JPS61141566A - Ink jet head - Google Patents

Abstract

PURPOSE:To reduce the size of a piezoelectric element without adverse effects such as a rise in a driving voltage and to contrive a reduction in the size of a printing head, by a construction wherein the acoustic capacity of a vibration system consisting of a piezoelectric element and a vibrating plate is set to be not higher than a specified value, and the inertance and acoustic resistance of liquid-ejecting passage are limited. CONSTITUTION:In the equivalent electric circuit of a printing head shown, (m) is inertance, c is acoustic capacity, r is acoustic resistance, 10 is the vibration system consisting of the piezoelectric element 11 and the vibrating plate 12, 1 is a pressure chamber, 2 is a supplying part and 3 is a nozzle part. Generally, the ink droplet diameter D1 is preferably about 50-150mum, and particularly in the case of high-density printing such as the case of 24 nozzles, an excessively large droplet diameter is unfavorable in view of printing quality. When m3 and r3 expressed in mks units of the SI system satisfy the relationship r3>10<4>Xm3, ink droplets with a diameter of not larger than 150Xm can be ejected, and the value of C0 which gives the minimum voltage is in the range of 1X10<-18>m<5>/N<=C0<=9X10<-17>m<5>/N.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ink-on-demand inkjet head, and more particularly to a compact print head.

Ink-on-demand inkjet reduces the volume of the pressurized chamber by deforming the piezoelectric element, communicates with the pressurized chamber, and then ejects liquid ink from the next nozzle.
It is attracting attention because it can be made into multiple nozzles. The structure of ink ejection is extremely simple, but ink ejection is performed in a transient situation, and the print head itself is small, making it difficult to measure pressure, flow rate, etc. , the theoretical analysis is far from complete.

In integrated multi-nozzle heads with 24 or more nozzles required for kanji printers, etc., it is desirable that the individual pressure chambers and piezoelectric elements be as small as possible. Due to completeness, the lower limit of the size of the pressurized chamber was not clear. And for general KFi, the thickness of the piezoelectric element t p 4'' (L S wH
A piezoelectric element having a diameter of about 5 m11 or more was used.

Also, if the piezoelectric element is made smaller, the driving force will be smaller, so
The voltage had to be increased, which was considered to be disadvantageous in practice. For example, st@ma, etc. is 1111
Transacton on 1! electrom
D@vices, ID-2041, 14 (1975)
, the above tp = α31111, D = 5 M
Matsuyama et al. described an example of tp=l13. states that Arashi has the highest deformation efficiency. In this case, the size of the piezoelectric element is approximately 2. Xi5. It is estimated that the size of the head will be reduced.

It's not necessarily a titration thing, it's a hole. In addition, as the area of the piezoelectric element increases, the cost of the substrate that makes up the piezoelectric element head body increases accordingly.Especially in highly integrated heads, the effect of using a large number of piezoelectric element tubes is large. In the case of a multi-nozzle, when the piezoelectric element becomes larger, the distance from the nozzle tip to the pressurizing chamber becomes longer due to the arrangement, and the flow path resistance increases. Since it is necessary to further increase the driving force of the piezoelectric element in response to the increase in resistance, there is a disadvantage that the area of the piezoelectric element must be increased.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to reduce the size of the print head without increasing the drive voltage.

Another object of the present invention is to obtain an efficient multi-nozzle head without increasing flow path resistance.

Yet another object of the invention is to reduce the cost of printheads.

As mentioned above, the theoretical analysis of the ink-on-demand print head is cool, but the inventors of the present invention conducted an analysis based on the equivalent electric circuit model of the print head, and as a result, the drive voltage increased. It has been discovered that piezoelectric elements can be miniaturized without any negative effects such as.

FIG. 1(a) shows the equivalent electrical circuit of the K print head.

m is inertance, and C is acoustic capacitance*rti acoustic resistance. fs1 figure (1)) shows the outline of the print head, 10
represents a vibration system consisting of piezoelectric element 11 and vibration @12,
11Ii is a pressurizing chamber, 2 is a supply section, and 3Fi is a nozzle section. Note that the subscripts in FIG. 1(a) represent the respective parts shown in FIG. The subscript 0 represents the vibration system 10t.The unit is pressure crab ψ (N/j), volume velocity:
u(11//8).

