Sand Testing Methods - Sand Testing Equipment

Methods of Sand testing:

The moulding sand after it is prepared should be properly tested to see that require properties are achieved. Tests are conducted on a sample of the standard sand. The moulding sand should be prepared exactly as it is done in the shop on the standard equipment and then carefully enclosed in a container to safeguard its moisture content.

Sand tests indicate the moulding sand performance and help the foundry men in controlling the properties of moulding sands. Sand testing controls the moulding sand properties through the control of its composition.

The following are the various types of sand control tests:

1. Moisture content test

2. Clay content test

3. Grain fitness test

4. Permeability test

5. Strength test

6. Refractoriness test

7. Mould hardness test

Moisture content test:

Moisture is the property of the moulding sand it is defined as the amount of water present in the moulding sand. Low moisture content in the moulding sand does not develop strength properties. High moisture content decreases permeability.

Procedures are:

1. 20 to 50 gms of prepared sand is placed in the pan and is heated by an infrared heater bulb for 2 to 3 minutes.

2. The moisture in the moulding sand is thus evaporated.

3. Moulding sand is taken out of the pan and reweighed.

4. The percentage of moisture can be calculated from the difference in the weights, of the original moist and the consequently dried sand samples.

Percentage of moisture content = (W1-W2)/(W1) %

Where, W1-Weight of the sand before drying,

W2-Weight of the sand after drying.

Clay content test:

Clay influences strength, permeability and other moulding properties. It is responsible for bonding sand particles together.

Procedures are:

1. Small quantity of prepared moulding sand was dried

2. Separate 50 gms of dry moulding sand and transfer wash bottle.

3. Add 475cc of distilled water + 25cc of a 3% NaOH.

4. Agitate this mixture about 10 minutes with the help of sand stirrer.

5. Fill the wash bottle with water up to the marker.

6. After the sand etc., has settled for about 10 minutes, Siphon out the water from the wash bottle.

7. Dry the settled down sand.

8. The clay content can be determined from the difference in weights of the initial and final sand samples.

Percentage of clay content = (W1-W2)/(W1) * 100

Where, W1-Weight of the sand before drying,

W2-Weight of the sand after drying.

Grain fitness test:

The grain size, distribution, grain fitness are determined with the help of the fitness testing of moulding sands. The apparatus consists of a number of standard sieves mounted one above the other, on a power driven shaker.

The shaker vibrates the sieves and the sand placed on the top sieve gets screened and collects on different sieves depending upon the various sizes of grains present in the moulding sand.

The top sieve is coarsest and the bottom-most sieve is the finest of all the sieves. In between sieve are placed in order of fineness from top to bottom.

Procedures are:

1. Sample of dry sand (clay removed sand) placed in the upper sieve

2. Sand is vibrated for definite period

3. The amount of same retained on each sieve is weighted.

4. Percentage distribution of grain is computed.

Permeability test:

The quantity of air that will pass through a standard specimen of the sand at a particular pressure condition is called the permeability of the sand.

Following are the major parts of the permeability test equipment:

1. An inverted bell jar, which floats in a water.

2. Specimen tube, for the purpose of hold the equipment

3. A manometer (measure the air pressure)

Steps involved are:

1. The air (2000cc volume) held in the bell jar is forced to pass through the sand specimen.

2. At this time air entering the specimen equal to the air escaped through the specimen

3. Take the pressure reading in the manometer.

4. Note the time required for 2000cc of air to pass the sand

5. Calculate the permeability number

6. Permeability number (N) = ((V x H) / (A x P x T))

Where,

V-Volume of air (cc)

H-Height of the specimen (mm)

A-Area of the specimen (mm²)

P-Air pressure (gm / cm²)

T-Time taken by the air to pass through the sand (seconds)

Strength test:

Measurements of strength of moulding sands can be carried out on the universal sand strength testing machine. The strength can be measured in compression, shear and tension.

The sands that could be tested are green sand, dry sand or core sand. The compression and shear test involve the standard cylindrical specimen that was used for the permeability test.

a. Green compression strength:

Green compression strength or simply green strength generally refers to the stress required to rupture the sand specimen under compressive loading. The sand specimen is taken out of the specimen tube and is immediately (any delay causes the drying of the sample which increases the strength) put on the strength testing machine and the force required to cause the compression failure is determined. The green strength of sands is generally in the range of 30 to 160 KPa.

b. Green shear strength:

With a sand sample similar to the above test, a different adapter is fitted in the universal machine so that the loading now be made for the shearing of the sand sample. The stress required to shear the specimen along the axis is then represented as the green shear strength. It may vary from 10 to 50 KPa.

c. Dry strength:

This test uses the standard specimens dried between 105 and 1100 C for 2 hours. Since the strength increases with drying, it may be necessary to apply larger stresses than the previous tests. The range of dry compression strengths found in moulding sands is from 140 to 1800 KPa, depending on the sand sample.

Steps involved are:

1. Specimen is held between the grips

2. Apply the hydraulic pressure by rotating the hand wheel

3. Taking the deformation use of the indicators.

Refractoriness test:

The refractoriness is used to measure the ability of the sand to withstand the higher temperature.

Steps involved are:

1. Prepare a cylindrical specimen of sand

2. Heating the specimen at 1500 C for 2 hours

3. Observe the changes in dimension and appearance

4. If the sand is good, it retains specimen share and shows very little expansion. If the sand is poor, specimen will shrink and distort.

Mould hardness test:

Hardness of the mould surface can be tested with the help of an “indentation hardness tester”. It consists of indicator, spring loaded spherical indenter.

The spherical indenter is penetrates into the mould surface at the time of testing. The depth of penetration w.r.t. the flat reference surface of the tester.

Mould hardness number = ((P) / (D – (D²-d²))

Where,

P- Applied Force (N)

D- Diameter of the indenter (mm)

d- Diameter of the indentation (mm)