JPH0764059B2 - Inkjet recording device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an inkjet recording apparatus, and more particularly to an inkjet recording apparatus suitable for halftone recording capable of obtaining high quality recording by modulating the volume of ink droplets.

[Conventional technology]

By utilizing the deformation of the piezoelectric element that occurs when a voltage pulse is applied to the piezoelectric element, a pulse pressure is applied to the ink in the pressure chamber of the ink jet head (hereinafter referred to as the head), and a nozzle communicating with this pressure chamber is used. The so-called drop-on-demand type inkjet recording technology, which ejects ink to perform printing or image recording, has recently been actively applied to practical printers.

FIG. 5 shows a cross-sectional view of an example of a conventionally used head, and the structure will be described with reference to the drawing. The head 1 has a piezoelectric actuator 10 using a cylindrical piezoelectric element having electrodes 8 and 9 on the outer surface and the inner surface. The inside of the piezoelectric actuator 10 is a pressure chamber for applying pressure to the ink.
11, the ink is introduced from the supply port 12 and ejected from the nozzle 4. A check valve 13 for preventing backflow of ink to the supply port side is provided between the pressure chamber 11 and the supply port 12, and a fluid resistance element is provided between the pressure chamber 11 and the nozzle 4.
14 are provided. When a pulse voltage is applied between the electrodes 8 and 9 through the lead wire 15, the ink in the pressure chamber 11 is ejected to the nozzle side. At this time, the ink flow is due to the fluid resistance element.
Limited by 14.

It is widely known that this inkjet technology can modulate the volume of an ink droplet to some extent by changing the amplitude and pulse width of a voltage pulse applied to a piezoelectric element. For example, IEICE Technical Committee, IE83-59 (September 1983
30th), pp. 31-36, there is described an inkjet technique for modulating the ink droplet volume by changing the pulse width of the voltage pulse to obtain gradation image recording.

[Problems to be Solved by the Invention]

In the conventional inkjet recording apparatus described above, when the ink is ejected from the nozzle, the surface tension and the viscosity of the ink act so as to prevent the ink from separating from the nozzle.
Therefore, there is a limit to the minimum value of the ink droplet volume that can be changed only by controlling the pulse width applied to the piezoelectric element.

In order to eject the ink droplets from the nozzles of the head, it is necessary to give kinetic energy to the ejected ink that is sufficiently larger than the energy consumed when the ink is separated from the nozzles. Especially when the volume of the ink droplet is reduced, it becomes necessary to apply sufficient kinetic energy within a shorter time. However, in order to apply kinetic energy to the ink by the pressure generation mechanism using the piezoelectric difference, a constant transient time is required. Therefore, if the time width of the voltage pulse is made shorter than this transient time, the ink droplet There was a problem that the speed suddenly decreased and the recorded image was distorted. Therefore, there is a limit in reducing the ink droplet volume only by controlling the pulse time width. By controlling the amplitude of the voltage pulse together with the pulse time width control, it is possible to further reduce the volume of the ink droplet, but even in this case, there is a limit to the minimum volume of the ink droplet. Further, the electric circuit configuration of the pulse voltage applying means becomes very complicated, which causes problems such as a decrease in reliability of the recording device and an increase in cost.

For the above reasons, it has been difficult for the conventional inkjet technology to record the entire area of recording density, particularly the area of low recording density, only by volume modulation. Therefore, as described in the above-mentioned reference, in order to record a low-density portion, the minimum volume of ink droplets is used, and the number of ink droplets ejected per unit surface area of the recording paper is changed to apparently The method of changing the concentration of the was taken. The above gradation expression method has a drawback in that, when an attempt is made to obtain a large number of gradations, the unit pixel for determining the density becomes large and the resolution is lowered. On the contrary, in order to increase the number of gradations while keeping the pixel size constant, it is necessary to reduce the volume of ink droplets correspondingly, and the number of ink droplets that are ejected to the recording paper increases as a whole.
There was a problem that the recording time would be long. That is, the conventional technology has a problem that it is difficult to simultaneously satisfy the high gradation number, the high resolution, and the high speed recording.

It is an object of the present invention to provide an ink jet recording apparatus capable of modulating an extremely fine ink droplet volume so that a recording density range required only by ink droplet volume modulation can be expressed.

