Metal Casting Technologies : MCT DEC 2017 (4TH QRT)
28 www.metals.rala.com.au Preparation of standard test specimens controlled mass of moulding sand rammed to a specific size is called a standard test specimen. As the strength of a moulding sand depends greatly on its degree of ramming, the conditions of ramming the standard sample must be carefully controlled. 1. The most commonly used standard test specimen is the 2-inch diameter by 2-inch long specimen. 2. The standard specimen tube controls the diameter. The specimen tube is closed at one end with a pedestal; a quantity of sand is placed into the tube and rammed according to the procedure for the specimen being used. The most common procedure is to use three rams. 3. The specimen rammer is a devise that provides a controlled weight falling a prescribed distance in order to compact the sand in the tube. See Figure 6. 4. After ramming, the specimen may be removed by means of a stripping post, which fits inside the tube, or it may be retained in the tube depending on the test being conducted. Determination of permeability Permeability is expressed as the volume of air in cubic centimetres that will pass per minute, under a pressure of 1 gram per square centimetre, through a specimen of sand 1 square centimetre in cross sectional area and 1 centimetre in height. 1. The permeability test is performed with the standard 2-inch diameter by 2-inch high specimen retained in the specimen tube. 2. The tube with specimen is placed on the permeability meter and a selected amount of air is forced through the specimen under controlled conditions. One end of the specimen tube is sealed to the instrument so that the air must pass through the sand specimen in order to escape. See Figure 7. 3. The rate of flow of air is timed in order to calculate a permeability number. The relationship of the variables can be expressed as follows: P= vh/pat Where: P = permeability number v = volume of air passing through specimen in cubic centimetres. h = height of specimen in centimetres. p = pressure of air in grams per square centimetres. a = cross sectional area of specimen in square centimetres. t = time for air passage in minutes. The standard procedure requires 2,000 cubic centimetres of air to pass through a specimen of sand 5.08 centimetres (2 inches) in height and 20.268 square centimetres (3.1416 square inches) in area. These known values can be substituted for v, h and a. The permeability metre is designed to produce a constant pressure of 10 grams per square centimetre, regardless of rate. Substituting these known METAL Casting Technologies 4th Quarter 2017 29 BacktoBASICS Casting Solutions Pty Ltd PO Box 131, Moorebank, NSW, 2170 Australia Tel: +61 2 9792 3782, Fax: +61 2 9792 3782, Email: email@example.com, Mob: 0412 178 895 Contact: Jeff Meredith – Director Category: Foundry Consultants Products: SOLIDCast, FLOWCast, OPTICast simulation software. www.finitesolutions.com Services: l metallurgical l solid modelling l flow, solidification and optimisation modelling of castings l casting methoding service l defect analysis l training FIGURE 7. Permeability meter. Sampling and testing of silica foundry sands - Part 2 Part 1 of this article appeared in the September 2017 edition of MCT. Jeff Meredith Casting Solutions Pty Ltd A FIGURE 6. Standard sand rammer, specimen tube and stripping post. PERMEABILITY IS EXPRESSED AS THE VOLUME OF AIR IN CUBIC CENTIMETRES THAT WILL PASS PER MINUTE, UNDER A PRESSURE OF 1 GRAM PER SQUARE CENTIMETRE, THROUGH A SPECIMEN OF SAND 1 SQUARE CENTIMETRE IN CROSS SECTIONAL AREA AND 1 CENTIMETRE IN HEIGHT.
MCT MAR 2018 (1ST QRT)