EP0342684B1 - Double layer tissue for the forming section of a paper machine - Google Patents

Abstract

A double-layer fabric for the sheetforming section of a papermaking machine which is referred to as a sheet forming fabric or a dewatering fabric consists of an upper layer (1) of transverse threads (3) and a lower layer (2) of transverse threads (4) and (8) which are interwoven with a single system of longitudinal threads (5). Successive transverse threads (4, 8) of the lower layer (2) form pairs and within each pair the lowest points (7) of the transverse thread floats (6) are in alignment. The lowest point (7) of the float of the one transverse thread (4) of a pair is offset from the center of the float (6) in the direction opposite to the direction in which the lowest point (7) of the float (6) of the other transverse thread (6) is offset.

Description

The invention relates to a double-layer covering for the sheet formation area of a paper machine, a so-called sheet forming screen or dewatering screen. A double-layer covering is understood to mean a covering made of a fabric in which the transverse threads are arranged in a lower and an upper layer and are generally one above the other in pairs. The cross threads are interwoven with a single system of longitudinal threads.

When the sheet is formed in the paper machine, the water is withdrawn from the aqueous fiber suspension through the dewatering screen until a nonwoven fabric is formed on the dewatering screen that is sufficiently strong to be able to remove it from the screen and introduce it into the press section.

The drainage screen must meet a wide variety of requirements, namely high drainage performance, fine and even surface structure on the paper-carrying side, good fiber retention, high longitudinal and transverse stability and high abrasion resistance.

In addition, the drainage screen must have good running stability, ie it must not warp and must also run perfectly straight at speeds of 1000 to 1500 m / min and must not drift or run sideways.

The problem of the drainage screen drifting off or running to the machine side does not occur with all weaves. It mainly depends on the symmetry of the transverse thread floats on the running side. In the case of double-layer screens in particular, the running side is formed by the transverse thread floatings because this improves the abrasion resistance and increases the running time.

A covering for the sheet formation area of a paper machine according to the preamble of claim 1 is known from EP-A-0 245 851, FIG. 1. This drainage sieve is double-layered and the transverse thread floatations on the running side are asymmetrical, ie the lowest point of the transverse thread floatations is shifted from the center to one side. The asymmetrical cross thread floats lead to an asymmetrical support of the dewatering screen on the paper machine, which at high speeds has the consequence that the dewatering screen drifts to the side. Lateral drifting is strongest when vacuum is applied to the nipples to pull the remaining water out of the nonwoven. The vacuum increases the force with which the dewatering screen is pressed down against the paper machine, which in turn has a greater impact on the asymmetry of the cross thread floats. In order to keep the dewatering screen in the paper machine, the roller of the screen controller is inclined. If this is not sufficient, additional rollers in the paper machine must be tilted, which creates transverse forces that counteract the lateral drifting of the dewatering screen. If, for operational reasons, the vacuum of the suction cups is reduced or switched off, the dewatering screen shifts in the opposite direction due to the effect of the rollers, which are still inclined, and is thereby Impact against the paper machine frame often damaged. These difficulties are particularly pronounced during the first working days of the dewatering screen, since the asymmetry of the cross thread floats is still completely present during this time. Since the abrasion on the running side of the dewatering sieve starts at the lowest points of the cross thread floatation, the asymmetry of the cross thread floatation becomes smaller the longer the dewatering sieve is in use.

The problem of lateral drifting could be solved by using ties with symmetrical transverse thread floats for the dewatering screen. However, these bindings generally have the disadvantage that the monoplanarity differences between the longitudinal thread floatations and the transverse thread floatations are smaller on the running side. Large differences in monoplanarity are advisable on the running side, since this increases the running time of the drainage screen. Large differences in monoplanarity allow the use of thicker transverse threads and the complete consumption of the transverse thread floats by abrasion before the longitudinal threads are exposed to the abrasion.

The asymmetrical cross thread floating is caused by the fact that several longitudinal threads act together on one point of the transverse thread floating. On the one hand, this creates a large difference in monoplanarity, and on the other hand, the transverse thread floating becomes asymmetrical if this point is not in the middle of the transverse thread floating.

From DE-A-33 07 144 it is known to weave the fabric in the two halves of the drainage screen to the left and right of the longitudinal center line form a mirror image, so that the binding diagonal has a V-shape. Here, however, there is the difficulty that in the middle of the drainage screen the longitudinal threads have to be woven in differently from the rest of the binding in order to avoid excessively long cross thread floating on the running side.

The invention has for its object to prevent lateral drifting of the fabric in double-layer fabrics with asymmetrical transverse thread floatation on the running side.

