JP4543647B2 - Driving method of droplet discharge head - Google Patents

Description

  The present invention relates to a method for driving a droplet discharge head that discharges droplets from a nozzle, and in particular, can suppress the bending of the tail of the droplet discharged from the opening of the nozzle and improve the landing accuracy of the droplet. The present invention relates to a method for driving a droplet discharge head.

  In a liquid droplet ejection head that ejects liquid droplets from a nozzle, such as an inkjet recording head for recording an image using minute ink droplets, the liquid droplets are ejected from the nozzle by applying pressure to the pressure generating chamber. Land on a recording medium such as recording paper.

  There are various means for applying pressure to the pressure generating chamber. Here, a liquid droplet discharge head in the case where the partition of the pressure generating chamber is constituted by a piezoelectric element and ink droplets are discharged from the nozzle by deforming the piezoelectric element. Will be briefly described with reference to FIGS.

  FIG. 1 is a diagram showing a schematic configuration of a shear mode (share mode) type ink jet recording head (hereinafter simply referred to as a recording head), which is an embodiment of a droplet discharge head, and FIG. FIG. 4B is a cross-sectional view showing a state in which an ink supply unit is provided. FIG. 2 is a diagram illustrating the operation, FIG. 3 is a diagram illustrating a droplet discharge operation, and FIG. 4 is a diagram illustrating a drive waveform.

  In FIG. 1, 1 is an ink tube, 2 is a nozzle forming member, 3 is a nozzle, S is a partition, 6 is a cover plate, 7 is an ink supply port, and 8 is a substrate. As shown in FIG. 2, an ink channel A that is a pressure generation chamber is formed by the partition wall S, the cover plate 6, and the substrate 8.

  FIG. 1B shows a cross-sectional view of one ink channel A having one nozzle 3, but the recording head H operating in an actual shear mode is shown in FIG. 2A. In addition, a plurality of partition walls S, that is, a plurality of ink channels A1, A2,... An separated by S1 + 1 are formed between the cover plate 6 and the substrate 8. One end of the ink channels A1, A2,... An (hereinafter sometimes referred to as nozzle end) communicates with the nozzle 3 formed on the nozzle forming member 2, and the other end (hereinafter sometimes referred to as manifold end). ) Is connected to an ink tank (not shown) by a supply port 7 and an ink tube 1 constituting an ink supply unit, and a meniscus is formed in the nozzle 3 by ink.

  Each of the partition walls S1, S2,... Is composed of partition walls Sa (S1a, S2a,...) And Sb (S1b, S2b,...) Made of two piezoelectric elements having different polarization directions as indicated by arrows in FIG. The partition wall S1 is provided with electrodes Q1 and Q2 formed in close contact, and the partition wall S2 is provided with electrodes Q3 and Q4 formed in close contact. Similarly, electrodes are formed in close contact with the partition walls, and the electrodes Q1, Q2,... Are electrically connected to a drive pulse generating circuit.

  In the state shown in FIG. 2 (a), for example, when the recording head H connects the electrodes Q1 and Q4 to the ground and applies a driving pulse consisting of the rectangular wave shown in FIG. 4 to the electrodes Q2 and Q3, first, The first rising edge (P1) of the drive pulse generates an electric field in a direction perpendicular to the polarization direction of the piezoelectric elements constituting the partition walls S1 and S2, and both the partition walls S1a and S1b cause a deformation in the joint surface of the partition walls. , S2b is similarly deformed in the opposite direction, and as shown in FIG. 2B, the partition walls S1a, S1b and the partition walls S2a, S2b are deformed outwardly to expand the volume of the ink channel A1. As a result, a negative pressure is generated in the ink in the ink channel A1, and the ink flows. At the same time, the pressure starts to increase from the manifold end and the nozzle end, and the acoustic wave is transmitted toward the center of the ink channel. Thereafter, the acoustic wave reaches the opposite end, and the inside of the ink channel becomes positive pressure.

  When the potential is returned to 0 after a predetermined time has elapsed from the application of the first drive pulse (P2), the partition walls S1 and S2 return from the enlarged position to the neutral position shown in FIG. 2A, and high pressure is applied to the ink.

  Subsequently, as shown in FIG. 2C, when the drive pulse is applied to deform the partition walls S1a, S1b and S2a, S2b in opposite directions (P3), and the volume of the ink channel A1 is reduced, the ink is reduced. A positive pressure is generated in channel A1. As a result, the meniscus in the nozzle 3 due to a part of the ink filling the ink channel A1 changes in the direction pushed out from the nozzle 3, and the ink column 100 protrudes from the nozzle 3 (see FIG. 3A).

