Foundry bentonite mainly includes sodium based bentonite and calcium sodium bentonite. Sodium based bentonite is processed from natural sodium based bentonite or artificial sodium based bentonite, and is popular in the foundry industry due to its good reusability and high wet compression strength. Due to its good plasticity, it can prevent sand inclusion, scarring, chipping, and sand mold collapse in castings. In addition, it has strong formability and high cavity strength, making it easy for the metal industry to cast wet or dry molds. It is the preferred sand binder for precision castings. Calcium sodium bentonite is processed from natural calcium sodium bentonite and is a widely used molding sand binder for lightweight castings. It is popular with users for its practicality and easy sand cleaning of castings.
Compared with other binders, bentonite has an important characteristic that it has a certain heat resistance. As long as the heating temperature is not too high, the free water removed bentonite can still restore its bonding ability as long as water is added. The cohesive ability of bentonite can only be shown after adding water. The loss of adhesion of bentonite is also related to its dehydration. Up to now, it is believed that there are three forms of water in bentonite: one type of water is free water, that is, water adsorbed by bentonite particles. When heated to above 100 ℃, free water can be removed. Bentonites that have removed free water have no impact on their binding ability; The second type of water is firmly bound water. When heated for a long time at 110 ℃, bentonite can completely remove free water, but it will not remove firmly bound water. Bentonite that has completely removed free water will continue to lose weight when heated at higher temperatures (such as 200 ℃, 300 ℃), indicating that there is still water loss. After the bentonite is heated and dehydrated in this way, as long as water is added, the adhesive ability can be completely restored; The three types of water are lattice water, also known as structural water. Lattice water can only be partially or completely removed at relatively high temperatures. After the lattice water of bentonite is removed, it loses its bonding ability and becomes a dead clay.
Different bentonite has different temperatures for loss of adhesion. According to the differential thermal analysis experiment, generally speaking, the failure temperature of natural sodium bentonite is 638 ℃, and that of calcium bentonite is 316 ℃. Due to different activation conditions and failure temperatures, artificially activated sodium bentonite is lower than natural sodium bentonite. It can be seen that there are two types of bentonite in clay bound sand, one is effective bentonite, and the other is dead clay. The more dead clay, the worse the performance of clay bonded sand.
When sodium bentonite is heated below 600 ℃, its bonding ability is basically unaffected. When the heating temperature exceeds 600 ℃, the adhesive ability is rapidly lost. When heated above 700 ℃, the adhesive ability is almost completely lost. When calcium bentonite is above 100 ℃, it slowly loses its bonding ability. The higher the heating temperature, the more obvious the loss of adhesion. When artificially activated sodium bentonite is not good, its failure temperature is only slightly lower than that of natural sodium bentonite during initial use. After several repeated heating, it is similar to calcium bentonite.
After the molten metal is injected into the mold, if the molding sand close to the surface of the casting is heated to above 800 ℃ (some non ferrous alloys cannot reach this temperature). No matter what kind of bentonite you use, the bentonite in this part of the molding sand will become dead clay. Some of these dead clays, along with molding sand, are attached to the surface of the casting and taken away by the casting, while some remain in the recycled old sand. In addition to manufacturing large castings, most molding sand cannot reach this temperature during the casting process. However, due to different foundry manufacturers using bentonite produced by different manufacturers, the temperature and rate of lattice water removal are different. If using bentonite that is easy to remove lattice water, even in molding sand that is not directly adjacent to the casting, there will be more bentonite that fails and becomes dead clay. If a clay that is not easy to remove lattice water is used, less dead clay will be produced. In short, the smaller the amount of bentonite added, the better. The higher the effective bentonite content in clay bound sand, the more bentonite will fail due to heating.
Post time: Mar-21-2023