EP0325167B1 - Double-layer screen for the forming section of a paper machine - Google Patents

Abstract

In a double-layered wire-forming fabric for the sheet-forming area of a paper-making machine, having an upper layer of cross strands (1) and a lower layer of cross strands (2, 3) and having a single system of longitudinal strands (4) which are interwoven with both layers of cross strands (1; 2, 3), the cross strands (2, 3) of the lower layer are woven-in in parallel and in pairs and the cross strands (1) of the upper layer are arranged over a pair of cross strands and, if desired, between them. <IMAGE>

Description

The invention is based on a double-layer covering for the sheet formation area of a paper machine with two layers of transverse threads which are interwoven with a single system of longitudinal threads.

Fabrics for the sheet formation area of a paper machine, in which the transverse threads are arranged in two layers and are woven with a single system of longitudinal threads, are generally referred to as double-layered sheet-forming fabrics or short, double-layered sieves. Double-layer screens brought a significant improvement in sheet formation properties compared to single-layer screens. The high degree of filling of the longitudinal threads of 95 - 110% has significantly improved the longitudinal stability. The drainage properties of these screens are also more favorable because there are no large meshes running through in the vertical direction and the water is drained from the pulp through numerous fine, slit-like meshes, which are predominantly arranged in the oblique direction.

In the case of double-layer screens, it is possible to choose the dimensions, material properties and shape of the two shooting systems differently. For marking reasons, fine transverse threads with a relatively short float length can be used for the paper side, while thick transverse threads with long floats can be used for the running side. It is also possible for to use other materials on the barrel side. It is common for the transverse threads on the running side to consist of polyester and of abrasion-resistant polyamide.

A two-layer sieve, on the other hand, is a covering for the sheet formation area of a paper machine, which consists of an upper sieve and a lower sieve, both of which are complete woven fabrics of longitudinal threads and transverse threads, the upper sieve and the lower sieve either by means of special binding threads or by weaving in the Longitudinal or transverse threads of the top wire are connected to the bottom wire or vice versa. The top sieve is fine-meshed. It consists of many thin threads and is not very stable in itself. The bottom wire is coarse and consists of thick threads. It gives the fabric the required longitudinal and transverse stability. The binding threads can run in the longitudinal direction, in the transverse direction or simultaneously in both directions.

The weak point of these two-layer screens is the binding thread. It is often destroyed prematurely by wear on the barrel side or by the internal friction between the two fabric layers. A remedy for this is provided by DE-C-3 329 739. This sieve has transverse threads arranged in pairs on the running side and the binding thread is no longer on the running surface, but is clamped in by a pair of transverse threads and held against the longitudinal thread crossing over it. In the case of two-layer sieves, the connection between the two layers is not strong enough to avoid mutual relative movement. When the screen is alternately deflected over the outside and inside guide rollers, there is an internal friction between the two fabric layers. The abrasive particles of the filler get through the high proportion of fillers in writing and printing papers between the two fabric layers. Due to the relative movement and the filler, the surface between the two layers of fabric is also heavily ground down. The binding threads at the points of contact with the fabric layers are often worn through early. As a result, the fabric layers separate and the sieve becomes unusable.

EP-A-0 224 276 describes a sieve which is similar to the double-layer sieve, but has the finer surface structure of a two-layer sieve. The paper-side structure is very finely designed by a special longitudinal thread guide and by double the number of transverse threads. In contrast, the running side of this sieve is rough and very open. Successive cross threads on the paper side are also supported in different ways by carrying a cross thread saddle-shaped from a longitudinal thread so that it lies exactly in the transverse direction in the fabric, while the next cross thread is supported by two adjacent longitudinal threads, one of the longitudinal threads on the screen surface rises and the other descends, so that the support of the transverse thread is scissor-shaped and the transverse thread float is rotated so that it lies at an angle to the floats of the saddle-supported transverse thread. In this way, the marking properties of the screen are significantly improved. With this sieve, the number of transverse threads on the paper side is twice as large as on the running side. As a result, significantly higher number of transverse threads can be achieved on the paper side than with conventional, double-layer sieves (with the same longitudinal thread count). In the case of the double-layered sieves, the maximum possible number of transverse threads is limited in terms of weaving technology, because the longitudinal threads always change between the upper and lower layers, so that only a limited total amount of transverse threads can be wrapped. The sieve according to EP-A-0 224 276 has the same total number of the cross threads more threads on the paper side than a conventional screen, because the thread count on the running side is only half as high as that on the paper side.

