18-07-2013, 02:14 PM
Metal Casting
Metal Casting.pdf (Size: 5.59 MB / Downloads: 200)
Pattern and Mould
Mould cavity
which contains molten metal is essentially a negative of the final product
Mould material
should posses refractory characteristics and withstand the pouring
temperature
When the mold is used for single casting, it made of sand and
known as expendable mold
When the mold is used repeatedly for number of castings and is
made of metal or graphite are called permanent mould
For making holes or hollow cavities inside a casting, cores made of
sand are used.
Melting and Pouring
Several types of furnaces are available for melting
metals
Furnace selection depends on
the type of metal,
the maximum temperature required and
the rate and the mode of molten metal delivery.
Before pouring, provisions are made for the escape
of dissolved gases.
The gating system should be designed to minimize
the turbulent flow and erosion of mould cavity.
The other important factors are the pouring
temperature and the pouring rate.
Solidification & Cooling
The properties of the casting significantly depends
on the solidification time cooing rate.
Shrinkage of casting, during cooling of solidified
metal should not be restrained by the mould
material, otherwise internal stresses may develop
and form cracks in casting.
Proper care should be taken at the design stage of
casting so that shrinkage can occur without casting
defects.
Mechanism of Solidification
Pure metals solidifies at a constant temp. equal to its
freezing point, which same as its melting point.
The change form liquid to solid does not occur all at
once. The process of solidification starts with nucleation,
the formation of stable solid particles within the liquid
metal.
Nuclei of solid phase, generally a few hundred atom in
size, start appearing at a temperature below the freezing
temperature.
A nuclei, more than a certain critical size grows, and
causes solidification.