Droplet deposition method and apparatus

ES 2 365 026 T3

DESCRIPTION

Droplet deposition procedure and apparatus

[0001] This invention relates to methods and to a droplet deposition apparatus in which the droplets are expelled from a chamber through a nozzle.

[0002] In known apparatus (see for example EP-A-0 277 703 and EP-A-0 590 278) an elongated ink chamber has one or more longitudinally extending walls formed of piezoelectric material. By applying an electric field in a direction appropriate to the polarization of that piezoelectric material, the wall can be made to move in and out of the ink chamber to establish longitudinal acoustic waves in the ink. At the right moment of the actuation waveform and with adequate acoustic reflection at the ends of the chamber, one or a controlled succession of drops can be expelled through the nozzle.

[0003] The nozzle may be located at one end of the elongated ink chamber in the so-called "end pull" arrangement or towards the center of the chamber in the "side pull" arrangement.

[0004] In a printer or other droplet deposition apparatus, obviously care is taken to avoid contamination with (or to remove from the ink) dirt or bubbles, which could cause a nozzle to block. The presence of dirt or bubbles, however, cannot be completely avoided; some debris can be generated through manufacturing irregularities within the print head and some bubbles can inevitably form in the print head as a direct result of fluid pressure changes that accompany droplet ejection.

[0005] To overcome this problem, it has been suggested to provide in the side pull and end pull configurations, a continuous flow of ink past the nozzle in an attempt to sweep any dirt or bubbles out of the nozzle, resulting in otherwise they could cause a blockage in the nozzle. This continuous flow occurs while the printer is printing and while the printer is not printing, so that the continuous flow is preferably greater, and has been suggested up to ten times greater, than the maximum flow rate through the nozzle.

[0006] The continuous or persistent flow of ink through the channel can provide significant improvements for uniformity and reliability of operation. Before printing, the flow can be used to purge any debris or air from the nozzle, channel, ink collectors or ink supply system, and where appropriate the system can include thermal control. Before printing, it is often necessary for the system to achieve thermal stability. During printing, and depending on the pattern that is formed, different parts of the actuator are likely to operate in different functions, which without constant flow are known to lead to different operating temperatures, increasing the risk of minor and catastrophic image defects.

[0007] The constant recirculating side puller is known to reduce the impact of certain defects, either by reducing the time the channel and nozzle are exposed or by providing an automatic printing mechanism. Some of these are as follows:

Cause of Failure - Effect

Vibration - Displacement or tear of the nozzle meniscus

Dirt (dust) - Causes local viscosity distortion that can interrupt flow. It can inhibit the expulsion of fluid at the nozzle

Dirt (air) - Large air bubbles block fluid channel / nozzle. Small air bubbles reduce acoustic efficiency (increased compliance)

Bubbles in the channel will grow due to rectified diffusion

Intake air - Accumulated air is sucked into the nozzle

Viscosity - Changes in fluid viscosity on a macro scale, due to flocculation or slight contamination, for example

[0008] Providing a continuous flow past the nozzle is relatively straightforward in the side pull configuration. Reference is directed in this regard to EP-A-1 140 513. In this earlier proposal, the two ends of the ink chamber remain open, simplifying the provision of a relatively high flow rate continuously beyond the nozzle. This flow through the nozzle is orthogonal to the direction along which the droplets are ejected and therefore particularly effective in sweeping dirt and bubbles out of the nozzle.

[0009] Providing a continuous flow through the ink chamber is not easy in an end pull configuration. In an earlier proposal (see, for example, US 6 705 704) a barrier divides the ink chamber longitudinally. In use, a continuous flow of ink is established in a U-shaped path in the chamber: towards the nozzle on one side of the barrier, through the nozzle, and away from the nozzle on the other side of the barrier. East

ES 2 365 026 T3 system has advantages, but is not appropriate in all circumstances.