[Means for Solving the Problems]

The ink jet recording apparatus of the present invention according to claim 1 uses a head that ejects ink from nozzles by the action of a pressure pulse, and changes the time width in which the pressure pulse acts to change the volume of the ejected ink for halftone recording. In the inkjet recording apparatus for performing, a vibrating unit is provided for applying ultrasonic vibration to the head after the ink is ejected from the nozzle by the action of the pressure pulse and before the ejected ink is separated from the nozzle. Is configured.

The inkjet recording apparatus of the present invention according to claim 2 is the inkjet recording apparatus according to claim 1,
Instead of a vibrating means for applying an ultrasonic vibration to the head, an ultrasonic wave generating means for emitting an ultrasonic wave toward the ejected ink is provided.

[Action]

In the present invention, kinetic energy and ultrasonic energy are applied to the ink ejected from the nozzle. The ultrasonic energy newly applied by the present invention cannot contribute to the ejection or ejection of ink droplets at all, but is consumed for the ejected ink to separate from the nozzle. That is, when a fine wave of ultrasonic waves is generated on the surface of the ejected ink, the direction in which the surface tension acts is dispersed, and the force for returning the ink into the nozzle is significantly weakened. Also, the ultrasonic energy causes the ink temperature to rise sharply and the viscosity of the ink to significantly decrease, so that the ink is likely to be ejected from the nozzle. Furthermore, since the ultrasonic energy causes the viscosity to drop instantly during the jetting operation, the transient time for applying kinetic energy to the ejected ink is greatly shortened even for voltage pulses of the same amplitude, and only pulse time width control is required. It is possible to form an ink droplet having a sufficiently small volume.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of a head portion according to an embodiment of the present invention.
A head 1 having the same structure as the conventional example shown in FIG. 5 is fixed to a vibrating table 2. The head 1 is connected to a conduit 3 for guiding ink from an ink container (not shown), and ejects the supplied ink as a droplet 5 from a nozzle 4. A vibrator 7 is coupled to the foot 6 of the vibrating table 2 and constitutes a vibrating means. A piezoelectric element, an electrostrictive element, a magnetostrictive element or the like is suitable for the vibrator 7, but in the present embodiment, a so-called laminated piezoelectric actuator element constituted by alternately laminating piezoelectric layers and electrode layers is vibrated. The case where it is used as the child 7 is shown.

FIG. 2 shows the two types of operation when the pulse voltage is applied to the head 1 and the high frequency voltage is applied to the vibrator 7 to apply vibration to the head 1 in the operating state of the embodiment shown in FIG. It is a relationship diagram which shows the relative positional relationship in the time-axis of voltage. FIG. 2A shows an example of a pulse voltage waveform applied to the head 1. The ink is ejected from time t ₁ to t ₂ , and the ejected ink is ejected from time t ₂ to t _3. Is separated from the nozzle and ink is supplied from the supply port. On the other hand, FIG. 2B is an example of a high-frequency voltage applied to the vibrator 7, which is the time t ₄ from the time when the ink is ejected from the nozzle to be separated to the time t ₅ after the ink is separated from the nozzle. Is applied continuously. As a result, the ink ejected from the nozzle is constantly vibrated at the tip of the nozzle, the influence of the surface tension and viscosity of the ink is reduced, and the ink is easily separated from the nozzle. It is desirable that the application of the high frequency voltage be finished immediately after the ink is separated from the nozzle. If the application of the high-frequency voltage is further continued, air bubbles will be taken in from the nozzle during ink replenishment, and problems such as inability to eject ink will occur.

As an example of the dimensions of the head 1 in the above embodiment, the piezoelectric actuator 10 has an outer diameter of 1.2 mmφ, an inner diameter of 0.9 mmφ and a length of 18 mm.
mm, and the nozzle has an inner diameter of 40 μm. The optimum value of the pulse voltage applied to the head 1 with a time width from t ₁ to t ₂ was 28V. Vibration frequency of oscillator is 50 ~ 500kH
The optimum range is z, and the time width from t ₁ to t ₂ is changed in the range of 5 to 200 μs. When the vibrator is not operated, the minimum value of the time width that enables normal ink ejection operation is 15 μs. When the time width is changed between 15 and 80 μs, the recording dot size is 80 160 μmφ
The range has changed. On the other hand, when the oscillator is vibrated at 100kHz, the minimum time width can be reduced to 5μs,
It was possible to change the diameter of the recording dot size in the range of 40 to 160 μmφ with respect to the change of the time width of 5 to 80 μs.

