Patent Publication Number: US-9834015-B2

Title: Printing machine

Description:
TECHNICAL FIELD 
     The present invention relates to a printing machine which performs printing on a sheet. 
     RELATED ART 
     There is known a printing machine which performs duplex printing. 
     As the printing machine which performs duplex printing, there is disclosed a printing machine of the type which performs printing on one surface of the sheet by a printing unit configured to perform printing while transferring the sheet by a transfer belt and which then performs printing on the other surface of the sheet by reversing the sheet upside down and refeeding the sheet to the printing unit while transferring the sheet along a circulation route, in Patent Document 1. 
     The printing machine disclosed in Patent Document 1 realizes high productivity by performing duplex printing at productivity per simplex printing which is equivalent to productivity during simplex printing. In order to realize the above-mentioned productivity in various sheet sizes, in the printing machine disclosed in Patent Document 1, there is provided, in the circulation route, a high-speed section in which the sheet is transferred at a circulation transfer speed which is higher than a printing transfer speed which is a transfer speed in the printing unit. The circulation transfer speed is set in accordance with each sheet size. Thereby, refeeding is possible at a timing according to the productivity in the printing unit. 
     In the high-speed section in the circulation route of the printing machine disclosed in Patent Document 1, a roller pair which transfers the sheet while nipping the sheet is driven at the circulation transfer speed. In the circulation route, the sheet is transferred at the printing transfer speed until the sheet reaches the high-speed section. When the sheet reaches the high-speed section, the sheet is pulled out from a roller pair in an upstream-side section in which the sheet is transferred at the printing transfer speed by the roller pair in the high-speed section. Thereby, the sheet is accelerated from the printing transfer speed to the circulation transfer speed. 
     The sheet accelerated is transferred at the circulation transfer speed in the high-speed section and then is reversed upside down by being switched back on the downstream side of the high-speed section. Then, the sheet reversed upside down is refed to the printing unit. 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2009-46303 
     SUMMARY 
     However, in the printing machine disclosed in Patent Document 1, since acceleration is performed by pulling out the sheet from the roller pair in an upstream-side section by the roller pair in the high-speed section, a heavy load is imposed on the sheet and the roller pairs. When the heavy load is imposed on the sheet and the roller pairs, the sheets may be damaged and the roller may be easily deteriorated. 
     The present invention has been made in view of the above problem. An object of the present invention is to provide a printing machine that can reduce the load on a sheet and a sheet transfer mechanism. 
     In order to attain the above-mentioned object, a printing machine according to the present invention includes a printing unit which performs printing while transferring a sheet at a printing transfer speed, a circulation transfer unit which receives the sheet after simplex printing and transfers the sheet by reversing the sheet upside down, during duplex printing; and a paper refeed unit which refeeds, to the printing unit, the sheet transferred by the circulation transfer unit after simplex printing, wherein the circulation transfer unit includes a reversing unit which receives the sheet at the printing transfer speed, reverses the sheet upside down by switching back the sheet and accelerates the sheet up to a circulation transfer speed which is higher than the printing transfer speed when sheet transfer is restarted after temporal stop in the switching back, and a high-speed transfer unit which transfers the sheet switched back by the reversing unit, at the circulation transfer speed. 
     Furthermore, in the printing machine according to the present invention, the reversing unit may be configured to switch back the sheet and to accelerate the sheet up to the circulation transfer speed, by a single roller pair. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described with reference to the accompanying drawings wherein: 
         FIG. 1  is a schematic configuration diagram of a printing machine according to an embodiment; 
         FIG. 2  is a control block diagram of the printing machine illustrated in  FIG. 1 ; 
         FIG. 3  is an explanatory diagram illustrating a printing schedule during duplex printing; and 
         FIG. 4  is a diagram illustrating transition of a transfer speed of a sheet when being switched back by a reversal roller pair. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will be described below with reference to the drawings. The same or equivalent numerals are attached to the same or equivalent portions and constitutional elements in the drawings. 
     The embodiment which will be described in the following merely illustrates a device and the like for embodying technical ideas of the present invention, and the technical ideas of the present invention do not specify a material quality, a shape, a structure, an arrangement and the like of each constituent component as ones which will be described below. The technical ideas of the present invention can be variously modified within a range of the scope of patent claims. 
