There are mainly two categories: sand casting and special casting.

1. Ordinary sand mold casting uses sand as mold material, also known as sand casting and sand turning, including wet sand mold, dry sand mold and chemical hardening sand mold, but not all sand can be used for casting. The advantage is lower cost, because the sand used in the mold can be reused; The disadvantage is that the manufacturing of the mold is time-consuming, the mold itself can not be reused, and the finished product can be obtained only after it is damaged.

1.1 sand mold (core) casting method: wet sand mold, resin self hardening sand mold, sodium silicate sand mold, dry mold and surface dry mold, 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 divided into manual core making and machine core making.

2. According to the molding materials, special casting can be divided into special casting with natural mineral sand as the main molding materials (such as investment casting, mud mold casting, shell mold casting, negative pressure casting, full mold casting, ceramic mold casting, etc.) and special casting with metal as the main molding materials (such as metal mold casting, pressure casting, continuous casting, low-pressure casting, centrifugal casting, etc.).

2.1 metal mold casting method

The mold is made of metal with high melting point. It is subdivided into gravity casting, low-pressure casting and high-pressure casting.

The metal that can be cast is also limited by the melting point of the mold.

2.2 Dewaxing casting method

This method can be outer film casting method and solid casting method.

First copy the object to be cast with wax, then immerse it in the pool containing ceramic (or silica sol) and let it dry, so that the copy made with wax is covered with a layer of ceramic outer film, and repeat the steps until the outer film is enough to support the casting process (about 1 / 4 inch to 1 / 8 inch), then melt the wax in the mold and pull it away from the mold. After that, the mold needs to be high-temperature for many times, and can be used for casting after enhancing the hardness.

This method has good accuracy and can be used for casting high melting point metals (such as titanium). However, due to the high price of ceramics and the need for multiple heating and complex production, the cost is quite expensive.  

Molding process

1. Gravity casting: sand casting, hard mold casting. The molten metal is poured into the cavity by the gravity of the metal itself.

2. Pressure casting: low pressure casting, high pressure casting. The molten metal is instantaneously pressed into the casting cavity by additional pressure.

The casting process usually includes

① Mold preparation (container for making liquid metal into solid casting). Mold can be divided into sand mold, metal mold, ceramic mold, mud mold, graphite mold, etc. according to the materials used, and can be divided into disposable mold, semi permanent mold and permanent mold according to the times of use. The quality of mold preparation is the main factor affecting the quality of castings;

② The melting and pouring of casting metal. Casting metal (casting alloy) mainly includes all kinds of cast iron, cast steel and cast non-ferrous metals and alloys;

③ Casting processing and inspection. Casting processing includes removal of foreign matters on the core and casting surface, removal of pouring and riser, shoveling and grinding of burrs, drape joints and other protrusions, as well as heat treatment, shaping, antirust treatment and rough machining.

The casting process can be divided into three basic parts: casting metal preparation, mold preparation and casting treatment. Cast metal refers to the metal materials used for casting in casting production. It is an alloy composed of one metal element as the main component and other metal or non-metallic elements. It is conventionally called cast alloy, mainly including cast iron, cast steel and cast non-ferrous alloy.

Metal smelting is not only a simple melting, but also a smelting process, so that the metal poured into the mold meets the expected requirements in terms of temperature, chemical composition and purity. Therefore, various inspections and tests for the purpose of quality control shall be carried out in the smelting process, and the pouring of liquid metal can be allowed only after reaching various specified indexes. Sometimes, in order to meet higher requirements, the molten metal must be treated outside the furnace after being discharged, such as desulfurization, vacuum degassing, refining outside the furnace, inoculation or modification. The common equipment for smelting metal includes cupola, electric arc furnace, induction furnace, resistance furnace, reflection furnace, etc.

Advantages: 1. It can produce parts with complex shape, especially blanks with complex inner cavity;

2. Wide adaptability. Metal materials commonly used in industry can be cast, ranging from a few grams to hundreds of tons;

3. Wide sources of raw materials and low prices, such as scrap steel, scrap parts, chips, etc;

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 the weight of agricultural machinery and 70% ~ 80% of the weight of machine tools are castings.

Disadvantages: 1. The mechanical properties are not as good as forgings, such as coarse structure, many defects, etc;

2. Sand casting, single piece, small batch production, high labor intensity of workers;

3. The casting quality is unstable, there are many processes, the influencing factors are complex, and it is easy to produce many defects.

Casting defects have an important impact on the quality of castings. Therefore, in order to lay a good foundation for the selection of casting alloys and casting methods, we should start with the quality of castings and combine the formation and prevention of main defects of castings.