Centrifugal casting pours molten metal into a rotating mold and lets centripetal acceleration do what gravity can't: pack the metal against the mold wall, force impurities inward, and produce hollow cylindrical parts with no core. The mold spins at 300–3,000 RPM depending on diameter, generating 60–100 G's of effective gravity at the bore.

Why spin instead of pour? In a static mold, gas bubbles, slag, and oxides get trapped randomly throughout the casting. Under high G-load, density wins: dense iron flings outward against the mold, while lighter impurities (sulfides, dross, gas) migrate to the inner surface. That inner layer is later machined away, leaving a clean, dense bore. The outer surface has near-forging grain density with no shrinkage porosity.

Three flavors:

• True centrifugal: horizontal axis, no core. Used for pipes, cylinder liners, gun barrels, bearing sleeves. Wall thickness is set by the volume of metal poured, not by tooling.
• Semi-centrifugal: vertical axis with a central core. Used for symmetric parts like wheels, pulleys, flywheels — the spin densifies the rim where stress concentrates.
• Centrifuging: multiple part cavities arranged radially around a central sprue. Spin forces metal outward into each cavity, filling intricate shapes (jewelry, dental crowns, turbine blades) that wouldn't fill under gravity alone.

Real-world example: ductile iron sewer and water pipe up to 60" diameter is almost exclusively centrifugally cast. A 36" pipe spins around 400 RPM in a water-cooled steel mold; the cast pipe is ejected in under two minutes and the mold immediately reused. The process produces 20-foot lengths with wall tolerance under 1/16" and a microstructure that handles 350 psi working pressure with no welds or seams.

Rule of thumb — the G-factor: centripetal acceleration at the bore should be 60–75 G's for sound iron castings, higher for thinner walls. Calculate as:

G = (RPM/30)² × r / g, where r is bore radius in meters, g = 9.81 m/s².

For a 0.3 m radius bore at 500 RPM: G = (16.67)² × 0.3 / 9.81 ≈ 8.5 — too low. Bump to 1,500 RPM: G ≈ 76. That's why small-diameter parts spin faster than big ones.

Limits: only axisymmetric shapes, only the outer profile is precise (inner bore needs machining), and unequal density alloys can segregate — high-tin bronze bearings sometimes show tin-rich inner zones that hurt fatigue life if not managed with cooling rate.