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conclusions


The present measurements reveal the complex nature of the gas evolution from PUNB bonded sand during heating and cooling. The TGA experiments provide the detailed variation of the binder mass with increasing temperature. The binder is found to be fully pyrolyzed at temperatures above 710C (1310F). At that temperature, 82% (by mass) of the original binder has become gaseous. Up to 100°C/ min (180°F/min), the measured binder mass decrease does not appear to be a strong function of the heating rate. The observed differences in the TGA measurements among the different heating rates are likely due to sample non-isother- mality, and not due to chemical kinetics. The present GED measurements for a heating rate of 2°C/min (3.6°F/min) show that the molecular weight of the gas evolved from PUNB bonded sand decreases in a complex manner from 375 g/mol at 115C (239F) to 33.3 g/mol at 898C (1648F). When the binder is fully pyrolyzed, continued chemical reactions within the gas cause its molecular weight to de- crease from 48 g/mol at 710C (1310F) to values as low as 17.4 g/mol at 1350C (2462F) and beyond. In this tempera- ture range, the binder gas does not condense during subse- quent cooling to room temperature. When the bonded sand is not heated to temperatures above about 510C (950F), the binder gas partially condenses during subsequent cooling. This condensation starts to occur at approximately 165C (329F). For easy use in casting simulations that include calculation of gas evolution in the mold and cores, the pres- ent data are fit to polynomials that describe the binder gas mass and molecular weight variations with temperature.


Since the present TGA measurements reveal that the binder gas mass evolution is not a strong function of the heating rate up to 100°C/min (180°F/min), it may be concluded that the present data can be used to describe the gas evolu- tion behavior at any distance from the mold-metal inter- face. Such a conclusion may be erroneous and additional measurements at very high heating rates, corresponding to locations very near the mold-metal interface26


, are needed


to investigate the rate dependency. Additional TGA mea- surements are also needed to investigate the dependence of the binder decomposition on the composition of the at- mosphere. The present gas evolution measurements extend only up to about 900C (1652F), with the extrapolation to higher temperatures based purely on previous data. This extrapolation should be verified by additional measure- ments. From a fundamental point of view, it would be de- sirable to further investigate the exact cause of the inverse peak in the measured binder gas molecular weight near 585C (1085F). The nature of the binder gas condensation observed at low temperatures is also an item that requires additional research attention, since such condensation can affect the flow of gases in outer portions of the mold. While the present study focused on PUNB bonded sand, the mea- surement of gas evolution for different binder and sand systems would be highly valuable.


International Journal of Metalcasting/Spring 2012


acknowledgements


This work was supported by the U.S. Department of Energy under grant number DE-FG36-06GO86029 through a sub- contract with the University of Northern Iowa. The authors would also like to express their gratitude to Professor Scott Giese of the University of Northern Iowa Department of In- dustrial Technology for his experimental support, Mr. Jerry Thiel and the staff of the University of Northern Iowa Metal Casting Center for their help with making the bonded sand specimens, Mr. Peter Hatch of the University of Iowa De- partment of Chemistry for his expertise in constructing the GED and all other quartz components, and Professors Allan Guymon and Gary Aurand of the University of Iowa Depart- ment of Chemical and Biochemical Engineering for the use of their TGA equipment.


references


1. Campbell, J., Castings, 2nd ed., pp. 99-116, Butterworth-Heinemann, Oxford, United Kingdom (2003).


2. Rahmoeller, K.M., “Mold Binder Decomposition: Prime Source of Cast Iron Defects Part 4 of 4,” Modern Casting, vol. 83, no. 10, pp. 36-39 (1993).


3. Bates, C.E., and Burch, R., “Core and Mold Gas Evo- lution: Porosity in Castings,” Foundry Management & Technology, vol. 135, no. 5, pp. 17-18 (2007).


4. Gibbs, S., “Illuminating Core Gas,” Modern Casting, vol. 98, no. 10, pp. 34-37 (2008).


5. Worman, R.A., Nieman, J.R., “A Mathematical System for Exercising Preventive Control over Core Gas Defects in Gray Iron Castings,” AFS Transactions, vol. 81, pp. 170-179 (1973).


6. Levelink, H.G., Julien, F.P.M.A., De Man, H.C.J., “Gas Evolution in Molds and Cores as the Cause of Casting Defects,” AFS International Cast Metals Journal, vol. 6, pp. 56-63 (March 1981).


7. Naro, R.L., Pelfrey, R.L., “Gas Evolution of Synthetic Core Binders: Relationship to Casting Blowhole Defects,” AFS Transactions, vol. 91, pp. 365-376 (1983).


8. Naro, R.L., “Porosity Defects in Iron Castings from Mold-Metal Interface Reactions,” AFS Transactions, vol. 107, pp. 839-851 (1999).


9. Monroe, R., “Porosity in Castings,” AFS Transactions, vol. 113, pp. 519-546 (2005).


10. Rogers, C., “Numerical Mold and Core Sand Simulation,” in Modeling of Casting, Welding, and Advanced Solidification Processes-X, eds. D.M. Stefanescu, J.A. Warren, M.R. Jolly, and M.J.M. Krane, TMS, Warrendale, PA, 2003, pp. 625-632.


11. Kimatsuka, A., Ohnaka, I., Zhu, J.D., Sugiyama, A., and Kuroki, Y., “Mold Filling Simulation for Predicting Gas Porosity,” in Modeling of Casting, Welding and Advanced Solidification Processes-XI, eds. C.A. Gandin and M. Bellet, TMS, Warrendale, PA, 2006, pp. 603-610.


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