According to EP-A-0 085 363, the marking properties and the retention of the fibers are improved in that an additional supporting thread is inserted without binding in a single-layer or double-layer screen on the paper side after each structural thread. This reduces the mesh size and the fibers of the paper pulp are better supported. For technical reasons of weaving, however, it is not possible to weave in further additional threads if the maximum possible thread density of the double-layer sieve has already been reached. The number of transverse threads in the original weave must be reduced by approximately one third in order to achieve approximately the same total number of transverse threads with additional threads.

In practice it has been shown that the sieves according to EP-A-0 085 363 and EP-A-0 224 276 significantly improve the sieve marking and the retention. When weaving these double-layer fabrics, however, care must be taken to ensure that the transverse threads are perfectly superimposed. This causes difficulties because the position of the upper and lower transverse threads relative to one another changes as a result of the overturning processes during fixing. During weaving, the transverse threads lay largely uncranked in the composite of the longitudinal threads. The longitudinal threads loop around the two layers of the inserted transverse threads and initially form the outermost points of the fabric both on the paper side and on the running side. Only when they are fixed with a very high longitudinal tension and the effect of heat do the longitudinal threads stretch and force the transverse threads to bend. For example, the long transverse thread floatation is formed on the running side, that in the finished sieve protrudes from the fabric plane and forms the abrasion surface of the sieve. (One speaks then of the transverse thread runner.) At the same time, the longitudinal thread is pressed into the interior of the fabric by the high longitudinal tension. As a result, it is protected against abrasion on the running side or at least only reached by abrasion when the volume of the transverse threads has already been largely used up. At the same time as these overturning operations, the transverse threads slide over one another, and it is extremely difficult to weave the fabric in such a way that all the transverse threads lie perfectly over one another over the entire length and width of the sieve.

With a sieve 9 m wide and 70 longitudinal threads / cm, the sieve has a total of 63,000 longitudinal threads. With a cross thread count of 30 cross threads in the top layer and 30 cross threads in the bottom layer / cm, the screen has a total of 135,000 cross thread pairs with a screen length of 45 m. With a 7-thread weave, there are 1215 million cross thread floats, all of which must lie cleanly on top of each other. If there is only a single point where the upper transverse thread slips off the lower one, this point will cause an inadmissible marking of the paper web at the high operating speeds of the sieves and the entire sieve becomes unusable.

With so many longitudinal threads and so many transverse thread pairs, it is extremely difficult to actually achieve a constant, superimposed layer. Already during weaving, the fabric tension over the entire width of the loom and of course over the entire length of the sieve must be precisely maintained. In addition, all cross threads must have exactly the same properties, both in the cold state during weaving and at the high temperatures during the fixing process. So that the fabric is resistant to abrasion alternately polyester and polyamide threads are woven into the barrel. These two materials behave very differently, especially when fixing. If a good superimposition of the upper transverse thread over the lower polyester transverse thread is achieved, then the next upper transverse thread slips off the lower polyamide transverse thread. Even slight dislocations cause marked marks in the paper.

In addition, the polyamide monofilament bends differently under tension and heat than the polyester monofilament. This depends on the temperature and voltage. As a result, the longitudinal thread embeds less deep into the fabric when the polyamide thread is tied in than when the polyester thread is tied. By selecting the most favorable physical properties of both materials, these differences in offset can be reduced, but it is not possible to completely avoid the differences in the internal structure of the screen.

These problems arise in the manufacture of all double-layer screens. They are particularly serious in the sieve constructions according to EP-A-0 085 363 and EP-A-0 224 276, in which, due to the absence of every second transverse thread on the running side, the internal structure is more unbalanced from the outset. The slipping of the cross threads is particularly pronounced in these fabrics.

