Abnormal Problems and Analysis of Electrical Discharge Machining in Die and Mould Manufacturing

1 Introduction

EDM is a very important process in the manufacture of molds, especially in plastic mold manufacturing. Most plastic mold parts usually use EDM to finish the final finishing process. The quality of the machining directly affects the assembly performance or molding accuracy of the mold parts. Abnormal problems caused by processing may cause some unnecessary processing methods, which may cause the entire workpiece to be scrapped, prolong the cycle of mold manufacturing, increase the cost of mold manufacturing, and reduce the quality of the mold. Therefore, it is of great significance to prevent the occurrence of abnormal processing. Machining anomalies include abnormalities in processing and quality problems after processing.

2 Common EDM problems and analysis

(1) Measured dimensions of the machined part after machining of the mold part are unqualified. The parts that can be achieved by EDM are usually able to achieve an accuracy of about 0.005mm. The requirements for machining accuracy in different parts of the mold parts are not the same, and there are some parts with high accuracy that require strict dimensional tolerance control. If the processing size is not within the tolerance range, it is the defective size. The defective size has more than the maximum limit size and less than the minimum limit size. There are several factors that affect the size of the processing:

a. Effect of electrode size 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 scale of the electrode is called the electrode scaling in production. During machining, the actual mismatch between the spark gap and the scaling of the electrode will directly affect the accuracy of the machining dimensions. In the absence of electrode translation processing, if the generated spark gap is smaller than the electrode scaling, the machined size 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. In the absence of electrode translation processing, to determine the size of the finishing spark should first consider the electrical parameters used in order to achieve the predetermined surface roughness to be selected, clear the conditions of the spark between the size of a gap and then determine the size of the electrode spark. The spark position of the finished part of the forming part is generally unilateral 0.04-0.08mm, and the structural part is unilateral 0.02-0.06mm. When determining the size of rough machining sparks, it is a standard to consider the machining speed and to reserve the appropriate margin for finishing. Generally, the single side is 0.15-0.25mm. When the electrode is used for translational processing, the spark position takes the standard value minus the spark gap and the momentum, and the electrode spark position in the finishing process generally takes about 0.12 mm on one side. When determining the electrode spark bit size, the processing performance of the processing site should also be considered in detail. If the slag discharge is good, it is not easy to form secondary discharge and the spark gap produced is relatively small. The blind hole processing is not very smooth due to slag discharge, and there are more chances of secondary discharge and sparks. The gap becomes larger. Large electrode scaling Smaller electrode scaling is relatively small. Because the discharge energy is distributed uniformly when the large electrode discharges, the small electrode discharges energy when it is discharged.

b. The actual size of the electrode, the impact of the control of the amount of translation. In the absence of electrode translation processing, the actual size of the electrode plays a decisive role in the size of the finished part. After the electrode spark position is correctly determined, reasonable processing methods should be used to ensure the accuracy of the electrode. In the use of electrode translation processing, the control of the amount of translation plays a decisive role in the processing dimensions. The correct amount of translation should be determined according to the actual measured size of the electrode to ensure that the processing dimensions meet the requirements.

c. The effect of electrode calibration accuracy. The size after processing has a great relationship with the calibration accuracy of the electrode. The deviation of the calibration of the electrode will increase the projected area of ​​the vertical plane in the processing feed direction, making the size of the processing site larger than normal. Therefore, there is usually a case where the side gap after processing with the small electrode is slightly larger than the side gap after processing with the large electrode. Because the correction accuracy of the small electrode cannot be as high as the large electrode. The calibration accuracy of the electrode is the most important part of the initial stage of EDM, and it is one of the important conditions for ensuring the qualified size of the processing.

d. Influence of electrical parameter adjustment factors. Electric parameter adjustment is directly related to the actual spark position in processing. Changing the parameters of the electrical parameters will affect the size of the spark gap. The most obvious effect on the spark gap is the current. As the current increases, the spark gap increases accordingly. The same applies to the pulse width. The increase of the pulse gap will make the spark gap smaller, but the effect is not obvious. Other relevant parameters also affect the size of the spark bit indirectly. Therefore, when adjusting the electrical parameters, it must be selected reasonably. When changing the electrical parameters, it is necessary to clarify the influence on the processing dimensions.

e. Effect of electrode loss during processing. Inevitably there is electrode loss during processing, and the electrode loss makes the finished size smaller than the standard value. (The reason for abnormally large electrode loss is described in detail below.) It is necessary to properly control the electrode loss to ensure that the size of the processing meets the standard.

f. The influence of processing depth control. The depth of the machining feed direction in the machined dimensions is a particularly important dimension. The depth of control accuracy is related to whether the processing size is qualified. The factors affecting the precision of depth control are firstly the influence of the accuracy of the tool before machining. If there is any debris between the electrode and the workpiece during tool setting, the tool offset will be generated, and the depth of processing will usually be less than the standard value. Therefore, when the knife is set, it must be ensured that the two poles are clean. The second is the impact of reserved machining allowances. The dimensional control of the side of the processing site is determined by the spark position of the electrode. Depth control, on the other hand, is determined by the depth of the machine to be machined. The selection of reserves is the same as the selection principle of electrode sparks. Again, it is the effect of the accuracy of the tool reference. The location of the electrode used to set the knife must be a clear reference and the reference surface should be clean and flat. Finally, we must pay attention to the effect of thermal expansion of the electrode during rough machining, make it longer and exceed the margin reserved for finishing, so that the depth of processing is deeper and there is no light during finishing.

