JP5125004B2 - Method for discharging minute ink droplets - Google Patents

Description

  The present invention relates to a method for driving an ink jet head that ejects ink droplets from nozzles communicating with an ink pressure chamber by increasing and decreasing the pressure of the ink in the ink pressure chamber. In particular, the present invention relates to a driving method suitable for discharging minute ink droplets.

  2. Description of the Related Art Conventionally, a drop-on-demand ink jet technique is known in which a drive voltage waveform is applied to a piezoelectric element and ink droplets are ejected only when dots are to be recorded. Ink jet printers of this type have the disadvantage that graininess is particularly noticeable in highlights because all colors are represented by a collection of dots formed on a medium with a limited number of colors. . Therefore, in order to create a high-quality print without graininess, it has been studied to reduce the size of the ejected ink droplets that can reduce the size of the dots. Furthermore, in recent years, it has also been studied to create circuit boards by patterning with conductive ink using inkjet technology, and to form various types of thin films. It is expected to be effective for the formation of wiring and extremely thin uniform thin films.

  In order to make the discharged ink droplets into small ink droplets, it is easy to reduce the diameter of the nozzle openings. However, if the nozzle opening is made smaller, the accuracy is increased and the manufacturing cost is increased, and the nozzle opening is likely to be clogged due to adhesion of foreign matter or ink, which causes non-ejection.

  On the other hand, the ink droplet ejection method based on vibration control of the ink surface of the nozzle opening (hereinafter referred to as meniscus) can eject ink droplets smaller than the size of the nozzle opening.

  Conventionally, there is a method of realizing small ink droplet ejection by causing ink to bounce at the center of the meniscus by suddenly pulling and stopping the meniscus and ejecting it as it is (see, for example, Patent Document 1). In addition, there is a method of realizing small ink droplet ejection by causing a thin ink column to be generated only from the center of the meniscus by slightly pushing out the meniscus and then ejecting it slightly (see, for example, Patent Document 2). ). Furthermore, after pulling out the meniscus and extruding it to generate an ink column from the nozzle opening, there is a method of reducing the amount of ink to be ejected and reducing the ejected ink droplets by pulling in the meniscus again (for example, patents) Reference 3 and Patent Document 4).

Japanese Patent Publication No. 4-36071 Japanese Patent No. 3491187 JP 2000-141642 A Japanese Patent No. 3159188

  However, in these conventional methods, the amount of ink droplets that can obtain an average droplet speed of about 5 m / s or more necessary for stable flight over a distance of about 1 mm is 1 pl (picoliter) at a nozzle opening with a diameter of 30 μm. ) Degree is the limit. On the other hand, in industrial applications such as high-density wiring using conductive ink using ink jet technology, further miniaturization of ink droplets is required.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for ejecting sub-picoliter minute ink droplets by ink jet technology.

In order to achieve the above object, the present invention includes an ink pressurizing chamber and an ink pressurizing / depressurizing unit, and applying an electric signal to the pressurizing / depressurizing unit to increase / decrease the ink in the ink pressure chamber. In a method of driving an ink jet head that ejects ink droplets from a nozzle opening communicating with an ink pressure chamber, a first step in which the tip of the generated first ink column is separated to form minute ink droplets, and a nozzle opening A second ink column that is pushed out from the nozzle and returned to the nozzle opening again, catches up with the ink column or ink droplet separated from the minute ink droplets, merges, and pulls back into the nozzle opening. And a second step of controlling the volume velocity.

  Next, the first step includes a step of drawing the meniscus abruptly so as to cause the meniscus to rebound.

  The first step includes a step of drawing the meniscus abruptly enough to cause the meniscus to rebound, and a step of further drawing the meniscus.

  The first step includes a step of drawing a meniscus, a step of pushing out, and a step of drawing the meniscus again.

  Furthermore, when the ink column generated in the first step is ejected, at least one ink droplet larger than the leading ink droplet flies continuously.

  Furthermore, at least the periphery of the nozzle opening is characterized by ejecting ink droplets using an ink jet head provided with a nozzle plate having a contact angle of 30 degrees or less with respect to the ink.

  Furthermore, the control amount and the control timing of the second step are changed according to the change of the ink viscosity.

  Furthermore, a temperature adjustment mechanism is provided to keep the temperature of the ink substantially constant.

  According to the present invention, the ink column or ink droplet that is about to fly from the back of the minute ink droplet formed in the first step is combined with the ink column generated in the second step and pulled back into the nozzle opening. Only ink droplets can fly. Therefore, sub-picoliter minute ink droplets can be discharged, and high-density wiring using conductive ink can be realized. In addition, since the ejection principle of the micro ink droplet according to the present invention is not affected by the natural vibration period of the pressure chamber, the ink head is large even in an ink jet head suitable for ejecting a large ink droplet having a large volume of the ink pressure chamber and a long natural vibration period. Ink droplets can be ejected, and ink droplets having a volume difference of 100 times or more can be separated by one head.

