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Properties required in molding sand and core sand

It describes the basic properties required in molding sand and sand core as under.

1) refractoriness. Refractoriness is defined as the ability of molding sand to withstand high temperatures without breaking down or fusing which facilitates to find sound solutions. It is a very important characteristic of sand molding. Refractoriness increase can only to a limited extent. Can refractoriness molding sand with a bad burn on to the casting surface and one can not get rid of surface solutions. The amount depends on the SiO2 refractoriness ie quartz material, and the shape and grain size of the particle. The higher the SiO2 content and the volumetric composition coarse grain the higher the refractoriness of the molding sand and core sand. Refractoriness is measured at the point sinter the sand rather than melting.

2) Permeability. It is also termed as the molding sand porosity to allow the escape of any air, gas or moisture present or generated in the mold when the molten metal is poured into it. All the gaseous generated during pouring and solidification process must escape becomes otherwise the faulty solutions. Permeability is a function of grain size, grain shape, and moisture and clay content in the sand molding. Size of ramming the sand directly affect the permeability of the mold. Mold permeability can be further increased by using rods loose vent

3) coherence. It is the property of molding sand by virtue of interacting particles and sand grains attract each other within the molding sand. Thus, the capacity of the molding sand gets enhanced binding to take to increase the green, dry and warm properties and core strength molding sand.

4) Green strength. The green sand after water mixed into it, must have sufficient strength and toughness to allow for and handle the model. So the sand grains must be adhesive, ie they must be able to attach themselves to another body. therefore, will and grain of sand is high adhesiveness cling to the side of the molding box. Also, have the grains of sand are the property called coherence ie the ability of sand grains to stick together. By virtue of this property, the pattern can be taken out of the mold without breaking the model and also the mold wall surfaces erosion does not occur during the flow of molten metal. The green strength also depends on the grain shape and size, amount and type of clay and moisture content.

5) Dry strength. Once the molten metal is poured into the mold, have the moisture in the layer of sand near the hot metal gets evaporated and the layer of dry sand sufficient strength to shape in order corrosion wall mold to avoid during the flow molten metal.

6) Flowability or plasticity. It is the ability to obtain the compacted sand and behave like a fluid. It will flow uniformly to all parts of a pattern when rammed and evenly distributed on the pressure ramming all around in all directions. Generally resist sand particles move around corners or projections. In general, increases flow capacity to reduce green strength, a reduction of grain size. Does the flow capacity as well as moisture and clay content.

7) adhesiveness. It is the property of molding sand stick to obtain or comply with such sticking foreign matter of sand molding to molding inner wall of the box.

8) collapsibility. After the molten metal gets the solidified mold, sand mold must be collapsible so free shrinkage occurs on the metal and this would naturally avoid the tearing or cracking of the metal contracting. In the absence of this property that the contraction of the metal barrier at the mold and thus results in tears and cracks in casting. This property is highly required in cores.

9) Miscellaneous properties. In addition to the above requirements, the molding sand should not comply with the resolution and should not react chemically with the metal. Moulding sand should be free and easily available. Should be reusable for economic reasons. Should its relatively low coefficients of expansion.

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Kinds of casting molding sand

Molding sand can also be classified according to their use in a number of varieties which are described below.

1) Green sand also known as tempered or natural sand is just prepared mixture of silica sand with 18 to 30 percent clay, by 6-8% moisture content. The clay and water to bring the band to the green sand. It is fine, soft, light, and porous. Green sand humidity is, when pressure in the arm and it retains the shape and sense to him under pressure. Models prepared by the sand not requiring support and thus is known as green sand molds. The sand is easily available and it possesses a low cost. It is commonly employed for the production of ferrous and non-ferrous castings.

2) Dry sand, green sand is dried or baked in an oven suitable after the mold and cores, called dry sand. It possesses greater strength, rigidity and thermal stability. It is primarily suitable for larger castings. Mold are prepared in the sand dry sand molds.

3) sand Loam, Loam is a mixture of sand and clay with water to thin plastic paste. Held high clay Loam sand as much as 30-50% and 18% water. Patterns not used for molding loam and mold shape for brushing. This is especially employed for molding loam used for large gray iron castings.

