JPH1044409A - Ink jet recording head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a smooth and high quality multilevel halftone image by an arrangement wherein each nozzle has such a cross-section as the jetting side is narrower than the ink cavity side, each head section has a nozzle of different cross-section and the head for jetting ink is switched appropriately. SOLUTION: The nozzle 28 at each of head sections 12, 14 has such a cross- section as the jetting side narrower than the ink cavity side 26 and the nozzle 28a at the large diameter head section 12 has a cross-section different from that of the nozzle 28b at the small diameter head section 14. Consequently, a pressurized ink 24 has a relatively low pressure loss at the time of passing through the nozzle 28a and a larger droplet is jetted from the nozzle 28a than from the nozzle 28b. When the large diameter head section 12 or the small diameter head section 14 is selected appropriately depending on an image signal and an applying voltage to be applied to a piezoelectric member 42 is varied, a smooth multilevel halftoning is realized and a high quality halftone image like a photograph is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drop-on-demand system in which ink in an ink cavity is pressurized according to an image signal to eject ink droplets from nozzles and adhere to a recording medium to record an image. The present invention relates to an inkjet recording head.

[0002]

2. Description of the Related Art Conventionally, as a method of creating a gradation image with an ink jet recording head, a large-diameter nozzle and a small-diameter nozzle having different sizes are formed on a head, and a large ink droplet is ejected from the large-diameter nozzle. Is known to perform gradation expression by appropriately discharging small ink droplets according to an image signal.

[0003]

However, changing the nozzle diameter alone does not greatly widen the gradation range, and is insufficient for producing a smooth halftone image such as a photograph.

[0004]

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of the above problems, and has been made in view of the above circumstances. An ink jet recording head is provided with a plurality of heads, each of which is provided with an ink cavity containing ink, and an ink cavity. A pressurizing means for pressurizing the ink in the cavity, and a nozzle for discharging the pressurized ink as ink droplets, each nozzle having a cross-sectional shape in which the discharge side is narrower than the ink cavity side, The cross-sectional shape of the nozzle differs depending on the head.

In the ink jet recording head according to the present invention, the cross section of the nozzle is formed so as to be gradually narrowed, and the length of the narrowest portion located on the discharge side in the direction along the nozzle center axis is determined by the head portion. The nozzle section may be made different, or the nozzle section may be formed so as to be tapered, and the taper angle of the inner surface of the nozzle with respect to the nozzle center axis may be made different depending on the head portion.

[0006]

In the ink jet recording head according to the present invention, the nozzle has a sectional shape in which the ejection side is narrower than the ink cavity side, and the sectional shape differs depending on the head portion. The flow path resistance of the nozzle varies depending on the difference in the cross-sectional shape, and the pressure loss when the pressurized ink passes through the nozzle also varies. Therefore, even if the same pressing force is applied to the ink in the ink cavity by the pressurizing means, small ink droplets are ejected from the head portion having a nozzle having a cross section having a large flow path resistance,
Conversely, a large ink droplet is ejected from a head having a nozzle having a small flow path resistance. Therefore, a high-quality halftone image having smooth and wide multi-step tones can be obtained by appropriately switching the head section for ejecting ink according to the image signal.

[0007]

Embodiments of the present invention will be described below with reference to the accompanying drawings. 1 to 4 show an ink jet recording head 10 according to the present invention. The head 10 includes a large-diameter ink droplet ejection head unit 12 (hereinafter, referred to as a “large-diameter head unit 12”) and a small-diameter ink droplet ejection head unit 14 (hereinafter, referred to as a “small-diameter head unit 14”). The large diameter head section 12 and the small diameter head section 14
Are the top plate 16, the partition 18, the diaphragm 20, and the substrate 22
Are superposed on each other to form an integral unit.

The top plate 16 is made of metal, synthetic resin, ceramic, or the like. The portion facing the partition 18 is finely processed by a method such as electroforming or photolithography to form a large diameter head portion 12.
And a small-diameter head unit 14, an ink cavity 26 for accommodating ink 24, an ink supply chamber 30 for accommodating replenishment ink 24, an ink inlet 32 for communicating each ink cavity 26 with ink supply chamber 30, Nozzle 28 for discharging ink 24 in cavity 26
Are formed. Hereinafter, the nozzles 28 of the large-diameter head unit 12 and the small-diameter head unit 14 will be appropriately distinguished using reference numerals 28a and 28b.

As shown in FIG. 1, the ink cavities 26 of the large-diameter head portion 12 and the small-diameter head portion 14 are formed in parallel and in a long groove shape extending along the direction in which the head portions 12 and 14 face each other. . The ink supply chamber 30
Are formed on the opposite side of the center line 34 across the ink cavity 26, and are connected to an ink tank (not shown).