       FIG. 1  is a schematic configuration diagram of a printing machine according to an embodiment of the present invention.  FIG. 2  is a control block diagram of the printing machine illustrated in  FIG. 1 . In the following description, a direction orthogonal to the plane of paper in  FIG. 1  will be referred to as a front-back direction and a paper plane front direction will be referred to as the front. In addition, the top and bottom and the left and right of the paper plane in  FIG. 1  will be respectively referred to as a top-bottom direction and a left-right direction. 
     In  FIG. 1 , a route indicated with a thick line is a transfer route along which a sheet which is a printing medium is to be transferred. In the transfer route, a solid line route is a printing route RP, a dashed line route is a circulation route RC, a broken line route is a paper discharge route RD and two-dot chain lines are, respectively, an external paper feed route RS 1  and an internal paper feed route RS 2 . The upstream and the downstream in the following explanation respectively mean the upstream and the downstream of the transfer route. 
     As illustrated in  FIGS. 1 to 2 , a printing machine  1  according to the present embodiment includes a paper feed unit  2 , a printing unit  3 , a circulation transfer unit  4 , a paper discharge unit  5 , a controller  6 , a housing  7  which houses or holds respective units. 
     The paper feed unit  2  (a paper refeed unit) feeds the sheet P on which printing is not yet performed to the printing unit  3 . Furthermore, the paper feed unit  2  refeeds the sheet P after simplex printing to the printing unit  3 , during duplex printing. The paper feed unit  2  is arranged on the most upstream side of the transfer route. The paper feed unit  2  includes an external paper feed tray  11 , an external paper feed roller pair  12 , two internal paper feed trays  13 , two internal paper feed roller pairs  14 , two internal paper feed motors  15 , three internal paper feed transfer roller pairs  16 , an internal paper feed transfer motor  17 , a vertical transfer roller pair  18 , a vertical transfer motor  19 , a registration roller pair  20 , and a registration motor  21 . 
     The external paper feed tray  11  is a tray on which the sheets P used for printing are to be stacked. The external paper feed tray  11  is installed so as to be partially exposed to the outside of the housing  7 . 
     The external paper feed roller pair  12  takes out the sheets P stacked on the external paper feed tray  11  one by one and transfers the sheets P taken out to the registration roller pair  20  one by one. The external paper feed roller pair  12  includes a scraper roller  12   a  and a pick-up roller  12   b.    
     The scraper roller  12   a  separates the highest-order (the uppermost) sheet P from the sheets P stacked on the external paper feed tray  11 . The scraper roller  12   a  is arranged above a right end part of the external paper feed tray  11 . 
     The pick-up roller  12   b  strips the sheet P separated by the scraper roller  12   a  from the sheets P stacked on the external paper feed tray  11 , between a not illustrated stripper plate and the scraper roller  12   a , and transfers the sheet P rightwards. The pick-up roller  12   b  is arranged adjacent to the downstream side (the right-hand side) of the scraper roller  12   a.    
     The internal paper feed tray  13  is a tray on which the sheets P used for printing are to be stacked. The internal paper feed tray  13  is arranged within the housing  7 . 
     The internal paper feed roller pairs  14  each takes out the sheets P stacked on the internal paper feed tray  13  one by one and transfers the sheets P so taken out to the internal paper feed transfer roller pairs  16 . The internal paper feed roller pair  14  includes a scraper roller  14   a  and a pick-up roller  14   b.    
     The scraper roller  14   a  separates the highest-order sheet P from the sheets P stacked on the internal paper feed tray  13 . The scraper roller  14   a  is arranged above a left end part of the internal paper feed tray  13 . 
     The pick-up roller  14   b  strips the sheet P separated by the scraper roller  14   a  from the sheets P stacked on the internal paper feed tray  13 , between a not illustrated stripper plate and the scraper roller  14   a , and transfers the sheet P leftwards. The pick-up roller  14   b  is arranged adjacent to the downstream side (the left-hand side) of the scraper roller  14   a.    
     Each of the two internal paper feed motors  15  drives an internal paper feed roller pair  14 . 
     Each of the internal paper feed transfer roller pairs  16  transfers the sheet P taken out from the internal paper feed tray  13  by the internal paper feed roller pair  14 , to the vertical transfer roller pair  18 . The internal paper feed transfer roller pairs  16  are arranged along the internal paper feed route RS 2 . 
     The internal paper feed transfer roller pair  16  is constituted by a pair of internal paper feed transfer rollers  16   a  and  16   b . The internal paper feed transfer roller pair  16  transfers the sheet P while nipping the sheet P between the internal paper feed transfer rollers  16   a  and  16   b.    
