What is Die casting?
Pressure casting refers to a casting method in which a molten or semi-molten metal is injected into a metal mold at a high speed and crystallized under pressure, which is referred to as Die casting. The common injection pressure is 30-70 MPa, the filling speed is about 0.5-50 m/s, and the filling time is 0.01-0.2 s.
Pressure casting is a liquid metal or alloy containing a liquid or semi-solid metal or alloy, or a phase containing a reinforcing phase, which is filled into a cavity of a die-casting mold at a high speed under high pressure, and the metal or alloy is under pressure. A method of casting that solidifies to form a casting. 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 fundamental differences between Die casting and other casting methods, and are also important features. In 1838, the first time Americans used the pressure casting method to produce printed letters, and the next year there was a pressure casting patent. After the 1860s, the pressure casting method has been greatly developed. It can not only produce tin-lead alloy die-casting parts, zinc alloy die-casting parts, but also aluminum alloy, copper alloy and magnesium alloy die-casting parts. After the 1930s, the steel pressure casting method was tested.
The principle of pressure casting is mainly the principle of injection molding of molten metal. Usually, the casting conditions are set by the speed, pressure, and speed switching positions on the Die casting machine, and others are selected in the Die casting type. 
1. Advantages of pressure casting
1) High productivity, easy mechanization and automation, and the production of thin-walled castings with complex shapes. The minimum wall thickness of the die-cast zinc alloy is only 0.3 mm, the minimum wall thickness of the die-cast aluminum alloy is about 0.5 mm, and the minimum cast hole diameter is 0.7 mm.
2) The casting has high dimensional accuracy and small surface roughness. The dimensional tolerance of die-casting parts can reach CT3~CT6, and the surface roughness is generally 0.8-3.2μm.
3) The castable parts in the die-casting parts not only save precious materials and machining man-hours, but also replace the assembly process of the parts, which can save the assembly process and simplify the manufacturing process.
2, the disadvantages of pressure casting
1) The filling speed of liquid metal during Die casting is high, the gas in the cavity is difficult to completely remove, and the casting is prone to defects such as pores and cracks and oxidized disasters, and the die casting is usually not heat treated.
2) The die-casting mold has a complicated structure, a long manufacturing cycle, and high cost, and is not suitable for small-quantity casting production.
3) The Die casting machine has high cost and large investment, and is limited by the clamping force of the die-casting machine and the size of the mold. It is not suitable for the production of large-scale die-casting parts.
4) The type of alloy is limited, and non-ferrous alloys such as zinc, magnesium and copper. 
In Die casting production, Die casting machines, Die casting alloys and Die casting are three major factors. The Die casting process is a process in which the three elements are combined and used. Make various process parameters meet the needs of Die casting production. The choice of injection specific pressure should be determined according to the structural characteristics of different alloys and castings. For the selection of filling speed, generally for castings with high thick wall or internal quality requirements, lower filling speed and high supercharging pressure should be selected; for castings with high wall thickness or surface quality and complex castings, should be selected Higher ratio and higher filling speed.
The pouring temperature refers to the average temperature of the liquid metal from the time of pressing into the cavity. Since the measurement of the temperature of the liquid metal in the pressure chamber is inconvenient, it is generally expressed by the temperature in the holding furnace.
The pouring temperature is too high, the shrinkage is large, the casting is prone to cracks, the grain size is large, and the sticking type can be caused; the pouring temperature is too low, and the defects such as cold partition, surface pattern and insufficient pouring are easily generated. Therefore, the pouring temperature should be considered simultaneously with the pressure, the Die casting temperature and the filling speed.
Die casting temperature
The cast type should be preheated to a certain temperature before use, generally using gas, blowtorch, electric appliance or induction heating.
In continuous production, the temperature of die-casting tends to increase, especially for die-casting high-melting alloys, which rises rapidly. Excessively high temperatures, in addition to making the liquid metal sticky, the castings cool slowly, making the grains coarse. Therefore, when the temperature of the die-casting type is too high, cooling measures should be taken. It is usually cooled with compressed air, water or a chemical medium.
Filling, holding and opening time
1) Filling time
The time required for the liquid metal to enter the cavity to fill the cavity is called the filling time. The length of filling depends on the size and complexity of the casting. For large and simple castings, the filling time is relatively long, and the filling time for complex and thin-walled castings is shorter. The filling time is closely related to the cross-sectional area of the gate or the width and thickness of the gate, and must be correctly determined.
2) Holding pressure and opening time
When the liquid metal filling cavity is completely solidified, the duration of the injection under the action of the injection punch 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 taken out. The time from the end of the injection to the opening of the Die casting is called the opening time, and the opening time should be controlled accurately. The opening time is too short. Due to the low strength of the alloy, it may cause deformation when the casting is ejected and the self-pressing mold falls. However, if the opening time is too long, the casting temperature is too low, the shrinkage is large, and the core is drawn and the casting is ejected. The resistance is also big. Generally, the opening time is calculated according to the wall thickness of the casting 1 mm, and then it is adjusted for 3 seconds.
Die casting coating
In the die-casting process, in order to avoid the welding of the casting and the die-casting type, the frictional resistance of the casting is reduced, and the coating is prevented from being excessively heated by the die-casting type. Requirements for coatings:
1) Good lubricity at high temperatures;
2) The volatilization point is low, and the diluent can be quickly evaporated at 100-150 ° C;
3) No corrosion to die-casting and die-casting parts;
4) Stable performance In the air, the thinner should not volatilize excessively and thicken;
5) No harmful gases will be precipitated at high temperatures;
6) No fouling will occur on the surface of the die-cast cavity.
The cleaning of castings is a very arduous task, and the workload is often 10 to 15 times that of Die casting. Therefore, as the productivity of the Die casting machine increases, the output increases, and the mechanization and automation of the casting cleaning work is very important.
1) Cut the gate and flash
The equipment used for cutting gates and flashing is mainly punching machines, hydraulic presses and friction presses. Under a large number of production parts, special molds can be designed according to the structure and shape of the castings, and the cleaning tasks can be completed once on the punching machine.
2) Surface cleaning and polishing
For surface cleaning, ordinary multi-angle rollers and shock-buried cleaning devices are used. For small and small parts that are not large in size, the multi-angle cleaning roller can be used, and the decorative parts with high surface requirements can be polished by cloth or leather polishing wheel. For the mass production of castings, a screw-type vibration cleaner 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.
Due to the advantages of Die casting, it has been widely used, mainly for the production of non-ferrous alloy castings in large quantities. Among the die-casting parts, the largest proportion is aluminum alloy die-casting parts, which is 30% to 50%; followed by zinc alloy die-casting parts; copper alloy die-casting parts account for 1% to 2%. The most applied Die castings are automobile and tractor manufacturing, followed by instrument manufacturing and electronic instrument industry, and again for agricultural machinery, defense industry, computer, medical equipment and other manufacturing industries. Parts produced by Die casting include engine cylinder blocks, cylinder heads, transmission cases, hoods, gauges and camera housings and brackets, pipe joints, gears, and the like.
In recent years, high technology has been used in the field of Die casting. If a three-stage injection mechanism is used, the pressure, the injection speed and the internal gas are controlled. Develop special die-casting processes (such as vacuum die-casting, directional pressure casting, oxygen-filled die-casting, etc.) and apply computer control technology to effectively remove pores and increase the density of castings. At the same time, develop new mold materials and new heat treatment processes to extend the life of the molding. There has been some progress in ferrous metal Die casting.
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