Abstract:
A method and apparatus for printing individual sheets gathers the sheets in groups and feeds the sheets past a printer unit. Paper paths are provided in two ring shapes and two printers are provided to permit printing on one side in one color, both sides in one color, one side in two colors and two sides in two colors or one color on one side and two colors on the other side.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to a method for printing individual sheets in a printer or a copier, whereby a both-sided printing process on individual sheets ensues. 
     2. Description of the Related Art 
     High-performance printers can be operated in what is referred to as duplex mode, which is also called duplex color spot mode. In this operating mode, individual sheets are printed on both sides. When only one print ensues on the one side and two prints with different colors ensue on the other side, then this is called the duplex printing mode with three-fold printing. When respectively two prints ensue on both sides of the individual sheet, then this is called duplex printing mode with four-fold printing. 
     Various duplex printing systems with ate least two printing units are disclosed by U.S. Pat. No. 4,591,884 A. Another high-performance printer device with two printing units is disclosed by WO 91/13386 A1. It comprises two printing units and turnover channels for turning the individual sheets over. A transfer printing transport path is allocated to each printing unit. 
     When individual sheets are supplied such in such a high-performance printer device with two printing units that the spacing between two individual sheets is greater than the length of an individual sheet viewed in transport direction, then a safety margin is created between two individual sheets that allows a further individual sheet to be transferred in upon transport of the individual sheets in gaps between two successive individual sheets at intersections of the sheet transport paths. Given this procedure, however, the sheet throughput is relatively low due to the relatively large sheet spacings. 
     U.S. Pat. No. 5,159,395 discloses a printer device having only a single printing unit wherein spaces are likewise created between two successive sheets of a sheet stream in order to subsequently transfer further individual sheets into the sheet stream. 
     U.S. Pat. No. 5,337,135 discloses a further printer device with only a single printing unit wherein a first printing operating condition with a first, relatively close printing spacing is provided and a second operating condition with a second, larger sheet spacing [is provided], whereby following sheets are again transferred into an existing sheet stream in the second operating condition. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to specify a method for printing individual sheets in a printer or in a copier in the duplex printing operating mode, whereby a high throughput of individual sheets given low wear and reduced risk of jams are achieved. 
     This object is achieved by the features of claim  1 . Advantageous ddevs are recited in the dependent claims. 
     In the invention, the individual sheets are conducted in groups on the transport path within the printer. In this way, a slight spacing can be set between the individual sheets given transfer printing speed. The drive elements for the transport are not stressed by an additional start-stop mode, so that the wear on the overall transport system is reduced. The control outlay is also reduced since path and switching tolerances do not occur and corresponding buffer zones are not required. The throughput of individual sheets through the printer or, respectively, the copier is increased, since the group of individual sheets can be conducted with maximum speed on the transport path. 
     According to a further aspect of the invention, a method for printing individual sheets is recited wherein a predetermined plurality of single sheets have a predetermined distance from one another at transfer printing speed that is smaller than the length of an individual sheet as viewed in transport direction, the individual sheets of this group are supplied successively to a first printing unit for printing with a first color on the first side, the individual sheets of the first group are successively re-supplied, turned over, to the first printing unit for printing with the first color on the second side, and wherein, subsequently, the individual sheets of the group are supplied to the second printing unit for printing with the second color on the second side and are then output. 
     This aspect of the invention relates to the duplex printing mode with three-fold printing. The individual sheets are conducted past two printing units in groups in order to apply one print on one side and two differently colored prints on the other side. Given this aspect of the invention, too, the spacing between two individual sheets can be minimal and the transport speed can be maximum. A high throughput given low wear for the drive elements is thus achieved. 
     In a preferred exemplary embodiment of the invention, the first individual sheet of a following group of individual sheets is supplied following the last individual sheet of a preceding group of individual sheets after said preceding group has been supplied twice to the first transfer printing path. As a result of these measures, a throughput that is high overall is achieved given a multitude of individual sheets, since the spacing between the last individual sheet of a preceding group and the individual sheet of a following group can be kept slight. 
