JP2007223050A - Method for delivering ink and inkjet type image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for delivering ink capable of preventing deterioration of image quality caused by deviation of depositing positions of a main droplet and a sub-droplet. <P>SOLUTION: The main droplet of ink droplets delivered from a printing head 22K1 is represented by K1m and the sub-droplet delivered therefrom is represented by K1s. In the similar way, the main droplet of the ink droplets delivered from the printing head 22K2 is represented by K2m and the sub-droplet delivered therefrom is represented by K2s. In this case, a raster L1 is constituted of the main droplets K1m of the ink droplets delivered from a plurality of nozzles on the printing head 22K1. One main droplet K1m forms one pixel and the raster L1 is constituted of a plurality of the main droplets K1m arranged in the lateral direction. Similarly, a raster L2 is constituted of the main droplets K2m. A raster L3 is constituted of the sub-droplets K1s and the sub-droplets K2s. One pixel is formed of one sub-droplet K1s and one sub-droplet K2s (i.e., two sub-droplets K1s and K2s). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ink ejection method and an inkjet image forming apparatus that eject ink from a plurality of nozzles formed on a print head.

  2. Related Art Inkjet image forming apparatuses (inkjet printers) that form images by ejecting ink (ink droplets) from a plurality of nozzles formed on a print head onto a recording medium are widely used. As a technique for ejecting ink droplets from a nozzle, a technique is known in which thermal energy corresponding to a drive pulse is supplied to ink in a nozzle to form bubbles due to film boiling, and ink droplets are ejected from the nozzles by the bubbles. . An image is formed on the recording medium by ejecting a large number of ink droplets corresponding to the image to be formed from the nozzles onto the recording medium.

  In order to improve the image recording speed (image forming speed) among the ink jet printers adopting the above technique, a multi-nozzle head in which a plurality of nozzles including ink discharge ports and ink flow paths are integrated in the recording medium conveyance direction. There is a type (line printer) that has a plurality of line heads arranged orthogonally and discharges ink simultaneously from the ink discharge ports of a plurality of nozzles in accordance with the conveyance of the recording medium.

  In general, an image forming apparatus that forms an image on a recording medium is required to form a high-resolution image with high image quality at high speed. However, by using an inkjet printer such as the above-mentioned line printer, Can be satisfied. Further, the ink jet printer has an advantage that it can perform very stable image recording because the print head (recording head) and the recording medium are not in contact with each other during image formation (image recording). .

  However, in the case of an ink jet printer, since ink that is a fluid is handled, various problems in hydrodynamics occur in the print head. Further, since ink is a liquid, the viscosity changes as needed depending on the environmental temperature, the standing time, and the like. Such a change in viscosity greatly affects image recording. Further, as a characteristic of ink droplets ejected from a nozzle, it is known that one ink droplet ejected from a nozzle is separated into a main droplet and a sub-droplet (satellite). The main droplet is separated from the ink discharge port of the nozzle before the sub droplet, and the sub droplet is separated from the ink discharge port after the main droplet.

  By the way, when comparing an industrial inkjet printer with a consumer printer (household inkjet printer), an industrial inkjet printer has a printing speed (recording medium transport speed) and a print duty (recording medium The number of ink droplets landed per unit area) has increased remarkably. When the conveyance speed of the recording medium is increased (for example, about 150 m / min), the position where the main droplet of one ink droplet ejected from the nozzle lands on the recording medium (landing position) and the sub-droplet lands on the recording medium. The landing position may deviate. As described above, when the landing positions of the main droplet and the sub-droplet constituting one ink droplet are deviated, one ink droplet is landed at different positions, so that the image quality may be deteriorated.

  In order to prevent the deviation of the landing positions of the main droplet and the sub droplet as described above, the interval from the nozzle to the recording medium is shortened (the nozzle and the recording medium are brought close to each other). However, if this interval is too short, the recording medium comes into contact with the nozzles, causing problems in ink ejection. Further, when the image forming speed (printing speed) is further increased, it is not possible to prevent the landing positions of the main droplet and the sub-droplet from being shifted only by shortening the interval from the nozzle to the recording medium.

  In view of the above circumstances, an object of the present invention is to provide an ink ejection method and an ink jet type image forming apparatus that prevent deterioration of image quality due to a shift in landing positions of a main droplet and a sub droplet.

In order to achieve the above object, an ink discharge method of the present invention is an ink discharge method for discharging ink from a plurality of print heads.
(1) Ink is ejected from the plurality of print heads such that sub-drops of the ink droplets ejected from the plurality of print heads form one pixel.

Further, an ink discharge method of the present invention for achieving the above object is an ink discharge method for discharging ink from a plurality of print heads.
(2) The ink is ejected from the plurality of print heads such that the sub-drops of the ink droplets ejected from the plurality of print heads land on one pixel region.

here,
(3) In discharging ink droplets from the plurality of print heads, a negative pressure acting on the ink in any one of the plurality of print heads is changed to a negative pressure acting on the ink in another print head. May be different.

further,
(4) When ejecting ink droplets from the plurality of print heads, the distance from the main droplet to the sub droplet of the ink droplets ejected from any one of the plurality of print heads is ejected from another print head. It may be longer than the distance from the main drop to the sub-drop of the ink drop.

Furthermore,
(5) The distance from the main droplet to the sub droplet of the ink droplets ejected from any one of the plurality of print heads is the distance from the main droplet to the sub droplet of the ink droplets ejected from the other print head When the length is longer, the negative pressure acting on the ink in any one of the print heads may be lower than the negative pressure acting on the ink in the other print head.

Furthermore,
(6) Ink may be ejected from the plurality of print heads toward the recording medium being conveyed.

Furthermore,
(7) The plurality of print heads may be immovable to eject ink.

Furthermore,
(8) Bar codes may be formed by ejecting ink from the plurality of print heads onto a recording medium.

  Here, the pixel means a minimum unit of an image. The pixel area refers to an area corresponding to a pixel before ink is landed. Furthermore, the raster means that pixels are arranged in a line in the horizontal direction (here, in a direction orthogonal to the transport direction).

  According to the present invention, since one pixel is formed by sub-drops of a plurality of ink droplets, the sub-drops are prevented from landing and becoming “dirt” on a portion where there is no print data (a portion where no image is formed). it can. Accordingly, it is possible to prevent the image quality from being deteriorated due to the difference between the landing positions of the main droplet and the sub droplet.

  The present invention has been realized in an inkjet image forming apparatus that forms an image by ejecting ink from a print head onto a recording medium.

    The present invention has been realized in an ink jet printer that forms an image by ejecting ink onto a recording medium from a plurality of nozzles formed in a print head.

  With reference to FIG. 1, an example of a printer employing the ink ejection method of the present invention will be described.

  FIG. 1 is a front view schematically showing an example of a printer that employs the ink ejection method of the present invention.

