JP2004299121A - Piezoelectric inkjet head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel piezoelectric inkjet head which can surely suppress residual oscillation of a driving part with a transverse oscillation mode piezoelectric element while a flying speed decrease of ink droplets is suppressed. <P>SOLUTION: In the piezoelectric inkjet head, a natural oscillation period T<SB>2</SB>of the driving part is set to be approximately 10% of a natural oscillation period T<SB>1</SB>of a volume velocity of the ink inside the head, or smaller than this. More preferably, the natural oscillation period T<SB>2</SB>of the driving part is set to be not larger than 0.1 times of the natural oscillation period T<SB>1</SB>of the volume velocity. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a piezoelectric inkjet head that can be suitably used particularly for a printer, a copier, a facsimile, and a multifunction peripheral thereof.
[0002]
[Prior art]
For example, in a piezoelectric inkjet head that uses an electrostrictive effect of a piezoelectric element as a drive source, which is used for an on-demand type inkjet printer or the like, a driving unit including the piezoelectric element and a vibration plate that supports the piezoelectric element has a piezoelectric element. By transmitting the generated force to the ink in the pressurized chamber as pressure, it plays a role as a drive source for ejecting ink droplets from a nozzle communicating with the pressurized chamber. At the same time, the driving unit also has a role as an elastic body against the vibration of the ink in the head because the vibration plate is bent by receiving the pressure of the ink in the pressurizing chamber.
[0003]
When a voltage is applied to the piezoelectric element to generate a force, the ink in the head vibrates due to the pressure received from the driving unit via the diaphragm. This vibration occurs due to the elasticity of the driving unit and the pressurizing chamber, the supply port for supplying ink to the pressurizing chamber, the nozzle flow path connecting the pressurizing chamber and the nozzle unit, and the nozzle unit as inertia. In this vibration, the natural oscillation period of the volume velocity of the ink in the head is determined by the dimensions of the above-described parts and the physical properties of the ink, and the dimensions and the physical properties of the driving unit.
[0004]
In the piezoelectric inkjet head, ink droplets are generated by using the vibration of the ink meniscus in the nozzle portion due to the vibration of the ink.
Therefore, as described in Patent Document 1, the pulse width of the drive voltage waveform of the drive voltage applied to the piezoelectric element has been conventionally determined according to the natural oscillation period of the ink volume velocity. For example, in a so-called pull driving method, a half of the natural vibration period of the volume velocity of the ink in the head is set as a reference. The pulse width of the drive voltage waveform has been set on the basis of the double.
[0005]
This will be described in further detail with reference to an example of a driving method using a pulling method.
5, the drive voltage waveform of the drive voltage V _P applied to the piezoelectric element in the driving method of the fill-before-fire (indicated by dashed line in bold line), when this driving voltage waveform is applied, the ink in the nozzle unit 6 is a graph showing a simplified relationship with a volume velocity (shown by a bold solid line, where the positive direction is the ink ejection direction).
In the fill-before-fire method of driving methods as seen in the figure, the standby (formerly than t ₁ in FIG. 5), for example in the case of the piezoelectric elements of the lateral vibration mode, the driving voltage V _P applied to the predetermined value V _H By maintaining ( _VP = _VH ) and continuing to shrink the piezoelectric element in the plane direction, the diaphragm is continuously deflected to maintain a state where the volume of the pressurizing chamber is reduced. Keeps the ink in a stationary state, that is, the volume velocity of the ink at the nozzle portion is zero.
[0006]
By ejecting ink droplets to form dots on the paper surface from the nozzle portion, first at the time of t ₁ immediately before the discharge driving voltage V _P from the predetermined value V _H and (V P _{= 0)} is allowed to the piezoelectric element The flexure of the diaphragm is released by extending in the plane direction.
Then, the ink in the nozzle portion is in a state where the ink meniscus is drawn toward the pressurizing chamber as much as the volume of the pressurizing chamber increases. Volume velocity at that time, once as shown in the portion between the t ₁ and t ₂ in FIG. 5, after the increased negative side, gradually approaching zero eventually decreases. This is the natural vibration period T ₁ of the volume velocity of the ink indicated by the thick solid line, corresponding to approximately a half cycle.