  After holding this drive pulse application for a predetermined time, the potential is returned to 0 (P4), and when the partition walls S1 and S2 are returned from the contracted position to the neutral position, the volume of the ink channel A1 is expanded and the meniscus is drawn and protruded. Since the rear end of the ink column 100 is pulled back, the ink column 100 separates from the meniscus and flies as a droplet 101 (see FIG. 3B).

  In this recording head H, as described above, positive and negative pressure is applied to the ink in the ink channel A by deformation of the partition wall S, and this partition wall S constitutes a pressure applying unit.

  By the way, as shown in FIG. 3B, the liquid droplet immediately after being ejected from the nozzle 3 is composed of a substantially spherical main droplet 101a and a tail portion 101b having a long tail from the rear end of the main droplet 101a. . However, after that, the tail portion 101b is split into minute secondary droplets 101c called satellites, and the substantially spherical main droplet 101a and the secondary droplets 101c called satellites fly toward the recording medium 200, and they land. By doing so, an image is recorded. At this time, if the flight direction of the main droplet 101a and the flight direction of the secondary droplet 101c are the same, they land on the same place and do not affect the image. However, if the flying direction of the secondary droplet 101c is different from that of the main droplet 101a, it will land on the periphery of the main droplet 101a as shown in FIG.

  The reason why the flight direction of the secondary droplet 101c differs from the flight direction of the main droplet 101a in this manner is that the original flight as viewed in the direction of the arrow in FIG. 3B is applied to the tail portion 101b of the droplet 101 immediately after being ejected from the nozzle 3. This is because bending occurs in a direction different from the direction.

  For example, Patent Document 1 and Patent Document 2 disclose that the volume of the pressure generation chamber is reduced to improve the deterioration of the image due to the bending of the tail of the droplet. Increasing the pressure causes the ink column to protrude from the nozzle, and after leaving it for a short period of time, immediately removes the deformation of the pressure generation chamber, thereby shortening the tail length of the ejected droplets and A technique for improving the separation action of the liquid droplets and making the flying direction of the tail of the droplet constant is disclosed.

Further, in Patent Document 3, the first pulse is applied to cause the ink column to protrude, and then the second pulse is applied before the droplets are separated to cause the meniscus to protrude from the nozzle. Discloses a technique for preventing tail bending by separating droplets.
JP-A-4-290748 Japanese Patent No. 2693656 JP-A-2-215537

  It is known that the cause of the bending of the tail of the droplet discharged from the nozzle is the non-uniformity of the inner surface of the nozzle. For example, as shown in FIG. 5, if the inclination of the inner surface of the nozzle is not uniform and the inclination is partially different, an imbalance occurs in the surface tension of the meniscus M on the inner surface of the nozzle, and the tail is perpendicular to the original flight direction. Force acting in one direction is applied, causing the tail to bend immediately after the droplet is separated from the meniscus M. Therefore, the accuracy of the shape of the inner surface of the nozzle greatly affects the stable discharge in which the bending of the tail of the droplet is suppressed.

  According to the techniques described in Patent Document 1 and Patent Document 2, the length of the tail portion of the droplet discharged from the nozzle is shortened to improve the breakage during the separation of the droplet, and the influence of the shape of the inner surface of the nozzle is affected. Although it is made difficult to receive, since it is made to separate early in order to shorten the length of the tail, the droplet is separated at the stage where the meniscus has not returned to the vicinity of the nozzle opening, There is a problem that it takes a long time before the liquid droplets can be ejected and the driving frequency is lowered. Moreover, in recent years, as the use of the droplet discharge heads has become diversified, the properties of the liquid used have also changed, and depending on the liquid, a long tail is still generated. For this reason, the long tail portion is easily affected by the shape of the inner surface of the nozzle, and the tail portion is bent.

  In order to prevent the tail of the droplet from being bent, it is desirable that the inner surface of the nozzle be formed in a perfect circle and symmetrical with respect to the center of the nozzle, but the level of accuracy required for the inner shape of the nozzle is extremely high. However, it is extremely difficult to process and form a perfect circle up to the inner surface of the nozzle and symmetrically with respect to the center of the nozzle, and it is difficult to meet the demand.

  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 this regard, according to the technique described in Patent Document 3, 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 that causes the ink column to protrude. Further, since it is necessary to apply the second pulse for causing the meniscus to protrude, it is necessary to cancel the vibration generated by the second pulse, and there is a problem that the driving frequency is lowered accordingly.

  Therefore, the present invention is a droplet ejection head driving method capable of stably ejecting droplets without being affected by the shape of the inner surface of the nozzle and suppressing the bending of the tail without lowering the driving frequency. It is an issue to provide.

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

  The above problems are solved by the following inventions.

