JP2015231679A - Ink jet recording device and ink jet recording method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink jet recording device and an ink jet recording method by wich a discharge quantity of an ink can be so set as to be a predetermined quantity according to a recording speed of an image and/or a fineness of an image.SOLUTION: In an ink jet recording device, when an image is recorded on a recording medium in the state such that a discharge quantity of an ink, which is discharged from a nozzle formed on an ink jet head, is so set as to be a first droplet value, a pressure acting on a liquid level of the ink in the nozzle is so adjusted as to be a first water head pressure value, and a drive voltage of a first voltage value is outputted. When an image is recorded on the recording medium in the state such that a discharge quantity of the ink, which is discharged from the nozzle, is so set as to be a second droplet value lower than the first droplet value, a pressure acting on the liquid level of the ink in the nozzle is so adjusted as to be a second water head pressure value lower than the first water head pressure value, and a drive voltage of a second voltage value lower than the first voltage value is outputted.

Description

  The present invention relates to an ink jet recording apparatus and an ink jet recording method.

  Techniques related to ink jet recording have been proposed. For example, Patent Document 1 discloses an ink jet recording apparatus and an adjustment method thereof. In the ink jet recording apparatus, a plurality of head units are provided. The head unit is mounted on the carriage and reciprocated in the width direction of the recording medium. The head unit has a recording head and a sub tank. The recording head ejects ink droplets onto the recording medium. The sub tank receives ink supply from the main tank and supplies ink to the recording head. In Patent Document 1, this head unit is supposed to enable the following. That is, the water head difference between the ink surface in the sub tank and the nozzle formation surface of the recording head can be adjusted. For example, it is possible to make the amount of ink ejected from the nozzles uniform even if fine adjustment for each recording head is not possible when considering common use of the drive waveform of the ink jet recording apparatus. Recording accuracy can be further increased.

  Patent Document 2 discloses an ink jet recording apparatus. An ink jet recording apparatus forms an image by ejecting ink droplets from an ink ejection head toward a recording medium. The ink jet recording apparatus includes pressure changing means. The pressure changing means changes the internal pressure of the ejection head. The internal pressure of the ink ejection head is controlled by a plurality of set values corresponding to a plurality of usage states of the ink ejection head.

  Patent Document 3 discloses an ink jet recording apparatus. The ink jet recording apparatus performs a printing operation by discharging ink from a print head. The ink jet recording apparatus includes an ink supply device, a pressure adjusting pump, a heater, a head recovery unit, and a CPU. The ink supply device supplies ink to the print head. The pressure adjustment pump adjusts the pressure of the ink in the print head. The heater adjusts the temperature of the ink in the print head. The head recovery means executes a recovery operation for discharging ink from the print head. The CPU executes a test print operation. The test print operation is executed after the recovery operation. In the test print operation, a predetermined pattern is printed by changing the ink pressure. The CPU changes the printing operation based on the ink information obtained by the test print.

  The applicant has proposed a technique relating to an ink jet recording method in Patent Document 4.

JP 2003-48315 A JP 2005-231351 A JP 2008-1000048 A JP 2012-87504 A

  The ink jet recording apparatus can record an image at high speed and can record an image with high definition. For example, when an image is recorded at a high speed, the ink may be ejected from nozzles formed on the ink jet head under recording conditions in which the ink ejection amount is increased. On the other hand, when recording an image with high definition, it is preferable to eject ink from nozzles formed on the inkjet head under recording conditions in which the amount of ink discharged is reduced.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an ink jet recording apparatus and an ink jet recording method capable of setting an ink ejection amount to a predetermined amount corresponding to an image recording speed and / or image definition.

  One aspect of the present invention is an ink tank in which a storage chamber for storing ink is formed, a nozzle that discharges the ink supplied from the ink tank, an internal flow that is connected to the nozzle. And an ink jet head including a piezoelectric element that deforms the internal flow path and discharges the ink from the nozzle, and the ink tank and the ink jet head are connected. A connection portion through which the ink supplied to the head flows, a head drive portion that outputs a drive voltage applied to the piezoelectric element, and an adjustment portion that adjusts the pressure acting on the liquid level of the ink in the nozzle. And the adjustment unit includes the ink in the nozzle when the discharge amount of the ink discharged from the nozzle is a first droplet value. When the pressure acting on the liquid surface is adjusted to the first head pressure value and the discharge amount of the ink discharged from the nozzle is set to the second droplet value smaller than the first droplet value, The pressure acting on the liquid level of the ink is adjusted to a second hydraulic head pressure value lower than the first hydraulic head pressure value, and the head driving unit controls the ejection amount of the ink ejected from the nozzles to the first droplet. When the value is a value, a driving voltage having a first voltage value is output, and when the ejection amount of the ink ejected from the nozzle is the second droplet value, a second voltage value lower than the first voltage value is set. An inkjet recording apparatus that outputs a drive voltage.

  According to this, it is possible to manage the ink discharge amount to the first droplet value or the second droplet value based on the pressure acting on the ink surface in the nozzle and the driving voltage. Image recording can be executed in a state where the ink ejection amount is managed. It is also possible to manage the ink discharge amount based only on either the pressure acting on the ink level in the nozzle or the drive voltage. However, in this case, the difference between the first droplet value and the second droplet value is smaller than in the case based on the two conditions described above. Accordingly, the range of the ink discharge amount that can be set by the ink jet recording apparatus is narrowed. In the ink jet recording apparatus that manages the ink discharge amount based on the pressure and drive voltage acting on the ink level in the nozzle, the setting range of the ink discharge amount can be widened. It is possible to flexibly cope with image recording speed and image definition. Even when the ink ejection amount is reduced to the second droplet value, the speed of the ink droplets during ejection can be increased. For this reason, it is possible to suppress the influence of the disturbance acting on the flying ink droplets and to suppress the accuracy of the position where the ink droplets land. Examples of the disturbance that acts on the flying ink droplet include an air flow.

  In this ink jet recording apparatus, the nozzle has a taper shape such that the nozzle diameter on the ejection surface side of the ink jet head facing the recording medium on which an image is recorded is smaller than the nozzle diameter on the side of the tributary section. May be. According to this, the flow path resistance in the nozzle can be suppressed. Therefore, the nozzle length can be set long. By increasing the nozzle length, it is possible to easily form an ink meniscus in the nozzle. For example, as compared with a straight nozzle, the difference between the first droplet value and the second droplet value can be increased, and the setting range of the ink discharge amount can be further widened. The nozzle length is a distance from the boundary position between the branch portion and the nozzle to the ejection surface in the direction in which the ink flows in the nozzle.

  Another aspect of the present invention provides an ink tank in which a storage chamber for storing ink is formed, a nozzle for discharging the ink supplied from the ink tank, and an interior in which the ink flows and is connected to the nozzle. And an ink jet head including a piezoelectric element that deforms the internal flow path and discharges the ink from the nozzle, and connects the ink tank and the ink jet head. A recording step of recording an image on a recording medium by operating an ink jet recording apparatus including a connection portion through which the ink supplied to the head flows and a head driving portion that outputs a driving voltage applied to the piezoelectric element. And the recording step includes performing the recording in a state where the ejection amount of the ink ejected from the nozzle is a first droplet value. When recording an image on a medium, the pressure acting on the liquid level of the ink in the nozzle is adjusted to the first hydraulic head pressure value, and the driving voltage of the first voltage value is output from the head driving unit When the image is recorded on the recording medium in a state where the ejection amount of the ink ejected from the nozzle is set to a second droplet value smaller than the first droplet value, The pressure acting on the ink surface is adjusted to a second hydraulic head pressure value lower than the first hydraulic head pressure value, and a driving voltage having a second voltage value lower than the first voltage value is output from the head driving unit. Ink jet recording method executed in

  According to this, it can be set as the inkjet recording method which has the function mentioned above. In the ink jet recording apparatus in which this ink jet recording method is executed, the nozzles may be tapered as described above.

