JP4474988B2 - Driving method of droplet discharge head - Google Patents

Description

  The present invention relates to a method for driving a droplet discharge head, and more particularly, a liquid capable of suppressing the bending of a tail of a droplet discharged from a nozzle and improving the landing accuracy of the droplet without reducing the driving frequency. The present invention relates to a driving method of a droplet discharge head.

  As an example of a droplet discharge head that discharges liquid as droplets from a nozzle, an inkjet recording head for recording an image by discharging minute ink droplets onto a recording medium is well known.

  Such an ink jet recording head applies pressure in a channel filled with ink to cause the ink column to protrude from the nozzle, and then pulls back the trailing end of the protruding ink column by pulling the meniscus. Separate the ink column from the meniscus. Thereby, as shown in FIG. 11, the ink droplet 200 flies from the nozzle 400 toward the recording medium 300.

  As shown in the figure, the ink droplet 200 immediately after being ejected from the nozzle 400 includes a substantially spherical main droplet 201 and a tail portion 202 having a long tail from the rear end of the main droplet 201. However, after that, the tail 202 is divided into minute secondary droplets 203 called satellites, and the main droplet 201 and the secondary droplet 203 fly toward the recording medium 300 and land on them, thereby recording an image. Is done. At this time, if the flight direction of the main droplet 201 and the flight direction of the secondary droplet 203 are the same, they land on the same place and do not affect the image. However, if the flight direction of the secondary droplet 203 is different from that of the main droplet 201, it will land on the periphery of the main droplet 201 as shown in FIG.

  Thus, the reason why the flying direction of the secondary droplet 203 differs from the flying direction of the main droplet 201 is that the original flying direction indicated by the arrow in FIG. 11 is applied to the tail portion 202 of the ink droplet 200 immediately after being ejected from the nozzle 400. This is because bending in different directions occurs.

  It is known that the cause of the bending of the tail portion of the ink droplet ejected from the nozzle is the non-uniformity of the inner surface of the nozzle. For example, as shown in FIG. 12, if the inclination of the nozzle inner surface 401 is not uniform and the inclination is partially different, the surface tension of the meniscus M on the nozzle inner surface 401 is unbalanced, and the ink protrudes from the nozzle 400. A force that causes the tail portion 501 of the column 500 to be offset in one direction perpendicular to the original flight direction acts, causing the tail portion to bend immediately after the ink droplet is separated from the meniscus M. Therefore, the accuracy of the shape of the inner surface of the nozzle greatly affects the stable ejection in which the bending of the tail of the ink droplet is suppressed.

  However, the level of accuracy required for the inner surface shape of the nozzle is very high, and it is extremely difficult to process and form a perfect circle and right / left symmetrically to the inner surface of the nozzle, and it is not easy to meet the requirement. In addition, if a foreign substance adheres to the inner surface of the nozzle during use, it is difficult to remove, and this may cause the tail to bend. For this reason, it is required to stably discharge the droplets without bending the tail by a method other than the accuracy of the nozzle inner surface shape.

In order to improve such bending of the tail of the ink droplet, it has been studied in the past. In Patent Document 1, the first pulse is applied to cause the ink column to protrude from the nozzle, and then the ink column is separated from the meniscus. Then, a technique is disclosed in which the second pulse is applied before being ejected as an ink droplet to cause the meniscus to protrude from the nozzle and be separated as an ink droplet at the apex of the convex meniscus, thereby preventing the tail from bending. Yes.
JP-A-2-215537

  According to the technique described in Patent Document 1, since the droplet is separated after the meniscus protrudes from the nozzle, it is not affected by the shape of the inner surface of the nozzle, but in addition to the first pulse for protruding the ink column, the meniscus is further increased. Since it is necessary to reapply the second pulse for projecting the image, the drive frequency is lowered accordingly, and there is a problem that the recording speed is lowered. In addition, since the next discharge cannot be started without canceling the residual pressure wave due to the second pulse, it is necessary to separately apply a new pulse for canceling the residual pressure wave, and further decrease the drive frequency. There is a problem that invites.

  Therefore, an object of the present invention is to provide a driving method of a droplet discharge head that can stably discharge a droplet without a tail bend without reducing the driving frequency.

  Other problems of the present invention will become apparent from the following description.

  The above problems are solved by the following inventions.

  The invention according to claim 1 has a plurality of channels separated by a side wall formed at least partly of a piezoelectric material, and among the plurality of channels, the channels separated from each other by sandwiching two channels are provided. As a group, the channel is divided into three groups, and a voltage pulse for ejecting droplets is applied to the electrodes formed on the side walls, so that each group is driven in time division sequentially. A method for driving a droplet discharge head, wherein the side wall is subjected to shear deformation, and the liquid in the channel is discharged as a droplet from the nozzle by the pressure at the time of shear deformation, and the application of a voltage pulse for discharging the droplet The driving by the first step of expanding the volume of the channel, the second step of maintaining the expanded state, and reducing the volume of the channel to cause the liquid column to protrude from the nozzle. A third step, a fourth step for maintaining a state in which the volume of the channel is reduced, and a fifth step for expanding the volume of the channel; and driving the first step immediately before Starts before the liquid column protruding from the nozzle of the adjacent channel is separated from the meniscus and discharged as a droplet, and when the meniscus formed after the liquid column protrudes substantially returns to the tip of the nozzle opening. The liquid column is separated from the meniscus and ejected as droplets.

  According to a second aspect of the present invention, the volume of the channel when reduced by the third step is smaller than the volume before the channel is enlarged by the first step, and is enlarged by the fifth step. 2. The method of driving a droplet discharge head according to claim 1, wherein the volume of the channel after being made is the same as the volume before the channel is expanded by the first step.

