JP2009126031A - Ink discharge method for ink jet-system image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ink discharge method capable of preventing image failure due to a change in meniscus position, which is easily caused in a high density printing in which adjacent nozzles discharge ink simultaneously, and to provide an ink jet-system image forming apparatus. <P>SOLUTION: In the ink jet-system image forming apparatus that forms an image by discharging ink onto a recording medium from the ink discharge port of each nozzle in which an ink meniscus is formed, the position of ink meniscus in the nozzle is altered based on image data. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an ink ejection method for ejecting ink droplets from an ink ejection port of an inkjet image forming apparatus onto a recording medium.

  2. Description of the Related Art Inkjet image forming apparatuses (inkjet printers) that form images by ejecting ink (ink droplets) onto a recording medium from ink ejection ports (nozzle exits) of a plurality of nozzles formed on a print head are widely used. . As a technology for ejecting ink droplets from the ink ejection port of the nozzle, a technology for supplying thermal energy corresponding to the drive pulse to the ink in the nozzle to form bubbles due to film boiling, and ejecting ink droplets from the nozzles by these bubbles. Are known. 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 plurality of 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 from ink discharge ports of a plurality of nozzles in accordance with conveyance of a recording medium.

  In general, an image forming apparatus is required to form a high-resolution image with high image quality at a high speed. However, the use of an inkjet printer such as the above-described line printer can satisfy these requirements. it can. In addition, the ink jet printer has an advantage that very stable image recording can be performed because the print head (recording head) and the recording medium are not in contact with each other at the time of image formation (image recording). .

  In the ink jet type image forming apparatus as described above, in order to stabilize the ink ejection operation from the print head, the inside of the print head is maintained at a constant negative pressure (the pressure acting on the ink in the print head is maintained at a constant negative pressure). Pressure). For this reason, in general, a negative pressure generating means is provided in an ink supply system for supplying ink to the print head, and ink to which negative pressure is applied by the negative pressure generating means is supplied to the print head.

  As such negative pressure generating means, a configuration is known in which negative pressure is generated by utilizing the capillary action of a sponge-like ink absorber housed in an ink tank (see, for example, Patent Document 1). As another negative pressure generating means, a configuration including a flexible ink bag and a bow spring is also known (see, for example, Patent Document 2). Furthermore, as another negative pressure generating means, a configuration in which an ink tank is disposed below the print head and a negative pressure is applied to the ink using a water head difference is also known (see, for example, Patent Document 3). .)

  The ink to which a certain negative pressure is applied by the negative pressure generating means as described above is transferred from the ink tank into the print head due to a differential pressure from the negative pressure in the print head that rises as the ink is ejected from the print head. Supplied to be drawn. As a result, the inside of the print head is kept at a constant negative pressure. When the ink in the print head is maintained at a constant negative pressure as described above, an ink meniscus is formed in the nozzle. Since a constant negative pressure is acting on the ink in the print head, the position where the ink meniscus is formed is constant.

Japanese Patent Laid-Open No. 2002-1988 Japanese Patent Application Laid-Open No. 06-198904 JP 2003-11380 A

  By the way, when ink droplets are ejected from the nozzles, a force for ejecting the ink droplets from the ejection port in the direction of the recording medium is applied to the ink by the foaming and defoaming of the ink due to the temperature rise and fall of the heating element in the nozzle. However, at this time, a force due to foaming / defoaming of the ink is also applied in the direction opposite to the ejection port. This force affects the nozzle adjacent to the ejected nozzle, and changes the meniscus position of the ink. In addition, when the ink meniscus position of the nozzle adjacent to the nozzle ejected under this situation comes outside the nozzle opening, the ink meniscus is not formed on the nozzle opening but on the ink ejection port forming surface. To do. When ink is ejected from the nozzle in this state, the force required for ejection is dispersed by the meniscus of the ink that is formed, making it impossible to eject. As a result, an image defect occurs. In addition, this phenomenon is likely to occur in a situation where the printing density at which adjacent nozzles are easy to eject is high.

  In view of the above circumstances, an object of the present invention is to provide an ink ejection method and an inkjet image forming apparatus that prevent image defects due to fluctuations in the meniscus position.

In order to achieve the above object, the ink ejection method of the present invention provides a recording medium in an ink jet type image forming apparatus that forms an image by ejecting ink onto a recording medium from an ink ejection port of a nozzle on which an ink meniscus is formed. In an ink ejection method for ejecting ink droplets from the ink ejection port based on image data carrying information of an image to be formed,
The position of the meniscus of the ink in the nozzle is changed based on the image data to prevent image defects.