     The internal paper feed transfer motor  17  drives the three internal paper feed transfer roller pairs  16 . 
     The vertical transfer roller pair  18  transfers the sheet P transferred by the internal paper feed transfer roller pairs  16  along the internal paper feed route RS 2 , to the registration roller pair  20 . Furthermore, the vertical transfer roller pair  18  transfers the sheet P after simplex printing, transferred along the circulation route RC, to the registration roller pair  20 , during duplex printing. The vertical transfer roller pair  18  is arranged along the internal paper feed route RS 2  on the downstream side of a spot where the circulation route RC joins the internal paper feed route RS 2 . 
     The vertical transfer roller pair  18  is constituted by a pair of vertical transfer rollers  18   a  and  18   b . The vertical transfer roller pair  18  transfers the sheet P while nipping the sheet P between the vertical transfer rollers  18   a  and  18   b . A printing surface of the sheet P after simplex printing comes into contact with the right-hand side vertical transfer roller  18   a  in the vertical transfer rollers  18   a  and  18   b , during duplex printing. Therefore, the vertical transfer roller  18   a  is composed of a roller subjected to antifouling treatment. For example, the vertical transfer roller  18   a  is composed of a roller the surface of which is coated with ceramic powder. The vertical transfer roller  18   b  is composed of a roller not subjected to antifouling treatment. 
     The vertical transfer motor  19  drives the vertical transfer roller pair  18 . Furthermore, the vertical transfer motor  19  drives the external paper feed roller pair  12 . The vertical transfer motor  19  is connected to the vertical transfer roller pair  18  and the external paper feed roller pair  12 , respectively, via not illustrated one-way clutches. Thereby, the vertical transfer roller pair  18  is driven by rotation of the vertical transfer motor  19  in one direction and the external paper feed roller pair  12  is driven by rotation of the vertical transfer motor  19  in the other direction. 
     The registration roller pair  20  temporarily stops movement of the sheet P which has been transferred by the external paper feed roller pair  12  or the vertical transfer roller pair  18  and corrects skew of the sheet P, and then transfers the sheet P to a belt platen  31  of the later described printing unit  3 . The registration roller pair  20  is arranged on the printing route RP in the vicinity of the downstream side of a spot where the external paper feed route RS 1  joins the internal paper feed route RS 2 . 
     The registration roller pair  20  is constituted by a pair of registration rollers  20   a  and  20   b . The registration roller pair  20  transfers the sheet P while nipping the sheet P between the registration rollers  20   a  and  20   b . The printing surface of the sheet P after simplex printing comes into contact with the lower-side registration roller  20   b  in the registration rollers  20   a  and  20   b , during duplex printing. Therefore, the registration roller  20   b  is composed of a roller subjected to antifouling treatment. The registration roller  20   a  is composed of a roller not subjected to antifouling treatment. 
     The registration motor  21  drives the registration roller pair  20 . 
     The printing unit  3  prints an image on the sheet P while transferring the sheet P. The printing unit  3  is arranged on the downstream side of the paper feed unit  2 . The printing unit  3  includes the belt platen  31 , a belt platen motor  32 , and a head unit  33 . 
     The belt platen  31  transfers the sheet P transferred by the registration roller pair  20  at a printing transfer speed Vg while sucking and retaining the sheet P on its own belt. The belt platen  31  is arranged on the downstream side of the registration roller pair  20 . 
     The belt platen motor  32  drives the belt of the belt platen  31 . 
     The head unit  33  ejects ink onto the sheet P transferred by the belt platen  31  and prints an image on the sheet P. The head unit  33  is arranged above the belt platen  31 . The head unit  33  includes a plurality of inkjet heads (not illustrated) with a plurality of nozzles being arrayed along a direction (a front-back direction) which is orthogonal to a transfer direction of the sheet P. The head unit  33  ejects ink from the nozzles of the inkjet heads. 
     The circulation transfer unit  4  transfers the sheet P after simplex printing along the circulation route RC and delivers the sheet P to the vertical transfer roller pair  18 , during duplex printing. The circulation route RC is a route which descends from a downstream end of the printing route RP, passes under the belt platen  31  and joins with the internal paper feed route RS 2  in the vicinity of the upstream side of the vertical transfer roller pair  18 . The circulation transfer unit  4  includes an intermediate transfer unit  41 , a reversing unit  42 , and a horizontal transfer unit  43  (a high-speed transfer unit). 
