Patent Application
20080238980
1. Field of the Invention
The present invention relates to a liquid circulation apparatus, an image forming apparatus and a liquid circulation method, and more particularly, to technology for circulating liquid in the vicinity of a plurality of nozzles of a liquid ejection head which ejects ink droplets from the nozzles.
2. Description of the Related Art
An inkjet recording apparatus has been known which performs recording by ejecting ink droplets toward a recording medium from a plurality of nozzles which are formed in an inkjet head (hereinafter, called a “recording head” or simply “head”). The inkjet recording apparatus has been commonly used because of its little noise during operation, low running cost and capability of recording a high quality image on a wide variety of recording medium. The ink ejection method may be a piezoelectric method in which ink droplets are ejected from nozzles by utilizing the displacement of piezoelectric elements to pressurize the ink inside pressure chambers, or a thermal method in which ink droplets are ejected from nozzles due to the pressure created by the growth of gas bubbles which are generated inside pressure chambers by means of the thermal energy created by heating elements, such as heaters, or the like.
In an inkjet recording method, if an ink having a solvent that is liable to evaporate at the operational temperature and humidity conditions is used (for example, an ink which uses water as a solvent, or the like), then during printing and during standby for printing, a phenomenon occurs whereby the solvent in the ink evaporates from the nozzles, the concentration of solvent in the ink in the vicinity of the nozzles becomes lower, and the viscosity of the ink rises. When the ink viscosity in the vicinity of the nozzles rises, then the fluid resistance inside the nozzles becomes greater, and ejection defects arise due to the occurrence of variations in the volume of the ejected ink droplets or in the direction of flight of the droplets, or ejection failures may occur. Consequently, this may give rise to displacement of the dot positions on the print medium, error in the size of the dots, and omission of dots.
If this situation proceeds further, then it becomes impossible to perform ejection completely, and maintenance known as “nozzle cleaning” becomes necessary.
In response to this, according to experimentation carried out by the present inventors and others, it has been confirmed that in a state where no particular countermeasures are implemented, then in the case of an ink which uses water as a solvent, the solvent starts to evaporate from the nozzles and within five or six seconds, ejection defects arise and extremely serious problems occur even under conditions of normal temperature and normal pressure.
In order to prevent this problem, in a piezoelectric type of head which uses an actuator (piezoelectric element), such as a piezo element, a vibration of a level which does not cause ejection of ink from the nozzles is also applied to the ink in the non-ejecting nozzles (non-operational nozzles) which are not performing ink ejection. The ink in the vicinity of the nozzles is thereby mixed up with the ink inside the pressure chambers, and the fall in the solvent concentration of the ink in the nozzle sections is restricted. Thus, control is implemented which suppresses increase in the viscosity of the ink in the nozzle sections. Below, control of this kind is called “meniscus shaking”.
However, even with this method, if a long period of time elapses, then there is a decline in the concentration of ink solvent in the whole of the pressure chambers and the nozzle sections, and consequently, this can lead to ejection defects. Therefore, before ejection defects arise, the ink in the whole of the pressure chambers needs to be expelled by dummy ejection or by a suctioning operation, and needs to be replaced with fresh ink.
In a head based on a thermal method in which it is difficult to mix up the ink by applying a vibration of a level that does not cause ejection, to the ink, as described above, the ejection force is inherently a strong force, and therefore it takes a long time until ejection defects such as those described above arise. However if left without taking any particular countermeasures, ejection defects will arise, and therefore control is implemented in a similar fashion in order to expel ink which has risen in viscosity in the vicinity of the nozzles.
Furthermore, a method has also been adopted in a thermal type of head, whereby ink is circulated through a common flow channel, the volume of the pressure chambers is made as small as possible, and the distance between the common flow channel and the nozzles is shortened, in such a manner that decline in the solvent concentration of the ink in the vicinity of the nozzles is delayed by the effects of the diffusion of solvent from the common flow channel. However, in order completely to prevent increase in the viscosity of the ink in the vicinity of the nozzles by means of this method, it is necessary for the length from the supply path to the nozzle, via the pressure chamber, to be no more than several tens of microns, and therefore, in practice, the increase in viscosity is not suppressed completely, and control for expelling ink of raised viscosity is still needed.
In inkjet printers using either of the aforementioned kinds of actuator, printing cannot be carried out while ink is being expelled, and therefore, the ink of raised viscosity is expelled by moving the head to a position that is distanced from the printing region, or alternatively, if the print medium is a cut paper, or the like, ink is expelled by providing a medium for receiving the ink expelled in the interval between respective sheets of the print medium.
In other words, it is not possible to prevent increase in the viscosity of the ink due to evaporation of the solvent, either by shaking the meniscus, or by employing a diffusion effect by reducing the volume of the pressure chambers, or the like, and therefore ink of raised viscosity is required to be expelled and discarded, giving rise to wasted ink.
Even if this ink is reused rather than being discarded, a filtering process is required since there is a high probability that dust, or the like, will have entered into the ink having been expelled from the nozzles.
Since it is essentially impossible to carry out printing during the expulsion of ink, whichever of the printing systems described above is adopted, then there is a problem in that productivity declines.
In response to problems of this kind, technology has been proposed for preventing decline in the concentration of ink solvent in the vicinity of nozzles by constantly circulating the ink in non-ejecting nozzles and ejecting nozzles, during printing (see, for example, Japanese Patent Application Publication No. 63-41152, Japanese Patent Application Publication No. 1-108056, Publication of Japanese translation of PCT Application No. 2000-512233 and Publication of Japanese translation of PCT Application No. 2003-505281.)
However, there are problems of the following kinds associated with these ink circulation technologies in the related art.
(1) If it is sought to maintain a good printing state for all of the printing conditions, then this implies the most severe conditions in terms of the volume of circulated ink, and therefore the total volume of reused ink after circulation becomes very large, and the amount of added solvent also becomes large.
(2) If a filter is used to deal with any possible infiltration of foreign matter, then the lifespan of the filter is very short.
(3) In the case of UV-curable ink, if circulation is continued then the ink becomes less readily curable, due to the effects of the oxygen and moisture in the air (in the case of a radical type of UV-curable ink, the presence of oxygen causes the radicals to be captured by the oxygen, thereby inhibiting the curing reaction, and in the case of a cationically-curable ink, the presence of moisture makes curing difficult to achieve). Furthermore, the ink is degraded and may become unrecoverable (irreversibly changed), due to chemical changes caused by the effects of heating due to the temperature adjustment of the head, or light of trace levels, or the like.
(4) Since air becomes dissolved in the ink that makes contact with the air in the nozzle sections, then if the circulation volume is high, the amount of dissolved air in the ink increases, the compliance of the ink changes, and the ejection characteristics hence change. Moreover, the time taken for the air bubbles to disappear also becomes longer in the event that air bubbles do enter into the ink, and the restoration time for the effects caused by the air bubbles becomes longer.
Consequently, it is desirable for the amount of circulated ink to be as small as possible.
The present invention has been contrived in view of the circumstances described above, an object thereof being to provide a liquid circulation apparatus, an image forming apparatus and a liquid circulation method whereby ejection defects are prevented by circulating the ink in the vicinity of the nozzles, as well as being able to reduce the circulated ink volume which is recycled or discarded.
In order to attain the aforementioned object, the present invention is directed to a liquid circulation apparatus, comprising: a plurality of liquid ejection elements each of which includes: a nozzle; a pressure chamber which is connected to the nozzle and accommodates liquid; and a piezoelectric element which displaces a wall of the pressure chamber to eject the liquid in the pressure chamber through the nozzle; a plurality of individual supply channels which are respectively connected to the liquid ejection elements; a common supply channel which is connected to the individual supply channels, the liquid being supplied from the common supply channel to the liquid ejection elements through the individual supply channels; a plurality of individual circulation channels which are respectively connected to the liquid ejection elements; a common circulation channel which is connected to the individual circulation channels, the liquid being circulated from the liquid ejection elements to the common circulation channel through the individual circulation channels; and a control device which controls a circulation volume of the liquid circulated from the liquid ejection elements to the common circulation channel, by adjusting a supply volume of the liquid supplied from the common supply channel to the liquid ejection elements in accordance with an ejection volume of the liquid ejected from the liquid ejection elements.
