3D printing has mainly been confined to parts made out of various kinds of plastics up till now. Metal has been hard to deal with, because at some point you have to melt it, and that requires temperatures higher than any printing-type device could cope with.

Now a company called “Desktop Metal” http://www.theregister.co.uk/2017/11/10/metal_3d_printing_at_desktop_metal/ has come up with a scheme involving binding layers of metal powder, laid down in the right shape, using a temporary compound which is then removed when the part is melted in a furnace so it becomes a proper solid piece. Obviously this has to be done without losing its shape.

The process can also do clever things like make a hinge, where there is no metal between the parts, so once the binding compound is removed, you have two solid parts that can freely rotate without falling apart.