What is Molding Sand?
Molding sand, also known as foundry sand, is sand that when moistened and compressed or oiled or heated tends to pack well and hold its shape. It is used in the process of sand casting for preparing the mold cavity.
The principal raw material used in molding is molding sand because it provides several major characteristics that may not be obtained from other materials. Molding sand is defined as granular particles resulting from the breakdown of rocks, due to the action of natural forces, such as frost, wind, rain, heat, and water currents. Rocks have a complex composition and sand contains most of the elements of the rocks.
Due to this reason, molding sand differs considerably in different parts of the world. In nature, it is found on the bottom and banks of rivers and lakes. Molding sand is classified into different categories according to the nature of its origin.
The principal constituents of molding sands are as follows:
- Silica (SiO2)—86 to 90%,
- Alumina (Al2O3)—4 to 8%,
- Iron oxide (Fe2O3)—2 to 5% with smaller amounts of the oxides of Ti,
- Ca, and some alkaline compounds.
Natural Sand/Green Sand
It is also called green sand and is collected from natural resources. It contains water as the only binder. It has the advantage of maintaining moisture content for a long time, having a wide working range of moisture content, permitting easy patching and finishing of molds.
Greensand is an aggregate of sand, bentonite clay, pulverized coal, and water. Its principal use is in making molds for metal casting. The largest portion of the aggregate is always sand, which is often a uniform mixture of a form of silica.
There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and the ability of the hot molten metal to degas. The coal typically referred to as sea coal, which is present at a ratio of less than 5%, partially combusts in the surface of the molten metal leading to off-gassing of organic vapors.
Sand casting is one of the earliest forms of casting practiced due to the simplicity of materials involved. It still remains one of the cheapest ways to cast metals because of that same simplicity. Other methods of casting, such as those using coquille, boast a higher quality of surface finish but have a higher cost.
Greensand (like other casting sands) is usually housed in what foundry workers refer to as “flask”, which are nothing other than boxes without a bottom or lid. The box is split into two halves which are stacked together in use. The halves are referred to as the cope and drag flask respectively.
Not all Greensand is green in color. But considered “green” in the sense that it is used in a wet state (akin to green wood). According to the Cast Metals Federation website, an alternative casting method is to heat-dry the molded sand before pouring the molten metal. This dry sand-casting process results in a more rigid mold better suited to heavier castings.
It is artificial sand obtained by mixing relatively clay-free sand, binder (water and bentonite), and other materials as required. It is better to mold sand as its properties can be easily controlled by varying the mixture content.
The composition of green synthetic sand for steel castings is as under.
New silica sand — 25%, old sand — 70%, Bentonite — 1.5%, Dextrine — 0.25%, and moisture — 3 to 3.5%.
The composition of dry synthetic sand for steel castings is as under.
Net silica sand — 15%, old sand 84%, Bentonite — 0.5%, and moisture — 0.5%.
In addition to it, there are certain varieties of special sands such as Zirconite, Olivin etc. These special sands are more expensive than silica and are, therefore, used only where their use is justified.
Types of Molding Sand
Molding sands can also be classified into various types according to their use are backing sand, core sand, dry sand, facing sand, green sand, loam sand, parting sand, system sand.
1. Backing sand or floor sand
Backing sand or floor sand is used to back up the facing sand and is used to fill the whole volume of the molding flask. Backing sand is sometimes called black sand because of old, repeatedly used molding sand is black in color due to the addition of coal dust and burning on coming in contact with the molten metal.
2. Core sand
Core sand is used for making cores and it is sometimes also known as oil sand. Core sand is highly rich silica sand mixed with oil binders such as core oil which is composed of linseed oil, resin, light mineral oil, and other bind materials. Pitch or flours and water may also be used in large cores for the sake of the economy.
3. Dry sand
Greensand that has been dried or baked in a suitable oven after the making mold and cores is called dry sand. It possesses more strength, rigidity, and thermal stability. Dry sand is mainly used for larger castings. Mold prepared in this sand is known as dry sand molds.
4. Facing sand
Facing sand forms the face of the mold. It is next to the surface of the pattern and it comes into contact with molten metal when the mold is poured. The initial coating around the pattern and hence for mold surface is given by facing sand. Facing sand has high strength refractoriness. Facing sand is made of silica sand and clay, without the use of already used sand.
Different forms of carbon are used in facing sand to prevent the metal from burning into the sand. A facing sand mixture for green sand of cast iron may consist of 25% fresh and specially prepared and 5% sea-coal.
They are sometimes mixed with 6-15 times as much fine molding sand to make facings. The layer of facing sand in a mold usually ranges between 20-30 mm. From 10 to 15% of the whole amount of molding sand is the facing sand.
