PRINTING PLATE FOR OFFSET PRINTING

Abstract: The invention relates to a printing plate (1) for offset print­ ing comprising a number of printing patterns (5), preferably alike in their outer contour, which are located on the printing plate (1) parallel to one another and adjoining one another but displaced in longitudinal direction in relation to one another. The printing plate (1) is made in one piece with a width which corrresponds to the combined width of the individual printing patterns (5) and the printing plate (1) is provided along its front edge (3) and its rear edge (2) with a stepped edge contour. The printing plate (1) is arranged around a carrier cylinder (11) which is fully surrounded by the printing plate (1), parts of the printing plate (1) being accommodated in a holding device (24) arranged within the contour of the carrier cylinder (11).

Description: The present invention relates to a printing plate for offset printing comprising a number of printing patterns, preferably alike in their outer contour, which are located on the printing plate parallel to one another and adjoining one another along their longitudinal sides but displaced in longitudinal direction in relation to one another. The invention also relates to a printing cylinder comprising a carrier cylinder which is provided with a printing plate in accordance with the invention. In the printing of blanks for packing containers, for example, material webs of paper or cardboard are used which are wide enough for a number of package blanks to be arranged side by side. The printing of such a wide web naturally brings with it an increased capacity and improved economy. Since package blanks in particular frequently are provided with lugs and thus do not have a straight edge contour, it is necessary in order to avoid wastage to displace the blanks in relation to one another so that a minimum of wastage is obtained. Hence it is endeavoured to displace the blanks so in relation to one another that a projecting lug from one blank fits into a corresponding recess in the adjoining blank and, since the blanks are identical, it is often possible by mutual displacement of the blank placements to achieve a production of blanks completely free of wastage or, in any case, presenting a minimum of wastage. This implies though, that the printing plates for printing of the blanks have to be displaced in relation to one another as the print has to be adapted to the placing of the blank. This problem has been solved previously in such a maner that a number of separate printing plates were mounted on a carrier cylinder with a mutual displacement corresponding to the desired displacement of the pattern on the printed web. This printing plate mounting work is very demanding and time-consuming, since the individual printing plates have to be placed with very great precision in relation to one another which, among other things, requires exceptional professional skill in the person carrying out the mounting of the printing plates on the carrier cylinder. This procedure also has given rise to large quantities of material having to be rejected because of insufficient precision in the mounting of the printing plates. The present invention proposes a method to avoid these difficulties and is characterised in that the printing plate is made in one piece with a width which corresponds to the combined width of the individual printing patterns, and that the printing plate is provided along both its front and rear edges with a stepped edge cutting so as to form a number of substantially rectangular coherent parts, each having a length corresponding to, or slightly exceeding, the length of a printing pattern. The invention is also characterized by a printing cylinder comprising a carrier cylinder, each of the said printing plate parts being arranged around and surrounding the carrier cylinder, the front edge of the printing plate parts being accommodated in a holding device arranged inside the contour of the carrier cylinder this holding device presenting a plane top side which is located level with the surface of the carrier cylinder and that the rear edge of the printing plate parts is fixed to the plane top side of the said holding device. The invention will be described in the following with reference to the enclosed schematic drawing, wherein Fig.1 shows a printing plate in accordance with the invention, Fig.2 shows a carrier cylinder, Fig.3 shows a carrier cylinder with printing plate fitted, Fig.4 shows a partial cylinder of the carrier cylinder in accordance with Fig.2, and Fig.5 shows the attachment of the printing plate to the carrier cylinder. It is well-known that offset printing plates are manufactured most frequently with the help of a photographic process in the course of which a text and decoration present on a drawing or film is transferred to the printing plate surface which subsequently is etched or treated in some other manner so that light-exposed parts are removed. This can be done, for example, in that the surface of the printing plate has a coating which after exposure to light undergoes a chemical change so that it can be removed whilst unexposed portions remain intact. It is easy on a drawing to indicate exactly the printing decoration for a number of package blanks arranged side by side even if these are to be displaced in relation to one another and it is also easy to transfer text and markings from such a drawing to a coherent printing plate, the position of the individual decorations in relation to one another being accurately defined. Thereafter the printing plate can be edge-cut in the manner as shown in Fig.1 where a number of partial printing plates 6, 7, 8 and 9 can be distinguished, each of which is of substantially square shape (with the exception of rounded portions and rounded corners). The printing plate parts 6, 7, 8 and 9 thus hang together as a unit and they are of the same length. Each of the printing plate parts presents front edges 3 and rear edges 2 and so-called steering holes 4 which are used to achieve an exact placing of the printing plates on the carrier cylinder. The length of each partial printing plate is such that it corresponds to, or slightly exceeds, the circumference of the carrier cylinder onto which the printing cylinder is to be mounted whilst the combined width of the printing plate parts corresponds to the width of the said carrier cylinder. As is evident from Fig.2 the carrier cylinder 11 consists of a cylinder which is mounted on an axle 13. In the case shown here the cylinder consists of a number of separate elements 12 which are threaded onto the axle 13 in such a manner that the cylinder elements 12 are fixed in their position in relation to the axle 13. This is achieved as shown in Fig.4 by providing the centre holes 17 of the partial cylinders 12 with longitudinal ridges which can be fitted into grooves 14 on the axle 13 of the carrier cylinder 11. The partial cylinders 12 thus can be mounted on the axle 13 in mutually displaced positions which are determined by the pitch between the grooves 14 and the ridges 18. The cylinder parts 12 are provided, moreover, with holding grooves 15 in which the printing plates are retained when they are mounted on the cylinder. As is evident from Fig.2 the partial cylinders 12 are mounted on the axle 13 in such a manner that the grooves 15 are displaced in relation to one another and this displacement corresponds to the desired displacement between parts of the printing plates. As is evident from Fig.5 the mounting is done in such a manner that the front edges 3 of the printing plate parts 6, 7, 8 and 9 are introduced into the grooves 15 in which is also introduced a holding device consisting of movable, wedgelike heels 19, 20 and one or more wedgelike elements 21 introduced between the heels which are fastened down with the help of a screw 22 between the heels 19, 20 so that the latter are pressed out against the side edges of the groove 15, so that the front edge portion 3 of the printing plate parts introduced is pressed home and is retained in fixed position. The printing plate 1 is made to surround the carrier cylinder 11, the rear edge 2 of the printing plate parts 6, 7, 8 9, being thus located over the upper plane end surface of the holding device 24 which is level with the surface of the carrier cylinder. The length of the printing plate parts is adapted so that it therefore corresponds to the circumference of the carrier cylinder, except for the part of the printing plate which is used up for fastening in the holding device 24. The ends of the rear printing plate edge 2 will thus join up to the front edge of the printing plate and the rear edge parts 2 are fixed to the upper surface 23 of the holding device 24 with the help of an adhesive or e.g. a double-adhesive tape. The finished printing cylinder is illustrated in Fig.3 and it can be seen that after mounting on the carrier cylinder the front and rear edges of the printing plate 1 have been made to join up along a stepped line 16 which extends alongside the printing cylinder. It has been found that the arrangement in accordance with the invention is very practical and can bring about great savings, since it need never be feared that the individual printing plate parts are fitted imperfectly in relation to one another and the stepped attachment of the printing plate has been found to provide a secure attachment. Owing to the fact that the partial cylinders of the carrier cylinder with their holding groove can be displaced in relation to one another with very great accuracy it is possible to achieve high precision in the mounting of the printing plate.

