JP2018138357A - Ink jet printer - Google Patents

Abstract

An inkjet printing apparatus capable of reducing a decrease in print image quality is provided. A test pattern print control unit 52 performs a test with an inkjet head 31 of a head unit 20 at which a sheet arrives at each timing when the leading end of the sheet reaches a print conveyance roller pair other than the most upstream print conveyance roller pair. In addition to printing the pattern image, the test pattern image is printed by the inkjet head 31 of the head unit 20 where the paper has not been removed at each timing when the trailing edge of the paper reaches the print conveyance roller pair other than the most downstream print conveyance roller pair. Control as follows. The print control unit 55 controls the drive frequency of the inkjet heads 31A and 31B based on the drive frequency of the inkjet heads 31A and 31B corresponding to each section calculated based on the length of each section divided by the test pattern image. To do. [Selection] Figure 4

Description

  The present invention relates to an inkjet printing apparatus that prints by ejecting ink from an inkjet head.

  2. Description of the Related Art A line-type inkjet printing apparatus is known that prints an image by ejecting ink from an inkjet head onto a sheet conveyed by a pair of conveyance rollers (see, for example, Patent Document 1).

  Some line-type ink jet printing apparatuses have a plurality of ink jet heads arranged at different positions in the sub-scanning direction (paper transport direction). For example, in order to enlarge the printable range in the main scanning direction, a plurality of ink jet heads are arranged in a staggered pattern shifted in the main scanning direction and the sub-scanning direction. In addition, for example, there are some in which a plurality of inkjet heads that eject inks of different colors are arranged in parallel in the sub-scanning direction for color printing.

  Here, in the ink jet printing apparatus that prints on the paper transported by the transport roller pair with the ink jet head as described above, it is necessary to reduce the distance between the transport roller pair so as not to decrease the transport accuracy of the paper. For this reason, in the case of a configuration having a plurality of ink jet heads arranged at different positions in the sub-scanning direction, in addition to the upstream side of the most upstream ink jet head and the downstream side of the most downstream ink jet head, between each ink jet head A pair of conveyance rollers is arranged.

Japanese Patent Laying-Open No. 2015-199552

  When a sheet is conveyed by a plurality of conveyance roller pairs, the conveyance roller pair nips the sheet at the timing when the leading edge of the sheet reaches each conveyance roller pair and at the timing when the trailing edge of the sheet exits each conveyance roller pair. The combination changes. When the combination of the conveyance roller pair that nips the paper changes, the paper conveyance speed may change due to a change in the conveyance load. The change in the paper conveyance speed causes expansion and contraction of the printed image by the ink jet head.

  Here, in the configuration having a plurality of inkjet heads arranged at different positions in the sub-scanning direction as described above, printing is performed at a later timing as the downstream inkjet head is at the same position in the sub-scanning direction. Do. Further, since the pair of transport rollers is arranged between the respective ink jet heads, the combination of the pair of transport rollers that nip the paper is changed during printing by the respective ink jet heads at the same position in the sub-scanning direction. For this reason, in the print image by each inkjet head with respect to the same position in the sub-scanning direction, image expansion / contraction of different degrees may occur for each inkjet head.

  As a result, the print image quality may be reduced. For example, in a configuration in which a plurality of ink jet heads are arranged in a staggered manner, a print image may be displaced due to a difference in the degree of expansion or contraction of the print image between the ink jet heads at the joint positions of the ink jet heads, and the print image quality may be deteriorated. Further, in a configuration in which a plurality of inkjet heads that eject different color inks are arranged in parallel in the sub-scanning direction, the landing positions of the inks of the respective colors may be shifted, and the print image quality may be deteriorated.

  The present invention has been made in view of the above, and an object of the present invention is to provide an ink jet printing apparatus that can reduce a decrease in print image quality.

  In order to achieve the above object, a first feature of an ink jet printing apparatus according to the present invention includes a single ink jet head or a plurality of ink jet heads arranged linearly in the main scanning direction, respectively. A plurality of head portions arranged at intervals in the paper conveyance direction orthogonal to the direction, and the upstream side of the most upstream head portion in the paper conveyance direction, between the head portions, and the most downstream head portion. At a timing at which the leading edge of the paper reaches a pair of transport rollers other than the most upstream pair of transport rollers and the transport roller pair other than the most upstream transport roller pair. A test pattern image is printed by the ink-jet head of the head unit, and each of the transport rollers other than the transport roller pair at the most downstream side At each timing when the trailing edge of the paper arrives, a test pattern print control unit that controls to print a test pattern image with the inkjet head of the head unit where the paper has not come off, and a test on the paper on which the test pattern image is printed Based on the length of each section divided by the pattern image, calculated for each head unit, based on the driving frequency of the inkjet head corresponding to each section, the driving frequency of each inkjet head at the time of print processing And a printing control unit for controlling.

  The second feature of the ink jet printing apparatus according to the present invention is that the ink jet heads of the adjacent head portions are arranged in a staggered manner, and the plurality of transport roller pairs are as fast as the downstream transport roller pairs. It is to transport the paper at a speed.

  A third feature of the ink jet printing apparatus according to the present invention is that a paper re-feed transport unit that circulates and transports the paper that has passed through the head unit and re-feeds the paper to the head unit, and a paper on which a test pattern image is printed. And a conveyance control unit that re-feeds the head unit by the re-feed conveyance unit.

  According to the first feature of the ink jet printing apparatus according to the present invention, the test pattern printing control unit is configured such that the paper arrives at each timing when the leading edge of the paper reaches each conveyance roller pair other than the most upstream conveyance roller pair. Print the test pattern image with the inkjet head of the head section, and test with the inkjet head of the head section where the paper is not missing at each timing when the trailing edge of the paper reaches each conveyance roller pair other than the most downstream conveyance roller pair Control to print a pattern image. The print controller prints based on the driving frequency of the inkjet head corresponding to each section, calculated for each head section based on the length of each section divided by the test pattern image on the paper on which the test pattern image is printed. The drive frequency of each inkjet head during processing is controlled. As a result, image expansion and contraction due to a deviation from the theoretical value of the actual sheet conveyance speed during printing of each section corresponding to each section divided by the test pattern image is suppressed. As a result, the difference in the degree of expansion / contraction of the print image between the ink jet heads is reduced, so that a decrease in print image quality can be reduced.

