Types of Moulding Sands:
According to their use in the foundry, moulding sands are classified into following categories:
1. Foundry Green sand:
- It is a sand used in wet condition for making the mould. It is a mixture of silica sand with 15-25 per cent clay and 6-8 per cent water
- As explained earlier, green sand moulds are not dried, when the metal poured in them in the wet condition
- Being damp the sand can be easily worked with hand to give it any desired shape
- This sand is used for producing small to medium sized moulds which are not very complex
2. Dry sand:
- Dry sand is the green sand that dried or baked after preparing the mould.
- Drying sand gives strength to the mould so that it used for larger castings
3. Loam sand:
- Loam sand containing up to 50 % clay which worked to the consistency of builder mortar.
- This sand used for moulds for making very heavy castings usually with the help of sweeps and skeleton patterns.
4. Parting sand:
- This sand used during making of the mould to ensure that, green sand does not stick to the pattern. The cope and drag parts easily separated for removing the pattern without causing any damage to the mould.
- Parting sand consists of fine grained clay free dried silica sand, sea sand or burnt sand with some parting compounds.
- The parting compounds used include charcoal, ground bone and limestone, groundnut shells, talc and calcium phosphate.
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5. Facing sand:
- Facing sand is the sand which covers the pattern all around it. The remaining box filled with ordinary floor sand.
- Facing sand forms the face of the mould and comes in direct contact with the molten metal when it poured.
- High strength and refractoriness required for this sand.
- It made of silica sand and clay without the addition of any used sand.
- Graphite, mollases, plumbago etc. added additionally to the facing sand. Thickness of the sand layer varies from 20 to 30 mm.
6. Backing sand:
- Backing sand is the bulk sands, used to back up beside the facing sand and to fill up remaining volume of the flask.
- It consists mainly of old, repeatedly used moulding sand which is generally black in colour due to addition of coal dust and burning on contact with hot metal.
- Because of the colour, backing sand also called as black sand.
- The main purpose for the use of backing sand is to reduce the cost of moulding.
7. System sand:
- This is the sand used in mechanized foundries for filling the entire flask.
- No separate facing sand in used in a mechanized foundry.
- Sand, cleaned and reactivated by the addition of water and binders used to fill the flask. Because of the absence of any fresh sand, system sand must have more strength, permeability and refractoriness compared to backing sand.
8. Core sand:
- Core sand is the sand used for making cores. This is silica sand mixed with core oil. That is why it is also called as oil sand.
- The core oil consists of linseed oil, resin, light mineral oil with some binders.
- For larger cores, sometimes pitch / flour and water used in saving the cost.