WO2019077295A1

WO2019077295A1 - Reducing crosstalk in liquid ejection

Info

Publication number: WO2019077295A1
Application number: PCT/GB2018/051762
Authority: WO
Inventors: Peter Brown; Matthias Ediger; Tristan DOWNING; Richard Janse Van Rensburg; James LAZARUS
Original assignee: Ttp Plc.
Priority date: 2017-10-16
Filing date: 2018-06-22
Publication date: 2019-04-25
Also published as: GB201717000D0

Abstract

A device is provided for ejecting liquid as droplets or jets. The device comprises a chamber for holding liquid to be ejected. A plurality of fingers is disposed at a surface of the chamber. Each finger comprises at least one hole passing the finger. A plurality of actuators are provided, each being associated with a respective finger of the plurality of fingers and adapted to generate oscillatory motion of the finger, thereby causing liquid to be ejected from the chamber through the hole in the finger when in use. A plurality of rigid baffles is provided, each being arranged between neighbouring fingers of the plurality of fingers such that regions of the chamber in the vicinity of each finger are at least partially separated from each other. The chamber is at least partially bounded by a floor surface disposed opposite the plurality of fingers. Each of the rigid baffles is mechanically coupled to at least one of fingers. Each rigid baffle is directly attached to the floor surface.

Description

REDUCING CROSSTALK IN LIQUID EJECTION

The present disclosure relates to a liquid ejection apparatus. More specifically, an aspect relates to a liquid ejection apparatus for use in a printhead.

BACKGROUND

There are many known devices that can be used for ejecting liquid. A typical use of a liquid ejection device is as a printhead for depositing ink to form images.

In application WO 99/54140, a device and method are described for ejecting liquid as jets or droplets from multiple nozzles formed in a material layer. The nozzles are formed in a material layer that incorporates a nozzle with an associated actuator with liquid being supplied to an inner end of the nozzles. By stimulating the actuator to generate movement of the finger, liquid is ejected from the nozzle.

Previous devices of the type described above tend to suffer from both fluidic and mechanical crosstalk.

Fluidic crosstalk is an effect whereby an actuated finger creates motion in a neighbouring finger due to the generation of pressure within the fluid under the driven finger that subsequently propagates under the neighbouring finger. This is highly undesirable, particularly when used in a printhead as it has a negative impact on print quality. Ideally, each finger should behave completely independently of the other fingers.

Mechanical crosstalk is similar fluidic crosstalk, but is defined as the movement generated in a finger when a different finger is driven whereby the motion is generated purely by vibrations transmitted through the materials used to make the device. Though usually of a lower magnitude than fluidic crosstalk, mechanical crosstalk can limit print quality in the same way as fluidic crosstalk. Fluidic and mechanical crosstalk can significantly limit the droplet ejection rate one can achieve in devices such as those described in WO 99/54140 without observing a decrease in print quality. This is because if one actuates a finger before the effects of crosstalk caused by the previous actuation have dampened, then the fingers in the head will be in an unknown state dependent on the history of use of the fingers. As a result, subsequent actuations of the finger will have an unknown effect on the liquid to be ejected and may, for example, result in the creation of a spray event from the nozzle instead of the ejection of a single, largely spherical droplet.

Patent application WO 2008/044071 addresses the problem of fluidic crosstalk in such printheads. The application teaches that the addition of walls, hereinafter called baffles, between each finger reduces the effect of fluidic crosstalk and teaches a number of restrictions on the design of the baffles that are understood to be necessary to reduce crosstalk.

Baffles work by creating a longer pathway for the fluid to flow from under an actuated finger to neighbouring fingers, allowing the magnitude of the pressure wave to dampen naturally as it flows.

However, the baffles must also be sufficiently well anchored and stiff such that they cannot move and/or bend in response to a pressure difference across a baffle. For example, an eardrum can transmit sound waves (pressure waves) across it because it is flexible. When adding baffles to a device, one must also ensure that this mode of pressure transmission is not enabled.

While devices of the type taught in WO 2008/044071 can reduce crosstalk in comparison with previously known devices, the interaction between ejection events at different fingers remains significant enough to cause problems.

