What is Molding Sand?- Types And Properties

What is Molding Sand?

Molding sand, otherwise known as foundry sand, is sand that, when damp and pressed, oil-coated, or heated, tends to pack and retain its form. Sand casting uses molding sand to form and create the cavity for the mold.

Molding sand is the principal raw material used in molding because it has some important characteristics that cannot come from other materials.

Molding sand is defined as granular particles formed when rocks break down from physical forces, such as frost, wind, rain, heat, and water currents. Rocks have a very complex composition and sand has most of the components in the rocks.

For this reason molding sand varies widely in different regions around the world. In nature, molding sand can be found on the bottom of rivers or on the banks of lakes. Molding sand is classified by the type of sand collected and is classified based on where it comes from.

The primary constituents of molding sands are:

• Silica (SiO2)—86 to 90%,
• Alumina (Al2O3)—4 to 8%,
• Iron oxide (Fe2O3)—2 to 5% with small amounts of Ti, Mn, Ca, & alkali oxides.

Natural Sand/Green Sand

It is a natural sand, also called green sand, obtained from natural sources. It has as the only binder water. The green sand has the benefits of retaining moisture for a long period, has a wide range of moisture content that is acceptable for working with it, can be easily repaired or patched, and can be finished easily.

Greensand is a mixture of sand, bentonite clay, pulverized coal, and water. The main use of greensand is to manufacture molds for metal casting purposes. The bulk of the greensand aggregate is always sand, usually a uniform mixture of some sort of silica.

There are a multitude of recipes for the proportions of clay, and there are usually different ratios that go into their mixes based on considerations of moldability, surface finish, and degassing that the hot molten metal undergoes as it cools.

The coal typically called sea coal, which is in the mix ratio of less than 5%, mostly burns on the surface of the molten metal, leading to organic vapor off-gassing.

Sand casting was one of the earliest forms of casting due to the simplicity of the materials. Hence, it continues to be one of the least expensive metal casting methods. There are molds, for example, contrived from a coque and take kwodo to make a more significant quality surface finish, but they are more luxurious ways to mold a metal.

Greensand (and other casting sands as well) is usually kept in what foundry people refer to as a “flask”, which is nothing more than a bottomless, lidless box. The box is made in two halves that mate together in use, referred to as a cope and drag flask.

Not all greensand is green in color, but it is “green” in the sense that it is used wet (similar to green wood). Either way, according to the Cast Metals Federation website, another method of sand casting is to heat-dry the mold sand before pouring the molten metal.

The whole process is the same (with the heat-dried sand being poured just like its wet counterpart) but will yield a more rigid mold that is more appropriate for heavier castings.

Synthetic Sand

Synthetic sand is artificial sand and is made by mixing relatively clay-free sand, binder (water and bentonite), and an admixture if required. The advantage of synthetic as opposed to natural sand is that the properties of synthetic sand can be easily controlled by changing the mixture proportions.

The following is the composition for green synthetic sand for steel castings.

• New silica sand — 25%,
• old sand 70%,
• Bentonite — 1.5%,
• Dextrine — 0.25%, and
• moisture — 3 to 3.5%.

The following is the composition of dry synthetic sand for steel castings.

• New silica sand — 15%,
• old sand 84%,
• Bentonite — 0.5%, and
• moisture — 0.5%.

In addition, there are various types of specialty synthetics, like Zirconite, Olivin, etc., but every one of these types of specialty sand will be more expensive than silica and will only be used where they are justified.

Types of Molding Sand

Molding sands can also be separated into various types according to their end use pre, backing sand, core sand, dry sand, facing sand, green sand, loam sand, parting sand, system sand.

1. Backing sand or floor sand

Backing or floor sand is used to support (back-up) the facing sand and is used to fill the whole volume of the molding flask. Backing sand is sometimes referred to as black sand since it was old, repeatedly used molding sand black due to the addition of coal dust and burnt after contact with liquid metal.

2. Core sand

Core sand is used to make cores, and is know to sometimes alternate as oil sand. Core sand is very rich silica sand that is mixed with oil binders such as core oil, composed of linseed oil, resin, light mineral oil and other binders. In big cores, pitch or flours and water may also be used for cost purposes.

3. Dry sand

After making a mold and cores with greensand, greensand can be dried or baked in a suitable oven to remove moisture is known as dry sand. It is of higher strength and rigidity, and thermal stability, and is primarily used for larger castings. Mold made with this sand is called dry sand molds.

4. Facing sand

Facing sand is the face if the mold, next to the pattern surface, which would contact molten metal, when the mold is poured. The first coating next to the pattern and thus the mold surface, is made with facing sand.

The strength refractoriness should be high, made with silica sand and clay, no reused sand. Different forms of carbon is added to the facing sand, to prevent the metal from burning into the sand.

A facing sand mixture for cast iron greensand may include up to 25% prepared sand and another 5 % of sea-coal. It is sometimes combined with fine molding sand from 6-15 times the amount of facing sand to form facing sand molds.

The thickness of the facing sand in the mold is generally located between 20-30 percent of the displayed dimension and represents 10-15 percent of the total volume of molding sand.

