- February 28, 2005, Timothy J. Walker, TJWalker & Assoc. Inc.
Whatever happened to scrolls? Scrolls were good business until about 200 A.D. when the book came along. What is it about books that made them better than scrolls? With scrolls, you’d be reading along, look up and wave at someone, and whoops, your scroll would curl back up and you’d lose your place. You could use a couple of rocks to hold your scroll open. With books, you didn’t need rocks; they stayed open. Curl killed the scroll.
Curl will kill your product, too. Curled sheets and label stock will jam a printer or copier. Curled book covers are unattractive and prone to damage. Curled sticky tapes will stick to themselves before they get to where you want them.
How do webs end up curly? Let’s go over the top causes and consider how to eliminate them.
Yielding from stress of a tight radius. Any time you form a flat sheet into a curved shape, whether winding a roll, going over to a roller, or dragging over a bar, you will induce stress and strain change through the thickness of the web. The inside of the curved web will go into or move toward compression. The web’s outside will see increased tension. In many cases, the web springs back with no induced curl, but there are limits.
The amount of stress change from forming a curve is directly proportional to the web thickness divided by the radius of curvature.
Take a sheet of paper. Roll it up into a 5-in.-dia cylinder and unroll it. There is probably no evidence of induced curl. Take the same sheet and roll it into a 0.5-in.-dia cylinder. Now let go. I bet it turned curly on you. The tighter radius curvature created enough stress on the paper’s outside edge to cause it to yield. Try this with thicker and thinner paper and you will see that thicker papers will curl at a larger radius of curvature.
Stop curvature-induced curl by (A) not forming your flat web into small-radius curves; or (B) reversing the process, forming a curve in the opposite direction. Option B is known as decurling.
Strain mismatch in laminate. Rule #1 in laminating is to match the strains of web A and B. If we laminate two materials with different strains, they will relax unequally when tension is removed, resulting in curl to the overly strained side. Set the tensions of the pre-laminate webs to have equal strains, so when tension is removed you get equal recover and maintain flatness.
Moisture variations in paper products. Papers will grow with increased moisture content. I’ve seen bond paper shrink 0.5% in width as it dries. If you coat and dry on a paper substrate, you will drive moisture out of both the coating and the paper. If you then laminate the dry paper before it can return to normal moisture content, you are asking for a curled product. The key to scroll-free paper products is controlling moisture by re-moisturizing either before or after laminating.
Expansion or contraction from temperature. Films and foils will expand with temperature. Laminating to a hot film or foil is the opposite of laminating to dry paper. After laminating, the film or foil moves to room temperature, and you are now entering Scroll-ville.
Density changes in film or coatings. As a coating dries, it may shrink, pulling the web toward the drying side. Freshly made polymer films may shrink subtly in the first few hours after quenching as the polymer chains try to alleviate internal residual stresses. For the more powerful density change of a thick coating, web tension stiffening can prevent machine direction curl but not the more common problems of cross-web curl. The induced machine-direction curvature of arc or air flotation ovens may provide the shape stiffening needed to fight cross-web curl.
Laminate bond develops while curved. If you want to form a curved hull on a sailboat, you soak the wood in water, shape it, and then bond it in place. Converters try to avoid this by bonding the web in the flat footprint of a nip or the flatness of a web span. If a product with poor green strength is transported over a small-radius roller too soon after laminating, the layers may slide and bond in the curved shape. Avoid this curly fate by avoiding small-radius turns until your bond is strong.
Curls are okay for cocker spaniels, ribbon bows, and building biceps, but flat is where it’s at for converted products.
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; firstname.lastname@example.org; tjwa.com.