DESCRIPTION

TECHNICAL FIELD [0001] The present invention relates to methods and machines for producing rolls of web material wound around tubular cores made for example of plastic or cardboard.

BACKGROUND TO THE INVENTION

[0002] In many industrial sectors rolls of continuous web material are produced, wound around tubular winding cores made for example of cardboard, plastic or other material. Specifically, in the field of tissue paper converting, rolls of toilet paper, kitchen towels or the like are produced by winding a tissue paper web, made of one or more plies, around tubular winding cores made for example of cardboard.

[0003] The rolls are wound by means of rewinding machines. In modern converting lines, either peripheral or combined peripheral-central rewinding and winding machines are used for winding. In peripheral winding machines (winders, rewinders) winding is carried out by transmitting, through a plurality of peripheral winding members, a rotation torque to a roll or log being formed, due to the friction between the roll and the winding members. The winding members are typically motorized rollers. In addition to peripheral winding members, the combined machines also have members transmitting rotation torque to the roll being formed through the tubular winding core or through a forming mandrel.

[0004] The leading edge of the web material is usually glued to the tubular winding core. This has many disadvantages, as glue is a consumable affecting the overall cost of the finished product. Furthermore, glue is a polluting agent, soils the machines and poses risks of jamming or of having defects on the finished products. For example, the last piece of web material that the user touches when unwinding the finished roll may be of poor quality or completely unusable due to glue used for anchoring it to the tubular core.

[0005] Rewinders have been therefore produced, wherein winding is carried out without the need of gluing the free leading edge of the web material around the tubular winding core. Examples of rewinders that do not require glue are disclosed in US 7.931.226. In these rewinders, winding begins without using glue, but using a mechanical member for facilitating the winding of the first turn of web material around the tubular winding core. Machines of this type are very advantageous, as it is possible to avoid the use of glue completely. However, these machines can be further improved.

[0006] In fact, the rolls of web material wound with the machines of the type disclosed in US 7.931.226 have a drawback, i.e. the tubular core in not efficiently held inside the roll and tends to slip out, with consequent inconveniences both in producing and packaging steps as well as during use. Moreover, at the end of winding the last piece of web material prematurely detaches from the tubular core and is lost or not used.

[0007] It would be therefore beneficial to have winding systems that, even if not using glue, allow a stronger joining between wound material and tubular winding core.

SUMMARY OF THE INVENTION

[0008] According to an aspect, a method is provided for producing rolls of web material wound around tubular winding cores, wherein a tubular winding core, having a longitudinal cut extending between a first end and a second end thereof, is inserted into a peripheral winding machine. A web material leading edge is anchored to the tubular winding core by inserting at least a portion of said leading edge between two edges delimiting the longitudinal cut. Once the web material has been anchored, a length of web material is wound around the tubular core.

[0009] In this way, the leading edge of the web material is anchored to the tubular winding core due to the interaction between the edges of the longitudinal cut. The edges may be temporarily moved away from each other so as to insert the web material therebetween, and then moved towards each other again, so that they touch, or press against, each other, to pinch the web material and to hold it.

[0010] Even a small and limited pinching effect allows improving the adhesion between tubular winding core and wound web material, thus reducing or eliminating the drawbacks mentioned above, but keeping the advantage of not using glue. [0011] “Free leading edge” of the web material means, in general, a portion of web material close to the end edge but not necessarily coinciding with, or including, the head edge. For instance, the free leading edge pinched between the opposite edges of the longitudinal cut can be a portion of web material spaced from the end edge by a few centimeters.

[0012] “Longitudinal cut” means a cut generically extending between the two ends of the tubular core. As it will be better explained below, the longitudinal cut may be parallel to the longitudinal axis of the tubular winding core, but this is not strictly necessary. The longitudinal cut may be inclined, V-shaped, undulating or zigzag, but always extending, in general, longitudinally.

[0013] The longitudinal cut may be also continuous. However, as it will be explained below with reference to some embodiments, the longitudinal cut may be discontinuous, i.e. it may have some segments of material joining the opposite edges of the longitudinal cut. These segments or bridges may be torn, cut or, in general, severed, partially or completely, during the step of anchoring the web material to the tubular winding core and of starting winding.

