Northwestern University researchers have developed a hybrid 3D printing process that combines extrusion with projection lithography to pattern thick, particle-filled inks. Called direct ink additive lithography, or DIAL, the method is described in Science Advances and was led by Pranav Kaarthik. Its developers position it between digital light processing and direct ink writing, two approaches that have forced trade-offs between printable materials and production speed, resolution and scale.
Digital light processing typically requires low-viscosity resins that scatter light weakly. Direct ink writing can handle functional yield-stress inks, but its speed is limited by filament deposition and its resolution by nozzle diameter. DIAL seeks to separate material choice from those constraints.
How the process works
For each layer, a slot die extrudes ink onto a substrate, and a doctor blade spreads it into a sheet as thin as 25 microns. A 405-nanometer projector then moves over the sheet and crosslinks selected regions. Ink that is not exposed remains in place and acts as support. Because light passes only through a thin sheet, opaque and heavily filled inks remain printable. The projector's 50-micron pixel pitch sets the planar resolution.
The team tested the process with a model ink made from polyurethane-based resin thickened with fumed silica. It printed a 60 mm by 40 mm log-pile structure in 30 minutes, compared with 124 minutes for direct ink writing, and produced fins about 150 microns thick. By rastering projection across overlaid ink sheets, the researchers also made a multimaterial lattice covering four times the projector's illumination area.
Demonstrators and reported performance
Beyond the log pile, the researchers printed interlocking chain mail with struts about 250 microns thick. They produced 15 gyroid structural electrolyte lattices from an ionogel ink in a single 15-minute run. They also made hydrogel cell-culture inserts containing 0.5-mm-deep inverted pyramids. These examples span filled, functional and hydrogel inks and show the process in laboratory demonstrators.
Ryan Truby, a materials science and engineering professor at Northwestern, said manufacturers have had to choose between needed materials and desired printing speed, resolution and scale. He described DIAL as a foundation for eliminating that compromise. The authors characterized their reported print times as conservative because tool changes had not been tuned for speed. They also said the absence of suction and peel forces found in vat-based printing should make it easier to expand build area and volume.
Truby linked the work to broader manufacturing goals, saying that rebuilding American manufacturing will depend on new ways to make advanced materials. Because DIAL can print many complex functional parts at once, the team sees 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 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.