JPS58114961A - Ink jet recording head - Google Patents

Abstract

PURPOSE:To provide an ink jet recording head of such a system that can be driven by a very low voltage, which is composed of an air discharging outlet provided in a space wherein an abrupt change of pressure slope due to the bending of air current exists and an ink discharging outlet drilled in a thin plate as an orifice provided so as to oppose to the air discharging outlet. CONSTITUTION:An air discharging outlet 1 is drilled in an insulation substrate 8 as a nozzle plate and an electrode 9 of copper of about 30mum thickness is formed around the air outlet. An ink discharging outlet 4 is formed by drilling a hole in an orifice plate 13. In a space from the ink discharging outlet 4 to the air discharging outlet 1, the direction of air flow is sharply changed in the vicinity of the air discharging outlet 1, and due to this, an abrupt change of pressure slope exists. Ink meniscus generated by the impression of signal pulse is elongated and then broken into pieces by said change of air pressure slope and ink droplets are caused to fly and recording is made. It is desirable that the diameter, d, of the ink discharging outlet satisfy the following formula by assuming the viscosity of ink to be q: q<l>/d<2=2.2X10<3>cp/cm, where; l is the length of the ink discharging outlet.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ink jet recording head. FIG. 1 shows an ink jet recording apparatus proposed by the same applicant as described in Japanese Patent Application No. 56-8428. In the figure, a conductive nozzle plate 2 has an air outlet 1 perforated therein, a wall 3 is disposed parallel to the nozzle plate 2, and a wall 3 is provided with a wall 3 opposite to the air outlet 1. An ink discharge port 4 is perforated. An air flow is sent from the periphery to the air layer 7 created by the nozzle plate 2 and the wall 3, and flows out from the air outlet 1.

Since the direction of the air flow changes rapidly near the air outlet 1, a rapid change in pressure gradient occurs in the space from the ink outlet 4 to the air outlet 1.

On the other hand, a constant pressure is applied to the ink inside the ink ejection port 4, and when the inkjet recording head is not driven, the air pressure near the ink ejection port 4 and the ink pressure inside the ink ejection port 4 are Adjustment is made so that the ink meniscus within the ink ejection port 4 is maintained stationary at approximately the same level.

The signal source 5 is electrically connected to a conductive ink supply pipe 6 communicating with the nozzle plate 2 and the ink discharge ports 4 so that a potential difference is generated between the nozzle plate 2 and the ink in the ink discharge ports 4. There is. When a potential difference is generated between the nozzle plate 2 and the ink within the ink discharge port 4 due to the signal source 6, the meniscus generated in the ink discharge port 4 is stretched in the direction of the air discharge port 1 due to the electric field caused by this potential difference. A rapid change in pressure gradient occurs in the space from the ink discharge port 4 to the air discharge port 1, and the degree of change is greater as it approaches the air discharge port 1. When the ink meniscus is stretched beyond a certain length, it is torn off by a change in pressure gradient and flies out of the air discharge port 1 as ink droplets.

As explained above, the inkjet recording head shown in FIG. 1 changes the ink meniscus generated at the ink ejection port 4 by electrostatic force, and ejects ink droplets by the sudden change in pressure gradient generated at the tip of the meniscus. It is.

There are various configurations of the structure that causes the sudden change in pressure gradient, and the one that is judged to be suitable from the viewpoint of assembly process etc. in Japanese Patent Application No. 56-8428 is described in Fig. 1. It has such a configuration that an air flow is sent from around the air outlet 1 through a thin circular air layer 7 that communicates with a surrounding annular groove.

Two constant pressures PN are generated on the meniscus surface of the ink generated at the ink ejection port 4 due to the bending of the air flow. Constant pressure P
N is closely related to the change in pressure gradient that occurs near the ink discharge port 4, and in the same size structure, 2 constant pressure PN
The larger the value, the greater the change in pressure gradient.

As will be described later, even in an inkjet recording head in which a relatively effective change in pressure gradient can be obtained by examining various dimensional structures, it takes approximately o, o3Ky / to reach a constant pressure.
The results of careful experiments have shown that it is necessary to exceed crit. When the constant pressure PN is about 0.03 Ky/d or less, the change in the pressure gradient caused by the bending of the air flow is small, so there is not enough force to tear off the ink liquid, and the ink 6 - 7 is broken by electrostatic force. Even if the meniscus ′ is stretched, it cannot fly to the paper surface as an ink droplet.

