WO2013156912A1

WO2013156912A1 - Blank for making round boxes

Info

Publication number: WO2013156912A1
Application number: PCT/IB2013/052924
Authority: WO
Inventors: Mauro Mazzinghi
Original assignee: Emmeci S.P.A.
Priority date: 2012-04-17
Filing date: 2013-04-12
Publication date: 2013-10-24
Also published as: ITBO20120212A1

Abstract

A blank for making round boxes comprises at least one panel (2, 2') which can be folded along a curved trajectory so as to define a side wall (P) of a round box and has the form of a parallelogram with a first edge and second edge (3, 4; 3', 4') situated opposite each other and a third edge and fourth edge (5, 6; 5', 6') also situated opposite to each other and transverse to the first edge and second edge (3, 4; 3', 4'). The first edge and second edge (3, 4; 3', 4') have respective outer profiles which complement each other and are formed so that, following folding along said curved trajectory, the first edge and second edge (3, 4; 3', 4') can be brought into end-to-end contact with each other along a helical joining line (G).

Description

DESCRIPTION

BLANK FOR MAKING ROUND BOXES

Technical field

The present invention relates to a blank for making round boxes, in particular for forming the round side wall of said type of boxes. Background art

"Round box" is understood as meaning, in general, a box with a rounded side wall.

More particularly, "round box" is understood as meaning a box with a cylindrical side wall which does not have sharp corners and has a given axial height.

It is currently known to form the aforementioned round walls using sections of tubes which are suitably cut to the desired length corresponding to the axial height of the box.

These tubes, which have a constant length and weight, are in turn formed by winding two or more continuous paper strips, which are staggered relative to each other, around a mounting spindle with a spiral movement.

This operation is performed using special machines, called "tube-forming machines" or "spiralling machines" which operate continuously unwinding the strips from respective reels.

An example of such a technical solution is provided by patent document

US3982686A.

The spiralling machines, therefore, owing to their particular nature, operate continuously and therefore form a continuous tube with a diameter and thickness which are adjustable as required (typically from a diameter of a few millimetres and thickness of a few tenths of a millimetre to diameters of tens of centimetres with thicknesses of several millimetres). The continuous tube output from the spiralling machine is divided into sections of predetermined length depending on the subsequent applications, which may consist for example in immediate use for manufacturing boxes (and in this case the cutting length will be in relation to the desired length of the box or multiples thereof) or transportation on lorries to users who then further divide up the tubes into shorter sections (and in this case the cutting length will be compatible with transportation in the loading compartment of a lorry).

In any case, the manufacture of round boxes from sectioned tubes involves problems and high transportation and storage costs because of the large dimensions compared to the small amounts of material. In fact, the transportation or storage of the tubes results in occupation of very large volumes albeit with small-size objects.

Moreover, since the strips must be wound around the mounting spindle, the currently known tube-manufacturing machines cannot be easily adapted to a change in format (in particular to a variation in the diameter of the tube) since this would requires stoppage of the machine, replacement of the spindle with a spindle of different diameter and replacement of the corresponding specific machining components for a predetermined diameter, together with restarting of the machine. There exist therefore evident logistical and production-related difficulties associated with these format-changing operations, in particular where the manufacture of small batches is involved.

Furthermore, an increase in the formats required results in an increase in the number of spindles which must be readily available. This means that, on the one hand, it is difficult to ensure an optimum production flexibility based on a wide range of formats, while on the other hand it is technically impossible to choose, as required, the format which is dependent on the diameters alone of the spindles equipping the machine.

These limitations and consequent disadvantages associated with the tube- forming spiralling machines and therefore the tubes produced have a negative impact on the systems for manufacturing round boxes which are dependent on the supply of tubes from which the sections for making the boxes are to be obtained.

In particular, the logistical difficulties involved in the storage and handling of tubes of such a size and the lack of versatility of the existing formats means that it is preferable to find an alternative solution to that of manufacturing round boxes using tubes.

Other technologies aimed at producing cylindrical paper walls, for example in the sector of cigarette-manufacturing machines, are known. In this case also continuous processes are used, these being based on a continuous strip of single-layer paper which is gradually closed up by winding it in the form of a cylinder having an axis parallel to the length of the strip with joining together of the two side edges of the strip along a generatrix of the cylinder itself.

Clearly these technologies do not apply to the box-manufacturing sector. Moreover, even if these technologies were to be applied to the box- manufacturing sector in order to manufacture continuous tubes with round (i.e. cylindrical) walls, the problems already mentioned with regard to spirally wound tubes would inevitably arise.

