specific variable under examination. Te sand variables tested for their effect on dimensional change included grain fineness, shape, screen distribution, reclaimed sand and sand additives. Te test results showed finer sand
developed smaller bridges with greater surface area-to-volume ratios, allow- ing for more rapid evaporation of solvents (Fig. 3). Variations in sand grain shapes and particle distributions also affected the number and size of the resin bridges. Te silica sand cores examined shrank more in the testing period than the land sand due to silica sand’s rounded geometry. Cores made with sand with additives shrank more than those made with sand with no additives. New sand and mechani- cally reclaimed sand had similar shrinkage levels. Te variation in shrinkage caused by using different sands is around 50% of the total average shrinkage.
DIAL INDICATOR TEST ACCURATELY MEASURES DIMENSIONAL CHANGES
The dial indicator’s measurement method, which can be used in any metalcasting facility, has been in use for many years to test dimensional change in cores.
The method measures the linear shrinkage of a 1 x 1 x 8 in. (25 x 25 x 200 mm) core using a dial indicator. A core is produced and immediately placed into a fixture with a spring-loaded dial indicator. The indicator is zeroed, and as the core shrinks, negative values are recorded over time. Researchers have tested this method and been able to pro- duce repeatable results. A recent study confirmed the test method is capable of detecting core dimensional changes over time with coremaking process and measurement method errors only contributing a small por- tion of the total changes.
The dial indicator measurement method is a valid testing process that can produce accurate shrinkage recordings in any metal- casting facility.
October 2011 MODERN CASTING | 37
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