Folding machine with suction means for folding a continuous web material and relative folding method Description Technical field The present invention relates to a folding machine to fold a web material along transverse folding lines. More specifically, the invention relates to a folding machine of the type comprising a pair of adjacent counter-rotating folding cylinders having parallel axes, each of which has at least one gripping member to grasp the web material along transverse lines and make folds along said lines. The invention also relates to a means to perform folding of a continuous web material according to transverse zigzag lines. State of the art In many folding machines used in the field of paper converting to produce, for example, folded paper napkins, a continuous web material - optionally previously folded along a longitudinal line - is fed to a pair of counter- rotating folding cylinders, disposed adjacent to each other to define a nip through which the web material passes and having parallel axes. Disposed on each cylinder are members that make the fold in the web material. The aforesaid members are disposed and controlled so that the web material is folded in a zigzag manner, adhering alternately first to one and then to the other of the two counter-rotating folding cylinders. The pack formed of the zigzag folded web material is then cut by a blade and divided into two piles of paper napkins or similar folded products. A machine of this type is described for example in WO-A-9728076 and in WO-A-0162651. Other examples of folding machines are described in US patent 3.195.882, in US patent 3.229.974, in US patent 3.820.774, in US patent 3.689.061 , in German patent 4.446.753 and in German patent 429.288. The folding members of these folding machines include, disposed on each folding cylinder, a gripping member, which grips the web material along a folding line at each turn of the cylinder. To insert the web material into the gripping member, respective folding blades or wedges are disposed on the two folding cylinders, in positions angularly staggered with respect to the gripping members. To make a fold a folding blade of one of the two folding cylinders and a gripping member of the other folding cylinder are disposed in angular positions so that they correspond with each other in the nip defined between the two folding cylinders, so that the web material is pushed by the folding blade inside the gripping member. Typically, each of the two folding cylinders has at least one folding blade and one gripping member, so that for each complete turn of the pair of folding cylinders at least two folds are made on the web material. The presence of folding blades -and gripping members on the counter- rotating folding cylinders makes these machines particularly complex from a mechanical viewpoint. Moreover, the mechanical action of the folding blade or wedge on the web material tends to damage it. The folding blades are subject to rapid wear with consequent maintenance costs. Folding machines of another type and with a different configuration to those to which the present invention relates are described in US-A-5,356,131; US-A-4,921 ,235; WO-A-03016184. Objects and summary of the invention The object of the present invention is to produce a folding machine of the aforesaid type, which is simpler, while at the same time efficient and reliable. This and further objects and advantages, which shall be apparent to those skilled in the art by reading the text hereunder, are obtained essentially with a folding machine comprising: a pair of counter-rotating folding cylinders, each provided with at least one gripping member to fold a continuous web material in a zigzag manner; a cutting member disposed in front of said folding cylinders to cut the zigzag folded web material and divide it into two piles of folded articles; characterized in that each gripping member is a suction member which retains the web material by suction at the respective folding line. The continuous web material (typically and preferably tissue paper for producing handkerchiefs or napkins and the like) is thus engaged alternately on one and on the other of the counter-rotating folding cylinders by suction, and said suction which engages the web material in equidistant areas causes the zigzag folding thereof, without the need to use complex mechanical members to grip the material with relative cam mechanisms to control them. This simplifies the machine, also making it efficient and reliable. To facilitate folding and, optionally, to obtain particularly reliable operation even at high speeds, according to an advantageous embodiment of the invention each of the folding cylinders can be provided with a wedge element in phase with the gripping member of the opposed cylinder, to produce a line which facilitates folding in the web material, at the level of said gripping member. The line which facilitates folding is in substance a sort of crease and can be