Extrusion process optimisation of recycled polypropylene composites reinforced with spent coffee grounds biochar for additive manufacturing filaments

This study explores the optimisation of extrusion process parameters for the fabrication of sustainable composite filaments from spent coffee ground (SCG) biochar and recycled polypropylene (rPP) for material extrusion (MEX) additive manufacturing. A Taguchi L18 experimental design was employed to investigate the effects of biochar composition, biochar treatment, type of additive, extrusion temperature, and screw speed on filament performance. The grey relational grade (GRG) values obtained from grey relational analysis (GRA) were used to optimise the production of composite filaments with multiple performance characteristics, including material density, dimensional accuracy, and mechanical properties. The results showed that the optimal parameter combination obtained through multi-response optimisation consisted of 3% alkali-treated SCG biochar composition with polyethylene glycol (PEG) at an extrusion temperature of 150/170℃ and a screw speed of 5 rpm. Under these conditions, the confirmation test demonstrated improved filament performance, with diameter deviation decreasing from 0.205 mm to 0.147 mm, tensile strength increasing from 10.90 MPa to 18.3 MPa, and density changing from 1.190 g/cm 3 to 1.05 g/cm 3 , indicating enhanced dimensional stability, structural homogeneity, and load transfer efficiency. Furthermore, scanning electron microscope (SEM) observations confirmed that the incorporation of maleic anhydride grafted polypropylene (MAPP) or PEG, together with alkali treatment of biochar, improved interfacial bonding, reduced void formation, and enhanced structural homogeneity within the composite filament. This study provides convincing evidence for refining the material composition and process parameters in the manufacture of sustainable composite filaments for additive manufacturing application.