This object is achieved in that the incorporation of the longitudinal threads into the lower layer of the transverse threads is carried out in such a way that successive transverse threads of the lower layer form pairs, the deepest points of the transverse thread floatations being aligned in the longitudinal direction and the lowest point of the floatation of one within a pair The transverse thread of a pair is offset in the opposite direction from the center of the float, as is the lowest point of the float of the other transverse thread of this pair.

In the covering according to the invention, the lateral thrust caused by the asymmetry of the transverse thread floatations balances within a pair of transverse threads. The opposite asymmetry of the cross thread floatations of a pair of cross threads can be e.g. in that each longitudinal thread is woven twice into the lower layer within a weave repeat, in such a way that the weave diagonal is broken on the running side.

Each longitudinal thread is expediently woven into the lower layer in such a way that, coming from above, it wraps around a transverse thread on the underside, runs over two transverse threads, again a cross thread on the underside and then runs between the two layers or is integrated into the upper layer.

In a double-layer dewatering screen, each longitudinal thread alternates between the top layer of the cross threads and the bottom layer of the cross threads. For example, he can have a stretch of five transverse threads woven into the upper layer, then a stretch of three transverse threads between the two layers, then a stretch of four transverse threads into the lower layer and finally a stretch of two transverse threads between the two layers , whereupon the course of the longitudinal thread is repeated and the longitudinal thread is woven a distance of five transverse threads into the upper layer, etc. Layer is woven, is called the corresponding lower binding distance. The stretch that a longitudinal thread runs between the two layers is called the intermediate stretch. In the dewatering screen according to the invention, it is generally the case that the upper setting sections on the paper side and the lower setting sections are regularly distributed on the running side. In the case of a 7-strand binding, for example, the course of adjacent longitudinal threads can always be offset by six transverse threads in one direction, ie the so-called count number is 6. This is equivalent to a doubled 3/4 atlas. Each setting section generally consists of several setting points. Looking at the individual binding points, however, the weave is generally irregular, ie the binding points do not progress regularly in the longitudinal and transverse directions. The regular arrangement of the tie lines with an irregular arrangement of the The individual binding points apply to both the paper side and the running side. If a 7-strand weave is selected for the drainage sieve according to the invention, it generally also applies that the longitudinal threads run asymmetrically, ie successive intermediate sections, as previously defined, are of different lengths. For the course of a longitudinal thread it is therefore not possible to find a straight line which runs perpendicular to the drainage screen and to which the course of the longitudinal thread is symmetrical.

For most types of paper, it is important that the drainage screen does not leave any marks in the paper. An essential prerequisite for the freedom from marking of a drainage screen is its monoplanarity on the paper side. Monoplane drainage screens are generally woven flat. This is followed by a heat setting in which a high tension is exerted on the longitudinal threads, which leads to a change in offset, that is to say the initially severely bent longitudinal threads projecting on the paper side receive a less bent course, while the transverse threads currently lying in the fabric are on the tying points are bent off from the longitudinal threads. Monoplanarity is achieved when the highest points of the longitudinal threads and the transverse threads lie in one plane. A very high tension must be exerted on the longitudinal threads so that the longitudinal threads on the running side are drawn into the interior of the fabric to such an extent that they are not exposed to abrasion. In order to avoid that due to the high longitudinal tension exerted during heat setting, the longitudinal threads on the paper side are also displaced into the interior of the fabric, the longitudinal threads on the paper side have floats which extend over two transverse threads.

In the case of drainage screens for the production of tissue paper, the paper side should not be monoplane, but should have a pronounced transverse structure. This is achieved in that the longitudinal threads on the paper side have short floats of only a single transverse thread. In the manufacture of tissue paper, the pronounced transverse structure of the paper side improves sheet removal.

The asymmetrical cross thread floatings can generally only be achieved with drainage sieves that have a density of the longitudinal threads of over 90%. This is generally the case with double-layer drainage screens. Only with such a high density of the longitudinal threads is it possible for several longitudinal threads to act on one point of a transverse thread float and to press the transverse thread downwards on the running side at this point.

The longitudinal and transverse threads suitably consist of plastic monofilaments. The transverse threads of the lower layer have the same or a lower density than the transverse threads of the upper layer. To increase the volume of abrasion, transverse threads with a larger diameter are generally used in the lower layer than in the upper layer. To improve the abrasion resistance, all or part of the transverse threads of the lower layer can also be made of polyamide, while the other threads are generally polyester monofilaments.