[0014] The longitudinal cut may extend for the whole length of the tubular winding core, but this is not mandatory. In some embodiments described below, the longitudinal cut may terminate at a given distance, for example a few millimeters or centimeters, from the ends of the tubular winding core.

[0015] The longitudinal cut may be formed on an already complete tubular structure, that is cut after having been produced. For example, it is possible to produce a continuous tube, by means of a core winder or in any other manner, and to cut the continuous tube longitudinally before dividing it into single tubular winding cores. In other embodiments, a plurality of tubular cores may be produced, for example by transversally cutting a continuous tube; the tubular winding cores thus obtained may be subsequently cut longitudinally.

[0016] However, the possibility is not excluded of producing the tubular cores either from discontinuous flat sheets or from a continuous web material. In both cases, a sheet may be curled, for example by means of a mechanical or thermal curling device, whose effect is to curl the sheet. In this way, two sheet edges orthogonal to the curling direction form the edges of the longitudinal cut in the tubular winding core.

[0017] Peripheral rewinders of various type may be used for peripheral winding. Here below, a structure of a possible rewinder will be described with reference to the attached drawing. For implementing the method described herein, it is also possible to use other peripheral rewinders, for example provided with rollers. Examples of peripheral rewinders are disclosed in US20170210584; US9856102; US9365379; US9352920; US9079737; US20110133015; US7891598; US7942363; US7404529; US6948677; and US5979818.

[0018] Further features and embodiments of the method briefly illustrated above will be described below with reference to the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0019] The invention will be better understood by following the description below and the attached drawing, showing a non-limiting embodiment of the invention. More specifically, in the drawing:

Fig. l is a diagram of a rewinder according to the present invention;

Fig. 2 is a cross-section of a tubular winding core;

Figs. 3, 4, and 5 are diagrams of tubular winding cores according to the present invention;

Figs. 6 to 11 show an operation sequence of the rewinder of Fig. 1;

Fig. 9A shows an enlargement of the detail IX of Fig. 9;

Figs. 12 to 13 are diagrams of forming devices for forming tubular winding cores by curling pieces of sheet material obtained from a continuous sheet;

Fig. 14 shows an enlargement of an embodiment of an operating anchoring member;

Fig. 15 is a diagram of a forming device for forming tubular winding cores by curling pieces of sheet material taken from a ream of cut sheets;

Fig. 16 shows a core winder with a system for making a longitudinal cut in a continuous tube;

Fig. 17 is a schematic section for longitudinally cutting a tubular winding core while inserting it into a rewinder; and

Figs. 18 and 19 are schematic views of tubular winding cores with longitudinal cuts not parallel to the axis.

DETAILED DESCRIPTION OF EMBODIMENTS

[0020] Briefly, a novel method is disclosed for producing rolls of web material wound around tubular winding cores, without using glue. To ensure a better anchoring of the web material to the tubular winding core, this latter has a longitudinal cut. When the winding cycle begins, the free leading edge of the web material is anchored to the tubular winding core by inserting it between edges of the longitudinal cut. In practice, the tubular core is deformed, so that the two edges delimiting the longitudinal cut move away from each other. A portion of the free leading edge of the web material is inserted into the space formed by the edges moving away from each other. When the force, which caused the tubular winding core to deform and the two edges of the longitudinal cut to move away from each other, ends, the two edges tend to join again, thus pinching and anchoring the web material. In this way, a stable and safe anchoring is obtained without the need of glue.

[0021] With reference to the drawing, Fig. 1 schematically shows a rewinder 1, comprising a peripheral winding head 3. The winding head 3 comprises a first winding roller 5, a second winding roller 7 and a third winding roller 9. The three winding rollers 5, 7, and 9 are adequately motorized in a known manner. A web material N, for example tissue paper, that shall be wound in rolls R around tubular winding cores A, is driven around the first winding roller 5. As known to those skilled in the art, the third winding roller 9 may be mounted on a pivoting arm 10, so as to move towards, and away from, the first winding roller 5 and the second winding roller 7 to allow a roll or log R to gradually grow in the winding cradle formed by the winding rollers 5, 7, 9.

