CASTING

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process.

               Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.

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Casting Terms

1.     Flask or moulding box: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag - lower molding flask, cope - upper molding flask, cheek - intermediate molding flask used in three piece molding.

2.     Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.

3.     Parting line: This is the dividing line between the two molding flasks that makes up the mold.

4.     Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.

5.     Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish         to the castings.

6.    Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.

7.    Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.

8.    Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold.

9.    Runner: The channel through which the molten metal is carried from the sprue to the gate.

10.  Gate: A channel through which the molten metal enters the mold cavity.

11.  Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic  force.

12.   Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as "feed head".

13.   Vent: Small opening in the mold to facilitate escape of air and gases.

 

Advantages of the casting process

Certain advantages are inherent in the metal casting process. These often form the basis for choosing casting over other shaping processes such as machining, forging, welding, stamping, rolling, extruding, etc. Some of the reasons for the success of the casting process are:

1.The most intricate of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.

2. Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housing, valves, and hydroelectric plant parts weighing up to 200 tons illustrate this advantage of the casting process.

3.   Metal casting is a versatile process.  It is equally adaptable to the requirements of all kinds of production systems. It is suitable to mass production, batch production or jobbing type of production.

4.   Construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.

5. Because of their physical properties, some metals can only be cast to shape since they cannot be hot-worked into bars, rods, plates, or other shapes from ingot form as a preliminary to other processing

6.  Some engineering properties are obtained more favorably in cast metals. Good bearing qualities are obtained in casting metals. Strength and lightness in certain light metal alloys can be produced only as castings.

7. Casting produces more uniform properties from a directional standpoint; i.e., cast metals exhibit the same properties regardless of which direction is selected for the test piece relative to the original casting. This is not generally true for wrought metals.

8. Casting can produce close tolerances of the order of 0.1 mm and surface finish of a few tenths of microns.

9. Since metal can be located at exact location in a casting, it results in most economic design as for yield is concerned.

10. Since cheaper materials are used for moulds, patterns etc. and power requirement is minimum, casting offers least cast among other manufacturing processes.

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STEP OF CASTING :

1. Casting design

2. Drawing

3. Pattern making

4. Moulding

5. Pouring-Melting

6. Fettling

7. Inspection

8. Shipping

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CLASSIFICATION OF CASTING PROCESS :

1.  Metallic moulds

      I.  Permanent die casting

      II.Pressure die casting

          A.Hot chamber die casting

          B.Cold chamber die casting

  2.No metallic moulds

      I.Sand casting

      II.Shell moulding

      III.Plaster moulding

      IV.Investment moulding

       V.Centrifugal casting

      VI.Ceramic moulding