In this aspect of the present invention, the liquid circulation volume is controlled by adjusting the liquid supply volume in accordance with the liquid ejection volume of the plurality of liquid droplet ejection elements. By this means, when the liquid ejection volume is low, it is possible to increase the liquid circulation volume and thereby prevent ejection defects caused by increase in the viscosity of the liquid in the vicinity of the nozzles. On the other hand, when the liquid ejection volume is high, then by lowering the liquid circulation volume (and desirably, reducing the liquid circulation volume to zero), the liquid inside the common circulation channel is caused to return to the ejecting nozzle side by the liquid ejection operation at the ejecting nozzle, and this liquid is ejected from the ejecting nozzle. At the same time, a flow of liquid is also created from the non-ejecting nozzles towards the common circulation channel, and therefore it is possible to prevent ejection defects in the non-ejecting nozzles. Consequently, the amount of circulated liquid that is to be recycled or discarded can be reduced, and costs can be reduced.
Preferably, the control device adjusts the supply volume by changing a pressure differential of the liquid between in the common supply channel and in the common circulation channel.
In this aspect of the present invention, it is possible to simplify the control of the liquid circulation volume.
Preferably, the control device adjusts the supply volume to be greater than the ejection volume when the ejection volume is smaller than a prescribed value, and adjusts the supply volume to be equal to the ejection volume when the ejection volume is greater than the prescribed value.
Since the liquid circulation volume is reduced to zero when the liquid supply volume is greater than the prescribed value, the volume of circulated ink that is to be recycled or discarded can be reduced, and therefore further cost savings can be achieved.
In this aspect of the present invention, the “prescribed value” may be the minimum ejection volume required to prevent ejection defects in the nozzles, or it may be a volume which is larger than the minimum required ejection volume by a prescribed margin.
Preferably, when the ejection volume is smaller than the prescribed value, the control device keeps the supply volume constant regardless of the ejection volume.
Preferably, when the ejection volume is smaller than the prescribed value, the control device adjusts the supply volume to increase gradually as the ejection volume increases.
Preferably, the control device adjusts the supply volume to be greater than the ejection volume but to approach the ejection volume gradually as the ejection volume increases.
In this aspect of the present invention, the differential between the liquid supply volume and the liquid ejection volume can be controlled to be inversely proportional to the number of printed dots, for example. Accordingly, the control procedure can be simplified.
Preferably, each of the liquid ejection elements further includes a nozzle channel which connects the pressure chamber with the nozzle; one of the individual supply channels is connected to the pressure chamber; and one of the individual circulation channels has an opening end which opens to the nozzle channel.
In this aspect of the present invention, it is possible effectively to prevent ejection defects caused by increase in the viscosity of the liquid in the vicinity of the nozzles.
Preferably, each of the liquid ejection elements further includes a nozzle channel which connects the pressure chamber with the nozzle; one of the individual supply channels has an opening end which opens to the nozzle channel; and one of the individual circulation channels is connected to the pressure chamber.
In this aspect of the present invention, it is possible to prevent ejection defects caused by increase in the viscosity of the liquid in the vicinity of the nozzles, and it is also possible to achieve faster refilling of the liquid.
Preferably, the nozzle channel has a flow speed regulating section between the pressure chamber and the opening end of the one of the individual circulation channels, the nozzle channel tapering in the flow speed regulating section toward the nozzle.
Preferably, the nozzle channel has a flow speed regulating section between the pressure chamber and the opening end of the one of the individual supply channels, the nozzle channel tapering in the flow speed regulating section toward the nozzle.
In these aspects of the present invention, it is possible to achieve a flow speed distribution which is substantially symmetrical in terms of the nozzle axis, in the liquid flowing through the nozzle flow channel.
Preferably, the nozzle channel is defined by an inner surface having a plurality of openings including the opening end of the one of the individual circulation channels, the openings being arranged at positions that are rotationally-symmetric in terms of an axis of the nozzle.
Preferably, the nozzle channel is defined by an inner surface having a plurality of openings including the opening end of the one of the individual supply channels, the openings being arranged at positions that are rotationally-symmetric in terms of an axis of the nozzle.
In these aspects of the present invention, it is possible to achieve a flow speed distribution which is substantially symmetrical about the nozzle axis, in the liquid flowing through the nozzle flow channel.
Preferably, each of the individual flow channels has a branching section which has a first end connected to the common circulation channel and a second end connected to at least two of the liquid ejection elements.
In this aspect of the present invention, it is possible effectively to suppress increase in the viscosity of the liquid in the pressure chambers.
In order to attain the aforementioned object, the present invention is also directed to a liquid circulation apparatus, comprising: a plurality of pairs of first and second liquid ejection elements, each of the first and second liquid ejection elements including: a nozzle; a pressure chamber which is connected to the nozzle and accommodates liquid; and a piezoelectric element which displaces a wall of the pressure chamber to eject the liquid in the pressure chamber through the nozzle; a plurality of first individual supply channels which are respectively connected to the first liquid ejection elements; a first common supply channel which is connected to the first individual supply channels; a plurality of second individual supply channels which are respectively connected to the second liquid ejection elements; a second common supply channel which is connected to the second individual supply channels; a plurality of individual circulation channels each of which connects the first and second liquid ejection elements with each other in one of the pairs of the first and second liquid ejection elements; and a control device which controls a pressure differential of the liquid between in the first common channel and in the second common channel according to an ejection volume of the liquid ejected from the liquid ejection elements.
In this aspect of the present invention, it is possible to circulate liquid between the first and second liquid droplet ejection elements, via the circulation flow channels, by changing the pressure differential between the first and second common flow channels in accordance with the liquid ejection volume from the plurality of liquid droplet ejection elements. Consequently, the liquid of increased viscosity in the vicinity of the non-ejecting nozzles can be circulated towards the ejecting nozzles, and can be ejected from the ejecting nozzles, and therefore it is possible to reduce the volume of circulated liquid that is to be recycled or discarded, and cost savings can be made.
In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus including one of the liquid circulation apparatuses as described above.
In this aspect of the present invention, it is possible to improve the image quality without causing ejection defects in the nozzles.
In order to attain the aforementioned object, the present invention is also directed to a liquid circulation method for a liquid circulation apparatus which includes: a plurality of liquid ejection elements each of which includes a nozzle, a pressure chamber which is connected to the nozzle and accommodates liquid, and a piezoelectric element which displaces a wall of the pressure chamber to eject the liquid in the pressure chamber through the nozzle; a plurality of individual supply channels which are respectively connected to the liquid ejection elements; a common supply channel which is connected to the individual supply channels, the liquid being supplied from the common supply channel to the liquid ejection elements through the individual supply channels; a plurality of individual circulation channels which are respectively connected to the liquid ejection elements; and a common circulation channel which is connected to the individual circulation channels, the liquid being circulated from the liquid ejection elements to the common circulation channel through the individual circulation channels, the method comprising the steps of: determining an ejection volume of the liquid ejected from the liquid ejection elements; and controlling a circulation volume of the liquid circulated from the liquid ejection elements to the common circulation channel, by adjusting a supply volume of the liquid supplied from the common supply channel to the liquid ejection elements in accordance with the ejection volume determined in the determining step.