  Further, for the purpose of preventing the image defect, when the ink meniscus position in the nozzle is changed, the ink meniscus position may be changed by changing the pressure acting on the ink in the nozzle. Good.

  Furthermore, when an image with a high printing density is formed, the ink meniscus may be further away from the ink discharge port than when an image with a low printing density is formed.

  Further, the drive pulse width may be changed to make the ejection amount constant regardless of the printing density.

  The inkjet image forming apparatus may include a plurality of print heads each having a plurality of nozzles.

  Furthermore, the position of the meniscus of the ink and the drive pulse may be changed for each of the plurality of print heads.

  According to the present invention, since the ink meniscus position can be changed based on the image data, the ink meniscus position can be moved backward to prevent image defects when the image has a high printing density.

  Hereinafter, an example of a printer in which the present invention is employed will be described with reference to FIG.

  FIG. 1 is a front view schematically showing an example of a printer in which the present invention is adopted.

  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 22 </ b> K, 22 </ b> C, 22 </ b> M, and 22 </ b> Y are arranged side by side in the conveyance direction (arrow A direction) of the recording medium (roll paper in this case). The four print heads 22K, 22C, 22M, and 22Y eject black, cyan, magenta, and yellow inks, respectively. These four print heads 22K, 22C, 22M, and 22Y 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 22K, 22C, 22M, and 22Y 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 22K, 22C, 22M, and 22Y are fixed and do not move during image formation (is in a non-moving state). An example of such a printer is a business card printer that produces a large number of business cards at high speed.

  A recovery unit 40 is incorporated in the printer 10 so that ink can be stably ejected from the four print heads 22K, 22C, 22M, and 22Y. By recovering the four print heads 22K, 22C, 22M, and 22Y using the recovery unit 40, the ink discharge states from the four print heads 22K, 22C, 22M, and 22Y are restored to the initial ink discharge state. In the recovery unit 40, during the recovery operation, ink is supplied from the ink discharge port forming surfaces (face surfaces) 22Ks, 22Cs, 22Ms, and 22Ys on which the respective ink discharge ports of the four print heads 22K, 22C, 22M, and 22Y are formed. A capping mechanism 50 is provided for removing water. The capping mechanism 50 is provided independently for each of the print heads 22K, 22C, 22M, and 22Y. In the example of FIG. 1, six colors (that is, six capping mechanisms 50) are shown. The color component is a preliminary mechanism when a print head is added. The capping mechanism 50 includes a known blade 150 (see FIG. 7 and the like), 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 22K, the print head is based on the recording data (image information). Black ink is selectively ejected from 22K. Similarly, each color ink is ejected in the order of the print head 22C, the print head 22M, and the print head 22Y to form a color image on the roll paper P. The printer 10 includes main tanks 28K, 28C, 28M, and 28Y for storing ink supplied to the print heads 22K, 22C, 22M, and 22Y, and print heads 22K, 22C, and 22M. , 22Y are provided with various pumps (see FIG. 3 and the like) for supplying ink and performing a recovery operation.

  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 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 renders the image data of each color component of the recording data by developing a bitmap on the image memory 106. 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 22K, 22C, 22M, and 22Y 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 color from the image memory 106, and controls the print heads 22K, 22C, 22M, and 22Y for the read data. Transfer via the 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 22K, 22C, 22M, and 22Y, 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, the ink supply device incorporated in the printer 10 will be described.

  FIG. 3 is a schematic diagram illustrating an ink supply device incorporated in an inkjet image forming apparatus. Although FIG. 3 shows an ink supply device for supplying ink to the print head 22K and for recovering the print head 22K, the ink supply device having the same configuration is also used for the other print heads 22C, 22M, and 22Y. Is provided. In FIG. 3, the same components as those shown in FIGS. 1 and 2 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 22K. 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, a sub tank 80 disposed in the middle of an ink supply path 62 that connects the ink tank 70 and the print head 22 </ b> K, and the like. A print head 22K is disposed under the sub tank 80.

  The sub tank 80 and the print head 22K are connected (connected) by two ink flow paths 64 and 66. The sub-tank 80 to the pressurizing valve 67 and the sub-tank 80 to the standby valve 69 are fixed to the main body frame of the printer 10, and some of the ink flow paths 64 and 66 are made of a flexible tube. For this reason, the print head 22K can move. A cleaning pump 68 that operates when the print head 22K 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 pressure acting on the ink in the ink flow channel 66 is attached to a portion of the ink flow channel 66 between the pressure valve 67 and the pressure adjustment pump 82 described below. The pressure detected by the pressure detection sensor 81 corresponds to the pressure acting on the ink in the nozzle 22Kn.

