JP2004114434A - Inkjet recording head and inkjet recording method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet recording head which can increase the number of nozzles in comparison with the prior art without greatly changing a conventional head size and a manufacturing technique, greatly improves a recording speed and can record highly fine images close to photographic images at a high speed. <P>SOLUTION: The inkjet recording head jets ink within many ink chambers formed in a substrate as ink droplets from a nozzle formed at one end of each ink chamber. In the inkjet recording head, two nozzles are formed for each one of the above ink chambers. The ink within the ink chamber is jetted simultaneously from each of the two nozzles, so that two pixels are formed simultaneously on a recording medium. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ink jet recording head, an ink jet recording apparatus, and an ink jet recording method, and more particularly, to an ink jet recording head capable of recording a high-quality image at high speed without significantly changing the structure and manufacturing technique of a conventional recording head. The present invention relates to an inkjet recording method.
[0002]
[Prior art]
At present, an image by ink jet recording is recorded at about 300 to 600 dpi. In 360 dpi, the size of one pixel is 70 μm × 70 μm, and dots are visible at an observation distance of 30 cm, so that an image having a rough feeling is produced in a highlight portion. In addition, since jaggies are seen in oblique lines, high-quality images cannot be formed. However, when the pixel density is set to 720 dpi or more, the granularity is greatly improved, the dots in the highlight portion cannot be visually recognized, and there is no roughness, and the jaggedness cannot be seen even in oblique lines. High quality images can be obtained.
[0003]
As described above, in order to obtain a high-quality image close to a photograph, it is necessary to increase the resolution, and for that purpose, it is effective to reduce the dot diameter. If the dot diameter is reduced, the number of dots per unit area increases, so that the number of matrices for gradation expression can be increased. At 720 dpi, the size of one pixel is 35 μm × 35 μm.
[0004]
An image formed by inkjet recording is formed by circular dots. On the other hand, since the pixels are square, unless the dot diameter of the ink droplet is made larger than the pixel pitch so that adjacent dots can overlap each other, white streaks will occur in the recorded image. For example, at 360 dpi, the size of one pixel is 70 μm × 70 μm. To fill this with circular dots, the dots are circles circumscribing square pixels, and a dot diameter of 100 μm is required. On the other hand, in order to record a high-definition, high-quality image at a pixel density of 720 dpi, the size of one pixel is 35 μm × 35 μm. To fill this with circular dots, the dot diameter must be 50 μm. There is a need to.
[0005]
The relationship between the nozzle diameter, the ink ejection amount, and the dot diameter is such that when an ink droplet is ejected, an ink column having the same thickness as the nozzle diameter is formed, which breaks into ink droplets, lands on the recording medium, and spreads slightly. Form dots.
[0006]
FIG. 6 shows an example in which the relationship between the ejected ink droplet amount and the dot diameter is measured. The dot diameter changes depending on the degree of bleeding on the recording medium, but this is the result of discharging onto coated paper with little bleeding that can achieve a photographic image. For example, in order to obtain a dot diameter of 50 μm on a recording medium in order to record an image with a pixel density of 720 dpi, about 4 to 10 pl (picoliter) may be ejected, and to record an image with a pixel density of 360 dpi. It has been found that to obtain a dot diameter of 100 μm on a recording medium, it is sufficient to eject 40 to 50 pl.
[0007]
That is, in a 360 dpi recording head, if one droplet of 50 pl is ejected, a dot of 100 μm can be formed to cover one pixel. In addition, when one droplet of 10 pl is ejected by a 720 dpi recording head, a dot of 50 μm is formed to form one pixel. Comparing with the same covering area, the area that can be covered by one 50 pl ink drop must be ejected by four or more drops with a 10 pl ink drop. For this reason, when printing a high-definition image with a pixel density of 720 dpi, if the driving frequency is the same, the printing speed is reduced to 1/4 or less as compared with the case where the pixel density is 360 dpi. Even if the driving frequency is doubled, the recording speed is reduced to half or less.
[0008]
However, when the recording head is driven at a high frequency, heat is generated and the heat is transmitted to the ink, whereby the viscosity of the ink is reduced and the ejection speed of the ink droplet is increased. For example, when the temperature of the ink rises by 5 ° C., the speed of the ink droplet increases by 1 m / sec or more. When the print head is ejected while moving in the main scanning direction, the landing position of the ink droplet shifts by 18 μm or more in the main scanning direction. become. This is unfavorable because an image having a pixel density of 720 dpi is shifted by half a pixel, thereby deteriorating the image quality.
