Abstract:
A printing unit of a sheet-fed printing press includes two cooperating screen-printing cylinders which are in direct contact. These two screen-printing cylinders define a printing gap through which the sheets to be printed pass during their printing step.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a printing unit with a screen-printing cylinder. The screen printing cylinder has a movable doctor blade device. Two directly cooperating screen-printing cylinders, each with at least one working doctor blade, may also be provided. 
     BACKGROUND OF THE INVENTION 
     EP 07 23 864 B1 has disclosed a printing unit for a rotary printing press. A first screen-printing cylinder, together with a second cylinder, forms a printing nip, in which a print stock is printed. 
     This known printing unit is only suitable for one-sided printing. 
     DE 26 38 344 A1 discloses two cooperating screen-printing cylinders. 
     JP 63-071350 A discloses two opposing printing cylinders, each with a doctor blade. A separation between a doctor blade and a counter-pressure device is not provided. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a printing unit with a screen-printing cylinder. 
     The object is attained according to the invention by the provision of at least one screen-printing cylinder with a movable doctor blade. Two directly cooperating screen-printing cylinders can also be used. Each has a working doctor blade and a counter-pressure device. The counter-pressure device of one of the screen-printing cylinders supports the working doctor blade of the other screen-printing cylinder. 
     The advantages that can be achieved with the present invention are comprised particularly in the fact that the printing press permits first forme printing and second forme printing in the screen-printing process with matching registers to be executed in a single printing procedure. 
     To this end, the present invention provides that the second cylinder is also a screen-printing cylinder so that each screen-printing cylinder is used to print a respective side of a print stock being fed through the printing nip which is defined by the two screen-printing cylinders. 
     In order to permit an exact, register-matching printing, even at the edge of a sheet-like print stock, preferably at least one of the two screen-printing cylinders is equipped with a sheet gripping mechanism for the sheet-like print stock. 
     In order for this sheet gripping mechanism to be able to pass through the printing nip, it is necessary for at least one of the two screen-printing cylinders to have an indentation on its circumference surface. A doctor blade device disposed inside the screen-printing cylinder can preferably be moved radially so that it can move out of the way of the indentation. 
     It is also preferable for the doctor blade devices of the two screen-printing cylinders to each have a working doctor blade, each of which working doctor blades which touches the interior of the screen of its respective screen-printing cylinder when pressing ink through it, the respective working doctor blades of the two doctor blade devices being exactly aligned with each other, so that the working doctor blade of each doctor blade device compensates for a pressure exerted on its screen by the working doctor blade of the other. This arrangement provides the simple assurance that there is a sufficient pressure in the printing nip to press the ink required for the printing through the screens and to transfer it onto the print stock. On the other hand, an undesirable deformation of each screen by the pressure of the doctor blade device associated with it is prevented because the respective other doctor blade device exerts a corresponding counter-pressure. 
     Alternatively to this, each doctor blade device can also have a counter-pressure device spaced apart from its associated working doctor blade in the circumference direction of the screen-printing cylinder. Each such counter-pressure device is aligned with the respective working doctor blade of the other doctor blade device and compensates for the pressure which this other doctor blade device exerts. 
     In addition to their previously well-known and customary task of applying ink to the screen-printing cylinder, the doctor blade devices thus also perform the task of the counter-pressure cylinder that is usually provided. 
    
    
     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 representation of a printing press with a printing unit in accordance with the present invention; 
     FIG. 2 shows a schematic section through the printing unit of the machine of FIG. 1; 
     FIGS. 3 a  and  3   b  respectively show the printing nip and partial regions of the screen-printing cylinders that form the printing nip, in accordance with a first preferred embodiment of the printing unit of the present invention, in two phases of its rotary motion; 
     FIGS. 4 and 5 show two configurations of an indented section of the circumference surface of the screen-printing cylinder; and 
     FIG. 6 shows the printing nip and partial regions of the screen-printing cylinders that form the printing nip, in accordance witha second preferred embodiment of the printing unit of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, there is shown a schematic view, e.g. of 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 . The printing unit  4  takes the sheets, one at a time, prints them, and outputs them to a second chain conveyor  6 , which, in the case of multicolor printing, feeds them to other printing units like the printing unit  4  or, as shown here, feeds them directly to an output stack  5 . 
