1. Overview
The problem of prolonging the life of die-casting molds has always been a concern of application companies. Reasonable selection of materials for die-casting molded parts has been valued by the majority of die-casting mold manufacturers and ordering parties. Because the choice of die casting material is the basic condition for the life of the die casting die.
Since the 1950s, the 3×2B8φ steel of the former Soviet Union, that is, the domestic grade 3Cr2W8V steel, has been widely used in China. Generally, the die-casting cracks have begun to occur in the cavity of 1 to 20,000 times of die-casting, and the life of the die-casting mold is 50,000 to 60,000 times.
In the late 1970s, when we translated the DCSP1 aluminum alloy and DCSP2 zinc alloy die-casting standards formulated by the Japan Die Casting Association, we compared the die-casting mold materials specified in the standard with the mold steels of the United States, West Germany, the United Kingdom, Sweden, and the Soviet Union. Analysis shows that chrome molybdenum vanadium steel is widely used in foreign countries, because it can be seen from the research report that there is no difference in the durability of molybdenum steel and tungsten steel, and tungsten steel is more likely to form thermal alternating crack than molybdenum steel. Therefore, we selected 4Cr5MoSiV1 steel for experimental research, please Fushun Steel Plant according to the technical conditions of our research (in principle, according to the chemical composition of the United States H13ASTM681-76, the requirements of the mold raw materials specified in the Japanese DCS), the key point as follows:
First, the chemical composition and impurity content should meet the requirements.
The second is to use electroslag smelting to avoid segregation and shrinkage defects caused by steel ingots. The steel ingots have high density and purity, low harmful impurities and non-metallic inclusions, pure grain boundaries, and uniform composition and structure.
The third is to require steel forging ratio to reach 4S or more.
With the steel and strict mold manufacturing process, the life of the die-casting mold produced is more than 250,000 times.
It can be said that H13 steel is one of the fine steel grades imported from the United States. Therefore, China has promoted steel grades as a key point when formulating national standards for die-casting molds.
Up to now, the steel grade and the approximate steel grade, such as: 40CrMoV5 of the International Organization for Standardization (ISO), SKD61 of Japan (JIS), STD61 of Korea (KS), X40CrMoV5-1-1 of the European Community (EN), X40CrMoV5-1-1KU from Italy (UN1), X40CrMoV5-1 from Germany (DIN), BH13 from the United Kingdom (BS), X40CrMoV5 from France (NF), 4X5MφIC from Russia, and some commercial grades 8407, QR90, GS-344M, etc.
H13 steel is a kind of steel with air-cooling type hot work die which is both strong and tough. The main features are as follows:
(1) It has high hardenability and toughness.
(2) Excellent resistance to thermal cracking, which can be water-cooled in the workplace.
(3) It has medium wear resistance, and can also use carburizing or nitriding process to improve its surface hardness, but slightly reduce the resistance to thermal cracking.
(4) Because of its low carbon content, the secondary hardening ability in tempering is poor.
(5) It has softening resistance at higher temperatures, but the hardness decreases rapidly when the temperature is higher than 540 ° C (1000 ° F) (ie, it can withstand an operating temperature of 540 ° C).
(6) The deformation of the heat treatment is small.
(7) Good machinability.
(8) Medium resistance to decarburization.
H13 steel has a carbon content (mass fraction) of 0.5% or less, and contains alloy elements such as Cr, W, Mo, and V. The steel has a Cr content of about 5%, which is supplied to the steel together with other carbide forming elements. High hardenability and good softening resistance, so the steel can be hardened under air cooling conditions, and its maximum quenching hardness is about 55HRC.
To extend the life of the die-casting mold, in addition to the selection of high-quality steel, the following problems should also be noted.
2. Casting and die casting design should be reasonable
Due to the unreasonable design of the casting structure, there is a thin cross section in the mold, which becomes the root of the fracture. If the slope value is unreasonable, it will cause scratches during core pulling, mold opening or picking; the chamfer at the junction of the cavity wall surface will be slightly leaky, which will cause stress concentrated crack; in the design of the gating system, if the flow direction Improper control of cross-sectional area, shot speed, etc., will cause erosion of the wall or core. The eddy current formed after the molten metal enters the cavity, because the flow velocity of the vortex core portion is infinite, plays a strong eclipse effect on the die face, causing local pulling, and the rigidity of the die is insufficient. Due to the one-sided emphasis on saving steel, early deformation or fracture occurs. If the matching accuracy level of each component is improperly selected, or due to the presence of clearance, the thermal conductivity is lowered, and thermal fatigue is generated prematurely. Or because the assembly size is too tight, pre-stress is formed, and the mold bursts during the die-casting process. The rapid ejection mechanism has been considered in modern mold structures, on the one hand to improve production efficiency; on the other hand, to reduce the mold retention time of the casting and to remove the thermal load of the mold material. In the design of the die-casting mold, the inlay is reduced as much as possible to enhance the rigidity of the cavity.
