Patent Publication Number: US-11642902-B2

Title: Printing apparatus and printing method

Description:
The present application is based on, and claims priority from JP Application Serial Number 2021-013267, filed Jan. 29, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a printing apparatus and a printing method. 
     2. Related Art 
     In serial printers, printing is applied to a sheet transported by a paper feed roller pair and a paper discharge roller pair by a method in which ink is ejected from a printing head provided between both roller pairs. Ink ejection by the printing head when a carriage with the printing head mounted thereon is moved in the main scanning direction, and paper feed by each roller being driven is alternately performed, whereby printing to a sheet progresses (see JP-A-2004-175082). 
     In this type of serial printers, when the trailing end of a sheet leaves the paper feed roller pair, a kicking phenomenon occurs in which the trailing end is kicked (flicked) by the rotating paper feed roller pair. Due to this kicking phenomenon, the actual paper feed amount becomes greater than the paper feed amount assumed in the designing, resulting in decreased sheet position accuracy in the transport direction. A method is known in which, in order to suppress this kicking phenomenon from occurring, a low-velocity region is set in a range where the trailing end of the sheet passes through the paper feed roller pair, and the paper feed velocity is changed to a velocity slower than the normal velocity when the trailing end of the sheet passes through this low-velocity region. 
     When the paper feed velocity of the printing medium is slowed for a certain period of time in order to suppress the kicking phenomenon, the drying time of ink in the process up to print completion will be different region by region in the printing medium. This difference in drying time will be visible as region-by-region difference in density in the printing result, that is, density unevenness. There is a demand for devising ways to make such density unevenness inconspicuous as much as possible. 
     SUMMARY 
     A printing apparatus includes: a printing head configured to execute a pass for ejecting liquid while moving forward along a main scanning direction and a pass for ejecting liquid while moving backward along the main scanning direction, a transport unit configured to transport a printing medium in a transport direction intersecting the main scanning direction using a first transport member and a second transport member, and a control unit configured to perform multi-pass printing in which a raster line along the main scanning direction is printed in a plurality of passes on the printing medium by controlling the printing head and the transport unit, wherein the first transport member is a roller pair disposed upstream of the printing head in the transport direction, the second transport member is disposed downstream of the printing head in the transport direction, and provided that a region of the printing medium including a region that is a target of printing by the printing head while a trailing end, which is an upstream end portion of the printing medium in the transport direction, passes through the first transport member and the printing medium is transported by the second transport member is a first region, a region downstream of the first region in the printing medium is a second region, and a region downstream of the second region in the printing medium is a third region, the control unit, in a first period in which the first region is a target of printing by the printing head, causes a velocity of transport of the printing medium executed by the transport unit between the pass and a pass next to the pass to be slower than in a third period in which the third region is a target of printing by the printing head, in the first period, causes a standby time of the printing head between the pass and a pass next to the pass to be longer than in the third period, and in a second period in which the second region is a target of printing by the printing head, changes the standby time so that the standby time approaches the standby time in the first period from the standby time in the third period as the first region approaches the printing head. 
     A printing method includes: a printing control step for performing multi-pass printing in which a raster line along a main scanning direction is printed in a plurality of passes on a printing medium by controlling a printing head configured to execute a pass for ejecting liquid while moving forward along the main scanning direction and a pass for ejecting liquid while moving backward along the main scanning direction and a transport unit configured to transport, using a first transport member disposed upstream of the printing head and a second transport member disposed downstream of the printing head in a transport direction intersecting the main scanning direction, the printing medium in the transport direction, the first transport member being a roller pair and in the printing control step, provided that a region of the printing medium including a region that is a target of printing by the printing head while a trailing end, which is an upstream end portion of the printing medium in the transport direction, passes through the first transport member and the printing medium is transported by the second transport member set as a first region, a region downstream of the first region in the printing medium is a second region, and a region downstream of the second region in the printing medium is a third region, in a first period in which the first region is a target of printing by the printing head, a velocity of transport of the printing medium executed by the transport unit between the pass and a pass next to the pass is slower than in a third period in which the third region is a target of printing by the printing head, in the first period, a standby time of the printing head between the pass and a pass next to the pass is longer than in the third period, and in a second period in which the second region is a target of printing by the printing head, the standby time is changed so as to approach the standby time in the first period from the standby time in the third period as the first region approaches the printing head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating an apparatus configuration in a simplified manner. 
         FIG.  2    is a view illustrating a relationship between a printing medium and a printing head as seen from above. 
         FIG.  3    is a view illustrating a relationship between a printing medium and a printing head as seen from a viewpoint facing a main scanning direction. 
         FIG.  4    is a flowchart illustrating printing control processing. 
         FIG.  5    is a view illustrating a printing medium and a printing head, of which the relative positions in a transport direction change. 
         FIG.  6 A  is a view showing an example of a correspondence relationship between the paper feed count and the standby time in a graph. 
         FIG.  6 B  is a view showing another example of a correspondence relationship between the paper feed count and the standby time in a graph. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Note that each of the drawings is merely illustrative for describing the present embodiment. Since the drawings are illustrative, proportions and shapes may not be precise; the drawings may not match each other; and some portions may be omitted. 
     1. Apparatus Configuration 
       FIG.  1    illustrates a configuration of a printing apparatus  10  according to the present embodiment in a simplified manner. 
