Web Lines | Curl Mechanics Pt. 1—Machine Direction Curl

In this first part of a four part series on curl, web handling expert Tim Walker covers curl in coated and laminated products

Curl is common in coated and laminated products. Curl is created when two materials are bonded together at differential strains. Laminates curl toward the side with the shorter dimension.

Machine direction (MD) curl is the easiest to understand. Tension will elongate a web. The percent elastic strain in a web will be proportional to the web tensile stress (force of tension divided by cross-sectional area of thickness times width, stress = F/tw).

For polyester webs, MD strain will be a small number—less than one percent. For a typical polyester, the strain will be 0.2% for 1 lb/in. tension on a 1 mil web (assumes the elastic modulus is 500,000 psi). Lower tension will have lower MD strain. Thicker webs will have lower MD strain.

Rule No. 1 of Laminating

Making a flat laminate in the machine direction starts with MD strain matching. If you laminate two 1-mil polyester webs at 1 PLI tension, each will have 0.2% strain when they are bonded together, and each will want to relax 0.2% when tension is removed.

If one of the 1-mil webs had higher tension, it would have higher strain at bonding, relax more when tension is removed, and the sample would curl toward the side of the higher tension, higher strain web.

If 1-mil and 2-mil polyester webs were laminated with each having 1PLI tension, the thinner 1-mil web would have higher strain at bonding, relax more when tension is removed, and the sample would curl toward the 1-mil side.

If the 1-mil and 2-mil polyester webs were laminated with 1PLI and 2PLI respectively, then they would be bonded at the same strain, relax the same when tension is removed, and have no MD curl. The 1- and 2-mil webs also would make MD flat laminates at any combination of tensions where the 2-mil tension is two times the 1-mil tension.

Tim WalkerWeb handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.

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