[0010] As shown in FIG.
Has a cross-sectional shape that is narrower on the ink ejection side than on the ink cavity 26 side. The nozzle 28a of the large diameter head 12 and the nozzle 2 of the small diameter head 14
8b has a different cross-sectional shape.

As shown in FIGS. 3 (a) and 3 (b), each of the nozzles 28a and 28b
a and 38b are formed so that the cross-sectional shape gradually decreases toward the discharge side.
Reference numeral 28b also denotes first regions 31a and 31b having an inner diameter D which is a wide portion located on the ink cavity 26 side, and second regions 29a and 29b having an inner diameter d which is a nozzle narrowest portion located on the ejection side.
It consists of However, the second region 2 of the nozzle 28a
Direction length L ₁ along the nozzle center axis 38a of the 9a is in the direction along the nozzle center axis 38b of the second region 29b of the nozzle 28b is formed shorter than the length L _2. Ink second region 29a of the nozzle 28, the flow path resistance when passing through 29b is proportional to the contact area between the ink if the inner diameter d is the same ([pi] d × L), the length L ₁ is Nozzle 28a which is formed short and has a relatively small contact area with ink
The flow path resistance at the time of ink ejection is smaller in the nozzle than in the nozzle 28b.

As shown in FIG. 2, the partition 18 is made of a thin film of metal, synthetic resin, or the like, and is fixed between the top plate 16 and the diaphragm 20. It is desirable that the partition wall 18 be fixed with a predetermined tension applied thereto.

The diaphragm 20 is made of a well-known piezoelectric material, and has a conductive metal layer (not shown) used as a common electrode and an individual electrode on its upper and lower surfaces, respectively.
8 and the substrate 22. In addition, diaphragm 2
2 and FIG. 4, a vertical groove 58 and a horizontal groove 60 are formed by dicing to divide and separate the piezoelectric member 42 from the piezoelectric member 42 (pressing means) corresponding to each ink cavity 26. Partition wall 4 located between the piezoelectric members 42
4 and a surrounding wall 46 surrounding them. Each piezoelectric member 42 is polarized by applying a high voltage between the upper and lower common electrodes and the individual electrodes at a high temperature. In addition, the piezoelectric member 42 is not limited to the single-layer type, and a laminated piezoelectric member formed by laminating a plurality of thin film piezoelectric sheets so as to sandwich the metal electrode layer may be used.

The substrate 22 is made of ceramic, metal, synthetic resin or the like, and is fixed to the diaphragm 20.

In the head 10 configured as described above, the ink 24 is supplied from an ink tank (not shown) to the ink supply chamber 30, and is supplied to and stored in each of the ink cavities 26 via the ink inlet 32. In this state, when a voltage is applied to the piezoelectric member 42 from an image signal control circuit (not shown), the piezoelectric member 42 is instantaneously deformed and the partition 18 is pushed into the ink cavity 26. Thereby, the ink 24 in the ink cavity 26 is pressurized, and the ink droplet is ejected from the nozzle 28.

Large diameter head section 12 and small diameter head section 14
Are formed in the same size,
If the applied voltage is the same, the pressure applied to the ink 24 in the ink cavity 26 by the deformation is the same. However, as described above, the flow path resistance of the nozzle 28a of the large diameter head portion 12 is smaller than that of the nozzle 28b of the small diameter head portion 14. Therefore, the pressure loss when the pressurized ink 24 passes through the nozzle 28a is relatively small, and the nozzle 28a
A larger ink droplet is ejected than that ejected from 8b. For this reason, the large-diameter head unit 1 according to the image signal
2 and the small-diameter head portion 14 are appropriately selected, and by changing the voltage applied to the piezoelectric member 42, a smooth and wide multi-step tone expression can be achieved. As a result, a high-quality halftone image such as a photograph is obtained. be able to.

The large diameter head 12 and the small diameter head 1
No. 4, the nozzle diameter d is the same except that the lengths L ₁ and L ₂ of the second regions 29 a and 29 b of each nozzle 28 are different, so that the nozzle hole processing is easier as compared with a head having nozzles of different diameters. become.

In this embodiment, each nozzle 28a, 2
Although the diameters of the second regions 29a and 29b of 8b are set to the same diameter d, if the large-diameter head portion 12 is formed to be larger in diameter than the small-diameter head portion 14, the gradation width is further increased.