  The printer 10 is connected to a host PC (personal computer) 12 that sends image information to the printer 10. In the printer 10, four (four) print heads 22K1, 22K2, 22K3, and 22K4 are arranged side by side in the conveyance direction (arrow A direction) of the recording medium (roll paper in this case). Black ink is ejected from each of the four print heads 22K1, 22K2, 22K3, and 22K4. These four print heads 22K1, 22K2, 22K3, and 22K4 are so-called line heads, and extend in a direction orthogonal to the paper surface of FIG. 1 (a direction orthogonal to the arrow A direction). The lengths of these four print heads 22K1, 22K2, 22K3, and 22K4 are slightly longer than the maximum width (the length in the direction perpendicular to the paper surface of FIG. 1) of the recording medium that can be printed by the printer 10. Further, these four print heads 22K1, 22K2, 22K3, and 22K4 are fixed and do not move during image formation (are in a non-moving state).

  A recovery unit 40 is incorporated in the printer 10 so that ink can be stably ejected from the four print heads 22K1, 22K2, 22K3, and 22K4. By this recovery unit 40, the ink discharge state from the print heads 22K1, 22K2, 22K3, and 22K4 is recovered to the initial ink discharge state. The recovery unit 40 includes a capping mechanism 50 that removes ink from the ink discharge port formation surfaces 22K1S, 22K2S, 22K3S, and 22K4S of the four print heads 22K1, 22K2, 22K3, and 22K4 during the recovery operation. The capping mechanism 50 is provided independently for each of the print heads 22K1, 22K2, 22K3, and 22K4. In the example of FIG. 1, six colors (that is, six capping mechanisms 50) are shown. Minute is a preliminary mechanism when the print head is added. The capping mechanism 50 includes a known blade, an ink removing member, a blade holding member, a cap, and the like.

  The roll paper P is supplied from the roll paper supply unit 24 and is conveyed in the direction of arrow A by the conveyance mechanism 26 incorporated in the printer 10. The transport mechanism 26 includes a transport belt 26a for loading and transporting the roll paper P, a transport motor 26b for rotating the transport belt 26a, and a roller 26c for applying tension to the transport belt 26a.

  When an image is formed on the roll paper P, after the recording start position of the roll paper P being conveyed reaches below the black print head 22K1, the print head is based on the recording data (image information). Black ink is selectively ejected from 22K1. Similarly, each color ink is ejected in the order of the print head 22K2, the print head 22K3, and the print head 22K4 to form a color image on the roll paper P. In addition to the components and members described above, the printer 10 supplies ink to the main tank 28K that stores ink supplied to the print heads 22K1, 22K2, 22K3, and 22K4, and to the print heads 22K1, 22K2, 22K3, and 22K4. Various pumps (see FIG. 4 and the like) for carrying out recovery and recovery operations are provided. The above-described main tank 28K, various pumps, and the like constitute the ink supply device of the present invention.

  The electrical system of the printer 10 will be described with reference to FIG.

  FIG. 2 is a block diagram showing an electrical system of the printer of FIG.

  Recording data (image data) and commands transmitted from the host PC 12 are received by the CPU 100 via the interface controller 102. The CPU 100 is an arithmetic processing unit that performs overall control such as reception of recording data of the printer 10, recording operation, handling of the roll paper P, and the like. After analyzing the received command, the CPU 100 rasterizes the image data and barcode data of the components of the recording data by rasterizing them and assigns them to the image memory 106 as data to be drawn by each print head.

  As an operation process before recording, the capping motor 122 and the head up / down motor 118 are driven via the output port 114 and the motor driving unit 116, and the print heads 22K1, 22K2, 22K3, and 22K4 are separated from the capping mechanism 50 for recording. Move to position (image forming position).

  Subsequently, the roll motor (not shown) that feeds the roll paper P through the output port 114, the motor drive unit 116, the transport motor 120 that transports the roll paper P at a low speed, and the like are driven to drive the roll paper P. -The paper P is conveyed to the recording position. The leading end position of the roll paper P is detected by a leading edge detection sensor (not shown) for determining the timing (recording timing) at which ink starts to be discharged onto the roll paper P conveyed at a constant speed. Thereafter, in synchronism with the conveyance of the roll paper P, the CPU 100 sequentially reads the recording data of the corresponding print head from the image memory 106, and the read data (barcode data 125 and image data 126) is used as the print head. The data is transferred to each print head 22K1, 22K2, 22K3, and 22K4 via the control circuit 112.

  The operation of the CPU 100 is executed based on a processing program stored in the program ROM 104. The program ROM 104 stores processing programs and tables corresponding to the control flow. A work RAM 108 is used as a working memory. During the cleaning and recovery operations of the print heads 22K1, 22K2, 22K3, and 22K4, the CPU 100 drives the pump motor 124 via the output port 114 and the motor drive unit 116, and controls ink pressurization and suction.

  With reference to FIG. 3, a conventional ink ejection method that does not use the ink ejection method of the present invention will be described for comparison.

  3A is a schematic diagram illustrating a state in which the recording medium P is conveyed in the conveyance direction (arrow A direction) under the four fixed print heads, and FIG. FIG. 6 is an explanatory diagram in which ink droplets ejected from four print heads and each pixel are associated with each other when printing a barcode.

  In the printer 10 shown in FIG. 1, as shown in FIG. 3A, four print heads 22K1, 22K2, 22K3, and 22K4 are arranged in the transport direction (arrow A direction) of the recording medium P, and each print head 22K1, 22K2, 22K3, and 22K4 extend in a direction perpendicular to the transport direction (a direction perpendicular to the paper surface of FIG. 3 and referred to as a print head longitudinal direction).

  Image data (print data) carrying an image to be printed (formed) on the recording medium P is raster-divided as described above. The raster referred to here is a pixel in which pixels, which are the smallest unit of an image, are arranged in a line in the horizontal direction (here, in a direction perpendicular to the conveyance direction of the recording medium (print head longitudinal direction)). In order to form a plurality of pixels constituting the raster by ejecting ink from one print head, each of the plurality of rasters constituting the image corresponds to one of the print heads 22K1, 22K2, 22K3, and 22K4. It means to divide. In the example described with reference to FIG. 2, raster division is executed by the CPU 100 and assigned to the image memory 106 as data to be drawn by the four print heads 22K1, 22K2, 22K3, and 22K4. Note that when image data is sent from the host PC 12 to the printer 10, image data that is raster-divided by a driver (not shown) or the like of the host PC 12 may be sent to the printer 10.

  A case will be described in which the barcode shown in FIG. 3B (in the region of the chain line) is formed by raster division.

  The barcode in the area of the one-dot chain line is composed of a thick line B1, a slightly thick line B2, and a thin line B3. The thick line B1 is composed of five rasters L1, L2, L3, L4, and L5. The slightly thick line B2 is composed of four rasters L7, L8, L9, and L10. The thin line B3 is composed of three rasters L13, L14, and L15. Between the thick line B1 and the slightly thick line B2, which corresponds to the raster L6, this is a portion where there is no print data. Further, between the slightly thick line B2 and the thin line B3, which corresponds to the two rasters L11 and L12, this is also a portion where there is no print data.

