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Finding the
source of curl
sometimes
requires test
ing of several
web process
variables.
Curl problems may occur on many products because of adhesive shrinkage as the adhesive cures and/or cross-links during the laminating step. Or it may occur because the webs being laminated do not have the same planer elongation at the time they are joined together in the laminating nip. The difference in planer elongation of the two webs may be due to excessive unwind tension or greater thermal expansion on one of the webs. Finding the source of curl sometimes requires testing of several web process variables.
Web curling is illustrated in the following figure:
Two webs that are laminated together with an adhesive that shrinks during curing exhibit curl if the following relationship is not true:
where t is web thickness and M is Young's modulus for the webs. There are two planer orientations to web curl-MD and TD. The modulus for one direction may be somewhat different than the...