Tensile and flexural performance of FDM 3D printed Harakeke (NZ Flax) fiber -PLA composites for lightweight structural applications

• A novel harakeke (New Zealand flax) fiber–reinforced PLA composite was developed and fabricated using FDM 3D printing, targeting lightweight and sustainable structural applications. • A Response Surface Methodology (RSM) framework (Central Composite Design) was employed to model and optimize the combined influence of fiber content (0–20 wt.%), raster angle (45–90°), and raster width (0.5–1.0 mm) on mechanical properties. • The optimal printing parameters were identified as 10 wt.% fiber content, 0.5 mm raster width, and a 45° raster angle, producing the best balance of stiffness and strength. • The optimized composite achieved a Young’s modulus of 4453.85 MPa and a flexural stress of 73.395 MPa, outperforming other parameter combinations. • Raster angle was found to be the most influential parameter, with a 45° orientation consistently improving load transfer and stress distribution. • Excessive fiber loading (>10 wt.%) reduced tensile and flexural performance due to fiber agglomeration, increased melt viscosity, and compromised fiber–matrix interfacial adhesion. • Narrow raster widths (0.5 mm) improved interlayer fusion and deposition density, contributing significantly to enhanced composite stiffness. • The study confirms that controlled fiber dispersion and optimized FDM settings are crucial for unlocking the structural potential of PLA–harakeke biocomposites. • Results establish a validated RSM-based predictive model (desirability = 0.91) to guide process optimization for sustainable, high-performance 3D-printed materials. Current research on the shift toward sustainable materials has intensified interest in biodegradable alternatives polymers, with poly-lactic acid (PLA) emerging as a leading candidate. With various advantages, including biodegradability and processability, PLA's tensile and flexural strength can be further enhanced to increase its use in lightweight structural applications. This study introduces a novel integration of harakeke (New Zealand flax) fibers into PLA, a material pairing that has not been comprehensively investigated for additive manufacturing-based components. The research uniquely employs a Response Surface Methodology (RSM)-based optimization framework to systematically analyze and model the combined effects of fiber content (0-20 wt.%), raster angle (45-90°), and raster width (0.5-1.0 mm) on the composite’s tensile and flexural performance. The findings reveal that a raster width of 0.5 mm, raster angle of 45°, and a flax infill of 10% by weight provide the best synergy of stiffness and strength. The maximum values ​​of Young's modulus are 4453.85 MPa, and the flexural stress is 73.395 MPa. Increased fiber loadings above 20 wt.% reduce performance due to fiber agglomeration. Among orientations, the 45° raster is preferable to 90° due to increased load transfer and stress distribution, and narrower raster widths facilitate greater interlayer bonding and deposition density.