Casting metal in sand is an ancient technology.
The first metalsmiths pored melted bronze into holes dug into a sandy place in the earth. Modern foundries still use
holes in the sand as recepticles for molten metal.
Sand casting is a process in which molten metal is poured into a sand mold to produce an object of a desired shape. It is by far the most widely used method of producing castings in quantities. The sand casting process is exceptionally flexible. A variety of metals can be sand cast, and the process can be used for unlimited designs of castings in sizes from ounces to many thousands of pounds.
Pattern costs for sand castings are lower than for any other casting method. Also, the tolerances on sand castings are usually wider than for other methods. These factors combined have considerable effect on making sand castings the least costly and most commonly used process for casting metals.
Advantages of the sand casting process
A casting may be defined as a
" metal object obtained by allowing molten metal to solidify in
a mold ",
the shape of the object being determined by the shape of the mold cavity.
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:
- 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.
- 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.
- Construction may be simplified. Objects may be cast in a single piece
which would otherwise require assembly of several pieces if made by
Metal casting is a process highly adaptable to the requirements of mass
production. Large numbers of a given casting may be produced very
rapidly. For example, in the automotive industry hundreds of thousands of cast
engine blocks and transmission cases are produced each year.
- 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.
Some engineering properties are obtained more favorably in cast metals.
- 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.
- Strength and lightness in certain light metal alloys, which can
be produced only as castings.
- Good bearing qualities are obtained in casting metals.
- A decided economic advantage may exist as a result of any one or a
combination of points mentioned above. The price and sale factor is
a dominant one which continually weighs the advantages and limitations
of the process used in a competitive enterprise.
There are many more advantages to the metal-casting process; of course
it is also true that conditions may exist where the casting process
must give way to other methods of manufacture, when other processes may
be more efficient. For example, machining produces smooth surfaces
and dimensional accuracy not obtainable in any other way; forging aids
in developing the ultimate of fiber strength and toughness in steel;
welding provides a convenient method of joining or fabricating wrought
or cast products into more complex structures; and stamping produces
lightweight sheet metal parts. Thus the engineer may select from a
number of metal processing methods that one or combination, which is
most suited to the needs of his work.
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