JP2013244712A - Inkjet printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inkjet printer which can suppress deterioration of print image quality.SOLUTION: An inkjet printer 1 includes: a conveying part 2 which conveys a paper in a conveying direction; an inkjet head 3K which has a plurality of head modules arranged in an approximately orthogonal direction to the conveying direction and discharging a droplet of ink on the paper conveyed by the conveying part 2 and carries out printing which represents a density by the number of droplets of the ink discharged on each pixel; and a control part 6 which controls the conveying part 2 and the inkjet head 3K. The control part 6 executes correction processing for making print density by the predetermined number of droplets of the respective head modules uniform when there is a crossing region, which includes pixels of the predetermined number of droplets at a rate higher than a threshold, is larger than a predetermined size, and crosses over a region to be printed of an adjacent head module, in image data to be printed.

Description

  The present invention relates to an ink jet printing apparatus that performs printing by ejecting ink onto a print medium from an ink jet head.

  In an inkjet printing apparatus, various factors can affect print image quality. For example, the ink ejection efficiency varies depending on the environmental temperature, and therefore the print image quality may deteriorate.

  Therefore, for example, Patent Document 1 discloses a ratio of dots of each size according to the environmental temperature in an ink jet printing apparatus that expresses gradation by the size of dots formed by ejecting ink droplets on paper. A technique for adjusting each nozzle row is disclosed. As a result, the ink jet printing apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-228561 aligns the ratio of the landing density of the droplets ejected from each nozzle row on the paper as designed, and suppresses the deterioration in print image quality.

JP 2005-225199 A

  In an inkjet printing apparatus, satellites may be generated. The satellite is an unnecessary minute droplet generated accompanying the main droplet discharged from the inkjet head. The satellite may land on a position shifted from the landing position of the main droplet on the paper that is the print medium, and may affect the print image quality. For this reason, even when ink ejection is performed with the landing density as designed as in the ink jet printing apparatus of Patent Document 1, the print image quality may deteriorate due to the influence of satellites.

  Incidentally, a line-type inkjet printing apparatus in which an inkjet head is configured by a plurality of head modules is conventionally known. In such an ink jet printing apparatus, the size of the head gap and the state of the airflow accompanying the conveyance of the paper may be different for each head module. As a result, the amount of satellite generated may differ from head module to head module.

  If the amount of satellite generated varies from head module to head module, even if ink is ejected from each head module so as to print at the same density, a difference in print density may occur between the head modules due to the influence of the satellite. In particular, when the print density is low, the difference in print density between the head modules is more conspicuous as density unevenness than when the print density is high. As a result, the print image quality may be degraded.

  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 capable of suppressing a decrease in print image quality.

  In order to achieve the above object, a first feature of the ink jet printing apparatus according to the present invention is a transport unit that transports a print medium in the transport direction, and a transport unit that is arranged in a direction substantially orthogonal to the transport direction. An inkjet head that has a plurality of head modules that eject ink droplets onto a printed medium, and that performs printing that expresses the density according to the number of ink droplets ejected to each pixel, the transport unit, and the inkjet A control unit that controls a head, and the control unit is an area of a predetermined size or more that includes pixels of a predetermined number of droplets at a ratio equal to or greater than a threshold in image data to be printed, and is adjacent to the head When there is a straddling region that is a region straddling the print target region of the module, it is a supplement for making the print density by the predetermined number of droplets of each head module substantially uniform. It is to execute the process.

  A second feature of the ink jet printing apparatus according to the present invention is that the correction process includes at least one of a drive voltage of the head module, a drive waveform of the head module, and a conveyance speed of the print medium by the conveyance unit. There is to include as.

  A third feature of the ink jet printing apparatus according to the present invention is that the control unit is based on the setting content of each correction item determined in advance so that the print density according to the predetermined number of droplets of each head module becomes substantially uniform. The correction process is executed.

  According to the first feature of the ink jet printing apparatus according to the present invention, the control unit is an area having a predetermined size or more including pixels having a predetermined number of droplets at a ratio equal to or higher than a threshold in the image data to be printed. When there is a straddling region that is a region straddling the print target region of the adjacent head module, a correction process is performed to make the print density of each head module according to a predetermined number of droplets substantially uniform. Thereby, the inkjet printing apparatus can suppress the occurrence of density unevenness in the printed image, and can suppress the deterioration of the printing image quality.

