Advantages of three types of vacuum casting

Vacuum casting is a process that uses ventilated molds. Molten metal flows into the mold using air pressure, which then clears the air, creating a vacuum. This casting method is mainly used for small parts or jewelry with fine details.

Vacuum modeling is a physical modeling method that combines vacuum technology with sand mold casting. The cavity surface and back of the sand mold are sealed with a plastic film, and a vacuum pump is used to extract air to generate negative pressure, causing internal and external damage to the sand mold. The pressure difference causes the mold sand to tighten into shape. After placing the sand core, closing the mold, and casting, after the casting solidifies, the negative pressure is released or the air pumping is stopped, and the mold sand collapses to obtain the casting. According to the current application fields, it can be mainly divided into vacuum sealing molding (V method) and full vacuum casting (FV method). Its advantages are:

(1) Moderate vacuum degree, high mold cavity tightness, and high casting quality.

(2) Vacuum casting combined with computer mold flow analysis and simulation can predict the occurrence of casting defects, which can greatly save time and improve production efficiency for the design of molding plans.

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 extract the air from the mold to create a certain negative pressure in the mold, causing the molten metal to be sucked into the mold cavity. When the inner runner of the casting solidifies, remove the negative pressure and allow the unsolidified molten metal in the vertical runner to flow back into the molten pool. Its advantage is that it improves the filling ability of the alloy liquid, the minimum wall thickness of the suction casting casting can reach 0.2mm, and the casting area is 300mm2, while reducing defects such as pores and slag inclusions. It is suitable for producing thin and fine small precision castings, especially cast steel (including stainless steel) parts. Its advantages are:

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

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

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

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

2. Vacuum low pressure casting

The vacuum low-pressure casting method evacuates the mold during the pressure filling process. After filling, the pressure is maintained so that the casting crystallizes and solidifies under constant pressure. 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 and Mg alloy castings. Its advantages are as follows:

(1) The existence of vacuum negative pressure can compact the mold sand and increase the strength of the mold.

(2) Increase the vacuum degree appropriately to shorten the filling time, which not only speeds up the filling but also makes it smooth.

(3) Vacuum low-pressure casting can speed up the fluidity of aluminum alloys at low temperatures, avoid aluminum alloy air absorption caused by high-temperature casting, and increase pinhole defects in castings.

(4) The casting has good formability, which is beneficial to the formation of castings with clear outlines and smooth surfaces and large thin-walled castings.

(5) The casting structure is dense and has high mechanical properties.

(6) Generally, no riser is needed, which greatly improves the yield of molten metal, usually up to 90%.

(7) Good working environment and simple equipment are conducive to mechanization and automation.

3. Vacuum differential pressure casting

Vacuum differential pressure casting mainly seals the resistance holding furnace and the casting mold, and passes compressed air with a high pressure such as 500kPa into the sealing cover. At this time, because the pressure inside the casting mold and the crucible are equal, the molten metal will not rise. , and then add 50kPa pressure to the metal liquid surface, and the metal liquid will rise to fill the mold cavity. It is especially suitable for producing complex thin-walled castings, but the equipment is relatively large and the operation is troublesome. It is only used when special requirements are required. Its advantages are as follows:

(1) Compared with traditional gravity casting and vacuum suction casting, vacuum differential pressure casting shows good mold filling ability on 1mm thick thin specimens.

(2) The casting has a denser crystalline structure and mechanical properties. The strength of the casting is about 20% to 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%.