Fused Deposition Modeling of Industrial Waste LDPE-Polystyrene Blends: Comparing Low and Medium Melt Flow Index of LDPE Waste

This study explores the potential of additive manufacturing (AM) to upcycle industrial waste low-density polyethylene (LDPE) by investigating the effect of LDPE waste's melt flow index (MFI) on the printability of LDPE/polystyrene (PS) blends via fused deposition modeling (FDM) and the corresponding mechanical properties and fracture behaviors of the printed components. Two industrial LDPE waste grades with MFI values (0.8 and 4 g/10 min) were blended with PS and compatibilized using a styrene–ethylene–butylene–styrene (SEBS) copolymer. The printability of various LDPE/PS blend ratios was evaluated for their mechanical, rheological, thermal, and morphological properties. Based on this analysis, selected blend ratios were used for FDM printing. The mechanical properties and fracture behaviors of the 3D printed LDPE/PS were first assessed. The practical printability of the blends in challenging FDM conditions was then evaluated by fabricating complex, unsupported geometries, such as bridges, overhangs, and thin-walled cubes. The results revealed that LDPE with a higher MFI showed improved flowability but compromised ductility, while lower MFI improved structural integrity. An MFI of 4 g/10 min was identified as optimal, offering a balance between flowability and strength, enabling successful 3D printing. Adding SEBS compatibilizer showed enhanced interfacial adhesion, improved filament quality, and enhanced printability in bridging and overhang tests. However, these blends exhibited limitations in printing thin-walled structures as demonstrated by warping and layer delamination, caused by thermal gradients between the extrusion temperature and ambient conditions and uneven shrinkage during cooling. This study demonstrates the feasibility of converting industrial LDPE waste into functional materials for FDM through optimized MFI selection and blend design. The findings provide a pathway to integrate mixed plastic waste into additive manufacturing workflows.