Forming fabric for use in a paper machine, and method and apparatus for manufacturing such a forming fabric

Abstract

The present invention relates to a forming fabric (2), for use in the sheet-forming section of a paper machine, having or comprising a textile planar structure in which, in order to enhance inherent stability, crossing yarns (5, 6, 7) are engaged into one another at crossing points (14) and in which yarns (5, 7) additionally are fused to one another, which is characterized in that the planar structure comprises crossing first and second yarns (5, 7), the first yarns (7) having the property that they absorb laser energy and can be brought, by absorbed laser energy, to melting temperature at least at the surface; and that first and second yarns (5, 7) are fused to one another at at least some of their crossing points (14).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM TO PRIORITY

This application is related to application number 05 017 006.7, filed Aug. 4, 2005 with the European Patent Office, which is incorporated herein by reference, and to which priority is claimed.

FIELD OF THE INVENTION

The invention relates to a forming fabric, for use in the sheet-forming section of a paper machine, having or comprising a textile planar structure in which crossing yarns are engaged into one another at crossing points and in which, in order to enhance inherent stability, yarns are fused to one another at crossing points. The invention further relates to a method for manufacturing such forming fabrics, in which method a textile planar structure is manufactured from yarns that intersect one another and are engaged into one another at crossing points, and in which yarns are then fused to one another at crossing points by heating to melting temperature. Lastly, the invention also refers to an apparatus for manufacturing a forming fabric of this kind.

BACKGROUND OF THE INVENTION

Forming fabrics are long, wide belts that circulate in the first part of a paper machine, called the sheet-forming section, forming a flat upper run. At the beginning of the upper run, the previously prepared fiber pulp is applied onto the forming fabric and dewatered through the forming fabric, so that a paper web still having a high liquid content is gradually formed. In subsequent sections of the paper machine, the paper web is further dewatered mechanically and thermally.

Single- or multiple-layer woven fabrics are generally used as forming fabrics. A woven fabric (or even a knitted fabric) obtains its inherent stability or diagonal stability from the fact that crossing yarns are engaged into one another, forming a weave pattern. Particularly in the context of large stresses such as those that occur in a paper machine, the inherent stability of the textile planar structure is not sufficient to ensure stable and problem-free circulation of the forming fabric through the sheet-forming section. Additional measures have therefore been taken in order to improve the dimensional stability, in particular the diagonal stiffness, of such textile planar structures.

One of these measures consists in adhesively bonding the yarns to one another at the crossing points, by the fact that the fabric structure is equipped with adhesive polymers. This method is cost-intensive because the dispersion must be applied very evenly, and because drying consumes a great deal of time and energy. Permeability is furthermore considerably decreased, which has a negative effect on the sheet-forming process. A further disadvantage is the fact that the adhesion at all crossing points resulting from this method causes a stiffening that is often not desired.

U.S. Pat. No. 5,888,915 A proposes, in order to improve the dimensional stability of such textile planar structures, to use bicomponent yarns in which the melting temperature of the outer casing is lower than that of the core. A woven or knitted fabric or yarn layer equipped with bicomponent yarns of this kind is then heated, in a continuous furnace, to a temperature that is above the melting temperature of the outer casing of the bicomponent yarns, but below the melting temperature of the core of those yarns, so that the casing melts and a fused or adhesive bond to other yarns is produced in this fashion at the crossing points.

Forming fabrics having good dimensional stability can be manufactured using this method. Manufacture is costly, however, since the bicomponent yarns are expensive, and heating of the entire forming fabric in a continuous furnace is energy-intensive.

Also known are forming fabrics having or comprising a textile planar structure that is formed from a yarn layer in which the yarns are not engaged with one another, i.e. not woven or interlinked with one another. Instead, transverse yarns that extend parallel to and at a distance from one another are laid onto a layer of longitudinal yarns that are likewise parallel to and at a distance from one another, and the longitudinal yarns are then joined to the transverse ones. Only thereby does the yarn layer acquire inherent stability. Joining can occur according to the method according to U.S. Pat. No. 5,888,915 A, using bicomponent yarns.

