POSITIONING APPARATUS FOR FLEXIBLE PRINTING PLATES

Abstract: The invention concerns an apparatus for the locating of fle­ xible printing plates (27) on printing cilinders (5). The flexible printing plates (27) to be located on a printing cilinder (5) each comprise two marks (16), and when those two marks are correctly located, the printing plates are positioned correctly. Two light sources (13) are provided each emitting a narrow beam to­ wards the printing plate, so that two markings are projected on the locations of the marks of the printing plate to be fi­ xed. Thus the location of the printing plate can easily take place. Further the projected markings are used to measure a length of adhesive foil such that said length fits exactly on the circumference of the printing cilinder.

Description: The present invention relates to the field of flexible printing plates and in particular to the fixing of such printing plates on printing cylinders. The flexible printing plates, often used in modern printing techniques have to be fixed to a printing cylinder to be used in the printing process. Of course, the location thereof has to take place very accurate as the picture to be produced by the printing plate has to be provided exactly on the right location on the material to be printed. The requirements for accuracy become even greater, when the material to be printed has to be printed subsequently with different colours. Hence, it is of utmost importance that such flexible printing plates are positioned as accurate as possible on the printing cylinder. Hitherto, such a positioning often took place by hand in which the positioning of the printing plate was judged with the eye and was adapted where necessary. Often, for instance a pencil line, located on the cylinder, was used as a help, such that the side of the printing plate to be positioned was applied parallel thereto. Also trial prints were made to determine whether the printing plates located on the printing cilinders, were correctly positioned. It will be clear that this procedure, as it was used until now, involved a substantial inaccuracy and besides, it took a lot of time. US-A-3616055 shows an apparatus for positioning a flexible printing plate, comprising at least two position marks on a printing cylinder, the apparatus comprising means for rotatably supporting the printing cylinder and at least two light sources movable in a direction parallel to the axis of the printing cylinder for projecting at least two light markings onto said printing cylinder. According to this specification a square pattern is projected onto the plate which pattern can be relocated such, that one line thereof co-incides with a line provided onto the printing cylinder. Subsequently, the square pattern is referred to the line on the printing cylinder and the printing plate can be referred to the square patters, so that it can be positioned such, that the depictions to be printed are correctly positioned relative to the square pattern. The plate onto which the printing plate is present is moved in a forward direction and subsequently the printing plate is applied onto the printing cylinder. US-A-3186060 describes an apparatus for providing curved rigid printing plate onto printing cylinders. These plates are put into register by entering the position of the first printing plates by positioning two lamps, so that the markings protected by said lamps co-incide with two marks provided on the printing plates. Subsequently, the printing plates for the other colours are positioned such that the marks present thereon co-incide with the markings projected by the lamps. It should be noted, however, that there is question of semi-cylindrical rigid printing plates, so that the positioning thereof only has to be made in two dimensions. In the radial direction the position of the printing plate is determined by the diameter of the cylinder. EP-A-0089015 describes a method for attaching flexible printing plates on a printing cylinder in which two television cameras have been provided, each recording the image of a printing plate to be positioned on the printing cylinder, and this plate is via a monitor. By combining the signals of both cameras onto the screen the words of position of the printing plates can be recorded and they can be corrected such, that relevant images are in register after which the printing plates are fixed onto the printing cylinders. For printing with more than two colours the images of more than two video cameras are combined. The present invention provides an apparatus according to which the printing plates can be applied very accurately on the printing cylinder. This aim is reached by an apparatus of the kind, referred to above, comprising: means for rotatably supporting the printing cylinder; at least two light sources movable in a direction parallel to the axis of the printing cylinder for projecting at least two light markings onto said printing cylinder. As use is made of the marks, being present on nearly every flexible printing plate, which were until now used for determining whether the printing plates were on the right spot, this apparatus can also be used for printing plates already existing. Thus it becomes possible to fix printing plates on the printing cylinder