  According to the second feature of the inkjet printing apparatus according to the present invention, at least one of the driving voltage of the head module, the driving waveform of the head module, and the conveyance speed of the print medium by the conveyance unit that affects the print density is corrected. Included as a correction item. Thereby, the inkjet printing apparatus can perform density correction with high accuracy.

  According to the third feature of the ink jet printing apparatus according to the present invention, the correction process can be easily executed during printing by executing the correction process based on the preset content of each correction item.

It is a block diagram which shows the structure of the inkjet printing apparatus which concerns on embodiment. It is a schematic block diagram of the conveyance part and inkjet head of the inkjet printing apparatus shown in FIG. It is a top view of a conveyance part and an inkjet head. It is a perspective view which shows the schematic structure of the head module in an inkjet head in a partial cross section. FIG. 5 is a partial cross-sectional view of the head module along the line AA in FIG. 4. It is a figure which shows the drive waveform of a head module. It is explanatory drawing of the discharge operation in a head module. It is explanatory drawing of the discharge operation in a head module. It is explanatory drawing which shows the dot formed on the paper when a satellite generate | occur | produces. It is explanatory drawing which shows the dot formed on the paper when a satellite generate | occur | produces. It is explanatory drawing of the influence on printing density by a satellite. It is a flowchart for demonstrating the operation | movement which determines the setting content of each correction item in a correction process. It is explanatory drawing which shows the mode of the landing of a main droplet and a satellite. It is explanatory drawing which shows the mode of the landing of the main droplet and a satellite in case a conveyance speed is slower than FIG. It is a figure which shows a test pattern print image. It is a flowchart for demonstrating the operation | movement at the time of printing. It is explanatory drawing of a straddle area | region.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Throughout the drawings, the same or equivalent parts and components are denoted by the same or equivalent reference numerals. However, it should be noted that the drawings are schematic and different from the actual ones. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Further, the embodiment described below exemplifies an apparatus or the like for embodying the technical idea of the present invention, and the technical idea of the present invention is to arrange the components and the like as follows. It is not something specific. The technical idea of the present invention can be variously modified within the scope of the claims.

  FIG. 1 is a block diagram showing a configuration of an inkjet printing apparatus according to an embodiment of the present invention, FIG. 2 is a schematic configuration diagram of a conveyance unit and an inkjet head of the inkjet printing apparatus shown in FIG. 1, and FIG. FIG. 4 is a perspective view showing a schematic configuration of the head module in the inkjet head in a partial cross-section, and FIG. 5 is a partial cross-sectional view of the head module along the line AA in FIG. . In the following description, the direction orthogonal to the paper surface of FIG. 2 is the front-rear direction, and the front surface direction is the front. Further, as shown in FIG. 2, when viewed from the front, the top, bottom, left, and right are the top, bottom, left, and right directions. Further, a path indicated by a broken line in FIG. 2 is a transport path R along which the paper P as a print medium is transported, and a direction from left to right is a transport direction.

  As shown in FIG. 1, the inkjet printing apparatus 1 according to the present embodiment includes a transport unit 2, inkjet heads 3 </ b> K, 3 </ b> C, 3 </ b> M, and 3 </ b> Y, a head drive unit 4, an image reading unit 5, and a control unit 6. With.

  The transport unit 2 transports the paper P in the transport direction. As shown in FIGS. 1 and 2, the transport unit 2 includes a transport belt 11, a driving roller 12, driven rollers 13 to 15, and a motor 16.

  The conveyor belt 11 is an annular belt that is stretched around the driving roller 12 and the driven rollers 13 to 15. A number of belt holes for attracting and holding the paper P are formed in the transport belt 11. The transport belt 11 sucks and holds the paper P on the transport surface (upper surface) by the suction force generated in the belt hole by driving a fan (not shown). The transport belt 11 rotates in the clockwise direction in FIG. 2 by driving the driving roller 12, thereby transporting the paper P sucked and held on the transport surface 11 a in the right direction. The conveyance surface 11 a is an upper surface of the conveyance belt 11 that is substantially horizontal between the driving roller 12 and the driven roller 13.

