Pressure die casting is a kind of liquid or semi-solid metal or alloy, or liquid metal or alloy containing reinforcement phase, filled into the cavity of a die-casting mold under high pressure at a higher speed, and the metal or alloy is under pressure die casting method of solidification to form castings.
The pressure commonly used in die casting is 4 to 500 MPa, and the metal filling speed is 0.5 to 120 m/s. Therefore, high pressure and high speed are the fundamental difference between die casting and other casting methods, and they are also important features.
The principle of pressure die casting is mainly the principle of injection molding of molten metal. Usually set casting conditions are adjusted by the speed, pressure, and speed switching position on the die casting machine, and the others are selected in the die casting line.
In the production of die-casting, die-casting machine, die-casting alloy and die-casting mold are the three major elements. The die-casting process is a process of combining the three elements with power and applying them. Make various process parameters meet the needs of die casting production. The choice of injection pressure should be determined according to the structural characteristics of different alloys and castings. For the selection of filling speed, generally for thick-walled or high-quality castings, lower filling speed and high boost pressure should be selected; for thin-walled or high-quality castings and complex castings, you should choose High ratio and high filling speed.
The pouring temperature refers to the average temperature of the liquid metal when it enters the cavity from the pressure setting. Because it is inconvenient to measure the temperature of the liquid metal in the pressure chamber, it is generally expressed by the temperature in the holding furnace.
Too high pouring temperature and large shrinkage make the castings prone to cracks, large grains, and stickiness; too low pouring temperature, easy to produce defects such as cold barrier, surface pattern and insufficient pouring. Therefore, the pouring temperature should be considered at the same time as the pressure, die-casting temperature and filling speed.
Die casting mold temperature
The casting mold must be preheated to a certain temperature before use. Generally, gas, blowtorch, electrical appliances or induction heating are used.
In continuous production, the temperature of the die-casting mold tends to rise, especially for die-casting high melting point alloys. Excessive temperature not only causes the liquid metal to produce stickiness, but also slows down the cooling of the casting, which makes the crystal grains coarse. Therefore, cooling measures should be adopted when the die temperature is too high. Compressed air, water or chemical media are usually used for cooling.
Filling, holding pressure and opening time
1) Filling time
The time required for the liquid metal to enter the cavity until the cavity is filled is called the filling time. The filling time depends on the volume and complexity of the casting. For large and simple castings, the filling time is relatively long, and for complex and thin-walled castings, the filling time is shorter. The filling time is closely related to the cross-sectional area of the inner gate or the width and thickness of the inner gate and must be determined correctly.
2) Holding pressure and opening time
The duration from the filling of the cavity with liquid metal to the complete solidification of the inner gate is called the holding time. The length of the holding time depends on the material and wall thickness of the casting.
After holding the pressure, the casting should be opened and taken out. The time from the end of the injection to the opening of the die is called the opening time, and the opening time should be controlled accurately. The mold opening time is too short, because the alloy strength is still low, it may cause deformation when the casting is ejected and the self-die casting mold is dropped; but the mold opening time is too long, the temperature of the casting is too low, and the shrinkage is large. The resistance is also great. Generally, the opening time is calculated according to the casting wall thickness of 1 mm, which takes 3 seconds, and then adjusted by trial.
Die casting coating
In the die-casting process, in order to avoid the welding of the casting and the die-casting mold, reduce the friction resistance of the casting and avoid excessive heating of the die-casting mold, paint is used. Requirements for coatings:
1) At high temperature, it has good lubricity;
2) The volatilization point is low, and the diluent can quickly volatilize at 100～150℃;
3) There is no corrosive effect on die casting molds and die castings;
4) Stable performance The diluent should not become thick due to excessive volatilization in the air;
5) No harmful gas will be precipitated at high temperature;
6) No fouling on the surface of the die-casting cavity.
The cleaning of castings is a very heavy work, and the workload is often 10 to 15 times that of die casting. Therefore, as the productivity of the die casting machine increases and the output increases, it is very important to realize the mechanization and automation of the casting cleaning work.
1) Cut off the gate and flash
The equipment used to remove gates and flash is mainly punches, hydraulic presses and friction presses. Under mass production of parts, special molds can be designed according to the structure and shape of the castings, and the cleaning task can be completed at one time on the punching machine.
2) Surface cleaning and polishing
Surface cleaning mostly uses ordinary polygonal rollers and vibration buried cleaning devices. For simple and small pieces with small batches, multi-angle cleaning rollers can be used. For decorations with high surface requirements, they can be polished with cloth or leather polishing wheels. For mass-produced castings, a spiral shell type vibration cleaning machine can be used.
The cleaned castings can also be surface treated and impregnated according to the requirements of use to increase gloss, prevent corrosion and improve air tightness.