What is Casting? Advantages, Limitations, Defects, Types

What is casting?

Casting is a manufacturing process in which molten metal is poured into a specially designed cavity. When metal cools down and becomes solid, it takes the shape of cavity. This cooled solid metal is called casting. After this we take out that casting of desired shape and size from the cavity.

We can further do some work (machining) in this casting to make it fit for our use.

Advantages of casting process

1. Any intricate shape either internal or external can be made.
2. It is practically possible to cast any material.
3. Tools required for casting processes are generally inexpensive compared to other manufacturing processes.
4. Cooling of casting is generally uniform from all directions hence it is generally does not have directional properties.
5. Many materials can only be processed by casting processes because of their metallurgical considerations.
6. It is practically possible to make casting of any size, even up to 200 tons.
7. It is generally cheaper compared to other manufacturing processes.

Limitations of casting process

1. Generally Dimensional accuracy of casting products (specially in case of sand casting) are not so good and can not be used for final use. Hence, further processing of casting is required before final use.
2. Sand casting process is labor intensive.
3. With some materials it is not possible to overcome defects arising due to moisture present in sand.
4. It has poor surface finish.

Casting defects

1. Gas defects

• Blowholes and open blows
• Air inclusions
• Pinhole porosity
• Shrinkage cavities
• Shrinkage cavities

2. Moulding material defects

• Cuts and washes
• Metal penetration
• Fusion
• Run out
• Rat tails and buckles
• Swell
• Drop

3. Pouring metal defects

• Misruns and cold shuts
• Slag inclusions

4. Metallurgical defects

• Hot tears
• Hot spots

Special casting processes

1. Shell moulding

2. Precision investment casting

3. Permanent mould casting

4. Die casting

• Hot chamber process
• Cold chamber process

5. Vacuum die casting

6. Low pressure die casting

7. Centrifugal casting

• True centrifugal casting
• Semi centrifugal casting
• Centrifuging

8. Continuous casting

9. Squeeze casting

10. Slush casting

11. Vacuum casting

Out of several casting methods discussed above, die casting and sand casting are most popular. Let us discuss advantages of die casting over sand casting.

• Rate of production is high
• Casting defects are less
• Comparably close tolerances can be obtained
• Thin sections with complex shapes can be easily obtained
• Provides better surface finish
• Requires relatively less floor space
• Due to good surface finish obtained, subsequent machining cost gets reduced

Types of sands used in casting processes

• CO2 sand
• Parting sand
• Loam Sand
• Facing sand
• Baking sand
• Molding sand

Below is the table showing casting defects and suggesting their probable causes and remedies.

Casting Defect	Probable Cause	Suggested Remedy
Warpage	Cooling is non-uniform	Follow methods of casting cooling which allow uniform cooling
Sponging	Impurities in molten metal	Remove impurities from molten metal
Fins, run out and flash	Faulty mold casing	Correct molding cases or use new ones
Swells	Poor strength of sand of mold walls	Proper hardening of the molding sand should be done
Drops	Poor green strength of molding sand	Increase green strength of molding sand
Fusion	Poor refractoriness of molding sand	Improve refractoriness of molding sand
Misrun	Faulty mold design or poor fluidity of molten metal	Improve mold design or adjust pouring temperature of the molten metal
Porosity	High moisture and low permeability of molding sand	Molding sand should have adequate moisture and should be sufficiently porous
Shrinkage	Improper chilling	Chilling should be proper and uniform
Blow holes	Sufficient ventilation is not present in mold	Mold should have sufficient ventilation

Image source

Attribution: By Ab5602 – File:Casting.jpg on en.wikipedia (Original: Own work), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=579552