Results of the advantages of graphite materials for the tire mold EDM based on the analysis of graphite electrode materials performance and characteristics , and there are different processing results with different graphite materials after the comparison test which a variety of grades of graphite are used in the actual EDM of tire molds.
In the mid-1960s, graphite began to be used as an electrode material for EDM. Due to the coarse grain size, loose structure and poor mechanical strength of the graphite powder at that time, the electrode is easy to degranulate and collapse during electrode fabrication and EDM, resulting in low processing precision, poor surface roughness, unstable processing, and easy pull arc burns, and due to the different composition directions of graphite particles, the conductivity isotropic properties are different during electrical discharge machining. Subsequently, high-quality isotropic graphite was developed by cold isostatic pressing (CIP) method, which gradually replaced copper as the mainstream of electro-processed electrode materials. At present, more than 90% of the electrode material in Europe is graphite, which is the same in China. It has become an ideal electrode material and has played an important role in the rapidly developing EDM industry.
EDM tire mold processing machine is a special machine tool specially designed for EDM of tire mold. It is a new type of EDM machine that has appeared in recent years. Most of the tire mold manufacturers in China use graphite as the electrode material. It lies in the many advantages of graphite electrodes and the progress of graphite processing technology.
Compared with copper, graphite has the following characteristics:
(1) Good processability
Graphite is a granular structure, and its mechanical cutting resistance is small, only about 1/5 of that of copper, so cutting is easy. Under certain conditions, the cutting efficiency of graphite is even 5 times that of copper, which can significantly shorten the electrode fabrication cycle and improve the production efficiency of machining equipment.
(2) Higher thermal stability
In the process of EDM (especially high current machining), the temperature of the electrode body is high, and the slight deformation of the copper material at this time may cause the dimensional accuracy of the cavity to deteriorate. The thermal expansion coefficient of the graphite electrode is less than 0.00000 005/K at 20-200 °C, and its thermal stability is much better than that of copper, so thermal deformation does not occur. Especially in the processing of the sheet or the fine forming, the problem that the cavity precision is deteriorated due to the deformation of the electrode can be effectively overcome.
(3) Small proportion
The bulk density of graphite is smaller than that of copper, which is only 1/4 of that of copper. In the case of EDM machining of large workpieces (such as automobile cover molds, household appliance shell molds, etc.), the electrodes are large. If copper is used as the electrode, the weight is considerable, which is a big burden on the spindle performance of the EDM equipment. And it is bound to increase the investment cost of equipment, such as the use of graphite materials can greatly reduce the weight of the electrode.
(4) No need to deburr
The graphite electrode has no burrs, and the surface is simply treated with sandpaper, which avoids the shape distortion caused by the external force of the shape and size of the electrode, and reduces the manufacturing process of the electrode, eliminating the auxiliary time.
(5) Lower unit volume cost
The price of copper is high and fluctuates greatly, while the price of isotropic graphite is lower and stable than copper. Under the same volume, the price of graphite for electrical processing is 30% to 60% lower than that of copper.
(6) Wide choice
Graphite manufacturers can provide different grades of graphite materials according to different properties. Users can select the corresponding grade materials according to different usage conditions and requirements, in order to achieve the best processing results.
(7) High current density
The high thermal stability of the graphite material, the small coefficient of thermal expansion and the good electrical conductivity make the graphite electrode able to withstand large current densities in EDM. In the same processing area, the graphite electrode can withstand a processing current of 10% to 30% larger than that of the copper electrode, thereby greatly improving the processing efficiency.
It can be seen from the above analysis that graphite materials have obvious advantages in electrical discharge machining.
Application of Graphite Electrode in EDM of Tire Mould
In recent years, graphite electrodes have been widely used in the electrical discharge machining of tire molds. The reason is that in addition to the characteristics of the graphite material analyzed above, there are also factors of the tire mold itself. The tire mold is characterized by a multi-cavity and a large variety of cavities, and there are 7-8 patterns on a pair of tire molds. There are 4 to 5 types of cavities, and 3 to 4 electrodes are usually prepared for each cavity, which are used for coarse, medium and fine processing respectively. Therefore, the amount of electrodes is large, especially for medium and large tire molds. Such as motorcycle tire molds, cars, trucks and truck tire molds, etc., because of the large block area, the use of electrodes is large, the cost of electrode materials is a problem that must be considered. The factors related to the tire mold itself are:
(1) The cavity is large, the electrode area is also large, and the electrode weight is also large. The graphite electrode can effectively reduce the load weight of the spindle head of the EDM machine to obtain better spindle dynamic performance.
(2) A large number of electrodes need to be equipped with sufficient machining equipment, and the use of graphite materials can greatly shorten the electrode fabrication time and improve the electrode fabrication efficiency of a single machining device. In order to ensure the continuity of the process, there is no blockage of a certain process. In general, the ratio of the CNC milling machine used for electrode fabrication to the EDM equipment is 1:4 or 1:5 when using a copper electrode. After the graphite material is used, the ratio can be reduced to 1:8 or 1:10, and the efficiency is more than doubled.
(3) The tire mold manufacturing cycle is very tight. The general enterprise starts work at 24 h. In order to further increase the speed, the method of maximum current density is usually used in roughing to obtain the fastest removal rate. By using a graphite electrode, a current of about 20% larger than that of the copper electrode can be applied under the same area condition, thereby effectively improving the production efficiency.
It is based on the above several practical factors that domestic tire mold manufacturers are increasingly prefer graphite as the electrode material for EDM, which makes the application of graphite materials in the tire mold manufacturing industry more and more extensive.
Author: Lu Zhiliang , Wu Qiang , Ni Minmin , Xu Qingyu
