PROPERTIES OF MOULDING SAND
Important properties of moulding sand are :
(ii) Green strength
(iii) Dry strength
(iv) Hot strength
It may be defined as the ability of the moulding material to sustain the high temperatures of the molten metal so that it does not cause fusion.
The moulding sand which contains moisture is called green sand. The strength of the green sand which should be sufficient enough to maintain the shape of the mould is called green strength.
The moulding sand which does not contain any moisture is called dry sand. The strength of the dry sand (by hot metal effect) which should retain the shape of the mould cavities is called dry strength.
After all, the moisture is eliminated and the metal in the mould is in liquid state, the temperature of the sand becomes very high. The strength of the sand which is required to hold the shape of the mould cavity in that condition is called hot strength.
Permeability / Porosity
During solidification, large amount of gases come out from the mould. These are the gases which are absorbed by the metal in the furnace, from atmosphere or gases involved by the moulding and core sand. If these gases are not allowed to escape, these will be trapped inside the casting and cause defects.
For this reasons, the moulding sand should be sufficiently porous so that the gases are allowed to escape out. The gas evolution capability of the moulding sand is called permeability/ porosity.
Besides all the above properties, the moulding sand should have collapsibility so that during contraction of the solidified casting, it does not provide any resistance which may result in cracks in casting. Moulding sand should be reusable and having good thermal conductivity so that heat from casting is quickly transferred to the sand.