Material extrusion additive manufacturing of recycled discontinuous carbon fibre reinforced thermoplastic composites with different fibre lengths: Through-process microstructural evolution and mechanical property loss

This paper investigates the evolution of recycled fibres and their influence on the mechanical property loss in material extrusion additive manufacturing of discontinuous carbon fibre reinforced polyamide-6 thermoplastic composites. For the first time, recycled carbon fibres in a broad range of 100 µm - 2.8 mm lengths with a consistent fraction of 20 wt.% were produced in composite flakes as feedstock with specific sizes, which were screw-extruded into filament for material extrusion additive manufacturing. Comprehensive characterisations were carried out to trace the evolution of material microstructure throughout the recycling and manufacturing processes. The void content, fibre orientation, fibre length and its breakage were analysed. Tensile tests show that the printed composites with the longest fibre length of 433 µm have the highest tensile strength of 126 MPa and tensile modulus of 6.4 GPa, which are 37% and 63.2% higher than the sample with a conventional 0.1 mm length of milled fibres. Specifically, it was found that the fibre length in 1.4 mm flakes was reduced to 750 µm by screw-extrusion (with customised auger teeth and extruder nozzle) and further reduced to 324 µm by material extrusion (with a nozzle diameter of 1 mm). It has been experimentally identified that the tensile strength of the current 3D printed composites using the initial fibre length of 1.4 mm in coupon can be further increased by more than 55.6% if properly reducing the air voids, fibre breakage and improving the fibre orientation.