Filtering the Dielectric

In order for the dielectric to perform its flushing function in the best possible way, eroded particles from the workpiece and the electrode, as well as the cracked parts of the dielectric itself, must be removed. In addition the dielectric, which heats up during spark erosion, has to be cooled down again to a normal working temperature of 20° C – 30° C. If it is too hot, there will be inaccuracies in the work and much of the dielectric will be lost through evaporation. For this reason every spark erosion machine has a filtering plant, which has the following functions to perform:

  • Storing the dielectric
  • Cleaning the dirty dielectric coming from the work tank
  • Providing the required amount of clean fluid and the necessary pressure for rapid filling as well as for pressure and suction flushing
  • Cooling the dielectric (by air, water or cooling plant)
  • Processing backwashed fluid and filtrate

Cartridge filter system
In practice cartridge filter systems have proved very effective for filtering dielectric in smaller spark erosion plants, in which up to approx. 450 mm2/min are eroded. Cartridge filter systems are simple, and, as far as the cost of acquisition is concerned, inexpensive apparatuses. In the main they consist of a storage tank, filter pump, machine pump, cartridge filter, cooler and the requisite piping. The plant is operated manually.

The filter element itself is housed in a pressure resistant container and consists of a piece of paper, folded like a star and arranged around a central pipe. The filter cartridge is not reusable. Once it has attained its maximum capacity for retaining dirt, it has to be replaced by a new one. The fineness of the filtering effect of such a plant lies between 1 and 5 μm, depending on the paper used. Under normal conditions the dielectric IME can be used with a paper filter plant for about 1 – 2 years.

Precoated filter system
In big spark erosion plants it is advisable to mount a so called precoated filter system. In these systems the filter elements are coated with an even layer of filter aid, before filtering begins. This layer may consist of diatomite, Rixid or cellulose. After precoating is completed, the filter cycle of the plant is started, either by hand or by machine.

After a maximum differential pressure has been reached, the entire filter system is flushed back and all the dirt on the filter elements, plus the filter aid, are expelled via a mud valve into the after-filter. After the flushing back process is completed, the filter plant can be precoated anew and the filter cycle restarted. The filter area should be large enough, so that all the dirt accumulating during one shift can be absorbed, before flushing back becomes necessary. A fineness of up to 1 μm can be achieved with precoated filter systems.

On the average 1 kg of diatomite or 0.5 kg Rixid are required for 1 m2 filtering area. The residual moisture of a dry sludge cake discharged from a precoated filter system lies between about 20 % and 30 % of the weight, depending on the type of dielectric used. The service life of the dielectric in precoated filter systems is very long, since diatomite and Rixid not only have a mechanical cleaning effect, but also filter out acid components from the dielectric to a certain extent.

In precoated filter systems bleaching earth may also be used as a filter aid, in order to clean the dielectric even more thoroughly. There is data available from precoated filter systems, which were filled twenty years ago with a quantity of the dielectric IME, which is still fully operative today. Merely the amounts lost through drag-out and evaporation had to be replaced.

The Transor filter system
The Transor filter system is able to produce a filtering effect of 1 μm without the use of filter aids by employing the edge filter principle. Filtering rods, on which thousands of extremely fine special paper discs are mounted, are installed in a pressure tank. The dirty dielectric is pumped into the pressure tank and pressed through the filtering rods from the outside to the inside.

As this system works without filter aids, no precoating is necessary. The gaps between the paper discs are so narrow, that all particles that are larger than 1 μm are deposited on the surface of the filter rods. When the rods are dirty, backflushing occurs, and the dielectric, which has already been filtered, is pressed back through the filter rods in the opposite direction.

The dirt layer on the filter rods is blasted off and can be taken out of a sludge tank. There is little sludge in comparison to the precoated filter system, because no filter aids are used. The service life of the filter rods is on the average about 8,000 working hours. In a Transor filter system one must make sure that the viscosity of the dielectric does not excede 4.0 cSt at 20° C.

Diagram of a filter system for dielectrics working according to the edge filter principle
a) filter container,
b) filter rods,
c) filter pump,
d) sludge tank,
f) clean oil tank,
g) machine pump,
h) oil air cooler,
i) water trap and reducing valve for compressed air,
j) central valve with single-lever operation

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