Curl/Moisture Control: Coated/Printed Grades

Steam-Foil® System provides superior curl control in hundreds of converting applications.

Features/Benefits

The Need for a New Solution

Traditional methods of controlling curl through water application, drying restraint and mechanical deformation, are no longer considered sufficient, given their limited effectiveness and the emergence of quality as a driving force in the marketplace. With increased competitiveness comes increased importance in searching out new, more effective means of curl control.

What is Curl?

Curl can be defined as the tendency for a flat piece of paper to distort in a cylindrical fashion. Curl is caused by the uneven expansion and contraction of fibers and/or coating, which causes one side of a piece of paper to "grow" more than the other. Once the curl memory state is set, a change in the exhibited web curl can usually be attributed to a change in its moisture content. This is due to the fibers growing with an increase in moisture, usually more in the cross-machine direction (CD) than the machine-direction (MD). Processing techniques which can cause curl include:

Solving the Problem

When steam is applied to paper it condenses, imparting both heat and moisture to the paper. This heat and moisture softens and re-moisturizes the fibers in addition to loosening the fiber bonds. Subsequent growing of the fibers and evaporation reset the curl memory state of the restrained paper. The objective of curl control is to reset the curl memory state of the paper so that the paper lies flat throughout a range of conditions.

The Steam-Foil Solution

Technological developments in nozzle and actuator design have led to the development of the Steam-Foil. Through its unique construction, the Steam-Foil System can selectively and precisely control the amount of steam it applies across the width of board, providing cross-machine direction curl control. At the heart of the system is the Coanda nozzle which applies steam counter and parallel to the web in the gap created between the board and the Steam-Foil. This steam flow pattern enhances the uniformity and efficiency of heat transfer by maximizing the time of steam engagement and minimizing entrained air traveling with the web. Cross-machine direction curl control is made possible by individual steam-flow control nozzles which can be located as close as 3 inches (6 in. standard) from one another across the web. This spacing also allows for selective control of the steam. The outer surfaces of the Steam-Foil are heated to prevent moisture from collecting and dripping onto the web. This design feature allows the actuator to be mounted in any orientation, including above the web.

Application Considerations

When controlling the curl of coated or printed paper, the Steam-Foil is typically located after the coating oven and chill rolls. When used in an uncoated application, the Steam-Foil is typically located before the calender stack. In both cases, for optimum performance, web temperature in the range of 80°F to 100°F is desired. The Steam-Foil has proven effective in controlling curl in hundreds of applications, including the manufacture of:

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