The disadvantages of the aforementioned method are eliminated by a method that is evident from EP 1 359 251 A1. In this method, the longitudinal and transverse yarns are fused to one another at crossing points as a consequence of a heating to melting temperature that is confined to those crossing points. The heating can be applied in single-point fashion by means of high-frequency, inductive, and/or laser energy. As an alternative thereto, however, the energy can also be applied in planar fashion if the crossing points are first equipped with an additive that promotes absorption of the energy and that concentrates energy uptake at the crossing points despite the planar application, so that only those points are heated to melting temperature and consequently fused to one another. When a laser is used, the additive should be a light-absorbing dye, e.g. black dye, or a photoactive substance. The additive can be applied between the yarns or onto the yarns. It is also proposed instead to add the additive to the yarn material during the extrusion operation.

SUMMARY OF THE INVENTION

It is the object of the invention to manufacture a forming fabric of the kind cited initially more economically, preferably without producing structural changes. A further object is to make available a method suitable for manufacture, and an apparatus therefor.

The first object is achieved, according to the present invention, by a forming fabric in which the planar structure comprises crossing first and second yarns, the first yarns having the property that they absorb laser energy and can be brought, by absorbed laser energy, to melting temperature at least at the surface; and in which first and second yarns are fused to one another at at least some of their crossing points. For many situations it is sufficient if only some of the first and second yarns are welded to one another, and those only at some of their crossing points.

The basic idea of the invention is therefore to use for some of the yarns, in the context of a forming fabric, particular yarns which are distinguished by the fact that they absorb laser light. In this fashion, the textile planar structure can be additionally stabilized by the fact that the first yarns are heated by means of a laser to melting temperature, and at at least some of the crossing points (if not at all) a fused join is thus produced with the second yarns, which absorb only little or no laser light. This manner of producing the fused join is substantially less time-consuming and energy-intensive than the known methods, especially since the first yarns themselves require little additional cost. A further advantage consists in the fact that the dimensional stability can be individually adapted to particular requirements by correspondingly varying the number of first yarns as well as the number of crossing points at which welding or fusing is performed. This is because in many cases, a forming fabric that is too stiff and therefore insufficiently adaptable is also disadvantageous.

In an embodiment of the invention, provision is made for the yarns otherwise not to be joined to one another, i.e. for no further joint to exist beyond the mutual engagement of the yarns and their single-point welding at crossing points.

To allow the first yarns to absorb the laser light, they can contain an additive that imparts the ability to absorb laser light. Examples of such additives are near-infrared-active (NIR-active) substances that absorb, for example, in the region of the wavelengths 808 nm, 940 nm, 980 nm, or 1064 nm. Suitable for this are, for example, carbon or colorless additives such as Gentex's Clearweld® or BASF's Lumogen® IR. The additive preferably extends over the entire length of the first yarns and is evenly distributed over the length and cross section. The additive can be incorporated into the first yarns and/or applied onto the surface of the first yarns and/or introduced at the crossing points between the first and second yarns. If the additive is incorporated, the proportions by weight should be approximately 0.10% to 2.5%.

In a further embodiment of the invention, provision is made for the first yarns to be bicomponent yarns, only one of the two components containing the additive. The bicomponent yarns should preferably comprise a core and a casing surrounding it, the additive then being contained only in the casing.

Planar structures suitable according to the present invention are those in which crossing yarns are engaged with one another, as is the case, for example, with woven and knitted fabrics. The planar structure should preferably comprise longitudinal and transverse yarns, in which context the first yarns can extend only in the longitudinal direction, only in the transverse direction, or in both directions. Depending on the dimensional stability requirements, only some of the longitudinal and/or transverse yarns can then also be embodied as first yarns. The first yarns should preferably be part of the weave of the yarns in the planar structure, i.e. should not have been additionally introduced into the existing woven fabric, knitted fabric, etc., in order not to disrupt the desired yarn distribution and structure. It is definitely useful if the first yarns are distributed in the planar structure in a consistently regular pattern.

The first yarns are, if possible, advantageously engaged into the planar structure in such a way that they do not reach as far as the paper side of the forming fabric. If the planar structure is embodied with multiple plies, the first yarns should be engaged only in an internally located layer and/or a roller-side layer.

