Vacuum modeling is a physical modeling method, which combines vacuum technology with sand mold casting. The cavity surface and back surface of the sand mold are sealed by plastic film, and the negative pressure is generated by vacuum pump suction, resulting in the pressure difference between the inside and outside of the sand mold to make the sand mold firmly formed. After placing the sand core, closing the mold, casting, and waiting for the solidification of the casting, the negative pressure is lifted or the suction is stopped, and the sand mold will collapse to obtain the casting. According to the current application fields, it can be divided into vacuum sealing modeling (V method) and solid vacuum casting (FV method).
The final quality of the workpiece may also be higher, as lower turbulence can lead to free float of the slag at the top of the container. Since the molten material is extracted from the bottom of the pool surface, the slag is usually left. Sand mold vacuum casting is a variant of forming plastic mold by vacuum, which is usually used with various metals. A thin plastic is usually placed on the mold, and then the sand is sucked into the mold by vacuum. Sand contacts the plastic sheath rather than the mold itself, but presents the shape of the mold. The molten metal will then contact with the sand. On this point, it will show the mold shape when the vacuum is removed.
Its advantages are:
• Moderate vacuum degree, high cavity tightness and high casting quality.
• Vacuum casting combined with computer simulation of mold flow analysis can predict the generation of casting defects. The design of modeling scheme can greatly save time and improve production efficiency.
Vacuum casting can be roughly divided into vacuum suction casting, vacuum low-pressure casting and vacuum differential pressure casting:
- Vacuum suction casting
The mold is placed in a closed container, and the air in the mold is extracted, which causes a certain negative pressure in the mold and causes the metal liquid to inhale the mold cavity. When the internal runner of the casting is solidified, the negative pressure is removed, and the unsolidified metal liquid in the vertical runner flows back to the molten pool. Its advantage is that the filling ability of alloy liquid is improved, the minimum wall thickness of suction casting can reach 0.2 mm, the casting area is 300 mm², and the defects such as porosity and slag inclusion are reduced. It is suitable for producing thin and fine small precision castings, especially steel castings (including stainless steel).
Its advantages are:
- Vacuum suction casting is conducive to the elimination of gas in the casting mold, inhibiting the generation of turbulent flow and gas entrapment, overcoming the disadvantages of low-pressure casting and differential pressure casting, and significantly improving the filling ability of metal liquid.
- Due to the improvement of filling ability in vacuum suction casting, the casting temperature of metal liquid can be 20 ~ 30 °C lower than that of gravity casting.
- Castable castings with large wall thickness difference, thin wall and high-quality requirements.
· Stable filling effect can be obtained by selecting the appropriate change rate of vacuum degree and controlling the speed of metal liquid entering the cavity.
- Vacuum low pressure casting
The vacuum low-pressure casting method is to vacuum the casting mold in the process of pressure filling. After the filling, the pressure is kept to make the casting crystallize and solidify under constant pressure, and the casting is fully filled and contracted, so the casting structure 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:
- The existence of vacuum negative pressure can make the mold sand tight and increase the strength of the mold.
- Appropriately increase the vacuum degree to shorten the filling time, not only the filling speed is accelerated but also stable.
- Vacuum low pressure casting can accelerate the flow of aluminum alloy at low temperature, avoid the phenomenon of aluminum alloy suction caused by high temperature casting, and increase the pinhole defects of the casting.
- The casting has good form-ability, which is conducive to the formation of clear outline, smooth surface castings and large thin-walled castings.
- The casting has dense structure and high mechanical properties.
· In general, no riser is needed, so that the yield of metal liquid is greatly improved, usually up to 90 %.
3. Vacuum differential pressure casting
Vacuum differential pressure casting mainly seals the resistance heat preservation furnace and the casting mold, and injects compressed air with high pressure, such as 500 kPa, into the sealing cover. At this time, because the pressure inside the casting mold and the crucible is equal, the metal liquid will not rise. Then, 50kPa pressure is added to the metal liquid surface, and the metal liquid will rise to fill the mold hole. Especially suitable for the production of complex thin-walled castings, but the equipment is large, troublesome operation, only when special requirements are applied.
Its advantages are as follows:
- Compared with the traditional gravity casting and vacuum suction casting, vacuum differential pressure casting has a good filling ability on the thin sheet sample of 1mm thickness.
- The casting has relatively dense crystal structure and mechanical properties. Compared with vacuum suction casting and gravity casting, the strength performance of the casting increases by about 20 %–25 %, and the elongation increases by about 50 %.
- Vacuum die casting can effectively reduce the porosity of die castings and increase the density of castings by more than 1%.