A liquid-ejection apparatus, including: a head configured to eject a liquid from a plurality of liquid-ejection openings; a main tank configured to accommodate the liquid supplied to the head; a liquid-supply pump configured to supply the liquid in the main tank to the head; a sub-tank in which are formed (a) a connection opening for connecting the sub-tank to the head and (b) an air-discharge opening for communicating the sub-tank with ambient air; a first channel extending from the liquid-supply pump to the sub-tank via the head and the connection opening; a second channel extending from the sub-tank to the liquid-supply pump; a third channel extending from the air-discharge opening of the sub-tank to the ambient air; an air-discharge valve provided in the third channel so as to be openable and closable; and a controller configured to control the air-discharge valve such that the air-discharge valve is temporarily opened in a liquid introducing operation in which the liquid is introduced from the main tank to the liquid-supply pump and an air discharging operation in which air separated from the liquid in the sub-tank by causing the liquid to be flowed through the first channel by driving of the liquid-supply pump is discharged through the third channel, and such that the air-discharge valve is closed in times other than the liquid introducing operation and the air discharging operation.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2008-248099, which was filed on Sep. 26, 2008, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid-ejection apparatus including a sub-tank which discharges air in a channel including a liquid-ejection head.

2. Description of the Related Art

As an ink-jet printer as an example of a liquid-ejection apparatus, there is conventionally known a technique, with reference to Patent Document 1 (U.S. Pat. No. 7,399,075 B2 corresponding to JP-A-2005-306005), that a sub-tank is provided, in addition to a main tank for accommodating an ink supplied to a liquid-ejection head, in order to discharge air in a channel including the head to an outside, for example. The sub-tank accommodates the ink and is communicated at an upper portion thereof with ambient air. When an ink with which air bubbles are mixed is supplied from the head into the sub-tank, the air bubbles are moved upward in the sub-tank to be discharged from the upper portion thereof to the ambient air.

SUMMARY OF THE INVENTION

According to the above-described Patent Document 1, an ambient-air-communicated valve which communicates the sub-tank with the ambient air is temporarily closed in a predetermined operation, but is kept opened in times other than the predetermined operation in order to resolve a pressure difference between the sub-tank and the head. Thus, since the ink in the channel including the head is exposed to the ambient air for a relatively long time, a viscosity of the ink rises, thereby causing a problem that the ink is ejected unstably.

This invention has been developed in view of the above-described situations, and it is an object of the present invention to provide a liquid-ejection apparatus which is configured to include a sub-tank for discharging air in a channel including a liquid-ejection head and which is allowed to assure ink-ejection stability.

The object indicated above may be achieved according to the present invention which provides a liquid-ejection apparatus, comprising: a head configured to eject a liquid from a plurality of liquid-ejection openings; a main tank configured to accommodate the liquid supplied to the head; a liquid-supply pump configured to supply the liquid in the main tank to the head; a sub-tank in which are formed (a) a connection opening for connecting the sub-tank to the head and (b) an air-discharge opening for communicating the sub-tank with ambient air; a first channel extending from the liquid-supply pump to the sub-tank via the head and the connection opening; a second channel extending from the sub-tank to the liquid-supply pump; a third channel extending from the air-discharge opening of the sub-tank to the ambient air; an air-discharge valve provided in the third channel so as to be openable and closable; and a controller configured to control the air-discharge valve such that the air-discharge valve is temporarily opened in a liquid introducing operation in which the liquid is introduced from the main tank to the liquid-supply pump and an air discharging operation in which air separated from the liquid in the sub-tank by causing the liquid to be flowed through the first channel by driving of the liquid-supply pump is discharged through the third channel, and such that the air-discharge valve is closed in times other than the liquid introducing operation and the air discharging operation.

In the liquid-ejection apparatus constructed as described above, since the controller controls the air-discharge valve to be kept closed in the times other than a predetermined period, the liquid in a channel including the head is not exposed to the ambient air for a relatively long time. Thus, a problem that the liquid is ejected unstably owing to a rise of a viscosity of the liquid does not arise, thereby assuring ink-ejection stability.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, there will be described a preferred embodiment of the present invention by reference to the drawings.

Initially, there will be explained a structure of an ink-jet printer1as an embodiment of the present invention with reference toFIG. 1.

