Moulding Sand | Silica Sand | Foundry Sand

Moulding Sand:

Most moulding sand casting operations use Silica sand (SiO₂). Usually, sand used to manufacture a mould for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be 89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when manufacturing parts by sand casting.

Molding Sand Constituents and Composition

Molding sand is composed mostly of silica sand, a binder, moisture content, and additives, among other things.

Silica sand

Silicon sand in the form of granular quartz is the most important ingredient of moulding sand. Silica sand’s high thermal conductivity and refractoriness allow it to provide strength, durability, and porosity to the moulding and core sand. However, minor quantities of iron oxide, alumina, lime stone (CaCO3), magnesia, soda, and potash are present as impurities in addition to silica.

The chemical composition of silica sand provides an indication of the presence of impurities such as lime, magnesia, alkalis, and other elements. Extremely high concentrations of iron oxide, alkali oxides, and lime may significantly reduce the fusing point, which is undesirable. Specifications for silica sand may be determined by the sand particle size, as well as the form (angular, sub-angular, and rounded) of the silica sand.

Binder

Binders are substances that may be either inorganic or organic in nature. Clay sodium silicate and portland cement, among other inorganic binders, are examples of inorganic binders. In a foundry shop, the clay serves as a binder, and the types of clay used include Kaolinite, Ball Clay, Fire Clay, Limonite, Fuller’s earth, and Bentonite, among others.

Dextrin, molasses, grain binders, linseed oil, and resins such as phenol formaldehyde, urea formaldehyde, and other formaldehyde derivatives are examples of organic binders. The majority of organic group binders are utilised in the core-making process. Among all of the binders listed above, the bentonite form of clay is the most frequently seen.

However, without the presence of moisture content in the moulding sand and core sand, this clay will not be able to form bonds between sand grinning on its own.

Moisture Content

The quantity of moisture present in the moulding sand ranges between 2 and 8 percent. This quantity is added to a clay and silica sand combination in order to aid in the development of bonding. This is the quantity of water necessary to completely fill the pores between the particles of clay without separating them from one another. A large quantity of water is kept tightly in place by the clay, and it is this amount of water that is primarily responsible for producing the strength in the sand.

The combined action of clay and water reduces permeability as the amount of clay and moisture in the soil increases. Initially, the green compressive strength improves as the amount of clay in the soil increases, but beyond a certain point, the strength begins to decline. In order to improve the qualities of moulding sand, various materials other than the basic ingredients are added, and these extra materials are referred to as additives.

Additives

Generally, additives are elements that are added to the moulding and core sand combination in order to give the sand a certain feature that is not present in the sand alone. For example, coal dust, maize flour, dextrin, sea coal, pitch, wood flour, and silica flour are all regularly used additions in moulding and core sands to improve the qualities of the sand.

Coal dust

The addition of coal dust is mostly for the purpose of creating a reducing environment during the casting process. As a consequence of the decreasing environment, any oxygen present at the poles becomes chemically bonded, rendering it incapable of oxidising the metal. It is often used in the manufacturing of grey iron and malleable cast iron castings, where it is added to the moulding sands used in the mold-making process.

Corn flour

Molding and core sand are made more collapsible by adding corn flour, which is a starch family of carbohydrates. Corn flour is utilised to achieve this effect. It is entirely volatilized by the heat generated in the sand mould, resulting in the formation of space between the sand grains. This enables for the free movement of sand grains, which ultimately results in mould wall movement and a reduction in mould expansion, as well as a reduction in casting fault rates. When corn sand is mixed with moulding sand and core sand, the strength of the mould and core is considerably increased.

Dextrin

A member of the starch family of carbohydrates, dextrin acts in a way similar to that of maize flour in terms of its behaviour and appearance. The dry strength of the moulds is increased by the addition of dextrin.

Sea coal

Sea coal is a fine powdered bituminous coal that finds its way into the pores of the silica sand grains in moulding sand and core sand, where it fills in the gaps between the grains. When sea coal is heated, it transforms into coke, which plugs the pores and is unaffected by moisture.

