One approach to 3-D printing is direct 3-D printing. Direct 3-D printing uses inkjet technology, which has been available for 2-D printing since the 1960s [source: Gibson, et al.]. Like in a 2-D inkjet printer, nozzles in a 3-D printer move back and forth dispensing a fluid. Unlike 2-D printing, though, the nozzles or the printing surface move up and down so multiple layers of material can cover the same surface. Moreover, these printers don't use ink; they dispense thick waxes and plastic polymers, which solidify to form each new cross-section of the sturdy 3-D object.
Rapid prototyping (RP), which we described earlier in the article, has been a major factor in the growth of direct 3-D printing. In 1994, the ModelMaker, a machine produced by a company known as Solidscape, became the first commercially successful technology to apply the inkjet approach to RP [source: Gibson, et al.]. Other commercial RP products have followed, and all of them use waxy compounds. For example, today's advanced rapid prototyping products use technologies such as multi-jet modeling (MJM), which creates wax prototypes quickly with dozens of nozzles working simultaneously [source: G.W.P.].
Binder 3-D printing, like direct 3-D printing, uses inkjet nozzles to apply a liquid and form each new layer. Unlike direct printing, though, binder printing uses two separate materials that come together to form each printed layer: a fine dry powder plus a liquid glue, or binder [source: Gibson, et al.]. Binder 3-D printers make two passes to form each layer. The first pass rolls out a thin coating of the powder, and the second pass uses the nozzles to apply the binder. The building platform then lowers slightly to accommodate a new layer of powder, and the entire process repeats until the model is finished.
MIT's 3DP process, mentioned earlier, uses this binder approach. MIT licenses companies to develop products that use 3DP, but to qualify, the company must use some unique combination of powder and binder materials.
Binder 3-D printing has a few advantages over direct 3-D printing. First, it tends to be faster than direct printing because less of the material is applied through the nozzles. Another advantage is that you can incorporate a wider variety of materials in the process, including metals and ceramics, as well as color.