Who Likes Film?

Web Lines

Customers like films because they are cool. Films can be clear, thin, shiny, clean, and elastic. Product designers like films because they are functional. Films can improve a product’s break strength, tear resistance, and gas barrier performance. Process people like films because…wait. Do process people like films?

Ask a process person whether he or she likes to run films or papers and many will answer "paper." The first run of a thin film on a line designed for paper will be fraught with waste—waste from scratching, winding, and wrinkling. Yikes! Maybe process people don’t like films. But since customers and product designers like films, process people better figure out a way to deal with the challenges of film processing.

In many cases, films tend to be flimsy compared to the paper product they replace. Films can be as stiff as paper, but many have lower elastic modulus, especially if heated. Generally, films are smoother and tend to announce abrasion like scratches in a car’s paint. On the positive side, films are less sensitive to moisture (except nylon film), and their high tear resistance (for most films) means less web breaks.

How do these differences translate into process challenges?

Film Challenge #1: Films will have more scratching and other slip-related problems. The very film properties we like in product design come back to haunt us in film handling. Nonporous means air is trapped between the web and roller. Low tension means less driving force to squeeze air out. Smooth surfaces provide less "tread" and quicker onset of lubrication.

To add to these problems, many films have naturally lower initial coefficients of friction to roller surfaces. When rollers do slip, the smooth surface and low hardness of films will abrade quickly and call attention to even the most subtle scratch.

Solution #1: Counter to the initial fear a rough roller surface will create more scratching, it will not. Slip is the cause of scratching, not roughness. Fight the anti-slip battle in two ways: Use textured or grooved rollers to maintain traction, and improve idler roller performance to spin more easily.

Film Challenge #2: Films will have more winding and wound roll problems, such as core crushing, blocking winding-induced bagginess, and telescoping. The telescoping problems are related to the same factors that cause increased slip and scratching defects.

High tension defects are more common because films tend to have a greater tourniquet effect in winding. Since films form wound rolls with relatively hard radial stiffness, there is little hoop tension relief from radial compression. Film rolls are rarely described as "squeezably soft." The reduced radial compression also will create more diameter difference in rolls after slitting, causing problems with lock shaft and shared nip winding.

Optimize tension, tension taper, and winding nip loads. Shorten roll length or increase cores size to reduce roll diameter and stress buildup. Use individual winding nips and differential winding to compensate for post-slitting diameter variations.

Film Challenge #3: Films will have more wrinkling and creasing problems. Films are not inherently more prone to wrinkles and creases, but since films usually are thinner and baggier than the material they replace, they will be relatively more prone to wrinkling.

Understand your wrinkle causes. Improve equipment alignment. Apply anti-wrinkle web path geometry and spreading rollers.

Film Challenge #4: Films will have more tension control problems. In an initial run of a new film, you may find your tension control system is sluggish, having trouble both running low enough tension and having too much tension variation. A film with lower thickness and modulus has a lower spring constant than the product it replaces.

Solution #4: Running thin films may require you to upgrade your tension feedback system, whether transducer or dancers, to work in a lower range with less noise or hysteresis. You also may need to re-tune the tension control loop gain to adjust for the lower web spring constant.

Do process people like films? Let’s say they enjoy the challenges of films. If you learn to run films and overcome their process hurdles, you will gain the benefits your customers want and a competitive advantage in process knowledge. Now there’s something to like about films.



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 651/686-5400; tjwalker@tjwa.com; tjwa.com.


To read more of Timothy J. Walker’s Web Lines columns, visit our Web Lines Archives.


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