JP4114014B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of preventing landing interference between dots when an image composed of a plurality of dots is formed.

  The image forming apparatus has an inkjet head (droplet ejection head) in which a large number of nozzles are arranged, and ejects ink from the nozzles toward the recording medium while relatively moving the inkjet head and the recording medium. Thus, an ink jet printer (ink jet recording apparatus) that records an image on a recording medium is known.

  In such an ink jet printer, when dots formed by ejecting droplets from a nozzle to a recording medium overlap with each other or are adjacent to each other, the dot shape of the dots on the recording medium is deformed. There was a problem of so-called landing interference.

  In order to prevent such landing interference, a predetermined output waiting time (specifically, drum rotation) is performed before droplet ejection of adjacent dots in the main scanning direction or sub-scanning direction among a plurality of droplet ejections. There has been proposed a system in which a waiting time for n laps) is entered (see Patent Document 1).

  In the case where droplets of different colors (for example, yellow and magenta) are ejected to the same location on the recording medium, there has also been proposed a method in which droplet ejection is performed with different rotations of the drum (see Patent Document 2). ). In the case of two colors, the object is achieved by increasing the time until the dots of both inks are overlapped by at least one rotation of the drum.

It has also been proposed that the time (color overlap time) T until dots of different colors are brought into contact or overlap at the landing position satisfies T ≧ 10 msec (see Patent Document 3).
JP 2001-129882 A (particularly columns 0019 to 0026) Japanese Patent Laid-Open No. 11-042799 (particularly column 0014) JP 2002-120361 A

  However, the conventional technique has a problem that even if the landing interference can be eliminated, an image cannot be formed at high speed.

  Further, Patent Documents 1, 2, and 3 do not describe how to specifically prevent landing interference between adjacent dots that overlap each other in the sub-scanning direction.

  The present invention has been made in view of such circumstances, and provides an image forming apparatus that can prevent landing interference between adjacent dots that overlap each other in the sub-scanning direction and can form an image at high speed. For the purpose.

  In order to achieve the above-mentioned object, the present invention has image signal input means for inputting image data that is an object of image formation and information indicating output resolution, and a plurality of nozzles in the main scanning direction, A droplet discharge head that selects a desired nozzle from the plurality of nozzles according to the image signal and discharges the droplet onto a predetermined recording medium; and the droplet discharge head is applied to the recording medium a plurality of times. Relative movement means for relatively moving the recording medium and the droplet discharge head in the sub-scanning direction so that the adjacent dots in the sub-scanning direction are overlapped with each other and are ejected, and the recording medium. A fixing time specifying means for specifying a fixing time for each dot, and when the number of dots overlapping each other in the sub-scanning direction on the recording medium is Vn, the V is based on the output resolution. A subtracting dot row in the sub-scanning direction by dividing the dot row in the sub-scanning direction into N groups by using the Vn calculated by the calculating unit as a basic unit. The droplet ejection order setting means for variably setting the droplet ejection order of the dots in the sub-scanning direction so that droplets are ejected every other, and the droplet ejection time difference between adjacent dots in the sub-scanning direction is equal to or more than the fixing time for each dot As described above, a droplet ejection time difference setting unit that sets a droplet ejection time difference between adjacent dots in the sub-scanning direction is provided.

  With this configuration, the fixing time for each dot on the recording medium is specified, and the dot ejection order of the dots in the sub-scanning direction is set based on at least the overlapping degree (Vn) of adjacent dots in the sub-scanning direction. Since the droplet ejection time difference between adjacent dots in the direction is set to be equal to or greater than the fixing time for each dot, landing interference between adjacent dots that overlap each other in the sub-scanning direction is prevented and images are formed at high speed. can do.

  In the present invention, the recording medium is wound around a rotating drum and directly ejected onto the recording medium to form dots on the recording medium, and dots are formed on the rotating drum as an intermediate transfer medium. Both aspects of transfer to a medium are included.

  Further, the droplet ejection order setting means may perform dot placement in the sub-scanning direction based on the fixing time for each dot, the output resolution in the sub-scanning direction, and the overlapping degree of adjacent dots in the sub-scanning direction. There is also an aspect in which the droplet order is set.

  This configuration takes into account the fixing time for each dot and the output resolution in the sub-scanning direction to set the dot ejection order in the sub-scanning direction, thus preventing landing interference more appropriately and at a higher speed. In addition, an image can be formed with high image quality.

  Further, the droplet ejection order setting means sets the dot rows in the sub-scanning direction to N groups with M as a basic unit, where M is an integer greater than or equal to the overlapping degree of adjacent dots in the sub-scanning direction and N is a natural number. There is also an aspect in which the droplet ejection order in the sub-scanning direction is set so as to divide and eject every (M-1).

  With this configuration, an integer M equal to or greater than the overlapping degree of adjacent dots in the sub-scanning direction is grouped as a basic unit, and droplets are ejected every (M−1), so that the difference in droplet ejection time between adjacent dots is almost equal. Since it becomes uniform, fixing unevenness is eliminated.

  Further, the relative moving means includes a rotating body, and the rotating body corresponds to the fixing time for each dot, the output resolution in the sub-scanning direction, the ejection cycle of the nozzle, and the basic unit M. There is also an aspect in which the circumference is configured.

  With this configuration, an image is formed at a higher speed by a rotating body having an appropriate circumference.

