There has been a long-standing, unresolved debate over whether graphite or copper is a better EDM electrode material. The preference is most always the same in any one geographical region. In North America , the preferred electrode material has shifted from copper to graphite. In Europe and Asia, many argue that copper is the preferable material. Perhaps it’s time to take a closer look at both materials to identify the differences and help you select the one that is most appropriate for your EDM applications.
Electrode Material Considerations
Material variety. Graphite is produced with a wide range of material characteristics, enabling you to match the electrode material properties to the EDM application. Less-critical applications with electrode features such as a large radius, an open tolerance or minimal EDM requirements would use an economically-priced graphite material with large particles and lower strengths. However, a highly detailed electrode with critical features, extreme tolerances and stringent EDM requirements would entail a more premium graphite to fit the needs of this application.
On the other hand, due to the high purity value required for efficient EDMing, the types of copper available on the market and used in EDM applications are limited. This minimizes the ability to match material characteristics to the EDM application. The most commonly used types are electrolytic copper and tellurium copper, which vary slightly in elemental composition and are both cast as a solid.
Cost. Copper is often considered a commodity material and can be less costly than graphite in the bulk or blank stage. Many graphite materials are considered specialty materials and, as such, can be more costly. In addition, copper is recyclable whereas graphite is limited in its ability to be reclaimed. However, with the wide range of graphite materials available on the market today, it is possible to find some low-quality EDM grades that are more economical than copper. To establish the true cost of copper versus graphite, the value of machining the electrode must also be considered. Even with the more expensive graphite materials, the machining costs often offset any savings that are realized with the copper.
For example, a quote was recently solicited for a simple electrode blank with a ground finish on the top and bottom. The quote was received with copper at $4.68 per cubic inch, while a premium grade of graphite was quoted at $6.80 per cubic inch, or 45 percent more costly. However, when the cost of machining a simple, finished electrode was included, the story changed. In this case, the finished graphite electrode was quoted at $15.50 each, whereas the copper electrode (“oxygen free”) was quoted as $95 each. Due to the soft, ductile characteristic of copper, it is often gummy, and conventional machining practices (such as feeds and speeds) must be altered to accommodate. This can be mitigated with tellurium copper, but EDM performance may be jeopardized. On the other hand, graphite is not gummy and can be conventionally machined very easily. However, machining graphite generates significant dust, which must be collected through an effective vacuum system with HEPA-type filtration.
Detail. In EDM applications, the electrical current tends to concentrate at sharp edges and corners. Copper’s low melting point does not allow it to handle current density as effectively as graphite, so features on a single copper electrode must be similar in detail. It is not wise to EDM with varying details on the same copper electrode. This is because the higher current required to efficiently EDM larger electrode details exceeds the current allowable for the smaller detail.
With copper electrodes, high current density on smaller, more intricate detail increases the electrode wear. Also, when machining intricate detail or EDMing at high current densities, a copper electrode may move or flex as material stress is relieved. This limits the ability to maintain the very tight tolerances required on many of today’s molds. Graphite performs very well at a high current density even with complex geometry. Graphite electrodes allow for efficient EDMing, even with varying details on the same electrode. Therefore, the number of graphite electrodes required to perform a job can be significantly reduced.
EDM Performance
Metal removal rates. An electrode material’s thermophysical properties determine its ability to process the energy of the EDM cut and remove metal. In generating a spark, peak current is discharged only after the gap between the electrode and workpiece has been ionized. At this point, the electrode emits electrons that collide with the molecules of the dielectric fluid. As a result, the fluid is vaporized and an energy channel is formed that allows the spark to take place. For this to happen, the electrode material must be hot enough for electrons to absorb enough energy to escape and work to create the energy channel.
For copper electrodes to release electrons in the gap, the temperature must be high enough for these electrons to absorb sufficient energy.1 As a result, these high temperatures tend to burn some of the copper electrode away. In order to generate this heat, the on times for copper electrodes are generally much higher than for graphite electrodes. However, due to its carbon base, a graphite electrode is able to emit these electrons at much lower temperatures and does not require the extended on times for electrons to release and create the energy channel. Therefore, the time required to form the energy channel is considerably less. Since the graphite initializes the spark faster, significantly higher metal removal rates are the result.
Wear. Electrode wear is a constant concern because excessive wear results in adding electrodes or redressing electrodes more often. Graphite is able to achieve electrode wear of less than 1 percent in relation to the depth of cut at machine parameters much more aggressive than copper electrodes. This is achieved through a “replating” process during which molten particles from the workpiece may penetrate into the structure of the graphite electrode and reduce the amount of wear caused by the EDM process. The melting temperature of most standard work metals being EDMed is around 1,500°C. This exceeds that of copper, which is around 1,100°C. Any molten particles being ejected out of the EDM cut are likely to carry a thermoelectric charge and have temperatures that do not permit them to affix to the copper electrode. In this case, the molten particles may actually create secondary discharging and erode the copper electrode away.
This means that the high amperage and long on times of a roughing condition (and the process of replating) actually preserve the graphite electrode but is detrimental to the copper electrode which erodes away at these settings. On the contrary, in the finishing stages, with low amperage and short on-times, the graphite electrode has a tendency to wear at a faster rate than copper. However, since electrode wear is a ratio of the amount of material removed in the EDM cut, the actual wear on either a graphite or copper electrode is minimal in the finishing stage and sometimes immeasurable.
Surface finish. Because it is cast as a solid with no porosity, it goes without saying that copper electrodes provide very fine surface finishes. However, with the sophistication of today’s EDM sinker technology, the surface finish gap between graphite and copper has narrowed significantly. Fine-grain graphite electrodes are now able to deliver surface finishes similar to copper with comparable electrode wear. With the proper electrode material selection and machine parameters, graphite is able to achieve near mirror finishes without the use of a powder additive and mirror-like finishes with the additive.
