The most critical factor in the development of graphite electrodes is the grade of petroleum coke used, as higher grades of the petroleum coke produce higher quality electrodes. In addition to the grade of petroleum coke, several other factors can impact the quality of the graphite electrode and its ability to carry a current such as length, diameter, bulk density, resistance and porosity. For example, lower grades of coke do not allow for an electric current to pass through as easily.
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There are two different types of graphite electrodes available. One type is called “SDGE,” which stands for small diameter graphite electrodes. These electrodes are most commonly used for melting scrap metal and other raw materials and are used in Electric Arc Furnaces (EAF). These electrodes carry a current that creates an arc between the electrode and the raw material, causing it to melt.
The other electrode type is the “LDGE” or large diameter graphite electrode. These types of graphite electrodes are most commonly used for steel melting in very large EAF’s requiring very high-temperature and high-intensity applications. Unlike small diameter electrodes, whose current carrying capacity ranges from 15,000 to 70,000 amps, the current capacity of LDGEs varies between 60,000 to 160,000 amps.
SDGE vs. LDGE
SDGEs are usually manufactured from petroleum coke, regardless of the grade. Some SDGEs are manufactured with high-grade coke, others with a lower grade coke. In fact, some SDGEs are a blend of several different grades. LDGEs on the other hand, are more commonly manufactured with premium, 100% high grade needle coke.
There are different grades specified within both electrode types, such as:
HP – High Power
HD – High Density
UHP – Ultra High Power
SHP – Super High Power
There are also other grades including regular power (RP), normal power (NP), and medium power or (MP). However, these grades are less commonly used.
Sinker EDM burning into H13 tool steel with a graphite electrode. Images courtesy of Toyo Tanso Ltd.
Diagram of the graphite manufacturing process. Diagram courtesy of Toyo Tanso Ltd.
Lloyd Booth from Aimmco in Woodland, WA measuring a fine rib of graphite.
High-speed milling centers feeding an EDM cell at M&M Mold.
Ken at M&M Mold putting a rib into a connector mold with an electrode on an EDM machine.
Moldmaking in the present day has seen many technological advances throughout the realm of EDM. We have seen significant upgrades in sinker machines that compensate for potential deficiencies in running conditions. The implementation of automation is commonplace to combat rising labor costs. Quick change tooling aids in rapid turn-around. One thing operators rarely consider as an area of technological significance is the graphite consumables they use.
Graphite takes roughly six months to manufacture from beginning to end. During this process a meticulous observation of quality needs to take place throughout every step of production. This begins in the selection of raw materials. Different sources for coal tar pitch and coke materials result in different final properties for the graphite that is produced.
These raw materials are crushed and sieved numerous times to ensure the con-sistency of the resultant particles. The mix-ing then occurs to ensure the particle distribution is correct. This mixture is placed in rubber sleeves and put into an isostatic press. Isostatic means constant pressure. This piece of equipment is crucial to ensure the properties in the block are isotropic. Iso-tropic generally means that the graphite will have homogeneous or similar properties throughout the block and consistent quality.
After the blocks are removed from the press they are still in the green stage—meaning they need to be transformed through heat treatment from carbon to graphite. Each graphite grade has a particular heat treat recipe that aids in the final formulation and will help determine the properties it will possess.
Graphite is baked at a very slow rate to ensure the gases within the blocks are not released too rapidly. That would cause an increased rate of cracked blocks. Some grades can be re-impregnated with pitch at pressure to ensure higher densities and baked a second time. The last step is a high temperature graphitization process that will complete the transformation from carbon to graphite by adjusting the crystalline structure of the material.
It is imperative that during every step along the manufacturing process quality checks are performed to ensure the graphite blocks meet quality standards. It also is important that the graphite manufacturer works to improve the performance of the graphite it makes. Employing a large staff of engineers who constantly develop grades for the ever-changing EDM marketplace is essential.
Now that we know generally how graphite is made, how do we choose the correct grade? In order to maintain cost effectiveness, an EDM shop needs to choose EDM graphite that provides dependable quality at a good price. The incorrect selection of consumables can cause the operation to take longer than planned or cost more than what was budgeted. If an improper grade was chosen, the customer may experience undesired results or pay too much for a grade that they did not require.
When looking at roughing grades they generally have a particle size around 10 to 8 microns. They are used for applications that require no detail and no sharp edges or corners. Because of the molecular and bonding structure in this grade class, you will experience high removal rates at the expense of high electrode wear. Although this grade class machines somewhat easily—due to its structure—there is an issue with it chipping and breaking with small electrodes.
