Abstract:
An ink jet recorder includes an ink jet head having a nozzle, which can be capped with a cap and a suction pump for purging the nozzle through the cap. The recorder further includes a pressure control unit interposed between an ink supply and the head to control the pressure on the ink in the nozzle. At least just before purging finishes, the control unit applies positive pressure to the ink in the nozzle to prevent the air bubbles, the foreign substances and/or the like sucked into the cap by the pump from flowing back into the head. The control unit may include an ink tank and a height adjuster for adjusting the height of the tank relative to the front end of the nozzle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet printer or recorder for forming an image by ejecting ink from its ink jet head. Specifically, the invention relates to an ink jet printer which makes it possible to suck and remove bad or defective ink, foreign substances, air bubbles, and/or the like from the ink jet head with the head nozzle capped. 
     2. Description of Related Art 
     A conventional ink jet printer of this type includes an ink jet head, which has a nozzle for ejecting ink and ink chambers storing ink behind the nozzle. By generating pressure vibration in one or more the ink chambers, it is possible to eject ink from the nozzle to form an image on a recording medium. 
     The printer also includes a cap for covering the outer side of the nozzle airtightly. The printer further includes a suction device, which may be a suction pump, for developing negative pressure in the cap in airtight contact with the nozzle to suck ink from the nozzle. The negative pressure developed in the cap covering the nozzle can suck and remove bad ink, foreign substances, air bubbles and/or the like from the nozzle. It is therefore possible to prevent defective ejection of ink from the nozzle so that a clear image can be formed. 
     On the other hand, negative pressure is applied always to the ink chambers and the nozzle. This negative pressure prevents ink from leaking from the nozzle, and allows ink to be ejected only when pressure vibration develops in one or more of the chambers. This prevents recording media from staining or spotting. The negative pressure may, however, worsen the suction and removal with the pump for the reason stated below. 
     FIGS. 5A-5D of the accompanying drawings show the suction and removal process as part of the maintenance of a conventional ink jet printer. The printer includes an ink jet head  73 , which includes a nozzle plate  75  having a nozzle  75   a  for ejecting ink. The head  73  has ink chambers  77   a  storing ink behind the nozzle plate  75 . The chambers  77   a  are connected to the nozzle  75   a.  The head  73  also includes actuators  77  including piezoelectric elements. Each of the actuators  77  is associated with one of the chambers  77   a.  The head  73  further includes a manifold  79  connected to the chambers  77   a.  The manifold  79  is also connected through a tube (not shown) or the like to an ink tank (not shown) for supplying the chambers  77   a  with ink. When one or more of the actuators  77  are energized to generate pressure vibrations in the associated chambers  77   a,  ink is ejected from the nozzle  75   a.    
     This printer also includes a cap  83  for compressively contacting the nozzle surface  75   b  of the nozzle plate  75  to airtightly cover the outer side of the nozzle  75   a.  The cap  83  has a suction port  83   a  connected to a suction pump (not shown). When the pump is driven, negative pressure develops in the cap  83 . 
     As shown in FIG. 5A, air bubbles  99  may be produced in the ink chambers  77   a.  As shown in FIG. 5B, it is possible to suck and remove the bubbles  99  together with ink by capping the nozzle  75   a  with the cap  83  and driving the pump. 
     On the other hand, negative pressure is applied always to the ink chambers  77   a  and nozzle  75   a.  Immediately after the pump stops, as shown in FIG. 5C, this negative pressure may draw back into the ink jet head  73  some of the bubbles  99  sucked already into the cap  83 . Even after the suction ends and the cap  83  separates from the head  73 , as shown in FIG. 5D, air bubbles  99  remain in the head, and may cause defective ejection of ink from the nozzle  75   a.  Not only the bubbles  99  but also foreign substances and/or solidified ink may be drawn back into the head  73 , and cause defective ejection of ink. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an ink jet recorder in which air bubbles etc. can be removed securely from the ink jet head with the nozzle capped airtightly, to prevent the head from ejecting ink defectively. 
     In accordance with the invention, an ink jet recorder is provided, which includes an ink jet head having a nozzle for ejecting ink. The head also has ink passages formed in it and communicating with the nozzle. The nozzle can be capped with a suction cap. The recorder also includes a suction pump for sucking ink from the nozzle through the cap to purge the nozzle. The recorder further includes an ink supply for supplying the head with ink. A pressure control unit is provided or interposed between the supply and the head to control the pressure applied to the ink in the nozzle. 
     The pressure control unit applies positive pressure to the ink in the nozzle at least just before purging finishes, that is to say, before the suction cap is separated from the ink jet head. This prevents the air bubbles, the foreign substances and/or the like sucked already into the cap by the suction pump from flowing back into the head. Consequently, the ink jet recorder can prevent defective ejection of ink, particularly just after the purging. 
