What are the properties of Molding Sand?
The basic properties of molding sand and core sand are adhesiveness, cohesiveness, collapsibility, flowability, dry strength, green strength, permeability, refractoriness.
1 Adhesiveness
Adhesiveness is a property of molding sand to get the stick or adhere to foreign material such as sticking molding sand with the inner wall of the molding box.
2 Cohesiveness
Cohesiveness is the property of molding sand by the virtue of which the sand grain particles interact and attract each other within the molding sand. Thus, the binding capability of the molding sand gets enhanced to increase the green, dry and hot strength property of molding and core sand.
3 Collapsibility
After the molten metal in the mold gets solidified, the sand mold 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 collapsibility 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 required in cores.
4 Dry strength
The moisture in the sand layer adjacent to the hot metal gets evaporated as soon as it comes in contact with the molten metal and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of the mold wall during the flow of molten metal. The dry strength also prevents the enlargement of the mold cavity caused by the metallostatic pressure of the liquid metal.
5 Flowability or plasticity
Flowability or plasticity is the ability of the sand to get compacted and behave like a fluid. It will flow uniformly to all portions of the pattern when rammed and distribute the ramming pressure evenly all around in all directions. Flowability increases with a decrease in the grain size of sand. The flowability also varies with moisture and clay content in sand.
6 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 mold. For this, the sand grains must be adhesive, i.e. they must be capable of attaching themselves to another body and. By virtue of this property, the pattern can be taken out from the mold without breaking the mold, and also the erosion of mold wall surfaces does not occur during the flow of molten metal.
7 Permeability
Permeability is also termed as the porosity of the molding sand in order to allow the escape of any air, gases, or moisture present or generated in the mold when the molten metal is poured into it. All these gaseous generated during the pouring and solidification process must escape otherwise the casting becomes defective. Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand.
8 Refractoriness
Refractoriness is defined as the ability of molding sand to withstand high temperatures without breaking down or fusing thus facilitating to get sound casting. Molding sand with poor refractoriness may burn onto the casting surface and no smooth casting surface can be obtained.
9 Miscellaneous properties of molding sand
In addition to the above requirements, the molding sand should not stick to the casting and should not chemically react with the metal. Molding sand needs to be economically cheap and easily available in nature. It needs to be reusable for economic reasons. Its coefficients of thermal expansion need to be sufficiently low.
