MOLD SAND, SAND ADDITIVES & SAND PROPERTIES Collection and Selection by Dr. Mehdi Divandari IUST Castin g 1 Flow Chart of a Metal Casting System Sand Casting of Metals Mold Materials Topics covered: Molding sand Constituents of molding sand Property requirements of molding sand Testing of sand properties 1 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Mold Material The mold material is the one out of which the mold is made. The mold material should be such that casting should be able to retain its shape till the molten metal has solidified. Types of molds: Permanent molds: They are made up of ferrous metals and alloys (Steel, Grey CI, etc.). Temporary refractory molds: They are made of refractory sands and resins Molds made of wax, plastic, Plaster of Paris , carbon, ceramics are also employed. 2 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Mold Material Permanent molds Temporary refractory molds Since they are made of refractory sands, the They are employed for casting low melting temporary refractory molds employed for point materials casting high melting point materials They are costly. They are cheaper. They are employed to produce objects They are employed to produce objects bigger smaller in size. in size. The surface finish, quality and dimensional They produce casting with better surface accuracy of the casting produced by finish, quality and dimensional accuracy. temporary molds is poor. 3 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) 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. 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. 4 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Properties of Molding Material 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. 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. 5 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Properties of Molding Material Hot Strength - It is strength of the sand above 212oF. 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. 6 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Properties of Molding Material 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. 7 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Molding Sands: Sources, Types, Ingredients Sources: Types: Ingredients: River beds Natural sands Refractory sand grains Sea Synthetic sands Binders Lakes Loam sands Water (moisture) Desert Additives 8 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) 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. 9 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Synthetic Sands A synthetic sand consists of natural sand with or without clay, binder and moisture. The organic matter is not present in synthetic sand. Synthetic sand is a formulated sand, formulation is done to impart certain desired properties not possessed by natural sand. Possesses good refractoriness, high permeability, uniform grain size as compared to natural sand. It is more suitable for mass production and mechanized foundries. 10 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Loam Sands It contains much more clay (50% or more) as compared to ordinary sand. The ingredients of Loam sand may be fine sand, clay, finely ground refractories, graphite and fibrous reinforcement. 11 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Refractory Sands 1. Silica sand 2. Magnite 3. Zircon 4. Dolomite 5. Olivine 6. Silimanite 7. Graphite / Carbon 12 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Properties of Refractory Sands They maintain their shape and other characteristics at high temperature. When packed to produce the mold cavity, they remain sufficiently porous or permeable to give out gases produced during solidification of molten material. They can be molded into intricate shapes. They are chemically immune to molten metals. They can be used repeatedly to make molds. They are inexpensive. They can be made available without much difficulty. 13 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Molding Sand Composition Base Sand Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite 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, Clay binders, Organic binders and 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 2SiO2 2H2O) and Bentonite (Al2O3 4SiO2 nH2O) Bentonite can absorb more water than fire clay which increases its bonding power. Water (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. 14 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Molding Sand Composition Typical Composition of Molding Sand Molding Sand Constituent Weight (%) Silica sand 92 Clay (Sodium Bentonite) 8 Water 4 15 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Grain Size The grain shape and size of molding sand defines the total surface area of grains contained in unit mass. Total surface area is known as Specific Surface. Specific surface gives rough idea of amount of binder needed to coat the grains of molding sand. 16 Compiled by Prof. Amruta A. Rane (Asst. Prof., DJSCE) Grain Size The grain size and distribution influences many sand properties such as permeability, flowability, refractoriness, surface fineness and strength. The finer the sand grains, the finer is the molding sand as whole.