EP2653299A1

EP2653299A1 - A single ply core manufacturing method, a ply and a single ply core

Info

Publication number: EP2653299A1
Application number: EP12002798.2A
Authority: EP
Inventors: René SIGWALT
Original assignee: SCA Tissue France SAS
Current assignee: Essity Operations France SAS
Priority date: 2012-04-20
Filing date: 2012-04-20
Publication date: 2013-10-23

Abstract

A method of manufacturing a single ply core (3) under the form of a cylindrical tube comprising the steps of:
- forming a cardboard sheet of substantially constant thickness (ep) and having two longitudinal edge portions defined relatively to a central portion;
- corrugating or embossing (S2, S2A) at least the central portion (9) of said sheet continuously along the length of said sheet so as to form a ply (5) having two flat longitudinal edge portions (6, 7) relatively to a corrugated or embossed central portion (9);
wherein said sheet is corrugated or embossed such as to create a central portion thickness being substantially equal or superior to the double thickness (2ep) of said sheet.

Description

FIELD OF THE INVENTION

An aspect of the invention relates to a core manufacturing method. Another aspect of the invention relates to a core, namely a cylindrical tube for roll product. Such a core finds a particular, though non exclusive, application in the tissue paper industry where a web of tissue paper is wound onto a core made of a cardboard cylindrical tube, for the purpose of, as an example, manufacturing paper towel or toilet tissue rolls.

BACKGROUND OF THE INVENTION

FIG. 1 is a perspective view schematically illustrating a paper towel roll 1. The paper towel roll 1 comprises paper towel or toilet tissue 2 wound onto a core 3. The paper towel or toilet tissue 2 may be provided with pre-cuttings transversal lines 4 for ease of detaching individual sheets of paper towel or toilet tissue.
FIG. 2 is a perspective view schematically showing a core 3 under the form of a cylindrical tube. Usually, tubes are produced by winding many plies of cardboard (also called paperboard) about an axis in such a manner that a cylindrical body wall is formed having a plurality of layers making up the radial thickness of the wall. Alternatively, a spirally wound tube core may be formed by spirally winding one ply 5 of cardboard onto a circular cylindrical mandrel at a given spiral wind angle. The ply is wound such that one external edge 6 is overlapped upon the internal edge 7 and adhered together in this overlapping joint zone 8 to build up the cylindrical tube. The overlapping zone 8 has a typical width of 10 mm.
FIG. 3 is a detailed cross-section view schematically showing a core under the form of a cylindrical tube (of radius R) and illustrating the problematic of manufacturing a single ply core tube. The cylindrical tube is thicker in the overlapping joint zone 8. The helical seam has at least a thickness that is double 2ep compared to the thickness ep of the cardboard ply. When the cardboard ply is stiff (due to its low intrinsic elasticity), the stretch of the ply 5 at the overlapping joint zone 8 may result in either a break, or a fragmentation, or a crumpled aspect of the ply.

There is a need to prevent such effects when manufacturing a tube core made of a single ply.
The document US 5,573,638 describes a continuous paperboard sheet adapted to be slit longitudinally into a plurality of continuous paperboard plies, and to improved paperboard plies for forming single layer, paperboard tubes. The paperboard sheet has a substantially constant width and includes a plurality of thick longitudinal sections and a plurality of thin longitudinal sections. The thick and thin sections are arranged in alternating relation across the width of the paperboard sheet. The sheet is slit longitudinally along the thin sections to provide a plurality of tube-forming paperboard plies.
However, the manufacturing process and the tube obtained are not satisfactory because, in practice, the edges of plies have been treated to decrease their thickness for the formation of cardboard tubes, by a grinding or compressing process. During this process the edges of the cardboard ply are ground with an abrasive wheel, or compressed between compressing rollers to decrease its thickness. As a result, the intrinsic functional properties of the grounded edges of the cardboard ply are affected. Further, such a grinding or compressing process may not give satisfactorily result in term of decreasing the thickness when the tube is made of cardboard sheet.

