In this article we are going to learn about What is Moulding sand, Properties of moulding sand and types of moulding sand.
The principal material used in the foundry shop for moulding is the sand. This is because it possesses the properties vital for foundry purposes.
All sands are formed by the breaking up of rocks due to the action of natural forces such as frost, wind, rain, heat and water currents. Rocks, however, are very complex in their composition, and sands contain most of the elements of the rocks of which they are fragments. For this reason, moulding sands in different parts of the world vary considerably. Today, sand is obtained from places which probably once were bottoms and banks of rivers and sand dunes.
In India, foundry sands are found in Damodar and Barakar area, Santhal Pargana (Bihar), Batala (Gurdaspur, Punjab), Bhavnagar (Saurashtra), Londha (Maharashtra), Avadi and Veeriyambakam (Madras), Kanpur, Jabalpur, Rajkot, Guntur, in Ganges and many other places.
The principal ingredients of moulding sands are : (1) silica sand grains (2) clay (3) moisture and (4) miscellaneous materials.
Silica in the form of granular quartz, itself a sand, is the chief constituent of moulding sand. Silica sand contains from 80 to 90 per cent silicon dioxide and is characterized by a high softening temperature and thermal stability.
It is a product of the breaking up of quartz rocks or the decomposition of granite, which is composed of quartz and feldspar. The feldspar, when decomposed, becomes clay (hydrous aluminium silicate). However, silica sand grains impart refractoriness, chemical resistivity, and permeability to the sand. They are specified according to their average size and shape.
Clay is defined as those particles of sand (under 20 microns in diameter) that fail to settle at a rate of 25 mm per minute, when suspended in water. Clay consists of two ingredients : fine silt and true clay. Fine silt is a sort of foreign matter or mineral deposit and has no bonding power.
It is the true clay which imparts the necessary bonding strength to the mould sand, so that the mould does not lose its shape after ramming. True clay is found to be made up of extremely minute aggregates of crystalline, usually flake-shaped, particles called clay minerals. Most moulding sands for different grades of work contain 5 to 20 per cent clay, Moisture, in requisite amount, furnishes the bonding action of clay.
When water is added to clay, it penetrates the mixture and forms a microfilm which coats the surface of flake-shaped clay particles. The bonding quality of clay depends on the maximum thickness of water film it can maintain. The bonding action is considered best if the water added is the exact quantity required to form the film. On the other hand, the bonding action is reduced and the mould gets weakened if the water is in excess. The water should be between 2 and 8 per cent.
Miscellaneous materials that are found, in addition to silica and clay, in moulding sand are oxide of iron, limestone, magnesia, soda, and potash. The impurities should be below 2 per cent.
Moulding sands types : It may be classified generally into three different types : (1) natural moulding sands, (2) synthetic or high silica sands, and (3) special sands.
1. Natural Moulding Sands
Natural moulding sands, called green sands, are taken from river beds or are dug from pits. They posses an appreciable amount of clay which acts as a bond between the sand grains and are used as received with water added. The quantity and type of clay mineral present affect the strength, toughness and refractoriness of the sand.
These are also obtained by crushing and milling soft yellow sandstone, carboniferous rocks, etc. During the milling operation, clay aggregates break down and clay particles get uniformly distributed over the sand grains. The grain shape of these sands is required to be sub angular to round.
Due to their ease of availability, low cost, and high flexibility of operation natural moulding sands are used for most of the ferrous and nonferrous light castings. The requirements of these sands are satisfied by IS: 3343-1965, which has classified them into three grades A, B and C, according to their clay content and sintering temperature.
2. Synthetic Sands
Synthetic sands are basically high silica sands containing little (less than 2 per cent) or no binder (clay) in natural form. They occur as loose or poorly consolidated deposits of sedimentary origin, dunes blown inland from the coast or accumulated deposits in estuaries and rivers along the coast.
They are also made in foundry by first crushing quartzite sandstones and then washing and grading these to yield a sand grade of requisite shape and grain distribution. The desired strength and bonding properties of these sands are developed by separate additions such as bentonite, water and other materials.
