Properties of Good Moulding Sand:

In order to provide satisfactory and consistent results, the sand utilized to make moulds should be carefully prepared. Ordinary silica sands are compounded along additives to meet following four needs which are necessary to moulding sand.

 (a) Refractoriness: this is defined like the ability to withstand high temperatures. So higher the pouring temperature higher shall be the needed refractoriness. For lower pouring temperatures even the lower value shall work. Refractoriness is provided by the fundamental nature of the sand.

 (b) Cohesiveness (also referred to as bond): this is defined as the ability to achieve a given shape. Cohesiveness of sand is ascertained through amount of bonding materials exist such as clay in presence of moisture.

 (c) Permeability: this is the ability to allow gases to escape through it. The rate in millimeter per minute at which air shall pass through the sand under a standard condition of pressure is utilized as index of permeability. This is based on the size of the sand particles, the amount and kinds of clay or the moisture content, bonding agent, and the compacting pressure. The mould should be enough porous to allow the gases to escape and ignore defects because of entrapped gases.

 (d) Collapsibility: This is the ability of disintegration of the cohesive mould as a consequence of metal shrinkage. Generally Collapsibility is attained by adding organic material, such as cereals, cellulose etc., that burn out while these are exposed to hot metal. The combustion decreases both the strength and volume of the restraining sand.