Centrifugal Casting | Diagram | Types | Application | Steps | True Centrifugal Casting | Semi-Centrifugal Casting | Centrifuging Casting | Mechgrace |

Centrifugal Casting

What is the centrifugal casting process?

Centrifugal Casting

The centrifugal casting process is a special casting process to produce reliable dense casting products by rotating the mould around its own axis until the metal solidifies.

This is the only casting process where mould is rotated as compared to another casting process where mould is static.

Due to the rotation of the mould, the centrifugal force applied to the mould is 100 times larger than the gravitational force.

Dense, longer, high yield, neat net size, corrosion resistance, superior quality, tough, high strength and good surface finished casting can be produced in large numbers using single moulds.

Casting products applications such as tubes, pipes and cylinders are difficult to manufacture at this accuracy and dimensional tolerances with die casting.

Centrifugal casting can be done in a vacuum creating porosity-free casting in the absence of air for the application of castings in space technology, aerospace equipment and military.

The less-dense impurities, oxides, foreign material, inclusions and slag gravitate towards the centre which is removed contently during the secondary manufacturing process.

Casting with lengths up to 1500 cm and diameters up to 600 cm can be easily produced. Wall thickness from 5 mm to 10 mm is possible.

Process parameters that control the quality of centrifugal castings are rotational speed, the viscosity of the liquid metal, pouring temperature, pouring rate, pre-heating mould temperature, mould coating, solidification rate, shrinkage and cooling rate of metal dies.

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What are the horizontal and vertical centrifugal casting processes?

If the process is done on horizontal moulds it is called horizontal centrifugal casting process and if the process is done on vertical mould the process is called vertical centrifugal casting process.

This casting can be done horizontally and vertically. For horizontal casting components length of the casting should be larger than the diameter of the casting.

This is a more cost-effective method than the forging process to produce tabular components.

When casting is done vertically height of the final casting should be less than twice the width and diameter of the casting.

What is centrifugal casting also called?

Centrifugal casting is also called roto-casting or centrifugal casting technique (CCT).

What are metals used in the centrifugal casting process to produce centrifugal castings?

The material used in this process to make casting is beryllium copper, silver, stainless steel, copper alloy, magnesium, cast iron, tin, lead, Inconel,  manganese bronze, phosphor bronze, silicon bronze, aluminium bronze, brass, iron, heat resistance steel, mild steel, copper, nickel alloy, aluminium, gunmetal, high steel alloy, bearing alloy, titanium, duplex material, carbon alloy, grey iron, chil iron, super alloy and low alloy steel.

What are parts of centrifugal casting machines?

Parts of centrifugal casting machines are as follows:

• Ladle: Ladle is used to pour the molten metal into the mould. For semi-centrifugal and centrifugal casting machines ladle is used to pour molten metal directly into the mould via the pouring basin. However, for the true-centrifugal casting process, a ladle is used to pour molten metal into the pouring tub first.
• Pouring basin: Basin where molten metal is poured from the ladle so that molten metal flows smoothly through the sprue without any turbulence.
• Sprue: Gating components which allow molten metal to pass through the pouring basin to the runner. All types of centrifugal casting have sprue but centrifugal casting sprue is special as it connects the sprue to multiple mould cavities to mass-produce casting.
• Runner: The runner is the horizontal gating elements that connects the sprue to the in-gate.
• Gate: Centrifugal moulds have an in-gate that controls the molten metal that enters the mould cavity. The true-centrifugal moulding machines and semi-centrifugal moulding machines do not have gates molten metal enters the mould cavity directly.
• Motors, Tramsission shafts And Rollers: Motors are used to rotate the transmission shaft that rotates the rollers allowing the mould to rotate around its own axis. These motors, rollers and gearbox control the rpm of the mould. The mould rotates up to 300 rpm during the solidification process depending upon the size, shape thickness, temperature, types of metal used and final casting application.
• Vacuum Pump: This pump creates a vacuum in the mould for the vacuum centrifugal casting process.
• Vacuum Chamber: Chamber where centrifugal casting can be manufactured in the vacuum for porosity-free casting. 
• Vacuum Leakage Detector: If the vacuum pump fails or there is any leakage in the mould the vacuum pump detector activates and indicates the failure of vacuum in the mould.
• Robotic Arms: To take casting out the split die moulds are opened and automatic robotic arms are used to take out casting from the centrifugal casting machine safely.
• Oxygen Indicator Sensor: This sensor sends a signal to the control panel indicating any oxygen in the mould in case of a vacuum centrifugal casting process.
• Thermocouple and pyrometer: These sensors are used to measure temperature in the mould, furnace and pouring tank respectively.
• Speed Sensor And Controller: When the mould rotates at a higher speed this sensor is used to indicate the rpm of the rotating mould on the control panel indicating the correct rpm of the mould to control the speed limits.

