JP7500973B2 - Discharge Head - Google Patents

Description

The present invention relates to an ejection head that ejects liquid.

Conventionally, in ejection heads such as inkjet heads that are provided with multiple nozzles for ejecting liquid such as ink, pressure dampers are provided midway along the supply path that supplies ink to a common flow path, which is a common liquid chamber connected to all nozzles, and the return path that returns ink from the common flow path to a storage tank, to suppress pressure fluctuations within the common liquid chamber (see, for example, Patent Document 1).

In addition, other conventional ejection heads have a pressure damper positioned for each nozzle so that it faces closely to the opening of the pressure chamber on the common liquid chamber side, which is provided between the common liquid chamber and the nozzle, and the pressure damper buffers the back pressure from the pressure chamber when ink is ejected (see, for example, Patent Document 2).

JP 2010-201698 A JP 2015-044324 A

However, in the ejection head of Patent Document 1, since the pressure damper is provided outside the common flow path, it is possible to suppress the influence of pressure fluctuations outside the head, but it is not possible to suppress the pressure head inside the head.
Furthermore, while the ejection head of Patent Document 2 is capable of suppressing back pressure during liquid ejection, it is unable to suppress, for example, a sudden drop in pressure in the common liquid chamber when large amounts of ink are ejected from many nozzles at once, or a sudden rise in pressure in the common liquid chamber due to refill ink when ejection is stopped all at once after ejecting a large amount of ink.

The objective of the present invention is to provide an ejection head that effectively suppresses pressure fluctuations within the head.

In order to achieve the above object, the ejection head of the present invention comprises:
A common liquid chamber for containing a liquid;
a plurality of pressure chambers communicating with the common liquid chamber;
A plurality of nozzles communicating with the plurality of pressure chambers;
an actuator that changes the volume of the pressure chamber;
a damper member disposed inside the common liquid chamber and configured to absorb pressure of the liquid;
the damper member has a volume of 0.3 mL or more, and both ends thereof are supported by inner walls of the common liquid chamber so as to be perpendicular to the droplet ejection direction of the nozzle;
the common liquid chamber has a volume excluding the damper member of 2 mL to 5 mL,
When the maximum flow rate of liquid ejected from all of the nozzles is F [mL/sec], the amount of volume change in the damper member relative to pressure is 3×F [uL/kPa] or more.

As described above, the present invention makes it possible to effectively suppress pressure fluctuations within the head.

1 is a diagram showing a schematic configuration of an inkjet printing apparatus according to an embodiment of the present invention. FIG. 2A is a schematic diagram of the internal configuration of the head unit when viewed from the front, and FIG. 2B is a schematic diagram of the internal configuration of the head unit when viewed from the conveyor belt side. 3 is a cross-sectional view of an ink flow path in the recording head as viewed from the front. 11 is a graph showing changes in ink pressure in a common ink chamber when simultaneous ejection and simultaneous cessation of ink ejection from all nozzles is repeated every few seconds in a conventional recording head that does not have a damper member. This is a test pattern image formed in a conventional recording head that does not have a damper member, when ejection is resumed after stopping from large-volume ejection by gradually increasing the number of nozzles ejecting without waiting for the internal pressure to stabilize. FIG. 13 is a diagram showing the results of measuring the pressure amplitude generated in a common ink chamber when simultaneous ink ejection and simultaneous cessation are repeatedly performed every few seconds in each of Example 1 and Comparative Examples 1 to 4. 1 shows a test pattern image formed by the print head of Example 1 when ejection is restarted by gradually increasing the number of nozzles immediately after stopping a large amount of ejection.

Below, an embodiment of an inkjet recording device to which the ejection head of the present invention is applied will be described with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an inkjet recording apparatus 1 according to an embodiment of the present invention.
The inkjet recording apparatus 1 includes a transport unit 10 and a head unit 20 .