  The driving roller 12 and the driven rollers 13 to 15 are for the conveyance belt 11 to be stretched over. The driving roller 12 rotates the conveyor belt 11. The driven rollers 13 to 15 are driven by the driving roller 12 via the conveyor belt 11.

  The driven roller 13 is substantially the same height as the drive roller 12 and is spaced from the drive roller 12 by a predetermined distance in the left-right direction. The driven rollers 14 and 15 are disposed at substantially the same height below the driving roller 12 and the driven roller 13 and spaced apart from each other by a predetermined distance in the left-right direction.

  The motor 16 drives the drive roller 12 to rotate.

  The inkjet heads 3K, 3C, 3M, and 3Y print an image by ejecting ink onto the paper P conveyed by the conveyance unit 2. The inkjet heads 3K, 3C, 3M, and 3Y eject inks of colors K (black), C (cyan), M (magenta), and Y (yellow), respectively. The inkjet heads 3K, 3C, 3M, 3Y are arranged in parallel in the left-right direction above the transport unit 2.

  The inkjet heads 3K, 3C, 3M, and 3Y are line-type inkjet heads, each having a plurality of head modules 21A, 21B,. In the present embodiment, as shown in FIG. 3, each of the ink jet heads 3K, 3C, 3M, and 3Y includes six head modules 21A to 21F. In each of the inkjet heads 3K, 3C, 3M, 3Y, the six head modules 21A to 21F are arranged in a direction (front-rear direction) substantially orthogonal to the paper P transport direction (left-right direction), and every other one. The positions in the transport direction are shifted. In the present embodiment, the alphabetical suffixes in the reference numerals of the head modules 21A to 21F may be omitted and collectively described.

  The head module 21 is a share mode type. The head module 21 can change the number of ink droplets (drop number) ejected from one nozzle to one pixel, and prints the density expressed by the number of droplets (for example, 1 to 7 drops). Do. As shown in FIGS. 4 and 5, the head module 21 includes a plurality of ink chambers 22 and a nozzle plate 23.

The ink chamber 22 holds the supplied ink and discharges ink from nozzles 28 of a nozzle plate 23 described later. The plurality of ink chambers 22 are arranged side by side in the main scanning direction (front-rear direction). The partition wall 24 that separates the ink chambers 22 includes a first piezoelectric member 25 and a second piezoelectric member 26 that have opposite polarization directions. The first piezoelectric member 25 and the second piezoelectric member 26 are made of a known piezoelectric material such as PZT (PbZrO ₃ —PbTiO ₃ ), for example. An electrode 27 is formed in close contact with the surface of the partition wall 24 constituting the side surface of the ink chamber 22. When the drive voltage is applied to the electrode 27, the volume of the ink chamber 22 and the pressure in the ink chamber 22 are changed by shearing the partition wall 24. Thereby, ink in the ink chamber 22 is ejected from the nozzle 28.

  The nozzle plate 23 is a plate-like member in which a plurality of nozzles 28 communicating with each ink chamber 22 are formed. The nozzle plate 23 is disposed at the lower end of the ink chamber 22.

  The head driving unit 4 drives the inkjet heads 3K, 3C, 3M, and 3Y to discharge ink. Specifically, the head driving unit 4 applies a driving voltage to the electrode 27 of the head module 21, thereby deforming the partition wall 24 to change the volume of the ink chamber 22 and the pressure in the ink chamber 22, and the nozzle 28. Ink is ejected from the ink.

  The image reading unit 5 optically reads image data of a document and generates image data.

  The control unit 6 controls the operation of the entire inkjet printing apparatus 1. The control unit 6 includes a CPU, RAM, ROM, hard disk, and the like.

  At the time of printing, the control unit 6 generates image data in a drop data format corresponding to printing by the ink jet heads 3K, 3C, 3M, and 3Y from the print data, and based on this, the head driving unit 4 causes each ink jet head 3K to generate. , 3C, 3M, 3Y are driven. The drop data is data indicating the number of ink droplets ejected by the head modules 21A to 21F of the inkjet heads 3K, 3C, 3M, and 3Y for each pixel. The control unit 6 may receive image data in the drop data format from the outside, and drive the ink jet heads 3K, 3C, 3M, and 3Y by the head driving unit 4 based on the image data.

