Control the Curl

Published: May 22, 2025

A Sustainable Solution for Coating & Laminating

By David Maddux, Paper Converting Sales Manager for North America, Valmet Joonas Kääriäinen, Business Manager, Paper/Board Converting & Nonwoven, Valmet

From cost control and production optimization to quality assurance, converters face a range of curling-related challenges in their coating and laminating operations. Even as they deal with obstacles, converters are increasingly challenged to make operations more sustainable and more profitable. Regulatory alignment, organization sustainability goals and even end-user preferences drive the need for eco-friendly processes - without sacrificing operational efficiency or quality.

The Persistent Problem of Curling

During coating and laminating processes, specific moisture ranges, as well as variations in moisture levels, cause the fibers to swell and shrink. This leads to a deviation from the intended plane, or curling. In release liners, curling is often caused by asymmetry between the top and bottom surfaces. Glassine papers are particularly prone to this because their high density and low thickness make them less stiff. Silicone application and curing can make moisture changes in fibers worse, resulting in cross-directional curling.

Beyond the aesthetic issues caused by curling, it can cause problems in downstream production processes. For example, it can trigger misalignment during printing, image distortion and poor print registration between colors. And the uneven moisture profile that can lead to curling can cause brittle paper to break or tear during printing or negatively impact ink absorption and toner adherence.

Steam Moisturizing as the Standard

The key to improving lay flatness, or the degree to which the material is perfectly level, is to uniformly and thoroughly wet all the fibers. Traditionally, converters have relied on steam moisturizing as a go-to method to eliminate curl and maintain quality. While many converters and laminators still depend on this technology, steam moisturizing has disadvantages, including its limited capacity and lack of profiling capability, plus its significant impact on an operational costs and carbon footprint.

Producing steam requires a tremendous number of resources, such as water and fossil fuels, like natural gas, coal or oil, to convert the water into steam. Boilers and related equipment, like web chillers, also take up valuable space, increasing the length of production lines — and the cost of the energy used for material handling. In addition, steam technology isn't easily controlled, which can lead to the waste of raw materials if the product doesn't meet quality standards.

In addition to requiring valuable energy resources, as the speed of the coating and laminating machines increases, moisturizers provide limited capacity. This creates production bottlenecks, either slowing down production or requiring the addition of more steam boxes and chilling capacity, which adds length to the machine and typically requires a costly major rebuild.

Next-Generation Solution: Steam-Free, Air-Atomized Droplet Moisturizing with Precision Control

Closed-loop moisture and curl control systems offer a more sustainable alternative to traditional steam moisturizing. In this method, a two-stage spray head with multiple air-water nozzles creates air-atomized microdroplets that are applied to the web.

Originally developed for fast-running paper machines in harsh production conditions, closed-loop moisture and curl control systems transform water into a highly atomized, full cone spray. Staggered nozzle rows dispense the fine mist spray to ensure full fiber coverage.

The foundation of this type of steam-free, air-atomized droplet moisturizer is advanced real-time measurement and control technology. Data from high-resolution measurements of adhesives, silicones, moisture and more feeds into the control system, which then determines how much water the nozzle applies in cross direction (CD) and machine direction (MD). Since each nozzle has its own actuator, this system can apply moisture at the right intensity and consistency, avoiding the local and small-scale tension differences that cause curling.

Using the data collected, this technology can control the profile every 50 millimeters to produce a better profile. For example, if the system detects that the edges are drier than the middle, it adds moisture to the edge areas to achieve a flat profile. if the system detects curling on the edge of reels, it will add more moisture to edge areas without over-wetting the center. The two-stage spray head system, combined with control technology is highly precise, avoiding some of the disadvantages, like spray quality and CD controllability, of other liquid-based systems, like steam humidification and hydrophilic rollers.

Closed-loop moisture and curl control offers key advantages over traditional steam moisturizing:

Reduce operational costs: Compared to steam moisturizing, closed-loop moisture and curl control systems can significantly reduce utility costs. It takes 0.8 megawatts per cubic meter (MW/m3) of natural gas to produce one ton of steam, and steam moisturizing typically consumes 250 tons of steam per month. If the price of natural gas is 200€/MW, then the utility cost to produce steam for steam moisturizing can run about 40,000€ per month. Replacing steam moisturizing with closed-loop moisture and curl control systems can result in natural gas savings of 480,000€ per year due to steam reduction. This also results in an annual reduction of 484,800 kilograms of carbon dioxide produced, which is the equivalent of driving 3,380,753 kilometers.
Reduce carbon footprint while enhancing quality: By eliminating the need for boilers, steam humidifiers and chillers for cooling, this technology helps operations reduce energy usage and carbon emissions to meet regulatory and organization requirements while providing end-users with the sustainable solutions they want. In one case study, a laminator eliminated the generation of 1,260 tons of steam per month, while enhancing lay flatness at higher production speeds and producing more stable laminate quality.
Improve capacity: This type of remoistening system has more capacity and has been shown to surpass conventional systems, like steam, by up to five times, making it one of the most efficient moisture and control solutions on the market.
Lower maintenance requirements: Compared to maintenance-heavy steam-based systems, the closed-loop moisture and curl control system requires yearly visual inspection and nozzle cleaning. Actuators only need to be replaced when necessary and the actuator O-rings every five years. Integrated tuning tools also allow for easy troubleshooting.
Gain user-friendly control: As converters and laminators struggle with a shrinking labor force and a growing skilled labor gap, the automated measurement and control system allows operators of many skill levels to ensure that the material stays flat after each speed, grade or other process change without the need for manual adjustments.

As converters and laminators evaluate current processes and consider next-generation moisturizing technology, it's essential to work with a partner with innovative solutions, converting and laminating expertise and commitment to improving sustainability.

Strategic Investment in Steam-free, Air-atomized Droplet Moisturizing Technology

While regulations, organization goals and even end-us-er preferences evolve, converters and laminators need practical solutions to reduce emissions without sacrificing quality or efficiency. With advanced technology like closed-loop moisture and curl control, converters and laminators can reduce their carbon footprint and eliminate costly, carbon-emitting steam moisturizing systems. A steam-free, water spray approach reduces waste while controlling moisture and curl more effectively, helping converters and laminators stay competitive with solutions that make a meaningful difference on operations and the environment.

About the Author

Employed by Valmet for six years, David Maddux is the Paper Converting Sales Manager for North America. He has worked for both paper companies and automation suppliers in the pulp and paper industry throughout his career.

With a background in specialty papers production and technical customer service management of Glassine, SCK and packaging papers, Joonas Kääriäinen oversees the Quality Management System Business for Paper & Board converting at Valmet.