Patent Application
20230415489
The present invention relates to a liquid droplet ejection device and a liquid feeding method.
A conventional liquid droplet ejection device ejects a liquid such as ink from nozzles provided on a liquid droplet ejection head and causes the liquid to land at desired positions on a recording medium to form an image or the like. The liquid droplet ejection head of the liquid droplet ejection device has channels (pressure chambers) connected to the nozzles and ejects liquid droplets from the nozzles by varying the pressure of the liquid in the channels.
If gas bubbles are present in the channels, pressure is not applied normally to the liquid in the channels, resulting in poor ejection of liquid from the nozzles and degraded image quality.
To address this issue, there is a technology for suppressing the occurrence of defects caused by gas bubbles by providing a degassing device to remove gas bubbles and dissolved gases in the liquid and supplying the degassed liquid to the channels (for example, Patent Literature 1).
There is also a technology in which a degassing channel branches off partway along a channel for supplying liquid to the nozzles, and gas bubbles in the liquid are discharged from the degassing channel to the outside of the liquid droplet ejection head (for example, Patent Literature 2).
In recent years, the nozzles in liquid droplet ejection heads are increasing in number and are being driven at faster speeds in response to the desire for more high-definition images and higher productivity, and the volume of liquid droplets ejected from the nozzles has increased accordingly. In a liquid droplet ejection head with a high liquid droplet ejection volume, high channel resistance in the channel of the liquid causes large pressure variations during ejection, making stable ejection impossible. Therefore, a design that lowers the channel resistance, that is, the pressure loss in the liquid droplet ejection head, is desired.
However, in a configuration provided with a degassing device as in Patent Literature 1, a large pressure loss occurs because of the high channel resistance in the degassing device.
Also, in a configuration provided with a degassing channel as in Patent Literature 2, the pressure loss in the liquid droplet ejection head increases because of the need to increase the inflow volume of liquid into the liquid droplet ejection head to generate a flow that discharges gas bubbles in the degassing channel.
In this way, there is a problem with the above technologies of the related art in that it is difficult to effectively suppress the occurrence of defects caused by gas bubbles while also lessening an increase in pressure loss.
An objective of the invention is to provide a liquid droplet ejection device and a liquid feeding method that can effectively suppress the occurrence of defects caused by gas bubbles while also lessening an increase in pressure loss.
To achieve the above objective, the invention of a liquid droplet ejection device as in claim 1 is provided with:
According to the invention as in claim 2, in the liquid droplet ejection device as in claim 1,
According to the invention as in claim 3, in the liquid droplet ejection device as in claim 1 or 2,
According to the invention as in claim 4, in the liquid droplet ejection device as in any one of claims 1 to 3,
According to the invention as in claim 5, in the liquid droplet ejection device as in any one of claims 1 to 4,
Also, to achieve the above objective, the invention of a liquid feeding method as in claim 6 is
According to the present invention, the occurrence of defects caused by gas bubbles can be suppressed effectively while also lessening an increase in pressure loss.
Hereinafter, embodiments related to the liquid droplet ejection device and the liquid feeding method of the present invention will be described on the basis of the drawings.
<Configuration of Liquid Droplet Ejection Device>
The liquid droplet ejection device 1 is provided with, among other things, a conveyor 2 and a head unit 3. The liquid droplet ejection device 1 of the present embodiment is an inkjet recording device that ejects droplets of ink as a liquid onto a recording medium M to form an image.
The conveyor 2 is provided with two conveyor rollers 2a, 2b that rotate about an axis of rotation extending in the Y direction of
The recording medium M may be a sheet of paper cut to a certain size. The recording medium M is supplied onto the conveyor belt 2c by a paper feeding device not illustrated, and after ink is ejected from the head unit 3 and an image is recorded, the recording medium M is delivered from the conveyor belt 2c into a predetermined delivery receptacle. Note that roll paper may also be used as the recording medium M. In addition, besides paper such as plain paper and coated paper, various types of media to which ink landing on the surface thereof can be fixed, such as woven fabric or sheet resin, can also be used as the recording medium M.
The head unit 3 records an image onto the recording medium M conveyed by the conveyor 2 by ejecting ink at appropriate timings on the basis of image data. In the liquid droplet ejection device 1 of the present embodiment, four head units 3 corresponding to each of the four ink colors yellow (Y), magenta (M), cyan (C), and black (K) are arranged at predetermined intervals in the order of the colors Y, M, C, K from the upstream side of the conveying direction of the recording medium M. Note that the number of head units 3 may also be three or less, or five or more.
In the liquid droplet ejection head 100, a plurality of nozzles N are arranged at equal intervals from each other in a direction intersecting the conveying direction of the recording medium M (in the present embodiment, the lateral direction orthogonal to the conveying direction, or in other words, the Y direction). In the present embodiment, each liquid droplet ejection head 100 has four lines (nozzle lines) of nozzles N arranged one-dimensionally at equal intervals in the Y direction. These four nozzle lines are staggered from each other in the Y direction so that the positions of the nozzles N in the Y direction do not overlap. Note that the number of nozzle lines included in the liquid droplet ejection head 100 is not limited to four, and may also be three or less, or five or more.