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

  FIG. 1 is a perspective view showing the configuration of an inkjet head to which an embodiment of the present invention is applied. An ink pressure chamber 12 is formed in the ink flow path forming member 11, and one end of the ink pressure chamber 12 communicates with a nozzle opening 14 opened in the nozzle plate 13. The other end of the ink pressure chamber 12 communicates with the common ink flow path 16 via a restrictor 15 that is a position where the ink flow path is narrowed in order to suppress a decrease in pressure applied to the ink.

  The pressure increasing / decreasing means for the ink in the ink pressure chamber 12 is based on the piezoelectric element 17 having a laminated structure. The piezoelectric element 17 is fixed to the piezoelectric element support substrate 18 provided in the stacking direction, and generates pressure by using piezoelectric expansion and contraction in the d33 direction. The piezoelectric element 17 contracts when the voltage applied to the positive electrode 19 decreases and is discharged, decompresses the ink pressure chamber, and expands and pressurizes the ink pressure chamber when the voltage applied to the positive electrode 19 increases and is charged. It works as follows. The positive electrode 19 is an electrode common to each piezoelectric element, is disposed on one surface of the piezoelectric element support substrate 18 and is connected to a drive voltage generation circuit (not shown). The negative electrode 20 is an electrode for each piezoelectric element, is disposed on the other surface of the piezoelectric element support substrate 18, and is grounded through a drive nozzle selection circuit (not shown). In the drive nozzle selection circuit, a diode (not shown) is inserted in parallel with a drive nozzle selection switch (not shown) so that a current flows in the ground direction. Only charging is done. The piezoelectric element support substrate 18 and the ink flow path forming member 11 are fixed to a housing (not shown) and are relatively hardly moved.

  The surface of the piezoelectric element 17 that is not fixed to the piezoelectric element support substrate 18 is fixed to the elastic film 21. The elastic film 21 forms a part of the wall of the ink pressure chamber 12, and has a structure in which the volume of the ink pressure chamber 12 changes when the elastic film 21 is deformed by expansion and contraction of the piezoelectric element 17.

  The inkjet head to which the present invention is applied has the above-described structure, and a plurality of nozzle openings 14 are arranged in a line at 1/100 inch intervals.

  Next, the ink droplet ejection principle of the inkjet head will be described.

  Only a nozzle for ejecting ink droplets is grounded by a drive nozzle selection circuit (not shown) connected to the negative electrode 20 of the piezoelectric element 17, and charging / discharging of the piezoelectric element is performed by a voltage applied to the positive electrode 19. . The piezoelectric element that is not grounded is not discharged. When not driven, a DC voltage is applied to the positive electrode 19, the piezoelectric element 17 is charged and extends in the stacking direction from the natural length, and the elastic film 21 is pushed into the ink pressure chamber 12. When the applied voltage of the positive electrode 19 decreases during driving, only the grounded piezoelectric element is discharged and contracts in the stacking direction. As a result, the elastic film 21 is pulled to depressurize the pressure chamber 12, and ink is supplied from the common ink flow path 16 through the restrictor 15 to the ink pressure chamber 12. Subsequently, when the voltage applied by the positive electrode 19 rises, the grounded and discharged piezoelectric element is charged, extends in the stacking direction, and pushes the elastic film 21 into the ink pressure chamber 12. As a result, the ink in the pressure chamber 12 is pressurized, and the ink is pushed out from the nozzles 14 communicating with the pressure chamber 12 to form ink droplets 22.

  In the ink jet head to which the present invention is applied, the fluid resistance of the nozzle opening 14 is larger than the fluid resistance of the restrictor 15, and the inertia (liquid inertia component) of the nozzle opening 14 is smaller than the inertance of the restrictor 15. Designed to. Therefore, if the piezoelectric element 17 is discharged so that the volume acceleration (rate of change) of the fluid in the ink pressure chamber 12 becomes small and the volume of the ink pressure chamber 12 is changed slowly, the fluid resistance becomes dominant, and the nozzle Ink flows more easily from the restrictor 15 having a relatively low fluid resistance to the ink pressure chamber 12 than when the opening 14 draws air from the outside. Conversely, when the piezoelectric element 17 is charged so that the volume acceleration (rate of change) of the fluid in the ink pressure chamber 12 is increased and the volume of the ink pressure chamber 12 is rapidly changed, the inertance becomes dominant, and the list Rather than returning ink from the rectifier 15 to the common ink flow path 16, it becomes easier to eject ink droplets from the nozzle opening 14 having a small inertance. Further, since the nozzle opening 14 is formed so that the diameter on the ink pressure chamber 12 side is larger than the diameter on the side (external) from which the ink droplet 22 is ejected, the ink pressure chamber 12 is in the process of depressurization. The surface tension acting on the meniscus is larger than in the pressurizing process, and it is difficult to draw air in the depressurizing process of the ink pressure chamber 12, and it is easy to eject ink droplets in the pressurizing process of the ink pressure chamber 12. It has become.