     The intermediate transfer unit  41  transfers the sheet P after simplex printing from the belt platen  31  to the reversing unit  42  at the printing transfer speed Vg, during duplex printing. The intermediate transfer unit  41  is arranged on the downstream side of the belt platen  31 . The intermediate transfer unit  41  includes two intermediate transfer roller pairs  46 , and an intermediate transfer motor  47 . 
     Each of the intermediate transfer roller pairs  46  receives, at the printing transfer speed Vg, the sheet P sent from the belt platen  31  after simplex printing and transfers the sheet P to a later described reversal roller pair  48  at the printing transfer speed Vg. The two intermediate transfer roller pairs  46  are arranged along the circulation route RC between the belt platen  31  and the reversal roller pair  48 . 
     The intermediate transfer roller pairs  46  each is constituted by a pair of intermediate transfer rollers  46   a  and  46   b . The intermediate transfer roller pairs  46  each transfers the sheet P while nipping the sheet P between the intermediate transfer rollers  46   a  and  46   b . The printing surface of the sheet P after simplex printing comes into contact with the right-hand side intermediate transfer roller  46   a  in the intermediate transfer rollers  46   a  and  46   b , during duplex printing. Therefore, the intermediate transfer roller  46   a  is composed of a roller subjected to antifouling treatment. The intermediate transfer roller  46   b  is composed of a roller not subjected to antifouling treatment. 
     The intermediate transfer motor  47  drives the two intermediate transfer roller pairs  46 . Furthermore, the intermediate transfer motor  47  drives later described two paper discharge roller pairs  57 . 
     The reversing unit  42  reverses the sheet P upside down after simplex printing. The reversing unit  42  is arranged on the downstream side of the intermediate transfer unit  41 . The reversing unit  42  includes the reversal roller pair  48 , and a reversal motor  49 . 
     The reversal roller pair  48  receives, at the printing transfer speed Vg, the sheet P transferred by the intermediate transfer roller pair  46 , reverses the sheet P upside down by switching back the sheet P, accelerates the sheet P up to a circulation transfer speed Vr and delivers the sheet P to the horizontal transfer unit  43 . The reversal roller pair  48  is arranged on the downstream side of the intermediate transfer roller pairs  46  along the circulation route RC. The circulation transfer speed Vr is a transfer speed of the sheet P in the horizontal transfer unit  43 . The circulation transfer speed Vr is higher than the printing transfer speed Vg. 
     The reversal roller pair  48  is constituted by a pair of reversal rollers  48   a  and  48   b . The reversal roller pair  48  transfers the sheet P while nipping the sheet P between the reversal rollers  48   a  and  48   b . The printing surface of the sheet P after simplex printing comes into contact with the right-hand side reversal roller  48   a  in the reversal rollers  48   a  and  48   b , during duplex printing. Therefore, the reversal roller  48   a  is composed of a roller subjected to antifouling treatment. The reversal roller  48   b  is composed of a roller not subjected to antifouling treatment. 
     The reversal motor  49  drives the reversal roller pair  48  in normal and reverse rotation directions. Normal rotation drive is a drive which rotates the reversal rollers  48   a  and  48   b  in a direction in which the reversal roller pair  48  transfers the sheet P downwards. Reverse rotation drive is a drive which rotates the reversal rollers  48   a  and  48   b  in a direction in which the reversal roller pair  48  transfers the sheet P upwards. 
     The horizontal transfer unit  43  transfers the sheet P switched back by the reversing unit  42 , to the vertical transfer roller pair  18  of the paper feed unit  2 . The horizontal transfer unit  43  is arranged on the downstream side of the reversing unit  42 . The horizontal transfer unit  43  includes four horizontal transfer roller pairs  50 , and two horizontal transfer motors  51 . 
     Each of the horizontal transfer roller pairs  50  receives the sheet P from the reversal roller pair  48  at the circulation transfer speed Vr, and transfers the sheet P to the vertical transfer roller pair  18  at the circulation transfer speed Vr. The three upstream-side horizontal transfer roller pairs  50  are arranged along a horizontal section of the circulation route RC under the belt platen  31 . The most downstream-side horizontal transfer roller pair  50  is arranged along an ascending section on the downstream side of the horizontal section of the circulation route RC. 