According to the present invention, the liquid circulation volume is controlled by adjusting the liquid supply volume in accordance with the liquid ejection volume of the plurality of liquid droplet ejection elements. By this means, when the liquid ejection volume is low, it is possible to increase the liquid circulation volume and thereby prevent ejection defects caused by increase in the viscosity of the liquid in the vicinity of the nozzles. On the other hand, when the liquid ejection volume is high, then by lowering the liquid circulation volume (and desirably, reducing the liquid circulation volume to zero), the liquid inside the common circulation channel is caused to return to the ejecting nozzle side by the liquid ejection operation at the ejecting nozzle, and this liquid is ejected from the ejecting nozzle. At the same time, a flow of liquid is also created from the non-ejecting nozzles towards the common circulation channel, and therefore it is possible to prevent ejection defects in the non-ejecting nozzles. Consequently, the amount of circulated liquid that is to be recycled or discarded can be reduced, and costs can be reduced.
The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein:
Firstly, an inkjet recording apparatus which forms an image forming apparatus according to an embodiment of the present invention will be described.
In
In the case of the configuration in which roll paper is used, a cutter 28 is provided as shown in
In the case of a configuration in which a plurality of types of recording paper can be used, it is preferable that an information recording medium such as a bar code and a wireless tag containing information about the type of paper is attached to the magazine, and by reading the information contained in the information recording medium with a predetermined reading device, the type of paper to be used is automatically determined, and ink-droplet ejection is controlled so that the ink-droplets are ejected in an appropriate manner in accordance with the type of paper.
The recording paper 16 delivered from the paper supply unit 18 retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper 16 in the decurling unit 20 by a heating drum 30 in the direction opposite from the curl direction in the magazine. The heating temperature at this time is preferably controlled so that the recording paper 16 has a curl in which the surface on which the print is to be made is slightly round outward.
The decurled and cut recording paper 16 is delivered to the suction belt conveyance unit 22. The suction belt conveyance unit 22 has a configuration in which an endless belt 33 is set around rollers 31 and 32 so that the portion of the endless belt 33 facing at least the ink ejection face of the print unit 12 and the sensor face of the print determination unit 24 forms a plane.
The belt 33 has a width that is greater than the width of the recording paper 16, and a plurality of suction restrictors (not shown) are formed on the belt surface. A suction chamber 34 is disposed in a position facing the sensor surface of the print determination unit 24 and the ink ejection surface of the print unit 12 on the interior side of the belt 33, which is set around the rollers 31 and 32, as shown in
The belt 33 is driven in the clockwise direction in
Since ink adheres to the belt 33 when a marginless print job or the like is performed, a belt-cleaning unit 36 is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt 33. Although the details of the configuration of the belt-cleaning unit 36 are not shown, examples thereof include a configuration in which the belt 33 is nipped with cleaning rollers such as a brush roller and a water absorbent roller, an air blow configuration in which clean air is blown onto the bet 33, or a combination of these. In the case of the configuration in which the belt 33 is nipped with the cleaning rollers, it is preferable to make the line velocity of the cleaning rollers different than that of the belt 33 to improve the cleaning effect.
The inkjet recording apparatus 10 can comprise a roller nip conveyance mechanism, in which the recording paper 16 is pinched and conveyed with nip rollers, instead of the suction belt conveyance unit 22. However, there is a drawback in the roller nip conveyance mechanism that the print tends to be smeared when the printing area is conveyed by the roller nip action because the nip roller makes contact with the printed surface of the paper immediately after printing. Therefore, the suction belt conveyance in which nothing comes into contact with the image surface in the printing area is preferable.
A heating fan 40 is disposed on the upstream side of the print unit 12 in the conveyance pathway formed by the suction belt conveyance unit 22. The heating fan 40 blows heated air onto the recording paper 16 to heat the recording paper 16 immediately before printing so that the ink deposited on the recording paper 16 dries more easily.
The print unit 12 is a so-called “full line head” in which a line head having a length corresponding to the maximum paper width is arranged in a direction (main scanning direction) that is perpendicular to the paper conveyance direction (sub scanning direction).
The recording respective heads 12K, 12C, 12M, and 12Y forming the print unit 12 is constituted by a line head, in which a plurality of ink ejection ports (nozzles) are arranged along a length that exceeds at least one side of the maximum-size recording paper 16 intended for use in the inkjet recording apparatus 10 (see
The recording heads 12K, 12C, 12M, and 12Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side (left-hand side in
The print unit 12, in which the full-line heads covering the entire width of the paper are thus provided for the respective ink colors, can record an image over the entire surface of the recording paper 16 by performing the action of moving the recording paper 16 and the print unit 12 relative to each other in the paper conveyance direction (sub-scanning direction) just once (in other words, by means of a single sub-scan). Higher-speed printing is thereby made possible and productivity can be improved in comparison with a shuttle type head configuration in which a recording head moves reciprocally in the direction (main-scanning direction) that is perpendicular to the paper conveyance direction.
Although the configuration with the KCMY four standard colors is described in the present embodiment, combinations of the ink colors and the number of colors are not limited to those. Light inks and dark inks can be added as required. For example, a configuration is possible in which the recording heads for ejecting light-colored inks such as light cyan and light magenta are added. Furthermore, there are no particular restrictions of the sequence in which the heads of respective colors are arranged.
As shown in
The print determination unit 24 has an image sensor (line sensor, and the like) for capturing an image of the ink-droplet deposition result of the print unit 12, and functions as a device to check for ejection defects such as clogs of the nozzles in the print unit 12 from the ink-droplet deposition results evaluated by the image sensor.
The print determination unit 24 of the present embodiment is configured with at least a line sensor having rows of photoelectric transducing elements with a width that is greater than the ink-droplet ejection width (image recording width) of the recording heads 12K, 12C, 12M, and 12Y. This line sensor has a color separation line CCD sensor including a red (R) sensor row composed of photoelectric transducing elements (pixels) arranged in a line provided with an R filter, a green (G) sensor row with a G filter, and a blue (B) sensor row with a B filter. Instead of a line sensor, it is possible to use an area sensor composed of photoelectric transducing elements which are arranged two-dimensionally.
The print determination unit 24 reads a test pattern image printed by the recording heads 12K, 12C, 12M, and 12Y for the respective colors, and the ejection of each head is determined. The ejection determination includes the presence of the ejection, measurement of the dot size, and measurement of the dot deposition position.
A post-drying unit 42 is disposed following the print determination unit 24, The post-drying unit 42 is a device to dry the printed image surface, and includes a heating fan, for example. It is preferable to avoid contact with the printed surface until the printed ink dries, and a device that blows heated air onto the printed surface is preferable.
In cases in which printing is performed with dye-based ink on porous paper, blocking the pores of the paper by the application of pressure prevents the ink from coming contact with ozone and other substance that cause dye molecules to break down, and has the effect of increasing the durability of the print.
A heating/pressurizing unit 44 is disposed following the post-drying unit 42. The heating/pressurizing unit 44 is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller 45 having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface.
The printed matter generated in this manner is outputted from the paper output unit 26. The target print (i.e., the result of printing the target image) and the test print are preferably outputted separately. In the inkjet recording apparatus 10, a sorting device (not shown) is provided for switching the outputting pathways in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to paper output units 26A and 26B, respectively. When the target print and the test print are simultaneously formed in parallel on the same large sheet of paper, the test print portion is cut and separated by a cutter (second cutter) 48. The cutter 48 is disposed directly in front of the paper output unit 26, and is used for cutting the test print portion from the target print portion when a test print has been performed in the blank portion of the target print. The structure of the cutter 48 is the same as the first cutter 28 described above, and has a stationary blade 48A and a round blade 48B. Although not shown in
The structure of a print head will be described. The print heads 12K, 12C, 12M and 12Y of the respective ink colors have the same structure, and a reference numeral 50 is hereinafter designated to any of the print heads.
Next, the ink circulation system of the inkjet recording apparatus 10 will be described.
