JP2005125762A - Image forming apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and method in which possibility of occurrence of troubles incident to increase in the number of nozzles (number of recording elements) can be reduced while realizing high definition image recording. <P>SOLUTION: In an ink jet recorder employing ink of three or more colors including cyan (C), magenta (M) and yellow (Y), the number of nozzles of Y is reduced as compared with that of other color (C, M, or the like), and the inter-nozzle distance of Y is increased (nozzle density is decreased). Furthermore, image formation is controlled to perform printing while lowering the record density of ink dots of Y on a print image. For lowering in density of Y due to lowering of record density, a required image density is attained by combining a method for increasing the dot size, a method for increasing the ink density, and the like. Nozzle density of Y is set not higher than 1/2 (preferably, not higher than 1/3) of the nozzle density of other color, and dot density of Y on the print result image is set in the range of 300-600 dpi in the arranging direction of nozzles. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus and method, and more particularly to a structure of a recording head suitable for an ink jet recording apparatus that forms a color image using a plurality of colors of ink and a recording control technique thereof.

  When printing a color image, the inkjet printer uses at least three inks of cyan (C), magenta (M), and yellow (Y), and further, black (Bk), light cyan (LC), and light color. An image is formed using magenta (LM), dark yellow (DY), special color (SPC), or the like.

  In general, printers that support high-quality (photo-level image quality) print output often use six or more colors of ink. In this type of ink-jet printer, the nozzle density on the head is usually set to be equal for all colors. In addition, there is an example in which only the number of Bk nozzles is larger than other colors and the density is high in consideration of the document printing speed. In this case, the same nozzles are used for all colors other than Bk. The density is set. That is, generally, as the number of colors increases, the number of nozzles also increases accordingly.

  Further, in the conventional ink jet printer, in order to shorten the printing time, the ink ejection time interval of the recording head is shortened (the ejection frequency is increased) and the number of ink ejection nozzles of the recording head is increased. It was. Increasing the discharge frequency has been achieved by increasing the upper limit of the response frequency of a discharge mechanism (a pressure device such as a piezo or a heater) or by refilling ink promptly after ink discharge. In addition, the increase in the number of discharge nozzles has been achieved by improving the head processing / manufacturing technology, miniaturization, and high density, and even an inexpensive ink jet printer has a total number of nozzles of several thousand.

  While such improvements can shorten the printing time, the following problems also arise. In other words, the increase in the number of nozzles described above increases the total number of nozzles and increases the total extension distance of the flow path in the head for supplying ink to these nozzles, as well as an increase in apparatus cost. There is a problem that the possibility of occurrence of discharge defects increases.

  This is not just an increase in the probability of failure, but it is characteristic of inkjet, such as bubbles that are clogged in the ink flow path and normal ejection cannot be performed, and ink viscosity increases near the nozzle and ejection becomes impossible. This means that the possibility of malfunctions increases.

  In particular, it is not a shuttle scan type in which an inkjet head performs reciprocating scanning to print, but a single pass type in which a head having a length longer than the print image width is fixedly installed, and paper is conveyed in a direction perpendicular to the head length direction for printing. In an ink jet printer, the number of nozzles per color of ink may exceed 10,000, and it is a very serious problem that the possibility of occurrence of the above-described problems is increased.

  Although the problem of occurrence of such a problem is contrary to the direction of improvement of image quality improvement, if the number of nozzles can be reduced, the possibility of the problem can be reduced.

  In connection with the reduction in the number of nozzles, Patent Document 1 discloses Bk, C, M, Y inks, and thinning inks in an inkjet printer using a head in which ink ejection nozzles are arranged in a matrix (staggered arrangement). The cleaning liquid can be supplied to an arbitrary sub tank of the head, two nozzle rows are assigned to M and C, one nozzle row is assigned to Y and Bk, and the number of nozzles of two colors Y and Bk is set to C and M. A technique for halving the number is disclosed.

Another Patent Document 2 discloses a technique for reducing the resolution of only yellow colors and a technique for reducing the resolution of a color with a small number of nozzles when the number of nozzles varies depending on the color.
JP 2000-94717 A JP 2003-127438 A

  Patent Document 1 discloses an embodiment in which Y and Bk are set to 300 dpi and C and M are set to 600 dpi. When the number of Y nozzles is smaller than other C and M, an appropriate balance with other inks is disclosed. There is no disclosure about the Y ink droplet ejection method for obtaining the density, the Y ink dot size, and the ink density.

  Further, the technique proposed in Patent Document 2 reduces the scanning operation to ½ by reducing the resolution at the time of color image formation even in the case of a head configuration in which the number of nozzles of other color inks is half that of Bk. The aim is to reduce the speed, and at that time, how to print the data for two pixels of color ink together into one pixel, and to the light ink and normal ink at the nozzle position shifted by staggered arrangement This is a technique for allocating print data, and does not solve the problem of reducing the possibility of occurrence of problems associated with the increase in the number of nozzles.

  The present invention has been made in view of such circumstances, and an image forming apparatus and method capable of reducing the possibility of occurrence of problems associated with an increase in the number of nozzles (number of recording elements) while realizing high-quality image recording. The purpose is to provide.

  In order to achieve the above object, an invention according to claim 1 is an image forming apparatus for forming an image on a recording medium using color materials of a plurality of colors including at least cyan, magenta, and yellow. A cyan recording head having a plurality of cyan recording elements for forming cyan recording pixels on the recording medium; a magenta recording head having a plurality of magenta recording elements for forming magenta recording pixels on the recording medium; and A yellow recording head having a configuration in which a plurality of yellow recording elements for forming yellow recording pixels are arranged at a density lower than the recording element density of each of the cyan recording element and the magenta recording element; and The recording density of yellow recording pixels formed on the recording medium is adjusted by the cyan recording head to the recording medium. Recording control means for controlling a recording operation so as to be lower than a recording density of cyan recording pixels formed on the recording medium and a recording density of magenta recording pixels formed on the recording medium by the magenta recording head; It is characterized by having.

  According to the present invention, the recording element density of the yellow recording head is made lower than that of the cyan recording head and the magenta recording head based on the difference in detection ability of the human eye with respect to cyan, magenta, and yellow, and yellow recording is performed on the recording medium. Since the pixels are formed at a recording density lower than that of other colors, the number of yellow recording elements can be reduced as compared with cyan and magenta recording heads. As a result, it is possible to reduce the size of the yellow recording head, thereby reducing the overall size and cost of the apparatus, reducing the consumption of color materials and energy necessary for forming the recording pixels, and reducing the occurrence rate of recording defects. it can.

  The “coloring material” means a coloring material or a coloring material, and includes various modes such as dyes, pigments, paints containing them, inks, color photographic dyes, and coloring substances in the coloring layer.

  A second aspect of the present invention relates to the image forming apparatus according to the first aspect, wherein the yellow recording density is at least in the cyan direction in the direction in which the number of recording elements in the yellow recording element array in the yellow recording head is large. The recording density is lower than the recording density of magenta.

