Abstract:
The present invention provides an image forming apparatus for forming an image by discharging liquid from a recording head onto a recording medium, comprising a carriage to which the recording head and a sub-tank for storing the liquid to be supplied to the recording head are mounted and which can be shifted reciprocally, a main tank disposed out of the carriage and adapted to store the liquid, a liquid flow passage connected between the main tank and the sub-tank in order to supply the liquid from the main tank to the sub-tank, a flow passage opening/closing valve disposed in the liquid flow passage on the way and adapted to open and close the liquid flow passage, and control means for controlling opening/closing of the flow passage opening/closing valve in accordance with an image forming condition during image formation using the recording head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus for forming an image by discharging liquid from a liquid discharge head onto a recording medium. 
     2. Description of Related Art 
     In conventional general image forming apparatus, an image is formed on a recording medium by discharging liquid such as ink and treatment liquid for adjusting a fixing ability of the ink from discharge ports of a liquid discharge head disposed in a spaced apart relationship from a surface of the recording medium by a proper distance. An ink tank for containing the ink to be discharged is provided on a carriage together with the liquid discharge head or is disposed out of the carriage. 
     FIG. 5 is a view showing a schematic construction of an image forming apparatus in which the ink tank is provided on the carriage. 
     A carriage  101  on which a liquid discharge head (not shown) for forming an image by discharging liquid such as ink onto a recording medium  102  and an ink tank (not shown) for storing the ink to be supplied to the head are mounted is secured to a part of a belt  104  mounted between and wound around two pulleys  106   a  and  106   b . The belt  104  is shifted by rotatingly driving one  106   a  of the pulleys reversibly by means of a motor  107 , with the result that the carriage  101  is reciprocally shifted in directions A along a shaft  103 . A signal and an electric power for causing the liquid discharge head to discharge the ink are inputted from control means (not shown) of the image forming apparatus via a cable  105 . 
     Further, the image forming apparatus is provided with convey means (not shown) for conveying the recording medium  102  in a direction B, and the image is formed on the entire recording medium  102  by repeating a conveying operation for the recording medium  102  effected by the convey means and an image forming operation effected by the liquid discharge head mounted on the carriage reciprocally shifting with respect to the recording medium  102 . 
     The ink tank can be exchanged with respect to the carriage  101  in accordance with consumption of the ink or new ink can be replenished into the ink tank. Depending upon kind of device of the image forming apparatus, a head cartridge in which an ink tank and a liquid discharge head are integrally formed is used so that the ink tank and the head can be exchanged integrally or collectively. In this type, since an ink storing amount is determined by a dimension of the carriage  101 , it is disadvantageous when a large amount of ink is consumed. 
     FIG. 6 is a view showing a schematic construction of an image forming apparatus in which the ink tank is disposed out of the carriage. 
     An operation of the carriage  101  in the image forming apparatus shown in FIG. 6 is substantially the same as that of the carriage shown in FIG.  5 . However, the image forming apparatus shown in FIG. 6 is designed so that the ink is supplied to a sub-tank  109  (refer to FIG. 7) on the carriage  101  through a tube  108  from a main tank  106  disposed out of the carriage  101 . 
     FIG. 7 is a sectional view showing an internal construction of the carriage shown in FIG.  6 . 
     The sub-tank  109  for storing a small amount of ink is mounted on the carriage  101 , and ink flow passages are communicated from the sub-tank  109  to nozzles  110  of a liquid discharge head. The sub-tank  109  also acts as a buffer for replenishing new ink from the main tank  106  (FIG. 6) and for collecting a bubble generated in the flow passage. 
     As such, by designing so that the main tank  106  is disposed out of the carriage  101  and the ink is supplied to the sub-tank  109  on the carriage  101  through the tube  108 , limitation regarding the dimension of the ink tank occupying the space on the carriage  101  is relaxed and, thus, the ink reserving amount of the entire apparatus can be increased. Therefore, recording regarding a recording medium  102  having a large size and recording regarding a large number of recording media  102  become possible. 
