Mesostructure, Toolpaths and the Finite-Element Prediction Gap in the Flexural Response of Planar and Non-Planar Fused Filament Fabrication

The flexural response of a fused filament fabrication (FFF) part depends on the polymer, on the mesostructure the toolpath builds from it, and on how that mesostructure is oriented to the bending load. This review examines these factors for polylactic acid (PLA), polyethylene terephthalate glycol (PETG) and acrylonitrile butadiene styrene (ABS) printed with planar and non-planar toolpaths, and how well finite-element models predict the result. Because the interlayer bond forms while the polymer cools, crystallinity, annealing, moisture, ageing and test temperature shift flexural strength and stiffness, in directions that differ between polymers and treatment windows. The toolpath is a primary mechanical variable, with effects that depend on the test conditions. Of 84 finite-element studies, 26 are compared quantitatively with a flexural experiment, 23 independently. Only 1 of those 26 treats curved-layer or non-planar deposition, and the shortfall lies in the small number of non-planar studies that predict a mechanical response, not in a lower standard of validation. None of the validated models makes the polymer constants depend on temperature, moisture or loading rate, or gives the bond between layers of one polymer its own interface law, leaving thermal, hygroscopic and time-dependent behaviour, with non-planar bending, as the clearest gaps.