Patent Publication Number: US-11648780-B2

Title: Printing apparatus and printing method

Description:
The present application is based on, and claims priority from JP Application Serial Number 2020-112569, filed Jun. 30, 2020, 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 
     Technology has been disclosed of an inkjet type printing apparatus in which a test pattern is printed on a print sheet by a recording head, the test pattern is read by a scanner, interpolation processing is performed on read data, and a nozzle abnormality is determined on the basis of the interpolated read data (see JP-A-2007-54970). 
     However, when a medium is selected, as a printing medium used for printing, in which ink bleed-through occurs relatively easily, the ink bleeds through and spreads, the shape of each of elements configuring the test pattern becomes uneven, and as a result, inspection of the nozzle on the basis of the read data of the test pattern cannot be appropriately performed. Thus, there is a need for a suitable test pattern for appropriately performing the inspection of the nozzle. 
     SUMMARY 
     A printing apparatus includes a printing head including a nozzle configured to discharge ink, and a control unit configured to control the printing head to print a test pattern on a printing medium, for inspecting a state of ink discharge by the nozzle. The test pattern includes a pattern element formed by a plurality of dots of the ink, and the control unit causes the printing head to print the test pattern where a number of the dots forming the pattern element on a second printing medium is smaller than a number of the dots forming the pattern element on a first printing medium, the second printing medium being more susceptible to bleed-through of the ink than the first printing medium. 
     A printing method includes a printing step of printing a test pattern on a printing medium, using a printing head including a nozzle configured to discharge ink, for inspecting a state of ink discharge by the nozzle. The test pattern includes a pattern element formed by a plurality of dots of the ink, and the printing step causes the printing head to print the test pattern where a number of the dots forming the pattern element on a second printing medium is smaller than a number of the dots forming the pattern element on a first printing medium, the second printing medium being more susceptible to bleed-through of the ink than the first printing medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating a device configuration in a simplified manner. 
         FIG.  2    is a diagram illustrating a specific example of a configuration including a transport unit and a printing head. 
         FIG.  3    is a view illustrating a relationship between the printing medium and the printing head, as seen from above. 
         FIG.  4    is a flowchart illustrating a flow from TP printing to an inspection of nozzles. 
         FIG.  5    is a diagram illustrating an example of TP image data. 
         FIG.  6    is a diagram illustrating an enlarged portion of a TP. 
         FIG.  7    is a diagram illustrating an example of a medium-specific dot count table. 
         FIG.  8    is a diagram illustrating an example of a dot count table. 
         FIG.  9    is a diagram illustrating TP image data according to a modified example. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the present disclosure will be described below with reference to the accompanying drawings. Note that each of the drawings is merely illustrative for describing the embodiment. Since the drawings are illustrative, proportions and shapes may not be precise, match each other, or some may be omitted. 
     1. APPARATUS CONFIGURATION 
       FIG.  1    illustrates a configuration of a printing apparatus  10  according to the embodiment, in a simplified manner. 
     The printing apparatus  10  is provided with a control unit  11 , a display unit  13 , an operation receiving unit  14 , a communication IF  15 , a transport unit  16 , a carriage  17 , a printing head  18 , a reading unit  19 , and the like. IF is an abbreviation for interface. The control unit  11  is configured to include, as a processor, one or more ICs including a CPU  11   a , a ROM  11   b , a RAM  11   c , and the like, another non-volatile memory, and the like. 
     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 lib, the other memory, or the like, using the RAM  11   c  or the like as a work area, to realize various functions such as a printing control unit  12   a , a reading control unit  12   b , an inspection unit  12   c , and the like. Note that the processor is not limited to the single CPU, and a configuration may be adopted in which the processing is performed by a hardware circuit such as a plurality of CPUs, an ASIC, or the like, or a configuration may be adopted in which the CPU and the hardware circuit work in concert to perform the processing. 
     The display unit  13  is a device for displaying visual information, and is configured, for example, by a liquid crystal display, an organic EL display, or the like. The display unit  13  may be configured to include a display and a drive circuit for driving the display. The operation receiving unit  14  is a device for receiving an operation by a user, and 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 be part of the configuration of the printing apparatus  10 , or 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 coupling the printing apparatus  10  with the outside in a wired or wireless manner, in accordance with a prescribed communication protocol including a known communication standard provide. 
     The transport unit  16  is a device for transporting the printing medium, and includes a roller, a motor for rotating the roller, and the like. The printing head  18  ejects ink from nozzles onto the printing medium, using an inkjet method, to perform printing. The reading unit  19  is a device for reading a printing result on the printing medium. The reading unit  19  is also referred to as a scanner. However, the printing apparatus  10  may have a configuration that does not include the reading unit  19 . 
     The carriage  17  is a mechanism capable of reciprocating along a predetermined direction as a result of receiving power from a carriage motor (not illustrated). The predetermined direction in which the carriage  17  moves is referred to as a main scanning direction. As illustrated in  FIG.  2    and  FIG.  3   , the printing head  18  is mounted on the carriage  17 . 
     The configuration of the printing apparatus  10  illustrated in  FIG.  1    may be realized by a single printer, or may be realized by a plurality of communicatively coupled devices. 
     In other words, the printing apparatus  10  may be the printing system  10  in actuality. The printing system  10  includes, for example, an information processing device that functions as the control unit  11 , and a printer including the transport unit  16 , the carriage  17 , the printing head  18 , and further, the reading unit  19 . A printing method according to the embodiment is realized in this way by the printing apparatus  10  or the printing system  10 . 
     Further, a portion of the control unit  11  that functions as the printing control unit  12   a  and a portion of the control unit  11  that functions as the reading control unit  12   b  and the inspection unit  12   c  may be separate information processing devices. 
