EP4666878A1

EP4666878A1 - Apparatus for crimping a flat web, method for making a crimped web and continuous semi-finished web

Info

Publication number: EP4666878A1
Application number: EP25184004.7A
Authority: EP
Inventors: Marco Esposti; Nicola Baldanza
Original assignee: GD SpA
Current assignee: GD SpA
Priority date: 2024-06-19
Filing date: 2025-06-19
Publication date: 2025-12-24

Abstract

An apparatus (100) for crimping a flat web (N) comprising a first and a second roller (200, 300) respectively rotatable about a first and a second axis of rotation (X1, X2) parallel to each other. At least either said first or second roller (200, 300) is a crimping roller comprising a plurality of circumferential protrusions positioned in succession along the corresponding axis of rotation (X1, X2) and separated by respective grooves. The protrusions have respective peripheral edges lying in a peripheral surface, each protrusion has at least one respective recess defining on the web (N) an interruption (I) with reduced or absent crimping. The recesses of the protrusions are distributed along at least one interruption line (F) lying on said peripheral surface of the roller and having a helical extension about the axis of rotation (X1, X2) of the crimping roller (200, 300) with a helix angle equal to or greater than 30° so that said interruptions (I) are aligned with one another to define on the web (N) at least one interruption zone (Z) with reduced or absent crimping extending along an inclined direction, with respect to the longitudinal direction (L) of the web (N) and forming an angle of inclination, with respect to the longitudinal direction (L), equal to or greater than 30°.

