Patent Publication Number: US-6991313-B2

Title: Ink jet printer and main maintenance method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ink jet printer in which maintenance processing is conducted in a state that air pressurized by an air pump is supplied to air chambers of respective ink cartridges. 
   2. Description of the Related Art 
   Various ink jet printers have been put in practical use in which color printing is generally conducted on a sheet in such a manner that a print head having a large number of ink jet nozzles that are arranged in columns jet out ink supplied from ink cartridges into ink droplets bit by bit. In those ink jet printers, a maintenance mechanism having a head cap and a cleaning blade, a suction pump, and other members are provided on a main body side thereof. To eliminate air bubbles from ink jet nozzles or to conduct a suction purge at the occurrence of nozzle clogging, the head cap is fitted to the nozzles and the ink in the nozzles is sucked by negative pressure generated by the suction pump. As a result, the air bubbles are removed or the nozzle clogging is solved. 
   Each ink cartridge is equipped with an ink accommodation chamber and an air chamber for exerting air pressure on the ink in the ink accommodation chamber. In printing or maintaining the ink jet nozzles, a pressure purge is performed in such a manner that pressurized air is introduced from the air pump to each air chamber via an air supply tube and ink that is pushed out of the ink accommodation chamber by the expansion of the air chamber is supplied to the corresponding nozzles. 
   For example, in an ink jet printer disclosed in Japanese Patent No. 2,703,647, a carriage is equipped with a color print head, a plurality of ink tanks and buffer tanks connected to the head, and other members. A pressurization pump, a waste fluid tank, a maintenance mechanism having a head cap and a cleaning blade, and other members are provided on a main body side. Air pressurized by the pressurization pump is supplied to the ink tanks via a relief valve, thereby pressurizing the ink within the buffer tanks and the ink passages of the head. 
   In an ink jet recording apparatus disclosed in JP-A-10-138506, a carriage is equipped with a recording head and a subtank unit. Ink cartridges that are connected to the head via supply tubes, an air pump for supplying pressurized air to the ink cartridges, a pressure regulator, a switching valve, etc., are provided on a main body side. When the residual amount of ink in a subtank becomes small, the switching valve switches so that the air pump comes to communicate with ink pouches of the ink cartridges. Air pressurized by the air pump is supplied to the ink pouches via the pressure regulator and the switching valve, and then ink is supplied from the ink pouches to the recording head and the subtank. 
   In conducting a suction purge, the ink within the nozzles is sucked in a state that the head cap is tightly attached to the nozzles. After completion of the suction purge, since the tightly attached head cap is removed from the nozzles in a state where negative pressure is developed in the head cap, the pressure in the head cap abruptly changes from the negative pressure to the ambient pressure. With that pressure change, the ink that has been sucked so far goes into the nozzles. In this situation, other inks that are stuck to portions around nozzles or air may go into nozzles. This raises a problem that color contamination or loss of a color may occur in the subsequent printing operation. 
   On the other hand, in the ink jet printer disclosed in Japanese Patent No. 2,703,647 and the ink jet recording apparatus disclosed in JP-A-10-138506, the relief valve or the pressure regulator is provided between the pressurization pump (air pump) and the ink tanks (ink cartridges). The relief valve or the pressure regulator adjusts the pressure of pressurized air that is supplied from the pressurization pump (air pump) to the ink tanks (ink cartridges). Therefore, a space for accommodating the large-size relief valve or pressure regulator is required, thereby leading to problems that the ink jet printer becomes large in size and its manufacturing costs become high. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the above problems, and therefore one object of the present invention is to reliably prevent color contamination and loss of a color in printing conducted after purge processing. 
   Another object of the invention is to conduct, easily at a low cost, a pressure adjustment of pressurized air that is supplied to ink cartridges. 
   According to an aspect of the invention, there is provided an ink jet printer including: a print head having a plurality of ink jet nozzles arranged in plural columns; an ink cartridge including an ink accommodation chamber having a deformable wall and an air chamber for exerting via the wall air pressure on ink accommodated in the ink accommodation chamber; an ink supply tube connecting the ink cartridge to the print head; an air pump for producing pressurized air for changing a state of ink that is located at a tip portion of each of the ink jet nozzles; an air supply tube for guiding the pressurized air to the air chamber of the ink cartridge; and a maintenance unit including a cap member for covering the print head, wherein the maintenance unit opens the cap member in a state that the pressurized air is supplied to the air chamber of the ink cartridge. 
