What is Molding Sand? What are its Properties? (Explained!)

Molding Sand

What is Molding Sand?
Molding Sand (or foundry sand) is a type of sand which can hold its shape and it tends to pack very well when it is moistened and compressed.
In the metal casting process, Moulding sand is used for preparing the mould cavity in which the molten metal is poured.

In most of the casting operations, the silica sand (SiO2) is used as a moulding sand.
The mould sand is generally mixed with water and clay. This mixture contains around 89% sand, 7% clay and  4% water.

Having said that, let’s discuss the Properties of molding sand used in the metal casting process.

Properties of Molding Sand

13 Properties of Molding Sand (or foundry sand) are mentioned below.

  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. Cohesiveness
  11. Reusability
  12. Ease of preparation and control
  13. Conductivity

Let’s discuss each of these properties of molding sand one by one.

#1) Strength

  • Strength is one of the important properties of the moulding sand.
  • Moulding sand (or foundry sand) should have good strength in its green state, dry state and hot state.
  • The strength possessed by the sand in its green state is known as green strength. This strength is required for proper moulding of the sand during the mould preparation.
  • The moulding sand should have enough strength to resist the heat of the molten metal when the molten metal is poured in the mould cavity.
  • When the molten metal is poured in the mould cavity, it should resist the flow of the metal and it should also withstand the pressure of the metal at such high temperatures. Otherwise the mould may expand and develop cracks.
  • The strength of molding sand depends on;
    • Grain size and shape
    • Moisture content
    • Density of sand after ramming
  • If the grain size is less, then the strength of the moulding sand will be more.
  • After ramming, if the density of the sand is more, then the strength will be more.
  • If the clay content is more, then the strength will be more.
  • The strength of the sand decreases if the moisture content increases beyond the desired value.

#2) Permeability

  • The molten metal always contains some amount of dissolved gases, which are released when it solidifies.
  • Also when the molten metal is poured in the mould cavity, the moisture is released from the mould cavity because of the extreme heat of the molten metal. Some gases are released due to heating of coal dust and other similar materials.
  • It is essential to remove all these gases and moisture from the mould cavity, otherwise they will form pores and gas holes in the casting.
  • Hence the sand used in molding should have porosity which helps the gases and moisture to escape from the mould cavity.

#3) Grain size and shape

  • Grain size and grain shapes are very important properties of moulding sand which describes the application of moulding sands in various foundries.
  • The grain sizes are of three sizes;
    • Fine grains
    • Medium grains
    • Coarse grains
  • If the casting is very small and detailed, then the fine grain sand is used to prepare the mould.
  • Medium sand is used for benchmark and light floor works.
  • If the large size castings are to be prepared, then coarse sand is used.
  • Grains of fine sand are arranged very well as a closed packed structure and it also gives better surface finish on the casting.

#4) Thermal stability

  • You already know that the temperature of molten metal is very high.
  • So when the molten metal is poured in the mould cavity, the sand surrounding the molten metal should resist the extreme heat of the molten metal. Otherwise cracks may develop in the mould and the shape of the final casting will be not accurate.
  • Thermal resistance helps the sand to prevent its expansion due to extreme heat.
  • Hence, the foundry sand should have good thermal resistance.

#5) Refractoriness

  • Refractoriness is a property of a moulding sand due to which it can withstand the higher temperature of molten metal.
  • Due to high refractoriness, the moulding sand does not get fused with the molten metal.
  • Silica sand has the highest refractoriness.

#6) Flowability

  • Flowability (also known as plasticity) is very necessary for the moulding sand so that the sand can be easily rammed in the mould and it can take the perfect replica of the pattern in the mould.
  • Flowability of the moulding sand increases as the clay and the moisture content or the water proportion increases in the sand.
  • If the flowability of the moulding sand is good, then the proper replica of the pattern can be achieved in the mould.
  • Hence, flowability is one of the important properties of molding sand.

#7) Sand texture

  • Sand texture is responsible for the good surface finish, as well as it also gives the path to the moisture and gases to escape from the mold.
  • A sand texture should be such that it provides higher strength, good surface finish as well as porosity to the mould.

#8) Collapsibility

  • The moulding sand should easily collapse when the metal contracts during its solidification in the mold.
  • The moulding sand should not provide any resistance to the contraction of metal.
  • This property of molding sand is known as collapsibility.

#9) Adhesiveness

  • Adhesiveness is the property of the object due to which it sticks with the other different particles.
  • Because of the adhesive nature of the sand, it remains attached to the sandbox and the entire box can be lifted and flipped easily.
  • Thus, adhesiveness helps the sand particles to remain attached with the moulding box.

#10) Cohesiveness

  • The ability of the sand particles to remain attached with the other sand particles is known as cohesiveness.
  • In other words, sand particles stick with each other due to cohesiveness.
  • In green state, this is termed as green strength and in dry state, it is termed as dry strength.
  • Clay and moisture content present in the moulding sand also affects the cohesiveness of the mold sand.

#11) Reusability

  • In the foundry, a large amount of sand is used for making castings.
  • By reusing the used sand, the cost of sand can be minimised a lot.
  • Also the disposal problems of the sand can be prevented.

#12) Ease of preparation and control

  • The sand should lend itself to easy preparation and control by mechanical equipment.

#13) Conductivity

  • As discussed above, the molten metal has a very high temperature and the surrounding sand also gets heats up due to this extreme heat.
  • The moulding sand should have good conductivity which can help in removing the heat from the mould.

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