Description

The present invention relates to an apparatus for crimping a flat web, e.g., a web usable in the tobacco or smoking article industry.
The present invention further relates to a method for making a crimped web.
The present invention further relates to a continuous semi-finished web, in particular usable in the tobacco or smoking article industry for making a segment forming part of a smoking article or aerosol generator.
As known, to date, there is an increasing need to appropriately crimp a web, e.g., a paper or tobacco web, intended to be inserted into smoking articles. Indeed, crimping facilitates, by virtue of the creation of folding lines on the web, the transverse "gathering" of the web to form a continuous rod intended to define a segment of a smoking article after cutting.
Furthermore, the use of a crimped web makes it possible to obtain segments, the draw resistance of which may be adjusted on the basis of the level of crimping of the web.
The crimping process commonly involves the use of two rotating cylindrical rollers between which the material web is pressed.
At least one of the rollers has grooves and ridges on its outer surface. On the latter, recesses are made forming zones in which crimping is reduced or absent. In such a situation, when the web is passed through the pair of rollers, the ridges present on the roller make crimping lines on the web separated by interruption zones (corresponding to the zones in which crimping of the roller is absent or reduced), in which the lines are absent or have a limited depth (meaning the distance between a ridge and a respective groove).
An example of these rollers is described in EP3609352B1 , in which a roller of the pair of rollers comprises a plurality of circumferential ridges and a plurality of interruption recesses made on the ridges and forming at least one interruption line on the web. The interruption line extends inclined with respect to the axis of the roller so that said interruptions define an interruption zone on the web, in which crimping is reduced or absent, inclined by 25° at most with respect to the longitudinal extension of the web.
Therefore, it is a technical task of the present invention to provide an apparatus for crimping a flat web, a method for making a crimped web, and a continuous semi-finished web which can provide an alternative to the known art.
In particular, it is the object of the present invention to provide an apparatus for making available a flat web which is structurally less complex than the known apparatuses.
It is a further object of the present invention to provide a method for making an efficient and reliable crimped web.
It is a further object of the present invention to provide a continuous semi-finished web which can be easily collected in a transverse manner. Furthermore, it is the object of the present invention to provide a continuous semi-finished web which makes it possible to obtain segments with an adequate draw resistance.
The specified technical task and the specified objects are substantially achieved by an apparatus for crimping a flat web, a method for making a crimped web, and a continuous semi-finished web comprising the technical features set forth in one or more of the appended claims. The dependent claims correspond to particular embodiments of the invention. In an aspect of the invention, the specified technical task and the specified objects are achieved by an apparatus for crimping a flat web extending along a longitudinal direction.
The apparatus comprises a first and a second roller rotatable respectively about a first and a second axis of rotation, respectively, which are parallel to each other.
At least either the first or second roller is a crimping roller comprising a plurality of circumferential protrusions positioned in succession along the corresponding axis of rotation and separated by respective grooves. The protrusions have respective peripheral edges lying on a peripheral surface (preferably cylindrical). Each protrusion has at least one respective recess defining an interruption with reduced or absent crimping on the web.
The recesses of the protrusions are distributed along at least one interruption line lying on said peripheral surface and having a helical extension about the axis of rotation of the crimping roller with a helix angle equal to or than greater 30° so that said interruptions are mutually aligned with one another to define at least one interruption zone on the web with reduced or absent crimping extending along an inclined direction, with respect to the longitudinal direction of the web and forming an angle of inclination, with respect to the longitudinal direction, equal to or greater than 30°.
Preferably, the helix angle is constant for the entire extension of the interruption line.
Preferably, the helix angle is equal to or less than 60°.
Preferably, the at least one interruption line extends substantially for the entire axial dimension of the crimping roller.
Alternatively, the at least one interruption line extends for a width less than the axial dimension of the crimping roller and, along the axial dimension of the crimping roller, there is only one interruption line (possibly replicated along the angular extension of the crimping roller, e.g., for a total of two or three angularly distributed interruption lines). This solution may be provided when only a central band of the crimping rollers is suitable to contact the web to be crimped.
Preferably, the apparatus comprises a plurality of interruption lines angularly distributed about the axis of rotation of the crimping roller and axially offset from one another.
Even more preferably, the interruption lines have, along the axis of rotation, dimensions which are identical or substantially identical to one another. Alternatively, the interruption lines have, along the axis of rotation, dimensions which are different from one another.
In an embodiment, a final end of one of the interruption lines is substantially aligned, in the circumferential direction, with a corresponding initial end of the next interruption line.
Preferably, in such embodiment, the interruption lines are arranged in succession from one axial end of the crimping roller to the opposite end, more preferably according to a helical trend.
Preferably, the interruption lines are angularly distributed with a constant pitch about the axis of rotation of the crimping roller.
Preferably, the interruption lines have the same helix angle.
