High pressure and high speed mold filling casting are two characteristics of die casting. Its common injection specific pressure ranges from several thousand to tens of thousands of kPa, even as high as 2× 105kPa. The filling speed is about 10 ~ 50m/s, and sometimes even more than100m/s. The filling time is very short, generally in the range of 0.01~ 0.2s. ..
The dimensional accuracy of castings is high, generally equivalent to 6~7 grades, and even up to 4 grades; Good surface finish, generally equivalent to the die casting workshop.
Grade 5~8; The strength and hardness are higher, the strength is generally 25 ~ 30% higher than that of sand casting, but the elongation is reduced by about 70%; Stable size and good interchangeability; Die-casting thin-walled complex castings. For example, the minimum wall thickness of zinc alloy die casting can reach 0.3mm； at present; Aluminum alloy castings can reach 0.5mm；; Although die casting with the minimum casting aperture of 0.7m has many advantages, there are also some shortcomings to be solved. Such as: 1). Due to the high filling speed and unstable flow pattern of liquid metal during die casting, the castings are easy to produce blowholes and cannot be heat treated by ordinary die casting methods. 2) It is difficult to die-cast complex concave castings; 3) High melting point alloys (such as copper and ferrous metals) have short die-casting life; 4) Not suitable for small batch production. The main reasons are high manufacturing cost of die casting, high production efficiency of die casting machine and uneconomical small batch production. 2. Application scope and development trend of die casting Die casting is one of the most advanced metal forming methods and an effective way to achieve less chips and no chips. It is widely used and develops rapidly. At present, die-casting alloys are no longer limited to non-ferrous metals such as zinc, aluminum, magnesium and copper, but also gradually expand to die-casting cast iron and steel castings. The size and weight of die casting depend on the power of die casting machine. Due to the increasing power of die casting machine, the shape and size of castings can range from a few millimeters to1~ 2 m; The weight ranges from a few grams to dozens of kilograms. Foreign aluminum castings with a diameter of 2m and a weight of 50kg can be die-cast. Die casting is no longer limited to automobile industry and instrument industry, but gradually extends to other industrial fields, such as agricultural machinery, machine tool industry, electronics industry, national defense industry, computers, medical devices, watches, cameras and daily hardware. In the aspect of die casting technology, new technologies such as vacuum die casting, oxygen die casting, precision rapid dense die casting and the application of soluble cores have emerged.
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Types of die casting machines
Die casting machines are generally divided into hot-pressing chamber die casting machines and cold-pressing chamber die casting machines. Cold chamber die casting machine can be divided into horizontal die casting machine and vertical die casting machine (including full vertical die casting machine) according to its chamber structure and arrangement. The pressure chamber of the hot air die casting machine (referred to as hot air die casting machine for short) is immersed in the liquid metal of the insulated melting crucible, and the injection part is not directly connected with the base, but installed on the crucible. The advantages of this die casting machine are simple production process and high efficiency; Less metal consumption and stable process. However, the press chamber and injection punch are immersed in liquid metal for a long time, which affects the service life. It is easy to increase the iron content of the alloy. At present, the autoclave die casting machine is mostly used for die casting low melting point alloy castings such as zinc alloy, but some of them are also used for die casting small aluminum-magnesium alloy castings. The pressure chamber of the cold chamber die casting machine is separated from the holding furnace. In the process of die casting, liquid metal is taken out of the holding furnace and poured into the pressure chamber for die casting.
Selection of die casting machine
In actual production, not every die casting machine can meet the needs of various die casting products, but should be selected according to the specific situation. Generally, the following two aspects should be considered: 1) When organizing multi-variety and small-batch production according to different varieties, a die casting machine with simple hydraulic system, strong adaptability and rapid adjustment should be generally selected, and when organizing large-batch production with few varieties, a high-efficiency die casting machine equipped with various mechanized and automatic control mechanisms should be selected; Special die casting machine can be selected for single variety castings produced in large quantities. 2) According to the casting structure and process parameters, the casting shape size, weight, wall thickness and other parameters have an important influence on the selection of die casting machine. The casting weight (including gating system and overflow tank) should not exceed the rated capacity of the die casting machine, but it should not be too small to avoid wasting the power train of the die casting machine. The rated capacity of a general die casting machine can be found in the instruction manual. The die casting machine has a certain maximum and minimum distance, so there are certain restrictions on the thickness of die casting and the height of casting. If the die casting thickness or casting height is too large, the casting may not be taken out.