[0022] The first winding roller 5 and the second winding roller 7 form a nip 11 therebetween, through which the tubular winding cores move and the web material N is fed when it is wound around a roll being formed in the winding cradle formed by the winding rollers 5, 7, 9.

[0023] Each roll or log R produced by the rewinder is then cut into a plurality of rolls of smaller axial length, that are packaged and sold to the consumer. The ends of each roll or log R are removed as trimmings or waste, to remove the defective part from the finished product.

[0024] The winding head 3 further comprises a stationary rolling surface 13 extending around the first winding roller 5 and spaced therefrom so as to form a channel 15 for introducing the tubular winding cores A.

[0025] The rewinder 1 further comprises a cutting or severing member 17 to cut the web material N once a roll R has been completely wound, to generate a tail edge, winding around the finished roll, and a leading edge that shall be anchored to a new tubular winding core as described below.

[0026] The reference number 19 indicates a core feeder for inserting tubular winding cores inside the channel 15 towards the winding cradle.

[0027] The rewinder described above is a mere example. Other embodiments of winding machines and winding heads for these machines, compatible with the features of the new winding method described herein, are disclosed, for example, in the publications cited in the introductory part of this description and in other documents cited therein.

[0028] The reference number 21 indicates a forming device for forming tubular winding cores. In the example of Fig. 1, with the device 21 there is associated a reel B of sheet material F, from which single pieces are cut to form tubular winding cores. Some exemplary embodiments of the forming device 21 are described below. In general, the device 21 produces tubular winding cores A that, differently from the traditional tubular winding cores, have a longitudinal cut that may extend through the whole thickness of the tubular wall of the tubular winding core. Fig. 2 schematically illustrates a cross-section of a tubular winding core A of the type described herein. The tubular winding core is constituted by a sheet F having a cut T extending for the whole longitudinal extension of the tubular winding core A, i.e. for the whole length of the tubular core in the direction of the longitudinal axis A-A of the tubular core.

[0029] Adequately, the cut T crosses the whole thickness of the sheet F forming the tubular winding core A. [0030] As shown in Fig. 8, the cut T is defined and delimited by two edges Bl, B2 of the sheet F. Advantageously, the edges Bl, B2 have respective surfaces extending through the thickness of the sheet F and that are inclined by an angle a with respect to a radial direction. The orientation of the angle a is such as to facilitate the moving of the edges Bl, B2 away from each other when the tubular core A is deformed due to a radial load. More in particular, the inclination of the two surfaces of the cut T delimiting the edges Bl, B2 is such as to facilitate the movement of the edge Bl towards the inside of the tubular winding core A.

[0031] The cut T has an overall longitudinal extension, i.e. it extends from one to the other of the two opposite ends of the tubular winding core A, as shown in Figs. 3, 4 and 5, which illustrate three embodiments of the cut T, given just by way of non limiting example. In Fig. 3 the cut T is undulating, in Fig. 4 it extends like a V with a very large vertex angle and the vertex arranged on the centerline of the tubular core A, whilst in Fig. 5 the simplest embodiment is shown, where the cut T is parallel to the longitudinal axis A-A of the tubular winding core A.

[0032] When the tubular core A has a cut T as described above, it can be deformed by a force having at least a radial component, so as to move the edges Bl and B2 away from each other. When the tubular core is stressed in this way, it is deformed so that one edge enters inside the theoretical cylindrical outer surface delimiting the outer surface of the tubular core A with no deformation, whilst the other edge tends to project radially outwards. In this way the cut T opens, thus facilitating the insertion of a leading edge of the web material N, to anchor the web material N to the tubular winding core A. When the deformation stress ends, the tubular core tends to take the round cylindrical shape (Fig. 2) again, with the edges Bl and B2 tending to press against each other. If the web material leading edge has been inserted between the edges Bl, B2, it is pinched therebetween.

[0033] This ability of the tubular winding core A to be deformed is used to anchor the leading edge of the web material to the tubular winding core A without using glue. The process of anchoring the web material and starting winding will be clearly apparent from the description below of an operation cycle, illustrated in the sequence of Figs. 1 and 5 to 11. [0034] Fig. 1 shows the step where a roll R is almost complete and is in the winding cradle between the winding rollers 5, 7 and 9. A new tubular winding core has been taken by the core feeder 19 and is ready to be inserted into the channel 15 between the first winding roller 5 and the rolling surface 13.

