Patent Publication Number: US-2021170765-A1

Title: Liquid ejecting apparatus

Description:
The present application is based on, and claims priority from JP Application Serial Number 2019-221940, filed Dec. 9, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a liquid ejecting apparatus. 
     2. Related Field 
     JP-A-2019-1115 discloses a liquid ejecting apparatus that includes an image forming unit, an irradiation unit, and an inert gas supplying unit. The image forming unit ejects a UV curable ink onto a surface of a substrate. The irradiation unit is disposed at a position downstream of the image forming unit in a substrate transporting direction and irradiates the substrate with active energy rays. The inert gas supplying unit has a discharge nozzle located between the image forming unit and the irradiation unit in the substrate transporting direction, and the discharge nozzle discharges an inert gas downstream onto the substrate. 
     The liquid ejecting apparatus disclosed in JP-A-2019-1115, however, may lower the curing performance of the UV curable ink since the oxygen concentration in the UV curable ink may increase while the substrate is transported from a region in which the image forming unit supplies the UV curable ink to the substrate to a region in which the discharge nozzle for discharging the inert gas is disposed. 
     SUMMARY 
     According to an aspect of the present disclosure, a liquid ejecting apparatus includes a liquid ejecting unit configured to eject a liquid curable by light irradiation onto a medium, an irradiation unit configured to emit light onto the medium on which the liquid is ejected, and an oxygen concentration reduction mechanism configured to lower an oxygen concentration to a level below an oxygen concentration of the atmosphere in an ejection region formed between the liquid ejecting unit and the medium when the liquid ejecting unit ejects the liquid and also in an irradiation region formed between the irradiation unit and the medium when the irradiation unit emits light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating an overall configuration of a liquid ejecting apparatus according to a first embodiment. 
         FIG. 2  is a schematic diagram illustrating a configuration of a liquid supply mechanism and a printing section of the liquid ejecting apparatus. 
         FIG. 3  is a schematic diagram illustrating a configuration of a liquid supply mechanism and a printing section of a liquid ejecting apparatus according to a second embodiment. 
         FIG. 4  is a schematic diagram illustrating a configuration of a liquid supply mechanism and a printing section of a liquid ejecting apparatus according to a third embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     1. First Embodiment 
     A liquid ejecting apparatus  10  according to a first embodiment will be described with reference to  FIGS. 1 and 2 . The following description covers an overall configuration of the liquid ejecting apparatus  10 , a liquid supply mechanism  2 , a printing section  70 , and a maintenance mechanism  91  in this order. The liquid ejecting apparatus  10  described in the present embodiment is a serial-type ink jet printer that performs printing by ejecting a liquid onto a medium M, such as a sheet of paper, and the liquid is curable by light irradiation. In the following description, the liquid curable by light irradiation is referred to as a “UV curable ink” or simply referred to as a “liquid”. 
     1-1 Overall Configuration of Liquid Ejecting Apparatus 
     In the following description, assuming that the liquid ejecting apparatus  10  is placed on a horizontal surface, the Z-axis extends in the vertical direction, in other words, in the direction in which gravity acts, and the X-axis and the Y-axis extend in directions parallel to the horizontal surface that perpendicularly intersects the vertical direction. The X-axis, the Y-axis, and the Z-axis orthogonally intersect each other. The pointed end of each arrow that represents each axis points toward the “plus side” on the axis, whereas the other end of the arrow points toward the “minus side” on the axis. In the following description, a direction parallel to the X-axis may be referred to as the “width direction”, and a direction parallel to the Y-axis may be referred to as the “depth direction”. In the liquid ejecting apparatus  10 , the +z side along the Z-axis may be referred to as a “bottom side” or a “lower side”, and the −z side along the Z-axis may be referred to as a “top side” or an “upper side”. 
     As illustrated in  FIG. 1 , the liquid ejecting apparatus  10  includes a pair of legs  11 , a housing  12 , a feed section  13 , a guide section  14 , a winding section  15 , a tension-imparting mechanism  16 , and an operation panel  17 . The housing  12  is joined to upper portions of respective legs  11 . The feed section  13  feeds a medium M wound in a roll toward the inside of the housing  12 . The guide section  14  guides the medium M discharged from the housing  12  toward the winding section  15 . 