  In addition, the relative moving unit includes a rotating body, and the rotating body includes a fixing time for each dot based on a combination of the most frequently used recording medium and the most frequently used ink, and a sub scanning direction. There is also an aspect in which the circumference has a length corresponding to the maximum value of the output resolution, the minimum value of the discharge period of the nozzle, and the degree of overlap of the dots in the high quality mode.

  With this configuration, it is possible to achieve the maximum image forming speed even when an image is formed in the high image quality mode in the combination of the most used recording medium and the most used ink.

  Further, there is an aspect in which the basic unit M of the group is configured to be equal to the overlapping degree of the dots.

  With this configuration, N can be set large, so that an image can be formed at a higher speed.

  The droplet ejection order setting means may be configured to set the droplet ejection order using the overlapping degree of dots at the largest dot diameter when droplets with different dot diameters are mixed and ejected. is there.

  With this configuration, it is possible to stably form a high-quality image while completely reducing landing interference while reducing the calculation load of the control system.

  Further, there is also an aspect in which the relative moving means is a rotating drum that rotates by winding the recording medium on the surface thereof.

  With this configuration, it is possible to simplify the traveling of the recording medium a plurality of times.

  The relative movement unit may be a rotary transfer drum that functions as an intermediate transfer recording medium, and may include a transfer unit that pressurizes and transfers the rotary transfer drum and the recording medium.

  With this configuration, it is possible to form an image with high speed and high image quality without being affected by the penetration characteristics of the recording medium.

  According to the present invention, it is possible to prevent landing interference between adjacent dots that overlap each other in the sub-scanning direction, and to form an image at high speed.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out an image forming apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1A is an overall configuration diagram showing an outline of an example of an ink jet recording apparatus as an image forming apparatus according to the present invention.

  As shown in FIG. 1A, the inkjet recording apparatus 10 includes a plurality of droplet discharge heads 50 (50K, 50C, 50M, 50Y) provided for each ink color, and each droplet discharge head 50K. Ink storage / loading unit 14 (14K, 14C, 14M, 14Y) for storing ink to be supplied to 50C, 50M, and 50Y, paper feeding unit 18 for supplying recording paper 16, and curling of recording paper 16 are removed. The decurling unit 20 for cutting, the cutter 28 for cutting the recording paper 16, the paper discharging unit 26 for discharging the recording paper 16, and the droplet discharge head 50 are scanned relative to the recording paper 16 a plurality of times, A rotary drum 33 (relative movement means) that relatively moves the recording paper 16 and the droplet discharge head 50 in the sub-scanning direction so that adjacent dots in the sub-scanning direction overlap each other and are ejected. A paper winding / unwinding member 300 that functions as a conveyance path for winding the sheet-like recording paper 16 around the rotating drum 33 and unwinding the recording paper 16 from the rotating drum 33, and the recording paper 16 Synchronous detection sensor for synchronizing the relative movement of the UV irradiation light source 42 for irradiating UV (ultraviolet light) to the recording paper 16 and the droplet discharge head 50 and the droplet discharge from the droplet discharge head 50. 43.

  In FIG. 1A, 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.

  In the case of an apparatus configuration using roll paper, a cutter 28 is provided as shown in FIG. 1A, and the roll paper is cut into a desired size by the cutter 28. When only cut paper is used, the cutter 28 is not necessary.

  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.

  FIG. 1A also shows a rotating drum 33 that moves the recording paper 16 around the circumference as relative movement means for moving the recording paper 16 relative to the droplet discharge head 50. The rotary drum 33 is generally a vacuum suction type or electrostatic suction type.

  In the present invention, the relative movement means is not particularly limited to the rotary drum 33, and instead of the rotary drum 33, the recording paper 16 is relatively positioned relative to the droplet discharge head 50 (for example, horizontal). A belt that moves in a direction) may be provided. The belt generally 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.

  The droplet discharge head 50 is a so-called line-type print head in which a line-type head having a length corresponding to the maximum paper width is arranged in a direction (main scanning direction) orthogonal to the paper transport direction (sub-scanning direction). Yes.

  In each of the droplet discharge heads 50K, 50C, 50M, and 50Y, a plurality of nozzles (ink discharge ports) are arranged over a length that exceeds at least one side of the maximum size recording paper 16 that is the target of the inkjet recording apparatus 10.

  Along the transport direction (paper transport direction) of the recording paper 16 from the upstream side (left side of FIG. 1A) to black (K), cyan (C), magenta (M), and yellow (Y) in order of each color ink. Corresponding print heads 50K, 50C, 50M and 50Y are arranged. A color image can be formed on the recording paper 16 by discharging the color inks from the print heads 50K, 50C, 50M, and 50Y while conveying the recording paper 16.

  As described above, according to the line head in which the droplet discharge heads 50 that cover the entire paper width in the main scanning direction are provided for each ink color, the recording paper 16 and the droplet discharge in the paper transport direction (sub-scanning direction). An image can be recorded on the entire surface of the recording paper 16 by performing an operation of moving the head 50 relatively a plurality of times (that is, by a plurality of sub-scans).

  Further, in this example, the configuration of KCMY standard colors (four colors) 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. May be. For example, it is possible to add a print head that discharges light ink such as light cyan and light magenta.

  FIG. 1B is an overall configuration diagram showing an outline of another example of an ink jet recording apparatus 100 as an image forming apparatus according to the present invention.

  The same components as those of the ink jet recording apparatus 10 shown in FIG. 1A are denoted by the same reference numerals, and the description thereof will be omitted because it has already been described.

