MIT Media Lab: 3-D printing with variable densities

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Over the years, the three MIT researchers and one of the companies that licensed the MIT patent, Z Corp., added new variations, including the ability to include colors in printed objects and to use a variety of materials. The ability to print metal objects, in particular, extended the technology from just a way of visualizing new designs to a means of manufacturing metal molds used for the injection molding of plastic parts.

Samuel Allen SM ’71, PhD ’75, the POSCO Professor of Physical Metallurgy and chair of the MIT faculty, spent a decade developing the metal-printing process. In producing molds for injection molding, he says, “the plastic shapes can be quite complicated, with round surfaces and thin walls.” In addition to the shapes of the finished parts, the molds need to have channels for the plastic material to be injected, and they have to be designed so that the resulting pieces can cool uniformly without warping. The 3DP process made it possible to make “parts you could not make through conventional machining,” Allen says.

Manufacturing companies took a strong interest in this work because it enabled “doing a complete design for a tool in days, rather than months,” he adds. “That means you can afford to go through more design iterations.”

Printing better materials

Another variant underway now is a system being developed by Neri Oxman PhD ’10, the Media Lab’s Sony Corporation Career Development Assistant Professor of Media Arts and Sciences, and her graduate student Steven Keating for “printing” concrete. Their ultimate aim: printing a complete structure, even a whole building.

Why do that, instead of the tried-and-true method of casting concrete in wooden forms that dates from the heyday of the Roman Empire? In part, Oxman explains, because it opens up new possibilities in both form and function. Not only would it be possible to create fanciful, organic-looking shapes that would be difficult or impossible using molds, but the technique could also allow the properties of the concrete itself to vary continuously, producing structures that are both lighter and stronger than conventional concrete.

To illustrate this, Keating uses the example of a palm tree compared to a typical structural column. In a concrete column, the properties of the material are constant, resulting in a very heavy structure. But a palm tree’s trunk varies: denser at the outside and lighter toward the center. As part of his thesis research, he has already made sections of concrete with the same kind of variations of density.

“Nature always uses graded materials,” Keating says. Bone, for example, consists of “a hard, dense outer shell, and an interior of spongy material. It gives you a high strength-to-weight ratio. You don’t see that in man-made materials.” Not yet, at least.

concrete density

Concrete samples made by hand to illustrate the concept of density gradient in concrete. A team from the MIT Media Lab hopes to be able to print such materials with a 3-D printer.
Photo: Steven Keating, Timothy Cooke and John Fernández

Variable-density printing is not just about large-scale objects. For example, Oxman has used a similar system to produce a glove with sections that are stiff and others that are flexible, designed to help prevent the wearer from developing carpal tunnel syndrome. She has also designed a chair made of different polymers, produciMACE POLICE MODEL PEPPER SPRAYng stiff areas for structural support and flexible areas for comfort, all printed out as a single unit.

Peter Schmitt, now a visiting scientist at the Media Lab, is pushing the technology in an even more sci-fi direction, trying to “build machines that could build machines,” he says. So far, he’s succeeded in making machines that can make many of the parts for another machine, but there remain many obstacles in establishing connections among these — and it’s still more of an intellectual exercise than a practical system, he concedes. “There are better ways to make the parts,” he says. “But at some point, these kinds of things will happen.”


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