JP3384797B2 - Ink jet recording head and ink jet recording apparatus - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ink jet recording head and an ink jet recording apparatus, and more specifically, one to a plurality of ink droplets, which are separated and independently fly, are attached to the same position on a recording medium. The present invention relates to an inkjet recording head and an inkjet recording apparatus in which pixels having different pixel diameters are formed by changing the number of ink droplets attached to the same location.

[0002]

2. Description of the Related Art The non-impact recording method has been attracting attention for office use and the like because noise generation during recording is so small that it can be ignored. inside that,
The so-called inkjet recording method, which enables high-speed recording and can perform recording on plain paper without requiring a special fixing process, is an extremely powerful recording method.
Various methods have been proposed so far, or have already been commercialized and put into practical use.

In such an ink jet recording method, recording is performed by ejecting small droplets (ink droplets) of a recording liquid, so-called ink, and attaching the ink droplets to a recording medium. For example, the applicant has disclosed in Japanese Patent Publication No. 56-9429. To summarize the invention disclosed in this Japanese Patent Publication No. 56-9429, heating the ink in the liquid chamber to generate bubbles causes a pressure rise in the ink, and this ink is applied to the fine nozzle tip. The recording is performed by ejecting from the ink ejection port.

After that, many inventions were made by utilizing this principle, and one of them was disclosed in, for example, JP-A-59-2072.
The invention disclosed in Japanese Patent Publication No. 65 is known. this is,
It shows a method for performing gradation recording, in which a group of pulse signals is applied to one heater to eject one ink droplet. That is, in the present invention, the number of ejected ink droplets changes according to the number of applied pulse signals, but these ink droplets fly in a state of being combined with each other and adhere to the same position on the recording medium. It is a thing.

The invention disclosed in Japanese Patent Laid-Open No. 63-53052 is known. The present invention is a method for performing gradation recording by ejecting a series of ink droplets that fuse on the recording medium within the wetting time of the recording medium (paper). According to this, the ink droplets fly at a high speed without being combined with other ink droplets. Then, when the ink droplets reach the recording medium, they fuse with each other within the wetting time of the recording medium. The dot (pixel) size on the recording medium is
It is increased by increasing the number of ink droplets coalescing on the recording medium within the wetting time of the recording medium.

Further, the invention disclosed in Japanese Patent Publication No. 59-43312 is known. According to the present invention, in order to sufficiently enhance the responsiveness and stability of the output with respect to the pulse signal input to generate bubbles, the input period of the pulse signal at the highest ink droplet formation frequency is set to the half width of the peak energy of the pulse signal. Is input as at least three times.

[0007]

Problems to be Solved by the Invention JP-A-59-2072
In the invention disclosed in Japanese Patent Laid-Open No. 65, since a plurality of ink droplets remain combined during flight,
They have to fly at low speed. Then, the trajectory of the ink droplets flying at a low speed is bad, and the reliability of printing is deteriorated. Further, the ink droplets are susceptible to defects of the inkjet recording head and changes in the moving speed, and if the moving speed of the inkjet recording head is high, a group of ink droplets flying at a low speed will have a circular shape when attached to the recording medium. No pixels are formed and the resulting image is blurred.

In the invention disclosed in Japanese Patent Laid-Open No. 63-53052, the time between the complete collapse of air bubbles generated when ejecting an ink drop and the heating of the resistance element for ejecting the next ink drop. Is only in the range of 0.1 μs to 1.0 ms. Specifically, under what kind of conditions should the ink droplets be ejected, or what kind of structure is the ink jet recording head? It was impossible to realize because there was no mention of whether to use.

In the invention disclosed in Japanese Patent Publication No. 59-43312, only the stabilization of ink droplet ejection by turning on / off a pulse signal is described.
The gradation recording method (gray scale printing) is not described at all, and only the stabilization conditions for binary recording are shown.

The present invention has been made in view of the above problems, and an object thereof is to propose an ink jet recording head and an ink jet recording apparatus which perform gradation recording by changing a pixel diameter in accordance with image density information. Another object of the present invention is to propose an ink jet recording head and an ink jet recording apparatus which form very small ink droplets with a small energy which is not available in the past and change the pixel diameter by changing the number of the ink droplets to perform gradation recording. . Another object of the present invention is to propose an ink jet recording head and an ink jet recording apparatus that stably form such extremely minute ink droplets at a high frequency that has never been seen in the past.

[0011]

The invention according to claim 1 is
A liquid chamber for storing ink, a plurality of ink discharge ports communicating with the liquid chamber via liquid passages, and a heater provided in the liquid passage, and a pulse signal is input to the heater to Bubbles are generated in the ink in the liquid path, and the action force caused by the expansion of the bubbles causes the ink to flow through the ink discharge port.
Ink droplets are ejected to separate multiple ink droplets from one
Flying in a state of
Recorded material is formed in the shape of a slender column whose length dimension is three times or more.
If one pixel is formed by adhering to almost the same place above
Image density is the number of ink droplets that adhere to almost the same location.
Change the pixel size by changing according to the information
In the ink jet recording head in so that, 該I
The drive frequency of the ink jet recording head is 10 kHz
Inkjet recording head used in the range of 75 kHz
If one pixel is formed by one ink drop,
Discharge the amount of ink droplets so that adjacent pixels do not overlap.
The ink discharge port was formed in a large size .

According to a second aspect of the present invention, there is provided a liquid chamber for storing ink, a plurality of ink discharge ports communicating with the liquid chamber via liquid channels, and a heater provided in the liquid channels. ,
A pulse signal is input to the heater to generate a bubble in the ink in the liquid path, and an ink drop is ejected from the ink ejection port by an action force associated with the expansion of the bubble,
Fly from one ink droplet to multiple ink droplets separately
The length of the ink droplet is 3 times or more of the diameter.
The pixel is formed by forming it in the shape of a slender column as shown above, and forming one pixel by adhering it to almost the same position on the recording medium and changing the number of ink drops that adhere to almost the same position according to the image density information. in the ink jet recording head to vary the size, the ink jet recording
The drive frequency of the head is in the range of 10 kHz to 75 kHz
Inkjet recording head used in , when one pixel is formed by one ink droplet, the pixel of one pixel
The diameter is less than √2 times the distance between the center points of adjacent pixels.
The ink ejection port was formed in such a size as to eject a certain amount of ink droplets.

According to a third aspect of the present invention, in the ink jet recording head according to the first and second aspects, the plurality of ink droplets are 30 or less.

According to a fourth aspect of the invention, in the ink jet recording head according to the first and second aspects, the plurality of ink droplets is 20 or less.

The invention according to claim 5 is the invention as defined in claims 1, 2 and
In scanner with inkjet Symbol RokuSo location of an image reading apparatus provided in the ink jet recording apparatus using an ink jet recording head 3, 4, wherein, in accordance with the image information obtained from the image reading apparatus, onto a recording medium, a pixel The number of ejected ink droplets for forming the ink is variable.

The invention according to claim 6 is the invention according to claims 1, 2 and
In the ink jet recording heads described in 3 and 4, the electrical signal groups for forming the adjacent pixels are set to have substantially equal intervals.

The invention according to claim 7 is the invention according to claim 1,
3. The inkjet recording head according to 3, 4, or 6, wherein an opening area of an ejection port is less than 500 μm ² , an ink droplet is ejected from the ink ejection port, and is ejected to adhere to a recording medium to form a pixel for recording. In the ink jet recording head for performing the above, the pixel is an ink jet recording head for performing gradation recording by performing superposition on one to a plurality of pixels at substantially the same position on a recording medium according to image information. The number of times of driving to approximately the same position on the recording medium is set to be a number of times of density 1.5 or less, which is a value at which the image density on the recording medium is almost saturated.

[0018]

According to the first aspect of the present invention, one ink drop produces one
When pixels are formed, adjacent pixels do not overlap with each other, so that a white background region where ink drops do not adhere is formed between pixels on the printing surface, and the entire printing surface has a light color. Then, as the number of ink droplets forming one pixel is increased, the pixel diameter of each pixel is gradually increased and the white background area is gradually reduced, and accordingly, the entire printing surface is gradually darkened, and gradation recording is performed.

