Patent Publication Number: US-2012026254-A1

Title: Ink supply device of inkjet printer and backflow shutoff mechanism of the same

Description:
TECHNICAL FIELD 
     The present invention relates to an ink supply device for supplying ink to a print head which ejects ink droplets and a backflow shutoff mechanism mounted on the ink supply device. 
     BACKGROUND ART 
     Inkjet printer is an apparatus which comprises a print head having a large number of nozzles formed therein and which ejects particulate ink droplets from the nozzles to deposit the ink droplets on a print medium while moving the print head relative to the print medium, thereby forming image of information such as characters graphics, patterns, and photographs on a printed surface of the print medium. Since ink is consumed according to the ejection of the ink in the inkjet printer, an ink tank (ink cartridge) of a volume according to the application is mounted on a carriage of the print head or a printer main body. In case of a large-sized commercial inkjet printer for printing large-sized billposters, labarums, and the like, a large amount of ink is consumed in a relatively short time. Therefore, in such a commercial inkjet printer, a large-volume ink tank is generally mounted on a printer main body and is connected to a print head by a tube or the like so that ink is supplied to the print head from the ink tank according to the ejection of ink. 
     As the inner pressure of the print head exceeds atmospheric pressure, ink is forced out of the nozzles and drops onto the print medium, i.e. so-called “dribbling or drooling of liquid” problem occurs. Therefore, in the inkjet printer, an ink supply device is designed in such a manner as to keep the inner pressure of the print head slightly lower than atmospheric pressure. As a conventional ink supply device, there is known an ink supply device of a “negative-pressure generating type” which comprises an ink tank (main tank) mounted on the printer main body and a sub tank having a small-volume ink chamber between the ink tank and the print head mounted on a carriage, wherein the pressure in the ink chamber of the sub tank is reduced so as to keep the inner pressure of the print head at a slightly negative pressure (see, for example, Patent document 1). 
     In order to prevent absence of ink supply to the nozzles, the ink supply device of the aforementioned type is controlled such that a predetermined amount of ink is stored in the ink chamber of the sub tank according to, the ejection of ink from the nozzles. As a sample of methods for such control, there is a method of detecting a liquid surface level of ink in the ink chamber and controlling the ink supply according to the detected liquid level of ink. Specifically, control is conducted such that ink is supplied to the ink chamber of the sub tank from the main tank when the liquid surface level of ink lowers to a predetermined lower limit level because of the ejection of ink from the nozzles. As means of detecting a liquid surface level of liquid stored in a container, for example, Patent document 2 discloses an arrangement comprising afloat having a magnet and a sensor (hall element) capable of detecting magnetism from a magnet confronting the same, wherein the float is floated on liquid surface movably in the vertical direction and the sensor (hall element) is disposed at a predetermined level (for example, the lower limit level). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent document 1: JP-A-2004-284207 
     Patent document 2: JP-A-2001-141547 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Since replenishment of ink to the sub tank is performed in a state that the sub tank is decompressed as mentioned above, there are advantages that the replenishment of ink is securely performed even when a small-size ink feeding means (pump) is used and that ink never leaks or dribbles from the nozzles of the print head. However, there is a drawback that ink in the ink chamber flows out (flows backward) to a gas flow passage connected to the pressure control means when the replenishment of ink is not stopped due to failure of the level detection sensor or the like and is continued even after the surface level of ink in the ink chamber rises to exceed a predetermined level. In addition, there is a problem that component devices composing of the pressure control means are damaged due to this backward flowing ink. Therefore, there has been taken a measurement of providing a backflow prevention section comprising a sealing float which rises together with the surface level of ink to close an opening formed in the ink chamber connected to the aforementioned gas flow passage when the surface level of ink in the ink chamber of the sub tank rises to exceed the predetermined level. However, even with the backflow prevention section in the sub tank, there are disadvantageous cases, for example, that the sealing float does not keep up with the rise of the surface level of ink because the rise of the surface level of ink is too fast and that the sealing float affixes itself to an inner surface of the ink chamber so that the sealing float does not perform its backflow prevention function. 
     The present invention is made to address the aforementioned problems and it is an object of the present invention to provide an ink supply device of an inkjet printer and a backflow shutoff mechanism of the same which securely, prevents ink in a sub tank from flowing backward when the ink flows out (flows backward) into a gas flow passage connected to a pressure control means. 
     Means for Solving the Problems 
     To solve the aforementioned problem, a first invention provides an ink supply device for an inkjet printer comprising: a sub tank which is connected to a print head for ejecting ink and which stores the ink; a main tank which is connected to said sub tank and in which the ink to be supplied to said sub tank is stored; an ink sending means (for example, the ink sending unit  115  in the following embodiments) for sending the ink stored in said main tank to said sub tank; and a pressure control means (for example, the sub tank depressurizing unit  140 , the sub tank pressurizing unit  150 , and the air pump  160  in the following embodiments) for adjusting the inner pressure of said sub tank, wherein said ink supply device further comprises a backflow shutoff means (for example, the backflow shutoff mechanisms  200 ,  300  in the following embodiments) which is disposed on an air passage (for example, the line  177  of the converging route  171  in the following embodiments) which connects said pressure control means and said sub tank to allow said pressure control means to adjust the inner pressure of said sub tank, and closes said air passage to shutoff inflow of the ink into said pressure control means side when the inner pressure of said sub tank is reduced to a negative pressure by said pressure control means through said air passage and the ink stored in said sub tank flows out into said air passage. 
     In the ink supply device having the aforementioned structure, it is preferable that said backflow shutoff means comprises: a housing member having an air flowing space (for example, the inner spaces  201 ,  301  in the following embodiments) formed inside thereof, a first an introduction passage (for example, the introduction passage  204   a  of the upper-side tube connector  204  and the introduction passage  302   a  of the housing  302  in the following embodiments) which is connected to said air passage on said pressure control means side to allow communication between said air passage and said air flowing space at an upper portion of said air flowing space, and a second air introduction passage (for example, the derivation passage  205   a  of the lower tube connector  205  and the derivation passage  304   a  of the shield member  304  in the following embodiments) which is connected to said air passage on said sub tank side to allow communication between said air passage and said air flowing space; and a float member (for example, the backflow shutoff floats  203 ,  303  in the following embodiments) which is disposed within said air flowing space and moves vertically because of buoyancy relative to the ink when the ink flows into said air flowing space, wherein when said float member moves upwardly according to the rise of the liquid surface of the ink flowing into said air flowing space through said second air introduction passage, said float member closes an opening on the air flowing space side of said first air introduction passage. 
     Also in the ink supply device having the aforementioned structure, it is preferable that said backflow shutoff means is formed in said sub tank and is connected to an air introduction port (for example, the shutoff mechanism connector  109  in the following embodiments) composing said air passage on said sub tank side. 
     To solve the aforementioned problem, a second invention provides a backflow shutoff mechanism (for example, the backflow shutoff mechanisms  200 ,  300  in the following embodiments) installed in an ink supply device for an inkjet printer, wherein said ink supply device comprises a sub tank which is connected to a print head for ejecting ink and which stores the ink, a main tank which is connected to said sub tank and in which the ink to be supplied to said sub tank is stored, an ink sending means (for example, the ink sending unit  115  in the following embodiments) for sending the ink stored in said main tank to said sub tank, and a pressure control means (for example, the sub tank depressurizing unit  140 , the sub tank pressurizing unit  150  and the air pump  160  in the following embodiments) for adjusting the inner pressure of said sub tank, and said backflow shutoff mechanism is disposed on an air passage (for example, the line  177  of the converging route  171  in the following embodiments) which connects said pressure control means and said sub tank to a low said pressure control means to adjust the inner pressure of said sub tank, and comprises: a housing member (for example, the housings  202 ,  302  in the following embodiments) having an air flowing space (for example, the inner spaces  201 ,  301  in the following embodiments) formed inside thereof, a first air introduction passage (for example, the introduction passage  204   a  of the upper-side tube connector  204  and the introduction passage  302   a  of the housing  302  in the following embodiments) which is connected to said air passage on said pressure control means side to allow communication between said air passage and said air flowing space at an upper portion of said an flowing space, and a second air introduction passage (for example, the derivation passage  205   a  of the lower tube connector  205  and the derivation passage  304   a  of the shield member  304  in the following embodiments) which is connected to said air passage on said sub tank side to allow communication between said air passage and said air flowing space; and a float member (for example, the backflow shutoff floats  203 ,  303  in the following embodiments) which is disposed within said air flowing space and moves vertically because of buoyancy relative to the ink when the ink flows into said air flowing space, wherein when said float member moves upwardly according to the rise of the liquid surface of the ink flowing into said air flowing space through said second air introduction passage, said float member closes an opening on the air flowing space side of said first air introduction passage. 
     In the backflow shutoff mechanism having the aforementioned structure, it is preferable that said housing member is formed in said sub tank and is connected to an air introduction port (for example, the backflow mechanism connector  109  in the following embodiments) composing said air passage on said sub tank side. 