When designing the mold, the heat balance of the mold should be fully considered to maintain a stable and reasonable working temperature. Because in the die casting process, molten metal is injected into the mold cavity, a large amount of heat is released in a very short time, which promotes the temperature of the mold. At the same time, the mold passes through conduction, convection, radiation, and mold surface spraying and cooling water absorption. Part of the heat causes the mold temperature to drop. After a period of time, the mold temperature reaches an equilibrium point. At this time, the temperature of the mold does not change greatly. The temperature at this equilibrium point is important for stable production, and the casting quality and productivity are very large. The extent depends on the thermal control capabilities of the mold. When considering the heat balance of the mold, first determine the temperature of the equilibrium point. Since there is a temperature gradient in each part of the mold and it changes continuously at a cycle temperature, and the difference in the wall thickness of the casting, the equilibrium temperature is generally controlled at 40 of the alloy casting temperature. %~50%.
Therefore, some molds should be designed with a cooling system.
The position, shape, pipe diameter, cooling water temperature and flow rate of the cooling water pipe have an important influence on the temperature distribution of the mold, thereby affecting the life of the mold. If the cooling water pipe is close to the surface of the mold, the thermal fatigue life of the mold can be improved, but the tensile stress on the inner surface of the water channel is increased. Some factory experience is:
(1) The gate and the splitter cone are cooled when the total amount of cast alloy (including the pouring system) is less than 150g.
(2) When the total amount of cast alloy is 150~200g, one set is placed near the gate of the sprue bushing, the diverting cone and the moving die? 6.35mm (1/4in) cooling water channel or 2 points cooling.
(3) When the total amount of cast alloy is 250~500g, add one more near the inner gate of the moving mold? 6.35mm (1/4in) cooling channel or 2~3 points cooling.
(4) When it is larger than 500g, increase the cooling water channel or change the shape of the cooling water channel accordingly. Turn the straight channel into an L shape or a U shape, and change the pipe diameter to ? 9.5mm (3/8in) to increase the cooling area.
(5) Large sliders on the mold and >25mm heat-receiving cores should also be provided with straight cooling water channels or point cooling.
(6) The distance between the cooling water pipe and the mold cavity or runner is generally 20~32mm, which is close to the heat concentration of the splitter cone and the inner runner, but the minimum is also larger than the diameter of the cooling pipe.
(7) In order to reduce the tendency of cracking, the cooling water can be heated to about 50 ° C, but not lower than 20 ° C. At the same time, in order to avoid excessive cooling of the mold, the flow rate of water should be adjusted in time.
3. Guarantee the processing quality of the mold
During the grinding process, the friction heat generated by the grinding wheel is not sharp enough, which may cause grinding cracks on the surface. In addition, due to the presence of grinding stress, the thermal fatigue capability of the mold is also reduced.
The surface of the cavity, especially the surface roughness of the runner or the surface of the profile with a small amount of scratches and scribe marks, are the source of the crack. The matching precision between the mold insert and the sleeve is improperly selected, or the heat transfer efficiency is affected by excessive looseness, or the pre-stress is generated due to over-tightening to cause the sleeve to be broken. Mounting accuracy between the mold and the machine, including parallelism and perpendicularity, can affect premature wear of the guide. For today's application of EDM molds, during the processing, the tempering zone under the surface layer is formed due to local high temperature. The structure and chemical composition of the zone are different from those of the matrix, and the hardness is high. In addition, there are residual stresses on the surface. After processing, the profile is easy to form fine cracks and must be polished.
4. Adopt the best mold heat treatment specification
The material of the die-casting mold should have high heat strength and tempering stability, so that it is possible to obtain high thermal fatigue resistance and wear resistance.
When die-casting zinc, aluminum or magnesium alloys, the parts in contact between the die-casting mold and the molten metal are generally made of 4Cr5MoV1, 3Cr2W8V or foreign materials. The heat treatment requires a hardness of 44~48HRC, and the hardness of the die-cast copper alloy is 38~42HRC. The die casting mold needs to be subjected to stress relief treatment before use. Generally, after the new die-casting die-casting is performed 5000-10000 times, the first stress-free tempering is performed, and after each 10000-20,000 times of die-casting, the stress-relieving temperature is 30~50 °C lower than the original tempering temperature.
