JP2658244B2 - Ultrasonic generator for inkjet printhead - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-reliability and low-cost ultrasonic generator for a multi-nozzle ink jet print head capable of stably generating ink droplets.

2. Description of the Related Art Various types of inkjet recording printers that eject ink from a large number of thin nozzles, control ink droplets, and perform direct printing have been proposed and put into practical use.

A so-called charge-amount-controlled printer that continuously ejects ink from a large number of nozzles, controls the amount of charge, performs recording, collects unnecessary ink, and reuses it. It is most frequently used along with the on-demand type in which an ink droplet is applied and ejected.

 FIG. 15 shows an outline of the printer of the charge amount control system.

High pressure (3-4 kg / cm ² ) ink is supplied to the nozzle
And the nozzle is vibrated by the ultrasonic vibrator 31 (several tens KHz to several hundred KH).
z), the ink flow is separated on the way to ink drops 8. When a voltage (recording signal) of various values is applied to the charging electrode 27 at the moment of this separation, a charge corresponding to the voltage is given to the separated ink particles, and the deflection electrode 28 (constant voltage) is applied.
Gives a deflection in accordance with the charge amount of the particles, and the recording direction is recorded on the paper 29 by swinging the flight direction of the particles up and down or left and right. Since the ink flies continuously, unnecessary ink is collected and reused.

[Problems to be Solved by the Invention] In such a conventional charge control type ink jet print head, since the vibrator is provided behind the nozzle, the ultrasonic vibrator makes the excitation ultrasonic wave uniform over the entire ink jet width. It is difficult to apply the ink droplets, the separation length of the ink droplets is not constant, and the variation in the voltage region that does not generate satellite particles that hinder proper charging of each particle becomes large between each jet. There is a problem that it is difficult to control the charge amount.

Means for Solving the Problems Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art, and has as its object to generate ink droplets given to a large number of nozzles. It is an object of the present invention to provide an ultrasonic generator for an ink jet print head capable of generating ink droplets suitable for printing by making the separation length of ink droplets uniform by keeping the ultrasonic pressure constant.

That is, the present invention has an ink flow path along the longitudinal direction inside, and is attached to one end of an ink jet print head body having a plurality of nozzles communicating with the ink flow path on the side surface thereof, and the ink in the ink flow path is provided. In an ultrasonic generator for an ink jet print head for separating ink jetted from the nozzles into ink droplets by applying ultrasonic vibration to the nozzles, an ultrasonic wave attached to one end of the ink jet print head body A pair of piston members that sandwich the piezoelectric vibrator and that one of the pair of piston members is located inside the ultrasonic generator block that communicates with the ink flow path of the ink jet print head body. Attach the piston member to the ultrasonic generator block and The wave generator block is provided with an ink inflow passage for supplying ink into the ink flow path of the ink jet print head body, and is configured to apply ultrasonic waves to the ink flow path through the piston member to propagate a traveling wave. Have been.

[Operation] In the present invention, a voltage is applied to the piezoelectric vibrator held between the piston members, and the piezoelectric vibrator generates ultrasonic vibration. Then, the ultrasonic vibration generates a traveling wave in the ink in the ink flow path through a piston member disposed in the ink flow path. Since the traveling wave of the ultrasonic wave has a uniform period and the sound pressure of the ultrasonic wave with respect to each jet portion, the ultrasonic wave is uniformly applied to the ink jetted from the nozzle of the print head main body, Ink droplets can be generated stably.

Further, since the present invention is configured so that the ultrasonic generator block is attached to one end of the ink jet print head main body, it is not necessary to directly mount the ultrasonic generator on the ink jet print head main body. A cylindrical member or the like can be used as the ink jet print head body, a plurality of nozzles can be formed directly and easily on the ink jet print head body by electric discharge machining or the like, and the cost of the head body can be reduced. .

Further, the present invention is configured such that an ultrasonic generator block is attached to one end of an ink jet print head main body, and the ultrasonic generator block is provided with an ink inflow passage for supplying ink into an ink flow path of the ink jet print head main body. As compared with the case where the ink inflow passage is provided in the ink jet print head main body, the ink flowing into the ink flow path of the ink jet print head main body is stabilized, and the traveling wave of the ultrasonic wave in the head main body is stabilized. It is possible to effectively prevent the ink droplet from becoming unstable at the end of the head.