  According to the present invention, it is possible to obtain an ink jet recording apparatus and an ink jet recording method capable of setting the ink discharge amount to a predetermined amount corresponding to the image recording speed and / or the image definition.

1 is a perspective view illustrating a schematic configuration of an ink jet recording apparatus. It is a fragmentary sectional perspective view which shows schematic structure of an inkjet head, a sub tank, and a pressure reduction part. FIG. 3 is a sectional view taken along the line D-E-F-G shown in FIG. 2. It is an expanded sectional view showing a nozzle. It is a figure which shows an experimental result.

  Embodiments for carrying out the present invention will be described with reference to the drawings. The present invention is not limited to the configurations described below, and various configurations can be employed in the same technical idea. For example, some of the configurations shown below may be omitted or replaced with other configurations. Other configurations may be included.

<Inkjet recording apparatus>
The ink jet recording apparatus 10 will be described with reference to FIGS. 1-3, illustration of the electrical wiring which connects each predetermined part is abbreviate | omitted suitably. In the inkjet recording apparatus 10, the recording medium 14 is conveyed and an image is recorded on the recording surface 15 of the recording medium 14. In the ink jet recording apparatus 10, images are recorded on various recording media 14. Examples of the recording medium 14 include fabrics and building materials. In the embodiment, a case where the recording medium 14 is a long fabric will be described as an example. The ink colors used for image recording are four colors of black, magenta, yellow and cyan. The ink color may be a color other than these. The number of ink colors may be 3 colors or less or 5 colors or more. The halftone dot pattern attached to the portion of the recording medium 14 illustrated on the downstream side in the transport direction in FIG. 1 corresponds to the image recorded on the recording medium 14.

  As shown in FIG. 1, the inkjet recording apparatus 10 includes a transport unit 20, a recording unit 30, a moving unit 40, main tanks 50K, 50M, 50Y, and 50C, and supply channels 51K, 51M, 51Y, and 51C. Sub tanks 52K, 52M, 52Y, 52C and a control device 120 are provided. Furthermore, the ink jet recording apparatus 10 includes a decompression unit 140 (see FIG. 2). In FIG. 1, the decompression unit 140 is not shown. In FIG. 2, hatching indicates a cross-sectioned portion.

  The transport unit 20 transports the recording medium 14 along the longitudinal direction of the recording medium 14. The recording medium 14 transported by the transport unit 20 passes through the recording unit 30. In the embodiment, the direction in which the recording medium 14 is transported by the transport unit 20 is referred to as “transport direction”. The transport direction is orthogonal to the short direction of the recording medium 14. The short side direction of the recording medium 14 is the width direction of the recording medium 14. The short side direction of the recording medium 14 coincides with the main scanning direction. The main scanning direction is a direction in which the recording unit 30 is reciprocated by the moving unit 40 as described later. The conveyance direction and the opposite direction may be referred to as “sub-scanning direction” with respect to the main scanning direction described above. In the embodiment, a direction orthogonal to each of the transport direction and the opposite direction is referred to as a “main scanning direction”. The conveyance direction and the opposite direction are referred to as “sub-scanning direction”.

  The recording unit 30 includes inkjet heads 31K, 31M, 31Y, 31C and a carriage 34. The inkjet head 31K is an inkjet head that ejects black ink and records an image with the black ink. A plurality of nozzles 32 for discharging black ink are formed in the inkjet head 31K. The inkjet head 31M is an inkjet head that discharges magenta ink and records an image with the magenta ink. A plurality of nozzles 32 that discharge magenta ink are formed in the inkjet head 31M. The inkjet head 31Y is an inkjet head that discharges yellow ink and records an image with the yellow ink. A plurality of nozzles 32 for discharging yellow ink are formed on the inkjet head 31Y. The ink jet head 31C is an ink jet head that discharges cyan ink and records an image with the cyan ink. A plurality of nozzles 32 that discharge cyan ink are formed in the inkjet head 31C.

  The inkjet heads 31K, 31M, 31Y, and 31C have the same configuration. In the embodiment, the inkjet heads 31K, 31M, 31Y, and 31C are not distinguished or are collectively referred to as “inkjet head 31”. Inkjet heads 31K, 31M, 31Y, and 31C are mounted on the carriage 34 in a state where they are positioned at predetermined positions. As shown in FIG. 1, the inkjet heads 31K, 31M, 31Y, and 31C are mounted on the carriage 34 in a state of being adjacent to each other in the main scanning direction. The order of arrangement of the inkjet heads 31 for each color may be different from that shown in FIG. These arrangement orders are appropriately determined in consideration of various conditions.

  The moving unit 40 reciprocates the recording unit 30 in the main scanning direction. As shown in FIG. 1, the moving unit 40 includes two pulleys 41 and 42, a timing belt 43, and a motor 44. The two pulleys 41 and 42 are respectively provided on both sides in the main scanning direction with the transport unit 20 as a reference. A motor 44 is connected to the pulley 41. The pulley 42 is rotatably supported. The pulley 41 rotates as the motor 44 rotates. The timing belt 43 is stretched over pulleys 41 and 42 in a state where tension is applied. The recording unit 30 is attached to the timing belt 43 via a support unit 46 fixed to the carriage 34.

  The main tank 50K stores black ink. The supply flow path 51K connects the main tank 50K and the sub tank 52K. A supply valve 53K is provided in the middle of the supply flow path 51K. The supply valve 53K is an electromagnetic valve that opens and closes the supply flow path 51K. When the supply valve 53K is opened, the supply flow path 51K is opened. When the supply valve 53K is closed, the supply channel 51K is in a closed state. The supply valve 53K is opened when black ink is supplied to the sub tank 52K, and is closed when supply is completed. The black ink flows out of the main tank 50K, flows through the supply channel 51K and the supply valve 53K, and is supplied to the sub tank 52K. The black ink supplied to the sub tank 52K flows through the connection portion 54K and is supplied to the inkjet head 31K. That is, the sub tank 52K is an ink tank that stores black ink supplied to the inkjet head 31K as a secondary. The black ink supplied to the sub tank 52K is stored in a storage chamber 55 (see FIGS. 2 and 3) formed inside the sub tank 52K. The connecting portion 54K is a flow path that connects the sub tank 52K and the inkjet head 31K, and flows black ink supplied from the sub tank 52K to the inkjet head 31K.

  The main tank 50M stores magenta ink. The supply flow path 51M connects the main tank 50M and the sub tank 52M. A supply valve 53M is provided in the middle of the supply flow path 51M. The supply valve 53M is an electromagnetic valve that opens and closes the supply flow path 51M. When the supply valve 53M is opened, the supply flow path 51M is opened. When the supply valve 53M is closed, the supply flow path 51M is closed. The supply valve 53M is opened when the magenta ink is supplied to the sub tank 52M, and is closed when the supply is completed. The magenta ink flows out of the main tank 50M, flows through the supply flow path 51M and the supply valve 53M, and is supplied to the sub tank 52M. The magenta ink supplied to the sub tank 52M flows through the connecting portion 54M and is supplied to the inkjet head 31M. In other words, the sub tank 52M is an ink tank that secondarily stores magenta ink supplied to the inkjet head 31M. The magenta ink supplied to the sub tank 52M is stored in a storage chamber 55 (see FIGS. 2 and 3) formed inside the sub tank 52M. The connection portion 54M is a flow path that connects the sub tank 52M and the inkjet head 31M, and flows magenta ink supplied from the sub tank 52M to the inkjet head 31M.

