Effects of wood fibre geometry and fabrication-induced thermo-mechanical loads on dimensional changes and mechanical performance of thermoplastic composites

Wood fibre dimensional instability induced by thermomechanical loads during polymer composites processing affects the performance of such composites, highlighting the need to understand the combined effects of fibre geometry and process parameters. This study employed a Taguchi-based experimental design to investigate the effects of wood fibre geometry and fabrication-induced thermomechanical loads, establishing links between fibre structure, compounding, and moulding processes. Two fresh spruce fibre types (fine and coarse), rheomixer rotor speed and temperature, and injection moulding screw speed and temperature profile were systematically investigated against the evolution of fibre geometry at various processing stages and the mechanical performance of the wood plastic composites (WPC). Fibre measurements and morphological analyses revealed that fibre dimensional instability is cumulative, with the most significant reductions in fibre length and length-to-diameter (L/D) ratio occurring during rheomixing, while granulation and injection moulding showed a lower magnitude of change. Evaluation of mechanical properties and variables’ interaction plots indicates that optimal performance could be achieved with rheomixer rotor and injection moulding screw speeds (20 r/min and 100 mm/s or 40 r/min and 50 mm/s), rheomixer temperature (180°C), injection moulding temperature profile (40/170/175/175/185/185°C), and initial fine wood fibre. Sample X11_Y11_Q1 exhibited better mechanical properties and incorporated several of the identified favourable parameters, indicating approximately optimal conditions within the investigated windows. This is linked to good fibre wetting and fibre-matrix interfaces, limited interface degradation, and retention of higher length and L/D ratio. This study demonstrates the structure–processing–structure–property relationship in an integrated optimisation fabrication process of WPC.