  The raster L1 is formed by ejecting ink from the print head 22K1. That is, when a portion of the recording medium P corresponding to the raster L1 is positioned below the print head 22K1, ink is ejected from all the nozzles of the print head 22K1, thereby forming an image corresponding to the raster L1. The raster L2 is formed by ejecting ink from the print head 22K2. That is, when a portion of the recording medium P corresponding to the raster L2 is positioned below the print head 22K2, ink is ejected from all the nozzles of the print head 22K2, thereby forming an image corresponding to the raster L2. Similarly, the raster L3 is formed by ejecting ink from the print head 22K3, and the raster L4 is formed by ejecting ink from the print head 22K4. The raster L5 is formed again by ejecting ink from the print head 22K1. Further, since the portion corresponding to the raster L6 is a portion having no print data, no ink is ejected from the print head 22K2 to this portion. The raster L7 is formed by ink ejection from the print head 22K3, and the raster L8 is formed by ink ejection from the print head 22K4.

  As described above, the four print heads 22K1 to 22K4 are sequentially associated with the plurality of rasters L1 to L15 to form a bar code including the thick line B1, the slightly thick line B2, and the thin line B3. When a change or the like occurs in the thickness of each line or the width of the portion where there is no print data, the four print heads 22K1 to 22K4 are appropriately controlled to correspond.

  By the way, usually, when the printing speed is increased or the distance from the print head to the recording medium is increased, the distance from the landing position of the main droplet of the ink droplet ejected from the printing head to the landing position of the sub-droplet is Longer (the landing position of the main drop and the landing position of the sub-drop are separated). For this reason, for example, the sub-drops of the ink droplets ejected from the print head 22K1 to form the raster L5 land on the portion corresponding to the raster L6 (the portion without print data), and a plurality of small dots are formed in this portion. (In FIG. 3B, this is indicated by a plurality of small points). Such a plurality of points are also formed in a portion corresponding to the raster L10. In this way, the image quality deteriorates due to the landing of the sub-drop on the portion where there is no print data (the portion where no image is formed). Therefore, as will be described later, in the present invention, an ink ejection method has been found in which a sub-droplet does not land on a portion without print data. An ink supply apparatus capable of realizing such an ink discharge method will be described with reference to FIG.

  FIG. 4 is a schematic diagram illustrating an ink supply device incorporated in the printer 10. FIG. 4 shows an ink supply device for supplying ink to the print head 22K1 and for recovering the print head 22K1, but the ink supply devices having the same configuration are used for the other print heads 22K2, 22K3, and 22K4. Is provided. In FIG. 4, the same components as those shown in FIGS. 1, 2, and 3 are denoted by the same reference numerals.

  The printer 10 (see FIG. 1) incorporates an ink supply device 60 that supplies ink to the print head 22K1. The ink supply device 60 includes an ink tank 70 that can be attached to and detached from the main body of the printer 10 and a sub tank 80 that is disposed in the middle of an ink supply path 62 that connects the ink tank 70 and the print head 22K1. A print head 22 </ b> K <b> 1 is disposed under the sub tank 80.

  The sub tank 80 and the print head 22K1 are connected (connected) by the two ink flow paths 64 and 66. The sub tank 80 is fixed to the main body frame of the printer 10, and a part of the ink flow paths 64 and 66 is made of a flexible tube. For this reason, the print head 22K1 can move. A cleaning pump 68 that operates when the print head 22K1 is cleaned and a standby valve 69 that opens and closes the ink channel 64 at a predetermined timing are attached to the ink channel 64. On the other hand, a pressure valve 67 that opens and closes the ink channel 66 at a predetermined timing is attached to the ink channel 66. A pressure detection sensor 81 for detecting the ink pressure in the ink channel 66 is attached to a portion of the ink channel 66 between the pressurizing valve 67 and the pressure adjustment pump 82 described below.

  Inside the sub tank 80, a pressure adjusting pump 82 for applying an appropriate pressure to the multiple nozzles 22K1n of the print head 22K1 is disposed. The pressure adjusting pump 82 is located slightly above the bottom surface of the sub tank 80 and is separated from the bottom surface by a predetermined distance. The pressure adjusting pump 82 is immersed in the ink stored in the sub tank 80. A drive unit 83 for driving the pressure adjusting pump 82 is disposed above the sub tank 80, and this drive unit is controlled by the CPU 100 (see FIG. 2). Further, an air release valve 84 for making the internal pressure of the sub tank 80 atmospheric pressure is fixed to the upper wall of the sub tank 80. By opening the atmosphere release valve 84, the internal pressure of the sub tank 80 becomes equal to the atmospheric pressure.

  The sub tank 80 is provided with a well-known liquid level detection sensor 86 for detecting the liquid level of the ink (stored ink) stored in the sub tank 80. When the liquid level detection sensor 86 detects that the ink level in the sub tank 80 has become below a certain level, the supply pump 72 starts to operate and ink is sucked from the ink tank 70 and supplied to the sub tank 80. On the other hand, when the liquid level detection sensor 86 detects that the ink level in the sub tank 80 has reached a predetermined upper limit level, the supply pump 72 is stopped and the ink supply is stopped.

  A detection sensor (not shown) that detects the presence or absence of ink in the ink tank 70 is attached to the ink tank 70. An air release valve 74 for bringing the internal pressure of the ink tank 70 to atmospheric pressure is attached to an air flow path connected when the ink tank 70 is attached to the main body of the printer 10.

  A technique for adjusting the pressure in the print head 22K by the pressure adjusting pump 82 will be described with reference to FIGS.

  FIG. 5 is an enlarged view showing the sub tank and the print head in detail. FIG. 6 is a top view showing blades of the pressure adjusting pump. FIG. 7 is a graph showing the relationship between the number of blade rotations shown in FIG. 6 and the pressure acting on the ink in the print head. In these drawings, the same components as those shown in FIG. 4 are denoted by the same reference numerals.

  As the pressurizing valve 67, the standby valve 69, and the air release valve 84, as shown in FIG. 5, an electromagnetic valve that shuts off the ink flow path by a valve sheet 132 integrated with a solenoid plunger 130 is used. Although adopted, these methods are not limited in the present invention, and there is no problem even if other methods are adopted.

  Appropriate negative pressure is applied to the print head 22K1 at the time of recording (necessary to apply pressure to the ink so that an ink meniscus M (see FIG. 9) is formed at the ink discharge port of the print head 22K1 (the outlet of the nozzle 22K1n)). In this case, the pressurizing valve 67 and the air release valve 84 are in an open state, and the standby valve 69 is in a sealed state, and the pressure adjustment pump 82 is driven in this state to move in the direction of arrow C in FIG. The blade 82a of the pressure adjusting pump 82 rotates, and the ink receives a centrifugal force from the center C of the blade 82a along the blade surface 82b (in the direction of the one-dot chain line). Since the peripheral portion of the center C has a relatively negative pressure, a negative pressure can be applied to the print head 22K from the suction port 80a of the sub tank 80 via the ink flow path 66. The suction port 80a is formed in the bottom wall of the sub-tank 80, and the pressure adjusting pump 82 is disposed above the suction port 80a at a predetermined interval (for example, several millimeters). , Controlled by the CPU 100 (see FIG. 2).

  As described above, the pressure adjustment pump 82 sucks the ink in the print head 22K1 from the ink channel 66 by the centrifugal force generated when the blade 82a is rotated in the direction of arrow C by driving the pressure adjustment pump 82. As a result, the ink in the print head 22K1 is negatively pressured (pressure lower than the atmospheric pressure around the outside of the ink discharge port). As a result, an ink meniscus M (see FIG. 9 and the like) is formed at the ink discharge port. When the pressure adjustment pump 82 is driven so that the blade 82a rotates in the direction opposite to the arrow C direction, the ink in the sub tank 82 tends to be pushed out from the suction port 80a. A positive pressure (pressure higher than the atmospheric pressure around the outside of the ink discharge port) is applied to the ink. As a result, ink is discharged from the ink discharge port.

