JPH0684073B2 - Driving method for inkjet recording head - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to an on-demand type ink jet recording head that uses an electro-mechanical conversion element such as a piezoelectric element as a drive source for ejecting ink, and particularly to a method of driving an ink jet recording head for improving ink ejection efficiency.

[Prior art]

A rectangular pulse voltage is generally applied to drive this type of ink jet recording head, and the applied voltage should be as low as possible if the volume and speed of the ink droplets at the time of ink ejection are kept to the necessary minimum. That is, it is desired to reduce the supplied energy as much as possible to improve the printing energy efficiency.

[Problems to be Solved by the Invention]

On the other hand, in the ink jet recording head in which the flow path shapes such as the ink supply path and the ink nozzle are fixed, it is necessary to increase the area of the piezoelectric element in order to maintain the driving force while lowering the driving voltage. However, when the area of the piezoelectric element is increased, the flow path length must be increased due to the arrangement, and when the flow path length is increased, the flow path resistance increases and the ink supply becomes insufficient, and the low voltage There is a problem that it is difficult to make it. Further, if the shape of the piezoelectric element becomes large, there is a problem that it becomes difficult to reduce the size and increase the degree of integration when the piezoelectric element is multi-sized. It is an object of the present invention to provide a method for driving an ink jet recording head in which the applied pulse voltage is lowered and the energy efficiency is improved without changing the shape of the piezoelectric element or the ink flow path.

[Means for Solving the Problems]

