Abstract:
A printing unit includes two screen-printing cylinders and two transfer cylinders that cooperate with the screen-printing cylinders and which define a printing gap or nip. At least one of the screen-printing cylinders has a screen surface and an interior doctor blade. A support element is provided for handling radially outwardly directed pressure exerted by the doctor blade on the screen surface in one section of the screen surface.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a printing unit with first and second screen-printing cylinders. Transfer cylinders cooperate with the screen-printing cylinders. 
     BACKGROUND OF THE INVENTION 
     EP 07 23 864 B1 discloses a printing unit for a rotary printing press. This printing unit has a first screen-printing cylinder and a printing nip formed by two cylinders, in which a print stock is printed. 
     A screen-printing cylinder of this prior printing unit forms a printing nip in cooperation with a counter-pressure cylinder, which has circumferential sections, each with a reduced radius in which sheet grippers are disposed. On the interior of the screen-printing cylinder, a doctor blade device is provided, and is oriented toward the printing nip. This doctor blade device presses against the screen of the screen-printing cylinder and pushes ink through it. In order to prevent the screen from being deformed, when a circumference section with a reduced radius passes through the printing nip, a mechanism is provided which pulls the doctor blade device back from the screen at these times. 
     This prior art printing unit is only suitable for one-sided printing. The mechanism for retracting the doctor blade device is complex. 
     JP 11-129599 A discloses a screen-printing press with two screen-printing cylinders and a sheet-feeding transfer cylinder. 
     EP 0 949 069 A1 describes an offset printing press with cooperating transfer cylinders, which can be preceded by a screen-printing unit. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a printing unit. 
     The object of the present invention is attained according to the invention by the provision of a printing unit with first and second screen-printing cylinders. Each screen-printing cylinder is associated with its own transfer cylinder. The two transfer cylinders cooperate to form a printing nip. 
     The advantages that can be achieved with the present invention are comprised particularly in the fact that it permits first forme printing and second forme printing, in the screen-printing process, with matching registers to be executed in a single printing procedure. The screen-printing cylinder is distinguished in particular by its uncomplicated construction. 
     In the printing unit in accordance with the present invention, at least one of the two screen-printing cylinders prints a first side of the print stock indirectly by use of a transfer cylinder. This transfer cylinder is the second of the two cylinders that form the printing nip and can therefore simultaneously serve as a counter-pressure cylinder for printing the second side of the print stock. 
     Preferably, the first of the two cylinders that form the printing nip is also a transfer cylinder. This results in an essentially symmetrical configuration of the printing unit of the present invention, with the same printing properties on both sides of the print stock. 
     In order to also permit an exact, register-matching printing, in relation to the edge of a sheet-like print stock, preferably at least one of the two cylinders that form the printing nip is equipped with a holding mechanism for the sheet-like print stock. 
     To be able to apply ink to the screen-printing cylinders, these screen-printing cylinders are each suitably provided with a doctor blade device on its interior, which doctor blade device is for use in pressing ink through a screen mounted on the screen cylinder. A device for preventing the doctor blade device from deforming the screen that it presses against is also advantageously provided in a specific circumferential section of the screen. This device prevents the screen from being excessively deformed when it is pressed against the holding mechanism or when it is pressed into a channel, which is complementary to the holding mechanism and which is disposed on the respective other cylinder forming the printing nip. This device thus prevents the screen from wearing too rapidly or from being damaged. 
     A device of this kind can be or can include an adjusting mechanism, which retracts the doctor blade device radially inward when the specific circumferential section of the screen-printing cylinder passes in front of the doctor blade device. In accordance with a second feature of the present invention, the screen deformation preventing device is a support element, which extends axially on the screen-printing cylinder in the vicinity of the specific circumferential section and serves to absorb a pressure exerted by the doctor blade in a radially outward direction on the specific circumferential section. 
     In order to prevent synchronization errors of the screen-printing cylinder, when the doctor blade device comes into contact with the support element and/or leaves contact with it, the support element is preferably provided with leading and trailing edges for the doctor blade device. These leading and trailing edges extend at an angle to a generatrix of the circumferential surface of the screen cylinder. Since the doctor blade device necessarily extends parallel to such a generatrix, the doctor blade device does not come into contact or leave contact with the support element over its entire span at one time, but only at certain points. In addition, braking forces acting on the screen-printing cylinder therefore remain low and are distributed over a finite section of the circumference of the screen-printing cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are shown in the drawings and will be described in detail below. 