  The main tank 50Y stores yellow ink. The supply flow path 51Y connects the main tank 50Y and the sub tank 52Y. A supply valve 53Y is provided in the middle of the supply channel 51Y. The supply valve 53Y is an electromagnetic valve that opens and closes the supply flow path 51Y. When the supply valve 53Y is opened, the supply flow path 51Y is opened. When the supply valve 53Y is closed, the supply flow path 51Y is in a closed state. The supply valve 53Y is opened when yellow ink is supplied to the sub tank 52Y, and is closed when supply is completed. The yellow ink flows out from the main tank 50Y, flows through the supply channel 51Y and the supply valve 53Y, and is supplied to the sub tank 52Y. The yellow ink supplied to the sub tank 52Y flows through the connecting portion 54Y and is supplied to the inkjet head 31Y. In other words, the sub tank 52Y is an ink tank that stores yellow ink supplied to the inkjet head 31Y as a secondary storage. The yellow ink supplied to the sub tank 52Y is stored in a storage chamber 55 (see FIGS. 2 and 3) formed inside the sub tank 52Y. The connecting portion 54Y is a flow path that connects the sub tank 52Y and the inkjet head 31Y, and flows yellow ink supplied from the sub tank 52Y to the inkjet head 31Y.

  The main tank 50C stores cyan ink. The supply channel 51C connects the main tank 50C and the sub tank 52C. A supply valve 53C is provided in the middle of the supply channel 51C. The supply valve 53C is an electromagnetic valve that opens and closes the supply flow path 51C. When the supply valve 53C is opened, the supply flow path 51C is opened. When the supply valve 53C is closed, the supply channel 51C is in a closed state. The supply valve 53C is opened when cyan ink is supplied to the sub tank 52C, and is closed when the supply is completed. The cyan ink flows out of the main tank 50C, flows through the supply channel 51C and the supply valve 53C, and is supplied to the sub tank 52C. The cyan ink supplied to the sub tank 52C flows through the connection portion 54C and is supplied to the inkjet head 31C. That is, the sub tank 52C is an ink tank that stores the cyan ink supplied to the inkjet head 31C as a secondary storage. The cyan ink supplied to the sub tank 52C is stored in a storage chamber 55 (see FIGS. 2 and 3) formed inside the sub tank 52C. The connecting portion 54C is a flow path that connects the sub tank 52C and the inkjet head 31C, and flows cyan ink supplied from the sub tank 52C to the inkjet head 31C.

  In FIG. 1, the supply flow paths 51K, 51M, 51Y, and 51C are illustrated in a simplified state. The supply flow paths 51K, 51M, 51Y, and 51C are provided in a state that can accommodate the reciprocating movement of the recording unit 30 in the main scanning direction. In the embodiment, the main tanks 50K, 50M, 50Y, and 50C are not distinguished or are collectively referred to as “main tank 50”. The supply channels 51K, 51M, 51Y, 51C are referred to as “supply channels 51” when they are not distinguished or are collectively referred to. The sub tanks 52K, 52M, 52Y, and 52C are referred to as “sub tank 52” when they are not distinguished or are collectively referred to. The supply valves 53K, 53M, 53Y, and 53C are referred to as “supply valves 53” when they are not distinguished or are collectively referred to. The connection parts 54K, 54M, 54Y, 54C are referred to as “connection part 54” when they are not distinguished or are collectively referred to. Switching of the supply valve 53 for each color is performed according to a control signal from the control unit 122 described later. The explanation regarding this point will be described later.

  The ink jet recording apparatus 10 includes a pump (not shown) in addition to the above-described units. Four pumps are provided corresponding to the number of ink colors. Four pumps are provided in the supply flow paths 51K, 51M, 51Y, and 51C, respectively. The ink of each color stored in the main tanks 50K, 50M, 50Y, 50C is supplied to the sub tanks 52K, 52M, 52Y, 52C by the pumps for each color. The supply of ink of each color to the sub tanks 52K, 52M, 52Y, and 52C may be performed by a known liquid feeding unit different from the pump. For example, the main tanks 50K, 50M, 50Y, and 50C may be pressurized by pressing. The liquid feeding unit is also provided in a known ink jet recording apparatus. The ink jet recording apparatus 10 can employ the same liquid feeding unit as that of a known ink jet recording apparatus. Therefore, the description regarding a liquid feeding part is abbreviate | omitted.

  The control device 120 includes a control unit 122, a head drive unit 124, a switch unit 126, a voltage input unit 128, and a pressure input unit 130. In FIG. 1, the control unit 122, the head drive unit 124, the switch unit 126, the voltage input unit 128, and the pressure input unit 130 are illustrated as a group as the control device 120. However, these units included in the control device 120 may not be provided in a single housing such as a control box. For example, the switch unit 126 may be provided at a position close to the inkjet head 31.

  The control unit 122 includes, for example, a CPU, a storage unit, and a RAM. The CPU executes arithmetic processing and controls various types of processing executed by the inkjet recording apparatus 10. The storage unit is configured by, for example, a nonvolatile memory and / or a hard disk. The storage unit stores various processing programs executed by the inkjet recording apparatus 10. For example, a program related to image recording is stored. The RAM serves as a work area when the CPU executes the program stored in the storage unit. The RAM stores predetermined data during the execution of processing.

  The head drive unit 124 is a head drive circuit that is electrically connected to an electrode plate 80 provided in the inkjet head 31 described later. The head drive unit 124 outputs a drive voltage (drive signal) having a predetermined voltage value at a predetermined cycle. The drive voltage output from the head drive unit 124 is applied to the electrode plate 80. The drive voltage (drive signal) is an electric pulse signal having a predetermined voltage value.

  The switch unit 126 is a switch circuit that is electrically connected to each of a plurality of piezoelectric elements 70 included in the inkjet head 31 described later. The switch unit 126 includes a switch corresponding to all or part of the plurality of piezoelectric elements 70. For example, it is assumed that 50 piezoelectric elements 70 are provided in one inkjet head 31. Assume that one switch unit 126 includes 25 switches. In this case, a total of two switch units 126 are provided for one inkjet head 31.

  The voltage input unit 128 receives an input of a voltage setting value for the drive voltage. The operator operates the voltage input unit 128 and inputs a voltage setting value. The control unit 122 acquires the voltage setting value received by the voltage input unit 128. The control unit 122 outputs a voltage value command corresponding to the acquired voltage setting value to the head driving unit 124. The head driving unit 124 outputs a driving voltage having a voltage value according to the voltage value command at a predetermined cycle.

  The pressure input unit 130 receives an input of a pressure set value for the pressure in the storage chamber 55 of the sub tank 52. The operator operates the pressure input unit 130 and inputs a pressure set value. The control unit 122 acquires the pressure set value received by the pressure input unit 130. The control unit 122 controls the decompression unit 140 according to the acquired pressure setting value, and adjusts the pressure in the storage chamber 55 to the pressure setting value or a range corresponding to the pressure setting value. The explanation regarding this point will be described later.