     EFFECT OF THE INVENTION 
     The ink supply device for an inkjet printer according to the first invention comprises a backflow shutoff means which is disposed on an air passage which connects the pressure control means and the sub tank to adjust the inner pressure of the sub tank, and closes the air passage to shutoff inflow of the ink into the pressure control means side when the inner pressure of the sub tank is reduced to a negative pressure by the pressure control means and the ink stored in the sub tank flows out into the air passage. Therefore, even when the backflow prevention section in the sub tank fails to work for any reason, the backflow of the ink is securely shut off by the backflow shutoff means disposed on the air passage when the ink in the sub tank flows (backward) into the aforementioned air passage. As a result of this, the respective components composing the pressure control means are prevented from being damaged with ink flowing backward. 
     In the aforementioned ink supply device, it is preferable that the backflow shutoff means comprises a housing member having an air flowing space formed inside thereof, a first air introduction passage which is connected to the air passage on the pressure control means side to allow communication between the air passage and the air flowing space at an upper portion of the air flowing space, and a second air introduction passage which is connected to the air passage on the sub tank side to allow communication between the air passage and the air flowing space; and afloat member which is disposed within the air flowing space and moves vertically because of buoyancy relative to the ink when the ink flows into the air flowing space, wherein when the float member moves upwardly according to the rise of the liquid surface of the ink flowing into the air flowing space through the second air introduction passage, the float member closes an opening at the air flowing space side of the first air introduction passage. According to this structure, when the ink in the sub tank flows backward, the float member moves upward according to the liquid surface of the ink flowing into the air flowing space of the housing through the second air introduction passage and closes the opening on the air flowing space side of the first air introduction passage before the ink reaches the opening, thereby preventing the ink from reaching the air passage on the pressure control means side through the first air passage. Therefore, backflow of ink can be securely shut off by the backflow shutoff means having a simple structure as mentioned above without the necessity of using a complex arrangement composed of a sensor for detecting backflow of ink, an electromagnetic shutoff valve which closes the air passage according to a signal from the sensor indicating the occurrence of backflow. 
     In the aforementioned ink supply device, it is preferable that the backflow shutoff means is formed in the sub tank and is connected to an air introduction port composing the air passage on the sub tank side. According to this structure, ink flowing backward from the sub tank to the air passage reaches the backflow prevention means soon and the backflow of the ink is shut off by the backflow shutoff means as mentioned above, thereby minimizing the range of the air passage contaminated with ink flowing backward. Therefore, the number of parts which will be forced to be replaced because of being contaminated when backflow of ink occurs is reduced. As a result of this, an effect of reducing the maintenance cost of the inkjet printer is obtained. 
     The backflow shutoff device according to the second invention comprises a housing member which is disposed on the air passage for connecting the pressure control means and the sub tank and adjusting the inner pressure of the sub tank and has an air flowing space formed inside thereof, a first air introduction passage which is connected to the air passage on the pressure control means side to allow communication between the air passage and the air flowing space at an upper portion of the air flowing space, and a second air introduction passage which is connected to the air passage on the sub tank side to allow communication between the air passage and the air flowing space; and a float member which is disposed within the air flowing space and moves vertically because of buoyancy relative to the ink when the ink flows into the air flowing space, wherein when the float member moves upwardly according to the rise of the liquid surface of the ink flowing into the air flowing space through the second air introduction passage, the float member closes an opening on the air flowing space side of the first air introduction passage. According to this structure, the float member moves upward according to the liquid surface of the ink flowing into the air flowing space of the housing through the second air introduction passage and closes the opening on the air flowing space side of the first air introduction passage before the ink reaches the opening, thereby preventing the ink from flowing out through the first air passage. Therefore, backflow of ink can be securely shut off by the backflow shutoff device having a simple structure as mentioned above without the necessity of using a complex arrangement composed of a sensor for detecting inflow of ink, an electromagnetic shutoff valve which closes the first air introduction passage according to a signal from the sensor indicating the inflow of ink. As a result of this, the respective components composing the pressure control means are prevented from being damaged with ink flowing backward. 
     In the aforementioned backflow shutoff device, it is preferable that the housing member is formed in the sub tank and is connected to an air introduction port composing the air passage on the sub tank side. According to this structure, ink flowing backward from the sub tank to the air passage reaches the backflow shutoff device soon and the backflow of the ink is shut off by the backflow shutoff device as mentioned above, thereby minimizing the range of the air passage contaminated with ink flowing backward. Therefore, the number of parts which will be forced to be replaced because of being contaminated when backflow of ink occurs is reduced. As a result of this, an effect of reducing the maintenance cost of the inkjet printer is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external perspective view showing a printer apparatus as an application example of the present invention as seen diagonally from the front. 
         FIG. 2  is an external perspective view showing the printer apparatus as seen diagonally from the back. 
         FIG. 3  is a front view showing main components of an apparatus body of the printer apparatus. 
         FIG. 4  is a perspective view showing a carriage and peripheries thereof in the printer apparatus. 
         FIG. 5  is a system diagram of an ink supply device. 
         FIG. 6  is an external perspective view showing a sub tank and a backflow shutoff mechanism disposed on the carriage. 
         FIG. 7  is a schematic block diagram of the ink supply device. 
         FIGS. 8(   a )- 8 ( b ) are illustrations showing a structure of the backflow shutoff mechanism according to the present invention, wherein  FIG. 8(   a ) is a side view of the backflow shutoff mechanism (some parts are omitted) and  FIG. 8(   b ) is a perspective view of the backflow shutoff mechanism (some parts are omitted). 
         FIG. 9(   a ) is a sectional view of the backflow shutoff mechanism taken along a line IX-IX of  FIG. 8(   a ) and  FIG. 9(   b ) is a side view of the backflow shutoff mechanism (some parts are omitted) in a state where a backflow shutoff float is moved vertically. 
         FIG. 10  is a side view of a second backflow shutoff mechanism (some parts are omitted) according to the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, preferred embodiments of the present invention will be described with reference to attached drawings. As an example of inkjet printers to which the present invention is applied, a structural example of an inkjet printer (hereinafter, referred to as “printer apparatus”) is employed in the following description. The structural example has orthogonal axes extending along a print surface of which one is used for moving a print medium and the other one is used for moving print heads, and is of a UV curable type using ultraviolet curable inks (hereinafter, referred to as “UV inks”) which are cured by an irradiation with ultraviolet light. A perspective view of a printer apparatus P of this embodiment as seen diagonally from the front is shown in  FIG. 1 , a perspective view showing the same as seen diagonally from the back is shown in  FIG. 2 , and main components of an apparatus body  1  of the printer apparatus P are shown in  FIG. 3 . First, the entire structure of the printer apparatus P will be outlined with reference to these drawings. In the following description, the directions indicated by arrows F, R, and U in  FIG. 1  will be forward, rightward, and upward directions, respectively. 
     The printer apparatus P mainly comprises an apparatus body  1  for conducting the image forming function, a feeding mechanism  3  which is disposed in front of and behind a supporting portion  2  supporting the apparatus body  1  to feed a print medium M as a non-printed material in a rolled state, and a winding mechanism  4  for winding up the print medium M which has been printed. 
     The apparatus body  1  comprises a frame  10  forming the framing structure. The frame  10  has a landscape window-like medium through portion  15  which is formed at a middle portion in the vertical direction of the frame  10  and through which the print medium M is passed in the anteroposterior direction. The frame  10  comprises a lower frame  10 L, which is positioned on the lower side of the medium through portion  15  and is provided with a platen  20  for supporting the print medium M and with a medium moving mechanism  30  for moving the print medium M supported by the platen  20  in the anteroposterior direction, and an upper frame  10 U, which is positioned on the upper side of the medium through portion  15  and is provided with a carriage  40  holding the print heads  60  and with a carriage moving mechanism  50  for moving the carriage  40  in the lateral direction. The apparatus body  1  is provided with a control unit  80  for controlling the operations of respective components of the printer apparatus P such as the anteroposterior movement of the print medium M by the medium moving mechanism  30 , the lateral movement of the carriage  40  by the carriage moving mechanism  50 , the ink ejection by the print heads  60 , and the ink supply by an ink supply device  100  as will be described later. In addition, a control panel  88  is disposed in front of the apparatus body  1 . 
     The platen  20  is mounted on the lower frame  10 L to extend in the anteroposterior direction below the medium through portion  15  and has a medium supporting portion  21  for supporting the print medium M horizontally which is an image forming area of a band-like shape extending in the lateral direction for the print heads  60 . The medium supporting portion  21  has a large number of small suction holes formed therein which communicate with a decompression chamber (not shown) formed below the medium supporting portion  21 . When the decompression chamber is set to have a negative pressure by the action of a vacuum generator, the print medium M is sucked to stick to the medium supporting portion  21  so as to prevent displacement of the print medium M during printing. 