According to the first aspect of the present invention, an opening of a nozzle for discharging a droplet, a pressure generating chamber capable of storing a liquid therein and communicating with the opening of the nozzle, and a pressure for changing the pressure in the pressure generating chamber A method of driving a droplet discharge head comprising an application unit, the step of causing the liquid in the pressure generation chamber to protrude from the opening of the nozzle by increasing the pressure in the pressure generation chamber by the pressure application unit; The meniscus formed at the base of the liquid after the liquid is protruded from the opening of the nozzle recedes in the direction opposite to the liquid protruding direction from the front end surface on the droplet discharge side in the opening of the nozzle. the meniscus open or the nozzle distance d becomes less than 1/2 of the nozzle radii between tip surface moves toward the distal end surface of the liquid droplet ejection side of the opening of the nozzle At the time when a state of protruding from, by lowering the pressure of the pressure generating chamber, a droplet discharge, characterized in that and a step of separating the liquid being projected from the opening of the nozzle as a droplet This is a head driving method.

According to a second aspect of the present invention, an opening of a nozzle for discharging droplets, a pressure generation chamber capable of storing liquid therein and communicating with the opening of the nozzle, and a volume in the pressure generation chamber are enlarged or reduced. A method of driving a droplet discharge head including a pressure applying unit for causing a pressure generating unit to expand a volume of the pressure generating chamber by the pressure applying unit, and the pressure applying unit after the first step. A second step of reducing the volume of the pressure generating chamber by the step of causing the liquid in the pressure generating chamber to protrude from the opening of the nozzle, and the liquid after the liquid has been protruded from the opening of the nozzle by the second step. of meniscus formed at the base is, the protruding direction of the further liquid from the tip surface of the liquid droplet ejection side of the opening of the nozzle retracts in the opposite direction, then the meniscus our the opening of the nozzle Moves toward the distal end surface of the droplet discharge side when the distance d is in a state of protruding from, or opening of the nozzle became less than 1/2 of the nozzle radii between distal end surface that, the A third step of separating the liquid protruding from the opening of the nozzle by the second step as a droplet by enlarging the volume of the pressure generating chamber by a pressure applying means, This is a method for driving a droplet discharge head.

  According to a third aspect of the present invention, the volume of the pressure generating chamber when reduced by the second step is smaller than the volume before the pressure generating chamber is expanded by the first step, and the first step The volume of the pressure generating chamber when expanded by the step 3 is substantially the same as the volume before the pressure generating chamber is expanded by the first step. This is a method for driving a droplet discharge head.

  According to a fourth aspect of the present invention, the pressure applying means is driven such that the volume in the pressure generating chamber is changed by applying a voltage, and different pressures are applied to the pressure generating chamber by applying different voltages. When the voltage applied to the pressure applying means in the first step is a (V), and the voltage applied to the pressure applying means in the third step is b (V), 4. The method of driving a droplet discharge head according to claim 2, wherein | a |> | b |.

  According to a fifth aspect of the present invention, in the method of driving a droplet discharge head according to the fourth aspect, the voltage a and the voltage b are | a | / | b | = 2.

  According to a sixth aspect of the invention, the voltage ratio | a | / | b | is adjusted according to the duration of the second step. is there.

  According to a seventh aspect of the invention, in the method of driving a droplet discharge head, the voltage ratio | a | / | b | is increased as the duration of the second step is longer. is there.

  The invention according to claim 8 is the method of driving a droplet discharge head according to any one of claims 1 to 7, wherein the pressure applying means includes a piezoelectric element.

  The invention described in claim 9 is the method of driving a droplet discharge head according to claim 8, wherein the piezoelectric element is deformed in a shear mode by applying an electric field.

  According to a tenth aspect of the present invention, in the method for driving a droplet discharge head according to any one of the second to ninth aspects, the duration of the second step is controlled according to the viscosity of the liquid. is there.

  An eleventh aspect of the invention is the method of driving a droplet discharge head according to the tenth aspect, wherein the duration of the second step is extended for a liquid having a high viscosity.

  According to a twelfth aspect of the present invention, in the method for driving a droplet discharge head according to any one of the second to ninth aspects, the duration of the second step is changed according to a change in head temperature. is there.

  A thirteenth aspect of the present invention is the method for driving a droplet discharge head according to any one of the first to twelfth aspects, wherein the liquid has a viscosity of 5 cp to 15 cp.

  According to a fourteenth aspect of the invention, the duration of the second step is controlled in accordance with the surface tension of the liquid. The method for driving a droplet discharge head according to any one of the second to ninth aspects It is.

  A fifteenth aspect of the present invention is the method of driving a droplet discharge head according to the fourteenth aspect, wherein the duration of the second step is extended for a liquid having a low surface tension.

