With the continuous improvement of industrial automation, the application of molds is becoming more and more widespread. Low mold life and poor accuracy of the working part will not only affect product quality, but also cause a huge waste of costs such as mold materials and processing hours, greatly increasing the cost of the product and reducing production efficiency, which seriously affects the competitiveness of the product. The research shows that the service life of the mold is related to many factors such as improper heat treatment, inappropriate material selection, irrational mold structure, irrational machining process, poor mold lubrication, and poor equipment level. According to the analysis and statistics of a large number of failed molds, among the various factors that cause mold failures, improper heat treatment accounts for about 45%, improper materials selection, mold structure irrationality accounts for about 25%, process problems account for about 10%; lubrication problems, equipment problems And other factors accounted for about 20%. Therefore, in the mold design and manufacturing process, the selection of appropriate materials, the rational design of the mold structure, the selection of a reasonable heat treatment process, the proper arrangement of the processing process of the mold parts, and the improvement of the mold's working conditions are all conducive to improving the quality of the mold and Service life.
1.1 Selection of mold materials
When selecting mold materials, they should be selected according to different production batches, process methods and processing objects. In large-scale production, long-life mold materials should be selected, such as hard alloys, high-strength, high-wear-resistant mold steels (such as YG15, YG20); for small batches or new product trials, zinc alloys, bismuth tin alloys, etc. can be used. Mold materials; For general molds that are easily deformed and easily fractured, materials with high strength and toughness (such as T10A) need to be selected; hot forging molds should be selected with good toughness, strength, wear resistance, and resistance to cold and hot fatigue Materials (such as 5CrMnMo); die-casting molds should use alloy steel with high thermal fatigue resistance and high temperature strength (such as 3Cr2W8V); plastic molds should choose materials that are easy to cut, dense in structure, and good in polishing performance. In addition, when designing male and female molds, molds with different hardness or different materials should be selected to match, such as: tool steel for male molds (such as T10A), and high-carbon high chromium steels for female molds (such as Cr12, Cr12MoV). The service life of the mold can be increased by 5 to 6 times.
1.2 Reasonable mold structure
The principle of mold design is to ensure sufficient strength, stiffness, concentricity, neutrality and reasonable blanking clearance, and reduce stress concentration to ensure that the parts produced by the mold meet the design requirements. Therefore, the main working parts of the mold (such as the convex and concave dies of the die, the moving and fixed dies of the injection mold, and the upper and lower dies of the forging die) are required to have high guiding accuracy, good concentricity and centering, and blanking. The clearance is reasonable.
When designing the mold, we should focus on:
① When designing the punch, attention must be paid to guide support and centering protection. In particular, the self-guided structure is used when designing the punching die for small holes, which can extend the life of the die.
② For weak parts such as angles and narrow grooves, in order to reduce the stress concentration, the arc transition is required. The arc radius R can be 3 to 5 mm.
③ The inlay structure is used for the concave mold with complicated structure, which can also reduce the stress concentration.
④ Reasonably increase the clearance, improve the stress state of the working part of the punch, reduce the punching force, unloading force and push force, and reduce the wear on the cutting edge of the punch and die.
Mold heat treatment process
According to the analysis of mold failure, 45% of mold failures are caused by improper heat treatment. As we all know, wear and adhesion occur on the surface, and fatigue and fracture often start from the surface, so the machining quality requirements for the mold surface are very high. However, due to the existence of processing marks, the surface oxidation and decarburization during heat treatment is also inevitable. Therefore, the surface performance of the mold is inferior to that of the substrate. The use of new heat treatment technology is an important economic and effective measure to improve mold performance. The mold heat treatment process includes substrate toughening and surface strengthening treatment. The strength and toughness of the matrix lies in improving the strength and toughness of the matrix, and reducing fracture and deformation. The main purpose of surface strengthening is to improve the wear resistance, corrosion resistance and lubrication performance of the mold surface.
2.1 The overall strengthening and toughening process of the mold
The mold must have excellent overall toughening performance and excellent cavity surface properties, so as to improve the service life of the mold. In order to meet this requirement, it is necessary to strengthen the overall surface of the mold and then strengthen the surface. Various treatment processes: For ordinary cold-working die steels, low-temperature quenching and low-temperature tempering can be used to obtain good effects of enhancing toughness, reducing brittleness and disassembly. For hot-working die steels, high-temperature quenching and high-temperature tempering are used. Treatment can significantly improve the toughness and thermal stability of hot work die steel. For example, for a die-casting mold made of 3Cr2W8V material, two pre-normalizations at 400 ° C to 500 ° C and 800 ° C to 850 ° C followed by high-temperature quenching and tempering treatment can improve toughness by 40% and mold life by twice .
In addition, deformation heat treatment can also be used. Deformation heat treatment is a toughening process that combines the strengthening of steel with phase transformation. The nature of strengthening and toughening in deformation heat treatment is to obtain fine austenite grains, refinement of martensite increases dislocation density in martensite and forms cellular substructures, and at the same time promotes the dispersion hardening of carbides.
2.2 Surface heat treatment of mold
The mold surface strengthening treatment process mainly includes gas soft nitridation method, ion nitridation method, electric spark surface strengthening method, boronizing method, TD method, CVD method, PVD method, light surface strengthening method, ion implantation method, and plasma spraying method. and many more.
