JP2009051063A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus making density unevenness, which occurs over the whole of a recording area by a conveyance error of a recording medium or a displacement error in the main scanning direction of a recording head, inconspicuous for improving quality of an image finally formed on the recording medium. <P>SOLUTION: This image forming apparatus is provided with recording heads 121-124 wherein a plurality of nozzles 121a-124a jetting ink toward the recording medium are arranged. The recording heads 121-124 are divided into first and second nozzle groups NS1 and MS2 comprising nine nozzles 121a-124a respectively. The first and second nozzle groups NS1 and NS2 form an image by thinned patterns MP1 and MP2 constructed by a predetermined ratio and a thinning method. The thinned patterns MP1 and MP2 are constructed so that blank dots are compensated mutually and dot formation density is periodically varied in the main scanning direction and the sub scanning direction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an inkjet recording type image forming apparatus that forms an image by ejecting ink from a plurality of nozzles onto a recording medium.

  Conventionally, an inkjet that forms an image by ejecting ink while displacing a recording head provided with a plurality of nozzles in a direction (main scanning direction) orthogonal to the conveyance direction of the recording medium while intermittently conveying the recording medium 2. Description of the Related Art Recording type image forming apparatuses, so-called ink jet printers, are known. In general, an inkjet printer simultaneously forms images on a number of main scanning lines corresponding to the number of nozzles by one main scanning by a recording head in which a plurality of nozzles are arranged in the conveyance direction of the recording medium. In this case, an image formed on each main scanning line is represented by a set of dots (dot rows) formed by ink droplets ejected from the nozzles.

  One of the image forming methods using an ink jet printer is an overlap recording method. The overlap recording method is a recording method in which an image on one main scan line is formed using different nozzles in a plurality of main scans. For example, as shown in FIG. 10 (A), among the recording head 2 constituted by 16 nozzles 1, it is constituted by a nozzle group 1a constituted by four nozzles 1 on the lower side in the drawing and a nozzle 1 on the upper side in the drawing. An image of the same recording area is completed with the nozzle group 1b. That is, an image is completed by performing two main scans on the same recording area on the recording medium. In this case, an image is formed by two thinned dot rows which are dot rows formed by halving the dot rows formed on one main scanning line and complement each other with empty dots.

  As the thinning patterns 3a and 3b, which are a set of thinning dot rows formed simultaneously by 16 nozzles 1 for each main scanning, a checkered pattern in which dots are formed for every vertical and horizontal dot has been used. It was. In this case, the thinning pattern 3a and the thinning pattern 3b are patterns with opposite arrangements, and complement each other's empty dots. When an image is formed on the recording medium, after the thinning pattern 3a is formed by the lower nozzle group 1a in the first main scan, the recording medium is conveyed under the nozzle group 1b. The image is completed by forming the thinning pattern 3b using the nozzle group 1b in the second main scanning.

However, for example, as shown in FIG. 10B, when the recording medium is not transported by 0.5 dots in the direction of the arrow in the conveyance of the recording medium immediately before the third main scanning, the first main scanning is performed. A part of the thinning pattern 3a formed by the third main scanning overlaps the thinning pattern 3a formed by the third main scanning and the thinning pattern 3b formed by the second main scanning. And a thinning pattern 3a formed by the third main scanning may cause a gap, resulting in density unevenness. As the density unevenness, there are a black line 4 of dark color unevenness caused by overlapping dots and a white line 5 of thin color unevenness due to a gap generated between the dots. In particular, the density unevenness 4a at the boundary between the thinning pattern 3a formed by the first main scanning and the thinning pattern 3a formed by the third main scanning appears remarkably due to the difference in dot formation time (time difference). . In order to prevent such density unevenness, for example, the following patent document 1 and the following patent document 2 describe the dot formation density at the boundary part of the thinning pattern formed in the adjacent regions by different main scans. An ink jet printer is disclosed in which density unevenness at the boundary portion is prevented by lowering toward the portion.
Japanese Patent Laid-Open No. 06-143618 JP 2002-103580 A