[0010] WO 89/02577 discloses a linear array of ink-supplied ink jet ports of alternately disposed pressure chambers on opposite sides of the array to allow close spacing of the ink jet ports. At the opposite end of the inkjet port, each pressure chamber communicates with a low acoustic impedance chamber to reflect negative pressure pulses from the back pressure chamber through the chamber in the form of positive pulses and to prevent pressure pulses from being transmitted to the ink supply. In addition, a high impedance passageway between the low acoustic impedance chamber associated with a pressure chamber and the adjacent ink jet pressure chamber provides a continuous flow through the passage from the ink supply port for jetting. ink to the ink supply port for an adjacent inkjet. This allows continuous thermal convection circulation of the ink when ink injections are not in use to prevent pigment settling in a pigmented ink and for the transport of ink containing dissolved air from the inkjet port region to a degassing passage.

[0011] It is an object of this invention to provide an improved droplet deposition process and apparatus in which the beneficial effects of a relatively high flow rate of liquid past the nozzle can be achieved in a "so-called" end pull configuration.

Accordingly, the present invention consists of an aspect of a droplet deposition apparatus comprising an elongated fluid chamber for containing droplet deposition liquid; a nozzle associated with one end of the chamber for the expulsion of drops; a channel of high impedance compared to the impedance of the fluid chamber communicating with the chamber at said end; drive means associated with the chamber to effect the expulsion of drops through the nozzle by generating longitudinal acoustic waves in the fluid chamber; and fluid supply means adapted to supply fluid to the chamber through the high impedance channel, adapted so that in use, a flow of liquid through the high impedance channel in the chamber is at least equal to the maximum flow rate at through the nozzle in the expulsion of the drops.

[0013] Preferably, the high impedance channel has an outlet immediately adjacent to the mouthpiece.

[0014] Suitably, the high impedance channel is directed orthogonal to the length of the fluid chamber.

Advantageously, the high impedance channel communicates between the chamber and a supply manifold in which a constant volume remains in the expulsion of the drops.

[0016] Preferably, the high impedance channel is directed orthogonally to the direction of droplet ejection through the nozzle.

In one form of the invention, the impedance of the high impedance channel is at least five, and preferably at least ten times, greater than that of the fluid chamber.

In one form of the invention, the cross-sectional area of ​​the fluid chamber is at least five, and preferably at least ten times, greater than that of the high impedance channel.

[0019] The flow of liquid through the high impedance channel in the chamber can be at least two times, at least five times or at least ten times the maximum flow rate through the nozzle during droplet expulsion. .

[0020] The liquid flow rate from the high impedance channel through the nozzle can be at least equal, at least twice, at least five times or at least ten times the maximum flow rate through nozzle during droplet expulsion.

[0021] The present invention consists, in another aspect, in a method of deposition of drops of an elongated fluid chamber containing liquid for deposition of drops and having at one end a nozzle associated with the chamber for the expulsion of drops; comprising the steps of establishing in the chamber a continuous flow of droplet deposition liquid along the chamber in a direction away from the nozzle, the flow entering the chamber adjacent to the nozzle through a channel having a cross-sectional area substantially less than that of the fluid chamber; and generating longitudinal acoustic waves in the chamber to effect droplet ejection through the nozzle, wherein the liquid flow through the high impedance channel is at least equal to the maximum flow rate through the nozzle during ejection of the drops.

[0022] Suitably, the outlet flow of the channel is directed orthogonally to the direction of expulsion of the drops through the nozzle.

[0023] Preferably, the liquid flow through the high impedance channel is at least two times, preferably at least five times, and more preferably at least ten times, the maximum flow rate through the droplet ejection nozzle.

ES 2 365 026 T3

Advantageously, the velocity of the liquid flow from the high impedance channel through the nozzle is at least equal, at least twice, at least five times or at least ten times, the maximum velocity flow through the nozzle during droplet expulsion.

[0025] Surprisingly, the droplets can be efficiently expelled by the generation of acoustic waves in the fluid chamber despite the presence of a channel in the vicinity of the nozzle providing a high speed flow past the nozzle. This is accomplished by forming the channel of high impedance compared to the impedance of the fluid chamber. By providing the high impedance channel with a cross section that is small compared to that of the fluid chamber, it can be positioned (even with a continuous flow rate, which is equal to or not much greater than the maximum flow rate through the nozzle during droplet ejection) so that a high velocity flow is established at the nozzle to sweep dirt and bubbles out.

[0026] One of the advantages of establishing this flow from the channel to the chamber (and not the other way around) is that there is no tendency for bubbles or dirt in the chamber to block the channel.

