slight dip in the retained room temperature elastic modu- lus at 150C (302F), but it is within the standard devia- tion of the measurements. This indicates that the urethane bond breakage, which occurs up to about 300C (572F) (Figure 10), is fully reversible upon cooling. However, for hold temperatures above 275C (527F) the retained room temperature elastic modulus declines first gradually up to about 350C (662F), and then steeply to zero at about 400C (752F). This degradation in the stiffness can be ex- pected because above about 350C (662F) the binder starts to break down to polymer aromatics (Figure 10), which is an irreversible chemical reaction. Figure 15 clearly shows that bonded sand heated for a prolonged period of time at temperatures above 400C (752F) does not regain any stiffness upon cooling.
Conclusions
The present three-point bend elastic modulus measure- ments for PUNB bonded sand reveal a complex behavior during heating and cooling. Previous studies have only reported the steady-state elastic moduli after extended holding of the bonded sand at elevated temperatures. The present measurements indicate that, for temperatures above 100C (212F), the variation of the elastic modu- lus with temperature is a strong function of the heating rate and, hence, time. Upon cooling from temperatures above about 275C (527F), the permanent degradation of the binder prevents the bonded sand from regaining its original stiffness.
Before the present data can be used in stress simulations of a casting process, additional experiments are needed for higher heating rates. Within 2.54 cm (1 in) of the mold- metal interface, the heating rates are greater than 50C/ min (90F/min) (for a steel casting). Once elastic modulus measurements at higher heating rates are available, all data need to be correlated as a function of both temperature and heating rate. Furthermore, the variation during cool- ing deserves greater attention. In order to create a complete model for the elastic modulus of bonded sand, it appears necessary to develop a quantitative description of the tem- perature and time dependent chemical changes the binder undergoes during heating and cooling. Then, the elastic modulus could be correlated directly with the chemical state of the binder, rather than with temperature and rates of temperature change. Additional study of the effect of different binder compositions on the elastic modulus would also be of great value.
Acknowledgements
This article is based upon work supported by the U.S. Department of Energy under Award No. DE-FC36- 04GO14230. The authors would like to thank the Steel Founders’ Society of America for their support of this work. The authors would also like to express their grati- tude to Mr. Jerry Thiel and staff of the University of North- ern Iowa for their help with making the dump box and test specimens, and to Prof. Colby Swan of the University of Iowa for the use of lab equipment.
Figure 14. Steady-state elastic modulus as a function of the hold temperature.
International Journal of Metalcasting/Fall 10
Figure 15. Retained room temperature elastic modulus as a function of the hold temperature prior to cooling.
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