     FIG. 1 is a schematic side elevation view of a printing press with a printing unit in accordance with the present invention; 
     FIG. 2 is a schematic side elevation view of the printing unit of the machine depicted in FIG. 1; 
     FIG. 3 is a schematic side elevation view of a printing unit in accordance with a second preferred embodiment of the present invention; 
     FIGS. 4 a  and  4   b  each show a detail of a screen-printing cylinder and a transfer cylinder cooperating with it, in two phases of the rotational movement of the cylinders; 
     FIG. 5 is a side elevation view of a modification of the screen-printing cylinder depicted in FIG. 4 a ; and 
     FIG. 6 is a side elevation view of a portion of a screen-printing cylinder and a transfer cylinder according to another preferred embodiment of a printing unit in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, there may be seen a schematic view of, for example a sheet-fed rotary printing press in which a printing unit  4 , in accordance with the present invention, is used. The printing press has a sheet feeder  1  with a sheet stacker  2 , from whose top, which is automatically kept at a constant height, sheets to be printed are fed individually, or in a continuous stream, by a belt conveyor  3  to the printing unit  4 , which takes the sheets one at a time, prints them, and outputs them to a chain conveyor  6 , which, in the case of multicolor printing, feeds them to other printing units like the printing unit  4  or, as shown in FIG. 1, feeds them directly to an output stack  5 . 
     The sheets pass through the printing unit  4  from top to bottom. A larger scale depiction of the printing unit  4  is provided in FIG.  2  . 
     Two transport cylinders  7 , the upper one of which is shown only partially in FIG. 2, are each provided with sheet grippers in a respective section  8  of their circumferences in order to take the edges of sheets, which are to be printed, from the first belt conveyor  3  in a register-matching manner. The lower one of the two transport cylinders  7  rolls in contact with a first transfer cylinder  9 . The first transfer cylinder  9  is embodied, for example, as a first rubber blanket cylinder  9  and is likewise equipped with a sheet gripper in a section  11  of its circumference for use in receiving the sheets from the lower transport cylinder  7 . The first transfer cylinder or rubber blanket cylinder  9  also rolls in contact with a first screen-printing cylinder  12 , so that a printing pattern is transferred from the first screen-printing cylinder  12  to it, which printed pattern is printed on a sheet conveyed in a first nip  13  between the lower transport cylinder  7  and the first rubber blanket cylinder  9 . The structure of the first screen-printing cylinder  12  will be discussed in more detail below. 
     After passing through the first nip  13 , a sheet to be printed reaches a second nip  14  between the first rubber blanket cylinder  9  and a second transfer cylinder  16 , which is also embodied as a rubber blanket cylinder  16 . In this second nip  14 , the sheet is printed on its second side with a pattern that has been transferred to the second transfer or rubber blanket cylinder  16  from a second screen-printing cylinder  17 . 
     The second rubber blanket cylinder  16  does not have a sheet gripper assembly. Instead, a section  18  of its circumference is provided with a channel, which permits the sheet gripper assembly  11  of the first rubber blanket cylinder  9  to pass through the nip  14 . 
     Since the first and second transfer or rubber blanket cylinders  9  and  16  and with them, the first and second screen-printing cylinders  12  and  17  as well, can rotate in a coupled fashion, it is easy to achieve an exact and durable register matching of the printing on the front and back sides of the sheets by executing a manual or automatic fine adjustment of the printing formes mounted on the first and second screen-printing cylinders  12  and  17  in relation to each other in the axial direction and in the circumference direction of the cylinders  9 ;  16 ;  12 ;  17 . 
     The various cylinders  7 ;  9 ;  16 ;  12 ;  17  discussed above are each supported at their ends in lateral side frames or mounts  19 ;  21 ;  22 , a seen in FIG.  2 . In the printing unit shown in FIG. 2, these lateral side frames or mounts  19 ;  21 ;  22  are comprised of a central module, which supports the transport cylinders  7  and the two rubber blanket cylinders  9 ;  16 , as well as two lateral modules, which each support one of the screen-printing cylinders  12 ;  17 . The end plates of each module are connected to each other so that they form a rigid frame unit, which can be removed from the printing unit  4  along with the screen-printing cylinder  12  or  17  that it supports. A module of this kind can be replaced, for example, by another module, which contains a forme cylinder, e.g. for conventional flat offset printing or the like. This permits the printing units, such as printing unit  4  to be easily adapted to a variety of requirements in order to print documents, in which different printing techniques are used for the front and back sides, with matching registers in a single pass. 
     FIG. 3 depicts a simplified configuration of the printing unit shown in FIG.  2 . In this simplified configuration, elements that correspond to those of the printing unit from FIG. 2 are provided with the same reference numerals. In the configuration of the printing unit of FIG. 3, the second rubber blanket cylinder  16  is omitted and instead, the second screen-printing cylinder  17  forms the second printing nip  14  directly with the first rubber blanket cylinder  9 . The first rubber blanket cylinder  9  consequently performs the function of a counter-pressure cylinder for the second screen-printing cylinder  17 . 
