In 20th century, copper was widely used as an electrode material in EDM. At that time, graphite materials would become an alternative consideration only when large electrodes were made and mold processing requirements were not high or rough processing. Some technicians who had been in contact with graphite electrodes in the early days had a common impression: graphite materials are dirty, easy to slag, surface effects are not good, processing efficiency is not high, and so on.
With development of high and new technology, manufacturing process of graphite materials has been continuously improved, graphite materials that can meet needs of different EDM are emerging in an endless stream. Graphite high-speed milling machine tools emerged on the market, graphite discharge performance of CNC EDM machine tools has also been comprehensive improved. Today, in Europe, where manufacturing technology is leading, more than 90% of electrode materials of mold companies use graphite. A wide range of molds such as aerospace, automobiles, home appliances, and electronics have adopted graphite as electrode material. This significant change is attributed to many advantages that graphite electrodes can achieve high efficiency and high quality processing.

 

But in China, most mold companies still use copper as their main electrode material. According to characteristics of graphite materials, the following comprehensive analysis of its advantages and disadvantages of EDM, introduces how to select graphite materials, main points of electrode production and electrical discharge machining, provides practical application cases.

Graphite is a non-metallic material with a very high melting point of 3650°C, while copper has a melting point of 1083°C, so graphite electrodes can withstand higher current setting conditions. The greater scaling of discharge area and electrode size, the more significant superiority of graphite material for high-efficiency rough machining. Thermal conductivity of graphite is 1/3 of that of copper, heat generated during discharge process can be used to remove metal materials more effectively. Therefore, processing efficiency of graphite is higher than that of copper electrodes in medium and fine processing. According to processing experience, under correct conditions of use, EDM speed of graphite electrodes is 1.5 to 2 times faster than that of copper electrodes as a whole.

Graphite electrodes have characteristics of being able to withstand high current conditions. In addition, carbon particles in corrosion products produced during processing of steel workpieces containing carbon elements and decomposition products produced by working fluid at high temperatures will adhere to electrode surface due to polar effect. Formation of a protective layer ensures that loss of graphite electrode during rough machining is extremely small, even "zero loss". Main electrode wear in EDM comes from rough machining. Although wear rate of fine machining setting conditions is high, machining allowance is not much, that is, machining erosion is less, and overall wear is also less. In general, loss of graphite electrodes in rough machining with high currents is less than that of copper electrodes, and loss in finishing machining may be slightly greater than that of copper electrodes. Electrode loss of the two is equivalent.

Particle diameter of graphite material directly affects surface roughness of EDM, the smaller diameter, the lower surface roughness value can be obtained. A few years ago, graphite materials with a particle diameter of φ5 μm were used, and best surface for EDM can only reach VDI18 (Ra0.8 μm). Nowadays, particle diameter of graphite materials can reach within φ3 μm, and best surface for EDM can stably reach VDI12 (Ra0.4 μm) or a finer grade, but graphite electrodes cannot be mirrored EDM. Resistivity of copper material is low, and structure is dense. EDM is easy to obtain a stable processing state, and it can be processed stably under difficult conditions. 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 extremely fine surfaces, it is more appropriate to use copper materials as electrodes, which is main advantage of copper electrodes over graphite electrodes. However, under conditions of high current setting, surface of 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 requirement of about VDI26 (Ra2.0 μm), a graphite electrode can be used to complete machining process from coarse to fine, achieving a uniform texture effect and no defects on surface. In addition, due to difference in structure of graphite and copper, corrosion points on the surface of graphite electrode are more regular than that of copper electrode. Therefore, when processing same surface roughness of VDI20 and above, surface granularity of workpiece processed by graphite electrode is more distinct, effect of this kind of grain is better than discharge surface effect of copper electrode.

 

Thermal expansion coefficient of graphite materials is small, and 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, more stable and reliable machining accuracy can be obtained. Especially when processing deep and narrow ribs, local high temperature is likely to cause copper electrode to bend and deform, while graphite electrodes do not. For copper electrodes with a large depth-to-diameter ratio, a certain thermal expansion value needs to be compensated to correct size during processing settings, while graphite electrodes do not need to.

