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At present, the mirror EDM technology is gradually promoted in the manufacture of precision cavity molds. This article focuses on the key link of mirror EDM application in the enterprise. Flourishing legend has many years of experience in mold processing, CNC processing, and precision injection molding. Combined with years of practice, it analyzes the factors that affect the performance of mirror processing. By controlling each process link, high-quality and high-efficiency mirror EDM can be effectively realized. It has practical application value for mold manufacturing companies to absorb and digest the current advanced EDM technology.
At present, the rapid development of mold manufacturing technology has given higher requirements for EDM. Surface roughness is an important technical index of EDM. At present, through the improvement of machine tools and the development of new process methods, mirror EDM has been realized. The so-called mirror EDM generally refers to the EDM with the surface roughness Ra<0.2μm, and the processed surface has a mirror reflection effect. This technology is mainly used in the processing of complex profiles of precision molds. The high-gloss surface obtained by EDM does not need to be manually polished, which avoids the defects of shape and size accuracy caused by manual polishing, which is of practical significance.
The key technologies for the application of mirror EDM are discussed below based on practical processing experience.
Due to the different chemical composition and organizational structure of the workpiece material, different processing effects will be produced under the same processing conditions. Some materials can get a good mirror effect, but some materials can't get a mirror effect, and even there are abnormalities such as stripes on the surface. Table 1 lists the EDM mirror machining performance of a variety of common workpiece materials. It should be noted that the material with poor EDM mirror effect does not mean that it is not a mirror material. For example, S136 ESR steel itself has excellent polishing performance, but the mirror effect of EDM is not ideal. This is because EDM and polishing are two completely different processing methods. The surface of EDM is composed of many overlapping disc-shaped pits. These tiny pits form a shining mirror under the reflection of light. The appearance of microscopic pits produced by EDM of different materials is different, so The glossiness of the surface is also different.
After many times of research and clinical processing experience by Flourish, the following situations are some of the reasons why the workpiece material affects the effect of mirror processing:
(1) Coarse raw carbides or coarse crystal particle additives containing large particle sizes are not conducive to the processing of mirror surfaces, resulting in microcracks;
(2) The silicon-containing component is conducive to obtaining a better mirror effect;
(3) Sulfur S containing fast cutting components is not good for processing mirror surfaces, and it is easy to produce surface streaks;
(4) Material manufacturing process factors such as the rolling direction of the material are unfavorable to the processing of the mirror surface, resulting in surface stripes;
(5) Metal inclusions, bubbles, oxides and other factors form pinholes and holes.
Generally speaking, most plastic mold steels can meet the requirements of mirror EDM, while most cold work mold steels cannot.
When the performance of the material meets the performance of mirror EDM, the higher the hardness, the better the effect of EDM mirror. Generally, it is required that the steel material for mirror EDM should be heat treated and hardened to HRC>50.
Since the surface roughness of EDM mainly depends on the energy of a single pulse, a reflective surface with a low surface roughness value can be produced by reducing the spark discharge energy of a single pulse, but the mirror processing at this time is limited to small area processing. As the processing area increases, the parasitic capacitance between the electrode and the workpiece increases accordingly. When a single small energy discharge pulse acts between the two poles, it cannot cause spark discharge between the two poles, and the gap voltage at this time increases greatly. Slow, the pulse energy is stored in the parasitic capacitance, and only when multiple discharge pulses arrive and enough energy is stored in the parasitic capacitance, the gap voltage gradually rises to the breakdown voltage, causing spark discharge, but the discharge at this time The energy is equivalent to the superposition of multiple discharge pulse energies, the depth of the generated discharge pit will be greatly increased, the surface roughness value will become larger, and the processed surface will lose the mirror effect.
In order to achieve large-area mirror EDM, adding a certain amount of powder to the working fluid will significantly improve the surface roughness after EDM, achieve a mirror-like effect, and obtain faster processing speed (making Finishing time is shortened by 20%-30%).