Possible materials for the yarns are any type of thermoplastic material that is suitable for the respective application, i.e. that permanently withstands the respective ambient conditions in the paper machine. For cost reasons, at least the first yarns, but better yet all the yarns, can be embodied as single-component yarns that can additionally be fiber-reinforced; i.e. individual yarns, or all the yarns, can contain a fiber reinforcement.

Forming fabrics have a finite length, with ends joinable via a seam. In the region of the two ends, i.e. in the seam region, first yarns should be present that extend in the transverse direction and are welded to second yarns extending in the longitudinal direction. In order to achieve particularly high strength there, the first yarns should be present in a higher concentration in the seam region than in the remaining region of the forming fabric, and the first and second wires should be welded to one another at as many crossing points as possible. The longitudinal yarns inserted in correctly woven fashion into the respectively opposite end during the stitching process are then fused to the first yarns. This creates the possibility of shortening the seam region without thereby impairing the strength of the seam. In this fashion the seam region can be reduced from a usual extension of, for example, 100 mm in the longitudinal direction to, for example, 60 mm, i.e. the seam region can be shortened by 20-60% in the machine direction.

A laser beam that has a power output of 20 to 200 W, preferably 50 to 150 W, should be used for welding.

The second object is achieved, according to the invention, by a method in which first and second yarns are used in the manufacture of the planar structure, in which context the first yarns can absorb laser energy; and in which first and second yarns are fused by means of laser energy at at least some of their crossing points or at all their crossing points.

Welding of the first and second yarns at the crossing points can occur in a consistently regular pattern, but also in stochastically distributed fashion. The possibility exists of guiding the laser over the forming fabric in parallel longitudinal tracks, the laser and forming fabric being moved relative to one another in the longitudinal direction of the forming fabric by the fact that either the laser is moved two-dimensionally over the forming fabric stretched in stationary fashion, or the forming fabric is moved along in the longitudinal direction below the laser, in which context the laser can additionally be displaced laterally. As an alternative thereto, the possibility exists of guiding the laser over the forming fabric in parallel transverse tracks along a transverse yarn. For this, the forming fabric can be alternately moved and then stopped, so that the laser is guided along every transverse yarn, or even every second, every third, or every tenth transverse yarn.

A further possibility is that of guiding the laser over the forming fabric in a diagonal direction, the angle between the diagonal direction and the transverse direction being selected so that the first and second yarns are fused to one another at as many crossing points as possible. The laser can follow the weave ridges of the fabric weave. The distances between the laser tracks can be selected, in the longitudinal direction, depending on the desired embodiment. Regardless thereof, it is not excluded for the laser to be guided over the forming fabric in spiral tracks.

Provision is also made according to the invention for the laser to be controlled in such a way that it is displaced to those crossing points of first and second yarns designated for joining. For joining, the laser first shines through the second yarn before striking the first yarn. The concentration of the additive in the first yarns, and the energy of the laser, should be correlated in such a way that the first yarns are melted only at the surface facing toward the laser, so that there is only a slight negative effect on the structure and shape of the yarns.

The third part of the object is achieved, according to the present invention, by an apparatus that comprises a tensioning device with which the forming fabric, rendered endless, is stretchable; and such that a laser device having at least one laser head is associated with the tensioning device in such a way that at least one laser beam is directable onto the forming fabric in the stretched state; and that the tensioning device and laser device are embodied in such a way that a relative motion is producible between the forming fabric and laser beam. With the aid of this apparatus, first and second yarns can be welded to one another by means of the at least one laser head.

Particularly suitable as a tensioning device are two spaced-apart tensioning rollers with which a longitudinal tension is impartable to the forming fabric pulled onto the tensioning rollers, for example by the fact that the spacing of the tensioning rollers is modifiable. At least one of the tensioning rollers should be connected to a drive motor in such a way that a forming fabric pulled onto the tensioning rollers can be caused to circulate continuously or in steps, in which context the drive motor can also be embodied reversibly.