There is a need, therefore, for a device that is capable of reducing fluidic and mechanical crosstalk between separate fingers that provides improved reduction of crosstalk in comparison to the devices of WO 2008/044071 . SUMMARY OF INVENTION

There is provided in the following disclosure a liquid ejection device that overcomes the problems of known devices.

According to a first aspect, there is provided a device for ejecting liquid as droplets or jets, the device comprising: a chamber for holding liquid to be ejected; a plurality of fingers disposed at a surface of the chamber, each finger comprising at least one hole passing therethrough; a plurality of actuators, each being associated with a respective finger of the plurality of fingers and adapted to generate motion of the finger, thereby causing liquid to be ejected from the chamber through the hole in the finger when in use; a plurality of rigid baffles, each being arranged between neighbouring fingers of the plurality of fingers such that regions of the chamber in the vicinity of each finger are at least partially separated from each other; wherein the chamber is at least partially bounded by a floor surface disposed opposite the plurality of fingers; and wherein each of the rigid baffles is mechanically coupled to at least one of fingers; and wherein each rigid baffle is directly attached to the floor surface. The above arrangement provides a device in which mechanical and fluidic crosstalk in reduced by improving the stiffness of the baffles, reducing the degrees of freedom of movement of the baffles, and increasing the fluidic path in the chamber between neighbouring sub-regions of the chamber. In some examples, all of the fingers are formed from a single sheet of material.

In some examples, the device comprises a plurality of support members, each support member being disposed between neighbouring fingers of the plurality of fingers, wherein each of the rigid baffles is directly attached to a respective support member.

In some examples, all of the support members are formed from the same sheet of material as the fingers, and the mechanical coupling of each of the rigid baffles to at least one of fingers is an indirect coupling via the support members and the sheet of material.

In some examples, the sheet of material comprises one of electroformed nickel, kapton, stainless steel, or glass.

In some examples, each finger is partially separated from a neighbouring support member by a slot in the sheet of material. In some examples, the chamber is bounded on at least one side by a rigid side wall; and each of the rigid baffles and each of the fingers is directly coupled to the rigid side wall, and the mechanical coupling of each of the rigid baffles to at least one of fingers is an indirect coupling via the rigid side wall. In some examples, the floor surface comprises at least one fluidic channel to provide a path for liquid to flow into or out of the chamber.

In some examples, the floor surface comprises a first fluidic channel and a second fluidic channel, wherein the first fluidic channel is an inlet to the chamber and the second fluidic channel is an outlet from the chamber.

In some examples, at least one rigid baffle extends partially into at least one fluidic channel. In some examples, a flow restricting element is disposed in at least one fluidic channel.

In some examples, the fingers, the baffles, the side walls and the floor surface are integrally formed from a single piece of material.

In some examples, the fingers, the baffles, the side walls and the floor surface are formed of separate sections that are bonded using an adhesive.

In some examples, each actuator comprises a piezoelectric element. In some examples, the rigid baffles have a rectangular cross-section.

In some examples, the rigid baffles have a trapezoidal cross-section. In some examples, each of the actuators is adapted to generate an oscillatory motion of the finger with which it is associated.

According to a second aspect, there is provided a printhead comprising the liquid ejection device of the first aspect.

According to a third aspect, there is provided a printing system comprising one or more of the printheads of the second aspect.

BRIEF DESCRIPTION OF THE FIGURES

Aspects of the present invention will now be described by way of example with reference to the accompanying figures. In the figures:

Figure 1 is a plan view of an example of a liquid ejection apparatus.

Figure 2a is a plan view of an example of a liquid ejection apparatus in accordance with the present disclosure.

Figure 2b is a side view of the liquid ejection apparatus of Figure 2a.

Figure 3a is a plan view of another example of a liquid ejection apparatus in accordance with the present disclosure.

Figure 3b is a cross-sectional view of the liquid ejection apparatus of Figure 3a. Figure 4a is a plan view of another example of a liquid ejection apparatus in accordance with the present disclosure.

Figure 4b is a cross-sectional view of the liquid ejection apparatus of Figure 4a. Figure 5 is a side view of the liquid ejection apparatus in of Figures 4a and 4b. Figure 6 is a plan view of another liquid ejection apparatus in accordance with the present disclosure.

