Vacuum precision casting technology
Vacuum casting can be divided into direct casting and special vacuum casting
(1) Direct casting
Taking CLA method as an example, the shell made by ordinary investment process is put in the sealing chamber, the sealing chamber is lowered, and the sprue is inserted with liquid metal. Start the vacuum pump to pump the sealing chamber into a vacuum, and the liquid metal is filled at the same time. After the liquid metal in the mould shell solidifies, the air is connected to the sealing chamber to eliminate the vacuum, and the liquid metal which has not yet solidified in the runner flows back to the crucible. Finally, the mold shell was taken out and the casting was obtained. In addition to CLA method, there are also tilting inversion vacuum casting, that is, after the vacuum casting filling is completed, the mold is inverted, and the remaining metal liquid in the sprue provides feeding for the solidification of the casting. This method is easy to realize continuous production and has high production efficiency.
(2) Special vacuum casting method
This kind of vacuum casting method mainly adds some special operations to the vacuum casting process according to the casting process requirements or alloy types. For example, in order to enhance the feeding capacity of vacuum casting. After the upper seal chamber is vacuumed and the mold cavity is filled with liquid metal, the lower seal chamber is then filled with compressed gas to improve the pressure difference between the upper and lower seal chambers, so as to enhance the feeding capacity of molten metal during the crystallization and solidification of castings.
There is also vacuum casting with inert gas protection. This method is mainly used for the vacuum melting and pouring casting of superalloys and easily oxidized alloys (also known as CLV method). After the metal is melted in vacuum, inert gas is injected into the vacuum melting chamber and casting chamber at the same time, and the same pressure is maintained. The shell runner or riser is inserted into the molten metal, and then the casting chamber pressure is reduced for casting. After holding for a certain time, the molten metal in the sprue flows back to the crucible after pressure relief.
Vacuum casting is suitable for investment casting, ceramic mold casting, shell casting, gypsum casting, graphite casting and sand casting. It can be used to produce complex thin-walled castings with average wall thickness less than 5mm such as aluminum alloy, magnesium alloy, titanium alloy, stainless steel and high-temperature heat-resistant alloy, and castings with casting weight less than 100kg.