Vacuum casting is a process that uses ventilated casting molds. The molten metal relies on air pressure to flow into the mold, which is then cleared to create a vacuum. This casting method is mainly used for small parts or jewelry with delicate details.
Vacuum molding is a kind of physical molding method, which combines vacuum technology with sand mold casting, seals the hole surface and back of the sand mold by plastic film, and generates negative pressure by vacuum pump pumping, resulting in the difference of internal and external pressure of the sand mold to tighten the mold sand. After placing the sand core, closing the mold, casting, and waiting for the casting to solidified, remove the negative pressure or stop pumping, the mold sand will collapse and obtain the casting. According to the current application of the field can be divided into vacuum seal molding (V method) and solid vacuum casting (FV method). Its advantages are:
(1) Moderate vacuum, high die-hole compactness, and high casting quality.
(2) Vacuum casting with computer mold flow analysis and simulation, can predict the generation of casting defects, for the design of mold scheme, can greatly save time and improve production efficiency.
Vacuum casting can be roughly divided into vacuum suction casting, vacuum low pressure casting, vacuum differential pressure casting three kinds:
1, vacuum casting
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The mold is placed in a closed container, and the air in the mold is drawn out, so that a certain negative pressure is caused in the mold, resulting in the suction of metal liquid into the mold hole. When the inner runner of the casting is solidified, the negative pressure is removed, so that the unsolidified metal liquid in the sprue flows back to the melting pool. Its advantage is to improve the filling capacity of the alloy liquid, the minimum wall thickness of the suction casting can reach O.2mm, while the casting area is 300mm2, and the defects such as porosity and slag inclusion can be reduced. Suitable for the production of thin and fine small precision castings, cast steel (including stainless steel) is especially suitable. Its advantages are:
(1) Vacuum casting is conducive to the elimination of gas in the mold, inhibits turbulence and the generation of enrolling gas, overcomes the disadvantages of low pressure casting and differential pressure casting, and significantly improves the filling ability of liquid metal.
(2) Due to the improvement of filling capacity during vacuum suction casting, the casting temperature of liquid metal can be 20~30ºC lower than that of gravity casting.
(3) Castings with large wall thickness difference, thin wall and high quality requirements can be cast.
(4) By selecting the appropriate vacuum degree change rate and controlling the speed of metal liquid entering the mold hole, a smooth filling effect can be obtained.
2, vacuum low pressure casting
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Vacuum low pressure casting method is to vacuum the casting mold in the process of pressure filling. After filling the mold, the pressure is kept so that the casting crystallizes and solidifies under constant pressure, and the casting is fully supplemented, so the casting is dense and the mechanical properties are improved. This method is mostly used for precision casting of Al and Mg alloy castings. Its advantages are as follows:
(1) The presence of vacuum negative pressure can make the mold sand compact and increase the strength of the mold.
(2) moderately improve the vacuum degree, so that the filling time is shorter, not only the filling speed is accelerated and stable.
(3) Vacuum low-pressure casting can speed up the fluidity of aluminum alloy at low temperatures, avoid the suction phenomenon of aluminum alloy caused by high temperature casting, and increase the pinhole defects of the casting.
(4) The casting formability is good, which is conducive to the formation of castings with clear outline and smooth surface and large thin-wall castings.
(5) The casting has dense structure and high mechanical properties.
(6) Under normal circumstances, no riser is required, so that the yield of liquid metal is greatly improved, usually up to 90%.
(7) Good working environment, simple equipment, conducive to mechanization and automation.
3, vacuum differential pressure casting
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Vacuum differential pressure casting is mainly to seal the resistance holding furnace and casting mold, and pass compressed air with a larger pressure such as 500kPa into the sealing cover. At this time, the metal liquid will not rise because the pressure inside the mold and the crucible is equal, and then add 50kPa pressure on the metal liquid surface, the metal liquid will rise and fill the mold hole. Especially suitable for the production of complex thin-wall castings, but the equipment is large, the operation is difficult, and it is only applied when special requirements are required. Its advantages are as follows:
(1) Compared with traditional gravity casting and vacuum suction casting, vacuum differential pressure casting reflects a good filling ability on 1mm thick thin slice samples.
(2) The casting has a relatively dense crystalline structure and mechanical properties, in the strength of the casting, compared with vacuum casting and gravity casting about 20% to 25%, the elongation is increased by about 50%.
(3) Vacuum die casting can effectively reduce the porosity of die casting parts and increase the density of castings by more than 1%.
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