Abstract
The invention relates to a method for forming a three-layer board web, preferably a folding box board (FBB) or a solid bleached (sulfate) board (SBS), in a forming section (300) comprising a multilayer headbox (100) and a forming unit (200). In the method the layers of the three-layer board web are first formed of pulp suspension in the multilayer headbox (100) and fed to only one forming unit (200), a gap former where the pulp suspension from the multilayer headbox is fed to a gap between lower and upper wires for water removal and for joining the layers of the three-layer board web and where water is first removed by a non-pulsating forming shoe (37, 39).
Claims
1. A method for forming a three-layer board web, in a forming section comprising a multilayer headbox and a forming unit, the method including the steps of first forming the layers of the three-layer board web of pulp suspension in the multilayer headbox and then feeding the output from the multilayer headbox to only one forming unit, the forming unit being a gap former where the pulp suspension from the multilayer headbox is fed directly into a gap between lower and upper wires for joining the layers of the three-layer board web and for first water removal by a curved non-pulsating forming shoe adjacent the gap where dewatering happens on the curved non-pulsating forming shoe in both directions, wherein, after the curved non-pulsating forming shoe, pulp suspension between the lower wire and the upper wire is guided below a suction unit located inside a loop of the upper wire.
2. Method according to claim 1, wherein in the method white-water is fed between at least two layers of the three-layer board web in an Aqua-headbox.
3. Method according to claim 1, wherein the non-pulsating forming shoe comprises cross machine direction lists which are arranged after each other with cross machine direction lists width in machine direction being 4-15 mm and distance between lists being 4-15 mm.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) In the following the invention is explained in detail with reference to the accompanying drawing to which the invention is not to be narrowly limited.
(2) In FIG. 1 is shown schematically an advantageous example of a multilayer headbox of a forming section according to the invention.
(3) In FIG. 2 is shown schematically another advantageous example of a multilayer headbox of a forming section according to the invention.
(4) In FIG. 3 is shown schematically an advantageous example a forming section according to the invention.
(5) In FIG. 4 is shown schematically an advantageous example of a forming section according to the invention.
(6) During the course of the following description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention and its advantageous examples. In the figures some repetitive reference signs have been omitted for clarity reasons.
DETAILED DESCRIPTION
(7) In FIG. 1 is shown an example of a multilayer headbox 100 of the forming section. The multilayer headbox comprises headers 10S, 10M, from which the pulp suspensions for each layer of a three-layer board web are fed to manifold tubes 11 to an equalization chamber 13 and further via a turbulence generator 14 to slice channels 15 ending to a slice opening 16. Advantageously, the header 10M for the middle layer of the three-layer board web comprises a dilution control device 9. Also, the headers 10S for the surface layers of the three-layer board web may comprise a dilution control. In each slice channel 15 separation elements 17 and turbulence elements 18 are located. The separation and turbulence elements 17, 18 can be for example lamellas, vanes or wedges. In the slice channels 15 length of the separation elements 17 between each layer is longer than length of the turbulence elements 18 within each layer. Advantageously, the separation elements 17 extend close to the slice opening 16, the distance between end tip 19 of the separation element 17 and the slice opening 16 is advantageously 1-50 mm and turbulence elements 18 are typically 10-300 mm shorter than separation elements 17. Separation elements 17 end tip 19 is thin, advantageously tip thickness is less than 0.5 mm. Separation elements 17 keeps different stock suspensions separate and by thin tip stock suspension mixing is minimized.
(8) In FIG. 2 is shown an example of a multilayer headbox 100 of the forming section. The multilayer headbox comprises headers 10M, 10S, from which the pulp suspensions for each layer of a three-layer board web are fed to manifold tubes 11 to an equalization chamber 13 and further via a turbulence generator 14 to slice channels 15 ending to a slice opening 16. Advantageously, the header 10M for the middle layer of the three-layer board web comprises a dilution control 9. Also, the headers 10S for the surface layers of the three-layer board web may comprise a dilution control device 20. In each slice channel 15 separation elements 17 and turbulence elements 18 are located. The separation and turbulence elements 17, 18 can be for example lamellas, vanes or wedges. In the slice channels 15 length of the separation elements 17 between each layer is longer than length of the turbulence elements 18 within each layer. Advantageously, the separation elements 17, extend close to the slice opening 16, the distance between end tip 19 of the separation element 17 and the slice opening 16 is advantageously 1-50 mm and turbulence elements 18 are typically 10-300 mm shorter than separation elements 17. Separation elements 17 end tip 19 is thin, advantageously tip thickness is less than 0.5 mm. Separation elements 17 keeps different stock suspensions separate and by thin tip stock suspension mixing is minimized. The multilayer headbox according to this example of the FIG. 2 is a so called Aqua-headbox, of which one example is described in EP-patent publication 2784213. Aqua-headers 10A are connected to corresponding distribution header 11A, which feeds white-water between the layers of the three-layer board web for forming an Aqua-layer between the layers of the three-layer board web. Additionally, the white-water may contain chemicals, fillers and/or fine substances. By the Aqua layer the layer purity is improved. Additionally, the inner strength of the three-layer board web can be improved ty the chemical, filler and/or fine substance additions between the layers of the three-layer board web. The aqua layer is combined to the stock suspension after the separation elements and by that way white-water prevents mixing of the stock suspensions.