As shown inFIG. 1, the ink-jet printer1has a body1ahaving a rectangular parallelepiped shape. On an upper portion of a top plate of the body1a, there is formed a sheet-discharge portion131which receives each recorded recording sheet P having been subjected to recording and discharged from an opening130. An inside space of the body1ais separated into spaces A, B, C in order from above. In the space A, there are disposed four ink-jet heads10which respectively eject inks (i.e., liquids) of four colors, namely, magenta, cyan, yellow, and black, and a sheet-feed unit122which feeds each recording sheet P. Each of the heads10are disposed such that a longitudinal direction thereof coincides with a main scanning direction in which the head10reciprocates, and the sheet-feed unit122feeds each sheet P in a sub-scanning direction. In the spaces B and C, there are respectively disposed a sheet-supply unit1band an ink tank unit1cattachable to and detachable from the body1ain the main scanning direction.

The ink tank unit1cincludes four main tanks21accommodating the respective inks of the four colors respectively corresponding to the four heads10. As shown inFIG. 2, the main tanks21are respectively connected to the heads10via tubes32or the like. The sheet-supply unit1bincludes a sheet-supply tray123which can accommodate a plurality of the sheets P, and a sheet-supply roller125attached to the sheet-supply tray123. The sheets P accommodated in the sheet-supply tray123are supplied one by one from an uppermost one of the sheets P by the sheet-supply roller125and fed to the sheet-feed unit122while being guided by guides127a,127band being nipped between a pair of feed rollers126.

The sheet-feed unit122includes (a) two belt rollers6,7, (b) an endless sheet-feed belt8wound around the rollers6,7so as to bridge the rollers6,7, (c) a tension roller9which applies tension to the sheet-feed belt8by being biased downward while contacting with an inner peripheral surface of the sheet-feed belt8at a lower portion thereof, and (d) a support frame11supporting the rollers6,7,9such that the rollers6,7,9are rotatable. When the belt roller7as a drive roller is rotated in a clockwise direction inFIG. 1, the sheet-feed belt8is rotated, whereby the belt roller6is also rotated in the clockwise direction inFIG. 1.

An upper portion of the sheet-feed belt8is supported by a platen19such that a belt surface of the sheet-feed belt8at the upper portion is distant by a predetermined distance from lower surfaces10aof the respective heads10(each of which functions as an ink-ejection surface in which a plurality of ink-ejection openings for ejecting the corresponding ink are formed and such that the belt surface extends parallel to the lower surfaces10a. The four heads10are arranged side by side in the sub-scanning direction and supported by the body1avia a frame3.

Under the sheet-feed unit122, there is disposed a safety plate12which is bent in a shallow V-shape and holds foreign materials fallen or dropped from the sheet-feed belt8or the like.

On a front surface of the sheet-feed belt8is formed a silicone layer having a weak viscosity. The sheet P fed to the sheet-feed unit122is pressed onto the front surface of the sheet-feed belt8by a pressing roller4, and then fed in the sub-scanning direction along boldface arrow while being held by and on the front surface of the sheet-feed belt8owing to the viscosity thereof. When the sheet P is fed or passed through just under the four heads10, the inks are sequentially ejected onto an upper surface of the sheet P from the ink-ejection surfaces10aof the respective heads10, thereby forming a desired color image on the sheet P. Then, the sheet P is peeled from the front surface of the sheet-feed belt8by a peeling plate5and fed upward while being guided by guides129a,129band being nipped between two pairs of feed rollers128. Then, the sheet P is discharged to the sheet-discharge portion131from the opening30formed in the upper portion of the body1a.

There will be next explained, with reference toFIG. 2, ink-supply systems in the ink-jet printer1inFIG. 1. The ink-supply systems are provided respectively for the four heads10and each includes the corresponding main tank21, an ink-supply pump (liquid-supply pump)22which supplies the ink in the main tank21to the corresponding head10, the head10, and a sub-tank50which separates the ink and air from each other. Hereinafter, there will be explained an ink-supply system corresponding to one of the heads10, but a content of the explanation is common to the ink-supply system of each of the heads10.

The main tank21and the ink-supply pump22are connected to each other by a tube31. The ink-supply pump22and the head10are connected to each other by the tube32. The head10and the sub-tank50are connected to each other by a tube33. The sub-tank50and the ink-supply pump22are connected to each other by a tube34.FIG. 2is a view in which the printer1inFIG. 1is seen in a direction along the sub-scanning direction. The sub-tank50, the ink-supply pump22, the tubes31,32,33,34, a controller100, and so on are disposed in the body1a(with reference toFIG. 1) so as to have positional relationships in a vertical direction shown inFIG. 2.

The main tank21includes an ink bag21afilled with the ink, a pressure-apply plate (pressure-apply portion)21bdisposed on an upper portion of the ink bag21ain a state the pressure-apply plate21bcan press the ink bag21a, and a biasing member21cwhich biases the pressure-apply plate21bdownward. The ink bag21a, the pressure-apply plate21b, and the biasing member21care disposed in a tank casing. When the pressure-apply plate21bis moved downward by the control of the controller100, a pressure is applied to the ink bag21aby pressing of the pressure-apply plate21b, whereby the ink in the ink bag21ais introduced to the ink-supply pump22. That is, the main tank21is disposed such that the ink in the ink bag21aapplies a positive pressure to the ink-supply pump22in “ink introduction” or a liquid introducing operation which will be described below.

