Understanding the process-microstructure-property relationships in material extrusion additive manufacturing of polylactic acid microcellular foams

Process-microstructure-property relationships are reported for microcellular foams printed via material extrusion (MEX) additive manufacturing (AM) process. Polylactic acid and thermally expandable microspheres were mixed and extruded to prepare the filament. In situ foaming during MEX AM process was then conducted to investigate the impact of nozzle temperature, flow rate, and print speed on the cellular morphology, mesostructure, part density, and the mechanical behavior of the foams. The temperature and the residence time were identified as the two key factors governing the foaming behavior and thus the resultant microstructure and properties. Too excessive temperature and residence time resulted in the deformed, wrinkled, or collapsed microspheres due to gas loss and contraction. On the other hand, too low temperature and residence time caused limited number of microspheres to expand, due to insufficient energy and time. Both overly activated and partially unexpanded microspheres provided nonuniform cellular morphologies and higher densities and thus adversely affected the tensile properties. The foam expansion/shrinkage behavior during the MEX AM process was demonstrated as a function of a combined process variable that unifies temperature, flow rate, and print speed. Upon the utilization of the optimum process variables, foam samples with uniform morphology, low density, and high toughness were achieved. The results shed light on the understanding and advancing of MEX AM process as a novel manufacturing approach to produce quality foams.