JP2008068576A - Ink discharging method, and inkjet type image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ink discharging method and an inkjet type image forming apparatus which can prevent the slippage in the hitting positions of main droplets and subsidiary droplets from occurring even when a carrying speed is changed. <P>SOLUTION: The position of a meniscus M is changed in such a manner that even when the carrying speed of a rolled paper P is changed, the hitting positions of the main droplet (m) and the subsidiary droplet (s) of one ink droplet Id may become the same position of the rolled paper P. When the position of the meniscus M is changed, the magnitude of a negative pressure which is imparted to a recording head 22K, i.e., the magnitude of a negative pressure which is applied to the ink in a nozzle 22Kn is changed by changing the rotating number of a pressure adjusting pump 82 (shown in Fig.3). The position of the meniscus M is changed in response to the negative pressure. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an ink ejection method and an inkjet image forming apparatus for ejecting ink (ink droplets) to a recording medium from ink ejection ports of a plurality of nozzles formed on a recording head.

  2. Related Art Inkjet image forming apparatuses (inkjet printers) that form images by discharging ink (ink droplets) from a plurality of nozzles formed on a recording 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. . In this technique, an image is formed on a 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 recording head (recording head) and the recording medium are not in contact with each other during image formation (image recording). .

  On the other hand, since the ink jet printer handles ink that is a fluid, various problems in hydrodynamics occur in the recording 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 120 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 droplets and the sub-drops as described above, the ink discharge port surface on which a plurality of ink discharge ports are formed is inclined to match the landing positions of the main droplets and the sub-drops. Has been proposed. However, in this technique, the inclination angle of the ink discharge port surface is constant and the discharge angle of the main droplet and the sub-droplet is fixed. Therefore, when the conveyance speed of the recording medium is changed, the main droplet and the sub-droplet are changed. The landing positions may not match.

  SUMMARY OF THE INVENTION 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 can prevent the landing position of a main droplet and a secondary droplet from being displaced even if the conveyance speed is changed.

In order to achieve the above object, an ink ejection method of the present invention is an inkjet image forming apparatus that ejects ink onto a recording medium from an ink ejection port of a cylindrical nozzle on which an ink meniscus is formed. In an ink ejection method for ejecting ink from the ink ejection port to a recording medium being conveyed,
(1) It has a planar ink discharge port surface that is inclined so as to face the upstream side in the transport direction in which the recording medium is transported, and has a plurality of ink discharge ports, and with respect to the axial direction in which the nozzles extend. Using a recording head in which the ink discharge port surfaces are not orthogonal,
(2) The ink discharge port is changed by changing the position of the meniscus so that the landing position where the main droplet and the sub droplet constituting one ink droplet discharged from the ink discharge port land on the recording medium is the same position. Ink is discharged from the ink.

In order to achieve the above object, another ink ejection method of the present invention is an ink jet type image in which an image is formed by ejecting ink onto a recording medium from an ink ejection port of a cylindrical nozzle on which an ink meniscus is formed. In an ink discharge method for discharging ink from the ink discharge port to a recording medium being conveyed in a forming apparatus,
(3) Using a recording head having a plurality of ink discharge ports and a planar ink discharge port surface inclined so as to face the upstream side in the transport direction in which the recording medium is transported.
(4) The ink discharge port is changed by changing the position of the meniscus so that the landing position where the main droplet and the sub droplet constituting one ink droplet discharged from the ink discharge port land on the recording medium is the same position. Ink is discharged from the ink.

here,
(5) The position of the meniscus may be changed based on the conveyance speed at which the recording medium is conveyed.

Also,
(6) When the discharge speed of the main droplet is Vm, the discharge speed of the sub-droplet is Vs, the conveyance speed of the recording medium is Vf, and the distance from the ink discharge port to the recording medium is h, The angle δ formed by the main droplet ejection direction and the axial direction is:
(1 / Vs) −1 / (Vm · cos δ) = tan δ / Vf
You may make it satisfy | fill.

Furthermore,
(7) In changing the position of the meniscus, the position of the meniscus may be changed by controlling the pressure acting on the ink in the nozzle.

Furthermore,
(8) When the landing positions are the same, the sub-droplet may be ejected in a direction different from the direction in which the main droplet is ejected.

Furthermore,
(9) The sub-droplet ejection direction may be directed toward the downstream side of the recording medium conveyance direction with respect to the main-droplet ejection direction.

Furthermore,
(10) The ink may be ejected from the ink ejection port by causing the heating element arranged in the nozzle to generate heat and foaming in the ink in the nozzle.

Furthermore,
(11) The pressure acting on the ink in the nozzles may be controlled based on the conveyance speed of the recording medium on which the ink droplets ejected from the ink ejection ports land.

Furthermore,
(12) The nozzle may eject ink in a stationary state.