Embodiment 1 First Embodiment The present invention will be described below based on an illustrated embodiment.

FIG. 1 shows an ink jet print head to which an ultrasonic wave generator for an ink jet print head according to the present invention can be applied. In the figure, H is an ink jet print head, and this print head H
Ink is introduced from a pump (not shown) through the introduction hole 2, and a large number of nozzles 4 are provided along the longitudinal direction.
Ink is jetted from 4... In the form of a jet 7 to be separated into ink droplets 8 and fly as described later. This ink drop 8
Is charged by a charging electrode (not shown) in response to an image signal, is deflected by an electric field generated by a deflecting electrode (not shown), and flies onto a recording sheet to record an image. I have. In the drawing, reference numeral 6 denotes a discharge hole for discharging ink as required.

Further, the ink jet print head H is configured as follows. As shown in FIG. 2, the inkjet print head H includes a cylinder 1 as a print head body made of a square material, and the cylinder 1 has a cross-section as shown in FIG. A circular ink flow path R is formed in a penetrating manner by gun drilling or the like. The cylindrical body 1 is provided with a number of small holes 3, 3,.
Penetrates to the ink flow path R. The small holes 3, 3
Are provided, for example, at a pitch of 3 mm. further,
A nozzle plate 5 is fixed on the surface of the cylindrical body 1 by means such as bonding, and the nozzle plate 5 is provided with nickel electroforms at positions corresponding to the small holes 3, 3,. And the like. As the nozzle plate 5, for example, 600 dots / inch (24 dots / mm)
Those having considerable nozzles 4, 4,... Are used.

As shown in FIG. 2, a driving device (driver) 10 and an ultrasonic absorbing member (terminator) 9 are provided at both ends of the cylindrical body 1.
Are provided respectively. Then, ultrasonic waves are generated by the driving device 10, the traveling waves of the ultrasonic waves are propagated in the ink flow path R, and ultrasonic vibrations are applied to the ink jets 7 ejected from the nozzles 4, 4,. The ink droplet 8 is generated by being separated into droplets. The ultrasonic wave propagated in the ink flow path R is absorbed by the ultrasonic absorbing member 9 so that no reflected wave is generated.

FIG. 3 shows the above driving device as one embodiment of the ultrasonic generator according to the present invention. This drive 10
In the same manner, disk-shaped PZT ceramics 13 and 13 and cylindrical pistons 12 and 12 made of stainless steel are arranged at both ends of a disk-shaped electrode 11 so as to sandwich the electrode, and these members are bonded or screwed. They are configured to be pressed and fixed to each other by means such as stoppers.

A driver block 15 for mounting is sandwiched between one of the PZT ceramics 13 and the piston 12, and
The driver device 15 is attached to the end of the cylinder 1 by holding the driver block 15 between the end of the cylinder 1 and an attachment member (not shown). As a result, one piston 12 (the right one in FIG. 3) is arranged in the ink flow path R. At this time, the driving device 10 performs centering of the piston 12 and prevention of ink leakage by the O-ring 14 fitted to the outer periphery of the piston 12.

On the other hand, an ultrasonic absorbing member (terminator) 9 is provided at the other end of the cylindrical body 1. This ultrasonic absorbing member 9
Is formed of a cylindrical silicon rubber, and the tip 9b is formed in a knife-edge shape so that reflected waves are hardly generated. Then, the ultrasonic wave is absorbed by the ultrasonic wave absorbing member 9 and the ultrasonic wave is absorbed so that a reflected wave is not generated and a standing wave is not generated in the ink flow path R.

The material of the cylinder 1 is preferably as acoustically hard as possible so as not to absorb sound pressure (for example, stainless steel, nickel alloy, etc.), and the terminator 9 is preferably acoustically soft so as not to reflect ultrasonic waves as much as possible. .
The tip shape of the terminator 9 is preferably such that the incident angle of the sound wave is as small as possible as shown in FIG.

Another example of the cylinder 1 is shown in FIGS. 4 (a) and (b).

