Properties of Moulding Sand
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| Properties of Moulding Sand |
To manufacture good casting in the sand casting process, moulding used for producing sand mould should have good properties.
I have discussed 18 different properties of moulding sand used in sand casting with diagrams in detail below.
- Permeability of moulding sand.
- Cohesiveness or strength.
- Adhesiveness.
- Green strength.
- Dry strength.
- Hot strength.
- Refractoriness.
- Plasticity.
- Flowability.
- Compatibility.
- Friability.
- Collapsibility.
- Mould hardness.
- Chemical resistivity.
- Conductivity.
- Reusability.
- Durability.
- Co-efficient of expansion.
Permeability Property
What is the permeability property of moulding sand?
Permeability of foundry sand can be defined as the property of moulding sand to allow and escape hot gasses from the mould to pass out during the casting process.
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| (a) Permeability Property |
Shown above in diagram (a) is how gas escapes between the sand grains out of the mould.
Permeability can also be defined as the "quantity" of air that passes through the standard specimen of the sand at a certain pressure under controlled conditions.
The property of sand due to which gas and steam escape from sand is called permeability.
Permeability is also called porosity.
When molten metal comes in contact with the mould, gasses such as water vapour, steam, hydrogen and carbon dioxide are generated that need to be removed from the mould.
If these gases are present in the mould they result in the formation of porosity and blowhole gas defects.
If these gases are not removed they will penetrate the molten metal giving gas defects in sand casting.
Permeability is the function of the following elements.
- Grain size.
- Moisture content.
- Type of binders such as clay.
- Quality and quantity of binders.
- Degree of compacting.
Sand casting uses a combination of fine, medium and coarse grain sizes to maintain the surface finish, porosity and permeability together.
Permeability increases by using sand grains in the following combination.
- Course shape grain.
- Angular shape grain.
- Larger grain size.
This sand combination gives better permeability as it has more space in between them for escaping gases.
Angular grains randomly oriented gives better permeability than spherical grains.
Permeability decreases by using the following combination with sand grain.
- Fine sand.
- Hard ramming of sand.
- Excess use of clay.
Fine and smaller size grains will give less permeability.
Permeability ∝ size of the grain.
Excess use of clay reduces permeability during the sand casting process.
Permeability property also depends on ramming.
Ramming is a method of compacting sand.
Hard ramming reduces overall spaces between the grains making mould more compact.
Hard ramming reduces the permeability of sand mould.
Very less permeability gives pinhole defects in sand casting.
What will happen if gaps between the sand are large?
- The strength of the mould will be reduced.
- High surface roughness as metal penetrates in two sand gaps.
- Permeability reduces allowing less gas to pass out of the mould if clay content is high.
- The surface finish of the final casting will be reduced considerably.
Factors influencing the permeability of moulding sand are as follows.
- Grain shape- Higher angular and coarser grains increase the permeability.
- Grain distribution- Sand grain will different sizes reduces permeability. Sand with the same size improves permeability.
- Grain fineness number (GFN)- Grain with higher GFN will increase permeability while small GFN will reduce the permeability.
- Additives- Additives increase permeability but after 2% moisture content permeability reduces.
- Binder- Binders such as active clay increase permeability but after 4% of clay content permeability reduces.
- Moisture- Moisture content increases permeability but up to a certain number. Above 2.5% moisture content permeability reduces.
Which test is done to determine the permeability of sand?
Permeability of moulding sand test is done to determine permeability before we start the casting process.
Cohesiveness Property
What is the cohesiveness property of moulding sand?
Cohesiveness can be defined as the ability of sand to hold itself together during the mould-making and casting process.
Cohesiveness is also called strength.
The cohesiveness of moulding sand property depends upon the following factors-
- Binders used.
- Moisture content.
- Sand ramming.
- Sand density.
- Shape and size of a sand grain.
If the strength property of moulding sand is low mould will not hold itself together and fall during mould making as mould is turned, inverted and moved to make cope and drag.
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| (b) Cohesiveness Property Diagram |
As shown above, in diagram (b) sand falls from the mould as it has less cohesiveness or strength properties.
The cohesiveness property is also important when the pattern is removed from the mould and when molten metal flows in the mould.
If the strength of the mould is low mould cavity will expand due to metallostatic forces acting on the mould resulting in oversized casting.
Denser the mould with binders (clay) and course grains better the cohesiveness property.
Finer grain gives less strength to the mould.
Adhesiveness Property
What is the adhesive property of moulding sand?
The ability and property of the sand mixture to stick/cling to the flask or mould box during the sand moulding process is called the adhesiveness of sand.
If sand does not have adhesiveness moulding sand will fall from the flask.
The mould is turned and inverted when making a mould to make cope and drag.
Moulding sand should adhere or cling to the flask and not get separated during this process.
Sand should adhere to the mould and should not fall during operations of making cope and drag mould.
Adhesiveness is highly dependent on the type and quantity of binder used in the sand mixture.
Cohesiveness vs. Adhesiveness of moulding sand.
| Cohesiveness | Adhesiveness |
|---|---|
| Cohesiveness is the property of sand to stick together to achieve strength for the mould. | Adhesiveness is the property of sand to cling/stick to the flask during the mould-making process. |
Green Strength Property
What is the green strength property of moulding sand?
