The present invention relates to a method for the production of a liner comprising at least a top layer and a base layer. The invention also relates to a paper machine for realisation of the method.
A liner, a form of multilayer board, comprises frequently a top layer, which is intended for printing. This top layer must display a high smoothness in order to achieve good printability. For production of the surface layer bleached pulp is preferably used, most of the times pulp which is bleached to high brightness and which contains short fibres.
Different methods and machines for the production of multilayer board are described in a number of different documents in the patent literature, e.g. EP 1 076 606, US 5 681 431, US 5 178 732, EP 0 511 186, WO 92/06242, US 4 961 824, EP 0 511 185, US 5 074 964, EP 0 233 058 and SE 506 611. None of these documents teaches how to produce multi-layer board, i.e. a liner, wherein a layer intended for printing can acquire a very good printability in a high speed process.
US patent No. 5 792 320 discloses a board machine having a multi-layer web former. The machine also includes a press section. A transfer belt is used to transfer the board web from the forming section and into the first press nip.
US patent No. 4 197 158 discloses a machine with a multi-ply board former and a press section comprising a number of nips formed between rolls. In one of the embodiments, the former has three headboxes. All nips are described as being double-felted nips.
It is an object of the present invention to eliminate or at least minimise the problem mentioned above, which is achieved with a method for the production of a liner according to claim 1.
Thanks to the use of this solution the great advantage is achieved that the top layer intended for printing acquires a very good printability, wherein the fibre web simultaneously lets itself be transferred through the press section with a higher speed than can be considered to be conventional.
In accordance with further aspects of the method:
The invention also relates to a paper machine for production of a liner, according to claim 12.
In accordance with further aspects of the paper machine:
In the following, the invention will be described more in detail with reference to the annexed figures, in which:
In Fig. 1 shows a preferred embodiment in accordance with the invention. A forming section 1 is illustrated. The forming section 1 consists of a fourdrinier wire, with two forming units, which in a known manner among other things comprises a first 1A and a second 1B headbox. The construction shown is entirely known to the man skilled in the art and is therefore not described in detail. By means of the forming section 1 a fibre web w is created, which consists of a base layer and a top layer. After the forming section 1, the fibre web w is conveyed into a press section P, by being transferred on a first press felt 2 by means of a pick-up suction roll 3. The press section comprises three roll nips, which are formed by only four rolls 6,7,9,11, whereby a compact arrangement can be achieved. To be able to optimise the overall height of the section, the rolls are suitably placed somewhat laterally in relation to each other, so that they can be said to be placed in the proximity to an oblique (non-vertical) line. (see also Fig. 3). The press felt 2 runs from the pick-up suction roll 3 to a press roll 7 (suitably a suction roll) and then around guide rolls (not shown). The suction roll 7 constitutes the upper roll in a first nip which is achieved against a second press roll 6 placed towards the bottom. This press roll 6 is co-operating with a lower press felt 5, which is co-operating with two guide rolls, 4, 8. Consequently a double-felted nip is created between the first two press rolls 6, 7. A second nip is created between the first suction roll 7 and a third press roll 9. Also this nip is double-felted, in that a third press felt 19 runs around the third press roll 9.
Finally, a third roll nip is shown, between the third press roll 9 and a shoe press roll 11. Apart from the third felt 19, one impermeable transfer belt 30 is being fed through this third nip. This transfer belt is at least arranged with one smooth surface 30A, which is turned away from the shoe press unit 11 and against the third press felt 19. The transfer belt 30 passes a first guide roll 10 before the nip and around a second guide roll 12 after the nip. Immediately before (seen in the machine direction) the second guide roll 12, a fourth felt 14 (or drying wire), which runs around a suction roll 13 and a guide roll 15, abuts the bottom side 30A of the transfer belt 30. Thereafter, a drying section D follows, which is only shown to a smaller extent. A drying wire 16, a vacuum roll 17 and a drying cylinder 18 are also shown. The vacuum roll 17 is arranged so that the drying wire 16 runs against the felt 14 between the suction roll 13 and the guide roll 15.
The above-described set of machines is, in compliance with the invention, intended to manufacture with high efficiency a multilayer liner with a top layer with high printability. The machine shown operates in the following manner. A base layer is applied on the fourdrinier wire by means of a first headbox 1A. Thereafter, a top layer is spread (in a known manner) by means of a second headbox 1B. The base layer consists essentially of pulp containing long fibres, preferably containing fir and/or pine fibres. The top layer is suitably formed by a pulp, which at least partly contains short fibres. (It will be understood that the determinations regarding the weight-% of fibres, which are mentioned, relates to the end product.) In most applications, preferably fibres from easily bleached kinds of wood as birch or eucalyptus are used, which are bleached to a relatively high brightness, preferably over 85 ISO, more preferred to a brightness exceeding 90 ISO. The fibre web w formed is then fed over to the press section P by means of a pick-up suction roll 3 and the felt 2 that runs around this. Consequently, the top layer abuts against this felt 2. The base layer then meets the lower press felt 5 and the fibre web w is then fed through the first roll nip 6, 7, which accordingly is double-felted. The fibre web then follows the felt 2 around the first suction roll 7 into the second roll nip 7, 9. In this roll nip, the top layer will abut against the press felt 2, while the base layer will abut against a third smoother press felt 19. The web follows thereafter the smoother third press felt 19, around a third press roll 9, which preferably consists of a grooved roll. Then, the web runs into the third roll nip 9, 11, in which the top layer meets the smooth surface 30A in the impermeable transfer belt 30. The web then follows the impermeable transfer belt 30 until it encounters the drying wire/felt 14 which runs around the suction roll 13. The web then follows the felt 14 until it encounters the drying wire 16 which it follows around the vacuum roll 17, then around the drying cylinder 18 and further into the drying section D.
