Rapid prototyping systems can’t yet produce parts in a wide enough range of materials, at a fast enough rate, to match the enormous spectrum of requirements of industry and science. Conventional processes such as molding and casting are still the only means available to do that, but RP is often the starting point for making these processes faster, cheaper and better.
Rapid prototyping is used in two ways to accomplish this: Molds may be directly fabricated by an RP system, or RP-generated parts can be used as patterns for fabricating a mold through so-called indirect or secondary processes.
Indirect or Secondary Processes
Typically a part made by the RP system is used as a pattern or model in these processes. While more than two dozen of them are in various stages of development, just a few are common and commercially important today.
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. As in the case of indirect or secondary processes, there are a large number of technologies being explored, but only a few are commercially important today.
The principal ways of using RP to generate thermoplastic parts 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 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,
may have part size and geometry limitations,
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.
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.
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.)
3D KeltoolTM.
A good choice for small, accurate parts that must be made in quantities from about 100 to a million or more. The main limitation is on the size of the part, about a 6 inch cube. The process is available from 3D Systems and a number of their
licensees.
Direct Fabrication of Injection Molds
Direct AIMTM.
These are tools made directly from photopolymer on a stereolithography machine. The process is essentially a proprietary stereolithography build style available from 3D Systems. There are substantial limitations to this soft tooling, but it’s useful
for unfilled, low temperature plastics in quantities up to about 50 pieces. The main advantage over RTV tools is that real plastics can be used, but the parts won’t be identical to those made with hard tooling.
Soft Tooling From Metals.
The Copper Polyamide process is available from 3D Systems as is the similar Direct Metal Laser Sintering (DMLS) process from EOS GmbH. These processes permit the manufacture of several hundred to several thousand parts
from molds which can be run much closer to conditions used for hard tooling.
Hard Tooling From Metals.
RapidToolTM from 3D Systems Corp. produces a fully-dense mold with about 70% steel content. EOS offers a similar process called DirectToolTM. These processes offer the greatest benefit for small, complex geometry parts that would be difficult to machine. Conformal cooling channels can be incorporated into the molds which should last for hundreds of thousands of shots of almost
any plastic. A new process called MoldFusionTM from D-M-E, a division of Cincinnati Milacron, will closely compete with these methods.
Processes based on Laser Engineered Net Shaping (LENS) technology are in early commercialization stages. Optomec Design Corp. and Precision Optical Manufacturing (POM) offer methods that create fully-dense, hard tools in multiple materials and with conformal cooling.
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