  An inkjet recording apparatus 100 shown in FIG. 1B includes a rotary transfer drum 33b that functions as an intermediate transfer recording medium, and a pressure transfer member 350 that presses and transfers an image formed on the rotary transfer drum to the recording paper 16. Have

  More specifically, at the time of printing from the print head 50 to the rotary transfer drum 33b, the pressure transfer member 350 and the recording paper 16 are separated from the rotary transfer drum 33b, and all the images are transferred to the rotary transfer drum 33b. At the timing after the end of the recording, the pressure transfer member 350 presses and transfers the recording paper 16 to the rotary transfer drum 33b.

  Here, terms used in the following description will be explained.

  “Landing interference” means that when dots that have landed on a recording medium are overlapped and dropped, the droplets of the dots on the surface of the recording medium coalesce or mix before fixing the dots after landing. The shape is deformed or the colors of different color inks are mixed non-uniformly to cause image degradation.

  “Dot overlap degree” is a physical quantity indicating the degree to which adjacent dots overlap.

  In this embodiment, the number of overlapping dots (also referred to as “overlap number”) is used as the “dot overlap degree”.

  For example, as shown in FIG. 2A, when two dots overlap in the sub-scanning direction, but every other dot does not overlap, that is, a distance Pt between the centers of adjacent dots. And the dot diameter D is D / 2 ≦ Pt <D, the degree of overlap Vn = 2.

  Also, for example, as shown in FIG. 2B, when D / 3 ≦ Pt <D / 2 dots overlap in the sub-scanning direction, the overlapping degree Vn = 3.

  When a plurality of types of dot diameters are used, the degree of dot overlap when the largest dot diameter is used is used.

  “Dot fixing” means that ink droplets on the surface of the recording medium are solidified (or hardened) first (surface solidification type), and second, ink droplets on the surface of the recording medium are inside the recording medium. In any case, it means that no droplets exist on the surface of the recording medium.

  In the case of “penetration type”, the fixing time is determined by the permeation characteristic of the ink into the recording medium. Specifically, the fixing time is determined mainly depending on the combination of the ink type and the recording medium type.

  If ink droplets on the surface of the recording medium do not exist due to penetration, the ink solute (coloring material) is fixed to the image receiving layer in the recording medium even if the ink solvent that has penetrated into the recording medium is not completely dry. As a result, it has been proved by experiments that landing interference hardly occurs. Therefore, in the present invention, the fixing time in the case of the penetrating type is defined as the time until the surface ink droplets finish penetrating. Even if the solvent in the recording medium is not dried, there is no relation to landing interference.

  In the case of the “surface solidification type”, the fixing time is determined by solidification (curing) characteristics such as ink drying characteristics and energy curing characteristics. The fixing time is determined mainly by the type of ink, UV (ultraviolet) irradiation energy, thermal energy, environmental conditions such as temperature and humidity, and the like.

  If there is no ink droplet on the surface of the recording medium, there is almost no landing interference, so the ink droplet may not be completely solidified but may be in a semi-solid solution state. Therefore, in the present invention, the definition of the fixing time in the case of the surface solidification type is a solidification (curing) time until no surface droplets exist.

  FIG. 3 is a block diagram showing a functional configuration of an embodiment of the image forming apparatus according to the invention.

  In FIG. 3, the image forming apparatus 10 mainly includes a relative moving unit 33, a droplet discharge head 50, a storage unit 81, a recording medium identification information reading unit 82, an ink identification information reading unit 83, and an image signal input. A means 84, an image processing means 85, a fixing time specifying means 91, a droplet ejection order setting means 92, a droplet ejection time difference setting means 93, a relative movement control means 94, and a droplet ejection control means 95. ing.

  The droplet discharge head 50 has at least a plurality of nozzles arranged in the main scanning direction, selects a desired nozzle from the plurality of nozzles according to an image signal, and discharges droplets onto a predetermined recording medium.

  The relative moving means 33 scans the recording medium and the droplet discharge head 50 such that adjacent droplets in the sub-scanning direction overlap each other by causing the droplet discharge head 50 to scan the recording medium a plurality of times. Are relatively moved in the sub-scanning direction.

  Examples of the relative moving unit 33 include a rotating drum (rotating body) that moves the wound recording medium relative to the droplet discharge head 50 by moving the recording medium on a predetermined circumference, a belt, and the like.

  The storage unit 81 stores information related to image formation. For example, the table information described later in detail necessary for specifying the fixing time for each dot is stored.

  The recording medium identification information reading unit 82 reads identification information (ID) that can identify the type of the recording medium from a medium storage magazine that stores the recording medium.

  The ink identification information reading unit 83 reads identification information (ID) that can identify the type of ink from an ink cartridge that stores ink.

  Reading of each identification information by the recording medium identification information reading means 82 and the ink identification information reading means 83 is various, such as wireless reading from a wireless tag (also referred to as RFID), optical reading from a barcode, magnetic reading, etc. There are reading modes.

  The image signal input means 84 receives an image signal from a host computer (not shown) or the like. The image signal includes image data that is an object of image formation and information indicating the output resolution.

The image processing unit 85 performs various types of image processing on the image data input to the image signal input unit 84. As a result of this image processing, the output resolution may be changed. Further, the image processing means 85 calculates the degree of dot overlap based on the output resolution (or dot pitch) and the desired density gradation expression.

  The fixing time specifying unit 91 specifies the fixing time for each dot (unit) on the recording medium based on the table information stored in the storage unit 81.

  Specifically, the recording medium identification information read by the recording medium identification information reading means 82, the ink identification information read by the ink identification information reading means 83, the dot diameter, etc. are used as parameters for each dot corresponding to these parameters. Specify the fixing time.

