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It can be seen from Figure 3 that good repeatability in the bonded sand decomposition is achieved at each heating rate. There are increasing delays (in terms of temperature) in the decomposition with increasing heating rate. The binder de- composition reactions can be expected to be time dependent in nature, and an increase in the sample heating rate may then shift the decomposition to higher temperatures. More likely, the delays are simply caused by greater non-isother- mality of the bonded sand samples for higher heating rates. For high heating rates, the center of the bonded sand sam- ples can be expected to be somewhat cooler than the surface. This implies that the lower heating rate results better reflect the “true” decomposition behavior at a given temperature. Overall, however, the decomposition behavior is fairly simi- lar between heating rates and, to a first approximation, may be assumed to be the independent of the heating rate within the range studied. It was later found that the heating rate of the GED needed to be low in order to ensure reasonable uniformity in temper- ature during the binder gas molecular weight measurements (see below). In light of this, only the 2°C/min (3.6°F/ min) measurements of the fraction of original binder mass remaining were fit to a set of piecewise polynomials for use in the binder gas molecular weight calculations. Additional modifications were made such that the fitted fraction of original binder mass was set to be


exactly equal to unity at or below 50C (122F) and to 0.1807 (the average binder mass fraction remaining at the conclu- sion of the decomposition) above 710C (1310F). The poly- nomial fit of the fraction of original binder mass remaining is superimposed on the corresponding measurements in Figure 3, and the equations of the polynomials are listed in Table 3.


Metal-only expansion tests


Figure 4 shows the measured metal-only expansion in the GED during heating at constant rates ranging from 2°C/min (3.6°F/min) to 15°C/min (27°F/min). The heating rate was always constant to within ±0.4°C/min (0.7°F/min). There is excellent repeatability in the height change measurements regardless of heating rate. It can be seen that the expansion


Table 3. Piecewise Polynomial Fitted to the Fraction of Original Binder Mass Remaining Measured during TGA Pyrolysis of PUNB Bonded Sand Samples Heated at a Rate of 2°C/min with a Total Argon Gas Flow of 25 cm3


/min.


Figure 3. Measured fraction of original binder mass remaining as a function of temperature during heating of PUNB bonded sand samples at rates of 2°C/min (3.6°F/ min), 10°C/min (18°F/min), and 100°C/min (180°F/min) with a total argon gas flow of 25cm3


/min. The piecewise


polynomial fitted to the measurements corresponding to the 2°C/min (3.6°F/min) heating rate is also shown.


International Journal of Metalcasting/Spring 2012


Figure 4. Comparison of measured and predicted metal- only height change as a function of temperature during heating. A constant volumetric expansion coefficient (obtained from separate experiments) was used to predict the metal expansion.


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