This allows greater flexibilities in the content of properties such as green and dry strength, permeabilities, and others that can be easily varied at will. In fact, therefore, synthetic sands are more expensive than natural sands.
IS: 1987-1974 covers the requirements of high silica sand for use in foundries and classifies it into three grades according to the silica content .
3. Special Sands
Special sands are ideal in getting special characteristics, which are not ordinarily obtained in other sands. Zircon, olivine, chamotte, chromite, and chrome-magnesite are often used as special sands. Zircon sands are suitable for cores of brass and bronze castings. Some foundries use olivine sand for nonferrous castings of an intricate nature. Chamotte is valuable for heavy steel castings.
Chromite and chrome-magnesite sands are particularly useful where the chilling tendency is to be increased to control solidification. They are also suitable as facing materials in moulds for steel castings.
Types of Moulding Sand
Moulding sands may again be classified, according to their use, into number of varieties. These are described below.
Green Sand Moulding
It is a mixture of silica sand with 18 to 30 per cent clay, having a total water of from 6 to 8 per cent. The clay and water furnish the bond for green sand. It is fine, soft, light, and porous. Being damp, when squeezed in the hand, it retains the shape, the impression giver to it under pressure. Moulds prepared in this sand are known as green sand moulds.
Dry Sand Moulding
Green sand that has been dried or baked after the mould is made is called dry sand. They are suitable for larger castings. Moulds prepared in this sand are known as dry sand moulds.
Loam sand is high in clay, as much as 50 per cent or so, and dries hard. This is particularly employed for loam moulding usually for large castings.
Facing sand forms the face of the mould. It is used directly next to the surface of the pattern and it comes into contact with the molten metal when the mould is poured. Consequently, it is subjected to the severest conditions and must possess, therefore, high strength and refractoriness.
It is made of silica sand and clay, without the addition of used sand. Different forms of carbon are used to prevent the metal from burning into the sand. They are sometimes mixed with 6 to 15 times as much fine moulding sand to make facings.
The layer of facing sand in a mould usually ranges from 20 to 30 mm. From 10 to 15 per cent of the whole amount of moulding sand used in the foundry is facing sand. A facing sand mixture for green sand moulding of cast iron may consist of 25 per cent fresh and specially prepared sand, 70 per cent old sand, and 5 per cent sea coal.
Backing sand or floor sand is used to back up the facing sand and to fill the whole volume of the flask. Old, repeatedly used moulding sand is mainly employed for this purpose.
The backing sand is sometimes called black sand because of that old, repeatedly used moulding sand is black in color due to the addition of coal dust and burning on coming in contact with molten metal.
In mechanical foundries where machine moulding is is employed a so-called system sand is used to fill the whole flask. In mechanical sand preparation and handling units, no facing sand is used. The used-sand is cleaned and reactivated by the addition of water binders and special additives. This is known as system sand. Since the whole mould is made of this system sand the strength, permeability and refractoriness of the sand must be higher than those of backing sand.
Parting sand is used to keep the green sand from sticking to the pattern and also to allow the sand on the parting surface of the cope and drag to separate without clinging. This is clean clay-free silica sand which serves the same purpose as parting dust.
Sand used for making cores is called core sand, sometimes called oil sand. This is silica sand mixed with core oil which is composed of linseed oil, resin, light mineral oil and other binding materials. Pitch or flours and water may be used in large cores for the sake of economy.
Properties of Moulding Sand
What are the properties of moulding sand ?
Proper moulding sand must possess six properties. Moulding sand properties are : It must have porosity, flowability, collapsibility, adhesiveness, cohesiveness or strength, and refractoriness.
- Porosity :- The most important property of moulding sand is porosity, also known as permeability. It refers to the moulding sand’s ability to allow gasses to pass through. During the pouring of molten metal into the sand cavity, gases and steam are produced. This property is affected not only by the shape and size of the sand particles, but also by the amount of clay, binding material, and moisture in the mixture.
- Flowability :- The ability of the sand to fill small gaps in the pattern. High flowability produces a more precise mold and is thus useful for detailed castings.
- Collapsibility :- The sand mixture’s ability to collapse under force. Greater mold collapsibility allows the metal casting to shrink freely as it solidifies, avoiding hot tearing or cracking.