The transport unit 10 includes a ring-shaped transport belt 103 whose inner side is supported by two transport rollers 101 and 102 that rotate around a rotation axis extending in the X-axis direction in Fig. 1. In the transport unit 10, with a recording medium M placed on the transport surface of the transport belt 103, the transport roller 101 rotates in response to the operation of a transport motor (not shown), causing the transport belt 103 to move in a circular motion, thereby transporting the recording medium M in the movement direction of the transport belt 103 (transport direction; Y-axis direction in Fig. 1).
The recording medium M is a sheet of paper or a long sheet material cut to a certain size. The recording medium M is supplied onto the conveyor belt 103 by a supply device (not shown), and after an image is recorded by ejecting ink, which is the liquid to be ejected, from the head unit 20, the recording medium M is ejected from the conveyor belt 103 to a predetermined ejection section.
As the recording medium M, in addition to paper such as plain paper or coated paper, various media capable of fixing the ink that has landed on the surface, such as fabric or sheet-like resin, can be used.

The head units 20 eject ink at appropriate timing based on image data onto the recording medium M being transported by the transport unit 10 to record an image. In the inkjet recording device 1 of this embodiment, four head units 20 corresponding to the four colors of ink, yellow (Y), magenta (M), cyan (C), and black (K), are arranged at predetermined intervals from the upstream side in the transport direction of the recording medium M in the order of Y, M, C, and K. The number of head units 20 may be three or less, or five or more.

Figure 2 is a diagram showing the internal configuration of the head unit 20. Figure 2(a) is a schematic diagram of the internal configuration when the head unit 20 is viewed from the front. Figure 2(b) is a schematic diagram of the internal configuration when the head unit 20 is viewed from the conveyor belt 103 side. Note that when the head unit 20 is viewed from the front, this refers to when the head unit 20 is viewed from the conveying direction of the recording medium M.

The head unit 20 has a plurality of recording heads 22 serving as ejection heads for ejecting ink.
In the recording head 22, a plurality of nozzles 241 are arranged in two rows along the X-axis direction to form two nozzle rows, and these two nozzle rows are arranged offset from each other by half the nozzle pitch in the X-axis direction.

In the head unit 20, two recording heads 22 are combined to form a head module 22M (ink ejection section), and the head modules 22M are arranged in a staggered pattern. In each head module 22M, the recording heads 22 are arranged in a positional relationship such that the nozzles 241 of the two recording heads 22 are arranged alternately in the X-axis direction. The number of head modules 22M provided in the head unit 20 is not particularly limited, but in this embodiment, it is set to twenty-four. With such an arrangement of the recording heads 22, the arrangement range in the X-axis direction of the nozzles 241 included in the head unit 20 covers the width in the X-axis direction (recordable width) of the area in which an image can be formed on the recording medium M transported by the transport belt 103. The head unit 20 is used with its position fixed when recording an image, and ejects ink sequentially at predetermined intervals (transport direction intervals) to different positions in the transport direction according to the transport of the recording medium M, thereby recording an image by a single pass method.

FIG. 3 is a cross-sectional view of the ink flow path in the recording head 22 as viewed from the front.
The recording head 22 is a shear mode type inkjet head, and includes a head chip 23, a nozzle plate 24 as a nozzle substrate bonded to the front side of the head chip 23 (the direction in which ink is ejected is defined as the front), a wiring board (not shown) bonded to the head chip 23, a flexible board (not shown) connected to an end of the wiring board and including a drive circuit for controlling ejection, a common ink chamber 25 as a common liquid chamber provided on the rear side of the head chip 23, and the like.

The nozzle plate 24 is adhered by an adhesive to the front side of the head chip 23. The nozzle plate 24 is a flat plate along the XY plane, and is made of a material that has lyophilicity with respect to ink, for example, a resin such as polyimide.
A plurality of nozzles 241 each consisting of a through hole aligned along the Z-axis direction are formed in the nozzle plate 24. The plurality of nozzles 241 constitute a nozzle row aligned at a constant pitch in the X-axis direction, and the nozzle rows are formed in a row in the Y-axis direction.

The head chip 23 is made of a hexahedron, and has a plurality of channels 231 formed as pressure chambers penetrating in the Z-axis direction. The channels 231 are aligned in the X-axis direction to form a channel row, and a plurality of channel rows are formed aligned in the Y-axis direction.
Each channel 231 is arranged so as to communicate with each nozzle 241 of the nozzle plate 24 when the nozzle plate 24 is attached to the front surface of the head chip 23 .

