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Rapid Prototyping is the process of going directly from CAD model to prototype part via some fast automatic process so that the prototype design can be seen, held, and possibly tested; the rapid prototyped part is frequently not made of the correct material (i.e., plastic or wax instead of metal or composite) or is not full scale, and so isn't the final part. Rapid prototyping in some cases can be part of a rapid manufacturing process.

Typically, rapid prototyping is used for making single parts for demonstration or testing. Examining a physical model in your hand and turning it around and looking at it from all sides is much more meaningful than looking at flat 2-d drawings of the same, and is still somewhat superior to even 3d models on the computer screen. Rapid prototyped models are typically less accurate or less durable than the final product, but still good enough for initial fit checking and some mechanical and physical testing.

Rapid prototyping is typically more expensive and slower than standard manufacturing techniques used for mass production, but cheaper and faster (and less accurate) than machining a single part.

Thus, rapid prototyping is most well suited to rapid manufacturing for small production runs of less than 50 parts, or small early production runs before tooling is complete, where time is more important than cost. Also, rapid prototyping could be used as part of the tooling process-- by creating preliminary molds for plastic injection molding or helping to form the investment in investment casting, etc.

This technology has been around since 1982 and is probably 50 years away from the Star Trek replicator technology or the matter compiler from The Diamond Age. Ongoing research in nanotechnology, materials science, miniturization, and other fields will both feed and drive rapid prototyping closer to this goal.

 

There are several methods of rapid prototyping, including:

Stereo Lithography (SLA)
The part is built one layer at a time by exposing the top surface of a photoset polymer with a laser.
Selective Laser Sintering (SLS)
A thermoplast powder (liquid, metal, glass) is melted (sintered) together one layer at a time with a laser to form a completed part. Metal parts are typically porous, and can be dipped in a liquid metal with a lower melting point to create a more solid part.
3D Printing
inkjet print head technology or similar is used to spray thermoplast polymer, wax, glue, or starch one layer at a time to form a part.
Laminated Object Manufacturing (LOM)
sheets of paper or plastic film are attached to previous layers by either sprayed glue, heating, or embedded adhesive, and then the desired outline of the layer is cut by laser or knife. Finished product typically looks and acts like wood.
computer aided machining
A computer controlled milling machine is used to carve a part out of a block of metal. CAM is typically not considered a rapid prototyping process, as they are historically very labor intensive, slow, and restrict part shape. However, modern emedded CNC systems are becoming faster and more automated.
fused deposition modeling
A semi-liquid material (from a plastic filament or metal wire) is extruded from a nozzle, sort of like toothpaste or like a glue gun to build the part.
microfabrication or lithography process
a material (like silicon wafers) is repeatedly photoetched and recoated to form a part in layers
Laser Engineered Net Shaping (LENS)
A laser is used to melt metal powder and deposit it on the part directly. This has the advantage that the part is fully solid (unlike SLS) and the metal alloy composition can be dynamically changed over the volume of the part.
Much like Neal Stephenson's matter compiler, all of these methods have a cavity size that limits the maximum part size, and have mechanical and material complications that limit speed. In both cases, a more expensive machine can be made to make larger parts faster.

A few companies that make products related to rapid prototyping include:

3d Systems
Cubital America
DTM Corporation
Solid Concepts
Conceptual Reality
Kodak

Rapid prototyping is a developing field with new methods being invented every year. Current systems are expensive and/or messy to run, and material properties of parts are not always ideal. The future is coming!

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