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EDM experience in precision mold factory

Posted by: ANDY Minghe 2021-10-12 Comments Off on EDM experience in precision mold factory

Electric discharge machining (EDM) is an important process for plastic mold manufacturing. Some technical misunderstandings in the mold factory will cause the CNC EDM to fail to meet the expected accuracy, surface, efficiency and other requirements. The following common misunderstandings are analyzed and guided.

Misunderstanding 1. Use the electrode to touch the workpiece to divide the center, often “offset”

Guidance: The method of using the electrode to directly touch the workpiece is a surface contact. There are inevitably more or less fine objects between the contact surfaces, and there are also clamping accuracy errors on the contact surfaces, which will directly affect the accuracy of edge finding and centering. Using this method, it is strictly required to wipe the contact surface clean, but due to the presence of human factors, the accuracy will be unstable.

For the CNC electric discharge machine, it is recommended to use the method of benchmark ball centring, which is a necessary method for the electric discharge of the mold factory. The usual practice is:

  • Clamping the workpiece;
  • Put a reference ball on the workbench;
  • Install the probe on the spindle head;
  • Use the probe to center the workpiece;
  • Use the probe to center the reference ball;
  • Remove the probe and install the electrode;
  • The electrodes after that are all centered on the reference ball

Since the centering process is all point-to-point perceptual contact, high-precision positioning accuracy of μm level can be achieved. In addition, the moving distance of the electrode in the process of dividing the reference ball is reduced, the stroke of the machine tool can be fully utilized, and the efficiency is also improved.

Misunderstanding 2. Always use the same electrode material

Guidance: Most domestic mold companies use copper as electrode material. In the pursuit of high-efficiency processing today, have you ever investigated the processing advantages of graphite electrodes? Perhaps you will simply think that graphite electrodes are only suitable for large-scale mold processing or rough machining. In fact, this understanding is one-sided or still stays in the traditional concept of mold making.

At present, more and more mold companies are beginning to use graphite electrodes to greatly shorten the mold manufacturing cycle. Because whether it is a milling electrode or an electric discharge machining process, it can save about 50% of the time, which is a significant advantage of graphite electrodes. In addition, the large graphite electrode is light in weight, the narrow slit is not easy to deform, CNC milling has no burrs, and the overall electrode can be designed to reduce the number of electrodes, etc., which fully reflects the advantages of graphite materials. Of course, graphite processing is not suitable for fine surface processing requiring Ra0.4μm or less.

For micro processing, extremely low electrode loss is required. At this time, it is necessary to choose high-quality copper electrodes or Luo copper electrodes. For the electrical discharge machining of high value-added parts, the more expensive copper-tungsten alloy can be used to obtain smaller electrode loss, especially in the processing of cemented carbide workpieces.

Misunderstanding 3. The electrode spark position is made too small, which greatly reduces the processing efficiency

Guidance: Most companies upgrade from using traditional electric discharge machines to CNC electric discharge machines. When many factories use CNC electric discharge machines, the electrode spark position process still refers to traditional electric discharge machines. , The finishing electrode is 0.06mm on one side.

The small electrode spark position greatly limits the CNC electric discharge machine can not use larger current for high-speed machining. In fact, after high-speed cutting processing, the side surface of the cavity can be quickly trimmed only by translational processing. This is a process method to achieve the perfect effect of discharge surface, efficiency, and accuracy indicators. Here is a reference. The rough machining electrode spark position of the CNC electric discharge machine is 0.3~0.15mm on one side, and the fine machining electrode is 0.15~0.1mm on one side. It is necessary to refer to the discharge area and the machining volume. If the area is allowed, the spark position should be made as large as possible, so that the machining efficiency can be even up to several times.

Misunderstanding 4. Manual chuck is still used to install and adjust electrodes

Guidance: For reasons of strength or cost, companies use traditional manual chucks to install and adjust electrodes. This method is simple and practical, and is widely used. However, some companies have purchased hundreds of thousands of CNC electric discharge machines and are still using traditional chucks.

Using traditional manual chucks, the actual utilization rate of the machine tool is not high, and it can only spend more capital investment to increase the discharge machine tool when the production efficiency cannot be met. In fact, a good horse needs to be equipped with a good saddle. The CNC machine should be equipped with a 3R quick clamping positioning fixture, which can save the manual metering process, reduce the frequent standby of the machine tool, and improve the production efficiency.

Misunderstanding 5. Use CNC machine tools instead of side punching and oblique punching

Guidance: CNC electric discharge machine can realize side punching and multi-axis machining. The picture shows a molding insert of an injection mold. There are relatively thin and deep glue positions around it. This position is suitable for side punching.

The R angle of the tool remaining after the electric discharge cleaning cutting is a relatively common type of machining. If the X, Y, and Z three-axis linkage method is used, that is, the oblique machining, it can avoid the unstable discharge due to the small area of ​​the machining part. The phenomenon of local electrode wear.

Misunderstanding 6. Large-area high-finish processing is difficult to meet the requirements

Guidance: If the company’s mold EDM is of a large area (above 30 square centimeters) type, and the surface is required to be below VDI18, it requires a uniform spark texture, like a TV remote control type cavity. Then the electrical discharge machining is a headache, and the texture is often trimmed over and over again, and the processing efficiency is also very low.

If it is to process large-area and large-cavity molds in batches, mixed powder processing technology should be considered, which can greatly improve the processing efficiency and make it easier to obtain large-area fine textures or mirrors.

Misunderstanding 7. Incorrect surface quality control of electrical discharge machining

Guidance: In some mold companies, the requirements for the molds they manufacture are not very high, and the discharge parts must be polished afterwards. In this case, the electrical discharge machining of the mold is pursuing the requirements of VDI18 (Ra0.8μm) or even the mirror surface processing, and at the same time complaining that the discharge speed is too slow and the delivery time is too late.

Enterprises should correctly control the discharge surface quality according to the different requirements of the mold, and distinguish whether the priority of discharge is efficiency or quality. For most parts to be polished later, EDM can reach VDI22 (Ra1.25μm) or above. For fine parts, in order to avoid polishing deformation, it can be processed more finely. What needs to be emphasized here is that when pursuing the requirements of high-quality matte surface below VDI22, the discharge time will be greatly increased, and the electrode loss will also increase.

Misunderstanding 8. Misunderstanding of mirror electric discharge machining

Guidance: For mold companies that have not been exposed to mirror electric discharge machining, they will be interested in this technology. But unfortunately, due to lack of practical experience, some of their incorrect cognition led to cases of processing failure.

In fact, it is not difficult to realize the mirror surface processing for the CNC electric discharge machine, and the sub-mirror surface like the VDI7 (Ra0.2μm) level is extremely difficult to process. Whether a high-quality mirror effect can be achieved, in addition to the selected processing parameters, it largely depends on the material of the workpiece. Some materials such as SKD11, DC53, and counterfeit S136 cannot achieve a good mirror effect anyway, so we must Determine the material and then decide to perform mirror discharge, otherwise it may waste time and fail to meet the requirements. For the electrode, there is no such strict requirement, and it is not that the electrode must be mirror polished.

The main experience of mirror processing is time control. How much area and how long should be set, experienced masters can flexibly realize high-efficiency mirror production. Without experience, they can only rely on the expert system of CNC machine tools to perform electrical discharge machining slowly. Quality is guaranteed anyway. of…

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