Element substrate and liquid discharging head

An element substrate includes a discharge port from which liquid is discharged, an energy generating element configured to generate energy for discharging the liquid from the discharge port, a heating element having a shape with a longitudinal axis and including at least two heating surfaces exposed to the liquid, a first support portion configured to support one end portion of the heating element, a second support portion configured to support the other end portion of the heating element, and a third support portion configured to support a portion between the one end portion and the other end portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an element substrate that discharges liquid by applying discharge energy to the liquid, and to a liquid discharging head including the element substrate.

2. Description of the Related Art

A liquid discharging head that discharges liquid is required to discharge small droplets having a volume of 2 pl or less. By applying such small droplets onto a printing medium at high density, a high-definition image can be obtained. With downsizing of the droplets, the number of discharges dramatically increases. When increasing the number of discharges, it has a limitation to simply increase the discharge frequency. Further, as the discharge frequency increases, the discharging speed sometimes decreases. To prevent the decrease in discharging speed and to discharge a predetermined amount of liquid in a shorter time, there has been adopted an element substrate in which multiple discharge ports are arranged at high density.

The element substrate that discharges liquid has a problem of the increase in viscosity of liquid due to the decrease in temperature of the liquid. To suppress such a problem, a method is adopted which heats liquid before supplying the liquid to action chambers that apply discharge energy to the liquid. However, in the element substrate that discharges small droplets, there is found a problem in that the discharge characteristics are deteriorated by the increase in viscosity with the increase in temperature of the liquid. That is, heated liquid evaporates via the discharge ports even when it stays in the action chambers. In the element substrate that discharges small droplets, the amount of liquid to be discharged from each discharge port is small. Hence, even when a small amount of solvent evaporates, the viscosity of the liquid easily increases. Further, since the discharge ports and the action chambers are relatively small, the liquid is likely to be affected by the increase in flow resistance due to the increase in viscosity. Such a problem is pronounced particularly in pigment ink that is likely to aggregate and high-function ink having a high content of additive resin.

The increase in flow resistance deteriorates the discharge characteristics of the element substrate. When the discharge characteristics are deteriorated, the element substrate is sometimes brought into a state in which it cannot discharge the liquid unless a recovery process is performed for liquid supply channels that reach the discharge ports and the action chambers.

U.S. Patent Application Publication No. 2009/160896 A1 discloses an element substrate that is controlled so that liquid in action chambers is not heated more than necessary.

In the element substrate described in the above publication, heating resistance elements serving as energy generating elements preheat the liquid in the action chambers to a predetermined temperature, and then discharge the liquid by boiling the liquid. However, it is difficult for the heat amount of preheating to rapidly heat the liquid. For this reason, in a low-temperature environment, a long standby time is required to heat the liquid to the predetermined temperature by preheating. This decreases the throughput of the element substrate.

In this way, the element substrate disclosed in the above publication cannot evenly cope with the increase in viscosity of the liquid within the usual use range. Further, to cope with the increase in viscosity of the liquid, it has been proposed to add heating elements different from the heat generating elements in the action chambers and to heat the liquid in the action chambers by the heating elements only by the necessary amount when needed. In such a case in which the heating elements are mounted in the substrate, most heat generated from the heating elements is transferred to the substrate, and this lowers the efficiency. Further, since the heat transferred to the substrate is stored in a liquid discharging head, it may continue heating the liquid even after the heating elements stop heating. For this reason, the heating state continues for a long time, and more liquid than necessary sometimes evaporates.

SUMMARY OF THE INVENTION

The present invention provides an element substrate and a liquid discharging head that efficiently heat liquid and achieve good discharge characteristics.

An element substrate according to an aspect of the present invention includes a discharge port from which liquid is discharged, an energy generating element configured to generate energy for discharging the liquid from the discharge port, a heating element having a shape with a longitudinal axis, including at least two heating surfaces exposed to the liquid, and configured to heat the liquid to an extent such as not to discharge the ink, a first support portion configured to support one end portion of the heating element, a second support portion configured to support the other end portion of the heating element, and a third support portion configured to support a portion between the first end portion and the other end portion.

