There are many bases for the selection of graphite electrode materials, but the main four standards are:
The average particle diameter of the material
The average particle diameter of the material directly affects the discharge condition of the material. The smaller the average particle size of the material, the more uniform the discharge of the material, the more stable the discharge condition, and the better the surface quality.
For forging and die-casting molds with low requirements for surface and precision, materials with coarser particles, such as ISEM-3, are usually recommended. For electronic molds with high requirements for surface and precision, it is recommended to use materials with an average particle size of less than 4μm to ensure the precision and surface finish of the processed molds. The smaller the average particle size of the material is, the less the material loss will be, and the greater the interaction force between each ion group will be. For example: It is usually recommended that in terms of precision die-casting molds and forging molds, ISEM-7 is sufficient to meet the requirements; However, when customers have particularly high requirements for precision, it is recommended to use TTK-50 or ISO-63 materials to ensure less material loss, thereby guaranteeing the precision and surface roughness of the mold.
Meanwhile, the larger the particles are, the faster the discharge rate will be and the smaller the loss during rough machining will be. The main reason is that the current intensity during the discharge process varies, resulting in different magnitudes of discharge energy. However, the surface finish after discharge also varies with the change of particles.
2. Flexural strength of the material
The flexural strength of a material is a direct manifestation of its strength, indicating the tightness of its internal structure. Materials with high strength have relatively better wear resistance during discharge. For electrodes with high precision requirements, materials with better strength should be chosen as much as possible. For example: TTK-4 can meet the requirements of general electronic connector molds. However, for some electronic connector molds with special precision requirements, TTK-5 material with the same particle size but slightly higher strength can be selected.
3. Shore hardness of the material
In the subconscious perception of graphite, it is generally regarded as a relatively soft material. However, the actual test data and application situations show that the hardness of graphite is higher than that of metallic materials. In the special graphite industry, the common hardness testing standard is the Shore hardness measurement method, and its testing principle is different from that of metals. Due to the layered structure of graphite, it has extremely superior cutting performance during the cutting process. The cutting force is only about one-third of that of copper materials, and the surface after mechanical processing is easy to handle.
However, due to its relatively high hardness, during cutting, the wear on the cutting tool will be slightly greater than that of the cutting tool for metal. Meanwhile, materials with high hardness perform relatively well in controlling discharge losses. In our company's EDM material system, for the materials of the same particle size that are widely used, there are two types of materials to choose from, one with slightly higher hardness and the other with slightly lower hardness, to meet the needs of various customers with different requirements. For example: Materials with an average particle size of 5μm include ISO-63 and TTK-50; The materials with an average particle size of 4μm include TTK-4 and TTK-5. The materials with an average particle size of 2μm are TTK-8 and TTK-9. The main consideration is the emphasis of various types of customers on discharge and mechanical processing.
4. The inherent resistivity of the material
According to our company's statistics on the characteristics of materials, if the average particle size of the materials is the same, the discharge speed of the material with a higher resistivity will be slower than that of the material with a lower resistivity. For materials with the same average particle size, the strength and hardness of materials with lower resistivity will also be slightly lower than those of materials with higher resistivity accordingly. That is, the speed and loss of discharge will be different. Therefore, it is very important to select materials according to the needs of actual applications. Regarding the resistivity of graphite electrodes, the higher the grade of the graphite electrode, the lower the resistivity. For instance, the resistivity of UHP graphite electrodes is lower than that of HP graphite electrodes and RP graphite electrodes.
Due to the particularity of powder metallurgy, for each parameter of each batch number material, there is a representative value of the material with a certain fluctuation range. However, for graphite materials of the same grade, their discharge effects are very similar, and the differences in application effects caused by various parameters are very small.
The selection of electrode materials is directly related to the discharge effect. To a large extent, whether the material selection is appropriate determines the final situation of the discharge speed, processing accuracy and surface roughness.
These four types of data represent the main discharge performance of the material and directly determine its service performance.
