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Nov 1, 2007 12:00 AM
Variations in web tension often lead to web lift-off from the surface of the chill roll. In the boundary layer between the web and the roll, condensed ink solvent that has not been flashed off in the oven builds up on the exterior surface of the chill rolls, forming a condensate. The problem usually begins to develop on the first chill roll, its surface becoming glossy over time. This solvent will transfer, or “sling,” onto the printed web, causing resoftening of the ink on the printed web and creating print defects in the form of streaks on the finished product. Common terms for this problem are “chill roll marking,” “streaking” or “condensate buildup.”
This problem often occurs on jobs with heavy ink coverage, such as magazines with many photos, and on presses that have extra printing units (cover presses). Printers producing heavy coverage retail coupons or time-sensitive inserts for newspapers will have marking issues. Printers trying to run jobs at high speeds on presses with “short ovens” also experience marking. Paper grade and quality of the heatset ink can contribute to the problem.
If the condensate is not removed from the chill roll, the only practical way to eliminate marking is to reduce press speed.
These are mounted on the back side of the first chill. Periodically, the wiper assembly, which includes a cloth wrapped sponge-wiper called a tube or sock, is indexed to the chill roll and wipes its exterior surface as it rotates. This wipes the solvent off the first chill. Once the tube or sock is saturated, it must be removed and replaced.
An air knife is installed across the chill roll at the end of the smoke tunnel near the wrap point, with the continuous nozzle very close to the web. A heavy duty blower motor provides high-pressure air to the air knife. The high-pressure, low-volume air stream applied to the web near the wrap point displaces the buildup of a boundary air layer in the gap between the chill roll and the printed web.
These systems achieve good web hold-down, ensuring contact across the chill roll. And, they reduce or eliminate solvent buildup, which causes chill roll marking at speeds up to 2,000 fpm. However, high energy costs (25- to 50-hp motors are required), the high noise level (up to 85db) and the need for regular filter replacements could limit the use of these systems.
Using electrostatic force is, arguably, the most cost-effective way to eliminate chill roll marking. A typical electrostatic chill roll tacking system consists of a high-voltage charging generator and a charging bar. The bar is mounted over the first chill roll where the web comes into contact with it. When energized, the ion current flows to the web, and the resulting charge pins the web to the chill roll surface, displacing the air layer.
Once the generator output has been adjusted for the charging current to provide optimal pinning (confirmed by the web temperature drop), the operators engage the “Constant Current” mode, where the generator automatically adjusts the voltage to maintain the set current. In this mode, the system maintains stable and strong pinning power even as ambient conditions change or the ionizing electrodes get contaminated over time. Condensate streaking is eliminated, allowing the operators to increase press speeds and improve print quality. Press speed increases of five to 20 percent have been observed.
After the web exits the chill stand, it passes a static neutralizing system to eliminate any downstream processing problems. One factor to consider with electrostatic chill roll tacking is paper weight/thickness. The best results are observed with web basis weights of 28 to 40 lbs.
The added benefits of electrostatic tacking include better thermal transfer of the heat from the web stock to the chill roll. Reduced temperature readings after the first chill roll confirm the web is not “floating” anymore. Other benefits noted by users are better web tension and less web weave.
Electrostatic chill roll tacking systems are not very expensive; they cost approximately half what a typical chill wipe or blower air knife system costs, they have no consumable parts or materials, and they use about the same energy as a light bulb. The only maintenance required is a regular cleaning of ionizing electrodes using a metal bristle brush.
Many considerations go into selecting the best method for controlling chill roll marking, and printers should seek advice from equipment manufacturers and other printers with experience using different systems.
Ken Troemel, Andy Grzesik and Mark Blitshteyn of MKS Ion Industrial have many years of combined experience in creating effective electrostatic solutions for various printing and converting problems. Contact them via www.mksinst.com.
Part 1 of the “Electrostatics from the pressroom to the bindery” exclusive online spotlight at www.americanprinter.com