In Fig. 2 a modification in accordance with the invention is shown. In large parts, the set of machines is the same as according to with Fig. 1, and therefore merely the modifications will be described. The considerable difference in relation to Fig. 1 is that five rolls are used to create the three nips, instead of four as according to Fig. 1, which allows larger flexibility for the set of rolls. It is shown that the first roll nip is created by means of a shoe press roll 21 which is pressing against a lower press roll 6. Thereafter follows a suction press roll 23, which constitutes the second nip with the last but one press roll 9. A guide roll 22 is arranged above the third press roll 23 in order to carry the felt 2. The lower press felt 5 runs as previously through the first roll nip and above that around the third press roll 23. In this embodiment, the transfer belt 30 runs around the last but one press roll 9. As in Fig. 1, the last press unit 11 is formed of a shoe press roll. A press felt 19 runs around this shoe press roll 11 by means of guide rolls 24, 25. In conformity with Fig. 1, the top layer will abut against the upper the press felt 2 in the first nip 6, 21, and is thereafter liberated from contact with the press felt 2, by means of the suction roll 23. It is also possible to liberate the web from the upper press felt 2 directly after the first nip, since the guide roll 22 preferably is arranged movable, whereby the web w can be allowed to run towards the second nip only in contact with the lower press felt 5. In the second nip 23, 9, the top layer encounters the smooth surface 30A of the transfer belt 30 and follows thereafter the transfer belt 30 around the last but one press roll 9 through the last nip 9, 11, and thereafter, as in Fig 1, it is transferred to a drying wire/felt 14 which runs towards the drying section D. Consequently, also in this set the top layer is pressed in the last nip against the smooth surface 30A of the transfer belt.
In Fig. 3 a further embodiment in accordance with the invention is shown. In contrast to what is shown in Figs. 1 and 2, a twin wire former with a first 1A and a second 1B headbox is used here for creating a web w with a top layer and a base layer. In conformity with Fig. 1, the press section is formed of only four press rolls. The first roll nip consists of one first press roll 21 and one suction roll 6. Another similarity with Fig. 2 is that the transfer belt 30 runs around the last but one press roll 9 in order to form both the last but one nip 6, 9 and the last nip 9, 11. Also here, the last press unit constitutes of a shoe press roll 11. Also as in Fig. 1 and Fig. 2, a transfer from the transfer belt 30 to a drying wire/felt 14 is done in accordance with Fig. 3 over the suction roll 13 and then to a drying wire, not shown.
In Fig. 4 still another modification in accordance with the invention is shown, wherein only two roll nips are used. In principle, the forming section 1 is the same as in accordance with Figs. 1 and 2. Also the transferring of the web from forming section 1 and to drying D section is substantially the same as what has earlier been shown. The first nip 6, 21 is substantially similar to what has already been described with reference to Fig. 2, with the difference that the lower press felt 5 according to Fig. 4 only runs through one roll nip and that the upper press felt 2 preferably consists of a smooth felt. The first nip 6, 21 can, however, just as well be a press. In the last roll nip 9, 11, in resemblance with all earlier shown embodiments, the top layer encounters the smooth surface 30A of the transfer belt 30. The web w is transferred in a known manner by means of a suction roll 24 from the upper the felt 2 to the third press felt 19 and is conveyed in that manner into the last the nip 9, 11.
In Fig. 5 there is shown a further modification according to the invention. In Fig. 5 the main portion of the press section P is identical to the press section shown in Fig. 2. The transfer zone from the press section to the drying section D of Fig. 5 is identical with what is shown in Fig. 4. The modification of the embodiment shown in Fig. 5 resides in the arrangement of a further press roll nip 6', 21' at the beginning of the press section P. It is shown that the fibre web W is transferred from the forming section 1 to the press section P by means of a press felt 2' and a suction pick up roll 3', which conveys the web on to the press felt 2'. Thereafter the web W encounters a second press felt 5' from below, such that a double felted nip is created between the first pairs of rolls 6', 21' forming the first roll nip. As can be seen from Fig. 5 this first nip is a press roll nip without the use of any shoe press roll. Thereafter the web W follows the lower press felt 5' into contact with a suction roll 3 which transfers the web W on to a third press felt 2. Thereafter the press section is identical with the one shown in Fig. 2 and accordingly will not be explained more in detail. However, it should be noted that also this press section, including four roll nips, the top layer of the web will be pressed against the smooth surface 30A of the transfer belt in the last nip.
The invention is not limited to what is shown above, but can be varied within the scope of the following claims. Consequently, it is realised that for instance more than four roll nips can be used, if desired for certain installations. In addition, it is realised that still more constellations of sets of rolls are feasible for utilising the beneficial principles in accordance with the invention. Further, it is realised that a liner consisting of more than two layers can be produced in accordance with a method according to the invention, for instance triple layers, four layers, etc., wherein different mixtures of pulp can be used to manufacture desired products. It is also realised that in certain cases mixtures of pulp entirely without short fibres are preferred for the top layer.
The skilled man also knows that there exist several different options of impermeable transfer belts to be used, but a preferred such impermeable transfer belt is TRANSBELT® (trademark belonging to Albany International).