JP2008100526A - Image forming method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming method and an apparatus whereby the visibility of image quality degradation because of defective nozzles can be reduced without undergoing steps such as the detection of no discharges. <P>SOLUTION: In the image forming method of performing tone expression by a dot collective composed of a dot arrangement determined from a droplet hitting ratio calculated on the basis of image data of a drawing object, a main scan direction dot line with dots arranged overlapping one another in a main scan direction by a predetermined overlapping ratio is formed in a region of a high droplet hitting ratio. A skew direction dot line with a predetermined tilt angle to the main scan direction is formed in a region of a low droplet hitting ratio. When a minimum dot diameter is made Dmin and a pitch between dots adjacent to each other in the main scan direction is made Pt, Dmin/2≥Pt is satisfied. The tilt angle of the skew direction dot line is changed within the one image according to density so that an arrangement direction of dot trains of the skew direction dot lines becomes nearer to a direction parallel to a sub scan direction as the droplet hitting ratio becomes lower. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming method and apparatus, and more particularly to an ink jet recording apparatus including an ejection head having a nozzle row in which a plurality of droplet ejection ports (nozzles) are arranged along a length corresponding to the entire width of a recording medium. The present invention relates to a droplet ejection control technique suitable for reducing image quality deterioration caused by nozzle ejection failure in an image forming apparatus.

  In an ink jet printer, there may occur a situation where ink cannot be ejected from a nozzle due to various factors. When a specific nozzle in a nozzle group fails to eject, the dots that should have been ejected by that nozzle are missing, so an unintended streak-like defect in the image formed on the recording medium ( (Straight stripe) occurs, and the stripe shape becomes very noticeable.

  In particular, in the case of an apparatus configuration in which printing is completed by one sub-scanning using a line-type recording head in which a plurality of nozzles are arranged, unlike the shuttle (multi) scanning method, the non-ejection nozzles are separated by another nozzle. Since it is difficult to cover the droplet ejection position (so-called shingling), the unevenness due to the non-ejection nozzle becomes noticeable and causes serious image quality degradation. For this reason, measures have been proposed for making it difficult for image quality deterioration to be noticeable when a certain nozzle fails to eject (Patent Documents 1 and 2).

  Japanese Patent Application Laid-Open No. H10-228707 proposes that the dots are complemented by using nozzles of other color heads instead of the nozzles that cannot perform recording due to non-ejection. For example, a non-ejection portion of the cyan head is subjected to complementary droplet ejection by a magenta head nozzle.

Patent Document 2 proposes a method of reducing the visibility of uneven stripes by ejecting a large amount of printability-improving ink in and around the non-ejection portion and attracting ink around the non-ejection nozzle line (on both sides).
Japanese Patent Laid-Open No. 2002-19101 JP 2002-67297 A

  However, any of the supplementation (correction) methods proposed in Patent Documents 1 and 2 are treatments after specifying that the nozzle is non-ejection by some method. In other words, the “non-ejection detection” step, which is not directly required for printing, is taken, which is inefficient.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide an image forming method and apparatus capable of reducing the visibility of image quality deterioration due to a defective nozzle without going through steps such as non-ejection detection. Objective.

In order to achieve the above object, the present invention discharges droplets from a recording head having a plurality of nozzles toward a recording medium, conveys at least one of the recording head and the recording medium, and An image forming method for forming an image on a recording medium by relatively moving the recording medium, wherein a plurality of images to be rendered are provided in a unit of a predetermined area including a plurality of pixels in each of a main scanning direction and a sub-scanning direction. When the image is output to the predetermined density within each area, the number of dots actually ejected by the nozzles relative to the maximum number of dots that can be ejected by one nozzle in that area When the ratio is referred to as “droplet ejection rate”, the droplet ejection rate of each nozzle in each region is substantially the same, and is determined based on the droplet ejection rate calculated based on the image data to be drawn. Dot placement And it performs gradation expression by droplet deposition density of et consisting dot aggregate, a droplet ejection rate calculation step of calculating a droplet ejection rate of the nozzles based on the image data to be drawn,
A dot placement determination step for determining a dot placement pattern based on the drop placement rate calculated in the drop deposition rate calculation step, and a droplet placement by the recording head to realize the dot placement determined in the dot placement determination step A droplet ejection control step for controlling the operation, and in a region where the droplet ejection rate calculated in the droplet ejection rate calculation step is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value. In accordance with the rate, a dot line in the main scanning direction is continuously formed in the main scanning direction substantially orthogonal to the relative movement direction, and dots are overlapped and arranged at a predetermined overlapping rate, and the hitting calculated in the droplet ejection rate calculating step is performed. In a region where the droplet rate is lower than a predetermined determination reference value, dots are continuously arranged on a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the relative movement direction according to the droplet ejection rate. Heavy at a given overlap rate When the minimum dot diameter of the dots constituting the dot line is Dmin and the pitch between adjacent dots in the main scanning direction is Pt, Dmin / 2 ≧ Pt is formed. As the droplet ejection rate is lower than a predetermined determination reference value, the image data to be drawn is arranged so that the direction of the dot rows of the oblique direction dot lines approaches a direction parallel to the sub-scanning direction. The arrangement direction of the dot rows of the oblique direction dot lines is changed in the one image according to the density to be displayed.

In the image forming method according to an aspect of the present invention, when n is an integer of 2 or more and k is a positive integer, droplet ejection is performed at pixel positions adjacent to the main scanning direction when the droplet ejection rate is 1 / n. The oblique dot lines are formed by shifting the droplet ejection positions from adjacent nozzles by shifting k pixels in the sub-scanning direction parallel to the relative movement direction, and the distance between dot lines x ₀ = n Among the k values such that / (k ² +1) ^1/2 is smaller than a predetermined threshold value, the minimum value is determined as k.

  Further, the image forming apparatus according to the present invention conveys at least one of the recording head in which a plurality of nozzles for discharging droplets are formed and the recording head and the recording medium so that the recording head and the recording medium are relative to each other. An image forming apparatus having a moving unit that moves the image to be drawn into a plurality of regions in a predetermined region unit including a plurality of pixels in each of the main scanning direction and the sub-scanning direction. The ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle in the area when the image is output at a predetermined density is called “droplet ejection rate”. In this case, the droplet ejection rate of each nozzle in the same region is substantially the same, and the level is determined according to the droplet ejection density of the dot assembly having the dot arrangement determined based on the droplet ejection rate calculated based on the image data to be drawn. It is a key expression A droplet ejection rate calculating unit that calculates a droplet ejection rate of a nozzle based on image data to be drawn, and a dot arrangement that determines a dot arrangement pattern based on the droplet ejection rate calculated by the droplet ejection rate calculating unit And a droplet ejection control unit that controls a droplet ejection operation of the recording head to realize the dot arrangement determined by the dot arrangement determination unit, and the droplet ejection calculated by the droplet ejection rate calculation unit In a region where the rate is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value, dots are continuously overlapped in a main scanning direction substantially orthogonal to the relative movement direction according to the droplet ejection rate. The droplet ejection control is performed so as to form dot lines in the main scanning direction that are overlapped and arranged at a ratio, and in the region where the droplet ejection rate calculated by the droplet ejection rate calculating unit is lower than a predetermined determination reference value, The relative movement method according to the rate The droplet ejection control is performed so as to form diagonal dot lines in which dots are continuously overlapped with each other at a predetermined overlapping ratio on a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the main scanning direction. Yes, when the minimum dot diameter of the dots constituting the dot line is Dmin and the pitch between adjacent dots in the main scanning direction is Pt, Dmin / 2 ≧ Pt is satisfied, and the droplet ejection rate is greater than a predetermined determination reference value. In accordance with the density indicated by the image data to be drawn, the diagonal direction in the image is adjusted so that the row direction of the diagonal dot lines approaches the direction parallel to the sub-scanning direction. The arrangement direction of the dot line of the dot line is changed.

