By Pillar Technologies
Corona Treating Continues To Become A Process That Is Being Employed By An Ever Increasing Number Of Converters.
The increased use of U.V. and waterbased inks, in conjunction with polyethylene and polypropylene is making corona treating a vital part of the converting process. As a result, corona treater manufacturers have been challenged to advance the state of technology to meet the increasing demands resulting from the expanding use of corona treaters.
Recent developments in corona treating designs have reflected the influence of the expanded role of corona treating in converting with the introduction of universal treating systems. These systems normally feature IGBT based variable frequency power supplies, which are designed to be able to perform on a wide range of treater station configurations. Proving to be as reliable as their SCR based predecessors, many of the wider ranged variable frequency IGBT designs, are capable of matching in to a wider array of treater station configurations.
Treater stations are becoming more universal in design to allow the converter the flexibility of changing station configurations from bare roll to covered roll to duel dielectric, in order to meet the specific needs of the job they are running. Although there are a variety of approaches intended to achieve this flexibility, they all involve the ability to change electrode and roller configurations.
Electrode configurations are most commonly changed by replacing electrode magazines or by rotating different electrodes mounted on to a common support bar, into position. Treater roll replacement is not as easy, since the roll design is dependent on the amount of power and the type of covering being used. It is for this reason that it becomes necessary for the converter to be more aware about the types of dielectrics and their performance strengths and limitations.
Definitions:
Before addressing the actual dielectric material used on the covered rolls, it might be helpful to review some of the critical parameters that, when combined, determine the overall effectiveness of a material as a dielectric.
Dielectric constant: Material that is able to concentrate a charge is expressed as a “dielectric constant.” Deficient treat levels can usually be overcome by using a material with a higher dielectric constant. Choosing a higher dielectric constant based material, such as epoxy or ceramic frequently enables a power supply to load into an existing system easier.
Ceramic rollers can provide structural strength that is sometimes insufficient in a bare roll w/ceramic tube electrode system. Therefore, to maximize the voltage storage capacity of the dielectric, it is desirable to have a high dielectric constant.
Dielectric strength: The ability to withstand excessive voltages that sometimes causes pinholing in dielectric coverings is a major concern when higher treat levels are being utilized. The electrode’s wall thickness covering material is determined by its strength. The lower the strength of the dielectric, the thicker the wall has to be. A thicker wall requires more power to produce efficient corona.
Power supplies provide a certain amount of voltage for which a dielectric covering material should be able to withstand twice that voltage amount. If a generator provides 20,000 volts, the dielectric materials should be able to handle up to 40,000 volts. Insufficient dielectric strength shows up as failures in the form of pinholes in the covering material itself. Primary reasons for “pinholing" include operating at too high a temperature or excessive voltage being used in the treatment process. Proper material selection is important to avoid roller failures.
Obviously, materials with high dielectric strength are preferable for corona treating. The wall thickness of the electrode’s covering materials is determined by its dielectric strength. Materials with high dielectric strengths can have thinner wall thickness. As the thickness of the wall is reduced, less power is required to produce corona treatment.
Ozone resistance: Heat and ozone generated on the dielectric surface have an erosion effect on most materials. This erosion reduces wall thickness and can ultimately result in insufficient dielectric strength and abrupt failure of the system. Dielectric materials used in corona treating applications must have high resistance to heat and ozone.
Cut resistance: Corona treaters operate in harsh environments that result in physical abuse that can lead to dielectric failure. Knife cuts, splices, friction from the web, drops, and banging all can have detrimental effects on the dielectric covering. It must be hardy enough to stand up to that abuse.
Heat dissipation: As the dielectric-covered roll rotates, it loses heat by radiation and convection. A material that readily dissipates heat is essential. If a dielectric material cannot dissipate heat normally, or if there is insufficient cooling due to the corona system design, burning is possible. Bare roll systems use a forced air cooling method vs. the convection cooling seen on conventional conductive stations. Properly sized dielectric surfaces determined by the correct choice of material are a necessary requirement to avoid heat dissipation.
Porosity: Dielectrics materials commonly have problems with porosity despite of the electrode covering. Porosity refers to the air entrapped in the coating, which can be a primary cause of roll failure. Porosity also relates to absorption of moisture, which can result in tracking of the electrical discharge to the ground, causing shutdown of the power source.
If a porous coating is used you may have a premature failure show up sometime within the first or first two weeks of usage depending on the degree of the problem. Porosity can prevent regrinding of rolls that were not covered properly.
