Suppression of Filament Defects in Embedded 3D Printing

Embedded 3D printing enables the manufacture of soft, intricate structures. In the technique, a nozzle is embedded into a viscoelastic support bath and extrudes filaments or droplets. While embedded 3D printing expands the printable materials space to low-viscosity fluids, it also presents new challenges. Filament cross-sections can be tall and narrow, have sharp edges, and have rough surfaces. Filaments can also rupture or contract due to capillarity, harming print fidelity. Through digital image analysis of in situ videos of the printing process and images of filaments just after printing, we probe the effects of ink and support rheology, print speeds, and interfacial tension on defects in individual filaments. Using model materials, we determine that if both the ink and support are water-based, the local viscosity ratio near the nozzle controls the filament shape. If the ink is slightly more viscous than the support, a round, smooth filament is produced. If the ink is oil-based and the support is water-based, the capillary number, or the product of the ink speed and support viscosity divided by the interfacial tension, controls the filament shape. To suppress contraction and rupture, the capillary number should be high, even though this leads to trade-offs in roughness and roundness. Still, inks at nonzero interfacial tension can be advantageous, since they lead to much rounder and smoother filaments than inks at zero interfacial tension with equivalent viscosity ratios.