The partitions located between the multiple channels 231 aligned in the X-axis direction are driving walls made of piezoelectric elements acting as actuators. The piezoelectric elements of the driving walls are made of, for example, PZT (lead zirconate titanate).
A drive electrode is formed on the inner surface of each drive wall, and a drive signal of a predetermined voltage is applied through the wiring electrodes formed on the wiring board and the flexible board, which causes shear deformation of the drive wall sandwiched between the pair of drive electrodes, applying a pressure change for ejection to the ink supplied into the channel 231, and ejecting ink droplets from the nozzle 241 of the nozzle plate 24 that communicates with the channel 231.

As shown in FIG. 2(a), the recording head 22 has an inlet 251 through which ink flows into the recording head 22, and an outlet 252 through which ink flows out of the recording head 22.

The common ink chamber 25 is a hollow box that contains ink therein, and has an inlet 251 that communicates with the interior as an ink supply flow path, and an outlet 252 that communicates with the interior as an ink discharge flow path.
The rear surface of the head chip 23 is attached to the front surface (the lower side in Figure 3) of the common ink chamber 25 by adhesive, and communication holes 253 are formed through the interior of the common ink chamber 25, individually connecting it to each channel 231 of the head chip 23.
The front surface of common ink chamber 25 is set to be the same size as or slightly larger than the rear surface of head chip 23, and the entire rear surface of head chip 23 is in close contact with the front surface of common ink chamber 25. Therefore, all channels 231 of head chip 23 communicate with the interior of common ink chamber 25 via communication holes 253.

A damper member 26 having a generally flat plate shape along the XY plane is disposed inside the common ink chamber 25 .
The damper member 26 comprises a main body 261 consisting of a hexahedral box with one of its flat surfaces (the front side) being completely open and the inside being hollow, and an elastic film member 262 attached to the main body 261 so as to completely block the open front side of the main body 261.
The inside of the main body 261 of the damper member 26 is sealed by an elastic film member 262, and a compressible gas (e.g., air) is sealed in the sealed space. Note that a foam material such as a compressible sponge may be sealed in the sealed space of the main body 261 together with the gas.

The width of the damper member 26 in the X-axis direction or the Y-axis direction is set to be equal to the width of the interior of the common ink chamber 25 in the X-axis direction or the Y-axis direction, and both ends of the damper member 26 in the X-axis direction or the Y-axis direction are supported by being adhered or the like to the inner wall of the common ink chamber 25 .
The damper main body 261 may be integral with the common ink chamber 25 at both ends in the X-axis direction or the Y-axis direction.
As described above, the damper member 26 has the elastic film member 262 provided on the front side and has a compressible gas sealed inside, so that when pressure fluctuations occur due to the inflow or outflow of ink inside the common ink chamber 25, the elastic film member 262 elastically bends inward or outward, thereby suppressing sudden pressure fluctuations.
The elastic film member 262 is formed from, for example, polyphenylene sulfide resin (PPS), liquid crystal polymer (LCP), polyimide resin, or the like.

An inlet 251 and an outlet 252 of the recording head 22 are connected to an ink tank (not shown) via a tube 254 serving as a supply pipe and a tube 255 serving as a discharge pipe, respectively.
A pump (not shown) is provided between the ink tank and the recording head 22 , and ink is circulated and supplied between the ink tank and the common ink chamber 25 through tubes 254 and 255 .

The ink used is not particularly limited in type, but may contain, in addition to a dispersion medium, solid particles having a larger specific gravity than the dispersion medium. Examples of solid particles include ceramic particles in ceramic ink and pigment particles such as titanium oxide.
In addition, it is preferable to use ink that does not volatilize when dried at normal temperature and pressure. Here, "not volatilizing" refers to ink that contains 10% or less, preferably 5% or less, of a substance whose vapor pressure at normal temperature is greater than that of water. When such ink is used, the increase in viscosity caused by the evaporation of volatile components, which occurs when using volatile inks such as water-based inks, does not become a problem. Examples of such inks include UV inks and oil inks.

[Example of Discharge Head]
A more specific example of the recording head 22 as the ejection head will be described using numerical values and the like.

