Name changes among the top three calendering equipment suppliers have not stopped technology developments over the past 12 months in this area

Catching up with the changes in calendering

This year's review shows that calendering is advancing along a broad front, such as has never been seen before. Not only is this giving mills a wide choice of equipment, but the different approaches are providing an increasingly clear picture of the theoretical background behind the technology.

The breakup of Beloit enabled Küsters to purchase all of Beloit's shares in the Küsters Beloit joint venture. The new company's name reverts to Küsters and the unit is completing all orders placed through the joint venture. The same range of calendering equipment is available and both the Krefeld and Spartanburg, SC, service centers have been extended and given a facelift to meet customers' requirements.

The latest Küsters startup is at Appleton Papers' Locks Mill in the US on a PM producing coated papers. The combination of a Peritherm Roll (at 200°C) and a Multi HV Roll (with correction potential in the range 5-300 kN/m) is giving a cross direction (CD) caliper profile with a 2-sigma value of 0.5 µm. (1 kN/m = 1.02 kg/cm = 5.71 lb/in).

A company name change has also resulted from Sulzer reducing its share in the joint venture Voith Sulzer, with the company now being called Voith Paper. After delivering over 20 Janus Concepts MK 1, Voith has now started up three MK 2 units, which are distinctive for their 45° reclining position. The units are producing supercalendered (SC) papers - SC-A and SC-B - as well as lightweight coated (LWC) grades, using 6-10 rolls and line loads going up to 350-500 kN/m. All three units are located within a paper machine and Voith is placing a high priority on increasing the running speed of the Janus in line with ever faster PM speeds. Voith is also developing both hard rolls with new surface properties and flexible rolls with improved covers.

Operating experience with the first Janus MK 2 unit to start up, at Myllykoski's Lang Papier, Ettringen, benefited from an earlier MK 1 unit at the mill1. The eight-roll MK 2 calender is designed for a maximum linear load of 550 kN/m, and has three thermo rolls that can reach a surface temperature of 170°C. Newsprint production at 2,000 m/min uses only one nip, which allows the rolls not being used to be changed during production. An hydraulic cylinder pushes a roll out of the stack, so that it can be removed by the crane. SC grades use all seven nips and are run up to 1,500 m/min. Four steam boxes are located ahead of nips one, two and four, in order to achieve the desired SC smoothness and gloss, and there is a further steam box ahead of nip seven to control curl. The quality of the SC-B grade achieved on PM 4 with the MK 1 unit (ie 35% Hunter gloss, 1.50 µm PPS smoothness) was soon surpassed by PM 5. A few months after startup, PM 5 was producing superior Hunter gloss (45%) and PPS smoothness (1.19 µm), with a two-sidedness of about 7%. Bendtsen porosity dropped to 17 ml/min. Calendering conditions were a linear load of 400 kN/m and a thermo roll surface temperature of 170°C at a machine speed of 1,300 m/min.

Shoe presses were initially applied to board grades because of concern over possible loss of bulk and stiffness. The same is happening in shoe press calendering, although Valmet's OptiDwell Shoe calender has already moved on to printing grades, making a total of ///// units installed or on order.

KCL’s pilot plant checks out new techniques in calendering

Confidence to go inline

Valmet's orders for its OptiLoad calender also reflect growing confidence toward inline installations. The first inline unit was the eighth OptiLoad calender to be installed and that happened in 1998. When all the orders received by the second quarter of 2000 have been installed (excluding rebuilds and conversions), eight of the 21 units will be inline.

Several mills running OptiLoad units have installed a Sensodec 6S, from Neles Automation, to continuously monitor and analyze its mechanical condition online. Besides allowing roll maintenance to be scheduled, the Sensodec can identify the roll causing calender vibration and warn of calender barring. The accuracy of roll grinding can also be checked. The system is sufficiently sensitive to detect small pieces of paper or splicing tape rotating in a nip, so that they can be removed before they cause any damage. Some mills use the equipment to compare the performance of roll covers from different suppliers.

Last year's review announced the arrival of Valmet's new model of soft calender - the Optisoft SlimLine (PPI November 1999, pg. 24). By giving each stack of rolls its own individual frame, about 30% less space is required relative to open frame construction. The modular configuration also reduces installation time and contributes to the shorter roll change time. Of the four SlimLine calenders sold, one will be on a new off-machine coater at the Modo Husum mill, where it will give matt grades their final finish after the four coating stations. The glossy grades will be produced by a separate downstream OptiLoad calender. Another SlimLine, this time with 2x2 rolls, is being installed on Holmen Paper's Hallsta PM 12 newsprint machine. All this calender's main rolls are fitted with the SymSlide sliding bearings used to ensure smooth running at speeds above 1,500 m/min.

As improvements in calendering allow the higher SC paper grades to be produced on-machine, so mills with supercalenders are improving their own quality level in a bid to stay ahead. Lake Superior Paper Industries in Duluth, US, was the first mill in North America to produce SC-A Plus, and is using supercalenders. The mill's rotogravure and offset SC-A grades have a Hunter gloss range of 42-48% (at TAPPI brightness 67%), and the Expedition SC-A Plus grade has a gloss of 50-52% (at brightness 70%).