(9) In FIG. 3 is shown an example of the forming section 300 comprising a multilayer headbox 100 and a forming unit 200. The forming unit 200 of the example of the FIG. 3 is a so-called blade gap former. In the blade gap former first dewatering element is stationary forming shoe. The forming unit formed as a twin-wire forming unit comprising a lower wire 30 and an upper wire 20, each comprising rolls 32, 22 for guiding and driving the wire as an endless loop. The pulp suspension from the multilayer headbox 100 is first fed to the gap between the lower wire 30 and the upper wire 20 between the rolls 32A, 22A and onto the area of a forming shoe 37, which is advantageously under-pressured has advantageously a configured surface, for example curved, such that the forming shoe 37 does not cause substantial pressure pulses i.e. the forming shoe 37 is a so called non-pulsating forming shoe 37. Non-pulsating forming shoe 37 can be formed by cross machine direction lists which are arranged after each other with small distance. Advantageously lists width in machine direction is 4-15 mm and distance between lists is 4-15 mm. This kind of arrangement doesn't cause pressure pulsations, which could mix stock suspension. Dewatering happens on the forming shoe 37 on both directions and by that good layer purity of three-layer web is ensured. First dewatering on forming shoe 37 is gently, but effective and by that way dewatering capacity and web layer purity is ensured. Thereafter the pulp suspension between the lower wire 30 and the upper wire 20 is guided below a suction unit 23, located inside the loop of the upper wire. The forming shoe 37 removes water by suction, which provides for exact control of the headbox flow, so that water is sucked through the lower wire 30 but pulp suspension is not bouncing on the wire 30 as there is no pulsating water removal. The further water removal means of the forming unit 200 inside the loop of the lower wire 30 are suction boxes 35.
(10) In FIG. 4 is shown an example of the forming section 300 comprising a multilayer headbox 100 and a forming unit 200. The forming unit 200 of the example of the FIG. 4 is a so-called combi former. Combi former contains short one wire section following twin-wire section. The pulp suspension from the multilayer headbox 100 is fed to the forming unit 200, in which at the beginning of the forming unit 200 is a short, advantageously 200-1500 mm long, substantially horizontal or inclined one-wire section comprising a forming shoe 39 removing water by suction, which provides for exact control of the headbox flow, so that water is sucked through the lower wire 30 but pulp suspension is not bouncing on the wire 30 as there is no substantially pulsating water removal. Length D of the one-wire section is measured from the beginning of the forming shoe 39 and between starting point of the twin-wire forming unit. The forming unit formed thereafter as a twin-wire forming unit comprises a lower wire 30 and an upper wire 20, each comprising rolls 32, 22 for guiding and driving the wire as an endless loop. The pulp suspension is led between the lower wire 30 and the upper wire 20 onto the area of a forming shoe 37, which is advantageously under-pressured has advantageously a configured surface, for example curved, such that the forming shoe 37 does not cause substantial pressure pulses i.e. the forming shoe 37 is a so called non-pulsating forming shoe 37. Non-pulsating forming shoe 37 can be formed by cross machine direction lists which are arranged after each other with small distance. Advantageously lists width in machine direction is 4-15 mm and distance between lists is 4-15 mm. This kind of arrangement doesn't cause pressure pulsations, which could mix stock suspension. Dewatering happens on the forming shoe 37 on both directions and by that good layer purity of three-layer web is ensured. Advantageously forming shoe 39 structure corresponds forming shoe 37 structure, but advantageously forming shoe 39 surface is straight. One wire section length is so short that web top side consistency is essentially at the headbox consistency when it is entering in the twin-wire forming unit. Thereafter the pulp suspension between the lower wire 30 and the upper wire 20 is guided below a suction unit 23, located inside the loop of the upper wire. The further water removal means of the forming unit 200 inside the loop of the lower wire 30 are suction boxes 35. This type of forming unit 200 allows lower running speeds, for example 300-400 m/min, as the pulp suspension flow from the multilayer headbox 100 is not flown first to the gap. An example of this type forming unit is disclosed in EP-patent publication 2841643.
(11) In the description in the foregoing, although some functions have been described with reference to certain features and examples, those functions may be performable by other features and examples whether described or not. Although features have been described with reference to the certain examples, those features may also be present in other examples whether described or not.
(12) Above only some advantageous examples of the inventions have been described to which examples the invention is not to be narrowly limited and many modifications and alterations are possible within the invention.