     The printing apparatus  10  includes a control unit  11 , a display unit  13 , an operation receiving unit  14 , a communication IF  15 , a printing unit  16 , and the like. The printing unit  16  includes a transport unit  17 , a carriage  18 , a printing head  19 , a maintenance unit  27 , and the like. IF is an abbreviation for interface. The control unit  11  includes one or a plurality of integrated circuits (IC) including a central processing unit (CPU)  11   a  as a processor, a read-only memory (ROM)  11   b , and a random access memory (RAM)  11   c , as well as other components such as a non-volatile memory. 
     In the control unit  11 , the processor, that is, the CPU  11   a  executes arithmetic processing in accordance with one or more programs  12  stored in the ROM  11   b  or other components such as a memory using the RAM  11   c  or the like as a work area, thereby controlling the printing apparatus  10 . Note that the processor is not limited to a single CPU. Processing may be performed by a plurality of CPUs or a hardware circuit such an application-specific integrated circuit (ASIC), and processing may be performed by a CPU and a hardware circuit working in concert. 
     The display unit  13  is a means for displaying visual information. The display unit  13  is constituted, for example, by a liquid crystal display, an organic electroluminescence (EL) display, or the like. The display unit  13  may include a display and a driving circuit for driving the display. The operation receiving unit  14  is a means for receiving an operation by a user. The operation receiving unit  14  is realized, for example, by a physical button, a touch panel, a mouse, a keyboard, or the like. Of course, the touch panel may be realized as a function of the display unit  13 . 
     The display unit  13  and the operation receiving unit  14  may constitute part of the configuration of the printing apparatus  10 , but may be peripheral devices externally coupled to the printing apparatus  10 . The communication IF  15  is a generic term for one or a plurality of IFs for connecting the printing apparatus  10  with the outside in a wired or wireless manner compliant with a predetermined communication protocol including a known communication standard. 
     The printing unit  16  is a mechanism for performing printing by an ink-jet method. 
     The transport unit  17  is a means for transporting a printing medium such as a sheet in a predetermined transport direction. The transport unit  17  includes a roller, a motor for rotating the roller, and the like. The printing medium may be a medium formed of a material other than paper as long as the medium is printable by liquid. Upstream and downstream in the transport direction may also be simply referred to as upstream and downstream. The printing head  19  includes a plurality of nozzles  20 . The printing head  19  ejects or does not eject a dot of liquid such as ink from the nozzles  20  based on print data generated by the control unit  11  for printing an image, thereby printing the image to a printing medium. The printing head  19  is capable of ejecting ink of various colors such as a cyan (C) ink, a magenta (M) ink, a yellow (Y) ink, and a black (K) ink, for example. Of course, the printing head  19  may also eject ink or liquid having a color other than CMYK. 
     The carriage  18  is a mechanism capable of reciprocating along a predetermined main scanning direction as a result of receiving power from a carriage motor (not illustrated). The main scanning direction intersects the transport direction. To intersect herein may be understood as to be orthogonal or substantially orthogonal. The printing head  19  is mounted on the carriage  18 . In other words, the printing head  19  reciprocates, along with the carriage  18 , along the main scanning direction. 
       FIG.  2    illustrates a relationship between a printing medium  30  and the printing head  19  in a simplified manner as seen from above. Along with the carriage  18 , the printing head  19  mounted on the carriage  18  moves from one end to the other end of a main scanning direction D 1  (forward movement) and moves from the other end to the one end (backward movement). 
       FIG.  2    illustrates an example of an arrangement of the nozzles  20  in a nozzle surface  21 . The nozzle surface  21  is the lower surface of the printing head  19 . Each small circle in the nozzle surface  21  is a nozzle  20 . In a configuration in which ink of each color is supplied from a liquid holding means (not illustrated) that is called an ink cartridge, an ink tank, and the like and ejected from the nozzles  20 , the printing head  19  includes a nozzle row  26  for each ink color.  FIG.  2    illustrates an example of the printing head  19  that ejects CMYK inks. The nozzle row  26  including the nozzles  20  that eject the C ink is a nozzle row  26 C. Similarly, the nozzle row  26  including the nozzles  20  that eject the M ink is a nozzle row  26 M, the nozzle row  26  including the nozzles  20  that eject the Y ink is a nozzle row  26 Y, and the nozzle row  26  including the nozzles  20  that eject the K ink is a nozzle row  26 K. The nozzle rows  26 C,  26 M,  26 Y, and  26 K are aligned along the main scanning direction D 1 . 
     Each of the nozzle rows  26  is constituted by a plurality of nozzles  20  having a constant or substantially constant nozzle pitch, which is an interval between nozzles  20  in a transport direction D 2 . The direction in which a plurality of nozzles  20  constituting a nozzle row  26  are aligned is a nozzle row direction D 3 . In the example illustrated in  FIG.  2   , the nozzle row direction D 3  is parallel with the transport direction D 2 . In a configuration in which the nozzle row direction D 3  is parallel with the transport direction D 2 , the nozzle row direction D 3  and the main scanning direction D 1  are orthogonal to each other. However, the nozzle row direction D 3  need not be parallel with the transport direction D 2 , and may diagonally intersect the main scanning direction D 1 . 
     The operation in which the printing head  19  ejects liquid such as ink in association with forward movement of the carriage  18  along the main scanning direction D 1  and the operation in which the printing head  19  ejects liquid such as ink in association with backward movement of the carriage  18  along the main scanning direction D 1  are called a main scanning or a pass. A pass of forward movement may also be called a forward pass, while a pass of backward movement may also be called a backward pass. The printing unit  16  combines passes and transport of the printing medium  30  in the transport direction D 2  by a constant amount by the transport unit  17  (hereinafter, paper feed) to perform printing to the printing medium  30 . More specifically, the printing unit  16  executes a single paper feed in the period of time after a single pass is ended until the next pass is started. For passes, the printing unit  16  alternately executes a forward pass and a backward pass. The period of time after a pass is ended until the next pass is started represents a standby time for the carriage  18  and the printing head  19 . The standby time for the carriage  18  and the printing head  19  is also simply referred to as the standby time. 
