JP2014061624A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a favorable image by preventing impact deviation of ink caused by transportation air stream even when the impact deviation of ink is influenced by an autogenous airflow.SOLUTION: In an image formation device, when an ink jet density of a nozzle 38 increases, a degree of an autogenous airflow W1 generated between the nozzle 38 and a sheet PA in an ink discharge direction becomes stronger. When an autogenous airflow degree indicating the degree of the autogenous airflow becomes higher, a degree of the ink flowed by transportation air stream W2 generated from an upstream side of a transportation direction to a downstream side in accordance with transportation of the sheet PA becomes weaker, an ink impact position is deviated in the upstream side of the transportation direction. A discharge quantity of the ink from each of the nozzles 38 is controlled by a controller on the basis of the ink jet density. Therefore, in a case that the degree of the discharged ink flowed to the downstream side of the transportation direction by the transportation air stream W2 is changed by influence of the autogenous airflow W1 generated in the ink discharge direction, the deviation of the ink impact position in the transportation direction W2 is prevented.

Description

  The present invention relates to an image forming apparatus that forms an image by ejecting ink from an inkjet head onto a recording medium that is transported on a transport path.

  In an inkjet printing apparatus that forms an image by ejecting ink from a nozzle of an inkjet head onto a recording medium that is conveyed on a conveyance path, an ink chamber that communicates with the nozzle is provided inside the inkjet head, and the volume of the ink chamber is determined by a drive signal. The ink droplets are ejected from the nozzles by changing (expanding and contracting).

  By the way, when the recording medium is transported directly below the ink jet head, an air flow is generated from the upstream to the downstream in the transport direction. Further, when the recording medium is sucked onto the conveying belt with a negative pressure and conveyed at a high speed, the air flow caused by the negative pressure generates an air flow directly under the ink jet head.

  Therefore, in a non-contact printing method in which ink droplets are ejected from a nozzle to a recording medium, the ink droplets are affected by these air currents (hereinafter referred to as conveyance air currents), and the ink droplets are moved downstream in the recording medium conveyance direction. This causes a so-called landing deviation in which the recording medium is displaced from the target trajectory and adheres to the recording medium, causing deterioration in image quality (white stripes, black stripes, color image color change, etc.) due to density unevenness. .

  There exists patent document 1 with respect to such a problem, for example. In the technique of Patent Document 1, when an ink droplet is ejected while relatively moving an inkjet head having a plurality of nozzles and a recording medium in a direction intersecting the nozzle arrangement direction, a droplet having a smaller droplet amount is ejected. Control to increase the speed. Thereby, the landing deviation of the ink droplet due to the conveying airflow is suppressed.

  Further, the cause of density unevenness is in addition to the above-described carrier airflow. For example, in a share mode type ink jet head in which a plurality of nozzles and ink chambers are provided in the ink jet head, ink discharge characteristics of each nozzle vary due to individual differences (characteristic differences) such as piezoelectric members that change the volume of the ink chamber. May occur, causing density unevenness.

  Therefore, in order to eliminate individual differences (characteristic differences) in the ink discharge amount of each nozzle, it is known to correct the ink discharge amount by the nozzle with correction contents corresponding to variations in the ink discharge characteristics of each nozzle. (For example, Patent Document 2).

  In the share mode type ink jet head, vibration due to a change in volume generated in the ink chamber near the center in the main scanning direction is transmitted to the ink chamber near the end in the main scanning direction. Then, the vibrations transmitted from the other ink chambers are accumulated in the ink chamber at the end in the main scanning direction, which may affect the characteristics of the ink ejection amount and cause density unevenness.

  Therefore, in order to eliminate the change in the ink discharge amount characteristic due to the vibration transmitted and accumulated from other ink chambers, the ink discharge amount correction by the nozzle is corrected with the correction content according to the nozzle arrangement in the main scanning direction. It is known to perform (for example, patent document 3).

  Further, the ink jet head may be composed of a plurality of head blocks arranged along the main scanning direction orthogonal to the recording medium conveyance direction. In this case, the two adjacent head blocks are arranged in a staggered manner with their positions shifted in the conveyance direction (sub-scanning direction) of the recording medium so that the end portions thereof partially overlap each other.

  At the joint where the ends of two head blocks overlap in the main scanning direction, the spacing between adjacent nozzles in the main scanning direction of each head block depends on the positioning accuracy of both head blocks, and the nozzle spacing in the same head block May be different. If the nozzle interval in the joint portion is different from the nozzle interval in the other portion, the dot interval of the landed ink is shifted, and white stripes and black stripes in the transport direction are generated in the formed image.

  Therefore, in order to prevent the occurrence of white and black stripes at the joint portion, it is known to correct the ink discharge amount by the nozzle at the joint portion with the correction content according to the interval between the nozzles at the joint portion ( For example, Patent Document 4).

  As described above, in addition to the control for suppressing the density unevenness due to the air flow, the control for suppressing the density unevenness has been proposed. Therefore, it is conceivable to perform the various corrections described above in combination in accordance with the cause of density unevenness existing in the ink jet printing apparatus.

JP 2010-173178 A JP 2007-19998 A JP 2006-326958 A JP 2004-154950 A

  Incidentally, the ink droplets ejected from the nozzles of the inkjet head toward the recording medium may cause a self-air flow. This self-airflow is directed directly under the nozzle in the same direction as the ink droplets are ejected from the nozzle.

  Since this self-airflow includes a directional component that assists the straightness of the ink ejected from the nozzles, depending on the strength, the degree of deviation of the landing of the ink in the conveyance direction of the recording medium due to the above-described conveyance airflow is determined. It can bring about change.

  However, the conventional control for suppressing the above-described various density non-uniformities other than the density non-uniformity due to the air flow is in response to an event that has no relation to the self-air flow described above, and thus occurs due to the air flow. It has no effect on alleviating the influence of self-stream on ink landing deviation.

