Investigation of the effect of bentonite origined at Cankiri Eldivan region on casting mold sand properties

Bentonite is a type of clay which constitutively consists of montmorillonite. Bentonites may contain various auxiliary minerals in addition to montmorillonite. These additional clay minerals can be different clay minerals such as kaolin, mica, illite and also these additives can be non-clay minerals such as quartz, feldspar, calcite and gypsum. Bentonite is a clay batch that has a large water absorption capacity, causing its structure to expand and swell. Bentonites show swelling properties when they interact with water. The swelling feature supports the separation of bentonites into three groups as well as ion exchange. Calcium bentonite (Ca-B) is not too much swollen and sodium bentonite (Na-B) is very swollen. Intermediate bentonite (Ca-B) and (Na-B) mixed bentonite show moderate swelling. Calcium and sodium bentonites are used in casting sands. The two bentonite species also impart different properties to the sand mixtures. Calcium bentonites provide high green compression strength, low dry compression strength. The sodium bentonites provide higher dry compression strength and lower green compressive strength than the calcium bentonites. In addition to its swelling properties, its colloidal properties, its high plasticity and its most important binding properties, allows bentonite to be used in many sectors. One of the most important usage area of bentonite is the foundry industry. Bentonite is commonly used with silica sand which has very common usage area among the casting sands, in metal forming process by casting method. Bentonite gives high strength and plasticity to the molding sand mixture. As a binder in the foundry industry, bentonites have outstanding importance during the preparation of moulding sand mixtures. Bentonites, originated from the deposits in Turkey, have important attitude in the applications of foundry industry. In the scope of the thesis, it is aimed to indicate the usability of Çankırı-Eldivan region bentonite in terms of foundry industry and to determine the most suitable usage conditions for casting molds. Chemical composition analysis (XRF), acid reaction test with HCl, watch-glass test, swelling capacity test and density were characterized to determine the chemical and physical properties of the bentonite originated from Çankırı-Eldivan region. After the determination of the properties of the bentonite, the mixture prepared by adding bentonite (3, 7, 10%) and moister (3, 7, 9%) to the casting mold sand which has the AFS grain number 41.09. Green compression strength, dry compression strength, gas permeability, shatter index and moldability index were determined. The results obtained were compared with the results in the literature and evaluated. The density of bentonite of Çankırı-Eldivan region was found 2.8 g / cm3 and the swelling capacity test showed that swelling capacity of this bentonite is 19 times more than original volume after 30 days of swelling. This result, obtained from the swelling test, can be expressed as that the bentonite of Çankırı-Eldivan region is a commercially good type of bentonites in the foundry industry. According to the watch-glass test, it was observed that bentonite, showing swelling as a result of water contact, showed a star-type splitting during drying. As a result, the bentonite of Çankırı-Eldivan region used in the experiment was determined as a bentonite with swelling properties. The green compression strength of the molding sand mixture having bentonite addition in 3%, 7% and 10% rates showed an increase with increase of the moisture proportion. As a result of this observation, the maximum value of the green compression strength among the molding sand mixtures is the molding mixture which has the highest moisture content, 9%. Dry compression strength is the strength required to protect the shape of the mold after it is dried. Dry compression strength in the mixtures prepared with the bentonite of Çankırı region in the rate of 7% and 10% showed an increase in parallel with the increase in moisture content. When the mixture containing 3% bentonite and 7% moisture, the maximum dry compression strength was obtained among the molding sand mixtures having lowest bentonite (%3) ratio. When different bentonite ratios were compared with a constant moisture content, it was determined that the molding sand mixture having the lowest dry compression strength at the 3% moisture content is the mixture containing maximum bentonite ratio which is 10%. Beside of this, at the highest moisture content, 9%, the mixture containing 10% bentonite has the highest dry compression strength value compared to other molding sand mixtures at the same moisture ratio. Molding sand is capable of allowing the passage of air, gas and water vapor during casting. Gas permeability increases in direct proportion to the sand gaps between each other. Gas permeability is the numerical value obtained by the velocity of passage of air passing under standard