SUMMARY OF THE INVENTION

It is an object of the invention to propose a core made of a single ply that overcomes the above mentioned drawback, and in particular maintain the intrinsic functional properties of the edge portion of the ply during the manufacturing process.
The invention proposes to corrugate / emboss the thick central portion of the ply before winding said single ply of cardboard about a longitudinal axis in such a manner that a cylindrical body wall is formed having a single layer making up the radial thickness of the wall of the core.
According to one aspect, there is provided a method of manufacturing a single ply core under the form of a cylindrical tube comprising the steps of:

forming a cardboard sheet of substantially constant thickness and having two longitudinal edge portions defined relatively to a central portion;
corrugating or embossing at least the central portion of said sheet continuously along the length of said sheet so as to form a ply having two flat longitudinal edge portions relatively to a corrugated or embossed central portion;

The corrugation/embossing step may further include:

corrugating or embossing the whole width of said sheet continuously along the length of said sheet so as to form a ply being totally corrugated or embossed;
compressing only the longitudinal edge portions relatively to the corrugated or embossed central portion continuously along the length of said sheet so as to form a ply having two flat longitudinal edge portions relatively to the corrugated or embossed central portion.

The single ply core manufacturing method may further comprises moistening said sheet before the corrugation/embossing step, and drying the ply after the corrugation/embossing step.
One side of the central portion may be flush with an external edge of the two flat longitudinal edge portions. Alternatively, a longitudinal axis of each of the two flat longitudinal edge portions may be symmetrically positioned at half the thickness of said ply. Alternatively, a longitudinal axis of each of the two flat longitudinal edge portions may be asymmetrically positioned, one external edges being flush with one side of the central portion, the other external edges being flush with the other side of the central portion.
The single ply core manufacturing method may further comprise spirally winding said single ply about a longitudinal axis in such a manner that a cylindrical body wall is formed having a single layer making up the radial thickness of the wall of the core.
Said single ply may be spirally wound about a longitudinal axis by being engaged by a winding belt cooperating with a mandrel, and advancing said single ply along the mandrel in a screw fashion.
The single ply core manufacturing method may further comprise longitudinally forming said single ply about a longitudinal axis in such a manner that a cylindrical body wall is formed having a single layer making up the radial thickness of the wall of the core.
Said single ply may be longitudinally formed about a longitudinal axis by being engaged and gradually oriented by a train of press rollers cooperating with a mandrel from a flat sheet to a tube conforming the shape of the mandrel.
The single ply core manufacturing method may further comprise applying glue on an external flat longitudinal edge portion of said ply as a full line or as a dotted line, and applying the external flat longitudinal edge portion so as to adhere onto the internal flat longitudinal edge portion of said ply.
According to another aspect, there is provided a ply having two flat longitudinal edge portions relatively to a corrugated or embossed central portion arranged to be used for manufacturing a single ply core under the form of a cylindrical tube.
The corrugated central portion may have an undulation or wave-like profile. Alternatively, the corrugated central portion may have an embossing pattern profile having a shape chosen among the group of embossments comprising pyramids or cones, truncated pyramids or truncated cones, half-spheres, or truncated half-spheres.
According to a further aspect, there is provided a single ply core formed from a single ply having two flat longitudinal edge portions relatively to a corrugated or embossed central portion.
The roll of sheet material may be a web of tissue paper roll.
The invention is advantageous for cardboard having an intrinsic stretch at break that is below 4%, more particularly below 3%, for example 2.5%. The stretch at break is measured according to the French standard NF Q 03-004 (« Essais des papiers et cartons - Méthode de détermination de la résistance à la rupture par traction et de la longueur de rupture par traction » - Testing of paper and cardboard - Method of determination of tensile strength and stretch at break).
With the invention, it is possible to avoid at least greatly reduce the deformation of the sheet material when wound as a roll of sheet material.
The invention further enables minimizing the cost of producing the core, e.g. by using less glue compared to core made of multiple plies glued together to form the thickness of the core wall.
Other advantages will become apparent from the hereinafter description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of examples and not limited to the accompanying drawings, in which like references indicate similar elements:

