Critical Review of Granule-Fed 3D Printing Applied to Elastomers: Processes, Materials, and Design Strategies

This systematic review examines scientific evidence published between 2014 and 2024 on 3D printing of elastomers incorporating recycled materials, focusing on thermoplastic polyurethane (TPU) and pellet-extrusion technologies. Following PRISMA guidelines, comprehensive searches were performed in Scopus, Web of Science, ScienceDirect, and SpringerLink. From 142 records, 37 studies met the inclusion criteria. The findings reveal that adding recycled fillers such as rubber powder slightly decreases mechanical strength while increasing surface hardness and significantly lowering environmental impact. A growing trend toward topology optimization and generative design is also observed to enhance structural performance. Overall, the literature supports pellet-based 3D printing of recycled elastomers as a sustainable manufacturing strategy with promising applications in footwear, orthotics, and impact-absorbing components, although interfacial compatibility and industrial scalability remain key challenges.