     When the suction cap is separated from the ink jet head during the purging, the suction pump may be either stopped or kept driven to suck the ink remaining in the cap (sucking ink and air to keep the head clean). 
     While the nozzle is not capped, the pressure control unit may apply negative pressure to the ink in the nozzle to maintain the menisci of ink in the nozzle. 
     The pressure control unit may include an ink tank in liquid communication with the ink passages in the ink jet head. The unit may also include a tank height adjuster for adjusting the height of the tank relatively to the front end of the nozzle. By using the tank and the height adjuster, it is possible to simplify the structure of the control unit and make the pressure control easy. The height adjuster can make the ink surface in the tank higher than that in the nozzle to produce a head difference for applying positive pressure to the ink in the nozzle so that the ink can be discharged from the nozzle. The adjuster can also make the ink surface in the tank lower than that in the nozzle to apply negative pressure to the ink in the nozzle so that the menisci of ink can be maintained well in the nozzle. 
     The pressure control unit may instead be a pump provided between the ink supply and the ink jet head. 
     The ink jet recorder may also include a controller for controlling the driving of the tank height adjuster and the suction pump. The controller may be a microcomputer for batch control of the operation of the recorder. 
     The ink jet recorder may further include a detector for detecting the ink level in the ink tank. On the basis of the result of the detection by the detector, the controller may control the tank height adjuster in such a manner that the ink surface in the ink tank is positioned at a predetermined level relative to the front end of the nozzle. 
     The ink supply may be an ink cartridge, which can be replaced by the user. The cartridge may contain ink and be sealed. The cartridge may be larger in volume than the ink tank. The ink tank may be open so that the ink in it may be exposed to the atmosphere. In this case, it is preferable that the tank be as small as possible in volume to prevent the ink from oxidizing or altering otherwise in quality. If the tank is small, however, the ink jet recorder can record only a small number of images continuously. By making the cartridge (main ink tank) larger than the tank, it is possible to replenish ink all times from the cartridge and therefore perform continuous recording even if the tank is small. Because the cartridge is closed, the ink in it is inhibited from altering in quality. 
     The side walls of the ink passages may be made of piezoelectric material. The invention can be also applied to bubble jet type ink jet recorders. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention is shown in the accompanying drawings, in which: 
     FIG. 1 is a schematic view in vertical section of an ink jet printer according to the embodiment; 
     FIGS. 2A and 2B are a top plan and a front view, respectively, of the head difference adjuster of the printer; 
     FIG. 3 is a block diagram of the control system of the printer; 
     FIGS. 4A-4D are views similar to FIG. 1, but showing the suction and removal process of the printer; 
     FIGS. 5A-5D are schematic views showing the suction and removal process of a conventional ink jet printer. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 1, an ink jet printer  1  according to the invention includes an ink jet head  3  mounted on a known carriage  11  (FIG.  3 ), which can move along a known cylindrical platen  13  (FIG.  3 ). The head  3  includes a nozzle plate  5  at its bottom, which has a nozzle  5   a  for ejecting ink. The head  3  has ink chambers  7   a  storing ink and positioned over the nozzle plate  5 . The head  3  also includes actuators  7  including piezoelectric elements. The actuators  7  are each associated with one of the chambers  7   a.  The head  3  further includes a manifold  9  at its top, which is connected to the chambers  7   a.  The manifold  9  is also connected through a tube  19  to an open ink tank  21  for supplying the head  3  with ink  97 . When one or more of the actuators  7  are energized to generate pressure vibrations in the associated chambers  7   a,  ink is ejected from the nozzle  5   a.    
     The surface  97   a  of the ink  97  stored in the tank  21  is exposed to the atmospheric pressure. The tank  21  is supported vertically movably by a head difference adjuster  23 . The tank  21  is connected through a tube  27  to a closed main tank  29 . The tube  27  is fitted with an ink pump  25  in its middle for supplying ink from the closed tank  29  to the open tank  21 . The closed tank  29  contains more ink than the open tank  21 . 
     The printer  1  further includes a maintenance mechanism  31  (FIG. 3) for cleaning the ink jet head  3  in a predetermined maintenance position. This mechanism  31  includes a cap  33  which can be moved into compressive contact with the nozzle surface  5   b  of the nozzle plate  5  by a moving device (not shown) to cover the outer side of the nozzle  5   a  airtightly. The cap  33  has a suction port  33   a  connected to a suction pump  35  (FIG.  3 ). By capping the nozzle  5   a  with the cap  33  and driving the pump  35 , thereby developing negative pressure in the cap  33 , it is possible to suck and remove bad ink and/or the like out of the head  3 . The maintenance mechanism  31  also includes a known wiper (not shown) etc. 