  When there is a straddling region corresponding to one drop of K (black) in the image data in the drop data format, the control unit 6 makes the print density by one drop of each of the head modules 21A to 21F of the inkjet head 3K substantially uniform. Correction processing is performed to The straddle region corresponding to one drop of K (black) is a region having a size larger than a predetermined size including pixels of one drop of K (black) at a ratio of a threshold (for example, 50%) or more, and the inkjet head 3K. This is a region straddling the print target region of the adjacent head module 21 in FIG. In such an area, as will be described later, density unevenness may be conspicuous in the printed image, and therefore correction processing described later is performed. The correction process is performed by the control unit 6 controlling the drive voltage V of the head modules 21A to 21F, the drive waveform of the head modules 21A to 21F, and the transport speed S by the transport unit 2.

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

  First, a basic printing operation will be described.

  At the time of printing, the control unit 6 drives the motor 16 to transport the paper P by the transport unit 2 at a predetermined transport speed (standard speed) according to the print resolution, and performs inkjet by the head drive unit 4 based on the image data. The heads 3K, 3C, 3M, 3Y are driven. In order to drive the inkjet heads 3K, 3C, 3M, and 3Y, the control unit 6 causes the head drive unit 4 to apply a voltage having a drive waveform as shown in FIG. Ink is ejected onto the paper P by driving the inkjet heads 3K, 3C, 3M, and 3Y, and an image is formed on the paper P.

  Here, the ejection operation in the head module 21 when the voltage as shown in FIG. 6 is applied by the head driving unit 4 will be described. A case where the central ink chamber 22 in FIG. 5 performs the ejection operation will be described.

  From the steady state shown in FIG. 5, the head drive unit 4 grounds the electrode 27 of the ink chamber 22 adjacent to both sides of the central ink chamber 22 to be ejected, and connects the electrode 27 of the central ink chamber 22 to the electrode 27 of FIG. An expansion pulse P1 having a negative voltage (−Va) shown in FIG. Then, an electric field in a direction perpendicular to the polarization direction of the first piezoelectric member 25 and the second piezoelectric member 26 constituting the partition wall 24 on both sides of the central ink chamber 22 is generated. As a result, the joint surface between the first piezoelectric member 25 and the second piezoelectric member 26 is deformed, and the partition walls 24 on both sides of the central ink chamber 22 are deformed in directions away from each other, as shown in FIG. The volume of the central ink chamber 22 is expanded. As a result, a negative pressure is generated in the ink in the central ink chamber 22, and the ink flows into the ink chamber 22 from an ink inlet (not shown).

  The pulse length (application time) of the extension pulse P1 is 1AL. AL (Acoustic Length) is a time until a pressure wave caused by the ink flowing into the ink chamber 22 whose volume is expanded propagates through the entire area of the ink chamber 22 and reaches the nozzle 28. AL is determined depending on the structure of the head module 21, the density of ink, and the like.

  After the application of the expansion pulse P1, the head driving unit 4 returns the voltage applied to the electrode 27 of the central ink chamber 22 to the ground potential as shown in FIG. As a result, the partition walls 24 on both sides of the central ink chamber 22 return from the state of FIG. 7 to the neutral position of FIG. As a result, the ink in the central ink chamber 22 is rapidly pressurized, and the ink is ejected from the corresponding nozzle 28.

  When the 1 AL rest time has elapsed after the voltage applied to the electrode 27 in the central ink chamber 22 is returned to the ground potential, the head drive unit 4 applies a contraction pulse of a positive voltage (Va) to the electrode 27 in the central ink chamber 22. P2 is applied. The pulse length of the contraction pulse P2 is 1AL. By applying the contraction pulse P2, as shown in FIG. 8, the partition walls 24 on both sides of the central ink chamber 22 are deformed so as to approach each other, and the volume of the central ink chamber 22 contracts.

  The ink is ejected when the pressure in the ink chamber 22 reaches a peak immediately after the application of the expansion pulse P1. After the pressure reaches a peak, a negative pressure is generated in the ink chamber 22. By applying the contraction pulse P2 to contract the volume in the ink chamber 22 to generate the applied pressure, the negative pressure in the ink chamber 22 after ink ejection is suppressed, and the residual vibration of the ink in the ink chamber 22 is reduced. Attenuated. Thereby, the next discharge operation can be performed stably.