  A sixteenth aspect of the present invention is the method for driving a droplet discharge head according to any one of the first to fifteenth aspects, wherein the surface tension of the liquid is 20 dyne / cm or more and 30 dyne / cm or less. .

  The invention according to claim 17 is the method of driving a droplet discharge head according to any one of claims 1 to 16, wherein the liquid is ink.

  The invention according to claim 18 is characterized in that the drive waveform applied to the pressure applying means in order to change the volume in the pressure generating chamber is a rectangular wave. This is a method for driving a droplet discharge head.

  According to the present invention, droplets can be stably ejected without being affected by the shape of the inner surface of the nozzle and without suppressing the bending of the tail without lowering the drive frequency.

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

  The driving method according to the present invention includes a nozzle opening for discharging a droplet, a pressure generating chamber communicating with the nozzle opening, and a pressure applying means for changing the pressure in the pressure generating chamber. Any liquid discharge head can be used as long as it is a discharge head, and the liquid filled in the pressure generation chamber may be any liquid. A shear mode type ink jet recording shown in FIGS. 1 and 2, which is a droplet discharge head that includes pressure applying means for changing the pressure by enlarging or reducing the volume, and using ink as the liquid filled in the pressure generating chamber. This will be described using the head H.

  FIG. 6A shows an example of a drive waveform for realizing the drive method according to the present invention, and FIG. 6B shows the pressure applied to the ink in the ink channel A by the drive waveform. FIG. 7 shows a state of meniscus and droplet discharge in the nozzle by the driving method according to the present invention. The numbers in parentheses in FIGS. 6 and 7 and the following description correspond to each other in time.

  In the present specification, the “ink column” means an ink whose front end protrudes from the opening of the nozzle 3 but whose rear end is connected to the meniscus in the nozzle 3 and has not yet been separated from the meniscus. The term “droplet” refers to ink in a state where the rear end is completely separated from the meniscus in the nozzle 3.

  (1); First, the partition wall S is expanded from the neutral state of FIG. 2A as shown in FIG. 2B to expand the volume of the ink channel A (Draw), and the ink is introduced into the ink channel A. (First step). When the driving waveform does not change, the pressure in the ink channel A repeats reversal every 1 AL time. Therefore, if this duration is continued for 1 AL time, the drawn meniscus M is moved to the droplet discharge side at the opening of the nozzle 3. (Hereinafter, the droplet discharge side tip surface of the opening of the nozzle 3 is referred to as the “return position” of the meniscus M), and the ink pressure is reversed to a positive pressure. When the expanded ink channel A is returned to the original neutral state (Release), high pressure is applied to the ink in the ink channel A. The ink pressure in the nozzle 3 changes with a slight delay from the change in the drive waveform, and the change in the meniscus M further delays a little.

  Here, “AL” is ½ of the acoustic resonance period of the ink channel. The duration is defined as 10% of the start of voltage rise or fall to the start of the next step. This AL measures the speed of the ink droplet ejected by applying a rectangular wave voltage pulse to the partition wall S, and changes the duration of the rectangular wave while keeping the rectangular wave voltage value constant. It is calculated as the time when the flight speed is maximized. Further, the rectangular wave is a waveform in which both the rise time and fall time between 10% and 90% of the voltage are preferably within ½ of AL, more preferably within ¼. is there.

  (2); Subsequently, as shown in FIG. 2C, the volume of the ink channel A is reduced, a higher pressure is applied to the ink (Reinforce), and the ink column 10 protrudes from the opening of the nozzle 3 (second step). ).

  (3): When 1 AL time has elapsed, the ink pressure is reversed and becomes negative pressure, so that the neck of the protruding ink column 10 is constricted as shown by an arrow in FIG.

  (4); When 0.5 AL time has passed, the negative pressure becomes maximum, the meniscus M is drawn into the deepest position in the direction opposite to the opening of the nozzle 3, and the meniscus M appears clearly.

  (5); When 0.5 AL time further elapses, the pressure is reversed to become positive pressure, and the meniscus M moves toward the return position.

  (6); When a further time elapses, the meniscus M returns to the return position. The meniscus M drawn deeply into the nozzle 3 starts to move rapidly toward the return position when the capillary force of the ink and the positive ink pressure are combined. When the meniscus M returns to the return position, the ink column 10 is not yet separated from the meniscus M, and the tail portion 10 b is connected to the meniscus M.

  (7); When the meniscus M substantially returns to the return position after 4 AL hours have passed since the high pressure was applied to the ink in (2) above, the partition wall S is brought to the neutral position as shown in FIG. Returning, the volume of the ink channel A is expanded from the previous reduced state.