[0009]
Therefore, it is effective to increase the number of nozzles of the recording head in order to increase the recording speed of the ink jet recording apparatus. However, if the width of the recording head is increased to increase the number of nozzles and the number of ink chambers is increased, the recording head becomes large, and there is a problem that manufacturing becomes difficult due to bending or distortion due to a difference in thermal expansion. It is also known to increase the number of nozzles by overlapping the same recording head. However, it is impossible to overlap two or more nozzles due to heat generation of the head, and the number can be increased only to at most a double.
[0010]
For this reason, it is desired that the number of nozzles be further increased and the high-definition image close to a photographic image can be recorded at high speed without increasing the width of the recording head as compared with the related art. .
[0011]
2. Description of the Related Art Conventionally, there has been known a technique of forming a gradation image by providing a plurality of nozzles in one ink chamber and changing the ink ejection amount (Patent Documents 1 to 7).
[0012]
Patent Document 1 discloses that when a plurality of nozzles having different diameters are provided in one ink flow path and the flow resistance in the nozzles decreases in inverse proportion to the square of the diameter, the energy generated by the heater is small. When the energy is increased, the ink is ejected from both the large and small diameter nozzles, thereby changing the amount of ink ejected and changing the dot area and shape on the recording medium. A method for creating an image having a halftone using an inkjet capable of recording only a value image is described.
[0013]
However, this method uses a method in which a heater is heated to generate vapor bubbles, and the ink is ejected with that force.Therefore, unlike the piezo method, the ejection amount of ink droplets greatly changes even when the driving conditions are changed. It is not intended to record high-definition images at high speed.
[0014]
Patent Document 2 discloses a recording head that discharges ink using the same vapor bubbles as described above. A plurality of nozzles having the same size are provided in one ink chamber, and a plurality of small droplets are discharged from a plurality of discharge ports. This is a method of discharging and merging on a recording medium. However, this is because, when an image is formed by combining a plurality of small droplets, rather than forming an image with one large droplet, the resolution is high, the fixability is good, and the shape of the dot approaches from a circle to a square, This is because the image density is increased and a high-quality image can be formed, and it is not intended to record a high-definition image at a high speed.
[0015]
Patent Document 3 discloses that in a recording head in a shear mode using a piezoelectric element, two large and small nozzles having different flow path resistances are provided in one ink chamber, and when the ejection pressure is low, ejection is performed only from a large nozzle. When the ink droplets ejected from both large and small nozzles are high, the ink droplets ejected from the small nozzles land on the ink droplets ejected from the large nozzles and coalesce to form ink droplets of different sizes by the same ejection. A method of discharging is described. However, there is no mention of recording a high-definition image at a high speed.
[0016]
Patent Document 4 discloses that, in a print head that discharges a vibrating plate by vibrating the vibrating plate with static electricity, when a plurality of nozzles are formed in one ink chamber and one pixel is formed by a plurality of ink droplets, one nozzle is clogged. Describes that the reliability is improved because no pixel is missing. However, there is no mention of recording a high-definition image at a high speed.
[0017]
Patent Literature 5 describes a method in which N droplets are ejected at intervals of 1 / N of the pitch of a recording unit in response to one recording signal and are filled without gaps. However, there is no mention of recording a high-definition image at a high speed.
[0018]
Patent Documents 6 and 7 disclose techniques for using an inkjet head having a plurality of orifices in one ink chamber for barcode recording. This is a method in which a large number of nozzles are provided in one ink chamber to reduce the number of head channels. However, since it is for barcode recording, the nozzle density is as low as 140 to 200 dpi, and cannot be used for forming high-quality images.
[0019]
[Patent Document 1] JP-A-10-166585
[Patent Document 2] JP-A-7-32596
[Patent Document 3] Japanese Patent Application Laid-Open No. 10-315463
[Patent Document 4] JP-A-9-57966
[Patent Document 5] Japanese Patent No. 2735852
[Patent Document 6] US Pat. No. 4,901,093
[Patent Document 7] US Pat. No. 4,714,934
[0020]
[Problems to be solved by the invention]
According to the present invention, the number of nozzles can be increased as compared with the conventional one without largely changing the size and manufacturing technology of the conventional head, and the recording speed is greatly improved, and a high-definition image close to a photographic image is recorded at a high speed. An object of the present invention is to provide an ink jet recording head that can perform the above-described operations.
