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Multi-wavelength additive manufacturing produces multi-material items

18 Mar 2019

University of Wisconsin-Madison develops optical platform able to solidify different resins in one component.

Additive manufacturing (AM), whereby the action of a laser solidifies liquid polymers or other suitable raw materials into a three-dimensional fused shape layer by layer, is a potentially revolutionary fabrication technique.

While the process has been extensively investigated for the processing of single raw materials, methods to successfully process different feedstock materials in the same item have proven challenging, and routes to achieve multi-material control along all three axes of printing operation are limited.

A project from the University of Wisconsin-Madison has now developed a technique that promises to enable exactly that kind of versatility, through a platform that combines visible long-wavelength and ultraviolet short-wavelength illumination in the same operation. The findings were published in Nature Communications.

"As amazing as 3D printing is, in many cases it only offers one color with which to paint," said Andrew Boydston of UW-Madison. "The field needs a full color palette."

Previous approaches to multi-material AM have included using the additive technique to add fresh material to items already part-manufactured by another route, or exchanging the photo-resin material in use during the progress of an AM operation. But these methods fall short of the ideal, in which AM technologies would be fully integrated with chemical synthesis.

The project's breakthrough exploited advances in digital light processing (DLP) and the ability of micromirrors to accurately manipulate light in three dimensions. Its optical platform was able to control chemical composition along all three axes of an object by projecting more than one light source into a vat of photoresin, under the direction of the DLP mirrors.

In trials this DLP-AM approach was able to employ visible and UV illumination to create multi-material and spatially controlled compositions as a function of input wavelength, in formulations of acrylate and epoxide-based photoresins.

A shift in 3D-printing

The project also investigated "4D-printing" applications, whereby the AM-produced component is designed to undergo spatially controlled swelling when immersed in suitable fluid, effectively creating multi-material actuators.

Efficient methods to produce such complex shapes from more than one material, each having different expansion properties, could be an effective way to produce spatial-control actuators for several applications.

"By correlating light inputs from DLP projection with chemical composition, multi-material and spatial-control AM (MASC-AM) enables one to rapidly dictate spatially-controlled heterogeneity throughout the entire volume of a printed part," commented the project in its published paper. "The disparate materials combinations have manifested heterogeneous imagery and mechanical anisotropy through spatially controlled swelling, each from single resin vats simply by inputting controlled combinations of light."

Eventually this technique could deliver advantages in a number of engineering contexts, including stress-focusing designs for mechano-responsive materials, simulated tissue models, controlled gradation in over-molded parts, dynamic optical materials, and autonomous actuators.

"This is a shift in how we think about 3D-printing with multiple types of materials in one object," Boydston said. "This is more of a bottom-up chemist’s approach, from molecules to networks. At this stage, we’ve only accomplished putting hard materials next to soft materials in one step. There are many imperfections, but these are exciting new challenges."

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