  The present invention has been made in view of the above points, and when an image is formed on a recording medium by ejecting ink from each nozzle of an inkjet head above the recording medium conveyed on the conveyance path, An object of the present invention is to provide an image forming apparatus capable of forming a good image in which landing deviation is suppressed even if the self-airflow affects the landing deviation of ink generated by the conveying airflow.

In order to achieve the above object, an image forming apparatus according to a first aspect of the present invention provides:
A plurality of nozzles are arranged on the inkjet head arranged above the conveyance path along a main scanning direction orthogonal to the conveyance direction of the recording medium conveyed on the conveyance path, and ink is ejected from each nozzle. In an image forming apparatus for forming an image on the recording medium,
Control means for correcting the amount of ink discharged from each nozzle based on the ink discharge density from each nozzle to the recording medium,
It is characterized by that.

  The image forming apparatus of the present invention described in claim 2 is the image forming apparatus of the present invention described in claim 1, wherein the amount of ink discharged from each nozzle to the recording medium is set to the discharged ink. Correction means for correcting according to density unevenness of the image formed on the recording medium by the control means, and the control means records the correction content corresponding to each nozzle by the correction means from each nozzle. Each correction is performed based on the discharge density of ink on the medium.

  Furthermore, the image forming apparatus of the present invention described in claim 3 is the image forming apparatus of the present invention described in claim 1 or 2, wherein the discharge density is orthogonal to the transport direction of the nozzle for discharging ink. It is characterized in that it is determined based on at least one of the number of continuous nozzles in the main scanning direction and the number of continuous lines in the transport direction of dots from which the same nozzle ejects ink.

  The image forming apparatus of the present invention described in claim 4 is the image forming apparatus of the present invention described in claim 3, wherein the discharge density of the ink discharged from the same nozzle to the same dot is the same. It is determined based on the number of drops.

  Furthermore, the image forming apparatus of the present invention described in claim 5 is the image forming apparatus of the present invention described in claim 1, 2, 3 or 4, wherein the head gap is an interval between the inkjet head and the recording medium. And a head gap adjusting unit that adjusts the amount of ink discharged from the nozzles according to the head gap.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, or fifth aspect of the present invention, the nozzle is formed according to the conveyance speed of the recording medium. The control content of the ink ejection amount by the control is adjusted.

  According to the present invention, when ink is ejected from each nozzle of an inkjet head above the recording medium transported on the transport path to form an image on the recording medium, the landing deviation of ink generated by the transport airflow Even if the self-air flow influences, a good image in which landing deviation is suppressed can be formed.

  That is, according to the image forming apparatus of the present invention described in claim 1, when the ink discharge density by the nozzles of the inkjet head increases, the ink discharge direction along the ink discharge direction between the nozzles and the recording medium (conveyance path). The degree of self-flow generated is increased. When the degree of self-airflow indicating this degree increases, the degree of ink flow caused by the conveyance airflow generated from the upstream side to the downstream side in the conveyance direction accompanying the conveyance of the recording medium decreases, and the ink landing position becomes upstream in the conveyance direction. It will shift to the side.

  In contrast, the amount of ink discharged from each nozzle is controlled by the control unit based on the discharge density of ink discharged from each nozzle. Therefore, even if the degree to which the ejected ink is caused to flow downstream in the transport direction by the transport airflow changes due to the influence of the self-stream generated along the ink discharge direction, the ink landing position is not shifted in the transport direction. It is possible to form a good image with suppressed landing deviation.

  Further, according to the image forming apparatus of the present invention described in claim 2, in the image forming apparatus of the present invention described in claim 1, density unevenness of an image formed on the recording medium by the ink ejected from each nozzle is reduced. When the correction means suppresses by correcting the ink discharge amount, even if the ink landing position changes to the downstream side in the transport direction due to the influence of the self-flow, the correction contents by the correction means are further corrected. By doing so, it is possible to suppress occurrence of landing deviation in the transport direction in the ink ejected from the nozzles, and it is possible to form a good image.

  Furthermore, according to the image forming apparatus of the present invention described in claim 3, in the image forming apparatus of the present invention described in claim 1 or 2, in the upstream nozzle row, the nozzles that eject ink are in the main scanning direction. If they are adjacent to each other and are continuous, a plurality of self-airflows are combined and developed in a band shape (wall shape). Thereby, the effect | action that an ink advances linearly against a conveyance airflow rather than one self-airflow becomes large. In addition, when dots that eject ink from the same nozzle continue over a plurality of lines in the transport direction, the period at which the nozzle ejects ink becomes shorter, so the stability of the self-stream generated by the ejection of ink increases, Increased self-flow rate.

  Therefore, the ink discharge density of each nozzle can be determined based on the number of continuous nozzles in the main scanning direction of the ink discharge nozzles and the number of continuous lines in the transport direction of dots from which the same nozzle discharges ink. Then, by adjusting the discharge amount of ink in consideration of the degree of self-flow according to the determined discharge density, it is possible to form a good image by suppressing the ink discharged from the nozzle from landing and shifting in the transport direction. Can do.

  According to the image forming apparatus of the present invention described in claim 4, in the image forming apparatus of the present invention described in claim 3, the level is determined by the number of ink drops ejected by the same nozzle to the same dot. When adjusting the tone, the greater the number of drops of ink ejected to the same dot, the longer the continuous generation time of the self-air flow. Becomes higher.

  Therefore, not only the number of continuous nozzles in the main scanning direction of the ink ejection nozzles and the number of continuous lines in the transport direction of dots from which the same nozzle ejects ink, but also the number of ink drops ejected by the same nozzle to the same dots In consideration of the above, it is possible to determine the ink discharge density of each nozzle. Based on the determined discharge density, the amount of ink discharged is adjusted in consideration of the degree of occurrence of self-airflow, and the ink discharged from each nozzle is suppressed from landing and shifting in the sub-scanning direction, thereby producing a good image. Can be formed.

