JPS6330868B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in an inkjet recording head as disclosed in Japanese Patent Application No. 56-8428 filed by the same applicant. FIG. 1 shows an inkjet recording head described in Japanese Patent Application No. 56-8428. In the figure, a conductive noise plate 2 has an air outlet 1 perforated therein, and a wall 3 is arranged parallel to this nozzle plate 2.
Further, an ink discharge port 4 is perforated in the wall 3 so as to face the air discharge port 1 . 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 approximately equal. Adjustment is made so that the meniscus of ink within the ink ejection port 4 is kept stationary. The signal source 5 connects the nozzle plate 2 so that a potential difference is generated between the nozzle plate 2 and the ink inside the ink ejection port 4.
It is electrically connected to a conductive ink supply pipe 6 communicating with the ink discharge port 4 . When a potential difference is generated between the nozzle plate 2 and the ink within the ink discharge port 4 by the signal source 5, a meniscus generated in the ink discharge port 4 is stretched in the direction of the air discharge port 1 due to an electric field caused by this potential difference. Ink discharge port 4
A rapid change in pressure gradient occurs in the space from the air outlet 1 to the air outlet 1, and the degree of this change is greater as the air approaches the air outlet 1. Therefore, the ink meniscus generated at the ink outlet 4 When it is stretched beyond a certain length, it is torn off by this 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.
Ink droplets are ejected by changing the meniscus of ink that occurs at the tip of the ink, and by causing a sudden change in the pressure gradient that occurs at the tip of the meniscus. There are various structures for producing the above-mentioned rapid change in pressure gradient, and detailed explanations are given in Japanese Patent Application No. 8428/1983. Among them, the one that is judged to be suitable in terms of performance and assembly process is the one with the configuration shown in Figure 1, in which the air layer 7 is connected to the surrounding annular groove through a thin circular air layer 7. hand,
This configuration is such that an air flow is sent from around the air outlet 1. A constant pressure P _N is generated on the meniscus surface of the ink generated at the ink discharge port 4 due to the bending of the air flow. This constant pressure P _N is closely related to the change in the pressure gradient that occurs near the ink ejection port 4, and in a structure with the same dimensions, the larger the constant pressure P _N is, the more
The change in pressure gradient is also large. As will be described later, even in an inkjet recording head in which relatively pressure gradient changes can be effectively obtained by examining various dimensional structures, a constant pressure
Careful experimental results have shown that P _N is required to be approximately 0.03 Kg/cm ² or more. Constant pressure P _N is approximately 0.03Kg/
cm ² or less, the change in 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 even if the ink meniscus is stretched by electrostatic force, the ink liquid It cannot fly to the paper surface as a droplet. When the constant pressure P _N is 0.03 Kg/cm ² , the flow velocity of the air flowing out from the air outlet 1 is about 40 m/s. Air outlet 1 is generally designed to have a structure with almost no pressure loss due to air flow, so a constant pressure is maintained.
