Researchers at Northwestern University have developed a hybrid 3D printing method called direct ink additive lithography (DIAL), designed to print thick, particle-filled extrusion inks at speeds and resolutions closer to light-based processes. The work, led by Pranav Kaarthik, was published in Science Advances.
The method addresses a trade-off between digital light processing (DLP) and direct ink writing (DIW). DLP requires low-viscosity, weakly scattering resins, while DIW can process functional yield-stress inks but is slowed 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 needed materials and desired printing speed, resolution and scale; DIAL, he said, lays a foundation for eliminating that compromise.
Thin ink sheets, projected crosslinking
In DIAL, a slot die extrudes ink onto a substrate for each layer, and a doctor blade scrapes it into a sheet as thin as 25 microns. A 405-nanometer projector then moves over the sheet and crosslinks selected regions. Uncured ink remains 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.
With a model ink made from 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 DIW, and fins about 150 microns thick. Rastered projection over overlaid ink sheets produced a multimaterial lattice four times the projector's illumination area.
Demonstrator parts and limits
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. They described their print times as conservative because tool changes had not been tuned for speed. They added that the absence of suction and peel forces seen in vat-based printing should make it easier to expand build area and volume.
Truby linked the work to broader manufacturing needs, saying that rebuilding American manufacturing would 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.