(2) The surface quality of the finished part is unqualified. Abnormal surface quality problems generally have carbon deposition, roughness does not meet the requirements, the surface metamorphic layer is too thick. The following specific analysis of these three issues.

a. Carbon deposition. Coking is the most serious surface quality problem and has a destructive effect on mold parts. It is a product of abnormal discharge in EDM. The main cause of abnormal discharge is the improper selection of electricity rules. The general coke deposition occurs in the finishing process because the discharge gap during finishing is small and slagging is not easy. Therefore, in the adjustment of electrical parameters to observe the stability of the discharge state as a standard, in the case of unstable discharge, the discharge time should be reduced, the height of the knife increases, the pulse width decreases, the pulse gap increases, the servo pressure decreases Small and so on. In roughing, the peak current should not be excessively large when the area to be machined is small. Secondly, the impact of oil also has a great influence. Inappropriate oil flushing methods and oil flushing pressure make it impossible to discharge electrocorrosion materials smoothly, causing arc discharge due to unstable discharge conditions. The commonly used oil flushing method is undershooting the oil, flushing the oil toward the opening, and soaking the oil. The oil pressure is controlled within the critical pressure range near the processing, and the spark oil used should be cleaner. During the entire processing process, it is necessary to monitor the stability of the machining at any time and evaluate the sparks, sounds, currents, and voltmeters. Take appropriate measures in time for abnormalities in processing.

b. Roughness does not meet the requirements. Finished surface roughness that does not meet the requirements is a common problem with surface quality anomalies. Some precision parts usually require a very fine surface to be polished and then polished. If it is rough, it will increase the amount of polishing, affecting the shape and dimensional accuracy. There are some products that require surface fire patterns, which require the surface roughness to meet the requirements, the overall uniformity. The above-mentioned improper selection of electricity standards and oil rushing factors are also important reasons for surface roughness not meeting the requirements. In addition there are electrode surface roughness, electrode material, processing retention and other factors. The surface roughness of the electrode directly affects the roughness of the machined surface. During machining, the surface of the electrode is copied to the surface of the workpiece, so the finished electrode is usually polished. Poor electrode material quality, non-uniform tissue, impurities, etc. will make the surface roughness of the finished workpiece uneven, not meet the predetermined requirements, the choice of electrode materials should be based on the requirements of the processing site reasonable selection. The processing allowance of rough machining has a great influence on the finishing effect. If you stay too little, there will be no finishing. Roughing is usually 0.15mm for finishing. Large electrodes can be less than the small electrodes. Some electrodes of the same cavity should be replaced in time when processing a plurality of numbers because the surface quality of the electrode will be deteriorated after the electrode is processed in multiple sites, and the surface roughness of the machined workpiece will be different.

c. The surface metamorphic layer is too thick. The instantaneous high temperature and pressure generated during discharge, and the rapid cooling action of the working fluid, causes the surface of the workpiece to produce an altered layer that is different from the characteristics of the raw material workpiece after the discharge. Under normal circumstances, the effect of surface metamorphic layers on the processing results is unfavorable. In particular, if the surface metamorphic layer is too thick, the wear resistance and fatigue resistance of the machined surface will be greatly reduced, and the service life of the workpiece will be adversely affected. The case where the surface metamorphic layer is too thick generally occurs at a relatively small area of ​​the processing site because the discharge energy of these processing sites is large. If the machined part has a small convex shape, a large current cannot be used for roughing. The thermal influence of excessive discharge energy will affect the surface quality of the workpiece.

(3) Processing position deviation. After the completion of processing, it was found that the electrode misalignment was a common problem in processing abnormalities. The method of clamping the workpiece and the electrode must be firm and does not loosen during processing. To ensure the electrode and workpiece calibration accuracy. When positioning, use accurate reference and accurate positioning method. Make sure the positioning base is free from burrs and debris. Carefully control the positioning accuracy during the operation. At the beginning of processing, it is necessary to carefully observe whether there is any deviation in the machining position, and whether the position of the machining position has to be checked in time during processing. Careful attention to these points can prevent the occurrence of abnormal position deviations.

