Liquid ejection apparatus and liquid processing method

In order to prevent, in an operation for circulating liquid, an ejection port from sucking air or the ejection port from pushing out liquid, the subtank for temporally storing ink to be supplied to the ink jet head includes the air communication passage opened or closed by the air communication valve. The ink jet head is communicated with the subtank by the first passage and the second passage to constitute one circulation passage. In order to circulate the ink in the subtank into the common liquid chamber, the main pump is energized while the air communication valve is being closed. When the circulation operation is stopped, the air communication valve is opened immediately after the stoppage of the main pump, thereby eliminating the differential pressure between the common liquid chamber and the subtank within a short period of time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid ejection apparatus incorporating a supplying liquid circulation system for circulating liquid in a liquid ejecting head and the liquid processing method thereof. In particular, the present invention is suitable as an ink jet apparatus using a full line-type ink jet head in which ejection ports are arranged over the entire width of a printing medium.

The term “print” described in the Specification includes, in addition to a case where significant information (e.g., characters, graphic) is formed, variety of cases such as a case where an image, marking, or pattern is formed on a printing medium or a case where the printing medium is processed (e.g., etching), regardless of the significance or non-significance and regardless of whether or not the information is elicited so as to be visually recognized by a person.

The term “printing medium” includes not only a paper used in a general print apparatus but also materials (e.g., cloth, resin film, metal plate, glass, ceramics, wood, leather) that can accept liquid and materials having a three-dimensional shape other than a sheet-like shape (e.g., sphere, cylindrical body).

The term “liquid” should be widely interpreted as in the case of the definition of the above term “print” and includes any liquids used for printing such as liquid applied to a printing medium to be used for the formation of an image, marking, pattern or the like, liquid for the processing of a printing medium (e.g., etching), or liquid for the processing of ink (e.g., liquid that can be used so that color material in ink applied to a printing medium has coagulation or encapsulation).

2. Description of the Related Art

In an ink jet print apparatus, ink is ejected from an ink jet head (hereinafter also referred to as “print head”) so that the ink is applied to a printing medium for printing, for example. The ink jet print apparatus is advantageous in that the print head can have a compact body in an easy manner, a high-definition image can be printed with a high speed, the running cost is low, the non-impact method reduces noise, and inks having a number of colors are used to print a color image in an easy manner, for example. The so-called full line-type ink jet head is particularly advantageous because a number of ejection ports are arranged over the entire width of the image formation region of a printing medium so that the ejection ports can eject ink simultaneously to form an image with a higher speed. The full line-type print head includes a number of ejection ports arranged in a longitudinal direction and thus a common liquid chamber for storing ink supplied to the respective ejection ports also has a long shape accordingly.

The full line-type print head as described above also has a number of heaters for ejecting ink. This causes a tendency where the ink in a common liquid chamber is heated by a heater to have a high temperature. To prevent this, a technique has been known in which the space in a common liquid chamber of a print head and a sub tank for storing ink supplied to the common liquid chamber are used as a circulation passage so that a pump provided to the passage is used to circulate ink, thus allowing the ink in the sub tank to be circulated in the common liquid chamber. Such a circulation of ink prevents the ink from having a high temperature to suppress the temperature increase of the print head.

The operation for circulating ink as described above also has, in addition to the purpose for suppressing the temperature increase of ink, another purpose for exhausting bubbles accumulated in the common liquid chamber to outside, for example.

FIG. 9is a cross-sectional view schematically showing a supplying ink circulation system disclosed in Japanese Patent Application Laid-Open No. 11-179932(1999).

As shown inFIG. 9, the supplying ink circulation system150has the ink jet head101, the subtank103temporally storing ink to be supplied to the ink jet head101, and the main tank102for storing ink. The supplying ink circulation system150is used by being provided to an ink jet printer (not shown).

The ink jet head101includes a plurality of ejection ports101afor ejecting ink, and one common liquid chamber126for storing ink to be supplied to the respective ejection ports101a. At a position at which the ink jet head101is opposed to a port opening surface, the cap108is provided for receiving ink pushed out of the ejection port101a.

The subtank103includes the first tank103aand the second tank103b. The first and second tanks103aand103bare divided to have an enclosed space, respectively. The first tank103aand the second tank103bstore ink while including therein a predetermined amount of air buffer. The existence of air buffer left in this manner absorbs the fluctuation of the flow rate of ink caused when the ink is circulated.

The first tank103ahas, at the upper face thereof, the air communication passage134for communicating air in the tank. The air communication passage134is attached with the air communication valve106dfor opening or closing this communication passage.

The main tank102has an ink cartridge-like shape so that the main tank102can be exchanged with a new one in an ink jet printer (not shown) and stores therein ink having a predetermined color.

The respective components as described above are appropriately connected by tube members. As a result, the ink jet printer can be operated with “ink supply mode”, “ink circulation mode”, “ink eject mode” or the like. Among these operations, the “ink circulation mode” will be described with regards to the configuration and operation.

In order to circulate ink in the common liquid chamber126, the common liquid chamber126has, at the upstream side and the downstream side, the first passage132and the second passage133communicated to each other, respectively.

The other end of the first passage132is communicated with the second tank103bof the subtank103while the other end of the second passage133is communicated with the first tank103a. The first and second tanks103aand103bare communicated to each other by a tube member. As described above, the supplying ink circulation system150has one circulation passage by the first passage132, the second passage133, and the tube member for communicating the first tank103ato the second tank103b.