     The configuration of the printing apparatus  10  illustrated in  FIG.  1    may be realized by a single printer, and may be realized by a plurality of apparatuses communicatively coupled to each other. 
     In other words, the printing apparatus  10  may be a printing system  10  in actuality. The printing system  10  includes, for example, a printing control apparatus that functions as the control unit  11 , and a printer corresponding to the printing unit  16  controlled by the printing control apparatus. A printing method according to the present embodiment is realized by such a printing apparatus  10  or such a printing system  10 . 
       FIG.  3    illustrates a relationship between the printing medium  30  and the printing head  19  in a simplified manner as seen from a viewpoint facing the main scanning direction D 1 . The nozzle surface  21  of the printing head  19  mounted on the carriage  18  is opposed to a platen  22 . The platen  22  supports the printing medium  30  that is transported at a position below the printing head  19 . The platen  22  constitutes part of a transport path for the printing medium  30 . 
     A transport roller pair  23  as a first transport member is disposed upstream of the printing head  19 . The transport roller pair  23  is a pair of rollers  23   a  and  23   b . The transport roller pair  23  rotates while sandwiching the printing medium  30  by the rollers  23   a  and  23   b , thereby transporting the printing medium  30 . One of the rollers  23   a  and  23   b  constituting the roller pair  23  is driven by a motor (not illustrated), while the other is rotated as a driven roller. Furthermore, a discharge roller pair  24  as a second transport member is disposed downstream of the printing head  19 . The discharge roller pair  24  is a pair of rollers  24   a  and  24   b . The discharge roller pair  24  rotates while sandwiching the printing medium  30  by the rollers  24   a  and  24   b , thereby transporting the printing medium  30 . One of the rollers  24   a  and  24   b  constituting the roller pair  24  is driven by the same motor (not illustrated) as that for the transport roller pair  23 , while the other is rotated as a driven roller. 
     The transport roller pair  23  and the discharge roller pair  24  each constitute part of the transport unit  17 . In the example of  FIG.  3   , the transport unit  17  transports the printing medium  30  in the transport direction D 2  using both or one of the transport roller pair  23  and the discharge roller pair  24 . Note that it is sufficient that the second transport member is a means capable of transporting the printing medium  30  downstream of the printing head  19 . The second transport member need not be limited to a roller pair. Furthermore, the transport unit  17  may also include, at a position upstream of the transport roller pair  23  or at a position downstream of the discharge roller pair  24 , a roller (not illustrated) or the like for transporting the printing medium  30 . 
     A printing medium sensor  25  is disposed upstream of the printing head  19 . In the example of  FIG.  3   , the printing medium sensor  25  is located slightly upstream of the transport roller pair  23 . The printing medium sensor  25  is capable of detecting the leading end or the trailing end of the printing medium  30  that is transported. In the present embodiment, with respect to the printing medium  30  and other objects, structures, and the like, the downstream end portion is called the leading end, while the upstream end portion is called the trailing end. In  FIG.  3   , the leading end of the printing medium  30  is not illustrated, whereas the trailing end of the printing medium  30  is indicated by reference numeral  31 . 
     In  FIG.  3   , a range from a predetermined position slightly upstream of the transport roller pair  23  to a predetermined position slightly downstream of the transport roller pair  23  is set as a low-velocity range L in the transport direction D 2 . The control unit  11  carries out control so that velocity VU&gt;velocity VL, wherein the velocity VU is the normal velocity of paper feed of the printing medium  30  by the transport unit  17  while the velocity VL is the paper feed velocity in a period in which the trailing end  31  of the printing medium  30  passes through the low-velocity range L. In other words, the control unit  11  drops the velocity of paper feed by the transfer unit  17  from the velocity VU to the velocity VL in a period in which the trailing end  31  passes through the low-velocity range L to prevent the kicking phenomenon described above from occurring when the trailing end  31  passes through the transport roller pair  23 . 
     2. Printing Control Processing 
       FIG.  4    illustrates printing control processing executed by the control unit  11  in accordance with the program  12  in a flowchart. This printing control processing includes a “printing control step” according to the present embodiment. Here, for ease of explanation, a scene is assumed in which a cut sheet, which is cut in advance on a page-by-page basis, is used as the printing medium  30 , and in which one page&#39;s worth of print data is printed to a single sheet of a cut sheet. When performing a plurality of pages&#39; worth of printing, it is sufficient that the flowchart of  FIG.  4    is repeatedly executed. In the present embodiment, the control unit  11  performs multi-pass printing in which a raster line along the main scanning direction D 1  is printed in a plurality of passes on the printing medium  30  by controlling the printing head  19  and the transport unit  17 . A raster line is a pixel row including a plurality of pixels aligned along the main scanning direction D 1  in the state of print data. 
     With the top-of-form positioning of the printing medium  30  by the transport unit  17  being ended, the control unit  11  starts the flowchart of  FIG.  4   . Top-of-form positioning is the processing for transporting the printing medium  30  to a predetermined position at which the printing medium  30  may be subjected to a first pass by the printing head  19 . The control unit  11  causes the transport unit  17  to start transport of the printing medium  30  loaded in a paper feed tray (not illustrated). With the timing when the leading end of the printing medium  30  is detected by the printing medium sensor  25  as a reference, for example, the control unit  11  causes the printing medium  30  to be transported therefrom by a predetermined distance, thereby ending the top-of-form positioning. 
