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Green Sand Metalcasting Foundry News

Determining the Essential Qualities of Green Sand: 6 Tests

Posted by Hill and Griffith Company on Aug 18, 2020 1:44:52 PM

Excerpt from the article on EngineeringNotes.com

The following points highlight the six main tests to determine the essential qualities of sand. The tests are:

  • Grain Fineness Test
  • Permeability Test
  • Sand Mold Strength Test
  • Moisture Content Test
  • Clay Content Test
  • Hardness Test

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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 shakes 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.

This percentage is again multiplied by a multiplying factor given in the example solved below:

The A.F.S. (American Foundry Society) grain fineness number will be,

Grain Fitness Formula

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 centimeters that will pass per minute under a pressure of 10 kg/m2 through a specimen of sand 1 square centimeter of cross-sectional area and one centimeter 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 molding sand.

(c) Dry permeability is the permeability measured in a specimen made of molding 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 baked at some temperature above 105°C.

Green Sand Grading

A permeability test is carried out by using a permeability meter consisting of an aluminum 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.

 

The sand specimen is placed at the base and is sealed with mercury. Lowering the floating tank forces air through the sand specimen. Air is passed through a nozzle to adjust the flow rate. For fine sand, flow rate should be slow.

Conduct the permeability test with a specimen usually of 20.26 cm2 cross-sectional area and 5.08 cm height. Then, place the specimen in the instrument cup. The cup provides a mercury seal. Finally, 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 passes through the specimen at constant pressure. Then permeability number is obtained by dividing 3007.2 by the time in seconds.

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:

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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.

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Table 3.3 shows the ranges for green permeability for moulding mixtures used for different metals.

 

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