obtained in a particularly efficient way by providing, for each wedge element, a corresponding notch produced on the opposite cylinder, associated with the suction gripping member. According to a different aspect, the invention relates to a method for producing folded articles from a continuous web material, advantageously and preferably in tissue paper, comprising the phases of: feeding the continuous web material into a nip defined between two counter-rotating folding cylinders, each provided with at least one gripping member; folding the web material in a zigzag manner by means of said folding cylinders, engaging the web material with the gripping members of one and of the other of said folding cylinders alternately; feeding the zigzag folded web material towards a cutting member (60); by means of said cutting member, cutting the zigzag folded web material into two piles of folded articles; characterized in that the web material is retained and folded in said folding cylinders by suction. Advantageously, the method can include a creasing phase of the web material, preferably obtained by means of said folding cylinders. Further advantageous characteristics and embodiments of the machine and of the method according to the invention are indicated in the appended claims and described in greater detail hereunder, with reference to an example of embodiment. Brief description of the drawings The invention shall be better understood by following the description and accompanying drawing, which shows a non-limiting example of embodiment of the invention. In particular, in the drawing: Figure 1 is a plan view of a folding machine incorporating the invention; Figure 2 is a front view of the folding cylinders of the machine; Figure 3 is a plan view according to Ill-Ill in Figure 2; Figure 4 is a longitudinal section of one of the folding cylinders; Figure 5 is a schematic cross section according to V-V in Figure 2 of the - A -

two folding cylinders; Figures 6A and 6B are enlargements, in two subsequent positions, of the gripping members of one of the folding cylinders in the point in which the folded web material starts to detach from the folding cylinder; Figure 7 is a section according to VII-VII in Figure 2; Figure 8 is an enlargement in longitudinal section of the suction area of one of the folding cylinders; Figure 9 is a plan detail of the assembly flange of the suction unit; and Figure 10 is an enlarged local section according to a plane orthogonal to the axes of rotation of the folding cylinders at the level of the folding area, in the instant in which the crease is formed on the web material. Detailed description of the preferred embodiments of the invention With initial reference to Figure 1 , the folding machine has a pair of folding cylinders 1 and 3 rotating according to the arrows f1 and f3 around respective vertical axes of rotation 1A and 3A, disposed parallel to each other and spaced so that the two folding cylinders 1 , 3 are adjacent to each other at the level of a nip 5. The folding cylinder 1 is supported by shanks 1C and 1D (Figure 2) in corresponding supports 7, 9. The folding cylinder 3 is supported in a similar way by shanks 3C and 3D in supports 11 and 13. The two folding cylinders 1 and 3 are made to rotate in opposite directions (arrows f1 and f3 in Figures 1 , 5 and 7) by means of a toothed wheel 14 which meshes with a toothed wheel 15 keyed onto the shaft of the folding cylinder 1, and which in turn meshes with a toothed wheel 17 keyed onto the shaft of the folding cylinder 3. An eccentric 19 is mounted at the upper end of the shaft 1 C of the folding cylinder 1 , and, by means of a connecting rod 21 , provides an alternate movement to a rocker 23 (see Figures 2 and 3). The rocker 23 is keyed onto a vertical shaft 25, parallel to the axes of the two folding cylinders 1 , 3 and carries integral therewith three shaped bars 26 which are inserted into corresponding annular grooves 27 of the folding cylinder 1 (see in particular Figure 2). Hinged to the rocker 23, at the opposite end to the end at which the connecting rod 21 is hinged, is a further connecting rod 29, the opposite end of which is hinged to a bracket 31 keyed onto a shaft 33, parallel to the shaft 25. Integral with the shaft 33 are three bars 35, analogous to the bars 26, which are inserted into corresponding grooves 37 provided on the folding cylinder 3. The number of bars 26, 35 can vary according to the height of the folding cylinders 1 , 3. As can be seen in particular in the cross section in Figures 5 and, limited to the folding cylinder 3, in the section in Figure 4, a longitudinal seat or cavity, indicated as a whole for both cylinders with 41 , is produced inside each of the folding cylinders 1, 3. The two cavities 41 are symmetrical to each other as can be seen in particular in Figure 5 and the folding cylinders are in phase with each other so that the cavities are in diametrically opposite positions. The cavities 41 emerge on the