FIG. 1 is a perspective view schematically illustrating a paper towel roll;
FIGS. 2 and 3 are a perspective view and a detailed cross-section view schematically showing a core under the form of a cylindrical tube and illustrating the problematic of manufacturing a single ply core tube tube of the state of the art, respectively;
FIGS. 4, 5 and 6 are cross-section views of a ply according to a first, second and third embodiment of the invention adapted to form a single ply core tube, respectively;
FIG. 7 illustrates the overlapping joint area of the single ply core tube according to the first, second and third embodiment of the invention of FIGS. 4 to 6;
FIG. 8 is a top or bottom view of a ply according to the first, second and third embodiment of the invention adapted to form a single ply core tube;
FIGS. 9 and 10 schematically illustrate two alternative methods for manufacturing single ply core tubes from a cardboard sheet;
FIG. 11 schematically illustrates a first machine for winding and cutting single ply core tube;
FIG. 12 schematically illustrates the single ply core tube obtained with the machine depicted in FIG. 11;
FIG. 13 schematically illustrates a second machine for forming and cutting single ply core tube;
FIG. 14 schematically illustrates the single ply core tube obtained with the machine depicted in FIG. 13; and
FIG. 15 schematically illustrates from top to bottom pyramids or cones, truncated pyramids or truncated cones, half-sphere, and truncated half-sphere embossing pattern profiles, respectively.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a cross-section view of a ply 5 according to a first embodiment. The ply 5 is adapted to form a single ply core tube. A sheet forming one ply 5 comprising two flat longitudinal edge portions 6A/6B (i.e. external edge 6A and internal edge 6B at one end) and 7A/7B (i.e. external edge 7A and internal edge 7B at the other end) relatively to a corrugated or embossed central portion 9 is manufactured. The central portion 9 of the ply 5 is corrugated or embossed such as to create a central portion thickness substantially equal or superior to the double thickness 2ep of the sheet (i.e. the thickness of the sheet is ep). In this embodiment, one side of the central portion 9 is flush with the external edges 6A, 7A of the two flat longitudinal edge portions.
FIG. 5 is a cross-section view of a ply according to a second embodiment. The second embodiment only differs from the first embodiment in that the sheet is corrugated or embossed in such a way that the two flat longitudinal edge portions (more precisely the longitudinal axis LX of said edge portions) are symmetrically positioned at half the thickness of the resulting ply 5.
FIG. 6 is a cross-section view of a ply according to a third embodiment. The third embodiment only differs from the first embodiment in that the sheet is corrugated or embossed in such a way that the two flat longitudinal edge portions (more precisely the longitudinal axis LX1/LX2 of said edge portions) are asymmetrically positioned, one external edges 6A/6B being flush with one side of the central portion 9, while the other external edges 7A/7B being flush with the other side of the central portion 9. This enables a natural overlapping of the adjacent edges during the winding process.
FIG. 8 illustrates a top or bottom view of a ply 5 according to the first, second or third embodiment adapted to form a single ply core tube. The ply 5 comprises two flat longitudinal edge portions 6, 7 on each side of the corrugated or embossed central portion 9.
FIG. 7 is a cross-section view of the ply according to the first, second and third embodiment (FIGS. 4 to 5) illustrating the overlapping joint area 8 of the single ply core tube.
