What Is Vacuum Precision Casting?

What Is Vacuum Precision Casting?

     Vacuum precision casting is a casting process in which metal is smelted, poured and crystallized in a vacuum chamber. It can reduce the gas content in the metal and prevent metal oxidation and trachoma. This method can produce very demanding castings. The cost of vacuum casting is high, and it is generally used for high-quality castings, and the mold should not contain volatiles. Vacuum precision casting is to insert the mold gate (or riser) into the molten metal, and then evacuate the mold to make the cavity of the mold under negative pressure. The riser pipe) rises and fills the mold, solidifies and forms, thereby obtaining a casting method.

   The process principle of vacuum precision casting is to put the investment shell in the sealed chamber, the sealed chamber is lowered, the sprue is immersed in the liquid metal, and then the vacuum pump is started to evacuate the sealed chamber, and the liquid is cast at the same time. After the casting in the mold shell is solidified, the vacuum state is released, the residual metal liquid in the runner is returned to the furnace, and the vacuum casting casting is obtained by sand cleaning. According to the difference of pressure difference established by vacuum casting process, vacuum precision casting can be divided into direct casting method and special vacuum method:

1. Direct casting method

     Taking the CLA method as an example, first put the shell made by the ordinary investment process in the sealed chamber, the sealed chamber descends, and the sprue is inserted into the liquid metal. The vacuum pump is started to evacuate the sealed chamber, and the liquid metal is filled at the same time. After the molten metal in the mold shell is solidified, the sealed chamber is connected to the atmosphere to eliminate the vacuum, and the liquid metal that has not solidified in the runner flows back into the crucible. Then take out the mold shell to clean the sand to obtain the casting. In addition to the CLA method, there is also the tilting and inverting method vacuum precision casting, that is, after the filling is completed, the mold is turned upside down, and the remaining molten metal 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 type of vacuum precision casting method is mainly based on the casting process requirements or alloy types to be poured, and some special operations are added during the casting process. For example, to enhance the feeding capability of vacuum precision casting. After the upper sealed chamber is evacuated; after the molten metal fills the cavity, the lower sealed chamber is then fed with compressed gas to increase the pressure difference between the upper and lower sealed chambers, thereby enhancing the liquid metal feeding capacity during the crystallization and solidification of the casting. The technological process is shown in Figure 2. There is also vacuum precision casting with inert gas protection. This method is mainly used to produce high-temperature alloys and easily oxidized alloys. The vacuum melting and casting method (also known as CLV method) is used to melt the metal under vacuum. Inert gas is introduced into the vacuum melting chamber and casting chamber at the same time, and they are kept at the same pressure. Insert the mold sprue or riser pipe into the molten metal, and then reduce the pressure in the casting chamber for casting. After holding for a certain period of time, the molten metal in the sprue flows back to the crucible after the pressure is relieved.

Process characteristics: (1) High yield, good casting quality, stable molten metal filling, less oxidized slag and splashes, reduced casting defects such as porosity and slag inclusion, and improved the yield. In addition, lower pouring temperature can be used for pouring, so that the crystal grain of the casting is refined and the mechanical properties are improved. (2) Good filling performance. In precision casting, the back pressure in the mold cavity is small and the filling speed is adjustable, so the filling ability is strong, and the thin part of the casting can reach 0.3mm. (3) Improve the utilization rate of molten metal and process yield. (4) Simplify the process and reduce the cost. (5) It is easy to realize mechanization and high labor productivity. Compared with the ordinary Investment Casting process, each module can be assembled with multiple wax molds, which can generally increase the output by 85% to 135%.

     Vacuum precision casting is suitable for investment molds, ceramic molds, shell molds, plaster molds, graphite molds and sand castings. It can be used to produce aluminum alloys, magnesium alloys, titanium alloys, stainless steels and high-temperature heat-resistant alloys and other complex thin films with an average wall thickness of less than 5mm. Wall castings, and castings with a single pouring weight less than 100kg.