Measures to improve the service life of stamping d

2022-07-25
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Measures to improve the service life of stamping dies

for deep drawing dies, adhesion wear is an important reason for the failure of deep drawing dies. Generally, adhesion tends to occur between materials with similar properties. Therefore, the corresponding die materials should be selected according to the different materials to be drawn. If the drawing material is non-ferrous metal, the die material can be cast iron, steel and cemented carbide; If the drawing material is ferrous metal, the die material shall be non-ferrous metal, hard alloy and steel material with small affinity

for cold extrusion dies, if the die bears a large unit extrusion pressure, materials with high hardenability such as matrix steel and high-speed steel should be used, otherwise the center of unhardened materials will cause plastic deformation of the die. If the punch is subject to large eccentric force, the material with high strength and toughness shall be selected. The shape of the extruded workpiece is complex, the production batch is large, or the strength of the extruded blank is high. Selecting cemented carbide or steel bonded cemented carbide can improve the service life

in the material selection of the cold heading die, attention should be paid to the original structure and chemical composition of the steel. The steel should not have original structure defects, such as segregation, inclusions and a small amount of shrinkage cavities. For the cold heading die working under high load, the steel used for the die should have high purity, and the sulfur and phosphorus content should be strictly controlled. Generally, the steel with carbon content of 0.8 ~ 0.9% has good toughness, the steel with carbon content of 0.95 ~ 1.05% is hard and tough, and the steel with carbon content of 1.05 ~ 1.15% is hard. The carbon content of large mold is the lower limit, and the carbon content of small mold is the upper limit

(1) reasonably design the die

on the premise of ensuring the quality of the blanking workpiece, the blanking die should select a larger blanking clearance as far as possible to reduce the blanking force and the wear of the die. In order to improve the rigidity of the punch and strengthen its ability to resist eccentric load, so as to prevent the punch from bending, deformation or breaking. Generally, the cross-sectional area of the head and the cross-sectional area of the tail handle of the punch are taken as about 2 and 4 times of the working area of the working end, which should be carried out by professionals. If necessary, the punch should be guided and protected. An elastic unloading plate can be used to exert a certain blank holder force on the sheet metal to reduce the force on the punch due to the slip or warpage of the sheet metal. In order to ensure uniform blanking clearance during stamping and avoid uneven wear of gnawing edge and cutting edge, accurate die guide device can be selected, such as ball guide column guide sleeve

the clearance design of punch and die of deep drawing die should be reasonable: if the clearance is too small, the increase of friction resistance will aggravate the wear of die; If the clearance is too large, the workpiece will wrinkle and the wear of the die will be increased; Uneven clearance will produce uneven internal stress during die operation, which will reduce the service life of the die. The working surface hardness of the die should be high to reduce wear. The surface roughness value of the die should be lower, and the surface roughness value of the drawn sheet should also be lower to reduce the friction resistance during deep drawing, which is conducive to the plastic forming of the drawn parts and improve the service life of the die

the structure of the cold extrusion die must have sufficient strength, rigidity, reliability and good guidance. Adopt the best punch shape, adopt the process shaft when conditions permit, and change the pure extrusion or back extrusion into compound extrusion to reduce the unit extrusion force. Extrusion punch is not easy to be too long to prevent longitudinal bending. The transition part of the working parts of the die shall be designed with a large enough fillet radius to avoid stress concentration caused by sharp corner transition. Split structure shall be adopted for the parts easy to crack laterally in the die to eliminate stress concentration. Engineers and developers can make correct decisions on the utilization of their products, raw materials and processes used in production, and adopt the prestressed composite die structure to prevent the longitudinal cracking of the inner die. Compared with the flat die of the same size, the stepped combined die has a greater ability to bear the radial internal pressure

due to the limited number of manufacturers planning to expand production, a large gradient fillet shall be set at the inlet of the female die of the cold heading die as far as possible to avoid stress concentration, and the draft angle shall be made in the direction of the die. In favor of new technology, it is always described as "revolutionary" to describe the flow of blank in the mold cavity and reduce the load of the mold. Hard alloy or steel bonded hard alloy cold heading dies have high hardness, good wear resistance and high precision of products. The combined structure of cemented carbide or steel bonded cemented carbide insert can be adopted, and the prestressing force can be applied by adding a sleeve to reduce or offset the cold heading force on the die, so as to improve the service life of the die. However, cemented carbide is very brittle. When the die shape is complex and works under high impact load, cemented carbide should not be used

improve the quality of die manufacturing

pay attention to the forging process of die billet, eliminate the distribution of banded and shaped carbides, and make the streamline perpendicular to the direction of impact force. In order to fully break the carbides in the blank and make them disperse evenly, the method of variable heading and drawing with high forging ratio should be adopted

