FI119943B - For forming a paper or board machine - Google Patents

Description

Paper or board forming machine forming unit

The invention relates to a paper or board machine web forming unit comprising a headbox and a wire adapted to slide over the first and second sliding surfaces of the headbox, between which the lip opening of the headbox is disposed, and the wire and the second sliding surface delimiting the forming area.

The invention also relates to a method of producing a multilayer web, comprising forming at least two web layers in separate web forming units and joining the separately formed layers to one another.

US 4,308,097 discloses a web forming unit comprising a curved press shoe 15 having a sliding surface and an inlet for feeding a fiber suspension to the sliding surface, and a wire and means for dissolving the wire past the sliding surface. The portion of the sliding surface downstream of the inlet is at a short distance from the sliding wire and the portion of the sliding surface upstream of the inlet extends closer to the wire than the portion downstream of the inlet. The wire and a portion of the sliding surface downstream of the feed 20 orifice define a wedge-tapered forming area therebetween. In order to provide a compact and compact space structure, the feed channel has a steep bend just before the inlet, as a result of which the flow has a smaller radius of curvature at the inlet than in the subsequent forming region. There is a risk that, at high travel speeds 25, secondary currents transverse to the direction of the main flow will be generated. They can lead to inhomogeneous fiber distribution and, in extreme cases, cavitation. In later patent US 4,416,730 by the same patentee, these problems are solved by moving the sliding surface upstream of the inlet to the opposite side of the wire, whereby the inlet can be made linear. At the same time, however, the compact design that was the advantage of the original solution, as well as the fully enclosed supported flow, are lost.

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Due to the steep bend before the inlet, the headbox of US 4,308,097 is not very suitable for high consistency masses. US 3,846,230 teaches that turbulence of high consistency pulp should be allowed to dampen in the headbox feed channel before the pulp is fed to the wire. For this purpose, the high-consistency stern-5 needle should be provided with such a long feed channel that the fiber particles can form a solid three-dimensional mesh structure before the fiber web is deposited on the wire.

WO 00/52261 discloses a web forming unit of the type disclosed above for high consistency masses. The headbox is pressed against the wire and a pulp suspension is introduced from the lip opening of the headbox into the forming region bordering the elongated upper lip and the wire sliding over it. The lip shower is open since the contact between the lower lip and the wire ends before the point where the pulp suspension spray hits the wire. The empty space between the lower lip tip and the lip jet 15 impact points easily generates additional eddy currents.

The known solutions have the problem that high pulp feed pressures run the risk of loosening the wire from the sliding surface control in the forming zone. Operation is disrupted and the web structure suffers if the pulp is allowed to flow upstream of the web 20 or if the rest of the forming region is not properly sealed.

The object of the invention is to solve the problems of the prior art. In particular, the object is a compact web forming unit suitable for use at high pulp consistency. A further object is a web forming unit that can be used to produce individual layers of a multilayer web from a thick pulp.

To achieve these and later objects, the web forming unit 30 of the invention is characterized by what is stated in the characterizing part of claim 1.

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In the solution according to the invention, there are two bending points in the headbox region at which the direction of travel of the wire changes relative to its straight or curved passage before its bending point. The first bending point may be, for example, at the leading edge of the first sliding surface, in which the direction of movement of the wire 5 is changed by a bending angle α, preferably of the order of 1-5 °. Preferably, the second bending point is located at the trailing edge of the second sliding surface, where the direction of travel of the wire is changed by a bending temperature β of preferably 1-8 °.

At the first bending point, the wire is tensioned against the trailing edge of the first sliding surface 10, sealing the entrance of the forming area so that the pulp suspension discharged from the lip duct cannot penetrate upstream between the wire and the first sliding surface. At the second bending point, the wire is tensioned toward the trailing edge of the second sliding surface, sealing the exit of the forming zone so that the pressure in the forming zone is maintained within the desired range. Bending points are provided with a rounded edge with a relatively small radius of curvature.

