Article reproduced from
Converting Magazine
September 2000
Experiments with 10-micron particle contaminations show which methods work best.
By Gary Larsen, President, and Frank Corrado, Managing Director, Polymag Tek, Inc.
Particulate contamination on base film is a major source of product waste in the converting industry. To reduce web and sheeted-product contamination, it’s common to install web-cleaning equipment ahead of the printing and/or coating stations. Conventional non-contact web cleaners such as ultrasonic, or air impingement types, are often not effective because they cannot overcome the boundary-layer air on the web surface which captures the dirt particles.
This article explains the benefits of today’s contact cleaning rolls (CCR) in the context of experiments using different cleaning methods for web decontamination.
The thin film of air attached to all moving webs is known as boundary-layer air. Small particles (less than 50 microns) on the web surface are typically trapped in this air layer. High-velocity air knives and ultrasonic devices can be used to remove this layer of air at low web speeds (usually less than 100 fpm). But for small particles at higher web speeds, non-contact methods of web cleaning are often not effective.
CCR efficiencies
Contact cleaning rolls are polymeric-covered rollers, which offer an efficient method of web and sheet cleaning. As the particles attached to the substrate make physical contact with the CCR, the particles transfer to the CCR surface. With a nip-style arrangement, the CCR simultaneously squeezes out the boundary-layer air attached to the substrate surface. As a result, it’s possible to achieve high-efficiency cleaning (more than 96 percent) of small particles (less than 50 micron) at higher web speeds.
Typical CCR installations take the form of either an idler roller, a turret arrangement or nip-style layout (See Figure 1). Each form provides a different level of cleaning efficiency depending on web tension (See Graph 1).
Contact pressure
An important factor in high cleaning efficiency is the contact pressure between the substrate being cleaned and the CCR surface. The turret and idle roller CCR arrangements rely on web tension and wrap angle to create the contact pressure between the web and the CCR. Graph 1 illustrates that for these configurations, lower web tension results in lower cleaning efficiency. In general, cleaning levels using a nipped-CCR arrangement are not affected by web tension.
Contact pressure can vary significantly depending on the CCR installation. For example, a nipped CCR can have 800 percent more contact pressure than a turret-style layout (2.0 psi compared to 0.25 psi).
The thick, compliant (soft) surface of the CCR attracts contamination without causing dimples or damage to fragile, thin substrates. The CCR “nip” arrangement, for instance, has been successfully used on photographic film, copper foil and magnetic tape products, to name a few.
Speed and traction curves
Even with CCR web cleaning, the effects of web speed and boundary-layer air must be considered. The idler roll and turret arrangements rely on web tension to squeeze out this air layer between the substrate and the CCR. At higher speeds, the boundary-layer air tends to float the web on the surface of the CCR, which reduces the contact area, and thus cleaning efficiency (See Figure 2).
A nip-style arrangement squeezes out the boundary-layer air so that cleaning efficiency is not reduced at higher speeds (See Graph 2). The nipped CCR has a much higher contact pressure between the web and the CCR, maintaining nearly 100 percent contact at all times. For idler roll and turret arrangements, as contact pressure declines and web speeds increase, contact area can be reduced by 40 to 90 percent. Such a significant reduction only dramatically lowers cleaning efficiency.
Cleaning calculations
For 10-micron particle contamination, CCR efficiency can be calculated from the data on Graphs 1 and 2.
Example 1:
CCR idler roll at 1 PLI tension (30-in. web at 30-lbs. tension)
Graph 1: 1 CCR @ 1 PLI = 50 percent cleaning efficiency.
Graph 2: 200 fpm @ 1PLI = 80 percent contact area.
Total efficiency = 50% X 80% = 40%
Example 2:
Nip-style CCR at 1PLItension (30-in. web at 30-lbs. tension)
Graph 1: Cleaning efficiency = 96%
Graph 2: 200 fpm @ 1 PLI nipped = 100 percent contact area
Total efficiency: 96% X 100% = 96%
CCR efficiency has been shown to extend even down to single-digit micron-sized particles. Scanning electron microscope photography (1,000X) from the Rochester Institute of Technology of a CCR surface reportedly reveals 1.6-micron spheres removed from a film sample.
Conclusions
Of the three types of contact cleaning-roll arrangements, nip-style CCR installations offer the highest cleaning efficiencies. The nip yields the highest contact pressure between the contact cleaning roll and the web, which eliminates the effects of web tension and speed without damage to fragile substrates.
Non-contact web cleaning devices may have lower small-particle cleaning efficiencies because of boundary-layer air.
Gary Larsen is president of Polymag Tek, Inc., Rochester, N.Y. He has a master’s degree in engineering from the State University of New York and 27 years’ experience in design and manufacturing. Frank Corrado, managing director, holds a BS in engineering from the University of Wisconsin. He has worked in product design, development and manufacturing for 37 years. Both can be reached at 585/235-8390, fax: 585/235-8395, email: info@polymagtek.com, www.polymagtek.com