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What is Casting Process?
The definition of casting process– Introduction of molten metal into a mould cavity, upon solidification, metal conforms to the shape of the cavity. This is known as the casting and process involved during casting, known as the casting process.
The casting is a complex process to make complex components in predefined shapes that can not be replaced by other processes.
Perhaps, we can not use any material to cast any required casting. The selection of material can be based on various factors.
Factors behind the Selection of material for casting-
- Melting temperature of the moulding and material to be casted
- Chemical behaviour between moulding material and casting material
- Chemical Combination between elements of the environment gases and moulding material with casting material
- Thermal conductivity and coefficient of expansion of material to be cast
The basic components required in the casting process-
- Molten material
Diagram of casting process–
Types of casting process-
Here are the classifications of casting given below-
- Sand casting
- Die Casting
- Investment Casting
- Centrifugal Casting
- Slush Casting
- Continuous Casting
Sand casting process-
The sand casting process is a widely used process in which silica sand is used in the moulding material. Silica sand is mixed with the binding material to make it more efficient. The binding material can be presented naturally inside or maybe need to mix additionally, according to need. Here, clay can be used in terms of binding material.
After mixing silica sand and binding material, water is added to provide moisture content in it and further develop plasticity in it.
Here, Silica sand is widely used due to the following reasons.
The cause behind using the Silica sand is-
- Silica sand is able to withstand very high temperature
- Marginally cheap
- High in collapsibility property
- High in green strength property
Sand casting is the most straightforward way to make metal objects with complex internal cavities, and medium quantities of objects with complex external profiles.
Diagram of sand casting process-
There are multiple steps involved to get the desired shaped casting which is given down.
Steps involve in sand casting process-
- Place half-pattern on horizontal moulding board
- Then drag is placed, drag is a full-size box which fixes accurately on the moulding board
- Now, the half- pattern will be filled with sand all the way up to the top of the drag.
- Ramming it down from the top of the drag to get high compactness and further fill the air gaps, if any.
- Once all is done, the whole compacted half pattern flips upside down.
- Now, it’s time to bring the second-half pattern on the board.
- The second half will be placed above the fliped compacted pattern along with the sprue pin.
- Sprue pin basically creates the entry gate or pathway for molten metal to go inside the mould.
- Now, the parting line is sprinkled with dry sand or general Telcom powder to avoid sticking during pouring of molten metal and further smooth the operation of separation of mould.
- Now, the second half of the pattern will be filled with sand and rammer down to get compact.
- After completing these steps, the drags, cope and sprue pin is removed and now it’s time to pour the molten metal.
- After pouring, the casted mould is kept for cooling and then the mould is separated to get the required casting inside.
Die Casting Process-
As the name suggests, Die casting is made of metal materials known as Die. The high-pressure(7 to 350MPa) molten metal is injected through the plunger via runner to mould or cavity.
These Dies can produce complex shapes with a high degree of accuracy and repeatability.
This Die is separated by two halves–
- Movable Die
- Fixed Die
Steps involve in Die casting process-
- In the die casting process, The reusable die or moulds are preheated and coated with the die-release agent to lubricate and protect the surfaces before each use.
- Next, a Premeasured amount of molten metal is injected into the die with extremely high pressure.
- Now, the newly formed part is then removed from the die and the cycle is repeated after the die cools.
- Here, Die must be stronger to stand out when high-pressure injection happens.
The common Die casting metals are-
- Zinc alloys
- Aluminium Alloys
- Magnesium Alloys
- Brass Alloys
As technology advances and computerized automation in this field increases, It is expected to see even more metal components in the future.
Investment casting process-
Investment casting is one of the oldest manufacturing processes, in which molten metal is poured into an expendable ceramic mould.
Investment casting is also called the lost-wax process or precision investment casting because the wax pattern is melted out of the mould after it has been formed.
The pattern is surrounded or invested into the ceramic slurry that hardens into the mould. Metals that are hard to machine or fabricate are good candidates for this process.
The mould is made by making a pattern using wax or some other material(disposable) that can be melted away. The term ‘investment’ derives from the fact that the pattern is invested with the refractory material.
The main characteristics of investment casting-
- Smooth surface finish
- Dimensional Accuracy
- A high degree of detail attainability
As the name suggests, the Centrifugal casting process works on the basis of centrifugal force where symmetric axis circular cylindrical or hollow types of casting are prepared.
The mould is rotated with the same axis of symmetric while pouring the molten material and due to centrifugal force and light density, the slag particle goes into inner parts and heavy metal parts go into outer parts of the mould.
After solidification, the prepared casting has full of slag inside which can be easily removed by the machining process.
