Types of Sand/Base
Sand:
•
The primary and basic
material used for preparing moulds is sand, due to its high refractoriness
•
Sand usually referred to
as 'base sand‘
•
Nearly 90 - 95 % of the
total moulding sand is occupied by sand and the remaining is binder and
additives.
Basic types of base sand
are
ü Silica Sand
ü Chromite sand
ü Zircon
ü Olivine sand
1)SILICA SAND:
•
Silica sand is
essentially silicon dioxide (Si02) found in nature on the bottoms and banks of
rivers, lakes and seashore
•
Silica deposits tend to
have varying degree of organic and contaminants like limestone, magnesia, soda
and potash that must be removed prior to its use, otherwise which affects
castings in numerous ways
•
Silica sand is available
in plenty, less expensive and possess favourable properties
•
Thermal expansion leads
to certain casting defects; the reason for which not being used in steel
foundries
•
However, silica sand when
mixed with certain additives like wood flour, (corn flour), saw dust etc.,
defects can be eliminated
•
These additives burn by the heat of the molten
metal thereby creating voids that can accommodate the sand expansion
2) Chromite sand: this is African sand with cost being much higher compared to other
sands
• Due to its superior thermal characteristics,
it is generally used in steel foundries for both mould and core making .
3)Zircon or Zirconium silicate: this sand possesses most stable thermal properties of all the above
sands. The choice for this type of sand arises when very high temperatures are
encountered and refractoriness becomes a consideration.
• But the major disadvantage is that, zircon
has trace elements of Uranium and Thorium which is hazardous in nature thereby
restricting its use in foundries.
4)Olivine sand: is typically used in non-ferrous foundries.
With its thermal expansion about half of that of silica sand, makes it suitable
for production steel castings .But the high cost restricts its wide use.
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