  According to the present invention, a recording head having a planar ink discharge port surface on which a plurality of ink discharge ports are formed and having an ink discharge port surface not orthogonal to the axial direction of the nozzle is used. Since the position of the meniscus is changed and the ink is ejected from the ink ejection port so that the landing position where the main droplet and the sub-droplet constituting one ink droplet ejected from the ejection port land on the recording medium is the same position, Even if the conveyance speed is changed, it is possible to prevent the landing positions of the main droplet and the sub-droplet from shifting.

  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 on a recording 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) recording heads 22K, 22C, 22M, and 22Y are arranged side by side in the conveyance direction (arrow A direction) of the recording medium (roll paper in this case). The four recording heads 22K, 22C, 22M, and 22Y eject black, cyan, magenta, and yellow inks, respectively. These four recording 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 recording 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 media that can be printed by the printer 10. Further, these four recording heads 22K, 22C, 22M, and 22Y 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 recording heads 22K, 22C, 22M, and 22Y. By the recovery unit 40, the ink ejection state from the recording heads 22K, 22C, 22M, and 22Y is restored to the initial ink ejection state. The recovery unit 40 includes a capping mechanism 50 that removes ink from the ink discharge port formation surfaces 22Ks, 22Cs, 22Ms, and 22Ys of the four recording heads 22K, 22C, 22M, and 22Y during the recovery operation. The capping mechanism 50 is provided independently for each of the recording heads 22K, 22C, 22M, and 22Y. In the example of FIG. 1, six colors (that is, six capping mechanisms 50) are shown. Minute is a preliminary mechanism when a recording 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 transported in the direction of arrow A by the transport 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 forming an image on the roll paper P, after the recording start position of the roll paper P being conveyed reaches below the black recording head 22K, the recording 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 recording head 22C, the recording head 22M, and the recording head 22Y to form a color image on the roll paper P. In this case, ink is ejected from the recording heads 22K, 22C, 22M, and 22Y onto the roll paper P while being conveyed without stopping. The printer 10 includes main tanks 28K, 28C, 28M, and 28Y for storing ink supplied to the recording heads 22K, 22C, 22M, and 22Y, and the recording heads 22K, 22C, and 22M, in addition to the components and members described above. , 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 recording 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 synchronization with the conveyance of the roll paper P, the CPU 100 sequentially reads out the corresponding color recording data from the image memory 106, and controls the recording heads 22K, 22C, 22M, and 22Y to read 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 recording heads 22K, 22C, 22M, and 22Y, the CPU 100 drives the pump motor 124 via the output port 114 and the motor driving 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. FIG. 3 shows an ink supply device for supplying ink to the recording head 22K and for recovering the recording head 22K, but the ink supply devices having the same configuration are used for the other recording 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 recording head 22K. The ink supply device 60 includes a main tank 28K that can be freely attached to and detached from the main body of the printer 10, and a sub tank 27K that is disposed in the middle of an ink supply path 62 that connects the main tank 28K and the recording head 22K. A recording head 22K is disposed under the sub tank 27K.

  The sub tank 27K and the recording head 22K are connected (connected) by two ink flow paths 64 and 66. The sub tank 27K to the pressurizing valve 67 and the sub tank 27K 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 recording head 22K can move. A cleaning pump 68 that operates when the recording head 22K is cleaned and a standby valve 69 that opens and closes the ink flow path 64 at a predetermined timing are attached to the ink flow path 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 27K, a pressure adjusting pump 82 for applying an appropriate pressure to the multiple nozzles 22Kn of the recording head 22K is disposed. The pressure adjusting pump 82 is located slightly above the bottom surface of the sub tank 27K, 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 27K. A drive unit 83 that drives the pressure adjusting pump 82 is disposed above the sub tank 27K, 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 27K atmospheric pressure is fixed to the upper wall of the sub tank 27K. By opening the atmosphere release valve 84, the internal pressure of the sub tank 27K becomes equal to the atmospheric pressure.

  Further, a well-known liquid level detection sensor 86 for detecting the liquid level of ink (stored ink) stored in the sub tank 27K is attached to the sub tank 27K. When the liquid level detection sensor 86 detects that the ink level in the sub tank 27K has become below a certain level, the supply pump 72 starts to operate and ink is sucked from the main tank 28K and supplied to the sub tank 27K. On the other hand, when the liquid level detection sensor 86 detects that the ink level in the sub tank 27K has reached a predetermined upper limit level, the supply pump 72 is stopped and the ink supply is stopped.

  A detection sensor (not shown) for detecting the presence or absence of ink in the main tank 28K is attached to the main tank 28K. In addition, an air release valve 74 for setting the internal pressure of the main tank 28K to atmospheric pressure is attached to an air flow path connected when the main tank 28K is mounted on the main body of the printer 10.

  A technique for adjusting the pressure in the recording 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 recording 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 rotational speed of the blade shown in FIG. 5 and the pressure acting on the ink in the recording 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.