     The winding section  15  winds the medium M guided by the guide section  14  into a roll. The tension-imparting mechanism  16  applies tension to the medium M to be wound by the winding section  15 . The operation panel  17  allows a user to input various operational conditions to be executed by the liquid ejecting apparatus  10 . 
     The liquid ejecting apparatus  10  also includes a main tank  20 . The main tank  20  is provided outside the housing  12 . The main tank  20  includes liquid containers  18  for storing liquids and a holder  19  that holds the liquid containers  18 . The liquid containers  18  are ink cartridges for accommodating inks, which are examples of the liquids. The holder  19  detachably holds the liquid containers  18 . 
     The liquid ejecting apparatus  10  also includes a control unit  100  configured to control operation of the liquid ejecting apparatus  10 . For example, the control unit  100  has CPU and memory. The CPU is an arithmetic processing unit for controlling operation of each unit of the liquid ejecting apparatus  10 . The memory is a storage device, such as RAM or EPROM, that has an area for storing a program to be executed by the CPU and also has a work area to be used during execution of the program. The CPU executes the program stored in the memory, and the control unit  100  thereby controls operation of the liquid ejecting apparatus  10 . 
     1-2 Liquid Supply Mechanism 
     As illustrated in  FIG. 2 , the liquid ejecting apparatus  10  includes the liquid supply mechanism  2  and print heads  80  that serve as a liquid ejecting unit. The liquid supply mechanism  2  includes, for example, a sub-tank  30  and a circulation line  31 . 
     The sub-tank  30  temporarily stores a liquid supplied from the main tank  20 . The sub-tank  30  of the present embodiment is an open tank. A liquid level of the sub-tank  30  is the height of the surface of the liquid in the sub-tank  30 . 
     Each print head  80  includes a nozzle face  80   a  and multiple nozzles  81  formed at the nozzle face  80   a  and arrayed along the Y-axis. The nozzles  81  eject the liquid. The vertical distance between the level of the nozzle face  80   a  and the liquid level of the sub-tank  30  is a head difference ΔH. 
     The circulation line  31  is a flow channel for the circulation of the liquid. The liquid circulating in the circulation line  31  is supplied from the sub-tank  30  to the print head  80  and is returned from the print head  80  to the sub-tank  30 . 
     The main tank  20  communicates with the sub-tank  30  through a replenishing line  21 . The replenishing line  21  is a flow channel for replenishing the sub-tank  30  with the liquid supplied from the main tank  20 . The upstream end of the replenishing line  21  is connected to the main tank  20 . The downstream end of the replenishing line  21  is connected to the sub-tank  30 . 
     A supply on-off valve  22  and a supply pump  23  are disposed in this order in the replenishing line  21  from the main tank  20  to the sub-tank  30 . The supply on-off valve  22  is, for example, a solenoid valve that opens and closes the replenishing line  21 . The supply pump  23  causes the liquid stored in the main tank  20  to flow toward the sub-tank  30 . 
     The sub-tank  30  has a liquid-level sensor  35 . The liquid-level sensor  35  detects the liquid level of the sub-tank  30 . The liquid-level sensor  35  determines whether the liquid level of the sub-tank  30  is not less than a first liquid level L 1 . The liquid-level sensor  35  determines whether the liquid level of the sub-tank  30  is not less than a second liquid level L 2  that is a level higher than the first liquid level L 1 . 
     The supply on-off valve  22  and the supply pump  23  supply or stop supplying the liquid from the main tank  20  to the sub-tank  30 . When the liquid level of the sub-tank  30  is less than the first liquid level L 1 , the supply on-off valve  22  and the supply pump  23  starts supplying the liquid. When the liquid level of the sub-tank  30  exceeds the second liquid level L 2 , the supply on-off valve  22  and the supply pump  23  stops supplying the liquid. Accordingly, the liquid level of the sub-tank  30  is kept between the first liquid level L 1  and the second liquid level L 2 . 
     Note that the supply on-off valve  22  and the supply pump  23  may supply the liquid when the print head  80  consumes the liquid. Moreover, the supply on-off valve  22  and the supply pump  23  may supply the liquid so as to control the pressure of the liquid in the print head  80  within a predetermined range. With this liquid replenishment method, the liquid pressure can be controlled at the nozzles  81  within an appropriate range while the liquid circulates in the circulation line  31 . In other words, the liquid can circulate in the circulation line  31  without disturbing a meniscus that is a gas-liquid interface formed in each nozzle  81 . 
