Mold costs account for about 35% of the production cost of profile extrusion. The quality of the mold and the reasonable use and maintenance of the mold directly determine whether the company can produce the profile normally and qualified. The working conditions of the extrusion die in the production of profile extrusion are very harsh. It not only needs to withstand severe friction and wear under high temperature and high pressure, but also needs to withstand periodic loads. This requires the mold to have high thermal stability, thermal fatigue, thermal wear resistance and sufficient toughness. In order to meet the above requirements, high-quality 4Cr5MoSiV1 (American brand H13) alloy steel is widely used in China, and vacuum heat treatment and quenching are used to make molds to meet the various requirements in the production of aluminum profiles.
However, in actual production, there are still some molds that fail to reach the expected output during extrusion. In severe cases, even less than 20 rods are extruded or the machine is scrapped in less than two times, resulting in the use of expensive mold steel. The mold is far from achieving its due benefits. This phenomenon is currently widespread in many domestic aluminum profile production enterprises. To investigate its causes, we need to start with the following aspects.
1. The 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, many industries have adopted aluminum profile instead of original materials. Due to the special profile of some profiles, the design and production of the mold are difficult due to the special profile section. If the conventional extrusion method is still used, it is often difficult to achieve the rated output of the mold, and a special process must be adopted to strictly control various production process parameters to perform normal production. In addition, some molds cannot be squeezed to the rated output due to the special profile section or quality problems of the mold itself. This requires the sales staff to fully communicate with the technical department and the mold factory when receiving the order. At the same time, the mold design and production department needs to continuously optimize mold design technology, improve mold production accuracy, and improve mold quality.
Second, select the appropriate extrusion model for production. Before extrusion production, it is necessary to fully calculate the profile section, and determine the tonnage of the extruder according to the complexity of the profile section, the wall thickness and the extrusion coefficient λ. Generally speaking, λ>7-10. When λ>8-45, the service life of the mold is longer and the profile production process is smoother. When λ>70-80, it is more difficult to extrude the profile, and the mold life is generally shorter. The more complex the product structure is, the more likely it is to cause insufficient local rigidity of the mold, and it is difficult for the metal flow in the mold cavity to tend to be uniform, and accompanied by local stress concentration. When the profile is produced, it is easy to plug the mold and the car or form a twisted wave, and the mold is prone to elastic deformation, and in serious cases, plastic deformation may cause the mold to be directly scrapped.
3. Reasonable choice of billet and heating temperature. The alloy composition of the extruded billet must be strictly controlled. At present, the general enterprise requires the grain size of the ingot to reach the first-class standard in order to enhance the plasticity and reduce the anisotropy. When there are pores, loose structure or central cracks in the ingot, the sudden release of gas during the extrusion process is similar to "blasting", which makes the local working zone of the mold suddenly unload and load again, forming a local huge impact load, which has a great impact on the mold. Big. Enterprises with conditions can homogenize the billet, and then forcefully cool it at 550-570C for 8 hours. The extrusion breakthrough pressure can be reduced by 7-10%, and the extrusion speed can be increased by about 15%.
Fourth, optimize the extrusion process. To scientifically extend the life of molds, the rational use of molds for production is an aspect that cannot be ignored. Due to the extremely harsh working conditions of the extrusion die, reasonable measures must be taken in the extrusion production to ensure the structure and performance of the die. (1) Adopt an appropriate extrusion speed. In the extrusion process, when the extrusion speed is too fast, it will cause the metal flow to be difficult to be uniform, the friction between the aluminum metal flow and the inner wall of the mold cavity will increase, and the wear of the mold working belt will accelerate, and the mold temperature will actually be higher. If the residual heat generated by metal deformation cannot be taken away in time at this time, the mold 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 25mm/s. (2) Reasonably choose the extrusion temperature. The extrusion temperature is determined by the mold heating temperature, the temperature of the ingot barrel and the temperature of the aluminum rod. Too low temperature of the aluminum rod will easily cause the extrusion force to increase or produce stuffiness. The mold is prone to local minor elastic deformation, or cracks in the stress-concentrated parts, which will lead to the early scrapping of the mold. Too high temperature of the aluminum rod will soften the metal structure, and cause it to adhere to the surface of the mold's working belt or even block the mold (in severe cases, the mold collapses under high pressure). The reasonable heating temperature of the uneven ingot is 460-520°C. The reasonable heating temperature of the ingot is 430-480°C.
5. In the early stage of use of the extrusion die, a reasonable surface nitriding treatment process must be carried out on the die. The surface nitriding treatment can greatly increase the surface hardness of the mold while maintaining sufficient toughness, so as to reduce the thermal wear when the mold is in use. It should be noted that the surface nitriding can not be completed at one time. During the mold service period, repeated nitriding treatments must be performed 3-4 times. Generally, the thickness of the nitriding layer is required to reach about 0.15mm. A more suitable nitriding process is the first nitriding after the mold enters the factory for inspection. At this time, since the structure of the nitrided layer is not stable, it should be nitrided again after 5-10 rods are extruded. After the second nitriding, 40-80 rods can be squeezed. It is advisable to use no more than 100-120 rods after the third nitriding. Before nitriding, the working belt must be polished, and the mold cavity must be cleaned, and no alkali slag or foreign particles should remain. Under normal circumstances, the number of nitriding of the mold does not exceed 4-5 times, because at this time, if the nitrided layer is not strained on the working belt, after repeated nitriding and extrusion production, the structure of the nitrided layer has been relatively stable. It should be noted that in the early nitriding, nitriding can be carried out through a suitable production process, and the number of nitriding should not be too frequent, otherwise the working belt will be easy to delamination.
