1. Send Casting: Silica Sand (SiO2) is used more universally for making casting than any other mould material and it is called sand casting.
It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent ( usually clay ) is mixed or occurs naturally with the sand. The mixture is moistened with water to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The resulting and mixture is casily prepared and moulded around various shapes to give satisfactory casting of almost any metal.
· Sand Casting defects
- Open blows and blow holes
- Entrained air and other gases
- Shrinkage defects
- Hot tears (cracks)
- Misruns and cold shuts
- Inclusions
- Gases in metal, pin hole porosity
- Cuts and washes
- Metal penetration
- Mold shift
· Methods for Inspecting Sand Casting Defects
- Process inspection
- Visual inspection
- Dimensional inspection
- Pressure testing
- Radiography
- Magnetic particle testing
- Penetrant testing method
- Ultrasonic testing method
2. Die Casting: This process employs a metal mould into which molten metal metal can be transferred with or without the use of externally applied pressure. In die casting process, pressure is used to inject molten metal into the die cavity. Die casting have excellent surface finish and dimensional accuracy, but volume production is necessary for low cost per casting.
3. Shell moulding: This is modification of sand casting in which relatively thin shell forms the mould cavity into which the molten metal is poured. This moulding involves the use of resinous material of the phenol formaldehyde or urea formaldehyde type as bonding agent for silica sand.
Casting produced by this process have better surface finish and closer dimensional tolerances than sand casting.
4. Investment Casting (low wax process): In this method, wax pattern of the part of mould is embedded (invested) in a fluid ceramic material that subsequently become solid. This mould is heated causing the wax to melt and flow out, leaving a cavity of desired shape. Molten metal is poured in to the mould cavity, and after the metal has solidified the mould material is broken away, leaving the final casting.
Investment casting have excellent surface and dimensional accuracy, and for this reason, they are used to part non-forgeable alloys, extremely complex section can produced by this method, since there are no problem of draft, and parting lines.
5. Centrifugal Casting: Casting that have rotational symmetry, such as long cylinders, are conveniently made by pouring the casting alloy into the metal, graphite, or sand mould rotating above its axis of symmetry. Non-metallic inclusions and slag particles, being less dense than the liquid metal, are forced to the inner surface of the casting and are removed in a latter machining operation.
· Modification Of Centrifugal Casting:
Three Types: 1) True centrifugal casting.
2) Semi centrifugal casting.
3) Centrifuging.
6. Plaster Casting: If the sand casting processed charged so that plaster of paris is substituted for sand as the moulding material, the method is called plastercasting.
7. Slush Casting: Hollow Castings, such as statues, can be made by pouring a low-melting point alloy into a bronze or a plaster mould and quickly pouring out the excess molten metal after a thin solid sheet has formed. The resulting slush casting can be finished by electroplating and lacquering.
8. Continuous Casting: Generally this is used for the production of rods, pipes, sheet metals and other particles known as semifinished products in an uninterrupted process.
In this process the molten metal is poured through a tower nearly 300 m high into a long mold. This process eliminates ingots, the removal of molds from ingots, the re-heating of ingots and their preliminary rolling. Cooled by water and the passage of the steel through the mold is so controlled that the metal emerges from other end is in the shape of the products of a primary rolling mill. Normally copper will be used for mold with sufficient cooling system.
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