[0007]
Then, when the volume velocity of the ink in the nozzle portion is close to 0 as possible, and the piezoelectric element is contracted in the surface direction is charged again to a predetermined value V _H of the drive voltage _{V P (V P = V H} ) Flex the diaphragm. This operation, as indicated by the dashed line in bold lines, a driving voltage V _P of the pulse width T ₃ has a driving voltage waveform which is 1/2 times the natural vibration period T _1, that are applied to the piezoelectric element Equivalent to.
At this time, the ink in the nozzle portion is in a state in which the ink meniscus is about to return in the positive direction from a stationary state in which the ink meniscus is drawn most toward the pressure chamber (a state in which the volume velocity is 0). By bending the diaphragm to reduce the volume of the pressure chamber, the ink pushed out from the pressure chamber is added. Therefore, the ink protrudes largely toward the positive side from the tip of the nozzle portion, and the ink droplet separates from the tip. Then, the separated ink droplets fly and reach the paper surface to form dots on the paper surface.
[0008]
Further, in Patent Document 2, in the pulling driving method, in order to suppress the residual vibration of the driving unit, the time constant at the time of the falling of the driving voltage waveform is set to 0.9 times or more of the natural oscillation period of the driving unit. It is preferable that the time constant is set to be 0.9 to 1.2 times the natural oscillation period of the driving unit.
[0009]
[Patent Document 1]
JP-A-2-192947 (page 8, upper left column, line 8 to page lower left column, line 12, FIG. 16 (a) to (c))
[Patent Document 2]
JP-A-5-318731 (Claims 3, 4, Column 0035)
[0010]
[Problems to be solved by the invention]
In the piezoelectric ink jet head, the period of the natural vibration of the drive unit is a small value of several tenths to one-tenth of the pulse width of the drive voltage waveform. However, as shown in FIG. 6, the above-described natural vibration is superimposed on the vibration of the ink volume velocity at the time of ink droplet formation as residual vibration. When the timing of the rise of the driving voltage waveform is shifted from the phase of the residual vibration, there arises a problem that the volume and the flying speed of the formed ink droplet fluctuate.
[0011]
That is, if the drive voltage waveform rises while the vibration speed of the residual vibration is increasing in the direction of the pressure chamber, the volume of the ink droplet is large and the flying speed is large. Conversely, if the drive voltage waveform rises while the vibration speed of the residual vibration is decreasing in the direction of the pressure chamber, the volume of the ink droplet is small and the flying speed is small.
Therefore, for example, if the pulse width of the drive voltage waveform fluctuates even slightly, the volume and the flying speed of the ink droplet fluctuate greatly.
[0012]
In addition, the thickness and conditions for bonding the piezoelectric element to the diaphragm are inevitably varied for each of the plurality of piezoelectric elements arranged on the piezoelectric inkjet head. Occurs. For this reason, even if all the pulse widths of the drive voltage waveform are kept constant, the volume of the ink droplet and the flying speed vary from nozzle to nozzle.
Therefore, in order to suppress the residual vibration of the drive unit, in Patent Document 2, as described above, the time constant at the time of the fall of the drive voltage waveform is 0.9 times or more the natural oscillation period of the drive unit, and the time constant at the time of the rise is driven. It is set to be 0.9 to 1.2 times the natural vibration period of the part.
[0013]
Certainly, by increasing the time constant of the rise / fall, the residual vibration of the drive unit can be suppressed.
However, if the falling / rising time constant is increased, the flying speed of the ink droplet is reduced. In particular, in Patent Document 2, the natural oscillation period of the driving unit is small because the piezoelectric element in the longitudinal vibration mode is used. However, in the case of the piezoelectric element in the lateral vibration mode, the natural oscillation period of the driving unit is large. If the falling / rising time constant is set to be as long as the natural oscillation period of the driving unit, the flying speed of the ink droplets is significantly reduced.
