Field of Technology The present invention relates to the processing of fibre webs. In particular, the invention relates to a method, according to the preamble of Claim 1, for removing moisture in a fibre web.

The invention also relates to a drying device for a moist, moving, fibre web, according to the preamble of Claim 18. Such a device generally comprises a heating unit, which is arranged to heat the fibre web, in order to remove water by vaporization at a temperature of at least 50 °C.

The invention also relates to use according to Claim 27.

Background of Technology

The fluting of paper as a phenomenon is a typical and characteristic problem in so-called heatset-offset printing. This printing method is the most-used in the world, if production is calculated in terms of paper consumption. Fluting appears in printed products as variations in shape, with a frequency of a few millimetres, in the direction of running. This phenomenon has been studied for a long time in several places, with no solution to it having yet been found, even though the reasons for it are known. However, because the printing method is, in terms of its price-quality relationship, the best for large and medium printing runs (50 000 - 500 000 items), the problem has had to be tolerated and it has also been accepted. The least satisfied have been advertisers and the publishers of high-quality journals and catalogues.

Fluting is due to the paper's moisture and the hot-air drying used in the printing method. Part of the surface of the paper web is covered in printing ink and part is not printed, depending on how images, texts, and other colour surfaces are designed and positioned in the product. Water evaporates more slowly from areas covered by printing ink than from unprinted surfaces, so that the paper's fibres shrink to a different extent in the cross direction of the paper web and the paper acquires a corrugated shape. The tensile stress in the paper can be adjusted during printing, but this cannot be done in the cross direction, even though it could prevent the web from shrinking.

Because the cause of the problem is water, the amount of it in paper should be reduced. There are two sources of water, i.e. the paper's own relative moisture content and the moisturizing water of the printing method based on lithography.

Water must be used to water the printing plates, so that the surfaces not to be printed will not take the oil-based ink. Due to the high running speed (10 - 15 m/s), this moisturizing water does not, in practice have time to be absorbed by the paper so that its fibres would begin to swell, before the paper web moves to the hot-air dryer. The major cause of fluting is actually the paper's relative moisture content.

Fluting has been less in so-called waterless offset, in which the printing plates and inks are more expensive. By drying the paper web with hot air just before the printing unit the paper dries and fluting is reduced significantly. However, the hot paper raises the temperature in the following print units and the printing ink, thus reducing the viscosity of the ink and causing the half-tone dots being printed to enlarge as the ink spreads more. This reduces the contrast of the impression and thus reduces its quality.

Brief summary of the invention

The present invention is intended to remove at least some of the drawbacks of the prior art and create a new type of solution for removing moisture from fibre webs, such as paper and board webs.

In particular, the task is to create a method for removing moisture from a paper or board web to be fed to a printing machine, in such a way that fluting is reduced at the same time as the quality of the printing result is maintained or even improved.

The invention is based on the idea of removing the moisture of a fibre web, such as an unprinted fibre web, in a partial vacuum. The removal of the solvent contained in a printing ink by vacuum is previously known. Publication DE 3 411 752 discloses a vacuum chamber arranged after a printing machine, the purpose of which is to remove the solvent contained in printing ink by a gas or a vacuum. GB patent publication 935 063 also discloses a solution, based on the use of a vacuum, for drying printing inks.

FR patent publication 1 397 222 discloses the drying of webs consisting of various materials, for instance, fabrics, leather, and paper, in a vacuum, the aim of which is to shorten the drying time, when the webs can also be stretched, if desired. Similarly, US patent publication 4 053 990 discloses the removal of solvents and solvent mixtures, such as mixtures of water and organic solvents, from web-shaped materials by means of a vacuum.

Other solutions for drying papers are disclosed in publications JP 2002166667 and WO 2005/053958.

The known methods are not, as such, suitable for the removal of moisture from

conventional fibre webs, nor is there any reference in the publications to the fact that they could be applied to the drying, before printing, of a fibre web to be fed to a printing press for printing.

In the present invention, it is stated that vacuum drying can be combined with heating of the fibre web and that it is best implemented in such a way that, after vacuum drying, the temperature of the fibre web is reduced to less than room temperature.