     Preferably, the path spacing between the last individual sheet of the preceding group and the first individual sheet of the following group is approximately the predetermined spacing. 
     According to a further exemplary embodiment of the invention, at least some of the individual sheets of the group are conveyed in alternation to the first ring and to the second ring via the connecting channel. In this way, the number of individual sheets of the group can be high since, due to the alternating conveying, individual sheets at the start of the group and individual sheets at the end of the group can be conveyed nearly simultaneously in the connecting channel and distributed onto both rings at the end of the conveying path in the connecting channel. A great number of individual sheets in a group leads to an improved utilization of the printing capacity offered by the two printing units. 
     In a preferred development, the alternating conveying of the individual sheets to the two rings and/or the transporting of the individual sheets into the proximity of the printing units ensues with increased speed compared to the transfer printing speed of the printing units. As a result of this increased conveying speed, the spacing between successive individual sheets of a group is enlarged. This spacing is used in order to convey individual sheets at the start of the group and individual sheets at the end of the group in alternation and distribute them onto the two rings. 
     A practical exemplary embodiment provides that, following the renewed pass of the individual sheets of the group past the first printing unit, these are output or the individual sheets are supplied to the second printing unit for printing with the second color on the second side and are then output. In the first version, a duplex printing mode with three-fold printing is realized, and a duplex printing mode with four-fold printing of the individual sheets is realized in the second version. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained below with reference to the drawing. Shown therein are: 
     FIG. 1 the schematic structure of a high-performance printer wherein the invention is realized; 
     FIGS. 2-9 operating phases of the duplex printing mode with four-fold printing; 
     FIGS. 10-17 operating phases of the duplex printing mode with three-fold printing; and 
     FIGS. 18-28 operating phases given four-fold printing and alternating delivery of individual sheets to the first or to the second transport ring. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a high-performance printer  10  that serves the purpose of fast printing of individual sheets of paper. The high-performance printer  10  contains a first, lower print unit D 1  as well as a second, upper printing unit D 2 . The two printing units D 1 , D 2  work according to the known electrographic method with the same transfer printing speed. The printing units D 1 , D 2  are followed by fixing devices that are schematically indicated in FIG. 1 by two roller pairs  12 ,  14 . A paper input  16  is connected to the high-performance printer  10 , this containing a plurality of reservoirs  18  through  24  with individual sheets as well as an external paper input channel  26  via which individual sheets can be supplied by the outside. Individual sheets are supplied to an input section  28  via a transport channel  27 . A paper output  30  that contains a plurality of output containers  32  through  36  is connected to the output side of the high-performance printer  10 . Further, two output channels  38 ,  40  are provided via which individual sheets can be output to further-processing stations. The high-performance printer  10  outputs the printed individual sheets via the output section  42 . 
     Transport paths for the transport of the individual sheets are arranged in the inside of the high-performance printer  10 , various operating modes of the high-performance printer being realized on the basis of these transport paths. Respective transfer printing transport paths  44 ,  46  are allocated to the printing units D 1 , D 2 , these being respectively set such by drives that the supplied individual sheets have their transfer printing speed at the printing units D 1 , D 2 . Both transfer printing transport paths  44 ,  44  are connected to one another via a connecting channel  48 . The transport path around the first printing unit D 1  is supplemented by a delivery channel  50  to form a ring R 1  via which individual sheets can also be supplied from the input section  28  to the second transfer printing transport path  46 . The transport path for the second printing unit D 2  is supplemented to form a ring R 2  with a discharge channel  52  in a similar way, the individual sheets printed by the printing unit D 1  being capable of being supplied to the output section  42  thereover. 
     A first shunt W 1  that makes it possible that individual sheets from the input section  28  are optionally supplied to the first transfer printing transport path  44  or to the delivery channel  50  is arranged between the input section  28 , the first transfer printing transport path  44  and the delivery channel  50 . A further version is comprised therein that individual sheets transported on the delivery channel  50  in the direction of the shunt W 1  can be supplied to the first transfer printing transport path  44 . 