DESCRIPTION OF THE EMBODIMENT

An embodiment of the present invention will be described below with reference to the drawings.FIG. 1is a perspective view of a liquid discharging head including an element substrate according to the embodiment of the present invention. As illustrated inFIG. 1, a liquid discharging head1includes an element substrate2that discharges liquid such as ink, a support member3that supports the element substrate2, and an electric wiring member4electrically connected to the element substrate2. The liquid discharging head1illustrated inFIG. 1can be mounted in a so-called full-line printing apparatus.

FIG. 2is a partly cutaway perspective view of the element substrate2illustrated inFIG. 1. To discharge inks of four colors of cyan, magenta, yellow, and black, the element substrate2includes discharge ports5. The discharge ports5corresponding to each of the four colors are arranged in two rows.

Ink is supplied from ink reservoirs (not illustrated) to the discharge ports5via common liquid chambers6common to discharge port rows each two of which correspond to each color. The discharge ports5adjacent in a row direction are arranged at an array density of 800 dpi (dots per inch). Further, the discharge ports5in different rows for the same color are shifted from each other by a half pitch. Therefore, the element substrate2can apply ink onto a printing medium at a printing density of 1600 dpi.

FIG. 3is a partly cutaway enlarged perspective view of a section near a discharge port5in the element substrate2illustrated inFIG. 2. As illustrated inFIG. 3, the element substrate2includes heating resistance elements7serving as energy generating elements that generate energy for discharging ink from the discharge ports5, and action chambers8that apply the energy from the heating resistance elements7to the ink.

The action chambers8communicate with the common liquid chambers6(seeFIG. 2), and the ink flows from the common liquid chambers6into the action chambers8. The ink supplied to each action chamber8causes film boiling by thermal energy received from the heating resistance element7disposed within the action chamber8, so that a bubble is produced in the ink. This bubble pushes and discharges the ink from the corresponding discharge port5.

The heating resistance element7preferably has a shape with a longitudinal axis (for example, a platelike shape, a columnar shape, and a prismatic shape). In the embodiment, the heating resistance element7is shaped like a plate like a strip. Both ends of the heating resistance element7are fixed to a wall of the action chamber8, and both surfaces of the heating resistance element7along the longitudinal axis are exposed to ink to be able to heat the ink. Therefore, heat can be applied to the ink from both surfaces of the heating resistance element7. This allows the ink to cause film boiling in a shorter time.

The element substrate2further includes partitions9provided on both sides of each heating resistance element7. The partitions9are formed of the same material as that of the heating resistance element7and by the same process as that for forming the heating resistance element7. Hence, as illustrated inFIG. 4, the distance between the heating resistance element7and a substrate19and the distance between a heating element15and the substrate19are substantially equal to each other. The heating resistance element7and the partitions9divide the action chamber8into an upper space provided on a side of the discharge port5and a lower space provided on a side of a supply port13(upstream side) (by a virtual plane intersecting the discharging direction). One of the partitions9extends to a bottom wall of the action chamber8(a wall opposed to a wall having the discharge port5), and divides the lower space of the action chamber8into a plurality of spaces.

The upper and lower spaces formed in the action chamber8communicate through gaps between the heating resistance element7and the partitions9, an aperture10formed in one of the partitions9, and a through hole11serving as an opening formed in the other partition9. The plural spaces formed in the lower space of the action chamber8communicate through an aperture12provided in the one partition9.

FIG. 4is a cross-sectional view of the element substrate2, taken along line IV-IV ofFIG. 3. As illustrated inFIG. 4, the substrate19provided at a position opposed to the discharge port5has a supply port13communicating with the action chamber8. More specifically, the supply port13penetrating the substrate19communicates with a space defined by the one partition9. The space is closed except for the apertures10and12. The one partition9functions as a rectifying element14that rectifies the flow of ink. In the embodiment, the element substrate2includes a plurality of action chambers8. The supply port13is provided for each of the action chambers8.

The other partition9includes a narrow portion, and the narrow portion is electrically connected to an electrode (not illustrated). The narrow portion generates heat by the application of voltage thereto via the electrode. That is, the narrow portion functions as a heating element (also referred to as a sub-heater)15. The sub-heater15can be driven independently of the heating resistance element7. Further, the sub-heater15is specified so that a bubble generating phenomenon does not occur even when the sub-heater15is driven alone. That is, the sub-heater15can heat the ink to an extent such as not to discharge the ink.