4 is a graph showing the change in pressure of ink in a common ink chamber when a simultaneous ejection and simultaneous cessation of ink ejection from all nozzles is repeated every few seconds in a conventional recording head that does not have a damper member. The pressure of ink in the common ink chamber was detected by a pressure sensor provided inside the outlet of the recording head.
As shown in the figure, the pressure of the ink in the common ink chamber of the recording head drops suddenly immediately after ejection, then maintains a constant low pressure state, and immediately after the simultaneous stopping of ejection, the pressure greatly exceeds the initial pressure before the ejection started, and then returns to the initial pressure.

When a large amount of ink is ejected and then stopped, the ink ejection stops in a state where the pressure in the common ink chamber is reduced, so a large amount of extra ink flows in from the inlet due to inertia, causing an increase in internal pressure.
In that case, ink bulges outward from each nozzle on the ejection surface of the nozzle plate.
For this reason, when ink ejection is restarted, the bulging ink may become an obstacle and prevent good ejection.

FIG. 5 shows a test pattern image formed in a conventional recording head that does not have a damper member, when ejection is resumed after stopping from a large amount of ejection by gradually increasing the number of nozzles ejecting without waiting for the internal pressure to stabilize.
As shown in the figure, immediately after the restart of ejection, the bulging ink that has accumulated at the ejection end of the nozzle causes the dot diameter to become unstable, some nozzles are unable to eject, and ink splashes occur.

The recording head 22 of the present invention is provided with a damper member 26 for mitigating pressure fluctuations in the common ink chamber 25, and by adjusting the setting conditions of each part of the recording head 22, the pressure fluctuations in the common ink chamber 25 are effectively reduced, achieving good ink ejection. The characteristic configuration of the recording head 22 will be described below.

First, the common ink chamber 25 has a volume of 2 mL to 5 mL excluding the damper member 26. More specifically, a volume of 4.5 mL will be exemplified here.
In contrast, the volume of the damper member 26 is set to 0.3 mL or more and 3 mL or less. More specifically, a case where the volume is 3 mL will be exemplified here.

Furthermore, the thickness of the elastic film member 262 of the damper member 26 is set to 30 [μm] or less. More specifically, a case where the thickness is set to 25 [μm] will be exemplified here.
The bulk modulus of the damper member 26 is set to be equal to or greater than 2 [GPa] and equal to or less than 6 [GPa]. More specifically, a case where the bulk modulus is set to be 4 [GPa] will be illustrated here.

Furthermore, in the recording head 22, when the maximum flow rate of liquid ejected from all nozzles 241 is F [mL/sec], the volume of the damper member 26 and the thickness and bulk modulus of the elastic film member 262 are adjusted so that the volume change amount of the damper member 26 relative to pressure is 3×F [uL/kPa] or more.
For example, if the print head 22 has 1024 nozzles 241, the ejection frequency is 11.7 kHz, and the amount of ink ejected from each nozzle 241 is 80 pL, then the maximum flow rate F is 1.0 mL/sec.
In contrast to this, a case will be illustrated in which the amount of volume change of the damper member 26 relative to pressure is 3.16 [uL/kPa] (≧3×F).

The inner diameter of each nozzle 241 of the recording head 22 is set to 35 μm or more, and more preferably, 40 μm or more and 50 μm or less. More specifically, a case of 35 μm will be exemplified here.
The ejection surface of the nozzle plate 24 is configured to have lyophilicity with respect to ink. Specifically, the nozzle plate 24 is formed from polyimide resin that has lyophilicity with respect to ink, and the outer surface of the ejection surface is not coated with a liquid repellent coating, leaving the surface made of polyimide resin exposed.
The standard for the liquid affinity of the ejection surface of the nozzle plate 24 is that the contact angle with respect to the ink is 90° or less.
The inner diameter of the tube 254 that supplies ink from the inlet 251 of the common ink chamber 25 and the tube 255 that discharges ink from the outlet 252 are set to 6 mm or more. More specifically, a case where the inner diameter is 6 mm will be exemplified below.