Inertance: m (odor/m'), acoustic capacity: C [m''7
N], acoustic resistance: r (N117m)'& used.

When we actually calculate each constant, we get “(16rl @ C2*
C8 etc. can be ignored, resulting in a simple equivalent circuit as shown in FIG.

Here l! 11=Kllkl, rl=krl
If we divide the pressure ψ into a step function and solve it, we get the damping coefficient: D = rl / 2 m2...
・・・・・・・・・As ■C=Co+CI ・・・・・・・・・・・・・・・・・・
It becomes a damped vibration represented by ・・・・・・・・・・・・・・・■.

From 2〇, the required pressure ψ can be expressed as the hole, Vm: required speed, and A: nozzle cross-sectional area.

Ma9 ink droplet volume qri However, tm=−…←・・…Conflict◆・…・−−−・・Fist・1+−...
It can be expressed as …………■.

9, Cp: piezoelectric element capacitance, K = constant, l' in the experiment
j Klj (L 1 to (Ll value). Also, the capacity pri Cp= g Bp /l p ・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・■It's a hole,
-: dielectric constant, 8p: piezoelectric element area, tp: piezoelectric element thickness.

The appeal constant is given as follows. However, the case where the piezoelectric element is a disk is shown.

52ηL r″″a1 °
−−−−−−−−−−−−−−−−.

Ep is the longitudinal elastic modulus of the piezoelectric element, lv is the longitudinal elastic modulus of the diaphragm, and I*x='' is a constant, and in experiments, 5°K! in rjxl is a value of about 1a to 2G, and 7j, a : radius of piezoelectric element, tp: thickness of piezoelectric element, tv: thickness of diaphragm, dC
: Depth of pressurized chamber, va: Sound velocity in ink, ρ: Ink density, IF: Ink viscosity, t: Channel length, 8: Channel cross-sectional area,
d: Channel diameter, equivalent diameter in case of rectangular cross section (2 in d
s/(b+c)), b.

C: Side of the flow path FIR surface.

The above constants are shown in FIGS. 5(a) and 5(b).

The proportion obtained using the above calculation formula is shown below.

Figures 4(a) and 4(b) show the nozzle portion of a nine-glass head made by etching. L indicated by dotted line sO
The flow path t from the pressurized chamber 31 to the nozzle 32 is approximated by the flow path shown by the solid line, and if we calculate 1 for 2 and @, then bl=a
oμm, cm=50μm, A1=250am
, b dew=5007All, cg=100μ@,
tg=2mm.

η=j, 5 CP, ρx100 (m when 1M4/m'
” x L 8 x 10” [1/II'
r"=&3X10"Ns/m'.

In addition, to find it with precision, you can integrate it along the flow path, or
All you have to do is find the division t-- and tp of the minute parts and add them.

Fig. 5 shows the vibration waveform (,) of the piezoelectric element of the actual print head used as a piezoelectric element, and the calculated vibration waveform (1).The constant is ax t25w,
k z t S , r3=4X 10” Ha/+
m s m5=15X10%/*', tp=tv=
lL15M, C1==122X-II l
-II fathom 10 m
/N, C@=A45X10 /m/N.

The vibration period is about 140 μa in actual measurement, but it is calculated to be about 1
Although it is not a perfect match, as it is 46 μS, it can be seen that the actual movement can be explained to a large extent by the above-mentioned theory. Note that, due to imperfections in the measurement method, displacements of less than 100 μm were not measured in the actually measured vibration waveform. Also,
The vertical axes of (,) and (su are not the same.

Next, an embodiment of the present invention will be described in which the piezoelectric element is miniaturized using the above calculation formula.

1. Middle 2 X 1 u'K4/m' s rs Middle 5
X 1 G' Ns/m, 1Vm=5m/a, person=
2.4 x 10-m”, K: α2.

ε=2070XIL854X10 F/! 11. IE
p-&9X1 (J”N7m”, I!V=7
X 1 (1” M/-, K1-44.

Kg-11, dcwall 113, Vs=146
0 m/a.

The calculation results of the required voltage V at time t are shown in the graph of FIG. 6 by setting k=1 and changing the thickness tp and radius a of the piezoelectric element as tp=tv.