  At the time of recording, it is necessary to apply an appropriate negative pressure to the recording head 22K (applying pressure to the ink so that an ink meniscus is formed at the ink discharge port (nozzle outlet) of the recording head 22K). 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 recording head 22K from the suction port 80a of the sub tank 27K through the ink flow path 66. can do. The suction port 80a is formed in the bottom wall of the sub tank 27K, 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 drives the pressure adjusting pump 82 to rotate the blade 82a in the direction of the arrow C, so that the pressure adjusting pump 82 causes the ink in the recording head 22K to be removed from the ink channel 66 and the suction port. As a result, a negative pressure (pressure lower than the atmospheric pressure around the outside of the ink discharge port) is applied to the ink in the recording head 22K. As a result, 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 from the ink discharge port.

  As shown in FIG. 6, the magnitude of the negative pressure generated by the pressure adjustment pump 82 varies according to the number of rotations when the blade 82a of the pressure adjustment 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 greater the negative pressure is generated, so that the ink in the recording 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 recording 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 recording 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 recording head 22K is reduced. That is, the magnitude of the negative pressure applied to the recording head 22K can be controlled in accordance with the number of rotations of the pressure adjustment pump 82, so that recording is performed with the ink flow channel 66 open by driving the pressure adjustment pump 82. The pressure in the head 22K is adjusted.

  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 in the pump according to the differential pressure. For example, when ink is ejected from the recording head 22K, the ink in the recording head 22K decreases, so the pressure between the recording head 22K and the pressure adjustment pump (centrifugal pump) 82 is reduced. In this case, the ink in the sub tank 27K is supplied to the recording head 22K via the ink channel 66. On the other hand, when a piston pump or the like 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 moves freely through the piston pump. In addition to this, outside air is easily sucked from the ink discharge ports of the nozzles 22Kn of the recording head 22K.

  The structure of the nozzle 22Kn will be described with reference to FIG. The nozzles of the other recording heads 22Y, 22M, and 22C have the same structure. FIG. 7 is a cross-sectional view showing the nozzle and its peripheral part. Although one nozzle 22Kn is shown in FIG. 7, a large number of nozzles are formed in the recording head 22K.

  In the recording head 22K, a large number of cylindrical (generally cylindrical) nozzles 22Kn for ejecting ink are formed side by side in the direction perpendicular to the paper surface of FIG. The large number of nozzles 22Kn are connected (communicated) to a common liquid chamber 150 in which ink is stored. The common liquid chamber 150 is connected to the sub tank 27K (see FIG. 3), and ink is supplied from the sub tank 27K to the common liquid chamber 150. Therefore, by changing the rotation speed of the pressure adjusting pump 82 (see FIG. 3 etc.) in the sub tank 27K, the magnitude of the negative pressure applied to the recording head 22K, that is, the negative pressure acting on the ink in the nozzle 22Kn. You can change the size.

  The nozzle 22Kn is provided with a heating element 152 that foams in the ink in the nozzle 22Kn. As will be described later, bubbles are generated in the ink in the nozzle 22Kn by generating heat from the heating element 152, and the ink is pushed out and discharged from the outlet (ink discharge port 22Km) of the nozzle 22Kn. The heating element 152 is formed on the silicon element substrate 156 by a known technique. A top plate nozzle 158 and a nozzle bank 160 are formed on the silicon element substrate 156 in order to make the ink wettability uniform in the vicinity of the meniscus M, which will be described later. It faces the inner wall of 22Kn. The top plate nozzle 158 and the nozzle bank 160 are covered with resin. The nozzle bank 160 is formed on the inner wall of the nozzle 22Kn in the vicinity of the ink discharge port 22Km, and narrows the nozzle 22Kn.

  The common liquid chamber 150 described above is also formed in the silicon element substrate 156. In addition, the valve seat 161 of the movable valve 162 that efficiently directs energy toward the ink discharge port 22Km when foaming by the heating element 152, and the flow path wall 164 that extends vertically from the top nozzle 158 toward the inside of the nozzle 22Kn are also silicon elements. It is formed on the substrate 156. The nozzle bank 160 is for preventing chipping when the top plate nozzle 158 is cut when a large number of nozzles 22Kn are produced. A sub-heater 166 is formed in a portion of the silicon element substrate 156 that faces the common liquid chamber 150. The sub-heater 166 maintains the temperature of the ink in the recording 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 the electric resistance layer and the 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 (bubbles) are generated in the ink around the heating element 152, and the ink is pushed out and discharged from the ink discharge port 22Km. 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 recording head 22K is determined according to the detected temperature.

