Discussion on the microscale geometry as the dominant factor for strength anisotropy in material extrusion additive manufacturing

This paper presents a discussion and interpretation of the findings in the review paper “Fused filament fabrication of polymer materials: A review of interlayer bond” by Xia Gao, Shunxin Qi, Xiao Kuang, Yunlan Su, Jing Li, Dujin Wang [Additive Manufacturing (2020): 101658]. This discussion draws different conclusions based on the microscale filament geometry of interlayer bonds as opposed to molecular-scale bonding (diffusion and entanglement of polymer chains), which is predominantly considered in the review. Four complementary arguments on the matter are proposed, demonstrating that microscale geometry rather than incomplete molecular bonding is the predominant cause of strength anisotropy in material extrusion additive manufacturing (MEAM). These arguments consider the evidence from studies that (i) factored microscale geometry into strength calculation; (ii) eliminated the influence of geometry; (iii) improved the geometry to reduce its impact on strength, and (iv) tested the effect of manually reproduced interlayer geometry in bulk material. Overall, this discussion suggests that the underlying cause of anisotropy in MEAM is filament-scale geometric features (grooves and voids between layers), not the deficient bonding as is often theorised. Drawing upon the evidence in the literature, this discussion proposes that specimens attain bulk material strength for a range of printing conditions and materials.