  According to a third aspect of the present invention, a voltage applied to the electrode formed on the side wall in the first step is a (V), and a voltage applied to the electrode formed on the side wall in the fifth step is b (V). 3) The method for driving a droplet discharge head according to claim 1, wherein | a |> | b |.

  According to a fourth aspect of the invention, there is provided the droplet discharge head driving method according to the third aspect, wherein | a | / | b | = 2.

  The invention according to claim 5 has a plurality of channels separated by a side wall formed at least in part by a piezoelectric material, and among the plurality of channels, the channels separated from each other by sandwiching two channels are provided. As a group, the channel is divided into three groups, and a voltage pulse for ejecting droplets is applied to the electrodes formed on the side walls, so that each group is driven in time division sequentially. Then, the side wall is subjected to shear deformation, and ink in the channel is ejected as droplets from the nozzle by the pressure at the time of shear deformation, and the electrode formed on the side wall of the non-recording channel in the one set of channels is applied to the electrode. A method of driving a droplet discharge head by applying a voltage pulse that does not discharge a droplet to slightly vibrate the meniscus of the nozzle without discharging the droplet. Driving by applying a voltage pulse of a degree includes a first step of expanding the volume of the channel, a second step of maintaining the expanded state, and a third step of reducing the volume of the channel. The first step is started before the liquid column protruding from the nozzle of the adjacent channel driven immediately before is separated from the meniscus and discharged as a droplet, and the meniscus formed after the liquid column protrudes Is a method of driving a droplet discharge head, wherein the liquid column is separated from the meniscus and discharged as droplets when it substantially returns to the tip of the nozzle opening.

  The invention described in claim 6 is characterized in that the volume before expanding the volume of the channel in the first step is the same as the volume after reducing the volume of the channel in the third step. 6. A method for driving a droplet discharge head according to claim 5.

  According to a seventh aspect of the present invention, in the liquid droplet ejection head according to the fifth or sixth aspect, the duration of the second step is 2AL (AL is 1/2 of the acoustic resonance period of the channel). This is a driving method.

  According to an eighth aspect of the present invention, the voltage pulse that does not discharge the droplet is applied to all the electrodes formed on the side walls of the one set of channels regardless of whether the droplet is discharged or not. The method of driving a droplet discharge head according to claim 5, 6, or 7.

  According to a ninth aspect of the present invention, the maximum meniscus push-out amount by applying a voltage pulse that does not eject the droplet is a nozzle radius or less. This is a method for driving a droplet discharge head.

  A tenth aspect of the present invention is the method of driving a droplet discharge head according to any one of the first to ninth aspects, wherein the voltage pulse is a rectangular wave pulse.

  An eleventh aspect of the invention is the method for driving a droplet discharge head according to any one of the first to tenth aspects, wherein the liquid has a viscosity of 5 cp to 15 cp.

  The invention according to claim 12 is the method of driving a droplet discharge head according to any one of claims 1 to 11, wherein the surface tension of the liquid is 20 dyne / cm or more and 30 dyne / cm or less. .

  ADVANTAGE OF THE INVENTION According to this invention, the drive method of the droplet discharge head which can discharge stably the droplet without a bending of a tail part without reducing a drive frequency can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  The driving method according to the present invention has a plurality of channels separated by a side wall formed at least partly of a piezoelectric material, and among the plurality of channels, channels that are separated from each other by sandwiching two channels. As a group, the channel is divided into three groups, and voltage pulses are applied to the electrodes formed on the side walls to drive each group sequentially in a time-sharing manner, thereby shearing the side walls. Any type of liquid droplet discharge head can be used as long as the liquid in the channel is discharged from the nozzle as liquid droplets by the pressure during shear deformation. The liquid droplets in the inside may be any liquid, but in the following description, a shear mode (shear mode) is a liquid droplet ejection head that uses ink for image recording as the liquid in the channel. Amodo) type ink jet recording head (hereinafter, simply described with reference to that.) Recording head.

  1A and 1B are diagrams showing a schematic configuration of a shear mode type recording head, in which FIG. 1A is a perspective view showing one surface in section, and FIG. 1B is a sectional view. FIG. 2 is a diagram showing the operation.

  The recording head 2 has a large number of channels 28 arranged in parallel between a cover plate 24 and a substrate 26 separated by a plurality of side walls 27A, 27B, 27C made of a piezoelectric material such as PZT. In FIG. 2, three (28A, 28B, 28C) which are a part of many channels 28 are shown. One end of the channel 28 is connected to the nozzle 23 formed in the nozzle forming member 22, and the other end is connected to an ink tank (not shown) by the ink tube 21 through the ink supply port 25. Electrodes 29A, 29B, 29C connected from the upper side of the side walls 27 to the bottom surface of the channel 28 are formed in close contact with the surface of the side wall 27 in each channel 28, and each electrode 29A, 29B, 29C is a drive signal generator. 100 is connected.

  The drive signal generation unit 100 generates a drive signal generation circuit that generates a series of drive signals including a plurality of voltage pulses for each pixel period, and a drive signal supplied from the drive signal generation circuit for each channel. A driving pulse selection circuit that selects a voltage pulse according to the data of each pixel and supplies it to each channel, and supplies a voltage pulse for driving the side wall 27 made of a piezoelectric material according to the data of each pixel. This voltage pulse includes an ejection pulse for ejecting ink droplets and a micro-vibration pulse that slightly vibrates the meniscus to the extent that ink droplets are not ejected from the nozzles 23.

