Corona treating is a surface modification technique that uses low-temperature corona discharge plasma to impact changes in the properties of a surface. With the increased use of UV and water-based inks with the conjunction of printing of polyethylene and polypropylene films makes corona treating an important part of the converting process.
As a result of which system manufacturers are challenged to advance corona treating technology to meet the increasing demands of converters. These systems normally feature IGBT-based variable-frequency power supplies which are designed to perform on a wide range of treater station configurations.
Electrode configurations are most commonly changed by replacing electrode magazines by rotating into position different electrodes on a common support bar. While treater roll replacement is not easy because the roll design is dependent on the amount of power and the type of dielectric covering like silicone sleeves. And for these obvious reasons, converters need to become more aware of different types of dielectrics and their performance strengths and limitations.
Here are some critical parameters that when combined can determine the overall effectiveness of a material as a dielectric.
Dielectric Constant
Dielectric constant is the material which is able to concentrate an electric charge. However, deficiency of treatment levels can usually overcome by using a material with a higher dielectric constant.
On the other hand, choosing a higher dielectric constant based material like epoxy or ceramic can enable the power supply to load into an existing system easier. In order to maximize the voltage storage capacity of the dielectric, it is desirable to select a high dielectric constant like silicone sleeves.
Ozone Resistance
As you already know that air and moisture generated on the dielectric surface can have an erosion effect on most materials. Due to this erosion, the thickness of the walls gets reduced and can ultimately result in insufficient dielectric strength and can fail the whole system. Although the dielectric materials used in corona testing must have high resistance to heat and ozone.
Cut Resistance
As corona treaters are operated in harsh conditions, it can result in dielectric failure due to knife cuts, splices, friction from the web, drops, and banging. So, the dielectric material should be tough enough to stand up to that abuse.
Heat Dissipation
Another feature that dielectric materials should have is the heat dissipation as the dielectrically covered roll rotates and loses heat by radiation and convection. If the material cannot dissipate heat easily then it could result in burning. While properly sized dielectric surfaces determined by the correct choice of material are a necessary requirement to avoid heat dissipation.
Ease of Field Repair
When an dielectric covering fails, the treater station is out of commission until the covering replaced or repaired. In most cases, replacement involves either having a spare dielectric roll in stock or sending the failed rollout for recovering.
Hardness
The surface hardness of the dielectric material prevents cuts and abrasion.
Maximum Service Temperature
As you know, high temperature can burn the dielectric covering which can be seen as carbon blackening on the roll surface.
While the temperatures on the roll are dissipated through the use of forced air-cooling, convection cooling, correct roll sizing at the time of station design and zero speed switches.
Porosity
There are many dielectric materials which have problems with the porosity despite the electrode covering. Porosity usually refers to the air that is trapped in the coating which can cause primary cause of roll failure.
However, porosity can also relate to the absorption of moisture resulting in the tracking of the electrical discharge to the ground causing a shutdown of the power source.
