Published June 20, 2001 20.50.00 +2 GMT
As the result of years of intensive development work and customer co-operation, Metso Paper is now ready to introduce a new manufacturing method for high quality coated board. The method is applicable both for new machines and as a rebuild. The concept is particularly attractive as a rebuild alternative, because it can increase the capacity of existing lines proportionally to the market growth.
The OptiConcept Rebuild for Coated Board is based on OptiDwell shoe nip calender and OptiCoat Jet coating application. OptiDwell shoe nip calender is used as a precalender replacing the conventional pretreatments such as yankee cylinder, wet-stack calendering or machine calendering. After OptiCoat Jet coating application the board surface is finished with a soft calender.
This ingenious new method, unlike the present methods, does not have any practical limitations in speed or production flexibility.
The process facilitates the development of new board grades and the improvement of coated board grades produced with machine calender pretreatment. It has a much wider range of parameters than the conventional methods, allowing the board properties to be varied according to the end user's needs.
A new application for OptiDwell shoe nip calender as a pretreatment before coating
Coated boards require high smoothness and a uniform, flat surface. Surface properties have been traditionally achieved with yankee cylinder or wet-stack calendering. Both of these methods are today reaching their limits due to increasing machines speeds, causing capacity and runnability problems.
OptiDwell shoe nip calender was introduced to the public in 1997. Shoe nip calendering is based on a long nip dwell time, high temperature and the use of an elastic belt. Low specific pressure preserves bulk, whilst the elastic belt provides uniform paper gloss and first class printability.
At the moment there are four shoe nip calenders in use, the majority of them are in board machines. There they are located in the final calendering position, replacing either a machine calender or a soft calender just before the reel. OptiDwell is thus a proven technology, making it possible to simultaneously improve board bulk and printing properties.
The new application of the OptiDwell as a precalender before coating removes the bottleneck in the critical finishing area of coated board manufacture. The dominant pretreatment method for folding boxbord, the yankee cylinder, has a relatively small operating window. Each basis weight must be produced at a certain fixed speed in order to maintain the moisture balance over the yankee cylinder. Thus, most folding boxboard machines can not utilize the full capacity of their entire machinery.
The wet-stack calender pretreatment produces a good board surface but reduces significantly the bulk and stiffness of the board. In addition, the process suffers from poor runnability, especially at lower basis weights.
The OptiDwell calender produces the smooth and flat surface by high temperature shoe nip calendering, preceded by moisture application. By creating a high moisture gradient at the surface, the calendering is concentrated at the board surface, leaving the inner parts of the board intact.
After the shoe nip calender, the board is coated by the proven OptiCoat Jet coating technology. OptiCoat Jet application method produces a minimal pressure pulse, pushing less water and color pigments into the structure. As a result, less fiber swelling occurs and the shoe nip calendered surface structure is preserved.
Color application with OptiCoat Jet is skip- and splash free due to the unique nozzle design giving an even board surface quality without streaks or other defects. The dwell time between nozzle and metering blade can be adjusted. In cases where ultimate coverage is needed, OptiCoat Duo coating station can be applied, where dwell time up to 1,6 m can be chosen.
After coating the the board surface is finished with a soft calender.
OptiConcept Rebuild for Coated Board produces the same quality board as the conventional methods, but without the capacity and runnability drawbacks of yankee cylinder and wet-stack calender. As a result, the production capacity of existing lines can be increased significantly, without compromising the end product quality.
A major step in coated board manufacturing
OptiConcept Rebuild for Coated Board is a major step towards boosting the efficiency and profitability of coated board manufacturing. Numerous pilot runs on various boards conducted in cooperation with board manufacturers have exceeded expectations.
Negotiations on the delivery of the first OptiConcept Rebuild for Coated Board are currently underway.