  As shown in FIG. 2, the decompression unit 140 includes an exhaust pipe 141, a vacuum pump 142, a buffer tank 143, a first adjustment valve 144, a second adjustment valve 145, and a pressure sensor 146. The exhaust pipe 141 connects the sub tank 52 for a predetermined color and the buffer tank 143, and connects the buffer tank 143 and the vacuum pump 142. A first regulating valve 144 is provided at a portion of the exhaust pipe 141 that connects the buffer tank 143 and the vacuum pump 142. That is, the storage chamber 55 of the sub tank 52 and the vacuum pump 142 are connected by the exhaust pipe 141 in a state in which the buffer tank 143 and the first adjustment valve 144 are provided on the way.

  The vacuum pump 142 sucks air present in the buffer tank 143 connected to the storage chamber 55 by the exhaust pipe 141. The buffer tank 143 is a buffer tank when the inside of the storage chamber 55 is set to a pressure (negative pressure) lower than the atmospheric pressure (1013 hPa). Air flows between the storage chamber 55 and the buffer tank 143 connected by the exhaust pipe 141. In the ink jet recording apparatus 10, the pressure in the storage chamber 55 is adjusted to a pressure setting value or a predetermined range corresponding to the pressure setting value by adjusting the pressure in the buffer tank 143 as described later. However, the buffer tank 143 is omitted, and the air existing in the storage chamber 55 is directly sucked so that the pressure in the storage chamber 55 is adjusted to a pressure set value or a predetermined range corresponding to the pressure set value. Good.

  The first regulating valve 144 is provided at a position in the middle of the exhaust pipe 141 between the buffer tank 143 and the vacuum pump 142. The first adjustment valve 144 is an electromagnetic valve that opens and closes the exhaust pipe 141. When the first adjustment valve 144 is opened, the exhaust pipe 141 is opened. When the first adjustment valve 144 is closed, the exhaust pipe 141 is closed. The second adjustment valve 145 is provided in the buffer tank 143. The second adjustment valve 145 is an electromagnetic valve that opens and closes the buffer tank 143. The side of the second adjustment valve 145 opposite to the buffer tank 143 leads to the atmosphere. That is, when the second adjustment valve 145 is opened, the buffer tank 143 is opened to the atmosphere. When the second regulating valve 145 is closed, the connection with the atmosphere is closed. The pressure sensor 146 measures the pressure in the buffer tank 143. The measurement value measured by the pressure sensor 146 is input to the control unit 122. The control unit 122 acquires a measurement value measured by the pressure sensor 146. The controller 122 adjusts the pressure in the buffer tank 143 according to the measurement value.

  This adjustment is performed, for example, by setting the pressure in the buffer tank 143 as a pressure set value, or setting the pressure in the buffer tank 143 within a predetermined range based on the pressure set value. In the embodiment, it is assumed that the pressure in the buffer tank 143 is adjusted within a predetermined range based on the pressure set value. The upper limit value of the predetermined range described above is referred to as “upper limit value”, and the lower limit value is referred to as “lower limit value”. The relationship between the pressure set value, the upper limit value, and the lower limit value is “lower limit value <pressure set value <upper limit value”. The upper limit value and the lower limit value are, for example, values within “± α%” with respect to the pressure set value. Examples of “± α%” described above include “± 10%” or “± 5%”. The upper limit value on the plus side of the above-described range is set so as to be a pressure lower than the atmospheric pressure. The pressure in the buffer tank 143 may be adjusted in a state where the pressure set value does not become the center value within a predetermined range. In addition, the pressure in the buffer tank 143 may be adjusted in a range where the pressure set value is the upper limit value and the pressure lower than the pressure set value by a predetermined value is the lower limit value. The pressure in the buffer tank 143 may be adjusted in a range where the pressure set value is a lower limit value and a pressure higher than the pressure set value by a predetermined value is an upper limit value.

<Inkjet head>
The inkjet head 31 will be described with reference to FIGS. In FIG. 3, the illustration of the portion of the exhaust pipe 141 connected to the sub tank 52 is omitted. The inkjet heads 31K, 31M, 31Y, and 31C have the same configuration as described above. Therefore, the inkjet head 31 will be described without distinguishing ink colors. 3 and 4, hatching indicates a cross-sectional portion.

  As shown in FIG. 3, the inkjet head 31 includes a flow path portion 60, a plurality of piezoelectric elements 70, an electrode plate 80, and an insulating sheet 90. The electrode plate 80 and the insulating sheet 90 form the vibration plate 100. The ink jet head 31 is provided below the sub tank 52 in the vertical direction. The inkjet head 31 is assembled in a state where the insulating sheet 90 is sandwiched in the vertical direction between the flow path portion 60 and the electrode plate 80. The plurality of piezoelectric elements 70 are provided in a state of being electrically connected to the electrode plate 80 on the upper surface side of the electrode plate 80. The upper surface of the electrode plate 80 is the surface of the electrode plate 80 that is the upper side in the vertical direction.

  As shown in FIG. 3, a plurality of nozzles 32, a supply port 61, and an internal flow path 63 are formed in the flow path portion 60. A connecting portion 54 is connected to the supply port 61. The ink flowing out from the sub tank 52 and flowing through the connection portion 54 passes through the supply port 61 and is supplied to the inkjet head 31. The plurality of nozzles 32 are formed on the ejection surface 35 in the state shown in FIG. 3, for example. That is, in the example shown in FIG. 3, a plurality of the nozzles 32 are arranged in the sub-scanning direction to form one nozzle row, and the nozzle rows are arranged in a state where two nozzle rows are provided in the main scanning direction. In this case, the two nozzle arrays respectively arranged on the first side and the second side in the main scanning direction are provided adjacent to each other in the main scanning direction while being shifted by a predetermined amount in the sub scanning direction. The ejection surface 35 is a surface on which the nozzles 32 are formed by the inkjet heads 31 for each color (see FIGS. 3 and 4).

  However, the arrangement of the plurality of nozzles 32 as described above is an example. Therefore, the arrangement of the plurality of nozzles 32 may be different from the arrangement described above. In the embodiment, a nozzle row including a plurality of nozzles 32 arranged in the sub-scanning direction on the first side in the main scanning direction is referred to as a “first nozzle row”. A nozzle row composed of a plurality of nozzles 32 arranged in the sub-scanning direction on the second side in the main scanning direction is referred to as a “second nozzle row”.

  The plurality of nozzles 32 all have the same shape. As shown in FIG. 4, the nozzle 32 has a tapered shape in which the nozzle diameter φ1 on the discharge surface 35 side is smaller than the nozzle diameter φ2 on the branch portion 67 side. The taper is represented by “(φ2−φ1) / Q”. The taper of the nozzle 32 is, for example, an inclination of about 1/2 to 3/5. In this case, the taper angle θ is about 14 ° to 17 °. “Q” in the above formula is the nozzle length of the nozzle 32 (see FIG. 4).

  The internal flow path 63 is a flow path for flowing ink flowing from the supply port 61 to each of the plurality of nozzles 32. Therefore, the internal flow path 63 connects the supply port 61 and each of the plurality of nozzles 32. As shown in FIG. 3, the internal flow path 63 includes a main flow portion 65 and a plurality of branch flow portions 67. The main flow part 65 is a flow path continuous from the supply port 61. The main flow portion 65 is formed in a state extending in the sub-scanning direction, like the first nozzle row and the second nozzle row. The ink that has flowed into the main flow portion 65 from the supply port 61 flows through the main flow portion 65 in the sub-scanning direction. The plurality of branch portions 67 are flow paths branched from the main flow portion 65. The plurality of branch portions 67 are respectively connected to the plurality of nozzles 32. The ink that has flowed from the main flow portion 65 into each of the plurality of branch portions 67 flows through each branch portion 67 and reaches each nozzle 32.