(4) Abnormalities in processing. Abnormal phenomena in EDM process make the whole process can not be carried out normally, and the common abnormal problems that often occur in processing are:

a. The processing efficiency is very low. The efficiency of EDM directly affects the efficiency of mold processing and should be properly controlled. Electrical parameters are the main reason for the processing speed. The size of the machining current is the most obvious condition in the electrical parameters. Generally, in the roughing process, the maximum current should be selected in the principle of guaranteeing a proper margin for finishing. This will not only improve the machining efficiency, but also facilitate the finish machining. If the current is selected to be too small during roughing, the efficiency of the roughing is affected first, and at the same time, the finishing machining efficiency is also reduced because of a larger amount of finishing. Increasing the pulse frequency, that is, increasing the pulse width and reducing the pulse gap, is also a method for improving the processing efficiency. Other auxiliary electrical parameters are also very important, such as increasing the discharge time and reducing the number of lifting times. However, the premise of processing stability is that if the working fluid in the processing area cannot be deionized in time, the electrocorrosion products cannot be discharged in time, which will result in a decrease in processing efficiency. For example, when finishing in large area, the length of discharge time should be set to be very short. Only in this way can the smooth processing be ensured, and in the case of stable discharge, the processing efficiency is substantially improved. The quality of the working fluid and the impact of the oil flushing method are also obvious. High-quality EDM fluids should be selected. A reasonable oil flushing method should be used to ensure smooth chip evacuation during oil flushing. It is recommended that large-electrode processing be performed with oil immersion. The processing site should be pre-processed, leaving the appropriate margin. Excessive machining allowance increases processing time and greatly reduces processing efficiency. The depth reserved for rough machining for rough machining should not be too much, especially in the case of large area, because the electric erosion ability of the finishing rule is very low, when the surplus is too much, it will increase the processing difficulty and affect the processing efficiency.

b. The electrode is very depleted. When the electrode loss is too large, it will seriously affect the accuracy and dimensional accuracy of the processing site. The cause of large electrode loss should be properly analyzed. The pulse width in the electrical gauge is the main parameter for controlling the electrode loss. In the case of large electrode loss, it should be considered whether the selected pulse width value is too small. In the case of high accuracy requirements, it should be considered whether the selected electrode material can achieve low-loss processing requirements. Check whether the polarity of the processing is reversed. Whether the loss is too large due to excessive processing current during small electrode machining. If the oil pressure is too high, the flow rate will also cause excessive local loss of the electrode and should be properly adjusted.

c. The discharge is unstable. The unstable discharge state is an abnormal phenomenon that can be detected during processing and should be dealt with in time to avoid the consequences of processing quality. When the discharge state is unstable, there will be a series of corresponding performances, such as bright red sparks, white smoke, low sound and stuffy, concentrated discharge in one place, uneven current and voltmeter hands, and swinging of the servo dial indicator. . When these conditions are discovered, the effects of electrical parameters should be considered first, such as whether the discharge time is too long, whether the slag discharge height is insufficient, whether the current, pulse width is too large, and whether the pulse gap is too small. Secondly, it should consider whether there are impurities or burrs on the processing site. Again, we should consider whether the use of oil pressure and methods is reasonable.

d. The electrode is deformed. Deformation of the electrode is more common in the processing of thin electrodes. This deformation is mainly thermal deformation. Followed by excessive servo pressure. For the processing of this type of electrode, the discharge energy must not be too large and the duration of the heat during the processing cannot be too long. Therefore, the current can not be selected too much, the discharge time should be short, and the oil should be sufficient. When designing the electrode, the electrodes of other processing parts should have sufficient strength except for the thin and small parts of the structure itself.

e. "Powdering" occurs during the processing of holes. The gas generated during the process is accumulated at the bottom of the electrode or inside the oil cup. When the gas is ignited by the electric spark, it will be discharged like a “shot gun” and it will easily displace the electrode and the die. This situation is more likely to occur during the pumping process. Therefore, in the use of oil cups for hole-making, special attention should be paid to the exhaust, appropriate lifting knife or open the gas tank around the top of the tank, exhaust hole, in order to facilitate the accumulation of accumulated gas.

(5) Abnormal processing caused by artificial carelessness. Due to human carelessness, processing quality abnormalities can occur during production. These phenomena mainly occur in the EDM operator, such as setting the wrong coordinates, setting the wrong depth, reversing the direction of the electrode clamping, taking the wrong electrode, misreading the drawings, making NC program errors, etc. These problems are not guilty of technicality but the lowest level of human error. But sometimes the consequences of some minor mistakes are very serious. Should try to avoid making mistakes and constantly improve their own technical level.

3 Conclusion

Preventing the occurrence of abnormal problems in EDM has important implications for improving the quality of the die. It is necessary to correctly analyze the influence of each link in the processing on the quality of processing, grasp each link, pay attention to each processing details, and ensure the final processing quality.