Evaluation of Fiber Orientation at Flow-Front Weld Lines in Particle-based Injection Molding Analysis

In response to environmental and energy concerns, there is increasing demand for the lightweighting of transportation equipment. Short fiber-reinforced plastics (SFRPs) have attracted significant attention due to their high specific strength, specific stiffness, and excellent moldability. However, controlling fiber orientation during injection molding remains a major challenge, as it significantly influences the mechanical performance of molded products. The objective of this study is to elucidate the fiber orientation mechanism at flow front weld lines using three-dimensional numerical simulations based on the Moving Particle Semi-implicit (MPS) method. The MPS method enables separate modeling of resin and fibers while accounting for their interactions. A T-shaped mold model, based on a section from a practical mold used in prior experiments, was developed to replicate the branching and merging of resin flow. The simulated flow trajectories and velocity distributions of the resin showed quantitative agreement with experimental results, validating the accuracy of the analysis model. Fiber tracking analysis revealed location-dependent orientation behaviors within the mold. Fibers in proximity to the mold walls tended to align in the flow direction due to shear flow. In contrast, fibers within the central region largely retained their initial orientation. In areas where shear and extensional flows acted in combination, changes in orientation were observed, ultimately resulting in a layered MD-TD-MD-TD-MD structure along the thickness direction of the molded part. This orientation pattern closely matched the experimental findings reported by Ogura et al. (2022). These results demonstrate that particle-based analysis is effective for predicting fiber orientation mechanisms in complex mold geometries and can contribute to improving the mechanical properties of SFRP products through optimized molding conditions and mold design.