[0021]
Another object of the present invention is to provide an ink jet recording method capable of recording a high-definition image with a gradation at high speed by using fine ink droplets.
[0022]
[Means for Solving the Problems]
The above object is achieved by the following inventions.
[0023]
1. In an ink jet recording head for discharging ink in a large number of ink chambers formed on a substrate as ink droplets from nozzles formed at one end of each ink chamber, two nozzles are formed for each of the one ink chamber, An ink jet recording head, wherein two pixels are simultaneously formed on a recording medium by simultaneously discharging room ink from the two nozzles.
[0024]
2. 2. The ink jet recording head according to claim 1, wherein two substrates on which the ink chambers are formed are overlapped so that the ink droplets are ejected in the same direction.
[0025]
3. 3. The ink jet recording head according to claim 2, wherein the two substrates are overlapped with each other with a shift of a half of a nozzle pitch with respect to an arrangement direction of ink chambers of each substrate.
[0026]
4. The substrate is formed by alternately forming a plurality of parallel grooves and side walls that become ink chambers in a polarized piezoelectric material, forming electrodes on the side walls, and applying an electric field to the electrodes to shear the side walls. 4. The ink jet recording head according to 1, 2, or 3, wherein the ink jet recording head is configured to be deformed so as to discharge ink in each ink chamber from a nozzle.
[0027]
5. By using the inkjet recording head according to any one of the above items 1 to 4, continuously ejecting the ink droplets from each nozzle to combine the continuously ejected ink droplets during flight or on a recording medium. An ink jet recording method characterized by the above-mentioned.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0029]
The ink jet recording head according to the present invention may be configured to discharge the ink in the ink chamber as ink droplets from a nozzle formed at one end of the ink chamber. The structure may be of any type, but here, the side walls constituting the ink chamber are formed of a polarized piezoelectric material, and an electric field is applied to the side walls to cause the side walls to undergo a shear deformation to discharge the ink in the ink chamber. This will be described using a so-called shear mode type recording head as an example.
[0030]
FIG. 1 is a longitudinal sectional view showing an example of such a recording head, and FIG. 2 is a sectional view taken along the line II-II in FIG.
[0031]
In the figure, H is a recording head, 1 is a piezoelectric material substrate, 2 is a nozzle plate, 3 is a nozzle, 4 is an ink channel, 5 is a side wall, 6 is a cover plate, 7 is an ink inlet, 8 is an FPC (flexible). Printed circuit board).
[0032]
In the piezoelectric material substrate 1, two piezoelectric materials 1a and 1b having different polarization directions are vertically bonded via an adhesive, and a groove serving as an ink chamber is formed from the upper piezoelectric material 1a by a diamond blade or the like. The plurality of ink channels 4 having the same shape are cut in parallel with the same shape, whereby adjacent ink channels 4 are defined by side walls 5 polarized in the direction of the arrow. In addition, the ink channel 4 has a deep groove 4a on the ink channel outlet side (left side in FIG. 1) of the piezoelectric material substrate 1, and gradually becomes shallower from the deep groove 4a toward the ink channel inlet side (right side in FIG. 1). And a shallow groove portion 4b.
[0033]
The piezoelectric material used for the piezoelectric material substrate 1 is not particularly limited as long as it is deformed by applying a voltage, and a known material may be used, and a substrate made of an organic material may be used. Preferably, a substrate made of a piezoelectric non-metallic material is used. As the substrate made of the piezoelectric non-metallic material, for example, a ceramic substrate formed through a process such as molding and firing, or formed without the need for molding and firing There are substrates and the like. Examples of the organic material include an organic polymer and a hybrid material of an organic polymer and an inorganic substance.
[0034]
PZT (PbZrO) ₃ -PbTiO ₃ ), PZT with a third component added, and Pb (Mg _1/2 Nb _2/3 ) O ₃ , Pb (Mn _1/3 Sb _2/3 ) O ₃ , Pb (Co _1/3 Nb _2/3 ) O ₃ And BaTiO ₃ , ZnO, LiNbO ₃ , LiTaO ₃ And the like.
