Screwless Extrusion for Natural Fiber Composites: A Critical Review of Legacy Data and Future Sustainability Implications

Screwless extrusion, leveraging viscoelastic normal stress effects rather than mechanical screw propulsion, presents a transformative approach for processing natural fiber-reinforced thermoplastics (NFRTCs). This review critically evaluates the Galea et al. (2004) legacy data through contemporary sustainability frameworks. The technique demonstrates exceptional fiber preservation (85% retention versus 30-55% for conventional methods), superior energy efficiency (50-67% reduction), and chemical-free processing enabling fiber loadings up to 40% without coupling agents. Statistical analysis reveals F-statistics exceeding 45.7 (p < 0.001) with effect sizes (η²) > 0.90, while mechanical performance achieves 87% of MAPP-enhanced composite properties without chemical additives. Comprehensive sustainability analysis demonstrates 42-48% reduction in total system impact, 92% recyclability, and economic viability with €1.0-1.2 million capital investment reductions. Scale-up challenges from laboratory throughput (10-15 g/min) to industrial requirements (100+ kg/h) are addressed through geometric scaling relationships and Industry 4.0 integration. This analysis positions screwless extrusion as a strategically relevant technology for sustainable composite manufacturing, enabling fully bio-based composite systems while achieving SDG alignment through 50-67% energy reduction and 35% carbon footprint reduction.