Lecture-3 Moulding sand.pptx DIFFERENT PROPERTIES OF SAND

SAND PROPERTIES

1. Sand Casting of Metals Topics covered: Molding sand and its types Constituents of molding sand Property requirements of molding sand

2. Flow Chart of a Metal Casting System

3. Moulding Sand The principal raw material used in moulding is the moulding sand because it provides several major characteristics that may not be obtained from other materials. Moulding sand is defined as granular particles resulting from the breakdown of rocks, due to the action of natural forces, such as frost, wind, rain, heat and water currents. Rocks have a complex composition and sand contains most of the elements of the rocks. Due to this reason, moulding sand differs considerably in different parts of the world. In nature, it is found on the bottom and banks of rivers and lakes. Moulding sand is classified into different categories according to the nature of its origin . Types of Moulding sand: Natural sand Artificial sand

4. Natural Sand: It is also called green sand and is collected from natural resources. It contains water as the only binder. It has the advantage of maintaining moisture content for a long time, having a wide working range of moisture content, permitting easy patching and finishing of moulds . Composition of green synthetic sand for steel castings is as under. New silica sand — 25%, old sand — 70%, Bentonite — 1.5%, Dextrine — 0.25%, and moisture — 3 to 3.5%. Synthetic Sand: It is an artificial sand obtained by mixing relatively clay free sand, binder (water and bentonite ) and other materials as required. It is a better moulding sand as its properties can be easily controlled by varying the mixture content.

5. Moulding sands may be classified, according to their use as under : ( i) Green Sand : When sand is in its natural (more or less moist) state, it is referred to as 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 fine, soft, light and porous . Being damp, it retains the shape given to it under pressure during squeezing . As the mould becomes dense by ramming, the structure is made porous by venting. Sharp edges are avoided in green sand moulding, because these being weak, break when hot metal is poured. Green sand is generally used for casting small or medium sized moulds. Coal dust is mixed in green sand to prevent defects in castings.

6. (ii) Dry Sand: Dry sand moulding is employed for large castings. The moulds prepared in green sand are dried or baked to remove, almost, all moisture of the moist sand . The structure in the moulding boxes after drying becomes stronger and compact . Venting is therefore necessary but not to that extent, as in the case of green sand mould . For larger heavy moulds, cow dung, horse manure, etc. are mixed with the sand of coarser grains.

7. (iii) Loam Sand: It is a mixture of clay and sand mixed with water to a thin plastic paste, from which, moulds are built up on a backing of soft bricks . Loam sand contains upto 50% clay and dries hard. It also contains fire clay. It must be sufficiently adhesive to hold on to the vertical surfaces of the rough structure of the mould. The moisture content is from 18 to 20%. It is used for casting larger regular shaped castings like chemical pans, drums, etc.

8. (iv) 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. Facing sand comprises 10 to 15% of the whole amount of mould sand.

9. (vii) Parting Sand: The moulding boxes are separated from adhering to each other by spreading a fine sharp dry sand called ‘parting sand’. Parting sand is also used to keep the green sand from sticking to the pattern. It is clean clay- free silica sand. Burnt core sand could also be used for this purpose. ( viii) Core Sand: It is used for making cores. It is silica sand mixed with core oil (linseed oil, rosin, light mineral oil and other binders). For the sake of economy pitch or flour and water may be used as core sand for large cores.

10. ( ix) CO 2 -Sand : In CO 2  sand, the silica grains, instead of being coated with natural clay, are coated with sodium silicate. This mixture is first packed around the pattern and then hardened by passing CO 2  through the interstices for about a minute. The sand thus sets hard and produces a strong mould. ( x) Shell Sands : Shell sands are synthetic sands coated with phenol or urea-formaldehyde resins and cured against a heated pattern to produce very strong, thin shell .

11. Properties of Molding Material Flowability – It is ability of molding sand to get compacted to a uniform density. Flowability assists molding sand to flow and pack all around the pattern and take up the required shape. The sand mold should response to different molding processes. Flowability increases as the clay and water content increases. Green Strength - The molding sand that contains moisture is termed as green sand. The strength of the sand in green or moist state is termed as green strength. A mold with adequate green strength will be able to retain its shape and will not distort or collapse. The green sand particles have the ability to cling to each other to impart sufficient strength to the mold .

12. Collapsibility - It is property due to which the sand mold automatically gets collapsed after casting solidifies. The molding sand should also have collapsibility so that during the contraction of the casting it does not provide any resistance, which may result in the cracks in the casting .

13. Dry Strength - It is the strength of the molding sand in dry conditions. When the molten metal is poured in the mold , the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must posses the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material. Dry sand strength is related to grain size, binder and water content.

14. Permeability - During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. The binder, additives, etc. present in the molding sand also produce steam and other gases. If these gases are not allowed to escape from the mold , they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous or permeable to provide path for the escape of gases. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity. Sand with Coarse grains exhibit more permeability. In absence of permeability the defects like surface blows, gas holes, etc. may be experienced.

15. Hot Strength - It is strength of the sand above 212 o F. As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength. In absence of hot strength the mold may enlarge, break, erode or get cracked. Durability – The molding sand should possess the capacity to withstand repeated cycles of heating and cooling during casting process Molding sand should be chemically immune to molten metals. Should be reusable. It should be easy to prepare and control.

16. Refractoriness - It is the ability of the molding material to withstand the temperature of the liquid metal to be poured so that it does not get cracked, fused with the metal or experience any major physical change. Refractoriness is essential while casting high melting point materials. The refractoriness of the silica sand is highest. Fineness – Finer sand molds resists metal penetration and produces smooth casting surface. Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results. Bench Life – It is ability of the molding sand to retain its properties during storage. Besides these specific properties the molding material should be cheap, reusable, coefficient of expansion, durability and should have good thermal conductivity.

17. Molding Sands: Sources, Types, Ingredients

18. Natural Sands It can be used as soon as received from source. It contains binding material (5-20%), water (5-8%) and considerable amount of organic matter. It can maintain moisture contain for long time. The finishing obtained on natural sand molds is good. It is cheaper compared to other sands. It has lesser refractoriness. It is employed for casting CI and non-ferrous metals. Molds made of natural sand can be easily repaired. When mixed with Bentonite , the properties of the sand gets improved and it gets properties like Synthetic sand.

19. Effect of moisture content on different properties of sand