The tube member for communicating the first tank103ato the second tank103bhas, at the intermediate position thereof, the first pump104for moving ink in the first tank103ainto the second tank103b. This first pump104is used to circulate ink.

The cap108is communicated with the collection passage135for collecting ink received by the cap108. The other end of the collection passage135is communicated with the space in the first tank103aof the subtank103. The collection passage135includes the filter152for capturing foreign matters in ink and the second pump109for sucking ink from the cap108.

The supplying ink circulation system150structured as described above is driven with the “ink circulation mode” as described below.

When the first pump104is driven while the air communication valve106dbeing closed, ink in the first tank103ais flowed into the second tank103b. As a result, the ink in the second tank103bis pressurized and is flowed via the first passage132to the common liquid chamber126(see the direction shown by the arrow in the drawing). In accordance with this, ink in the common liquid chamber126is partially pushed out into the second passage133and is returned to the first tank103avia the second passage133. The ink left in the common liquid chamber126is partially pushed out of the ejection port101aand is received by the cap108.

Then, the second pump109is driven in synchronization with the first pump104so that the ink received by the cap108is returned via the collection passage135to the first tank103a.

The following section will describe in detail the circulation operation as described above.

First, immediately after the start of the circulation operation, the first pump104is driven to flow ink into the second tank103band the space in the second tank103bis pressurized while the air buffer therein being compressed. The pressurization of the second tank103bin this manner pushes the ink in the tank toward the common liquid chamber126. On the other hand, ink in the first tank103ais sucked toward the second tank103band thus the tank has therein a negative pressure to inflate the air buffer. In the situation immediately after the start of the circulation operation as described above, the pressures in subtank103and in the common liquid chamber126are not stabilized yet and thus a relatively large amount of ink is pushed out of the ejection port101a. When a filter (not shown) for cleaning ink is provided at the side of the second passage133inFIG. 9in particular, a larger amount of ink is pushed out because the space in the common liquid chamber126tends to be pressurized due to the influence by the pressure loss of this filter.

When a certain period of time has passed since the start of the circulation operation, the pressures in the subtank103and in the common liquid chamber126are stabilized. Specifically, the inflation or contraction of the air buffer is stopped and the amount of ink pushed out of the ejection port101ais also reduced, thus causing the amount of ink flowing into the subtank103to be the same as that of ink flowing in the first pump104.

However, the circulation system as described above causes the subtank to be closed while the common liquid chamber being communicated with air via the ejection port even when the circulation operation is performed in the stabilized condition, thus causing the differential pressure between the common liquid chamber and the subtank. Due to this reason, the ink circulation operation may not be stopped in some cases, even when the pump is stopped. As a result, the common liquid chamber has therein a negative pressure. This has caused a case in which the negative pressure having a magnitude that exceeds an ink meniscus retention force in the ejection port causes air to be sucked via the ejection port. When the air sucked via the ejection port is collected as bubbles in the common liquid chamber, the ejection may not be provided to a correct manner.

The air suction phenomenon as described above tends to be caused as the pump has a larger flow rate or as the air buffer in the subtank has a larger capacity. The air suction phenomenon also tends to be caused when the exhaust side of the common liquid chamber has a filter and the filter has a larger pressure coefficient. Specifically, the prevention of the air suction as described above is desirable because it improves the freedom in the selection of a pump or a filter or the freedom in the selection of the setting of an air buffer.

The ink circulation operation may have, in addition to a defect caused by the air suction as described above, a defect in which the space in the common liquid chamber is pressurized immediately after the start of the circulation operation to cause the ink to be pushed out of the ejection port. The ink pushed out as described above is not particularly problematic in the configuration as shown inFIG. 9in which the pushed-out ink is again returned to the subtank103. However, the pushed-out ink is a problem in a configuration in which the pushed-out ink is collected by an independent waste ink collection tank.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a liquid ejection apparatus and a liquid processing method by which a circulation operation of liquid (e.g., ink) is prevented from causing air to be sucked via an ejection port or causing, on the contrary, liquid from being pushed out of an ejection port.

The first aspect of the present invention that can achieve the above objects is a liquid ejection apparatus, the liquid ejection apparatus comprises a head for ejecting liquid from an ejection port, a subtank for temporally storing liquid to be supplied to the liquid ejecting head, an air communication valve for opening or closing a passage for communicating air in the subtank, a circulation flow line for providing communication between the liquid ejecting head and the subtank for circulation, pumping means that is provided to this circulation flow line for circulating liquid between the liquid ejecting head and the subtank, and means for controlling the air communication valve and the pumping means to circulate liquid by closing the air communication valve while driving the pumping means and for opening, simultaneously with or immediately after the stoppage of the pumping means, the air communication valve to communicate air in the subtank.

In the liquid ejection apparatus according to the present invention, when the pumping means is energized to stop the circulation operation for circulating the liquid between the liquid ejecting head and the subtank, the controlling means opens, simultaneously with or immediately after the stoppage of the pumping means, the air communication valve to recover the negative pressure in the subtank within a short period of time, thereby eliminating the differential pressure between the subtank and the liquid ejecting head within a short period of time.

According to the liquid ejection apparatus of the present invention, the liquid ejection apparatus comprises the controlling means for communicating, when the circulation operation is stopped, air in the subtank simultaneously with or immediately after the stoppage of the pumping means. Thus, the differential pressure between the subtank and the liquid ejecting head is eliminated within a short period of time. This can suppress air from being sucked by the ejection port, thus improving the reliability of the liquid ejection apparatus.