  Here, the point of time when the ink column 10 is substantially restored refers to a state in which the position of the meniscus M after the ink column 10 protrudes is substantially located at the return position or protrudes from the opening of the nozzle 3. 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 opening of the nozzle 3, it is preferable to be positioned substantially at the return position in that the drive frequency can be further increased. The opening shape of the nozzle 3 is not limited to a perfect circle, but may be various shapes such as an ellipse. In the present invention, the nozzle radius is ½ of the longest diameter at the tip surface on the liquid discharge side in the opening of the nozzle 3. That is.

  At this time, the meniscus M is drawn from the opening of the nozzle 3, and the ink column 10 protruding from the opening of the nozzle 3 in the second step is separated from the meniscus M and ejected from the nozzle 3 as a droplet 11 (first). Step 3).

  The separation of the ink column 10 from the meniscus M is performed after the meniscus M has substantially returned to the return position, so that it is not affected by the non-uniformity of the inner surface of the nozzle 3 and the influence of the inner surface shape. This prevents the tail 11b of the droplet 11 from bending in a direction different from the flight direction.

  Here, a DRR (Draw-Release-Reinforce) method driving waveform (hereinafter also referred to as a DRR waveform) consisting of a rectangular wave is used, and the second step is continued for 4 AL hours, and then the third step. Is running. Although the residual pressure wave in the ink channel A is canceled by returning the volume of the ink channel A to the neutral position and expanding in the third step, the meniscus M substantially returns to the return position at this point. Therefore, the next drive can be started quickly and the drive frequency can be increased. In this embodiment, for example, when the duration for causing the ink column to protrude is 2 AL hours, the residual pressure is canceled at the meniscus M just before (the back side) from the return position. The return of the meniscus M to the return position is performed only by capillary force, and becomes extremely slow, making it difficult to increase the drive frequency.

  The duration of the second step is most preferably 4AL as described above, but is preferably 3.5 to 4.4AL hours.

  By the way, the ink column 10 protruding from the opening of the nozzle 3 in the second step is composed of the main droplet 10a on the tip side protruding greatly from the nozzle 3, and the main droplet 10a. It consists of a tail portion 10b that has a long tail from the rear end and is connected to the meniscus M in the nozzle 3. When the ink column 10 is separated as a droplet in the third step, the thickness of the ink column 10 at the tip surface on the droplet discharge side at the opening of the nozzle 3 that is the return position of the meniscus M is expressed by α (μm). ), When the maximum thickness of the ink column 10 is β (μm), it is preferable that α / β ≦ 1/3.

  In the third step, when the meniscus M that has substantially returned to the return position by enlarging the volume of the ink channel A is drawn, the thicknesses α and β of the ink column 10 satisfy α / β ≦ 1/3. In the state, the bending of the tail portion 10b can be further suppressed.

  The thicknesses α and β of the ink column 10 can be measured by stroboscopically measuring the ink column 10 protruding from the opening of the nozzle 3 using a CCD camera.

  In such driving, the volume of the ink channel A when reduced by the second step is smaller than the volume before the ink channel A is enlarged by the first step, and is enlarged by the third step. The volume of the ink channel A at this time is substantially the same as the volume before the ink channel A is expanded by the first step. By driving in this way, the drive waveform can be a simple waveform as shown in FIG. Moreover, in the final third step, the partition wall S returns to the initial state, and at this time, the residual pressure wave in the ink channel A is canceled, so the residual pressure wave to the meniscus M after substantially returning to the return position. There is no influence of this, and high-speed driving is possible.

  Further, as shown in the drive waveform of FIG. 6A, the voltage a (V) applied to the ink channel A in the first step and the voltage b (V) applied to the ink channel A in the third step. Is a relationship of | a |> | b |, the meniscus M is quickly restored, which is more preferable for high-speed driving. The voltage a and the voltage b are differential voltages.

  The voltage a and voltage b are more preferably | a | / | b | = 2, which makes it possible to achieve both high-speed driving and ejection stability.

  It is also preferable to adjust the voltage ratio | a | / | b | according to the duration of the second step. For example, since the residual pressure wave attenuates when the duration of the second process is extended, the voltage ratio | a | / | b | is increased as the duration of the second process is increased. The residual pressure wave can be effectively canceled according to the residual pressure wave, which is particularly preferable.

  By the way, the capillary penetration speed of the ink is expressed as {2 · (capillary radius) · (surface tension) · cos (contact angle)} / {8 · (viscosity) · (tube length)}. Largely affected by tension. For example, when an ink having a surface tension of 40 dyne / cm and a viscosity of 2 cp is compared with an ink having a surface tension of 28 dyne / cm and a viscosity of 10 cp, with the same capillary radius and the same tube length, the latter ink has a capillary penetration rate 1 / It will drop to 10.

