Misunderstanding of EDM Forming Process in Die & Mould Enterprises

1 Introduction

EDM is an important process method for mold making, and it plays a very important role in the manufacture of injection molds. At present, there are still many mold companies in the process level is not perfect, there are some errors in the process often lead to EDM parts of the mold can not reach the predetermined accuracy and efficiency requirements. The following is a detailed analysis of common misunderstandings of EDM in die and mould companies.

2 common misunderstandings of EDM

Misunderstanding of 2.1 positioning methods

The precision control and assurance of EDM depends largely on positioning accuracy. In the commonly used Z-axis machining, the X and Y axes guarantee the position of the cavity, and the Z-axis ensures the depth requirement of the processed cavity. There are many ways to achieve EDM positioning. The positioning method that uses electrodes and workpieces to sense contact (Figure 1) is widely used in mold companies.

Fig. 1 The direct contact and sensing of the electrode and workpiece

This positioning method is relatively simple and is very familiar to the operator. However, we can find that if the accuracy of EDM is very high, the positioning error must not exceed 0.01mm, like the processing of the Huff slider of the plastic mold, the matching of the upper and lower mold cores, and the insertion site, etc. The use of the above method will be very difficult, it is difficult to ensure a stable positioning accuracy, due to the positioning error caused by repeated repairs, scrap parts of the situation. Operators often complain about poor machine performance or poor positioning accuracy. In fact, the accuracy of current EDM machines in terms of displacement is quite high, and the situation of inaccurate positioning is that there are misunderstandings in the method of selecting positioning.

Use the electrode and the workpiece to directly touch the edge, because the contact area is large, the burr on the electrode or the workpiece, the presence of subtle debris, the flatness, parallelism, verticality and other factors of the electrode and the workpiece affect the positioning accuracy, even if the workpiece and the electrode Very clean and difficult to guarantee the positioning accuracy of 0.01mm. For workpieces (electrodes) that are unusual or have too large (too small) electrodes and require high accuracy, there are only indirect positioning methods that use indirect positioning with reference balls (Figure 2). The point contact is to eliminate the error in the largest range.

Figure 2 Indirect positioning of the datum ball

There are still many mold companies do not know how to use the reference ball positioning. The use of a reference sphere for positioning is not only easy to ensure accuracy, but also in the case of a complex workpiece shape, can easily achieve the center; the process of the electrode to the center of the reference ball to avoid the movement of the electrode for a larger stroke, saving the operating time; In the case of multiple workpiece processing, it is not necessary to use the electrode for multiple divisions, as long as the reference ball is divided into once, which saves a lot of operation time, and the advantage of multi-electrode processing is particularly obvious. Of course, here is not to emphasize the advantages of using the reference ball positioning method. For the general accuracy required processing, a small number of workpieces, and a small number of electrodes, in order to improve work efficiency, the electrode is usually used for positioning. In addition, the use of the method of positioning the reference sphere requires that the accuracy of the reference surface of the electrode and the workpiece be very high, otherwise, a larger positioning error will be generated.

2.2 Misunderstanding of electrode material selection

The choice of electrode material is directly related to the effect of discharge. To a large extent, the selection of materials is appropriate, which determines the final conditions of discharge rate, processing accuracy and surface roughness. Should be based on the actual needs of different types of mold processing, targeted selection of materials. EDM electrode materials mainly include copper, graphite, and copper-tungsten alloys. Many mold companies rarely consider the choice of electrode materials, and large and small electrodes are always accustomed to using the same type of electrode material. This practice will not find its drawbacks in normal processing, but in the extreme processing can find obvious problems, affecting the effect of EDM. If all the electrode materials are selected from copper, then high-precision machining often complains that the machine tool wears too much. It requires a large number of electrodes for processing. The large electrode also uses copper, which leads to a lot of time and high cost. This is the misunderstanding of electrode material selection in EDM.

Reasonable choice of electrode material can be considered from the following aspects: whether the electrode is easy to form and shape; how the electrode's electrical discharge machining performance; processing accuracy, surface quality; electrode material cost is reasonable; In many cases, the choice of different electrode materials has its own advantages and disadvantages, which requires the capture of the key elements of processing. If high-precision machining is performed, it is necessary to abandon the consideration of the cost of the electrode material. If high-speed machining is required, lower the machining accuracy.

Copper can be easily obtained in a stable processing state. A low-loss rule can be used in the finishing process to obtain a contoured cavity with a high surface finish. However, due to the low melting point of the material itself, it should not withstand a large current density. Generally, it cannot exceed 30A current processing, otherwise it will seriously damage the electrode surface, crack, affect the processing effect. The coefficient of thermal expansion is large, and in the processing of deep-narrow ribs, local high temperatures generated under large currents can easily deform the electrodes. Copper electrodes usually use low-loss processing conditions, and their productivity is not high due to the low average loss of processing.