In the liquid ejection apparatus according to the first aspect of the present invention, the circulation flow line may include a first passage for supplying liquid from the subtank into the liquid ejecting head, and a second passage for returning liquid from the liquid ejecting head into the subtank, and the liquid ejection apparatus further may comprise a switching valve that is provided to the second passage for opening or closing this second passage.

The liquid ejection apparatus may further comprise a main tank for storing liquid to be supplied to the subtank, a passage for supplying the liquid in this main tank into the subtank, and means for detecting the remaining amount of the ink in the subtank to output the amount to the controlling means, when the remaining amount of the liquid in the subtank detected by the detecting means is equal to or lower than a predetermined value, then the controlling means opens the air communication valve while liquid in the main tank is being supplied into the subtank.

Furthermore, when liquid in the main tank is supplied into the subtank, the liquid is preferably filled up in the subtank. The expression “liquid is filled up in the subtank” means a status in which a sensor for detecting the amount of the liquid in the subtank detects that the liquid is filled up. Thus, this expression includes a status in which the subtank is filled with liquid while including a predetermined amount of air buffer.

In the present invention, the liquid ejection apparatus preferably has a merging portion at which the supplying passage merges into the first passage and the pumping means is provided to the first passage between this merging portion and the subtank so that liquid can be flowed in both directions. Furthermore, the liquid ejection apparatus also may further comprise a valve for switching between a first status in which the communication between the subtank and the main tank is blocked to provide the communication between the subtank and the ink ejecting head and a second status in which the communication between the subtank and the ink ejecting head is blocked to provide the communication between the subtank and the main tank. The controlling means also may operate the switching valve to provide the communication between the subtank and the main tank, thereby supplying ink in the main tank into the subtank.

The second aspect of the present invention is a liquid processing method in a liquid ejection apparatus including a head for ejecting liquid from an ejection port, a subtank for temporally storing liquid to be supplied to the liquid ejecting head, an air communication valve for opening or closing a passage for communicating air in the subtank, and a circulation flow line for providing communication between the liquid ejecting head and the subtank for circulation, the liquid processing method comprises the steps of circulating liquid while the air communication passage is being closed, and opening, simultaneously with or immediately after the completion of the liquid circulation, the air communication passage to communicate air in the subtank.

In the liquid processing method according to the second aspect of the present invention, the liquid ejection apparatus further includes a main tank for storing liquid to be supplied to the subtank and a passage for supplying liquid in this main tank into the subtank, the liquid processing method further comprises, prior to the step for circulating liquid, a step of detecting the remaining amount of liquid in the subtank, and a step of supplying, when the remaining amount of liquid in the subtank is equal to or lower than a predetermined value, the liquid in the main tank into the subtank while the air communication valve is being opened. In this case, the liquid processing method may further comprise, prior to the step for supplying the liquid in the main tank into the subtank, a step of blocking the communication between the subtank and the liquid ejecting head while providing the communication between the subtank and the main tank.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Structure of Supplying Ink Circulation System

FIG. 1is a cross-sectional view schematically showing the structure of a supplying ink circulation system according to one embodiment of the present invention.

As shown inFIG. 1, the supplying ink circulation system50has the full line-type ink jet head1,the main tank2for storing ink supplied to the ink jet head1, and the subtank3that is provided between the ink tank2and the ink jet head1and that temporally stores ink supplied from the main tank2. The supplying ink circulation system50is used by being provided to an ink jet printer (not shown). The supplying ink circulation system50also includes, in an independent manner, the waste ink collection tank10for storing ink (waste ink) pushed out of the ink jet head1.

The supplying ink circulation system50mainly has two passages (which will be described later). One of the passages is a circulation passage for the circulation among the ink jet head1, the main tank2, and the subtank3. The other of the passages is a collection passage in which ink pushed out of the ink jet head1is received by the cap8and is collected in the waste ink collection tank10.

The ink jet head1has a plurality of ejection ports1afor ejecting ink, and one common liquid chamber26for storing ink supplied to the respective ejection ports1a. A printing operation is performed by causing ink supplied from the main tank2via the subtank3to the common liquid chamber26to be ejected from the ejection port1a.

The main tank2is a flexible ink bag for storing ink that can be exchanged in the supplying ink circulation system50. The main tank2partially has a supply port (not shown) for supplying ink to the exterior that is provided by an elastic member (e.g., rubber). This supply opening is inserted with the ink communication needle2aso that the main tank2is connected to the supplying ink circulation system50.

The subtank3is configured as an airtight container that stores ink while including therein a predetermined amount of air buffer3f. The upper face of the subtank3is connected with the air communication passage34for communicating air to the interior of the subtank3. The air communication passage34is attached with the air filter13for preventing dust from intruding into the subtank3and the air communication valve6dfor opening or closing the air communication passage34. The air communication valve6dis the same as other switching valves6ato6c(which will be described later) and the details will be described later.

The subtank3has, at the side face thereof, the detecting section12for detecting the remaining amount of ink in the subtank3. The detecting section12includes, as shown inFIG. 2, the fill-up detection sensor12afor detecting when the ink in the subtank3is filled up and the empty detection sensor12bfor detecting when the ink in the tank is used up. Any of the sensors12aand12bmay be an optical sensor provided with LED, optical prism, and photosensor or the like. The term “fill-up” does not mean that the subtank3is filled with ink perfectly but means that the subtank3is filled with ink while including therein a predetermined amount of air buffer3f(seeFIG. 1). The position at which the fill-up detection sensor12ais attached is adjusted so that the fill-up can be detected while allowing the subtank3to have therein a predetermined amount of air as described above.

With reference toFIG. 1again, the ink passage of the supplying ink circulation system50will be described.