Centrifugal Casting Types

What are the types of centrifugal casting processes?

There are three types of centrifugal casting process

1. The true centrifugal casting process
2. Semi-centrifugal casting process
3. Centrifuging casting process

True Centrifugal Casting

What is the true centrifugal casting process? Explain its working, method, function and principle with a diagram.

(a) True Centrifugal Casting Diagram

The centrifugal casting process is a method of producing casting by pouring molten metal into a horizontal or vertical metal mould which rotates around its own axis pushing molten metal from the centre of the mould towards the circumference by centrifugal force until casting solidifies as shown above in diagarm (a).

Horizontal moulds are preferred for pipes and long cylinders while vertical moulds are best for producing metal rings and bearings.

The advantage of this casting process is that variation in the thickness of hollow sections is uniform along the length of the casting as equal centrifugal force is exerted on the molten metal in the mould cavity.

Casting of any diameter, wall thickness and length can be manufactured using this process in the foundry.

I have explained step-by-step the true centrifugal casting process with an illustrative diagram below in detail.

STEP 1: Heating Molten Metal In Furnace And Pre-Heating Centrifugal Mould

Moulds are pre-heated and start rotating around their axis even before the molten metal is poured into the mould.

Metals used for producing casting are nickel, stainless steel, copper, iron, steel, cast iron, aluminium and magnesium.

Centrifugal moulds are pre-heated to avoid soldering and sticking of metal to the mould surface.

STEP 2: Pouring Molten Metal Into The Centrifugal Mould

(b) True Centrifugal Casting Pouring Operation

The ladle is used to pour molten metal into the pouring basin as shown above in diagram (b). The pouring basin used here is movable and is removed after the pouring operation takes place.

Before pouring the molten metal care is taken that the mould is coated with refractory materials to improve the surface finish and discourage liquid metal from sticking to the die surface and producing surface defects.

As shown in diagram (b) below metal mould rotates around with the support of bottom and top rollers while the molten metal is poured into the mould through the mould opening at one end of the mould.

The rollers and bearings properly balance and absorb all the rotating forces exerted on them while the mould rotates around its own axis.

The volume of molten metal poured, rotating speed and time required for the solidification process define the thickness of the hollow casting sections. The mould is rotated up to 300 rpm during this whole process.

The pouring rate should be slow to promote directional solidification of the final casting considering vibration in the centrifugal machines, pre-heated mould and initial solidification of liquid metal skin at the mould wall to produce uni-equi distanced finer grain structure restricting the columbular grain castings.

STEP 3: Solidifying Molten Metal Into The Centrifugal Mould

(c) True Centrifugal Casting Solidification Process

This mould keeps on rotating continuously until the metal is solidified around the mould surface as shown in diagram (c) above, giving dense hollow castings such as pipes and tubes.

Solidification of the casting takes place from the outer layer from the mould side towards the centre of the mould or casting.

To produce casting with different lengths, sections and dimensions the entire mould needs to be replaced from the centrifugal casting machines with the new mould with the required specification for that desired casting.

Limitations of the true centrifugal casting process are that only axisymmetrical casting can be manufactured with concentric holes.

Double-layer centrifugal castings can be manufactured if the pouring and solidification operation is done right.

The first metal is poured into the mould and allowed to solidify then the next metal is poured and allowed to solidify over the previous metal creating a double layer centrifugal casting.

Care must be taken that the metallurgical bonding of two metals should be done properly.

STEP 4: Casting Ejection

At this stage, casting is ejected from the mould with ejector pins and casting is sent for secondary manufacturing processes such as machining, drilling, milling, grinding, painting and anti-rust surface coating.

Semicentrifugal Casting Process

(d) Semi-Centrifugal Casting Process Diagram

What is the semi-centrifugal casting process? What are its principles, working, machine and method? Explain with an illustrative diagram.

Shown above in diagram (d) illustrative image of the semi-centrifugal casting process.

The centrifugal casting process produces complex casting that cannot be cast through the true centrifugal casting process and centrifuging casting method.

The product produced from this casting process has less internal stress in the final casting as directional solidification of the molten metal in the mould cavity is uniformly distributed without any disturbance.

STEP 1: Core Making For Semi-centrifugal Casting Process

Sand and metal cores are manufactured first and placed in the metal semi-centrifugal metal mould perfectly before the foundry operator starts rotating the mould. 

These sand cores are broken once the casting is solidified and the desired casting cavity is obtained.

Cores can be moulded according to requirements based on shape, size, and complexity in a corebox using dry sand.

