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A 3-D printer created this one piece rocket-engine injector in 40 hours. It has previously months to make the same part by welding many pieces together. The left image shows the injector after it was removed from the melting printer and the right image shows the final version.
Image credit: NASA/MSFC
A computer file instructs the printer how use its laser to melt fine powder into its final shape. Of course, NASA wasn't satisfied with calling it 3-D printing so it's now additive manufacturing.
This paint brush shows the fine granules used in the process.