     The sheets pass through the printing unit  4  from top to bottom. The printing unit can be seen more clearly in FIG. 2 which gives a larger scale depiction of its structure. 
     Two transport cylinders  7 , the upper of which is shown only partially in FIG. 2, are each provided with sheet gripping mechanisms, each in a respective section  8  of their circumferences, in order to grip the leading edges of sheets, which are to be printed, from the first belt conveyor  3  in a register-matching manner. The lower of the two transport cylinders  7  rolls in contact with a first screen-printing cylinder  12 , which is likewise equipped with a sheet leading edge gripping mechanism  29  in a section  11  of its circumference. In particular, this sheet leading edge gripping mechanism  29  may be a rotatable shaft with sheet gripper fingers, as seen in FIG. 3 a  and which is adapted for taking the sheets from the lower transport cylinder  7 . The first screen-printing cylinder  12 , together with a second screen-printing cylinder  17 , forms a printing nip  14  through which the sheets held by the sheet leading edge gripper  29  of the first screen-printing cylinder  12  are conveyed. The two screen-printing cylinders  12  and  17  can be rotated in unison in such a way that with each pass through the printing nip  14 , the sheet gripper  29  of the first screen-printing cylinder  12  coincides with, or is aligned with, a channel-like indented section  18  of the second screen-printing cylinder  17 . 
     The operation of the screen-printing cylinders  12  and  17  and a first preferred embodiment of their construction will be described below, and taken in conjunction with FIGS. 3 a  and  3   b.    
     FIGS. 3 a  and  3   b  each show a view of a partial section of the two cooperating and coacting screen-printing cylinders  12  and  17  in the vicinity of the printing nip  14 . 
     At each of its axial ends, each screen-printing cylinder  12  and  17  has a support ring  22  whose outer circumference has a screen  23  stretched onto it. Each screen  23  is preferably made of silk or polyamide gauze or bronze wire mesh. On the interior of each screen-printing cylinder  12  and  17 , a doctor blade device  24  is provided, whose position in the radial direction of its associated screen-printing cylinder is controlled by a curved body, in this instance a guide slot  26  situated at the ends of the screen-printing cylinder  12  or  17 , and through which slot  26  a cylindrical guide projection  27  of the doctor blade device  24  extends. Outside the screen-printing cylinder  12  or  17 , the guide projection  27  is supported at both ends so that it can move in the direction of a line  28  extending between and connecting the rotation axes of the screen-printing cylinder  12  and the screen-printing cylinder  17 . FIG. 3 a  shows the doctor blade device  24  in a position in which the sheet gripper  29  of the screen-printing cylinder  12  is passing through the printing nip  14  between the two cylinders. In the vicinity of the sheet gripper  29  and opposite from it, 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 , which is not shown completely in FIG. 3 a , and 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 guide projection  27  during the rotation of the screen-printing cylinder  12 , the doctor blade device  24  is retracted radially inward so far that it exerts only a minimal pressure against the screen  23 , which minimal pressure produces no appreciable deformation of the screen  23  in the screen section  31 , or the doctor blade device  24  is retracted so far that it loses all contact with the screen  23  and consequently exerts no pressure on its screen section  31 , which pressure could otherwise deform this screen section  31  and damage it during the course of operation. 
     FIG. 3 b  shows the position of the doctor blade devices  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 devices  24  pressed against the inside of the screen  23  so that an ink  34  disposed against a working doctor blade  19  of the doctor blade device  24  is pressed through the open regions of the screen  23  and is thus applied to a sheet of print stock conveyed through the printing nip  14  defined by the two screen-printing cylinders  12  and  17 . 
