Solo Systems, Inc.

The function of the electrode is to deliver the generator power to the web.  It does this by presenting the high voltage to the air gap between the electrode and the insulated treater roll which carries the web.  The power stresses the air to the point at which it ionizes, corona is formed, and the web is treated.

The degree of treat is a function of the intensity and duration of the corona discharge per unit area.  A shoe-shaped electrode which is concentric with the surface of the treater roll exposes the film to corona longer than a bar or knife edge electrode, so treat levels increase.  Because the corona spreads more evenly, hot spots are reduced and damage to the web and roll insulation is avoided.

Electrode shoe size is based on the following formulas:

    Generator Frequency            Shoe area per kW
    3 kHz solid state                     70-100 inches squared
    9.6 kHz solid state                  30-47 inches squared
    17 - 18 kHz                                20-24 inches squared
    25 kHz                                        12-18 inches squared

The graph below shows the most effectice shoe size for different line conditions.  To read this graph, find the intersection of your treat width with your generator power rating.  The shoe width (in inches) is shown directly below this intersection at the bottom of the chart.  Please note the graph assumes a generator frequency of 9.6 kHz and .125" thick Solo Systems E-3 Epoxy treater roll coating.


The bare roll system is presently the most common option for treating conductive materials like foil.  The electrode in a bare roll treating system does double duty.  It not only delivers power to the web, it carries the nonconductive surface necessary for corona to form.  Therefore, the key to successful bare roll treating is in the electrode design and construction.

In any system, the electrode carries off approximately one half the heat produced by the corona.  Insulated electrodes run hotter than conventional electrodes.  The insulating material must be exceptionally heat resistant, and the system adequately cooled during operation for acceptable service life of the insulation.

Several manufacturers supply insulated electrodes made of glass, ceramic or composite materials.  The more heat resistant materials, such as glass or ceramic, are brittle and easily damaged by impact, such as  a splice in the web.  The composite materials are tougher; however, they must be void-free to prevent corona developing in air pockets inside the insulation and shortcircuiting the system.

Adequate bare roll treating systems are available.  They are typically more expensive and less durable for conventional applications.  However, it is possible to develop a successful, bare roll treating system for conductive web materials.

The gap between the roll and the electrode provides a controlled volume of air in which corona can be formed.  This happens because the air gap between the electrode and the roll coating has the least dielectric strength of the electrical components.  By applying the right output voltage to the electrode, the air can be maintained in the ionized state commonly called corona discharge.

The power delivered to the air gap is a function of the gap spacing, the applied electrode voltage, the dielectric constant and thickness of the coating, and the output frequency of the power supply.  If the web is thicker than 4 or 5 mils, its capacitance should be included in the electrical equation.

It is important to maintain an accurate and correctly proportioned air gap in order to deliver full-rated generator power to the web.  If the air gap is too wide the excess voltage required to start corona can damage the web or the roll coating.  Corona discharge becomes harder to start as the gap becomes wider.  If the air gap is too narrow, maintaining the gap evenly across the width of the web becomes too critical for production efficiency.

An uneven air gap across the width of the web will produce uneven treat levels across the web.  A constant gap will produce consistent treat measurements.