Preferably, the second sliding surface is curved with a radius of curvature of 200-1000 mm. The first sliding surface may be straight or curved.

Preferably, the geometry of the headbox is such that a pulp suspension jet discharging from the lip channel meets the wire at an angle of 20-70 °.

It is typical of thick pulp boxes that the pulp slurry is first well fluidized, after which turbulence is allowed to dampen while preventing the formation of large fiber blocks. In order to prevent the formation of fiber flocs, the lip canal may be provided, in a manner known per se, with various recesses, projections or bends that create small-scale turbulence in the flow. Alternatively, the lip channel may be smooth-walled, whereby the wall friction causes sufficiently small-scale turbulence, especially when the height of the flow channel 30 is small.

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Conventional open lip spray based bulk formulators find it difficult to obtain a web having good planar strength properties. In the former according to the invention, the problem can be solved by arranging different velocities of wire and pulp suspension flow. By adjusting the wire velocity to substantially greater than the velocity of the pulp suspension flow from the lip duct, the fiber layer is made to stretch and thinner in the forming region. Between the fixed sliding surface and the movable wire, the fibers of the pulp suspension are increasingly aligned with the flow, which improves the strength properties parallel to the web plane.

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The invention also relates to a method of producing a multilayer web, comprising forming at least two web layers in separate web forming units and joining separately formed web layers. The method is characterized in that at least one of the web layers is formed using a web forming unit according to the invention.

The invention enables the production of sufficiently good grades of paperboard and paper with substantially lower capital and operating costs. The solution is suitable for high headbox consistencies up to 5% consistency, but it can also be used in the normal 0.5-2% consistency range to replace open lip shower former concepts. On the basis of the test runs, the primary use of the web forming unit would appear to be single or multilayer board grades having a basis weight of 100-200 g / m2.

In the following, the invention will be described in detail with reference to the figures in the accompanying drawings, in which details, however, are not intended to be narrowly limited.

Figure 1 shows schematically the structure of a web forming unit according to the invention.

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Fig. 2 is a side view of a prior art two-layer formatter.

Fig. 3 is a side view of a two-layer forming machine in which the surface layer of the web is formed using a web forming unit according to the invention.

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Fig. 4 is a side view of a two-layer formatter in which both layers of the web are formed using a web forming unit according to the invention.

Fig. 5 is a side view of a three-layer forming machine in which the middle layer of the web is formed using a web forming unit according to the invention.

Fig. 6 is a side view of a three-layer forming machine in which all layers of the web are formed using a web forming unit according to the invention.

Fig. 7 shows a modification of the three-layer mold of Fig. 6.

Fig. 1 is a plan view of a web forming unit according to the invention comprising a head box 10 and a wire 13 adapted to slide over the head box sliding surfaces 15 and 16 while feeding a pulp suspension from the lip opening 12 between the slide pins 20 and 16.

Prior to lip opening 12, wire 13 slides along the curved surface of first sliding surface 15. The second sliding surface 16 is retracted inward with respect to the first sliding surface 15 so that a recess 25 is formed on the surface facing the wire 13 of the headbox, including a lip opening 12. After the lip opening 12, the wire 13 slides a short distance from the second sliding surface 16. 17 receiving a mass suspension flow from the lip opening 12. The feed pressure of the pulp suspension and the tension of the wire 13 cause the water to drain through the wire 13. The pulp suspension leaves the formation 30 region 17 in the form of web W.

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The lip passageway 11 preceding lip opening 12 is substantially linear over a relatively long distance. Prior to the linear portion of the lip duct 11, the pulp suspension is fluidized in a manner known per se. The lip duct 11 may be smooth-walled or may, in a manner known per se, be provided with small bends, projections, or recesses 18 that cause small-scale turbulence in the flow. The mass suspension flow discharged from the lip duct 11 through the lip opening 12 onto the wire 13 hits the wire 13 at an inlet angle of 20-70 °. Advantageously, the velocity Vw of the wire 13 is set higher than the velocity Vs of the pulp suspension jet, whereby the pulp suspension layer is stretched and thinned within the forming region 17. By adjusting the ratio of wire velocity Vw to pulp suspension Vs.