  As described above, in one recording head 22K, a plurality of nozzles 22Kn are formed side by side in a direction perpendicular to the paper surface of FIG. 7, and each of the plurality of nozzles 22Kn has a flat ink discharge port surface. (Face surface) 22Ks is formed in a line. The ink discharge port surface 22Ks extends in the direction perpendicular to the paper surface of FIG. Further, the ink discharge port surface 22Ks is formed in a portion of the recording head 22K that faces the roll paper P being conveyed on which the ink discharged from each ink discharge port 22Km is landed. The ink discharge port surface 22Ks is not orthogonal to the axis L1 (axial direction) of the nozzle 22Kn, and is inclined with respect to the roll paper P being conveyed in the arrow A direction. The axis line L1 here refers to a straight line passing through the center of the cross section of the cylindrical nozzle 22Kn, and is an imaginary straight line L1. The ink discharge port surface 22Ks is inclined at an angle θ so as to face the upstream side in the conveyance direction (arrow A direction) of the roll paper P, the position of the meniscus M is adjusted, and the main droplet constituting one ink droplet Even if the transport speed of the roll paper P is changed by controlling the ejection direction of the sub-drops, the landing positions where the main droplets and sub-drops constituting one ink droplet land on the roll paper P can be made the same. . The same applies to the other recording heads 22C, 22M, and 22Y. The angle θ will be described.

  As described above, the ink discharge port surface 22Ks is not orthogonal to the axis L1 of the nozzle 22Kn, and the normal line L2 and the axis L1 of the ink discharge port surface 22Ks form an angle θ. In other words, when the roll paper P being transported in the direction of arrow A is used as a reference, the ink discharge port surface 22Ks is inclined by an angle θ toward the upstream side in the transport direction (arrow A direction) of the roll paper P. Yes. That is, the ink discharge port surface 22Ks has an angle θ with a surface F0 (a surface parallel to the roll paper P being conveyed) orthogonal to the axis L1 toward the upstream side in the conveyance direction of the roll paper P (left side of the paper surface in FIG. 7). It is also a tilted surface. As will be described later, the magnitude of the angle θ is determined in consideration of the relative movement speed of the roll paper P and the recording head 22K.

Here, the discharge speed of the main drop m of one ink drop is Vm (m / sec), the discharge speed of the sub-drop s is Vs (m / sec), and the transport speed of the roll paper P is Vf (m / sec). ), And when the distance from the ink discharge port 22Km to the roll paper P is h (m), when the ink discharge port surface 22Ks coincides with the surface F0, the deviation amount of the landing positions of the main droplet m and the sub droplet s. d is represented by the following equation.
d = {(1 / Vs) − (1 / Vm)} × h × Vf

  When the ink discharge port surface 22Ks coincides with the surface F0, the discharge velocity Vm of the main droplet m, the discharge velocity Vs of the sub-drop s, the distance h from the ink discharge port surface 22Ks to the roll paper P, the transport speed of the roll paper P A shift amount d having a magnitude corresponding to Vf occurs, and the landing positions of the main droplet m and the sub-drop s of one ink droplet cannot be matched.

On the other hand, the ink ejection port surface 22Ks is inclined by an angle θ with respect to the roll paper P being conveyed so as to satisfy the following conditions, thereby matching the landing positions of the main droplet m and the sub droplet s of one ink droplet. Be made.
[(1 / Vs) − {(1 / Vm · cos θ)}] × h × Vf = h · tan θ
Organizing the above formula gives the following formula.
(1 / Vs) − {1 / (Vm · cos θ)} = tan θ / Vf

That is, the discharge speed of the main droplet of one ink drop is Vm (m / sec), the discharge speed of the sub-droplet is Vs (m / sec), the transport speed of the roll paper P is Vf (m / sec), When the distance from the ink discharge port 22Km to the roll paper P is h (m), the angle θ (°) is
(1 / Vs) −1 / (Vm · cos θ) = tan θ / Vf
Meet. When the conveyance speed Vf of the roll paper P is changed, the position of the meniscus M is adjusted by driving the pressure adjustment pump 82 as described later.

  In adjusting (changing) the position of the meniscus M, the rotational speed of the pressure adjusting pump 82 (see FIG. 3 and the like) is changed as described above. As a result, the magnitude of the negative pressure applied to the recording head 22K, that is, the magnitude of the negative pressure acting on the ink in the nozzle 22Kn can be changed (the negative pressure can be controlled), and the position of the meniscus M can be adjusted. .

  Here, from the foaming to the ink landing when the negative pressure is applied to the nozzle 22Kn only by the water head difference method without performing the negative pressure control as described above will be described with reference to FIGS.

  FIG. 8A is a schematic diagram showing a standby state before the heating element generates heat, and FIG. 8B is a diagram showing a state in which the heating element generates heat and bubbles are generated in the ink. It is a schematic diagram which shows the state pushed out, (c) is a schematic diagram which shows the state from which the large amount of ink was extruded from the ink discharge outlet following the state of (b), (d) is a bubble. FIG. 9 is a schematic diagram showing a state immediately before the ink droplet that has become smaller and ejected from the ink ejection port is separated into a main droplet and a sub-droplet, and (e) shows one ink ejected from the ink ejection port while defoaming FIG. 4 is a schematic diagram illustrating a state in which a droplet is separated into a main droplet and a sub-drop, FIG. 5F is a schematic diagram illustrating a state in which the main droplet has landed on a recording medium, and FIG. FIG. 4 is a schematic diagram illustrating a state where a droplet has landed on a recording medium. FIG. 9A is a schematic diagram showing the ejection direction of the main droplet, and FIG. 9B is a schematic diagram showing the contact angle between the ink and the ink ejection port surface.