     Further, a second shunt W 2  and a third shunt W 3  are arranged at the ends of the connecting channel  48  and respectively connect the adjoining transport paths  44 ,  48 ,  52  or, respectively,  46 ,  48 ,  50 . A fork shunt W 4  is located in the proximity of the output section  42  and connects the adjoining transport paths. The paper output  30  contains a fifth shunt W 5  that works as turn-over means. Further, an ejector means  54  should also be pointed out, reject individual sheets being supplied thereto via a shunt W 6 . 
     As a result of the arrangement described in FIG. 1, different operating modes of the high-performance printer  10  can be realized. These operating modes also include the operating modes of duplex printing mode with four-fold printing as well as the duplex printing mode with three-fold printing that are relevant here. 
     FIGS. 2 through 9 show various operating phases in the duplex printing mode with four-fold printing. For reasons of clarity, a plurality of components that were explained in the injunction with FIG. 1 have been omitted from the Figures. However, it can be easily seen how the operating sequences shown in FIGS. 2 through 9 are realized by the components shown in greater detail in FIG.  1 . 
     As mentioned, the front side and the back side of the individual sheets are printed with image patterns of different colors in the duplex printing mode. Of course, it is assumed for this purpose that the printing units D 1 , D 2  can print differently colored print images. In the first operating phase shown in FIG. 2, a group of six individual sheets B 1  through B 6  are drawn in via the input section  28  and the shunt W 1  (respectively see FIG. 1 with respect thereto) and are conducted along the first transfer printing transport path  44  past the lower printing unit D 1 , whereby the first side is printed with a first color, indicated by dots. While the first individual sheet B 1  has nearly reached the shunt W 2  and the connecting channel  48 , the last individual sheet B 6  of the group is still in or, respectively, in front of the input section  28 . 
     FIG. 3 shows that the sheets successively traverse the connecting channel  48  and are delivered via the second transfer printing transport path  46  to the second printing unit D 2  for printing. Upon passage through the connecting channel  48 , a higher transport speed than the transfer printing speed is preferably present in order to convey the individual sheets in an optimally short time on the distance between the two printing units D 1 , D 2 . Shortly before reaching the printing unit D 2 , the individual sheets B 1  through B 6  are in turn decelerated to transfer printing speed. A further print with a different color, for example red, is applied onto the side already printed by the printing unit D 1  at the printing unit D 2 . This second printing event is identified in FIG. 3 by longitudinal strokes. 
     FIG. 4 shows the operating phase wherein the first individual sheet B 1  is conveyed forward after being printed and has been turned over at the shunt W 4  having turn-over function. Subsequently, the individual sheets B 1  through B 6  are successively conveyed again along the discharge channel  52  in the direction of the first printing unit D 1 . It must be noted that the sixth individual sheet B 6  is still located at an adequate safety margin in front of the first sheet B 1  on the path in the direction of the connecting channel  48 . 
     FIG. 5 shows the operating phase wherein the sixth individual sheet B 6  is printed by the second printing unit D 2 , whereas the first individual sheet B 1  has already traversed the connecting channel  48  and is now being conveyed along the delivery channel  50 . All individual sheets B 1  through B 6  are turned over at the shunt W 4 . 
     FIG. 6 shows the first individual sheet B 1  shortly before the delivery to the first printing unit D 1 . The conveying speed for the individual sheet B 1  is in turn set to the transfer printing speed. The further individual sheets B 2  through B 6  are still conveyed with increased speed. 
     FIG. 7 shows that the individual sheets B 1 , B 2  are printed for the third time, i.e. the first printing unit prints the other side of the individual sheets, indicated by dots in FIG.  7 . The sixth individual sheet B 6  is still situated in the connecting channel  48 , but has an adequately great safety margin from the following individual sheet B 1 . 