Conducting test burns and tracking results will help determine the monetary impact that the electrode material has on an EDM application. Part 2 of this article will review the results of EDM testing conducted with both graphite and copper, and reveal the bottom-line cost for each. With all of this information, you will then be able make an informed decision on the electrode material type best suited for a specific EDM application.
Choosing the electrode material depends on the type of mold:
1) How to choose the electrode material for “middle-sized or large-sized mold of plastic / die-casting”
The element made much of is machining efficiency at the time of the electrode production and removal rate at the time of the electric discharge machining. Graphite is used as the electrode material for this reason. Many kinds of graphite for electrodes are widely used. As for the high quality type of small graphite particle size, the finishing quality at the time of EDM is good. On the other hand, machining efficiency at the time of electrode production by milling is extremely bad. For this reason, average quality graphite of relatively large particle size is used for molds of the middle and large size. Also, graphite has an advantageous characteristic of less bending at EDM even for the thin electrode for reinforcement area.
2) How to choose the electrode material for “small mold of plastic / die-casting”
The importance of surface quality of the core is small, but you need a large number of small electrodes
The important element is surface quality. For this reason, electrolytic copper is used as the electrode material. In EDM, electrode wear generally increases in the domain of fine surface roughness, but graphite has this tendency extremely much, and high quality graphite of small particle size is used for machining of small surface roughness. Still more electrode number is required than copper. On the other hand, the graphite which is high in machining efficiency at the time of electrode production is suitable for machining of core side because of a large number of electrodes.
3) How to choose the electrode material for “precision plastic molds”
The element made much of is machining accuracy. High machining accuracy is required at the time of electrode production and EDM. Generally, copper is used. And also, non-copper materials are used because of their advantages. For example, copper tungsten and graphite are used because of cutting performance at the time of electrode production and because of less thermal expansion at EDM, and graphite is used because of an advantage that an electrode is inflexible by heat at the time of EDM, and so on.
The electrode materials for electric discharge machining mainly include graphite, copper and tungsten-copper composite materials. I believe that the master of mechanical machining also knows that in EDM, the physical properties of the electrode material have a great influence on the discharge erosion amount, electrode loss, processing speed, machining accuracy and processing stability. Under different conditions, the use of different electrode materials will directly affect the processing quality.
EDM electrodes consist of highly conductive and/or arc erosion-resistant materials such as graphite or copper. EDM is an acronym for electrical discharge machining, a process that uses a controlled electrical spark to erode metal. EDM electrodes include components made from brass, copper and copper alloys, graphite, molybdenum, silver, and tungsten.
Electrical discharge machining (EDM) makes it possible to work with metal for which traditional machining techniques are ineffective. It only works (except by specific design) with materials that are electrically conductive. Using recurring electric discharge, it is possible to cut small, odd-shaped angles and detailed contours or cavities in hardened steel as well as exotic metals such as titanium and carbide.
People familiar with EDM operations’ workings only know too well that excess wear requires additional electrodes or its only other alternative, i.e., redressing it frequently. Graphite as an electrode material boasts of keeping electrode wear to less than 1% compared to how deep it cuts when used with particularly aggressive parameters. That translates to high ampere levels and greater on-times, which help to preserve the electrode. It is something that copper conspicuously lacks as an electrode material.
We can take the example of EDM use in aerospace, where their thinness and fine details characterize the electrodes. If we use copper in such situations, the applied pressure will result in often unnoticed movements. Copper is relatively fragile and doesn’t tolerate rough handling, and is vulnerable to physical damage. The copper movements resulting from pressure are likely to create significant issues with performance further down the line. EDM Graphite electrode material, in contrast, can easily be identified as being work-ready or being damaged. Such a characteristic dramatically reduces the production risks posed by the electrode.
Please don’t hold the misconception that it is not possible to wire erode Graphite. It is a fallacious view as many leading graphite electrode companies have indeed wire eroded Graphite without breaking it. Even the wire erosion process timings are comparable to that of copper EDM electrodes.
EDM electrodes consist of highly conductive and/or arc erosion-resistant materials such as graphite or copper. EDM is an acronym for electrical discharge machining, a process that uses a controlled electrical spark to erode metal. EDM electrodes include components made from brass, copper and copper alloys, graphite, molybdenum, silver, and tungsten
Electrical discharge machining (EDM) makes it possible to work with metal for which traditional machining techniques are ineffective. It only works (except by specific design) with materials that are electrically conductive. Using recurring electric discharge, it is possible to cut small, odd-shaped angles and detailed contours or cavities in hardened steel as well as exotic metals such as titanium and carbide.
EDM electrode materials need to have properties that easily allow charge and yet resist the erosion that the EDM process encourages and stimulates in the metals it machines. Alloys have properties which provide different advantages based on the needs of the application.
When selecting EDM electrodes, the most important considerations alongside its form and function are the material’s conductivity (or resistivity) and it’s erosion resistance.
Conductivity promotes cutting efficiency, since electric current is the “cutting tool”.
Erosion resistance (a factor of melting point, hardness, and structural integrity) gives the electrode a longer service life and lowers the frequency of replacement. These properties, which vary almost exclusively by the type of alloy or material used, must be the deciding factors when selecting an electrode.
Copper and copper alloys have better EDM wear resistance than brass, but are more difficult to machine than either brass or graphite. It is also more expensive than graphite. Copper is, however, a common base material because it is highly conductive and strong. It is useful in the EDM machining of tungsten carbide, or in applications requiring a fine finish.
Tellurium copper is useful in EDM machining applications requiring a fine finish. Tellurium copper has a machinability that is similar to brass and better than pure copper.