     The medium moving mechanism  30  comprises a cylindrical feeding roller  31  which is disposed such that an upper periphery is exposed to the platen and which extends in the lateral direction, a roller driving motor  33  for rotating the feeding roller  31  via a timing belt  32 , and the like. Above the feeding roller  31 , a plurality of roller assemblies  35 , each having a pinch roller  36  rotatable in the anteroposterior direction, are disposed to be aligned in the lateral direction. The roller assemblies  35  are adapted to be selectively have a clamping position where the pinch rollers  36  are pressed against the feeding roller  31  and an unclamping position where the pinch rollers  36  are spaced apart from the feeding roller  31 . By driving the roller driving motor  33  in a state that the roller assemblies  35  are set at the clamping position so that the print medium M is clamped between the pinch rollers  36  and the feeding roller  31 , the print medium M is fed for a distance corresponding to the rotational angle of the feeding roller  31  (a drive control value outputted from the control unit  80 ) in the anteroposterior direction. It should be noted that the state where the roller assemblies  35  are set at the clamping position and the state where the roller assemblies  35  are set at the unclamping position are both shown in  FIG. 3 . 
     A guide rail  45  is attached to the upper frame  10 U extending parallel to the feeding roller  31  and the carriage  40  is supported on the guide rail  45  via a slide block (not shown) such that the carriage  40  can freely move in the lateral direction. The carriage  40  is driven by a carriage driving mechanism  50  as will be described in the following. In the carriage  40 , the print heads  60  for ejecting UV inks are disposed such that nozzle faces as the lower faces of the heads are spaced apart from the medium supporting portion  21  of the platen  20  by a predetermined gap to face the same. Generally, the print heads  60  comprise print heads of which number corresponds to the number of inks used in the printer apparatus P and which are aligned in the lateral direction. For example, in case of a printer apparatus using UV inks of four basic colors, i.e. cyan (C), magenta (M), yellow (Y), and black (K) and having ink cartridges corresponding to the respective colors, four print heads  60  (a first print head  60 C, a second print head  60 M, a third print head  60 Y, and a fourth print head  60 K) corresponding to the respective ink cartridges are provided as shown in a perspective view of the periphery of the carriage  40  in  FIG. 4 . In the carriage  40 , sub tanks  120  (a first sub tank  120 C, a second sub tank  120 M, a third sub tank  120 Y, and a fourth sub tank  120 K) of the ink supply device  100  as will be described in detail later are provided to correspond to the print heads  60 C,  60 M,  60 Y, and  60 K, respectively. A tray-shaped ink tray  180  for receiving UV inks is placed below the print heads  60  ( 60 C,  60 M,  60 Y, and  60 K) in a state that the carriage  40  is set at the reference position (so-called “home position”) when the printer apparatus does not work. The method for driving the print heads  60  (the method of ejecting ink fine particles) may be the thermal method or the piezo method. 
     On the left and right sides of the carriage  40 , UV light sources for irradiating the UV inks ejected from the print heads to the print medium M with ultraviolet lights to cure the UV inks are arranged. The UV light sources are a left UV light source  70 L located on the left side of the carriage  40  and a right UV light source  70 R located on the right side of the carriage  40  so that the first through fourth print heads  60 C  60 M,  60 Y, and  60 K arranged in the carriage  40  are sandwiched from the left and right by the left and right UV light sources  70 L,  70 R. Each of the left UV light source  70 L and the right UV light source  70 R is a light source, for example a UV lamp or UV-LED, which emits ultraviolet light of which wavelength A is in a range of from about 100 to 380 nm. The on-off actions of the left and right UV light sources  70 L,  70 R are controlled by the control unit  80  according to the movement of the carriage  40  by the carriage driving mechanism  50  and the ejection of the inks from the print heads  60 . 
     As shown in  FIG. 3 , the carriage moving mechanism  50  comprises a driving pulley  51  and a driven pulley  52  which are disposed in left and right portions of the frame  10  such that the guide rail  45  is arranged between the driving pulley  51  and the driven pulley  52 , a carriage driving motor  53  for rotating the driving pulley  51 , and an endless belt-like timing belt  55  wound around the driving pulley  51  and the driven pulley  52  with some tension. The carriage  40  is connected and fixed to the timing belt  55 . By driving the carriage driving motor  53 , the carriage  40  supported by the guide rail is moved above the platen  20  in the lateral direction for a distance according to a rotational angle of the carriage driving motor  53  (a drive controlled value outputted from the control unit  80 ). 
     As shown in  FIG. 7 , the control unit  80  comprises a ROM  81  in which a control program for controlling the actions of the respective components of the printer apparatus P is written, a RAM  82  in which a print program for forming images on the print medium M and the like are temporarily stored, an arithmetic processing section  83  which conducts arithmetic processing based on the print program read from the RAM  82  and operational signals inputted through an operational panel  88  to control the actions of the respective components according to the control program, and the operational panel  88  on which a display panel for displaying the operational state of the, printer apparatus P and various operational switches are provided, whereby the control unit  80  controls the anteroposterior movement of the print medium M by the medium moving mechanism  30 , the lateral movement of the carriage  40  by the carriage moving mechanism  50 , the ejection of inks from nozzles of the print heads  60 , the supply of inks by the ink, supply device  100 , and the like. 
     For example, in case of forming images on the print medium M based on the print program read by the control unit  80 , the print medium M and the print heads  60  are moved relative to each other by combination of the anteroposterior movement of the print medium M by the medium moving mechanism  30  and the lateral movement of the carriage  40  by the carriage moving mechanism  50 . During this, inks are ejected onto the print medium M from the print heads  60  and the UV light source, positioned behind the carriage  40  in the moving direction, (for example, the left UV light source  70 L when the carriage is moved rightward) is turned ON, thereby forming image of information according to the print program. 
     In the printer apparatus P having the structure outlined in the above, UV inks are supplied to the print heads  60  disposed on the carriage  40  by the ink supply device  100 .  FIG. 5  is a system diagram of the ink supply device  100 ,  FIG. 6  is an external perspective view of the sub tank  120  and a backflow shutoff mechanism  200 , and  FIG. 7  is a schematic block diagram of the ink supply device  100 . 
     As shown in  FIG. 5 , the ink supply device  100  comprises the sub tanks  120  connected to the print heads  60 , main tanks  110  which are connected to the sub tanks  120  and in which UV inks to be supplied to the sub tanks  120  are stored, a sub tank depressurizing unit  140  for reducing the inner pressure of the sub tanks  120  to negative pressure, a sub tank pressurizing unit  150  for increasing the inner pressure of the sub tanks  120  to positive pressure, ink sending units  115  for sending the UV inks stored in the main tanks  110  to the sub tanks  120 , and the like. The sub tank depressurizing unit  140  and the sub tank pressurizing unit  150  have a common single air pump  160 . 
     The main tanks  110  are designed to store the UV inks of volume corresponding to the consumption quantities per a unit period of time in the printer apparatus P. In this embodiment, corresponding to the aforementioned four colors C. M, Y, and K, cartridge type main tanks  110  (a first main tank  110 C, a second main tank  110 M, a third main tank  110 Y, and a fourth main tank  110 K) of about 500 ml for the respective colors are used. These main tanks  110  are detachably attached to the back surface of the apparatus body  1  (see  FIG. 2 ). The form of the main tanks  110  may be another form such as a cylindrical vessel or a flexible envelope. The installation position of the ink tanks may be suitably set at the front face or the top of the apparatus body  1 , or a position separate from the apparatus body  1 . 
     As shown in  FIG. 6 , the sub tank  120  comprises a reservoir member  121  having a thin box-like shape which opens to one side (the right) and is long in the vertical direction as seen in a side view, and a lid member  122  for covering and closing an opening of the reservoir member  121 . Inside a tank which is formed by closing with the lid member  122 , an ink storage chamber  123  for storing UV ink is formed. In addition, a float receiving portion  124  is formed which is a groove-like portion extending vertically on the rear side of the ink storage chamber  123 . Inside the float receiving portion  124 , a disc-like level detecting float  134 , which has a magnet fixed to the center thereof and floats on the UV ink, is accommodated to freely move in the vertical direction. 
     As for the sub tank  120 , the lid member  122  is integrally attached to the reservoir member  121  by applying sealant or adhesive on the peripheries of the opening of the reservoir member  121  and is strongly connected by fastening means such as screws (not shown) so that the ink storage chamber  123  is held in the sealed state. At least one of the lid member  122  and the reservoir member  121  is made of a transparent or semi-transparent material for the purpose of observing the storing state of UV ink in the ink storage chamber  123  and the floating state of the level detecting float  134  on the UV ink from the outside. In this embodiment, the lid member  122  is made of a transparent material. 
