Centrifugal casting is an exceptional and versatile method for manufacturing cylindrical-shaped and axially symmetrical parts with remarkable integrity and durability.
It uses centrifugal force to spread molten material along the walls of a rotating mold, resulting in high-quality, dense, and fine-grained castings.
Guaranteeing the production of completely uniform cylindrical components without cores, centrifugal casting eliminates defects such as internal stress, gas bubbles, porosity, or voids, thereby promoting corrosion resistance, reliable performance, and longevity.
In this article, we’ll explain centrifugal casting, the three types, its step-by-step process, materials suitable for centrifugal casting, its advantages and disadvantages, and its applications in various industries.
What Is Centrifugal Casting?
Centrifugal casting is a manufacturing process whereby molten material is poured into a pre-heated mold subject to centrifugal force by rotating it on a central vertical or horizontal axis.
The continuous rotation throws molten material against the mold’s wall, directing fine grains and dense particles to the outer wall, resulting in a hollow inner wall.
As the finer particles move towards the outer wall, the less fine materials, impurities, gas, and pores move towards the inner wall, creating a dense cylindrical structure of superior quality.
Centrifugal casting is also known as rotocasting, spin casting, and rotational casting.
Centrifugal Casting Process
The process of centrifugal casting is completed in a centrifugal casting machine, as depicted in the figure below.
Even though centrifugal casting differs slightly between the three types, here are the general steps taken for a complete centrifugal casting process:
- Mold preparation: The mold in the centrifugal casting machine is prepared for casting. This is done by cleaning the mold and heating it to cure it. Subsequently, ceramic slurry is introduced to the mold while it is spinning.
- Pouring of molten material into the mold: The molten material is poured into the mold before the machine rotates or after rotation has commenced. The rotation is usually between 300-3000 RPM (revolutions per minute), depending on the desired dimensions.
- Centrifugal casting: The rotation causes centrifugal force to spread the molten material against the wall of the mold. This continuous spinning leads to a uniform coating of the mold cavity to form the desired shape of the component being cast.
- While the rotation continues, the fine grain of the material is coated along the outer wall, driving impurities, gas, and pores to the inner wall of the mold.
- Directional solidification: The rotation of the mold continues until the molten material cools and solidifies. Due to rotocasting and directional solidification, the cast dries from the outside wall to the inside wall of the cast.
- Removal and finishing: After solidification, the rotation stops and the cast is removed from the mold. Subsequently, machining commences to remove accumulated impurities from the inner walls.
Note that the inner walls of the cast are usually not the exact specification of the component. This is to give allowance for machining impurities. After machining, the final part can be subject to tests like heat testing.
Different Centrifugal Casting Types
The three centrifugal casting types are true centrifugal casting, semi-centrifugal casting, and centrifuging. We will explain them below:
1. True Centrifugal Casting
The steps outlined above best explain the true centrifugal casting process. It involves the rotation of molten material in a mold on a vertical or, more typically, a horizontal axis. The choice of horizontal centrifugal casting provides a final product with high integrity and strength.
True centrifugal casting diagram source: Engineering Product Design
This centrifugal casting method is beneficial compared to the other types because it creates parts with the highest integrity. Additionally, the process eliminates defects that cause internal stress. More so, the method is cost-effective, minimizes waste, and can produce large components easily.
The true centrifugal casting method is limited by its inaccurate production of inner surface diameter and the impossibility of creating thin-walled cylindrical parts. It is also limited to cylindrical and symmetrical objects.
The true centrifugal method of casting is used to manufacture tubes, pipes, bushings, and rings.
2. Semi Centrifugal Casting
Semi-centrifugal casting differs from true centrifugal casting in that it is primarily completed on a vertical axis and is suitable for axial symmetrical parts but with additional features like flanges, bosses, or other complex geometries.
This vertical centrifugal casting mold design incorporates the additional flares or protrusions desired in the final product. Unlike true centrifugal casting, it is used for shorter, thick-walled parts like gear banks, flywheels, impellers, and rotors.
Semi-centrifugal casting is limited to creating products with a 2:1 length-to-diameter ratio. Deviations from this can lead to uneven coatings along the mold’s wall leading to low-density components.
3. Centrifuging
Centrifuging, also known as centrifuge casting, is completed on a vertical axis and is mostly used for smaller products like jewelry, dental casting, sleaves, and small bushes.
Similar to investment casting, this vertical centrifugal casting incorporates a sprue and feeder system, with the molds arranged around the sprue.
