Researchers at Northwestern University have developed a hybrid additive manufacturing process that combines slot-die extrusion of thick, particle-filled inks with projection lithography, an approach intended to narrow the long-standing trade-off between the materials a printer can handle and the speed, resolution and scale it can achieve. The method, called direct ink additive lithography (DIAL), was led by Pranav Kaarthik and published in Science Advances.
The two established routes each carry limits. Digital light processing requires low-viscosity resins that scatter light only weakly, which rules out many heavily loaded functional formulations. Direct ink writing can deposit functional yield-stress inks, but its throughput is constrained by filament-by-filament deposition and its feature size by nozzle diameter.
"In 3D printing, manufacturers have had to choose between the materials they need and the printing speed, resolution, and scale they want," said Ryan Truby, June and Donald Brewer Junior Professor of Materials Science and Engineering at Northwestern. "With DIAL, we lay a foundation for eliminating that compromise."
How the process works
For each layer, a slot die extrudes ink onto the substrate and a doctor blade scrapes the deposit into a sheet as thin as 25 microns. A 405-nanometer projector then travels across the sheet and crosslinks only the selected regions, leaving uncured ink in place to act as support. Because the light has to pass through just a thin sheet rather than a deep vat, opaque and heavily filled inks remain printable. The projector's 50-micron pixel pitch sets the 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 by direct ink writing, and produced fins roughly 150 microns thick. Rastering the projection across overlaid ink sheets yielded a multimaterial lattice four times the projector's illumination area.
Demonstrated geometries
- Interlocking chain mail with struts about 250 microns thick.
- Fifteen gyroid structural electrolyte lattices printed from an ionogel ink in a single 15-minute run.
- Hydrogel cell-culture inserts containing 0.5-mm-deep inverted pyramids.
The authors described their reported print times as conservative, noting that tool changes had not been tuned for speed. They also argued that the absence of the suction and peel forces found in vat-based printing should make it easier to expand build area and volume.
Truby framed the work in manufacturing terms: "Rebuilding American manufacturing will depend on new ways to make advanced materials. Because DIAL can print many complex, functional parts at once, we see a path toward scaling it from the lab to larger-volume manufacturing of architected materials for applications spanning robotics, aerospace, healthcare, and much more."
Kaarthik and Truby are listed as coinventors on a U.S. provisional patent application. The work was funded by the National Science Foundation, the Air Force Office of Scientific Research and Northwestern's McCormick School of Engineering.