Figure 6. Core measurement with vernier height gauge. (a)
Figure 8. Testing fixtures.
(b)
Figure 7. Boxplots of core measurement without (left) and with (right) locator pins.
of the same magnitude as the standard deviation or variation within the measurements. This made detection problematic. In an effort to improve the measurement method, a metal screw was inserted into the top of the bar to provide a loca- tor pin as the measurement point. Again, two freshly made cores were produced and measured using the screw as the measurement point. After 24 hours, the cores were measured again. The use of the locator pin improved the accuracy of the measurements as shown on the boxplots for core 1, with no locator, and core 3, with a locator pin (see Figure 7).
Dial Indicator Measurement Method
A more precise method of measuring core shrinkage in- volves measuring the linear shrinkage of a 1 x 1 x 8-in. (25.4 x 25.4 x 203.2-mm) core using a dial indicator. A newly-
International Journal of Metalcasting/Summer 10
Figure 9. Core shrinkage of 8 cores from same mix, mm.
made core is 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. While this procedure has been in use for many years, it is not included in the AFS Sand Testing Handbook, and was only recent- ly reported in AFS Transactions.2
To evaluate the method,
eight cores from the same mix were blown and immediately placed in the fixtures as shown in Figure 8. Readings were taken over a period of 24 hours as shown in Figure 9.
Given the accuracy of the test, measurement of groups of four equal cores would show differences of 0.0008 in. (0.02 mm) or about 10% of the total shrinkage seen in 24 hours. The use of the dial indicator test fixture and groups of four of each type of core were selected as the test method for further evaluation of core shrinkage.
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