In the case of not using the powder mixing process, the larger the processing area, the more difficult it is to obtain a good mirror effect. In fact, due to the application of the translation process, the electrode is not in contact with the entire area. This partial discharge reduces the "capacitance effect". In addition, a uniform layer of carbon black is also produced between the electrode and the workpiece during the discharge process. Layer, its function is similar to the significance of powder mixing processing, so a larger processing area (such as 1600mm2) can also achieve a uniform mirror effect by optimizing the process parameters. For the case of using powder mixing processing technology, the mirror effect of large area (such as 10000mm2) can be easily realized.
From the processing shape point of view, simple shapes are easier to obtain mirror surfaces than complex shapes. The easiest shape to obtain a mirror effect is a circle, followed by a square shape with complex contours, and the most difficult one is the type of integral discharge of multiple cavities. The mirror effect of the corners and curved surfaces of complex shapes is more prone to abnormalities such as unevenness and ripples.
Judging the difficulty of cavity mirror processing can also refer to whether the carbon black layer generated by the discharge can evenly cover the surface of the electrode and the workpiece. For example, the bottom surface is easier to obtain a good mirror effect than the side surface. This is because the bottom surface is always in a trimmed state during the entire discharge process, and the carbon black layer can be evenly distributed on the processing surface, but the side surface is completed by translational processing. The black layer does not effectively cover the bottom surface. In addition, the deeper the cavity, the uniformity of the mirror effect will be affected because the processed part cannot cover the carbon black layer evenly.
The configuration content of the EDM process database mainly includes electrical specifications and reserved quantities. A mirror EDM database consists of as many as a dozen electrical standards. From the rough machining electrical gauge to the finishing electrical gauge, it is necessary to make a reasonable transition of the electrical gauge to gradually reduce the surface roughness. Each electrical standard should be able to discharge stably without causing bad surface conditions such as arcing and carbon deposition. At the same time, a reasonable margin needs to be reserved between every two electric standards, and the latter electric standard must be able to smooth out the discharge pits produced by the previous electric standard. Mirror processing can be realized in actual production only by perfectly matching electrical specifications and reserved quantities to build a processing database.
Electrical standards include parameters such as current, pulse width, pulse gap, polarity, voltage, and knife lift control. The electrical standard in the rough machining stage uses a larger peak current and pulse width, a smaller pulse gap, and uses positive polarity (the electrode is positive) to achieve high-efficiency and low-loss processing. The tool lifting speed should be fast to help chip removal . The electric current in the middle machining stage decreases in turn. In order to reduce the surface roughness value and achieve stable EDM, the pulse width is greatly reduced. Since most of the machining volume has been removed in the rough machining stage, the relative loss in the middle machining stage is not large. big. The finishing stage usually uses negative polarity processing (the electrode is the negative pole) below the VDI18 level (Ra0.8um) standard, uses a smaller pulse width, and discharges through capacitor energy storage, so that it can maintain a relatively high temperature in the finishing stage. High efficiency and discharge stability. In order to avoid defects such as dark spots on the surface, the electric gauge in the finishing stage uses large veins to achieve a uniform and consistent matte finishing surface.
When the finished surface reaches about VDI12 level (Ra0.4μm), it can switch to mirror processing. The effect of the mirror finish depends on the configuration of the last electrical standard. Theoretically speaking, the electrical standard configuration of mirror EDM should use small peak current, small pulse width, and large pulse gap; the smaller the peak current is, the smaller the pulse width is, and the larger the pulse gap is, the better the machining surface will be. The roughness value is smaller. In fact, with this parameter matching method, the processing efficiency is extremely low, and too small pulse energy cannot obtain a uniform mirror effect due to the inability to stabilize the discharge. For actual mirror processing, a larger peak current and a smaller pulse width should be selected. The larger peak current makes the discharge breakdown under the condition of small pulse width easier. Mirror EDM adopts negative polarity processing (the electrode is negative), and cannot be matched with capacitors. In addition, the selection of tool lifting control parameters is also very important, and there are some differences from the selection of conventional processing. For example, the discharge time should be set longer, the tool lifting height should not be too high, and the tool lifting speed should not be too fast. A uniform carbon black layer can be formed between the electrode and the workpiece to maintain a stable electric erosion process, and excessive knife lifting will interfere with the formation of the carbon black layer.