  Accordingly, the return timing of the meniscus M to the return position differs depending on the difference in ink viscosity, and the return of the meniscus M is delayed in the case of ink having a high viscosity, whereas the return of the meniscus M is accelerated in the case of ink having a low viscosity. Similarly, the return timing of the meniscus M to the return position differs depending on the difference in the surface tension of the ink, so that the return of the meniscus M is delayed with an ink with a low surface tension, and conversely with an ink with a high surface tension. The return of M is faster.

  As described above, when the return timing of the meniscus M to the return position varies depending on the difference in ink viscosity and surface tension, the meniscus M is not changed even if the second step is continued for 4 AL hours and then the third step is performed. Is assumed to have not yet substantially returned to the return position.

  For this reason, it is preferable to control the duration of the second step according to the viscosity of the ink. That is, for ink with a high viscosity, the duration of the second step is lengthened, and conversely for ink with a low viscosity. The duration of the second step can be changed by changing the setting of the drive pulse generation circuit shown in FIG.

  Specifically, since the ink viscosity varies depending on the composition of the ink, it is preferable to change the duration of the second step as described above when the ink viscosity varies depending on the ink composition. The change in the duration of the second step depending on the composition of the ink is performed by automatically changing the setting of the drive pulse generation circuit appropriately according to the type of ink supplied to the recording head H or manually by an operator or the like. Alternatively, by specifying the composition of the ink used for each recording head H, the setting of the drive pulse generation circuit may be different for each recording head H.

  In general, the droplet discharge head generates heat when the pressure applying unit is driven, and the heat at this time is transmitted to the ink, whereby the viscosity of the ink may change (decrease). For example, in the recording head H shown in FIGS. 1 and 2, since the partition wall S generates heat due to the shear deformation of the partition wall S, the heat directly acts on the ink in the ink channel A. Therefore, even for the same ink, the ink viscosity varies depending on the head temperature difference. Therefore, it is preferable to change the duration of the second step as described above in accordance with the change in the head temperature.

  The head temperature can be detected, for example, by providing a temperature sensor (not shown) so as to contact the cover plate 6 of FIG. The detection signal is output to the drive pulse generation circuit of the recording head H shown in FIG. 2, and the duration of the second step may be appropriately changed based on the detection signal in the drive pulse generation circuit.

  Similarly, it is also preferable to change the duration of the second step according to the surface tension of the ink. That is, for ink with low surface tension, the duration of the second step is lengthened, and conversely for ink with high surface tension.

  Since the surface tension of the ink also varies depending on the ink composition, it is preferable to change the duration of the second step as described above when the ink composition is different.

  Since the relationship between the ink viscosity or the surface tension of the ink and the behavior of the meniscus M differs for each recording head and ink, it correlates with the position of the meniscus M of the recording head H (the thickness α of the ink column 10 at the return position of the meniscus M). When the drive signal is applied, the nozzle 3 which is the return position of the meniscus M is obtained by performing an experiment of observing the actual movement of the meniscus M of the recording head H with a microscope or the like, or by a finite element method or the like. The movement mode of the meniscus M at the opening of the signal may be examined, and the signal application timing may be adjusted by electric circuit means.

  The driving method of the present invention exhibits a remarkable effect when the viscosity of the ink is 5 cp or more and 15 cp or less. Such inks have high viscosity, are less likely to be disturbed by the residual pressure during the meniscus recovery process, and the droplets are difficult to separate from the meniscus, and the tails are elongated so that tail bending is likely to occur. It is.

  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 it is difficult for the droplets to be separated from the meniscus, and similarly, the tail is elongated so that the tail is easily bent.

  In the above embodiment, the pressure applying means (partition wall S) is constituted by a piezoelectric element. The driving method of the present invention is preferable because the timing of lowering the pressure in the pressure generating chamber can be easily controlled when the pressure applying means is constituted by a piezoelectric element.

  In the above embodiment, a rectangular driving waveform is applied to the piezoelectric element. By using the rectangular wave, it becomes easy to set the timing when starting the third process at the timing when the meniscus M returns to the return position, and a strong negative pressure is generated by the third process, so that the droplets can be easily formed. It is preferable because it can be separated.

  In the above embodiment, a shear mode type piezoelectric element that deforms in a shear mode by applying an electric field is used as the pressure applying means. The shear mode type piezoelectric element is preferable because the rectangular waveform driving waveform shown in FIG. 6A can be used more effectively, the driving voltage is lowered, and more efficient driving is possible. However, the present invention is not limited to these, and other types of piezoelectric elements such as a single plate type piezoelectric actuator and a longitudinal vibration type stacked piezoelectric element may be used as the piezoelectric element. Also, other pressure applying means such as an electromechanical conversion element using electrostatic force or magnetic force, or an electrothermal conversion element for applying pressure using a boiling phenomenon may be used.