[0035]
The substrate formed without the need for molding and firing can be formed, for example, by a Solgel method, a laminated substrate coating, or the like. According to the solgale method, the sol is adjusted by adding water, an acid or an alkali to a homogeneous solution having a predetermined chemical composition to cause a chemical change such as hydrolysis. Further, by applying a treatment such as evaporation or cooling of the solvent, a sol in which a precursor of fine particles of a desired composition or a non-metallic inorganic fine particle is dispersed is prepared, and can be used as a substrate. A compound having a uniform chemical composition can be obtained, including the addition of a small amount of a different element, and a water-soluble metal salt or metal alkoxide such as sodium silicate is generally used as a starting material. A compound represented by the formula M (OR) n, in which the OR group has a strong basic property, is easily hydrolyzed, and changes into a metal oxide or a hydrate thereof through a condensation process like an organic polymer. .
[0036]
In addition, there is a method of vapor deposition from the gas phase as a laminated substrate coating, and there are two methods of producing a ceramic substrate from the gas phase, a vapor deposition method by physical means, and a production method by a chemical reaction of the gas phase or the substrate surface. The physical vapor deposition method (PVD) is further subdivided into a vacuum deposition method, a sputtering method, an ion plating method, and the like, and a chemical method includes a gas phase chemical reaction method (CVD), a plasma CVD method, and the like. is there. Vacuum evaporation as physical vapor deposition (PVD) is a method in which a target substance is heated and evaporated in a vacuum, and the vapor is deposited on a substrate. This method utilizes the sputtering phenomenon in which atoms and molecules on the target surface exchange momentum with the colliding particles, and are repelled from the surface. Further, the ion plating method is a method of performing deposition in an ionized gas atmosphere. In the CVD method, a compound containing atoms, molecules or ions constituting a film is made into a gaseous phase, and then guided to a reaction section with an appropriate carrier gas, and then reacted or deposited on a heated substrate to form a film. In the plasma CVD method, a gas phase is generated by plasma energy, and a film is deposited by a gas phase chemical reaction in a relatively low temperature range of 400 ° C. to 500 ° C.
[0037]
On the upper surface of the piezoelectric material substrate 1, a cover plate 6 is adhered via an adhesive so as to cover the deep groove portion 4 a over all the ink channels 4, and on the shallow groove portion 4 b of each ink channel 4, An ink inlet 7 into the ink channel 4 is formed.
[0038]
In the recording head H according to the present embodiment, as described above, the heads H1 and H2 having the same structure as the substrate constituted by the piezoelectric material substrate 1 are configured such that the ink inflow ports 7 are in opposite directions and the ink ejection is performed. Two sheets are superposed so that the directions face the same direction (left side in the figure), and one nozzle plate 2 having the nozzles 3 opened is bonded via an adhesive over the front end surfaces of the heads H1 and H2. Have been.
[0039]
The material of the cover plate 6 is not particularly limited, and may be a substrate made of an organic material, but is preferably a substrate made of a non-piezoelectric non-metallic material. It is preferable to be selected from at least one of aluminum nitride, zirconia, silicon, silicon nitride, silicon carbide, quartz, and PZT. This non-piezoelectric material substrate includes, for example, a ceramic substrate formed through a process such as molding and firing, or a substrate formed without the need for molding and firing, and is formed through a process such as firing. As a ceramic substrate, for example, Al ₂ O ₃ , SiO ₂ , Their mixtures, mixed melts and ZrO ₂ , BeO, AeN, SiC and the like can be used. Examples of the organic material include an organic polymer and a hybrid material of an organic polymer and an organic substance.
[0040]
As a material of the nozzle plate 2, a synthetic resin such as a polyimide resin, a polyethylene terephthalate resin, a liquid crystal polymer, an aromatic polyamide resin, a polyethylene naphthalate resin, and a polysulfone resin, and a metal material such as stainless steel can be used.