  According to the present invention, based on the droplet ejection rate calculated from the image data to be drawn, a dot arrangement pattern with low visibility of streak irregularities is selected, and adjacent dots are lined up overlapping each other at a predetermined overlapping rate. Therefore, the visibility of dot defects when a discharge failure has occurred for a certain nozzle can be significantly suppressed without performing non-discharge detection.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Overall configuration of inkjet recording apparatus]
FIG. 1 is an overall configuration diagram of an ink jet recording apparatus using an image forming apparatus according to an embodiment of the present invention. As shown in the figure, the ink jet recording apparatus 10 includes a plurality of ink jet heads (hereinafter referred to as “ink jet heads”) corresponding to black (K), cyan (C), magenta (M), and yellow (Y) inks. A printing unit 12 having 12K, 12C, 12M, and 12Y, an ink storage / loading unit 14 that stores ink to be supplied to each of the heads 12K, 12C, 12M, and 12Y, and a recording sheet as a recording medium 16 is disposed opposite to the decurling unit 20 for removing the curl of the recording paper 16 and the nozzle surface (ink ejection surface) of the printing unit 12 to improve the flatness of the recording paper 16. A suction belt conveyance unit 22 that conveys the recording paper 16 while holding it, a print detection unit 24 that reads a printing result by the printing unit 12, and a discharge that discharges the recorded recording paper (printed material) to the outside. And parts 26, and a.

  The ink storage / loading unit 14 has an ink tank that stores ink of a color corresponding to each of the heads 12K, 12C, 12M, and 12Y, and each tank has a head 12K, 12C, 12M, and 12Y through a required pipe line. Communicated with. Further, the ink storage / loading unit 14 includes notifying means (display means, warning sound generating means) for notifying when the ink remaining amount is low, and has a mechanism for preventing erroneous loading between colors. ing.

  In FIG. 1, a magazine for rolled paper (continuous paper) is shown as an example of the paper supply unit 18, but a plurality of magazines having different paper widths, paper quality, and the like may be provided side by side. Further, instead of the roll paper magazine or in combination therewith, the paper may be supplied by a cassette in which cut papers are stacked and loaded.

  When multiple types of recording paper are used, an information recording body such as a barcode or wireless tag that records paper type information is attached to the magazine, and the information on the information recording body is read by a predetermined reader. Thus, it is preferable to automatically determine the type of recording medium (media type) to be used and perform ink ejection control so as to realize appropriate ink ejection according to the media type.

  The recording paper 16 delivered from the paper supply unit 18 retains curl due to having been loaded in the magazine. In order to remove this curl, heat is applied to the recording paper 16 by the heating drum 30 in the direction opposite to the curl direction of the magazine in the decurling unit 20. At this time, it is more preferable to control the heating temperature so that the printed surface is slightly curled outward.

  In the case of an apparatus configuration that uses roll paper, a cutter (first cutter) 28 is provided as shown in FIG. 1, and the roll paper is cut into a desired size by the cutter 28. The cutter 28 includes a fixed blade 28A having a length equal to or greater than the conveyance path width of the recording paper 16, and a round blade 28B that moves along the fixed blade 28A. The fixed blade 28A is provided on the back side of the print. The round blade 28B is disposed on the printing surface side with the conveyance path interposed therebetween. Note that the cutter 28 is not necessary when cut paper is used.

  After the decurling process, the cut recording paper 16 is sent to the suction belt conveyance unit 22. The suction belt conveyance unit 22 has a structure in which an endless belt 33 is wound between rollers 31 and 32, and at least portions facing the nozzle surface of the printing unit 12 and the sensor surface of the printing detection unit 24 are horizontal ( Flat surface).

The belt 33 has a width that is wider than the width of the recording paper 16, and a plurality of suction holes (not shown) are formed on the belt surface. As shown in FIG. 1, a suction chamber 34 is provided at a position facing the nozzle surface of the print unit 12 and the sensor surface of the print detection unit 24 inside the belt 33 spanned between the rollers 31 and 32. The suction chamber 34 is connected to the fan 35.
The recording paper 16 is sucked and held on the belt 33 by suctioning to negative pressure.

  When the power of the motor (reference numeral 88 in FIG. 7) is transmitted to at least one of the rollers 31 and 32 around which the belt 33 is wound, the belt 33 is driven in the clockwise direction in FIG. The held recording paper 16 is conveyed from left to right in FIG.

  Since ink adheres to the belt 33 when a borderless print or the like is printed, the belt cleaning unit 36 is provided at a predetermined position outside the belt 33 (an appropriate position other than the print area). Although details of the configuration of the belt cleaning unit 36 are not shown, for example, there are a method of niping a brush roll, a water absorbing roll, etc., an air blow method of blowing clean air, or a combination thereof. In the case where the cleaning roll is nipped, the cleaning effect is great if the belt linear velocity and the roller linear velocity are changed.

  Although a mode using a roller / nip conveyance mechanism instead of the suction belt conveyance unit 22 is also conceivable, if the roller / nip conveyance is performed in the print area, the image easily spreads because the roller contacts the printing surface of the sheet immediately after printing. There is a problem. Therefore, as in this example, suction belt conveyance that does not bring the image surface into contact with each other in the print region is preferable.

  A heating fan 40 is provided on the upstream side of the printing unit 12 on the paper conveyance path formed by the suction belt conveyance unit 22. The heating fan 40 heats the recording paper 16 by blowing heated air onto the recording paper 16 before printing. Heating the recording paper 16 immediately before printing makes it easier for the ink to dry after landing.

  Each of the heads 12K, 12C, 12M, and 12Y of the printing unit 12 has a length corresponding to the maximum paper width of the recording paper 16 targeted by the inkjet recording apparatus 10, and the nozzle surface has a recording medium of the maximum size. This is a full-line type head in which a plurality of nozzles for ink discharge are arranged over a length exceeding at least one side (full width of the drawable range) (see FIG. 2).

  The heads 12K, 12C, 12M, and 12Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side in the recording paper 16 feed direction. 12K, 12C, 12M, and 12Y are fixedly installed so as to extend along a direction substantially orthogonal to the conveyance direction of the recording paper 16.

  A color image can be formed on the recording paper 16 by discharging different color inks from the heads 12K, 12C, 12M, and 12Y while transporting the recording paper 16 by the suction belt transporting section 22.

  As described above, according to the configuration in which the full-line heads 12K, 12C, 12M, and 12Y having nozzle rows that cover the entire width of the paper are provided for each color, the recording paper 16 and the printing unit in the paper feeding direction (sub-scanning direction). The image can be recorded on the entire surface of the recording paper 16 by performing the operation of moving the 12 relatively once (that is, by one sub-scan). Thereby, it is possible to perform high-speed printing as compared with a shuttle type head in which the recording head reciprocates in a direction orthogonal to the paper transport direction, and productivity can be improved.

  In this example, the configuration of KCMY standard colors (four colors) is illustrated, but the combination of ink color and number of colors is not limited to this embodiment, and light ink, dark ink, and special color ink are used as necessary. May be added. For example, it is possible to add an ink jet head that discharges light ink such as light cyan and light magenta. Also, the arrangement order of the color heads is not particularly limited.

  The print detection unit 24 shown in FIG. 1 includes an image sensor for imaging the droplet ejection result of the printing unit 12, and means for checking nozzle clogging and other ejection defects from the droplet ejection image read by the image sensor. Function as.

  The print detection unit 24 of this example is composed of a line sensor having a light receiving element array that is wider than at least the ink ejection width (image recording width) by the heads 12K, 12C, 12M, and 12Y. The line sensor includes an R sensor row in which photoelectric conversion elements (pixels) provided with red (R) color filters are arranged in a line, a G sensor row provided with green (G) color filters, The color separation line CCD sensor is composed of a B sensor array provided with a blue (B) color filter. Instead of the line sensor, an area sensor in which the light receiving elements are two-dimensionally arranged can be used.

  A test pattern or practical image printed by the heads 12K, 12C, 12M, and 12Y of each color is read by the print detection unit 24, and ejection determination of each head is performed. The ejection determination includes the presence / absence of ejection, measurement of dot size, measurement of dot landing position, and the like.

  The present invention has a droplet ejection arrangement in which image defects due to non-ejection are not noticeable even when there is a non-ejection nozzle, and can cover a certain degree of image defect without performing non-ejection detection. When the number of ejection nozzles increases extremely, it is necessary to perform non-ejection detection and recover the non-ejection nozzles.

  A post-drying unit 42 is provided following the print detection unit 24. The post-drying unit 42 is means for drying the printed image surface, and for example, a heating fan is used. Since it is preferable to avoid contact with the printing surface until the ink after printing is dried, a method of blowing hot air is preferred.

  When printing on porous paper with dye-based ink, the weather resistance of the image is improved by preventing contact with ozone or other things that cause dye molecules to break by pressurizing the paper holes with pressure. There is an effect to.