As shown inFIG. 1, the ink jet head1is connected to the subtank3by a pair of tube members to form a circulation passage. One of the pair of tube members is the first passage31for supplying ink in the subtank3, via the filter11a, into the common liquid chamber26of the ink jet head1. The other of the pair of tube members is the second passage32for returning the ink pushed out of the common liquid chamber26, via the filter11b, into the subtank. The first passage31and the second passage32are connected to the connecting portions of the common liquid chamber26. The connecting portions include the filters11aand11bfor capturing foreign matters in the ink as described above. The filters11aand11barranged as described above prevent foreign matter from intruding into the common liquid chamber26.

The passage33is used for supplying ink in the main tank2into the subtank3. The supplying passage33merges at the merging portion K at the intermediate position of the first passage31. InFIG. 1, a passage from this merging portion K to the subtank3is shown as the first passage31. The passage31between the above merging portion K in the first passage31and the subtank3is structured so as to be also used as a passage for supplying, by the action by the main pump4that can be driven in a reverse direction, ink in the main tank2into the subtank3. In this way, the passage from the ink jet head1is merged into the passage from the main tank2, thereby providing a simplified passage.

The first passage31includes the main pump4that can be operated in forward and backward directions so as to flow the ink in two directions, and the flowmeter7for measuring the flow rate of the moving ink.

The respective passages31to33include three switching valves6ato6cfor opening or closing these passages31to33. The first switching valve6ais provided to the first passage31, the second switching valve6bis provided to the second passage32, and the supply switching valve6cis provided to the supplying passage33. The supply switching valve6cand the first switching valve6aconstitute a switching valve of the present invention. More particularly, the first switching valve6ais in the vicinity of the merging portion K at which the supplying passage33is merged into the first passage31so as to be provided at the intermediate position between this merging portion and the ink jet head1.

The respective switching valves6ato6care controlled in an independent manner and are opened or closed with different manners to change the communication status between ink passages. For example, when the supply switching valve6cis closed and the first switching valve6ais opened, the subtank3is communicated with the ink jet head1, thus allowing the ink in the subtank3to be flowed into the ink jet head1. On the contrary, when the supply switching valve6cis opened and the first switching valve6ais closed, the main tank2is communicated with the subtank3, thus allowing the ink in the main tank2to be flowed into the subtank3.

The respective switching valves6ato6cand the above-described air communication valve6dhave the same structure and also may be provided, for example, as a solenoid on-off valve by providing a solenoid plunger with a sealing function. Although the respective switching valves6ato6dincluding the air communication valve6dmay have an initial status that is not particularly limited, the switching valves6ato6cand the air communication valve6din this embodiment as shown in the drawing have initial statuses in which the switching valve6ais opened, the switching valve6bis opened, the switching valve6cis closed, and the air communication valve6dis opened, respectively and, when a control signal is inputted, the switching valve6ais closed, the switching valve6bis closed, the switching valve6cis opened, and the air communication valve6dis closed, respectively.

The cap8provided to be opposed to the ink jet head1, the waste ink collection tank10for storing waste ink, the waste ink passage35for providing the communication between the cap8and the waste ink collection tank10, and the subpump9provided to the waste ink passage35are provided to the collection passage for collecting waste ink. In the collection passage structured as described above, the subpump9is driven to allow the ink received by the cap8to be collected via the waste ink passage35into the waste ink collection tank10. This collecting operation can be carried out by a known control method and thus the details will not be described.

The above main pump4and the subpump9may be a tube pump or may be a cylinder pump. Although the configuration shown inFIG. 1was provided such that the first passage31and the supplying passage33include two switching valves of the first switching valve6aand the supply switching valve6c, respectively, the present invention is not limited to the configuration in which two switching valves are provided. Another configuration also may be used in which one switching valve is provided by which a status in which the communication between the subtank3and the main tank2is blocked to provide the communication between the subtank3and the ink jet head1can be switched with a status in which the communication between the subtank3and the ink jet head1is blocked to provide the communication between the subtank4and the main tank2.

A control block diagram in this embodiment is shown inFIG. 3. Specifically, a signal detected by the detecting section12is outputted to the controller36. Then, the controller36controls, in accordance with a predetermined program, the above-described main pump4, the switching valves6ato6c, the air communication valve6d, and the subpump9for example.

The supplying ink circulation system50of this embodiment structured as described above is controlled by the controller36in accordance with various operating modes of an ink jet printer (not shown). Such operating modes include, for example, an “ink supply mode” for supplying ink into the subtank3, a “pressurization recovery mode” for forcedly pushing ink out of the ejection port1a, a “print mode” for ejecting ink from the ejection port1afor printing, and a “circulation mode” for circulating ink in the common liquid chamber26that is a characteristic part of the present invention. The respective modes will be described.

Ink Supply Mode

The “ink supply mode” is a mode for supplying ink in the main tank2into subtank3. For example, the ink supply mode is performed in an initial status of an ink jet printer (not shown) in which the subtank3stores no ink.

In the ink supply mode, the first switching valve6ais closed and the supply switching valve6cis opened to provide the communication between the subtank3and the main tank2while the main pump4being energized in the forward direction, thereby supplying ink into the subtank3. The ink supply mode may be separately performed or also may be performed while ink is being ejected from the ink jet head1(i.e., while the print mode is being performed). Thus, opening or closing of the second switching valve6bare appropriately determined depending on the operating mode.

A specific driving of the ink supply mode will be described with reference to the flowchart ofFIG. 4.

