topic of discussion in the metalcast- ing industry. In the 1960s and 1970s, industry experts debated whether core sand was a contamination in green sand. But core sand or other additives (new sand) are not considered contam- inations because these materials are added into the green sand by design, not by accident. Today’s sand casting facilities treat core sand and new sand as an additive that is monitored, and it is adjusted to compensate for varia- tions in the process. Te 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 fri- ability. Additionally, the sand’s emission characteristics at pouring, cooling and shakeout can and should be evaluated. Te influence of core sand and other additives affects the application of green sand in metalcasting facilities. Recent developments in wet tensile
T
strength, cone jolt toughness and fri- ability 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 Te application of wet tensile
strength has been used in the met- alcasting industry since the 1960s, but it has increased in popularity in the last 25 years. Tis test determines the tensile strength of the condensa- tion zone that is created in all clay bonded molds during and after metal is poured. Te wet tensile strength is determined by pulling a detachable
he dilution of core sand and other additives into green sand is a popular
ring in position when the loaded prepared molding sand is compacted into the specimen tube (Fig. 1). Since the resulting wet layer is the weak- est 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. Terefore, wet tensile testing, in cooperation with other testing methods, can indicate the influence of core sand and new sand additions.
Cone Jolt Toughness Te 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 speci- men with indentations on the top and bottom is prepared so it can be loaded into the test apparatus. Te 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) speci- men ruptures. Figure 2 shows a test apparatus with a prepared sand specimen, and Figure 3 shows the same apparatus after the test speci- men 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 pre- ferred.) The general opinion of metalcasting experts is that the cone jolt toughness has a tendency to decrease as higher levels of core
Fig. 1. Shown here is the methodology of the wet tensile strength test apparatus. July 2014 MODERN CASTING | 27
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