The sub-heater15of the embodiment is shaped like a plate having a longitudinal axis, and includes at least two heat generating surfaces. That is, both principal surfaces of the sub-heater15serve as heat generating surfaces. The heat generating surfaces are disposed at a predetermined distance from the substrate19, and are both exposed to ink in the action chamber8. In the embodiment, both ends of the sub-heater15are fixed to the wall of the action chamber8, and both surfaces of the sub-heater15in the longitudinal direction are exposed to ink to be able to heat the ink. Therefore, heat can be applied from the surfaces of the sub-heater15to the ink, and this suppresses heat dissipation to the substrate19. Hence, the ink can be heated to a predetermined temperature in a shorter time. The shape of the sub-heater15of the present invention is not limited to such a platelike shape, and may be, for example, a columnar shape or a prismatic shape.

The ink is supplied from the support member3(seeFIG. 1) for supporting the element substrate2to the common liquid chamber6(seeFIG. 2). The common liquid chamber6communicates with the supply port13, and the ink flows into the action chamber8from the common liquid chamber6through the supply port13. The element substrate2has a circuit (not illustrated) electrically connected to a liquid discharging apparatus body (not illustrated) in which the liquid discharging head1is mounted.

According to the embodiment, the ink in the action chamber8is heated by the sub-heater15when needed. For this reason, a state in which the temperature of the ink is high can be limited to the needed time in contrast to the related art in which the substrate is entirely heated. Hence, evaporation of the ink from the discharge port and evaporation of solvent in the ink can be suppressed more than before. Therefore, the increase in viscosity of the liquid can be suppressed. Further, since the sub-heater15is provided separately from the heating resistance element7, the ink in the action chamber8can be heated to the predetermined temperature at an arbitrary timing.

Next, with reference toFIG. 5, a description will be given of the flow of ink that reaches the discharge port5from the supply port13.FIG. 5is a cross-sectional view illustrating the flow of ink from the supply port13to the discharge port5.

When the ink is heated by the heating resistance element7, it boils on the surfaces of the heating resistance element7. Energy of ink boiling applies kinetic energy to the surrounding ink, and a bubble grows. Since the rectifying element14defines a closed space except for the apertures10and12, it functions as a channel resistance that efficiently directs discharge energy to the ink near the discharge port5.

After the ink in the action chamber8is discharged from the discharge port5, the pressure in the grown bubble becomes a negative pressure. When the inertial force of ink transmitted during boil forming falls below the negative pressure, the bubble rapidly disappears. As a result, a force acts to take the ink into a region where the bubble was present. By virtue of this force, the ink flows into the action chamber8from the supply port13.

At this time, the ink introduced from the supply port13first reaches the rectifying element14, and flows into the upper and lower spaces of the action chamber8from the apertures10and12of the rectifying element14, as shown by arrows inFIG. 5. A space on a back side of the heating resistance element7(a space opposite from the discharge port5) is filled with ink by this flow.

The liquid passing through the back side of the heating resistance element7further flows to a position near the sub-heater15, and flows into the space on the side of the discharge port5through the through hole11. In this specification, a channel through which the ink reaches the region on the side of the discharge port5through the through hole11is referred to a “bypass channel”. In other words, the through hole11forms a part of the bypass channel, and the sub-heater15serves as a channel wall of the bypass channel. The ink flows through the bypass channel and the aperture10, and is then supplied to a front side of the heating resistance element7(a side of the discharge port5).

Next, the structure of the sub-heater15will be described in detail.

The liquid discharging head of the related art mainly includes the sub-heater for the purpose of suppressing the increase in viscosity of ink in a low-temperature environment. In this case, the sub-heater is often provided on a member having high thermal conductivity (for example, a liquid discharging head board or a support member also having a heat dissipating function) in terms of the design such as to heat the entire liquid discharging head and advantageous arrangement layout. In such a structure, heat is dissipated from the liquid discharging head. Therefore, the sub-heater requires not only the heat quantity that heats the entire liquid discharging head, but also the heat quantity that covers the heat to be dissipated. As a result, a considerably large-sized sub-heater is needed. In such a large-sized sub-heater (provided on the member having high thermal conductivity), since it is difficult to finely control the temperature, a heating resistance element preheats ink to adjust the temperature of the ink in environments other than a normal temperature environment. This is because the discharge amount subtly changes with the decrease in viscosity in the high-temperature environment and the optical density (OD) of a printed image increases. This phenomenon is likely to be conspicuous in a high-duty image.

