Fused deposition modeling (FDM) concept model of a geneva mechanism.	Fused deposition modeling (FDM) concept model of a pin drive mechanism.	Three dimensional printing (3DP) concept model of a cell phone.
(Courtesy, Stratasys Corp.)	(Courtesy, The Innovative Manufacturing Centre, UK)	(Courtesy, Z Corporation)

Essentially all RP technologies can be used for Concept Modeling. The choice of a technology depends to some extent on what the application is, but it isn't critical.

If you would like to see what a part that will ultimately be injection molded will look and feel like, stereolithography is a typical choice. The results will generally be sufficiently accurate, although you may need to sand or polish the model to obtain the appearance or finish that's required. The parts can often be used for functional testing, as well, depending on the application and choice of materials

Fused deposition modeling (FDM) is another good choice. It may be a little slower than stereolithography, and a lot slower than 3DP, but the parts have good strength out of the machine and can also be used for functional testing within limitations.

Other technologies may offer just as good results and may even have some advantages in terms of speed of fabrication and costs. Certainly both of these characteristics apply to three dimensional printing (3DP). The trade-offs compared to stereolithography are materials and finish. These models don't look very plastic-like, and will have a sandy appearance and require secondary operations to strengthen them to withstand even a small amount of handling. However, it's also possible to use color for enhancement.

These are the technologies that provide the fastest iteration speeds for concept modeling. In this case the goal might be to see as quickly as possible what a change might look like and make another modification right away.

Three dimensional printing (3DP) is considerably faster than any other method, and also offers very cheap materials and can print in color. Jetted photopolymer technologies are pretty quick, too, while material costs are higher and less finishing might be required. Fused deposition modeling (FDM) is slower, but little or no secondary finishing might be required which may result in a shorter overall time delay, especially for small parts. Stereolithography might offer better finish, but secondary operations are required and the part is also likely to cost more.

Selective laser sintering (SLS) gets closest to final plastic materials, and fused deposition modeling (FDM) also offers a number of durable materials, but with somewhat lower resolution. Stereolithography is also a good choice, but the materials will emulate the final thermoplastics and are not the plastics themselves. They need to be chosen carefully. Many photopolymers used in stereolithography degrade with time resulting in poor long-term stability. Newer filled materials exhibit better performance, however.

Functional testing model with more than 30 parts made by fused deposition modeling (FDM).	Functional testing model made by fused deposition modeling (FDM).	Planetary gearhead made by fused deposition modeling (FDM)	Paintball mask fabricated in plastic by selective laser sintering (SLS).
(Courtesy, Stratasys Corp.)	(Courtesy, Stratasys Corp.)	(Courtesy, Alex den Ouden)	(Courtesy, Accelerated Technologies, Inc.)