  As shown in FIG. 7, the magnitude of the negative pressure generated by the pressure adjusting pump 82 varies according to the number of rotations when the blade 82a of the pressure adjusting pump 82 rotates in the direction of arrow C in FIG. As the blade 82a rotates faster in the direction of the arrow C (the greater the number of rotations per unit time), the larger negative pressure is generated, so that the ink in the print head 22K1 tends to be sucked into the sub tank 82 with a strong force. Thus, the negative pressure acting on the ink in the print head 22K1 (in the nozzle 22K1n) increases. Conversely, as the blade 82a rotates slower in the direction of the arrow C (the smaller the number of revolutions per unit time), only a smaller negative pressure is generated, so the ink in the print head 22K is sucked into the sub tank 82 with a weak force. As a result, the negative pressure acting on the ink in the print head 22K1 is reduced. That is, the magnitude of the negative pressure applied to the print head 22K1 can be controlled in accordance with the number of rotations of the pressure adjustment pump 82. Therefore, the pressure adjustment pump 82 is driven to print with the ink flow path 66 open. The pressure in the head 22K1 is adjusted.

  By the way, as the pressure adjusting pump 82, it is desirable to use a pump generally classified as a turbo type. Examples of the turbo-type pump include a centrifugal pump type, a mixed flow type, and an axial flow type employed in this embodiment. Since these can generate pressure without blocking (closing) the ink flow path (liquid flow path), the ink can move the pump according to the differential pressure. For example, when ink is ejected from the print head 22K1, the ink is reduced in the print head 22K1, and the pressure between the print head 22K1 and the pressure adjusting pump (centrifugal pump) 82 is reduced. In this case, the ink in the sub tank 80 is supplied to the print head 22K1 via the ink flow path 66. On the other hand, when a piston pump classified as a positive displacement type is used as the pressure adjusting pump 82, the ink flow path 66 is shut off in order to pump ink, so that the ink cannot freely move through the piston pump. In addition, outside air is easily sucked from the ink discharge port of the nozzle 22K1n of the print head 22K1.

  The structure of the nozzle 22K1n will be described with reference to FIG. The nozzles of the other print heads 22K2, 22K3, and 22K4 have the same structure. FIG. 8 is a cross-sectional view showing the nozzle and its periphery. Although one nozzle 22K1n is shown in FIG. 8, a number of nozzles are formed in a line in the print head longitudinal direction in the print head 22K1.

  In the print head 22K1, a large number of nozzles 22K1n for ejecting ink are formed side by side in the direction perpendicular to the paper surface of FIG. The large number of nozzles 22K1n are connected (communicated) with a common liquid chamber 150 in which ink is stored. The common liquid chamber 150 is connected to the sub tank 80 (see FIG. 4), and ink is supplied from the sub tank 80 to the common liquid chamber 150. Accordingly, the magnitude of the negative pressure applied to the print head 22K, that is, the magnitude of the negative pressure acting on the ink in the nozzle 22K1n is changed by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 4 and the like). it can.

  A heating element 152 that foams in the ink in the nozzle 22K1n is disposed in the nozzle 22K1n. By causing the heating element 152 to generate heat, bubbles are generated in the ink in the nozzle 22K1n, and the ink is pushed out and discharged from the outlet (ink discharge port 154) of the nozzle 22K1n. The heating element 152 is formed on the silicon element substrate 156 by a known technique. A silicon top plate 158 and nozzles l160 are formed on the silicon element substrate 156 in order to make ink wettability uniform in the vicinity of the meniscus M, which will be described later. The silicon top plate 158 and the nozzles l160 are formed of the nozzles 22K1n. Facing the inner wall. The silicon top plate 158 and the nozzle l160 are coated with resin. The nozzle l160 is formed on the inner wall near the ink discharge port 154 of the nozzle 22K1n, and narrows the nozzle 22K1n.

  The common liquid chamber 150 described above is also formed in the silicon element substrate 156. Further, a valve 162 that efficiently directs energy in the ink discharge direction (arrow D direction) when foaming by the heating element 152 and a flow path wall 164 that extends vertically from the silicon top plate 158 toward the inside of the nozzle 22K1n are also provided on the silicon element substrate. 156. The nozzle l160 is used to prevent chipping when the silicon top plate 158 is cut when a large number of nozzles 22K1n and the like are manufactured. A sub-heater 166 is formed in a portion of the silicon element substrate 156 facing the common liquid chamber 150, and the sub-heater 166 maintains the temperature of the ink in the print head 22K at a constant level and stabilizes the viscosity of the ink. The purpose is to print within the stable discharge range.

  The heating element 152 is formed by patterning an electric resistance layer and wiring. The heating element 152 generates heat by applying a voltage to the electric resistance layer through this wiring and passing a current. Due to this heat generation, bubbles are generated in the ink around the heating element 152, and the ink is pushed out and discharged from the ink discharge port 154. Further, a Di sensor (not shown) for detecting the temperature of the heat accumulated in the silicon element substrate 156 and the heating element 152 (heat storage temperature) is disposed on the silicon element substrate 156, and this Di sensor detects it. The drive condition of the print head 22K1 is determined according to the detected temperature.

  By changing the rotational speed of the pressure adjusting pump 82 (see FIG. 4 etc.) as described above, the magnitude of the negative pressure applied to the print head 22K1, that is, the magnitude of the negative pressure acting on the ink in the nozzle 22K1n. Can be changed. With reference to FIG. 9, a description will be given of foaming to ink landing when a negative pressure is applied to the nozzle 22K1n only by the water head difference method without performing such negative pressure control.

  FIG. 9A is a schematic diagram showing a standby state before the heating element generates heat, and FIG. 9B is a diagram showing that the heating element generates heat and bubbles are generated in the ink. (C) is a schematic diagram showing a state in which the ink droplets that have been defoamed and ejected from the ink ejection port are separated into main droplets and sub-droplets, and (d) FIG. 9E is a schematic diagram illustrating a state where the ink droplet has landed on the recording medium, and FIG. 9E is a schematic diagram illustrating a state where the sub-droplet has landed on the recording medium after the main droplet. In FIG. 9, the same components as those shown in FIG. 8 are denoted by the same reference numerals.

  In a standby state before the heating element generates heat, a negative pressure corresponding to a water head difference acts on the ink in the nozzle 22K1n. Therefore, as shown in (a), the ink discharge port 154 has a meniscus M. Is formed. In this standby state, print data (image information) is sent from the host PC 12 (see FIG. 1) to the printer 10, and this print data is sent to the print head 22K1 as a heat signal. As a result, the heating element 152 generates heat, and as shown in (b), bubbles B are generated (foamed) in the ink from the interface of the heating element 152, and at the same time, the ink in the vicinity of the ink discharge port 154 is transferred to the ink discharge port 154. Extruded from. When the pressure adjusting pump 82 (see FIG. 4 etc.) is used, the meniscus (M) moves backward by the distance t depending on the rotation conditions.