As a result, the sand grains become constrained and are unable to migrate into a dense packing arrangement as a result. As a result, sea coal lowers the movement of the mould wall and the permeability of the mould and core sand, resulting in a clean and smooth surface on the mould and core surface.

Pitch

Pitch is a soft coal that has been distilled. It may be used in mould and core sand in concentrations ranging from 0.02 percent to 2 percent. When heated, pitch improves hot strength and surface polish on mould surfaces, and it performs in a way that is identical to that of sea coal.

Wood flour

In wood flour, a fibrous element, such as wood, is combined with a granular ingredient, such as sand. Wood flour is made up of long, thin fibres that prevent the sand grains from coming into touch with one another and forming a ball. In mould and core sand, wood flour may be used in concentrations ranging from 0.05 percent to 2 percent.

When heated, wood flour volatilizes, causing the sand grains to expand and become more visible. Wood flour will enhance the mobility of the mould wall and reduce the number of expansion faults. Wood flour also improves the collapsibility of both the mould and the core of the mould.

Silica powder, also known as Silica flour

Pulverized silica is another name for silica flour. Pulverized silica may be simply added up to 3 percent, which boosts the hot strength of the moulds and cores as well as the finish on the mould and core surfaces. It also has the additional benefit of reducing metal penetration through the walls of the moulds and cores.

Natural Sand Alternative name for Green sand, and it is obtained via the use of natural resources. The only thing that holds everything together is water. Among its many advantages are the ability to keep moisture content for an extended period of time, a broad working range of moisture content, and the ease with which moulds may be patched and finished.

In addition to that, there are other types of specialized sands, such as Zirconite and Olivin, among others. They are more costly than silica, hence they are only utilised in situations when their usage is warranted by the circumstances.

Characteristics of Moulding Sand:

Important characteristics are:

1. These sands are refractory in nature and can withstand temperature of the metal being poured, without fusing.

2. The moulding sands do not chemically react or combine with molten metal and can therefore be used repeatedly.

3. The sands have a high degree of permeability and thus allow the gases formed during pouring to escape.

4. The strength, permeability and hardness of the sand mix can be varied by changing the structure or ingredients of sand.

Properties of moulding sands:

The important properties are:

1. Strength
2. Permeability
3. Grain Size and Shape
4. Thermal stability
5. Refractoriness
6. Flow ability
7. Sand Texture
8. Collapsibility
9. Adhesiveness
10. Reusability
11. Easy of preparation and control
12. Conductivity

1. Strength

· The sand should have adequate strength in its green, dry and hot states

· Green strength is the strength of sand in the wet state and is required for making possible to prepare and handle the mould.

· If the metal is poured into a green mould the sand adjacent to the metal dries and in the dry state it should have strength to resist erosion and the pressure of metal.

· The strength of the sand that has been dried or basked is called dry strength

· At the time of pouring the molten metal the mould must be able to withstand flow and pressure of the metal at high temperature otherwise the mould may enlarge, crack, get washed or break

· Strength of the moulding sand depends on:

1. Grain size and shape

2. Moisture content

3. Density of sand after ramming

· The strength of the mould increases with a decrease of grain size and an increase of clay content and density after ramming. The strength also goes down if moisture content is higher than an optimum value.

2. Permeability

· The moulding sand must be sufficiently porous to allow the dissolved gases, which are evolved when the metal freezes or moisture present or generated within the moulds to be removed freely when the moulds are poured. This property of sand is called porosity or permeability.

3. Grain size and shape

· The size and shape of the grains in the sand determine the application in various types of foundry. These are three different sizes of sand grains.

1. Fine

2. Medium

3. Coarse

· Fine sand is used for small and intricate castings. Medium sand is used for benchmark and light floor works. If the size of casting is larger coarse sand is used

· Sand having fine, rounded grains can be closely packed and forms a smooth surface. Although fine-grained sand enhances mould strength.

4. Thermal stability

· The sand adjacent to the metal is suddenly heated and undergoes expansion. If the mould wall is not dimensionally stable under rapid heating, cracks, buckling and flacking off sand may occur.