  A heating / pressurizing unit 44 is provided following the post-drying unit 42. The heating / pressurizing unit 44 is a means for controlling the glossiness of the image surface, and pressurizes with a pressure roller 45 having a predetermined surface uneven shape while heating the image surface to transfer the uneven shape to the image surface. To do.

  The printed matter generated in this manner is outputted from the paper output unit 26. It is preferable that the original image to be printed (printed target image) and the test print are discharged separately. The ink jet recording apparatus 10 is provided with a sorting means (not shown) for switching the paper discharge path in order to select the print product of the main image and the print product of the test print and send them to the discharge units 26A and 26B. Yes. Note that when the main image and the test print are simultaneously formed in parallel on a large sheet, the test print portion is separated by a cutter (second cutter) 48. The cutter 48 is provided immediately before the paper discharge unit 26, and cuts the main image and the test print unit when the test print is performed on the image margin. The structure of the cutter 48 is the same as that of the first cutter 28 described above, and includes a fixed blade 48A and a round blade 48B.

  Although not shown in FIG. 1, the paper output unit 26A for the target prints is provided with a sorter for collecting prints according to print orders.

[Head structure]
Next, the structure of the head will be described. Since the structures of the respective heads 12K, 12C, 12M, and 12Y for each color are common, the heads are represented by the reference numeral 50 in the following.

  FIG. 3A is a plan perspective view showing an example of the structure of the head 50, and FIG. 3B is an enlarged view of a part thereof. 3C is a plan perspective view showing another structure example of the head 50, and FIG. 4 is a cross-sectional view showing a three-dimensional configuration of one droplet discharge element (an ink chamber unit corresponding to one nozzle 51). FIG. 4 is a sectional view taken along line 4-4 in FIG.

  In order to increase the dot pitch printed on the recording paper 16, it is necessary to increase the nozzle pitch in the head 50. As shown in FIGS. 3A and 3B, the head 50 of this example includes a plurality of ink chamber units including nozzles 51 serving as ink droplet ejection openings, pressure chambers 52 corresponding to the nozzles 51, and the like. It has a structure in which (droplet discharge elements) 53 are arranged in a zigzag matrix (two-dimensionally), and is thereby projected so as to be arranged along the head longitudinal direction (direction perpendicular to the paper feed direction). High density of substantial nozzle interval (projection nozzle pitch) is achieved.

  The configuration in which one or more nozzle rows are configured over a length corresponding to the entire width of the recording paper 16 in a direction substantially orthogonal to the feeding direction of the recording paper 16 is not limited to this example. For example, instead of the configuration of FIG. 3 (a), short head blocks 50 ′ in which a plurality of nozzles 51 are two-dimensionally arranged are arranged in a staggered manner and connected as shown in FIG. 3 (c). A line head having a nozzle row having a length corresponding to the entire width of the recording paper 16 may be configured.

  The pressure chamber 52 provided corresponding to each nozzle 51 has a substantially square planar shape (see FIGS. 3 (a) and 3 (b)), and is connected to the nozzle 51 at both corners on a diagonal line. An outlet and an inlet (supply port) 54 for supply ink are provided.

  As shown in FIG. 4, each pressure chamber 52 communicates with a common flow channel 55 through a supply port 54. The common channel 55 communicates with an ink tank (not shown in FIG. 4, not shown in FIG. 6 and indicated by reference numeral 60) serving as an ink supply source, and the ink supplied from the ink tank 60 passes through the common channel 55 in FIG. Then, it is distributed and supplied to each pressure chamber 52.

  An actuator 58 having an individual electrode 57 is joined to a pressure plate (vibrating plate that also serves as a common electrode) 56 that forms the top surface of the pressure chamber 52. By applying a driving voltage to the individual electrode 57, the actuator 58 is deformed to change the volume of the pressure chamber 52, and ink is ejected from the nozzle 51 due to the pressure change accompanying this. For the actuator 58, a piezoelectric body such as a piezoelectric element is preferably used. After ink discharge, new ink is supplied from the common channel 55 to the pressure chamber 52 through the supply port 54.

  As shown in FIG. 5, the ink chamber unit 53 having such a structure is latticed in a fixed arrangement pattern along a row direction along the main scanning direction and an oblique column direction having a constant angle θ not orthogonal to the main scanning direction. The high-density nozzle head of this example is realized by arranging a large number in the shape.

  That is, with a structure in which a plurality of ink chamber units 53 are arranged at a constant pitch d along a certain angle θ with respect to the main scanning direction, the pitch P of the nozzles projected so as to be aligned in the main scanning direction is d × cos θ. Thus, in the main scanning direction, each nozzle 51 can be handled equivalently as a linear arrangement with a constant pitch P. With such a configuration, it is possible to realize a high-density nozzle configuration in which 2400 nozzle rows are projected per inch (2400 nozzles / inch) so as to be aligned in the main scanning direction.

When the nozzles are driven by a full line head having a nozzle row having a length corresponding to the entire printable width, (1) all the nozzles are driven simultaneously, (2) the nozzles are sequentially moved from one side to the other. (3) The nozzles are divided into blocks, and the nozzles are sequentially driven from one side to the other for each block, etc., and one line (1 in the width direction of the paper (direction perpendicular to the paper conveyance direction)) Driving a nozzle that prints a line of dots in a row or a line consisting of dots in a plurality of rows is defined as main scanning.

  In particular, when driving the nozzles 51 arranged in a matrix as shown in FIG. 5, the main scanning as described in (3) above is preferable. That is, nozzles 51-11, 51-12, 51-13, 51-14, 51-15, 51-16 are made into one block (other nozzles 51-21,..., 51-26 are made into one block, Nozzles 51-31,..., 51-36 as one block,...), And the nozzles 51-11, 51-12,. One line is printed in 16 width directions.

  On the other hand, by relatively moving the above-mentioned full line head and the paper, printing of one line (a line formed by one line of dots or a line composed of a plurality of lines) formed by the above-described main scanning is repeatedly performed. This is defined as sub-scanning.

  In implementing the present invention, the nozzle arrangement structure is not limited to the illustrated example. In the present embodiment, a method of ejecting ink droplets by deformation of an actuator 58 typified by a piezo element (piezoelectric element) is adopted. However, in the practice of the present invention, the method of ejecting ink is not particularly limited. Instead of the piezo jet method, various methods such as a thermal jet method in which ink is heated by a heating element such as a heater to generate bubbles and ink droplets are ejected by the pressure can be applied.

[Configuration of ink supply system]
FIG. 6 is a schematic diagram showing the configuration of the ink supply system in the inkjet recording apparatus 10. The ink tank 60 is a base tank that supplies ink to the head 50 and is installed in the ink storage / loading unit 14 described with reference to FIG. In the form of the ink tank 60, there are a system that replenishes ink from a replenishing port (not shown) and a cartridge system that replaces the entire tank when the remaining amount of ink is low. A cartridge system is suitable for changing the ink type according to the intended use. In this case, it is preferable that the ink type information is identified by a barcode or the like, and ejection control is performed according to the ink type. The ink tank 60 in FIG. 6 is equivalent to the ink storage / loading unit 14 in FIG. 1 described above.

  As shown in FIG. 6, a filter 62 is provided between the ink tank 60 and the head 50 in order to remove foreign matters and bubbles. The filter mesh size is preferably equal to or smaller than the nozzle diameter (generally about 20 μm). Although not shown in FIG. 6, a configuration in which a sub tank is provided in the vicinity of the head 50 or integrally with the head 50 is also preferable. The sub-tank has a function of improving a damper effect and refill that prevents fluctuations in the internal pressure of the head.

  Further, the inkjet recording apparatus 10 is provided with a cap 64 as a means for preventing the nozzle 51 from drying or preventing an increase in ink viscosity near the nozzle, and a cleaning blade 66 as a means for cleaning the nozzle surface 50A. . The maintenance unit including the cap 64 and the cleaning blade 66 can be moved relative to the head 50 by a moving mechanism (not shown), and is moved from a predetermined retracted position to a maintenance position below the head 50 as necessary.

The cap 64 is displaced up and down relatively with respect to the head 50 by an elevator mechanism (not shown). The cap 64 is raised to a predetermined raised position when the power is turned off or during printing standby, and the head 5
The nozzle surface 50 </ b> A is covered with the cap 64 by being brought into close contact with 0.