Constant pressure PN 0.03 KP / ,! At the time of
It is L/S. The air discharge port 1 is generally designed to have a structure with almost no pressure loss due to air flow, so a constant pressure P
The ratio of N and the air flow rate from the air outlet 1 is approximately 1:1. Therefore, the constant pressure PN is 0.03 KP/
'c1 or more is equivalent to requiring the air flow velocity from the air discharge port 1 to be approximately 401 L/S or more.

The diameter of the air discharge port 1 is selected so that the air flow is not disturbed and the ink liquid flow is not disturbed.

Such turbulence occurs when the air flow velocity is high and the diameter of the air outlet 1 is large. The air flow velocity from the air discharge port 1 is required to be approximately 40 m/s or more, but if the speed is higher, the pressure gradient near the ink discharge port 4 will become even steeper. However, when the air flow velocity exceeds approximately 150 m/s, the velocity of the ink liquid increases, causing problems such as the ink liquid arriving on the paper surface scattering, and the constant pressure PN also increases. Unfortunately, the state of the pressure balance created to keep the ink meniscus generated at the ink ejection port 4 stable is likely to fluctuate, making it difficult to maintain stability in performance.

Therefore, it is desirable that the air flow velocity flowing out from the air discharge port 1 is in the range of about 40 to 160 m/s. In this range, the diameter of the air outlet 1 is desirably about 250 μm or less; if the diameter is about 260 μm or more, turbulence occurs in the air flow, and convergence of the air flow is poor, resulting in a large radius radius.

Another factor regulating the diameter of the air outlet 1 is the structure that effectively produces a change in pressure gradient due to bending of the air flow. It is better that the bending of the air flow occurs near the ink ejection port 4, and that the change in pressure gradient is large only in the direction of ink liquid flight and small in the direction orthogonal to the direction of flight, the better for the ink meniscus. The force that tears it apart works effectively.

In order for the airflow to curve near the ink discharge port 4, the diameter of the air discharge port 1 must be the diameter of the ink discharge port 7B-9゛4, and the size of the droplet flowing out from the ink discharge port 4. ,
Alternatively, the size should be close to the size of the ink meniscus generated at the ink ejection port 4.

However, although it is a matter of course, the ink liquid does not reach air outlet 1.
The dimensions are taken into consideration so that the air can pass through the air outlet 1 without being blocked.

``The diameter of the ink ejection opening 4 is set so as to obtain a dot diameter suitable for the purpose on the recording material.

However, in the inkjet recording apparatus shown in FIG. 1, the retention force of the ink meniscus that occurs at 0.4 ink ejection becomes an important factor. For ink discharge port 4 with radius r and ink with surface tension T, the holding force is T
/ Proportional to r. Generally, the surface tension of ink has an upper limit, usually 20 to 50 dyV', and at least 70 ayn/min. Therefore, the smaller the ink discharge port 4, the greater this holding force becomes. For this reason, in order to obtain a holding force to keep the ink meniscus generated at the ink ejection port 4 stable, the diameter of the ink ejection port 4 is desirably about 10 μm or less.

The retention force of the ink meniscus is weak, and the ink meniscus responds sensitively to changes in air pressure, pressure changes due to impact, etc., resulting in an unstable inkjet recording head that is vulnerable to external disturbances.

The size of the air layer 7 is selected so that air can flow smoothly, and the change in pressure gradient generated near the ink ejection port 4 is large. Unless the thickness of the air layer 7 is 20 to 30 μm or more, the air flow will not flow smoothly. This is due to the fact that the flow path resistance in the air layer 7 is easily affected by the flow resistance and the adhesion of foreign matter or ink liquid. When the flow path resistance in the air layer 7 increases, the air pressure in the vicinity of the ink ejection port 4 decreases, resulting in a disadvantage that the change in pressure gradient becomes smaller. Furthermore, in order for the change in the pressure gradient occurring near the ink ejection port 4 to be large, a structure must be provided in which the air flow is curved near the ink ejection port 4. For this purpose, the smaller the thickness of the air layer 7, the better.

Normally, the ratio of the thickness of the air layer 7 to the air ejection opening 1 is preferably about 2.69 mm or less, and if it is 3 or more, the change in pressure gradient becomes small and the ejection force of ink droplets decreases.