The problems arising from the use of continuous tubes which are then divided up could be partly overcome by making boxes with round walls from rectangular blanks, winding them in the form of a cylinder having an axis parallel to the length of the strip with joining together of the two side edges of the strip along a generatrix of the cylinder itself.

However, in this case too there would be a number of drawbacks.

In fact, if on the one hand folding a rectangular blank in order to form the cylindrical wall of a round box would overcome the logistical and industrial difficulties associated with use based on continuous tubes which are then divided, on the other hand the problems associated with a change in format would not be solved and further problems would arise.

These rectangular blanks would be wound onto a spindle until the opposite edges of the blank mate with each other and these edges would then be sealed in order to obtain the side wall of the box. However, joining together of the said opposite edges would be seriously affected by problems of lack of continuity in the curvature along the joint of the edges. The result is that the tubular article thus obtained would assume a pear-like and not a circular cross-section as is instead desired. Moreover, the need to use a spindle would pose the same problems associated with the change in format mentioned above.

An example of such a typology of solutions is provided by patent document GB2298855A. Disclosure of the invention

In light of the above explanation, the object of the present invention is to provide a blank for making round boxes which overcomes the drawbacks of the prior art mentioned above.

In particular, the object of the present invention is to provide a blank for making round boxes which can be used to obtain boxes having a cylindrical cross-section with large dimensional and form-related tolerances, resulting in simplification of the logistical management (in particular as regards the storage of the parts to be used for making the boxes).

Another object of the present invention is to provide a blank for making round boxes which is suitable for use on production lines which have a high degree of manufacturing flexibility, in particular with regard to the change in format and in particular the production of small batches.

Said objects are fully achieved by the blank according to the present invention which is characterized by the contents of the claims indicated below.

In particular, the blank according to the invention is defined by a panel made of flexible material, preferably paper material, which can be folded along a curved trajectory so as to define a side wall of a round box and having a first edge and second edge situated opposite each other and a third edge and fourth edge also situated opposite each other and transverse to the first edge and second edge. The first edge and second edge have respective outer profiles which complement each other so as to make end-to-end contact with each other defining a joining line.

The blank according to the invention is characterized in that the outer profiles of the first edge and second edge are formed so that the joining line defined by them is at least partly helical.

Preferably, at least the third edge, and even more preferably also the fourth edge, is straight so as to assume, following folding along said curved trajectory, the configuration of a closed line lying in a plane, preferably a circumference.

In the blank according to the present invention, the helical form of the joining line is obtained by providing, at least a first section of each first and second edge, with a straight form inclined relative to the third edge.

Preferably, in an embodiment of the blank according to the invention, each first edge and second edge also has at least one second straight section oriented perpendicularly with respect to the third edge so that the joining line has at least one corresponding straight section arranged along a generatrix of the side wall of the round box. These second sections have the function of providing a locating abutment when the first edge and second edge are moved towards each other, preventing undesirable sliding along the first inclined sections. Moreover, the cooperation between the first section and second section of each first edge and second edge defines a profile which extends along a broken line, and these broken-line profiles complement each other and define in a specific manner the correct contact position of the first edge and second edge.

In particular, preferably, each one of said first and second edge is (completely) defined by said first straight section inclined (of a predetermined first angle) with respect to the third edge and said second straight section oriented perpendicularly with respect to the third edge. In a preferred embodiment, the blank comprises two panels which are arranged alongside each other along the respective third edge and can be folded over onto each other so as to obtain a superimposed configuration suitable for providing the side wall of the box with a multilayer structure. Regarding said second panel, preferably, each one of said first and second edge (of the second panel) is (completely) defined by said first straight section inclined (of a predetermined second angle, which is preferably different from said first predetermined angle) with respect to the third edge and said second straight section oriented perpendicularly with respect to the third edge.

Preferably, the first and second edge of the second panel are inclined with respect to the corresponding third edge by an angle which is different from the angle of inclination of the first and second edge of the first panel with respect to the corresponding third edge.

This feature allows the end portions of the first and second panel to match precisely, although the two panels are superposed and thus positioned at different distances from a folding axis (i.e. the axis around which the blank is folded).

Preferably, said two panels are connected together along a (first) section of the third edge defining a connecting portion of the panels.

In this respect, it is envisaged that the two panels are not connected along the remainder of the third edge.