  As described above, the parameters differ depending on the dot fixing mode (penetration type or surface solidification type) or the like. Therefore, different table information is prepared for each dot fixing mode, and after specifying the dot fixing mode based on the ink identification information, the type of parameter and the table information to be referenced are switched.

  The droplet ejection order setting unit 92 sets the dot ejection order in the sub-scanning direction based on at least the overlapping degree of adjacent dots in the sub-scanning direction.

  For example, the dot ejection order in the sub-scanning direction is set based on the fixing time for each dot, the output resolution in the sub-scanning direction, and the overlapping degree of adjacent dots in the sub-scanning direction.

  When the droplet ejection time difference setting unit 93 performs droplet ejection so that adjacent dots in the sub-scanning direction overlap each other, the droplet ejection time difference between adjacent dots in the sub-scanning direction is the dot specified by the fixing time specifying unit 91. A droplet ejection time difference between adjacent dots in the sub-scanning direction is set so as to be equal to or longer than each fixing time.

  The relative movement control means 94 is means for moving the recording medium relative to the droplet discharge head 50 by the relative movement means 33.

  Further, the relative movement control means 94 changes the setting of the relative movement speed of the relative movement means 33. For example, when the relative movement unit 33 is a rotary drum, the relative movement control unit 94 changes the setting of the rotation speed (also referred to as the number of rotations) of the rotation drum 33 based on the output resolution, the fixing time for each dot of the nozzle, and the like. I do.

  The droplet ejection control means 95 controls droplet ejection from the nozzles of the droplet discharge head 50 based on the image signal. During such droplet ejection, the droplet ejection control unit 95 performs the dot ejection sequence in the sub-scanning direction set by the droplet ejection sequence setting unit 92 and the sub-scanning direction set by the droplet ejection time difference setting unit 93. The droplet ejection from the nozzles of the droplet discharge head 50 is controlled based on the droplet ejection time difference between adjacent dots.

  The droplet ejection control means 95 changes the setting of the ejection cycle of the nozzles of the droplet ejection head 50 based on the output resolution, the fixing time for each nozzle dot, and the like.

  Next, table information stored in advance in the storage unit 81 will be described.

When the dot fixing is “penetration type”, mainly the ink type, the type of recording medium, the dot diameter, etc. are used as parameters, and the time required for penetration corresponding to these parameters is set as the fixing time. Information is created in advance and stored in the storage means in advance. Table information in which environmental conditions such as temperature and humidity are further added as parameters may be created and stored.

  Specifically, the penetration time (that is, the fixing time) determined by the combination of the ink type and the recording medium type is specifically the surface tension of the ink, the viscosity of the ink, the capillary radius of the recording medium, and the contact between the ink and the recording medium. Influenced by conditions such as angle (ink conditions and recording medium conditions). Therefore, for various inks and recording media used for image formation, the relationship between these conditions and the permeation time is investigated or experimented, and table information is created based on the results of such investigation or experiment.

  FIG. 4 shows the measurement results of the combination of the ink and the recording medium and the penetration time when the dot fixing is the penetration type.

  In FIG. 4, the horizontal axis indicates the type of ink, and the vertical axis indicates the average value of the permeation time measured a plurality of times for each combination of ink and recording medium. Measurements were performed on combinations of seven types of ink and three types of recording media. Note that there is a difference in the size of the droplets within the range of 120 to 190 pl depending on the ink.

  In inkjet paper and photographic paper, there was a difference of several times (about 2 to 9 times) in permeation time between dye-based ink (for example, ink C) and dispersed ink (for example, ink F). In addition, the measurement results show that ink E (pigment-based) and ink G (dispersed medium-viscosity) have a faster penetration time into photographic paper than with inkjet paper and recycled paper. It is considered that the state of staying on the surface of the paper was not measured.

  Recycled paper is considered to have been less affected by the size of the particle diameter of the dispersed ink because the paper has the largest gap diameter.

  When dot fixing is “surface solidification type”, ink type, recording medium type, dot diameter, energy required for solidification such as UV (ultraviolet) irradiation energy and thermal energy, environmental conditions such as temperature and humidity, etc. Table information is created in advance and stored in the storage unit 81 in advance, with the time required for solidification corresponding to these parameters as the fixing time.

  Next, the operation of the image forming apparatus according to the embodiment of the present invention will be described with reference to the flowchart of FIG.

  Hereinafter, a case where a solid image, which is the most severe condition for landing interference, is ejected will be described. The solid image is described as an example to facilitate understanding of the present invention. In an actual print image, ink other than the solid image is selectively ejected from the nozzle according to the image signal. Needless to say, an image can also be formed. Also, a droplet ejection algorithm that mainly prevents landing interference in the sub-scanning direction will be described. Although the case where the ink is a single color will be described, the same droplet ejection control can be performed for each color ink even when a plurality of colors of ink are used.

  First, an image signal is input from the host computer or the like to the image signal input means 84 (S2).

  The image signal generally includes data (image data) indicating an image to be formed on a recording medium and an output resolution Rs. There is a case where the output resolution is determined by editing the image data on the image forming apparatus 10 side.

  Next, the fixing time specifying unit 91 specifies the fixing time Tfix for each dot (S4).

  Specifically, using the table information stored in advance in the storage unit 81, the fixing time Tfix is specified based on parameters relating to image formation, such as ink type, recording medium type, and dot diameter.