Green sand cores are not preferred in this process as they are not strong enough to sustain the centrifugal force in this casting process. Dry sand cores result in better cavity sections without porosity defects.

STEP 2: Melting Ingots And Pre-Heating Semi-Centrifugal Mould

Production of this casting starts with melting metal in an induction electric furnace and pouring molten metal into a rotating centrifugal mould.

The mould used in this process rotates around its own axis with cores placed where engineers want cavities to form.

STEP 3: Pouring Molten Metal Into Rotating Mould

(e) Semi-Centrifugal Pouring Operation

After the mold is pre-heated the refractory material is used to coat the entire mould including gating elements to reduce soldering of the molten metal to the die surface to eliminate any surface defects in the final casting.

At this point once molten metal is heated to pouring temperature and the mould is pre-heated molten metal is poured into the mould using a ladle as shown in diagram (e) below.

This semi-centrifugal mould is different from true centrifugal mould in terms of the complexity of casting to be produced.

Pouring of molten metal need not be done at once but can be done in stages reducing the turbulence in the mould.

Molten metal is poured in stages and casting with different thicknesses can be achieved based on the volume of liquid metal poured in the mould cavity.

Feeding of the molten metal is kept continuous until the final required volume is filled in the mould.

STEP 4: Solidification of Casting

(f) Semi-Centrifugal Solidification Process

At this stage, molten metal is allowed to solidify as shown above in diagram (f).

In semi-centrifugal casting, the mould rotates, rotation of the mould is less than the centrifuging casting process.

Mould spinning gives a very dense structure to the molten metal and promotes directional solidification.

Solidification of metal starts from the molten metal that is in contact with the mould towards the mould's core uniformly while the mould spins.

The surface finish and texture obtained from this process are better than the sand moulding method as metal moulds are used along with molten metal solidified uniformly at centrifugal force.

Chilling and padding are provided in the casting process to achieve directional solidification of the large castings.

Mechanical properties obtained from this process are high. At times this process is of lower cost for manufacturing casting components than forging.

STEP 5: Casting Ejection

At this stage, semi-centrifugal castings are ejected from the mould with ejector pins and the casting is machined and kept ready to dispatch it for customers.

Centrifuging Casting Process

(g) Centrifuging Casting

What is centrifuging casting? Explain its working, method and machine with a diagram.

Centrifuging casting is also known as centrifuge casting and is shown above in diagram (g).

The centrifugal or centrifuge casting process is carried out for products having non-axi-symmetrical small shapes and size casting which cannot be cast using the true centrifugal casting method.

The process is similar to the other two processes with little difference in mould rotation, mould structure, type of complexity and shape of the final casting.

Pre-heating the die, applying coating refractory material and the process of solidification and taking out the solidified casting are also the same.

The difference between centrifugal and semi-centrifugal casting is the rotational speed of the mould.

Mould is rotated at a higher speed and molten metal is ready to pour as shown in the diagram (h) below.

(h) Centrifuging Casting Pouring Operation

Molten metal is then allowed to flow in all sections of the mould cavity using centrifugal force as shown in the diagram below (i).

(i) Centrifuging Casting Solidification Operation

This molten metal is poured into the mould through a common sprue connected to all sections of the mould cavity through runners and every mould cavity as an individual gate.

This arrangement looks like a spoke of a wheel, where the casting mould has a centre which is connected radially to all sections of the mould cavity through runners.

This mould cavity is placed uniformly all around the circumference of the mould and all castings are solidified uniformly while the mould rotates at a higher speed.

Mass production of dense, complex, non-axi-symmetrical castings is possible in the centrifugal casting process.

The limitation of this casting process is the length of the casting, casting with a larger length cannot be produced as we can manufacture using the true-centrifugal casting method.

Dry sand and metal cores are used in this process to produce the desired hollow cavities by placing the cores in the centrifugal mould cavities.

Casting diameters from 100mm to 250mm and heights from 120mm to 400mm are possible using this process.

Centrifugal Casting Advantages And Disadvantages

Centrifugal Casting Process Advantage

What are the advantages, merits and cons of the centrifugal casting process?

