What Is Molding Sand?

Posted on Oct 30, 2020

Molding sand

Molding sand, also known as foundry sand, is a specific type of sand and it’s used for making of castings. It is moistened and pressed then heated by oil and usually can be densely packed and maintain a certain geometry.

It is used in the sand casting process to prepare the mold cavity. Commonly called green sand is an aggregate of sand, bentonite clay, coal dust and water. Its main use is in making metal casting molds. The greatest part of the aggregate is always sand, which is often a homogeneous mixture of silica form.

There are many recipes for clay ratios, but they all strike a balance between formability, surface finish, and the ability of hot molten metal to degass. Coal, usually referred to as sea carbon, which is present in an amount less than 5%, partially burns at the surface of the molten metal, leading to the degassing of organic vapors.

Sand casting is one of the earliest forms of casting practiced due to the simplicity of the materials used. Due to the same simplicity, it still remains one of the cheapest methods of metal casting.

Other casting methods, such as those using coquille, have a higher quality surface finish but have a higher cost. Green sand (like other foundry sands) is usually stored in foundries known as "flasks", which are nothing but bottomless or lidless boxes.

The box is divided into two halves that are stacked together for use. The halves are called the cap and molding flask, respectively.

A specialized container of bronze is poured into Green Sand that is a continuous hollow shpe in the mold. That below can be filled with molten material and turned into a finished casting. Not all green sand is actually green. But considered "green" in the sense that it is used when wet (akin to green wood).

According to some online sources, other casting methods and methods are hot-drying the molded sand before pouring the molten metal. This dry casting process results in a stiffer mold better suited to heavier castings.


What Is the History of Molding Sand

There are no explicit records in history that would attribute the first metal casting and the use of foundry sand, but ancient artifacts and writings date back to around 3200 B.C. in ancient Mesopotamia. The history of molding sand is difficult to study as many of these practices date back to pre-writing.

Molding sand was used exclusively for bronze castings that were pioneered by the ancient Chinese. Another important advance occurred in India in 500 BC. when crucible steel was created. Eventually Sir Humphrey Davy first made aluminum castings in Britain around 1808.

Currently, the production capacity in the United States is over 8 million tons of iron casting, 1.4 million tons of steel casting, 1.7 million tons of aluminum casting and 321,000 tons of copper casting.

Much of the modern metal industry has innovated due to the huge increase in sand quality due to better tooling, as well as metallurgy and a better understanding of the properties of metals.


Sources of Molding Sand

The general sources of receiving molding sands are sea shores, rivers, lakes, deserts and granular rock elements. Molding sands can be mainly divided into two types: natural or synthetic.

Natural molding compounds contain a sufficient amount of binder. On the other hand, synthetic molding sands are prepared artificially with the use of basic molding ingredients (quartz sand 85-91%, binder 6-11%, water or moisture content 2-8%) and other additives in appropriate weight proportions with perfect mixing and grinding appropriate equipment.

:: Read more : Casting Sand For Industrial Use


Ingredients of Molding Sand

The main components of the molding sand are quartz sand, binder, moisture content and additives. Granular quart silica sand is the main component of foundry sand with sufficient refractoriness to impart strength, stability and permeability to molding sand and core sand.

But along with silica, small amounts of iron oxide, alumina, limestone (CaCO3), magnesium oxide, soda and potash are present as impurities. The chemical composition of the quartz sand gives an idea of ​​the impurities present, such as lime, magnesium oxide, alkali etc.

The presence of excessive amounts of iron oxide, alkali oxides and lime can significantly lower the melting point, which is undesirable. Quartz sand can be determined by the grain size of the sand and its shape (angular, oblique and rounded).


Binder of Molding Sands

The binders can be inorganic or organic substances. Binders included in the inorganic group are sodium silicate clay, Portland cement etc. In the foundry the clay acts as binder which may be Kaolinite, Ball Clay, Fire Clay, Limonite, Fuller’s earth and Bentonite.

Binders included in the organic group are dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc. Binders of organic group are mostly used for core making. Among all the above binders, the bentonite variety of clay is the most commonly used.

However, this clay alone can’t develop bonds among sand grins without the presence of moisture content in molding sand and core sand.


Content of Moisture in Molding Sands

The moisture content of the molding sand varies from 2 to 8%. This amount is added to the mixture of clay and quartz sand to induce bonding. This is the amount of water needed to fill the pores between the clay particles without separating them.

This amount of water is held rigidly by the clay and is mainly responsible for increasing the strength of the sand. The action of clay and water reduces permeability as the clay content and moisture increase. The compressive strength in the raw state first increases with increasing clay content, but begins to decrease when a certain value is reached.

In order to improve the properties of the molding sand, other additives, known as additives, are added to the basic ingredients.


Additives and Coal Dust

Additives are materials usually added to the mixture of molding sand and core sand to obtain the special properties of sand. Some commonly used additives in order to improve the properties of molding and core sand, they are coal dust, corn meal, dextrin, sea coal, tar, wood flour, quartz flour.

Coal dust is mainly added to create a reducing atmosphere during the casting process. This reducing atmosphere causes any oxygen in the poles to be chemically bound so that it cannot oxidize the metal. It is usually added to molding sands to make molds for the production of gray and malleable cast iron.


Other Additives for Molding Sand

Corn flour belongs to the starchy carbohydrate family and is used to increase the incidence of molding sand and core sand. It is completely evaporated by the heat in a sand form, thus leaving a space between the sand grains.

This allows the sand grains to move freely, which ultimately causes the walls of the mold to move and reduces the expansion of the mold and hence casting defects. Corn sand added to molding and core sand greatly improves mold and core strength.


Molding Sand with Sea Coal

Sea coal is a fine powdered bituminous coal that places its place in the pores of the quartz sand grains in the molding and core sand. When heated, the sea coal turns into coke which fills the pores and is resistant to water.

Due to this, the sand grains become finite and cannot develop into a dense packing pattern. Thus, the sea carbon reduces the movement of the mold walls and the permeability to the molds and the core sand, thereby keeping the mold and core surface clean and smooth.

Tar is a distilled form of soft coal. It can be added from 0.02% to 2% in the molding sand and core. Tar increases the hot strength, surface finish on the mold surface and behaves in exactly the same way as sea coal.


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