According to the second aspect of the invention, when one pixel is formed by one ink drop, the pixel diameter of one pixel is smaller than the square root times the distance between the center points of the adjacent pixels, and therefore, on the printing surface. There is a white background area between the pixels in which ink drops do not adhere, and the entire printing surface has a light color. And 1
As the number of ink droplets forming a pixel is increased, the pixel diameter of each pixel is gradually increased and the white background region is gradually decreased, and accordingly, the entire printing surface is gradually darkened, and gradation recording is performed.

According to the third aspect of the invention, the pixel diameter on the recording medium can be changed according to the number of ink droplets, and high-quality gradation recording can be performed.

According to the fourth aspect of the invention, the pixel diameter on the recording medium can be changed relatively greatly according to the number of ink droplets, and the pixel forming speed (recording speed) is not so sacrificed. High-quality gradation recording is possible.

According to the fifth aspect of the invention, the number of ink droplets ejected to form one pixel on the recording medium is made variable according to the image information obtained from the image reading apparatus, so that the number of ink droplets is thin. The original can be printed dark, or the dark original can be printed thin, and the shade of the print result can be freely controlled.

According to the sixth aspect of the invention, even if one to a plurality of ink droplets are ejected to form one pixel, the electric signal groups for forming the adjacent pixels are formed at substantially the same interval, so that they are formed. The distances between the centers of adjacent pixels become almost equal, and high-quality recording is realized.

According to the invention described in claim 7, a large gradation change rate can be obtained, and the printing speed can be prevented from slowing down.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. First, FIG. 17 is a side view for explaining one embodiment of the heating element substrate of the first inkjet recording head according to the present invention. On the substrate 1, the first electrode 2, the insulating layer 3, the heater 4, The two electrodes 5 and the protective layer 6 are sequentially formed in a laminated state. In addition, one end (A portion) of the first electrode 2 is a portion for taking out the lead wire, and the other end (B portion) is a portion to which one end of the heater 4 is connected.

FIGS. 18 (a) to 18 (d) are diagrams showing the procedure for obtaining the structure shown in FIG. 17. First, as shown in FIG. Form on top. FIG. 3B shows an insulating layer 3 which covers the first electrode 2 and both ends (A and B) of the first electrode 2 are not covered with the insulating layer 3. The same figure (c) shows a heater 4 formed on a part of the insulating layer 3, and the same figure (d) shows a second electrode 5 formed on a part of the insulating layer 3. It is connected to the portion B of the one electrode 2, and one end of the second electrode 5 is connected to the other end of the heater 4.

Here, the material used in the present invention is, for example, Al, Au or the like as the material of the electrodes 2 and 5, and is given by using techniques such as vapor deposition, sputtering and plating, and is well known. As described above, the pattern is formed by the photolithography technique. As the material of the insulating layer 3, SiO ₂ , Si ₃ N ₄ or the like is applied and patterned by the same method. The heater 4 is formed in the same manner, and examples of the material thereof include tantalum nitride, nichrome, hafnium boride and the like.

In the present invention, only the minimum necessary structure has been described for simplifying the description. For example, as a method of forming the electrodes 2 and 5, after Au or Al is thinly vapor-deposited, Au plating is performed. It is conceivable that the insulating layer 3 has a two-layer structure formed by thickening, and the insulating layer 3 may also have a multi-layer structure. Further, it may be necessary to provide the substrate 1 with a heat storage layer that does not allow the heat of the heater 4 to escape.

FIG. 19 is a diagram showing an embodiment in which the first electrode 2 shown in FIG. 18 is a common electrode for the plurality of second electrodes 5 and the heater 4.

In this way, the Applicant has proposed that the heater 4
A heating element substrate having an arrangement density of 48 / mm (equivalent to 1200 dpi) was manufactured (note that the heater 4 on one substrate 1
256).

Next, in order to complete this heating element substrate as an ink jet recording head, it is necessary to form an ink liquid path and an ink ejection port. For example, the ink jet recording head is completed by bonding the lid substrate 9 having the groove 7 and the recess 8 as shown in FIG. 20 to the above-mentioned heating element substrate. Array density of liquid channels is 24 / mm, 32 /
Since the density is very high, such as 48 mm / mm, the recording head was formed by the photolithography technique in order to obtain a highly accurate liquid path pattern accuracy.

Next, an example of an ink jet recording head formed by the photolithography technique will be described with reference to FIGS. 21 to 28. FIG. 21 shows a heating element substrate, on which a heater 11 and a thin film 12 covering the heater 11 are formed. In this step, a required number of heaters 11 are arranged on a substrate 10 made of a material such as Si, glass or ceramics,
Further, if necessary, for the purpose of imparting ink resistance and electric insulation, a thin film 12 of SiO ₂ , Ta ₂ O ₅ , glass or the like is coated. Although not shown, electrodes for pulse signal input are connected to the heater 11.

In the step shown in FIG. 22, the surface of the thin film 12 obtained through the step of FIG. 21 is cleaned and dried, and then a liquid photoresist is applied by spin coating and prebaking (for example, 80). 30 at ℃
Minutes). The liquid photoresist can also be favorably applied by roll coating, dip coating, or the like. It has been conventionally proposed to use a dry film photoresist to form a liquid path pattern, but the dry film photoresist is used for its original purpose (for forming a pattern on a printed circuit board).
Therefore, it is difficult to form a high density pattern. Currently, up to 16 lines / mm can be formed,
Higher density array than that is difficult, like the present invention,
It is not suitable for forming a high-density pattern. In the present invention, the liquid photoresist is BMR manufactured by Tokyo Ohka Kogyo Co., Ltd.
The thickness of the photoresist layer 13 to be formed is 7 to 30 by using S-1000 and changing the rotation speed during spin coating in the range of 500 to 2500 rpm.
I could change to μm.

Next, a photomask 14 having a predetermined pattern is overlaid on the photoresist layer 13, and then exposure is performed from above the photomask 14. At this time, the heater 11 and the pattern of the photomask 14 are aligned by a known method.

FIG. 23 shows a process in which the unexposed portion of the exposed photoresist layer 13 is dissolved and removed by a developing solution made of a predetermined organic solvent such as trichloroethane. Then, after the unexposed portion is dissolved and removed by a developing solution, in order to improve the ink resistance of the exposed portion of the photoresist layer 13 left on the substrate 10, a heat curing treatment (for example, at 150 to 250 ° C. for 30 minutes) is performed. ~ 6 hours heating), or
Ultraviolet irradiation (for example, an ultraviolet intensity of 50 to 200 mW / cm ² or higher) is performed to sufficiently advance the polymerization and curing reaction. It is also effective to perform both the above-mentioned heat curing treatment and ultraviolet irradiation.

FIG. 24 shows a lid substrate that covers the grooves 15 and the recesses (not shown) formed in the photoresist layer 13, and is a flat plate member made of a material that transmits electromagnetic waves, for example, a transparent ultraviolet light material. A dry film photoresist 17, which is a photosensitive resin film, is laminated on one surface of 16.
The dry film photoresist 17 is laminated using a commercially available laminator so that air does not enter between the flat plate member 16 and the dry film photoresist 17. In the present invention, as the dry film photoresist 17, SY-325 manufactured by Tokyo Ohka Kogyo Co., Ltd. was used.

FIG. 25 shows a step of pressing and adhering the dry film photoresist 17 on the lid substrate shown in FIG. 24 and the photoresist layer 13 on the heating element substrate shown in FIG. In addition, at the time of this pressure sticking, ultraviolet irradiation (for example, 50 to 200 mW) in a non-oxygen atmosphere is performed.
/ Cm ² or higher UV intensity)
Dry film photoresist 17 from above
Fully cure. Further, a heat curing treatment (for example, 13
It is also effective to heat at 0 to 250 ° C. for 30 minutes to 6 hours.

