Industry Application and Key Points for Attention of Magnesium Alloy Die Casting Die Thermostat
Key Points for Safe Operation of Aluminum Alloy Die Casting Die Thermostat and Hazards of Poor Magnesium Alloy Die Casting Equipment
With the rise and development of magnesium alloy die casting mold warming, the technological characteristics and safety operation points of magnesium alloy die casting mold warming are discussed, which is conducive to safe and high-quality production. According to the different characteristics of magnesium alloy mold heaters, special protective measures and equipment should be taken. Some factories still use conventional cold chamber die casting machines to produce magnesium alloy die castings, which poses potential risks and hidden dangers in production.
1. Die casting process
The die casting process of magnesium alloys is similar to that of other alloys, but due to the different characteristics of magnesium alloys, there are differences in the application of pressure, speed, temperature, and coatings.
1.1 Pressure
The magnesium alloy die casting mold has two types of temperature machine: a hot chamber and a cold chamber. The pressure during die casting is also different. The specific pressure of the hot chamber machine is around 40 MPa, while the specific pressure of the cold chamber machine is higher than that of the hot chamber machine, with a typical specific pressure of 40-70 MPa
Another important point is the pressurization and pressure build-up time. Due to the low solidification latent heat of magnesium alloys, the solidification time of magnesium alloys in the mold is much shorter than that of aluminum alloys. If the pressurization time is too late, the molten metal in the gate and mold cavity has solidified, and pressurization also has no significance. Therefore, pressure build-up time is an important factor to measure the performance of magnesium alloy die casting mold thermometers, and the pressurization and pressure build-up time of most die casting mold thermometers is more than 60 ms, At this time, the magnesium alloy at the gate has solidified, and the pressurized pressure cannot be transmitted to the mold cavity. The pressure build-up time of an excellent injection system is usually within 20 ms
1.2 Speed
Magnesium alloys have low inertia due to their low density (only 2/3 of aluminum alloys). At the same time, due to the rapid solidification of magnesium alloys, the entire mold cavity needs to be filled before the metal solidifies, so the injection speed of magnesium alloys needs to be fast. The injection speed of magnesium alloy in the hot chamber can reach 6 m/s, while the temperature rise speed of the cold chamber die casting mold is higher, reaching 8 m/s. High injection speeds also produce high gate speeds. For example, the gate speed of zinc alloy and aluminum alloy die-casting molds is about 40 m/s to 60 m/s, otherwise mold erosion may occur. The gate speed of thin-walled magnesium alloy castings often exceeds 80 m/s. Due to the low heat resistance of magnesium alloy and its low weldability to mold steel, the erosion of die-casting molds is not as severe as aluminum alloy.
1.3 Temperature
Temperature is a thermal factor in the die casting process. In order to provide good filling conditions, ensure the molding quality of the die casting, and control and maintain thermal stability, corresponding temperature specifications must be selected, mainly referring to the pouring temperature of the alloy and the mold temperature.
The material pot of the hot chamber die casting mold is in the furnace, and the heat loss during injection is small. Therefore, the temperature of magnesium alloy for hot chamber die casting is lower, usually around 640 ℃. The temperature of the cold chamber die casting mold temperature machine is higher, generally around 680 ℃. One thing worth noting about the magnesium alloy die casting mold temperature machine is that if the molding of the product is not ideal, it can be improved in other aspects, such as injection speed, mold temperature, and so on. It is not allowed to blindly increase the alloy pouring temperature, because the protective gas used for magnesium alloy furnaces today will lose its effectiveness when the temperature is too high (above 710 ℃).