     The inside of the sub-tank  30  is open to the atmosphere when the liquid ejecting apparatus  10  performs printing. Opening the sub-tank  30  to the atmosphere is performed by adjusting the internal pressure of the sub-tank  30 . The internal pressure of the sub-tank  30  is adjusted without breaking the meniscus formed in each nozzle  81 . The internal pressure of the sub-tank  30  is, for example, −3500 Pa or more and −1000 Pa or less relative to atmospheric pressure. The adjustment of the internal pressure of the sub-tank  30  can stabilize the meniscus of the nozzle  81 . 
     Note that the adjustment of internal pressure of the sub-tank  30  may be performed by adjusting the head difference ΔH. For example, the supply on-off valve  22  and the supply pump  23  may adjust the liquid level of the sub-tank  30  so as to cause the head difference ΔH to be 190 mm. 
     The sub-tank  30  communicates with a pressurizing module  36  through an air channel  37 . Air is supplied to or discharged from the inside of the sub-tank  30  through the air channel  37 . The pressurizing module  36  pressurizes the liquid in the sub-tank  30  by supplying air through the air channel  37  and depressurizes the liquid by discharging air through the air channel  37 . 
     The pressurizing module  36  is used, for example, for performing pressurized cleaning. In the pressurized cleaning, the liquid supplied to the nozzles  81  is pressurized so that the liquid is forcibly discharged from the nozzles  81 . The pressurized cleaning is performed to discharge foreign matter, such as bubbles, contained in the liquid out of the print head  80 . In the pressurized cleaning, the pressurizing module  36  increases the internal pressure of the sub-tank  30  so as to break the meniscus of the nozzle  81 . 
     The pressurizing module  36  may be used to adjust the internal pressure of the sub-tank  30  when, for example, the liquid ejecting apparatus  10  performs printing. The pressurizing module  36  controls the internal pressure of the sub-tank  30 , for example, in a range of −2400 Pa or more and −1900 Pa or less relative to atmospheric pressure, so as not to break the meniscus of the nozzle  81 . The adjustment of the internal pressure of the sub-tank  30  by means of the pressurizing module  36  can also stabilize the meniscus of each nozzle  81 . 
     The circulation line  31  includes a liquid supply line  32  and a liquid discharge line  33 . The liquid is supplied from the sub-tank  30  toward print head  80  through the liquid supply line  32 . The upstream end of the liquid supply line  32  is connected to the sub-tank  30 . The downstream end of the liquid supply line  32  is connected to the print head  80 . 
     A portion of the liquid supplied to the print head  80  is returned toward the sub-tank  30  through the liquid discharge line  33 . In other words, a portion of the liquid supplied to the print head  80 , which is not discharged from the nozzles  81  of the print head  80 , is returned to the sub-tank  30  through the liquid discharge line  33 . The upstream end of the liquid discharge line  33  is connected to the print head  80 . The downstream end of the liquid discharge line  33  is connected to the sub-tank  30 . 
     The liquid supply line  32  is connected to one end portion of each print head  80 . The liquid discharge line  33  is connected to the other end portion of each print head  80 . 
     A diaphragm pump  40 , a heating unit  48 , a deaeration mechanism  49 , a filter  50 , and a damper  60  are disposed in this order in the liquid supply line  32  from the sub-tank  30  toward the print head  80 . 
     The diaphragm pump  40  is an example of a pump. The diaphragm pump  40  supplies the liquid to the print head  80  through the liquid supply line  32 . 
     It is preferable that at least part of the diaphragm pump  40  be positioned below the liquid level of the sub-tank  30 . It is more preferable that the center of diaphragm chamber of the diaphragm pump  40  in the vertical direction be positioned below the liquid level of the sub-tank  30 . In the case of the inlet of the diaphragm pump  40  being positioned below the liquid level of the sub-tank  30 , the occurrence of cavitation can be suppressed and the liquid supply of the diaphragm pump  40  can be thereby stabilized. 
