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repeatedly dipping the “tree” into a thin refractory slurry. After dipping, a refractory ag- gregate, such as silica, zircon or aluminum silicate sand, is rained over the wet slurry coating. After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The required thick- ness of this shell depends on the size of the castings and temperature of the metal to be poured. After the ceramic shell is complete, the entire assembly is placed into an autoclave oven to melt and remove most of the wax. The majority of investment castings


First, a gel is formed in a


pattern and stripped from the mold. The mold then is heated to a high tempera- ture until rigid. After the mold cools, molten metal is poured into it, with or without preheating. The ceramic molding


Rubber plaster mold casting helps save in production time and tooling costs with reusable rubber molds.


weigh less than 5 lbs., but larger castings in the 10-30-lb. range are becoming more common. Castings weighing up to 800 lbs. have been poured in this process. Some of the advantages of investment casting include: • excellent surface finishes; • tight dimensional tolerances;


• machining elimination or reduction; • ability to produce titanium castings, as well as the other superalloys.


Ceramic Molding—Generally, these


processes employ a mixture of graded re- fractory fillers that are blended to a slurry consistency. Various refractory materials can be used as filler material. The slurry then is poured over a pattern that has been placed in a container.


processes have proven effec- tive with smaller castings in short- and medium-volume production runs. At the same time, the processes offer castings with excellent surface finish and good di- mensional tolerances. Plaster Molding—Plas-


ter molding is used to pro- duce castings of the lower


melting temperature metals, such as aluminum alloys. In the process, a slurry containing calcium sulfate, sometimes called gypsum, is poured into a flask that contains the pattern. After the slurry has set, the pattern and flask are re- moved, and the drying cycle to remove the moisture from the mold begins. After the mold has cooled, the cores and mold are assembled. After assem- bly, most molds are preheated before pouring. Because these molds have poor permeability in many cases, vac- uum-assistance or pressure is required during pouring. Plaster molding is well-suited to short run and prototype work.


RAPID PROTOTYPING Rapid prototyping (RP) is a general name that encompasses numerous meth- ods used to fabricate objects from CAD data. A number of different RP processes are available, and new developments are constantly being made. RP most commonly is used with in-


vestment casting, sand casting and plaster molding to produce actual cast parts to test for form, fit and function, as well as determine the approximate final proper- ties of the cast parts. Investment Casting—RP models


for investment casting are created by converting a 3-D CAD model into an .STL file. The file then is “printed” three- dimensionally using either photopolymer, thermopolymer, polystyrene or other ma- terials, depending on the RP method. The prototype models then can be attached to a gating system and processed through typical investment casting to produce cast prototype parts. Sand Casting—In sand casting, RP-


generated parts can be used as patterns for fabricating a sand mold. RP processes that


12 METAL CASTING DESIGN & PURCHASING 2011 CASTING SOURCE DIRECTORY


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