Contents Introduction ..................................................................................................................................... 2 Specimen Preparation ................................................................................................................. 3 Compression .................................................................................................................................... 4 Shear ................................................................................................................................................ 5 Flow ability ..................................................................................................................................... 6 Hardness .......................................................................................................................................... 7 Green Strength ................................................................................................................................ 8 Dry Strength .................................................................................................................................... 9 Hot Strength ................................................................................................................................. 10 Collapsibility ................................................................................................................................. 10 Plasticity ........................................................................................................................................ 10 List of Reference ............................................................................................................................ 11 1 Introduction The testing process is divided in to three stages: sampling of the bulk material, sample preparation, and testing. Sampling is usually done at three different points of the process: upon first arrival from the supplier, en transport for processing (usually on a conveyor), and after processing. In each situation it is important to take a representative sample by mixing the sand or by taking multiple samples in different locations. Also, the sample must be stored in an airtight container to keep from spoiling it. There are more than 25 basic tests, however only the important ones for the given casting process are used. The basic tests measure the following parameters: wet tensile strength, cone jolt, friability, moisture content, permeability, green compression strength, compact ability, loss on ignition, volatiles content, grain size & distribution, dust (dead clay) content, and active clay content. Each of these tests can lead you to obtain specific characteristics of sand which can be crucial quality of casting. Advanced testing tests for other parameters, such as splitting strength, shear strength, and high-temperature compression strength. 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 that are generally required in molding materials are : Specimen Preparation Compression Shear Flow ability Hardness Green Strength Dry Strength Hot Strength Plasticity Collapsibility 2 Specimen Preparation Since the permeability of sand is dependent to a great extent, on the degree of ramming, it is necessary that the specimen be prepared under standard conditions. To get reproducible ramming conditions, a laboratory sand rammer is used along with a specimen tube. The measured amount of sand is filled in the specimen tube, and a fixed weight of 6.35 to 7.25 Kg is allowed to fall on the sand three times from a height of 50.8 ±0.125 mm. The specimen thus produced should have a height of 50.8± 0.8 mm. To produce this size of specimen usually sand of 145 to 175 g would be required. After preparing a test sample of sand as described, 2000 cm of air are passed through the sample and time taken by it to completely pass through the specimen is noted. 3 Compression Sand moulds are supposed to take up all the load that is inflicted upon it. The load may be structural as applied by the mould material. Or it may also be due to the weight of the casting material when it is poured in to the mould. Although the moulds are designed with utmost care, taking established material property data and practices into consideration, it may not always be possible to obtain the desired characteristics due to the difference in processing the materials. Hence at the end of a design, it is customary to prepare a test specimen and obtain the property specific to that design or the batch. Compression test is carried out on symmetrical specimen which may either be cylindrical or cubical/rectangular. The size of the specimen is taken as per the prevailing standard usually accepted in the locality. There are some universally accepted test standards as laid down by ISO and BIS, too. These standards clearly specify the test method and specimen shape or dimensions to be used in the test. They also make the data acquisition and reporting method universal so that reports from different test labs or places can be compared and conclusions can be arrived upon with accuracy. 4 Shear For green shear, test heads are placed in lower position on the pusher arm and weight of the Universal Sand Strength Machine as for the compression test. When the heads are placed in the upper position the breaking force is increased five time, and the scale designated “dry shear” is read. Shear strength tests up to 220 psi (AFS) or 1530 kN/m2 (DIN) can be made by using these test heads in the High Dry Strength Accessory . 5 Flow ability “Flowability” is the property which enables a sand to flow readily into a mould and fill up the interstices when subjected to ramming. It is influenced by moisture, permeability, grain shape, fineness and clay content. The indicator is a specially constructed dial gauge calibrated to read in percentage flowability, and may be readily attached to the Sand Rammer. The stem rests on top of the plunger shaft and the movement between the fourth and fifth drop of the weight is registered as flowability. The further the plunger descends, indicating incomplete flow the lower the percentage flowability. When the Sand Rammer is used for ramming test specimens, the indicator is rotated out of position. The flowability indicator can be supplied to fit either an imperial (AFS) rammer or a metric (DIN) rammer. 6 Hardness The mould hardness is measured by a method similar to the Brinell hardness test. A spring loaded steel ball with a mass of 0.9 Kg is indented into the standard sand specimen prepared. The depth of indentation can be directly measured on the scale which shows units 0 to 100. When no penetration occurs, then it is mould hardness of 100 and when it sinks completely, the reading is zero indicating a very soft mould. Besides these, there are other tests to determine such properties as deformation, green tensile strength, hot strength, expansion, etc 7 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. This is an alternative tester for determining green compression strength. When fitted with the low capacity balance it is specially suited for testing unbaked oilsand core mixtures and uncured resin bonded sands of very low green strength. The test specimen is rammed in the Split Specimen Tube and stripped directly on to a special carrying plate . 8 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 moldings and must possess 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. The High Dry Strength Accessory provides the means for determining the dry compression and dry shear strengths of moulding and core-sand mixtures by increasing the capacity of the Universal Sand Strength Machine by a factor of three. It is of sturdy design and produces accurate results. 9 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 . Plasticity It is that property of sand due to which it flows to all portions of the moulding box or flask. The sand must have sufficient plasticity to produce a good mould. 10 List of Reference FOUNDRY SAND TESTING EQUIPMENT CATALOGUE NO. 750 http://engineeringhut.blogspot.com/2010/11/moulding-sand-properties-and-itstypes.html http://www.mechanicalengineeringblog.com/2950-moulding-sand-silica-sandfoundry-sand/ http://www.scribd.com/doc/18096886/Moulding-Sand-Properties 11