     In step S 100 , the control unit  11  controls the carriage  18  and the printing head  19  to execute a single pass on the printing medium  30 . The pass executed in step S 100  is a pass in a third period. The third period will be described later. 
     In step S 110 , the control unit  11  determines whether the paper feed to be executed after the pass executed in step S 100  is a paper feed for the “downstream adjustment region”. 
     Here, regions including the downstream adjustment region in the printing medium  30  will be described with reference to  FIG.  5   .  FIG.  5    illustrates a portion of the printing medium  30  including the trailing end  31 , and the printing head  19  of which the relative position with the printing medium  30  in the transport direction D 2  changes. As illustrated in  FIG.  2   , the printing head  19  is mounted on the carriage  18  and has the nozzle rows  26 . However.  FIG.  5    illustrates the printing head  19  in a simplified manner as a simple rectangle. 
     The printing heads  19  illustrated in  FIG.  5    all represent one and the same printing head  19 . The position of the printing head  19  in the transport direction D 2  is actually unchanged. In other words,  FIG.  5    illustrates how the relative position between the printing medium  30  and the printing head  19  in the transport direction D 2  changes each time a single paper feed of the printing medium  30  is carried out in the transport direction D 2 . According to  FIG.  5   , the transport amount by a single paper feed is “F”. In  FIG.  5   , numbers such as #N enclosed in parentheses and accompanying the respective printing heads  19  stand for pass numbers. A pass number is a number indicating the ordinality of a pass in a single sheet of the printing medium  30 . In other words, the printing head  19  with a pass number of #N represents the printing head  19  when the Nth pass is executed. In  FIG.  5   , due to space constraints, only the pass number is given for printing heads  19  having a pass number of #N+5 or greater, with reference sign  19  being omitted. 
     The transport amount F corresponds, for example, to ¼ of the length in the transport direction D 2  of the nozzle rows  26 . Furthermore, in  FIG.  5   , it is simply understood that the length in the transport direction D 2  of the printing head  19  equals the length in the transport direction D 2  of the nozzle rows  26 . In such an example, the printing head  19  prints the same region of the printing medium  30  in four passes. For example, one region  45  in the printing medium  30  is printed in a total of four passes with a pass number from N+1 to N+4. Based on print data, an image in which a plurality of raster lines are aligned in the transport direction D 2 , that is, a bundle of raster lines are printed to the region  45 . Each raster line is printed in a plurality of passes, and in four passes in the example of  FIG.  5   . Of course, in multi-pass printing, the number of passes required for print completion of a raster line is not limited to four. 
       FIG.  5    illustrates the printing medium  30  as being divided into regions  40 ,  41 ,  42 ,  43 , and  44  in the transport direction D 2 . The region  42  is called a “low-velocity printing region  42 ”. The low-velocity printing region  42  is a region that is printed by the printing head  19  in a period in which the trailing end  31  passes through the low-velocity range L. In other words, the low-velocity printing region  42  is a specific example of the “first region” of the printing medium  30  including a region that is a target of printing by the printing head  19  while the trailing end  31  passes through the first transport member and the printing medium  30  is transported by the second transport member. For the control unit  11 , the low-velocity range L in the transport direction D 2  and the position and size of the printing head  19  are known information. Furthermore, when printing to the printing medium  30  is started, the size of the printing medium  30  is also known information for the control unit  11 . Accordingly, based on these pieces of information, the control unit  11  can calculate and identify the low-velocity printing region  42  of the printing medium  30 . The control unit  11  sets, for example, a range from a position in the printing medium  30  corresponding to the leading end of the nozzle rows  26  when the trailing end  31  is positioned at the trailing end of the low-velocity range L to a position in the printing medium  30  corresponding to the trailing end of the nozzle rows  26  when the trailing end  31  is positioned at the leading end of the low-velocity range L as the low-velocity printing region  42 . 
     Furthermore, as described above, in a situation where the transport amount F by a single paper feed by the transport unit  17  is known, the control unit  11  can calculate, for example, with the time when the top-of-form positioning is ended as a reference, what the ordinalities of passes that correspond to the period in which the low-velocity printing region  42  is to be printed by the printing head  19  (hereinafter, the first period) are. Here, with reference to  FIG.  5   , it is assumed that the control unit  11  judges, for example, that each pass with a pass number from #N+5 to #N+8 corresponds to the first period. 
     The control unit  11  sets an adjustment region  41  downstream of the low-velocity printing region  42  in the printing medium  30  and an adjustment region  43  upstream of the low-velocity printing region  42  in the printing medium  30 . Here, it is assumed that one page&#39;s worth of print data used for printing to the printing medium  30  is data for printing some sort of image over the substantially entire surface from the leading end to the trailing end of the printing medium  30 . The adjustment region  41  corresponds to the “second region,” while the adjustment region  43  corresponds to the “fourth region”. Hereinafter, the adjustment region  41  is called a downstream adjustment region  41 , while the adjustment region  43  is called an upstream adjustment region  43 . The control unit  11  makes the length in the transport direction D 2  of each of the downstream adjustment region  41  and the upstream adjustment region  43  the same as the length in the transport direction D 2  of the low-velocity printing region  42 , for example. 
     Furthermore, the control unit  11  sets the region downstream of the downstream adjustment region  41  in the printing medium  30  as a normal region  40 , and the region upstream of the upstream adjustment region  43  as a normal region  44 . The normal regions  40  and  44  each correspond to the “third region”. Hereinafter, the normal region  40  is called a downstream normal region  40 , while the normal region  44  is called an upstream normal region  44 . 
