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.