Application of Graphite Electrode in Die Making EDM
1 Introduction
In the 20th century, copper was widely used as an electrode material in electric discharge machining. At that time, graphite materials would only be considered as an alternative if large electrodes were to be fabricated and mold processing requirements were not high or rough machining. Some early-experienced graphite electrode technicians had the general impression that the graphite material was dirty, it was easy to drop the slag, the surface was not good, and the processing efficiency was low. With the development of high-tech, the manufacturing process of graphite materials has been continuously improved, and graphite materials that can meet different EDM requirements have emerged in endlessly. Graphite high-speed milling machine tools have emerged on the market, and the graphite discharge performance of CNC EDM machines has also become comprehensive. Promote. Today, in Europe where manufacturing technology is leading, mold materials use more than 90% of the electrode materials in graphite. The graphite, as an electrode material, has been widely used in molds for aerospace, automobiles, home appliances, and electronics. This remarkable change can be attributed to the many advantages of graphite electrodes such as high efficiency and high-quality processing. However, in China, most mold companies still use copper as the main electrode material. According to the characteristics of graphite materials, the following comprehensive analysis of its advantages and disadvantages of EDM, describes how to correctly choose the main points of graphite materials, electrode manufacturing and EDM, and provide practical applications.
2. Electrical discharge characteristics of graphite materials
2.1 discharge processing speed
Graphite is a non-metallic material with an extremely high melting point of 3,650°C and a melting point of 1,083°C, so the graphite electrode can withstand more current setting conditions. When the discharge area and the size of the electrode are scaled, the superiority of high-efficiency roughing of the graphite material is more significant. The thermal conductivity of graphite is 1/3 that of copper. The thermal energy generated during discharge can be used more effectively to remove metal materials. Therefore, the processing efficiency of graphite is higher than that of copper electrodes in medium and fine processing. According to the processing experience, under the correct use conditions, the discharge processing speed of the graphite electrode is 1.5 to 2 times faster than that of the copper electrode.
2.2 Electrode loss
Graphite electrodes have the characteristics of being able to withstand high current conditions. In addition, under appropriate rough setting conditions, carbon-containing steel workpieces produce carbon at the time of processing and decomposing carbon produced in working fluids at high temperatures. Particles, under the effect of polarity, some of the erosions, carbon particles will adhere to the electrode surface to form a protective layer, ensuring that the graphite electrode in the roughing of the loss is minimal, even "zero loss." The main electrode loss in EDM is from rough machining. Although the wear rate of the finishing conditions is higher, the spare machining allowance is not much, that is, the machining erosion is small, and the total loss is also less. . In general, the graphite electrode will have less loss than the copper electrode in the rough machining of large current, and the loss may be slightly larger than that of the copper electrode in the finishing process, and the electrode loss of both electrodes is equivalent.
2.3 Surface quality
The particle diameter of the graphite material directly affects the surface roughness of EDM machining, and the smaller the diameter, the lower the surface roughness value. A few years ago, using a graphite material with a particle diameter of φ5 μm, the best surface for electric discharge machining could only reach VDI18 (Ra0.8 μm). Nowadays, the particle diameter of graphite material can reach within φ3 μm, and the best surface for EDM machining. Stable to reach VDI12 (Ra0.4 μm) or finer grade, but the graphite electrode can not be mirror EDM. The copper material has a low resistivity and a compact structure. The EDM finish is easy to obtain a stable machining state. It can also be stably processed under difficult conditions. The surface roughness can be less than Ra0.1 μm, and mirror electric discharge machining can be performed. . It can be seen that if the electric discharge machining pursues an extremely fine surface, it is more appropriate to use a copper material as the electrode, which is the main advantage of the copper electrode over the graphite electrode. However, under the condition of high current setting, the copper electrode may become rough and even cracked. However, the graphite material does not have such a problem. For a cavity with a surface roughness requirement of about VDI26 (Ra 2.0 μm) Processing, using a graphite electrode to complete the process from coarse to fine, to achieve a uniform texture effect, the surface will not have defects. In addition, due to the difference in the structure of graphite and copper, the corrosion point on the surface of the graphite electrode discharge is more regular than that of the copper electrode. Therefore, when processing the same surface roughness of VDI20 and above, the particle surface of the workpiece surface processed with the graphite electrode is more distinct. The grain surface effect is better than the discharge surface effect of the copper electrode.
