Volumes 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.

Speed

Slow, but it varies depending on the material that is used and the size, shape, and desired wall thickness of the part.

Surface

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.

Scale

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. Semi-centrifugal casting and centrifuging produce smaller parts.

Tolerances

The tolerances can be as good as 1/50 inch on the outer diameter when using metal molds.

Relevant materials

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.

Typical products

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.

Similar methods

Rotational molding (p.137), although in centrifugal casting the mold is rotated at much higher speeds.

Sustainability issues

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.

Further information

www.sgva.com/fabrication_processes/rna_centrif.htm

www.acipco.com

www.jtprice.fsnet.co.uk

Electroforming

This process has changed very little since the early nineteenth century, when simple electroplating—a way of plating metals from their salts—was developed out of the initial work of British scientist Sir Humphry Davy on passing currents through electrolytes. Situated somewhere between a surface coating and a form-making technique, electroforming is a fairly unusual process. It is perhaps best explained by way of comparing it to growing a “skin” over a shape. The “skin” ultimately becomes the final component once it is lifted off the mold. Essentially, it is a step on from electroplating, in which the layer of metal acts only as a coating for the original shape.

Electroforming is based on the electro-depositing of metal onto molds. The shape that would, in simple electroplating, be coated (the cathode), in electroforming becomes a mold onto which the metalizing source (the anode) is grown, in a solution of electrolyte. An electrical current forces metal ions from the anode onto the cathode. Once a sufficient buildup of metal has been achieved—and this is where it differs radically from electroplating—the component is separated from the mold. The mold does not necessarily have to be made from metals—it can be made from any nonconductive material, which can be coated with a conductive outside layer before plating.

The usefulness of electroforming lies in the fact that intricate flat and three-dimensional patterns can be easily reproduced without the need for expensive tooling, because the detail is created on the mold. The process is unique in that it creates a uniformly thin layer of material around the mold, unlike press forming (see metal cutting p.59) and sheet-metal forming (see p.50), which stretches the metal and, in so doing, leaves it an uneven thickness.

–  Excellent definition in detailing.

–  Generates a uniform thickness of metal.

–  Low tooling costs.

–  An easy way to replicate existing products.

–  High tolerance.

–  Fairly slow and thus expensive.