Once upon a time, the only way to make a three-dimensional object was by hand. One could carve wood or bone, shape clay, blow glass, and pound metal into shape. Unfortunately, while such techniques are extraordinarily versatile, they are not good at making many identical duplicates of a single object. Molding and casting can make duplicates that seem identical to the human eye, but they nevertheless vary on the microscopic scale, which can be quite important if the duplicates are intended to function as interchangeable parts in complex machines. Machine tools such as lathes improve the situation enough to make interchangeable parts for relatively simple machines such as rifles, and indeed Eli Whitney's development of interchangeable parts for muskets helped create mass production and make America's westward expansion possible in the nineteenth century.
But early machine tools were human-controlled. It took a great deal of expertise to use them, and a fair amount of time to make each duplicate. In the late 1940s, John Parsons (head of a company that produced helicopter rotors) devised a way to make punch-card-operated electromechanical calculators generate templates for human-operated machine tools to follow. He then envisioned an extension of the system that would have automated machine tools follow the templates on their own. He became known as the father of numerical control technology and was awarded the National Medal of Technology in 1985.
As computers developed, numerical control became quite sophisticated. It even became possible to design an object on a computer and feed the design to automated equipment to make the object; this is CAD/CAM (computer-aided design/computer-aided manufacturing). But the process remained expensive--"machine tools" meant drills and lathes capable of working hard metal--and it remained impossible to make hollow objects.
Both of these problems are now being addressed. "Rapid-prototyping" and "3D-printing" tools have been built from the basic idea that a thin layer of powder (plastic, metal, or ceramic) can be fused with a laser, or a thin film of liquid (plastic) can be polymerized (hardened) with a laser. A second layer can then be laid down atop the first and similarly fused or hardened. The trick is to build a machine that can produce and harden layers on demand. When such layers amount to slices through a 3D object (even ones with interior spaces), the accumulation of layers produces the object, as described in Ivan Amato, "Instant Manufacturing," Technology Review (November 2003). It is already finding much use in building prototypes (rapid prototyping) of items to be manufactured by more conventional means, as well as special items such as gears and machine parts, bone implants, and form-fitting items such as hearing aids. The equipment tends to be expensive, but the price is expected to decrease. Will it ever reach a level that the home consumer would feel able to afford? That is a very interesting question, as is the closely related question of what that consumer would use 3D printing for.
One new version of the technology is essentially an inkjet printer that can build up small shapes. Another version offers a 3D printer than can almost reproduce itself! (See Celeste Biever, "3D Printer to Churn Out Copies of Itself," New Scientist news service, March 18, 2005.) Still another version "prints" a biodegradable gel and distributes cells; the aim is to produce custom-designed tissues and organs for use in transplants. Researchers have already begun to develop techniques for "printing" skin and blood vessels, and the future should be very interesting. See Rebecca Camber, "Tailor-Made Skin from 'Ink' Printer," Manchester University (January 19, 2005), and Kate Green, "Printing Blood Vessels," Technology Review (January 20, 2006).
Will 3D printing "emerge"? It already has a place in industry, so one can say it has already done so in one sense. But as Amato mentions, there is enormous potential in the realm of "on-demand manufacturing," personal customization (think of case-modding!), and so on, if the equipment can be brought down enough in price. If this happens, a great many products may no longer be sold in physical form, but as computerized design files. Consumers will "print" the files to obtain the physical products.
Will consumers choose to print rather than fetch? If they do, there will be negative effects on all those companies that make and distribute the objects that can now be sold in the new form. The positive effects will be on new companies that generate and distribute design files, as well as on consumers who can get products more cheaply or can get products that can no longer be found in physical form at all (such as parts for classic cars!). The net effect is what will determine the emergence of the technology as a consumer technology.
And the technology does not stop here! Nanotechnology advocates have been talking for some time now about the "nanofactory," which will be able to manufacture on demand almost anything from a basic supply of atoms and molecules. If nanofactories are even possible (and some do question their practicality), they are much further off than home 3D printers.