Influence of hottest grinding on die life

2022-08-24
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The influence of grinding on mold life

China's mold industry has indeed made great progress today, but there is still a large gap between large, precision and long-life molds and foreign advanced levels. This paper gives some opinions on mold life and related failure problems

1. Comparison of mold life between China and foreign countries

mold category domestic level foreign level

die casting mold:

zinc tin die casting mold 10000 times

aluminum die casting mold 200000 times 1000000 times

copper die casting mold 10000 times 100000 times

ferrous metal die casting mold 15million times

plastic injection mold:

non quenched steel mold 10000 times 10000 times

quenched steel mold 10000 times

die:

alloy steel die assembly Service life 10000 times 10000 times

total service life of cemented carbide punching die 100 million times/grinding once 100 million times/grinding once

forging die:

ordinary forging die 10000 times or so 25000 times

precision forging die 0 Ten thousand times 50000 times

10000 times of glass mold

2 The basic formation of mold damage

in order to improve the service life of the mold, the analysis of the failed mold shows that the main forms of mold damage are: plastic deformation, wear, fatigue, cold and hot fatigue, fracture and cracking, corrosion. During the service process of the mold, there may be many forms of damage at the same time, and various damages penetrate and promote each other, and develop separately. Until the products produced by the mold are scrap, the mold will fail. In order to improve the service life of dies, it is necessary to carefully analyze the causes of die damage and various influencing factors, and formulate methods and measures to overcome them

3. Factors affecting mold failure:

(1) mold structure

(2) mold materials

(3) manufacturing process of cold and hot machining a, forging B, heat treatment C, cutting D, grinding, electrical machining

(4) mold working conditions

(1) during grinding, the grinding wheel and the workpiece are in arc contact. When the grinding wheel is cutting, the workpiece produces 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 inside the workpiece material generate relative movement pairs, forming internal friction and heating, and 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 transmitted to the workpiece and grinding debris, and 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, the surface layer produces secondary quenching, part of the residual austenite is transformed into martensite, and the specific volume of martensite is large, With the increase of specific volume, the surface will generate compressive stress. If the grinding cooling is not good, or no coolant is used, the surface will produce tempering, martensitic transformation and tensile stress (such as γ- Fe transforms into α- When Fe is used, the volume of iron will expand by 1%. These stresses (residual stress can reach MPa, i.e. kg/mm ‰ 2). 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, after tempering, there is no return fire, and there is too much martensite or residual austenite. During grinding, phase transformation will occur, and the stress will cause cracks on the surface of the workpiece. Grinding crack is a kind of very fine surface crack, which is mostly perpendicular to the grinding direction, sometimes in shape, and its depth is within 0.03mm

(2) incorrect grinding and heat treatment processes during grinding cause annealing, burns, grinding cracks and residual stresses on the surface of the workpiece after grinding, resulting in workpiece deformation. The main causes of die fatigue failure are stress concentration and cyclic load. The mold is normally in service under the condition of high strength and low plastic. Under the action of cyclic loading, the microcrack expands and finally leads to fatigue failure

(3) measures to reduce grinding defects. 245 there are many factors that cause grinding cracks and residual stresses in pile foundations, mainly from the process system

first is to reasonably select the grinding parameters, and the grinding depth is the main factor affecting the grinding heat. Increasing the speed of the workpiece and the grinding wheel can also reduce the occurrence of cutting heat. Second is to reasonably select and trim the grinding wheel. The grinding wheel with white corundum is better than when the error and indication repeatability are abnormal in the verification process. Its performance is hard and brittle, and it is easy to produce new cutting edges, so the cutting force is small, and the grinding heat is small, It is better to use medium grain size in grain size, such as No. 46 and No. 60. Medium soft and soft (Zr  1, Zr  2 and R  1, R  2) are used in the hardness of grinding wheel, that is, the cutting heat can be reduced only when the grinding wheel with coarse grain size and low hardness has good self excitation. In grinding, it is also necessary to be diligent in dressing the grinding wheel to keep the grinding edge sharp. Then there is the rational use of cooling and lubricating fluid, which plays the three major roles of cooling, washing and lubrication, and keeps the cooling and lubrication clean, so as to control the increase of grinding heat and make the grinding heat within the allowable range, so as to prevent the thermal deformation of the workpiece. The fourth aspect is to 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

in a word, the grinding process should be paid full attention to when the mold is manufactured, and the grinding microcracks and residual stress should be reduced to a minimum to improve the service life of the mold

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