Researchers at Northwestern University have developed a hybrid additive manufacturing method, direct ink additive lithography (DIAL), intended to print 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.
The method targets a trade-off between two established routes. Digital light processing requires low-viscosity resins that scatter light only weakly, while direct ink writing can handle functional yield-stress inks but is slowed by filament-by-filament deposition and limited by nozzle diameter.
Ryan Truby, 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 speed, resolution and scale they want, and that DIAL lays a foundation for removing that compromise.
How a layer is formed
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 over the sheet and crosslinks only the selected regions, with uncured ink left in place to serve as support. Because light has to pass through only a thin sheet, opaque and heavily filled inks remain printable, the researchers report. Planar resolution is set by the projector's 50-micron pixel pitch.
Reported build results
Using a model ink made from a 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, with fins about 150 microns thick. Rastering the projection across overlaid ink sheets produced a multimaterial lattice four times the projector's illumination area.
The group 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.
Stated limits and next steps
The authors described their reported print times as conservative, noting that tool changes had not been tuned for speed. They added that the absence of the suction and peel forces seen in vat-based printing should make it easier to enlarge build area and volume.
Truby said rebuilding American manufacturing will depend on new ways to make advanced materials, and that because DIAL can print many complex, functional parts at once, the team sees a path to scaling the process from the lab toward larger-volume production of architected materials for robotics, aerospace, healthcare and other applications.
Kaarthik and Truby are 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.