  When forming an image while relatively moving the recording head and the recording medium, the recording density is reduced at least in the direction in which the number of recording elements in the array of recording elements is large (in the case of a line head, the main scanning direction) More preferably, it is desirable to reduce the recording density also in the relative movement direction.

  A third aspect of the present invention relates to the image forming apparatus according to the first or second aspect, wherein the yellow recording density is 1 / n times the cyan recording density and the magenta recording density (where n is 2 or more). The number of

  The recording density of yellow on the recording medium is 1 / n times the recording density of cyan and magenta (where n ≧ 2, preferably n is a number exceeding 2), and preferably 1/3 or less.

  From the visibility of yellow, image quality can be ensured without causing image deterioration by such low density.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the first, second, or third aspect, the size of the yellow recording pixel is larger than the sizes of the cyan and magenta recording pixels.

  Since yellow dots are harder to recognize the granularity than cyan and magenta, even large dots do not cause image degradation and can ensure image quality.

  When the recording density of yellow is 1 / n (n ≧ 2) of the recording density of cyan and magenta, the recording pixel size on the recording medium is n times the recording pixel size of cyan and magenta. It is preferable that the recording conditions be determined so as to increase the yellow recording pixel size as “n” increases, such as √n times.

  A fifth aspect of the present invention relates to the image forming apparatus according to any one of the first to fourth aspects, wherein the density of yellow recording pixels formed on the recording medium by the yellow recording head is set to the cyan and the magenta. It is characterized by being higher than the density of the recording pixels.

  A mode in which the density of the yellow recording pixels is increased so as to compensate for the density decrease due to the decrease in recording density is preferable. Thereby, a required image density can be secured.

  A sixth aspect of the present invention relates to the image forming apparatus according to any one of the first to fifth aspects, wherein the yellow recording head has a plurality of yellow recording elements extending over a recording width of the recording medium. It is a full-line type recording head arranged.

  By applying the present invention to a high-density recording head, in particular, a long full-line type recording head in which a large number of recording elements are arranged, the total number of recording elements can be greatly reduced, which is very effective. It is.

  The “full-line type recording head” is usually arranged along a direction orthogonal to the relative feeding direction (relative movement direction) of the recording medium, but is in a predetermined direction with respect to the direction orthogonal to the relative movement direction. There may also be a mode in which the recording head is arranged along an oblique direction having the angle. Further, the arrangement form of the recording elements in the recording head is not limited to a single line arrangement, but may be a matrix arrangement composed of a plurality of columns. Furthermore, a configuration is also possible in which a plurality of short recording head units having recording element arrays that are less than the length corresponding to the entire width of the recording medium are combined to form a recording element array corresponding to the entire width of the recording medium as these units as a whole. possible.

  The “recording medium” is a medium (which can be called a print medium, an image forming medium, a recording medium, an image receiving medium, or the like) that receives an image by the action of a recording head, and is a continuous sheet, a cut sheet, a seal sheet, It includes various media regardless of the material and shape, such as a resin sheet such as an OHP sheet, a film, a cloth, a printed board on which a wiring pattern or the like is formed by an inkjet recording apparatus. In this specification, the term “printing” represents the concept of forming an image in a broad sense including characters.

  The moving means (conveying means) for moving the recording medium and the recording head relatively moves the recording medium relative to the stopped (fixed) recording head, and moves the recording head relative to the stopped recording medium. Either the aspect or the aspect in which both the recording head and the recording medium are moved is included.

  A seventh aspect of the present invention relates to the image forming apparatus according to any one of the first to sixth aspects, wherein the yellow recording head has a liquid for fixing the ink used as the color material on the recording medium, A liquid discharge head having a nozzle for discharging at least one of a coating liquid for protecting the colorant pigment and a liquid for forming a protective film for protecting the colorant from friction and abrasion is provided. It is characterized by.

  By utilizing the space created by the reduction in the number of recording elements of the yellow recording head, it is possible to arrange nozzles that discharge liquid having different functions.

  An eighth aspect of the present invention relates to the image forming apparatus according to any one of the first to seventh aspects, wherein the yellow recording pixel is formed on the recording medium by controlling at least one of recording pixel size modulation and density modulation. It is characterized by being recorded on.

  As a technique for expressing the gradation of an image, recording image size modulation for changing the size (size) of recording pixels, density modulation for changing density, or a combination thereof can be used.

  A ninth aspect of the present invention relates to the image forming apparatus according to any one of the first to seventh aspects, wherein the yellow recording pixels are recorded on the recording medium by controlling area modulation. .

  As a technique for expressing the gradation of an image, an aspect using area modulation represented by an error diffusion method or a dither method is also possible.

  A tenth aspect of the present invention relates to the image forming apparatus according to any one of the first to ninth aspects, wherein the yellow recording density is not less than 300 dpi and not more than 600 dpi, and the cyan and magenta recording densities are not less than 1200 dpi. It is characterized by being.

  By realizing a recording density that satisfies the conditions of 300 dpi to 600 dpi for yellow and 1200 dpi for cyan and magenta, it is possible to form a high-quality image and reduce the size by reducing the number of yellow recording elements. Cost reduction and improved reliability can be achieved.

  An eleventh aspect of the present invention relates to the image forming apparatus according to any one of the first to tenth aspects, wherein a recording element density of the yellow recording element provided in the yellow recording head is equal to the density of the cyan recording head. The recording element density is less than or equal to ½ of the recording element density of the cyan recording element and the recording element density of the magenta recording element in the magenta recording head.

  When each cyan and magenta recording head is provided with a high-density recording element array that realizes a resolution of 1200 dpi or more, sufficient image quality can be obtained even with a recording element density of 1/2 or less for the yellow recording head.

  A twelfth aspect of the present invention relates to the image forming apparatus according to any one of the first to eleventh aspects, wherein the cyan recording element, the magenta recording element, and the yellow recording element each eject ink of a corresponding color. A nozzle, an ink chamber communicating with the nozzle and filled with ink to be ejected from the nozzle, and pressure generating means for applying an ejection force by pressurizing the ink in the ink chamber, and ejecting from the nozzle A recording pixel of each color is formed on the recording medium by the formed ink droplet. That is, the present invention is suitable for an ink jet recording apparatus, and has the benefits peculiar to ink jet, such as a reduction in the occurrence rate of defective ejection and an improvement in maintainability.

  A thirteenth aspect of the invention relates to the image forming apparatus according to the twelfth aspect, wherein the nozzle diameter of the yellow recording element is larger than the nozzle diameters of the cyan recording element and the magenta recording element.

  If the nozzle diameter is increased, ink ejection abnormality is less likely to occur, and the frequency of nozzle maintenance is reduced.

  A fourteenth aspect of the present invention relates to the image forming apparatus according to the twelfth or thirteenth aspect, wherein the yellow recording element is an ink determined from a relative speed between the recording medium and the yellow recording head and the yellow recording density. The ejection drive is performed at a frequency higher than the ejection frequency, and one recording pixel is formed by a plurality of ink droplets.