  However, since the conveyance error of the recording medium as shown in FIG. 10B occurs for all the nozzles constituting the recording head, density unevenness occurs in the entire recording area including the boundary portion of each thinning pattern. . Further, as shown in FIG. 10C, the density unevenness is caused not only by the conveyance error of the recording medium but also by the displacement error of the recording head. That is, the density unevenness similarly occurs when the thinning pattern is shifted in the main scanning direction of the recording head (in the case of the figure, the dot formation position is shifted by 0.5 dots on the right side in the drawing). Furthermore, in addition to the above-mentioned recording medium transport error and recording head displacement error, density unevenness is also caused by variations in the size and direction of ink droplets ejected from the nozzles of the recording head, and the floating of the recording medium. Will occur. As described above, the density unevenness generated in the entire recording area including the boundary portion of each thinning pattern cannot be solved by the ink jet printers disclosed in Patent Document 1 and Patent Document 2. For this reason, there is a problem that the quality of the image finally formed on the recording medium is lowered.

  Here, in the overlap recording method, the dot density on the recording medium when the dot is formed at the correct position, and the recording medium on the recording medium when the dot formation position is shifted by 0.5 dot in the main scanning direction. FIGS. 11A to 11C show the dot density and the dot density on the recording medium when the dot formation position is shifted by one dot in the main scanning direction. In each figure, (a) shows the relationship between the dot formation position and density in the first main scan, (b) shows the relationship between the dot formation position and density in the second main scan, and (c) ) Shows the relationship between the dot formation position and density after the second main scan.

  In the case where dots are formed at the correct positions in the first and second main scans, the density of the image that is completed on the same main scan line is compensated by complementation of dot formation due to the thinning pattern. It becomes constant (see FIGS. 11A and 11C). On the other hand, if a displacement error occurs in the recording head during the second main scanning and the dot formation position is shifted by 0.5 dots to the right in the main scanning direction, the image is completed on the same main scanning line. As for the density of, the density of the part where the dots overlap is increased, and the density of the part where the dots are not formed becomes lower, resulting in density unevenness (see FIGS. 11B and 11C). Further, when a displacement error occurs in the recording head during the second main scanning and the dot formation position is shifted by one dot to the right in the main scanning direction, the second time is formed on the dot formed by the first main scanning. Therefore, the density unevenness appears on the same main scanning line larger than the density unevenness when the dot formation position is shifted by 0.5 dots in the main scanning direction.

  The present invention has been made to address the above problems, and its purpose is to improve the quality of an image finally formed on a recording medium by making the density unevenness generated in the entire recording area of the recording medium inconspicuous. An object of the present invention is to provide an image forming apparatus that can be used.

  In order to achieve the above object, the present invention is characterized by a recording head in which a plurality of nozzles for forming dots by ejecting ink on a recording medium are arranged, and a direction orthogonal to the direction in which the nozzles in the recording head are arranged. Scanning drive means for displacing the recording head relative to the recording medium in the main scanning direction, and feed driving for displacing the recording medium relative to the recording head in the sub-scanning direction in which the nozzles of the recording head are arranged Recording is performed so as to complete an image on the same main scanning line on the same recording medium by means of a means and a set of thinned dot lines that are thinned out at a predetermined rate and complement each other to form an image formed on the recording medium. In an image forming apparatus comprising a recording head driving means for controlling ink ejection from the head, the recording head drive Means is to control the ejection of ink from the recording head such that the formation density of dots in the thinning-out dot array is periodically changed in at least one direction of the main scanning direction and the sub-scanning direction.

  In this case, for example, the recording head driving means periodically changes the dot formation density in the thinned dot row for each nozzle group obtained by dividing the recording head into a plurality of regions, and the same recording region in the recording medium is used. It is preferable to complete the image with the different nozzle groups. In these cases, the dot formation density is preferably changed in a sine wave shape.

  According to the feature of the present invention configured as described above, the dot formation density in the thinned dot rows that are thinned out at a predetermined rate and complement each other is periodically changed. It is supposed to be. According to this, the image formed on the recording medium has a high (or low) dot row formation density in addition to a recording region formed by a set of thinned dot rows in which the dot row formation densities are substantially equal to each other. The recording area is formed by a set of thinned dot rows having the same formation density as that of the thinned dot rows. That is, when an image is formed on a recording medium, even if the recording head is shifted by several dots in the main scanning direction and / or the sub-scanning direction with respect to the recording medium, the thinning of the dot rows having different density is reduced. In a recording region where an image is formed by a set of dot rows, density unevenness becomes inconspicuous due to the thinned dot rows having a high dot row formation density. As a result, the quality of the image finally formed on the recording medium can be improved.

  Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic perspective view showing an entire image forming apparatus 100 (so-called inkjet printer) that forms a desired image by adhering ink to the surface of a single-sheet recording medium WK that is an image formation target. . FIG. 2 is a block diagram of a control system for controlling the operation of the image forming apparatus 100.

  The image forming apparatus 100 includes a platen 101 having a planar portion and both ends of the planar portion curved in the front-rear direction of the image forming apparatus 100. The platen 101 is a mounting table on which the recording medium WK is mounted on the flat portion, and is formed to extend in the horizontal direction in the figure. A cylindrical grid roller 102 is provided at the center of the platen 101 along the horizontal direction in the drawing with its upper surface exposed. The grid roller 102 is rotationally driven by an X-axis direction feed motor 103 whose operation is controlled by a controller 130 described later.

  A long guide rail 104 is provided above the platen 101 so as to be parallel to the platen 101. Under the guide rail 104, two pinch rollers 105a and 105b each having a cylindrical portion facing the grid roller 102 are provided in a displaceable manner along the guide rail 104. The grid roller 102 and the pinch rollers 105a and 105b convey the sheet-like recording medium WK in the front-rear direction shown in the figure while being sandwiched from the vertical direction in the figure. That is, the X-axis feed motor 103 that rotationally drives the grid roller 102 corresponds to the feed driving means according to the present invention. Note that the front-rear direction (X-axis direction) in the drawing in which the recording medium WK is conveyed is the sub-scanning direction, and the left-right direction (Y-axis direction) orthogonal to the X-axis direction is the main scanning direction.

  The guide rail 104 supports the recording head unit 120 via the linear motion rail 106 and a linear motion block (not shown). The linear motion rail 106 is a single rail fixed to the guide rail 104 along the main scanning direction (Y-axis direction in the drawing). The linear motion block is a moving body that slides along the linear motion rail 106, and is fixed to each back surface of the recording head unit 120. That is, the recording head unit 120 is guided in the main scanning direction along the linear motion rail 106 (guide rail 104).

  The recording head unit 120 includes four recording heads 121, 122, 123, and 124 inside a casing having a substantially U-shaped cross section. These recording heads 121 to 124 are supplied with four different colors (black (BK), cyan (CY), magenta (M), and yellow (Y)) supplied from an ink tank (not shown) as recording media. A plurality of nozzles 121a to 124a for discharging toward the WK are provided. That is, the recording heads 121 to 124 form full-color images on the surface of the recording medium WK by a combination of four different colors of ink. As shown in FIG. 3, the nozzles 121 a to 124 a are arranged in a staggered manner at a constant interval D along the sub-scanning direction (X-axis direction in the drawing) that is the conveyance direction of the recording medium WK. The interval D between these nozzles 121a to 124a is set to a positive integer multiple of the interval between dots in the recording resolution in the sub-scanning direction.

  In addition, a part of the drive belt 107 laid in the main scanning direction (Y-axis direction in the drawing) is fixed to the upper back of the recording head unit 120. The drive belt 107 is connected to a Y-axis direction scan motor 108 whose drive is controlled by a controller 130, and is displaced in the main scanning direction by the rotational drive of the Y-axis direction scan motor 108. That is, the recording head unit 120 is displaced in the main scanning direction by driving the Y-axis direction scanning motor 108 via the driving belt 107. The Y-axis direction scan motor 108 corresponds to scan drive means according to the present invention.

  Above the recording head unit 120, a long upper cover 109 that constitutes an upper casing of the image forming apparatus 100 is provided. At both ends of the upper cover 109 and the platen 101, flat side frames 110R (not shown) and 110L for supporting the guide rails 104 are respectively provided in a state of standing vertically. Side covers 111 </ b> R and 111 </ b> L constituting a housing on each side surface of the image forming apparatus 100 are provided on the outer sides of the side frames 110 </ b> R and 110 </ b> L, respectively. Among these, on the front surface of the side cover 111R, an operation panel 112 for giving an instruction to the image forming apparatus 100 and displaying information from the image forming apparatus 100 is provided. Further, below the platen 101, a stand 113 that supports the image forming apparatus 100 and can be moved to a position desired by the operator is provided.

