JP2015174234A - Printer and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a printer capable of preventing a boundary portion of a printing path from being conspicuous when performing printing by a multipath system.SOLUTION: A printer includes a head part for performing printing to a medium by a multipath system, and a control part. A printing density with which printing is performed in an initial printing path by k times is set to be lower than a printing density with which printing is performed in a (k+1)th printing path; and in a nozzle row 204 of the head part, a printing density with which each of a plurality of nozzles discharging ink droplets for a k-th printing path performs printing, is set to be higher gradually toward a head rear end side.

Description

  The present invention relates to a printing apparatus and a printing method.

  Conventionally, ink jet printers that perform printing by an ink jet method have been widely used. Further, as a method of performing printing with an inkjet printer, for example, a method of performing printing in a multi-pass method in which printing is performed with a plurality of printing passes at each position of a medium (media) is known (for example, non-patent literature). 1).

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  When printing with a multi-pass method in an inkjet printer, stripes, streaks, or the like may occur at the boundary of the print pass. As a result, the print quality may deteriorate. For this reason, conventionally, it has been desired to perform printing by a more appropriate method by appropriately suppressing stripes and streaks generated at the boundary portion of the print path. Accordingly, an object of the present invention is to provide a printing apparatus and a printing method that can solve the above-described problems.

  In an ink jet printer, the state of a printing result is determined according to various conditions. For this reason, for example, even if a printing defect having a certain state occurs, it is not easy to determine the cause.

  On the other hand, the inventor of the present application has, due to diligent research, regarding the stripes and streaks that occur at the boundary portion of the print path, because the density difference of the print density is abruptly generated at the boundary portion of the print path. I found out. Therefore, the inventor of the present application firstly performs printing at a lower density for a printing pass that performs printing first for each position of the medium among a plurality of printing passes that are printed by the multipass method. Thought. With this configuration, for example, it is possible to suppress changes in density at the boundary portion of the print pass. Therefore, the inventor of the present application initially thought that such a configuration can appropriately suppress stripes, streaks, and the like.

  However, the inventor of the present application has shown that the boundary of the printing pass may become conspicuous only by making the density of the first printing pass, etc., lower than the density of other printing passes, by further earnest research. I found it. In addition, it has been found that the cause of the change is greatly related to how the density of the printing pass is changed. More specifically, for example, when the density of each print pass is simply changed in units of print passes, the print density is stepped between the first print pass and the next print pass. Will change. However, in the printing result by the ink jet method, for example, when the density greatly changes across a specific boundary, the boundary becomes conspicuous. For this reason, it is considered that the boundary of the printing pass becomes conspicuous if the density of the first printing pass or the like is simply set to a lower density than the other printing passes.

  Therefore, the inventor of the present application has considered that the density of the printing pass is further changed gradually in the printing pass instead of simply changing the printing pass in units of steps. In addition, it has been found that by changing the density in this way, the boundary of the print path is prevented from being noticeable, and printing can be performed more appropriately. In order to solve the above problems, the present invention has the following configuration.

  (Configuration 1) A printing apparatus that performs printing by an inkjet method, and includes a head unit having a nozzle row in which a plurality of nozzles that eject ink droplets to a medium are arranged, and ink droplets while moving in a preset main scanning direction. A main scanning drive unit that causes the head unit to perform a main scanning operation to be ejected, a sub-scanning driving unit that moves the head unit relative to the medium in a sub-scanning direction orthogonal to the main scanning direction, and a main unit by the head unit A plurality of nozzles arranged in the sub-scanning direction in the nozzle row of the head unit, and the head unit performs a plurality of main scanning operations on the same region of the medium. Performing printing on a medium in a multi-pass method, and performing a main scanning operation corresponding to each of N preset printing passes (N is an integer of 2 or more) for the same area on the medium; Control unit At least the density of printing performed in the first k printing passes (k is an integer less than or equal to 1 and less than N) among the N printing passes performed on the same area on the medium is set to the (k + 1) th printing. Ink droplets for the Nth printing pass from nozzles that discharge ink droplets for the first printing pass in N printing passes in the nozzle row of the head unit, and lower than the density of printing performed in the pass. If the direction toward the nozzle that discharges ink is the head rear end side, the density of printing performed by each of the plurality of nozzles that discharge ink droplets for the kth printing pass in the nozzle row of the head portion is Set gradually higher toward the rear end.

  When configured in this way, for example, the density of the first k printing passes including the first printing pass can be appropriately set to a lower density than the subsequent printing passes. In addition, for example, a change in density at the boundary portion of the print pass can be appropriately suppressed.

  Further, when configured in this way, for example, for the k-th print pass in which the print density is lowered as compared with the immediately subsequent print pass, the density of the print pass is not reduced uniformly, but the density of the print pass is not reduced. The density of printing performed by each of the plurality of nozzles that discharge the ink droplets is gradually set higher toward the head rear end side. Thereby, for example, it is possible to appropriately prevent the printing density from changing greatly in a stepwise manner in units of print passes at the boundary between the kth print pass and the (k + 1) th print pass.

  Therefore, with this configuration, for example, stripes and streaks that occur at the boundary portion of the print path can be appropriately suppressed. In addition, for example, when printing is performed by the multi-pass method, it is possible to prevent the boundary of the print path from being noticeable and perform printing more appropriately.

  Note that the density of printing performed by each of the plurality of nozzles that eject ink droplets for the kth printing pass is set to be gradually higher toward the head rear end side, for example, toward the head rear end side. The printing density corresponding to each nozzle is set so that the density becomes high. In this case, the density is not necessarily different for all nozzles, but the same density as that of the adjacent nozzles may be set for some nozzles, for example. For example, the printing density by each nozzle may be gradually changed in units of a plurality of preset nozzles. Further, the printing density by each nozzle may be gradually changed more finely in units of one nozzle.

  (Configuration 2) The head unit ejects ink droplets of ultraviolet curable ink from the nozzle. If comprised in this way, when using an ultraviolet curable ink, for example, the printing by an inkjet system can be performed more appropriately with high precision.

  (Configuration 3) The control unit at least sets the density of printing performed in the first printing pass among the N printing passes performed on the same area on the medium to be higher than the density of printing performed in the second printing pass. The density of printing performed by each of a plurality of nozzles that discharge ink droplets for the first printing pass in the nozzle row of the head unit is gradually set higher toward the head rear end side. .

  In order to appropriately suppress stripes, streaks, and the like that occur at the boundary portion of the print pass, it is particularly preferable to reduce the print density for the first print pass in which printing is first performed on the medium. Therefore, with this configuration, for example, stripes and streaks that occur at the boundary portion of the print path can be more appropriately suppressed.

  (Configuration 4) The control unit has a printing density performed by each of the plurality of nozzles in the nozzle row of the head unit in a direction opposite to the head rear end side with the central portion of the nozzle row in the sub-scanning direction as a center. The density change method is set to be symmetrical between the direction toward the head front end and the direction toward the head rear end.

  When printing by the multi-pass method, it is necessary to match the density obtained by summing up the printing densities of the respective printing passes to a predetermined density set in advance at each position on the medium. Therefore, for example, when the density of one of the print passes is lowered, it is necessary to increase the density of the density of the other print passes by that amount. In addition, the density setting is not simply set for each printing pass, but when the printing density by a plurality of nozzles that eject ink droplets for any printing pass is set to gradually change, In other printing passes, it is necessary to set the density so as to compensate for this change.

