It is special kind of molding sand. Keeping the above mentioned objectives in view, the special considerations should be given while selecting core sand. Those considerations involves
(i) The cores are subjected to a very high temperature and hence the core sand should be highly refractory in nature
(ii) The permeability of the core sand must be sufficiently high as compared to that of the molding sands so as to allow the core gases to escape through the limited area of the core recesses generated by core prints
(iii) The core sand should not possess such materials which may produce gases while they come in contact with molten metal and
(iv) The core sand should be collapsible in nature, i.e. it should disintegrate after the metal solidifies, because this property will ease the cleaning of the casting.
The main constituents of the core sand are pure silica sand and a binder. Silica sand is preferred because of its high refractoriness. For higher values of permeability sands with coarse grain size distribution are used. The main purpose of the core binder is to hold the grains together, impart strength and sufficient degree collapsibility. Beside these properties needed in the core sand, the binder should be such that it produces minimum amount of gases when the molt metal is poured in the mould. Although, in general the binder are inorganic as well as organic ones, but for core making, organic binders are generally preferred because they are combustible and can be destroyed by heat at higher temperatures thereby giving sufficient collapsibility to the core sand. The common binders which are used in making core sand as follows:
1. Cereal binder
It develops green strength, baked strength and collapsibility in core. The amount of these binders used varies from 0.2 to 2.2% by weight in the core sand.
2. Protein binder
It is generally used to increase collapsibility property of core.
3. Thermo setting resin
It is gaining popularity nowadays because it imparts high strength, collapsibility to core sand and it also evolve minimum amount of mold and core gases which may produce defects in the casting. The most common binders under this group are phenol formaldehyde and urea formaldehyde.
4. Sulphite binder
Sulphite binder is also sometimes used in core but along with certain amount of clay.
It is commonly added in core sand for increasing collapsibility and baked strength of core.
It is widely used to increase the hot strength of the core.
It is generally used as a secondary binder to increase the hardness on baking. It is used in the form of molasses liquid and is sprayed on the cores before baking.
8. Core oil
It is in liquid state when it is mixed with the core sand but forms a coherent solid film holding the sand grains together when it is baked. Although, the core drying with certain core oils occurs at room temperature but this can be expedited by increasing the temperature.
That is why the cores are made with core oils and are usually baked. Copied from A Textbook of a Basic Manufacturing Processes and Workshop Technology by Rajender Singh.