  The controller 130 includes a microcomputer including a CPU, a ROM, a RAM, and the like, and in accordance with instructions from an external computer device 140 connected via an interface 131, the X-axis direction feed motor 103 and the Y-axis direction scan motor 108. And the operation of the recording head unit 120 (recording heads 121 to 124), respectively. That is, the controller 130 corresponds to a recording head driving unit according to the present invention. The external computer device 140 is a personal computer (so-called personal computer) including an input device 141 composed of a keyboard and a mouse and a display device 142 composed of a liquid crystal display. The external computer 140 stores in advance a computer program for generating image data representing an image to be formed on the recording medium WK (hereinafter referred to as “image data generation program”).

  Next, the operation of the image forming apparatus 100 configured as described above will be described. First, the operator connects the external computer device 140 and the image forming apparatus 100 via the interface 117, and turns on the external computer device 140 and the image forming apparatus 100, respectively. As a result, the external computer device 130 enters a standby state in which it waits for the input of a command from the operator by executing a predetermined program (not shown). Also, the image forming apparatus 100 waits for an instruction from the external computer device 140 after returning the origin of the recording head unit 120 by executing a predetermined program (not shown) stored in advance in the ROM in the controller 130. It will be in a standby state. In this case, the origin of the recording head unit 120 is one of the displaceable limit positions (the rightmost end in the drawing) within the displaceable range of the recording head unit 120 in the main scanning direction.

  Next, the operator positions the recording medium WK between the grid roll 102 and the pinch rolls 105a and 105b and sets the recording medium WK on the platen 101, and then operates the input device 141 of the external computer device 140. The image forming apparatus 100 is instructed to print an image. In response to this instruction, the external computer device 140 executes the image forming program (not shown), thereby generating image data corresponding to the image to be printed stored in the external computer device 140 to generate an image. Output to the forming apparatus 100 (controller 130). The external computer device 140 outputs data representing the overlap amount and data representing the thinning pattern to the image forming device 100 together with the image data.

  Here, the data representing the overlap amount is the amount of overlap (long) of the recording heads 121 to 124 in the same recording area in the recording medium WK in order to complete an image on one main scanning line by a plurality of main scannings. Data). In the present embodiment, in order to facilitate understanding, the total pitch amount of nine nozzles 121a (122a, 123a, 124a) is used. Further, the data representing the thinning pattern is a ratio and a thinning method of thinning dots constituting an image formed on one main scanning line in order to complete an image on one main scanning line by a plurality of main scans. Is defined in units of a plurality of nozzles 121a to 124a. There are as many thinning patterns as the number of times main scanning is performed on one main scanning line, and the thinning patterns in one main scanning line complement each other with empty dots. Note that the data representing the overlap amount and the data representing the thinning pattern may be set in advance in the controller 130 of the image forming apparatus 100.

  The thinning pattern in this embodiment will be described in detail. In the present embodiment, as shown in FIG. 3, the number of nozzles 121 a to 124 a of the recording heads 121 to 124 is 18 and the nine nozzles 121 a to 124 a on the upstream side in the transport direction of the recording medium WK are the first ones. With the nozzle group NS1 and the remaining nine nozzles 121a to 124a on the downstream side in the transport direction of the recording medium WK as the second nozzle group NS2, thinning patterns MP1 and MP2 are prepared for the respective nozzle groups NS1 and NS2. . That is, the thinning patterns MP1 and MP2 are a set of dot rows for forming an image on the main scanning line for each of the nozzles 121a to 124a constituting the nozzle groups NS1 and NS2. Each of these thinning patterns MP1 and MP2 has a complementary relationship that completes an image on the recording area by main scanning the recording heads 121 to 124 a total of twice in the same recording area on the recording medium WK. is there. In other words, in the present embodiment, the ratio of thinning out dots constituting an image formed on one main scanning line is about 50%. In FIG. 3, the dot formation results in the present embodiment are shown in a solid color. Further, although the shape of each dot formed on the recording medium is originally circular, it is shown in the upper shape for convenience of explanation.