  Here, each side wall 27 is constituted by two piezoelectric materials 27a and 27b having different polarization directions as indicated by arrows in FIG. 2, but the piezoelectric material may be only a portion 27a, for example. 27 may be present in at least a part of 27.

  When a voltage pulse as an ejection pulse is applied to the electrodes 29A, 29B and 29C formed in close contact with the surface of each side wall 27 by the control of the drive signal generator 100, an ink droplet is ejected from the nozzle 23 by the operation exemplified below. . In FIG. 2, the nozzle is omitted.

  First, when no voltage pulse is applied to any of the electrodes 29A, 29B, and 29C, none of the side walls 27A, 27B, and 27C is deformed, but the electrodes 29A and 29C are grounded in the state shown in FIG. In addition, when a voltage pulse as shown in FIG. 3 is applied to the electrode 29B, an electric field in a direction perpendicular to the polarization direction of the piezoelectric material constituting the side walls 27B and 27C is generated, and the piezoelectric materials 27a and 27b are applied to the side walls 27B and 27C, respectively. As shown in FIG. 2B, the side walls 27B and 27C are deformed toward each other, and the volume of the channel 28B is expanded. This creates a negative pressure in the channel 28B, causing ink to flow (Draw).

  When this state is continued for a certain time and the potential of the voltage pulse is returned to 0, the side walls 27B and 27C return from the enlarged position shown in FIG. 2B to the neutral position shown in FIG. High pressure is applied to the ink (Release). Subsequently, as shown in FIG. 2C, a voltage pulse is applied so as to deform the side walls 27B and 27C in opposite directions to reduce the volume of the channel 28B (Reinforce). Pressure is generated. As a result, the meniscus in the nozzle due to a part of the ink filling the channel 28B changes in the direction pushed out from the nozzle, and the ink column is ejected from the nozzle. The other channels operate in the same manner as described above by applying voltage pulses.

  Such a driving method is called a DRR driving method and is a typical driving method for a shear mode type recording head.

  Thus, when driving the recording head 2 having a plurality of channels 28 separated by the side walls 27 at least partially formed of the piezoelectric material, when the side walls 27 of one channel 28 are deformed, the adjacent channel 28 is changed. Therefore, among the plurality of channels 28, the channels 28 that are separated from each other by sandwiching the two channels 28 are combined into one set, and the channels 28 are divided into three sets. The drive control is sequentially performed in time division for each group (hereinafter, this may be referred to as three-cycle drive). As described above, in the case where the drive control is sequentially performed for each group in a time-division manner, a driving method in which the voltage pulse is applied to the next adjacent group after the voltage pulse applied to one group is completed. Adopted.

  The driving method of the present invention for performing such three-cycle driving will be further described with reference to FIG. In the example shown in FIG. 4, the recording head is described as having nine channels 28 of A1, B1, C1, A2, B2, C2, A3, B3, and C3. In addition, FIG. 5 shows an example of a timing chart of voltage pulses applied to the respective channels 28 of A, B, and C at this time. FIG. 6 schematically shows the behavior of the ink and the meniscus in the nozzles 23 of the A sets of channels 28 shown in FIG. The numbers (1) to (5) in parentheses in FIGS. 5 and 6 and the following description correspond to each other.

  In this specification, “liquid column” or “ink column” is a state in which the front end protrudes from the opening of the nozzle, but the rear end is connected to the meniscus in the nozzle and is not yet separated from the meniscus. The term “droplet” or “ink droplet” refers to a liquid or ink that has a trailing edge that is completely separated from the meniscus in the nozzle.

  In the shear mode type recording head, the deformation of the side wall 27 occurs due to a voltage difference applied to the electrodes 29 provided on both sides of the wall. Therefore, instead of applying a negative voltage pulse to the electrode 29 of the channel 28 for ink ejection, The same operation can be performed by grounding the electrode 29 of the channel 28 for discharging ink and applying a positive voltage pulse to the electrode 29 of the adjacent channel 28. According to this method, it can be driven only by a positive voltage, but in this specification, a case of driving by both positive and negative voltages is shown.

  Here, the voltage pulse for ejecting the ink droplet applied to each set of channels 28, that is, the ejection pulse is an ejection pulse by the DRR driving method, and this is a first step of expanding the volume of the channel 28. A second step of maintaining the enlarged state, a third step of reducing the volume of the channel 28 to project the ink column from the nozzle 23, and a fourth step of maintaining the state of reducing the volume of the channel 28. And a fifth step of expanding the volume of the channel 28.

  (1); First, an ejection pulse is applied to the electrodes of each channel of the A group (A1, A2, A3), and the electrodes of the adjacent channels are grounded. That is, when a positive voltage pulse (+ Von) is applied to the A set of channels 28, the side walls 27 of the A set of channels 28 to be discharged are deformed outward, and the volume of the channels 28 is expanded (first step of the A set). ). As a result, a negative pressure is generated in the channel 28, and ink flows from the ink tank into the A set of channels 28 due to the negative pressure.

  The pressure in the channel 28 repeats inversion every 1 AL when the driving waveform does not change. Therefore, the expanded state of the volume of the channel 28 due to the application of the positive voltage pulse is maintained for 1 AL (second set of A set). Then, the drawn meniscus M moves toward the opening tip of the nozzle 23, and the ink pressure is reversed to a positive pressure.