  However, the concentration setting for performing such complementation is not always easy and may be complicated. For example, when the printing density by each nozzle is gradually changed, it may be difficult to match the total printing density by a plurality of printing passes.

  On the other hand, when configured as in configuration 4, for example, by giving symmetry to the density change method, the print density by each nozzle is appropriately set between the head rear end side and the head front end side. Can be complemented. For this reason, if configured in this way, for example, the printing density of the first printing pass or the like can be set more appropriately.

  (Configuration 5) The control unit is configured such that the density of printing performed by the nozzles in the central portion of the nozzle row in the sub-scanning direction is higher than the density of printing performed by the nozzles at the end of the nozzle row, and gradually increases as the distance from the central portion increases. The density of printing performed by each of the plurality of nozzles is set so as to be low. With this configuration, for example, a low density can be appropriately set for the printing density of the first printing pass or the like.

  In this configuration, the head unit may include a plurality of inkjet heads arranged in a staggered shape. In this case, each of the plurality of inkjet heads has a nozzle row in which nozzles are arranged in the sub-scanning direction, for example. In this case, the nozzle row of the head unit may be, for example, a nozzle row in which nozzle rows in each of the plurality of inkjet heads are virtually connected in the sub-scanning direction.

  (Configuration 6) The control unit sets the printing density to the same density for a plurality of nozzles arranged continuously including the nozzle in the central portion in the nozzle row, and the plurality of nozzles arranged continuously in the central portion The density of printing performed by each of a plurality of other nozzles is set so that the density gradually decreases as the distance from the center portion increases.

  For example, when the graph is drawn with the position of the nozzles in the nozzle row as the horizontal axis and the printing density corresponding to each nozzle as the vertical axis, the density is changed to a trapezoid. It is a setting. With this configuration, for example, a low density can be appropriately set for the printing density of the first printing pass or the like.

  In the case of providing symmetry in the density change method, for example, if the printing density corresponding to the nozzle at the end of the nozzle row is lowered, the corresponding density is complemented, so that it corresponds to the nozzle in the central portion of the nozzle row. The concentration becomes higher. In particular, for example, when the highest density (peak density) is set for only one nozzle in the central portion, the high density areas are concentrated in one area. In this case, for example, if a medium in which ink dots are likely to spread is used, bleeding or unevenness may occur easily.

  On the other hand, when configured in this way, for example, the nozzle that performs printing at the peak density is not only one nozzle but a plurality of nozzles in the central portion. Therefore, if comprised in this way, it can prevent appropriately that a location with a high density | concentration concentrates on one place, for example. Accordingly, for example, even when a medium in which ink dots are easily spread is used, bleeding, unevenness, and the like can be appropriately suppressed.

  In this configuration, the plurality of nozzles arranged continuously including the nozzles in the central portion may include, for example, all the nozzles assigned to the printing pass performed by the nozzles in the central portion among N printing passes. . Further, all nozzles assigned to a plurality of continuous printing passes including a printing pass performed by the nozzles in the central portion may be included. If comprised in this way, the setting of the density | concentration of the several nozzle which arranges continuously including the nozzle of a center part can be performed more simply and appropriately, for example.

  (Configuration 7) The head unit includes a plurality of inkjet heads arranged in a staggered shape, each of the plurality of inkjet heads includes a nozzle row in which nozzles are arranged in the sub-scanning direction, and the control unit includes each inkjet head. As for the density of printing performed by a plurality of nozzles included in the nozzle row, the density of printing performed by the nozzles in the central portion of the nozzle row in the sub-scanning direction is high, and the density gradually decreases as the distance from the central portion increases. Set. With this configuration, for example, a low density can be appropriately set for the printing density of the first printing pass or the like.

  Further, in an inkjet head, the nozzles at the end of the nozzle row are usually more likely to be displaced in the landing position than the nozzle at the center of the nozzle row. On the other hand, when configured in this manner, for example, in each of the staggered inkjet heads, the density of printing by the nozzles is set to be low for the nozzles at the end of the nozzle row. Therefore, for example, for each inkjet head, the influence of the nozzles at the end of the nozzle row can be appropriately reduced. This also makes it possible to appropriately suppress the influence on the printing result even when, for example, a landing position shift occurs in the nozzles at the end of the nozzle row. Therefore, with this configuration, for example, the density of each printing pass can be set more appropriately according to the configuration of a plurality of inkjet heads arranged in a staggered manner.

  (Configuration 8) A printing method for performing printing by an ink jet method, in which ink droplets are moved while moving in a preset main scanning direction on a head portion having a nozzle row in which a plurality of nozzles that eject ink droplets to a medium are arranged. A main scanning operation for discharging and a sub-scanning operation that moves relative to the medium in a sub-scanning direction orthogonal to the main scanning direction are performed, and a plurality of nozzles in the nozzle row of the head section move in the sub-scanning direction. By controlling the main scanning operation by the head unit, the head unit performs printing on the medium by a multi-pass method in which the main scanning operation is performed a plurality of times on the same area of the medium, and the medium The main scanning operation corresponding to each of N preset printing passes (N is an integer equal to or greater than 2) is performed on the same area in FIG. Among the N print passes performed for the same area, the print density performed in the first k print passes (k is an integer of 1 or more and less than N) is the print performed in the (k + 1) th print pass. And a nozzle that ejects ink droplets for the Nth printing pass from a nozzle that ejects ink droplets for the first printing pass in N printing passes in the nozzle row of the head unit. When the heading direction is the head rear end side, the density of printing performed by each of a plurality of nozzles that eject ink droplets for the k-th printing pass in the nozzle row of the head portion is the head rear end side. Set gradually higher towards. If comprised in this way, the effect similar to the structure 1 can be acquired, for example.

  According to the present invention, for example, when printing is performed by a multi-pass method, it is possible to prevent printing path boundaries from being noticeable and perform printing more appropriately.

1 is a diagram illustrating an example of a printing apparatus 10 according to an embodiment of the present invention. FIGS. 1A and 1B are a front view and a top view illustrating an example of a configuration of a main part of the printing apparatus 10. 3 is a diagram illustrating an example of a configuration of a head unit 12. FIG. FIG. 2A shows an example of the entire configuration of the head unit 12 together with the ultraviolet irradiation unit 20. FIG. 2B shows an example of the configuration of a plurality of inkjet heads 202 that eject ink droplets of the same color ink in the head unit 12. FIG. 10 is a diagram illustrating an example of a print density setting for each print pass. It is a figure which shows an example of the state after performing one main scanning operation | movement. FIG. 4A is a diagram illustrating an example of density setting. FIG. 4B shows a printing result when one main scanning operation is performed. FIG. 6 is a diagram illustrating an example of a state after performing a main scanning operation for the number of printing passes in a multi-pass method. FIG. 5A is a diagram for explaining a printing operation in the multi-pass method. FIG. 5B shows a printing result when the main scanning operation for the number of printing passes is performed in the multi-pass operation. It is a figure explaining the problem of the blur which may arise according to the characteristic of a medium. FIG. 6A is a diagram illustrating an example of the setting of the printing density by each nozzle. FIG. 6B is a photograph showing a state where local bleeding has occurred on the medium. An example of a configuration that lowers the discharge density of ink droplets at a density peak will be described regarding a modification example of density setting. FIG. 7A shows a first modification of density setting. FIG. 7B shows a second modification example regarding density setting. FIG. 7C shows a third modification for setting the density. It is a figure explaining density setting in the case of using a stagger head. FIG. 8A shows an example in which the density is set so that the density peak is at the center of the entire stagger head. FIG. 8B is a diagram illustrating a fourth modification example regarding density setting. FIG. 6 is a diagram for explaining the influence of printing density at both ends of an inkjet head. FIG. 9A shows an example of a print result when the same density is set for all nozzles. FIG. 9B is an example of a print result when the density is set as in the fourth modification. It is a figure which shows an example of the result of having printed using the mask with a low spatial frequency.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. FIG. 1 shows an example of a printing apparatus 10 according to an embodiment of the present invention. FIGS. 1A and 1B are a front view and a top view illustrating an example of a configuration of a main part of the printing apparatus 10. In this example, the printing apparatus 10 is an inkjet printer that performs printing by an inkjet method, and includes a head unit 12, a main scanning drive unit 14, a sub-scanning drive unit 16, a platen 18, an ultraviolet irradiation unit 20, and a control unit 22. . Except for the points described below, the printing apparatus 10 may have the same or similar configuration as a known inkjet printer. For example, except for the points described below, each of the above-described configurations may have the same or similar characteristics as a known inkjet printer. The printing apparatus 10 may further include other configurations that are the same as or similar to those of a known inkjet printer, in addition to the above-described configurations.

