SAND CASTING EQUIPMENT.

1. Principle and characteristics of centrifugal casting
1.1 Basic principles:
Centrifugal casting is a technology and method that injects liquid metal into a high-speed rotating mold, so that the molten metal fills the mold and forms a casting under the action of centrifugal force.
The functions of centrifugal force are:
Make the liquid metal fill the mold well in the radial direction and form the free surface of the casting;
A cylindrical inner hole can be obtained without using a core;
Contribute to the elimination of gas and inclusions in liquid metal;
Affect the crystallization process of the metal, thereby improving the mechanical and physical properties of the casting.
1.2 Classification:
According to the spatial position of the rotation axis of the mold, common centrifugal casting can be divided into horizontal centrifugal casting and vertical centrifugal casting.
Centrifugal casting when the rotation axis of the mold is in a horizontal state or when the angle with the horizontal line is small (<4°) is called horizontal centrifugal casting. Centrifugal casting when the rotation axis of the mold is in a vertical state is called vertical centrifugal casting. Centrifugal casting in which the rotation axis of the mold has a large angle with the horizontal and vertical lines is called inclined axis centrifugal casting, but it is rarely used.
1.3 Features:
1.31 Advantages:
1.311 Castings have high density, few defects such as pores and slag inclusions, and high mechanical properties;
1.312 Cores can be used when producing hollow castings, so when producing long tubular castings, the metal filling ability can be greatly improved, the ratio of the wall thickness of the castings to the length or diameter can be reduced, and the production process of sleeve and pipe castings can be simplified;
1.313 There is almost no metal consumption in the gating system and the riser system, which improves the production rate of the process;
1.414 It is convenient to manufacture barrel and sleeve composite metal castings, such as steel-backed copper sleeves, bimetallic rolls, etc.; when forming castings, centrifugal force can be used to improve the metal filling capacity, so thin-walled castings can be produced.
1.32 Disadvantages:
1.321 castings are prone to segregation of specific gravity, so they are not suitable for alloys that are prone to segregation of specific gravity (such as lead bronze), especially not suitable for casting alloys with impurities larger than molten metal. However, in recent years, this feature of centrifugal casting has also been used for production. The situation of gradient composites;
1.322 The diameter of the inner hole of the casting is inaccurate, the surface of the inner hole is relatively rough, the quality is poor, and the machining allowance is large;
1.323 has certain limitations when used in the production of special-shaped castings.
1.4 Application:
1.41 The castings with large output produced by the centrifugal casting method are:
1.411 Iron pipe: Nearly 1/2 of the total annual output of ductile iron in the world is iron pipe produced by centrifugal casting
1.412 diesel engine and gasoline engine cylinder liner
1.413 Various types of steel sleeves and steel pipes
1.414 Bimetallic steel back copper sleeve, bearing bushes of various alloys
1.415 Paper machine drum.
1.42 Castings with significant benefits produced by centrifugal casting are:
1.421 bimetal cast iron roll;
1.422 heating furnace bottom heat-resistant steel roller table;
1.423 special steel seamless steel pipe;
1.424 Brake drums, piston ring blanks, copper alloy worm wheels;
1.425 special-shaped castings such as impellers, metal dentures, gold and silver mesons, small valves and cast aluminum motor rotors.
Centrifugal casting was first used to produce cast pipes, and then this process has been rapidly developed. At present, the centrifugal casting process is used in metallurgy, mining, transportation, irrigation and drainage machinery, aviation, national defense, automobile and other industries at home and abroad to produce steel, iron and non-ferrous carbon alloy castings. Among them, the production of castings such as centrifugal cast iron pipes, internal combustion engine cylinder liners and shaft sleeves is the most common. For some forming tools and gear castings, centrifugal force pouring can also be used for the molten mold shell, which can not only improve the accuracy of the casting, but also improve the mechanical properties of the casting.

