1 Introduction <br> <br> EDM is an important part of the process in the mold manufacturing, especially in plastic manufacturing is more important. Most plastic mold parts usually use EDM to complete the final finishing. The quality of the finished processing directly affects the assembly performance or molding accuracy of the mold parts. The abnormal problem of processing causes some unnecessary treatment methods, which causes the whole workpiece to be scrapped, prolongs the mold manufacturing cycle, increases the mold manufacturing cost, and reduces the mold quality. Therefore, it is of great significance to prevent the occurrence of abnormal processing. Abnormal processing problems include abnormalities in processing and quality problems after processing.
2 Common abnormal problems and analysis of EDM (1) The measured dimensions of the machined parts after the completion of the machining of the mold parts are unqualified. The accuracy of the part completed by EDM is usually about 0.005mm. The machining accuracy requirements of different parts of the mold parts are different, and some parts with high precision requirements have strict dimensional tolerance control. If the machined size is not within the tolerance, it is a defective size. The unqualified size is greater than the maximum limit size and less than the minimum limit size. There are several factors that affect the size of the processing:
a. The effect of the amount of electrode size scaling. There is a spark gap between the two poles during EDM. In order to process the required size, the electrode is scaled to the appropriate size for processing. The scaling of the electrodes is referred to as the amount of electrode scaling in production. When machining, the actual mismatch between the spark gap and the amount of electrode scaling will directly affect the accuracy of the machining dimensions. When the electrode translation processing is not used, if the generated spark gap is smaller than the electrode scaling amount, the processed size will be smaller than the standard value. Conversely, an electrode scaling that is less than the actual spark gap will result in a processed size that is greater than the standard value. Therefore, correctly determining the size of the electrode scaling is a prerequisite for ensuring the processing size. When determining the amount of electrode scaling, it is reasonable to choose depending on the processing location. Plastic mold processing parts are generally divided into structural parts and molding parts. The structural part plays a role in matching and positioning in the mold. The surface roughness of these parts is not strictly required, but the size is required to be processed in one position to ensure that the dimensions after processing meet the requirements. The size of the spark gap actually generated during processing is taken when determining the size of the spark bits in these portions. The molded part is a part for directly molding a plastic part. The processing dimensions and surface roughness of such parts have corresponding requirements. The forming part of the electric discharge machining generally adopts a polishing method to remove the fire trace to achieve a predetermined surface roughness requirement after the processing is completed, so the polishing allowance should be accurately determined when determining the amount of electrode scaling of such a molding portion. Generally, the polishing allowance is about 4Ra+0.005mm (Ra: surface roughness value after EDM machining), and the sum of the actual spark gap and the polishing margin is taken when calculating the electrode scaling amount. The EDM process generally uses different sizes of electrodes to process from coarse to fine using different electrical specifications. The size after processing is mainly determined by the control of finishing. When the electrode translation processing is not used, the electrical parameter condition to be used to achieve the predetermined surface roughness should be considered first. The size of the gap between the sparks should be determined and the size of the spark position of the electrode should be determined. The spark position of the finished part is generally 0.04-0.08mm on one side and 0.02-0.06mm on one side. When determining the size of the roughing spark position, taking into account the processing speed and the appropriate margin for finishing, the standard is 0.15-0.25mm on one side. In the case of electrode translation, the spark position takes the standard value minus the spark gap and the momentum, and the spark position of the electrode in the finishing process is generally about 0.12 mm on one side. When determining the size of the spark position of the electrode, the processing performance of the processed part should also be considered in detail. For example, in the case of good slag discharge in the through hole, it is not easy to form a secondary discharge, and the spark gap generated is relatively small, and the blind hole type processing is not very smooth due to slag discharge, and the chance of secondary discharge is relatively large, and the spark is caused. The gap becomes larger. The larger the amount of large electrode scaling, the smaller the amount of electrode scaling. Because the discharge energy distribution is uniform when the large electrode is discharged, the energy is concentrated when the small electrode is discharged.
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