Properties, Classification & Ingredients of Moulding Sand (Casting)

Moulding Sand:

Sand is the principal moulding Material in the foundry shop. It is used for all types of castings, non-ferrous metals, iron, steel. It can withstand the high temperature of the molten metal. It does not react chemically with molten metal.

Properties of Moulding Sand

Collapsibility:

After the molten metal in the mould gets solidified, the sand mould must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. In absence of this property the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly desired in cores.

Adhesiveness:

It is property of molding sand to get stick or adhere with foreign material such sticking of molding sand with inner wall of molding box.

Green strength:

The green sand after water has been mixed into it, must have sufficient strength and toughness to permit the making and handling of the mould. For this, the sand grains must be adhesive i.e., they must be capable of attaching themselves to another body. Grains having high adhesiveness will cling to the sides of the molding box, also the sand. Grains must have the property known as cohesiveness i.e. ability of the sand grains to stick.

Porosity [Permeability]:

A porous material. Gap between sand particles provides passage for ﬂue gases.

Plasticity:

Acquires the shape of pattern. Deforms to required shape.

Flowability:

It ﬂows into corners and deeper into moulding box.

Cohesiveness:

Ability of sand particles to stick each other.

Refractoriness:

Withstands high temperature. Withstands heat of molten metal.

Classification of Moulding Sand

Green sand:

It is a mixture of silica sand with 18-30% clay, having a total water of 6-8%. Clay and water furnish the bond for green sand. It is fine, soft, light and porous. It is used for making simple, small and medium size castings. Dampening, squeezed in hand, retain it’s shape, the impression given to it under pressure.

Dry sand:

Green sand that has been dried or baked after the mould is made called dry sand. It is suitable for large castings. This mould has greater strength and rigidity. These mould does not cause defects which occurs due to moisture. Engine blocks, large gears, big housings, construction parts are making by using dry sand mould.

Loam sand:

Loam sand is high in clay and dries hard. This is particularly employed for loam moulding. Usually for large castings like bell, roller, pulleys. It contains silica, graphite, clay and water, fireclay and gainster. Loams are gritty, moist and retain water easily. Loam soil feels soft and rich and is easy to work over wide range.

Facing sand:

It forms the face of the mould. It is next to the surface of the pattern and it comes into contact with molten metal when the mould is poured. It is made of silica and clay without addition of used sand. Layer of facing sand in mould usually 20-30mm. 10-15% of whole amount of moulding sand is facing sand.

Backing sand:

It is used to back up the facings sand and to fill whole volume of the ﬂask. It is also called as ﬂoor sand and some times called blacksand. Moulding sand is in black due to addition of coal dust and burning on coming in contact with molten metal. Backing sand is reconditioned sand used for ramming main part of mold after pattern has been covered with facing sand. In foundries where machine moulding is done by using backing sand. The used sand is cleaned and reactivated by the addition of water binders and special additives. Strength, refractoriness and permeability of this sand is more than backing sand.

Parting sand:

It is use to keep the green sand from sticking to the pattern. To allow the sand on the parting surface of cope and drag to separate without clinging. This is clean clay-free silica sand which serves the same purpose as parting dust. It is the mixture of silica and brick powder.

Ingredients in Foundry Sand

Sand:

Molding sands may be of two types namely natural or synthetic. Natural molding sands contain sufficient binder. Sand in form of granular quarts is the main constituent of molding sand having enough refractoriness which can impart strength, stability and permeability to molding and core sand. The chemical composition of sand has the impurities like lime, magnesia, alkalis etc. The presence of excessive amounts of iron oxide, alkali oxides and lime can lower the fusion point to a considerable extent which is undesirable.

Binder:

In general, the binders can be either inorganic or organic substance. The inorganic group includes clay sodium silicate and port land cement etc. In foundry shop, the clay acts as binder which may be Kaolonite, Ball Clay, Fire Clay, Limonite, Fuller’s earth and Bentonite. Binders included in the organic group are dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc. Organic binders are mostly used for core making. Among all the above binders, the bentonite variety of clay is the most common. However, this clay alone can not develop bonds among sand grains without the presence of moisture in molding sand and core sand.

Moisture:

The amount of moisture content in the molding sand varies generally between 2 to 8 percent. This amount is added to the mixture of clay and silica sand for developing bonds. This is the amount of water required to fill the pores between the particles of clay without separating them. The effect of clay and water decreases permeability with increasing clay and moisture content. The green compressive strength first increases with the increase in clay content but after a certain value, it starts decreasing.

Additives:

Additives are the materials generally added to the molding and core sand mixture to some special property in the sand. Some common used additives for enhancing the properties of molding and core sands are discussed below.

Coal dust:

Coal dust is added mainly for producing a reducing atmosphere during casting. This reducing atmosphere results in any oxygen in the poles becoming chemically bound so that it cannot oxidize the metal.

Wood ﬂour:

This is a fibrous material mixed with a granular material like sand; its relatively long. Thin fibers prevent the sand grains from making contact with one another.

Silica ﬂour:

It is called as pulverized silica and it can be easily added up to 3% which increases the hot strength and finish on the surfaces of the molds and cores. It also reduces metal penetration in the walls of the molds and cores.

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Properties, Classification & Ingredients of Moulding Sand (Casting)