  Furthermore, according to the image forming apparatus of the present invention described in claim 5, in the image forming apparatus of the present invention described in claim 1, 2, 3 or 4, for example, a recording medium having a large thickness such as an envelope is used. In order to prevent the recording medium from colliding with the ink jet head, the head gap between the ink jet head and the recording medium may be widened by the head gap adjusting means.

  When the head gap increases and the interval between the inkjet head and the recording medium increases, the airflow of the carrier easily flows between the inkjet head and the recording medium. In addition, since the head gap is widened, the flying distance and the flying time of the ink are increased, so that the amount of ink that flows is increased. That is, the ink ejected from each nozzle is likely to be caused to flow in the sub-scanning direction by the transport airflow.

  Therefore, even if the head gap is adjusted according to the thickness of the recording medium, etc., by adjusting the control content of the ink discharge amount by each nozzle according to the head gap, By controlling the amount of ink ejected from the nozzles according to the content, it is possible to suppress the ink ejected from each nozzle from landing and shifting in the sub-scanning direction, thereby forming a good image.

  Further, according to the image forming apparatus of the present invention described in claim 6, in the image forming apparatus of the present invention described in claim 1, 2, 3, 4 or 5, for example, when the conveyance speed of the recording medium increases. A change corresponding to the increase in the speed of the transport airflow occurs to the extent that the speed of the transport airflow increases and the self-airflow generated by each nozzle advances the ink straight.

  Therefore, even if the recording medium conveyance speed is changed by adjusting the control content of the ink ejection amount by each nozzle according to the recording medium conveyance speed, the nozzles can be changed according to the conveyance speed. As a result, the amount of ink discharged by the nozzle is adjusted to prevent the ink discharged from each nozzle from landing and shifting in the sub-scanning direction, and a good image can be formed.

1 is a schematic configuration diagram of an inkjet printing apparatus according to a first embodiment of the present invention. It is a block diagram which shows the structure of the control system of the inkjet printing apparatus of FIG. FIG. 2 is a partial enlarged cross-sectional view of a conveyance belt and a plastic template in FIG. 1. It is explanatory drawing which shows arrangement | positioning of the head block of FIG. (A) is explanatory drawing which shows the relationship between the nozzle in the head block of FIG. 1, and the ink discharge amount of each nozzle, (b) is offset correction memorize | stored in the control part of FIG. 2 corresponding to the nozzle of (a). It is explanatory drawing which shows an example of a table. (A) is explanatory drawing which shows the relationship between the nozzle arrangement in the head block of FIG. 1, and the ink discharge amount of each nozzle, (b) is the arrangement memorize | stored in the control part of FIG. 2 corresponding to the nozzle of (a). It is explanatory drawing which shows an example of another correction table. It is explanatory drawing of the self-air flow which generate | occur | produces in the head block of FIG. FIG. 2 is an explanatory diagram illustrating landing deviation according to the ejection density of ink ejected from the nozzles of the head block of FIG. 1. It is explanatory drawing which shows an example of the additional correction table corresponding to the offset correction table of FIG.5 (b). It is explanatory drawing which shows an example of the additional correction table corresponding to the correction table classified by arrangement | positioning of FIG.6 (b). It is explanatory drawing which shows arrangement | positioning of the ink head which has a joint nozzle, and the image formed with the ink head. It is explanatory drawing which shows an example of the test pattern used when determining the correction content of the additional correction table of FIG.9 and FIG.10. It is a flowchart for demonstrating the procedure at the time of determining the correction content of the additional correction table of FIG. 11 is a flowchart for explaining the operation of the ink jet printing apparatus when correcting the ink discharge amount of the correction target nozzle by applying the correction content of the additional correction table of FIG. 9 or FIG. 10. (A) is explanatory drawing which shows the nozzle which ejected the ink in the experiment which shows the relationship between the amount of landing deviations, and the distance between a paper and an ejection surface, (b) is the landing deviation in the subscanning direction in the said experiment. It is explanatory drawing which shows the relationship between a quantity and the distance between a paper and an ejection surface.

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

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

  FIG. 1 is a schematic configuration diagram of an inkjet printing apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing a configuration of a control system of the inkjet printing apparatus of FIG. 1, and FIG. FIG. 4 is an explanatory view showing the arrangement of the head block of FIG.

  In the following description, the paper surface direction in FIG. Further, as shown in FIG. 1, the top, bottom, left, and right sides of the paper are defined as the top, bottom, left, and right directions. Further, a path indicated by a broken line in FIG. 1 is a transport path R along which the paper PA that is a recording medium is transported. In the following description, upstream and downstream mean upstream and downstream in the transport path R, respectively.

  As shown in FIGS. 1 and 2, the inkjet printing apparatus 1 includes a paper feeding unit 2, a transport unit 3, a printing unit 4, a control unit 5, and a reading unit 6.

  The paper feeding unit 2 feeds paper PA. The sheet feeding unit 2 includes a sheet feeding table 11, a sheet feeding roller 12, and a registration roller 13. The paper feed table 11 is used for loading paper PA used for printing.

  The paper feed roller 12 picks up the paper PA stacked on the paper feed tray 11 one by one and conveys the paper PA toward the registration roller 13. The paper feed roller 12 is disposed on the upper side of the paper feed tray 11. The paper feed roller 12 is rotationally driven by a motor (not shown).

  The registration roller 13 temporarily stops the paper PA conveyed by the paper supply roller 12 and then conveys the paper PA toward the conveyance unit 3. The registration roller 13 is disposed on the downstream side of the paper feed roller 12. The registration roller 13 is rotationally driven by a motor (not shown).

  The transport unit 3 transports the paper PA transported from the registration rollers 13. The transport unit 3 includes a transport belt 21, a drive roller 22, driven rollers 23 to 25, a belt drive motor 26, a plastic template 27, and a fan 28.