     Formed in the bottom side of the sub tank  120  is a short cylindrical connecter portion  125  projecting downwardly from a bottom wall  121   b  of the reservoir member  121 . Above the connector portion  125 , a block-like duct portion  126  is formed to extend from the bottom wall  121   b  into the inside of the ink storage chamber  123  upwardly. A first derivation passage  127   a  is formed to penetrate vertically the bottom wall  121   b  to connect the bottom of the ink storage chamber  123  and the connecter portion  125  and a second derivation passage  126   b  is formed to penetrate vertically the duct portion  126  and the bottom wall  121   b  to connect the top  126   a  of the duct portion  126  and the connector portion  125 . Therefore, the ink storage chamber  123  of the sub tank  120  and the ink chamber of the print head  60  are connected to each other via the first derivation passage  127   a  and the second derivation passage  126   b.  It should be noted that disposed between the tube  69  and the print head  60  is a filter  61  for filtering UV ink passing therethrough. 
     On the rear side of the sub tank  120 , a sub tank reserve detecting unit  130  for detecting the reserved state of the UV ink in the ink storage chamber  123  is provided. The sub tank reserve detecting unit  130  comprises the level detecting float  134  which is accommodated in a float receiving portion  124  extending in the vertical direction such that the level detecting float  134  can freely move in the vertical direction and thus moves in the vertical direction according to the surface of the UV ink in the ink storage chamber and a level detection plate  135  which detects the liquid surface level of the UV ink by detecting the level detecting float  134 . 
     Formed in a rear wall  121   r  of the reservoir member  121  is a plate receiving portion  131  which has a dovetail groove-like shape extending in the vertical direction. The level detection plate  135 , to which a plurality of magnetic sensors  136  ( 136 H,  136 L) are attached, is installed and fixed to the plate receiving portion  131 . That is, the level detection plate  135  is disposed to face the level detecting float  134  via the rear wall  121   r.  The magnet fixed to the level detecting float  134  in the ink storage chamber  123  (the float receiving portion  124 ) is detected by the magnetic sensors  136 , thereby detecting the vertical position of the level detecting float  134 , that is, detecting the surface level of the UV ink retained in the ink storage chamber  123 . 
     This embodiment employs such an arrangement that, as the magnetic sensors  136 , two magnetic sensors, that is, a Hi detection sensor  136 H for detecting that the ink storage chamber  123  is filled with UV ink so that the surface of the UV ink is at a reference level for filling and a Lo detection sensor  136 L for detecting that the UV ink in the ink storage chamber  123  is consumed and is thus at a level lower than a predetermined value are attached to the level detection plate  135 . It should be noted that an arrangement in which three or more magnetic sensors  136  are attached so as to sequentially detect changes in the surface level in the ink storage chamber according to changes in magnetism may be employed. An output signal from the level detection plate  135  is inputted into the control unit  80 . 
     On the front side of the sub tank  120 , an ink introduction passage is formed at a middle position in the vertical direction to penetrate the front wall  121   f  of the reservoir member  121  in the anteroposterior direction and a tube connector  128  is connected to the ink introduction passage. On the upper side of the sub tank  120 , an air introduction passage is formed to penetrate the top wall  121   t  of the reservoir member  121  and a tube connector  129  is connected to the air introduction passage. Inside the ink storage chamber  123  below the air introduction passage (the tube connector  129 ), a backflow prevention section  132  is attached to the top wall  121   t.    
     The backflow prevent on section  132  comprises float supporting members  132   a  which are paired as front and rear members and which extend downwardly from the top wall  121   t  and are folded forward and backward, and a sealing float  133  which is vertically movably accommodated in a sealing float accommodation space formed between the front and rear float supporting members  132   a  so that the sealing float  133  freely moves in the vertical direction together with the liquid surface of the UV ink in the ink storage chamber. When the sealing float  133  moves to the uppermost position in the sealing float accommodation space together with the liquid surface of the UV ink and reaches the uppermost position, the sealing float  133  comes in contact with the top wall  121   t  and closes the lower end opening of the aforementioned air introduction passage. The sealing float  133  is structured such that the sealing float  133  is floated on the liquid surface of the UV ink and moves vertically together with the liquid surface of the UV ink, but the sealing float  133  is not moved vertically due to suction force when air in the ink storage chamber  123  is sucked through the air introduction passage by the sub tank depressurizing unit  140  or the like. 
     As shown in  FIG. 5 , each of the ink sending units  15  is composed of a main supply route  116  connecting the main tank  110  and the sub tank  120 . The main supply route  116  comprises an ink suction line  117   a  of which one end is connected to the main tank  110  and the other end is connected to a feed pump  118 , an ink delivery line  117   b  of which one end is connected to the feed pump  118  and the other end is connected the tube connector  128  of the sub tank  120 , and the feed pump  118  which is disposed in the apparatus body  1  between the main tank  110  and the sub tank  120  to suck UV ink stored in the main tank  110  through the ink suction line  117   a  to supply the UV ink to the sub tank  120  through the ink delivery line  117   b.    
     The feed pump  118  is a pump capable of sucking the UV ink from the main tank  110  and sending the UV ink into the sub tank  120  even in a state that the ink suction line  117   a  is not filled with the UV ink, that is, the UV ink is mixed with air and also capable of cutting off the pressure from the ink suction line  117   a  and the ink delivery line  117   b.  For example, a tube pump or a diaphragm pump may be preferably used as the feed pump  118 . 
     The sub tank depressurizing unit  140  is composed of a negative pressure route  141  connecting the sub tank  120  and an inlet  161  of the air pump  160 . The negative pressure route  141  comprises an air chamber  142  composed of a sealed vessel, a pressure sensor  144  for detecting pressure of the negative pressure route  141 , a negative pressure control valve  145  for opening and closing the negative pressure route  141 , and lines  147  ( 147   a,    147   b,    147   c,    147   d ) composed of tubes connecting these components to connect the inlet  161  of the air pump and the sub tank  120 , the main components being shown and surrounded by a frame A in  FIG. 5 . It should be noted that components surrounded by a frame C in  FIG. 5  are disposed in the carriage  40  and components outside of the frame C are disposed in the apparatus body  1 . 
     The air chamber  142  is connected to the inlet  161  of the air pump through the line  147   a  so that air in the chamber is discharged by the action of the air pump  160  so as to reduce the pressure of the air chamber  142  into a negative pressure state. The air chamber  142  is provided with an air introduction line  147   i  for introducing air into the chamber of which pressure is reduced into a negative pressure. The air introduction line  147   i  has a flow regulating valve  143   a  for adjusting the flow rate of air and an air filter  143   b  for dust removal. 
     In a state that the air pump  160  and the sub tank  120  are connected via the negative pressure route  141 , the flow regulating valve  143   a  keeps the inner pressure of the air chamber  142  constant by adjusting the flow rate of air entering into the air chamber  142 . Therefore, the inner pressure of the ink storage chambers  123  is set to be a predetermined value (for example, −1.2 kPa: hereinafter referred to as “preset negative pressure”) in a range of from about −1 to −2 kPa which is suitable for meniscus formation at the nozzles of the print heads  60 . As mentioned above, the air chamber  142  functions as a buffer tank which absorbs pulsation in air suction by the action of the air pump  160  and keeps the inner pressure of the sub tanks  120  at the constant preset negative pressure. 
     The negative pressure control valve  145  is an electromagnetic value which is positioned between the air chamber  142  and the sub tanks  120  and is disposed in the carriage  40  and which switches the line  147   c  on the air chamber  147  side and the line  147   d  on the sub tank  120  side between the connected state and the disconnected state and. In this embodiment, a three-way valve is employed as the negative pressure control valve  145  so that the line  147   c  is connected to a common port (COM) of the negative pressure control valve  145 , the line  147   d  is connected to a normal open port (NO) of the negative pressure control valve  145 , and a normal closed, port (NC) of the negative pressure control valve  145  is opened to atmosphere via a line  147   x  and a silencer  148 . 
     Therefore, when the negative pressure control valve  145  is in the OFF state (during normal operation such as printing or waiting), the line  147   c  and the line  147   d  are connected so as to set the negative pressure route  141  in the communicating state so that the inlet  161  of the air pump  160  and the sub tanks  120  are connected via a converging route  171  as will be described later. On the other hand, when the negative pressure control valve  145  is in the ON state (such as during the ink filling or cleaning), the line  147   c  and the line  147   d  are disconnected so that the negative pressure route  141  is shut off and, at the same time, the line  147   c  is connected to the line  147   x  so as to open a route on the inlet side of the air pump  160  to the atmosphere. The negative pressure control valve  145  is connected to the control unit  80  so that the ON/OFF of the negative pressure control valve  145  is controlled by the control unit  80 . 
     The pressure sensor  144  is a pressure sensor of a gauge pressure type which has a detection range about ±5 kPa and is disposed between the air chamber  142  and the negative pressure control valve  145 . The pressure sensor  144  detects the pressure of the line  147  near the sub tanks. Specifically, in a state that the negative pressure control valve  145  is turned OFF so that the air pump  160  and the sub tanks  120  are connected via the negative pressure route  141 , a pressure (for example, a pressure of about −1.3 kPa) obtained by adding a pressure loss due to the line reaching the sub tanks  120  to aforementioned preset negative pressure is detected by the pressure sensor  144 . Paradoxically, the inner pressure of the ink storage chambers  123  is set to the preset negative pressure by setting the flow regulating valve  143   a  (default setting) such that the pressure detected by the pressure sensor  144  becomes to the aforementioned pressure value. Therefore, it is possible to detect whether or not the pressure in the ink storage chambers  123  is set to the preset negative pressure by monitoring the detected pressure of the pressure sensor  144 . The detection signal of the pressure sensor  144  is inputted into the control unit  80 . 