  The head unit 12 is a portion having a nozzle row in which a plurality of nozzles that eject ink droplets are arranged, and performs printing on the medium 50 by ejecting ink droplets onto the medium (medium) 50 to be printed. In this example, the head unit 12 includes a plurality of inkjet heads, and ejects ink droplets of ultraviolet curable ink onto the medium 50 from each nozzle of the nozzle row of each inkjet head. A more specific configuration of the head unit 12 will be described in detail later.

  The main scanning drive unit 14 is configured to cause the head unit 12 to perform a main scanning operation for ejecting ink droplets while moving in a preset main scanning direction (Y direction in the drawing). In this case, causing the head unit 12 to perform the main scanning operation means, for example, causing the inkjet head in the head unit 12 to perform the main scanning operation. In this example, the main scanning drive unit 14 includes a carriage 102 and a guide rail 104. The carriage 102 holds the head unit 12 in a state where the nozzle row faces the medium 50. The guide rail 104 is a rail that guides the movement of the carriage 102 in the main scanning direction, and moves the carriage 102 in the main scanning direction in accordance with an instruction from the control unit 22. In this example, the main scanning drive unit 14 causes the head unit 12 to perform a main scanning operation in each of a forward direction set in advance in the main scanning direction and a return direction opposite to the forward direction.

  The sub-scanning drive unit 16 is configured to cause the head unit 12 to perform a sub-scanning operation that moves relative to the medium 50 in the sub-scanning direction (X direction in the drawing) orthogonal to the main scanning direction. In this case, causing the head unit 12 to perform the sub-scanning operation means, for example, causing the inkjet head in the head unit 12 to perform the sub-scanning operation. Further, in this example, the sub-scan driving unit 16 is a roller that transports the medium 50, and causes the head unit 12 to perform the sub-scanning operation by transporting the medium 50 between main scanning operations. In this case, more specifically, the sub-scanning drive unit 16 determines the interval between the main scanning operation in the forward direction and the main scanning operation in the backward direction, the main scanning operation in the backward direction, and the forward direction. In each interval between the main scanning operations, the head unit 12 is moved relative to the medium 50 by a preset printing pass width.

  The configuration of the printing apparatus 10 is, for example, a configuration in which the sub-scanning operation is performed by moving the inkjet head side with respect to the medium 50 whose position is fixed without conveying the medium 50 (for example, XY). It is also possible to use a table type machine. In this case, as the sub-scanning driving unit 16, for example, a driving unit that moves the ink jet head by moving the guide rail 104 in the sub-scanning direction can be used.

  The platen 18 is a table-like member on which the medium 50 is placed, and supports the medium 50 so as to face the head portion 12. The ultraviolet irradiation unit 20 is an ultraviolet light source that irradiates ultraviolet rays onto the ink dots formed on the medium 50. As the ultraviolet irradiation unit 20, for example, a UVLED can be suitably used. The ultraviolet irradiation unit 20 is held by the carriage 102 together with the head unit 12 and moves together with the head unit 12 during the main scanning operation. Thereby, the ultraviolet irradiation unit 20 cures the ink on the medium 50 during the main scanning operation.

  Moreover, in this example, the ultraviolet irradiation part 20 is arrange | positioned at the both sides of the head part 12 in the main scanning direction. In the main scanning operation in each of the forward direction and the backward direction, the ultraviolet irradiation unit 20 on the rear side of the head unit 12 in the moving direction of the head unit 12 irradiates the ink on the medium 50 with ultraviolet rays. . If necessary, for example, ultraviolet light may be irradiated from the front ultraviolet irradiation unit 20 in addition to the rear ultraviolet irradiation unit 20. More specifically, for example, in a case where the total irradiation amount is satisfied by irradiating ultraviolet rays with a plurality of ultraviolet irradiation units 20, it is considered to irradiate ultraviolet rays from the front ultraviolet irradiation unit 20 as well. It is done.

  The control unit 22 is a CPU of the printing apparatus 10, for example, and controls the operation of each unit of the printing apparatus 10 according to an instruction from the host PC, for example. Accordingly, the control unit 22 causes the head unit 12 to perform a main scanning operation, a sub scanning operation, and the like.

  More specifically, in this example, the control unit 22 causes the printing apparatus 10 to perform a multi-pass printing operation. Further, the control unit 22 can set the density level and the gradient for each nozzle in the nozzle row with respect to the density at which the ink droplets are ejected from the inkjet head. Accordingly, the control unit 22 sets the density to be printed in each printing pass in the multi-pass printing operation. This density setting will be described in more detail later.

  Except for the points described above and below, the control unit 22 performs the same or similar operation as, for example, the control unit in a conventional inkjet printer. For example, the control unit 22 may receive an image to be printed from the host PC and perform image forming processing such as RIP processing. Further, according to the image formed by the image forming process, the control unit 22 determines an operation to be performed in each multi-pass printing pass, for example.

  With the above configuration, according to this example, for example, it is possible to appropriately print each area of the medium 50 by the multipass method. Further, in this case, by performing the sub-scanning operation after the main scanning operation in each of the forward path and the backward path, ink dots are formed on the same area of the medium 50 by different nozzles in the head portion in each of the forward path and the backward path. can do. Therefore, according to the present example, for example, the nozzle characteristics can be more appropriately uniformed, and printing with high accuracy can be performed more appropriately.

  Next, a more specific configuration of the head unit 12 will be described in detail. FIG. 2 shows an example of the configuration of the head unit 12. FIG. 2A shows an example of the entire configuration of the head unit 12 together with the ultraviolet irradiation unit 20. FIG. 2B shows an example of the configuration of a plurality of inkjet heads 202 that eject ink droplets of the same color ink in the head unit 12.

  In this example, the head unit 12 is a color printing head unit that ejects ink droplets of each of a plurality of colors (for example, CMYK colors), and the ultraviolet irradiation unit 20 on one side and the other side in the main scanning direction. In between, a plurality of inkjet heads 202 for each color are provided. The plurality of inkjet heads 202 for each color are arranged in a staggered shape. The plurality of inkjet heads 202 arranged in a staggered manner means, for example, that they are arranged in the sub-scanning direction while alternately shifting their positions in the main scanning direction as shown in the figure. Further, as shown in the drawing, the inkjet heads 202 having different colors are arranged side by side in the main scanning direction with the corresponding inkjet heads 202 of other colors aligned in the sub-scanning direction. The arrangement of the inkjet heads 202 for each color may be, for example, a color stagger arrangement.