[0014]
As described above, at present, optimal ink ejection cannot be realized only by adjusting the drive voltage waveform.
SUMMARY OF THE INVENTION An object of the present invention is to provide a novel piezoelectric inkjet head capable of reliably suppressing a residual vibration of a driving unit including a piezoelectric element in a transverse vibration mode while suppressing a decrease in a flying speed of an ink droplet. is there.
[0015]
Means for Solving the Problems and Effects of the Invention
To solve the above problems, the inventor and the natural vibration period of the drive unit (and T _2), the ink in the head as described above, was examined the relationship between the natural vibration period T ₁ of the volume velocity.
As a result, the natural vibration period T ₂ of the drive unit, the ink in the head, to as small as possible with respect to the natural vibration period T ₁ of the volumetric rate, while suppressing the decrease in the flying speed of the ink droplet, the driving unit Has been found to be effective in suppressing residual vibrations of.
[0016]
Specifically, the natural vibration period T ₂ of the drive unit, the ink in the head, when the about 10% or driver designed to be less than the natural vibration period T ₁ of the volume velocity, driver Can reduce the effect of residual vibration. And even if the pulse width varies, also be the natural period T ₂ of the drive unit slightly varies for each nozzle, it is possible to make it difficult variation of flight speed and volume of ink droplets.
Therefore, the invention described in claim 1 is
(A) a pressurized chamber filled with ink;
(B) a nozzle portion communicating with the pressurizing chamber;
(C) The piezoelectric element in the lateral vibration mode, and the piezoelectric element deflects by contraction in the surface direction due to the application of the drive voltage waveform to reduce the volume of the pressurizing chamber, thereby allowing the ink in the pressurizing chamber to pass through the nozzle section. A driving unit including a diaphragm for discharging ink droplets,
A piezoelectric inkjet head comprising:
A piezoelectric ink jet head, wherein a natural oscillation cycle of the driving unit is set to about 10% or less of a natural oscillation cycle of a volume velocity of the ink in the head.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described.
FIG. 1 is a plan view showing an example of a piezoelectric inkjet head according to the present invention before a driving section including a piezoelectric element and a diaphragm is attached.
The piezoelectric ink jet head of the example shown in the figure has a plurality of dot forming sections including a pressurizing chamber 2 and a nozzle section 3 communicating therewith arranged on a single substrate 1.
[0018]
FIG. 2A is an enlarged cross-sectional view showing one dot forming unit in a state where a driving unit is mounted in the piezoelectric ink jet head of the above example, and FIG. 2B is a diagram showing one dot forming unit. It is a perspective view which shows the overlapping state of each part which comprises.
The nozzle units 3 of each dot forming unit are arranged in a plurality of rows in the main scanning direction indicated by white arrows in FIG. In the example shown in the figure, the dots are arranged in four rows, the pitch between the dot forming parts in the same row is 90 dpi, and 360 dpi is realized as a whole of the piezoelectric inkjet head.
[0019]
Each dot forming part is formed on the upper surface side of the substrate 1 in FIG. 2A, and has a pressurizing chamber 2 having a planar shape in which semicircular ends are connected to both ends of a rectangular central part; A nozzle section 3 formed at a position on the lower surface side of one end of the pressurizing chamber 2 on one end side overlapping with the center of the semicircle has a nozzle flow having the same diameter as the semicircle at the end and a circular cross section. Each pressurizing chamber 2 is connected to the other end of the pressurizing chamber 2 through a supply port 5 formed at a position overlapping with the center of a semicircle. It is configured by connecting to a common supply path 6 (shown by a broken line in FIG. 1) formed so as to connect the forming sections.
[0020]
In the example shown in the figure, each of the above-described units includes a first substrate 1 a having a pressurized chamber 2 formed therein, a second substrate 1 b having an upper part 4 a of a nozzle flow path 4 and a supply port 5, and a lower part of the nozzle flow path 4. The third substrate 1c on which the common supply path 6 is formed and the fourth substrate 1d on which the nozzle unit 3 is formed are laminated and integrated in this order.