       FIG.  2    illustrates a specific example of a configuration mainly including the transport unit  16  and the printing head  18 , which are part of the printing apparatus  10 . In  FIG.  2   , the specific example is illustrated using a perspective orthogonal to a transport direction D 2  of a printing medium  30 . 
     The transport unit  16  is provided with a feeding shaft  22  upstream in the transport direction, and a winding shaft  25  downstream in the transport direction. Upstream and downstream in the transport direction are simply denoted using upstream and downstream. The long printing medium  30  wound in a roll shape around the feeding shaft  22  and the winding shaft  25  is stretched along the transport direction D 2 . The printing medium  30  is transported in the transport direction D 2 . The printing medium  30  may be a paper sheet or may be a medium made from a material other than paper. 
     In the example illustrated in  FIG.  2   , the printing medium  30  wound around the feeding shaft  22  is fed downstream by the feeding shaft  22  rotating in the clockwise direction. A front driving roller  23  is provided at a position downstream of the feeding shaft  22 , and a rear driving roller  24  is provided at a position upstream of the winding shaft  25 . By rotating in the clockwise direction, the front driving roller  23  transports downstream the printing medium  30  fed out from the feeding unit  22 . A nip roller  23   n  is provided with respect to the front driving roller  23 . The nip roller  23   n  comes into contact with the printing medium  30  so as to clamp the printing medium  30  between the nip roller  23   n  and the front driving roller  23 . 
     By rotating in the clockwise direction, the rear driving roller  24  transports further downstream the printing medium  30  transported downstream by the front driving roller  23 . Note that a nip roller  24   n  is provided with respect to the rear driving roller  24 . The nip roller  24   n  comes into contact with the printing medium  30  so as to clamp the printing medium  30  between the nip roller  24   n  and the rear driving roller  24 . 
     The printing head  18  that discharges ink onto the printing medium  30  from above is disposed between the front driving roller  23  and the rear driving roller  24 . As illustrated in  FIG.  2   , the printing head  18  is mounted on the carriage  17 . The printing head  18  is capable of discharging a plurality of colors of ink, such as cyan (C), magenta (M), yellow (Y), black (K), light cyan (LC), and light magenta (LM), for example. 
     Each of the nozzles of the printing head  18  is open in a nozzle surface  20 , of the printing head  18 , facing the printing medium  30 , and the printing head  18  discharges or does not discharge the ink from the nozzles on the basis of print data. The ink discharged by the nozzle is also referred to as an ink droplet, or as a dot. The printing head  18  may also be referred to as a print head, an inkjet head, a liquid discharging head, a recording head, and the like. 
     As a result of the winding shaft  25  rotating in the clockwise direction, the winding shaft  25  takes up the printing medium  30  after printing that is transported by the rear driving roller  24 . 
     The feeding shaft  22 , the winding shaft  25 , each of the rollers, the motor (not illustrated) for rotating these members as appropriate, and the like are a specific example of the transport unit  16  that transports the printing medium  30 . A number and arrangement of the rollers provided along the transport path for transporting the printing medium  30  is not limited to the mode illustrated in  FIG.  2   . Further, the colors of the inks discharged by the printing head  18  are not limited to the colors described above. It goes without saying that a flat platen or the like, which supports, from below, the printing medium  30  that receives the ink discharge from the printing head  18 , may be provided between the front driving roller  23  and the rear driving roller  24 . Further, the portion of the printing medium  30  on which the printing by the printing head  18  has been performed need not necessarily be wound into the roll shape by the winding shaft  25 , and may be cut away from the printing medium  30  that is upstream of the printed portion, using a cutter (not illustrated). 
     In the example illustrated in  FIG.  2   , the reading unit  19  is provided at a position downstream of the carriage  17  and the printing head  18  and upstream of the rear driving roller  24 . Using an image sensor, the reading unit  19  optically reads the printing medium  30  on which the printing has been performed by the printing head  18 , and outputs image data as a reading result. The reading unit  19  may be configured to read the printing medium  30  while being moved by the carriage in a similar manner to the printing head  18 , or may be configured to read the printing medium  30  in a stationary state. 
       FIG.  3    illustrates a relationship between the printing medium  30  and the printing head  18  in a simplified manner, as seen from above. The printing head  18  mounted on the carriage  17  moves, together with the carriage  17 , from one end of a main scanning direction D 1  to the other end (a forward movement) and from the other end to the one end (a return movement). The main scanning direction D 1  and the transport direction D 2  intersect each other. The intersection may be understood to be orthogonal. Accordingly,  FIG.  2    illustrates the printing head  18  and the like from a perspective facing in the main scanning direction D 1 . However, due to various errors in the printer as a manufactured product, for example, the main scanning direction D 1  and the transport direction D 2  may not be precisely orthogonal. 
     In  FIG.  3   , an example is illustrated of an array of nozzles  21  in the nozzle surface  20 . Each of small circles in the nozzle surface  20  is the nozzle  21 . The printing head  18  is provided with a plurality of nozzle rows  26  in a configuration in which each color of the inks is discharged from the nozzles  21  after being supplied from a liquid holding unit (not illustrated) that is referred to as an ink cartridge, an ink tank, or the like. The nozzle row  26  including the nozzles  21  that discharge the C ink is also described as a nozzle row  26 C. Similarly, the nozzle row  26  including the nozzles  21  that discharge the M ink is also described as a nozzle row  26 M, the nozzle row  26  including the nozzles  21  that discharge the Y ink is also described as a nozzle row  26 Y, the nozzle row  26  including the nozzles  21  that discharge the K ink is also described as a nozzle row  26 K, the nozzle row  26  including the nozzles  21  that discharge the LC ink is also described as a nozzle row  26 LC, and the nozzle row  26  including the nozzles  21  that discharge the LM ink is also described as a nozzle row  26 LM. The nozzle rows  26 C,  26 M,  26 Y,  26 K,  26 LC, and  26 LM are aligned along the main scanning direction D 1 . 