When the ply 5 is formed and two adjacent edges are superposed and adhered together (e.g. by means of glue 10) over their respective adjacent flat longitudinal edge portions, for example 6B, 7A, the thickness is such that the overlapping of the two flat longitudinal edge portions results in an overall thickness substantially equal or superior to the double thickness 2ep of the sheet. As explained hereinafter with respect to FIGS. 9, 10, 11 or 13, the ply is formed over a mandrel (i.e. a metallic cylinder) in such a manner that, in the overlapping joint area 8, the respective adjacent flat longitudinal edge portions 6B, 7A are deformed to be flush with the external sides of the central portion 9 whatever the embodiment. For the third embodiment, there is no deformation because the adjacent edges are naturally positioned in complementary fashion relatively to each other.
The resulting stretch/deformation of the ply 5 at the overlapping joint zone 8 is either null or sufficiently low so that neither break, nor fragmentation, nor crumpled aspect of the ply is observed. Further, the whole process enables avoiding compressing or grinding the ply (in the Z direction), resulting in that the longitudinal edge portions stay untouched. Thus, this enables maintaining the intrinsic functional features of the longitudinal edge portions.
FIG. 9 schematically illustrates a first alternative method for manufacturing single ply core tubes from a cardboard sheet.
The cardboard sheet has a grammage (weight per unit area) of 80 to 1000 g/m², more particularly 160 to 500 g/m². The cardboard sheet has an intrinsic stretch at break that is below 4%, more particularly below 3%, for example 2.5% (measured according to the French standard NF Q 03-004 hereinbefore cited).
In a first step S1, the flat sheet forming the ply 5 of cardboard may be passed through a moistening unit 11. The ply 5 is moistened in a moistening bath 12. A moistened ply is obtained. This enables loosing the fiber bonds forming the ply 5 of cardboard. Moistening the ply is an optional step that eases the subsequent corrugating/embossing process.
In a second step S2A/S2B, the ply 5 is passed through a corrugating/embossing unit 13 and a compressing unit 16. The corrugating/embossing unit 13 comprises a pair of opposed cylinders 14, 15 (steel/steel or steel/rubber cylinders) comprising mating/combined protuberances that deform the ply 5. The ply 5 is corrugated/embossed on the overall surface. Then, the longitudinal edge portions 6, 7 are flattened by the respective press 17, 18 of the compressing unit 16. During this flattening step, the thickness which was 2ep as a result of the corrugation/embossing step is returned to the original thickness ep. Thus, there is not any compression or grinding affecting the original thickness of the ply. In other word, the compression only affects the shape (from the corrugated/embossed shape to a flat shape) and not the structure/properties of the edge portions. The sheet is centered relatively to the respective press 17, 18 before being flattened. The vertical positions of the respective press 17, 18 determine the vertical positions of the flat longitudinal edge portions relatively to the central portion to be obtained so as to define the ply according to the first, second or third embodiment (FIGS. 4 to 6).
In a third step S3, the ply 5 comprising the corrugated/embossed central portion 9 and flat longitudinal edge portions 6, 7 is dried in a drying unit 19. This step is performed when a moistened ply is involved in the process, e.g. because of the optional first moistening step S1. The drying step is performed until an acceptable moisture content is reached (e.g. around 10% percent by weight).
As a result, the sheet forming one ply 5 comprising two flat longitudinal edge portions relatively to a corrugated or embossed central portion is manufactured.
In a fourth step S4, the tube core 3 is spirally wound in a mandrel winding unit 20 and then cut to the desired length in a cutting unit 30. This step will be explained in details in relation with FIGS. 11 and 13. Alternatively, the overall process may not be continuous as depicted in FIGS. 9 and 10, meaning that the sheet forming one ply 5 may be manufactured and then temporarily stored as roll (this is not shown) before being wound as tube cores in the mandrel winding unit.