     Examples of suitable configurations of the screen-printing cylinder or cylinders  12  and  17 , for use in the printing unit of the present invention will be described below in conjunction with FIGS. 4 a ,  4   b , and  5 . 
     FIGS. 4 a  and  4   b  each show a partial section of the first screen-printing cylinder  12  in the vicinity of the first nip  13 , which it forms with the first rubber blanket cylinder  9 . The structures described here, however, can also be used in the same way for the configuration of the second screen-printing cylinder  17 . 
     The first screen-printing cylinder  12  has a support ring  20  at each of its axial ends, whose outer circumference has a screen  23  stretched onto it. Screen  23  may be made of silk or polyamide gauze or bronze wire mesh. On the interior of the screen-printing cylinder  12 , a doctor blade  24  is provided, whose position in the radial direction is controlled by a curved body, in this instance a curved guide slot  26  located at the ends of the screen-printing cylinder  12 , and through which a cylindrical guide projection  27  of the doctor blade  24  extends. Outside of the screen-printing cylinder  12 , the guide projection  27  is supported at both ends so that it can move in the direction of a line  28  connecting the rotation axes of the two cylinders  9  and  12 . FIG. 4 a  shows the doctor blade  24  in a position in which the sheet gripper mechanism  29  of the first rubber blanket cylinder  9  is passing through the first nip  13  between the two cylinders  9  and  12 . Opposite from the sheet gripper mechanism  29 , the screen  23  has a screen section  31  that is indented radially inward. The guide slot  26  has an arc-shaped guide slot section  33 , not shown completely in FIG. 4 a , which is concentric to the cylindrical outer surface of the screen  23 , and an inwardly indented guide slot section  32 , whose curvature corresponds to that of the screen section  31 . The curvature of the guide slot section  32  is selected so that when the guide slot section  32  moves past the cylindrical doctor blade guide projection  27  during the rotation of the first screen-printing cylinder  12 , the doctor blade  24  is retracted radially inward so far that it exerts only a minimal pressure against the screen  23 , which produces no appreciable deformation of the screen  23  in the screen section  31 , or is retracted so far that it loses all contact with the screen  23  and consequently exerts no pressure on screen section  31 , which could deform this screen section and could damage it during the course of operation. 
     FIG. 4 b  shows the position of the doctor blade  24  after the guide slot section  32  has passed the guide projection  27 . The arc-shaped section  33  of the guide slot  26  keeps the doctor blade  24  pressed against the inside of the screen  23  so that ink  34  disposed against the doctor blade  24  is pressed through the open regions of the screen  23  and is thus transferred to the first rubber blanket cylinder  9 . 
     FIG. 5 shows a modification of the first screen-printing cylinder  12  depicted in FIGS. 4 a  and  4   b . Elements that correspond to those of the first screen-printing cylinder  12  described above are provided with the same reference numerals and will not be described again. The structure of the screen-printing cylinder  12  depicted in FIG. 5 is simplified through the elimination of the guide slot  26 . A spring device, which is not specifically shown in FIG. 5, keeps the doctor blade  24  pressed against the screen  23 . The doctor blade  24  consequently remains in contact with the screen  23  even when the indented screen section  31  passes the doctor blade  24  during the course of the rotational motion of the screen-printing cylinder  12 . When the indented screen section  31  passes in front of the doctor blade  24 , the doctor blade  24  is pushed back counter to the force of the spring device toward the axis of the screen-printing cylinder  12 . In order to prevent the doctor blade  24  from deforming the screen  23  in the screen section  31 , the screen  23  is supported by an externally situated support element  36 . The support element  36 , as seen in FIG. 5 has the shape of a basin or of a trough that is uniformly curved in cross section, and is, for example, made of sheet metal or a rigid plastic, and is anchored at its two axial ends to the ends of the screen-printing cylinder  12 . Preferably, the leading and trailing ends of the screen  23  are also disposed in the screen section  31  and are covered by the support element  36 . The support element  36  can be adjusted in the radial direction, as indicated by arrow  37  in order to set the tension of the screen  23 . 
     During the rotation of the cylinders  9  and  12  of the printing unit, since the support element  36  respectively coincides with the sheet gripper holding mechanism  29  of the rubber blanket cylinder  9  and consequently no ink can be accepted from the screen-printing cylinder  12  in the vicinity of the screen section  31 , it is useful for the support element  36  to be embodied as a closed plate, which does not permit any ink to pass through the screen section  31  and reach the exterior of the screen-printing cylinder  12 . 
