Q. Describe the complete procedure of testing permeability of moulding sand. How is permeability influenced by various parameters of the moulding sand ?
Ans. Permeability Test : The permeability expressed as the volume of air in cu. Cm that will pass per minute under a pressure of 1 gm/cm2 through a specimen of sand 1 cm2 in cross-sectional area and 1 cm in height. There are following four recognized conditions of permeability.
(i) Basic permeability : It is the permeability measured with a specimen of packed dry sharp sand.
(ii) Green permeability : It is the permeability measured with a specimen made of moist moulding sand.
(iii) Dry permeability : It is the permeability measured with a specimen made of moulding sand and dried at 105ºC to 110ºC.
(iv) Baked permeability : It is the permeability measured with a specimen made of sand with thermosetting binders and baked at some temperature above 110ºC.
The test is carried out as following :
(a) A specimen of size 5.08 cm by 5.08 cm is placed in an instrument.
(b) A selected amount of air is forced through the specimen under controlled conditions.
(c) The rate of flow of air is timed to calculate the permeability number.
(d) The permeability number is calculated by the following formula :
where P= Permeability number to be determined,
v= Volume of air passing through the test specimen.
h= Height of specimen in cm,
p= Pressure of air in gm/cm3
a= Cross sectional area of specimen in cm2 and
t= Time taken by air to pass in minutes.
(e) The standard procedure requires 2,000 cu cm of air to be forced through a specimen of sand 5.08 cm in height and 20.268 cm2 in area. The drum in the permeability meter is designed to produce a constant pressure of 10 gm/cm2. The formula may now be simplified by substituting these values.
P = 2000 x 5.08 P = 50.12
10 x 20.268 x t t
P = 3007.2 (t being in seconds)
Permeability of moulding sand influenced by following parameters :
(i) Size of the grain (Varying over the wide range of 50 microns to 3360 microns)
(ii) Shape of the grain (round, angular, subangular or compound), the round shape is more favorable for porosity.
(iii) Compactness density has also a bearing on the permeability.
(iv) Moisture content in the moulding sand effects permeability since the excess moisture tends to collect in the interstices.
(v) Bonding content also affects the porosity of moulding sand through the interstitial structure.
The above figure shows how permeability is affected by various factors.