New calendering technologies including shoe calendering, winding and roll wrapping trends highlighted at Finishing/Converting Conference

Advances in Calendering Continue to Offer New Options for Papermakers

By Harold M. Cody, Editorial Director

The major developments covered at the 1998 Finishing and Converting Conference held in Minneapolis in October were roll handling, roll wrapping, large roll winding, and the continued evolution of calendering. The conference, sponsored by TAPPI, had 415 pre-registrants, including an estimated 140 participants employed by paper producers, converters, printers, and distributors.

Within this article we’ll highlight just a few of the developments outlined at the conference. A number of presentations that won’t be reviewed here discussed such aspects of converting as: winder rider roll drives, unwind systems, quality improvements, waste handling, and automated evaluation of cut-sheet edge quality. The results of research studies on topics such as heat transfer during calendering and modeling paper strain in a calender nip were also presented at the conference.

Mill Based Calendering Technology. Calendering, both on- and off-machine, and using both traditional single-nip or multi-roll systems, continues to be a centerpiece of developments in web finishing technology. On-machine soft calendering continues to make progress as this relatively widespread technology evolves. A limited number of practical papers highlighted recent developments and are summarized below.

On-machine calendered rotogravure. Artur Stockler, mill manager of Haindl Papier, Schongau, Germany, discussed the use of on-machine calendering to produce uncoated rotogravure printing papers at the mill. Three paper machines (228-, 335-, 236-inch trim) produce uncoated wood containing grades including newsprint, improved news, SCB and directory grades. The paper covered a rebuild of 228-inch trim PM 6, which was not competitive in the long run on commodity newsprint grades. It was rebuilt to make rotogravure printing papers based on 100% deinked pulp (DIP). The machine was well-suited for the conversion, as it already had a gap former and modern 4-nip press section. The project entailed: (1) DIP peroxide bleach plant, (2) new winder, and (3) replacement of steel roll calender stack with soft-nip calender. The rebuild was designed to make a wide range of grades, with the two extremes ranging from standard newsprint (PPS of 3.5um) up to rotogravure quality printing papers (PPS 1.6 to 1.8 um).

A Kusters-Beloit Mat online unit with two single-nip units in tandem was installed. The calender has hydro-vario rolls and provides a wide load range from 5 to 440 kN/m. All rolls in the calender have blades, and in the first nip, the heated roll is on the bottom and a zone controlled roll with soft cover is on top. In the second nip these are reversed. A gloss profiler precedes the stack that runs on manual control. Due to the high temperature the paper runs wider than the nip to protect the soft cover. This extra width is trimmed using a water jet cutting system that replaces the previous knife system.

Soft rolls have been exchanged after 40 days run time. The machine produces rotogravure grades, with a 2-sigma deviation in basis weight of only 0.5 gsm. Since April 1998, precipitated calcium carbonate is used as the filler. Date for the SOGA Top T, 52 gsm grade are ash, 30%; caliper 50 um; PPS smoothness, 1.1/1.7 um; gloss (Kajaani), 28/28.

Mr. Stockler reported that they grind the thermo rolls on a 40-day cycle to avoid decreases in sheet quality. Also, the mill switched from kaolin to PCC recently and the higher brightness of the PCC (96%) allowed them to cease bleaching, and saved about one million marks annually. Use of PCC also led to better dewatering of the sheet allowing the machine to run about 50 m/min faster.

Results from online multi-roll calenders. A paper by Vesa Malmsten, Finishing Supt. at UPM-Kymmene’s Rauma mill, discussed the multi-roll calenders used for the new LWC production line. The mill evaluated five potential calendering alternatives including: supercalender, filled rolls; supercalender, polymer rolls; 2-12 roll multi-nip; 2-10 roll multi-nip; and 2-2x5 roll multi-nip. The mill chose to purchase 2-12 roll calenders from Valmet, with a 1450 m/min design speed.

Among the operational results reported were the following points: Polymer roll grinding interval is at 1300 hours and rising, with the target interval set at 2000 hours. Initially, targets for quality were not met, and based on pilot testing the mill increased the water circulation temperature to 130ºC, which led to a nip load drop from 300 down to 200 kN/m. The result was better opacity and bulk at the same gloss level.

A dusting problem in the top metal roll was occurring but was solved by adjusting the coating formula. Automatic splicing is still improving and the success rate is currently 80%. The target is 90% and improvements continue to reach this. Current quality values on 60 g/m² paper obtained at the mill were reported as follows: brightness, 75%; opacity, 91%; gloss, 60%; and density 1130 kg/m³.

Shoe calendering. A paper by Martti Tuomisto of Valmet highlighted the first commercial installation of a new concept called shoe calendering, based on the first and only production unit running at a Swedish board mill since 1994. The Korsnas mill in Gavle, Sweden produces liquid packaging board on a 283-in. wide, 280,000 tpy machine. The calender provides improved bulk, stiffness, and printability. Design data is as follows: max load 350 kN/m (normal running load of 250); speed, around 500 m/min; temperature of 200ºC.

