EDM is an important process method in mold manufacturing, especially in injection mold manufacturing. However, some misunderstandings in EDM process of mold factory often cause machining accuracy, surface, and efficiency to fall short of requirements. Following will analyze common misunderstandings of EDM in mold factory.

 

Method of directly touching workpiece with an electrode is a surface contact. There are inevitably more or less fine objects between contact surfaces, and there are also clamping accuracy errors on contact surfaces, which will directly affect accuracy of edge finding and centering. With this method, contact surface must be wiped clean, but accuracy may be unstable due to human factors.
For CNC discharge machines, method of benchmark ball is recommended, which is necessary method for mold factory discharge. Usual approach is:
Clamping workpiece;
Place a reference ball on workbench;
Mount probe on spindle head;
Use probe to center workpiece;
Use probe to score reference ball;
Remove probe and install electrodes;
Subsequent electrodes all center reference ball.
Since centering process is point-to-point sensing contact, it can achieve μm level high-precision positioning accuracy. In addition, moving distance of electrode in process of dividing reference ball becomes smaller, stroke of machine tool can be fully utilized, and efficiency is also improved.
Of course, if production process is more complete, eccentricity of electrode can be measured outside machine through three-coordinate measurement, and eccentricity value is transmitted to EDM machine. There is no need to divide on EDM machine, which can greatly improve machine's productivity and improve overall production efficiency of EDM.

Most domestic mold factory use copper as electrode material. In pursuit of high-efficiency processing, have you ever examined processing advantages of graphite electrode? Perhaps you would simply think that graphite electrode are only suitable for large mold machining or rough machining. In fact, this understanding is one-sided or still stuck in traditional concept of molding.
At present, more and more mold factory have begun to use graphite electrodes to greatly shorten mold manufacturing cycle. Because whether it is milling electrodes or electrical discharge machining, processing efficiency can be greatly improved, which is a significant advantage of graphite electrode. In addition, graphite is a large electrode with light weight, and it is not easy to deform in slit processing. CNC milling has no burrs. Overall electrode can be designed to reduce number of electrodes, etc., which fully reflects advantages of graphite materials. Of course, when fine surface processing is required below Ra0.4μm, graphite processing is not suitable.
For fine processing, extremely low electrode losses are required. At this time, it is necessary to choose high quality copper electrode or chrome copper electrode. For electrical discharge machining of high value-added parts, use of more expensive copper-tungsten alloys can achieve smaller electrode losses, especially in machining of cemented carbide workpieces.

 

Most companies are upgrading from using traditional electric dischargers to numerically controlled electric dischargers. When many factories use CNC discharge machines, electrode spark level process still refers to traditional electric discharge machines, such as rough machining electrode spark position taking 0.15mm on one side, and finishing electrode taking 0.05mm on one side.
Small electrode spark level has greatly restricted CNC electric discharge machine from using higher currents for high-speed machining. In fact, after high-speed cut-in processing, side of cavity can be quickly repaired only through translational processing. This is a process method to achieve perfect effect of discharge surface, efficiency, and accuracy indicators. Here is a reference. Rough machining electrode spark position of CNC electric discharge machine is 0.3 ~ 0.15mm, and finishing electrode is 0.15 ~ 0.05mm. It is necessary to refer to discharge area and processing amount. If area allows, make spark level as large as possible, and achieve processing efficiency as high as several times.

For reasons of strength or cost, companies use traditional manual chucks to install and adjust electrodes. This method is simple and practical, it is widely used. But some companies have purchased hundreds of thousands of CNC discharge machines and still use manual chucks.
With traditional manual chuck, actual utilization rate of machine tool is not high. In the case that it cannot meet production efficiency, more capital can only be invested to increase discharge machine. In fact, a good horse needs to be equipped with a good saddle. CNC machine should be equipped with a 3R fast clamping positioning fixture, which can save process of manual metering, reduce frequent standby of machine, and improve production efficiency.

 

NC electric discharge machine can realize side, oblique and multi-axis linkage processing. For example, for some molding inserts for injection molds, there are relatively thin and deep glue positions around these parts, which are very suitable for side hits.
Clearance of tool R angle remaining after cutting is a more common machining type. If X, Y, and Z three-axis linkage method is used, that is oblique machining, it can avoid phenomenon of unstable discharge and local electrode loss due to small area of processed part.
For oblique gate processing on molds, many factories process molds in a Z-vertical manner by tilting molds. In fact, it can be done by using oblique tapping function of CNC EDM machine. Setting start point and end point can realize processing of oblique gate. Need to design electrode according to oblique method when designing electrode.
Some factories are equipped with high-end CNC electric discharge machine tools, and machine tools are also equipped with C-axis. However, C-axis function is not used when processing horn gate of mold insert. In order to achieve horn gate processing, insert is divided into two halves for inserting. In fact, it can be done using C-axis servo machining.

 

If company's mold EDM is a large area (more than 30 square centimeters) type, and surface is required to be below VDI18, uniform spark texture is required, like TV remote control type cavity. Then EDM is a headache. It is often repeated for texture, and processing efficiency is very low.
If it is batch discharge machining of large-area, large-cavity molds, use of mixed powder processing technology should be considered, which can greatly improve processing efficiency, and it is easier to obtain large-area fine texture or mirror surface.

Some mold factory do not have high requirements for molds they make, discharge parts are basically polished afterwards. In this case, mold EDM is pursuing requirements of VDI18 (Ra0.8μm) or even mirror surface processing, while at the same time complaining that discharge rate is too slow and delivery date is too late.
Enterprise should correctly control quality of discharge surface according to different requirements of mold, and clearly distinguish whether priority of discharge is efficiency or quality. For most of subsequent processing parts to be polished, electrical discharge machining can reach VDI22 (Ra1.25 μm) or more. For fine parts, you can work finer to avoid polishing deformation. What needs to be emphasized here is that when pursuing high-quality matte surface requirements below VDI22, discharge time will increase significantly, and electrode loss will also increase.

For mold factory that have not been exposed to mirror discharge machining, this technology will be quite interested. But unfortunately, due to lack of practical experience, some of their incorrect perceptions can easily lead to cases of processing failure.
In fact, it is not difficult for CNC discharge machines to realize mirror surface processing, and sub-mirror surfaces like VDI7 (Ra0.2μm) are extremely difficult to process. Whether high-quality mirror effect can be achieved, in addition to selected processing parameters, it depends to a large extent on material of workpiece. Certain materials such as SKD11, DC53, and counterfeit S136 cannot achieve good mirror effect anyway, so you must judge material before deciding to perform mirror discharge, otherwise time may be wasted and requirements may not be met.
Main experience of mirror finishing is time control. How large area is and how much time should be set, experienced masters can flexibly achieve high-efficiency mirror production.