Density of graphite material is lower than that of copper, and weight of graphite electrode with same volume is only 1/5 of that of copper electrode. It can be seen that graphite material is very suitable for larger electrode, which greatly reduces load on spindle of EDM machine tool. Electrode will not cause problems such as inconvenient clamping and yaw displacement during processing due to heavy weight. It can be seen that it is meaningful to use graphite electrodes in large-scale mold processing.

 

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

Graphite material has good adhesion. It is possible to use conductive glue to bond graphite to fixture for milling electrodes and electric discharge machining, which can save process of machining screw holes on 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 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 workpiece to be scrapped.

Copper materials are non-renewable resources, price trend will become more and more expensive, while price of graphite materials tends to stabilize. In recent years, price of copper materials has continued to rise, and major graphite manufacturers have continuously improved process of making graphite to make it more cost-effective. Now, price of general graphite electrode materials is equivalent to that of copper electrode materials under same volume. High-efficiency processing that can be achieved by graphite saves a lot of man-hours compared to use of copper electrodes, which is equivalent to directly reducing production costs.
In summary, among 8 EDM characteristics of graphite electrodes, its advantages are obvious: efficiency of milling electrodes and electrical discharge machining are significantly better than copper electrodes. Large electrode has small weight and good dimensional stability. Thin electrode is not prone to deformation, and surface texture is better than copper. Disadvantage of graphite materials is that they are not suitable for fine surface electrical discharge machining below VDI12 (Ra0.4 μm), and efficiency of using wire EDM to make electrodes is low. But from a practical point of view, an important reason that affects effective promotion of graphite materials in China is that milling electrode requires a special graphite processing machine, which puts forward new requirements for processing equipment of mold companies, some small companies may not have this condition. In general, advantages of graphite electrode characteristics cover most of processing occasions of EDM, it is worthy of promotion and application. Long-term benefits are considerable. Deficiencies of its fine surface processing can be compensated by use of copper electrodes.

For graphite materials, following 4 indicators directly determine performance of material:

Average particle diameter of material directly affects discharge status of material. The smaller average particle size of graphite material, the more uniform discharge of material, the more stable discharge condition, the better surface quality, and the smaller loss; the larger average particle size, the better removal rate can be obtained in rough machining, but effect of finishing surface is poor and electrode loss is larger.

Flexural strength of a material is a direct manifestation of strength of material, showing tightness of internal structure of material. High-strength materials have relatively good discharge resistance performance. For electrodes with high precision requirements, materials with better strength should be selected as much as possible.

Hardness of graphite is higher than that of metal materials, and tool wear during cutting is greater than that of metal. At the same time, graphite materials with high hardness have better control of discharge loss.

Graphite materials with large intrinsic resistivity have a slower discharge rate than those with low resistivity. The higher intrinsic resistivity, the smaller electrode loss, but when intrinsic resistivity is high, stability of discharge will be affected.
At present, different suppliers of world-renowned graphite suppliers have many different grades of graphite to choose from. It is usually classified according to average particle diameter of graphite materials. 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 particle diameter, the more expensive material, appropriate graphite material can be selected according to requirements and cost of EDM.

 

Graphite electrodes in mold manufacturing are mainly made by milling. From perspective of processing technology, graphite and copper are two different materials, and their different cutting characteristics should be mastered. If process of processing copper electrodes is used to process graphite electrodes, problems will inevitably occur. If flakes often break, this requires use of suitable cutting tools and cutting parameters.
Tool wear for processing graphite electrodes is larger than that of copper electrodes. In terms of economy, it is the most economical to choose carbide tools. Choosing diamond-coated tools (called graphite tools) is more expensive, but diamond-coated tools have a long service life, high machining accuracy, and good overall economic benefits. Size of rake angle of tool also affects its service life. A tool with a rake angle of 0° will have up to 50% longer service life than a tool with a rake angle of 15°. Cutting stability is also better, but the larger bevel angle, the better machined surface, and the best machined surface can be achieved by using a 15° bevel tool. Cutting speed during processing can be adjusted according to shape of electrode, usually 10m/min, which is similar to processing aluminum or plastic. During rough processing, tool can be directly moved up and down on workpiece, phenomenon of corner chipping and chipping is easy to occur in finishing process, method of light knife and fast walking is often used.
Graphite electrodes will generate a lot of dust during cutting. In order to prevent graphite particles from being sucked into spindle and screw of machine tool, there are currently two solutions. One is to use a special graphite processing machine, and the other is to modify an ordinary processing center with a special vacuum device. Specialized graphite high-speed milling machine tools on the market have high milling efficiency, can easily complete manufacture of complex electrodes, with high electrode accuracy and good surface quality.
If you need to use wire EDM to make graphite electrodes, it is recommended to use fine graphite materials with smaller particle diameters. Performance of wire EDM of graphite material is poor, and the smaller particle diameter, the higher cutting efficiency can be obtained, occurrence of abnormal problems such as frequent wire breakage and surface streaks can be avoided.