The biasing member21cis shown as a spring inFIG. 2, but may be an actuator using a solenoid, a pressing mechanism using a ling mechanism, and so on. In any case, an amount of displacement of the biasing member21cis controlled by the controller100.

The sub-tank50has a hollow cylindrical shape. In a lower portion of a side face of the sub-tank50is formed a connection opening51for connecting the sub-tank50to the head10. In an upper end of the sub-tank50is formed an air-discharge opening52for communicating the sub-tank50with the ambient air. In a lower end of the sub-tank50is formed a circulation opening53for connecting the sub-tank50to the ink-supply pump22. Further, on another side face of the sub-tank50is provided a sensor (detecting portion)55which detects a position of a liquid surface in the sub-tank50.

The air-discharge opening52of the sub-tank50is connected to one end of a tube35. In the other end of the tube35, there is formed an ambient-air-communicating opening35ato which air in the sub-tank50is discharged from the air-discharge opening52by driving of an air-discharge pump80disposed on the tube35. Further, a waste-ink receiver90is provided near the ambient-air-communicating opening35asuch that where a small amount of the ink in the sub-tank50is accidentally discharged in discharging of the air in the sub-tank50, the ink does not flown into the printer1.

In the printer1, there are formed a first channel41, a second channel42, and the third channel43. The first channel41extends from the ink-supply pump22to the sub-tank50via the tube32, the head10, the tube33, and the connection opening51. The second channel42extends from the sub-tank50to the ink-supply pump22via the tube34and the circulation opening53. The third channel43extends from the sub-tank50to the ambient air via the air-discharge opening52, the tube35and the ambient-air-communicating opening35a. In the tube34partly constituting the second channel42is provided an open-close valve24. In the tube35partly constituting the third channel43is provided an openable and closable air-discharge valve25such that the air-discharge pump80is disposed at a position further from the air-discharge opening52than the air-discharge valve25.

The head10includes the ink-ejection surface10ain which the plurality of the ink-ejection openings are formed, and is disposed such that the ink-ejection surface10ahorizontally extends and is located at a position higher in the vertical direction than the air-discharge valve25, an upper end of the sub-tank50, and the open-close valve24in the vertical direction. In other words, the ink-ejection surface10ais located above the air-discharge valve25in the vertical direction, the upper end of the sub-tank50, and the open-close valve24. Further, in the present embodiment, a water head difference of the ink-ejection surface10awith respect to the open-close valve24falls within a range from −100 mmAq to −20 mmAq. That is, “H1” indicated inFIG. 2(a height difference between the ink-ejection surface10aand the open-close valve24in the vertical direction) falls within the range from 20 mmAq to 100 mmAq.

There will be next explained, with reference toFIGS. 3,4, and5, operations of the controller100configured to control various operations of the components of the printer1. More specifically, there will be explained (a) a “ink introduction control” (corresponding toFIG. 3) in which the ink is introduced from the main tank21into the ink-supply pump22, and (b) an “air discharging control” or an air discharging control (corresponding toFIGS. 4 and 5) in which the ink is flowed through the first channel41by driving of the ink-supply pump22, and air separated from the ink in the sub-tank50is discharged from the ambient-air-communicating opening35ato the ambient air through the third channel43.

Here, the “ink introduction control” is performed in an initial setting of the printer1, and the “air discharging control” includes a “first discharging control” performed after the ink introduction in the initial setting of the printer1and a “second discharging control” which is performed when a specific condition is satisfied after the initial setting and in which the ink is flowed through the first channel41in a state that the ink exists in the first channel41. For example, the initial setting is performed when the ink is initially introduced into the printer1, and when the main tank21is replaced. Further, the specific condition is a condition required for the air discharging because an amount of air in the channels41,42is larger than a prescribed amount on the basis of an elapsed time from the initial setting, an elapsed time from a previous air discharging operation, a temperature change, and the like. In detecting the condition, there may be used a timer, a temperature sensor provided in the printer1, the sensor55provided on the sub-tank50, and the like.

