Method for producing plastics cylinders for printing in general, in particular for rotogravure and flexography, and the cylinder obtained

Abstract: The method according to the invention consists essentially of (a) preparing an inner cylindrical tube of self-supporting rigid material, advantageously epoxy resin reinforced with glass or carbon filament or fabric, (b) coating said cylindrical tube with an expanded plastics material, advantageously rigid polyurethane, (c) making the outer surface of said coating perfectly smooth, (d) making said outer smooth surface of the expanded material electro-conductive by applying metallic materials in any manner, and (e) making said electro-conductive surface engravable, advantageously by applying electroplated copper or applying engravable materials or photopolymer type.

Description: This present invention relates to a method for producing plastics cylinders as described in the preamble of the main claim. The patent also relates to the cylinder obtained by said method. Many methods have been proposed for producing plastics printing cylinders, however such known methods are extremely complicated and difficult to implement, in particular with regard to coating the basic plastics cylinder with metal, as is apparent for example from USA patent 4,503,769 in the name of Andersen Dennis. In addition, cylinders produced by conventional methods are of poor reliability or indeed unreliable. As is well known, currently used cylinders of good reliability are those of steel construction, and rarely of aluminium, and thus various problems arise including those of their weight and their need for dynamic balancing. Further drawbacks are well known to the expert of the art. NL-A-8 302 811 describes a pressure or printing cylinder comprising a core cylinder, a layer of flexible material around such a cylinder, a shell located around the layer and carrying a plurality of clichés or print forms (print moulds). The core cylinder has shaft-trunnions on which bearings and a drive-toothed wheel are mounted. All these parts are made of non-deformable material such as metal or synthetic material. The layer around the core is made of slightly flexible material such as foam like a material, e.g. polyurethane foam with open cells; the layer is connected to the core by means of a glue. The shell around the layer is made of not deformable material such as metal or synthetic material. It supports the print forms made of rubber elastic material or photopolymeric material, said print forms are joined to the shell by means of double-sided glue tape. The shell is not connected to the synthetic foam layer but is tightly arranged around it with a slight contact pressure and is kept in place by means of friction forces which generate during the use of the pressure cylinder. The prior solution has many drawbacks. In particular, it has a plurality of parts (core, layer, shell) separated each other in a very well defined way and this can cause dangerous relative movements between the parts during the use of the cylinder which are detrimental to the quality of printing. An object of the present invention is to provide a method for producing cylinders for printing in general, and in particular rotogravure and flexography, which is very simple to implement using any suitable apparatus. A further object is to provide a method of the aforesaid type which enables plastics printing cylinders to be produced which are of maximum reliability and low weight, so that they can be easily handled and stored. A further object is to provide a method which enables a plastics printing cylinder to be produced, the diameter of which can be varied at will during its production, without encountering particular difficulty. These and further objects of the invention, that will be apparent to the expert of the art from the following description, are obtained by a method as set in the characterizing part of the main claim. The stages of the method according to the invention will be apparent from the following example. A glass fibre fabric or yarn impregnated with epoxy resin is wound about a perfectly cylindrical steel mandrel having a ground, lapped and chromium-plated surface, until a thickness of between 1.5 and 2 mm is obtained. The structure is then polymerized in conventional manner at a temperature of between 80° C and 120° C. The resultant tube is then mounted on the central shaft of a casting mould consisting of a cylinder in which said shaft is centrally positioned and which has its inner surface perfectly ground and lapped. The mould is tightly closed and a quantity of polyurethane is introduced such that the final expanded product will have a density of 0,480 kg/dm³, said polyurethan being prepared by mixing isocyanate and polyol in stoichiometric proportion in a mixing machine. Both the components and the mould are temperature-controlled at 20-35° C, and preferably at 30° C. On removing the structure consisting of the tube and expanded coating from the mould the surface of this coating is perfectly smooth due to the action of the inner mould surface. The structure is checked for perfect cylindricity and if necessary can be ground until of correct geometry. The structure obtained can be made electrically conductive by applying a silver or nickel-based paint by spray or brush to a thickness of 200 µc. Copper is then applied in an electroplating tank to a thickness sufficient to enable the required engraving of the cylinder surface to be effected, this thickness being advantageously 500 µC. The thickness of the expanded polyurethane layer can be varied according to requirements without varying the dimensions of the internal tube, to enable cylinders of variable outer diameter to be mounted on the same standard support shaft.

Claims: A method for producing plastic cylinders for printing of the type comprising an inner cylindrical core or tube of not-deformable material, an intermediate layer of flexible materials such as expanded material located around the core, and an outer layer or shell, said cylinders being in particular for rotogravure and flexography, characterized by comprising the following stages: a) preparing the inner tube by winding a glass or carbon yarn or fabric impregnated with epoxy resin about a cylindrical steel mandrel having a ground and lapped surface until a thickness of between 1.5 and 2 mm is obtained, the structure being then polymerized in conventional manner at a temperature of between 80°C and 120°C; b) coating said inner tube with expanded material, the coating being obtained by locating the tube on the central shaft of a casting mould comprising a cylinder in which said shaft is centrally positioned and having its inner surface perfectly ground and lapped, the mould being tightly closed and a quantity of expanding material, suitably poliurethane, being introduced, the latter intimately connecting to the inner tube; c) applying an electroconductive paint onto the expanded material layer or intermediate layer; and d) electroplating an engravable surface to said intermediate layer through said electroconductive paint so to define the outer layer or shell, the inner tube, the intermediate layer and the outer layer defining a substantially single piece body. A method as claimed in Claim 1, in which the intermediate layer has a density of 0,480 kg/dm³. A method as claimed in claim 1, in which the inner tube, the expanding material and the mould are temperature-controlled at 20-35°C, and preferably at 30° C. A method as claimed in claim 1, in which the structure comprisig the intimately-connected inner tube and expanded material layer is checked for perfect cylindricity and, when necessary, is grounded until the correct geometry is attained. A method as claimed in claim 1, in which the application of the electroconductive paint is made by spraying or brushing it onto the expanded material layer. A method as claimed in claim 1, in which the connection of the engravable surface to the expanded material layer is obtained according to galvanic process per se known by applying said surface in an electroplating tank. A cylinder for printing, in particular for rotogravure and flexography, obtained according to the method claimed in claim 1, characterized by comprising an inner cylindrical tube made of glass or carbon yarn or fabric, an expanded plastic material layer connected to said tube and engravable surface connected to said layer, said tube, layer and surface being intimately connected to each other so to define a substantially one piece body.

Inventors: ROSSINI FELICE, ROSSINI, FELICE
Applicants: ROSSINI ERMINIO SPA, ERMINIO ROSSINI S.P.A.
Application year: 1990
Main classification: B41C1
IPC codes: B41C1, B41N1