FIG. 10 schematically illustrates a second alternative method for manufacturing single ply core tubes from a cardboard sheet. The second alternative method differs from the first alternative method only with respect to the second step S2. The first moistening step S1, third drying step S3, and fourth winding/cutting step S4 are identical to the one of the first embodiment and therefore will not be further described.
In the second step S2, the ply 5 is passed through a corrugating/embossing unit 13. The corrugating/embossing unit 13 comprises a pair of opposed cylinders 14, 15 (steel/steel or steel/rubber cylinders) comprising mating/combined protuberances that deform the ply 5. The sheet is centered relatively to the cylinders before being corrugated or embossed. The ply 5 is corrugated/embossed only in the central portion 9. The longitudinal edge portions 6, 7 are unaffected by the corrugating or embossing process and are, therefore, flat. As a result, the sheet forming one ply 5 comprising two flat longitudinal edge portions relatively to a corrugated or embossed central portion is manufactured.
FIG. 11 schematically illustrates a first machine for winding and cutting single ply core tube. The tube core 3 is spirally wound in the winding unit 20 and then cut to the desired length in the cutting unit 30.
The winding unit 20 comprises a gluing depositing nozzle 21, a mandrel 22 and a winding belt 23.
The gluing depositing nozzle 21 is positioned upstream the mandrel 22 and the winding belt 23. The gluing depositing nozzle 21 is applying glue 10 on the external flat longitudinal edge portion 7A of the single ply 5 as a full line (as shown) or as a dotted line (not shown).
The single ply core tube is formed by spirally winding the single ply 5 onto the mandrel 22. The mandrel 22 may be an elongate metallic cylinder. The winding angle may be 45°, but any other angle may be acceptable.
The winding belt 23 engages the single ply 5 and forms a single ply core tube 3. As a result of this engagement, the single ply core tube advances along the mandrel in a screw fashion. The external flat longitudinal edge portion 7A is applied so as to adhere onto the internal flat longitudinal edge portion 6B of the single ply 5 (see FIG. 7 for details). This results in forming a well defined single ply core tube 3 downstream the winding belt 23.
The cutting unit 30 is positioned downstream the winding unit 20. The cutting unit 30 cuts single ply core tube 3 of the desired length L. The cutting unit 30 may comprise a circular saw, a blade, a crushing device, etc... FIG. 12 schematically illustrates the single ply core tube 3 that is obtained with the machine depicted in FIG. 11.
FIG. 13 schematically illustrates a second machine for forming and cutting single ply core tube. The tube core 3 is longitudinally formed in the formation unit 20A and then cut to the desired length in the cutting unit 30.
The formation unit 20A of the second machine comprises a gluing depositing nozzle 21, a mandrel 22 and a train of press rollers 24, 25.
The gluing depositing nozzle 21 may be positioned upstream the mandrel 22 and the train of press rollers 24. The gluing depositing nozzle 21 is applying glue 10 on the external flat longitudinal edge portion 7A of the single ply 5 as a full line (as shown) or as a dotted line (not shown).
The single ply core tube is formed by longitudinally wrapping the single ply 5 onto the mandrel 22 by means of the train of press rollers 24. The mandrel 22 may be an elongate metallic cylinder.
