There are various casting processes being implemented
now days. The oldest amongst them is sand casting. Spin casting is also
widely used. They can be described as follows:
Sand Casting: Sand
casting involves formation of mold from a mixture of sand and to pour a
casting liquid, most probably, a molten metal into mold. The metal is
then allowed to solidify and the removal of mold, takes place. Sand
molding consists of two types: green sand method and air set method.
The first one consists of mixture of clay, moisture, silica, and many
other additives. The second one consists of mixture of dry sand and
other materials, not moist clay. They are mixed with the help of a
quick curing adhesive. The collective use of these materials is called
'air set'.
At times, there is temporary plug placed to pour the
fluid which is to be molded. Air-set molds usually form molds
consisting of two parts-the bottom and the top. The mixture of sand
gets tamped down after its addition. It does not generate any
by-product. After the solidification and cooling of metal, the mold
gets usually destroyed. This is because its removal involves a lot of
breaking and cracking. The casting accuracy depends a great deal on the
sand and the process of molding used. Castings composed of green sand
result into formation of rough texture on casting surface, and this
characteristic makes them easily recognizable. Air-set molds produce
smoother castings.
Many a times, the casting process results in
losing of components of sand mixture. It is possible to reuse green
sand by the way of adjusting the composition to get the lost additives
and moisture replenished. The entire pattern itself is eligible to be
reused for producing novel sand molds. The method of reuse can be
continued for an indefinite period. In 1950, casting process got
automated partially. They have been in great demand for developing
production lines since then.
Spin Casting: Spin Casting is better
known as Centrifugal Rubber Mold Casting (CRMC). It implies utilization
of centrifugal force for producing castings out of rubber mold. As a
customary practice, a mold having shape of a disc gets spun through its
'central axis' at a pre-decided speed. The material used for casting is
usually thermoset plastic in the liquid form or a molten metal. It gets
poured into the mold through the opening at its centre. Corresponding
to the solidification of metal, or the setting of thermoset plastic,
the spinning of the filled mold takes place.
Normally, organic
rubber or vulcanized silicone is used as a mold-making substrate in
spin casting. Vulcanization takes place in the middle of process of
mold-making. After the successful completion of vulcanization process,
venting and gating must be undergone by the mold. This implies carving
of channels for ensuring proper material flow and air during the course
of casting. A scalpel or knife is used to carry out the above two
processes. The mold complexity is directly proportional to the time
required in implementation of venting and gating.
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