  The cleaning blade 66 is made of an elastic member such as rubber, and can slide on the ink discharge surface (nozzle plate surface) of the head 50 by a blade moving mechanism (not shown). When ink droplets or foreign substances adhere to the nozzle plate, the nozzle plate surface is wiped by sliding the cleaning blade 66 on the nozzle plate to clean the nozzle plate surface.

  During printing or standby, when a specific nozzle is used less frequently and the ink viscosity in the vicinity of the nozzle increases, preliminary discharge is performed toward the cap 64 to discharge the deteriorated ink.

  When air bubbles are mixed in the ink in the head 50 (in the pressure chamber 52), the cap 64 is applied to the head 50, and the ink in the pressure chamber 52 (ink in which the air bubbles are mixed) is removed by suction with the suction pump 67. Then, the sucked and removed ink is sent to the collection tank 68. In this suction operation, the deteriorated ink that has increased in viscosity (solidified) is sucked out when the ink is initially loaded into the head 50 or when the ink is used after being stopped for a long time.

  If the head 50 is not ejected for a certain period of time, the ink solvent near the nozzles evaporates and the viscosity of the ink near the nozzles increases. Will not discharge. Therefore, before this state is reached (within the viscosity range in which ink can be discharged by the operation of the actuator 58), the actuator 58 is operated toward the ink receiver to discharge ink in the vicinity of the nozzle whose viscosity has increased. “Preliminary discharge” is performed. In addition, after the dirt on the surface of the nozzle plate is cleaned by a wiper such as a cleaning blade 66 provided as a means for cleaning the nozzle surface 50A, the foreign matter is prevented from entering the nozzle 51 by the wiper rubbing operation. Also, preliminary discharge is performed. Note that the preliminary discharge may be referred to as “empty discharge”, “purge”, “spitting”, or the like.

  In addition, if bubbles are mixed into the nozzle 51 or the pressure chamber 52 or if the viscosity increase of the ink in the nozzle 51 exceeds a certain level, ink cannot be ejected by the preliminary ejection, and the suction operation described below is performed.

  That is, when bubbles are mixed in the ink in the nozzle 51 or the pressure chamber 52, or when the ink viscosity in the nozzle 51 rises to a certain level or more, the ink can be ejected from the nozzle 51 even if the actuator 58 is operated. Disappear. In such a case, a suction means for sucking ink in the pressure chamber 52 with a pump or the like is brought into contact with the nozzle surface 50A of the head 50, and an operation of sucking ink mixed with bubbles or thickened ink is performed.

  However, since the above suction operation is performed on the entire ink in the pressure chamber 52, the ink consumption is large. Therefore, when the increase in viscosity is small, it is preferable to perform preliminary discharge as much as possible.

[Explanation of control system]
FIG. 7 is a principal block diagram showing the system configuration of the inkjet recording apparatus 10. The inkjet recording apparatus 10 includes a communication interface 70, a system controller 72, an image memory 74, a ROM 75, a motor driver 76, a heater driver 78, a print control unit 80, an image buffer memory 82, a head driver 84, and the like.

  The communication interface 70 is an interface unit that receives image data sent from the host computer 86. As the communication interface 70, a serial interface such as USB (Universal Serial Bus), IEEE 1394, Ethernet (registered trademark), a wireless network, or a parallel interface such as Centronics can be applied. In this part, a buffer memory (not shown) for speeding up communication may be mounted.

  Image data sent from the host computer 86 is taken into the inkjet recording apparatus 10 via the communication interface 70 and temporarily stored in the image memory 74. The image memory 74 is a storage unit that temporarily stores an image input via the communication interface 70, and data is read and written through the system controller 72. The image memory 74 is not limited to a memory made of a semiconductor element, and a magnetic medium such as a hard disk may be used.

  The system controller 72 includes a central processing unit (CPU) and its peripheral circuits, and functions as a control device that controls the entire inkjet recording apparatus 10 according to a predetermined program, and also functions as an arithmetic device that performs various calculations. . That is, the system controller 72 controls each part such as the communication interface 70, the image memory 74, the motor driver 76, the heater driver 78, etc., performs communication control with the host computer 86, read / write control of the image memory 74, and the like. A control signal for controlling the motor 88 and the heater 89 of the transport system is generated.

  The ROM 75 stores programs executed by the CPU of the system controller 72 and various data necessary for control. The ROM 75 may be a non-rewritable storage unit, or may be a rewritable storage unit such as an EEPROM. The image memory 74 is used as a temporary storage area for image data, and is also used as a program development area and a calculation work area for the CPU.

  The motor driver 76 is a driver (drive circuit) that drives the motor 88 in accordance with an instruction from the system controller 72. The heater driver 78 is a driver that drives the heater 89 such as the post-drying unit 42 in accordance with an instruction from the system controller 72.

  The print control unit 80 has a signal processing function for performing various processing and correction processing for generating a print control signal from the image data in the image memory 74 according to the control of the system controller 72, and the generated print It is a control unit that supplies data (dot data) to the head driver 84. Necessary signal processing is performed in the print control unit 80, and the ejection amount and ejection timing of the ink droplets of the head 50 are controlled via the head driver 84 based on the image data. Thereby, a desired dot size and dot arrangement are realized.

  The print control unit 80 includes an image buffer memory 82, and image data, parameters, and other data are temporarily stored in the image buffer memory 82 when image data is processed in the print control unit 80. In FIG. 7, the image buffer memory 82 is shown in a mode associated with the print control unit 80, but it can also be used as the image memory 74. Also possible is an aspect in which the print controller 80 and the system controller 72 are integrated and configured with one processor.

  The head driver 84 drives the actuators of the heads 12K, 12C, 12M, and 12Y of the respective colors based on the print data given from the print control unit 80. The head driver 84 may include a feedback control system for keeping the head driving conditions constant.

  Image data to be printed is input from the outside via the communication interface 70 and stored in the image memory 74. At this stage, RGB image data is stored in the image memory 74.

  The image data stored in the image memory 74 is sent to the print control unit 80 via the system controller 72, and is converted into dot data for each ink color by the method disclosed in the present invention. The That is, the print control unit 80 performs processing for converting the input RGB image data into KCMY four-color dot data. The dot data generated by the print controller 80 is stored in the image buffer memory 82.

  The head driver 84 generates a drive control signal for the head 50 based on the dot data stored in the image buffer memory 82. By applying the drive control signal generated by the head driver 84 to the head 50, ink is ejected from the head 50. An image is formed on the recording paper 16 by controlling the ink ejection from the head 50 in synchronization with the conveyance speed of the recording paper 16.

  As described with reference to FIG. 1, the print detection unit 24 is a block including a line sensor, reads an image printed on the recording paper 16, performs necessary signal processing, and the like to perform a print status (whether ejection is performed, droplet ejection And the detection result is provided to the print control unit 80.

  The print controller 80 performs various corrections on the head 50 based on information obtained from the print detector 24 as necessary. Further, the system controller 72 performs control to perform a predetermined recovery operation such as preliminary ejection, suction, or the like based on information obtained from the print detection unit 24.

(Drip ejection control method)
Next, a droplet ejection control method in the ink jet recording apparatus configured as described above will be described. For convenience of explanation, the nozzle array of the head 50 is simplified (modeled) and replaced with one nozzle array arranged in a straight line along the main scanning direction. However, the actual nozzle arrangement is shown in FIG. As described in (a) to (c), a two-dimensional arrangement structure is formed.

  In the ink jet recording apparatus 10, a pseudo continuous tone image is formed by changing the droplet ejection density and dot size of fine dots with ink (coloring material) to the human eye. It is converted into a dot pattern using a halftoning algorithm that reproduces the gradation (shading of the image) as faithfully as possible.

  (Droplet ejection method 1) FIG. 8 is a schematic diagram showing an example of an image (dot arrangement) recorded at a droplet ejection rate of 1/2. In the figure, it is assumed that the recording medium is conveyed from the bottom to the top of the sheet. By controlling the conveyance of the recording medium and the timing of ejecting ink from the nozzles 51, one dot row arranged in the main scanning direction is sequentially formed.

  Black dots (reference numeral 100) and white circles (reference numeral 102) shown in the dot arrangement of FIG. 8 indicate positions (pixel positions) at which each nozzle 51 can eject droplets. A black circle 100 indicates the position of the pixel where the droplet is actually ejected, and a white circle 102 indicates the position of the pixel where the droplet is not ejected (thinned out). A circle (reference numeral 104) drawn around the black circle 100 represents a dot formed by spreading of the ink that has landed at the position indicated by the black circle 100. In addition, as a standard of the dimension in the same figure, the distance between nozzles is about 10 micrometers and the dot diameter is about 30 micrometers, for example. 10 to 15 other than FIG. 8 are the same as the description rule of FIG.