with a substantial amount of accuracy. Subsequently, the present invention will be elucidated with the help of the accompanying drawings in which: fig. 1: is a schematic perspective view, partially broken away of an apparatus according to the present invention; and fig. 2: is a schematic perspective view, partially broken away of the apparatus depicted in fig. 1 and comprising means for fixing double-sided-adhesive tape on the printing cylinder. The apparatus depicted in fig. 1 comprises a frame 1 with two guiding shafts 2 onto which two movable supports 3 have been provided. Each of the supports 3 comprises two suspension wheels 4 onto which a printing cylinder 5 can be located. The printing cylinder 5 can be fixed onto the suspension wheels 4 by means of fixation wheels 6 which have been provided through a bracket 7, a rack 8 and a guiding element 9. The printing cylinder 5, located on the supporting wheels 4, can rotate freely. At the upper side of the frame two plates 10 have been fixed, through which two guiding shafts 11 have been provided such, that these are parallel to the shafts of the printing cylinder 5. Two yokes 12 are movable along these guiding shafts 11 and in each of the yokes 12 a laser light source 13 has been provided. Each of the laser light sources 13 is driven by means of a spindle shaft 14 which is driven by means of a motor 15. When switched on, both laser light sources emit a narrow ray of laser light downwards unto the printing cylinder 5. Thus, two for instance cross-shaped positioning markings 16 are projected on the printing cylinder 5. Each of the motors 15 is connected with a control unit 17, which can dictate the position of both laser light sources 13 by means of the motors 15 and the spindle shafts 14. A pressing roller 18 has been provided to the frame such by means of two yokes 19, that this pressing roller 18 can be located adjacent with the printing cylinder 5 in which account is taken of printing cylinders with different diameters by the movability of the pressing roller 18. The position of the pressing roller 18 is adjustable by means of two spindle shafts 20 which are actuable simultaneously. Both spindle shafts 20 are driven by means of a hand wheel, a driving shaft 22 and several conical gear transmissions 23. At the outer side of one of the yokes 19 a coupling apparatus 24 has been provided with which, when the pressing roller 18 is adjacent to the printing cylinder 5, both rollers are connected for rotation. Furhter, the pressing roller 18 comprises a driving motor 25 which drives the pressing roller 18 through a worm-wheel transmission 26. Subsequently, the action of the apparatus described above will be elucidated. Initially, the position of the marks 28 located on the printing plate 27 are fed in the control unit 17. It is, of course, also possible that only the number of the printing plate 27 has to be entered and that the control unit 17 comprises a memory in which, except for the number of the relevant printing plates, also the co-ordinates of the marks belonging thereto have been stored. Subsequently, the control unit 17 will make the motors 15 drive the laser light sources such, that the laser light sources 13 both take such a position, that the required positioning markings 16 are projected on the printing cylinder 5 on the required position. Subsequently, the printing plate 27 can be positioned such on the printing cylinder 5, that the markings projected co-incide with the marks 28. When this stage is reached, the pressing roller 18 is driven by means of the motor 25 through which the coupling apparatus 24, the printing cylinder 5 is driven and the printing plate 27 is provided in the right position on the printing cylinder 5. Of course, it is possible that the back layer of the printing plate 27 or the circumference of the printing cylinder 5 has been provided with suitable means of fixation. In the embodiment, shown in fig. 2, a printing cylinder 5 has been provided of which the diameter is considerably larger than that of the printing cylinder 5, depicted in fig. 1. Further, it is shown that the pressing roller 18 is moved until it is adjacent the printing cylinder 5. Further, this embodiment comprises a shaft, extending parallel to the axis of the printing cylinder 5 onto which a coil 31 of couble-sided-adhesive foil has been provided. At the upper side this foil comprises a layer of protective paper. On a shaft 32, also being provided parallel to the axis of the printing cylinder 5, a cutting organ 33 has been provided which is movable along this shaft and which can cut the piece34 of double-sided-adhesive foil leading from the coil 31 towards the printing cylinder 5. In the worm-wheel transmission 26 an angle rotation sensor 35 has been provided, which is connected with the signal generator 36. The angle rotation sensor 35 serves to divide the circumfrence of the printing cylinder 5, which is of importance to determine the initial position of the printing plate departing from an initial or zero-position of the printing cylinder 5. Further, this is necessary, when the printing plate 27 does not extend over the whole circumference and when several printing plates 27 have to be provided on the circumference. To provide a layer of double-sided-adhesive