What is Centrifugal Casting?
Centrifugal casting forms a hollow, rotationally symmetric metal part by pouring molten metal into a rotating mold. Rotation holds the metal against the mold wall while it solidifies from the outside diameter toward the bore. Pipes, sleeves, bushings, rings and cylinder liners are typical candidates. The process is valuable when a dense working outside region and a machinable inner surface suit the design; it is not a general route for arbitrary complex shapes.
The true centrifugal casting principle
The mold rotates around the casting axis in a horizontal or vertical arrangement selected for geometry and equipment. Metal enters, spreads around the circumference and remains against the mold under rotational acceleration. Heat leaves primarily through the mold wall, so solidification advances inward. The finished bore is created without a conventional central core, although it normally requires machining allowance.
Rotation speed is not a universal recipe. Mold diameter, metal density, section thickness, pour rate, temperature and desired solidification behavior interact. Too little rotational force can produce poor distribution or an irregular bore. Excessive or unstable conditions can increase vibration, segregation, mold loading or surface problems. Qualification must use the actual alloy, diameter and section.
Do not confuse the process families
True centrifugal casting makes its own hollow cylindrical interior. Semi-centrifugal casting uses rotation to feed a solid axisymmetric part such as a wheel or gear blank, usually with a central feed system. Centrifuge casting places multiple shaped cavities around a rotating axis and uses centrifugal force to distribute metal into those cavities. These routes have different tooling, metal flow and quality relationships.
A supplier proposal should name the specific route. Calling all three centrifugal casting can obscure whether the part requires a core, where inclusions collect and what machining stock is necessary.
Why the bore is usually sacrificial
As the casting freezes inward, lower-density oxides, slag, gas-related discontinuities and compositionally segregated liquid may concentrate nearer the inner diameter. The degree and mechanism depend on alloy and process. Designers commonly treat part of the bore as removal stock, then machine to sound material and final size.
This is a useful process feature only when the machining allowance is established from evidence. Too little stock can leave an unacceptable inner layer. Too much wastes alloy and machining time. Sectioned first articles, macroetching, chemical checks or nondestructive examination can help establish the removable zone for the specific casting.
Candidate geometry and limitations
Part condition | Centrifugal-casting implication | Buyer question |
|---|---|---|
Long hollow cylinder | Horizontal equipment may support a uniform axial section | How are pour progression and axial chemistry controlled? |
Short ring or flange blank | Vertical equipment may fit the geometry | How are end effects and bore taper managed? |
Complex external details | Mold features may be possible, but extraction and balance constrain them | Which features remain cast and which are machined? |
Non-axisymmetric internal passages | True centrifugal casting is generally a poor fit | Would sand, investment or pressure casting be more direct? |
Length-to-diameter relationship, wall profile, flanges, external steps and required bore shape affect feasibility. A tube with simple machining may fit well. A housing with cross-passages, deep pockets and offset bosses may require extensive machining that defeats the process advantage. Compare alternatives through a metal casting route review.
Alloy behavior and segregation
Ferrous, nickel, copper and some aluminum alloy systems can be candidates on appropriate equipment, but commercial capability must be confirmed grade by grade. Density differences among phases and the outside-to-inside freeze pattern can affect segregation. The chemistry and microstructure at the working outside diameter may differ from the removable bore region.
State where composition, hardness, tensile, corrosion or wear requirements apply. A certificate for the melt does not alone prove local structure through the wall. Sampling position and orientation should match the service load and governing material specification.
Mold, pour and solidification controls
Mold material, coating, preheat, rotational stability, pour temperature and pour rate shape the outer surface and solidification front. The machine and mold must remain dynamically stable with the changing mass of liquid metal. End closures, bearings and drive components are part of the process risk, especially for long or heavy castings.
Control records should connect each casting to alloy heat, mold setup, rotation profile, pour sequence and cooling conditions. A supplier should also define restart rules after interruption. The first casting after a thermal upset may not represent steady production.
Defects and their verification
Possible nonconformities include an irregular bore, axial laps, hot tears, shrinkage, inclusions, segregation bands, outer-surface defects and dimensional runout. Rotation can relocate some lower-density material toward the bore; it does not eliminate dirty metal or incorrect solidification. Claims such as void-free require a defined flaw type, location, size and examination method.
Use dimensional inspection for diameters, wall variation, straightness and machining stock. Select ultrasonic, radiographic, penetrant, magnetic-particle or other examination according to material, geometry and defect orientation. Mechanical and metallurgical samples should come from agreed locations. Final pressure or rotation tests may be necessary when the part's function demands them.
Machining and cost model
Include outer cleanup, bore removal, end facing, grooves, holes, heat treatment, balancing and inspection in the delivered cost. Centrifugal casting can avoid a central core and can place sound metal near the mold wall, but the sacrificial bore and end allowances are real material and machining costs. Compare them with forged hollow stock, rolled-and-welded tube, static casting and machining from solid.
Volume, alloy, diameter range, tooling reuse and machine availability determine economics. A lower tooling line does not guarantee lower unit cost if machining removes a large fraction of the casting or if the demand does not fit the equipment's productive range.
What to include in an RFQ
Send finished and as-cast geometry, alloy and specification, annual volume, working outside and inside surfaces, machining allowances, pressure, wear, corrosion, temperature and fatigue duties. Mark required property locations, NDT zones, defect acceptance, heat treatment, traceability and documentation. State whether imbalance, straightness or runout affects operation.
Request the proposed orientation, mold route, rotation and pour-control strategy, expected bore removal, test-coupon locations, machining sequence and validation plan. Engineering review should prove that outside-to-inside solidification puts acceptable material where the part works and removable material where machining can reach it. That is the central sourcing test for centrifugal casting.
