Principle of low pressure casting

Low-pressure casting is a casting method in which the liquid metal is pressurized to fill the cavity and solidify the casting. Because the pressure used is low (20~60kPa), so it is called low-pressure casting. The process is: in the sealed crucible (or sealed can), pass into the dry compressed air, the metal liquid in the gas pressure, along the rising liquid pipe into the straight pouring channel rise, through the inner pouring channel smoothly into the cavity, and keep the crucible liquid surface gas pressure, until the casting is completely solidified. Then the gas pressure on the liquid surface is lifted, so that the unsolidified metal liquid in the rising liquid pipe flows back to the crucible, and then the mold is opened and the casting is taken out.

The low pressure casting production process includes the following four basic processes:

（1）Metal melting and preparation of mold or casting pattern.

（2）Preparation before pouring: including crucible sealing (assembling sealing cover), slagging in the ascending liquid pipe, measuring liquid level height, sealing test, mold matching, tightening mold or casting, etc.

（3）Pouring: including liquid rising, mold filling, pressurization, solidification, pressure relief and cooling, etc.

（4）Demolding: including loosening the mold and taking out the casting.

Advantages

(1) Pure liquid metal filling improves the purity of castings. As the slag generally floats on the surface of the liquid metal, low-pressure casting is realized by the liquid metal in the lower part of the crucible through the ascending tube, which completely avoids the possibility of the slag entering into the cavity of the casting model.

(2) The liquid metal filling is smooth, which reduces or avoids the tumbling, impact and splashing phenomenon of liquid metal when filling, thus reducing the formation of oxidized slag.

(3) casting good molding, metal liquid filling under pressure, can improve the fluidity of the metal liquid, conducive to the formation of clear contours, surface finish castings, for the molding of large thin-walled castings is more favorable.

(4) castings under pressure crystallization and solidification, can be fully complementary shrinkage, castings dense organization.

(5) Improve the rate of liquid metal, generally do not need a riser, and rising liquid tube in the unconsolidated metal can be returned to the crucible, re-use, so that the rate of liquid metal greatly improved, the rate of up to 90%, generally.

Disadvantages

(1) slow production speed is low pressure die casting faces a big challenge. Slow speed greatly affects the efficiency of mass production. Also, the process is unable to produce wall thicknesses less than 3 mm, which limits its use in certain designs.

(2) In the production of aluminum alloy castings, the crucible and liquid-raising tube are in contact with the metal liquid for a long period of time, which makes them susceptible to erosion and scrap, as well as deterioration of the performance of the metal liquid by increasing iron.