- Jun 05, 2024
Vacuum casting is a process that uses a ventilated mold. The molten metal flows into the mold under air pressure, and then the air is removed to form a vacuum. This casting method is mainly used for small parts or jewelry with delicate details.
Vacuum molding is a physical molding method that combines vacuum technology with sand mold casting. The mold cavity surface and back surface of the sand mold are sealed with a plastic film. The vacuum pump is used to evacuate the air to generate negative pressure, which causes a pressure difference between the inside and outside of the sand mold to tighten the mold sand. After placing the sand core, closing the mold, and casting, the casting is waited for the casting to solidify. The negative pressure is released or the air is stopped, and the mold sand collapses to obtain the casting. According to the current application field, it can be mainly divided into vacuum sealed molding (V method) and full mold vacuum casting (FV method). Its advantages are:
(1) Appropriate vacuum degree, high mold cavity compactness, and high casting quality.
(2) Vacuum casting combined with computer mold flow analysis simulation can predict the occurrence of casting defects, which can greatly save time and improve production efficiency for the design of molding schemes.
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 mold in a closed container, extract the air in the mold, and 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 capacity of the alloy liquid. The minimum wall thickness of the suction casting can reach 0.2mm, and the casting area is 300mm2. At the same time, it reduces defects such as pores and slag inclusions. It is suitable for the production of thin and fine small precision castings, especially cast steel (including stainless steel) parts. Its advantages are:
(1) Vacuum suction casting is conducive to the removal of gas in the mold, inhibiting the generation of turbulence and air entrainment, overcoming the disadvantages of low-pressure casting and differential pressure casting, and significantly improving the filling capacity of the molten metal.
(2) Due to the improvement of the filling capacity during vacuum casting, the casting temperature of the molten metal can be 20~30℃ lower than that during 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 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 evacuate the mold during the pressure filling process, and maintain the pressure after filling so that the casting crystallizes and solidifies under constant pressure. The casting is fully compensated for shrinkage, so 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. Its advantages are as follows:
(1) The existence of vacuum negative pressure can make the mold sand compact and increase the strength of the mold.
(2) Properly increase the vacuum degree to shorten the filling time, which not only speeds up the filling speed but also stabilizes it.
(3) Vacuum low-pressure casting can accelerate the fluidity of aluminum alloy at low temperature, avoid the aluminum alloy air absorption phenomenon caused by high-temperature casting, and increase the pinhole defects of the casting.
(4) The casting has good formability, which is conducive to the formation of castings with clear contours and smooth surfaces and large thin-walled castings.
(5) The casting has dense structure and high mechanical properties.
(6) Under normal circumstances, no riser is required, 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 conducive to mechanization and automation.
3. Vacuum differential pressure casting
Vacuum differential pressure casting mainly involves sealing the resistance insulation furnace and the mold, and introducing compressed air with a high pressure of 500kPa into the sealing cover. At this time, since the pressure inside the mold and the crucible is equal, the molten metal will not rise. Then, 50kPa pressure is added to the surface of the molten metal, and the molten metal will rise to fill the mold cavity. It is particularly suitable for the production of complex thin-walled castings, but the equipment is large and the operation is difficult. 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 has shown good filling ability on 1mm thick thin sheet samples.
(2) The casting has a denser crystal 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 increased by about 50%.
(3) Vacuum die casting can effectively reduce the pores of die castings and increase the density of castings by more than 1%