  The conveyor belt 21 is an annular belt that is stretched around the driving roller 22 and the driven rollers 23 to 25. As shown in FIG. 3, the conveyor belt 21 is formed with a number of belt holes 21a that are through holes for attracting and holding the paper PA. The conveyance belt 21 sucks and holds the paper PA on a paper holding surface (corresponding to a medium holding surface in claims) 21b by the suction force generated in the belt hole 21a by driving the fan 28. The sheet holding surface 21 b is an upper surface of the conveyance belt 21 that is substantially horizontal between the driving roller 22 and the driven roller 23.

  The conveyor belt 21 rotates in the clockwise direction in FIG. Accordingly, the transport belt 21 moves endlessly, thereby transporting the paper PA sucked and held on the paper holding surface 21b in the right direction.

  The driving roller 22 and the driven rollers 23 to 25 are for the conveyance belt 21 to be stretched over. The driving roller 22 is driven to rotate by a belt driving motor 26 and rotates the conveyor belt 21. The driven rollers 23 to 25 are driven by the driving roller 22 via the conveyance belt 21. The driven roller 23 is substantially the same height as the drive roller 22 and is spaced from the drive roller 22 by a predetermined distance in the left-right direction. The driven rollers 24 and 25 are disposed at substantially the same height below the driving roller 22 and the driven roller 23 and spaced apart from each other by a predetermined distance in the left-right direction. The belt drive motor 26 drives the drive roller 22 to rotate.

  The plastic template 27 is disposed below the conveying belt 21 between the driving roller 22 and the driven roller 23, and supports the lower surface of the conveying belt 21 so as to be slidable. The plastic template 27 includes a plurality of concave portions 27a dug down from the upper surface toward the lower surface at a portion where the belt hole 21a passes, and a plurality of suction holes 27b penetrating from the bottom surface of the concave portion 27a to the lower surface of the plastic template 27. Have

  The fan 28 generates a downward airflow. As a result, the fan 28 sucks air through the suction holes 27b and the concave portions 27a of the plastic template 27 and the belt holes 21a of the conveying belt 21 to generate a negative pressure in the belt holes 21a. Adsorb on 21b. The fan 28 is disposed below the plastic template 27.

  The printing unit 4 performs printing on the paper PA conveyed by the conveyance unit 3. The printing unit 4 is provided on the upper side of the transport unit 3. The printing unit 4 is fixed in a housing (not shown) of the inkjet printing apparatus 1. The printing unit 4 includes inkjet heads 31 </ b> C, 31 </ b> K, 31 </ b> M, and 31 </ b> Y, a head holder 32, and a head gap adjustment unit (corresponding to the head gap adjustment means in the claims) 33. Note that alphabetic suffixes (C, K, M, Y) indicating colors in codes may be omitted when there is no need to distinguish colors.

  The inkjet heads 31C, 31K, 31M, and 31Y discharge cyan (C), black (K), magenta (M), and yellow (Y) inks, respectively. The inkjet heads 31C, 31K, 31M, 31Y are arranged in parallel in the left-right direction above the transport unit 3. The inkjet heads 31C, 31K, 31M, and 31Y are line-type inkjet heads, each having six head blocks 35 as shown in FIG.

  In each of the inkjet heads 31C, 31K, 31M, and 31Y, the six head blocks 35 are arranged in a staggered manner. Specifically, the six head blocks 35 are arranged in the front-rear direction (main scanning direction), and every other head block 35 is arranged by shifting the position in the left-right direction (sub-scanning direction).

  As shown in FIG. 1, the head holder 32 holds the head block 35 of the inkjet head 31 above the transport unit 3. The head holder 32 is formed in a hollow, substantially rectangular parallelepiped shape.

  Each head block 35 has a plurality of nozzles 38 on the discharge surface 35a, which is the lower surface, as shown in the explanatory view of FIG. The nozzles 38 are arranged at regular intervals with a predetermined pitch P in the main scanning direction. Each head block 35 can change the number of droplets (drop number) of ink ejected from one nozzle 38 to one pixel, and expresses the density by the number of droplets (for example, 1 to 7 drops). Perform gradation printing.

  The head gap adjustment unit 33 adjusts the head gap H. As shown in FIG. 3, the head gap H is a distance between the paper holding surface 21 b of the transport belt 21 and the ejection surface 35 a of the inkjet head 31. The head gap adjustment unit 33 includes a head gap adjustment mechanism 41, a lift motor 42, and a connection member 43.

  The head gap adjustment mechanism 41 moves the transport unit 3 up and down with respect to the inkjet head 31. Two head gap adjusting mechanisms 41 are provided apart in the front-rear direction. The head gap adjusting mechanism 41 includes a pair of pulleys 46 and 47, a shaft 48, and wires 49 and 50.

  Pulleys 46 and 47 wind and unwind wires 49 and 50, respectively. The pulleys 46 and 47 are supported in the head holder 32 so as to be spaced apart from each other in the left-right direction.

  The shaft 48 connects a pair of pulleys 46 and 47 to each other. The shaft 48 is made of a long member extending in the left-right direction, and one end is fixed to the pulley 46 and the other end is fixed to the pulley 47. Thereby, a pair of pulleys 46 and 47 are rotated synchronously.

  The wires 49 and 50 suspend and support the transport unit 3. One ends of the wires 49 and 50 are connected to the transport unit 3, and the other ends are wound around pulleys 46 and 47. As the wires 49 and 50 are wound and fed by the rotation of the pulleys 46 and 47, the transport unit 3 moves up and down, and the head gap H changes.

  The lifting motor 42 rotates the pulleys 46 and 47. The connection member 43 is a member that connects the head holder 32 and the transport unit 3. The connecting member 43 is configured such that the length in the vertical direction can be adjusted according to the head gap H.

