WO2004110913A1

WO2004110913A1 - Folding unit for a sheet-like material and folder apparatus making use of said unit

Info

Publication number: WO2004110913A1
Application number: PCT/EP2004/006459
Authority: WO
Inventors: Jozsefne Fuvesi
Original assignee: Aest-Trading Llc
Priority date: 2003-06-16
Filing date: 2004-06-16
Publication date: 2004-12-23
Also published as: ITFI20030169A1

Abstract

A folding unit for a sheet-like material cooperates with a first folding cylinder (1) to carry out a first transversal folding of a sheet (F2), the unit comprising a second folding cylinder (2), cooperating tangentially with the first cylinder (1) and provided with first gripping means (9) suitable for lifting the sheet from the first cylinder (1), the second folding cylinder (2) also comprising second gripping means (13), suitable for engaging with the sheet (F2) brought to the second cylinder (2) along a second folding line, parallel to the first folding line. The unit also comprises a third folding cylinder (3), cooperating tangentially with the second cylinder (2) and provided with sheet holding means for lifting and holding a front flap of the same sheet. Following a coordinated rotation of the second and third cylinders (2, 3), while the second gripping means (13) of the second cylinder (2) remain engaged with the sheet along the second folding line, the front flap is overturned about the second folding line and, downstream of the third cylinder (3), the sheet is held on the second cylinder (2) in a double transversal fold configuration.

Description

TITLE

FOLDING UNIT FOR SHEET-LIKE MATERIAL AND FOLDER APPARATUS

MAKING USE OF SAID UNIT

DESCRIPTION Field of the Invention

This invention relates to the field of folding apparatus for sheet-like materials, and particularly paper, tissue paper or the like, for the manufacture of multi-ply paper articles, such as for instance paper wipes, handkerchiefs, napkins. Namely, it relates to an apparatus, and more precisely to a unit of said apparatus, for the execution of transverse folds in a material fed in web form.

Background of the invention According to a solution known in the art, an apparatus of this type comprises a pair of counter- rotating cylinders, arranged adjacently with parallel axes so as to be substantially tangential with one another. A first folding cylinder tangentially receives the paper web material, and cooperates with a cutting roller to separate the material into distinct sheets. The cut is made along a generatrix of mutual tangency between the cutting roller and the first cylinder, in turn mutually parallel and counter-rotating. For the purpose, the working surface of the cutting roller has at least two radially-projecting cutters, arranged along respective generatrices equally spaced on the circumference . The working surface of the first cylinder has a corresponding distribution of counter-cutting grooves, so that following to the coordinated rotation between the roller and the first cylinder, the grooves and the cutters match together in the area of tangency, thereby cutting the material . During and after the cut, the paper material is held on the surface of the first cylinder by a vacuum system. Following to the rotation of the first cylinder, the material (now cut in sheets) is conveyed to the area of tangency with the second cylinder, where a fold is made along a transversal line, i.e. parallel to the cutting line .

In greater detail, the folding is a result of the concurrent action of radial deviation devices and paper material gripping devices, distributed respectively along the working surfaces of the first cylinder (in positions alternating with the counter-cutting grooves) and of the second cylinder. Here again, the rotation of the two cylinders is mutually coordinated, so that a deviation device and a gripping device correspond in the area of tangency, at opposite sides of the sheet of paper. Upon operation of the deviation device, the sheet is raised slightly off the surface of the first cylinder and tucked in the gripping device, which is thus able to grasp the material. A known device which, among many others, adopts a solution of this type is for example illustrated in U.S. Patent No. 4,822,328.

As rotation continues, the second cylinder gripping the paper sheet carries along the same, overcoming the resistance of the suction system of the first cylinder. The sheet is thus arranged, transversely folded on the surface of the second cylinder, and may be picked up from the cylinder for any further treatment, or directly for packaging . If a further transversal folding of the sheet is required, the same sheet is transferred to another pair of cylinders, downstream of, and operating in exactly the same way as, the pair just described for making the first fold. An evident complication thus arises, also and especially in terms of overall bulk of the apparatus.

Summary of the Invention The essential object of the present invention is to solve this problem, by providing a folding unit which, when used in an apparatus of the type mentioned above, enables a second transversal fold to be obtained substantially at the same time as the first fold, without the additional bulk and the complications deriving from the need to transfer the sheet from the second folding cylinder to an additional folding unit downstream, as in the case of the known art .

This object is achieved with the unit according to the present invention, the essential characteristics of which are defined in the first of the accompanying claims .