Here, FIG. 26 is a perspective view showing a state after the step shown in FIG. 25 is completed. By covering the groove 15 with a lid substrate, a liquid passage 18 through which ink flows is formed, and a concave portion is formed. A liquid chamber 19 for storing ink is formed by covering the container with a lid substrate. In addition, an ink introduction hole 21 to which an ink supply pipe 20 (illustrated in FIG. 28) for supplying ink into the liquid chamber 19 is connected is formed in the lid substrate. Then, the tip end side portion of the liquid passage 18 is cut along the line AA and the cut surface is smoothed to form the ink ejection port 22 (shown in FIG. 28). The ink jet recording head 23 is completed by attaching the ink supply pipe 20. The cutting along the line AA is performed in order to optimize the distance between the heater 11 and the ink ejection port 22, and the cutting area is appropriately determined. For this cutting, a dicing method usually used in the semiconductor industry is adopted.

Here, FIG. 27 shows BB in FIG.
28 is a sectional view taken along the line A, while FIG. 28 is a sectional view showing an inkjet recording head 23 completed by cutting along the line AA and mounting the ink supply tube 20. is there.

The inventor of the present invention changes the thickness of the photoresist layer 13 by the above-described method, so that the ink ejection ports 22 and the liquid paths 18 can be changed from 24 lines / mm to a maximum of 48 lines.
The ink jet recording head was completed by forming with an array density of book / mm.

The size of the ink discharge ports 22 at this time is 22 × 22 when the arrangement density is 24 lines / mm.
μm, 17 × 1 when the array density is 32 lines / mm
7 μm, 14 × when the array density is 48 lines / mm
It is 14 μm.

In such a configuration, in FIG. 1, ink droplets 24 are continuously ejected by using the ink jet recording head 23 formed as described in FIGS. 21 to 28, and these ink droplets 24 are recorded. Body (for example,
This shows a state in which one pixel 26 is formed by adhering it to the same position on a sheet 25. The important point here is
In the present invention, each ink droplet 24 is separately ejected and ejected according to the input of the pulse signal to the heater 11 and adheres to the recording medium 25 (Japanese Patent Laid-Open No. 59-20726).
In the invention disclosed in Japanese Patent Publication No. 5, the ink droplets are ejected and fly while being connected to each other). In addition, each ink droplet 24 flies in the form of an elongated column (Japanese Patent Laid-Open No. 63-5305).
In the invention disclosed in Japanese Patent Publication No. 2, each ink droplet flies in a spherical shape). In addition, the size of the ink droplet 24 in the form of an elongated column is 3 to 10 times as long as its diameter.

Here, the conditions for the ink droplets 24 to fly in such an elongated columnar shape are that the flying speed of the ink droplets 24 is fast and that they are not easily affected by disturbance (for example, the flow of ambient air). That is. Therefore, the ink drop 24
Between the shape of the ink droplet and the flight speed, and a plurality of ink drops 2
Each ink drop 2 when 4 forms one pixel 26
The relationship between No. 4 and the amount of deviation from the target position on the recording medium 25 was investigated and shown in Table 1.

[0044]

[Table 1]

The ink jet recording head 23 used here has the size of the ink ejection port 22 of 17 × 17 μm, the size of the heater 11 of 14 × 84 μm, and the resistance value of the heater 11 of 75Ω. When measuring the shape of the ink column and the flight speed of the ink droplet, a vehicle having the following components (a transparent liquid obtained by removing the dye component from the ink) was used instead of the ink.

Glycerin 18.0% Ethyl alcohol 4.8% Water 77.2% On the other hand, the following ink components were used to measure pixel position accuracy.

Glycerin 18.0% Ethyl alcohol 4.8% Water 75.0% C.I. I. Direct black 154 2.2% As the recording medium 25 to which the ink droplets 24 are attached, PPC paper 6200 manufactured by Ricoh Co., Ltd. is used, and the frequency of the pulse signal input to the heater 11 is 20 kHz.
And

From Table 1, I_L/ I_D, That is, ink drops
24 diameter dimensions (I_D) Length of ink drop 24
Law (I_L) Ratio of 2.8 or less, the ink droplet 24
Flight speed is slow (does not reach 5.0 m / s)
It is possible to attach the ink droplet 24 to a target position on 25.
It is not possible (the image quality deteriorates if it is shifted by 1 dot or more),
It turns out to be impractical. In other words, flying ink drops
24 is an elongated column (I _L/ I_DIs 3 or more)
It turns out that it is necessary to discharge under the conditions. That
The flying speed of the ink droplet 24 at that time is 5 to 10 m /
s or more, strong against external disturbances, direct to flight
Adheres well to the target position with good progress.
You can

Next, FIG. 2 shows the shape of the ink droplet 24 in more detail. The ink droplet 24 shown in FIG. 9A has the most ideal shape, but as shown in FIG. 9B, it flies with mist-like minute ink droplets called satellites 24a. There are two ink droplets 24 (three in some cases) as shown in FIGS.
In some cases, it may fly separately. These differences depend on the size of the ink ejection port 22, the physical properties of the ink (viscosity, surface tension), the shape of the pulse signal, and the like, but in any case, only one pulse signal is input to the heater 11. Therefore, here, even when what should originally be one ink drop 24 separates and flies as shown in FIGS. 7C and 7D, one ink drop 24 is applied to one pulse signal.
Are treated as formed. Even if the satellites 24a are thus separated or fly with the satellites 24a, if the flying speed is 5 to 10 m / s or more as described above, the satellites 24a and the separated ink droplets 2
Since No. 4 adheres to substantially the same position on the recording medium 25, the formed pixel 26 is close to a perfect circle, and the problem of deterioration of image quality does not occur.

Next, in FIG. 3, one pixel 2 is changed by changing the number of pulse signals continuously input to the heater 11.
6 shows how the size of the pixel 26 formed by changing the number of the ink droplets 24 forming 6 and changing the number of the ink droplets 24 is changed. In FIG. 9A, one pulse signal is input to the heater 11, one ink droplet 24 is ejected from the ink ejection port 22, and one ink droplet 24 adheres to the recording medium 25. Then, the pixel 26 is formed. In FIG. 2B, three pulse signals are input, three ink droplets 24 are ejected, and one pixel 26 is formed by the three ink droplets 24. In the same figure (c), five pulse signals are input, five ink droplets 24 are ejected, and five ink droplets 2 are ejected.
4 form one pixel 26. In FIG. 7D, eight pulse signals are input and eight ink droplets 24 are ejected, and one pixel 26 is formed by the eight ink droplets 24. Ink drop 2
Pixels 26 formed become larger as the number of 4 increases.

In the present invention, in order to obtain a large pixel 26, the number of pulse signals continuously input to the heater 11 is increased and the number of ink droplets 24 continuously discharged is increased. However, increasing the number of pulse signals inevitably increases the time required to form one pixel 26. Therefore, the flying ink droplets 24 are
If the ink droplets 24 are connected to each other as disclosed in Japanese Patent Laid-Open No. 207265/1984, as described above, the flight loci of the ink droplets deteriorate, and the reliability of printing deteriorates. It is an important point to increase the recording speed to generate the ink droplets 24 as frequently as possible in a range that does not connect.

Therefore, the present inventor manufactured the following ink jet recording head and investigated the conditions thereof in order to investigate to what extent the formation frequency of the ink droplets 24 which were not connected to each other could be increased. Ink ejection port size 17 × 17 μm Heater size 14 × 84 μm Heater resistance value 75Ω Ink ejection port array density 32 lines / mm (about 800 dp
equivalent to i) Number of ink ejection ports 256

Using this ink jet recording head 23 and a vehicle having a surface tension of 49.3 dyn / cm and a viscosity of 1.39 cp, the driving voltage of the heater 11 is 6 V, the pulse width of the pulse signal is Pw = 4 μs, and the pulse is applied. When the frequency of the signal was set to 20 kHz and the vehicle was ejected, good ejection was continuously performed. The ejection speed at this time was 11.7 m / s at a position 0.5 mm away from the ink ejection port 22.

The state of the bubbles at this time is observed from above the transparent flat plate member 16 (shown in FIGS. 24 to 28) and its temporal change is examined. FIG. 4 shows the input pulse. The signal and the appearance of bubbles are shown along the time axis.
According to it, when the drive voltage is turned on and the pulse signal is input, it is slightly delayed (0.2 μs) from the input of the pulse signal.
After) bubble generation starts, the bubble gradually expands, and the bubble continues expanding even after the drive voltage is turned off (after 4 μs), and the time when the bubble reaches the maximum value is 4.9 μs later. It was After that, the bubbles started to shrink and disappeared completely 14.7 μs after the driving voltage was turned on.