  In the above description, an inkjet recording head for performing image recording is used as the droplet discharge head. However, the present invention is not limited to this, and the nozzle opening for discharging droplets communicates with the nozzle opening. The invention can be similarly applied as long as it includes a pressure generating chamber and a pressure applying means for changing the pressure in the pressure generating chamber.

Example 1
Using a shear mode recording head with a nozzle pitch of 180 dpi and an output droplet volume of 15 pl, the DRR waveform has a Draw to Reinforce voltage ratio (| a | / | b |) of 2/1 and Reinforce (second Driving was performed with the duration (R time) of the step) being 18 μs, and droplets were discharged. The position of the meniscus at that time and the occurrence of bending of the tail of the discharged droplet were observed.

  Meniscus position measurement: The position of the meniscus in the nozzle was estimated by observing the movement of the meniscus pushed out from the nozzle opening immediately after droplet discharge by strobe measurement using a CCD camera, and confirmed by simulation.

  Presence or absence of bending of tail of droplet: Visual observation was made with a CCD camera. In addition, the presence or absence of bending was judged by whether or not the end of the tail immediately before separation was parallel to the flying direction of the droplet.

Ink used: Oil-based ink (10 cp, 28 dyne / cm)
Drive voltage: 20V

(Comparative Example 1)
Example 1 was the same as Example 1 except that the duration (R time) of Reinforce (second step) of the DRR waveform was set to 9 μs.

  The results of Example 1 and Comparative Example 1 are shown in Table 1.

  In Example 1, since the droplets were separated in the third step after the meniscus returned, discharge without bending of the tail portion was obtained.

  In Comparative Example 1, as a result of performing the third step before returning to the meniscus, the tail was bent and emitted.

(Examples 2 to 4)
In Example 1, droplet ejection stability and high-speed driving were evaluated when | a | / | b | of the DRR waveform was changed as shown in Table 2, respectively.

The emission stability was evaluated according to the following criteria.
○: When ejected at a droplet velocity of 8 m / s, it is ejected stably.
Δ: Some droplet speed fluctuations are observed, but the light is emitted without generating a shortage.
X: Outgoing lack occurs.

Further, high speed driving was evaluated according to the following criteria.
○: When the light is emitted with a driving period of 25 μs, the light is emitted stably.
Δ: Some fluctuations in the droplet velocity are observed, but the light is emitted without occurrence of missing light.
X: Outgoing lack occurs.

(Comparative Example 2)
In Example 1, droplet ejection stability and high-speed driving were evaluated in the same manner as described above when | a | / | b | of the DRR waveform was 1/1. At this time, the meniscus did not return to the tip surface on the droplet discharge side in the nozzle opening.

  The results of Examples 2 to 4 and Comparative Example 2 are shown in Table 2.

  In Examples 2, 3, and 4 shown in Table 2, the droplets were separated after the meniscus returned, and none of the tails bent.

  Furthermore, in Examples 2 and 3, the residual pressure was effectively canceled, and stable ejection was obtained even when the droplet speed increased.

  In Examples 3 and 4, the meniscus returned quickly and the ink was filled in time, and stable ejection was obtained even at high speed driving.

  On the other hand, in Comparative Example 2, the meniscus did not return and stable discharge could not be obtained while taking the same R time (duration of the second step) as in the example.

  From the above, it can be seen that when | a | / | b | is 2/1, more stable and faster driving becomes possible.

2A and 2B are diagrams illustrating a schematic configuration of a shear mode type ink jet recording head, in which FIG. 1A is a perspective view illustrating a partial cross-section, and FIG. (A)-(b) is a figure which shows the operation | movement of a recording head. (A) (b) is explanatory drawing which shows the mode of the droplet discharge by the conventional drive method The figure which shows the drive waveform in the conventional drive method The figure which shows the mode of the ink column which protrudes from the nozzle (A) is a figure which shows the drive waveform for implement | achieving the drive method which concerns on this invention, (b) is a figure which shows the pressure provided to the ink in an ink channel by a drive waveform. The figure which shows the mode of the meniscus and droplet discharge in the nozzle by the drive method which concerns on this invention The figure explaining the positional relationship between the nozzle opening and the meniscus The figure which shows the mode of the ink column by the drive method which concerns on this invention