     In this phase of the rotation of the two screen-printing cylinders  12  and  17 , the respective lips  16  of the two working doctor blades  19  touching the screens  23  are oriented toward each other and rest exactly in a plane defined by the axes of the two screen-printing cylinders  12  and  17 , which plane is represented by the line  28  in FIG. 3 b    
     The parallelism of the lips  16  is of great importance for satisfactory printing results. If the lips  16  are not parallel to each other, but rather intersect at an angle, there is the possibility that away from the intersecting point of the two respective doctor blade lips  16 , the screen  23  will move away from the pressure exerted against its inside by the doctor blade device  24 , with the result that little or no ink  34  is pushed through the screen  23 , and this ink, for lack of effective pressure, is transferred either incompletely or not at all to a sheet being fed through the printing nip  14 . 
     It is easy to see that the requirement for the lips  16  to be parallel is greater, the narrower the zone is in which the lip  16  and screen  23  contact each other. It is consequently useful to select the elasticity of the material of the working doctor blades  19  and the pressure of the doctor blade device  24  against the screen  23  to be great enough for a sufficient width of the contact zone of the lips  16  to be produced. With an axial length of the screen-printing cylinders  12  and  17  of approximately 800 to 900 mm, it is desirable for the contact zone to extend at least 0.05 to 1 mm, preferably approx. 0.3 to 0.4 mm in the circumference direction of the screen-printing cylinders  12  and  17 . With axial lengths that differ from this, a proportionally larger or smaller width of the contact zone can be selected. 
     In accordance with a second preferred embodiment of the present invention, as shown in FIG. 4, the doctor blade device  24  of the screen-printing cylinder  17  remains in contact with the screen  23  even during its passage through the indented screen section  31 . In this case, in order to prevent the doctor blade device  24  from deforming the screen  23  in the screen section  31 , the screen  23  is supported on the outside of the screen  23 , in the area of the screen section  31  by a support element  36 . The support element  36  here has the shape of a basin or a trough that is uniformly curved in cross section. The support element  36  may be, 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  17 . Advantageously, the leading and trailing ends of the screen  23  are also disposed in the screen section  31 , where they are covered by the support element  36 . The support element  36  can be adjusted in the radial direction, which is indicated by the arrow  37  in FIG. 4, in order to set the tension of the screen  23 . 
     The screen-printing cylinder  12 , which cooperates with the above-described screen-printing cylinder  17  has a corresponding support element  36  in its indented screen section  31 . This may also be seen by referring to FIG.  4 . 
     Since no ink can be transferred from the screen-printing cylinders  12 ;  17  to the print stock in the indented screen sections  31  supported by the support element  36 , it is useful for the support element  36  to be a closed plate which does not permit any ink  34  to pass through and reach the exterior of the screen-printing cylinder  12 ;  17 . 
     FIG. 5 depicts an alternative construction of the screen-printing cylinder  17  of the present invention. 
     The support element  36  depicted in FIG. 5 is affixed radially inside the screen  23 . The leading end  42  and the trailing end  43  of the screen  23  overlap each other in the indented screen section  31  which is supported by the support element  36 . In this manner, the sensitive connection between the two ends  42  and  43  of the screen  23 , which two screen ends  42  and  43  can be welded to each other, for example, is protected from contact with the lip  16  of the doctor blade device  24  and thus from premature wear. 
     Also in this embodiment of a screen-printing cylinder  17 , as shown in FIG. 5, an adjusting element can be provided for adjusting the doctor blade device  24  in the radial direction, for example in the guide slot  26  shown in FIGS. 3 a ,  3   b . However, since in this embodiment, the inner radius of the surface of screen  23  that is wiped by the doctor blade device  24  only fluctuates minimally, it is sufficient to compensate for these minimal fluctuations solely by the provision of a radially flexible support, which is not specifically shown, for the doctor blade device  24 . 