FIG. 3 is a perspective view of an embodiment of ultrasonic wave generating means for applying high frequency vibration of MHz or more to the ink at the nozzle end in order to further improve the separability of the ink from the nozzle.
A ring-shaped piezoelectric element 18 having an external electrode 16 and an internal electrode 17 is used as a vibrator. An ultrasonic lens 20 is embedded in the piezoelectric element 18 in order to focus the ultrasonic wave on the nozzle tip portion 19 or in the vicinity thereof.

FIG. 4 is a sectional view of the ultrasonic lens 20. Ultrasonic lens
20 is made of an inorganic material such as metal or ceramics, and its cross-sectional shape is such that the inner surface 21 is formed into a spherical surface, and the nozzle end portion 19 or its vicinity is located at the center thereof. The ultrasonic waves emitted from the spherical surface are converged in the vicinity of the nozzle end 19 as shown by 22 in the figure, and give a strong vibration to the ejected ink surface.

As a typical dimension example, the ring-shaped piezoelectric element 18 has an outer diameter of 7 mm.
φ, the inner diameter is 5 mmφ, and the length is 2 mm, and the diameter of the inner surface of the ultrasonic lens 20 is 3 mmφ. The same head 1 as that used in the above-mentioned embodiment was used under the same operating condition as that in the above-mentioned embodiment, and the nozzle end thereof was arranged at the center of the ultrasonic lens. Immediately after applying the pulse voltage to the head 1, the external electrode of the piezoelectric element 18
A high frequency voltage was applied between the electrodes 16 and the internal electrode 17 to generate ultrasonic vibration of 5 MHz and continue for 120 μs. As a result, it was possible to eject ink with a sufficiently high ejection speed even if the time width of the pulse voltage was shortened to 3 μs. Further, the recording dot size in this minimum time width could be reduced to about 30 μmφ. From the above results, it was confirmed that by increasing the vibration frequency, the separability of the ink ejected from the nozzle from the nozzle end was further enhanced.

As the drop-on-demand type inkjet head, various types of structures are known today, and any type of head can be used in the inkjet recording apparatus according to the present invention.

〔The invention's effect〕

As described above, according to the present invention, it is possible to control that the viscosity and the surface tension of the ink act in the direction in which the separation of the ink ejected from the nozzle from the nozzle is prevented, and the ink much smaller than the conventional ink is used. Allowed to eject drops. As a result, the ink drop volume can be modulated over a wide range, and almost the entire density area of gradation recording of about 32 gradations can be expressed only by dot size modulation. In the conventional technique, a low density area which is 1/4 of the total density area cannot be expressed by dot size modulation.

[Brief description of drawings]

FIG. 1 is a perspective view of a head portion of an embodiment of the present invention, FIG. 2 is a relationship diagram showing a relative positional relationship of a pulse voltage and a high frequency voltage on a time axis, and FIG. 3 is another ink jet recording apparatus. FIG. 4 is a sectional view of an embodiment of the vibrating means, and FIG. 5 is a sectional view of a head portion of a conventional ink jet recording apparatus. 1 ... Inkjet head, 2 ... Shaking table, 3 ... Conduit, 4 ... Nozzle, 5 ... Ink drop, 6 ... Shaking table foot, 7 ... Transducer, 8 ... Outer surface electrode, 9 ... ... inner electrode,
10 ... Piezoelectric actuator, 11 ... Pressure chamber, 12 ... Supply port, 13 ... Check valve, 14 ... Fluid resistance element, 15 ... Lead wire, 16 ... External electrode, 17 ... Internal electrode, 18 ... Piezoelectric element, 19 ... Nozzle tip, 20 ... Ultrasonic lens, 21 ...
Inside.

Claims (2)

[Claims] 1. An inkjet recording apparatus for performing halftone recording by using an inkjet head for ejecting ink from a nozzle by the action of a pressure pulse, and changing the time width of the pressure pulse to vary the volume of the ejected ink. Inkjet recording, characterized by comprising a vibrating means for applying ultrasonic vibration to the inkjet head between the time when ink is ejected from the nozzle by the action of a pressure pulse and the time when the ejected ink is separated from the nozzle. apparatus. 2. The ink jet recording according to claim 1, further comprising ultrasonic wave generating means for emitting ultrasonic waves toward the ejected ink, instead of vibrating means for applying ultrasonic vibration to the ink jet head. apparatus.