     However, when the ink is supplied from the main tank  106  disposed out of the carriage  101  to the liquid discharge head through the tube  108  during the recording operation, there arises a problem that pressure of the ink within the sub-tank  109  is greatly changed to affect an influence upon a discharging ability of the liquid discharge head, thereby worsening appearance quality of a recorded image. In general, it is preferable that the pressure within the sub-tank  109  is within a proper negative pressure range for balancing with discharge energy of the liquid discharge head. If the pressure is increased above such a proper range, i.e., if the pressure becomes near a positive pressure condition or it attains a positive pressure, the ink is apt to be dropped from the nozzles  110 . On the other hand, if the pressure is decreased below such a proper range, i.e., if the negative pressure is further increased, the ink is hard to be discharged from the nozzles  110  and the ink discharging amount tends to be decreased. 
     Main factors for causing the change in pressure inherent to the case where the ink is supplied from the main tank  106  disposed out of the carriage  101  to the liquid discharge head through the tube  108  during the recording operation are (1) dynamic pressure of the tube, (2) pressure loss in the tube and (3) difference in posture. 
     Here, the “dynamic pressure of the tube” is a phenomenon that the tube  108  connected to the sub-tank  109  on the carriage  101  is vibrated as the carriage  101  is reciprocally shifted, with the result that inertia of the ink within the tube  108  acts to increase or decrease the pressure with respect to the sub-tank  109 , which phenomenon is generated in accordance with acceleration/deceleration of the carriage and vibration due to such acceleration/deceleration. 
     Further, the “pressure loss in the tube” is flow resistance in the tube  108 . If a consumed amount of ink due to ink discharging from the liquid discharge head becomes greater than the amount of ink supplied from the tube  108 , the pressure in the sub-tank  109  will gradually be decreased, i.e., the negative pressure will be further increased. When an inner diameter of the tube  108  is sufficiently great, such a phenomenon is not noticeable, but, since the ink amount existing within the tube  108  is increased, the influence of the dynamic pressure of the tube tends to increase, contrary to the influence of the pressure loss in the tube. 
     Further, the “difference in posture” is inclination of the installed recording apparatus. If the recording apparatus is inclined with respect to the scanning direction of the carriage  101 , since at both stroke ends of the reciprocating carriage  101 , height of the carriage  101  with respect to the main tank, that is, height of the nozzles  110  is differentiated, change in pressure is generated due to difference in water head. 
     As countermeasure for such change in pressure, in the conventional image forming apparatuses, for example, as proposed in Japanese Patent Application Laid-open No. 1-281948 (1989), a fluidity suppressing member for suppressing fluidity of ink is added to each flow passage, or, as proposed in Japanese Patent Application Laid-open No. 5-201015 (1993), a sub-tank has a pressure dampening function as a pressure damper. In the image forming apparatus, in addition to consideration of such countermeasure, effort is generally made to provide a head in which use conditions mainly including a recording speed condition are stipulated to suppress the change in pressure within the sub-tank and to permit possible pressure change. 
     Further, as countermeasure for dampening the influence of the pressure change of the ink, it is considered that an open/close valve is provided in a flow passage between the main tank and the sub-tank so that the valve is opened to suppress the change of pressure within the tube during the recording operation and the valve is opened to supply the ink intermittently during a waiting condition. 
     However, in the above-mentioned countermeasure for the change in pressure, there were the following drawbacks. 
     That is to say, when the ink is supplied during the recording operation, since the apparatus is designed under the condition imposed regarding the pressure change, these are determined limits to the appearance quality of the recorded image and the discharging ability, so that abilities of constructural elements of the apparatus cannot be demonstrated adequately. Further, in the apparatus in which the open/close valve is closed to suppress the pressure change during the recording operation and the valve is opened to supply the ink during the waiting condition, since a continuous recording amount is determined by the volume of the sub-tank, the merit obtained by the design in which the ink is supplied from the main tank to the sub-tank through the tube cannot be sufficiently utilized. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus in which change in pressure within a sub-tank caused by dynamic pressure of a tube is selectively excluded and maximum ability can be achieved in accordance with an image forming condition to which the user desires. 