- Adhesiveness :- Adhesiveness is the property of sand that causes sand particles to adhere to the sides of the moulding box. The sand’s adhesion allows for the proper lifting of the cope along with the sand.
- Cohesiveness :- Sand’s cohesiveness is the ability of its particles to stay together. It is the tensile strength of the moulding sand. This property is critical in retaining the intricate shapes of the mold. Inadequate strength may cause the mould particles to collapse during handling, turning over, or closing. The addition of clay and bentonite improves the cohesiveness.
- Refractoriness :- Sand’s refractoriness refers to its ability to withstand high temperatures of molten metal without fusion or softening. Moulding sands with low refractoriness may catch fire when molten metal is poured into the mold. Sand moulds should typically be able to withstand temperatures of up to 1650°C.
- Strength :- The strength of stand that has been dried or baked is called dry strength. It must have then strength to withstand erosive forces you to molten metal, and retain its shape.
Sand Moulding Process
Sand casting, also known as sand moulding casting or Sand Moulding Process, is a casting-based manufacturing process that uses a sand mould. Metal items and components of various sizes and forms are made with it. To put its prevalence into perspective, sand casting accounts for over 60% of all metal castings. The six major phases of sand casting are described in detail below.
Step 1 : Put the mould pattern in the sand
The placing of the mould pattern in sand is the initial stage in sand casting. The mould has a direct impact on the size and shape of the casting. As a result, manufacturers must develop new moulds in order to produce metal items and components in specified sizes and shapes.
Step 2 : Establish a Gating System
A gating system is used in almost every casting process, and sand casting is no exception. It funnels the molten mould into the mould cavity using a pouring cup and tunnels or “gates” to the mould. Manufacturing businesses will put up a gating system like this after setting the mould design in sand.
Step 3 : Mold Pattern Should Be Removed
Manufacturing enterprises can then remove the mould pattern from the sand after the gating system is in place. At this time, the mould pattern is irrelevant. The sand takes the shape of the mould pattern when it is placed inside it. As a result, it is possible to remove the mould pattern.
Step 4 : Pour molten metal into the cavity of the mould
It’s now time to pour the molten metal into the cavity of the mould. Iron, steel, aluminium, bronze, magnesium, zinc, and tin are just a few of the metals and alloys that can be cast in sand. Manufacturing organizations may need to heat metals or alloys up to 3,000 degrees Fahrenheit, depending on the exact metal or alloy employed. The metal or alloy is poured into the mould cavity once it has transformed from a solid to a liquid condition.
Step 5 : Wait for the metal to cool down
Manufacturing companies must wait for the molten metal to cool once it has been put into the mould hole. Again, different types of metal require varied amounts of cooling time. The molten metal will cool and transition from a liquid to a solid form.
Step 6 : Break Open the Mold to Remove the Metal Casting
Breaking open the mould to extract the newly generated metal casting is the sixth and last step in sand casting. Mold patterns are usually reusable, but moulds themselves are not. As a result, each time a manufacturing company wants to make a new metal product or component using sand casting, they must develop a new mould.
Collapsibility of Moulding Sand
Collapsibility of Moulding Sand refers to the sand’s ability to be easily removed from the casting once it has solidified. Sands with a low collapsibility will stick to the casting tenaciously. A sand with poor collapsibility will induce cracking and hot tearing in metal castings that flex a lot while cooling or have long freezing temperature ranges. To improve collapsibility, certain additives might be introduced.
1. Permeability can be defined as the property of moulding sand
A. to hold sand grains together
B. to allow gases to escape easily from the mould
C. to withstand the heat of melt without showing any sign of softening
D. none of the above
Answer: Option B
2. Why should the moulding sand be porous ?
A. For gases to enter
B. For water to enter
C. For gases to escape
D. For water to escape
Answer: Option C
Explanation : A moulding sand is defined as a sand that has holes in it and can contain a certain amount of moisture. A moulding should contain pores and be porous so that gases can escape freely.
This was all about Moulding Sand where we have covered all the topics such as What is Mouldind Sand, describe properties of moulding sand, green sand moulding process, dry sand, Collapsibility and some MCQs based on these topics.
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