From this result, when printing with the lowest pressure, when m+r of the flow path system is constant, there is an optimal radius a for the thickness tp of the piezoelectric element, and under that condition, the minimum voltage is almost constant. It turns out that it takes a value.

To explain this from another perspective, if the flow path system of m+r is fixed at Cot- under certain conditions, CI
＜＜ CO therefore C* C@ and Nasu, 2〇 to CO
If is constant, E also takes a nearly constant value.

Therefore, 9 from equation (2) also takes a constant value. From the above formula ■, in the case of a disk, Cp-g・πa ” / t p・・・・・・・・・・・・
・・・・・・・・・・・・・・・■'. On the other hand, the formula ■
If tl) = tv, then 1/l'' becomes constant from co constant, and cp% becomes constant from equation ■'.In the end, if Cot- is fixed in equation If it is 1, it can be seen that the other constants are approximately constant values and V% does not change.

The above description shows that if the value of a''/lp is within a certain range, the piezoelectric element can be made smaller without increasing the voltage.

Furthermore, it can be seen from 20 that the ink droplet volume q also becomes approximately constant if the COO value is constant.

Next, it is used for towing in a normal flow path, but W&3 is
” ” ” Kf/vm' to S X 1 G ” K
l/groove', change from rl lj1%10''Na/- to 12 X 1 G llN m 7- and at 9 o'clock,
COO value t that minimizes V of 20 under the same conditions as in Figure 6
, shown in FIG. In other words, when the flow path system is determined, if the vibration system is selected so that CO as shown in diagram H7 is obtained, the driving voltage can be minimized.

The particle size D1t at this time is shown in FIG. Generally, the ink diameter D1 is preferably about 50 μm to 150 A groove,
Particularly in the case of high-density printing using 24 nozzles or the like, if the particle size is too large, it is not desirable in terms of print quality. Therefore, for example, if we set the condition that D1≦150μ in Figure 8, purchase @ = I
X 10 ” In the case of Kg's', rl≧I
1 G” Ha/n*, @, x2×l Q”
In the case of V groove 4, rl≧2 X 10' Ha/wt
In the case of mg = 3 × 10 Ill/ss', rl≧3 × 10 Ill/ss'. In other words, the range shown by the solid line in FIG. 7 is desirable. That is, expressed in aX system mka units, the relationship between nine-time solution 3 and r3 is rl≧104X, and if 3, ink of φ150 μm or less can be ejected.

Therefore, in the range of the flow path system shown in Fig. 7, C (the value of 1 is the graph error sr H IXlo 7N≦06≦9×10 inert/N...
... is in the range of [phase]. However, if higher density printing is required, a head with a smaller co value and smaller particle size is desirable.

There is a % optimal relationship between tv and tp, and according to experiments If you do, you will get good results.

Substituting into the expression ◎ 會 [phase], we get is obtained.

Substituting the formula [stock] into the formula OK, K, = 4.4, IEp =
5.9
−・@ is obtained.

tp=[lL2. Then, 1wm≦a≦2-2asip
= a1s, , (Lea≦a≦2.ol1mtp
=11m, (L7.≦a≦t6-).

From this result, when the flow path system is determined, if ft-Cot is selected as appropriate for iIk, the voltage can be minimized.
Cotla'/l"p, t, friend a"/t
It can be seen that it is determined by the value of p. For a typical flow path system shown in FIG. 7, there is an optimum value of a within the range of the equation 0t-9.

It can be seen that in order to reduce the radius a of the piezoelectric element, it is sufficient to reduce the thickness tpi.

On the other hand, the lower limit of the thickness tp of the piezoelectric element is said to be about 11 mm in terms of processing and about a15 ms in terms of strength in assembly and handling, for example in the case of 1'ZT. Figure 6 @1 = 2X 1
r: J"K4/m', r3==e3 X 1 G"
Under the condition of lfa/m, Cot giving the lowest voltage
j, 2 out of 00 from Figure 7. I X I U""@' 7
N, and a=1123V'TT is obtained from the equation [phase].

i p=(L 15. for a:t5mtp=(
For 11g, a m 1.2 wa is the radius that gives the lowest voltage. Radius a that gives the lowest voltage in Figure 6
However, in the TorerilIi6 diagram, tv=tp, whereas in the above calculation, xl=t4. gp=a9xtot・, K,!
11, 1y=7X1 Ol” -ii Substituting, tv
This is due to the fact that =17tp. It should be noted that from the formula [stock], a should be smaller if ty; Conversely, in tv((tp, 2〇 does not hold t, and the voltage increases.) This is because the deformation of the piezoelectric element effectively works to improve the diaphragm and makes it stiffer.

