Abnormal problems and analysis of EDM in the hotte

2022-08-21
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Abnormal problems and analysis of EDM in mold manufacturing

Abstract: according to the practical experience of EDM in mold enterprises for many years, this paper fully expounds the abnormal problems often occurred in EDM, and analyzes the causes and methods to prevent the problems, which plays a guiding role in improving the quality of EDM

key words: EDM; Mold quality; Abnormal; reason; Preventive measures

Chinese Library Classification Number: tg661 document identification code: b

problems in Mount EDM and Analysis on why

abstract: the problems are generally occurred in EDM was discussed in detail according to many years of practical experiences in EDM processing The whys were analyzed and the method to prevent was explored. It can be some references to the upgrading of EDM.

Key words: EDM; mould quality; problem; whys; Countermeasure

1 Introduction

EDM is a very important process in mold manufacturing, especially in plastic mold manufacturing. Most plastic mold parts usually use EDM to complete the final fine machining. Special attention should be paid to the effective stroke of electronic tensile machine and sample fixture. The quality of machining directly affects the assembly performance or forming accuracy of mold parts. The abnormal problems in processing may cause some unnecessary treatment methods at least, or the overall scrap of the workpiece, which prolongs the mold manufacturing cycle, increases the mold manufacturing cost, and reduces the mold quality. Therefore, it is of great significance to prevent abnormal processing problems. Abnormal processing problems include abnormal processing as a new kind of energy-saving, environmental protection, recycling, low-carbon material phenomenon and quality problems after processing

2 common abnormal problems and analysis of EDM

(1) the measured size of the machined part is unqualified after the machining of mold parts. The accuracy of the parts machined by EDM is usually about 0.005mm. The machining accuracy requirements of different parts of mold parts are different, and the dimensional tolerance control of some parts with high accuracy requirements is very strict. If the machining dimension is not within the allowable tolerance, it is an unqualified dimension. The unqualified dimensions are larger than the maximum limit dimension and smaller than the minimum limit dimension. There are several factors affecting the size of machining size:

a. the influence of electrode size scaling. There is a spark gap between the two poles during EDM. In order to machine the required size, scale the electrode to an appropriate size for machining. The scaling size of the electrode is called the electrode scaling amount in production. During machining, the mismatch between the actual spark gap and the electrode scaling will directly affect the accuracy of machining dimensions. When electrode translation machining is not used, if the spark gap generated is less than the electrode scaling amount, the machined size will be less than the standard value. On the contrary, if the electrode scaling is smaller than the actual spark gap, the size after machining will be larger than the standard value. Therefore, correctly determining the size of electrode scaling is the premise to ensure that the machining size is qualified. When the electrode scaling amount is determined, it should be selected reasonably according to the different processing parts. Plastic mold processing parts are generally divided into structural parts and molding parts. The structural part plays the role of coordination and positioning in the mold. There are no strict requirements for the machined surface roughness of these parts, but the size is required to be machined in place at one time to ensure that the processed size meets the requirements. When determining the spark position of these parts, take the actual spark gap generated during machining. The forming part is the part used to directly form plastic parts. The processing size and surface roughness of such parts have corresponding requirements. Generally, the formed parts of EDM are polished after machining to remove spark marks and meet the requirements of predetermined surface roughness. Therefore, the polishing allowance should be accurately determined when determining the electrode scaling amount of such formed parts. Generally, the polishing allowance is about 4ra+0.005mm (RA: the surface roughness value after EDM), and the sum of the actual spark gap and the polishing allowance is taken when calculating the electrode scaling amount. EDM process is generally completed by using electrodes of different sizes and different electrical standards from coarse to fine. The size after machining mainly depends on the control of finishing. When electrode translational machining is not used, when determining the size of finish machining spark position, the electrical parameter conditions to be selected to achieve the predetermined surface roughness should be considered first, and the size of 1 gap between sparks under this condition should be determined before determining the size of electrode spark position. The spark position of finishing machining of forming parts is generally 0.04-0.08mm on one side, and that of structural parts is 0.02-0.06mm on one side. When determining the size of rough machining spark position, consider the machining speed and reserve appropriate allowance for finish machining as the standard, and generally take 0.15-0.25mm on one side. When electrode translational machining is adopted, the standard value of spark position minus spark gap and flat momentum is taken. In precision machining, the spark position of electrode is generally taken as about 0.12mm on one side. When determining the size of electrode spark position, the machining performance of machining parts should also be considered in detail. For example, in the case of good through-hole slagging, it is not easy to form a secondary discharge, and the spark gap is relatively small, while the blind hole machining has more opportunities for secondary discharge because the slagging is not very smooth, which makes the spark gap larger. The large electrode scaling amount is smaller, and the electrode scaling amount is relatively small. Because the discharge energy distribution is uniform when the large electrode discharges, and the energy concentration when the small electrode discharges

b. the influence of actual electrode size and translation control. When the electrode translation processing is not used, the actual size of the electrode plays a decisive role in the size of the machined part. After correctly determining the spark position of the electrode, a reasonable machining method should be adopted to ensure the accuracy of the electrode. When electrode translation processing is adopted, the control of translation quantity plays a decisive role in the processing size. The correct translation amount shall be determined according to the measured size of the electrode to ensure that the processing size meets the requirements

