Continuous Natural‑Fiber Reinforced FFF: Understanding Deposition Geometry and Interfacial Behavior

Abstract

Natural fibers offer a sustainable route to lightweight composite materials, but their integration into fused filament fabrication (FFF) remains constrained by complex, unresolved interactions among bead geometry, melt flow, and the polymer-fiber interface. These coupled effects can introduce quality integrity issues that may consequently reduce the load-bearing performance of the resulting heterogeneous composites. This paper reports a mechanistic baseline for continuous natural-fiber FFF in the inverse nozzle-filament regime, where a full-diameter (1.75 mm) pre-embedded composite filament is extruded through a substantially smaller nozzle under a strict 1:1 feed discipline. With this configuration, formulated as geometric overfill deposition, the imposed dimensional constraint produces stable, near-filament bead geometries and repeatable overfilled-bead stacking, enabling continuous-fiber deposition without specialized printheads. Baseline mechanical tests show that overfilled PLA beads maintain the tensile performance of conventional FFF, demonstrating that the overfill geometry does not intrinsically diminish matrix strength. However, cross-sectional and fracture analyses reveal that low-fraction flax cores (∼2 %) experience only superficial impregnation and generate moisture-driven and interfacial voids that dominate failure via tow pull-out. These results identify interfacial quality (not overfill bead geometry) as the primary limitation to reinforcement in current pre-embedded natural fiber filaments, and they establish a reproducible, stock-hardware-compatible platform for future advances in drying, impregnation, and fiber-surface modification to enable high-performance continuous natural-fiber composites.