However, it should be noted that, preferably, the two panels are connected together also along a second section of the third edge, defining a further connecting portion of the panels; the panels are not connected along the remainder of the third edge (along the portion of the third edge situated outside of the said first and second connecting sections).

Preferably, the two panels and the connecting portion (or portions) are made as one piece by means of cutting of a sheet of flexible, preferably paper material.

Preferably, the panels are connected together along at least one end portion of the respective third edge; more preferably, the panels are connected together along both the end portions of the respective third edge.

In order to facilitate the connection together of the first and second edges of the panels, the panels are staggered relative to each other along the third edge so that, in the superimposed configuration, each panel covers only partially the other panel, leaving an end portion of the other panel exposed. The two exposed end portions can therefore be superimposed on each other so as to form, in that zone also, a superimposed zone in which connection of the two portions is obtained by joining together (for example by means of gluing) the two exposed end portions arranged overlapping each other.

This means that sealing together of the first edge and second edge may not be performed solely on the front section of the panel along the first edge and second edge, but advantageously the surface area of the sealing surface may be increased.

In the non-superimposed condition of the panels, the first inclined sections of a panel are inclined in the opposite direction to the corresponding first inclined sections of the other panel. This provides the blank with an "arrow-like" form when the panels are in a position not folded onto each other.

Brief description of drawings

The technical features of the invention, in accordance with the abovementioned object, may be clearly determined from the contents of the claims provided below and the advantages thereof will emerge more clearly from the detailed description which follows, with reference to the accompanying drawings which illustrate a purely exemplary and non- limiting embodiment thereof, where:

- Figures 1A, 2A and 3A show three simplified embodiments of a blank according to the present invention;

- Figures 1 B, 2B and 3B show three side walls for round boxes obtained, respectively, from the blanks according to Figures 1A, 2A and 3A; - Figure 4 is a plan view of a blank according to the present invention in accordance with a fourth embodiment;

- Figures 5 to 8 show the blank according to Figure 4 in different configurations assumed during successive steps of a folding method for obtaining a side wall of a round box.

Detailed description of preferred embodiments of the invention

In accordance with the accompanying figures, 1 denotes a blank according to the present invention.

The blank 1 consists of a flat body, preferably made of paper and having a mainly elongate surface.

As can be seen in Figures 1A, 2A, 3A, the blank 1 comprises at least one panel 2 having advantageously the form of a parallelogram with a first edge and second edge 3, 4 situated opposite each other and a third edge and fourth edge 5, 6 also situated opposite to each other and transverse to the first edge and second edge 3, 4.

The panel 2 can be folded along a curved trajectory so as to define a side wall "P" of a round box.

Figures 1 B, 2B, 3B show the blank 1 in the folded configuration defining the said side wall "P".

The first and second edges 3, 4 have outer profiles complementing each other so that, once the blank 1 has been folded, these edges 3, 4 make end-to-end contact with each other along their entire extension, in particular along a continuous joining line.

Advantageously, the outer profiles of the first edge and second edge 3, 4 are formed so that, following folding along the said curved trajectory, they may be brought into end-to-end contact with each other along a joining line "G" which is substantially helical, in other words a joining line which extends spirally along the axis of extension of the side wall "P".

This enables a stronger joint to be obtained, not subject to the "pear-like" deformation which instead occurs in the case of joints arranged along a generatrix of the cylindrical side wall "P".

Preferably, at least the third edge 5 is straight. This allows the third edge 5 to assume, following folding of the blank 1 along the said curved trajectory, the configuration of a closed line lying in a plane, in particular a circumference, suitable for defining a base "B" of the side wall "P".

Preferably, as shown in the embodiments of Figures 1A, 2A, 3A, the fourth edge 6 is also straight and is parallel to the third edge 5.

In the embodiments shown in Figures 1A, 2A, 3A, the first edge and second edge 3, 4 have at least one first straight section 3a, 4a, inclined at a predetermined angle "a" relative to the third edge 5 so as to define, following folding along said curved trajectory, the aforementioned helical joining line "G".

It should be noted that the expression "inclined" is understood as meaning: not parallel and not orthogonal, namely, forming an angle greater than 0 and smaller than 90 degrees (or the supplement of said angle).

Preferably, the first edge and second edge 3, 4 are parallel.

