<p>Willow bark fiber (WBF) was used as a filler in 3D printing of K resin composite filaments. WBF was extracted from willow bark (WB) using a chemical degumming procedure with NaOH and MgSO₄·7H₂O solution. The morphological characteristics and surface functional groups of both raw willow bark (WB) and extracted willow bark fiber (WBF) were systematically characterized using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. Analytical results demonstrated that the degumming process effectively increased the cellulose content while reducing hemi-cellulose and lignin components in WBF, simultaneously eliminating the inherent fiber aggregation present in raw WB. Contact angle measurements indicated enhanced hydrophilicity with increasing WBF content. Melt flow index (MFI) analysis showed a corresponding decrease in melt fluidity. These results suggested that WBF incorporation modifies both the surface and rheological properties of the K resin matrix. Furthermore, mechanical characterization included tensile testing of solid specimens and compression testing of porous scaffolds was conducted. The composite with 5% (by weight) WBF exhibited a significant improvement in fracture strain, reaching 421% when compared to a neat K resin. The tensile yield properties remained stable across the 15–25% (by weight) WBF concentration range, demonstrating consistent mechanical performance at higher filler loadings. In compression testing, WBF increased the yield strength from 126.67&#xa0;MPa (neat K resin) to a maximum of 177.03&#xa0;MPa at 10% (by weight) loading. These results highlighted WBF's dual role as both a mechanical reinforcement agent and cost-reducing filler for K resin. The developed K resin/WBF composites showed particular promise for fabricating lightweight structural components with complex geometries, offering potential applications across various engineering fields.</p> Graphical abstract <p></p>