     Another object of the present invention is to provide an image forming apparatus for forming an image by discharging liquid from a recording head onto a recording medium, comprising a carriage to which the recording head and a sub-tank for storing the liquid to be supplied to the recording head are mounted and which can be shifted reciprocally, a main tank disposed out of the carriage and adapted to store the liquid, a liquid flow passage connected between the main tank and the sub-tank in order to supply the liquid from the main tank to the sub-tank, a flow passage opening/closing valve disposed in the liquid flow passage on the way and adapted to open and close the liquid flow passage, and control means for controlling opening/closing of the flow passage opening/closing valve in accordance with an image forming condition during image forming using the recording head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic constructural view showing an image forming apparatus according to an embodiment of the present invention; 
     FIG. 2 is a flow chart for explaining a controlling operation of the image forming apparatus according to the present invention; 
     FIG. 3 is a flow chart for explaining another controlling operation of the image forming apparatus according to the present invention; 
     FIG. 4 is a flow chart for explaining a further controlling operation of the image forming apparatus according to the present invention; 
     FIG. 5 is a schematic constructural view of a conventional ink jet recording apparatus designed to have an ink tank on a carriage; 
     FIG. 6 is a schematic constructural view of a conventional ink jet recording apparatus designed to have an ink tank disposed out of a carriage; and 
     FIG. 7 is a sectional view showing an internal construction of the carriage shown in FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. 
     FIG. 1 is a schematic constructural view showing an image forming apparatus according to an embodiment of the present invention. 
     Similar to the apparatus shown in FIGS. 6 and 7, the image forming apparatus according to the illustrated embodiment comprises a carriage  1  on which a liquid discharge head (not shown) for forming an image by discharging liquid such as ink onto a recording medium  2  and a sub-tank (not shown) for storing the ink to be supplied to the head are mounted, a belt  4  a part of which is secured to the carriage  1  and which is wound around two pulleys  6   a  and  6   b , a motor  7  for rotatingly driving one  6   a  of the pulleys reversibly, and a cable  5  for inputting a signal and an electric power for causing the liquid discharge head to discharge the ink, from control means (not shown) of the image forming apparatus to the liquid discharge head. The carriage  1  is reciprocally shifted in directions A along a shaft  103  by shifting the belt  4  by rotatingly driving the pulley  6   a  reversibly by means of the motor  107 . 
     The image forming apparatus includes convey means (not shown) for conveying a recording medium  2  in a direction B, and the image is formed on the entire recording medium  2  by repeating a conveying operation for the recording medium  2  effected by the convey means and an image forming operation effected by the liquid discharge head mounted on the carriage reciprocally shifting with respect to the recording medium  2 . 
     Further, a main tank  9  is disposed out of the carriage  1 , and the ink is supplied from the main tank to the sub-tank on the carriage  1  via a tube  8 . Further, a flow passage opening/closing valve  10  for opening and closing a liquid passage within the tube  8  is provided on the way of the tube  8  connecting between the main tank  9  and the sub-tank mounted on the carriage  1 . Further, the image forming apparatus includes control means (not shown) for controlling the opening/closing of the flow passage opening/closing valve  10 . 
     Incidentally, for example, the flow passage opening/closing valve  10  may be an electromagnetic valve capable of being switched between an opening condition and a closing condition. Pressure in the sub-tank is set to be within a negative pressure range which permits the discharging of the ink statically both in an opened condition and a closed condition of the flow passage opening/closing valve  10  so long as the amount of the reserved ink is proper. 
     (First Embodiment) 
     FIG. 2 is a flow chart showing a first embodiment of an image forming apparatus according to a first embodiment. 
     When the control means (not shown) of the image forming apparatus receives image data from an information equipment (not shown) as an external device for the image forming apparatus (step S 1 ), the control means judges whether a recording mode of the image data is an “appearance quality preferential mode” or a “speed preferential mode” (step S 2 ). For example, such judgement is effected on the basis of information regarding an image forming condition included in the image data sent from the external information equipment (not shown). 
     If the recording mode is the “speed preferential mode” which allows to generate distortion of the appearance quality of the recorded image due to the pressure change of the ink, i.e., if the appearance quality of the recorded image is lower than a predetermined appearance quality, the flow passage opening/closing valve is opened (step S 3 ) and the recording is executed continuously (step S 4 ). After the recording is completed (step S 5 ), the flow passage opening/closing valve is maintained to the opened condition. 
     On the other hand, if the recording mode is the “appearance quality preferential mode”, i.e., if the appearance quality of the recorded image is higher than the predetermined appearance quality, the flow passage opening/closing valve is closed to suppress the pressure change of the ink during the recording operation (step S 6 ), and the recording is executed in such a condition (step S 7 ). After the recording is completed (step S 8 ), the flow passage opening/closing valve is opened (step S 9 ), thereby supplying the ink from the main tank to the sub-tank. 