  After applying the contraction pulse P2, the head driving unit 4 sets the voltage applied to the electrode 27 of the central ink chamber 22 to the ground potential, and returns to the state shown in FIG. As described above, a single (one drop) discharge operation is completed. When ejecting a plurality of droplets to one pixel, the voltage of the driving waveform in FIG. 6 is continuously applied to the electrode 27, and the above-described ejection operation is repeated.

  In a printed image, when the number of droplets in each pixel is small, the print density is easily affected by satellites. For example, in the case of printing with one drop for each pixel, as shown in FIG. 9, the area of the marginal portion around the dot 31 formed on the paper P by the main droplet is large. For this reason, the dot 32 by the satellite is formed in the margin part, so that the area occupied by the dot fluctuates greatly, and the print density is relatively greatly affected. On the other hand, for example, in the case of printing with 7 pixels for each pixel, as shown in FIG. 10, the dots 33 formed on the paper P by the seven main droplets occupy a large area. For this reason, the dot 32 by satellite has a large overlapping portion with the dot 33, and even if the dot 32 is formed in the margin, the change of the area occupied by the dot is small and the influence on the printing density is small.

  In the inkjet heads 3K, 3C, 3M, and 3Y including the plurality of head modules 21A to 21F, the size of the head gap (the distance between the lower surface of the head module 21 and the conveyance surface 11a) and the conveyance of the paper P are accompanied. The state of the airflow may be different between the head modules 21A to 21F. As a result, the amount of satellite generated may differ between the head modules 21A to 21F. If the amount of satellite generated differs between the head modules 21 </ b> A to 21 </ b> F, a difference in print density may occur between the head modules 21. For example, in FIG. 11, the print density of the head module 21 printed on the right side of the head module 21 printed on the right side is higher than that of the central one-dot chain line due to the occurrence of many satellites.

  Further, the difference in print density between the head modules 21 is conspicuous in K (black) ink which is a dark color.

  Therefore, as described above, the inkjet printing apparatus 1 performs a correction process for correcting the print density by one drop in the inkjet head 3K when there is a straddling region corresponding to one drop of K (black).

  The inkjet printing apparatus 1 determines in advance the setting contents of the driving voltage V of the head module 21, the driving waveform of the head module 21, and the conveyance speed S by the conveyance unit 2, which are correction items in the correction processing described above.

  The operation of determining the setting contents of each correction item in this correction process will be described with reference to the flowchart of FIG.

  First, in step S10 of FIG. 12, the control unit 6 prints a one-drop test pattern by the inkjet head 3K under the setting conditions of all combinations of setting candidates for each correction item. The 1-drop test pattern is a print pattern for discharging 1 drop to each pixel.

  Here, it is assumed that the setting candidates for the driving voltage V of the head module 21 are V1 to V3. The drive voltage V is the magnitude of the voltages of the expansion pulse P1 and the contraction pulse P2 in FIG. For example, when Va is a standard voltage (recommended voltage), V1 = Va−ΔV, V2 = Va, and V3 = Va + ΔV.

  Further, the driving waveform setting candidates of the head module 21 are the first waveform to the third waveform. For example, a drive waveform with an AL value of 0.8AL is a first waveform, a drive waveform (standard waveform) of 1AL is a second waveform, and a drive waveform of 1.2AL is a third waveform.

  In addition, the setting candidates for the conveyance speed S are S1 to S3. For example, when Sa is a standard speed, S1 = Sa−2ΔS, S2 = Sa−ΔS, and S3 = Sa.

  The control unit 6 prints the test pattern under the setting conditions of a total of 27 combinations obtained by combining the three setting candidates for each correction item described above. For example, the control unit 6 first prints a one-drop test pattern using the conveyance speed S1, the drive voltage of all the head modules 21A to 21F of the inkjet head 3K as V1, and the drive waveform as the first waveform. Next, the control unit 6 prints a one-drop test pattern with the transport speed S1, the drive voltage of all the head modules 21A to 21F of the inkjet head 3K as V1, and the drive waveform as the second waveform. In this way, the control unit 6 executes printing of a 1-drop test pattern under 27 setting conditions. Although the number of setting candidates for each correction item is three here, the number is not limited to this number.