Fig. 1

mit Blick von unten die Laufseite eines Entwässerungssiebes mit wechselnder Asymmetrie der Querfadenflottungen;

Fig. 2

im Querschnitt den Verlauf eines Querfadens der unteren Lage bei einem Entwässerungssieb nach Figur 1;

Fig. 3

im Querschnitt den Verlauf eines anderen Querfadens mit entgegengesetzter Asymmetrie zu dem in Figur 2 gezeigten Querfaden;

Fig. 4

im Längsschnitt durch das Entwässerungssieb den Verlauf eines Längsfadens;

Fig. 5

in einer schematischen von oben gesehenen Darstellung die Anordnung der Bindungspunkte, wobei ein schwarz ausgefülltes Quadrat bedeutet, daß der Längsfaden über einem Querfaden der oberen Lage verläuft, ein Kreuz bedeutet, daß der Längsfaden unter einem Querfaden der unteren Lage verläuft, und die leeren Quadrate bedeuten, daß an dieser Stelle der Längsfaden zwischen beiden Lagen verläuft, was gleichbedeutend damit ist, daß an dieser Stelle auf der Papierseite und auf der Laufseite die Querfäden sichtbar sind, und

Fig. 6 bis 10

Darstellungen entsprechend denen von Figur 1 bis 5, jedoch mit einer anderen Gewebebindung.

Embodiments of the invention are explained below with reference to the drawing. Show it:

Fig. 1

with a view from below the running side of a dewatering screen with changing asymmetry of the cross thread floats;

Fig. 2

in cross-section the course of a transverse thread of the lower layer in a drainage screen according to Figure 1;

Fig. 3

in cross section the course of another transverse thread with opposite asymmetry to the transverse thread shown in Figure 2;

Fig. 4

in longitudinal section through the drainage sieve the course of a longitudinal thread;

Fig. 5

in a schematic view from above, the arrangement of the tie points, with a black filled square means that the longitudinal thread runs over a transverse thread of the upper layer, a cross means that the longitudinal thread runs under a transverse thread of the lower layer, and the empty squares mean that at this point the longitudinal thread runs between the two layers, which is equivalent to the fact that the transverse threads are visible at this point on the paper side and on the running side, and

6 to 10

Representations corresponding to those of Figures 1 to 5, but with a different weave.

As can be seen most clearly in FIG. 4, the dewatering screen of FIGS. 1 to 5 consists of an upper layer 1 and a lower layer 2 of transverse threads 3 or 4 and 8, which are interwoven with a single system of longitudinal threads 5. The binding is repeated in the transverse direction after seven longitudinal threads 5 and in the longitudinal direction after fourteen transverse threads 3 of the upper layer 1. Each transverse thread 3 of the upper layer 1 lies above a transverse thread 4, 8 of the lower layer 2, so that the transverse thread density in the upper layer 1 and in the lower layer 2 is the same.

Each longitudinal thread 5 is woven twice per repeat into the upper layer 1 in such a way that it runs over two transverse threads 3, under one transverse thread 3 and then again over two transverse threads 3 and is then guided to the lower layer 2. The binding of the top layer 1 and thus the structure of the paper side is carried out in accordance with DE-A-36 15 304. In the upper layer 1 there is therefore a heterogeneous support of the transverse threads 3, a transverse thread 3 always being supported by a single longitudinal thread 5 as in a saddle and therefore running exactly in the transverse direction. The adjacent transverse threads 3 are supported in scissors-like fashion by two successive longitudinal threads 5, one longitudinal thread 5 descending to the lower layer 2 after completion of the floatation and the other longitudinal thread 5 rising straight from the lower layer 2 to form the floatations on the paper side. Both types of support for the transverse threads 3 alternate on the paper side. Successive transverse threads 3 therefore do not form parallel floats on the paper side, but the floats lie at an angle to one another, as a result of which the marking properties of this drainage screen are improved. After heat setting, all floats of the longitudinal threads 5 and the transverse threads 3 lie in one plane on the paper side.

After incorporation in the upper layer 1, the longitudinal thread 5 runs an intermediate section of 3 transverse threads between the layers 1, 2 and is then also interwoven twice with the transverse threads 4, 8 of the lower layer, the longitudinal thread 5 between these two binding points inside the Fabric runs over two transverse threads 4, 8. In the lower layer 2, the longitudinal thread 5 thus wraps around a transverse thread 8 on the underside, lies over two transverse threads 4, 8 and in turn wraps around a transverse thread 4 on the underside. This is followed by an intermediate stretch of 2 cross threads until the next integration in the upper layer 1.

