If you want to see what centrifugal casting looks like, peek into your washing machine when it’s on its spin cycle. See how all the clothes have been thrown against the wall of the drum? Well centrifugal casting does the same thing, only instead of clothes it uses molten metal.
In conventional casting, metal is poured into a cavity or mold and allowed to solidify, (“freeze” in the technical jargon,) where it takes on the shape of the mold. In centrifugal casting the outer wall of the drum forms the mold that sets the shape of the cast piece, but there’s no need for an inner wall because the circular motion throws the metal to the outside where it freezes.
This makes centrifugal casting an ideal process for creating hollow metal pieces such as pipes. A second major advantage is that the cast material tends to be free of impurities and defects.
When metal is in its molten or liquid state impurities float to the surface. Usually this is skimmed off as “slag” before the metal is poured into a mold. Alternatively, metal is tapped from the bottom of the crucible. Both methods ensure that only “clean” metal is used.
Centrifugal casting takes advantage of the tendency of the impurities to be less dense than the metal. Whenever something is rotated its mass causes it to want to continue in a straight line. (This is Newton’s Second Law of Motion.) In practice this means that the heavier metal moves to the outside, leaving the lighter impurities, along with any entrapped gas bubbles, on the inside of the casting where they can easily be machined away. This ensures that the finished casting is free from flaws like porosity, (another word for trapped gas bubbles,) and impurities.
There are other advantages too. Material utilization can be better than for other casting processes because there is no need for “runners” and “risers.” (These are the metal that flows into and out of the mold as it is filled.) This can make centrifugal casting quite cost-effective. It also has advantages in terms of the properties of the cast metal. The rapid cooling of the metal that comes into contact with the drum results in a fine grain metallurgical structure which tends to be corrosion-resistant This can be an important factor when the process is used to make pipes.
It’s also possible to cast bimetallic structures. In other words, it’s possible to have one metal for the outside surface and a second metal for the inside. This is useful when different properties are needed inside and out.
Centrifugal casting isn’t ideal for every application, but when hollow parts are needed, and particularly if they can have a circular cross-section, it’s a process worth considering.