5. Greensand

Greensand with clay is also referred to as tempered or natural sand, which is a prepared mixture of silica sand, clay 18 to 30%, and moisture 6 to 8%. Water and clay provide bond to the greensand. It is soft, fine, light, and porous.

Greensand is damp when squeezed in the hand and retains the impression to be made under compression. Molds made from greensand do not require backing and hence are referred to as greensand molds.

Greensand has high availability, and low cost; therefore, greensand is usually used for ferrous as well as non ferrous castings.

6. Loam sand

Loam sand is a mixture of sand and clay mixed to a thin plastic paste with water. Loam sand can contain very high clay up to 30-50%, and at least 18% moisture. In loam molding there are no patterns and a shape is produced by sweeps. Loam sand is predominately used in loam molding of large grey iron castings.

7. Parting sand

Parting sand is the loose, dry sand without binder and moisture that has a dual purpose; it prevents the green sand from sticking to the pattern and it provides a separation surface the cope and drag can separate without the sand clinging to the surface. Parting sand is clean clay-free silica sand, which is synonymous to parting dust.

8. System sand

In mechanized foundries where machine molding is used. System sand fills the entire molding flask. There is no use of facing sand in mechanical sand preparation and handling systems. Used sand is reclaimed, reactivated by adding water and special additives.

As system sand is used to make an entire mold all the properties of the molding sand such as strength, permeability, and refractoriness must be higher than backing sand.

Properties of Molding sand

There are basic properties that must be present in molding sand and core sand such as; adhesiveness, cohesiveness, collapsibility, flowability, dry strength, green strength, permeability, refractoriness which are discussed as follows.

1. Adhesiveness

Adhesiveness is a property of molding sand to get the stick or adhere to foreign material. An example would be sticking molding sand to the inner wall of the molding box.

2. Cohesiveness

Cohesiveness is the property of molding sand by virtue of which the sand grain particles interact, attracting one another resulting in a greater binding capacity of the molding sand, thus increasing the green, dry and hot strength of the molding sand and core sand.

3. Collapsibility

Since the molten metal in the mold will have a tendency to shrink after it has solidified, the sand mold should be collapsible to allow for the free contraction of the metal as it cools. This way, the contracted metal may collapse into the mold with little likelihood of tearing or cracking.

When the property of collapsibility does not exist the mixture may restrict contraction of the metal and thus causes an increased amount of tears and cracks in the castings. The property collapsibility is very important in cores.

4. Dry strength

After pouring molten metal into a mold, the moisture in the sand layer nearest the hot metal will evaporate and this dry layer of sand will have to have sufficient dry strength to maintain its shape to avoid erosion of the mold wall during the flow of molten metal. Dry strength also prevents enlargement of the mold cavity due to the metallostatic pressure of the liquid metal.

5. Flowability or plasticity

Flowability (or plasticity) refers to the ability of sand to be resistant to compaction and flow. It has the ability to flow uniformly into all portions of the pattern when rammed, and the ability to distribute the ramming pressure uniformly about the pattern in all directions.

In a general sense, sand particles will have a resistance to flow over corners or projections. In a general manner, flowability is expected to increase with a decrease in green strength; and flowability will increase with a decrease in grain shape. The flowability also varies with moisture and clay content in sand.

6. Green strength

Green sand with water must provide sufficient strength and tough enough to allow it to be made and handled as a mold. Also, the sand grains must be adhesive, i.e. able to stick to another body, and as such, sand grains which are adhesive will cling to the sides of a molds box.

In addition, sand grains must have the property called cohesiveness, i.e. they should stick to each other. Through this property, the pattern can be removed from the mold without breaking the mold, while at the same time while molten metal is flowing into the mold, erosion of mold wall surfaces will not take place.

Further, the green strength also relies on the grain shape and size as well as the amount and types of clay and moisture.

7. Permeability

Permeability may also be called the porosity of the molding sand to allow air, gases or moisture to escape in a mould when molten metal is poured into it. Because gaseous will be generate during the pouring process and solidification process, if they do not escape properly while in the mold the casting will be defective.

Permeabilty is a function of grain size, grain shape and moisture and clay contents in molding sand. The more or less firmly it is rammed will directly effect the permeability of the mold, and permeability of mold may also be increased with venting with vent rods.

8. Refractoriness

Refractoriness is the property of molding sand which permits factors of high temperature without breaking down or causing them to fuse together and aids in obtaining sound castings. Refractoriness is very important property to molding sands. Refractoriness (in chemicals) can be increased only to a limit.

Molding sand with a poor refractoriness might burn onto the casting surface and thus no smooth casting surface is possible. It depends on the SiO2 i.e. quartz content and the shape and grain size of the particle.

The more SiO2 and the rougher the grain volumetric composition for a molding sand and core sand the greater the refractoriness. Refractoriness is measure in respect to sintering of sand rather than melting.

9. Miscellaneous properties of molding sand

In addition to the above needs, molding sand will not stick to casting, nor chemically react with metal. Molding sand must be inexpensive and accessible in nature and the sand needs to be reused for economic purposes. Its coefficients of thermal expansion should not be too high.

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