  In the example shown in FIG. 8A, droplets are deposited most densely without thinning the droplet ejection positions (pixels) in the main scanning direction, and droplets are ejected every other pixel line in the sub-scanning direction. It has been broken. This is an example of droplet ejection when the dots are arranged at a rate of 50% in the sub-scanning direction with respect to the densest dot arrangement that can actually be ejected, and the “droplet ejection rate” is 1/2. .

  In this embodiment, an image output method is adopted in which an image to be created is divided into a certain area, and the density is set to a predetermined density (a predetermined density determined based on data of the image to be created) in the area. Shall. Within this region, the operating rate of each nozzle is substantially the same. That is, in this region, every nozzle strikes dots approximately the same number of times. At this time, the ratio of the number of times the nozzle actually hits the dot to the maximum number (= the number of pixels in the sub-scanning direction of the area) in which the nozzle can hit the dot, The maximum number (= number of pixels in the sub-scanning direction of the area) that the nozzle can strike dots within the area is defined as “droplet ejection rate”. Since the operation rate of each nozzle is substantially the same, the droplet ejection rate is equal to “the number of dots in the region / the total number of pixels in the region”.

  Note that the droplet ejection rates of the nozzles in the same region do not have to be exactly the same. A range up to ± 10% with respect to the reference nozzle ejection rate is substantially allowed.

  Further, in the present embodiment, when performing droplet ejection of the maximum number of dots that can be ejected (so-called all solid images), a ratio display in which the droplet ejection rate is “1” (maximum droplet ejection rate) is used. The expression form of the drop rate may be a percentage.

  FIG. 8B shows an example of an image when a certain nozzle (reference numeral 51-NG) in the head 50 fails to eject under the droplet ejection method of FIG. 8A. The pixel position indicated by the hatched circle (reference numeral 106) in FIG. 8B becomes non-ejection, and the column of dots 108 indicated by the dotted line is missing. However, in the case of the dot arrangement as shown in the figure, even when a specific nozzle 51-NG fails to eject, if there are dots around it, the nozzle 51-NG that has failed to eject should originally eject. Ink (specifically, surrounding dots) is also present in the area, and streaks can be prevented to some extent.

  That is, as shown in FIG. 8, it can be said that a dot line in which dots are continuously solidified in the main scanning direction (at least partially overlap each other) and densely present has a strong deterrence of non-ejection unevenness. . As shown in FIG. 8A, an array of dots that are most tightly (densely) present in the main scanning direction is formed by ejecting all pixels in the main scanning direction to form a single dot line (dots in the main scanning direction). Line). This droplet ejection method is referred to as “first droplet ejection method”.

  The larger the degree of overlapping between adjacent dots (overlapping ratio), the larger the area that can cover the area of the missing dot 108 with the adjacent dots. The dots formed by the ink droplets have a relatively high density at the center of the dot and a relatively low density at the periphery, so that the center of the missing dot 108 can be covered by the adjacent dots. It is preferable to overlap the dots.

That is, as shown in FIG. 9, when the minimum dot diameter to be ejected by the nozzle 51 is Dmin and the inter-dot pitch is Pt, the following equation (1)
Dmin / 2 ≧ Pt (1)
It is preferable to satisfy. When the condition of the above equation (1) is satisfied, the center of the missing dot can be covered with the adjacent dot.

  Note that in the droplet ejection arrangement by the first droplet ejection method described above, the phases of the dots arranged in a line along the main scanning direction are slightly shifted in the sub scanning direction (smaller than the recording density in the sub scanning direction). The same effect can be expected.

Further, in FIG. 8, the dot missing due to the non-ejection has been described as an example, but the ejection failure of the nozzle 51 includes not only non-ejection (recording impossible) but also an abnormal ejection amount (dot size), dot recording position (landing position) ) And the like. In any case, it can be considered in the same manner as in the case of non-ejection, in that the ink is insufficient in the area of the dots that should be ejected.

  By the way, when the droplet ejection rate is high to some extent, it can be dealt with by forming a dot line in the main scanning direction as described in FIG. 8, but when the droplet ejection rate is low, that is, when the dot density is sparse. Therefore, in this droplet ejection, the interval between the dot rows is excessively widened, thereby causing periodic unevenness that can be visually recognized in the sub-scanning direction.

  FIG. 10 is a schematic diagram in which dot lines in the main scanning direction are formed when the droplet ejection rate is 1/4. As shown in the figure, the interval between the main scanning direction dot lines 110 is increased, and periodic unevenness occurs.

  In order to avoid the situation as shown in FIG. 10, the ink jet recording apparatus 10 according to the present embodiment employs a droplet ejection method described below in a low droplet ejection rate region.

  (Droplet ejection method 2) FIG. 11 is a schematic diagram showing an example of dot arrangement by the second droplet ejection method. This is because the dot lines that are parallel to the main scanning direction in FIG. 8A are tilted in the sub scanning direction, that is, as shown in FIG. This is a droplet ejection method in which droplets are ejected while being shifted in the sub-scanning direction, and oblique dot lines are formed in which dots are continuously overlapped on a straight line having a predetermined inclination angle with respect to the main operation direction.

  FIG. 11 shows a case where the droplet ejection rate is 1/4. As shown in FIG. 11B, even if a nozzle 51-NG fails to eject, the center of the dot 118 that the nozzle 51-NG originally should have ejected is the dot from the adjacent nozzle 51. (In FIG. 11B, these adjacent dots are ejected at the upper left and lower right as viewed from the non-ejection dots 118) and the color does not fall out. Further, as indicated by the hatched lines in FIG. 11B, the region 122 where the color is lost is the peripheral portion of the non-ejection dots 118, and therefore the normal (normal) ejection shown in FIG. In comparison, it is considered that there is almost no change in density.

  For comparison, FIG. 12 shows another droplet ejection arrangement (staggered arrangement) example of a droplet ejection ratio of 1/4. In a staggered droplet ejection arrangement as shown in FIG. 12A, when a certain nozzle does not eject, the dot center portion 124 having the highest density is empty as shown in FIG. Compared to FIG. 11, stripes are more noticeable.

  In relation to the dot arrangement in FIG. 11, a case where the droplet ejection rate is 1 / n (all nozzles eject droplets once every n times) will be considered with reference to FIG. As shown in FIG. 13, it is considered that diagonal dot lines are formed by ejecting droplets from adjacent nozzles with a shift of k (pixel) in the sub-scanning direction. That is, the example of FIG. 11 corresponds to the case of k = 1. Here, the minimum distance between pixels that can be ejected is expressed in units of 1 pixel.

  As shown in FIG. 13, droplet ejection from nozzles 51 adjacent to each other in the main scanning direction is sequentially ejected with a shift of k (pixel) in the sub scanning direction, and each nozzle 51 n times (n pixels) in the sub scanning direction. ) Once, a dot row is formed in which dots are arranged on an oblique straight line 140 having a relationship of changing 1 (pixel) in the main scanning direction to k (pixel) in the sub-scanning direction. If the inclination angle of the dot row with respect to the main scanning direction is ψ, there is a relationship of tan ψ = k, and the value of k corresponds to a value that defines the arrangement direction (inclination angle) of the dot row.

Thus, the distance x ₀ between the dot rows ejected in parallel with each other at a certain inclination k can be calculated by the following equation (2):
x ₀ = n / (k ² +1) ^1/2 (pixel) (2)
It becomes. The x ₀ represents the repetition period of the oblique direction dot line. Is the most high concentration center of the dot, the dot peripheral portion since the density is lower, x ₀ represents the period of shading due to the oblique direction dot lines. Therefore, under the condition that the x ₀ is not visually recognized, the slope of the dot row, i.e. to determine the value of k in as small as possible, it performs droplet ejection in accordance with the value of the k.

  (Droplet ejection method 3) FIG. 14 is a schematic view showing the droplet placement by the third droplet ejection method. As shown in the drawing, a plurality of dots (m, where m is an integer of 3 or more) are arranged in the main scanning direction, and these m dot groups are represented as G (m). This G (m) is arranged while being shifted in the sub-scanning direction depending on the position in the main scanning direction. FIG. 14 shows an example in which the droplet ejection rate is 1/4 and m = 3.

