It is well known that fluids cannot transmit tension, only pressure can occur, and any small shear forces can cause fluid to flow or hinder fluid flow. The force acting on the fluid mainly has mass force and surface force. The surface force is mainly pressure. The fluid pressure is caused by the pressure difference ΔP at both ends. When the degree of vacuum is large, the back pressure in the casting cavity is smaller. The greater the pressure difference ΔP at both ends of the molten metal, it is advantageous for the molten metal to fill the cavity. If the degree of vacuum is too small, the back pressure in the mold is large, and the molten metal is not easy to fill the cavity, and may cause defects such as insufficient pouring and pores in the casting.
Vacuum is an important parameter affecting the filling ability of castings, which is the basis of the whole process. The degree of vacuum not only affects the residual pressure in the mold, but also affects the establishment of the filling power.
The effects of vacuum on filling are mainly:
1 reduce the back pressure in the casting cavity, thereby reducing the resistance during filling, the easier it is to fill;
2 The greater the degree of vacuum, the easier it is to establish a larger filling pressure difference, which is conducive to the filling of molten metal;
3 When the degree of vacuum is high, there is only a small amount of gas in the mold, which can effectively avoid oxidation of the molten metal and generation of oxidation inclusions. As far as the quality of the casting is concerned, the greater the degree of vacuum, the stronger the filling ability, the smaller the possibility that the molten metal produces pores, oxidized inclusions and insufficient pouring, and the higher the quality of the casting.
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