     U.S. Pat. Nos. 5,450,105, 5,486,854 and 5,570,116 disclose maintenance mechanisms each including a suction pump which can be used in the printer  1 . The disclosure of the patents is incorporated herein by reference. 
     As shown in FIGS. 2A and 2B, the head difference adjuster  23  includes a square sill or base plate  37  and a square horizontal movable table or plate  39 . A pair of vertical cylindrical guides  41  stand on the sill  37  near its two adjacent vertexes or corners. The guides  41  extend loosely through the table  39 . A drive unit  43  is mounted on the sill  37  midway between the other corners. The drive  43  may include an AC motor, a DC motor, a stepping motor or another rotating actuator. A vertical ball screw  45  extends from the drive  43  and can be rotated by it. The table  39  is in engagement with the screw  45  and kept horizontal. When the screw  45  rotates, the table  39  moves vertically. 
     As shown in FIG. 3, the printer  1  further includes a carriage motor  47  for moving the carriage  11  through a belt etc., a platen motor  49  for turning the platen  13  to feed a recording sheet (not shown) as a recording medium, an interface  51  for sending signals to and receiving signals from an external computer, and a liquid level sensor  53  for detecting the level of the ink surface  97   a  relative to the adjuster sill  37 . The sensor  53  may detect the ink level on the basis of the electric conduction between terminals, because ink is conductive. These components  47 ,  49 ,  51  and  53  are connected to an electronic control circuit  55 , which is connected to the actuators  7 , the pumps  25  and  35 , and the drive  43 . 
     The control  55  is a microcomputer including a CPU  55   a,  a ROM  55   b  and a RAM  55   c.  On the basis of the signals input from the interface  51  and the liquid level sensor  53 , the control  55  controls the driving of the actuators  7 , the pumps  25  and  35 , the drive  43  and the motors  47  and  49  as follows. 
     If image data are input, the control  55  performs a process of image formation, which includes driving the platen motor  49  to feed a recording sheet, driving the carriage motor  47  to move the ink jet head  3  to a desired position, and subsequently energizing one or more of the actuators  7  to eject ink from the nozzle  5   a.  This process is repeated to form an image on the sheet in accordance with the data. 
     During the process of image formation, the control  55  energizes the drive  43  on the basis of the signal from the liquid level sensor  53  to position the ink surface  97   a  at a predetermined distance below the nozzle  5   a.  The nozzle  5   a  is kept at a constant height relative to the adjuster sill  37  by the carriage  11  for holding the ink jet head  3  thereon. Because the ink chambers  7   a,  the tube  19  and the open ink tank  21  are filled with ink, the head difference h 1  (FIG. 4A) between the surface  97   a  and the nozzle  5   a  applies negative pressure to the ink in the nozzle. Concave menisci of ink are formed in the holes of the nozzle  5   a,  and their surface tension balances with the negative pressure. This keeps ink from leaking out of the nozzle  5   a  while the actuators  5  are not energized. It is therefore possible to eject ink only to desired spots, well preventing recording sheets from being stained. 
     The control  55  can detect a decrease in the ink in the open tank  21  on the basis of the amount of driving of the drive  43  and the signal from the liquid level sensor  53 . During the process of image formation, if the control  55  detects an ink decrease in the open tank  21 , it drives the ink pump  25  to supply this tank  21  from the main tank  29 . 
     The control  55  performs a maintenance process at a predetermined time during the process of image formation, a predetermined time after the printer  1  is switched on, or another predetermine time. The maintenance process is shown in FIGS. 4A-4D. 
     At one the predetermined times, the control  55  energizes the carriage motor  47  to move the ink jet head  3  to the maintenance position, as shown in FIG. 4A, where the nozzle  5   a  faces the cap  33 . At this stage or point, the ink surface  97   a  is positioned below the nozzle surface  5   b.    
     Subsequently, the control  55  performs a suction and removal process as shown in FIGS. 4B-4D. First, as shown in FIG. 4B, the cap  33  is moved into compressive contact with the nozzle surface  5   b.  In the meantime, the open tank  21  is lifted until the ink surface  97   a  is positioned above the nozzle surface  5   b.  Thereafter, the suction pump  35  (not shown in FIGS. 4A-4D) is driven to suck and remove liquid ink  97 , solidified ink, air bubbles  99 , foreign substances, etc. from the ink chambers  7   a.  After the suction pump  35  is stopped with the ink surface  97   a  kept above the nozzle surface  5   b  as shown in FIG. 4C, the cap  33  is separated from the nozzle surface. Thereafter, as shown in FIG. 4D, the open tank  21  is lowered. This completes the suction and removal process. 
