How the 3D printer prints these slices to form the final model varies. Most consumer models are filament printers, which use a thin string of plastic that is melted in the extruder, where the molten plastic drops onto the building surface. More-advanced (and expensive) 3D printers use a resin, and the most-expensive models use a powder that is melted or liquefied (and then quickly hardens) to bind to the layer below it.