Influence of infill density, pattern, and contour width on the mechanical behavior of 3D-printed PLA components

Fused Deposition Modeling (FDM) is widely used in additive manufacturing, yet establishing robust process–structure–property relationships remains challenging due to the complex interplay between deposition parameters, porosity, and filament bonding. This work investigates the influence of infill pattern (Triangle, Zigzag, Tri-hexagon), infill density (30%–70%), and the number of contour lines (1–3) on the mechanical behavior of FDM-printed Polylactic Acid (PLA). A Taguchi L 9 orthogonal array combined with analysis of variance and Response Surface Methodology quantitatively identifies the statistically significant contributions of each parameter. Tensile testing supported by Digital Image Correlation provides elastic modulus, Poisson’s ratio, yield stress, and ultimate tensile strength. Infill density is shown to dominate stiffness and strength, while the number of contour lines primarily governs the elastic modulus. At 100% infill density, increasing the number of contour lines from 0 to 20 increases the elastic modulus from about 2.7 to 3.25 GPa. Beyond 16 contour lines, the stiffness plateaus because the gauge section is essentially fully occupied by longitudinal filaments. Dynamic mechanical analysis and cyclic loading confirm a predominantly elastic response with negligible viscous effects. A newly developed cryogenic fracture protocol enables brittle failure and preservation of the pristine printed architecture, allowing high-resolution scanning electron microscopy to more clearly separate manufacturing-induced porosity from deformation-induced damage. Quantitative void analysis confirms that tensile loading increases the apparent void surface fraction from 1.67% in the cryogenic reference state to 5.00% after deformation. The width-wise distribution of void surface area further shows that this damage amplification is mainly localized in the inner printed material, where the apparent void fraction reaches approximately 7.94% after tensile loading, while the shell regions remain close to 1.1%. These results suggest that inter-filament voids in the inner architecture govern damage initiation, cavity growth, and crack propagation, thereby providing a quantitative microstructural basis for the observed mechanical trends and for the process-mesostructure-performance relationship.