     The heating unit  48  includes a heated-water tank having a heater and a thermometer, a heated-water circulation line, a heated-water pump, and a heat exchanger. The heated-water tank stores heated water having a temperature adjusted in a predetermined range. The heated-water circulation line is a flow channel routing from the heated-water tank to the heated-water tank via the heat exchanger. The heated-water pump circulates the heated water in the heated-water circulation line. The heat exchanger exchanges heat between the heated water flowing through the heated-water circulation line and the liquid flowing through the circulation line  31 . 
     The heating unit  48  heats the liquid flowing through the circulation line  31  to a predetermined temperature. The predetermined temperature is a temperature at which the liquid supplied to the print head  80  has a viscosity suitable for ejection from the print head  80 , which is 35° C. or more and 40° C. or less, for example. The heating unit  48  reduces the likelihood of a highly viscous liquid not suitable for ejection being supplied to the print head  80 . 
     The deaeration mechanism  49  deaerates the liquid supplied to the print head  80 , in other words, the liquid flowing through the circulation line  31 . The deaeration mechanism  49  has a deaeration module and a decompressor. For example, the deaeration module has a plurality of hollow fiber membranes. The decompressor reduces the pressure outside the hollow fiber membranes, and the liquid flowing through the hollow fiber membranes is deaerated. The deaeration mechanism  49  reduces the likelihood of the bubble-containing liquid being supplied to the print head  80 . 
     The filter  50  disposed in the liquid supply line  32  is positioned above the level of the nozzle face  80   a  of the print head  80  in the vertical direction. The filter  50  is detachably disposed in the liquid supply line  32 . The liquid from which foreign matter such as bubbles is removed at the filter  50  flows downstream into the liquid supply line  32 . 
     The damper  60  reduces pressure fluctuation of the liquid flowing through the circulation line  31 . For example, the damper  60  is a permeable membrane having rubber elasticity. 
     A deaeration line  58 , which is different from the liquid supply line  32 , is connected to an upstream portion of the filter  50 . The deaeration line  58  connects the filter  50  and the sub-tank  30 . A discharge valve  59  is disposed at an intermediate portion of the deaeration line  58 . The deaeration line  58  is connected to a nearly uppermost portion of the filter  50  in the vertical direction. 
     The discharge valve  59  switches the state of the deaeration line  58  between a communicating state and a non-communicating state. Bubbles captured at the filter  50  are discharged to the sub-tank  30  through the deaeration line  58  while the discharge valve  59  in the deaeration line  58  is open. 
     1-3 Printing Section 
     The liquid ejecting apparatus  10  includes the printing section  70  for performing printing on a medium M supplied from the feed section  13  to the inside of the housing  12 . The printing section  70  includes print heads  80  as described above, a carriage  71 , irradiation units  79 , an ejection region  72 , irradiation regions  73 , a depressurizing mechanism  90  that serves as an oxygen concentration reduction mechanism, and a platen  75 . The carriage  71  is shaped like a box of which the bottom side is open, and the bottom side is sealed by an oxygen permeable membrane  76 , which will be described later. The print heads  80 , the irradiation units  79 , and part of the depressurizing mechanism  90  are mounted in the carriage  71  that is located above the platen  75 . The carriage  71  is guided by a guide shaft  74  disposed along the X-axis and is configured to move reciprocally in the X direction, which is the main scanning direction. The irradiation units  79  are a first irradiation unit  79   a  and a second irradiation unit  79   b.  The first irradiation unit  79   a,  the print heads  80 , and the second irradiation unit  79   b  are mounted in the carriage  71  in this order from the +x side to the −x side of the X-axis. The platen  75  is disposed so as to oppose the carriage  71  that moves reciprocally. The platen  75  supports the medium M from below. The medium M is transported in a transporting direction that extends from the +y side to the −y side of the Y-axis. 
     Each print head  80  ejects a UV curable ink onto the medium M. The printing section  70  of the present embodiment has four print heads  80  that are arrayed along the X-axis. Each print head  80  has a nozzle face  80   a  at which multiple nozzles  81  for ejecting the UV curable ink are arrayed along the Y-axis. For example, the four print heads  80  correspond to respective four colors, in other words, cyan (C), magenta (M), yellow (Y), and black (K). Ejection of the UV curable ink from each print head  80  is controlled by the control unit  100 . Note that the number of print heads  80  mounted in the printing section  70  may be three or less or may be five or more. 