Graphite electrodes are particularly suitable for machining larger cavity sizes, enabling low-loss, high-speed rough machining in large area machining situations, such as the use of large-scale plastic molds, forging dies, and die-casting dies for EDM. Its unique processing advantages. It has good mechanical strength under high temperature and low thermal expansion coefficient, which is very suitable for high-precision processing of narrow slits. Because of its light weight, it is often used in the manufacture of large electrodes. Good electrical conductivity, high processing speed, can save a lot of discharge time, the price is generally 1/2 copper material. The disadvantage is that the discharge stability is poor during the finishing process, and it is easy to transition to the arc discharge. Only processing conditions with large loss can be selected for processing. The roughness of the processing fine surface is slightly poor, and it is easy to fall off and dregs during processing. The production of graphite electrodes requires special processing machines.

Copper-tungsten alloy materials are rarely used in normal machining and are only used in high-precision molds and EDM processes for special applications. Due to the high content of tungsten, it can effectively resist the loss during EDM, can ensure extremely low electrode loss, and can achieve stable processing under extremely difficult processing conditions. The disadvantage is that it is expensive and the source of the material is difficult. When processing high-precision molds for electronic connectors, strict requirements are imposed on the shape of the subtle parts (such as long, straight walls, and complex small cavities). This requires that the electrode losses during processing must be extremely small. The use of copper-tungsten alloys for the manufacture of electrodes is a process. The basic requirements of technology. Copper tungsten electrode is for tungsten steel, high carbon steel, high temperature resistant superhard alloy metal, because common electrode loss is large, the speed is slow, copper tungsten electrode is the preferred material.

2.3 Mistakes in the selection of electrode scaling

Spark gaps exist between the two poles during EDM, and the electrodes are scaled to an appropriate size for processing to meet the required dimensions. The size of the electrode scaling affects the dimensional accuracy, profiling accuracy, processing efficiency, and surface quality of the process. Most of the mold companies choose a fixed pattern for the scaling of the electrodes. For example, the common roughing electrode scales 0.2 mm on one side, and the precision machining electrode scales 0.07 mm on one side. If the area of ​​the electrode is large, due to the small amount of scaling of the electrode, it is restricted that a large discharge parameter cannot be selected in the EDM process, which reduces the processing efficiency. This is the misunderstanding of the selection of the electrode scaling amount.

The selection of electrode scaling must take into account various factors. There are two methods of EDM machining: translational processing and non-rotary processing. CNC EDM machines can generally use translational processing, while conventional EDM machines cannot perform translational processing without a translational head. There are differences in the selection of the scaling of the electrodes in these two processing methods.

In the absence of translational machining, if the generated spark gap is smaller than the electrode scaling, the machined dimensions will be smaller than the standard value. Conversely, if the electrode scaling is smaller than the actual spark gap, the machined size will be larger than the standard value. Therefore, the correct determination of the size of the electrode scaling is a prerequisite to ensure that the processing size is qualified. To determine the size of the electrode scaling depends on the different parts of the processing and a reasonable choice. Plastic mold processing parts are generally divided into structural parts and molding parts. The structural parts play the role of coordination and positioning in the mold. The roughness of the machined surface of these parts is not strictly required, but the dimensions are required to be processed in one place to ensure that the dimensions after processing meet the requirements. The size of the spark gap actually produced during machining is taken into account when determining the size of these spark positions. The molding part is a part for directly molding the plastic part. The machining size and surface roughness of such parts have corresponding requirements. EDM machined parts are usually polished after the completion of the process to remove the fire traces to achieve the required surface roughness requirements, so in determining the amount of such molding site scaling should accurately determine the polishing margin. The general polishing allowance is about 4Ra+0.005mm (Ra: surface roughness value completed by electric discharge machining), and the sum of actual spark gap and polishing allowance is taken when calculating the amount of electrode scaling. The EDM process is generally performed with different sizes of electrodes using different electrical specifications from coarse to fine finish. The size after processing mainly depends on the control of finishing. When determining the size of the finishing spark, it is necessary to first consider the electrical parameter conditions to be used to achieve the predetermined surface roughness, and to determine the size of the spark gap and then determine the scale of the electrode. The spark position of the finished part of the forming part is generally 0.04 to 0.08 mm on one side and 0.02 to 0.06 mm on the single side of the structural part. When determining the size of the rough machining sparks, taking the machining speed into account and setting an appropriate margin for finishing, it is generally 0.15 to 0.25 mm on the single side.

