Casting-melting metal, making a mold, pouring the molten metal into the mold, and solidifying to obtain a metal part blank with a certain shape, size and performance.
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 shaped, it can achieve the purpose of no processing or a small amount of processing, which reduces the cost and production time to some extent. Casting is one of the basic processes in modern device manufacturing industry. There are two main categories: sand casting and special casting.
1 sand used for ordinary sand mold casting is also called sand mold casting and sand turning, including three types: wet sand mold, dry sand mold and chemically hardened sand mold, but not all sand can be used for casting. The advantage is that the cost is lower, because the sand used in the mold can be reused; The disadvantage is that the mold is time-consuming, and the mold itself cannot be reused, and the finished product can only be obtained after it is destroyed.
1. 1 sand mold (core) casting method: wet sand mold, resin self-hardening sand mold, sodium silicate sand mold, dry type and surface dry type, full mold casting and negative pressure molding.
1.2 manufacturing method of sand core: it is selected according to the size, shape, production batch and specific production conditions of sand core. In production, it can be generally divided into manual core making and machine core making.
Special casting can be divided into two types according to molding materials: special casting with natural ore as the main molding material (such as investment casting, mud casting, shell casting, 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. ).
2. 1 permanent mold casting method
Make molds with metals with higher melting point than raw materials. Among them, it is subdivided into gravity casting, low pressure casting and high pressure casting.
Limited by the melting point of the mold, the metal that can be cast is also limited.
2.2 dewaxing casting method
This method can be external film casting method and solid casting method.
First, copy the articles to be cast with wax, then immerse them in a jar containing ceramics (or silica sol) to dry, so that the wax copy is covered with a ceramic outer film, and repeat the above steps until the outer film is enough to support the casting process (about 1/4 inches to 1/8 inches), and then melt the wax in the mold and take it out of the mold. After that, the mold needs to be heated several times to enhance the hardness before casting.
This method has good accuracy and can be used for casting high melting point metals (such as titanium). However, because ceramics are expensive, they need to be heated many times, and the production is complicated and the cost is quite expensive. 1. Gravity casting: sand casting and hard mold casting. The molten metal is injected into the cavity by the gravity of the metal itself.
2. Pressure casting: low pressure casting, high pressure casting. The molten metal is instantly pressed into the casting cavity by extra pressure.
The casting process usually includes
(1) Prepare the mold (container for making liquid metal into solid castings). According to the materials used, molds can be divided into sand mold, metal mold, ceramic mold, clay mold, graphite mold and so on. According to the times of use, it can be divided into disposable molds, semi-permanent molds and permanent molds. Mold quality is the main factor affecting the quality of castings;
(2) Melting and pouring of casting metals, casting metals (casting alloys) mainly include all kinds of cast iron, cast steel and casting nonferrous metals and 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.
Advantages: 1, can produce parts with complex shapes, especially blanks with complex cavities;
2, wide adaptability, all metal materials commonly used in industry can be cast, ranging from a few grams to hundreds of tons;
3. Raw materials are widely available and cheap, such as scrap steel, waste parts and fragments.
4. The shape and size of the casting are very close to the parts, which reduces the cutting amount and belongs to non-cutting processing;
5. It is widely used. 40%~70% of agricultural machinery and 70%~80% of machine tools are castings.
Disadvantages: 1, mechanical properties are not as good as forgings, such as coarse structure and many defects;
2. Sand casting, single piece, small batch production, and the labor intensity of workers is high;
3. The quality of castings is unstable, there are many processes, and the influencing factors are complex, which is easy to produce many defects.
Casting defects have an important influence on casting quality. Therefore, we should start with the quality of castings and combine the formation and prevention of main defects of castings to lay a good foundation for the selection of casting alloys and casting methods.