Greensand is also known as tempered or natural sand which is a just prepared mixture of silica sand with 18 to 30% clay, having moisture content from 6 to 8%. The clay and water furnish the bond for green sand. It is fine, soft, light, and porous.
Greensand is damp when squeezed in the hand and it retains the shape and the impression to give to it under pressure. Molds prepared by this sand are not requiring backing and hence are known as green sand molds.
Greensand is easily available and it possesses low cost. Greensand is commonly employed for the production of ferrous and non-ferrous castings.
6. Loam sand
Loam sand is a mixture of sand and clay with water to a thin plastic paste. Loam sand possesses high clay as much as 30-50% and 18% of water. Patterns are not used for loam molding and shape is given to mold by sweeps. Loam sand is particularly employed for loam molding used for large grey iron castings.
7. Parting sand
Parting sand without binder and moisture is used to keep the green sand not to stick to the pattern and also to allow the sand to the parting surface the cope and drag to separate without clinging. Parting sand is clean clay-free silica sand that serves the same purpose as parting dust.
8. System sand
In mechanized foundries where machine molding is employed. System sand is used to fill the whole molding flask. In mechanical sand preparation and handling units, facing sand is not used. The used sand is cleaned and re-activated by the addition of water and special additives.
This is known as system sand. Since the whole mold is made of this system sand, the properties such as strength, permeability, and refractoriness of the molding sand must be higher than those of backing sand.
Properties of Molding sand
The basic properties required in molding sand and core sand are adhesiveness, cohesiveness, collapsibility, flowability, dry strength, green strength, permeability, refractoriness described as under.
Adhesiveness is a property of molding sand to get the stick or adhere to foreign material such as sticking molding sand with the inner wall of the molding box.
Cohesiveness is the property of molding sand by virtue of which the sand grain particles interact and attract each other within the molding sand. Thus, the binding capability of the molding sand gets enhanced to increase the green, dry and hot strength properties of molding and core sand.
After the molten metal in the mold gets solidified, the sand mold must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. In absence of collapsibility property, the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly required in cores.
4. Dry strength
As soon as the molten metal is poured into the mold, the moisture in the sand layer adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of the mold wall during the flow of molten metal. The dry strength also prevents the enlargement of the mold cavity caused by the metallostatic pressure of the liquid metal.
5. Flowability or plasticity
Flowability or plasticity is the ability of the sand to get compacted and behave like a fluid. It will flow uniformly to all portions of the pattern when rammed and distribute the ramming pressure evenly all around in all directions.
Generally, sand particles resist moving around corners or projections. In general, flowability increases with a decrease in green strength and vice versa. Flowability increases with a decrease in the grain size of sand. The flowability also varies with moisture and clay content in sand.
6. Green strength
The green sand after water has been mixed into it, must have sufficient strength and toughness to permit the making and handling of the mold. For this, the sand grains must be adhesive, i.e., they must be capable of attaching themselves to another body and. therefore, and sand grains having high adhesiveness will cling to the sides of the molding box.
Also, the sand grains must have the property known as cohesiveness i.e., the ability of the sand grains to stick to one another. By virtue of this property, the pattern can be taken out from the mold without breaking the mold, and also the erosion of mold wall surfaces does not occur during the flow of molten metal.
The green strength also depends upon the grain shape and size, amount and type of clay and moisture content.
Permeability is also termed as the porosity of the molding sand in order to allow the escape of any air, gases or moisture present or generated in the mold when the molten metal is poured into it. All these gaseous generated during the pouring and solidification process must escape otherwise the casting becomes defective.
Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand. The extent of ramming of the sand directly affects the permeability of the mold. Permeability of mold can be further increased by venting using vent rods.
Refractoriness is defined as the ability of molding sand to withstand high temperatures without breaking down or fusing thus facilitating to get sound casting. It is a highly important characteristic of molding sands. Refractoriness can only be increased to a limited extent.
Molding sand with poor refractoriness may burn onto the casting surface and no smooth casting surface can be obtained. The degree of refractoriness depends on the SiO2 i.e., quartz content, and the shape and grain size of the particle.
The higher the SiO2 content and the rougher the grain volumetric composition the higher is the refractoriness of the molding sand and core sand. Refractoriness is measured by the sintering point of the sand rather than its melting point.
9. Miscellaneous properties of molding sand
In addition to the above requirements, the molding sand should not stick to the casting and should not chemically react with the metal. Molding sand needs to be economically cheap and easily available in nature. It need to be reusable for economic reasons. Its coefficients of thermal expansion need to be sufficiently low.