[0035] In Fig. 6 the core feeder 19 has inserted the new tubular winding core A into the channel 15. The web material N is cut by means of the cutting member 17. In a known manner, the cutting member has pads adapted to pinch the web material against the outer surface of the first winding roller 5. The operation of the cutting member 17 may be, for example, similar to what disclosed in the patent documents cited in the introductory part of this description.

[0036] The cutting member 17 can enter the channel 15, as shown in Fig. 6, to pinch the web material N against the outer cylindrical surface of the first winding roller 5 and to cause the cut thereof, thus forming a tail edge Lf of the roll R and a leading edge Li of web material to be wound around the new tubular winding core A. The severing or cutting member 17 enters into the channel 15 thanks to the fact that the rolling surface 13 is formed by a comb structure, with parallel teeth spaced from one another, to allow the cutting member 17 to pass. The cutting member penetrates into the channel 15 for example rotating in the same direction as the winding rollers, so as to enter the channel 15 from opposite side with respect to the opening where the new tubular winding core is inserted by the core feeder 19. The rotation direction of the cutting member 17 is indicated with P7.

[0037] In this way, the leading edge Li forms a sort of bag between the new tubular winding core inserted by the core feeder 19 and the point where the web material N is pinched between the first winding roller 5 and the cutting member 17.

[0038] In the position of Fig. 6 the web material N is pinched in a point between the first winding roller 5 and the new tubular winding core A, and in a second point between the first winding roller 5 and the cutting member 17.

[0039] As the distance between the rolling surface 13 and the cylindrical surface of the first winding roller 5 is advantageously smaller than the diameter of the tubular winding core A, this latter tends to be deformed by a radial force. The radial force tends to move the edges B 1 and B2 of the cut T away from each other. At the same time, due to the compressive force acting onto the tubular winding core A, this latter is angularly accelerated and starts rolling along the stationary rolling surface 13.

[0040] Fig. 7 shows the same position of Fig. 6 in enlarged scale. In Fig. 7, the new tubular core A inserted into the channel 15 takes an oval shape, and the edges Bl, B2 of the cut T are moved away from each other. The edge Bl has penetrated the inside of the tubular core, sliding along the inner surface of the wall formed by the sheet F constituting the tubular core.

[0041] In Fig. 8 a subsequent instant is shown, when the tubular winding core A is rolling along the rolling surface 13 due to the torque applied by the first winding roller 5. The leading edge Li of the web material, formed, i.e. cut or torn, by the severing or cutting member 17, rests against the rolling surface 13 and the tubular winding core start to roll on it, so that the leading edge Li start to form a partial winding turn of the web material around the new tubular winding core.

[0042] The roll R, formed in the winding cradle between the winding rollers 5, 7, 9, moves away according to the arrow fR thanks to the different speed of the winding rollers, in particular thanks to a reduction in the peripheral speed of the second winding roller 7.

[0043] While the winding core moves forward rolling along the channel 15, the cut T rotates around the axis A-A of the tubular winding core A. In Fig. 9, the cut T and the edges Bl, B2 delimiting it have passed the point of contact between tubular winding core A and stationary rolling surface 13, said point representing the axis of instant rotation of the tubular winding core A during the rolling movement.

[0044] When the cut T has passed the point of contact between tubular winding core A and rolling surface 13, the thrust in approximately radial direction generated in the point of contact between tubular winding core A and rolling surface 13 pushes the edge B l inside the tubular core, enlarging the cut T. In this way, the leading edge Li, or a portion thereof, can enter in the cut between the two edges Bl, B2.

