Sustainable Polymer Additive Manufacturing Across Scales: A Critical Review of Pallet-Scale Structural Opportunities and Membrane Feed Spacers

Additive manufacturing (AM) can support more sustainable polymer production, but the benefit is conditional on process energy, material chemistry, build yield, post-processing, service life, repair, and end-of-life recovery. This critical integrative review compares three polymer AM routes: stereolithography (SLA), digital light processing (DLP), and fused deposition modelling (FDM) through two deliberately contrasting application scales: pallet-scale load-bearing structures and membrane feed spacers. The review distinguishes direct application evidence from design opportunities inferred from adjacent AM literature. For pallet-scale structures, the literature currently supports large-format thermoplastic extrusion, zoned cellular architectures, modular repair, and controlled recycled feedstock as plausible translation routes, but direct peer-reviewed evidence for fully additively manufactured transportation pallets remains very limited. For membrane feed spacers, direct studies provide stronger quantitative evidence: published 3D-printed designs have reported approximately threefold pressure-drop reduction with doubled specific water flux, pressure-drop gradients as low as 0.091 bar m−1 under reported test conditions, and a 16% increase in permeate flux with a thinner fouling layer for a honeycomb geometry. Process-energy evidence also shows that results depend strongly on the functional unit and machine state; reported desktop values span 24.8–85.7 kJ cm−3 for FFF and 10.8–21.5 kJ cm−3 for SLA, while post-processing and machine utilization can materially change the lifecycle result. Recycled polymers likewise involve a performance–circularity trade-off: some post-consumer PLA studies report strength losses of about one-third or more, whereas controlled blends and recycling strategies can retain a much larger fraction of virgin-material performance. The synthesis therefore treats geometry, process parameters, material state, operational performance, lifecycle impact, and cost as one coupled design problem rather than assuming that AM, recycled content, or bio-based chemistry is inherently sustainable.