Large-format material extrusion additive manufacturing of PLA, LDPE, and HDPE compound feedstock with spent coffee grounds

As an abundant commodity, coffee production generates significant scraps and by-products, increasing the potential pollution hazards. Circular economy and bioeconomy can help valorize Spent Coffee Grounds (SCGs), e.g., as fillers in polymer-based composites using conventional manufacturing, i.e., injection molding. Large-format additive manufacturing with pellet extruders offers a further option for new applications, reducing costs for the valorization of biomass waste. However, its use in this context remains unexplored, especially for applications with complex geometries or critical overhangs. This work investigates new biomass waste-based materials for large-format additive manufacturing with direct feeding extrusion, fabricating self-supported complex overhang geometries through nonplanar slicing. The thermal, rheological, and mechanical properties of three novel polymer-based pellet compounds with post-industrial SCGs, i.e., injection molding grade polylactic acid/SCGs, recycled low-density polyethylene/SCGs, and high-density polyethylene/SCGs, were herein investigated to evaluate their printability, defining their extrusion temperatures (190 °C and 170 °C). Results showed suitable viscosity ranges (133.6–839.7 Pa∙s) and accurate tensile values comparable with literature, e.g., ~ 1–3% minimum relative standard deviations (polylactic acid/SCGs), or conventional manufacturing, e.g., elastic moduli of 107.4 MPa (recycled low-density polyethylene/SCGs) and 587 MPa (high-density polyethylene/SCGs). Their use with large-format 3D printers was assessed thanks to nonplanar samples with complex overhang geometries, reaching a maximum curvature angle of 32° and fabricating overhangs up to 25° without supports. According to the tests, especially polylactic acid/SCGs, a bio-based compound, and recycled low-density polyethylene/SCGs, a fully recycled material, can be used for large-format 3D printing applications with complex geometries, e.g., furniture, interior, and exhibition design. This work paves the way for new materials for large-format additive manufacturing, reducing the need for 3D printing grade feedstock, cutting costs and consumption from filament processing, and fostering material waste reduction practices.