  In this example, each inkjet head 202 has a nozzle row 204 in which nozzles are arranged in the sub-scanning direction. In this case, for example, as shown in FIG. 2B, the nozzle rows 204 in the plurality of inkjet heads 202 for the same color are shifted in the main scanning direction according to the positions of the inkjet heads 202 while moving in the sub-scanning direction. line up. Therefore, when only the positions in the sub-scanning direction are viewed for each nozzle row 204, it can be considered that they are aligned in a straight line as shown on the right side of FIG. In this case, a nozzle row 206 in which the nozzle rows 204 in each of the plurality of inkjet heads 202 for the same color are virtually connected in the sub-scanning direction is considered, and this nozzle row 206 is considered as the nozzle row of the head unit 12. Can do. Therefore, hereinafter, the nozzle row 206 in which the nozzle row 204 is virtually connected in the sub-scanning direction is referred to as the nozzle row 206 of the head unit 12.

  In FIG. 2, for convenience of explanation, a configuration in the case of having three inkjet heads 202 for each color used for printing is shown. However, the number of inkjet heads 202 for each color may be other than three. For example, the number of inkjet heads 202 for each color may be one. The head unit 12 may further include one or a plurality of inkjet heads 202 for other colors (for example, colors other than CMYK). For example, the head unit 12 may further include a part or all of the inkjet heads 202 of each color such as W (white), CL (clear), and PR (primer) in addition to each color of CMYK.

  Next, regarding the printing operation performed by the multi-pass method, the setting of the printing density for each printing pass will be described. In this example, the printing apparatus 10 performs a main scanning operation corresponding to each of N preset printing passes (N is an integer of 2 or more) for the same area in the medium 50 (see FIG. 1). Do. In this case, the plurality of nozzles 208 arranged in the nozzle row 206 of the head unit 12 function as nozzles 208 that eject ink droplets for each printing pass from the head front end side toward the head rear end side. In this case, the head rear end side is a direction from the nozzle that ejects ink droplets for the first printing pass to the nozzle that ejects ink droplets for the Nth printing pass. The head front end side is the side opposite to the head rear end side. N may be an integer of 3 or more.

  FIG. 3 shows an example of setting the print density for each print pass. In the case illustrated in FIG. 3, the printing apparatus 10 performs printing in 12 printing passes. In this case, as shown in the figure, the nozzles 208 in the nozzle row 206 of the head unit 12 are assigned for the respective first to twelfth print passes from the head front end side to the head rear end side.

  As described with reference to FIG. 2, in this example, the nozzle row 206 of the head unit 12 is composed of the nozzle rows 204 of the three inkjet heads 202. Therefore, in this case, more specifically, the nozzles in the nozzle row 204 of the inkjet head 202 located closest to the front end of the head are assigned for the respective first to fourth printing passes. Further, the nozzles in the nozzle row 204 of the inkjet head 202 that is second from the head front end side are assigned for the respective printing passes of the fifth pass to the eighth pass. Then, the nozzles in the nozzle row 204 of the inkjet head 202 located closest to the head rear end are assigned for the respective 9th to 12th printing passes.

  In FIG. 3, for convenience of illustration, the arrangement of the nozzles 208 is appropriately simplified, such as reducing the number of nozzles 208 corresponding to one printing pass. In the actual configuration, the plurality of nozzles 208 constituting the nozzle row 204 of each inkjet head 202 are arranged at a pitch of 300 dpi resolution in the sub-scanning direction, for example. Further, in the multi-pass printing operation, the sub-scan driving unit 16 may use a feed amount that is shifted by a distance less than the pitch of the nozzles 208 for the feed amount of the medium 50 in each sub-scan operation. More specifically, for example, it can be considered that the feed amount of the medium 50 in one sub-scanning operation is set such that a deviation of half the pitch of the nozzles 208 occurs. In this case, the printing resolution in the sub-scanning direction is 600 dpi, which is twice the resolution corresponding to the pitch of the nozzles 208. It is also conceivable to set the feed amount of the medium 50 in one sub-scanning operation so that a shift corresponding to 1/3 of the pitch of the nozzle 208 occurs. In this case, the printing resolution in the sub-scanning direction is 900 dpi, which is three times the resolution corresponding to the pitch of the nozzles 208.

  In this example, the control unit 22 (see FIG. 1) performs at least the first k times (k is an integer less than or equal to 1 and less than N) among N print passes performed on the same area of the medium. The density of printing performed in the printing pass is set lower than the density of printing performed in the (k + 1) th printing pass. In this case, the density of printing performed in each printing pass is, for example, the density corresponding to the density of dots of ink formed in the printing pass within the band region of the printing pass width. Further, the density corresponding to the density of the ink dots may be, for example, a density appropriately normalized according to the density of the ink dots.

  Further, the control unit 22 gradually increases the density of printing performed by each of the plurality of nozzles that eject ink droplets corresponding to the k-th printing pass in the nozzle row 206 of the head unit 12 toward the head rear end side. Set to high. In this case, the density of printing performed by each of the plurality of nozzles is, for example, a density corresponding to the density of ink dots formed by one main scanning operation using the nozzles. In this case, the density of the ink dots may be, for example, the density of the ink arrangement in the main scanning direction.

  More specifically, the control unit 22 sets the density corresponding to each printing pass, for example, as shown in the right part of FIG. Thereby, for example, the control unit 22 sets the density of printing performed in the first printing pass (first pass), which is the first printing pass, to be higher than the density of printing performed in the second printing pass (second pass). Set low. In addition, the control unit 22 sets the density of printing performed by each of the plurality of nozzles that eject ink droplets for the first printing pass in the nozzle row 206 of the head unit 12 toward the head rear end side. And gradually set higher.

  In this example, as shown in the drawing, the first to sixth printing passes, which are the first half printing passes, are performed in the respective printing passes (for example, the first to fifth printing passes). The printing density is set to be lower than the density of printing performed in the next printing pass (for example, the second to sixth printing passes). Further, the density of printing performed by each of the plurality of nozzles that eject ink droplets for each printing pass is set gradually higher toward the head rear end side.

  When configured in this way, for example, the density of the first print pass can be appropriately set to a density lower than the subsequent print pass. In addition, for example, a change in density at the boundary portion of the print pass can be appropriately suppressed.

  Also, in this case, a plurality of nozzles that eject ink droplets for the printing pass instead of uniformly reducing the density of the entire printing pass for a printing pass that lowers the printing density compared to the immediately following printing pass. The density of printing performed by each of the above is gradually increased toward the head rear end side. In this case, for example, the print density does not change greatly in a stepped manner in units of print passes.

  Therefore, if configured as described above, for example, it is possible to appropriately prevent the boundary of the print path from being noticeable. Further, for example, it is possible to appropriately suppress stripes, streaks, and the like generated at the boundary portion of the print path. Therefore, according to this example, for example, when printing by the multi-pass method, it is possible to prevent the boundary of the print path from being noticeable and perform printing more appropriately.

