METAL-CASTING
PROCESSES AND EQUIPMENT
Fundamental of Metal Casting
Casting is a
solidification process in which molten metal is poured into a mold and allowed to cool to
ambient temperature. Fluid flow and heat transfer are important consideration, as is the
design of the mold and gating system to ensure proper flow of the metal into to the mold
cavities.
Because metal contract during
solidification and cooling cavities can form in the casting. Porosity caused by gases
evolving during solidification is a signification problem, particularly because of its
adverse effect on the mechanical properties of castings. The grain structure of castings
can be controlled by various means to obtain the desired properties. The choice of
furnaces is important and involves economic consideration. Various defects can develop in
castings from lack of control of material and process variable.
The casting process basically
involves pouring molten metal into a mold patterned after the part to be manufactured,
allowing it to cool, and removing the metal from the mold. As with all other manufacturing
processes, certain fundamental relationships are essential to the production of good
quality and economical castings. Knowledge of these relationships helps us establish
proper technique for mold design and casting practice. The objective is to produce casting
that is free from detects and that meets such requirements as strength, dimensional
accuracy and surface finish.
Metal-Casting Processes
The most common processes in the
first group are sand, shell-mold, plaster, ceramic-mold and investment casting. Compared
to permanent-mold casting, casting made by these processes usually involve relatively low
mold and equipment costs. However, they generally tend to produce castings having high
porosity and low dimensional accuracy at a low production rate.
The molds used in permanent-mold
casting are made of metal or graphite and are used repeatedly to produce much part.
Because metal are good heat conductors but not allow gases to escape, permanent molds
perform in fundamentally different ways from sand and other aggregate mold materials.
Processes that used permanent molds
are pressure, slush, die, centrifugal, and squeeze casting. Die and equipment costs are
relatively high, but the processes are economical for large production run. Scarp loss is
low and dimensional accuracy is relatively high, with good surface details.
Melting practices are also important
in casting operations. These include proper melting of the metals, preparation for
alloying, removal of slag and dross, and pouring the molten metal into the molds.
CASTING DESIGN, MATERIALS, AND ECONOMICS
General principles have been
established to aid designers in producing castings that are free from detects and meet
tolerances and service requirements. This principles concern shape of casting and various
techniques to minimize hot spot that could lead shrinkage cavities. Because of the large
number of variables involved, close control of all parameters is essential, particularly
those related to the nature of liquid metal flow into the molds and dies and the rate of
cooling in different region of the mold or die.
Several nonferrous and ferrous
casting alloys are available. They have a wide range of properties, casting
characteristics, and applications. Because many casting are designed and produced to be
assembled with other mechanical components and structures, various other considerations,
such as weldability, machinability, and surface conditions, are also important.
Within the limits of good
performance, the economic aspects of casting are just as important as technical
considerations. Factors affecting the overall cost are the cost of the materials, molds,
dies, equipment, and labor, each of which varies with the particular casting process. An
important parameters is the cost per casting, which, for large production runs, can
justify large expenditures for automates machinery.
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