First, the detecting section12is driven to detect the remaining amount of ink in the subtank3(Step S1).

Then, when the fill-up detection sensor12adetects that ink is filled up, then the empty detection sensor12bdetects whether the subtank3is empty or not (Step S2). When the empty detection sensor12bdetects that the subtank3is not empty, it means that the ink is filled up in the subtank3and thus there is no need to supply ink, thus completing the set of steps of ink supply modes. On the other hand, when the empty detection sensor12bdetects that the subtank3is empty, then this detection result is inconsistent with the detection result by the fill-up detection sensor12, showing a possibility where any or both of the sensors12aand12bmay have a failure. In this case, a user is notified that the detecting section12has an abnormality (Step S3). Then, the ink supply mode is completed.

When Step S1shows that the fill-up detection sensor12adetects that ink is not filled up, then ink is supplied from the main tank2into the subtank3by the procedure as described below. First, the first switching valve6ais closed and the supply switching valve6cis opened as described above to provide the communication between the subtank3and the main tank2via the supplying passage33and the first passage31(Step S4).

Next, the main pump4is driven in the forward direction to supply ink in the main tank2into the subtank3via the switching valve6c, the pump4, and the first passage31(Step S5). The main pump4may have a flow rate of 1 ml/sec., for example. While the main pump4is being driven, pressure loss by the ink communication needle2ainserted to the main tank2causes a negative pressure in the supplying passage33and the first passage31between the main tank2and the main pump4.

The main pump4is driven until ink is filled up in the subtank3. Specifically, in order to generate a timing at which the main pump4is stopped, the detecting section12is driven to detect the amount of ink in the subtank3while the main pump4is being driven (Step S6).

Next, when ink is filled up in the subtank3, the main pump4is stopped (Step S7). At this point of time, the supplying passage33has therein a negative pressure as described above. This may cause a possibility where, when the first switching valve6ais opened immediately after the stoppage of the main pump4, a negative pressure is also caused via the first passage31in the common liquid chamber26of the ink jet head1, thus causing the ejection port1ato suck air. To prevent this, this embodiment provides Step S8for providing, after the stoppage of the main pump4(Step S7), a predetermined time (e.g., 2 seconds) for recovering a pressure in the supplying passage33to an atmospheric pressure.

Next, the respective switching valves6ato6dare provided to have an initial status (Step S9) and to subsequently wait for a predetermined time (Step S10). Thereafter, the set of steps of the ink supply mode are completed.

Pressurization Recovery Mode

The “pressurization recovery mode” is a mode for pressurizing the space in the common liquid chamber26of the ink jet head1to eject ink in the ejection port1ain a forced manner. Such a forced ejection of ink is performed for the purpose of pushing out ink having an increased viscosity or for pushing out bubbles mixed in ink.

The “ink having an increased viscosity” is caused, for example, when a print operation is repeated for a long time to increase the temperature of ink in the ejection port1ato cause the moisture in the ink to evaporate from the ejection port1a. When the ink having an increased viscosity as described above is left in the ejection port1a, the ejection port1ais sealed by the ink, which may cause a failure in the ejection. The “bubbles mixed in the ink” are caused, for example, when the common liquid chamber26has therein a negative pressure to cause air to be sucked into the ejection port1aand is also caused when small bubbles dissolved in the ink are united. Any of the ink having an increased viscosity or the mixed bubbles as described above causes a failure in the ejection. In order to prevent them, the ink having an increased viscosity or mixed bubble must be pushed out.

In the pressurization recovery mode, the first switching valve6ais opened, the second switching valve6bis closed, the supply switching valve6cis closed, and the air communication valve6dis opened to provide the communication between the subtank3and the ink jet head1via the first passage31while the main pump4is being driven, thereby supplying ink in the subtank3into the ink jet head1via the first passage31, the switching valve6a, the first passage31, the filter11a, and the common liquid chamber26to eject ink from the ejection port1ain a forced manner.

A specific driving in the pressurization recovery mode will be described with reference to the flowchart ofFIG. 5.

First, the above-described ink supply mode (seeFIG. 4) is performed so that ink is filled up in the subtank3(Step S11). Next, the switching valve6ais opened, the switching valve6bis closed, the switching valve6cis closed, and the air communication valve6dis opened, respectively (Step S12). This provides the communication between the subtank3and the ink jet head1to close the second passage32. The air communication passage34is opened.

Next, the main pump4is driven in a backward direction (Step S13) to continue this driving status for a predetermined time (e.g., T seconds) (Step S14). This will be described with reference toFIG. 1. The ink in the subtank3is supplied from the subtank3into the common liquid chamber26via the first passage31, the switching valve6a, and the first passage31. More specifically, the ink in the subtank3is supplied by the action by the main pump4via the first passage31into the common liquid chamber26. In accordance with this, the ink in the common liquid chamber26is pressurized so that the ink having the same amount as that of the supplied ink is pushed out from the ejection port1a. At this point, the subtank3is being communicated with air as described above and thus the subtank3sucks outside air as ink is being supplied into the common liquid chamber26, thereby providing ink supply in a smoother manner.

By the forced ejection of ink as described above, the bubbles mixed in the ink in the common liquid chamber26or the ink having an increased viscosity in the ejection port1ais pushed out to outside, thereby recovering the function of the ink jet head1.

Next, the main pump4is stopped (Step S15), thus completing the forced ejection of ink. Thereafter, the second switching valve6bis provided to have an initial status (open) (Step16).

