On the influence of grinding on die life

Brief discussion on the influence of grinding on mold life

mold manufacturing is the continuation of mold design and the process of verifying the correctness of design. Advanced, efficient and high-precision machine tools and automatic production technology are used in modern mold production. The grinding workload will account for 25% - 45% of the total manufacturing hours of the mold. China's mold industry has made great progress today, but there is still a big gap with the advanced level of foreign countries. Manufacturing is the continuation of mold design and the process of verifying the correctness of design. Advanced, efficient and high-precision machine tools and automatic production technology are used in modern mold production. The grinding workload will account for 25% - 45% of the total manufacturing hours of the mold. China's mold industry has made great progress today, but there is still a large gap with the advanced level of foreign countries. See the attached table for the comparison of mold life. The quality of finished products in mold manufacturing is closely related to the precision of mold manufacturing, especially with the precision and surface roughness of mold cavity surface

in actual production, the factors affecting die failure are:

① die structure

② mold material

③ manufacturing process of cold and hot machining (forging, heat treatment, cutting, grinding, electric machining, etc.)

④ mold working conditions

in order to improve the service life of the mold, we must carefully analyze the causes of mold damage and various influencing factors, and formulate ways and measures to overcome it

first, to improve the accuracy and quality of mechanical processing and electrical processing, so as to minimize the workload of manual finishing, such as the development of high-precision machine tools and high-speed forming milling machines and their processing technology, which lays the foundation for the development of this process route

second, it focuses on the polishing and grinding process in finishing, and its processing time is almost the same as that of machining and EDM. A mold is composed of many parts. The quality of the parts directly affects the quality of the mold, and the final quality of the parts is guaranteed by finishing

in most domestic mold manufacturing enterprises, the methods used in the finishing stage are generally grinding, electric machining and fitter processing

the impact of grinding on the service life of the die has not been paid enough attention to. Due to the incorrect grinding process, the workpiece surface burns, grinding cracks, grinding marks and grinding stress are caused, resulting in the initiation source of cracks in the subsequent process and the mechanical fatigue and cold and hot fatigue of the die during service, which seriously affects the service life of the die

the purpose of this paper is to study and discuss how to improve the grinding quality, improve the service life of dies, prolong the service time, and promote the adoption of new die technology

1. Grinding of molds

the essence of the grinding process is that the workpiece is ground on the metal surface, which deforms under the instantaneous extrusion and friction of countless abrasive particles, and then turns into debris, forming a smooth surface. The whole process of grinding is represented by force and heat

① in the grinding process, the machined surface will produce thermal expansion under the action of cutting heat. At this time, the temperature of the base metal is low, so the surface will produce thermal stress. When the grinding is finished, the surface temperature of the workpiece decreases. Due to the thermal plastic deformation of the surface and the limitation of the matrix, the surface generates residual tensile stress and the inner layer generates residual compressive stress

② we should always pay attention to the problems found in the use process of the experimental machine. During grinding, the grinding wheel and the workpiece are in arc contact. When the grinding wheel is cutting, the workpiece will produce plastic deformation and intense friction resistance between the grinding wheel and the workpiece, so as to form a grinding force with equal size and opposite direction between the grinding wheel and the workpiece. At the same time, due to the plastic deformation of the surface material, the metal molecules in the workpiece material will produce relative displacement, forming internal friction and heating, The external friction between the grinding wheel and the workpiece also generates heat. This grinding heat will produce a local instantaneous high temperature of 1000 ℃ in the grinding area, and the grinding wheel is not easy to transfer heat, so 80% of the heat is transferred to the workpiece and grinding debris, while the metal in the solid state changes from one lattice to another with the change of temperature, resulting in the transformation of metallographic structure. When grinding hardened steel, it is fully cooled, and the surface layer produces secondary quenching, Some residual austenite is transformed into martensite, and the specific volume of martensite is large, and the specific volume increases, resulting in compressive stress on the surface. If the grinding cooling is not good, or no coolant is used, the surface will be tempered, martensite transformation will occur, and tensile stress will occur on the surface (e.g γ Fe transforms into α－ When Fe is used, the volume of iron will expand by 1%, and these stresses (residual stress can reach 500 ~ 1000 MPa). If the yield limit of the material is exceeded, grinding cracks will occur. In addition, after heat treatment and quenching, the mold is not tempered immediately, the quenching temperature is too high, there is like carbonization, and there is too much martensite or residual austenite that does not fire back after tempering. During grinding, phase transformation will occur, and stress will occur, causing cracks on the surface of the workpiece. Grinding crack is a kind of very fine surface crack. There are generally three types of grinding crack: parallel line crack, complex crack and splay crack. The occurrence direction of the crack is related to the mold shape, and the development direction of the crack is related to the grinding direction of the grinding wheel on the workpiece surface, and its depth is within 0.03 mm