To assist supercalender users, Honeywell (the new name of Honeywell-Measurex since the Allied Signal merger) has extended application of its Calcoil to supercalenders and thus lessened several of their shortcomings. This external induction heating system has been around for some time, but was confined to hard and soft nip calenders both on and off-machine. Only chilled iron rolls are heated, and the paper both effectively insulates the opposing synthetic or filled rolls and also removes some heat. A base heating load is applied uniformly across the roll face and the power is varied locally to improve caliper and gloss profile.

Back to basics

Following the mainly mechanical improvements in calendering technology since the mid-1990s, pilot plant researchers are investigating fundamentals to see how the art can be further refined. Temperature gradient calendering - whereby high calender roll temperatures (130-200°C) soften the first 5-10 µm (0.2-0.4 thou) of the paper surface and thus preserve caliper, opacity and strength in the paper's interior - has a counterpart in moisture gradient calendering. The two softening mechanisms are possibly similar, in that either a high temperature heats both the paper polymers (cellulose, lignin, etc) and any coating adhesive polymers to their glass transition temperature, or moisture reduces the level of these transition temperatures. Of the two, temperature gradient calendering has been the easier to harness, due to the difficulty of spraying a 1-2 µm thick layer of water at high speeds without drop marking or other problems.

The situation has recently been reassessed for SC papers in a joint investigation by Valmet and VIB Apparatebau2. The only practical control variable in creating a moisture gradient is imbition (ie drinking up) time. In their pilot plant calender trials, imbition time was varied by the web run length between the spray units and the first calender nip. The trials were carried out on a range of European SC papers that were given a moisture gradient, with an average moisture content of 10%, by spraying 2.25 g/m‰ on each side on the pilot calender. The control papers were the same SC grades, but they had been given their 10% average moisture content several days before being run through the pilot calender.

The calendering trials showed that both Hunter gloss and Bendtsen porosity initially dropped significantly with increasing imbition time, and traced a hyperbolic curve that started to level out after an imbition time of about 0.5 s. Paper bulk was essentially insensitive to imbition time, but its value after moisture gradient calendering was superior to that of the control. Smoothness appeared insensitive to imbition time using the Parker Print-Surf, but the STFI profilometer showed moisture gradient calendered papers to be significantly smoother at small scale wavelengths (below 200 µm). All existing suppliers appear to employ a 1 s imbition time, so that by working on the plateau they are failing to harness the quality benefits potentially available from shorter imbition times. Further work is required, which must include re-expressing a single imbition time value in terms of the spraying normally applied at several nips, and also the influence on printability.

Finland's KCL has effectively complemented this work by looking at the main influences on mill-made LWC when calendered in their pilot plant and subsequently heat set web offset (HSWO) printed. They found that:

• linear load was the main variable influencing the interior of the paper (and thus bulk and smoothness), whereas temperature was the main determinant of surface properties (and thus gloss). However, even at high temperatures, an adequate linear load is required to achieve good smoothness levels

• a sufficiently high moisture content before calendering permits high smoothness, even at fairly low linear loads. The latter permit better opacity

• the lower the moisture content after calendering, the higher the gloss and smoothness of the HSWO paper

• at a high moisture content, water evaporates rapidly during the drying of HSWO printed paper. This breaks the paper structure, particularly where there is an impermeable printing ink layer on the paper surface. This "blistering roughening" is significantly greater than that caused by the water applied to paper surface during offset printing. Some paper smoothening may occur in the printing nips.

New rolls

Valmet's range of polymer roll covers also benefited from the breakup of Beloit, with the merged supply now falling into three reorganized product families. These are: Dura rolls, for the new generation of multinip calenders; the Summit Series for soft calenders; and the Supertex Series for supercalenders, where Dura rolls can be used for demanding applications.

The new DuraGloss roll cover was introduced at the same time. Designed for multinip calenders and other demanding applications, this tough and flexible polymer is said to be especially tolerant of impacts and as a result is less vulnerable to web breaks. It also develops less heat than most covers, which makes the roll cover particularly resistant to hot spots.

Stowe Woodward has not only developed a new family of composite covers for calender rolls, called Jalon, but also several new testing methods that better approximate to cover performance under mill conditions3. These test methods include cover wear, load limit, impact behavior, and long term dynamic fatigue performance (as in a hot spot).

Traditional filled supercalender rolls continue to fight back against polymer roll coverings with the availability of the Endurance roll from Richard Hough of Bolton, UK. Belonging to a new generation of filled rolls, it is said to run for up to 1,000hr between grinds, be more resistant to surface marking, and require a lower capital cost.

1 Palm, C. and Cedra, I. Compact PM 5 the first online SC-A line. Preprints of PAPTAC 86th Annual Meeting, Montreal, Feb 2000, pg. B93.

2 Heikkinen, A. and Linnonmaa, P. Moisture gradient calendering of woodcontaining papers. ibid, pg. B181.

3 Haupert, F. et al. Optimisation of composite compound covers for calender rolls in the paper industry. ibid, pg. C179.

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