  If the drive voltage applied to the positive electrode 19 of the piezoelectric element is raised or lowered in a short time or greatly raised or lowered even in the same time, the ink volume velocity in the ink pressure chamber 12 becomes faster and the speed of the ejected ink droplet becomes faster. If the drive voltage applied to the positive electrode 19 of the piezoelectric element is raised or lowered over a long period of time or is lowered or lowered even during the same period of time, the ink volume velocity in the ink pressure chamber 12 becomes slower and the velocity of ejected ink droplets becomes slower. Thus, the ink volume velocity of the ink pressure chamber can be controlled by the drive voltage waveform applied to the positive electrode 19 of the piezoelectric element.

  FIG. 10 is an example of examining the drive voltage waveform applied to the positive electrode 19 of the piezoelectric element implemented by the inventors in the course of studying the present invention for the purpose of ejecting small ink droplets. Step A to Step E are the first step, and Step F to Step G are the second step. In step A, the meniscus is pulled in, in step B, the voltage is held for a certain period, and in step C, the meniscus is pushed out to generate ink columns. In step D, the voltage is maintained for a certain period of time, and in step E, the meniscus is drawn to reduce the amount of ink columns to be ejected to form ultrafine ink columns and eject small ink droplets. In step F, the voltage is maintained for a certain period, and in step G, the voltage reduced in step E is raised to the original voltage.

  As shown in FIG. 5, the ultra fine ink column (FIG. 5 (2)) generated in the first step is separated into minute ink droplets with the passage of time, and the separated minute ink is generated. The droplet starts to fly (FIG. 5 (3)). On the other hand, the ink column existing on the nozzle opening 14 side of the minute ink droplet starts to fly as a small ink droplet or a plurality of ink droplets of small ink droplets and minute ink droplets (FIGS. 5 (4) and (5)). ). A group of a plurality of ink droplets that have started to fly is a small ink droplet, which lands on almost the same position on the recording medium and forms a small dot.

  In the process G, by increasing the time Gt of the process G or decreasing the voltage Gv of the process G, the process G prevents a large amount of ink from being pushed out from the nozzle, or the process G is the first. The residual vibration is set so as to cancel, for example, it is applied at a timing at which a vibration having a phase opposite to that of the residual vibration generated in the process is generated. For this reason, the vibration of the meniscus is suppressed, and in the conventional system, the ink column and ink droplets that have been generated in the second step and combined with the previously generated ink column and ink droplets are not large enough to be pulled back to the nozzle opening.

  On the other hand, FIG. 2 shows an embodiment of the drive voltage waveform applied to the positive electrode 19 of the piezoelectric element 17 according to the present invention. Step A or Step A to Step C is the first step, and Step D to Step E are the second step. In step A, the meniscus is rapidly pulled in, and in step B, the pulling is stopped to generate an extra fine ink column by the bounce of the meniscus. In step C, the voltage is lowered until the potential difference required for step E is obtained. In order to make the ultrafine ink column generated in the first step thinner by the step C, the ink amount of the ink column to be ejected is reduced by setting the time Bt of the step B to about 0.5 μs to 4 μs. be able to. In step D, the voltage is maintained for a certain period, and in step E, an ink column that is pushed out of the nozzle opening 14 and returns to the nozzle is generated.

  With the drive voltage waveform, as shown in FIG. 6, the ultrafine ink column (FIG. 6 (2)) generated in the first step is separated into fine ink droplets with the passage of time, and the separated fine ink is separated. The droplet 80 starts to fly (FIG. 6 (3)). On the other hand, the ink column 81 present on the nozzle opening 14 side of the minute ink droplet 80 attempts to start flying as a small ink droplet or a plurality of ink droplets of small ink droplets or minute ink droplets. The generated ink column 82 catches up with and merges with the ink column 81 present on the nozzle side with respect to the first minute ink droplet to fly (FIG. 6 (4)), and pulls it back into the nozzle opening together (FIG. 6). 6 (5)), it becomes possible to fly only the minute ink droplets separated from the head of the ultrafine ink column (FIG. 6 (6)).

  On the other hand, the drive voltage waveform may show the behavior shown in FIG. 7 depending on the physical properties (viscosity and surface tension) of the ink. That is, the ultra fine ink column 90 (FIG. 7 (3)) generated in the first step is separated into the minute ink droplets 91 with the passage of time (FIG. 7 (4)), and the separated minute ink droplets 91 are separated. Starts flying (FIG. 7 (5)). On the other hand, the ink column 92 present on the nozzle opening 14 side of the minute ink droplet 91 starts to fly as a small ink droplet or a plurality of ink droplets of the small ink droplet 93 and the minute ink droplet 94. The generated ink column 95 catches up with and merges with the small ink droplets 93 and the minute ink droplets 94 existing on the nozzle side of the leading minute ink droplet 91 which is about to fly (FIGS. 7 (7) to (8)). ) And pulling back into the nozzle opening together (FIG. 7 (9)), it becomes possible to fly only the minute ink droplets separated from the head of the ultrafine ink column (FIG. 7 (10)).