P _N and the air flow velocity from air outlet 1 are approximately 1:
Corresponds to 1. Therefore, the constant pressure P _N is 0.03Kg/
The fact that it is required to be at least cm ² is equivalent to the fact that the air flow velocity from the air outlet 1 is required to be at least about 40 m/s. 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 outlet 1 is required to be approximately 40 m/s or more, but if the velocity is higher, the pressure gradient near the ink outlet 4 will become even steeper. however,
If the air flow velocity exceeds about 150 m/s, the velocity of the ink liquid will increase, causing problems such as the ink liquid arriving on the paper surface scattering, and the above-mentioned constant pressure P _N
As a result, the state of the pressure balance, which is established 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 velocity flowing out from the air outlet 1 is in the range of about 40 to 150 m/s. In this range, the diameter of air outlet 1 is approximately
A value of 250 μm or less is desirable, and if the value is about 250 μm or more, turbulence occurs in the air flow, or the air flow becomes poorly converged and becomes a wide-spread flow. 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. Where airflow bends,
It is better to be near the ink ejection port 4, and if the change in pressure gradient is large only in the ink liquid flying direction and small in the direction orthogonal to the flying direction, the force for tearing off the ink meniscus works more effectively. In order for the airflow to curve near the ink discharge port 4, the diameter of the air discharge port 1 must be determined by the diameter of the ink discharge port 4, the size of the droplet flowing out from the ink discharge port 4, or the diameter of the air discharge port 4. The size should be close to the size of the ink meniscus that occurs in . However, as a matter of course, the ink liquid is not blocked by the air outlet 1 and
Consider the dimensions that will allow it to pass through. The diameter of the ink ejection opening 4 is set so as to obtain a dot diameter suitable for the intended use on the recording material. However, in the inkjet recording apparatus shown in FIG. 1, the retention force of the ink meniscus formed at the ink ejection opening 4 is an important factor. For ink discharge port 4 with radius r and ink with surface tension T, this holding force is proportional to T/r. In general, the surface tension of ink has an upper limit, usually 20 to 50 dyn/cm, and at least
It is 70dyn/cm or less. 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 100 μm or less. If the diameter of the ink ejection port 4 exceeds approximately 100 μm, the retention force of the ink meniscus is weak, and the ink meniscus responds sensitively to changes in air pressure or pressure due to shock, making it unstable and vulnerable to external disturbances. This makes it an excellent inkjet recording head. The air layer 7 has dimensions that allow air to flow smoothly,
In addition, the dimensions are selected such that the change in the pressure gradient generated near the ink discharge 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 because the flow path resistance in the air layer 7 increases and the air layer is susceptible to adhesion of foreign matter or ink liquid. air layer 7
When the flow path resistance 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 that occurs near the ink ejection port 4 to be large, the structure must be such that 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. Usually, the ratio of the thickness of the air layer 7 to the air ejection opening 1 is preferably about 2.5 or less, and if it is more than 3, the change in pressure gradient becomes small and the ejection force of ink droplets decreases. The thickness of the nozzle plate 2 is determined based on factors such as ease of perforation of the air outlet 1, easy passage of ink droplets, or sufficient rigidity of the nozzle plate 2. 1/2 of the diameter
A size approximately 5 times larger is selected. Table 1 shows an example of the inkjet recording head shown in FIG. 1 manufactured for the reasons detailed above.

[Table] The inkjet recording head shown in Table 1 is capable of ejecting ink liquid when the potential difference caused by the signal source 5 is in the range of approximately 200V to 1KV. This is an inkjet recording head that can be driven with a much lower voltage than a recording device. The reason why low-voltage driving is possible is that the sudden change in pressure gradient caused by the bending of the airflow is the source of energy for the flight of ink droplets. , the distance between the nozzle plate 2 and the ink ejection opening 4 is very narrow, and a large electrostatic force acts with a low potential difference. etc. are possible. Now, assuming that the air flow does not flow out from the air outlet 1 in the inkjet recording head shown in FIG. Although the meniscus is stretched to the vicinity of the air outlet 1, it is further stretched and does not fly beyond the air outlet 1. This is because the distance between the air outlet 1 and the ink outlet 4 is very short.
In addition to the fact that the ink liquid stretched from within the ink ejection port 4 cannot be given enough speed to pass through the air ejection port 1, even if it were to pass through the air ejection port 1, the ink liquid would be charged with electricity. This is because the pulling force in the opposite direction that occurs when the ink is removed does not cause the ink to fly. Furthermore, since the structure for causing a rapid change in pressure gradient due to air flow is achieved by a very small dimensional structure, the air layer 7 and the air outlet 1 are affected by the surface tension of the ink. Due to capillarity, the ink discharge port 4 is easily filled with ink. This filled ink cannot be easily cleared without air flow, i.e.