     The sub tank pressurizing unit  150  is composed of a positive pressure route  151  connecting the sub tanks  120  and an outlet  162  of the air pump  160 . The positive pressure route  151  comprises a flow regulating valve  153   a  for adjusting the flow rate of air flowing into the positive pressure route  151 , an air filter  153   b  for removing dust from air flowing toward the sub tanks  120 , a pressure sensor  154  for detecting the pressure of the positive pressure route  151 , a positive pressure control valve  155  for opening and closing the positive pressure route  151 , and lines  157  ( 157   a,    157   b,    157   c,    157   d ) composed of tubes connecting these components to connect the outlet  162  of the air pump and the sub tanks  120 , the main components being shown and surrounded by a frame B in  FIG. 5 . 
     The flow regulating valve  153   a  is a valve for preventing the inner pressure of the ink storage chambers  123  from rising to a value exceeding a predetermined value by adjusting the flow rate of air flowing through the positive pressure route  151  in a state where the air pump  160  and the sub tanks  120  are connected via the positive pressure route  151 . The flow regulating valve  153   a  adjusts the flow rate such that the inner pressure of the sub tanks  120  becomes about 20 kPa. 
     The positive pressure control valve  155  is an electromagnetic value which is positioned between the flow regulating valve  153   a  and the sub tanks  120  and is disposed in the carriage  40  and which switches the line  157   c  and the line  157   d  between the connected state and the disconnected state. In this embodiment, a three-way valve is employed as the positive pressure cont of valve  155  so that the line  157   c  is connected to a common port (COM) of the positive pressure control valve  155 , the line  157   d  is connected to a normal closed port (NC) of the positive pressure control valve  155 , and a normal open port (NO) of the positive pressure control valve  155  is opened to atmosphere via a line  157   x  and a silencer  158 . 
     Therefore, when the positive pressure control valve  155  is in the OFF state (during normal operation such as printing or waiting), the line  157   c  and the line  157   d  are disconnected so that the positive pressure route  151  is shut off and, at the same time, the line  157   c  is connected to the line  157   x  so as to open the positive pressure route on the outlet side of the air pump  160  to the atmosphere. On the other hand, when the positive pressure control valve  155  is in the ON state (such as during the ink filling or cleaning), the line  157   c  and the line  157   d  are connected so as to set the positive pressure route  151  in the communicating state so that the outlet  162  of the air pump  160  and the sub tanks  120  are connected via the converging route  171 . The positive pressure control valve  155  is connected to the control unit  80  so that the ON/OFF of the positive pressure control valve  155  is controlled by the control unit  80 . 
     The pressure sensor  154  is a pressure sensor of a gauge pressure type which has a detection range about ±50 kPa and is disposed in the carriage  40 . The pressure sensor  154  detects the pressure of the line  157  near the sub tanks. Specifically, in a state that the positive pressure control valve  155  is turned ON so that the air pump  160  and the sub tanks  120  are connected via the positive pressure route  151 , a pressure applied to the sub tanks  120  is detected. Therefore, it is possible to detect whether or not the pressure in the ink storage chambers  123  is set to the preset positive pressure by monitoring the detected pressure of the pressure sensor  154 . The detection signal of the pressure sensor  154  is inputted into the control unit  80 . 
     The air pump  160  is a pump which sucks air from the negative pressure route  141  connected to the inlet  161 , and discharges the sucked air into the positive pressure route  151  connected to the outlet  162  and which is thus in a form of producing a predetermined positive pressure and a predetermined negative pressure at the outlet  162  and the inlet  161 , respectively. That is, the air pump  160  produces a predetermined negative pressure at the inlet  161  when the negative pressure route  141  is closed, while the air pump  160  produces a predetermined positive pressure at the outlet  162  when the positive pressure route  151  is closed. For example, as this pump, a diaphragm pump capable of producing positive and negative pressures of about ±40 kPa is preferably employed. 
     The negative pressure route  141  and the positive pressure route  151  converge on the way to the sub tanks  120  so that the converging route  171  is formed. The converging route  171  comprises a line  177  which is connected to the sub tanks and on which the line  147   d  of the negative pressure route and the line  157   d  of the positive pressure route are converged, and a converging route switch valve  175  which is provided on the line  177  for opening and closing the converging route  171 . The converging route switch valves  175  are provided to correspond to the sub tanks  120 , respectively. In this embodiment, the converging route  171  (the line  177 ) is branched into four routes at the converging route switch valve  175  so that the converging route switch valve  175  is designed to open and close the branched converging routes (lines  177 C,  177 M,  177 Y, and  177 K, numerals of some of which are omitted), respectively. 
     That is, the converging route switch valve  175  is an electromagnetic valve of manifold type having a common input port connected to the line  177 , four valves and output ports corresponding to the four sub tanks so that the first through fourth converging switch valves  175 C,  175 M,  175 Y, and  175 K corresponding to the first through fourth sub tanks  120 C,  120 M,  120 Y, and  120 K can independently open and close the converging route  171 . The operation of the converging route switch valve  175  is controlled by the control unit  80 . 
     The number of branches of the converging route  171  may be arbitrarily set according to the number of the print heads  60 . For example, in case of a structure having a single print head  60 , the converging route switch valve  175  may use a single electromagnetic shut-off valve. In case of a printer apparatus having eight print heads, the converging route switch valve  175  may use an eight-port type electromagnetic valve (or two four-port type electromagnetic valves) as shown in  FIG. 5 . 
     Each line  177  connecting the converging route switch valve  175  and the sub tank  120  is provided with a backflow shutoff mechanism  200  which shuts off the line  177  when the UV ink flows out from the sub tank  120  toward the converging route switch valve  175  through the line  177  (this phenomenon will be called “backflow of UV ink”) (see  FIG. 5  and  FIG. 6 ). That is, in this embodiment, the lines  177 C,  177 M,  177 Y,  177 K connecting the first through fourth sub tanks  120 C,  120 M,  120 Y,  120 K and the first through fourth converging route switch valves  175 C,  175 M,  175 Y,  175 K are provided with the first through fourth backflow shutoff mechanisms  200 C,  200 M,  200 Y,  200 K, respectively. 
     As shown in  FIG. 8 , the backflow shutoff mechanism  200  mainly comprises a cylindrical housing  202  having an inner space  201  and a backflow shutoff float  203  which is vertically movably accommodated in the inner space  201  and which moves vertically together with the liquid surface of the UV ink entering in the inner space  201 . In  FIGS. 8(   a ),  8 ( b ) and  9 ( b ), the housing  202  is shown by chain double-dashed lines for the purpose of showing the structure inside of the housing  202 . 
     In an upper portion of the backflow shutoff mechanism  200 , a connector mounting hole is formed to vertically penetrate the top wall of the housing  202  and an upper tube connector  204  is fitted in the connector mounting hole so that a lower portion of the upper tube connector  204  projects into the inner space  201 . The line  177  on a side of the converging route switch valve  175  is connected to an upper portion of the upper side tube connector  204  so that the converging route  171  and the inner space  201  of the housing are connected through an introduction passage  204   a  vertically penetrating the upper tube connector  204 . Also in a lower portion of the backflow shutoff mechanism  200 , a connector mounting hole is formed in the bottom of the housing  202  and a lower tube connector  205  is fitted in the connector mounting hole so that an upper portion of the lower tube connector  205  is exposed to the inner space  201 . The line  177  on a side of the sub tank  120  is connected to a lower portion of the lower tube connector  205  so that the inner space  201  of the housing and the sub tank  120  are connected through the line  177  and a derivation passage  205   a  vertically penetrating the lower tube connector  205 . 
     In the inner space  201  of the housing  202 , a cylindrical supporting member  206  opening in the vertical direction is disposed on the upper end of the lower tube connector  205  exposed to the inner space  201  at the bottom of the housing  202 . The backflow shutoff float  203  is disposed on the supporting member  206 . The supporting member  206  has through holes  206   a  formed in a peripheral surface thereof. Normally (when no backflow UV ink is stored in the inner space  201 ), the communication between the inner space  201  of the housing and the derivation passage  205   a  of the lower tube connector is allowed through the through holes  206   a.    
