The scene is too familiar to everyone who is involved in EDM machining, as well as electrical discharge machining. The script looks like this:
You spend hours designing a core or cavity of an injection mold, days of CNC milling and grinding to get the required size and shape. Only EDM machining of parts remains. You have graphite electrodes that take a couple more days, and finally, everything is set up on your EDM CNC machine.
As a rule, you can make 4 cavities that will produce 4 exactly identical plastic parts when everything is completely finished. Thing goes well, you launched the first 6 different electrodes in the process, now there is another one. Only 16 more hours, and you will be made in the next project.
So, you set it all up for the whole night, and you feel confident that everything is as it should be, you can sleep well without waking up at the beginning, wondering if you did this or that before you go home.
Next morning
You check the work the next morning and check it while you are still in the car, everything is checked, so you remove it, clean it and give it to the polishing machine to make it shiny and beautiful. Soon he enters and says that he needs to show something. Oh oh ...
Of course, right at the bottom of the deepest part of the rib, which is there to make the plastic part stronger, is a hole. Another name for this pit is a DC arc, or zit, or some kind of abusive one, which is non-printing. This means that there is a small hole or crater in the most inaccessible area of your mold, which looks like a bomb when you look through a microscope.
Now you need to be creative and fix it if you can. Usually there is a somewhat confusing way to fix this by cutting out the bad part and inserting an insert to replace the dimpled area. Sometimes, although it is simply impossible to recover, and the piece must be revised!
How to prevent DC arcs
DC arches are primarily caused by one problem: poor flushing. Even with central washing or side washing, dead zones can form where an EDM precipitate forms; the lower part of the ribs is a favorite place to hide.
The slurry acts as a part of the electrode and connects the sparks that pass through the dielectric fluid and accurately reproduce their shape in steel. If this continues for some time, you get a hole. The longer it lasts, the larger the pit.
Dirty oil is also the usual culprit in the DC arc. This is due to the fact that the filters are full or not good enough to filter out EDM sediment and sand. It should be able to filter to a size of 1 micron to ensure no pitting due to dirty oil.
Some manufacturers claim that their machines have software that prevents direct current arcs, and to a large extent they do. This is mainly the removal of the electrode from the incision, so flushing may occur. Some of them have a high-speed oscillating effect, which also improves flushing. Then there are some that have improved cutting parameters that will adapt when the machine perceives a DC arc.
All these methods are really aimed at improving the flushing. Even high-tech, new EDM machines will be broken. I know because I had to polish many EDM surfaces with these machines! You will find a beautiful surface that is not so easy to detect until you start to remove the first layer of redistribution.
Then you will find a lot of holes that are not large enough to give up a part, but, of course, large enough to make the work of polishers much more difficult and, perhaps, change the integrity of the dimensions of the steel.
Save time and money
It may seem like a waste of time to make sure you have adequate flushing, but in the end it will save you time and money. Even when using new EDM machines, you should keep this in mind. Regardless of whether you deal with it according to the scheme or the mechanics of the orbit or the filter, make sure that you take care of the washing
The electrical isolation is an amazing machine, of great importance for manufacturing. This is perhaps the least known and most used machine for producing an infinite variety of plastic products that we use and appreciate every day. Just look around!