Naturally, it is difficult to cause the ink liquid to fly if there is only a potential difference applied to the ink within the nozzle plate 2 and the ink discharge ports 4. The nozzle plate 2 has an air discharge port 1 and also serves as an electrode that generates a potential difference and stretches the ink liquid. Japanese Patent Application No. 56-35711 or Japanese Patent Application No. 56-35713 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. has been done. FIG. 2 is an enlarged view of the air discharge ports 1 and ink discharge ports 4 of the inkjet recording head described in Japanese Patent Application No. 56-35711. The configuration shown in Fig. 2 uses a glass epoxy laminate plate 8 with a thickness of about 150 μm as a nozzle plate, in which an air discharge port 1 is perforated, and an electrode 9 is made of copper with a thickness of about 30 μm around the hole.
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. Next, the ink ejection ports 4 will be explained in detail. FIG. 3 shows the air outlet 1 and electrode 9 connected to the second
It is constructed as shown in the figure, and has a structure in which ink discharge ports 4 are bored in an orifice plate 13, and is an improvement on the structure shown in FIG. The main reason why the ink ejection ports 4 are formed by perforating the orifice plate is to improve the 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. has a concave shape 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, making ejection virtually impossible. 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. The restoring time due to this restoring force depends on the magnitude of the restoring force, It 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
Letting d, l, and the viscosity of the ink be η, the relationship is Pr∝ηl/d ² (1). Therefore, in order to improve the responsiveness to repetition of signal pulses, it is necessary to keep ηl/d ² below a certain value and shorten the restoration time. In general, the characteristics of an inkjet recording head are such that it is not practical unless the responsiveness to the signal pulse repetition frequency is at least 1KHz.
We conducted experiments on the response to signal pulses of Hz and driving voltages of 0.2 to 1 KV.

[Table] The experimental results are shown in Table 2, and the range in which ink liquid can be ejected in response to a repetition frequency of 1KHz is that ηl/d ² is 1.7 to 2.2×10 ³ cp/cm or less. It was hot. As mentioned above, in order to shorten the restoration time of the ink meniscus, in addition to reducing the value of ηl/d ² , there are also methods to increase the restoring force or to reduce the dent that occurs in the ink meniscus. In reality, the surface tension of ink has a narrow range of variation, being at least 70 dyn/cm or less, and the diameter of the ink ejection port 4 is mainly adjusted so that characters, images, etc. can be recorded with a predetermined size and density. Since the liquid is determined to be discharged, it is difficult to obtain a sufficient effect without considering the value of ηl/d ² , and it is difficult to satisfy ηl/d ² ≦2.5×10 ³ cp/cm. desirable. As is clear from the above explanation, the ink ejection port 4
The shorter the length of the ink outlet, the better. Therefore, in the configuration shown in FIG.
The one with perforations is used. The orifice plate 13 may be made of an insulating material, but it is advantageous to use a metal material that allows easy drilling of the ink discharge ports 4 and is rigid, and stainless steel is used here. In the configuration shown in FIG. 3, even if a potential difference is created between the electrode 9 and the ink supply pipe 6, and even if a potential difference is created between the electrode 9 and the orifice plate 13, the same ink liquid can be ejected. . When the potentials of the electrode 9, the orifice plate 13, and the ink supply tube 6 are different, 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 tube 6 becomes irrelevant. From this, it can be seen that the orifice plate 13 and the ink supply pipe 6 may be electrically connected, and the body 10 may be made of a conductive material. In the configuration shown in FIG. 3, the thickness of the insulating substrate 8 is approximately
250 μm, the diameter of air discharge port 1 is approximately 100 μm, the thickness of air layer 7 is approximately 85 μm, and the diameter of ink discharge port 4 is approximately 65 μm.
When we prototyped a large number of inkjet recording heads with a dimensional structure in which the thickness of the orifice plate 13 was approximately 50 μm, the minimum driving voltage was approximately 200 V.
