The essence of force casting (referred to as die casting) is a method of filling the die-casting mold cavity with liquid or semi-liquid metal at a high speed under high pressure, and forming and solidifying under pressure to obtain castings. .
Die-casting characteristics
High-pressure and high-speed filling of die-casting molds are the two major characteristics of die-casting. Its commonly used injection specific pressure ranges from several thousand to tens of thousands of kPa, or even as high as 2×105kPa. The filling speed is about 10~50m/s, and sometimes it can even reach more than 100m/s. The filling time is very short, generally in the range of 0.01~0.2s. Compared with other casting methods, die casting has the following three advantages:
Advantages:
1. Good product quality
The dimensional accuracy of castings is high, generally equivalent to Level 6~7, even up to level 4; good surface finish, generally equivalent to level 5~8; high strength and hardness, the strength is generally 25~30% higher than that of sand casting, but the elongation is reduced by about 70%; dimensional stability , good interchangeability; can die-cast thin-walled and complex castings. For example, the current minimum wall thickness of zinc alloy die-casting parts can reach 0.3mm; the minimum wall thickness of aluminum alloy castings can reach 0.5mm; the minimum casting hole diameter is 0.7mm; and the minimum thread pitch is 0.75mm.
2. High production efficiency
The machine has high productivity. For example, the domestic JIII3 horizontal cold air die casting machine can perform an average of 600 to 700 die castings in eight hours, and the small hot chamber die casting machine can perform an average of 600 to 700 die castings per hour. It can die-cast 3,000 to 7,000 times in eight hours; the die-casting mold has a long life. A pair of die-casting molds and die-casting bell alloys can have a life of hundreds of thousands or even millions of times; it is easy to realize mechanization and automation.
3. Excellent economic effect
Due to the advantages of precise size and smooth surface of die-casting parts. Generally, it is no longer mechanically processed but used directly, or the processing amount is very small, so it not only improves the metal utilization rate, but also reduces a lot of processing equipment and man-hours; the price of castings is easy; combined die-casting can be used to make other metal or non-metallic materials . Saves both assembly time and metal.
Although die casting has many advantages, it also has some disadvantages that have yet to be solved.
Disadvantages
For example:
1). Due to the high speed of liquid metal filling the cavity and unstable flow state during die casting, the general die casting method is used to It is easy to produce pores and cannot be heat treated;
2). For castings with complex concavities, die casting is more difficult;
3). High melting point alloys (such as copper, ferrous metals), The life of the die-casting mold is relatively low;
4). It is not suitable for small batch production. The main reason is that the manufacturing cost of the die-casting mold is high, the production efficiency of the die-casting machine is high, and small batch production is uneconomical.
Die-casting application scope and development trend
Die-casting is one of the most advanced metal forming methods. It is an effective way to achieve less and no chips. It is widely used and is developing rapidly. At present, die-casting alloys are no longer limited to non-ferrous metals such as zinc, aluminum, magnesium and copper, but are also gradually being used to die-cast cast iron and steel castings. The size and weight of die-casting parts depend on the power of the die-casting machine. As the power of die-casting machines continues to increase, the size of castings can range from a few millimeters to 1 to 2 meters; the weight can range from a few grams to tens of kilograms. Overseas, aluminum castings with a diameter of 2m and a weight of 50kg can be die-cast.
Die castings are no longer limited to the automobile industry and instrument industry, and are gradually expanding to other industrial sectors, such as agricultural machinery, machine tool industry, electronics industry, defense industry, computers, medical equipment, clocks, cameras and Dozens of industries such as daily hardware, including: auto parts, furniture accessories, bathroom accessories (bathroom), lighting parts, toys, shavers, tie clips, electrical and electronic parts, belt buckles, watch cases, metal buckles , locks, zippers, etc. In terms of die-casting technology, new processes such as vacuum die-casting, oxygenated die-casting, precision die-casting, and the application of soluble cores have emerged.
Selection of die-casting machine
In actual production, not every die-casting machine can meet the needs of die-casting various products, but must be selected according to the specific situation. Generally, the following two Consider the following aspects:
1) Select according to different varieties and batches
When organizing multi-variety and small-batch production, hydraulic systems are generally selected for their simplicity, strong adaptability, and rapid operation. For adjusted die-casting machines, when organizing mass production of small varieties, a high-efficiency die-casting machine equipped with various mechanization and automated control mechanisms should be selected; for castings produced in large quantities of a single variety, a special die-casting machine can be used.
2) Select according to the casting structure and process parameters
The dimensions, weight, wall thickness and other parameters of the casting have an important impact on the selection of the die-casting machine.
The weight of the casting (including the 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 of the die-casting machine. Generally, the rated capacity of a die-casting machine can be found in the manual.
The die-casting machine has a certain maximum and minimum distance, so the thickness of the die-casting mold and the height of the casting must have a certain limit. If the thickness of the die-casting mold or the height of the casting is too large, the casting may not be taken out.