  When the drive voltage V and the drive waveform (AL value) of the head module 21 change, the appropriate amount of liquid for one drop and the discharge speed change. As a result, the size of dots formed on the paper P and the occurrence of satellites change, and the printing density changes. Further, when the transport speed S is relatively fast, as shown in FIG. 13, the main droplet 35 and the satellite 36 are separated and land on the paper P. On the other hand, when the transport speed S is slowed, as shown in FIG. In addition, the satellite 36 overlaps with the main droplet 35 or lands on the vicinity of the main droplet 35. For this reason, the print density varies depending on the conveyance speed S.

  In addition, as described above, the amount of satellite generated in each of the head modules 21A to 21F may differ depending on the size of the head gap of each of the head modules 21A to 21F and the state of the airflow accompanying the conveyance of the paper P.

  Accordingly, the print density may be different between the head modules 21A to 21F even under the same setting condition, and the print density may be different when the setting condition is different even in the same head module 21.

  As shown in FIG. 15, test pattern printing images having densities corresponding to the head modules 21 </ b> A to 21 </ b> F under the setting conditions for each combination are obtained by printing the test patterns under the 27 setting conditions described above.

  When the printing of the test pattern is completed, the user performs an operation for causing the image reading unit 5 to read each printed matter corresponding to the setting condition of each combination. In response to this, the control unit 6 causes the image reading unit 5 to read the image of each printed matter in step S20. The image reading unit 5 outputs image data obtained by reading an image of each printed matter to the control unit 6.

  Next, in step S <b> 30, the control unit 6 detects the print density of each head module 21 </ b> A to 21 </ b> F under each combination setting condition based on the image data generated by the image reading unit 5. For example, the control unit 6 detects, from the image data, a ratio (dot area ratio) between the area of the portion where the dots are formed and the area of the blank portion (dot area ratio) in the print region of each of the head modules 21A to 21F as the print density. .

  Next, in step S40, the setting content of each correction item in the correction process is determined based on the print density of each head module 21A to 21F in the setting condition of each combination detected in step S30. Specifically, the control unit 6 determines the setting contents of the correction items corresponding to the head modules 21A to 21F so that the print densities of all the head modules 21A to 21F are substantially uniform.

  For example, it is assumed that the test pattern print images for each of the head modules 21A to 21F surrounded by a thick line frame in FIG. In this case, the control unit 6 determines, for each of the head modules 21A to 21F, the drive voltage V and the drive waveform corresponding to the test pattern print image surrounded by the thick line frame as the setting contents of the correction item in the correction process. Further, the conveyance speed S1 is determined as the setting content of the conveyance speed S in the correction process. Here, the conveyance speed S in the correction process is determined so as to be common to the head modules 21A to 21F. That is, the control unit 6 corrects the drive voltage V and the drive waveform corresponding to the test pattern print images having substantially the same density in all the head modules 21A to 21F in the test pattern print images having the same transport speed S. The setting contents of the correction item are determined. This is because the conveyance speed S during printing cannot be individually changed for the head modules 21A to 21F.

  In addition, when there are a plurality of combinations of correction item setting contents so that the test pattern print images in the head modules 21A to 21F have substantially the same density, the control unit 6 determines the combination that minimizes the density difference. To do.

  When the setting content of each correction item in the correction process is determined, the control unit 6 stores the setting content in a hard disk or the like. The processing of the flowchart of FIG. 12 may be performed, for example, every predetermined period to update the setting content of each correction item.

  When the inkjet printing apparatus 1 can print at a plurality of print resolutions, a standard speed Sa of the transport speed S is provided for each print resolution. In this case, the control unit 6 executes test pattern printing for each printing resolution under the setting conditions of each combination as described above, and determines the setting content of each correction item in the correction processing.

  Next, the operation during printing will be described with reference to the flowchart of FIG.

  In step S110 of FIG. 16, the control unit 6 determines whether or not a straddle region corresponding to one drop of K (black) exists in the image data in the drop data format to be printed.