     In this way, by controlling the opening/closing of the flow passage opening/closing valve  10  depending upon whether the recording mode of the recording information is the “appearance quality preferential mode” or the “speed preferential mode”, the recording operation of the image forming apparatus can be optimized in accordance with the user&#39;s desire. 
     (Second Embodiment) 
     FIG. 3 is a flow chart showing a second embodiment of an image forming apparatus according to the present invention. 
     When the control means (not shown) of the image forming apparatus receives image data from an information equipment (not shown) as an external device for the image forming apparatus (step S 11 ), the control means judges whether a recording medium on which the image data is to be recorded is a high appearance quality paper or a plain paper (step S 12 ). 
     If the recording medium on which the recording is executed is the plain paper or the like which allows to generate distortion of the appearance quality of the recorded image due to the pressure change of the ink, i.e., if the recording medium is not a recording medium on which an image having appearance quality higher than a predetermined appearance quality is recorded, the flow passage opening/closing valve is opened (step S 13 ) and the recording is executed continuously (step S 14 ). After the recording is completed (step S 15 ), the flow passage opening/closing valve is maintained to the opened condition. 
     On the other hand, if the recording medium on which the recording is executed is the high appearance quality paper, i.e., if the recording medium is a recording medium on which an image having appearance quality higher than a predetermined appearance quality is recorded, the flow passage opening/closing valve is closed to suppress the pressure change of the ink during the recording operation (step S 16 ), and the recording is executed in such a condition (step S 17 ). After the recording is completed (step S 18 ), the flow passage opening/closing valve is opened (step S 19 ), thereby supplying the ink from the main tank to the sub-tank. 
     Incidentally, for example, the judgement regarding the recording medium is effected on the basis of information regarding the recording medium included in the image data sent from the information equipment (not shown) or is effected by discriminating the kind of the recording medium set in the image forming apparatus by means of a medium discriminating sensor provided in the image forming apparatus. 
     In this way, in the illustrated embodiment, by controlling the opening/closing of the flow passage opening/closing valve in accordance with the recording medium on which the recording is executed, the recording operation of the image forming apparatus can be optimized in accordance with the user&#39;s desire. 
     (Third Embodiment) 
     FIG. 4 is a flow chart showing a third embodiment of an image forming apparatus according to the present invention. 
     When the control means (not shown) of the image forming apparatus receives image data from an information equipment (not shown) as an external device for the image forming apparatus (step S 21 ), the control means judges or determines a shifting speed of the carriage corresponding to a recording mode for recording the image data (step S 22 ). 
     If the shifting speed of the carriage is smaller than a predetermined speed, the flow passage opening/closing valve is opened (step S 23 ) and the recording is executed continuously (step S 24 ). After the recording is completed (step S 25 ), the flow passage opening/closing valve is maintained to the opened condition. Incidentally, if the shifting speed of the carriage is smaller than the predetermined speed, distortion of the recording appearance quality caused by the pressure change of the ink becomes to the extent which can be allowed. 
     On the other hand, if the shifting speed of the carriage is greater than the predetermined speed, the flow passage opening/closing valve is closed to suppress the pressure change of the ink during the recording operation (step S 26 ), and the recording is executed in such a condition (step S 27 ). If the shifting speed of the carriage is greater than the predetermined speed, although the pressure change of the ink affecting an influence upon the recording appearance quality is generated, by closing the flow passage opening/closing valve during the recording operation as mentioned above, such an influence can be suppressed. After the recording is completed (step S 28 ), the flow passage opening/closing valve is opened (step S 29 ), thereby supplying the ink from the main tank to the sub-tank. 
     In this way, according to the illustrated embodiment, by controlling the opening/closing of the flow passage opening/closing valve in accordance with the shifting speed of the carriage during the recording operation, the recording operation of the image forming apparatus can be optimized in accordance with the user&#39;s desire. 
     As mentioned above, in the image forming apparatus according to the present invention, since the flow passage opening/closing valve for opening and closing the liquid flow passage is provided on the way of the liquid flow passage and the opening/closing of the liquid flow passage during the image forming operation effected by the liquid discharge head is controlled in accordance with the image forming conditions, the change in pressure within the sub-tank due to the dynamic pressure of the tube can selectively be excluded, and the ability of the image forming apparatus can be achieved at the maximum in accordance with the image forming condition to which the user desire.