What Is Casting?

Written by: Finn Orfano • Edited by: Lamar Stonecypher
Published Jun 22, 2009 • Related Guides: Mold

What is a casting and the defects encountered in this technique. Every technology produces fine products, but possibly not without some deficiencies. Casting produces unique type of defects related to the science of metallurgy.

Introduction

Casting is an engineering manufacturing process generally used for mass production in which materials in a molten state are poured into a mold where they solidify. In this process, complex parts can be manufactured economically and rapidly that otherwise would involve a lot of time if produced by other methods like shaping or cutting. Casting process can be utilized to produce a large variety of parts that are used in different industries, ranging from a small plastic toy to a large gas turbine blade.

Selection Of Casting Processes

Different casting processes are available for utilization, and their suitability is influenced by a number of factors including the following:

Quantity of castings
Manufacturing cost
Product material
Dimensional accuracy required
Surface finish required

Casting Processes

The major casting processes used in the manufacturing industry are the following:

Sand casting

Sand casting is normally used for the production of large parts

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, by filling a molten metal into the mold cavity that has been shaped from natural or synthetic sand. The cavity is created by the utilization of a pattern, generally made of wood or metal that is of the same shape and dimensions as the actual part. The pattern is prepared slightly oversize due to which the cavity is also a little larger and compensates for the contraction of molten metal during cooling. Surface of the sand castings is normally rough with surface impurities for which a machining allowance is included.

Die Casting

In this process metal is forced into the mold

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at a high pressure that ensures production of identical parts, a better surface finish, and an increased dimensional accuracy. Some parts produced by die casting even do not require machining after casting, or may require only a light machining to achieve the desired dimensions. Defects of porosity are found more often in large castings because of entrapped air and the solidification of melt before it reaches the boundaries of the cavity. Parts with a uniform wall thickness can be more accurately produced by die casting. Die casting molds are expensive since these are made from hardened steel and because a longer time duration is required for their production.

Centrifugal Casting

This is a casting technique that has an extensive range of industrial applications including the casting of machine fittings where durability of the finished product is important. Television picture tubes, spherical glass objects, pipes, flywheels, and boilers are also produced by centrifugal casting. As the molten metal is poured, a perman

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ent mold spins at high speeds around its axis. The molten metal moves towards the mold walls due to centrifugal force, solidifies after cooling, producing a fine part. The external diameter of the casting has fine grains, and is resistant to atmospheric corrosion.

Investment Casting

Investment casting is an ancient manufacturing process used for metals that are difficult to be machined or fabricated. It is also used for the manufacture of parts that cannot be formed by usual manufacturing techniques like turbine b

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lades or components of airplane that are subjected to high temperatures. This process provides an excellent dimensional accuracy and surface finish. The pattern is made of wax or other substance that is melted, leaving behind a cavity which is filled with the material of the part being produced.