Injection speed of die casting process
During the injection process, the injection speed is directly affected by the pressure, and together with the pressure, it plays an important role in the internal quality, surface quality, and contour clarity of the casting. In production, the expression of speed is usually divided into two types: punch speed (injection speed) and gate speed.
1. Injection speed
The speed at which the injection punch in the pressure chamber moves the metal is called the injection speed (also known as the punch speed).
The injection speed is divided into two levels. The first level of injection speed is also known as the slow injection speed. This level of speed refers to the movement speed of the punch from the start of action until the punch enters the metal in the chamber into the inner gate. In this stage, it is required to fill the pressure chamber with the metal liquid in the pressure chamber. Under the principle of not only reducing the temperature of the alloy liquid too much but also facilitating the elimination of gas in the pressure chamber, the speed in this stage should be as low as possible, typically 0.3 meters per second. Level II injection speed is also known as fast injection speed. This speed is determined by the characteristics of the die casting machine. The maximum injection speed given by die casting machines is generally within the range of 4-5 meters per second. The injection speed of older die casting machines is relatively low, while modern die casting machines are relatively high, even reaching 9 meters per second.
(1) The role and influence of fast injection velocity
By increasing the injection speed and converting kinetic energy into heat energy, the fluidity of the alloy melt is improved, which is conducive to eliminating defects such as flow marks and cold shuts, and improving mechanical properties and surface quality; However, when the speed is too fast, the alloy molten liquid mixes with the gas in a mist shape, resulting in severe eddy current aeration and reduced mechanical properties.
(2) Selection considerations for fast injection speed
① Characteristics of die casting alloys: melting latent heat, specific heat of the alloy, thermal conductivity, and solidification temperature range.
② When the mold temperature is high, the injection speed can be appropriately reduced. Taking into account the mold thermal conductivity, mold design structure, manufacturing quality, and improving mold life, the injection speed can also be appropriately limited.
③ Casting quality requirements: high surface quality requirements and thin-walled complex parts, using a higher injection speed.
2. Inner gate speed
Under the action of the punch movement, molten metal passes through the transverse gate to the inner gate, and then fills the mold cavity. When the performance of the machine's injection system is excellent, the speed at which molten metal passes through the inner gate can be considered constant (or slightly variable). This constant speed, which is the linear speed at which molten metal passes through the inner gate and enters the mold cavity, is called the inner gate speed. The commonly used inner gate speed range is 15 to 70 meters/second. When molten metal flows (fills) into various parts of the mold cavity after passing through the inner gate, the flow speed changes at any time due to various factors such as the shape and thickness of the mold cavity (the wall thickness of the casting), the mold thermal state (temperature field distribution), and the like. The speed of this change is called the filling speed. Generally, only constant speed is selected to measure the process parameters, so the inner gate speed is one of the important process parameters. The speed of the inner gate has a great impact on the mechanical properties of the casting. If the inner gate speed is too low, the strength of the casting will decrease; Increase in speed and strength; The speed is too high and the intensity decreases again.
3. Relationship between punch speed (injection speed) and inner gate speed (filling speed)
According to the principle of continuity, the relationship between the inner gate speed and the injection speed can be expressed by the following formula:
V inner gate=F injection chamber × V injection/F inner gate
Where: V inner gate - inner gate speed (m/s - m/s)
F Injection chamber - Injection chamber cross-sectional area (cm2 cm2)
V injection - injection velocity (m/s - m/s)
F Inner gate - sectional area of inner gate (cm2 cm2)
Therefore, the higher the injection speed of the punch, the faster the metal flows through the inner gate.
4. Selection of speed
In die casting production, speed and pressure together play an important role in the internal quality, surface requirements, and contour clarity of castings.
In summary, if high requirements are placed on the mechanical properties of die castings, such as tensile strength and compactness, excessive gate speeds should not be selected, which can reduce the vortices caused by turbulent flow, which contain air and gases volatilized by the coating. With the increase of air and steam trapped in the vortex, the microstructure of the die casting exhibits porosity and its mechanical properties deteriorate significantly. If the structure of a die casting is a complex thin-walled part and requires high surface quality, it is entirely necessary to choose a higher injection speed and internal gate speed. According to different situations of castings, the recommended values in the following table can be selected to calculate the injection speed and conduct pressure test correction.
Recommended values for filling speed of various parts of the pouring system
Sprue
Filling speed (m/s) 15 ～ 25 20 ～ 35 30 ～ 60
Table of recommended relationship between average wall thickness of castings and gate speed
Average wall thickness of casting (mm) Inner gate speed (m/s) Average wall thickness of casting (mm) Inner gate speed (m/s)
1 46~55 5 32~40
1.5 44~53 6 30~37
2 42~50 7 28~34
2.5 40~48 8 26~32
3 38~46 9 24~29
3.5 36~44 10 24~27
4 34~42
The inner gate speed is related to the injection speed, the pressure chamber diameter, and the inner gate cross-sectional area, and can be adjusted through the following aspects:
(1) Adjusting the speed of the injection punch
(2) Change the diameter of the injection chamber
(3) Changing the sectional area of the inner gate
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