Sand Casting

Sand casting is a process where molten metal is cast in a mould made from a sand mixture. In the past, the process was cost effective only for small volume production. But nowadays it is also suitable for high volume production thanks to automated equipment for making sand moulds.
Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. Medium to large parts such as valve bodies, crankshafts and engine blocks are typically manufactured by this process. The dimensional accuracy and surface finish of the casting depend on the type of sand and the moulding process.
There are two main types of sand used for moulding: green sand and dry sand. Green sand consists of silica sand, clay, moisture and other additives. Dry sand is a mixture of sand and fast curing adhesive. When dry sand is used, it is often referred to as no bake mould casting or air set sand casting. Castings made from coarse green sand typically have a rough surface whilst air-set moulds can produce castings with much smoother surfaces.
A pattern is made in the shape of the desired part, but enlarged to account for shrinkage and machining allowances in the final casting. The pattern is typically made from wood or plastic, and can be reused to produce new sand moulds. Sand cores can be used to form the inside shape of hollow parts of the casting.
A metal box (the flask) is used for making the sand mould. It typically consists of two halves to form the top and bottom parts of the mould, termed cope and drag respectively. The pattern is placed in the flask, and then packed with sand mixture forming a mould cavity. If necessary, a temporary plug is placed to form a channel for pouring the cast metal (so-called a sprue). After that, the pattern and channel plug are removed leaving the mould cavity and pouring channel. The cope and drag are prepared separately and assembled afterwards to form a mould.
The molten metal is poured into the mould through a sprue, which transmits the molten metal via runners into the mould cavity. After the cast metal has solidified and cooled, the casting is taken out from the sand mould. Generally, no mould release agent is used and the sand mould is destroyed in the removal process. Green sand can be reused after adjusting its composition by compensating the lost moisture and additives.

Porosity or Permeability
It is the ability of sand by which it allows the gases to pass through it easily.
Some gases gets dissolved in molten metal and when this molten metal starts to solidify, these dissolved gases comes out of the molten metal and try to escape out of the moulding sand. If the sand is not enough porous than these gases will not be able to go out of the mould and gets trapped into the casting and produces holes and pores in metal casting. Also if the molten metal comes in contact with the moist sand, steam or water vapor is produced. This steam or vapors also results in the formation of holes in the casting if they do not able to escape out of the mould. So it is advised to use sufficiently porous moulding sand to eliminate the porosity defect in metal casting.

Flowability 
The ability of moulding sand to behave like a fluid when it is rammed is called flowability.
Due to this property the sand can easily occupy the space in molding box and take up its shape. This allows the sand to compress to a compact density and let it pack around the pattern. The sand should be of high flowability, so that it can be easily compacted for uniform density and to obtain a good impression of the pattern in the mould. The flowability of the sand can be increases as we increases the clay and water content in the sand.

Collapsibility 
The ability of the moulding sand to collapse after solidification of the molten metal is called collapsibility.
After the solidification of molten metal, the sand should get collapse for free contraction of the metal. If free contraction of the metal will happen than if eliminates naturally the tearing or cracking of the contracting metal.

Adhesiveness 
The ability of the sand particles to get stick with another body is called adhesiveness.
The sand should have sufficient adhesiveness so that it can easily get cling to the sides of the moulding boxes and does not fall out to the box when it is removed.

Cohesiveness or Strength
The ability of the sand particles to stick with each other is called cohesiveness.
The strength of the sand depends upon how cohesive the sand particles are. The sand should have sufficient strength so that it can easily capable to retain its shape during conveying, turning or closing and pouring. If it is not of appropriate strength than it will not be able to hold its shape and the mould may damage during pouring of molten metal. Low strength sand leads to pouring casting defects in metals. To avoid pouring defects, the sand should be of sufficient strength to produce mold of desired shape and also retain this shaped even when the molten metal is poured in the moulding cavity.
The sand strength can be of two types
Green strength: The strength of sand possessed by it in its green or moist state is called green strength. The mould with adequate green strength retains its shape and do not collapse even when the pattern is removed from the moulding box.
Dry strength: The strength possessed by the sand in its dry or baked state is called dry strength. Enough dry strength allows the sand to withstand erosive forces due to molten metal and helps to retain its shape.

Refractoriness
The ability of the moulding sand to withstand the high temperature of the molten metal without fusing into it is called refractoriness.
The moulding sand must have enough refractoriness property to produce excellent quality of casting free from defects. The sand with lack of refractoriness melts and gets fuse in the casting and spoils the quality of the cast metal. The refractoriness is the measure of sinter point of the sand not its melting point.