Towards in situ and real time characterization of flow-induced phenomena during material extrusion of polymer composites using 3D X-ray microtomography

This study investigates the flow-induced phenomena occurring during additive manufacturing by material extrusion of short-fiber reinforced thermoplastic composites. For that purpose, in situ interrupted and real-time printing experiments coupled with 3D synchrotron X-ray microtomography observations were conducted. Various types of composites, namely glass fiber-reinforced polyamide 6, glass-fiber reinforced polycarbonate and wood fiber-reinforced polylactic acid, as well as various nozzles were investigated. Using 3D images acquired during the static printing experiments, it was possible to quantify the evolution of several key microstructure descriptors (e.g., pore and fiber volume fractions, porosity shape and size, fiber length and orientation) in various locations in the tested nozzles. All the experiments revealed that drastic microstructural changes (e.g., pore appearance/disappearance, orientation and shortening of fibers) occurred in various zones of the nozzles during material extrusion. In addition, the results highlighted the central role of the nozzle geometry (e.g., convergent angle, output channel) on the pore formation, transport and disappearance as well as on the orientation and shortening of fibers. Besides, the 3D images acquired during the dynamic (real-time) printing experiments confirmed the observations made during the interrupted tests but also emphasized the complex kinematics of pores in the cavity of the nozzles and the mechanisms at the origin of their disappearance through the convergent. The knowledge acquired during these experiments will undoubtedly lead to the manufacturing of optimized nozzles.