  Further, in this example, with respect to the density of printing performed by each of the plurality of nozzles 208 in the nozzle row 206 of the head unit 12, the control unit 22 more specifically centers on the central portion of the nozzle row 206 in the sub-scanning direction. Thus, the method of changing the density is set to be symmetrical between the direction toward the head front end side and the direction toward the head rear end side. For example, as shown in the right part of FIG. 3, the control unit 22 increases the density of printing performed by the nozzles 208 in the central part of the nozzle row 206 in the sub-scanning direction, thereby performing printing performed by the nozzles 208 in the central part. Is set higher than the density of printing performed by the nozzle 208 at the end of the nozzle row 206. Further, the density of printing performed by each of the plurality of nozzles 208 is set so that the density gradually decreases as the distance from the central portion increases. With this configuration, it is possible to appropriately set a low density for the print density of the first print pass or the like.

  Here, when printing by the multi-pass method, it is necessary to match the density obtained by summing up the printing densities of the respective printing passes with a predetermined density set in advance. Therefore, for example, when the density of one of the print passes is lowered, it is necessary to increase the density of the density of the other print passes by that amount.

  In addition, the density setting is not simply set for each printing pass, but as in this example, the density is set for each nozzle, and printing by a plurality of nozzles that eject ink droplets for one printing pass is performed. When the density is set so as to change gradually, it is necessary to set the density so as to complement this change in other printing passes. However, the concentration setting for performing such complementation is not always easy and may be complicated.

  On the other hand, in this example, for example, by giving the above-described symmetry to the density change method, the print density by each nozzle 208 is set between the head rear end side and the head front end side. Can be complemented appropriately. This also makes it possible to appropriately reduce the printing density in the first printing pass or the like.

  In the above description, the density of printing performed in each printing pass and the density of printing performed by each of the plurality of nozzles 208 are more specifically when the medium is filled with a density preset in the printing apparatus. The concentration may be This density may be, for example, a 100% density preset in the printing apparatus. Further, the density may be defined as, for example, 200% or 300% according to the setting of the printing apparatus.

  Further, the density of printing performed by each of the plurality of nozzles 208 that eject ink droplets for the printing pass such as the first printing pass is gradually set higher toward the head rear end side. The printing density corresponding to each nozzle is set so that the density increases toward the side. In this case, the density is not necessarily different for all nozzles, but for example, the same density as the adjacent nozzles may be set for some nozzles. For example, the printing density by each nozzle may be gradually changed in units of a plurality of preset nozzles. In this case, the printing density may change stepwise, for example. Also in this case, for example, the density change can be appropriately and sufficiently gradual as compared with a case where the printing pass is changed in units of steps. In addition, this makes it possible to appropriately prevent the print path boundary from being noticeable. Further, the printing density by each nozzle may be gradually changed more finely in units of one nozzle. If comprised in this way, it can prevent more appropriately that the boundary of a printing path is conspicuous, for example.

  Further, when the density of printing performed by each nozzle 208 is reduced in the first printing pass, etc., the positions of a plurality of ink dots formed on the same line in the sub-scanning direction are, for example, a dither method or an error. Dispersion based on a certain rule determined using a diffusion method or the like. If comprised in this way, the position of the dot to form can be disperse | distributed appropriately about the nozzle 208 which prints with a low density, for example.

  In this example, as described above, by reducing the density of the first print pass or the like, for example, the density change at the boundary portion of the print pass is suppressed, and the boundary of the print pass is prevented from being noticeable. However, when printing by the multi-pass method, the print quality may be deteriorated due to another cause.

  For example, when printing by the multi-pass method, if the shape of the ink dots formed on the surface layer of the ink layer is not uniform, a striped pattern such as a light stripe may occur. In particular, in the case of using ultraviolet curable ink, the occurrence of such a striped pattern may be a serious problem when high-precision printing is to be performed at high speed. Further, the non-uniformity of the ink dot shape is caused, for example, by the connection of uncured ink dots on the medium.

  On the other hand, in this example, since the above-described symmetry is given to the density change method, the density change corresponding to the seventh to twelfth print passes, which are the latter half print passes, is the first half. It becomes symmetrical with the change of the density of the printing pass. More specifically, the control unit 22 determines the density of printing performed in each printing pass (for example, each of the eighth to twelfth printing passes) for the latter printing pass, for example, the previous printing pass (for example, It is set lower than the density of printing performed in each of the seventh to eleventh printing passes. Further, the density of printing performed by each of the plurality of nozzles that eject ink droplets for each printing pass is set gradually lower toward the head rear end side.

  In this case, the print density in the twelfth print pass, which is the last print pass, is low, as is the print density in the first print pass. The last printing pass is a printing pass for forming ink dots on the surface layer of the ink layer formed on the medium.

  Therefore, in this example, for example, the density of ink dots to be formed can be lowered in the surface layer portion of the ink layer. In this case, since the distance between dots of ink formed in the same printing pass is increased, it is difficult for dots to be connected. Therefore, according to this example, for example, the shape of the ink dots can be made uniform in the surface layer portion of the ink layer. In addition, for example, it is possible to appropriately prevent the generation of a striped pattern such as a light stripe, and to perform high-quality printing more appropriately.

  Next, the results of actual printing using the printing apparatus 10 (see FIG. 1) of this example will be described. FIG. 4 shows an example of a state after performing one main scanning operation. In the following description, unless otherwise specified, the vertical direction in the figure is the main scanning direction, and the horizontal direction is the sub-scanning direction.

  FIG. 4A is a diagram showing an example of density setting, and shows an example of density setting for the nozzles at each position in the nozzle row. In FIG. 4 (a), the diagram on the left side denoted by reference symbol A is a diagram showing an example in which the density is set with a configuration different from this example, depending on the position in the nozzle row. An example in which the same density is set for all nozzles without giving a density gradient is shown. This density setting is an example when the printing pass is controlled by a known method, for example. The density setting may be a setting performed using a known mask in each printing pass.

  Also, in FIG. 4A, the right-hand side diagram denoted by reference character B is a diagram showing an example of density setting in the configuration of this example, and according to the position in the nozzle row. An example of density setting when a density gradient is provided is shown. This density setting is the same as or similar to the setting described with reference to FIG. 3, for example, except as described below.

  In the configuration described below, the number of printing passes is set to 6 for convenience of experiments and the like. Therefore, when the same density is set for all the nozzles as indicated by symbol A, the density of each print pass is 16.7%. In this case, the density of each printing pass is, for example, the ratio of the printing result of each printing pass to the printing density after all the printing passes are completed. The printing density corresponding to 100% is the printing density after printing for all printing passes.

  Further, when the density is set according to the configuration of this example as indicated by reference numeral B, the density of each print pass is 5.6%, 16.8%, 27.6 in order from the first print pass. %, 27.6%, 16.8%, 5.6%. In this case, if the density corresponding to the nozzle that ejects ink droplets with the highest density (peak density) is defined as 100% ejection density in the density setting according to the configuration of this example, the same applies to all nozzles. When the density is set (in the case of reference A), the density corresponding to each nozzle is 50%.

  FIG. 4B shows a printing result when one main scanning operation is performed using the density setting shown in FIG. In FIG. 4B, the print results indicated by reference signs A and B are print results corresponding to the settings indicated by reference signs A and B in FIG. 4A.

  As shown in the figure, when the same density is set for all the nozzles as in the setting indicated by the symbol A in FIG. 4A, the printing result is not dependent on the position in the sub-scanning direction. Uniform concentration. On the other hand, when the density gradient is given according to the position in the nozzle row as in the setting indicated by B in FIG. 4B, printing is performed corresponding to the density gradient setting. The result is also a gradation in which the density gradually changes in the sub-scanning direction.