FIG. 4 (a) shows an integrated molding by micro-punching, which does not require bonding of a nozzle plate, and FIG. 4 (b) shows a configuration in which a nozzle plate 5 made of nickel electroform is bonded to a tubed tube 1. Therefore, gun drilling is not required. In addition, combinations of these may be considered. The inside of the cylindrical body has a circular cross section so that sound waves can be easily propagated, but some deformation is of course allowed.

Similarly, as for the terminator 9, a resin terminator block 9a having a structure bent at the end of the cylindrical body as shown in FIG. 5 can be used in order to effectively use the print head width.

Further, instead of performing the jet ejection by the ink pressurization, it is also possible to perform the ink droplet generation by replacing the pressurization with suction air suction or the like.

With the above configuration, the ultrasonic generator according to this embodiment generates an ultrasonic wave as follows. That is, as shown in FIG. 2, the electrode 11 of the driving device 10
High frequency voltage is applied by the power supply G, and PZT ceramic 1
Ultrasonic vibration is generated by 3 and 13. The ultrasonic vibration is such that the PZT ceramics 13 and 13 vibrate in the thickness direction, and the vibration of the PZT ceramics 13 and 13 is
Propagated to 2, 12, the pistons 12, 12 vibrate in the shape of a piston, as in characters. Therefore, the above ultrasonic vibration
Propagating as an ultrasonic traveling wave to the ink in the ink flow path R via 12 and 12, the jet-shaped ink 7 ejected from the nozzles 4, 4,. It is made to let.

The ultrasonic vibration propagated in the ink flow path R to the other end is absorbed by the ultrasonic absorbing member 9 and
Inside, no reflected wave is generated.

As described above, the driving device 10 generates the ink droplets 8, 8 by applying the ultrasonic traveling wave to the ink in the ink flow path R. By the way, the traveling wave of the ultrasonic wave makes the acoustic shape of the ink flow path R uniform,
Since the period and the acoustic intensity can be kept constant with high accuracy, the pressure of the ultrasonic wave applied to the ink jet 7 ejected from the nozzles 4, 4,... Can be made uniform. .. Can be separated stably and uniformly.

Next, the present inventors actually manufactured this device as a prototype and performed an experiment for generating ink droplets. FIG. 6 shows the result of observing the operation (the relationship between the jet number and the ink droplet separation length).

In FIG. 6, the separation length on the vertical axis is the length from the ink jet to the separation of the ink droplet as shown in FIG. (That is, one wavelength (λ): see FIG. 7 (b)).

As shown by the solid line in FIG. 6, the dispersion of the separation length was 2 wavelengths or less, and it was found that the conventional device had sufficient superiority to the dispersion of about 5 wavelengths. Such a small variation in the separation length means that the charging efficiency can be sufficiently compensated, which leads to a reduction in printing disturbance, and the thickness of the charging electrode can be reduced, so that the charging electrode can be far from the nozzle electrode, Therefore, troubles due to a short circuit between the nozzle plate and the electrode can be reduced.

In addition, when the driving frequency was changed for one jet to see the feature of using only traveling waves,
As shown by the solid line in FIG. 8, it was confirmed that the sound pressure of the ultrasonic wave calculated from the ink droplet separation length had a stable region in a specific frequency range (150 KHz to 300 KHz). The fact that the sound pressure is stable against a frequency change means that the instability of the ink droplet generation state, which is considered to be caused by a change with time of the physical properties of the ink and a change in the environment, which has been a problem in the past, has been solved.

Further, when the phase angle of the ink droplet separation point was measured in order to grasp the physical properties of the present example and plotted for each jet, good linearity was exhibited as shown in FIG. When the sound speed was calculated from the relationship between this angle and the nozzle pitch, it almost coincided with the sound speed in water generally reported.

Therefore, it was also confirmed that the expected one-way sound wave propagation and sound wave absorption by the terminator were realized.

Since there is a linear relationship between the jet nozzle portion and the phase as shown in FIG. 9, the timing of ink droplet generation at each nozzle can be predicted, so that phase detection at each nozzle portion is not required and accurate It has become possible to easily perform charging control.