In order to achieve the above object, the present invention changes the volume of the ink pressurizing chamber by applying a pulse voltage to an electro-mechanical conversion element that constitutes the vibration system of the ink pressurizing chamber, and this occurs. In a method of driving an ink jet recording head in which ink is ejected from an ink nozzle communicating with the ink pressurizing chamber by a pressure wave, the time from the fall of a pulse voltage to the rise of the next pulse voltage is The natural vibration period is (2n-1) / 2 times, (n = 1,2,3 ...). The details will be described below. FIG. 3 is a schematic sectional view showing a general configuration of the on-demand type ink jet recording head, and the mechanism of ink ejection is as follows. Ink is supplied from the ink tank 6 to the ink pressurizing chamber 1 through the ink supply path 2, while the piezoelectric element 5 attached to the vibrating plate 4 is passed through a lead wire from an external power source (not shown) and between the front and back surfaces thereof. A pulse voltage is applied. When a pulse voltage is applied, a stress that tends to contract in the direction of arrow 7 is generated in the piezoelectric element 5. However, since the vibrating plate 4 does not contract, both the vibrating plate 4 and the piezoelectric element 5 bend inward, and the volume of the ink pressurizing chamber 1 is reduced. At this time, the internal pressure of the ink pressurizing chamber 1 rapidly rises, and the ink is ejected from the ink nozzle 3 as an ink droplet. By the way, theoretical analysis of an on-demand type ink jet recording head has been performed from various fields in recent years. Among them, a method of replacing the constituent elements of the ink jet recording head with an acoustic system and analyzing it with an equivalent electric circuit model obtained by electroacoustic conversion. Are being studied. As a result of detailed examination of the ink ejection characteristics of the inkjet recording head based on this analysis method, the present inventors have found a method of driving the inkjet recording head that reduces the applied pulse voltage without changing the shape of the piezoelectric element or the flow path. I found it. FIG. 4 is a diagram in which the ink jet recording head shown in FIG. 3 is replaced with an equivalent electric circuit based on the above analysis method. In FIG. 4, m is inertance, C is acoustic capacity, r is acoustic resistance, u is the ink volume velocity of the ink nozzle portion, and the subscripts are the respective portions in FIG. However, the subscript 0 represents a vibration system including the vibration plate 4 and the piezoelectric element 5. That is, the subscripts indicate 0: vibration system, 1: ink pressurizing chamber, 2: ink supply path, 3: ink nozzle.
As a unit, pressure: P (N / m ² ), ink volume velocity: u: (m ³ /
s], inertance: m (kg / m ⁴ ), acoustic capacity: C (m ⁵ / N),
Acoustic resistance: r [NS / m ⁵ ] is used. m ₀ , r ₀ , C ₂ and C ₃ can be ignored in the actual calculation process, and the equivalent circuit of FIG. 4 can be approximated as shown in FIG. Here, in order to further simplify the calculation, each constant is regarded as m ₂ = km ₃ , r ₂ = kr _3, and the pressure P is used as a step function to determine the ink volume velocity u ₃ of the ink nozzle portion at time t ≧ 0. When the transient response characteristic is obtained, the following damping vibration is obtained. _{u 3 = {PC 0 / m} 3 (C 0 + C 1) β} exp (-αt) sinαt ......
(1) Attenuation coefficient α = r ₃ / 2m ₃ …… (2) Further, the relationship between the voltage V applied to the piezoelectric element and the ink pressure P is as follows using the approximate expression proposed by Stemme et al. (E.Stemms, S.Lras-son, IEEE Transaction
Devices, Vol.ED-20 No.1,1973, P14-19.). V (t) = P (t) / C (4) C = {24t _P d ₃₁ (1 + α) αη} / {S ₁₁ ^E × a ₂ (1-ν _P ) (7 + ν _P ) (1 + αη)} ...... (5) where, t _P: thickness of the piezoelectric element t _V: the thickness of the diaphragm _{α: t V / t P S} 11 E: compliance of the piezoelectric element E _V: Young's modulus of the diaphragm eta: S _{^{11 E · E V (1-}} ν P) 2 / (1-ν V) 2 ν P: Poisson's ratio of the piezoelectric element [nu _V: Poisson's ratio of the diaphragm d _31: Side effect piezoelectric constant a: radius above the piezoelectric element FIG. 6 is a plot of the ink volume velocity u ₃ versus time when t = 0 and a step voltage of V = V ₀ is applied based on the equations (1) and (4). Umax in the figure is the maximum value of the volume velocity of the ink droplet ejected from the ink nozzle, and the ejection velocity of the ink droplet can be obtained by multiplying this by the ink nozzle cross-sectional area. T is the natural vibration period of the ink volume velocity in the ink nozzle section,
From the equation (3), it can be obtained by T = 2π / β. The volume Q of the ink droplet becomes a value obtained by integrating the equation (1) at t = 0 to t _1, represented by the following formula. Q = {PC ₀ / (1 + 1 / k)} {1 + exp (-απ / β)} ...
(6) FIG. 7 is a plot of the vibration of the ink volume velocity in the ink nozzle when the rectangular pulse voltage is applied to the piezoelectric element by the conventional method. pulse width
If T ₀ is set to be longer than 1/2 of the natural vibration period, the velocity and volume of the ejected ink droplet are hardly affected by the pulse width T ₀ . Generally, this pulse width T ₀ is T / 2
It is often decided within the range of ~ 3T / 2. However, the inventors have found that rectangular pulse response analysis
Paying attention to the vibration behavior after the pulse voltage falls at T ₀ , the vibration that contributes to ink ejection at the next pulse voltage rise is utilized by utilizing the vibration at the time of the fall that has a phase that is completely opposite to that at the rise. I thought I couldn't strengthen. That is, the pulse voltage is always lowered T / 2 before the next rise of the pulse voltage so that the peaks and peaks of the vibration at the rising and falling edges are overlapped with each other to increase the max at ink ejection. FIG. 1 is a plot of the vibration of ink volume velocity u ₃ when a pulse voltage is applied by this method with respect to time. T _W in the figure is 1/2 of the natural vibration period T, that is, t _W = T
At / 2, the pulse width T ₀ is equal to the pulse repetition period T ₁ minus T _W. T ₀ = T ₁ −t _W (7) As is clear from the figure, the maximum value of vibration umax caused by the rise of the second pulse voltage and the maximum value of vibration caused by the fall of the first pulse voltage It can be seen that the value umax ′ overlaps by shifting the phase by T / 2 and increases the ink volume velocity at the time of ink ejection. This effect is most significant when t _W = T / 2, but it is effective in all cases where t _W = (2n-1) T / 2, (n = 1,2,3 ...) It is thought that there is. By this method, (umax + umax ') is
If a pulse voltage is applied so as to correspond to umax, umax ′
It is possible to drive by lowering the voltage by a considerable amount. An example of calculating umax and umax ′ based on the above theoretical analysis is shown below. First, each constant of the equivalent circuit is calculated by the following equation. C ₀ = πa ⁶ / K ₁ (E _P t _P ³ + E _V t _V ³ ) …… (8) C ₁ ＝ πa ² d _C / ρv ² …… (9) r = 32 ηl / Sd ² …… (10 ) m = lρ / S ...... ( 11) where, E _P, E _V: piezoelectric elements, vibration Young's modulus of the plate K _1: constant, experiments K ₁ = 12.5 a: radius of the piezoelectric element t _P, t _V : Radius of piezoelectric element and diaphragm d _C : Depth of ink pressurization chamber v: Sound velocity in ink η: Viscosity of ink l: Flow path length S: Flow path cross-sectional area d: Flow path diameter b, c Is the side length of the cross section of the flow path, d≈2
S / (b + c) ρ: Ink density Then, the following constants were substituted into the above equations (5) and (8) to (11) to calculate umax and (umax ′). Ink nozzle part: l = 0.3mm, d = 0.05mm Ink pressurizing chamber: d _C = 0.3mm, d = 2.5mm Vibration system: E _P ＝ 5.5 × 10 ¹⁰ N / m ² E _V ＝ 7.4 × 10 ¹⁰ N / m ² a ＝ 2.0mm, t _P ＝ 0.2mm t _V ＝ 0.16mm, d ₃₁ ＝ 245m / V Ink supply path: k ＝ 1.6 Applied voltage: V ₀ ＝ 50V Ink: η ＝ 2.0cp, ρ ＝ 1.0g The calculation result of / cm ³ is umax = 1.6 × 10 ⁻⁸ m ₃ / sec (umax + umax ′) = 2.5 × 10 ⁻⁸ m ³ / sec, and the ink volume velocity is about 56% when driven by the method of the present invention. It was estimated to increase. This means that when driven at the ink volume velocity of umax, the applied voltage can be lowered by 36% for driving.