6. The working belt must be ground and polished before the mold is put on the machine. The working belt is generally required to be polished to a mirror surface. Check the flatness and verticality of the mold work belt before assembly. To a certain extent, the quality of nitriding determines the polished finish of the working belt. The mold cavity must be cleaned with high-pressure air and a brush, and there must be no dust or foreign matter. Otherwise, it is easy to pull the working belt driven by the metal flow, and the extruded profile product will have defects such as rough surface or scribing.
7. The holding time of the mold during extrusion production is generally about 2-3 hours, but not more than 8 hours, otherwise the hardness of the nitride layer of the mold working belt will be reduced, which will cause the surface of the profile to be rough when the machine is not wear-resistant, and it will cause serious problems. Defects such as marking. When using the mold, there must be mold supports, mold sleeves and supporting pads that are matched with the mold to avoid that the contact surface between the mold outlet surface and the supporting pad is too small due to the large inner hole of the supporting pad, which may cause the mold to deform or break. The mold, the extrusion cylinder and the extrusion shaft are concentric, and the concentricity is within ±3mm, otherwise it is easy to produce eccentric load and change the design flow speed of each part of the mold, which will affect the shape of the profile.
8. Use the correct alkaline washing (mold cooking) method. After the mold is unloaded, the mold temperature is above 500°C. If it is immediately immersed in alkaline water, the temperature of the alkaline water is much lower than the mold temperature. If the mold temperature drops rapidly, the mold is prone to cracking. The correct method is to wait for the mold to be unloaded, place the mold in the air at 100°-150°C and then immerse it in alkaline water. Ordinary split combined molds are drawn before mold unloading, which can greatly reduce the workload of mold cooking and shorten the time of mold cooking. The specific method is that after the end of the extrusion, the extrusion rod retreats before the extrusion barrel, and the residual pressure remains in the extrusion barrel, and then the extrusion barrel retreats, and at the same time, part of the residual aluminum in the mold shunt hole can be pulled out with the residual pressure, and then Then carry out alkaline cooking. Some split-flow combination mold core heads are extremely small, even thinner than a pen. This type of mold is not allowed to be drawn after extrusion. The molder must see the mold structure clearly before opening the mold, and must wait for the residual aluminum in the mold cavity Basically boil them to open the mold. Otherwise, the core head will be broken if you carelessly, and the mold will be scrapped.
Nine, the mold uses the strength from low to high to low. When the mold just enters the service period, the internal metal structure performance is still in the floating stage. During this period, a low-strength operation plan should be adopted to make the mold transition to a stable period. In the middle of the mold's use, since the performance of the mold is basically in a stable state, it is similar to a car that has just passed the running-in period, and the use strength can be appropriately increased. In the later stage, the metal structure of the mold has begun to deteriorate, and the fatigue strength, stability and toughness have begun to fall into a downward curve after long-term production and service. At this time, the use strength of the mold should be appropriately reduced until the mold is scrapped.
10. Strengthen the use and maintenance records of molds in the extrusion production process, and improve the tracking record files and management of each set of molds. Extrusion molds are scrapped after they are inspected in the factory. The intermediate time can be as short as a few months and can be as long as more than one year. Basically, the usage record of the mold also records the various processes of profile production. Extrusion molds are large in number and varieties. The management of each set of molds is helpful to help mold library administrators, mold users, and mold design and manufacturing personnel to understand the true situation of each set of molds in stock.
The mold tracking record includes:
(1) Mold manufacturing information, including design drawings, production records, inspection records (precision value, hardness value), etc. of each set of molds.
(2) The process information of each extrusion die on the machine, such as heating time, aluminum rod temperature, mold temperature, extrusion speed, extrusion force, breakthrough pressure, aluminum rod length, number of qualified products, profile linear density, Yield rate and so on.
(3) The first three mold repair plans, nitriding treatment time, time in and out of the mold warehouse, scrap or return to the mold factory for repairs and reasons, etc. for each set of molds. The collection of these records is useful for improving mold management, accounting for mold costs, and optimizing molds. Designing and repairing molds, judging the quality of molds, improving the stability of extrusion production, using molds rationally, and determining the minimum inventory of molds all have a direct impact.
The increasing competition in the aluminum profile market has forced various aluminum profile manufacturers to invest huge energy in the procurement, use, maintenance and management of extrusion dies. This requires companies to change their own concepts while changing the previous extensive production management. Only by grasping it and doing a good job of statistical analysis and cost consumption management of molds can we adapt to the new market situation and seize the opportunity in the market.