As shown in FIG. 1, a common supply path 6 formed in the third substrate 1c is connected to the first substrate 1a and the second substrate 1b on the upper surface side of the substrate 1 with a pipe from an ink cartridge (not shown). A through-hole 11a for forming a joint portion 11 for connection is formed.
[0021]
Further, each of the substrates 1a to 1d is made of, for example, a resin or a metal, and is formed of a plate having a predetermined thickness provided with through holes serving as the above-described portions by etching using a photolithography method or the like.
On the upper surface side of the substrate 1, one diaphragm 7 having the same size as the substrate 1 and one thin-film common electrode 8 having a size covering at least each dot forming portion, And a thin plate-like piezoelectric element 9 of a transverse vibration mode having a substantially rectangular planar shape, which is individually provided at a position overlapping with the center of the pressurizing chamber 2 of each dot forming section as indicated by a dashed line. A driving unit is configured by stacking the individual electrodes 10 having the same planar shape formed on the element 9 in this order.
[0022]
The piezoelectric element 9 is integrally formed to have a size extending over the pressurizing chambers 2 of several dot forming sections, and only the individual electrode 10 is provided as shown by a dashed line in FIG. 2 may be individually provided at positions overlapping the central portion.
Among the above, the diaphragm 7 is a plate-like plate having a predetermined thickness made of a single metal such as molybdenum, tungsten, tantalum, titanium, platinum, iron, nickel, or an alloy of these metals, or a metal material such as stainless steel. It is formed in. The diaphragm 7 has a through-hole 11b that forms the joint portion 11 together with the through-hole 11a of the substrate 1 described above.
[0023]
Each of the common electrode 8 and the individual electrode 10 is formed of a metal foil having excellent conductivity such as gold, silver, platinum, copper, and aluminum, a plating film made of these metals, and a vacuum deposition film. Note that the diaphragm 7 may be formed of a highly conductive metal such as platinum, and the common electrode 8 may be omitted.
As a piezoelectric material for forming the piezoelectric element 9, for example, lead zirconate titanate (PZT) or one or more oxides of lanthanum, barium, niobium, zinc, nickel, manganese, and the like are added to the PZT. Examples thereof include PZT-based piezoelectric materials such as PLZT. In addition, those containing, as main components, lead magnesium niobate (PMN), lead nickel niobate (PNN), lead zinc niobate, lead manganese niobate, lead antimony stannate, lead titanate, barium titanate, etc. You can also.
[0024]
The thin plate-shaped piezoelectric element 9 can be formed in the same manner as in the related art.
For example, a chip having a predetermined planar shape obtained by polishing a sintered body formed by sintering the above-described piezoelectric material into a thin plate shape is bonded and fixed at a predetermined position on the common electrode 8, and a so-called sol-gel method is used. (Or MOD method), a paste formed from an organometallic compound serving as a base of the piezoelectric material is printed in a predetermined planar shape on the common electrode 8 and formed through a process of drying, preliminary baking, and baking. Alternatively, by forming a thin film of a piezoelectric material into a predetermined planar shape on the common electrode 8 by a vapor phase growth method such as a reactive sputtering method, a reactive vacuum deposition method, or a reactive ion plating method, The element 9 can be formed.
[0025]
In order to set the piezoelectric element 9 in the transverse vibration mode, the polarization direction of the piezoelectric material is oriented in the thickness direction of the piezoelectric element 9, more specifically, in the direction from the individual electrode 10 to the common electrode 8. For this purpose, conventionally known polarization methods such as a high-temperature polarization method, a room-temperature polarization method, an AC electric field superposition method, and an electric field cooling method can be adopted. In addition, the piezoelectric element 9 after polarization may be subjected to an aging process.
The polarization direction of the piezoelectric material is oriented in the direction of the piezoelectric element 9, while grounding the common electrode 8, by applying a positive driving voltage V _P from the individual electrode 10, a plane perpendicular to the polarization direction To shrink. However, since the piezoelectric element 9 is fixed to the diaphragm 7 via the common electrode 8, as a result, the piezoelectric element 9 and the diaphragm 7 bend in the direction of the pressure chamber.
