SE428575B - SET AND DEVICE FOR SHAPING A PAPER COAT - Google Patents

Description

The thicknesses of the stock layer do not have time to grow to the same extent as on a flat wire machine.

The dewatering on a flat wire machine, until the individual fibers are fixed in a fiber bed, takes place by negative pressure with the help of different types of dewatering elements, forming tables, register rollers, foils, wet suction boxes.

Common to these, in addition to the primary task of dewatering, is that they introduce a greater or lesser degree of disturbance into the stock layer.

As an example, the dewatering using foils can be described. A foil strip is placed at a certain angle relative to the wire so that it forms a diverging space with the wire in the machine direction. As the wire with the stock layer rushes over the strip, a negative pressure is formed in the diverging space, which causes the dewatering. A greater or lesser amount of the drained water follows the wire on its underside until the next foil strip. At the front edge of this strip, the water is scraped off. The scraping of the water gives rise to a pressure pulse directed towards the wire and the already formed sheet on top. The magnitude of the pressure pulse is a function of the amount of water scraped off, the angle of scraping and the speed of the wire. For reasons previously discussed, there is often an unacceptable flocculation condition in the stock out of the drawer for the formation of the paper.

The pressure pulses produced at the leading edge of the foil strips introduce shear forces into the stock on top of the wire, which at an early stage during sheet formation gives a positive deflocculating effect. However, this effect is difficult to control and too large pressure pulses in a slightly later sheet-forming stage can break an already formed fiber network on the wire with a negative effect on the sheet formation.

Various methods and constructions for solving the problems described above have been proposed. It is known, for example, to use a nozzle on a headbox with an upper nozzle lip which extends forwards over the wire in its direction of movement and over a drainage means located below the wire. The intention is to provide a converging space adapted to the dewatering speed between the upper lip and the wire in order thereby to be able to keep the flow rate of the stock constant in this space. As a result, during most of the irrigation process, a well-defined stock layer is obtained from the extended upper lip and wire, where hydrodynamic disturbances generated in the headbox are not allowed to develop. Where applicable, the converging space between the elongated upper lip and the wire is defined in its shape by the upper lip being rigid and where the wire is supported by a drainage means which gives the wire a given stretch.

The drainage means may be a suction breast roller or a flat suction box. The appearance of the suction box can vary. The open area in the suction box lid can be a hole pattern or slits running across the machine. Common to all suction drawers- 20 15 20 25 30 35 QUALITY 8106648-2 3 luck is that the open area or landarcan is arranged so that the product is supported in a way that means a minimum of nvd increase 5 the suction zones. The suction cup can be sectioned so that varying vacuum levels can be applied in the different sections. Through this arrangement, the dewatering speed can be controlled so that it is adapted to the converging forming space. However, as discussed above for foils, a support under the wire during a dewatering stage means that pressure pulses are directed towards the wire which can have a degrading effect on the formed sheet. Across the land areas, the shaped fiber network is also not affected by stabilizing suction forces, which further aggravates the situation.

To eliminate these problems, the elongated upper lip has been made flexible while the underlying part of the wire has been arranged unsupported at least during the final stage of the molding. The dewatering is then effected by means of negative pressure in an open suction box arranged under the wire.

This still means that seals must be arranged along the edges of the suction box, which results in disturbances in the edge zones of the paper web. Furthermore, the dewatering speed is limited by the available negative pressure in the suction box.

The present invention has for its object to further improve and simplify the forming of a paper web. This is achieved by the dewatering of the pulp suspension (stock) taking place by means of a static pressure from a flexible upper lip as stated in the claims.

The negative pressure on the underside of the wire can thus be reduced. Suitably the dewatering is carried out completely without negative pressure. As a result, no seals are required along the edge zones of the paper web. Disturbances from any dewatering means such as strips, foils, register rollers or the like during the forming zone can be tolerated to a greater extent if the dewatering is mainly caused by an overpressure against the web in the forming zone instead of a negative pressure on the dewatering side. The overpressure should be at least 3 kPa.

According to the invention, in a paper machine the hydrodynamic disturbances can be effectively attenuated over the entire width of the paper web, which means that the basis weight of the web can be kept more even. The stock concentration in the headbox can be high without disturbing the forming process. which is particularly advantageous in the production of high bulk paper. Because the dewatering pressure can be set as desired, the dewatering capacity and thus the machine speed can be increased. The invention will be described in the following in the form of some embodiments with reference to the drawings, in which Figs. 1 and 2 show flat-wire paper machines and Fig. 3 shows a round-wire machine. 10 15 20 25 30 35 40 s1oes4s-2 4 Fig. 1 shows a nozzle 1 on a headbox (not shown). The stock 2 is sprayed through the nozzle onto a wire 3 which passes over a breast roller 4.

