What Is Centrifugal Casting?
Centrifugal casting or rotocasting is a casting technique that is typically used to cast thin-walled cylinders. It is typically used to cast materials such as metals, glass, and concrete.
A high quality is attainable by control of metallurgy and crystal structure. Unlike most other casting techniques, centrifugal casting is chiefly used to manufacture rotationally symmetric stock materials in standard sizes for further machining, rather than shaped parts tailored to a particular end-use.
Centrifugal casting is a method used to produce metal products that are tubular, cylindrical, or annular in shape that require high quality, purity, and structural uniformity.
Molten metal is poured into a spinning mold which forces the molten metal to the mold wall. The process is precisely monitored and controlled to assure the correct volume of metal is poured at the calculated flow rate and temperature.
As the metal is poured, it conforms to the inside diameter of the mold and solidifies as it cools. Once solidified, the casting can be heat-treated, cut, and machined to customer specifications.
The centrifugal casting process inherently produces high quality tubes with uniform properties and higher mechanical strength than provided by other casting methods.
Centrifugal force causes impurities and lighter oxides to migrate to the inside diameter of the casting which, upon solidification, are easily removed by CNC machining and processing.
Materials for Centrifugal Casting
Typical materials that can be centrifugal cast are metals, cements, concretes, glass, and pottery materials. Typical metals cast are iron, steel, stainless steels, and alloys of nickel, aluminum, and copper, magnesium.
Two materials can be combined by introducing a second material during the process. A common example is cast iron pipe coated on the interior with cement.
In this century, industrial part manufacturers employ centrifugal casting as a process for creating components from a variety of metals and metal alloys. Some popular raw materials for this type of casting include iron, stainless steel, steel, aluminum, copper, and nickel.
Facilities may invest significant sums in the creation of the permanent molds used during centrifugal casting.
Since manufacturing with this process involves both high temperatures and the need to rotate molds at controlled rates of speed, plants may incur comparatively higher energy production costs than with some other casting processes.
Centrifugal casting lends itself to automation, so despite the initial investment it frequently offers a very efficient manufacturing process, especially for firms engaging in high volume production runs.
Process For Casting Metal
The centrifugal casting method is the method to produce pipes by pouring molten metal into a rapidly spinning cylindrical mold in which centrifugal force from the rotation exerts pressure on the molten metal. In 1952, Kubota developed its first centrifugal casting technology for steel.
The centrifugal casting process starts with liquid metal being filled with a high-temperature die that rotates. The die might be either on a vertical or flat pivot contingent upon the detailed specifications of the design.
During the process, centrifugal power acts to disseminate the liquid metal in the shape at pressure moving toward multiple times the power of gravity.
This applied pressure ensures that there are no cracks or other microscopic and macroscopic flaws. This is very similar to the regular die casting process where applied pressure reduces the overall flaws in the product.
As the die fills, the thicker liquid metal is compelled to the mass of the turning die. Moreover, directional cementing of sound metal advances from the outer diameter towards the drag, while the less thick material along with impurities floats to the outer diameter.
Once the solidification is completed, the complete part is taken out from the die and the remaining debasements in the boundaries of the part are removed through the machining process. Therefore, you are left with a high-quality material without any internal cavities or air bubbles.
Here’s a simple breakdown of the basic centrifugal casting process when your mold is ready.
- The start is just like any casting process, where you heat the metal beyond its melting point
- The second step is rotating the mold to prepare it for pouring. Generally, the rate of rotation is between 300 to 3000 RPM. However, the actual number depends on the specifics of the project.
- The next step begins with pouring. This is a relatively simple process as it doesn’t require any special preparation. However, you need to be careful because the process will take place while the mold is rotating.
- Once the pouring is completed, the next step automatically starts. The rotation facilitates the cooling process but creates enough pressure within the mold to ensure that there are no chances of flaws associated with the casting process.
- After cooling, the next step is similar to how any casting process goes. All you need to do is to remove the mold from the roller and extract the material from the casting.
- Finally, refine the final product. The centrifugal rotation pushes the impurities in the material to the edges and you can machine them out to get the quality you need.
Types of Centrifugal Casting
There are two types of centrifugal casting processes – vertical and horizontal. In addition, some manufacturers offer near-net shaping which combines the benefits of centrifugal casting with O.D shaping, perhaps even with the finished detail of an investment casting.
Vertical Centrifugal Casting
Some manufacturers produce centrifugal components, including some with O.D shaping, in dies rotating about the vertical axis.