     The medium M on which the UV curable ink has been ejected is irradiated with light by a corresponding irradiation unit  79 . Each irradiation unit  79  includes a light source that emits light for curing the UV curable ink to the medium M. Various types of the light sources are usable, such as a light emitting diode (LED), a laser diode (LD), a mercury lamp, a metal halide lamp, a xenon lamp, or an excimer lamp. The length of the irradiation unit  79  along the Y-axis is such as to cover the entire nozzles  81  that are disposed in each print head  80  and arrayed along the Y-axis. In the first embodiment, two irradiation units  79  are provided. The number of the irradiation units  79  may be two or more. For example, one irradiation unit  79  may be provided for each print head  80 . When the carriage  71  moves toward the +x side of the X-axis, the second irradiation unit  79   b  located downstream of the four print heads  80  is activated. On the other hand, when the carriage  71  moves toward the −x side of the X-axis, the first irradiation unit  79   a  located downstream of the four print heads  80  is activated. The irradiation units  79  are controlled by the control unit  100 . 
     The ejection region  72  is a space formed between the print heads  80  and the medium M when each print head  80  ejects the UV curable ink. The irradiation regions  73  are spaces formed between respective irradiation units  79  and the medium M when the irradiation units  79  emit light. A first irradiation region  73   a  corresponds to the first irradiation unit  79   a,  and a second irradiation region  73   b  corresponds to the second irradiation unit  79   b.  The size of the ejection region  72  and the sizes of the first and second irradiation regions  73   a  and  73   b  are not specifically limited here. The ejection region  72  and the first and second irradiation regions  73   a  and  73   b  move over the medium M when the carriage  71  moves in the X direction. When the carriage  71  moves toward the +x side of the X-axis, the ejection region  72  first moves toward +x side, and subsequently the second irradiation region  73   b  comes to the previous position of the ejection region  72  to cure the UV curable ink. On the other hand, when the carriage  71  moves toward the −x side of the X-axis, the ejection region  72  first moves toward −x side, and subsequently the first irradiation region  73   a  comes to the previous position of the ejection region  72  to cure the UV curable ink. 
     The depressurizing mechanism  90  serves to lower oxygen concentration in the ejection region  72  and also in the irradiation regions  73  to a level below the oxygen concentration of the atmosphere. The depressurizing mechanism  90  has the oxygen permeable membrane  76 , a depressurizing pump  77 , and a pipe  78 . 
     The oxygen permeable membrane  76  can transmit oxygen selectively. For example, the oxygen permeable membrane  76  may be a dense film that utilizes electroconductivity to pass specific ions selectively with the pressure difference in oxygen concentration serving as the drive force. The oxygen permeable membrane  76  covers the bottom side of the carriage  71  entirely except for the bottom surfaces of the print heads  80  and of the irradiation units  79 . The space surrounded by the oxygen permeable membrane  76  and the carriage  71  is a depressurized region  69 . In other words, the oxygen permeable membrane  76  serves as a partition that separates the depressurized region  69  from the ejection region  72  and from the irradiation regions  73 . Note that in the present embodiment, the oxygen permeable membrane  76  forms the entire partition. However, the oxygen permeable membrane  76  may be configured to form part of the partition. More specifically, the oxygen permeable membrane  76  may be disposed at least upstream of the ejection region  72  in the moving direction of the carriage  71 . 
     The depressurizing pump  77  is a pump that can reduce the pressure in the depressurized region  69 . For example, the depressurizing pump  77  may be a diaphragm pump. A plurality of the depressurizing pumps  77  may be provided. It is sufficient that the depressurizing pump  77  is able to lower the pressure at least below atmospheric pressure. The depressurizing pump  77  is connected to the depressurized region  69  via the pipe  78 . The pipe  78  may be connected to any portion of the carriage  71  that forms the depressurized region  69  in collaboration with the oxygen permeable membrane  76 . 