  The reading unit 6 includes a document feeding device and a document image reading device (both not shown), and reads the image by the reading device while transporting the document set on the tray of the feed device and converts it into data. To do.

  By the way, inside each head block 35, a plurality of ink chambers (not shown) communicating with the respective nozzles 38 are provided. Then, by changing the volume of each ink chamber under the control of the control unit 5, ink in the ink chamber is ejected from the corresponding nozzle.

  In such a share mode type head block 35, an ink discharge amount is caused by an individual difference (characteristic difference) of an element (for example, a piezoelectric member constituting a partition wall of each ink chamber) for changing the volume of each ink chamber. Variations occur in the characteristics.

  In the example of the arrangement shown in FIG. 5A, the dot diameters of the inks ejected by the nozzles 38 of the head block 35 with the same ejection amount vary for each nozzle 38, thereby causing density unevenness in the image. .

  Therefore, in order to eliminate the occurrence of such density unevenness, the control unit 5 uses, for example, an offset correction table stored in the hard disk, as in the example of the offset correction table shown in FIG. The amount of ink discharged from each nozzle 38 of the head block 35 is offset-corrected with the correction content specified according to the above.

  In the share mode type head block 35, vibration due to the change in volume generated in the ink chamber near the center in the main scanning direction is transmitted to the ink chamber near the end in the main scanning direction. Then, the vibrations transmitted from the other ink chambers are accumulated in the ink chamber at the end in the main scanning direction, which affects the characteristics of the ink ejection amount.

  In the example of the arrangement shown in FIG. 6A, the dot diameter of the ink ejected by each nozzle 38 of the head block 35 with the same ejection amount becomes larger as it approaches the end portion than the center in the main scanning direction. Density unevenness occurs in which the image density is higher than that of the surrounding area.

  This is because vibration generated by the volume change of each ink chamber (not shown) is sequentially transmitted from the center to the end in the main scanning direction, and the ink chamber closer to the outermost end due to the transmitted vibration is actually ink. This is a phenomenon that occurs when a volume change equal to or greater than the volume change according to the discharge amount occurs and the ink discharge amount increases as shown in the graph in the figure.

  Therefore, in order to eliminate the occurrence of density unevenness due to vibration transmitted and accumulated from other ink chambers, the control unit 5 determines the arrangement of the nozzles 38 in the main scanning direction in, for example, an arrangement-specific correction table stored in a hard disk. The amount of ink discharged from each nozzle 38 of the head block 35 is corrected for each arrangement with the correction content defined accordingly.

  In the example of the arrangement-specific correction table shown in FIG. 6B, correction coefficients are defined for the fifth nozzles 38 from both ends of the head block 35 in the main scanning direction. A correction coefficient of 0.9 is defined for both the first (# 1) and the 300th (# 300) nozzles 38 at the extreme end where the ink discharge amount increases most. For other nozzles 38 (# 2 to # 5, # 296 to # 299), correction coefficients of 0.92 to 0.97 that are lower as they are closer to the end in the main scanning direction are defined. ing.

  In addition to the offset correction and the arrangement-specific correction described above, the control unit 5 corrects the ink ejection amount corresponding to the self-stream generated by the ink ejected from the nozzles 38.

  That is, at the time of printing by the inkjet printing apparatus 1 of the present embodiment, as shown in FIG. 7, the ink droplets 52 are ejected from the head block 35 of the inkjet head 31, so that the self toward the paper PA from the head block 35. Airflow W1 is generated. Further, the conveyance air flow W <b> 2 that is the air flow in the conveyance direction is generated by the conveyance of the paper PA by the conveyance unit 3 and the suction of the air by the fan 28.

  The transport air flow W2 acts to cause the ink droplets 52 to flow downstream in the transport direction and shift landing. On the other hand, the self-air flow W1 described above increases the straightness of the ink droplets 52 toward the paper PA, and reduces the degree of landing of the ink droplets 52 on the downstream side in the transport direction by the transport airflow W2. work.

  The self-air flow W1 becomes stronger when a plurality of nozzles 38 that are continuous in the main scanning direction discharge ink. The self-air flow W1 also increases when ink is repeatedly ejected to a plurality of lines of dots where the same nozzle 38 is continuous. Further, the self-air flow W1 is high when the same nozzle 38 repeats ink ejection to the same dot.

  That is, when there is a high discharge density region where the density per unit area (discharge density) of the ink discharged from the nozzles 38 to the paper PA is high, the degree of the self-air flow W1 generated by the ink discharge becomes strong. Therefore, the self-airflow degree indicating the degree of the self-airflow W1 affects the degree to which the ink droplet 52 is landed and shifted to the downstream side in the transport direction of the paper PA by the transport airflow W2.

  For example, as shown in the explanatory diagram of FIG. 8, when ink is ejected from the nozzles of the cyan (C) and magenta (M) head blocks 35 to the same pixel of the paper PA, # 5 to ## Since the cyan (C) and magenta (M) inks are ejected to the pixels corresponding to the seventh nozzle 38 at the same density, there is no ink landing deviation between the colors. However, since the cyan (C) and magenta (M) inks are ejected to the pixels corresponding to the nozzles # 1 to # 4 at different densities, cyan (C) with respect to the magenta (M) ink. ) Ink landing position is shifted in the sub-scanning direction.

  Although FIG. 8 shows a case where the discharge densities of cyan (C) and magenta (M) inks are different in the sub-scanning direction, the case where the discharge densities are different in the main scanning direction or the discharge density for each dot is shown. A similar situation occurs in different cases.

  When the situation shown in FIG. 8 occurs, the magenta (M) of the same pixel is increased by increasing the discharge amount of cyan (C) ink corresponding to the pixel having a low discharge density to increase the degree of the self-air flow W1. The landing deviation from the ink can be suppressed.