After machining, the final product which is symmetrically circular and cylindrical appears with higher metal composition with a smooth surface finish.
The process is used to make castings of hollow cylindrical shapes. The materials used to prepare moulds can be cast iron, steel, sand or graphite(For non-ferrous castings).
Slush casting is a process in which hollow castings are produced without the use of cores.
The slush casting process is used to make statues that should be hollow inside and without any axis of symmetry. A metal mould is used to prepare the casting which is generally made of Bronze or plaster of Paris(POP).
The slush casting process is not suitable to produce objects for engineering use. Although, it is used to produce objects like statues, toys, lamp bases, and candlesticks.
Steps involve in Slush casting process-
In the slush casting process, the molten metal is poured into a metallic mould and kept for a short period of time.
Solidification happens at mould walls. When a shell of desired thickness is formed, the mould is inverted and the metal which is still in the liquid state is drained off.
When the mould halves are separated, the external wall of casting comes with a great finish but with variable wall thickness inside of the casting.
Continuous casting is a process in which the operation of pouring, solidification and withdrawal of casting from an open mould is carried out continuously. It is also known as Strang Casting.
This casting process is mostly used in casting steel materials.
This casting process is mostly used to cast steel billets and it was invented by Henry Bessemer in 1858.
Process involved in continuous casting-
- Molten metal or steel molten is passed through laddle, then it goes to the tundish, a kind of reservoir which hold the molten metal.
- Now, It goes to the mould where further solidification happens down the mould. After solidification, A finished slab can be obtained at the end.
- Cylindrical slab. Square slabs, Hexagonal slabs can be cast With the help of a continuous casting process.
Advantages of casting process-
Here are some major advantages of the casting process-
The flexibility of size & weight-
Any size big or small can be manufactured with casting which made the casting method so flexible. The size of an object doesn’t matter for casting. Practically possible to make a casting of any size even up to 200 tons of weight.
Formation of complex shape-
The complex shapes can be manufactured with the casting method which can be very difficult for other manufacturing processes.
In this process, molten metal can flow into small sections of the mould cavity easily. This makes it very easy to form a complex casting shape.
Simple & inexpensive tools-
Tooling and equipment costs are low compared to other metal manufacturing processes. Tools required for casting processes are generally inexpensive. This process doesn’t require costly equipment. The process is also very simple in general.
Flexibility in materials choices-
Almost every material in the world can be used for casting. No restrictions on types of materials. Mostly all types of engineering Such as aluminium, magnesium, copper alloys, tin, zinc, lead alloys, iron & steel materials can be cast through this process
Low wastage of material and cost-
As extra materials can be reused for further casting requirements, hence Wastage of materials is very less and further the parts made by the casting process has the lowest possible cost compared to other manufacturing processes.
Wide range of materials-
There are certain materials that cannot be used in any other manufacturing process. But it can be successfully used for the casting process.
Short production lead time-
o consideration in the production, the casting process has a short lead time for complex parts when compared to other manufacturing processes and hence this process is ideal for short production runs especially while manufacturing complex parts like automobile cylinders, machining tools. crankshaft, camshafts, IC engines parts etc.
High production rate-
The production rate of this process depends on various factors, usually, it is from 5-200 parts per hour, this depends on factors like the size and complexity of the part.
Disadvantages of casting process-
Here are some limitations of the casting process–
This process requires a lot of labour work with skilled manpower and here, Sand casting requires higher levels of handcrafting and is more labour intensive than other casting methods. Hence this process comes under labour intensive process.
Poor Dimensional Accuracy-
Because of shrinkage in the casting, the dimensional accuracy might be at risk. proper care must be taken and suitable allowance must be provided. In this process, there needs to do machining & finishing operations to get the required dimension. So, this can become poor dimensional accuracy and this is the biggest limitation of this process.
Poor surface finish-
Most of the time casting suffers from a poor surface finish. However, this was the problem in the older days. Nowadays, Methods are developed such as investment casting, shell moulding casting, vacuum-sealed moulding process has got the excellent surface finish.
Just casting a part is not enough. It requires secondary machining operations in order to improve the surface finish and this needs to follow the sequence of machining operations to make the final part.
Modern casting processes can be automated to some extent but automation can be considered when production is medium to large.
Low in strength-
The parts made by the casting process have low strength when compared to machined parts or forged parts. This is due to the high porosity in parts made by the casting process.
The forged parts are more reliable and stronger than casting due to the better grain structure.
Cooling of casting is generally uniform from all directions, hence it generally doesn’t have directional properties.
The above-described Casting processes are widely used in mechanical industries as per requirement or need. I hope you liked spending time here and gained some value in return,
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Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.