Acoustic capacity CorjsIt is expressed as the ratio of the volume change in the pressurizing chamber and the pressure when pressure is applied to the pressurizing chamber at 9 o'clock, but it depends on the shape of the print head, the method of layering the piezoelectric element, and the bonding method and material of the loss plate. For example, it may take a slightly different value from the formula [phase], and for example, it may be better suited to the experiment. In the experiment, constant: x
1 out of 5. tL4 or X in I2! I was in 1st year of junior high school.

When using the formula [phase]', if we consider %, tv=tp, we can think of the same way as when using the formula @.

6, it is possible to select a value smaller than the radius at-, since there is no sudden voltage increase near the radius a that provides the lowest voltage for a given piezoelectric element thickness tp.

For example, in Fig. 6, the optimal radius ai for tpxα15u
j about 1.75. However, the driving voltage is about aOV to 10
If it is allowed to rise to 0V, it can be set to 1.2■ in a.Similarly, for t=11 battles, & *
19 g. Further, in the example shown in FIG. 7, if tv=α7tp, alz can be made even smaller than the above value.

Next, another study will be made regarding the lower limit of the thickness tp of the piezoelectric element. The aforementioned lower limit is the lower limit in terms of the strength of the piezoelectric element, but it is necessary to consider the lower limit based on the withstand voltage.

Figure 9 shows the electric field strength V/1pt under the same conditions as Figure 6.
- Calculate and show the following results. Generally, the dielectric breakdown strength of PZT is said to be about 3KV/ to 4KV/9.
From the figure, it can be used even if t p = 25 μm or t p = 50 μm. Therefore, due to advances in manufacturing technology,
If piezoelectric elements of 25 μm or 50 μm are used, it is possible to further reduce the radius ai. It is also possible to obtain a print head as small as half a by using a thin film such as pz'r formed by 'tt vaporization* or sputtering. Generally, the withstand voltage decreases due to increases in humidity, etc., and in order to safely eject ink even under high humidity conditions, the electric field must be IKV/,
It is preferable to use it below. Under this condition, from FIG.
tjα8 ax ≦! For L≦L 2 B, tp=12 fins, it is necessary that α8111J1≦a≦λ6u.

To summarize the above, once the t flow path system is determined, there is Co that makes the drive voltage t-the lowest.

2. a@ is one/ipK! Determined. Therefore at
- Make it smaller Kr1tpt Just make it smaller.

5. The lower limit of tp may be 25 μm from the viewpoint of withstand voltage. However, considering the influence of humidity, etc., tp≧11
Fighting is desirable.

4. The lower limit for processing and handling is tp=11. or tp
= (115g. On the safer side, tp = 1
It is 20.

! L, tp=α12u+, 1 fight a≦12藺, t
L9. against p=115. ≦＆＜L Om,tp
= α7 for α111m ≦ a < 1.6 wa K
There is an optimal a.

& Slight voltage increase'I! - If allowed, it is possible to choose at- a small range as described in & above.

In the above detailed explanation, the piezoelectric element and the pressurizing chamber are assumed to be disk-shaped, but the same concept can be applied to ellipses, polygons, etc. Of course, it is necessary to transform the expression @bed according to the shape. In addition, if a long rectangular pressure element is used, in order to increase the rigidity and reduce Co, the thickness must be made 7 times thinner compared to a disk or square pressure element, or conversely, the area must be increased. It is disadvantageous in terms of quality.

Therefore, even in the case of a rectangle, the width and length l: tl: 2
t- It is desirable not to hear it.

FIG. 10 shows an embodiment of a glass print head which is miniaturized according to the present invention and has a pressurized chamber. In this example, PZ
'l' 100 radius a = t 25, thickness tp =
cL1s, , as shown in the figure, a disk-shaped pressurizing chamber 10
By combining 1 alternately, it becomes even more like a hut.

