Closing the Loop Locally: A Systematic Review and Framework for Distributed Recycling of Post-Consumer Plastics with a Focus on HDPE and PP

This systematic review evaluated technical evidence and system-level conditions for Distributed Recycling and Additive Manufacturing (DRAM) of post-consumer high-density polyethylene (HDPE) and polypropylene (PP). Peer-reviewed journal articles, conference papers, and reviews published from 2020 to 2025 were eligible when they addressed recycled-polymer preparation, feedstock conversion, extrusion-based additive manufacturing, printing performance, or circularity-related implementation; studies focused only on virgin polymers, outside the publication period, or lacking relevance were excluded. Scopus and Web of Science were searched on 17 October 2025 using four thematic strategies. Methodological completeness and applicability were appraised across six domains for the HDPE/PP material-specific evidence. Findings were synthesized narratively and thematically; meta-analysis was not performed because of substantial heterogeneity in feedstock, formulation, equipment, specimens, test methods, and outcomes. The final corpus comprised 119 unique sources: 28 material-specific sources (11 HDPE and 17 PP) and 91 contextual or system-level sources. Of the material-specific subset, 24 provided direct or partially direct recycled experimental evidence and four provided indirect or secondary evidence. Most appraised material-specific sources showed methodological concerns related to feedstock provenance, processing history, parameter reporting, replication, or test conditions. Recycled HDPE and PP could be converted into extrusion-based additive manufacturing feedstocks under study-specific conditions, but the evidence did not support universal processing windows, property ranges, degradation rates, cost competitiveness, environmental superiority, or generalized industrial readiness. Most evidence remained at laboratory or demonstrator scale. The Circular DRAM Viability Framework provides an evidence-informed structure intended to support application-specific diagnosis, monitoring, and qualification.