Researchers at Northwestern University have developed a hybrid 3D printing method called direct ink additive lithography, or DIAL, designed to print thick, particle-filled extrusion inks at speeds and resolutions closer to those of light-based processes. The work was led by Pranav Kaarthik and published in Science Advances.
The project addresses a trade-off 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. 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 speed, resolution and scale they want. He said DIAL lays a foundation for eliminating that compromise.
A layer-by-layer process
For each layer, a slot die extrudes ink onto the substrate, and a doctor blade scrapes the material into a sheet as thin as 25 microns. A 405-nanometer projector then travels over the sheet and crosslinks selected regions. Uncured ink remains in place and acts as support. Because light only has to pass through a thin sheet, opaque and heavily filled inks remain printable. The projector's 50-micron pixel pitch sets the planar resolution.
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 for direct ink writing, and produced fins about 150 microns thick. Rastered projection over overlaid ink sheets also produced a multimaterial lattice four times the projector's illumination area.
Printed demonstrations
The researchers printed interlocking chain mail with struts about 250 microns thick. They also produced 15 gyroid structural electrolyte lattices from an ionogel ink in a single 15-minute run. In addition, they made 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 absence of suction and peel forces seen in vat-based printing should make it easier to expand build area and volume.
Outlook for manufacturing
Truby linked the work to broader manufacturing goals, saying that rebuilding American manufacturing will depend on new ways to make advanced materials. He said DIAL can print many complex, functional parts at once, which the team sees as a path toward scaling the process 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.