Characteristics of wet mold and dry mold sand casting

Clay sand mold can be divided into wet mold, dry mold and surface drying mold. The main differences among the three are as follows: the wet mold is made by pouring the sand mold directly into the high-temperature metal liquid without drying; the dry sand mold is to send the whole sand mold into the kiln for drying before closing the box and pouring; the surface drying sand mold only uses appropriate methods to dry the surface of the mold cavity before pouring to a certain depth (generally 5-10 mm, larger than 20 mm).

At present, green sand is the most widely used and convenient molding method, accounting for about 60-70% of all sand mold use, but this method is not suitable for large or thick castings. The ratio of surface drying type to dry type can save oven, fuel and power, shorten production cycle, so it has been popularized in the production of medium and large iron castings. Generally, coarser sand particles (with high permeability), more clay and water are added, and sometimes 1-2% wood chips are added to the molding sand (to improve the ability of anti sand inclusion and scab), and the surface of the cavity must be coated with coating.

The dry mold is mainly used for heavy iron castings and some steel castings. In order to prevent the mold from cracking during drying, the bentonite is usually added together with the common clay. Dry mold mainly depends on coating to ensure the surface quality of castings. The quality of sand and sand mold is easy to control, but the production cycle of sand mold is long, special drying equipment is needed, and the dimensional accuracy of castings is poor. Therefore, in recent years, most of the dry mold, including the surface drying clay political mold, has been replaced by the chemical bonded self hardening sand mold.

The basic characteristic of wet casting is that the sand mold (core) does not need to be dried and there is no hardening process. Its main advantages are high production flexibility, high productivity, short production cycle, easy to organize flow production, easy to realize the mechanization and automation of production process, low cost of materials, saving drying equipment, fuel, power and workshop production area, and prolonging the service life of sand box, etc. However, some casting defects, such as sand inclusion, scarring, rat tail, sticking sand, porosity, sand hole, expanding sand and so on, can be easily produced by wet casting. With the development of casting science and technology, the understanding of the interaction principle between metal and mold is more profound; the control of molding sand quality is more effective; in addition, modern sand processing equipment ensures the quality of molding sand to a certain extent; advanced molding machinery makes the molding sand compact evenly, the molding is stable, and the quality of the mold is high, which promotes the expansion of the application scope of the wet casting method. For example, in automobile, tractor, diesel engine and other industries, the thin-walled iron castings with a mass of less than 300-500kg have been successfully cast by wet casting. Modern modeling methods include: general machine modeling, micro vibration compaction modeling, multi contact high-pressure modeling, injection pressure modeling, impact modeling and static pressure modeling.

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