  In order to achieve the required recording density, it is necessary to eject ink at a predetermined ink ejection frequency calculated from the relative speed of the recording medium and the head. However, in order to increase the yellow recording pixel size or increase the density, one recording pixel may be formed by a plurality of ejected ink droplets. In such a case, there is a mode in which ink is ejected at a frequency higher than the calculated predetermined ink ejection frequency, and a plurality of ink droplets are ejected at substantially the same position (substantially one dot position). Alternatively, there is an aspect in which a plurality of ink droplets are ejected at positions where the plurality of ink droplets contact each other on the recording medium, and the plurality of ink droplets are aggregated by surface tension to form one recording pixel.

  The invention according to claim 15 provides a method invention for achieving the object. That is, the invention described in claim 15 is an image forming method for forming an image on a recording medium using a plurality of color materials including at least cyan, magenta, and yellow, wherein cyan recording pixels are formed on the recording medium. Forming a cyan recording head having a plurality of cyan recording elements, a magenta recording head having a plurality of magenta recording elements for forming magenta recording pixels on the recording medium, and forming a yellow recording pixel on the recording medium A yellow recording head having a configuration in which a plurality of yellow recording elements to be arranged are arranged at a density lower than the recording element density of each of the cyan recording element and the magenta recording element. The recording density of yellow recording pixels formed on the recording medium is formed on the recording medium by the cyan recording head. Recording pixels of each color are formed on the recording medium so as to be lower than the recording density of the cyan recording pixels and the recording density of the magenta recording pixels formed on the recording medium by the magenta recording head. Features.

  According to the present invention, in an image forming apparatus that forms an image on a recording medium using a plurality of color materials including at least cyan, magenta, and yellow, the recording element density of the yellow recording head is set to the cyan recording head and the magenta recording. Since the recording density of yellow is lower than that of the head and yellow recording pixels are formed on the recording medium at a recording density lower than that of other colors, the number of yellow recording elements can be reduced, and the size and cost of the head can be reduced. It is possible to realize reduction of consumption of color materials and energy necessary for forming the recording pixel, improvement of reliability, and the like.

  Further, in the present invention, it is possible to form a high-quality image by ensuring the yellow recording density and density required within a range that does not cause image deterioration from the relationship with yellow visibility.

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 according to an embodiment of the present invention. As shown in the figure, the inkjet recording apparatus 10 includes a printing unit 12 having a plurality of printing heads (corresponding to recording heads) 12Bk, 12C, 12M, and 12Y provided for each ink color, and each printing head. 12Bk, 12C, 12M, 12Y Ink / protective liquid storage / loading unit 14 for storing ink and protective liquid to be supplied, a paper supply unit 18 for supplying recording paper 16, and curling of the recording paper 16 are removed. A decurling unit 20, an adsorption belt conveyance unit 22 that is arranged to face the nozzle surface (ink ejection surface) of the printing unit 12 and conveys the recording paper 16 while maintaining the flatness of the recording paper 16, and a printing unit 12 and a paper discharge unit 26 that discharges recorded recording paper (printed matter) to the outside.

  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. Therefore, it is preferable to automatically determine the type of paper to be used and perform ink ejection control so as to realize appropriate ink ejection according to the type of paper.

  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. Then, the suction chamber 34 is sucked by the fan 35 to be a negative pressure, whereby the recording paper 16 on the belt 33 is sucked and held.

  When the power of a motor (not shown in FIG. 1, not shown in FIG. 9) is transmitted to at least one of the rollers 31 and 32 around which the belt 33 is wound, the belt 33 rotates in the clockwise direction in FIG. , And the recording paper 16 held on the belt 33 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.

  The printing unit 12 is a so-called full line type head in which line type heads having a length corresponding to the maximum paper width are arranged in a direction orthogonal to the paper feed direction (see FIG. 2). Although a detailed structural example will be described later (FIGS. 3 to 5), each of the print heads 12Bk, 12C, 12M, and 12Y has a maximum size recording paper that is the target of the inkjet recording apparatus 10 as shown in FIG. The line head includes a plurality of ink discharge ports (nozzles) arranged over a length exceeding at least one side of 16.

  A print head 12Bk corresponding to each color ink in the order of black (Bk), cyan (C), magenta (M), and yellow (Y) from the upstream side along the feeding direction of the recording paper 16 (hereinafter referred to as the paper transport direction). , 12C, 12M, and 12Y are arranged, and color images can be formed on the recording paper 16 by ejecting the color inks from the print heads 12Bk, 12C, 12M, and 12Y while conveying the recording paper 16, respectively.

  As described above, according to the printing unit 12 in which the full line head that covers the entire paper width is provided for each ink color, the operation of relatively moving the recording paper 16 and the printing unit 12 in the sub-scanning direction is performed. The image can be recorded on the entire surface of the recording paper 16 only by performing it once (that is, by one sub-scan). Thereby, it is possible to perform high-speed printing as compared with a shuttle scan type head in which the print head reciprocates in the main scanning direction, and productivity can be improved.

  In this example, the configuration of standard colors (4 colors) of Bk CMY is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light ink and dark ink are added as necessary. Also good. For example, it is possible to add a print head that discharges light ink such as light cyan and light magenta.

  Although details will be described later, in this embodiment, only the Y color print head (hereinafter referred to as Y head) 12Y nozzle has a lower density than the other color heads. For example, in this embodiment, Bk, C, and M are nozzle densities corresponding to 2400 npi (nozzle per inch), and Y is a nozzle density corresponding to 600 npi. In addition, using the reduced space obtained by reducing the number of nozzles that discharge Y ink, the Y head 12Y is also provided with nozzles that discharge protective liquid. The nozzle for the protective liquid has a nozzle density equivalent to 150 npi, for example.

  As shown in FIG. 1, the ink / protective liquid storage / loading unit 14 has tanks for storing inks and protective liquids of colors corresponding to the print heads 12Bk, 12C, 12M, and 12Y. The print heads 12Bk, 12C, 12M, and 12Y communicate with the corresponding print heads via the illustrated pipelines. The ink / protective liquid storage / loading unit 14 includes notifying means (display means, warning sound generating means) for notifying when the remaining amount of ink or protective liquid is low, and prevents erroneous loading between colors. It has a mechanism to do.

  In the present embodiment, the Y ink printing dots ejected by the Y head 12Y have a larger ink dot diameter than other colors and penetrate relatively deeper in the recording paper 16. Therefore, the Y ink has a low color density (luminous efficiency) with respect to the amount of ejected ink. Accordingly, in order to obtain a required color density, it is necessary to apply Y ink more than other colors, and it is desirable that the Y ink tank be larger than the other colors.