  According to the second feature of the ink jet printing apparatus according to the present invention, it is possible to reduce the deterioration of the print image quality due to the shift of the print image at the joint position of the ink jet heads arranged in a staggered pattern. In addition, since the sheet is conveyed at a higher speed as the pair of conveying rollers on the downstream side, the occurrence of sagging of the sheet can be suppressed. Thereby, since the fluctuation | variation of a head gap is suppressed, the fall of printing image quality can be reduced more.

  According to the third feature of the ink jet printing apparatus according to the present invention, the paper on which the test pattern image is printed is re-fed to the head portion by the re-feed conveyance unit. Since it can be reused for printing, paper waste can be reduced.

1 is a schematic configuration diagram of an inkjet printing apparatus according to an embodiment. It is a schematic block diagram of the printing part of the inkjet printing apparatus shown in FIG. It is a principal part top view of the printing part of the inkjet printing apparatus shown in FIG. It is a control block diagram of the inkjet printing apparatus shown in FIG. It is a flowchart for demonstrating head drive frequency calculation operation | movement. (A), (b) is explanatory drawing of the printing operation of a test pattern image. (A), (b) is explanatory drawing of the printing operation of a test pattern image. (A), (b) is explanatory drawing of the printing operation of a test pattern image. (A), (b) is explanatory drawing of the printing operation of a test pattern image. It is explanatory drawing of the reference length of the area divided | segmented by the test pattern image. (A), (b) is a figure which shows the drive frequency of an inkjet head.

  Embodiments of the present invention will be described below with reference to the drawings. Throughout the drawings, the same or equivalent parts and components are denoted by the same or equivalent reference numerals.

  The following embodiments exemplify devices for embodying the technical idea of the present invention, and the technical idea of the present invention is the material, shape, structure, arrangement, etc. of each component. Is not specified as follows. The technical idea of the present invention can be variously modified within the scope of the claims.

  FIG. 1 is a schematic configuration diagram of an inkjet printing apparatus according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a printing unit of the inkjet printing apparatus shown in FIG. FIG. 3 is a plan view of the main part of the printing unit. FIG. 4 is a control block diagram of the inkjet printing apparatus shown in FIG. In addition, let the direction orthogonal to the paper surface of FIG. 1 be the front-back direction, and let a paper surface direction be the front. Also, the top, bottom, left, and right of the paper surface in FIG.

  In FIG. 1, a path indicated by a thick line is a transport path through which the paper P that is a print medium is transported. Of the transport routes, the route indicated by the solid line is the printing route RP, the route indicated by the one-dot chain line is the circulation route RC, the route indicated by the broken line is the paper discharge route RD, and the route indicated by the two-dot chain line is the paper feed route RS. In the following description, upstream and downstream mean upstream and downstream in the transport direction of the paper P on the transport path.

  As shown in FIGS. 1 and 3, the inkjet printing apparatus 1 according to the present embodiment includes a paper feeding unit 2, a printing unit 3, a circulation transport unit 4, a reading unit 5, and a paper discharge unit 6. And a control unit 7.

  The paper feeding unit 2 feeds the unprinted paper P to the printing unit 3. In addition, the paper feeding unit 2 refeeds the paper P after the front surface printing to the printing unit 3 at the time of duplex printing. The sheet feeding unit 2 includes a sheet feeding table 11, a sheet feeding roller 12, and a registration roller pair 13.

  The paper feed table 11 is used for loading paper P used for printing.

  The paper feed roller 12 takes out the paper P one by one from the paper feed tray 11 and conveys it toward the registration roller pair 13 along the paper feed path RS. The paper feed roller 12 is rotationally driven by a motor (not shown).

  The registration roller pair 13 temporarily stops the paper P conveyed by the paper feed roller 12 or the circulation conveyance unit 4 and then conveys the paper P toward the printing unit 3. The registration roller pair 13 is rotationally driven by a motor (not shown). The registration roller pair 13 constitutes a part of the refeed conveyance section.

  The printing unit 3 prints an image on the paper P while conveying the paper P. As shown in FIG. 2, the printing unit 3 includes a print transport roller pair (corresponding to the transport roller pair in claims) 16A to 16C, a print transport motor 17, drive belts 18A to 18C, platens 19A and 19B, Head units 20A and 20B and a paper sensor 21 are provided. In some cases, alphabetical suffixes are omitted from the reference numerals of the print conveyance roller pairs 16A to 16C and the like.

  The print transport roller pairs 16A to 16C transport the paper P in the direction from left to right while nipping the paper P.

  The print conveying roller pair 16 includes a pair of a driving roller 26 and a driven roller 27, and conveys a sheet while nipping the driving roller 26 and the driven roller 27. The driving roller 26 is rotationally driven by the driving force of the print transport motor 17. The driven roller 27 rotates following the driving roller 26.

  The print conveyance roller pairs 16 </ b> A to 16 </ b> C are arranged at intervals in the conveyance direction (sub-scanning direction) of the paper P. The print conveying roller pair 16A is arranged on the upstream side of the head portion 20A. The print conveyance roller pair 16B is disposed between the head unit 20A and the head unit 20B. The print conveyance roller pair 16C is disposed on the downstream side of the head portion 20B. The distance between the pair of print transport rollers 16A and the pair of print transport rollers 16C is shorter than the length in the sub-scanning direction for the paper size having the smallest length in the sub-scanning direction among the paper sizes used in the inkjet printing apparatus 1. .

