Overview of Centrifugal Casting
Centrifugal casting is a process that delivers components of high material soundness. As a result, it is the technology of choice for applications like jet engine compressor cases, hydro wear rings, many military products, and other high-reliability applications. It has also proven to be a cost-effective means of providing complex shapes with reduced machining requirements and lower manufacturing costs as compared to forgings and fabrications.
The centrifugal casting process steps begin with molten metal being poured into a preheated, spinning die. The die may be oriented either on a vertical or horizontal axis depending on the configuration of the desired part.
By spinning a mold while the molten metal is poured into it, centrifugal force acts to distribute the molten metal in the mold at pressures approaching 100 times the force of gravity. The combination of this applied pressure and the engineering mechanics of controlled solidification and secondary refining produces components of superior quality.
As the die begins to fill, the more dense molten metal is forced to the wall of the spinning die. Directional solidification of sound metal progresses from the O.D. towards the bore, while the less dense material, including impurities, &#;floats&#; to the I.D.
Once the casting has solidified, the part is removed from the die and residual impurities in the I.D. are machined away, resulting in a defect-free structure without cavities or gas pockets.
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.
Check out our video on the vertical centrifugal casting process here.
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.
Check out our video on the horizontal centrifugal casting process here.
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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.
Directional Solidification
A key dynamic in the centrifugal process is that solidification is managed to occur in one direction. The exterior size and shape of the casting is determined by the die (mold) surface, which acts as a chill to initiate solidification. With the aid of exothermal materials that may be added to the I.D. after pouring, along with the heat sink of the mold at the O.D., a temperature gradient results in directional solidification from O.D. to I.D. Liquid metal is continually available to feed the casting, promoting sound metal until the I.D. if finally solidified.
Exterior machining is typically performed to remove surface roughness and maintain concentricity with the bore. However, in some applications, an as-cast O.D. surface is acceptable. Any shrinkage occurring in a centrifugal is at the I.D., which will always be removed by machining.
When to Choose Centrifugal Casting
Centrifugal casting stands as a unique and versatile casting technique. It has demonstrated its cost-effectiveness in producing complex shapes with reduced machining needs and lowered manufacturing expenses when contrasted with forgings and fabrications (e.g. stamping). Centrifugal castings are used for components such as bushings, rings, engine cylinder liners, large pipes, and many more.
This article will discuss the process of centrifugal casting, shedding light on its advantages, disadvantages, and its wide array of applications.
Centrifugal casting is a manufacturing process used to create cylindrical components with a central axis of rotation. This technique involves rotating a mold around a central axis at speeds typically ranging from 300 to rpm while pouring molten material into the mold. The centrifugal force generated by the rotation spreads the molten material against the inner surface of the mold, resulting in the desired shape of the finished product. This method is often employed for producing large pipes, cylinder liners, and hollow cylindrical components cast from a variety of materials, including: metals, concrete, and glass. While centrifugal casting maintains tight dimensional tolerances on the outer surface, the inner diameter is typically cast slightly undersized, allowing for subsequent machining to achieve the precise inner diameter required.
The initial patent for centrifugal casting was granted to Anthony Ekhardt, an Englishman, in . Within a few years, additional patents were issued in Germany and America. However, progress in perfecting the technique remained slow and inconsistent for more than a century. A significant turning point in the method's evolution took place in the aftermath of World War I, particularly in the casting of pipes. During this period, centrifugal casting of pipes became a viable and competitive manufacturing approach.
Before the widespread adoption of centrifugal casting for pipes, pipes were typically made using labor-intensive methods such as: wrought iron or steel welding, clay shaping and firing, wooden stave construction, and cast iron casting. These methods had limitations in terms of seam strength, corrosion resistance, and size and shape variability. Centrifugal casting became competitive with these traditional methods in the early 20th century. This marked a significant advancement in pipe manufacturing.
Variations and specific applications within the centrifugal casting method have led to the use of other terms such as "spin casting" and "rotocasting." "Spin casting" is often applied to describe specific adaptations of centrifugal casting, particularly those involving rubber molds or processes geared toward specific industries like jewelry making. "Rotocasting" may refer to variations of centrifugal casting used in certain applications, but it is less commonly encountered in general casting terminology. These terms are used to specify certain subtypes or adaptations of the centrifugal casting process.
Centrifugal casting is important for its ability to yield components characterized by high material integrity and soundness. The centrifugal forces that distribute the molten metal to the walls of the mold achieve a uniform material distribution around the circumference of the part. It also reduces porosity, which improves the mechanical properties of the resulting casting.
An essential aspect that sets centrifugal casting apart is its specific focus on producing rotationally symmetric stock materials in standard sizes. Centrifugal casting primarily produces components that are further refined through machining. This approach not only minimizes material waste but also streamlines production cycles and contributes to cost efficiency.
Centrifugal casting works by introducing molten metal into a heated rotating mold. The mold's rotation can be either vertically or horizontally aligned, depending on the desired configuration of the final part. Centrifugal (or, more accurately, centripetal) force due to the rapid rotation of the mold distributes the molten metal onto the walls of the mold at pressures that can reach nearly 100 times the force of gravity. The controlled solidification advances from the outer diameter toward the bore. During controlled solidification, the denser components, with greater momentum, move towards the wall, displacing less dense elements and impurities. Additionally, the rapid rotation prevents the less dense components from being flung as far outward, resulting in their concentration towards the inner diameter. After the casting has solidified, the part is extracted from the mold. Any remaining impurities within the inner diameter are then machined away. This results in a nearly flawless structure without cavities, defects, or gas and shrinkage porosity.
Centrifugal casting's uniquely valuable features include the production of fully dense castings with good mechanical properties; the ability to process parts ranging from the small to the very large; the range of materials that can be cast, including not just most metals, but also concrete, ceramics and glass; and the ability to cast one layer of material inside another (for example, concrete-lined cast iron pipe). Unlike most casting methods, which rely on gravity or applied pressure to fill a mold, centrifugal casting spins the mold while liquid metal is poured into it, and the rotational forces fill the mold. This distinctive approach eliminates the need for gating systems and risers. To learn more, see our guide on Types of Casting Processes.
The process of centrifugal casting is illustrated in Figure 1 below:
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