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Materials

Sand in the foundry industry is the most commonly used, but least understood substance. In a sand casting foundry the sands which are used for making moulds or cores are mainly of following types:

Green sand

Resin coated sand

Molasses

Cold box sand

Hot box sand

CO2 sand

Chemically bonded sands

Testing

The testing of sands mainly can be divided in to three different stages:

Sampling from bulk material

Sample preparation for testing (Applicable for tests which require samples of standard size)

Testing

Each of these steps can be achieved with standard equipments which are available in market.

Sampling of foundry sands

Free flowing un-bonded sands is a kind of material which segregates itself very quickly when moved from place to place or poured from one container to another or on application of vibrations. If one takes a sample for testing from the surface, it might not be a representative sample on account of the natural tendency of segregation. A sample of sand can be taken from bulk of sand to have representative properties if proper sampling methods are followed. Sand in foundry can be found in three different states:

Stationary in heap

Moving on belt conveyor

Prepared sand

Most of the times un-bonded sand which arrives from sand supplier or which is lying in foundry premises is in the form of heap. Sample should be taken at various depths and mixed to have a proper representative sample. This can be easily done with a standard sand sampler. This equipment is dug in the heap to depth enough to cover the gap, the handle is rotated to get the sample stored in side the sampler. The equipment then is taken out from the sand. Handle is removed to get the sample which is further stored in an air tight container to retain properties which are based on volatile matters and water.

Sand moving on belt conveyor does not leave more choice than collecting required amount of sample at any point of time from the belt, still effort can be made to collect the sample by taking complete cross-section of sand moving on the belt. This sample is further kept in air tight container to retain properties.

A sample can be taken from prepared sand sample either during mixing or after mixing as per requirements of testing. Great amount of care is taken by transferring the sample to Air tight container immediately to retain properties for testing.

Green sand testing

Green sand testing comprises more than 25 different tests. The sand technologists in the foundries adapts the tests that are relevant for the casting defects observed due to sand. The basic testing parameters are:

Moisture content

Permeability

Green compression strength

Compactability

Wet tensile strength

Mouldability

Friability

Loss on ignition

Volatiles content

Grain size & distribution

Dust (dead clay) content

Active clay content

Each of these tests can lead you to obtain specific characteristics of sand which can be crucial quality of casting.

Basics of controlling green sand

Though green sand is widely used through out the world, testing and control for the same is not popularly understood. Green sand is made up of, basic sand (shell sand), bentonite (or proprietary binding material equivalent to bentonite), pitch powder (or coal dust) and uninvited dust. Green sand properties cannot be standardized for all foundries and castings as such, yet place to place and job to job the specifications can be set to maintain minimal amount of rejection. Basic set of parameters to test for Green Sand would be:

Fineness number (grain size/AFS Number) of the base sand

Moisture content in the mixture (ranges from 2-7% depending on the casting method)

Permeability (ability of compacted mould to pass air through it)

Total clay content (Dust content)

Active clay content (Presence of active bentonite/clay which can readily bond)

Compressive strength

For parameters 1,2,4,5 standard bulk material sampling methods can be applicable or sampling can be done with help of sand muller, sand sampler and sand splitter to do it in a standardized manner. Other than these parameters the tests which can be carried out.

Advanced Green Sand testing

Advanced green sand testing is a term generally used when parameters additional to the basic parameters are affecting the casting quality and need to be measured. The parameters are as following:

Wet tensile strength

Mouldability

Friability

Splitting strength

Shear strength

High temperature compression strength

See also

Sand rammer

Molding sand

Foundry

Casting

Cope and drag

Hand mould

Pattern (casting)

References

^ Indian Standards IS1918:1966 Physical testing of foundry sands

^ Mold & Core Test Handbook, published by the American Foundry Society, ISBN 0-87433-228-1

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