Method of making a plate cylinder for a web printing press
The invention relates to a method for producing a plate cylinder for web printing presses, in particular for intaglio printing. 
     The main requirements which must be fulfilled by a plate cylinder for web printing presses, i.e. for printing endless paper webs, are exact concentricity for achieving true running, in order that the contact pressure during the printing process and, in the case of intaglio printing, above all during wiping, remains constant, also, a printing plate surrounding the periphery of the plate cylinder virtually without a joint as well as high stability such that the contact forces exerted in particular during the intaglio printing method by the printing cylinder do not cause any deformation of the casing of the plate cylinder. Methods known hitherto, with which these requirements could be fulfilled in a general satisfactory manner, consist essentially in that the printing design is produced on the surface of the plate cylinder either by the transfer method, i.e. by rolling a roller provided with profiles corresponding to the printing design against the surface of the plate cylinder, or, however, by a galvanic method. Both known methods are expensive and complicated. 
     The invention intends to provide a simpler method, by which it is possible to produce high quality plate cylinders, in particular for intaglio printing of bank notes and postage stamps. 
     This object is fulfilled according to the invention due to the fact that a cylinder casing of ductile material is prepared and also printing plates are cut and bent such that they surround the outer periphery of the cylinder casing virtually without a joint, that these printing plates are arranged on the cylinder casing with the interposition of a layer of hardenable adhesive, that this cylinder casing provided with the printing plates is inserted in a hollow cylindrical multi-part mould, with an inner diameter corresponding to the outer diameter of the plate cylinder to be produced and is expanded radially to such an extent that the printing plates are pressed against the hollow cylindrical mould at a predetermined pressure, whereupon the adhesive is left to harden and after hardening, the cylinder casing supporting the printing plates is removed from the hollow cylindrical mould. 
     This method according to the invention has the advantages that firstly, in the normal manner, printing plates, in particular engraved intaglio printing plates, can be produced independently of the subsequent plate cylinder and then these plates can be attached by sticking to the smooth cylinder casing in the manner afore-described, in which case the cylindrical hollow mould acting as an abutment on the one hand makes it possible to exert an adequate sticking pressure due to radial expansion of the cylinder casing and on the other hand guarantees the exact concentricity of the latter. Therefore, the method according to the invention is simpler and more economical than production methods known hitherto. 
     A cylinder casing is preferably produced with its inner periphery forming a hollow cone and is expanded radially in the hollow cylindrical mould by pressing a conical core into the hollow cone. In this way, the radial contact pressure or radial expansion of the cylinder casing can be achieved in a particularly simple manner and accurately to the desired extent. 
     Advantageously, a film of a commerically available epoxide based adhesive can be used as the adhesive layer, in which case, for the purpose of hardening the adhesive film, when the hollow cylindrical mould is closed, the entire arrangement is heated and kept at a predetermined temperature for a certain period of time. 
     The printing plates to be located on the cylinder casing may preferably be engraved nickel plates which, after being attached to the cylinder casing, i.e. after completing the plate cylinder, are chromium-plated in a suitable gavanic bath. 
     The cylinder casing and the conical core preferably consist of steel or a steel alloy. 
     The apparatus for carrying out the method according to the invention is characterised by a two-part hollow cylindrical mould with an exactly concentric inner periphery adapted to the outer diameter of the plate cylinder to be produced and by a conical core whose conicity corresponds to that of the hollow cone of the cylinder casing. 
     A plate cylinder produced according to the method of the invention is characterised by the fact that it comprises a cylinder casing and curved printing plates stuck thereto, which plates surround the cylinder casing virtually without a joint. 
    
    
     The invention is described in detail with reference to the drawings as regards one example of carrying out the method and one embodiment of the corresponding apparatus: 
     FIG. 1 is an axial section through the closed hollow cylindrical mould, in which the cylinder casing with the printing plates located thereon, with the interposition of an adhesive layer and the conical core inserted in the hollow cone of the cylinder casing are located, 
     FIG. 2 is an end view of the arrangement according to FIG. 1 and 
     FIG. 3 is a perspective view of the closed hollow cylindrical mould. 
    
    
     As a support for the printing plates of the plate cylinder, a cylinder casing 2 of a suitable ductile material, preferably a steel alloy, is produced, the inner periphery of which is constructed as a hollow cone 4, the conicity amounting to 2° for example. Furthermore, a conical core 6 is produced, likewise from a suitable steel alloy, whose conicity corresponds to that of the hollow cone 4 of the cylinder casing 2. The inner periphery of the cylinder casing 2 and the outer periphery of the conical core 6 are cleaned with a suitable solvent and then the conical core 6 is pushed into the hollow cone 4 of the cylinder casing 2 to such an extent that it is held in the latter with a slight interference fit. 
     On the other hand, two printing plates 1a and 1b, in particular engraved nickel plates, are produced according to known methods and cut and bent such that their radius corresponds to that of the cylinder casing 2 and they surround the outer periphery of the latter virtually without a joint. 