  The ink type is obtained by, for example, reading identification information indicating the ink type from an ink cartridge (not shown) that can be attached to and detached from the image forming apparatus 10. The type of the recording medium is obtained, for example, by reading identification information indicating the type of the recording medium from the recording medium. There are various reading modes of the identification information indicating the type of ink and the identification information indicating the type of the recording medium. For example, the identification information is read wirelessly, magnetically, or optically. The dot diameter is specified by a nozzle drive signal generated through image processing from image data, while the discharge amount (discharge volume) from the nozzle is determined by the ink and the recording medium. Even if the nozzle, the ink, and the recording medium are the same, the dot diameter can be switched by switching the ejection mode from the nozzle.

  Next, the dot ejection order setting unit 92 groups the dot rows formed in the sub-scanning direction on the recording medium into N × M based on the dot overlap degree Vn (S6).

  Specifically, first, a plurality of dot rows in the sub-scanning direction formed on the recording medium when droplets are ejected from the nozzles of the droplet discharging head 50 while relatively moving the droplet discharging head 50 and the recording medium. Divide into groups. More specifically, the dot row in the sub-scanning direction is divided into N groups with M dots arranged continuously as a basic unit. The division of the dot row in the sub-scanning direction into N groups by the basic unit M dots is hereinafter referred to as “N × M grouping”.

  Here, “N × M grouping” will be described by taking, as an example, a dot row having an overlap degree Vn (overlap number) of “3” as shown in FIG.

  In FIG. 6A, the first dot 101 which is the head of the dot row overlaps with the second dot 102 and the third dot 103, and overlaps with the fourth dot 104. Does not occur. That is, the i-th dot from the head of the dot row overlaps with the subsequent (Vn−1) dots, but does not overlap with the (i + Vn) -th dot.

  FIG. 6B is an explanatory diagram showing the dot row shown in FIG. 6A so that the dots do not overlap for convenience. It should be noted that the dot row shown in FIG. 6 (b) is simply represented so that the dot rows do not overlap for the sake of convenience. Actually, as shown in FIG. 3 ".

  Here, since the overlapping degree Vn is “3”, one group is formed for every “3” dots from the top, and N groups are formed. That is, an N × M group is formed with the basic unit M = Vn. For example, the first to third dots 101 to 103 from the head of the dot row form a first group, the fourth to sixth dots 104 to 106 form a second group, and the seventh to ninth dots. The dots 107 to 109 form a third group, and N groups are formed by sequentially combining three dots arranged in succession. That is, a group having Vn dots is formed in order from the top of the dot row. When the total number of dots in the dot row is not divisible by the dot overlap degree Vn, the number of dots belonging to the last group (Nth group) is less than D (here, 1 or 2). Such dots that do not actually exist in the last group are hereinafter referred to as dummy dots.

  In the dot row that has been subjected to the N × M grouping as described above, the droplet ejection order of each dot is determined as follows. First, the first droplet ejection group consisting of only the first dots (101, 104, 107,...) In each group from the first group to the Nth group is sequentially ejected, and then from the first group. A second droplet ejection group consisting of only the second dots (102, 105, 108,...) In each group up to the Nth group is sequentially ejected sequentially, and finally, from the first group to the Nth group. The M-th droplet ejection group consisting of only the M-th dot (M = 3) (103, 106, 109,...) In each group is assumed to be sequentially ejected. That is, the M-th droplet ejection group consisting only of the m-th dot (1 ≦ m ≦ M) in each group is formed, and droplets are ejected in order from the first droplet ejection group to the M-th droplet ejection group. Within each droplet ejection group, droplet ejection is performed every (M-1) dots.

  As shown in FIG. 6A, when the overlap degree Vn is “3” and the basic unit M = Vn is grouped into N × M, FIG. 7A shows the first droplet group. The dots (101, 104, 107,...) To be performed are shown by solid lines, and FIG. 7B shows the dots (102, 105, 108,...) Constituting the second droplet ejection group by solid lines, and FIG. ) Indicates the dots (103, 106, 109,...) Constituting the third droplet ejection group (that is, the Mth droplet ejection group) by a solid line.

  Note that the dot overlap degree Vn may be variable depending on the output image even if the ink and the recording medium are the same. For example, the dot overlap degree Vn varies depending on the output resolution Rs (which is the reciprocal of the dot pitch Pt). Accordingly, grouping may be performed directly based on the output resolution Rs (or dot pitch Pt), and the present invention includes such a case.

  Further, the description of the dot rows in the main scanning direction is omitted, but it goes without saying that the dot rows in the main scanning direction may be grouped.

As described above, after specifying the fixing time Tfix for each dot (S4) and grouping the dot rows in the sub-scanning direction (S6), the droplet ejection time difference setting means 93 overlaps and adjoins in the sub-scanning direction. A droplet ejection time difference Td (a droplet ejection time difference between adjacent dots) is set (S8).

Specifically, the droplet ejection time difference Td between adjacent dots is set based on the fixing time Tfix for each dot and the dot overlap degree Vn. In order to realize high-speed printing, the droplet ejection time difference Td between adjacent dots is set so as to be as small as possible.

For convenience of explanation, first consider the case where the fixing time Tfix for each dot is not considered.
The droplet ejection time difference Td between adjacent dots is expressed by (Formula 1).

(Formula 1)
Td ≧ Tjet × N + α
Here, Tjet is the minimum discharge period of the nozzle. N is the number of groups set by grouping in step S6. Α is the minimum rotation time of the rotating drum 33 corresponding to the total distance between the distance around the rotating drum 33 where the recording medium is not wound and the distance between the margin where the image on the recording medium is not formed. Value.

