As an electrode material, copper is widely used in EDM in mold factories. Many people believe that graphite materials will only be considered as an alternative when making large electrodes and the mold processing requirements are not high or it is the 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 is the comprehensive analysis of the advantages and disadvantages of EDM.
(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 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 processing experience, under the correct conditions of use, the EDM speed of the electrode de graphite is 1.5 times faster than that of the copper electrode as a whole.
(2) Electrode loss
Graphite electrodes have the characteristics of being able to withstand high current conditions. In addition, under suitable rough machining conditions, there will be corrosion in the processing of carbon containing steel workpiece and carbon particles in the decomposition produced by the working fluid at high temperature. Under the effect of the polarity effect, part of the corrosion 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 during rough processing is extremely small, even "zero loss".
The main electrode wear in EDM comes from rough machining. Although the wear rate under 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 loss of graphite electrode during rough machining with high current is less than that of copper electrode, and the loss during finishing machining may be slightly larger than that of 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, and the smaller the diameter, the lower the surface roughness value can be obtained. A few years ago, using graphite materials with a particle diameter of φ5μm, the best surface for EDM can only reach VDI18 (Ra0.8μm). Nowadays, the particle diameter of graphite materials can reach within φ3μm, and the best surface for EDM can reach VDI12 (Ra0.4μm) or a finer level.
The resistivity of copper material is low, and the structure is compact. 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 Ra0.1μm, 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 the electrode, which is the main advantage of copper electrode over graphite electrode. 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 the cavity processing in which surface roughness is required to be about VDI26 (Ra2.0μm), one graphite electrode can be used to 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 are more regular than that of the copper electrode. Therefore, when processing the same surface roughness of VDI20 and above, the surface particle size of the workpiece processed by the graphite electrode 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 materials is small, and the thermal expansion coefficient of copper materials is 4 times that of graphite materials. Therefore, in electrical discharge machining, graphite electrodes are 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 is likely to cause the copper electrode to bend and deform, while the graphite electrode does not. For copper electrodes with a large aspect ratio, a certain thermal expansion value needs to be compensated during processing settings so as to correct the size, which is not required for graphite electrodes.
(5) Electrode weight
The density of graphite material is lower than that of copper, and the weight of graphite electrode 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 on the spindle of the EDM machine tool. The electrode will not cause problems such as inconvenient clamping and deflection or displacement during processing due to the heavy weight. It makes sense to use graphite electrodes in mold processing.
(6) Difficulty in making electrodes
The machining performance of graphite materials is good, and the cutting resistance is only 1/4 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 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 convenient to remove burrs, eliminating the need for manual polishing of the electrode and avoiding shape changes, size error, etc. caused by polishing.
It should be noted that because graphite is a dust accumulation, and a large amount of dust will be generated when milling graphite. Therefore, the milling machine must have a sealing and dust suction device. If you need to use WEDM cutting to process graphite electrodes, its processing performance is not as good as that of copper materials, and the cutting speed is about 40% slower than that of copper.
(7) The installation and use of electrode
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 for 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.
Copper material is a non-renewable resource, and the price will become higher, while the price of graphite material tends to stabilize and the cost of graphite is stable. 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 well-known graphite suppliers in the world can provide many different grades of graphite to choose from. It is usually classified according to the average particle diameter of graphite materials. Those with a particle diameter ≤ φ4μm are defined as fine graphite, particles with a particle diameter of φ5-φ10μm are defined as medium graphite, and particles above 10μm are defined as coarse graphite. The smaller the particle diameter, the more expensive the material. The appropriate graphite material can be selected according to the requirements and cost of EDM.