[0026]
For this reason, the force at which the deflection occurs is transmitted as a pressure change to the ink in the pressurizing chamber 2, and the pressure change causes the supply port 5, the pressurizing chamber 2, the nozzle flow path 4, and the The ink vibrates. Then, as a result of the vibration speed going out of the nozzle portion 3, the ink meniscus in the nozzle portion 3 is pushed out to form an ink column.
The ink column eventually absorbs the ink meniscus in the nozzle portion 3 due to the speed of the vibration going inward in the nozzle portion. At this time, the tip portion of the ink column is cut off as described above to form an ink droplet. And fly in the direction of the paper to form dots on the paper.
[0027]
The amount of ink reduced by the flying ink droplets is transferred from the ink cartridge to the piping of the ink cartridge, the joint portion 11, the common supply path 6, the supply port 5, the pressurized portion by the surface tension of the ink meniscus in the nozzle portion 3. The nozzle portion 3 is refilled via the chamber 2 and the nozzle flow path 4.
In this example, the vibration plate 7 of the respective units, the common electrode 8, the natural vibration period T ₂ of the driving unit including at piezoelectric element 9 and the individual electrodes 10, the ink in the head as a volume velocity about 10% of the natural period T ₁ of the sets or below it.
[0028]
Thus, it is possible to reliably suppress the residual vibration of the driving unit while suppressing the drop in the flying speed of the ink droplet as described above.
Note in order to more reliably suppress the residual vibration of the drive section, the natural vibration period T ₂ of the drive unit, in particular also in the above-mentioned range, the ink in the head, 0 of the natural vibration period T ₁ of the volume velocity. More preferably, it is set to 1 or less.
The natural vibration period T ₂ of the drive unit in order to adjust the range of the above, each part of the material and dimensions constituting the driving unit may be changed or the like shape.
[0029]
The piezoelectric inkjet head of the present invention may be driven by any of the above-described pulling and pushing driving methods.
[0030]
【Example】
Hereinafter, the present invention will be described based on examples.
1. Fabrication of Piezoelectric Ink Jet Head The structure shown in FIG. 1 and FIGS. 2A and 2B is used, and the area of the pressure chamber 2 is 0.2 mm ² , the width is 200 μm, the depth is 100 μm, and the diameter of the nozzle portion 3 A piezoelectric inkjet head having a length of 25 μm, a length of 30 μm, a diameter of the nozzle flow path 4 of 200 μm, a length of 800 μm, a diameter of the supply port 5 of 25 μm, and a length of 30 μm was produced.
[0031]
The ink in the head, the natural vibration period _{T 1} of the space velocity was 7.2Myusec.
As the drive unit, the material and size of each part constituting the driving portion as described above, by changing the like shape, the natural vibration period T ₂ is a 1.26μsec (0.18 times the T ₁₎ and stuff, the natural vibration period _{T 2} is using two but a 0.63μsec (0.09 times the _{T 1).}
[0032]
The natural period _{T 2} has a head B of the piezoelectric ink jet head using the driving unit is a piezoelectric ink jet head of the head A, the natural vibration period _{T 2 0.63μsec} using the driving unit is 1.26Myusec.
Preparation of Electrical Equivalent Circuit With respect to the heads A and B, the above-described components were approximated by lumped constants, and an electrical equivalent circuit of an acoustic system shown in FIG. 3 was prepared.
[0033]
In the electric equivalent circuit shown in the figure, the drive unit can be equivalently represented by the acoustic capacitance Ca, the inertance Ma, and the acoustic resistance Ra, and the pressurizing chamber 2 can be represented by the acoustic capacitance Cc.
The supply port 5 can be represented by inertance Ms and acoustic resistance Rs, and a water head pressure based on a difference between the liquid surface of the ink meniscus of the nozzle portion 3 and the liquid surface of the ink in the ink cartridge (not shown) acts. are doing.
