FI84088C - Method and plant for reducing and smoothing the transverse shrinkage of the paper web at the drying portion of a paper machine - Google Patents

Description

84088

Method and apparatus for reducing and smoothing the transverse shrinkage of a paper web with a drying section of a paper machine For the purpose of reducing the cross-sectional shrinkage of a paper machine

One of the main goals in papermaking is to achieve the most uniform paper properties possible. In drying, the paper web shrinks more from the edge areas than from the center area. This can even lead to the edge paper being unusable or compensating for the shrinkage of the edge areas adversely affecting other parts of the paper path. From the point of view of paper quality, it would be advantageous for the shrinkage of the web 15 to be small and even.

The prior art solutions have avoided this problem by increasing the vacuum of the suction rollers. By increasing the vacuum, the track shrinks less in the middle region, but the difference between the edge and the middle region may even increase. The result is therefore not desirable.

One solution to the problem is presented in patent application 895928. In the solution according to the invention, the vacuum profile smoothing the problem is applied to the drying section only to a part of the paper path. Patent application 8601291 discloses an edge belt solution which handles shrinkage prevention and smoothing along the entire path. The disadvantages of the method are the wear of the edge belts, their retention in the groove and the unusability of the edge band.

The method according to the invention takes advantage of the fact that, with high-speed machines, the paper passes through the drying section, supported by a wire. Thus, shrinkage can be reduced and smoothed by creating an adhesive force between the wire and paper edge regions in the critical drying step. Mechanical, chemical or electrical methods can be applied to the edge region to provide an adhesive force. One of the features of the invention is that the holding profile provided on the wire in single wire export reduces and compensates for the transverse shrinkage of the web. An added benefit is improved runnability.

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At the point of change of the drying group, i.e. the wire, it would be advantageous to reduce the strength of the adhesion. However, if the adhesion force is chosen to be suitable, it is possible to carry out the group change by means of a suction roll or a similar device.

Electrostatic forces can be utilized for the purpose of the invention by exploiting the property of cellulosic fibers to generate a negative electrical charge on the surface of a paper web. By selecting the material of the drying wire from the edges or the entire width of the web so that the wire 10 is statically charged or charged with a positive electric charge, an electrical attraction is created between the paper and the wire.

The method according to the invention is mainly characterized in that the method is provided with adhesive means at its edge areas, whereby the adhesive means provide an adhesive force between the wire and the edge areas of the paper during the drying step, thus preventing transverse shrinkage of the paper web.

The device solution according to the invention is mainly characterized in that the apparatus comprises in the drying section of the paper machine means for producing an adhesive at least at the edge areas of the wire, the adhesive increasing the holding force

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The invention will now be described with reference to some preferred embodiments of the invention shown in the figures of the accompanying drawings, to which, however, the invention is not intended to be exclusively limited.

Figure 1 illustrates the invention axonometrically. Shown is the wire export of the drying section of the paper machine.

Figure 2 shows a schematic representation of another embodiment of the method according to the invention with a drying section.

Figure 3 shows a separate friction surface in the edge areas of the wire.

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Figure 4 shows the width position of the wire on the horizontal axis and the adhesion force on the vertical axis. The curve shows the adhesion profile transmitted to the paper web when using adhesion-increasing spacers in the edge regions of the wire.

Figure 5 shows the shrinkage as a percentage of the web width as a function of the track width position. The diagrams show the behavior of the web with a broken line if the adhesion increase according to the invention in the edge region has not been used and the solid line shows the behavior of the web when the increase of adhesion according to the invention has been used in the edge region.

Figure 1 shows axonometrically the method and apparatus according to the invention. The wire H is brought into contact with the web W via the drive wheel 10. Prior to the nip N, an adhesive is introduced from the adhesion source 11 to both edge regions of the wire 12. The adhesive 11 may be an adhesive which, after the nip N, is tightly bonded to the surface of the wire H. Thus, a greater holding force is applied to the edge regions of the paper web W by means of the adhesive 11 20 than to other widths of the web W. Thus, the transverse shrinkage of the web is prevented by holding the web firmly on the wire surface by means of the adhesive 11. The structure of the adhesive may be such that it evaporates or dries as the drying progresses, whereby a holding force is obtained at those points in the drying section where edge holding is necessary.

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Figure 2 schematically shows a drying section in which the drying cylinders are arranged vertically. The adhesive 11 is produced from the adhesive source 12 to the wire surface between the paper web W and the wire H. When the web separates in the drying group Rx from the wire Hlt, the adhesive 30 is removed at the post station A, e.g. with a scraper K from the surface of the wire.