  In this manner, by ejecting droplets by shifting G (m) in the sub-scanning direction in accordance with the position in the main scanning direction, temporarily, as shown in FIG. Even in the case of the edge of), the color loss due to non-ejection is in the periphery of the dot, and the density hardly changes compared to the case of normal (normal) ejection (FIG. 14A). In addition, when the dot at the center of G (m) is missing due to non-ejection, the center of the missing dot is covered with the adjacent dot as in the example described with reference to FIG.

  In the head 50, when it is assumed that any nozzle 51 is not ejected with substantially equal probability, it is desirable to eject droplets while shifting the position of G (m) in the main scanning direction. On the other hand, in the case where a non-ejection nozzle is known in advance, a method of changing the overall droplet ejection arrangement so that the nozzle is located at the center of G (m) and making the uneven stripes inconspicuous is also conceivable. In this droplet ejection arrangement, the same effect can be expected even if the phase of dots aligned in the main scanning direction is slightly shifted in the sub scanning direction (smaller than the recording density in the sub scanning direction).

Although FIG. 14 illustrates m = 3, the value of m is designed as appropriate. In the third droplet ejection method described with reference to FIG. 14, as shown in FIG. 15, periodic density unevenness occurs in a direction substantially perpendicular to a straight line 150 connecting the centers of G (m) connected in a staircase pattern. It is thought to get. This interval x is expressed by the following equation (3):
x = m × n / (m ² +1) ^1/2 (pixel) (3)
It becomes. Here, m represents the number of dots arranged in the main scanning direction, and 1 / n represents the droplet ejection rate.

The threshold interval streaks by the density unevenness is visually recognized as a x _t, it is preferable to employ a maximum m in which x does not exceed the threshold value x _t.

  Since the discharge interval of one nozzle 51 is not always n (pixel), it may be slightly different from the calculation formula of the value of x.

[About the threshold considering the visibility of image degradation]
In the case of the ink jet recording apparatus 10 according to the present embodiment, in the region where the droplet ejection rate is high, dots are densely arranged in parallel to the main scanning direction as shown in FIG. As shown in FIG. 14, dots are arranged in an oblique direction having an inclination angle with respect to the main scanning direction, or m dots are arranged in the main scanning direction as shown in FIG. 14, and the m dot group (G (m )) Is performed in accordance with the position in the main scanning direction.

As described above, when switching the form of droplet ejection within one image according to the droplet ejection rate, it is possible to distinguish between a high droplet ejection rate (high density) and a low droplet ejection rate (low density) and to switch the droplet ejection method. The threshold value (determination reference value) is set to a droplet ejection rate at which periodic unevenness is visually recognized in the sub-scanning direction. Specifically, the value of the droplet ejection rate is determined from the practical print.

  FIG. 16 shows the human eye spatial frequency characteristics (Visual Transfer Function; VTF) and actual observation results. The “observation result” here is based on visual observation of a sample in which dots are ejected in a straight line parallel to the main scanning direction as shown in FIG. 8 at a nozzle density of 1200 npi (nozzles per inch) × droplet ejection density of 2400 dpi (dots per inch). It is an observation result. The observation distance is 350 mm. As shown in FIG. 16, it can be seen from the actual visual results that low frequency unevenness can be seen from about 7 cycles / mm, which is in good agreement with the VTF results. However, this result is considered to vary greatly depending on the ink density, dot diameter, droplet ejection position error, and observation distance.

It should be noted that appropriate values are set for the x ₀ threshold described in FIG. 13 and the x threshold described in FIG. 15 from the same viewpoint as described above.

  In the above embodiment, a configuration in which a plurality of full-line heads are arranged for each color has been described. However, when implementing the present invention, a head configuration in which nozzle rows are formed for each color in a multi-color integrated head is also possible. It is.

  14 and 15 exemplify a polygonal dot line formed by connecting m dot groups G (m) in a staircase pattern in the main scanning direction, but in implementing the present invention, the dot group G ( m) may be connected to the “W” character type to form a dot line.

[Appendix]
The present specification also discloses the following invention.

  (Invention 1): Droplets are ejected from a recording head having a plurality of nozzles toward a recording medium, and at least one of the recording head and the recording medium is conveyed to relatively move the recording head and the recording medium. An image forming method for forming an image on the recording medium by dividing an image to be drawn into a plurality of regions, and outputting the image at a predetermined density in each region. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle is called “droplet rate”, the droplet ejection rate calculated based on the image data to be drawn is The droplet ejection rate calculation step for calculating the droplet ejection rate of the nozzle based on the image data to be drawn, and the droplet ejection rate calculation step. Calculated in A dot placement determining step for determining a dot placement pattern based on the droplet ejection rate, and a droplet ejection control step for controlling a droplet ejection operation by the recording head to realize the dot placement determined in the dot placement determination step; In the region where the droplet ejection rate calculated in the droplet ejection rate calculating step is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value, the relative movement direction and the relative ejection direction are determined according to the droplet ejection rate. An image forming method comprising: forming a dot line in a main scanning direction in which dots are continuously overlapped with each other at a predetermined overlapping ratio in a substantially orthogonal main scanning direction.

  According to the first aspect, when the image to be drawn is divided into a plurality of areas and the dot arrangement is realized so that each area has a predetermined density determined from the image data, the calculated droplet ejection rate can be ejected. In an area that is lower than the maximum droplet deposition rate that means the maximum number of droplet ejections (so-called all solid images) and higher than a predetermined determination reference value, the dot lines are arranged in the main scanning direction according to the droplet ejection rate. In this case, adjacent droplets are overlapped with each other at a predetermined overlapping rate and ejected. For this reason, even if a certain nozzle becomes defective in ejection, a part of defective dots is covered by adjacent dots ejected from other nozzles, and uneven stripes due to defective ejection are less noticeable. Further, in the first aspect, it is not necessary to go through a process of detecting a discharge failure of the nozzle, and the visibility of dot defects when a discharge failure occurs can be reduced.

  In the dot arrangement determining step of the present invention, it is preferable that the gradation expression is performed so that the droplet ejection ratios of all the nozzles that eject droplets in each region are substantially equal.

  The predetermined determination reference value is set as a boundary condition as to whether or not stripes between main scanning direction dot lines are visually recognized.

  (Invention 2): According to an aspect of the image forming method described in Invention 1, when the minimum dot diameter of the dots constituting the main scanning direction dot line is Dmin, and the pitch between adjacent dots in the main scanning direction is Pt, An image forming method characterized by satisfying Dmin / 2 ≧ Pt.

  When this condition is satisfied, the center of a defective dot due to ejection failure is covered with an adjacent dot, which is a preferable aspect in that the visibility of image quality deterioration can be further reduced.

  (Invention 3): Droplets are ejected from a recording head having a plurality of nozzles toward a recording medium, and at least one of the recording head and the recording medium is conveyed to relatively move the recording head and the recording medium. An image forming method for forming an image on the recording medium by dividing an image to be drawn into a plurality of regions, and outputting the image at a predetermined density in each region. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle is called “droplet rate”, the droplet ejection rate calculated based on the image data to be drawn is The droplet ejection rate calculation step for calculating the droplet ejection rate of the nozzle based on the image data to be drawn, and the droplet ejection rate calculation step. Calculated in A dot placement determining step for determining a dot placement pattern based on the droplet ejection rate, and a droplet ejection control step for controlling a droplet ejection operation by the recording head to realize the dot placement determined in the dot placement determination step; , And in a region where the droplet ejection rate calculated in the droplet ejection rate calculating step is lower than a predetermined determination reference value, according to the droplet ejection rate, with respect to the main scanning direction substantially orthogonal to the relative movement direction An image forming method comprising forming diagonal dot lines in which dots are continuously overlapped with each other at a predetermined overlapping ratio on a straight line having a predetermined inclination angle.

  According to the present invention, in a region where the droplet ejection rate is lower than a predetermined determination reference value, an oblique line-shaped dot line having an inclination angle with respect to the main scanning direction is formed according to the droplet ejection rate. At that time, adjacent droplets are overlapped with each other at a predetermined overlapping ratio and ejected. For this reason, even if a certain nozzle becomes defective in ejection, a part of defective dots is covered by adjacent dots ejected from other nozzles, and uneven stripes due to defective ejection are less noticeable. Further, in the present invention, it is not necessary to go through a process of detecting a nozzle discharge failure, and the visibility of dot defects when a discharge failure occurs can be reduced.