In an embodiment, the recesses of the protrusions are distributed along at least two interruption lines lying on said peripheral surface and having respective helical extensions about the axis of rotation of the crimping roller with a helix angle equal to or greater than 30°, in which the at least two interruption lines have opposite winding directions about the axis of rotation. Preferably, the at least two interruption lines converge towards a vertex point, defining an "arrow" or "cusp" trend. Preferably, the vertex point is oriented in a direction consistent with the direction of rotation of the crimping roller.
It is understood that the apparatus as described above may be comprised in a machine for making a continuous rod, in particular in the tobacco industry. In particular, the machine comprises: feeding means of a continuous web; an apparatus for crimping a web according to the foregoing description; collecting means of the web positioned downstream of the apparatus for crimping the web to collect the web and impart to it a smaller transverse width than the transverse width of the web upstream of the crimping apparatus; feeding means of a wrapper intended to wrap the collected crimped web; a forming beam, which receives the collected (or at least partially collected) crimped web and the wrapper to wrap the wrapper about the collected crimped web, imparting a rod shape to it.
The specified technical task and specified objects are further achieved by a method for making a crimped web, in particular a web for making a segment of a smoking article or article for generating aerosols.
The method is preferably operated by means of an apparatus in accordance with the above.
The method comprises a step of preparing a web which extends along a longitudinal direction and a step of operating a crimping of the web generating on the web longitudinal crimping lines parallel to one another. The crimping lines are interrupted lines having respective interruptions with reduced or absent crimping. In other words, the crimping lines are weakening/deforming lines of the web; these crimping lines are longitudinal, i.e., extend in the longitudinal direction of extension of the web. Said crimping lines are mutually parallel and each have at least one interruption, i.e., a zone in which the material is less weakened or not weakened at all. Such interruptions of each crimping line define interruption zones with reduced or absent crimping on the web.
The interruptions are aligned with one another so as to form on the web a succession of interruption zones with reduced or absent crimping, each of which extending along a direction inclined with respect to the longitudinal direction of the web and forming an angle of inclination, with respect to the longitudinal direction, equal to or greater than 30° and preferably equal to or less than 60°.
Preferably, the interruption zones are arranged in succession along the longitudinal direction of the web.
Even more preferably, the interruption zones are arranged in an equidistant manner, and each interruption zone extends substantially across the entire width of the web.
Preferably, the interruption zones are arranged in succession along the longitudinal direction of the web and have respective extensions, measured along the width of the web, less than the width of the web. Preferably, the lengths of the interruption zones are substantially identical to one another.
The interruption zones are further offset from one another along the width of the web.
In this situation, a final end of one of the interruption zones is substantially aligned, in the circumferential direction, with a corresponding initial end of the next interruption zone.
Preferably, the interruption zones are arranged in succession from a lateral edge of the web to the opposite lateral edge, in particular according to an inclined trend, preferably rectilinear.
The specified task and specified objects are further achieved by a continuous semi-finished web, in particular for making a segment of a smoking article or article for generating aerosols.
The web comprises a plurality of longitudinal crimping lines mutually parallel to one another. The crimping lines are interrupted lines having respective interruptions with reduced or absent crimping. The interruptions are aligned with one another so as to form on the web a succession of interruption zones with reduced or absent crimping, each of which extending along a direction inclined with respect to the longitudinal direction of the web and forming an angle of inclination, with respect to the longitudinal direction, equal to or greater than 30° and preferably equal to or less than 60°.
Preferably, the interruption zones are positioned in succession along the longitudinal direction of the web, in particular, preferably in an equidistant manner, and each interruption zone extends substantially for the entire width of the web.
Preferably, the interruption zones are arranged in succession along the longitudinal direction of the web and have respective extensions, measured along the width of the web, less than the width of the web. Preferably, the lengths of the interruption zones are substantially identical to one another.
Furthermore, in an embodiment, the interruption zones are further offset from one another along the width of the web.
In an embodiment, the interruptions are aligned with one another so as to form on the web a succession of pairs of interruption zones, each of which extends along a direction inclined with respect to the longitudinal direction of the web and forming an angle of inclination, with respect to the longitudinal direction, equal to or greater than 30° and preferably equal to or less than 60°. In this embodiment, the interruption zones of each pair are preferably inclined in opposite directions and more preferably converging with one another at a vertex point. This solution defines an "arrow" or "cusp" trend of the interruptions on the continuous web.
In a different embodiment, each interruption line extends across the entire width of the web, continuously and without interruptions or skips.
In this situation, a final end of one of the interruption zones is substantially aligned, in the circumferential direction, with a corresponding initial end of the next interruption zone.
Preferably, the interruption zones are arranged in succession from a lateral edge of the web to the opposite lateral edge, preferably according to an inclined trend, preferably rectilinear.
Further features and advantages of the present invention will become clearer from the indicative, and therefore non-limiting, description of an embodiment of an apparatus for crimping a flat web, a method for making a crimped web, and a continuous semi-finished web.
This description will be presented below with reference to the appended drawings, provided for indicative purposes only and therefore non-limiting in which:

Figure 1 shows a perspective view of a crimping apparatus of the present invention;
Figure 2 shows a front view of the apparatus in figure 1;
Figure 3 shows a plan view of a crimped web of the present invention;
Figures 4 and 5 show respectively a perspective view and a front view of a further embodiment of the apparatus of the present invention.

With reference to the accompanying figures, reference numeral 100 indicates an apparatus for crimping a flat web "N" extending along a longitudinal direction "L".
According to a preferred embodiment, the web "N" is a web employable in the tobacco or smoking article or aerosol generator industry. In detail, the web "N" may be made of a cellulose-based material (e.g., paper) and/or tobacco and/or non-woven fabric. Furthermore, the web "N" may be a single layer web or formed by multiple layers superimposed on one another (said layers may be equal to one another or made of different materials).
The apparatus 100 comprises a first and a second roller 200, 300 rotatable respectively about a first and a second axis of rotation "X1", "X2", respectively, which are parallel to each other.
At least either the first or the second roller 200, 300 is a crimping roller and comprises a plurality of circumferential protrusions positioned in succession along the corresponding axis of rotation "X1", "X2" and separated by respective grooves.
The protrusions have respective peripheral edges lying on a peripheral, preferably cylindrical, surface.
Furthermore, each protrusion has at least one respective recess defining an interruption "I" with reduced or absent crimping on the web "N".
In use, once the web "N" is passed through a passage gap defined between the first and the second roller 200, 300, a crimping pattern is recognizable on the surface of the web "N". This crimping pattern has longitudinal crimping (i.e., material weakening) lines separated by some interruption zones "Z", in which the lines are interrupted (i.e., crimping is absent) or in which the lines are less pronounced (i.e., the distance between a groove and a protrusion is less than the distance between the groove and the protrusion along their remaining longitudinal extension).
In detail, the recesses of the protrusions are distributed along at least one interruption line "F" lying on said peripheral surface of the roller and having a helical extension about the axis of rotation "X1, X2" of the crimping roller "200, 300" with a helix angle equal to or than greater 30° so that said interruptions "I" are aligned with one another to define on the web "N" at least one interruption zone "Z" with reduced or absent crimping that extends along an inclined direction, with respect to the longitudinal direction "L" of the web "N" and forming an angle of inclination, with respect to the longitudinal direction "L", equal to or greater than 30°.
In other words, if the shell of the roller 200, 300 were arranged or "spread out" on a plane, it would be noted that the interruption line "F" is inclined by an angle equal to or greater than 30° with respect to the axis of rotation "X1", "X2" of the crimping roller 200, 300, i.e., with respect to a horizontal direction.
In the accompanying figures, the protrusions are shown only on an axial stretch of the crimping roller 200, 300 so as to better highlight the position of the interruption lines "F" on the surface of the crimping roller 200, 300. In the preferred embodiment, the helix angle is constant for the entire extension of the interruption line "F".
Preferably, the helix angle is equal to or less than 60°.
Preferably, the at least one interruption line "F" extends substantially for the entire axial dimension of the crimping roller 200, 300.
In other words, the at least one interruption line "F" extends from one edge of the crimping roller 200, 300 to the opposite edge.
In a preferred embodiment, the apparatus 100 comprises a plurality of interruption lines "F" angularly distributed about the axis of rotation "X1", "X2" of the crimping roller 200, 300 and axially offset from one another. Preferably, the interruption lines "F" are arranged one after another from one axial end of the crimping roller 200, 300 to the opposite end, according to a helical trend.
Preferably, the interruption lines "F" are angularly distributed with a constant pitch about the axis of rotation "X1", "X2" of the crimping roller 200, 300.
Preferably, the interruption lines "F" have the same helix angle, more preferably a helix angle "α" equal to or greater than 30° (this angle is shown in figure 3 by means of the corresponding angle on the crimped web).
In the illustrated embodiment, the interruption lines "F" have, along the axis of rotation "X1", "X2", dimensions which are identical or substantially identical to one another.
The term "substantially identical" means that their length has a variation comprised within the range of ± 10%.
Alternatively, the interruption lines "F" could have, along the axis of rotation "X1", "X2", dimensions which are different from one another.
In the preferred embodiment, the at least one interruption line "F" is replicated two or more times, preferably at regular intervals, along the angular extension of the crimping roller 200, 300.
Further, the present invention relates to a method for making a crimped web "N", in particular a web for making a segment for the tobacco or smoking article industry, preferably operated by means of an apparatus 100 according to the foregoing description.
