Sand Casting Materials
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| Sand Casting Material |
Sand mould materials are used in the sand moulding process for producing sand mould. Sand moulds are made from silica sand, binders, moisture and additives mentioned below.
The entire process of making sand mould takes place in a foundry under skilled experienced operators to produce good quality green, dry sand skin-dried sand moulds for manufacturing sound casting.
Material Used For Making Sand Mould
- Mould sand.
- Binders.
- Additives.
Mould sand
a) Silica Sand
Silica sand is the most common sand used for making sand-casting moulds.
It is found in its natural form and can be purchased at the lowest cost from beaches and river beds in abundance or obtained from crushing sandstones.
100% pure silica has a fusion point of 1800 ℃ or 3272 ℉ and can be used for various metals but the problem is sand readily available has some impurities.
Based on impurities in the sand application silica sand changes for various metals.
Silica sand should be used with 2% clay in it.
Silica sand can be found in various forms, list of silica sand
and its applications are explained below based on their
purity.
| Casting Metal | % of Silica Sand |
|---|---|
| 1) Steel | 98% pure silica sand |
| 2) Cast iron and non-ferrous metal | 98% to 94% of pure silica sand |
The drawbacks of using silica sand are as follows:
Thermal expansion is high in silica sand and thermal conductivity is low at high casting temperatures causing casting defects.
b) Chromite Sand
Has a fusion point of 1890 ℃ or 3434 ℉ with high thermal conductivity used for expensive alloys as the cost is on the higher side too.
c) Zircon Sand
Ziron has a fusion point of 2500 ℃ or 4532 ℉ with less thermal expansion than silica sand and better thermal conductivity giving a better surface finish used for many alloys and metals.
d) Olivine Sand
Olivine sand is obtained by mixing orthosilicates and dunite from iron and magnesium respectively.
It is free from silica and does not have drawbacks like silica sand.
It overcomes all drawbacks of silica sand giving a lower thermal expansion of sand and high thermal conductivity at higher temperatures reducing defects in the final casting.
Binders
Binders are agents used in sand mould to bind binding agent and sand together.
Binders can be of two types, organic and inorganic binders.
Organic binders are obtained naturally and mixed with mould sand, while Inorganic binders are best for making core.
Using excess binders impacts the permeability of sand and blocks moisture from leaving the mould.
Binders used in sand casting mould making are as follows-
a) Clay
Clay is the most used binder agent in sand moulding. Clay is mixed with water and sand to form sand mould. Two types of clay binder agents are-
- Kaolinite.
- Bentonite.
- Fire clay.
- Fire clay.
- Illite.
- Sodium silicate.
- Oil.
To bind sand with clay oils made up of vegetables, minerals, fish oil, resin, pitch, soyabean oil, and linseed are used.
These oils need to be cured at high temperatures making sure that
overheating does not cause brittleness.
b) Sodium Silicate
Sodium silicate also called water glass is used for making cores and moulds.
It is a high-strength binder than clay.
As we use water with clay, sodium silicate requires carbon dioxide for
curing along with some heat. At times, a heat gun is used for binding moulding
sand.
Sodium silicate also has a few drawbacks such as it becomes challenging to remove patterns from the mould and hot tears defect.
c) Resin
Resin can be in natural or synthetic form with high melting point temperature, giving better collapsibility and surface finish.
Two types of resin gums are-
- Phenol formaldehyde.
-
Methylene diphenyl diisocyanate.
-
Urea-formaldehyde.
Additives, binders and sand are mixed together to form strong sand mould.
Catalyst-based resin binders are also used without the application of heat.
Additives
Additives are added to sand to improve cushioning properties of sand (sand expands creating a cushion in mould) and improve overall refractoriness, additives give a better surface finish to casting, reduce metal penetration, hot tears and hot crack also reduces collapsible and improves dry strength of the sand mould.
Additives can be in the form of powder or in form of liquid such as fuel oil
The list of most commonly used additives in making gree, dry and skin-dry moulds is as follows-
a) Sawdust, Wood flour, Peat, Straw & Husk
All the above materials are used in mould to improve the green strength, and density of the mould and reduce collapsibility of the mould.
Improves flowability in the mould and eliminates expansion defects in the casting.
Eliminates deformation of the mould and cores.
Retain green strength during the mould-making process as it retains moisture for a longer period of time.
b) Coal Dust Or Sea Coal, Creosote, Pitch & Silica Flour Powder
The above material improves the surface finish of the casting by not allowing molten metal to fuse and adhere to the sand.
The material mentioned burns and forms a protective layer of gas between sand and molten metal reducing metal penetration.
c) Iron Oxide And Silica Flour
Iron oxide and silica flour improve glazing and hot strength by reducing metal penetration in the mould giving a better surface finish.
Silica four is very fine and used for making facing sand improving the hot strength of the mould when molten metal is poured.
Overusing iron oxide can make it difficult for shakeout operation in the mould.
Overusing iron oxide also reduces the green strength, collapsibility and permeability of the mould.
d) Dextrin & Starch
Expansion defects such as scabs and deformation of mould and cores are possible by using dextrin and starch.
e) Molasses & Cereals
Cereals made from corn are used in mould to prevent sand from drying out. It improves the green and dry strength of mould.
Molasses are purchased from the sugar industry from foundries as it improves collapsibility.