     The depressurizing mechanism  90  lowers the oxygen concentration in a manner as described below. Due to the depressurizing pump  77  depressurizing the depressurized region  69 , the oxygen content of the air inside the depressurized region  69  is lowered relative to that in the ejection region  72  and in the irradiation regions  73 . The oxygen content per unit volume becomes different between the depressurized region  69  and the ejection region  72  and the irradiation regions  73 . Due to this difference in the oxygen content serving as the drive force, oxygen in the ejection region  72  and in the irradiation regions  73  moves to the depressurized region  69  through the oxygen permeable membrane  76 . Thus, the oxygen concentration becomes low in the ejection region  72  and in the irradiation regions  73 . 
     1-4 Maintenance Mechanism 
     The liquid ejecting apparatus  10  includes the maintenance mechanism  91  for the maintenance of the print heads  80  to stabilize liquid ejection onto the medium M. 
     The maintenance mechanism  91  is formed of a flushing box  92 , a wiper  93 , caps  94 , and a support member  95  for movably supporting these components together. In wiping, the wiper  93  having a wiping face  93   a  and a wiping face  93   b  moves in a wiping direction Dw with the wiping face  93   a  or the wiping face  93   b  being in contact with the nozzle faces  80   a.  In capping, the caps  94  come into contact with corresponding nozzle faces  80   a  so as to cover the nozzles  81  at a position at which the caps  94  overlap the print heads  80  as viewed in plan along the Z-axis. Moreover, the carriage  71  periodically moves to a position at which the carriage  71  overlaps the flushing box  92  as viewed in plan along the Z-axis, and each print head  80  ejects liquid into the flushing box  92 . The maintenance mechanism  91  is provided at a position different from the position of the platen  75  along the X-axis. Along the Z-axis, the maintenance mechanism  91  may be positioned differently from the platen  75 . In addition, when the maintenance mechanism  91  performs maintenance of the print heads  80  or when the carriage  71  is positioned vertically above the maintenance mechanism  91 , the depressurizing mechanism  90  may stop operation. 
     The maintenance mechanism  91  also includes a maintenance activation mechanism to drive the wiper  93  and the caps  94 . The maintenance activation mechanism is configured to move the wiper  93  and the caps  94  along the Z-axis. The maintenance activation mechanism also moves the wiper  93  along the X-axis and raise or lowers the caps  94  along the Z-axis. 
     Note that in the present embodiment, the deaeration mechanism  49  is disposed between the heating unit  48  and the filter  50 . The deaeration mechanism  49 , however, may be disposed inside the carriage  71 . Since the inside of the carriage  71  is depressurized by the depressurizing pump  77 , the deaeration mechanism  49  can be formed only of the deaeration module. In other words, the depressurizing pump  77  of the depressurizing mechanism  90  can serve as the decompressor of the deaeration mechanism  49 , which can reduce the number of pumps included in the liquid ejecting apparatus  10 . 
     The liquid ejecting apparatus  10  according to the present embodiment can provide advantageous effects as described below. 
     The liquid ejecting apparatus  10  includes the oxygen concentration reduction mechanism that lowers oxygen concentration in the ejection region  72  and in the irradiation regions  73  to a level below the oxygen concentration of the atmosphere. In other words, the oxygen concentration is lowered in the ejection region  72  in which the UV curable ink is ejected and in the irradiation regions  73  in which light for curing the UV curable ink is emitted, which can improve the curing performance of the UV curable ink. 
     The liquid ejecting apparatus  10  includes the depressurizing mechanism  90  that serves as the oxygen concentration reduction mechanism. The depressurizing mechanism  90  reduces the oxygen content in the depressurized region  69  using the depressurizing pump  77  and moves oxygen from the ejection region  72  and the irradiation regions  73  to the depressurized region  69  using the oxygen permeable membrane  76 . The oxygen concentration can be thereby lowered advantageously in the ejection region  72  and in the irradiation regions  73  to a level below the oxygen concentration in the atmosphere. 
     The liquid ejecting apparatus  10  includes the deaeration mechanism  49  that deaerates the liquid to be supplied to the print heads  80 . This reduces the amount of bubbles contained in the liquid ejected from the print heads  80 . In other words, this reduces the oxygen content dissolved in the UV curable ink. As a result, the curing performance of the UV curable ink can be improved. 