  As shown in FIG. 8A, in the state before the heating element 152 is not energized and the ink in the nozzle 22Kn is heated, the ink near the ink discharge port 22Km forms a meniscus M. As shown in FIGS. 8B and 8C, when the heating element 152 is energized and the ink is heated by the heat generated by the heating element 152, bubbles (bubbles) B are generated with the film boiling of the ink. Arise. In this case, the propagation direction of the pressure based on the generation of the bubbles B is guided to the ink discharge port 22Km when the movable valve 162 is displaced using the valve seat 161 as a fulcrum. As the meniscus M starts to advance in the ejection direction along with the propagation of the pressure generated by the foaming, the ink near the ink ejection port 22Km is transferred to the nozzle bank on the ink ejection port surface 22Ks as shown in FIG. 9B. The ink is ejected from the ink ejection port 22Km while maintaining the same contact angle α with respect to 160 and the top plate nozzle 158. At this time, the angle formed between the direction A1 in which the ink is pushed out and the axis L1 of the nozzle 22Kn is an inclination angle θ of the ink discharge port surface 22Ks as shown in FIG. That is, the ink in the vicinity of the ink discharge port 22Km is pushed in the direction of the arrow A1 along the normal line L2 orthogonal to the ink discharge port surface 22Ks, as shown in FIGS. 8B, 8C, and 9. It becomes. On the other hand, as shown in FIGS. 8B, 8C, and 8D, the ink positioned near the heating element 152 is pushed out in the direction of the arrow A2 along the direction of the axis L1 of the nozzle 22Kn.

  Thereafter, as shown in FIG. 8 (d), the bubble B enters the contraction process, so that the ink in the vicinity of the ink ejection port 22Km is drawn into the nozzle 22Kn. Therefore, as shown in FIG. 8 (e), the main droplet m And subdrops s are formed. Since the main droplet m and the sub droplet s have different directions pushed out by foaming, as shown in FIG. 8E, the main droplet m is in the direction of the arrow A1 that forms an angle θ with the axis L1 (normal line shown in FIG. 9). L2 direction), and the sub-drop s flies in the direction of arrow A2 (axis L1 direction). This arrow A2 direction is directed to the downstream side in the transport direction (arrow A direction) than the arrow A1 direction.

  After the ink droplet Id (the main droplet m and the secondary droplet s) is ejected from the ink ejection port 22Km, the position of the meniscus M retracted is shown in FIGS. 8F and 8G by the capillary force (refill) of the nozzle 22Kn. Return to the standby state as shown. On the other hand, the main droplet m of the ejected ink droplet Id lands on the roll paper P being conveyed. After the main droplet m has landed on the roll paper P, the roll paper P is conveyed (moved) in the arrow A direction (conveyance direction) until the sub-drop s reaches the roll paper P. However, since the discharge direction of the sub-drop s is the direction of the arrow A2, as shown in FIG. 8G, the main droplet m (absorbed by the roll paper P) on the roll paper P is at the same position as the landing. The secondary droplet s reaches. As described above, since the landing positions of the main droplet m and the sub-drop s are the same, it is possible to prevent deterioration in image quality due to the landing positions of the main droplet m and the sub-drop s of one ink droplet Id being different, Image quality can be improved.

When the conveyance speed of the roll paper P is changed, the landing positions of the main droplet m and the sub droplet s of one ink droplet Id may be different.
As a result, the main droplet m and the sub-drop s are displaced in the landing positions. The distance (interval) of this deviation becomes longer (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, paying attention to the change in the ejection direction of the main droplet m depending on the position of the meniscus M, even if the transport speed of the roll paper P is changed, the main droplet m and the sub droplet s of one ink droplet Id. The position of the meniscus M is changed so that the position where the ink reaches the roll paper P becomes the same position. In changing the position of the meniscus M, the rotation speed of the pressure adjusting pump 82 (see FIG. 3 and the like) is changed to act on the magnitude of the negative pressure applied to the recording head 22K, that is, the ink in the nozzle 22Kn. The magnitude of negative pressure to be changed was changed. The position of the meniscus M is changed according to the negative pressure.

  FIG. 10 is an explanatory diagram when the position of the meniscus M is changed according to the conveyance speed of the roll paper.