[0045] This phenomenon is illustrated in particular in Fig. 9 and in the enlargement of Fig. 9A. An anchoring member, for example a mechanical, pneumatic, or pneumatic-mechanical anchoring member, may facilitate the insertion of the leading edge Li, or part thereof, into the cut between the edges Bl, B2 that are spaced from each other due to the deformation of the tubular winding core A. To this end, in Fig. 9 an anchoring member is shown, comprising a mechanical member 31. The mechanical member 31 may be mounted so as to pivot around an axis 31.1, substantially parallel to the rotation axes of the winding rollers 5, 7, 9, which are parallel to one another. In some embodiments, the mechanical member 31 may be elastically stressed by means of a spring, for example a pneumatic spring 31.2, to remain in idle position. The mechanical member 31 has an appendix, or more precisely a plurality of appendices 31.4, that are usually withdrawn with respect to the channel 15, i.e. below the stationary rolling surface. This position can be kept thanks to the spring 31.2.

[0046] In some embodiments, the mechanical member may have a plurality of fingers 31.3 that are in the channel 15 when the mechanical member 31 is in idle position. When the tubular winding core A moves forward along the channel 15, it interferes with the fingers 31.1 and causes the mechanical member 31 to pivot around the axis 31.1, thus causing the actuation of the mechanical member 31 without the need for an actuator. An embodiment of this type is disclosed in detail in US 7.931.226, to which reference can be made for further details. When the tubular winding core A has passed downstream of the mechanical member 31, the elastic return member 31.2 brings the mechanical member 31 to the idle position again.

[0047] In other embodiments, the fingers 31.3 may be omitted and the mechanical member 31 may be actuated through an actuator, for example an electronically controlled electric actuator. The actuation of the mechanical member 31 may be synchronized with the position of the tubular winding core A along the channel 15, for example using a sensor detecting the passage of the tubular core A and sending an actuation signal to the actuator controlling the mechanical member 31.

[0048] As shown in Fig. 10, the pivoting movement of the mechanical member 31 may continue whilst the tubular winding core A moves forward, so that the ends of the appendices 31.4 follow the tubular core A and ensure the insertion of the portion of free leading edge Li between the edges Bl, B2 of the cut T. The leading edge Li, or part thereof, may be inserted between the two edges Bl, B2 up to project within the inner volume of the tubular winding core A. [0049] The action of the mechanical member 31 terminates when the tubular core has moved forward by a given length along the channel 15, so that the edges Bl, B2 of the cut T may press against each other again, pinching and holding the free leading edge Li of the web material N. The further forward movement of the tubular winding core A along the channel 15 results in the complete winding of the first turn of web material and then the winding of the following turns. At the same time, the mechanical member 31 exits from the tubular core A and returns to the idle position of Fig. 8.

[0050] At this point, the winding process continues in a known manner, whilst the core passes through the nip 11 between the first winding roller 5 and the second winding roller 7 and enters the winding cradle between the winding rollers 5, 7 and 9.

[0051] The described process allows to anchor the free leading edge Li of the web material to the tubular winding core A without using glue, thanks to the mechanical pinching effect generated by the co-action between the edges Bl, B2 of the longitudinal cut T of the tubular winding core A.

[0052] Fig. 11 shows an intermediate step of the winding cycle of the roll R, wherein the roll being formed is in contact with the winding rollers 5, 7, 9 before a new tubular winding core is inserted and the formed roll is discharged (Fig. 6).

[0053] The tubular winding core A may be formed in various ways. In some embodiments, the tubular winding core A may be formed by means of an usual core winder, arranged outside the converting line, where the rewinder is located. The tubular core formed by the core winder may be cut forming the longitudinal cut T when it is still in the core winder or during a following step, for example when it is engaged by the core feeder 19 before being inserted into the channel 15. In this way, the correct angular position of cut T is ensured.

[0054] The tubular winding core A may be cut for example with a cutting blade or, alternatively, a laser system.

[0055] The core winders are bulky machines and require the use of glue for making the turns of one or more strips of cardboard adhere to one another, to obtain a tube. As the method described herein does not require a whole tubular core, in some advantageous embodiments the tubular core may be formed from a piece of sheet material, adequately curled so as to take a round cylindrical shape, with a longitudinal cut.

[0056] Different methods can be used for curling a sheet made of paper or cardboard, for example. In general, the curling method comprises the step of exerting a differential action on the two faces of the sheet. The action can be mechanical, or thermal, or a combination thereof. The mechanical action can comprise the application of a surface force onto the sheet. The thermal action can comprise surface heating of the sheet.