(Invention 4): According to an aspect of the image forming method described in Invention 3, when n is an integer of 2 or more and k is a positive integer, when the droplet ejection rate is 1 / n, adjacent to the main scanning direction. The oblique direction dot lines are formed by ejecting droplets by shifting the droplet ejection positions from adjacent nozzles that can eject droplets to the pixel position to be shifted by k pixels in the sub-scanning direction parallel to the relative movement direction. A value of k is determined so that the distance x ₀ = n / (k ² +1) ^1/2 is smaller than a predetermined threshold value.

  For example, the predetermined threshold value is set as a boundary condition that allows density unevenness due to diagonal dot lines to be visually recognized. According to this aspect, density unevenness in the direction perpendicular to the oblique direction dot lines is also less noticeable.

  The “adjacent nozzle” here is not limited to the meaning of a nozzle arranged in a physically adjacent positional relationship in the nozzle array of the recording head, but is a positional relationship in which dots that are substantially adjacent on the recording medium can be hit. Means a nozzle. For example, in the case of a configuration in which a large number of nozzles are two-dimensionally arranged at high density, dots may be formed at adjacent pixel positions on the recording medium by nozzles that are not necessarily adjacent to each other in the nozzle array. In this way, nozzles that can eject droplets at adjacent pixel positions on the recording medium are collectively referred to as “adjacent nozzles” for convenience.

  (Invention 5): Droplets are ejected from a recording head having a plurality of nozzles toward a recording medium, and at least one of the recording head and the recording medium is conveyed to relatively move the recording head and the recording medium. An image forming method for forming an image on the recording medium by dividing an image to be drawn into a plurality of regions, and outputting the image at a predetermined density in each region. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle is called “droplet rate”, the droplet ejection rate calculated based on the image data to be drawn is The droplet ejection rate calculation step for calculating the droplet ejection rate of the nozzle based on the image data to be drawn, and the droplet ejection rate calculation step. Calculated in A dot placement determining step for determining a dot placement pattern based on the droplet ejection rate, and a droplet ejection control step for controlling a droplet ejection operation by the recording head to realize the dot placement determined in the dot placement determination step; In a region where the droplet ejection rate calculated in the droplet ejection rate calculating step is lower than a predetermined determination reference value, m in the main scanning direction substantially orthogonal to the relative movement direction according to the droplet ejection rate (Where m is a positive integer) a dot group is formed by overlapping and arranging dots at a predetermined overlapping ratio, and the dot group consisting of the m dots is arranged in the main scanning direction while being shifted in the sub-scanning direction to form a polygonal line An image forming method comprising forming a dot line.

According to the present invention, in a region where the droplet ejection rate is lower than the predetermined determination reference value, a predetermined number (m) of dots are adjacent in the main scanning direction according to the droplet ejection rate, and these dot groups are mutually connected. It is assumed that a polygonal dot line is formed in the main scanning direction so as to be displaced in the sub-scanning direction, and at that time, adjacent droplets are overlapped with each other at a predetermined overlapping ratio and ejected. For this reason, even if a certain nozzle becomes defective in ejection, a part of defective dots is covered by adjacent dots ejected from other nozzles, and uneven stripes due to defective ejection are less noticeable. Further, in the present invention, it is not necessary to go through a process of detecting a nozzle discharge failure, and the visibility of dot defects when a discharge failure occurs can be reduced. Note that m is preferably an integer of 3 or more.

  (Invention 6): According to an aspect of the image forming method described in Invention 1, according to the droplet ejection rate in a region where the droplet deposition rate calculated in the droplet ejection rate calculation step is lower than a predetermined determination reference value. An oblique direction dot line is formed on a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the relative movement direction, and the dots are continuously overlapped with each other at a predetermined overlapping ratio.

  A mode in which the droplet ejection control is switched so as to form a main scanning direction dot line for the high droplet ejection rate region and an oblique direction dot line for the low droplet ejection region is preferable.

  (Invention 7): According to an aspect of the image forming method described in Invention 1, in a region where the droplet ejection rate calculated in the droplet ejection rate calculation step is lower than a predetermined determination reference value, the droplet ejection rate is determined according to the droplet ejection rate. A dot group is formed by forming a dot group in which m dots (where m is a positive integer) are overlapped and arranged at a predetermined overlapping ratio in the main scanning direction substantially orthogonal to the relative movement direction. Are arranged in the main scanning direction while shifting in the sub-scanning direction to form a polygonal dot line.

  There is also an aspect in which droplet ejection is switched so that a dot line in the main scanning direction is formed for a region with a high droplet ejection rate and a polygonal dot line is formed for a low droplet ejection region.

  (Invention 8): a recording head in which a plurality of nozzles for discharging liquid droplets are formed, and a conveying unit that conveys at least one of the recording head and the recording medium to relatively move the recording head and the recording medium; In the image forming apparatus, when the image to be drawn is divided into a plurality of areas and the image is output with a predetermined density in each area, one nozzle can eject droplets in that area. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots is referred to as the “droplet rate”, the dot arrangement is determined based on the droplet deposition rate calculated based on the image data to be drawn. Gradation is expressed by a dot aggregate, and a droplet ejection rate calculating unit that calculates a droplet ejection rate of a nozzle based on image data to be drawn, and a droplet ejection rate calculated by the droplet ejection rate calculating unit Based on the dot arrangement pattern And a droplet ejection control unit that controls the droplet ejection operation of the recording head to realize the dot arrangement determined by the dot arrangement determination unit, and is calculated by the droplet ejection rate calculation unit. In a region where the applied droplet ejection rate is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value, dots are continuously arranged in the main scanning direction substantially orthogonal to the relative movement direction according to the droplet ejection rate. The image forming apparatus is characterized in that droplet ejection control is performed so as to form dot lines in the main scanning direction that overlap with each other at a predetermined overlapping ratio.

  (Invention 9): a recording head in which a plurality of nozzles for discharging droplets are formed, and a transport unit that transports at least one of the recording head and the recording medium to relatively move the recording head and the recording medium; In the image forming apparatus, when the image to be drawn is divided into a plurality of areas and the image is output with a predetermined density in each area, one nozzle can eject droplets in that area. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots is referred to as the “droplet rate”, the dot arrangement is determined based on the droplet deposition rate calculated based on the image data to be drawn. Gradation is expressed by a dot aggregate, and a droplet ejection rate calculating unit that calculates a droplet ejection rate of a nozzle based on image data to be drawn, and a droplet ejection rate calculated by the droplet ejection rate calculating unit Based on the dot arrangement pattern And a droplet ejection control unit that controls the droplet ejection operation of the recording head to realize the dot arrangement determined by the dot arrangement determination unit, and is calculated by the droplet ejection rate calculation unit. Continuously in a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the relative movement direction in a region where the applied droplet ejection rate is lower than a predetermined determination reference value. An image forming apparatus, wherein droplet ejection control is performed so as to form an oblique direction dot line in which dots are overlapped and arranged at a predetermined overlapping ratio.

  (Invention 10): a recording head in which a plurality of nozzles for discharging droplets are formed, and a transport unit that transports at least one of the recording head and the recording medium to relatively move the recording head and the recording medium; In the image forming apparatus, when the image to be drawn is divided into a plurality of areas and the image is output with a predetermined density in each area, one nozzle can eject droplets in that area. When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots is referred to as the “droplet rate”, the dot arrangement is determined based on the droplet deposition rate calculated based on the image data to be drawn. Gradation is expressed by a dot aggregate, and a droplet ejection rate calculating unit that calculates a droplet ejection rate of a nozzle based on image data to be drawn, and a droplet ejection rate calculated by the droplet ejection rate calculating unit Dot placement pattern based on A dot placement determining means for determining, and a droplet ejection control means for controlling the droplet ejection operation of the recording head to realize the dot placement determined by the dot placement determining means, and calculating in the droplet ejection rate calculating step In a region where the applied droplet ejection rate is lower than a predetermined determination reference value, m dots (where m is a positive integer) in the main scanning direction substantially orthogonal to the relative movement direction according to the droplet ejection rate. Droplet control is performed so as to form a dot group that is arranged in an overlapping manner at a predetermined overlapping ratio and to arrange a dot group consisting of m dots in the main scanning direction while shifting the dot group in the sub-scanning direction to form a polygonal dot line. An image forming apparatus that is performed.