Note:- The name green strength comes from the fact that mould still has moisture.
There is no connection between the colour green and the colour of moulding sand.
Sand that has moisture in it is simply called green sand.
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| (c) Green Strength Property |
As shown above, diagram (c) of green strength mould.
While mould making and before pouring molten metal this strength is very important.
Green strength can be defined as the ability of sand to retain its shape when moisture is present in the mould during the mould-making process.
Mould is made up of sand, moisture and clay. All these three important elements hold the mould together.
If we add the same quantity of clay and binder the green strength with fine grain size has higher strength as compared to green sand with a coarser structure. Thus, the more the GFN more the green strength.
Round grains give better green strength compared to angular grains.
Muller is used for mixing sand, clay and moisture together.
The more the mulling time more we get greenth strength, and after some point, there is no significant change in the green strength.
Which test is done to determine the green strength of moulding sand?
Green compression strength is used to determine the green strength of the moulding sand.
Dry Strength Property
What is the dry strength property of moulding sand?
Dry strength can be defined as the property of sand to withstand strength after moisture has been evaporated after molten metal comes in contact with the sand.
This causes mould to be dry called dry strength.
As shown below diagram (d) of dry strength where all moisture is evaporated from the mould and the moulding sand can hold together even at a higher temperature.
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| (d) Dry Strength Property |
A mould having dry strength should retain its shape and should be able to withstand the erosion of sand during the pouring operation.
When metal is poured into the mould metal gets in contact with the mould immediately evaporating all moisture from the sand.
At this point, the entire mould dries up.
The ability of mould to withstand itself when mould gets dried completely without moisture is called dry strength.
Dry strength can be measured in two ways-
- Dry strength during compression.
- Dry strength during shearing.
Which test is done to determine the dry strength of moulding sand?
A dry compression strength test is done to determine the dry strength of moulding sand.
Hot Strength Property
What is the hot strength property of moulding sand?
After dry strength, we are going to learn the hot strength of the moulding sand.
When metal is poured into the mould all moisture escapes and the mould starts getting heated up resulting in the mould being dry.
Mould at this point should be able to withstand high temperatures till the solidification of the entire casting.
Hot strength can be defined as the ability of mould to bear the heat of molten metal when the metal starts heating the mould at a higher temperature is called hot strength.
Hot strength can be increased by mixing additives like pitch.
Shown below in diagram (e) strength of mould at high temperatures.
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| (e) Hot Strength Property |
Difference between green strength, dry strength and hot strength properties of moulding sand.
| Green Strength | Dry Strength | Hot Strength |
|---|---|---|
| Green strength is the ability of sand to retain its strength, structure and shape while a mould is freshly made and full of moisture. | Dry strength is the strength of the mould after all moisture evaporates after molten metal is poured into the green mould. | Hot strength is the strength of mould after mould gets heated up and is at peak temperature. |
Refractoriness Property
What is the refractoriness property of moulding sand?
When molten metal is poured into the mould the metal is at a high temperature.
The ability of the moulding sand mixture to withstand the high temperature of the molten metal without fusing it to the high temperature of the metal is called refractoriness.
The refractory property ensures that sand does not burn, crack and melt when the molten metal comes in contact with the moulding sand.
Shown below in diagram (f) is how moulding sand fails and fuses to sand grain if sand does not have refractoriness properties.
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| (f) Refractoriness Property |
If moulding sand lags refractoriness molten metal will be fused to the moulding sand giving surface defects to the final casting.
High temperatures can cause cracks, burns and thermal deformation in the sand not having refractoriness properties.
Refractoriness is measured in sintering point rather than melting point.
Refractoriness is dependent on the size and shape of the sand, SiO2 (quartz) and purity of sand particles.
Compatibility Property
What is the compatibility property of moulding sand?
Compatibility can be defined change in the height of the loose mass of sand during the controlled compacting.
As shown below diagram (g) compatibility of moulding sand. Sand should be able to get compacted by the ram force acting on it.
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| (g) Compatibility Property of Moulding Sand |
The compatibility property indicates the water-tempering degree of the green sand mould.
Higher compatibility improves dimensional value and gives a better surface finish.
Wet sand will compact more than dry sand.
Compatibility is measured in percentage (%).
Less compatibility will give oversized casting, cuts and wash defects will cause the metal to penetrate the moulding sand and result in roughcasting.
Factors influencing compatibility are as follows.
- Mixing time in a muller.
- Quality of clay.
- Additive used.
- Moisture content.
Which test is done to determine the compatibility of moulding sand?
The compatibility of moulding sand test is done to determine the compatibility of moulding sand.
Plasticity Property
What is the plasticity property of moulding sand?
Plasticity can be defined as the ability to mould sand to retain its shape around the pattern once the pattern is taken out.
Moulding sand that has insufficient plasticity will cause a change in impression caused by pattern.
Flowability Property
What is the flowability property of moulding sand?