The first sliding surface 15 may be curved as in Figure 1 or it may be straight. The leaving edge of the sliding surface 15 forms a bending point A, in which the direction of travel of the wire 13 changes by an angle a. change the running direction of the wire seals fas-dostusalueen 17 of the input side, so that the pulp suspension does not penetrate between the upstream of the wire 13 and the first sliding surface 15.

The second sliding surface 16 is curved such that its radius of curvature R is 200-1000 mm. The radius of curvature R may be constant over the entire length of the slide surface 16 or it may be different at different portions of the slide surface 16. The leaving edge of the second sliding surface 16 forms a bending point B where the direction of travel of the wire 13 changes by an angle β. The angle β is preferably from 1 to 8 °. This second change in the direction of travel of the wire seals the leaving side of the forming region 17.

The molded structure, in which the direction of travel of the wire 13 changes slightly as it enters and exits the forming zone 17, can efficiently seal the introduction of the pulp suspension from the headbox 10 to the wire 13. In addition, the wire 30 behaves more stably and good web formation is achieved. Also, the web strength at higher blinds remains almost at the same level as at lower consistency.

The web forming unit of Figure 1 is particularly well-suited for use at high 2-5% pulp consistency, but nothing prevents it from being used at normal 0.5-2% pulp consistency as well. The web forming unit is ideally suited for the production of thick grades of paper and board. It can also be used to form individual layers of a multilayer web, as will be described below.

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Figure 2 shows a prior art double-layer formatter. It comprises a lower wire unit for forming a web base layer and an upper wire unit for forming a web surface layer and means for connecting the webs to one another. The bottom wire unit comprises a first headbox 8 and a bottom wire 20, which is guided by guide rollers 21 and forms an endless loop. The diluted pulp suspension is fed from the headbox 8 as an open jet to the lower wire 20, the flat wire portion of which is dewatered through the wire 20 to form the bottom layer of the web. The upper wire unit comprises a second headbox 9 and an upper wire 30 which, guided by rolls 31,32, forms an endless loop which touches the lower wire 20. The diluted pulp suspension is fed from the second headbox 9 as an open jet to dewater the web. The formed web is then guided over the bottom layer on the bottom wire 20 and the layers are joined together by a felt roll 32. At the end of the flat wire portion of the lower wire 8, the connected web 25 is moved by a catch roll 50 to the press section. Both headboxes 8 and 9 are conventional dilute mass headboxes having a consistency of the pulp suspension in the range of 0.5-1.0%.

Fig. 3 shows a double-layer reformer in which the top wire unit is replaced by a forming unit corresponding to Fig. 1. The lower wire unit is similar to Figure 2. The upper wire unit comprises a head box 10 which is pressed against the wire 30 8. The wire 30 initially initially slides along the first sliding surface 15 and, after the lip opening 12, advances at a short distance from the second sliding surface 16. A forming region remains between the wire 30 and the second sliding surface 16 to drain water from the pulp suspension through the feed pressure and wire tension. The consistency of the pulp fed from the headbox 10 5 is preferably in the range of 2-5%. In this case, the flat wire portion of the upper wire loop 30 required to achieve the desired dry matter content may be significantly shorter than in the conventional upper wire unit of Figure 2.

Also, in the bilayer fender of Figure 4, the bottom wire assembly 10 comprises a former arrangement according to the invention in which the headbox 10 with curved sliding surfaces 15,16 is pressed against the bottom wire 10. Thus, both layers of the web can be formed using web forming units according to the invention.