     In order to prevent synchronization errors in the screen-printing cylinder  17  with such a construction, when the working doctor blade  19  passes a leading edge  46  or a trailing edge  47  of the support element  36 , the subject 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  16  of the doctor blade device  24 , but extend at a slight angle with respect to that generatrix or lip. For example, a sawtoothed, rafter-shaped, or sinusoidal curve of the edges  46  and  47  is possible. Preferably the edges  46  and  47  each represent a helix with a pitch that is a multiple of the axial length of the screen-printing cylinder  17 . When the doctor blade device  24  is running onto them or off of them, 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 smoothes the torque required to drive the screen-printing cylinder  17  and prevents synchronization errors. 
     FIG. 6 is an axial section view in the vicinity of a printing nip  14  of another preferred embodiment of the printing unit  4  according to the present invention, in the same phase of its rotational movement as the one shown in FIG. 3 b . Elements which are depicted in FIG. 6, and correspond to those that have already been described in relation to FIGS. 3 a  and  3   b , are provided with the same reference numerals in FIG.  6  and will not be described again in the following discussion. 
     The difference between this depicted embodiment of the sheet-fed printing press with screen-printing cylinders, in accordance with the present invention, and the ones described with reference to FIGS. 3 a  and  3   b ,  4 , and  5  lies in the configuration of the doctor blade devices  24 . The printing unit  4  depicted in FIG. 6 has two differently embodied doctor blade devices  24   a  and  24   b . Each of them has a working doctor blade  19   a ,  19   b , respectively which is made of an elastically deformable material with a lip  16 , which, during the course of the rotary motion of the screen-printing cylinder  17 , sweeps along the inside of the screen  23  in order to press ink  34  through screen  23 . Separate from the lips  16  and parallel to them, each doctor blade device  24   a  and  24   b  has a counter-pressure device  21   a  or  21   b , respectively in the form of a bridge that extends, in the axial direction of the screen-printing cylinder  17 , over the same length as the working doctor blade  19   a  or  19   b  and which has an end surface  13  which sweeps along the inside of the screen  23 . Each counter-pressure device  21   a  or  21   b  is respectively aligned so that an end surface  13  of each such counter-pressure device  21   a  or  21   b  is disposed opposite from the lip  16  of the working doctor blade  19   b  or  19   a  of the respective opposing doctor blade device  24   b ,  24   a . The purpose of these counter-pressure devices  21   a  and  21   b  is to provide a buttress or a counter-acting support for the pressure exerted by the opposing working doctor blade  19   b  or  19   a , respectively, which buttress or support assures that a sufficient quantity of ink passes through the screen  23  and is printed with high quality on print stock being fed through the printing nip  14 . The end surface  13  of each of the counter-pressure devices  21   a  and  21   b  can have a width of several millimeters in the circumference direction. This assures that, even when the two doctor blade devices  24   a  and  24   b  are slightly out of parallel, the pressure of each lip  16  is compensated for over its entire length by the cooperating counter-pressure device. As a result, a uniform printing quality is achieved over the entire width of the print stock. 
     In the doctor blade device  24   a , the working doctor blade  19   a  is disposed before the counter-pressure device  21   a  in the rotation direction of the screens  23 . In the doctor blade device  24   b , this is reversed. The doctor blade device  24   b  can therefore be embodied as a chamber doctor blade in which the counter-pressure device  21   b  constitutes a closing doctor blade. The chamber defined by the working doctor blade  19   b  and counter-pressure device  21   b  can be subjected to pressure in order to control the flow rate of the ink provided in the chamber defined by the working doctor blade  19   b  and the counter-pressure device  21   b.    
     Although not specifically shown in FIG. 6, in this embodiment of the screen-printing cylinders  12  and  17 , the indented screen sections  31  can be equipped with support elements  36 , as shown in FIGS. 4 and 5. 
     While preferred embodiments of a sheet-fed printing press with screen-printing cylinders 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 drives for the cylinders, the types of sheets being printed, and the like can be made without departing from the true spirit and scope of the present invention which is to be limited only by the following claims.