What Is Centrifugal Casting?
Centrifugal casting, sometimes referred to as rotocasting, is a widely used method for producing thin-walled cylinders from materials such as metals, glass, or even concrete. This technique stands out for its ability to deliver consistent quality, largely thanks to the careful control of both the metallurgy and the resulting crystal structure throughout the process.
Unlike many other casting approaches that focus on creating parts shaped for a specific application, centrifugal casting is mainly employed to produce rotationally symmetrical stock—think standard-sized tubes or cylinders—that are later finished through machining. In other words, rather than ending up with a finished component right out of the mold, the result here is a high-quality starting material, ready to be adapted as needed.
The process itself is straightforward but highly precise. Molten metal is poured into a mold that’s spinning at high speed. This rotation pushes the liquid metal outward, pressing it firmly against the inner wall of the mold. By carefully monitoring variables like pour rate, volume, and temperature, operators ensure that the finished product meets strict standards for both structure and uniformity.
As the metal cools, it takes on the shape of the mold’s interior, forming a solid tube or ring. Once the casting has fully solidified, it can be further refined—through heat treatment, cutting, or machining—to match whatever specifications are required.
One notable advantage of centrifugal casting is the quality of the resulting tubes: they tend to have a uniform composition and mechanical strength that surpasses what’s typically achieved through other casting methods. Additionally, because of the centrifugal force at play, impurities and lighter oxides are driven toward the inside diameter of the casting. This makes them much easier to remove during later machining, leading to a final product with excellent purity and performance.
Materials for Centrifugal Casting
Centrifugal casting is a versatile process, suitable for a wide range of materials such as metals, cements, concretes, glass, and pottery. When it comes to metals, the process is often used with iron, steel, stainless steel, and various alloys that include nickel, aluminum, copper, and even magnesium.
It’s also possible to combine two materials within the same casting process by adding a second material at the right moment. One familiar example is the production of cast iron pipes that have an interior lining of cement.
Today, many industrial manufacturers rely on centrifugal casting to produce parts from an assortment of metals and metal alloys. Common choices for raw materials in this process include iron, stainless steel, steel, aluminum, copper, and nickel, each offering distinct advantages depending on the application.
Of course, the process isn’t without its challenges. Setting up for centrifugal casting often requires a significant investment in permanent molds. On top of that, because the process demands high temperatures and precise control over the rotation of molds, energy costs can run higher compared to some other casting techniques.
Even so, centrifugal casting is particularly well-suited to automation. For companies that need to produce large quantities, the initial investment tends to pay off, as the process enables efficient, high-volume manufacturing.
Process For Casting Metal
Centrifugal casting is a technique used to manufacture pipes by pouring molten metal into a fast-rotating cylindrical mold. The rotation generates centrifugal force, which presses the metal outward against the mold’s inner surface. Kubota first introduced this method for steel production in 1952.
The process begins by filling a pre-heated die with liquid metal while the die spins at high speed. Depending on the specific design requirements, the axis of rotation may be either vertical or horizontal.
As the mold spins, centrifugal force distributes the molten metal evenly along the inner wall, applying pressures several times greater than gravity alone. This intense pressure helps to eliminate the kinds of cracks and other flaws—both microscopic and macroscopic—that might otherwise develop during casting. The effect is similar to traditional die casting, where applied pressure serves to minimize imperfections in the finished product.
During filling, the denser metal is driven toward the wall of the rotating die, while lighter materials and impurities tend to collect away from the central axis. Solidification progresses from the outer diameter inward, causing purer metal to form the pipe wall and pushing less desirable material toward the bore.
Once the casting has fully solidified, the finished piece is removed from the mold. Any impurities or unwanted material at the boundaries are then removed through machining. The result is a high-quality pipe, free from internal cavities or trapped air, with consistent properties throughout its cross-section.
Let’s walk through the essential steps of the centrifugal casting process, starting from the point when the mold is ready.
- As with most casting techniques, everything begins by heating the metal until it reaches a liquid state well past its melting point.
- Once the metal is ready, the mold is set in motion. The speed typically ranges from 300 to 3,000 revolutions per minute, but the precise rate is chosen based on the requirements of the specific application.
- With the mold spinning, the molten metal is poured inside. This step doesn’t demand a complicated setup, but steady hands are important since pouring occurs while the mold is already rotating.