The thickness of the nozzle plate 2 is determined so that the air discharge ports 1 can be easily perforated, ink droplets can easily pass through, or
This is determined based on the fact that the nozzle plate 2 has sufficient rigidity, and the size is usually selected to be about 1/2 to 6 times the diameter of the air discharge port 1.

An example of the inkjet recording head shown in FIG. 1, which was manufactured for the reasons detailed above, is as shown in Table 1. , the ink liquid can be ejected when the potential difference caused by the signal source 6 is in the range of approximately 200V to 1kV, and it can be driven at a very low voltage compared to other types of inkjet recording devices that use electrostatic force. This is a type of inkjet recording head. The reason why low-voltage driving is possible is that: 1. The sudden change in pressure gradient caused by the bending of the air flow is the source of energy for the flight of ink droplets. (2) It is conceivable that the distance between the nozzle plate 2 and the ink ejection openings 4 is very narrow, and a large electrostatic force acts due to the low potential difference.

Now, assuming that the air flow does not flow out from the air discharge port 1 in the inkjet recording head shown in FIG.
When a potential difference is applied between the nozzle plate 2 and the ink discharge port 4, the meniscus of ink in the ink discharge port 4 is stretched to the vicinity of the air discharge port 1, but it is further stretched and the ink flies from the air discharge port 1. Never. This is because the distance between the air outlet 1 and the ink outlet 4 is extremely short.
The ink liquid stretched from inside the ink discharge port 4 is 11,
-〇In addition to not being able to provide enough speed to pass through the air outlet 1, even if the ink liquid were to pass through the air outlet 1, a pulling force in the opposite direction generated by the ink liquid being charged acts on the ink liquid. This is because it is not enough to fly. Furthermore, since the structure of the reservoir that causes a sudden change in pressure gradient due to the air flow is achieved by a very small dimensional structure, the air layer 7 and the air outlet 1 are affected by capillary action due to the surface tension of the ink. However, the ink discharge port 4 is easily filled with ink. This filled ink cannot be easily cleared without an air flow. In other words, it is naturally difficult for the ink liquid to fly just by the potential difference applied to the ink within the nozzle plate 2 and the ink discharge port 4. .

The nozzle plate 2 has an air discharge port 1 and also serves as an electrode for generating a potential difference and stretching the ink liquid. Patent application 1986-35711 or patent application 1973
No. 6-35713 describes an invention related to the structure of the nozzle plate 2, and presents a method for creating the nozzle plate 2 using an electrical insulator and a conductor.

Figure 2 is a special request from 1930! FIG. 3 is an enlarged view of the air discharge ports 1 and ink discharge ports 4 of the inkjet recording head described in Japanese Patent No. 35711. In the configuration shown in FIG. 2, an insulating substrate 8, for example, a glass epoxy laminate with a thickness of about 150 μm, is used as a nozzle plate, an air discharge port 1 is perforated in this, and an electrode 9 is formed around the hole using copper with a thickness of about 30 μm. It was formed.

In this configuration, the air discharge port 1 is easily perforated, and electrodes of various shapes can be easily formed using techniques such as etching, so that multi-nozzle formation is easy.

The present invention provides an improved configuration of the ink ejection ports 4 in the inkjet recording head configured as described above. A detailed explanation will be given below based on examples.

FIG. 3 shows a structure in which the air outlet 1 and the electrode 9 are configured according to FIG. 2, and the ink outlet 4 is bored in the orifice plate 13. In the figure, the same parts as in FIGS. 1 and 2 are designated by the same reference numerals 13, -6-, and the explanation thereof will be omitted.

The main reason for forming the ink ejection ports 4 in the orifice plate 13 is to improve responsiveness to repeated signal pulses.

When a signal pulse is applied, the ink meniscus in the ink ejection port 4 is stretched, and is torn off by a sudden change in pressure gradient, causing the ink liquid to fly. However, immediately after the ink liquid is torn off, the ink meniscus is The shape becomes concave compared to the initial state. If the ink meniscus has a large concavity, the ink meniscus cannot be stretched without a very large electrostatic force, and ejection becomes impossible until the ink meniscus returns to its initial state. Therefore, the time it takes for the ink meniscus depression that occurs after the ink liquid is ejected to return to its initial state affects the responsiveness to repetition of signal pulses.