     It can be seen at the right in the Figure that the first individual sheet B 1  of the following group is already being supplied in the paper input  16  in order to be transferred into the first transfer printing transport path  44  at the shunt W 1  following the passage of the individual sheet B 6 . 
     FIG. 8 shows the renewed delivery of the individual sheet B 1  as well as of the further individual sheets B 2  through B 6  of the first group to the printing unit D 2 . The fourth print is applied at this printing unit D 2 . It can be seen at the right in the Figure that the individual sheets B 1 ′ B 2 ′, B 3 ′ of the following group at the shunt 
     W 1  are delivered to the first transfer printing transport path  44 . The distance between the last individual sheet B 6  of the first group and the first individual sheet B 1  of the second group corresponds to the predetermined spacing a of the individual sheets at transfer printing speed. 
     FIG. 9 shows an operating phase wherein the first group of individual sheets B 1  through B 6  are being printed by the second printing unit D 2  (identified by horizontal strokes in FIG. 9) and are subsequently conveyed via the output section  42  into the paper output  30  and are deposited there at. The second group of individual sheets B 1 ′ through B 6 ′ is already is being subsequently printed by the first printing unit D 1 . 
     Various modifications for the group-by-printing of individual sheets in the duplex printing mode with four-fold printing are possible. Thus, the turnover need not necessarily ensue at the shunt W 4  but can also be implemented at the shunts W 2 , W 3  or, given suitable equipment at the shunt W 1  as well, given the assumption that the appertaining drive elements can implement a turnover function. Let a turnover event be explained below with reference to the shunt W 4 . The respective individual sheet is first transported past the shunt W 4  on a first transport path in a conveying direction in the direction of the shunt B 5 . Subsequently, the transport direction is reversed and the respective individual sheet is conveyed in the direction of the discharge channel  52 . A similar functioning is possible for the further shunts W 1 , W 2 , W 3 . 
     Another modification is comprised therein in selecting the path of the group of individual sheets differently, for example in that the group is first supplied to the printing unit D 2 , then to the printing unit D 1 , is resupplied to the printing unit D 2  after following turnover, is then supplied to the printing unit D 1  and the individual sheets are then discharged via the discharge channel  52 . 
     The duplex printing mode with three-fold printing is explained below with reference to FIGS. 10 through 17. FIG. 10 shows the delivery of the first group of individual sheets B 1  through B 6  from the paper input  16  to the first printing unit D 1  that prints the individual sheets B 1  through B 6  with a first color (illustrated with dots), for example with a black color. The delivery speed can be higher than the transfer printing speed; however, this higher speed must be reduced to the transfer printing speed when the first printing unit D 1  is reached. 
     FIG. 11 shows the turnover of the individual sheets at the shunt W 2  (see FIG.  1 ), whereby the individual sheet B 1  is initially conveyed in the direction of the shunt W 4 , the conveying direction is then reversed, and the individual sheet B 1  is transported in the direction of the connecting channel  48 . A higher transport speed can again be selected when transporting outside the printing units D 1 , D 2 . 
     FIG. 12 shows that the individual sheets B 1  through B 6  are conveyed along the first, closed transport path R 1 . 
     FIG. 13 shows the printing of the back side of the individual sheets B 1  through B 6  by the printing unit D 1 . 
     FIG. 14 shows the conveying of the individual sheets B 1  through B 6  via the connecting channel  48  to the second transport path  46 , whereby no turnover ensues. It can be seen at the right in FIG. 14 that the individual sheets B 1 ′, B 2 ′ of the following group are already being offered via the paper input  16 . 
     FIG. 15 shows the printing of the one side of the individual sheets B 1  through B 6  by the second printing unit D 2 . The individual sheets of the following group are already being supplied to the first printing unit D 1 , i.e. the first individual sheet B 1 ′ of the following group follows the last individual sheet B 6  of the first group. 