  Subsequently, the heat generation of the heating element 152 is completed, and the generated bubble B disappears (disappears), and at the same time, as shown in (c), the meniscus M moves backward to a position behind the nozzle 22K1n. On the other hand, the ejected ink droplet Id is separated into a main droplet m and a sub droplet (satellite) s. The satellite s is affected by the backward movement of the meniscus M as shown in (c), and is caused by the vector force in the direction opposite to the ink ejection direction (direction of arrow D) (foam defoaming on the heater and water head difference). Since the vector is combined with the negative pressure), the discharge speed is slower than that of the main droplet m. After the ink droplet Id is ejected from the ink ejection port 154, the position of the retracted meniscus M returns to the standby state as shown in (d) by the capillary force (refill) of the nozzle 22K1n. On the other hand, the main droplet m of the ejected ink droplet Id lands on the recording medium P being conveyed. Since the recording medium P is conveyed (moved) in the direction of arrow A (conveying direction) after the main droplet m has landed on the recording medium P and until the satellite s reaches the recording medium P, (e) As shown in FIG. 3, the sub-drop s lands on the upstream side in the arrow A direction from the position where the main droplet m (absorbed by the recording medium P) of the recording medium P lands. As a result, the main drop m and the sub-drop s cause a deviation L in the landing position. The distance (interval) of the deviation L increases (increases) as the conveyance speed of the recording medium P increases. As described above, when the main droplet m and the sub droplet s of one ink droplet Id land on different positions on the recording medium P, the image quality deteriorates.

  Therefore, in the present invention, attention is paid to the fact that the vector force in the direction opposite to the ink discharge direction (arrow D direction) (the force affecting the sub-droplet discharge speed) can be changed by the pressure adjusting pump 82 or the like. The pressure acting on the meniscus M was changed so that the position (L) where the main droplet m and the sub droplet s of the droplet Id landed on the recording medium P was variable. The landing position is changed by changing the ejection speed of the plurality of sub-drops s arranged in a line in the transport direction, and one pixel is formed by the plurality of sub-drops s. Thereby, the subdrop s is made inconspicuous. In changing the pressure acting on the meniscus M, the magnitude of the negative pressure applied to the print head 22K1, that is, the ink in the nozzle 22K1n is changed by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 4 and the like). The magnitude of the negative pressure acting on the was changed. The position of the meniscus M is changed according to the negative pressure.

  Referring to FIG. 10, an example in which the position of the meniscus M is changed by changing the number of rotations of the pressure adjusting pump 82 (see FIG. 4, etc.) and changing the magnitude of the negative pressure applied to the print head 22 </ b> K <b> 1. explain.

  FIG. 10A is a schematic diagram showing a standby state before the heating element generates heat, and FIG. 10B is a diagram in which the heating element generates heat and bubbles are generated in the ink, and at the same time, the ink is pushed out from the ink discharge port. (C) is a schematic diagram showing a state in which the ink droplets that have been defoamed and ejected from the ink ejection port are separated into main droplets and sub-droplets, and (d) FIG. 9E is a schematic diagram illustrating a state where the ink droplet has landed on the recording medium, and FIG. 9E is a schematic diagram illustrating a state where the sub-droplet has landed on the recording medium after the main droplet. 10, the same components as those shown in FIG. 9 are denoted by the same reference numerals.

  In a standby state before the heating element generates heat, the negative pressure applied to the print head 22K1 by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 4 and the like), that is, the negative pressure acting on the ink in the nozzle 22K1n. And the position of the meniscus M2 is moved backward from the ink ejection port 154 by a distance t2, as shown in FIG. In this standby state, print data (image information) is sent from the host PC 12 (see FIG. 1) to the printer 10, and this print data is sent to the print head 22K1 as a heat signal. As a result, the heating element 152 generates heat, and as shown in (b), bubbles B are generated (foamed) in the ink from the interface of the heating element 152, and at the same time, the ink in the vicinity of the ink discharge port 154 is transferred to the ink discharge port 154. Extruded from.

  Subsequently, the heat generation of the heating element 152 is completed, and the generated bubble B disappears (defoaming), and at the same time, as shown in (c), the meniscus M2 moves backward. On the other hand, the ejected ink droplet Id is separated into a main droplet m and a sub droplet (satellite) s. The velocity of the main droplet m is the same as that of the main droplet m in FIG. 9 because it receives the force in the energy direction during foaming with respect to the ejection direction, while the satellite s is defoamed as shown in (c). Under the influence of the negative pressure acting on the meniscus M2 by the backward movement of the meniscus M2 and the pressure adjusting pump 82 (see FIG. 4 etc.), the vector force in the direction opposite to the ink discharge direction (arrow D direction) is received. The discharge speed is slower than that of the sub-drop s in FIG.

  After the ink droplet Id (the main droplet m and the secondary droplet s) is ejected from the ink ejection port 154, the position of the meniscus M2 that has retracted is in a standby state as shown in FIG. 4D due to the capillary force (refill) of the nozzle 22K1n. Return. On the other hand, the main droplet m of the ejected ink droplet Id lands on the recording medium P being conveyed. After the main droplet m has landed on the recording medium P, the recording medium P is transported (moved) in the direction of arrow A (transport direction) until the sub-drop s reaches the recording medium P. Compared to FIG. 9, the landing distance difference L <b> 2 between the main droplet m and the sub-drop s when landing on the printing surface is widened because the discharge speed of the sub-drop is slow.

  As described above, the negative pressure acting on the ink in the nozzle 22K1n is adjusted by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 4 etc.) to adjust the landing positions of the main droplet m and the sub droplet s. The test print is performed at the time of factory shipment to determine the rotation speed of the pressure adjusting pump 82 and is stored in, for example, the program ROM 104 (see FIG. 2). In this case, the pressure detection sensor 81 indicates the ink pressure (corresponding to the pressure (negative pressure) acting on the ink in the nozzle 22K1n) in the ink flow channel 66 when the main droplet m and the sub droplet s reach the desired landing positions. (See FIG. 4) and the detected value (detected pressure) is stored in the program ROM 104, for example, and the rotational speed of the pressure adjusting pump 82 (see FIG. 4 etc.) is changed so as to be the detected value. Also good. Further, even after the printer 10 (see FIG. 1) is started, test printing may be performed to determine the rotation speed of the pressure adjusting pump 82 and stored in the program ROM 104 (see FIG. 2), for example. . The negative pressure acting on the ink in the nozzle 22K1n is changed by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 4 and the like) based on the conveyance speed of the recording medium P and the temperature of the ink. The position may be adjusted to determine the discharge speed of the sub-drop s.

  With reference to FIGS. 11 and 12, the relationship between the negative pressure acting on the ink in the print head and the ejection speeds of the main droplets and satellites will be described.

  FIG. 11 is a graph showing the relationship between the negative pressure acting on the ink in the print head and the ejection speeds of the main and sub-drops. The vertical axis represents the ejection speed of the main and sub-drops, and the horizontal axis represents Represents the negative pressure acting on the ink in the print head. FIG. 12 is an explanatory diagram schematically showing the discharge state of the main droplet and the sub-droplet.