Figure 7 is a side view of another liquid ejection apparatus in accordance with the present disclosure.

Figure 8 is a side view of another liquid ejection apparatus in accordance with the present disclosure. Figure 9 is a graph showing the effect of the motion of a driven finger on neighbouring fingers according to a computer model.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled in the art to make and use the system, and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art.

The devices of the present disclosure comprise a plurality of fingers that can be individually actuated to cause liquid to be ejected from a chamber through holes ("nozzles") of the fingers. The liquid may be an ink and the liquid projection device may form part of a printhead of a printing apparatus. A plurality of rigid baffles is used to separate regions of a chamber that supplies liquid to be ejected in order to reduce crosstalk between the fingers.

As described in the background section, previously known devices that use rigid baffles to reduce crosstalk between fingers of a liquid ejection device impose strict limitations on the form and arrangement of the baffles. In particular, it was understood that rigid baffles must be mechanically isolated from the material layer/fingers in order to avoid introduction of mechanical crosstalk between adjacent transducers. Figures 1 shows an example of a liquid ejection apparatus 100 in which the material layer/fingers 101 are isolated from each of a plurality rigid baffles 102 by a plurality of slots 105 in the material layer, each of which entirely surrounds the rigid baffle 102. Vibration of the rigid baffles 102 is decoupled from the material layer/fingers.

Surprisingly, and in contrast to the established prejudices of the skilled person at the priority date of the present application, the applicant has demonstrated that baffle structures can be created that provide both improved dampening of mechanical and fluidic crosstalk by securely attaching the baffle structures in novel structures to the driven material layer.

Figures 2a and 2b show an example of a liquid ejection device in which the rigid baffles 102 are coupled to the fingers 101 via direct attachment to a material layer 101 in which the fingers 101 are formed.

In the device of Figures 2a and 2b, a chamber 206 is provided for holding a liquid to be ejected. The chamber 206 is bounded on a lower side by the floor surface 217 of a floor 207. The chamber is bounded on an upper side by a material layer 1 1 1 formed of a single sheet of material.

A plurality of fingers 101 are provided in a material layer 1 1 1 . Each finger 101 comprises a hole 104 (or "nozzle") passing through its thickness. Each finger 101 is coupled to an actuating element 103 configured to produce oscillations of the finger 101 in a direction transverse to the surface 121 of the material layer 1 1 1 . The hole 104 is preferably located at the motional anti-node of the transducer. The actuating element 103 is preferably formed from a piezoelectric material.

When the chamber 206 is filled with a liquid, the liquid is in contact with the material layer 1 1 1 , including the fingers 101 . When a finger 101 is actuated by applying a voltage waveform to the actuating element 103, an impulse applied to the fluid by the material layer shown at 4 induces positive pressure excursions in the liquid resulting in emergent liquid through nozzle 104 in the direction of oscillation (i.e. away from the surface 121 of the material layer 1 1 1 ).

Rigid walls 102 ("baffles") are disposed between neighbouring fingers 101 in order to prevent pressure fluctuation in the vicinity of one finger 101 from producing further pressure fluctuations in the vicinity of neighbouring fingers 101 . The baffles 102 extend through the chamber 206 from the material layer 1 1 1 to the floor 107. The rigid baffles 102 subdivide the chamber 206 into a plurality of sub-regions 206a that are at least partially separated from each other by the rigid baffles 102. The baffles 102 reduce fluidic crosstalk between neighbouring fingers 101 as they reduce the amount of pressure that is transmitted from the liquid in the sub-region 206a of the chamber 206 beneath an actuated finger 101 to liquid in a different sub-region 206a of the chamber beneath a neighbouring finger 101 . The material layer 1 1 1 comprises a plurality of slots 105 that partially separate the rigid baffles 102 from the fingers 101 . The slots 105 allow the fingers 101 sufficient freedom to oscillate without being limited by the baffles 102. Unlike the device of Figure 1 , the slots 105 do not entirely surround the baffles 102. Instead, the rigid baffles 102 are directly attached to the material layer 1 1 1 at the top surfaces 122 of their distal ends 1 12. As such, the rigid baffles 102 are coupled to the fingers 101 , which are formed in the material layer 1 1 1 . The baffles 102 are directly attached at their bottom ends to the floor 207 of the chamber 206.

