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at 200C (392F). Then, the molecular weight is approximately constant until 270C (518F), after which it decreases at a de- creasing rate to 51.6 g/mol at 500C (932F). After decreasing slightly further above 500C (932F), the binder gas molecular weight steeply decreases from 47.7 g/mol at 550C (1022F) to 30.3 g/mol at 585C (1085F) and then steeply increases to 47.2 g/mol at 630C (1166F). This steep decrease and increase in the binder gas molecular weight measurements corresponds to the peak in the height change measurements mentioned in connection with Fig. 6. The molecular weight remains essen- tially constant from 630C (1166F) to 750C (1382F). Recall that no additional binder gas mass is generated at tempera- tures above 710C (1310F). Beyond 750C (1382F), the mea- sured binder gas molecular weight gradually decreases to 33.3 g/mol at 898C (1648F), indicating that the binder gas compo- nents continue to react with each other after the solid binder decomposition is complete.


It can be seen from Figure 7 that the molecular weight measurements have excellent repeatability across the en- tire temperature range. It is also noteworthy how the mo- lecular weight curves from different experiments agree at 300C (572F). The volume of gas evolved from the larger sample masses of bonded sand is large by 300C (572F), which ensures that the corresponding binder gas molecular weight measurements at this temperature are accurate. Figure 7 shows that the gas measurement cut-off (i.e., the point where the molecular weight measurements become reliable) when smaller amounts of bonded sand are employed corresponds to temperatures near 300C (572F). The good agreement between the binder gas mo- lecular weight measurements at 300C (572F) for different amounts of bonded sand proves that the imposed gas mea- surement cut-off provides an effective means to differen- tiate between reliable and unreliable binder gas molecular weight measurements.


When the binder gas molecular weight was determined dur- ing the cooling portion of the tests, the binder gas mass was assumed to be constant and equal to the value at the test’s maximum temperature. This assumption is valid only if the binder gas does not condense during cooling. The binder gas molecular weight cannot be calculated if condensation occurs, since the mass of the gas then decreases in an unknown man- ner. Figure 7 shows binder gas molecular weight measurements during cooling for those tests that clearly exhibited no binder gas condensation (dashed lines). It can be seen that the binder gas is composed of fixed gases for all tests where the GED was heated to temperatures above about 710C (1310F). Interestingly, this temperature corresponds to the one where the decomposition of the bonded sand samples ceases based on the TGA measurements.


34


Figure 7. Binder gas molecular weight measurements as a function of temperature during heating and cooling of different PUNB bonded sand sample masses. The color scheme of the curves follows that of Figure 6. The heating rate was 2°C/min (3.6°F/min), and measurements during cooling are shown for tests that clearly exhibited no binder gas condensation.


Table 4. Primary Sources of Error in the Binder Gas Molecular Weight Measurements


The estimated error in the measurement of the binder gas molecular weight during heating was determined through a detailed root-sum-squares error analysis. The primary sources of error in the binder gas molecular weight mea-


surements are shown in Table 4, with average values re-


β ). The significance of each primary error source on the molecular weight measurements is related to the total ini- tial PUNB bonded sand sample mass and the measurement temperature. For example, when the bonded sand sample mass is large, more binder gas mass is evolved and the er- ror in the fraction of original binder mass remaining has a greater impact on the molecular weight measurements. The average error in the present binder gas molecular weight measurements between 115C (239F) and 898C (1648F) is estimated to be 6%.


ported for parameters that are temperature-dependent (f and eff m


International Journal of Metalcasting/Spring 2012


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