Prototype the Tool, Not the Part
American Industrial Casting, Inc. (AIC), a manufacturer of precision investment cast components, has for a long time offered customers free engineering design assistance in the early stages of product development.
This has helped customers reduce time to market by avoiding changes to tooling that might have added to the normal eight to ten weeks required to produce new tools and samples from a new design. It also enabled AIC to enhance its services to customers by bringing the customers' design for manufacture and assembly practices into its own operation.
To further increase its service to customers, AIC added a variety of new capabilities. Included among these were the transfer of designs by e-mail, fax, and modem; use of powerful software to view CAD files; and the timely production of rapid prototype castings from customers' three-dimensional CAD models.
Since 1992, AIC has produced acceptable castings from the patterns built by several of the leading rapid prototyping processes offered by rapid prototyping design service bureaus.
However, AIC was not satisfied with the overall speed, quality, and cost of providing sample investment cast parts for many accounts. The company felt that most of the rapid prototyping processes could not meet the resolution and surface finish requirements of the intricate, fine-featured castings for which AIC was noted.
They also found that some of the pattern materials from those processes can react in detrimental ways with the materials used in the solid mold and shell mold investment casting processes. This made it difficult, risky, and expensive to achieve the high quality required by investment casting customers.
Doing It In-house
Early in 1994, AIC studied the various rapid prototyping systems and decided to purchase a low cost, desk-top, rapid prototyping system called the ModelMaker™ System from Sanders Prototype Inc. They found that this system would allow them to build patterns with the dimensional accuracies and surface finishes their customers demanded.
The Sanders ModelMaker™ System (Model MM-6B) uses a combination of plotter/miller/inkjet printing technology to produce precise intricate prototypes of a thermoplastic material that serves as the wax pattern in investment casting. The system uses a dual printhead to deposit a sequence of thin layers, or slices, of the pattern being built. One printhead deposits a thermoplastic material to produce the pattern; the other deposits a wax material to support the pattern's overhangs and cavities during the build.
The finished pattern is immersed in a solvent bath to dissolve away the support wax, leaving the pattern ready for investment casting without the need for further manual post-processing steps.
AIC soon found that its ability to produce exact investment-ready patterns with the new system was a major time-saving advantage. There were no scaffolding support structures to be snapped off or broken away. Thus, there is no manual touch-up or repair needed, as sometimes required on models made by other types of rapid prototyping processes.
The company discovered several other advantages of this three- dimensional printing process. Dimensional tolerances were maintained by precision X-Y plotter technology to within 0.001 inches. In the Z-axis, dimensions were maintained by a horizontal milling technique that accurately maintains slice layer thickness from 0.0005 to 0.005 inch. AIC uses the thinnest layer of 0.0005 inch to construct and cast extremely finely detailed parts with features such as 4-40 NF male and female screw threads, or cutting edges.
Another use for the finest 0.0005 inch layer is building any pattern where smooth, curved surfaces are required, such as jet engine turbine blades, medical cutters and piercers, fine jewelry filigree, and the like. More intermediate (0.0015 inch) and coarse settings (0.005 inch) are used to produce tooling grade patterns and concept models, respectively.
Since AIC started rapid prototyping in-house, they have modeled and shipped numerous parts as cast components in a week or two. They usually use the customer's three-dimensional CAD designs supplied as .STL files. The process has been ideal for producing a single unit, or a few small patterns, for intricate cast components.
The constructed parts are precise, with tolerances of 1 to 1.5 times normal, dimensional resolution of 0.002 inch, and surface finishes as normal production quality at 80 to 100 micro-inches, RMS. The process is capable of producing parts of up to six inches on a side. However, the build time for large patterns with thinly sliced layer thickness becomes longer and more expensive.
Temporary Mold Prototypes
When customers have needed a dozen or more larger parts to be prototyped economically or quickly, AIC has occasionally used temporary elastomeric molds made by quick processes, with moderate success. The approach is to rapidly prototype a single appropriately scaled model of a part. The prototyped part is investment cast as a beryllium/copper alloy master part, which is used to make any one of three kinds of temporary molds of rubber, RTV, or epoxy.
Rubber molds can be made in about one day for under $100, with tolerances of 1/16 inch, but with considerable part distortion in the process. On the other hand, RTV molds can be built in about two to three weeks for typically $500 to $1000, with tolerances of 1/32 inch. RTV molds exhibit less dimensional distortion than do rubber molds.
For best results and minimum distortion, hard epoxy molds, sometimes even with core elements, can be produced in three to five weeks for $1500 to $2500. They have tolerances of 0.005 inch or better. The lives of such molds are somewhat limited, and yield life of waxes for investment casting from these molds is about 50 pieces for rubber, 100 pieces for RTV, and 200 pieces for epoxy.
Modeling Tools, Not Parts
To overcome time, cost, and accuracy constraints in making molds from a beryllium/copper part master, AIC established a single step process to directly prototype the part's mold, instead of the part itself.
In this process, negative images of the part's surface are modeled. These negative images become molds to create wax patterns of the part for investment casting.
As a test, a sample part of moderate complexity was chosen from those in normal production at AIC.
Virtual Concepts Design Inc., of North Attleboro, MA, was asked to design mold components, starting from an .STL file for the chosen part. Parting lines were created at agreed upon places, and three-dimensional drawings of the mold components were made by working into negative space from the surfaces of the part. After the mold components were designed, a separate .STL file was produced for each.
Each mold component was scaled, oriented advantageously, placed beside one another, and combined into a single .STL file for the best build profile. Then the patterns for all the components were rapid prototyped in a single overnight session using a low melting point polymer that behaves like wax.
On the third day, using the standard solid mold processing technique, the rapid prototypes were investment cast in a beryllium copper alloy, which perfectly reproduced every detail of the prototype models. These cast components underwent the regular processes of knockout, grind, finish, inspection, and solution heat treatment by the fifth day.
The final mold components were put together and used to hand injection-mold several wax patterns of the final part by the seventh day. Those wax patterns of the final parts were investment cast in solid molds on the eighth day. Solution heat treating and layout inspection only took one day more.
AIC's lessons-learned from these rapid prototyping ventures have helped to add value to their services by decreasing the company's response time. Computerized rapid prototyping technologies and processes have now become an integral part of the company's operations.
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