A pump 112 is provided in the flow channel 110 which connects the ink tank 100 with the sub tank 102. The ink inside the ink tank 100 is supplied to the sub tank 102 by means of the pump 112. The pump 112 is controlled in such a manner that the amount of ink inside the sub tank 102 is uniform. A heater and cooler for ink temperature adjustment 114 is provided in the sub tank 102, and by adjusting the temperature of the ink inside the sub tank 102 to a prescribed temperature, by means of this heater and cooler for ink temperature adjustment 114, the viscosity of the ink is maintained at a uniform value. For example, there is a mode in which a temperature sensor (not illustrated) for determining the temperature of the ink inside the recording head 50 is provided, and the heater and cooler for ink temperature adjustment 114 is controlled in such a manner that the temperature of the ink inside the recording head 50 assumes a prescribed temperature (for example, 55° C.) (in other words, in such a manner that the ink assumes a desired viscosity).
The sub tank 102 and the recording head 50 are connected with each other by means of a first flow channel 116 and a second flow channel 118. The first flow channel 116 is connected via a first supply port 54 which is formed at one end of a common flow channel (common supply channel) 52 formed in the recording head 50, and the second flow channel 118 is connected via a second supply port 56 which is formed at the other end of the common flow channel 52. The first flow channel 116 is an individual supply channel for supplying ink to the recording head 50 from the sub tank 102, and it is provided with a pump 120 and a filter 122. On the other hand, the second flow channel 118 is a circulation flow channel for returning a portion of the ink supplied to the recording head 50, to the sub tank 102, and it is provided with a pump 124.
The ink inside the sub tank 102 is supplied from the first flow channel 116, via a filter 122, to the recording head 50, by the action of the pump 120. It is preferable that the fineness (mesh size) of the filter 122 should be smaller than the nozzle diameter, since this makes it possible to prevent in advance any blockages in the nozzles caused by foreign material which has entered into the recording head 50 from the sub tank 102. For example, a filter having a mesh size that is smaller than the nozzle diameter by about 10% is used.
A portion of the ink supplied to the recording head 50 is returned from the second flow channel 118 to the sub tank 102, via the common flow channel 52, by the pump 124. Although not shown in the drawing, there is also a mode in which a vacuum deaeration apparatus is installed in the second flow channel 118, to the upstream side of the pump 124 (the side of the recording head 50).
Nozzle flow channels 62 which are connected to the nozzles 64 are provided respectively for the pressure chambers 58 which are connected to the common flow channel 52. An individual circulation channel 72 is provided in each nozzle flow channel 62, and the nozzle flow channel 62 is connected via this individual circulation channel 72 to a common circulation channel 70. The common circulation channel 70 is connected to collection ports 74 via a connecting flow channel (not illustrated here, but indicated with reference numeral 71 in
The pressure chambers 58 are connected to the common flow channel 52 via individual supply channels 60, respectively. The ink is supplied from the common flow channel 52 to each of the pressure chambers 58 via a corresponding one of the individual supply channels 60. The individual supply channels 60 also function as supply restrictors which prevent reverse flow from the pressure chamber 58 to the common flow channel 52. Furthermore, the nozzles 64 are respectively connected to the pressure chambers 58 via nozzle flow channels 62.
A piezoelectric element 68 is provided on top of the diaphragm 66 which constitutes a wall of each of the pressure chambers 58. When a drive voltage is applied to the piezoelectric element 68, the volume of the pressure chamber 58 changes in accordance with the deformation of the diaphragm 66. When the diaphragm 66 deforms in the direction which increases the volume of the pressure chamber 58, then the meniscus formed in the nozzle 64 is pulled in toward the ink inflow side (the side of the pressure chamber 58), and the ink in the common flow channel 52 is sucked into the pressure chambers 58 via the individual supply channels 60, thereby performing refilling. On the other hand, when the diaphragm 66 is deformed in the direction which reduces the volume of the pressure chamber 58, the meniscus of the nozzle 64 is pushed out toward the ink ejection side (the opposite side to the pressure chamber 58), and an ink droplet is ejected from the nozzle 64. In particular, it is desirable that the interval between the pulling and pushing actions should be ¼ of the fluid resonance period of the pressure chamber 58 and the ink, whereby the pulling and pushing vibrations become mutually superimposed, a large displacement is obtained, and ejection of ink can be achieved readily.
When ink is being ejected, the ink inside the pressure chamber 58 not only flows through the nozzle flow channel 62, which is arranged on the ink ejection side, but also, a portion of the ink flows through the individual supply channel 60, which is arranged on the ink supply side. The ink flow volume from the pressure chamber 58 toward the nozzle flow channel 62 and the ink flow volume toward the individual supply channel 60 are determined by the ratios between the resistances and the inertances of the respective flow channels. In a typical inkjet head, the dimensions of the respective sections are determined in such a manner that the ratios are substantially 1 to 1.
Furthermore, as shown in
As shown in
Furthermore, a plurality of common circulation channels 70 are provided in the head unit 51, each of the common circulation channels 70 corresponding to each column 59 of pressure chambers. Each of the common circulation channels 70 is connected to the pressure chambers 58 which belong to the corresponding one of the pressure chamber columns 59. More specifically, as shown in
As shown in
In the recording head 50 having the above-described composition, as shown in
The temperature distribution of the interior of the recording head 50 (and in particular, the temperature distribution of the ink) becomes uniform due to the flow of ink created in the common flow channel 52, and furthermore, even if air bubbles become mixed into the common flow channel 52, it is possible to remove these air bubbles swiftly from the second supply port 56 on the low pressure side. Moreover, due to the flow of ink created in the direction from the common flow channel 52 toward the common circulation channel 70, via the pressure chambers 58 and the like, it is possible to circulate the ink of raised viscosity in the vicinity of the nozzles, and it is possible to prevent ejection defects. The control of the circulation of ink, which is one of the characteristic features of the present invention, will be described later.
The flow volume per unit time of the ink which flows in the common flow channel 52 can be determined from the pressure differential (P1-P2) in the ink between the first supply port 54 and the second supply port 56, and the fluid resistance of the common flow channel 52. It is preferable that the flow volume in the common flow channel 52 be an amount such that the temperature change due to the heat generated by the recording head 50 can be controlled, and it is preferable that the flow volume in the common flow channel 52 be set so as to cause a flowing movement of any air bubbles that may have entered into the common flow channel 52. These two requirements can be met when the flow volume in the common flow channel 52 is adequately large. At the same time, it is also necessary to set the flow volume to a range which does not create turbulence in the common flow channel 52. This is not likely to present an irreconcilable situation, given the amount of heat generated in a typical inkjet head, and the dimensions of a typical inkjet head.
For example, a practicable flow speed is some 10 to 20 times the amount of ink consumed per unit time in a state of full ejection from the head (namely, ejection in a case where ejection for printing is continued at maximum frequency and at maximum ejection volume). If a head which is ejecting 2 (pl) at 40 (kHz) has a nozzle density of 1200 (dpi), and a length of 2 inches per one head unit, then the ink consumption will be 2×2×1200×40000 (pl/sec)=0.192 (ml/sec), and therefore the ink volume flowing in the common flow channel 52 is set to approximately 2 to 4 (ml/sec).
The pressures P1 and P2 applied to the respective supply ports 54 and 56 by the pumps 120 and 124 are weak negative pressures, in such a manner that the meniscus formed at the opening sections of the nozzles 64 of the recording head 50 is slightly pulled in, and these pressures are set to −20 through −60 (mmH2O) with respect to the atmospheric pressure.
