Theory

Permeability of sand is a physical property of the physical sand mixture, which allows gases to pass through it easily.

It can be understood as when the molten metal is poured in the mould, it comes in contact with the moist sand, and it generates steam or water vapour, also molten metal contains certain amount of dissolved gases which emerges when the metal starts freezing. So, if gases and water vapour released by the molten metal and sand, do not find opportunity to escape completely through the mould, they will get entrapped and form gas holes or pores in the casting. The sand must therefore be sufficiently porous to allow the gases and water vapour to escape out. This property of sand is referred to as permeability.

The American Foundry Men Society (AFS) defines permeability as “the number obtained by passing 2000cc of air through a standard specimen under a pressure of 10 gm/cm² for a given time in minutes”.

Permeability is one of the most important properties affecting the characteristic of moulds and it depends upon several factors such as the grain size, grain shape, grain distribution, binder content, moisture level and degree of compactness.

Permeability of sand is measured in number, and it is called as permeability number. And the permeability is calculated by the following equation:

$$P_N = \frac{VH}{PAt}$$

Where V = Volume of air passing through the specimen, 2000 cc
H = Height of the specimen = 50.8 mm (standard value)
P = Pressure as read from the manometer in gm/cm²
A = Area of the specimen = (πd²)/4 where d = 50.8 mm (standard value)
t = time in minutes for 2000 cc of air passed through the sand specimen