  Each of these thinning patterns MP1 and MP2 is configured such that the dot formation density changes periodically (sine curve) in the main scanning direction and the sub-scanning direction. Specifically, the dot formation density in each of the thinning patterns MP1 and MP2 changes by 9 dots in the main scanning direction and the sub-scanning direction for one cycle. In other words, the data relating to the thinning pattern in the data representing the thinning pattern has a sine wave shape between a value having the lowest dot formation density (for example, “0”) and a value having the same density (for example, “100”). It consists of multi-value data that changes continuously. In addition, since the thinning patterns MP1 and MP2 shown in FIG. 3 have a small number of dots constituting the thinning pattern, it is difficult to understand the periodicity of the dot formation density. Therefore, FIG. 4A shows the periodicity of the dot formation density in the dot row in the main scanning direction in the thinning patterns MP1 and MP2, and FIG. 4 shows the periodicity of the dot formation density in the dot row in the sub-scanning direction. Shown in B).

  4A, the dot formation density in the main scanning direction in the thinning pattern MP1 is “dense”, “rough”, “dense”, “rough”, “dense” from the left end in the main scanning direction to the right end in the drawing. ”In this order. The dot formation density in the main scanning direction in the thinning pattern MP2 is a reverse period that complements empty dots in the thinning pattern MP1, specifically, “rough” and “dense” from the left end to the right end in the main scanning direction. ”,“ Coarse ”,“ dense ”, and“ coarse ”. On the other hand, in FIG. 4B, the dot formation density in the sub-scanning direction in the patterns MP1 and MP2 is “rough”, “dense”, “rough”, “dense” from the lower end in the sub-scanning direction toward the upper end in the drawing. , In the order of “coarse”. Further, the dot formation density in the sub-scanning direction in the thinning pattern MP2 is the reverse period of complementing the empty dots in the thinning pattern MP1, specifically “dense” and “rough” from the lower end in the sub-scanning direction toward the upper end in the drawing. ”,“ Dense ”,“ Coarse ”, and“ Dense ”. That is, an image is completed by a set of thinning patterns MP1 and MP2 that complement these empty dots.

  4A and 4B, the “rough” or “dense” regions exist at both ends of the thinning patterns MP1 and MP2, because the “rough” or “dense” period starts in the middle of the phase. It depends on. The dot formation density originally changes continuously from “rough” to “dense” and from “dense” to “rough”, but the number of dots constituting the thinning pattern in this embodiment is small, Since the formation is binary information on whether or not to form dots, the change is not smooth. A part of the thinning pattern actually used by the present inventor is shown in FIG. According to the thinning pattern shown in FIG. 5, it can be confirmed that the dot formation density periodically changes in the main scanning direction and the sub-scanning direction.

  The controller 130 that has input the image data, the data representing the overlap amount, and the data representing the thinning pattern specifies the number of main scans necessary for forming the entire image on the recording medium WK using the data related to the image data and the overlap amount. Then, image data for each main scan is generated. Next, the controller 130 generates thinned image data obtained by thinning image data for one main scan of the recording head unit 120 based on data representing a thinning pattern. Specifically, the controller 130 calculates each logical product of the image data for one main scan and the thinning patterns MP1 and MP2 to thereby perform one main scanning for each thinning pattern MP1 and MP2. Generate thinned image data. Then, the controller 130 starts image formation, that is, printing based on the generated thinned image data. Specifically, the controller 130 controls the operations of the X-axis direction feed motor 103, the Y-axis direction scan motor 108, and the ink heads 1322a to 132d based on the thinned image data, and prints an image represented by the thinned image data. To start.

  The controller 130 generates thinned image data for each main scan of the recording head unit 120, conveys the recording medium WK for each main scanning of the recording head unit 120, and forms an image on the recording medium WK. In this case, the controller 130 completes the image by performing the main scanning of the recording head unit 120 twice based on the thinned image data that complement each other with respect to the same recording area in the recording medium WK.

  When dots are formed on the recording medium WK, the density of dots formed by the thinning patterns MP1 and MP2 changes periodically. The change in the dot formation density appears as a change in image density on the recording medium WK. FIGS. 6A to 6C show how the density of the formed image changes in the main scanning direction on a certain main scanning line on the recording medium WK. In FIG. 6A, a curve indicated by a thin solid line indicates a change in density when dots are formed by the thinning pattern MP1, and a curve indicated by a thin broken line indicates a change in density when dots are formed by the thinning pattern MP2. Is shown. In any case of density change, the density continuously changes in the opposite period. A straight line indicated by a dark solid line corresponding to the total value of these curves indicates a change in the density of the image completed by the thinning patterns MP1 and MP2 on the same main scanning line. In this case, the density of the image completed on the same main scanning line becomes constant by complementing the excess or deficiency of dot formation by the thinning patterns MP1 and MP2.