Although it was constant, the minimum pulse width at which ink liquid could be ejected varied within a range of approximately 50 μs to 400 μs. The shorter the minimum pulse width at which the ink liquid can be ejected, the better, and in order to enable high-speed recording, the minimum pulse width should be made small. After carefully investigating the cause of the variation in minimum pulse width, we found that ink ejection port 4
It was found that this was caused by differences in the fine structure of The ink discharge ports 4 were used by removing burrs from an orifice drilled by drilling, but the ink discharge ports 4 produced in large numbers had slightly different states of burrs or sagging caused during polishing. Ta. Among these, the shape of the ink ejection opening 4 of the one having a particularly large minimum pulse width was a so-called clean-shaped orifice without burrs or sag. On the contrary, the shape of the ink ejection opening of the ink having a small minimum pulse width was a so-called clean orifice with many burrs and sag. This is because when the ink ejection port 4 has a perfect circular shape, the ink meniscus generated at the ink ejection port 4 will also have a shape close to a beautiful spherical surface, and the shape fluctuation (or depression) caused by ink liquid ejection will be large. ,
On the other hand, if the ink ejection port 4 has a so-called dirty shape, ink will adhere to sag or burrs, and the ink meniscus formed at the ink ejection port 4 will have a complicated shape, resulting in difficulty in ejecting ink droplets. This is because the accompanying shape variation becomes smaller. However, it can be said that it requires advanced technology to artificially form the sag or burr on the ink ejection port 4 into a certain shape. The present invention provides an inkjet recording head with uniform characteristics as described above, and will be described in detail below with reference to Examples. FIG. 4 shows an embodiment of the present invention, in which the air outlet 1 has a diameter of about 120 μm, and the ink outlet has five orifices 16 with a diameter of about 30 μm and an orifice plate 17. This uses holes that are densely perforated. Considering the viscous resistance at the ink discharge port in FIG. 4, Pr∝ηl/Nd ² ...(2) is obtained from equation (1). However, N indicates the number of orifices 16. Therefore, in the configuration shown in Figure 3, ηl/d ² is
It needed to be 2.5×10 ³ cp/cm or less, but the fourth
In the configuration shown in the figure, ηl/Nd ² should be 2.5×10 ³ cp/cm or less. Therefore, regarding viscous resistance, the fourth
The ink ejection port shown in the figure is equivalent to having one orifice with a diameter of about 67 μm. In addition, the ink meniscus formed at the ink ejection port in Fig. 4 is approximately 30 μm in diameter through the orifice 1.
6 and has a diameter of about 67 μm, it is held by a surface tension that is more than twice that of the orifice, and fluctuations associated with ink liquid ejection are also small. As described in detail above, the present invention provides an inkjet recording head with low viscous resistance and small fluctuations in the ink meniscus by providing a plurality of minute orifices as ink ejection ports. In the experimental example shown in FIG. 4, the driving voltage was about 800 V, and the minimum pulse width that enabled _PP droplet ejection was about 70 μs, allowing stable ejection.

[Brief explanation of the drawing]

1 is a sectional view of a conventional inkjet recording head, FIG. 2a is a sectional view showing a partial configuration example, FIG. 2b is a plan view, FIG. Figure a is a sectional view of an inkjet recording head in one embodiment of the present invention, and figure b is a plan view thereof. 1... Air discharge port, 2... Nozzle plate, 3...
Wall, 4... Ink discharge port, 5... Signal source, 6...
Ink supply pipe, 7... air layer, 8... insulating substrate,
9... Electrode, 10... Body, 13, 17... Orifice plate, 16... Orifice.

Claims (1)

[Claims] 1. An air nozzle plate in which air discharge ports are perforated, and a plurality of ink discharge ports arranged approximately parallel to the air nozzle plate, and in which a plurality of ink discharge ports are perforated in approximately the same axial direction near the axis of one air discharge port. an ink nozzle plate,
Air flow is sent out from the air outlet while creating a sharp bend in the gap between the ink outlet and the air outlet, creating a pressure gradient in which the air pressure decreases in the direction from the ink outlet to the air outlet. a first means for forming a variable space; and a second means for selectively creating a potential difference between the air nozzle plate and the ink in the ink ejection port in response to a signal; Stretching the ink meniscus generated at the ink ejection port by an attractive force due to the potential difference, and applying an accelerating force of the air flow generated in the space where the pressure gradient changes by the first means,
An inkjet recording head characterized in that ink within an ink ejection port is ejected outward through an air ejection port.