Claims: A printing plate (1) for offset printing comprising a number of printing patterns (5) preferably alike in their outer contour, which are located on the printing plate (1) parallel to one another and adjoining one another along their longitudinal sides but displaced in longitudinal direction in relation to one another, characterized in that the printing plate (1) is made in one piece with a width which corresponds to the combined width of the individual printing patterns (5) and that the printing plate (1) is provided along both its front edge (3) and its rear edge (2) with a stepped edge contour so as to form a number of substantially rectangular, coherent parts (6-9), each of which has a length coresponding to, or slightly exceeding, the length of a printing pattern (5). A printing cylinder for offset printing comprising a carrier cylinder (11) and a printing plate (1) in accordance with claim 1 arranged on the carrier cylinder (11), characterized in that each of the said printing plate parts (6-9) is arranged around the carrier cylinder (11) which is thus surrounded by the said printing plate parts (6-9), the front edge (3) of the printing plate parts (6-9) being accommodated in a holding device (24) arranged inside the contour of the carrier cylinder (11) this holding device presenting a plane top side which is located level with the surface of the carrier cylinder (11) and that the rear edge (2) of the printing plate parts (6-9) is fixed to the plane top side of the said holding device (24). A printing cylinder in accordance with claim 2, characterized in that the holding device (24) is constituted of two heels (19, 20) displaceable from each other which are arranged in longitudinal grooves (15) of the carrier cylinder (11) and that the said heels (19, 20) are adapted to be displaced from each other by means of one or more wedgelike elements (21) which are introduced between the heels (19, 20) and through a clamping effect engage and retain the front edges (3) of the printing plate parts (6-9) between one of the said heels (20) and the adjoining groove edge of the carrier cylinder (11). A printing cylinder in accordance with claims 2 and 3, characterized in that the carrier cylinder (11) is constituted of a number of circular, cylindrical partial cylinders (12) arranged on a joint axle (13), each comprising longitudinal holding grooves (15) and arrangements (14, 18) for fixing the portions (12) in position in relation to one another and to the axle (13) A printing cylinder in accordance with claim 2, characterized in that the surface of the carrier cylinder (11) is fully covered by the printing plate (1) arranged around the carrier cylinder (11) whose connection between the front and rear edge parts (2, 3) of the printing plate parts (6-9) forms a coherent, stepped line along the surface of the carrier cylinder (11) A printing cylinder in accordance with claim 2, characterized in that the rear edge (2) of the printing plate parts (6-9) is fixed to the upper plane surface of the holding part (24) by means of a double-adhesive tape.

Inventors: HOLSTROEM, HAKAN, Holström, Hakan
Applicants: AB TETRA PAK
Application year: 1985
Main classification: B41N1
IPC codes: B41C1, B41F13, B41N1, B41F27