Metal Casting Technologies : MCT-3RDQRT-2017
36 www.metals.rala.com.au placed in a mixing device such as that shown in Figure 4, and treated with a standard sodium hydroxide solution under controlled conditions. 2. Agitate for 5 minutes with a mechanical stirrer, then dilute with water to a height of 150mm and allow settling for 10 minutes. 3. Siphon off 125mm of water, dilute again to 150mm height, and allow settling for 10 minutes. 4. Siphon off 125mm of water, dilute again to 150mm height, and allow settling for 5 minutes. 5. Repeat step 4 enough times so that, after standing for 5 minutes, the water is clear. 6. Remove the remaining sand from the bottle, dry and weigh. Loss of weight is a measure of clay substance and may be expressed in per cent by weight. The clay must be removed from all sands containing more than 1per cent clay if it is intended to perform sieve analysis on the sand. Sampling of sand for testing ll sand tests should be performed on samples, which are representative of the parent material. The fundamental objective in sampling is to obtain a random, representative sample. This usually requires taking three samples: one each from the front, rear and centre of the sand heap, at a depth of not less than 150mm, to yield one representative sample when mixed. Samples may also be taken from the mixer, conveyor belt, or sand hopper of a sand system. Figure 1 shows the use of a sampling tool to extract a sample from a sand heap. Due to segregation during transportation, handling and flow through silos, the taking of samples from silos cannot be considered to be representative even if samples are taken at different heights in the silo. A single sample taken directly from the silo outlet is equally unrepresentative. A reasonably representative sample from a silo is only possible by the mixing of several individual samples taken from the outlet over an extended time period. Determination of moisture content As moisture content affects so many properties of the sand, it is one of the most important and frequently performed tests. Moisture content may be determined by loss of weight after evaporation. One type of instrument designed for this purpose is shown in Figure 2. This instrument utilises a heating lamp enclosed within a cabinet. The sand sample is placed on a balance scale mounted tray below the lamp. As moisture is lost by evaporation, the percentage of weight loss is indicated on the dial connected to the balance scale. Moisture content may also be determined by heating a sample of sand in an oven. A typical procedure would be: 1. A 50-gram sample of tempered sand is weighed, dried at 110°C for two hours, cooled to room temperature in a desiccator, and reweighed. 2. Moisture content is expressed in per cent by weight and is the loss of weight noted after drying the sample. Moisture content may also be determined by means of a chemical reaction. 1. A 10-gram sample is weighed and placed in a sealed pressure vessel along with a measured amount of powdered calcium carbide. The reaction between the moisture in the sand and the calcium carbide will produce an amount of acetylene gas proportional to the amount of moisture in the sand. The instrument used is known as the Speedy Moisture Tester and is shown in Figure 3. 2. Moisture content is read directly from a calibrated scale on the instrument. Determination of clay content Clay content is determined by loss of weight after washing a sample. The AFS clay content of a sand is defined as “particles which fail to settle one inch per minute when suspended in water. These are usually less than 20 microns in diameter”. 1. A 50-gram sample of previously dried sand is weighed, METAL Casting Technologies 2nd Quarter 2017 37 BacktoBASICS Sampling and testing of silica foundry sands - Part 1 J. F. Meredith Casting Solutions Pty Ltd A FIGURE 1. Using a sampling tool to obtain a representative sample. FIGURE 3. Speedy Moisture Tester. Casting Solutions Pty Ltd PO Box 131, Moorebank, NSW, 2170 Australia Tel: +61 2 9792 3782, Fax: +61 2 9792 3782, Email: firstname.lastname@example.org, 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 2. Evaporative moisture determination instrument.
MCT DEC 2017 (4TH QRT)