Ester cured alkali phenolic resin is a phenolic resin formed by condensation polymerization of phenol and formaldehyde catalyzed by strong alkali. It is named alkaline phenolic resin because its pH value reaches 12-14 and is strongly alkaline. As early as 1952, it was found that organic esters can be crosslinked and cured by adding them to the aqueous solution of basic methyl phenolic resin. In 1975, Japanese Nakazawa and others developed it as casting binder. In the 1980s, British Borden company invented alkaline phenolic resin ester cured self hardening sand (abbreviated as α- Set method) and apply this method to the foundry industry. The resin used is a water-soluble grade a phenolic resin polycondensated under alkaline conditions. The addition amount in molding sand is 1.5 ~ 2.5% of the weight of sand. The curing agent used is generally liquid esters such as low molecular lactone, low molecular carbonate and glycerol acetate, and its dosage is 20 ~ 30% of the weight of the resin. The amount of curing agent can not change the hardening speed, but can only change the hardening speed by adjusting the type of ester. Compared with furan resin self hardening sand, ester cured alkaline phenolic resin self hardening sand has its own unique advantages, in addition to the smooth surface, high dimensional accuracy and stability of sand castings with furan resin self hardening sand:
① The curing agent and binder do not contain nitrogen, sulfur, phosphorus and other elements harmful to sand castings, which eliminates the defects of sand castings caused by sulfur, phosphorus and nitrogen infiltration on the surface of sand castings; It can also effectively prevent the adverse defects of spheroidization of nodular cast iron caused by sulfur and phosphorus. It can be well used in the production of alloy steel, such as low alloy steel, stainless steel, manganese steel, etc. it is very suitable for the production of sand castings such as box and shell with thin wall and uneven wall thickness.
② Because the molding sand still maintains a certain plasticity during mold lifting, the mold lifting performance is good, the molding sand is not easy to adhere to the mold, the sand surface is relatively smooth, and the mold lifting slope on the pattern can also be small.
③ At room temperature, the organic ester can only make most of the alkaline phenolic resin cross-linking reaction, so it not only has a certain strength, but also has a certain plasticity. When pouring at high temperature, the resin without crosslinking continues the polycondensation reaction, which is called “secondary curing”. Secondary curing produces high thermal strength, which is the unique property of alkaline phenolic resin sand, which is not possessed by other resin sands. In the process of pouring and cooling, the explosion thermal expansion of alkaline phenolic resin sand is very small, and the high-temperature concession is good. It can greatly reduce the thermal crack and hairy wing defects of sand castings. It can also avoid premature collapse due to low sand mold strength at high temperature, resulting in sand flushing, sand inclusion and deformation of sand castings. It can effectively improve the product quality. It is a kind of resin self hardening sand favored by people in the industry in the production of steel castings.
④ The binder is not only suitable for ordinary silica sand, but also suitable for special sand, such as chromite sand, forsterite sand, jade bead sand, etc.
⑤ Sand mold has good collapsibility. After pouring, the alkaline phenolic resin self hardening sand is easy to collapse under the action of high temperature, which is conducive to preventing cracks in steel castings with complex shapes, and improving the sand falling performance of the mold after pouring.
⑥ The content of free formaldehyde and free phenol in the binder is relatively low, the pungent smell is small in the process of sand mixing, molding and pouring, and the pollution to the environment and harm to human body are small. It is an ideal environment-friendly casting binder.
Alkaline phenolic resin sand process has a wide range of applications, which is more suitable for carbon steel, high manganese steel, alloy steel, aluminum copper alloy and nodular cast iron. Therefore, it has been highly valued by casting workers at home and abroad. It is a resin self hardening method with good development prospects. Alkaline phenolic resin ester cured self hardening sand process has been used in the UK since 1980, and has gradually been recognized all over the world. The application of ester cured phenolic resin sand process has increased rapidly. In 1992, the alkali phenolic resin ester cured self hardening sand in the UK accounted for 40% of the cold hardening sand Market, and increased to more than 50% in 1994. Shenyang Foundry Research Institute first developed a-hardening method in China in 1986. It has been gradually popularized and applied in valve, water pump, petroleum machinery and heavy machinery industries. At present, some manufacturers in Jiangsu, Chongqing, Shenyang, Shandong and Shaanxi have built production lines to solve some major casting defects of steel castings. Because the alkaline phenolic resin sand process started late in China, its application has not been popularized in a large area, and the production of cast steel motor shell by this process in China is still in the experimental stage.
Although alkali phenolic resin sand has many advantages, it also has some disadvantages, which restrict its wide application: the specific strength of alkali phenolic resin self hardening sand is low. Compared with furan resin binder, the normal temperature strength of ester cured alkali phenolic resin binder is low, and the addition amount of binder is too large (furan resin 0.8-1.2%, alkali phenolic resin 1.4-2.0%), which makes the modeling and core making cost high. Due to the high alkalinity of alkali phenolic resin self hardening sand, the resin contains more alkali. During pouring, the alkali in the resin is easy to interact with silica sand to form silicate with low melting point. The molten silicate is firmly adhered to the sand particles and is not easy to remove, resulting in difficulty in the regeneration of old sand. The regeneration rate of furan sand combined with mechanical regeneration method can only reach more than 70-90%. The regeneration performance of alkaline phenolic resin self hardening sand is not satisfactory. The reuse of recycled sand will reduce the strength of self hardening sand. The product of resin cross-linking reaction is alkali metal carbonate, which is heated to more than 510 degrees, and the alkali metal salt is transformed into strong alkaline metal oxide. This oxide can interact with silica sand to form glassy alkali metal composite silicate film on the surface of sand particles. This film is glassy, which is not easy to be wetted by resin, When the resin is added to the recycled sand for re mixing, the resin is difficult to be evenly attached to the film on the surface of the sand, so the re bonding performance is poor, and the strength of the prepared self hardening sand decreases significantly. For alkaline phenolic resin self hardening sand, the residual alkali metal silicate film on the sand surface has a great influence on the re bonding. When dry friction regeneration is adopted, the bonding strength of recycled sand is significantly reduced. At present, the reuse amount of reclaimed sand in all countries in the world generally does not exceed 80%. The shortcomings of alkaline phenolic resin self hardening sand process need to be further studied by scholars, experts and technicians.