pressure from the standard test sample. Among the molding sand mixtures examined, it was observed that the gas permeability value decreased when the moisture rate increased from 3% to 9%. No standard trends in gas permeability values were observed, proportionally to the increase in the ratio of bentonite. The mold sand mixture containing 7% bentonite at the moisture content of 3% has the highest gas permeability while the mold sand mixture containing 7% bentonite at the moisture content of 9% had the lowest gas permeability value. In every case, there are features that affect the performance of the mold sand as well as the properties that affect the yield. Among the most important properties that affect this performance of the molding sand mixture, is seen as a moldability index. Moldability index values of all samples, containing 3%, 7% and 9% moisture and 3% and 7% bentonite content, were 20 or less. In the case of a moldability index value below 60, the molding sand mixtures could have some difficulties to operate. Excessive voids will be visible on the separation surface between model and the mold and the vertical walls of the mold cavity will be compacted inadequately. The molding sand mixture prepared with 10%bentonite and %3 moisture content has a optimum value of moldability index compared to value range from the literature. The moldability index of all the other sand mixtures prepared were determined very low also compared to the same value range from the literature. In all molding sand mixtures containing 3% and 7% bentonite content prepared at three different moisture rates (%3, %7, %9), the shatter index values increased with the increased moisture content, while the shatter index first decreased and then increased with increasing moisture content in the sand mixtures having 10% bentonite. The bentonite originated from Çankırı and moisture ratios which can give the optimum mold characteristics for steel casting pieces over 100 kg in the molding sand mixtures among the mixture having 3 to 10 % bentonite rates and 3- to 9 % moisture content are the 7% moisture - 7% bentonite and 3% moisture – 10% bentonite. The values of the dry compression strength, gas permeability and shatter index of the molding sand mixture having 7% moisture - 7% bentonit experimentally determined are in the optimum value range for sand casting of steel parts over 100 kg in the literature. In order to increase the moldability index value of this mixture having 7% moisture and 7% bentonite and remove it to the standard range value it needs to be had, fuel oil addition or graphite powder addition is required to the sand mixture. It has been observed that the molding sand mixtures prepared with the addition of fuel oil and graphite powder have lower values of moldability index than the molding sand mixture containing only 7% moisture and 7% bentonite, as opposed to the required optimum moldability value range. The moldability values of the molding sand mixtures prepared with 0.1% fuel oil, 1% graphite powder and 2% graphite are,respectively, 3, 3,75 and 3,25. The values of the green compression strength, moldability index and gas permeability of the molding sand mixture having 3% moisture - 10% bentonit experimentally determined are in the optimum value range for sand casting of steel parts over 100 kg in the literature. It is required to add corn flour or dextrin to this molding sand mixture which needs to be increased in dry compression strength value. The addition of cereal has been determined to be able to extract the shatter index property of the mixture to the appropriate value range in the literature. To increase the dry compression strength value, the addition of cereals, such as corn-flour was performed for the molding sand mixture including 3% moisture and 10% bentonite. As a result, the dry compression strength value was increased as expected, but this value was not enough for the required range. Cereal addition into the moldimg sand mixture can be used to increase the shatter index value to the optimum value range from the literature. It has been observed from the mixture prepared with corn-flour that corn flour increases the shatter index value of molding sand mixture as a result of the addition of the that type of cereal. The value of dry compression strength and shatter index obtained as a result of the corn-flour addition to the molding sand mixture containing 3% moisture - 10% bentonite originated at Çankırı-Eldivan region could not reached the required value ranges from the literature. Na(sodium) bentonite addition is reccommended in order to improve dry compression strength and shatter index properties of molding sand mixtures which is used in casting industry. It is advisable to mix Ca(calcium) bentonite with Na (sodium) bentonite in order to have intermediate type bentonite for optimizing the dry compression strength and shatter index properties of sand mixture instead of using just Ca bentonite. The addition of Na affects the dry compression strength values in the positive direction.