The restoring force that restores the ink meniscus dent to its initial state is exerted by the surface tension of the ink and the pressure difference generated inside and outside the ink ejection port 4. , is determined by the degree of depression and the magnitude of viscous resistance at the ink ejection port 4. In particular, the viscous resistance of the ink ejection port 4 has a large effect on the restoration time. The viscous resistance is Pr.

The diameter and length of the ink discharge port 4 are respectively d.

Let l be the viscosity of the ink, and η be the viscosity of the ink. There is a relationship.

Therefore, in order to improve the responsiveness to repetition of signal pulses, it is necessary to keep ηl/d2 below a certain value and shorten the restoration time.

In general, the characteristics of an inkjet recording head are as follows:
Since it is not practical unless the responsiveness to the repetition frequency of the signal pulse is at least 1 kHz, experiments were conducted on the responsiveness to a signal pulse with a pulse width of 500 μs, a repetition frequency of 1 kHz, and a drive voltage of 0.2 to 1 kV.

Table 2 Table 2 shows the experimental results, and the range in which ink liquid can be ejected in response to a repetition frequency of 1 kHz is ηl.
/d2 was 1.7 to 2.2 x 103 cp, less than /; drift.

As mentioned above, to reduce the restoration time of the ink meniscus, ηl! /'d In addition to decreasing the value of 2, there are also methods to increase the restoring force or to decrease the dent that occurs in the ick meniscus, but in practice,
The surface tension of the ink has a narrow range of variation and is at least 70 a
yn/ca or less, and the diameter of the ink ejection port 4 is
Mainly, since it is determined that the ink liquid is ejected so that the character 9 image etc. can be recorded with a predetermined size and density, η
It is difficult to obtain sufficient effects without considering the value of l/d2.

Although there are other methods to reduce the concavities that occur in the ink meniscus, even if such methods are considered, it is desirable to satisfy ηl/d2≦2.2×103 c p /an.

As is clear from the detailed explanation above, the shorter the length of the ink ejection ports 4, the better. Therefore, in FIG. 3, a thin orifice plate 13 with ink ejection ports 4 perforated is used. . The orifice plate 13 may be an insulator, but
It is advantageous to use a metal material that is easy to form the ink discharge port 4 and is rigid, such as stainless steel.

In the configuration shown in FIG. 3, even if a potential difference is created between the electrode 9 and the ink supply tube 6, the electrode 9 and the orifice plate 13
Even if a potential difference is created between them, the same ink liquid can be ejected. Electrode 9, orifice plate 13, and ink supply tube 6
In the case where the potentials of the 17 vanes' are different from each other, the ink liquid is ejected according to the potential difference between the electrode 9 and the orifice plate 13, and the potential of the ink supply pipe 6 becomes irrelevant. From this, it can be seen that the orifice plate 13 and the ink supply tube 6 may be electrically connected, and the body 10 may be made of a conductive material.

In the present invention, an ink discharge port is formed in a thin orifice plate, and the right side of the above equation (1) is 2.2X103 cp/crn.
If the configuration is as follows, the configuration will be improved, and the responsiveness to repetition of the signal pulse for ejecting ink droplets will be improved.

[Brief explanation of drawings]

It is a diagram. 1... Air discharge port, 2... Nozzle plate,
3... Wall, 4... Ink discharge port, 6...
...Signal 18 page 7 Source, 6...Ink supply tube, 7...Air layer, 8...Insulating substrate, 9... electrode, 1
o...Body, 13...Orifice plate. Name of agent Patent attorney Toshio Nakao and one other person (α
ν (b) 113 Figure v;y VJ

Claims (2)

[Claims] (1) An air flow having a constant flow velocity is ejected from an air outlet with a sharp curve, and the air flow is opposed to the air outlet in a space having a sudden change in pressure gradient caused by the curve of the air flow. An ink ejection port is provided in the ink ejection port, the shape of an ink meniscus formed at the ink ejection port is changed by an electric field force, and an ink droplet is ejected and stopped by a change in a pressure gradient generated in the meniscus. An inkjet recording head characterized in that ink ejection ports are formed by orifices bored in a thin plate. (2) If the diameter of the ink discharge port is di, the length is l, and the viscosity of the ink is η, then the ink discharge port is ηA'/
The inkjet recording head according to claim 1, which satisfies di2≦2.2×103 cp 7cm.