Properties of Moulding Materials/Sand – Permeability, Collapsibility, Refractoriness and strength
There is a large variety of materials are available for manufacturing of moulds. The commonly used moulding materials in foundries are moulding sand, sand binders, and sand additives. The main constituent is moulding sand, so generally moulding sand is referred as a moulding material. The mould material should have certain qualities, and it depends on the processing properties of moulding material. The properties generally required for moulding material are the following.
RefractorinessThe Refractoriness of a moulding material is defined as the ability of the material to withstand at a high temperature of molten metal.
Read: Desirable properties of refractory material
Types of Patterns Used For Casting
Porosity/PermeabilityThe molten metal used for casting absorbs air, steam and other gases in the furnace. During solidification, these gases expelled from mould along with the gases generated from mould and core Martials. These gases should be allowed to escape from the mould. Otherwise, it causes mouding defects. The moulding sand should be sufficiently porous to allow these gas to escape from mould. This property of mould material is known as permeability.
Permeability expressed in permeability number (pn)
V = Volume of air passing through the specimen
H = Height of standard specimen
P = Pressure of air passing through the specimen
A = Cross sectional area of cylindrical specimen
T = Time required to air pass through the specimen
Green strengthThe molding sand that contains moisture is known as green sand. The green sand should have enough strength, so that constructed mold retains its shape.
Dry strengthWhen molten metal is poured into the mould cavity, the sand around the cavity is quickly gets dried. The moisture content of sand evaporates. At this stage, the moulding sand should have enough strength to retain the shape of mould cavity as well as withstand metallostatic forces. This strength is known as Dry strength.
Green strength and Dry strength again divide into Green/Dry compressive strength and Green/Dry shear strength.
Hot strengthThe molten metal in mold cavity solidifies very slowly, during this time the mould sand reaches high temperature and all moisture contents will get evaporated. Hot strength is defined as the strength of mould sand to retain the shape of the cavity, after the all moisture content is fully eliminated.
Plasticity/FlowabilityIt is the property of that allows mould sand to flow to all portions of moulding flask. The sand should have enough plasticity to get uniformly compacted mould. It is the property that allows the reproduction of shape during moulding.
Adhesiveness and cohesivenessThey are the properties that ensure the strength of mould. Cohesiveness is the property that allows the sand grains to stick together, while adhesiveness allows sand to adhere or stick to sides of moulding box.
Read: Difference between Adhesion and Cohesion
CollapsibilityWhen molten metal solidifies, the volume of solid decreases. If mould creates the resistance this contraction, cracks appear on casting. So it is desirable to mold material to have low resistance to shrinkage of cast. This is known as collapsibility.
To get required collapsibility wood powder is added to moulding sand. When molten metal poured to mould, wood powder burns to ashes due to heat, that allows easy collapsing of mould face near to casting during shrinkage.
Some other important properties of moulding material are
- Sand grain shape
- Sand texture
- Clay content