  Here, the process C occurs in the second process with the position where the minute ink droplets are separated from the head of the ultrafine ink column being a position close to the nozzle opening 14 in either case of FIGS. 6 and 7. The caused ink column also plays a role of facilitating coalescence with ink droplets that are about to fly from behind the minute ink droplets.

  The time Dt of the process D, the time Et of the process E, and the voltage Ev of the process E are applied to the minute ink droplets that are separated from the head of the extra fine ink column generated by the first process. To the extent that it cannot catch up, Dt is lengthened to delay the generation timing of the ink column generated by the second step, or Et is lengthened and Ev is decreased to decrease the volume velocity of the ink column generated by the second step. In addition, after the head of the ultra fine ink column generated in the first step is separated into fine ink droplets, the ink columns or ink droplets that are present on the nozzle side with respect to the fine ink droplets are caught up and merged and pulled into the nozzle openings. To the extent that it can be returned, Dt is shortened, the generation timing of the ink column generated in the second step is advanced, or Et is shortened and Ev is increased to increase the volume velocity of the ink column generated in the second step. The present invention is established by setting Dt, Et, and Ev that satisfy both of these conditions.

  FIG. 8 is a diagram illustrating a relationship between the time Et of the process E and the voltage Ev of the process E. If the time Et is too long, the ink columns generated in the second step cannot catch up with the ink columns or ink droplets to fly from behind the minute ink droplets formed in the first step, and can be pulled back into the nozzle openings. However, the ink column or ink droplet which is going to fly from behind the minute ink droplet formed in the first step becomes a small ink droplet and flies. If the time Et is too short, the ink column generated in the second step also flies to become a large ink droplet, or catches up to the head of the ultra fine ink column generated in the first step and merges it back into the nozzle opening. And ink droplets will not fly. If the voltage Ev is too small, the ink column generated in the second step cannot catch up with the ink column or ink droplet that is about to fly from behind the minute ink droplet formed in the first step, and cannot be pulled back into the nozzle opening. Thus, the ink column or ink droplet that is about to fly from behind the minute ink droplet formed in the first step becomes a small ink droplet and flies. If the voltage Ev is too large, the ink column generated in the second step will also fly and become a large ink drop, or catch up to the head of the ultra fine ink column generated in the first step and merge into the nozzle opening. It will be pulled back and the ink droplet will not fly. A hatched portion in FIG. 8 indicates an appropriate area. By appropriately setting the time Et and the voltage Ev of the process E, the ink column or the ink column existing on the nozzle side from the micro ink droplet after the head of the ultra fine ink column generated in the first process is separated into the micro ink droplets or Since the ink column generated in the second step catches up with the ink droplet and merges with it, it is pulled back into the nozzle opening together, so that only the minute ink droplet can fly.

  The appropriate region in FIG. 8 falls below the graph when the time Dt of the process D becomes short, and rises above the graph when the time Dt of the process D becomes long. In addition, the width of the appropriate region also changes depending on the value of the time Dt of the process D, and the appropriate region may disappear even if Dt is too long or too short.

  When the temperature of the ink changes due to a change in temperature or the like, the viscosity of the ink changes, and the value of the appropriate area shown in FIG. 8 changes, so that the time Dt of the process D and the process E It is necessary to change any one value or a plurality of values of the time Et and the voltage Ev of the process E, or to keep the ink viscosity change within a certain range. Therefore, an electric circuit that changes the values of Dt, Et, and Ev so that the set value does not deviate from the appropriate range due to a change in temperature or a change in ink viscosity, or a temperature adjustment mechanism that keeps the ink viscosity within a certain range. It is desirable to provide it.

  In the present invention, the voltage Av = 23.6 V in the process A, the time At = 0.2 μs in the process A, the time Bt = 3 μs in the process B, the time Ct = 1 μs in the process C, the time Dt = 20 μs in the process D, and the process E When an ink having a viscosity of 10 mPa · s and a surface tension of 31 mN / m is used with a drive voltage waveform of a voltage Ev of 39.4 V, a time E of process E = 20 μs, and a surface tension of 31 mN / m, the behavior of FIG. From the nozzle opening, it was possible to stably fly a 0.4 pl minute ink droplet at a rate of about 1.5 mm from the nozzle opening surface at 7 m / s. Further, it was possible to stably fly a 0.5 pl ink droplet at a high speed of 14 m / s and a distance of about 2 mm from the nozzle opening surface. It should be noted that the present invention can be implemented without the step C by shortening the time Et of the step E. The advantage of providing the process C is that the ultra-thin ink column generated in the first process can be made thinner, and the voltage Ev of the process E can be increased, so that the time Et of the process E is matched with the Helmholtz oscillation period. Thus, it is possible to reduce residual vibration after ink droplet ejection. Also, the behavior of FIG. 7 can be shown by changing the physical properties of the ink.