  Next, the result of printing in the multi-pass method with the density setting shown in FIG. 4 will be described. FIG. 5 is a diagram showing an example of a state after performing the main scanning operation for the number of printing passes in the multi-pass method, and printing is performed with the sub-scanning operation in between using the density setting shown in FIG. A state after six main scanning operations corresponding to the number of passes is shown.

  FIG. 5A is a diagram for explaining the printing operation in the multi-pass method, and shows an area through which the head unit 12 (see FIG. 2) passes in the main scanning operation corresponding to each printing pass. FIG. 5B shows a printing result when the main scanning operation for the number of printing passes is performed in the operation of the multi-pass method shown in FIG.

  In FIG. 5 (a), the diagram indicated by reference numerals A and B corresponds to the setting indicated by reference symbols A and B in FIG. 4 (a). Further, in FIG. 5A, for convenience of illustration, the head unit 12 corresponding to each printing pass is displayed by shifting the position in the main scanning direction, and the position in the sub scanning direction is displayed in each main scanning operation. The position of the head part 12 is shown. More specifically, FIG. 5A shows a plurality of head portions 12 arranged in the main scanning direction by sequentially shifting the positions in the sub-scanning direction in the drawing given the reference numerals A and B. The positions in the sub-scanning direction of the head unit 12 in the first to sixth printing passes are shown in order from the left side to the right side. In addition, the print density graphs shown in the drawings with the reference signs A and B are performed for each position on the medium at the timing when six main scanning operations corresponding to the number of print passes are completed. The density of printing is shown.

  Further, in FIG. 5B, the print results indicated by the symbols A and B are the print results corresponding to the operations indicated by the symbols A and B in FIG. is there. First, a printing result corresponding to the operation denoted by reference symbol A in FIG. This print result is a print result in the case where the same density is set for all the nozzles as in the setting indicated by the symbol A in FIG. In this case, in a state where one main scanning operation is performed, the density is uniform regardless of the position in the sub-scanning direction, as indicated by reference numeral A in FIG.

  However, when printing is performed using the multi-pass method, as indicated by the symbol A in FIG. 5B, the density of each area on the medium is equal to the number of printing passes performed for that area. Concentrate according. In this case, the density in the band-like area (band area) corresponding to each printing pass is the same density. Therefore, in this case, a sudden density difference occurs at the boundary portion of the print pass. Such a discontinuous and rapid density change is likely to cause stripes, streaks, and the like that occur at the boundary of the print pass.

  Subsequently, a printing result corresponding to the operation denoted by reference numeral B in FIG. This print result is a print result in the case where a density gradient is given according to the position in the nozzle row as in the setting indicated by the reference symbol B in FIG. In this case, in a state where one main scanning operation is performed, a gradation is obtained as indicated by reference numeral B in FIG.

  When printing is performed by the multi-pass method with such density setting, the density of each area on the medium is the same as that of the area where printing has been performed, as indicated by reference numeral B in FIG. A gradation is formed in which the density gradually changes in the sub-scanning direction throughout. Further, in this case, since the density continuously changes even at the boundary portion of the print path, the boundary portion of the print path is inconspicuous as compared with, for example, the case where the symbol A is attached in FIG. Therefore, in this case, stripes, streaks, and the like at the boundary portion of the print path are less likely to occur.

  As described above, also from the result of actual printing, it can be seen that the present example can appropriately suppress the stripes and stripes at the boundary portion of the printing pass. Therefore, according to this example, for example, it is possible to appropriately prevent the boundary of the print path from being noticeable. Thereby, for example, printing by an inkjet method can be performed more appropriately with high accuracy.

  Subsequently, regarding the density setting and the like performed in this example, a modified example other than the specific configuration described above will be described. First, problems related to the modification described later will be described.

  FIG. 6 is a diagram for explaining a bleeding problem that may be caused by the characteristics of the medium. FIG. 6A is a diagram illustrating an example of the setting of the printing density by each nozzle. This density setting is, for example, the same setting as the density setting indicated by symbol A in FIG. In this case, the print density is maximum (density peak) at the center position of the nozzle row (position indicated by a dotted circle in the drawing). As a result, in the gradation-like printing result printed in each main scanning operation, ink droplets are ejected at a high ejection density near the center of the gradation.

  Further, when printing is performed by an inkjet method, printing may be performed on media of various materials by the same printing apparatus 10. More specifically, for example, the printing apparatus 10 may use a medium (for example, KAPPA medium) on which ink dots are likely to spread. When such a medium is used, if there is a portion where the ink droplet discharge density is high, for example, local bleeding may occur, which may cause unevenness. Further, for example, when a plurality of ink jet heads arranged in a staggered arrangement (hereinafter referred to as a staggered head) are used as the ink jet heads for the respective colors, the position of the density peak and the boundary of the ink jet heads in the stagger head overlap. There is also a possibility that stripes generated due to the boundary of the line are conspicuous.

  FIG. 6B is a photograph showing a state where local bleeding has occurred on the medium. As described above, in the case where the density setting indicated by the symbol A in FIG. 4A is used, the ink droplet ejection density is highest in the region 402 near the center of the gradation. Become. In this case, if a medium in which ink dots are easy to spread is used, blurring may occur as shown in a region 404 of a photograph showing an enlarged region 402, for example.

  Here, for such a problem, for example, it is conceivable to lower the discharge density of the ink droplets at the density peak. Accordingly, an example of a configuration for lowering the ejection density of ink droplets at the density peak will be described with respect to a modification example of density setting. Except for the points described below, each modification has the same or similar features as the configuration described with reference to FIGS.

  FIG. 7 shows an example of a configuration for lowering the ejection density of ink droplets at a density peak, regarding a modification of density setting. FIG. 7A shows a first modification of density setting.

  When lowering the discharge density of ink droplets at the density peak, for example, considering that the printing density in each printing pass is mutually complemented, other parts are used to compensate for the decrease in density at the density peak. Therefore, it is necessary to increase the discharge density. Therefore, more specifically, for example, as shown in FIG. 7A, compared to the density setting as shown in FIG. It is conceivable to lower the discharge density of the nozzle and increase the discharge density by the nozzles at both ends of the nozzle row. With this configuration, for example, the discharge density of the ink droplet at the density peak can be appropriately lowered. Accordingly, for example, even when a medium in which ink dots are easily spread is used, bleeding, unevenness, and the like can be appropriately suppressed.

  FIG. 7B shows a second modification example regarding density setting. In order to reduce the discharge density of the ink droplet at the density peak, for example, the density is changed to a trapezoidal shape as shown in FIG. 7B, for example, without concentrating the density peak position at one point. It is also possible. In this case, for example, in the nozzle row, the control unit 22 (see FIG. 1) sets the printing density to the same density for a plurality of nozzles arranged continuously including the nozzle in the central portion. Further, the density of printing performed by each of a plurality of nozzles other than the plurality of nozzles continuously arranged in the central portion is set so that the density gradually decreases as the distance from the central portion increases.

  With this configuration, for example, a low density can be appropriately set for the printing density of the first printing pass or the like. In this case, for example, the nozzles that perform printing at the peak density are not only one nozzle but a plurality of nozzles in the central portion. Therefore, if comprised in this way, it can prevent appropriately that a location with a high density | concentration concentrates on one place, for example. Accordingly, for example, even when a medium in which ink dots are easily spread is used, bleeding, unevenness, and the like can be appropriately suppressed.

