(Figures 5 to 7 and Table 3). The mechanical properties will recover once the solvents and additives are lost, which re- sults in the observed increase of strength at 300C (572F), although this can also be attributed to enhancement in cross- linking occurring at the higher temperature.14


The gas evolved while heating samples of AFS GFN 52 sand with various mixtures of resin is shown in Figures 8 and 9. The graphs show a tendency for gas to evolve at a continuous rate until it reaches a steady state. Heating the samples at ei- ther 70C or 150C (158 and 302F) contributes to the reduction in the amount of gas produced. Figure 15 plots the values of gas produced during the interval of 200 to 250 seconds from the various samples. It is found that the amount of gas from samples prepared with either 0.80 or 0.90% of resin B is 20% higher than that evolved in samples prepared with similar amounts of resin A. These results may be attributed to the higher losses of additives of low molecular weight that were detected in the TGA of samples of resin B. It is not clear the reason for the reduction in the amount of gas from samples prepared with 1.15% of resin B in comparison with resin A.


Conclusions


The study compares two resin binders used to produce cores for automotive parts. The results indicate that the best char- acteristics are obtained with resin A, as the cores produced exhibit higher strength and the amount of gas evolved is lower than in cores produced with resin B. The only charac- teristic in which resin B shows better results is with respect to bench life. The lower amount of gas produced by resin A may result in the increase of surface quality of the pieces produced, as the incidence of trapped gas may be reduced.


The work shows the increase in strength that results by aug- menting the resin used to make the cores, but the increment


of gas evolved may correspond to the increment of defective parts due to generation of bubbles and other gas-associated defects. The use of finer sand contributes to the increase in strength, so it would be possible to reduce the amount of resin in cores that are subjected to higher stresses without impairing their characteristics. This effect can be used to produce high quality castings. While the use of fine sand is avoided due to the occurrence of gas-trapped defects, if the amount of resin is reduced, the amount of gas produced will be reduced as well.


Acknowledgements


The authors would like to thank the National Council for Science and Technology (CONACYT), Mexico for their fi- nancial support.


REFERENCES


1. Norris, P.M., Hastings, M.C., and Wepfer, W.J., “Heat Transfer Regimes in the Coolant Passages of a Diesel Engine Cylinder Head,” J. Exp. Heat Trans., vol. 7, issue 1, pp. 43-53 (1994).


2. Campbell, J., Castings, 2nd Heinemann, Oxford (2003).


Ed., Butterworth-


3. Gundlach, R.B., Ross, B., Hetke, A., Valtierra, S., and Mojica, J.F., “Thermal Fatigue Resistance of Hypoeutectic Aluminum-Silicon Casting Alloys”, Transactions of the American Foundrymen’s Society, vol. 102, pp. 205-223 (1994).


4. P.M. Norris, M.C. Hastings and W.J. Wepfer, J. Exp. Heat Trans., 7, 43 (1994).


5. Wile, L.E., Strausbaugh, K., Archibald, J.J., Smith, R.L. and Piwonka, T.S., “Coremaking”, ASM Handbook, Casting, vol. 15, p. 238, ASM International, Materials Park, OH (1988).


Figure 14. Strength of cores prepared with different amounts of resin tested at room temperature and at 150C and 300C.


International Journal of Metalcasting/Winter 11


Figure 15. Gas evolved in samples of sand with different amounts of resin.


47


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87