Patent Application
20240026607
The invention relates to a press felt in accordance with the preamble of claim 1.
In the production of paper or cardboard, coverings in the form of press felts are used for transporting and dewatering the fibrous material web in the region of the press section. The main components of such felts are a load-bearing base structure and nonwoven plies, which are generally needled to the base structure. In most cases, woven fabrics are used for the base structures.
To increase the volume for liquid absorption and also to increase strength, the load-bearing base structure can have a plurality of fabric plies which are arranged one above the other. Felts with such base structures are described in EP 0 425 523 or EP 0 672 784 B1, for example. It is likewise also possible, as described in EP2160495 B1, to provide several plies of nonwoven fibers having different fiber counts.
During operation, a press felt is subjected to repeated loads in one or more press nips. In the press nip, the felt is compressed, and, after passage through the press nip, the felt expands again substantially to its original thickness. Since this process is carried out extremely frequently, compaction of the felt occurs after only a short time. According to the prior art, the explanation for this is compression of the nonwoven plies. Compaction of the fabric plies with the formation of a layer of reduced permeability also occurs. This changes important properties of the felt, such as, for example, permeability. In order to compensate for this, it is known, e.g. from EP 2 678 472, to provide a particularly fine nonwoven ply which is abraded during operation of the felt. As a result of the abrasion of the fine nonwoven fibers, the permeability of the felt is increased while the permeability is simultaneously being reduced by compaction. Although a largely constant permeability of the felt can thereby be achieved, the provision of the “sacrificial nonwoven ply” is associated with additional costs and effort.
It is therefore an object of the invention to propose a press felt in which the tendency for compaction is reduced. It is furthermore an object of the present invention to propose a felt which provides a large volume for liquid absorption.
According to the invention, the object is achieved by means of an embodiment as claimed in the independent claim. Further advantageous embodiments of the present invention can be found in the dependent claims.
Here, the proposal is for a press felt for a machine for producing a fibrous material web, comprising a woven base structure and a nonwoven overlay fixed thereon, wherein the base structure comprises a first fabric ply and a second fabric ply. According to the invention, it is envisaged that at least the first fabric ply has longitudinal threads and transverse threads which intersect at intersections, wherein the longitudinal threads and the transverse threads of the first fabric ply are welded to one another at at least 5% of the intersections, in particular at at least 10% of the intersections.
In this context, the terms “longitudinal threads” and “MD threads” as well as “transverse threads” and “CD threads” are synonyms.
As mentioned, base structures which have a plurality of fabric plies are advantageous as regards the provision of a large volume for liquid absorption (“void volume”). However, it has been found that such felts also have a relatively high tendency for compaction. The inventor has recognized that some of this compaction effect is due to the fact that the various fabric plies are partially pressed into one another by the loads in the nip. This is promoted or reinforced by the fact that the threads of the fabrics can be displaced to a certain extent. Continued loading in the nip can result in displacement of the individual MD or CD threads in such a way that threads of one ply are pressed into interspaces of the other ply, thereby increasing compaction of the felt and also reducing permeability.
In order to prevent this effect, or at least to significantly reduce it, it is proposed by the inventor that, at least in the first fabric ply, parts of the longitudinal threads and transverse threads that intersect at intersections are welded to one another. It has proven to be advantageous here if the longitudinal threads and the transverse threads of the first fabric ply are connected to one another in an integrally bonded manner, in particular welded to one another, at at least 5% of the intersections, in particular at at least 10% of the intersections. In this way, the displacement of the threads is prevented or made more difficult, and the above-described compacting effect is suppressed.
With an increase in the proportion of the welded intersections, e.g. to 15%, 20%, 25%, 30%, 35%, 40% or more, the fixing of the threads and the suppression of displaceability increases. However, this also increases the stiffness of the base structure and thus of the felt as a whole. This is usually only possible or desired up to a certain extent. Thus, it is often advantageous if the longitudinal threads and the transverse threads of the first fabric ply are welded to one another at less than 60%, in particular at less than 50%, of the points of intersection.
The advantageous effect of the invention can be increased if the second fabric ply also has longitudinal threads and transverse threads which intersect at intersections, wherein the longitudinal threads and the transverse threads of the second fabric ply are connected to one another in an integrally bonded manner, in particular welded to one another, at at least 5% of the intersections, in particular at at at least 10% of the intersections. This reduces the mobility of the threads of the two plies relative to one another even further. In the second ply too, it is the case that, with an increase in the proportion of the welded intersections, e.g. to 15%, 20%, 25%, 30%, 35%, 40% or more, the fixing of the threads and the suppression of displaceability increases and that it is often advantageous if the longitudinal threads and the transverse threads of the second fabric ply are welded to one another at less than 60%, in particular at less than 50%, of the points of intersection.
Even if, in most of the examples described, the base structure has exactly two fabric plies, it is also possible to provide embodiments in which the base structure also comprises one or more further plies, in particular one or more further fabric plies.
The integral bond at the points of intersection can be produced in various ways.
It is possible, for example, to use two-component fibers (“BiCo fibers”). Bi-component fibers consist of two components, e.g. a core and a sheath. Here, the two polymers have different softening or melting temperatures. The melting temperature of the core is higher than in the case of the sheath, and therefore the sheath can be melted when a certain temperature is applied and thus creates connection points between the respective core fibers in the mixture of the matrix present.
