Excerpt from February 2009 Foundry Management & Technology article by Nobuyoshi Sasaki.
A novel concept uses sodium and silica to increase the heat resistance of organically bonded molds and cores, which increases hot strength and allows the production of thin-wall castings.
Historically, foundries worldwide have been forced to rely on organic-based core and mold binders, and the use of refractory coatings, along with sand additives, when the hot strengths of the binders are insufficient to produce salable castings.
In 1995, Cadic Corp., and its subsidiary CTI Inc., introduced the Cadic Convert Process, which uses a liquid mixture of silicon oxide and sodium in alcohol to treat resin-bonded molds and cores. In practice, cores and molds produced using organic heat- and amine-cured resins were dipped or immersed in the ethanol solution, dried, and subsequently fired in a kiln to convert the organic-bonded article to an inorganic bonded mold or core.
This post-production treatment of organic-bonded aggregates resulted in the formation of organo-silicates that were converted to ceramics when fired at elevated temperatures, and poured hot to produce thin-wall (2 mm or less) stainless steel castings.
Laboratory evidence, supported by foundry trials, confirmed that the silica compound formed a crystal lacuna surrounding the sodium (Na) at room temperatures and allowed the Na ion to retain its high degree of reactivity.
The subsequent high-temperature treatment resulted in the conversion of quartz to tridymite, thereby increasing the high-temperature properties of the core or mold.
Because of the high degree of reactivity, the Na ion is utilized in this process as the alkali oxide Na2O. In this new process, the nano silica composition allows the Na ion to maintain its reactivity from room temperature to the boiling point of the compound.
With the elimination of a post-production treatment for the core or mold using ethanol as a carrier, the resulting new development makes the Cadic Convert Process considerably simpler, because the nano silica compounds can be added to the sand mix prior to the production of the core or mold in any organic resin-bonded processes, such as phenolic urethanes and shell sand mixtures.
Formation of silica compounds
Preparation of nano silica compounds is accomplished by blending alcoholic solutions of silicon oxide and sodium under controlled conditions. The resulting product is α-trydimite when reacted at 117°C.
The Na ion is captured within the lacuna or shell of the crystal formed without losing its reactivity of metallic sodium. When exposed to temperatures of approximately 800°C, the alkali oxide is formed and as the crystal begins to melt the Na ion reaches its boiling point, and begins to evaporate.
As the temperature is increased to 867°C, the silica crystal is converted into β-tridymite by the alkali oxide. As the temperature increases above 867°C, the Na ion evaporates and the black silica composition becomes a white tridymite crystal.
Thermal analysis of the SiO2-Na compound confirms that SiO2 is deoxidized at 778.91°C, and that the Na ion oxidizes at 882.91°C, generating Na2O. Further investigation confirms that the Na ion evaporates at 1,044.96°C.
Crystalline silica formation
Both TEM at room temperature and X-ray diffraction at 900°C confirm the formation of the trydymite crystal.
The quartz crystal converts into α-tridymite at lower temperatures but the conversion of quartz to β-trydimite at higher temperatures passes through cristobalite in the intermediate phase. The conversion of the crystal of the silica composition is similar to the conversion of quartz to tridymite.
The crystal lacuna, or shell, of α-tridymite has a radius of approximately 1.2 Å. The Na ion of the crystalline shell acts as a catalyst in the formation of the larger crystalline shell, as it expands during formation to 1.75 Å in the 800°C temperature range.
The Na ion, with a 1.1 Å radius expands and deoxidizes the SiO2, with the result that Na2O is liberated and forced out of the shell as a black by-product. Further temperature increases up to the boiling point of Na results in the distillation of Na, and the remaining silica crystal is converted to β-Tridymite at 867°C by the generated oxidizing alkali.
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