As shown inFIG. 3, in the “ink introduction control”, the controller100initially changes in S1the air-discharge valve25in a closed state in which the air-discharge valve25is closed to an open state in which the air-discharge valve25is opened, and then drives in S2the pressure-apply plate21bin a state in which the air-discharge valve25and the open-close valve24are opened (i.e., in their open state). As a result, the ink in the ink bag21aof the main tank21is introduced into the ink-supply pump22. In this time, a speed of the introduction of the ink may be adjusted by adjusting resistance of the air-discharge valve25, thereby preventing generation of air bubbles owing to sudden and strong introduction of the ink, for example. Further, where the ink is accommodated in the sub-tank50in the introduction, the ink is introduced from the circulation opening53of the sub-tank50into the ink-supply pump22via the tube34. The ink is also introduced not only into the ink-supply pump22but also into the channels extending from the ink-supply pump22toward the air-discharge valve25via the tubes32,34. When the controller100has detected that the ink has reached a height of the open-close valve24on the basis of a signal from, e.g., a sensor configured to detect a liquid surface in the second channel42, for example (S3: YES), that is, a liquid surface height becomes equal to the height of the open-close valve24, the controller100judges that the “ink introduction” has been completed in S4. Then, the controller100changes in S5the open-close valve24to a closed state in which the open-close valve24is closed, and completes the ink introduction control. Next, the “first discharging control” is continuously performed.

As shown inFIG. 4, in the “first discharging control”, the controller100initially drives in S11the ink-supply pump22while maintaining the closed state of the open-close valve24and the open state of the air-discharge valve25, and thereby introduces in S12the ink into the first channel41including the head10and the sub-tank50along black arrow inFIG. 2. In this time, since the open-close valve24is in the closed state, the ink-supply pump22uses the ink in the main tank21without using the ink in the sub-tank50to introduce the ink into the first channel41. That is, the ink-supply pump22introduces the ink in the main tank21into the first channel41, thereby enabling the ink introduction in a state in which the air (i.e., the air bubbles) is not included in the ink. In the sub-tank50, with the liquid surface of the ink rising, the ink and the air are separated from each other, so that the air is discharged from the ambient-air-communicating opening35ato the ambient air via the tube35. Here, since the connection opening51is provided on the lower portion of the sub-tank50, the ink is flowed from a portion of the sub-tank50which is lower than the ink liquid surface thereof in height. Thus, unnecessary air bubbles are not generated when the ink is flowed into the sub-tank50. The controller100receives a detection signal relating to a height of the liquid surface of the ink in the sub-tank50from the sensor55, and when the liquid surface of the ink in the sub-tank50is lowered to reach a predetermined height (for example, a height of the upper end of the sub-tank50) (S13: YES), the controller100stops in S14the driving of the ink-supply pump22on the basis of the signal. Then, after S15in which the air-discharge valve25is changed to the closed state, and the open-close valve24is changed to an open state in which the open-close valve24is opened, the “first discharging operation” is completed.

After the operation relating to the initial setting of the printer1in times other than predetermined period in the “second discharging control” which will be described below, the controller100controls the air-discharge valve25to be kept in the closed state. That is, each of the channels41,42including the head10and so on in the present embodiment is of what is called a airtight state or airtight type in which each of the channels41,42is interrupted from the ambient air in the times other than predetermined operations.

As shown inFIG. 5, in the “second discharging control”, the controller100initially drives in S21the ink-supply pump22while maintaining the open state of the open-close valve24and the closed state of the air-discharge valve25, whereby the ink is circulated along the first channel41extending along the black arrow inFIG. 2and the second channel42extending from the sub-tank50to the ink-supply pump22via the tube34. As a result, the ink and the air are separated from each other in the sub-tank50, so that the air is accumulated in an upper portion of the sub-tank50. The controller100receives the detection signal relating to the height of the liquid surface of the ink in the sub-tank50from the sensor55, and when the liquid surface of the ink in the sub-tank50is lowered to reach a predetermined height (for example, a position slightly higher than the connection opening51) within a first predetermined time (S22: YES), the controller100stops in S23the driving of the ink-supply pump22on the basis of the signal. Here, where the predetermined value of the liquid surface in S22is set to the position slightly higher than the connection opening51, there can be prevented that the unnecessary air bubbles are generated while the ink is introduced into the sub-tank50.

After S23, the controller100changes in S24the open-close valve24to the closed state and the air-discharge valve25to the open state, and drives the air-discharge pump80in S25. As a result, the accumulated air is discharged from the ambient-air-communicating opening35ato the ambient air via the tube35along white arrow inFIG. 2. Then, when the controller100has judged that a second predetermined time (determined by an air-discharge speed at which the air is discharged by the air-discharge pump80and an air discharged volume to the upper end of the sub-tank50from the liquid surface of the ink in the sub-tank50at a timing in which the controller100has judged that the result in S22is “YES”) has passed from start of driving of the air-discharge pump80(S26: YES), the controller100stops in S27the driving of the air-discharge pump80. Then, after the controller100has changed in S28the air-discharge valve25to the closed state and the open-close valve24to the open state, a processing of the “second discharging operation” returns to S21, and the controller100restarts the “second discharging operation”.

