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98 MoldMaking Technology July 2015 EDM 98 MoldMaking Technology July 2015 FOR MORE INFORMATION: Poco Graphite, an Entegris company / 940-393-4252 jerry_mercer@entegris.com / poco.com Pages 91 to 98 Graphite or Copper? By Jerry Mercer There has been a long-standing, unresolved debate over whether graphite or copper is a better EDM electrode material. Perhaps it's time to take a closer look at both materials to iden- tify the differences and help you select the one that is most appropriate for your EDM applications. Material variety. Graphite is produced with a wide range of material characteristics, enabling you to match the elec- trode material properties to the EDM 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. 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. However, with the wide range of graphite materials available on the market today, it is pos- sible to find some lower-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. 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. High current density on smaller, more intricate detail also increases the electrode wear. Also, when machining intricate detail or EDMing at high current densi- ties, a copper electrode may move or flex as material stress is relieved. This limits the ability to maintain the very tight toler- ances required on many of today's molds. Graphite performs 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 per- form a job can be significantly reduced. Metal removal rates. An electrode material's thermophysi- cal properties determine its ability to process the energy of the EDM cut and remove metal. Copper electrodes require higher temperatures and longer on times, and therefore have lower metal removal rates than graphite. Wear. Electrode wear is a constant concern because exces- sive wear results in adding electrodes or redressing electrodes Image courtesy of Poco Graphite. This photo illustrates the number of copper electrodes required for one specifc job compared with the graphite equivalent. 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. The high amperage and long on times of a roughing condi- tion actually preserve the graphite electrode but are detrimen- tal to the copper electrode, which erodes away at these set- tings. On the contrary, in the finishing stages, with low amper- age 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 poros- ity, it goes without saying that copper electrodes provide very fine surface finishes. However, with the sophistication of today's EDM sinker technology, 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. CONTRIBUTOR Jerry Mercer is an EDM applications manager for Poco Graphite.