Abstract:
A proof press for the mounting of flexographic printing plates, comprising a printing cylinder, which is covered by a blanket for the mounting of at least one printing plate with double adhesive tape interposed, the printing cylinder being movable at right angles to its own axis in order to be placed in contact, along a generatrix, with a respective impression roller for performing proof prints, an optical system for collimating points of the impression roller with respective points of the printing plate by direct viewing, and a compression device for producing the forced adhesion of the printing plate to the double-adhesive tape and to the blanket.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a proof press for mounting flexographic printing plates. 
     As is known, an apparatus for the flexographic printing of corrugated cardboard is provided with one or more flexographic printing plates, which are fixed, by way of double adhesive tape, to corresponding mounting and supporting blankets. Generally, these printing plates are mounted on a curved surface, i.e., on a blanket cylinder of a suitable machine. 
     Printing plates with gradually increased dimensions were used, which gave rise to unwelcome phenomenon such as separation of the printing plate from the blanket, with infiltration of air and formation of pockets, particularly when the blanket with the corresponding printing plate is spread flat. This is due substantially to the generation of two opposite actions: a traction on the blanket and a compression on the thin polyester film provided on the lower face of the printing plate. These opposite actions can reach intensities that overcome the adhesive action of the tape, causing unacceptable bulges in one or more points. 
     However, it has been noted in this regard that the separation of the printing plate from the blanket is less evident if blanket cylinders are used, having a large diameter and in which therefore the ratio between the length of the printing plate and the length of the circumference of said cylinder is low. 
     In order to obviate the problem of the separation of the printing plate from the blanket, machines for mounting the printing plate on the blankets in a flat configuration have been devised: in this case, no significant separations are observed when the blanket and the printing plate are laid flat, but when they are wound in a curved configuration onto a cylinder to perform a proof print, one or more bulges of the blanket with respect to the printing plate are observed. A compression on the blanket and a traction on the printing plate are generated and can overcome the adhesion forces produced by the presence of the tape. 
     Although in this last mounting situation the phenomenon is in practice less conspicuous than observed in conventional mounting on a curved surface, it is known that machines for mounting printing plates in a flat configuration are unable to perform proof prints, which can be performed exclusively on proof presses provided with a blanket cylinder. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide a proof press that is suitable for the mounting of a flexographic printing plate in a curved configuration without the drawbacks noted above, i.e., that is capable of performing proof prints without causing separations of the printing plate from the blanket. 
     Within this aim, an object of the present invention is to provide a structure that is simple, relatively easy to provide in practice, safe in use, effective in operation, and relatively low in cost. 
     This aim and this object are achieved by the present proof press for the mounting of flexographic printing plates, comprising a printing cylinder, which is covered by a blanket for mounting at least one of said printing plates by means of double adhesive tape and is movable at right angles to its own axis in order to be placed in contact, along a generatrix, with a respective impression roller for performing proof prints, and optical means for collimating points of said impression roller with respective points of said printing plate viewed directly, characterized in that it comprises a compression device that is adapted to produce the forced adhesion of said printing plate to said double-adhesive tape and to said blanket, so as to provide uniform fixing over the entire contact surface, avoiding the onset of separation or bulging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will become better apparent from the detailed description of a preferred but not exclusive embodiment of a proof press for mounting flexographic printing plates according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: 
     FIG. 1 is a transverse sectional view of a detail of the separation of the flexographic printing plate from the double adhesive tape for fixing to the mounting blanket, if the printing plate is fixed to the blanket on a cylindrical surface; 
     FIG. 2 is a partially sectional side elevation view of a printing roller, showing the separation of the mounting blanket of the flexographic printing plate, which is fixed to said blanket on a flat surface; 
     FIG. 3 is a transverse sectional view of a detail of the correct configuration for mounting the printing plate on the blanket; 
     FIG. 4 is a partially sectional side elevation view of the proof press according to the invention; 
     FIG. 5 is a partially sectional side elevation view of a detail of another embodiment of the device for compressing the printing plate against the blanket; 
     FIG. 6 is a schematic partially sectional side elevation view of a detail of the machine during the mounting of the flexographic printing plate on the blanket; 
     FIG. 7 is a partially sectional top view of a detail of the proof press according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 4, the reference numeral  1  generally designates a proof press for mounting flexographic printing plates according to the invention. 
     For better comprehension, reference should be made first to FIG. 1, in which a printing plate  2  (made of a material such as photopolymers), lined on its lower face with a polyester film  3  for dimensional stabilization, is fixed, by way of double adhesive tape  4 , to a blanket  5  for mounting on a printing cylinder  6 . As described earlier, mounting the printing plate  2  onto the blanket  5  along a curved surface and then laying said printing plate and said blanket onto a flat surface, the polyester film  3  of the printing plate  2  is seen to separate and bulge with respect to the double adhesive tape  4 . 
     FIG. 2 clearly illustrates the separation and bulging of the blanket  5  with respect to the printing plate  2 , previously assembled on a flat surface, after fixing to a printing cylinder  6 . 
     FIG. 3 instead illustrates the correct configuration for mounting the printing plate  2  on the blanket  5 , i.e., without separations or bulges. 
     As shown in FIG. 4, the machine comprises a footing  7  that rises vertically on opposite sides with two parallel side walls  8 . The side walls  8  are provided at the front with guides  9  for the vertical sliding of respective sliders  10 , which are associated with translational motion means, which are not shown in the figures because they are conventional; each slider  10  forms an upper extension  11  for accommodating, at the top, rolling bearings  12  for rotatably supporting a printing cylinder  6 , which has a horizontal axis and on which a blanket  5  with the corresponding printing plate  2  is mounted. 