After the driving of the ink-supply pump22is restarted in S21, where the liquid surface of the ink in the sub-tank50is lowered to reach the predetermined height within the first predetermined time (S22: YES), the operations described above are repeated. On the other hand, where the height of the liquid surface of the ink in the sub-tank50has not reached the predetermined value even where the first predetermined time has passed (S22: NO and S29: YES), the controller100stops in S30the driving of the ink-supply pump22at a timing at which the first predetermined time has passed. Here, the first predetermined time is set to a time more than the second predetermined time. Next, the controller100changes in S31the open-close valve24to the closed state and the air-discharge valve25to the open state, and drives the air-discharge pump80in S32. Then, when the controller100has judged that a third predetermined time (determined by the air-discharge speed by the air-discharge pump80and the air discharged volume to the air-discharge valve25from the liquid surface of the ink in the sub-tank50at a timing in which the controller100has judged that the result in S22is “NO”) has passed from the start of the driving of the air-discharge pump80(S33: YES), the controller100stops in S34the driving of the air-discharge pump80, and changes the air-discharge valve25to the closed state and the open-close valve24to the open state. At a time in S34, a distal end of the ink in the third channel43has reached a position of the air-discharge valve25or a position slightly nearer to the ambient-air-communicating opening35athan the air-discharge valve25. As a result, there is formed an ink-supply system of what is called the airtight state or airtight type in which no air (no air bubbles) exists in channels including the first channel41, the second channel42, and a part of the third channel43which extends from the sub-tank50to the air-discharge valve25.

In the present embodiment, the controller100changes the open-close valve24to the closed state in S24and S31which are performed before the start of the driving of the air-discharge pump80, but the present invention is not limited to this configuration, that is, the open-close valve24may be in the open state. Like in the present embodiment, where the air-discharge pump80is driven after the controller100changes the open-close valve24to the closed state in S24and S31, the ink is supplied from the first channel41to the third channel43via the sub-tank50. On the other hand, where the air-discharge pump80is driven in a state in which the open-close valve24is in the open state, the first channel41and the second channel42are communicated in parallel with the sub-tank50. Thus, even where the air is accumulated in the second channel42, the air is reliably discharged via the sub-tank50. Further, in this case, since a channel resistance from the main tank21to the sub-tank50is lowered, a negative pressure applied by the air-discharge pump80to the head10is lowered. Thus, a meniscus of the ink formed in the ink-ejection openings of the head10is less broken, so that the air-discharge speed by the air-discharge pump80can be made higher accordingly.

In the present embodiment, the water head difference of the ink-ejection surface10aof the head10with respect to the open-close valve24falls within the range from −100 mmAq to −20 mmAq, but the present invention is not limited to this configuration. For example, a water head difference of the ink-ejection surface10aof the head10with respect to the liquid surface of the ink in the sub-tank50may fall within the range from −100 mmAq to −20 mmAq. That is, a height difference between the ink-ejection surface10aof the head10and the liquid surface of the ink in the sub-tank50falls within the range from 20 mmAq to 100 mmAq. Further, a water head difference of the ink-ejection surface10aof the head10with respect to the air-discharge valve25may fall within the range from −100 mmAq to −20 mmAq. That is, a height difference between the ink-ejection surface10aof the head10and the air-discharge valve25falls within the range from 20 mmAq to 100 mmAq.

It is noted that, in each of the above-described operations, the controller100controls the components such that the meniscus of the ink formed in the ink-ejection openings of the head10is not broken.

As described above, according to the present embodiment, since the controller100controls the air-discharge valve25to be temporarily in the open state and to be kept in the closed state in the times other than the predetermined period, the ink in the channels41,42including the head10is not exposed to the ambient air for a relatively long time. Thus, a problem that the ink is ejected unstably owing to a rise of a viscosity of the ink does not arise, thereby assuring ink-ejection stability.

The main tank21is disposed such that the ink in the main tank21applies the positive pressure to the ink-supply pump22in the “ink introduction”, and the introduction of the ink into the ink-supply pump22is performed not by the driving of the ink-supply pump22but by the pressure of the ink in the main tank21. Thus, since the driving of the ink-supply pump22in a state in which no ink exists in the ink-supply pump22can be avoided, there do not occur problems such as wear deterioration of the ink-supply pump22, generation of foreign materials, and the like caused by idle running of the ink-supply pump22. Further, since the driving of the ink-supply pump22in a state in which the air and the ink are mixed in the ink-supply pump22can be avoided, problems such as generation of the air bubbles by agitation, and the like do not occur. Further, compared to a case in which the “ink introduction” is performed by the driving of the ink-supply pump22, the ink can be introduced stably, and it can be prevented that the ink is discarded more than necessary. In addition, even where a non-self-priming pump (e.g., an impeller pump and a turbopump) which normally requires a means for introducing priming is employed as the ink-supply pump22, the means for introducing priming need not be provided. Further, since a choice of options of the ink-supply pump is increased by the above-described configuration, problems such as generation of the foreign materials by sliding in using a pump of a volume-change type do not occur by avoiding employment of the pump of the volume-change type, and it is possible to employ, as the ink-supply pump22, the impeller pump which simplifies, e.g., construction of the channels and is economical compared to, e.g., a tube-type pump.

