Ceramic electrodes are one of the most important advances in the art of surface treating during the last century. Ceramic electrodes make it possible to treat any substrate, even metalized films and foils.
Today's unique and complex packaging alternatives incorporate metallized substrates and these products demand reliable surface treatment.
Ceramic electrodes offer a remarkable increase in treating range and have become the standard for many converting applications.
Although rugged, protecting ceramic electrodes from excessive heat and physical shock is of paramount importance. The first step in understanding these enemies of the ceramic electrode is understanding the construction of the electrode itself.
The main components of a typical ceramic electrode are (refer to drawing below):
- filling material used to eliminate air pockets inside the tube.
- an active element
- a high voltage lead
- a rectangular ceramic tube with outside mounting tabs
Corona is generated by applying high voltage electrical energy to the active element inside the electrode and then discharging this energy through the dielectric ceramic to the ground roll. This process generates a significant amount of heat. The more power applied the greater the heat generated.
Most electrode failures are caused by an excessive buildup of heat. Excessive heat build-up from insufficient airflow is caused by improper system installation or lack of system maintenance.
As the heat intensifies the ceramic tube begins to expand. This is normal and the electrode is designed to handle this expansion.
If the airflow around the electrode is insufficient because of improper exhaust installation or lack of maintenance, the heat can become severe and stress the ceramic tube. When the heat reaches excessive levels, the stress may cause hairline cracks or even fractures to develop in the ceramic.
These heat related ruptures typically run down the length of the ceramic tube. Ruptures compromise the dielectric strength of the ceramic wherever they occur. This results in a higher current discharge in those areas. Ultimately, the dielectric properties of the ceramic will completely fail at these points causing "pin-holes" - points where the high voltage current will arc directly to ground.
The key to preventing this type of failure is to ensure proper airflow across the electrodes. You can determine if your airflow is adequate by examining the following:
- Is the exhaust blower properly sized and installed?
- Do you know the CFM and water column at the station?
- Is the ductwork made from the correct material, properly sized and properly installed?
- Is the airflow path around the electrode and through the plenum of the electrode assembly clean?
Dirt and dust in the surrounding air, contamination on the web and bits of film resulting from web breaks sometimes find their way behind the electrodes and become an impediment to airflow. Enercon's unique hinged electrode design makes the process of finding and eliminating such obstructions a "piece of cake" (see article for more information).
The next most frequent source of damage to ceramic electrodes is physical shock.
Ceramic electrodes must be handled more carefully than metal electrodes. They must be protected from collisions with large substrate splices, tools and any other abrupt contacts. Damage resulting from physical shock is usually manifested in a significant break around the circumference of the ceramic tube. This type of damage is almost always fatal to the electrode and it must be immediately replaced.
High Voltage Lead Failure
Occasionally a failure will occur in the high voltage lead. This usually results from improper installation of the electrode, but could also result from a failure of the lead itself. This type of failure will be evident from the discoloration of the insulation on the high voltage lead. In very rare cases the ceramic used to manufacture the ceramic tube may be defective.
When properly installed and maintained ceramic electrodes will provide years of reliable performance. Electrode failures don't happen that often... but when they do, we want to help make sure that you experience the best possible outcome.