  In addition, AL (Acoustic Length) is 1/2 of the acoustic resonance period of a channel. This AL measured the speed of an ink droplet ejected by applying a rectangular wave voltage pulse to the side wall 27 formed of a piezoelectric material, and changed the pulse width of the rectangular wave while keeping the rectangular wave voltage value constant. Sometimes it is determined as the pulse width that maximizes the flying speed of the ink droplets. A pulse is a rectangular wave having a constant voltage peak value. When 0V is 0% and a peak voltage is 100%, the pulse width is 10% of the voltage from 0V and the peak voltage. It is defined as the time between 10% of the falling edge. Furthermore, the rectangular wave here refers to a waveform in which both the rise time and fall time between 10% and 90% of the voltage are within 1/2, preferably within 1/4 of AL.

  (2): After maintaining the enlarged state for 1 AL as described above, when the electrode is grounded at this timing, the deformation of the side wall 27 is restored and the volume of the A group of channels 28 is reduced, and the high pressure is applied to the A group. The ink pillar 10 protrudes from the nozzle 23 due to the ink in the channel 28. Further, when a negative voltage pulse (−Voff) is applied to the electrode 29 of each channel 28 of the A group at the same timing, the side wall 27 is deformed inward as shown in FIG. The ink column 10 is further reduced and a higher pressure is applied to the ink, and the ink column 10 is further pushed out from the nozzle 23 (set A of the third step).

  (3); This reduced state is maintained for 2AL (set A, fourth step). After the first 1AL, the pressure is reversed and the pressure in the channel 28 becomes negative, and when 1AL further passes, the pressure in the channel 28 is reversed and becomes a positive pressure. At this time, the meniscus M in the nozzle 23 is moved to the nozzle 23. Move toward the tip of the opening.

  (4); Next, when the electrode is grounded, the deformation of the side wall 27 is restored and the volume of the channel 28 is expanded again (Fifth set of steps A). At this time, the residual pressure wave in each of the channels A is canceled. At this point, the ink column 10 has not yet been separated from the meniscus M, and its tail 10 b is connected to the meniscus M.

  In this way, after the ejection pulses including the first to fifth steps are applied to each channel 28 of the A group, the B groups (B1, B2,. Similarly, the ejection pulses including the first to fifth steps are applied to the respective channels 28 of B3).

  As shown in FIG. 5, the ejection pulse is applied to each channel 28 of the B set in this case after the ejection pulse is applied to each channel 28 of the A set, and then protrudes from the nozzle 23 of each channel 28 of the A set. In this example, the ink column 10 is performed at the timing when the first step of the ejection pulse is started after 4AL, where the ink column 10 is not yet separated from the meniscus M.

  (5): After driving each channel 28 of the A set, the ink column 10 protruding from the nozzle 23 of each channel 28 of the A set is adjacent when it has not yet separated from the ink meniscus M in the nozzle 23. A voltage pulse is applied to the electrode 29 of each channel 28 of the B group, and the electrodes 29 of both adjacent channels are grounded. That is, when a positive voltage pulse (+ Von) is applied to the B sets of channels 28, the side walls 27 of the B sets of channels 28 to be discharged are deformed outward, and the volume of the channels 28 is expanded (first step of the B sets). ).

  At this time, the side wall 27 on one side of each channel 28 in the A group adjacent to each channel 28 in the B group is deformed so as to reduce the volume of each channel 28 in the A group. The meniscus M in the nozzle 23 is applied with pressure in the direction in which it is pushed out in the ejection direction of the nozzle 23 with the tail portion 10b of the ink column 10 connected, and the opening tip of the nozzle 23 (hereinafter referred to as the opening tip of the nozzle 23). May be referred to as the “return position” of the meniscus M.) Then, when the meniscus M formed after the ink column 10 protrudes substantially returns to the return position, the ink column 10 is separated from the meniscus M in the nozzle 23, and from the nozzle 23 of each channel 28 of the A set. It is ejected as ink droplets.

  Thus, the timing of the first step of the ejection pulse starts before the ink column 10 protruding from the nozzle 23 of the adjacent channel 28 driven immediately before is separated from the meniscus M and ejected as an ink droplet. When the meniscus M formed after the ink column 10 protrudes substantially returns to the return position, the ink column 10 is separated from the meniscus M in the nozzle 23, and the nozzle 23 of each channel 28 of the A set Therefore, when the tail portion 10b of the ink column 10 is separated from the meniscus M, it is not affected by the inner surface shape of the nozzle 23. Since there is no possibility of bending in different directions, the effect of suppressing tail bending can be increased, and the drive frequency and landing accuracy can be improved. Rukoto can.

  Here, when the meniscus M substantially returns to the return position, the position of the meniscus M after the ink column 10 protrudes is substantially located at the return position or protrudes from the opening tip of the nozzle 23. State. The term “substantially located” means that the distance d between the meniscus M and the return position is 1/2 or less, preferably 1/4 or less of the nozzle radius, as shown in FIG. Rather than projecting from the front end of the opening of the nozzle 23, it is preferable that it is substantially located at the return position in that the drive frequency can be further increased. The opening shape of the nozzle 23 is not limited to a perfect circle, but may be various such as an ellipse. In the present invention, the nozzle radius is ½ of the longest diameter at the opening end surface on the discharge side of the nozzle 23. It is.