When the sound attenuation in the ink jet nozzle direction was measured, the value was 4 db or less for 300 mm in all cases. Therefore, it was confirmed that the acoustic non-uniformity due to the sound attenuation in the jet array direction was sufficiently within the allowable value (12 db).

FIGS. 10 and 11 further show the configuration of the first embodiment. In the first embodiment, an ultrasonic generator block is attached to one end of an ink jet print head body, and a piezoelectric vibrator is held therebetween. The pair of piston members is attached to the ultrasonic generator block such that one piston member is located inside the ultrasonic generator block communicating with the ink flow path of the ink jet print head body. In addition, the ultrasonic generator block is provided with an ink inflow passage for supplying ink into the ink flow path of the ink jet print head body. It is generally difficult to integrally form a small-diameter nozzle on a square body. However, as a method of forming a nozzle directly on a cylindrical tube, a method of forming the nozzle on a stainless steel cylindrical tube by electric discharge machining (FDM) is possible. is there.

Reference numeral 1 denotes an ink flow path and nozzle tube (tubular body). An ultrasonic signal is input to a vibrator built in the ultrasonic generator block 17 to generate a sound wave for drop generation. The ink flowing from the ink inflow path is ejected from each of the nozzles 4, 4,. The jets ejected from the nozzles 4, 4,... Generate ultrasonic ink vibrations, and the jets generate ink droplets according to the initial vibrations due to the propagation of sound waves in the direction of the center axis of the tube (from left to right in the figure). After arriving at the right end, the sound wave is absorbed by the terminator in the sound wave absorber block 18, and has a structure with almost no reflected wave. The cross section of the sound wave propagation path is circular in order to minimize attenuation of sound wave propagation. In other words, forming the ink flow path R with a cylindrical tube is consistent with the advantage that the acoustic wave is efficiently propagated in addition to the ease of manufacturing.

The cylindrical tube 1 may be provided with a nozzle array formed by electroforming, for example, by taking measures against ink corrosion.

In the example of Fig. 10, the ultrasonic wave source
Although PZT is used, it is also possible to use a material other than PZT, for example, a polymer piezoelectric material. A drop generation method using an electrostatic field is also possible. In addition, if proper charging can be performed in a state where the drop generation varies, the ultrasonic absorbing material is not required.

In this example where the nozzle is formed directly on the cylindrical tube, the degree of freedom of rotation of the tube, if the jet directionality is biased up or down as a whole, it is possible to correct by rotating the tube, This is a feature not found in the flat nozzle.

FIG. 12 shows the configuration of the ultrasonic generator block. The ultrasonic generator block 17 is attached to one end of the cylindrical tube 1. The cylindrical tube 1 is fitted and fixed to an ultrasonic generator block 17, and the tip of the cylindrical tube 1 is
The outer periphery of the outer periphery is prevented by an O-ring 19 from leaking ink with the block 17. The driving device 10 is attached to an end of the ultrasonic generator block 17. The driving device 10 is configured such that the electrodes 11 are made of PZT ceramics 13, 13 and the piston 1
These members are connected and fixed by connecting screws 23 in a state of being sandwiched by 2 and 12.

In the drive device 10, a flat PZT presser 22 is held between the piston 12 and the PZT ceramic 13, and the PZT presser 22 is held between the piston 12 and the ultrasonic generator block 17 by the rear block 21. The rear block 21 is attached to the ultrasonic generator block 17 by screwing with the rear screw 24. Further, the driving device 10 includes a PZT presser 22.
O-ring interposed between the ultrasonic generator block 17
19 prevents the ink from leaking and centers the piston 12. Further, an ink introduction hole 2 is formed in the ultrasonic generator block 17, and the ink introduced from the ink introduction hole 2 is supplied into the space 1 between the cylinders through the outer periphery of the piston 12. It has become.

By doing so, the piston 12 is configured so as not to touch the cylindrical tube 1 and the O-ring 14 for free oscillation. A driving ultrasonic wave is supplied to the high-voltage electrode 11 sandwiched between the sandwich-shaped PZT ceramics 13, 13, and the piston 12 applies drop generation vibration to the ink in the cylindrical tube 1.