   The ink in the ink accommodation chamber, at least part of which is the deformable wall, of the ink cartridge is supplied via the ink supply tube to each of the many ink jet nozzles that are arranged in plural columns in the print head. In maintenance processing, the air pump operates and pressurized air produced by the air pump is introduced into the air chamber of the ink cartridge via the air supply tube. Therefore, the ink accommodation chamber of the ink cartridge receives air pressure. 
   As a result, the ink in the ink accommodation chamber is supplied to the print head via the ink supply tube, thereby the state of the ink at the tip of each ink jet nozzle is changed, that is, the ink expands there. A purge is performed in this state. When the purge is performed, the print head is covered with the cap member. However, since the cap member is opened in a state that the pressure in the cap member is not a negative one, none of other inks that are stuck to portions around ink jet nozzles, dust, and air go into nozzles. This reliably prevents color contamination or loss of a color that might otherwise occur in the subsequent color printing operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a perspective view showing a multifunctional apparatus according to an embodiment of the present invention; 
       FIG. 2  is a plan view showing internal mechanisms of an ink jet printer; 
       FIG. 3  is a side view of a print mechanism section and a vertical side sectional view taken along line III—III in  FIG. 2 ; 
       FIG. 4  is a plan view showing the print mechanism section; 
       FIG. 5  is a vertical front sectional view taken along line V—V in  FIG. 2 ; 
       FIG. 6  illustrates an ink supply section and an air supply section; 
       FIGS. 7A  to  7 E illustrate how maintenance processing is conducted by supplying pressurized air to ink jet nozzles, in which 
       FIG. 7A  illustrates a printing enable state, 
       FIG. 7B  illustrates a pressure purge in which a head cap is in operation in a pressurized state, 
       FIG. 7C  illustrates a starting state of wiping with a blade in the pressurized state, 
       FIG. 7D  illustrates a completion state of the wiping with the blade, and 
       FIG. 7E  illustrates a maintenance completion state; and 
       FIG. 8  shows a modified example of the ink supply section and the air supply section shown in FIG.  6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be hereinafter described with reference to the accompanying drawings. 
   This embodiment is such that the invention is applied to a multifunctional apparatus having a telephone function etc. in addition to a printer function, copier function, a scanner function, and a facsimile function. 
   As shown in  FIG. 1 , a multifunctional apparatus  1  is equipped with a sheet feeder  2  on a back side thereof. A document reading device  3  for the copier function and the facsimile function is disposed so as to occupy a top portion of a section in front of the sheet feeder  2 . An ink jet printer  4  as an implementation of the printer function is disposed so as to occupy the entire portion under the document reading device  3 . A table  5  for ejection of printed sheets is disposed in front of the ink jet printer  4 . 
   The document reading device  3  is structured as follows (not shown in FIG.  1 ). The document reading device  3  can be swung vertically around a horizontal axis that is located at the rear end. If a top cover  3   a  is opened upward, a user can see a document placement glass plate. An image scanning device for document reading is disposed under the glass plate. By opening the document reading device  3  upward by hand, the user can replace ink cartridges  40 - 43  of the ink jet printer  4  or maintain a print mechanism section  10 . That is, the ink jet printer  4  is disposed in front of the sheet feeder  2  in a manner as shown in FIG.  2 . 
   Subsequently, the ink jet printer  4  will be described with reference to  FIGS. 2-5 . 
   The ink jet printer  4  includes the print mechanism section  10  for printing on a sheet (e.g., A4-sheet) supplied from the sheet feeder  2  by jetting ink droplets from a print head  23 P, a maintenance mechanism section  11  for performing maintenance processing on the print head  23 P, an ink supply section  12  for supplying inks from the ink cartridges  40 - 43  to the print mechanism section  10 , an air supply section  13  for supplying pressurized air to the ink cartridges  40 - 43 , and other sections. First, the print mechanism section  10  will be described. 