  FIG. 10A is a schematic diagram showing a standby state before the heating element generates heat, and FIG. 10B is a diagram showing a state in which the heating element generates heat and bubbles are generated in the ink. It is a schematic diagram which shows the state pushed out, (c) is a schematic diagram which shows the state from which the large amount of ink was extruded from the ink discharge outlet following the state of (b), (d) is a bubble. It is a schematic diagram which shows the state just before the ink droplet discharged from the ink discharge port is separated into the main droplet and the sub-droplet as it becomes small, and (e) is the main droplet when the ink droplet discharged from the ink discharge port disappears (F) is a schematic diagram showing a state where the main droplet has landed on the recording medium, and (g) is a schematic diagram showing that the sub-droplet is delayed from the main droplet. It is a schematic diagram which shows the state which landed on.

  As shown in FIG. 10A, the ink meniscus M in the nozzle 22Kn is located behind the position of the meniscus M shown in FIG. When the meniscus M starts to advance toward the ink discharge port 22Km along with the propagation of the pressure generated by the foaming, the ink is pushed out while having the inclination angle θ of the ink discharge port surface 22Ks in FIG. 8B. In FIG. 10B, since the meniscus M is retracted, the ink moves in the nozzle 22Kn. Subsequently, as shown in FIG. 10C, the meniscus M reaches the ink discharge port 22Km, so that the ink is pushed out while having the inclination angle θ of the ink discharge port surface 22Ks.

  However, since the ink is pushed out along the axis L1 direction (arrow A2 direction) of the nozzle 22Kn as shown in FIG. 10B, the ink will be pushed out while having the inclination angle θ of the ink discharge port surface 22Ks. However, the force in the direction of the axis L1 works, and the ink is pushed out in the direction (arrow A3 direction) in which the forces in the arrow A1 direction (see FIG. 8) and the arrow A2 direction are combined. Here, (the angle formed by the arrow A1 direction and the arrow A2 direction)> (the angle formed by the arrow A2 direction and A3). On the other hand, the ink existing in the vicinity of the heating element 152 is pushed out in the direction of the arrow A2 along the direction of the axis L1 of the nozzle 22Kn as in the case of FIG.

  Thereafter, as shown in FIG. 10 (d), the bubble B enters the contraction process, whereby the ink in the vicinity of the ink discharge port 22Km is drawn into the nozzle 22Kn, and as shown in FIG. A sub-drop s is formed. Since the main droplet m and the sub-drop s have different directions pushed out by foaming, as shown in FIG. 10E, the main droplet m flies in the direction of the arrow A3, which is a combination of the arrow A1 direction and the arrow A2 direction. The subdrop s flies in the direction of arrow A2. Assuming that the angle formed by A3 and A2 is δ, an equation in which the angle θ in the above-described equation is replaced with δ holds for this δ.

  As described above, even if the transport speed of the roll paper P is changed, by changing the position of the meniscus M according to the transport speed, δ changes, and the main droplet m and the sub droplet s have the same landing position. Become position. For this reason, it is possible to prevent the image quality from being lowered due to the difference in landing positions of the main droplet m and the sub-drop s of one ink droplet Id, and the image quality can be improved.

  With reference to FIG. 11, the relationship between the negative pressure of the recording head 22Kn, the position of the meniscus M, and the ejection direction of the main droplet will be described.

  FIG. 11 is a graph showing the relationship between the distance from the ink discharge port surface to the meniscus and the negative pressure in the recording head, and the relationship between the main droplet discharge angle and the negative pressure of the recording head. The negative pressure (head internal pressure) acting on the head (ink in the nozzle) is represented, the right vertical axis represents the main droplet ejection angle, and the left vertical axis represents the distance from the ink ejection port surface to the meniscus (meniscus). Retreat position). On the horizontal axis, the negative pressure in the head increases toward the right (below atmospheric pressure). Note that 1 mmAq = 9.8 pa.

  In the figure, a line segment X represents the relationship between the head internal pressure and the main droplet ejection angle. As represented by the line segment X, at the head internal pressure within the range of (a), the ejection angle of the main droplet is the same and does not change, but the head internal pressure within the range of (b) (within the range of (a)) At an internal pressure lower than the head internal pressure (large negative pressure), the discharge angle δ (see FIG. 8 and the like) of the main droplet m increases as the head internal pressure decreases. That is, as the negative pressure acting on the ink in the nozzle 22Kn becomes larger (lower than the atmospheric pressure), the main droplet m is ejected downstream in the transport direction (the direction of arrow A in FIG. 8). Note that P1 in the figure is the internal pressure of the head when the meniscus M starts to retreat into the nozzle.

  In the figure, a line segment Y represents the relationship between the head internal pressure and the position where the meniscus M is retracted. As represented by the line segment Y, the meniscus M moves backward (moves to the back of the nozzle 22Kn) as the head internal pressure decreases (the negative pressure increases) (the negative pressure acting on the ink in the nozzle 22Kn increases). . However, if the negative pressure (meniscus holding force) at which the meniscus M is formed by the nozzle 22Kn is too large (the negative pressure is too large) (range (c)), the meniscus M moves rearward from the nozzle bank 160. Therefore, the behavior proposed in the present invention cannot be taken.

