Table 2. Average (mm) and Standard Deviation Values from DOE Trials over Time
dition followed by blow pressure, the level of fused silica, and benchlife. Resin levels, resin type, and gas time had minimal effects. As expected, the water addition reduced core weight, likely because of reduced flowability of the sand. Higher blow pressure increased weight, more fused silica reduced weight because of the lower density of the material compared to the core sand, and extended benchlife decreased core weight, again likely because of decreased flowability.
Core strength effects also were as expected. The water ad- dition caused a very significant loss in strength while the binder type and percentage also impacted strength. The fused silica level, blow pressure, benchlife, and gas time had more minor effects.
The effects of the different variables on core shrinkage were less clear. This was first indicated by the shrinkage graphs for each mix shown in Figure 10. The graphs are not parallel but intersect over time. This suggests the effects of one variable may be strong early but become weaker over time. Or in some cases, the effects of the variables may reverse over time, with the lower level producing more shrinkage in one time period and less in another. Figure 11 shows the ANOVA tables and shrinkage level averages for the different variables at 1, 4, and 24 hours. At 1 hour, benchlife, binder level, and binder type seem to have the greatest effects with the fresh mix, lower binder level, and older binder system producing more shrinkage. At 4 hours, the same three variables continue to dominate, but the ef- fects and differences between levels are less. By 24 hours, the benchlife effects are minimal while the effect of the water addition has become the most important factor, re- versed from the 1-hour results.
International Journal of Metalcasting/Summer 10
Second DOE
Because of the unexpected results for the binder level, benchlife and water addition, a second, smaller DOE was conducted to look at only those variables. An L4 array was constructed as shown in Table 3. To reduce the chance of errors associated with handling or day to day environmental changes, all the mixes were produced the same day, and two cores from each mix were placed into the test fixture and measured over the same 24 hour period. Additional mea- surements also were taken at 48, 72, and 168 hours to ensure the dimensional changes had truly stabilized. The shrinkage results are shown in Table 4 and Figure 12.
Once again the graphs did not remain parallel and inter- sected over time. This confirmed the relative effects of each variable were not constant. The level averages at 1, 4, and 24 hours were again calculated and graphed in Figure 13. The apparent results were somewhat contradictory to the first DOE. The 1-hour shrinkage was greater with higher binder levels, a 2-hour benchlife, and no water addition. This was opposite of the results of the original DOE. At 24 hours, higher binder levels still showed more shrinkage, but benchlife showed little effect. The water addition showed more rather than less shrinkage.
Discussion of DOE Results
The DOE results are contradictory and confusing. This may be an indication of interactions between the variables that were not taken into account in the DOE design and con- founded the results. It also could indicate environmental variables affected the results, even though the trials were conducted under laboratory conditions with controlled tem-
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