The direct attachment of the baffles 102 to both the material layer 1 1 1 and the floor 207 stiffens the baffles 102 their motion, which improves their resistance to motion caused by pressure variations in the sub-regions 206a caused by ejection events. This is a significant improvement over devices in which the baffles 102 are free at either the top end or the bottom end. When the baffles 102 are directly attached to both the material layer 1 1 1 and the floor 207, there are few available paths through which liquid can flow around the baffles 102 from one sub-region 206a to a neighbouring sub-region 206a. The fluidic pathway between neighbouring sub-regions is lengthened, thus decreasing fluidic cross-talk between neighbouring fingers 101 .

In particular, by directly attaching the baffles 102 to the floor 207 of the chamber 206, liquid flow underneath the baffles 102 is avoided without the need for baffles 102 of large length (in the direction of finger oscillation). Where baffles 102 are sufficiently flexible, either because they are relatively thin or are have unanchored edges that have freedom to move, they can move and bend in response to a pressure difference between two neighbouring sub regions 206a, therefore transmitting the pressure difference from one sub region to the neighbouring sub region. This effect is reduced by, for example, forming baffles from a stiffer material; changing the cross-section shape of the baffle in a favourable manner by, for example, making them thicker; or by anchoring the edges of the baffles to other surfaces. In this case this is achieved by the closing of liquid flow paths beneath the baffles 102 when the baffles 102 are directly attached to the floor 207 of the chamber 206. Figures 3a and 3b show an example of a liquid projection apparatus in which baffles 102 are directly attached to the material layer 1 1 1 along their entire upper surface 122. Figure 3b shows a cross-sectional view along the line indicated in Figure 3a.

The material layer 1 1 1 has been constructed such that there are now two slots 305 between each finger 101 , leaving a support member 302 ("dummy finger") onto which a baffle 102 has been attached. The support members 302 and the fingers are formed from the same material layer 1 1 1 which is made of a single sheet of material. Again, the baffles 102 are no longer mechanically isolated from the fingers 101 . The applicant has found that the baffles 102 perform a much better job at reducing fluidic crosstalk and mechanical crosstalk in comparison with mechanically isolated baffles as the bonded baffles 102 cannot sway from side to side and the material layer 1 1 1 is anchored firmly to the floor of the fluid pathway.

Figures 4a and 4b show a liquid ejection device in which baffles 102 and fingers 101 are coupled via their direct attachment to end walls 401 of the chamber 206. Figure 4b shows a cross-sectional view along the line indicated in Figure 4a

The chamber 206 is bounded on its sides by two end walls 401 . Baffles 102 are directly attached at their distal end 1 12 to the end walls 401 of the chamber 206. Actuated fingers 101 are also directly attached at their ends to the same end walls 401 . The baffles 102 are directly attached to the floor 207 of the chamber 206 and extend to the height of the fingers 101 . The anchoring of the baffles 102 and the fingers 101 to the side walls stiffens the fingers 101 and the baffles, reducing mechanical and fluidic crosstalk between fingers 101 . Furthermore, the fluidic path between neighbouring sub-regions 206a is increased due the closing of the path between the end walls 401 and the baffles 102, thus further reducing fluidic crosstalk.

In the example shown, the fingers 101 are formed from distinct sections of material. In other examples, the fingers 101 and the end walls 401 may be formed from one single piece of material. In further examples, the baffles 102 and the end walls 401 may be formed from a single piece of material. In yet a further example, the fingers 101 , the end walls 401 and the baffles 102 may all be formed from a single piece of material. In a further example, the fingers 101 , the end walls 401 , the baffles 102 and the floor 207 may all be formed from a single piece of material.

Figure 5 shows a side view of the liquid ejection device shown in Figure 4. Two side channels 502 are disposed between the floor 207 and the end walls 401 . One of the side channels 502 may act as an inlet, supplying liquid to the chamber 206, while the other side channel may act as an outlet, removing liquid from the chamber 206, thus allowing for recirculation of ink. This recirculation can enable continuous filtering of the ink, or create agitation sufficient to prevent sedimentation of large particles in the ink.

