The calendering system on Rauma's PM 4 has allowed the mill to produce quality LWC at record-breaking speeds

PM 4 at Rauma makes a record start

UPM-Kymmene's lightweight coated (LWC) PM 4 at Rauma has just set another world record, this time on the machine's calendering section. In August this year, the new Finnish PM captured the 24-hour world record for on-machine LWC production with a speed of 1,578 m/min. That record has since been succeeded by PM 6 at the Kaipola mill (1,610 m/min), but Rauma has again positioned itself in the record books with the highest production speed for off-machine multinip calendering.

Last month, the PM's two multinip calenders ran at 1,450 m/min during regular production, achieving "normal" paper quality figures. The operation lasted for two and a half hours, during which time the nip loads were increased from 190 kN/m to 210 kN/m and the temperature from 130C to 146C.

"Calendering is not a bottleneck in production volume or quality," according to Vesa Malmstén, finishing superintendent on the giant 400,000 ton/yr capacity PM. "Calendering technology is entering a high speed era with the polymer rolls and new nip loading method."

Rauma's new production line has a wire width of 10.1 m and the web is 9.31 m on the reel. The average paper machine speed was 1,530 m/min in August and even at this speed, noise levels are surprisingly low in the machine hall - less than 85 dB in the finishing area.

PM 4 started up 7 January this year and during its first eight months produced over 130,000 tons of LWC offset in a basis weight range of 54-60 g/m.

Nip on

Calendering LWC is a challenging task. The paper is sold on the basis of good printability and print gloss characteristics. Smoothness and opacity are critical, in addition to even profiles and shipping roll quality. It is generally assumed that every LWC mill uses supercalenders to achieve the gloss and smoothness required. But instead of supercalenders, Rauma's PM 4 has two Valmet OptiLoad units. This technology is part of a new range of products which includes the likes of Voith Sulzer's Janus and Küsters Beloit's ProSoft systems. These products seek to deliver higher speed production at lower investment costs compared with traditional supercalenders, but with the same gloss and smoothness. Calenders of this type are called "multinip" calenders, to avoid confusion with the traditional supercalenders on paper rolls.

As Malmstén points out, "The reason for buying the OptiLoads was really to save on investment and production costs. If traditional supercalenders were selected, three units would have been needed. With this new technology, we believe we are also gaining higher quality, which is offered by the new flexibility in the nip load control of these calenders."

Under normal operations, Rauma's two multinips have run at 1,100 m/min, but the record run shows that additional speed potential exists if needed.

Material question

Pilot tests highlighted the differences between the supercalenders and polymer roll multinip calenders. Polymer rolls with surface temperatures of more than 110C offered better print gloss and lower fiber roughening than the supercalenders using paper rolls. UPM-Kymmene also selected a 12-roll multinip system, rather than a 10 or 8-roll version to maximize gloss and smoothness quality.

Soft roll life has proven to be good at Rauma. Today, the soft rolls are changed after 1,000 running hours - practically two to two and half months. The target is to reach more than 1,400 working hour intervals between planned grindings. Part of the reason for the extended life is that polymer rolls do not lose smoothness during ordinary calendering. Indeed, the opposite is true.

Rauma uses three different types of soft cover - Valmet's Dura, Beloit's Supertex and Stowe's Jalon 9607. They all have similar grinding intervals, although the Dura rolls do have a thicker polymer cover. The thermo rolls are warmed via steam-heated water at a maximum 155C. Nip loads range from 50 kN/m to 400 kN/m at Rauma. The required maximum is about 100 kN/m lower than conventional supercalenders as the nip loads are equal.

The nip loads on the calenders are controlled by hydraulic cylinders mounted to each roll end in the stack. The cylinders are used to relieve the desired proportion of roll weight through a pivot mechanism. There are none of the spindles, slides or screws found in traditional supercalenders. The system makes it possible to achieve the same load in all calender nips.

Another important design feature is roll dimensioning. Using the roll relieving technique to produce desired nip loads allows the freedom to design the roll diameters without concern for roll weights. The soft and hard rolls are large diameter units, dimensioned to have exactly the same nominal deflection. This feature eases controllability. When the cross-machine deflection curve is the same in all the rolls, there is no need for additional CD adjustments under normal circumstances. With equal loading, deflection in all the rolls compensates each other.

According to Malmstén, the multinips at Rauma always run with equal loads in all the nips. "In this way, we save on roll covers. The maximum nip pressure is much lower than in the traditional supercalenders, where the bottom rolls have the whole stack weight with the extra load by the control system," he says.

The load relieving system on the multinip is a combination of two cylinders. The larger takes care of the loading and the smaller diameter unit carries out the constant and rapid stack movements, such as quick nip opening and automatic positioning. The hydraulics have worked well so far as maintenance has been minimal and little leakage has been seen.

The unwind and rewind stands are equipped with oscillation devices. At the unwind, an edge sensor ensures that the web is accurately located in the center of the stack to avoid excessive edge heat generation. If the temperature does get too high, an edge cooling system automatically starts blowing cool air onto the roll ends.

PM 4's multinips have an automatic web threading arrangement. This features a flat belt installed on one end of the roll stack. A triangular fabric with a rope in one comer is fastened by adhesive to the web tail. In use, the operator ties the rope to the transfer belt, which carries the sheet through the machine. The operators no longer need to work in the lifts at the stack to thread the sheet through the nips. Consequently, there is no need for any finger guards on the calenders. Splicing on the unwind stand is also automatic.

On the rewind stand, the reel change system resembles the paper machine pope operation. A major difference is that there is no center roll. Instead, there is a pendulum roll to take care of the wound-in tension for reel hardness control. The full parent is transferred to the storage position at the end of the run at crawling speed, and the new empty parent is lowered to the reeling location. The web, running from the previous parent below, is simply blown around the new reel spool.

The splicing system works well at the unwind. But the rewind stand still requires some adjustment work by the supplier before it will meet the 97% success rate target. The reel change time is also felt to be too long and these are two of the major issues for improvement. But as Malmstén points out, these issues have not created any problems in terms of production capacity or quality. Indeed, they are considered normal during the startup optimization process. And as Malmstén says, "We believe that with this new technology we will be able to develop our sheet quality
more efficiently than with the traditional technologies."