Abstract
Embedded 3D printing is an additive manufacturing technique wherein a nozzle extrudes continuous filaments into a support bath, which is usually a yield stress fluid such as a polymeric hydrogel or granular microgel. Because the bath takes care of form holding, embedded 3D printing enables the fabrication of inks that are not self-supporting, including low-viscosity bio-inks and functional materials. Moreover, embedded 3D printing allows for complex printed geometries via freeform print paths because it is not limited to layer-by-layer printing. However, defects including sharp edges, anisotropy, filament roughness, contraction, rupture, curling, and poor inter-filament fusion can compromise the mechanical integrity and geometric fidelity of printed parts. Here, we discuss ways to control common filament-scale defects via feedstock materials and printing parameters. We use OpenFOAM, a computational fluid dynamics solver, to simulate the extrusion of single filaments. By simulating theoretical materials, we can separate viscous dissipation, yielding, shear thinning, and interfacial tension effects. Additionally, simulations provide insights into cell survival inside the nozzle and the shape of the flow field, which governs sharp edges and anisotropy. We use in-situ imaging experiments to monitor the extrusion of horizontally- and vertically-printed filaments. By examining a wide material space and processing space, we identify key scaling relationships that govern defect formation. Critically, the local ink viscosity to support viscosity ratio near the nozzle governs sharp edges, anisotropy, and roughness in low interfacial tension systems. In high interfacial tension systems, the capillary number, or the relationship between extrusion speed, local support viscosity, and interfacial tension, governs anisotropy, contraction, and droplet formation. By printing at a moderate viscosity ratio or high capillary number, we can produce high-quality soft structures.