In addition, the number of transverse threads on the running side is lower in these sieves, and thus the distances between the transverse threads are considerably larger than in double-layered sieves with the same number of transverse threads in the upper and lower layers. Because at the high speed of today's paper machines for printing papers of more than 1000 m / min, hydrodynamic pressure waves emanating from the running side have a decisive influence on the marking, the large spacing of the transverse threads on the running side has an adverse effect on the marking. The strength of the pressure waves depends on the distance between successive transverse threads. When using fabrics with only half the number of transverse threads on the running side, it sometimes happens that unacceptably strong transverse thread markings become visible in the paper web. The density of the lines of the marking in the paper web does not correspond to every lower transverse thread, but only to every second transverse thread on the running side; this is the polyamide cross thread. The reasons for this are the differences in the layering (slipping) described above and the differences in the offset between polyamide and polyester. In this way, the otherwise interesting advantages of these screens are sometimes nullified.

Another negative factor of fabrics with a low number of transverse threads on the running side is water dragging. These fabrics are closed on the paper side, but very open at the bottom. Large amounts of water are retained in the open space of the mesh and then flung over the guide rollers during the deflection.

The invention has for its object to design a double-layer paper machine screen so that the risk of slipping of the upper transverse threads compared to the lower transverse threads is significantly reduced.

According to the invention, this object is achieved in that the lower transverse threads are woven in parallel in pairs and a transverse thread of the upper layer is arranged over each pair of transverse threads in the lower layer. In addition, a transverse thread of the upper layer can also be arranged between each pair of transverse threads of the lower layer.

The paper machine sieve according to the invention is produced in such a way that, with the structural features remaining unchanged, instead of a lower transverse thread, two lower transverse threads each with a diameter of approximately 20% to 30% smaller are woven in. The number of transverse threads in the lower layer is thus doubled compared to the sieves according to EP-A-0 085 363 and EP-A-0 224 276. Surprisingly, doubling the number of transverse threads in the lower layer does not result in a reduction in the number of transverse threads in the upper layer . Each pair of cross threads in the lower layer therefore only has the same effect as a single cross thread on the total number of transverse threads that can be achieved.

In the manufacture of the paper machine screen according to the invention, the weaving sequence is such that a lower transverse thread of a pair, then an upper transverse thread and finally the other transverse thread of this pair are woven in. The two transverse threads of the lower layer are therefore not woven in one after the other, but the upper transverse thread is woven in between them.

In addition to improving the stability of the sieve structure, the abrasion resistance is also increased in the paper machine sieve according to the invention, since the total volume of the transverse threads of the lower layer available for the abrasion is higher. E.g. Instead of a 0.22 mm thick lower transverse thread, two transverse threads each 0.18 mm thick are woven in, so the material volume of the lower transverse threads is increased by a total of about 34%. This leads to an improvement in the abrasion resistance and thus an increase in the running time.

In general, the diameter of the lower transverse threads is at least equal to that of the upper transverse threads and that it can be up to 30% larger.

As mentioned at the beginning, it is customary for paper machine screens to alternately use transverse threads made of a different material on the running side, in particular alternating between polyamide and polyester. In the paper machine screen according to the invention, each pair of transverse threads preferably consists of a polyester thread and a polyamide thread, generally a polyester monofilament and a polyamide monofilament. As a result, each longitudinal thread always ties in the same cross thread combination with each winding of the cross threads on the running side. As a result, there are no longer any differences between the individual longitudinal thread offsets on the running side and the uniformity of the running side is significantly improved. The different properties of the cross thread materials, namely polyamide and polyester, no longer have an adverse effect. Due to the smaller thickness of the lower transverse threads compared to a paper machine screen, otherwise unchanged design features, the thickness of the lower layer is also reduced and at the same time the open area of the lower layer is reduced. The distances between the cross thread pairs also become smaller, which further reduces the risk of marking.

The paper machine screen according to the invention and the upper fabric of the multi-layer paper machine screen according to the invention also have the typical features of a double-layer paper machine screen. In particular, the upper transverse threads are finer than the lower transverse threads, although the difference is not as pronounced as in the prior art, since in the paper machine screen according to the invention each individual lower transverse thread is thinner than usual. The longitudinal and transverse threads are generally monofilaments. The upper transverse threads and the longitudinal threads are generally polyester monofilaments. The paper machine screen can be flat or endless be woven. In particular in the case of endless weaving, the longitudinal threads can be multifilaments, for example. The paper machine screen according to the invention can be woven in any number of shafts. A seven-year, eight-year, ten-year, fourteen-year or sixteen-year bond is particularly useful.