- After the pour, the process flows right into cooling. The continued rotation serves a dual purpose: not only does it help the casting cool, but it also generates enough internal pressure to reduce the risk of casting defects.
- When the material has cooled and solidified, the next phase is straightforward—removing the mold from its rollers and separating the finished piece from the casting.
- The last touch involves cleaning up the product. Thanks to centrifugal force, most impurities are driven out toward the outer edge, making it easier to machine them away and achieve a high-quality result.
Types of Centrifugal Casting
Centrifugal casting can be classified into two main types: vertical and horizontal processes. Additionally, some manufacturers have developed near-net shaping techniques, which blend the advantages of centrifugal casting with the precision of outer diameter (O.D.) shaping. In some cases, this approach achieves a level of finished detail comparable to that seen in investment casting.
Vertical Centrifugal Casting
Some manufacturers take advantage of vertical-axis dies to produce centrifugal components, including those that require specific shaping on the outer diameter.
In these setups, it’s not uncommon to see graphite, sand, or ceramic molds inserted into the die, which helps form the desired O.D. features right from the start. This approach goes a long way in cutting down the need for secondary processes like machining or fabrication later on.
Interestingly, the outer surface of the casting doesn’t always have to be a perfect circle. By building flanges or bosses into the mold’s inner diameter, it’s possible to introduce more complex shapes directly during casting.
In practice, the final part itself may not end up symmetrical; however, to keep things running smoothly while the mold spins, the casting mold itself sometimes needs to maintain a balanced design.
As for the dimensions, the inside diameter—and by extension, the wall thickness—are determined by both the amount of molten metal poured into the spinning mold and how much is later machined away. It’s also worth noting that, with vertical casting, the height of the finished piece usually ends up being less than twice its width.
Horizontal Centrifugal Casting
Some centrifugal casting machines are designed exclusively for horizontal casting, meaning the die rotates around a horizontal axis. This method tends to be quite economical when it comes to manufacturing tubular components that require high quality.
It works particularly well for producing long, cylindrical parts those where the overall length far exceeds the outside diameter. Typical examples include straight tube sections, elongated cylinders with end flanges, or even shorter items like rings and flanges, as several of these smaller parts can be machined efficiently from a single straight cylinder.
In practice, the process involves spinning a lengthy steel mold at a high speed while it remains in a horizontal position. This rapid rotation is necessary to counteract the effects of gravity during casting.
To keep the molten metal securely inside, covers are placed at both ends of the mold, and a pour funnel is used to introduce a measured amount of metal into the mold’s cavity.
Similar to what occurs in vertical centrifugal casting, the inner dimension of the mold sets the outer diameter of the final part, while the volume of metal poured determines the inner diameter.
Vacuum Centrifugal Casting
When it is essential to maintain precise part details and carefully control atmospheric exposure, centrifugally casting metals in a vacuum becomes especially important. This is particularly true for alloys such as nickel-cobalt superalloys, which tend to react with oxygen.
Beyond the usual benefits that come with vacuum casting, centrifugal casting itself contributes significantly to metal quality. This method encourages directional solidification, eliminates porosity, and allows for more accurate shaping of the final product.
Thanks to this combination, vacuum centrifugal casting is known for producing components with a high level of reliability qualities that make it a preferred choice in demanding fields like aerospace and military manufacturing.
What Are Three Forms Of Centrifugal Casting?
Centrifugal casting can be classified into three main types: true centrifugal casting, semi-centrifugal casting, and centrifuging. Additionally, this process is further categorized as either vertical or horizontal, based on the orientation of the equipment used.
True Centrifugal casting
In this casting technique, molten metal is introduced into a rotating horizontal mold to create tubular components such as pipes, tubes, rings, and bushes. While molds can rotate around either a horizontal or vertical axis, horizontal rotation is generally preferred in practice.
The speed at which the mold turns is a key factor, as it significantly influences both the success of the casting process and the final quality of the parts produced.
Typically, these molds are constructed from materials like steel, iron, or graphite, and they are often lined with a refractory coating to enhance durability. The design of the mold surfaces allows for the production of pipes with various exterior profiles.