This adhesive is further transferred back to the adhesive application point at the beginning of the Group B Rx wire export. Thus, the transition of the web from one group space Rj to another is effectively facilitated. Between groups, the paper web W should not be subjected to the same holding force to allow a smooth transition from one group to another.

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According to the invention, it is also possible to electrically fasten the paper web W to the wire H. According to the invention, electrostatic forces are utilized by utilizing the property of cellulose fibers to generate a negative electric charge on the surface of the paper web. According to the invention, the material of the drying wire 5 can be chosen at the edges to be statically and specifically charged with a positive electric charge during wire running. According to the invention, it is also possible to actively apply a positive charge to at least the edge regions of the wire and / or the web. Thus, an electrical attraction is obtained between the paper and the wire, with which the web 10 is attached to the wire. This electrically prevents the web from shrinking transversely.

Figure 3 shows a wire H of a drying section of a paper machine provided according to the invention with a friction surface from the edge portions of the wire, preferably a roughening 13. The main principle is that the wire has good friction properties at its width positions. According to the invention, the above-mentioned can be made possible e.g. so that a patterning is applied to the edge areas of the wire during the wire-making step, whereby the patterning allows the edge areas of the wire to have better adhesion properties to the web than the central areas of the wire. The wire may comprise an application at its edge areas, i.e. a separate coating, preferably a pattern coating.

Figure 4 shows graphically the holding profile transmitted to the paper web W when using the method and apparatus according to the invention. The horizontal axis shows the position of the width of the wire and the vertical axis the adhesion force transmitted to the wire and thus to the paper. The width ranges of the wire are indicated by the numbers 1 and 4. Between the width widths 1 and 2 and 3 and 4 of the wire there is an adhesive in the wire, e.g. an adhesive placed as shown in Fig. 1. The dashed line shows the holding profile transmitted to the paper web W.

Figure 5 shows the shrinkage of the paper web as a percentage of the web width as a function of the wire / web width position. The dashed line shows the case where no additional adhesion means have been used, and the solid line shows the case where the adhesion promoter has been used, especially in the edge areas of the wire. It can be seen from the figure that the percentage of shrinkage has decreased at all web / wire width positions.

Claims (10)

A method for exporting a paper web (W) to reduce the transverse shrinkage of a paper web (W) and to smooth it with a drying section of a paper machine, characterized in that the method provides the wire (H) with adhesive means at its edges, whereby the adhesive means H) and between the edge regions of the paper (W), thereby preventing shrinkage across the paper web. 10 Method according to Claim 1, characterized in that glue is added to the edge areas of the wire (H). Method according to Claim 2, characterized in that the adhesive (11) added to the wire (H) is removed from the wire surface at a stage when the adhesive force provided by the adhesive means is no longer required. Method according to one of the preceding claims, characterized in that electrical means are used as the adhesion means, the method providing an electrical charge at least to the edge areas of the wire (H) and / or the web (VJ) such that the paper web (W) is attached to the edge areas by electrostatic forces. using tightly to the wire (H). 25 Method according to the preceding claim, characterized in that the method charges the entire wire (H) in its width direction with an electrical charge of a certain sign, different from the charge in the paper web (W), thereby providing an electrostatic force to the wire (H) and the paper web (W) between. Apparatus for exporting a paper web (W) to reduce and smooth transverse shrinkage of a paper web, characterized in that the apparatus comprises in the drying section of the paper machine means 35 for producing an adhesive (11) at least on the edge areas of the wire (H), increasing the holding force , with which the 7 84088 paper web is held on the surface of the wire, effectively preventing transverse shrinkage of the paper web. Apparatus according to claim 7, characterized in that the apparatus comprises means (K) for removing the adhesive from the wire surface towards the wire exit (point A) where the adhesive is no longer needed. Apparatus according to claim 7, characterized in that the apparatus comprises, as a source of adhesive, electrical means for providing a charge on the wire (H) at least at its edge areas different from the charge on the paper web (W), thereby providing electrostatic attraction to the paper web (W) and the wire (H), in particular to its edge areas where the electric charge is produced in the wire 15. Apparatus according to one of the preceding claims 6 to 8, characterized in that a wire is used as the adhesion means (11), the edge area of which is provided with a separate friction surface (13), whereby the edge area of the wire (H) has a higher web adhesion than the wire. (H) other areas. Apparatus according to claim 9, characterized in that the wire (H) comprises at its edge areas an application (13), i.e. a separate coating, preferably a pattern coating or the like, which effectively adheres to the web surface in the edge areas of the web (U). required to prevent transverse shrinkage of the paper web (W). 30 and 84088