Green sand testing aptitude: a combination of tests can determine how core sands and recycled additives impact the properties of green sand.
The testing of green sand with core sand dilution and other additives is based largely on work from a variety of industry experts in the last 25 years. In that time, testing has come a long way. It can be a dynamic process that checks physical properties such as wet tensile strength, cone jolt toughness and friability. Additionally, the sand's emission characteristics at pouring, cooling and shakeout can and should be evaluated. The influence of core sand and other additives affects the application of green sand in metalcasting facilities.
Recent developments in wet tensile strength, cone jolt toughness and friability have broadened the industry's understanding of the influence of core sand and other additives. Still, a single test cannot be used to completely understand the influence of these additives. Instead, a combination of testing protocols has generated the greatest amount of information to support the application.
Wet Tensile Strength
The application of wet tensile strength has been used in the metalcasting industry since the 1960s, but it has increased in popularity in the last 25 years. This test determines the tensile strength of the condensation zone that is created in all clay bonded molds during and after metal is poured. The wet tensile strength is determined by pulling a detachable ring in position when the loaded prepared molding sand is compacted into the specimen tube (Fig. 1). Since the resulting wet layer is the weakest area, the specimen breaks at that point when a load is applied by the testing apparatus.
Metalcasting experts generally agree that wet tensile strength has a tendency to decrease as higher levels of core sand or new sand increase due to the dilution of the bonding mechanism and not because of a chemical interference. After a few cycles, the core sand dilution or new sand addition will return to a higher wet tensile strength, because the bentonite bonding system has the opportunity to develop. Therefore, wet tensile testing, in cooperation with other testing methods, can indicate the influence of core sand and new sand additions.
Cone Jolt Toughness
The cone jolt toughness test is a good tool to evaluate the ability of a prepared molding sand to resist breakage (toughness). In casting applications, prepared molding sand that does not have the ability to "draw pockets" results in broken mold defects. A green sand specimen with indentations on the top and bottom is prepared so it can be loaded into the test apparatus. The apparatus lifts the specimens with a plumb bob weight and drops them 1/32 in. (0.8 mm) using a cam rotation (measured in jolts) specimen ruptures. Figure 2 shows a test apparatus with a prepared sand specimen, and Figure 3 shows the same apparatus after the test specimen has broken and the evaluation is complete.
The optimum characteristics of this test are measured in the number of jolts needed to break the specimen. (Higher is preferred.) The general opinion of metalcasting experts is that the cone jolt toughness has a tendency to decrease as higher levels of core sand or new sand is added. Again, this decrease is not the result of a chemical interference with the clay bonding mechanism but due to the dilution of the bonding mechanism. After a few cycles, the core sand dilution or new sand addition will return to the higher toughness of the prepared molding sand. Cone jolt toughness testing, in cooperation with other green sand methods, can indicate the influence of core sand and new sand additions.
The friability test evaluates molding sand's resistance to scuffing. Friable sand leads to erosion and inclusion defects. Measuring friability is critical in selecting molding sand and bonding clay. In this investigation, a Dietert Model 875 friability and moldability tester was used (shown in Figure 4). Test results are measured in the percentage of sand that is scuffed off the surface of two prepared 2 x 2-in. (50.8 x 50.8mm) specimens that have been abrading against each other in a circular cage. The optimum characteristics of this test are measured in the lowest percentage (so lower is better).
Industry researchers generally agree that higher levels of core sand or new sand in green sand increases friability. Similar to the previous two testing methods, this increase is not the result of a chemical interference with the clay bonding mechanism but simply a dilution of the bonding mechanism. After a few cycles, as in the previous two cases, the core sand dilution or new sand addition will return the prepared molding sand to a less friable state. With adequate mulling or mixing, the bentonite bonding system has the opportunity to develop improved characteristics.
The topic of testing core sand dilution and other additives (including new sand) would not be complete without evaluating the emission characteristics of green sand. Research has shown that core sand does not significantly impact green sand's emission characteristics, which contradicted earlier perceptions. The greatest impact resulted from additives (organic materials, such as seacoal) that were added to maintain green sand properties. Additionally, the addition of the phenolic urethane cold binders minimally affected most green sand properties. Friability and cone jolt toughness tests reflected the greatest change. Physical property testing improved the understanding of green sand's emissions, which could be influenced by the dilution. Regardless, the addition of core sand does have a slight impact on green sand emission, so any inclusion requires monitoring and possible dilution with new sand.
Green sand's physical properties need to be evaluated before adding new sand to dilute any influence of core sand. Testing also can supplement the need to maintain a desired level of molding sand. Both core sand dilution and new sand addition need to be controlled because both additives will not fully bond with bentonite and other clay additives when initially introduced into the molding sand operations.
Wet tensile strength, cone jolt toughness and friability testing have improved the industry's understanding of the impact of core sand dilution and addition of new sand. These enhanced testing methods for green sand have given laboratories a quantifiable way to determine if a green sand molding process has increased in variability.
This article was based on Paper 14-005, "Testing of Green Sand with Core Sand Dilution and Other Additives," which was presented at the 118th Metalcasting Congress.
VICTOR LAFAY, S&B INDUSTRIAL MINERALS, CINCINNATI
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|Date:||Jul 1, 2014|
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