Cross threads woven in pairs are known in single-layer and two-layer or multi-layer paper machine screens. With these paper machine screens, however, there is no risk of displacement of transverse threads arranged vertically one above the other, or such a displacement does not have a serious influence on the marking properties. Multi-layer paper machine screens with cross threads running in pairs in parallel in one of the layers are known from US-A-4,636,426 and DE-C-3,329,739. In the former case, the two transverse threads of a pair are bound together over their entire length by a hydrophilic, water-absorbing polyester, so that the two transverse threads act like a single thread with a flat cross-section. In the second case, it is a two-ply paper machine screen, in which the transverse threads of the lower layer are guided in pairs in parallel in order to enclose the binding cross thread between them and the crossing longitudinal thread, so that this is largely removed from abrasion.

From DE-A-31 46 385 it is known to improve the retention in a double-layer paper machine sieve in that the longitudinal threads on the paper side and / or on the running side are guided in pairs at least in sections. Due to the parallel guidance of the longitudinal threads, the fibers of the paper web are supported by a flat structure, whereby the striking impressions of individual longitudinal threads on the paper web are avoided.

EP-A-0 117 856 describes a two-layer paper machine screen in which the warp threads of the lower layers are woven in pairs in parallel over their entire course. This makes it possible, on the one hand, to produce double-layer paper machine sieves and, on the other hand, two-layer paper machine sieves with warp threads of the same diameter in the same warp thread density.

From DE-A-34 45 367 a two-ply paper machine screen is known in which, in order to avoid high tensions in the binding threads, the lower fabric contains two groups of transverse threads, of which the first group in 1: 1 binding and the second group in 1: n weave are interwoven with the warp threads of the lower fabric, successive transverse threads of different types of weave, which form double offsets on the inside of the lower fabric, touch, and wherein the long n-offsets form a plane of wear on the outside. n means an odd number greater than l.

• Fig. 1 im Längsschnitt ein Papiermaschinensieb mit paarweise parallel eingewobenen unteren Querfäden;
• Fig. 2 einen Ausschnitt von Fig. 1 in Vergrößerung;
• Fig. 3 im Längsschnitt eine andere Ausführungsform eines doppellagigen Papiermaschinensiebes mit paarweise parallel eingewobenen unteren Querfäden;
• Fig. 4 eine Ansicht der Laufseite des Papiermaschinensiebes nach Fig. 1;
• Fig. 5 und 6 im Quer- bzw. im Längsschnitt ein weiteres Ausführungsbeispiel eines doppellagigen Papiermaschinensiebes mit paarweise parallel eingewobenen unteren Querfäden; und
• Fig. 7 im Längsschnitt ein Ausführungsbeispiel eines mehrlagigen Papiermaschinensiebes, bei dem die obere Lage ein doppellagiges Gewebe ist, das dem in Fig. 1 gezeigten Papiermaschinensieb entspricht.

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

• 1 shows in longitudinal section a paper machine screen with pairs of lower transverse threads woven in parallel;
• FIG. 2 shows a detail of FIG. 1 on an enlarged scale;
• 3 shows in longitudinal section another embodiment of a double-layer paper machine screen with lower transverse threads woven in parallel in pairs;
• FIG. 4 is a view of the running side of the paper machine screen according to FIG. 1;
• 5 and 6 in cross-section or in longitudinal section a further embodiment of a double-layer paper machine screen with paired lower cross threads woven in parallel; and
• Fig. 7 in longitudinal section an embodiment of a multi-layer paper machine screen, in which the upper layer is a double-layer fabric, which corresponds to the paper machine screen shown in Fig. 1.