The unit according to the present invention offers the advantage of performing the second transversal fold at the same time as the first, on the second folding cylinder itself. In a preferred embodiment, the second cylinder comprises two parts angularly displaceable with respect to each other, thus offering the further important advantage of permitting to adjust at will the position of the second fold with respect to the first. This increases the flexibility and versatility of the apparatus, meaning that the folding requirements of all the main types of paper articles on the market can be satisfied, with a single item of apparatus.

Brief description of the drawings The characteristics and advantages of the folding unit for sheet-like material and of the folder apparatus making use of said unit according to the present invention will be apparent from the description that follows of embodiments thereof, provided solely as a non-restrictive example, with reference to the accompanying drawings in which: - figure 1 is a schematic perspective view of an apparatus with a folding unit according to a first embodiment of the present invention, exploded and with parts omitted for the sake of clarity; figures 2, 3 and 4 represent the apparatus of figure 1, cross-sectioned normally to the axes of rotation of the cylinders in an intermediate position, in three distinct stages of the folding operation;

- figures 5 and 6 illustrate the apparatus according to the views/sections taken respectively along lines V-V of figure 3 and VI-VI of figure 2. figure 7 is a schematic perspective view of a folding unit in accordance with a second embodiment of the present invention, exploded and with parts omitted for the sake of clarity; - figures 8 and 9 are views in the radial and axial direction, respectively, of an outer cage of the second folding cylinder used in the unit of figure 7;

- figures 10 and 11 are cross-section taken along lines X-X and XI-XI of figures 8 and 9; - figures 12 and 13 are views in the radial and axial directions, respectively, of a core of the same folding cylinder as referred to in figures 7 to 11;

- figures 14 and 15 are cross-section views taken along lines XIV-XIV and XV-XV of the figures 12 and 13; and

- figure 16 schematically depicts a suction timing flange, associated with the folding cylinder according to the embodiment of figures 7 to 15.

Detailed description of the invention

With reference to figures 1 to 6, the folding apparatus according to the invention, as regards the area where the cutting and the first fold are carried out, has the typical overall configuration adopted by the known art and briefly outlined in the introductory part of this description. In particular, a first folding cylinder 1 is visible, tangentially cooperating at one side with a cutting roller 4 and, on the opposite side, with a second folding cylinder 2. The cylinders 1, 2 and the cutting roller 4 rotate integrally to respective shafts Ia, 2a, 4a, pivotally supported at their ends by a frame 10 (figures 5 and 6) . The roller 4 is driven by a motor, not shown in the figures. The operation is transmitted to the first cylinder 1 through a gear drive operating between the shafts 3a and Ia, outside the frame 10. Of this gear drive, only a crown gear integral with the driven shaft Ia, indicated at the numeral 12, is shown schematically in figures 5 and 6. The transmission between the first and the second folding cylinder will be described later. The web material to be cut and folded is indicated at the letter M in figures 2 and 3, and is fed to the point of tangency between the cutting roller 4 and the first folding cylinder 1, by means of a deviation roller R (figures 2 and 3) .

Two cutters 5 are visible on the cutting roller 4, projecting along diametrically-opposed generatrices for cooperating with respective counter-cutting grooves 6 formed, according to an analogous arrangement, in the first folding cylinder 1. The first folding cylinder 1 also comprises two radial deviation devices 7 placed in an intermediate position between the grooves 6. The devices 7 adopt known technical solutions and will consequently not be described in detail. Furthermore, it is clear from figures 2 and 3 that the first cylinder 1 is internally hollow, defining a central chamber Ib that communicates with the outside thanks to a distribution of radial holes 8 formed in the working surface of the cylinder, i.e. the nominal cylindrical surface over which the sheet-like material is held. A suction force is in fact exerted through the holes 8 so as to hold and transport the web material M during and after cutting, as represented in figures 2 and 3. In figure 2, in particular, there can be noticed a sheet Fl on the first cylinder 1, completely cut and transported to the folding area. Suction means, not shown, are provided to obtain a vacuum within the chamber Ib, communicating pneumatically with the same chamber through the shaft Ia, which is also hollow. The second folding cylinder 2 comprises, here again in a manner substantially comparable to the known art, four gripping devices 9 equally spaced along the working surface, suitable for cooperating with the deviation devices 7, as already mentioned above. In particular, figure 2 shows the step in which a deviation device 7 and a gripping device 9 cooperate to fold the sheet Fl, while a sheet F2 (already folded by the gripping device that has operated previously) has been conveyed to the second cylinder 2. Figure 3, on the other hand, refers to an immediately subsequent phase, in which the sheet Fl is leaving the first cylinder 1. The gripping devices 9, here again according to a known solution, are operated by a cam 11, visible in figures 5 and 6, integral with the frame 10 coaxially with the shaft 2a. In fact, the devices 9 comprise rotating shafts 9a, the ends of which protrude axialIy from the cylinder 2 and come into contact with the cam 11 through respective driving arms 9b. The cam 11 is profiled in such a way as to bring the shafts 9a into a gripping position along a predetermined arc of rotation of the cylinder 2, obviously corresponding to the sector in which the sheets are transferred from the first folding cylinder 1.

