Morphology prediction in injection-molded fiber-reinforced thermoplastic composites: a technical review of modeling frameworks

Fiber orientation, fiber length distribution, and spatial morphology gradients govern the mechanical performance of injection-molded fiber-reinforced thermoplastic composites. Predicting these features accurately requires modeling frameworks that span flow kinematics, fiber interaction mechanics, and process–structure linkage. This paper reviews the principal modeling approaches used for morphology prediction in short-fiber (SFRT) and long-fiber reinforced thermoplastics (LFRT), from foundational Jeffery-based orientation theory through Folgar–Tucker formulations, reduced-strain and anisotropic diffusion closures, coupled orientation–length frameworks, and physics-informed hybrid approaches. Classical models offer computational efficiency and are well integrated into commercial solvers but assume constant fiber length, limiting their applicability in LFRT systems where breakage, evolving aspect ratio, and orientation–length coupling dominate morphology. Coupled frameworks improve fidelity but remain research-level tools. Illustrative worked examples, calibrated against published experimental data are used to convey the practical magnitude of model disagreement for both SFRT and LFRT geometries. Industrial deployment barriers including mesh sensitivity, closure instability, and parameter non-transferability are examined. The review also addresses the comparatively under-explored linkage between predicted morphology and mechanical property estimation, and identifies weld-line morphology and interfacial sizing effects as open research gaps.