[0057] Methods and machines for curling a paper sheet through a mechanical action are disclosed, for example, in US 20030205235 A1 and US 3962957.

[0058] Fig. 12 schematically shows an embodiment of the forming device 21 for forming tubular winding cores A from a reel B of a continuous sheet F made of paper or cardboard, for instance. A curling device 35 is arranged along an unwinding path of the continuous sheet F; the device has a corner, around which the sheet F is guided, forming a return angle substantially smaller than 180°. The traction on the sheet F causes a friction force between the corner of the curling device 35 and the surface of the sheet F in contact with the curling device 35. This mechanical stress (that is different for the two opposite surfaces or faces of the sheet F) results in a mechanical work causing the sheet F to curl.

[0059] In the diagram of Fig. 12 two traction rollers 37 are also schematically shown, which are provided downstream of the curling device 35, and at least one of which is motorized. Upstream of the curling device 35 a brake may be provided for generating sufficient traction in the sheet F. The brake may be so arranged as to act on the axis of the reel B.

[0060] A cutting device 39 is provided downstream of the traction rollers 37, dividing the sheet F into single pieces, each of which has such a length to form a tubular winding core A. Due to the action exerted by the curling device 35, the piece of cut sheet F tends to curl, taking the cylindrical shape of the tubular winding core A that is taken by the core feeder 19 to be inserted into the channel 15.

[0061] As mentioned above, the curling effect may be obtained also in different ways. A further curling device that can be used in this context is disclosed, for example, in US 5.928.124.

[0062] Fig. 13 shows a different embodiment of the device 21 for forming the tubular winding cores A. The same numbers indicate equal or equivalent parts to those described with reference to Fig. 12. In the embodiment of Fig. 13, the curling device 35 comprises two rollers 35 A, 35B that can be motorized or idle. The two rollers 35 A, 35B are kept at different temperatures, for example one may be heated and the other may be chilled. The different temperatures, to which the two opposite surfaces of the sheet F are subjected, result in the sheet tending to curl when it is cut into pieces by the cutting device 39. The rollers 35A, 35B may act as traction rollers. Alternatively, or in combination, traction rollers 37 may be provided, as shown in Fig. 12.

[0063] In some embodiments, the anchoring member, facilitating the insertion of the free edge Li into the cut T of the tubular winding core A to anchor the web material N to the tubular core, may be controlled by an actuator, rather than by the passage of the tubular core A. Moreover, the anchoring member may be provided with a movement different than the pivoting movement of the mechanical member 31.

[0064] For instance, Fig. 14 shows an anchoring device comprising a mechanical member, indicated again with the reference number 31, comprising a movable element 31.5 controlled by an actuator 31.6, for example a rotary or linear electric motor. The movable element may be provided with a movement of insertion into, and removal from, the channel 15 and may be actuated in synchronized manner with the passage of the tubular winding core A, so as to penetrate into the channel 15 when the tubular core A rolls on the rolling surface 13 at the position where the anchoring member is. As in the sequence described above with reference to Figs. 2 to 11, the tubular winding core A is inserted into the channel 15 with such an angular position that, rolling along the rolling surface 13, it reaches the position of Fig. 14 with the cut T facing downwards. The lifting of the movable element 31.5, controlled, for example, by a signal of a sensor detecting the passage of the tubular winding core A, causes the movable element 31.5 to penetrate into the cut T and, therefore, a portion of the leading edge Li of web material N to be inserted into the cut T.

[0065] Once the leading edge Li has been inserted between the edges Bl, B2, the element 31.5 may be removed. The edges B 1 , B2 of the cut T move towards each other again, and pinch the leading edge Li, holding it.

[0066] In some embodiments, the process described above of inserting the leading edge Li into the cut T between the edges Bl, B2 may be facilitated by the tubular winding core A being deformed due to the dimension of the channel 15, that is smaller than the diameter of the tubular core A.

[0067] Alternatively, the movement of the edges B 1 , B2 away from each other may be caused by the penetration of the movable element 31.5, without the need for a deformation of the tubular winding core A by radial compression. In this case, the channel 15 may have a dimension in radial direction, i.e. a distance between first winding roller 5 and rolling surface 13, equal to, or slightly lower than, the diameter of the core, in order to generate enough friction on the tubular winding core A to cause the angular acceleration and the start of rolling thereof.

