Centrifugal casting is a casting method that pours liquid metal into a rotating mold, so that the liquid metal fills the mold and solidifies under the action of centrifugal force.
To realize the above process, a centrifugal casting machine must be used to create conditions for the mold to rotate. According to the different spatial positions of the mold rotation axis, two types of centrifugal casting machines are commonly used: vertical centrifugal casting machines and horizontal centrifugal casting machines.
The mold on the vertical centrifugal casting machine rotates around the vertical axis. It is mainly used to produce ring-type castings with a height less than the diameter. Sometimes this centrifugal casting machine can also be used to cast special-shaped castings.
The mold of the horizontal centrifugal casting machine rotates around the horizontal axis. It is mainly used to produce sleeve and pipe castings with a length greater than the diameter.
Since the liquid metal fills the mold and solidifies under rotation during centrifugal casting, centrifugal casting has the following characteristics:
1) Liquid metal can form a hollow cylindrical free surface in the mold, so that hollow castings can be cast without a core, greatly simplifying the production process of sleeves and pipe castings; 2) Due to the centrifugal force generated by the liquid metal during rotation, the centrifugal casting process can improve the ability of metal to fill the mold, so some alloys with poor fluidity and thin-walled castings can be produced by centrifugal casting; 3) Due to the centrifugal force, the shrinkage compensation conditions are improved, and gas and non-metallic inclusions are also easily discharged from the liquid metal, so the structure of centrifugal castings is denser, and there are fewer defects such as shrinkage cavities (shrinkage), pores, and inclusions; 4) Eliminate or greatly save metal consumption in the pouring system and riser; 5) Castings are prone to segregation, and the inner surface of the casting is rough. The size of the inner surface is not easy to control.
The first patent for centrifugal casting was proposed by the British Erchardt in 1809. It was not until the early 20th century that this method was gradually adopted in production. In the 1930s, my country also began to use centrifugal tubes, cylinder castings such as iron pipes, copper sleeves, cylinder sleeves, bimetallic steel back copper sleeves, etc. Centrifugal casting is almost a major method; in addition, centrifugal casting is also very effective in the production of heat-resistant steel rollers, some special steel seamless steel pipe blanks, paper machine drying drums, etc. At present, highly mechanized and automated centrifugal casting machines have been produced, and mechanized centrifugal casting workshops for mass production have been built.
Almost all casting alloys can be used for centrifugal casting production. The minimum inner diameter of centrifugal castings can reach 8 mm, the maximum diameter can reach 3m, the maximum length of castings can reach 8m, and the weight of centrifugal castings ranges from a few Newtons to tens of thousands of Newtons (a few tenths of a kilogram to more than ten tons).
2. Centrifugal Casting Process
1) Selection of Centrifugal Casting Mold Speed
When selecting the speed of the centrifugal casting mold, two issues should be considered: (1) The speed of the centrifugal casting mold should at least ensure that the liquid metal can form a cylindrical color and rotate around the axis immediately after entering the mold; (2) Make full use of the centrifugal force to ensure good internal quality of the casting and avoid shrinkage cavities, shrinkage, inclusions and pores in the casting.
When using sand molds for centrifugal casting, it is also necessary to pay attention to the fact that the liquid metal has too much centrifugal pressure on the mold wall, which may cause defects such as sand sticking and swelling in the casting.
2) Centrifugal Casting Mold
There are two major types of molds used in centrifugal casting, namely metal molds and non-metal molds. Non-metal molds can be sand molds, shell molds, investment shell molds, etc. Since metal molds have a series of advantages in mass production and batch production, they are widely used in centrifugal casting.
The metal molds on the horizontal cantilever centrifugal casting machine can be divided into single-layer metal molds and double-layer metal molds according to the structural characteristics of their main body. In a single-layer metal mold, the mold wall consists of one layer. The single-layer metal mold has a simple structure and is easy to operate. However, after it is damaged, a new mold needs to be made before production can begin. Only castings with a single outer diameter can be cast in this mold. In a double-layer metal mold, the mold wall consists of two layers, and the casting is formed on the surface of the inner mold. Although the structure of the double-layer metal mold is complex, centrifugal castings with various outer diameters can be cast by simply changing the working surface size of the inner mold. After long-term work, the old mold can be used as a new mold by simply replacing the inner mold with a simpler structure.
3) Paint
During centrifugal casting of metal molds, it is often necessary to spray paint on the working surface of the metal mold. The requirements for paint for centrifugal casting metal molds are the same as those for general metal mold casting. In order to prevent the casting from bonding with the metal mold and the occurrence of white spots on the cast iron, the paint layer on the centrifugal metal mold is sometimes thicker. Most of the paint for centrifugal casting uses water as a carrier. Sometimes it is also used for state paint, such as graphite powder, so that the casting can be easily removed from the mold.
When spraying paint, attention should be paid to controlling the temperature of the metal mold. When producing large castings, if the heat of the mold itself is not enough to dry the coating, the mold can be placed in a heating furnace to heat it and maintain the working temperature of the mold, waiting for pouring. When producing small castings, especially when using a cantilever centrifugal casting machine, it is hoped that the heat of the mold itself can be used as much as possible to dry the coating and wait for pouring.
4) Pouring
During centrifugal casting, the pouring process has its own characteristics. First of all, since the inner surface of the casting is a free surface, and the control of the thickness of the casting is entirely determined by the amount of liquid metal poured, the quantitative requirements for the poured metal are relatively high during centrifugal casting. In addition, since the pouring is carried out under the rotation of the mold, in order to eliminate the phenomenon of metal splashing as much as possible, the direction of the metal entering the mold must be well controlled.
The quantitative method of liquid metal includes weight method, volumetric method and fixed free surface height (liquid metal thickness) method. The volumetric method uses a certain volume of ladle to control the amount of liquid metal poured. This method is simpler, but it is affected by the temperature of the metal, slag, etc., and the quantitative is not very accurate. It is used more in production.
In order to eliminate metal splashing during pouring as much as possible, it is necessary to control the flow direction of liquid metal when it enters the mold.