     The description now returns to  FIG.  4   . In step S 110 , the control unit  11  determines whether the paper feed to be executed after the pass executed in step S 100  is a paper feed for the downstream adjustment region  41 . With reference to  FIG.  5   , it is assumed that the control unit  11  judges, for example, that each pass with a pass number from #N−2 to #N+4 corresponds to the period in which the downstream adjustment region  41  is to be printed by the printing head  19  (hereinafter, the second period). In this case, if the paper feed to be executed after the pass executed in step S 100  is a paper feed between a pass and another pass in the second period, a “Yes” determination may be made in step S 110  to proceed to step S 130 . On the other hand, if the paper feed to be executed after the pass executed in step S 100  is a paper feed for the downstream normal region  40 , a “No” determination is made in step S 110  to proceed to step S 120 . As in the example described above, assuming that each pass with a pass number from #N−2 to #N+4 is for the second period, the control unit  11  treats each pass with a pass number from #1 to #N−3 as a pass for the period in which the downstream normal region  40  is to be printed by the printing head  19  (hereinafter, the third period). Thus, if the paper feed to be executed after the pass executed in step S 100  is a paper feed between a pass and another pass in the third period, a “No” determination may be made in step S 110 . 
     The control unit  11  similarly identifies each of the passes corresponding to the period in which the upstream adjustment region  43 , which is the fourth region, is to be printed by the printing head  19  (hereinafter, the fourth period), and each of the passes corresponding to the period in which the upstream normal region  44 , which is the upstream third region, is to be printed by the printing head  19 . It can be said that the period in which the upstream normal region  44  is to be printed by the printing head  19  is also a type of the third period because it is a printing period for a normal region. However, in order to distinguish from the period in which the downstream normal region  40  is to be printed by the printing head  19  (third period), the period in which the upstream normal region  44  is to be printed by the printing head  19  is called a fifth period for convenience. In other words, in the time series, printing progresses in the order of the third period, the second period, the first period, the fourth period, and the fifth period. Note that just like the relationship between the last pass of the third period and the first pass of the second period, for example, the control unit  11  may treat, of two continuous periods (regions), the paper feed carried out between the last pass of one period (region) and the first pass of the other period (region) either as a paper feed for the one period (region) or as a paper feed for the other period (region). 
     In step S 120 , the control unit  11  controls the transport unit  17  so as to cause the same to execute a single paper feed at the velocity VU. Furthermore, in association with the paper feed, the control unit  11  causes the carriage  18  and the printing head  19  to stand by until the next pass execution. The standby time in step S 120  is called the “normal standby time”. The normal standby time corresponds to the standby time in the third period. No particular mention is made as to the details of the normal standby time. However, the time required for a single paper feed at the velocity VU may be simply grasped as the normal standby time. Following step S 120 , the control unit  11  performs step S 100 . 
     In step S 130 , a standby time for the downstream adjustment region is set. The standby time for the downstream adjustment region corresponds to the standby time in the second period. In the second period, the control unit  11  changes the standby time so that the standby time approaches the standby time in the first period (low-velocity standby time) from the standby time in the third period (normal standby time) as the first region (low-velocity printing region  42 ) approaches the printing head  19 . 
       FIG.  6 A  shows a correspondence relationship between the paper feed count and the standby time in a graph. A paper feed count is a number indicating the ordinality of a paper feed, with the paper feed immediately after the first pass on the printing medium  30  counted as the first. For example, the paper feed executed between a pass with a pass number of #N+1 and a pass with a pass number of #N+2 is a paper feed with a paper feed count of N+1. TU stands for normal standby time, while TL stands for low-velocity standby time. As is clear from  FIG.  6 A , TU&lt;TL, wherein TU is the normal standby time, and TL is the low-velocity standby time. 
     A range Ra is a range of the paper feed count in the third period. The standby time associated with the paper feed executed between passes in the third period is each time the normal standby time TU. A range Rc is a range of the paper feed count in the first period. The standby time associated with the paper feed executed between passes in the first period is each time the low-velocity standby time TL. No particular mention is made as to the details of the low-velocity standby time. However, the time required for a single paper feed at the velocity VL may be simply set as the low-velocity standby time. 
     A range Rb is a range of the counts of paper feeds executed in the second period. As illustrated in  FIG.  6 A , the control unit  11  makes the standby time associated with the paper feed executed between passes in the second period longer so that the standby time approaches the low-velocity standby time TL from the normal standby time TU as the paper feed count increases. 
       FIG.  6 B  shows a correspondence relationship between the paper feed count and the standby time in a graph different from that of  FIG.  6 A .  FIG.  6 B  differs from  FIG.  6 A  only in terms of how the standby time changes in the range Rb and a range Rd. The standby time associated with the paper feed executed between passes in the second period need not necessarily be longer than the standby time associated with the preceding paper feed. As shown in the range Rb of  FIG.  6 B , the standby time may change stepwise. In other words, in the second period, the standby time associated with one paper feed may be the same as the standby time associated with the preceding paper feed. In either case, according to such  FIG.  6 A or  6 B , in step S 130 , the control unit  11  sets a standby time equal to or greater than the standby time associated with the preceding paper feed as the standby time for the downstream adjustment region. 
     In step S 140 , the control unit  11  controls the transport unit  17  so as to cause the same to execute a single paper feed at the velocity VU. Furthermore, in association with the paper feed, the control unit  11  causes the carriage  18  and the printing head  19  to stand by until the next pass execution. The standby time in step S 140  is the standby time set in step S 130 . 