  When ink droplets are ejected from the nozzle 22K1n, if the ink supply to the nozzle 22K1n is sufficient (in time), the print head 22K1 has a vector force in the direction opposite to the ink ejection direction (arrow D direction). Even when the negative pressure (refers to the negative pressure acting on the ink in the print head, the same applies hereinafter) becomes low (even if it becomes lower than atmospheric pressure), the ejection speed of the main droplet m hardly changes (see FIG. 11 is represented by a curve m in FIG. On the other hand, as the negative pressure of the print head 22K1 decreases, the ejection speed of the sub-drop s decreases (decreases) as shown by the curve s in FIG.

When the velocity of the main droplet discharged from the nozzle 22K1n is Vm, the velocity of the sub-droplet is Vs, the distance h from the print head 22K1 to the recording medium P (printing surface), and the printing speed (conveying speed) A, The distance from the landing position of the droplet m to the landing position of the sub-drop s is expressed by Equation 1.

  According to Equation 1, the lower the negative pressure of the print head, the slower the discharge speed of the sub-drop s according to the result of FIG. 11, so the distance from the landing position of the main drop m to the landing position of the sub-drop is longer. Become. On the contrary, the higher the negative pressure of the print head (closer to the atmospheric pressure), the faster the ejection speed of the sub-drop s, so the distance from the landing position of the main drop m to the landing position of the sub-drop s becomes shorter.

  By controlling the negative pressure acting on the print head 22K1 as described above, the landing position of the sub-drop s is controlled, and sub-drops of ink droplets ejected from a plurality of print heads (for example, the print heads 22K1, 22K2) are used. A method for forming one pixel will be described.

  First, with reference to FIG. 13, the relationship of the distance from the main droplet to the sub droplet with respect to the negative pressure of the print head will be described.

  FIG. 13 is a graph showing the relationship of the distance from the main droplet to the sub-drop with respect to the negative pressure acting on the ink in the print head, the vertical axis represents the distance from the main droplet to the sub-drop, and the horizontal axis represents It represents the pressure acting on the ink in the print head.

  According to the graph of FIG. 13, the distance from the main droplet to the sub droplet increases as the negative pressure of the print head increases. On the contrary, as the negative pressure of the print head decreases, the distance from the main droplet to the sub droplet decreases. For example, when the negative pressure is P1 (the absolute value is larger than P2), the distance from the main drop to the sub-drop is L1, and this distance is long, but when the negative pressure decreases from P1 to P2, The distance decreases from L1 to L2. As a result, if you want to shorten the distance from the main drop to the sub-drop (if you want to make the sub-drop closer to the main drop), press the pressure so that the negative pressure acting on the ink in the print head is lowered (close to atmospheric pressure). The rotational speed of the adjustment pump 82 (see FIG. 5 and the like) is decreased. Conversely, when it is desired to increase the distance from the main drop to the sub-drop (when the sub-drop is further away from the main drop), the negative pressure acting on the ink in the print head is increased (lower than the atmospheric pressure). The rotational speed of the pressure adjusting pump 82 (see FIG. 5) is increased.

  Next, an ink ejection method for forming one pixel with two subdrops will be described with reference to FIG.

  FIG. 14 is a schematic diagram illustrating an example of an ink ejection method in which one pixel is formed by two subdrops.

  Here, in order to give a relatively simple example among the ink discharge methods in which a plurality of sub-drops form one pixel, two sub-droplets discharged from two print heads form one pixel. To do. An example will be described in which raster L1, L2, L3, and L4 data is printed on a recording medium that is transported (moved) in the transport direction (arrow A direction). The raster L1 is composed of a large number of pixels (represented by one circle in FIG. 14) arranged in a direction orthogonal to the direction of the arrow A (the horizontal direction in FIG. 14), and the other raster L2, The same applies to L3 and L4.

  A main droplet of ink droplets ejected from the print head 22K1 (see FIG. 1 and the like) is K1m, and a sub droplet is K1s. Similarly, the main droplet of ink droplets ejected from the print head 22K2 (see FIG. 1 and the like) is K2m, and the sub droplet is K2s. In this case, the raster L1 is composed of main droplets K1m of ink droplets ejected from a plurality of nozzles of the print head 22K1. One main drop K1m forms one pixel, and a plurality of main drops K1m arranged in the horizontal direction constitute a raster L1. Similarly, the raster L2 is constituted by the main droplet K2m. The raster L3 is composed of a sub-drop K1s and a sub-drop K2s. One pixel is composed of one sub-drop K1s and one sub-drop K2s (that is, two sub-drops K1s and K2s). The nozzles of the two print heads 22K1 and 22K2 are arranged in a line in the head longitudinal direction (direction orthogonal to the arrow A direction).

  The landing positions of the sub-drops K1s and K2s constituting the raster L3 are determined by changing the meniscus M (see FIG. 12 and the like) by controlling the negative pressure acting on the ink in the print heads 22K1 and 22K2. As shown in FIG. 13, this control is performed so that the distance between the main droplet K2m and the sub-drop K2s is L2 when the distance between the main droplet K1m and the sub-drop K1s is L1. In this way, the ink ejection is controlled so that the two sub-drops K1s and K2s land on the pixel area Gs3 in the same raster. That is, ink is ejected from the print heads 22K1 and 22K2 so that one pixel is formed by two subdrops K1s and K2s instead of one main droplet. The two sub-drops K1s and K2s are ejected from the nozzles arranged in the arrow A direction among the plurality of nozzles of the two print heads 22K1 and 22K2. The exact landing position is obtained in advance by experiments or the like.

  Here, the landing positions of the two sub-drops K1s and K2s are set in the pixel area Gs3 (within the range) of the same raster, but the landing positions of three or more sub-drops are set to the same raster pixel. Ink ejection from the print head may be controlled so as to be in the region Gs3. After the two sub-drops K1s and K2s have landed on the same raster pixel area Gs3, the sub-droplets K1s and K2s are slightly diffused to form a pixel having the same area as the pixel area Gs3. The main droplet K1m lands on the pixel region Gs1, and after landing, the main droplet K1m is slightly diffused to form a pixel having the same area as the pixel region Gs1. Similarly, the main droplet K2m is landed on the pixel region Gs2, and after landing, the main droplet K2m is slightly diffused to form a pixel having the same area as the pixel region Gs2. A raster L4 indicates a portion where there is no print data.

  With reference to FIG. 15, an example in which barcodes are formed by ejecting ink from four print heads 22K1n, 22K2, 22K3, and 22K4 onto a recording medium will be described.

  FIG. 15A is a schematic diagram showing a state in which the recording medium P is conveyed in the conveyance direction (arrow A direction) under the four fixed print heads, and FIG. FIG. 6 is an explanatory diagram in which ink droplets ejected from four print heads and each pixel are associated with each other when printing a barcode.

  In the printer 10 shown in FIG. 1, as shown in FIG. 15A, four print heads 22K1, 22K2, 22K3, and 22K4 are arranged in the conveyance direction (arrow A direction) of the recording medium P, and each print head 22K1, 22K2, 22K3, and 22K4 extend in a direction perpendicular to the transport direction (a direction perpendicular to the paper surface of FIG. 3 and referred to as a print head longitudinal direction).