     FIG. 6 is a detailed depiction of another preferred embodiment of a screen-printing cylinder  17 . 
     The screen-printing cylinder  17  depicted in FIG. 6 is cylindrical over its entire circumference, and is without an indented section. It is therefore suitable for cooperating with a second cylinder, which has no radially outward-protruding elements such as sheet gripping mechanisms for a print stock. With reference to the printing unit shown in FIG. 1, the second screen-printing cylinder  17  can have the configuration shown in FIG.  6 . Hence in the description that follows, the screen-printing cylinder is identified at  17  and the cylinder that cooperates with it is identified at  16 . 
     As was the case of the screen-printing cylinder depicted in FIGS. 4 a ,  4   b , and  5 , a doctor blade  24  is disposed on the inside of the screen-printing cylinder  17  shown in FIG. 6, and pushes a paste-like ink  34  through the screen  23  stretched on the screen-printing cylinder  17 . Doctor blade  24  exerts a radially outward pressure on the screen  23 . In the cooperating rotation of the screen-printing cylinder  17  and the rubber blanket cylinder  16 , as long as the screen  23  of the screen-printing cylinder  17  touches the surface of the rubber blanket cylinder  16 , this cylinder  16  supplies a counter-pressure, which prevents the doctor blade  24  from deforming the screen  23 . In order to prevent such a deformation of the screen  23 , even in the vicinity of the channel  38  of the cylinder  16 , in which the screen  23  is not in contact with the rubber blanket cylinder  16 , a support element  39  is disposed radially inside the screen  23 . The support element  39  extends in the circumference direction of the screen-printing cylinder  17  over a section  41  of the screen  23 , which corresponds to the span of the channel  38  on the rubber blanket cylinder  16 . The support element  39  is configured as a closed plate made of metal or rigid plastic, which is curved in the form of a cylinder segment. 
     In this configuration depicted in FIG. 6, the support element  39  is attached radially inside the screen  23  and the leading end  42  and trailing end  43  of the screen  23  overlap each other in the section  41  of the screen  23  which is supported by the support element  39 . Thus, the sensitive connection between the two ends  42  and  43  of the screen  23 , which can be welded for example, is protected from contact with the doctor blade  24  and is therefore protected from premature wear. 
     A spring element for moving the doctor blade  24  in the radial direction could also be provided in this embodiment of a screen-printing cylinder  17  depicted in FIG.  6 . However, since in this embodiment, the inner radius of the surface of the screen  23  that the doctor blade  24  sweeps across, is uniform or continuous, the configuration of the screen-printing cylinder  17  can be further simplified by eliminating the adjustability of the doctor blade  24  so that the fluctuations in the radius are compensated for solely through an elastic deformation of an elastic lip  44  of the doctor blade  24  touching the screen  23 . 
     In a configuration of this kind, in order to facilitate the transition of the doctor blade  24  from the screen  23 , onto the support element  39 , and back onto the screen  23  again, and in order to avoid synchronization errors of the screen-printing cylinder  17 , the support element  39  is provided with a leading edge  46  and a trailing edge  47 , which are beveled in the circumference direction of the screen-printing cylinder  17 . In addition, the present invention provides that these edges  46  and  47  do not extend exactly parallel to a generatrix of the outer surface of the screen-printing cylinder  17  or to the lip  44  of the doctor blade  24 , but extend at a slight angle to them. For example, a sawtoothed, rafter-shaped, or sinusoidal curve of the edges  46  and  47  is conceivable. Preferably, the edges  46  and  47  each are configured as a helix with a pitch that is a multiple of the axial length of the screen-printing cylinder  17 . When the doctor blade  24  is running onto or off these support element edges, such a curvature of the edges  46  and  47  prevents braking or acceleration forces acting on the screen-printing cylinder  17  from being exerted only at a certain point in time and at a particular angular position of the screen-printing cylinder  17 . Instead, these forces are distributed over a circumferential section of the screen-printing cylinder  17  which, depending on the dimensions of the screen-printing cylinder  17 , can be from several millimeters up to a few centimeters wide. This force distribution smoothes the torque required to drive the screen-printing cylinder  17  and prevents synchronization errors. 
     While preferred embodiments of a printing unit in accordance with the present invention have been set forth fully and completely hereinabove, it will be apparent to one of skill in the art that various changes in, for example the specific drives for the various cylinders, the specific sheet gripping mechanisms and the like can be made without departing from the true spirit and scope of the present invention which is accordingly to be limited only by the following claims.