Low-pressure casting liquid metal is then filled in the cavity under pressure to form a casting method. As used in low pressure,

so called low-pressure casting. The process (see Figure 1) is: in a sealed crucible (or sealed cans), the pass into the dry air,

liquid metal 2 in the gas under pressure, fluid along the tube 4 liters rose steadily through the gate 5 into the cavity 8, and

keep the crucible on the gas pressure within the liquid, until the complete solidification of the casting so far. Then lift the

gas pressure on the surface, so that is not open solidification of liquid metal tube flow crucible, and then by the cylinder 12

to open and release type casting.

The unique advantages of low pressure die casting in the following areas:

1. Liquid metal filling is relatively stable;

2. Casting shape is good, clear contours conducive to the formation, smooth surface of the casting, for large thin-walled

castings forming more favorable;

3. Cast compact structure, high mechanical properties;

4. Improve the process of liquid metal yield, under normal circumstances no riser, so that the yield of liquid metal has greatly

improved the recovery rate is generally up to 90%.

In addition, good working conditions; equipment simple and easy to mechanization and automation, but also highlight the

advantages of low-pressure casting.

8.2 low-pressure casting process design

Used in low pressure die casting mold, there are two types of metallic and non-metallic type. Metal used for large, mass

production of non-ferrous castings, non-metallic mold used for single and small batch production, such as sand, graphite, ceramic

and shell melting model can be used for low-pressure casting, etc., and more used in the production or sand. However, the shape

of low-pressure sand casting material permeability and strength should be high when the proportion of force casting, cavity in

the gas, thanks to the exhaust discharge and sand porosity.

To take full advantage of low-pressure casting liquid metal under pressure feeding the bottom up, casting, during the process

design should take into account the

Casting away from the site of the first gate solidification, so that the final gate solidification, the casting during

solidification has been feeding through the gate to achieve solidification. Often used the following measures:

1. Gate in the thick part of the casting, leaving thin-walled parts away from the gate;

2. Castings with wall thickness adjustment processing margin to adjust the directional solidification of castings;

3. Change the casting cooling conditions.

For the difference of the casting wall thickness, with the above general measures are difficult to get care order freezing

conditions, we can use some special methods, such as thick wall castings for local cooling, in order to achieve solidification.

8.3 low-pressure casting process

Low-pressure casting process specifications including filling, pressure, mold preheat temperature, pouring temperature, and the

cast of the coatings.

(1) filling and pressurization

Liters of fluid pressure is increased when the metal surface to the gate, with the required pressure. Liters of liquid metal

fluid tube in the rate of increase should slow as possible, in order to facilitate the discharge of the gas cavity, while

allowing the liquid metal when entering the gate will not have a splash.

(2) filling pressure and filling velocity

Filling pressure Pa is ordered liquid metal filling the mold up to the top of the pressure required. In the filling stage, the

speed boost on the metal surface is filling velocity.

(3) booster and boost speed

Cavity filled with liquid metal, and then continue to boost, the crystallization of the casting solidification in a certain size

under the effect of pressure, when pressure is called the crystallization pressure. Greater crystallization pressure, feeding the

better, and finally casting organizations get more dense. However, by increasing the pressure to improve the crystalline quality

of casting, not under any circumstances can be used.

(4) The holding time

Cavity pressure to crystallization pressure, and crystallization under pressure for some time, until completely solidified

casting time required for call holding time. If the holding time is not enough, the casting is not completely solidified on the

relief, the cavity in the molten metal will flow back into the batch to donate all or part of, resulting in casting \"shorting\"

Waste: If you dwell too long, the gate remains too long This process not only reduces yield, but can also cause the gate to

\"freeze\" the casting out the type of difficulty, so production must select a suitable dwell time.

(5) mold temperature and pouring temperature

Low pressure die casting mold can be a variety of non-metallic type of temperature is generally room temperature, the first

special requirements, and operating temperature of the metal have certain requirements. Such as low-pressure cast aluminum, the

metal working temperature is controlled at 200 ~ 2500C, when casting thin-walled complex parts can be as high as 300 ~ 3500C.

Pouring temperature of the alloy, proved in practice to ensure the casting forming the premise should be the smaller the better.

(6) paint

Such as low pressure casting of metal type, in order to improve their life and casting quality, must paint brush; coating should

be uniform coating thickness according to the casting surface finish and casting structures to decide.

8.4 Low pressure casting equipment

Low pressure casting equipment generally by holding furnace and its equipment, mold opening and closing system and air supply

system consists of three parts. Connection by way of casting and holding furnace, can be cast into the top side of low pressure

casting machine and cast low-pressure casting machine types.

(1) holding furnace and ancillary equipment

It consists of furnace body, bath, closures and tubes are composed of liquid l is the basic part of the low-pressure casting

machine. Holding furnace of the furnace a lot, such as coke oven, gas stove, electric resistance furnace, induction furnace and

so on. But widely used is the resistance furnace, followed by electric reverberatory furnace.

(2) supply system

In the low-pressure casting, casting the right to control the filling and pressurization is critical to obtain good castings,

controlled entirely by the supply system to achieve. Depending on the casting, casting different requirements, gas supply system

should be able to be adjusted, stable and reliable work to the structure to make maintenance easy.

8.5 Special low-pressure casting process

Low pressure casting of some special structure or special performance requirements of the casting, it is difficult to meet the

time in the low-pressure casting, based on the development of special low-pressure casting process.

(1) low-pressure casting method

Some of the casting of high internal quality requirements, hope high pressure crystallization, but the general low-pressure

casting of the crystal can not be too much. Thus the development of a low pressure casting method. The process is all the

resistance holding furnace and the mold is sealed within a learned person in a sealed cover as 500kPa of air pressure, this time

due to mold and increase the pressure inside the pot equal to the liquid metal does not increase, then liquid metal up to add

50kPa pressure, liquid metal rises filling the cavity. While also casting molten metal within the crystal under high pressure

freezing. This approach allows the casting to be more compact crystal structure, improve the mechanical properties of castings.

According to the information on this process compared with the general method of casting, the casting strength is increased by

about 25% elongation increased by about 50%. But the equipment is relatively large, operational problems, only when the

application specific requirements.

(2) low-pressure vacuum casting

Or thin-walled castings of complex large-scale, low-pressure casting process using the aforementioned, it is difficult to meet,

the low-pressure vacuum casting can be easily solved. Its low-pressure casting device and the differential pressure method are

similar. Before pouring the first cavity in the gas pouring out then, then casting speed can be improved, no defects such as

oxide inclusions and porosity.