2.4 Processing accuracy
The coefficient of thermal expansion of the graphite material is small, and the coefficient of thermal expansion of the copper material is four times that of the graphite material. Therefore, the graphite electrode is less likely to be deformed than the copper electrode in the electric discharge machining, and more stable and reliable machining accuracy can be obtained. Especially in the processing of deep-narrow ribs, the local high temperature tends to cause bending deformation of the copper electrode, while the graphite electrode does not; for the copper electrode with a large depth-to-diameter ratio, it is necessary to compensate for a certain thermal expansion value during processing. Fix size while graphite electrode does not need.
2.5 electrode weight
Graphite materials are less dense than copper, and the same volume of graphite electrode weighs only 1/5 of the copper electrode. It can be seen that the use of graphite material for the larger electrode is very suitable, which greatly reduces the load on the spindle of the EDM machine tool. The electrode does not cause problems such as inconvenient clamping and displacement during machining due to heavy weight. It can be seen in the large-scale mold processing. It makes sense to use graphite electrodes.
2.6 electrode production difficulty
The mechanical processing performance of graphite materials is good, and the cutting resistance is only 1/4 of that of copper. Under the correct processing conditions, the efficiency of milling graphite electrodes is 2 to 3 times that of copper electrodes. The graphite electrode is easy to clear the corners, and the workpiece that is usually completed by a plurality of electrodes can be designed as an integral electrode to be processed. The unique grain structure of graphite material makes the electrode burr-free after milling, and it is not easy to remove the burr for complicated styling. It directly eliminates the need for artificial electrode polishing and avoids the shape change caused by polishing. , size error, etc. It should be noted that since graphite is a dust deposit, a large amount of dust is generated when the graphite is milled. Therefore, the milling machine must have a sealed and dust-collecting device. If it is necessary to use a wire electrodeion cutting graphite electrode, its processing performance is not as good as copper material, and the cutting speed is about 40% slower than that of copper.
2.7 electrode installation and use
The graphite material has good adhesion, and the method of bonding the graphite and the jig with a conductive adhesive can be used to mill the electrode and perform the electric discharge machining, and the process of processing the screw hole on the electrode material can be omitted, and the working time can be saved. Graphite materials are relatively brittle, especially small, narrow and long electrodes, which are easily broken when subjected to external forces during use. However, it is immediately known that the electrodes have been damaged. If it is a copper electrode, it will only bend without breaking. This condition is very dangerous and difficult to find during use, and it is easy to cause the workpiece to be scrapped.
2.8 price
Copper materials are non-renewable resources, and the price trend will be more and more expensive, while the price of graphite materials tends to be stable. In recent years, the price of copper materials has continuously increased, and major graphite manufacturers have continuously improved the process of making graphite to make them more cost-effective. Today, the price of isotropic graphite materials is lower than that of copper materials. Under the same volume, general-purpose graphite electrodes The price of the material is 30%~50% lower than the price of the copper electrode material, which is equivalent to the use of ordinary copper material to obtain high-quality fine graphite material.
In summary, among the 8 EDM features of the graphite electrode, the advantages are obvious: the milling electrode and the EDM are significantly better than the Cu electrode. The large electrode has a small weight and good dimensional stability. The thin-film electrode is not prone to occur. Deformed, the surface texture is better than copper and the price is lower. The disadvantage of graphite materials is that it is not suitable for fine surface discharge machining below VDI12 (Ra0.4 μm), and the efficiency of using electrodes for wire cutting is low. However, from an actual point of view, an important reason for the inefficient use of graphite materials in the country is that milling electrodes require special graphite processing machines. This places new demands on the processing equipment of mold companies. Some small companies may not have the necessary requirements. This condition. In general, the advantages of graphite electrode properties cover most of the EDM machining applications and are worthy of promotion and application. The long-term benefits are considerable. The deficiency of fine surface processing can be compensated by using copper electrodes.