Die-casting process
In die-casting production, die-casting machine, die-casting alloy and die-casting mold are the three major elements. The die-casting process is a process of combining and utilizing three major elements. Make various process parameters meet the needs of die-casting production.
Selection of pressure and speed
The selection of injection specific pressure should be determined based on the structural characteristics of different alloys and castings. Table 1 is the empirical data.
Table 1 Specific pressure (kPa) of commonly used die-casting alloys
Alloy casting wall thickness 3mm
Simple structure Complex structure Simple complex structure
< p>Zinc alloy 30000 40000 50000 60000Aluminum alloy 30000 35000 45000 60000
Aluminum-magnesium alloy 30000 40000 50000 65000
Magnesium alloy 30000 40000 50000 60000
p>Copper alloy 50000 70000 80000 90000
For the selection of filling speed, generally for castings with thick walls or high internal quality requirements, a lower filling speed and a high boost pressure should be selected ; For castings with thin walls or high surface quality requirements and complex castings, a higher specific pressure and a higher filling speed should be selected.
Pouring temperature
Pouring temperature refers to the average temperature of the liquid metal when it enters the cavity from the press. Since it is inconvenient to measure the temperature of the liquid metal in the pressure chamber, it is generally used in a holding furnace. temperature representation.
The pouring temperature is too high and the shrinkage is large, which makes the casting prone to cracks, large grains, and mold sticking; the pouring source is too low, which is prone to defects such as cold insulation, surface patterns, and insufficient pouring. . Therefore, the pouring temperature should be considered together with the pressure, die-casting mold temperature and filling speed.
The temperature of the die-casting mold
The die-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 die-casting mold temperature often rises, especially for die-casting high melting point alloys, which rises very quickly. In addition to causing the liquid metal to stick to the mold, if the temperature is too high, the casting will cool slowly, causing the grains to become coarse. Therefore, when the temperature of the die-casting mold is too high, periodic cooling measures should be taken. Compressed air, water or chemical media are usually used for cooling.
Filling, pressure holding and mold opening time
1) Filling time
From the time when liquid metal begins to enter the mold cavity to the time when the mold cavity is filled, the required The time is called filling time. The length of filling time depends on the size and complexity of the casting. For large and simple castings, the filling time should be relatively long, and for complex and thin-walled castings, the filling time should be 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) Pressure holding and mold opening time
The duration from when the liquid metal fills the cavity to when the inner gate is completely solidified, and continues to be under the action of the injection punch, is called Pressure holding time. The length of the pressure holding time depends on the material and wall thickness of the casting.
After holding the pressure, the mold should be opened and the casting taken out. The time from the end of injection to the opening of die casting is called the mold opening time, and the mold opening time should be controlled accurately. If 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 falls; but if the mold opening time is too long, the casting temperature will be too low and the shrinkage will be large, which will affect the core pulling and ejection castings. The resistance is also great. Generally, the mold opening time is calculated based on the casting wall thickness of 1 mm, which takes 3 seconds, and then can be adjusted after trial.
Coatings for die-casting
During the die-casting process, coatings are used in order to avoid welding between the casting and the die-casting mold, reduce the frictional resistance of the ejection of the casting and prevent the die-casting mold from being overheated. Requirements for coatings:
1) Good lubricity at high temperatures;
2) Low evaporation point, the diluent can evaporate quickly at 100~150℃ ;
3) No corrosive effect on die-casting molds and die-casting parts;
4) Stable performance. The diluent should not evaporate excessively and become thickened in the air;
< p>5) Harmful gases will not be precipitated at high temperatures;6) No fouling will occur on the surface of the die-casting cavity.
Cleaning of castings
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 die-casting machines increases and output increases, it is very important to mechanize and automate casting cleaning work.
1) Removal of gates and flashes
The equipment used to remove gates and flashes is mainly punches, hydraulic presses and friction presses. In the case of mass production of parts, the castings can be Special molds are designed for structure and shape, and the cleaning task can be completed in one go on the punch.
2) Surface cleaning and polishing
Surface cleaning mostly uses ordinary polygonal rollers and vibrating embedded cleaning devices. For simple small parts with a small batch size, a multi-angle cleaning roller can be used. For decorations with high surface requirements, a cloth or leather polishing wheel can be used for polishing. For mass-produced castings, a spiral shell vibration cleaning machine can be used.
The cleaned castings can also be surface treated and impregnated according to usage requirements to increase gloss, prevent corrosion, and improve air tightness.