The train of press rollers 24, 25 comprises a plurality of press rollers pairs (on each side of the ply, only one side press rollers being depicted in FIG. 13 for a mere clarity reason) having different orientations. For example, six to eight pairs of motorized press roller may be necessary. The pairs of press rollers engage the single ply 5 and gradually orient the ply from a flat sheet to a tube conforming the shape of the mandrel 22. The plurality of press rollers pairs enables forming a single ply core tube 3. As a result of this engagement, the single ply core tube advances longitudinally along the mandrel. The external flat longitudinal edge portion 7A is applied by means of a terminal press roller 25 so as to adhere onto the internal flat longitudinal edge portion 6B of the single ply 5 (see FIG. 7 for details). This results in forming a well defined single ply core tube 3 downstream the train of press rollers 24, 25.
The cutting unit 30 is positioned downstream the formation unit 20A. The cutting unit 30 cuts single ply core tube 3 of the desired length L. The cutting unit 30 may comprise a circular saw, a blade, a crushing device, etc... FIG. 14 schematically illustrates the single ply core tube 3 that is obtained with the machine depicted in FIG. 13.
The drawings and their descriptions hereinbefore illustrate rather than limit the invention.
It is to be noted that the central portion 9 can be corrugated or embossed longitudinally, transversally or even according to another direction provided that the central portion thickness is substantially equal or superior to the double thickness 2ep of the sheet. The corrugation creates an undulation or wave-like profile on the sheet. Nevertheless, a similar technical effect can be obtained by embossing, namely creating an embossing pattern profile on the sheet. The embossing pattern profile may have the shape of pyramids or cones 9A, truncated pyramids or truncated cones 9B, half-spheres 9C, truncated half-spheres 9D, etc... some examples being schematically illustrated in FIG. 15. This embossing pattern may be advantageous in order to limit the radial compression effect when the single ply is wound onto the mandrel by the winding belt or wrapped by the train of press rollers. The above mentioned embossing pattern profiles create walls within the thickness of the cardboard that have a better resistance to said radially inward oriented compression than the undulation/corrugation. This result in the ply not being flattened or at least only in a limited fashion during the winding process (through the action of the winding belt or the train of press rollers).
The numbers, densities, angles and positions of the undulation/corrugation/embossments in the depicted embodiments are non limitative examples. The skilled person will readily recognize that these numbers, densities, angles and positions may be changed if desired or deemed necessary with respect to the required radial compression resistance, the cardboard ply intrinsic elasticity, or to adjust the width of the ply, etc....
The core of the invention may be used in winding sheet material as "rolls of sheet material". This has a large meaning encompassing, as examples, the rolls of paper towels, toilet tissues, plastic sheets or the like, metal sheets (e.g. aluminum) sheets or the like, food preservation bags, wraps, etc... The sheet material may be rolled as a continuous non-perforated sheet or pre-perforated sheets. The "roll of sheet material" may be used for residential or commercial applications. A dispenser assembly may dispense the sheets of material from the roll of sheet material either manually (a user may pull the sheet of material with a hand) or motor assisted (a motor may assist in automatically dispensing the sheet of material on wish). Such dispenser assembly and mechanisms are not germane to the present invention and will not be further described in details.
Any reference sign in a claim should not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements than those listed in a claim. The word "a" or "an" or "at least one" preceding an element does not exclude the presence of a plurality of such element.