  Image data (print data) carrying an image to be printed (formed) on the recording medium P is raster-divided as described above. The raster and raster division are the same as those described with reference to FIG. In the example described with reference to FIG. 2, raster division is executed by the CPU 100 and assigned to the image memory 106 as data to be drawn by the three print heads 22K1, 22K2, and 22K3. As will be described later, the print head 22K4 is handled separately from the print head assigned by raster division. Note that when image data is sent from the host PC 12 to the printer 10, image data that is raster-divided by a driver (not shown) or the like of the host PC 12 may be sent to the printer 10.

  A case will be described in which the barcode shown in FIG. 15B (in the region of the chain line) is formed by raster division and the ink ejection method of the present invention.

  The barcode in the area of the one-dot chain line is composed of a thick line B1, a slightly thick line B2, and a thin line B3. The thick line B1 is composed of five rasters L1, L2, L3, L4, and L5. The slightly thick line B2 is composed of four rasters L7, L8, L9, and L10. The thin line B3 is composed of three rasters L13, L14, and L15. Between the thick line B1 and the slightly thick line B2, which corresponds to the raster L6, this is a portion where there is no print data. Further, between the slightly thick line B2 and the thin line B3, which corresponds to the two rasters L11 and L12, this is also a portion where there is no print data.

  When ink is ejected from the print heads 22K1, 22K2, 22K3, and 22K4, it acts on the ink in the print head 22K4 rather than the pressure (negative pressure) that acts on the ink in the three print heads 22K1, 22K2, and 22K3. Keep the pressure low (the absolute value of negative pressure is large). For example, as shown in FIG. 13, the pressure acting on the ink in the three print heads 22K1, 22K2, and 22K3 is set as P2, and the pressure acting on the ink in the print head 22K4 is set as P1. Such negative pressure control is executed by the pressure adjusting pump 82 (see FIG. 3 and the like) of the sub tank 80 (see FIG. 3 and the like) connected to the print heads 22K1, 22K2, 22K3 and 22K4. With the above settings, all rasters are printed in sequence using the print heads 22K1, 22K2, and 22K3 in principle, and only the necessary rasters are printed at 22K4.

  Specifically, the main droplets of the ink droplets ejected from the print head 22K1 to form the raster L1 are landed on the pixel region Gs1 (see FIG. 14) of the raster L1, and the sub droplets are landed on the pixel region of the raster L2. The pressure adjusting pump 82 is driven to land on Gs2 (see FIG. 14). This main droplet forms a raster L1, and this sub-drop overlaps with the main droplet of the ink droplets ejected from the print head 22K2 to form the raster L2. Since the four print heads 22K1 to 22K4 eject black ink, the deterioration of the image quality due to the sub-drops of the ink droplets ejected from the print head 22K1 to form the raster L1 is prevented. The same applies to the sub droplets of the ink droplets ejected from the print head 22K2 in order to form the raster L2. Next, in order to form the raster L3, the print head 22K3 is to be used from the order, but here, ink is ejected from the print head 22K4. The negative pressure acting on the ink in the print head 22K4 is controlled by the pressure adjustment pump 82 (see FIG. 3 and the like) so that the sub-drops of the ejected ink droplets land on the pixel area of the raster L5. Accordingly, the main droplet of ink ejected from the print head 22K4 to form the raster L3 forms the raster L3, but the sub-droplet lands on the pixel area of the raster L5. The main droplets of the ink droplets ejected from the print head 22K1 to form the raster L4 land on the pixel region of the raster L4, but the sub droplets land on the pixel region of the raster L5. That is, the sub-drop of ink droplets ejected from the print head 22K4 and the sub-drop of ink droplets ejected from the print head 22K1 land on the pixel area of the raster L5, and these two sub-drops overlap. As a result, the raster L5 is formed from two subdrops instead of one main drop. Since the portion corresponding to the raster L6 is a portion having no print data, ink is not ejected from the print head 22K3 for ejecting ink to this portion.

  Similarly to the raster L1, the raster L7 is formed from the main droplets of the ink droplets ejected from the print head 22K1 in the order of the raster division, and the sub droplets land on the pixel area of the raster L8. For the raster L8, the print head 22K2 is conventionally used in the order of raster division, but instead, the print head 22K4 is used similarly to the raster L3. A raster L8 is formed by the main droplets of the ink droplets ejected from the print head 22K4, and the sub-droplets land on the pixel area of the raster L10. The raster L9 is formed from the main droplets of ink droplets ejected from the print head 22K3, and the sub-droplets land on the pixel area of the raster L10. That is, a sub-drop of ink droplets ejected from the print head 22K4 and a sub-drop of ink droplets ejected from the print head 22K3 land on the pixel area of the raster L10, and these two sub-drops overlap. As a result, the raster L10 is formed from two subdroplets instead of one main droplet. Since the portions corresponding to the rasters L11 and L12 are portions without print data, ink is not discharged from the print heads 22K2 and 22K3 for discharging ink to these portions.

  The raster L13 is formed from the main droplets of the ink droplets ejected from the print head 22K4, and the sub droplets land on the pixel area of the raster L15, similarly to the raster L8. The raster L14 is formed from a main droplet of ink droplets ejected from the print head 22K2, and the sub-droplet lands on the pixel area of the raster L15. That is, the sub-drop of ink droplets ejected from the print head 22K4 and the sub-drop of ink droplets ejected from the print head 22K2 land on the pixel area of the raster L15, and these two sub-drops overlap. As a result, the raster L15 is formed from two subdroplets instead of one main droplet.

  As described above, the sub-drops of ink droplets ejected from the print heads 22K1, 22K2, and 22K3 land on the pixel area of the next raster, and the sub-drops of ink droplets ejected from the print head 22K4 are The negative pressure acting on the ink in each of the print heads 22K1 to 22K4 is adjusted by the pressure adjusting pump 82 (see FIG. 3 and the like) so as to land on the pixel area of the second raster, and the two subdrops are removed. One pixel is formed by landing on one pixel region. Thereby, since the sub-droplet does not land on the portion where there is no print data like the raster L6, the image quality can be prevented from deteriorating.

  The procedure for forming the image shown in FIG. 14 by adjusting the pressure inside the print head and ejecting ink will be described with reference to the flowchart of FIG.

  FIG. 16 is a flowchart showing an example of the procedure of the ink ejection method.

  This control is started when the satellite control sequence is activated. First, it is determined whether or not there is print data from the host PC (see FIG. 1) (S1601). If there is print data, refer to the graph of the paper transport speed and the main droplet-satellite distance and head internal pressure shown in FIG. Thus, the pressure in the head for each of the print heads 22K1 to 22K3 and 22K4 is set (S1602), and the rotation speed of the pressure adjusting pump 82 is determined (S1603). The print heads 22K1 to 22K3 are set to the same pressure, and the print head 22K4 is set to have a lower internal pressure (higher negative pressure) than that in order to land at a longer distance by lowering the satellite discharge speed.

