at even the shortest dip time. In order for wetting to oc- cur, air in the pores of the sand disk must be displaced. The time required to displace this air is the time required for wetting to occur. Complete wetting occurs when all air is displaced. Since the disks were all dipped to the same depth, and the porosity of the sand cores can be considered relatively constant, the differences observed can only be contributed to differences in coating proper- ties (viscosity and surface tension). Comparison of the viscosity data would indicate that the 0.35% surfactant level should wet faster due to its lower initial viscos- ity, but this is not the case, indicating that surface ten- sion forces override the effects of viscosity on capillary forces. The surfactant level had an effect on the overall thickness of the coating deposited on the specimens. As the surfactant level decreased so did the amount of proud and pene- trated coating deposited (Figure 6).
Surfactants reduce the surface ten- sion of the liquid components in the coating, which increases its ability to penetrate between the sand grains. As the surfactant level was reduced, the coating deposited fewer solids (dry coat) on the specimens. The dip time did not have a great effect on the wet coat weight (Figure 7) for the medium and high surfactant lev- el coatings. However, dip time had some effect on the wet coat weight for the low surfactant level coating. Figure 8 also shows clear differences in the measured dry coat weights at different dip times within the low surfactant level coating. There was a significant difference between the dry and wet coat weights among the three surfactant levels for the same dip time. This is an important finding in demonstrating the effectiveness of surfactants in reducing the time nec- essary to deposit the same amount of coating on a mold or core.
Dry Coat Vs. Wet Coat
An important part of this study was determining the relationship between the amounts of coating deposited on the specimen as it relates to dip time and percent surfactant in the coating. As Figure 7 and 8 illustrate, a decrease in the wet and dry coat weights was observed as the surfactant level decreased. This
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indicates that an increase in surfactant level will not only promote penetration by the liquids in the coating into the sand specimen, it will also deposit more solids on the proud layer of the specimen. Previous research in the foundry coatings area has found an increase in the proud layer provides some anti-veining characteristics on resin-bonded sand molds.2
Figure 6. Coating penetration at various surfactant levels.
Figure 7. Relationship between the wet coat weight and surfactant levels at various dip times (avg. of three replicates).
Figure 8. Relationship between the dry coat weight and surfactant levels at various dip times (avg. of three replicates).
International Journal of Metalcasting/Spring 11
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