  On the other hand, in the case of forming an image on the same main scanning line, when dots are formed by the thinning pattern MP2, the change in image density when the recording head unit 120 is shifted by 0.5 dots in the main scanning direction is shown. 6 (B). In the figure, a thin solid line indicates a change in density when dots are formed by the thinning pattern MP1. Further, a curve indicated by a thin broken line indicates a change in density when dots are formed by the thinning pattern MP2 in a state where the recording head unit 120 is shifted by 0.5 dots in the main scanning direction. A straight line indicated by a dark solid line corresponding to the total value of these curves indicates a change in density of an image completed on the same main scanning line by thinning patterns MP1 and MP2. According to the figure, due to the displacement of the recording head unit 120, the thinned image by the thinned pattern MP1 and the thinned image by the thinned pattern MP2 do not completely complement each other, but the density of the image completed by each thinned image is gentle and continuous. Changes. When the density change according to the present invention shown in FIG. 6B is compared with the density change according to the conventional example shown in FIG. 11B, the change in the dot density according to the present invention is small and continuous. It is clear that For this reason, density unevenness (black streaks, white streaks) with clear contrast as in the prior art becomes inconspicuous.

  Further, FIG. 6C shows an image when the recording head unit 120 is shifted by one dot in the main scanning direction when forming dots by the thinning pattern MP2 when forming an image on the same main scanning line. Shows the change in concentration. In the figure, a thin solid line indicates a change in density when dots are formed by the thinning pattern MP1. Further, a curve indicated by a thin broken line indicates a change in density when dots are formed by the thinning pattern MP2 in a state where the recording head unit 120 is shifted by one dot in the main scanning direction. A straight line indicated by a dark solid line corresponding to the total value of these curves indicates a change in the density of the image completed by the thinning patterns MP1 and MP2 on the same main scanning line. Even in this case, due to the displacement of the recording head unit 120, the thinned image by the thinned pattern MP1 and the thinned image by the thinned pattern MP2 do not completely complement each other, but the density of the image completed by each thinned image is gentle and continuous. Changes. If the density change according to the present invention shown in FIG. 6C is compared with the density change according to the conventional example shown in FIG. 11C, the amount of change in the dot density according to the present invention is small and continuous. It is clear that For this reason, density unevenness (black streaks, white streaks) with clear contrast as in the prior art becomes inconspicuous. 6B and 6C show the change in image density when the recording head unit 120 is displaced in the main scanning direction. However, the recording head unit 120 is sub-scanned due to the conveyance error of the recording medium WK. Similarly, the density unevenness becomes inconspicuous even when the direction is shifted.

  Here, a thinning pattern in which the dot recording density is not periodically changed, that is, an image formation example in the case where an image is formed using a thinning pattern according to the prior art, and the dot recording density is periodically changed. FIGS. 7 and 8 show examples of image formation when an image is formed using a thinning pattern, that is, a thinning pattern to which the present invention is applied. In these drawings, an image is shown when the recording head unit 120 is displaced by one dot in the sub-scanning direction during main scanning indicated by an arrow. According to these figures, it can be clearly confirmed that the density unevenness is less noticeable in the image formed by the thinning pattern to which the present invention is applied than in the image formed by the thinning pattern according to the prior art. it can.

  When dots are formed for all of the image data output from the external computer device 140, the controller 130 returns the recording head unit 120 to the origin position and ejects the recording medium WK forward to again. It will be in a standby state. On the other hand, the operator removes the recording medium WK from the platen 101 and finishes the processing operation. Thereby, the image printing operation on the recording medium WK is completed.