Thus, according to the invention, the fibre web is brought to a temperature of more than 50 °C, after which it is subject to vacuum evaporation, in connection with which the temperature of the web is reduced to less than 20 °C. The apparatus according to the invention comprises a vacuum evaporator, which is arranged after the heating unit in the direction of movement of the fibre web, and which is arranged to remove water by evaporating it at a temperature of less than 20 °C. The method described can be used to pre-dry a printing base, such as paper, in order to reduce the fluting of paper printed in heatset-offset printing.

More specifically, the method according to the invention is mainly characterized by what is stated in the characterizing portion of Claim 1.

The apparatus according to the invention is, for its part, characterized by what is stated in the characterizing portion of Claim 18. The use according to the invention is characterized by what is stated in Claim 27.

Significant advantages are gained with the aid of the invention. Thus, by means of the disclosed vacuum drying a significant energy saving can be made in paper and board manufacture. In addition to this, by pre-drying the printing base, i.e. paper or board, fluting is reduced in heatset-offset printing. At the same time, the so-called "blistering" phenomenon is eliminated and the contrast and colour reproduction of the impression are improved.

In an especially preferred solution, pre-heating preceding the vacuum drying is combined with the hot-air circulation of a dryer and/or reheating, which are arranged after the printing press, when the web being fed to the vacuum unit can be heated using recirculated residual heat and finally dried using vacuum. The web can be cooled when the final moisture evaporates, so that the printing ink attaches and adheres to it well, dot

enlargement being also reduced.

Vacuum evaporation together with pre-heating is an easy and cheap solution, because the paper does not yet have printing ink in it, which could smudge on the rolls in the closed vacuum chamber. Preferred embodiments and the advantages gained by them are described below.

Brief description of the drawings Figure 1 shows a schematic diagram of four-colour printing, in which, before a heatset- offset printing press, a vacuum dryer according to one embodiment of the invention is arranged, by which moisture is removed immediately before printing; and

Figure 2 shows a side view of the schematic construction of a vacuum chamber according to one embodiment of the invention.

Description of the embodiments As stated above, in the method presented here for removing moisture from a fibre web, the fibre web is first brought to a temperature of over 50 °C, after which it is subjected to vacuum evaporation, in connection with which the web's temperature is reduced to under 20 °C. By reducing the web's temperature particularly to under 10 °C, dot enlargement is significantly reduced in printing.

Operation preferably takes place in such a way that vacuum evaporation is performed immediately after the heating stage, so that the temperature of the fibre web does not drop substantially before vacuum evaporation. In terms of equipment, this can be implemented by arranging the vacuum chamber immediately after the heating unit.

The solution described is generally suitable for drying paper and board, when vacuum evaporation is combined with the drying section of the paper or board machine.

In one embodiment, the solution presented here can be arranged after the drying cylinders as the final drying stage before the finishing of the fibre web. In a second embodiment, one or more conventional drying cylinders can be replaced using the present solution. It has been shown that it is possible to replace a significant part, of up to half of the drying cylinders of a conventional drying section. In practice, 1/20 - 1/3 of the drying cylinders are replaced. The solution can thus be used to reduce the energy consumption of drying. Especially advantageously, the vacuum drying described in this case is arranged at the end of the drying section. In practice, the moisture content of the paper or board web brought to the vacuum drying is at most 50 %, for example, at most 40 %, most suitably at most 30 % or at most 25 % or at most 20 %. In both embodiments, vacuum drying can be used to reduce the moisture of the paper or board web to its desired final value, e.g. to about 2 - 8 %, especially to about 4 - 6 %.

In one especially preferred embodiment, vacuum evaporation is performed in connection with the printing press. In such an embodiment, the fibre web then comprises a printing base, such as the paper or board web to be printed, and this is brought to vacuum evaporation immediately before it is fed to the print unit of the printing press.

According to the technology presented here, it is advantageous to cool the paper after drying and before printing, because, for example, on the basis of winter frost experiences it had been noted that water in a solid state, i.e. as ice, does not cause the fluting

phenomenon. However, the storage and handling of frozen paper rolls and the

condensation of the relative moisture in the indoor air on the paper hinder production. Also the condensation wets the paper, reduces its strength, and increases fluting. On the basis of these observations, in the technology presented here it has been decided on a solution, in which cooling is arranged just (immediately) before the printing stage, i.e. before the print unit. Due to the speed of the paper web, printing starts a few milliseconds after drying and cooling. In one embodiment, the printing press is an offset printing press and the vacuum

evaporation is performed immediately before the first print unit of the printing press, seen in the direction of movement of the printing base.