     FIG. 16 shows the delivery of the individual sheets B 1  through B 6  of the first group into the paper output  30 , whereby no turnover and what is referred to as a face-down deposit ensue. The individual sheets B 1 ′ through B 6 ′ of the following group are in an operating phase as shown in FIG.  11 . 
     FIG. 17 shows the further deposit of the individual sheets B 1  through B 6  of the first group and the printing of the individual sheets B 1 ′ through B 6 ′ of the following group. 
     A number of modifications are also conceivable given this duplex printing mode with three-fold printing. For example, the group of the individual sheets can be supplied first to the second printing unit D 2  and printed a first time; then, the individual sheets are resupplied along the closed, second transport path R 2  to the second printing unit D 2 , whereby the individual sheets have been previously turned over, for example in the shunt W 4 ; then, the individual sheets are delivered without turnover to the first printing unit D 1  via the connecting channel  48  and are printed. Subsequently, the individual sheets printed three times are output. 
     Various operating phases in the duplex printing mode with four-fold printing are shown in FIGS. 18 through 28, whereby individual sheets are conveyed in alternation to the first transport ring R 1  or to the second transport ring R 2 . For reasons of clarity, a number of components that were explained in conjunction with FIG. 1 have been omitted from the Figures. However, it can be easily seen how the operating sequences shown in FIGS. 18 through 28 are realized by the components shown in the more detailed illustration of FIG.  1 . 
     As mentioned, the front side and the back side of the individual sheets are printed with image patterns of different colors in the duplex printing mode. Of course, it is assumed for this purpose, that the printing units D 1  and D 2  can print differently colored print images. In the first operating phase shown in FIG. 18, a group of eleven individual sheets B 1  through B 11  from the paper input  16  are drawn in via the input section  28  and the shunt W 1  (see respectively FIG. 1 for this purpose) and are conducted past the lower printing unit D 1  along the first transfer printing path  44 , whereby the first side is printed with a first color, indicated by dots. Whereas the first individual sheet B 1  has nearly reached the shunt W 2  and the connecting channel  48 , the last individual sheet B 11  of the group is still located in front of the input section  28  in the paper input  16 . 
     FIG. 19 shows that the individual sheets successively traverse the connecting channel  48  and are delivered via the second transfer printing transport path  46  to the second printing unit D 2  for printing. When passing through the connecting channel  48 , a higher transport speed is present than the transfer printing speed in order to convey the individual sheets in the shortest possible time on the path between the two printing units D 1 , D 2 . Shortly before reaching the printing unit D 2 , the individual sheets B 1  through B 11  are in turn decelerated to the transfer printing speed. A further print with a different color, for example red, is applied onto the side already printed by the printing unit D 1 , being applied at the printing unit D 2 . This second printing event is identified in FIG. 3 by longitudinal strokes. 
     FIG. 20 shows the operating phase wherein the first individual sheet B 1  is further-conveyed after having been printed and has been turned over at the shunt W 4  with turnover function. Subsequently, the individual sheets B 1  through B 11  are in turn successively conveyed in the direction of the first printing unit D 1  within the ring R 2  along the discharge channel  52 . It must be noted that the eleventh individual sheet B 11  is still in the unprinted condition preceding the input section  28  and the individual sheet B 6  as well as the following individual sheets B 7 , B 8 , B 9  have not yet past through the connecting channel  48 . 
     FIG. 21 shows an operating phase wherein the first individual sheet B 1  at the shunt W 2  is threaded in between the individual sheets B 6  and B 7  and is conveyed upward in the connecting channel  48 . As mentioned, the individual sheets B 1  through B 11  are conveyed with a speed higher than the transfer printing speed after leaving the printing unit D 1 , being conveyed to the printing unit D 2 . As a result thereof, the distance between the individual sheets increases. This distance is utilized in the invention for threading the individual sheets in that derive from the printing unit D 2  and are arranged at the start of the group. 