The phenomenon in which the optical density of the printed image is increased by the increase in temperature is more conspicuous in the above-described environment in which small droplets are discharged. This is because the ratio of the increase in discharge amount with respect to discharge droplets increases.

Further, as described above, the high-temperature environment in discharging of small droplets also causes thickening due to evaporation. That is, the high-temperature environment in discharging of small droplets causes the decrease in viscosity resulting from the increase in kinetic energy of the liquid in the short term, and then causes the increase in viscosity resulting from evaporation of moisture in the ink in the long term. In this way, in the element substrate for discharging small droplets, it is seriously difficult to control the viscosity.

Accordingly, in the embodiment, the sub-heater that performs heating with front and back surfaces is provided within the action chamber8. To efficiently heat only a required amount of ink when needed, it is preferable that the ink between the discharge port and the heating resistance element should be heated and the ink upstream of the heating resistance element should be hardly heated. In this case, heating in a short time is possible, the ink does not evaporate more than necessary, and the backward resistance is ensured on the upstream side of the heating resistance element. This enhances the discharge efficiency. When the heating element15serving as the sub-heater is disposed on the downstream side of the heating resistance element7in the flow direction of liquid in the action chamber8, heating of the ink upstream of the heating resistance element7is minimized. From a different viewpoint, as illustrated inFIG. 4, the substrate19has the supply port13. The distance between the supply port13and the sub-heater15is larger than the distance between the supply port13and the heating resistance element7. Because of this relationship, the liquid on the further downstream side (side closer to the discharge port5) in the flowing direction of liquid can be heated to decrease the viscosity, and discharging can be performed properly. In more detail, the distance between the supply port13and the sub-heater15is smaller than the distance between the supply port13and the discharge port5and larger than the distance between the supply port13and the heating resistance element7.

When the heating resistance element and the sub-heater are provided in the member having high thermal conductivity, heat generated from the heating resistance element and the sub-heater is transferred not only to the ink, but also to the member having high thermal conductivity. Therefore, the member having high thermal conductivity is sometimes unsuitable for the element substrate that discharges small droplets, for example, because a sub-heater having a certain size is needed and a large area is required between the heating resistance element and the discharge port.

In addition, heat transferred to the member having high-thermal conductivity is stored in other members within the liquid discharging head. For this reason, even after the sub-heater stops heating the ink, the heat stored in the other members is likely to heat the ink. As a result, the heating state continues for a long time, and more ink than necessary easily evaporates.

In the embodiment, the sub-heater15is shaped like a long plate, and is provided within the action chamber8with its front and back surfaces being exposed to the ink. More specifically, shorter-side portions of the long plate are fixed to the wall of the action chamber8. Therefore, heat generated by the sub-heater15is unlikely to be transferred to the wall of the action chamber8. Further, since the front and back surfaces of the sub-heater15are exposed to the ink, the ink can be heated efficiently.

Moreover, according to this structure, the amount of ink to be heated is far smaller than in the liquid discharging head of the related art. Therefore, it is unnecessary to change the dimension between the heating resistance element7and the discharge port5to add the sub-heater15. Further, most heat generated by the sub-heater15is transferred only to the ink to be discharged. Therefore, the ink is unlikely to evaporate, and the increase in viscosity can be minimized. In addition, since the ink within the action chamber is heated, the temperature can be adjusted speedily. Therefore, the standby time during a warm-up can be made shorter than in the element substrate of the related art in which only preheating is performed by the heating resistance element. Since the element substrate2of the embodiment can expand the range of temperature adjustment of the ink and can speedily adjust the temperature, it is easier to correct discharging variation during printing.

To more effectively enjoy the effect of temperature adjustment with the sub-heater15, the heating resistance element7preferably adopts a structure for performing heating with front and back surfaces, similarly to the sub-heater15. When a protective layer is provided on the surface of the heating resistance element7, extra energy is needed, and accumulated heat in the protective layer sometimes affects the temperature adjustment. From the viewpoint of temperature management in the action chamber8, the heating resistance element7and the sub-heater15are preferably formed of a material having sufficient durability without using the protective layer. Such a material of the heating resistance element7and the sub-heater15is, for example, an amorphous-based high-resistance material mainly composed of a high-melting-point metal such as TiAlN. Alternatively, the material may be a laminate of TiAlN and TiAl.

