You spend hours designing an injection mold core or cavity
You check on you job the next morning and inspect it while still in the machine, it all checks out, so you remove it, clean it up and give it to the mold polisher to make it all shiny and nice. Soon, he comes in and tells you he needs to show you something.
DC arcs are primarily caused by one thing: bad flushing. Even with central flushing or lateral flushing, there can be dead spots where EDM sludge builds up; the bottom of ribs are a favorite hiding spot.
The sludge acts like part of the electrode and attracts sparks that arc across the dielectric fluid and faithfully reproduce their shape in the steel. If this continues for a length of time, you get a pit. The longer it persists, the bigger the pit.
Sure enough, right at the bottom of the deepest part, a rib that is there to make the plastic part stronger, is a pit. Another name for this pit is a DC arc, or zit, or some expletive that is unprintable. What this means is that there is a small hole, or crater at the most inaccessible region of your mold that looks like it was bombed when you view it through a microscope.
Dirty oil is also a common culprit to DC arcing. This happens because the filters are full, or not fine enough to filter out the EDM sludge and grit. It should be able to filter down to a 1 micron size in order to ensure no pitting due to dirty oil.
Some manufacturers claim that their machines have software that prevents DC arcing, and to a great extent they do. Mostly this is by retracting the electrode out of the cut so flushing can occur. Some have a high speed oscillating effect that improves the flushing as well. Then there are some that have improved cutting parameters that will adapt when the machine senses a DC arc.
Then you will find many, pits, not big enough to scrap the part, but certainly big enough to make the polishers job much more difficult and possibly changing the dimensional integrity of the steel.
It might seem like a waste of time to make sure you have adequate flushing, but in the long run, it will save you time and money. Even with the high end, new EDM machines, you should keep this in mind. Whether you deal with it through the circuitry or mechanics of the orbiting or the filter, make sure you take care of the flushing.
All of these methods really get down to improving flushing. Even high tech, new EDM machines will pit. I know because I have had to polish many surfaces EDM'd by these machines! You will find a nice looking surface that is not so easy to detect until you begin removing the first layer of recast.
EDM is an amazing machine tool with tremendous importance for manufacturing. It is possibly the least known and most used machine for producing the endless variety of plastic products we use and appreciate everyday.