5. Refractoriness

· Refractoriness is the property of withstanding the high temperature condition moulding sand with low refractoriness may burn on to the casting

· It is the ability of the moulding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the Silica sand is highest.

6. Flowability

· Flowability or plasticity is the property of the sand to respond to the moulding process so that when rammed it will flow all around the pattern and take the desired mould shape. High flowability of sand is desirable for the sand to get compacted to a uniform density and to get good impression of the pattern in the mould.

· Flowability is also very important in machine moulding

· Flowability of sand increases as clay and water content are increased.

7. Sand texture

· As mentioned earlier the texture of sand is defined by its grain size and grain size distribution.

· The texture chosen for an application should allow the required porosity, provide enough strength and produce the desired surface finish on the casting.

8. Collapsibility

· The moulding sand should collapse during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the moulding material should be cheap, reusable and should have good thermal conductivity

9. Adhesiveness 

· It is the important property of the moulding sand and it is defined as the sand particles must be capable of adhering to another body, then only the sand should be easily attach itself with the sides of the moulding box and give easy of lifting and turning the box when filled with the stand.

10. Reusability 

· Since large quantities of sand are used in a foundry it is very important that the sand be reusable otherwise apart from cost it will create disposal problems

11. Easy of preparation and control 

· Sand should lend itself to easy preparation and control by mechanical equipment

12. Conductivity 

· Sand should have enough conductivity to permit removal of heat from the castings..

Types of Moulding Sands:

According to their types of moulding sands use in the foundry, moulding sands are classified into following categories:

1. Foundry Green sand

• It is a sand used in wet condition for making the mould. It is a mixture of silica sand with 15-25 per cent clay and 6-8 per cent water
• As explained earlier, green sand moulds are not dried, when the metal poured in them in the wet condition
• Being damp the sand can be easily worked with hand to give it any desired shape
• This sand is used for producing small to medium sized moulds which are not very complex

2. Dry sand

• Dry sand is the green sand that dried or baked after preparing the mould.
• Drying sand gives strength to the mould so that it used for larger castings

3. Loam sand

• Loam sand containing up to 50 % clay which worked to the consistency of builder mortar.
• This sand used for moulds for making very heavy castings usually with the help of sweeps and skeleton patterns.

4. Parting sand

• This sand used during making of the mould to ensure that, green sand does not stick to the pattern. The cope and drag parts easily separated for removing the pattern without causing any damage to the mould.
• Parting sand consists of fine grained clay free dried silica sand, sea sand or burnt sand with some parting compounds.
• The parting compounds used include charcoal, ground bone and limestone, groundnut shells, talc and calcium phosphate.

5. Facing sand

• Facing sand is the sand which covers the pattern all around it. The remaining box filled with ordinary floor sand.
• Facing sand forms the face of the mould and comes in direct contact with the molten metal when it poured.
• High strength and refractoriness required for this sand.
• It made of silica sand and clay without the addition of any used sand.
• Graphite, mollases, plumbago etc. added additionally to the facing sand. Thickness of the sand layer varies from 20 to 30 mm.

6. Backing sand 

• Backing sand is the bulk sands, used to back up beside the facing sand and to fill up remaining volume of the flask.
• It consists mainly of old, repeatedly used moulding sand which is generally black in colour due to addition of coal dust and burning on contact with hot metal.
• Because of the colour, backing sand also called as black sand.
• The main purpose for the use of backing sand is to reduce the cost of moulding.

7. System sand

• This is the sand used in mechanized foundries for filling the entire flask.
• No separate facing sand in used in a mechanized foundry.
• Sand, cleaned and reactivated by the addition of water and binders used to fill the flask. Because of the absence of any fresh sand, system sand must have more strength, permeability and refractoriness compared to backing sand.

8. Core sand

• Core sand is the sand used for making cores. This is silica sand mixed with core oil. That is why it is also called as oil sand.
• The core oil consists of linseed oil, resin, light mineral oil with some binders.
• For larger cores, sometimes pitch / flour and water used in saving the cost.