  In the flow channel part 60, a part of the internal flow channel 63 is in an open state without its inner wall. The portion of the internal flow path 63 in the open state is a portion of the branch portion 67 corresponding to each piezoelectric element 70 facing each of the plurality of piezoelectric elements 70 in the completed inkjet head 31 shown in FIG. For example, the flow channel unit 60 may be formed by abutting the main body unit and the nozzle plate. A supply port 61 and an internal flow path 63 are formed in the main body. A plurality of nozzles 32 are formed on the nozzle plate. The nozzle plate is formed of a thin flat plate having a thickness of about 20 μm to 1 mm.

  The plurality of piezoelectric elements 70 provided on the first side in the main scanning direction are provided corresponding to the respective branch portions 67 respectively connected to the plurality of nozzles 32 forming the first nozzle row. The plurality of piezoelectric elements 70 provided on the second side in the main scanning direction are provided corresponding to the tributaries 67 respectively connected to the plurality of nozzles 32 forming the second nozzle row. As shown in FIG. 3, the piezoelectric element 70 is in a state of facing the tributary portion 67 connected to each nozzle 32 in the vertical direction with the diaphragm 100 interposed therebetween. Electrical wiring (not shown) is individually made on the upper surface of each piezoelectric element 70. The plurality of piezoelectric elements 70 are electrically connected to the plurality of switches included in the switch unit 126 via the respective electric wirings. The piezoelectric element 70 is a drive element that ejects ink from the nozzle 32 connected to the corresponding branch portion 67. The piezoelectric element 70 may be referred to as a piezo element.

  The electrode plate 80 is a thin flat plate having a predetermined thickness. The plate thickness of the electrode plate 80 is about several tens of μm to several hundreds of μm. The electrode plate 80 is made of a conductive material such as a metal material. On the upper surface of the electrode plate 80, a plurality of piezoelectric elements 70 are electrically connected in a state of being aligned in the sub-scanning direction on each side in the main scanning direction. The electrode plate 80 is electrically connected to the head driving unit 124 as described above. The drive voltage output from the head drive unit 124 is applied to the electrode plate 80 at a predetermined cycle.

  The insulating sheet 90 is a thin flat plate having a predetermined thickness. The plate thickness of the insulating sheet 90 is about several tens μm to several hundreds μm. The insulating sheet 90 is formed of an electrically insulating material such as a resin material. The upper surface of the insulating sheet 90 is in contact with the electrode plate 80. The lower surface of the insulating sheet 90 is in contact with the flow path portion 60. The lower surface of the insulating sheet 90 covers and seals the portion of the branch portion 67 that is open without the inner wall described above. Accordingly, the insulating sheet 90 forms a part of the inner wall of each branch portion 67. As described above, the electrode plate 80 and the insulating sheet 90 form the diaphragm 100 in a state where they overlap each other. As the piezoelectric element 70 is deformed, the portion of the diaphragm 100 that contacts the deforming piezoelectric element 70 is deformed. This deformation is elastic deformation. Therefore, when the piezoelectric element 70 returns to the original state, the deformed portion of the diaphragm 100 also returns. That is, when the piezoelectric element 70 returns to the original state, the deformed electrode plate 80 and the insulating sheet 90 also return to the original state.

<Inkjet recording method>
In the inkjet recording apparatus 10, when an image is recorded on the recording medium 14, an inkjet recording method is executed. The ink jet recording method includes a recording process and a conveying process. The transport process is a process of transporting the recording medium 14 by the transport unit 20. The recording step is a step of recording an image on the recording medium 14 being transported by the transport unit 20. The ink jet recording method is controlled by the control unit 122. The ink jet recording method is started as follows. That is, the inkjet recording method is started at the timing when an image recording start command is input to the control device 120 and the control unit 122 acquires the command. The control unit 122 that has acquired the start command controls the moving unit 40, the recording unit 30, and the transport unit 20. Accordingly, the recording process and the conveyance process are executed in an interlocked state.

  In the recording process, in the moving unit 40, the motor 44 connected to the pulley 41 reciprocally rotates in the clockwise and counterclockwise directions. As the motor 44 rotates, the pulley 41 rotates back and forth on both the left and right sides. An “arc-shaped arrow” shown inside the pulley 41 in FIG. 1 indicates the rotation direction of the pulley 41. When the pulley 44 rotates counterclockwise as the motor 44 rotates in a predetermined direction, the timing belt 43 also rotates counterclockwise. The pulley 42 rotates following the timing belt 43 that rotates counterclockwise. Accordingly, the recording unit 30 moves from the second side in the main scanning direction to the first side. When the recording unit 30 reaches the first moving end in the main scanning direction, the rotation direction of the motor 44 is reversed and the pulley 41 rotates to the right. When the pulley 41 rotates to the right, the timing belt 43 also rotates to the right. The pulley 42 rotates following the timing belt 43 that rotates clockwise. Accordingly, the recording unit 30 moves from the first side in the main scanning direction to the second side. When the recording unit 30 reaches the second moving end in the main scanning direction, the rotation direction of the motor 44 is reversed again, and the pulley 41 rotates counterclockwise again.

  In the recording process, in the recording unit 30, ink of each color is ejected toward the recording medium 14 from the nozzles 32 formed in the inkjet heads 31 </ b> K, 31 </ b> M, 31 </ b> Y, and 31 </ b> C. Each color ink is ejected at a predetermined timing when the recording unit 30 moves in the main scanning direction in accordance with the operation of the moving unit 40 described above. Ink droplets of each color ink ejected from the nozzles 32 of the ink jet heads 31K, 31M, 31Y, and 31C land on the recording surface 15 of the recording medium 14. An image is recorded on the recording surface 15 of the recording medium 14 by the landed ink droplets of each color. The recording surface 15 of the recording medium 14 is a surface facing the ejection surface 35 of the inkjet head 31 for each color when the recording medium 14 passes through the recording unit 30. A description of ink ejection from the nozzles 32 will be described later.

  If the image recording is continued, in the sub tank 52, the amount of ink in the storage chamber 55 becomes smaller than a predetermined amount. The amount of ink of each color stored in the sub tank 52 for each color is managed by, for example, a liquid level sensor (not shown). For example, two liquid level sensors are provided for one sub tank 52. The first liquid level sensor detects the lower limit value of the ink amount. The second liquid level sensor detects the upper limit value of the ink amount.

  The first liquid level sensor provided in the sub tank 52 for a predetermined color outputs a lower limit signal at a timing when the amount of ink stored in the sub tank 52 falls below the lower limit value. The lower limit value signal is input to the control unit 122, and the control unit 122 acquires the lower limit value signal. The control unit 122 that has acquired the lower limit signal outputs an open signal to the supply valve 53. The supply valve 53 that is the output destination of the release signal is a supply valve provided in the supply flow path 51 connected to the sub tank 52 provided with the first liquid level sensor that outputs the lower limit signal. The opening signal output to the supply valve 53 is a control signal for switching the supply valve 53 to an open state. The output supply valve 53 switches to an open state in response to the open signal. Accordingly, the supply channel 51 described above is in an open state. The ink flows from the main tank 50 side through the supply valve 53 to the sub tank 52 side and is supplied to the sub tank 52.