  Further, the protective liquid serves as a protective film for protecting the ink dye and the surface of the recording paper 16 from oxygen, ozone, other gases, light rays such as visible light and ultraviolet light, and external forces such as friction and abrasion. The protective liquid is naturally dried after being applied onto the recording paper 16, and at the same time, dry fixing is also performed by the processing of the post-drying unit 42 and the heating / pressurizing unit 44 in the subsequent stage.

  The print detection unit 24 includes an image sensor for imaging the droplet ejection result of the print unit 12, and functions as a means for checking nozzle clogging and other ejection defects from the droplet ejection image read by the image sensor. The print detection unit 24 is composed of a line sensor or an area sensor having a light receiving element array that is wider than at least the ink discharge width (image recording width) by the print heads 12Bk, 12C, 12M, and 12Y.

  The print detection unit 24 reads the test pattern printed by the print heads 12Bk, 12C, 12M, and 12Y for each color, and detects the ejection of each head. The ejection determination includes the presence / absence of ejection, measurement of dot size, measurement of dot landing position, and the like.

  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 blocking the paper holes by pressurization. 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) that switches the paper discharge path so as 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.

  Further, although not shown in the drawing, the paper is reversed on the paper output unit 26A for the main image, and the paper is reversed and returned from the back of the first cutter 28 to the suction belt conveyance unit 22 for printing. A printing unit is provided.

[Print head structure]
Next, the structure of the print head will be described. Since the print heads 12Bk, 12C, and 12M other than the Y head 12Y have the same structure, hereinafter, the print head is indicated by reference numeral 50 as a representative of them.

  FIG. 3 (a) is a plan perspective view showing an example of the structure of the print head 50, and FIG. 3 (b) is an enlarged view of a part thereof. 3C is a perspective plan view showing another example of the structure of the print head 50, and FIG. 4 is a cross-sectional view showing the three-dimensional configuration of the ink chamber unit (along line 4-4 in FIG. 3A). FIG.

  In order to increase the dot pitch printed on the recording paper surface, it is necessary to increase the nozzle pitch in the print head 50. As shown in FIGS. 3A to 3C and FIG. 4, the print head 50 of this example includes a nozzle 51 from which ink droplets are ejected, a pressure chamber (ink chamber) 52 corresponding to each nozzle 51, and the like. A plurality of ink chamber units (recording elements) 53 are arranged in a staggered matrix to achieve a high density of the apparent nozzle pitch.

  That is, the print head 50 according to the present embodiment has a plurality of nozzles 51 for ejecting ink in a direction substantially perpendicular to the feeding direction of the recording paper 16 as shown in FIGS. 3 (a) and 3 (b). A full line head having one or more nozzle rows arranged over a length corresponding to the entire width.

  However, the arrangement structure of the nozzles is not limited to the illustrated example in carrying out the present invention. Further, as shown in FIG. 3C, a length corresponding to the full width of the recording paper 16 is obtained by arranging short head units 50 'in which a plurality of nozzles 51 are arranged two-dimensionally in a staggered manner. You may comprise the full line head which has the nozzle row of.

  As shown in FIGS. 3 (a) to 3 (c), the pressure chamber 52 provided corresponding to each nozzle 51 has a substantially square planar shape, and nozzles are formed at both corners on the diagonal line. 51 and a supply port 54 are provided.

  Further, as shown in FIG. 4, each pressure chamber 52 communicates with a common channel 55 through a supply port 54. The common flow channel 55 communicates with an ink tank as an ink supply source, and the ink supplied from the ink tank is distributed and supplied to each pressure chamber 52 via the common flow channel 55.

  An actuator 58 having an individual electrode 57 is joined to the pressure plate 56 constituting the top surface of the pressure chamber 52, and the actuator 58 is deformed by applying a driving voltage to the individual electrode 57, and the nozzle 51 Ink is ejected. When ink is ejected, new ink is supplied from the common channel 55 to the pressure chamber 52 through the supply port 54.

  As shown in FIG. 5, a large number of ink chamber units 53 having such a structure are arranged in a row direction along the main scanning direction and an oblique column direction having a constant angle θ that is not orthogonal to the main scanning direction. The structure is arranged in a lattice pattern. 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. .

  That is, 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 the number of nozzle rows projected so as to be aligned in the main scanning direction is 2400 per inch (2400 npi). Hereinafter, for convenience of explanation, it is assumed that the nozzles 51 are arranged in a straight line at a constant interval (pitch P) along the longitudinal direction of the head (direction aligned in the main scanning direction).

  When the nozzles are driven by a full line head having a nozzle row corresponding to the full width of the paper, (1) all the nozzles are driven simultaneously, (2) the nozzles are sequentially driven from one side to the other (3) ) The nozzle is divided into blocks, and each block is sequentially driven from one side to the other. One line is formed in the width direction of the paper (direction perpendicular to the paper conveyance direction). Alternatively, the driving of the nozzle that prints a line composed of a plurality of rows of dots) is defined as main scanning.

  In particular, when the nozzles 51 arranged in the matrix as shown in FIG. 5 are driven, the main scanning as described in the above (3) 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.

  With the matrix structure described above, the substantial projection nozzle pitch in the main scanning direction (line head direction) is achieved to be about 10 to 20 μm.

  On the other hand, in the Y head 12Y, a nozzle for discharging Y ink is formed so as to be 600 npi with a configuration similar to that of the print head 50 described above, and a nozzle for discharging protective liquid is formed in the same head unit at 150 npi. The

  Naturally, the liquid supply path is different between the nozzle for discharging Y ink and the nozzle for discharging protective liquid, and further, two liquids are ejected by the operation of a cleaning blade (denoted by reference numeral 66 in FIG. 8) described later. Each nozzle row is arranged at an appropriate distance so as not to mix on the plate.

  FIG. 6 is a schematic view showing the configuration of the print head 50 other than Y in comparison with the configuration of the Y head 12Y. 6A shows the print head 50 other than Y, and FIG. 6B shows the Y head 12Y. In the figure, the recording paper 16 is conveyed along the direction of the arrow from the bottom to the top of the paper.

  As shown in FIG. 6B, the Y head 12Y has a Y ink discharge portion 12Y-y in which a plurality of Y ink discharge nozzles 51y are arranged and a plurality of protective liquid discharge nozzles 51p. And a protective liquid discharger 12Y-p. The nozzle row of the protective liquid discharge unit 12Y-p is disposed on the downstream side of the nozzle row of the Y ink discharge unit 12Y-y with respect to the conveyance direction of the recording paper 16.

  The nozzle density of the Y ink discharge section 12Y-y is about 1/4 of the nozzle density (for example, 600 npi) as compared with the print head 50 (for example, 2400 npi) of colors other than Y shown in FIG. It has become. Further, the nozzle density of the protective liquid discharger 12Y-p is further about 1/4 of the nozzle density (for example, 150 npi) compared to the Y ink discharger 12Y-y.