2. Centrifugal casting process
In centrifugal casting production, mold speed, pouring system, pouring quantitative, solidification under the slag, metal filtration, paint use, pouring temperature, casting de-molding, etc. are process problems that must be determined or resolved, because they directly affect the quality and quality of castings. Productivity.
2.1 The casting speed is an important process factor in centrifugal casting. Different castings and different casting processes have different casting speeds when the casting is formed.
Too low casting speed will lead to poor filling of molten metal during vertical centrifugal casting, liquid metal rain during horizontal centrifugal casting, and also cause looseness, slag inclusion, and uneven inner surface of the casting defects in the casting;
If the casting speed is too high, defects such as cracks and segregation are easy to appear on the castings. The outer surface of the sand centrifugal castings will form defects such as expansion boxes, which will also cause large vibrations, increased wear and tear, and excessive power consumption of the machine.
Therefore, the selection principle of the casting speed should be to select the smallest value on the premise of ensuring the quality of the casting.
2.2 Pouring system; the pouring system during centrifugal casting mainly refers to the pouring cup that accepts the metal and the pouring trough connected to it, and sometimes also includes the runner in the mold. When designing the gating system, the following principles should be paid attention to:
2.21 When pouring long-length and large-diameter castings, the pouring system should enable the molten metal to spread quickly and evenly on the inner surface of the mold;
2.22 When pouring easy-to-oxidize molten metal or using centrifugal sand molds, the pouring trough should enable the molten metal to fill the mold in a balanced manner to minimize splashing of the molten metal and reduce the erosion of the sand mold;
2.23 When pouring forming castings, the runner in the mold should enable the molten metal to flow smoothly into the cavity;
2.24 After pouring is finished, there should be no metal and slag left in the pouring cup and pouring trough. If there is residual metal and slag, it should also be easy to remove.
2.3 Quantitative pouring; the inner diameter of centrifugal castings is often determined by the amount of molten metal poured, so during centrifugal pouring, the amount of molten metal poured into the mold must be controlled to ensure the size of the inner diameter.
In recent years, when pouring large castings, a digital display remote crane scale is used for quantitative pouring. The installation of pressure sensors on the pouring ladle shelf for centrifugal pouring and automatic quantitative pouring of the electromagnetic pump of the holding induction furnace has also been applied in production.
2.4 Utilization of molten slag; in order to overcome the defects of subcutaneous shrinkage caused by the two-way solidification of thick-walled centrifugal castings, the slag-forming agent can be poured into the mold together with the molten metal during casting. Heat dissipation on the surface creates a sequential solidification condition from outside to inside, eliminating subcutaneous shrinkage holes. At the same time, the slagging agent can also play a role in refining molten metal.
The method of pouring the slagging agent is: sprinkle the powdery slagging agent in the casting trough during pouring; pour the molten slag and the molten metal into the mold together.
2.5 Filtration of molten metal; some alloy liquids have more slag that is difficult to remove. Various filters can be placed in the pouring system to remove the slag, such as foam ceramic filters, glass wire filters, etc.
2.6 The use of paint; the composition of the paint for centrifugal metal mold is similar to that of gravity metal mold casting. When pouring slender centrifugal castings, since it is difficult to remove the residual paint on the working surface of the casting mold, the residual strength of the binder in the paint composition after high temperature work should be as low as possible to facilitate removal.
2.7 Pouring temperature; centrifugal castings are mostly tubular, sleeve-shaped, and ring-shaped parts. The resistance encountered when molten metal is filled is small, and there is centrifugal pressure or centrifugal force to strengthen the filling capacity of molten metal, so the pouring temperature during centrifugal casting It can be 5~10°C lower than that of gravity pouring.
2.8 De-molding of castings; in order to improve production efficiency, the de-molding of castings should be carried out as soon as possible under the premise of ensuring quality. Sometimes in order to prevent the cracking of the casting, the casting after de-molding should be immediately put into a holding furnace or buried in a sand pile to cool down. For some castings that are not easy to release and require slow cooling to prevent cracking, the mold with castings can be removed from the centrifugal casting machine immediately after the mold stops rotating, and buried in the sand pile to cool down slowly, and it will be released when it reaches room temperature. type.

3. Brief introduction of centrifugal casting machine
According to different production objects, centrifugal casting machines are divided into general-purpose and special-purpose types. The special centrifugal casting machine is a centrifugal casting machine designed according to the special casting structure or the size of the production batch. Sometimes the structure of the machine is simpler than the general-purpose centrifugal casting machine. General centrifugal casting machines include horizontal cantilever centrifugal casting machines, horizontal drum centrifugal casting machines and vertical centrifugal casting machines.

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