Including true-and semicentrifugal casting, and centrifuging
Centrifugal casting is a process that is based on a specific use of gravity. The same force that is at work when lettuce leaves are spun in a salad spinner, or when people are rotated in a waltzer at a fairground, is employed to thrust a heated liquid material horizontally against the inside of a mold. Once the liquid has cooled, the finished part is taken out of the mold. In industrial manufacturing, centrifugal casting is most often used to make large-scale metal cylinders that require specific surface properties within the metal component.
Centrifugal casting for metals can be broken down into three main variants: true centrifugal casting, semicentrifugal casting, and centrifuging. As you may well have guessed, each process uses a centrifugal force to throw molten metal against the inside wall of a mold to produce a variety of shapes.
True centrifugal casting is used to make pipes and tubes, and it involves molten metal being poured into a rotating cylindrical mold. The mold defines the outside surface of the final component, while the wall thickness of the final tube or pipe is determined by the amount of material that is poured in. This type of casting solves one of the problems traditionally associated with metal, because the outer surface of the component is of such a fine grain that it is resistant to atmospheric corrosion (which is a common issue with pipes), while the internal diameter is rougher, with more impurities.
In semicentrifugal casting, either permanent or disposable molds are employed for making symmetrical shapes such as wheels and nozzles. It involves a vertical spindle around which the mold is held, like a spinning top. It also involves a slower rotation than true centrifugal casting and parts can be “stacked”—in other words, more than one part can be made at a time because multiple molds can be attached to the spindle. Because the material nearest the center (that is, nearest the spindle) rotates at a slower rate than the material farthest away, small air pockets can occur in the component.
Centrifuging is similar to semicentrifugal casting in as much as the spinning occurs around a vertical spindle, but it is used to produce small multiple components. The metal is forced into the various mold cavities (which are only a short distance from the spindle) to produce fine details.
- Parts can be produced with good mechanical properties in all directions, because the process results in nondirectional grain orientation.
- The strength of centrifugal castings is close to that of wrought metal.
- With true centrifugal casting, the outer surface has a fine grain, which makes it more resistant to corrosion.
- Can achieve economical production over short runs.
- Limited production base.
- Limited shapes achievable.
Volume of production
From a relatively simple setup in a jewelry workshop, to large-scale industrial production, these are processes that can be used for batch rather than mass-production.
Unit price vs. capital investment
Depends on the specific type of production: low-cost graphite molds can be used for small production runs (up to about 60 pieces), while more expensive permanent steel molds are used for larger runs of, perhaps, several hundred.
Slow, but it varies depending on the material that is used and the size, shape, and desired wall thickness of the part.
True centrifugal casting produces an outer surface of fine-grain quality. Due to the slower rotation speed of semicentrifugal casting, the forces in the center of the casting are small, so gaps and porosity generally occur that need to be machined away after forming. Centrifuging enables fine details to be produced.
Types/complexity of shape
True centrifugal casting produces only tubular shapes. Semicentrifugal casting produces parts that are axisymmetric (symmetrical around the vertical spindle) in shape only. Centrifuging is more versatile, and can produce more complex shapes.
True centrifugal casting can be used to form massive tubes up to 10 feet in diameter and 50 feet long. Wall thickness can be between 1/8 and 5 inches. Semicentrifugal casting and centrifuging produce smaller parts.
The tolerances can be as good as 1/50 inch on the outer diameter when using metal molds.
Most materials that can be cast by other methods, including iron, carbon steels, stainless steels, bronze, brasses, and alloys of aluminum, copper, and nickel. ‘Two materials can be cast simultaneously by introducing a second material during the process. Glass and plastics can also be used.
The casting of metals is based in heavy industry, where it is used for hollow parts with large diameters. Typical parts made by true centrifugal casting are pipes for the oil and chemical industries, and water-supply components. The process is also used in the production of poles for lighting and other street furniture. Semicentrifugal casting produces axisymmetric parts, such as storage containers for wine and milk, boilers, pressure vessels, flywheels, and cylinder liners. Jewelers use centrifuging for more modestly sized metal and plastic parts.
Rotational molding (p.137), although in centrifugal casting the mold is rotated at much higher speeds.
Each of the centrifugal casting techniques relies upon continuous rotation throughout each cycle, which combined with the heat required to melt the material is energy intensive. However, no waste is produced as the molten metal is added only until the required thickness is reached, keeping material consumption to a minimum. The fine finish of the outer surface attained through casting also provides metals with several years of excellent wear- and corrosion-resistance.