The side channels 502 may extend along the length of the liquid ejection device, such that each side channel 502 is in fluid communication with all of the sub- regions 206a.

In the example shown in Figure 5, the baffles extend to the surface 217 of the floor, but do not extend into the side channels 502.

Figure 6 shows a device that combines features of the device of Figures 3a and 3b with the device of Figures 4a, 4b and 5.

A material layer 1 1 1 formed of a single sheet of material comprises support members 302 and fingers 101 . The support members 302 are separated from the fingers 101 by slots 605 on each side of each support member 302. The support members 302 are bonded to rigid baffles 102.

The material layer 1 1 1 and the rigid baffles 102 are both directly attached to side walls 401 . The fingers 101 are coupled to the baffles 102 via both the material layer 1 1 1 and the side walls 401 . Though the slots 605 are shown extending over the edge of the end walls 401 , in some examples the slots 605 do not extend over the edge of the end walls 401 . It is preferable that the slots 605 extend at least up to the end walls 401 . In some examples, the ends of the slots are not curved but take other shapes.

Figure 7 shows an example that is similar to the device of Figure 5 but in which the baffles 102 extend down the side channels 502 formed between the end walls 401 and the floor 207. The extension of the baffles 102 into the side channels 502 further increases the length of the fluid path between neighbouring sub-regions 206a, thus further reducing the effects of fluidic crosstalk. The baffles 102 can be extended as far down the channels 502 as is required to reduce the effects of fluidic crosstalk. Figure 8 shows an example that is similar to the device of Figure 7 but in which restrictions 801 are placed in the side channels 502 between the floor 207 and the end walls 401 . The restrictions 801 may be provided as valves or other obstructions to the flow of liquid. The restrictions 801 increase the resistance to flow of liquid between sub- regions 206a. In some examples, a restriction 801 is provided in only one of the two side channels 502. The restriction 801 may be provided in the inlet channel, the outlet channel or both. Figure 9 shows the results of COMSOL modelling that show the effect of baffles on mechanical and fluidic crosstalk.

The COMSOL model assumes that the head is driven with a continuous sinusoidal drive waveform and the amplitude of motion of the finger is taken as the steady-state amplitude once the transient effects have damped down. The ratio, X, of the amplitude of motion of a driven finger to the amplitude of motion generated in a neighbouring finger was calculated for a number of different device arrangements. X was calculated for devices loaded with liquid and for empty devices. X is plotted for the following arrangements:

A. A printhead similar to that shown in Figure 6, but where there are no baffles in the printhead.

B. A printhead similar to that shown in Figure 6, but where the baffles are only as wide as the central floor section.

C. A printhead similar to that shown in Figure 6 where the baffles have the same design as that shown in Figure 5 and extend the full width of the device.

D. A printhead similar to that shown in Figure 6 where the baffles have the same design as that shown in Figure 7 and extend the full width of the device and extend into the side channels.

E. A printhead similar to that shown in Figure 6 where the baffles have the same design as that shown in Figure 8 and extend the full width of the device, extend into the side channels and have a restriction in the side channels.

By plotting results for when the devices are both loaded with liquid and empty, it is possible to separate the effects of fluidic and mechanical crosstalk, as fluidic crosstalk is not present when the device is empty.

It is clear from this data that as the baffles are made longer, such that the fluidic pathway is increased, that fluidic crosstalk is reduced.

Furthermore, as baffles are introduced, and the level of mechanical contact with the fingers, material layer and end walls is increased, the mechanical crosstalk is reduced. This is in direct contrast to the expectations of the skilled person and the teaching of documents in the art.

Whilst the results shown here are correct for sinusoidal drive waveforms, the results are generally true for pulsed drive waveforms of the type used to generate individual drops on demand. An advantageous method of manufacturing a liquid projection device according to the above examples comprises providing a material layer, a plurality of baffles, two end walls and a central floor as separate components. The separate components are then bonded together with adhesive. The use of an adhesive increases the damping into the device with reduces the time a finger may mechanically ring after the drive waveform has been applied.