In an inkjet head, the ink in the nozzle sections is typically set to a slightly negative pressure with respect to the atmospheric pressure, in order that the ink does not flow out from the nozzles which are not performing ejection. If this negative pressure is too strong, then the surface tension of the meniscus is overcome by the negative pressure and air is sucked in through the nozzles. For example, if using an ink having a surface tension of 35 mN/m for nozzles having a diameter of 18 μm, the maximum value of the surface tension will be 1.98×10−6 (N), and therefore the surface tension per unit surface area of the nozzles will be 8 (kN/m2). This value corresponds to 81 (gf/cm2) through unit conversion, and therefore the meniscus is balanced when the negative pressure is at −810 (mmH2O), and the meniscus breaks down if the negative pressure exceeds this value. However, in an actual bead, since there are a large number of nozzles, then there are many cases where the meniscus breaks down at a back pressure which is lower than this calculated value, due to factors such as the manufacturing precision and surface roughness of the nozzle sections, defects in the hydrophobic treatment on the nozzle sections, the occurrence of vibrations, or the like. Although it was difficult to obtain stable results through actual experiments due to the causes of instability described above, it has been found that the meniscus broke down at −100 through −400 (mmH2O) in many cases. From this experimental result, a margin is allowed and the upper limit of the back pressure is set to −60 (mmH2O) in the present embodiment. On the other hand, the lower limit is set to −20 (mmH2O), in such a manner that ink does not leak out despite the application of a back pressure, due to the effects of vibrations, or environmental changes in the air pressure, temperature or the like. Neither of these values is determined logically, but rather they indicate a range in which stable performance can be achieved on the basis of experimental results.
Returning to
In the flow channel 136 which connects the reserve tank 134 with the sub tank 102, a solvent concentration detector 104, a solvent addition apparatus 106, a deaeration apparatus 108, a pump 138 and a filter 140 are provided, in this sequence, from the upstream side (the side of the reserve tank 134) toward the downstream side (the side of the sub tank 102).
When the ink collected into the reserve tank 134 is returned to the sub tank 102 via the flow channel 136, firstly, the concentration of ink solvent is determined by the solvent concentration detector 104, on the basis of the ink density, viscosity, flow speed variation, electrical conductivity, or other properties. Thereupon, solvent from a solvent tank 144 is added to the ink in the flow channel 136 by the solvent addition apparatus 106, in accordance with the determination results obtained by the solvent concentration detector 104. By this means, it is possible to restore the ink which has been circulated via the pressure chambers 58 and the nozzle flow channels 62, and in particular, the ink which has increased in viscosity in the vicinity of the nozzles, to a suitable viscosity. As described hereinafter, the solvent concentration determined by the solvent concentration detector 104 is sent to a solvent concentration control unit 196 (see
Moreover, a process (deaeration process) is also carried out in order to reduce the amount of dissolved air in the ink, by means of the deaeration apparatus 108, which is connected to a vacuum pump 146. If a vacuum deaeration apparatus is provided on the upstream side (the side toward the recording head 50) from the pump 124 in the second flow channel 118 which connects the sub tank 102 with the recording head 50, then this deaeration apparatus 108 may be omitted.
The ink which has been deaerated by the deaeration apparatus 108 is then returned to the sub tank 102 through a filter 140, by means of the pump 138. Thereupon, the ink is supplied again to the recording head 50, together with ink supplied from the ink tank 100.
According to the composition of the ink circulation system shown in
In general, heat is generated in the recording head 50 due to the operation of the actuators (piezoelectric elements 68), and therefore the ink which is circulated as described above also serves to remove the heat thus generated in the recording head 50. Therefore, it is desirable that the temperature of the circulated ink should be adjusted when it is being recycled or when it is being supplied again.
Next, the control system of the inkjet recording apparatus 10 will be described.
The communication interface 170 is an interface unit for receiving image data sent from a host computer 186. A serial interface or a parallel interface may be used as the communication interface 170. A buffer memory (not shown) may be mounted in this portion in order to increase the communication speed.
The image data sent from the host computer 186 is received by the inkjet recording apparatus 10 through the communication interface 170, and is temporarily stored in the image memory 174. The image memory 174 is a storage device for temporarily storing images inputted through the communication interface 170, and data is written and read to and from the image memory 174 through the system controller 172. The image memory 174 is not limited to a memory composed of semiconductor elements, and a hard disk drive or another magnetic medium may be used.
The system controller 172 is a control unit which controls the respective sections, such as the communications interface 170, the image memory 174, the motor driver 176, the heater driver 178, and the like. The system controller 172 is made up of a central processing unit (CPU) and peripheral circuits thereof, and as well as controlling communications with the host computer 186 and controlling reading from and writing to the image memory 174, and the like, it generates control signals for controlling the motors 188 and heaters 189 in the conveyance system.
The motor driver (drive circuit) 176 drives the motor 188 in accordance with commands from the system controller 172. The heater driver (drive circuit) 178 drives the heater 189 of the post-drying unit 42 or other units in accordance with commands from the system controller 172.
The print controller 180 has a signal processing function for performing various tasks, compensations, and other types of processing for generating print control signals from the image data stored in the image memory 174 in accordance with commands from the system controller 172 so as to supply the generated print control signal (dot data) to the head driver 184. Prescribed signal processing is carried out in the print controller 180, and the ejection amount and the ejection timing of the ink droplets from the respective recording heads 50 are controlled via the head driver 184, on the basis of the print data. By this means, prescribed dot size and dot positions can be achieved.
The print controller 180 is provided with the image buffer memory 182; and image data, parameters, and other data are temporarily stored in the image buffer memory 182 when image data is processed in the print controller 180. The aspect shown in
The head driver 184 generates drive signals for driving the piezoelectric elements 68 (see
The print determination unit 24 is a block that includes the line sensor as described above with reference to
According to requirements, the print controller 180 makes various corrections with respect to the recording head 50 on the basis of information obtained from the print determination unit 24.
Moreover, the inkjet recording apparatus 10 according to the present embodiment comprises a pressure control unit 190, a pressure determination unit 192, a pump driver 194, a solvent concentration control unit 196, and the like.
The pressure determination unit 192 determines the pressure in the common flow channel of the recording head 50 (at the position where the supply port is formed), and the pressure in the common circulation flow channels (at the position where the collection port is formed), and it supplies pressure signals including these determination results to the pressure control unit 190.
The pressure control unit 190 receives information indicating the number of dots to be printed, from the print control unit 180, and calculates the total ink ejection volume for the recording head 50 (or for the head unit 51), and furthermore, it also supplies pump drive control signals to the pump driver 194 on the basis of the pressures determined by the pressure determination unit 192, in such a manner that a desired ink circulation volume is achieved by adjusting the ink supply volume in accordance with the calculated ink ejection volume. The pump driver 194 drives the respective pumps 112, 120, 124, 132 and 138 on the basis of the pump drive control signals supplied from the pressure control unit 190.
Furthermore, the pressure control unit 190 sends instructions regarding a solvent concentration value to the solvent concentration control unit 196, in accordance with the ink circulation volume. The solvent concentration control unit 196 drives the solvent addition apparatus 106 on the basis of the solvent concentration value determined by the solvent concentration detector 104, in such a manner that the solvent concentration instructed by the pressure control unit 190 is achieved. Accordingly, a suitable volume of solvent is added to the ink which has been collected and circulated.
Firstly, the relationship between the ink volume required to prevent ejection defects and the ejection volume used by printing will be described.
With regard to the phenomenon of evaporation of solvent from the nozzles which causes increase in the viscosity of the ink and leads to ejection defects, provided that a nozzle always performs ejection at or above a certain frequency, then this is equivalent to continuously doing away with the ink which has increased in viscosity due to evaporation of the solvent, and therefore ejection defects can be prevented by means of continuous ejection of that frequency.
The threshold frequency of “at or above a certain frequency” depends on various conditions, such as the ambient temperature and humidity conditions, the state of air flow in the periphery of the head, the solvent component of the ink, and the volumes of the respective components of the ink, and the like. Through experimentation carried out by the present inventors, it was found that if ink droplets of 4 pl are ejected in one ejection action, using an ink containing a solvent of water, under normal temperature and normal humidity conditions, then the frequency of ejection must be once or more every 0.1 seconds approximately.
