graphite electrode rod

Die electrical discharge machining, which is better, copper electrode or graphite electrode rod?

Copper is widely used as an electrode material in EDM in mold factories. Many people think that graphite material will only be considered as an alternative when making large electrodes and the mold processing requirements are not high or rough processing.

Today, in Europe, where manufacturing technology is leading, more than 90% of the electrode materials of mold companies use graphite. But in China, most mold companies still choose copper as their main electrode material. According to the characteristics of graphite materials, the following comprehensive analysis of its advantages and disadvantages of EDM.

EDM characteristics of graphite materials

(1) Electric discharge machining speed

Graphite is a non-metallic material with a very high melting point and can withstand higher current setting conditions. The greater the scaling of the discharge area and electrode size, the more significant the superiority of the graphite material for high-efficiency rough machining. The thermal conductivity of graphite is 1/3 of that of copper, and the heat energy generated during the discharge process can be used to remove metal materials more effectively. Therefore, its processing efficiency is higher than that of copper electrodes in medium and fine processing. According to the machining experience, under the correct conditions of use, the electric discharge machining speed of the graphite electrode rod is 1.5~2 times faster than that of the copper electrode as a whole.

(2) Electrode loss

The graphite electrode rod has the characteristics of being able to withstand high current conditions. In addition, under the appropriate rough machining setting conditions, the corrosion products generated during the processing of carbon-containing steel workpieces and the decomposition products generated by the working fluid at high temperatures Carbon particles, under the effect of polarity, part of the ablation and carbon particles will adhere to the surface of the electrode to form a protective layer, which ensures that the loss of the graphite electrode rod during rough processing is extremely small, even “zero loss” . The main electrode wear in EDM comes from rough machining. Although the wear rate of the setting conditions for finishing machining is relatively high, the amount of machining erosion is small due to the small amount of machining allowance reserved for parts, and the overall wear is also small. . In general, the graphite electrode rod will lose less than the copper electrode in rough machining with high current, and the loss in the finish machining may be slightly greater than that of the copper electrode, and the electrode loss of the two is equivalent.

(3) Surface quality

The particle diameter of graphite material directly affects the surface roughness of EDM, the smaller the diameter, the lower the surface roughness value can be obtained. A few years ago, graphite materials with a particle diameter of φ5 μm were used, and the best surface for EDM can only reach 0.8 μm). Nowadays, the particle diameter of graphite materials can reach within φ3 μm, and the best surface for EDM can stably reach 0.4 μm) or finer grades.

The resistivity of copper material is low, and the structure is dense. EDM is easy to obtain a stable processing state, and it can be processed stably under difficult conditions. The surface roughness can be less than Ra0.1 μm, and it can be mirrored EDM. . It can be seen that if electrical discharge machining pursues an extremely fine surface, it is more appropriate to use copper as an electrode. This is the main advantage of copper electrode over graphite electrode rod. However, under the conditions of high current setting, the surface of the copper electrode is prone to become rough and even cracks, while graphite materials do not have this problem. For cavity processing with a surface roughness of about 2.0 μm, use 1 A graphite electrode rod can complete the machining process from coarse to fine, achieving a uniform texture effect, and there will be no defects on the surface. In addition, due to the difference in the structure of graphite and copper, the corrosion points on the surface of the graphite electrode rod are more regular than that of the copper electrode. Therefore, when the surface roughness is the same as above, the surface graininess of the workpiece processed by the graphite electrode rod is more distinct. This texture effect is better than the discharge surface effect of copper electrodes.

(4) Machining accuracy

The thermal expansion coefficient of graphite material is small, and the thermal expansion coefficient of copper material is 4 times that of graphite material. Therefore, in electrical discharge machining, graphite electrode rod is less prone to deformation than copper electrodes, and more stable and reliable machining accuracy can be obtained. Especially when processing deep and narrow ribs, local high temperature will easily cause the copper electrode to bend and deform, while the graphite electrode rod will not be like this; for copper electrodes with a large depth-to-diameter ratio, a certain thermal expansion value needs to be compensated when processing settings To correct the size, the graphite electrode rod does not need.

(5) Electrode weight

The density of graphite material is lower than that of copper, and the weight of graphite electrode rod with the same volume is only 1/5 of that of copper electrode. It can be seen that the graphite material is very suitable for the larger electrode, which greatly reduces the load of the EDM machine tool spindle. The electrode will not cause problems such as inconvenience in clamping and yaw displacement during processing due to the heavy weight. This can be seen in the processing of large molds. It makes sense to use graphite electrode rod in this article.

(6) Difficulty in making electrodes

The machining performance of graphite material is good, and the cutting resistance is only 1/4 of that of copper. Under the correct processing conditions, the efficiency of milling graphite electrode rod is 2 to 3 times that of copper electrode. The graphite electrode rod is easy to clean the corners, and the workpiece that is usually completed by multiple electrodes can be designed as a whole electrode for processing. The unique particle structure of the graphite material prevents burrs from being produced after milling and shaping of the electrode. It directly meets the requirements for complex shapes that are not easy to remove the burrs, eliminating the need for manual polishing of the electrode and avoiding the shape change caused by polishing. , Size error, etc. It should be noted that because graphite is a dust accumulation, a large amount of dust will be generated when milling graphite, so the milling machine must have a sealing and dust suction device. If you need to use EDM wire cutting to process the graphite electrode rod, its processing performance is not as good as that of copper material, and the cutting speed is about 40% slower than that of copper.

(7) Electrode installation and use

The graphite material has good cohesiveness. The electrode can be milled and the electric discharge machining can be used to bond the graphite to the fixture with conductive glue, which can save the process of machining screw holes on the electrode material and save working time. Graphite materials are relatively fragile, especially small, narrow and long electrodes, which are easy to break when subjected to external force during use, but it can be immediately known that the electrode is damaged. If it is a copper electrode, it will only bend and not break. This situation is very dangerous and difficult to find during use, and it is easy to cause the workpiece to be scrapped.

(8) Price

Copper material is a non-renewable resource, and the price trend will become more and more expensive, while the price of graphite material tends to stabilize. In recent years, the price of copper materials has continued to rise, and major graphite manufacturers continue to improve the process of making graphite to make it more cost-effective.

At present, different suppliers of world-renowned graphite suppliers have many different grades of graphite to choose from. Graphite materials are usually classified according to the average particle diameter. Those with a particle diameter ≤ φ4 μm are defined as fine graphite, particles with a particle diameter of φ5~φ10 μm are defined as medium graphite, and particles above 10 μm are defined as coarse graphite. The smaller the particle diameter, the more expensive the material, and the appropriate graphite material can be selected according to the requirements and cost of EDM.

In summary, among the 8 EDM characteristics of graphite electrode rod, its advantages are obvious:
The efficiency of milling electrode and electrical discharge machining is significantly better than that of copper electrode, and the production efficiency is high;
Large electrodes are very suitable;
The dimensional stability is good, and the thin electrode is not easy to deform;
Disadvantages of graphite materials:
Milling electrodes require a special graphite processing machine
Not suitable for fine surface electrical discharge machining below 0.4 μm);
The superiority of graphite electrode rod materials will gradually be recognized and accepted by the domestic mold manufacturing industry. The correct selection of graphite electrode rod materials and the improvement of related process links will bring high efficiency and low cost benefits to mold manufacturing enterprises.