c. influence of electrode correction accuracy. The size after machining has a great relationship with the correction accuracy of the electrode. The correction deviation of the electrode will increase the projection area of the vertical plane in the processing feed direction, and make the size of the processing part larger than the normal value. Therefore, generally, the side clearance after machining with small electrode is slightly larger than that after machining with large electrode. Because the correction accuracy of small electrode cannot be as high as that of large electrode. Ensuring the correction accuracy of the electrode is the most important link in the initial stage of EDM, and it is one of the important conditions to ensure that the machining dimension is qualified

d. influence of electrical parameter adjustment factors. The adjustment of electrical parameters is directly related to the actual spark position in machining. Changing electrical parameters and conditions will affect the size of spark gap. The most obvious influence on the spark gap is the current, and the spark gap increases with the increase of the current. The same is true of pulse width. The increase of pulse gap will make the spark gap smaller, but the effect is not very obvious. Other relevant parameters also indirectly affect the size of spark level. Therefore, when adjusting the electrical parameters, we must choose a reasonable one, and when changing the electrical parameters, we must make clear the impact on the processing size

e. influence of electrode loss in machining. Electrode loss is inevitable in the processing process, which makes the finished size smaller than the standard value. (the reasons for abnormal excessive electrode loss are detailed below) electrode loss must be properly controlled to ensure that the processing size meets the standard

f. influence of machining depth control. In machining dimension, the depth of machining feed direction is a particularly important dimension. The control accuracy of depth is related to whether the machining dimension is qualified. The first factor that affects the accuracy of depth control is the influence on the tool accuracy before machining. During tool setting, if there are sundries between the electrode and the workpiece, it will cause deviation in tool setting, which will usually make the finished depth of machining less than the standard value. Therefore, when setting the knife, we must ensure that the two poles are clean. The second is the influence of reserved machining allowance. The size control of the side of the machining part depends on the spark position of the electrode. The depth control depends on the reservation of the depth dimension to be processed. The selection principle of the reserved amount is the same as that of the electrode spark position. The third is the influence on the accuracy of the tool benchmark. The part of the electrode used for tool setting must be a clear reference, and the reference plane should be smooth and flat. Finally, we should also pay attention to the influence of electrode thermal expansion in rough machining, so that it becomes longer and exceeds the allowance reserved for finishing, so that the machining depth is too deep, and there is a phenomenon of uneven finishing during finishing

(2) the surface quality of the processed part is unqualified. The abnormal problems of surface quality generally include carbon deposition, the roughness does not meet the requirements, and the surface metamorphic layer is too thick. The following is a specific analysis of these three problems

a. carbon deposition. Carbon deposition is the most serious problem of abnormal surface quality, which has a destructive effect on mold parts. It is the product of abnormal discharge in EDM. The main cause of abnormal discharge is the improper selection of electrical rules. Generally, carbon deposition occurs in finishing, because the discharge gap is small during finishing, and it is not easy to discharge slag. Therefore, when adjusting electrical parameters, it is necessary to observe that the discharge state is stable. In the case of unstable discharge, the discharge time should be shortened, the tool lifting height should be increased, the pulse width should be reduced, the pulse gap should be increased, and the servo pressure should be reduced. During rough machining, pay attention to the peak current when the machining area is small. Secondly, flushing oil also has a great impact. Improper oil flushing method and oil flushing pressure make it impossible for the electric corrosion to be discharged smoothly, making the discharge state very unstable and causing arc discharge. Generally, the oil flushing method is to flush oil downward, flush oil towards the opening, spray oil, etc. The oil flushing pressure should be controlled within the range close to the critical pressure of machining, and the spark oil used should be cleaner. During the whole machining process, we should monitor the machining stability at any time, and evaluate it by looking at sparks, listening to sound, observing current and voltmeter. Take corresponding measures in time for abnormal phenomena in processing

b. the roughness does not meet the requirements. It is a common problem of abnormal surface quality that the machined surface roughness does not meet the requirements. Some precision parts usually require very fine surfaces to be machined and then polished. If they are rough, the polishing amount will be increased and the machining shape and dimensional accuracy will be affected. There are also some products that require fire patterns on the surface, which requires that the processed surface roughness meet the requirements and be uniform as a whole. The improper selection of electrical standards and oil flushing factors mentioned above are also important reasons for the surface roughness not meeting the requirements. In addition, there are factors such as electrode surface roughness, electrode material, machining allowance and so on. The roughness of electrode surface directly affects the roughness of machined surface. During machining, the surface of the electrode is copied to the surface of the workpiece, so the precision machined electrode is usually polished. Poor quality of electrode material, uneven structure, impurities, etc. will make the surface roughness of the processed workpiece uneven, which can not meet the predetermined requirements. When selecting electrode material, it should be selected reasonably according to the requirements of the processing part. The processing allowance of rough processing has a great impact on the effect of refined products with good thermal insulation function and flame retardant and fire prevention function. If too little is left, it will produce the phenomenon of finishing and polishing. Generally, 0.15mm is reserved for finishing for rough machining. The large electrode can be appropriately left less than the small electrode. The electrode should be replaced in time when processing several numbers of the same cavity, because the electricity

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