     Each of the nozzle rows  26  is configured by the plurality of nozzles  21  for which a nozzle pitch, which is an interval between the nozzles  21  in the transport direction D 2 , is constant or substantially constant. The direction in which the plurality of nozzles  21  configuring the nozzle row  26  are aligned is referred to as a nozzle row direction D 3 . In the example illustrated in  FIG.  3   , the nozzle row direction D 3  is parallel with the transport direction D 2 . In the 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. However, the nozzle row direction D 3  need not necessarily be parallel with the transport direction D 2 , and a configuration may be adopted in which the nozzle row direction D 3  obliquely intersects the main scanning direction D 1 . 
     The respective positions of the nozzle rows  26 C,  26 M,  26 Y,  26 K,  26 LC, and  26 LM in the transport direction D 2  match each other. The printing apparatus  10  prints an image on the printing medium  30  by performing a combination of transport of the printing medium  30  in the transport direction D 2 , and ink discharge by the printing head  18  in accordance with movement of the carriage  17  along the main scanning direction D 1 . The operation of the ink discharge by the printing head  18  in accordance with the forward movement and the return movement of the carriage  17  is referred to as a “scan” or a “pass”. The movement of the printing head  18  in the main scanning direction D 1  by the carriage  17  corresponds to one of relative movements between the printing head  18  and the printing medium  30 . 
     2. TEST PATTERN PRINTING 
       FIG.  4    illustrates, using a flowchart, a flow executed by the control unit  11  in accordance with the program  12 , from printing of a TP to an inspection of the nozzles  21  on the basis of the TP. TP is an abbreviation for test pattern. The flowchart consists, in overview, of TP printing processing (step S 100 ), acquisition of a reading result of the printed TP (step S 200 ), and an inspection based on the reading result of the TP (step S 300 ). Step S 100  corresponds to a TP printing step. In  FIG.  4   , step S 100  is illustrated in detail while being divided into steps S 110  to S 150 . 
     At step S 110 , the printing control unit  12   a  acquires TP image data, which is image data representing the TP, from a storage source such as a predetermined memory or storage device with which the control unit  11  can communicate. The TP image data is, for example, image data in a bitmap format defining the color of each of pixels in a predetermined color system. As the color system referred to here, for example, there are various color systems, such as an RGB (red, green, blue) color system, a CMYK color system, or the like. 
     At step S 120 , the printing control unit  12   a  sets TP printing conditions. The printing control unit  12   a  sets the printing conditions when performing normal printing as the TP printing conditions. The normal printing refers to a process in which an object such as a photo, text, CG, or the like chosen by the user is printed, rather than the TP. The user can set the printing conditions for the normal printing by operating the operation receiving unit  14  while viewing a user interface (UI) screen displayed on the display unit  13 . The printing conditions include, for example, the type of the printing medium  30  and a printing quality. 
     There are various types of the printing medium  30  such as, for example, coated paper to which a glossy coating material is applied and having a high degree of glossiness, matte paper having a lower degree of glossiness than that of the coated paper, plain paper, and the like. The coated paper is also referred to as gloss coated paper, or as glossy paper. The user sets the desired type of the printing medium  30  in the transport unit  16 , and sets the type of the set printing medium  30  as an item of the printing conditions. In the embodiment, when the type of the printing medium  30  is referred to as a “first printing medium”, the printing medium  30  that is more susceptible to ink bleed-through than the first printing medium, is referred to as a “second printing medium”. In other words, in the embodiment, the difference in the type of the printing medium  30  is captured as a difference in the susceptibility of the printing medium  30  to the bleed-through of the ink. As an example, when the coated paper is the first printing medium, that matte paper corresponds to the second printing medium. Note that the printing control unit  12   a  may set the type of the printing medium  30  as the item of the printing conditions by automatically determining the type of the printing medium  30  on the basis of data acquired by sensing the printing medium  30  set on the printing apparatus  10 , using a sensor or a scanner. 
     The print quality is presented to the user as subjective options, such as high resolution, normal, fast, and the like, and the printing control unit  12   a  sets each of items necessary for execution of the printing in accordance with the selected printing quality, such as a movement velocity of the carriage  17 , a transport velocity of the transport unit  16 , a waveform of drive signals used to drive the nozzles  21 , and a driving period of the nozzles  21  in the pass. Further, if a default setting is provided for the printing conditions, and the user does not specifically change the default setting, the printing control unit  12   a  applies the default setting to the TP printing or the normal printing. 
     The order of execution of steps S 110  and S 120  may be reversed from that illustrated in  FIG.  4   , or may be substantially simultaneous. 
     At step S 130 , the printing control unit  12   a  generates the print data for the TP printing, from the TP image data. The printing control unit  12   a  generates the print data that prescribes ink discharge (dot on) or ink non-discharge (dot off) for each of the pixels and each of the ink colors, by performing predetermined image processing, such as color conversion processing and halftone processing, on the TP image data. As illustrated in  FIG.  3   , assuming that the printing head  18  uses the six colors of ink of CMYKLCLM, at step S 130 , the printing control unit  12   a  generates the print data prescribing the dot on and off for each of the pixels and for each of CMYKLCLM, based on the TP image data. 
       FIG.  5    illustrates an example of TP image data  40  acquired at step S 110 . The TP image data  40  is image data representing a TP  41 . In  FIG.  5   , and  FIG.  6    to be described below, a correspondence relationship between the TP image data  40  and the directions D 1  and D 2  is also illustrated. The TP  41  includes a TP for each of the ink colors. According to  FIG.  5   , a TP  41 C is the TP represented by the color C. Similarly, a TP  41 LC is the TP of the LC color, a TP  41 M is the TP of the M color, a TP  41 LM is the TP of the LM color, a TP  41 Y is the TP of the Y color, and a TP  41 K is the TP of the K color. 