The pressure-apply plate21bwhich applies the pressure to the ink in the main tank21, and, in the ink introduction, the controller100changes the air-discharge valve25to the open state and controls the pressure-apply plate21bsuch that the ink in the main tank21is introduced into the ink-supply pump22. Thus, the above-described effects in the ink introduction can be obtained more reliably.

The printer1includes the air-discharge pump80which discharges the air in the sub-tank50from the air-discharge opening52, and, in the air discharging shown inFIGS. 4 and 5, when the air is discharged from the sub-tank50by changing of the air-discharge valve25to the open state, the controller100drives the air-discharge pump80(in S15and S26). By the driving of the air-discharge pump80, the discharging of the air from the sub-tank50is promoted, thereby avoiding problems caused by the air in the channels more reliably.

The printer1includes the open-close valve24provided in the second channel42, and the controller100changes the open-close valve24to the open state in the ink introduction. Thus, the ink in the sub-tank50can be introduced into the ink-supply pump22, thereby leading to lower cost of the printer1.

The controller100controls the open-close valve24to be kept in the closed state in the first discharging control shown inFIG. 4, and changes the open-close valve24to the open state in S15after the operation has been completed. By keeping the closed state of the open-close valve24in the first discharging control as thus described, the ink-supply pump22introduces the ink only from the main tank21. Thus, the sub-tank50needs only relatively small volume, thereby enabling a downsizing of the sub-tank50. Further, a problem that the ink with which the air bubbles in the sub-tank50are mixed is introduced into the ink-supply pump22can be avoided. Further, in a case where a liquid in the channel is replaced with another liquid, e.g., in a case where the ink is replaced with the ink of another color, a mixture of an unnecessary liquid can be prevented, thereby realizing the replacement in a relatively short time.

In the second discharging control shown inFIG. 5, the controller100changes the open-close valve24to the closed state (in S24and S31) before the air is discharged from the sub-tank50by changing of the air-discharge valve25to the open state, and the controller100changes the air-discharge valve25to the closed state and the open-close valve24to the open state (in S28and S34) after the air is discharged. Here, at least one of the second predetermined time in S26and the third predetermined time in S33may be set to a time in which at least a part of the ink in the sub-tank50is discharged from the ambient-air-communicating opening35ato the ambient air. Thus, the ink in the sub-tank50whose viscosity has risen can be discarded with the air without mixing with the ink accommodated in the main tank21.

Like the present embodiment, in the case where the controller100judges that the “ink introduction” has been completed when the ink has reached the height of the open-close valve24, since the ink-ejection surface10aof the head10is located above the open-close valve24, there is prevented leakage of the ink from the ink-ejection openings in the ink introduction.

The printer1includes the sensor55which detects the position of the liquid surface of the ink in the sub-tank50, and the controller100controls the opening and closing of the air-discharge valve25(with reference to S13inFIG. 4and S23inFIG. 5) on the basis of the detection of the sensor55. Thus, the air can be discharged via the air-discharge valve25at a proper timing on the basis of the liquid surface detected by the sensor55.

While the preferred embodiment of the present invention has been described, it is to be understood that the present invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the present invention.

For example, in the above-described embodiment, there is provided the sensor55which detects the liquid surface of the ink in the sub-tank50, but the present invention is not limited to the sensor which detects only an inside of the sub-tank50. That is, there may be provided a sensor (the detecting portion) which detects a liquid surface of the ink in a channel extending from the sub-tank50to the air-discharge valve25. It is noted that the sensor55is not essential in the present invention and may be omitted.

The open-close valve24and/or the air-discharge pump80may be omitted, and the controls of the controller100for the omitted at least one of the open-close valve24and the air-discharge pump80may be omitted. As modifications of the ink-jet printer1according to the above-described embodiment,FIG. 6shows an ink-jet printer201from which the open-close valve24is omitted whileFIG. 7shows an ink-jet printer301from which the open-close valve24and the air-discharge pump80are omitted.