The eight liquid droplet ejection heads 100 in the head unit 3 are arranged in a staggered grid so that the arrangement range of the nozzles N is continuous in the Y direction. The arrangement range, in the Y direction, of the nozzles N included in the head unit 3 covers the width, in the Y direction, of the available image recording area of the recording medium M conveyed by the conveyor belt 2c. The head unit 3 records an image with a single-pass system, in which the head unit 3 is used by being fixed in place while the image is being recorded, and ink is ejected from the nozzles N at each position at predetermined intervals (conveying direction intervals) in the conveying direction according to the conveyance of the recording medium M.
In the present embodiment, a water-based ink is used as the ink to be ejected from the liquid droplet ejection head 100. A water-based ink, for example, contains water as a dispersant and a pigment or dye as a colorant, and may also contain any of various types of water-soluble organic solvents, hydrophobic polymers, and the like.
Note that the ink to be ejected from the liquid droplet ejection head 100 is not limited to a water-based ink, and a solvent ink that uses an organic solvent as a dispersant, a UV-curing ink that is cured by UV irradiation, or the like may also be used.
The liquid droplet ejection head 100 is provided with, among other things, an ejection actuator 10, a liquid storage tank 20, and a cover member 30.
The ejection actuator 10 includes the nozzles N, and the bottom side (−Z direction side) is the nozzle aperture surface 100a where the apertures of the nozzles N are arranged. The ejection actuator 10 causes a liquid, in this case ink, supplied from the liquid storage tank 20 to be ejected from the nozzles N. Also, the ejection actuator 10 can cause the ink not ejected from the nozzles N from among the supplied ink to be discharged into the liquid storage tank 20. Furthermore, the ejection actuator 10 is provided, among other things, ink channels 151 (see
The cover member 30 fits with the ejection actuator 10 and internally houses circuitry and the like for supplying a drive signal to the pressure varying means of the ejection actuator 10.
The liquid storage tank 20 is attached at a position covering a part of the outside of the cover member 30 on the opposite side (+Z direction side) from the nozzle aperture surface 100a side (ejection surface side) of the ejection actuator 10. The liquid storage tank 20 includes, among other things, a supply port 21 (inlet) for ink supplied from an external ink tank or the like, a main body 20a provided with a liquid container 23 (see
Inside the liquid storage tank 20, an ink channel which leads from the supply port 21 through the liquid container 23 to the outflow port 25 and which allows the passage of ink to be supplied to the ejection actuator 10 and an ink channel which leads from the inflow port 26 to the discharge port 28 and which allows the passage of ink to be discharged from the ejection actuator 10 are each provided. The outflow port 25 is connected to the ink inflow port 11 of the ejection actuator 10, and the inflow port 26 is connected to the ink outflow port 17 of the ejection actuator 10. This arrangement forms a continuous ink channel (liquid channel) from the supply port 21 to the discharge port 28 of the liquid storage tank 20 in the liquid droplet ejection head 100. The outflow port 25 and the inflow port 26 are provided on legs protruding out from the main body 20a. The liquid storage tank 20 is removably secured by screwing these legs to the ejection actuator 10 with screws S.
The liquid storage tank 20 has a shape that is long in the Y direction and thin in the X direction when viewed from the top side (the side looking down on the supply port 21 and the discharge port 28), or in other words, a plan view (as seen from the +Z direction) in this case. The supply port 21 and the discharge port 28 are located separately near both ends of the liquid storage tank 20 in the longitudinal direction (Y direction). Similarly, the outflow port 25 and the inflow port 26 are also located separately near both ends of the liquid storage tank 20 in the longitudinal direction (Y direction).
Hereinafter, the upper part or upper end means the highest position in the +Z direction (position with the largest Z coordinate). Also, the lower part or lower end means the lowest position in the +Z direction (position with the smallest Z coordinate).
The liquid container 23 provided in the main body 20a of the liquid storage tank 20 and storing ink is divided into a front chamber 23a and a rear chamber 23b by an internally provided filter 231 (
For the filter 231, it is possible to use, for example, a structure (hereinafter referred to as a “through-hole filter”) in which a tabular member of resin, metal, or the like is provided with many fine through-holes that allow the passage of ink, or a structure (hereinafter referred to as a “porous plate filter”) internally having fine three-dimensional channels through which liquid can pass. Examples of porous plate filters include three-dimensionally woven fibers of metal or the like, and a porous member produced by sintering resin particles such as polyethylene resin.
In the present embodiment, a filter having a mesh diameter smaller than the aperture diameter of the nozzle N (the diameter of the circle formed by the aperture of the nozzle N) is used as the filter 231.
In the case in which the filter 231 is a through-hole filter, the mesh diameter of the filter 231 is the diameter of the through-holes.
In the case in which the filter 231 is a porous plate filter, the mesh diameter of the filter 231 is the particle size indicated as the absolute filtration rating of the filter 231 (or if not indicated, the particle size corresponding to the absolute filtration rating). Here, the absolute filtration rating is the minimum value of X that satisfies the condition that the filter 231 is capable of capturing at least 99.9% of particles of particle size X.
