| The subject of this dissertation is a study of the physics and chemistry of sodium bonding in foundry sands using three methods of curing sodium silicate. The dissertation also includes the application of inorganic compounds to improve the sodium silicate bond integrity in humid atmospheres and to promote easy core shakeout after the solidification of a casting.;Sodium atoms and hydroxyl groups present in the sodium silicate bond in cured compacted sand are primarily responsible for the hygroscopicity of the bond and its degradation. Sodium oxide (Na(,2)O) contributes to the poor core collapsibility of sodium silicate bonded sands because in combination with silicon dioxide, it forms an eutectic composition with a low melting point: 793 C (1459 F) for Na(,2)O(.)3SiO(,2) sodium silicate composition and 871 C (1600 F) for Na(,2)O(.)2SiO(,2) sodium silicate composition. Upon cooling, the glassy eutectic compositions form strong bonds that create difficulties in core shakeout.;Inorganic additives such as lithium silicate (Li(,2)SiO(,3)), lithium carbonate (Li(,2)CO(,3)), zinc carbonate (Z(,n)CO(,3)), calcium carbonate (CaCO(,3)), and magnesium carbonate (MgCO(,3)) improved upon the sodium silicate bond integrity in humid atmospheres and the shakeout characteristics of sodium silicate bonded sands. Among these additives, Li(,2)CO(,3), Li(,2)SiO(,3), and ZnCO(,3) proved to be the best additives for retaining the bond strength of sodium silicate in humid atmospheres. Similarly, ZnCo(,3) and CaCO(,3) additives impart shakeout characteristics to the sodium silicate binder as good as those of organic binders, if not superior. Therefore, blends of Li(,2)CO(,3) with ZnCO(,3) and CaCO(,3) provide both an excellent resistance to atmospheric humidity and an improved core shakeout characteristic. A blend of Li(,2)CO(,3) and corn flour cereal also improves the humidity resistance and the core shakeout of sodium silicate bonded sand. The Wedron 5010 silica sand was found to have the strongest bond with equivalent amounts of sodium silicate, followed by chromite sand, Michigan silica sand, New Jersey silica sand, olivine sand, and Michigan Bank sand, in that order.;The three distinct curing methods adopted in this research were the CO(,2) Process, the Microwave Energy Process, and a Hot Air Process. The CO(,2) process has been used to cure sodium silicate bonded sand since 1948. The microwave energy has been used to cure sodium silicate cores for the past five years; but the hot air process is new with this research. This hot air process is offered as a viable alternative for curing sodium silicate bonded sand. Cores cured by microwave energy and hot air are stronger than those cured by CO(,2). In terms of the initial core-strength and the retained bond-strength after exposure to 97 percent relative humidity for a period of 24 hours, however, the cores cured by hot air at 150 C (302 F) for 10 minutes are slightly superior to those cores cured by microwave energy at a power level of 6 kw for five minutes. |