When the pressurized ink was supplied from the ink circulation system and the jet was jetted, the directivity was within ± 0.5 degrees over 100 jets. In addition, when the variation in drop separation length was observed, the maximum and minimum differences were within 2 wavelengths for 100 jets (wavelength = drop speed /
Generation frequency). Therefore, the jet direction characteristics equivalent to the head constituted by the gun drill and the flat nickel electroform nozzle described in the first embodiment,
And the ink droplet formation characteristics were obtained.

Second Embodiment When charging ink droplets by the ink-jet printing method, if satellite particles are generated, the particles may be combined with other ink droplets, thereby preventing proper charging.
To suppress the generation of satellite particles, there is a method of lowering the viscosity of the ink, but this method leads to deterioration of the ink characteristics. It is conceivable to raise the temperature of the ink for the same purpose, but the apparatus becomes complicated. In addition, when these measures are taken, the timing of ink droplet separation from the ink jet becomes earlier, and therefore the drop separation length becomes shorter. This necessitates bringing the charging electrode for charging closer to the head, and tends to cause a short circuit between the charging electrode and the nozzle portion.

Therefore, in this embodiment, the piezoelectric element is driven by an ultrasonic wave in which a second or higher harmonic wave (harmonic wave) is superimposed on the basic waveform, the ink droplet separation length is kept long, and the charging plate is separated as much as possible. As a state, generation of citrite was suppressed.

As a piezoelectric element, for example, polyvinylidene fluoride (PV
The conventional inkjet printhead that excites from behind the nozzle using F ₂ ) or the like uses a standing wave, so controlling the applied ultrasonic waveform is not very effective in suppressing satellite particles. However, in this example, since the sound wave is propagated in only one direction, it is very effective to superimpose the second or higher harmonics on the oscillator driving basic waveform to suppress satellite particles. In addition, there is an advantage that it can be driven at a low voltage.

As the inkjet print head H, a multi-nozzle print head having the same configuration as that shown in FIG. 1 and having 100 holes corresponding to 600 dots / inch at a pitch of 3 mm was used.
The drop formation was observed using a 1.8 cps ink at a viscosity of 25 ° C. Use a sine wave for the basic drive waveform, 200K
When driven at Hz, the satellite obtained the state where the satellite merged with the preceding drop at the vibrator drive voltage of 150 V _PP , but the distance was 3λ to 4λ from the ink drop separation point. It was not possible to obtain a state in which charging could be performed (FIG. 13 (a)).

The drop separation length at this time was about 0.7 mm.

Then, when the ink was heated to a viscosity of 1.3 cps at 35 ° C., a state without satellite was obtained as shown in FIG. 13 (b). However, the separation length became 0.5 mm, and the charge leaked to the nozzle plate. It became easier. When the second harmonic was superimposed again at 25 ° C. in the state shown in FIG. 13A, a satellite-free state was obtained at a vibration drive voltage of 100 V _PP and a separation length of 0.9 mm. This is a phenomenon that cannot be realized in a conventional print head having a structure in which a second harmonic using a standing wave is canceled out, and is first realized in the present configuration. Control of satellites based on forced cutting of drops by applying a second harmonic has been reported previously (Japanese Patent Publication 55
No. 43713), the second harmonic is superimposed, but it was found that the application of the third harmonic is more preferable in this embodiment.

Specifically, the control of the satellite for one jet is the second or third for the fundamental wave.
This is achieved by adjusting the phase difference and intensity of the higher harmonics. Observing other jets by adjusting the harmonics of a certain jet to make it non-satellite, it was found that the satellite combined with the fundamental wave only at 3 to 4λ was 4 to 5λ.
It turned out that there was something that was rather degraded.
Observation of all jets shows that such an undesired state repeats as 7 jet cycles when the second harmonic is applied and 10 jet cycles when the third harmonic is applied (see FIG. 14). Therefore, this phenomenon is considered to be repetition due to a difference in phase difference based on a difference in sound speed in ink between the fundamental wave and the second and third harmonics.

Then, when the temperature of the ink was raised in this state, the satellite coalescence distance of 4 to 5λ was able to be reduced to 0 to 1λ coalescence distance at 35 ° C. due to the decrease in viscosity.