   As shown in  FIGS. 2 and 4 , the print mechanism section  10  is housed compactly in a box-shaped print unit frame  20  whose top plate has a generally elliptical opening. The right ends and the left ends of a rear guide shaft  21  and a front guide rail  22  that are provided in the print unit frame  20  so as to extend in the right-left direction are fixed to a right side wall  20   a  and a left side wall  20   b , respectively. A carriage  23  is supported by the guide shaft  21  and the guide rail  22 , and can be reciprocated in the right-left direction along the guide shaft  21  and the guide rail  22  by a carriage driving motor  24  (directed to a front side) via a wire (not shown). The carriage  23  itself also serves as the print head  23 P. 
   As shown in  FIGS. 2 and 4 , a number of ink jet nozzles (hereinafter referred to as “nozzles”)  23   a - 23   d  are arranged on the bottom surface of the print head  23 P in four columns so as to correspond to four ink colors (the columns extend in the front-rear direction). The nozzle columns  23   a - 23   d  for black, cyan, magenta, and yellow, respectively, are arranged in the stated order from the left side in such a manner that the black nozzle column  23   a  and the cyan nozzle column  23   b  are disposed close to each other and the magenta nozzle column  23   c  and the yellow nozzle column  23   d  are disposed close to each other. The nozzles  23   a - 23   d  are provided with respective piezoelectric elements (not shown), and very small amounts of ink are jetted from the nozzles  23   a - 23   d  having the piezoelectric element energized toward a sheet. 
   A main transport roller  25  called “registration roller” is disposed under the guide shaft  21  and rotatably supported by the right side wall  20   a  and the left side wall  20   b . The main transport roller rotates in a prescribed direction by a sheet feed motor  26  via a gear mechanism  27  to transport a sheet that is supplied from the sheet feeder  2  toward the front side (i.e., in a sheet feed direction) while moving the sheet approximately horizontally right under the print head  23 P, and to eject the sheet to the ejection table  5 . 
   Subsequently, the maintenance mechanism section  11  will be described briefly, which is the same as used in ordinary ink jet printers. 
   As shown in  FIG. 4 , a thin-plate-shaped wiper blade  31  (corresponds to a wiper member) is disposed vertically in a maintenance case  30  that occupies a right end portion of the print unit frame  20 . A pair of rubber head caps  32  (corresponds to a cap member) are disposed upward on the right of the wiper blade  31 . When a maintenance motor  33  that is attached to the rear wall of the maintenance case  30  rotates in a normal direction, the wiper blade is moved upward and downward via a blade elevation mechanism (not shown). When the maintenance motor  33  rotates in a reverse direction, the head caps  32  are moved upward and downward via a cap elevation mechanism (not shown). 
   Subsequently, the ink supply section  12  will be described. 
   In front of the ink supply section  12 , a black ink cartridge  40 , a cyan ink cartridge  41 , a magenta ink cartridge  42 , and a yellow ink cartridge  43  are arranged in the stated order from the left side so as to be at the same height and parallel with each other, so that the ink cartridges  40 - 43  are accommodated in a small space. As shown in  FIG. 3 , a flexible film member  43   a  (corresponds to a deformable wall), which is stretched inside the cartridge case of the yellow ink cartridge  43  so as to cover most of its entire area, partitions the cartridge case into a bottom ink accommodation chamber  43   b  and a top air chamber  43   c.    
   A yellow ink YI is accommodated in the ink accommodation chamber  43   b , and the air chamber  43   c  communicates with the ambient air. The other three ink cartridges  40 - 42  are configured in the same manner as the ink cartridge  43 . That is, flexible film members  40   a - 42   a  partition the cartridge cases into bottom ink accommodation chambers  40   b - 42   b  and top air chambers  40   c - 42   c , respectively. A black ink BI, a cyan ink CI, and a magenta ink MI are accommodated in the ink accommodation chambers  40   b - 42   b  of the black ink cartridge  40 , the cyan ink cartridge  41 , and the magenta ink cartridge  42 , respectively. 
   As shown in  FIGS. 2 ,  3  and  5 , ink needles  44  are disposed in the rear of the respective ink cartridges  40 - 43  so as to project toward the front side. The proximal portions of the ink needles  44  are connected to the print head  23 P via dedicated ink supply tubes  45 - 48 , respectively. Portions closer to the print head  23 P of the black and cyan ink supply tubes  45  and  46  are bundled so as to be arranged in the vertical direction, and portions closer to the print head  23 P of the magenta and yellow ink supply tubes  47  and  48  are also bundled so as to be arranged vertically. 