In an especially preferred embodiment, the offset printing press is a heatset-offset printing press, such as a four-colour heatset-offset printing press.

Most suitably, the fibre web is brought to a low enough temperature to be able to the viscosity of the printing ink brought to the fibre web. In one embodiment, in which the fibre web comprises a printing base, the temperature of this is reduced to the range -10.. +2 °C before it is taken to the printing unit. When operating in the manner described above, the fibre web raises the viscosity of the printing ink on the fibre web in the press stage of the print unit and reduces dot

enlargement, thus improving the colour reproduction and the contrast. An increase in the ink's stickiness on the printing base, and not on the ink-transfer rolls, the press plate, or the rubber-fabric cylinder, improves the adhesion in the printing base, and does not cause tensile forces to act on the surface of the paper (the stickiness is on the "right" side).

Figure 1 shows a heatset printing press 7 equipped with a vacuum-drying unit 3 - 6.

The apparatus comprises, in order (i.e. in the direction of movement of the paper or board web), an unwinder 1, which rolls the paper open, a feed device, by which the paper web is fed and guided to the printing press, a moisture-removal unit 3, in which the paper is heated 4, and which as a unit is connected without a gap to a vacuum chamber 6, in which the desired vacuum is formed by vacuum suction 5. As can be seen from the drawing, vacuum evaporation is performed in the vacuum chamber, which is essentially airtight. The vacuum chamber is closed to be at least substantially airtight by press rolls, between which the paper web travels into the unit and, dried and cooled, out of it. One embodiment of the construction of the vacuum chamber is shown in greater detail in Figure 2.

The vacuum chamber 6 is arranged in connection with the printing press, immediately before the print units 7. Having travelled through the vacuum chamber 6, the paper web transfers immediately to the first print unit 7 of the printing press, in the case of the figure, the black-ink print unit. In the direction of movement of the web, the print units are: Black, Cyan, Magenta, and Yellow.

Most suitably, after the vacuum chamber the fibre web's temperature is not able to rise, or rises at most by about 0.1 - 10 °C, for example, at most by about 0.5 - 5 °C before the first print unit. As described below, in one preferred embodiment the temperature of the fibre web is kept under the freezing point of water over the whole distance of the printing press's print units (i.e. in the area indicated by the reference number 7).

The fibre web travels printed through the four print units 7 to the hot-air dryer 8, where the solvent component of the printing ink is removed by heating the impression. In the case of the figure, there are three dryer units and the drying temperature is highest in the first and lowest in the last. Thus, in the first unit, the temperature in the present example is about 240 - 270 °C, in the second 180 - 210 °C, and in the third 150 - 170 °C.

After the heating unit 8, cooling cylinders 9 are arranged, and after them the web's creasing device, or, when printing packaging, a re-reeler. The temperature after the drying unit can be determined, for example, using an IR thermometer. In the case of the example shown in the figure, the temperature is typically about 130 °C.

The fibre web is heated by hot-air heating. Figure 1 shows the circulation of hot air from the hot-air dryer 8 to the moisture-removal unit 3, by a line with an arrow, which is drawn to run under the apparatus, from right to left.

On the basis of the above description, in one embodiment the drying device for a moist, moving, fibre web comprises, in a combination:

- a heating unit, which is arranged to heat the fibre web to remove water by

vaporization at a temperature of at least 50 °C, and

- a vacuum evaporator, which is arranged after the heating unit, in the direction of travel of the fibre web, and which is arranged to remove water by evaporating it to a temperature of less than 20 °C.

If the device described is arranged as part of the drying section of a paper or board machine, at least one drying cylinder preceding the device operates most suitably as the heating unit of the device.

