Fig. 1. Data points from 3-ram tests indicated compactibility began fluctuating more than normal in July 2014.
sand is drying out or losing compact- ibility from muller to mold. Te current variation in a sand
system can be quantified by defin- ing it with a standard deviation. Fig. 1 shows a standard deviation for compactibility using the 3-ram testing method at Grede-St. Cloud. Each data point represents the average of 30 3-ram tests. Tirty tests are used because that is considered a statistically sound sample size. As seen in the chart, the metalcasting operation was running in control in May and early June
but then veered out of control in July. Tis alerted Grede to investigate for equipment malfunctions. Grede found and repaired the mechanical issues in the system, after which the system went back into control. Te chart plotted from the automatic tester over the same period confirmed the
It’s important to build a history of normal ranges to be able to
determine when something is not operating regularly.
variation trend indicated by the 3-ram tests (Fig. 2). Hartley tests and results can be used in the same way. Te histogram from a Hartley test in Fig. 3 shows how many tests ran in an eight-hour time period. In this case, 197 tests ran in eight hours, 94 of which were +/-2 points from the target and 192 of which were +/-3 points from the target. Tests on compact- ibility should be per- formed daily to quantify and identify variation within the system. Compactibility can
be improved by reducing
Fig. 2. Data points from the automatic tester at the muller confirmed the fluctuation revealed by the 3-ram tests. 32 | MODERN CASTING March 2015
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