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This original work investigates the influence of infill design, printer selection, and part orientation on the mechanical properties and production cost of parts fabricated using material extrusion additive manufacturing systems. Flexural test specimens are fabricated in both production-grade (Fortus 250mc) and entry-level (MakerBot Replicator 2X) material extrusion systems with varying infill densities (1 mm to 10 mm spacing between rasters). In addition, solid infill specimens are printed in three orientations to establish baseline mechanical stiffness and strength. Finite element simulations and a simplified analytical model based on Euler-Bernoulli beam theory are developed. Results show reasonable agreement between analytical, simulation, and experimental results; 10-20% and 10-40% deviation for production-grade and entry-level specimens specimens, respectively. There is a 40% reduction in stiffness and strength between the solid XY specimen and 1 mm infill specimen. As infill density is further decreased, stiffness and strength asymptotically reduces by 60-70% when compared to solid specimens. This effect is more pronounced in specimens fabricated using entry-level printers, which indicates that printer selection plays a role in printing highly sparse parts. Cost analysis suggests that up to 40% savings can be achieved with highly sparse structures. However, for structural parts, it is recommended that parts be printed with solid infill and with the loading direction aligned in the XY plane to achieve high stiffness, high strength, and reasonable cost. Findings from this study show that there is minimal cost savings but high reduction in mechanical stiffness and strength when sparse infills are used in both production-grade and entry-level printers. Hence, it is recommended that solid infill should be used in all regions of parts that carry significant mechanical stress and sparse infill be used solely to support internal geometries and overhangs.

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