In order to guarantee the quality of castings, the cores used in sand casting are generally dry cores.Depending on the binder used in the core, the core is divided into clay sand core, oil sand core and resin sand core.
1. Clay Sand Core
A simple core made of clay sand.
2. Oil Sand Core The core sand made with drying oil or semi-drying oil as binder is more widely used. The viscosity of oil is low, the core sand is good fluidity, the core is easy to tighten. But just made of core strength is very low, generally to use the shape of the core Board to undertake, and then bake in the oven for several hours in the 200~300℃, air oil oxidation and make it hardened.
The disadvantage of this method is: Mold Core in the release, handling and baking process prone to deformation, resulting in reduced dimensional accuracy of castings, long baking time, more energy.
3. Resin Sand Core Various cores made of resin sand. After hardening the core in the core box and then removing it, the shape and size of the core can be guaranteed. According to the hardening method, the manufacture of the resin sand core is divided into three methods: core-core, shell-core and cold-core box core-making. ① Hot Core Box core: appeared in the late 50. Furan resins are usually used as core-sand binders, which also include a potential hardener (such as ammonium chloride). When making the core, the core box is kept in 200~300℃, the core sand is injected into the core box, the ammonium chloride reacts with the free formaldehyde in the resin at a higher temperature to produce acid, so that the core is quickly hardened. It takes about 10-100 seconds to build the release strength. With hot core box, core size accuracy is higher, but the process device is complex and expensive, energy consumption, the expulsion of irritating gas, workers working conditions are very poor. ② Shell core is covered with sand thermal process core, the core strength is high and the quality is good; ③ cold Core Box Core: appeared in the late 60. The urethane resin was used as binder for core sand. When using this method, the core box is not heated and the amine steam is blown into it for a few seconds to harden the core. This method is superior to the hot core box method in energy, environment, production efficiency and so on. In the middle of the 70, there was a cold core-box method of furan resin blowing sulfur dioxide hardening.
The hardening mechanism is completely different from that of the urethane cold-box method, but the characteristics of the process, such as fast hardening, high core strength, and the urine is similar to the cold-core box method.
4. Water Glass Sand Core
The sand cores made with water glass as binder can be divided into the following types: Water glass CO2 method, ester hardened water glass self-hardening method, water glass methyl ester cold core box method.