Figure 5 illustrates a multilayer forming machine in which the bottom layer and the top layer 15 of the web are formed from a low consistency mass by conventional technique and the middle layer is formed from a high consistency mass using a web forming unit according to the invention. To form the bottom layer of the web, a dilute pulp suspension is fed as an open jet from the conventional headbox 8 to the lower wire 30. To form the middle web layer, a thick pulp suspension from the headboard 20 to the first upper wire 30 is arranged to slide along the headbox sliding surfaces 15 and 16. To form the surface layer of the web, a dilute pulp suspension is fed as an open jet from one conventional headbox 9 to another upper wire 40. which is guided by the guide rollers 41,42 to form an endless loop. The first top-25 wire and the second top-wire 40 are guided together to provide a two-wire zone in which the middle and surface layers of the web are joined. The composite web is then guided over the bottom layer of the web on the lower wire 20 and all layers of the web are joined together by a felt roll 42 in the two-wire zone formed by the lower wire 20 and the second upper wire 40 together.

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Fig. 6 shows a multilayerformer comprising three web forming units according to the invention. Each of the three layers of the web is formed from a thick pulp using a thick pulp base box 10 according to the invention printed on a respective wire 20, 30 or 40. The middle web layer formed on the first 5 upper wire 30 is then connected to the bottom layer of the web 20 on the lower wire 20 and finally the web top layer formed on the second upper wire 40 is joined to the already existing web layers on the lower wire 20.

Fig. 7 shows an alternative solution in which the middle web layer formed on the first top fabric 30 and the web top layer formed on the second top fabric 40 are first joined together in the biaxial zone formed by said wires, and then these layers are joined to the bottom layer web.

Many variations of the invention are possible within the scope defined by the following claims.

Claims (10)

A web forming unit in a paper or cardboard machine, which unit comprises an inlet carriage (10) and a wire (13) arranged to slide over a first and a second sliding surface (15,16) on the inlet tray (10), between the surfaces of the inlet slide (10) lip opening (12), and in which the wire (13) and the second slide surface (16) define between themselves a forming region (17) which receives the pulp suspension flow discharged through the lip opening (12), characterized in that in the area of the inlet drawer (10) there are two bending points (A, B), in which the direction of movement of the wire (13) changes in relation to the straight or curved course thereof before the bending point (A, B) and that the first inflection point (A) is located before the lip opening (12) in the running direction of the wire (13) and the second inflection point (B) is located after the lip opening (12). A web forming unit according to claim 1, characterized in that the trailing edge of the first sliding surface (15) forms the first bending point (A), at which the running direction of the wire (13) changes with a bending angle a, the size of which is advantageously 1-5 °. 3. The web forming unit according to claim 1 or 2, characterized in that behind the edge of the second sliding surface (16) forms the second bending point (B), at which the running direction of the wire (13) changes with a bending angle β, the size of which is advantageously 1- 8 °. 25 4, web forming unit according to any of the preceding claims, characterized in that the second sliding surface (16) is curved in such a way that its radius of curvature (R) is 200-1000 mm. 5. The web forming unit according to any of the preceding claims, characterized in that the pulp suspension flow meets the wire (13) at an angle whose size is 20-70 °. 6. A web forming unit according to any of the preceding claims, characterized in that the lip opening (12) is preceded by a substantially linear lip channel (11), the walls of which show surface shapes (18) which produce turbulence on a small scale in the flow. The web forming unit according to any of claims 1-5, characterized in that the lip opening (12) is preceded by a substantially linear lip channel (11), the walls of which are solid and parallel. 8. A web forming unit according to any of the preceding claims, characterized in that the inlet carriage is suitable for use with pulp consistencies of 0.5-5%. 9. Teaching for the preparation of a multilayer web, in which the present at least two web layers are formed into separate web forming units and the separately formed layers are joined together, characterized in that at least one of the web layers is formed by using a web forming unit according to some of the web layers. claims. Teaching according to claim 9, characterized in that a pulp suspension, the consistency of which is 0.5-5%, preferably 2-5%, is used in said web forming unit. 25