When the main pump4is being driven in the pressurization recovery mode, the action by the main pump4causes a differential pressure between the ink jet head1and the subtank3(specifically, the common liquid chamber26is being pressurized). To prevent this, this embodiment provides Step S17for waiting for a predetermined time (e.g., 1 second) in order to eliminate the differential pressure between the ink jet head1and the subtank3so that this differential pressure is provided to have an initial status (status in which the differential pressure equals to a water head differential pressure).

Next, the ink supply mode (seeFIG. 4) is performed in which ink having an amount that is the same as that supplied into the common liquid chamber26is supplied from the main tank2into the subtank3(Step S18), thereby completing the set of steps of the pressurization recovery mode. An amount of ink in this mode is measured by the flowmeter7.

Print Mode

The “print mode” is a mode in which the second switching valve6bis opened without driving the main pump4to provide the communication between the subtank3and the common liquid chamber26while ejecting ink from the ejection port1aof the ink jet head1for printing. When ink is ejected from the ejection port1a, ink having the same amount as that of the ejected ink is sucked by a capillary force from the subtank3into the common liquid chamber26.

A specific driving of the print mode will be described with reference to the flowchart ofFIG. 6.

First, the first switching valve6ais opened, the second switching valve6bis opened, the supply switching valve6cis closed, and the air communication valve6dis opened to provide the communication between the subtank3and the common liquid chamber26while providing air communication to the subtank3. Then, ink in this status is ejected from the ejection port1ato perform a printing (Step S21). In this status, the port opening surface of the ink jet head us opposed to a printing medium.

While ink is being ejected from the ink jet head1, the detecting section12is driven to detect the remaining amount of ink in the subtank3(Step S22).

When the amount of ink in the subtank3is sufficient, the print operation of Step S21is continued. On the other hand, when it is detected that the subtank2is empty, the printing operation of Step S21is continued while the ink supply mode (seeFIG. 4) is being performed to supply ink from the main tank2into the subtank3(Step S23) during which the second switching valve6bis being opened.

When Step S21judges that the print operation is finished, ink is supplied as required into the subtank3(Step S24), thus completing the set of steps of the print mode.

Circulation Mode

The “circulation mode” is a mode performed for the purpose of cooling the ink jet head1having a high temperature by the print operation or for exhausting bubbles to the exterior that are not dissolved in ink in the common liquid chamber26and are collected, as described above.

In the circulation mode, the first switching valve6ais opened, the second switching valve6bis opened, the supply switching valve6cis closed, and the air communication valve6dis closed to allow the subtank3and the ink jet head1to provide one circulation passage while the main pump4is being driven in the backward direction, thereby circulating the ink in the subtank3into the common liquid chamber26via the first passage31, the switching valve6a, the first passage31, the filter11a, the common liquid chamber26, the filter11b, the second passage32, and the switching valve6b.

A specific driving in the circulation mode will be described with reference to the flowchart ofFIG. 7.

First, in order to provide the air buffer3fin the subtank3having a predetermined amount, the above-descried ink supply mode (seeFIG. 4) is performed so that ink is filled up in the subtank3(Step S31).

Next, in order to prevent the subtank3during the circulation from sucking outside air, the air communication valve6dis closed (Step S32). In this status, the switching valve6ais opened, the switching valve6bis opened, and the switching valve6cis closed, respectively.

Next, the main pump4is driven in the backward direction (Step S33) simultaneously with the driving of a timer (not shown) for measuring the time of the circulation operation (Step S34). When the main pump4is driven, the ink has a circulating flow to cause ink to be supplied, as shown by the arrow in the drawing, from the filter11ainto the common liquid chamber26. Then, the ink is circulated via the second passage32toward the subtank3. The main pump4in this status may have a flow rate of 2 ml/sec., for example.

When the main pump4is driven, the filter11aside of the ink jet head1(upstream side of the circulating flow) is directly transmitted with the action by the pump while the filter11bside of the ink jet head1(downstream side of the circulating flow) is not directly transmitted, due to the action by the air buffer3f, with the action by the pump. Specifically, the action by the pump causes the subtank3immediately after the driving of the main pump to have therein a negative pressure. A part of this negative pressure is used for inflating the air buffer3fand thus this buffering action prevents the filter11bside from being directly transmitted with the action by the pump. As a result, the common liquid chamber26has such an ink input/output balance in which an excessive amount of ink is inputted into the common liquid chamber26, thus allowing the ejection port1aimmediately after the driving of the pump to push out ink in a relatively easy manner. In order to minimize this, the amount of the air buffer3fand the flow rate of the circulating flow may be reduced.

When a certain period of time has passed since the driving of the main pump4, the inflation of the air buffer3fis stopped. As a result, ink flowed out of the common liquid chamber26has the same flow rate as that in the main pump4and the ink in the common liquid chamber26is communicated with air via the ejection port1aand the pressure thereof is gradually close to the atmospheric pressure. Thus, the amount of ink pushed out of the ejection port1ais also gradually reduced.

The driving of the main pump4is performed for “T” second(s). This duration for “T” second(s) is judged by Step S35based on the time “t” measured by the above timer. When Step S35judges that the duration for “T” second(s) has passed, then the main pump4is stopped (Step S37).

There is a case in which, even when the duration for “T” second(s) is not yet reached in the circulation operation, a user cancels the instruction for the circulation operation. Thus, Step S36is provided as a step for judging this instruction for stoppage. Specifically, when the user inputs the instruction for stoppage even when the duration for “T” second(s) is not yet reached in the circulation operation, then the instruction is judged by Step S36and the main pump4is stopped (Step S37).