  FIG. 3 shows another embodiment of the drive voltage waveform applied to the positive electrode 19 of the piezoelectric element according to the present invention. Step A to Step E are the first step, and Step F to Step G are the second step. In step A, the meniscus is pulled in, in step B, the voltage is held for a certain period, and in step C, the meniscus is pushed out to generate ink columns. In step D, the voltage is held for a certain period of time, and in step E, the meniscus is drawn, and the amount of ink columns to be ejected is reduced to form ultra-thin ink columns. In order to obtain an extra fine ink column, the time Dt of the process D is desirably about 4 μs or less. In step F, the voltage is maintained for a certain period, and in step G, an ink column that is pushed out of the nozzle and returns to the nozzle is generated.

  The very fine ink columns (FIGS. 6 (2) and 7 (3)) generated in the first step are separated into minute ink droplets with the passage of time (FIG. 7 (4)), and the separated minute ink is separated. The droplet starts to fly (FIGS. 6 (3) and 7 (5)). On the other hand, the ink column existing on the nozzle side from the minute ink droplets starts to fly as a small ink droplet or a plurality of ink droplets of small ink droplets and minute ink droplets, but the ink generated by the second step It catches up with the ink column or ink droplet that is closer to the nozzle opening 14 than the top minute ink droplet to fly by the column (Fig. 6 (4), Fig. 7 (7) to (8)), By pulling them back into the nozzle opening 14 together (FIGS. 6 (5) and 7 (9)), it is possible to fly only the minute ink droplets separated from the head of the ultrafine ink column (FIG. 6 (6)). FIG. 7 (10)).

  The time Ft of the process F, the time Gt of the process G, and the voltage Gv of the process G are applied to the minute ink droplets that are separated from the head of the extra fine ink column generated by the first process. To the extent that it cannot catch up, the generation timing of the ink column generated by the second step is increased by increasing Ft, or the volume velocity of the ink column generated by the second step is decreased by increasing Gt and decreasing Gv. In addition, after the head of the ultra fine ink column generated in the first step is separated into fine ink droplets, they catch up with the ink columns or ink droplets existing on the nozzle opening 14 side of the fine ink droplets, and merge. The Ft is shortened so that the ink column generated in the second step is generated earlier, and the volume velocity of the ink column generated in the second step is increased by shortening Gt and increasing Gv. To do. By setting Ft, Gt, and Gv that satisfy both of these conditions, the present invention is established.

  FIG. 9 is a diagram illustrating a relationship between the time Gt of the process G and the voltage Gv of the process G. If the time Gt is too long, the ink column generated in the second step cannot catch up with the ink column or ink droplet to fly from behind the minute ink droplet formed in the first step, and can be pulled back into the nozzle opening. However, the ink column or ink droplet which is going to fly from behind the minute ink droplet formed in the first step becomes a small ink droplet and flies. If the time Gt is too short, the ink column generated in the second step also flies to become a large ink droplet, or catches up to the head of the ultra fine ink column generated in the first step and merges it back into the nozzle opening. As a result, ink droplets cannot be discharged. If the voltage Gv is too small, the ink column generated in the second step cannot catch up with the ink column or ink droplet that is about to fly from behind the minute ink droplet formed in the first step, and cannot be pulled back into the nozzle opening. Thus, the ink column or ink droplet that is about to fly from behind the minute ink droplet formed in the first step becomes a small ink droplet and flies. If the voltage Gv is too large, the ink column generated in the second step will also fly and become a large ink drop, or catch up to the head of the ultra fine ink column generated in the first step and merge into the nozzle opening. It will be pulled back and the ink droplet will not fly. A hatched portion in FIG. 9 indicates an appropriate area. By appropriately setting the time Gt and the voltage Gv of the process G, the ink that is present on the nozzle opening 14 side of the minute ink droplet after the head of the ultrafine ink column generated in the first process is separated into the minute ink droplets. Since the ink column generated in the second step catches up with the column or ink droplet and merges with the column or ink droplet, the ink column is pulled back into the nozzle opening 14, so that only a minute ink droplet can fly.

  The appropriate region in FIG. 9 falls below the graph when the time Ft of the process F becomes short, and rises above the graph when the time Ft of the process F becomes long. Further, the width of the appropriate region also changes depending on the value of the time Ft in the process F, and the appropriate region may disappear if Ft is too long.

  In the present invention, the time A of the process A = 2.8 μs, the time Bt of the process B = 2.2 μs, the voltage Cv of the process C = 23 V, the time Ct of the process C = 2.2 μs, the time D of the process D = 0 t. An ink having a viscosity of 10 mPa · s and a surface tension of 31 mN / m with a driving voltage waveform of a process Et time Et = 2 μs, a process F time Ft = 0 μs, a process G voltage Gv = 23 V, and a process G time Gt = 2 μs. Using a nozzle having a diameter of 28 μm, a 0.2 pl micro ink droplet could be stably fly at a distance of 1.5 mm from the nozzle surface at 7 m / s.

