Views: 48 Author: Site Editor Publish Time: 2022-01-07 Origin: Site
Vacuum casting is a process that uses a ventilated mold. Molten metal is forced into the mold by air pressure, and the air is purged, creating a vacuum. This casting method is mainly used for small parts or jewelry with fine details.
Vacuum molding is a physical molding method, which combines vacuum technology with sand mold casting. The surface and back of the sand mold are sealed by plastic film, and the negative pressure is generated by vacuum pumping, causing the inside and outside of the sand mold. The pressure difference makes the moulding sand tightly formed. After placing the sand core, clamping the mould, casting, and waiting for the casting to solidify, the negative pressure is released or the air extraction is stopped, and the moulding sand will collapse to obtain the casting. According to the current application fields, it can be mainly divided into vacuum sealing molding (V method) and solid vacuum casting (FV method). Its advantages are:
(1) Moderate vacuum degree, high mold cavity compactness, and high casting quality.
(2) Vacuum casting cooperates with computer mold flow analysis and simulation, which can predict the occurrence of casting defects. For the design of mold making plan, it can greatly save time and improve production efficiency.
Vacuum casting can be roughly divided into three types: vacuum suction casting, vacuum low pressure casting, and vacuum differential pressure casting:
1. Vacuum suction casting
Place the casting mold in a closed container, and draw out the air in the casting mold to create a certain negative pressure in the casting mold, causing the molten metal to be sucked into the mold cavity. When the inner runner of the casting is solidified, the negative pressure is removed, so that the unsolidified metal liquid in the vertical runner flows back into the molten pool. The advantage is that the filling ability of the alloy liquid is improved, the minimum wall thickness of the suction casting casting can reach 0.2mm, and the casting area is 300mm2, and the defects such as pores and slag inclusions are reduced at the same time. It is suitable for the production of thin and fine small precision castings, especially cast steel (including stainless steel). Its advantages are:
(1) Vacuum suction casting is conducive to the removal of gas in the mold, inhibits the generation of turbulent flow and entrained air, overcomes the shortcomings of low-pressure casting and differential pressure casting, and significantly improves the filling ability of molten metal.
(2) Due to the improvement of mold filling ability during vacuum suction casting, the casting temperature of molten metal can be 20~30 °C lower than that of gravity casting.
(3) Castings with large difference in wall thickness, thin wall and high quality requirements can be cast.
(4) By selecting an appropriate vacuum degree change rate and controlling the speed of the molten metal entering the mold cavity, a stable filling effect can be obtained.
2. Vacuum low pressure casting
The vacuum low pressure casting method is to vacuumize the casting mold during the process of pressurizing and filling. After filling, the pressure is maintained to make the casting crystallize and solidify under constant pressure, and the casting is fully fed, so the structure of the casting is dense and the mechanical properties are improved. This method is mostly used for precision casting of Al, Mg alloy castings. Its advantages are as follows:
(1) The existence of vacuum negative pressure can make the molding sand compact and increase the strength of the casting mold.
(2) Appropriately increase the degree of vacuum, so that the filling time is shortened, and the filling speed is not only accelerated but also stable.
(3) Vacuum low pressure casting can accelerate the fluidity of aluminum alloy at low temperature, avoid the phenomenon of inhalation of aluminum alloy cast at high temperature, and increase the pinhole defect of the casting.
(4) The casting has good formability, which is conducive to the formation of castings with clear outlines and smooth surfaces and large thin-walled castings.
(5) The structure of the casting is dense and the mechanical properties are high.
(6) Under normal circumstances, no riser is needed, which greatly improves the yield of molten metal, usually up to 90%.
(7) The working environment is good and the equipment is simple, which is helpful for mechanization and automation.
3. Vacuum differential pressure casting
Vacuum differential pressure casting is mainly to seal the resistance holding furnace and the casting mold, and pass compressed air with a high pressure such as 500kPa into the sealing cover. At this time, since the pressure inside the casting mold and the crucible is equal, the molten metal will not rise. , and then add 50kPa pressure on the metal liquid surface, the metal liquid will rise to fill the mold cavity. It is especially suitable for the production of complex thin-walled castings, but the equipment is large and the operation is troublesome, so it is only used when there are special requirements. Its advantages are as follows:
(1) Compared with traditional gravity casting and vacuum suction casting, vacuum differential pressure casting shows a good filling ability on 1mm thick sheet samples.
(2) The casting has a denser crystalline structure and mechanical properties. The strength of the casting is about 20%~25% higher than that of vacuum suction casting and gravity casting, and the elongation is about 50% higher.
(3) Vacuum die casting can effectively reduce the pores of die castings and increase the density of castings by more than 1%.