Making sure you're covered

Pulp & Paper, Mar 2005 by Butterfield, Bill

THREE MAJOR TRENDS DRIVING roll cover development in the paper industry are the desire for wider nips; the need for valueadded productivity and longevity features such as wear resistance, toughness, hardness stability, and handling of high loads and speeds; and the development of engineering tools to improve production and assist the papermaker with producing a uniform and trouble-free product. Each of these trends will lead to continuous improvement by providing higher machine efficiencies while improving sheet properties. Providing covers with higher reliability and quality will continue to be emphasized.

Extending the nip

Wider nips provide several benefits. Lower peak pressures in the nip result in less densiflcation of both the sheet and the press felt. The increased sheet bulk results in a host of improved properties including bending stiffness for packaging grades and softness for tissue. While these properties manifest themselves more readily in heavier grade sheets, lighter grades have seen a dramatic rise in the use of shoe press technology. Reduced densiflcation of the press felt leads direcdy to increased life.

The wider nip width also leads to slower drainage velocities in the nip. These lower drainage velocities reduce the risk of crushing, which can initially lead to a reduction in burst strength, but progressively deteriorates virtually all properties.

Roll cover manufacturers have been addressing the trend to wider nips in many areas. First, a computer modeling program (such as PressManager) enables papermakers to predict the dewatering effects of various covers by taking into account press type, clothing design, and parameters of the sheet before the covers are actually installed. By providing cover stress and heat generation results, the papermaker can select covers to obtain the benefits of wider nips without exceeding the allowable stress or temperature of the cover. These programs will continue to develop and in the future will model die effects on paper properties.

Shoe press sleeve development will continue to address the need of wider nip widths with future developments toward higher nip temperatures. A quarter century ago, shoe presses were first implemented on board machines. But as benefits were realized, they have been progressively implemented on other grades. Shoe press sleeves are essentially roll covers without a core that have a very wide nip.

The third area of importance for wider nips is in venting technology. Important developments in venting technology, such as the use of Dri-Press venting, will continue. Venting geometry becomes even more crucial as papermakers move toward wider nips, since the covers tend to soften and become more susceptible to the vented area closing.

Grooved soft covers may experience groove closure if the grooves are not designed and cut properly. Blind drilled and blind drilled and grooved covers may become more important as these longer nip trends continue to increase. Roll cover manufacturers will have to develop proven designs and manufacturing techniques that work well with the softer covers present with wide nips.

Value-added covers and tools

The second trend for covers is the use of value-added features that lead to such improvements as sheet release, hardness stability, wear and barring resistance, and increased toughness, all of which lead to higher paper machine productivity. The last half of the 1990s saw a doubling of roll cover life between regrinds as the result of these developments.

Breakthrough polyurethane technology has led to the generation of such low heat build-up (hysteresis) that these covers can be operated in long nip pressing (LNP up to 2,200 pli) applications while turning the water cooling completely off. This development of high-temperature adhesion systems has led to the shift in development from base/adhesion systems to top stocks that will withstand higher operating temperatures without property degradation. This trend will continue.

In addition, covers that are now operating for much longer times between regrinds have become increasingly susceptible to chemical attack and heat aging that can lead to cover surface degradation. The trend here will be development of more ageresistant compounds to withstand the affects of longer run times. Age hardening resistance will continue to be emphasized in high-temperature applications such as tissue pressure roll applications. Finally, increased emphasis on film pressing " will bring about the continued development of covers that provide excellent film split and film coating properties.

The third trend is development of engineering tools that assist in uniform production and overall cover longevity. The invention of the electronic nip impression system has improved the measurement of nip uniformity and has made this process fast and efficient. Further development from continuous sensors to real-time SMART roll technology will allow the papermaker to monitor nip conditions in real time.

BILL BUHERFIELD is vice president, Global Rolls Technology, Stowe Woodward and Mount Hope, Middletown, Va.