Alternatively, one may make any combination of the components of the head out on the same piece of material. This would increase the level of mechanical coupling between the named components that, according to the previously accepted teaching, would increase mechanical crosstalk. However, as this conclusion has been disproven, manufacturing two or more of the named components of a printhead out of the same material, such as glass, ceramic, silicon and the like, confers an advantage during the manufacturing process as there will be fewer aligning and bonding steps to complete to finish the printhead.

The liquid ejection device of any of the above examples may be used as a printhead. Where the device is a printhead, the liquid ejected by the device may be an ink containing any of ceramic pigments, decorative pigments, security pigments, taggants, metals, dyes, glass frit, polymers, monomers, paint, adhesive, hard coating, soft coating, electronic materials, or any combination thereof. Where the device is a printhead, the liquid ejected by the device may be an ink where the main carrier fluid is any of: water; alcohols such as methanol, ethanol, isopropanol, Dowanol or similar; ketones, such as acetone, methyl ethyl ketone or similar; or acetates, such as ethyl acetate, butyl acetate or similar. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. It is intended that the specification and examples be considered as exemplary only.

Claims

CLAIMS:

1 . A device for ejecting liquid as droplets or jets, the device comprising:

a chamber for holding liquid to be ejected;

a plurality of fingers disposed at a surface of the chamber, each finger comprising at least one hole passing therethrough;

a plurality of actuators, each being associated with a respective finger of the plurality of fingers and adapted to generate motion of the finger, thereby causing liquid to be ejected from the chamber through the hole in the finger when in use;

a plurality of rigid baffles, each being arranged between neighbouring fingers of the plurality of fingers such that regions of the chamber in the vicinity of each finger are at least partially separated from each other;

wherein the chamber is at least partially bounded by a floor surface disposed opposite the plurality of fingers;

and wherein each of the rigid baffles is mechanically coupled to at least one of fingers;

and wherein each rigid baffle is directly attached to the floor surface.

The device of claim 1 , wherein all of the fingers are formed from single sheet of material.

3. The device of claim 1 or claim 2, comprising

a plurality of support members, each support member being disposed between neighbouring fingers of the plurality of fingers,

wherein each of the rigid baffles is directly attached to a respective support member.

The device of claim 3, wherein all of the support members are formed from the same sheet of material as the fingers,

wherein the mechanical coupling of each of the rigid baffles to at least one of fingers is an indirect coupling via the support members and the sheet of material.

5. The device of any of claims 2 to 4, wherein the sheet of material comprises one of electroformed nickel, Kapton, stainless steel, or glass.

6. The device of any of claims 2 to 5, wherein

each finger is partially separated from a neighbouring support member by a slot in the sheet of material. 7. The device of any preceding claim, wherein

the chamber is bounded on at least one side by a rigid side wall; and

wherein each of the rigid baffles and each of the fingers is directly coupled to the rigid side wall, and

wherein the mechanical coupling of each of the rigid baffles to at least one of fingers is an indirect coupling via the rigid side wall.

8. The device of any preceding claim, wherein

the floor surface comprises at least one fluidic channel to provide a path for liquid to flow into or out of the chamber.

9. The device of claim 8, wherein the floor surface comprises a first fluidic channel and a second fluidic channel, wherein the first fluidic channel is an inlet to the chamber and the second fluidic channel is an outlet from the chamber.

10. The device of claim 8 or claim 9, wherein

at least one rigid baffle extends partially into at least one fluidic channel. 1 1 . The device of any of claims 8 to 10, comprising a flow restricting element disposed in at least one fluidic channel. The device of claim 7, wherein the fingers, the baffles, the side walls and the floor surface are integrally formed from a single piece of material.

The device of claim 7, wherein the fingers, the baffles, the side walls and the floor surface are formed of separate sections that are bonded using an adhesive.

The device of any preceding claim, wherein each actuator comprises a piezoelectric element.

The device of any preceding claim, wherein the rigid baffles have a rectangular cross-section.

The device of any of claims 1 -15, wherein the rigid baffles have a trapezoidal cross-section.

The device of any preceding claim, wherein each of the actuators is adapted to generate an oscillatory motion of the finger with which it is associated.

A printhead comprising the device of any of claims 1 -18.

19. A printing system comprising one or more of the printheads of claim