May 22, 2014

An Overview Of Metal Casting Process

Casting is a process in which molten metal flows by gravity or other forces into a mold where it solidifies in the shape of mold cavity. The term casting is also applied to the part made by this process

Metal casting processes divide into two main categories, based on the mold type

Permanent mold casting process
Expandable mold casting process

Permanent mold casting

In permanent mold casting, the mold is reused many times. Uses a metal mold constructed of two sections designed for easy, precise opening and closing. Molds used for casting lower melting point alloys are commonly made of steel or cast iron. Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures. Permanent mold casting has nine basic types 

1. Slush Casting 

Slush casting is a variation of permanent mold casting that is used to produce hollow parts in manufacturing industries .It is one of the most important process use in manufacturing technology In this method neither the strength of the part nor its internal geometry can be controlled accurately. This metal casting process is used primarily to manufacture toys and parts that are ornamental in nature, such as lamp bases and statues.

2. Pressure Casting 

Pressure casting, also known in manufacturing industry as low pressure casting or pressure pouring, is another variation of permanent mold casting. Instead of pouring the molten metal into the casting and allowing gravity to be the force that distributes the liquid material through the mold, pressure casting uses air pressure to force the metal through the gating system and the metal casting's cavity. This process can be used to cast high quality manufactured parts. Often steel castings are cast in graphite molds using this process. For example, in industry, steel railroad car wheels are cast with this method.

3. Vacuum Permanent Mold Casting 

Vacuum permanent mold casting is a permanent mold casting process employed in manufacturing industry that uses the force caused by an applied vacuum pressure to draw molten metal into and through the mold's gating system and casting cavity. This process has a similar name to vacuum mold casting discussed in the expendable mold process section; however these are two completely different manufacturing processes and should not be confused with each other.

4. Die Casting 

Die casting is a permanent mold manufacturing process that was developed in the early 1900's. Die casting manufacture is characteristic in that it uses large amounts of pressure to force molten metal through the mold. Since so much pressure is used to ensure the flow of metal through the mold, metal castings with great surface detail, dimensional accuracy, and extremely thin walls can be produced. Wall thickness within castings can be manufactured as small as .02in (.5mm). The size of industrial metal castings created using this process vary from extremely small to around 50lbs. Typical parts made in industry by die casting include tools, toys, carburetors, machine components, various housings, and motors.

5. Hot Die Casting 
Hot chamber die casting is one of the two main techniques in the manufacturing process of die casting. This section will primarily discuss the specific details of the hot chamber process and contrast the differences between hot chamber die casting and cold chamber die casting, which is the other branch of die casting manufacture.

6. Cold Die Casting 
Cold chamber die casting is the second of the two major branches of the die casting manufacturing process. This section will discuss cold chamber die casting specifically and contrast it with the hot chamber process discussed previously. For a basic view of die casting in general see die casting manufacture.

7. True Centrifugal Casting 
The manufacturing process of centrifugal casting is a metal casting technique, that uses the forces generated by centripetal acceleration to distribute the molten material in the mold. Centrifugal casting has many applications in manufacturing industry today. The process has several very specific advantages. Cast parts manufactured in industry include various pipes and tubes, such as sewage pipes, gas pipes, and water supply lines, also bushings, rings, the liner for engine cylinders, brake drums, and street lamp posts. The molds used in true centrifugal casting manufacture are round, and are typically made of iron, steel, or graphite. Some sort of refractory lining or sand may be used for the inner surface of the mold.

8. Semi centrifugal Casting 
Semi centrifugal casting manufacture is a variation of true centrifugal casting. The main difference is that in semicentrifugal casting the mold is filled completely with molten metal, which is supplied to the casting through a central sprue. Castings manufactured by this process will possess rotational symmetry. Much of the details of the manufacturing process of semicentrifugal casting are the same as those of true centrifugal casting. For a better understanding of this process and centrifugal casting manufacture in general see true centrifugal casting. Parts manufactured in industry using this metal casting process include such things as pulleys, and wheels for tracked vehicles

9. Centrifuge Casting Ingot Casting

Centrifuge casting is the third main branch of centrifugal casting processes used for industrial manufacture of cast parts. For more detailed information on the other two manufacturing processes that fit into the category of centrifugal casting see, true centrifugal casting and semicentrifugal casting. Developing an understanding of these techniques will greatly assist in learning about centrifuge casting, since the main principles that govern centrifuge casting are the same for all centrifugal casting processes. Centrifuge casting is different in that castings manufactured by the centrifuge casting process need not have rotational symmetry. With centrifuge casting, metal castings of desired shapes can be manufactured with all the distinct benefits of castings produced by a centrifugal casting process.

Expandable mold casting process

In Expandable mold casting process mold is sacrificed in order to remove the casting part. There are six expandable mold casting process used worldwide which are discus below

1. Sand Casting

Most widely used casting process, accounting for a significant majority of total tonnage cast. Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel and titanium. Parts ranging in size from small to very large. Production quantities from one to millions

2. Vacuum Molding

The term "vacuum" refers to mold making rather than casting operation itself. Vacuum molding Uses sand mold which is held together by vacuum pressure rather than by a chemical binder. Vacuum molding was developed in Japan around 1970

3. Expanded Polystyrene Process

Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold. Other names: lost-foam process, lost pattern process, evaporative-foam process, and full-mold process. Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed). Mold does not have to be opened into cope and drag sections

4. Investment Casting (Lost Wax Process)

A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal. "Investment" comes from one of the less familiar definitions of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern. It is a precision casting process capable of castings of high accuracy and intricate detail

5. Plaster Mold Casting

Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H2O). In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set. Wood patterns not generally used due to extended contact with water. Plaster mixture readily flows around pattern, capturing its fine details and good surface finish


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