The supply port 21 and the front chamber 23a are connected by a first supply channel 22 (
The open end 232 and the open end 233 are provided at diagonally opposite positions in the liquid container 23 (
Here, the supply port 21 and the outflow port 25 are located on the same side in the Y direction while the inflow port 26 and the discharge port 28 (discharge channel 27) are located on the opposite side (in this case, the +Y side), and the open end 233 is located on the side opposite the supply port 21 and the outflow port 25 in the Y direction (
The inflow port 26 and the discharge port 28 are located on the same side in the Y direction while the supply port 21 and the outflow port 25 are located on the opposite side (in this case, the −Y side), and the inflow port 26 and the discharge port 28 are connected by the discharge channel 27 extending in the Z direction (
Multiple, in this case two, inflow ports 26 are provided to match the multiple ink outflow ports 17a, 17b (
Between the discharge port 28 and the confluence of the discharge channels 27a, 27b in the discharge channel 27, a check valve 271 (
The front chamber 23a and the discharge channel 27 are connected by a communicating channel 29 (
The ejection actuator 10 is provided with an ink manifold 16 that includes the ink inflow port 11 and the ink outflow port 17 (17a, 17b), and a head chip 15 affixed to the lower surface (the surface on the −Z direction side) of the ink manifold 16 (
The ink manifold 16 is provided with the common ink chamber 12 that communicates with the ink inflow port 11 and the ink outflow port 17a (
The ink manifold 16 is also provided with a second common discharge channel 18 isolated from the common ink chamber 12 (
The head chip 15 is provided with the nozzles N, the ink channels 151 that communicate with the nozzles N, and individual discharge channels 152 that branch off from the ink channels 151 (
The head chip 15 has a configuration in which a nozzle plate 15a, a channel substrate 15b, and a pressure chamber substrate 15c are layered in the Z direction.
The nozzle plate 15a is a tabular member provided with through-holes to serve as the nozzles N. The nozzle N in the present embodiment has a straight part Ns and a tapered part Nt. The straight part Ns is a portion with a cylindrical, which is to say straight, shape provided in a predetermined range in the Z direction from the aperture (ejection port) of the nozzle N. The tapered part Nt is connected to the end of the straight part Ns on the +Z direction side, and the cross-sectional area perpendicular to the ink ejection direction (Z direction) decreases closer to the aperture of the nozzle N (that is, closer to the straight part Ns).
A meniscus m (liquid surface) of ink inside the nozzle N (in this case, the straight part Ns) is slightly drawn inward into the nozzle N, or in other words, raised upward in
As the pressure inside the nozzle N is lowered, the meniscus m ruptures at a certain pressure and gas bubbles are mixed inside the nozzle N. The meniscus pressure of the nozzle when the meniscus m ruptures is referred to as the second meniscus break pressure (meniscus break pressure of the nozzle N). Provided that P2 [Pa] is the second meniscus break pressure, dn [m] is the aperture diameter of the nozzle N, and σ [N/m] is the surface tension of the ink, the relation P2=4σ/dn holds.
The ink channels 151 and the individual discharge channels 152 are formed in the channel substrate 15b and the pressure chamber substrate 15c.
One ink channel 151 is provided with respect to one nozzle N. The ink channel 151 penetrates through the channel substrate 15b and the pressure chamber substrate 15c in the Z direction, with the upper end communicating with the lower surface of the common ink chamber 12 and the lower end communicating with one nozzle N. Ink supplied to the common ink chamber 12 is supplied to the nozzle N through this ink channel 151.
The material of the pressure chamber substrate 15c forming a part of the wall surface of the ink channel 151 is a ceramic piezoelectric body (a member that deforms in response to the application of a voltage). Examples of such a piezoelectric body include lead zirconate titanate (PZT), lithium niobate, barium titanate, lead titanate, and lead metaniobate. Also, the inner wall surface of the pressure chamber substrate 15c is provided with driving electrodes not illustrated. In response to the application of a drive signal from the circuitry described above to the drive electrode, the side walls dividing adjacent ink channels 151 undergo shear-mode displacement, which causes the pressure of the ink inside the ink channel 151 to vary. In response to this variation in pressure, the ink inside the ink channel 151 is ejected from the nozzle N. In this way, the liquid droplet ejection head 100 of the present embodiment performs shear-mode ink ejection. The side wall and driving electrodes of the ink channel 151 form the pressure varying means described above.
Note that an air chamber lacking an ink inflow channel may also be provided instead of the ink channel 151 at the formation location of every other ink channel 151 in the Y direction in
The individual discharge channel 152 has a horizontal part 152a that branches off from the end on the nozzle N side of the ink channel 151 and extends in the −X direction, and a vertical part 152b that bends in the +Z direction from the end of the horizontal part 152a and communicates with the second common discharge channel 18. One individual discharge channel 152 is provided with respect to one ink channel 151. The horizontal part 152a of the individual discharge channel 152 is a trench provided in the surface on the −Z direction side of the tabular channel substrate 15b, and the vertical part 152b is a through-hole provided in the channel substrate 15b and the pressure chamber substrate 15c.
