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With the development and progress of materials science, in order to meet the needs of some special situations, a large number of mold materials with high strength, high hardness, high toughness, high brittleness, high temperature resistance and other special properties have been produced one after another, thus bringing traditional Many new difficulties and problems are difficult to meet the requirements with traditional processing methods. For workpieces with complex structures, such as complex cavities, molds with concave model holes are often difficult to implement using traditional cutting methods. Therefore, some new methods have been developed in production, and some have been widely used. . Among them, EDM processing method is one of them, which is widely used in the mold manufacturing industry for the processing of the hole and the cavity.
First, the mechanism and characteristics of EDM
Electric discharge machining is a method of machining a workpiece by electrolytic erosion of a pulse discharge between a tool electrode and a workpiece electrode in a certain medium. The principle of EDM is shown in the figure below.
The automatic feed adjustment device 3 here is a hydraulic cylinder and a piston to maintain a small discharge gap between the tool and the workpiece. When the pulse voltage is applied between the two poles, it is at a minimum relative to a certain gap or dielectric strength under the current conditions. The weakest part breaks through the medium, and a spark discharge is generated in the part. The instantaneous high temperature causes the surface of the tool and the workpiece to be partially melted, and even vaporized and evaporated to electrically erode a small portion of the metal, each forming a small pit. After the end of the pulse discharge, after the pulse interval time is passed, the working fluid is restored to the insulation, and the second pulse voltage is applied to the two poles again, and the discharge is discharged at the time when the distance between the poles is relatively closest or the insulation strength is the weakest, and the electrical erosion is performed. Make a small pit. The entire machined surface will consist of numerous small pits. This discharge cycle is repeated thousands of times to tens of thousands of times per second, forming many very small pits on the surface of the workpiece, called electro-erosion. As the tool electrode is continuously fed, the contour dimensions of the tool electrode are accurately "copied" on the workpiece for the purpose of forming.
EDM can process a variety of materials with high melting point, high hardness, high strength, high purity and high toughness. The processing mechanism is completely different from the cutting method, and has the following characteristics:
1. The energy density of pulse discharge is high, which is convenient for processing special materials and complex shapes of workpieces that are difficult to machine or cannot be processed by ordinary machining methods. Not affected by material hardness, not affected by heat treatment conditions. 2. The pulse discharge duration is extremely short, the heat conduction diffusion range generated during discharge is small, and the material is affected by heat. 3. When machining, the tool electrode is not in contact with the workpiece material, and the macro force between the two is extremely small. The tool electrode material does not need to be harder than the workpiece material, so the tool electrode is easy to manufacture. 4, can reform the structure of the workpiece, simplify the processing technology, improve the service life of the workpiece, reduce the labor intensity of workers.
Second, the main factors affecting the quality of mold EDM
The processing quality of the mold mainly includes the processing precision and the quality of the etched surface. Since the input and output parameters in the EDM process are many and complicated, the process factors affecting the machining accuracy of the mold involve the manufacturing precision of the machine itself, the clamping accuracy of the workpiece, the manufacturing and clamping precision of the electrode, the electrode loss, and the discharge gap. , processing slope and many other aspects, which have a large impact on electrode loss and discharge gap.
1. Influence of electrode loss on machining accuracy
In the process of die electric discharge machining, the pulse discharge will cause the tool electrode to be corroded and damaged, so that the electrode loss law can be understood and mastered, and various measures are taken to minimize the loss of the tool electrode, which can ensure the higher processing of the mold. Precision. In EDM, the loss of the different parts of the tool electrode is different. Generally, the electric field strength of the convex part such as the sharp corner and the edge is high, and the tip discharge is easy to form, so the loss is faster than the flat part, and the unevenness is uneven. Loss will inevitably lead to a decrease in machining accuracy.
At the same time, the loss of the electrode is also affected by the thermal physical constant of the electrode material. The melting point, boiling point, specific heat capacity, melting and vaporization latent heat of the electrode material are higher, and the higher the thermal conductivity, the better the corrosion resistance and the stronger the heat transfer capability, thereby reducing the loss of the electrode.
2. Influence of discharge gap on machining accuracy
In the EDM mold, the pulse discharge between the tool electrode and the workpiece needs to maintain a certain discharge gap, so that the processed workpiece hole size is different from the electrode size, and a discharge gap is different along the machining contour. The discharge gap mainly determines the processing stability, and generally increasing the pulse discharge gap time can improve the processing stability. Increasing the peak current will increase productivity, but the electrode loss will increase.
The surface metamorphic layer is deepened and the roughness becomes large. In order to keep the discharge gap stable, it is necessary to keep the electrical parameters of the pulse power supply stable. In addition, the accuracy and rigidity of the machine tool should be kept stable, and special attention should be paid to the influence of the secondary discharge caused by the electrolytic corrosion product on the discharge gap.
3. Influence on the surface roughness of the machined surface
The roughness evaluation parameter Ra of the etched surface increases as the pulse width and current peak increase. Under certain processing conditions, the increase of pulse width and current peak will increase the energy of a single pulse and increase the cross-sectional size of the etch pit. Therefore, the surface roughness mainly depends on the energy of a single pulse. To reduce the value of the surface roughness Ra, the energy of a single pulse must be reduced.
