However, in actual production, some dies can't reach the expected output during extrusion, and even less than 20 bars are extruded less than twice in advance or scrapped, which makes the dies made of expensive die steel far from achieving the due benefits. This phenomenon is common in many domestic aluminum production enterprises. Investigate its reason, need to start from the following aspects.
1. The cross section of aluminum profile itself is ever-changing, and the aluminum extrusion industry has developed to this day. Aluminum alloy has important advantages such as light weight and good strength. At present, aluminum profiles are used in many industries to replace the original materials. Because of the particularity of some profiles and the particularity of section, it brings difficulties to the design and manufacture of dies. If the conventional extrusion method is still used, it is often difficult to achieve the rated output of the die. Only by adopting special technology and strictly controlling various production process parameters can normal production be achieved. However, some molds cannot be squeezed to the rated output due to the special section of their own profiles or the quality problems of the molds themselves, which requires the sales staff to fully communicate with the technical department and the mold factory when taking orders. At the same time, mold design and production departments need to constantly optimize mold design technology, improve mold production accuracy and improve mold quality.
Second, choose the appropriate extruder for production. Before extrusion production, the section of the profile should be fully calculated, and the tonnage of the extruder should be determined according to the complexity of the section, wall thickness and extrusion coefficient λ. Generally speaking, λ >; 7- 10。 When λ >; 8-45, the die life is longer and the profile production process is smoother. When λ >; It is difficult to extrude profiles after 70-80, and the die life is generally short. The more complex the product structure, the more likely it is to lead to insufficient local stiffness of the mold, and the metal flow in the mold cavity is difficult to be uniform, accompanied by local stress concentration. In profile production, it is easy to jam, traffic jam or form torsional waves, and the die is prone to elastic deformation, and serious plastic deformation will lead to the die being scrapped directly.
3. Reasonable selection of billet and heating temperature. The alloy composition of extrusion ingot should be strictly controlled. At present, general enterprises require ingot grain size to reach the first-class standard to enhance plasticity and reduce anisotropy. When there are pores, loose structures or central cracks in the ingot, the sudden release of gas in the extrusion process is similar to "blasting", which makes the local working area of the die suddenly unload and reload, forming a local huge impact load, which has a great influence on the die. Conditional enterprises can homogenize the billet and keep it at 550 ~ 570℃ for 8 hours before forced cooling. The extrusion breakthrough pressure can be reduced by 7- 10%, and the extrusion speed can be increased by about 15%.
Fourthly, optimize the extrusion process. In order to scientifically prolong the service life of molds, it is an important aspect to use molds reasonably for production. Because the working conditions of extrusion die are extremely bad, reasonable measures must be taken to ensure the structure and performance of the die in extrusion production.
(1) Adopt appropriate extrusion speed. In the process of extrusion, when the extrusion speed is too fast, it will make the metal flow difficult to be uniform, and the friction between the aluminum metal flow and the inner wall of the die cavity will increase, which will lead to the accelerated wear of the die working area and the actual temperature of the die will be high. If the residual heat generated by metal deformation cannot be taken away in time at this time, the die may fail due to local overheating. If the extrusion speed is appropriate, the above adverse consequences can be avoided, and the extrusion speed should generally be controlled below 25 mm/s.
(2) Reasonable selection of extrusion temperature. Extrusion temperature is determined by mold heating temperature, spindle barrel temperature and aluminum bar temperature. Too low temperature of aluminum bar is easy to cause extrusion pressure rise or truck phenomenon, and the die is easy to have a small amount of local elastic deformation, or cracks at stress concentration parts, which leads to premature scrapping of the die. If the temperature of the aluminum bar is too high, the metal structure will soften, causing it to stick to the surface of the working area of the mold, and even blocking the mold (in severe cases, the mold will collapse under high pressure). The reasonable heating temperature of uneven ingot is 460-520℃, and the reasonable heating temperature of homogenized ingot is 430-480℃. ..
5. Before the extrusion die is used, it is necessary to carry out reasonable surface nitriding treatment on the die. Surface nitriding treatment can greatly improve the surface hardness of the die while maintaining sufficient toughness, thus reducing the thermal wear of the die when it is used. It should be noted that surface nitriding can not be completed once, and it must be repeated 3-4 times during the service of the mold. Generally, the thickness of nitrided layer is required to reach about 0. 15 mm, and the more suitable nitriding process is the first nitriding after the mold enters the factory for inspection. At this time, due to the unstable structure of nitrided layer, it should be nitrided again after extruding 5- 10 bar. After the second nitriding, 40-80 bars can be extruded. After the third nitridation, it is recommended to use a rod not exceeding 100- 120. Before nitriding, the working area must be polished, and the cavity of the mold must be cleaned, and no alkali residue or foreign particles can be left. In general, the number of nitriding times of the die should not exceed 4-5 times, because at this time, if the working zone is not pulled, the nitrided layer has been relatively stable after repeated nitriding and extrusion production. It should be noted that nitriding can only be carried out after proper production technology in the early stage of nitriding, and the nitriding times should not be too frequent, otherwise the working area will be easily stratified.