4) Facing sand, sand Facing have just prepared and the face of the model forms, surface finish gives solutions. It is directly adjacent to the surface of the pattern and it comes in contact with molten metal when the mold poured. A wide front shield pattern and thus from the surface by the sand mold. The sand under the severest conditions and must, therefore, high strength refractoriness. It is made of silica sand and clay, without the use of sand used.

5) Support sand or sand floors used to support the front and sand used to fill the entire volume of the flask shape. Used molding sand is mainly employed for this purpose. The sand is supported sometimes called black sand because of age, molding sand repeated use black in color due to excess dust and burning coal is coming into contact with the molten metal.

6) Separation sand without binder and moisture are used to keep the green sand without following the pattern and also to allow the separation of sand on the surface of the handle and pull the separation without clinging.

7) Core sand used for making cores and is sometimes also known as oil sands. The high silica rich sand mixed with oil binders such as a central oil consists of linseed oil, resin, light mineral oil and other bound materials.

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The molding sand types of sand castings

Depending on the purity and other ingredients are present, sand is classified into (i) Natural sand. (ii) synthetic sand, (iii) sand loam.

(i) Natural sand: natural sand is used directly for molding and 5-20% of clay as binding material. It needs 5-8% for mixing water before making the mold. Natural sand is quite wide working range of moisture and are able to retain the moisture content for long. The main disadvantage is that it is less refractory compared with synthetic sand. There are many natural sand molding weak properties. The sand is restored by small amounts of a binding material such as bentonite to improve their properties and are called semi-synthetic sand mix.

(II) synthetic Sands: synthetic sand consists of silica sand with or without clay, binder or moisture. It is a combined sand ie sand made by various constituents. Sand formulations made more certain desired properties possessed by natural sand. The sand is better casting properties such as permeability and refractoriness and is ideal for casting non-ferrous and ferrous materials. The symptoms can be controlled by mixing different ingredients. Synthetic sand used for making heavy castings.

(III) Loam Sand: sand loam has many ingredients, such as fine particles of sand, finely ground refractories, clay, graphite and fiber reinforced. In many cases, the material can be clay the order of 50% or more. When mixed with water, the mixture of materials such as mortar and consistency to be hard after drying. Large molds for casting made of brick framework lined with sand loam and dried. Sweeping etc used for making large castings like big bells by using sand loam.

Refractory sand grains: sand grain size and shape have a significant effect on the properties of molding sand. The unique surface rough idea of ​​the amount needed for molding sand binder.

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Constituents of molding sand

Involves the main ingredients of silica sand molding sand, binder, the moisture content and additives.

Silica sand: sand silica in the form of quarts granular the main constituent of sand molding is enough refractoriness can taste strength, stability and permeability of molding and core sand: Besides silica small amounts of iron oxide, alumina, stone lime, magnesia, soda and potash are present as impurities. The chemical composition of silica sand thinking of impurities like lime, magnesia, alkalis present etc. The presence of excessive amounts of iron oxide, can alkali oxides and lime to lower the fusion point to a large extent are undesirable. Moisture: The size varies moisture content in molding sand generally between 2-8 percent. The following the combination of clay and silica sand for developing bonds. This is the amount of water needed to fill the pores between soil particles without separating them. The amount of water held tightly by soil and is largely responsible for developing the strength in the sand. Reduces the effect of clay and water to increase permeability clay and moisture content. The green compressive strength initially increases with the increase in clay content, but after a certain value, it starts to decline. Add: Add the following topics in general with the molding and core sand mixture in the sand some special property development. Some common additives used to improve the properties of molding and core sand.

(i) Coal dust: coal dust is added primarily for producing a reducing atmosphere during casting. This reduction results in no oxygen in the atmosphere becoming bound chemical poles so it can oxidize the metal. (ii) dextrin: Dextrins family relates to starch carbohydrates. It increases the molds dry strength. (iii) Pitch: It is distilled form of soft coal. It can be added from 0.02% to 2% in mold and sand core. It offers hot strengths, surface finish on model surfaces. (iv) Wood flour: This is the fibrous material mixed with granular material like sand; a relatively long thin fibers prevent sand grains from contact with each other. It can be added from 0.05% to 2% in mold and sand core. It increases both mold collapsibility and heart.

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