  The drive roller 26 of the print transport roller pair 16B has a larger diameter than the drive roller 26 of the print transport roller pair 16A. Further, the driving roller 26 of the print conveying roller pair 16C has a larger diameter than the driving roller 26 of the printing conveying roller pair 16B. In other words, the diameter of the drive roller 26 is larger in the downstream side of the print conveyance roller pair 16. Accordingly, the downstream side print conveyance roller pair 16 is configured to convey the paper P at a higher speed. Thereby, sagging of the paper P is prevented.

  The drive roller 26 is a one-way roller. Thus, the paper P is transported at the paper transport speed by the print transport roller pair 16B from when the paper P starts to be nipped by the print transport roller pair 16B until it reaches the print transport roller pair 16C, and the print transport roller pair 16A I will go around with P. Further, in a state where the paper P is nipped by the print transport roller pair 16C, the paper P is transported at a paper transport speed by the print transport roller pair 16C, and the print transport roller pairs 16A and 16B are rotated with the paper P.

  The print transport motor 17 rotates and drives the drive rollers 26 of the print transport roller pairs 16A to 16C. The print transport motor 17 has an output shaft 17 a that outputs a rotational driving force for rotationally driving the driving roller 26.

  The driving belts 18 </ b> A to 18 </ b> C transmit the rotational driving force of the print transport motor 17 to the driving rollers 26 and rotate the driving rollers 26. The drive belt 18A is stretched between the rotation shaft 26a of the drive roller 26 of the print conveyance roller pair 16A and the rotation shaft 26a of the drive roller 26 of the print conveyance roller pair 16B. The drive belt 18B is stretched between the rotation shaft 26a of the drive roller 26 of the print conveyance roller pair 16B and the rotation shaft 26a of the drive roller 26 of the print conveyance roller pair 16C. The drive belt 18 </ b> C is stretched between the rotation shaft 26 a of the drive roller 26 of the print conveyance roller pair 16 </ b> C and the output shaft 17 a of the print conveyance motor 17.

  The platens 19A and 19B support the paper P transported by the print transport roller pairs 16A to 16C. The platen 19A is disposed between the print conveyance roller pair 16A and the print conveyance roller pair 16B. The platen 19B is disposed between the print transport roller pair 16B and the print transport roller pair 16C.

  The head units 20A and 20B print an image by ejecting ink onto the paper P transported by the print transport roller pairs 16A to 16C. The head portions 20A and 20B are arranged with a space therebetween in the sub-scanning direction. The head portion 20A is disposed between the print conveyance roller pair 16A and the print conveyance roller pair 16B, and the head portion 20B is disposed between the print conveyance roller pair 16B and the print conveyance roller pair 16C. The head units 20A and 20B have inkjet heads 31A and 31B, respectively.

  The inkjet head 31 prints an image by ejecting ink onto the paper P. The inkjet head 31 has a plurality of nozzles (not shown) arranged along the main scanning direction (front-rear direction) orthogonal to the transport direction of the paper P, and ejects ink from the nozzles.

  The inkjet heads 31A and 31B are arranged in a staggered manner as shown in FIG. That is, the inkjet head 31B is arranged at a position shifted to the rear side with respect to the inkjet head 31A in the main scanning direction, and is arranged at a position shifted to the downstream side with respect to the inkjet head 31A in the sub-scanning direction. . With such an arrangement, the inkjet head 31A prints in the area on the front side of the sheet P in the main scanning direction, and the inkjet head 31B prints in the area on the back side of the sheet P.

  The paper sensor 21 detects the paper P being conveyed. The paper sensor 21 is disposed in the vicinity of the upstream side of the print conveying roller pair 16A. The paper sensor 21 is composed of, for example, an optical sensor having a light emitting unit and a light receiving unit.

  The circulatory conveyance unit 4 conveys the paper P, which has been printed on the surface by passing through the head unit 20, along the circulation path RC and returns it to the registration roller pair 13 during duplex printing. The circulation conveyance unit 4 includes two intermediate conveyance roller pairs 36, a reverse roller pair 37, two horizontal conveyance roller pairs 38, an encoder 39, and two vertical conveyance roller pairs 40. In addition, the circulation conveyance part 4 comprises a part of re-feed conveyance part of a claim.

  The intermediate conveyance roller pair 36 conveys the paper P sent out from the printing unit 3 to the reverse roller pair 37. The intermediate conveyance roller pair 36 is disposed along a circulation path RC between the downstream end of the printing path RP and the reverse roller pair 37. The intermediate transport roller pair 36 is rotationally driven by a motor (not shown).

  The reverse roller pair 37 switches back the paper P conveyed by the intermediate conveyance roller pair 36 and conveys it to the horizontal conveyance roller pair 38. The reverse roller pair 37 is disposed on the downstream side of the intermediate transport roller pair 36 along the circulation path RC. The reverse roller pair 37 is rotationally driven by a motor (not shown) so as to be able to rotate forward and backward.

  The horizontal conveyance roller pair 38 conveys the paper P switched back by the reverse roller pair 37 to the vertical conveyance roller pair 40. The horizontal conveyance roller pair 38 is disposed along the upstream portion of the circulation path RC between the reversing roller pair 37 and the junction point of the circulation path RC to the sheet feeding path RS. The horizontal conveyance roller pair 38 is rotationally driven by a motor (not shown).

  The encoder 39 is connected to the upstream horizontal conveyance roller pair 38 and outputs a pulse signal at every predetermined rotation angle of the horizontal conveyance roller pair 38.

  The vertical conveyance roller pair 40 conveys the paper P conveyed by the horizontal conveyance roller pair 38 to the registration roller pair 13. The vertical conveying roller pair 40 is disposed along the downstream portion of the circulation path RC between the reverse roller pair 37 and the junction point of the circulation path RC with the sheet feeding path RS. The vertical conveying roller pair 40 is rotationally driven by a motor (not shown).