Next, the frequency of the pulse signal is set to 10 kHz.
Similarly, the profile of bubble generation was examined at z, 30 kHz, and 40 kHz.
There is almost no difference in the time when the bubbles disappear (the time until the bubbles become maximum is 4.8 to 5.1 μs, the time when the bubbles disappear is 14.7 to 15 μs), and the bubble generation profile is a pulse. It turns out that it does not depend on the frequency of the signal.

Then, when the frequency of the pulse signal was further increased and the limit of stable ejection of the ink droplet 24 was examined, it was found that stable ejection was performed up to a frequency of 51 kHz. Ink drops 24 at this time
Had a flight speed of 12.5 m / s. When the frequency was further increased to 55 kHz, ink drops 2 for 2-3 seconds.
After the discharge of 4, the discharge was stopped.

In order to investigate the reason, the frequency is 50 to 5
When the profile of bubble generation was carefully examined at a frequency of 5 kHz, it was found that the bubbles were generated and disappeared in a pattern as shown in FIG. 4 up to a frequency of 51 kHz. On the other hand, when the frequency is 52 kHz, the first few seconds are shown in FIG.
Although the generation and disappearance of the bubbles were performed in the pattern as shown in FIG. 7, the bubbles that did not disappear after that covered the upper part of the heater 11, and the pattern of the generation, expansion, contraction and disappearance of the bubbles was not performed. Therefore, ink drop 2
The discharge of No. 4 was also stopped.

Therefore, the upper limit frequency of the pulse signal for stably repeating the pattern until the bubbles are generated and disappeared, in other words, the upper limit frequency of the pulse signal for the stable ejection of the ink droplets 24. Is 51 kHz
It turns out that.

Here, the frequency of the pulse signal is 51 kHz.
The driving voltage and the state of bubbles at the time are shown in FIG. 5 with their time axes aligned. In FIG. 5, “T” is the time from when the pulse signal is input until the bubble becomes maximum (in this case, T = 4.
9 μs). Further, from FIG. 5, in order to stably generate the bubbles in the ink jet recording head 23 used in this experiment for the second time and thereafter, after inputting the previous pulse signal, “4T (= 19.6 μs) ) ”And the subsequent pulse signals can be input. still,
A pulse signal with a frequency of 51 kHz has a cycle of one pulse signal of 1 / (51 × 1000) seconds, that is, 19.6 μ.
s.

From another point of view, if the time "Ti" from the disappearance of the previous bubble to the start of the generation of the next bubble is set to be larger than the above "T", it is almost stable. ,
Moreover, bubbles are generated at the highest frequency and ink drops 2
It can be seen that 4 discharges can be performed.

The above results indicate that the size of the ink ejection ports 22 is 17 × 17 μm and the array density of the ink ejection ports 22 is 32.
This is the result of examining the profile of bubble generation in an inkjet recording head of book / mm.
Table 2 shows the profile of bubble generation in another ink jet recording head in which the size and arrangement density of No. 2 are changed.
Each time shown in Table 2 is the time after the pulse signal is input. The frequency of this pulse signal was all 5 kHz.

[0062]

[Table 2]

Next, the frequency of the pulse signal in these ink jet recording heads was gradually increased from 5 kHz, and the bubble generation limit, or in other words, the ejection limit of the ink droplet 24 was examined. As a result, with an inkjet recording head of 48 lines / mm, the upper limit is about 75 kHz, and the flying speed of the ink droplets 24 at that time is 11.1 m / s.
Met. In the case of an inkjet recording head of 24 lines / mm, the upper limit was about 46 kHz, and the flight speed of the ink droplets 24 at that time was 10.7 m / s. And
In these inkjet recording heads, when the frequency of the pulse signal is further increased, the ejection of the ink droplets 24 is stopped, and the above-described inkjet recording heads (ink ejection port size: 17 × 17 μm, array density: 32 /
As seen in the experiment with the head (mm head), the air bubbles covered the entire surface of the heater.

On the other hand, in the ink jet recording head of 16 lines / mm, when the frequency of the pulse signal is increased to 9 to 9.5 kHz, the ejection of ink droplets is stopped, and in the ink jet recording head of 8 lines / mm, the frequency of the pulse signal is increased. 6 to
At the time when the frequency was raised to 7 kHz, the ejection of ink droplets stopped.
Then, when the heater of these ink jet recording heads was observed, it was found that the heater was broken.

That is, it was found that in all ink jet recording heads, when the frequency of the pulse signal was raised, ink droplets were not ejected at a certain point, but the cause was different. In the ink jet recording head having the arrangement density of the ink discharge ports of 24 lines / mm and 48 lines / mm, as in the above-mentioned ink jet recording head of 32 lines / mm, the cycle of bubble generation by the heater reaches the limit and ink drops While the ejection was stopped, in the ink jet recording head in which the array density of the ink ejection ports was 16 lines / mm and 8 lines / mm, the ejection of ink drops was stopped due to the destruction of the heater.

It is considered that the cause depends on the size of the bubbles generated. It is generally known that when a bubble collapses and disappears, a very large impact force is exerted by a cavitation effect. The larger the generated bubbles, the stronger the impact force on the heater when the bubbles disappear. According to the above experimental result, the heater breaks when the frequency of the pulse signal is gradually increased in the ink jet recording head having the ink ejection port arrangement density of 8 lines / mm and 16 lines / mm, due to the impact force. It is considered to be a thing. In other words, although there was no abnormality when the frequency of the pulse signal was 5 kHz, since the frequency was gradually increased, the number of repetitions of the impact force due to cavitation gradually increased, and the heater could not withstand the impact force and was destroyed. It is believed that

On the other hand, the arrangement density of the ink discharge ports is 24 /
It is considered that the reason why the heater of the inkjet recording head of mm / 48 lines / mm was not destroyed is because the bubbles generated are very small and therefore the impact force acting on the heater is also small.

In order to investigate this point in more detail, the inventor of the present invention inputs a pulse signal in air and a vehicle to heaters of various ink jet recording heads having different arrangement densities of ink ejection ports and determines the frequency thereof. Instead, the durability of those heaters was investigated. The ink jet recording head used has an ink ejection port array density of 8 lines / mm, 16 lines / mm, 24 lines / mm, 32 lines / mm,
The driving voltage and the pulse width of the pulse signal were the same as those when the above-mentioned bubble generation profile was examined. As a result, when driving in air, the pulse signal frequency was 100 kHz and driving was performed for 3 hours (10 ⁹ pulse signals were input), but no abnormality was found in the heaters of all inkjet recording heads. On the other hand, when the same test was performed in the vehicle,
The results shown in Table 3 were obtained.

[0069]

[Table 3]

That is, the arrangement density of the ink discharge ports is 8 /
When the bubbles generated by the heater are large, such as mm and 16 lines / mm, the heater is destroyed at the frequency of the pulse signal before reaching the upper limit of the bubble generation cycle. Array density of 2
If the heater is small and the generated bubbles are small, such as 4 lines / mm, 32 lines / mm, and 48 lines / mm, the durability of the heater of 10 ⁹ or more even when driven near the upper limit of the bubble generation cycle. It turned out that there is a nature. The size of bubbles generated is 380 μm when the length in the longitudinal direction is 380 μm, and 195 μm when 16 bubbles / mm,
The number of 24 lines / mm was 115 μm, the number of 32 lines / mm was 90 μm, and the number of 48 lines / mm was 60 μm.

From the above results, in the ink jet recording head in which the ink ejection openings are small to a certain extent and arranged in high density, the upper limit for ejecting ink droplets at a high frequency is that bubbles are maximum after the pulse signal is input. It can be seen that, when the time until it becomes "T" is set to "T", the time to input the next pulse signal should be "4T" or later after the input of the previous pulse signal. From another point of view, if driving is performed so that the time from the disappearance of the previous bubble to the start of the generation of the next bubble is larger than “T”, the ink droplets are almost stable and have the highest occurrence frequency. It can be seen that the discharge can be performed.