     In the TP image data  40 , the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K for each of the ink colors are aligned in the main scanning direction D 1 , and the positions thereof in the transport direction D 2  are the same as each other. Each of the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K for each of the ink colors is a collection of a plurality of “pattern elements”. In the example illustrated in  FIG.  5   , each of the pattern elements is a ruled line parallel to the main scanning direction D 1 . Each one of the pattern elements is an image printed using one of the nozzles  21  of the corresponding ink color. 
       FIG.  6    illustrates an enlarged portion of the TP  41  represented by the TP image data  40 . Specifically,  FIG.  6    illustrates portions of the TP  41 C and the TP  41 LC, respectively. The TP  41 C is configured by a plurality of pattern elements  42 C arranged at equal intervals in the transport direction D 2 , and the TP  41 LC is configured by a plurality of pattern elements  42 LC arranged at equal intervals in the transport direction D 2 . In  FIG.  6   , for ease of understanding, as well as the TPs  41 C and  41 LC, a portion of each of the nozzle rows  26 C and  26 LC used for printing the TPs  41 C and  41 LC are also illustrated. In other words, each of the pattern elements  42 C is disposed at a spacing similar to the nozzle pitch in the transport direction D 2 , such that each one of the pattern elements  42 C configuring the TP  41 C is printed by one of the nozzles  21  configuring the nozzle row  26 C. Similarly, each of the pattern elements  42 LC is arranged at a spacing similar to the nozzle pitch in the transport direction D 2 , such that each one of the pattern elements  42 LC configuring the TP  41 LC is printed by one of the nozzles  21  configuring the nozzle row  26 LC. 
     Further, in the example illustrated in  FIG.  6   , in order to easily verify each one of these pattern elements  42 C at the time of inspection, each of the pattern elements  42 C is disposed with a position thereof being offset in the main scanning direction D 1 , such that the positions thereof in the main scanning direction D 1  coincide every three cycles. Similarly, each of the pattern elements  42 LC is also disposed with a position thereof being offset in the main scanning direction D 1 , such that the positions thereof in the main scanning direction D 1  coincide every three cycles. However, the pattern elements configuring the TP corresponding to one of the ink colors may all have the same position in the main scanning direction D 1 . 
     Further, in the example illustrated in  FIG.  6   , the pattern elements  42 C and the pattern elements  42 LC are disposed with the positions thereof offset in the main scanning direction D 1  in order to reduce the bleed-through of each one of the pattern elements  42 C and the pattern elements  42 LC. For example, when the pattern element  42 C is printed in two passes, the dots printed in a first pass are disposed at odd-numbered pixel positions in the main scanning direction D 1 , and the dots printed in a second pass are disposed at even-numbered pixel positions in the main scanning direction D 1 . Further, for example, when the pattern elements  42 LC are printed in two passes, the dots printed in the first pass are disposed at the odd-numbered pixel positions in the main scanning direction D 1 , and the dots printed in the second pass are disposed at the even-numbered pixel positions in the main scanning direction D 1 . However, when printing on the printing medium  30  in which bleed-through is not likely to occur, the dots may be formed in all of the pixels in each pass. 
     The print data generated at step S 130  is image data in which the TP  41  represented by the TP image data  40  is expressed using the dot on and off. Each of the pattern elements configuring each of the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K for each of the ink colors is formed of dots of the corresponding ink color only. 
     At step S 140 , the printing control unit  12   a  determines, depending on the type of the printing medium  30 , a number of passes and a thinning ratio when printing the TP. The type of the printing medium  30  referred to here is the type of the printing medium  30  set as the item of the printing conditions at step S 120 . The printing control unit  12   a  determines a common number of passes and thinning ratio for the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K. 
       FIG.  7    illustrates an example of a medium-specific dot count table  50 . The medium-specific dot number table  50  is stored in advance in a memory or a storage device, either in or outside the printing apparatus  10 , so as to be accessible by the control unit  11 . The medium-specific dot count table  50  is a table defining parameters used to determine, directly or indirectly, the number of dots for printing the TP depending on the type of the printing medium  30 . According to  FIG.  7   , the medium-specific dot count table  50  prescribes the number of passes and the thinning ratio for each of the coated paper and the matte paper. It goes without saying that the medium-specific dot count table  50  may be a table prescribing the number of passes and the thinning ratio for other types of the printing medium in addition to the coated paper and the matte paper. At step S 140 , the printing control unit  12   a  refers to the medium-specific dot count table  50 , to determine the number of passes and the thinning ratio depending on the type of the printing medium  30  set at step S 120 . 
     The number of passes is the number of the passes used to print the TP. For example, if the number of passes is 2 for a given type of the printing medium  30 , this does not mean that the TP  41  represented by the print data generated at step S 130  is printed in two passes, but that the pass to print the TP  41  represented by the print data is repeated twice. Thus, the greater the number of passes, the greater the number of dots forming the pattern elements in the TP  41  reproduced on the printing medium  30 . According to the medium-specific dot count table  50 , the printing control unit  12   a  determines that the number of passes is 2 when the type of the printing medium  30  is the coated paper, and determines that the number of passes is 1 when the type of the printing medium  30  is the matte paper. 