From the viewpoint of enhancement of discharging efficiency, the sub-heater15is preferably disposed at a position such as to speedily heat the ink near the heating resistance element7and such as to be out of the area between the discharge port5and the heating resistance element7that may affect discharging. Such a position is, for example, a position on the partition9, which forms the bypass channel, on the side of the heating resistance element7.

Further, from the viewpoint of heating efficiency of the sub-heater15, it is preferable that the electric resistance of the sub-heater15should be higher. Although the electric resistance can be increased by selecting a material having high specific resistance, when the same material is used, the electric resistance can also be increased by decreasing the cross-sectional area of the sub-heater15, that is, by decreasing the thickness of the sub-heater15.

When the thickness of the sub-heater15is decreased, the mechanical strength of the sub-heater15decreases. Since the sub-heater15repeats heating, it suffers structural fatigue due to thermal stress. Further, the sub-heater15itself vibrates owing to, for example, a Karman vortex caused by movement of the ink through the through hole11of the sub-heater15and its surroundings, and accumulation of vibration may cause structural fatigue. If the sub-heater15is used in the state of structural fatigue, it is broken, and this causes trouble with the element substrate2and the liquid discharging head1.

In the embodiment, as illustrated inFIG. 6, the element substrate2further includes first and second support portions16and17, and a third support portion18provided separately of the first and second support portions16and17. The first and second support portions16and17are provided in one end portion and the other end portion of the sub-heater15, respectively, and are formed by bending parts of a member that forms the sub-heater15to support both end portions of the sub-heater15. Voltage can be applied between the one end portion and the other end portion of the sub-heater15. Similarly, the third support portion18is also formed by bending a part of the sub-heater15to support a portion of the sub-heater15between both end portions. In this way, parts of the sub-heater15are bent to form bent parts serving as support portions on the substrate19.

According to this structure, even when both surfaces of the heat generating part of the sub-heater15are separate from the substrate19and are exposed to the ink, rigidity of the entire sub-heater15can be increased, breakage resulting from thermal stress and structural fatigue due to vibration can be prevented, and the operating life can be lengthened. Further, the sub-heater15can be prevented from being broken by vibration or unexpected fall during transportation of the liquid discharging head1or a liquid discharging apparatus (not illustrated) incorporating the liquid discharging head1.

The third support portion18is preferably connected to the wall of the action chamber8with an insulator being disposed therebetween. By electrically insulating the third support portion18from the wall of the action chamber8, thermal diffusion to the wall of the action chamber8(substrate main body and discharge-port forming member) due to free electrons can be prevented, and the decrease in efficiency of the sub-heater15can be avoided. Further, when the third support portion18is electrically insulated from the wall of the action chamber8, it becomes an electrically suspended component, and does not greatly change the total resistance of the sub-heater15. Therefore, the sub-heater15can ensure an amount of heat generation similar to that when the third support portion18is not provided. As illustrated inFIG. 6, the sub-heater15and the heating resistance element7extend along each other and along the surface of the substrate19.

In the embodiment, the supply port13is provided in each action chamber8, and the channel length of the supply port13is longer than the channel length from an exit end of the supply port13to the discharge port5. This structure can make the backward resistance of the ink high and can make the forward resistance low. Thus, the discharge efficiency can be further enhanced in combination with the liquid resistance at the supply port13having a length more than or equal to the predetermined length.

While the heating resistance element7is used as the energy generating element in the embodiment, a piezoelectric element having a vibrating plate may be used as the energy generating element. When the energy generating element is the piezoelectric element, the sub-heater15is preferably disposed at a position such as to be opposed to the piezoelectric element within the action chamber8and such as to be shifted to a fixed portion of the vibrating plate on a side of the discharge port5.

While the present invention is applicable to any kind of ink, it is advantageous particularly for pigment ink that is likely to aggregate and high-function ink having a high content of additive resin. The present invention is, of course, not limited to the element substrate that discharges ink, and is also applicable to an element substrate that discharges liquid.

While the heating element15serving as the sub-heater is provided within the action chamber8in the above-described embodiment, the present invention is not limited thereto. The heating element may be provided at any position as long as it is exposed to liquid in the liquid discharging head. For example, the heating element may be provided within the supply port13, the channel communicating between the supply port13and the action chamber8, or the common liquid chamber.

This application claims the benefit of Japanese Patent Application No. 2014-126412, filed Jun. 19, 2014, which is hereby incorporated by reference herein in its entirety.