The following points highlight the six main tests done to determine the essential qualities of sand. The tests are: 1. Grain Fineness Test 2. Permeability Test 3. Sand Mould Strength Test 4. Moisture Content Test 5. Clay Content Test 6. Hardness Test.
1. Grain Fineness Test:
Granular particles of various sizes and shapes provide variable interstices (space between grains) and hence, are directly responsible for permeability and compactness of the sand. Granular particles have higher- strength but lower permeability, whereas round grains have high permeability and lower strength.
To carry out this test, a sample of dry sand weighing 50 grams, free from clay is placed on the topmost sieve bearing U.S. series equivalent number 6. A set of standard testing sieves having U.S. Bureau of Standard Meshes 6, 12, 20, 30, 40,50, 70,100,140,200 and 270 are mounted on a mechanical shaker. The above sample is shaking for about 15 minutes. After this, weight of the sand retained on each sieve can be obtained.
To obtain the grain-fineness, weight of the sand retained by each sieve is multiplied by 2, which gives the percentage of weight retained by each sieve.
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This percentage is again multiplied by a multiplying factor given in the example solved below:
The A.F.S. (American Foundry Men’s Society) grain fineness number will be,
2. Permeability Test:
Permeability is a condition of porosity and thus is related to the passage of gaseous materials through the sand. It is expressed as the volume of air in cubic centimetres that will pass per minute under a pressure of 10 kg/m2 through a specimen of sand 1 square centimetre of cross-sectional area and one centimetre in height.
There are four conditions of permeability:
(a) Base permeability is the permeability measured in a specimen of packed dry sharp sand.
(b) Green permeability is the permeability measured in a specimen made of moist moulding sand.
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(c) Dry permeability is the permeability measured in a specimen made of moulding sand and dried at about 100 to 110°C.
(d) Baked permeability is the permeability measured in a specimen made of sand with thermo-setting binder and backed at some temperature above 105°C.
Permeability test is carried out by using a permeability meter consisting of an aluminium casting in the form of a water tank and a base. A balanced tank floats inside the water tank. A specimen tube extends down to the specimen and opens into the air space.
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The sand specimen is placed at the base and is sealed with mercury. Lowering of the floating tank makes air to pass through the sand specimen. Air is passed through a nozzle to adjust the flow rate. For fine sand, flow rate should be slow.
Permeability test is conducted with the specimen usually of 20.26 cm2 cross-sectional area and 5.08 cm height, placed in the instrument cup, which provides a mercury seal, and a predetermined amount of air is forced through the specimen under controlled conditions.
The permeability reading is taken by noting the time in which 2000 c.c. of air is passed through the specimen at constant pressure. Then permeability number is obtained by dividing 3007.2 by the time in seconds.
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This permeability number is a relative number. It does not necessarily tell the permeability of a mould made with the same sand, which depends on the compactness of the sand. The unit can be made direct reading, if an electric timer unit and a direct reading dial are provided.
The permeability number P can be found mathematically, by the formula given below:
P = permeability number to be determined.
v = volume of air passing through the specimen in cm3.
h = height of the specimen in cm (5.08 cm).
p = pressure of air in gm/cm2 (10 gm/cm2).
a = cross-sectional area of specimen in cm2. (A standard value of 20.26 cm2 is generally adopted).
t = time for air to pass in minutes.
Table 3.3 shows the ranges for green permeability for moulding mixtures used for different metals.
3. Sand Mould Strength Test:
Permeability and strength, the most important factors, depend on size and shape of sand grains provided, the correct quantity of water is used in mixing the sand and also on the degree of hardness to which the sand in rammed.
If the sand hardness number does not exceed 85, it is observed that the product of hardness number and permeability number remains constant.
To find out the holding power of various bonding materials in green and dry sand moulds, strength tests are performed. Compression tests are most commonly performed, although tensile, shear and transverse tests are also sometimes performed. A typical strength tester is shown in Fig. 3.26.
The strength test is performed on the horizontal hydraulic press. The specimen of cylindrical shape, whose strength is to be found out is placed on the lugs and pressure is applied, slowly by hand wheel until the specimen breaks. The reading of the needle on high pressure and low pressure manometer indicates the compressive strength of the specimen.
These manometers are graduated in four different scales each for compressive, shear, tensile and bending in kg/cm2. Dial gauge is used, when the deformation tests are to be performed. Table 3.4 gives the compressive strength of the sand to be used for different casting materials.
4. Moisture Content Test:
Moisture content may be determined by the loss of weight, after evaporation. A sample of tempered sand weighing 50 gm. is dried at 110 to 115°C, and then cooled to room temperature in desiccator and re- weighed. The difference in weight, before and after heating gives the moisture content in the sample. The moisture content can be expressed in percentage by the formula % Moisture content
Other methods of finding the moisture content are based on the principle of reaction and electrical conductivity. In the former method, weighed amounts of sand sample and calcium carbide are mixed by shaking and the resultant pressure of acetylene gas generated indicates the percentage of moisture.
5. Clay Content Test:
Clay content is determined by finding the loss of weight after washing of sample. A 50 gm sample of previously dried sand is weighed, placed in the mixing device and treated with a standard sodium hydroxide solution consisting of 475 c.c. of water and 25 c.c. of NaOH standard solution under controlled conditions. Such a testing apparatus consists of jar, which is securely covered and sealed, and rotated at 60 rpm by electric motor for about an hour.
After it, the sand adhering to cover or sides is washed into jar. After thorough washing, residue is dried and reweighed. Actually, residue is obtained only after ensuring that it settles into water depth of 125 mm in 5 minutes time. The material, which is not able to settle through 125 mm depth in 5 minutes is clay and should be removed. Loss of weight in the sample before and after it is dried gives the clay content.
6. Hardness Test:
Mould and core hardness can be found out by the hardness-tester, which is based on the same principle as Brinell hardness tester. A steel ball of 50 mm diameter weighing 237 gm is pressed on the mould surface.
The depth of penetration of steel ball will give the hardness of mould surface on the direct reading dial. This hardness test is useful in finding out the mould uniformity.
The following are the moulding hardness numbers for moulding:
