Choosing the Correct Dielectric in Corona Treating

From: Converting Magazine, Dec. 1999 v17 i12 88(3)
Author: Opad, Jeff

Full Text

The increased use of UV and water-based inks, in conjunction with the printing of polyethylene and polypropylene films, is making corona treating a vital part of the converting process. As a result, system manufacturers are challenged to advance corona treating technology to meet the increasing demands of converters.

The introduction of universal treating systems is a recent development reflecting the influential role that corona treating now has in converting. These systems normally feature IGBT-based variable-frequency power supplies, which are designed to perform on a wide range of treater station configurations. They have proved to be as reliable as their SCR-based predecessors.

Treater stations are becoming more universal in design. This allows converters the flexibility of changing station configurations from bare roll to covered roll to dual dielectric, 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 into position different electrodes mounted on a common support bar. Treater roll replacement is not as easy because the roll design is dependent on the amount of power and the type of dielectric covering being used. For this reason, converters need to become more aware of the types of dielectrics and their performance strengths and limitations.

Ten Critical Parameters

The following section reviews 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 an electric charge is expressed as a "dielectric constant." Deficient treatment 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 with 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 cause pinholing in dielectric coverings is a major concern when higher treatment levels are used. The electrode's wall thickness covering material is determined by its strength. The lower the dielectric strength, the thicker the wall must be. A thicker wall requires more power to produce an efficient corona.

Power supplies provide a certain amount of voltage, and a dielectric covering material should be able to withstand twice that voltage amount. If a generator provides 20,000 volts, the dielectric material should be able to handle up to 40,000 volts. Insufficient dielectric strength shows up as failure 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.

Obviously, material 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 walls. 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 can result in 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: Dielectric materials commonly have problems with porosity despite 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, a premature failure may show up sometime within the first two weeks of use, depending on the degree of the problem. Porosity can prevent regrinding of rolls that were not covered properly. The most common remedy for a porosity problem is to rely on suppliers' technical knowledge in recommending the best dielectric/roller covering for a particular 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 involves 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, ease of repair becomes important. Ceramic electrodes cannot be repaired.

Maximum service temperature: High temperature can cause burn-through of the dielectric covering. This is usually seen as carbon blackening on the roll surface. Temper-atures 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: Surface hardness prevents cuts and abrasion.

Cost: There is almost always a tradeoff between desirable features and low cost. Each application should be analyzed to determine the best material for a particular job

The Materials

The variety of dielectric materials available that can be used for coverings can be broken into three main types. Elastomerics include silicone, bonded silicone and hypalon. Some special blends are available with proprietary formulations; inorganics include glass, glassed steel, quartz, and ceramics; and plastics such as unsaturated polyester and epoxy.

The specific applications and priorities of the converter determine which dielectric materials will be used for which jobs. The strengths and weaknesses of each material follow.

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 also susceptible to knife cuts. It is most often used on applications in which an extremely long roll face or large diameters are required.

Epoxy is a hard plastic coating that was the primary replacement for Hypalon. It stands up to knife cuts and mechanical abuse better than the Hypalon, is less susceptible to pinholing, and is easily repaired. It has a tendency for heat buildup.

Silicone is unique in that it can be offered in a sleeve design, which greatly eases roll repair. Currently, sleeves are available in 4-, 6- and 8-in. diameters. Silicone sleeves can be slipped off the roll and a replacement sleeve slipped on. Downtime is reduced to a matter of minutes on a properly designed station. A spare roll is not normally required. These advantages, coupled with the electrical strengths of silicone, make this a popular dielectric coating.

Bonded silicone, unlike silicone sleeves, is applied directly to the mandrel and vulcanized. It normally features very good temperature resistance and a high dielectric strength; however, bonded silicone can often exhibit a much lower tear strength and is 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 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 the 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, a glass covering 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. With good dielectric strength and an excellent resistance to ozone, glass rolls make a very good corona treater roll covering. One 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 affect treatment in very high-speed applications. Finally, glass rolls are usually expensive, although costs are more competitive on larger rolls.

Trends Ahead

The converting industry continues to look for flexibility and reliability. The choice of dielectric coverings is no different. Applications that do not involve frequent changeover and cannot afford downtime tend to employ the more expensive inorganic coverings such as ceramic and glass. Most other applications look toward the flexibility and reduced cost of sleeves. In general, converters will continue to seek coatings that offer the reliability of the inorganic group but also offer the financial and handling benefits of sleeves.

DielectricDielectric ConstantOzone Strength (v/Mil)Ozone ResistanceCut ResistanceHeat DissipationPorosityField RepairabilityMaximum Service Temp(°F)Hardness (Shore A)Cost
Hypalon 5 to 6 400 Good Poor Fair Excellent Fair 150-300 60-90i Low
Epoxy 3 to 4 450 Good Excellent Fair Good Excellent 190-250 70-80 Low
Silicone 4 to 5 450 Good Poor Good-Excellent Excellent Fair 250 60-90 Medium
Bonded Silicone 3 to 5 - Good Poor Good-Excellent Excellent Fair 350 60-70 Medium
Ceramic 8 to 10 500-plus Excellent Excellent Excellent Good Poor 350 55 (Rockwell C) iiHigh
Glass 7 to 8 500-plus Excellent Excellent Excellent Excellent Poor 800 50 (Rockwell C) High

Source: Pillar Technologies

Copyright © 2000 Cahners Business Information


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