Next, in order to recover the subtank3having a negative pressure, the air communication valve6dis opened (Step S38). Step S38is performed immediately after the stoppage of the main pump4.

During the circulation operation, the subtank3has therein a negative pressure due to the action by the main pump4while the space in the common liquid chamber26has, by being communicated with air via the ejection port1aas described above, a pressure that is almost equal to the atmospheric pressure. When there is a differential pressure between the subtank3and the common liquid chamber26as described above, there is a possibility where, when the main pump4is stopped, the negative pressure in the subtank3is transmitted to the common liquid chamber26, causing the ejection port1ato suck air.

In this embodiment, a step is provided for opening the air communication valve6dimmediately after the stoppage of the main pump4(Step S38). As a result, the pressure in the subtank3is recovered to the atmospheric pressure within a short period of time to eliminate the differential pressure between the subtank3and the common liquid chamber26, thus preventing the ejection port1afrom sucking air immediately after the stoppage of the circulation operation.

Next, in order to stabilize the status in the subtank3and in order to provide the differential pressure between the subtank3and the common liquid chamber26to be equal to a negative pressure of a water head differential pressure in an initial status, a sufficient period of time (e.g., two seconds) is waited (Step S39).

Next, ink in an amount that was ejected in the circulation operation is supplied into the subtank3(Step S40), thus completing the set of steps of the circulation mode.

As described above, when the ink circulation operation in this embodiment is stopped, the air communication valve6dis opened immediately after the stoppage of the main pump4. As a result, a negative pressure in the subtank is recovered within a short period of time to eliminate the differential pressure between the subtank3and the common liquid chamber26. Therefore, ink is prevented from continuously flowed due to the above differential pressure, thereby suppressing the ejection port1afrom sucking air.

The existence of Step S31prior to the circulation operation for filling up ink in the subtank3allows the air buffer3fin the subtank3to have an amount when ink is filled up. Specifically, an amount of the air buffer3fof the subtank3is minimized and thus can reduce an amount of ink pushed out of the ejection port1aeven immediately after the start of the circulation operation in which ink tends to be pushed out of the ejection port1ain a relatively easy manner. This means that an amount of waste ink is reduced when the waste ink collection tank10is independently provided as in this embodiment. Thus, an advantage is provided to that unnecessary exhaust of ink can be suppressed, thus reducing the running cost.

The circulation operation also may be provided as shown inFIG. 8.FIG. 8is a flowchart of the circulation operation according to Embodiment 2. The circulation operation according to Embodiment 2 includes, in addition to the steps shown in the flowchart ofFIG. 7, a step for controlling a switching valve immediately after the stoppage of the main pump4(Step S48), a step for subsequently opening the second switching valve6b(Step S50), and a step for subsequently waiting for a predetermined time (Step S51). The same steps as those ofFIG. 7will not described further.

In Embodiment 2, a control is provided as in the first embodiment in which the circulation operation is performed for “T” seconds to subsequently stop the main pump4(Step S47) and then the air communication valve6dis opened and the second switching valve6bis closed in order to eliminate the differential pressure between the subtank3and the common liquid chamber26. By closing the second switching valve6bto block the communication between the subtank3and the common liquid chamber26as described above, the negative pressure in the subtank3is not transmitted to the common liquid chamber26to prevent ink from being continuously flowed from the common liquid chamber26into the subtank3, thereby preventing the ejection port1afrom sucking air.

This means that the air filter13can use a dust-proof material. Specifically, a dust-proof material generally has a high pressure loss and thus such a dust-proof material used in the air filter13may prevent the subtank3from sucking outside air even when only the air communication valve6dis opened as in the first embodiment. In such a case, some period of time is required for the subtank3to have the atmospheric pressure, thus causing a possibility where ink in the common liquid chamber26may, during this period of time, be continuously flowed toward the subtank3. To prevent this, a control is provided as in this embodiment in which the air communication valve6dis opened and the second switching valve6bis closed, thereby preventing, even when the air filter13uses a dust-proof material, the ejection port1afrom sucking air.

Next, as in Step S39ofFIG. 7, a sufficient time (e.g., two seconds) is waited in order to stabilize the status in the subtank3(Step S49). Next, the second switching valve6bis again opened (which is the initial status) (Step S50). Then, a sufficient time (e.g., one second) is waited in order to stabilize the differential pressure between the subtank3and the common liquid chamber26(Step S51).

Next, as in the first embodiment, ink having the same amount as that of ink ejected from the ejection port in the circulation operation is supplied into the subtank3(Step S52), thereby completing the set of steps of the circulation mode.

According to this embodiment, the main pump4is stopped to subsequently open the air communication valve6dand to close the second switching valve6b(Step S48). As a result, an action is provided to which the air communication valve6dis opened to recover the pressure in the subtank3and another action is provided to which the second switching valve6bis closed to block the communication between the subtank3and the common liquid chamber26, thereby preventing ink from continuously flowed from the common liquid chamber26into the subtank3to minimize the air sucked by the ejection port1a.

It is noted that a time between Step S47for stopping the main pump4and Step S48for opening the air communication valve6dis preferably determined, in an appropriate manner, depending on the characteristics of the respective components of the supplying ink circulation system50so that the ejection port1ais prevented from sucking air or from having ink leakage therefrom.

For example, when the main pump4has a characteristic in which the main pump4after receiving a stop signal is continuously driven by inertia and when the second switching valve6bfor blocking the communication between the ink jet head1and the subtank3is closed too soon, the action by the main pump4continuously driven by inertia may pressurize the space in the supply liquid chamber26to push out ink therefrom. To prevent this, a step for waiting a predetermined time (e.g., 0.5 seconds) may be provided depending on the characteristic of the main pump4.