     Preferably, the printing cylinder  6  has respective pivot-like ends  13 , each of which rests on respective pairs of rolling bearings  12 . 
     The footing  7  supports rotatably, in its upper portion, an impression roller  14 , whose axis is parallel to the axis of the printing cylinder  6 ; the impression roller  14  is designed to make contact, along a generatrix, with the printing cylinder  6  in order to perform proof prints. 
     The machine has, at the top, optical means  15 , known per se are disclosed in EP 0 728 580 by the same Applicant, which are adapted to produce the collimation of specific points of the impression roller  14  with respective points of the printing plate  2  that are viewed directly. 
     The optical means  15  comprise a semitransparent mirror  16  and a television camera  17 , which are located at the lateral surface of the impression roller  14  and are functionally connected to a monitor  18  that is supported at the top of the footing  17  of the machine. The impression roller  14  is designed to be covered with a suitable sheet of paper provided by suitable tracings of lines and/or dots, provided by means of a writing device  19 ; during the fine-tuning of the machine, said dots are made to collimate with respective dots of the printing plate  2  that are viewed directly by virtue of the optical means  15 . 
     The machine comprises, according to the invention, a device  20  for compressing the printing plate  2  against the blanket  5 , which is fixed onto the printing cylinder  6 , while said printing cylinder is shifted into a suitable downward position. The printing cylinder  6  can therefore perform a translational motion along the guides  9  between a lower position for compressing the printing plate  2  against the double adhesive tape  4  and against the blanket  5 , an intermediate position for mounting the printing plate  2 , and a raised position in which it is in contact with the impression roller  14 . The compression device  20  is suitable to produce the forced adhesion of the printing plate  2  to the double-adhesive tape  4  and accordingly to the blanket  5 : this facilitates the elimination of air pockets, which cause bulges and are present between the polyester film  3  and the double adhesive tape  4 , accordingly achieving uniform fixing over the entire contact surface. 
     The compression device  20  comprises a compaction roller  21 , whose axis is parallel to the axis of the printing cylinder  6 ; said compaction roller is associated with actuation means  22  so that it can move from an inactive configuration to an active configuration in which it is in contact, along a generatrix, with the printing cylinder  6 , in order to produce the uniform adhesion, in mutual rolling, of the printing plate  2  on the double adhesive tape  4  (FIG.  6 ). 
     The compaction roller  21  is constituted by an elongated tubular core  23 , along which a sleeve  24  is keyed; said sleeve is made of substantially flexible material, preferably soft rubber or the like. 
     The actuation means  22  comprise at least one pair of cranks  25 , each of which has a first end  26  that is pivoted, about a first articulation axis A, to a first bracket  27  that is fixed to the footing  7 , and a second end  28  that is pivoted, about a second articulation axis B, to the stem  29  of a linear actuator  30 , preferably of the pneumatic type; in turn, said actuator has, at its other end, its cylinder pivoted to a second bracket  31  that is rigidly coupled to the footing  7 , about a third articulation axis C. The cranks  25  are affected, substantially in their central portion, by respective seats  32  for bearings  33  for rotationally supporting the tubular core  23  of the compaction cylinder  21 . Each crank  25  and the respective linear actuator  30  are arranged so that the respective longitudinal axes form an angle of less than 180° between them. Each one of the cranks  25  can rotate about the first articulation axis A by actuation of the respective actuator  30  so as to move the compaction roller  21  from the inactive configuration to the active configuration, in which it is in forced contact with the printing cylinder  6 . 
     Advantageously, the compaction roller  21  is supported, in the specific case (as shown in FIG.  9 ), by a plurality of cranks  25 , which are provided at the footing  7  substantially axially equidistant and are associated with respective pneumatic linear actuators  30 , which operate conveniently in step with each other, as clearly shown in FIG.  7 . In this manner, the compression thrust on the printing cylinder  6  and the load on the footing  7  are distributed uniformly. The sleeve  24  is divided for this purpose into a plurality of portions of equal length, which are keyed on the tubular core  23  between each crank  25  and the directly adjacent crank. 
     In practical operation, after fixing the flexographic printing plate  2  to the printing cylinder  8  in the intermediate position, said printing cylinder is moved into the lower position, and then the pneumatic linear actuators  30  are actuated simultaneously, in order to move the compaction roller  21  from the inactive configuration to the active configuration, in which it is in contact with the printing cylinder  8 , by rotating the cranks  25  about the first articulation axis A. Then the rotation of the printing cylinder  8  is started, so that the substantially radial thrust applied by the compaction roller  21  allows complete and uniform adhesion of the printing plate  11  to the blanket  10 , expelling any interstitial air pockets: in this manner, the printing plate  11  is fixed to the blanket  10  in an optimum manner, without separation or bulging. 
     It has thus been shown that the invention achieves the intended aim and object. 
     The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims. 
     In particular, FIG. 5 illustrates an important detail of another embodiment of the machine according to the invention, which provides for means  22  for actuating the compaction roller  21  which are constituted by compression springs  34  that can be adjusted appropriately and can be actuated mechanically instead of the pneumatic linear actuators  30 . 
     All the details may be replaced with other technically equivalent ones. 
     In practice, the materials used, as well as the shapes and the dimensions, may be any according to requirements without thereby abandoning the scope of the protection of the appended claims. 
     In the preferred embodiments described by way of examples, individual characteristics, presented in relation to specific examples, may actually be interchanged with other different characteristics that exist in other examples of embodiment. 
     Moreover, it is noted that anything found to be previously known is understood not to be claimed.