  Inside the sub tank 80, a pressure adjusting pump 82 for applying an appropriate pressure to the ink in the multiple nozzles 22Kn of the print head 22K 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. 4 is an enlarged view showing the sub tank and the print head in detail. FIG. 5 is a top view showing blades of the pressure adjusting pump. FIG. 6 is a graph showing the relationship between the number of blade rotations shown in FIG. 5 and the pressure acting on the ink in the print head. In these drawings, the same components as those shown in FIG. 3 are denoted by the same reference numerals.

  As the above-described pressurizing valve 67, standby valve 69, and atmosphere release valve 84, as shown in FIG. 4, 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 22K during recording (pressure is applied to the ink in the nozzle 22Kn so that a meniscus of ink is formed at the ink discharge port of the nozzle 22Kn (exit of the nozzle 22Kn) of the print head 22K). There is a need. In this case, the pressurization valve 67 and the atmosphere release valve 84 are in an open state, and the standby valve 69 is in a sealed state. By driving the pressure adjustment pump 82 in this state, the blade 82a of the pressure adjustment pump 82 rotates in the direction of arrow C in FIG. 5, and the ink receives a centrifugal force from the center C of the blade 82a along the blade surface 82b. . That is, since the central portion of the rotation shaft of the pressure adjusting pump 82 (the peripheral portion of the center C) has a relatively negative pressure, a negative pressure is applied to the print head 22K from the suction port 80a of the sub tank 80 through the ink flow path 66. can do. 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. The rotational speed of the blade 82a is controlled by the CPU 100 (see FIG. 2).

  As described above, the pressure adjusting pump 82 sucks the ink in the print head 22K 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 adjusting pump 82. Attempts to suck into the sub-tank 80 via the port 80a (almost never actually sucked), and as a result, negative pressure is applied to the ink in the print head 22K (pressure lower than the atmospheric pressure around the outside of the ink discharge port). Thus, an ink meniscus 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 (extruded) from the ink discharge port.

  6 is a graph showing the relationship between the rotational speed of the blade shown in FIG. 5 and the pressure acting on the ink in the print head. The rotational speed when the blade 82a of the pressure adjusting pump 82 rotates in the direction of arrow C in FIG. Accordingly, the magnitude of the negative pressure generated by the pressure adjusting pump 82 varies (see C1). As the blade 82a rotates faster in the direction of the arrow C (the greater the number of rotations per unit time), the greater the negative pressure is generated, so that the ink in the print head 22K tends to be sucked into the sub tank 82 by a strong force. Thus, the negative pressure acting on the ink in the print head 22K (in the nozzle 22Kn) increases. Conversely, as the blade 82a rotates slower in the direction of the arrow C (the smaller the number of rotations 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 22K is reduced. That is, the magnitude of the negative pressure applied to the print head 22K 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 22K (pressure acting on the ink in the nozzle 22Kn) 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 in accordance with the differential pressure. For example, when ink is ejected from the print head 22K, the ink is reduced in the print head 22K, and the pressure from the print head 22K to 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 22K via the ink flow channel 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 ports of the nozzles 22Kn of the print head 22K.

  A technique for changing the position of the ink meniscus by controlling the rotation speed of the pressure adjusting pump 82 will be described with reference to FIGS.

  FIG. 7 is a cross-sectional view schematically showing a schematic configuration of the nozzles of the print head. FIG. 8 is a graph showing the relationship between the rotation speed of the pressure adjustment pump and the ink meniscus position. The horizontal axis represents the rotation speed of the pressure adjustment pump, and the vertical axis represents the ink meniscus position. C2 in the figure is a line showing this relationship. The rotation speed of the pressure adjustment pump 82 increases (the rotation speed increases) as it goes to the right of the horizontal axis, and the rotation speed of the pressure adjustment pump 82 decreases (the rotation speed decreases) as it goes to the left. The ink meniscus position is closer to the ink discharge port (nozzle outlet) as it goes up the vertical axis, and the ink meniscus is located deeper in the nozzle as it goes down. When the ink meniscus position is closer to the ink ejection port than the position indicated by L1, the ink meniscus is not formed and ink falls from the ink ejection port (ink drops). The rotation speed of the pressure adjustment pump 82 when the ink meniscus is positioned at L1 is r1.