Note that the horizontal part 152a of the individual discharge channel 152 is not limited to being a trench provided in the channel substrate 15b and may also penetrate through the channel substrate 15b and be a trench provided in the nozzle plate 15a. Also, the connection location of the individual discharge channel 152 in the ink channel 151 is not limited to the end on the nozzle N side, and the individual discharge channel 152 can also be made to branch off from any location in the ink channel 151.
The individual discharge channel 152 guides, to the second common discharge channel 18, ink not discharged from the nozzle N among the ink supplied to the ink channel 151. This flow of ink causes tiny gas bubbles 62 and foreign matter inside the ink channel 151 to also be discharged to the second common discharge channel 18. The ink discharged to the second common discharge channel 18 passes through the ink outflow port 17b, the inflow port 26b, and the discharge channel 27, and is discharged from the discharge port 28.
Of the configuration of the liquid droplet ejection head 100 described above, the supply port 21, first supply channel 22, liquid container 23, second supply channel 24, common supply channel 13, common ink chamber 12, and ink channel 151 form a supply channel 101 (see
Also, the first common discharge channel 14, individual discharge channel 152, second common discharge channel 18, discharge channel 27 (27a, 27b), and discharge port 28 form a discharge channel 102 (see
A flow of ink leading from the supply port 21 of the liquid droplet ejection head 100, through the supply channel 101 and the discharge channel 102, to the discharge port 28 can be generated by an ink circulation mechanism 9 included in the liquid droplet ejection device 1.
The ink circulation mechanism 9 is provided with, among other things, a supply sub-tank 91, a reflux sub-tank 92, a main tank 93, ink channels 94 to 97, and pumps 98 and 99.
The supply sub-tank 91 stores ink to be supplied to the liquid droplet ejection head 100.
The supply sub-tank 91 is connected to the supply port 21 by the ink channel 94.
The reflux sub-tank 92 is connected to the discharge port 28 by the ink channel 95 and stores ink discharged from the discharge port 28.
The supply sub-tank 91 and the reflux sub-tank 92 are connected by the ink channel 96. Ink can then be returned from the reflux sub-tank 92 to the supply sub-tank 91 by the pump 98 provided in the ink channel 96.
The main tank 93 stores ink to be supplied to the supply sub-tank 91. The main tank 93 is connected to the supply sub-tank 91 by the ink channel 97. Also, ink is supplied from the main tank 93 to the supply sub-tank 91 by the pump 99 provided in the ink channel 97.
The supply sub-tank 91 is provided at a position such that the liquid surface therein is higher than the nozzle aperture surface 100a of the ejection actuator 10 by a height H1. Also, the reflux sub-tank 92 is provided at a position such that the liquid surface therein is lower than the nozzle aperture surface 100a by a height H2. With this arrangement, when the pressure inside the nozzle N (≈atmospheric pressure) is taken to be a reference pressure, the pressure at the supply port 21 is a positive pressure Pin relative to the reference pressure due to the hydraulic head differential, and the pressure at the discharge port 28 is a negative pressure Pout relative to the reference pressure due to the hydraulic head differential. This pressure difference between the pressure Pin and the pressure Pout generates a flow of ink from the supply port 21, through the supply channel 101 and the discharge channel 102, to the discharge port 28. By changing the position of the liquid surface in each sub-tank, the pressure Pin and the pressure Pout can be adjusted, and therefore the ink flow rate can be adjusted.
The ink circulation mechanism 9 corresponds to a “liquid feeder”. Also, the operation by the ink circulation mechanism 9 for circulating ink through the supply channel 101 and the discharge channel 102 corresponds to a “liquid feeding operation”. Here, the liquid feeding operation includes the ink pumping operations by the pumps 98 and 99.
The liquid droplet ejection device 1 is provided with, among other things, the head unit 3 described above, a controller 40, a conveyor driver 51, and a communicator 52, these being interconnected by a bus 53. Of these, the head unit 3 includes ahead driver 200 and the liquid droplet ejection head 100. Also, the controller 40 includes a central processing unit (CPU) 41, random access memory (RAM) 42, read-only memory (ROM) 43, and storage 44.
The CPU 41 reads out programs and settings data for various types of control stored in the ROM 43, stores the programs and data in the RAM 42, and executes the programs to perform various types of computational processing. Additionally, the CPU 41 centrally controls the operations of the liquid droplet ejection device 1 as a whole.
The RAM 42 provides a memory workspace to the CPU 41 and stores temporary data. The RAM 42 may also include non-volatile memory.
The ROM 43 stores, among other things, programs and settings data for various types of control to be executed by the CPU 41. Note that rewritable non-volatile memory such as electrically erasable programmable read-only memory (EEPROM) or flash memory may also be used in place of the ROM 43.
In the storage 44, a print job and image data related to the print job which are inputted from an external device via the communicator 52 are stored. A hard disk drive (HDD) or the like is used as the storage 44, for example.
