Since the invention of the coated sand shell process by Johannes Cronin of Germany in the 1940s, the application of coated sand has increased year by year. Although silica sand is the most widely used foundry sand, its angular coefficient and thermal expansion rate are large, and it is easy to be broken. If the temperature is too high in the regeneration process, phase transformation will occur, and it is easy to be powdered, which will affect the quality of reclaimed sand. In order to solve the above problems, people began to look for other foundry sand to replace silica sand as the raw sand of coated sand. Ceramic foundry sand is a new type of artificial sand. It has good comprehensive properties, such as round shape, controllable particle size, and good regeneration effect because of its low expansion coefficient and hard particles. The surface precision of the casting obtained by using fine particle resin-coated ceramic sand in shell mold casting is as high as that of precision casting.
The old sand of coated sand is usually regenerated by thermal method. The purpose is to make the coated resin film coking and burning to complete removal under high temperature.
Advantages of Recycled Sand over New Sand
The surface of new sand is easy to absorb various impurities under the influence of external environment, which reduces its surface activity. Thermal regeneration mainly activates the sand surface. Because the reclaimed sand is obtained by high-temperature thermal action, in addition to the stress generated in the sand, the organic matter on its surface is oxidized and burned, and some inorganic matter is decomposed, which makes the sand regain high surface energy and increases the affinity between the sand and the binder. Under the action of surface tension, more adhesive can gather at the bonding bridge, increasing the bonding bridge area and improving the strength.
Regeneration Mechanism of Resin-coated Ceramic Foundry Sand
The binder of Resin-coated Ceramic Foundry Sand is thermoplastic phenolic resin, which is an organic binder formed by condensation reaction of phenol and formaldehyde. The phenolic resin cured at high temperature will decompose. The basic decomposition process of phenolic resin is that disproportionation reaction will occur in the intermediate products of resin curing process above 250 ℃. When the temperature reaches 300 ℃, in the presence of oxygen, the unsaturated methylene bridge -ch2, - in its molecular structure is destroyed, converted into hydroperoxide, and finally formed alcohols and ketones. Ketones are prone to free radical fracture, releasing a large amount of CO2, H2O, phenol and alkylphenol, and the decomposition process can last up to 600 ℃. With the increase of shrinkage above 600 ℃, the increase of conductivity will continue to emit gases and aromatic compounds.