[0041]
In each ink channel 4, a metal electrode 9 is formed from both side surfaces to the bottom surface, and the metal electrode 9 extends to the rear surface 1c of the piezoelectric material substrate 1 through the shallow groove 4b. An FPC 8 is adhered to each metal electrode 9 via an anisotropic conductive film (not shown) on the rear surface 1c. By applying a drive voltage to each metal electrode 9 from a drive circuit (not shown), the side wall 5 is formed. Shear deformation is performed, and the ink in the ink channel 4 is ejected from the nozzle 3 formed on the nozzle plate 2 by the pressure at the time of the deformation.
[0042]
As the metal used for the metal electrode 9, platinum, gold, silver, copper, aluminum, palladium, nickel, tantalum, and titanium can be used. In particular, from the viewpoint of electrical characteristics and workability, gold, aluminum, copper , Nickel is preferable, and is formed by plating, vapor deposition, and sputtering.
[0043]
Such a recording head H of the shear mode type can constitute a main part of the head simply by forming the ink channel 4 in the piezoelectric material 1 and forming the metal electrode 9 on the side wall 5 as described above. This is a preferable mode for performing high-definition image recording because the manufacture is simple and a large number of ink channels 4 can be arranged at high density.
[0044]
The nozzle plate 2 of the recording head H is formed with two nozzles 3a and 3b for one ink channel 4. In the present invention, when performing image recording on a recording medium using this recording head H, As shown in FIG. 5 (a), by simultaneously ejecting ink from these two nozzles 3a and 3b in parallel, each ink droplet lands on a recording medium, and the two ink droplets form two pixels. Are formed at once. As compared with the method of ejecting one ink droplet from one nozzle 30 and filling one pixel as in the recording head HA shown in FIG. And an image without highlight roughness can be obtained.
[0045]
Further, in the present embodiment, since the two heads H1 and H2 are overlapped as shown in FIGS. 1 and 2, the number of the ink channels 4 is further doubled without increasing the width of the recording head H. And image recording can be performed at a higher speed. Moreover, since the structure and the size are almost the same as those of the conventional recording head, the production is easy.
[0046]
The shape of the nozzles 3a and 3b is preferably a so-called conical nozzle having a large inlet diameter and a small outlet diameter as shown in FIG. 1 in order to reduce the discharge resistance and make it difficult to suck air from the nozzle openings.
[0047]
In order to eject fine ink droplets of 5 to 10 pl for performing high-definition image recording, it is preferable that the nozzle diameter is small. Since the ejection amount is determined by the diameter of the ink column formed by the nozzle portion and the ejection pressure at the time of ejection, a small ink droplet can be ejected as the diameter of the nozzle is smaller, but if it is too small, the nozzle is easily clogged. The outlet diameter is preferably about 15 to 25 μm, and more preferably 23 μm. If it is smaller than this, nozzle clogging is likely to occur, and if it is larger than this, it becomes impossible to discharge minute ink droplets of 10 pl or less, which are necessary for printing a high-definition image with a pixel density of 720 dpi.
[0048]
Further, it is preferable that the inlet diameter of the nozzles 3a and 3b is 30 to 40 μm. However, in the case of a 180 dpi head, since the width of the ink channel 4 is only 70 μm, two or more nozzles cannot be arranged. How to arrange the two nozzles 3a and 3b is determined by the inlet diameter. It is preferable that the distance between the two nozzles 3a and 3b be 35 μm in the main scanning direction and 35 μm in the sub-scanning direction (see FIG. 3) in accordance with the pixel density of 720 dpi.
[0049]
The nozzles 3a and 3b can be formed by piercing a resin nozzle plate using an excimer laser or piercing a stainless nozzle plate with a punch. For example, the polyimide resin plate may be irradiated with an excimer laser through a mask. The opening shape of the nozzles 3a and 3b is not particularly limited, and may be any shape such as a perfect circle, an ellipse, and a square.
[0050]
FIG. 3 shows an arrangement configuration in which the nozzles 3a and 3b of the nozzle plate 2 are viewed from the front side. Since the recording head H is provided with two nozzles 3a and 3b for one ink channel 4, one nozzle (nozzle 3a located on the upper side in the drawing) in one ink channel 4 is set to α, and the other (lower side in the drawing). Assuming that the nozzle 3b) is located at β, α holes and β holes may be alternately drilled, or the operation of drilling several α holes and then drilling β holes several times may be repeated.