     The backflow shutoff float  203  is formed in a disk-like shape made of a material capable of floating in the UV ink and is put on the supporting member  206  to close the upper open end of the supporting member  206 . A disk-like sealing rubber  207  is attached to the upper surface of the backflow shutoff float  203 . When the backflow shutoff float  203  moves to the uppermost position together with the liquid surface of the UV ink, the sealing rubber  207  comes in contact with the lower end of the upper tube connector  204  projecting from the top wall into the inner space  201  of the housing  202  to close the lower open end of the introduction passage  204   a  of the upper tube connector. While the backflow shutoff float  203  floats n the UV ink and vertically moves together with the liquid surface of the UV ink as mentioned above, the backflow shutoff float  203  does not move vertically due to force of sucking air within the inner space  201  of the housing through the introduction passage  204   a  of the upper tube connector by the sub tank depressurizing unit  140 . 
     At the center of the upper surface of the backflow shutoff float  203 , a rod-like guide member  208  extending upwardly from the upper surface to a location near the lower end opening of the introduction passage  204   a  of the upper tube connector. The outer diameter of the guide member  208  is smaller than the diameter of the introduction passage  204   a.  The vertical movement of the backflow shutoff float  203  is guided as follows. When the backflow shutoff float  203  moves upwardly together with the liquid surface of the UV ink, the guide member  208  enters into the introduction passage  204   a  so that the sealing rubber  207  securely closes the lower end opening of the introduction passage  204   a  when the backflow shutoff float  203  reaches the uppermost position. The vertical movement of the backflow shutoff float  203  is guided so that the backflow shutoff float  203  securely closes the upper end opening of the supporting member  206  when the backflow shutoff float  203  reaches the lowermost position (i.e. normally). 
     As mentioned above, the negative pressure route  141  and the positive pressure route  151  are connected to the air pump  160  so as to form a single continuous route and distal ends of both the routes are converged so as to form a closed-loop-like pressurizing and depressurizing circuit. The converging route  171  to which the both routes are converted is connected to the sub tanks  120  through the converging route switch valve  175  and the backflow shutoff mechanisms  200  so that the ink storage chambers  123  of the sub tanks  120  are switchable between the depressurized state and the pressurized state by controlling the switching between the negative control valve  145  and the positive control valve  155 . 
     In the ink supply device  100  having the aforementioned structure, the operations of the feed pumps  118 , the negative pressure control valve  145 , the positive pressure control valve  155 , and the air pump  160  are controlled by the control unit  80  in the following manner. As apparent from the aforementioned description, the four systems (C, M, Y, and K) as systems for supplying UV inks have the same structures so that description will be made as regard to a single system by omitting subscripts representing respective systems. 
     (Control During Normal Operation) 
     As the main power switch of the printer apparatus P is turned ON, the control unit  80  reads out the control program stored in the ROM  81  and controls the operation of respective components of the printer apparatus according to the read control program. In the ink supply device  100 , electric power is supplied to the air pump  160  to set the air pump  160  to the rotational driven state and all of the converging route switch valves  175  are turned ON. At this point, the negative pressure control valve  145  and the positive pressure control valve  155  are both kept in the OFF state. Therefore, in the negative pressure route  141 , the communication between the line  147   c  and the line  147   d  is allowed so as to connect the inlet  161  of the air pump  160  and the ink storage chamber  123  of the sub tank  120  through the line  147  and the line  177 . In the positive pressure route  151 , the line  157   c  and the line  157   x  are connected so as to open the route on the outlet side of the air pump  160  to atmosphere. 
     Accordingly, air in the line  147  connected to the inlet  161  of the air pump is sucked to reduce the inner pressure of the air chamber  142  to a negative pressure so that the inner pressure of the air chamber  142  is stabilized at a substantially constant value defined according to the balance between the flow rate of entering air adjusted by the flow regulating valve  143   a  and the amount of air sucked by the air pump  160 . As mentioned above, the inner pressure is set to be a predetermined negative pressure value (for example, a preset negative pressure of −1.2 kPa) in a range of from about −1 to −2 kPa which is suitable for meniscus formation at the nozzle portion of the print head  60 , that is, the inner pressures of all of the ink storage chambers  123  of the four sub tanks are stably held at the same preset negative pressure. 
     At this time, in the positive pressure route  151 , the route on the outlet side of the air pump  160  is opened to atmosphere so that air sucked from the negative pressure route  141  is released to atmosphere through the silencer  158 . Therefore, the pressure (back pressure) of the positive pressure route  151  does not rise so that the suction efficiency of the air pump  160  is not lowered, thereby keeping the stable negative pressure state. 
     According to the detected signal inputted from the pressure sensor  144  to the control unit  80 , it can be determined whether the inner pressure of the ink storage chamber  123  is kept at the preset negative pressure. When it is determined that the pressure detected by the pressure sensor  144  is out of a certain range from the preset negative pressure, for example, the detected pressure exceeds ±20% relative to the preset negative pressure, the control unit  80  may alarm that the pressure is out of the range (the pressure singularity of the negative pressure route). In this case, it is checked whether the respective components of the negative pressure route are normal. When all of these are normal, the inner pressure of the ink storage chamber is set to the proper preset negative pressure by adjusting the flow regulating valve  143   a.    
     In operation, normally, some degree of UV ink is stored in the ink storage chamber  123  of the sub tank  120 . In accordance with the start of the print program or the like, the UV ink retained in the ink storage chamber  123  is ejected from the nozzles of the print head  60  and is thus consumed so that the UV ink retained is gradually reduced. Since the ink supply device  100  is provided with the sub tank reserve detecting unit  130 , the UV ink stored in the main tank  110  is supplied to the sub tank by the ink sending unit  115  when the amount of the UV ink retained in the ink storage chamber  123  becomes a predetermined amount or less, thereby replenishing the sub tank with the UV ink. 
     Specifically, When the UV ink retained in the ink storage chamber  123  is reduced and the residual amount of the UV ink becomes a predetermined value or less, the level detecting float  134  moving vertically together with the liquid surface of the UV ink is detected by the Lo detection sensor  136 L which is disposed on the level detection plate  135 . The control unit  80  receives the detection signal of the Lo detection sensor  136 L from the level detection plate  135  and actuates the feed pump  118  in a state that the inner pressure of the ink storage chamber  123  is reduced to be a negative pressure. The UV ink sent from the main tank  110  by the feed pump is supplied to the ink storage chamber  123  through the line  117   b  and the tube connector  128  so as to increase the amount of the ink stored in the ink storage chamber. When the level detecting float  134  is detected by the Hi detection sensor  136 H, the feed pump  118  is stopped, thereby completing the replenishment of the UV ink to the ink storage chamber  123 . 
     In this manner, since the replenishment of the UV ink to the sub tank  120  is conducted in the state that the pressure in the ink storage chamber  123  is reduced, the UV ink is securely sent even with the small-size feeding pump and ink never leaks or dribbles from the nozzles of the print head  60  during the replenishment of the UV ink. If the pressure in the ink storage chamber  123  rises during the replenishment of the UV ink according to the relationship between the volume of the ink storage chamber  123  and the volume of the air chamber  142 , the pressure in the ink storage chamber  123  is kept constant without being increased by increasing the rotation speed of the air pump  160  or reducing the opening degree of the flow regulating valve  143   a,  or conducting both of these according to the pressure detected by the pressure sensor  144 . As the printer apparatus P is actuated, the air pump  160  is continuously operated so that the inner pressure of the sub tank  120  is always kept at the preset negative pressure whenever the print program is conducted even when waiting. 
     As mentioned above since the replenishment of the UV ink is conducted in the state that the ink storage chamber  123  is decompressed, there is an advantage that the replenishment of the UV ink is securely conducted and ink never leaks or dribbles from the nozzles of the print head  60 , while there is a possibility that the UV ink flows out (flows backward) from the air introduction passage formed in the top wall  121   t  of the reservoir member  121  to the converging route switch valves  175  through the tube connector  129  and the line  177  when, for example, the feed pump  118  is not stopped due to failure of the Hi detection sensor  136 H or the like so that the replenishment of the UV ink is continued even after the liquid surface of the UV ink in the ink storage chamber  123  rises to exceeds the reference level for filling. For this, the ink storage chamber  123  is, provided with the backflow prevention section  132  of which the sealing float  133  rises together with the liquid surface of the UV ink and comes in contact with the top wall  121   t  when the liquid surface of the UV ink in the ink storage chamber rises and exceeds the reference level for filling, thereby closing the, opening at the lower end of the aforementioned air introduction passage and thus preventing the backflow of the UV ink. 
     However, there is a possibility that the backflow prevention section  132  fails to work because the rise of the liquid surface is too fast to allow the sealing float to catch up with the rise of the liquid surface of the UV ink or because the sealing float  133  is stuck to the float supporting member  132   a  and thus does not rise together with the liquid surface of the UV ink. For this, the backflow shutoff mechanism  200  is disposed on the line  177  connecting the sub tank  120  and the converging route switch valve  175 , whereby the backflow of the UV ink is shut off by the backflow shutoff mechanism  200  when the UV ink flows backward from the sub tank  120  through the line  177 . 