  The reading unit 5 reads a test pattern image on a sheet P on which a test pattern image to be described later is printed. The reading unit 5 is arranged in the vicinity of the downstream side of the upstream horizontal conveyance roller pair 38. The reading unit 5 includes test pattern detection sensors 41A and 41B.

  The test pattern detection sensors 41A and 41B detect test pattern images printed on the paper P by the inkjet heads 31A and 31B, respectively. The test pattern detection sensors 41A and 41B are arranged in parallel in the front-rear direction (main scanning direction), the test pattern detection sensor 41A is arranged on the front side, and the test pattern detection sensor 41B is arranged on the rear side. The test pattern detection sensors 41A and 41B are made of, for example, a CIS (Contact Image Sensor).

  The paper discharge unit 6 discharges the printed paper P. The paper discharge unit 6 includes a switching unit 46, two paper discharge roller pairs 47, and a paper discharge table 48.

  The switching unit 46 switches the transport path of the paper P between the paper discharge path RD and the circulation path RC. The switching unit 46 is disposed at a point where the print route RP branches into the paper discharge route RD and the circulation route RC. The switching unit 46 is driven by a solenoid (not shown).

  The paper discharge roller pair 47 conveys the paper P printed by the printing unit 3 to the paper discharge table 48. The paper discharge roller pair 47 is disposed along the paper discharge path RD. The paper discharge roller pair 47 is rotationally driven by a motor (not shown).

  The paper discharge table 48 is for stacking the paper P discharged by the paper discharge roller pair 47. The paper discharge table 48 is disposed at the downstream end of the paper discharge route RD.

  The control unit 7 controls the operation of the entire inkjet printing apparatus 1. The control unit 7 includes a conveyance control unit 51, a test pattern print control unit 52, a reference value storage unit 53, a drive frequency calculation unit 54, and a print control unit 55. Each unit of the control unit 7 can be realized in software or hardware by a CPU, RAM, ROM, hard disk, or the like.

  The conveyance control unit 51 controls the sheet conveyance operation by the paper feeding unit 2, the printing unit 3, the circulation conveyance unit 4, and the paper discharge unit 6.

  The test pattern printing control unit 52 controls the inkjet heads 31A and 31B to print a test pattern image in a head driving frequency calculation operation described later.

  The reference value storage unit 53 stores the reference length of each section divided by the test pattern image on the paper P on which the test pattern image is printed. Further, the reference value storage unit 53 stores reference drive frequencies of the ink jet heads 31A and 31B corresponding to each section. The reference length and reference drive frequency of each section will be described later.

  The drive frequency calculation unit 54 calculates the drive frequencies of the inkjet heads 31A and 31B corresponding to each section based on the length of each section divided by the test pattern image on the paper P on which the test pattern image is printed.

  The print control unit 55 controls the ink jet heads 31 </ b> A and 31 </ b> B to print an image on the paper P. The print control unit 55 controls the drive frequencies of the inkjet heads 31A and 31B during the printing process based on the drive frequencies corresponding to the sections calculated by the drive frequency calculation unit 54 in the head drive frequency calculation operation.

  Next, the head drive frequency calculation operation in the inkjet printing apparatus 1 will be described.

  The head drive frequency calculation operation is an operation for calculating the drive frequency of the ink jet heads 31A and 31B according to the change in the paper conveyance speed due to the change in the conveyance load in the printing conveyance roller pairs 16A to 16C of the printing unit 3.

  Here, in the printing unit 3, the combination of the print transport roller pair 16 that nips the paper P changes during the paper transport.

  Specifically, from the time when the leading edge of the paper P reaches the print transport roller pair 16A to the time when it reaches the print transport roller pair 16B, the paper P is nipped only by the print transport roller pair 16A in the printing unit 3. From the time when the leading edge of the paper P reaches the print conveyance roller pair 16B to the time when it reaches the print conveyance roller pair 16C, in the printing unit 3, the paper P is nipped only by the print conveyance roller pairs 16A and 16B. From the time when the leading edge of the paper P reaches the print conveyance roller pair 16C to the time when the trailing edge of the paper P reaches the print conveyance roller pair 16A, the printing unit 3 nips the paper P between the print conveyance roller pairs 16A to 16C. ing.

  Further, until the trailing edge of the paper P reaches the print conveyance roller pair 16A and reaches the print conveyance roller pair 16B, the paper P is nipped only by the print conveyance roller pairs 16B and 16C in the printing unit 3. From the time when the trailing edge of the paper P reaches the print transport roller pair 16B to the time when it reaches the print transport roller pair 16C, in the printing unit 3, the paper P is nipped only by the print transport roller pair 16C.

  In the printing unit 3, the paper P is transported at a paper transport speed by the most downstream print transport roller pair 16 among the print transport roller pairs 16 that nip the paper P.

  Specifically, in a state where the paper P is nipped only by the print transport roller pair 16A, the paper P is transported at a paper transport speed by the print transport roller pair 16A. In a state where the paper P is nipped only by the print transport roller pair 16A, 16B, the paper P is transported at a paper transport speed by the print transport roller pair 16B. The paper P is transported at the paper transport speed by the print transport roller pair 16C until the trailing edge of the paper P reaches the print transport roller pair 16C after the leading edge of the paper P reaches the print transport roller pair 16C. .

  However, when the paper P is nipped by the plurality of print transport roller pairs 16, the actual paper transport speed may be reduced from the theoretical value due to the back tension applied to the paper P. In addition, after the leading edge of the paper P reaches the print conveying roller pair 16C, the back tension applied to the paper P changes before and after the trailing edge of the paper P passes through the printing conveying roller pair 16A. The sheet conveyance speed may change.

  Even in a state where the paper P is nipped only by the print conveyance roller pair 16A and a state where the paper P is nipped only by the print conveyance roller pair 16C, the actual condition is affected by the dimensional error in the paper conveyance roller pair 16A, 16C. The paper transport speed may deviate from the theoretical value.