  According to the experiment, the C color and the M color are easy to feel the granularity when observed with the human eye, and even if the thin ink having a density of 1/4 to 1/6 of the normal ink is used in combination, it is about 1200 dpi or more. Preferably, an image without good image quality, particularly no graininess, could not be obtained without a resolution of about 1440 dpi. In this case, the size of the C and M color dots is about 1.4 to 1.5 times the pixel size of 1440 dpi, and the entire droplet ejection range on the paper surface is filled with ink dots. It was made a condition.

  On the other hand, as is well known, the Y color is difficult to recognize the shape of the dots when viewed with the human eye, and it is difficult to feel the granularity even with large dots, as compared to the C and M colors. That is, even if the Y color is printed with larger dots than the other colors, image deterioration is unlikely to occur. According to the experiment of making a simulation image, the boundary of whether or not it causes a problem in image quality was 300 to 400 dpi. In this case, the size of the Y color dot is about 1.3 to 1.5 times the pixel size of 300 to 400 dpi.

  Therefore, in the present embodiment, among the inkjet heads of the printing unit 12 that ejects a plurality of colors of ink, the number of Y color nozzles is reduced from other colors (C, M, etc.), and the Y color nozzle distance is reduced. The configuration is large (nozzle density is low). Further, not only the nozzle arrangement but also the printed image is controlled to reduce the recording density of Y ink dots at the time of image formation.

  To reduce Y density due to low recording density, the necessary image density can be obtained by a combination of methods such as increasing the size of each dot or increasing the ink density. At this time, the nozzle density of Y is set to be ½ or less of the nozzle density of other colors, and the Y dot density in the nozzle arrangement direction on the image of the print result is approximately 400 dpi or more, preferably 600 dpi. Make the above prints.

  The difference in the visibility of the dots in each of the colors Y, M, and C is due to the difference in the detection ability of the shade change (in this case, different colors) by the human eye.

  According to the experiment, with respect to M and C, Y has a density discrimination capability of almost ½ at a spatial frequency with the highest sensitivity to the density change (it cannot be distinguished without a density difference of about 2 times). In addition, the spatial frequency with high sensitivity was about 1/4 of the spatial frequency with the highest sensitivity in the case of M and C. (On the print, the low spatial frequency has a long dimension. In terms of dots, the spatial frequency is sparse. is there).

  In the embodiment of the present invention, not the density difference but the dot density is considered. From the above experiment, the number of Y nozzles (number of dots) is 1/2 or less, more preferably 1/3 or less of other colors. Is desirable.

  Such a color difference is caused by the structure of the retina of the human eye. That is, in the retina of the human eye, there are three types of cone cells for detecting color and rod cells for detecting brightness. The three types of cone cells correspond to the three primary colors of light, red, green, and blue, but the ratio of the number of these three types of cells is generally red: green: blue = 40: 20: 1. It is said.

  Since this is a number on the retina, it is considered that an approximate cell spacing is obtained by taking the square root to obtain the average distance between cells for each color. That is, red: green: blue = √40: √20: 1≈6: 4: 1.

  The colors (wavelengths of light) that the actual cone cells have sensitivity overlap, so the spatial resolution is not simply determined by the above ratio, but the spatial resolution of yellow (Y), which is the complementary color of blue, is not The fact that it is about 1/4 of M and C can be qualitatively explained from the above. Note that the above-described characteristics of the human eye are also used for a technique for efficient thinning out of color signals, for example, by encoding a television signal (NTSC system).

  In a normal shuttle scan type ink jet printer, since dots of each color are shot simultaneously, the difference in dot density on the image of the printing result becomes the difference in nozzle density as it is.

  On the other hand, in the single-pass inkjet printer, when considering the nozzle density as viewed from the paper transport direction, the difference in dot density is the same as the difference in nozzle density on the printed image, as in the shuttle scan inkjet printer. become.

  Most high-quality printers have a recording density of 1200 dpi or higher, so if Y is 600 dpi and other colors are 1200 dpi, the nozzle density of Y is ½ that of other colors. If the lengths of the nozzles of each color are the same, the difference in nozzle density is the difference in the number of nozzles. When the nozzle density is expressed in npi (nozzle per inch), for example, the nozzles of C, M, and Bk are 1200 npi and 1200 nozzles per inch respectively, and the Y nozzle is 600 npi and 600 nozzles per inch.

  In the case of the single pass type, for example, if a 12-inch printing width corresponding to A3 size paper is printed at 1200 dpi, C, M, and Bk are printed at 14400 nozzles, and Y is printed at 600 dpi to 7200 nozzles. In other words, Y has fewer 7200 nozzles than other colors, which can greatly reduce the total number of nozzles.

  6 (a) and 6 (b), if the printing width of 12 inches is printed at 2400 dpi, C, M, Bk will be 28800 nozzles, Y will be printed at 1200 dpi and 14400 nozzles. Become. In other words, Y has fewer 14400 nozzles than other colors. As described above, the higher the performance (high density) single-pass configuration, the greater the effect of reducing the number of nozzles.

  When Y is reduced in density as described above, the number of dots applied to the unit area of Y color is smaller than that of other colors, and the dot density is reduced. Therefore, the conventional method cannot obtain the necessary density. Therefore, in the present embodiment, a mode in which the ink droplets ejected at one time are enlarged to increase the dot size, a mode in which the number of ink droplets is increased when hitting dots corresponding to one pixel, and the dot size is increased, Y The color ink density itself is made darker than the other colors, and a required density is obtained by, for example, a mode of hitting dots. These methods may be used alone, or a plurality of methods may be used in appropriate combination.

  As shown in FIG. 6B, the Y ink ejection nozzle 51y has a larger nozzle diameter than the other color nozzles 51 shown in FIG. 6A, and is ejected all at once. Ink droplet size is larger than other colors.

  The nozzle diameter depends on the physical properties of the ink used, such as clay, and the ink absorption characteristics of the recording medium, but the C, M, Bk nozzle 51 that prints at 2400 dpi preferably has a diameter of about 25 μm. On the other hand, it is desirable that the Y nozzle 51y for printing at 600 dpi of the present application has a diameter of about 60 μm.

This may be determined in proportion to the cube root of the value obtained by squaring the recording density ratio of each color (in this case, 4 ^2/3 ≈2.52).

  With this nozzle diameter, the Y ink droplet volume is about 16 times the C, M, Bk ink droplet volume, and printing can be performed with almost no gap on a recording medium used for high-quality printing.

  The size of the ejected ink droplets is affected not only by the nozzle diameter but also by the balance of the cross-sectional area with the supply port 54 of each pressure chamber 52 when the ink has the same physical property value. This is because the ink volume is proportional to the cube of the diameter ratio.

  However, when the recording density in the sub-scanning direction is different from the recording density in the main scanning direction, the nozzle diameter ratio as described above is not always desirable.

  In a single-pass inkjet recording apparatus, it is generally easy to increase the recording density in the sub-scanning direction, and therefore the Y recording density in the sub-scanning direction can be higher than 600 dpi. Thus, when the recording density in the sub-scanning direction is higher than the recording density in the main scanning direction, it is desirable that the Y nozzle diameter is determined between the diameter of 60 μm and the C, M, Bk nozzle diameters of 25 μm.