     The liquid ejecting apparatus  10  includes the maintenance mechanism  91  for the maintenance of the print heads  80 . The liquid adhered to the maintenance mechanism  91  during the maintenance may be cured by light escaped from the irradiation units  79 . However, the depressurizing mechanism  90  stops operation during the maintenance, which reduces the likelihood of the liquid adhered to the maintenance mechanism being cured. 
     2. Second Embodiment 
     A liquid ejecting apparatus  110  according to a second embodiment will be described with reference to  FIG. 3 . Note that elements similar to those described in the first embodiment will be denoted by the same reference numerals, and repeated descriptions will be omitted. The liquid ejecting apparatus  110  according to the present embodiment has an oxygen concentration reduction mechanism configured differently from that of the liquid ejecting apparatus  10  described in the first embodiment. 
     The liquid ejecting apparatus  110  includes an inert gas supply mechanism  120  serving as the oxygen concentration reduction mechanism. For example, the inert gas is nitrogen gas. The inert gas supply mechanism  120  is configured to supply the inert gas to the ejection region  72  and the irradiation regions  73 . The inert gas supply mechanism  120  has an inert gas generator  125  serving as the source of the inert gas and an inert gas supply pipe  132 . One end of the inert gas supply pipe  132  is connected to the inert gas generator  125 . The inert gas supply mechanism  120  is provided to inhibit the UV curable ink from absorbing oxygen in the ejection region  72 . The inert gas supply mechanism  120  reduces the oxygen concentration of the air in the irradiation regions  73 . 
     The other end of the inert gas supply pipe  132  has discharge orifices  133  that are positioned upstream of the print heads  80  in the moving direction of the print heads  80  relative to the medium M. More specifically, the inert gas supply mechanism  120  is provided at positions upstream of the carriage  71  in the carriage movement direction when the carriage  71  reciprocally moves along the X-axis. The inert gas supply pipe  132  may be disposed inside or outside of the carriage  71 . The inert gas supply pipe  132  according to the present embodiment is formed of a first inert gas supply pipe  132   a  having a first discharge orifice  133   a  and a second inert gas supply pipe  132   b  having a second discharge orifice  133   b.  The first discharge orifice  133   a  is disposed between the first irradiation unit  79   a  and the print heads  80  and the second discharge orifice  133   b  is disposed between the second irradiation unit  79   b  and the print heads  80 . When the carriage  71  moves toward the +x side of the X-axis, the first discharge orifice  133   a  discharges the inert gas. When the carriage  71  moves toward the −x side of the X-axis, the second discharge orifice  133   b  discharges the inert gas. The inert gas supply mechanism  120  is controlled by the control unit  100 . Note that the inert gas supply mechanism  120  may be configured to discharge the inert gas simultaneously from the first discharge orifice  133   a  and from the second discharge orifice  133   b  irrespective of the movement direction of the carriage  71 . In addition, when the maintenance mechanism  91  performs maintenance of the print heads  80  or when the carriage  71  is positioned vertically above the maintenance mechanism  91 , the inert gas supply mechanism  120  may stop operation. 
     The liquid ejecting apparatus  110  according to the present embodiment can provide advantageous effects as below. 
     As described above, the liquid ejecting apparatus  10  includes the inert gas supply mechanism  120  that serves as the oxygen concentration reduction mechanism. The inert gas supply mechanism  120  supplies the inert gas to the ejection region  72  and the irradiation regions  73 . This can lower the oxygen concentration advantageously in the ejection region  72  and in the irradiation regions  73  to a level below the oxygen concentration in the atmosphere. 
     3. Third Embodiment 
     A liquid ejecting apparatus  210  according to a third embodiment will be described with reference to  FIG. 4 . Elements similar to those described in the first embodiment will be denoted by the same reference numerals, and repeated descriptions will be omitted. The liquid ejecting apparatus  210  of the present embodiment is an ink jet printer of a line-printing type. 
     The liquid ejecting apparatus  210  includes the printing section  270  for performing printing on a medium M supplied from the feed section  13  to the inside of the housing  12 . The printing section  270  includes print heads  280 , an irradiation unit  279 , an ejection region  272 , an irradiation region  273 , a depressurizing mechanism  290  and an inert gas supply mechanism  220 , both of which serve as the oxygen concentration reduction mechanisms, and medium transport rollers  230 . The print heads  280  and the irradiation unit  279  are accommodated in an accommodation box  271  that is shaped as a box of which the bottom side is open. The irradiation unit  279  is disposed downstream of the print heads  280  in a transporting direction of the medium M transported from the +y side toward the −y side of the Y-axis. The accommodation box  271  is formed as a cuboid having a longitudinal length greater than the width of the medium M. The accommodation box  271  is fixed inside the housing  12 . 
