3D printing, also known as stereo modeling or rapid prototyping, starts with a computer-aided design (CAD) [digital drawing] file of an object. The CAD file is then converted to a 3D printer file, which represents the object in layers and directs the printer to deposit layer upon layer of material to build an object from bottom to top. See also: Computer-aided design and manufacturing

For more than twenty years, the printing of 3D objects from digital models has been in various stages of development, ranging from printing industrial prototypes to fabricating bioengineered tissue-engineering scaffolds. Most of these 3D-printed objects were composed of single materials. However, recently is has become possible to print multiple materials with differing materials properties, such as hard, rubbery, conductive, or insulating, leading the way from rapid prototyping toward rapid production. See also: Prototype

Common methods of 3D printing include extruding thin layers of thermoplastic polymers, such as acrylonitrile butadiene styrene (ABS), poly(lactic acid), nylon, and polyethylene, and inkjet printing of liquid photopolymerizable materials that solidify on exposure to UV light or metal powders that are fused by laser melting or sintering. See also: Extrusion; Inkjet printing; Polyacrylonitrile resins; Polyamide resins; Polyester resins; Polymer; Polyolefin resins; Powder metallurgy; Sintering; Ultraviolet radiation

A quick search of the recent (2012) patent literature revealed some potential applications for producing products by 3D printing multiple materials. U.S. patent 8,323,122 describes a method of making golf club heads by 3D printing multiple metal powders, such as stainless steel for the club head and titanium for the club face. And U.S. patent 8,254,608 describes a hearing-aid device in which the housing for holding the microphone, amplifier, earphone, and battery and the conductive and insulating layers for connecting the electrical components are fabricated using a 3D-printer with multiple printing heads. See also: Hearing aid

Another development, analogous to the development of silk-screen printing which enabled 2D stencil printers to print open spaces (think of the letter O), 3D printers are now using water-soluble poly(vinyl alcohol) [PVA] as a temporary support over which another material, such as ABS, is printed. After printing is completed, the PVA is dissolved in water, leaving an object with open spaces, overhangs, and so on. Likewise, polymers that can be removed by heating can be used to produce undercuts and other features in 3D metal-printing applications. See also: Polyvinyl resins