  FIG. 4 shows still another embodiment of the drive voltage waveform applied to the positive electrode 19 of the piezoelectric element according to the present invention. Step A to Step C are the first step, and Step D to Step E are the second step. In step A, the meniscus is pulled in, in step B, the voltage is held for a certain period, and in step C, the meniscus is pushed out to generate a thin ink column. In step D, the voltage is maintained for a certain period, and in step E, an ink column that is pushed out of the nozzle opening 14 and returns to the nozzle opening 14 is generated.

  The thin ink column (FIGS. 6 (2) and 7 (3)) generated in the first step is separated into fine ink droplets with the passage of time (FIG. 7 (4)), and the separated micro ink is separated. The droplet starts to fly (FIGS. 6 (3) and 7 (5)). On the other hand, the ink column present on the nozzle side of the minute ink droplets starts to fly as a medium ink droplet or a plurality of ink droplets of medium ink droplets and small ink droplets, but the ink generated in the second step It catches up with the ink column or ink droplet existing on the nozzle side with respect to the minute ink droplet to fly by the column and merges (FIG. 6 (4), FIGS. 7 (7) to (8)), and the nozzle opening together. 14 (FIG. 6 (5), FIG. 7 (9)), it becomes possible to fly only the minute ink droplets separated from the head of the ultrafine ink column (FIG. 6 (6), FIG. 7 (FIG. 7). 10)).

  The time Dt of the process D, the time Et of the process E, and the voltage Ev of the process E are applied to the minute ink droplets that are separated from the head of the ultra fine ink column generated by the first process. To the extent that it cannot catch up, Dt is lengthened to delay the generation timing of the ink column generated by the second step, or Et is lengthened and Ev is decreased to decrease the volume velocity of the ink column generated by the second step. In addition, after the head of the extra fine ink column generated in the first step is separated into fine ink droplets, they catch up with the ink columns or ink droplets existing on the nozzle side with respect to the fine ink droplets, and merge. The Dt is shortened so that the ink column is generated in the second step, and the generation timing of the ink column generated in the second step is shortened, or the Et column is shortened and Ev is increased to increase the volume velocity of the ink column generated in the second step. . The present invention is established by setting Dt, Et, and Ev that satisfy both of these conditions.

  FIG. 8 is a diagram illustrating a relationship between the time Et of the process E and the voltage Ev of the process E. Similarly to the first embodiment, the time Et and the voltage Ev of the process E are set so as to be within the appropriate region of FIG.

  The appropriate region in FIG. 8 falls below the graph when the time Dt of the process D becomes short, and rises above the graph when the time Dt of the process D becomes long. In addition, the width of the appropriate region also changes depending on the value of the time Dt of the process D. If the Dt is too long, the appropriate region may be lost, and if it is too short, the appropriate region may be lost.

  In the present invention, at least the periphery of the nozzle opening 14 on the surface of the nozzle plate 13 is used by using ink with high wettability so that the contact angle with respect to the ink is 30 degrees or less, or by performing a parent ink treatment on the nozzle surface. It is easier to collect ink around the nozzle opening 14. The reason will be described below.

  11 and 12 are explanatory diagrams showing the difference in the behavior of the ink depending on the presence or absence of the ink reservoir 55. 11 shows a case where there is an ink reservoir 55 around the nozzle opening 14, and FIG. 12 shows a case where there is no ink reservoir around the nozzle opening 14. 11 and 12 show a state in the vicinity of the nozzle opening 14 when only the driving waveform of the first step of the present invention is applied.

  As shown in FIGS. 11A to 11E and FIGS. 12A to 12E, the fine ink droplets 50 and 60 formed in the first step are meniscus pulled into the nozzle opening 14. 11 (c), (d), FIG. 12 (c), and (d)), the minute ink droplets 50 and 60 are formed regardless of the presence or absence of ink accumulation around the nozzle opening 14. Are discharged. That is, the minute ink droplets 50 and 60 are hardly affected by the accumulated ink, and even if the ink reservoir 55 exists, it flies in the same manner as when the ink reservoir does not exist.

