These artificial hip-joint castings are made using a foam pattern known as FoPat, which makes it possible to produce castings with no pattern-removal step, eliminating the use of steaming to remove wax patterns.
A shroud segment showing wax (bottom) and FoPat (top) patterns.
For thousands of years, wax in various forms has been used for patterns in the lost-wax or investment casting process. A material developed only recently makes it possible to replace wax as the pattern material.
At the Investment Casting Institute conference held in Milwaukee in October, Richard Harrington of Harrington Product Development Center (www.harringtonpdc.com) presented a progress report on using FoPat (that is, foam pattern ) material to form investment casting patterns.
The new technique uses a special formulation of polymers in conjunction with a modified reaction injection molding (RIM) process for producing patterns. RIM is a plastic fabrication technique involving the extremely rapid impingement mixing of two chemically reactive liquid streams that are injected into a mold, resulting in simultaneous polymerization, cross-linking, and formation of the desired shape.
The foam pattern is invested in a mold, as in conventional investment mold production. The major difference is that no pattern removal step is required, which eliminates the need for a steam autoclave used to melt and remove wax patterns. Instead, the pattern material evaporates when the molten metal is poured into the investment-casting mold.
There are several benefits offered by FoPat over wax patterns. For one thing, the pattern is very stable, dimensionally, and durable even in thin sections, which means that the patterns are readily shipped. The problem of shell cracking due to expansion of wax patterns is eliminated. The FoPat patterns can withstand temperatures up to 200ºF during handling. No chills are needed, as required in production of some wax patterns. FoPat is cost-effective for a given pattern, and its use also may result in lower tooling costs.
In his presentation to the ICI, Harrington detailed Phase I and Phase II work sponsored by the U.S. Dept. of Energy Inventions and Innovations program, and the U.S. Dept. of Defense Small Business Innovative Research program. He showed multiple samples of patterns with dimensional, cosmetic, and processing details. The photographs shown here compare wax and FoPat patterns for casting shroud segments, and two sizes of castings for an artificial hip, with their respective FoPat patterns.
Development work on FoPat began in the late1990s, but it was accelerated recently through the efforts of companies working with government-sponsored programs. The project would not have happened but for the insight and funding by the DoE and DoD. Over the past two years, almost $1 million in funding has been allocated by the two departments to help complete the feasibility and viability analysis, followed by scale-up, tooling, and process optimization work. FoPat LLC (www.nowaxpatterns.com) was created to commercialize the new technology.
Joining Harrington Product Development Center in the FoPat development effort are Cleveland Tool and Machine and S&A Consulting Group LLP, both of Cleveland, and BuyCastings.com (www.buycastings.com). According to Neil Chaudhry of BuyCastings, the team expects to begin commercialization by mid-2007.