At first sight, this appears to be a relatively low frequency, and it can be imagined that no particular problems would be created in the printed object if ink were to be ejected at a frequency satisfying this condition during printing, regardless of the image being printed. However, this ejection frequency is in fact highly likely to give rise to problems in the printed object, unless the image is one that is printed by ejecting at a substantially uniform rate from all of the nozzles, or unless the printing speed is extremely fast and the volume of ink discarded in this manner during the printing of one sheet is extremely small. For example, if printing is carried out on one sheet of A4 paper in 0.5 seconds from a fixed head, then the aforementioned condition implies ejecting five times from all of the nozzles during the printing of one sheet. Since ejection of this kind is necessary particularly for blank areas where ejection is not performed from the nozzles for a prescribed period of time and where there are few dots, then the density at the originally white blank surface will inevitably be raised, even if the five ejections are performed from each of the nozzles so as to create the largest possible gaps between the dots.
Here, although the term “white blank surface” is used, for each of the inks of respective colors used in this inkjet recording apparatus, the region where no ink of that color is printed is considered as a “white blank surface” for that color, In other words, in a case where the whole areas of the printed object are covered with inks, although it may be regarded that there is no blank surface, since the problem discussed here must be considered in terms of each color of the inks, then the blank surface is in fact liable to exist in respect of any of the colors.
Conversely, under conditions where there is no blank surface in respect of any of the colors, then this means that each of the colored inks is being ejected at a certain frequency, and therefore in the whole printed object (and not just locally) there is a presence of gray to black colored regions. This also includes objects which are printed by spacing out the inks of the respective colors to create the greatest possible intervals between dots in order that the inks are not mutually superimposed, and in printed objects of this kind, the overall appearance to a visual observer is a gray color. Needless to say, the aforementioned ejection frequency condition can also be satisfied by ejecting the inks of the respective colors locally, in an independent fashion, but in this case the resulting printed object will obviously differ from the originally intended image. Moreover, in a low-humidity environment, the stated frequency of “once every 0.1 seconds” is increased, and therefore the density of the blank surface is raised even further.
In other words, it depends on the image being printed whether or not beneficial effects can be obtained in respect of discarding ink which has risen in viscosity, by means of the method described here. Moreover, the greater the amount of blank surface in the printed object and therefore the greater the need to discard ink, the more conspicuous the discarded ink becomes and the harder it becomes to discard the ink. Therefore, practical implementation of this method becomes more difficult.
Consequently, the method of discarding ink which has increased in viscosity, by ejection in portions other than the original image, is limited in terms of the conditions under which it can be implemented, and therefore it is impracticable.
However, when viewed in terms of the whole print medium, provided that the image is not a completely blank sheet, then ejection of some volume of ink can be expected per unit surface area, as described below. For example, in a printed object which has a high ejection volume, including a photograph, or the like, the average ink ejection volume per unit surface area is approximately 1 cc/M2, and this is equivalent to a print rate of approximately 22% (the surface area covered with ink when recording with an ink droplet size of 2 pl at a resolution of 1200 dpi). Furthermore, even with an A4-sized printed object bearing black text characters and having a print rate of 5%, which is specified as a standard sheet, if the print medium is conveyed in the breadthways direction with respect to a fixed inkjet head having a printing width of 297 mm, and if recording is carried out using an ink droplet size of 2 pl at a resolution of 1200 dpi, then the average ejection volume per nozzle will be 992 pl. Supposing conditions where one sheet of A4-size medium is printed in 0.5 seconds, as described previously, this means that 198 ejections are performed every 0.1 seconds, and hence this ejection frequency is considerably higher than the condition of ejection “at or above a frequency of once every 0.1 seconds approximately” as stated above.
However, even in the case of ejection at a high average frequency in this way, as mentioned previously, it depends on the print image which of the nozzles eject ink, and hence there may be a print image in which particular nozzles do not perform printing at all. Moreover, if the same content is printed on a plurality of sheets, then the particular nozzles continue in a state of not performing printing at all, for a long period of time.
Consequently, although ink may be ejected at a level which would avoid problems in the case of normal printing, if the average value for the whole head is considered, since there are in fact variations in the ejection volumes of nozzles, then there is no guarantee that the conditions for avoiding problems are satisfied at all times. Furthermore, as stated above, there are also problems with the implementation of a method which performs the minimum necessary ejection to prevent ejection defects, by distributing ejection throughout the image. Consequently, various countermeasures such as those described in the related art are necessary.
However, provided that good and effective use is made of the circumstances described above in which a sufficient volume of ink is ejected in average terms, then it is possible significantly to improve the problem of ejection defects.
Focusing on this point, the present invention circulates the ink of increased viscosity in the vicinity of the nozzles through the common circulation channels in cases where the overall ink ejection volume is low. Moreover, in cases where the overall ink ejection volume is high, the ink supply volume is set to be equal to the ink ejection and the degraded ink which has been circulated through the common circulation channels is returned to the ejecting nozzle side and is ejected from the ejecting nozzles together with fresh ink supplied from the common flow channel side. The control of ink circulation according to the present invention is described more specifically below.
In normal meniscus shaking, in order to prevent increase in the viscosity of the ink in the vicinity of the nozzles due to evaporation of the solvent from the nozzles, the meniscus is shaken at a relatively large amplitude of a level which does not cause the ink to be ejected from the nozzles, and the ink in the vicinity of the nozzles is thereby mixed and exchanged with the ink inside the pressure chambers. By this means, the ink viscosity in the nozzle sections is prevented from increasing for a certain time.
However, since the volume of the pressure chambers is limited, then if meniscus shaking is continued, the ink viscosity in the whole of the pressure chamber eventually rises and ejection defects inevitably occur.
In response to this, in the present embodiment, the ink circulation side (the common circulation channel 70 side) is set to a relative negative pressure with respect to the ink supply side (the common flow channel 52 side), and a sufficiently low flow speed compared to the average flow speed of the ink supply accompanying the ink ejection, in other words, a slow circulation flow, is created. In this case, the meniscus is shaken at a low frequency which is 1/2 to 1/100 (and more desirably, 1/20 to 1/100) the frequency of normal meniscus shaking, in such a manner that ink which has started to rise in viscosity is caused to move to the vicinity of the opening section of the individual circulation channel 72 (the opening in the nozzle flow channel 62, or the junction port). Due to this meniscus shaking at a low frequency of 1/2 to 1/100, the ink which has started to increase in viscosity in the vicinity of the nozzles is mixed together with the ink in the vicinity of the opening section of the individual circulation channel 72, and this ink is drawn into the individual circulation channel 72 together with the ink which flows in from the pressure chamber 58 due to the ink circulation flow.
Meniscus shaking is carried out at a low frequency of 1/2 to 1/100 compared to normal meniscus shaking in this way in order to minimize the change in the viscosity of the ink inside the pressure chambers 58.
In a state of this kind, the solvent concentration acquires a gradient, whereby the solvent concentration becomes lower on the nozzle 64 side compared to the individual circulation channel 72 side, from the surface section (opening section) of the nozzle 64 toward the vicinity of the opening section of the individual circulation channel 72. In other words, the vicinity of the opening section of the individual circulation channel 72 assumes a state of substantially fresh ink due to the circulation of ink.
In the state described above, the ink can be ejected from the nozzles 64. The ink at the surface section (opening section) of the nozzle 64 has relatively high viscosity, and therefore the speed of diffusion of solvent in the ink is slow, the replenishment of solvent to the surface section of the nozzles 64 is slow. Consequently, a state which suppresses the evaporation of solvent is achieved and the total volume of evaporated solvent can be reduced.
