Functions of the Dielectric

Insulation:

One important function of the dielectric is to insulate the workpiece from the electrode. The disruptive discharge must take place across a spark gap which is as narrow as possible. In this way efficiency and accuracy are improved.

Ionization:

As quickly as possible optimum conditions for the production of an electrical field must be created and then a spark path must be provided. After the impulse the spark path must be de-ionized quickly so that the next discharge can be made. The dielectric ought to constrict the spark path as much as possible, so that high energy density is achieved, which increases discharge efficiency at the same time.

Cooling:

The spark has a temperature of 8,000-12,000° C when it punctures the workpiece and so the dielectric must cool both the electrode and the workpiece. Overheating of the electrode must be avoided, so that excessively high electrode wear cannot occur. It must be possible for the metal gases which develop during spark erosion to condense in the liquid.

Removal of waste particles:

Metal particles that have been eroded away must be removed from the area of erosion by the dielectric to avoid disruptions in the process.

Requirements for Dielectric

Theoretically all insulating liquids can be used as dielectrics. However, due to the requirements set out below, only de-ionized water (for polishing) and hydrocarbons are used for this purpose today. These hydrocarbons can either be produced by distillating and refining mineral oil, or synthetically by processing gases in a synthetizing oven with the help of a catalyst. Synthetically produced hydrocarbons are characterized by otherwise unparalleled purity. In addition, precisely those chains of hydrocarbon molecules can be synthetized which have the best possible erosive effect as well as offering optimum protection against electrode wear. In this way they are far superior to those mineral.oil products which are produced from certain mineral oil fractions.

Criteria for assessing Dielectrics

The following criteria are generally used today to assess different dielectric fluids:

a)    Degree of metal removal and electrode wear
b)    Effects on health:
           skin irritation
           toxicity
           smoke
           odours
c)    Flash point
d)    Density
e)    Viscosity
g)    Conductivity
h)    Dielectric constant
i)     Disruptive
j)     Particle suspension
k)    Filterability
l)     Compatability with other machine components (engine parts, varnish, sealing material)
m)   Aging stability
n)    Constancy of quality
o)    Availability
p)    Price

In general it can be said that it is easy to develop a product which achieves excellent results according to one or another of the above criteria. However, it is important for the utilized product to achieve an optimum in them all, if possible. Thus it is possible for a product of the highest mechanical efficiency, combining high metal removal with low electrode wear, to be unusable in practice, because of physiological reasons, or because it eats into engine parts.

Effects on health:

In the present, and certainly even more so in future, the effects of industrially used hydrocarbon fluids on health are becoming increasingly important. Smoke, odours and skin irritation have a decisive influence on working conditions at spark erosion machines.

Skin irritation:

Products, which are so pure that they are unharmful from a dermatological point of view, should always be given preference over others. As far as possible these products ought to consist of completely saturated hydrocarbons and should contain as few aromatic compounds as can be. An aromatic content of less than 1 % in vol. is desirable. Hydrocarbons from the normal paraffin series Of C₁₂ to C₁₄ often cause skin irritation and ought not to be used. If at all possible only such products ought to be used which have been proven to be unharmful to the skin by independent medical tests.

Toxicity:

There are as yet no legal provisions i.ro. toxicity (or rather physiological properties) for the industrial utilization of dielectric fluids. Low aromatic content in an unused product is not on its own an indication of good quality. Far more important is the question, to what extent there is a tendency for aromatic compounds to develop during erosion (aging stability). Even after the product has been in use for some time it must not develop any polycyclic aromatics (e.g. benzpyrene), which are today considered to be carcinogenic.

Smoke:

The amount of smoke given off during erosion is largely dependent on the varying rates of metal removal. Thin-bodied dielectrics usually give off less smoke than more viscous ones. The higher the flow of the dielectric over the place of erosion, the less it smokes. (According to German engineering guidelines - VDI 3400 - the dielectric level must be at least 40 mm above the place of erosion.) A ventilator should always be provided at a spark erosion machine, unless it is used exclusively for fine work.

Odours:

The unused dielectric should be odourless and should not begin to smell, even when heated. After it has been used for some time, it is quite usual for a faint ozonic smell, caused by the electrical discharges, to develop. A sour, acrid smell, however, is often an indication that the dielectric ought to be renewed.

Flash point (German standard - DIN 51755):

The flash point is the lowest temperature at which a dielectric gives off sufficient vapours to produce an inflammable mixture of air and gases in a standardized apparatus. The higher the flash point, the safer is the use of the dielectric. Dielectrics are divided into different danger classes according to their differing flash points.

Danger class:        A I        under 21° C         e.g. benzine
                            A II       21-550° C           e.g. crude petroleum, white spirit
                            A III      55-1000° C          e.g. diesel, light fuel oil

According to German engineering guidelines - VDI 3400 - substances with flash points below 21° C may not be used in spark erosion machines. It must also be pointed out that crude petroleum and white spirit are in danger class A 11 and that special safety regulations must therefore be complied with when they are used.
Most of the dielectrics in use today are in danger class A Ill. Dielectrics whose flash point is over 100° C are not considered to be inflammable as defined by German law. No special safety measures are therefore needed for them.
To determine the flash point of fluids in accordance with the German legal provisions for industrial substances, flash points up to 50° C must be measured with the Abel-Pensky apparatus, while flash points of over 50° C must be measured with the Pensky-Martens apparatus (FIp. PM). It is not permissible to use an open cup apparatus, such as the one developed by Cleveland.