  Thereafter, when the application of the ejection pulse to each channel 28 of the B set is completed, the C set (C1, C2, C3) adjacent to the B set is also similar to the above as shown in FIG. Further, the A group adjacent to the C group is driven in the same manner as described above. That is, the first step of the C set is started before the ink column 10 protruding from the nozzle 23 of each channel 28 of the adjacent B set driven immediately before is separated from the meniscus M, and the ink column 10 protrudes. When the meniscus M formed thereafter is substantially returned to the return position, the ink column 10 is separated from the meniscus M and ejected as droplets. Further, the first step of the A set is started before the ink column 10 protruding from the nozzle 23 of each channel 28 of the adjacent B set driven immediately before is separated from the meniscus M, and the ink column 10 protrudes. When the meniscus M formed thereafter is substantially returned to the return position, the ink column 10 is separated from the meniscus M and ejected as droplets. Therefore, the ink and meniscus in the nozzles 23 of the channels 28 of the B group and the C group also exhibit the same behavior as described above, and the tail portion 10b is not affected by the inner surface shape of the nozzles 23.

  In general, since the residual pressure wave in the channel 28 is canceled by the fifth step of expanding the volume of the channel 28 performed subsequent to the fourth step of maintaining the state of reducing the volume of the channel 28, in general, Although the return of the meniscus M in the nozzle 23 to the front end of the opening is delayed, as described above, before the ink column 10 protruding from the nozzle 23 is separated from the meniscus M, the volume of the adjacent channel 28 to be driven next is increased. By enlarging the first step of the ejection pulse, the meniscus M quickly moves to the return position, and when the meniscus M formed after the ink column 10 protrudes, the ink is substantially returned to the return position. Since the column 10 is separated from the meniscus M and ejected from the nozzle 23 as ink droplets, the bending of the tail is reduced without reducing the drive frequency. It is possible to win to become.

  Incidentally, as shown in the present embodiment, when the driving by the ejection pulse includes the first step to the fifth step, the volume of the channel 28 is reduced, and the reduction is performed by the third step of projecting the ink column 10 from the nozzle 23. The volume of the channel 28 when the channel 28 is expanded is smaller than the volume before the channel 28 is expanded by the first step, and the volume of the channel 28 after the expansion by the fifth step is determined by the first step. If the volume is the same as before the channel 28 is expanded, the ejection pulse drive waveform can be made simple as shown in FIG. In addition, since the discharge pulse has the side wall 27 returned to the initial state in the final fifth step in the series of steps, the residual pressure wave generated in the third step can be effectively canceled in the fifth step. It is more preferable in that high frequency driving is performed.

  In the first step, the voltage applied to the electrode 29 formed on the side wall 27 is a (V), and the voltage applied to the electrode 29 formed on the side wall 27 in the fifth step is b (V). In this case, as shown in FIG. 5, if | a |> | b | is satisfied, the meniscus M recovers more quickly, which is more preferable for high-frequency driving. The voltage a and the voltage b are differential voltages.

  When the voltage a and voltage b are | a | / | b | = 2, the residual pressure wave generated in the third step can be canceled more effectively in the fifth step. It is more preferable because both can be achieved.

  Next, in the case of the same three-cycle driving, the meniscus M is slightly vibrated to such an extent that no ink droplets are ejected to the electrode 29 formed on the side wall 27 of the non-ejection channel 28 in one set of channels 28. A driving method for applying a voltage pulse, that is, a fine vibration pulse for causing the meniscus M of the nozzle 23 to vibrate without discharging an ink droplet will be described.

  FIG. 7 is a timing chart of voltage pulses (ejection pulses and micro-vibration pulses) showing an example of such a driving method. FIG. 8 schematically shows the state of ink and meniscus in the nozzles 23 of the A channels 28 of FIG. The numbers (1) to (8) in parentheses in FIGS. 7 and 8 and the following description correspond to each other.

  Here, the voltage pulse as a fine vibration pulse for finely vibrating the meniscus M of the nozzle 23 without ejecting ink droplets applied to each set of channels 28 is a first step of expanding the volume of the channel 28. And a second step of maintaining the expanded state, and a third step of reducing the volume of the channel 28.

  (1); First, a micro-vibration pulse is applied to the electrode 29 of each channel 28 of the A group (A1, A2, A3), and the electrodes 29 of the adjacent channels 28 are grounded. That is, when a positive voltage pulse (+ Von) is applied to the A set of channels 28, the side walls 27 of the A set of channels 28 are deformed to the outside and the volume of the channels 28 is expanded (first step of the A set). As a result, a negative pressure is generated in the channel 28, and the meniscus M in the nozzle 23 of each channel 28 of the A group is drawn into the nozzle 23 by this negative pressure.

  (2); The pressure in the channel 28 is inverted every 1 AL when the driving waveform does not change. Therefore, the expanded state of the volume of the channel 28 by the application of the positive voltage pulse is maintained for 2 AL (second set of A set). Then, the drawn meniscus M is once pushed out of the opening of the nozzle 23 and then moves again into the nozzle 23.

  (3); Next, when the A set of channels 28 are grounded, the deformation of the side walls 27 of the A set of channels 28 is restored to reduce the volume of the channels 28 (the third step of the A set). As a result, a positive pressure is generated in the channel 28, and the meniscus M in the nozzle 23 of each channel 28 of the A set returns to the return position due to the positive pressure. By applying a series of fine vibration pulses from the first process to the third process, the meniscus M in the nozzle 23 is given a fine vibration that does not eject ink droplets.

  Here, after the application of the micro-vibration pulse, the ejection pulse is applied to the A channels 28 based on the image data. The ejection pulse here is the same as the ejection pulse illustrated in FIG.

  (4): After 1 AL from the end of the application of the micro-vibration pulse, when a positive voltage pulse (+ Von) as a discharge pulse is applied to the A set of channels 28, the side wall 27 of the A set of channels 28 to be discharged is deformed outward. Thus, the volume of the channel 28 is expanded.