     Specifically, the UV ink, flowing out from the air introduction passage of the top wall  121   t  through the tube connector  129  and the line  177  because the backflow prevention section  132  in the ink storage chamber fails to work, flows into the housing  202  through the derivation passage  205   a  of the lower tube connector  205  connected to the line  177  and is initially stored in the inner space  201  of the housing through the through holes  206   a  formed in the peripheral surface of the supporting member  206  (see arrows in  FIG. 9(   a )). Then, when the liquid surface of the UV ink in the inner space  201  exceeds the upper end of the supporting member  206 , the backflow shutoff float  203  is pushed upwardly by the UV ink flowing into the inner space  201  through the opening formed in the upper end of the supporting member  206  so as to rise together with the liquid surface of the UV ink, so that the sealing rubber  207  attached to the upper surface of the float comes in contact with the lower surface of the upper tube connector  204  so as to close the opening at the lower end of the introduction passage  204   a  of the upper tube connector  204  (see  FIG. 9(   b )). 
     As mentioned above, when the UV ink flows backward from the sub tank  120  and reaches the backflow shutoff mechanism  200 , the backflow shutoff float  203  rises together with the liquid surface of the UV ink in the inner space  201  and the opening at the lower end of the introduction passage  204   a  of the upper tube connector is closed by the sealing rubber  207 , thereby preventing the UV ink from flowing out of the upper, tube connector  204  toward the converging route switch valve  175 . By this backflow shutoff mechanism  200  having the simple structure as mentioned above, the backflow of the UV ink is securely shut off. As a result of this, the converging route switch valve  175 , the negative pressure control valve  145 , and the like are prevented from being damaged with UV ink flowing backward from the sub tank  120 . Since the backflow shutoff float  203  is in contact only with the upper edge of the cylindrical supporting member  206  in the housing  202 , the backflow shutoff float  203  never be stick to the other member, thereby ensuring the performance of the backflow shutoff action. 
     (Control During Ink Filling) 
     At the time of the initial filling of UV ink or the start up after nozzle cleaning with cleaning liquid, there is a case that any UV ink does not exist in the ink chamber of the print head  60  and the line  117  of the main supply route. At the restart after it has been left for a predetermined period of time or the restart after replacement of the main tank, there is a case that air bubbles are mixed in the UV ink. In such a case, according to the ink filling command inputted from the operational panel  88  into the control unit  80 , the control for the ink filling is carried out as follows. 
     As a command for carrying out the ink filling is inputted into the control unit  80  by pushing a function key or the like of the operational panel  88  to select an “ink filling” process and specify one of the print heads  60 , the arithmetic processing section  83  carries out a process of turning ON the converging route switch valve corresponding to the print head, of which ink filling is required, and turning OFF the other converging route switch valves in the state the inner pressure of the sub tank is kept to be the preset negative pressure for the normal operation (that is, the negative pressure control valve  145  and the positive pressure control valve  155  are both in the OFF state) (negative pressure keeping step). For example, in case that only the first print head  60 C is selected as the print head, of which the ink filling is required, by the operational panel  88 , only the first converging route switch valve  175 C corresponding to the first print head  60 C is turned ON and the second through fourth converging route switch valves  175 M,  175 Y,  175 K corresponding to the second through fourth print heads are turned OFF (hereinafter, description will be made with reference to this case). 
     The ink is sent from the first main tank  110 C to the first sub tank  120 C of which inner pressure is reduced, thereby filling the first sub tank  120 C with the ink (ink replenishment step). That is, only the feed pump  118 C corresponding to the first sub tank  120 C is actuated, whereby the UV ink stored in the first main tank  110 C is supplied to the first sub tank  120 C. The feed pump  118 C is stopped when the level detection float  134  is detected by the Hi detection sensor  136 H. Accordingly, an enough amount of the UV ink is stored in the ink storage chamber  123  of the first sub tank  120 C. If the level detection float  134  is already detected by the Hi detection sensor  136 H so that it is determined that UV ink is stored to reach the reference level for filling at the start of the ink replenishment step, the ink replenishment step is skipped and the next step (print head ink filling step) is conducted. 
     Then, the negative pressure route  141  is shut off and the inner pressure of the first sub tank  120 C is increased into a positive pressure by the sub tank pressurizing unit  150 , thereby dropping a part of the UV ink stored in the first sub tank  120 C from the first print head  60 C (print head ink filling step). Specifically, the control unit  80  turns ON the negative pressure control valve  145  to shut off the communication between the line  147   c  and the line  147   d  and connect the line  147   c  to the line  147   x  so as to open the route on the inlet side of the air pump  160  to the atmosphere. In addition, the control unit  80  turns ON the positive pressure control valve  155  to allow the communication between the line  157   c  and the line  157   d  so as to connect the outlet  162  of the air pump and the ink storage chamber  123  of the first sub tank  120 C. 
     By this switch control, the communication between the air pump  160  and the first sub tank  120 C through the negative pressure route  141  is shut off while the communication between the air pump  160  and the first sub tank  120 C through the positive pressure route  151  is allowed so that air discharged from the outlet  162  of the air pump  160  is supplied to the ink storage chamber  123  of the first sub tank  120 C through the line  157 , the line  177 , the line  177 C, and the first backflow shutoff mechanism  200 C. As mentioned above, the feed pump  118  is a pump capable of shutting off the pressure applied from either of the ink suction line  117   a  and the ink sending line  117   b  on the both sides of the feed pump  118 . Therefore, the UV ink in the first sub tank  120 C never flows backward to the first main tank  110 C so that the inner pressure of the first sub tank  120 C is increased and becomes in the positive pressure state having a pressure (for example, about 20 kPa) set by the adjustment of the flow regulating valve  153   a.  As a result, the UV ink stored in the ink storage chamber  123  of the first sub tank  120 C is forced through the first derivation passage  127   a  in a lower portion of the tank and the second derivation passage  126   b  and is supplied to the first print head  60 C. Then, the UV ink dropping from the nozzles of the first print head  60 C is received by the ink tray  180 . 
     During this, since the route on the inlet side of the air pump  160  in the negative pressure route  141  is opened to atmosphere, the air pump  160  is operated with little or no load on the inlet side. Therefore, the suction pressure of the air pump never be reduced so that the discharge efficiency also never be reduced, thereby securely filling the ink chamber of the first print head  60 C with the UV ink. In addition, the first sub tank  120 C can be switched between the depressurized state and the pressurized state by simple control of turning on the negative control valve  145  and the positive control valve  155  with keeping the rotation of the air pump  160  in a certain direction. 
     The print head ink filling step is continued until the ink chamber is filled with UV ink and the UV ink drops from the nozzles at the lower surface of the head even when the ink chamber of the first print head  60 C is empty. For example, a timer is set for duration of this step (time setting) based on the time required for supplying the UV ink to the first print head  60 C of which the ink chamber is empty by pressurizing the first sub tank  120 C until a certain amount of UV ink flows out of the nozzles. Alternatively, it is defined (according to the amount of ink) that it is a time when the UV ink stored in the ink storage chamber  123  flows out and the level detection float  134  is detected by the Lo detection sensor  136 L disposed on the level detection plate  135 . 
     By the print head ink filling step, the areas from the ink storage chamber  123  of the first sub tank  120 C to the nozzles of the first print head  60 C are filled with the UV ink. At this point, the air bubbles, if any, in the lines are forced out through the nozzles so that the area from the first sub tank  120 C to the first print head  60 C is filled with the UV ink. Then, the process proceeds to the next step (sub tank filling step). At this point, the converging route switch valves  175  other than the first converging route switch valve  175 C are in the closed state so that the inner pressures of the second through fourth sub tanks are held in the initial negative pressure. 
     Then, the positive pressure route  151  is shut off and the inner pressure of the first sub tank  120 C is reduced to a negative pressure by the sub tank depressurizing unit  140 . The ink is sent from the first main tank  110 C into the first sub tank  120 C with the reduced pressure by the ink sending unit  115 , thereby filling the first sub tank  120 C with the UV ink (sub tank ink filling step). That is, the control unit  80  turns OFF the positive pressure control valve  155  to shut off the communication between the line  157   c  and the line  157   d  and connect the line  157   c  to the line  157   x  so as to open the route on the outlet side of the air pump  160  to the atmosphere. In addition, the control unit  80  turns OFF the negative pressure valve  145  to allow the communication between the line  147   c  and the line  147   d  and connect the inlet  161  of the air pump to the ink storage chamber  123  of the first sub tank  120 C. 
     By this switch control, the communication between the air pump  160  and the first sub tank  120 C is shut off in the positive pressure route  151 , while the air pump  160  and the first sub tank  120 C are connected in the negative pressure route  141  so that air in the ink storage chamber  123  of the first sub tank is sucked by the air pump  160  through the first backflow shutoff mechanism  200 C, the line  177 C, the line  177  and the line  147 . Accordingly, the inner pressure of the first sub tank  120 C is reduced from a positive pressure to a negative pressure. The control unit  80  actuates the feed pump  118 C when the pressure detected by the pressure sensor  144  becomes a negative pressure below a predetermined value (for example, −0.8 kPa or less) and stops the feed pump  118 C when the level detection float  134  is detected by the Hi detection sensor  136 H. Accordingly, the UV ink stored in the first main tank  110  is supplied into the ink storage chamber  123  of the first sub tank  120 C and the ink storage chamber  123  of the first sub tank  120 C is filled with the UV ink until the liquid level reaches the reference level for filling. 
