Fused filament deposition of PLA: the role of interlayer adhesion in the mechanical performances

A criteria to enhance mechanical performances of a standard specimens (Type V, ANSI D368) made of polylactic acid (PLA) was proposed. Fused PLA deposition was conducted, with nozzle tem-perature ranging from 180°C to 230°C, and deposition plate temperature ranging from 70°C to 110°C. Optical microscopy, elastic modulus analysis and density measurement allowed empha-sizing the effect of temperature field, also measured during the process, on the morphology and the mechanical characteristics of the specimen. Atomic force microscopy revealed a morphology typical of amorphous samples, with globular structures. Poor interlayer adhesion was detected in the topmost part of the specimen, showing an elastic modulus, 220 MPa, lower than those measured in the central part, 500 MPa. The specimen crystallinity degree was below 3%. The molecular weight between entanglements was adopted as a measure of the interlayer molecular diffusion. A successful diffusion and re-entanglement of the polymer melt at the interface was the key for improving mechanical per-formances. A mathematical model describing the transient heat transfer during the fused PLA deposition and accounting for solidification and the non-isothermal crystallization kinetics was introduced. Sim-ulated temperature evolutions were consistent with the experimental ones. They were related to the mechanical performances, the morphology, and the molecular weight between entanglements of the parts.