  The number of rotations of the pressure adjusting pump 82 (see FIG. 3 and the like) is controlled based on the relationship between the nozzle retraction position and the negative pressure of the recording head, and the relationship between the main droplet ejection angle and the negative pressure of the recording head. The negative pressure acting on the ink in the nozzle 22Kn is determined.

  Referring to FIG. 12, an example of the procedure of an ink ejection method for ejecting ink by determining the negative pressure acting on the ink in the nozzle 22Kn by controlling the rotation speed of the pressure adjusting pump 82 (see FIG. 3 etc.). explain.

  FIG. 12 is a flowchart showing an example of the procedure of the ink ejection method.

  In the ink ejection method, the position of the meniscus M (see FIG. 8 and the like) is changed by controlling the rotation speed of the pressure adjusting pump 82 (see FIG. 3 and the like) according to the conveyance speed of the roll paper P, and the main droplet m (see FIG. 8). The ink is ejected by controlling the ejection angle θ (see FIG. 7 etc.). This control is started when the main droplet control sequence is activated. First, it is determined whether or not there is print data from the host PC (see FIG. 1) (S1201). If print data is present, a simulation of the head internal pressure in the print data is calculated by counting the print duty or the like (S1202). . Based on the calculated data, the main droplet ejection angle θ corresponding to the printing speed (recording medium conveyance speed) is calculated with reference to the graph shown in FIG. 11, and the meniscus corresponding to the angle θ is calculated. The reverse position of M is determined (S1203). In order to retract the meniscus M, the rotational speed of the pressure adjusting pump 82 (see FIG. 3 and the like) is determined (S1204), and printing is started (S1205). During printing, the head internal pressure fluctuates due to a change in print duty or the like (S1206). When the head internal pressure increases, referring to the graph shown in FIG. 11 (S1207), the pressure adjustment pump 82 ( The number of revolutions is increased (S1208). On the other hand, when the head internal pressure decreases, the rotational speed of the pressure adjustment pump 82 (see FIG. 3 and the like) is decreased with reference to the graph shown in FIG. 11 (S1209) (S1210).

  The above control is a control for changing the position of the meniscus M in order to obtain the optimum main droplet ejection angle in accordance with the transport speed of the roll paper P, regardless of whether the head internal pressure increases or decreases. If there is print data, the above sequence is continuously executed (S1211).

  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.), or a device composed of a single device (for example, a copier, a facsimile machine, etc.) ). In the above ink ejection method, a storage medium in which a program code of software that realizes the functions of the above-described embodiments is supplied to a system or apparatus, and the 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 program code stored in the. In this case, the program code itself read from the storage medium realizes the function of the above embodiment. As a storage medium for supplying the program code, for example, a floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM, or the like is used. be able to.

  Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on the instruction of the program code is actually used. A case where part or all of the processing is performed and the functions of the above-described embodiments are realized by the processing is also included. 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.

Table 1 shows an example of the head internal pressure, the main droplet discharge angle δ, and the roll paper P conveyance speed.

  As shown in Table 1, the negative pressure in the head is reduced (the meniscus M is brought closer to the ink discharge port surface) as the conveyance speed of the roll paper P is increased, and the main droplet discharge angle θ is increased. On the contrary, the negative pressure in the head is increased (the meniscus M is moved away from the ink discharge port surface) as the transport speed of the roll paper P is decreased, and the main droplet discharge angle θ is decreased. Thus, the image quality can be improved by changing the position of the meniscus M in accordance with the transport speed of the roll paper P so that the landing positions of the main droplet and the sub droplet of one ink droplet coincide. In Table 1, 1 mmAq = 9.8 Pa.

  Here, with reference to FIG. 13, a method for measuring the ejection speed of the main droplet and the sub-droplet will be described.

  FIG. 13 is a schematic diagram showing a method for measuring the ejection speed of the main droplet and the sub-droplet.

  The strobe 200 is fired twice, and one main droplet m is photographed on the same screen by a CCD camera. On this image, since the distance and time of two droplets (one main droplet m before and after ejection) are known, the ejection speed of the main droplet (or sub-droplet) can be calculated. This method is known as an automated test apparatus (tool).