Using the principle of centrifugal casting, the mold around the sprue is rotated and the molten metal is poured. This causes the molten material to spread into the molds while driving impurities inward towards the central sprue.
One major benefit of centrifuging is that it is perfect for small, geometrically intricate parts requiring minor post-processing. However, it is limited by the choice of materials that can be used and the additional cost of machining the runner and sprue.
What Materials Are Used for Centrifugal Casting?
- Steel: Suitable for applications requiring high integrity, toughness and wear resistance, like pipes, tubes, and rings used in oil and gas (pipeline components), automotive (brake rotors and drums), and power generation industries (turbine rings).
- Iron: Similar to steel, this material, specifically gray and ductile iron, is used for rotocasting because of its excellent wear resistance and machinability. It is used for products like engine cylinder liners.
- Aluminum: This material is recommended for lightweight, yet strong parts like those needed in aerospace, and automotive parts.
- Copper: Specifically known for its excellent thermal and electrical conductivity, copper is used for electrical components like heat exchanger tubes in HVAC systems.
- Bronze: Bronze is used for centrifugal casting because of its corrosion resistance and remarkable frictional properties. It is perfect for parts such as propeller shafts and fittings for marine hardware and sculpture and architectural structures.
- Alloys: Alloys like nickel-based alloys and stainless steels are used for centrifugal casting due to the high temperature and corrosion resistance needed in some products. Examples of such products are pipes for chemical processing, aerospace engine parts, and medical devices.
- Glass: Even though cautiously used, glass is used in centrifugal casting for products like precision lenses, and chemical-resistant containers in industrial settings.
- Concrete: Concrete can be subject to spin casting when making construction and infrastructure materials like precast concrete pipes, and support columns.
- Ceramics: Ceramics are used when there is a need to cast high-temperature resistant parts, such as thermal-resistant tubes for industrial furnaces.
Advantages and Disadvantages of Centrifugal Casting
Centrifugal Casting Advantages
- High quality: Centrifugal casting allows the production of parts with high density and fine grain structure, thereby lending to its overall strength and integrity
- Eliminates defects: In centrifugal casting, the impurities are pushed from the outer wall to the inner wall of the cast. This helps to prevent trapped gas, pores, or other irregularities in the casting material. The impurities collected on the inner walls can be machined away.
- Uniform casting: The continued rotational movement of the mold along the central axis until solidification enables a uniform, 360° distribution of materials for the product.
- Prevents midwall defects: Unlike gravity and static casting, where the solidification occurs both inside and outside simultaneously, centrifugal casting dries from the outer wall inward, minimizing the risk of midwall defects.
- It enables a clean surface finish with dimensional accuracy
- Versatility: A wide range of materials can be employed for centrifugal casting. It can also cast both large and small parts.
- Bi-metal casting: Spin casting can be used to cast two material components.
- Cost-effectiveness: Its reduced need for machining and high dimensional accuracy reduces cost.
Centrifugal Casting Disadvantages
Some of the limitations of centrifugal casting are:
- Centrifugal casting is limited to cylindrical and symmetrical shapes
- In some cases, additional machining is needed to achieve an accurate surface finish.
- It cannot produce parts with intricate inner layers and geometric shapes.
- The setup of equipment and operations is considerably complex, leading to a high cost of production.
- Vertical centrifugal casting, particularly semi-centrifugal casting, is limited to a length-to-diameter ratio of 2:1, deviations from which can cause poor production.
Applications of Centrifugal Casting
Even though some of the applications of centrifugal casting have been touched on under materials used for centrifugal casting above, we have compiled a further list of centrifugal casting applications. They are as follows:
- Automotive industries: brake drums, rotors, gear blanks and cylinder liners
- Power generation plants: boiler tubes, turbine rings, and sleeves
- Chemical processing: piping systems and reactor vessels
- Oil and gas industries: pipes, tubes, and drill collars
- Medical devices: prosthetic components and surgical instruments
- Aerospace industry: turbine blades, compressor rings of jet engines, and landing gear components
- Water treatment and supply: water pipes, pump housing, and impellers
- Mining industry: conveyor rollers and wear-resistant liners for grinding mills and crushers
- Engine and industrial machinery: flywheels, large bearings and bushings
Final Thoughts!
Centrifugal casting, also known as roto casting and spin casting, is perfect for axial symmetrical parts, usually cylindrical, which are strong and high density.
It is remarkable because of its directional solidification due to centrifugal force, which causes the denser and fine-grained materials to solidify outside, pushing all impurities inward for machining.
The superior mechanical properties of the products of this casting method have made it invaluable in industries where precision and strength are required.