  When the dot size is increased by increasing the ink droplets, or when the dot size is increased by increasing the number of ink droplets, the dot size on the paper to be used is from 1.3 times the Y-color dot density to 1. It is desirable to set the ink amount to be about 5 times.

  In addition, when the Y ink density is increased to obtain a necessary density, the STATUS A reflection is performed with Y color dots on all the pixels on the paper (when the Y color dots are fully printed on the paper). It is appropriate to select an ink density such that the density is 1.8 to 2.0.

  However, if the dot size is smaller than the pixel density, the white background of the paper will be exposed, and even if dark ink is used, the required density may not be obtained. It is desirable to use together.

  Further, by reducing the density of the Y-color nozzles 51y in this way, the Y head can be smaller in size than the heads of the other colors, so that in the reduced space portion of the Y head 12Y, It is possible to arrange a nozzle for discharging a liquid having another function.

  In the present embodiment, an example in which a nozzle 51p that discharges a protective liquid is added is shown. Examples of functional liquids other than ink include liquids for fixing ink, coating liquids for protecting ink pigments from oxygen, ozone, and ultraviolet rays, and protective film liquids for protecting ink from friction, abrasion, etc. is there.

  As described above, Y ink is shot with dots larger than other color inks or with darker ink, but density modulation that expresses gradation by changing the size or density of each dot ( The image may be formed by density gradation), or may be formed by area modulation (area gradation) such as error diffusion or dither that expresses the gradation depending on whether or not each dot is present.

  Further, when an image is formed by error diffusion or other area modulation, the Y ink droplet size can be set to one type, and the drive circuit and the control method can be simplified. At this time, other colors may be dot-size modulated or density-modulated, or may be area-modulated similarly to Y.

  When a plurality of ink droplets are ejected at the same position on the recording paper 16 or when a plurality of ink droplets are ejected at close positions where they contact each other on the recording paper 16, the surface tension of the droplets Since the liquid aggregates on the recording paper, the dot shape is close to the dot shape formed by one ink droplet, and a dot having a larger dot size and higher density than the dot formed by one ink droplet is formed. .

  When an image is formed by dot size modulation or density modulation of a color other than Y, the number of types of dot sizes on the Y ink print object (recording paper 16) is the same as that on one or more other ink print objects. This is smaller than the number of types of dot sizes.

  In this case, in particular, it is possible to simplify the drive circuit and control method for Y ink by using one type of droplet size for Y ink, and it is possible to make non-ejection difficult to occur by always firing large droplets. Thus, reliability is further improved.

  It is desirable that the dot size on the Y ink to be printed is larger than the smallest dot size on the other ink to be printed. Further, it is desirable that the dot size on the Y ink print object is equal to or larger than the largest dot size on the other ink print object.

  Further, it is desirable that the discharge frequency of Y ink is not more than ½ times the discharge frequency of at least one of C and M. Furthermore, it is desirable that the discharge frequency of Y ink is 1 / n times the discharge frequency of at least one of C and M (where n is an arbitrary number satisfying n ≧ 2).

  The recording pixel density on the object to be printed with three or more types of ink including C, M, and Y is approximately 2540 dpi or less, and sufficient quality can be obtained.

  FIG. 7 shows an example of C, M, Bk dot arrangement (1440 dpi) and Y dot arrangement (400 dpi). In the figure, the horizontal direction of the paper surface indicates the longitudinal direction of the head, and the vertical direction of the paper surface is the paper transport direction.

  In the example shown in the figure, the dot diameter of colors other than Y is about 30 μm (FIG. 7A), and the dot diameter of Y ink is about 108 μm, which is larger than this (FIG. 7B). )).

  The recording density of the dots arranged in the head longitudinal direction corresponds to the nozzle density projected so as to be arranged in the same direction. The recording density of dots in the paper transport direction is defined by the transport speed of the recording paper 16 and the ink ejection frequency. In the figure, the discharge frequency of Y ink is 1/3 of the number of discharge frequencies of other color inks.

  As shown in FIG. 7 (c), these dot arrangements having different recording densities (FIGS. 7 (a) and (b)) are overlapped on an actual printed object to realize color. An image is formed.

  As described above, it is possible to form an image with large dots only for Y by a relatively simple image processing calculation.

  On the other hand, there is also an aspect in which Y ink is ejected at the same dot size as C and M ink as follows. For example, the nozzle density of C, M, Bk is 1440 npi, and only the Y nozzle is 720 npi. Since the Y ink dots are formed at 720 dpi in the main scanning direction, the dot density in the main scanning direction is 1/2 that of 1440 dpi for the other C, M, Bk inks. Therefore, by setting the Y ink ejection frequency to twice the C, M, Bk ink ejection frequency and the Y dot density in the sub-scanning direction to twice the C, M, Bk dot density in the sub-scanning direction. It becomes possible to bring the two-dimensional Y dot density close to the other C, M, Bk dot densities. Thereby, the dot density can be made substantially equal to other inks with a small number of Y nozzles.

  In other words, in a configuration in which the nozzle density of the Y nozzle is ½ or less of the nozzle density of the C and M nozzles, in order to make the two-dimensional dot density of each color substantially equal, the Y ink ejection frequency is C And at least one of the ejection frequencies of at least one of M and M is desirable. Furthermore, it is desirable that the discharge frequency of Y ink is n times the discharge frequency of at least one of C and M (where n is an arbitrary number satisfying n ≧ 2, more preferably a number exceeding 2).

[Configuration of ink supply system]
FIG. 8 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 for supplying ink, and is installed in the ink / protective liquid 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 of FIG. 8 is equivalent to the ink / protective liquid storage / loading unit 14 of FIG. 1 described above.

  As shown in FIG. 8, a filter 62 is provided in the middle of the conduit connecting the ink tank 60 and the print heads 50 and 12Y to remove foreign substances 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. 8, a configuration in which a sub tank is provided in the vicinity of the print heads 50 and 12Y or integrally with the print heads 50 and 12Y 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 nozzle surface cleaning means.

  The maintenance unit including the cap 64 and the cleaning blade 66 can be moved relative to the print heads 50 and 12Y by a moving mechanism (not shown), and if necessary, maintenance is performed below the print heads 50 and 12Y from a predetermined retraction position. Moved to position.

  The cap 64 is displaced up and down relatively with respect to the print heads 50 and 12Y 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 is brought into close contact with the print heads 50 and 12Y, thereby covering the nozzle surface with the cap 64.

  The cleaning blade 66 is made of an elastic member such as rubber, and can slide on the ink discharge surfaces (surfaces of the nozzle plates) of the print heads 50 and 12Y 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.