The double-layer paper machine screen shown in FIGS. 1 and 4 has upper transverse threads 1 and lower transverse threads 2 and 3 woven in parallel in pairs, which are interwoven with longitudinal threads 4. The upper transverse threads 1 together with the longitudinal threads 4 form the paper side 5 of the paper machine screen, on which the sheet is formed. The lower transverse threads 2 and 3 form the running side 6 of the paper machine screen, with which the paper machine screen faces most of the rollers and the guide elements of the paper machine. The longitudinal threads 4 are generally not involved in the formation of the running side, since they are decisive for the longitudinal stability of the paper machine screen and should therefore be exposed to the abrasion as little as possible. This is achieved in the manufacture of the paper machine screen in that a correspondingly high longitudinal tension is exerted on the paper machine screen during heat setting, so that the lower offsets of the longitudinal threads 4 are pulled upward.

The paper machine screen shown in FIGS. 1 and 4 is essentially identical to the paper machine screen according to FIGS. 1 and 2 and example 1 of EP-A-0 224 276 same, but with the exception of the double cross threads 2, 3. The paper machine fabric was produced with the following data, the values after the weaving and heat setting being indicated where relevant:

If the same paper machine screen is manufactured with a single lower cross thread of 0.22 mm diameter instead of a pair of cross threads of 0.18 mm diameter, the fabric thickness is 0.716 mm and the free distance between the lower cross threads is 0.405 mm. It can be seen from this that in the paper machine screen according to the invention the lower layer is significantly thinner and is no longer so extremely open.

Figure 3 shows an embodiment with a different weave, in which the upper layer of the transverse threads 1 alternately consists of polyester monofilaments of 0.11 and 0.18 mm in diameter. The other design features are unchanged in both the state after weaving and the state after heat setting.

Figures 5 and 6 relate to an embodiment in which the double-layer paper machine wire known from Figures 5a and 5b of EP-A-0 245 851 has been modified in accordance with the present invention by each of the transverse threads of the lower layer being replaced by a pair of transverse threads woven in parallel was replaced.

The present invention also covers embodiments in which the uppermost fabric 7 of a multi-layer paper machine wire is formed by a fabric which corresponds to one of the double-layer paper machine wires described above (FIG. 7). The upper fabric 7 is connected to the lower fabric 8 in that the one transverse thread 2 of a parallel cross thread pair is interwoven with a longitudinal thread 9 of the lower fabric 8. At the binding points 10 between the two fabrics 7, 8, the structure of the upper fabric 7 is not impaired, since the longitudinal thread 4 of the upper fabric 7 has its normal course due to the other transverse thread 3, which has remained unchanged in the course. This also means that there are no troughs on the paper side 5 at the binding points, which would lead to a noticeable marking in the paper. It is also possible to use a double-layer fabric as the lower fabric 8 instead of the single-layer fabric shown in FIG.

Claims (6)

1. A double-layer clothing for the sheet forming section of a papermaking machine, comprising an upper layer of transverse threads (1) and a lower layer of transverse threads (2, 3) and comprising a single system of longitudinal threads (4), which are interwoven with both layers of transverse threads (1; 2, 3), characterized in that said transverse threads (2, 3) of said lower layer are woven in in parallel pairwisp, and in that one transverse thread (1) of the upper layer is positioned above each transverse thread pair (2, 3).

2. A double layer clothing as according to claim 1, characterized in that the one transverse thread of each transverse thread pair (2, 3) is a polyester-monofilament and the other transverse thread is a polyamide-monofilament.

3. A double-layer clothing as according to claim 1 or 2, characterized in that the number of upper transverse treads (1) is equal to the number of lower transverse threads (2, 3).

4. A double-layer clothing as according to claim 3 characterized in that the diameter of said lower transverse threads (2, 3) is about 30 % larger than that of said upper transverse threads (1).

5. A double-layer clothing as according to claim 1 or 2, characterized in that the number of upper transverse threads (1) is half as large as that of said lower transverse threads (2, 3), and in that said upper transverse treads (1) and said lower transverse threads (2, 3) have about the same diameter.

6. A multi-layer clothing for the sheet forming section of a papermaking machine, comprising an upper woven fabric (7) and at least one lower woven fabric (8) connected therewith, characterized in that said upper woven fabric is designed in accordance with the double-layer clothing according to one of claims 1 to 5, and in that said upper woven fabric (7) is connected to the subsequent, lower woven fabric (8) in that one (2) of the transverse threads of a pair of transverse threads of said upper woven fabric (7) is interwoven with a longitudinal thread (9) of said lower woven fabric (8).

Images