[Work]

As described above, according to the present invention, the vibration at the falling edge of the pulse voltage is superimposed on the vibration at the rising edge of the next pulse voltage by shifting the phase by an odd multiple of a half cycle, and the peaks and peaks of the vibration are overlapped. The vibration that contributes to the injection is strengthened.

【Example】

Based on the results of the above theoretical analysis, an ink jet recording head having the cross-sectional shape shown in FIG. 3 was manufactured from glass and a piezoelectric ceramic plate, and the ejection characteristics were measured. The results are shown below. Borosilicate glass is used for the ink pressurizing chamber, the vibration plate, the ink nozzle section, and the ink supply path, and the shape and dimensions shown in the above calculation example are obtained by a known processing method, that is, etching, dicing saw, and ultrasonic processing. Processed to be.
For the joining, a method was used in which low melting point glass was used and fusion was performed while applying pressure at 440 ° C. for 2 hours. Piezoelectric ceramics were used with silver paste baked on the front and back surfaces.To take out the lead, solder the lead wire for one of the electrodes.
The other electrode was formed by depositing an ITO conductive thin film on the vibration plate in advance and adhering it to it with a conductive adhesive. In the evaluation of umax and (umax + umax ′), assuming that the flight distance L of the ink droplet is L = Ku (K is a constant) (12), T _{0 in} FIG. 7 and t _{W in} FIG.
And L _MAX = Kumax, and L _MAX ′ = Ku _MAX ′, and the ink droplet flying distance L was measured. FIG. 2 shows the result. The curve indicated by the solid line in the figure was driven by the method of the present invention and exhibited the behavior as predicted by the analysis. The dotted line in the figure is the measured value of the flight distance when T ₀ is changed. (L _MAX + L _MAX ′) / L _MAX ≈1.5 when t _W = T / 2,
(Umax ＋ umax ′) / umax≈1.6 estimated by the above analysis
It became a value close to. That is, it is considered that the validity of the driving method of the present invention has been proved. According to the present invention, if the flight distance is the same as the conventional one, the driving voltage can be lowered.

【The invention's effect】

According to the present invention, the vibration at the time of the rise of the pulse voltage is superimposed on the vibration at the time of the next rise of the pulse voltage by shifting the phase by an odd multiple of a half cycle so that the peaks and peaks of the vibration are superposed and the energy at the rise of the pulse voltage is increased. The energy efficiency is remarkably improved, and if the ink jet recording head is used with the same characteristics, the driving voltage can be reduced accordingly.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a pulse voltage applying method of the present invention, FIG. 2 is a diagram showing ink flight distance characteristics in an embodiment of the present invention, and FIG. 3 is a general diagram of an on-demand type ink jet recording head. FIG. 4 is a schematic cross-sectional view showing the structure, FIG. 4 is an equivalent electric circuit diagram when an electro-acoustic conversion is performed on the ink jet recording head, FIG. 5 is an equivalent electric circuit diagram obtained by simplifying FIG. 4, and FIG. FIG. 7 is a diagram illustrating a general step response characteristic of ink volume velocity, and FIG. 7 is a diagram illustrating a step response characteristic of ink volume velocity in an ink nozzle portion when driven by a conventional method. 1: Ink pressurizing chamber, 2: Ink supply path, 3: Ink nozzle, 4:
Vibration plate, 5: Piezoelectric element, 6: Ink tank.

Claims (1)

[Claims] 1. A volume of the ink pressurizing chamber is changed by applying a pulse voltage to an electro-mechanical conversion element forming a vibration system of the ink pressurizing chamber, and the ink pressurizing is performed by a pressure wave generated at this time. In a method for driving an inkjet recording head that ejects ink from an ink nozzle that communicates with a chamber, the time from the fall of a pulse voltage to the next rise of the next pulse voltage is defined as (2n− 1) / 2 times, (n = 1,2,3 ・
・ ・) The method for driving an ink jet recording head, characterized in that