Also, unlike the nozzle cross-sectional shape of the present embodiment, as shown in FIGS.
The cross section of each of the nozzles a and 28b is formed in a tapered shape gradually narrowing toward the discharge side, and the taper angles θ ₁ and θ ₂ of the nozzle inner surfaces 40a and 40b with respect to the nozzle center axes 38a and 38b are changed, so that the flow path of each nozzle is changed. The resistance may be different. In this case, the nozzle 28 of the large-diameter head unit 12
a, the taper angle θ ₁ of the nozzle inner surface 29a of the small diameter head 14 is changed to the taper angle θ ₂ of the inner surface 29b of the nozzle 28b of the small diameter head 14.
By making it larger, the flow path resistance of the nozzle 28a can be made smaller than that of the nozzle 28b.

Further, instead of gradually reducing the cross section of each of the nozzles 28a and 28b,
Nozzle holes having the same diameter d are formed so as to communicate with each other from the 6th side to the discharge side.
It may be different depending on. For example, as shown in FIG. 6, instead of the top plate 16, a cavity forming member 62 for forming a space region such as the ink cavity 26, and a nozzle plate for the large-diameter head portion 12 covering the upper portion thereof, as shown in FIG. Using the nozzle plate 64 and the nozzle plate 66 for the small-diameter head section 14, the nozzles 28a and 28b having the same diameter d are formed in the respective nozzle plates 64 and 66, and the thickness of the nozzle plate 64 is made thinner than the nozzle plate 66. Due to the difference in thickness between the nozzle plates 64 and 66, the length of the inner surface of the nozzle in the direction along the central axis of each of the nozzles 28a and 28b is larger than that of the small-diameter head 14.
2 is shorter and the flow path resistance of the nozzle 28a of the large diameter head 12 can be made smaller than that of the nozzle 28b of the small diameter head.

Here, the relationship between the voltage applied to the piezoelectric member and the dot diameter formed by ink droplets ejected from the large-diameter head portion and the small-diameter head portion was examined using the above-described ink jet recording head.

First, in Experiment 1, the diameter d of the second regions 29a and 29b of the nozzles of each head portion was 35 μm, and the length L (L ₁ , L ₂ ) of the second regions was L ₁ = L _{1 in the} large diameter head portion. 10μ
m, L ₂ = 20 μm in the small diameter head portion, and the applied voltage is 1
The diameter of the ink dots formed on the recording medium was measured while changing from 0 or 20 V to 60 V in steps of 10 V. The results are shown in the graph of FIG. From this graph,
It can be seen that even if the voltage applied to the piezoelectric member is the same, the ink dots formed in the large diameter head are larger than those formed in the small diameter head.

Subsequently, the second nozzle of each head section
The length L (L ₁ , L ₂ ) of the region is set to 10 μm, and the second
With the nozzle diameter in the region set to 50 μm for the large diameter head portion and 35 μm for the small diameter head portion, the change in ink dot diameter when the applied voltage was changed in the same manner as described above was examined. The results are shown in the graph of FIG. From this graph, it is clear that the larger the nozzle diameter on the ejection side, the larger the ink dot diameter. Therefore, in consideration of the result of Experiment 1, the gradation width can be further increased by combining the respective changes in the length and diameter of the second region of the nozzle.

Next, in Experiment 2, the change in the ink dot diameter when the flow path resistance of the nozzle was varied by changing the angle θ of the nozzle inner surface with respect to the nozzle center axis in a nozzle having a tapered cross section was examined. Was. In this experiment, the diameter d of the nozzle orifice of each head portion was 35 μm and the length L of the nozzle inner surface was 20 μm, while the taper angle θ ₁ of the nozzle inner surface of the large diameter head portion was 45 °, and the small diameter head portion was The taper angle θ ₂ was set to 0 °, and the voltage applied to the piezoelectric member was changed from 20 V to 60 V in increments of 10 V, and the diameter of the ink dots formed on the recording medium was measured. The results are shown in the graph of FIG. From this graph, it can be seen that even when the same voltage is applied to the piezoelectric member, ink droplets larger than those of the small-diameter head portion are ejected from the large-diameter head portion having a small flow path resistance of the nozzle, and large-diameter ink dots are formed. I understand.

Subsequently, this time, the taper angle θ of the nozzle inner surface
(= 45 °) and its length L (= 20 μm) are common to each head portion, while the diameter d of the nozzle discharge port is 35 μm for the large diameter head portion and 20 μm for the small diameter head portion, and the application to the piezoelectric member is performed. The change in the ink dot diameter was examined by changing the voltage. The results are shown in the graph of FIG. The ink dots formed by the small-diameter head portion when a voltage of 50 or 60 volts is applied have a dot shape that cannot be put to practical use due to a gap at the time of impact, and thus are not adopted as a measured value of the dot diameter. From this graph, it is clear that if the diameter d of the nozzle ejection port is increased, an ink dot having a larger diameter is formed even at the same voltage. Therefore, considering the results of Experiment 2, it can be seen that the gradation width can be further increased by combining the change in the angle θ of the nozzle inner surface and the change in the diameter d of the nozzle discharge port.

