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

Vacuum suction molding refers to placing the mold in a closed container and extracting the air inside the mold to cause a certain negative pressure in the mold, which causes the molten metal to be sucked into the mold cavity. When the inner runner of the casting solidifies, the negative pressure is removed, and the unsolidified molten metal in the vertical runner flows back into the molten pool. The utility model has the advantages that the filling ability of the alloy liquid is improved, the minimum wall thickness of the suction casting is up to 0.2 mm, and the casting area is 300 mm 2 , and the defects such as pores and slag inclusion are reduced. Suitable for the production of small and fine small precision castings, cast steel (including stainless steel) parts are particularly suitable. Its advantages are:

(1) Vacuum suction casting is beneficial to the elimination of gas in the mold, inhibits the generation of turbulent flow and gas generation, overcomes the disadvantages of low pressure casting and differential pressure casting, and significantly improves the filling ability of the molten metal.

(2) Due to the increase in filling ability during vacuum suction casting, the casting temperature of the molten metal can be 20 to 30 ° C lower than that during gravity casting.

(3) Castings with large wall thickness, thin wall and high quality requirements can be cast.

(4) By selecting an appropriate degree of vacuum change rate and controlling the speed at which the molten metal enters the cavity, a smooth filling effect can be obtained.

2. Vacuum low pressure casting

The vacuum low-pressure casting method vacuums the mold during the pressurization filling process. After the filling is completed, the casting is allowed to crystallize and solidify under a constant pressure, and the casting is fully filled, so that the casting structure is dense and the mechanical properties are improved. This method is mostly used for precision casting of Al and Mg alloy castings. The advantages are as follows:

(1) The presence of vacuum negative pressure can make the mold sand tight and increase the strength of the mold.

(2) Moderately increase the degree of vacuum, so that the filling time becomes shorter, not only the filling speed is accelerated but also stable.

(3) Vacuum low-pressure casting can accelerate the fluidity of aluminum alloy at low temperature, avoid the phenomenon of aluminum alloy suction by high-temperature casting, and increase the pinhole defects of castings.

(4) The castings have good formability, which is favorable for forming castings with clear outlines and smooth surfaces and large thin-walled castings.

(5) The castings are densely organized and have high mechanical properties.

(6) Under normal circumstances, no riser is required, so that the yield of molten metal is greatly improved, usually up to 90%.

(7) The working environment is good and the equipment is simple, which contributes to mechanization and automation.

3. Vacuum differential pressure casting

Vacuum differential pressure casting, mainly to seal the electric resistance holding furnace and the mold, and to introduce compressed air with a pressure of 500 kPa in the sealed cover. At this time, the molten metal does not rise because the pressure inside the mold and the crucible is equal. Then, the pressure of 50 kPa is added to the molten metal surface, and the molten metal rises to fill the cavity. It is especially suitable for the production of complex thin-walled castings, but the equipment is large and the operation is troublesome. It is only applied when there are special requirements. The advantages are as follows:

(1) Compared with conventional gravity casting and vacuum suction casting, vacuum differential pressure casting has a good filling ability on a 1 mm thick sheet sample.

(2) The casting has a dense crystal structure and mechanical properties. In terms of strength properties of the casting, it is about 20% to 25% higher than vacuum suction casting and gravity casting, and the elongation is increased by about 50%.

(3) Vacuum die casting can effectively reduce the pores of the die-casting parts and increase the density of the castings by more than 1%.