  Note that N = K / M. Here, K is the total number of dots in the sub-scanning direction, and M is a basic unit set by grouping in step S6. Therefore, (Formula 1) is expressed by (Formula 2).

(Formula 2)
Td ≧ Tjet × K / M + α
Since the dot overlap degree Vn is used as the basic unit M of the group, M = Vn, and (Expression 2) is expressed by (Expression 3).

(Formula 3)
Td ≧ Tjet × K / Vn + α
On the other hand, in consideration of the fixing time Tfix for each dot, a condition for preventing landing interference is expressed by (Formula 4).

(Formula 4)
Td ≧ Tfix
Here, Tfix is the fixing time for each dot specified in step S4.

  Therefore, the droplet ejection time difference Td between adjacent dots is set to a numerical value that satisfies the above (Formula 3) and satisfies the above (Formula 4). If high-speed printing is important, set the minimum value that satisfies both equations.

  Describes the case where the image to be formed is a solid image, and the length of the recording medium in the sub-scanning direction is also fixed (that is, the recording medium of the same size moves relative to the droplet discharge head in the same direction) And the maximum number of dots K in the sub-scanning direction is considered as a fixed value. Α is also considered as a fixed value. Then, the droplet ejection time difference Td between adjacent dots can be calculated using the fixing time Tfix for each dot and the dot overlap degree Vn as variable parameters.

  If there is a variable parameter other than the fixing time Tfix for each dot and the dot overlap degree Vn, the droplet ejection time difference Td between adjacent dots may be calculated in consideration of such a variable parameter. Needless to say. For example, if the image is not a solid image, K is generally variable. Further, if the image size and the recording medium size are different, α is variable in addition to K.

  Further, when the maximum rotation period of the rotary drum 33 is “Tjet × N + α”, that is, when the paper length in the sub-scanning direction of the recording medium is large, for example, Note that in the case where the rotational performance is low, it is necessary to calculate the droplet ejection time difference Td by further considering the maximum rotation period (or the maximum rotation speed [rpm]).

  Further, the dot overlap degree Vn has been described as a variable parameter. However, when the dot overlap degree Vn is fixed regardless of the output resolution Rs or the like, the dot overlap degree Vn may be handled as a fixed value instead of a variable parameter. Needless to say, that's good.

  As described above, by performing droplet ejection from the nozzle to the recording medium based on the droplet ejection order set by the grouping in step S6 and the droplet ejection time difference Td between adjacent dots set in step S8. Then, an image is formed on the recording medium (S10).

  Each step of the image forming process described above is actually executed by a microcomputer according to a program stored in advance in the storage unit 81.

  By the way, as described below, by forming the rotating drum 33 with the circumferential length L of the rotating drum 33 set to an optimum value, it is possible to further increase the speed of image formation.

  For convenience of explanation, it is first assumed that a solid image is formed on the recording medium without generating a margin in the sub-scanning direction, and that there is no portion on the rotating drum 33 around which the recording medium is not wound. In other words, when the above α is ignored, the following (Formula 5) is established based on (Formula 2) and (Formula 4).

(Formula 5)
Tfix ≦ Td = Tjet × K / M
On the other hand, the length Ldmin (minimum drum circumference) of the portion where the solid image is ejected is expressed by (Formula 6).

(Formula 6)
Ldmin = K × Pt
Here, Pt is a dot pitch.

  By applying (Equation 6) to (Equation 5) and solving for Ldmin, (Equation 7) is obtained.

(Formula 7)
Ldmin ≧ Tfix × M × Pt / Tjet
In such (Equation 7), the minimum value that can be ejected by the nozzles is set in the ejection period Tjet of the nozzles in order to cope with the higher speed of image formation. In addition, the dot overlap degree Vn (the number of overlaps) is set for the basic unit M of the group in order to cope with high image quality in image formation. The dot pitch Pt (which is the reciprocal of the output resolution Rs) is set to a minimum value in order to correspond to the high image quality mode. The fixing time Tfix for each dot is set to the fixing time corresponding to the combination of the most frequently used recording medium and the most frequently used ink.

The minimum drum circumferential length Ldmin obtained by (Equation 7) and the length Lp of the recording medium in the sub-scanning direction
The actual circumference Ld of the rotary drum 33 is set by comparing with an integral multiple of (recording medium length). For example, if the minimum drum circumferential length Ldmin is smaller than the recording medium length Lp, the circumferential length Ld of the rotary drum 33 is set to the recording medium length Lp + β. Here, β is the length of the portion around the rotating drum 33 where the recording medium is not wound.

  In this way, based on the fixing time Tfix for each dot, the basic unit M of the group, the output resolution Rs (or dot pitch Pt = 1 / Rs), and the nozzle discharge cycle Tjet, the circumference of the rotary drum 33 is determined. The length Ld is set.

  Hereinafter, the drum circumferential length Ld will be described in detail in two cases (case A and case B). Case A and Case B differ in dot fixing time Tfix and output resolution Rs, but have the same dot overlap degree Vn. In brief, Case A is high-resolution output and high-speed fixing, and Case B is low-resolution output and low-speed fixing.

[Case A]
Dot fixing time Tfix = 30ms
Dot overlap degree Vn = 3
Output resolution Rs = 2400 dpi (dot pitch Pt = 10.6 μm)
Length of recording medium (A4) in the sub-scanning direction Lp = 300 mm
Total number of dots in the sub-scanning direction of the recording medium (A4) K = Lp / Pt = 28302 dots
Nozzle discharge cycle Tjet = 40μsec (25kHz)
In such a case A, the fixing time Tfix for each dot is specified, and N × M grouping is performed.