  Therefore, the control unit 5 in FIG. 2 adjusts the ejection amount of the ink droplets 52 ejected by the nozzles 38 that are the targets of the above-described offset correction and placement correction of the head block 35 with respect to the dots in the high ejection density region. Therefore, each additional correction table is stored in advance.

  Each additional correction table includes correction contents to be added to the ink discharge amount correction contents respectively defined in the above-described offset correction table and the arrangement-specific correction table in accordance with the degree of occurrence of the self-airflow W1. 5 is a table that is defined according to the ejection density of ink that affects the degree of self-flow that affects the air flow.

  Here, the ink ejection density can be represented by, for example, a printing rate indicating an area ratio of an actually printed region to a printable region of the paper PA. The printing rate can be defined in a one-dimensional or two-dimensional range by one or both of the number of continuous ink ejection dots in the main scanning direction and the number of ink continuous ejection dots in the sub-scanning direction.

  The control unit 5 of the present embodiment is defined by an offset correction table or an arrangement-specific correction table in accordance with the printing rate defined in a two-dimensional range by the number of continuous ejection dots of ink in both the main scanning and sub-scanning directions. Each additional correction table in which the correction contents of the ink ejection amount to be added to the correction contents are defined and stored is stored in the control unit 5 and used.

  The correction content defined in each additional correction table may be classified according to the ink ejection density (printing rate) in consideration of the number of ink drops ejected to the same dot by the same nozzle 38.

  In the example of the additional correction table shown in FIG. 9 corresponding to the offset correction table, the correction contents divided into three according to the respective printing rates are defined for each nozzle 38 of the head block 35. Specifically, correction factors 0 to +0.05 are defined as additional correction contents for a printing rate of “˜0.5 (less than 0.5)”. Similarly, correction factors 0 to +0.02 are defined as additional correction contents for a printing rate of “0.5 to 0.7 (0.5 or more and less than 0.7)”. For a printing rate of 1.0 (0.7 to 1.0), a correction coefficient of 0 is defined as additional correction content.

  Further, in the example of the additional correction table shown in FIG. 10 corresponding to the arrangement-specific correction table, the nozzles 38 (# 1 to # 5, # 296 to # 300) near the both ends of each head block 35 in the main scanning direction are used. On the other hand, correction contents divided into three according to the respective printing rates are defined. Specifically, a correction coefficient of 0 to +0.06 is defined as additional correction content for a printing rate of “˜0.5 (less than 0.5)”. Similarly, correction factors 0 to +0.03 are defined as additional correction contents for a printing rate of “0.5 to 0.7 (0.5 or more and less than 0.7)”. For a printing rate of “1.0 (0.7 or more and 1.0 or less)”, a correction coefficient of 0 is defined as additional correction content.

  In the inkjet printing apparatus 1 of the present embodiment having the above-described configuration, six head blocks 35 are arranged in a staggered manner to configure the inkjet heads 31C, 31K, 31M, and 31Y for each color. Therefore, in the portion where the head blocks 35 overlap in the main scanning direction so that dots for one line in the main scanning direction are formed on the paper PA by the ink ejected from the nozzles 38 of the six head blocks 35. The nozzles 38 that eject ink are switched from one head block 35 to the other head block 35.

  That is, as shown in FIG. 11, in the portion where the adjacent head blocks 35 overlap in the main scanning direction, that is, at the joint b2, both head blocks 35 and 35 are positioned in the main scanning direction between the respective nozzles 38. Are arranged to match.

  In addition, in the head block 35 (“head 1” in FIG. 11) on the upstream side in the sub-scanning direction (conveyance direction), ink is ejected from the nozzles 38 up to # 297 (# 297). On the other hand, in the head block 35 (“head 2” in FIG. 11) on the downstream side in the sub-scanning direction (conveyance direction), ink is ejected from the nozzles 38 from # 3 (# 3) onward. Thus, the dots of ink ejected from the nozzles 38 of the two head blocks 35 are arranged on the paper PA in a straight line at equal intervals in the main scanning direction.

  However, if the density of ink ejected from the nozzles 38 of the upstream and downstream head blocks 35 and 35 is different, the landing positions of the ink ejected from the nozzles 38 of both head blocks 35 are in the sub-scanning direction. Shift.

  FIG. 11 shows the case where the ink discharge densities from the nozzles 38 of both head blocks 35 are different in the sub-scanning direction. However, the case where the discharge density is different in the main scanning direction or the discharge density for each dot is shown. A similar situation occurs when the two are different.

  When the situation shown in FIG. 11 occurs, the amount of ink discharged from the nozzles 38 of the head block 35 corresponding to the pixels having a low discharge density is increased to increase the degree of the self-air flow W1, thereby reducing the discharge density. Landing deviation of ink from the nozzles 38 of the head block 35 corresponding to high pixels can be suppressed.

  In the joint b2 of the head blocks 35 adjacent in the main scanning direction, both head blocks 35 and 35 may be arranged with the positions of the nozzles 38 in the main scanning direction shifted. When ink is ejected from each nozzle 38 in this state, the distance between the dot formed by the ejected ink and the adjacent dot becomes shorter or longer. When the dot interval is shortened, black stripes are generated, and when the dot interval is increased, white stripes are generated, both of which deteriorate print quality.

  Therefore, the amount of ink discharged from the nozzle 38 located at the joint b2 of one or both head blocks 35, 35 is adjusted according to the correction content defined by a joint correction table (not shown) stored in the control unit 5 (joint correction). The generation of black stripes and white stripes is suppressed.

  In this case, since the ink discharge amount from the nozzle 38 that is the target of seam correction is changed from the original ink discharge amount, the degree of the self-air flow W1 generated by the ink discharged from the nozzle 38 changes. Also in this case, by changing the ink discharge amount as described above, it is possible to suppress the landing deviation of the discharged ink by the other nozzles 38.

