Two-phase numerical modeling of interfacial dynamics in overmolding of thermoplastic composite inserts with short fiber-reinforced thermoplastics

Understanding the interfacial interactions between molten short fiber-reinforced thermoplastics and pre-molded continuous fiber composite inserts during overmolding is essential for optimizing adhesion, stress distribution, and the mechanical performance of the final part. This study introduces a novel and streamlined numerical approach to model the formation of this complex interface during the overmolding process. A two-dimensional, two-phase flow model is developed by solving the Stokes equations in both the low-viscosity short fiber polymer suspension and the highly viscous thermoplastic composite phases. This strategy eliminates the high computational cost typically associated with traditional fluid–solid interaction (FSI) models. A single equation is solved across the entire domain with only varying viscosities in the two phases.To capture fiber–flow coupling effects within each phase, the model employs a modified constitutive equation that incorporates a fourth-order orientation tensor. This tensor represents the influence of fiber orientation on flow behavior and is computed by directly solving the Fokker–Planck equation using the Streamline-Upwind/Petrov–Galerkin (SUPG) finite element method, enabling the dynamic tracking of fiber orientation over time. The model is validated through simulations investigating how variations in fiber concentration and orientation influence the deformation of the composite insert.The results show that this method achieves improved accuracy while significantly reducing computational cost, making it a promising tool for advancing research in fiber-filled polymer processing. Moreover, the approach provides a foundation for more sophisticated simulations of real-world overmolding scenarios involving fiber-reinforced thermoplastics.