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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Research Team Prints Robust Structural Glass

By Kevin Wilcox

A new 3-D printing process yields elegant transparent glass shapes with dramatic patterns of refracted light that can hold its own against structural forces.

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The process creates a coiled rope effect in the 3-D printed glass structures that is both beautiful and strong. Andy Ryan

October 20, 2015—A team of researchers working at the Massachusetts Institute of Technology (MIT) has developed and successfully tested an additive manufacturing printer capable of withstanding the extreme temperatures of molten glass to create transparent, three-dimensional glass objects. The breakthrough holds the potential to bring significant changes to the fields of architecture and engineering.

The research is a collaboration between the Mediated Matter Group at the MIT Media Lab, MIT's Department of Mechanical Engineering, MIT's Glass Lab, and Harvard's Wyss Institute for Biologically Inspired Engineering. Developing the system was an iterative process that focused on the need to keep the temperature of the molten glass high and consistent before and during printing and to slow and control the rate of cooling after printing. This resulted in a printer that employs a ceramic crucible surrounded by a kiln to improve the gravity flow of the molten glass. The material flows through an independently heated ceramic nozzle, the assembly's movements controlled digitally on the x, y, and z axes. The glass is deposited into a heated annealing chamber that slows the cooling process to remove residual stress.

In the team's experiments, structural elements that would be exposed to extreme heat are fabricated from square steel tubing, and aluminum is used for elements that would not be exposed to high temperatures. Three motors drive lead screw-gantry systems that move the print head in accordance with a computer model of the item to be printed.

The resulting 3-D-printed glass shapes have a high degree of transparency and a coiled rope appearance, each layer of the printing process visible in the shape. The temperature-control measures result in good adhesion between the layers—a concern in many 3-D printing applications, including concrete and steel. "Printing directly into a lower kiln at the glass-annealing temperature definitely improved adhesion between layers to the point that when the shapes were load-tested, they did not break along the layer boundary," explained Peter Houk, the director of MIT's Glass Lab, in written answers to questions posed by Civil Engineering online. "After each part is printed, it is then immediately transferred into a separate annealing oven, where residual stress is removed from the glass through slow cooling."

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By maintaining consistent heat during printing and managing and slowing the cooling process, the system produces good adhesion between the layers of molten glass. Steven Keating

The process is creating a structurally superior product. "The printed parts appear to be highly resistant in compression," noted Giorgia Franchin, a Ph.D. candidate in the Mediated Matter Group at the MIT Media Lab. Although strength testing is in the early stages, Franchin explained that a printed tube 85 mm in diameter, 7.3 mm in thickness, and 70 mm tall did not break under a load of 50kN, though it did show damage.

"Delamination between the different layers is a function of the strength of adhesion; bars tested via a three-point bending test did not break along the boundary layer," Franchin added. "But tension and bending remain critical loading situations because of the stress concentration at the recess between two layers."

The coiled rope effect of the process and the complex geometric forms that can be created through the printing process combine to create spectacular patterns of refracted light when the glass is lit from within. "The optical properties of the 3-D-printed glass structures are one of the most promising aspects of the project," said John Klein, a research assistant in the Mediated Matter research group at the MIT Media Lab. "We've demonstrated that depositing material in varying geometrical configurations results in highly complex, caustic patterns. We believe that further developing the relationship between geometry and light transmission can lead to multifunctional architectural envelopes, with the capacity to store thermal energy and transmit light into interior volumes to enhance the atmospheric and spatial qualities." Klein was a colead author of the paper "Additive Manufacturing of Optically Transparent Glass," published recently in the September 2015 issue of 3D Printing and Additive Manufacturing. Michael Stern, a member of the technical staff in the MIT Lincoln Laboratory's Rapid Prototyping Group, and Franchin are the other colead authors.

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The combination of layering and complex geometric shapes created dramatic caustic patterns of refracted light. Andy Ryan

In addition to those previously mentioned, researchers on this project include Markus Kayser and Chikara Inamura, both research assistants in the Mediated Matter research group at the MIT Media Lab, Shreya Dave, a graduate student, and James Weaver, Ph.D., a senior research scientist at Harvard University's Wyss Institute. The team was inspired by architect Ludwig Mies van der Rohe, who in 1922 developed the concept for a largely glass skyscraper to be sited in Berlin. Mies developed a model and found that the extensive use of glass created "a great game of reflections of light." Mies went on in the 1940s to design the Resor House, in Jackson Hole, Wyoming, and the Farnsworth House in Plano, Illinois. Both are sleek, modernist summer homes that make extensive use of glass.

"It was Mies who professed that the material is only what we make of it, that everything depends on how we use a material, not on the material itself," said Neri Oxman, Ph.D., an associate professor of Media Arts and Sciences at the MIT Media Lab, who is also on the team. "Mies' glass skyscraper was, to us, more than an inspiration because at its core is the belief that technological innovation can drive form and function.

"Because glass is at once structural and transparent, it's relatively easy to consider the integration of structural and environmental building performance within a single integrated skin, especially if multifunctionality can be achieved as part of a single process," she said. That might include, for example, embedding the openings for electrical or other utility services into the printed structural glass elements.

"Consider printable optoelectronics, or the possibility of combining optical fibers for high-speed data transmission by light, combined within printed glass building facades," Oxman said. "Or channel networks built into the architectural skin containing photosynthetic media for the production of biofuels and electricity. Consider a single, transparent building skin that can integrate multiple functions and can be shaped to tune its performance.

"That's what's next."


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