5. Mold high energy density surface strengthening treatment
High energy density surface strengthening is the application of extremely high energy to the surface of the material to cause physical-chemical changes for reinforcement purposes. Its main features are: simple process, rapid process, small deformation of parts and high production efficiency. The use of the electric spark surface strengthening process is one of the effective ways to reduce the surface erosion, prevent the metal from mating with the die surface, and improve the service life. The principle is to use the full discharge of the pulse circuit to connect the electrode made of cemented carbide (Y68) to the positive pole of the power supply, the metal workpiece to the negative pole of the power supply, and the two are periodically contacted in the air to cause air gap discharge. , forming sparks and high temperatures. Under the action of high temperature, tungsten carbide is released from the electrode, and the process of remelting, depositing, diffusing, compounding and hardening is generated on the surface of the workpiece, so that the surface of the coated workpiece forms a uniform composition, compact structure and hardness. The high tungsten carbide deposit has a surface hardness of 1100~1400HV. The reinforcing layer is firmly bonded to the substrate, and is resistant to impact and does not peel off. During the intensive treatment, the workpiece is in a cold state, the discharge point is extremely small, the time is short, and there is no annealing and deformation. The strengthened mold has good results in terms of heat resistance, corrosion resistance and wear resistance.
6. Operating specifications for die casting
(1) Preheating of the mold and maintaining the heat balance The magnitude of the stress in the mold is proportional to the temperature gradient in the mold, so it is understandable to appropriately increase the preheating temperature. However, the excessive preheating temperature makes the contact temperature of the cavity surface high, which is detrimental to the yield strength of the material, and is disadvantageous to the thermal fatigue resistance of the mold. In addition, the mold is always in thermal equilibrium during the service process, which has become an important means to improve the life of the mold, increase production efficiency and ensure the compactness of the casting. The arrangement of the cooling water channel, as well as the application of the heating oil heating and cooling control device, is based on measuring the distribution of the temperature field of the mold. In addition, the rhythm and continuity of the operation, coupled with the use of the mold temperature control device, can create conditions for maintaining the optimal balance of the mold.
(2) The film coating property of the mold coating paint plays an important role in protecting the mold material. Where the paint is not evenly covered, there is a risk of erosion. In the United States, the form of the droplets and the size of the scattering area during the spraying of the mold surface were discussed. There are similar reports in Germany. They believe that the droplets of paint sprayed onto the high temperature mold will produce high vapor pressure, which will not be easily adsorbed on the profile, which may cause detachment and slumping, and will not fully wet the profile, which will affect the life of the mold. According to the mechanism analysis, the droplets of the coating should first be in contact with the profile surface in a spherical shape, and then flattened into an elliptical shape to increase the contact area, and then further flatten the film to achieve uniform coverage. For this reason, it is required that the mold temperature should not exceed the "wetting temperature" range. The best solution is to increase the spraying pressure, so that the droplets get higher kinetic energy, suppress the condensation phenomenon of the droplets, and achieve the purpose of not rapidly gasifying.
In the selection of die-casting coatings, it is necessary to use coatings that are less corrosive to the mold cavity to reduce corrosion on the mold.
(3) Inspection and maintenance of the mold is essential for the observation and maintenance of the mold during the service. Deformation of the support plate, the sleeve forming module, the core pulling mechanism, and the pushing mechanism in the production, as well as the loosening of the fastening bolts, are important causes of early damage of the mold.
(4) Operation and installation procedures for die-casting molds
1 Before using the die-casting mold, you must understand the basic structure of the mold.
2 Open the moving and fixed mold to remove the oil seal and wipe off the dirt and rust on the mold.
3 Check the movable part of the mold, such as the position of the slider, the push rod, the reset rod, etc., whether there is any skew or not. Fixed parts shall not be relatively turbulent; inspection of profiles, cores, guide columns and diagonal tie rods shall be free of defects such as bumps, dents, cracks, etc., and the assembly position shall be correct.
4 There is a mold for the water cooling system, the cooling waterway should be unblocked, there should be no leakage, pay attention to the inlet and outlet without mistakes; if there is a hydraulic core pulling mold, check whether the hydraulic extractor is installed correctly and the movement is flexible. And adjust the core distance.
5 When the mold is hoisted, the strength of the lifting ring screw should meet the requirements of GB825. When the crane and the mold are moving, be careful not to damage the die casting machine and the control cabinet. When installing the mold, pay attention to the pouring sleeve and the cup of the pressing machine (the die casting machine is shot) Part of the height can be adjusted according to the size of the mold). To install the platen and bolts of the mold, the mold must be fastened and not loose.
6 Before the casting, the mold is preheated and pressed and casted, and the alloy liquid cannot be directly used for hot stamping. Spray the paint reasonably and use the mold temperature machine as much as possible for the cooling of the mold.
7 It is strictly forbidden to hit the mold with hard steel rods, iron hammers, etc. When disassembling the movable parts of the mold and the small core, only soft copper, aluminum, lead rods or wooden hoes can be used.
8 When the mold is glued with aluminum and then polished with oil stone, be careful not to grind the mold cavity and core into a slope or a depression.
9 When the mold is not used for a long time, it should be cleaned before storage. All parts should be coated with anti-rust agent to prevent bumping during transportation and ensure that the mold is intact.
After 10 casting a certain number of castings, the mold should be subjected to stress relief treatment.
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