Extending from the upper part of the nozzle is a flexible upper lip 5 which is attached to the nozzle by means of a fastening device 6. The nozzle opening can be adjusted by means of an adjusting device 7. The nozzle 1 is directed so that the angle between the stock jet and the wire becomes small. It should suitably be less than 1s °.

Between the flexible upper lip 5 and the wire, a forming space 8 for the paper web is formed. This part of the wire is unsupported in its entire width and this unsupported part extends a distance past the upper lip 5 up to a support roller 9. The length of this piece is at least 10% of the length of the forming space 8. As a result, no disturbances occur when the paper web leaves the forming space 8.

The flexible upper lip 5 is subjected to a static pressure by a pressure device 10. Between the pressure device 10 and the upper lip 5 a means 11 is placed to distribute the pressure on the upper lip. This member 11 may be of elastic material, for example foam plastic or air cushions.

By regulating the pressure, the drainage can be regulated.

The thus controlled formation of the paper web in the space 8 means that the stock concentration in the headbox can amount to å - 1% in the production of paper with a low basis weight and 3 - 5% in the production of pulp sheets.

According to this embodiment, the wear on the wire can be minimized by the wire being freely laid between two rollers in the forming zone.

The flexible upper lip shown has a smooth underside. To achieve an increased microturbulence in the stock at the beginning of the forming space, the upper lip on the underside can be provided with smaller surface irregularities. These surface irregularities introduce shear forces into the flow, which has a deflocculating effect on the stock and thus the formation of the sheet can in some cases be further improved.

Another way of causing such microturbulence is to arrange a number of strips or the like under the wire, suitably only at the beginning of the forming space. At the same time, this provides a relaxing effect on the already formed fiber network, which can facilitate the continued dewatering. Since the dewatering is achieved by an overpressure against the forming space and not by suction from the underside, it is thus possible in some cases to arrange strips, foils or the like without the web forming being disturbed too much. Such an embodiment is shown in Fig. 2 where the wire is supported by strips 12. Otherwise, the reference numerals correspond to Fig. 1.

Fig. 3 shows an embodiment where the web forming takes place on a breast roll 14 formed with dewatering means 13. The wire 3 is supported here under the forming space (8) by the breast roll 14.

The invention is of course not limited by the above embodiments but can be varied within the scope of the inventive concept.

Claims (9)

10 15 20 25 30 5 8106648-2 P a t Ueälilarftl A method of forming a paper web in a paper machine where a pulp suspension flows out on a wire (3) through a nozzle (1) and is formed in a space (8) between a flexible upper lip (5) projecting from the nozzle by the dewatering of the pulp and wire (3), characterized suspension (2) takes place by means of a static pressure from the flexible upper lip (5). Method according to claim 1, the pressure is distributed over substantially the entire flexible upper lip (5). 3. A method according to claim 1 or 2, characterized in that the shaping takes place between the Flexible upper lip (5) and at least during the knowing that the final stage of the static shaping is an unsupported part of the wire (3). Device for forming a paper web in a paper machine comprising a nozzle (1) for applying a pulp suspension (2) to a wire (3) and a flexible upper lip (5) projecting from the upper part of the nozzle, wherein a forming space ( 8) is defined between the upper lip and the wire (3), characterized in that the flexible upper lip (5) is loaded by a pressure device (10) which provides a static pressure for dewatering the pulp suspension in the forming space (8). k ä n n e t e c k n'a d of printing Device according to claim 4, the device (10) is arranged to load substantially the entire flexible upper lip (5). Device according to claim 4 or 5, an elastic member (11, 15) is arranged to distribute the pressure from the pressure sensor by the device (10). Device according to one of Claims 4 to 6, in that the forming space (8) is defined by the flexible upper lip (5) and an unsupported part of the wire (3) at least at the end of the space (8). of that viran k ä n n e t e c k nga d Device according to claim 7, characterized in that the molding space (8) is freely laid between two rollers (4, 9). Device according to one of Claims 4 to 6, characterized in that the wire (3) under the forming space (8) is at least partially supported.