Measures to increase the output of extruded aluminum
In the production of aluminum and aluminum alloy semi-finished products, extrusion is one of the main forming processes. Extrusion products (profiles, tubes, and a small amount of wire rods and rods) account for about 30% of all semi-finished products. In 1998, the global output of aluminum semi-finished products was approximately 19,100 kt, of which the extruded product was 5,820 kt and the variety was nearly 40,000. There are more than 5500 aluminum extrusion presses in the world, more than 2,500 in mainland China and about 190 in Taiwan. However, the extrusion pressure of 98% of the extruders in industrialized countries exceeds 15MN, but the extrusion pressure of 90% of China's extruders is below 15MN. The lack of operating rate of extruders is a global problem. The average capacity utilization rate is about 65%, of which Japan's high is 91.6%, North America and Western Europe are about 70%, and China's is about 35%.
In this situation, how to increase the productivity of the extruder has become a top priority. There are many measures to increase productivity, such as increasing the installed capacity, improving the quality of workers, and improving management standards. This article focuses on some of the more specific measures for improving production and quality.
1 Advanced Equipment and High-Quality Workers are the Prerequisites for Increasing Production
To improve the output of extruded products, advanced equipment, high-quality workers, and modern scientific management play a crucial role. In China's existing more than 2,500 extrusion presses, only about 25 sets are world class, ie about 1%, such as the 14.5MN (1600UStf.) extruder and 25MN (2750UStf.) at Nanping Aluminum Plant. Extrusion machine, 55MN (5500UStf.) extruder imported from Italy by Tianjin Nonferrous Metals Group Corporation, and 21 MN2 (2200/2500Ustf.) imported from SMS Sutton by Yuhua Aluminum Co., Ltd. Extrusion machines, 21MN (2350Ustf.) and 16.3MN (1800Ustf.) extruders imported from the Ube Japan Production Corporation (UBE) by Xi'an Aircraft Company.
China's aluminum extrusion industry is in the stage of structural adjustment, and it should avoid low-level repeated introduction and construction. It is better to transform the existing remodeling extruder into a modern high-level extruder. The newly introduced positive extrusion machine shall be equipped with a computer aided positive extrusion system (CADEX—Computer Aided Direct Extrusion) in order to make the actual extrusion time as short as possible during the isothermal extrusion; Ultra-fine hydraulic oil filter equipped with continuous monitoring of hydraulic oil quality and temperature instruments; equipped with various process parameter display and monitoring electronic system PICOS (Process Information & Control System); also should have MIDIS system (management information and diagnosis Display Information - Diagnostic Indication System (Management Information and Diagnostic Indication System) to display all kinds of information about process parameters and production conditions, as well as to issue warnings and display signals to the equipment in a timely manner to guide the maintenance and repair, and to shorten the parking time; etc. Wait.
2 Temperature Control and Increasing Production
In general, if there is no unplanned downtime, the larger production depends mainly on the extrusion speed, while the latter is subject to four factors, three of which are fixed and the other is variable. The first factor is the extrusion force of the extruder. The extrusion force can be extruded smoothly when the temperature of the billet is low. The second factor is the design of the mold. The friction between the metal and the mold wall during extrusion can usually be achieved. The temperature of the passing aluminum alloy is increased by 35-62°C. The third factor is the characteristics of the extruded alloy, which is an uncontrollable factor that limits the extrusion speed. The exit temperature of the profile generally cannot exceed 540°C, otherwise, the material surface The quality will drop, the pattern marks will obviously increase, and even sticky aluminum, gravure, micro cracks, and tears will appear. The latter factor is the temperature and its degree of control.
If the extrusion force of the extruder is not enough, it is difficult to smoothly squeeze or even appear plug phenomenon and can increase the billet temperature, but the extrusion speed should be lower to prevent the material outlet temperature high. Each alloy has its particular preferred extrusion (got) temperature. Production practice has proved that the billet temperature is better maintained at around 430°C (when the extrusion speed is ≥16mm/s). The temperature of 6063 alloy profile should not exceed 500°C, the higher outlet temperature of 6005 alloy should be 512°C, and the better of 6061 alloy should not exceed 525°C. Subtle changes in mold temperature can also affect the product's yield and quality.
It is also important to extrude the barrel temperature. In particular, attention should be paid to the temperature rise in the preheating stage. Excessive thermal stress between the layers should be avoided. It is better to simultaneously raise the barrel and the bushing to the working temperature. The warm-up temperature should not exceed 38°C/h. The preheating specification is better: raise to 235°C, hold for 8 hours, continue to heat up to 430°C, and keep warm for 4 hours before putting it into operation. This will not only ensure uniform internal and external temperatures, but also have enough time to eliminate all internal thermal stresses. Of course, heating the barrel in the furnace is the preferred method of preheating.
During the extrusion process, the extrusion barrel temperature should be 15 to 40°C lower than the ingot temperature. If the extrusion speed is so fast that the temperature of the barrel rises above the billet temperature, it is necessary to try to lower the temperature of the barrel. This is not only a troublesome task, but also the yield is reduced. During the increase of production speed, sometimes the heating elements controlled by the electric couple are cut off, but the temperature of the barrel is still rising. If the barrel temperature is higher than 470°C, the extrusion waste will increase. The ideal barrel temperature should be determined based on the different alloys.