The method comprises a step of preparing a web "N" which extends along a longitudinal direction "L" and a step of operating a crimping of the web "N" generating on the web "N" longitudinal crimping lines parallel to one another.
The crimping lines are interrupted lines having respective interruptions "I" with reduced or absent crimping.
The interruptions "I" are aligned with one another so as to form on the web "N" a succession of interruption zones "Z" with reduced or absent crimping, each of which extends along a direction inclined with respect to the longitudinal direction "L" of the web "N" and forming an angle of inclination "β", with respect to the longitudinal direction "L", equal to or greater than 30° and preferably equal to or less than 60°.
Preferably, the interruptions "I" have a length, in the longitudinal direction "L", comprised between 2 mm and 6 mm. Even more preferably, the interruptions "I" have a length, in the longitudinal direction "L", comprised between 3 mm and 5 mm.
As shown in the accompanying figures, the interruption zones "Z" are arranged in succession along the longitudinal direction "L" of the web "N". Preferably, the interruption zones "Z" are arranged along the longitudinal direction "L" in an equidistant manner, and each interruption zone "Z" extends substantially for the entire width "G" of the web "N".
Alternatively, the interruption zones "Z" are arranged in succession along the longitudinal direction "L" of the web "N" and have respective extensions, measured along the width "G" of the web "N", less than the width "G" of the web "N".
Preferably, the interruption zones "Z" have lengths which are substantially identical to one another, i.e., their length may vary, e.g., by ± 10%.
The interruption zones "Z" are further offset from one another along the width "G" of the web "N".
In this situation, a final end of one of the interruption zones "Z" is substantially aligned, in the circumferential direction "L", with a corresponding initial end of the next interruption zone "Z".
Alternatively, a final end of one of the interruption zones "Z" could be substantially aligned, along the longitudinal direction "L", with a corresponding central portion (comprised between the initial end and the final end) of the next interruption zone "Z".
Preferably, the interruption zones "Z" are reciprocally arranged in succession from a lateral edge of the web "N" to the opposite lateral edge, e.g., according to an inclined trend, preferably rectilinear.
Preferably, the interruption zones "Z" are offset from one another, in the longitudinal direction "L", by a constant pitch "p" preferably comprised between 6 and 30 mm and more preferably comprised between 8 and 16 mm. The constant pitch "p" is similarly observable on the crimping roller, as a circumferential pitch between adjacent interruption lines.
In the preferred embodiment, the interruption zones "Z" are replicated in succession at preferably regular intervals along the longitudinal direction "L" of the web "N".
It is a further object of the present invention a continuous semi-finished web "N", in particular for making a segment of a smoking article or for generating aerosols.
The web "N" comprises a plurality of longitudinal crimping lines mutually parallel to one another.
The crimping lines are interrupted lines having respective interruptions "I" with reduced or absent crimping.
The interruptions "I" are aligned with one another so as to form on the web "N" a succession of interruption zones "Z" with reduced or absent crimping, each of which extends along a direction inclined with respect to the longitudinal direction "L" of the web "N", forming an angle of inclination, with respect to the longitudinal direction "L", equal to or greater than 30° and preferably equal to or less than 60°.
In the preferred embodiment, the interruption zones "Z" are arranged in succession along the longitudinal direction "L" of the web "N".
Preferably, the interruption zones "Z" are arranged in an equidistant manner, and each interruption zone extends substantially for the entire width "G" of the web "N".
Alternatively, the interruption zones "Z" are arranged in succession along the longitudinal direction "L" of the web "N" and have respective extensions, measured along the width "G" of the web "N", less than the width "G" of the web "N".
In this situation, a final end of one of the interruption zones "Z" is substantially aligned, in the circumferential direction "L", with a corresponding initial end of the next interruption zone "Z".
Preferably, the lengths of the interruption zones "Z" are substantially identical to one another.
The interruption zones "Z" are further offset from one another along the width "G" of the web "N".
Preferably, the interruption zones "Z" are arranged in succession from a lateral edge of the web "N" to the opposite lateral edge, in particular according to an inclined trend, preferably rectilinear.
The present invention achieves the proposed objects by providing an alternative solution with respect to the prior art.
In particular, the present invention provides a reliable apparatus for crimping a web.
The present invention further provides a method for making an efficient and reliable crimped web.
The present invention further provides a continuous semi-finished web, in particular for making a segment of smoking articles or for generating aerosols, which can be easily collected and makes it possible to obtain segments with an adequate draw resistance.