   As shown in  FIG. 3 , the print head  23 P is positioned higher than the ink cartridges  40 - 43  so as to generate a head difference H, which is a difference between the heights of the ink needles  44  and the nozzles  23   a - 23   d  of the print head  23 P. When the ink cartridges  40 - 43  are mounted at their prescribed mounting positions, the tip portions of the ink needles  44  penetrate through the rear end portions of the film members  40   a - 43   a  and reach the ink accommodation chambers  40   b - 43   b , respectively, whereby the inks BI, CI, MI, and YI in the ink accommodation chambers  40   b - 43   b  are supplied to the print head  23 P via the dedicated ink supply tubes  45 - 48 , respectively. 
   Therefore, the nozzles  23   a - 23   d  of the print head  23 P are filled with inks BI, CI, MI, and YI thus supplied and a negative pressure corresponding to the head difference H develops there, whereby meniscuses suitable for printing are formed at the tips of the nozzles  23   a - 23   d  so as to be curved inward as shown in FIG.  7 A.  FIGS. 7A  to  7 E show a black nozzle  23   a  and a cyan nozzle  23   b  that is adjacent to the black nozzle  23   a.    
   Now, the air supply section  13  will be described. 
   As shown in  FIGS. 2 and 5 , a pump motor  50  is disposed on the left of the mounting portion for the black ink cartridge  40  so as to be directed downward. Under the pump motor  50 , a cylindrical gear member  51  having a bottom wall is rotatably supported by a support shaft  52 . The inside surface of the cylindrical portion of the cylindrical gear member  51  is formed with a gear. A drive gear  53  that is fixed to a drive shaft of the pump motor  50  meshes with the gear of the cylindrical gear member  51  from inside. A top portion of the cylindrical gear member  51  is integrally formed with a plate-like brim  51   a  that has a slit and extends outward from the periphery of the top portion of the cylindrical gear member  51 . A bottom portion of the cylindrical gear member  51  is integrally formed with a cylindrical eccentric cam  51   b.    
   A left end portion of a connecting rod  54  is slidably fitted with the eccentric cam  51   b  of the cylindrical gear member  51 , and a right end portion of the connecting rod  54  is connected to a diaphragm  56  that is provided inside an air pump  55 . The air pump  55  is equipped with an exhaust valve and an inlet valve (both not shown), and a left end portion of a flexible air supply tube  57  is connected to the air pump  55 . Four T-shaped branching members  58  are attached to the air supply tube  57  at positions that are separated from each other by approximately the same, prescribed length. As shown in  FIG. 6 , a pressure contact pad  60  that is elastically urged by a coil spring  59  is attached to a branch end portion of each branching member  58   
   An orifice  61  is fixed to the air supply tube  57  at a position close to its air pump  55  side end via a branching member  58 . The inner diameter of the air supply tube  57  is about 1 mm. The orifice  61  has a passage whose inner diameter (e.g., about 0.5 mm) is smaller than the inner diameter of the air supply tube  57 . The passage of the orifice  61  always communicates with the ambient air. Therefore, when the ink cartridges  40 - 43  are mounted at their respective mounting positions, pressurized air is supplied to the air chambers  40   c - 43   c  of the ink cartridges  40 - 43  via the air supply tube  57  and the pressure contact pads  60  that are urged elastically, respectively. 
   When the air pump  55  is not in operation, atmospheric pressure acts on the air chambers  40   c - 43   c  via the air supply tube  57  and the pressure contact pads  60 . When the pump motor  50  is driven during maintenance processing, the diaphragm  56  is reciprocated in the right-left direction via the drive gear  53 , the cylindrical gear member  51 , and the eccentric cam  51   b . The air pump  55  thus operates to produce pressurized air having an increased pressure of about 100 mmAq, which acts on the air chambers  40   c - 43   c . The pressure of this pressurized air exceeds the negative pressure of the head difference H. 