  (5): After maintaining the above expanded state for 1 AL, when the electrode 29 is grounded at this timing, the deformation of the side wall 27 is restored, the volume of each channel 28 of the A group is reduced, and a high pressure is applied to the A group. The ink pillar 10 protrudes from the nozzle 23 due to the ink in the channel 28. Further, when a negative voltage pulse (−Voff) is applied to the electrode 29 of each channel 28 of the A set at the same timing, the side wall 27 is deformed inward, the volume of the channel 28 is further reduced, and a higher pressure is applied to the ink. The ink column 10 is further pushed out from the nozzle 23.

  (6); If this contracted state is maintained for 2AL, after the first 1AL, the pressure is reversed and the pressure in the channel 28 becomes negative, and when 1AL further passes, the pressure in the channel 28 is reversed and becomes positive. At this time, the meniscus M in the nozzle 23 moves toward the return position.

  (7); Next, when the electrode 29 of each channel 28 of the A set is grounded, the deformation of the side wall 27 is restored and the volume of the A set of channels 28 is expanded again. At this time, the residual pressure wave is canceled. At this point, the ink column 10 has not yet been separated from the meniscus M, and its tail 10 b is connected to the meniscus M.

  After applying the micro-vibration pulse and the ejection pulse to each of the channels A in the above-described group A, the micro-vibration pulse is continuously applied to each channel 28 in the group B (B1, B2, B3) adjacent to each channel 28 in the group A. And an ejection pulse is applied. However, the ejection pulse is applied based on the image data, and FIG. 7 shows a case where the ejection pulse is not applied to each channel 28 of the B set.

  As shown in FIG. 7, the application of the micro-vibration pulse to each channel 28 of the B set is performed here from the nozzle 23 of each channel 28 of the A set after the application of the ejection pulse to each channel 28 of the A set is completed. An example is shown in which the protruding ink column 10 is performed at the timing at which the first step of the micro-vibration pulse is started after 4AL, which has not yet separated from the meniscus M.

  (8): After the driving of the channels A of the group A, when the ink column 10 protruding from the nozzles 23 of the channels 28 of the group A has not yet separated from the ink meniscus M in the nozzle 23, the group B When a positive voltage pulse (+ Von) is applied to the channels 28, the side walls 27 of the B sets of channels 28 are deformed outward, and the volume of the channels 28 is expanded (first step of the B sets).

  At this time, the side wall 27 on one side of each channel 28 in the A group adjacent to each channel 28 in the B group is deformed so as to reduce the volume of each channel 28 in the A group. The meniscus M in the nozzle 23 is applied with pressure in the direction in which it is pushed out in the ejection direction of the nozzle 23 with the tail 10b of the ink column 10 connected, and moves quickly to the return position. Then, when the meniscus M formed after the ink column 10 protrudes substantially returns to the return position, the ink column 10 is separated from the meniscus M in the nozzle 23, and from the nozzle 23 of each channel 28 of the A set. It is ejected as ink droplets.

  Thereafter, the C set (C1, C2, C3) adjacent to the B set and the A set adjacent to the C set are also driven in the same manner as described above. However, in this example, the B group is not ejected, so the ink column is not ejected and is omitted. The first step of the micro-vibration pulse applied to each channel A of the set A is performed before the ink column 10 protruding from the nozzle 23 of each adjacent channel C 28 driven immediately before is separated from the meniscus M. The ink column 10 is separated from the meniscus M and ejected as droplets when the meniscus M formed after the ink column 10 protrudes substantially returns to the return position. The ink column 10 protruding from the nozzle 23 of each channel 28 of the C set also exhibits the same behavior as described above, and the tail portion 10b is not affected by the inner shape of the nozzle 23.

  In this way, by taking the execution timing of the first step of the fine vibration pulse, the volume of the adjacent channel 28 to be driven next is expanded before the ink column 10 protruding from the nozzle 23 is separated from the meniscus M. As a result, the meniscus M quickly returns to the return position, and when the meniscus M formed after the ink column 10 protrudes substantially returns to the return position, the ink column 10 is separated from the meniscus M and the ink. Since the droplets are ejected from the nozzle 23, the tail portion 10b of the ink column 10 is not easily affected by the inner surface shape of the nozzle 23, and the possibility that the tail portion of the ink droplet bends in a direction different from the flight direction due to the inner surface shape is reduced. 3-cycle drive does not require useless operation, and therefore driving without lowering the tail frequency is possible without lowering the drive frequency. To become. In addition, the use of the micro-vibration pulse can prevent the tail from being bent even in a non-ejection channel, so that the above effect can be obtained regardless of the image data, and the meniscus M is subject to micro-vibration. Therefore, clogging of the nozzle 23 can be suppressed.

  In such a micro-vibration pulse, the volume of the channel 28 before being expanded in the first step is the same as the volume after the channel 28 is reduced in the third step, as shown in FIG. The fine vibration pulse is preferable because it can have a simple waveform. Further, this fine vibration pulse is preferable in that high-frequency driving is performed because the side wall 27 returns to the initial state in the final third step in the series of fine vibration steps.

  As shown in FIG. 7, the micro-vibration pulse has a duration of the second step of maintaining the expanded state of the volume of the channel 28 as 2AL. Thereby, the residual pressure wave generated in the first step can be easily canceled in the third step, which is preferable in terms of high frequency driving.

