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Die discharge machining, copper electrode and graphite electrode which is better?

Jun 12, 2019

In the mold processing, precision discharge is an indispensable process and a very important process. It is well known that the electrode must be used for discharge. The electrode has two common materials, one copper and one graphite. Then we Let's take a look at the difference between copper and graphite as electrodes!

 

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

Today, in Europe, where manufacturing technology is leading, more than 90% of the electrode materials used in mold companies use graphite. But in China, most mold companies still use copper as the main electrode material. According to the characteristics of graphite materials, the advantages and disadvantages of EDM are analyzed below.

 

1. EDM characteristics of graphite materials

(1) Discharge machining speed

Graphite is a non-metallic material with a very high melting point that can withstand larger current setting conditions. When the discharge area and the electrode size are larger, the superiority of the high efficiency rough processing of the graphite material is more remarkable. The thermal conductivity of graphite is 1/3 of that of copper. The heat generated during discharge can be used to remove metal materials more effectively. Therefore, in medium and fine processing, the processing efficiency is higher than that of copper electrodes. In general, under the correct conditions of use, the electrical discharge processing speed of the graphite electrode is 1.5 to 2 times faster than the overall copper electrode.

 

(2) Electrode loss

Graphite electrodes have the characteristics of being able to withstand large current conditions. In addition, under suitable roughing setting conditions, the carbonaceous steel workpieces are processed during the etching and the working fluid is produced at high temperatures. Under the action of the polarity effect, some of the etchants and carbon particles adhere to the surface of the electrode to form a protective layer, which ensures that the loss of the graphite electrode in the rough processing is extremely small, even "zero loss". The main electrode loss in EDM is from roughing. Although the wear rate is higher, the machining allowance is higher, but the machining allowance is less, the machining loss is less, and the total loss is less. . In general, the graphite electrode will lose less than the copper electrode in roughing of large currents; the loss in finishing may be slightly larger than that of the copper electrode.

 

(3) Surface quality

The particle diameter of the graphite material directly affects the surface roughness of the EDM, and the smaller the diameter, the lower the surface roughness value. A few years ago, using graphite materials with a particle diameter of φ5 μm, the surface of EDM can only reach VDI18 (Ra 0.8 μm). Nowadays, the particle diameter of graphite materials can reach φ3 μm, and the surface of EDM can be stably reached VDI12. (Ra 0.4 μm) or a finer level.

The copper material has low resistivity, compact structure, easy to obtain stable processing state by EDM finishing, stable processing under difficult conditions, surface roughness of less than Ra 0.1 μm, and mirror electric discharge machining . It can be seen that if the electrical discharge machining pursues an extremely fine surface, it is more suitable to use a copper material as the electrode, which is the main advantage of the copper electrode compared to the graphite electrode. However, under the condition of large current setting, the surface of the electrode is easily roughened and even cracked, but the graphite material has no such problem. The cavity with the surface roughness requirement of VDI26 (Ra2.0 μm) is required. Processing, using a graphite electrode to complete the process from coarse to fine, to achieve a uniform pattern effect, the surface will not be defective? In addition, due to the difference in the structure of graphite and copper, the corrosion point of the surface discharge of the graphite electrode is more regular than that of the copper electrode. Therefore, when the same surface roughness of VDI20 and above is processed, the surface of the workpiece processed by the graphite electrode is more distinct. The grain surface effect is better than the discharge surface effect of the copper electrode.

 

(4) Machining accuracy

The thermal expansion coefficient of the graphite material is small, and the thermal expansion coefficient of the copper material is four times that of the graphite material. Therefore, in the electrical discharge machining, the graphite electrode is less likely to be deformed than the copper electrode, and a more stable and reliable processing precision can be obtained. Especially in the processing of deep and narrow ribs, the local high temperature tends to bend and deform the copper electrode, but the graphite electrode does not. For a copper electrode with a large aspect ratio, it is necessary to compensate for a certain thermal expansion value during processing. The size is corrected, and the graphite electrode is not required.

 

(5) Electrode weight

The graphite material has a lower density than copper, and the same volume of graphite electrode weighs only 1/5 of the copper electrode. It can be seen that the larger volume of the electrode is very suitable for the use of graphite material, which greatly reduces the load on the main shaft of the EDM machine. The electrode does not cause inconvenient clamping and yaw displacement during processing due to the large weight, which can be seen in the processing of large molds. It makes sense to use a graphite electrode.

 

(6) Difficulty in electrode fabrication

The mechanical properties of graphite materials are 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 2-6 times that of copper electrodes. The graphite electrode is easy to clear the angle, and the workpiece which is usually completed by a plurality of electrodes can be designed as a single electrode to be processed. The unique granular structure of the graphite material makes the electrode not burr after milling, which is not convenient for removing the burr in complex modeling. It directly eliminates the requirement of polishing the electrode 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 milling graphite, so the milling machine must have a sealing and vacuuming device. If you need to use graphite wire to cut graphite electrodes, the processing performance is not as good as copper, and the cutting speed is about 40% slower than copper.

 

(7) Electrode installation and use

The graphite material has good adhesion, and the method of welding the graphite and the jig by the conductive adhesive can be used to mill the electrode and discharge machining, thereby eliminating the process of processing the screw hole on the electrode material, thereby saving working time. Graphite materials are relatively brittle, especially small narrow and long electrodes, which are easy to break when subjected to external force during use, but it is immediately known that the electrodes are damaged. If it is a copper electrode, it will only bend and will not break. This condition is very dangerous and difficult to find during use, and it is easy to cause the workpiece to be scrapped.

 

(8) Price

Copper materials are non-renewable resources, the price trend will become more and more expensive, and the price of graphite materials tends to be stable. The price of copper materials has been rising in recent years, and major graphite manufacturers are constantly improving the process of making graphite to make it more cost-effective.

 

Different suppliers of the world's leading graphite suppliers currently have a variety of different grades of graphite to choose from. Usually classified according to the average particle diameter of the graphite material, the particle diameter ≤ φ 4 μm is defined as fine graphite, the particles are defined as medium graphite at φ 5 φ 10 μm, and the coarse particles are defined at 10 μm or more. The smaller the particle diameter, the more expensive the material, and the appropriate graphite material can be selected according to the EDM requirements and cost.

 

In summary, among the eight EDM characteristics of graphite electrodes, the advantages are obvious: 

Milling electrode efficiency is significantly better than copper electrode;

EDM processing efficiency is better than copper electrode;

The large electrode is light and very suitable;

Good dimensional stability, the sheet electrode is not easily deformed;

Milling electrodes have no burrs, automation is preferred.

Die discharge machining, copper electrode and graphite electrode which is better


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