Properties of Mouldings Sands

Refractoriness: Refractoriness is a property of moulding sand due to which it can withstand high temperature of molten metal without fusing and burning.

Permeability: Permeability is ability of moulding sand to escape vapour and gases formed during pouring the molten metal into the mould cavity. Due to lack of permeability there may be casting defects like blowhole, porosity, and pinholes.

Flowability or Plasticity: Flowability is that property of moulding sand due to which it flows uniformly into the moulding box during ramming.

Adhesiveness: Adhesiveness is the adhering ability of the sand particles to other materials due to which the heavy sand mass is successfully held in a moulding flask without any danger of its falling down.

Cohesiveness: This is a property of sand particles due to which it binds together firmly so that it can easily be withdrawn from moulding box without damage to the mould surfaces and edges.

Collapsibility: It is property of the moulding sand due to which mould collapses automatically after solidification of the casting to allow free contraction of the metal.

Binders Used in Moulding Sand

Various types of binders used in moulding sand are given as below:

  • Clays: Fire clay 2SiO2 · Al2O3 · 2H2O.
  • Southern bentonite: Al1.67 Mg0.33 Ca0.35, O3 · 4SiO2 · 2H2O.
  • Western bentonite: Al1.67 Mg0.33 Na0.33, O3 4SiO2 · H2O.
  • Secondary mica clays: K2O · 6SiO2 · 3Al2O3 · 2H2O.
  • Oils: (Hardened by baking.) Vegetable oil (linseed oil), marine animal (whale oil), and mineral oil (used as diluting oil).
  • Synthetic resins (thermosetting plastic): Urea formaldehyde, phenol formaldehyde.
  • Cereal binders made from corn: Gelatinized starch (made by wet milling contains starch and gluten), gelatinized corn flour (made by dry milling), and dextrin (made from starch, a water soluble sugar).
  • Wood products binder: Natural resin (e.g., rosin, thermoplastic), sulphite binders (contain lignin, produced in paper-pulp process), water soluble gums, resins, and organic chemicals.
  • Protein binders (contains nitrogen): Glue, casein.
  • Other binders: Portland cement, pitch (coal tar products), molasses, cements, sodium silicate (water glass, CO2).
  • Additives used in moulding sands are coal dust, sea coal, cereals or corn flour, silica flour, wood flour, pitch, dextrin and molasses, and fuel oil.

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