Effect of Microscale Fabrication on Multi-Directional Mechanical Properties of Additively Manufactured Poly Lactic Acid With Grid Infills

Abstract

In additive manufacturing, infill patterns have a significant impact on both printing time and mechanical performance, creating a necessary trade-off between the two from an industrial perspective. This study aims therefore to find an easy-to-handle procedure for rapid evaluation of the influence of infill density and raster angle on the elastic properties of 3D-printed components, from the perspective of their adoption in the industrial process of component design. In particular, the study's goal is to predict the elastic modulus in three directions. Tensile tests were carried out on bulk specimens according to ISO 527 to determine the elastic properties of 3D-printed PLA necessary for the numerical analysis. Cubic specimens were then manufactured with three densities (20%, 40%, and 60%) and two raster angles (−45°/45° and 0°/90°). Quasi-static compression tests were conducted on those specimens to assess their homogenized elastic behavior in three directions. One important result of the experimental phase was the relationship between Young's modulus (E) in the three directions. The average of E in directions 1 and 2 (build plate) is named E_1,2 and on the build-up directions is E₃, for α = 0°/90° was E_1,2 = 0.8E₃ and for α = −45°/45° was E_1,2 = 0.28E₃. Three finite element models were developed and run with the elastic properties determined by tensile tests, namely: (a) a shell model (SHL) where the internal and external walls of the specimens were modeled using shell elements with the nominal geometry; (b) a solid model (SLD) with the nominal geometry and (c) a nonuniform section model (NUS) in which the geometry was taken from microscope image to account for manufacturing imperfections. The difference between simulation and experiment for SHL was 19%, SLD was 15%, and NUS was 13%, indicating an overall good correspondence and, at the same time, that the real geometry resulting from the manufacturing process has a non-negligible impact on the homogenized value. Besides validating the values and relationships, FEM elucidated which sections of the cubes experienced stress and contributed to stiffness under various patterns and loading scenarios.