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Rapid Prototyping  is a term used for the process of taking computer information stored in electronic files (usually 3d models), and making a three-dimensional physical model by the use of special machines.  These precision machines interpret the computer information and actually "build" a model which can be accomplished by several different processes.  There are many reasons why rapid prototyping is used, here are a few......
1.  Designs can be made quickly and inexpensively, guaranteeing companies the best product possible, in the shortest time possible. 2.  Save Money - Using rapid prototypes will greatly reduce design time, production, & tool costs. Having a working model at the time of quotes also greatly improves business for many companies.
3.  Catch Errors - Companies can save thousands of dollars by discovering costly errors before the product goes into production. 4.  Sell Products - It always helps companies to be able to show clients a working model 3-D model. Prototypes also allow sales teams to pitch new products before they're manufactured.

 

Stereolithography (SL) is probably the oldest process of rapid prototyping available today.  Stereolithography uses an epoxy resin as the building material.  Parts are built by directing a laser in the shape of the part cross-sections over the surface of a vat of this liquid resin. This pre-cures one slice of the part. The part is then lowered into the vat by the slice distance (usually around .004"), and the liquid is "swept" smooth over the top surface of the part by a blade. The next slice of the part is then traced with the laser, and so on. The laser "cure depth" is set so that the consecutive layers are cured into the preceding layers, providing a dense prototype. After the part is finished building on the SL machine, it must have any support structures cut away, have the excess resin on the "skin" of the part cleaned off, and then be cured in an ultraviolet curing oven. There are some precautions when dealing with the uncured resin, and it must be run in a well-ventilated area.  The odors that are given off smell bad and can be dangerous to the nervous system.  Below are examples of test pieces that were made using the SL process.  On the right is a pic of an SL facility. 

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Laminated Object Manufacturing (LOM)   is another process of prototyping.  The LOM process uses paper or plastic media (with ceramics and metals on the way soon), and works by laminating the slices of material together and cutting out the cross-sections with a laser. Parts come out with a wood-like texture and can be used as injection tools, concept models, or even casting models.  LOM currently is one of the largest RP process used for production
Sanders Prototyping (also known as SP) was developed by Sanders Prototype, Inc. in Wilton, NH.  This process of prototyping fires wax microdroplets from 2 ink-jets to build cross-sections of a part, much like a printer uses ink. The droplets are printed into the cross section of the part in layers usually 0.0005" thick, then a mill passes over the part to ensure the proper  height is obtained. The final part comes out as a "brick" of support and build material, which must be placed into a solvent to carefully remove the support material. The SP process is more popular with people who deal in small, intricate models (i.e. jewelers). Models built with this machine tend to have higher accuracy and better surface finish than other processes, however larger parts may take a long time to build. The finished parts can be used as conceptual models, or may be shelled and cast for a usable part.