In the printer201inFIG. 6as the modification, a controller200judges that an “ink introduction” has been completed when having detected that the ink has reached a height of the sub-tank50(i.e., the upper portion thereof) on the basis of a signal from, e.g., a sensor which detects the liquid surface. Although a first discharging control and a second discharging control following an ink introduction control are respectively similar to those in the above-described embodiment, there is a difference that the opening and closing of the open-close valve24are not performed in this modification. In this modification, since the first channel41and the second channel42are communicated in parallel with the sub-tank50, the air is less accumulated in the second channel42. Further, since the channel resistance from the main tank21to the sub-tank50is lowered, the negative pressure applied by the air-discharge pump80to the head10is lowered. Thus, the meniscus of the ink formed in the ink-ejection openings of the head10is less broken, so that the air-discharge speed by the air-discharge pump80can be made higher accordingly. Further, in the printer201inFIG. 6, a water head difference of the ink-ejection surface10aof the head10with respect to the liquid surface of the ink in the sub-tank50falls within a range from −100 mmAq to −20 mmAq. That is, “H2” indicated inFIG. 5(a height difference between the ink-ejection surface10aand the liquid surface of the ink in the sub-tank50in the vertical direction) falls within the range from 20 mmAq to 100 mmAq. It is noted that, as to a setting of the water head difference, the water head difference of the ink-ejection surface10aof the head10with respect to the air-discharge valve25may fall within the range from −100 mmAq to −20 mmAq. A height difference between the ink-ejection surface10aand air-discharge valve25in the vertical direction may fall within the range from 20 mmAq to 100 mmAq.

In the printer301inFIG. 7as the modification, a controller300judges that an “ink introduction” has been completed when having detected that the ink has reached a height of the air-discharge valve25on the basis of a signal from, e.g., a sensor which detects the liquid surface. Further, in the printer301, a water head difference of the ink-ejection surface10aof the head10with respect to the air-discharge valve25falls within a range from −100 mmAq to −20 mmAq. That is, “H3” indicated inFIG. 7(a height difference between the ink-ejection surface10aand the air-discharge valve25in the vertical direction) falls within the range from 20 mmAq to 100 mmAq. Here, a first discharging control and a second discharging control following an ink introduction control are respectively different from those in the above-described embodiment and are relatively simple. Specifically, in the second discharging control, the controller301initially changes the air-discharge valve25to the closed state, and then the ink-supply pump22is driven to start a circulation of the ink. In this time, the air in the head10is flowed into and accumulated in the sub-tank50with the ink. In this modification, a volume of a portion of the sub-tank50which is higher than the connection opening51is larger than that of channels (including the head10) higher than the air-discharge valve25. Thus, where a specific time has passed, increase of the air in the sub-tank50is stopped. Where, in this state, the air-discharge valve25is changed to the open state, the ink in the main tank21is pressed by the pressure-apply plate21band thus the liquid surface of the ink in the sub-tank50rises and reaches the height of the air-discharge valve25. In this time, the controller301has judged that the ink has reached the height of the air-discharge valve25on the basis of the signal from, e.g., the sensor which detects the liquid surface, and changes the air-discharge valve25to the closed state. As a result, the ink-supply system of the airtight state or airtight type is formed. It is noted that, as to the setting of the water head difference, the water head difference of the ink-ejection surface10aof the head10with respect to the liquid surface of the ink in the sub-tank50may fall within the range from −100 mmAq to −20 mmAq. That is, a height difference between the ink-ejection surface10aand the liquid surface of the ink in the sub-tank50in the vertical direction may fall within the range from 20 mmAq to 100 mmAq.

In the construction of the printer as each of the modifications shown inFIGS. 6 and 7, the above-described water head difference is set within the range, thereby preventing the ink from being discharged more than necessary even where there occurs a failure of the air-discharge valve25, a malfunction of the sensor, and the like.

The present invention is not limited to the construction of the above-described embodiment and each modification, and a water head difference of the ink-ejection surface10awith respect to the liquid surface of the ink introduced into at least one of the first channel41, the second channel42, and the third channel43in the ink introduction may fall within the range from −100 mmAq to −20 mmAq. That is, a height difference in the vertical direction between the ink-ejection surface10aand the liquid surface of the ink introduced into the at least one of the first channel41, the second channel42, and the third channel43in the ink introduction may fall within the range from 20 mmAq to 100 mmAq. Alternately, this water head difference may not be formed.

The controller100may judge that the “ink introduction” has been completed when the ink has reached a back-pressure setting position or a position at which a pressure balance is achieved, which positions are other than the above-described positions. In this case, the height of the liquid surface of the ink at a completion of the ink introduction is determined by, e.g., an ability of the ink-supply pump22, and the controller may judge that the “ink introduction” has been completed on the basis of, e.g., a change of a driving current of the ink-supply pump22without performing the operation (e.g., S3) for detecting the liquid surface of the ink like the above-described embodiment or each modification.