[0051]
In addition, when ejection is performed from a large number of nozzles, the ink droplets ejected between the nozzles have variations, and the ink droplets are slightly misaligned and landed, so that horizontal stripes appear in the recorded image. In addition, there is also a problem that white or black horizontal stripes (banding) occur due to uneven feeding of the recording medium. In order to prevent these defects, the printing width of the printing medium is set to の of the dot diameter, and the printing is performed with the rows superimposed on each other or the dots are thinned out appropriately at the first printing, and the printing medium is slightly fed ( For example, a method is adopted in which a thinned portion is printed by another nozzle, and printing is performed so that dots of ink droplets ejected from different nozzles are arranged.
[0052]
However, although the variation between nozzles can be corrected by such a recording method, it cannot be completely corrected if the flight direction of the ejected ink droplet itself is largely bent. It is necessary to measure the angle that is bent in the scanning direction and the sub-scanning direction. In actuality, as shown in FIG. 4, a row of ejected ink droplets is projected onto one plane along the main scanning direction, and is expressed by an angle θ measured at a point 0.5 mm away from the nozzle and the nozzle.
[0053]
Usually, if the angle θ is in the range of 1 to 1.5 °, the landing deviation on the recording medium is 20 μm or less, which is acceptable. However, if an ejection curve in which the angle θ between the ejected ink droplets and the normal ejection direction is 1.5 ° or more occurs, adverse effects such as streaks appearing on the image occur. In the conventional method of filling one pixel at a time by ejecting one ink droplet, the ink droplet of one pixel is ejected, the ink droplet permeates the recording medium, and then another ink droplet is ejected to the next pixel. To fill in.
[0054]
Generally, when manufacturing a print head, a nozzle plate with perforated nozzles is bonded to the head, and a discharge test of ink droplets is performed to eliminate defective products.In this case, the head assembled halfway must be discarded. However, there is a problem that the loss increases. As in the present invention, two ink droplets are simultaneously discharged from two nozzles 3a and 3b of the α hole and the β hole of one ink channel 4 to fill two pixels at a time. Land at the same time on the recording medium and attract each other. Therefore, even if the ejection bend in which the angle θ is 2 ° occurs, the image is not greatly affected. Therefore, according to the present invention, there is an effect that the yield at the time of manufacturing the recording head is significantly improved.
[0055]
The heads H1 and H2 are overlapped with each other by shifting the nozzle pitch by １ ／ with respect to the direction in which the ink channels 4 are arranged. For example, in the case of a 720 dpi recording head, assuming that the nozzle pitch P of each of the heads H1 and H2 is 140 μm, the shift pitch between the nozzles of the heads H1 and H2 is P / 2 = 70 μm.
[0056]
Thus, for example, if each of the heads H1 and H2 is a 180 dpi head, it is possible to use the head as a 360 dpi recording head by shifting the nozzle pitch by 互 い に, and the width of the recording head H (sub The number of channels can be increased without increasing the length along the scanning direction), and a high-density recording head can be obtained.
[0057]
According to the recording head H, two ink droplets are simultaneously ejected from the two nozzles 3a and 3b arranged in the sub-scanning direction to one ink channel 4, whereby two main scans are performed in one main scan. Scanning will be performed. Generally, when printing an image at a resolution of 720 dpi, printing is performed by stopping the movement of the printing medium and moving the printing head at a speed of Vc = 25.4 × Fmax / 720 in the main scanning direction. It is necessary to move the printhead 35.2 μm in the scanning direction and stop it, move the recording head in the main scanning direction at a carriage speed of Vc, and repeat this. However, according to the recording head H according to the present invention, the speed of Vc in the main scanning direction is , The recording medium is moved by 70 μm in the sub-scanning direction, stopped, and then moved again at the speed of Vc in the main scanning direction for recording, so that the resolution becomes 720 dpi × 360 dpi. However, since each pixel is formed of 720 dpi ink droplets, the apparent resolution is 720 dpi × 720 dpi, and a high-definition image can be recorded.
[0058]
Also, as compared with an image having a resolution of 360 dpi, there is a problem that the recording speed is reduced to 1/4 or less at 720 dpi because the resolution is doubled. However, with the recording head H according to the present invention, two drops per channel are required. Since the ink droplets are ejected at the same time, the recording speed is not significantly reduced. Moreover, as shown in the present embodiment, by superposing the two heads H1 and H2 having the same configuration, a double recording speed can be achieved.
