Rotating die extrusion of continuous fiber-reinforced polymer filaments

Continuous fiber-reinforced polymers (CFRPs) offer high strength-to-weight ratios, which makes them an attractive choice for applications in transportation, biomedical devices, and sports equipment. Additive manufacturing presents new opportunities for producing CFRPs with improved geometric freedom and digital fabrication flexibility. However, achieving adequate fiber impregnation and strong interfacial bonding remains a major challenge. This paper presents a novel rotating impregnation die, patented by some of the authors, designed to produce fiber-reinforced polymer filaments at a die speed of 15 rad/s. These filaments, characterized by a final diameter of 0.65 mm and a fiber volume fraction of 5.4%, are compatible with fused deposition modelling for 3D printing. The die is engineered to improve polymer–fiber interaction during filament fabrication. Specifically, its rotating geometry induces a swirling flow pattern in the molten polymer, which enhances fiber wetting and promotes partial fiber interlacing. The performance of the system was evaluated through both numerical simulations and experimental tests. In the computational fluid dynamics analysis, an inlet velocity of 5 mm/s was imposed, showing that the rotational motion generates a tangential velocity component that improves fiber-polymer interaction and locally reduces viscosity at the fiber surface, leveraging the shear-thinning behaviour of the polymer. This results in improved impregnation efficiency without affecting the internal pressure of the die. Two filament configurations were produced for comparison: one using the rotating impregnation die PLAGF-B (PolyLactic Acid – Glass Fiber Braided) and one using a static die PLAGF-UB (PolyLactic Acid – Glass Fiber UnBraided). The produced filaments consisted of three glass-fiber bundles impregnated with PLA resin and were subjected to standard tensile testing, after being pulled at a controlled speed of 6 mm/s. The PLAGF-B samples exhibited higher tensile strength (~ 70 MPa vs. ~60 MPa) and elongation at break (~ 0.023 mm/mm vs. ~0.018 mm/mm), attributed to enhanced twisting and compaction induced by the die’s rotation.