① Gas soft nitridation: After nitrogen decomposes at the nitriding temperature, active nitrogen atoms are generated, absorbed by the metal surface, penetrated into the steel, and continuously diffused from the surface to form a nitrided layer. After the mold is nitrided, the surface hardness can reach HV950 ～ 1200, which makes the mold have high red hardness and high fatigue strength, and improves the surface finish and anti-seizure ability of the mold.
② Ion nitriding: Place the mold to be processed in a vacuum container, fill it with a nitrogen-containing gas (such as nitrogen or a mixture of nitrogen and hydrogen) at a certain pressure, and then use the mold to be treated as the cathode and the cover wall of the vacuum container as For the anode, a DC voltage of 400 to 600 volts is applied between the cathode and anode, and a glow discharge is generated between the cathode and anode. The gas in the container is ionized, and a large number of electrons and ions are generated in the space. Under the action of the electric field, positive ions rush towards the cathode, bombard the surface of the mold at a high speed, and heat the mold. High-energy positive ions rush into the surface of the mold, obtain electrons, and become nitrogen atoms that are absorbed by the surface of the mold and diffuse inward to form a nitrided layer. The application of ion nitridation can improve the wear resistance and fatigue strength of the mold.
③Spark Surface Strengthening: This is a process that directly uses the high energy density of electrical energy to strengthen the surface of the mold. It uses the effect of spark discharge to infiltrate the conductive material as the electrode into the surface layer of the metal workpiece, thereby forming an alloyed surface strengthening layer, which improves the physical, chemical and mechanical properties of the working surface. For example, the surface of high-speed steel or alloy tool steel is strengthened with hard alloy electrode materials such as WC and TiC, which can form a wear-resistant, corrosion-resistant and red-hardened reinforcement layer with a microscope hardness of HV1100 or more, which significantly improves the service life of the mold. The advantages of EDM surface strengthening are that the equipment is simple, the operation is convenient, and the wear resistance of the mold after treatment is significantly improved; the disadvantages are that the strengthening surface is rough, the thickness of the strengthening layer is thin, and the efficiency of the strengthening treatment is low.
④ Boronizing: Because the boronizing layer has good red hardness and abrasion resistance, the hardness of the mold surface (up to HV1300 ~ 2000) and abrasion resistance can be significantly improved by boronizing. It can be widely used for mold surface strengthening, especially suitable for processing. Mold under abrasive wear conditions. However, the boronizing layer often has greater brittleness, which also limits its application.
⑤ TD heat treatment: Put a crucible made of heat-resistant steel in an air furnace or a salt tank, heat the borax in the crucible to 800 ° C to 1200 ° C, and then add the corresponding carbide to form a powder (such as titanium, barium, niobium, Chromium), and then immerse the steel or hard alloy workpiece in the crucible for 1 to 2 hours. Adding elements will diffuse to the surface of the workpiece and react with the carbon in the steel to form a carbide layer. The resulting carbide layer has a very High hardness and abrasion resistance.
⑥CVD method (chemical vapor deposition): After the mold is placed in hydrogen (or other protective gas) and heated to 900 ° C to 1200 ° C, it is used as a carrier gas to vaporize low-temperature compound gas such as titanium tetrachloride ( TiCI4) and toluene CH4 (or other hydrocarbons) vapors are brought into the furnace, so that the titanium in TiCI4 and the carbon in the hydrocarbons (and the carbon on the steel surface) undergo a chemical reaction on the surface of the mold to form a layer of Requires metal compound coating (such as titanium carbide).
⑦PVD method (physical vapor deposition): The metal atoms for strengthening are evaporated in a vacuum chamber, or by the bombardment of charged particles, under the action of a current bias, they are attracted and deposited on the surface of the workpiece to form a strengthening layer. PVD method can be used to deposit titanium carbide, titanium nitride, alumina and other compounds on the surface of the workpiece.
Holmium laser surface strengthening: When a laser beam with a certain power is irradiated to the blackened mold working surface at a certain scanning speed, the mold working surface will be rapidly heated due to the absorption of laser energy in a short time. When the laser beam is removed, the working surface of the mold is rapidly cooled by the substrate itself to form a surface-reinforced layer with certain properties. Its hardness can be increased by 15-20%. In addition, it has a small quenched structure, high wear resistance, Significant energy saving effects and improved working conditions.
Thorium ion implantation: A small, low-energy ion accelerator is used to ionize the atoms of the element to be ionized in the ion source of the heater, and then it is heated by the high voltage electric field of the ion heater to become a high-speed ion current, which is then subjected to magnetic analysis After the device is refined, the ion beam is forced into the working surface of the mold placed in the target chamber, thereby changing the micro hardness and roughness of the mold surface, reducing the surface friction coefficient, and ultimately increasing the service life of the workpiece.
3.Machining process of mold
The machining process of the mold is an important link that directly affects the service life of the mold and product quality. Due to the various shapes of mold parts and high precision requirements, in addition to ordinary machining equipment such as lathes, milling machines, planers, honing machines, and grinders, various advanced equipment such as EDM machine tools are also required during processing , EDM machine tools and precision grinding machines. At present, for the molds with complex structures and special process requirements, a new processing method different from traditional machining-mold special processing (electric processing) has also been developed rapidly. With this method, the tool material is not required to be harder than the workpiece material, and no obvious mechanical force needs to be applied during processing. Instead, the workpiece is directly processed using electrical energy, chemical energy, light energy and acoustic energy to achieve a certain Shape size and surface roughness requirements. Machining practice has proved that: using the correct machining technology to double the cavity surface roughness of high-precision molds can increase the mold life by 50%. This is especially important for plastic molds.
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