You've got it covered

PPI, Aug 2007 by Radtke, Mike

New developments in roll covers continue to improve runnability, efficiency and overall paper characteristics

PAPERMAKERS ARE EXPERIENCING increasing pressure to reduce costs, improve efficiency and maximize product quality. With energy and fiber costs on the rise, mills are forced to use lower quality fiber sources, more fillers and steam boxes to stay competitive. This places heavier demands on the paper machine.

Since roll covers have a major impact on machine efficiency and the quality characteristics of the end paper product, papermakers are finding recent developments in roll cover technology to be beneficial in alleviating the mounting demands. Advanced roll cover materials are helping to increase press dryness, felt life and life between grinds; prevent barring and vibration; improve doctor blade life; and increase the overall life of the roll - all factors leading to more efficient paper production.

Voith Paper Rolls has introduced several such technologies in the past 12 montiis. This article provides a summary of three new technologies, guidelines for their application, and actual mill results.

THERMAL SPRAYED CERAMIC COATINGS

Sophisticated rubber and resin coatings were developed in the mid 1980s as an alternative to granite. However, the new materials could not replace the release characteristics of a granite roll. This led to the development of thermal sprayed coatings mat can be applied to a steel or cast iron shell and can match the release properties of granite. This type of coating continues to evolve.

Voith introduced TerraSpeed in 2006, with the goal of pushing the limits of the press section and achieving and maintaining sheet release without agents or on-machine roughening of the roll surface. TerraSpeed offers improved release characteristics and contaminant suspending micro turbulences. In addition, it is easier and more forgiving to doctor.

The TenaSpeed coating uses specially engineered ceramic powders and coating parameters that are combined and adjusted to meet a variety of papermaking demands. Compared with earlier generations of thermal coatings, TenaSpeed further improves release properties (Figure 1) and maintains the release over extended running periods, resulting in consistent paper production. It also provides cost savings by increasing doctor blade life and often eliminates the need for release agents. The coating can be continuously doctored with microabrasive or coated steel blades.

TerraSpeed is suitable for high-speed graphic machines and specialty machines with various furnishes, as well as some lower speed machines in abrasive environments.

COMPOSITE CALENDER COVERS

Another recent cover development is the Vantis M, a fiber-reinforced, synthetic composite material introduced in September 2006. Vbith's goal in creating me Vantis M was to improve cover surface properties, extending in-machine operating times and expanding the usable limits of the cover. Developed originally for use in multi-nip calenders, the attributes of this cover are also performing well in soft-nip and supercalender applications.

One of the major innovations applied to the cover is a nano-particle filler system with a very narrow particle size distribution. The difficulty in using nano-particle fillers is preventing tiieir agglomeration into larger particles, which would negate the benefits seen from the uniform distribution of the particles.

By optimizing the filler type and blend, the direct benefits of the nano-particle technology are manifested in several cover properties. The fine size and blend allow for a smoother calender cover surface that will improve during operation due to a self-polishing effect. This typically results in roughness values of only 5-6 Ra (0.15 µm), Figure 2.

The Vantis M also incorporates improvements in mechanical and thermal properties that allow the cover to run safely in extreme conditions. The mechanical loss factor (tan delta) is reduced, meaning less of the applied energy is converted into heat in the cover. This enables the Vantis M to run at increased line loads and higher speeds.

Thermally, the Vantis M has improved heat conduction and a lower thermal expansion coefficient. This allows the cover to withstand higher surface temperatures and lowers the internal stress generated by repeated heating and cooling cycles during operation.

In addition to the property benefits of Vantis, Voith uses an exclusive bonding system to ensure optimal runnability of the paper machine. Results indicate diat diis adhesive system has significantiy improved the resistance to mechanical damage and reduced the problem of delamination between the cover and the core. Testing has shown diat even in cases of severe mechanical overload no delaminations were generated.

The combination of better impact resistance, a smoother and more wear-resistant surface, and improved thermal and dynamic properties all provide increased value to the papermaker. Fewer shutdowns for roll maintenance, reduced losses for paper quality, reduced grind losses and improved impact resistance all lead to lower total cost of ownership.