A core should have the following characteristics:
(i) Permeability:
A core is supposed to be more permeable than the mould itself and for this reason, coarse sand with large grain size (special washed silicon sand free from clay) mixed with molasses is used for core making. Vents are also provided in core for giving easy escape of hot gases that are generated during casting.
(ii) Amount of Gas:
Core should produce minimum amount of gas when in contact with molten metal.
(iii) Collapsibility:
A core should be capable of collapsing shortly after the molten metal has solidified around the core. Collapsibility provides freeness in the contraction of the metal.
(iv) Thermal Stability:
According to service conditions to which a core is subjected, core sand should be highly refractory, and able to withstand high temperatures of molten metal otherwise defects like rough surface and metal penetration etc. may occur.
(v) High Strength:
Core must have high strength and be capable of being handled after drying. If required, wires may be inserted in the core while it is being moulded to give added strength. It must be able to withstand force of molten metal.
Uses of Core Moulding:
A core is used for the following purposes:
To obtain the desired cavities and recesses, which otherwise could not be obtained by normal moulding operation. In pit moulding, the entire mould is made of cores. Sometimes the cores are also used to reduce metal erosion in gates and runners (splash cores), to retard foreign matter in the melt (strainer cores), provide a cup or basin on the top of mould (pouring basin cores).
