Basic knowledge of die casting

One. Die casting process;

Die casting process is the die casting machine. Die-casting mold and die-casting alloy three elements, the organic combination and comprehensive use of the process.

In die casting, the process of filling the cavity with metal is to apply pressure. Speed. Temperature. And time and other process parameters to be unified. At the same time. These process parameters influence each other. mutual restraint. And complement each other, only the correct selection and adjustment of these parameters. Make it coherent. to get the desired effect. Therefore. In the die casting process. Not only should pay attention to the process of casting structure. The advanced nature of the mold, the excellent performance and structure of the die-casting machine, the adaptability of the selection of die-casting alloys and the standardization of the melting process. More attention should be paid. Pressure. Speed. Temperature. The important role of process parameters such as time and time on the quality of castings.

Section 1; Pressure: The presence of pressure is the main feature of the die-casting process, which is different from other casting methods. Pressure is an important factor in making the tissue dense and clear. In die casting production. Pressure is expressed in the form of; injection force. and injection ratio pressure.

injection force;

The injection force is the force that pushes the injection piston in the injection mechanism of the die-casting machine.

Usually expressed in; P. or F. It is a main 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 pressure chamber is the pressure on the area;

. p = f/s p-specific pressure s-sectional area f of pressure chamber-injection force

　　F= P 3.14D2/4

The specific pressure can be divided into two kinds;

Filling ratio filling pressure; the force on each part of a metal filling a cavity. (also known as the shot pressure)

2. Supercharging specific pressure; the pressure in the supercharging stage is called supercharging specific pressure;

The specific pressure of these two stages is determined according to the injection force;

The injection force of the two stages of the existing die casting machine is different, so the specific pressure is also different.

The filling pressure is to overcome the resistance of the gate system and the cavity; especially the resistance of the inner gate.

The pressurization specific pressure determines the pressure on the solidifying metal. and at this time

The magnitude of the resulting expansion force.

The specific pressure increases. Fine crystalline. Thickening of fine crystalline layer. Improved due to filling characteristics. Improved surface quality. The stomatal effect is reduced. Increased tensile strength. But the elongation is reduced. The molten metal fills the cavity under high specific pressure. The alloy temperature rises.

Liquidity changes. It is beneficial to improve the quality of castings.

Factors affecting stress;

1. The higher the temperature, the greater the effective specific pressure

2. The mold temperature is too low. Increased pressure loss.

3 casting structure and gating system design. The greater the filling resistance. The pressure decreases greatly. Also affecting the pressure; the performance of the machine tool. Sensitivity of hydraulic systems. Tightness. The pressure of nitrogen. Viscosity fluctuations caused by changes in oil temperature. The fit between the injection head and the barrel.

V; speed in the process of injection;

The general value range is: 0.3 m/S at one speed; High speed; 1 --- 5 m/S; Pressure building time; 0.03 --- 0.05 m/s. After the shot. Maintain boost pressure. until the casting is completely solidified.

Six; the choice of specific pressure

Castings are generally divided into two types; strength requirements and no strength requirements.

The choice of specific pressure should be selected according to the wall thickness;

Under normal circumstances. When casting thin arm castings. The flow resistance of the liquid metal in the cavity is large. Because the gate is also thin, there is greater resistance, so a larger filling ratio pressure is required to ensure that the required gate speed is reached.

Pair with thick wall castings

On the one hand, the selected inner gate speed is lower, and the solidification time of the metal is longer, so a smaller filling ratio can be used.

On the other hand, in order to make the casting have a certain density, it is necessary to have sufficient pressure ratio to meet the requirements.

For castings with complex shapes, the filling ratio should be higher.

. But consider the type of alloy; such as alloy. The speed of the gate. The closing of the press

factors such as mold force

The size of the filling pressure is mainly based on the selected inner gate speed.

Calculated. The size of the supercharging specific pressure is mainly based on different alloy categories. When the mold exhaust is good and the inner gate and casting arm thickness are properly designed, a small point can be selected, otherwise a large point should be selected.

Air tightness is required. Big face. The wall thickness should generally be; 50 --- 60MPa

Section 2 Injection Speed

In the process of injection, 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 together with the pressure. The speed is often expressed in the form of injection speed and in-gate speed,

Shot speed.

The speed at which the injection head pushes the metal forward is called the injection speed.

The injection speed is divided into multiple levels (generally there are; slow speed. One fast. Two fast)

Slow; also called slow shot speed. It means that the punch starts to move until it seals the melt.

The speed of the cup

One fast; Also called the first-level shot speed; it refers to the speed from the end of the slow speed to the time when the metal fills the gate.

Second fast; also called secondary shot speed; also called fast shot speed; it refers to from

The speed at which the metal fills the cavity completely.

As soon as possible, the liquid metal in the pressure chamber is required to fill the pressure chamber. Under the principle of not only lowering the temperature of the bottom metal but also eliminating the gas in the pressure chamber, the speed at this stage should be as low as possible, generally at O.3M/S,

Second, the speed is determined by the characteristics of the die-casting machine. The existing domestic die-casting machine is generally 4 ----- 10 m/s, which is one of the main parameters of the die-casting machine. However, on the premise of ensuring the internal and external quality of the casting, the lower the speed, the better. This facilitates the sequential filling. Reduce the presence of stomata.

Two quick effects;

(1) the effect 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 insulation. Can change the mechanical properties and surface quality.

But when the speed is too fast. The alloy liquid is mixed with the gas in the form of mist, resulting in severe turbulence. Decreased mechanical properties. The casting produces pinholes locally.

(2); the effect of injection speed on filling characteristics;

Increasing the injection speed can change the injection conditions. Can be die-cast to produce high-quality complex thin-walled castings. But when the speed is too fast. Deterioration of filling conditions, most notably in thick-walled castings.

Second, the choice of speed and the factors to be considered;

1; To consider the melting latent heat, solidification temperature range.

2; high mold temperature. The injection speed can be properly reduced, and the injection speed can be properly limited to improve the life of the mold.

3 When the casting wall is thin, the shape is complex and the surface requirement is high. A higher injection speed should be used.

Ingate speed

Under the action of the punch, the molten metal passes through the runner to the inner gate, and then enters the cavity. It's called ingate velocity. The commonly used gate speed range is 15 ----- 70 m/s.

This change in the flow of molten metal into the cavity is complicated by the shape of the cavity. The thickness is different. The influence of the temperature gradient of the mold is not equal. The speed of flow changes at any time. This speed is called the filling speed.

The high bottom of the inner gate speed has a great influence on the mechanical properties of the casting. The gate speed is too low. The strength of the casting will decrease. Increased gate speed. the intensity will rise. And too high will lead to a decrease in strength.