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Design is a matter of balance: weight vs. strength, cost vs. quality, speed vs. accuracy, and so on. As development progresses, prototyping is an essential part of the balancing process, and prototyping itself presents the designer with choices.
The Options
In plastic part design, technology has given us a variety of prototyping options. Rapid prototyping (RP) includes stereolithography, selective laser sintering, fused deposition modeling, laminated object manufacturing, and three dimensional printing. Each of these techniques builds parts, one-by-one, from 3D-CAD models, joining layers of material to create the finished prototype.
Rapid tooling (RT) uses rapid prototyping to create an initial part and then creates, from that part, a mold in which additional parts can be made. Mold materials can range from silicone rubber to composites. A third prototyping option is rapid injection molding (RIM), which works directly from a 3D-CAD model, using CNC machining to mill aluminum molds in which true injection-molded parts can be made. Finally, there is traditional injection molding, which is used primarily for production, but could conceivably be used to create prototypes.
Each method has strengths and weaknesses.
Rapid prototyping is the quickest, and can reproduce very complex shapes. With no up-front tooling costs, it can be inexpensive as long as only a few parts are needed. However, because each part is made from scratch, RP offers no economies of scale and its costs rise rapidly with quantity. Parts can only be made from a limited range of materials and are typically left with a coarse finish.
Rapid tooling can sometimes produce better quality parts than rapid prototyping, though materials choice is still somewhat limited. It is also slower and more costly due to the extra step required to create a tool from the original prototype. The need to create molds also increases up-front cost and can limit the complexity of shapes that can be effectively duplicated.
Rapid injection molding uses metal molds to produce truly functional parts with good finish and in a wide variety of resins. It is similar to traditional injection molding (though far faster and much less costly). It is competitive with rapid tooling for speed and offers better economies of scale than rapid prototyping or rapid tooling.
Traditional injection molding can produce the ultimate in part complexity and finish, but is generally considered too slow and expensive for prototyping, though it may be used when there is a high likelihood that the molds will go directly into large-scale production.
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