     The thinning ratio is the thinning ratio in a single pass. For example, when the thinning ratio relating to the type of the printing medium  30  is 50%, in one pass, during a period for printing the TP  41  represented by the print data generated at step S 130 , regardless of whether the original dot is dot on or dot off, for 50% of the pixels, dot off is forcibly applied and the ink is not discharged. Therefore, the higher the thinning ratio, the fewer the number of dots forming the pattern elements in the TP  41  reproduced on the printing medium  30 . Further, it can be said that the higher the thinning ratio, the more a discharge rate of the ink by the nozzle  21  is reduced, and the lower the thinning ratio, the more the discharge rate of the ink by the nozzle  21  is increased. Thus, by changing the thinning ratio depending on the type of the printing medium  30 , the discharge rate of the nozzles  21  can be controlled for each type of the printing medium  30 . According to the medium-specific dot count table  50 , the printing control unit  12   a  determines that the thinning ratio is 50% when the type of the printing medium  30  is the coated paper, and determines that the thinning ratio is 66% when the type of the printing medium  30  is the matte paper. When the thinning ratio is 0%, this means that the printing is performed as per the print data for each of the passes. 
     Here, in the print data generated at step S 130 , it is assumed that all of the individual pattern elements that configure any of the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K for each of the ink colors is configured by approximately the same number of dots. According to step S 140  at which the medium-specific dot count table  50  is referred to, when the type of the printing medium  30  is the coated paper that is the first printing medium, the number of passes is determined to be 2 and the thinning ratio is determined to be 50%. When the type of the printing medium  30  is the matte paper that is the second printing medium, the number of passes is determined to be 1 and the thinning ratio is determined to be 66%. As a result, when comparing the number of dots of the pattern elements printed on the matte paper with the number of dots of the pattern elements printed on the coated paper based on the determination at step S 140 , the number of dots of the C ink forming the one pattern element  42 C printed on the matte paper, for example, is approximately one third the number of dots of the C ink forming the one pattern element  42 C printed on the coated paper. 
     At step S 150 , the printing control unit  12   a  prints the TP  41  on the printing medium  30  by controlling the movement of the carriage  17  and the ink discharge by the printing head  18 , in accordance with the printing conditions set at step S 120 , the print data generated at step S 130 , and the number of passes and the thinning ratio determined at step S 140 . 
     Specifically, when the type of the printing medium  30  is the coated paper, based on the medium-specific dot count table  50 , the printing head  18  performs two passes to print the TP  41  on the printing medium  30 . In each of the two passes, the printing head  18  discharges the CMYKLCLM inks from the respective nozzles  21  of the nozzle rows  26 C,  26 LC,  26 M,  26 LM,  26 Y, and  26 K to print the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K on the basis of the print data and at the thinning ratio of 50%. 
     On the other hand, when the type of the printing medium  30  is the matte paper, the printing head  18  performs one pass to print the TP  41  on the printing medium  30 . The printing head  18  discharges the CMYKLCLM inks from the respective nozzles  21  of the nozzle rows  26 C,  26 LC,  26 M,  26 LM,  26 Y, and  26 K to print the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K on the basis of the print data and at the thinning ratio of 66%. 
     As a result, when performing a comparison at a pattern element level, the printing control unit  12   a  has printed the TP  41  such that the number of dots forming the pattern element on the second printing medium is less than the number of dots forming the pattern element on the first printing medium. Note that the printing control unit  12   a  does not cause the transport unit  16  to transport the printing medium  30  during a period of time from a first pass for the printing head  18  to print the TP  41  to the end of a last pass. 
     The above is a description of step S 100 . Steps S 200  and S 300  will be briefly described. 
     At step S 200 , the reading control unit  12   b  controls the reading unit  19  to read the printing medium  30  on which the TP  41  has been printed at step S 100 , and retrieves the image data from the reading unit  19  as the reading result. It goes without saying that the transport unit  16  performs the transport necessary for the reading unit  19  to read the printing medium  30  after the printing. 
     However, at step S 200 , it is sufficient that the reading result of the printing medium  30  on which the TP  41  has been printed can be acquired. Thus, the user may cause an external scanner to read the printing medium  30  on which the TP  41  has been printed, and the printing apparatus  10  may acquire the reading result via the communication IF  15 . 
     At step S 300 , the inspection unit  12   c  inspects a state of the ink discharge by the nozzles  21  of the printing head  18 , based on the image data acquired as the reading result at step S 200 . The state of the ink discharge is divided into normal and abnormal. Abnormal applies to a discharge failure in which the dot cannot be discharged, landing position displacement in which the landing positions of the dots are displaced from ideal landing positions, and the like. The inspection unit  12   c  inspects whether each of the nozzles  21  is normal or abnormal by analyzing the image data and identifying a density and position of each of the pattern elements for each of the ink colors and for each of the nozzles  21 , and stores inspection results as data. 
     The flowchart illustrated in  FIG.  4    ends here. 
     3. SUMMARY AND DESCRIPTION OF EFFECTS 
     As described above, according to the embodiment, the printing apparatus  10  includes the printing head  18  including the nozzles  21  that discharge the ink, and the control unit  11  that, by controlling the printing head  18 , causes the TP to be printed on the printing medium  30  for the inspection of the state of the ink discharge by the nozzles  21 . The TP includes the pattern elements formed of the plurality of dots of the ink. Then, the control unit  11  reduces the number of dots forming the pattern element in the TP printed by the printing head  18  on the second recording medium for which the bleed-through of the ink is likely to occur in comparison to the number of dots forming the pattern element in the TP printed by the printing head  18  on the first printing medium. In other words, the control unit  11  causes the printing head  18  to print the TP so as to reduce the number of dots forming the pattern element on the second printing medium in comparison to the number of dots forming the pattern element on the first printing medium. 