In the present embodiment, the ink of increased viscosity in nozzles 64 of a low ejection frequency flows from the individual circulation channel 72 to the common circulation channel 70 due to the slow circulation flow. The flow speed of the ink is adjusted whereby, although the ink of increased viscosity has higher viscosity than in the initial state, the ink does not increase in viscosity to the extent that the ink ejection is impossible. For example, the flow volume is 400 (pl/sec per nozzle), which is much smaller than the full ejection volume (the amount of ink ejected by continuous ejection for printing at maximum frequency and maximum ejection volume) and represents an extremely small amount compared to the ink circulation volume which is implemented generally in the related art.
Next, the ink circulation volume is described with reference to
In
In a recording head which does not have an ink circulation function, the relationship between the ink ejection volume and the ink supply volume is indicated by the straight line 200 shown in
On the other hand, in a recording mode which has the ink circulation function used in the related art, the ink supply volume Y6 may be sufficiently greater than the ink supply volume Y5 (=X5) at full ejection, regardless of the ink ejection volume X, in such a manner that there is no shortage of ink even when performing full ejection, as indicated by the straight line 206 which is parallel to the horizontal axis shown in
Therefore, in the present invention, if the ink ejection volume is less than the minimum ink ejection volume required to prevent ejection defects, then the ink supply volume is adjusted to be greater than the ink ejection volume. An ink flow is thereby created from each of the nozzles 64A, 64B and 64C toward the common circulation channel 70, and the degraded ink in the nozzles 64A, 64B and 64C is circulated into the common circulation channel 70, as shown in
More specifically, in
The ink circulation control according to the present invention which was described above is based on ideal conditions. If the ink ejection volume X is X1, which is the minimum ink ejection volume required to prevent ejection defects, then the ink supply volume Y1 is set to be equal to the ink ejection volume X1. Thus, there is no margin with respect to any slight variations, such as environmental changes, and there is a possibility that ejection defects, such as ink shortages, may occur.
In the recording head 50 according to the present embodiment, if the ink ejection volume is less than the ink ejection volume X2 (>X1), then the ink supply volume is made greater than the ink ejection volume, and the ink supply volume is gradually increased as the ink ejection volume increases. On the other hand, if the ink ejection volume is greater than the ink ejection volume X2, then the ink supply volume is set to be equal to the ink ejection volume. If the ink ejection volume is equal to the ink ejection volume X2, then either option is possible. In the present embodiment, for the sake of convenience, the ink supply volume is set to be equal to the ink ejection volume whenever the ink ejection volume is equal to or greater than X2. In other words, in
In implementing the present invention, the ink supply volume when the ink ejection volume is less than X2 as described above may of course also be set to a uniform volume (in other words, a volume not less than Y2), regardless of the ink ejection volume.
The ink flows into the liquid droplet ejection element 80 including the pressure chamber 58, and the like, from the common flow channel 52 and from the common circulation channel 70. The ratio between these ink flows is determined by the ratios of the flow resistance and the inertance between the individual supply channel 60 and the individual circulation channel 72, and the pressure differential between the common flow channel 52 and the common circulation channel 70. Therefore, in the present embodiment, taking the resistance of the individual supply channel 60 to be Rs, taking the inertance thereof to be Ls, taking the resistance of the nozzle flow channel 62 to be Rn, taking the inertance thereof to be Ln, taking the resistance of the individual circulation channel 72 to be Rr and taking the inertance thereof to be Lr, (see
Rs+Rr≈Rn
Ls+Lr≈Ln
Rs>>Rr (Rr is approximately 1/10 to 1/100 of Rs)
Ls>>Lr (Lr is approximately 1/10 to 1/100 of Ls)
Furthermore, in
X
1
/X
2
=R
r
/R
n.
In other words, the volume of ink that is supplied from the individual circulation channel 72 to the nozzle flow channel 62 during ink ejection is determined by the ratio of the resistances between the individual circulation channel 72 and the nozzle flow channel 62. By this means, it is possible to reduce the effects caused by the individual circulation channel 72 during ink ejection.
For example, when a head having dimensions shown in Table 1, the relationship between Rr and Rn is as follows:
X
1
/X
2
=R
r
/R
n=0.835/9.7=1/11.6.
Therefore, if X1=400 (pl/sec), then X2=4640 (pl/sec).
Moreover, the pressures P1, P2 and P3 (see
P3<−20 through −60 (mmH2O)<P2<P1
The relationship between the pressures P1 and P2 is determined by the resistance of the common flow channel 52 and the required flow speed.
Firstly, when not ejecting (when the ink ejection rate X =0), the pressure P3 is set in such a manner that the following conditions are satisfied:
−(P2−P3)/(Rs+Rr)=I,
where I is the minimum ink ejection volume required to prevent ejection defects and it corresponds to the ink ejection volume X1 in
Furthermore, during ejection (when the ink ejection volume X>0), the pressures are set as follows. If the average ejection volume (which is represented as “Ia”) is equal to or greater than the ink ejection volume X2 (in other words, Ia≧X2), then pressures are set so as to satisfy conditions of P2=P3. Furthermore, if the average ejection volume Ia is less than X2 (in other words, if 0<Ia<X2), then pressures are set in such a manner that the following relationship is satisfied:
P3=(Ia/X2) (P2−P3n)+P3n,
where P3n is the value of P3 during non-ejection.
The average ejection volume is calculated by dividing the total “Ia” of the ejection volume per unit time from all of the nozzles which are subject to control, by the total number of nozzles. For example, if there are 1000 nozzles and of these, 500 nozzles each perform continuous ejection of 2 (pl) of ink at 10 (kHz), then the average ejection volume can be calculated as follows:
Ia=500×2×10000/1000=10000 (pl/sec per nozzle).
Furthermore, although the resistances Rcs and Rcr of the common flow channel 52 and the common circulation channel 70 are so small as to be ignorable, it is convenient to suppose a relationship of Rcs≈Rcr, since this means that the pressure gradients in the common flow channel 52 and the common circulation channel 70 are the same, and therefore the ink circulation volume is virtually uniform in each of the pressure chambers 58.
By setting the respective parameters as described above, in
According to the ink circulation control of this kind, if the ink ejection volume in the recording head 50 is less than the ink ejection volume X2 which is obtained by incorporating a margin into the minimum required ink ejection volume X1, then the ink supply volume is adjusted to be greater than the ink ejection volume (in other words, the ink circulation volume is increased), and hence it is possible to prevent ejection defects caused by increase in the viscosity of the ink in the vicinity of the nozzles. On the other band, if the ink ejection volume is equal to or greater than the ink ejection volume X2, then by adjusting the ink supply volume to be equal to the ink ejection volume (in other words, by setting the ink circulation volume to zero), the degraded ink which has been circulated into the common circulation channel 70 is made to return toward the ejecting nozzles due to the ejection operation (and in particular the ink supply operation) at the ejecting nozzles, and this degraded ink can thereby be ejected from the ejecting nozzles together with fresh ink which has been supplied from the common flow channel 52. At the same time as this, since a flow of ink is created from the non-ejecting nozzles towards the common circulation channel 70, then it is also possible to prevent ejection defects in the non-ejecting nozzles.
By this means, it is possible to reduce the circulated ink volume (the volume of collected ink) which requires readjustment in respect of the solvent concentration and the amount of dissolved gas therein. Furthermore, if the printed object involves printing dots at or above a prescribed amount, then the volume of collected ink can be set to zero. Consequently, it is possible to reduce the costs of the consumables (including ink) and the devices which are required in order to achieve circulation of ink.
For instance, the following is an example of a calculation for determining the level of the ejection (printing) frequency at which the amount of the collected ink is zero. Firstly, the various conditions which are premises of this calculation are as follows.
When the total circulation volume and the maximum ejection volume in one nozzle are calculated on the basis of these conditions, then the following values are obtained:
Total circulation volume: 400×1000=400000 (p/sec); and
Maximum ejection volume in one nozzle: 40×103×2=80000 (pl).
Accordingly, (total circulation volume)/(maximum ejection volume in one nozzle) is calculated to be 5.