When molten metal is poured into the mould, sand should flow smoothly as if water is flowing. The ability of the sand to flow in all parts of the mould and on the pattern flawlessly is called flowability.
Sands having good flowability properties will give a uniform density of mould and a better impression of pattern in the sand.
The flowability of sand plays an important role in machine moulding.
Factors influencing flowability are as follows.
- Sand shape- Round grain size increases flowability.
- Additives- Additives reduce flowability.
- Clay content- Clay in the moulding sand decreases flowability.
Difference between plasticity and flowability property of moulding sand.
| Plasticity of moulding sand. | Flowability of moulding sand. |
|---|---|
| Plasticity is the ability of sand to retain its shape and structure after the pattern is taken out. | Flowability is the ability of sand to "flow" on the pattern to form a cavity, shape and structure. |
Durability Property
What is the durability property of moulding sand?
During sand casting, sand moulds have to go through cycles of heating and cooling.
The ability of sand to withstand these hot and cool cycles during casting operations is called the durability property.
This sand has durable properties that can be reused again to make the mould again.
Friability Property
What is the friability property of moulding sand?
The ability of moulding sand to crumble or get crushed after the solidification of the casting is done is called friability.
Friability is an important property of core sand as sand cores need to be crumbled or crushed to separate casting from the sand.
Additives such as dextrin reduce friability. This property becomes important when casting sections are thin and fragile and it becomes important to separate delicate parts of casting from the sand without applying extensive pressure on the casting sections.
Collapsibility Property
What is the collapsibility property of moulding sand?
Collapsibility can be defined as the ability of moulding sand to decrease its volume during the shrinkage of casting due to compressive forces acting on it during the solidification process.
Collapsibility can also be defined as the ability of sand to collapse in simple words.
Collapsibility is also important when the solidified casting is to be taken out of the mould.
When the final casting is solidified mould needs to be broken quickly to remove the casting.
The ability of sand to collapse/break after the casting is taken out of the mould is called the collapsibility of sand.
Collapsibility is exactly the opposite property of cohesiveness.
Collapsibility is important during shake-out and knock-out operations during solidification and while removing the final casting out of the mould.
The collapsibility property is also essential for cores as cores are compressed by compression forces from all directions.
Collapsibility property depends on the type and amount of quartz sand and binder used.
Additives such as wood floors increase the collapsibility of the moulding sand.
Lack of collapsibility results in the formation of cracks in the casting while solidifying.
Cracks are also formed during the removal of solidified casting from mould in thinner casting sections.
Sand should retain cohesiveness while removing patterns but should also have collapsibility properties while removing the final casting out.
| Compatibility | Collapsibility |
|---|---|
| The compatibility of sand is the property of sand to compact during the mould-making process. | Collapsibility property is the property of sand to compress itself while compressive forces act on it in the mould during the solidification of molten metal. |
Hardness Property
What is the hardness property of moulding sand?
Once the shape is given to the mould cavity by pattern mould should resist any inadvertent and unwanted deformation.
The ability of sand to resist any inadvertent and unwanted deformation is called mould hardness or hardenability of sand.
Hardness depends upon clay content, moisture in sand and ramming.
The ability of mould to hold hardness is called hardenability.
The hardness of mould is measured by BHN test.
Which test is done to determine the hardness of moulding sand?
A hardness test is done to determine the hardness of moulding sand.
Reusability Property
What is the reusability property of moulding sand?
The ability of moulding sand to be reused is called reusability.
This sand is taken from the broken mould and reused as backing sand in the mould to support the mould.
This property is important as it reduces the cost of manufacturing sand casting in the foundry as sand is reused which is one step forward towards keeping the environment friendly.
In the sand-resin casting process resin is separated from the sand and reused again for manufacturing castings.
Chemical Resistivity
What is the chemical resistivity of moulding sand?
The ability of moulding sand to not react chemically with molten metal is called chemical resistivity.
If sand reacts with the molten metal it will cause surface defects on the final casting surface.
Conductivity Property
What is the conductivity property of moulding sand in the sand-casting process?
The ability of sand to remove heat during the sand-casting process is called conductivity.
Sand moulds should have a conductive property to remove the heat from the mould during the casting process during the solidification operation of the casting to achieve directional solidification of the molten metal in the mould.
This property is important for the structural grain growth of the metal during the solidification process during the sand casting process and the lost form casting process.
Coefficient Of Expansion Property
What is the coefficient of expansion property of moulding sand?
The coefficient of expansion of sand should be small to promote accurate sand casting.
If moulding sand expands there will be a change in the dimension of the mould cavity and the position of the core in the mould changing the accuracy of the final sand casting.
In order to produce good sand-casting, moulding sand should have a good coefficient of expansion characteristics and properties.
Properties Of Moulding Sand Summary
The properties of moulding sand are permeability, cohesiveness, adhesiveness, green strength, dry strength, hot strength, refractoriness, plasticity, flowability, compatibility, friability, collapsibility, mould hardness, chemical resistivity, conductivity, reusability, durability and co-efficient of expansion property.
It becomes essential to study the properties of sand and the factors influencing these properties which I have explained above.
Having good properties of moulding sand results in good defect-free final casting.