiber-reinforced compo- nents. In general, the degree of change in the physical and mechanical properties produced by a weld line will depend on the ability of the two flow fronts to unite in the most homogeneous way possible. It has been proved that discontinuous fibers tend to increase the discontinuity at the weld region insofar as they are oriented parallel to the weld plane. 5–8 In practice, weld lines are virtually impossible to avoid, but the deterioration in the mechanical proper- ties of injected components caused by its presence can be reduced by selecting the most favorable processing conditions—i.e. that which results in the least weld line—. 9–11 A problem arises due to the fact that micro- structural characteristics induced by processing—such as fiber orientation or the presence of flow defects—are not always visible to the naked eye, and its evaluation requires, in most cases, the destruction of the pieces. Instead, they can be predicted by simulation. The stan- dard analysis of mold filling stage, performed by pro- cess simulation software Moldex3D, provides information about the flow patterns, flow-induced fiber orientation distribution, and weld lines as Instituto de Investigaciones en Ciencia y Tecnolog  ıa de Materiales (INTEMA), Universidad Nacional de Mar del Plata, Buenos Aires, Argentina Corresponding author: Maria C Quintana, Universidad Nacional de Mar del Plata, Av. Juan B. Justo 4302, Mar del Plata B7608FDQ, 7600 Buenos Aires, Argentina. Email: cquintana@fi.mdp.edu.ar Journal of Reinforced Plastics and Composites 0(0) 1–12 ! The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0731684419883941 journals.sagepub.com/home/jrp