Going into further detail about the second folding cylinder 2, according to the invention its structure comprises two parts, angularly displaceable with respect to each other in an adjustable manner around the rotation axis of the cylinder. A first part consists of a core, indicated at the numeral 23 in figure 1. The core 23 is integral with the shaft 2a and comprises, in addition to two side flanges 23a lying on planes normal to the axis of rotation, four beams 23b extending axially between the flanges 23a, in an angularly equally spaced manner, each beam 23b accommodating a gripping device 9.

The rotational operation from the first cylinder is actually transmitted to the core 23 of the second folding cylinder. In the depicted embodiment, such transmission comprises gears 23d formed in the periphery of the two side flanges 23a of the core 23, gears that engage with respective crown gears 15 (outlined only in figures 5 and 6), projecting radially from, and integrally with, the shaft Ia of the first folding cylinder 1.

If the circumferential length of the core 23 is followed, the four beams 23b are alternated with wide passages, in correspondence with which four profiled windows 23c are formed on both side flanges 23a. More precisely, as shown clearly in figures 2 to 4, the windows 23c are formed in positions immediately consecutive to the beams 23b, if the direction of rotation of the cylinder is assumed as a reference, and they are circumferentially aligned with the same beams. The width of the windows 23c, measured in the radial direction, is roughly the same as that of the beams 23, and the windows extend circumferentially so as to cover a sector of a few tenth degrees (typically around 40°) .

The second folding cylinder 2 comprises a second part, arranged coaxially with the core 23, namely a cage 24, supported by the core itself so that the former is free for rotation with respect to the latter. The cage 24 in its turn has two side flanges 24a externally overlapping the side flanges 23a of the core 23. In this case, the side flanges 24a are annulus shaped, centrally defining respective apertures 24e that accommodate corresponding thickened portions of the side flanges 23a. The flanges 24a have a complete circular figure, with the same diameter as the flanges 23a, thus obstructing their windows 23c.

Between the two side flanges 24a of the cage 24, four further beams 24b extend axially, substantially in the same fashion as the beams 23b of the core 23 and each slidingly inserted within a pair of mutually opposed windows 23c. The width of the beams 24b is significantly narrower than the angular extension of the windows, so that the above-mentioned reciprocal mutual angular displacement between flanges 23a and 24a can take place; the displacement corresponds precisely to a translation of the beams 24b of the cage 24 along the circumference of the cylinder.

The beams 24b accommodate respective second gripping devices 13, which are similar to the first gripping devices 9, and which are driven by a second cam 14, shown in figures 5 and 6. The second cam 14 is integrally attached to the frame 10, coaxially with the shaft 2a, at the end opposite to the first cam 11 used for driving the first gripping devices 9. The second gripping devices 13 also include rotating shafts 13a, the ends of which protrude axialIy from the cylinder 2 and make contact with the cam 14 through respective driving arms 13b (figure 5) . The second cam 14 is profiled in such a way as to bring the shafts 13a into a gripping position along a given arc of rotation of the cylinder 2, as will be better explained below.

A number of slots 25a, 25b are formed in each side flange 24a of the cage 24, having a curved outline reproducing the curved edge of the flange. The slots are equally spaced along the said edge and run in the circumferential direction to much the same extent as the windows 23c. A group of slots 25a slidingly house the ends of respective shafts 9a of the first gripping devices 9. Further slots 25b, on the other hand, accommodate respective screws 26 which, inserted axially from the outside, engage in threaded seats formed in the flange 23a of the core 23. The flanges 23a and 24a are solidly attached to each other following a coordinated tightening of the screws 26; on the other hand, by removing said tightening, they are able to perform mutual angular displacements, within the limits set by the circumferential extension of the windows 23c (and of the slots 25a and 25b) .

