How does 3d printing work? Does it print stuff made out of paper? Or will it print metal objects?
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.