Once you enter the finishing/detail aspect of the grade selection process, you will find that the grades generally have particle sizes from 8 to 5 microns. The most common application for this grade class is forging dies and die cast molding dies. They also can be used in less complex powdered and sintered metal applications, and for larger plastic injection parts. The downfall to these grades is that they are limited on strength and you cannot produce thin fragile electrodes, or superior surface finishes either.
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If the workpiece requires smaller and more intricate features, sharper corners, thin ribs or fine detailed smaller electrodes, another grade class usually is required. The fine/detail category will provide the necessary results. These grades usually have a particle size from 3 to 5 microns.
When the application calls for wire cutting, aerospace applications, blow molding, plastic injection, threading electrodes or medical applications, the fine/detail category will meet all of your needs. These applications require that the graphite provides sharp corners, superior surface finishes, and exhibits high strength for small and thin electrode applications.
There are applications that require ultra fine surface finishes, extremely small parts, intricate details on the electrode, best wear conditions, precision or thin wire cutting and superior electrode wear. If this is the prerequisite then the precision class will work best for them. The particle size for this grade class ranges from 3 to 1 micron. It works exceptionally when applied to exotic aerospace type metals (high nickel and copper alloys) and carbide applications.
Finally, there also are copper-impregnated grades that provide an advantage over non-impregnated grades. Typically companies impregnate two classes of graphites: (1) the finishing/detail grades and (2) the fine detail grades.
When graphite is impregnated with copper, micron size particles of copper fill in the holes (otherwise known as porosity) of the graphite structure. By impregnating the graphite with copper it increases the graphite’s electrical resistivity and strength.
Now the electrode manufacturer has the ability to make extremely thin and fragile electrodes, such as a very thin deep rib electrode. Copper-impregnated graphites also are the preferred grades for aerospace applications because the added copper helps to stabilize machining of exotic, copper and nickel alloys.
If you require the finest finish available, impregnated graphites provide the absolute finest finish. Copper-impregnated grades also allow for stable machining when nonfavorable conditions exist—such as poor flushing or when the operator is not very experienced with the application.
With all of the different grades of graphite available, if an incorrect decision is made, and the wrong grade is used in an application, the result could be costly in both profitability and delivery. However, if educated on what to expect from each grade class and manufacturer, the venture should prove to be a profitable and timely success.
Each of our EDM graphite grades has been tailored for a specific range of electrode applications with benchmarked performance characteristics.
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Ultra-high power graphite electrodes, by replacing copper electrodes with graphite electrodes for mold manufacturing, significantly shorten the mold manufacturing cycle, enhance labor productivity, and reduce the mold manufacturing cost. In recent years, with the introduction of precision molds and high-efficiency molds (with increasingly shorter mold cycles), people's requirements for mold production have been getting higher and higher. Due to the various limitations of copper electrodes themselves, It has increasingly failed to meet the development requirements of the mold industry. Graphite, as an EDM electrode material, has been widely used in the mold industry due to its advantages such as high machinability, light weight, fast forming, extremely low expansion rate, low loss and easy dressing. It is inevitable that it will replace copper electrodes.
1. Characteristics of Graphite Electrode Materials
CNC machining features fast processing speed, high machinability and easy dressing. The processing speed of graphite machines is 3 to 5 times that of copper electrodes, and the precision processing speed is particularly outstanding. Moreover, its strength is very high. For ultra-high (50 to 90mm) and ultra-thin (0.2 to 0.5mm) electrodes, they are not prone to deformation during processing. Moreover, in many cases, products need to have a very good texture effect. This requires that when making electrodes, they should be made as integral male electrodes as possible. However, there are various hidden corner clearings during the production of integral male electrodes. Due to the easy trimming property of graphite, this problem can be easily solved and the number of electrodes can be greatly reduced, which copper electrodes cannot achieve.
