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Factors Affecting Treater Roll Durability
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The film manufacturing and converting industry creates an environment that is very tough on rollers and roller coverings. The continued drive for increased productivity, faster line speeds and improved product quality place added stress and wear on rollers and roller surfaces.
For lines that include a treatment process (corona, plasma, flame or chemical), the manufacturing environment becomes even harsher on roller surfaces. Ozone, heat, humidity, film residue and cleaning chemicals place additional wear and tear on roller surfaces. For treater rolls, the effect of these operating conditions ultimately results in either a mechanical breakdown of the roll or a dielectric failure (electrical breakdown). In either case, the roll failure results in expensive downtime and customer service problems. Prior to a failure, uneven and worn treater roll surfaces can also cause undesirable outcomes including unacceptable treatment, backside treatment and web damage.
Avoiding the conditions that lead to either premature roll failure or unacceptable product quality is paramount for film manufacturer’s and converters. This white paper examines how the durability and longevity of a treater roll cover is directly affected by two main variables; 1.) the manufacturing environment and 2:) the roll coating characteristics
1. Heat
Prolonged exposure to high temperatures will cause roll surfaces to wear more quickly than if the roll coating was operating at room temperature or slightly elevated temperatures. The chemical structure and composition of the coating as well as the design of the treater roll will determine how well a treater roll dissipates heat. Some treater roll surfaces, like silicone, do not dissipate heat very well. Other surfaces, like glassed steel, can dissipate heat very well. Rolls that do not dissipate heat well sometimes require a cooling system to effectively treat the film – this adds cost and complexity to the manufacturing process. Unless lower operating temperatures can be achieved to prolong roll life, companies should choose a roll coating that is able to dissipate heat effectively.
2. Humidity
Humid air not only causes moisture to accumulate on rolls, it also presents an avenue for premature dielectric failure to occur, especially where roll surfaces are constructed of porous materials.
3. Ozone
Ozone, especially in combination with heat, will degrade roll surfaces. Uneven and worn treater roll surfaces will result in backside treatment, uneven treatment and overall poor product quality.
4. Corona Discharge
Operating with generators, transformers, power supplies, electrodes or rolls that are not properly maintained and adjusted can lead to a “spiky” or uneven corona discharge. A spiky discharge occurs at higher voltages, leaving the dielectric roll surface more susceptible to dielectric failure. Properly tuning the main components of a treater station will help extend the life of a treater roll covering.
5. Operator
Well trained line operators will understand how to set the corona treater station properly, avoiding undue stress on the treater roll surface. Other conditions that the operator has influence over include the wrap angle and tension. Obviously, a specified wrap angle and tension must be maintained for proper treatment and production. However, excessive, unnecessary tension will cause grooving of porous roll coverings like ceramic. When wrap-arounds occur in the treater station, the operator must be very careful when cutting these so as to not damage the treater roll surface. To fail-safe this situation, companies should use treater roll surfaces that cannot be cut with utility knives.
6. Cleanliness
Roll surfaces that are not properly cleaned will decrease roll life and cause other treatment problems like backside treatment. Essentially, any leftover dirt or cleaning chemical will act as an abrasive on the roll surface, wearing the surface prematurely. In covered roll applications, metallic residue or chips (i.e. from filings or lathe work) will be cause for an immediate short.
7. Film Residue
Depending on resins used and the formulation for the film being run, there will be a tendency for film residue to build up on the roll surface. Some porous roll surfaces “attract” residue more than others. Excessive film residue build-up requires more frequent cleaning, further degrading the roll surface. Finding a smooth, hard, non-porous non-sticky roll surface is important when choosing a corona treater roll.
1. Surface Type (Organic or Inorganic)
Roll coatings are made from two types of basic materials: organic and inorganic. In general, organic materials will degrade as they are subjected to heat, ozone and cleaning chemicals, thus decreasing roll life. Some rolls, such as ceramic rolls, are a combination of inorganic materials with an organic sealer. The inorganic material will not wear, however, the organic sealer will degrade and wear. Rolls that are made up entirely of inorganic materials will outlast organic roll surfaces. Inorganic materials are also better at handling heat and are essentially immune to the effects of ozone and cleaning chemicals.
2. Porosity
Roll surfaces that contain porosity will wear or fail much sooner than a treater roll surface that does not contain porosity. Porosity on a roll surface not only reduces the dielectric strength of a surface (subjecting it to dielectric failure), but also allows a coating to abrade more easily as the web travels over the roll face.
3. Surface Finish
Smooth, non-sticky surfaces will prevent residue from accumulating on a roll surface and will require less cleaning and maintenance. Surfaces that can be ground and polished to a specific Ra will help with the function of the coating as well as extend its life.
4. Dielectric Strength
One of the most critical functions of a treater roll surface is its ability to operate effectively when subjected to continuous high voltage. Materials with high dielectric strength are better equipped to operate over long periods of time at high voltages. Periodically, high voltage spikes can occur during the treatment process. The ability of a roll surface to handle these spikes (its dielectric strength) directly impacts the longevity of the coating. Materials with lower dielectric strength will also require thicker coatings in an effort to prevent dielectric failure. The thicker a dielectric coating, the less capacitance. So not only are materials with lower dielectric strength more susceptible to breakdown, they operate inefficiently when compared to rolls with high dielectric strength.
5. Journal Construction/Bearing Surfaces
Although not the roll coating itself, the roll journals and/or bearing surfaces of a treater roll will also impact the life of a roll coating. Roll concentricity and straightness is critical to have even surface wear across the roll face. Manufacturers that are able to provide the roller coating and do the finish machining of the roll journals and bearing surfaces will be able to control the quality of the overall roll.
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