  On the other hand, when the position of the ink meniscus is located behind the nozzle than the position indicated by L2, the ink meniscus is not formed (the ink meniscus is dropped). The rotational speed of the pressure adjustment pump 82 when the ink meniscus is positioned at L2 is r2. FIG. 9 is a graph showing the relationship between the rotation speed of the pressure adjustment pump and the discharge amount, where the horizontal axis represents the rotation speed of the pressure adjustment pump, and the vertical axis represents the size of the ink droplet. C3 in the figure indicates the relationship. The rotation speed of the pressure adjustment pump 82 increases (the rotation speed increases) as it goes to the right of the horizontal axis, and the rotation speed of the pressure adjustment pump 82 decreases (the rotation speed decreases) as it goes to the left. The discharge amount increases as it goes above the vertical axis, and the discharge amount decreases as it goes down. When the rotation speed of the pressure adjustment pump 82 is r1 (immediately before ink drops), the discharge amount is W1 (maximum), and when the rotation speed of the pressure adjustment pump 82 is r2 (immediately before ink meniscus drops), discharge is performed. The amount is W2 (minimum).

  In the print head 22K (see FIG. 3 and the like), a number of nozzles 22Kn arranged in a direction orthogonal to the recording medium conveyance direction (the direction of arrow A in FIG. 1) are formed. As shown in FIG. 7, an ink meniscus M is formed at the outlet (ink discharge port) 22Kd of the nozzle 22Kn. As shown in FIG. 7, a heating element 22Kh that foams in the ink in the nozzle 22Kn is disposed in the nozzle 22Kn. By causing the heating element 22Kh to generate heat, bubbles are generated in the ink in the nozzle 22Kn, and the ink is pushed out and discharged from the outlet (ink discharge port) 22Kd of the nozzle 22Kn.

  As described above, the magnitude of the negative pressure applied to the print head 22K can be controlled according to the rotational speed of the pressure adjusting pump 82 (see FIG. 3 and the like). As the blade 82a of the pressure adjustment pump 82 rotates faster in the direction of arrow C in FIG. 5 (the more the number of rotations per unit time), the ink in the print head 22K tends to be sucked into the sub tank 82 with a stronger force. Thus, the negative pressure acting on the ink in the print head 22K (in the nozzle 22Kn) increases (becomes lower than the atmospheric pressure). As a result, as shown in FIG. 8, as the number of rotations of the pressure adjusting pump 82 increases, the ink meniscus M is formed deeper in the nozzles 22Kn. On the contrary, as the blade 82a of the pressure adjusting pump 82 rotates slowly in the direction of arrow C in FIG. 5, the ink in the print head 22K is sucked into the sub tank 82 only with a weak force. The negative pressure acting on the ink (within 22Kn) becomes small (close to atmospheric pressure). As a result, as shown in FIG. 8, the ink meniscus M is formed closer to the ink discharge port 22Kd of the nozzle 22Kn as the rotation speed of the pressure adjusting pump 82 is reduced.

  As described above, the position of the ink meniscus M can be changed by changing the pressure acting on the ink in the nozzle 22Kn. A method for preventing the occurrence of image defects by changing the nozzle position in the nozzle 22Kn will be described with reference to FIGS.

  FIG. 10 shows the cause of image failure due to crosstalk. Here, when one of the many nozzles 22Kn in the print head 22K is 22Kn-1, the nozzle adjacent to this nozzle is 22Kn-2. Further, when the discharge port and the heating element of the nozzle 22Kn-1 in the nozzle 22Kn-1 are 22Kd-1 and 22Kh-1, respectively, the discharge port and the heating element of the nozzle 22Kn-2 in the nozzle 22Kn-2. Are also 22Kd-2 and 22Kh-2, respectively. Here, the arrow 24 in the figure indicates the flow of ink. (A) shows the position of the meniscus of the ink before printing, and (b) shows the ink of the nozzles 22Kn-1 and 22Kn-2 when the ink is foamed by heating the heating element 22Kd-2. (C) shows the position of the meniscus when the ink is defoamed when the heating element 22Kd-2 stops generating heat and at the same time when the ink is foamed by causing the heating element 22Kd-1 to generate heat. The ink meniscus positions of 22Kn-1 and nozzle 22Kn-2 are shown. Here, in the state of (b), since the heat generating element 22Kh-2 generates heat and the ink is foamed, the ink is pushed out not only in the ejection port direction but also behind the nozzle. At this time, the ink pushed rearward of the nozzle pushes the ink in the adjacent nozzle Kn22-2 toward the ejection port 22Kd-2. Thus, when the ink meniscus position is in front of the head face surface, the ink forms a liquid film on the head face surface.