As a result of this, it is possible to eject the total volume of circulated ink for all of the nozzles by means of five nozzles performing ejection at the maximum rate. If an ejection volume ten times this amount is considered in order to allow some spare margin, then a total of 50/1000 (1/20=5% ) of the nozzles need to perform ejection at 40 kHz.
A normal printed object has at least a dot presence of approximately 5% on the medium surface. Consequently, there is a spare margin of exactly ten times.
In the present embodiment, as shown in
In the present embodiment, as shown in
Next, a second embodiment of the present invention will be described. Below, portions which are common with the first embodiment are not explained further, and the following description centers particularly on characteristic features of the second embodiment.
The second embodiment is the same as the first embodiment except that the second embodiment is different from the first embodiment in respect of the connection arrangement of the individual circulation channels 72. More specifically, in the recording head 50B according to the present embodiment, the mutually adjacent pressure chambers 58A and 58B are connected by a T-shaped individual circulation channel 72 as shown in
According to the second embodiment, it is possible efficiently to suppress increase in the ink viscosity inside the pressure chambers 58, even by performing meniscus shaking, and therefore ejection defects can be prevented reliably over a long period of time and greater reliability can be achieved.
Next, a third embodiment of the present invention will be described. Below, portions which are common with the first and second embodiments described above are not explained further, and the following description centers on characteristic features of the third embodiment.
More specifically, as shown in
As shown in
Supply ports 54A and 54B are formed respectively in the first and second common flow channels 52A and 52B, and if the pressure of these supply ports 54A and 54B are taken to be Ph and P1 respectively, then the pressure Ph at the supply port 54A is set or controlled by means of a pump (not illustrated) so as to be higher than the pressure P1 of the supply port 54B (in other words, Ph>P1). More specifically, since the first common flow channel 52A is on the high-pressure side and the second common flow channel 52B is on the low-pressure side, then as indicated by the arrow in
The pressures Ph and P1 have a relationship of −60 (mmH2O)<P1<Ph<−20 (mmH2O). Due to the pressure differential (Ph-P1) between the pressures Ph and P1, a flow of ink is created from the pressure chamber 58A on the high pressure side to the pressure chamber 58B on the low pressure side.
In the present embodiment, similarly to the first embodiment, taking the resistance of the individual supply channel 60 to be Rs, taking the inertance thereof to be Ls, taking the resistance of the nozzle flow channel 62 to be Rn, taking the inertance thereof to be Ln, taking the resistance of the individual circulation channel 72 to be Rr and taking the inertance thereof to be Lr, (see
Rs+Rr≈Rn;
Ls+Lr≈Ln;
Rs>>Rr (Rr is approximately 1/10 through 1/100 of Rs) ; and
Ls>>Lr (Lr is approximately 1/10 through 1/100 of Ls).
According to the present embodiment, the pressure chambers 58B on the low pressure side function as a common circulation channel. Furthermore, since ink which has increased in viscosity to some extent flows from the pressure chambers 58A on the high pressure side, to the pressure chambers 58B on the low pressure side, then increase in the viscosity can be suppressed by means of greater flow of ink in comparison with the first embodiment. Furthermore, it is also possible to eliminate the space required for installing flow channels for circulating the ink.
Furthermore, if the circulation volume is large, then there is a possibility that some degree of cross-talk (variation in the ejected ink volume) will occur between two opposing pressure chambers 58A and 58B (two pressure chambers 58A and 58B that are mutually connected through one of the individual circulation channels 72), and in this case, it is desirable that the dot arrangement should be selected in such a manner that simultaneous ejection from the two opposing pressure chambers 58A and 58B is avoided as far as possible. The effects of cross-talk are especially visible in low-density to medium-density regions, and therefore simultaneous ejection should be avoided particularly in such regions. Avoiding simultaneous ejection means either ejecting from one pressure chamber only, or performing ink supply to one chamber while ejecting from the other. Alternatively in the step of determining the dot arrangement, a dot arrangement which takes account of this variation in the ink volume may be determined. More specifically, the amount of variation in the ejected ink volume due to cross-talk can be predicted at the design stage, depending on whether both of the two opposing pressure chambers 58A and 58B perform ejection virtually simultaneously, or whether only one chamber performs ejection. The dot arrangement can therefore be determined in accordance with the variation in the ejected ink volume that has been predicted.
Density variations caused by variations in the ink volume are not readily visible in dark solid regions, and therefore the dot arrangements for dark colored lines or text characters (and especially, long lines which extend in the direction of alignment of the two pressure chambers) can be determined without paying particular attention to cross-talk.
Moreover, the present embodiment has been described with reference to an example where two separate common flow channels 52A and 52B are provided, but it is also possible to provide three or more separate common flow channels, provided that a composition is adopted in which a pressure differential is generated between the pressure chambers 58A and 58B which are connected via an individual circulation channel 72.
Next, a fourth embodiment of the present invention will be described. Below, portions which are common with the first to third embodiments described above are not explained further, and the following description centers on characteristic features of the fourth embodiment.
In the present embodiment, taking the resistance of the individual supply channel 60 to be Rs, taking the inertance thereof to be Ls, taking the resistance of the nozzle flow channel 62 to be Rn, taking the inertance thereof to be Ln, taking the resistance of the individual circulation channel 72 to be Rr and taking the inertance thereof to be Lr, then it is preferable that the following conditions are satisfied:
Rs+Rr≈Rn;
Ls+Lr≈Ln;
Rs<<Rr (Rs is approximately 1/10 through 1/100 of Rr) ; and
Ls<<Lr (Ls is approximately 1/10 through 1/100 of Lr).
According to the fourth embodiment, since ink is supplied from the common flow channel 52 to the vicinity of the nozzles, then increase in the viscosity of the ink in the vicinity of the nozzles is prevented, and furthermore faster ink refilling can be achieved.
An opening section 62a which serves as a connection with the individual supply channel 60 is formed in the nozzle flow channel 62, and hence there is a possibility that the flow speed distribution during ink ejection becomes asymmetrical (i.e., the flow speed distribution is distorted), particularly in cases where this opening section 62a is formed in the vicinity of the nozzle. In other words, the flow speed distribution in the nozzle flow channel 62 is symmetrical as shown in
In order to resolve problems of this kind, compositional examples such as those shown in
The compositions shown in
Next, a fifth embodiment of the present invention will be described. Below, portions which are common with the first to fourth embodiments described above are not explained further, and the following description centers on characteristic features of the fifth embodiment.
In the fifth embodiment, ink circulation control of the following kind is implemented in the recording head 50A similar to the first embodiment.
When not ejecting (when the ink ejection volume X=0) (see
−(P2−P3)/(Rs+Rr)=I,
where I is the minimum ink ejection volume required to prevent ejection defects and corresponds to the ink ejection volume X1 shown in
On the other hand, during ejection (when the average ejection rate Ia>0), the following relationship is satisfied:
P3=(Ia/X4)(P2−P3n)+P3n,
where P3n is the value of P3 during non-ejection.
By setting the pressures P1, P2 and P3 in this way, in
According to the present embodiment, the ink supply volume is made greater than the ink ejection volume, and furthermore, a differential between the ink supply volume and the ink ejection volume is reduced (i.e., the ink supply volume approaches the ink ejection volume) as the ink ejection volume increases. The ink circulation volume may be controlled so as to be inversely proportional to the ink ejection volume. In this case, since the ink circulation can be controlled so as to be inversely proportional to the number of printed dots, for example, then the control procedure can be simplified.
Although the liquid circulation apparatus, the image forming apparatus and the liquid circulation method according to the present invention have been described in detail above, the present invention is not limited to the aforementioned embodiments, and it is of course possible for improvements or modifications of various kinds to be implemented, within a range which does not deviate from the essence of the present invention.
It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-095507 | Mar 2007 | JP | national |