Improper mold temperature can lead to dimensional instability of castings, and may produce defects such as mold adhesion, deformation, and underflow. In order to heat the mold, some die casting factories often use the method of torch heating. This method is not suitable for magnesium alloys because it is difficult to achieve uniform mold temperature, and mold life can also be prone to cracks due to the impact of supercooling and overheating. Due to the low latent heat of magnesium alloys, in production, especially for thin-walled parts, in order to maintain better filling conditions and stable production, it is usually necessary to continuously heat the mold to maintain a balance of mold temperature. In magnesium alloy die casting, the most ideal heating method is to use a mold temperature machine to heat the mold through hot oil, which continuously passes through the pipes inside the mold and heats the mold from the inside to achieve a stable equilibrium state of the mold. As heat transfer oil can not only heat but also cool the mold like water, it functions like a heat exchanger, keeping the mold temperature within a certain range. With this heating method, the temperature of the mold is stable and uniform, which can effectively extend the life of the mold, improve product quality, and stabilize the production rhythm. The temperature of the magnesium alloy die casting mold is generally maintained between 180 ℃ and 280 ℃ during the warm-up process.
1.4 Paint
The role of the coating is to provide an effective isolation and protection layer between the die casting alloy mold thermostat and the mold, to prevent the metal liquid from directly scouring the mold cavity, maintain the fluidity of the metal liquid, and also to cool the mold, reducing the tendency to adhere to the mold. Compared with aluminum alloy, magnesium alloy is lighter in mold and weldability than aluminum alloy, but due to its higher injection speed than aluminum alloy
Welding may occur when certain components of high-speed impact molds are used. The use of suitable mold coatings can reduce this trend. The most commonly used lubricant is water based. Due to the small heat capacity of magnesium alloys, there is no need to use lubricants for cooling media, and the use time should be as short as possible, typically 50% of that of aluminum alloys. To reduce water content, higher concentrations of paint are typically used.
Before carrying out operations, operators must wear protective equipment according to the above requirements. Personnel who do not wear protective equipment should not approach the operation area and cannot operate.
2.2 Equipment safety protection
"Magnesium alloy furnaces cannot use stainless steel containing nickel, nor can smelting tools use stainless steel containing nickel.". The furnace body is preferably designed with a double layer structure. When the inner layer of the crucible breaks, the magnesium liquid can flow into the interlayer between the inner and outer layers, and an alarm is given to stop heating, so that the molten magnesium liquid does not flow outside, causing danger. Frequently inspect the furnace for rust, and timely clean it if any
During the warm machine production process of magnesium alloy die casting molds, the most important thing is to keep the site dry and clean. Before starting each time, the mold should be preheated to above 150 ℃. During spraying, do not spray too much paint to avoid water accumulation in the mold cavity, which may cause danger. In addition, try not to use water cooling for the cooling of the punch and mold. Air cooling can be used for cooling the punch, and high-temperature resistant oil is generally used for heating and cooling the mold. The warm press punch speed of the magnesium alloy die casting mold is also higher than that of the aluminum alloy die casting. In order to avoid flying materials injuring people, flying material baffles are sometimes installed on the parting face of the mold.
2.3 Magnesium alloy accident handling
The fire extinguishing agents used for magnesium alloys include dry sand, covering agent, and Class D fire extinguishers. These fire extinguishing equipment should be placed in a conspicuous place and convenient for emergency use on site. Dry sand and covering agent should be stored in containers to prevent moisture, and should be regularly inspected.
For combustion of a small amount of magnesium, it is possible to
3. Disasters, accidents, safety and quality issues that may be caused by poor quality magnesium alloy die casting mold warming equipment
Due to the flammability and explosion of magnesium alloys and their high pressure and high speed during injection, there is a high requirement for die casting equipment. Poor quality die casting equipment has potential hazards.
During die casting mold warm-up operations, molten magnesium liquid is injected into the mold cavity for molding at a speed of 70-100 meters per second (at the gate). Due to the flammable and explosive nature of molten magnesium liquid, it combusts violently in the presence of oxygen, explodes in the presence of water, reacts violently in the presence of rust, water containing aggregates, and silicon containing refractory materials, and is difficult to extinguish in a fire. "A large fire cannot be extinguished, so the performance, reliability, and safety requirements of its die casting complete equipment are extremely high. Poor quality equipment is extremely prone to disasters and accidents.". In the process of exploring applications, many major safety accidents caused by equipment problems have occurred both at home and abroad