     According to the configuration, when printing the TP on the second printing medium, the pattern element is printed using fewer dots than when the TP is printed on the first printing medium. In this way, as a printing result of the TP on the second printing medium, the TP can be provided including each of the pattern elements in which the bleed-through is suppressed. As a result, when printing the TP on the printing medium  30  for which the ink bleed-through is likely to occur, a problem is avoided that is caused by a shape of the pattern element becoming uneven due to the ink bleed-through, and the inspection on the reading result cannot be appropriately performed. Specifically, with the pattern elements whose shape has become uneven due to the ink bleed-through, since it is difficult to accurately identify a position and the like when performing the inspection based on the reading result, the above-described determination as to whether the ink discharge is normal or abnormal cannot be performed with a high degree of accuracy. However, by performing the printing of the TP at step S 100  of the embodiment, whichever of the printing medium  30  is used, the inspection of the nozzles  21  using the pattern elements can be performed with a high degree of accuracy. 
     Further, according to the embodiment, the printing head  18  is capable of performing the scan that discharges the ink from the nozzles  21  as the printing head  18  moves in the predetermined direction, and the control unit  11  reduces the number of scans for printing the pattern element on the second printing medium in comparison to the number of scans for printing the pattern element on the first printing medium. In other words, the control unit  11  causes the printing head  18  to print the TP with a reduced number of scans for printing the pattern element on the second printing medium, in comparison to the number of scans for printing the pattern element on the first printing medium. 
     According to the configuration, by causing the number of scans for printing the pattern elements on the second printing medium to be fewer than the number of scans for printing the pattern elements on the first printing medium, the control unit  11  can easily print the TP in which the number of dots forming the pattern elements on the second printing medium are fewer than the number of dots forming the pattern elements on the first printing medium. 
     Further, according to the embodiment, the control unit  11  may reduce the discharge rate of the ink by the nozzles  21  for printing the pattern elements on the second printing medium in comparison to the discharge rate of the ink by the nozzles  21  for printing the pattern elements on the first printing medium. In other words, the control unit  11  causes the printing head  18  to print the TP while lowering the ink discharge rate of the ink by the nozzles  21  for printing the pattern elements on the second printing medium in comparison to the discharge rate of the ink by the nozzles  21  for printing the pattern elements on the first printing medium. 
     According to the configuration, by causing the discharge rate of the ink by the nozzles  21  for printing the pattern elements on the second printing medium to be lower than the discharge rate of the ink by the nozzles  21  for printing the pattern elements on the first printing medium, the control unit  11  can easily print the TP in which the number of dots forming the pattern elements on the second printing medium are fewer than the number of dots forming the pattern elements on the first printing medium. 
     The number of passes and numerical values of the thinning ratio for each of the types of the printing medium  30  in the medium-specific dot number table  50  illustrated in  FIG.  7    are merely examples. Further, the medium-specific dot count table  50  may be, for example, a table in which the number of passes is the same regardless of the type of the printing medium  30 , and the difference between the first printing medium and the second printing medium is provided in terms of the thinning ratio. Alternatively, the medium-specific dot count table  50  may be a table in which the thinning ratio is the same regardless of the type of the printing medium  30 , and the difference between the first printing medium and the second printing medium is provided in terms of the number of passes. 
     Further, according to the embodiment, the printing conditions when printing the TP are the same as the printing conditions when performing the normal printing. 
     In other words, the control unit  11  sets a velocity of a relative movement between the printing head  18  and the printing medium  30  when printing the TP to be the same as the velocity of the relative movement when performing the normal printing. According to the above description, the velocity of the relative movement referred to here is the movement velocity of the carriage  17  when performing the pass. 
     Further, the control unit  11  sets a waveform of a drive signal used to drive the nozzle  21  when printing the TP to be the same as the waveform of the drive signal used to drive the nozzle  21  when performing the normal printing. The drive signal used to drive the nozzle  21  is a pulse wave, and the drive signal is applied to a driving element of each of the nozzles  21  in accordance with the dot on information, thus causing the dot to be discharged from the nozzle  21 . If the waveform of the drive signal is different, a size of the dot discharged by the nozzle  21  in a single drive is also different. 
     In this way, by setting the velocity of the relative movement and the drive signal to be the same for the printing of the TP and for the normal printing, the TP suitable for performing the inspection of the nozzle  21  can be printed under the same conditions as when performing the normal printing. 
     The embodiment also discloses an invention of each of categories, such as a method other than the printing apparatus  10  and the printing system  10 , and the program  12 . 
     The printing method includes the printing step of printing the TP on the printing medium  30 , using the printing head  18  including the nozzles  21  that discharge the ink, for inspecting the state of the ink discharge by the nozzles  21 . The TP includes the pattern elements formed by the plurality of dots of the ink, and the printing step causes the printing head  18  to print the TP where the number of the dots forming the pattern element on the second printing medium is smaller than the number of the dots forming the pattern element on the first printing medium. 
     4. OTHER EMBODIMENTS 
     The embodiment is not limited to the modes described above. 
     Specific examples of the first printing medium and the second printing medium are not limited to the types described above. For example, when the matte paper is assumed to be the first printing medium, the plain paper may be the second printing medium. Further, for the coated paper, the matte paper, the plain paper, and other printing media, the printing apparatus  10  may print the TP such that the number of dots per pattern element is reduced the more the printing medium used is susceptible to the bleed-through of the ink. 
     The printing apparatus  10  need not necessarily be a so-called serial inkjet printer in which the printing head  18  is mounted on the carriage  17  that moves in the main scanning direction D 1 , as described above. 
     A so-called line type inkjet printer for discharging the ink may be assumed, using the printing head  18  including the nozzle rows  26  for each of the ink colors, where the nozzle rows  26  extend in the main scanning direction D 1  intersecting the transport direction D 2  and are long enough to cover the width of the printing medium  30 . In the line type inkjet printer, the nozzle row direction D 3  may be understood to be parallel with the main scanning direction D 1  rather than with the transport direction D 2 . 