  As described above, the straddle region corresponding to one drop of K (black) is a region having a predetermined size or more including pixels of one drop of K (black) at a ratio equal to or higher than a threshold, and This is an area that straddles the print target area of the adjacent head module 21. As shown in FIG. 17, if the areas printed by the head modules 21A to 21F in the image data are the print target areas 41A to 41F, the straddle area corresponding to one drop of K (black) is, for example, the area 42 It exists in such a position. When the region 42 is a straddling region corresponding to one drop of K (black), one drop of K (black) is included in the entire region 42 at a ratio equal to or higher than the threshold value.

  In order to detect the straddle region corresponding to one drop of K (black), for example, the control unit 6 sets a square region of a predetermined size as the attention region in the image data. The control unit 6 calculates the ratio Rb1 of one drop pixel of K (black) in the attention area while scanning the attention area. When the ratio Rb1 is equal to or greater than the threshold and the boundaries of the print target areas 41A to 41F are included in the attention area, the control unit 6 determines that there is a straddling area corresponding to one drop of K (black). .

  When there is a straddle region corresponding to one drop of K (black) in the image data, there is a possibility that density unevenness occurs in the print image in the straddle region. For example, when the area 42 in FIG. 17 is a straddle area corresponding to one drop of K (black), there is a difference in print density between the part belonging to the print target area 41B and the part belonging to the print target area 41C. And density unevenness may occur.

  Therefore, when it is determined that a straddling region corresponding to one drop of K (black) exists (step S110: YES), the control unit 6 executes correction printing in step S120. Correction printing is printing performed while performing the above-described correction processing.

  Specifically, the control unit 6 causes the transport unit 2 to transport the paper P at the transport speed S in the correction process determined in advance as described above. Further, the control unit 6 causes the head driving unit 4 to drive the head modules 21A to 21F of the inkjet heads 3K, 3C, 3M, and 3Y based on the image data, and discharges ink onto the conveyed paper P. As a result, an image is printed on the paper P. Here, at the time of discharging one drop of K (black), the control unit 6 drives the head modules 21A to 21F of the inkjet head 3K with the driving voltage V and the driving waveform in the correction process determined in advance as described above. For ejection other than one drop of K (black), the controller 6 drives the head modules 21A to 21F of the inkjet heads 3K, 3C, 3M, and 3Y with the standard voltage Va and the standard waveform.

  Ink ejection in the head module 21 is performed for each line in the main scanning direction, but one drop pixel and two or more drop pixels may coexist in the same line in one head module 21. . In this case, the control unit 6 drives the ink chamber 22 corresponding to the one-drop ejection nozzle 28 with the driving voltage V and the driving waveform in the correction process, and the ink chambers 22 corresponding to the other nozzles 28 have the standard voltage Va. And drive with standard waveform.

  If it is determined that there is no straddle region corresponding to one drop of K (black) (step S110: NO), in step S130, the control unit 6 executes normal printing that is not correction printing. In normal printing, the control unit 6 drives the head modules 21A to 21F of the inkjet heads 3K, 3C, 3M, and 3Y with the standard voltage Va and the standard waveform while transporting the paper P by the transport unit 2 at the standard speed Sa. Let

  The control unit 6 performs correction printing or normal printing for each page by the processing of the flowchart of FIG.

  Here, in the image data to be printed, even if there is an area of a predetermined size or more including one (1) drop pixel of K (black) at a ratio equal to or greater than the threshold value, the area is not a straddle area but straddles another. When there is no area, normal printing is performed. For example, it is assumed that the area 43 in the print target area 41D and the area 44 in the print target area 41E shown in FIG. 17 are solid areas of 1-drop pixels of K (black). In this case, the print density may be different between the region 43 and the region 44. However, since the region 43 and the region 44 do not straddle the plurality of head modules 21, density unevenness is not recognized. Therefore, the control unit 6 does not perform the correction process if there is no straddle area even if areas such as the areas 43 and 44 exist.

  As described above, in the inkjet printing apparatus 1, when there is a straddling region corresponding to one drop of K (black) in the image data to be printed, one drop of each of the head modules 21A to 21F of the inkjet head 3K is performed. Correction processing for making the printing density substantially uniform is executed. Thereby, the inkjet printing apparatus 1 can suppress the occurrence of density unevenness in the printed image and can suppress the deterioration of the print image quality.