     After the end faces of the cylinder casing 2 provided with the conical core 6 have been covered with dust caps, the surfaces which are to be subsequently stuck together, i.e. the outer periphery of the cylinder casing 2 and the inner surfaces of the curved printing plates 1a and 1b are sand-blasted or shot-blasted and then de-greased with perchloroethylene. In order to prevent excess adhesive material from subsequently sticking to the printing plates, their outer printing surfaces are covered with a mould parting compound. 
     Immediately after de-greasing of the cylinder casing 2, a layer 3 of hardenable adhesive is applied to the periphery of the latter. For this purpose, it has been found that a commerically available epoxy resin based adhesive film with a thickness of 0.02 to 0.05 mm is particularly suitable. Tests were carried out for example with adhesive films having a thickness of 0.02 and 0.4 mm, which are sold under the trade names REDUX 312 UL by Ciba-Geigy and FM 123/5 by the U.S. company Cyanamid. Whilst avoiding air pockets, the adhesive film hardening in the heat is rolled around the cylinder casing 2 so that the edges overlap. The overlap is then cut with a knife such that the overlap region of the adhesive film can be removed by forming a perfect butt joint. 
     In addition, a hollow cylindrical mould preferably of steel is prepared, which consists of two semi-cylindrical partial moulds 5a and 5b. In the assembled state, this mould has an exactly concentric inner periphery and an inner diameter corresponding to the outer diameter of the plate cylinder to be produced, generally amounting to between 20 and 30 cm. In order to strengthen this hollow cylindrical mould, flanges 10a and 10b are provided on the end faces of the partial moulds 5a and 5b, which flanges in the assembled state, form a square flange. Furthermore, on both end faces, the flanges are interconnected by strengthening ribs 11, which are formed on the periphery of the partial moulds 5a and 5b. Diametrically opposed ribs serve to receive securing bolts 7, by which the two partial moulds 5a and 5b are secured one to another. 
     After the inner periphery of the partial moulds 5a and 5b has been cleaned and covered with a mould parting compound, the first semi-cylindrical curved printing plate 1a is inserted in the first partial mould 5a arranged horizontally and with its concave inner surface directed upwards. Then, the cylinder casing 2 covered with the adhesive film 3 is lowered onto this printing plate 1a, the second printing plate 1b is placed on the metal cylinder 2 and finally the second partial mould 5b is put in position and fixed to the lower partial mould 5a by means of the securing bolts 7. 
     After the completion of this arrangement, the conical core 6 is pressed axially into the hollow cone 4 of the cylinder casing 2 in the direction of the arrows in FIG. 1, in which case the cylinder casing 2 is expanded radially to such an extent that the printing plates 1a and 1b are pressed with a predetermined pressure against the hollow conical mould 5a and 5b. In the example in question, this expansion causes an increase in the diameter of approximately 0.2 mm and the contact pressure produced in this way, which thus provides the necessary adhesive force for perfect connection of the printing plates 1a and 1b to the surface of the cylinder casing 2, amounts to 200-350 N/cm 2  (Newtons per square centimeter). Therefore, the dimensions of the hollow cylindrical mould 5a, 5b are such that its inner diameter is approximately 0.2 mm greater than that of the cylinder casing supporting the printing plates before its expansion. 
     Subsequently, to harden the adhesive layer 3, the arrangement is heated to a temperature of 120° C. and maintained at this temperature for approximately 30 minutes. In order to obtain uniform heating, in the example in question, in the vicinity of its periphery, both the conical core 6 as well as the partial moulds 5a and 5b of the hollow cylindrical mould are provided with axis-parallel heating elements 8, which are preferably electrical heating elements. After hardening is completed, the entire arrangement is cooled, for example by means of a coolant flowing through the central bore 9 of the core 6, the conical core 6 is removed and the hollow cylindrical mould 5a, 5b is opened for the removal of the cylinder casing 2 provided with the printing plates 1a, 1b. 
     Then, if necessary, any excess adhesive is removed, the surfaces of the printing plates 1a and 1b are polished and carefully de-greased with perchloroethylene. 
     The last stage of the treatment consists of immersing the cylinder casing 2 supporting the printing plates 1a, 1b in an electrolytic chromium plating bath and in this way of providing the nickel plates with a layer of chromium having a depth of approximately 25 microns, whereupon the plate cylinder which is now complete is washed. 
     The invention is not limited to the afore-described stages of the method and the afore-described apparatus for carrying out the method, but may have numerous variations. Thus, in particular the radial expansion of the cylinder casing for producing adequate contact pressure of the printing plates against the hollow cylindrical mould and thus for producing secure adhesion of the printing plates on the cylinder casing may also be brought about with means other than those described. Therefore, secure adhesion of the printing plates to the cylinder casing is necessary, because during the printing operation and in the case of intaglio printing, also during wiping, the printing plates are subjected to considerable tangential forces however without their being any danger of displacement of the printing plates on the periphery of the cylinder casing or plate cylinder.