in the manufacturing process, the dimensional and shape accuracy of the mold must be strictly guaranteed to avoid leaving machining tool marks; The transition part shall be smooth without minor defects to prevent stress concentration cracks during use. Tempering shall be carried out after electric machining and grinding to eliminate machining stress

the operation direction of the final polishing process of the drawing die shall be consistent with the flow direction of the blank metal, and the concave mold cavity shall be polished by longitudinal reciprocating rather than circular motion. Pay attention to cooling during polishing to prevent overheating from reducing the hardness of the mold

the cold extrusion punch shall be symmetrical in shape after machining, and the working part must be in the same axis, otherwise the punch is easy to break under the force on one side. The lower the surface roughness of the forward extrusion or reverse extrusion die, the better. The method of grinding and polishing after grinding can be adopted to reduce wear and improve the service life of the die

the quenching hardness and hardened layer depth shall be properly selected according to the working conditions and material properties of the cold heading die to prevent early failure. During heat treatment, attention shall be paid to full tempering. The tempering time is insufficient and the stress is not completely eliminated. Even if the hardness meets the requirements, chunking will still occur. The tempering time is generally more than 1.5 hours

(2) correct selection of die materials

when the production batch of blanking die is large, high-performance die materials with high strength, good toughness and good wear resistance should be selected. Because the working condition of punch is worse than that of die, the wear resistance of punch material can be higher than that of die material

(3) adopting die strengthening and toughening treatment and surface strengthening treatment

adopting strengthening and toughening treatment and surface strengthening treatment technology to make the die obtain excellent overall strength and toughness and excellent surface hardness, wear resistance and adhesion resistance, which is an effective way to improve the service life of various dies

(4) reasonable use and maintenance of the die

during the operation of the blanking die, the depth of the punch into the concave die shall be strictly controlled to avoid aggravation of wear. After a period of use, the cutting edges of punch and die will inevitably appear wear and wear grooves. At this time, repairing the die in advance can reduce friction, prevent cracks caused by wear grooves, avoid additional bending moments caused by uneven clearance between convex and concave dies after wear, and improve the service life of the die. After the male and female dies are grinded again, the cutting edge shall be carefully grinded and polished with a fine oilstone to remove the grinding burr, make the surface roughness Ra 0.10 m and eliminate the potential damage. When the mold is stored, the upper and lower mold should keep a certain gap to protect the cutting edge

appropriate lubricant must be applied between the drawing die and the sheet metal to be drawn, so that the die and the sheet metal do not contact directly, and the conditions of adhesion and occlusion are eliminated. During deep drawing, the relative motion of the contact surface between die and sheet metal becomes the relative motion between lubricant molecules, which can greatly reduce friction and friction heat, and effectively reduce or prevent wear. The thickness, hardness and microstructure of the drawn sheet shall be uniform; The surface shall be kept clean and free of impurities, oxide skin and rust, so as to avoid premature wear of the die due to uneven stress. If the surface roughness of the mold becomes worse after use, it shall be polished in time

the drawing machine with lower drawing speed shall be selected to facilitate the flow of metal materials to be drawn and reduce the friction on the die surface. The drawing speed of the double action press is slow, and the force is more stable and uniform than that of the ordinary punch, which is conducive to extending the service life of the drawing die

the cold extrusion die shall also be reasonably lubricated during operation, and phosphating treatment and lubrication shall be adopted during extrusion of ferrous metal. In the process of cold extrusion, the temperature of the die rises rapidly, so it should be cooled regularly. For heavy-duty dies, stress relief tempering treatment (160 ℃ ~ 180 ℃ for two hours) should be carried out after thousands of extrusion, which can effectively improve the service life of the die. For the repeatedly used outer or middle prestressed ring, after being pressed out for many times, it shall be subject to 180 ℃ heat preservation for two hours to stress tempering treatment to prevent the outer ring from cracking. At low temperature in winter, it is best to preheat the die before use to prevent cold brittle fracture of the punch. A complete maintenance system must be established, and a special person must be appointed to adjust and repair the press and mold in time. During the storage and transportation of molds, anti rust measures shall be taken, and the upper and lower mold bases shall be protected by limited blocks

cold heading die in order to reduce the friction coefficient during operation and prevent the die from adhering and seizing, the cold heading blank shall be phosphated or copper plated. In most cases, the blank is preheated before cold heading. Preheating can improve the machinability of the material, reduce the possibility of cracks, and improve the service life of the die. Lubrication should also be carried out during cold heading. Good lubrication can reduce the surface roughness of products and improve the service life of dies. Especially for the cold heading of the workpiece with complex shape, lubrication is more important

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