In this article, we will see the casting process type- Centrifugal Casting types, Working principle, Centrifugal Casting advantages, disadvantages and application of Centrifugal Casting
In the centrifugal casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force. It is employed for mass production of circular casting as the castings produced by this process are free from impurities. Due to centrifugal force, the castings produced will be of a high-density type and of good strength. The castings produced promote directional solidification as the colder metal (less temperature molten metal) is thrown to outside of casting and molten metal near the axis or rotation. The cylindrical parts and pipes for handling gases are most adaptable to this process
The Casting Process involves pouring liquid metal into a mold cavity and allowing it to solidify to obtain the final casting. The flow of molten metal into the mold cavity depends on several factors like minimum section thickness of the part, presence of corners, a non-uniform cross-section of the cast, and so on. The casting processes can be broadly classified into expendable mold casting and permanent mold casting processes.
Types of Centrifugal Casting
Centrifugal casting processes are mainly of three types which are as follows.
(1) True centrifugal casting
- Water down method :
- De-lavand method
- Sand spun process
(2) Semi-centrifugal casting and
(3) Centrifuged casting
True centrifugal casting:
In this process mould is rotated rapidly about its central axis as the metal is poured into it. Centrifugal force is utilized to distribute liquid metal over the outer surface of the mould. Centrifugal force tends the poured metal and the freezing metal to fly outward, away from the axis of rotation, and this tendency creates high pressure on the metal or casting while the lighter slag, oxides, and other inclusions being lighter, get pushed towards the center. The axis may be horizontal, vertical, or inclined. Casting cools and solidifies from outside towards the axis of rotation; so it results in good directional solidification.
Types of True Centrifugal casting:
- Water down method: Water town method is a method of which, a heavy cast-iron rotor is used as a chill mould for the manufacturing of larger gun tubes. The mould is coated with refractory wash and normally rotates at 1300 rpm. A weighed amount of liquid metal is poured into a pouring basin provided with a short spout in the mould where the diameter of the spout can be varied and rate of entry of melt can be adjusted so that the melt can evenly distribute into the mould cavity without cracking the solidification.
- De-lavand method : In De-lavand method, a thin-walled metal mould is provided with a water jacket for cooling, is used for making of soil pipes and similar tubular like objects. The rotating machine and the mould are mounted on a plane inclined at a slight angle to the horizontal.
- Sand spun process : In the process of sand spun, the chilling rate of the metal is lower than other
methods because of the sand lining rammed inside the steel mould. The thermal protection barrier provided by the sand layer provided inside the mould as lining reduces heat shock lessens checking of the mould surface and prevents surface laps on castings caused by premature freezing of the metal.
Horizontal Centrifugal Casting
- In this process, the mold is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the center.
- This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are axisymmetric with a concentric hole.
- Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole.
- The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously.
- The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.
Vertical Centrifugal Casting :
- In centrifugal casting, centrifugal force plays a major role in shaping and feeding of the casting.
- In this process mould is rotated rapidly about its central axis as the metal is poured into it.
- Centrifugal force is utilized to distribute liquid metal over the outer surface of the mould. Hollow cylinders and other annular shapes are formed in this way. Centrifugal force tends the poured metal and the freezing metal to fly outward, away from the axis of rotation, and this tendency creates high pressure on the metal or casting while the lighter slag, oxides, and other inclusions being lighter, get pushed towards the centre.
- The axis may be horizontal, vertical, or inclined. Casting cools and solidifies from outside towards the axis of rotation; so it results in good directional solidification. Hence castings are free from shrinkage. It may be produced in metal or sand lined mould, depending largely upon the quantity desired.
- It is similar to true centrifugal casting but only with a difference that a central core is used to form the inner surface. Semi- centrifugal casting setup is shown in Fig. .
- This casting process is generally used for articles that are more complicated than those possible in true centrifugal casting but are axisymmetric in nature. A particular shape of the casting is produced by mold and core and not by centrifugal force.
- The centrifugal force aids proper feeding and helps in producing the castings free from porosity. The article produced by this process is shown in Fig. Symmetrical objects namely wheel having arms like flywheel, gears and back wheels are produced by this process
Centrifuge Casting :
When a group of small molds are arranged in a circle (to balance each other) around the central vertical axis of the flask and the flask is rotated about the vertical axis, the process is called centrifuge casting. It is clear that the molds are not symmetrical about the axis of rotation, that is, the axis of casting and the axis of rotation do not coincide with one another. Here again the centrifugal force is used to obtain higher pressure on the metal and get more dense castings. The molten metal will flow to all the molds under centrifugal force from a central feeding sprue.
Centrifugal Casting Process Parameter :
- The mold rotation speeds (rpm)
- The pouring temperature ( ºC )
- The mold initial preheated temperature prior to pouring
- Centrifugal Force ( N )
- The spinning period of time ( min. )
- The G factor – G-factor (GF) as the ratio of centrifugal force to weight.
Advantages of Centrifugal casting :
1. Relatively very light impurities move inwards towards the center. So they can be removed easily thus helping in producing sound castings.
2. Gates and risers are not needed.
3. This technique is best suited for the mass production of symmetrical objects and Castings yield is very high in some cases it is even equal to 100%.
4. Castings acquire high density, high mechanical strength, and fine-grained structure.
5. Inclusions and impurities are lighter.
6. These castings have a directional solidification starting from outside to inside.
Disadvantages of Centrifugal Casting :
1. Skilled labors are to be employed for this process.
2. An inaccurate diameter of the inner surface of the casting.
3. Only some shapes can be generated by this casting process.
4. Not all alloys can be cast in this way.
5. Centrifugal castings require very high investments.
Applications of Centrifugal Casting :
1. Bush bearings.
2. Clutch plates.
3. Paper making rollers.
4. Piston rings.
5. Cylinder liners.
6. Pipes of water gas sewage.
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