SHOE CALENDERING

The NipcoFlex™ Calender for Bulky Boards

Dr. Jörg Rheims

Many current board machines use a Yankee dryer [Figure. 1].

Figure 1
Figure 1

Yankee dryer- a bottleneck

It has the potential for bulk-preserving surface treatment, but at the expense of a narrow operational window. Furthermore, it does not allow any significant increase in speed and hence in productivity. This bottleneck has been overcome in some of the latest board machines by installing a softnip calender after the dryer section instead. Pushing the idea of gentle, bulk-saving calendering action to its limits leads to an extended nip calender, a so-called shoe calender. "NipcoFlex™ calender" is the brand name Voith Paper uses for this device.

Developing the NipcoFlex™ Calender

Voith Paper successfully started developing the shoe calendering technology in the early 1990s. Basic research was completed and several patents were granted, but due to market requirements in the decade's 2nd half Voith focused on developing the Janus™ calender instead. In 2000 work was resumed and since January 2002 the NipcoFlex™ pilot calender is in operation.

Basic Design and Working Principle

The NipcoFlex calender consists of a heated roll acting against a shoe with a concave shape [Figure 2].

Figure 2
Figure 2

The shoe is covered by a sleeve (QualiFlex™ Cal). Between the sleeve and the heated roll the paper or board web is calendered.

The following parameters are especially important for shoe calendering:

  • The sleeve and its properties: structure, material, surface roughness, and hardness
  • The shoe itself: nip length, MD-load profile and lubrication system between shoe and sleeve
  • The heated roll: Shoe calendering is especially effective at temperatures well above 200 °C. Oil-based heating systems cannot be used at this temperature level anymore. Instead, Voith Paper furnishes the NipcoFlex™ calender with rolls heated by induction.
  • The shoe and the sleeve are mounted in the NipcoFlex™ roll. The design of this roll was adopted from the well-established NipcoFlex™ press, as well as the basic design of the QualiFlex™ Cal sleeves. Using this proven technology ensures trouble-free operation of the NipcoFlex™ calender. Beside these parameters, the parameters wellknown in every calendering technology are also utilized in shoe calendering, namely, pressure, temperature and moisture.

The principle of shoe calendering is based upon the effects of moisture and temperature gradients [Figure. 3].

Figure 3
Figure 3

Due to the elongated nip, their influences increase, enhanced by the high temperature level. This allows significant reductions in line load i.e. pressure, which in turn leads to the desired gains in bulk.

NipcoFlexTM Pilot Calender

Voith Paper's NipcoFlex™ pilot calender is depicted on Figures 4 and 5. Typically, the NipcoFlex™ roll is installed in the bottom position of stack 2 (configuration 1). The heated roll is placed on top so that the top side is calendered.

Figure 4
Figure 4

Figure 5
Figure 5

In the pilot calender, this roll is heated from inside and, additionally, by an external inductive heating unit. This permits higher temperatures of the heated roll and compensates for the high energy transfer from the heated roll to the web during shoe calendering compared to conventional roll calendering, caused by the prolonged dwell time. Furthermore, it helps to quickly stabilize temperature making possible efficient trials. The first stack can be equipped with hard- or softnips so that pre-calendering or profiling can be performed online. The system is completed by moisturizing units (steam or water). The pilot calender can also be arranged in a reversed order, configuration 2, with the NipcoFlex™ calender in the first stack, thus permitting online post-calendering.The NipcoFlex™ pilot calender can be operated at speeds up to 1500 m/min, linear loads up to N/mm and roll surface temperatures up to some 260 °C, the latter depending on grade, basis weight and speed. Nip lengths vary between and 250 mm, where the shorter nips are mainly used for graphic grades and the longer nips for board grades.

Trials with NipcoFlex™ Calender

A huge number of trials were conducted since January 2002, mainly focusing on folding boxboard and white lined chipboard [Figure 6].

Figure 6
Figure 6

Trials covered a wide range of basis weights from folding boxboard at 370 g/m² down to LWC paper at 47.4 g/m². To save time and costs, and to utilize the most recent methods in trial planning, evaluation and presentation, a process optimization software package was used. Basically, all these trials have proven that with a NipcoFlex™ calender the correlation of surface properties to bulk or bending stiffness can be improved. Out of this huge collection of data, the following results were selected to demonstrate the potential of the NipcoFlex™ calender. Figures 7-10 give a very brief overview. First, a comparison is made for folding boxboard produced conventionally with a Yankee dryer with board treated with the NipcoFlex™ pilot calender after pre-drying and before coating. The NipcoFlex™ calender was operated with different nip lengths and at varying calender settings, at speeds 40% above the current production speed with a Yankee dryer.

Figure 7
Figure 7

Figure 7 shows the surface micro roughness of the board before and after coating, measured as PPS S10, vs. bulk. With the NipcoFlex™ calender, the same level of PPS is obtained as with a Yankee dryer, but at up to 4 % more bulk and at higher speed. Figures 8 and 9 show results from white lined chipboard, evaluated with process optimization software.

Figure 8
Figure 8

In Figure 8, bulk is shown as function of linear load, roll surface temperature, and the amount of steam fed to the calendered side of the web. Load and temperature have a similarly high effect on bulk, whereas the effect of steam is much less. Linear load and temperature also have a huge impact on surface micro roughness (PPS S10), steam can be used to support surface smoothing [Figure 9].

Figure 9
Figure 9

The effects of linear load and steam level off at high settings. The process optimization software can also be used to derive an operational window, as is shown for liquid packaging board in Figure 10.

Figure 10
Figure 10

Data was derived from trials with a conventional soft nip calender compared to a NipcoFlex™ calender. In this final stage of data evaluation, threshold values can be defined for every parameter. Combining these requirements, it can be sought  for calender settings where they all are met or even exceeded. In the present case, no operational window remains for the board treated with soft nip calender: requirements for surface properties are met, but bulk is too low. With the NipcoFlex™ calender, on the other hand, all requirements are fulfilled in the range marked in yellow. Furthermore, it can be seen that with increasing temperature the linear load can be reduced. The increase of bending resistance with increasing temperature is somewhat surprising, but it was observed many times. It is most likely due to an hornification effect due to high temperatures, which increases bending resistance of the board. Trials Voith has conducted over the last two years have shown that best results are obtained at high temperatures and with the application of steam (temperature and moisture gradient calendering), thus permitting a reduction in load. [Figure  11]. 

Figure 11
Figure 11

However, "optimum temperature" is not equivalent to "maximum temperature". The appropriate level has to be found , considering issues of economy and technology. The nip length also is a crucial factor for optimum results, it has to be adopted to the calendered grade. With the experience gained so far, Voith Paper is certainly able to make profound recommendations for the best operating conditions for shoe calendering.

Summary and Outlook

Summing up, the NipcoFlex™ calender provides a new, very interesting alternative for the production of paper or board [Figure 12].

Figure 12
Figure 12

It can be installed in new machines and, above all, it is an attractive, economically and technically feasible option for rebuilds, for example, if machine speed is limited by the Yankee dryer. At the same level of surface properties more bulk is obtained with the NipcoFlex™ calendar than with a Yankee dryer or a soft nip calender.These savings can be traded for fibre savings and, inherently, for cost reductions. Operation conditions and nip lengths, balancing the influences and dwell times of temperature and load, are crucial for successful shoe calendering.

The first shoe calender installed in a folding boxboard machine worldwide started up at Stora Enso Baienfurt on February 1st, 2004. It has given and will further give production experience in shoe calender operation. Voith Paper's future R&D work on extended nip calendering will also consider light-weight graphic papers.