     In this example, although the cap  33  has been moved into compressive contact with the nozzle surface  5   b  before the open tank  21  is lifted to apply the positive pressure to the nozzle  5   a,  the open tank may be lifted to apply the positive pressure to the nozzle  5   a  before the cap is covered with the nozzle surface  5   b.  In this case, the positive pressure may be controlled so that the ink is not leaked out of the nozzle  5   a.    
     The maintenance process also includes wiping the nozzle surface  5   b  with the wiper (not shown) after the suction and removal process. When the nozzle surface  5   b  is wiped, negative pressure is kept applied to the ink in the nozzle  5   a  to maintain an appropriate meniscus. It is therefore possible to clean the nozzle surface  5   b  well. 
     Thus, in the maintenance process, after the suction pump  35  is driven and until the cap  33  is separated from the nozzle surface  5   b  to decap the nozzle  5   a,  the ink surface  97   a  is kept above the nozzle surface. While the ink surface  97   a  is kept above, the head difference h 2  (FIG. 4B) between it and the nozzle surface  5   b  applies positive pressure to the ink in the nozzle  5   a.  This keeps the pressure in the capped nozzle  5   a  higher than the pressure in the cap  33  after the suction pump  35  stops. Therefore, even after the pump  35  stops, the bubbles  99  etc. sucked into the cap  33  do not flow back into the ink jet head  3 . It is consequently possible to remove the bubbles  99  etc. securely from the head  3 , thereby preventing the head well from ejecting ink defectively. 
     The open tank  21  might be lifted just before the suction pump  35  stops. In this case, it is possible to remove air bubbles  99  etc. likewise, and reduce the amount of sucked ink so that ink is saved. If, as stated earlier, the open tank  21  is lifted before the suction pump  35  is driven, it is possible to reduce the load on this pump and remove the bubbles  99  rapidly. 
     The main tank  29  contains a large amount of ink  97 . With ink supplied from this tank  29  to the open tank  21 , the printer  1  can form an image continuously. Because the main tank  29  is closed, the ink in it alters little in quality. This enables the open tank  21  to be smaller relatively in volume, well preventing the ink in it from altering in quality. This also makes it possible to form a number of images continuously. 
     The head difference adjuster  23  is an example of the tank height adjuster. The electronic control circuit  55  and the driving of the adjuster  23  by it is an example of the pressure control unit. The invention is not limited to the foregoing embodiment, but various modifications may be made without departing from the spirit and scope of the invention. 
     The head difference adjuster  23  changes the level of the ink surface  97   a  to apply either positive or negative pressure to the nozzle  5   a.  Pressure might instead be applied to the nozzle  5   a  by either forced pressurization or forced pressure reduction with a pump or the like. However, the adjuster  23  makes very simple the system for changing the ink level, and can therefore make the apparatus simpler in structure. It might otherwise be possible to change the ink level by either inflating or deflating a balloon in the open tank  21 . However, the adjuster  23  makes it easier to adjust the ink level, and simplifies the apparatus more in structure. Therefore, the foregoing embodiment can make the structure of the printer  1  and the control process simpler. 
     In the head different adjuster  23  of the above embodiment, the movable table  39  has been moved by the drive unit  43  and the vertical ball screw. However, it is possible to control the movable table  39  or the open tank  21  itself so as to move upward or downward without the drive unit  43  and the vertical ball screw in response to a movement of the carriage  11 . For this purpose, a spring member which is connected to the movable table  39  and the sill  37  at the respective ends and a linking mechanism for linking the action of the movable table  39  to the movement of the carriage  11  may be provided. The movable table  39  may be urged downwardly by the spring member. The linking mechanism may lift the movable table  39  against the spring force of the spring member when the carriage comes into the maintenance area for maintenance operation from the printing area. The linking mechanism may comprise a swing or rotatable arm which can swing or rotate about an axis. When the carriage  11  moves into the maintenance area, a portion of the carriage  11  can contacts with one end of the arm to swing or rotate the arm such that the other end of the arm can lift the movable table  39  against the downwardly urging force of the spring member. Once the carriage  11  returns to the printing area, the arm swings or rotates back to the original state and thereby the force for lifting the movable table  39  is released. As a result, the movable table  39  returns to the original height position by means of the spring member. In the above modification, the spring member may be connected directly to the open tank  21  and the movable stage may be a part of the open tank  21 . 
     The ink jet head  3  is connected through the tube  19  to the open tank  21 . Therefore, the invention can also be applied suitably to a printer in which the ink jet head is not moved by a carriage, but mounted in a fixed position, and only a recording sheet can move.