Different Types of Centrifugal Casting

cENTRIFUGAL CASTING | CENTRIFUGING PROCESS | CENTRIFUGAL CASTING TYPES

Centrifugal Casting:

Centrifugal casting is done by pouring molten metal into a rotating mould. The centrifugal force acting on the mould helps in feeding and positioning the metal in the mould. Mould rotation is continued till after the metal is solidified.

Centrifugal casting results in denser and cleaner metal as heavier metal is thrown to parts of the mould away from the centre of rotation and the lighter impurities like slag, oxides and inclusion are squeezed out to the centre.

The castings produced have a close grain structure, good detail, high density and superior mechanical properties. Elaborate gating and risering systems are not required as very simple systems will do the job. There is also a considerable saving of material.

Types of centrifugal casting:

Centrifugal casting can be divided into three categories namely

1. True centrifugal casting,
2. Semi centrifugal casting
3. Centrifuging.

1. True centrifugal casting:

The true centrifugal method of casting is used to produce hollow castings with a round hole. The characteristic feature of this process is that the hole is produced by the centrifugal force alone and no cores are used.

Vertical True centrifugal casting 

The mould is rotated about the axis of the hole with the axis held horizontal, inclined or vertical. The outside surface of the job may be round, square, hexagonal etc. and should be symmetrical with the whole axis. The central hole should be round to be formed without cores.

Long castings like cast iron soil pipes are cast with the moulds rotated about a horizontal axis. Castings with relatively short lengths are poured with moulds rotated about an inclined or vertical axis. Rotation about the vertical or inclined axis is convenient but the central hole produced will be slightly parabolic with smaller diameter at the bottom because the metal has a tendency to settle down due to gravity. The speed of rotation for true centrifugal casting should be high enough to hold the metal on to the mould wall till it solidifies.

 A low speed of rotation would result in raining or slipping of the metal inside the mould. Too large a speed of rotation on the other hand may result in internal stresses and possible hot tears. A speed which would provide a centrifugal force of 60 to 75 times the force of gravity on horizontal moulds and 100 times force of gravity for vertical moulds is found to be suitable. The moulds used for the process may be metal moulds or refractory or sand lined moulds. Common products produced by true centrifugal casting include pipes, oil engine cylinders, piston ring stock, gear blank stock, bearing bushes and the like.

2. Semi-centrifugal casting:

In semi-centrifugal casting process no attempt is made to produce a hole without a core. The centrifugal force resulting from rotation of the mould is used to properly feed the casting to produce a close grained clean casting.

The process is suitable for large axis-symmetrical castings like gear blanks, fly wheels and track wheels. Any hole round or otherwise is made with the use of a core. The mould is clamped to a turn table with casting axis along the axis of rotation.

The metal is poured along or near the axis to feed the points farthest from the axis of rotation under pressure. If made solid the central portion tends to be porous and with inclusion which are removed in subsequent machining.

3. Centrifuging:

Centrifuging or centrifuge casting is employed to force metal under pressure into moulds of small castings or castings not symmetrical about any axis of rotation. The moulds are made around a central axis of rotation, to balance each other.

The metal is poured along this axis of rotation through a central sprue and made to flow into mould cavities through radial ingates cut on the mould interface. Centrifuging helps in proper feeding of castings resulting in clean, close grained castings.

CENTRIFUGAL CASTING PROCESS | MANUFACTURING PROCESS | MECHANICAL (2017)

CENTRIFUGAL CASTING PROCESS

In Centrifugal Casting , A Permanent Metal Mold is rotated continuously on its own axis at high speed as the molten metal is poured.

The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling.

The Casting is usually a fine rained casting.
Vertical Centrifugal Casting Machine is ideal for Gearblanks.

Centrifugal casting or rotocasting is a casting technique that is typically used to cast thin-walled cylinders. It is used to cast such materials as metal, glass, and concrete.

Unlike most other casting techniques, centrifugal casting is chiefly used to manufacture stock materials in standard sizes for further machining, rather than shaped parts tailored to a particular end-use.

Typical materials that can be cast with this process are iron, steel, stainless steels, glass, and alloys of aluminum, copper and nickel.

   Steps Involved in Centrifugal Casting

1. Starting with a rotating die, liquid metal is introduced into the die.
2. Because the die is rotating, the g-forces drive the liquid metal against the die walls, which simultaneously begin to freeze the castings from the outside in.
3. Shrinkage voids are filled under high pressure from centrifugal force. Slag, dirt and dross, being of lower densities than the metals, spin to the core or center.
4. The tubes are machined to remove these non-metallic impurities that have spun to the center.

    Types of Centrifugal Casting

Centrifugal casting can be divided into three categories namely 

1. True centrifugal casting,
2. Semi centrifugal casting
3. Centrifuging.

(clear explanation of all above types will be in next blog)

    Features of Centrifugal casting

• Castings can be made in almost any length, thickness and diameter.
• Different wall thicknesses can be produced from the same size mold.
• Eliminates the need for cores.
• Resistant to atmospheric corrosion, a typical situation with pipes.
• Mechanical properties of centrifugal castings are excellent.
• Only cylindrical shapes can be produced with this process.
• Size limits are up to 6 m (20 feet) diameter and 15 m (50 feet) length.
• Wall thickness range from 2.5 mm to 125 mm (0.1 - 5.0 in).
• Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15 in).
• Surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.

Advantages of Centrifugal casting

Centrifugal casting has certain advantages such as 

1. High wear resistance
2. High metal density.
3. No blisters.
4. Centrifugal castings have no non metallic inclusions and slag.
5. This method is cost effective.
6. Both ferrous and non-ferrous metals can be used.
7. The equipment can be used for multiple types of metals without sacrificing quality.
8. Impurities are pulled toward the inside surface and can be easily machined.
9. Castings with good dimensional accuracy and quality are produced.
10. Can produce castings with up to 10 feet in diameter and 50 feet in length.
11. High rate of productivity.

   Disadvantages of Centrifugal Casting

Centrifugal casting has certain disadvantages such as 

1. Cannot produce complex geometric shapes.
2. Segregation and Banding: “zones of segregated low melting point constituents such as eutectic phases and sulphide and oxide inclusions.
3. The mold rotating too slowly or the pouring rate too fast can result in the metal falling down from the top of the rotation onto the bottom.
4. Vibration defects due to improper mounting and faulty equipment.