     Each of the horizontal transfer roller pairs  50  is constituted by a pair of horizontal transfer rollers  50   a  and  50   b . Each of the horizontal transfer roller pairs  50  transfers the sheet P while nipping the sheet P between the horizontal transfer rollers  50   a  and  50   b . The printing surface of the sheet P after simplex printing comes into contact with the upper side horizontal transfer roller  50   a  in the horizontal transfer rollers  50   a  and  50   b , during duplex printing. Therefore, the horizontal transfer roller  50   a  is composed of a roller subjected to antifouling treatment. The horizontal transfer roller  50   b  is composed of a roller not subjected to antifouling treatment. 
     One of the two horizontal transfer motors  51  drives the two upstream-side horizontal transfer roller pairs  50 . The other horizontal transfer motor  51  drives the two downstream-side horizontal transfer roller pairs  50 . 
     The paper discharge unit  5  receives the printed sheet P from the printing unit  3 , transfers the sheet P along the paper discharge route RD and discharges the sheet P. The paper discharge unit  5  includes a switching unit  55 , a solenoid  56 , the three paper discharge roller pairs  57 , a paper discharge motor  58 , and a paper receiving tray  59 . 
     The switching unit  55  switches the transfer route of the sheet P, between the paper discharge route RD and the circulation route RC. The switching unit  55  is arranged on a spot corresponding to a downstream end of the printing route RP and upstream ends of the paper discharge route RD and the circulation route RC. The paper discharge route RD is a route which ascends from the downstream end of the printing route RP and extends to the paper receiving tray  59 . 
     The solenoid  56  drives the switching unit  55 . 
     Each of the paper discharge roller pairs  57  each receives and transfers the sheet P transferred from the belt platen  31 , and discharges the sheet P onto the paper receiving tray  59 . The paper discharge roller pairs  57  are arranged along the paper discharge route RD. 
     The paper discharge roller pairs  57  each is constituted by a pair of paper discharge rollers  57   a  and  57   b . Each of the paper discharge roller pairs  57  transfers the sheet P while nipping the sheet P between the paper discharge rollers  57   a  and  57   b.  One surface and the other surface of the sheet P subjected to duplex printing, respectively, come into contact with the paper discharge rollers  57   a  and  57   b.  Therefore, both of the paper discharge rollers  57   a  and  57   b  are composed of rollers subjected to antifouling treatment. 
     The paper discharge motor  58  drives the most downstream-side paper discharge roller pair  57 . Note that the two upstream-side paper discharge roller pairs  57  are driven by the intermediate transfer motor  47 . 
     The paper receiving tray  59  is a tray obtained by stacking the sheets P discharged by the paper discharge roller pairs  57 . The paper receiving tray  59  is arranged on a downstream end of the paper discharge route RD. 
     The controller  6  controls operations of respective units of the printing machine  1 . The controller  6  is constituted by including a CPU, a RAM, a ROM, a hard disk and the like. 
     Next, the operation of the printing machine  1  during simplex printing will be described. 
     First, the controller  6  causes the belt platen motor  32  to start driving of the belt platen  31 . The controller  6  performs control such that the transfer speed of the sheet P by the belt platen  31  reaches the printing transfer speed Vg. Here, the printing transfer speed Vg is set on the basis of: a maximum number of drops per pixel which is defined in accordance with each sheet type and the like; printing resolution; and the like. 
     Furthermore, the controller  6  causes the intermediate transfer motor  47  and the paper discharge motor  58  to start driving of the three paper discharge roller pairs  57 . 
     Next, the controller  6  controls the paper feed unit  2  so as to feed the sheets P to the printing unit  3  every print time Tps for single-sheet printing in the printing unit  3 . 
     The print time Tps for single-sheet printing in the printing unit  3  is expressed by the following Numerical formula. 
     
       
         
           
             
               
                 
                   
                     T 
                     ps 
                   
                   = 
                   
                     
                       
                         L 
                         p 
                       
                       + 
                       
                         L 
                         g 
                       
                     
                     
                       V 
                       g 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Numerical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
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     Here, Lp is a sheet length and Lg is an inter-sheet distance. 
     The sheet length Lp is a length in the transfer direction of the sheet P. The sheet length Lp is determined in accordance with each sheet size. 
     The inter-sheet distance Lg is a distance between a trailing end of the preceding sheet P and a leading end of the succeeding sheet P on the belt platen  31  in the transfer direction of the sheets P. The shorter the inter-sheet distance Lg is, the larger the number of sheets to be output per unit time becomes. In the present embodiment, the inter-sheet distance Lg is set to a minimum value which is feasible under conditions such as the performance of the inkjet head of the head unit  33  and the like in order to attain high productivity. 
