Fig. 2. A prepared specimen is positioned underneath the testing apparatus’s weight.


Fig. 3. The apparatus repeatedly drops the specimen until it ruptures.


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 tough- ness testing, in cooperation with other green sand methods, can indicate the influence of core sand and new sand additions.


Friability


Te friability test evaluates molding sand’s resistance to scuff- ing. 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 percent- age 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. Te opti- mum 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


28 | MODERN CASTING July 2014


Physical property testing has improved the industry’s understanding of core sands and additives in green sand.


two testing methods, this increase is not the result of a chemical interfer- ence with the clay bonding mecha- nism 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 mix- ing, the bentonite bonding system has the opportunity to develop improved characteristics.


Emission Control


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 character- istics, 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 addi- tion of the phenolic urethane cold binders minimally affected most green sand properties. Friability and cone jolt tough- ness tests reflected the greatest change. Physical property testing improved the understanding of green sand’s emis- sions, which could be influenced by the dilution. Regardless, the addition of core


sand does have a slight impact on green sand emission, so any inclu- sion requires monitoring and pos- sible dilution with new sand.


Conclusions


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 con- trolled 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 understand- ing of the impact of core sand dilu- tion and addition of new sand. Tese enhanced testing methods for green sand have given laboratories a quan- tifiable way to determine if a green sand molding process has increased in variability. Tis article was based on Paper


Fig. 4. Friability testing abrades two samples off one another to determine resistance to scuffing.


14-005, “Testing of Green Sand with Core Sand Dilution and Other Addi- tives,” which was presented at the 118th Metalcasting Congress.


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