foil on the printing cylinder 5 with this embodiment one handles as follows: initially, the diameter of the printing cylinder 5 is fed into the signal generator 36. Of course, it is possible that, when entering the number of the required printing plate into the control unit 17, the diameter belonging to the printing cylinder is directly fed to the signal generator 36. Subsequently, two positioning markings are being projected on the printing cylinder 5 by means of the laser light sources 13. Afterwards, so much foil is unwound from the coil 31, that the free end of the piece34 co-incides with the projected positioning markings 16. Then the pressing roller 18 is driven by means of the motor 25 in which also the printing cylinder 5 is driven through the coupling apparatus 24 and the gear 29 provided at the printing cylinder, so that the foil is fixed on the printing cylinder 5. This is continued until the signal generator 36 generates a signal stopping the motor 25. Then the piece 34 of foil is cut through by means of the cutting organ 33, after which the pressing process is continued by means of the motor 25, until the piece 34 cut off is completely located on the printing cylinder. The dimensioning of the cutting organ 33 and of the required angle of rotation of the printing cylinder 5 and thus of the pressing roller 18 which are sensed by the angle rotation sensor 35 is such, that the pieceof the piece 34 of foil cut off suits exactly the circumference of the printing cylinder 5. This avoids that a part of the printing cylinder is covered with a double layer of foil, or that a slit developes in the layer of foil. Subsequently, the layer of covering paper can be removed from the foil, and the printing plate can be fixed by means of the method described with the help of fig. 1. In the embodiment shown of the present invention use is made of laser light sources, as the ray of light emitted by these sources is so parallel, that also with printing cylinders with different diameter a projection which is focussed sufficiently, and which is thus accurately, is obtained. When applying printing cylinders with one single diameter, a classical, not coherent light source will suffice. It is also possible to provide video cameras and to record the projected laser marks and to display these through a displaying apparatus of a large size. This allows small relocations of the printing plate to be displayed on an enlarged scale, so that the positioning process can take place more accurately.

Claims: Apparatus for positioning a flexible printing plate (27) comprising at least two position marks (28) on a printing cylinder (5), the apparatus comprising: means (3,4) for rotatably supporting the printing cylinder (5); at least two light sources (13) movable in a direction parallel to the axis of the printing cylinder (5) for projecting at least two light markings (16) on to said printing cylinder, characterized by: a memory (36) for storing the co-ordinates of the marks (28) of the printing plates (27); a control unit (14,15,16) for controlling the movement of the light sources (13) in dependance of the co-ordinates of the marks (28) of a printing plate present in the memory (36). Apparatus according to claim 1, characterized by a rotation sensor (35) for supplying to the signal generator (36) a signal indicating the rotation position of the printing cylinder (5). Apparatus according to claim 1 or 2, characterized by a drive unit (18,25,26) for rotatably driving the printing cylinder (5) to positions, as calculated by the control unit (36) in dependance of the number of printing plates (27) to be mounted on the printing cylinder (5), the size of those printing plates (27) and the positions of the marks (28) on the printing plates (27) to be mounted. Apparatus according to one of the preceding claims, characterized in that at least one camera directed to each of the positioning markings (16) has been provided, and that at least one monitor, representing the image of the camera, has been provided. Apparatus according to one of the preceding claims, characterized by two movable supports (3,4) for the printing cylinder (5) which movable supports (3,4) are movable parallel to the axis of the printing cylinder (5), and a pressing roller (18) which pressing roller (18) is movable until adjacent to the printing cylinder (5), and by a drive unit (24,25,26,29) for the printing cylinder (5) and the pressing roller (18), driving both simultaneously. Apparatus according to one of the preceding claims, characterized by a shaft (30), provided parallel to the axis of the printing cylinder (5) for storing a storage roller (31) of the double-sided-adhesive foil, a cutting organ (33) movable parallel to the shaft (30), and provided on a fixed distance thereto; a rotation sensor (35) for measuring the angle of rotation of the printing cylinder (5); and a signalling generator (36) for generating a signal, when the rotation sensor (35) has registered a pre-determined rotation; and such that the piece of foil (34) cut by the cutting organ (33) is equal to the circumference of the relevant printing cylinder (5).

Inventors: VAN DER HORST, HANS
Applicants: ANDERSON & VREELAND B.V.
Application year: 1989
Main classification: B41F27
IPC codes: B41F27