  FIG. 11 shows the ink meniscus position of the nozzle 22Kn-1 when the nozzle 22Kn-2 in FIG. 10 ejects. The horizontal axis represents the elapsed time t (μsec) since the heating element 22Kh-2 started to generate heat, and the vertical axis represents the meniscus position of the nozzle 22Kn-1 ink at that time. C5 in the figure is a line indicating this relationship. The ink meniscus position is closer to the ink discharge port (nozzle outlet) as it goes up on the vertical axis, and the ink meniscus is located at the back of the nozzle as it goes down. Then, when the ink meniscus position of the nozzle 22Kn-1 comes forward from the position indicated by L, a liquid film is formed on the head face surface. When the heating element 22Kd-1 is caused to generate heat and ink is discharged, the force to be discharged is dispersed by the liquid film on the head face surface on which the force is formed. For this reason, ink cannot be ejected, resulting in an image defect. This is likely to occur when the adjacent nozzles have a high probability of discharging at short intervals and the print duty is high.

  FIG. 12 shows a state in which the meniscus position is controlled in the configuration of FIG. 10 and the meniscus M is placed behind the nozzle. (A) shows the position of the meniscus of the ink before printing, and (b) shows the meniscus of the ink of the nozzles 22Kn-1 and 22Kn-2 when the ink is foamed by heating the heating element 22Kd-2. (C) shows the position of the nozzle 22Kn-1 when the ink is foamed by causing the heating element 22Kd-1 to generate heat while the heating element 22Kd-2 stops generating heat and the ink is defoamed. The ink meniscus position of the nozzle 22Kn-2 is shown. As in FIG. 10, the heating element 22Kh-2 generates heat, and the ink foams to push the ink of the adjacent nozzle Kn22-1 in the direction of the ejection port Kd22-1 so that the ink meniscus position moves forward. . At this time, in FIG. 10- (c), the meniscus position of the ink protrudes to the front side from the discharge port and forms a liquid film on the head face surface, but in FIG. 12- (c), the ink meniscus position. By retreating the lens, the liquid film is not formed on the face surface and the head face surface, thereby preventing image defects. FIG. 13 shows the ink meniscus position of the nozzle 22Kn-1 when the nozzle 22Kn-2 in FIG. 12 ejects. C4 in the figure is a line indicating this relationship. It can be seen that the meniscus as shown in FIG. 11 protrudes forward from L and no liquid film is formed on the head face surface.

  FIG. 14 shows the relationship between the ink meniscus position and the drive pulse when the ejection amount is constant. The vertical axis indicates the ink meniscus position, and the horizontal axis indicates the pulse width. C6 in the figure indicates the discharge amount with respect to the meniscus position. As shown in FIG. 9, by changing the ink meniscus position, the ink volume (weight) between the ejection port 22Kd of the nozzle 22Kn and the heating element 22Kh changes. For this reason, when the heat quantity of the heating element for foaming the ink is the same, the volume (weight) of the ink to be ejected is different, so the amount of ink that can be ejected changes. Therefore, the discharge amount can be kept constant by changing the amount of heat applied to the heating element.

  With reference to FIG. 15, an example of a procedure for preventing image defects by ink meniscus control will be described. FIG. 15 is a flowchart illustrating an example of a procedure for preventing image defects by ink meniscus control.

  This procedure is executed by the CPU 100 in accordance with a program stored in the program ROM 104 shown in FIG. First, it is determined whether or not print data (image data) is transmitted from the host PC 12 (see FIG. 1) (S1401). When print data is transmitted, the print duty of the print data is confirmed (checked). (S1402). Subsequently, in order to make the position of the ink meniscus M (see FIG. 7) constant, the ink meniscus position-pump rotational speed profile shown in FIG. 8 is referred to (S1403), and the pressure adjusting pump 82 (FIG. 3) is obtained. Etc.) is determined (S1404). Next, it is determined whether or not there is an instruction to change the position of the meniscus (S1405).

