User login

Dynamic Controls for Static

(December 2012) posted on Tue Feb 12, 2013

Learn about what static electricity is, the factors that affect it, and the methods available to counteract it.


By Screen Printing's Solution Sourcebook

Passive eliminators A charged object generates an electric field between itself and any surrounding earthed object (or any object of differing voltage). In the case of a passive eliminator, the field is between the surface and the tips of the carbon-fiber or stainless-steel earthed brush. The fine point at the end of the individual bristles causes the electric field to be highly concentrated at this point. Ionization of the air molecules surrounding the tip occurs when the strength of this electric field reaches a sufficient value.

Radioactive eliminators Radioactive eliminators use polonium-210 or another low-level radioactive source. Alpha particles are emitted to the surrounding atmosphere in the process of radioactive decay. These high-speed particles collide with the air molecules and, in doing so, cause the air to become ionized. This ionized air then neutralizes nearby surfaces in similar fashion to the passive eliminators.
AC eliminators AC Eliminators operate at supply frequency. The main voltage (110, 240, etc.) is greatly increased using a ferro-resonating transformer to generate voltages of between 4.5-7 kV. This high voltage is fed to the ionizing pins, and the casing of the bar is connected to earth.

If we look at the positive cycle of the input waveform, we will see that the electrode pin is at a positive voltage compared to the casing. This generates a strong electric field between the two that is highly concentrated at the sharp point of the electrode pin. This, in similar fashion to the passive bar, generates positive ions at the pin point. These molecules are then repelled from the pin due to their like charge. As the ionization at the bar is not dependent upon the surface charge and ions are produced regardless of the proximity of a surface charge, complete neutralization of a surface can be achieved. This is a significant advantage over the passive eliminators.

Pulsed DC eliminators Pulsed DC eliminators, like their AC counterparts, produce ionized air by using high voltage. Whereas the AC units operate at supply frequency the pulsed DC units operate at lower frequencies, often between 0.5-20 Hz. The low frequency of operation lends pulsed DC equipment to long-range neutralization. The ionizing bar consists of a series of emitters connected alternately to the negative and positive outputs. The casing of the bar is made of plastic, so there is no proximity earth.

The output from the power supply is effectively a square wave switching from negative to positive at the chosen frequency. Looking at the positive half of the wave form, the controller switches on the high output voltage connected to the positive emitters. This then sets up an electric field between the emitter and the surrounding earthed objects. At the sharp point of the emitter this field is extremely strong, and in similar fashion to the AC eliminators, positive ions are produced. The similar charge of the ion and the emitter drives the ions away from the bar. Long-range pulsed DC bars can be very effective in screen-printing production by eliminating electrostatic charges from screens without causing drying.


Terms:

Did you enjoy this article? Click here to subscribe to the magazine.