The term Rapid Prototyping (RP) refers to a class of the technologies that can automatically construct physical models from the CAD (Computer-Aided Design) data. The main advantage of the system is that almost any shape can be produced. The Rapid Prototyping Technologies can be divided into two basic techniques: the material removal (subtractive) RP and the material addition (additive) RP. The first category comprises machining, primarily milling and drilling, using a related CNC machine. The second category includes the technologies of incremental layer addition of the material. The computer model is sliced into thin layers and the component is fabricated by adding a layer upon a layer. The use of Rapid Prototyping for the production of patterns significantly reduces the time required for production preparation and the prototype production of moulds.
Stereolithography (SLA) was the first RP process introduced in 1988 by the 3D Systems Inc. SLA uses a low-power, highly focused UV laser to produce a 3D object in a vat of liquid photosensitive polymer. It can produce highly accurate polymer parts of the minimal layer thickness from 0.02 mm.
Fused deposition modelling (FDM) was developed by Stratasys. In this process, a long filament of wax or polymer is extruded onto the existing part surface from a workhead to complete each new layer of the minimal layer thickness from 0.15 mm.
Selective laser sintering (SLS) was patented in 1989. The basic concept of SLS is similar to that of SLA. It uses a moving laser beam to selectively sinter the powdered polymer, ceramics or metal composite materials. After each layer is completed, a new layer of loose powders is spread across the surface. In the areas not sintered by the laser beam, the powders are loose and can be poured out of the completed part. The minimal layer thickness is from 0.1 mm.
Laminated Object Manufacturing (LOM) – the main components of the system are a feed mechanism that advances a sheet over a build platform (the sheet material is usually supplied as rolls with adhesive backing), a heater roller to apply pressure to bond a layer with the layer below, and a laser to cut the outline of the part in each sheet layer. After each cut is completed, the platform lowers by a depth equal to the sheet thickness (0.05 – 0.5 mm). The laser cuts the outline and the process is repeated until the part is completed. After a layer is cut, extra material remains in place to support the part.
3D Printing (3DP) technology was developed at the MIT (the Massachusetts Institute of Technology) and licensed to several corporations. An ink-jet printing head deposits a liquid binder that binds the material. The binder holds the powders together to form a solid part, while the unbonded powders remain loose to be removed later. Material options are somewhat limited but inexpensive relative to other additive processes. 3D printing is quite fast, typically 2 – 4 layers per minute. However, the accuracy, surface finish, and part strength are not as good as those of some other additive processes. To further strengthen the part, a sintering step can be applied to bond the individual powders.
Direct metal laser sintering (DMLS) technology was developed jointly by Rapid Prototyping Innovations (RPI) and EOS Gmbh in 1994. It was the first commercial RP-method to produce metal parts in a single process. Metal powder (20 μm in diameter) without binder is completely melted by a high-power laser beam scanning. Density of a produced part is about 98 %. An advantage of DMLS is small size of the particles, which enables production of very fine details.