     When describing the embodiment assuming that the printing apparatus  10  is the line type inkjet printer, the TP  41  is printed on the printing medium  30  such that the TP  41  has an orientation in which each of the pattern elements, which is the ruled line, is parallel with the transport direction D 2  rather than with the main scanning direction D 1 . Further, the plurality of passes of the printing head  18  described above are achieved using back feed by the transport unit  16 . The back feed is processing in which the transport unit  16  transports the printing medium  30  from downstream to upstream. In other words, when the printing medium  30  passes under the printing head  18  in the process of transporting the printing medium  30  from upstream to downstream, printing is performed once on the printing medium  30 . Thereafter, the transport unit  16  returns the portion of the printing medium  30  that has once been printed, back to a position upstream of the printing head  18 , and once more starts transporting the printing medium  30  downstream. By repeating this, the TP  41  can be repeatedly printed in the same manner as the serial inkjet printer prints the TP  41  in the superimposed manner by the plurality of passes. 
     When the printing apparatus  10  is the line type inkjet printer, the transport of the printing medium  30  by the transport unit  16  during the printing period by the printing head  18  corresponds to the relative movement between the printing head  18  and the printing medium  30 . In other words, when the printing apparatus  10  is the line type inkjet printer, the transport velocity of the transport unit  16  during the printing period by the printing head  18  is the same when printing the TP and when performing the normal printing. 
     It goes without saying that the printing medium  30  need not necessarily be the continuous sheet wound into the roll, as exemplified in  FIG.  2   , or the like. The printing medium  30  may be a single sheet cut into page units, or the like. 
     5. MODIFIED EXAMPLES 
     In addition to the type of printing medium  30  described above, the printing apparatus  10  may print the TP by varying the number of dots configuring the pattern element in accordance with a difference in ink color or a difference in dot size. 
     The TP includes a “first ink pattern element” formed by a plurality of dots of a “first ink” and a “second ink pattern element” formed by a plurality of dots of a “second ink” that has a higher degree of brightness than the first ink. For example, assuming that the six color inks of CMYKLCLM are used, the CMYK inks corresponds to the first ink, and the LC and LM inks correspond to the second ink. In  FIG.  5   , the TPs  41 C,  41 M,  41 Y, and  41 K are the TPs printed using the first ink, and the TPs  41 LC and  41 LM are the TPs printed using the second ink. Further, in  FIG.  6   , the pattern element  42 C corresponds to an example of the first ink pattern element, and the pattern element  42 LC corresponds to an example of the second ink pattern element. However, the Y ink that has a higher degree of brightness than the CMK inks may be treated as the second ink rather than the first ink. In such a case, the control unit  11  may cause the printing head  18  to print the TP in which the number of dots of the second ink forming the second ink pattern element is higher than the number of dots of the first ink forming the first ink pattern element. 
     Further, each of the plurality of nozzles  21  may be capable of discharging the dots of a plurality of sizes in which a volume per droplet is different. In the following description, the dot having a given size is referred to as a “first size” dot, and the dot having a size smaller than the first size is referred to as a “second size” dot. For convenience, the first size dot is referred to as a “large dot”, and the second size dot is referred to as a “small dot”. The size of each of the large dot and the small dot is set in terms of design considerations. In such a case, the TP includes a “first size pattern element” formed by a plurality of the dots of the first size and a “second size pattern element” formed by a plurality of the dots of the second size. The control unit  11  may cause the printing head  18  to print the TP in which the number of dots of the second size forming the second size pattern element is higher than the number of dots of the first size forming the first size pattern element. The “large dot,” which is the dot of the first size, may be formed by discharging a plurality of the “small dots,” which are the dots of the second size. 
       FIG.  8    illustrates dot count tables  51  and  52 , which are referred to by the printing control unit  12   a  at step S 140  in the modified example. The dot count tables  51  and  52  are tables defining parameters for directly or indirectly determining the number of dots for printing the TP, and are stored in advance in a memory or a storage device, either in or outside the printing apparatus  10 , so as to be accessible by the control unit  11 . The dot count table  51  prescribes the number of passes for each type of the printing medium  30 , and for each of the ink colors and for each of the dot sizes. On the other hand, the dot count table  52  prescribes the thinning ratio for each type of the printing medium  30 , and for each of the ink colors and for each of the dot sizes. 
       FIG.  9    illustrates the TP image data  40  acquired at step S 110  in the modified example. The TP image data  40  is the image data representing the TP  41 . Of the TP  41 , a TP  41 C 1  printed using the C ink, a TP  41 LC 1  printed using the LC ink, a TP  41 M 1  printed using the M ink, a TP  41 LM 1  printed using the LM ink, a TP  41 Y 1  printed using the Y ink, and a TP  41 K 1  printed using the K ink are printed with the large dots of the respectively corresponding colors, and may be understood to be the same as the TPs  41 C,  41 LC,  41 M,  41 LM,  41 Y, and  41 K illustrated in  FIG.  5   . On the other hand, a TP  41 C 2  printed using the C ink, a TP  41 LC 2  printed using the LC ink, a TP  41 M 2  printed using the M ink, a TP  41 LM 2  printed using the LM ink, a TP  41 Y 2  printed using the Y ink, and a TP  41 K 2  printed using the K ink are printed with the small dots of the respectively corresponding colors, In other words, in the modified example, the TP for each of the ink colors and for each of the dot sizes is printed on the printing medium  30  of the type set by the user. 
     In  FIG.  9   , each of the pattern elements configuring the TPs  41 C 1 ,  41 M 1 ,  41 Y 1 , and  41 K 1  corresponds to the first ink pattern element and corresponds to the first size pattern element. Each of the pattern elements configuring the TPs  41 C 2 ,  41 M 2 ,  41 Y 2 , and  41 K 2  corresponds to the second ink pattern element and corresponds to the second size pattern element. 
     Each of the pattern elements configuring the TPs  41 LC 1  and  41 LM 1  corresponds to the second ink pattern element and corresponds to the first size pattern element. 