The margin between electrical gauges affects processing speed and surface roughness. A small reserve amount is efficient in processing, but it will result in poor surface repair. The most ideal processing condition is that after the first condition is processed, the subsequent processing is only to repair the surface unevenness formed by the first processing condition, without destroying new materials. However, in actual processing, considering the ever-changing constraints on the discharge status, a safe reserve should be considered. The processing efficiency of the electrical standards above VDI18 mainly depends on the reserved amount between the various standards. The electrical standards of VDI18 and below, due to their weak electric corrosion ability, basically only play the role of polishing. If they are strictly in accordance with the specified If the size is processed, the time required for processing may be very long. In fact, as long as the required surface roughness is processed, the processing can be finished. Therefore, the CNC EDM machine tool can effectively control the processing efficiency through the timing processing function for the finishing standard, that is, set the modification for each electric standard. light time.
The electrical standard configuration method of the advanced CNC EDM machine tool is intelligent, and the processing data can be automatically generated through the expert system. When programming, select the material pair, application type, surface roughness value, input the processing area, processing depth, electrode size scaling, etc., and the machine tool can configure the processing electrical specification.
The translational function of the CNC EDM machine tool, the electrode is continuously processed along the target processing shape while shaking, eliminating the stagnation of the electrode, and achieving a uniform surface effect on the bottom and side of the cavity. In the processing without translation process, even a small area with a diameter of 10mm is difficult to achieve a uniform high-gloss mirror effect. The application of the translation process avoids the concentrated contact discharge of the entire electrode area, and the mirror surface processing of a larger area can be realized through periodic partial discharge.
Mirror EDM needs to choose a suitable translation method. For the mirror EDM of most cavities, the translational amount of each standard can change with the depth at the same time during the machining process, that is, the depth gradually increases, and the translational amount also increases accordingly. The bottom surface and the side are in the three-axis synchronous processing linkage method. This translation method significantly improves the stability of machining in small-energy finishing. Since the bottom surface and the side surface are processed at the same time, the timing processing function can be effectively enabled, and the surface of the side surface and the bottom surface can be achieved within the specified processing time. Even and consistent effect.
Mirror EDM adopts the process method of multi-electrode replacement. It is necessary to determine how many electrodes are used for processing according to the processing volume of the processing part. Generally, two electrodes are used for roughing and finishing. The size scaling of the roughing electrode is 0.25mm on one side, and the scaling amount of the finishing electrode is 0.15mm on a single side. When the processing area is small or the profiling accuracy is high, it is better to be smaller. The rough machining electrode adopts a large discharge energy electric standard to quickly etch a large amount of metal, and the surface can reach VDI28 (Ra2.5μm), usually leaving a margin of about 0.07mm; the finishing electrode is gradually thinned, and finally the mirror surface is processed . If the cavity has been pre-milled and the processing volume is not large, one electrode can be used to complete the mirror EDM from rough to fine.
Rough and fine machining require good electrode consistency, high manufacturing accuracy, repeated clamping and positioning accuracy for electrode replacement. High-speed milling can be used to manufacture electrodes, the positioning method using reference ball measurement, and the 3R quick clamping positioning system for repeated positioning can be used to meet high requirements.
Mirror EDM requires the use of high-purity copper as the electrode material (such as Japan's Sanbao red copper), which has good processing performance, is not prone to arc discharge, and can obtain a uniform processed surface. If the purity of the copper electrode material used is not enough, it will lead to undesired phenomena such as uneven mirror surface and local defects.