Next, another feature of the present invention will be described. In the present invention, ink droplets are ejected with much smaller energy than in the past. Further, since the ink ejection port of the ink jet recording head used in the present invention is also very small, it is conventional, for example, Japanese Patent Publication No. 59-43.
Ejection of ink droplets becomes considerably difficult as compared with the case of ejecting ink droplets from a large ink ejection port of 50 × 40 μm as disclosed in Japanese Patent Laid-Open No. 312. The main reason for this is that the ink ejection port becomes smaller and the fluid resistance increases, but it is also possible to perform stable ink droplet ejection by finding the optimum conditions.

Therefore, the present inventor investigated how much energy required to eject ink droplets was required per unit area of the ink ejection port. This is described below. The ink jet recording head used here has an ink ejection port array density of 24 lines / mm and 32 lines / mm, respectively.
mm, 48 lines / mm, and the size of each ink ejection port is 22 × 22 μm, 17 × 17 μm, 14 × 1
It is 4 μm, and the other dimensions, the components of the vehicle used in the experiment, and the like are the same as those in the experiment conducted previously. As the contents of the experiment, the driving voltage was changed in order to change the energy required for ejecting the ink droplet, and the flight speed Vi (m / s) of the ejected ink droplet was measured. The frequency of the pulse signal input to the heater of each inkjet recording head was about 10% lower than the usage limit (upper limit) of each inkjet recording head. That is, the inkjet recording head of 24 lines / mm was set to 40 kHz, the inkjet recording head of 32 lines / mm was set to 45 kHz, and the inkjet recording head of 48 lines / mm was set to 65 kHz. Also, the pulse width of the pulse signal is fixed, and for each inkjet recording head,
It was set to 4.5 μs, 4 μs, and 3 μs. Table 4 shows the result.

[0074]

[Table 4]

As a result, the ratio of the energy required for ejecting ink droplets to the area of the ink ejection port "E / S (J /
cm ² ) "is less than 0.3, the ink droplets are spherical and the flying speed is slow, which is unstable and impractical. On the other hand, when it is more than 3, the heater reaches the endurance limit of the heater. Was found to be destroyed and unusable.

As another point of view, a minute ink ejection port (14 × 14 μm to 22 × 2) which is not in the prior art as in the present invention is used.
2 μm) to very high frequencies (at least 10 kHz)
In order to stably eject ink droplets when ejecting ink droplets with the above pulse signals, Table 4 shows that in the ink jet recording head in which the arrangement density of the ink ejection ports is 24 / mm, 1. 46μJ (at 5V drive voltage)
˜15.0 μJ (at a driving voltage of 16 V), 0.90 μJ (at a driving voltage of 4.1 V) to 8.74 μJ for an ink jet recording head having an ink ejection port array density of 32 lines / mm.
(At a drive voltage of 12.8 V), an ink jet recording head having an ink ejection port array density of 48 lines / mm is 0.62 μm.
J (at drive voltage 3.8 V) to 5.97 μJ (drive voltage 1
1.8 V), and 0.6 μJ to 1 in total.
It is preferable to use it in the range of about 5.0 μJ.

In the present invention, the size of the pixel formed on the recording medium (for example, paper) is 1 to 1 very small ink drop generated at a very high frequency of 10 to 75 kHz.
It is changed by attaching a plurality of them to substantially the same place. Image density information is input to the ink jet recording head in order to attach the one or more ink droplets to substantially the same location, and one or more image signals are provided by one or more pulse signals corresponding to the pixel density information. However, there is a limit to the range in which the pixel diameter can be changed when a plurality of ink droplets are adhered to the recording medium, and many ink droplets can be printed indefinitely. It does not have to be attached to a place.

In view of this point, the present inventor investigated the relationship between the number of ink drops to be generated and the size of the pixel formed on the recording medium. The ink jet recording head used here has an ink ejection port size of 17 × 17 μm, an ink ejection port array density of 32 lines / mm, and other dimensions are the same as those of the ink jet recording head used in the above experiment. The composition of the used ink is glycerin 18.0% ethyl alcohol 4.8% water 75.0% C.I. I. Direct black 154 is 2.2%. Further, the ejection conditions are as follows: drive voltage is 6V, pulse width of pulse signal is 4 μs, and frequency of pulse signal is 45
kHz. Under this condition, the number of pulse signals is 1, 2, 3,
.. was input up to a maximum of 50, and the pixel diameter on the recording medium formed according to each pulse signal number was measured.
As the recording medium, PPC paper 620 manufactured by Ricoh Co., Ltd.
0 and Mitsubishi matte coated paper NM were used. The results are shown in FIG. In the graph of FIG. 6, the horizontal axis represents the number of ink droplets (pulse signal number) ejected to form one pixel, and the vertical axis represents the pixel diameter of the formed pixel.

As can be seen from the above, the pixel diameter can be increased as the number of ink droplets attached to substantially the same position on the recording medium is increased, but when the number of ink droplets exceeds a certain value, the pixel diameter is increased. Makes less contribution to increasing. Here, the frequency of ink droplet generation, that is, the frequency of the pulse signal is set to 45 kHz, but since one pixel is formed by a plurality of ink droplets,
The actual pixel formation frequency depends on the number of ink droplets forming the pixel, but is much slower than 45 kHz.

Assuming that n ink droplets are used to obtain the maximum number of pixels, the pixel formation frequency is 45 kHz.
z / n. The pixel formation frequency is the same whether one ink droplet forms one pixel or n ink droplets form one pixel, and n ink droplets form one pixel. The case is the rule condition. FIG. 7 shows the relationship between the frequency of ink drop generation and the frequency of pixel formation.

In the example shown in FIG. 7, pixels having different sizes are formed by changing the number of ink droplets within the range of 1 to 22. If the frequency is set to 22 kHz, the pixel formation frequency drops to 1 kHz. Since the pixel formation frequency determines the speed at which one print is executed, it should be as fast as possible. Therefore, if the number of ink droplets for forming one pixel is increased more than necessary, the printing speed becomes slow, which is not preferable. From this point of view, FIG.
The result shows that when the number of ink drops is less than 20, the pixel diameter changes relatively greatly according to the number of ink drops, but when it falls within the range of 20 to 30, the pixel diameter changes. It becomes slightly dull. Further, when the number of ink droplets is 30 or more, the pixel diameter hardly changes even if the number of ink droplets is increased, and it can be seen that it is meaningless to attach more ink droplets to the same place on the recording medium.

That is, in order to record one pixel by changing its diameter, it is desirable to record by changing the number of ink droplets within a range of at most 30, preferably within a range of up to 20. It is better to change the number of ink drops with, and optimally 10
It is recommended to change the number of ink drops within the range of up to 10. As is clear from the graph of FIG. 6, this is because the change rate of the pixel diameter is the best in the range where the number of ink drops is 10 or less.
This is because the rate of change is slightly lower in the range of 0 to 20, the rate of change is further lower in the range of 20 to 30, and almost no change occurs in the range of 30 or more.

From another point of view, the present invention is characterized by a pulse signal of 10 kHz or more, which could not be realized by an ink jet recording head having an ink ejection array arrangement density of about 16 ink droplets / mm in the conventional ink droplet forming frequency. To demonstrate
Since the upper limit is 75 kHz, the pixel forming frequency is 0.3 to 7.5 kHz.

Here, the result of actual recording will be described. In addition, the arrangement density of the ink discharge ports is 32 / m
Four inkjet recording heads each having m (the number of ink ejection ports is 256) are used, and yellow, magenta, and
A4 size paper (matte coated paper NM manufactured by Mitsubishi Paper Mills) was used by filling inks having different colors of cyan and black.
Various recording conditions are as follows. The frequency of the pulse signal was 45 kHz, and the number of ink droplets for forming one pixel was set in the range of 1 to 15 according to the image density information.
Therefore, the pixel formation frequency is 3 kHz. Furthermore, the gradation expression is not limited only by changing the pixel diameter by the method of the present invention, but by combining a 4 × 4 matrix,
It was decided to change up to 256 gradations. Therefore, although the pixel density was 32 / mm, the pixel density was 8 / mm. In order to perform recording on A4 size paper under the above conditions, 34 scans (one-way recording) were performed, and the process was completed in about a little less than 2 minutes. The obtained image had an image quality comparable to that of a silver halide photograph.