  In the example shown in FIG. 7, the fine vibration pulse is applied regardless of ejection or non-ejection, and in this way, even in the channel 28 that performs recording by ejecting ink droplets, Ink droplets can be reliably ejected after a slight vibration is applied to the meniscus M, and even if the adjacent channel is not ejected, as described above, the adjacent channel immediately before it is formed by the first step of the fine vibration pulse. Since the meniscus M can be moved to the vicinity of the return position at an early stage, bending of the tail portion of the ink droplet can be prevented. The fine vibration pulse may be applied only to the non-ejection channel 28. In this case, power consumption can be suppressed and heat generation of the recording head can be suppressed.

  As described above, it is preferable that the maximum push-out amount of the meniscus M when the meniscus M is vibrated slightly is equal to or less than the radius of the nozzle 23. If the meniscus push-out amount due to slight vibrations is large, the meniscus M cannot return to the return position by the immediately following ink discharge timing, and stable discharge becomes difficult. In this case, the meniscus M push-out amount is set to the nozzle 23. By suppressing the radius to less than this radius, stable ejection can be performed even immediately after the meniscus M is slightly vibrated.

  The maximum extrusion amount is the maximum value of the meniscus M extrusion amount from the opening tip of the nozzle 23 in one extrusion operation. The extrusion amount of the meniscus M from the nozzle 23 can be measured by stroboscopic synchronization using, for example, a digital microscope “VH-6300” manufactured by KEYENCE. As shown in FIG. 9, the amount of extrusion is a value obtained by measuring the amount of protrusion of the meniscus M from the tip of the opening of the nozzle 23 at a substantially central portion in a direction substantially perpendicular to the nozzle forming member 22.

  In the present invention, the ejection pulse applied to the electrode 29 of each channel 28 and the voltage pulse as the fine vibration pulse are preferably rectangular wave pulses as shown. In the rectangular wave pulse, the deformation of the side wall 27 is more extreme than in the case of a trapezoidal wave in which the rising and falling of the voltage are inclined, and a strong reduction pressure is generated in the adjacent channel 28 by the pulse that enlarges the volume of the channel 28. Thus, the meniscus can be substantially returned to the tip of the nozzle opening more reliably. Further, since the rectangular wave can be easily generated by using a simple digital circuit, there is an advantage that the circuit configuration can be simplified as compared with the trapezoidal wave.

  The drive method of the present invention exhibits a remarkable effect of suppressing tail bending when the viscosity of the ink is 5 cp or more and 15 cp or less. Such an ink has a high viscosity, and the fluctuation due to the residual pressure wave in the returning process of the meniscus M is small. Further, the ink column 10 is difficult to be separated from the meniscus M, and the tail portion 10b extends long. This is because bending tends to occur.

  In addition, the driving method of the present invention exhibits a remarkable effect even when the surface tension of the ink is 20 dyne / cm or more and 30 dyne / cm or less. This is because such an ink has a low surface tension, so that the ink droplets are not easily separated from the meniscus M, and the tail portion 10b is also elongated so that the tail portion 10b is easily bent.

  The application timings of the ejection pulse and the micro-vibration pulse shown in FIG. 5 and FIG. 7 are merely examples, and the ease of ink droplet separation and the speed at which the meniscus returns is different for each print head and ink. In the invention, an experiment of observing the actual meniscus movement of the recording head with a microscope or the like, or a simulation by a finite element method or the like, to investigate the movement form of the meniscus at the nozzle opening tip which is the return position of the meniscus. It is preferable to adjust the timing of signal application by electric circuit means.

The effects of the present invention are illustrated below by examples.
(Example)
As a droplet discharge head, the following shear mode type recording head was used.

Specifications Nozzle pitch: 180 dpi
Nozzle diameter: 30 μm
Ejected ink droplet volume: 15 pl
Ink: Oil-based pigment ink
Viscosity at 25 ° C .: 7 cp
Surface tension at 25 ° C .: 28 dyne / cm

Discharge pulse The discharge pulse by the DRR driving method shown in FIG. 5 was used.
Voltage ratio between the first step and the fifth step of the ejection pulse (| a | / | b |): 2/1
Holding time for the second step of the ejection pulse: 1 AL
Holding time of the fourth step of the ejection pulse: 2AL

Evaluation Method 3 Interval of discharge pulse between cycles in cycle driving (interval from the end of the fifth step of the discharge pulse of the immediately preceding cycle to the start of the first step of the discharge pulse of the next cycle of the adjacent channel) is 4AL Then, the occurrence of bending of the tail of the ink droplet ejected at that time was observed and evaluated according to the following evaluation criteria.
◎: Tail bending is not seen at all ○: Tail bending is hardly seen ×: Tail bending is seen considerably

(Comparative example)
Evaluation was performed in the same manner as in Example except that the interval of the ejection pulse between cycles in the 3-cycle driving was set to 7AL.

  The examples and comparative examples are shown in Table 1 above.

  The presence or absence of bending of the tail of the ink droplet was visually observed with a CCD camera. The presence or absence of bending was determined by whether or not the end of the tail immediately before separation from the meniscus was parallel to the direction of ink droplet flight.

  In Table 1, “separation of ink droplets” means that at the start of the first step of the ejection pulse, the ink columns protruding from the nozzles of the adjacent channel in the immediately preceding cycle are separated from the meniscus and the ink droplets. Whether or not it is discharged.

  Furthermore, the “meniscus state” in Table 1 means that at the start of the third step of the ejection pulse, the meniscus after the ink column protrudes from the nozzle of the adjacent channel in the immediately preceding cycle is the tip of the nozzle opening. It is said whether or not it has substantially returned.

  As in the embodiment, in the case where the first step of the ejection pulse of the next cycle of the adjacent channel is started before the ink droplet is separated, pressure is applied in the direction in which the meniscus returns, so that the meniscus is at the tip of the nozzle opening. In this state, the ink column is separated from the meniscus, so that the tail portion is not bent at all and ink droplets can be ejected satisfactorily.

