Effect of material extrusion process parameters on filament geometry and inter-filament voids in as-fabricated high solids loaded polymer composites

Fully dense structures of high solids loaded polymer composites printed via material extrusion additive manufacturing are of interest across many applications, including biomedical scaffolds and energetic materials. It is common to observe defects such as interfilament voids in space-filling structures, particularly during material extrusion printing with high solids inks, which exit the nozzle in a circular structure and require applied forces to deform. An understanding of the impact of process parameters on the void volume and structure, which can ultimately affect material performance, is necessary. Here we examine the effects of four key process parameters, layer height, extrusion rate, print speed and nozzle diameter, on the filament geometry and interfilament void formation. The layer height, extrusion rate and print speed all impact the width of the printed lines, but the layer height plays the most significant role in the line aspect ratios and void formation in printed structures. We find that in material extrusion of high solids loaded inks there is a critical layer height; below the critical height, the ink will spread to fill in gaps between filaments, and above it the filament will maintain a rounded shape and lead to formation of interfilament voids. The results illuminate a pathway to control porosity of fully dense structures with high solids loaded inks using the layer height parameter while maintaining geometric accuracy in the printed design.