     Then, the inner pressure of the first sub tank  120 C detected by the pressure sensor  144  is reduced to be a value near the preset negative pressure (for example, about −1.0 kPa). When the inner pressure reaches this value or less, the second through fourth converging route switch valves  175 M,  175 Y, and  175 K which have been closed until now are opened so that all of the first and fourth sub tanks are kept at the preset negative pressure (negative pressure keeping step). Then, the ink filling is terminated. In this manner, the first print head  60 C selected by the operation panel  88  is filled with ink and all sub tanks including the first sub tank are set at the preset negative pressure and are held in the standby state. It should be noted that, in case of carrying out the ink filling process onto a plurality of print heads, the same process as mentioned above will be carried out by turning ON the converging route switch valves corresponding to the print heads of which ink filling is required. 
     Now, as another embodiment of the backflow shutoff mechanism of the ink supply device  100 , different from the aforementioned backflow shutoff mechanism  200 , a backflow shutoff mechanism  300  will be described. It should be noted that, in the following description, components the same as the components of the aforementioned ink supply device  100  are marked with the same numerals so that the explanation about those components will be omitted. 
     The backflow shutoff mechanisms  300  are disposed between the converging route switch valves  175  and the sub tanks  120  in the ink supply device  100  to correspond to the first through fourth sub tanks  120 C,  120 M,  120 Y,  120 K, respectively. As shown in  FIG. 10 , each backflow shutoff mechanism  300  is directly connected to a shutoff mechanism connector  109  which is formed to communicate with the air introduction passage of the top wall  121   t  at the upper portion of the sub tank  120 . The backflow shutoff mechanism  300  mainly comprises a housing  302  which has a release mechanism  302   b  and is thus detachably attached to the shutoff mechanism connector  109 , and a backflow shutoff float  303  which is accommodated in an inner space  301  formed in the housing  302  and which vertically move together with the liquid surface of UV ink flowing into the inner space. It should be noted that in  FIG. 10 , the housing  302  is shown virtually by two dot chain lines for the purpose of showing the structure inside of the housing  302 . 
     The housing  302  is provided with the inner space  301  which extends in the vertical direction and opens the lower surface of the housing  302 . A cylindrical shield member  304  is fitted in the opening at the lower surface, thereby keeping the inner space  301  in the sealed state. The line  177  connected to the converging switch valve  175  is connected to the upper portion of the housing  302  so that the converging route  171  and the inner space  301  of the housing are connected through an introduction passage  302   a  extending from the upper end to the inner space  301  of the housing. The shield member  304  keeps the inner space  301  in the sealed state and is also in contact with the upper surface of the shutoff mechanism connector  109  to prevent air from leaking out of this contact portion when the backflow shutoff mechanism  300  is attached to the shutoff mechanism connector  109 , whereby the inner space  301  and the shutoff mechanism connector  109  are connected through a derivation passage  304   a  penetrating through the shield member  304  in the vertical direction. 
     In the inner space  301  of the housing  302 , a cylindrical supporting member  305  bored in the vertical direction is mounted on the upper surface of the shield member  304  and the backflow shutoff float  303  and a guide member  306  are disposed on the supporting member  305 . In the peripheral surface of the supporting member  305 , a penetration slit  305   a  which penetrates through the peripheral surface and extends to the top is formed. Normally (when no backflow UV ink is stored in the inner space  301 ), the communication between the inner space  301  of the housing and the derivation passage  304   a  of the shield member  304  is allowed through the penetration slit  305 . 
     The backflow shutoff float  303  is formed in a spherical shape made of a material capable of floating in the UV ink and is put on the supporting member  305  to close the upper open end of the penetration slit  305   a  of the supporting member  305 . The diameter of the backflow shutoff float  303  is larger than the diameter of the introduction passage  302   a  of the housing. The guide member  306  is formed in a cylindrical shape using a net frame. The guide member  306  accommodates the backflow shutoff float  303  inside thereof and extends from the upper surface of the supporting member  305  to a portion near the lower open end of the introduction passage  302   a  to guide the vertical movement of the backflow shutoff float  303 . 
     As for the backflow shutoff mechanism  300  having the aforementioned structure, the UV ink, flowing out from the air introduction passage of the top wall  121   t  through the shutoff mechanism connector  109  because the backflow prevention section  132  in the ink storage chamber  123  of the sub tank  120  fails to work as mentioned above, flows into the housing  302  through the derivation passage  304   a  of the shield member  304  connected to the shutoff mechanism connector  109  and is initially stored in the inner space  301  of the housing through the penetration slit  305   a  formed in the peripheral surface of the supporting member  305 . Then, when the liquid surface of the UV ink in the inner space  301  exceeds the upper end of the supporting member  305 , the backflow shutoff float  303  is pushed upwardly by the UV ink flowing into the inner space  301  through the opening formed in the upper end of the supporting member  305  so that the backflow shutoff float  303  is guided by the guide member  306  to rise together with the liquid surface of the UV ink and come in contact with the portion near the lower open end of the introduction passage  302   a  of the housing to close the lower open end. 
     As mentioned above, when the UV ink flows backward from the sub tank  120  and reaches the backflow shutoff mechanism  300 , the backflow shutoff float  303  rises together with the liquid surface of the UV ink in the inner space  301  and the opening at the lower end of the introduction passage  302   a  of the housing is closed by the backflow shutoff float  303 , thereby preventing the UV ink from flowing out of the introduction passage  302   a  toward the converging route switch valve  175  and thus securely shutting off the backflow of the UV ink in the backflow shutoff mechanism  300 . Since the backflow shutoff mechanism  300  is directly attached to the shutoff mechanism connector  109  formed on the sub tank  120 , the UV ink flowing backward from the sub tank  120  reaches the backflow shutoff mechanism  300  immediately and the backflow of ink is shut off by the backflow shutoff mechanism  300  as mentioned above, thereby preventing the line  177  from being contaminated by the UV ink flowing backward. Therefore, the number of parts which will be forced to be replaced because of being contaminated when backflow of ink occurs is reduced. As a result of this, an effect of reducing the maintenance cost of the printer apparatus P is obtained. Since the backflow shutoff float  303  having the spherical shape is in contact with the other member (for example, the supporting member  306  and the housing  302 ) by a small area, the backflow shutoff float  303  never be stick to the other member, thereby ensuring the performance of the backflow shutoff action. 
     Though the preferred embodiments of the present invention have been described in the above, the range of the present invention is not limited to the aforementioned embodiments. For example, though as one example of the inkjet printer to which the present invention is applied, an inkjet printer which conducts the printing process by moving a print head (carriage) relative to a print medium held on a platen in a direction of one axis (Y axis) of two orthogonal axes extending horizontally and feeding the print medium on the platen in a direction of the other axis (X axis) during the printing process is employed in the aforementioned embodiment, the present invention can be applied to an inkjet printer of another type, such as an inkjet printer which conducts a printing process onto a print medium fixedly held on a supporting table (so-called, inkjet printer of a flat bed type), and an inkjet printer which conducts a printing process by putting a print medium such as a CD onto a pallet and moving the pallet by a belt conveyer. 
     EXPLANATION OF REFERENCE SIGNS IN DRAWINGS 
     
         
         P printer apparatus (inkjet printer) 
           60  print head ( 60 C: first print head,  60 M: second print head,  60 Y: third print head,  60 K: fourth print head) 
           100  ink supply device 
           109  shutoff mechanism connector (air introduction port) 
           110  main tank ( 110 C: first main tank,  110 M: second main tank,  110 Y: third main tank,  110 K: fourth main tank) 
           115  ink sending unit (ink sending means) 
           120  sub tank ( 120 C: first sub tank,  120 M: second sub tank,  120 Y: third sub tank,  120 K: fourth sub tank) 
           140  sub tank depressurizing unit (pressure control means) 
           150  sub tank pressurizing unit (pressure control means) 
           160  air pump (pressure control means) 
           171  converging route (air passage) 
           200  backflow shutoff mechanism ( 200 C: first backflow shutoff mechanism,  200 M: 
       
    
     second backflow shutoff mechanism,  200 Y: third backflow shutoff mechanism,  200 K: 
     fourth backflow shut off mechanism) (backflow shutoff means, backflow shutoff mechanism)
       201  inner space (air flowing space)     202  housing (housing member)     203  backflow shutoff float (float member)     204   a  introduction passage (first air introduction passage)     205   a  derivation passage (second air introduction passage)     300  backflow shutoff mechanism (backflow shutoff means, backflow shutoff mechanism)     301  inner space (air flowing space)     302  housing (housing member)     302   a  introduction passage (first air introduction passage)     303  backflow shutoff float (float member)     304   a  derivation passage (second air introduction passage)