  For this reason, even if the drive frequency of the ink jet heads 31A and 31B is changed to a value corresponding to the theoretical value of the paper transport speed in accordance with the change in the combination of the print transport roller pair 16 that nips the paper P, the actual The print image expands or contracts due to a deviation from the theoretical value of the paper conveyance speed.

  Then, the inkjet head 31A and the inkjet head 31B perform printing at the same timing on the paper P in the sub-scanning direction at different timings of the combination of the print transport roller pair 16 that nips the paper P. For this reason, a difference in the degree of expansion / contraction of the print image with respect to the same position in the sub-scanning direction occurs between the inkjet head 31A and the inkjet head 31B. As a result, the print image shifts at the joint position between the inkjet head 31A and the inkjet head 31B.

  Therefore, the inkjet printing apparatus 1 performs a head drive frequency calculation operation in order to obtain the drive frequencies of the inkjet heads 31A and 31B in accordance with the deviation from the actual paper transport speed in the printing unit 3 from the theoretical value.

  FIG. 5 is a flowchart for explaining the head drive frequency calculation operation. The head drive frequency calculation operation is performed before the actual printing process is performed in the inkjet printing apparatus 1.

  In step S <b> 1 of FIG. 5, the transport control unit 51 and the test pattern print control unit 52 execute a test pattern image printing operation.

  Specifically, the conveyance control unit 51 causes the printing conveyance motor 17 to start driving the printing conveyance roller pairs 16 </ b> A to 16 </ b> C, and then causes the paper feeding unit 2 to feed the paper P to the printing unit 3. The paper P is transported in the printing unit 3 by the print transport roller pairs 16A to 16C.

  Then, the test pattern printing control unit 52 has the head unit 20 to which the paper P has arrived at each theoretical timing at which the leading edge of the paper reaches the print transport roller pairs 16B and 16C other than the most upstream print transport roller pair 16A. A test pattern image is printed by the inkjet head 31. In addition, the test pattern print control unit 52 has the head unit 20 in which the paper P is not removed at each theoretical timing when the trailing edge of the paper reaches the print transport roller pairs 16A and 16B other than the most downstream print transport roller pair 16C. A test pattern image is printed by the inkjet head 31.

  Specifically, the test pattern print control unit 52 prints a test pattern image by the inkjet head 31A at the theoretical timing when the leading edge of the paper P reaches the print transport roller pair 16B as shown in FIG. Control to do. As a result, the test pattern image 61A is printed on the paper P as shown in FIG. Here, the test pattern printing control unit 52 determines the theoretical timing at which the leading edge of the paper P reaches the print conveying roller pair 16B based on the elapsed time after the leading edge of the paper P is detected by the paper sensor 21.

  Next, the test pattern print control unit 52 prints a test pattern image by the ink jet heads 31A and 31B at the theoretical timing when the leading edge of the paper P reaches the print conveying roller pair 16C as shown in FIG. 7A. Control as follows. Accordingly, test pattern images 61B and 62A are newly printed on the paper P as shown in FIG.

  Next, as shown in FIG. 8A, the test pattern print control unit 52 prints a test pattern image by the ink jet heads 31A and 31B at the theoretical timing when the trailing edge of the paper P reaches the print conveying roller pair 16A. Control to do. As a result, as shown in FIG. 8B, test pattern images 61C and 62B are newly printed on the paper P.

  Next, as shown in FIG. 9A, the test pattern print control unit 52 prints the test pattern image by the inkjet head 31B when the theoretical timing at which the trailing edge of the paper P reaches the print conveying roller pair 16B is reached. Control. As a result, a test pattern image 62C is newly printed on the paper P as shown in FIG. Here, the test pattern images 61A to 61C and 62A to 62C are images composed of one or more dots.

  Returning to FIG. 5, in step S <b> 2, the conveyance control unit 51 and the test pattern detection sensors 41 </ b> A and 41 </ b> B perform a reading operation of the test pattern images 61 and 62 printed on the paper P.

  Specifically, the conveyance control unit 51 causes the switching unit 46 to introduce the paper P on which the test pattern images 61 and 62 are printed from the printing route RP to the circulation route RC. Thereafter, the paper P is switched back by the reverse roller pair 37 and is transported by the horizontal transport roller pair 38.

  When the paper P passes below the test pattern detection sensors 41A and 41B, the test pattern detection sensors 41A and 41B detect the test pattern images 61A to 61C and 62A to 62C, respectively. The test pattern detection sensors 41A and 41B also detect the leading edge and the trailing edge of the paper P.

  At this time, the drive frequency calculation unit 54 measures the lengths La1 to La4 and Lb1 to Lb4 of the sections 71A to 71D and 72A to 72D shown in FIG. The sections 71A to 71D are sections divided in the sub-scanning direction by the test pattern images 61A to 61C on the paper P on which the test pattern images 61 and 62 are printed. The sections 72A to 72D are sections divided in the sub-scanning direction by the test pattern images 62A to 62C on the paper P on which the test pattern images 61 and 62 are printed.

  Here, when the paper P passes below the test pattern detection sensors 41A and 41B, the conveyance control unit 51 reverses the reverse roller so that the paper P sags between the reverse roller pair 37 and the upstream horizontal conveyance roller pair 38. The pair 37 and the upstream horizontal conveyance roller pair 38 are driven and controlled. Further, after the leading edge of the sheet P reaches the downstream horizontal conveyance roller pair 38, the conveyance control unit 51 determines that the sheet conveyance speed by the downstream horizontal conveyance roller pair 38 is the sheet by the upstream horizontal conveyance roller pair 38. Drive control is performed so as not to be faster than the conveyance speed. This is to avoid a decrease in detection accuracy of the lengths La1 to La4 and Lb1 to Lb4 in the sections 71A to 71D and 72A to 72D due to the back tension of the paper P.

  Returning to FIG. 5, in step S3, the drive frequency calculation unit 54 uses the lengths La1 to La4 and Lb1 to Lb4 of the sections 71A to 71D and 72A to 72D, and inkjets corresponding to the sections 71A to 71D and 72A to 72D. The drive frequency of the heads 31A and 31B is calculated.