  Further, when such trapezoidal density is set, the portion where the highest density is set is not the position where one peak is reached, but is shown by a dotted circle in FIG. 7B, for example. This is a part where the density does not change within a certain range, such as the part where Therefore, for example, when a staggered head is used, even if the boundary of the inkjet head overlaps the portion with the highest density, the influence of the boundary (cut) of the inkjet head is less noticeable. Therefore, with this configuration, for example, when a stagger head is used, high-precision printing can be performed more appropriately.

  In addition, considering that the influence of the boundary of the ink jet head in the stagger head becomes less conspicuous regarding the setting of the trapezoidal density, for example, even when the ink droplet discharge density at the density peak is not lowered, the trapezoidal density It is also possible to use a setting. FIG. 7C shows a third modification for setting the density. The third modified example is an example in the case where the density peak density is higher than that of the second modified example and a trapezoidal density setting is used. Also in this case, for example, when a stagger head is used, high-precision printing can be performed more appropriately.

  As described above, when a stagger head is used, depending on how the density is set, the influence of the boundary of the ink jet head is conspicuous, and the print quality may deteriorate. Therefore, when a stagger head is used, it may be possible to set a density more suitable for the stagger head. Then, subsequently, a modified example of such a configuration will be described. In the following, in order to simplify the description, a configuration in the case of using a stagger head in which two ink jet heads are arranged in a stagger shape will be described.

  FIG. 8 is a diagram for explaining density setting when a stagger head is used. FIG. 8A shows an example in which the density is set so that the density peak comes to the center of the entire stagger head without considering the configuration of the inkjet head 202 in the stagger head. FIG. 8B is a diagram showing a fourth modified example of density setting, and shows an example of density setting according to the configuration of the inkjet head 202 in the stagger head.

  When the density is set as shown in FIG. 8A, as can be seen from the figure, the density peak that changes in a gradation and the boundary of the inkjet head 202 in the stagger head overlap. Therefore, in this case, high density printing is performed by the nozzles near the boundary of the inkjet head 202. Therefore, in this case, the influence of printing by the boundary portion of the inkjet head 202 becomes large.

  However, in the inkjet head 202, the nozzles at the end of the nozzle row are usually more likely to shift the landing position than the nozzles at the center of the nozzle row. In the stagger head, for example, there may be a deviation in the adjustment of the position of each inkjet head 202 (adjustment between the heads). For this reason, when the stagger head is used, if the portion where the printing density is high and the boundary of the inkjet head 202 overlap, the influence of the boundary of the inkjet head 202 is conspicuous, and the print quality may be deteriorated.

  On the other hand, in the fourth modified example shown in FIG. 8B, the control unit 22 (see FIG. 1), for example, sets the density of each inkjet head 202 to a gradation, thereby changing the ink jet head 202. The density is set so that the density becomes low at the boundary portion. In this case, more specifically, for example, the control unit 22 performs printing performed by the nozzles in the central portion of the nozzle row in the sub-scanning direction with respect to the density of printing performed by a plurality of nozzles included in the nozzle row in each inkjet head 202. The density is set to be high and the density gradually decreases as the distance from the center portion increases.

  When configured in this way, in each of the staggered inkjet heads 202, the density of printing by the nozzles is set low for the nozzles at the end of the nozzle row. Therefore, with this configuration, for example, a low density can be appropriately set for the printing density of the first printing pass or the like.

  In this case, for example, the influence of the nozzles at the end of the nozzle row can be appropriately reduced for each inkjet head 202. As a result, for example, even when a landing position shift or the like occurs in the nozzles at the end of the nozzle row, or when there is a shift in the position adjustment of the inkjet head 202, the influence on the printing result can be appropriately suppressed. . Therefore, with this configuration, for example, when a stagger head is used, the density of each printing pass can be set more appropriately according to the configuration of the inkjet head 202.

  In the ink jet head 202, for example, when the temperature of the ink or the ink jet head 202 changes, the ink spreading method (dot gain) on the medium and the ejection speed may change. Further, when the discharge speed changes, the landing position shifts. Therefore, for example, when a difference in temperature of the ink or the ink jet head occurs depending on the position of the nozzle, a difference in dot gain or discharge speed may occur depending on the position of the nozzle, resulting in density unevenness. Also, due to such a cause, for example, a difference in dot gain or ejection speed is likely to occur particularly at both ends of the inkjet head 202.

  On the other hand, when the density is set as in the fourth modification, the density of printing at both ends of each inkjet head 202 can be appropriately reduced. Therefore, with this configuration, for example, density unevenness caused by the above-described causes can be appropriately suppressed.

  FIG. 9 is a diagram for explaining the influence of the printing density at both ends of the inkjet head 202. In FIG. 9, the case where the same density is set for all nozzles and the case where the density is set as in the fourth modification are compared for the result of printing by one main scanning operation. And show.

  FIG. 9A shows an example of a print result when the same density is set for all nozzles. This setting is an example in the case where the density is set in the same or similar manner as in the case where the symbol A is attached in FIG.

  In this case, in the printing result by one main scanning operation, the printing density is uniform in the entire region indicated by parentheses A in the drawing. However, in this case, as described above, dot gain and landing position deviations are easily noticeable in the portions where printing is performed at both ends of each inkjet head 202. As a result, density unevenness or the like tends to occur.

  FIG. 9B is an example of a print result when the density is set as in the fourth modification. In this case, areas printed by the nozzles at the end of each inkjet head 202 are areas indicated by parentheses B, C, and D in the drawing. Further, by using the density setting shown in FIG. 8B, the density of printing in these areas is lower than that in other portions. Therefore, in this case, compared to the case shown in FIG. 9A, for example, it is possible to appropriately suppress the influence of dot gain and landing position deviation at both ends of the inkjet head 202. Thereby, for example, occurrence of density unevenness or the like can be appropriately suppressed.

  Here, in each of the configurations described above with reference to FIGS. 1 to 9, the density of the first printing pass or the like is set to a low density, and the density change method has symmetry. For this reason, the density of the last pass or the like is low as well as the density of the first print pass or the like. In addition, as described above with reference to FIG. 3 and the like, for example, the density of the ink dots formed on the surface layer portion of the ink layer is lowered, and the dots are connected. Can be difficult. Therefore, the matters related to this point will be described in more detail below.

  When printing by the multi-pass method, for example, for the ink droplets ejected to the surface layer portion of the ink layer in the latter half of the printing pass, the occurrence of light fringes and the like is suppressed by increasing the distance between the formed ink dots. be able to. In this case, for example, it is preferable to lower the print density and increase the distance between dots in the latter half of the print pass including the last print pass. In this case, the surface finish can be made into a matte shape by separating the distance between the dots in the surface layer portion. In this case, it is preferable that the surface becomes a mat in the main scanning operation in either the forward direction or the backward direction.

The conditions for suppressing the occurrence of light fringes, that is, the distance at which the ink dots do not contact can be obtained by the following calculation.
(Print settings)
Dot diameter: 75 μm Landing position error in the main scanning direction (Y landing error): ± 40 μm
Number of colors: 4 (CMYK) Head resolution: 300 dpi / color

  In this case, the required distance between dots = dot diameter 75 μm + Y landing error 40 μm × 2 = 155 μm. Further, the distance between dots in consideration of other colors = required distance between dots 155 μm × 4 colors = 620 μm.

