Rising from the ooze

ADDITIVE manufacturing, or 3D printing as it is popularly called, can make complex objects out of many materials. The technology, though, has a problem: speed. Fusing together tiny filaments of plastic or melting successive layers of metallic powder into a solid shape takes time. This means even a small object may require hours to emerge from a 3D printer. Things can be printed simultaneously, in batches, which speeds things up a bit. But what is needed to carry the technology beyond specialist and low-volume manufacturing into mass production is a step-change in speed. In March a group of researchers led by Joseph DeSimone of the University of North Carolina, Chapel Hill, showed, in a paper in Science, how that might be done.

Their idea harks back to the way 3D printing began, with a process called stereolithography that was invented in 1986. Stereolithography uses ultraviolet (UV) light to cure plastic into the desired shape. A pattern of light played onto the surface of a vat of liquid polymer creates a solid layer out of part of that surface. As in other additive-manufacturing methods, the process is then repeated to cure another layer of polymer on top of the first, and then another, and another, with the resulting shape being lowered steadily into the liquid until it is complete.

Dr DeSimone and his colleagues have turned stereolithography on its head. Their 3D printer works from the bottom of the vat rather than the top, and cures continuously as the growing object is gradually extracted from the liquid.

Doing things continuously, rather than one layer at a time, is much faster. The top-down approach generates layers a twentieth to a tenth of a millimetre thick. Each layer needs several seconds to complete, and it can take an hour or so to make just a few millimetres-worth of an object. The continuous liquid interface production (CLIP) process, as the new method is called, is able to build items such as a 10cm tall model of the Eiffel Tower (illustrated) in that amount of time. Other shapes have been printed even faster, at up to 50cm an hour.

The process is not, however, just a matter of turning stereolithography upside down. For a start, the UV that cures the required shape has to be projected through a window in the bottom of the vat containing the liquid polymer. This window must also be permeable to oxygen, letting the gas pass into the zone between the window and the area being cured. Oxygen inhibits polymerisation and is used in plastics production to slow down the process of curing. By using it to create a “dead zone” in this way, it prevents the cured polymer sticking to the base of the vat. As the shape is steadily formed above the dead zone, the printer raises the object until, eventually, it emerges complete.

Dr DeSimone and his colleagues have established a startup called Carbon3D to commercialise the CLIP process. At the moment it would be no rival to printing metals with a laser or electron beam, a technique that a number of aerospace companies are now using. But CLIP could give existing ways of printing plastics a run for their money. The researchers think it would be particularly good at making soft, elastic objects, and that it could be adapted for ceramics. It might also be used to make medical devices and even, in the long run, biological tissues—which one day may be printed on demand when patients need a transplant.


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