Sands Used In sand casting process
Types of base sands
Base sand is the type used to make the mold or core. Because it does not have a binder it will not
bond together and is not usable in this state.
I.Silica sand:
Silica (SiO2) sand is the sand found on a beach and along river beds and is also the most
commonly used sand in casting.
Silica sand is the most commonly used sand because of its great abundance and is available at
low cost. Its disadvantages are high thermal expansion, which can cause casting defects with
high melting point metals, and low thermal conductivity, which can lead to unsound casting. It
also cannot be used with certain basic metals because it will chemically interact with the metal,
forming surface defects.
II. Olivine sand
Olivine is a mixture of orthosilicates of iron and magnesium
Olivine sand is typically used in non-ferrous foundries.
• With its thermal expansion about half of that of silica sand, makes it suitable for production
steel castings .
• But the high cost restricts its wide use
III. Chromite sand
One of the expensive base sands used in foundries, Chromite sand is a solid solution of spinel, a
group of minerals. It has a very high fusion point and thermal conductivity and low thermal
expansion and can be easily used for alloy steel casting. It has a low percentage of silica. The only
disadvantage is that Chromite sand is expensive.
IV. Zircon or Zirconium silicate
Two-third zircon oxide and one-third silica make Zircon sand one of the best casting sands for
foundries. It has an extremely high fusion point, high thermal conductivity, and low thermal
expansion. Zircon sand is really expensive and hard to find but it is still frequently used for
casting expensive alloys because of its inherent qualities.
Binders:
Binders are added to a base sand to bond the sand particles together
Clay and water
A mixture of clay and water is the most commonly used binder. There are two types of clay
commonly used: bentonite and kaolinite, with the former being the most common.
Oil
Oils, such as linseed oil, other vegetable oils and marine oils, used to be used as a binder,
however due to their increasing cost; they are not used
Resin
Resin binders are natural or synthetic high melting point gums.
Organic resins (e g , phenolic resins)
Inorganic binders (e g , sodium silicate and phosphate)
Additives:
Additives are sometimes combined with the mixture to improve: surface finish, dry strength,
refractoriness (ability of sand to withstand the temperature of liquid metal casted with break
down) and "cushioning properties
Examples:
Up to 5% of reducing agents, such as coal powder, and fuel oil, may be added to the molding
material to prevent wetting (prevention of liquid metal sticking to sand particles,), improve
surface finish, ,
Up to 3% of "cushioning material", such as wood flour, saw dust, and straw, can be added to
reduce hot crack casting defects when casting high temperature metal. They also increase
collapsibility and reduce shakeout time.[21]
Up to 2% of cereal binders, such as dextrin, starch, and molasses, can be used to increase dry
strength (the strength of the mold after curing) and improve surface finish.
Up to 2% of iron oxide powder can be used to prevent mold cracking and metal penetration,
essentially improving refractoriness.
Types of Moulding Sand:
Moulding sands may be classified, according to their use as under:
Green Sand:
It is a mixture of silica sand, with 18 to 30% clay and 6 to 8% water. The clay and water give
bonding strength to green sand.
It is slightly wet when squeezed with hand. It has the ability to retain the shape and
impression given to it under pressure.
It is easily available and has low cost.
It is commonly used for producing ferrous and non-ferrous casting
Green sand is generally used for casting small or medium sized moulds.
The mould which is prepared in this sand is called green sand mould.
Dry Sand:
After making the mould in green sand, when it is dried or baked is called dry sand.
It is suitable for making large castings.
The moulds which is prepared in dry sand is known as dry sand moulds.
If we talk about the physical composition of the dry sand, than it is same as that of the green
sand except water.
Loam sand:
Loam sand is the mixture of sand and clay with water. It contains up to 50% clay and 18% water.
This sand is used for big size casting. This sand does not use pattern and the mold made by
sweep or skeleton pattern
Facing Sand:
It is used directly next to the surface of the pattern and it comes into contact with the molten
metal. Since, it is subjected to the most severe conditions, it must possess high strength and
refractoriness. It is made of silica sand and clay, without the addition of used sand.
Different forms of carbon known as facing materials, (e.g., plumbago powder, ceylon lead or
graphite) are used to prevent the metal from burning into the sand.
Facing sand layer in a mould, usually ranges from 20 to 30 mm.
Backing Sand:
This sand is used to back up the facing sand. The total flask in poured by backing sand after
pouring facing sand. It is mainly the used sand so it has blackish in color. It is used to support
facing sand.. It is weak in bonding strength because the sharp edges of sand grain become
rounded due to high temperature of molten metal and burning of clay content.
Parting sand:
This silica sand is free from the moisture and clay content.
Parting compounds:
To get the pattern out of the mold, prior to casting, a parting compound is applied to the pattern
to ease removal. They can be a liquid or a fine powder
Common powders include talc, graphite, and dry silica; common liquids include mineral oil and
water-based silicon solutions. The latter are more commonly used with metal and large wooden
patterns.
Core Sand:
The sand which is used to make core is called core sand.
It is also called as oil sand.
It is a mixture of silica sand and core oil. Core oil is mixture of linseed oil, resin, light
mineral oil and other binding materials.
SODIUM SILICATE—CO 2 MOLDING
Molds can also be made from sand that receives its strength from the addition of 3% to 6%
sodium silicate, an inorganic liquid binder, commonly known as water glass. It remains soft
and moldable until it is exposed to a flow of CO 2 gas. It then hardens in a matter of seconds
by the reaction:
Na 2 SiO 3 + CO2 Na2CO3 + SiO2 (colloidal)
The CO2 gas is nontoxic, nonflammable, and odorless, and no heating is required to
initiate or drive the reaction. The sands achieve a tensile strength of about 40 psi (0.3
MPa) after five seconds of CO 2 gassing, with strength increasing to 100-200 psi (0.7-1.4
MPa) after 24 hours of aging. The hardened sands, however, have extremely poor
collapsibility, making shakeout and core removal quite difficult.