Here, we prepared Example 1, which is the recording head 22 designed with the above-mentioned specific set values, and Comparative Examples 1 to 4, which are recording heads designed with different values for the thickness of the elastic film member 262 of the damper member 26 and the amount of volume change in response to pressure of the damper member 26. Note that the design conditions other than the thickness of the elastic film member 262 of the damper member 26 and the amount of volume change in response to pressure of the damper member 26 were the same for Comparative Examples 1 to 4 and Example 1.
In all of Comparative Examples 1 to 4, the thickness of the elastic film member 262 was 50 [μm]. Furthermore, Comparative Example 1 had a volume change of 1.24 [uL/kPa], Comparative Example 2 had a volume change of 1.35 [uL/kPa], Comparative Example 3 had a volume change of 1.95 [uL/kPa], and Comparative Example 4 had a volume change of 2.24 [uL/kPa].

6 shows the results of measuring the pressure amplitude generated in the common ink chamber when simultaneous ink ejection from all nozzles and simultaneous cessation of ejection were repeatedly performed every few seconds for each of Example 1 and Comparative Examples 1 to 4, as in the case of Fig. 4. The pressure amplitude is the pressure difference between the minimum pressure generated immediately after simultaneous ink ejection from all nozzles and the maximum pressure generated immediately after simultaneous cessation of ejection.
In FIG. 6, P1 to P4 indicate the pressure amplitudes of Comparative Examples 1 to 4, respectively, and P5 indicates the pressure amplitude of Example 1.

As shown in FIG. 6, the pressure amplitudes of Comparative Examples 1 to 4 were large, at 2.0 [kPa], 1.1 [kPa], 0.4 [kPa], and 0.8 [kPa], respectively, whereas the pressure amplitude of Example 1 could be reduced to 0.03 [kPa].
L1 in Fig. 6 is a curve showing the relationship between pressure amplitude and compliance (volume change relative to pressure) by plotting the results of Comparative Examples 1 to 4 and Example 1. According to this curve L1, it can be seen that if the volume change relative to pressure is set to 3.0 (=3 x F) [uL/kPa], the pressure amplitude can be reduced to about 0.05 [kPa].

FIG. 7 shows a test pattern image formed by the recording head 22 of Example 1 when, as in the case of FIG. 5, the number of nozzles is gradually increased and ejection is resumed immediately after stopping from large-volume ejection.
As can be seen by comparing with Figure 5, in the recording head 22 of Example 1, the instability of dot diameter immediately after ejection is resumed, the occurrence of non-ejecting nozzles, and the occurrence of ink scattering are suppressed, and it can be seen that good ejection is being performed.

Technical Effects of the Invention Embodiments
As described above, the recording head 22 of the inkjet recording device 1 has a volume change amount against the pressure of the damper member 26 of 3×F [uL/kPa] or more for the maximum flow rate F [mL/sec] of liquid ejected from all nozzles. Therefore, even in cases where the pressure amplitude in the common ink chamber 25 would have been large in the past, such as when all ejection is stopped, the pressure amplitude can be dramatically reduced.
This prevents ink from swelling out of the ejection surface of the nozzle 241, and allows ink dots to be formed stably in a desired size in good condition. It also prevents ejection defects and enables stable ejection. Furthermore, it also prevents ink from scattering.

Furthermore, in the recording head 22, the volume of the common ink chamber 25 excluding the damper member 26 is set to 2 mL or more and 5 mL or less. This makes it possible to achieve the above-mentioned good ejection even when the common ink chamber 25 is made smaller in size.
Furthermore, by making the volume of the damper member 26 0.3 mL or more, it is possible to ensure a large amount of volume change in the damper member 26 in response to pressure relative to the volume of the common ink chamber 25, even when the common ink chamber 25 is made smaller, and it becomes possible to easily achieve the condition of 3×F [uL/kPa] or more.

Furthermore, the recording head 22 has a structure in which one surface of the damper member 26 is formed from an elastic film member 262 and gas is sealed inside the main body portion 261, so that it is possible to obtain a damper effect for reducing pressure amplitude with a simple structure.
Furthermore, by setting the thickness of the elastic film member to 30 μm or less, a large amount of volume change relative to pressure can be ensured, making it possible to easily achieve the condition of 3×F [uL/kPa] or more.
In particular, if the bulk modulus of the damper member 26 is set to 2 [GPa] or more, a large amount of volume change relative to pressure can be ensured, making it easier to achieve the condition of 3×F [uL/kPa] or more.

