Ceramic molding process helps metalcasting facilities remain competitive.
Precision castings can be made by conventional sand technologies but often require considerable skills. At best, overall accuracy can be very good but surface finish leaves much to be desired. No cast component produced in a sand mold can yield the type of surface finish one would expect of a typical investment casting. However, the costs and logistics of converting a conventional metalcasting facility to an investment casting firm are often prohibitive.
The answer is a method of casting that lies halfway between traditional sand techniques and investment casting that is commonly referred to as ceramic molding. Increasing demand tot greater ease of operation and better accuracy has led to more recent developments by Unicast Development Co., New Haven, Conn., which develops and distributes the Unicast Process worldwide. This technology emphasizes ease of molding with minimal need for additional skills. There are few limitations on size or alloy, and castings are being produced in all materials from aluminum and copper-based alloys and iron, to most carbon and stainless steel alloys.
The ceramic molds are formed by pouring a quick setting refractory slurry over the pattern, usually within a removable flask. The slurry is prepared as needed using a simple mixer to combine the graded dry ceramic with a liquid organo-silicate binder. There is a choice of numerous ceramic blends and binders depending on the ultimate mold of core strength needed and the degree of permeability and surface finish required. Cores ate produced using a standard type corebox but with a ceramic slurry variation that permits good venting, high strength and ease of removal. As with mold patterns, coreboxes need to be accurately constructed and well finished to yield optimum internal surfaces in the finished casting.
Slurry setting time is usually within 3-5 min., and the set section can be removed manually or with a simple stripping machine, with the pattern or corebox removed, the molded ceramic is chemically stabilized then quickly flame dried. This is followed by rapid high temperature curing, usually to about 1,800F (1,000C) in a kiln-type furnace. Depending on the alloys being cast, molds can be poured hot, immediately after furnace removal, of can be cooled to lower temperature.
The latest applications for the process continue to extend beyond the original concepts to include cores and other applications. By using ceramic molding technology, ceramic cores that can be set into a conventional sand mold for pouring can be produced Where greater overall accuracy and surface finish is needed, the cope and drag mold halves as well as the core can be produced entirely in ceramic. Figure 1 shows a typical shell arrangement for producing impellers.
[FIGURE 1 OMITTED]
With the process, there is no penalty for short or single run castings, allowing wide diversification of applications. An entirely independent market that often uses short run castings is cast-to-size tooling. This includes cast cavities, molds and inserts for plastics molding, diecasting and forging dies, even foundry patterns and coreboxes. Complex shapes and intricate geometry can often be cast at much lower cost than equivalent machined sections. Figure 2 illustrates a cast-to-size beryllium-copper mold used for producing plastic toy moldings.
[FIGURE 2 OMITTED]
Ceramic slurry costs ate higher than sand, but the difference is not significant on small molds. It becomes more of a concern on larger molds and is compensated for by forming a thin ceramic shell backed by a low-cost supporting sand mold.
Through increasing ceramic technology, the Unicast Process has allowed smaller, conventional metalcasting facilities to recoup the work they have lost over the past decade.
Select No. 091 at www.moderncasting.com/info
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|Title Annotation:||New Product|
|Date:||May 1, 2004|
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