At that time, the drop separation length was about 0.8 mm, the fundamental wave driving voltage was 100 V _PP , and the second harmonic was 50 V _PP . Therefore, it has been found that the separation length is kept long by applying a harmonic, and a satellite-free state is obtained.

Although the multi-nozzle type having 100 jets has been described above, the present invention can be applied to a head having a shorter length. If the jet array is contained in a cycle in an unfavorable state caused by a difference in sound speed, all the jets can be made into a satelliteless state even when the ink viscosity is considerably high, so that a heating device is not required.

[Effects of the Invention] According to the present invention, a piezoelectric vibrator is sandwiched between a pair of piston members, one of the piston members is arranged in an ink flow path, and ultrasonic waves are applied to the ink flow path through the piston member. Since the traveling wave is propagated, the ultrasonic pressure for generating ink droplets given to a number of nozzles is made constant to make the separation length of ink droplets uniform, and ink droplets suitable for printing are generated. be able to.

In addition, since the present invention is configured to attach the ultrasonic generator block to one end of the ink jet print head main body, it is not necessary to directly mount the ultrasonic generator on the ink jet print head main body, and thus has a plurality of nozzles. A cylindrical member or the like can be used as the ink jet print head body, a plurality of nozzles can be formed directly and easily on the ink jet print head body by electric discharge machining or the like, and the cost of the head body can be reduced. .

Further, the present invention is configured such that an ultrasonic generator block is attached to one end of an ink jet print head main body, and the ultrasonic generator block is provided with an ink inflow passage for supplying ink into an ink flow path of the ink jet print head main body. As compared with the case where the ink inflow passage is provided in the ink jet print head main body, the ink flowing into the ink flow path of the ink jet print head main body is stabilized, and the traveling wave of the ultrasonic wave in the head main body is stabilized. It is possible to effectively prevent the ink droplet from becoming unstable at the end of the head.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of an ultrasonic generator for an ink jet print head according to the present invention, FIG. 2 is an exploded sectional view of the same part, FIG.
(A) and (b) are cross-sectional views of a modified example of the cylindrical body, and FIG.
FIG. 6 is a sectional view of a modified example of the terminator section, FIG. 6 is a graph showing the relationship between the ink jet and the ink droplet separation length in the head according to the present invention and the conventional head, and FIGS. 7 (a) and 7 (b) are vertical in FIG. FIG. 8 is a graph showing the relationship between the applied frequency and the sound pressure for the head to which the present invention is applied and the conventional head, and FIG. 9 is a graph showing the relationship between the separation point of the ink droplet and the phase angle. FIG. 10 is a partially exploded perspective view showing a first embodiment of the present invention, FIG. 11 is a front view of FIG. 10, FIG. 12 is a sectional view showing an ultrasonic generator block, and FIG. FIGS. 14A and 14B are explanatory diagrams of the state of ink droplet generation, FIG. 14 is a graph showing the behavior of satellite particles when a second harmonic is applied to the fundamental wave, and FIG. It is a schematic block diagram. [Explanation of Symbols] H ... Inkjet print head R ... Ink flow path 1 ... Cylinder 4 ... Nozzle 7 ... Inkjet 8 ... Ink droplet 10 ... Driving device 12 ... Piston 13 ... PZT ceramic

Claims (1)

(57) [Claims] An ink jet print head having an ink flow path extending in a longitudinal direction therein and a plurality of nozzles communicating with the ink flow path on one side thereof is attached to one end of an ink jet print head body. In an ultrasonic generator for an ink jet print head for separating ink jetted from the nozzle into ink droplets by applying ultrasonic vibration, an ultrasonic wave generator attached to one end of the ink jet print head body A pair of piston members sandwiching the piezoelectric vibrator, so that one of the pair of piston members is located inside the ultrasonic generator block communicating with the ink flow path of the ink jet print head body. While attaching the piston member to the ultrasonic generator block,
Providing an ink inflow passage for supplying ink into the ink flow path of the ink jet print head body in the ultrasonic generator block, and applying ultrasonic waves to the ink flow path through the piston member to propagate a traveling wave. An ultrasonic generator for an ink jet print head, comprising: