PROPERTIES OF MOULDING SAND

BY – NILESH BHANDARI

A large variety of moulding materials is used in foundries for manufacturing moulds and cores. They include moulding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of moulding materials is based on their processing properties. The basic requirement of moulding sand is that it should produce sound castings free from defects. The properties that are generally required in moulding materials are: PERMEABILITY DRY STRENGTH GREEN STRENGTH HOT STRENGTH FLOWABILITY REFRACTORINESS COLLAPSIBILTY ADHESIVENESS COHESIVENESS FINENESS BENCH LIFE COEFFICIENT OF EXPANSION AND

DURABILITY

PERMIABILITYDuring pouring and subsequent solidification of a casting, a large amount of gases and steam is generated.These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the moulding and core sand. If these gases are not allowed to escape from the mould, they would be entrapped inside the casting and cause casting defects. To overcome this problem the moulding material must be porous. Proper venting of the mould also helps in escaping the gases that are generated inside the mould cavity.

DRY STRENGTH

When the molten metal is poured in the mould, the sand around the mould cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the moulding sand must posses the sufficient strength to retain the exact shape of the mould cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material.

GREEN STRENGTH

The moulding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mould. The green sand must have enough strength so that the constructed mould retains its shape.

HOT STRENGTH

It is the strength of the mould in hot conditions generally above 1500 degree Celsius.When molten metal gets poured in the mould, the mould gets heated. The mould must be sufficiently strong to withstand the temperature and pressure of the molten metal.It must not enlarge,break,rode or crack in the hot conditions and must retain its strength at high temperatures.

FLOWABILITY

It is that property of the sand due to which it flows during ramming to all portions of the moulding flask and packs properly around the pattern to acquire the desired shape and distributes the ramming pressure evenly to all parts of the mould. It increases with the addition of clay and water content and reduction of green strength and grain size.

REFRACTORINESS

It is the ability of the moulding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal.It is also defined as the ability of the sand to withstand high temperatures without fusion or other physical changes.

 The moulding sand owes this property mainly due to the presence of quartz content as Silicon Oxide and the shape and size of the grains. Rough and larger grains lead to higher refractoriness. On the other hand iron oxides, calcium and sodium and potassium react chemically during pouring of molten metal to form compounds which have very low melting points. Thus their presence in the sand reduces its refractoriness in those sands.

ADHESIVENESS

It is that property of the sand due to which it is capable of adhering to the surface of other materials. The heavy sand flask is successfully held in the moulding flask due to this property.

COHESIVENESS

It is the property of moulding sand owing to which it enables the sand particles to stick among itself.

Coarse grain sand gives greater cohesiveness then round grain particles.

BENCH LIFEIt is the ability of moulding sand to retain its property during storage that may be caused due to delay at any stage of production.

FINENESSFineness and permeability are in conflict with each other and must be balanced for optimum results.

Fine sand particles show a good surface but are poor in permeability.

On the other hand course sand shows good permeability but have poor surface finish.

Fine sand particles also resist metal penetration.

COEFFICIENT OF EXPANSION

Moulding sand should possess a low coefficient of thermal expansion.

DURABILITYMoulding sand should be durable and capable of withstanding repeated cycles of heating and cooling during casting operations.

Moulding sand should be reusable and chemically immune to molten materials.

COLLAPSIBILITY

The moulding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings.

Besides these specific properties the moulding material should be cheap, reusable and should have good thermal conductivity

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