Moulding

INTRODUCTION The quality of castings depends to a large extent on the technology of mould making and characteristics of moulding materials. According to the production data on an average 40-70 percent of casting defects arises due to improper mould properties. Thus, it is imperative for the foundry men to have the accurate selection of the moulding sand composition to get the desired properties.

MOULDING SAND A high grade of molding sand should be fat, i. e., strongly mixed with clay. Naturally the molds of this sand should be employed only in a perfectly dry state. The fat molding sand is prepared artificially from quartz sand (fine sprinkling sand), fat clay, free from lime and ferric oxide (red ocher). The molding sand is fixed by breaking up the loose pieces in which it is partly dug; next it is passed through a fine sieve and mixed up to onethird of its volume with charcoal dust, or, better still, with lampblack, which, owing to its looseness and fatness, does not detract so much from the binding qualities of the sand. The utility of the sand may be tested by pressing the finger into it, whereupon the fine lines of the skin should appear sharply defined; its binding power is ascertained by dropping a lump pressed together with the hand from a height, which is increased until it breaks.

WATER BONDED SAND Water bonded sand or Green Sand has been around forever and is the traditional molding sand. It is made with fine sand, Bentonite Clay and water.

OIL BONDED SAND Commercially sold as Petrobond and homemade as K-bond. Fine sand is combined with Benton Clay, oil and a Catalyst. If you’re in a hurry to get casting/ want excellent results / have the budget, use Petrobond.

SUFFICIENT POROSITY Sufficient porosity to allow the escape of gas and steam generated in the body of the mould by the heat of the molten metal. The greater the bulk of metal the coarser the sand used, and the greater the need for careful venting.

SUFFICIENT PLASTICITY Sufficient plasticity and tenacity to hold its form in the mould and to resist the erosive action of hot metal. These qualities are due to clay, which is more or less detrimental, because it fills the spaces, or pores, between the silica grains. A high enough fusing point not to melt and stick to the face of the casting. There are many trade names for foundry sands, but all must fulfill the requirements mentioned. Vegetable or other combustible matter, sea salt, lime, and substances easily decomposed by heat should not exist naturally in moulding sand, as they may cause failure in casting. Brass sand is green sand of fine grain used in moulds for brass castings. Facing sand is also fine-grained sand placed next to the pattern in small moulds to make a smooth casting. Core sand is used for cores. Loam is a very course moulding sand made up for loam moulds.

PERMEABILITY IN GREEN SAND MOLDING

Sand moulds evolve volatile gases when filled with molten metal. Thus sufficient permeability is necessary to prevent the gases from developing high pressures and blowing into and through the metal. Permeability is the venting quality with which the sand permits free escape of gases through its pores between sand grains. Permeability therefore depends on the number and size of the pores, or on their number and total volume. The volume of pores in a granular material depends on the size, distribution of the grains and on the way in which they are packed. In addition to base sand, permeability also depends on the additives made to the sand mix, binder, water additions, mulling time and the bulk density of the sand mix. When water and clay content of sand increases density increases. Higher the density, lower is the permeability. Good mulling increases permeability, which should be as high as necessary, not as high as possible.

MOLDING MATERIAL AND PROPERTIES A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties those are generally required in molding materials

REFRACTORINESS It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest.

GREEN STRENGTH

The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape.

DRY STRENGTH 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 metalloid static pressure of the liquid material.

HOT STRENGTH 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.

COLLAPSIBILITY The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity.

MOLDING SAND COMPOSITION

The main ingredients of any molding sand • • •

Base sand, Binder, and Moisture

BASE SAND Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromites sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available.

BINDER Binders are of many types such as: 1.

Clay binders,

2.

Organic binders and

3.

Inorganic binders

Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are: Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O) Of the two the Bentonite can absorb more water which increases its bonding power.

MOISTURE

Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand

MOLDING SAND CONSTITUENT Silica sand

WEIGHT PERCENT 92

Clay (Sodium Bentonite)

8

Water

4

LATEST METHOD OF SAND PREPARTION The high quality of cast pieces requires the implementation of automation solution down to the foundry sand processing of any sand casting foundry. Operational analyses and technical literature show that approximately 45-55% of the cast piece rejections are due to inferior moulded material characteristics. The separation of casting and sand must commence as early on in the process as possible, so as to increase the remaining moisture in the moulded material whilst simultaneously reducing the temperature. The effectiveness of the processing facility can only be guaranteed in this way. In the following, systems and sensors for monitoring the processing from the shake out to the foundry machine, and supply the desired sand characteristics, with the aim of supplying the foundry facility with high-quality and homogeneous moulded material.

PERMEABILITY The rate of flow of air passing through a standard specimen under a standard pressure is termed as permeability number. The standard permeability test is to measure time taken by a 2000 cm3 of air at a pressure typically of 980 Pa to pass through a standard sand specimen confined in a specimen tube. The standard specimen size is 50.8 mm in diameter and a length of 50.8 mm. Then, the permeability number, P is obtained by

Where V = volume of air = 2000 cm3 H = height of the sand specimen = 5.08 cm p = air pressure, g/cm2 A = cross sectional area of sand specimen= 20.268 cm2 T = time in minutes for the complete air to pas through Inserting the above standard values into the expression, we get

PERMEABILITY IN GREEN SAND MOULDING

Sand moulds evolve volatile gases when filled with molten metal. Thus sufficient permeability is necessary to prevent the gases from developing high pressures and blowing into and through the metal. Permeability is the venting quality with which the sand permits free escape of gases through its pores between sand grains. Permeability therefore depends on the number and size of the pores, or on their number and total volume. The volume of pores in a granular material depends on the size, distribution of the grains and on the way in which they are packed. In addition to base sand, permeability also depends on the additives made to the sand mix, binder, water additions, mulling time and the bulk density of the sand mix. When water and clay content of sand increases density increases. Higher the density, lower is the permeability. Good mulling increases permeability, which should be as high as necessary, not as high as possible.

CONCLUSION

The percentage increment in permeability of moulding sand increases with increase in depth size of venting rod, grain size and number of venting holes. By keeping the depth and size of vent rod constant, the permeability is improved. This is achieved by increasing the number of holes. The sand used was found to be as good as those sands currently employed in some Nigerian Foundries for the casting of heavy brass or iron, steel green sand and steel dry facing sand. Permeability is found to be dependent on number of holes, depth of vent, position or distribution of vents on the jig and diameter of venting rods. Engineering properties of systems such as backing sand is useful in understanding materials applicable everyday life. This facilitates knowledge and technology transfer and should encourage active collaboration between industries and engineering research institutions including universities. This should will in no small measure contribute to improvement in engineering education especially in developing countries. In manufacturing processes especially in relation to sands some effects have been studied and Sand with improved foundry properties could find use in the making of computers, consumer Products, communications equipment, manufacturing industries, automobiles and by the military in some cases involving the fabrication of micro systems.