  As can be understood from the above description of operation, according to the above embodiment, the dot formation density in the thinning patterns MP1 and MP2 that thin out the dot rows constituting the image formed on the recording medium WK at a predetermined ratio and complement each other. Is supposed to change periodically. According to this, an image formed on the recording medium WK has a high (or low) dot row formation density in addition to a recording region formed by a set of thinned dot rows in which the dot row formation densities are substantially equal to each other. ) The recording area is formed by a set of thinned dot rows having the same formation density as the thinned dot rows. That is, when forming an image on the recording medium WK, even if the recording head unit 120 is displaced by several dots in the main scanning direction and / or the sub-scanning direction with respect to the recording medium WK, the dot row formation density In a recording area where an image is formed by a set of thinned dot rows MP1 and MP2 having different density, density unevenness becomes inconspicuous due to thinned dot rows having a high dot row formation density. As a result, the quality of the image finally formed on the recording medium WK can be improved.

  Furthermore, in carrying out the present invention, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the object of the present invention.

  In the above embodiment, the image is completed on the recording medium by the collection of the thinned images by the thinning patterns MP1 and MP2. However, the present invention is not limited to this as long as a desired image is completed by a set of periodically thinned images. In other words, the image may be completed by a thinned dot array in which the dot formation density is periodically changed in units of the nozzles 121a to 124a of the recording heads 121 to 124. Also by this, the same effect as the above-mentioned embodiment can be expected.

  In the above embodiment, the cycle of the thinning patterns MP1 and MP2 is 1 for 9 dots corresponding to the 9 nozzles 121a to 124a constituting the first nozzle group NS1 and the second nozzle group NS2. Period. However, the period in the thinning patterns MP1 and MP2 is appropriately set according to the resolution of the image formed on the recording medium WK, the nozzle pitch of the recording heads 121 to 124, and the like, and is not limited to the above embodiment. Absent. In this case, the cycle of the thinning patterns MP1 and MP2 in the main scanning direction may be (image resolution in the main scanning direction / recording density of the recording head in the main scanning direction) × 2 dots or more. The cycle of the thinning patterns MP1 and MP2 in the sub-scanning direction is (image resolution in the sub-scanning direction / recording density of the recording head in the sub-scanning direction (nozzle pitch)) × 2 dots or more and the nozzles of the recording head What is necessary is just to make it into 1/2 or less of a number. Also by these, the same effect as the above embodiment can be expected.

  In the above embodiment, the dot row formation density in the thinning patterns MP1 and MP2 is configured to periodically change in the main scanning direction and the sub-scanning direction of the recording head unit 120, respectively. This is to make the density unevenness caused by the deviation in the main scanning direction and the sub-scanning direction of dots formed on the recording medium WK inconspicuous. Therefore, it is not always necessary to configure the dot row formation density in the thinning patterns MP1 and MP2 so as to have periodicity in the main scanning direction and the sub-scanning direction, and the period in either the main scanning direction or the sub-scanning direction. The thinning pattern may be configured to have a property. Specifically, the main scanning direction or the sub-scanning direction in the thinning patterns MP1 and MP2 corresponding to either the main-scanning direction deviation or the sub-scanning direction deviation of the dots formed on the recording medium WK. What is necessary is just to comprise so that it may have periodicity in the formation density of this dot row. According to this, it is possible to make the density unevenness due to the deviation of the dots in the direction in which the dot row formation density in the thinning patterns MP1 and MP2 has a periodicity inconspicuous.

  In the above embodiment, the image is completed by performing the main scanning twice for the same recording area in the recording medium WK. However, the present invention is not limited to this as long as the image is completed by performing a plurality of main scans on the same recording area in the recording medium WK. In other words, the image may be completed by performing three or more main scans on the same recording area in the recording medium WK. For example, as shown in FIG. 9, when the interval D in the sub scanning direction of the nozzles 121a to 124a of the recording heads 121 to 124 is 360 dpi, and the recording resolution of the image in the sub scanning direction of the recording medium WK is 720 dpi, the recording medium An image can be completed by four main scans of the recording heads 121 to 124 twice in the main scanning direction and the sub-scanning direction for the same recording area in the WK. Also by this, the same effect as the above-mentioned embodiment can be expected.