Do not think that preheating the barrel is a waste of time and energy. In one factory, once for production tasks, it was heated on the one hand by internal resistance elements and on the other hand by liquefied gas burners. In this case, the temperature can not be measured and controlled, it will produce a huge thermal stress, the liner temperature is high, the expansion is faster than the jacket, so that the squeeze tube cracks, and hear the "broken" sound. This is the author's own experience.
During the working process, the extrusion shaft will accumulate internal stress, and this kind of stress will generate fatigue cracks to a certain extent, and will break once it is subjected to a non-axial radial force. Therefore, after the accumulated working time of the extrusion shaft reaches 4500h, the stress relieving treatment is preferably performed once. The temperature is kept at 430-480°C for 12 hours, and then the furnace is cooled to below 50°C. Unfortunately, few factories in our country handle this matter.
In the production of high-quality surface construction profiles, the extrusion pad temperature should also be strictly controlled to reduce the amount of inconsistent surface waste. The mass of the fixed extrusion pad is much better than that of the activity, and it can accumulate more heat, so it can reduce the temperature of the billet end, reduce impurities into the profile, and help increase the output. Castool of the United States used compressed air to cool the extrusion pad and the extrusion shaft to reduce the temperature to about 50°C[1].
The mold temperature plays an important role in obtaining a high yield, generally not lower than 430°C. On the other hand, it must not be too high. Otherwise, not only the hardness may decrease, but also oxidation occurs, mainly in the working belt. During the heating of the mold, close contact between the molds should be avoided to obstruct air circulation. It is better to use a grid box furnace, and each mold is placed in a separate box.
The temperature increase of the billet in the extrusion process can reach about 40°C or more, and the amount of increase is mainly determined by the mold design. In order to obtain larger output, the temperature must not be ignored, and each temperature should be recorded and controlled strictly to find out the relationship between the larger output of the machine and various temperatures.
Later, the employees of the extrusion plant should bear in mind that the precise control of the temperature is crucial to increase production.
3 Optalex Control System
The Optalex control system developed by Alumac of Alleroed, Denmark, can be mounted on a conventional isothermal extruder and can interface with most PLC systems. The constant temperature of the outlet of the material is always controlled. The extrusion cycle of root ingots is shortened by 5% to 35%, so that the extrusion work can be optimized. It can not only increase the output, but also improve the product quality, and the performance is always consistent.
The Optalex system consists of three components: an infrared camera, a control panel and a process control system. The temperature measurement system is non-contact and based on the multi-wavelength infrared scanning principle of the target. The camera is equipped with a laser device, the emitted laser is always aligned with the center of the target surface. The camera is positioned as close as possible to the die exit so that the profile temperature can be accurately determined. The control panel is specially designed using standard industrial hardware, and the software system is designed based on the data provided by the user (mold type and number, type of alloy, higher extrusion speed and temperature). All production data is stored on disk and can be extracted at any time.
The process control system has three independent process controls that perform their duties; information transmission, control and temperature measurement. The system performs closed-loop control calculations based on the data captured by the camera and some of the information transmitted, and automatically controls the relevant parameters so that the temperature of the extrusion die of the profile is kept constant.
4 Measures for Increasing Production of 31.5MN Extruder from Swedish Profiles Group
In 1998, Profil Gruppen put into operation a 31.5 MN extrusion press designed and manufactured by Cometal Engineering in Italy for the extrusion of automotive industrial sections and building profiles [3]. The production line has two major characteristics: it has adopted a series of measures to reduce waste products to increase production; environmental pollution has dropped to a lower level in the world today, and it can be described as a “green†model extrusion line.
Extruded ingots with a diameter of 250 mm can produce extrusions with 45 to 50 profiles. The profile has a linear density of 2.46 to 2.216 kg/m. The non-working (assisting) time is 15s. Gradient heating is used to achieve isothermal extrusion and the exit temperature of the profile is strictly controlled to achieve a reasonably large extrusion speed.
Since the output and quality of products mainly depend on the quality of workers, rigorous training of operators and maintenance workers was conducted before production was put into operation. The specific measures to reduce the amount of waste products are as follows:
- Lengthening of the billet, from the previous 900m to 1100mm, while the residual thickness is still 25mm.
- The use of heat shear can increase the metal yield by 4%.
- sawing at the weld of the profile. Therefore, the length of scrap produced on 43m long material is only 1m, while the length of conventional material is 44m, and the waste generated is as long as 2.5m. The result of this change is a 3.3% increase in the yield. At the same time, due to careful control of the extrusion speed, the surface quality of the profile is also increased. In addition, special attention should be paid to the movement of the profile on the rear equipment, and scratches and scratches should be avoided as much as possible to increase the yield by 0.5%.
- Minimize the wear of parts and components, use automatic lubrication to shorten the maintenance and repair time, extend the production effective time, and adopt the precise structure as far as possible.
The plant has paid special attention to environmental protection during the design and construction process. Sweden is a Scandinavian country and it is one of the regions with stricter environmental protection requirements in the world. The factory's large noise is 70dbA. Therefore, the circular saw and the tractor are all located in a soundproof room. The larger amount of NOx in the flue gas emitted from the heating furnace is 55 mg/MJ, and the larger amount of CO is 100×10-6. It also has a heat exchange system that uses waste heat to heat domestic water so that the temperature of flue gas discharged into the atmosphere is less than 150°C. Parts of all equipment in the factory are cleaned with water. Sawdust is received by special devices and will not fall onto the ground. All equipment has undergone safety analysis. The Swedish Profile Group can be said to be the typical "green" aluminum extrusion plant in the world, representing the development direction of the aluminum plant.
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