6. The working belt must be polished and polished before the mold is put on the machine. Generally, the working belt is required to be polished to a mirror surface. Check the flatness and verticality of the mold working belt before assembly. The quality of nitriding determines the polishing smoothness of the working zone to a certain extent. The mold cavity must be cleaned with high-pressure air and brush, and there must be no dust or impurities. Otherwise, the working area will be easily pulled by metal flow, and the extruded profile products will have defects such as rough surface or scribing.
7. In extrusion production, the heat preservation time of the die is generally about 2-3 hours, but not more than 8 hours. Otherwise, the hardness of the nitrided layer in the working area of the die will be reduced, which will lead to the rough surface of the profile when it is not worn by the machine, and will seriously cause defects such as scratches. When using the mold, there should be a mold bracket, a mold sleeve and a support pad matched with the mold, so as to avoid the deformation or fracture of the mold due to too large inner hole of the support pad and too small contact surface between the mold outlet surface and the support pad. The die, the extrusion cylinder and the extrusion shaft are concentric, and the concentricity is within 3 mm, otherwise it is easy to change the eccentric load and design flow rate of each part of the die and affect the profile forming.
Eight, use the correct alkali washing (cooking) method. After the mold is taken out, the temperature of the mold is above 500℃. If the mold is immediately immersed in alkaline water, the temperature of alkaline water is much lower than that of the mold. If the mold temperature drops rapidly, the mold is easy to crack. The correct method is to place the mold in air at100-150 C, and then immerse it in alkaline water. The common split-flow combined die can be pulled out before demoulding, which can greatly reduce the workload and shorten the boiling time. The specific method is as follows: after extrusion, the extrusion rod retreats in front of the extrusion cylinder, leaving residual pressure in the extrusion cylinder, and then the extrusion cylinder retreats, so that part of residual aluminum in the die shunt hole is pulled out at the same time with residual pressure, and then alkali cooking is carried out. The core of some split-flow combined dies is very small, even thinner than a pen. After this kind of mold is extruded, it is not allowed to pull out the mold. When opening the mold, the mold opener must see the mold structure in advance and wait until the residual aluminum in the mold cavity basically boils before opening the mold. Otherwise, if you are not careful, the core will be broken and the mold will be scrapped.
Nine, the use of the mold adopts the use intensity from low to high and then to low. When the die just entered the service period, the internal metal structure and properties were still in the floating stage. During this period, the low-intensity operation scheme should be adopted to make the die transition to the stable period. In the middle period of the use of the mold, the performance of the mold has been basically in a stable state, which is similar to that of the car just after the running-in period, so the use strength can be appropriately improved. In the later stage, the metal structure of the die has begun to deteriorate, and the fatigue strength, stability and toughness have begun to enter a downward curve after long-term production and service. At this time, the use intensity of the mold should be appropriately reduced until the mold is scrapped.
Ten, strengthen the use and maintenance records of molds in the extrusion production process, and improve the tracking records and management of each set of molds. The time from the acceptance of extrusion die to the end of die use ranges from a few months to more than a year. Basically, the use record of the mold also records the various processes of profile production. There are many kinds of extrusion dies. Managing the use process of each set of molds is helpful to help mold library administrators, mold users and mold design manufacturers understand the real situation of each set of molds in stock. The tracking records of molds include:
(1) Mold manufacturing data, including design drawings, manufacturing records and inspection records (precision values and hardness values) of each set of molds.
(2) Process information of each die extrusion, such as heating time, aluminum bar temperature, die temperature, extrusion speed, extrusion pressure, breakthrough pressure, aluminum bar length, qualified product quantity, profile linear density, yield, etc.
(3) The first three repair plans, nitriding treatment time, time for entering and leaving the mold warehouse, time and reasons for scrapping or returning to the mold factory for maintenance of each set of molds, etc. The collection of these records has a direct impact on improving mold management, accounting mold cost, optimizing mold design and repair, judging mold quality, improving the stability of extrusion production, rationally using molds and determining the minimum inventory of molds.
The increasingly fierce competition in the aluminum profile market forces aluminum profile production enterprises to invest huge energy in the procurement, use, maintenance and management of extrusion dies, which requires enterprises to change their own concepts while changing the extensive production management in the past, start with details, do a good job in statistical analysis and cost management of dies, so as to adapt to the new market situation and seize the market initiative.
This article comes from Zhihu: How to maintain the extrusion die for aluminum profile?