  On the other hand, the behavior differs greatly depending on the presence of the ink column and the ink reservoir 55 that are to fly from behind the minute ink droplets 50 and 60 formed in the first step. When the ink reservoir 55 is present, as shown in FIGS. 11F to 11H, ink is supplied from the accumulated ink to the ink column 51 and the accumulated ink is stored. Since the ink column 51 is pulled by viscosity, it is difficult for the ink column 51 to be separated from the ink on the nozzle opening 14 side, and as a result, the ink column 51 is difficult to be ejected as an ink droplet. On the other hand, when there is no ink reservoir, the ink column 61 is easily separated from the ink on the nozzle opening 14 side and discharged as shown in FIGS. . Therefore, when the ink reservoir 55 is present, the limit point (the range of the appropriate region shown in FIGS. 8 and 9) that can be pulled back by the ink column generated in the second step is wider than when the ink reservoir is absent. That is, FIG. 11 (h) shows a limit point at which the ink column or ink droplet that is about to fly from behind the minute droplet can be pulled back in the second step when the ink reservoir exists. FIG. 12 (g) shows the limit points at which ink columns or ink droplets that are about to fly from behind the minute droplets when they do not exist can be pulled back in the second step. Assuming that the distance to the head of the ink column or ink drop is h and h ′, h> h ′, and the ink reservoir can be pulled back farther. Also with respect to the passage of time, FIG. 11 (h)> FIG. 12 (g), and there is an opportunity for the ink reservoir to be pulled back later.

  As described above, by providing an ink reservoir around the nozzle opening 14 with a contact angle of 30 degrees or less with respect to the ink around the nozzle opening 14, fine droplets that are desired to be ejected can be ejected without any problem. Ink columns or ink droplets that are about to fly are easily pulled back by the second step. In addition, when the contact angle is 30 degrees or more, an ink reservoir suitable for the present invention does not occur. That is, if the contact angle is large, the ink pool is biased, and the ink droplet ejection direction is bent or non-ejection.

  As described above, when continuous ejection is performed so that the peripheral portion of the nozzle opening 14 has a contact angle of 30 degrees or less with respect to the ink, the ink oozes out and the ink may be accumulated to the extent that the ejection failure occurs. In order to avoid this, as shown in FIG. 13, it is preferable to provide a barrier 70 around the nozzle opening 14 to suppress the wetting and spreading of the ink like the ink reservoir A or the ink reservoir B.

  In each of the above-described embodiments, the drive voltage waveform using the displacement of the piezoelectric element in the vertical direction (d33 mode) for ejection has been described. However, the displacement of the piezoelectric element in the horizontal direction of the electrode (d31 mode) is used for ejection. The present invention can also be applied to an inkjet head or an inkjet head that uses a displacement in a deflection mode (such as a shear mode or a bend mode) for ejection.

  In addition, the present invention has described a method for driving an ink jet head that discharges ink droplets by generating pressure by expansion and contraction of a piezoelectric element. However, the ink body is removed from the ink body by a method such as bubble expansion force, electrostatic force, or magnetic force. Thus, the present invention can be applied to all ink droplet ejection devices that fly.

  Furthermore, in the present invention, when a physical property such as the viscosity of the ink changes, a mechanism for changing the control amount and the control timing of the second step is provided so that the control is always performed so that the ink enters the appropriate region. It is desirable to provide a temperature adjustment mechanism and keep the temperature of the ink substantially constant so that physical properties such as viscosity do not change so much.

  For example, if the physical properties of the ink change, even if the same drive waveform is used, there is a possibility that it will deviate from the appropriate region shown in FIG. In order to prevent this, as shown in FIG. 14, a configuration for changing the drive voltage waveform in accordance with the change in the viscosity of the ink may be added. Specifically, a waveform table is provided in an FPGA (Field Programmable Gate Array) in a drive voltage generation circuit of a print control unit that performs print control on the printer side. In this waveform table, ink at each predetermined temperature is stored. Data defining the relationship between physical properties and drive voltage waveforms is stored in advance (the waveform table may be stored in ROM). The drive voltage waveform is digitized and stored in the waveform table. The print data developed by the host device is input to the FPGA in the drive voltage generation circuit of the print control unit. In the FPGA, the drive waveform of the print data is output to the positive electrode 19 of the piezoelectric element of the print head section via a drive voltage generation circuit described later. On the other hand, print data relating to necessity of printing is output directly from the FPGA to the drive nozzle selection circuit. Thereby, it is possible to discharge ink while controlling to an optimum driving waveform.

  Hereinafter, control in the drive voltage generation circuit will be described. Since the viscosity of the ink varies depending on the temperature of the ink, a thermistor is attached to the inkjet head, and data relating to the temperature of the inkjet head, that is, the temperature of the ink is input to the FPGA. In the FPGA, the temperature data input from the thermistor is compared with the waveform table held in advance, the drive voltage waveform is determined from the waveform table, and is output to the D / A converter as several bits of digital parallel data. . The D / A converter converts the input digital parallel data into a continuous current change and outputs it to the current / voltage converter. Furthermore, the current change is converted into a voltage change by a current / voltage converter, and the voltage change is amplified to a desired voltage and current by an amplifier, and then output to the print head unit.

  In the control circuit described above, the drive voltage waveform may be controlled so that the voltage change amount is increased when the ink temperature is decreased, and the voltage change amount is decreased when the ink temperature is increased.

