A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 2 5 6 3D PRINTSHOP WOOD SHEETS SELF-MORPH INTO 3D OBJECTS Researchers from the Hebrew University of Jerusalem are taking lessons from nature and the way cut wood warps as it dries and are using the idea to program 3D printing of a wood-like material. A few years ago, the team developed an environmentally friendly water-based ink composed of wood-waste microparticles known as “wood flour” mixed with cellulose nanocrystals and xyloglucan, which are natural binders extracted from plants. The researchers then began using the ink in a 3D printer. They recently discovered that the way the ink is laid down dictates the morphing behavior as the moisture content evaporates from the printed piece. For instance, a flat disk printed as a series of concentric circles dries and shrinks to form a saddle-like structure, and a disk printed as a series of rays emanating from a central point turns into a dome or cone-like structure. The ultimate shape of the object can also be controlled by adjusting print speed, the team found. That’s because shrinkage occurs perpendicular to the wood fibers in the ink, and print speed changes the degree of alignment of those fibers. A slower rate leaves the particles more randomly oriented, so shrinkage occurs in all directions. Faster printing aligns the fibers with one another, so shrinkage is more directional. The scientists learned how to program the print speed and pathway to achieve a variety of final shapes. They found that stacking two rectangular layers that are printed in different orientations yields a helix after drying. In their latest work, they found that they can program the printing pathway, speed, and stacking to control the specific direction of shape change, such as whether rectangles twist intoahelix that spirals clockwise or counterclockwise. Further refinement will allow the team to produce objects with complicated final shapes, such as a chair. Ultimately, it could be possible to make wood products that are shipped flat to the end user, says Doron Kam, graduate student. “Then, at the destination, the object could warp into the structure you want.” Eventually, it might be feasible to license the technology for home use so consumers could design and print their own wooden objects with a regular 3D printer. The team is also exploring whether the morphing process could be made reversible. “We hope to show that under some conditions we canmake these elements responsive—to humidity, for example—when we want to change the shape of an object again,” says Eran Sharon, one of the project’s principal investigators. A video on the research is available at www.acs.org/printingwood. IN SITU CURING OF THERMOSET COMPOSITES A new 3D-printing technique cures carbon-fiber-reinforced thermoset composites without postcuring or postprocessing steps. A team from the Walter Scott, Jr. College of Engineering at Colorado State University have discovered a way to print carbon fiber composites in a very short amount of time with minimal heat. Most industrial production of carbon fiber requires intense heating of molds that spend 6 to 10 hours in huge ovens the size of semi-trucks. Thermoset polymers are liquids that turn solid through a one-way chemical reaction, meaning they won’t turn back into liquids upon reheating. Epoxy that is used in woodworking is an example of a thermoset polymer. Upon extrusion and deposition of the composite ink from a printing nozzle, the ink is cured via frontal polymerization, leading to rapid printing of high-quality composites. Tailoring the processing conditions allows for freeform or rapid, supported printing of 3D composite objects with zero void content and highly oriented carbon fiber reinforcements. colostate.edu. Carbon fiber material hardens on its own as it unfolds on a heated surface. Courtesy of Colorado State University. Wood ink printed as a flat rectangle is programmed to form a complex shape after drying and solidifying. (Ruler is marked in centimeters.) Courtesy of Doron Kam.
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