Transition from M-shaped to Gaussian fiber alignment governed by inlet orientation in fused filament fabrication

Precise control of fiber orientation in fused filament fabrication (FFF) is essential for tailoring the anisotropic properties of printed composites. However, orifice-embedded nozzles, which are used to promote transverse alignment, often produce a non-uniform “M-shaped” orientation profile in which centerline fibers remain aligned with the flow. The origin of this persistent alignment has not been fully understood. In this study, we investigate the role of inlet fiber alignment in governing orientation evolution using combined flow visualization experiments and computational fluid dynamics simulations coupled with the Advani–Tucker model. By systematically perturbing the inlet alignment, the resulting alignment profiles were quantified. These results show that the final orientation topology is jointly determined by shear-driven Jeffery rotation within the finite-length orifice channel and the subsequent response of this preconditioned orientation state to the expansion flow. Critical inlet-alignment thresholds are identified as a function of fiber aspect ratio, above which the centerline-aligned state is destabilized and the profile transitions from M-shaped to Gaussian-like. These findings demonstrate that alignment topology in FFF is governed by both nozzle geometry and inlet orientation, providing a new pathway to control fiber alignment and enhance anisotropic performance in printed composites.