Uniform infill strategies overlook how infill density and pattern interact across the thickness of additively manufactured polymer parts. This study examined the mechanical performance of three-zone PLA specimens, with each zone assigned an infill density (30%, 60%, or 90%) and a configuration combining pattern geometry and raster orientation. An L18 Taguchi design guided fabrication, and a general linear model quantified factor contributions. Core density was the dominant predictor of ultimate tensile strength, accounting for 54.7% of the explained variation, while core configuration contributed 7.2% (p < 0.001). Configuration effects depended on density: honeycomb at 0° favored strength at moderate core densities, whereas rectilinear at ±45° improved stiffness at high densities. Abrupt transitions between zones had a smaller, statistically nonsignificant effect, although smoother gradients were qualitatively associated with better strength retention. Segmented specimens exhibited tortuous crack paths and ductile post-yield behavior, unlike the brittle fracture observed in uniform solid references. The best segmented design approached reference tensile strength within 5–13%, while reducing material use by approximately 30% and printing time by 50%. Multiobjective optimization yielded a composite desirability of 0.593. These findings identify core density as the primary design variable for lightweight, functionally graded FDM parts and highlight the importance of separately evaluating pattern geometry and raster orientation.