Sand Grains :
The shape and size of the sand grains has a remarkable effect on the physical properties of the foundry sand. The sand grains may have smooth, conchoidal or rough surfaces. Out of these the first type i.e., smooth, is preferred for moulding for the reason that such a surface renders higher permeability, sinter point and plasticity to the sand mass, but the percentage of binder required is also equally high.
Similarly the sand grains may have different shapes. The commonly formed shapes are rounded, sub-angular, angular and compound. The rounded grains do not bind together two well when rammed and, hence, render the sand mould highly permeable but the strength of the mould is also reduced.
Sub-angular grains give a relatively stronger bond than above but the permeability is reduced. Angular or
sharp grains produce a much stronger bond and a low permeability when rammed. Thus they enable a mould of greater strength. Sand grains which are cemented together such that they do not separate when screened are called compound. They may consist of one, two or a combination of all the above three shapes. They are not much preferred.
Like the shape the size of sand grains also effects the mould structure and its characteristics. Large, regular and uniform grains increase permeability. Smaller grains increase smoothness on mould surfaces.