  The dot overlap degree Vn (= 3) is assigned to the grouping basic unit M [dot]. Then, the number N of groups is as shown in (Formula 8).

(Formula 8)
N = K / M = 28302/3 = 9434
The minimum drum circumference Ldmin is as shown in (Equation 9).

(Formula 9)
Ldmin = Tfix × M × Pt / Tjet = 0.030 × 3 × 0.0000106 / 0.000040 = 23.9 [mm]
In case A, since Ldmin <paper length Lp (300 mm), the actual drum circumferential length Ld may be the paper length Lp + β. Note that β is the length of the portion around the rotary drum 33 where the recording medium is not wound. When β = 30 mm, the actual circumferential length Ld of the rotary drum 33 is Ld = 300 + 30 = 330 [mm]. The rotating drum 33 having the circumferential length Ld thus obtained is formed and provided in the image forming apparatus.

 For reference, in case A, the number of rotations of the rotary drum 33 is 159 rpm when the total number of dots K = 9434 in the sub-scanning direction of the paper is calculated as one round at the nozzle discharge period Tjet = 40 μsec. In this case, the peripheral speed of the rotary drum 33 is peripheral length Ld × rotational speed = 330 mm × 159 rpm / 60 sec = 0.874 m / sec.

[Case B]
Dot fixing time Tfix = 60ms
Dot overlap degree Vn = 3
Output resolution Rs = 240 dpi (dot pitch Pt = 106 μm)
Length of recording medium (A4) in the sub-scanning direction Lp = 300 mm
Total number of dots in the sub-scanning direction of the recording medium (A4) K = Lp / Pt = 2833 dots
Nozzle discharge cycle Tjet = 40μsec (25kHz)
In case B as described above, the fixing time Tfix for each dot is specified, and N × M grouping is performed.

  The dot overlap degree Vn (= 3) is assigned to the grouping basic unit M [dot]. Then, the number N of groups is as shown in (Formula 10).

(Formula 10)
N = K / M = 2830/3 = 944
The last two dots in the last group (Nth group) form a group as dummy dots.

  The droplet ejection time difference Td between adjacent dots is set based on the above-described (Equation 3) and (Equation 4).

  The minimum drum circumference Ldmin is as shown in (Formula 11).

(Formula 11)
Ldmin = Tfix × M × Pt / Tjet = 0.060 × 3 × 0.000106 / 0.000040 = 477 [mm]
In Case B, the minimum required drum circumferential length Ldmin is compared with the paper length Lp (300 mm). Since Lp <Ldmin <Lp × 2, the actual drum circumferential length Ld = 2 × Lp + β is preferable. If the drum peripheral length Ld = Lp + β, landing interference will occur unless the nozzle discharge period Tjet is increased. If the nozzle discharge cycle Tjet is increased,
Although landing interference does not occur, the image forming speed (printing speed) is lowered.

  For reference, in case B, the rotational speed of the rotary drum 33 is 1589 rpm when it is calculated assuming that the total number of dots K = 944 dots in the sub-scanning direction of the paper is rotated once in the nozzle ejection cycle Tjet = 40 μsec. In this case, the peripheral speed of the rotating drum 33 is the peripheral length Ld × the number of rotations = 630 mm × 1589 rpm / 60 sec = 17 m / sec.

  In the image forming apparatus using both the case A and the case B described above, for example, the drum circumference Ld is set by giving priority to the one with the highest use frequency.

  When the drum circumference Ld is set with priority given to case A (high resolution output, high-speed fixing), for example, the following setting change 1 or 2 is performed when droplets are ejected with low resolution output.

(Setting change 1)
The nozzle discharge period Tjet is fixed at 40 μsec, and the rotation speed of the rotary drum 33 during image formation is changed from 159 rpm to 1589 rpm.

(Setting change 2)
The rotation speed of the rotary drum 33 during image formation is fixed to 159 rpm, and the nozzle discharge period Tjet is changed from 40 μsec to 400 μsec.

  The flow of the image forming process when such setting change is performed is shown in the schematic flowchart of FIG.

  In FIG. 8, first, an image signal is input (S2). Next, using the table information stored in the storage unit 81, a fixing time Tfix for each dot is specified based on parameters relating to image formation, such as ink type, recording medium type, and dot diameter (S4). Next, the dot rows formed in the sub-scanning direction on the recording medium are grouped into N × M based on the dot overlap degree Vn (S6). Here, the dot overlap degree Vn is calculated by the image processing unit 85 based on the output resolution Rs (or dot pitch Pt), a desired density gradation expression, and the like. Next, based on the output resolution Rs (or dot pitch Pt) and the like, the setting of the nozzle ejection cycle Tjet or the rotational speed of the rotary drum 33 is changed (S7). Next, a droplet ejection time difference Td between adjacent dots in the sub-scanning direction is set based on the fixing time Tfix for each dot and the dot overlap degree Vn (S8). Based on the droplet ejection order set by the grouping in step S6 and the droplet ejection time difference Td between adjacent dots set in step S8, droplets are ejected from the nozzle onto the recording medium, thereby recording an image. It is formed on the medium (S10).

  Hereinafter, the image formation time of this embodiment will be considered.