[0068] Whilst in the embodiments described above the tubular core is obtained by cutting a continuous sheet F unwound from a reel B, in other embodiments already cut individual sheets can be used, that are curled before being inserted into the channel 15. Fig.15 shows a diagram of a forming device for forming tubular winding cores A, indicated again with the reference number 21. RF indicates a ream of single sheets, which are individually taken by a conveyor, for example a belt conveyor schematically indicated with 41. The single sheets are fed to a curling device 42, which may comprise, for example, a pair of rollers 43, kept at two different temperatures to apply two different thermal actions on the opposite surfaces of the sheet F, analogously to what described with reference to Fig. 13. The curled sheet F forming the tubular winding core, once exited from the nip between the two rollers 43 is inserted into the core feeder 19.

[0069] In the embodiments described above, a device 21 is provided for making tubular winding cores, which is arranged directly along the converting line where the rewinder 1 is provided. In this way, there is no need for a core winder outside the converting line and at the side thereof, thus eliminating the production costs (including those for glue) of the tubular cores. Moreover, the overall footprint of the machinery associated with the converting line is reduced. [0070] It is however possible to use tubular cores formed outside the converting line, with machines adjacent to the converting lines or even separated and distinct from the converting line where the rewinder 1 is located. For example, traditional core winders can be used for producing tubular cardboard cores, by helically winding cardboard strips. The tubular cores may be stored in a storage space and inserted individually into the rewinder. In other embodiments, it is possible to use winding cores made of extruded plastic or other material. For example, compostable materials can be used, or water-soluble materials, or other non-polluting, easy-to-dispose materials.

[0071] Independently of the production process used for the tubular winding cores, according to the invention longitudinal cuts T are made in the cores before inserting them into the winding head 3. The longitudinal cut can be made, for example, as a step of the winding process of the tubular core in a core winder. Fig. 16 schematically shows a core winder 51 for producing tubular winding cores. The core winder 51 may be built in any known manner, for example it can be a core winder of the type disclosed in US 9.068.595, to which reference shall be made for further details. With respect to a known core winder, the core winder 51 has also a disk-shaped cutting blade 53 or any other cutting device, for making longitudinal cuts in the tube T being formed around a forming mandrel 55. Instead of a disk-shaped blade 53, a laser cutting system may be provided, or a water-jet cutting system, or any other adequate cutting member. The cutting member 53 co-acts with the forming mandrel 55 acting as a counter-blade. As the tube T is usually formed by rotating it around the mandrel axis, the disk-shaped blade 53 can be installed on an annular structure surrounding the mandrel and which can move along a circular trajectory so as to follow the rotating tube T.

[0072] Core winders are also provided, forming a continuous tube by means of a longitudinal movement, as disclosed for example in US 5.593.375 and US 2016/0082686. In core winders provided with longitudinal movement instead of helical movement, a cutting device, for example a rotating disk-shaped blade, can be arranged in a stationary position.

[0073] Instead of a blade, a laser, ultrasound, or water-jet cutting system may be used, or any other system adapted to cut the material forming the tube and of which the tubular winding cores are made. In general, this material is not limited to cardboard: it can also be, for example, a plastic material, preferably a compostable or recyclable material.

[0074] In other embodiments, the cutting member may be inserted as an integral part of a feeding unit feeding tubular winding cores to the winding head 3 of the rewinder 1.

[0075] Systems for inserting tubular winding cores into a rewinder are known in the art, see for example the system disclosed in EP 0306092. This known system provides for an apparatus for applying glue according to a longitudinal line during the insertion of the core into the rewinder. An insertion system of this type can be modified and adapted to include a longitudinal cutting device, which replaces the glue applying apparatus. Fig. 17 shows a diagram of a system for inserting a tubular winding core, indicated with the letter A, and for cutting it longitudinally, in a cross-section according to a plane orthogonal to the axis of the tubular winding core A, and therefore orthogonal to the insertion direction. This figure schematically shows the tubular winding core A which moves forward parallel to the axis thereof, resting on a stationary sliding support 61. The forward movement may be imparted through motorized rollers or wheels, whose rotation axes are arranged at an angle of 90° with respect to the tubular winding core. In the embodiment schematically illustrated in Fig. 17, analogously to what described in above mentioned EP 0306092, the tubular winding core A is kept between two belts 63, 65; only a cross-section of the active branch of each belt touching the tubular winding core A is shown in the figure.