• Dense casting can be produced than die casting, sand, investment casting and slush casting.
• Centrifugal moulds are permanent metal moulds which means they can be reused for at least 1,00,000 production cycles.
• This casting process does not have gating elements such as a runner, riser or gates reducing the scrap rate and increasing the casting yield. 
• All inclusions, foreign material, slag and oxides are pushed towards the centre of the casting and are removed easily through machining.
• Hollow casting sections such as pipes can be easily manufactured without using a core of any kind.
• Quality, density, surface finish, casting cavity, production rate and casting length are better than the sand casting process.
• True centrifugal casting is best for producing longer hollow tube axis-symmetrical castings.
• The thickness of the casting and directional solidification of the casting can be adjusted properly.
• Suitable only for large-scale production casting volumes as the cost of making the mould, maintenance, setup and control panel is high.
• Lengthier hollow casting can be produced than the slush casting process without the use of cores.
• The semi-centrifugal casting process can produce casting with complex shapes that are difficult to manufacture using a true centrifugal casting process.
• Posoity in this casting process is least as metal is pushed towards the circumference making final casting products dense and porosity-free.
• Because of the centrifugal force and uniform cooling rate of the mould, equi-axial grain distribution and finer grain structure of the grains in the casting are better.
• Near net shape, high-quality castings can produced without air pockets and holes in the the vacuum chamber.
• The process can be carried out on the x-axis and y-axis depending on the shape, size, complexity and foundry space available without casting having any parting line.

Centrifugal Casting Disadvantages

What are the cons, demerits and disadvantages of the centrifugal casting process?

• Casting production is expensive because of the use of metal moulds, rotating machine setups and motors.
• Maintenance of centrifugal casting machines is high compared to sand casting and permanent mould casting machines.
• Possibility of shrinkage in the centre of true centrifugal casting during the solidification process if the solidification process is uniform.
• Initial investment in centrifugal casting machines is high.
• Weak sand cores in semi-centrifugal and centrifuging castings can break producing defective casting products.
• The machining cost of inner diameter is high as it becomes important to remove slag, inclusions and impurities from the casting.
• Limitations over the size of castings with sharp cuts, edges, corners and non-circular profiles
• Certain alloys and high-temperature metals damage the metal mould reducing the life of metal dies.
• The lead time of this process is high.
• The inner surface or diameter of the casting is inaccurate, rough and needs extra machining and operator time.
• Thin castings can be manufactured but not a lesser wall thickness as compared to the slush casting process. 

Centrifugal Casting Application

What are the uses, examples, products and applications of the centrifugal casting process?

• Hollow tube sections.
• Railway wheels.
• Hollow small bushes.
• Hydrowear, impeller and case wear rings.
• Steel sleeve valves.
• Machine wheels and pully wheels.
• Jet engine case components.
• Precise bimetallic internally clad pipes.
• Hollow pipes and nozzles.
• Missile tank barrels.
• Pistons, discs, thin cylinders and liners.
• Dental implants.
• Industrial boilers and pressure vessels.
• Flywheels, brake drums. and flanges.
• Telescope tubes and worm gear.
• Axial components and sprockets.
• Wind turbine pipes, water and sewage pipes.
• Pump housing, elbow and fittings.
• Parts of the valve housing, gate valves, valve cages and ball valves.
• Furnace tubes, seats, switch gears and flowmeter components.
• Half bearing and sliding bearing.
• Beautiful ornaments and jewellery such as gold and silver rings using silversmithing.
• Parts of the transmission system, propellers, compressors, heat exchangers and steam turbines.
• Components used in military products, aerospace, drilling, mining, medical and chemical industries.

Centrifugal Casting Conclusion

This process is best for producing small-size castings such as busing and sleeves in large volumes using rotating dies giving complex shapes high-density homogeneous casting with better mechanical properties and metallurgical colour grain structure than sand casting.

Flexibility in producing any external shape such as circular, hexagonal, pentagon, square, or octagon keeping the inside diameter of the casting circular or round.

Application of these products are in industries like marine, aerospace, missiles and military where the product is subjected to high forces, temperature and external pressure.

Centrifugal castings can produce large barrels with highly dense strong materials with higher compressive, tensile, shear and bending stress acting on them during operations.

Casting design elements that are possible are draft angle (1°-2°), length of 1500mm and thickness from 2mm to 120mm for metals up to 2000℃.

All three types of centrifugal casting such as true centrifugal, semi-centrifugal and centrifugal casting processes can produce casting applications with different shapes and sizes based on the customer's demand without any macroscopic and microscopic defects as metal solidifies when castings are dense.

These castings are produced without blowholes, air pockets and porosity due to high centrifugal forces acting on the casting. 

The advantage of this casting process is that impurities, oxides, slag and unwanted substances accumulate in the inner diameter of the casting and can be removed through CNC machining.

Low-cost casting stack moulding can be used to produce casting with multiple layers from this process. These products are stronger than products manufactured from the forging process and have longer life as they are dense. 

This same concept of rotating or spinning metal in the mould and solidifying it while the mould rotates can also be applied to materials such as plaster, cement, resin, and metal material.

In this process, the G-factor goes between 10 to 15 which is not observed in any other casting process. This is the only casting process with the spinny mould with gives exceptional high-quality casting.