Density (German standard - DIN 51757):

Irrespective of viscosity, the influence of density is greater during the finishing process than in rough cut operations. "Heavy" products remove more metal. The density of a substance is the ratio of its mass to its volume (usually measured at a temperature of 15° C). Dielectrics normally used today have a density of 0.750-0.820. The shorter the chain of hydrocarbon molecules, t e lower usually is its specific gravity. Changes in the specific gravity of a dielectric before and after use indicate that alien substances, such as hydraulic fluid, have entered it. Density increases in a dielectric which was blended from different fractions show to what extent the more volatile parts have evaporated. Density can easily be checked with a densimeter (hydrometer). This is a floating glass instrument with a density scale (units of 0.001) also containing a thermometer.

Evaporation number (German standard - DIN 53170):

The evaporation number (VD) is the ratio of evaporating time for the dielectric to that for ether. Dielectrics for polishing work should have an evaporation number of 500-1000. For economic reasons, substances that evaporate more quickly (e.g. Petroleum VD 260) are not suitable as dielectrics.

Viscosity (German standard - DIN 51562):

Viscosity is the property of a fluid whereby it tends to resist the displacement of two neighbouring layers. The physical unit of measurement of absolute viscosity is the Pascal second. One mPa.s is equal to one Centipoise (cP). The ratio of absolute viscosity to density is called kinematic viscosity. The unit of measurement is the square metre per second (M2/ s). A centistoke (cSt) is equal to 1 mm2/s. The viscosity of thin-bodied dielectrics is usually measured at a temperature of 20 0 C.

Dielectrics of 2 to 3.5 cSt at a temperature of 20° C are suitable for polishing work. 4 to 6. 5 cSt at 20° C is suitable for rough cut operations. The disadvantage of dielectrics which have been produced from two fractions of differing viscosity is that the more volatile, less viscous components evaporate more quickly, leaving behind a dielectric which is so viscous after prolonged use that it is suitable only for rough cut operations. The surface roughness of the processed workpiece is also dependent on viscosity. Thus a narrow spark gap can be used with a thin-bodied dielectric, leading to a finer finish. When more viscous dielectrics are used, a larger spark gap must be chosen to avoid flushing difficulties. This leads to greater roughness in the processed workpiece (see fig. 1).

Table of recommended viscosity as a function of Hmax

Conductivity:

Conductivity is equal to the reciprocal of volume resistivity. The unit is the Siemens. A conductivity AC bridge on the Whetstone bridge principle, at frequencies of 50 or 3000 Hz, is used for measurement. Hydrocarbon dielectrics for industrial use have a conductivity of about 2x 10^-14 ohmxcm^-1 when new.

Dielectric constant (German standard - DIN 53483):

The relative dielectric constant (DK) of a particular dielectric shows to what extent the capacitance of an empty capacitor is increased by introducing that dielectric. A (dielectric constant) DK-meter is used to measure the dielectric constant. The capacity of a capacitor is measured by connecting it to a high frequency resonant circuit, both when filled with dielectric and when empty. The dielectric constant is the ratio of the two different values obtained. A dielectric suitable for spark erosion ought to have a dielectric constant of 2-2.5.

Disruptive voltage (German standard - DIN 53481 / German electrical guidelines - VDE 0303):

The voltage required to disrupt a 2.5 mm layer of dielectric between two spherical electrodes is called disruptive voltage. Good dielectrics should have a disruptive voltage of 50-60 kv when new. It must be noted that the least amount of moisture added to the dielectric (e.g. condensation water) will have a negative influence on this value.

Particle suspension:

Waste particles eroded away from the workpiece and the electrode, as well as carbon particles resulting from electrical discharges, are impurities in the working substance. The dielectric must remove these particles from the work area. Adequate particle suspension is necessary for this task. However, particle suspension must not be too high, otherwise these impurities will not separate from the dielectric during filtration. Too many impurities lead to arcing. On the other hand, a dielectric will only function in the best possible way if a few micro-particles are to be found in the dielectric, as this is conducive to ionization. These tiny particles can even be added to the dielectric artificially when it is new to improve erosion from the start.

Compatability with other machine components:

Dielectric fluids in industrial use must remain neutral towards other machine components with which they come into contact, e.g. sealing material, tubes and varnish used in containers. The dielectric must not cause these materials to swell up, shrink or dissolve.

Aging stability:

Aging stability in dielectrics is very important for economic reasons. The longer a product can be used, the better is the relationship of price to performance. In ordinary erosion practice it ought to be possible to use a dielectric with paper filtration for one or two years. When using precoated filters, dielectrics have now been known to last for almost 20 years without having been renewed. In these cases nothing more was done than to replenish the dielectric tank as the need arose. Age can be assessed by means of infrared spectrographic analysis, but the alternative method, by which neutralization value is determined (NZ/German standard DIN 52558), has also proved to be reliable up to the present. Dielectrics with an acid number of more tha 1 mg/KOH/g ought to be renewed as soon as possible.

Quality and availability:

The producer of a dielectric must be able to guarantee its quality for an adequate period of time. In addition, the quality of a dielectric sold under the same name in different countries must always be the same. Dielectric fluids for industrial use ought to be available in those quantities, in those localities, and within those time periods, in which they are required.

Prices:

When prices are compared, all the above criteria must be taken into consideration, as the dielectric which is cheapest at first is often the most expensive in the long run.