In the previous article, we have discussed what are the different manufacturing processes. The manufacturing process turns the raw material into finished products. There are different manufacturing process which uses different technologies and machinery to get the final products from the raw material. In this article, we discussed the different casting processes.
Casting is oldest manufacturing process in which liquid molten metal is poured into the casting cavity. Allow the liquid metal to solidify to defined shape, after solidification the defined casting metal is taken out by breaking the mould.
The sequence of steps involved in casting are
- Pattern making
- Mould and core making
- Pouring and solidification
1. Pattern Making
- The pattern is the replica of casting to be made.
- The size of the pattern is slightly greater than the casting by an amount called allowances
- Pattern size = Casting ± Allowances
- Shrinkage Allowance
- Machining Allowance
- Draft Allowance
- Shake Allowance
- Distortion Allowance
Different types of patterns are used depends upon the design casting, complexity of shape, number of castings required, moulding process, surface finish and accuracy. They are as follows
- Solid or single piece pattern
- Split pattern
- Match plate pattern
- Gated pattern
- Sweep pattern
- Skelton pattern
- Loose piece pattern
- Follow board pattern
- Shell pattern
Following are the different materials used for pattern making.
2. Mould and core making
a) Mould Making
Mould making is the process of manufacturing by shaping liquid raw material using a rigid frame called a mould. Moulding sand will be used for manufacturing of the mould. Consists of 3 basic elements
- Silica sand particles
- Water or sodium silicate
This moulding sand basically follow the below properties
- Porosity property
- Cohesiveness property
Many methods are present for mould making. As follows
a) Hand Moulding
- Human hands are used for the force required for ramming or compressing the moulding sand is called hand moulding
- It is a cheaper methodology.
b) Machine moulding
Ramming or compressive force is produced by the machine is called as machine moulding. Processes are below:
- Jolting and Squeezing
- Sand slinging
b) Core Making
This is used for making cavities and hollow projections which cannot normally produce by the pattern alone. Cores are normally made by CO2 moulding. Properties that core material should consist are
- To avoid the bond formation between core and casting metal it should be non-metal.
- It should have high collapsibility.
- It should be moisture free.
- It should have a high strength to withstand the self-weight and buoyancy force.
3. Pouring and solidification
a) Pouring Process
In this casting process to avoid the oxidation, the molten metal is always poured into the casting cavity using a system called as gating system. It consists of mainly 4 basic elements
- Pouring basin
Characteristics of idle gating system:
- The gating system must be designed such that time taken for pouring or filling of molten metal into the cavity should be as minimum as possible.
- This is to ensure that no part of casting cavity should start to solidify before complete filling of the cavity.
- The velocity of molten metal in gating system should be as high as possible within the limit of laminar flow.
- It should separate the impurities present in the molten metal.
Classification of the gating system
1. Based on pressure above the molten metal in the pouring basin.
- Non-pressurized gating system
- Pressurized gating system
2. Based on the position of in-gate
- Top gating system
- Bottom gating system
- Parting gating system
- Step gating system
Gating ratio = sprue area: runner area: in-gate area
b) Solidification Process
The heat transfer from molten metal to surroundings through the mould is considered as unsteady state heat conduction with infinite wall thickness problem.
- The method is used for breaking the mould and taking out casting from the mould is called as fettling operation.
- When the vibrations are applied to the mesh, the mould is getting broken slowly.
- The broken piece of the mould is falling into the pit leaving the mould boxes.
a) Dimensional Inspection
In the casting process, first casting will be inspected dimensionally and the pattern is qualified after that proceeds for the random inspection.
b) Defective Inspection
following are the different methods of inspection:
- Visual inspection
- Pressure test
- Magnetic particle inspection
- Dye penetrant inspection
- Radiographic Examination
- Ultrasonic Inspection
Special casting processes:
- Shell moulding
- Investment casting process
- Die casting process
- Centrifugal casting process