These vertical castings may achieve that O.D. shaping by inserting graphite, sand, or ceramic molds into the die – resulting in significantly reduced post-processing, like machining or fabrication.
Details on the outside surface of the casting may be modified from the true circular shape by the introduction of flanges or bosses to the inner diameter of the mold.
The finished part need not be symmetrical but, in some cases, the casting mold does to maintain balance while spinning.
The inside diameter and therefore the wall thickness of the casting are functions of the amount of metal poured into the rotating mold and the quantity machined away. When casting vertically, the height of the casting will typically be less than twice the width.
Horizontal Centrifugal Casting
Some centrifugal casters produce only horizontal castings where the die rotates about the horizontal axis. This is a cost-effective method for producing high-quality tubular components.
This process is especially suited for long cylindrical parts where the casting length is significantly longer than its outside diameter.
This includes straight tube sections, long cylinders with end flanges, or short parts such as rings or flanges where multiple parts can be machined effectively from a straight cylinder.
A long steel casting mold is spun at high speed while positioned horizontally. The rotational speed of the mold is high, to offset gravitational forces.
Covers are fixed at each end of the mold to contain the molten metal and a pour funnel is used to deliver a specified weight of metal inside the mold.
Just as in vertical casting, the interior dimension of the mold determines the O.D. size of the part, while the amount of metal poured into the mold determines the I.D. size.
Vacuum Centrifugal Casting
Centrifugally casting in a vacuum is used when part detail and control of exposure to the atmosphere is critical since some alloys, including nickel-cobalt super alloys, are reactive to oxygen.
In addition to the advantages of casting in a vacuum, the inherent high metal integrity delivered by centrifugal casting is realized, including directional solidification, absence of porosity, and net-shaping.
Vacuum centrifugal casting provides products with very high reliability, often used in aerospace and military applications.
What Are Three Forms Of Centrifugal Casting?
True centrifugal casting, semi-centrifugal casting, and centrifuging are the three forms of centrifugal casting. The process is further divided into, Vertical and Horizontal, depending on their construction.
True Centrifugal casting
Molten metal is poured into a rotating horizontal mould to produce tubular parts such as pipes, tubes, rings, and bushes in this casting process.
Mould rotation can occur on a horizontal or vertical axis, with the former being more common. The revolving speed of the horizontal true centrifugal casting is critical to the effectiveness of the casting process and part quality.
Moulds are made of steel, iron, or graphite and can be coated with a refractory lining to extend their life. The surfaces of the moulds are designed to allow for pipe casting with a range of outside styles.
The inner surface of the casting remains cylindrical due to centrifugal forces that evenly distribute the molten metal.
Castings produced by true centrifugal casting have a high density, particularly in the outer portions of the component where centrifugal force is highest.
Because centrifugal force constantly reallocates molten metal toward the mould wall during freezing, solidification shrinkage outside the cast tube is not an issue.
Any imperfections in the casting tend to be on the inner wall and, if necessary, can be eliminated by machining.
Semicentrifugal Casting
Semicentrifugal casting uses centrifugal force to produce solid casting rather than tubular casts. The image below shows an example of semi-centrifugal casting where the moulds have their riser at the axis of rotation to feed the molten metal.
Expendable moulds are common and used to make parts such as spoked wheels, pulleys, gear blanks, brass bush and nozzles.
The exterior regions of items formed by semi-centrifugal casting have a higher density than the centre of the rotating axis.
This casting procedure makes things like spoked wheels that have rotational symmetry and can remove the casting centre. Removing the centre section of the cast also removes the cast’s lowest-density component.
The quality of the final casting is affected by factors such as rotating speed, component diameter, pouring temperature, pouring speed, mould temperature, and cooling rate.
Centrifuging
Centrifuging, also known as centrifuge casting, involves the placement of mould cavities of any shape at a certain distance from the axis of spin.
The molten metal is poured from the centre, and centrifugal forces push it into the mould cavity through the sprue and the runner.
Like true centrifugal casting, the characteristics of the castings might change with distance from the spin axis. This method is used for small parts such as jewellery, small bushes and sleaves.
Centrifugal casting machines such as Neycraft, Tulsa, and Kerr, used to make jewellery and dental casting, are all based on the centrifugal casting technique.
Applications of Centrifugal Castings
Centrifugal casting has a wide range of applications in the industry and is used to make parts such as bushings, engine cylinder liners, rings, brake drums, water supply lines, sewage pipes, street lamp posts and gas pipes.