  Specifically, the drive frequency calculation unit 54 calculates the drive frequencies Fa1 to Fa4 of the inkjet head 31A corresponding to the sections 71A to 71D, respectively, using the following formulas (1) to (4). Further, the drive frequency calculation unit 54 calculates the drive frequencies Fb1 to Fb4 of the inkjet head 31B corresponding to the sections 72A to 72D, respectively, using the following formulas (5) to (8).

Fa1 = (La1 / Lka1) × Fka1 (1)
Fa2 = (La2 / Lka2) × Fka2 (2)
Fa3 = (La3 / Lka3) × Fka3 (3)
Fa4 = (La4 / Lka4) × Fka4 (4)
Fb1 = (Lb1 / Lkb1) × Fkb1 (5)
Fb2 = (Lb2 / Lkb2) × Fkb2 (6)
Fb3 = (Lb3 / Lkb3) × Fkb3 (7)
Fb4 = (Lb4 / Lkb4) × Fkb4 (8)
Here, Lka1 to Lka4 are reference lengths of the sections 71A to 71D (theoretical values of the lengths of the sections 71A to 71D), respectively. Lkb1 to Lkb4 are reference lengths of the sections 72A to 72D (theoretical values of the lengths of the sections 72A to 72D), respectively.

  Fka1 to Fka4 are reference drive frequencies of the inkjet head 31A corresponding to the sections 71A to 71D, respectively. Fkb1 to Fkb4 are reference drive frequencies of the inkjet head 31B corresponding to the sections 72A to 72D, respectively.

  The reference lengths Lka1 to Lka4 and Lkb1 to Lkb4 of the sections 71A to 71D and 72A to 72D are the sections 71A to 71D and 72A to 72D when the sheet transport speeds by the print transport roller pairs 16A to 16C are the theoretical values, respectively. Is the length of

  Specifically, as shown in FIG. 10, the reference length Lka1 of the section 71A is a distance between the ink discharge position (nozzle position) of the inkjet head 31A and the nip point of the print conveying roller pair 16B. The reference length Lka2 of the section 71B is a distance between the nip point of the print conveyance roller pair 16B and the nip point of the print conveyance roller pair 16C. The reference length Lka3 of the section 71C is a distance between the rear end of the paper P and the nip point of the print conveyance roller pair 16A when the leading edge of the paper P is at the nip point of the print conveyance roller pair 16C. The reference length Lka3 is determined according to the length of the paper P. The reference length Lka4 of the section 71D is a distance between the nip point of the print conveying roller pair 16A and the ink discharge position of the inkjet head 31A.

  The reference length Lkb1 of the section 72A is a distance between the ink ejection position of the inkjet head 31B and the nip point of the print conveying roller pair 16C. The reference length Lkb2 of the section 72B is the same value as the reference length Lka3 of the section 71C described above. The reference length Lkb3 of the section 72C is a distance between the nip point of the print conveyance roller pair 16A and the nip point of the print conveyance roller pair 16B. The reference length Lkb4 of the section 72D is a distance between the nip point of the print conveying roller pair 16B and the ink discharge position of the inkjet head 31B.

  The reference drive frequencies Fka1 to Fka4 of the inkjet head 31A corresponding to the sections 71A to 71D are sections corresponding to the sections 71A to 71D when the sheet transport speeds by the print transport roller pairs 16A to 16C are as theoretical values. This is the drive frequency corresponding to the paper conveyance speed when the inkjet head 31A performs printing.

  Specifically, the reference drive frequency Fka1 of the ink jet head 31A corresponding to the section 71A is a drive frequency according to the theoretical value of the paper transport speed by the print transport roller pair 16A. The reference drive frequency Fka2 of the inkjet head 31A corresponding to the section 71B is a drive frequency according to the theoretical value of the paper transport speed by the print transport roller pair 16B. The reference drive frequency Fka3 of the ink jet head 31A corresponding to the section 71C is a drive frequency according to the theoretical value of the paper transport speed by the print transport roller pair 16C. The reference drive frequency Fka4 of the inkjet head 31A corresponding to the section 71D is a drive frequency according to the theoretical value of the paper transport speed by the print transport roller pair 16C, as with the above-described reference drive frequency Fka3. That is, the reference drive frequency Fka4 is the same value as the above-described reference drive frequency Fka3.

  The reference drive frequencies Fkb1 to Fkb4 of the inkjet head 31B corresponding to the sections 72A to 72D are sections corresponding to the sections 72A to 72D when the sheet transport speeds by the print transport roller pairs 16A to 16C are as the theoretical values. This is the drive frequency corresponding to the paper conveyance speed when the inkjet head 31B performs printing.

  Specifically, the reference drive frequency Fkb1 of the inkjet head 31B corresponding to the section 72A is a drive frequency according to the theoretical value of the sheet transport speed by the print transport roller pair 16B, as with the above-described reference drive frequency Fka2. That is, the reference drive frequency Fkb1 is the same value as the above-described reference drive frequency Fka2.

  The reference drive frequency Fkb2 of the inkjet head 31B corresponding to the section 72B is a drive frequency according to the theoretical value of the paper transport speed by the print transport roller pair 16C, as with the above-described reference drive frequencies Fka3 and Fka4. That is, the reference drive frequency Fkb2 has the same value as the above-described reference drive frequencies Fka3 and Fka4.

  The reference drive frequency Fkb3 of the inkjet head 31B corresponding to the section 72C is a drive frequency according to the theoretical value of the sheet transport speed by the print transport roller pair 16C, similarly to the above-described reference drive frequencies Fka3, Fka4, and Fkb2. That is, the reference drive frequency Fkb3 is the same value as the above-described reference drive frequencies Fka3, Fka4, and Fkb2.