     When the sheet P is fed to the printing unit  3 , the sheet P is transferred by the belt platen  31  at the printing transfer speed Vg. The controller  6  controls the head unit  33  so as to eject ink onto the sheet P which is transferred by the belt platen  31  and to print the image. 
     When the leading end of the sheet P reaches the switching unit  55 , the sheet P is guided to the paper discharge route RD by the switching unit  55 . The sheet P which has been printed and which has been guided to the paper discharge route RD is transferred by the paper discharge roller pairs  57  and is discharged onto the paper receiving tray  59 . 
     When the sheets P of the number corresponding to a designated number of sheets are discharged, the controller  6  stops the operations of the belt platen  31  and the paper discharge roller pair  57 . Thereby, a simplex printing operation is completed. 
     Next, a printing schedule during duplex printing will be described. 
     The printing schedule of the printing unit  3  during duplex printing is for realizing the productivity per one side which is equivalent to the productivity during simplex printing, by an interleaf control system. The interleaf control system is a system that printing is alternately performed on one surface (a front surface) of one sheet P on which printing is not yet performed and a non-printed surface (a back surface) of another sheet P after simplex printing while transferring the plurality of sheets P on the transfer route. 
     Specifically, the printing schedule during duplex printing is for alternately performing front-surface printing and back-surface printing at intervals of the print time Tps for single-sheet printing as illustrated in  FIG. 3 . In  FIG. 3 , a numeral in each sheet P indicates what the order of that sheet P is. In addition, a blank sheet P indicates that front-surface printing is to be performed on that sheet P and a dot-hatched sheet P indicates that back-surface printing is to be performed on that sheet P. 
     However, the sheets p are continuously subjected to front-surface printing until the first sheet P after simplex printing is refed to the printing unit  3  and is subjected to back-surface printing. In  FIG. 3 , a period of time when front-surface printing is being performed on the first to third sheets P corresponds to a period of time when front-surface printing is continuously performed. In this period of time, an idle time corresponding to the print time Tps for single-sheet printing is generated between printing on the preceding sheet P and printing on the succeeding sheet P. 
     In addition, after front-surface printing of the last sheet P, back-surface printing is continuously performed. In  FIG. 3 , a period of time when back-surface printing is performed on the sixth and succeeding sheets P corresponds to a period of time when back-surface printing is continuously performed. Also in this period of time, the idle time corresponding to the print time Tps for single-sheet printing is generated between printing on the preceding sheet P and printing on the succeeding sheet P. 
     In the interleaf system, substantially, the period of time when front-surface printing and back-surface printing are alternately performed corresponds to a processing target. In  FIG. 3 , the period of time when back-surface printing is performed on the first to third sheets P is a period of time and at the same time, front-surface printing is performed on the fourth and fifth sheets P is a period of time when the productivity per one side which is equivalent to the productivity during simplex printing is realized by the interleaf system. In this period of time, the productivity per one side which is equivalent to the productivity during simplex printing is realized. 
     The number of sheets (three sheets in the example in  FIG. 3 ) continuously subjected to front-surface printing until the first sheet P is refed to the printing unit  3  and is subjected to back surface printing, is determined in accordance with the sheet size (the sheet length Lp). 
     Next, the operation of the printing machine  1  during duplex printing will be described. 
     First, the controller  6  causes the belt platen motor  32  to start driving of the belt platen  31 . The controller  6  performs control such that the transfer speed of the sheet P by the belt platen  31  reaches the printing transfer speed Vg. 
     Furthermore, the controller  6  causes the intermediate transfer motor  47  and the paper discharge motor  58  to start driving of the two intermediate transfer roller pairs  46  and the three paper discharge roller pairs  57 . The controller  6  performs control such that the transfer speed of the sheet P by the intermediate transfer roller pairs  46  reaches the printing transfer speed Vg. 
     Moreover, the controller  6  causes the reversal motor  49  to start normal rotation driving of the reversal roller pair  48 . The controller  6  performs control such that the transfer speed of the sheet P by normal rotation driving of the reversal roller pair  48  reaches the printing transfer speed Vg. 
     In addition, the controller  6  causes the two horizontal transfer motors  51  to start driving of the four horizontal transfer roller pairs  50 . The controller  6  performs control such that the transfer speed of the sheet P by the horizontal transfer roller pairs  50  reaches the circulation transfer speed Vr. 