Centrifugal casting offers several unique advantages. Hence centrifugal casting is done for producing various parts for various applications.
Here are some of the most common applications of this process:
Pipes and Tubes:
Centrifugal casting is done for producing seamless pipes and tubes, including:
- Lamp posts
- Sewage pipes
- Water supply lines
- Oil and gas pipelines
- Boiler tubes Hydraulic cylinders
Rings and Wheels:
The centrifugal casting process is ideal for creating high-strength and dimensionally accurate rings and wheels, such as:
- Gears
- Bearings
- Flywheels
- Brake drums
- Railroad wheels
Cylindrical Parts:
This process is suitable for producing various cylindrical parts, including:
- Liners
- Shafts
- Rollers
- Sleeves
- Bushings
Other Applications:
The centrifugal casting process can also be used to create a variety of other parts, such as:
- Valve bodies
- Turbine blades
- Missile casings
- Pump housings
- Medical implants
- Aerospace components
Advantages Centrifugal Casting
Eliminates Mid-wall Defects
The centrifugal process does not rely on discrete risers or feed points to ensure the absence of trapped porosity.
Rather, the inner diameter remains fully liquid during the solidification process, resulting in a continuous supply of metal to feed areas of contraction.
Unlike conventional gravity or static castings that chill from both the inside and outside surfaces and risk trapping mid-wall shrinkage, centrifugal castings solidify from the outside surface inward.
The result is a part consistently free of shrinkage cavities, gas pockets, and blowholes.
Controls Impurities
Unique to the centrifugal process is the high force that effectively isolates less dense substances in the casting’s bore area.
These impurities are later machined away, leaving a defect free part. Molten metal is fed into a rotating mold where a centrifugal force of up to 100Gs is applied to the metal.
Centrifugal force is critical for this process. The Gs produced are dependent on the RPM and diameter of the die.
The centrifugal force pushes high density metal against the mold sidewall while lower density components migrate or “float” toward the I.D.
These lower density items include metal oxides, sulfides, gas, and other impurities that would otherwise have been inclusions in the casting. This phenomenon is termed “secondary refining”.
Greatly Reduces Micro Porosity
The combination of directional solidification, impurity migration to the I.D., and solidification under pressure produces a component with superior defect distribution and soundness compared to other manufacturing processes.
Disadvantages Of Centrifugal Casting
Length to Diameter Ratio and Cost
Vertical centrifugal castings are generally limited to a length (height) to diameter ratio of about 2:1. If this ratio of 2:1 is exceeded, the molten metal may not have sufficient centrifugal force to be held evenly against the mold wall.
Even under the best conditions, there is a parabola created in the roughcasting where the wall thickness at the bottom of the casting is larger than that of the top. This impacts the calculation for cast weight and machining cost.
While there is a practical limit to the I.D. size and how long the die may be, horizontal centrifugal castings do not face the same ratio restrictions that verticals do.
In many instances, shorter parts are cast in multiples and cut to the desired length. This has proven to be a very cost-effective method to produce numerous round parts from a single casting.
Shape and Mold (Die) Cost
Irregularly shaped outside diameters can be cast; however, the cost of a metal or graphite mold having the required shaping must be taken into account.
The quality of centrifugal and cost savings in post-processing including machining must be balanced with the tooling cost.
Centrifugal castings are very attractive in applications where the roundness of the finished application is important. Parts are cast round with no internal stresses, which will encourage the part to lose its roundness.
Small Inside Diameters, Safety, and Cost
Centrifugal casting with very small inside diameters can be a challenge. As the size of the inside diameter decreases, the beneficial effects of the centrifugal process also decreases.
For example, to cast a 2-½” inside diameter at 100 times the force of gravity, a mold speed of 1700 RPM would be required.
For example a 1″ inside diameter requires a mold speed of 2600 RPM to obtain the same 100Gs of force. Although these speeds are obtainable, other factors such as operational safety factors can limit the speed used.
Usually, small inside diameters indicate small castings. Centrifugal castings tend to provide greater value as part diameters and wall sections increase.
Good Foundry Practices Required
Although many claims are made concerning the superior properties obtainable from centrifugal castings, the process alone is not a guarantee of a superior product.
In cases where equipment and design permit the full utilization of deep chill, directional solidification from the mold face inward, and high feeding pressure, castings are consistent of very high quality.
When these principles are compromised, a wide range of properties can be expected. As a result, good foundry practice in all methods of casting metal is an essential factor in producing high-quality castings.