  When ink is supplied to the sub tank 52, the amount of ink stored in the sub tank 52 increases and reaches the upper limit value. The second liquid level sensor provided in the sub tank 52 outputs an upper limit signal at the timing when the amount of ink reaches the upper limit. The upper limit signal is input to the control unit 122, and the control unit 122 acquires the upper limit value signal. The control unit 122 that has acquired the upper limit signal outputs a closing signal to the supply valve 53. The supply valve 53 serving as the output destination of the closing signal is a supply valve provided in the supply flow path 51 connected to the sub tank 52 provided with the second liquid level sensor that outputs the upper limit value signal. The closing signal output to the supply valve 53 is a control signal for switching the supply valve 53 to a closed state. The output supply valve 53 is switched to a closed state in response to the closing signal. Accordingly, the supply channel 51 described above is in a closed state. The ink flow is stopped, and the supply of ink to the sub-tank 52 for a predetermined color is completed. Thus, the recording step includes a step of supplying ink to the sub tank 52. As will be described later, the recording process is performed in a state where the pressure in the storage chamber 55 is reduced by the pressure reducing unit 140. Therefore, the step of supplying ink to the sub tank 52 is also performed in a state where the pressure in the storage chamber 55 is reduced.

  In the transport process, the transport unit 20 transports the recording medium 14 in the transport direction in response to the reciprocal movement of the recording unit 30 in the main scanning direction. That is, the conveyance of the recording medium 14 by the conveyance unit 20 is intermittently performed according to the reciprocation of the recording unit 30 in the main scanning direction.

  The moving unit 40 operates until image recording is completed. Therefore, the reciprocating movement of the recording unit 30 in the main scanning direction is repeated until the image recording is completed. For example, the conveyance unit 20 stops at a predetermined timing after the image recording is completed. As the transport unit 20 stops, transport of the recording medium 14 in the transport direction is also terminated. The ink jet recording method ends when the transport unit 20 stops.

<Ink ejection>
Ink ejection from the nozzles 32 executed in the recording process will be described. In the recording process, the control unit 122 controls the output of the driving voltage from the head driving unit 124. That is, the control unit 122 outputs a voltage value command corresponding to the acquired voltage setting value to the head driving unit 124. In the head drive unit 124, output of a drive voltage having a voltage value according to the voltage value command is started. The drive voltage is repeatedly output at a predetermined cycle. The output drive voltage is applied to the electrode plate 80 at a predetermined cycle as described above. The control unit 122 controls the switch unit 126 according to the image data stored in the storage unit included in the control unit 122. The image data is data corresponding to the image recorded on the recording medium 14. The image data is generated using a predetermined application program. The image data may be input from an external device connected to the control device 120. In the switch unit 126, a predetermined switch among the built-in switches is closed according to the control by the control unit 122 according to the image data.

  When a driving voltage is applied to the electrode plate 80 at a timing when a predetermined switch built in the switch unit 126 is closed, a plurality of piezoelectric elements 70 electrically connected to the electrode plate 80 on the upper surface of the electrode plate 80. Among them, a driving voltage is applied to the piezoelectric element 70 connected to the closed switch. As the piezoelectric element 70 to which the drive voltage is applied is deformed, the portion of the diaphragm 100 that is in contact with the deforming piezoelectric element 70 is deformed. As the vibration plate 100 is deformed, the volume of the tributary portion 67 in which the portion of the insulating sheet 90 forming the vibration plate 100 forms part of the inner wall is reduced. The pressure accompanying the reduction of the volume of the tributary portion 67 acts on the ink in the reduced tributary portion 67. Along with this, ink is ejected from the nozzle 32 connected to the branch portion 67.

  The piezoelectric element 70 is repeatedly deformed whenever the closing of the switch built in the switch unit 126 to which the piezoelectric element 70 is connected coincides with the application of a driving voltage having a predetermined period. When the deformation of the piezoelectric element 70 is repeated, the portion of the diaphragm 100 corresponding to the piezoelectric element 70 that repeatedly deforms is also repeatedly deformed. Along with this, ink is repeatedly ejected from the nozzle 32 connected to the branch portion 67 corresponding to the predetermined piezoelectric element 70. The configuration in which the drive voltage is applied to the piezoelectric element 70 via the switch unit 126 can simplify the control configuration. As a result, the cost of the inkjet recording apparatus 10 can be reduced.

  Ink discharge in the recording process is performed in a state where the inside of the storage chamber 55 is adjusted to a pressure lower than the atmospheric pressure. The adjustment of the pressure in the storage chamber 55 is realized by adjusting the pressure in the buffer tank 143 communicated with the storage chamber 55 as described above. The pressure in the buffer tank 143 is controlled by the control unit 122. In other words, the control unit 122 applies any or all of the supply valve 53, the vacuum pump 142, the first adjustment valve 144, and the second adjustment valve 145 according to the measurement value acquired from the pressure sensor 146 and the acquired pressure setting value. Output a control signal.

  For example, it is assumed that the measured value acquired from the pressure sensor 146 is higher than the upper limit value corresponding to the pressure set value. A state where the measured value is higher than the upper limit value means a state where the measured value is closer to the atmospheric pressure than the upper limit value. In this case, the control unit 122 outputs a start signal to the vacuum pump 142, outputs an opening signal to the first regulating valve 144, and outputs a closing signal to the second regulating valve 145. The start signal is a control signal for starting the operation of the vacuum pump 142. The release signal output to the first adjustment valve 144 is a control signal for switching the first adjustment valve 144 to an open state. The closing signal output to the second regulating valve 145 is a control signal for switching the second regulating valve 145 to a closed state. The vacuum pump 142 starts driving in response to the start signal. The first adjustment valve 144 is switched to an open state in response to the open signal, and the exhaust pipe 141 is opened. The second adjustment valve 145 switches to a closed state in response to the closing signal. As a result, the air in the buffer tank 143 is sucked into the vacuum pump 142, and the pressure in the buffer tank 143 decreases.

  It is assumed that the measured value acquired from the pressure sensor 146 becomes a lower limit value corresponding to the pressure set value acquired from the pressure input unit 130 as the air in the buffer tank 143 is sucked. In this case, the control unit 122 outputs a stop signal to the vacuum pump 142 and outputs a closing signal to the first adjustment valve 144. The stop signal is a control signal for stopping the operation of the vacuum pump 142. The closing signal output to the first regulating valve 144 is a control signal for switching the first regulating valve 144 to a closed state. The vacuum pump 142 stops driving in response to the stop signal. The first regulating valve 144 is switched to a closed state in response to the closing signal, and the exhaust pipe 141 is closed. Accordingly, the suction of air in the buffer tank 143 is stopped.

  It is assumed that the measurement value acquired from the pressure sensor 146 is lower than the lower limit value corresponding to the pressure setting value acquired from the pressure input unit 130 as the air in the buffer tank 143 is sucked. A state where the measured value is lower than the lower limit value means a state where the lower limit value is closer to the atmospheric pressure than the measured value. In this case, the control unit 122 outputs a stop signal to the vacuum pump 142, outputs a closing signal to the first regulating valve 144, and outputs an opening signal to the second regulating valve 145. The opening signal output to the second regulating valve 145 is a control signal for switching the second regulating valve 145 to the opened state. Similarly to the above, the vacuum pump 142 stops driving, and the first adjustment valve 144 is also switched to the closed state. The second adjustment valve 145 switches to an open state in response to the release signal, and the inside of the buffer tank 143 is opened to the atmosphere. Along with this, air flows into the buffer tank 143, and the pressure in the buffer tank 143 increases.