     As the circulation transfer speed Vr, a value which has been calculated such that it is possible to refeed the sheet P after simplex printing at a timing corresponding to the above-mentioned printing schedule during duplex printing is set. The circulation transfer speed Vr fluctuates depending on the sheet size (the sheet length Lp). 
     Next, the controller  6  controls the paper feed unit  2  so as to feed the non-printed sheet P at a timing when a time interval between paper feed timings of the respective sheets P reaches twice the time interval (that is, the print time Tps) between paper feed timings in simplex printing. Namely, the controller  6  controls the paper feed unit  2  so as to feed the non-printed sheets P at intervals of 2Tps. 
     When the non-printed sheets P is fed to the printing unit  3 , the sheet P is transferred by the belt platen  31  at the printing transfer speed Vg. The controller  6  controls the head unit  33  so as to eject ink onto one surface (the front surface) of the sheet P transferred by the belt platen  31  and to print the image on the front surface of the sheet P. 
     When the leading end of the sheet P reaches the switching unit  55 , the sheet P is guided to the circulation route RC by the switching unit  55 . The sheet P which has been guided to the circulation route RC after simplex printing is transferred by the intermediate transfer roller pair  46  in the intermediate transfer unit  41  of the circulation transfer unit  4 , at the printing transfer speed Vg. When the leading end of the sheet P reaches the reversal roller pair  48 , the sheet P is transferred by the reversal roller pairs  48  and the intermediate transfer roller pairs  46 , at the printing transfer speed Vg. 
     After the trailing end of the sheet P has slipped out from the downstream side intermediate transfer roller pair  46 , the controller  6  stops the reversal roller pairs  48  as illustrated in  FIG. 4 . Here, the controller  6  performs control such that the reversal roller pair  48  is brought into a state of nipping the sheet P at a position of a trailing end remaining amount Ls from the trailing end of the sheet P, in the transfer direction when the reversal roller pair  48  is driven to normally rotate at a time t 1  when the reversal roller pair  48  is stopped. The trailing end remaining amount Ls has a constant value not depending on the sheet length Lp. 
     When a predetermined temporary stop time elapses after the reversal roller pair  48  has been stopped, the controller  6  causes the reversal motor  49  to start reversal rotation driving of the reversal roller pair  48  at a time t 2 . Thereby, the sheet P begins to be transferred toward the horizontal transfer roller pairs  50 . 
     The controller  6  performs control such that the transfer speed of the sheet P by the reversal roller pair  48  reaches the circulation transfer speed Vr before the leading end of the sheet P reaches the most-upstream horizontal transfer roller pair  50 . Thereby, the sheet P reaches the most-upstream horizontal transfer roller pair  50  at the circulation transfer speed Vr. When the transfer speed of the sheet P by the reversal roller pair  48  reaches the circulation transfer speed Vr, the controller  6  performs control so as to maintain the circulation transfer speed Vr. 
     When the leading end of the sheet P reaches the most-upstream horizontal transfer roller pair  50 , the sheet P is transferred by the horizontal transfer roller pair  50  and the reversal roller pair  48 , at the circulation transfer speed Vr. When the trailing end of the sheet P slips out from the reversal roller pair  48 , the controller  6  stops reverse rotation driving of the reversal roller pair  48  and then starts normal rotation driving of the reversal roller pair  48  at the printing transfer speed Vg. 
     Before the sheet P which is transferred by the horizontal transfer roller pairs  50  reaches the vertical transfer roller pair  18 , the controller  6  causes in advance the vertical transfer motor  19  to drive the vertical transfer roller pair  18  at the circulation transfer speed Vr. When the leading end of the sheet P reaches the vertical transfer roller pair  18 , the sheet P is transferred by the vertical transfer roller pair  18  and the horizontal transfer roller pairs  50 , at the circulation transfer speed Vr. 
     Then, the controller  6  controls the vertical transfer roller pair  18  so as to decelerate the sheet P and to stop the operation by abutting the leading end of the sheet P on the registration roller pair  20 . After the vertical transfer roller pair  18  has been stopped, the controller  6  causes the registration motor  21  to start up the registration roller pair  20  at a predetermined timing according to the printing schedule and performs control so as to send the sheet P from the registration roller pair  20  to the belt platen  31 . Thereby, the sheet P after simplex printing is refed to the printing unit  3 . 
     Since the sheet P after simplex printing is switched back by the reversal roller pair  48 , refeeding is performed with its not-printed surface (the back surface) facing upward. The sheet P after simplex printing, which has been refed, is transferred by the belt platen  31  at the printing transfer speed Vg in the printing unit  3 . The controller  6  controls the head unit  33  so as to eject ink onto the not-printed surface of the sheet P transferred by the belt platen  31  and to print the image. 
