1. Die-casting process;
The die casting process is to transfer the die casting machine. The process of organic combination and comprehensive use of the three elements of die-casting mold and die-casting alloy.
When die casting, the process of filling the cavity with metal is to place the pressure. Velocity. Temperature. and time and other process parameters are unified. Meantime. These process parameters in turn affect each other. Mutual constraints. And complement each other only if these parameters are selected and adjusted correctly. Make it coordinated. in order to get the desired effect. Therefore. In the die casting process. Not only should we pay attention to the manufacturability of the casting structure. The advanced nature of the casting mold, the excellent performance and structure of the die-casting machine, the adaptability of the die-casting alloy selection and the standardization of the melting process. It should be paid more attention. Pressure. Velocity. Temperature. and time and other process parameters play an important role in the quality of castings.
Section I; Pressure: The presence of pressure is the main feature that distinguishes the die casting process from other casting methods. Stress is an important factor in making tissues dense and contoured. In die casting production. Expressions of pressure are; Injection force. and injection specific pressure.
ejection force;
The injection force is the force that pushes the motion of the injection piston in the injection mechanism of the die casting machine
Usually used ; P. or F denotes. It is a major parameter of the function of the reaction die casting machine. Its size is determined by the area of the injection cylinder and the working pressure of the system.
specific pressure;
The pressure on the area of the aluminum alloy in the press chamber is alone;
. P= F/S P---- Specific pressure S---- Cross-sectional area of the pressure chamber F--- ejection force
F= P 3.14D2/4
Specific pressure can be divided into two types;
Filling ratio filling pressure; The force on each part of the cavity when the metal fills the cavity. (also known as specific pressure of injection)
2. Pressurization specific pressure; The pressure in the booster phase is called the booster specific pressure;
The specific pressure of these two stages is determined according to the ejection force;
The injection force of the two stages of the existing die-casting machine is different, so the specific pressure is also different.
The specific pressure of the filling is to overcome the resistance of the gate system and the cavity; In particular, the resistance of the inner gate.
The specific pressure of the pressurization determines the pressure on the metal that is solidifying. And at this time
The magnitude of the formation of the inflating force.
The specific pressure increases. Fine crystals. Thickening of the fine-grained layer. Due to improved filling characteristics. The surface quality is improved. The stomatal effect is reduced. Tensile strength increases. But the elongation decreases. The molten metal fills the cavity under high specific pressure. The alloy temperature increases.
Liquidity changes. Beneficial with the improvement of the quality of castings.
factors influencing stress;
1. The higher the temperature, the greater the effective specific pressure
2. The mold temperature is too low. Increased pressure loss.
3. Design of casting structure and gating system. The greater the filling resistance. The pressure drops greatly. Also affecting stress are; The performance of the machine. Sensitivity of hydraulic systems. Tightness. Nitrogen pressure. Fluctuations in viscosity caused by changes in oil temperature. The fit between the injection head and the barrel.
Five; velocity during ejection;
The general value range is as follows; One speed 0.3m/s; High speed; 1---5m/S ; Pressurization time: 0.03---0.05m/s. After the injection is completed, the pressurization pressure is maintained until the casting is completely solidified.
Six; The choice of specific pressure
Castings are generally divided into two types; There are strength requirements and no strength requirements.
The choice of specific pressure should be selected according to the wall thickness;
In general. When die-casting thin arm castings. The resistance to the flow of molten metal in the cavity is greater. Because the inner gate is also thin, there is a large resistance, so there must be a large filling specific pressure to ensure that the required inner gate speed is reached.
to castings with thick walls
On the one hand, the selected inner gate velocity is low and the metal has a longer solidification time, so a small filling specific pressure can be used
; On the other hand, in order to make the casting have a certain density, it also needs to have enough pressurization specific pressure to meet the sufficient requirements
For castings with complex shapes, the filling specific pressure should be higher
. But consider the type of alloy; Such as alloys. The velocity of the inner gate. The closure of the press
mold force and other factors
The size of the filling specific pressure is mainly based on the selected inner gate velocity
Calculated. The size of the pressurization specific pressure is mainly based on the different types of alloys to choose different values. When the mold exhaust is good and the inner gate and the casting arm thickness are properly designed, a small point can be selected, otherwise a large point should be selected.
There are airtightness requirements. The surface is large. The wall thinness should generally be; 50---60MPa
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Section 2 Injection velocity
In the injection process, the injection speed is not only directly affected by the pressure, but also plays an important role in the internal quality, surface requirements and contour clarity of the casting. The velocity is often expressed in the form of injection velocity and inner gate velocity,
Injection velocity.
The speed at which the jet head pushes the metal forward is known as the jet velocity.
The injection velocity is divided into multiple stages (generally; Slow. One quick. Two fast )
Slow; It is also called slow jet velocity. It refers to the punch starting to move to seal the melt
The speed of the cup
One fast; It is also called the first-order injection velocity; It refers to the speed from the beginning of the slow end to the time when the metal fills the inner gate.
Two fast; It is also called the second-stage injection velocity; It is also called fast injection velocity; It refers to from
A speed that ends up to the time when the metal has completely filled the cavity.
A fast requires the molten metal in the pressure chamber to fill the pressure chamber, in the principle of not reducing the bottom metal temperature too much, and is conducive to the removal of gas in the pressure chamber, the speed of this stage should be as low as possible, generally should be the bottom of O.3M/S,
The speed is determined by the characteristics of the die-casting machine, the existing domestic die-casting machine is generally 4-----10m/s, the speed is one of the main parameters of the die-casting machine, but the lower the speed is better under the premise of maintaining the internal and external quality of the casting. This facilitates sequential filling. Reduces the presence of stomata.
the role of the second fast;
(1) Influence on the mechanical properties of castings;
Increase the injection speed. The kinetic energy is converted into heat energy, which can improve the fluidity of the alloy. It is beneficial to eliminate defects such as flow marks and cold separation. It can change the mechanical properties and surface quality.
But when the speed is too fast. The molten alloy is mixed with the gas in the form of a mist, resulting in severe turbulence. Mechanical properties decrease. Pinholes are generated locally in the casting.
(2); the effect of injection velocity on filling characteristics;
Increasing the injection speed can change the injection conditions. It can die cast complex thin-walled castings with excellent quality. But when the speed is too fast. The deterioration of filling conditions is most pronounced in thick-walled castings.
Second, the choice of speed and the factors to be considered;
1; It is necessary to consider the latent heat of melting and the range of solidification temperature.
2; When the mold temperature is high. The injection speed can be appropriately lowered to the bottom; In order to improve the life of the mold, the injection speed can also be appropriately limited.
3. When the wall of the casting is thin, the shape is complex and the surface requirements are high. A higher injection velocity should be used.
Inner gate velocity
Under the action of the punch, the molten metal passes through the cross runner to reach the inner gate, and then enters the cavity, and the speed of entering the cavity. It is called the inner gate velocity. The typical inner gate velocity range is 15-----70 m/s
With the change, this change of molten metal flows into the cavity by the complex shape of the cavity. The thickness is different. The mold temperature gradient is not affected by other factors. The speed of the flow changes at any time. This speed is called the filling speed.
The high bottom of the inner gate velocity has a great influence on the mechanical properties of the casting. The inner gate velocity is too low. The strength of the casting will decrease. Inner gate velocity increases. The intensity will go up. And too high will lead to a decrease in strength.