Diﬀerent types of rolls are the core of papermaking. Since the invention of the modern paper machine, rolls have played a major part in the papermaking process and its development. Depending on their function, from just simple felt guiding rolls to more demanding process rolls like press center or ﬁlm sizing soft rolls, rolls must always do what they are meant to and ensure eﬃcient and economical production.
Over the decades, paper machine running speed has increased dramatically from the walking speed of the early 19th century to today's speed, which is equivalent to driving a car on a highway. This has all been realized through the
developments in materials technology. Alongside general material development, better roll cover materials have been adopted. Rubber was introduced in the 1920s, polyurethanes in the 1970s and various composites in the 1980s. Chromium plating technology was already being used in hard coatings in the 1930s. Much later, in the 1980s, the groundwork was laid for novel carbide and ceramic coatings, which are manufactured with diﬀerent thermal spraying methods. This development enabled a revolutionary change from the original granite press center rolls to ceramic coated cast iron center rolls. Over the decades, Valmet has been in the vanguard of searching and developing new and better solutions for the papermaking industry.
Modern papermaking lines include dozens of rolls operating in diﬀerent positions. Each position has its own operating environment, which rolls and their covers must meet. The major technical parameters for roll covers to be considered in papermaking applications are local moisture level, temperature, mechanical load and the chemical environment. Good wet condition solutions may diﬀer markedly from solutions which work best in dry conditions.
Roll cover wear is also an issue to be considered in developing covers for diﬀerent applications. This is based on longstanding know-how in material testing in the laboratory, as well as experience of real life applications. Depending on roll position and cover type – hard ceramic or soft polyure-
thane, for example – the cover behavior and actual wear phenomena are diﬀerent, and ﬁnding an optimum solution requires speciﬁc know-how and understanding.
Valmet has been involved in roll cover development for decades. In many areas, Valmet has been the forerunner, and this legacy is still carried forward in today’s roll-related R&D. Our own material laboratory and pilot facility makes it possible to perform intensive material testing and pilot cover manufacturing before entering real scale application.
The continuous drive for increased raw material eﬃciency and low emission products means that Valmet has also been closely following the development of bio-based and recycled materials. In future, sustainable raw materials will constitute an increasing share of Valmet’s roll cover production.
Alongside the latest cover material solution, modern rolls may also include sensored intelligence. This is a relatively recent development, which began with the measurement and control of the CD proﬁles of parent reels and web tension. Pressure sensing has since spread throughout the paper machine – for example, to press rolls, size press rolls and calender rolls.
Roll measurements are based on pressure-sensing materials. These are embedded in the roll structure with polymer covers, with wireless signal transport from the running roll to the external control systems. For most process rolls, knowing the pressure proﬁle is vital for optimizing the papermaking process. For example, almost all paper machines have occasional reeling troubles, which create a lot of broke and may even result in temporary shutdowns. Measuring reel proﬁles aﬀords signiﬁcant advantages in resolving these issues. Another example is the size press, where the quality proﬁle of the paper may be critically aﬀected by the proﬁle of the application rod pressure.
Valmet rolls are based on our strong experience as the industry’s leading OEM player. New replacement rolls from Valmet are always designed and manufactured according to the latest speciﬁcations. And when it’s time to perform maintenance and reconditioning services for existing rolls, Valmet’s full-scope technological know-how is available.
Our R&D is constantly working to develop mechanical improvements and upgrades for diﬀerent roll types. Strong project R&D and engineering form a world-class team to bring roll technology forward.
Text: Jukka T. Heikkinen
Article published in Roll Services catalog 05/2019