JPS6330867B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is based on a patent application filed in 1983 by the same applicant.
This invention relates to an improvement in the inkjet recording head described in No. 8428. FIG. 1 shows an example of an inkjet recording apparatus described in Japanese Patent Application No. 8428/1983. 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 flow 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 the pressure gradient occurs in the space from the air outlet 1 to the air outlet 1, and the degree of change is greater as the air approaches the air outlet 1. Therefore, the ink meniscus generated at the ink outlet 4 is When it is stretched beyond a certain length, it is torn off by the change in pressure gradient and flies out of the air outlet 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. The 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 larger the change in the pressure gradient is. 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 as droplets to the paper surface. 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 will occur in the air flow, or the air flow will have poor convergence, resulting in 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 that tears off the ink meniscus 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 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, the holding force is proportional to T/r. In general, the surface tension of ink has an upper limit, usually 20~
Many are 50dyn/cm, at least 70dyn/cm
It is as follows. Therefore, the smaller the ink discharge port 4, the greater this holding force becomes. For this reason, in order to obtain the holding force required to keep the ink meniscus generated at the ink discharge port 4 stable, the diameter of the ink discharge 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, which was 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 at 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. 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 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 port 1 and ink discharge port 4 portion of the inkjet recording head described in Japanese Patent Application No. 56-35711. In the configuration shown in Figure 2, the insulating substrate 8 has a thickness of approximately 150 μm.
Using a glass epoxy laminate with a thickness of approximately 30μ
The electrode 9 was formed of copper with a thickness of m. This is an application of what is called a flexible printed circuit board, and those using resins such as Teflon, polyester, and polyimide as insulating substrates are already commercially available. In this example, a glass epoxy laminate is used, but it is of course possible to use other resins. When such a flexible printed circuit board is used, it is easy to make holes for the air outlet 1, and electrodes 9 of various shapes can be easily formed using techniques such as etching, so a multi-nozzle structure can be easily realized. . However, in an inkjet recording head using such a flexible printed circuit board, variations may occur in the amount of ink ejected or in the appropriate driving voltage value. This is because the air flow is constantly flowing out from the air outlet 1, and the air flow in the air layer 7 is 0.03 to 0.2
This is because the air pressure is Kg/cm ² and the insulating substrate 8 is displaced. The thickness of the air layer 7 is approximately 30
~400μm is possible, but this thickness variation is
If it is about 80 μm in a design with a large tolerance range, and if it is about 5 μm in a design with a small tolerance range, it will affect the ejection state of the ink liquid. Generally, when the variation in the thickness of the air layer 7 is about 10 to 30 μm or more,
Fluctuations occur in the air pressure in the air layer 7, particularly in the vicinity of the ink ejection ports 4, causing a change in the shape of the ink meniscus formed at the ink ejection ports 4, and the state of ink liquid ejection changes significantly. The present invention aims to eliminate such drawbacks, and one embodiment thereof will be described in detail below. FIG. 3 shows an embodiment of the present invention, which solves the above-mentioned difficulties by using a conductive substrate 13 in which an orifice can be easily formed. For the conductive substrate 13, metals such as iron, copper, aluminum, nickel, gold, silver, platinum, etc., alloys such as stainless steel, duralumin, etc., or semiconductor single crystals such as silicon, germanium, etc., which can be easily drilled with orifices, can be used. be. For metals and alloys, orifices can be easily formed by drilling, electrical discharge machining, or other means, and for semiconductor single crystals, orifices can be easily formed by etching. A conductive substrate 13 in which an orifice for the air outlet 1 is bored is coated with an insulating coating 12 in advance. The insulating film 12 can be formed of an oxide film such as aluminum oxide or silicon oxide film, or a polymer resin film. The electrode 9 has an insulating coating 12
It is provided near the upper air discharge port 1. According to a good example, an orifice is formed in aluminum with a thickness of about 250 μm, an oxide film of about 20 to 30 μm is formed by anodizing, and after surface polishing, an electrode 9 is formed by vapor deposition of copper. An inkjet recording head with stable characteristics was obtained. Recently, insulation methods using heat-curable polymer resins have been advanced in the field of integrated circuits, and naturally good results can be obtained by applying such materials to the insulation coating 12. Note that in the configurations shown in FIGS. 2 and 3, when multi-nozzle configuration is performed, a plurality of electrodes 9 are arranged in a fine configuration corresponding to the ink discharge ports 4 or the air discharge ports 1. In such a case, in order to protect the electrodes 9 and prevent discharge between the electrodes, it is necessary to form a protective film of an insulating material to cover at least the electrodes 9. This protective film is formed of the aforementioned oxide film, polymer resin, or the like. As explained above, in the present invention, a conductive substrate having air discharge ports perforated therein is coated with an insulating film, and an electrode is formed on this insulating film, which facilitates the formation of an orifice and provides stable performance. It is possible to provide an inkjet recording head having the following.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an inkjet recording head according to an earlier application of the present invention, FIGS. Figure 3 a, b
1 is a sectional view and a plan view of an air nozzle portion of an embodiment of an inkjet recording head according to the present invention. 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, 11...Jig, 1
2... Insulating coating, 13... Conductive coating.

Claims (1)

[Claims] 1. Install an air outlet on the same axis as the ink outlet and face it, and send air from the air outlet while creating a sharp bend in the gap between the ink outlet and the air outlet. a first means for forming a space in which the pressure gradient changes such that the air pressure decreases in the direction leading to the discharge port; and a first means for forming a space in which the pressure gradient changes such that the air pressure decreases in the direction reaching the discharge port; a second means for selectively generating a potential difference between an electrode provided around the air discharge port on the insulating coating and ink within the ink discharge port according to a signal; The meniscus of the ink generated at the ink ejection port is stretched by the attractive force caused by the potential difference, and the accelerating force of the air flow generated in the space where the pressure gradient changes is applied by the first means, so that the ink within the ink ejection port is drawn to the air ejection port. An inkjet recording head characterized by ejecting ink to the outside through a process.