In the use of electrode translation processing, the dimensional accuracy of the processing is determined by the control of the discharge gap, the amount of scaling of the electrode, and the amount of the momentum. Since the magnitude of the translational momentum is controllable, the translational momentum can be adjusted according to the size of the discharge gap, and the size of the processing can be controlled easily, and the scale of the electrode can be relatively large, especially for the finish machining. And can be selected flexibly according to some specific conditions. Generally, no translational processing is used in rough machining, and the scale of the electrode is 0.15-0.3 mm on one side. The finishing process uses multiple stages of processing conditions to improve chip evacuation conditions with a translational method, achieving stable processing and obtaining a more uniform surface roughness on the sides and the bottom surface. The scale of the electrode generally takes 0.05-0.15 mm. In ordinary processing, the amount of translational force is not large, and it will not affect the accuracy of processing. However, in the precision machining, because the shape of the translation method is different from the processing shape, in order to improve the accuracy of copying, the zooming amount of the electrode can not be too large, generally taking a unilateral edge of 0.05 mm or less.

When determining the amount of scaling of the electrode, the machining performance of the processing site should also be considered in detail. If the slag discharge is good, it is not easy to form a secondary discharge, and the scaling of the electrode may be smaller. However, the blind hole processing is not very smooth due to slag discharge, and there are more chances of secondary discharge. The scale of the electrode should be increased. Take larger ones; in large area processing, in order to obtain faster processing speed, the scale of the electrode may be larger; the discharge gap in mixed powder processing is larger than the discharge gap processed by common working fluid, and the scale of the electrode may be larger; Precision machining has smaller scaling than normal machining. However, it should be noted that for thin, sharp-shaped electrodes, the scaling should be chosen smaller, because such processing cannot select large processing conditions, otherwise it will deform the electrode during processing, and the larger the scale of the electrode is reduced. The strength of the electrode.

2.4 Mistakes in Process Methods

At present, the application of CNC EDM machine tools in mold enterprises is becoming more and more popular. Some manufacturers of mold enterprises have developed from the use of traditional EDM machine tools to the use of CNC EDM machines. These companies have formed a lot of processing experience due to the long-term use of traditional machine tools. In fact, because the operation of CNC EDM machines is different from that of traditional EDM machines, this has caused many misunderstandings in the use of CNC EDM machines by mold companies.

Traditional EDM machines can only use the Z axis for vertical servo feed processing. At present, most EDM machines can realize horizontal machining and multi-axis linkage machining. However, these functions have not been well applied in the mold companies. Many operators believe that they can use the Z-axis processing as long as the parts can be processed. In fact, if we can give full play to the functions of the machine tool, we can significantly improve the processing quality and improve the processing efficiency. Figure 3 shows a molding insert for an injection mold with relatively thin, deep glue spots around it. If Z-axis servo machining is used for this part, the partial discharge area will be small, the machining depth will be large, the discharge instability will occur during the processing, and then the electrode feed will be repeatedly retreated, the processing speed will be slow, and the electrode loss will be large. , Anomalies such as uneven surface roughness do not meet the processing requirements. If we improve the process method and use the horizontal servo function of the digital EDM machine to make the electrode for horizontal servo machining, we can solve the above problems and achieve obvious improvement. In particular, the machining speed can be improved by several times compared with the Z-axis servo processing.

Figure 3 Transverse servo processing

Figure 4 is the use of EDM to remove the tool R angle left after cutting, is a more common type of processing, also known as "clear angle processing." When this kind of processing is performed, if the X-, Y-, and Z-axis linkage methods are used, that is, diagonal machining, discharge instability due to the small area of ​​the processing part can be avoided.

Figure 4 Oblique angle processing

3 Conclusion

At present, some mold companies lack innovation in process methods and lack understanding of new technologies from the outside world, which makes it impossible for companies to increase the overall manufacturing level of molds. The technical analysis of the common misunderstandings in the EDM of die and mould enterprises was carried out in the above section, hoping to bring helpful hints to related companies. In addition, the current EDM technology has made great progress, such as the emergence of advanced CNC EDM machine, which can achieve high-efficiency precision machining; rapid clamping positioning system (EROWA, 3R) does not require repeated calibration of electrodes and workpieces , Positioning, greatly improved the processing efficiency; mixed powder processing makes large-area mirror processing become a reality, and so on. It can be seen that in the rapid development of mold manufacturing technology, mold enterprises should tailor-made a set of reasonable processes according to their own manufacturing needs, and constantly improve, so that enterprises are in an invincible position in the fierce competition.

Motors

Our Electric Motors included Ordinary Motors , Explosion-proof Motors, Variable Frequency Motors, High Voltage Motors, Wind Power Motors, Marine Motors. Our Motors are widely used as Metallurgy mill roller drive. Plastic Film Roller and Industrial Rollers drive. They are equipped with excellent quality Bearings.

Asynchronous Motor,Three Phase Motor,High Voltage Motors,Variable Frequency Motors,Explosion-proof Motors

Huatao Group Limited , https://www.rollerandreducer.com