  On the other hand, in the comparative example, since the first step of the ejection pulse is started after the ink droplet is separated from the nozzle of the adjacent channel driven immediately before, the tail portion of the ink droplet is affected by the inner shape of the nozzle. Then, the ejected ink droplet was bent.

(A) is a general perspective view showing an example of a recording head in a partial cross section, and (b) is a cross sectional view. (A)-(c) is a figure which shows the action | operation at the time of the ink discharge of a recording head. Waveform diagram showing an example of ejection pulses (A)-(c) is explanatory drawing of the time division operation | movement of a recording head. Timing chart of ejection pulses applied to each pair of channels A, B, C The figure which shows typically the behavior of the ink and meniscus in the nozzle of the channel of A group shown in FIG. Timing chart of micro-vibration pulse and ejection pulse applied to each set of channels of A, B, C The figure which shows typically the behavior of the ink and the meniscus in the nozzle of the channel of A group shown in FIG. Diagram explaining the amount of meniscus extrusion The figure explaining the return position of the meniscus The figure explaining the flying state of the conventional ink drop The figure explaining the state of the ink column which protruded from the conventional nozzle

Explanation of symbols

2: Recording head 21: Ink tube 22: Nozzle forming member 23: Nozzle 24: Cover plate 25: Ink supply port 26: Substrate 27: Side wall 28: Channel 29: Electrode 10: Ink column 10a: Main droplet 10b: Tail 100: Drive signal generator M: Meniscus

Claims (12)

A plurality of channels at least partially separated by side walls formed of a piezoelectric material, of which the channels separated from each other by sandwiching two channels are grouped together as a set; Is divided into three groups, and a voltage pulse for ejecting droplets is applied to the electrodes formed on the side walls, so that each of the groups is driven in a time-division manner, and the side walls are subjected to shear deformation. A method for driving a droplet discharge head that discharges liquid in a channel as droplets from a nozzle by pressure during shear deformation,
Driving by applying a voltage pulse for discharging the droplets is
A first step of expanding the volume of the channel;
A second step of maintaining the enlarged state;
A third step of reducing the volume of the channel and projecting a liquid column from the nozzle;
A fourth step of maintaining a state of reducing the volume of the channel;
A fifth step of enlarging the volume of the channel,
The first step is started before the liquid column protruding from the nozzle of the adjacent channel driven immediately before is separated from the meniscus and discharged as a droplet, and the meniscus formed after the liquid column protrudes A method of driving a droplet discharge head, characterized in that the liquid column is separated from the meniscus and discharged as droplets when it substantially returns to the tip of the nozzle opening.   The volume of the channel when reduced by the third step is smaller than the volume before the channel is enlarged by the first step, and the volume of the channel after being enlarged by the fifth step is 2. The method of driving a droplet discharge head according to claim 1, wherein the volume is the same as the volume before the channel is expanded by the first step.   When the voltage applied to the electrode formed on the side wall in the first step is a (V) and the voltage applied to the electrode formed on the side wall in the fifth step is b (V), | a | 3. The method of driving a droplet discharge head according to claim 1, wherein: | b |.   4. The method of driving a droplet discharge head according to claim 3, wherein | a | / | b | = 2. A plurality of channels at least partially separated by side walls formed of a piezoelectric material, of which the channels separated from each other by sandwiching two channels are grouped together as a set; Is divided into three groups, and a voltage pulse for ejecting droplets is applied to the electrodes formed on the side walls, so that each of the groups is driven in a time-division manner, and the side walls are subjected to shear deformation. The ink in the channel is discharged as a droplet from the nozzle by the pressure at the time of shear deformation, and the droplet is not discharged to the electrode formed on the side wall of the non-recording channel in the one set of channels. A method for driving a droplet discharge head, in which a voltage pulse is applied and the meniscus of the nozzle is finely oscillated without discharging a droplet,
Driving by applying a voltage pulse that does not discharge the liquid droplets,
A first step of expanding the volume of the channel;
A second step of maintaining the enlarged state;
A third step of reducing the volume of the channel;
The first step is started before the liquid column protruding from the nozzle of the adjacent channel driven immediately before is separated from the meniscus and discharged as a droplet, and the meniscus formed after the liquid column protrudes A method of driving a droplet discharge head, characterized in that the liquid column is separated from the meniscus and discharged as droplets when it substantially returns to the tip of the nozzle opening.   6. The droplet discharge according to claim 5, wherein a volume before the channel volume is expanded by the first step is the same as a volume after the channel volume is reduced by the third step. Head drive method.   7. The method of driving a droplet discharge head according to claim 5, wherein the duration of the second step is 2AL (AL is 1/2 of the acoustic resonance period of the channel).   6. The voltage pulse that does not eject the droplet is applied to all the electrodes formed on the side walls of the one set of channels regardless of whether the droplet is ejected or not. 8. A method for driving a droplet discharge head according to 6 or 7.   9. The method of driving a droplet discharge head according to claim 5, wherein the maximum meniscus push-out amount by applying a voltage pulse that does not discharge the droplet is equal to or less than the nozzle radius.   The method of driving a droplet discharge head according to claim 1, wherein the voltage pulse is a rectangular wave pulse.   The method of driving a droplet discharge head according to claim 1, wherein the viscosity of the liquid is 5 cp or more and 15 cp or less.   The method of driving a droplet discharge head according to claim 1, wherein the liquid has a surface tension of 20 dyne / cm or more and 30 dyne / cm or less.

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