Claims

A method of manufacturing a single ply core (3) under the form of a cylindrical tube comprising the steps of:
- forming a cardboard sheet of substantially constant thickness (ep) and having two longitudinal edge portions defined relatively to a central portion;

- corrugating or embossing (S2, S2A) at least the central portion (9) of said sheet continuously along a length of said sheet so as to form a ply (5) having two flat longitudinal edge portions (6, 7) relatively to a corrugated or embossed central portion (9);
wherein said sheet is corrugated or embossed such as to create a central portion thickness being substantially equal or superior to the double thickness (2ep) of said sheet.
The single ply core manufacturing method of claim 1, wherein:
- the corrugation/embossing step (S2A) further includes corrugating or embossing a whole width of said sheet continuously along the length of said sheet so as to form a ply (5) being totally corrugated or embossed;

- compressing (S2B) only the longitudinal edge portions relatively to the corrugated or embossed central portion continuously along the length of said sheet so as to form a ply (5) having two flat longitudinal edge portions (6, 7) relatively to the corrugated or embossed central portion (9).
The single ply core manufacturing method of claim 1 or 2, further comprising moistening (S1) said sheet before the corrugation/embossing step (S2, S2A), and drying (S3) the ply (5) after the corrugation/embossing step.
The single ply core manufacturing method according to anyone of the claims 1 to 3, wherein one side of the central portion (9) is flush with an external edge (6A, 7A) of the two flat longitudinal edge portions.
The single ply core manufacturing method according to anyone of the claims 1 to 3, wherein a longitudinal axis (LX) of each of the two flat longitudinal edge portions is symmetrically positioned at half the thickness of said ply (5).
The single ply core manufacturing method according to anyone of the claims 1 to 3, wherein a longitudinal axis (LX1, LX2) of each of the two flat longitudinal edge portions is asymmetrically positioned, one external edges (6A/6B) being flush with one side of the central portion (9), the other external edges (7A/7B) being flush with the other side of the central portion (9).
The single ply core manufacturing method according to anyone of the claims 1 to 6, further comprising spirally winding (S4) said single ply (5) about a longitudinal axis in such a manner that a cylindrical body wall is formed having a single layer making up the radial thickness of the wall of the core (3).
The single ply core manufacturing method of claim 7, wherein said single ply is spirally wound about a longitudinal axis by being engaged by a winding belt (23) cooperating with a mandrel (22), and advancing said single ply (5) along the mandrel (22) in a screw fashion.
The single ply core manufacturing method according to anyone of the claims 1 to 6, further comprising longitudinally forming (S4) said single ply (5) about a longitudinal axis in such a manner that a cylindrical body wall is formed having a single layer making up the radial thickness of the wall of the core (3).
The single ply core manufacturing method of claim 9, wherein said single ply is longitudinally formed about a longitudinal axis by being engaged and gradually oriented by a train of press rollers (24, 25) cooperating with a mandrel (22) from a flat sheet to a tube conforming the shape of the mandrel (22).
The single ply core manufacturing method according to anyone of the claims 7 to 10, further comprising applying glue (10) on an external flat longitudinal edge portion (7A) of said ply (5) as a full line or as a dotted line, and applying the external flat longitudinal edge portion (7A) so as to adhere onto the internal flat longitudinal edge portion (6B) of said ply (5).
A ply (5) of a cardboard sheet having two flat longitudinal edge portions (6, 7) relatively to a corrugated or embossed central portion (9) arranged to be used for manufacturing a single ply core (3) under the form of a cylindrical tube.
The ply of claim 12, wherein the corrugated central portion (9) has an undulation or wave-like profile.
The ply of claim 12, wherein the corrugated central portion (9) has an embossing pattern profile having a shape chosen among the group of embossments comprising pyramids or cones (9A), truncated pyramids or truncated cones (9B), half-spheres (9C), or truncated half-spheres (9D).
The ply according to anyone of the claims 12 to 14, wherein one side of the central portion (9) is flush with an external edge (7A, 7B) of the two flat longitudinal edge portions.
The ply according to anyone of the claims 12 to 14, wherein a longitudinal axis (LX1, LX2) of each of the two flat longitudinal edge portions is symmetrically positioned at half the thickness of said ply (5).
The ply according to anyone of the claims 12 to 14, wherein a longitudinal axis (LX) of each of the two flat longitudinal edge portions is asymmetrically positioned, one external edges (6A/6B) being flush with one side of the central portion (9), the other external edges (7A/7B) being flush with the other side of the central portion (9).
The ply according to anyone of the claims 12 to 17, wherein the cardboard sheet has a grammage of 80 to 1000 g/m², and an intrinsic stretch at break below 4 % measured according to the French standard NF Q 03-004.
The ply according to the preceding claim, wherein the cardboard sheet has a grammage of 160 to 500 g/m², and an intrinsic stretch at break below 3% measured according to the French standard NF Q 03-004.
A single ply core (3) formed from a single ply (5) having two flat longitudinal edge portions (6, 7) relatively to a corrugated or embossed central portion (9) according to anyone of the claims 12 to 19.
A roll of sheet material (1) comprising sheet material (2) wound onto a single ply core (3) having two flat longitudinal edge portions (6, 7) relatively to a corrugated or embossed central portion (9) according to anyone of the claims 12 to 19.
The roll of sheet material of claim 21, wherein the sheet material is a web of tissue paper.