This study explores how infill geometry and density affect the fracture behavior of thermoplastics made by material extrusion (ME) 3D printing. Using polylactic acid (PLA) as a model material, different infill patterns and densities were tested. Fracture toughness and fracture energy were measured through compact tension tests, while crack tip strain fields were tracked using digital image correlation. Two main findings emerged: (1) fracture toughness changes in a similar way with unit cell density for different infill patterns, and (2) the Kagome infill pattern shows much higher effective fracture energy than the square pattern, due to mechanisms like crack tip deflection and bifurcation. The results show that relying only on elastic fracture toughness is not enough to fully assess fracture resistance in 3D-printed thermoplastics with varying infill designs. This work highlights the importance of distinguishing between fracture toughness and effective fracture energy and presents scaling laws that help predict fracture behavior in ME-printed polymers.