  When it is determined that there is this instruction, the meniscus positions of the print heads K1 to K4 are referred to the profile of the combination of the print duty and the meniscus position stored in the program ROM 104 based on the print duty confirmed in S1402. decide. For the print head that is determined to eject with the meniscus position retracted, refer to the meniscus position-pump rotational speed profile shown in FIG. 8 (S1406), and the rotational speed of the pressure adjusting pump 82 (see FIG. 3). Is determined (S1407).

  At this time, if the meniscus position is changed from FIG. 9, the discharge amount changes, so the meniscus position-pulse width profile is referred to (S1408), and the pulse width is determined (S1409). In accordance with the rotational speed determined in this way, the pressure adjustment pump 82 in each sub tank 80 connected to each print head K1 to K4 is driven (S1410), and printing is started (S1411). Subsequently, it is determined whether or not all printing of the print data has been completed (S1412). If it is determined that printing has been completed, the print heads K1 to K4 and the like are shifted to the print standby mode to execute this flow. Exit. If it is determined that printing has not ended, the process returns to S1402. If it is determined in S1405 that there is no instruction to change the meniscus, the process proceeds to S1411 and printing is started by the conventional printing method.

  In the above-described embodiments, the case where the print head is fixed at a fixed position in the printing apparatus and the print head is applied to a so-called line printer type printing apparatus that forms an image by moving the print medium is described as an example. The invention can also be applied to a so-called serial printer type printing apparatus that performs a recording operation while moving the print head in a direction (main scanning direction) perpendicular to the recording medium conveyance direction.

1 is a front view schematically showing an example of a printer in which the present invention is adopted. FIG. 2 is a block diagram illustrating an electrical system of the printer of FIG. 1. 1 is a schematic diagram illustrating an ink supply device incorporated in an inkjet image forming apparatus. FIG. 3 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. 6 is a graph showing the relationship between the number of blade rotations shown in FIG. 5 and the pressure acting on the ink in the print head. It is sectional drawing which shows typically schematic structure of the nozzle of a print head. It is a graph which shows the relationship between the rotation speed of a pressure adjustment pump, and the position of a meniscus. It is a graph which shows the relationship between the rotation speed of a pressure adjustment pump, and the magnitude | size (size) of an ink drop. It is sectional drawing which shows typically the nozzle schematic structure of the print head in FIG. (A) is a state where neither 2Kd-1 nor 2Kd is foamed. (B) shows the flow of ink when 2Kd-2 is foamed. (C) shows the flow of ink when 2Kd-1 is foamed and 2Kd-2 is defoamed. The ink meniscus position of the nozzle 22Kn-1 when the nozzle 22Kn-2 in FIG. 10 ejects is shown. FIG. 11 shows the ink flow when the meniscus position in FIG. 10 is retracted. FIG. The ink meniscus position of the nozzle 22Kn-1 when the nozzle 22Kn-2 in FIG. 12 ejects is shown. The relationship between the meniscus and the drive pulse width when the discharge amount is fixed is shown. FIG. 15 is a flowchart showing an example of a procedure for preventing image defects by ink meniscus control.

Explanation of symbols

10 Printer 22K, 22C, 22M, 22Y, K1, K2, K3, K4 Print head 22Kn Nozzle 22Kd Ink ejection port 50 Capping mechanism M Meniscus

Claims (7)

An ink jet type image forming apparatus for forming an image by ejecting ink onto a recording medium from an ink ejection port of a nozzle on which an ink meniscus is formed, based on image data having image information formed on the recording medium In an ink ejection method for ejecting ink droplets from
An ink ejection method, wherein a position of a meniscus of the ink in the nozzle is changed based on the image data. 2. The ink ejection according to claim 1, wherein when changing the position of the meniscus of the ink in the nozzle, the position of the meniscus of the ink is changed by changing a pressure acting on the ink in the nozzle. Method. 3. The ink ejection method according to claim 1, wherein when an image having a high printing density is formed, an ink meniscus is further away from the ink ejection port than when an image having a low printing density is formed. 4. The ink ejection method according to claim 1, wherein the drive pulse width is changed in order to make the ejection amount constant regardless of the printing density. The inkjet image forming apparatus includes a plurality of print heads each having a plurality of nozzles formed therein.
5. The ink ejection method according to claim 1, wherein the position of the ink meniscus and the driving pulse are changed for each of the plurality of print heads. 6. An image forming method, comprising: forming an image by ejecting ink onto a recording medium by the ink ejecting method according to claim 1. An inkjet image forming apparatus, wherein an image is formed on a recording medium by the image forming method according to claim 6.

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