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corresponding to higher initial volumes for all temperatures. Hence, there is low accuracy in the molecular weight mea- surements when the change in gas volume is low, and this is especially true at lower temperatures. Therefore, a gas measurement cut-off criterion is imposed. The gas molecu- lar weight measurements are considered reliable when the

portion of height change in cylinder 2 from gas expansion g

∆h2 is greater than or equal to 0.8 cm (0.315 in). The gas cut-

offs for the pure gas expansion tests are shown in Figure 5 on each of the dimensionless molecular weight curves. It can be seen that the molecular weight measurements to the right of the gas cut-off are to within better than 5% of the true gas molecular weight. These “trusted” dimensionless molecular weight measurements also have good repeatability.

Figure 5 (a-b) indicates that the greatest deviation of the di- mensionless molecular weight from unity for either pure gas occurs between about 50C (122F) and 150C (302F). The GED was found to be the least isothermal within this tempera- ture range. Non-isothermality in the GED increases the error from equating the gas temperature with the metal tempera- ture, which subsequently increases the error in the molecular weight measurements. Figure 5 (a) shows that the error in the molecular weight measurements decreases with decreasing heating rate. The liquid metal acts as a large thermal mass, and lowering the heating rate reduces the temperature lag of the liquid metal compared to the other contents of the GED. It was concluded from these results that the binder gas tests were best performed at a low heating rate of 2°C/min (3.6°F/ min), which would maximize the isothermality of the GED and minimize the error in the molecular weight measure- ments. Figure 5 (a-b) also shows that dimensionless molecu- lar weight measurements for temperatures lower than 200C (392F) are closer to unity for the hydrogen tests compared to those for the argon tests. The initial volumes of hydrogen were somewhat greater than those for argon, which improved the accuracy of the hydrogen molecular weight measure- ments. In addition, the thermal conductivity of hydrogen is ten times greater than that of argon.32

This caused the hydrogen

to be more isothermal than the argon and further improved the accuracy in the hydrogen molecular weight measurements compared to argon. These findings further support the deci- sions to impose a gas measurement cut-off criterion and use a low heating rate of 2°C/min (3.6°F/min) for the binder gas evolution tests. Additional analysis of the GED isothermality and other experimental results for the expansion of pure gas in the GED may be found elsewhere.27

Binder Gas evolution tests

The measured height change as a function of temperature for all PUNB bonded sand tests performed using the GED are plotted in Figure 6. The heating rate was 2°C/min (3.6°F/min), and measurements during cooling (dashed lines) are shown when available. The different curves correspond to different initial bonded sand sample masses. Care must be taken when comparing them, since different initial sample masses result

International Journal of Metalcasting/Spring 2012

in different volumes of evolved gas. Figure 6 also shows the average metal-only height change for comparison. As ex- pected, decomposition of the PUNB bonded sand generates a significant amount of binder gas and dramatically increases the measured height change. In general, the height change increases monotonically with temperature. At approximately 585C (1085F), however, the height change rapidly increases and then suddenly decreases with increasing temperature. This substantial peak in the binder gas volume is reproducible and will be discussed in greater detail later in the discussion. After the height change rapidly decreases, it levels out and then continues to increase monotonically with increasing tem- perature. The maximum achievable temperature was limited by the maximum allowable height change and the maximum temperature of the furnace. Larger initial sample masses were used to achieve greater height changes and, hence, more ac- curate molecular weight measurements at low temperatures. Good repeatability in the height change measurements for similar sample masses can be observed. It can be seen that the height change during cooling decreases linearly for only the 0.637 g and 0.690 g tests. This indicates that the binder gas did not condense during the cooling portions of these tests. The binder gas behavior during cooling will be addressed in greater detail later.

Figure 7 shows the measured binder gas molecular weights for all PUNB bonded sand tests as a function of temperature. The testing conditions and curve coloring correspond to those of Figure 6, and only gas measurements meeting the gas cut- off criterion ( g

∆h2 ≥ 0.8 cm [0.315 in]) are included. It can be

seen that during heating, the binder gas molecular weight rap- idly decreases from 375 g/mol at 115C (239F) to 99.8 g/mol

Figure 6. Measured height change as a function of temperature during heating and cooling of different PUNB bonded sand sample masses. Solid lines indicate heating at a rate of 2°C/min (3.6°F/min), and dashed lines indicate cooling after the furnace was turned off.


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