     FIG. 22 shows the operating condition wherein the individual sheet B 1  is conveyed in the direction of the first printing unit D 1  after leaving the turnover W 3 . The individual sheet B 6 , by contrast, has been conveyed in the print direction of the printing unit D 2 . The following individual sheets B 7  is just being redirected by the shunt W 3  in the direction of the printing unit D 2 . The individual sheet B 2  has been threaded in between the individual sheets B 7  and B 8  and is conveyed up in the connecting channel  48 . 
     FIG. 23 shows the first individual B 1  shortly before delivery to the first printing unit D 1 . The conveying speed for the individual sheet B 1  is again set to the transfer printing speed. The further individual sheets B 2 , B 3 , B 4  in the ring R 1  are still being conveyed with increased speed. The individual sheet B 4  has been inserted between the individual sheets B 9  and B 10  at the shunt W 3 . 
     FIG. 24 shows that the individual sheets B 1  and B 2  are being printed for the third time, i.e. the first printing unit D 1  prints the other side of the individual sheets, illustrated in FIG. 8 by dots. The eleventh individual sheet B 11  is still located in the connecting channel  48  and is redirected in the direction of the printing unit D 2  for printing. The individual sheet B 6  is inserted between the individual sheets B 11  and B 1  and is conveyed up in the connecting channel  48 . 
     FIG. 25 show the renewed delivery of the individual sheet B 1  as well as of the further individual sheets B 2  and the following sheets B 3  through B 11  of the first group to the printing unit D 2 . The individual sheet B 7  is inserted between the individual sheets B 1  and B 2  in the connecting channel  48 . 
     FIG. 26 shows the operating phase wherein a first portion of the individual sheets B 1 , B 2 , B 3  of the first group is printed by the second printing unit D 2 , indicated by horizontal strokes. The individual sheets B 10 , B 11  are inserted between the individual sheets B 4  and B 5  or, respectively, B 5  and B 6  in the connecting channel in the fashion of a zipper system. 
     FIG. 27 shows that the individual sheets provided with a fourth print image, for example the individual sheets B 1 , B 2 , are conveyed via the output section  42  into the paper output  30  and are deposited thereat. A following group of individual sheets B 1 ′ through B 11 ′ is conveyed out of the paper input  16 . The individual sheet B 1 ′ immediately follows the last individual sheet B 11  of the first group . The group of individual sheets B 1 ′ through B 11 ′ then passes through the high-performance printer in the way described for the first group of individual sheets B 1  through B 11 . 
     FIG. 28 shows the common transport of individual sheets B 3  through B 11  of the first group as well as of individual sheets B 1 ′, B 2 ′ and B 3 ′ of the following, second group. Both printing units D 1  and D 2  are utilized to nearly 100% given the illustrated operating mode. 
     Numerous modifications for the group-by-group printing of individual sheets in the duplex printing mode with four-fold printing are possible. Thus, the turnover need not necessarily ensue at the shunt W 4  but can also be carried out at the shunts W 2 , W 3  or, given suitable equipment, at the shunt W 1  as well, given the assumption that the appertaining drive elements can implement a turnover function. Let a turnover function be explained below with reference to the shunt W 4 . The respective individual sheet is first transported past the shunt W 4  on a first transport path in a conveying direction in the direction of the shunt W 5 . Subsequently, the transport direction is reversed and the respective individual sheet is conveyed in the direction of the discharge channel  52 . A similar functioning is possible for the further shunts W 1 , W 2 , W 3 . 
     Another modification is comprised in selecting the path of the group of individual sheets differently, for example in that the group is first supplied to the printing unit D 2 , then to the printing unit D 1 , re-supplied to the printing unit D 2  after subsequent turnover, then the printing unit D 1 , and the individual sheets are output via the discharge channel  52 . 