Still in relation to the cage 24, it should also be noted that the relative side flanges 24a have, like the side flanges 23a of the core 23, respective geared profiles 24d for the transmission of the rotational operation downstream of the second cylinder 2. According to the invention, the apparatus includes a third folding cylinder 3, arranged adjacent to the second cylinder 2 so that it is substantially tangential thereto, downstream of the area of mutual tangency between the first and the second cylinder.

The third folding cylinder 3 is in turn made up of a core 33 and a cage 34, the latter mounted on the former substantially in the same way as already described for the second cylinder 2. The core 33 comprises two side flanges 33a, which rotate integrally with a hollow shaft 3a, here again supported by the frame 10. A cylindrical sector 33b extends axially between the two flanges 33a, integrally with the shaft 3a in its turn, in which a plurality of sheet-holding suction holes 33c are radially formed, aligned along a generatrix of the cylinder and communicating with the interior of the shaft 3a. A suction force is exerted, as already seen for the first folding cylinder 1, by creating a vacuum inside the shaft, thanks to suction means not depicted.

Also the flanges 33a exhibit respective geared profiles 33d, which engage with the geared profiles 23d on the periphery of the two side flanges 23a of the core 23. Thus, the core 33 of the third folding cylinder with the suction holes 33c, is kept timed with the first gripping devices 9 of the second cylinder 2, which in their turn are timed with the first cylinder 1, from which they receive their rotational operation.

Also the cage 34 comprises two side flanges 34a, arranged on the outside of the flanges 33a of the core 33 and mounted idle on the shaft 3a. Between the flanges 34a there extends a radial deviation device 36, identical to the radial deviation devices 7 of the first cylinder 1. The device 36 passes through slots 35 formed in the flanges 33a of the core 33, being slidingly received thereby so as to permit mutual angular displacements of a certain extent between core 33 and cage 34, all of this in a way substantially similar to what has already seen for the slots 25a mentioned above for the second cylinder 2.

Finally, the flanges 34a of the cage 34 have respective geared profiles 34d, which engage with the geared profiles 24d of the cage 24 of the second folding cylinder 2. The cage 34 of the third folding cylinder 3, and consequently the relative radial deviation device 36, can therefore be kept timed with the second gripping devices 13 of the second folding cylinder 2, whenever these, in order to adjust the position of the second fold

(as will be explained later on) , are brought closer to - or moved away from - the first gripping devices 9, by angularly displacing the cage 24 with respect to the core

23. With the apparatus according to the invention, and in particular with the unit comprising the second and the third folding cylinder 2, 3, the second transversal folding of the sheets is carried out in the following way. With reference to figure 2, the sheet F2 on which the first fold has already been made (as already described above) , is held and transported to the surface of the second cylinder 2 by the action of one of the first gripping devices 9, which stays in gripping engagement after picking the sheet from the first cylinder 1.

When the sheet F2 reaches the area of tangency with the third folding cylinder 3 (precisely the step illustrated in figure 2) , the generatrix along which the first gripping is being made, is timed with the suction holes 33c formed in the core 33. At this point, the device 9, under the action of the cam 11, disengages from the sheet F2 , the front flap of which, undergoing suction from the holes 33c, is held on the outer surface of the sector 33b, as can be clearly understood from figure 4.

Again with reference to the same figure, as the coordinated rotation of the two cylinders continues, the radial deviation device 36 integral with the cage 34 of the third cylinder 3 reaches the area of tangency, at the same time as the second gripping device 13 which, on the second cylinder 2, immediately follows the first gripping device 9 which has become disengaged from the sheet . In fact, as already mentioned, the cage 24 that supports the devices 13 is kept timed with the cage 34 thanks to the transmission between the geared profiles 24d and 34d. The sheet F2 , under the action of the deviation device 35, is tucked into the gripping device 13, which enters gripping engagement with the material due to the effect of the driving imparted by the second cam 14..

As can clearly be made out from figure 3 , with the mutual movement apart of the two points in engagement with the sheet F2 , the front flap of this sheet is overturned about the gripping line of the second gripping device, which extends transversally in an intermediate position. Finally, when the traction exerted by the gripping device 13 overcomes the resistance offered by the holes 33c, the front flap falls back towards the rear flap, thus achieving the desired second transversal folding. The sheet is thus doubly folded already on the surface of the second cylinder 2, and is picked by the latter for possible further processing or for packaging.

