Particulate contamination on base film is a major source of product waste in the converting industry. In order to reduce web and sheeted product contamination, it is common to install web cleaning equipment ahead of the coating/printing stations. Conventional non-contact web cleaners such as ultra-sonic, or air impingement types are not effective because they cannot overcome the boundary layer air on the web surface which captivates the dirt particles.
The thin film of air attached to all moving webs is known as boundary layer air. Small particles (< 50 microns) on the web surface are trapped in this air layer. High velocity air knives and ultra-sonic devices can be used to remove this layer of air at low web speeds (<100fpm). For small particles at higher web speeds, non-contact methods of web cleaning are not effective.
Contact Cleaning Rolls (CCR’S) 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 are transferred to the CCR surface. With a nip style arrangement, the CCR also squeezes out the boundary layer air attached to the substrate surface. As a result it is possible to achieve high efficiency cleaning (96%+) of small particles (<50 micron) at higher web speeds.
An important factor for 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. A nipped CCR cleaning efficiency is not affected by web tension.
Even with CCR web cleaning, the effects of web speed and boundary layer air must be considered. The idle roller and turret arrangements rely on web tension to squeeze out the boundary layer air between the substrate and the CCR. At higher speeds, the boundary layer air will tend to float the web on the surface of the CCR which reduces the contact area, and thus the cleaning efficiency. A nip style arrangement squeezes out the boundary layer air so that cleaning efficiency is not reduced at higher speeds.
In closing, nip style CCR installations offer the highest cleaning efficiencies. The nip offers the highest contact pressure between the CCR and the web, which eliminates the effects of web tension and speed without damage to fragile substrates. Non-contact web cleaning devices have lower small particle cleaning efficiencies because of boundary layer air.
Polymag Tek Inc.
(800) 787-0830; www.polymagtek.com