     The duplex printing mode with three-fold printing is explained below with reference to FIG.  1 . The group of individual sheets in this operating mode first passes through the first transfer printing transport path  44 , then the connection channel  48 , the second transfer printing transport path  46 , the discharge channel  52 , again traverses the connecting channel  48 , the delivery channel  50  and the first transfer printing transport path  44  and is then output in the output section  42 . Upon return of the individual sheets printed by the second printing unit D 2  to the first printing unit D 1 , individual sheets are threaded in at the shunt W 2  in the described way and are supplied at the shunt W 3  either to the first ring R 1  or to the second ring R 2 . 
     As can be seen with reference to the exemplary embodiment according to FIGS. 1 and 18 through  28 , what is achieved given group-by-group printing of the individual sheets is that the first printing unit D 1  has 100% usage, i.e. Interruption-free printing operation exists for the printing unit D 1 . 
     The maximum number of individual sheets of a group is dependent on the overall length of the transport path in the printing system. The predetermined number of individual sheets of a group derives according to          N   ≤     INT        {         L   44     +     L   46     +     L   48     +     L   50     +     L   52           L   B     +   a       }         ,                          
     whereby L 44  is the length of the first transfer printing transport path  44 , L 46  is the length of the second transfer printings transport path  46 . L 48  is the length of the connecting channel  48 , L 50  is the length of the delivery channel  50 , L 52  is the length of the discharge channel  52 , L B  is the length of the individual sheet viewed in transport direction, and A is the distance between two successive individual sheets at transfer printing speed. 
     Claim  19  describes a further calculating method for determining the maximum number of individual sheets of a group. The threading of the individual sheets into the connecting channel  48  can ensue triggered or untriggered. Given the untriggered operating mode, the individual sheets deriving from the ring R 2  are threaded into the connecting channel  48  without arresting the individual sheets. Given the assistance of FIG.  1  and FIGS. 20 and 21, this untriggered operating mode shall be explained in greater detail. As mentioned, the individual sheet B 1  is threaded in between the individual sheets B 6  and B 7  in the connecting channel and is conveyed up and is then supplied to the printer D 1 , whereas the individual sheets B 6  and B 7  are supplied to the printing unit D 2  (see FIGS.  20  and  21 ). In the untriggered operating mode, the distance between the individual sheets following the individual sheet B 6  is then dimensioned of such a size that the individual sheet B 1  to be threaded in is conveyed on the transport path  52  with a defined speed and can thread in between the individual sheets B 6  and B 7  without being arrested. The speed of the individual sheets on the transport path from the shunt W 4  up to the shunt W 6 , where the speed is in turn reduced to the transfer printing speed, remains approximately constant given this operating mode. 
     On the common path segment of the connecting channel  48 , the individual sheet B 1  threads between the individual sheets B 6  and B 7 , the individual sheet B 2  threads between the individual sheets B 7  and B 8 , the individual sheet B 3  threads between the individual sheets B 8  and B 9  in succession, etc. The shunt W 3  is actuated at every individual sheet upon departure of the individual sheets in order to deliver the successive individual sheets in alternation to the first or to the second printing unit D 1  or D 2 . 
     The individual sheet B 1  and the following individual sheets, for instance from the shunt W 6  up to the shunt W 2 , are transported with transfer printing speed, are printed for a third time at the printing unit D 1  and thread between the individual sheets B 6  and B 7  as well as the following individual sheets that have a corresponding distance from one another. After the threading at the shunt W 2 , the individual sheets B 1  and the following individual sheets are conveyed to the printing unit D 2  with elevated speed. After leaving the upper printing unit D 2 , the individual sheets are again transported with elevated speed via the shunt W 4  in the direction of the paper output  30 . At the shunt W 5 , the individual sheets are optionally turned over in order to be able to deposit them in the output compartments  32 ,  34 ,  36  face down or face up. In the interim, the last individual sheets B 11  of the first group of individual sheets has past the common path segment in the connecting channel  48 , and the first individual sheet B 1 ′ of the following group of elevens can be drawn in. The stream of individual sheets is thus closed. 
