Regulating Crystallization to Enhance Interfacial Bonding in Material-Extrusion 3D Printed Polylactic Acid Parts

Abstract Polylactic acid (PLA) components fabricated by fused filament fabrication (FFF) typically suffer from poor interlayer bond strength. Owing to PLA's intrinsically weak crystallization ability, the role of crystallization in interlayer bonding has rarely been investigated, limiting the development of interfacial enhancement strategies. To address this, we designed a PLA printing material with enhanced crystallization ability by incorporating polyethylene glycol (PEG-4K), and then systematically regulated the crystallization behavior in FFF-printed part via varying bed temperatures (Tb) and applying thermal annealing. When printed or annealed at 90 °C, both neat PLA and PLA/PEG-4K exhibited rapid crystallization with half-crystallization times (t1/2) below 9.8 min. Such extensive crystallization restricted chain diffusion across adjacent layers, resulting in interfacial bond strengths below 15 MPa. Conversely, decreasing Tb below 60 °C extended t1/2 to over 90 min, yielding the printed samples with low crystallinity and markedly increased bond strength. Notably, incorporating PEG-4K shortened the segmental relaxation time by 30% and decreased the melt viscosity, synergistically improving the bond strength to 24.7 MPa. These results confirmed that the kinetic competition between crystallization and chain interdiffusion determined the interlayer bonding. This work also established a quantitative correlation between crystallinity and interfacial bond strength, offering a theoretical basis for the mechanical optimization of FFF-printed PLA parts.