processes for specific shapes

Centrifugal Casting
- This casting process has been known for several hundred years, but its evolution into a sophisticated production method for other than simple shapes has taken place only in this century. Today, very high quality castings of considerable complexity are produced using this technique.

To make a centrifugal casting, molten metal is poured into a spinning mold. The mold may be oriented horizontally or vertically, depending on the casting's aspect ratio. Short, squat products are cast vertically while long tubular shapes are cast horizontally. In either case, centrifugal force holds the molten metal against the mold wall until it solidifies. Carefully weighed charges insure that just enough metal freezes in the mold to yield the desired wall thickness. In some cases, dissimilar alloys can be cast sequentially to produce a composite structure.

Molds for copper alloy castings are usually made from carbon steel coated with a suitable refractory mold wash. Molds can be costly if ordered to custom dimensions, but the larger centrifugal foundries maintain sizeable stocks of molds in diameters ranging from a few inches to several feet.

The inherent quality of centrifugal castings is based on the fact that most nonmetallic impurities in castings are less dense than the metal itself. Centrifugal force causes impurities (dross, oxides) to concentrate at the casting's inner surface. This is usually machined away, leaving only clean metal in the finished product. Because freezing is rapid and completely directional, centrifugal castings are inherently sound and pressure tight. Mechanical properties can be somewhat higher than those of statically cast products.

Centrifugal castings are made in sizes ranging from approximately 2 in to 12 ft (50 nlm to 3.7 m) in diameter and from a few inches to many yards in length. Size limitations, if any, are likely as not based on the foundry's melt shop capacity. Simple-shaped centrifugal castings are used for items such as pipe flanges and valve components, while complex shapes can be cast by using cores and shaped molds. Pressure-retaining centrifugal castings have been found to be mechanically equivalent to more costly forgings and extrusions.

In a related process called centrifuging, numerous small molds are arranged radially on a casting machine with their feed sprues oriented toward the machine's axis. Molten metal is fed to the spinning mold, filling the individual cavities. The process is used for small castings such as jewelry and dental bridgework, and is economically viable for both small and large production quantities. Several molding methods can be adapted to the process, and the unit costs of centrifuged castings will depend largely on the type of mold used.

Bronze Continuous Cast Bar



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