[0059]
In the shear mode head shown in this embodiment, as shown in FIG. 3, since the ink droplets are ejected by deforming the side walls 5 between the ink channels 4, all the ink channels 4 are A, B, It is necessary to divide into three groups of C and divide into three groups to discharge. For example, when it is desired to perform ejection from all channels, it is necessary to perform main scanning three times: ejection from group A, ejection from group B, and ejection from group C. Therefore, each time the main scanning is completed once, the recording medium is fed by a fixed length, so that the nozzle positions of the B group and the C group are respectively shifted from those of the A group in advance. This has the effect of preventing crosstalk.
[0060]
Next, a method of printing a high-definition image with gradation using the print head H will be described.
[0061]
When recording an image using the recording head H described above, the ink droplets are continuously ejected from each of the nozzles 3a and 3b, so that a plurality of continuously ejected ink droplets are combined during flight or on a recording medium. Let it. Thus, even if minute ink droplets are ejected, medium to large dots are formed by combining a plurality of ink droplets continuously ejected at a time, and an image having a high density gradation is recorded at high speed. It is possible. By changing the number of ink droplets to be continuously ejected, gradation can be expressed from the middle part to the shadow part, and a high-definition image close to a photograph can be recorded at high speed.
[0062]
Regarding the relationship between the resolution and the gradation, the smaller the dots, the more difficult it is to detect the density difference between the dots, so that the required number of gradations is reduced. For example, to achieve photographic quality at a pixel density of 360 dpi, 20 gradations are required, but at a pixel density of 720 dpi or more, only 4 to 8 gradations are required. Therefore, when recording an image at a pixel density of 720 dpi, it is sufficient to form eight gradations by continuously ejecting seven fine ink droplets and combining them during flight or on a recording medium. In the highlight portion, it is desirable to record a minute ink droplet at 720 dpi, and from the middle portion to the shadow portion, it is desirable to mix the minute droplet, the medium droplet, and / or the large droplet and record at approximately 360 dpi in order to increase the recording speed. This is because the sensitivity of the eyes is reduced in the high-density portion, and it becomes difficult to distinguish the density difference, so that the use of minute ink droplets is not so effective. However, if the recording head H according to the present invention is used, two pixels can be simultaneously filled, so that the recording speed does not decrease from the intermediate density portion to the shadow portion.
[0063]
By the way, as shown in the present embodiment, when ink is ejected from the ink channel 4 using a piezoelectric material, the acoustic wave generated by pressurizing the ink channel 4 is synchronized with the time for reciprocating the ink channel 4. By discharging ink, ink droplets can be discharged efficiently. For this reason, the shorter the length of the ink channel 4 is, the higher the frequency can be ejected. Further, the weight of the ink droplets ejected at this time is proportional to the amount of deformation of the side wall 5 of the ink channel 4, so that the shorter the ink channel 4, the smaller the droplets can be ejected. Therefore, by reducing the size of the ink channel, fine droplets can be ejected at a high frequency, and a higher-definition image can be recorded at a high speed.
[0064]
For example, to briefly explain, if the length of the ink channel 4 is halved, an ink droplet having half the size can be ejected at twice the frequency. Therefore, in order to discharge at a cycle of 10 kHz or more, the length of the ink channel 4 is preferably 1 mm or less. The reason is that if the length of the ink channel 4 is 1 mm and the speed of sound in the ink is 1000 m / sec, the time for a sound wave to reciprocate in the ink channel 4 is 2 μsec. The time required to eject one drop may be considered to be five times this. Therefore, when seven droplets are continuously discharged, the discharge time is 70 μsec, and the discharge can be performed at 14.3 kHz.
[0065]
In the conventional recording head, the length of the ink chamber was several millimeters and the ejection frequency was inevitably low so that a large ink droplet of about 50 pl could be ejected. By setting it to 2 or less, specifically 1 mm or less, the number of ejected ink droplets can be further increased without lowering the printing speed.
[0066]
Note that a drive waveform given to the recording head H for ejecting ink droplets is preferably a DRC waveform or a DRRC waveform.