Further, when air bubbles are mixed into the ink (pressure chamber) in the print head 50, the cap 64 is applied to the print heads 50 and 12Y, and the ink in the pressure chamber (ink mixed with air bubbles) 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 with increased viscosity (solidified) is sucked out when the ink is initially loaded into the head or when the ink is used after being stopped for a long time.

  That is, if the print heads 50 and 12Y are 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, and the ejection driving actuator 58 operates. Ink is not discharged from the nozzle 51. Therefore, before this state is reached (within the range of viscosity that allows ink to be ejected by the operation of the actuator), the actuator is operated toward the ink receiver to eject ink in the vicinity of the nozzle whose viscosity has increased. Discharge "is performed. In order to prevent foreign matter from entering the nozzle 51 by the wiper rubbing operation after the dirt on the nozzle plate surface is cleaned by a wiper such as a cleaning blade 66 provided as a nozzle surface cleaning means. 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, ink cannot be ejected from the nozzle 51 even if the actuator is operated. . In such a case, a suction means for sucking the ink in the pressure chamber 52 with a pump or the like is brought into contact with the nozzle surface of the print 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 amount of ink consumption is large. Therefore, when the increase in viscosity is small, it is preferable to perform preliminary discharge as much as possible.

  The cap 64 described with reference to FIG. 8 functions as a suction unit and can also function as a preliminary discharge ink receiver.

  In this embodiment, the Y ink has a small total nozzle number, a low nozzle density, and a large droplet size to be ejected. Therefore, the nozzle diameter is larger than other colors, and the ink flow path in the head is also thick ( The cross-sectional area is large). Therefore, ink clogging due to bubbles or the like is unlikely to occur, and it can be expected that the nozzle maintenance frequency is less than other colors.

[Explanation of control system]
Next, the control system of the inkjet recording apparatus 10 will be described.

  FIG. 9 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 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, IEEE 1394, Ethernet, and 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 is a control unit that controls each unit such as the communication interface 70, the image memory 74, the motor driver 76, and the heater driver 78. The system controller 72 includes a central processing unit (CPU) and its peripheral circuits, and performs communication control with the host computer 86, read / write control of the image memory 74, and the like, as well as a transport system motor 88 and heater 89. A control signal for controlling is generated.

  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 heaters 89 of the post-drying unit 42 and other units in accordance with instructions 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 a control signal (print data) to the head driver 84.

  The signals corresponding to Bk, C, M, and Y supplied to the head driver 84 are obtained by performing processing according to the nozzle density of each color head.

  Further, the signal for discharging the protective liquid is set so that the liquid is discharged over the entire surface of the recording paper 16, and an aspect in which the protective liquid is selectively output for a partial area on the surface of the recording paper. Is possible. For example, you can set the protective liquid to be ejected only to the part that ejected ink, or the protective liquid to be ejected only to the part that ejected less durable ink, or erase the image partially later For example, the protective liquid may be set to be discharged to the whole or a portion other than the portion “desirable or erasable”.

  Necessary signal processing is performed in the print control unit 80, and control of the discharge amount and discharge timing of ink droplets or protective liquid of each print head 12Bk, 12C, 12M, 12Y via the head driver 84 based on the image data. Is done. Thereby, desired dot size and dot arrangement and application of the protective liquid are realized. The recording control means of the present invention is provided in the print control unit 80.

  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. 9, the image buffer memory 82 is shown as being attached to the print control unit 80, but 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 a single processor.

  The head driver 84 drives the actuators 58 of the print heads 12Bk, 12C, 12M, and 12Y for 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.

  Unlike Y head 12Y, which is controlled and driven so that the other color heads eject only about 2 picoliters (pl) of fine droplets, it performs dot size modulation, so that it can eject ink of different liquid amounts. Is controlled and driven. Further, the protective liquid discharge unit 12Y-p provided in the Y head 12Y has a low nozzle density, but does not need to discharge different droplet amounts, and is controlled to discharge the same amount of protective liquid. .

  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 control unit 80 performs various corrections on the print heads 12Bk, 12C, 12M, and 12Y based on information obtained from the print detection unit 24 as necessary.

  FIG. 10 is an explanatory diagram including a flow of an example of image recording control in the inkjet recording apparatus 10 of the present example.

  When the original image data 100 to be printed (for example, RGB 8bit, 400dpi data) is input, first, in the color matching processing step 102, processing for converting the image data in accordance with the color reproduction characteristics of the apparatus is performed. Done. In this process, the resolution does not change, and the RGB data value representing the color changes. For example, the original image data illustrated above is converted into data of 10 bits and 400 dpi for each color of RGB.

  Thereafter, the process proceeds to the color separation processing step 104, and processing for converting RGB data into CMYBk data is performed. For example, in the above example, the data is converted into data of 10 bits and 400 dpi for each color of CMYBk. The multi-valued image data of each color generated (color-separated) by the color separation processing step 104 is denoted by reference numeral 106-i (where i = 1, 2, 3, 4) in FIG.

  Next, the multi-value image data 106-i (i = 1, 2, 3, 4) of each color is obtained by a halftoning process step 108-i (i = 1, 2, 3, 4) using an error diffusion method or the like. Converted to binary data. In this halftoning processing step 108-i (i = 1, 2, 3, 4), each color multi-value image data is binarized and resolution conversion is also performed. As a conversion method, an error diffusion method, a blue noise max, a threshold method, or the like can be used. The horizontal arrows in the figure indicate that the arrangement of each color is also taken into consideration. In the above example, for example, Bk, C, and M of CMYBk are converted to data of 1 bit and 2400 dpi for each color, and Y is converted to data of 1 bit and 600 dpi.

  The binary image data of each color (binarized) generated by the halftoning process step 108-i (i = 1, 2, 3, 4) is denoted by reference numeral 110-i (i = 1, 2, 3). , 4).

  Each color binary image data is converted into a data string (dot data) in consideration of the dot printing order in the printing order conversion processing step 112-i (i = 1, 2, 3, 4). That is, in the print order conversion processing step 112-i (i = 1, 2, 3, 4), the binary image data for each color is converted into a data sequence that matches the head configuration, structure, and print order.

  In this way, print data 114-i (i = 1, 2, 3, 4) for each color is generated. The print data 114-i (i = 1, 2, 3, 4) is image data for driving the head driver 116-i (i = 1, 2, 3, 4) of each color. Each color head driver 116-i (i = 1, 2, 3, 4) is synchronized with a clock signal (not shown) based on each color print data 114-i (i = 1, 2, 3, 4). 118-i (i = 1,2,3,4) is driven. The head drivers 116-i (i = 1, 2, 3, 4) for each color shown in FIG. 10 are equivalent to the head driver 84 described in FIG. 9, and the heads for each color shown in FIG. 118-i (i = 1, 2, 3, 4) is equivalent to the print heads 12Bk, 12C, 12M, and 12Y described in FIG.

  Thus, as shown in FIG. 10, ink droplets 120-i (i = 1, 2, 3, 4) are ejected from the heads 118-i (i = 1, 2, 3, 4) of the respective colors, and the recording medium An image is formed on 122 (corresponding to the recording paper 16 in FIG. 1).