  As a configuration example of the recording head in the image forming apparatus according to any one of Inventions 8 to 10, a full-line type inkjet head having a nozzle row in which a plurality of nozzles for ejecting ink are arranged over a length corresponding to the entire width of the recording medium. Can be used.

  In this case, a combination of a plurality of relatively short ejection head blocks having nozzle rows that are less than the length corresponding to the entire width of the recording medium, and connecting them together, the nozzles having a length corresponding to the entire width of the recording medium as a whole There is an aspect that constitutes a column.

  A full-line type inkjet head is usually arranged along a direction perpendicular to the relative feeding direction (relative conveyance direction) of the recording medium, but at a certain angle with respect to the direction perpendicular to the conveyance direction. There may also be a mode in which the inkjet head is arranged along an oblique direction with a gap.

  A “recording medium” is a medium that can record an image by the action of a recording head (a medium that can be called an ejection medium, a printing medium, an image forming medium, a recording medium, an image receiving medium, etc.), continuous paper, cut paper It includes various media regardless of materials and shapes, such as a sealing sheet, a resin sheet such as an OHP sheet, a film, a cloth, a printed circuit board on which a wiring pattern is formed by an ejection head, an intermediate transfer medium, and the like.

  The conveying means for relatively moving the recording medium and the recording head is an aspect in which the recording medium is conveyed with respect to the stopped (fixed) recording head, an aspect in which the recording head is moved with respect to the stopped recording medium, or Any of the modes in which both the ejection head and the recording medium are moved is included.

1 is an overall configuration diagram of an ink jet recording apparatus according to an embodiment of the present invention. FIG. 1 is a plan view of a main part around a printing unit of the inkjet recording apparatus shown in FIG. Plane perspective view showing structural example of head Enlarged view of the main part of Fig. 3 (a) Plane perspective view showing another configuration example of a full-line head Sectional view along line 4-4 in Fig. 3 (a) Enlarged view showing the nozzle arrangement of the head shown in FIG. Schematic diagram showing the configuration of the ink supply system in the inkjet recording apparatus of this example Main block diagram showing the system configuration of the inkjet recording apparatus of this example Schematic diagram showing a dot arrangement example (droplet ejection rate of 1/2) by the first droplet ejection method Explanatory drawing used to explain the overlap condition between adjacent dots Schematic diagram when dot lines parallel to the main scanning direction are formed at a droplet ejection rate of 1/4 Schematic diagram showing a dot placement example (droplet ejection rate of 1/4) by the second droplet ejection method Schematic diagram showing another dot arrangement example (staggered arrangement) with a droplet ejection rate of 1/4 Explanatory drawing used to explain the droplet ejection method when the droplet ejection rate is 1 / n Schematic diagram showing an example of dot placement by the third droplet ejection method Explanatory drawing used to explain the method of setting the number of dots m of the dot group G (m) in the third droplet ejection method Figure showing VTF and actual visual results

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Inkjet recording device, 12 ... Printing part, 12K, 12C, 12M, 12Y ... Head, 14 ... Ink storage / loading part, 16 ... Recording paper, 18 ... Paper feed part, 22 ... Adsorption belt conveyance part, 31, 32 ... Roller, 33 ... Belt, 34 ... Suction chamber, 35 ... Fan, 50 ... Head, 50A ... Nozzle surface, 51 ... Nozzle, 52 ... Pressure chamber, 58 ... Actuator, 72 ... System controller, 75 ... ROM, 80 ... Print Control unit

Claims (3)

The recording medium is configured to eject droplets from a recording head having a plurality of nozzles toward the recording medium, transport at least one of the recording head and the recording medium, and relatively move the recording head and the recording medium. An image forming method for forming an image on the image,
When the image to be drawn is divided into a plurality of areas in units of a predetermined area including a plurality of pixels in the main scanning direction and the sub-scanning direction, and when performing image output to set the density to a predetermined density in each area, When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle in the area is referred to as “droplet ejection rate”, the droplet ejection of each nozzle in the same area for each area The rate is substantially the same, and gradation representation is performed by the droplet ejection density of a dot assembly composed of dot arrangements determined based on the droplet ejection rate calculated based on the image data to be drawn,
A droplet ejection rate calculating step of calculating a nozzle droplet ejection rate based on image data to be drawn;
A dot placement determination step for determining a dot placement pattern based on the droplet ejection rate calculated in the droplet ejection rate calculation step;
A droplet ejection control step of controlling a droplet ejection operation by the recording head to realize the dot arrangement determined in the dot arrangement determination step,
In a region where the droplet ejection rate calculated in the droplet ejection rate calculation step is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value, the main droplet is approximately orthogonal to the relative movement direction according to the droplet ejection rate. Forming a dot line in the main scanning direction in which dots are continuously overlapped with each other at a predetermined overlapping rate in the scanning direction;
In a region where the droplet ejection rate calculated in the droplet ejection rate calculation step is lower than a predetermined determination reference value,
In accordance with the droplet ejection rate, diagonal dot lines are formed on a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the relative movement direction, and dots are continuously overlapped at a predetermined overlapping ratio. Is what
When the minimum dot diameter of the dots constituting the dot line is Dmin and the pitch between adjacent dots in the main scanning direction is Pt,
Dmin / 2 ≧ Pt
The filling,
As the droplet ejection rate becomes lower than a predetermined determination reference value, the density indicated by the image data to be drawn is set so that the arrangement direction of the dot rows of the oblique dot lines is closer to the direction parallel to the sub-scanning direction. Accordingly, the image forming method is characterized in that the arrangement direction of the dot rows of the oblique direction dot lines is changed in the one image. When n is an integer of 2 or more and k is a positive integer, when the droplet ejection rate is 1 / n, the relative droplet ejection positions from adjacent nozzles that can eject droplets to adjacent pixel positions in the main scanning direction are The oblique direction dot lines are formed by ejecting droplets shifted by k pixels in the sub-scanning direction parallel to the moving direction,
The minimum value among k values such that the distance between dot lines x ₀ = n / (k ² +1) ^1/2 is smaller than a predetermined threshold is determined as k. 2. The image forming method according to 1. An image forming apparatus comprising: a recording head on which a plurality of nozzles for discharging droplets are formed; and a conveying unit that conveys at least one of the recording head and the recording medium to move the recording head and the recording medium relative to each other. Because
When the image to be drawn is divided into a plurality of areas in units of a predetermined area including a plurality of pixels in the main scanning direction and the sub-scanning direction, and when performing image output to set the density to a predetermined density in each area, When the ratio of the number of dots actually ejected by the nozzle to the maximum number of dots that can be ejected by one nozzle in that area is called “droplet ejection rate”, the droplet ejection ratio of each nozzle in the same area is substantially the same. The gradation expression is performed by the droplet ejection density of the dot assembly composed of the dot arrangement determined based on the droplet ejection rate calculated based on the image data to be drawn,
A droplet ejection rate calculating means for calculating a nozzle droplet ejection rate based on image data to be drawn;
Dot placement determining means for determining a dot placement pattern based on the droplet ejection rate calculated by the droplet ejection rate calculating means;
Droplet ejection control means for controlling the droplet ejection operation of the recording head to realize the dot arrangement determined by the dot arrangement determination means,
In a region where the droplet ejection rate calculated by the droplet ejection rate calculating means is lower than the maximum droplet ejection rate and higher than a predetermined determination reference value, the main ejection layer is approximately orthogonal to the relative movement direction according to the droplet ejection rate. Droplet ejection control is performed so as to form a main scanning direction dot line in which dots are continuously overlapped with each other at a predetermined overlapping rate in the scanning direction,
In a region where the droplet ejection rate calculated by the droplet ejection rate calculating means is lower than a predetermined determination reference value,
In accordance with the droplet ejection rate, diagonal dot lines are formed on a straight line having a predetermined inclination angle with respect to the main scanning direction substantially orthogonal to the relative movement direction, and dots are continuously overlapped at a predetermined overlapping ratio. The droplet ejection control is performed to
When the minimum dot diameter of the dots constituting the dot line is Dmin and the pitch between adjacent dots in the main scanning direction is Pt,
Dmin / 2 ≧ Pt
The filling,
As the droplet ejection rate becomes lower than a predetermined determination reference value, the density indicated by the image data to be drawn is set so that the arrangement direction of the dot rows of the oblique dot lines is closer to the direction parallel to the sub-scanning direction. Accordingly, the arrangement direction of the dot rows of the oblique direction dot lines is changed within the one image.

Images