According to a further feature of the invention, provision is made for the at least one laser head to be movably guided transversely with respect to the forming fabric pulled onto the tensioning device, and preferably over the entire width of the tensioning device. As an alternative thereto, but preferably in combination therewith, the at least one laser head should also be movably guided longitudinally with respect to the forming fabric pulled onto the tensioning device. This can usefully be done in such a way that the at least one laser head is supported on a guide rail that extends transversely with respect to the forming fabric and is displaceable in the longitudinal direction of the forming fabric, the forming fabric being in each case pulled onto the tensioning device.

In order to allow the relative motions between laser beam and forming fabric, as proposed in accordance with the method according to the present invention, to proceed automatically, a programmable control device should be provided for controlling the tensioning device and laser device and their motors for moving, for example, the tensioning rollers and/or the laser head. This control device can additionally be combined with a sensor that is mounted on the laser device and serves to sense yarns of the forming fabric that differ from the other yarns of the forming fabric in terms of a property that can be sensed by the sensor. The sensor can be, for example, an optoelectronic sensor (photocell) if the first yarns of the forming fabric according to the present invention have a different color and/or a different brightness from the second yarns. It is also possible, however, to use a sensor that responds to the presence of the additive in the first yarns that imparts the ability to absorb laser light. In conjunction with the control device, the sensor allows the first yarns to be located, and allows the laser to be moved to the locations designated for welding.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in more detail in the drawing, with reference to an exemplifying embodiment. The drawing shows, in an oblique view, an apparatus for partial manufacture of forming fabric 2 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Forming fabric 2 was previously woven in a finite length, and its ends were then stitched to one another so that an endless structure was produced. Forming fabric 2 was then stretched between two rollers 3, 4 arranged at a distance from one another, one of the rollers being movably guided in such a way that forming fabric 2 acquires a specific longitudinal tension. At least one of rollers 3, 4 is driven clockwise in motorized fashion. Upon activation of the drive system, forming fabric 2 is moved at a predetermined speed in the direction of arrow A, while rollers 3, 4 execute a rotary motion in the direction of arrow B, C. It is understood that rollers 3, 4 are supported in an apparatus frame (not depicted in further detail) in which the drive system is also housed.

Forming fabric 2 was manufactured in a finite length, and was converted into the endless form shown by way of a seam joining the ends. Forming fabric 2 is made of a woven fabric that, in this embodiment, comprises longitudinal yarns (labeled 5 by way of example) extending in the machine direction (arrow A) and transverse yarns (labeled 6 by way of example) extending perpendicular thereto. Longitudinal and transverse yarns 5, 6 are produced from a thermoplastic that is usual for use in forming fabrics, and constitute second yarns for purposes of the present description. Longitudinal and transverse yarns 5, 6 are engaged into one another in accordance with a specific weave pattern.

Extending between each two transverse yarns 6 constituting second yarns is a respective further transverse yarn (labeled 7 by way of example) that is emphasized in the drawing. Transverse yarns 7 are engaged into longitudinal yarns 5 and are part of the weave pattern. They constitute first yarns for purposes of the present description. They contain an additive that makes them capable of absorbing laser energy, so that they can be brought to melting temperature with the aid of a laser beam.

A laser apparatus 8 is arranged above the plane of rollers 3, 4. Laser apparatus 8 has longitudinal rails 9, 10 (depicted only in shortened fashion here) that extend, parallel to one another, parallel to the plane of rollers 3, 4 and above them, and are immovably joined to the apparatus frame. Longitudinal rails 9, 10 have a spacing that is larger than the width of forming fabrics 2 that are to be processed in apparatus 1.

Mounted displaceably in the directions of double arrow D on longitudinal rails 9, 10 is a transverse rail 11. It extends perpendicular to longitudinal rails 9, 10 and thus parallel to the axes of rollers 3, 4. Mounted on transverse rail 11 via an arm 12 is a laser 13, which can be displaced back and forth on transverse rail 11 in the directions of double arrow E. It can furthermore be pivoted about the longitudinal axis of transverse rail 11 in the directions of double arrow F. The movements of transverse rail 11 relative to longitudinal rails 9, 10, and the movement of laser head 13 relative to transverse rail 11, are brought about by means of motors (not depicted here in further detail).