cylindrical surface of each of the two folding cylinders 1 , 3. Housed in each of the two seats or cavities of each folding cylinder 1 , 3 is a gripping member, indicated as a whole with 43, used to grasp and fold the web material N fed into the nip 5 between the two folding cylinders 1 , 3. The two gripping members 43 are symmetrical to each other and only one of them shall be described in detail hereunder with particular reference to Figure 4. The gripping member 43 is essentially composed of a block provided with a series of through holes 45 (disposed in the example shown on four rows parallel to the axis of the folding cylinder). The holes emerge on an essentially flat internal or rear surface 47 of the block 43 and on an essentially cylindrical outer surface 49 positioned flush with the cylindrical surface of the respective folding cylinder 3 (or 1). The cylindrical surface 49 is interrupted at the level of the grooves 37 (or 27) of the folding cylinders 1 and 3 and at the level of said annular grooves has lowered areas 51 , to allow passage of the bars 26, 35. The seat 41 of both cylinders 1 and 3 is in connection with a suction duct 53 extending coaxially to the axis of rotation of the respective cylinder and emerging on the upper front surface thereof, where (as shall be described hereunder) it is carried for a certain angle of rotation of the respective cylinder in communication with a suction line. In this way, through the holes 45 of each block 43 a suction is produced - for a predetermined angle - the purpose of which, as shown in greater detail hereunder, is to temporarily retain the web material to be folded along a transverse folding line. On the cylindrical front surface 49 of each block 43, a cut or notch 55 is produced (see enlargement in Figure 10), into which a wedge 57, produced on an insert 59 disposed on the opposed cylinder, penetrates. As can be seen in Figure 5, on each of the two folding cylinders 1 , 3 the block 43 and the insert 59 are disposed in diametrically opposed positions, so that during rotation of the two cylinders 1 , 3 in the nip 5 each notch 55 of a block 43 mounted on one of the two cylinders comes to correspond with the wedge 57 of the insert 59 mounted on the other cylinder. The machine described hereinbefore operates in the following way. When the folding machine is operating, a web material N, optionally folded longitudinally upstream of the folding cylinders 1 , 3 with systems known per se and not described herein, is fed (arrow fN, Figure 1) through the nip 5 between the folding cylinders 1 , 3. Here it is engaged along transverse folding lines alternately by one or by the other of the two suction gripping members composed of the blocks 43 with the relative suction holes 45. Suction through the holes 45 is activated, for each cylinder 1 , 3 before the line of holes 45 farthest forward with respect to the direction of rotation of the respective cylinder 1 , 3 reaches the nip 5, so that the web material is drawn and retained on the cylindrical surface of the respective folding cylinder. The plurality of lines of suction holes 45 guarantees efficient retention of the web material when this passes through the nip 5 and moves therebeyond, following the movement of rotation of the cylinder. Suction through the holes 45 is maintained until the block 43 is in an angular position approximately 45-60° farther forward than the position of the nip 5. In this way, the web material N, which advances at essentially the same peripheral speed as the folding cylinders 1 , 3, as a consequence of suction, continues to adhere to the folding cylinder 1 or 3 on which suction has been activated, producing a fold as shown in Figure 5 (in this case the fold formed by the cylinder 3 is shown). In the position in Figure 5 suction has been interrupted and the folded edge of web material N is detached from the folding cylinder 3, facilitated in this by the oscillatory movement of the bars 35, the oscillating movement of which is advantageously synchronized with the angular position of the folding cylinders 1 , 3. The fold produced by the gripping member 43 associated with the folding cylinder 3 is indicated with P. Continuing rotation in opposite directions of the two folding cylinders 1, 3, when the suction gripping member 43 is in proximity to the nip 5 suction is activated therethrough to form a fold on the material N, analogous to the fold P produced by the folding cylinder 3. With continual rotation of the folding cylinders 1 , 3 and alternate activation of suction through one or other of the two gripping members 43, a zigzag fold is obtained on the web material N. This thus forms a pack of material P which is fed (Figure 1) according to the arrow F towards and against a cutting member 60 with a cutting edge parallel to the axes of rotation 1A, 3A of the folding cylinders 1 , 3 and disposed on a vertical plane tangent to the two cylinders. The cutting member can be (in a per se known manner) a