In the first embodiment of the blank 1 , shown in Figure 1A, the winding angle of the joining line "G" about the axis of extension (symmetry) of the side wall "P", following folding of the blank 1 along said curved trajectory, is less than 360°. In fact, the length of the third edge and fourth edge 3, 4 is equal to the length of the circumference (erect line) of the base "B" of the side wall "P", while the projection "H" of the first edge 3 on the third edge 5 is smaller than this length and therefore only covers a section of the said circumference.

In the second embodiment of the blank 1 , shown in Figure 2A, the winding angle of the joining line "G", following folding of the blank 1 along said curved trajectory, is equal to 360°. In fact, in this case the length of the projection of the first edge 3 on the third edge 5 is equal to the length of the third edge 3 and therefore the joining line "G" (defined by the first edge 3) covers the entire circumference of the base "B".

In the third embodiment of the blank 1 , shown in Figure 3A, the winding angle of the joining line "G", following folding of the blank 1 along said curved trajectory, is greater than 360°. In fact, in this case the length of the projection of the first edge 3 on the third edge 5 exceeds the length of the third edge 3, and therefore the joining line "G" (defined by the first edge 3) has an extension greater than the circumference of the base "B".

Figure 4 shows a detailed view of a further embodiment of the blank 1 according to the present invention.

In this embodiment the blank 1 comprises two panels 2, 2', each of which in accordance with that described and illustrated above.

Therefore, the two panels 2, 2' have a form substantially of a parallelogram and, in the embodiment shown in Figures 4-8, the panels correspond to the embodiment shown in Figure 1A and therefore are suitable for forming a joining line "G" with a winding angle smaller than 360°.

In greater detail, the panels 2, 2' have a main direction of extension parallel to the third edge and fourth edge 5, 5', 6, 6' and face each other along the respective third edge 5, 5'.

Preferably, the panels 2, 2' are connected together along a section of the third edge 5, 5', while they are instead not connected along the remainder of the third edge 5, 5'. This section of the third edge 5, 5' therefore defines a connecting portion 7 of the panels 2, 2'.

Preferably, the two panels 2, 2' are connected together along an end portion of the respective third edge 5, 5' and the third edges 5, 5' are arranged alongside each other over most of their length.

For reasons which will be clarified below, it is preferable that the two panels 2, 2' are arranged staggered relative to each other along the third edge 5, 5', in particular staggered by an amount such as to define two steps 8, 8' (or "noses") positioned opposite each other in the main direction of extension of the panels 2, 2'.

Preferably, moreover, in the embodiment shown in Figures 4-8, the first edge and second edge 3, 4 of each panel 2, 2' also have at least one second straight section 3b, 3b', 4b, 4b' oriented perpendicularly with respect to the third straight edge 5, 5' and adjacent thereto.

In other words the first edge and second edge 5, 5' of each panel 2, 2' assume an outer profile defined by a broken line.

As shown in Figures 5 and 6, the two panels 2, 2' can be folded onto each other (by means of an overturning operation) about the respective third edges 5, 5', so as to obtain a superimposed configuration suitable for providing the side wall "P" of the box with a multilayer structure. In this way, a first panel defines the inner face of the side wall "P" of the box, while the other panel defines the outer face of the side wall "P".

Since the panels 2, 2' are staggered along the third edge 5, 5', in the superimposed configuration, each panel 2, 2' covers only partially the other panel 2, 2', leaving an end portion 9, 9' of the other panel 2, 2' exposed.

Advantageously, in this way the exposed end portion 9 of a panel 2 can be associated by means of superimposition on a corresponding, opposite, exposed, end portion 9' of the other panel 2' following folding of the panels 2, 2' along the curved trajectory (Figures 7 and 8).

By means of suitable gluing of the two exposed end portions 9, 9' the blank 1 can be fixed in the folded configuration defining the side wall "P" of the box in a multilayer configuration.

Moreover, the aforementioned joining line "G", following folding of the blank 1 along the said curved trajectory, has at least one straight section "R" defined by joining together of the said second straight sections 3b, 3b'; 4b, 4' of the first edge and second edge 3, 3'; 4, 4b'.

This straight section "R" therefore defined by the aforementioned steps 8, 8' of the panels 2, 2' has the dual function of strengthening the ends of the panels 2, 2' and ensuring frontal locking abutment between the ends of the panels 2, 2' during folding of the blank along the curved trajectory. This allows the mutual position of the first edge and second edge 3, 3'; 4, 4' to be precisely determined when sealing them in order to obtain the side wall "P", preventing damaging sliding movements which could arise if the second straight sections 3b, 3b'; 4b, 4b' were not present.

