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Web Cleaning: To Contact or Not?

The need for companies to improve productivity without investing in complete new lines or equipment has resulted in an increasing interest in technologies that reduce waste and downtime. High on the list are web cleaning and static control systems, and while the latter is often a solution in its own right, it would be pointless to install the former if static is not being controlled, because the substrate is likely to be recontaminated quickly.

Web cleaning systems fall under two headings — noncontact and contact. Within those two groups there are a variety of offerings, and as a general rule, the cost of a noncontact system is greater.

Cost aside, if the production line is used for substrates with a sensitive coating or that are prone to marking, then a noncontact system will be the best option. This is also the situation with companies operating in fields in which it is desirable to have as few objects as possible touching the material, e.g., pharmaceutical packaging and LCD and plasma screen industries, etc.

During the past six months, there has been a trend for companies to invest in noncontact web cleaning systems even if the press or production line currently does not fall into the criteria above, simply because they want the flexibility to handle any substrate in the future.

The majority of systems invariably rely on some form of vacuum, although this on its own is unlikely to be sufficient. Whatever the technology employed, to operate effectively a web cleaner must break up the boundary layer of air that is entrained by the moving web, holding contamination firmly to the substrate's surface (any fast moving object will create a boundary layer of air near the surface).

Contact Systems

Contact web cleaners fall under the following broad headings:

  • Vacuum Systems | Typically the web is in contact with a low friction part of the web cleaning manifold, which breaks the boundary layer of air, allowing the contaminants to be vacuumed from the surface. Systems commonly incorporate active AC static control bars before and after the vacuum slot. Depending on the system, the vacuum is generated by compact extraction units or three-phase fan units that include filtration ranging from F8 grade (90%-95% efficient at 0.4 micron) to 5 micron respectively.
  • Tacky (Elastomer) Rollers | These systems use a sticky roller, driven by contact with the moving web, to pull off debris and fibers. The roller is in contact with a second roller that employs more adhesive, which in turn takes up the contaminants. The systems are relatively inexpensive, easy to install, and on the whole provide adequate results. The main complaints from users are the cost of buying the replacement sheets for the second roller, the time it takes to fit them, and if the sheets are not replaced, then performance decreases as the roller becomes more saturated with contaminates.
  • Brush Systems | This style of system is probably the least popular among users due to the common design, which typically involves the web being “scrubbed” by a range of brushes. Even if the web is not susceptible to marking, the process can appear harsh. One of the main benefits is a relatively small cross section manifold.

Noncontact Systems

Noncontact systems can broadly be grouped as follows:

  • Boundary Layer | These systems are the latest breed. They function by creating a powerful layer of air that breaks the web boundary layer, which is then “sheared” into a vacuum chamber. Boundary layer systems have low running costs and no consumable costs. Systems include active AC static control bars. As with vacuum systems, a range of extraction units is available.
  • Ultrasonic | Reports show these systems are effective, but typically they are the most expensive style of web cleaner. These systems work by blasting the web with ultrasonic pulses from either side of a vacuum slot. These systems often include static eliminator bars located on the infeed of the system.
  • Blow and Vacuum | These systems include a combination of blowing and vacuum airflows to strip contamination from the surface. The systems can be very effective, but some reports show they can require a lot of ongoing adjustment of the airflows to achieve the desired cleaning results.

Installing a web cleaner can have a dramatic effect on profits. One European gravure printer reports a monthly sales revenue increase of about $12,000 following the recent commissioning of a noncontact web cleaner on a 48-in.-wide, ten-color Cerutti gravure press.

Prior to the installation, the Cerutti was stopped an average of 16 times per month because of damaged/worn doctor blades or cylinder rollers, debris in the ink pan, streaking and hickies, or web breaks. This equated to 12,000 ft of wasted material and a total loss of $20,000 per month. The web cleaner has reduced stoppages to an average of six per month and cut waste to 4,500 ft. The return on investment is expected to be within nine months.

Controlling Static

Some form of static control, preferably active, normally is incorporated as part of the cleaning process to neutralize any static charges that may be holding contamination to the web. It also can reduce the risk of recontamination caused by static attracting airborne particles from the environment.

Handheld static locators can be used to measure the voltage on the material at different areas of the machine or press. Where excessive voltages are found to be causing contamination, ionizers should be selected to suit the installation position. Modern AC ionizing bars are powerful ionizers that can cope with the fast web speeds found on modern printing and converting machinery. Mounted close to the web, the AC ionization neutralizes charges of either polarity.

Resistive coupling of the emitter pins, which makes the units safe to touch, is an important factor in operator safety. Often, excess voltage is found on unwinds and rewinds. The cumulative effect of layer upon layer of charge can produce frightening results. To combat this, pulsed DC ionizers have become popular for their ability to control charges over long distances without the need for moving air. To improve the neutralization of charges of known polarity, the output of pulsed DC ionizers can be biased to give faster and more accurate control.

Adjustment, however, requires intervention generally by busy operators. For a small extra investment, high-end pulsed DC systems can incorporate a closed-loop feedback facility that automatically adjusts output to attain the most accurate static control possible.

The positioning of the ionizers is critical, if the best results are to be obtained. Like most things to do with static control, experienced technicians are vital — it's not an area for amateurs.

Adam Battrick joined Meech Intl., Witney, UK, in 1997 and has held various sales and marketing positions within the company. Currently he is a business unit director responsible for the worldwide sales and marketing of the Meech range of cleaning systems, including the ShearClean and Tornado web cleaners. Contact him at sales@meech.com.

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The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).

Want To Learn More?

PFFC covers static each and every month in contributing editor Kelly Robinson's “Static Beat” column. Find it this month on p10 or online at http://pffc-online.com/static-beat.


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