A common problem faced by printers is the inability to run their web presses at rated speeds because of chill roll marking.
During printing, especially at higher speeds, 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 build-up."
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 who are 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 could experience marking. Paper grade and quality of the heatset ink also 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 the press speed, which is not a desirable solution.
Chill roll wipe systems: 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 the exterior surface of the chill roll as it rotates. This action wipes the solvent off the first chill. Once the tube or sock is saturated, it will need to be removed and replaced.
While chill roll wipe systems reduce or eliminate the solvent build-up that causes chill roll marking, the equipment is expensive, both in terms of the initial investment and the cost of cleaning supplies.
Chill roll air knife systems: 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 build-up 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 build-up, which causes chill roll marking at speeds up to 2,000 fpm. However, high energy costs (25 to 50hp motors are required), the high noise level (up to 85db) and the need for regular filter replacements could limit the use of these systems.
The electrostatic alternative
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. A charging bar is mounted directly over the first chill roll where the web comes into contact with it. When the bar is energized, the ion current flows to the web, and the resulting charge on the web pins it to the chill roll surface, displacing the air layer in the process.
The installation and setup of an electrostatic chill roll tacking system is rather straightforward. The charging bar is compact and integrates easily into a basic web press with existing smoke tunnels or into the most advanced presses with web-up chains and other forms of automation.
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 the ambient conditions change or the ionizing electrodes get contaminated over time. Condensate streaking is eliminated, allowing the operators to increase press speeds and improve the quality of the printed job. 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. Click here to view a chart depicting a typical electrostatic chill roll tacking installation.
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.
The 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.
One factor to consider with electrostatic chill roll tacking is the paper weight/thickness. The best results are observed with web basis weights of 28 to 40 lbs.
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.