According to an alternative embodiment, shown in the figures from 7 to 16, the gripping and radial deviation functions of the second cylinder 2 and the third cylinder 3 are carried out, instead of by mechanical devices, by pneumatic gripping systems. With reference to these figures, the second cylinder, indicated as a whole with numeral 102, includes a core 123 and a cage 124, again in this case angularly displaceable with respect to one another. The core 123, shown in particular in the figures from 12 to 15, similarly to the core 23 of the first embodiment, has two side flanges 123a integral with the rotation shaft 102a. However, in this case the flanges 123a have a smaller diameter than the nominal diameter of the cylinder. The nominal diameter is in fact defined by four beams 123b which extend axially between the two flanges 123a, angularly equally spaced. The beams 123b, seen in transversal section as in figure 14, have an elongated quadrilateral shape and protrude significantly beyond the edge of the flanges 123a. On its outer side, i.e. that corresponding to the working surface of the cylinder, each beam 123b has a group of suction holes 109a, formed radially in alignment along a generatrix of the cylinder. All the holes 109a of a beam are communicated with a channel 109b, formed axially inside the same beam, and having ends that open to the outside through the flanges 123a. The cage 124 (see, in particular, figures 8 to 11) also has two annulus shaped side flanges 124a, the central apertures 124e of which house respective flanges 123a of the core 123. The flanges 124a axially abut against the protruding beams 123b, the respective external peripheral edges of the flanges being substantially flush with the outer faces of the beams .

Four beams 124b axially join the two side flanges 124a, in a position that is adjacent and consecutive to the beams 123b of the core 123. Also the beams 124b of the cage 124 comprise groups of suction holes 113a, aligned along respective generatrices of the cylinder. A channel 113b extends along the central part of each beam 124b, at the base of the holes 113a and in communication therewith. The channels 113b are open at the ends, through the flanges 124a.

Mutual locking of core 123 and cage 124 in the suitable position, in this embodiment is permitted by means of screws 126, represented in figure 7 by the respective axes. The screws 126 are engaged in threaded holes, formed radially in the side flanges 124a of the cage 124, and are tightened until they become forced against the edge of the flanges 123a of the core 123.

The second folding cylinder 103 (figure 7) consists of a single body 133 integral with the rotation shaft 103a. Like the sector 33b of the core 33, the body 133 also features a distribution of aligned suction holes 133c, while the pneumatic flow passes along a channel 133b, formed axially in an off-center position and open at the two sides of the body 133. The absence of the radial deviation means is made possible by the fact that the pneumatic gripping (particularly that exerted by the second groups of suction holes 113a on the cage 124, for carrying out the second folding) does not require the tucking action that, on the other hand, is required by a mechanical gripping. As a result, it is no longer necessary to produce a third folding cylinder with a two- part structure.

A transmission gear between the shafts 102a and 103a, arranged externally to the support frame of the unit and not illustrated, provides for the driving from the second to the third folding cylinder. Obviously, thanks to this transmission, the third cylinder 103 is timed with the first groups of suction holes 109a formed in the core 123 of the second cylinder 102 (corresponding to the first gripping means of the previous embodiment) , as the mutual angular positioning of the same core and the cage 124 changes. Also the operation from the first to the second cylinder can in this case be transmitted directly on the relative shafts, externally to the support frame.

The suction via the holes 109a and 113a, respectively of the core and of the cage of the second cylinder 102, must be suitably timed, so that the gripping on the sheet is exerted during the required angle of rotation, while on the other hand the gripping is released in the remaining angle. The above, according to the drive timings already described for the first embodiment and determined by the outline of the two cams 11 and 14.

For the purpose, the sides of the second cylinder 102 are closed between two fixed timing flanges 111, one of which is shown schematically in figure 16. Each flange 111, provided with a central hole Ilia to permit the passage of the shaft 102a, obstructs at one side the ends of the channels 109b, 113b, ends which are open on the side flanges of the core and of the cage. However, along the circular paths defined by the channels 109b, 113b on each fixed flange 111, respective distributors 111b, 111c are formed, consisting of pass-through and concentric arc- shaped notches, each having a width substantially corresponding to the diameter of the channels. In particular, an inner, minor distributor 111b controls the channels 109ba of the core 123, whereas an outer, main distributor 111c controls the channels 113b of the cage 124.