Explanation of symbols

1: Ink tube 2: Nozzle forming member 3: Nozzle 6: Cover plate 7: Ink supply port 8: Substrate 10, 101: Ink column 10a, 101a: Main droplet 10b, 101b: Tail 101c: Secondary droplet 200: Recording medium H: recording head A, A1,...: Ink channel S, S1,...: Partition wall (electrical / mechanical conversion means)
Q, Q1, ...: Electrode M: Ink meniscus

Claims (18)

A droplet having a nozzle opening for discharging a droplet, a pressure generating chamber capable of storing liquid therein and communicating with the nozzle opening, and a pressure applying means for changing the pressure in the pressure generating chamber A method of driving a discharge head,
Causing the liquid in the pressure generating chamber to protrude from the opening of the nozzle by increasing the pressure in the pressure generating chamber by the pressure applying means;
The meniscus formed at the base of the liquid after the liquid is protruded from the opening of the nozzle recedes in the direction opposite to the liquid protruding direction from the front end surface on the droplet discharge side in the opening of the nozzle. the meniscus is protruded from the distance d or the opening of the nozzle became less than 1/2 of the nozzle radii between tip surface moves toward the front end face of the liquid droplet ejection side of the opening of the nozzle Separating the liquid protruding from the opening of the nozzle as droplets by lowering the pressure in the pressure generating chamber when the state is reached ;
A method for driving a droplet discharge head, comprising: A liquid provided with an opening of a nozzle for discharging droplets, a pressure generating chamber capable of storing liquid therein and communicating with the opening of the nozzle, and a pressure applying means for expanding or reducing the volume in the pressure generating chamber A method for driving a droplet discharge head,
A first step of expanding the volume of the pressure generating chamber by the pressure applying means;
After the first step, a second step of reducing the volume of the pressure generating chamber by the pressure applying means and causing the liquid in the pressure generating chamber to protrude from the opening of the nozzle;
The meniscus formed at the base of the liquid after the liquid is protruded from the nozzle opening in the second step is more opposite to the liquid protruding direction than the front end surface on the droplet discharge side in the nozzle opening. retracted in the direction, then the distance d is or wherein became half or less of the nozzle radii between tip face the meniscus moves toward the distal end surface of the liquid droplet ejection side of the opening of the nozzle When the state of projecting from the opening of the nozzle is reached, the volume of the pressure generating chamber is expanded by the pressure applying means, whereby the liquid projecting from the opening of the nozzle in the second step is formed as a droplet. A third step of separating;
A method for driving a droplet discharge head, comprising:   The volume of the pressure generating chamber when reduced by the second step is smaller than the volume before the pressure generating chamber is expanded by the first step and is expanded by the third step. 3. The method of driving a droplet discharge head according to claim 2, wherein the volume of the pressure generating chamber is substantially the same as a volume before the pressure generating chamber is expanded by the first step.   The pressure applying means is configured to change the volume in the pressure generating chamber by applying a voltage, and is configured to be able to apply different pressures to the pressure generating chamber by applying different voltages. When the voltage applied to the pressure applying means in the first step is a (V) and the voltage applied to the pressure applying means in the third step is b (V), | a |> | b | 4. A method for driving a droplet discharge head according to claim 2, wherein the droplet discharge head is provided.   5. The method of driving a droplet discharge head according to claim 4, wherein the voltage a and the voltage b are | a | / | b | = 2.   5. The method of driving a droplet discharge head according to claim 4, wherein the voltage ratio | a | / | b | is adjusted according to the duration of the second step.   7. The method of driving a droplet discharge head according to claim 6, wherein the voltage ratio | a | / | b | is increased as the duration of the second step is longer.   The method for driving a droplet discharge head according to claim 1, wherein the pressure applying unit includes a piezoelectric element.   9. The method of driving a droplet discharge head according to claim 8, wherein the piezoelectric element is deformed in a shear mode by applying an electric field.   The method for driving a droplet discharge head according to claim 2, wherein the duration of the second step is controlled according to the viscosity of the liquid.   The method of driving a droplet discharge head according to claim 10, wherein the duration of the second step is extended for a liquid having a high viscosity.   The method for driving a droplet discharge head according to claim 2, wherein the duration of the second step is changed according to a change in head temperature.   The method for driving a droplet discharge head according to claim 1, wherein the liquid has a viscosity of 5 cp to 15 cp.   The method for driving a droplet discharge head according to claim 2, wherein the duration of the second step is controlled according to the surface tension of the liquid.   15. The method of driving a droplet discharge head according to claim 14, wherein the duration of the second step is increased for a liquid having a low surface tension.   16. The method for 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.   The method of driving a droplet discharge head according to claim 1, wherein the liquid is ink.   18. The method of driving a droplet discharge head according to claim 2, wherein a drive waveform applied to the pressure applying unit in order to change the volume in the pressure generating chamber is a rectangular wave.

Images