Apparatus 1 comprises a programmable control device (likewise not depicted here in further detail), similar to a CNC controller, with which the individual motors for moving laser head 13 and rollers 3, 4 can be controlled, and laser head 13 can be activated. In the example shown, laser head 13 is moved only in the transverse direction via transverse rail 11. Rollers 3, 4 are halted when a first transverse yarn 7 comes to rest below laser head 13. Laser head 13 is then guided along transverse yarn 7 over the width of forming fabric 2, and activated at the positions designated for welding. As a result of the laser energy, transverse yarn 7 heats up at the surface to melting temperature, with the consequence that it fuses to longitudinal yarns 5 at crossing points 14, so that a welded join is produced there after cooling.

Claims

1. A forming fabric (2), for use in the sheet-forming section of a paper machine, having or comprising a textile planar structure in which, in order to enhance inherent stability, crossing yarns (5, 6, 7) are engaged into one another at crossing points (14) and in which yarns (5, 7) additionally are fused to one another,

wherein the planar structure comprises crossing first and second yarns (5, 7), the first yarns (7) having the property that they absorb laser energy and can be brought, by absorbed laser energy, to melting temperature at least at the surface; and first and second yarns (5, 7) are fused to one another at at least some of their crossing points (14).

2. The forming fabric according to claim 1, wherein the first and second yarns (5, 7) are fused to one another at all their crossing points.

3. The forming fabric according to claim 1, wherein the second yarns (5, 6) absorb less laser energy than the first yarns (7) or none at all.

4. The forming fabric according to claim 1, wherein the yarns (5, 6, 7) are otherwise not joined to one another.

5. The forming fabric according to claim 1, wherein the first yarns (7) contain an additive that imparts the ability to absorb laser light.

6. The forming fabric according to claim 5, wherein the additive is an NIR-active substance or an NIR-light-absorbing substance.

7. The forming fabric according to claim 5, wherein the additive is incorporated into the first yarns (7) and/or applied onto the surface of the first yarns (7) and/or introduced at the crossing points between the first and second yarns (5, 7).

8. The forming fabric according to claim 5, wherein the first yarns are bicomponent yarns, only one of the two components containing the additive.

9. The forming fabric according to claim 8, wherein the bicomponent yarns comprise a core and a casing surrounding it, the additive being contained only in the casing.

10. The forming fabric according to claim 1, wherein the planar structure comprises longitudinal and transverse yarns, and the first yarns extend in the longitudinal and/or the transverse direction.

11. The forming fabric according to claim 10, wherein the only some of the longitudinal and/or transverse yarns are first yarns.

12. The forming fabric according to claim 1, wherein the first yarns are part of the weave of the yarns in the planar structure.

13. The forming fabric according to claim 1, wherein the first yarns are distributed in the planar structure in a consistently regular pattern.

14. The forming fabric according to claim 1, wherein the first yarns are engaged in such a way that they do not reach as far as the paper side of the forming fabric.

15. The forming fabric according to claim 14, wherein the planar structure is embodied with multiple plies; and the first yarns are engaged only in an internally located layer and/or a roller-side layer.

16. The forming fabric according to claim 1, wherein the forming fabric (2) has a finite length with ends joinable via a seam, transverse yarns that are embodied as first yarns (7) being present in the region of both ends.

17. The forming fabric according to claim 16, wherein the forming fabric (2) comprises longitudinal yarns (5) that extend into the region of the ends and that in the region of the ends are fused to first yarns (7) that extend in the transverse direction.

18. A method for manufacturing a forming fabric (2) for use in the sheet-forming section of a paper machine, in which method a textile planar structure is manufactured from yarns (5, 6, 7) that intersect one another and are engaged into one another, and in which yarns (5, 7) are fused to one another at crossing points by heating to melting temperature,

wherein first and second yarns (5, 6, 7) are used in the manufacture of the planar structure, in which context the first yarns (7) can absorb laser energy; and first and second yarns (5, 7) are fused by means of laser energy at at least some of their crossing points (14).