The head driver 200 provides various controls signals and image data to the circuitry of the liquid droplet ejection head 100 at appropriate timings, on the basis of a control signal from the controller 40.
The ink circulation mechanism 9 performs the liquid feeding operation described above by operating the pumps 98 and 99 on the basis of a control signal from the controller 40.
The conveyor driver 51 supplies, on the basis of a control signal supplied from the CPU 41, a drive signal to the motor that drives the conveyor rollers 2a, 2b of the conveyor 2, thereby rotating the conveyor rollers 2a, 2b at a prescribed speed and timings to move the conveyor belt 2c in a loop.
The communicator 52 is a communication interface that controls communication operations with external equipment. The communication interface includes one or more LAN boards or LAN cards, for example, that support various communication protocols. The communicator 52 acquires image data to be recorded and settings data (job data) related to image recording from an external device on the basis of control by the controller 40 and transmits status information and the like to external equipment.
<Operations of Liquid Droplet Ejection Device>
Next, the operations of the liquid droplet ejection device 1 will be explained, with focus on operations related to ink circulation.
As illustrated in
Of these, the flow rate of ink that flows through the first common discharge channel 14, the second common discharge channel 18, and the communicating channel 29, into the discharge channel 27 and is discharged from the discharge port 28 is hereinafter referred to as the circulation flow rate. The circulation flow rate is constant, regardless of the ejection status (ejection volume) of ink from the nozzles N.
The ejection volume of ink from the nozzles N increases or decreases depending on the content of the image to be formed. According to ink ejection from the nozzles N, ink equal to the volume ejected is supplied to the common ink chamber 12. Therefore, the greater the ejection volume of ink per unit time from the nozzles N is, the higher the flow rate of ink in the supply channel 101, or in other words the ink passing through the filter 231 becomes. Hereinafter, the flow rate of ink in the supply channel 101 that corresponds to the maximum ejection volume of ink per unit time from the nozzles N is referred to as the maximum ejection flow rate. Consequently, the maximum flow rate of ink passing through the supply channel 101 is the sum of the maximum ejection flow rate and the circulation flow rate.
Generally, the relation Q=ΔP/R holds, where Q [m3/s] is the flow rate of ink flowing through the channel, ΔP [Pa] is the pressure difference (differential pressure) between the ends of the channel, and R [Pa·s/m3] is the channel resistance. Also, if the channel is a circular duct and the flow of ink is laminar, the following Hagen-Poiseuille equation holds.
R=(128·μ·L)/(π·d4)
where μ [Pa·s] is the viscosity of the ink, L [m] is the length of the channel, and d [m] is the diameter of the channel.
Gas bubbles 61 larger than the mesh diameter of the filter 231 are trapped by the filter 231 and remain in the front chamber 23a. These gas bubbles 61 may include gas bubbles that flow in from the supply port 21 in addition to gas bubbles created by dissolved gases in the ink due to pressure and temperature changes or the like. The gas bubbles 61 are guided with the ink through the communicating channel 29 to the discharge channel 27 and are discharged from the discharge port 28.
If the pressure loss in the filter 231, or in other words the pressure difference between the front chamber 23a and the rear chamber 23b, is equal to or greater than a certain pressure, the liquid surface (meniscus) of the gas bubbles 61 will rupture and split into smaller gas bubbles. The pressure difference at this time is referred to as the first meniscus break pressure (meniscus break pressure of the filter).
In the case in which the filter 231 is a through-hole filter, the first meniscus break pressure P1 [Pa] can be obtained by the relational expression P1=4σ/dt, where dt [m] is the aperture diameter of the through-holes of the filter 231 and σ [N/m] is the surface tension of the ink.
Also, in the case in which the filter 231 is a porous plate filter, the first meniscus break pressure P1 [Pa] can be obtained by the relational expression P1=4σ/da, where da is the value of the absolute filtration rating. Alternatively, the first meniscus break pressure can be obtained experimentally as follows. To be specific, when the rear chamber 23b of the filter 231 is filled with a liquid, the front chamber 23a is filled with air, and the front chamber 23a is pressurized, the pressure in the front chamber 23a at which the filter 231 breaks the meniscus can be measured to determine the first meniscus break pressure. In this case, the filter 231 breaking the meniscus refers to when gas bubbles begin to permeate (pass through) into the rear chamber 23b.
In the phenomenon whereby the meniscus of the gas bubbles 61 trapped in the filter 231 break (rupture) and gas bubbles pass through the filter 231, the gas bubbles after passing through the filter 231 may be larger than the mesh diameter of the filter 231 in some cases. This is because in this phenomenon, gas bubbles larger than the mesh diameter may deform to pass through the mesh of the filter 231, or a plurality of gas bubbles may unite after passing through the filter 231. If gas bubbles larger than the mesh diameter after passing through the filter 231 enter the ink channel 151 from the rear chamber 23b, there is a possibility that poor ejection of ink may occur.