     When the leading end of the sheet P reaches the switching unit  55 , the sheet P is guided to the paper discharge route RD by the switching unit  55 . The sheet P after duplex printing, which has been guided to the paper discharge route RD, is transferred by the paper discharge roller pair  57  and is discharged onto the paper receiving tray  59 . 
     When the sheets P of the number corresponding to the designated number of sheets are discharged, the controller  6  stops the belt platen  31 , the intermediate transfer roller pairs  46 , the reversal roller pair  48 , the horizontal transfer roller pairs  50  and the paper discharge roller pairs  57 . Thereby, a duplex printing operation is completed. 
     As described above, in the printing machine  1 , the reversal roller pair  48  of the reversing unit  42  switches back the sheet P and reverses the sheet P upside down, and the sheet P is accelerated up to the circulation transfer sped Vr by acceleration when sheet transfer is restarted after a temporary stop in the switchback operation. Thereby, it is possible to accelerate the sheet P without pulling out the sheet P from the roller pair in the low-speed section by the roller pair in the high-speed section. Consequently, it is possible to reduce the load on the sheets P and the sheet transfer mechanism such as the roller pairs. In addition, since the sheets P are not rubbed with the rollers, it is possible to reduce stains on the sheets P. 
     Furthermore, since acceleration to the circulation transfer speed Vr is started at the time of restarting transfer after the temporary stop in the switchback operation by the reversal roller pair  48 , the length of the route along which sheet transfer is performed by acceleration and at the circulation transfer speed Vr becomes constant not depending on the sheet size. Thereby, it is possible to reduce the length of the transfer route in the printing machine  1  and to miniaturize the main body of the printing machine  1 . In addition, it is possible to eliminate necessity of excessively increasing the transfer speed of the sheet P. 
     Moreover, in the printing machine  1 , the sheet P is switched back and is accelerated up to the circulation transfer speed Vr, by the single reversal roller pair  48 . Therefore, it is possible to suppress the load on driving sources (the motors) in order to accelerate the sheet P. 
     The present invention is not limited to the above-mentioned embodiment as it is and constituent elements can be modified and embodied within a range not deviating from the gist thereof in the implementation phase. In addition, it is possible to form various inventions by appropriately combining together the plurality of constituent elements disclosed in the above-mentioned embodiment. For example, some constituent elements may be deleted from all of the constituent elements described in the embodiment. 
     In addition, for example, each of the functions and the processing described above can be implemented by one or more processing circuits. The processing circuits include a programmed processor, an electric circuit and the like and further include devices such as an integration circuit (ASIC) for specific application, circuit constituent elements arranged so as to execute the described functions, and the like. 
     The present application claims the priority based on Japanese Patent Application No. 2015-068103 filed on Mar. 30, 2015 and the entire content of the present patent application is incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, the reversing unit accelerates the sheet up to the circulation transfer speed which is higher than the printing transfer speed at the time of restarting transfer after the temporal stop in the switchback operation. Thereby, it is possible to accelerate the sheet without pulling out the sheet from the roller pair in the low-speed section by the roller pair in the high-speed section. Consequently, it is possible to reduce the load on the sheets and the sheet transfer mechanism such as the roller pairs. 
     REFERENCE SIGNS LIST 
     
         
           1  printing machine 
           2  paper feed unit 
           3  printing unit 
           4  circulation transfer unit 
           5  paper discharge unit 
           6  controller 
           11  external paper feed tray 
           12  external paper feed roller pair 
           13  internal paper feed tray 
           14  internal paper feed roller pair 
           15  internal paper feed motor 
           16  internal paper feed transfer roller pair 
           17  internal paper feed transfer motor 
           18  vertical transfer roller pair 
           19  vertical transfer motor 
           20  registration roller pair 
           21  registration motor 
           31  belt platen 
           32  belt platen motor 
           33  head unit 
           41  intermediate transfer unit 
           42  reversing unit 
           43  horizontal transfer unit 
           46  intermediate transfer roller pair 
           47  intermediate transfer motor 
           48  reversal roller pair 
           49  reversal motor 
           50  horizontal transfer roller pair 
           51  horizontal transfer motor 
           55  switching unit 
           56  solenoid 
           57  paper discharge roller pair 
           58  paper discharge motor 
           59  paper receiving tray