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Casting is the process of melting metal into liquid that meets certain requirements and pouring it into the mold. After cooling, solidification and cleaning, castings with predetermined shapes, sizes and properties are obtained. Because the billet is almost formed, the purpose of no processing or a small amount of processing is achieved, and the cost and time are reduced to some extent. Casting is one of the basic processes of modern machinery manufacturing industry.
There are many kinds of casting, which are customarily divided into: ① ordinary sand casting, including wet sand mold, dry sand mold and chemically hardened sand mold. ② Special casting, according to the molding materials, can be divided into special casting with natural ore as the main molding material (such as investment casting, clay casting, shell casting in casting workshop, negative pressure casting, full mold casting, ceramic mold casting, etc. ) and special casting with metal as the main mold material (such as metal mold casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc. ). The casting process usually includes: ① Preparing molds (containers for making solid castings from liquid metal), which can be divided into sand molds, metal molds, ceramic molds, clay molds, graphite molds, etc. According to the times of use, it can be divided into disposable mold, semi-permanent mold and permanent mold, and the quality of mold making is the main factor affecting the quality of castings. (2) Melting and pouring of casting metals, casting metals (casting alloys) mainly include cast iron, cast steel and casting nonferrous alloys; (3) Casting processing and inspection. Casting treatment includes removing foreign bodies from the core and the surface of the casting, cutting off the riser, shoveling off burrs, overlapping seams and other protrusions, as well as heat treatment, shaping, rust prevention and rough machining.
The casting process can be divided into three basic parts, namely, casting metal preparation, mold preparation and casting treatment. Casting metal refers to the metal material used to pour castings in casting production. It is an alloy with a metallic element as the main component and other metallic or nonmetallic elements. Traditionally called cast alloy, it mainly includes cast iron, cast steel and cast nonferrous alloy.
Metal melting is not only a simple melting, but also a melting process, which makes the metal poured into the mold meet the expected requirements in temperature, chemical composition and purity. Therefore, in the process of smelting, all kinds of inspections and tests aimed at quality control should be carried out, and liquid metal can only be poured after reaching various specified indexes. Sometimes, in order to meet higher requirements, molten metal must be treated outside the furnace, such as desulfurization, vacuum degassing, refining outside the furnace, inoculation or deterioration. Commonly used metal smelting equipment includes cupola, electric arc furnace, induction furnace, resistance furnace and reverberatory furnace.
Different casting methods have different mold preparation contents. Taking sand casting, which is the most widely used, as an example, mold preparation includes two major tasks: molding material preparation and molding core making. All kinds of raw materials used for modeling and core-making in sand casting, such as casting sand, sand binder and other auxiliary materials, as well as molding sand, core sand and coatings made from them, are collectively called modeling materials. The task of preparing molding materials is to select suitable raw sand, binder and auxiliary materials according to the requirements of castings and the properties of metals, and then mix them into molding sand and core sand with certain properties according to a certain proportion. Commonly used sand mixing equipment includes drum mixer, counter-current mixer and blade slot mixer. The latter is specially designed for mixed chemical self-hardening sand, with continuous stirring and high speed.
On the basis of determining the modeling method and preparing modeling materials, modeling and core-making are carried out according to the requirements of casting process. The precision of castings and the economic effect of the whole production process mainly depend on this process. In many modern foundry workshops, modeling and core-making have been mechanized or automated. Commonly used sand mold core-making equipment includes high, medium and low pressure molding machine, sand throwing machine, boxless injection molding machine, core shooting machine, cold and hot core box machine and so on.
After the casting is taken out of the pouring cooling mold, there are gates, risers and metal burrs, and sand is still attached to the casting cast by sand mold, which must be cleaned up. The equipment for this kind of work includes shot blasting machine, gate and riser cutting machine, etc. Sand removal and cleaning of sand castings is a process with harsh working conditions, so when choosing the molding method, we should try our best to create convenient conditions for sand removal and cleaning. Due to special requirements, some castings need post-treatment, such as heat treatment, shaping, rust prevention, rough machining and so on.
Casting is an economical blank forming method, which can show its economy for parts with complex shapes. Such as cylinder block and cylinder head of automobile engine, propeller of ship and exquisite works of art. Some difficult-to-machine parts, such as nickel-based alloy parts of gas turbines, cannot be molded without casting.
In addition, the size and weight of castings are widely applicable, and the types of metals are almost unlimited; Parts not only have general mechanical properties, but also have comprehensive properties such as wear resistance, corrosion resistance and shock absorption, which cannot be achieved by forging, rolling, welding, stamping and other metal forming methods. Therefore, the number and tonnage of blank parts produced by casting in machinery manufacturing industry are still the largest.
The materials often used in casting production are various metals, coke, wood, plastics, gas and liquid fuels, modeling materials and so on. The required equipment includes various furnaces for smelting metals, sand mixers for mixing sand, molding machines for making cores, core-making machines, sand mixers for cleaning castings and shot blasting machines. There are also machines and equipment for special casting and many transportation and material handling equipment.
Casting production is different from other processes, such as wide adaptability, many materials and equipment, and great environmental pollution. Foundry production will produce dust, harmful gas and noise pollution to the environment, which is more serious than other mechanical manufacturing processes and needs to be controlled by measures.
The development trend of casting products is to require better comprehensive performance, higher precision, less allowance and smoother surface. In addition, the demand for energy conservation and the call for social restoration of the natural environment are getting higher and higher. In order to meet these requirements, new casting alloys will be developed, and new smelting processes and new equipment will also appear accordingly.
With the continuous improvement of mechanization and automation of foundry production, more flexible production will be developed to expand the adaptability to different batches and varieties of production. Give priority to the development of new technologies that save energy and raw materials, and give priority to the development of new technologies and equipment that are less polluting or pollution-free. Quality control technology will make new progress in detection, nondestructive testing and stress determination of each process.