Valmet’s selection of calender roll covers: A result of extensive expertise

Dec 19, 2023

Few roll cover manufacturers can offer as wide a range of calender covers as Valmet can today. Most attention is paid to high-performance covers addressing difficult conditions in the calender, such as vibration and barring in high-speed online calenders. But many calenders also run without vibration issues, and when such “anti-barring” covers are not required, other properties become more important.

Valmet Calender Roll Cover without vibration issues

Performance and cost requirements

Some covers must have a uniformly smooth surface for good calendering action to achieve the desired paper smoothness and gloss. Others must run at a low load to retain paper bulk and must therefore be relatively soft to continue to provide some nip width. Of course, all covers must be durable in their grinding intervals (wear and marking resistance or profile stability) and overall lifetime (impact and load strength). The choice of cover is never based on performance alone – it’s about finding the right balance between performance and cost. All Valmet roll covers have their specific strengths and appeal, and our specialists always work closely with customers to find the ideal solution for each requirement.

Durable covers in all conditions

The Valmet calender cover selection offers economical products for basic calendering needs. When the speed or load increases, the more advanced covers come into play. For these requirements, Valmet’s selection offers well-proven standard products, as well as the latest high-tech applications, activating the best material technology achievements.

The fit-for-purpose economical Valmet Calender Roll Cover Eco is a durable choice for calendering conditions in soft- and medium-load multinip calenders. This is a reliable high-performance cover that can run for a long time at today’s machine speeds. Valmet Calender Cover Eco is comprised of the durable 89–91 ShD hardnesses, and it is ideal for taking on the challenge, e.g., in soft calenders for newsprint manufacture, coated boards or various specialty papers. Its long list of references in all calender type grades is definitive proof of its durability and cost-effectiveness.

Valmet Calender Roll Cover Eco is a specially developed calender application for medium-load conditions.

Remarkable engineering features

The Valmet Calender Roll Cover CJ product family has outstanding performance at medium cost. It includes three cover variants, creating a standard calender cover offering for a wide range of calendering cases. With a superbly smooth surface, combined with good wear and impact resistance, CJ-S aims for special performance in high-end coated paper manufacture. It also has moderate cover hardness for good contact between the paper and the thermo roll. Meanwhile, CJ-W targets long running durability in doctoring for vibration or just normal conditions. This hard cover offers wear resistance at the highest level. When all these features are combined with its special structure, CJ-W enables very long runs when vibration is an issue in the calender. Valmet Calender Roll Cover CJ-EC has added durability for challenging calendering conditions. It incorporates a special thermal base layer, providing additional robustness in high-temperature applications.

Premium covers for the most demanding requirements – Valmet Calender Roll Cover CL family

Finally, Valmet is very proud to introduce the latest results from the world’s leading roll cover R&D: the new Valmet Calender Roll Cover CL family, offering all the properties required of a reliable high-performance calender cover. The Valmet Calender Roll Cover CL family consists of four roll cover products for the most demanding soft or multinip calenders. Combining the highest wear resistance with the best load-speed durability and impact resistance, these covers offer the longest regrinding intervals in the most challenging calendering conditions. An optimized anti-barring design, combining wear and deformation resistance, provides robustness and extended running times under vibration and doctoring.

The CL product family is the optimal cover choice for all specific demands such as use in the production of graphical papers, specialty papers, and container and packaging boards. It provides trouble-free papermaking and durable and robust covers for ultimate grinding intervals with excellent paper quality.

The Valmet Calender Roll Cover CL family is developed for the most demanding calender applications.

CL: Cost savings from reliable long runs with good smoothness and gloss, including in calenders with barring. A stable high-performance cover for all soft, super and multinip calender processes. The chosen cover for producers of, e.g., LWC, WFC, SC, coated boards and various specialty papers for more than a decade.

CL-S: A special cover for an elastic soft nip with maximal resistance to dents, markings and cracks caused by sheet breaks, wrap-ups and dirt-induced impacts. CL-S exceeds the performance of the previous best covers in impact resistance and elasticity. It therefore offers a reduced risk of sudden cover damage. These special properties are based on elastic additives mixed in the cover structure, providing better tolerance for sudden load peaks. The structure also allows a smooth, elastic, long-lasting nip for optimal paper quality characteristics.

CL-W: A long-running cover in harsh environments with extreme wear resistance and tolerance under vibration. This has been achieved by combining polymer technology with the latest material additives in a new composite formulation. Special surface properties have been achieved, with new filler additives also providing stable smoothness/roughness to the cover, e.g., for special paper applications, container board and WFU grades. This all leads to less profile wear and less susceptibility to barring.

CL-EC: The highest hot spot resistance and tolerance of high loads/speeds for a long service life with good smoothness and gloss. A special thermal base layer provides excellent heat transfer, protecting the cover in high load and temperature conditions. It also evens out temperature-related profile variations. The first choice for release paper and high nip-load SC-A paper grades.

Soft roll covers for special needs

Calender Roll Cover CPU, a special calender cover from Valmet. Instead of being based on a composite, it uses a much softer material: elastomer. Operating conditions used to be too severe for an elastomer material. Calender Roll Cover CPU now offers unprecedented opportunities for board and specialty papermakers by saving bulk, avoiding mottling and still achieving good smoothness. This cover is also optimal for thermal paper calendering, and it can also find its use in the soft calendering of technical specialty papers, as well as in bulky grades and tissue. The cover structure and material recipe has been optimized for use in calendering nip conditions for long running times.

Hard coatings for calender thermo rolls – Valmet Calender Roll Cover CO family

Valmet Calender Roll Cover CO series products are very hard and smooth roll covers for calender hard rolls. These covers provide long regrinding intervals, a high-quality paper and board surface finish, and stable production.

Calender Roll Cover CO covers prolong roll lifetime by preventing wear of the hard chilled layer, thus reducing the need for grinding.

These high-performance hard coatings reduce paper quality CD variation due to superior wear resistance. They provide extended grinding/superfinishing intervals, high paper smoothness and gloss due to a very smooth surface.

CO-W has been specially designed to provide maximal wear resistance while retaining roll profile and surface smoothness.

CO-S targets the maintenance of a very smooth surface, resulting in maximal paper surface quality and gloss.

CO-M is developed for hard nip calendering, keeping the cover profile and prolonging hard roll lifetime in harsh conditions.

Years of expertise and experience

In addition to offering the Valmet Calender Roll Cover for the most difficult calendering cases, Valmet can evaluate and optimize the cover structure for each calender issue if necessary. Valmet’s extensive papermaking and paper machine manufacture expertise and experience mean you are sure to find a cover for every calender type and paper grade. With continuous development and several new covers in the pipeline, we serve our cover customers with special cover needs in all global calendering requirements.