Next, another feature of the present invention will be described. This feature is that the maximum number of ink droplets 24 that can be attached to the same position on the recording medium 25 is variable. That is, in addition to the original recording mode of the recording apparatus, a mode (draft mode) in which the maximum number of the ink droplets 24 that can be attached to the same position on the recording medium 25 is changed is provided. . The original recording mode forms one pixel with one to ten ink droplets 24, while the draft mode forms one pixel with one to five ink droplets 24. If so, it is possible to print at twice the speed. It should be noted that such a draft mode is very convenient when it is desired to quickly obtain a rough image of the entire printed image even if the highest image quality is not obtained.

In addition, since the ink jet recording head is non-impact and non-contact recording, in principle, any recording medium (eg copy paper, recycled paper, O
It can be printed on a HP sheet, postcard, etc.).
However, the size of the pixel 26 formed on the recording medium 25 changes depending on the type of the recording medium 25.
Table 5 shows an example thereof. In addition, in Table 5,
A, B, and C are different types of recording media, and each has 1
One pixel 2 with 5 drops, 10 drops of ink 24
6 shows the ink mass and the pixel diameter when No. 6 is formed. The ink mass is actually measured by collecting 6 × 10 ⁵ ink droplets 24 (collecting by ejecting the ink droplets 24 for 30 seconds with a pulse signal of 20 kHz) and the weight thereof. The pixel diameter is the pixel 26 on the recording medium 25.
Was measured with an optical microscope with an XY stage,
The average value of 30 pieces is shown.

[0087]

[Table 5]

As is clear from Table 5, the pixel diameter of the pixel 26 formed on the recording medium A and the recording medium B is only slightly expanded on the recording medium B, but on the recording medium C. It can be seen that it is much wider than the recording bodies A and B. When the same image sample was printed using the recording materials A, B, and C under exactly the same conditions, the image on the recording material B was slightly darker than the image on the recording material A, and The image on the body C is much darker than the images on the recording bodies A and B. The largest pixel 26 on each recording medium A, B, C at that time is formed by 10 ink droplets 24.

Next, the maximum pixel 2 on the recording medium A is recorded.
When the same print sample was obtained by changing the number of the ink droplets 24 forming 6 to 11 droplets, an image having the same density as the image on the recording medium B was obtained. Furthermore, the recording medium A
When a similar printing sample was obtained by setting the number of ink droplets 24 forming the maximum pixel 26 to 14 droplets, an image having the same density as the image on the recording medium C was obtained. .

From the above results, even if the recording medium changes,
It can be seen that substantially the same image can be obtained by changing the maximum number of the ink droplets 24 attached to the same position on the recording medium according to the type of the recording medium. It should be noted that the number of ink droplets 24 attached to the same location does not change only when the largest pixel 26 is formed, and also changes in the case of pixels 26 of other sizes. Nor.

Further, in an ink jet recording apparatus equipped with a scanner which is an image reading apparatus, usually, a manuscript is read by a scanner and is faithfully output by an ink jet recording section. A dark copy may be printed, or a dark original may be printed lightly. At that time, by changing the maximum number of the ink droplets 24 attached to the same place, it is possible to easily obtain an image of an arbitrary density.

As described above, the method of adjusting the density of the image printed on the recording medium 25 by varying the maximum number of the ink droplets 24 attached to the same position on the recording medium 25 is as follows. The present invention is not limited to the thermal ink jet recording head of the type in which the heater 11 is heated to generate bubbles, but is also applicable to a continuous flow type ink jet head in which ink droplets are ejected by vibrating a piezoelectric vibrator. It is possible.

Next, another feature of the present invention will be described. First, when gradation recording is performed by changing the pixel diameter by changing the number of ink droplets 24 attached to the same location on the recording medium 25 as in the present invention, when the ink droplets 24 attached to the same location are recorded. It is desirable that the relationship between the number and the image density is a linear increase until the maximum density is reached as shown in FIG. However, when the relationship between the number of ink droplets 24 attached to the same place and the image density is measured, the linear increase does not occur, and the result shown in FIG. 9 is obtained. The ink jet recording head used in this measurement had a size of the ink ejection port 22 of 17 × 17 μm, a size of the heater 11 of 14 × 84 μm, a resistance value of the heater 11 of 77 Ω, and an array density of the ink ejection port 22 and the heater 11 of 800 dpi. In addition, the following components were used as the ink.

Glycerin 18.0% Ethyl alcohol 4.8% Water 75.0% C.I. I. Direct black 154 2.2% Further, as the recording medium 25 to which the ink droplets 24 adhere, PPC paper 6200 manufactured by Ricoh Co., Ltd. was used.
The number of ink drops 24 forming one pixel is 1, 2,
In order to be able to measure the concentration in case of 3, ..., ..., 20, 1
The whole surface is printed in a region of 0 × 10 mm, and each density is measured and plotted.

According to the measurement results shown in FIG. 9, in the range where the image density is low, the image density increases substantially linearly with the increase of the ink droplets 24, but as the image density increases, the saturation gradually approaches. Then, the increase of the image density becomes slower with the increase of the ink drops 24, and the required image density cannot be obtained unless the number of the ink drops 24 attached to the same position on the recording medium 25 is significantly increased. I understand.

Therefore, in the present invention, as shown in FIG.
And change the number of ink drops 24 so that the size is different.
D pixel₁, D_Two, D_Three,,,,, D₁₀When
And the number of ink drops 24 that make up those pixels.
1,2,3,4,5,6,7,8,9,10
Instead, for example, in the example of the present invention, 1, 2, 3, 4,
Set to 5, 6, 8, 10, 12, 20 drops
It By doing this, each pixel (D ₁~ D₁₀) Against the image
The concentration increases linearly as shown in FIG.
Image density can be easily obtained, and it is smooth and high quality.
Quality gradation recording is possible.

Next, another feature of the present invention will be described. This feature makes the center position of the pixel 26 formed by one to a plurality of ink droplets 24 substantially coincide with the center position of the region in which one pixel is to be formed, and the center-to-center distance between the adjacent pixels 26. They are made substantially equal to each other, and the center position intervals of adjacent pulse signal groups for forming one pixel are made substantially equal to each other. These characteristics will be described in detail below.

First, each square frame shown in FIG. 11 shows five areas in which one pixel is to be formed on the recording medium 25, and FIG. 12 shows binary values in each area. It shows a state in which a recording pixel 26 is formed. In the case of such binary recording, the center of the pixel 26 and the center of the region where the pixel is to be formed easily coincide with each other, and the center-to-center distance La of the region where the pixel is to be formed and the adjacent pixel 26. The center-to-center distance Lb substantially coincides.

Next, FIG. 13 shows a conventional example in which one pixel is formed by one to a plurality of ink droplets 24.
Depending on the number of ink droplets 24 forming one pixel, the center position of the pixel 26 and the center position of the region where one pixel is to be formed do not match, and further, the center-to-center distance Lc ₁ between adjacent pixels 26, There is a problem that Lc ₂ , Lc ₃ and Lc ₄ are divided, and finally the image quality is deteriorated.
This is because printing is performed while the ink jet recording head and the recording medium move relative to each other, and the number of ink droplets 24 forming one pixel changes in the range of 1 to a plurality of 1
This is because the time for forming pixels is different. Further, the central position intervals Ta ₁ , Ta ₂ , Ta ₃ , Ta ₄ of adjacent pulse signal groups for forming one pixel are also different.
Note that FIG. 13 shows the maximum ink droplet 24 that forms one pixel.
Is shown, and the pulse signal shown by the solid line is for ejecting the ink droplet 24.

FIG. 14 shows the features of the present invention.
When the number of ink droplets 24 forming a pixel is small (when forming a small pixel), a pulse signal for ejecting the ink droplet 24 is generated with a time delay. Specifically, in the case of forming one pixel with one ink droplet 24, the third pulse signal of the five pulse signals that can be generated is generated and one pixel is formed with two ink droplets 24. If so, the second and third pulse signals are generated. By doing so, the center position of each pixel 26 and the center position of the region in which one pixel is to be formed are substantially coincident with each other, and the center-to-center distances Ld ₁ and Ld of the adjacent pixels 26 are further increased.
d _{2, Ld} 3, _{Ld 4} is substantially coincident, the image quality is improved. Here, the expression “substantially the same” is used for the ink droplet 2
This is because a positional deviation of one pulse (one ink droplet) occurs depending on whether the number of pulse signals for generating 4 is an even number or an odd number. However, this positional deviation for one pulse is a value that can be almost ignored, and when forming one pixel with two ink droplets 24, the third and fourth pulse signals may be generated.