  FIG. 10 schematically shows the state of the ink droplets 120-i (i = 1, 2, 3, 4) ejected from the respective color heads 118-i (i = 1, 2, 3, 4). As shown in the figure, the recording density of the Y head 118-4 is relatively low compared to the heads 118-i of Bk, C, and M (where i = 1, 2, 3).

  In the above-described embodiment, a method of ejecting ink droplets by deformation of the actuator 58 typified by a piezo element (piezoelectric element) is employed, but 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.

  In the above embodiment, an inkjet recording apparatus using a page-wide full-line head having a nozzle row having a length corresponding to the entire width of the recording medium has been described. However, the scope of application of the present invention is not limited to this, and the short length The present invention can also be applied to an inkjet recording apparatus using a shuttle scan type head that records an image while reciprocating the recording head.

  In the above description, an ink jet recording apparatus is illustrated as an example of an image forming apparatus, but the scope of application of the present invention is not limited to this. Other than the ink jet system, the present invention can be applied to various types of image forming apparatuses such as a thermal transfer recording apparatus having a line head, an LED electrophotographic printer, and a silver salt photographic system printer having an LED line exposure head.

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 ink jet recording apparatus shown in FIG. Plane perspective view showing structural example of print head Enlarged view of the main part of Fig. 3 (a) Plane perspective view showing another structural example of the print head Sectional view along line 4-4 in Fig. 3 (a) Enlarged view showing the nozzle arrangement of the print head shown in FIG. 6A is a schematic plan view showing a configuration example of a print head other than Y, and FIG. 6B is a schematic plan view showing a configuration example of a Y print head. Explanatory drawing which showed the example of the dot arrangement | positioning implement | achieved by embodiment of this invention Schematic diagram showing the configuration of an ink supply system in the inkjet recording apparatus according to the present embodiment Main block diagram showing the system configuration of the inkjet recording apparatus of this example Explanatory drawing including the flow of the example of control of image recording in the ink jet recording apparatus of this example

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Inkjet recording device, 12Bk, 12C, 12M, 12Y ... Print head, 12Y-p ... Protection liquid discharge part, 12Y-y ... Y ink discharge part, 14 ... Ink / protection liquid storage / loading part, 16 ... Recording paper 50 ... Print head 51 ... Nozzle 52 ... Pressure chamber 54 ... Supply port 56 ... Pressure plate 58 ... Actuator 72 ... System controller 80 ... Print control unit

Claims (15)

An image forming apparatus that forms an image on a recording medium using a plurality of color materials including at least cyan, magenta, and yellow,
A cyan recording head having a plurality of cyan recording elements for forming cyan recording pixels on the recording medium;
A magenta recording head having a plurality of magenta recording elements for forming magenta recording pixels on the recording medium;
A yellow recording head having a configuration in which a plurality of yellow recording elements for forming yellow recording pixels on the recording medium are arranged at a density lower than the recording element density of each of the cyan recording element and the magenta recording element;
The recording density of yellow recording pixels formed on the recording medium by the yellow recording head is equal to the recording density of cyan recording pixels formed on the recording medium by the cyan recording head and the recording medium by the magenta recording head. Recording control means for controlling the recording operation so as to be lower than the recording density of the magenta recording pixels formed thereon;
An image forming apparatus comprising:   The yellow recording density is lower than the cyan recording density and the magenta recording density at least in a direction in which the number of recording elements in the yellow recording element array in the yellow recording head is large. Item 2. The image forming apparatus according to Item 1.   3. The image forming apparatus according to claim 1, wherein the recording density of yellow is 1 / n times the recording density of cyan and the recording density of magenta (where n is a number of 2 or more). .   4. The image forming apparatus according to claim 1, wherein the size of the yellow recording pixel is larger than the sizes of the cyan and magenta recording pixels.   The density of yellow recording pixels formed on the recording medium by the yellow recording head is higher than the densities of the cyan and magenta recording pixels. Image forming apparatus.   6. The yellow recording head according to claim 1, wherein the yellow recording head is a full-line type recording head in which the plurality of yellow recording elements are arranged over a length equal to or greater than a recording width of the recording medium. The image forming apparatus described.   The yellow recording head has a liquid for fixing the ink used as the color material on the recording medium, a coating liquid for protecting the coloring material of the color material, and a protective film for protecting the color material from friction and abrasion. The image forming apparatus according to claim 1, further comprising a liquid discharge head having a nozzle for discharging at least one of the liquids for forming the liquid.   The image forming apparatus according to claim 1, wherein the yellow recording pixels are recorded on the recording medium by controlling at least one of recording pixel size modulation and density modulation.   The image forming apparatus according to claim 1, wherein the yellow recording pixels are recorded on the recording medium by area modulation control. The image forming apparatus according to claim 1, wherein the recording density of the yellow is 300 dpi or more and 600 dpi or less, and the recording densities of the cyan and the magenta are 1200 dpi or more.   The recording element density of the yellow recording element provided in the yellow recording head is 1/2 of the recording element density of the cyan recording element in the cyan recording head and the recording element density of the magenta recording element in the magenta recording head. The image forming apparatus according to claim 1, wherein the image forming apparatus is:   The cyan recording element, the magenta recording element, and the yellow recording element each include a nozzle that ejects ink of a corresponding color, an ink chamber that communicates with the nozzle and is filled with ink ejected from the nozzle, and the ink chamber And pressure generating means for applying an ejection force by pressurizing the ink of the recording medium, and a recording pixel of each color is formed on the recording medium by the ink droplets ejected from the nozzle. The image forming apparatus according to any one of 1 to 11.   13. The image forming apparatus according to claim 12, wherein the nozzle diameter of the yellow recording element is larger than the nozzle diameters of the cyan recording element and the magenta recording element.   The yellow recording element is driven to be ejected at a frequency higher than the ink ejection frequency obtained from the relative speed between the recording medium and the yellow recording head and the yellow recording density, and one recording pixel is formed by a plurality of ink droplets. The image forming apparatus according to claim 12, wherein the image forming apparatus is an image forming apparatus. An image forming method for forming an image on a recording medium using a plurality of color materials including at least cyan, magenta, and yellow,
A cyan recording head having a plurality of cyan recording elements for forming cyan recording pixels on the recording medium;
A magenta recording head having a plurality of magenta recording elements for forming magenta recording pixels on the recording medium;
A yellow recording head having a configuration in which a plurality of yellow recording elements for forming yellow recording pixels on the recording medium are arranged at a density lower than the recording element density of each of the cyan recording element and the magenta recording element;
Use
The recording density of yellow recording pixels formed on the recording medium by the yellow recording head is equal to the recording density of cyan recording pixels formed on the recording medium by the cyan recording head and the recording medium by the magenta recording head. An image forming method comprising: forming recording pixels of each color on the recording medium so as to be lower than a recording density of magenta recording pixels formed thereon.

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