The distributors 111b, 111c thus open the channels to the outside only along an arc of their path, the length of which is determined by the circumferential extension of the notches. The opening of the channels 109b, 113b leads to the suction being established, since respective suction orifices, represented schematically in figure 16 and indicated at the numeral 114, are located on the outside of the distributors 111b, 111c. Conversely, during the remaining arc of rotation, the channels will be obstructed and thus closed by the flange 111. As a result, suction will cease.

A perfectly similar, though simplified, timing system is adopted to periodically interrupt suction by the holes 133c of the third cylinder 103, and thus assist the release of the sheets in the final step of the second folding operation. The same suction timing system, which in any case is widely known when considered as such, may also be used in the first embodiment, to make the suction of the holes 33c of the third cylinder 3 intermittent, and accordingly optimize the gripping and releasing action of the front flap of the sheet, during the second folding.

It will be readily understood how, thanks to the unit according to the invention, with the particular function accomplished by the second gripping means of the second cylinder, and by the third folding cylinder, there is no longer any need to transport the sheets to an extra unit/apparatus for the second folding. Dimensions are considerably compacted as a result, and the manufacturing process drastically simplified. In this way, the main object of the invention in fully achieved.

Moreover, with the preferred solution in which the second folding cylinder is built in two mutually displaceable parts, i.e. with the possibility of adjusting the reciprocal position of the first and second gripping means (keeping them constantly timed with the complementary means on the third folding cylinder) , it becomes possible to move the line of second folding closer to - or further away from - that of first folding. The adjustment, performed as mentioned by angularly moving the cage 24, 124 with respect to the core 23, 123 about the axis of rotation of the cylinder, allows the most varying production requirements to be satisfied with a single apparatus item.

The construction details adopted in the illustrated embodiment, in particular but not exclusively those related to the reversible locking between the core 23 and the cage 24, may undergo modifications within the scope of an equivalent working concept. For instance, the cage 24, i.e. the so-called "second" part of the second folding cylinder 2, may also be made of several, non-integral parts, i.e. each capable of locking on the core 23 in an independent manner. Finally, it must be pointed out that, with obvious adaptations with respect to what described above, the second cylinder 2 may be made in such a way as to include first and second gripping devices in a number less than - or greater than - four. At the same time, an adjustment of the configuration of the third cylinder 3 could obviously be required. Other variants and/or changes may be made to the folding unit for sheet-like material and to the folder apparatus employing said unit according to the present invention without departing from the protective scope of the invention itself.

Claims

1. A folding unit for sheet-like material (M), suitable for cooperating with a first folding cylinder (1) to carry out a first transversal folding on a sheet (F2) , said unit comprising: a second folding cylinder (2) , cooperating tangentially with said first cylinder (1) and comprising first gripping means (9) suitable for lifting the sheet (F2) off said first cylinder (1) , by gripping it along a first transversal folding line, and transporting the folded sheet (F2) to the second cylinder (2) ; control means (11) of said first gripping means (9) , for bringing the latter to a gripping position along a given angle of rotation of said second cylinder (2) ; and drive means (15, 23d) for transmitting to said second cylinder (2) a rotating operation opposite to, and coordinated with, the first cylinder (1) , the unit being characterized in that said second folding cylinder (2) also comprises second gripping means (13) , arranged adjacently and consecutively to said first gripping means (9) , for engaging with said sheet (F2) transported to the second cylinder (2) , along a second folding line parallel to the first folding line, control means (14) of said second gripping means (13) being also provided for bringing the latter to a gripping position along a given angle of rotation of said second folding cylinder (2) , the unit being further characterized in that it includes: a third folding cylinder (3), cooperating tangentially with the second folding cylinder (2) ; sheet-holding means (33c) formed in said third folding cylinder (3) , suitable for lifting and holding a front flap of said sheet (F2), when disengaged from said first gripping means (9) ; and drive means (23d, 24d, 33d, 34d) for transmitting to said third cylinder (3) a rotational operation opposite to, and coordinated with, said second cylinder (2) , so that following the coordinated rotation of the second and of the third cylinder (2, 3), while the second gripping means (13) of the second cylinder (2) remain engaged with said sheet (F2) along said second folding line, said front flap is overturned about said second folding line, and downstream of said third cylinder (3) said sheet (F2) remains held on said second cylinder (2) in a double transversal fold configuration.

2. The unit according to claim 1, wherein said holding means (33c, 133c) comprise a plurality of suction holes

(133c) formed in alignment along a generatrix of said third cylinder (3) , said holes (133c) being communicated with an axial channel (133b) , open at least at one end to communicate with suction means arranged externally to said third cylinder (103) .