More specifically, for example, when printing at a resolution of 600 × 900 dpi,
Main scanning direction (Y direction): 620 μm / 42 μm≈14.6 dots≈6.8%
Sub-scanning direction (X direction): 155 μm≈150 dpi → 300 dpi / 150 dpi = 50%
It becomes. Therefore, if ink droplets are ejected at a density of 6.8% or less, a distance where the dots do not come into contact with each other can be obtained. In this case, by preventing contact between the dots, for example, the shape of the ink dots can be appropriately uniformized.

  Further, in connection with a method for reducing light fringes and the like, the inventors of the present application have an effect of reducing light fringes and the like in a mask having a low spatial frequency by experiments or the like for a mask used in a multi-pass method. I found out. In this case, the mask used in the multi-pass method is, for example, data specifying pixels that eject ink droplets in each printing pass.

  FIG. 10 is a diagram illustrating an example of a result of printing using a mask having a low spatial frequency. As can be seen from the figure, in this case, at each location, ink dots are formed in a lump like a circle shown in the diagram. Further, the dot clusters are formed apart from each other, for example, as indicated by arrows in the drawing.

  In this case, for example, if the distance between dots is not a single dot but is captured on a larger scale, the distance as a lump of dots is separated, which is the same as the distance between individual dots is separated. It is thought that the effect is acquired. Further, for example, in a cluster of dots, a plurality of dots are formed close together, so that the dots are connected regardless of the time from landing to irradiation with ultraviolet rays, for example. Therefore, for example, even if there is a difference in the timing of ultraviolet irradiation to the same position on the medium between the main scanning operation in the forward direction and the main scanning operation to the sub-scanning operation, There will be no difference in state. Further, it is considered that the surface state of the ink layer can be uniformly made glossy.

  For this reason, in each configuration described above, for example, a mask having a low spatial frequency may be used. If comprised in this way, a light fringe etc. can be suppressed more appropriately, for example. Thereby, for example, high quality printing can be appropriately performed.

  As mentioned above, although this invention was demonstrated using embodiment, the technical scope of this invention is not limited to the range as described in the said embodiment. It will be apparent to those skilled in the art that various modifications or improvements can be added to the above embodiment. It is apparent from the description of the scope of claims that embodiments with such changes or improvements can be included in the technical scope of the present invention.

  The present invention can be suitably used for a printing apparatus, for example.

DESCRIPTION OF SYMBOLS 10 ... Printing apparatus, 12 ... Head part, 14 ... Main scanning drive part, 16 ... Sub-scanning drive part, 18 ... Platen, 20 ... Ultraviolet irradiation part, 22 ... Control unit 50... Medium 102 102 carriage 104 guide rail 202 ink jet head 204 nozzle row 206 nozzle row 208 nozzle 302 ... dots, 402 ... area, 404 ... area

Claims (8)

A printing apparatus that performs printing by an inkjet method,
A head portion having a nozzle row in which a plurality of nozzles for discharging ink droplets onto a medium are arranged;
A main scanning drive unit that causes the head unit to perform a main scanning operation of discharging ink droplets while moving in a preset main scanning direction;
A sub-scanning drive unit that moves the head unit relative to the medium in a sub-scanning direction orthogonal to the main scanning direction;
A control unit for controlling the main scanning operation by the head unit,
In the nozzle row of the head unit, the plurality of nozzles are arranged in the sub-scanning direction,
The head unit performs printing on the medium by a multi-pass method in which the main scanning operation is performed a plurality of times on the same area on the medium, and N is set in advance on the same area on the medium. Performing the main scanning operation corresponding to each of N times (N is an integer of 2 or more) printing passes,
The controller is
At least the density of printing performed in the first k times (k is an integer less than or equal to 1 and less than N) among the N times of printing passes performed on the same area of the medium is (k + 1) th. Lower than the density of printing performed in the printing pass,
In the nozzle row of the head portion, the nozzles ejecting ink drops for the first printing pass in the N printing passes go to the nozzles ejecting ink drops for the Nth printing pass. When the direction is the head rear end side, the density of printing performed by each of the plurality of nozzles that eject ink droplets corresponding to the kth printing pass in the nozzle row of the head unit is set to the rear side of the head. A printing apparatus, characterized by being set gradually higher toward the end side.   The printing apparatus according to claim 1, wherein the head unit ejects ink droplets of ultraviolet curable ink from the nozzle. The controller is
At least the density of printing performed in the first printing pass among the N printing passes performed on the same area of the medium is lower than the density of printing performed in the second printing pass,
And,
The density of printing performed by each of the plurality of nozzles that eject ink droplets corresponding to the first printing pass in the nozzle row of the head unit is gradually set higher toward the head rear end side. The printing apparatus according to claim 1, wherein the printing apparatus is a printing apparatus.   The control unit is opposite to the head rear end side with respect to the density of printing performed by each of the plurality of nozzles in the nozzle row of the head portion, centering on a central portion of the nozzle row in the sub-scanning direction. 4. The method according to claim 1, wherein the density change method is set to be symmetrical between a direction toward the head front end side and a direction toward the head rear end side. Printing device.   The control unit is configured such that the density of printing performed by the nozzles in the central portion of the nozzle row in the sub-scanning direction is higher than the density of printing performed by the nozzles at the end of the nozzle row and is further away from the central portion. The printing apparatus according to claim 4, wherein the density of printing performed by each of the plurality of nozzles is set so that the density gradually decreases.   In the nozzle row, the control unit sets the printing density to the same density for a plurality of nozzles arranged continuously including the nozzle in the central portion, and the plurality of nozzles arranged continuously in the central portion. The printing apparatus according to claim 4, wherein the density of printing performed by each of the plurality of nozzles other than the nozzles is set so that the density gradually decreases as the distance from the center portion increases. The head portion has a plurality of inkjet heads arranged in a staggered shape,
Each of the plurality of inkjet heads has a nozzle row in which the nozzles are arranged in the sub-scanning direction,
The control unit has a high density of printing performed by the nozzles in the central portion of the nozzle row in the sub-scanning direction with respect to the density of printing performed by the plurality of nozzles included in the nozzle row in each of the inkjet heads. The printing apparatus according to claim 4, wherein the density is set to gradually decrease as the distance from the center portion increases. A printing method for performing printing by an inkjet method,
In the head portion having a nozzle row in which a plurality of nozzles for discharging ink droplets to the medium are arranged,
A main scanning operation for ejecting ink droplets while moving in a preset main scanning direction;
A sub-scanning operation that moves relative to the medium in a sub-scanning direction orthogonal to the main scanning direction;
In the nozzle row of the head unit, the plurality of nozzles are arranged in the sub-scanning direction,
By controlling the main scanning operation by the head unit,
The head unit is made to perform printing on the medium by a multi-pass method in which the main scanning operation is performed a plurality of times on the same area on the medium, and the same area on the medium is set in advance. The main scanning operation corresponding to each of N times (N is an integer of 2 or more) printing passes is performed,
In controlling the main scanning operation,
At least the density of printing performed in the first k times (k is an integer less than or equal to 1 and less than N) among the N times of printing passes performed on the same area of the medium is (k + 1) th. Lower than the density of printing performed in the printing pass,
In the nozzle row of the head portion, the nozzles ejecting ink drops for the first printing pass in the N printing passes go to the nozzles ejecting ink drops for the Nth printing pass. When the direction is the head rear end side, the density of printing performed by each of the plurality of nozzles that eject ink droplets corresponding to the k-th printing pass in the nozzle row of the head unit is set to the rear side of the head. A printing method, characterized by being set gradually higher toward the end side.