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The research has been published in the journal Additive Manufacturing.

“The most common form of 3D printing is thermoplastic extrusion,” said Emre Gunduz, assistant research professor of mechanical engineering. “That’s usually good enough for prototypes, but for actual fabrication, you need to use materials with high strength, like ceramics or metal composites. But the precursor for these materials are extremely viscous, and normal 3D printers can’t deposit them, because they can’t be pushed through a small nozzle.”

Most proposed solutions to this problem involve changing the composition of the materials themselves. However, Gunduz’ solution is to apply high-amplitude ultrasonic vibrations to the nozzle itself. “We found that by vibrating the nozzle in a very specific way,” said Gunduz, “we can reduce the friction on the nozzle walls, and the material just snakes through.” Gunduz and his team have been able to print items with 100-micron precision, which is better than most consumer-level 3D printers.

It’s difficult to visualize the process, because the material is opaque and the surfaces are hidden inside the nozzle. So the team traveled to Argonne National Laboratory, outside Chicago, to conduct high-speed microscopic X-ray imaging. They were able to see inside the nozzle, and precisely measure the flow of the clay-like material for the first time. “The results were really striking,” said Gunduz. “Nobody has ever characterized a viscous flow through a channel this way. We were able to quantify the flow, and understand how our method was actually working.”

The research is being conducted at Zucrow Labs, the largest academic propulsion lab in the world. As such, the first practical application being explored is for solid rocket fuel.

“Solid propellants start out very viscous, like the consistency of cookie dough,” said Monique McClain, Ph.D. candidate in aeronautics and astronautics. “It’s very difficult to print, because it cures over time, and it’s also very sensitive to temperature. But with this method, we were actually able to print strands of solid propellant that burned comparably to traditionally cast methods.” McClain tested the combustion by printing two-centimeter samples, igniting them in a high-pressure vessel (up to 1,000 psi), and analyzing slow-motion video of the burn.

For solid rocket fuels, 3D printing offers the opportunity to customize the geometry of a rocket and modify its combustion. “We may want to have certain parts burn faster or slower, or something that burns faster in the center than the outside,” said McClain. “We can do that much more precisely with this 3D printing method.”

Beyond rocket propellants, Gunduz imagines numerous uses for this new viscous material printing process. “We can 3D print different textures of food,” said Gunduz. “Biomedical implants, like dental crowns made of ceramics, can be customized. Pharmacies can 3D print personalized drugs, so a person only has to take one pill, instead of ten. It’s very exciting that we can print materials that no one’s been able to print.””

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