  The control unit 122 repeatedly executes the control described above. By such control, the pressure in the buffer tank 143 is adjusted to a predetermined range based on the pressure set value acquired from the pressure input unit 130. In other words, the pressure in the storage chamber 55 is adjusted to a predetermined range based on the pressure set value acquired from the pressure input unit 130. Such pressure adjustment is performed with the supply valve 53 closed. When closing the supply valve 53, the control unit 122 outputs a close signal to the supply valve 53. As described above, the supply valve 53 is closed when it is detected by the second liquid level sensor that the amount of ink stored in the sub tank 52 has reached the upper limit value.

<Experimental results and discussion>
Experiments were conducted on the relationship between the ink ejection amount, the water head pressure, the driving voltage, the droplet velocity, and the recording quality. The experimental results will be described with reference to FIG. In the following, in order to clarify the correspondence between the ink jet recording apparatus used in the experiment and the ink jet recording apparatus 10 described above, the same reference numerals are given to the respective parts of the ink jet recording apparatus 10 for each part of the ink jet recording apparatus used in the experiment. This will be described with reference numerals.

  The recording condition 1 is a condition in which the ink discharge amount is set to “40 ng (nanogram)”. In the recording condition 1, the water head pressure was “−10 cm aq” and the drive voltage was “62 V”. The recording condition 2 is a condition in which the ink discharge amount is set to “40 ng”. In the recording condition 2, the water head pressure was “−3 cm aq”, and the drive voltage was “57 V”. The recording condition 3 is a condition in which the ink discharge amount is set to “80 ng”. In the recording condition 3, the water head pressure was “−3 cm aq” and the driving voltage was “80 V”. The recording condition 4 is a condition in which the ink discharge amount is set to “80 ng”. In the recording condition 4, the water head pressure was “−10 cm aq”, and the driving voltage was “88 V”. 1 cm aq is 98 Pa. In the recording conditions 1 to 4, the same ink jet recording apparatus was used, and the conditions other than the hydraulic head pressure and the driving voltage shown in FIG. 5 were the same. The gap between the ejection surface 35 and the recording surface 15 of the recording medium 14 was 5 mm. The ink ejection direction is the lower side in the vertical direction. The nozzle 32 has a tapered shape as shown in FIG. The dimensions of each part of the nozzle 32 are such that the nozzle diameter φ1 is 0.06 mm, the nozzle length Q is 0.1 mm, and the taper angle θ is 15 °. In this case, the nozzle diameter φ2 is “0.06+ (tan 15 ° × 0.1 × 2)” and is 0.11 mm.

  The “water head pressure” in the recording conditions 1 to 4 is a pressure acting on the ink surface in the nozzle 32. The water head pressure can be obtained from the height H (see FIG. 2) from the ink level in the nozzle 32 to the ink level in the storage chamber 55 and the pressure in the storage chamber 55. Regarding the setting of the water head pressure, for example, when the height H of the ink liquid level in the storage chamber 55 from the ink liquid level in the nozzle 32 is 10 cm, the pressure in the storage chamber 55 is reduced by 20 cm aq from the atmospheric pressure. Therefore, it can be set to “−10 cm aq”. The “droplet velocity” in the recording conditions 1 to 4 is an average velocity until the ejected ink droplet reaches a position 0.5 mm away from the ejection surface 35 in the vertical direction.

  Under the recording condition 1, the droplet velocity was 7.2 m / sec. With this droplet velocity, it was possible to land the ink droplet on the target position with high accuracy, and an image with suitable recording quality could be recorded. Furthermore, a high-definition image could be obtained by setting the ink discharge amount to 40 ng. As a result, it can be said that the recording condition 1 is a recording condition suitable when priority is given to the definition of an image.

  Under recording condition 2, the droplet velocity was 3.4 m / sec. Compared with the recording condition 1 where the ink ejection amount is the same, the droplet velocity is reduced to half or less. A drop in the accuracy of the ink droplet landing position was observed. As a result, the recording quality of the recorded image was low. Such a result is considered to be caused by a low droplet velocity during ejection. For example, it is considered that ink droplets having a small mass and flying at a low speed are easily affected by an air current generated between the ejection surface 35 and the recording surface 15. Ink droplets affected by the airflow are caused to flow in the direction of the airflow.

  Under recording condition 3, the droplet velocity was 7.5 m / sec. This droplet velocity makes it possible to land an ink droplet on a target position with high accuracy. As a result, an image with suitable recording quality could be recorded. Furthermore, by setting the ink discharge amount to 80 ng, the image recording time can be shortened as compared with the case where the ink discharge amount is 40 ng. As a result, it can be said that the recording condition 3 is a recording condition suitable for giving priority to the image recording speed.

  Under the recording condition 4, the droplet velocity was 14 m / sec. Compared with the recording condition 3 in which the ink ejection amount is the same, the droplet velocity increased by about 1.9 times. It was confirmed that the discharged 80 ng of ink was not collected in one drop. As a result, the recording quality of the recorded image was low. Such a result is considered to be caused by the droplet speed being too high during ejection.

  The present inventor was able to clarify the following as a condition for recording an image having a suitable recording quality in the recording process of the ink jet recording method by this experiment. That is, on the premise of securing suitable recording quality, when priority is given to the definition over the image recording speed, it is preferable to reduce both the head pressure and the driving voltage to reduce the ink discharge amount (see recording condition 1). ). In other words, on the premise of ensuring suitable recording quality, when priority is given to recording speed over image definition, it is preferable to increase both the head pressure and the driving voltage to increase the ink discharge amount (recording conditions). 3).

<Effect of embodiment>
According to the embodiment, the following effects can be obtained.

  (1) Based on the water head pressure and the driving voltage, the ink discharge amount is managed (see FIG. 5). Image recording can be executed in a state where the ink ejection amount is managed. It is also possible to manage the ink discharge amount based only on either the water head pressure or the drive voltage. However, in this case, the range of the ink discharge amount that can be set by the ink jet recording apparatus is narrower than in the case based on the two conditions described above. By managing the ink discharge amount based on the water head pressure and the drive voltage, the setting range of the ink discharge amount can be widened. It is possible to flexibly cope with image recording speed and image definition. As apparent from the above, the water head pressure is determined based on the height H (see FIG. 2) from the ink level in the nozzle 32 to the ink level in the storage chamber 55 and the pressure in the storage chamber 55. The water head pressure value can be set. The position of the ink level in the storage chamber 55 can be managed by adjusting the amount of ink in the storage chamber 55 to a predetermined range.

  (2) The nozzle 32 is tapered (see FIG. 4). For example, as compared with a straight nozzle having the same nozzle length Q, the resistance when the ink flowing into the nozzle 32 from the tributary portion 67 flows through the nozzle 32 can be reduced by using the tapered shape. Based on the above-described resistance, the nozzle length Q can be set longer in the tapered nozzle 32 than in the straight nozzle. Therefore, an ink meniscus can be formed in the nozzle 32 even if the pressure in the storage chamber 55 is lower. For example, as compared with a straight nozzle, the setting range of the ink discharge amount can be further widened.

<Modification>
The embodiment can also be performed as follows. Some configurations of the modifications shown below can be appropriately combined and employed. Hereinafter, points different from the above will be described, and description of similar points will be omitted as appropriate.