Connected to a printing press, particularly a heatset-offset press, the heating unit of the device is, in turn, connected to the hot-air circulation of the hot-air dryer of the printing press. Thus, in both cases the heat that usually arises in the apparatus, or even waste heat, is utilized to heat the fibre web. Figure 2 shows the schematic construction of a vacuum dryer according to one

embodiment. The drying section 20 can be closed to become airtight by means of press rolls 25a - 25d, between which the paper web 23 travels into the unit and, dried and cooled, or even frozen, out of it. In one embodiment, the fibre web moving in the airtight vacuum chamber is subject to an absolute pressure of about 0.001 - 0.1 bar.

A great deal of vacuum can be easily sucked through the suction pipes 21 and 22 using moderate pumping power, due to the closed system. The rolls 25a - 25d against the paper web are rubber surfaced, so that the compression becomes tight. Against the rubber rolls are idle rolls 24a - 24d, which in this case are chrome-plated. Against the surface of the idle rolls are ceramic or steel blades 26a - 26d (so-called doctor blades). The task of these blades is to press tightly against the idle rolls 24a - 24d with the aid of a vacuum. The bearings of the rolls are sealed at the end of the roll and against the vacuum chamber (sealing bearings).

The preheating section of the device (marked in Figure 1 with the reference number 2) is also closed by rolls, though the tightness demand is not important. The reason is that heat transfer is more efficient in a closed space and waste heat can be returned to preheating the air of the printing press' s actual printing-ink hot-air dryer.

The cooling of the paper web, even though all the water may not have evaporated, keeps the moisture in a solid form, i.e. ice, for about half a second of the time of four-colour printing 7, before final drying 8. In addition, the cooling increases the printing ink's viscosity on the paper during the press stage and reduces dot enlargement, which permits better colour reproduction and contrast. It has been shown that it is not worth raising the viscosity of the ink earlier, for example, when preparing the inks, which is due to the ink dosing and the set of ink rolls, and above all the surface strength of the paper.

Industrial Applicability

The present technology can be exploited in both printing, such as offset printing and flexo printing, and in paper and board manufacture.

The offset printing presses, for which the solution is particularly preferably suitable, can be not only heatset-offset but also "coldset", i.e. "non-heatset" -offset printing presses.

However, combining the present solution with heatset-offset printing achieves the significant advantage that the heat required for drying is obtained from heating after the printing press.

The present solution is also suitable for water-free offset printing.

By means of the invention, the fluting phenomenon in printed products, which relates mainly to heatset-offset printing and is a nuisance to, for instance, publishers and advertisers, is eliminated.

The reduction in dot enlargement, due to the cold paper web obtained from vacuum drying, is a further significant advantage and benefits mainly these print units, which determine the contrast of the impress, i.e. in the first dark inks. The need for cooling in the rubberized- fabric cylinders is also reduced.

In a paper or board machine, preheating is not needed, because the web is dried by the machine's hot cylinders. The vacuum apparatus by itself is enough and by using cylinder drying the heating energy can be reduced. In paper and board manufacture the benefit is precisely the saving in thermal energy. When manufacturing a new paper machine, it is also possible to reduce the number of drying cylinders, as described above.

On the basis of what is described above, one embodiment encompasses a method for removing moisture from a fibre web, in which method the fibre web is brought to a temperature of over 50 °C, after which it is subjected to vacuum evaporation, in connection with which the temperature of the web is reduced to less than 20 °C, when the said vacuum evaporation is performed in the drying section of the paper or board machine. In the embodiment described above, vacuum evaporation is arranged as part of the drying section of the paper or board machine, possibly at its end, so that it sets the web's final moisture content. Most suitably, the vacuum evaporation is arranged as part of the drying section of the paper or board machine, when it replaces some of the drying cylinders of the drying section of such a paper or board machine.

List of Reference Numbers

1 unwinder

2 feed device

3 moisture removal

4 heating

5 vacuum suction

6. vacuum chamber

7 print units (in order: Black, Cyan, Magenta, and Yellow)

8 hot-air dryer

9 cooling cylinder

10 folding device

20 vacuum-chamber wall

21, 22 suction pipe

23 paper web

24a - 24d chrome roll

25a - 25d rubber roll

26a - 26d compressing vacuum blade

List of Reference Publications

Patent literature

DE3411752

GB935063

FR1397222

US4053990

JP 2002166667

WO 2005/053958.