     In the untriggered operating mode, the advantage of the employment is comprised therein that no particular control outlay is required when threading in at the shunt W 2 . On the contrary, the return transport of the individual sheets from the upper printing unit D 2  to the lower printing unit D 1  can ensue with constant speed. What is disadvantageous is that tolerances in the distances between the individual sheets within the group cannot be compensated and, thus, a backup upon thread-in can arise. 
     The triggered operating mode is explained below. According to FIGS. 20 and 21, the individual sheet B 1  to be delivered from the printing unit D 2  to the printing unit D 1  is to be threaded in between the individual sheets B 6  and B 7  at the shunt W 2 . Given this operating mode, the distance between the individual sheets B 6  and B 7  as well as the following individual sheets is equal to the shortest nominal spacing. The individual sheet B 1  is transported in the direction of the shunt W 2  with such a high speed after leaving the shunt W 4  with turnover function that it arrives adequately early in front of the shunt W 2 , preferably before the individual sheet B 6  has past the shunt W 2 . At this shunt W 2 , the individual sheet B 2  is briefly arrested or is decelerated The continued transport of the individual sheet B 1  is then triggered by the individual sheet B 6 . Preferably, a signal is generated when the trailing end of the individual sheet B 6  leaves, this signal initiating the transport of the individual sheet B 1  into the connecting channel  48 , preferably at the same speed with which the individual sheet B 6  is being conveyed. The individual sheet B 1  thus threads between the individual sheets B 6  and B 7  with the same elevated speed as the transport speed of the individual sheet B 6  in the connecting channel  48 . Shortly before the individual sheet B 7  reaches the first drive roller pair in the connecting channel  48 , this roller pair and the one following thereupon in the connecting channel  48  is reduced in transport speed to the transfer printing speed, since the individual sheet B 7  is still partially located in the region of the first printing unit D 1 , for example in the decurler. Accordingly, the individual sheet B 1  is reduced in speed to the speed of the individual sheet B 7 . When the individual sheet B 7  leaves the printing region, preferably the region of the decurler, in the printing unit D 1 , it can again be transported with elevated speed. The individual sheet B 1  is then also transported forward with elevated speed and proceeds to the transport path  50 . The individual sheet B 1  is transported with constantly elevated speed up to the region at the shunt W 6  and is then stepped down to transfer printing speed. The point-in-time of the reduction of the transport speed of the individual sheet B 1  can be matched to the preceding individual sheet B 11 . The gap between the end of the individual sheet B 11  and the start of the individual sheet B 1  can then be set to a nominal spacing or slightly greater. The further individual sheets B 2 , B 3 , etc., within the group of eleven individual sheets are transported in a similar way as was set forth for the individual sheet B 1 . 
     Given the triggered operating mode, a controlled threading ensues at the shunt W 2 . The back-up risk during threading is thus reduced, and the distance between the individual sheets can be minimized. What is disadvantageous is the increased outlay for regulation and control during thread-in. 
     Up to now, the duplex printing mode has been described, whereby a predetermined number of individual sheets, preferably eleven, are combined to form a group and this group is printed on both sides by the printing units D 1  and D 2 . Another operating mode not covered by the attached claims is possible wherein no group-by-group transport of the individual sheets ensues; rather, a continuous delivery of individual sheets occurs. This operating mode is called continuous operating mode. In this continuous operating mode, only every second individual sheet is drawn in from the paper input  16 , the distance between two successive, drawn-in individual sheets is identical to the respective sheet length plus the nominal spacing. The gaps arising in this way between the individual sheets are successively filled by preceding individual sheets that have already been printed twice. In this way, a continuous individual stream is produced and the printing units D 1 , D 2  are optimally utilized. So that the sheets can be threaded in controlled fashion at the shunt W 2 , a second transport control path in the region of the shunt W 1  is required in addition to a transport control path in the region of the shunt W 2 . The sequences and transport speeds of the individual sheets from the input  16  up to the shunt W 4  coincide with those that were described in the untriggered mode. 
     Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.