A key characteristic of this method is that the inner surface of the casting remains consistently cylindrical, thanks to the centrifugal forces that distribute the molten metal evenly. As a result, components made by true centrifugal casting tend to have a high density, particularly along their outer sections where the centrifugal effect is most pronounced.
One notable advantage is that, since the molten metal is continuously forced toward the mold wall as it cools, issues like solidification shrinkage outside the cast tube are largely avoided. If there are any imperfections, they are most likely to appear along the inner wall of the casting, and these can usually be addressed through machining if necessary.
Semicentrifugal Casting
Semicentrifugal casting relies on centrifugal force to create solid castings, rather than hollow or tubular forms. In this process, as illustrated in the image below, the molds are designed with their risers positioned at the axis of rotation so they can properly feed the molten metal during solidification.
Typically, expendable molds are used for this method. It is a common technique for producing parts like spoked wheels, pulleys, gear blanks, brass bushings, and nozzles.
One characteristic of semicentrifugal casting is that the material toward the outer regions of the casting ends up denser than the areas near the central axis. This effect is especially noticeable in components that exhibit rotational symmetry, such as spoked wheels. By removing the center section after casting, manufacturers also eliminate the area of lowest density from the finished part.
Several factors can influence the final quality of the casting. These include the speed at which the mold rotates, the diameter of the component, the temperatures at which both the metal is poured and the mold itself is maintained, the rate at which the metal is poured, and how quickly the part cools after casting.
Centrifuging
Centrifuging, sometimes referred to as centrifuge casting, is a technique where mold cavities of varying shapes are positioned at a specific distance from the spinning axis. In this process, molten metal is introduced at the center. As the apparatus spins, centrifugal force drives the metal outward, guiding it through the sprue and runner to fill the mold cavity.
Much like what we observe in true centrifugal casting, the properties of the resulting casting can vary depending on how far a section is from the axis of rotation. This approach lends itself particularly well to producing smaller components, including items like jewellery, as well as small bushes and sleeves.
Jewellery and dental castings often rely on specialized centrifugal casting machines such as those produced by Neycraft, Tulsa, and Kerr. These machines all operate on the same fundamental principle of using centrifugal force to achieve precise and consistent results.
Applications of Centrifugal Castings
Centrifugal casting plays a significant role across various industries, finding its place in the production of components like bushings, engine cylinder liners, rings, brake drums, water and sewage pipes, street lamp posts, and gas lines.
Its versatility stems from the unique advantages this process offers, making it a go-to choice for manufacturing a broad spectrum of parts.
To give a clearer picture, here are some key areas where centrifugal casting is regularly employed:
- Pipes and Tubes: When it comes to producing seamless pipes and tubes, centrifugal casting stands out. You’ll commonly find this method used for lamp posts, sewage pipes, water supply lines, oil and gas pipelines, as well as boiler tubes and hydraulic cylinders.
- Rings and Wheels: If the goal is to achieve high strength and precise dimensions, especially for rings and wheels, centrifugal casting is often the method of choice. It’s relied upon to produce gears, bearings, flywheels, brake drums, and even railroad wheels.
- Cylindrical Components: The process is equally effective for various cylindrical parts. Examples include liners, shafts, rollers, sleeves, and bushings—all benefiting from the consistency and quality centrifugal casting provides.
- Additional Applications: Beyond these, centrifugal casting is also used for manufacturing items such as valve bodies, turbine blades, missile casings, pump housings, medical implants, and aerospace components.
In short, the adaptability and reliability of centrifugal casting make it a mainstay in both traditional and specialized industrial applications.
Advantages of Centrifugal Casting
Eliminates Mid-wall Defects
In the centrifugal casting process, there is no dependence on individual risers or specific feed points to prevent trapped porosity. Instead, the inner diameter stays in a fully liquid state throughout solidification, which means there is always molten metal available to compensate for any shrinkage as the metal contracts.
This stands in contrast to traditional gravity or static casting methods, where solidification tends to begin at both the inner and outer surfaces. That approach often leads to the risk of mid-wall shrinkage becoming trapped within the casting. With centrifugal casting, however, the metal solidifies gradually from the outer surface moving inward.
Because of this distinctive solidification pattern, components produced by centrifugal casting are consistently free from shrinkage cavities, gas entrapment, and blowholes.