In this example, 12 nozzles on one side are connected to 22 nozzles on both sides 11i.
It has a 24 nozzle head assembly with dimensions of WX18mm x 2 mist. In addition, a supply m 102 and an outflow path 10 that communicate with each pressurizing chamber
5 inertance m.

The acoustic resistance r is approximately the same between two pressurized chambers, such as length and width.
The ink speed, ink particle size, etc. for each nozzle are the same. Note that 104 is a filter that prevents dirt from entering, and 105fi is a high part that uniformizes the flow of ink in the pressurizing chamber 101, and is created at the same time as other flow channels by etching. If machined, thickness t
By selecting a piezoelectric element with a small p, the m! of the piezoelectric element can be increased without increasing the drive voltage. *r Can be made smaller.

In the above description, pzτ is used as the currently most desirable piezoelectric material, but it is conceivable to miniaturize the print head using the same concept as in the present invention using a negative piezoelectric material.

In addition, in the present invention, one piezoelectric element and one diaphragm constitute a vibration system, but it is also possible to construct an L vibration system with a plurality of piezoelectric elements, such as a bimorph L, or to apply pressure. It is also possible to make the print head even more compact by providing vibration systems on both sides of the chamber.

In the above embodiment, the volume of the pressurizing chamber is reduced by 1 for every 4g of printing and printing is performed.

In that case, increase the 'J product of the 19 pressurizing chamber in the print signal,
A method has also been proposed in which the volume of the pressurized chamber is restored by performing seven 1V printing operations using a vibration system or dynamic movement of fluid. If this method is used, there is also a possibility that the drive voltage will be lowered by the method L9 in which ink is directly ejected to reduce the volume, and if applied to this case, the voltage will further decrease, so the radius a
It becomes possible to make it even smaller than the t-optimal value.

As described above, according to the present invention, by selecting a vibration system suitable for the flow path system, the drive voltage tube can be lowered, and by making the piezoelectric element thinner, the area of the pressure 1E element can be reduced, and the entire print head can be reduced. By reducing the area and shortening the distance from the nozzle to the pressurizing chamber, the impedance of the flow path is lowered, which further helps in lowering the driving voltage. In addition, by using a small piezoelectric element and making the print head itself smaller, head manufacturing costs can be lowered, and the motor used to move the print head can be made smaller and cheaper. It has many advantages and can be widely applied not only to highly integrated multi-head serial printers, but also to various printers, plotters, facsimile machines, etc.

[Brief explanation of drawings]

Figures 1 (a) and (b), Figure 2 are equivalent electric circuit diagrams showing the concept of the present invention, Figures 3 (a) and (b), and Figure 4 (a) (
Figure 5(a) is a graph showing the constants used in the calculations of the present invention (→ is a graph showing the comparison between the calculations used in the invention and the actual situation), and Figure 6 is the graph showing the constants used in the calculations of the present invention. The voltage graph, T47 is a graph showing the optimum acoustic capacity tube calculated by the present invention, and the fsa diagram is a graph showing the particle size under the same conditions as in Fig. 7. Fig. 9 is the electric field graph under the same conditions as Fig. 6. Diagram showing strength,
FIG. 10 is a plan view showing an embodiment of a printing head to which the present invention is applied. co...Acoustic capacity of the vibration system C1...Acoustic capacity of the pressurized chamber Purchasing co...1-Status of the supply section, T2...
... Acoustic resistance m3 of the supply section ... Inertance r3 of the nozzle section ...
...Acoustic resistance of the nozzle section 11...Piezoelectric element 12...Vibration plate 1 [JO...PZT Above (b) Fig. 1 Fig. 2 O Fig. 6 %IO” N54s Figure 7

Claims (1)

[Claims] In an inkjet head of an ink-on-demand printing device that records on a recording medium by increasing the pressure in a pressurizing chamber and ejecting a liquid inter from a droplet ejecting path, the liquid ejecting path directly communicates with the pressurizing chamber and the pressurizing chamber. It consists of a supply path that directly communicates with the pressure chamber, a flat diaphragm forming the wall of the pressure chamber, and a flat piezoelectric element laminated on the diaphragm, and the piezoelectric element and the diaphragm. The volume capacity Co of the vibration system is 9×
10^1^7m^5/N or less, and SI type mKs
When expressed in units, the inertance m_ of the liquid injection path
3 and acoustic resistance r_3 is r_3≧10^4×m_3.