  The reference drive frequency Fkb4 of the inkjet head 31b corresponding to the section 72D is a drive frequency according to the theoretical value of the sheet transport speed by the print transport roller pair 16C, as with the above-described reference drive frequencies Fka3, Fka4, Fkb2, and Fkb3. . That is, the reference drive frequency Fkb4 is the same value as the above-described reference drive frequencies Fka3, Fka4, Fkb2, and Fkb3.

  When the drive frequencies Fa1 to Fa4 and Fb1 to Fb4 are calculated in step S3 in FIG. 5, the head drive frequency calculation operation is completed.

  Next, the operation at the time of printing of the inkjet printing apparatus 1 will be described.

  The paper P on which the test pattern images 61 and 62 are printed is transported by the horizontal transport roller pair 38 and the vertical transport roller pair 40 after the test pattern images 61 and 62 are read by the reading unit 5, and by the registration roller pair 13. The paper is fed again to the printing unit 3. Here, since the sheet P is reversed by the switchback by the reversing roller pair 37, the sheet P is re-fed to the head unit 20 with the surface on which the test pattern images 61 and 62 are printed facing downward. The The re-feeded paper P is transported by the print transport roller pairs 16A to 16C.

  The print control unit 55 drives the inkjet heads 31A and 31B based on the image data to be printed, and ejects ink onto the paper P conveyed by the print conveyance roller pairs 16A to 16C. As a result, an image is printed on the paper P.

  At this time, the print control unit 55 determines the drive frequencies of the inkjet heads 31A and 31B based on the drive frequencies Fa1 to Fa4 and Fb1 to Fb4 corresponding to the sections 71A to 71D and 72A to 72D calculated in the head drive frequency calculation operation. Control.

  Specifically, the print control unit 55 controls the drive frequency of the inkjet head 31A as shown in FIG. 11A and controls the drive frequency of the inkjet head 31B as shown in FIG.

  First, as shown in FIG. 11A, at time t1, which is the timing when the leading edge of the paper P reaches the print conveying roller pair 16A, the print control unit 55 sets the drive frequency of the inkjet head 31A to Fa1. Here, the print control unit 55 determines the timing at which the leading edge of the paper P reaches the print conveying roller pair 16 </ b> A based on the timing at which the leading edge of the paper P is detected by the paper sensor 21. Note that F0 in FIGS. 11A and 11B is an initial value of the drive frequency of the inkjet head 31. FIG. As the initial value F0, the reference drive frequency Fka1 is set according to the theoretical value of the sheet conveyance speed by the above-described print conveyance roller pair 16A.

  Next, at time t2, which is the theoretical timing when the leading edge of the paper P reaches the print conveying roller pair 16B, the print control unit 55 sets the drive frequency of the inkjet head 31A to Fa2, as shown in FIG. Set. At time t2, the print control unit 55 sets the drive frequency of the inkjet head 31B to Fb1, as shown in FIG.

  Next, at time t3, which is the theoretical timing when the leading edge of the paper P reaches the print conveying roller pair 16C, the print control unit 55 sets the drive frequency of the inkjet head 31A to Fa3 as shown in FIG. Set. At time t3, the print control unit 55 sets the drive frequency of the inkjet head 31B to Fb2, as shown in FIG. 11B.

  Next, at time t4, which is the theoretical timing at which the trailing edge of the paper P reaches the print conveying roller pair 16A, the print control unit 55 sets the drive frequency of the inkjet head 31A to Fa4 as shown in FIG. Set to. At time t4, the print control unit 55 sets the drive frequency of the inkjet head 31B to Fb3 as shown in FIG. 11B.

  Next, at time t5, which is the theoretical timing at which the trailing edge of the paper P reaches the print conveying roller pair 16B, the print control unit 55 sets the drive frequency of the inkjet head 31B to Fb4 as shown in FIG. Set to.

  Then, at time t6, which is the theoretical timing when the trailing edge of the paper P reaches the print conveying roller pair 16B, the print control unit 55 returns the drive frequencies of the inkjet heads 31A and 31B to the initial value F0.

  By controlling the drive frequency as described above, the inkjet head 31A is driven at the drive frequencies Fa1 to Fa4 when printing is performed in each section corresponding to the sections 71A to 71D. Further, the inkjet head 31B is driven at the drive frequencies Fb1 to Fb4 at the time of printing in each section corresponding to the sections 72A to 72D.

  Thus, image expansion and contraction due to deviation from the actual paper transport speed from the theoretical value during printing in the sections corresponding to the sections 71A to 71D and 72A to 72D can be suppressed. For this reason, the shift | offset | difference of the print image in the position of the joint of 31 A of inkjet heads and the inkjet head 31B is suppressed.

  In the case of single-sided printing setting, the paper P printed on one side by the printing unit 3 is guided to the paper discharge path RD by the switching unit 46 and is discharged to the paper discharge tray 48 by the paper discharge unit 6.

  In the case of duplex printing setting, the paper P printed on one side by the printing unit 3 is guided to the circulation path RC by the switching unit 46, switched back by the reverse roller pair 37, and turned upside down. Then, the paper P is transported by the horizontal transport roller pair 38 and the vertical transport roller pair 40, and is fed again to the printing unit 3 by the registration roller pair 13. The paper P that has been re-fed to the printing unit 3 is printed on the back side by the printing unit 3, and is then discharged to the paper discharge tray 48 by the paper discharge unit 6. Note that the back surface of the paper P is the surface on which the test pattern images 61 and 62 are printed, but the test pattern images 61 and 62 have a degree that hardly affects the printed image.

  As described above, in the inkjet printing apparatus 1, the test pattern print control unit 52 determines the timing at which the leading edge of the paper P reaches the print transport roller pair 16B, 16C, and the trailing edge of the paper P is the print transport roller pair 16A, 16B. The test pattern images 61A to 61C and 62A to 62C are printed by the ink jet heads 31A and 31B at the timing of reaching. The print control unit 55 calculates the drive frequencies corresponding to the sections 71A to 71D and 72A to 72D calculated based on the lengths of the sections 71A to 71D and 72A to 72D divided by the test pattern images 61A to 61C and 62A to 62C. Based on Fa1 to Fa4 and Fb1 to Fb4, the drive frequencies of the inkjet heads 31A and 31B are controlled.

