Effects of printed bead volume on thermal history, polymer degree of crystallinity and mechanical properties in large scale additive manufacturing

One of the promising innovations in additive manufacturing by material extrusion is the usage of large machines that allow building big parts in shorter times. In this case, though, the complex interactions between materials and processing (polymer plasticization, polymer chains inter-diffusion, inter-bead fusion, polymer crystallization etc.) are still not completely understood. The present work brings novel fundamental and quantitative knowledge that would contribute to the development of this technology. Specimens made of a semicrystalline polylactic acid were printed using different bead volumes with a single screw extruder mounted on a robot arm. A cascade effect was identified during production runs: variations in the bead volumes substantially impacted the thermal histories, which influenced polymer crystallization, and this in turn affected the mechanical properties. As a result, a substantial heterogeneity in the degree of crystallinity could be generated in large 3D printed parts, that would induce locally different mechanical properties. Thus, it is clear that not only inter-bead junctions must be considered for understanding the mechanical properties, but also the extent of crystallization of the polymer. Finally, thermal histories of the beads were precisely measured, and these valuable data could help the scientific community to better understand the evolutions of the bead temperature during the production runs.