Fused deposition modeled parts of polymeric origin exhibit inferior interlayer mechanical properties. To enhance interfacial weld stiffness, poly(lactide) blends containing low molecular weight polymers of chemically identical but enantiomerically different nature are explored. The enantiomeric composition of the low molecular weight fraction is either random or opposite, promoting molecular diffusion or nucleation respectively. The structure-relationship of the interfaces is studied using torsional stiffness, calorimetry, and rheology. Fully miscible, non-crystallizable low molecular weight additives of random L and D enantiomeric composition reduce melt viscosity and crystallization rate, promoting molecular diffusion. Nevertheless, incomplete entangling and crystallization upon interfacial mixing induce poor interfacial stiffening. Poly(lactide) stereocomplex enriched interfaces promote crystallization. Chains across weld interfaces may be mechanically anchored in crystals, but hindered diffusion limits molecular mixing and thus the extent of mechanical stiffening. Ultimately, combining melt plasticization and increased crystallization rates distinctly increases weld stiffness and thermodynamic/geometrical stability of fused deposition modeled poly(lactides).