     The printing section  270  of the present embodiment has four print heads  280  that are arranged along the Y-axis. Each print head  280  has a nozzle face  280   a  at which multiple nozzles  281  for ejecting the UV curable ink are arrayed along the X-axis. The length of each print head  280  along the X-axis is greater than the width of the medium M. The medium transport rollers  230 , which are disposed so as to oppose the print heads  280 , transport the medium M in the transporting direction extending from the +y side to the −y side of the Y-axis. 
     The ejection region  272  is a space formed between the print heads  280  and the medium M when each print head  280  ejects the UV curable ink. The irradiation region  273  is a space formed between the irradiation unit  279  and the medium M when the irradiation unit  279  emits light. A portion of the medium M located at the ejection region  272  moves to the irradiation region  273  as the medium M is transported. 
     The depressurizing mechanism  290  serves to lower the oxygen concentration in the ejection region  272  and in the irradiation regions  273  below the oxygen concentration of the atmosphere. The depressurizing mechanism  290  has an oxygen permeable membrane  276 , the depressurizing pump  77 , and the pipe  78 . 
     The oxygen permeable membrane  276  covers the bottom side of the accommodation box  271  entirely except for the bottom surfaces of the print heads  280  and the irradiation unit  279 . The space surrounded by the oxygen permeable membrane  276  and the accommodation box  271  is a depressurized region  269 . The depressurizing pump  77  is a pump that can reduce the pressure in the depressurized region  269 . The depressurizing pump  77  is connected to the depressurized region  269  via the pipe  78 . Due to the depressurizing mechanism  290  depressurizing the depressurized region  269 , oxygen in the ejection region  272  and in the irradiation region  273  moves to the depressurized region  269  through the oxygen permeable membrane  276 . Thus, the oxygen concentration becomes low in the ejection region  272  and in the irradiation region  273 . 
     The inert gas supply mechanism  220  is configured to supply the inert gas to the ejection region  272  and the irradiation region  273 . The inert gas supply mechanism  220  has the inert gas generator  125  and an inert gas supply pipe  231 . One end of the inert gas supply pipe  231  is connected to the inert gas generator  125 . The other end of the inert gas supply pipe  231  is a discharge orifice  233  that is positioned upstream of the print heads  280  in the transporting direction of the medium M. The inert gas supply pipe  231  is disposed so as to incline, for example, at 45 degrees at a position upstream of the accommodation box  271  in the transporting direction. 
     The inert gas supply mechanism  220  supplies the inert gas to the ejection region  272  and the irradiation region  273  and thereby reduces the oxygen concentration in the ejection region  272  and the irradiation region  273 . The inert gas supply mechanism  220  also serves, like an air curtain, to prevent air present around the medium M from entering the ejection region  272  and the irradiation region  273 . 
     Note that in the present embodiment, one discharge orifice  233  of the inert gas supply mechanism  220  is provided upstream of the accommodation box  271 . However, multiple discharge orifices  233  may be disposed along the X-axis. In addition, in the present embodiment, one irradiation unit  279  is provided downstream of the four print heads  280 . However, multiple irradiation units  279  may be disposed downstream of respective print heads  280 . Moreover, the irradiation unit  279  may have multiple light sources arranged along the X-axis. In addition, when the maintenance mechanism  91  performs maintenance of the print heads  80  or when the carriage  71  is positioned vertically above the maintenance mechanism  91 , the depressurizing mechanism  290  and the inert gas supply mechanism  220  may stop operation. 
     The liquid ejecting apparatus  210  according to the present embodiment can provide advantageous effects as below. 
     The liquid ejecting apparatus  210  includes the depressurizing mechanism  290  and the inert gas supply mechanism  220 , both of which serve as the oxygen concentration reduction mechanism. Accordingly, the oxygen concentration can be reduced effectively in the ejection region  272  and in the irradiation region  273 .