What is Centrifugal Casting Process?
Centrifugal Casting Process (also known as spin casting or rotocasting) is a casting process which uses centrifugal force to form cylindrical components.
There are 3 types of centrifugal casting process used in manufacturing industries.
- True Centrifugal Casting
- Semi Centrifugal Casting
- Centrifuge Casting
In any of these centrifugal casting processes, the force generated by centrifugal acceleration is used to equally distribute the molten material in the mold.
Let me tell you a few important things about each casting process along with steps.
#1) True Centrifugal Casting
Like all other centrifugal casting processes, the true centrifugal casting process also works on the principle of centrifugal force.
The molds used in the true centrifugal casting process are round and are made up of iron, steel or graphite.
The inner surface of the molds are coated with some refractory lining or sand.
Steps of true centrifugal casting process
The steps of true centrifugal casting process are mentioned below.
Step #1: Arrangement of mechanical rollers
In order to manufacture a casting by using a true centrifugal casting process, the mechanical equipment is needed to rotate the mold at high speed.
In industries, the rotation of the mold is achieved by using mechanical rollers.
The mold is rotated at a desired speed about its axis.
For manufacturing the smaller molds, they are generally rotated about the vertical axis.
However, in a true centrifugal casting process, the longer castings are made and they are rotated about the horizontal axis.
The gravity also affects the material during the metal casting process. Hence for longer castings, the force generated from horizontal rotation should be used for manufacturing the casting.
Step #2: Pouring of molten material
As shown in the diagram, the molten metal is introduced to the mold from the external source.
As the mold is rotating at a particular speed, the molten metal remains in touch with the outer wall of the mold.
The molten is either poured into the spinning mold or it is first poured and then the mold is spinned.
Step #3: Solidification
As the mold is spinning continuously, the solidification of molten metals begins.
You can see the solidified metal other diagram below.
True centrifugal casting process is basically suitable for manufacturing the hollow cylindrical tubes.
Step #4: Removing the casting
After the Solidification of molten is done, it is removed from the mold by opening the mold into two halves.
In the centrifugal casting process, a large amount of centrifugal force is generated which guarantees the good adhesion of material to the mold surface.
For achieving more thickness of the component, more molten material is poured in the mould.
It is not necessary that the outer surface of the final casting is always round. But many shapes like rectangle, square, or different polygons can also be casted by using a true centrifugal casting process.
But due to the nature of centrifugal casting process, the inner surface of the casting always remains rounded.
Step #5: Removing the impurities (if any)
Now the main thing to note down here is that the centrifugal force is greater at the surface away from the center axis.
And because of this, the outer surface of the final castings will have more density.
Generally the impurities present in the molten metal have less density as compared to the molten metal.
Hence, the impurities tend to remain in the inner regions of the casting (which is closer to the axis of rotation).
These impurities can be removed during the casting process, or they can also be removed by machining operations.
Advantages of true centrifugal casting process
The advantages of true Centrifugal Casting process are mentioned below.
- This process is suitable for manufacturing hollow parts.
- The wall thickness of casting can be controlled easily by adding a controlled amount of molten material in the mold.
- In true casting process, there is no need of sprue, riser or other elements of gating system.
- Good surface finish can be achieved by using a true centrifugal casting process.
- Better dimensional accuracy can be achieved as the centrifugal force generated is high.
- By using a true centrifugal casting process, the shrinkage of the metal can be avoided.
- Cylindrical metal castings with longer length can be casted easily using this process.
- The metals having higher melting temperatures can also be casted by applying a sand lining at the inner surface of the mold.
- By using a true centrifugal casting process, higher productivity can be achieved.
Disadvantages of true centrifugal casting process
The disadvantages of true centrifugal casting process are mentioned below.
- The large amount of force is required to rotate the casting at higher speed which will press the molten material towards the mold surface.
- The inner surface of the final casting shows some impurities (because the impurities are less dense than the molten metal, so they remain towards the inner surface of the casting).
#2) Semi Centrifugal Casting
Semi centrifugal casting process is a variation of true centrifugal casting.
The main difference is that, in the semi centrifugal casting process, the mold is completely filled with molten metal.
The castings manufactured by semi centrifugal casting process will have rotational symmetry.
In manufacturing industries, parts like pulleys and wheels are manufactured by using a semi centrifugal casting process.
As shown in the diagram, the semi centrifugal casting employs a permanent mold or expandable sand mold.
The example of wheel manufacturing by using semi centrifugal casting process is shown below.
The molten metal is poured from the center sprue so that it gets distributed equally in all the directions of the mold.
As the mold spins, the molten metal is pushed away from the center axis under the application of centrifugal force.
The shrinkage of the metal is avoided as the metal from the top of the sprue fills the vacancies of material.
The spinning is continued until the solidification of molten metal occurs.
After the solidification, the mold is opened and the casting is removed.
#3) Centrifuge Casting
Centrifuge casting is similar to the above two types of casting which we have discussed.
But the only difference is that the castings manufactured by centrifuge casting process need not have rotational symmetry.
By using centrifuge casting, the metals with desired shape and size can be manufactured which have all the properties of centrifugal casting.
As shown in the above picture, the mold is made around the central sprue.
The molten metal is introduced from the central sprue and the entire system is rotated about the centre axis as shown in the above diagram.
Because of the centrifugal force, the molten metal is forced away from the center sprue.
Correct amount of molten metal is poured which is further collected in the mold cavity due to centrifugal force exerted due to spinning of the system.
The casting is allowed to solidify during the spinning process only.
Finally after the solidification, the spinning is stopped and the casting is removed.
Casting of concrete pipes: Image by Anna Frodesiak, CC0, via Wikimedia Commons.