Session explored calendering’s past, present and future
By Pulp & Paper Canada
AS EARLY AS 1820, calenders were close to today's design. Thursday morning's session looked at not only the evolution of calendering but also its future through new and upcoming trends and concepts.To...
By Pulp & Paper Canada
AS EARLY AS 1820, calenders were close to today’s design. Thursday morning’s session looked at not only the evolution of calendering but also its future through new and upcoming trends and concepts.
Tony Pope, Voith Paper Technology in Appleton, WI, looked back at the beginning of calendering, in his keynote address. According to Pope, early 19th century, calenders were close to today’s machine calender, with paper pressed between two iron rolls.
Later in the 19th century, it was discovered that a softer nip created higher gloss and a more even surface. This was the advent of the supercalender, with the earliest “soft” rolls being rounded and smoothed wooden logs made from the “bowl” of the tree trunk. Filled cotton or paper rolls eventually replaced them. These cotton or paper rolls were made by stacking discs of cotton or cellulose fibre paper around a centre shaft, compressed to a high hardness and then machine wound into a round and polished roll.
Multi-nip supercalenders grew in popularity and to this day remain a tool of the papermaker in order to produce the smoothest and glossiest surface possible.
The desire to increase productivity by calendering in line and an attempt to emulate the cast coating process at higher speeds led to the development of new calendering techniques. Suppliers looked to develop a “soft” roll which was much more resistant to marking and which would operate well in a supercalender. This new type of roll came onto the market in 1972 but the improper operation of the supercalenders and very poor bonding of the synthetic material to the roll shell resulted in a very long problematic curve. However, towards the end of the 70’s, the problems were overcome and these new synthetic rolls began replacing the more traditional materials.
The early 80’s saw the dawn of new synthetic designs for on line calendering, replacing the old hard nip calenders. By the early 90’s single nip soft calenders in line, sometimes as many as four units, were used in an attempt to produce supercalender grades on the machine. Then in the mid 1990’s, it was decided that the high specifications required for SCA grades could not be produced by this generation of hot soft calenders and this resulted in the birth of the latest multi-nip calenders.
Mr. Pope noted that the new type of multi-nip units are now being installed on line in the paper machine. The benefit gained is a sheet coming off the dryer section with a high sheet temperature and moisture which allows the sheet to be calendered much more easily than with an off line configuration.
Long nip calendering is the future of calendering, according to Bo-Christer Aberg, from Albany Nordiskafilt AB, Halmstad, Sweden. Mr. Aberg said that calendering technology is undergoing a radical change. Yankee cylinders can no longer be used on board grades because of high productivity requirements. “Machine calenders and soft nip calenders have until now been able to balance the quality needs on printability for board and packaging paper. But the trend towards higher quality color printing of liquid packaging and linerboard to carry advertising, is making it necessary to explore new methods.”
Long nip calendering has been developed in order to improve bulk and surface uniformity of the paper web. The design is based on the traditional shoe press technology popular now. The calendering application makes it possible to independently control important variables such as nip dwell time, temperature and pressure.
An elastic process belt, which is the key parameter in this concept, contributes to the operation by offering a dynamic modulus 10 times lower than with soft calender covers. The result: optimum nip geometry for bulk preservation.
The soft process belt properties are designed to meet the need, from bulk sensitive board grades, at low nip pressures. For lighter weight grades requiring higher nip pressure, a harder process belt was proven best in pilot trials. A medium hard belt is also on the market for intermediate applications.
Research realized on both pilot and production machines on board grades shows that the new method gives increased surface uniformity with low calendering pressures. This allows for the bulk to develop favorably without compromising on printing quality. Consequently, consumption of fibres and coating colors can be reduced. Similar benefits can also be attained with lower basis weight grades.
“To date, there has been only one shoe calender operating since 1994. But soon we will see more mills rebuilding their calenders according to this new generation…concept,” Aberg predicted.
Michael Trefz, Voith Sulzer Papiermaschinen GmbH, followed with a look at new machine concepts for coated groundwood grades. Traditionally, LWC papers are blade coated and supercalendered. Many of today’s production lines were installed in the 1980’s and include a paper machine, an off-machine coater, re-reelers and two or three supercalenders. In the early 1990’s, improvements in the application of hot soft calendering made it possible to develop the first LWC machines with both coating and calendering on-line. Mr. Trefz used PM#5 in Port Alberni and a similar machine in Sweden as examples. In the late 90’s, another milestone in calendering technique made it possible to consider multi-nip-calenders with polymer covers on-line, thus increasing the quality of the finished product.
Voith Sulzer now proposes three machine concepts for coated groundwood grades. Concept A is the conventional off-line blade coater with a re-reeler between paper machine and coater. Concept B is the on-line coating section within the paper machine. Concept C is also an on-line coating section, but with only one coater station for simultaneous coating of both sides of the sheet. The coating section for concept A requires double the space as concept B. Most of the additional space is needed for reel transport from the PM to the re-reeler, the re-reeler itself and the unwind for the off-line coater. The difference between concepts B and C is the number of coating stations. In concept C, the coating is applied simultaneously to both sides of the sheet with a pre-metered film coater. Having only one coating station instead of two provides another reduction of investment costs.
Film coated LWC with on-line calendering has been produced since 1996. Film coating imposes less stress on the sheet during the coating process. The reduced paper stress allows also to reduce the amount of kraft fibre from a level between 40% to 50% down to about 20% and below. “This fundamental difference with traditional coating is also a major contribution to any return-on-investment calculation,” Trefz added. But with maximum possible paper machine speeds reaching 2000 m/min, film coating requires careful optimization of both the base paper properties and the coating color formulation. If you want to know more about film coated LWC, watch for PM#4 at Perlen Papier in Switzerland which is scheduled to start production in the summer of 2000.