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. 1 is a schematic diagram illustrating an ink supply device incorporated in an inkjet image forming apparatus. It is an enlarged view showing a sub tank and a recording 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 rotational speed of the blade shown in FIG. 5 and the pressure acting on the ink in the recording head. It is sectional drawing which shows a nozzle and its peripheral part. (A) is a schematic diagram showing a standby state before the heating element generates heat, and (b) is a case where a small amount of ink is pushed out from the ink ejection port at the same time as the heating element generates heat and bubbles are generated in the ink. (C) is a schematic diagram showing a state in which a large amount of ink has been pushed out from the ink discharge port following the state of (b), and (d) is a diagram in which bubbles are reduced. FIG. 4B is a schematic diagram showing a state immediately before the ink droplets discharged from the ink discharge port are separated into a main droplet and a sub-droplet, and FIG. FIG. 6 is a schematic diagram illustrating a state where the main droplet has landed on the recording medium, and FIG. 6G is a schematic diagram illustrating a state where the main droplet has landed on the recording medium. FIG. It is a schematic diagram which shows the state which carried out. (A) is a schematic diagram which shows the discharge direction of a main droplet, (b) is a schematic diagram which shows the contact angle of an ink and an ink discharge port surface. (A) is a schematic diagram showing a standby state before the heating element generates heat, and (b) is a case where a small amount of ink is pushed out from the ink ejection port at the same time as the heating element generates heat and bubbles are generated in the ink. (C) is a schematic diagram showing a state in which a large amount of ink has been pushed out from the ink discharge port following the state of (b), and (d) is a diagram in which bubbles are reduced. FIG. 4B is a schematic diagram showing a state immediately before the ink droplets discharged from the ink discharge port are separated into a main droplet and a sub-droplet, and FIG. FIG. 6 is a schematic diagram illustrating a state where the main droplet has landed on the recording medium, and FIG. 6G is a schematic diagram illustrating a state where the main droplet has landed on the recording medium. FIG. It is a schematic diagram which shows the state which carried out. 6 is a graph showing the relationship between the distance from the ink discharge port surface to the meniscus and the negative pressure in the recording head, and the relationship between the main droplet discharge angle and the negative pressure of the recording head. It is a flowchart which shows an example of the procedure of an ink discharge method. It is a schematic diagram which shows the method of measuring the discharge speed of a main drop and a subdrop.

Explanation of symbols

10 Printer 22K, 22C, 22M, 22Y Recording head 22Kn Nozzle 22Km Ink discharge port 22Ks Ink discharge port surface (face surface)
M Meniscus m Main drop s Secondary drop

Claims (11)

In an inkjet image forming apparatus that forms an image by ejecting ink onto a recording medium from an ink ejection port of a cylindrical nozzle on which an ink meniscus is formed, the ink is ejected from the ink ejection port onto a recording medium being conveyed In the ink ejection method,
It has a flat ink discharge port surface that is inclined so as to face the upstream side in the transport direction in which the recording medium is transported, and has a plurality of ink discharge ports formed therein. Use a recording head whose exit surface is not orthogonal,
The position of the meniscus is changed so that the landing position where the main droplet and the sub droplet constituting one ink droplet discharged from the ink discharge port land on the recording medium is the same position, and the ink is discharged from the ink discharge port. An ink discharge method comprising discharging. In an inkjet image forming apparatus that forms an image by ejecting ink onto a recording medium from an ink ejection port of a cylindrical nozzle on which an ink meniscus is formed, the ink is ejected from the ink ejection port onto a recording medium being conveyed In the ink ejection method,
Using a recording head having a planar ink discharge port surface that is inclined so as to face the upstream side in the transport direction in which the plurality of ink discharge ports are formed and the recording medium is transported,
The position of the meniscus is changed so that the landing position where the main droplet and the sub droplet constituting one ink droplet discharged from the ink discharge port land on the recording medium is the same position, and the ink is discharged from the ink discharge port. An ink discharge method comprising discharging. The ink ejection method according to claim 1, wherein the position of the meniscus is changed based on a conveyance speed at which the recording medium is conveyed. When the discharge speed of the main droplet is Vm, the discharge speed of the sub-droplet is Vs, the conveyance speed of the recording medium is Vf, and the distance from the ink discharge port to the recording medium is h, The angle δ formed by the discharge direction and the axial direction is:
(1 / Vs) −1 / (Vm · cos δ) = tan δ / Vf
The ink discharge method according to claim 1, wherein: 5. The position of the meniscus is changed by controlling the pressure acting on the ink in the nozzle when changing the position of the meniscus. 6. Ink ejection method. 6. The sub-droplet is ejected in a direction different from a direction in which the main droplet is ejected when the landing positions are the same. 6. 2. An ink discharge method according to 1. The discharge direction of the sub-droplet is directed to the downstream side in the recording medium conveyance direction with respect to the discharge direction of the main droplet, according to any one of claims 1 to 6. Ink ejection method. The ink is ejected from the ink ejection port by causing the heating element disposed in the nozzle to generate heat and foaming in the ink in the nozzle. The ink discharge method according to item. 9. The pressure applied to the ink in the nozzle is controlled based on the transport speed of the recording medium on which the ink droplet ejected from the ink ejection port is landed. The ink discharge method according to one item. The ink ejection method according to claim 1, wherein the nozzle ejects ink in a non-moving state. 11. 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 any one of claims 1 to 10.

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