Researchers at Northwestern University have demonstrated a hybrid additive manufacturing process that combines slot-die extrusion with projection lithography, aiming to remove the usual trade-off between the material a part needs and the speed, resolution and scale at which it can be printed. The method, called direct ink additive lithography (DIAL), is reported in Science Advances in work led by Pranav Kaarthik.
The two established routes each carry limits. Digital light processing requires low-viscosity, weakly scattering resins, while direct ink writing can handle functional yield-stress inks but is slowed by filament-by-filament deposition and constrained by nozzle diameter.
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-nanometre projector then travels over the sheet and crosslinks the selected regions, with uncured ink left in place to act as support. Because the light only has to pass through a thin sheet rather than a deep vat of material, opaque and heavily filled inks remain printable. Planar resolution is set by the projector's 50-micron pixel pitch.
Reported results
- 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, against 124 minutes with direct ink writing.
- The same work produced fins about 150 microns thick.
- Rastered projection across overlaid ink sheets yielded a multimaterial lattice four times the projector's illumination area.
- Further demonstrations included interlocking chain mail with struts about 250 microns thick and 15 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 were also printed.
Stated prospects and limits
The authors describe their reported print times as conservative, noting that tool changes had not been tuned for speed. They also point out that the process does not involve the suction and peel forces associated with vat-based printing, which they expect to make it easier to expand build area and volume. Those expectations concern future scaling rather than demonstrated production output.
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 printing speed, resolution and scale they want, and that DIAL lays a foundation for eliminating that compromise. He added that 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 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.