Controls Impurities
One of the defining aspects of the centrifugal casting process is its ability to use intense force to separate lighter substances toward the bore, or inner diameter, of the casting.
As the process continues, these less dense impurities, things like oxides, sulfides, trapped gases, and similar contaminants, naturally migrate inward. Once the casting cools, this central layer can be removed by machining, resulting in a part that’s free of internal defects.
The method works by pouring molten metal into a mold that’s spinning at high speed, sometimes generating forces as high as 100 times gravity. It’s this centrifugal force that really drives the whole process, and it all comes down to how fast the mold spins (RPM) and the mold’s diameter.
During spinning, the denser molten metal is thrown outward, forming a solid, clean structure against the mold wall, while those lighter impurities float inward. Left unchecked, these would otherwise remain trapped as inclusions, compromising the quality of the final product.
Interestingly, this self-purifying effect is known as “secondary refining,” a term that captures the process’s ability to deliver a cleaner, more reliable casting.
Greatly Reduces Micro Porosity
When these factors come together, directional solidification, the movement of impurities toward the inner diameter, and the influence of solidification under pressure, the result is a component that stands out for its defect distribution and overall integrity.
Compared to what is typically achieved through alternative manufacturing methods, this approach consistently yields parts with fewer flaws and better structural soundness.
Disadvantages Of Centrifugal Casting
Length to Diameter Ratio and Cost
Vertical centrifugal casting processes typically maintain a length-to-diameter ratio close to 2:1. When this proportion is exceeded, the centrifugal force generated during casting is often insufficient to keep the molten metal distributed evenly along the mold’s inner surface.
Even under ideal conditions, it is common to observe a parabolic profile in the rough casting; this means the wall thickness is generally greater at the bottom compared to the top. This variation must be taken into account when estimating both the casting’s final weight and the subsequent machining expenses.
It’s worth noting that, unlike vertical methods, horizontal centrifugal castings are not bound by the same length-to-diameter ratio constraints. The practical limitations of internal diameter and die length exist, of course, but the specific 2:1 ratio does not restrict horizontals in the same way.
A common and efficient approach for producing shorter components involves casting multiple parts together and then cutting them to length as needed. This technique has shown itself to be highly cost-effective for manufacturing several round pieces from a single casting.
Shape and Mold (Die) Cost
It is certainly possible to cast components with irregularly shaped outside diameters, but it is important to consider the expense involved in producing a metal or graphite mold that can achieve the desired contour.
When evaluating centrifugal casting, one needs to weigh the high quality of the resulting product and the potential savings in post-processing—such as reduced machining time—against the initial investment in tooling.
Centrifugal casting stands out, particularly in situations where maintaining precise roundness is essential. Because these parts are formed to be round and free of internal stresses, there is less risk of them losing their shape over time, making this process especially appealing for such applications.
Small Inside Diameters, Safety, and Cost
Casting components with very small internal diameters using the centrifugal process often presents notable challenges. As the inside diameter becomes smaller, the advantages typically associated with centrifugal casting—such as uniform density and reduced impurities—also diminish.
To illustrate, achieving a centrifugal force equivalent to 100 times gravity (100 Gs) for a casting with a 2.5-inch inside diameter requires a mold speed of approximately 1700 RPM.
However, if the diameter drops to just 1 inch, the mold must spin at about 2600 RPM to reach that same force. While such rotational speeds are technically achievable, operational and safety considerations may place practical limits on how fast the molds can actually be run.
In most cases, a smaller inside diameter also means the casting itself is relatively small. Centrifugal casting tends to deliver greater value and efficiency as the overall diameter and wall thickness of the part increase.
Good Foundry Practices Required
While centrifugal casting is often praised for its potential to yield superior material properties, it’s important to recognize that the process itself doesn’t automatically guarantee exceptional results.
The quality of the final product hinges on how well the foundry takes advantage of certain key factors, namely, effective use of deep chilling, ensuring that solidification proceeds inward from the mold surface, and maintaining sufficient feeding pressure throughout the process. When these elements are handled properly, the castings tend to show consistently high quality.
However, when any of these principles are overlooked or compromised, the properties of the resulting castings can vary considerably. This underscores a broader truth in metal casting: regardless of the specific method employed, good foundry practice remains a fundamental requirement for achieving high-quality results.