Nip width is controlled by shoe width. Shoes can be changed, but once the shoe is installed nip length is fixed. A belt roll, with a hydraulically loaded shoe to press the soft belt against a heated mating roll forms the shoe. The shoe calender produces a more uniform pressure distribution than any other technique, resulting in a more uniform gloss distribution and a higher bulk. The long nip dwell time helps preserve bulk, as the nip pressure can be lower. From a practical standpoint this could translate into the use of less fiber, with a potential 3% to 5% less fiber. A series of tables and charts compared bulk losses; stiffness loss and gloss variation and density from board finished with different calendering (soft, shoe, hard calender, uncalendered). Paperboard grades seem to be the most promising applications, according to Mr. Tuomisto, but printing and writing grades such as uncoated freesheet are potential future application areas. Belt life was reported at four belts per year. The heated roll is ground at a rate similar to soft calenders, Mr. Tuomisto reported, or about once per year.

“Winding Force as a Tool for Successful Winding of LWC-Paper,” by P. Tolonen, UPM-Kymmene, and J. Strengel, Valmet Inc., discussed the operation of two multi-station belt relieving winders at the Rauma mill. The winders are part of the new 400,000 tpy LWC PM 4 installed at the mill that started up in January 1998. The units were chosen because of their high capacity and fully automatic features. Each can handle wide and small reels, and is equipped with 14 winding stations. The maximum weight of the wound roll has been 8800 lbs with up to a 71-inch diameter. The first machine roll was wound on the winder at the end of November 1997. The belt concept is designed to relieve stress on the core and deflection of the core and its subsequent stress on the end of the roll. Also, the belted rider rolls apply a winding force to the surface of the roll that gives even roll tightness, more so than conventional center drives which lose their tightening capability as roll diameter increases. The winders have handled the high output of the new machine, which operated in June 1998 at 5085 ft/min (1550 m/min), a new world speed record for LWC.

Guide to winders. A presentation by Jan Gronewold, Beloit Winders, provided an update on a three year project to develop a list of winder types and functions available on the market. It’s designed as a book for training and operations. The table of contents includes: understanding the basics; different plateaus of winding; winder types by concept and industry; and categories. Sample pages illustrated the contents of a section on reels and a converting Simplex Slitter Rewinder.

Market development impact. Randy Lorenz, Evtec Inc.; discussed the growth in the small office/home office (SOHO) market for paper, and its impact on packaging methods (see table). Data on cut size demand over the 1995 to 2000 period by various market segments (SOHO, small office, medium office and large office) illustrated the rapid growth of SOHO markets. Compound annual growth rates for the large to small office segments were projected at 5.1% to 6.5% annually, while the rate of increase for the SOHO market was pegged at 10.3%. At this rate of growth 784,000 tons were forecast to be used in the SOHO market in the year 2000. This represents 13% of total cut sheet demand, vs. a 1997 level of 596,000 tons or 11.6% of demand. These data were sources to CAP Ventures of Norwell, Mass.

The paper pointed out that while remaining a small segment it, is a rapidly growing one. One of the major differences in smaller offices is how cut sheets are packaged. In large offices it’s essentially a 500-sheet ream wrapped in kraft paper. But in SOHO uses, five major packaging methods are used and sheet counts can range from 10 to 2500 sheets.

Vendor presentations. Valmet also highlighted its WinRoll winders and its multi-roll Optiload calenders at a breakfast seminar.

The WinRoll winders are a new multi-station winder introduced in 1997. Three units are currently running at European paper mills. The winder was developed to handle new developments such as super jumbo rotogravure rolls. New presses are now 135 inches wide and capable of using 55-inch rolls. The unit can handle rolls up to 22,000 lb. There are now 6 units running or on order, including (grade, width, startup date): UPM Voikka, machine finish coated, 205 in., 1997; UPM Rauma, LWC, 370 in., 2 units, 1998; Norske Skog, Golbey, newsprint, 370 in., 2 units 1999; and Steyermuhl, SC Roto, 260 in., 1999. It was noted that the design speed for the new Norske Skog machine in France, with startup scheduled for early 1999, is 6150 ft/min.

Valmet also covered its current orders for the Opti-Load multi-roll calender at its seminar. Fifteen units have been ordered to date, with eleven for offline operation and four online units. Currently there are two mills running the units offline, the first at Enso in Oulu, Finland and the early 1998 startup of two online units at UPM-Kymmene, Rauma, Finland. The first unit is scheduled for startup at Enso, Varkaus, Finland, on news and catalog grades with the second unit now scheduled for startup at Irving Paper in St. John, New Brunswick, on news and SC-B grades. Opti-loads have been ordered for coated woodfree grades by Shinmoorim Paper, Korea and Jiangsu Gold East, in China. Steyrermuhl has ordered an offline unit for its SCB mill in Austria. Mead Paper in Escanaba, Michigan has ordered the first rebuild application for two existing supercalenders for production of medium weight coated papers. Haindl has ordered one online unit for its new LWC machine, to be installed at its Augsburg, Germany mill.

Newest entry in multi-roll calenders. Beloit outlined its entry in the multi-roll calender product area by describing their ProSoft calender at a break-fast seminar and in a technical paper authored by Dr. P. Svenka, Dir. R&D, Kusters Beloit and M. Sorenson, Dir. of Op., Kusters Beloit. Currently a pilot calender is operating at its Krefeld, Germany research plant. The ProSoft pilot calender has an 8m width, 2000 m/min speed potential and can provide line load pressure of up to 500 kN/m.

Advantages of the new ProSoft system according to Beloit include elimination of horizontal roll deflection through nip power management. In conventional drive concepts (one main drive only) the driving forces are transferred such that horizontal roll deflection occurs that can lead to uneven CD profiles, an overload at the roll edges and a loss of tensile strength. ProSoft is designed to eliminate this, providing higher tensile strength in the paper, and increased plastic cover life. All rolls are driven in the unit.