An alternative to this is to connect the MD and CD fibers to one another via welded joints. To create the welded joint, various methods are possible, such as ultrasonic welding or transmission welding. NIR transmission welding is regarded as particularly advantageous. This is because the threads of polyamide which are usually used are largely transparent to light from the NIR range between about 780 nm and 1100 nm.
It is then advantageous to make provision for at least some of the longitudinal threads and/or transverse threads of the first ply—and/or of the second ply—to absorb laser light of a wavelength which is in the range between 780 nm and 1100 nm completely or to a significant extent. (Absorption of more than 30%, in particular more than 40%, of the corresponding light is regarded as significant absorption in this context. Such threads are referred to below as absorbent threads). Given suitable irradiation of the fabric with light from this wavelength range, the light penetrates through the non-absorbent threads relatively unhindered and is absorbed by the absorbent threads. As a result, the contact point of the two threads heats up to such an extent that welding takes place.
As an advantageous possibility, the absorbent threads can consist of the same polymer as the other threads, to which an absorber additive is additionally admixed. In this way, particularly durable welded joints can be achieved. Alternatively, instead of the same polymer, it is also possible to use compatible polymers, e.g. polyamide 6 and polyamide 6.6.
Furthermore, it is also possible to use quasi-simultaneous welding methods in order to produce spot welds. In this case, it is possible to dispense in part with the use of absorbers.
By selective irradiation of selected contact regions/points of intersection, it is possible to weld only these. It is possible, for example, for this selective irradiation to take place in the form of regular patterns, e.g. in the form of straight lines, wavy lines, dot patterns, etc. The width of these lines or the diameter of the points can, in particular, be selected to be so large that a plurality of points of intersection, in particular 2, 3, 4, 5 or more, are covered by them.
It is advantageous for the desired effect if the points of intersection having the integral bonds are not situated exclusively in a partial region of the covering—in particular a seam region—but are distributed, in particular uniformly, over the entire area of the covering.
Such a uniform distribution can be achieved, for example, in that absorbent threads are woven in as CD threads or else as MD threads according to a fixed, predetermined pattern. Thus, for example, provision can be made for every tenth CD thread to be an absorbent thread. This leads to a rather small number of connection points. If every fourth CD thread, every second CD thread or even every CD thread is woven in as an absorbent thread, the number of possible connection points is increased.
Similarly, BiCo threads can also be woven in according to the pattern described above.
There are very wide degrees of freedom for the fabric plies described here. Some examples are listed below:
In very advantageous embodiments, it can be provided that, for each intersection at which the longitudinal threads and the transverse threads are connected to one another in an integrally bonded manner in the first fabric ply and/or the second fabric ply, there is no integral bond at the adjacent intersections.
In this case, adjacent intersections are understood to mean the four intersections that are directly adjacent in the longitudinal and transverse directions.
Such an arrangement of the integral bonds, in particular of the welded joints, is advantageous since good fixing of the threads is possible here in this way, but the increase in the stiffness of the structure remains tolerable even with a comparatively high proportion of integrally bonded intersections (for example 30%, 40% or 50%).
Moreover, such a fabric ply is also simple to produce. Thus, for example, a fabric which is produced in plain weave can be used as the first fabric ply. For the fabric, it is possible to use longitudinal threads which are transparent to the light of a specific wavelength, while the transverse threads completely or partially absorb this wavelength. The integral bonds can then be implemented as welded joints by means of transmission welding with light of this wavelength.
If the fabric is irradiated from one side with light of this wavelength, e.g. with a laser, then there are points of intersection at which the transparent thread lies above the absorbent thread. At these points of intersection, the light penetrates through the transparent thread and is absorbed by the absorbent thread, as a result of which there is heating at the contact point and an integral bond is formed.
As a result of the plain weave, however, the absorbent thread now lies above the transparent thread at each of the four adjacent points of intersection. Therefore, the absorbent thread heats up only on its upper side, but not at the contact point. Thus, no integral bond is formed at these points of intersection.
If the entire fabric is irradiated with the laser using this method, then substantially 50% of the intersections are welded to one another.
However, it is also possible to irradiate only parts of the fabric with the laser. This results in a lower proportion of bonded intersections.
Further advantageous refinements of the invention are explained by means of exemplary embodiments with reference to the drawings. The features mentioned can be advantageously implemented not only in the combination shown but also individually in combination with one another. More specifically, the figures show:
The figures are described in greater detail below.
Here, either all of these intersections 5 can be welded, or else only some of them.
It should be noted here that, in the case of some of the intersections in
If irradiation is carried out only from one side, the fabric shown in
In the case of such a fabric ply 1, 2, both the displacement of the longitudinal threads 3 in the transverse direction and the displacement of the transverse threads 4 in the longitudinal direction are impeded or prevented.
Here,
Here too, the nonwoven overlay 7 is compressed. However, the longitudinal threads 3.1, 3.2 cannot be deflected in the transverse direction. The transverse forces are absorbed by the integral bonds at the points of intersection 5. As a result, penetration of the first fabric ply 1 and of the second fabric ply 2 is avoided or at least reduced. The void volume of the fabric plies 1, 2 is hardly reduced by penetrating threads 3, 4 of the other fabric ply 2, 1 and thus continues to be available for liquid absorption.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 121 627.0 | Aug 2020 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/067968 | 6/30/2021 | WO |