The most common remedy to a porosity problem that may show up as a premature failure is to rely on suppliers with a the technological knowledge to assist in recommending the best dielectric/roller covering for quality and service on your application.
Ease of Field Repair: When a dielectric covering fails, the treater station is out of commission until the covering is either replaced or repaired. In most cases, replacement involved either having a spare dielectric roll in stock or sending the failed roll out for recovering. If there is no spare and it is necessary to repair the roll until a new roll can be installed, the ease of repair becomes an important measurement. Ceramic electrodes cannot be repaired.
Maximum service temperature: High temperatures can cause burn-through of the dielectric covering. This is usually seen as carbon blackening on the surface of the roll. 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.
Hardness: Surfaces hardness prevents cuts and abrasion.
Cost: As with anything, there is usually a tradeoff between desirable features and low cost. Each application should be analyzed to determine the best material for that particular job
The Materials
The specific applications and priorities of the user will determine which dielectric materials will be used for which jobs. A brief description of the strengths and weaknesses of each material may help your decision.
The variety of dielectric materials available that can be used for coverings can be broken down in to three main sections:
- Elastomerics: These include silicone, bonded silicone and hypalon. Some special blends are available but are the rights of the company that formulated the material.
- Inorganics: This includes glass, glassed steel, quartz, and ceramics.
- Plastics: Which include unsaturated polyester and epoxy.
Hypalon is an elastomeric coating that was formerly one of the premier coatings for dielectric rolls. The disadvantages of Hypalon are that it pinholes rather easily and is highly susceptible to knife cuts. It is most often used on applications in which an extremely long roll face or large diameter is required.
Epoxy is a hard plastic coating that was the primary replacement for Hypalon. It stands up to knife cuts and mechanical abuse much better than the Hypalon, is less susceptible to pinholing, and is easily repairable. It has a tendency for heat buildup.
Silicone is unique in that it can be offered in a sleeve design, which means that roll repair is greatly enhanced. Currently the sleeves are readily available in 4 inch, 6 inch and 8 inch diameters. Instead of sending rolls out for rework when there is a failure, the silicone sleeve can easily be slipped off the roll and a relatively inexpensive replacement sleeve can be slipped on. Downtime is reduced to a matter of minutes on a properly designed station. In addition, a spare roll is not normally required. These advantages, coupled with the electrical strengths of silicone, make this a very popular dielectric coating.
Bonded Silicone unlike silicone sleeves, which are extruded tubes that are slid over a mandrel, bonded silicone is applied directly to the mandrel and vulcanized. Bonded silicones normally feature very good temperature resistance and a high dielectric strength; however, they often exhibit a much lower tear strength and are more susceptible to mechanical damage. Although bonded silicone requires a completely new roll in order to replace the covering, the coating can be patched as a temporary fix.
Ceramics are the most expensive dielectric materials, but they also provide the best performance. They have excellent electrical and mechanical properties. The extremely high dielectric strength of ceramic results in a thinner wall thickness compared to other materials. It is the material of choice when very high treatment levels are required. It is also excellent for extremely harsh environments and will last longer than any other dielectric material.
Glass material is well suited to the corona process. A glass treater roll may be manufactured by applying several layers of glass to a machined casting using a distinct procedure. Special carbon steel is utilized, as aluminum cannot withstand high temperatures needed to fuse glass to the base metal. It is fired after each layer until the desired thickness of glass has been achieved. The resulting covering is hard and dense with a fire-polished smooth finish. Unlike ceramic rolls, it does not require any organic materials to fill voids generated during application of the roll coating.
Glass rolls have performed through the years and have become one of the most reliable coatings available. Featuring a very good dielectric strength and an excellent resistance to ozone, the glass rolls make a very good corona treater roll covering. A limitation is that the glass rolls require a special steel core, which is heavy and expensive. In addition, the mechanical tolerances on the coating are not as tight as ceramic, which can effect treatment in very high-speed applications. Finally, glass rolls are usually very expensive, although they are more competitive on larger rolls.
The Trend
The industry is continuing to look for flexibility and reliability. The dielectric covering is no different. Applications that do not involve frequent changeover and cannot afford down-time, tend to employ the more expensive inorganic coverings such as the ceramic and glass. Most other applications look toward the flexibility and reduced cost of sleeves. The industry will continue to demand new coatings that offer the reliability of the inorganic group, but offer the financial and handling benefits of the sleeves.