  Further, in the inkjet printing apparatus 1, in the correction process, the drive voltage V of the head module 21 that affects the print density, the drive waveform of the head module 21, and the transport speed S of the paper P by the transport unit 2 are set as correction items. Thereby, the inkjet printing apparatus 1 can perform density correction with high accuracy.

  Further, in the inkjet printing apparatus 1, the correction process is executed based on the setting content of each correction item determined in advance so that the print density by one drop of each of the head modules 21A to 21F of the inkjet head 3K is substantially uniform. Thereby, the inkjet printing apparatus 1 can perform a correction process easily at the time of printing.

  In the present embodiment, when there is a straddle region corresponding to one drop of K (black) in the image data, a correction process for correcting the print density by one drop of K (black) is performed. . However, the number of drops to be corrected is not limited to one drop, but may be other drop numbers.

  Also, a plurality of drop numbers (for example, 1 drop and 2 drops) may be targeted for correction. In this case, the control unit 6 determines and stores the setting contents of each correction item in advance by printing a test pattern for each number of correction target drops. At this time, the transport speed S is determined so as to be common for each number of drops. At the time of printing, the control unit 6 determines the presence or absence of a corresponding straddle region for each correction target drop number in the image data. And the control part 6 performs the correction process with respect to the number of drops in which a corresponding straddle area exists.

  In addition, colors other than K (black) may be corrected. In this case, for each color to be corrected, the control unit 6 predetermines and stores the setting contents of each correction item by printing a test pattern for each correction target drop number. At this time, the transport speed S is determined so as to be common to each color and each drop number. At the time of printing, the control unit 6 determines whether or not there is a corresponding straddle region for each correction target color for each correction target drop number in the image data. And the control part 6 performs the correction process with respect to the color (inkjet head) in which a corresponding straddle area | region exists, and the number of drops.

  Further, the correction processing of the present embodiment may be applied to an inkjet printing apparatus having only one color (for example, K (black)) inkjet head.

  Further, any one or two of the three correction items (drive voltage V, drive waveform, transport speed S) in the present embodiment may be omitted. Thereby, it is possible to suppress a decrease in print image quality while simplifying the correction process. Other correction items may be adopted.

  In addition to the above-described correction process, a process for adjusting the density by changing the number of drops in the image data may be performed.

  In the present embodiment, the control unit 6 determines whether or not there is a straddling region for each page and executes the correction process for each page. However, the unit of the correction process is not limited to this. For example, the control unit 6 may determine the presence / absence of a straddling area in units of print jobs, and may perform correction processing in units of print jobs.

  It goes without saying that the present invention includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

DESCRIPTION OF SYMBOLS 1 Inkjet printing apparatus 2 Conveyance part 3K, 3C, 3M, 3Y Inkjet head 4 Head drive part 5 Image reading part 6 Control part 21A-21F Head module 22 Ink chamber 23 Nozzle plate 24 Partition 27 Electrode 28 Nozzle

Claims (3)

A transport unit that transports the print medium in the transport direction;
It has a plurality of head modules that are arranged in a direction substantially perpendicular to the transport direction and ejects ink droplets onto a print medium that is transported by the transport unit, and depends on the number of ink droplets ejected to each pixel. An inkjet head that performs printing to express density;
A controller that controls the transport unit and the inkjet head,
The control unit is an area having a predetermined size or more including pixels having a predetermined number of droplets at a ratio equal to or higher than a threshold in the image data to be printed, and straddling the print target area of the adjacent head module. An ink jet printing apparatus that executes a correction process for making the print density of each head module according to the predetermined number of droplets substantially uniform when there is a straddle region.   2. The inkjet according to claim 1, wherein the correction process includes at least one of a driving voltage of the head module, a driving waveform of the head module, and a conveyance speed of the print medium by the conveyance unit as a correction item. Printing device.   2. The control unit according to claim 1, wherein the control unit executes the correction process based on a setting content of each correction item that is determined in advance so that a print density according to the predetermined number of droplets of each head module is substantially uniform. 2. An ink jet printing apparatus according to 2.