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The principal ways of using RP to generate injection molds today are presented below in approximately increasing order of cost and part quantity. If only a few parts are needed, RTV silicone rubber tooling is often the best choice. Beyond about 50 parts, or to study the operation of a production mold or for other reasons, it will probably be advantageous to choose one of the other RP injection mold fabrication methods.
 Why Use Rapid Prototyping to Make Injection Molds?
Skilled craftspeople are in short supply, product complexity is increasing and product cycles are growing ever shorter. This means that an ever larger number of more precise tools have to be created by a declining population of toolmakers. There is therefore a great deal to gain from a process which provides both great time and labor savings and addresses these limitations head-on. In addition, RP offers the tantalizing prospect for improvement in mold performance beyond anything that can be accomplished with subtractive technologies. The ability to fabricate complex conformal cooling channels to provide better thermal performance, or to use multiple or gradient materials to optimize each portion of a mold for performance and cost, may ultimately lead to a revolution across the entire field.
What are the Limitations?
Rapid prototyping injection mold fabrication methods should be considered for projects in which the reduction of time to market is important, for prototype and short to medium volume production runs, and for parts which may be very hard to machine because of their geometry. The general limitations of RP methods compared to CNC today are:
- they produce somewhat less accurate and less durable tools,
- they may have part size and geometry limitations,
- they don't necessarily produce identical parts to hard tooling, and,
- RP-generated tools may not easily be modified or corrected using typical toolmaking techniques.
These limitations vary both as function of the specific RP technology used and for each individual case.
Selecting a Process.
Selection of the optimum RP-based process for each case is complex. Among the factors to consider are the final application, production volume, part size, accuracy and material requirements.
 Manual Part Fabrication Methods
RTV Silicone Rubber Tooling.
This is a popular method of making small quantities of polymer parts. Any rapid prototyping-generated part can be used as a pattern to make silicone rubber tooling. These tools can be used to mold small to medium quantities of parts in a large variety of urethane, epoxy or other polymers. If quantities greater than about 10 to 50 are needed, an injection mold may be the way to go. There are many suppliers for this process, as well as the very similar aluminum-filled epoxy, sprayed metal and kirksite tooling methods.
Injection Mold Fabrication Methods
Indirect or Secondary Processes that Utilize RP-generated Patterns.
Aluminum-filled Epoxy Tooling.
Aluminum-filled epoxy tooling is a good choice for short prototype or production runs for applications that require a final engineering thermoplastic. These tools are fabricated much like RTV silicone rubber tooling. Aluminum-filled epoxy tools work best for relatively simple shapes with tool life adequate for anywhere from 50 to 1,000 parts, depending on requirements. (Many suppliers.)
Spray Metal Tooling.
These tools and the methods for making them are very similar to aluminum-filled epoxy tooling. Tool life is about the same as well, but the method can accommodate larger parts. (Many suppliers.)
Kirksite Tooling.
Similar to, but less accurate than aluminum-filled epoxy or spray metal, but a good choice for more complex parts in quantities up to about 1,000. (Many suppliers.)
Direct Fabrication of Injection Molds
Soft Tooling From Metals
The EOS GmbH Direct Metal Laser SinteringTM (DMLS) SLS process can use a bronze alloy which offers a step up in soft tooling over epoxy-based stereolithography methods. As many as several thousand relatively simple parts have been produced from such DMLS molds.
Hard Tooling From Metals
3D Systems' selective laser sintering process for metals uses polymer-coated steel powders. The resultant green part is burned out, sintered and infiltrated with bronze in secondary furnace operations to produce a fully-dense mold with about 70% steel content. EOS's DMLS process for bronze alloys and steel powders doesn't require secondary sintering and burnout cycles in a furnace because the parts produced are already at 95% density. These steel-based processes offer the greatest benefit for small, complex geometry parts that would be difficult to machine.
There are several additional choices which are described in more detail in the Rapid Tooling & Metal Part tutorial section. Some of them are quite similar in concept to selective laser sintering. A process from ProMetal, a division of ExOne Company, is based on Three Dimensional Printing technology developed at MIT. Several processes based on laser powder forming technologies are also available. Optomec Corp., POM-Group and other companies offer methods that create fully-dense, hard tools in multiple materials and with conformal cooling.
Functional Parts and Tools From Rapid Prototyping...
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Detailed Rapid Tooling & Metal Parts Tutorial...
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