   In the situation, the orifice  61  performs a pressure adjustment by exhausting part of the pressurized air produced by the air pump  55 . The pressure-adjusted pressurized air acts on the air chambers  40   c - 43   c  with the same air pressure. The top portion of the cylindrical gear member  51  is integrally formed with the plate-like brim  51   a  that has a slit and extends outward from the periphery of the top portion of the cylindrical gear member  51 . An encoder  62  that is a photointerrupter is provided to detect the brim  51   a . The air pump  55  is reciprocated once every time the pump motor  50  makes four rotations, and one detection pulse signal is output from the encoder  62  every time the air pump  55  reciprocates once. 
   Subsequently, the operation and advantages of the ink jet printer  4  thus configured will be described. When the four ink cartridges  40 - 43  are mounted at their prescribed mounting positions shown in  FIG. 2 , the tip portions of the ink needles  44  penetrate through the rear end portions of the film members  40   a - 43   a  and reach the ink accommodation chambers  40   b - 43   b , respectively, whereby the inks BI, CI, MI, and YI in the ink accommodation chambers  40   b - 43   b  are supplied to the print head  23 P via the dedicated ink supply tubes  45 - 48 , respectively, and fill the nozzles  23   a - 23   d  of the print head  23 P. 
   At this time, as shown in  FIG. 7A , a negative pressure corresponding to the head difference H develops, whereby meniscuses suitable for printing are formed at the tips of the nozzles  23   a - 23   d  so as to be curved inward. Only a black nozzle  23   a  and a cyan nozzle  23   b  are shown in  FIGS. 7A  to  7 E. As shown in  FIGS. 7B and 7C , inks of other colors, dust, etc. are stuck to a surface of the print head  23 P in the vicinity of the nozzles  23   a  and  23   b.    
   To conduct purge processing before a start of printing, the print head  23 P is moved to a maintenance position shown in FIG.  2  and the print head  23 P is then capped tightly as shown in  FIG. 7B  by elevating the head cap  32  to its operating position by rotating the maintenance motor  33  in the reverse direction. The pump motor  50  is then driven in this state. 
   As a result, the air pump  55  operates in the above-described manner, whereby pressurized air having an air pressure p (about 100 mmAq) is supplied from the air pump  55  to the air chambers  40   c - 43   c  of the respective ink cartridges  40 - 43  via the air supply tube  57 . After a lapse of a prescribed period of time (e.g., about 5 seconds), all the air chambers  40   c - 43   c  are pressurized sufficiently by the pressurized air having the air pressure p to establish a state (i.e., a pressure purge processing completion state) that the inks BI, CI, MI, and YI assume convex shapes at the tips of the nozzles  23   a - 23   d.    
   At this time, as described above, part of the pressurized air produced by the air pump  55  is exhausted by the orifice  61 , so as to adjust the pressure of the pressurized air. When the pressure purge processing has completed in the above-described manner, the pressure in the head cap  32  is not negative. After a lapse of a prescribed period of time, the maintenance motor  33  is rotated in the normal direction, whereby the head cap  32  that has so far been attached tightly to the print head  23 P is removed and the wiper blade  31  is elevated to its operating position as shown in FIG.  7 C. 
   At this time, the inside of the head cap  32  has been in a pressurized state (not in a negative-pressure state) and the pressurized air having the air pressure p has caused the inks BI, CI, MI, and YI to assume convex shapes at the tips of the nozzles  23   a - 23   d , none of inks of other colors and dust that are stuck to portions around the nozzles  23   a - 23   d  and air enter the nozzles  23   a - 23   d.    
   Further, in this state, the print head  23 P is moved leftward as shown in  FIG. 7D , whereby the printing surface of the print head  23 P is wiped over with the wiper blade  31 . Finally, the maintenance motor  33  is driven to lower the wiper blade  31 . The driving of the pump motor  50  is stopped at the same time as the wiper blade  31  reaches its original standby position. The pressurized air having the air pressure p continues to act on the nozzles  23   a - 23   d  even during the wiping with the wiper blade  31 , so that none of wiped-off inks BI, CI, MI, and YI, dust, and air enter the nozzles  23   a - 23   d.    
   When the pressurized air having the air pressure p that has acted on the nozzles  23   a - 23   d  has been depressurized upon the stop of operation of the air pump  55 , a negative pressure due to the above-mentioned head difference H forms, at the tips of the nozzles  23   a - 23   d , meniscuses suitable for printing that are curved inward as shown in FIG.  7 E. After the maintenance processing has completed in the above manner, printing processing is performed on the basis of printing data, to thereby print a color image on a sheet that is supplied from the sheet feeder  2 . The maintenance mechanism section  11  corresponds to a maintenance unit. 