fixed or movable member and divides the pack P into two piles M1 , M2 of napkins or other folded articles, which are fed towards the unloading area of the machine, where members known and not described in detail herein are disposed to separate the individual packs of articles from each other. To facilitate forming of the folds at the level of the gripping members 43 wedges 57 cooperating with the notches 55 are provided. These two elements are inserted inside each other at the level of the nip 5 (Figure 10), deforming the web material N between them. This forms a line to facilitate folding, essentially in the form of a crease. To obtain this, it is not necessary for particularly high pressure to be exerted between the two elements 55, 57. However, it is advantageous for the notch 55 to be produced in a removable and replaceable portion of the block 43. This removable portion can advantageously be made of a yielding material, such as rubber, which may be relatively hard although with greater elasticity than the metal material of which the remaining part of the block 43 is composed. As can be seen in the drawing, for each block 43, the notch 55 is positioned at the level of the row of suction holes 45 farthest forward with respect to the direction of rotation of the respective folding cylinder 1 or 3. To connect the suction duct 53 of each of the two folding cylinders 1 , 3 to the suction, for example a line connected to a fan, an arrangement is provided of a connection cooperating with the upper front surface of each of the two cylinders, indicated with 1 F for the folding cylinder 1 and with 3F for the folding cylinder 3. This arrangement is shown in detail in Figures 7, 8 and 9 for the folding cylinder 3. A symmetrical arrangement is provided for the folding cylinder 1. By means of a plate 83 in a material with a low coefficient of friction, a shoe 81 rests on the front surface 3F of the cylinder 3. The shoe 81 is connected slidingly to two vertical guide columns 85 clamped in respective sleeves 87. These are in turn fixed on a flange 89 clamped on a portion 91 of the fixed structure of the machine. Two compression springs 93 (one of which can be seen in Figure 8) stress the shoe 81 against the front surface of the respective folding cylinder 1 or 3. The flange 89 is shown in a plan view in Figure 9. Two sets of three holes of the fixing screws 97 of the sleeves 87 to retain the guide columns 85, are indicated with 95. As can be seen in Figure 9, the flange 89 is provided with two curved slots 99. Two screws (one of which is indicated with 101 in Figure 8) are inserted in these slots to fix the flange 89 to the structure 91 of the machine. Thanks to the curved slots 99, by loosening the screws 101 the angular position of each flange 89 can be modified with respect to the axis of the relative folding cylinder 1 or 3. In this way it is possible to adjust the angular position of the respective shoe 81. The structure of the shoe 81 is shown in detail in Figures 7 and 8. It has a main body in which a lowered seat 103 is produced, inside which the plate 83 is housed. The main body and the plate 83 of the shoe 81 have a hole with an oblong section 105, which extends parallel to the axis of the folding cylinder 1 and 3. The oblong hole 105 is joined to a side hole 107 connected to a suction pipe 109 in turn connected to a fan or other suitable suction member, not shown. The oblong hole 105 extends angularly, corresponding to the angle of rotation of the cylinder 1 or 3 along which suction is implemented through the holes 45 of the block 43. With this arrangement, when the folding cylinder 1 or 3 rotates about the axis thereof, suction is produced inside the suction duct 53 for the entire rotation of the cylinder to which the suction duct 53 is connected with the oblong hole 105 produced in the shoe 81. The possibility to adjust the angular position of the flange 89 allows adjustment of the angular positions in which suction starts and terminates inside the suction duct 53. It is clear from the above description that with the suction system associated with the folding cylinders 1, 3 the web material N is folded (optionally with the aid of the crease line which can be produced by the wedge 57 cooperating with the notch 55) without the need for mechanical gripping members controlled and operated by particularly complex cam systems, as is the case in conventional devices. Compared to conventional machines (such as the type described in WO- A-0162651), the machine is consequently much simpler. It is understood that the drawing purely shows a possible embodiment of the invention, which may vary in forms and arrangements without however departing from the scope of the concept on which the invention is based. Any reference numerals in the appended claims are provided for the sole purpose of facilitating reading in the light of the description hereinbefore and the attached drawings and do not in any way limit the scope of protection.