  Alternatively, as shown in FIG. 15, there is a method of controlling the temperature change of the ink jet head to be small so as not to change the physical properties of the ink so that it does not deviate from the appropriate region shown in FIG. The basic control circuit is the same as that shown in FIG. 14, but in FIG. 15, the FPGA waveform table is replaced with a temperature comparator, and a Peltier element is mounted on the print head. Then, the temperature data obtained by the thermistor attached to the inkjet head is compared with the set temperature by a temperature comparator. If the temperature data is higher than the set value, the temperature of the Peltier element attached to the inkjet print head is lowered in the direction in which the head cools. If the temperature is lower than the set temperature, the operation is performed so that the temperature moves in the direction in which the head is heated, and the temperature of the ink jet head and thus the physical properties of the ink are controlled to be constant.

  In the present invention, after the minute ink droplets are separated from the head of the ink column (FIG. 5 (3)), at least one ink droplet larger than the minute ink droplet separated from the head portion flies back. When applied in such a way (FIGS. 5 (4) and 5 (5)), large ink droplets can be collected in the nozzle openings 14, and the flying ink droplets are small. The effect can be enhanced. In order to make such a jump, it is effective to make the ink column generated in the first step thinner. The reason is that since the surface area per unit volume is large, the ink column tends to be a sphere, and the minute ink droplets are separated at the head of the ink column. Further, when the viscosity or surface tension of the ink is increased, the ink column extends without being cut off in the middle, and tends to be a sphere at the tip of the ink column, so that such a fly is easily obtained.

1 is a perspective view illustrating a structure of an inkjet head to which the present invention is applied. The figure which shows the drive voltage waveform which becomes an example of this invention. The figure which shows the drive voltage waveform used as the other example of this invention. The figure which shows the drive voltage waveform used as the other example of this invention. FIG. 2 is an explanatory diagram showing a state of ink droplet ejection when a conventional drive voltage waveform is applied to the inkjet head of FIG. 1. FIG. 2 is an explanatory diagram showing how ink droplets are ejected when the drive voltage waveform of the present invention is applied to the inkjet head of FIG. 1. FIG. 6 is an explanatory diagram showing another example of how ink droplets are ejected when the drive voltage waveform of the present invention is applied to the inkjet head of FIG. 1. The figure which showed the example of the setting area | region of the drive voltage at the time of applying this invention in Example 1 and 3, and time. FIG. 6 is a diagram showing an example of setting regions of drive voltage and time when the present invention is applied in the second embodiment. The figure which shows the conventional drive voltage waveform. Explanatory drawing which shows the behavior of the ink when there is an ink reservoir around the nozzle opening. FIG. 4 is an explanatory diagram showing the behavior of ink when there is no ink reservoir around the nozzle opening. The perspective view of a section showing the modification of the composition near the nozzle opening of the present invention. The control block diagram which shows an example of the mechanism which changes the control timing of a piezoelectric element. The control block diagram which shows the structure at the time of providing a temperature control mechanism.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Ink flow path forming member 12 Ink pressure chamber 13 Nozzle plate 14 Nozzle opening 15 Restrictor 16 Common ink flow path 17 Piezoelectric element 18 Piezoelectric element support substrate 19 Positive electrode 20 Negative electrode 21 Elastic film 22 Ink droplet

Claims (7)

An ink pressure chamber and an ink pressure-increasing / decreasing unit are provided, and an electric signal is applied to the pressure-increasing / decreasing unit to increase / decrease the ink in the ink pressure chamber, whereby an ink droplet is discharged from a nozzle opening communicating with the ink pressure chamber. In the ink jet head driving method for discharging ink, a first step in which the tip of the generated first ink column separates to form a minute ink droplet, and a second step of pushing out from the nozzle opening and returning to the nozzle opening again . And a second step of controlling the ink volume velocity of the ink pressure chamber so as to generate an ink column, catch up with and merge with the ink column or ink droplet separated from the minute ink droplet, and pull back into the nozzle opening. A method for discharging fine ink droplets.   2. The method for ejecting micro ink droplets according to claim 1, wherein the first step includes a step of drawing the meniscus abruptly so that the meniscus rebounds.   2. The method for ejecting micro ink droplets according to claim 1, wherein the first step includes a step of drawing the meniscus abruptly so as to cause the meniscus to rebound, and a step of further drawing the meniscus.   2. The method for ejecting micro ink droplets according to claim 1, wherein the first step includes a step of drawing a meniscus, a step of pushing out, and a step of drawing the meniscus again.   5. Ink droplet ejection according to claim 1, wherein an ink droplet is ejected by using an inkjet head having a nozzle plate having a nozzle plate having a contact angle of 30 degrees or less with respect to the ink at least around the nozzle opening. Method.   6. The method for ejecting micro ink droplets according to claim 1, wherein the control amount and control timing of the second step are changed in accordance with a change in ink viscosity. 6. The method for ejecting fine ink droplets according to claim 1, wherein a temperature adjusting mechanism is provided to keep the temperature of the ink substantially constant.

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