     Note that the control unit  11  may execute the determination of step S 110  and the setting of step S 130  in parallel with the pass executed in step S 100 , and when the pass of step S 100  is ended, immediately execute step S 120  or step S 140 . Furthermore, such a configuration in which step S 100  and steps S 110  and S 130  are performed in parallel can be similarly applied to the relationship between step S 150  and steps S 160  and S 130 , the relationship between step S 180  and steps S 190  and S 200 , and the relationship between step S 220  and steps S 230  and S 200  to be described later. 
     Following step S 140 , in step S 150 , the control unit  11  controls the carriage  18  and the printing head  19  to execute a single pass on the printing medium  30 . The pass executed in step S 150  is a pass in the second period. 
     In step S 160 , the control unit  11  determines whether the paper feed to be executed after the pass executed in step S 150  is a paper feed for the low-velocity printing region  42 . 
     As described with reference to  FIG.  5   , it is assumed that the control unit  11  judges that each pass with a pass number from #N+5 to #N+8 corresponds to the first period. In this case, if the paper feed to be executed after the pass executed in step S 150  is a paper feed between a pass and another pass in the first period, a “Yes” determination may be made in step S 160  to proceed to step S 170 . On the other hand, if the paper feed to be executed after the pass executed in step S 150  is a paper feed for the downstream adjustment region  41 , that is, a paper feed between a pass and another pass in the second period, a “No” determination is made in step S 160  and step S 130  is executed again. Following a “No” determination in step S 260 , in step S 130 , as described above, the control unit  11  sets a standby time equal to or greater than the standby time set in step S 130  in the preceding time with reference to the range Rb of the graph in  FIG.  6 A  or  FIG.  6 B . 
     The velocity of paper feed of the printing medium  30  in step S 140  is the same velocity VU as in step S 120 . On the other hand, the standby time in step S 140  is longer than the normal standby time TU employed in step S 120 . Accordingly, in the relationship between step S 120  and the subsequent step S 100 , for example, when a paper feed is ended, a pass is immediately started; in contrast, in the relationship between step S 140  and the subsequent step S 150 , even after a paper feed is ended, the carriage  18  and the printing head  19  continue to stand by for a while, and then proceed to pass execution. 
     In step S 170 , the control unit  11  controls the transport unit  17  so as to cause the same to execute a single paper feed at the velocity VU. Furthermore, in association with the paper feed, the control unit  11  causes the carriage  18  and the printing head  19  to stand by until the next pass execution. The standby time in step S 170  is the low-velocity standby time TL. 
     Following step S 170 , in step S 180 , the control unit  11  controls the carriage  18  and the printing head  19  to execute a single pass on the printing medium  30 . The pass executed in step S 180  is a pass in the first period. 
     In step S 190 , the control unit  11  determines whether the paper feed to be executed after the pass executed in step S 180  is a paper feed for the upstream adjustment region  43 . If the paper feed to be executed after the pass executed in step S 180  is a paper feed between a pass and another pass in the fourth period, the control unit  11  may make a “Yes” determination in step S 190  to proceed to step S 200 . On the other hand, if the paper feed to be executed after the pass executed in step S 180  is a paper feed for the low-velocity printing region  42 , that is, a paper feed between a pass and another pass in the first period, a “No” determination is made in step S 190  and step S 170  is executed again. 
     In step S 200 , a standby time for the upstream adjustment region is set. The standby time for the upstream adjustment region corresponds to the standby time in the fourth period. In the fourth period, the control unit  11  changes the standby time so that the standby time approaches the standby time in the third period (normal standby time TU) from the standby time in the first period (low-velocity standby time TL) as the third region (upstream normal region  44 ) upstream of the first region (low-velocity printing region  42 ) approaches the printing head  19 . 
     Reference is again made to  FIGS.  6 A and  6 B . The range Rd is a range of the counts of paper feeds executed in the fourth period, while a range Re is a range of the counts of paper feeds executed in the fifth period. The standby time associated with the paper feed executed between passes in the fifth period is each time the normal standby time TU. As illustrated in  FIG.  6 A , the control unit  11  makes the standby time associated with the paper feed executed between passes in the fourth period shorter so that the standby time approaches the normal standby time TU from the low-velocity standby time TL as the paper feed count increases. However, the standby time associated with the paper feed executed between passes in the fourth period need not necessarily be shorter than the standby time associated with the preceding paper feed. As shown in the range Rb of  FIG.  6 B , the standby time may change stepwise. In other words, in the fourth period, the standby time associated with one paper feed may be the same as the standby time associated with the preceding paper feed. In either case, according to  FIG.  6 A or  6 B , in step S 200 , the control unit  11  sets a standby time equal to or less than the standby time associated with the preceding paper feed as the standby time for the upstream adjustment region. 
     In step S 210 , the control unit  11  controls the transport unit  17  so as to cause the same to execute a single paper feed at the velocity VU. Furthermore, in association with the paper feed, the control unit  11  causes the carriage  18  and the printing head  19  to stand by until the next pass execution. The standby time in step S 210  is the standby time set in step S 200 . 
     Following step S 210 , in step S 220 , the control unit  11  controls the carriage  18  and the printing head  19  to execute a single pass on the printing medium  30 . The pass executed in step S 220  is a pass in the fourth period. 
     In step S 230 , the control unit  11  determines whether the paper feed to be executed after the pass executed in step S 220  is a paper feed for the upstream normal region  44 . 
