Fused deposition method (FDM) is popular as a plastic 3D printing technique. One of the drawbacks of this technology is its low bonding strength between layers, reducing through-plane mechanical properties of a part compared with in-plane strength within the layers themselves. This study focuses on altering the molecular structure of a polylactic acid by chain extension to increase the adhesion strength between layers, quantified by peel testing, in order to increase overall part strength. Four different samples were prepared in a high shear mixer, processed into filament and printed by a FDM-type 3D printer. These samples were characterized for their thermal, rheological, mechanical and adhesion properties. The findings showed that the chain extender-modified resins exhibited higher layer-to-layer adhesion strength as well as increased viscosity corresponding to an increasing degree of branching. The interlayer diffusion and entanglement of newly created branch chain ends improved bonding between the printed layers resulting in higher tensile properties. POLYM. ENG. SCI., 59:E59-E64, 2019. (c) 2018 Society of Plastics Engineers