13 and 14, there is a gap between adjacent pixels 26,
This is to avoid complication of the drawing, and in the case of a continuous straight line or in the case of solid printing of the entire surface, the adjacent pixels 26 are overlapped with each other. Further, in FIGS. 13 and 14, when one pixel 26 is formed by a plurality of ink droplets 24, the pixel 26 is shown to be horizontally long, but this is also extremely large for convenience of description. It is a representation, and each pixel 26
Has a substantially round shape.

On the other hand, regarding the pulse signal group for forming one pixel, the center position interval T between the adjacent pulse signal groups is T.
b ₁ , Tb ₂ , Tb ₃ , and Tb ₄ are substantially constant.
It should be noted that the expression "substantially constant" is used here as well, as in the case described above, depending on whether the number of pulse signals for generating the ink droplets 24 is an even number or an odd number. This is because a positional deviation occurs, but this positional deviation for one pulse is a value that can be almost ignored.

Next, another feature of the present invention will be described with reference to FIGS. First, in an ordinary inkjet recording head that performs binary recording, when full surface printing (solid printing) is performed, adjacent pixels are partially overlapped with each other to eliminate a white background area where ink drops do not adhere. I have to. For example, as shown in FIG. 15, when the pixel diameter of each pixel 26 is D _d and the center-to-center distance of the adjacent pixels 26 is D _p , the white background area is set by setting D _d ≧ √2 · D _p. Disappear. To give specific numbers, in the case of a printing density of 400 dpi, D _p = 63.5μ
Therefore, if D _d ≧ 90 μm, the blank area disappears and the entire surface printing is realized. In order to obtain such a pixel diameter, for example, the above-mentioned Japanese Patent Publication No. 59-4.
In the so-called edge shooter type thermal ink jet recording head which ejects ink droplets in the horizontal direction with respect to the heater surface as disclosed in Japanese Patent No. 3312, the ink ejection port varies depending on the material of the ink or the paper. Is about 28 × 28 μm.

On the other hand, the present invention is an ink jet recording head in which gradation recording is performed by multi-value recording in which the size of the pixel is changed, and the ink discharge port 22 is 400 dp.
i are formed in a line at a density of i, and each ink ejection port 22 has 16
It is formed with a size of × 16 μm. The heater 11 is formed to have a size of 15 × 60 μm, and its resistance value is set to 61.7Ω.

Using this ink jet recording head, the ink of the above-mentioned composition (glycerin 18
%, Ethyl alcohol 4.8%, water 75%, C.I.
I. Direct black 154 (2.2%) was used to eject ink droplets, and as a result, the ink droplets were ejected with a stable frequency of the pulse signal input to the heater 11 up to 53 kHz.

Therefore, as a recording paper, P made by Ricoh Co., Ltd. is used.
Using the PC paper 6200, printing of all pixels in which ink droplets were ejected from all the ink ejection ports 22 was performed, and the pixel diameter of the formed pixels 26 was measured. Since the frequency of the pulse signal at this time is set to 48 kHz and the number of ink droplets for forming one pixel is changed within the range of 1 to 6, the pixel formation frequency is 8 kHz. FIG.
(A) is a state in which one pixel is formed by one ink droplet, and the pixel diameter is 32.1 μm. Similarly, FIG.
Has a pixel diameter of 6 when one pixel is formed with two ink drops.
3.8 μm, the figure (c) shows a pixel diameter of 72.5 μm when one pixel is formed by three ink droplets, and the figure (d) shows a pixel when one pixel is formed by four ink droplets. Diameter is 80.9
In the same figure (e), the pixel diameter is 88.8 μm when one pixel is formed by five ink droplets, and in the same figure (f) the pixel diameter is formed when one pixel is formed by six ink droplets. It is 96.2 μm. Note that the measurement of the pixel diameter when the adjacent pixels 26 are in contact with each other as shown in FIGS.
This was performed by printing only one pixel alone.

Here, when one pixel is formed by one ink droplet, the ink ejection port 22 is small and the heater 11 is also small accordingly, so that each ink ejection port 2
The amount of one ink droplet ejected from 2 is small, and the pixel diameter D _d of each pixel 26 is equal to the distance D _p between the center points of the adjacent pixels.
It becomes smaller than √2 times (that is, D _d <√2 · D _p ), and adjacent pixels are separated from each other. Therefore, even if all pixels are printed, a large amount of white area remains, and the entire printing surface becomes light gray. Then, as the number of ink droplets forming one pixel 26 is increased, FIG.
As shown in (d), the pixel diameter of each pixel 26 is increased, the overlapping portion of adjacent pixels is gradually increased, the white background region is gradually reduced, and the entire printing surface is gradually blackened. Then, in FIG. 16E, the pixel diameter becomes equal to √2 times the distance between the center points of the pixels 26 (that is, D _d
= √2 · D _p ), the white area disappears and the printing surface becomes completely black. In addition, in FIG. 16F, the pixel diameter is pixel 2
6 is larger than √2 times the distance between the center points, the overlapping portion of pixels is further increased, and the entire printing surface is darker black.

Next, when all pixels are printed by the usual ink jet recording head for performing the above-mentioned binary recording, the same result as in the case shown in FIG. 16 (e) is obtained, and the white background area on the printing surface is Disappear. Then, when one pixel was formed independently and the pixel diameter was measured, it was about 95.5 μm.
Met. Therefore, in order to perform gradation recording with this ink jet recording head, there is no choice but to thin out pixels and print, and gradation recording can be performed for a while.
Since the density of 0 dpi cannot be effectively used, the image has a very low resolution.

On the other hand, in the present invention, the pixel diameter when one pixel is formed by one ink drop is very small,
Adjacent pixels 26 do not overlap even when printed with a density of 0 dpi, and a sufficiently large white background area exists between the adjacent pixels 26. Therefore, gradation is recorded by changing the number of ink droplets forming one pixel to increase the pixel diameter so that the white background area is gradually filled, and finally eliminating the entire white background area. . In addition, it is not necessary to thin pixels as in the case of performing gradation recording by the above-described normal inkjet recording head that performs binary recording, and 400
Since the printing is performed by the pixel density of dpi, the resolution is not lowered, and it is possible to perform a very high quality printing.

[0110]

As described above, the invention according to claim 1 has the following advantages.
The liquid chamber that stores the ink is connected to this liquid chamber through a liquid path.
A plurality of ink discharge ports and heaters provided in the liquid passage.
And a pulse signal is input to the heater.
In addition to generating bubbles in the ink in the liquid storage path,
Ink is ejected from the ink ejection port by the action force associated with expansion.
Droplets are ejected to separate one or more ink droplets independently.
And eject the ink droplets for a long time with respect to the diameter.
The number of ink droplets is formed in the shape of a slender column whose size is three times or more, and is attached to almost the same location on the recording medium to form one pixel. in the ink jet recording head so as to vary the size of the pixel by varying, said in
The drive frequency of the cuddle recording head is 10 kHz to 7
Inkjet recording head used in the range of 5 kHz
A is, the case of forming one pixel by one ink droplet, since the formation of the ink discharge port sized to eject ink droplets of the amount adjacent pixels do not overlap, ink droplets forming one pixel It is possible to perform gradation recording by changing the number of pixels to change the size of the pixel and changing the white background area between the pixels. Moreover, gradation recording can be performed while ejecting ink droplets from all ink ejection ports. Therefore, there is an effect that the pixel density can be maintained at a high level and very high quality gradation recording can be performed.