4. Effect of surface change layer on mold surface quality
The surface of the mold after EDM processing will produce a surface change layer comprising a solidified layer and a heat affected layer. The solidified layer is a metal layer that melts under the action of the instantaneous high temperature of the pulse discharge and cannot be thrown. The metal layer is rapidly cooled and solidified after the end of the pulse discharge. The metal grain of the layer is very small and has a strong Corrosion resistance.
The heat-affected layer is located between the solidified layer and the workpiece matrix material, and the metallographic structure of the material changes due to the high temperature transmitted from the discharge point. In the process of processing, due to the selected electrical parameters, cooling conditions and the original heat treatment conditions of the workpiece material, the hardness of the changing layer will change differently. The thickness of the surface change layer is related to the electrical properties of the workpiece material and the pulse power source, and is thickened as the pulse energy increases, and the hardness of the solidified layer is generally high. Therefore, the surface wear resistance of the mold after EDM is better than that of ordinary machining.
Third, the application of EDM technology in mold processing
Due to the rapid development of the mold industry, in order to meet the needs of some special situations, a large number of mold materials with high strength, high hardness, high toughness, high brittleness, high temperature resistance and other special properties have been successively produced, thus bringing traditional metal cutting processing. Many new difficulties and problems are difficult to meet the requirements with traditional processing methods. At this time, EDM technology is widely used and mold processing. With the application of EDM technology to the mold industry, it has had a great impact on the traditional mold processing technology and changed the mold processing process. The conventional processing method is to perform milling after annealing; then heat treatment, grinding or electric discharge machining; finally, manual grinding and polishing, and the processing cycle is long. Dies are often scrapped due to quenching or cracking. The EDM technology can process the quenched template to avoid hot deformation defects and inlay structure, which not only simplifies the mold structure, but also improves the strength and life of the mold.
The electric spark modification technology has great application on the surface of the mold. In order to improve the quality of the mold surface, the working principle of high temperature is generated by electric spark pulse discharge. The hard alloy material such as YG8 is used as the electrode material to infiltrate the hard alloy material. On the working surface of the mold and wearing parts, a hard white alloy strengthening layer with high hardness, high strength, high wear resistance, high temperature resistance and no peeling is formed to change the physical and chemical properties of the surface. A very effective method of surface treatment. The electric spark strengthening layer is a new alloy layer formed by re-alloying the electrode and the workpiece material under high temperature and high pressure at the moment of discharge, and is not simply coating and stacking of the electrode material. A diffusion layer of nitrogen or the like between the alloy layer and the base metal is firmly bonded to the substrate and is extremely resistant to impact. In the intensive treatment, since the discharge time is very short, the area of ​​the discharge point is small, the thermal effect of the discharge only occurs in a small area on the surface of the workpiece, and the whole workpiece is still at a normal temperature state or the temperature rise is low, the workpiece is in a cold state, and the time is short. Does not cause annealing or thermal deformation. EDM modification is carried out in an air or liquid medium. It can be applied to the local surface of the part, as well as to the modification of the plane or surface of the general geometry, such as tools, molds, mechanical parts and so on. The electrode material can be freely selected according to the application. When repairing the worn machine parts, carbon, copper, brass and other materials can be used as the electrodes. These materials are widely used and the material consumption is also small. The modified layer of different thicknesses and surface roughness can be obtained by adjusting the electrical parameters and controlling the modification time. The method of operation is easy to master and does not require an operator with a high level of skill. It can use a small amount of material to achieve the effect that a large amount of expensive overall material is difficult to achieve, which can improve the hardness, wear resistance and high temperature red hardness of the product, significantly improve the service life of the product, and greatly reduce the consumption of valuable materials. Reduce production or maintenance costs.
The precision of EDM precision cavity mold is represented by three aspects: dimensional accuracy, profiling precision and surface quality. Processing precision cavity molds. The dimensional requirements of each part should be correctly analyzed, and the dimensional accuracy of different places should be flexibly controlled. Special attention should be paid to those parts with high dimensional requirements. The important dimensional tolerances in precision cavity molds can reach ±2~3 microns. The rigor of its size requirements can be seen. The precision of the profiling is also high, and the cavity should be clear and the edges and corners are clear. The surface roughness value is uniform and reaches the predetermined surface roughness value. Generally, the Ra value is required to be small, and even the mirror surface processing is required, and the thickness of the surface deterioration layer of the EDM portion is small.
Fourth, summary
Nowadays, with the rapid development of the economy, the great potential of EDM technology and the production potential has become an important technical field for the research of Germany, the United States and Japan. High-speed machine tools have also entered the market and are used in aircraft, automotive and mold manufacturing industries. However, in the mold industry, EDM technology has many factors affecting the quality of machining in the process of EDM. In the specific processing, it should be reasonable according to the processing requirements of the workpiece, the material of the electrode and the workpiece, and the processing index. Select processing parameters, and master the conversion rules through process testing and production practices to improve mold processing accuracy, processing productivity and economy. If these problems are solved properly, I believe that in the near future, EDM technology will have more and more applications in the mold industry, and it has more and better prospects.
Application of EDM Technology
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