  In the above embodiment, the nozzles 121a to 124a in the recording heads 121 to 124 are arranged in a staggered manner, but it is not always necessary to arrange them in a staggered manner. That is, they may be arranged on a straight line. Further, the number of recording heads 121 to 124 constituting the recording head unit 120 may be appropriately prepared according to the type of ink color to be used. For example, the recording head unit 120 may be configured with six recording heads when six different colors of ink are used, and with eight recording heads when eight different colors of ink are used. Further, in the case of using monochromatic ink, the recording head unit may be configured with one recording head. In these cases, the recording heads may be arranged side by side in the main scanning direction, or may be arranged in a staggered or stepwise manner. According to these, the same effect as the above embodiment can be expected.

1 is a perspective view illustrating an entire image forming apparatus according to an embodiment of the present disclosure. FIG. 2 is a block diagram of a control system that controls the operation of the image forming apparatus shown in FIG. 1. FIG. 2 is an explanatory diagram for explaining a relationship between a configuration of a nozzle of a recording head included in the image forming apparatus illustrated in FIG. 1 and a thinning pattern image formed by the nozzle. (A) is explanatory drawing which shows the thinning method in the main scanning direction in a thinning pattern, (B) is explanatory drawing which shows the thinning method in the subscanning direction in a thinning pattern. It is explanatory drawing which shows a part of thinning pattern actually used by this inventor. (A) is a graph showing density changes in the main scanning direction of an image formed on a certain main scanning line on the recording medium. (B) and (C) are graphs when the recording head is displaced in the main scanning direction. 5 is a graph showing a change in density in the main scanning direction of the same image. FIG. 10 is an explanatory diagram illustrating an example of image output according to the related art when the recording head is displaced in the main scanning direction. FIG. 10 is an explanatory diagram illustrating an example of image output according to the related art when the recording head is displaced in the main scanning direction. It is explanatory drawing which shows the image formation process by the thinning-out turn which concerns on the modification of this invention. (A) is explanatory drawing for demonstrating the relationship between the structure of the nozzle of the recording head based on a prior art, and the thinning pattern image formed with the nozzle, (B) is a recording head having shifted | deviated to the main scanning direction. FIG. 6C is an explanatory diagram showing an example of image formation according to the conventional technique at the time, and FIG. 8C is an explanatory diagram showing an example of image formation according to the conventional technique when the recording head is displaced in the sub-scanning direction. (A) is explanatory drawing which shows the density of the dot on the recording medium when the dot is formed at the correct position, and (B) is when the dot formation position is shifted by 0.5 dot in the main scanning direction. It is explanatory drawing which shows the density | concentration of the dot on a recording medium, (C) is explanatory drawing which shows the density | concentration of the dot on a recording medium in case the dot formation position has shifted | deviated only 1 dot in the main scanning direction.

Explanation of symbols

WK: Recording medium, MP1, MP2: Thinning pattern, NS1: First nozzle group, NS2: Second nozzle group, 100: Image forming apparatus, 101: Platen, 102: Grid roll, 103 ... X-axis direction feed motor 104 ... guide rails, 105a, 105b ... pinch rollers, 108 ... Y-axis direction feed motor, 120 ... recording head unit, 121-124 ... recording head, 121a-124a ... nozzle, 130 ... controller, 140 ... external computer device.

Claims (3)

A recording head in which a plurality of nozzles are arranged to form dots by ejecting ink on a recording medium;
Scan driving means for displacing the recording head relative to the recording medium in a main scanning direction orthogonal to the direction in which the nozzles in the recording head are arranged;
Feed driving means for displacing the recording medium relative to the recording head in a sub-scanning direction in which nozzles in the recording head are arranged;
The recording head is configured to complete an image on the same main scanning line of the recording medium by a set of thinning dot arrays that thin out dot arrays constituting an image to be formed on the recording medium at a predetermined ratio and complement each other. In an image forming apparatus provided with a recording head driving unit that controls ejection of ink from
The recording head driving unit controls ejection of ink from the recording head such that the dot formation density in the thinned dot row periodically changes in at least one of the main scanning direction and the sub-scanning direction. An image forming apparatus. The image forming apparatus according to claim 1,
The recording head driving means periodically changes the dot formation density in the thinned dot row for each nozzle group obtained by dividing the recording head into a plurality of areas, and is different from each other for the same recording area on the recording medium. An image forming apparatus that completes the image by the nozzle group. The image forming apparatus according to claim 1, wherein:
An image forming apparatus in which the dot formation density changes in a sine wave shape.