- April 01, 2007, By Timothy J. Walker, TJWalker & Assoc. Inc.
This is the first of four columns on baggy webs, starting with understanding the problems they create.
To most converters, the ideal web is one that is uniform in all aspects, especially geometry. It is uniform in thickness, width, and length. If you cut an ideal web, it will form into rolls, slit strands, or sheets that are identical and will delight customers. Ideal webs often are seen in elevation drawings of web lines, represented as perfectly straight lines that travel from one roller tangent point to another. When you see an ideal web running, you may confuse it for a pane of glass. That's the ideal (i.e., dream) world.
The nightmare that spoils the ideal dream for many converters is a baggy web, one with non-uniform geometry in the plane of the web, usually crossweb variations in length.
Web bagginess is one of the main quality complaints about webs bought in roll form. Instead of forming glass-like straight and flat web paths through the machine, baggy webs droop and flutter like sheets on a clothes line. When you see this web running, you'll think about things such as broken wings, rippled chips, and hammocks. The nightmares of baggy webs keep many a converter awake at night.
What problems do baggy webs cause?
Baggy webs look bad, at least when they are under no or low tension. It will not matter that some amount of bagginess is inherent in all webs or that a slight bagginess can be pulled out with tension and cause no measureable performance problems. For some people, looks are everything.
Most baggy webs, due to their crossweb length variations, will have crossweb tension and strain variations, leading to myriad potential problems.
In some precision coating methods, such as kiss gravure or fluid bearing dies, web tension directly affects coating thickness, so a baggy web leads to crossweb coating variations.
Low-tension lanes of a baggy web may see poor slit edge quality or failure to cut. Strands cut from low-tension lanes may go loose after slitting, leading to weave in wound roll or wrapped rollers and web breaks.
Wound roll tightness is highly sensitive to web tension. A baggy web will have crossweb tension variations that will lead to crossweb variations in roll tightness. Crossweb roll variations may be minor in winding a single wide roll but cause high waste in lock-bar winding after slitting (creating sales for differential winding shaft suppliers).
Registration to pre-printed web may be impossible when yielding within the wound roll produces baggy web and out-of-spec pattern dimensions.
Corona treatment errors
Lanes of baggy web that carry no or low tension will air-lubricate on rollers, which leads to unwanted backside treatment.
Webs with asymmetrical bagginess will have a weak but real tendency to shift toward their low tension sides, especially with long spans, low tension, and in air flotation ovens.
The last and biggest problem with baggy webs is that they lead to increased wrinkle waste. Baggy webs with loose lanes or centers will wrinkle in long spans, especially if combined with subtle roller misalignment. Unless sufficiently tensioned to avoid loose lanes, all baggy webs will have trouble running through nips without wrinkles.
I hate to leave you with these nightmares, but you must face your demons before you can exorcise them. Over the next three columns, I will try to make your baggy web nightmares go away by reviewing options to measure the problem, helping you understand root causes, and advising you on how to de-sensitize your process to baggy webs.