  It should be noted that each time the target nozzle 38 ejects ink, the correction contents to be applied to each nozzle 38 using the additional correction table are determined by the printing rate that is an index of the nozzle 38. The printing rate can be determined as follows, for example.

  That is, for example, in each head block 35, the ejection pattern (in which the nozzles 38 to be corrected and the nozzles 38 adjacent in the main scanning direction ejected to dots over the past predetermined lines (for example, 30 lines) ( Main scanning direction, sub-scanning direction, same dot). Of course, the target of the nozzle 38 that refers to the ink ejection pattern to determine the printing rate may be further expanded in the main scanning direction.

  In addition, the correction content of each classification according to the printing rate is, for example, from the test pattern according to the printing rate (ink discharge density) printed by the inkjet printing apparatus 1, the landing deviation of the ink in the sub-scanning direction by the self-air flow W1. The degree can be grasped and determined based on it.

  In the present embodiment, the correction content corresponding to the printing rate is determined for each nozzle 38. For this reason, as a test pattern to be printed by the inkjet printing apparatus 1 when determining the correction contents of each section according to the printing rate, all the nozzles 38 have different printing rates (discharges) in the main scanning direction and the sub-scanning direction. It is preferable that a large number of patterns in which ink is ejected at a density) are included.

  In this embodiment, since the number of ink drops ejected to the same dot by the same nozzle 38 is added to the printing rate (ejection density), a pattern in which the number of ink drops ejected to the same dot is different is included. It is preferable.

  Therefore, the test pattern shown in the explanatory diagram of FIG. 12 may be used. In this test pattern, there are three groups in which the number of continuous nozzles 38 that eject ink is large, medium, and small ("wide", "medium", and "narrow" in the figure). To do. Each group has three patterns in which the number of continuous ink ejections from each nozzle 38 in the sub-scanning direction is large, medium, and small. Then, two test patterns including such three groups are provided, one having N drops as the number of ink drops ejected to the same dot and one having N + 1 drops.

  By using such a test pattern, the self-airflow W1 can be changed according to the printing rate by adding the number of drops of ink ejected to the same dot to the two-dimensional printing rate (dot density) including both main scanning and sub-scanning directions. The degree of landing deviation of the ink in the sub-scanning direction due to the influence can be grasped, and the correction content can be determined based on the degree.

  Next, a procedure for determining the correction contents of the additional correction table of FIGS. 9 and 10 will be described.

  FIG. 13 is a flowchart for explaining the operation of the inkjet printing apparatus 1 when determining the correction content of the additional correction table. The process of the flowchart of FIG. 13 is started when the image data of the test pattern of FIG.

  In step S <b> 1 of FIG. 13, the control unit 5 prints, for example, a test pattern read from the hard disk (including a dot density large, medium, and small image portion) by the printing unit 4. At this time, the control unit 5 uses the offset correction table of FIG. 5B already stored in the hard disk, the layout-specific correction table of FIG. 6B, and the joint correction table (not shown) for each color. The ink discharge amount by the nozzle 38 to be corrected of the head block 35 is corrected.

  If the test pattern is printed on the paper PA, it is determined whether or not density unevenness has occurred in the portion of the ink ejected by the nozzle 38 to be corrected of the head block 35 on the test pattern image on the printed paper PA. Confirm (step S3).

  If density unevenness has occurred (YES in step S3), correction parameters for eliminating the density unevenness are updated and input to the inkjet printing apparatus 1 by an operation of a user (not shown) on the operation panel (step S5). Then, the correction contents of the offset correction table, the arrangement-specific correction table, and the joint correction table (not shown) are updated for each dot density of each color (in other words, for each printing rate, that is, for each ink ejection density). The correction content is calculated by the inkjet printing apparatus 1 and temporarily stored in a hard disk (not shown) of the control unit 5 (step S7), and the process returns to step S1.

  If density unevenness does not occur in step S3 (NO), the correction contents of the offset correction table, the arrangement-specific correction table, and the joint correction table (not shown) currently temporarily stored in the hard disk or the like are stored in the additional correction table. After the main contents are newly stored in the hard disk or the like as the correction contents (step S9), the procedure is terminated.

  The test pattern image is read by the reading device of the reading unit 6, the control unit 5 determines the presence or absence of density unevenness, and based on the result, the control unit 5 performs the update input of the correction parameter, so that the test pattern The control unit 5 may determine the correction content of the additional correction table without repeating the user's input operation or the like while repeating the printing and the image reading.

  Next, the operation of the inkjet printing apparatus 1 when correcting the ink ejection amount of the correction target nozzle 38 of the head block 35 by applying the correction content of the additional correction table illustrated in FIGS. 9 and 10 will be described.

  FIG. 14 is a flowchart for explaining the operation of the inkjet printing apparatus 1 when correcting the ink discharge amount of the correction target nozzle by applying the correction content of the additional correction table of FIG. 9 or 10. In the process of the flowchart of FIG. 14, image data to be printed is input to the inkjet printing apparatus 1, and drop data that is image data in a format corresponding to printing by the inkjet head 31 from image data in RGB format by the control unit 5. Is generated when is generated.

  In step S11 in FIG. 14, when the pixel corresponding to the correction target nozzle 38 of the head block 35 is set as the pixel of interest for each color, the control unit 5 sets the pixel and the periphery (both sides in the main scanning direction). Whether the dot density of the pixel of interest belongs to large, medium, or small is determined from the drop data with the pixel on the upstream side in the sub-scanning direction).

  Next, the control unit 5 determines the correction content corresponding to the dot density (large, medium, small) determined in step S11 of the target pixel, that is, the printing rate, using the additional correction table illustrated in FIG. 9 or FIG. To do. Then, the contents of the placement-specific correction performed by the control unit 5 with respect to the ink ejection amount of the nozzle 38 corresponding to the target pixel are the offset correction table in FIG. 5B, the placement-specific correction table in FIG. The correction content specified in the joint correction table (not shown) is switched to the content with the determined correction content added (step S13).

