Development of a custom commingled flax/PLA wrapped yarn for additive manufacturing of long-fibre biocomposites

• Continuous flax fibre/PLA biocomposites were manufactured using in-nozzle impregnation technique by Fused Filament Fabrication. • The yarn was designed via commingling and wrapping to pre-impregnate the filament and align fibres along the axis. • Microstructure of filaments was characterized to assess commingling quality and efficiency on porosity. • Printed composites longitudinal mechanical behaviour and properties were evaluated. • Positive impact on tensile performances was highlighted. Plant fibres are promising reinforcements for bio-composites in additive manufacturing, but their use as long fibres remains limited, often reduced to short particles that underuse their potential. This study presents a customised yarn design that not only maintains fibre alignment parallel to the yarn axis but also ensures core resin impregnation. Commingling and wrap spinning techniques were used to produce four flax/PLA yarns with varying compositions. The manufacturing process and printing of unidirectional composite specimens are detailed. Tomography revealed up to 3.3 times lower intra-yarn porosity thanks to commingling, and tensile tests showed a modulus increase by a factor of 2.1 compared to similar previous works using conventional twisted yarns. These results pave the way for broader use of long flax fibres in 3D printing.