More precisely, moreover, in the multilayer configuration shown in Figures 4-8 the blank 1 gives rise to two joining lines "G", one directed towards the outer face of the side wall "P" and one directed towards the inner face of the side wall "P". The two joining lines "G", which can be seen in Figure 8, are angularly staggered about the axis of extension of the side wall "P". As can be seen in Figure 4, in the non-superimposed configuration of the panels 2, 2', the first inclined sections 3a, 4a of a panel 2 are inclined in the opposite direction to the corresponding first inclined sections 3a', 4a' of the other panel 2' so that, on one side of the blank 1 , the first inclined sections 3a, 3a' converge, while, on the other side, they diverge. This provides the blank 1 with an "arrow-like" configuration.

As a result, by folding over one panel 2 onto the other panel 2', the first inclined sections 3a, 3a' arranged on the same side of the blank 1 are parallel or substantially parallel to each other.

In particular, it is possible that the angle "a" formed between the first inclined sections 3a, 4a of a first panel 2 (which, as described above, are parallel to each other) and the third edge 5 of the first panel 2 is different from the angle "α'" formed by the first inclined sections 3a', 4a' of the second panel 2 with respect to the third straight edge 5" of the second panel 2'.

In the embodiment shown in Figures 4-8, moreover, the second straight sections 3b, 4b of a panel 2 (which have the same length, since they must come into contact against each other) have a different length compared to the second straight sections 3b', 4b' of the other panel 2'.

A method of making round boxes, using a blank of the type described above, therefore essentially comprises a step of folding the panels 2, 2' along a curved trajectory (preferably a circumference), as can be seen in Figure 7, until the first edge and second edge 3, 4; 3', 4' of each panel 2, 2' make end-to-end contact with each other (Figure 8), and then joining together said first edge and second edge 3, 4; 3', 4', obtaining a round side wall "P" of a box.

This step of folding around the curved trajectory is preceded by a step of folding over the two panels 2, 2' onto each other about the third edges 5, 5' so as to obtain a superimposed configuration suitable for providing the side wall "P" of the box with a multilayer structure.

The step of folding over onto each other the two panels 2, 2' about the third edges 5, 5' results in superimposition, on top of each other, of the panels 2, 2' along their respective contact surfaces, except for the aforementioned exposed end portions 9, 9'.

Preferably the said contact surfaces intended to be superimposed are lined with a relatively slow setting glue (with a setting time for example of 3 minutes, preferably at least 2 minutes) suitable for allowing relative sliding of the two panels 2, 2' during folding along said curved trajectory. This folding operation in fact arranges the two panels 2, 2' along two curved paths with a different distance from a centre of instantaneous rotation and this inevitably results in the relative sliding of the two panels 2, 2' along their mutual contact surface.

This sliding action is moreover favoured precisely by the fact that the two panels 2, 2' are totally separate from each other apart from the small connecting portion 7.

With reference to the exposed end portions 9, 9', which are not superimposed, they are preferably lined with a fast setting or normal setting glue (with a setting time for example of 5 seconds, preferably 3 to 7 seconds).

In a preferred embodiment of the method, the step of applying the glue onto the exposed end portions is performed after the step of folding over of the panels onto each other and therefore after application of the slow setting glue.

Advantageously, the particular form of the panels 2, 2' allows them to be folded about the curved trajectory simply by gripping a front end of the blank 1 with the two panels 2, 2' which are already superimposed.

Continuation of the folding action may be assisted by a series of rollers which produce a curved or circular path and terminate in the mutually superimposed arrangement of the exposed end portions 9, 9'.

Advantageously, this operation may be performed without the use of a central mounting spindle and simply requires a system for supporting a gripper arranged on the side of the curved trajectory followed by the blank. Owing to the particular "arrow-like" form of the blank, in fact, following folding over of one panel onto the other one, a substantially parallelogram configuration of the blank may be obtained, this being able to be easily gripped laterally by means of a special gripper which does not interfere with any rollers for guiding the blank along the circular trajectory.

The present invention is particularly intended for making round boxes which have a diameter of between 40 and 330 millimetres and height of between 30 and 330 millimetres.

The invention thus conceived may obviously be implemented on an industrial level; moreover it may also be subject to numerous modifications and variations all falling within the scope of the inventive idea; all the details may be replaced moreover by technically equivalent elements. The invention therefore achieves the predefined objects, overcoming the problems associated with the prior art.