  Therefore, the control unit 5 adjusts and controls the ink discharge amount by the correction target nozzle 38 by changing the drop data in accordance with the correction content after switching (step S15).

  As a result, the ink discharged from the correction target nozzle 38 and subjected to the discharge amount control by the offset correction, the correction by arrangement, and the joint correction is caused to flow in the sub-scanning direction by the self-air flow W1, and FIG. As shown in FIG. 6A, it is possible to suppress the occurrence of ink landing deviation. Accordingly, it is possible to form a good image in which the occurrence of density unevenness is suppressed by approaching the landing state when there is no landing deviation in the sub-scanning direction due to the influence of the self-airflow W1.

  Incidentally, the amount of landing deviation increases as the distance between the paper PA and the ejection surface 35a increases. The distance between the paper PA and the ejection surface 35a corresponds to the distance obtained by removing the thickness of the paper PA from the head gap H. For the same type of paper PA (having the same thickness), the larger the head gap H, the greater the distance between the paper PA and the ejection surface 35a.

  However, for example, when a plurality of types of paper PAs having different thicknesses are mixed in the paper PA used for printing, the head gap H is maintained at a size that matches the thickness of the thick paper PA. Therefore, when the thin paper PA passes just below the ejection surface 35a, the distance between the paper PA and the ejection surface 35a may be increased.

  Here, FIG. 15 shows experimental results showing the relationship between the landing deviation amount and the distance between the paper PA and the ejection surface 35a. In FIG. 15A, a plurality of adjacent nozzles 38 that ejected ink in this experiment are shown in black. FIG. 15B shows the relationship between the average value of the amount of landing deviation in the sub-scanning direction at the nozzle 38 and the distance between the paper PA and the ejection surface 35a. As shown in FIG. 15B, as the distance between the paper PA and the ejection surface 35a increases, the amount of ink landing deviation in the sub-scanning direction increases.

  Therefore, the ink discharge amount correction contents defined in the additional correction table illustrated in FIGS. 9 and 10 are further classified according to the size of the head gap H. As the head gap H is larger, offset correction, correction by arrangement, and joint are performed. The correction content (correction coefficient) to be added to the correction content of the correction may be increased.

  Moreover, the conveyance air flow W2 becomes stronger as the wind speed is higher. Since the air velocity of the conveyance air flow W2 increases as the conveyance speed of the paper PA by the conveyance unit 3 increases, the correction content of the ink discharge amount specified by the additional correction table illustrated in FIG. 9 or FIG. Further, the correction content (correction coefficient) to be added to the correction content of the joint correction may be increased as the conveyance speed increases.

  In the above-described embodiment, in consideration of the change in the amount of landing deviation in the sub-scanning direction due to the influence of the self-airflow W1, correction details of the ink discharge amount used for eliminating density unevenness due to a different factor are added. I explained the case of changing in. However, the present invention can be widely applied to the case where the ink discharge amount is controlled in accordance with the degree of generation of the self-airflow W1 (self-airflow degree) based on the ink discharge density.

  In the above-described embodiment, the case where the inkjet head 31 is configured by arranging a plurality (six) of the head blocks 35 in a staggered manner in the main scanning direction has been described. However, the present invention is also applicable to the case where the nozzle row that covers the print width in the main scanning direction is provided in the single-unit inkjet head 31.

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

DESCRIPTION OF SYMBOLS 1 Inkjet printing apparatus 2 Paper feed part 3 Conveyance part 4 Printing part 5 Control part 6 Reading part 11 Paper feed stand 12 Paper feed roller 13 Registration roller 21 Conveyance belt 21a Belt hole 21b Paper holding surface 22 Drive roller 23-25 Followed roller 26 Belt drive motor 27 Plastic template 27a Recessed portion 27b Suction hole 28 Fan 31, 31C, 31K, 31M, 31Y Inkjet head 32 Head holder 33 Head gap adjusting unit 35 Head block 35a Discharge surface 38 Nozzle 41 Head gap adjusting mechanism 42 Lifting motor 43 Connection Member 46, 47 Pulley 48 Shaft 49, 50 Wire 52 Ink droplet b2 Joint portion H Head gap P Pitch PA Paper R Conveyance path W1 Self-air flow W2 Conveyance air flow

Claims (6)

A plurality of nozzles are arranged on the inkjet head arranged above the conveyance path along a main scanning direction orthogonal to the conveyance direction of the recording medium conveyed on the conveyance path, and ink is ejected from each nozzle. In an image forming apparatus for forming an image on the recording medium,
Control means for correcting the amount of ink discharged from each nozzle based on the ink discharge density from each nozzle to the recording medium,
An image forming apparatus.   A correction unit that corrects the discharge amount of the ink discharged from each nozzle to the recording medium according to the density unevenness of the image formed on the recording medium by the discharged ink; The image forming apparatus according to claim 1, wherein the correction contents corresponding to the nozzles by the correcting unit are respectively corrected based on the ejection density of ink from the nozzles to the recording medium.   The ejection density is at least one of the number of continuous nozzles in the main scanning direction orthogonal to the transport direction of the nozzles that eject ink and the number of continuous lines in the transport direction of dots from which the same nozzle ejects ink. 3. The image forming apparatus according to claim 1, wherein the image forming apparatus is determined based on one of the two.   The image forming apparatus according to claim 3, wherein the ejection density is determined based on a number of drops of ink ejected from the same nozzle to the same dot.   The image forming apparatus further includes a head gap adjusting unit that adjusts a head gap that is an interval between the inkjet head and the recording medium, and the control unit controls the ink discharge amount by the nozzles according to the head gap. 5. The image forming apparatus according to claim 1, 2, 3, or 4.   The image forming apparatus according to claim 1, wherein the control unit adjusts the control content of the ink ejection amount by the nozzles according to a conveyance speed of the recording medium.