Lecture No.
3 Casting
Casting process
3.1 casting process
Process in which molten metal flows by gravity or other force into a mold where it
solidifies in the shape of the mold cavity.
3.2 Advantages of casting:
Casting can be used to create complex part geometries, including both external
and internal shapes.
Some casting processes are capable of producing parts to net shape. No further
manufacturing operations are required to achieve the required geometry and
dimensions of the parts. Other casting processes are near net shape, for which
some additional shape processing is required (usually machining) in order to
achieve accurate dimensions and details.
Casting can be used to produce very large parts. Castings weighing more than
100 tons have been made.
The casting process can be performed on any metal that can be heated to the
liquid state.
Some casting methods are quite suited to mass production.
Disadvantages Of Casting:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for some processes; e.g.,
sand casting
Safety hazards to workers due to hot molten metals and Environmental
problems
Parts made by casting processes range in size from small components weighing
only a few ounces up to very large products weighing tons. The list of parts includes,
jewelry, statues, wood-burning stoves, engine blocks and heads for automotive
vehicles, machine frames, railway wheels, frying pans, pipes, and pump housings. All
varieties of metals can be cast, ferrous and nonferrous. Casting can also be used on
other materials such as polymers and ceramics.
1
Lecture No.3 Casting
3.3 Casting Processes
Discussion of casting logically begins with the mold. The mold contains a
cavity whose geometry determines the shape of the cast part. The actual size and
shape of the cavity must be slightly oversized to allow for shrinkage that occurs in the
metal during solidification and cooling.
Different metals undergo different amounts of shrinkage, so the mold cavity
must be designed for the particular metal to be cast if dimensional accuracy is critical.
Molds are made of a variety of materials, including sand, plaster, ceramic, and metal.
The various casting processes are often classified according to these different types of
molds.
To accomplish a casting operation, the metal is first heated to a temperature
high enough to completely transform it into a liquid state. It is then poured, or
otherwise directed, into the cavity of the mold. In an open mold, Figure2.1(a), the
liquid metal is simply poured until it fills the open cavity. In a closed mold, Figure
2.1(b), a passageway, called the gating system, is provided to permit the molten metal
to flow from outside the mold into the cavity
FIGURE 2.1 Two forms of mold: (a) open mold, simply a container in the shape of
the desired part; and (b) closed mold, in which the mold geometry is more complex
and requires a gating system (passageway) leading into the cavity.
2
Lecture No.3 Casting
Once the casting has cooled sufficiently, it is removed from the mold.
Depending on the casting method and metal used, further processing may be
required. This may include trimming the excess metal from the actual cast part,
cleaning the surface, inspecting the product, and heat treatment to enhance properties.
In addition, machining may require to achieve closer tolerances on certain part
features and to remove the cast surface
Two Categories of Casting Processes
1. Expendable mold processes – use an expendable mold which must be
destroyed to remove casting
Mold materials: sand, plaster, and similar materials, plus
binders
2. Permanent mold processes – use a permanent mold which can be used to
produce many castings
Made of metal (or, less commonly, a ceramic refractory
material) that can withstand the high temperatures of the
casting operation.
More intricate geometries are possible with expendable mold processes
Part shapes in permanent mold processes are limited by the need to open
the mold
3.4 Solidification of metals:
After pouring into the mold, the molten metal cools and solidifies. In this
section we examine the physical mechanism of solidification that occurs during
casting. Issues associated with solidification include the time for a metal to freeze,
shrinkage, directional solidification, and riser design.
Solidification involves the transformation of the molten metal back into the
solid state. The solidification process differs depending on whether the metal is a
pure element or an alloy. Pure Metals solidifies at a constant temperature equal to its
freezing point, which is the same as its melting point. The process occurs over time as
shown in the plot of Figure 10.4, called a cooling curve. The actual freezing takes
time, called the local solidification time in casting, during which the metal’s latent
3
Lecture No.3 Casting
heat of fusion is released into the surrounding mold. The total solidification time is
the time taken between pouring and complete solidification
FIGURE 2.2 Cooling curve for a pure metal
FIGURE 2.3 (a) Phase diagram for a copper– nickel alloy system and (b) associated
cooling curve for a 50%Ni–50%Cu composition during casting.