   As described above, in maintenance processing, pressure purge processing using the head caps  32  and wiping processing using the wiper blade  31  are conducted in a state where the pressurized air having the air pressure p produced by the air pump  55  acts on the nozzles  23   a - 23   d . Therefore, color contamination and loss of a color can be prevented reliably during printing that is performed after pressure purge processing. Further, since an air pressure adjustment of pressurized air to be supplied to the ink cartridges  40 - 43  is performed by exhausting, through the orifice  61 , part of the pressurized air produced by the air pump  55 , the air pressure adjustment can be performed easily at the low costs. 
   Subsequently, modifications of the above embodiment will be described. Components other than modified ones will be given the same reference symbols as used in the embodiment. 
   1) Instead of the flexible film member that is provided in each of the ink cartridges  40 - 43  to partition the cartridge case into the ink accommodation chamber and the air chamber, any film member or wall member can be used as long as it can serve as a deformable wall capable of transmitting air pressure acting on the air chamber to the ink in the ink accommodation chamber. 
   2) As shown in  FIG. 8 , an orifice  61 A may be provided at the opposite end of the air supply tube  57  to the air pump  55 . In this example, a pressure adjustment is performed by the orifice  61 A&#39;s exhausting part of pressurized air supplied from the air pump  55  and resulting pressure-adjusted pressurized air acts on the air chambers  40   c - 43   c.    
   However, in this case, the air pressure acting on the air chamber  40   c ,  41   c ,  42   c , or  43   c  of the ink cartridge  40 ,  41 ,  42 , or  43  is higher as the ink cartridge is closer to the air pump  55  though the pressure difference is small. In view of this, the ink viscosities are set in such a manner that black ink BI&gt;cyan ink CI&gt;magenta ink MI&gt;yellow ink YI. The black ink cartridge  40  with the highest ink viscosity is located at the position closest to the source of a flow of pressurized air, that is, at the leftmost position where the air pressure p of pressurized air supplied form the air pump  55  is highest. 
   The cyan ink cartridge  41  with the second highest ink viscosity is located next to the black ink cartridge  40 . The magenta ink cartridge  42  with the third highest ink viscosity is located next to the cyan ink cartridge  41 . The yellow ink cartridge  43  with the lowest ink viscosity is located at the rightmost position. As a result, the pressure of the black ink BI acting on the black nozzle  23   a  is made approximately the same as the pressure of the cyan ink CI, magenta ink MI, or yellow ink YI having the lower ink viscosity acting on the dedicated nozzle  23   b ,  23   c , or  23   d ; the pressure differences of pressurized air can be made uninfluential. 
   3) The inner diameter of the air supply tube  57  may be decreased gradually as the position goes away from the air pump  55 , that is, comes closer to the orifice  61 A. The air supply tube  57  that becomes thinner as the position goes away from the air pump  55  makes it possible to decrease the pressure inside the air supply tube  57  gradually. This in turn makes it possible to decrease the size of the orifice  61 A that is provided at the opposite end of the air supply tube  57  to the air pump  55 . The orifice  61 A can even be omitted when the degree of pressure decrease is high. 
   4) The air pump  55  is not limited to the one using a diaphragm, and may be any of various kinds of small air pumps. 
   5) The negative pressure acting on the ink accommodation chambers  40   b - 43   b  of the ink cartridges  40 - 43  need not always be produced by the head difference H that is caused by the difference between the heights of the print head  23 P and the ink cartridges  40 - 43 , and may be produced any of various negative pressure generating means by, for example, forcibly expanding the ink accommodation chambers  40   b - 43   b  by means of a certain member. 
   6) The invention is not limited to or by the above embodiment and modifications. Other various modifications are possible without departing from the spirit and scope of the invention and, as such, the invention can be applied to various ink jet printers. 