③ during grinding, the grinding wheel is not sharp, the feed rate is large, and the cooling is poor, which makes the temperature on the workpiece surface reach 300 ℃, causing burns on the workpiece surface

2. Measures to reduce grinding defects

① reasonably select the grinding amount, and adopt the fine grinding method with small radial feed, or even fine grinding. If the radial feed rate and the speed of the grinding wheel are properly reduced, and the axial feed rate is increased, the contact area between the grinding wheel and the workpiece is reduced, and the heat dissipation conditions are improved, so as to effectively control the increase of the surface temperature

② reasonably select and trim the grinding wheel. The white corundum grinding wheel is better. Its performance is hard and brittle, and it is easy to produce new cutting edges. Therefore, the cutting force is small, and the grinding heat is small. It is better to use the medium particle size on the particle size, such as 46 ~ 60 mesh. It is better to use the medium soft and soft (ZR1, Zr2 and R1, R2) on the hardness of the grinding wheel, that is, the coarse-grained and low hardness grinding wheel. Good self excitation can reduce the cutting heat. It is very important to choose an appropriate grinding wheel during fine grinding. In view of the high vanadium and high molybdenum condition of die steel, Gd single crystal corundum grinding wheel is more suitable. When machining cemented carbide and materials with high quenching hardness, diamond grinding wheel with organic binder is preferred. The organic binder grinding wheel has good self grinding performance, and the grinding hardness ranges from shore a hardness 20 to 75, and the workpiece roughness can reach Ra0.2 μ m. In recent years, with the application of new materials, CBN (cubic boron nitride) grinding wheels have shown very good machining effects, which are superior to other types of grinding wheels in precision machining on CNC molding grinding machines, coordinate grinding machines, CNC Internal and external cylindrical grinding machines. In the grinding process, it is necessary to pay attention to the timely dressing of the grinding wheel to maintain the sharpness of the grinding wheel. When the grinding wheel is passivated, it will slide and squeeze on the surface of the workpiece, causing the surface burn of the workpiece and the strength reduction

③ reasonably use the cooling lubricant, play the three roles of cooling, washing and lubrication, and keep the cooling and lubrication clean, so as to control the grinding heat within the allowable range, yield strength and hardness to prevent the thermal deformation of the workpiece. Improve the cooling conditions during grinding, such as using oil impregnated grinding wheel or internally cooled grinding wheel. When the cutting fluid is introduced into the center of the grinding wheel, the cutting fluid can directly enter the grinding area to play an effective cooling role and prevent workpiece surface burns

④ reduce the quenching stress after heat treatment to the minimum, because the quenching stress and the like carbonized structure produce phase transformation under the action of grinding force, which is very easy to cause cracks in the workpiece. For high-precision dies, in order to eliminate the residual stress of grinding, low-temperature aging treatment should be carried out after grinding to improve the toughness

⑤ to eliminate grinding stress, the mold can also be immersed in a salt bath at 260 ~ 315 ℃ for 1.5 min, and then cooled in oil at 30 ℃, so that the hardness can be reduced by 1hrc and the residual stress can be reduced by 40% - 65%

⑥ for the precision grinding of precision molds with dimensional tolerance within 0.01 mm, attention should be paid to the influence of ambient temperature, and constant temperature grinding is required. According to the calculation, when the temperature difference of 300 mm long steel is 3 ℃, the material is 10.8 μ About M, (10.8=1.2 × three × 3. 1.2 deformation per 100mm μ M/℃), and the influence of this factor should be fully considered in all finishing processes

⑦ adopt electrolytic grinding to improve mold manufacturing accuracy and surface quality. During electrolytic grinding, the grinding wheel scrapes off the oxide film instead of grinding metal, so the grinding force is small and the grinding heat is small. Grinding burrs, cracks, burns and other phenomena will not occur. Generally, the surface roughness can be better than ra0.16 μ m； In addition, the wear of the grinding wheel is set to be small. For example, when grinding cemented carbide, the wear amount of the silicon carbide grinding wheel is about 400% - 600% of the weight of the ground cemented carbide. When using electrolytic grinding, the wear amount of the grinding wheel is only 50% - 100% of the grinding amount of cemented carbide

3 conclusion

in a word, the grinding process should be paid full attention to during mold manufacturing, so as to minimize the grinding microcracks and residual stress, so as to improve the service life of the mold. Mold manufacturing is the continuation of mold design and the process of verifying the correctness of design. Advanced, efficient and high-precision machine tools and automatic production technology are used in modern mold production. Grinding workload will account for the total mold manufacturing automatic calibration: the system can automatically achieve the calibration of indication accuracy; 25% - 45% of working hours. China's mold industry has made great progress today, but there is still a large gap with the advanced level of foreign countries. See the attached table for the comparison of mold life. The quality of finished products in mold manufacturing is closely related to the precision of mold manufacturing, especially with the precision and surface roughness of mold cavity surface. (end)