As shown in FIG. 11, the cross section of the nozzle 70 is constituted by a combination of a region 72 having a constant nozzle diameter and a tapered region 74, and the length L of the region 72 and the taper angle of the region 74 Nozzle 7 with various θ
If a plurality of heads provided with 0 are provided, the gradation can be finely controlled and a smoother gradation image can be obtained.

In the ink jet recording head 10 according to the present embodiment, two head portions 12 and 14 are provided.
Three or more heads having different nozzle cross-sectional shapes may be provided.

In the above description, the partition is provided and the piezoelectric member and the ink are accommodated in separate rooms, respectively. However, the partition is not necessarily required. However, if the ink is brought into direct contact with the piezoelectric member, the ink penetrates into the piezoelectric member and the deformability decreases, so if no partition is provided, a film is formed on the piezoelectric member and the piezoelectric member comes into direct contact with the ink. It is preferable to prevent this.

Further, in the head of this embodiment, a piezoelectric member is used as a pressurizing means for pressurizing the ink in the ink cavity. However, the present invention is also applicable to a bubble jet type head using a heating element as the pressurizing means. it can.

[Brief description of the drawings]

FIG. 1 is a partial plan view of an ink jet recording head according to the present invention.

FIG. 2 is a sectional view taken along line II-II of FIG.

FIG. 3 is an enlarged view of a cross section of each nozzle of a large diameter head portion and a small diameter head portion in FIG. 2;

FIG. 4 is a sectional view taken along line IV-IV of FIG. 2;

FIG. 5 is a view showing a modification of the cross-sectional shape of each nozzle of the large-diameter head portion and the small-diameter head portion.

FIG. 6 is a partial cross-sectional view of a modified example of a head formed by covering a large diameter head portion and a small diameter head portion with nozzle plates having different thicknesses.

FIG. 7 is a graph showing the relationship between the voltage applied to the piezoelectric member and the dot diameter when the length of the second region is different in the nozzle cross-sectional shapes of the large-diameter head portion and the small-diameter head portion.

FIG. 8 shows the relationship between the voltage applied to the piezoelectric member and the dot diameter when the diameters of the nozzle discharge ports of the large diameter head section and the small diameter head section are made different while the length of the second area of each nozzle is made the same. 6 is a graph showing the relationship of.

FIG. 9 is a graph showing the relationship between the voltage applied to the piezoelectric member and the dot diameter when the taper angle of the inner surface of the nozzle with respect to the central axis of the nozzle is changed in the nozzle cross-sectional shapes of the large-diameter head portion and the small-diameter head portion. is there.

FIG. 10 shows the relationship between the voltage applied to the piezoelectric member and the dot diameter when the taper angle of the inner surface of the nozzle is made the same and the diameter of the nozzle ejection port differs between the large-diameter head portion and the small-diameter head portion. It is a graph shown.

FIG. 11 is a diagram showing a modified example in which a nozzle cross-sectional shape is configured by combining a region having the same nozzle diameter and a tapered region.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Ink jet recording head, 12 ... Large diameter head part, 14 ... Small diameter head part, 16 ... Top plate, 18 ... Partition wall, 2
0: diaphragm, 24: ink, 26: ink cavity,
28, 28a, 28b ... nozzle, 29a, 29b ... second
Area (the narrowest part of the nozzle), 38a, 38b: central axis of the nozzle, 42: piezoelectric member (pressing means).

Claims (3)

[Claims] 1. An ink cavity having a plurality of heads, each of which contains ink, a pressurizing means for pressurizing the ink in the ink cavity, and the pressurized ink forming an ink droplet. An ink jet recording head comprising: a nozzle for discharging; and a nozzle having a different cross-sectional shape depending on a head portion. 2. A method according to claim 1, wherein the plurality of heads include first and second heads having nozzles whose diameters on the ejection side are equal to or less than the diameters on the ink cavity side. 2. The ink jet recording head according to claim 1, wherein the ink jet recording head is formed so as to be gradually narrowed, and the length of the narrowest portion of the nozzle along the central axis of the nozzle is different between the first and second head portions. . 3. The plurality of head units include first and second head units having nozzles whose diameters on the ejection side are equal to or smaller than the diameters on the ink cavity side, and at least the nozzles of the first head unit have cross-sections thereof. 2. The ink jet recording head according to claim 1, wherein the ink jet recording head is formed so as to be tapered and the taper angle of the inner surface of the nozzle with respect to the central axis of the nozzle is different between the first and second head portions.