Properties of moulding sands:
The important properties are:
· The sand should have adequate strength in its green, dry and hot states
· Green strength is the strength of sand in the wet state and is required for making possible to prepare and handle the mould.
· If the metal is poured into a green mould the sand adjacent to the metal dries and in the dry state it should have strength to resist erosion and the pressure of metal.
· The strength of the sand that has been dried or basked is called dry strength
· At the time of pouring the molten metal the mould must be able to withstand flow and pressure of the metal at high temperature otherwise the mould may enlarge, crack, get washed or break
· Strength of the moulding sand depends on:
1. Grain size and shape
2. Moisture content
3. Density of sand after ramming
· The strength of the mould increases with a decrease of grain size and an increase of clay content and density after ramming. The strength also goes down if moisture content is higher than an optimum value.
· The moulding sand must be sufficiently porous to allow the dissolved gases, which are evolved when the metal freezes or moisture present or generated within the moulds to be removed freely when the moulds are poured. This property of sand is called porosity or permeability.
3. Grain size and shape:
· The size and shape of the grains in the sand determine the application in various types of foundry. These are three different sizes of sand grains.
· Fine sand is used for small and intricate castings. Medium sand is used for benchmark and light floor works. If the size of casting is larger coarse sand is used
· Sand having fine, rounded grains can be closely packed and forms a smooth surface. Although fine-grained sand enhances mould strength.
4. Thermal stability:
· The sand adjacent to the metal is suddenly heated and undergoes expansion. If the mould wall is not dimensionally stable under rapid heating, cracks, buckling and flacking off sand may occur.
· Refractoriness is the property of withstanding the high temperature condition moulding sand with low refractoriness may burn on to the casting
· 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. The refractoriness of the Silica sand is highest.
· Flowability or plasticity is the property of the sand to respond to the moulding process so that when rammed it will flow all around the pattern and take the desired mould shape. High flowability of sand is desirable for the sand to get compacted to a uniform density and to get good impression of the pattern in the mould.
· Flowability is also very important in machine moulding
· Flowability of sand increases as clay and water content are increased.
7. Sand texture:
· As mentioned earlier the texture of sand is defined by its grain size and grain size distribution.
· The texture chosen for an application should allow the required porosity, provide enough strength and produce the desired surface finish on the casting.
· The moulding sand should collapse 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
· It is the important property of the moulding sand and it is defined as the sand particles must be capable of adhering to another body, then only the sand should be easily attach itself with the sides of the moulding box and give easy of lifting and turning the box when filled with the stand.
· Since large quantities of sand are used in a foundry it is very important that the sand be reusable otherwise apart from cost it will create disposal problems
11. Easy of preparation and control:
· Sand should lend itself to easy preparation and control by mechanical equipment
· Sand should have enough conductivity to permit removal of heat from the castings.