Coating Matters: Steam-The Cure for Curl

Published: April 01, 2011, By By Mark Miller Contributing Editor

The following column, co-written by Joe Martin at Qualitek, Tucker, GA, is a preview of the paper I will be presenting at the Pressure Sensitive Tape Council's Week of Learning, May 9-13, in Orlando, FL.

In the world of coating, the substrate is sometimes overlooked when it comes to process improvement. Specifically, the coating of paper label stock can be trying because the substrate tends to curl within the process. I have gained an appreciation for steam remoisturization equipment when dealing with paper substrates.

In today's competitive marketplace, customers want label stock to “lay flat.” This is not just an aesthetic issue; it is so that customers can efficiently and cost effectively process label stock into a finished product. Curled stock can jam equipment, slow printing, and cause other inefficiencies in downstream processing. When faced with a choice of supplier, customers select manufacturers that can produce stable products to lay-flat specifications.

Manufacturers of labels and other coated paper products use various solutions to eliminate curl. However, the most common of these is remoisturization using steam applicators since they typically can add up to 3% additional moisture to the web with minimal coating contamination problems; provide consistent profile control of moisture in the cross direction; and operate efficiently with typical machine speeds of 600-1,200 fpm.

When choosing a steam applicator, several guidelines should be kept in mind:

• The remoisturizer should not allow fugitive steam to escape to the atmosphere and condense on the applicator or neighboring web handling equipment. Water dripping on a coated web is one of the main sources of contaminated product. This can be avoided by using an enclosed and sealed steam applicator, which safely removes excess steam and has a heated exterior shell to prevent condensation from occurring during start-ups.
• The remoisturizer should provide the required amount of moisture for the paper basis weights, type of coatings, and machine speeds that will be run. The amount of moisture that can be absorbed by the web is dependent upon the temperature of the web entering the steam applicator (ambient to 125 deg F), the density of the steam, and the time it is in contact with the web (dwell time). Suppliers should be able to provide production data for similar applications of their equipment to show how they were able to meet target moisture levels.
• The remoisturizer should be configurable to existing process lines. Some suppliers will custom engineer a remoisturizer solution for existing process lines so major modifications to web handling equipment are not necessary. This will keep the investment at a reasonable level and amplify the IRR.
• The remoisturizer should be fabricated from the highest quality materials to ensure maximum uptime and maintainability. The combination of stainless steel and titanium has proven to enable welds to accommodate thermal expansion. A 12-ft-long steam applicator can expand and contract ½ in. while temperature cycling, so it is important that the welds do not crack or leak. The steam valves also must be robust enough to withstand years of cycling without replacement.
• The remoisturizer should lend itself to automatic control. For example, using IR sensors to detect moisture levels across the web before coating and after remoisturizing also can provide closed loop control and recording of the finished moisture profile across the web, a key variable that process engineers want for SPC.

Case studies have shown the typical economic payback for a steam remoisturizer is between 12 and 18 months. The achievable revenue returned is dependent upon the specific application. However, key contributors to positive cash flow include incremental sales of products meeting lay-flat specifications, reduction in material scrap, and increased production efficiencies with improvements such as faster machine speeds and reduced maintenance downtime. Key contributors to a lower initial investment include having an existing steam source and being able to install a custom-engineered remoisturizer without having to perform expensive modifications to existing web handling equipment.

Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 715-456-9545; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.coatingtechservice.com.