“Dryer curl” often occurs in the coating of flexible substrates. Curling is caused when there are differences between the internal stresses in two layers (coated layer and substrate) or different internal stresses in different parts of the same layer. It can also result from materials “remembering” their shape from being stored in roll form for a long period of time.

Extensive experience with coating applications has allowed Carestream Contract Manufacturing to develop a library of solutions for preventing or correcting curl. Some of these solutions include:

• Keeping paper in a humidity-controlled environment before coating it. This allows the moisture content of the coating and paper to equilibrate and be evenly matched.
• Continuing to work on gradual acclimation by allowing the moisture to equalize between the paper and coating substrate, both before and after coating is applied. This gradual acclimation requires controlling humidity in the plant very carefully, in both coating and drying operations.
• If the paper is curled before coating, running it on small-diameter rolls at exact tension can help. This “breaks” fibers and relaxes tension. “Breaking paper” can also create other problems such as scratching, but using special types of rolls can prevent such issues.
• Using similar coatings on both sides of the substrate to balance stresses caused by water intake or differences in internal stresses between the coating and the substrate. If using an expensive coating on one side, it is typical to use a less costly version on the backside. Frequently this means using the same formula without the expensive ingredients.

A common curl issue with paper is the curl induced by adding a coating. The stresses added by the coating most commonly induce a curl towards the coating. In that case it’s possible to take advantage of the property of the paper cellulose fibers, in that the paper tends to curl toward the side that the moisture leaves last. The paper industry, as well as coaters, often use steam boxes or steam bars at the end of the drying process to control curl. The steam is applied to the back side of the paper and allowed to dry just before the paper exits the dryer. This sets up a counter-balancing curl tendency.

Roll set curl is a second type of coating-induced curl common to both papers and film-based substrates. Film-based products do not have moisture sensitivity, so do not respond to treatment by moisture or breaking as paper does. Roll set curl can be improved by counter-winding; however, curl primarily caused by the stresses of an added coating often need to be counter-balanced. In these instances, the strategy of a secondary coating on the back side is valuable. An additional consideration in choosing the backside coating is to consider a polymer system that will induce greater stress, but apply it in a thinner coating. The thickness of the backside coating can be manipulated to balance changes of the main side coating, when the main side coating is varied for some specific property.

There are some other causes for curl that are not so easily remedied. For instance, plastic creep is caused by internal stresses and the “memory” of plastic film. This is much more difficult to fix. One solution can be to roll the film in the opposite direction from the curl and allow it to sit. This fix is time and temperature dependent so it is best to do it right away, while re-rolling the film. It is costly unless done in conjunction with other coating, laminating, or slitting operations.

A final consideration is the material properties of the substrate. Here material choice for other properties may impact the thickness required to offset curl. Film thickness often must be selected to minimize curl effects. When that is the case, the use of an appropriate backside coating can help minimize the thickness of the selected base required.

In summary, substrate curl is caused by unbalanced stresses within or between the coating and the substrate. Solutions include relaxing the substrate and balancing the stresses within the product by coating both sides, using a method that fits the particular situation. Experience lends efficiency to an otherwise troublesome endeavor. Carestream’s technical team applies its advanced knowledge and skills in precision coating to solve curl issues and deliver the best possible results for its contract coating customers.