Accordingly, the ink circulation mechanism 9 that serves as the liquid feeder of the present embodiment performs the liquid feeding operation in such a way as to meet a condition under which the gas bubbles 61 do not rupture in the filter 231. In other words, the ink circulation mechanism 9 performs the liquid feeding operation in such a way as to meet the condition (hereinafter referred to as a first condition) stipulating that “the pressure loss in the filter 231 is smaller than the first meniscus break pressure at which the meniscus of the liquid ruptures in the filter 231”. The pressure Pin at the supply port 21 and the pressure Pout at the discharge port 28 are adjusted to meet this first condition. With this arrangement, the gas bubbles 61 stay in the front chamber 23a without rupturing and are discharged from the communicating channel 29.
Note that the first condition may also be met by adjusting the circulation flow rate of ink instead of the adjustment of the pressures Pin and Pout (or in addition to this adjustment). The circulation flow rate of ink can be adjusted by the shape of the channel, the area of the filter 231, and the like.
Also, if the range of increase or decrease of the pressure loss in the filter 231 is equal to or greater than the second meniscus break pressure, a pressure variation corresponding to the range of increase or decrease will occur in the meniscus m of the nozzle N, causing the meniscus m to break and gas bubbles to flow into the nozzle N. Here, the range of increase or decrease of the pressure loss that can occur in the filter 231 corresponds to the difference between the pressure loss when the ink ejection volume per unit time from the nozzle N is 0 and the pressure loss when the ink ejection volume per unit time is a maximum. Therefore, the range of increase or decrease of the pressure loss in the filter 231 is equal to the pressure loss that occurs in the filter 231 due to the ink at the maximum ejection flow rate described above.
Accordingly, the liquid droplet ejection device 1 of the present embodiment is configured to meet a condition (hereinafter referred to as a second condition) stipulating that “the pressure loss that occurs in the filter 231 due to the ink at the maximum ejection flow rate is smaller than the second meniscus break pressure”. That is, the area and mesh diameter of the filter 231 are determined so that the second condition is met. In addition, the ink circulation mechanism 9 performs the liquid feeding operation in such a way as to meet the second condition.
Incidentally, some of the gas bubbles present in the front chamber 23a are smaller than the mesh diameter of the filter 231 to begin with. The gas bubbles 62 of such size can pass through the filter 231 and may flow into the rear chamber 23b, as illustrated in
As illustrated in
Additionally, the portion of the second supply channel 24 illustrated in
Note that the descending portion is not limited to extending in the vertical direction and includes any portion where the liquid feeding direction has a vertically downward component.
As illustrated in
In general, gas bubbles in the ink channel 151 give rise to poor ejection because the gas bubbles absorb the pressure wave that the pressure varying means has generated inside the ink channel 151. Here, the smaller the aperture diameter of the nozzle N is, the greater the energy required for ink ejection is, and thus small gas bubbles can easily lead to poor ejection. More specifically, simulation results show that poor ejection occurs when the size of gas bubbles is equal to or greater than the aperture diameter of the nozzle N, and that gas bubbles smaller than the aperture diameter of the nozzle N are unlikely to lead to poor ejection.
In this way, in the liquid droplet ejection head 100 of the present embodiment, even if the gas bubbles 62 smaller than the aperture diameter of the nozzle N flow into the ink channel 151, poor ejection caused by these gas bubbles 62 is unlikely to occur. By utilizing this result and increasing the mesh diameter of the filter 231 somewhat, pressure loss in the liquid droplet ejection head 100 as a whole can be reduced while also suppressing the occurrence of poor ejection. From this standpoint, the mesh diameter of the filter 231 is, for example, preferably set to be equal to or greater than ⅓ the aperture diameter of the nozzle N, more preferably set to be equal to or greater than ½ the aperture diameter of the nozzle N.
<Effects>
As above, the liquid droplet ejection device 1 according to the present embodiment is provided with the liquid droplet ejection head 100 and the ink circulation mechanism 9 that serves as a liquid feeder. The liquid droplet ejection head 100 includes the nozzles N that eject ink, the supply channel 101 through which ink to be supplied to the nozzles N runs, the discharge channel 102 which communicates with the supply channel 101 and through which ink to be discharged without being ejected from the nozzles N runs, the filter 231 which is provided in the supply channel 101 and through which ink running through the supply channel 101 passes, and the communicating channel 29 that branches off from the supply channel 101 on the upstream side of the filter 231 in the liquid feeding direction of the ink and communicates with the discharge channel 102. The mesh diameter of the filter 231 is smaller than the aperture diameter of the nozzles N, and the ink circulation mechanism 9 performs the liquid feeding operation in such a way that the pressure loss in the filter 231 is smaller than the first meniscus break pressure at which the meniscus of the ink ruptures in the filter 231.
With this arrangement, gas bubbles of a size at least equal to or greater than the aperture diameter of the nozzles N can be captured by the filter 231, and these gas bubbles can be discharged to the outside via the communicating channel 29 and the discharge channel 102.
Moreover, since the pressure loss in the filter 231 can be made smaller than the first meniscus break pressure, the gas bubbles captured in the filter 231 do not rupture easily. Therefore, the captured gas bubbles can be discharged efficiently from the communicating channel 29, and it is possible to suppress the occurrence of poor ejection of ink due to the gas bubbles rupturing and flowing into the ink channel 151.