The operations in the “ink introduction” and the “air discharging” are not limited to the above-described operations and may be variously changed or modified. For example, the ink in the main tank21is not limited to be introduced also into the channels extending from the ink-supply pump22to the tubes32,34to reach the air-discharge valve25in the “ink introduction”, but may be introduced into only the ink-supply pump22. Further, in the “air discharging”, when the air in the sub-tank50is discharged from the air-discharge opening52, the controller100controls the driving of the air-discharge pump80in the above-described embodiment, but may control driving of at least one of the ink-supply pump22and the pressure-apply plate21bof the main tank21. In this case, the air in the sub-tank50can be discharged without providing the air-discharge pump80and the like, thereby simplifying the configuration of the printer.

As a means applying a pressure to the ink in the main tank21, the pressure-apply plate21bis employed in the above-described embodiment, but other members, mechanisms, and the like may be employed. Further, this means may not be provided.

The main tank21may not be disposed such that the ink in the main tank21applies the positive pressure to the ink-supply pump22in the “ink introduction”. It is noted that, in the above-described embodiment, in the second the second discharging operation shown inFIG. 5, where the ink-supply pump22is driven, the air-discharge valve25is closed, but the present invention is not limited to this configuration. That is, where the ink-supply pump22is driven, the air-discharge valve25may be opened.

The “second discharging control” in the above-described embodiment is for coping with a case where the air cannot be sufficiently discharged by one air-discharge operation. However, where the printer has a configuration in which the air can be sufficiently discharged by one air-discharge operation, a procedure of the air-discharge operation may be simplified. There will be explained a procedure in this case.

As shown inFIG. 8, in the “second discharging control”, the controller100drives in S40the ink-supply pump22while maintaining the open state of the open-close valve24and the closed state of the air-discharge valve25. Then, when the controller100has judged that a fourth predetermined time (i.e., a time in which when the ink is circulated by the ink-supply pump22, all air remaining in a circulation channel is accumulated in the sub-tank50) has passed from the start of the driving of the ink-supply pump22(S41: YES), the controller100stops in S42the driving of the ink-supply pump22. Then, the controller100changes in S43the open-close valve24to the closed state and the air-discharge valve25to the open state. Next, the controller100detects or measures in S44the height of the liquid surface of the ink in the sub-tank50by the sensor55. The controller100calculates an air discharged volume from the liquid surface of the ink in the sub-tank50to the air-discharge valve25on the basis of the signal from the sensor55, and calculates a time (a fifth predetermined time, a calculated value) of the driving of the air-discharge pump80on the basis of the air discharged volume and the air-discharge speed by the air-discharge pump80. Then, the controller100starts in S45the driving of the air-discharge pump80, and when the controller100has judged that the fifth predetermined time has passed from the start of the driving of the air-discharge pump80(S46: YES), the controller100stops in S47the driving of the air-discharge pump80and changes the air-discharge valve25to the closed state and the open-close valve24to the open state. At this time, the distal end of the ink in the third channel43reaches the position of the air-discharge valve25or the position slightly nearer to the ambient-air-communicating opening35athan the air-discharge valve25. As a result, there is formed the ink-supply system of what is called the airtight state or airtight type in which no air (no air bubbles) exists in the channels including the first channel41, the second channel42, and the part of the third channel43which extends from the sub-tank50to the air-discharge valve25.

In each of the above-described embodiment and the modifications, a wiping operation in which the ink-ejection surface10ais wiped by a wiper may be performed after the air-discharge operation has been completed, or a purging operation in which the ink is forced to be ejected from the ink-ejection openings after the air-discharge operation has been completed, and then the wiping operation may be performed. Although there is a possibility of a leakage of the ink from the ink-ejection surface10aduring the operations of the ink introduction and the air discharging, performing the operation such as the wiping operation and the purging operation can keep the ink-ejection surface10aclean and can prevent generation of an ink-ejection failure. Further, where image forming on the sheet P is not required thereafter, the ink-ejection surface10amay be covered with a cap, thereby preventing a thickening of the ink near the ink-ejection openings. On the other hand, where the image forming on the sheet P is required, an image forming processing for forming an image on the sheet P on the basis of image data is performed while the controller stops the driving of the ink-supply pump22and keeps the open-close valve24being in the open state.

The liquid-ejection apparatus according to the present invention is applicable to both of a line-type printer and a serial-type printer and also applicable to a facsimile, a copying machine, and the like without being limited to the printer. Further, the liquid-ejection apparatus is applicable to an apparatus configured to eject a liquid different from the ink.