[0067]
The DRC waveform is an abbreviation of Draw-Release-Cancel, and as shown in FIG. 7, the ink channel is expanded by deforming the side wall to the outside to expand the ink channel (D: Draw) and filling the ink channel with the ink. If this state is maintained for a certain period of time, for example, the AL period, and the deformation of the side wall is restored (R: Release), pressure is applied to the ink channel and ink is ejected. After the lapse of 2AL time, the ink channel is expanded again to cancel the remaining pressure (C: Cancel).
[0068]
As a result, one droplet is ejected from one nozzle. If this is repeated the required number of times, the ink droplet ejected later has the energy of the previously ejected ink droplet remaining, so that the ejection speed gradually increases. That is, the continuously ejected ink droplets can be combined during flight, and an image having a high density and a gradation can be recorded.
[0069]
Note that AL (Acoustic Length) is １ ／ of the acoustic resonance period of the ink channel, and when the length of the ink channel is L and the sound velocity in the ink is C, the time represented by L / C (Unit: μs). If the drive pulse width is set to AL, ejection can be performed most efficiently. However, since it is difficult to measure the speed of sound in the ink, the flying speed of the ink droplet is measured by changing the pulse width, and the pulse width at which the speed is maximized is defined as AL of the head.
[0070]
Further, in the present invention, in the high quality mode, it is preferable that dots of 5 to 35 pl (actually double this) are formed at a frequency of 10 to 15 kHz. In the high speed mode, it is preferable to form 10 pl ink droplets at 18 kHz.
[0071]
【The invention's effect】
According to the present invention, since two nozzles are provided in one ink chamber, the number of nozzles can be increased as compared with the related art without largely changing the size and manufacturing technology of the related art head, and the recording speed is greatly improved. In addition, it is possible to provide an ink jet recording head capable of recording a high-definition image close to a photographic image at a high speed.
[0072]
Further, by superposing two heads each having two nozzles in one ink chamber, the number of nozzles can be increased four times as compared with the related art, and a high-definition image close to a photographic image can be recorded at a higher speed. It is possible to provide an ink jet recording head which can be used.
[0073]
Further, according to the present invention, it is possible to provide an ink jet recording method capable of recording a high-definition image with a gradation at high speed by using fine ink droplets.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an example of a recording head according to the present invention.
FIG. 2 is a sectional view taken along the line II-II in FIG.
FIG. 3 is a partial front view showing the arrangement of nozzles of a recording head according to the present invention.
FIG. 4 is an explanatory diagram showing a state of ejection of ink droplets from nozzles.
FIG. 5A is a diagram illustrating a state of ejection of ink droplets of a recording head according to the present invention, and FIG. 5B is a diagram illustrating a state of ejection of ink droplets of a conventional recording head.
FIG. 6 is a graph showing a relationship between a dot diameter and a discharged droplet.
FIG. 7 is a diagram showing a DRC waveform;
[Explanation of symbols]
H: Recording head
H1, H2: Head
1, 1a, 1b: piezoelectric material substrate
2: Nozzle plate
3, 3a, 3b: Nozzle hole
4: Ink channel
5: Side wall
6: Cover plate
7: Ink inlet
8: FPC
9: Metal electrode

Claims (5)

In an ink jet recording head that ejects ink in a large number of ink chambers formed on a substrate as ink droplets from nozzles formed at one end of each ink chamber,
Two pixels are formed for each of the ink chambers, and two pixels are simultaneously formed on the recording medium by simultaneously discharging the ink in the ink chambers from the two nozzles. An ink jet recording head, characterized in that: 2. The ink jet recording head according to claim 1, wherein two substrates on which the ink chambers are formed are overlapped so that the ink droplets are ejected in the same direction. 3. The ink jet recording head according to claim 2, wherein the two substrates are overlapped with each other by shifting the nozzle pitch by 1/2 with respect to the direction in which the ink chambers of each substrate are arranged. The substrate is formed by alternately forming a plurality of parallel grooves and side walls serving as ink chambers in a polarized piezoelectric material, forming electrodes on the side walls, and shearing the side walls by applying an electric field to the electrodes. 4. An ink jet recording head according to claim 1, wherein the ink jet recording head is configured to be deformed so as to discharge ink in each ink chamber from a nozzle. An ink droplet is continuously ejected from each nozzle using the ink jet recording head according to any one of claims 1 to 4, so that the continuously ejected ink droplets are combined during flight or on a recording medium. An ink-jet recording method, characterized in that:

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