Also, by allowing some gas bubbles smaller than the aperture diameter of the nozzles N (the gas bubbles 62 smaller than the mesh diameter of the filter 231) to pass through the filter 231, it is possible to lower the channel resistance of the supply channel 101. With this arrangement, the pressure loss in the liquid droplet ejection head 100 as a whole, that is, the difference between the pressure of the ink at the supply port 21 and the pressure of the ink at the nozzles N, can be kept small, and large gas bubbles that would lead to poor ejection (gas bubbles larger than the aperture diameter of the nozzles N) can be discharged to the outside and kept from flowing into the ink channel 151.
Also, provided that the maximum ejection flow rate is the flow rate of ink in the supply channel 101 that corresponds to the maximum ejection volume of ink per unit time from the nozzles N, the pressure loss that occurs in the filter 231 due to the ink at the maximum ejection flow rate is smaller than the second meniscus break pressure at which the meniscus of the ink ruptures in the nozzles N.
With this arrangement, even if the ink ejection volume of ink from the nozzles N varies, rupturing of the meniscus m in the nozzles N due to such variation can be suppressed. Therefore, it is possible to suppress the occurrence of poor ejection caused by gas bubbles flowing in from nozzles N.
In addition, the supply channel 101 includes the ink channel 151 as a descending portion where the liquid feeding direction has a vertically downward component, and the ink circulation mechanism 9 performs the liquid feeding operation in such a way that the vertically downward component of the velocity of the ink in the ink channel 151 is greater than the velocity at which the gas bubbles 62 smaller than the aperture diameter of the nozzles N float upward by buoyant force.
With this arrangement, the gas bubbles inside the ink channel 151 can be made to flow downward against the buoyant force and be discharged from the individual discharge channels 152.
Also, the nozzle N has a tapered part Nt in which the cross-sectional area perpendicular to the ink ejection direction decreases closer to the aperture of the nozzle N.
By including the tapered part Nt in the nozzle N, the energy required to eject ink can be reduced. Therefore, it is possible to lower the likelihood of poor ejection caused by the inflow of gas bubbles into the ink channel 151.
Also, in the present embodiment, a water-based ink is used. In water-based inks, dissolved gases tend to bubble out at higher pressures compared to solvent inks and the like, and gas bubbles are easily generated by cavitation (the state of negative pressure inside the ink channel 151 after ink ejection). Accordingly, in the case of applying a water-based ink to the liquid droplet ejection device 1 of the present embodiment, the occurrence of defects caused by gas bubbles can be suppressed effectively.
Also, the liquid feeding direction according to the present embodiment includes a liquid feeding step that causes ink in the supply channel 101 and the discharge channel 102 to flow in a liquid feeding direction, wherein in the liquid feeding step, the ink is made to flow in such a way that the pressure loss in the filter 231 is smaller than the first meniscus break pressure at which the meniscus of the ink ruptures in the filter 231.
With this arrangement, the pressure loss in the liquid droplet ejection head 100 as a whole can be kept small, and large gas bubbles that would lead to poor ejection can be discharged to the outside and kept from flowing into the ink channel 151.
Next, experiments that were performed to confirm the effects of the above embodiment will be described.
A total of 11 experiments from Experiment 1 to Experiment 11 were conducted.
In each of the experiments, at least one from among the mesh diameter, aperture ratio, and area of the filter 231, the aperture diameter of the nozzles N, and the maximum ejection flow rate were different from each other.
Of these, the mesh diameter, aperture ratio, and area of the filter 231 were changed to adjust the level of channel resistance, pressure loss (a1), pressure loss (a2), and first meniscus break pressure (b) (denoted “MB pressure” in the table) of the filter. Here, the pressure loss (a1) is the pressure loss that occurs due to ink at the maximum flow rate obtained by combining the circulation flow rate and the maximum ejection flow rate, of which the pressure loss (a2) is the pressure loss that occurs due to ink at the maximum ejection flow rate. The pressure loss (a1) and the pressure loss (a2) were calculated from the calculated value of the channel resistance. Also, a porous plate filter was used as the filter 231, and the first meniscus break pressure (b) was obtained by calculation.
Also, the aperture diameter of the nozzles N was changed at the two levels of 40 [μm] and 20 [μm] to adjust the level of the second meniscus break pressure (c).
Also, the ink circulation flow rate was set to the two levels of 90 [ml/min] and 20 [ml/min], and the maximum ejection flow rate was set to the two levels of 80 and 60 [ml/min].
Through combinations of the above, the maximum flow rate combining the circulation flow rate and the maximum ejection flow rate was set to the three levels of 170 [ml/min], 150 [ml/min], and 80 [ml/min].
Note that the parameters common to Experiments 1 to 11 are as follows.
