Green Sand of Foundry
Immediately on pouring a mould, when the solidification starts, the moulding sand
adjoining the molten metal face get s heated and starts losing moisture which is
converted to steam.
This steam migrates between the sand grains to adjoining cooler r egions of the
mould towards the mould face away from the casting and condenses away from the
solidifying molten metal.
This condensation results in a thin layer of sand that i s saturated with water
The water content at this layer becomes al most 3 times that of the original moisture
content in the moulding sand.
This condensation zone is known as the wet layer.
Beyond this layer towards the mould face is a layer of warm sand with nor mal
moisture percentage and is not influenced by the ca sting process
While the sand in the mould prior to pouring was relatively unifor m after pouring,
the sand layers are of varying strengths
At the mould metal interface the sand surface layer is dry, hot and strong. Behind
this hot zone is the condensation layer that is wet and weak.
Following this condensation zone is a war m sand layer that is suffi ciently strong.
This condensation zone or layer has the lowest strength and this tensile strength is
referred to as Wet Tensile Strength.
The combination of quartz expansion at 3000 C- 575°C in the hot zone resulting in
compressive stresses and the weak wet layer causes a ruptur e between the two
layers, resulting in scabs, rattail, buckle and other expansion related defects.