Process–Structure Interactions in Stretching-Dominated Lattices: How Nodal Toolpath Fragmentation Limits Geometric Isotropy in MEX-AM

Stretching-dominated lattice metamaterials offer exceptional stiffness-to-weight efficiency, but their physical realisation via material extrusion additive manufacturing (MEX-AM) is fundamentally constrained by process-induced defects. This study investigates these process–structure interactions by evaluating Isomax (cubic+octet) against Kagome architectures. A simulation-led framework identified the Isomax topology as superior for load transfer, leading to the exclusion of the buckling-prone Kagome from physical testing. Closed-cell Isomax specimens were subsequently fabricated in PLA and evaluated under tension, flexure, and compression. Mechanical testing revealed severe performance asymmetry. Tensile and flexural strengths degraded by > 60% and ~ 50%, respectively, versus solid controls. Fractography confirmed this penalty is driven by Nodal Toolpath Fragmentation (NTF), a slicer-induced discretization of the extrusion path at structural intersections that forces premature, interface-dominated failure. Conversely, compression physically suppresses NTF via contact stabilisation. This yields near-isotropic behaviour, with orientation-dependent strength varying by only 1.4%, and limits the compressive manufacturing penalty to 14% relative to numerical baselines. This confirms geometry dictates compressive performance, while process defects govern tension. Deploying Maxwell-stable metamaterials requires process-aware design prioritising toolpath continuity. By isolating these bottlenecks, this work supports UN Sustainable Development Goals 9 and 12, optimising material efficiency for load-bearing structures.