     Each of the pattern elements configuring the TPs  41 LC 2  and  41 LM 2  corresponds to the second ink pattern element and corresponds to the second size pattern element. 
     At step S 140 , in the modified example, by referring to the dot count tables  51  and  52 , the printing control unit  12   a  determines the number of passes and the thinning ratio for printing the TP for each of the ink colors and each of the dot sizes depending on the type of the printing medium  30  set at step S 120 . 
     For example, it is assumed that the type of the printing medium  30  is the coated paper. At this time, according to the dot count tables  51  and  52 , for the K ink and the large dots, that is, for the printing of the TP  41 K 1 , the printing control unit  12   a  determines the number of passes to be 2 and the thinning ratio to be 50%. For the K ink and the small dots, that is, for the printing of the TP  41 K 2 , the printing control unit  12   a  determines the number of passes to be 6, and the thinning ratio to be 0%. Similarly, for the LC ink and the large dots, that is, for the printing of the TP  41 LC 1 , the printing control unit  12   a  determines the number of passes to be 4 and the thinning ratio to be 0%, for example. For the LC ink and the small dots, that is, for the printing of the TP  41 LC 2 , the printing control unit  12   a  determines the number of passes to be 10 and the thinning ratio to be 0%. 
     For example, it is assumed that the type of the printing medium  30  is the matte paper. At this time, according to the dot count tables  51  and  52 , for the K ink and the large dots, that is, for the printing of the TP  41 K 1 , the printing control unit  12   a  determines the number of passes to be 1 and the thinning ratio to be 66%. For the K ink and the small dots, that is, for the printing of the TP  41 K 2 , the printing control unit  12   a  determines the number of passes to be 4 and the thinning ratio to be 50%. Similarly, for the LC ink and the large dots, that is, for the printing of the TP  41 LC 1 , the printing control unit  12   a  determines the number of passes to be 2 and the thinning ratio to be 50%, for example. For the LC ink and the small dots, that is, for the printing of the TP  41 LC 2 . the printing control unit  12   a  determines the number of passes to be 8 and the thinning ratio to be 50%. 
     According to the dot count tables  51  and  52  described above, when the TP  41  is printed on the coated paper and the TP  41  is printed on the matte paper, under the same ink color and dot size conditions, the number of dots forming the pattern elements of the TP is fewer when using the matte paper. This is the same as the embodiment described with reference to  FIG.  7   . In addition, according to the dot count tables  51  and  52 , for the TP printed using the first ink, such as CMYK, and the TP printed using the second ink, such as LCLM, under the same conditions of the type of the printing medium  30  and the dot size, the number of dots forming the pattern elements of the TP is fewer when using the second ink. Furthermore, according to the dot count tables  51  and  52 , for the TP printed using the first size dots (the large dots), and the TP printed using the second size dots (the small dots), under the same conditions of the type of the printing medium  30  and the ink color, the number of dots forming the pattern elements of the TP is fewer when using the second size. 
     In this way, according to the modified example, the control unit  11  causes the number of dots forming the pattern element in the TP, which is printed by causing the printing head  18  to perform the printing on the second printing medium, to be fewer than the number of dots forming the pattern element in the TP, which is printed by causing the printing head  18  to perform the printing on the first printing medium, and causes the number of dots of the second ink forming the second ink pattern element to be greater than the number of dots of the first ink forming the first ink pattern element. 
     According to this configuration, the second ink pattern element printed using the ink of a color having a high degree of brightness can also be printed with a certain degree of density. As a result, even when a difference in brightness is small between the printing medium  30  and a portion, of the TP for inspecting each of the nozzles  21 , printed using some of the inks having the high degree of brightness, a failure resulting from not being able to appropriately perform the inspection based on the reading result of the reading can be eliminated. Specifically, for the pattern element having the small difference in brightness with the printing medium  30  that is white or of a color having a high degree of brightness, it is difficult to accurately identify the position and the like of the pattern element at the time of the inspection based on the reading result. Thus, the determination relating to normal or abnormal as described above cannot be performed with a high degree of accuracy. However, using the TP printed at step S 100  of the modified example, the nozzle  21  for any of the ink colors can be inspected with a high degree of accuracy based on the pattern element. 
     Further, according to the modified example, the control unit  11  causes the number of dots forming the pattern element in the TP printed by causing the printing head  18  to perform the printing on the second printing medium to be fewer than the number of dots forming the pattern element in the TP printed by causing the printing head  18  to perform the printing on the first printing medium, and causes the number of dots of the second size forming the second size pattern element to be greater than the number of dots of the first size forming the first size pattern element. 
     According to this configuration, the second size pattern element printed using the small dot size can also be printed with a certain degree of density. In this way, even when a difference in brightness is small between the printing medium  30  and a portion, of the TP for inspecting each of the nozzles  21 , printed using the relatively small dots, the failure resulting from not being able to appropriately perform the inspection based on the reading result of the reading can be eliminated. Specifically, for the pattern element having a small difference in brightness with the printing medium  30  that is white or of a color having a high degree of brightness, it is difficult to accurately identify the position and the like of the pattern element at the time of the inspection based on the reading result. Thus, the determination relating to normal or abnormal as described above cannot be performed with a high degree of accuracy. However, using the TP printed at step S 100  of the modified example, the inspection can be performed with a high degree of accuracy based on each of the pattern elements resulting from the discharge of the dots of differing sizes by the nozzles  21 . 
     Furthermore, according to the modified example, when causing the printing head  18  to perform the printing of the TP on the printing medium  30 , the control unit  11  causes the number of dots of the second ink forming the second ink pattern element to be greater than the number of dots of the first ink forming the first ink pattern element, and causes the number of dots of the second size forming the second size pattern element to be greater than the number of dots of the first size forming the first size pattern element.