Roots vacuum pump is a kind of rotary volume vacuum pump. Its structure is evolved from Roots blower. According to the different working pressure range of Roots vacuum pump, it can be divided into low vacuum Roots pump, medium vacuum Roots pump and high vacuum multistage Roots pump. Roots vacuum pump has been widely used in smelting, vacuum casting, petrochemical, electrical, electronic and other industries. Today I will talk about the benefits of Roots Vacuum Pump in Vacuum Casting.
Vacuum casting is a process using ventilated mould. The molten metal flows into the mould by air pressure, and then clears the air to form a vacuum. This casting method is mainly used for small parts or jewelry with exquisite details.
|1stage piston pump as backing pump||5×10-2|
|2 stage piston pump as backing pump||1×10-1|
|Max. allowable diff. pressure（Pa）||8×103||5×103|
|Motor rotary speed（RPM）||3000|
|Suitable motor power（kW）||2.2||4||7.5||11|
|Cooling water consumption（L/h）||120||150|
|Vacuum pump oil||100#|
Vacuum moulding is a kind of physical moulding method, which combines vacuum technology with sand moulding, seals the cavity surface and back surface of sand moulds by plastic film, produces negative pressure by Roots vacuum pump, causes the difference of internal and external pressure of sand moulds to make the moulds firm and form. After setting up sand, closing mould and casting, waiting for the casting to solidify, relieving negative pressure or stopping gas extraction, the moulds will then collapse. Get the castings. According to the current application fields, it can be divided into vacuum sealing moulding (V method) and mold vacuum casting (FV method). Its advantages are:
1. Moderate vacuum, high compactness of cavity and high quality of castings.
2. Vacuum casting with computer simulation of mold flow analysis can predict the production of casting defects. For the design of the mold scheme, it 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.
1. Vacuum suction casting
The mould is placed in a sealed container and the air in the mould is drawn out, which causes a certain negative pressure in the mould and causes the liquid metal to suck into the cavity. When the inner runner of the casting solidifies, the negative pressure is removed and the solidified metal liquid in the vertical runner flows back into the melting pool. Its advantage is to improve the filling capacity of alloy liquid, the minimum wall thickness of suction casting can reach O.2mm, and the casting area is 300mm2, while reducing the defects such as blowhole and slag inclusion. It is suitable for producing thin and fine small precision castings, especially steel (including stainless steel) castings. Its advantages are:
(1) Vacuum suction casting is conducive to the removal of gas in the casting mould, restraining turbulence and entrainment, overcoming the disadvantages of low pressure casting and differential pressure casting, and improving the filling capacity of liquid metal.
(2) Due to the improvement of filling capacity in vacuum suction casting, the casting temperature of molten metal can be 20-30 C lower than that in gravity casting.
(3) Castings with different wall thickness, thin wall and high quality requirements can be cast.
(4) By choosing the appropriate rate of vacuum change and controlling the speed of liquid metal entering the cavity, a stable filling effect can be obtained.
2. Vacuum Low Pressure Casting
Vacuum low pressure casting is to vacuum the casting mould in the process of pressure filling. After filling, holding pressure makes the casting crystallize and solidify under constant pressure, and the casting is fully compensated, so the structure of the casting is compact and the mechanical properties are improved. This method is mostly used in precision casting of Al and Mg alloy castings. Its advantages are as follows:
(1) The existence of vacuum negative pressure can make the mould sand compact and increase the strength of the mould.
(2) Moderately increasing the vacuum, shortening the filling time, not only accelerating the filling speed, but also stable.
(3) Vacuum low pressure casting can accelerate the fluidity of aluminium alloy at low temperature, avoid the phenomenon of air absorption of aluminium alloy cast at high temperature, and increase the pinhole defect of the casting.
(4) The castings have good formability, which is conducive to the formation of clear contour and smooth surface castings and large thin-walled castings.
(5) The casting has compact structure and high mechanical properties.
(6) In general, no riser is needed, which greatly improves the yield of liquid 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 is mainly to seal the resistance holding furnace and the casting mould. The pressure of 500 kPa compressed air is introduced into the sealing cover. At this time, because the pressure of the casting mould is equal to that of the crucible, the liquid metal will not rise, and then the liquid metal will rise to fill the cavity when the pressure of 50 kPa is added to the liquid metal surface. Especially suitable for the production of complex thin-walled castings, but the equipment is large and the operation is troublesome. Only when special requirements are applied. Its advantages are as follows:
(1) Compared with traditional gravity casting and vacuum suction casting, vacuum differential pressure casting has a good filling capacity on 1 mm thick sheet specimens.
(2) The castings have compact crystalline structure and mechanical properties. The strength and elongation of the castings are increased by 20%~25% and 50% respectively compared with vacuum suction casting and gravity casting.
(3) Vacuum die casting can effectively reduce the porosity of die castings and increase the density of castings by more than 1%.