It works by building up layers of material, each layer looks like what you’d get if you sliced the object at that level, or did a CAT scan.
Yes, one printer makes the layers out of paper, but the most common material is plastic extruded out of what amounts to a hot-melt glue gun. Pretty much anything that melts and refreezes at a reasonable temperature will do, up to and including chocolate!
Another technique is to expose a liquid polymer to a patterned or scanned UV light that hardens the liquid into a solid.
Still another selectively fuses layers of powder. One technique uses an inkjet printer head to squirt droplets of glue and/or ink onto the powder, another uses a scanned laser to melt select areas together. Materials include plastics, plaster, glass (sand), sugar, and a wide variety of metals, from copper through stainless steels, to exotic titanium alloys.
And lastly, a few companies build up the parts with repeated welding beads.
Interesting stuff. And scary. Apparently there’s nothing that can’t be done.
I remember back in the early 90s someone told me “One day you’ll be able to send and receive faxes from your car.” He was almost right. He missed the part about email being invented. And speaking of email, the first time I heard that word was Ross Perot talking about it on Larry King Live. It sounded very weird.
Around that same time I told my wife that one day there would be a box on top of the TV where you could slide a card and buy things from stores and the items would be delivered. All I missed were the boxes.