Effects of Infill Pattern and Infill Density on the Flexural Properties of Markforged Onyx

Fused Filament Fabrication (FFF) enables the production of lightweight polymer composite components with complex geometries; however, the effects of infill geometry on printing accuracy and mechanical behavior of unreinforced Markforged Onyx remains insufficiently understood. This parametric study investigates the influence of four infill patterns (gyroid, hexagonal, rectangular, and triangular) and three infill densities (30%, 40%, and 50%) on the post-print mass, dimensional accuracy, microstructure and flexural properties of Onyx specimens fabricated using a Markforged Mark Two printer. Experimental measurements were compared with Eiger slicing software predictions, and geometry- and mass-based area moment of inertia correction methods were evaluated. The results show that Eiger consistently overestimated part mass by approximately 8–10% with the largest deviations occurring for the hexagonal infill pattern. Increasing infill density increased flexural strength and stiffness for every infill pattern. The triangular pattern exhibited the highest mechanical performance, whereas the hexagonal pattern produced the lowest due to its larger internal voids and reduced effective material. The mass-based correction method produced flexural properties comparable to the geometry-based approach, providing an alternative method for predicting the mechanical performance of FFF-manufactured Onyx components.