3. Selection of graphite electrode materials for EDM
For graphite materials, the following four indicators directly determine the performance of the material:
(1) The average particle diameter of the material. The average particle diameter of the material directly affects the discharge of the material. The smaller the average particle size of the graphite material, the more uniform the discharge of the material, the more stable the discharge condition, the better the surface quality and the smaller the loss; the larger the average particle size, the better the removal rate can be obtained in the roughing process, but the finish The surface effect is poor and the electrode loss is large.
(2) The flexural strength of the material. The flexural strength of a material is a direct reflection of the strength of the material and shows the tightness of the internal structure of the material. High-strength materials have relatively good discharge loss resistance properties. For electrodes with high accuracy requirements, materials with better strength should be selected as much as possible.
(3) Shore hardness of the material. The hardness of graphite is higher than that of metal materials, and the loss of the tool during cutting is larger than that of cutting metal. At the same time, the graphite material with high hardness is superior in control of discharge loss.
(4) Intrinsic resistivity of the material. Graphite materials with a higher intrinsic resistivity will discharge at a slower rate than the resistivity. The higher the intrinsic resistivity, the smaller the electrode loss. However, when the intrinsic resistivity is high, the stability of the discharge is affected.
At present, different suppliers in the world-known graphite suppliers have a variety of different grades of graphite to choose from. Generally classified according to the average particle diameter of the graphite material, the particle diameter ≤ φ4 μm is defined as fine graphite, and the particle is defined as φ5 to φ10 μm as medium graphite, and the particle is defined as coarse graphite above 10 μm. The smaller the particle diameter is, the more expensive the material is, and the appropriate graphite material can be selected according to the EDM processing requirements and costs.
4. Production of graphite electrodes
Graphite electrodes in mold making are mainly produced by milling. From the processing point of view, graphite and copper are two different materials, and they should be able to grasp their different cutting characteristics. If a graphite electrode is processed using a process of processing a copper electrode, problems will inevitably occur, such as frequent breakage of the sheet, which requires the use of suitable cutting tools and cutting parameters. The graphite electrode is more wear than the copper electrode. For economic reasons, it is economical to use a carbide tool. The diamond-coated tool (called a graphite knife) is expensive, but the diamond-coated tool has a long service life. High processing accuracy, good overall economic benefits. The rake angle of the tool also affects the service life. The tool with 0° rake angle will have a service life up to 50% longer than the tool with rake angle of 15°. The stability of cutting is also good, but the larger the angle of inclination, the machining The better the surface, the best tooling surface can be achieved with a 15° angled tool. The processing speed can be adjusted according to the shape of the electrode, usually 10m/min. Similar to machining aluminum or plastic, the tool can be directly on and off the workpiece when roughing, and it is easy to cause chipping and chipping during finishing. Often use light knife to go fast way.
Graphite electrodes generate a large amount of dust during cutting. In order to prevent graphite particles from inhaling the machine tool spindle and screw, there are currently two solutions. One is to use a special graphite processing machine, and the other is to use an ordinary machining center. Modified, equipped with a special vacuum device. Special graphite high-speed milling machine on the market has high milling efficiency and can easily manufacture complex electrodes with high electrode accuracy and good surface quality.
If you need to use spark wire cutting to make graphite electrodes, finer graphite materials with smaller particle diameters are recommended. The performance of WEDM of graphite materials is poor, and the smaller the particle diameter, the higher cutting efficiency can be obtained, and frequent problems such as broken wires and surface stripes can be avoided.
5. Electrode machining parameters of graphite electrodes
There is a big difference in the selection of EDM parameters for graphite and copper materials. EDM parameters mainly include current, pulse width, pulse gap, and polarity. The following is the basis for the rational use of these main parameters.