Claims

An apparatus (100) for crimping a flat web (N) extending along a longitudinal direction (L), said apparatus (100) comprising a first and a second roller (200, 300) rotatable respectively about a first and a second axis of rotation (X1, X2) parallel to each other, and wherein at least one between said first and second roller (200, 300) is a crimping roller comprising a plurality of circumferential protrusions positioned in succession along the corresponding axis of rotation (X1, X2) and separated by respective grooves, said protrusions having respective peripheral edges lying in a peripheral surface, each protrusion having at least one respective recess defining on the web (N) an interruption (I) with reduced or absent crimping, wherein the recesses of the protrusions are distributed along at least one interruption line (F) lying on said peripheral surface and having a helical extension about the axis of rotation (X1, X2) of the crimping roller (200, 300) with a helix angle equal to or than greater 30° in such a way that said interruptions (I) are aligned with each other to define on the web (N) at least one interruption zone (Z) with reduced or absent crimping extending along an inclined direction, with respect to the longitudinal direction (L) of the web (N) and forming an angle of inclination, with respect to the longitudinal direction (L), equal to or greater than 30°.
The apparatus (100) according to claim 1, wherein said helix angle is constant for the entire extension of the interruption line (F).
The apparatus (100) according to claim 1 or 2, wherein said helix angle is equal to or less than 60°.
The apparatus (100) according to claim 1 or 2 or 3, wherein said at least one interruption line (F) extends substantially for the entire axial dimension of the crimping roller (200, 300).
The apparatus (100) according to claim 1 or 2 or 3, comprising a plurality of said interruption lines (F) angularly distributed about the axis of rotation (X1, X2) of the crimping roller (200, 300) and axially offset from each other.
The apparatus (100) according to claim 5, wherein said interruption lines (F) have, along the axis of rotation (X1, X2), dimensions which are identical or substantially identical.
The apparatus (100) according to any one of claims 4 to 6, wherein said interruption lines (F) are arranged one after another from one axial end of the crimping roller (200, 300) to the opposite end, according to a helical trend.
The apparatus (100) according to any of claims 4 to 7, wherein said interruption lines (F) have the same helix angle.
The apparatus (100) according to any one of claims 1 to 7, wherein the recesses of the protrusions are distributed along at least two interruption lines (F) lying on said peripheral surface and having respective helical extensions about the axis of rotation (X1, X2) of the crimping roller (200, 300) with a helix angle equal to or greater than 30°, said at least two interruption lines (F) having opposite winding directions about the axis of rotation (X1, X2) and preferably converging towards a vertex point preferably facing in a direction concordant with the direction of rotation of the crimping roller (200, 300).
The apparatus (100) according to any one of the preceding claims, wherein said at least one interruption line (F) is replicated two or more times, preferably at regular intervals, along the angular extension of the crimping roller (200, 300).
A method for making a crimped web (N), in particular a web for making a segment of the tobacco industry or smokers' articles, preferably using an apparatus according to any one of the preceding claims, comprising the steps of:
preparing a web (N) which extends along a longitudinal direction (L);

crimping the web (N) generating longitudinal crimping lines parallel to each other on the web, said crimping lines being interrupted lines having respective interruptions (I) with reduced or absent crimping;

wherein said interruptions (I) are aligned with each other in such a way as to form on the web (N) a succession of interruption zones (Z) with reduced or absent crimping, each of which extending along a direction inclined with respect to the longitudinal direction (L) of the web (N) and forming an angle of inclination, with respect to the longitudinal direction (L), equal to or greater than 30° and preferably equal to or less than 60°.
A continuous semi-finished web (N), in particular for making a segment of a smoking article or for generating aerosols, comprising a plurality of longitudinal crimping lines parallel to each other, said crimping lines being interrupted lines having respective interruptions (I) with reduced or absent crimping, and wherein said interruptions (I) are aligned with each other in such a way as to form on the web (N) a succession of interruption zones (Z) with reduced or absent crimping, each of which extending along a direction inclined with respect to the longitudinal direction (L) of the web (N) and forming an angle of inclination, with respect to the longitudinal direction (L), equal to or greater than 30° and preferably equal to or less than 60°.
The web (N) according to claim 12, wherein said interruption zones (Z) are positioned in succession along the longitudinal direction (L) of the web (N), preferably in an equidistant manner, and each interruption zone extends substantially for the entire width (G) of the web (N).
The web according to claim 12, wherein said interruption zones (Z) are positioned in succession along the longitudinal direction (L) of the web (N) and have respective extensions, measured along the width (G) of the web (N), which are less than the width (G) of the web (N) and preferably lengths which are substantially identical to each other, said interruption zones (Z) also being offset from each other along the width (G) of the web (N).
The web according to claim 14, wherein a final end of one of said interruption zones (Z) is substantially aligned, along said longitudinal direction (L), with a corresponding initial end of the next interruption zone (Z).
The web according to claim 14 or 15, wherein said interruption zones (Z) are arranged in succession from a lateral edge of the web (N) to the opposite lateral edge, preferably according to an inclined trend preferably rectilinear.
The web according to claim 12, wherein said interruptions (I) are aligned with each other in such a way as to form on the web (N) a succession of pairs of interruption zones (Z) with reduced or absent crimping, each interruption zone (Z) extending along a direction inclined with respect to the longitudinal direction (L) of the web (N) and forming an angle of inclination, with respect to the longitudinal direction (L), equal to or greater than 30° and preferably equal to or less than 60°, and wherein the interruption zones (Z) of each pair of interruption zones (Z) are inclined in opposite directions, preferably converging with each other at a vertex point.