Centrifugal casting — also widely known as spin casting — is a jewellery manufacturing technique in which molten metal is driven into an investment mould by the rotational force generated when the entire flask assembly is spun at high speed. Unlike gravity pouring or vacuum-assisted casting, centrifugal force acts continuously throughout the fill, pressing liquid metal into fine recesses and thin sections before solidification can begin. The result is a casting that is notably dense, largely free of shrinkage porosity, and capable of reproducing crisp surface texture from the original wax or resin model.

Mechanism and Equipment

The central piece of equipment is the broken-arm caster (sometimes called a spring-wound centrifugal casting machine), a counterbalanced arm mounted on a vertical pivot. One end carries the heated investment flask and a crucible or sprue button of molten metal; the other end bears an adjustable counterweight. When the arm is released, a pre-wound spring drives rapid rotation — typically two to four revolutions per second in the initial burst — generating centrifugal acceleration that forces metal from the crucible through the sprue and into every cavity of the mould.

Electric centrifugal casting machines offer variable speed control and are preferred in higher-volume production settings, allowing the operator to tune rotational velocity to the alloy being cast and the complexity of the model. The flask itself is a cylindrical steel tube packed with investment plaster that has been burned out in a kiln to remove the original wax pattern, leaving a precise negative cavity.

Relationship to Lost-Wax Casting

Centrifugal casting is most commonly employed as the final stage of the lost-wax (or cire perdue) process. A wax model — carved by hand, injected into a rubber mould, or produced by a resin printer — is attached to a wax sprue, invested in plaster, and placed in a burnout oven. Once the wax has been fully eliminated and the flask brought to the correct pre-heat temperature (which varies by alloy, typically 480–650 °C for gold), the flask is transferred immediately to the casting machine and the metal is introduced. The centrifugal force ensures that even delicate prong tips, milgrain edges, and fine filigree sections receive a complete fill before the metal freezes.

Metallurgical Advantages

The primary metallurgical benefit of centrifugal casting is the suppression of gas porosity and shrinkage voids. Because the rotational force maintains pressure on the solidifying metal, dissolved gases are less able to nucleate into bubbles, and shrinkage at the solidification front is compensated by additional metal drawn from the sprue reservoir. Castings produced by this method generally exhibit higher density than those produced by simple gravity pouring, translating to better polishability, improved mechanical strength in thin sections such as shank walls and claw tips, and reduced risk of failure during stone setting.

Alloys and Applications

Centrifugal casting is compatible with the full range of jewellery alloys: yellow, white, and rose gold in standard karatages (9 ct, 14 ct, 18 ct, and 22 ct); sterling and Argentium silver; platinum-group alloys; and palladium. Platinum casting requires particular care because of the metal's high melting point (approximately 1,768 °C) and its sensitivity to contamination; dedicated platinum crucibles and a clean casting environment are essential. For platinum and some white-gold alloys, vacuum-assisted or vacuum-centrifugal hybrid machines are often preferred, combining the fill pressure of centrifugal force with the degassing benefit of a partial vacuum.

In the trade, centrifugal casting is the dominant production method for rings, pendants, earring components, and intricate settings produced in small-to-medium batch sizes. It is particularly well suited to reproducing lost-wax models with undercuts, fine texture, and thin walls — characteristics that can defeat gravity casting entirely.

Limitations

The technique is not without constraints. Very large or heavy castings can exceed the capacity of standard spring-wound machines, and the rapid rotation demands careful counterbalancing to avoid vibration that degrades surface quality. Operator skill in timing the metal pour and releasing the arm at the correct moment remains important even on mechanised equipment. Additionally, the centrifugal force acts radially outward, meaning that the outermost portions of a flask receive the greatest pressure; in a flask containing multiple models on a tree, pieces positioned at different radii may experience slightly different fill conditions.