[0027] Preferably, the flow at the outlet of the high impedance channel is directed orthogonal to the direction in which the drops are expelled and orthogonal to the length of the fluid chamber. The outlet of the high impedance channel is preferably located immediately adjacent to the mouthpiece; even the cross section of the mouthpiece inlet can extend into the high impedance channel.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is an exploded view of a known inkjet print head;

Figure 2 is a longitudinal section of the inkjet print head shown in Figure 1;

Figure 3 is a longitudinal section of an inkjet print head according to one embodiment of the present invention; Y

Fig. 4 is a longitudinal section of an ink jet recording head according to another embodiment of the present invention.

[0029] Shown in Figure 1 is a conventional inkjet print head using the action of piezoelectric material to create longitudinal acoustic waves in ink channels having nozzles in the "end pull" configuration. The print head 1 is provided with a piezoelectric actuator 2 that cooperates with a cover plate 8 to form elongated ink channels 3. The elongated walls 9 of piezoelectric material are shared between adjacent channels and can enter or exit any of the channels to change the volume of that channel. Electrodes 6 are provided for establishing a driving electric field through at least part of the piezoelectric wall.

[0030] The nozzles 5 are provided on a nozzle plate 4 which is fixed to the piezoelectric actuator to close one end of each of the ink channels 3. A manifold 7 on the cover plate allows the replacement of the ink channels .

[0031] A longitudinal section through the droplet deposition apparatus as shown in Figure 1 is shown in Figure 2.

[0032] The effect of the transverse movement of one or both of the delimiting walls of each ink channel is to generate longitudinal acoustic waves which are shown by arrow 21. As described in more detail in documents EP-A-0 277 703 and EP-A-0 278 590, the drops are expelled through nozzle 5. The droplets can be expelled in binary form or in grayscale mode in which a plurality of droplets are mixed at the nozzle before being expelled to form droplets of different sizes. Ink ejected through nozzle 5 is replaced by a make-up flow from the channel shown by arrow 22, through manifold 7 into chamber 3.

[0033] A problem that has been identified with this construction is that dirt or bubbles in the ink are carried along channel 3 by the make-up flow of the channel and become trapped in the end of the channel adjacent to the plate. nozzle 4 and may cause temporary or permanent blockage of nozzle 5. It has been determined that even a relatively small bubble, if allowed to remain at the end of the channel adjacent to the nozzle plate, will cause clogging of the nozzle. This is because changes in ink pressure that accompany droplet ejection encourages bubble size growth.

[0034] An embodiment of this invention is illustrated in Figure 3, where the components remain essentially unchanged from the arrangement shown in Figure 2, maintaining the same reference numerals.

In this embodiment of the invention, an additional flow path is established which is shown by arrow 31. This flow is carried out in a channel 32 which extends in direct parallel with the length of the ink chamber 3. Channel 32 can be conveniently positioned below chamber 3, that is, out of the plane containing the

ES 2 365 026 T3 arrangement of the ink channels 3 so as not to increase the space between adjacent channels and, therefore, between adjacent nozzles. Flow 31 can be specific for an ink channel 3, with a lateral flow channel 32 for each ink channel 3; alternatively, a relatively wide channel 32 may serve all or some of the ink channels 3.

[0036] A high impedance channel 33 extends from channel 32 to channel 3, adjacent to the nozzle plate 5.

[0037] It should be noted that the position of nozzle 5 relative to the longitudinal axis of channel 3 has been adjusted so that the outlet of high impedance channel 33 is immediately adjacent to nozzle 5. In fact, the cross-sectional area of the mouthpiece inlet is seen to extend into the high impedance channel 33.

The skilled person will recognize that the representation of Figure 3 is somewhat schematic and that there exists, particularly in relation to the establishment of the lateral flow shown by arrow 31, a great variety of construction techniques by which one can establish an ink flow. It is important to recognize that channel 32 or other ink supply structure to high impedance channel 33 is passive, that is, its volume does not change during droplet ejection.