Furthermore, even when the nozzle diameter of the multiple nozzles 241 in the recording head 22 is increased, such as when the nozzle diameter is in the range of 35 μm to 50 μm, or even when the nozzle diameter is in the range of 40 μm to 50 μm, the ink can be prevented from swelling from the ejection surface of the nozzles 241, and good ink ejection is possible.
In addition, since the surface around the ejection side of the multiple nozzles 241 of the nozzle plate 24 is made liquid-philic to the ink, the ink spreads easily around the ejection side of the multiple nozzles 241 of the nozzle plate 24 due to its wettability, making it possible to suppress the ink from swelling out of the nozzles 241.

In addition, in the recording head 22, the inner diameter of the tube 254 that supplies ink to the common ink chamber 25 is set to 6 mm or more, and with this inner diameter, it is possible to suppress a sudden drop in pressure inside the common ink chamber 25 during large-volume ejection. Also, even when ink flows in due to inertia when ejection stops, the damper member 26 reduces the pressure amplitude, making it possible to eject ink well.

[others]
The above description of the embodiment is merely an example of the ejection head according to the present invention, and the present invention is not limited to this. The detailed configuration and operation of each part constituting the device can be appropriately changed without departing from the spirit of the present invention.
For example, a shear mode type inkjet head has been exemplified as the recording head 22 serving as an ejection head, but this is not limited thereto, and the configuration of the common ink chamber 25 and the damper member 26 can also be applied to other types of inkjet heads.
Furthermore, the liquid to be ejected is not limited to ink, and the configuration of the common ink chamber 25 (common liquid chamber) and the damper member 26 can be applied to an ejection head for any liquid that is required to be ejected as droplets.

In the above embodiment, the effect of suppressing the vibration amplitude when all nozzles of the recording head 22 are fully ejected and then completely stopped is described. However, this is not limited to the case where such a large pressure amplitude occurs. It is also possible to suppress the pressure fluctuations that occur in the common ink chamber 25 during normal ink ejection, and it goes without saying that good ejection performance can be achieved even in normal ejection conditions.

1 Inkjet recording device 10 Transport unit 20 Head unit 22 Recording head (ejection head)
22M head module 23 head chip 231 channel (pressure chamber)
24 Nozzle plate 241 Nozzle 25 Common ink chamber (common liquid chamber)
251 inlet 252 outlet 253 communication holes 254, 255 tube 26 damper member 261 main body portion 262 elastic film member M recording medium

Claims (8)

A common liquid chamber for containing a liquid;
a plurality of pressure chambers communicating with the common liquid chamber;
A plurality of nozzles communicating with the plurality of pressure chambers;
an actuator that changes the volume of the pressure chamber;
a damper member disposed inside the common liquid chamber and configured to absorb pressure of the liquid;
the damper member has a volume of 0.3 mL or more, and both ends thereof are supported by inner walls of the common liquid chamber so as to be perpendicular to the droplet ejection direction of the nozzle;
the common liquid chamber has a volume excluding the damper member of 2 mL to 5 mL,
An ejection head characterized in that, when the maximum flow rate of liquid ejected from all of the nozzles is F [mL/sec], the volume change amount of the damper member relative to pressure is 3 x F [uL/kPa] or more. 2. The ejection head according to claim 1 , wherein at least one surface of the damper member is made of an elastic film member, and a gas is sealed inside the damper member. 3. The ejection head according to claim 2 , wherein the elastic film member has a thickness of 30 [μm] or less. 4. The ejection head according to claim 1, wherein the damper member has a bulk modulus of elasticity of 2 GPa or more. 5. The ejection head according to claim 1, wherein the nozzle diameter of the plurality of nozzles is 35 [μm] or more and 50 [μm] or less. 6. The ejection head according to claim 5 , wherein the nozzle diameter is 40 [μm] or more. 7. The ejection head according to claim 1, wherein the peripheral surfaces of the ejection sides of the plurality of nozzles have liquid affinity with respect to the ejected liquid, with a contact angle of 90[deg.] or less. 8. The ejection head according to claim 1, wherein an inner diameter of a supply pipe that supplies liquid to the common liquid chamber is 6 mm or more.

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