Natural and synthetic molding sand and Properties of molding sand


According to the amount of clayey matter they contain, the molding sands are classified as:

1.       Silica Sand:
2.       Lean or weak sand :
3.       Moderately strong sand :
4.       Strong sand :
5.       Extra Strong sand (Loam sand) :


Sand casting

Natural / green sand for casting :


Natural sand is the one which is available from natural deposits. Only additives and water need be added to it to make it satisfactory for molding.
The clay content of most natural sands is slightly higher than desired so that new sand can be continuously added to the used sand to replenish that which is 


Natural sand or green sand

Synthetic sand for casting :


A synthetic sand is prepared by mixing a relatively clay free sand having specified type of sand grain, with specified type of clay binder as well as water and other additives.           

Advantages of Synthetic molding sand :


1.       Low san maintenance cost
2.       Improper permeability
3.       Lower moisture
4.       Easier to work on mass production of molding
5.       Semi skilled workers can work on machine molding
6.       No sand damping


Properties of molding sand:

The success of the casting process depends to a large extent on the making of a satisfactory mould. For this, the molding properties of the sand have to be controlled.
These properties include:

(i) Porosity or Permeability
(ii) Strength or cohesiveness,
(iii) Refractoriness, 
(iv) Plasticity, 
(v) Collapsibility and 
(vi) Adhesiveness 
(vii) Co-efficient of expansion etc.

Refractoriness:


It is the ability of the molding sand mixture to withstand the heat of melt without showing any signs of softening or fusion.
This property is greatly influenced by the purity of the sand particles and their size.
It increases with the grain size of sand and its content and with the diminished amount of impurities and silt.

Permeability:


Permeability or porosity of the molding sand is the measure of its ability to permit air to flow through it.
Molten metal always contains a certain amount of dissolved gases which try to leave it when the metal solidifies. If all these gases and vapors are not able to escape completely through the walls of the mould, they may penetrate the liquid metal where, after solidification, they form gas holes and pores. To avoid these defects, the molding sand should have good gas permeability.
Again, higher the silt contents of sand, the lower its gas permeability. If the mould is rammed too hand, its permeability will decrease and vice versa.

Cohesiveness:


It is defined as the property of holding together of sand grains. Molding sand should have ample strength so that the mould does not collapse or get partially destroyed during conveying, turning over or closing.
This property also enables the pattern to be removed without breaking the mould and to stand, the flow of molten metal when it rushes inside the mould.
The strength of the molding sand grows with density, clay content of the mix and decreased size of sand grains. So, it is clear that as the strength of the molding sand increases, its porosity decreases;

Adhesiveness:


This is the property of sand mixture to adhere to another body (here, the molding flasks). The molding sand should cling to the sides of the molding boxes so that it does not fall out when the flasks are lifted and turned over.
This property depends on the type and amount of binder used in the sand mix.

Plasticity or flow-ability:


It is the measure of the molding sand to flow around and over a pattern during ramming and to uniformly fill the flask. This property may be enhanced by adding clay and water to the silica sand.

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