Selection of EDM parameters for graphite and copper materials is quite different. EDM parameters mainly include current, pulse width, pulse gap, and polarity. Following introduces basis for rational use of these main parameters.
Current density of graphite electrodes is generally 10~12 A/cm2, which is much larger than that of copper electrodes. Therefore, within current range allowed by corresponding area, the larger current selection, the faster graphite discharge machining speed and the smaller electrode loss, but surface roughness will become coarser.
The larger pulse width is, electrode loss will decrease. But a larger pulse width will make processing stability worse, at the same time make processing speed slower and surface rough. In order to ensure low electrode loss during rough machining, a larger pulse width is usually used. When value is 100 to 300 us, it can effectively realize low loss machining of graphite electrodes. In order to obtain a fine surface and stable discharge effect during finishing, a smaller pulse width should be selected. Generally, pulse width of graphite electrode is reduced by about 40% than that of copper electrode.
Pulse gap mainly affects electrical discharge machining speed and processing stability. The larger value, the better processing stability, which helps to obtain better surface uniformity, but processing speed will decrease. In the case of ensuring stable processing, selecting a smaller pulse gap to obtain higher processing efficiency, but when discharge state is unstable, selecting a larger pulse gap to obtain higher processing efficiency. In graphite electrode electrical discharge machining, pulse gap and pulse width are usually set at 1:1, while in copper electrode machining, pulse gap and pulse width are usually set at 1:3. When graphite processing is stable, ratio of pulse gap to pulse width can be adjusted to 2:3. When pulse gap is small, it is beneficial to form a cover layer on electrode surface, which is helpful to reduce electrode loss.
Polarity of graphite electrode EDM is basically same as that of copper electrode. According to polarity effect of EDM, positive polarity processing is usually used when processing mold steel, that is, electrode is connected to positive electrode of power supply and workpiece is connected to negative electrode of power supply. Using larger current and pulse width, choosing positive polarity processing can get extremely low electrode loss effect, and choosing wrong polarity will cause electrode loss to become very large. Only when surface requirements are less than VDI18 (Ra0.8μm) for fine processing, using extremely small pulse widths, can negative polarity processing be used to obtain better surface quality, but electrode loss is greater.
Current CNC EDM machine tools are equipped with graphite EDM parameters. Use of electrical parameters is intelligent and can be automatically generated by expert system of machine tool. Usually when programming, select material pair, application type, surface roughness value, input processing area, processing depth, electrode size scaling, etc., machine tool can configure optimized processing parameters. Machine tool has a rich processing parameter library for electric discharge machining of graphite electrodes. In material type, coarse graphite, medium graphite, and fine graphite can be selected to correspond to a variety of workpiece materials. Application types are subdivided into standard, deep groove, sharp point, large area, large cavity, fine, etc., multiple processing priority options such as low loss, standard, and high efficiency are also provided.

Material S136 steel of mobile phone mold parts shown in figure below, processed shape contains complex ribs. Graphite electrode is used, TTK-4 graphite is used for finishing electrode, number of electrodes is 2, size of roughing electrode is 0.08 mm on one side, and size of finishing electrode is 0.08 mm on one side. Roughing time is 3H, finishing time is 53 min, and surface roughness is VDI18 (Ra 0.8μm). Processing instructions: Use characteristics of graphite to combine different shapes and many thin ribs of different heights on one electrode for electrical discharge processing, which can reduce input cost of electrode raw materials and fixtures, reduce number of electrode clamping times and workload of positioning, discharge time is shorter than using copper electrode processing 40%.

 

New graphite electrode material is worthy of vigorous promotion, its superiority will gradually be recognized and accepted by domestic mold manufacturing industry. Correct selection of graphite electrode materials and improvement of related process links will bring high efficiency, high quality and low cost benefits to mold manufacturing enterprises.