[0034]
Further, the nozzle portion 3 can be represented by inertance Mn and acoustic resistance Rn, and the surface tension of the ink meniscus of the nozzle portion 3 acts.
In the above-described electric equivalent circuit, when a driving voltage _VP is applied to the driving unit to generate pressure, ink flows in the nozzle unit 3 in a direction indicated by an arrow in the drawing. Then, the volume velocity can be obtained. Further, the flying speed of the ink droplet and the volume of the ink droplet can be obtained by calculation from the calculated volume speed, the diameter of the nozzle portion 3 and the surface tension of the ink.
[0035]
Flying speed of the ink droplet, and the volume of the operational head A, and to drive B at each pull fill-before-fire method of driving method, a driving voltage waveform shown in FIG. 5, and the predetermined value V _H of the drive voltage V _P is the driving voltage is 20V, while the pulse width _{T 3} from 2μsec of 5μsec in applying in a state of shaking in (the natural vibration period _{T 1} of the 0.28-fold to 0.7-fold during), the ink droplets The change in the flying speed [m / s] and the change in the volume [pl (picoliter)] of the ink droplet were calculated using the above-mentioned electric equivalent circuit. FIG. 4A shows the change in the flying speed of the ink droplet, and FIG. 4B shows the change in the volume.
[0036]
Results Conclusions Figure 4 (a) (b), the head A, the B both pulse width T ₃ of the drive voltage be varied, it was found that it is possible to suppress the variation of the flying velocity and the volume of the ink. Further, when comparing the two heads, the natural vibration period T ₂ of the drive unit, the ink in the head, the direction of the head B which is below 0.1 times the natural vibration period T ₁ of the volume velocity, as compared with the head A It has been confirmed that the fluctuations in the flying speed and volume of the ink can be more reliably suppressed.
[Brief description of the drawings]
FIG. 1 is a plan view showing an example of a piezoelectric inkjet head according to the present invention before a driving unit including a piezoelectric element and a diaphragm is attached.
FIG. 2A is an enlarged cross-sectional view showing one dot forming unit in a state where a driving unit is attached in the piezoelectric inkjet head of the example of FIG. 1, and FIG. It is a perspective view which shows the overlapping state of each part which comprises one dot formation part.
FIG. 3 is a circuit diagram showing an electric equivalent circuit created by approximating each part constituting the piezoelectric inkjet head manufactured in the embodiment of the present invention by a lumped constant.
FIG. 4 is a graph showing a result of simulating a change in a flying state of an ink droplet when a pulse width of a driving voltage is changed for two types of piezoelectric inkjet heads manufactured in the examples of the present invention. FIG. 3A is a graph showing a change in the flying speed of the ink droplet, and FIG. 3B is a graph showing a change in the volume of the ink droplet.
[5] and the driving voltage waveform of the drive voltage V _P applied to the piezoelectric element in the driving method of the fill-before-fire method, when this driving voltage waveform is applied, the relationship between the volume velocity of the ink in the nozzle unit is simplified FIG.
FIG. 6 is a graph showing a state in which, in a conventional piezoelectric ink-jet head, natural vibration of a driving unit rides on vibration of volume velocity of ink in a nozzle as residual vibration.
[Explanation of symbols]
2 Pressurizing chamber 3 Nozzle part 7 Vibrating plate 9 Piezoelectric element in lateral vibration mode

Claims (1)

(A) a pressurized chamber filled with ink;
(B) a nozzle portion communicating with the pressurizing chamber;
(C) The piezoelectric element in the lateral vibration mode, and the piezoelectric element deflects by contraction in the surface direction due to the application of the drive voltage waveform to reduce the volume of the pressurizing chamber, thereby allowing the ink in the pressurizing chamber to pass through the nozzle section. A driving unit including a diaphragm for discharging ink droplets,
A piezoelectric inkjet head comprising:
A piezoelectric ink jet head, wherein a natural oscillation cycle of the driving unit is set to about 10% or less of a natural oscillation cycle of a volume velocity of ink in the head.

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