  Next, the print heads 22K1 to 22K3 are assigned to each raster (S1604). In the print data, the presence / absence of print data is checked for each raster row (S1605). If the condition is that there is data in L3 and no data in L4 (S1606), the print assigned for each raster in S1604 The heads 22K1 to 22K3 are changed to the printing 22K4 (S1607). That is, in the portion where the condition in S1606 is met, the sub-droplet (satellite) discharged at the same time as the main droplet of the raster L1 needs to land on the raster L3, so the previous raster (L1) is discharged. The print head (22K1 to 22K3) is changed to the print head 22K4. Data of raster L3 is formed by satellites of main droplets forming rasters L1 and L2 (S1608), and printing is started without changing other data (S1609). If there is no print data, the process ends (S1610).

  As described above, the sub-drops of ink droplets ejected from the print heads 22K1, 22K2, and 22K3 land on the pixel area of the next raster, and the sub-drops of ink droplets ejected from the print head 22K4 are The negative pressure acting on the ink in each of the print heads 22K1 to 22K4 is adjusted by the pressure adjusting pump 82 (see FIG. 4 and the like) so as to land on the pixel area of the second raster, and the two subdrops are removed. One pixel is formed by landing on one pixel region. As a result, the sub-droplet does not land on the portion having no print data like the raster L4, so that the image quality can be prevented from deteriorating.

  Other factors that determine the internal pressure of the print head include the type of ink and changes in ink physical properties with respect to environmental changes. Therefore, in the above flow, the landing position of the satellite can be more accurately controlled by detecting the head internal pressure during printing and printing and adjusting the head internal pressure based on the detection result.

  The ink ejection method described above can be applied to a system composed of a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.). You may apply to. In the above ink ejection method, a storage medium storing software program codes for realizing the functions of the above embodiments is supplied to a system or apparatus, and a computer (or CPU or MPU) of the system or apparatus stores the storage medium. Needless to say, this can also be achieved by reading and executing the stored program code. In this case, the program code itself read from the storage medium realizes the function of the above embodiment. Examples of the storage medium for supplying the program code include a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, and a ROM. It is done.

  In addition, the functions of the above-described embodiments are realized by executing the program code read by the computer, and an OS (operating system) operating on the computer based on an instruction of the program code is actually processed. In some cases, the functions of the above-described embodiments are realized by performing part or all of the above. Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. This includes the case where the CPU or the like provided in the board or function expansion unit performs part or all of the actual processing, and the functions of the above embodiments are realized by the processing.

1 is a front view schematically illustrating an example of a printer in which an ink ejection method of the present invention is employed. FIG. 2 is a block diagram illustrating an electrical system of the printer of FIG. 1. (A) is a schematic diagram showing a state in which the recording medium P is conveyed in the conveying direction (arrow A direction) under the four fixed print heads, and (b) is a barcode on the recording medium P. 5 is an explanatory diagram in which ink droplets ejected from four print heads and each pixel are associated with each other when printing. FIG. 2 is a schematic diagram illustrating an ink supply device incorporated in the printer. It is an enlarged view showing a sub tank and a print head in detail. It is a top view which shows the blade | wing of a pressure adjustment pump. It is a graph which shows the relationship between the rotation speed of the blade | wing shown in FIG. 6, and the pressure which acts on the ink in a print head. It is sectional drawing which shows a nozzle and its peripheral part. (A) is a schematic diagram showing a standby state before heating the heating element, and (b) is a state in which the heating element generates heat and bubbles are generated in the ink, and at the same time, ink is pushed out from the ink discharge port. (C) is a schematic diagram showing a state in which the ink droplet discharged from the ink ejection port is separated into a main droplet and a sub-droplet, and (d) is a schematic diagram showing that the main droplet is recorded. It is a schematic diagram which shows the state which landed on the medium, (e) is a schematic diagram which shows the state which the subdroplet landed on the recording medium after the main drop. (A) is a schematic diagram showing a standby state before heating the heating element, and (b) is a state in which the heating element generates heat and bubbles are generated in the ink, and at the same time, ink is pushed out from the ink discharge port. (C) is a schematic diagram showing a state in which the ink droplet discharged from the ink ejection port is separated into a main droplet and a sub-droplet, and (d) is a schematic diagram showing that the main droplet is recorded. It is a schematic diagram which shows the state which landed on the medium, (e) is a schematic diagram which shows the state which the subdroplet landed on the recording medium after the main drop. It is a graph which shows the relationship between the negative pressure which acts on the ink in a print head, and the discharge speed of a main drop and a subdrop, A vertical axis | shaft represents the discharge speed of a main drop and a subdrop, and a horizontal axis is a print head. Represents the negative pressure acting on the ink. It is explanatory drawing which shows typically the discharge state of a main droplet and a subdrop. It is a graph which shows the relationship of the distance from a main drop to a subdrop with respect to the negative pressure which acts on the ink in a print head, a vertical axis | shaft represents the distance from a main drop to a subdrop, and a horizontal axis is a print head. It represents the pressure acting on the ink. FIG. 6 is a schematic diagram illustrating an example of an ink ejection method for forming one pixel with two subdrops. (A) is a schematic diagram showing a state in which the recording medium P is conveyed in the conveying direction (arrow A direction) under the four fixed print heads, and (b) is a barcode on the recording medium P. 5 is an explanatory diagram in which ink droplets ejected from four print heads and each pixel are associated with each other when printing. It is shown with the flowchart which shows an example of the procedure of the ink discharge method of this invention.

Explanation of symbols

10 Printer 22K1, 22K2, 22K3, 22K4 Print head 60 Ink supply device 82 Pressure adjusting pump L1, L2, L3, L4 Raster K1m, K2m Main droplet K1s, K2s Subdrop Gs1, Gs2, Gs3 Pixel area

Claims (9)

In an ink ejection method for ejecting ink from a plurality of print heads,
An ink ejection method, comprising: ejecting ink from the plurality of print heads such that sub-drops of the ink droplets ejected from the plurality of print heads form one pixel. In an ink ejection method for ejecting ink from a plurality of print heads,
An ink ejection method, comprising: ejecting ink from the plurality of print heads so that sub-drops of the ink droplets ejected from the plurality of print heads land on one pixel region. When discharging ink droplets from the plurality of print heads, the negative pressure acting on the ink in one of the plurality of print heads is different from the negative pressure acting on the ink in another print head. The ink discharge method according to claim 1, wherein the ink discharge method is an ink discharge method. In discharging ink droplets from the plurality of print heads, the distance from the main droplet to the sub droplet of the ink droplets discharged from any one of the plurality of print heads is set to the ink discharged from the other print heads. 3. The ink ejection method according to claim 1, wherein the distance is longer than the distance from the main droplet to the sub droplet. The distance from the main drop to the sub-drop of ink droplets ejected from any one of the plurality of print heads is longer than the distance from the main to the sub-drops of ink droplets ejected from the other print heads. 5. The ink ejection method according to claim 4, wherein the negative pressure acting on the ink in any one of the print heads is made lower than the negative pressure acting on the ink in the other print head. . 6. The ink ejection method according to claim 1, wherein ink is ejected from the plurality of print heads toward a recording medium being conveyed. The ink ejection method according to claim 1, wherein the plurality of print heads are in an immovable state to eject ink. The ink discharge method according to claim 1, wherein a bar code is formed by discharging ink from the plurality of print heads onto a recording medium. An ink-jet image forming apparatus, wherein an image is formed by ejecting ink onto a recording medium by the ink ejecting method according to claim 1.

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