Rapid Tooling

Rapid prototyping is used in two ways to make tooling: Molds or other tools may be directly fabricated by an RP system, or RP-generated parts can be used as patterns for fabricating a mold or tool through so-called indirect or secondary processes.

Direct Fabrication Processes
Specialized rapid prototyping processes have been developed to meet specific application and material requirements for molding and casting. These may be forms of basic RP processes, such as stereolithography or selective laser sintering, or may be unique RP methods developed for a specific application. A large number of technologies have been or are being explored, but only a relatively few are commercially important at present.

Indirect or Secondary Processes
Although the properties of rapid prototyping materials continue to improve and expand, their comparatively small number and a limitless array of applications means that there will always be a need to transfer parts fabricated in a material used in an additive fabrication process into yet another material. Consequently, numerous material transfer technologies have been developed. Typically a part made by the RP system is used as a pattern or model in these processes. As in the case of the direct fabrication processes discussed above, there are many secondary processes in various stages of development. However, of the more than two dozen such methods available or under investigation, just a few are common and commercially important today.

About RP-generated Patterns RP-generated patterns must undergo finishing operations before they can be used in any indirect or secondary process. No rapid prototyping technology today delivers surface finishes that are adequate for accurate applications such as injection mold tooling. Removal of the stair-stepping inherent in the process and other surface artifacts is necessary before parts will eject from a mold, and may lead to additional errors being introduced. The accuracy of most secondary processes is ultimately limited by the precision of the pattern after finishing. Rapid prototyping patterns are best for applications with just a few critical dimensions: If many tight tolerances must be held, it's generally still faster and cheaper to use CNC.

Choosing Isn't Easy
The net result is that there are a bewildering number of routes to get to a final functional part or tool starting from a CAD definition. The choice depends on:

• the application,
• volume of parts to be produced,
• final material and accuracy requirements,
• the availability of a particular rapid prototyping process,