The difference between casting and die casting
The difference between casting and die casting
Casting can be divided into gravity casting and pressure casting according to the pouring process of molten metal. Gravity casting refers to the process of injecting molten metal into a mold under the influence of the earth's gravity, also called casting. Gravity casting in a broad sense includes sand casting, metal mold casting, investment casting, lost foam casting, mud mold casting, etc. Gravity casting in a narrow sense refers specifically to metal mold casting. Pressure casting refers to a process in which molten metal is injected into a mold under the action of other external forces (excluding gravity). Pressure casting in a broad sense includes pressure casting and vacuum casting of die casting machines, low pressure casting, centrifugal casting, etc. Pressure casting in a narrow sense specifically refers to metal mold pressure casting of die casting machines, referred to as die casting. The precision casting factory has long been engaged in gravity casting of sand molds and metal molds. These casting processes are currently the most commonly used in non-ferrous metal casting and are also the least expensive.
Die casting is metal mold pressure casting performed on a die-casting machine. It is currently the most productive casting process. Die casting machines are divided into two categories: hot chamber die casting machines and cold chamber die casting machines. Hot chamber die-casting machines have a high degree of automation, less material loss, and higher production efficiency than cold chamber die-casting machines. However, due to the heat resistance of the machine parts, they can currently only be used to produce castings of low-melting point materials such as zinc alloys and magnesium alloys. . Aluminum alloy die castings that are widely used today can only be produced on cold chamber die casting machines due to their high melting points.
The main feature of die casting is that the molten metal fills the cavity under high pressure and high speed, and is formed and solidified under high pressure. The shortcomings of die casting are: because the molten metal fills the cavity under high pressure and high speed. During the process, the air in the cavity is inevitably trapped inside the casting, forming subcutaneous pores. Therefore, aluminum alloy die castings are not suitable for heat treatment, and zinc alloy die castings are not suitable for surface spraying (but they can be painted). Otherwise, when the pores inside the casting are heated during the above treatment, they will expand due to heat, causing the casting to deform or bubble. In addition, the mechanical cutting allowance of die castings should also be smaller, generally around 0.5mm, which can not only reduce the weight of the castings and the amount of cutting to reduce costs, but also avoid penetrating the surface dense layer and exposing subcutaneous pores, causing The workpiece is scrapped.
What is the difference between hot chamber die casting machine and cold chamber die casting machine?
Die casting machines are generally divided into two categories: hot press chamber die casting machines and cold press chamber die casting machines. Cold pressure chamber die-casting machines are divided into two types: horizontal die-casting machines and vertical die-casting machines (including fully vertical die-casting machines) according to their pressure chamber structure and layout.
The pressure chamber of the hot-pressure chamber die-casting machine (referred to as the hot-air die-casting machine) is immersed in the liquid metal of the heat-insulated melting crucible. The injection parts are not directly connected to the machine base, but are installed on the crucible. The advantages of this kind of die-casting machine are simple production process and high efficiency; low metal consumption and stable process. However, the pressure chamber and injection punch are immersed in liquid metal for a long time, which affects the service life. And it is easy to increase the iron content of the alloy. Hot press chamber die-casting machines are currently mostly used for die-casting zinc alloy and other low-melting-point alloy castings, but are also used for die-casting small aluminum and magnesium alloy die-castings.
The pressure chamber and holding furnace of the cold chamber die casting machine are separated. During die-casting, liquid metal is taken out of the holding furnace and poured into the pressure chamber for die-casting
With the rapid development of my country's automobile, motorcycle, home appliance and other industries, the appearance of industrial products must meet performance requirements while also It is becoming more and more complex, and the manufacturers of these products do not open molds, which requires the mold manufacturing industry to produce molds at the fastest speed, lowest cost, and highest quality. In order to meet the above requirements, mold companies can only remain invincible in the fierce market competition by using advanced management methods and integrated manufacturing technology. So in die-casting production, you will inevitably encounter problems of this kind. The following focuses on 2 points of knowledge for explanation.
⒈Why can’t the machining allowance in the hole of aluminum die-casting exceed 0.25mm?
In order to be suitable for die-casting, humans add a lot of silicon (SI) to the aluminum alloy used for die-casting. When the aluminum alloy solidifies in the mold, the silicon will float to the surface, forming a thin silicon film. This silicon film is very hard and wear-resistant. Some OEM designers take advantage of this feature. The inner surface of the die casting hole is directly counted as the bearing surface. This silicon surface layer is generally only 0.2 to 0.9mm. Too much machining will shorten the life of this bearing surface.
⒉Why do aluminum die castings have black spots when polished?
There are several reasons for this. There may be the formation of silicon oxide, or aluminum oxide. The solution is simple, use fresh aluminum ingots. However, the biggest possibility is from the release agent. Maybe, we sprayed too much release agent. It is also possible that the organic content of the release agent is too high. At the temperature of hot melted aluminum, some of these organic compounds are reduced to carbon elements, and some become organic macromolecular polymers. These mixtures of carbon molecules and polymers are contained in the surface layer as the aluminum casting is formed, becoming the dark spots we see. We can reduce the concentration of the spray agent, use another spray agent, or extend the blowing time after spraying.
To reduce the formation of carbon elements and prevent the accumulation of macromolecular polymers