3. The unit according to claim 1 or 2 , wherein said first and second gripping means comprise respective groups of suction holes (109a, 113a) formed in alignment along respective generatrices of said second cylinder (102) , said holes being communicated with respective axial channels (109b, 113b) , said channels being open at least at one end to communicate with suction means arranged externally to said second cylinder (102) .

4. The unit according to claim 3 , wherein said control means of said first and second gripping means of the second cylinder comprise at least one timing flange (111) arranged on one side of the cylinder (102) to obstruct the end apertures of said axial channels (109b, 113b) , and distributor notches (111c, 111b) formed in said at least one flange (111) along respective arc-shaped paths of said axial channels (109b, 113b) , respective suction orifices (114) of said suction means being placed on the outside of said distributor notches (111c, 111b) .

5. The unit according to claim 3 or 4 , wherein said second folding cylinder (2, 102) comprises two parts (123, 124), first groups (109a) of said suction holes being formed in a first part (123), whereas second groups (113a) of said suction holes are formed in a second part (124) of the cylinder (123), said first part and said second part (123, 124) being angularly displaceable with respect to one another about the axis of the cylinder (102) , locking means (126) being also provided for mutually locking the parts (123, 124), said locking means (126) being releasable so as to permit said mutual angular displacement .

6. The unit according to claim 5, wherein said parts consist respectively of a core (123) integral with a central rotating shaft (102a) of the second cylinder (102) , and a cage (124) arranged coaxially with, and externally to, said core (123), said core (123) and said cage (124) comprising respective pairs of side flanges (123a, 124a) lying orthogonally to the axis of rotation of the cylinder, mutually engaging in an angularly displaceable manner, a plurality of beams (123b, 124b) extending axially between said side flanges in a equally spaced way and defining respective portions of the working surface of the second cylinder (102), said first groups (109a) of suction holes with the relative axial channels (109b) being formed in respective beams (123b) of the core (123) , while said second groups (113a) of suction holes with the relative axial channels (113b) are formed in respective beams (124b) of the cage (124) .

7. The unit according to claim 6, wherein said side flanges (123a) of said core (123) have a diameter lower than the nominal diameter of the second cylinder (102) , said nominal diameter being defined by beams (123b) having an elongated quadrilateral cross section, protruding significantly in the radial direction beyond the edge of the relative side flanges (123a) , said side flanges (124a) of said cage (124) being annulus shaped, axially abutting against the beams (123b) of the core (123) and defining central apertures (124e) which accommodate respective flanges (123a) of the same core (123) .

8. The unit according to claim 7, wherein said locking means (126) between the parts of said second cylinder

(102) comprise a plurality of screws (126) engaging in threaded holes formed radially in the side flanges (124a) of the cage (124) , for being forced against the edge of the flanges (123a) of the core (123) .

9. The unit according to any of the claims from 3 to 8 , wherein said drive means of said third cylinder (3) comprise gear means (33d) engaging with gear means (23d) integral with the second cylinder (102), in turn engaging with gear means (15) integral with the first cylinder (101) .

10. The unit according to any of the claims from 5 to 9, wherein said drive means of said third cylinder comprise gear means (33d) engaging with gear means (23d) integral with the core (23) of the second cylinder (102) , in turn engaging with gear means (15) integral with the first cylinder (101) .

11. The unit according to claim 1 or 2 , wherein said first and second gripping means (9, 13) of said second cylinder (2) comprise respectively first and second mechanical gripping devices (9, 13) , said third folding cylinder (3) comprising at least one radial deviation device (36) suitable for cooperating with said second gripping devices (13) of said second cylinder (2) for assisting their engagement with said sheet (F2) along said second folding line.

12. The unit according to claim 11, wherein said control means of said first and second gripping means (9, 13) comprise respectively first and second fixed cams (11, 14) , arranged on the two sides of the second cylinder (2) , said first and second cams (11, 14) being contacted by the ends of rotating shafts (9a, 13a) respectively of the first and second gripping devices (9, 13), so that said gripping devices (9, 13) are brought into a gripping position in a given arc of rotation of the second cylinder (2) .

