Table 4. Photos of the Casting Surfaces Produced With the Different Mixes Sand Mix
NA Sand No additive EU Sand SA Sand
China (CN) Sand 1% Woodflour
between silica sand with no additive and with ESA1 and ESA2, and relative little contraction, followed by secondary expansion. It is believed this secondary expansion indicates the transformation of the silica from beta quartz to tridymite. Te expansion curves seem to con-
1% RIO
1% starch
2% BIO
2.5% ESA1
4% ESA2 6% ESA3
firm the results of the casting tests with the current theory for veining defects. If the initial contraction can be reduced, there is some small benefit as shown with the starch additive. However, the primary benefit seems to be the reduc- tion in contraction following the initial expansion. If the sand mix shows little contraction, or even a secondary expan- sion following the alpha-beta expansion, veining seems to be well controlled. Significant differences in the sands and binder compositions’ performance of on a regional basis indicate sand additive needs also vary regionally. Te high purity, round grain silica sand from North America has excellent mold- ing and coremaking properties, but has the greatest tendency for veining. Te Brazilian and Chinese systems provide lower strength and coremaking proper- ties, but are much more forgiving for veining defects. Te European system provides good strength, but with higher binder levels and moderate veining. Te dilatomer studies at UNI and
6% old ESA
to remain across the different systems. For instance, the starch additive was consistently worse than the ESAs, but performed better on the SA and China systems where veining was less severe on the cores with no additive. Tis indicates, as expected, that systems with lower veining tendencies can still achieve good results with less effective additives, but systems with more severe veining ten- dencies need more powerful additives. Much effort has gone into under-
standing and explaining the veining defect formation and the possible mechanism of anti-veining materials. Jerry Tiel and Sairam Ravi at the Univ. of Northern Iowa, Cedar Falls, Iowa, conducted dila- tometer studies for thermal expansion on
40 | MODERN CASTING April 2013
some of the sand mixes used in this study. As shown in earlier studies, the unbonded Wedron sand showed a rapid expansion corresponding to the alpha/beta transfor- mation of the silica, followed by a gradual then increasing contraction. Te addi- tion of the starch base additive, 1% AFX, reduced the amount of initial expansion, but was followed by contraction similar and parallel to the unbonded sand. Te ESA1 additive had a different
effect. It both reduced the initial expan- sion and minimized the contraction phase. Te ESA2 had a similar curve, but at a 4% additive level rather than the 2.5% of ESA1. Te older generation ESA with more history of antiveining perfor- mance had intermediate initial expansion
the resulting expansion curves seem to provide the reason for the good perfor- mance of the engineered sand additives. By reducing initial expansion and minimizing the subsequent contraction, the ESAs apparently minimize the for- mation of tensile stresses on the surface of the cores and prevent the formation of surface cracks and the resulting veining defects. Te correlation of the curves to test casting performance also supports the use of the dilatomer in the development of other new additives.
Tis article is based on a research paper presented at the 2012 AFS Metalcasting Congress.
ONLINE RESOURCE
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www.moderncasting.com to read the paper on which this article was based.
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