2. Rapid EDM forming, small thermal expansion and low loss: Due to the better electrical conductivity of graphite than that of copper, its discharge rate is faster than that of copper, being 3 to 5 times that of copper. Moreover, it can withstand a relatively large current during discharge, which is more advantageous for rough electrical discharge machining. Meanwhile, under the same volume, the weight of graphite is 1/5 times that of copper, which greatly reduces the load of EDM. It has great advantages in manufacturing large electrodes and integral male electrodes. The sublimation temperature of graphite is ℃, which is 3 to 4 times that of copper (the sublimation temperature of copper is ℃). At high temperatures, change
Ultra-high power graphite electrode
It is extremely small in shape (1/3 to 1/5 of copper under the same electrical conditions) and does not soften. The discharge energy can be transferred to the workpiece efficiently and with low consumption. Because the strength of graphite actually increases at high temperatures, it can effectively reduce the discharge loss (the loss of graphite is 1/4 of that of copper), ensuring the processing quality.
3. Light weight and low cost: In the production cost of a set of molds, the CNC machining time, EDM time, and electrode wear of the electrodes account for the vast majority of the total cost, and all these are determined by the electrode material itself. Compared with copper, the machining speed and EDM speed of graphite are both 3 to 5 times that of copper. Meanwhile, the feature of minimal wear and the production of the integral graphite electrode can both reduce the number of electrodes, thereby reducing the material consumption and machining time of the electrodes. All of these can significantly reduce the production cost of molds
2. Requirements and Characteristics of Mechanical and Electrical Processing of Graphite Electrodes
1. The production of electrodes: Professional graphite electrode production mainly uses high-speed machine tools for processing. The machine tools should have good stability, with uniform and stable three-axis movements without vibration. Moreover, the rotational accuracy of components like the main shaft should also be as good as possible. The electrode can also be processed on general machine tools, but the process of writing the tool path is different from that of copper electrodes.
2.EDM electrical discharge machining graphite electrodes are carbon electrodes. Because graphite has good electrical conductivity, it can save a lot of time in electrical discharge machining, which is also one of the reasons why graphite is used as an electrode.
3. Processing Characteristics of Graphite Electrodes: Industrial graphite is hard and brittle, causing relatively severe wear on tools during CNC machining. Generally, it is recommended to use tools coated with hard alloy or diamond. When rough machining graphite, the tool can be directly placed on and off the workpiece. However, during finish machining, to prevent chipping and cracking, a light tool and fast traverse method is often adopted.
Generally speaking, graphite rarely breaks when the cutting depth is less than 0.2mm, and a better surface quality of the side wall can also be obtained. The dust generated during CNC machining of graphite electrodes is relatively large and may invade the guide rails, lead screws and spindles of the machine tool, etc. This requires that the graphite processing machine tool has corresponding devices for dealing with graphite dust, and the machine tool's sealing performance should also be good because graphite is toxic. Graphite powder is a substance that is highly sensitive to chemical reactions. Its resistivity changes in different environments, meaning its resistance value varies. However, there is one thing that remains constant: graphite powder is one of the excellent non-metallic conductive materials. As long as the graphite powder is kept in an insulating object without interruption, like a thin thread, it will still be electrified. But what is the resistance value? There is no definite figure for this value either, because the fineness of graphite powder varies, and the resistance value of graphite powder used in different materials and environments will also be different.
You may not know that high-purity graphite powder also has conductive uses:
Generally, rubber is insulating. If electrical conductivity is required, conductive substances need to be added. Graphite powder has excellent electrical conductivity and lubricating demolding properties. Graphite is processed into graphite powder, which has excellent lubricating and conductive properties. The higher the purity of the graphite powder, the better its conductive performance. Many special rubber product factories need conductive rubber. Then, can graphite powder be added to rubber to conduct electricity? The answer is yes, but there is also a question: What is the proportion of graphite powder in rubber? Some enterprises use a proportion of no more than 30%, which is applied to wear-resistant rubber products such as car tires, etc. There are also special rubber factories that use a proportion of 100%. Only such products can conduct electricity. The basic principle of conductivity is that the conductor cannot be interrupted, just like a wire. If it is interrupted in the middle, it will not be electrified. The conductive graphite powder in conductive rubber is the conductor If the graphite powder is blocked by insulating rubber, it will no longer conduct electricity. Therefore, if the proportion of graphite powder is too low, the conductive effect is likely to be poor.
Graphite powder is a substance that is highly sensitive to chemical reactions. Its resistivity changes in different environments, meaning its resistance value varies. However, there is one thing that remains constant: high-purity graphite powder is one of the excellent non-metallic conductive materials. As long as the graphite powder is kept in an insulating object without interruption, like a thin thread, it will still be electrified. But what is the resistance value? There is no definite figure for this value either, because the fineness of graphite powder varies, and the resistance value of graphite powder used in different materials and environments will also be different.
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