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TESTING 1-2-3


Preventing Sand Mold and Core Defects


Researchers evaluated new mineral additives for veining, penetration, surface finish and core quality properties. A MODERN CASTING STAFF REPORT


metalcasting facilities try to use a local sand to control shipping costs, the dif- ferences also tend to be regional. Sand from Wisconsin will be different from the sand in Texas or New Jersey. Tis is even more true globally. Sands from the United States are expected to perform differently from European, South American or East Asian sands. Aggregates, iron oxides and organic


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additives have been used extensively to control veining. Other strategies include coating the mold prior to cast- ing. Recently, new mineral additives have been developed to reduce veining at relatively low levels. Researchers Ralph E. Showman


and Sean B. Harmon with ASK Chemicals, Dublin, Ohio, conducted testing to better understand these new additives’ performance characteristics versus other sand additives currently in use. Building on previous research, they published the results in the paper, “Further Evaluations of Anti-Veining Sand Additives.”


Question How do the three new types of


mineral additives recently introduced into metalcasting production compare to existing additives?


Background


Casting defects in general and specific sand expansion defects like veining depend on sand properties. Veining prob-


lems will vary based on the sand most April 2013 MODERN CASTING | 37


and performance characteristics vary depending on its chemistry, grain shape and grain size distri- bution. Because most


widely used in a geographic region. Te methods for dealing with those problems also are different. High purity silica sand can be


replaced with other aggregates such as lake or bank sand, zircon, chromite, oliv- ine, fused silica or manmade materials. Te lower and more uniform expan- sion of these materials can minimize or eliminate veining. Tey are more costly than silica sand and may present special problems with molding or coremaking. Metalcasters have used sand addi-


tives extensively to control veining. Tese fall into several categories depending on their chemistry and activity. Iron oxides were among the first sand additives. Tese create a small reduction in volume as they lose oxygen and have a “fluxing” or


ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.


“Further Evaluations of Anti-Veining Sand Additives,” Ralph E. Showman and Sean B. Harmon, ASK Chemicals, Dublin, Ohio


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Background—Numerous types and formulations of antiveining sand additives currently are used throughout the world. Three new types of mineral additives have been developed and introduced into metalcasting production recently. The researchers conducted a series of tests to compare the performance of these new additives to those previously in use. Procedure—Testing included casting evaluations for veining, penetration and surface finish. Core testing determined the impact on core tensile strength, benchlife, tensile strength at elevated humidity, and hot distortion. The tests included sands and phenolic urethane coldbox binders from several geo- graphic regions including Europe, China, North America and Brazil. Further testing was conducted to show the effects of some of the additives on the expansion curves of the mixed and bonded sand. Results and Conclusions—The tests confirmed different sands and binder compositions perform differently on a regional basis. This indicates sand addi- tive needs also vary regionally. The additives are effective because they both reduce the initial expansion at the alpha/beta transformation of the silica sand mix and reduce the amount of contraction following the expansion.


is somewhat coarser and may be used at 1-4% levels. Red iron oxide has proven effective when used in conjunc- tion with other sand additives. Organic materials like dextrin, starch


are typically used at levels of 1-2% but are very fine and may impact mold and core strength. Black iron oxide (Fe3


softening effect on the surface of the sand grains. Red iron oxides (Fe2


O3 ) O4 )


and wood flour are used at relatively low levels of 0.5-2%. At elevated tempera- tures, but below the alpha/beta transition for quartz, these will burn out, provide a volume reduction and “cushion” the expansion of the sand. Like iron oxides, these materials may have negative effects on mold/core strength because the fineness of the material increases resin demand and reduces strength.


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