1. Die casting process;
The die casting process is to put the die casting machine. The process of organically combining and comprehensively applying the three elements of die-casting mold and die-casting alloy
In die casting, the process of metal filling the cavity is to put the pressure. Speed. Temperature. and time and other process parameters to be unified. At the same time. These process parameters affect each other. Mutual restraint. And complement each other, only by correctly selecting and adjusting these parameters. Make it coordinated. In order to achieve the expected effect. Therefore. In the die casting process. Not only should we pay attention to the workmanship 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 selection of die casting alloy and the standardization of the melting process. More attention should be paid to. Stress. Speed. Temperature. and the important role of process parameters such as time on the quality of castings
section; Pressure: The presence of pressure is the main feature that distinguishes the die casting process from other casting methods. Pressure is an important factor in making tissues dense and well-defined. In die casting production. The expression of stress is; Compression force. and injection specific pressure
compression force;
Injection force is the force that drives the movement of the injection piston in the injection mechanism of the die casting machine, which is 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 aluminum alloy in the pressing chamber is the pressure on the area;
. P= F/S P---- Specific pressure S---- Cross-sectional area of the pressure chamber F--- Compression force
F= P 3.14D2/4
The specific pressure can be divided into two types.
filling ratio filling pressure; The force exerted by each part when the metal fills the cavity. (also known as injection specific pressure).
2. Boost specific pressure; The pressure in the boost stage is called the boost specific pressure;
The specific pressure of these two stages is determined by the pressure projectile;
The injection force of the two stages of the existing die casting machine is different, so the specific pressure is also different
The filling specific pressure overcomes the resistance of the gate system and the cavity; Especially the resistance of the inner gate
The boost specific pressure determines the pressure on the solidifying metal. And the magnitude of the upward force formed at this time
The specific pressure increases. Crystalline fine. Fine grain layer thickening. Due to improved filling characteristics. Surface quality improved. The stomatal effect is reduced. Tensile strength is improved. But the elongation rate is reduced. The metal liquid fills the cavity under the action of high specific pressure. The temperature of the alloy rises
Liquidity changes. It is beneficial to the improvement of casting quality
factors that influence stress;
1. The higher the temperature, the greater
the effective specific pressure 2. The mold temperature is at the bottom. Increased pressure loss
3. Design of casting structure and gating system. The greater the filling resistance. The pressure is greatly reduced. There are also factors that affect stress; Machine tool performance. Sensitivity of the hydraulic system. Sealing. Nitrogen pressure. The fluctuation of viscosity caused by the change of oil temperature. The cooperation between the injection head and the barrel
Five; speed during injection;
The general value range is; 1 speed 0.3m/s; high speed; 1---5m/S ; Build pressure time: 0.03---0.05m/s. After the injection is over, maintain the boost pressure until the casting is completely solidified
Six; The selection of castings for specific pressure is 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 metal liquid in the cavity is large. Because the inner gate is also thin, there is a large resistance, so it is necessary to have a large filling specific pressure to ensure that the required inner gate speed is reached
For thick-walled castings
, on the one hand, the selected inner gate speed is low, and the solidification time of the metal is longer, so a small filling specific pressure
can be used; On the other hand, in order to make the casting have a certain density, it is also necessary to have enough boost specific pressure to meet the sufficient requirements
. However, the type of alloy should be considered; Such as alloys. Speed of the inner gate. The clamping force of the press and other factors
of the filling specific pressure are mainly calculated according to the selected inner gate speed
.
The size of the boost specific pressure is mainly based on different values according to the category of different alloys. When the mold exhaust is good and the inner gate and casting arm thickness are properly designed, you can choose a small point, otherwise you should choose a large point
There are air tightness requirements. The face is large. The wall thinness should generally be; 50---60MPa
main figure 5.1.jpg
Section 2 Injection speed
During 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 expression of speed is often the injection speed and the inner gate speed, and
the injection speed
The speed at which the injection head pushes the metal forward is called the injection velocity
The injection velocity is divided into multiple stages (generally there are; Slow. Quick. 2. Fast
) slow; It is also called slow pressure firing speed. It refers to the speed at which the punch starts to move to seal the mouth of the molten
cup;
It is also called the first-order injection velocity; It refers to the speed from the end of the slow speed to when the metal fills the inner gate
2. Fast; It is also called the second-level injection velocity; It is also called fast pressure injection speed; It refers to the speed from
the end of the fast to when the cavity is completely filled with metal
Under the principle of not reducing the temperature of the bottom metal too much and helping to eliminate the gas in the pressure chamber, the speed of this stage should be as low as possible, generally at 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 speed is better under the premise of maintaining the quality inside and outside the casting. This is conducive to filling in order. Reduces the presence of stomata
2. The role of fast;
(1) Influence on the mechanical properties of castings;
Increase the injection speed. The kinetic energy is converted into thermal energy, which can improve the fluidity of the alloy. It is beneficial to eliminate defects such as flow marks and cold separation. Mechanical properties and surface quality can be changed
But when the speed is too fast. The alloy liquid mixes with the gas in the form of mist, creating severe turbulence. Mechanical properties decrease. The casting partially produces pinholes
(2); 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 filling conditions deteriorate, which is significant in thick-walled castings
2. The choice of speed and considerationfactors;
1; The latent heat of melting and the solidification temperature range should be considered
2; When the mold temperature is high. The injection speed can be appropriately lowered; In order to improve the life of the mold, the injection speed can also be appropriately limited
3. When the casting has thin walls, complex shapes and high surface requirements. A higher injection speed should be used
Inner gate velocity
: Under the action of the punch, the molten metal passes through the cross sprue to reach the inner gate, and then enters the cavity, and the speed of entering the cavity. It's called the inner gate speed. The usually used inner gate speed range is 15-----70m/s
with the same change, and when the molten metal enters the cavity flow, it is complex with the shape of the cavity. Different thicknesses. The influence of mold temperature gradient and other factors. The speed of flow can change at any time. This speed is called fill speed
The high bottom of the inner gate velocity has a great influence on the mechanical properties of the casting. The inner gate speed is too low. The strength of the casting will decrease. Increased inner gate speed. The intensity will rise. And too high will lead to a decrease in strength