The course of the longitudinal threads 5 is asymmetrical. The weave pattern is irregular in the lower layer 2 in that the first transverse thread 8a (FIGS. 1 and 5) of a repeat is tied off from the second longitudinal thread 5b, the second transverse thread 4b is tied from the fourth longitudinal thread 5d, and the third transverse thread 8c is not tied from the sixth 5f , but is tied from the seventh longitudinal thread 5g, the fourth transverse thread 4d from the ninth longitudinal thread 5b (= second longitudinal thread of the repeat that follows on the right) and the fifth transverse thread 8e is again not from the eleventh (= fourth) but from the twelfth (= fifth) Longitudinal thread 5e is tied. This results in the desired structure of the running side, as shown in FIG. 1, successive transverse threads 4, 8 viewed in cross section (FIGS. 2 and 3) having asymmetrical floats.

The longitudinal threads 5 have a diameter of 0.15 mm and consist of low-stretch polyester (Type 940 from Hoechst). Their density was 63 / cm. After fixation, the longitudinal thread density increased to 72 / cm. In the top layer 1, transverse threads 3 with a diameter of 0.15 mm were woven from soft polyester (Type 900 from Hoechst) with a density of 34 / cm. Cross threads 4, 8 with a diameter of 0.18 mm are woven into the lower layer 2. The transverse threads 4 are made of soft polyester (Type 900 from Hoechst), and the transverse threads 8 are made of polyamide PA 6. By fixing, the density of the transverse threads in the upper layer 1 and in the lower layer 2 was reduced to 32 / cm. After fixing, the uppermost points of all threads lie on one level on the paper side of the drainage screen. On the running side, the height difference between the transverse threads 4, 8 and the longitudinal threads 5 is 9.5 / 100 mm, so that when using the drainage screen, the transverse threads 4, 8 must be completely looped through before the longitudinal threads 5 loop through.

In the exemplary embodiment shown in FIGS. 6 to 10, the longitudinal thread 5 runs in the upper layer 1 over three transverse threads 3. The transverse threads 3 alternately have a diameter of 0.18 and 0.12 mm, in such a way that the central transverse thread 3 has the smaller diameter within a longitudinal thread float on the paper side. The differently thick transverse threads 3 also have a different course, namely the finer transverse threads 3 lie entirely on the paper side of the dewatering screen, that is, they are never looped around from above by the longitudinal wire 5 or, in other words, the warp threads 5 never run over a fine one Cross thread 3 and then between this cross thread 3 and the subsequent thicker cross thread 3 (EP-A-0 085 363).

In the lower layer 2, the longitudinal thread 5 binds the transverse threads 4, 8 in the same way as in the embodiment of FIGS. 1 and 5. Also in the lower layer 2, the transverse threads 4, 8 have a diameter of 0.20 mm and are also alternately made of polyester and polyamide, as in the exemplary embodiment in FIGS. 1 to 5.

Claims (5)

1. A double-layer clothing for the sheet forming section of a papermaking machine, comprising an upper and a lower layer (1, 2) of transverse threads (3, 4, 8) which are interwoven with a system of longitudinal threads (5), the transverse threads (4, 8) of the lower layer (2) forming transverse threads floats (6), the lowest point (7) of which is offset from the center towards one side, characterized in that successive transverse threads (4, 8) of the lower layer (2) form pairs and within a pair the lowest points (7) of the transverse thread floats (6) are in alignment in longitudinal direction, and the lowest point (7) of the float of the one transverse thread (4) of a pair is offset from the center of said float (6) in the direction opposite to the lowest point (7) of the float (6) of the other transverse thread (8) of the pair.
2. The clothing of claim 1, characterized in that the longitudinal threads (5) within each fabric weave repeat are woven twice into the lower layer (2), each longitudinal thread (5) extending, coming from the top, beneath a transverse thread (8), over two transverse threads (8, 4) and again beneath a transverse thread (4) of the lower layer (2) and then rising to the top again, and in that the distribution of the binding points is an atlas distribution on the paper side as well as also on the running side.
3. The clothing of claim 1 or 2, characterized in that the weave repeat is a 7-harness one and encompasses 14 transverse threads (3) of the upper layer (1) and fourteen transverse threads (4, 8) of the lower layer (2).
4. The clothing of one of claims 1 to 3, characterized in that the two transverse threads (4, 8) of the lower layer (2), over which the longitudinal thread (5) extends between the two points of interweaving form one of the pairs.
5. The clothing of one of claims 1 to 4, characterized in that said longitudinal threads (5) in said upper layer (1) have floats, which extend over two transverse threads (3), and in that said longitudinal threads (5) extend non-symmetrically.

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