  Thus, image expansion and contraction due to deviation from the actual paper transport speed from the theoretical value during printing in the sections corresponding to the sections 71A to 71D and 72A to 72D can be suppressed. For this reason, the shift | offset | difference of the print image in the position of the joint of 31 A of inkjet heads and the inkjet head 31B is suppressed. As a result, it is possible to reduce a decrease in print image quality.

  Further, the inkjet printing apparatus 1 is configured to convey the paper P at a higher speed as the downstream-side print conveyance roller pair 16. As a result, the occurrence of sagging of the paper P can be suppressed, so that fluctuations in the head gap, which is the distance between the paper P and the inkjet head 31, can be suppressed. As a result, it is possible to further reduce the decrease in print image quality.

  Further, in the inkjet printing apparatus 1, the paper P on which the test pattern images 61 and 62 are printed is circulated and conveyed along the circulation path RC and is re-fed to the head unit 20. Thereby, since the paper P on which the test pattern images 61 and 62 are printed can be reused for printing, waste of the paper P can be reduced.

  In the above-described embodiment, the test pattern images 61A to 61C and 62A to 62C are read by the test pattern detection sensors 41A and 41B in the inkjet printing apparatus 1, and the drive frequencies Fa1 to Fa4 and Fb1 to Fb4 are calculated. However, the reading of the test pattern images 61A to 61C and 62A to 62C and the calculation of the driving frequencies Fa1 to Fa4 and Fb1 to Fb4 may be performed by an external device.

  In the above-described embodiment, the configuration in which each of the two head units 20 includes one inkjet head 31 is shown, but the present invention is not limited thereto.

  For example, the two head units 20 each have a plurality of inkjet heads 31 arranged linearly along the main scanning direction, and the inkjet heads 31 of the two head units 20 are arranged in a staggered manner. But you can. Alternatively, a plurality of sets of two head portions 20 may be arranged in parallel, and the inkjet heads 31 of the adjacent head portions 20 may be arranged in a staggered manner. In this case, the print conveyance roller pair 16 is disposed upstream of the most upstream head unit 20, downstream of the most downstream head unit 20, and between each head unit 20.

  When the head unit 20 includes a plurality of inkjet heads 31, the test pattern image may be printed by at least one inkjet head 31 in each head unit 20. Then, the drive frequency of the inkjet head 31 corresponding to each section divided by the test pattern image is calculated for each head unit 20.

  Further, the arrangement of the inkjet head is not limited to the staggered arrangement. For example, a configuration may be adopted in which each of the plurality of head units has one inkjet head that covers the entire width of the paper P, and each inkjet head ejects ink of a different color. In such a configuration, when control of the driving frequency of the inkjet head in the above-described embodiment is not performed, the landing position shifts between the colors due to the difference in the degree of expansion and contraction of the print image by the ink of each color, and the print image quality is deteriorated. There is a risk. On the other hand, by controlling the drive frequency of the ink jet head in the above-described embodiment, it is possible to suppress the deviation of the landing positions of the inks of the respective colors and reduce the deterioration of the print image quality.

  In the above-described embodiment, the example in which the diameter of the driving roller 26 is as large as the downstream side print conveyance roller pair 16 and the sheet P is conveyed at a high speed has been described. However, the present invention can also be applied to a configuration in which all the print conveyance roller pairs 16 are designed to convey the paper P at the same speed.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiments. For example, some components may be deleted from all the components shown in the embodiment.

DESCRIPTION OF SYMBOLS 1 Inkjet printing apparatus 2 Paper feed part 3 Printing part 4 Circulation conveyance part 5 Reading part 6 Paper discharge part 7 Control part 16,16A-16C Print conveyance roller pair 17 Print conveyance motor 20,20A, 20B Head part 31,31A, 31B Inkjet head 39 Encoder 41A, 41B Test pattern detection sensor 51 Transport control unit 52 Test pattern print control unit 53 Reference value storage unit 54 Drive frequency calculation unit 55 Print control unit 61, 61A to 61C, 62, 62A to 62C Test pattern image 71A ~ 71D, 72A ~ 72D section

Claims (3)

A plurality of ink jet heads or a plurality of ink jet heads arranged linearly along the main scanning direction, and a plurality of head parts arranged at intervals in the paper transport direction perpendicular to the main scanning direction; ,
A plurality of conveying roller pairs arranged on the upstream side of the most upstream head unit in the sheet conveying direction, between the head units, and on the downstream side of the most downstream head unit, and conveys the sheet while nipping it;
At each timing when the leading end of the paper reaches each of the transport roller pairs other than the upstreammost transport roller pair, a test pattern image is printed by the ink jet head of the head unit where the paper has reached, and the most downstream of the transport roller pair A test pattern printing control unit that controls to print a test pattern image with the inkjet head of the head unit in which the sheet does not come out at each timing when the trailing end of the sheet reaches each pair of conveying rollers other than the pair of conveying rollers;
Based on the driving frequency of the inkjet head corresponding to each section, calculated for each head section based on the length of each section divided by the test pattern image on the paper on which the test pattern image is printed, An inkjet printing apparatus comprising: a printing control unit that controls a driving frequency of each inkjet head at the time. The inkjet heads of the adjacent head portions are arranged in a staggered manner,
The inkjet printing apparatus according to claim 1, wherein the plurality of conveyance roller pairs convey the sheet at a speed higher than that of the downstream conveyance roller pair. A paper refeeding and conveying unit that circulates and conveys the paper that has passed through the head unit and refeeds the paper to the head unit;
The inkjet printing apparatus according to claim 1, further comprising: a conveyance control unit that re-feeds the paper on which the test pattern image is printed to the head unit by the re-feed conveyance unit.