  (1) In the above, the case where the inkjet recording apparatus 10 is a serial type has been described as an example (see FIG. 1). In the serial type inkjet recording apparatus 10, as described above, ink is ejected while the recording unit 30 is reciprocally moved in the main scanning direction by the moving unit 40, and an image is recorded on the recording medium 14. The structure shown in FIGS. 2 to 4 can also be employed in a line-type inkjet head. In the line-type inkjet head, the plurality of nozzles are arranged in the main scanning direction that coincides with the short direction of the recording medium 14. The range in the main scanning direction in which the nozzles are arranged is, for example, not less than the width W of the recording medium 14 (see FIG. 1). The number of nozzle rows may be a plurality of rows as described above.

  (2) In the above description, the exhaust pipe 141 is used to connect the sub tank 52 for a predetermined color and one buffer tank 143, and the decompression unit 140 is connected to this buffer tank 143 and one vacuum pump 142 as an example. As described (see FIG. 2). For example, the decompression unit 140 may have a configuration in which the sub tanks 52K, 52M, 52Y, and 52C are connected to one buffer tank 143 through the exhaust pipe 141, and the buffer tank 143 and one vacuum pump 142 are connected. . In addition, for example, the decompression unit 140 connects one buffer tank 143 to each of the sub tanks 52K, 52M, 52Y, and 52C by the exhaust pipe 141, and connects four buffer tanks 143 and one vacuum pump 142. A connected configuration may be used. In this case, the four buffer tanks 143 may be connected to each other by the exhaust pipe 141. The first regulating valve 144 is provided in a portion of the exhaust pipe 141 that does not branch to the four buffer tanks 143. The pressure sensor 146 is provided in one predetermined buffer tank 143. As described above, the control unit 122 adjusts the pressure in the buffer tank 143 according to the measurement value from the pressure sensor 146.

  (3) The inkjet head 31 for each color may be formed by a group of head units arranged in the sub-scanning direction, or in the sub-scanning direction and the main scanning direction. For example, the inkjet head 31 may have a configuration in which a plurality of head units are arranged in the sub-scanning direction to form one unit row, and two unit rows are provided in the main scanning direction. In this case, the first unit row and the second unit row are provided adjacent to each other in the main scanning direction with a predetermined amount shifted in the sub scanning direction. The sub tank 52 is provided in each of the plurality of head units. One buffer tank 143 is provided for the plurality of sub tanks 52. That is, a plurality of sub tanks 52 are connected to one buffer tank 143 by the exhaust pipe 141. The pressure in each storage chamber 55 in the plurality of sub tanks 52 is adjusted by adjusting the pressure in one buffer tank 143 to which each sub tank 52 is connected, as described above.

  (4) In the above, the inkjet recording apparatus 10 in which the sub tank 52 is provided on the upper side of the inkjet head 31 in the vertical direction has been described as an example (see FIGS. 1 to 3). In this case, the liquid level of the ink in the storage chamber 55 is higher than the liquid level of the ink in the nozzle 32 in the vertical direction. In the ink jet recording apparatus, a tank for storing ink supplied to the ink jet head 31 such as the sub tank 52 may be provided below the ink jet head 31 in the vertical direction. In this case, the ink level in the storage chamber of the tank described above is lower than the level of the ink in the nozzle 32 in the vertical direction. In such an ink jet recording apparatus, the head pressure can be adjusted by adjusting the height of the ink level in the storage chamber of the tank. The adjustment of the height of the ink liquid level in the storage chamber of the tank is performed, for example, by the same control as the control related to the ink supply to the sub tank 52 using the liquid level sensor described above. You may make it adjust the height of this tank itself. In such an ink jet recording apparatus, the decompression unit 140 may be omitted.

DESCRIPTION OF SYMBOLS 10 Inkjet recording device, 14 Recording medium, 15 Recording surface 20 Conveyance part, 30 Recording part 31, 31K, 31M, 31Y, 31C Inkjet head 32 Nozzle, 34 Carriage, 35 Ejection surface 40 Moving part, 41, 42 Pulley, 43 Timing Belt 44 Motor, 46 Support 50, 50K, 50M, 50Y, 50C Main tank 51, 51K, 51M, 51Y, 51C Supply flow path 52, 52K, 52M, 52Y, 52C Sub tank 53, 53K, 53M, 53Y, 53C Supply Valve 54, 54K, 54M, 54Y, 54C Connection part 55 Storage chamber, 60 flow path part, 61 supply port 63 internal flow path, 65 main flow part, 67 tributary part 70 piezoelectric element, 80 electrode plate, 90 insulating sheet, 100 vibration Plate 120 control device, 122 control unit, 124 f Drive unit 126 switch unit 128 voltage input unit 130 pressure input unit 140 pressure reduction unit 141 exhaust pipe 142 vacuum pump 143 buffer tank 144 first adjustment valve 145 second adjustment valve 146 pressure sensor H height Q Nozzle length, W width θ Taper angle, φ1, φ2 Nozzle diameter

Claims (3)

An ink tank in which a storage chamber for storing ink is formed;
A nozzle that ejects the ink supplied from the ink tank and an internal flow path through which the ink flows and is connected to the nozzle are formed, and the internal flow path is deformed to eject the ink from the nozzle. An inkjet head comprising a piezoelectric element;
Connecting the ink tank and the inkjet head, and a connection portion through which the ink supplied from the ink tank to the inkjet head flows;
A head driving unit that outputs a driving voltage applied to the piezoelectric element;
An adjustment unit that adjusts the pressure acting on the liquid level of the ink in the nozzle,
The adjustment unit is
When the discharge amount of the ink discharged from the nozzle is a first droplet value, the pressure acting on the liquid surface of the ink in the nozzle is adjusted to the first hydraulic head pressure value,
When the discharge amount of the ink discharged from the nozzle is set to a second droplet value smaller than the first droplet value, the pressure acting on the liquid surface of the ink in the nozzle is determined from the first water head pressure value. Adjust to a low second head pressure value,
The head drive unit is
When the discharge amount of the ink discharged from the nozzle is the first droplet value, a driving voltage having a first voltage value is output,
An inkjet recording apparatus that outputs a driving voltage having a second voltage value lower than the first voltage value when the ejection amount of the ink ejected from the nozzle is the second droplet value.   2. The inkjet recording according to claim 1, wherein the nozzle has a taper shape in which a nozzle diameter on a discharge surface side of the inkjet head facing a recording medium on which an image is recorded is smaller than a nozzle diameter on a side of the tributary portion. apparatus. An ink tank in which a storage chamber for storing ink is formed, a nozzle for discharging the ink supplied from the ink tank, and an internal flow path through which the ink flows and is connected to the nozzle are formed, An ink jet head having a piezoelectric element that deforms the internal flow path to discharge the ink from the nozzle, and the ink tank and the ink jet head are connected, and the ink supplied from the ink tank to the ink jet head is A recording step of recording an image on a recording medium by operating an inkjet recording apparatus including a connecting portion that flows and a head driving unit that outputs a driving voltage applied to the piezoelectric element;
The recording step includes
When an image is recorded on the recording medium in a state where the ejection amount of the ink ejected from the nozzle is set to the first droplet value, the pressure acting on the liquid surface of the ink in the nozzle is a first water head. It is adjusted to a pressure value, and is executed in a state where a driving voltage of the first voltage value is output from the head driving unit,
When recording an image on the recording medium in a state where the ejection amount of the ink ejected from the nozzle is set to a second droplet value smaller than the first droplet value, the liquid level of the ink in the nozzle This is executed in a state where the pressure acting on the pressure is adjusted to the second hydraulic head pressure value lower than the first hydraulic head pressure value, and the driving voltage having the second voltage value lower than the first voltage value is output from the head driving unit. Inkjet recording method.

Images