In each of the experiments, continuous ejection from the nozzles N was performed for 10 minutes at the maximum ejection volume, and the presence or absence of defects due to non-ejection of ink from the nozzles N was determined. In the “Continuous ejection evaluation result” column in
Also, in each of the experiments, intermittent ejection was performed for 10 minutes by switching the ejection volume from the nozzles N between minimum (OFF) and maximum (ON) at 0.5 second intervals, and the presence or absence of defects due to non-ejection of ink from the nozzles N was determined. In the “Intermittent ejection evaluation result” column in
The results of the experiments show that a “circle” continuous ejection evaluation result was obtained in Experiments 2-4, 6, 7, and 9-11 that meet the condition (corresponding to the first condition described above) that “the pressure loss (a1) of the filter 231 is smaller than the first meniscus break pressure (b)”. Also, a “cross mark” continuous ejection evaluation result was obtained in Experiments 1, 5, and 8 that do not meet the first condition.
Also, a “circle” intermittent ejection evaluation result was obtained in Experiments 2-4, 6, 7, 10, and 11 that meet the condition (corresponding to the second condition described above) that “the pressure loss (a2) that occurs in the filter 231 due to the ink at the maximum ejection flow rate is smaller than the second meniscus break pressure (c)”. Also, a “cross mark” intermittent ejection evaluation result was obtained in Experiments 1, 5, 8, and 9 that do not meet the second condition.
<Modification>
Next, a modification of the liquid droplet ejection device 1 will be described.
In the liquid droplet ejection head 100 of the modification, the common ink chamber 12 has an upper layer 12a and a lower layer 12b located on the −Z direction side of the upper layer 12a. Also, the upper layer 12a and the lower layer 12b are partitioned by a filter 231 parallel to the XY plane. In this way, the filter 231 may be provided outside the liquid storage tank 20.
The upper layer 12a leads to the first common discharge channel 14, inflow port 26a, and discharge channel 27a described above. Also, the lower layer 12b leads to an inflow port 26c provided separately from the inflow ports 26a and 26b, and also leads to a discharge channel 27c provided separately from the discharge channels 27a and 27b. The discharge channel 27c converges with the discharge channels 27a, 27b and also communicates with the discharge port 28.
Ink running from the ink inflow port 11 to the common supply channel 13 first flows into the upper layer 12a of the common ink chamber 12. Some of the ink in the upper layer 12a flows together with gas bubbles and foreign matter through the first common discharge channel 14, the inflow port 26a, and the discharge channel 27a, and is discharged from the discharge port 28. In this modification, the portion from the first common discharge channel 14 to the discharge channel 27a corresponds to a “communicating channel” and functions as a degassing channel.
Also, some of the ink in the upper layer 12a passes through the filter 231 and flows into the lower layer 12b. Some of the ink in the lower layer 12b flows into the ink channel 151, of which a portion is ejected from the nozzles N while the remainder flows through the individual discharge channels 152, the second common discharge channel 18, the inflow port 26b, and the discharge channel 27b, and is discharged from the discharge port 28. Also, the portion of the ink in the lower layer 12b that did not flow into the ink channel 151 flows through the inflow port 26c and the discharge channel 27c and is discharged from the discharge port 28.
According to the configuration of this modification, too, the pressure loss in the liquid droplet ejection head 100 as a whole can be kept small, and large gas bubbles that would lead to poor ejection can be discharged to the outside and kept from flowing into the ink channel 151.
<Other>
Note that the present invention is not limited to the above embodiment and modification and may be subject to various changes.
For example, the liquid droplet ejection head 100 may eject a liquid other than ink, such as a functional liquid for forming circuit patterns and the like on a recording medium, for example.
Also, a communicating channel that branches off from the rear chamber 23b and communicates with the discharge channel 27 may be further provided, in addition to the communicating channel 29 that branches off from the front chamber 23a.
Also, a shear-mode liquid droplet ejection head 100 is illustrated as an example, but the configuration is not limited thereto. For example, a vent-mode liquid droplet ejection head 100 may also be used, in which ink is ejected by varying the pressure of the ink inside a pressure chamber connected to a nozzle by deforming a piezoelectric element (pressure varying means) affixed to the wall of the pressure chamber. In this case, individual discharge channels can be branched off from any location in the range from the pressure chamber to the nozzle.
Also, a channel including the individual discharge channels 152 branching off from the ink channel 151 and the second common discharge channel 18 that communicates with the individual discharge channels 152 is illustrated as an example of the discharge channel 102, but the configuration is not limited thereto, and the individual discharge channels 152 and second common discharge channel 18 may also be omitted.
Moreover, although a single-pass liquid droplet ejection device 1 is described as an example, the present invention may also be applied to a liquid droplet ejection device that records an image while scanning the head unit or the liquid droplet ejection head.
Moreover, the description uses the example of conveying the recording medium M with the conveyor belt 2c but is not intended to be limited thereto, and for example, the recording medium M may also be held and conveyed on the outer circumferential surface of a rotating conveyor drum.
Several embodiments of the present invention have been described, but the scope of the present invention is not limited to the embodiments described above and includes the scope of the invention as described in the claims and equivalents thereof.
The present invention can be used with a liquid droplet ejection device and a liquid feeding method.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/042799 | 11/17/2020 | WO |