Next, the actual result of the control based on the flowchart shown inFIG. 8will be described. This result is obtained by using the apparatus shown inFIG. 1to use the pressurization flow rate by the main pump4of 2.7 cc/sec. during the operation of the apparatus. As shown inFIG. 8, the second switching valve6bis opened and the air communication valve6dis closed while the main pump4is being operated, thereby circulating ink. After the circulation, Step47is switched to Step48with timing as described below.

Condition 1: After 0.3 seconds after the completion of the circulation supply stoppage of the main pump4), the air communication valve6dis opened and the second switching valve6bis closed.

Condition 2: Simultaneously with the completion of the circulation supply (stoppage of the main pump4), the air communication valve6dis opened and the second switching valve6bis closed.

Condition 3: After the completion of the circulation supply (stoppage of the main pump4), the second switching valve6band the air communication valve6dhave no change in the status. Specifically, the second switching valve6bis continuously opened and the air communication valve6dis continuously closed.

Under the conditions 1 to 3 as described above, the internal pressure of the subtank3and the internal pressure of the common liquid chamber26of the ink jet head1(shown by the broken line) were measured. The result under condition1is shown inFIG. 10AtoFIG. 10C. The result under condition2is shown inFIG. 11AtoFIG. 11C. The result under condition3is shown inFIG. 12AtoFIG. 12C. In these drawings, the solid line represents the internal pressure of the subtank3while the broken line represents the internal pressure of the common liquid chamber26.

FIG. 10Ashows the status in the subtank3and in the common liquid chamber26before the circulation is started and shows that there is no differential pressure therebetween.FIG. 10Bshows the status in which the pump operation is started and the ink circulation status is stabilized.FIG. 10Cshows that, after the main pump4is stopped to stop the ink circulation, the air communication valve6dis opened and the second switching valve6bis closed when 0.3 seconds have passed as a predetermined time, which causes the pressure in the common liquid chamber26to be slightly decreased as shown by “I” and causes the pressure in the subtank3to be increased as shown by “II”, immediately after which (i.e., within a time less than 1 second) the common liquid chamber26and the subtank3have almost the same pressure and are stabilized as shown by “III”.

FIG. 11AandFIG. 11Bshow the pressure change behaviors like those shown inFIG. 10AandFIG. 10B. InFIG. 11C, the air communication valve6dis opened and the second switching valve6bis closed simultaneously with the stoppage of the main pump4, thus causing the internal pressure in the common liquid chamber26to be increased, as shown by “IV”. This is caused because ink is sent by inertia even when the main pump4is stopped and thus ink cannot be flowed from the ink jet head1to the subtank3. When this pressure increase is high, meniscus formed in the ejection port1ais broken and thus ink is pushed out of the ejection port1a, thus causing the waste of ink. However, the phenomenon in which the meniscus is broken is not necessarily caused when the air communication valve6dis opened and the switching valve6bis closed.

FIG. 12AandFIG. 12Bshow the pressure change behaviors like those shown inFIG. 10AandFIG. 10B. InFIG. 12C, a control is provided to which, after the stoppage of the main pump4, the second switching valve6bis opened and the air communication valve6dis closed. As a result, due to an influence by the internal pressure of the subtank3having a large negative pressure as shown by “V”, the common liquid chamber26has therein a reduced internal pressure and is stabilized with a pressure that is lower than that ofFIG. 12A. When this pressure is increased to have a magnitude high enough to suck the meniscus of the ejection port1a, the meniscus formed in the ejection port1ais broken and air is sucked into the ink jet head1.

This tendency is always caused regardless of an amount of ink sent by the main pump4. When the ink circulation mode is stopped, the air communication valve6dis opened and the second switching valve6bis closed when a predetermined time has passed since the stoppage of the main pump4, thereby providing a control by which meniscus in the ejection port1ais not moved, ink is not pushed out, or air is not sucked.

FIG. 13AtoFIG. 15Cshow the results obtained by performing the conditions 1 to 3 when the subtank3includes therein air of about 10cc. AlthoughFIG. 13AtoFIG. 13CandFIG. 14AtoFIG. 14Cshow similar tendencies as those ofFIG. 10AtoFIG. 10CandFIG. 11AtoFIG. 11C, the pressure changes are rather slower due to the existence of air in the subtank.FIG. 15Cusing the condition3shown inFIG. 15AtoFIG. 15Cshows a particularly severe condition in which the common liquid chamber26has a reduced internal pressure (negative pressure) to cause meniscus in the ejection port1ato be broken to suck air as shown by “VI” (the sucked air is shown by the gradually-increasing negative pressure). Even under such a severe condition, a predetermined time can be waited after the stoppage of the main pump4in the circulation mode to subsequently open the air communication valve6dand to close the second switching valve6b, thereby providing a control in which the meniscus in the ejection port1ais prevented from being moved, ink is prevented from being pushed out, and air is prevented from being sucked.

The waiting time immediately after the stoppage of the main pump4is a parameter that changes depending on a system configuration and thus is difficult to be specified. However, it is important to determine such a waiting time by which the air communication valve6dcan be opened and the second switching valve6bcan be closed prior to the increase or reduction in the pressure after the stoppage of the main pump4that may break the meniscus in the ejection port1a.

This application claims priority from Japanese Patent Application Nos. 2004-085600 filed Mar. 23, 2004 and 2005-044246 filed Feb. 21, 2005, which are hereby incorporated by reference herein.