Can't Touch This (Web): Part 3
- Published: June 01, 2003, By Timothy J. Walker, TJWalker & Assoc. Inc.
Over the last two months, I have reviewed why some webs demand “U Can't Touch This” and web line options for true touch-free web handling. This month, let's cover qualified touch-free cases, where limited contact is allowed, including one-sided and edge-only contact.
One-sided web handling is like holding buttered toast or a DVD — touching one side is no problem, but touching the other side may cause a big mess or damage. A videotape cassette is an example of a backside-only web handling system where the magnetic coated face is the “untouchable” side. A videocassette unwinds, transports, guides, tensions, and rewinds the tape without faceside contact (not counting the contact within the winding roll). A videocassette is an example of a lazy C web line layout where the entire line from reel to reel will avoid face-side contact.
One-sided contact, whether for a short distance or an entire web line, is a game of wrap angle allocation. On many coaters or printers, the game begins once the web exits the coating station. The web exits the print station heading up and returns from the dryer heading down, leaving 180 deg of wrap for all one-sided web handling functions in between.
If the dryer has ten idler rollers, we have 18 deg for each. However, if we also need a single roller web guide (90 deg), a pull roller (90 deg), and a tension-measuring roller (30 deg), we won't have enough web wrap to go around.
Fortunately, we can change the rules of the game. We can increase the total wrap allotment by changing the start and end orientations. In the lazy C web line, we can increase the total one-sided wrap by changing how we enter or exit the winders. For more wrap angle, you can imagine a web path that resembles a loose scroll, spiraling in and out of the winders for increasing total wrap angle and true one-sided handling.
There are other ways to change the wrap game rules. We can reduce the wrap for various functions by using a vacuum pull roller, a more sensitive tension roller, or tendency drive idler rollers. We also could eliminate some functions by simplifying tension control or using chase guiding to eliminate actuating the web.
Air turns or suction boxes restart the wrap allocation game. Face-side air turns, discussed last month, reverse the web's direction, creating more wrap angle. If you fear face-side turns, a backside air bar can redirect the 90 deg. Suction boxes combined with a set of driven rollers can pull the web in an otherwise zero-wrap web path. A vacuum dancer system combines a suction box between two rollers with a position sensor to provide tension feedback in a zero-wrap position while also resetting the wrap allocation game.
The buttered toast and DVD analogies offer a third alternative to touch-free handling — edge-only contact. Undercut rollers, sprocket gears, and tenters take advantage of touchable edges that are uncoated or to-be-trimmed.
Undercut and sprocket rollers exert force on the web's edges, so they require a relatively stiff web to avoid buckling in the unsupported center. Movie film projectors use undercut rollers and sprockets to prevent scratching and maintain registration. Web stiffness required for undercut rollers comes from a combination of modulus, thickness, tension, radius of curvature, and width.
A tenter grips the web on either edge using a series of clips attached to a belt, chain, or screw drive. A tenter is an expensive, complex device, used primarily for polymer film orientation, rarely used for simple handling.
Whether your touch-sensitive web requires true touch-free, one-sided, or edge-only contact, there is a web handling technique for your application. If you have more questions about touch-free web handling, feel free to contact me directly.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; This email address is being protected from spambots. You need JavaScript enabled to view it.; or visit tjwa.com