[0039] In practice, a lateral ink flow 31 is established which is at least equal to and preferably greater than the maximum flow rate of ink through the droplet ejection nozzle. Ink passes through high impedance channel 33 and enters channel 3:

- Directly adjacent to the nozzle

- In a direction transverse to the expulsion of the drops

- At a relatively high speed

[0040] For these reasons, the flow is particularly effective in sweeping out of the nozzle dirt that could block the nozzle, and even small bubbles that, if left in position, could eventually block the nozzle. These bubbles and dirt then pass through chamber 3 and exit through collector 7.

[0041] The channel replacement flow after droplet expulsion, as illustrated by the arrow

22, is dominated by flow from the collector adjacent to the active channel due to its fluid impedance lower than that of channel 33. With the pressures generated in channel 3 being of the order of 1 or 2 atmospheres, liquid replacement can reach average time speeds approaching 0.1 ms-1.

[0042] In the event that the acoustic pressure waves in the fluid within the channel propagate simultaneously with the make-up flow and by approximately 500 ms-1. The make-up flow only occurs when the liquid is expelled according to the control of the pressure waves.

[0043] The magnitude of the lateral flow is chosen such that the exposure time of a channel to dirt (and others, see above) is kept below a certain level. For certain basic graphics applications, it is accepted that occasionally nozzle defects up to 1,000 pixels in length can be tolerated. Graphic images for primary applications will tolerate defects of no more than 40 pixels. The “photographic” quality image requires less than 20 pixels. Printing of function devices (eg PCBs, displays, electronics, etc.) will impose more stringent requirements.

[0044] A second consideration to the magnitude of the flow is the velocity of the fluid at the rear of the nozzle. Bubbles introduced during device operation will migrate into the channel and without intervention can become trapped and significantly increase the risk of ejection failure. Depending on the type of fluid and its conditioning cavitation it can act to accelerate an ejection failure. To minimize the time that dirt can cause ejection failure, lateral flow is arranged to provide a fluid velocity that causes fluid in the chamber to be entrained within the 1000 pixel ejection time of a single nozzle.

The lateral flow rate will depend on the flow through channel 33 and the relative cross-sectional areas of channel 33 and chamber 3.

[0046] If the flow through channel 33 is equal to the maximum flow through the nozzle (which will be greater than the average make-up flow time by an amount that depends on the duty cycle of the chamber and the data of print) and if the cross-sectional area of ​​channel 33 is one-tenth of the cross-sectional area of ​​chamber 3, then a ten times greater flow rate can be expected past the nozzle.

Advantageously, the lateral flow opposes the dominant make-up flow so that the active chamber is protected from the influence of dirt from the ink source due to the smaller size of the channel provided by the lateral flow.

[0048] One consideration in recirculating flow design is the negative pressure applied to the fluid, which if large can cause unwanted cavitation. The described embodiment requires that the side channel provide a

ES 2 365 026 T3 significant impedance, such that a large positive pressure must be applied to the associated manifold to generate the necessary flow velocity in the actuation chamber. Conveniently, the opposing manifold (which must provide a negative pressure for the nozzle to stay below atmospheric pressure) can be positioned to provide only a modest negative pressure (near atmospheric) so that the risk of cavitation is low.

[0049] The cross-sectional area of ​​the high impedance channel 33 is substantially less than the cross-sectional area of ​​channel 3. In one arrangement, the ink channels 3 have a height of 300 µm and a width of 75 µm. The high impedance channel can extend across the width of the ink chamber 3 with a dimension of 75 pm, with a thickness (in the direction of elongation of the ink channel 3) of 30 pm, with a cross-sectional area of one-tenth the cross-sectional area of ​​ink channel 3. In variations, the high impedance channel 33 may extend less than the full width of the ink channel and may extend a greater or lesser amount in the direction or elongation of the length of the channel 3.

[0050] A modification is illustrated in figure 4. In this case, the high impedance channel takes the form of a rebate 41 cut into the nozzle plate 4. The nozzle plate can be designed to be thicker, to accommodate this snare, and to provide a nozzle of the same length as in the embodiment described above.

Although the invention has been described in relation to a recording head, it will be understood that the invention applies more broadly to a droplet deposition apparatus. It will be similarly understood that the high impedance channel communicating with the chamber at the end of the nozzle can have a variety of shapes beyond those described, and the described walls of piezoelectric material are just one example of the means of drive associated with the chamber to expel the droplets through the nozzle by generating longitudinal acoustic waves in the fluid chamber.