13. The unit according to claim 11 or 12, wherein said second cylinder (2) comprises two parts (23, 24), said first gripping devices (9) being supported by a first part (23) , while said second gripping devices (13) are formed in the second part (24) of the cylinder (2) , said first part and said second part (23, 24) being angularly displaceable with respect to one another about the axis of the cylinder (2) , locking means (26) being also provided for mutually locking the parts (23, 24), said locking means (26) being releasable so as to permit said mutual angular displacement, said third cylinder (3) comprising in turn two parts (33, 34) angularly displaceable with respect to one another, said radial deviation device (36) and said suction holes (33c) being arranged respectively on the two parts (34, 33) of said third cylinder (3) .

14. The unit according to claim 13, wherein said parts of the second cylinder (2) consist respectively of a core

(23) integral with a rotating central shaft (2a) of the cylinder, and a cage (24) arranged coaxially with and externally to said core (23) , said core (23) and said cage (24) comprising respective pairs of side flanges (23a,

24a) lying orthogonally to the axis of rotation, reciprocally engaging in an angularly displaceable way, a plurality of beams (23b, 24b) extending axially in equally spaced manner between said side flanges (23a, 24a) and defining respective portions of the working surface of the second cylinder (2) , the beams (23b) of said core (23) supporting respective first gripping devices (9) , whereas the beams (24b) of said cage (24) support respective second gripping devices (13), said locking means (26) being arranged between said side flanges (24a) of said cage (24) and said side flanges (23a) of said core (23) .

15. The unit according to claim 14, wherein the side flanges (24a) of said cage (24) externally overlap the side flanges (23a) of said core (23) , the beams (24b) of the cage (24) passing through profiled windows (23c) formed in the side flanges (23a) of the core (23) .

16. The unit according to claim 15, wherein on each of said side flanges (24a) of said cage (24) two or more slots (25a) are formed, along a curved path in accordance with the peripheral edge of the flanges (24a) , said slots (25a) being arranged equidistantly along said edge, said locking means comprising means (26) for reversibly tightening said side flanges (23a) of said core (23) to said side flanges (24a) of said cage (24) , received slidingly by said slots (25a) .

17. The unit according to claim 16, wherein said locking means (26) comprise a plurality of screws (26) engaging in respective threaded seats formed in said side flanges (23a) of said core (23) , and abutting, when tightened, on the outside of said side flanges (24a) of said cage (24) .

18. The unit according to any of the claims from 14 to 17, wherein said parts of said third folding cylinder (3) consist respectively of a core (33) integral with a rotating central shaft of the cylinder (3a) , and a cage (34) arranged coaxially with and externally to said core (33) , said core (33) comprising a cylindrical sector (33b) in which said suction holes (33c) are formed, and a gap inside which said radial deviation device (36) is arranged supported by said cage (34) , said drive means of said third cylinder (3) comprising gear means (34d) integral with the relative cage (34) , engaging with gear means (24d) integral with the cage (24) of the second cylinder (2) , and gear means (33d) integral with the relative core

(33) , engaging with gear means (23d) integral with the core (23) of the second cylinder (2) , in turn engaging with gear means (15) integral with the first folding cylinder (1) .

19. The unit according to claim 18, wherein said cage

(34) and said core (33) of said third folding cylinder (3) comprise mutually overlapping pairs of side flanges (33a, 34a) , between which said sector (33b) and said radial deviation device (36) extend, said gear means (33d, 34d) consisting of geared profiles of said flanges (33a, 34a) , engaging with geared profiles (23d, 24d) formed in the side flanges (23a, 24a) of the second folding cylinder

(2) .

20. The nit according to any of the claims from 2 to 19, wherein said suction holes (33c) of said third cylinder

(3) are controlled by control means comprising at least one flange arranged on a side of the cylinder (3) to obstruct the end aperture of the relative axial channel

(133b) , and a distributor notch formed in said at least one flange along an arc shaped path followed by the channel (133b) , a suction orifice of said suction means being provided on the outside of the distributor notch.

21. A folder apparatus for sheet-like material (M), comprising: a first folding cylinder (1) to carry out a first transversal folding on a sheet (F2) ; a second folding cylinder (2) , cooperating tangentially with said first cylinder (1) and comprising first gripping means (9) suitable for lifting the sheet (F2) off said first cylinder (1) , by gripping it along a first transversal folding line, and transporting the folded sheet (F2) to the second cylinder (2) ; control means (11) of said first gripping means (9) , for bringing the latter to a gripping position along a given angle of rotation of said second cylinder (2); and drive means (15, 23d) for transmitting to said second cylinder (2) a rotating operation opposite to, and coordinated with, the first cylinder (1) , the apparatus being characterized in that said second folding cylinder (2) is part of a unit according to any of the previous claims.