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In die casting production, die casting machine, die casting alloy and die casting type are three major elements. Die casting technology is a process of combining these three elements and applying them. So that various process parameters can meet the needs of die casting production.
Selection of pressure and speed
The selection of injection specific pressure should be determined according to the structural characteristics of different alloys and castings. For the selection of filling speed, generally speaking, for castings with high wall thickness or internal quality requirements, lower filling speed and higher pressurization pressure should be selected; For thin-walled or complex castings with high surface quality requirements, higher proportion and higher filling speed should be selected.
Casting temperature
Pouring temperature refers to the average temperature of liquid metal when it enters the cavity from pressing. Because it is inconvenient to measure the temperature of liquid metal in the pressing chamber, it is generally expressed by the temperature in the holding furnace. When the pouring temperature is too high, the shrinkage is large, and the castings are prone to cracks, large grains and bonding; When the pouring source is too low, it is easy to produce defects such as cold preservation, surface pattern and insufficient pouring. Therefore, pouring temperature should be considered simultaneously with pressure, die casting temperature and filling speed.
Die casting mold temperature
The casting die should be preheated to a certain temperature before use, generally using gas, blowtorch, electrical appliances or induction heating. In continuous production, the temperature of die casting often rises, especially for die casting alloys with high melting point, which rises rapidly. Excessive temperature will not only lead to adhesion of liquid metal, but also lead to slow cooling and coarse grains of castings. Therefore, when the die casting temperature is too high, cooling measures should be taken. Usually, compressed air, water or chemical media are used for cooling.
Filling, pressure maintaining and mold opening time
1) the filling time is from the beginning of liquid metal entering the cavity to the end of filling, which is called the filling time. The filling time depends on the size and complexity of the casting. For large simple castings, the filling time is relatively long, and for complex thin-walled castings, the filling time is short. 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 time and mold opening time: When the cavity is filled with liquid metal and the inner gate is completely solidified, it will continue to be under the action of the injection punch, which is called holding time. The holding time depends on the material and wall thickness of the casting. After maintaining the pressure, the casting should be opened and taken out. The time from the end of injection to the opening of die casting is called opening time, which should be controlled accurately. The mold opening time is too short, because the alloy strength is still low, which may cause the casting to be ejected and deformed when it is self-die-cast; However, the mold opening time is too long, the casting temperature is too low, the shrinkage is large, and the core pulling and ejection resistance of the casting is large. Generally, the mold opening time is calculated according to the casting wall thickness 1mm, which takes 3 seconds, and then it is adjusted through trial operation.
Coating for die casting
In the process of die casting, in order to avoid the welding between castings and die castings, reduce the friction resistance of castings and avoid overheating of die castings, coatings are used. Requirements for coatings: 1) Good lubricity at high temperature; 2) The volatilization point is low, and the diluent can quickly volatilize at 100 ~ 150℃; 3) no corrosion to die castings and die castings; 4) Diluents with stable performance in air should not be thickened due to excessive volatilization; 5) No harmful gas will be precipitated at high temperature; 6) The surface of die-casting cavity will not produce scale.
Cleaning of castings
Casting cleaning is a very heavy job, and its workload is often 10 ~ 15 times that of die casting. Therefore, with the improvement of productivity and output of die casting machine, it is very important to realize the mechanization and automation of casting cleaning. 1) The equipment for removing gates and flash is mainly punch, hydraulic press and friction press. Under the condition of mass production of parts, a special mold can be designed according to the structure and shape of the casting, and the cleaning task can be completed on the punch at one time. 2) Ordinary polygonal rollers and vibration embedded cleaning devices are mostly used for surface cleaning and polishing. For small batches of simple small pieces, multi-angle cleaning rollers can be used, and for decorations with high surface requirements, polishing wheels made of cloth or leather can be used for polishing. The invention can be used for mass-produced castings. After the castings are cleaned, surface treatment and impregnation can be carried out according to the use requirements to increase luster, prevent corrosion and improve air tightness. m； The minimum spacing is 0.75mm.