3D printers are machines used to make a three-dimensional solid object from a digital model. They use an additive process where successive layers of material are laid down in different shapes to form the final piece. 3D printers are used for prototyping as well as limited manufacturing applications.
A typical 3D printer. Image credit: Gnomon School of Visual Effects
Three-dimensional printing (3DP) is a relatively complicated process. It begins by depositing a layer of powder at the top of a fabrication chamber. The upward incremental movement of a piston dispenses the measured quantity of powder from a supply chamber. A roller then distributes and compresses the powder at the top of the fabrication chamber. Subsequently, a multi-channel jetting head deposits a liquid adhesive in a two-dimensional pattern onto the powder layer. This bonds the powder together and forms a layer of the object. When a layer is completed, the fabrication piston moves downward by the thickness of the layer. This process is repeated until the entire object is formed within the powder bed. Upon completion, the object is elevated and the extra powder brushed away, leaving a "green" object. No external supports are required during fabrication since the powder bed supports overhangs. Three-dimensional printing offers speedy fabrication and low material cost. In fact, 3DP is probably the fastest of all rapid prototyping methods. Recently, color outputs have become available; however, there are limitations in terms of resolution, surface finish, part fragility and available materials.
The video below examines parts produced by a 3D printer and also examines the printing process itself.
Video credit: Objet Geometries