[0111] As described above the present invention of claim 2, wherein, in
The liquid chamber that stores the fluid is connected to this liquid chamber via a liquid path.
A plurality of ink discharge ports and a heater provided in the liquid passage
And a pulse signal is input to the heater.
Bubbles are generated in the ink in the liquid path and the bubbles expand.
Ink droplets from the ink ejection port
To separate and separate one to a plurality of ink droplets.
Flying in a state of
Formed in the shape of an elongated column whose size is three times or more, and adhered to almost the same position on the recording medium to form one pixel, and at the same time, change the number of ink droplets adhered to the same position according to the image density information. In an ink jet recording head in which the size of the pixel is changed by changing the ink
The drive frequency of the jet recording head is 10 kHz to 75
Inkjet recording head used in the range of kHz
There are, one case of forming one pixel by the ink droplets, the ink to a size in which the pixel size of one pixel is to eject ink droplets of smaller volume than √2 times between the center point distance of pixels adjacent Since the ejection port is formed, gradation recording can be performed by changing the number of ink droplets forming one pixel to change the size of the pixel and the white background area between the pixels. Since gradation recording is performed while ejecting ink droplets from the ink ejection ports, it is possible to maintain a high pixel density and perform extremely high quality gradation recording.

According to the third aspect of the present invention, the pixel diameter on the recording medium can be changed according to the number of ink droplets, and it is possible to perform high-quality gradation recording.

According to the fourth aspect of the present invention, the pixel diameter on the recording medium can be changed relatively greatly according to the number of ink droplets, and the pixel forming speed (recording speed) is not sacrificed so much. This has the effect of enabling high-quality gradation recording.

According to the fifth aspect of the invention, the number of ink droplets ejected to form one pixel on the recording medium is made variable according to the image information obtained from the image reading device. The original can be printed dark, or the dark original can be printed thin, and the shade of the print result can be freely controlled.

According to the sixth aspect of the present invention, even if one to a plurality of ink droplets are ejected to form one pixel, the electric signal groups for forming adjacent pixels are formed at substantially the same interval, so that they are formed. The distances between the centers of adjacent pixels become substantially equal, which has the effect of realizing high-quality recording.

According to the invention described in claim 7, there is an effect that a large gradation change rate can be obtained and the print speed can be prevented from slowing down.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a flying state of ink droplets in an example of the present invention.

FIG. 2 is an explanatory diagram showing in detail the shape of flying ink droplets.

FIG. 3 is an explanatory diagram showing the relationship between the number of pulse signals, the number of ejected ink droplets, and the size of pixels formed.

FIG. 4 is an explanatory diagram showing how a pulse signal is input and how bubbles are generated.

FIG. 5 is an explanatory diagram showing a pulse signal continuously input and a state of bubble generation.

FIG. 6 is a graph showing the relationship between the number of ink droplets forming one pixel and the pixel diameter.

FIG. 7 is an explanatory diagram showing an ink drop generation frequency, a pixel formation frequency, and a pixel size.

FIG. 8 is a graph showing an ideal relationship between the number of ink droplets attached to the same position on a recording medium and the image density.

FIG. 9 is a graph showing the results of measuring the relationship between the number of ink droplets adhering to the same location on the recording medium and the image density.

FIG. 10 is a graph showing the relationship between each pixel and image density.

FIG. 11 is a plan view showing five regions where one pixel is to be formed on a recording medium.

FIG. 12 is a plan view showing a state in which a binary recording pixel is formed in each area where one pixel is to be formed on a recording medium.

FIG. 13 is an explanatory diagram showing a positional relationship between an area in which one pixel is to be formed and a pixel and a pulse signal generation timing in a conventional example in which one pixel is formed by one to a plurality of ink droplets.

FIG. 14 is an explanatory diagram showing a positional relationship between a pixel and a region where one pixel is formed in the present invention in which one pixel is formed by one to a plurality of ink droplets, and a pulse signal generation timing.

FIG. 15 is a plan view showing pixels formed by printing all pixels of an ordinary inkjet recording head that performs binary recording.

FIG. 16 is a plan view showing the relationship between the number of ink droplets forming one pixel and the pixel diameter, and the relationship of a white background area between the pixels.

FIG. 17 is a side view showing a heating element substrate of one ink jet recording head.

FIG. 18 is a plan view showing a procedure for forming a heating element substrate.

FIG. 19 is a plan view showing a modified example of the heating element substrate.

FIG. 20 is a perspective view showing a lid substrate.

FIG. 21 is a front view showing a heating element substrate of the ink jet recording head.

FIG. 22 is a front view showing a step of forming a groove for flowing ink on the heating element substrate.

FIG. 23 is a front view showing a state after the formation of the groove on the heating element substrate is completed.

FIG. 24 is a front view showing a lid substrate.

FIG. 25 is a front view showing a state in which a heating element substrate and a lid substrate are pressed and attached.

FIG. 26 is a perspective view showing a state in which a heating element substrate and a lid substrate are pressed and attached.

27 is a cross-sectional view taken along the line BB in FIG.

FIG. 28 is a vertical sectional side view showing a completed inkjet recording head.

[Explanation of symbols]

4,11 heater 18 liquid channels 19 Liquid chamber 22 Ink ejection port 23 Inkjet recording head 24 ink drops 25 Recorded material 26 pixels

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Claims (7)

(57) [Claims] 1. A liquid chamber for storing ink, a plurality of ink discharge ports communicating with the liquid chamber via liquid channels, and a heater provided in the liquid channel, and a pulse signal to the heater. Is generated to generate bubbles in the ink in the liquid path, and ink drops are ejected from the ink ejection port by the action force caused by the expansion of the bubbles, so that one to a plurality of ink drops are separated and independent. Fly and eject the ink droplets directly.
Shaped in the shape of an elongated column whose length dimension is three times or more the diameter dimension
To form one pixel by adhering to approximately the same location on the recording medium and changing the number of ink droplets adhering to approximately the same location according to the image density information to change the pixel size. In the ink jet recording head, the ink jet recording head has a driving frequency
Is used in the range of 10 kHz to 75 kHz.
Inkjet recording head, characterized in that when one pixel is formed by one ink droplet, the ink ejection port is formed in a size that ejects an amount of ink droplet that does not overlap adjacent pixels. Recording head. 2. A liquid chamber for storing ink, a plurality of ink discharge ports communicating with the liquid chamber through liquid channels, and a heater provided in the liquid channel, and a pulse signal to the heater. Is generated to generate bubbles in the ink in the liquid path, and ink drops are ejected from the ink ejection port by the action force caused by the expansion of the bubbles, so that one to a plurality of ink drops are separated and independent. Fly and eject the ink droplets directly.
Shaped in the shape of an elongated column whose length dimension is three times or more the diameter dimension
To form one pixel by adhering to approximately the same location on the recording medium and changing the number of ink droplets adhering to approximately the same location according to the image density information to change the pixel size. In the ink jet recording head, the ink jet recording head has a driving frequency
Is used in the range of 10 kHz to 75 kHz.
In the case of a jet recording head, when one pixel is formed by one ink droplet, a size in which the pixel diameter of one pixel is smaller than √2 times the distance between the center points of adjacent pixels is ejected. In addition, an ink jet recording head characterized in that the ink discharge port is formed. 3. The ink jet recording head according to claim 1, wherein the plurality of ink drops is 30 or less. 4. The ink jet recording head according to claim 1, wherein the plurality of ink droplets is 20 or less. 5. An ink jet recording apparatus with a scanner, wherein the ink jet recording apparatus using the ink jet recording head according to any one of claims 1, 2, 3 and 4 is provided with an image reading apparatus, wherein the image information is obtained from the image reading apparatus. An ink jet recording apparatus, wherein the number of ink droplets ejected to form one pixel on a recording medium is variable. 6. The ink jet recording head according to claim 1, wherein the groups of electric signal groups for forming the adjacent pixels are substantially equal to each other. 7. The opening area of the ejection port is less than 500 μm ² , and ink droplets are ejected and ejected from the ink ejection port,
In an ink jet recording head for recording by forming pixels by adhering to a recording medium, one to a plurality of the pixels are overlapped on the recording medium at substantially the same position according to image information. In an inkjet recording head that performs gradation recording, the number of times of driving at approximately the same location on the recording medium is a density of 1.5 or less, which is a value at which the image density on the recording medium is almost saturated. The ink jet recording head according to claim 1, 2, 3, 4, or 6.