Choose sand casting for any part that is not a plain body of revolution, and centrifugal casting for tubes, sleeves, rings and liners whose value is in the soundness of the wall. If the drawing has a bolted flange, a side port, a mounting pad or a cored passage that does not run down the axis, it is a sand casting. If the drawing is essentially an OD, an ID and a length, and the run is long enough to pay for a steel mold, it is a centrifugal casting.
Volume and material spec settle the cases in between. A spinning steel mold is tooling that a short run cannot recover, so short runs of cylindrical parts are still sand cast around a core, and some product specifications — centrifugally cast pipe standards above all — name the process for you.
What Is Sand Casting?
Sand casting forms an expendable mold in bonded sand around a pattern, sets cores for the internal passages, and pours the metal through a gating system that feeds the casting from risers while it solidifies. The shape is constrained only by what a pattern can be drawn from and a core can be set into, which is why housings, bodies, brackets and counterweights are sand castings.
The price of that freedom is paid in the metal. Sand conducts heat poorly, so the casting freezes slowly and from whichever surfaces cool first rather than in one direction; the grain is coarser, and any isolated hot spot shrinks without feed metal unless a riser or chill was put there for it. Sound sand castings come from engineered gating and risering — chills, riser sizing, solidification simulation — not from the process itself.
What Is Centrifugal Casting?
Centrifugal casting pours the metal into a mold spinning about its own axis, and the rotation — set to hold the metal against the wall at roughly 60 to 80 times gravity — forms the bore without a core. The mold is usually a preheated, ceramic-coated steel tube with capped ends; sand molds are used for very large pieces or one-off runs.
Three things happen in that spinning wall that a static sand mold cannot reproduce. The metal freezes from the cold mold wall inward in one direction, so the grain is consistent through the wall. The liquid still on the bore feeds the solidification front, so shrinkage porosity is not trapped in the wall and there are no risers to size. And the lighter inclusions and gas are thrown toward the axis, where they collect on the bore.
That bore is then machined off. A centrifugal casting always carries machining stock on the ID, because the ID is where the process has put the dirt; the clean, dense wall you are buying is the wall that remains after that cut.
True centrifugal casting makes tubes and rings — tubes spun horizontally so the wall is uniform end to end, rings spun vertically. Semi-centrifugal casting spins a complete mold, hub and rim, around a central riser; wheels and pulleys are the textbook case, and the hub is the least dense part of the casting.
Differences Between Sand Casting and Centrifugal Casting
Every difference in the table follows from one fact: sand casting forms the whole shape with tooling and feeds it with risers, while centrifugal casting forms the bore with rotation and feeds the wall from its own liquid core. That is why one process wins on geometry and the other on wall soundness, and why neither is “more accurate” in a way a tolerance grade would capture.
| Factor | Sand Casting | Centrifugal Casting |
|---|---|---|
| Basic principle | Molten metal is poured into a sand mold | Molten metal is poured into a rotating mold |
| Typical part shape | Almost any geometry | Cylindrical or rotationally symmetrical parts |
| Mold | Disposable sand mold | Usually permanent metal mold |
| Cores | Often required for cavities | Often unnecessary for hollow cylindrical parts |
| Dimensional accuracy | Moderate | Good |
| Surface finish | Rougher | Better, especially on outer surfaces |
| Mechanical properties | Good, but depend strongly on process control | Often excellent due to dense structure |
| Porosity risk | Moderate to high | Generally lower in critical outer regions |
| Production rate | Low to medium | Medium to high |
| Tooling cost | Low | Higher |
| Part size flexibility | Very high | Primarily limited to rotational shapes |
| Best production volume | Prototypes to medium-volume production | Medium- to high-volume cylindrical components |
Two rows need qualifying before they go on a drawing. “Moderate” accuracy for sand casting means, on SFSA’s production data for steel castings, CT 10–12 for most parts in long-run production and CT 12–14 on large castings. “Good” accuracy for centrifugal casting has no grade behind it: the process-by-process tolerance tables, SFSA’s and ISO 8062’s, do not list the process at all.
The surface-finish row is similar. The as-cast OD of a centrifugal casting is genuinely better than a sand surface, but if your part needs a finished bore, you are machining it under either process; the difference is how much stock comes off and what comes off with it.
Size is not what limits centrifugal casting — shape is. Spun castings have been made 10 m long, 1.73 m in outside diameter and 36,000 kg, so a large plain cylinder is not pushed toward sand casting by its size alone. A large cylinder with a bolted flange at one end is a different matter.
When Should You Choose Sand Casting?
Choose sand casting when the geometry is more than an OD, an ID and a length, or when the run is too short to recover the cost of a spinning steel mold. A pump casing with a volute and a split flange, a valve body with side ports, a gearbox housing with bores on two axes, a counterweight with a cast-in lifting eye — each needs a core or a pattern feature that a rotating mold cannot form, and each is routinely poured in ductile iron, gray iron and carbon steel to ASTM A536, A48 and A216, none of which presumes a process.
Round parts fall on this side too when they carry anything off the axis or the volume is low. A flywheel with a hub and spokes or a bearing housing with a mounting flange can be spun as a semi-centrifugal casting, but the permanent mold only pays at volumes most OEM parts never reach; below that they are sand castings with a cored bore, and the bore carries machining stock either way.
When Should You Choose Centrifugal Casting?
Choose centrifugal casting when the part is a tube, sleeve, ring or liner whose critical requirement is a sound, inclusion-free wall — pressure-containing pipe, wear liners, bearing sleeves, roll shells, cylinder barrels — and the volume pays for a steel mold. Often the specification has already chosen for you: ASTM A426 and A451 are written for centrifugally cast ferritic and austenitic alloy steel pipe, and ANSI/AWWA C151/A21.51 for centrifugally cast ductile iron water pipe from 3 to 64 in. A drawing that calls out one of those is not a sand-casting RFQ, however a sand foundry might want to read it.
Expect to machine the bore in every case and usually the OD, and plan the geometry around the axis: rings go on a vertical machine, tubes on a horizontal one, and any flange, port or boss off the axis has to be welded on or machined out of a thicker wall. That last point is where the cost case quietly reverses. A plain tube that grows a bolted flange at one end is often cheaper as a sand casting with a cored bore than as a spun tube plus a welded flange — especially in ductile iron, where the weld is its own problem.
Conclusion
Shape decides first, volume second, properties third. A part that is an OD, an ID and a length, in a quantity that pays for a steel mold or under a spec written for spun pipe, belongs at a centrifugal caster; everything with a feature off the axis, and every short run, is a sand casting. The centrifugal advantages are real but confined — a denser, cleaner wall and a bore you will machine anyway — so do not pay for them on a part that does not need them, and do not accept a sand casting for a plain pressure tube where the spec says spun.