The current density of the graphite electrode is generally 10~12 A/cm2, which is much larger than that of the copper electrode. Therefore, the larger the current is in the allowable current range of the corresponding area, the faster the graphite discharge processing speed is and the smaller the electrode loss is. , but the surface roughness will become thicker.
The greater the choice of pulse width, the lower the electrode loss. However, a larger pulse width degrades the processing stability, slows the processing speed, and roughens the surface. In order to ensure low loss of the electrode during rough machining, a larger pulse width is generally used. When the value is 100 to 300 μs, low loss processing of the graphite electrode can be effectively achieved. When finishing, a smaller pulse width should be used to obtain a fine surface and a stable discharge effect. Generally, the width of the pulse selected by the graphite electrode is reduced by about 40% compared with the copper electrode.
The pulse gap mainly affects the discharge machining speed and processing stability. The larger the value is, the better the processing stability is, and the better the surface uniformity is, but the processing speed is reduced. In the case of ensuring stable processing, a smaller pulse gap can be used to obtain higher processing efficiency, but when the discharge state is unstable, a larger pulse gap can be selected to obtain higher processing efficiency. In the graphite electrode discharge machining, the pulse gap and the width of the pulse width are usually set at 1:1, and usually the pulse gap and the width of the pulse width are set at 1:3 in the copper electrode processing. In the case of stable graphite processing, the pulse gap and pulse width can be adjusted to 2:3. In the case of a small pulse gap, the formation of a coating layer on the surface of the electrode is helpful to reduce the electrode loss.
The polarity of the graphite electrode EDM is basically the same as that of the copper electrode. According to the polarity effect of EDM, the positive polarity processing is usually used when processing tool steel, that is, the electrode is connected to the positive electrode of the power supply, and the workpiece is connected to the negative electrode of the power supply. Using a larger current and pulse width, a positive polarity processing can be used to obtain extremely low electrode loss effects. If the polarity is wrongly selected, the electrode loss becomes very large. Only when the surface requirements are less than VDI18 (Ra0.8μm) fine processing, the use of a very small pulse width, the use of negative polarity processing to obtain a good surface quality, but the electrode loss is greater.
Current CNC EDM machines are equipped with graphite discharge machining parameters. The use of electrical parameters is intelligent and can be automatically generated by the machine's expert system. Usually, the material pair, application type, surface roughness value, input processing area, processing depth, and electrode size scaling amount are selected during programming, and the machine tool can configure optimized processing parameters. The machine tool has a rich processing parameter library for the electric discharge machining of graphite electrodes. The material types can be selected from coarse graphite, medium graphite, and fine graphite corresponding to a variety of workpiece materials. The application types can be subdivided into standards, deep grooves, sharp points, and large areas. , Large cavity, fine and so on, it also provides low-loss, standard, high-efficiency and other processing priority options.
6. Graphite electrode processing application case
As shown in the figure below, the material of the mobile phone mold part S136 is a steel material, and the processed shape includes a rib with a complex shape. Graphite electrodes were used, and TTK-4 graphite was used as the finishing electrode. The number of electrodes was 2. The size of the rough electrode was unilaterally 0.08 mm, and the size of the finished electrode was unilaterally 0.08 mm. Roughing time 3H, finishing time 53 min, surface roughness VDI18 (Ra 0.8 μm). Processing instructions: The use of graphite characteristics of different shapes, a variety of thin high-strength ribs combined in a single electrode for discharge machining, can reduce the input cost of electrode materials, fixtures, reduce the number of electrode mounting and positioning workload, discharge The time is 40% shorter than using copper electrodes.
7. Conclusion
The new graphite electrode material is worthy of promotion, and its superiority will gradually be recognized and accepted by the domestic mold manufacturing industry. The correct selection of graphite electrode materials, together with the improvement of related process steps, will bring high efficiency, high quality, and low cost benefits to mold manufacturing companies.
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