Die casting process introduction
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 speed is usually expressed in two types: the punch speed (injection speed) and the inner gate speed.
1. Injection speed
The speed at which the injection punch in the pressure chamber pushes the metal to move is called the injection speed (also known as the punch speed). The injection speed is divided into two levels. The first-level injection speed is also called the slow injection speed. This level of speed refers to the movement speed of the punch from the initial movement until the punch sends the metal in the chamber into the gate. In this stage, it is required to fill the pressure chamber with the molten metal in the pressure chamber. Under the principle that the temperature of the alloy liquid is not excessively reduced and the gas in the pressure chamber is eliminated, the speed in this stage should be as low as possible, generally 0.3 m/ second. The second-level injection speed is also called fast injection speed. This speed is determined by the characteristics of the die casting machine. The maximum injection speed given by the die-casting machine is generally in the range of 4 to 5 m/s. The old die-casting machine has a lower injection speed, while the modern die-casting machine has a higher injection speed, even reaching 9 m/s.
(1) The role and influence of fast injection speed
Increase the injection speed, convert kinetic energy into heat energy, improve the fluidity of the alloy melt, help eliminate defects such as flow marks and cold barriers, and improve the mechanical properties and surface quality; but when the speed is too fast, the alloy melt will become misty and The gas is mixed, resulting in serious vortex entrapment, and the mechanical performance is reduced.
(2) Considerations for the selection of fast injection speed
①Characteristics of die-casting alloy: latent heat of fusion, specific heat of the alloy, thermal conductivity and solidification temperature range.
②When the mold temperature is high, the injection speed can be appropriately reduced. Taking into account the heat conduction of the mold, the mold design structure and manufacturing quality, and the improvement of the mold life, the injection speed can also be appropriately limited.
③ Casting quality requirements: high surface quality requirements and thin-walled complex parts, using higher injection speed.
2. Inner gate speed
Under the action of the movement of the punch, the molten metal passes through the runner to reach the inner gate, and then fills the cavity. When the performance of the injection system of the machine is good, the speed of the molten metal through the inner gate can be considered unchanged (or little change) ), this constant speed, that is, the linear velocity at which molten metal is introduced into the cavity through the inner gate, is called the inner gate speed, and the usually adopted inner gate speed ranges from 15 to 70 m/s. After the molten metal passes through the inner gate, when it enters the cavity to flow (fill), due to the shape and thickness of the cavity (the wall thickness of the casting), the mold thermal state (temperature field distribution) and other factors, the flow The speed is changing at any time, and the speed of this change is called the filling speed.
Usually, only a 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 influence on the mechanical properties of the casting. If the speed of the inner gate is too low, the strength of the casting decreases; the speed increases, the strength increases; if the speed is too high, the strength decreases.
3.Inner gate speed and injection speed
The relationship between the punch speed (injection speed) and the 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 gate = F injection chamber × V injection / F gate
Where: V internal gate-internal gate speed (m/s-m/s)
F injection chamber-cross-sectional area of injection chamber (cm2-cm2)
V injection-injection speed (m/s-m/s)
F internal gate-internal gate cross-sectional area (cm2-cm2)
Therefore, the higher the injection speed of the punch, the higher the metal flowing through the gate.
4. Choice of speed
In die casting production, speed and pressure together play an important role in the inherent quality, surface requirements and contour definition of the casting.
In summary, if high requirements are placed on the mechanical properties of die castings, such as tensile strength and compactness, an excessively large internal gate speed should not be used, which can reduce the turbulence caused by turbulent flow. The vortex contains air and gases volatilized from the paint. With the increase of the air and steam involved in the vortex, the internal structure of the die casting is porous, and the mechanical properties are obviously deteriorated.
If the structure of the die-casting part is a complex thin-walled part, and higher requirements are put forward on its surface quality, it is absolutely necessary to select a higher injection speed and an inner gate speed.
According to the different conditions of castings, the recommended values in the following table can be selected, the injection speed can be calculated, and the pressure test can be corrected.