Electrodes for mirror surface processing must be finely polished (at least 1000# sandpaper) to achieve a surface with a low surface roughness. If there are ripples and defects on the electrode surface, it will not affect the visual brightness of the processed surface, but the measured surface roughness value will be greater than Ra0.2μm. In the translational EDM of the mirror surface, the micro-periodical movement of the electrode relative to the XY plane can effectively homogenize the microscopic defects on the electrode surface, which is beneficial to reduce the roughness value of the processed surface. According to processing experience, when the surface roughness of the electrode is Ra0.25μm, the mirror surface processing effect of Ra0.1μm can also be achieved.
The EDM working fluid plays the role of deionization, cooling, and elimination of electric corrosion products in the process of processing. The working fluid should be selected according to the requirements of EDM, mainly referring to the "viscosity" index, anti-oxidation performance and thermal stability are also critical, which determine the service life of the oil. Low viscosity is conducive to the flow of working fluid in the processing gap, taking away the heat generated by erosion products and processing, and making the processing gap return to normal state in time. The viscosity of the working fluid used in mirror surface EDM is recommended to be 2-3 mm2/s, and it is recommended to use EDM oil TOTAL EDM 22 (kinematic viscosity (40°C) 2.4mm2/s; flash point 102°C).
Mixed powder EDM adds a certain amount of powder particles to the working fluid. According to many technical reports on mixed powder processing at home and abroad, there are basically silicon powder, aluminum powder, magnesium powder, and graphite powder. The powder used by the FORM20 machine tool is a special powder mixing solution researched by GF Processing Solutions. Its main components are graphite particles, and the approximate length is less than 8um. The concentration of powder added is required to be 2g/L. It is necessary to filter and clean the spark oil before changing the powder mixing liquid every time, filter out the residue and invalid powder in the processing process, and then add the powder mixing solution to ensure that the processing effect can be used for 350 to 400 hours. When the concentration of the powder mixing solution is not enough, the glossy surface cannot be achieved in a short period of time. When the concentration of the powder mixing solution is too high, the sharp edges and corners of the cavity will be destroyed, and there will be more powder between the electrode and the workpiece. It will cause a short circuit of the flow, and in severe cases, it will cause scars on the surface of the workpiece.
Flourish reminds you that CNC EDM generally adds side flushing fluid with appropriate pressure on the basis of oil-immersion machining to help effectively remove chips and improve machining efficiency. However, the side flushing fluid cannot be used in the mirror EDM process, and the working fluid can be set to a slight circulation state. The scouring force of the side flushing liquid will destroy the carbon black layer between the electrode and the workpiece, which will affect the uniformity of the mirror surface processing effect.
During the process of mirror surface EDM, it can often maintain a high and stable discharge state, and generally there will be no carbon deposits. Therefore, try not to stop the machine during the processing, especially do not clean the powder layer on the workpiece processing surface, otherwise it will be serious. Affect the continuation of the mirror processing process.
When the mirror surface effect is found to be unsatisfactory after processing, do not remove the workpiece, and use the mirror electric gauge for finishing. During intensive repair, it will be found that the discharge state is very unstable, and the electrodes cannot enter the overall discharge state for a long time. In this case, first use an air gun to completely blow off the carbon black layer on the electrode and workpiece surface, then start the discharge, and increase the voltage value to 200V~250V. After the voltage is increased, the discharge breakdown becomes easier. In just a few minutes, it can be observed that the discharge state changes from partial discharge to overall discharge, and then the voltage value returns to the normal value. In this process, a carbon black layer will be regenerated between the electrode and the workpiece, and it is necessary to wait for a uniform carbon black layer to appear in the entire processing part before the processing can be completed.
After in-depth analysis by flourish legend, by selecting the appropriate workpiece material, high-purity copper electrode material, low-viscosity working fluid, using the appropriate translation process, and rationally configuring the process database, the processing process is correctly handled, and these key points are grasped. In the application link, high-quality and high-efficiency mirror EDM can be effectively realized, which has important guiding significance for improving the technological level of mold manufacturing enterprises.
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