     If the paper feed to be executed after the pass executed in step S 220  is a paper feed between a pass and another pass in the fifth period, the control unit  11  may make a “Yes” determination in step S 230  to proceed to step S 240 . On the other hand, if the paper feed to be executed after the pass executed in step S 220  is a paper feed for the upstream adjustment region  43 , that is, a paper feed between a pass and another pass in the fourth period, a “No” determination is made in step S 220  and step S 200  is executed again. Following a “No” determination in step S 230 , in step S 200 , as described above, the control unit  11  sets a standby time equal to or less than the standby time set in step S 200  in the preceding time with reference to the range Rd of the graph in  FIG.  6 A  or  FIG.  6 B . 
     The velocity of paper feed of the printing medium  30  in step S 210  is the same velocity VU as in step S 120 . On the other hand, the standby time in step S 210  is longer than the normal standby time TU. Accordingly, in the relationship between step S 210  and the subsequent step S 220 , even after a paper feed is ended, the carriage  18  and the printing head  19  continue to stand by for a while, and then proceed to pass execution. 
     In step S 240 , the control unit  11  controls the transport unit  17  so as to cause the same to execute a single paper feed at the velocity VU. Furthermore, in association with the paper feed, the control unit  11  causes the carriage  18  and the printing head  19  to stand by until the next pass execution. The standby time in step S 240  is the normal standby time TU. In other words, step S 240  represents the same processing as that of step S 120 . 
     Following step S 240 , in step S 250 , the control unit  11  controls the carriage  18  and the printing head  19  to execute a single pass on the printing medium  30 . The pass executed in step S 250  is a pass in the fifth period. As simply illustrated by a dashed line with an arrow in  FIG.  4   , the control unit  11  repeats steps S 240  and S 250  until the last pass based on one page&#39;s worth of print data used for printing to the printing medium  30  is executed. In response to the pass corresponding to the last pass being executed in step S 250 , the control unit  11  ends the flowchart of  FIG.  4   . Of course, the control unit  11  can control the transport unit  17  to transport the printing medium  30  for which the last pass is ended downstream of the printing head  19  so as to discharge to a paper discharge tray (not illustrated) or the like. 
     3. Description of Advantageous Effects 
     As described above, according to the present embodiment, the printing apparatus  10  includes: the printing head  19  configured to execute a pass for ejecting liquid while moving forward along the main scanning direction D 1  and a pass for ejecting liquid while moving backward along the main scanning direction D 1 ; the transport unit  17  configured to transport the printing medium  30  in the transport direction D 2  intersecting the main scanning direction D 1  using the first transport member and the second transport member; and the control unit  11  configured to perform multi-pass printing in which a raster line along the main scanning direction D 1  is printed in a plurality of passes on the printing medium  30  by controlling the printing head  19  and the transport unit  17 . The first transport member is a roller pair disposed upstream of the printing head  19  in the transport direction D 2 . The second transfer member is disposed downstream of the printing head  19  in the transport direction D 2 . In addition, the control unit  11 , with a region of the printing medium  30  including a region that is a target of printing by the printing head  19  while the trailing end  31 , which is an upstream end portion of the printing medium  30  in the transport direction D 2 , passes through the first transport member and the printing medium  30  is transported by the second transport member set as a first region, a region downstream of the first region in the printing medium  30  set as a second region, and a region downstream of the second region in the printing medium  30  set as a third region, in a first period in which the first region is a target of printing by the printing head  19 , makes a velocity of transport of the printing medium executed by the transport unit  17  between a pass and another pass slower than in a third period in which the third region is a target of printing by the printing head  19 , in the first period, makes a standby time of the printing head  19  between a pass and another pass longer than in the third period, and in a second period in which the second region is a target of printing by the printing head  19 , changes the standby time so that the standby time approaches the standby time in the first period (low-velocity standby time TL) from the standby time in the third period (normal standby time TU) as the first region approaches the printing head  19 . 
     According to the above-described configuration, in order to suppress the kicking phenomenon from occurring when the trailing end  31  of the printing medium  30  passes through the first transport member, the velocity of paper feed is decreased in the first period, which makes the standby time in the first period longer than in other periods. In a configuration in which multi-pass printing is performed, the standby time serves as a drying time until a dot of liquid ejected to the printing medium  30  in a previous pass is overlapped with or brought into contact with a dot of liquid ejected in a later pass. Such difference in the drying time gives rise to difference in density in printing results. In such a situation, the control unit  11  sets a second region downstream of the first region between the third region and the first region. In the second period in which the second region is a target of printing, the control unit  11  makes the standby time longer as the paper feed count increases. This makes the drying time gradually longer in the second region connecting the third region and the first region, causing the density of printing results to change in gradation. Therefore, density unevenness due to difference in density between the third region having a short drying time and the first region having a long drying time is made inconspicuous thanks to the effect of such density change in gradation, which practically reduces density unevenness. 
     Furthermore, according to the present embodiment, the control unit  11 , with a region upstream of the first region in the printing medium  30  set as a fourth region and a region upstream of the fourth region in the printing medium  30  set as a third region, in a fourth period in which the fourth region is a target of printing by the printing head  19 , further changes the standby time so that the standby time approaches the standby time in the third period (normal standby time TU) from the standby time in the first period (low-velocity standby time TL) as the third region upstream of the first region approaches the printing head  19 . 
     According to the above-described configuration, the control unit  11  sets a fourth region upstream of the first region between the third region and the first region. In the fourth period in which the fourth region is a target of printing, the control unit  11  makes the standby time shorter as the paper feed count increases. This makes the drying time gradually shorter in the fourth region connecting the first region and the upstream third region, causing the density of printing results to change in gradation. Therefore, density unevenness due to difference in density between the first region having a long drying time and the upstream third region having a short drying time is made inconspicuous thanks to the effect of such density change in gradation, which practically reduces density unevenness. 