If the present invention is not applied, assuming that the fixing time for each dot Tfix = 30 ms and the total number of dots in the sub-scanning direction K− = 28301 dots, the total time T1 in image formation using only monochrome ink is
T1 = 30 msec × 28301 = 849 sec.

The total time T4 in image formation using four color inks of C, M, Y, and K is
T4 = 849sec × 4 = 3396sec
It is.

On the other hand, when the present invention is applied, all the dots can be ejected with an ejection cycle Tjet = 40 μsec. Therefore, the total time T1 in image formation using only monochromatic ink is
T1 = 40μsec × 28301 = 1.13sec
Thus, it is possible to form an image at high speed while preventing landing interference.

The total time T4 in image formation using four color inks of C, M, Y, and K is
T4 = 1.13sec × 4 = 4.52sec
It becomes.

  The present invention is not limited to the examples described in the embodiments, and various design changes and improvements may be made without departing from the spirit of the present invention.

1 is an overall configuration diagram showing an outline of an example of an ink jet recording apparatus as an image forming apparatus according to the present invention. (A) is explanatory drawing in case the overlapping degree of a dot is "2", (b) is explanatory drawing in case the overlapping degree of a dot is "3". 1 is a block diagram illustrating a functional configuration of an embodiment of an image forming apparatus according to the present invention. It is a figure which shows the measurement result showing the relationship between the penetration | invasion time as a fixing time, the kind of ink, and the kind of recording medium in case dot fixation is a permeation | transmission type. 3 is a schematic flowchart illustrating an example of image forming processing in an embodiment of the image forming apparatus according to the present invention. (A) is explanatory drawing which shows the dot row | line | column of the subscanning direction in case the overlapping degree of a dot is "3", (b) is explanatory drawing drawn so that dots might not overlap for convenience. is there. (A) is an explanatory diagram in which dot rows in the sub-scanning direction are grouped into 3 × N, and the first hit group is indicated by a solid line, and (b) is an explanation that indicates the second droplet group by a solid line. (C) is an explanatory view showing a third droplet ejection group with a solid line. 6 is a schematic flowchart illustrating another example of the image forming process in the embodiment of the image forming apparatus according to the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10, 100 ... Inkjet recording apparatus (image forming apparatus), 50, 50Y, 50M, 50C, 50K ... Droplet discharge head, 14 ... Ink storage / loading unit, 16 ... Recording paper, 33 ... Rotating drum, 33b ... Rotation transfer Drum 42 ... UV irradiation light source 43 ... Synchronization detection sensor 81 ... Storage means 82 ... Recording medium identification information reading means 83 ... Ink identification information reading means 84 ... Image signal input means 91 ... Fixing time specifying means, 92: Droplet ejection order setting means, 93 ... Droplet ejection time difference setting means, 94 ... Relative movement control means, 95 ... Droplet ejection control means, 101 to 109 ... Dots, 300 ... Paper winding / unwinding member, 350 ... Addition Pressure transfer member

Claims (7)

Image signal input means for inputting image data including image data to be image-formed and information indicating output resolution;
A droplet discharge head having a plurality of nozzles in the main scanning direction, selecting a desired nozzle from the plurality of nozzles according to the image signal, and discharging droplets to a predetermined recording medium;
The recording medium and the droplet discharge head are scanned in the sub-scan so that adjacent droplets in the sub-scanning direction overlap each other by causing the droplet discharge head to scan the recording medium a plurality of times. Relative movement means for relative movement in the direction;
Fixing time specifying means for specifying the fixing time for each dot on the recording medium;
Calculating means for calculating Vn based on the output resolution when the number of dots overlapping each other in the sub-scanning direction on the recording medium is Vn;
The dot row in the sub-scanning direction is divided into N groups using Vn calculated by the calculating means as a basic unit, and droplets are ejected every (Vn−1) dot rows in the sub-scanning direction. A droplet ejection order setting means for variably setting the droplet ejection sequence in the sub-scanning direction;
Droplet ejection time difference setting means for setting a droplet ejection time difference between adjacent dots in the sub-scanning direction so that a droplet ejection time difference between adjacent dots in the sub-scanning direction is equal to or greater than the fixing time for each dot;
An image forming apparatus comprising: The relative movement means includes a rotating body,
2. The rotation body according to claim 1, wherein the rotating body has a perimeter corresponding to the fixing time for each dot, the output resolution in the sub-scanning direction, the ejection cycle of the nozzle, and the number of dots Vn. Image forming apparatus. The relative movement means includes a rotating body,
The rotating body has a circumference corresponding to the highest output resolution among the output resolutions in the sub-scanning direction used in the image forming apparatus, and when droplets are ejected at an output resolution lower than the highest output resolution. The image forming apparatus according to claim 1, wherein the number of rotations of the rotating body or the ejection cycle of the nozzles is changed.   2. The droplet ejection order setting means sets the droplet ejection order using the number of dots Vn at the largest dot diameter when droplets having different dot diameters are ejected in a mixed manner. The image forming apparatus according to claim 3.   5. The droplet ejection time difference setting means sets a droplet ejection time difference between adjacent dots in the sub-scanning direction based on the fixing time for each dot and the number of dots Vn. The image forming apparatus according to any one of the above.   6. The image forming apparatus according to claim 1, wherein the relative moving unit is a rotating drum that rotates by winding the recording medium on the surface thereof.   6. The relative movement unit is a rotary transfer drum that functions as an intermediate transfer recording medium, and includes a transfer unit that pressurizes and transfers the rotary transfer drum and the recording medium. The image forming apparatus described in 1.

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