[0076] In a suitable position, preferably in a position diametrically opposite to the support 61, a cutting device 53 may be provided, schematically represented as a laser cutting device emitting a laser beam FL. Since the laser beam is focused on the surface of the tubular winding core A, it cuts the tubular core along a line parallel to the axis of the same tubular core and does not damage the diametrically opposite portion of the tubular core.

[0077] By modulating the laser beam and, if necessary, by moving the laser source 53 transversally to the feed direction of the tubular winding core, discontinuous cuts can be obtained, and/or cuts non-parallel to the axis of the tubular winding core A, as described below.

[0078] When the tubular winding core A is cut after the formation of a closed tube structure, the cut T can be done incompletely, i.e. so that it has not the same longitudinal extension as the tubular winding core A. Fig. 18 shows, for example, a tubular winding core A with a first end A1 and a second end A2. The longitudinal cut, indicated again with T, does not extend for the whole longitudinal extension of the tubular core, and terminates at a given distance from the axial ends A1 and A2 of the tubular winding core, thus leaving two segment T1 and T2 in the wall of the tubular core A uncut. In this way, the tubular winding core better keeps the approximately cylindrical shape when handled, and does not tend excessively to deform. At the same time, the cut T, which can have the shape shown in Fig. 2, may stretch by the movement of the edges Bl, B2 away from each other, so that a portion of the leading edge Li of the web material N to be wound is inserted inside the cut and is anchored to the tubular winding core.

[0079] The portions of the tubular core A, where the uncut segments T1 and T2 are positioned, may be short enough to be contained in the parts of the finished roll R destined to be removed as trimmings.

[0080] In other embodiments, the longitudinal cut T may be discontinuous, as schematically shown in Fig. 19. In this case, the cut is in the form of a perforation, with cut segments T3 and uncut segments T4 in the tubular wall forming the tubular winding core A. The uncut segments T4 may cover an overall length substantially shorter, i.e. 10-100 times shorter, than the overall length of the cut segments T3. The interrupted cut of Fig. 19 may be obtained, for example, through pulsed control of a laser cutting device, or through a toothed disk-shaped blade 53. The uncut segments T4 may be arranged along the tubular winding core A so as to coincide with the cutting planes along which a severing machine will cut the roll into single rolls destined to be packaged.

[0081] The tubular winding core A with discontinuous cut T as Fig. 19 may be used exactly as described above. When the leading edge Li shall be inserted into the cut T, the bridges connecting the edges Bl, B2, represented by the uncut segments T4, can be broken easily by the anchoring member 31. To this end, the anchoring member 31 advantageously applies enough force to break or tear the connection bridges of the edges Bl, B2 of the cut T. Alternatively, the connection bridges or uncut segments T4 may remain unbroken, and the tissue paper can be inserted between pairs of uncut segments T4. If the uncut segments T4 are in phase with the cutting of the severing machine, this allows the severing machine to perform more uniform cuts on the whole surface of the roll to be cut, as usually occurs on rolls provided with a traditional (uncut) core.

[0082] The tubular winding core of Fig. 19 ensures shape stability of the tubular core up to the step of breaking or tearing the bridges and of anchoring the free leading edge Li.

[0083] In the embodiments described above, a pneumatic system may facilitate the insertion of the leading edge of web material between the edges of the longitudinal cut. For example, one or more nozzles may be provided, oriented so as to generate one or more air flows pushing the leading edge of the web material between the edges of the longitudinal cut. The nozzles may be either fixed or movable, for example they may be carried by the mechanical member 31, if any. In some embodiments, the leading edge of the web material may be inserted between the edges of the cut only by means of pressurized air blows, without using a mechanical member like the member 31. [0084] The description above illustrates some embodiments of the invention. It is clearly apparent to those skilled in the art that modifications, changes and omissions can be done to the invention without however departing from the protective scope as defined in the attached claims.