Lost-wax casting (also called "investment casting", "precision casting", is the process by which a duplicate metal sculpture (often silver, gold, brass or bronze) is cast from an original sculpture. Intricate works can be achieved by this method.
Precision lost-wax casting investment Casting Process
Casts can be made of the wax model itself, the direct method, or of a wax copy of a model that need not be of wax, the indirect method.
1. Model-making. Mould-maker creates an original model from wax, clay, or another material. Wax and oil-based clay are often preferred because these materials retain their softness.
2. Mouldmaking. A mould is made of the original model or sculpture. The rigid outer moulds contain the softer inner mould, which is the exact negative of the original model. Inner moulds are usually made of latex, polyurethane rubber or silicone, which is supported by the outer mould. The outer mould can be made from plaster, but can also be made of fiberglass or other materials.
3. Wax. Once the mould is finished, molten wax is poured into it and swished around until an even coating, usually about 3 mm (1⁄8 inch) thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is to fill the entire mould with molten wax and let it cool until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.
4. Removal of wax. This hollow wax copy of the original model is removed from the mould. The model-maker may reuse the mould to make multiple copies, limited only by the durability of the mould.
5. Chasing. Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the parting line or flashing where the pieces of the mould came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were moulded separately can now be heated and attached; foundries often use registration marks to indicate exactly where they go.
6. Spruing. The wax copy is sprued with a treelike structure of wax that will eventually provide paths for the molten casting material to flow and for air to escape.
7. Slurry. A sprued wax copy is dipped into a slurry of silica, then into a sand-like stucco, or dry crystalline silica of a controlled grain size.
8. Burnout. The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out.
9. Testing. The ceramic shell is allowed to cool, then is tested to see if water will flow freely through the feeder and vent tubes.
10. Pouring. The shell is reheated in the kiln to harden the patches and remove all traces of moisture, then placed cup-upwards into a tub filled with sand.
11. Release. The shell is hammered or sand-blasted away, releasing the rough casting.
12. Metal-chasing. Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, so that the casting now looks like the original model. Pits left by air bubbles in the casting and the stubs of the spruing are filed down and polished.
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