Researchers at Northwestern University have developed a hybrid 3D printing method called direct ink additive lithography, or DIAL, for printing thick, particle-filled extrusion inks at speeds and resolutions closer to those of light-based processes. The work, led by Pranav Kaarthik, was published in Science Advances. Ryan Truby, the June and Donald Brewer Junior Professor of Materials Science and Engineering at Northwestern, said manufacturers have had to choose between the materials they need and the printing speed, resolution and scale they want; he said DIAL lays a foundation for eliminating that compromise.
The method targets a gap between two established approaches. Digital light processing requires low-viscosity, weakly scattering resins, while direct ink writing can handle functional yield-stress inks but is slowed by filament deposition and limited by nozzle diameter. DIAL combines extrusion with projector-based crosslinking in an effort to retain the material range of direct ink writing while gaining speed and finer patterning.
For each layer, a slot die extrudes ink onto the substrate, and a doctor blade scrapes it into a sheet as thin as 25 microns. A 405-nanometer projector then travels over the sheet and crosslinks selected regions, leaving uncured ink in place as support. Because light only has to pass through a thin sheet, opaque, heavily filled inks remain printable. The projector's 50-micron pixel pitch sets planar resolution.
Using a model ink of polyurethane-based resin thickened with fumed silica, the team printed a 60 mm by 40 mm log pile structure in 30 minutes, compared with 124 minutes using direct ink writing, along with fins about 150 microns thick. Rastered projection over overlaid ink sheets produced a multimaterial lattice four times the projector's illumination area. The researchers also printed interlocking chain mail with struts about 250 microns thick, 15 gyroid structural electrolyte lattices from an ionogel ink in a single 15-minute run, and hydrogel cell culture inserts containing 0.5-mm-deep inverted pyramids.
The authors described their print times as conservative because tool changes had not been tuned for speed. They also said the lack of suction and peel forces seen in vat-based printing should ease expansion of build area and volume. Truby said DIAL can print many complex, functional parts at once, and he described a path toward scaling it from the lab to larger-volume manufacturing of architected materials for robotics, aerospace, healthcare and other applications.
Kaarthik and Truby have been listed as coinventors on a U.S. provisional patent application. Funding came from the National Science Foundation, the Air Force Office of Scientific Research and Northwestern's McCormick School of Engineering.