The manufacture of the blank with edges intended to be superimposed along a helical joining line helps overcome and eliminate the typical weakness of the straight joint arranged along a generatrix of the final cylindrical surface.

In fact, the helical arrangement of the joining line results in a greater rigidity and strength of the joint between the contact edges, ensuring an optimum cylindrical form of the resulting side wall.

Moreover, owing to the particular parallelogram form of the blank, as mentioned above, a simple folding action along a curved (circular) path may be performed by means of a lateral gripper, thereby making it possible to dispense with a central mounting spindle. As a result the blank proves to be very suitable for ensuring a high operational flexibility as well as a wide range of possible formats.

Claims

1 . A paper or cardboard blank for making round boxes, comprising at least one panel (2, 2') which can be folded along a curved trajectory so as to define a cylindrical side wall (P) of a round box, wherein said panel (2, 2') has a first edge and second edge (3, 4; 3', 4') situated opposite each other and a third edge and fourth edge (5, 6; 5', 6') also situated opposite to each other and transverse to said first edge and second edge (3, 4; 3', 4'), said first edge and second edge (3, 4; 3', 4') having respective outer profiles which complement each other, characterized in that said outer profiles of the first edge and second edge (3, 4; 3', 4') are formed so that said first edge and second edge (3, 4; 3', 4'), following said folding along said curved trajectory, can be brought into end-to-end contact with each other along a joining line (G) which is at least partly helical.

2. Blank according to claim 1 , wherein at least said third edge (5, 5') of the panel (2, 2') is straight so as to assume, following said folding along said curved trajectory, the configuration of a circular closed line lying in a plane and wherein each of said first and second edges (3, 4; 3', 4') has at least one first section (3a, 3a'; 4a, 4a') inclined relative to said third edge (5, 5') so as to define, following said folding along said curved trajectory, said helical joining line (G).

3. Blank according to claim 2, wherein said first inclined sections (3a, 3a'; 4a, 4a') of the first edge and second edge (3, 4; 3', 4') are straight and parallel relative to each other.

4. Blank according to claim 2 or claim 3, wherein said first edge and second edge (3, 4; 3', 4') have at least one respective second straight section (3b, 3b'; 4b, 4b') oriented perpendicularly with respect to said third straight edge (5, 5') so that said joining line (G), following said folding along said curved trajectory, has at least one straight section (R) defined by joining together of said second straight sections (3b, 3b'; 4b, 4b') of the first edge and second edge (3, 3'; 4, 4').

5. Blank according to any one of the preceding claims, comprising two of said panels (2, 2') which are arranged alongside each other along the respective third edge (5, 5') and can be folded onto each other so as to obtain a superimposed configuration suitable for providing said side wall (P) of the box with a multilayer structure.

6. Blank according to claim 5, wherein said panels (2, 2') are connected together along at least one section of said third edge (5, 5') and are not connected along the remainder of said third edge (5, 5').

7. Blank according to claim 6, wherein said panels (2, 2') are connected together along a first section and second section of said third edge (5, 5') and are not connected along the remainder of said third edge (5, 5').

8. Blank according to claim 6 or claim 7, wherein said panels (2, 2') are connected together along end portions of the respective third edge (5, 5').

9. Blank according to any one of claims 5 to 8, wherein said panels (2, 2') are staggered relative to each other along said third edge (5, 5') so that, in said superimposed configuration, each of said panels (2, 2') covers only partly the other panel (2, 2') leaving exposed an end portion (9, 9') of the other panel (2, 2').

10. Blank according to any one of claims 5 to 9, wherein at least said third edge (5, 5') of the panel (2, 2') is straight so as to assume, following said folding along said curved trajectory, the configuration of a circular closed line lying in a plane and wherein each of said first and second edges (3, 4; 3', 4') has at least one first section (3a, 3a'; 4a, 4a') inclined relative to said third edge (5, 5') so as to define, following said folding along said curved trajectory, said helical joining line (G), and

wherein in a non-superimposed condition of said panels (2, 2'), said first inclined sections (3a, 4a) of a panel (2) are inclined in the opposite direction to the corresponding first inclined sections (3a,', 4a') of the other panel (2').

1 1 . Blank according to claim 10, wherein said first inclined sections (3a,

4a) of a first one of said panels (2) form an angle (a), relative to the third edge (5) of the first panel (2), which is different from the angle (α') formed by the first inclined sections (3a', 4a') of the second panel (2') relative to the third straight edge (5') of said second panel (2').