Specifications and Testing of Moulding Sand:

Moulding sand is indicated in terms of the shape and size of the silica grains it have, the clay content, and the moisture content. These are find out as follows :

Maintaining constant sand quality is of little concern to the casting designer; however it is a important matter to the foundry employee, who is expected to deliver stable, high- quality products. Standard tests and process have been developed to assess clay content, moisture content, grain size and compactability, along with permeability, mould hardness, and strength.

Grain size is calculated by shaking a known amount of clean, dry sand downward from a set of 11 standard sieves of reducing mesh size. After shaking for 15 minutes, the amount remaining on each of sieve is weighed, and the weights are changed into an AFS (American Foundrymen's Society) grain fineness number.

Usually Moisture content is calculated by a special device that measures the electrical conductivity of a short sample of sand that is compressed among two prongs. Another method is to calculate the weight lost from a 50-g sample after this has been subjected to a temperature of around 230ºF (110ºC) for adequate time to drive off all the water.

Clay content may be determined by washing the clay from a 50-g sample of moulding sand in water that have sufficient sodium hydroxide to make it alkaline. Various cycles of agitation and washing might be needed to fully remove the clay. The remaining sand is then dried and weighed to find out the amount of clay in the original sample.

Strength and Permeability tests are conducted on a standard rammed specimen. An enough amount of sand is placed into a 2-inch-diameter steel tube so that after a 14-1b weight is reduced three times from a height of 2 inch, the final height of the specimen is within 1/32 of 2 inch.

Permeability is a determination of how simply gases can pass through the narrow voids among the sand grains. Air in the mould before pouring (plus the steam that is generated while the hot metal contacts the moisture in the sand) should be allowed to escape, instead of be trapped in the casting as porosity or blow holes. Throughout the permeability test, the sample tube having the rammed specimen is located on a device and subjected to an air pressure of 10g/cm2. Through either a flow rate measurement or determination of the pressure among the orifice and the sand, an AFS permeability number is measured. Most of the test devices are calibrated to provide a direct readout of the permeability number.