   According to a first aspect of the invention, in an ink jet printer having a print head, an ink cartridge, and an ink supply tube, the ink cartridge comprises an ink accommodation chamber at least part of which is a deformable wall and an air chamber for exerting, via the wall, air pressure on ink accommodated in the ink accommodation chamber. The ink jet printer further comprises an air pump for producing pressurized air for changing a state of ink that is located at a tip portion of each of the ink jet nozzles; an air supply tube for guiding the pressurized air produced by the air pump to the air chamber of the ink cartridge; and a maintenance unit comprising a cap member for covering a head surface of the print head. The maintenance unit opens the cap member in a state that the pressurized air is supplied to the air chamber of the ink cartridge. Therefore, the ink in the ink accommodation chamber, at least part of which is the deformable wall, of the ink cartridge is supplied to each ink jet nozzle via the ink supply tube. 
   In maintenance processing, the air pump operates and pressurized air produced by the air pump is introduced into the air chamber of the ink cartridge via the air supply tube. Therefore, the ink accommodation chamber of the ink cartridge receives air pressure. As a result, the ink in the ink accommodation chamber is supplied to the print head via the ink supply tube, whereby the state of the ink at the tip of each ink jet nozzle is changed, that is, the ink expands there. A purge can be performed in this state. However, since after the purge the cap member is opened in a state that the pressure in the cap member is not negative, none of other inks that are stuck to portions around ink jet nozzles, dust, and air go into nozzles. This reliably prevents color contamination or loss of a color that might otherwise occur in the subsequent color printing operation. 
   According to a second aspect of the invention, the maintenance unit further comprises a wiper member for wiping over the head surface of the print head, and the air pump supplies the pressurized air to the air chamber of the ink cartridge from a prescribed time before opening of the cap member to completion of wiping of the head surface with the wiper member. Therefore, the pressure in the cap member is not negative when the cap member is opened and this pressurized state continues also when the head surface is wiped over with the wiper member. Therefore, none of other inks that are stuck to portions around ink jet nozzles, dust, and air go into nozzles. This reliably prevents color contamination or loss of a color that might otherwise occur in the subsequent color printing operation. 
   According to a third aspect of the invention, the ink cartridge is a plurality of ink cartridges that accommodate inks of plural colors, respectively, and that are connected in parallel to the air supply tube in a horizontal plane. Since the plurality of ink cartridges have the same height and are arranged parallel with each other, the space occupied by the ink cartridges can be made small. 
   According to a fourth aspect of the invention, an orifice for exhausting part of the pressurized air is provided in the vicinity of an air-pump-side end of the air supply tube. Therefore, part of the pressurized air is exhausted by the orifice that is provided in the vicinity of the air-pump-side end of the air supply tube, whereby a pressure adjustment is made. 
   According to a fifth aspect of the invention, an orifice for exhausting part of the pressurized air is provided in the vicinity of an opposite end of the air supply tube to the air pump. Therefore, part of the pressurized air is exhausted by the orifice that is provided in the vicinity of the opposite end of the air supply tube to the air pump, whereby a pressure adjustment is made. 
   According to a sixth aspect of the invention, the plurality of ink cartridges are arranged in such a manner that an ink cartridge with a higher ink viscosity is located upstream of a flow of the pressurized air from the air pump to the orifice. That is, an ink cartridge with a higher ink viscosity is located at a position closer to the source of a flow of the pressurized air, that is, at a position where the air pressure of the pressurized air supplied from the air pump is higher. Therefore, an ink having a higher ink viscosity receives a higher air pressure and hence is jetted in the same manner as an ink having a lower ink viscosity. The pressure differences of the pressurized air acting on the ink jet nozzles can easily be made uninfluential. 
   According to a seventh aspect of the invention, the inner diameter of the air supply tube decreases as the position goes from the air pump to the orifice. The air supply tube that becomes thinner as the position goes away from the air pump makes it possible to decrease the pressure inside the air supply tube gradually along its length. This in turn makes it possible to decrease the size of the orifice that is provided at the opposite end of the air supply tube to the air pump. The orifice can even be omitted when the degree of pressure decrease is high. 
   According to an eighth aspect of the invention, the print head is located at a position higher than the ink cartridge by a prescribed length, whereby a negative pressure acts on each of the ink jet nozzles of the print head and a concave ink meniscus is thereby formed at a tip of each of the ink jet nozzles. Because of a head difference that is caused by the difference between the heights of the print head and the ink cartridge, a negative pressure acts on each ink jet nozzle of the print head and a proper concave ink meniscus is thereby formed at the tip of each ink jet nozzle. The printing quality can thus be increased. 
   The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.