     Note that at which position of the printing medium  30  printing to the printing medium  30  based on print data is ended, that is, on which region of the printing medium  30  the last pass proves to be depends on print data. Although not particularly illustrated in  FIG.  4   , depending on print data, any of the passes in steps S 100 , S 150 , S 180 , and S 220  can prove to be the last pass. Therefore, for example, a pass directed to the first region can prove to be the last pass for the printing medium  30 . When a pass directed to the first region proves to be the last pass for the printing medium  30 , it is of course unnecessary to control passes, paper feeds, or standby time directed to the fourth region or the third region upstream of the fourth region. 
     The present embodiment discloses a printing apparatus and a printing system. Further, the present embodiment discloses a method executed by this apparatus or this system, and the program  12  that causes a processor to execute this method. 
     The printing method includes a printing control step for performing multi-pass printing in which a raster line along the main scanning direction D 1  is printed in a plurality of passes on the printing medium  30  by controlling the printing head  19  configured to execute a pass for ejecting liquid while moving forward along the main scanning direction D 1  and a pass for ejecting liquid while moving backward along the main scanning direction D 1  and the transport unit  17  configured to transport, using a first transport member disposed upstream of the printing head  19  and a second transport member disposed downstream of the printing head  19  in the transport direction D 2  intersecting the main scanning direction D 1 , the printing medium  30  in the transport direction D 2 . The first transport member is a roller pair. In the printing control step, with a region of the printing medium  30  including a region that is a target of printing by the printing head  19  while the trailing end  31 , which is an upstream end portion of the printing medium  30  in the transport direction D 2 , passes through the first transport member and the printing medium  30  is transported by the second transport member set as a first region, a region downstream of the first region in the printing medium  30  set as a second region, and a region downstream of the second region in the printing medium  30  set as a third region, in a first period in which the first region is a target of printing by the printing head  19 , a velocity of transport of the printing medium executed by the transport unit  17  between a pass and another pass is slower than in a third period in which the third region is a target of printing by the printing head  19 , in the first period, a standby time of the printing head  19  between a pass and another pass is longer than in the third period, and in a second period in which the second region is a target of printing by the printing head  19 , the standby time is changed so as to approach the standby time in the first period (low-velocity standby time TL) from the standby time in the third period (normal standby time TU) as the first region approaches the printing head  19 . 
     4. Other Description 
     The length in the transport direction D 2  of the second region is a length close to the shorter length of the length in the transport direction D 2  of the first region and the length in the transport direction D 2  of the third region. 
     The first region printed in a period in which the paper feed velocity is temporarily slowed in order to suppress the kicking phenomenon from occurring represents only a portion of the printing medium  30 . Accordingly, in many cases, the first region is shorter in length in the transport direction D 2  than the third region. In the example of  FIG.  5    as well, the low-velocity printing region  42  is shorter in length in the transport direction D 2  than the downstream normal region  40 . In consideration of such a situation, in the example of  FIG.  5   , the control unit  11  makes each of the downstream adjustment region  41  and the upstream adjustment region  43  equivalent in length in the transport direction D 2  to the low-velocity printing region  42 . Note that each of the downstream adjustment region  41  and the upstream adjustment region  43  and the low-velocity printing region  42  need not have an identical length in the transport direction D 2 . By adopting such a configuration, in the printing result on the printing medium  30 , the region in which density changes in gradation is prevented from spreading out as much as possible. If the low-velocity printing region  42  is longer in length in the transport direction D 2  than the downstream normal region  40 , the control unit  11  may make each of the downstream adjustment region  41  and the upstream adjustment region  43  equivalent in length in the transport direction D 2  to the downstream normal region  40 . 
     As will be understood from the description above, in the first period and the second period, the standby time is longer compared with that of the third period. During the standby time, maintenance of the printing head  19  can be performed. Therefore, the control unit  11  makes the number of times maintenance is performed by the printing head  19  per unit time in the first period and the second period greater than the number of times maintenance is performed by the printing head  19  per unit time in the third period. Maintenance herein means, for example, cleaning to remove dirt on the nozzle surface  21 , flushing that forcibly causes liquid ejection from each nozzle  20  to improve ejection failure, and the like. The maintenance unit  27  constitutes at least part of a member necessary for such maintenance. The maintenance unit  27  is, for example, a wiper for cleaning the nozzle surface  21 , an absorbent material or a receptacle for receiving dots ejected from each nozzle  20  by flushing, and the like. According to such a configuration, a relatively long standby time in the first period and the second period can be effectively utilized to perform maintenance of the printing head  19 . 
     The control unit  11  need not necessarily cause the carriage  18  and the printing head  19  to execute forward passes and backward passes. For example, the control unit  11  may perform printing to the printing medium  30  only in forward passes. In this case, it is necessary to cause the carriage  18  to execute, between a forward pass and a next forward pass, backward movement as an empty pass in which no liquid ejection is performed. Alternatively, the control unit  11  may perform printing to the printing medium  30  only in backward passes. In this case, it is necessary to cause the carriage  18  to execute, between a backward pass and a next backward pass, forward movement as an empty pass in which no liquid ejection is performed. The control unit  11  causes backward movement or forward movement as an empty pass to be executed within a standby time associated with paper feed. In other words, standby time does not mean a period of time in which the carriage  18  and the printing head  19  are not moved at all. Standby time is also a period of time in which the carriage  18  or the printing head  19  is caused to execute maintenance-related operations described above other than printing, or operations that need to be done before the start of the next pass.