Passage TwoAddictive 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 to carry the technology beyond specialist and low volume manufacturing into mass production will require a step-change in speed. This week, 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 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. 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 thefluid.Doing things continuously is much faster. The top-down approach makes layers a twentieth to a tenth of a millimeter thick. Each needs several seconds to complete, and it can take an hour or so to make just a few millimeters-worth of product. The continuous liquid interface production (CLIP) process, as Dr DeSimone calls the new method, is able to build items such as a 10cm-tall model of the Eiffel Tower 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 polymerization and is used in plastics production to slow down the process of curing. By employing it to create a “dead zone” in this way, Dr DeSimone stops 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 start up called Carbon3D to commercialize CLIP. 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 biological tissues—which one day may be printed on demand when patients need a transplant.(51)Whatdoes the passage say about CLIP?

A. CLIP is a new method invented by Dr DeSimone by turning stereolithography upside down.
By using CLIP, the250px-tall model of the Eiffel Tower can be built within half an hour.
CLIP has already been put into market to make soft and elastic objects.
D. CLIP can speed up the printing process and lead to mass production.