Droplet deposition method and apparatus

[0001] The present invention relates to a method and apparatus for depositing droplets, in which droplets are ejected from the chamber via a nozzle.

[0002] In the known device (see for example EP-A-0 277 703 and EP-A-0 278 590) the elongated ink chamber has one or more longitudinally extending walls formed of piezoelectric material. By applying an electric field in a direction suitable for polarizing this piezoelectric material, this wall can be moved into and out of the ink chamber to cause longitudinal acoustic vibrations in the ink. With proper time synchronization of the resulting wave and with appropriate acoustic reflection at the ends of the chamber, one or controlled series of drops can be ejected through the nozzle.

[0003] The nozzle may be located at one end of the longitudinal ink chamber in the so-called "end-shooter" arrangement or along the center of the chamber in a "side shooter" arrangement.

[0004] For printers or other droplet deposition devices, of course, efforts are made to avoid dust and bubble contamination (or to remove them from the ink) as they can cause clogging of the nozzle. However, dust or bubbles cannot be completely avoided; some dust grains may be due to surface imperfections on the print head, while some bubbles may inevitably be formed inside the print head, which is a direct result of changes in fluid pressure that accompanies droplet ejection.

[0005] To solve this problem, it has been suggested to provide both a "side shooter" and "end shooter" continuous ink jet through the nozzle to flush any dust grains or bubbles from the nozzle that could otherwise cause nozzle blockage. This continuous stream occurs when the printer prints as well as when the printer does not print, whereby the continuous stream is preferably larger, with a stream up to ten times greater than the maximum stream through the nozzle being suggested.

[0006] A continuous or persistent stream through a channel can provide significant improvements in the uniformity and reliability of operations. Before printing, the stream can be used to flush any grain of dust or air from the nozzle, channel, ink manifolds or ink supply system, and wherever needed, the system may include temperature control. There is often a need for thermal stability before printing. During printing, and depending on the pattern to be created, different parts can be started that operate at different loads, and this without a continuous stream, as is known, leads to different operating temperatures, which increases the risk of both minor and serious image defects .

[0007] The side shooter ejection system with constant recirculation, as is known, reduces the occurrence of some defects by reducing the time in which the channel and nozzle are exposed or by being equipped with a self-priming mechanism. Some of them are listed below:

-2 Cause of Vibration malfunction

Dust grains (dirt)

Grains (air)

Air trapped. Viscosity

Effect

Nozzle meniscus dislocation or fracture

It causes local viscosity disorders, it can block the discharge of fluid from the nozzle

Large air bubbles deprive the nozzle / fluid channel, small air bubbles reduce acoustic performance (increased compatibility)

Bubbles in the channel may grow as a result of the sound field (fractional diffusion)

Accumulated air captured by the nozzle Changes in the viscosity of the fluid on a macro scale due to, for example, flocculation or contamination.

[0008] Providing a continuous jet through the nozzle is relatively simple in the side shooter ejection system. In this case reference is made to document EP-A-1 140 513. In this prior art concept, both ends of the ink chamber remain open, simplifying the application of a relatively high continuous jet speed through the nozzle. This jet through the nozzle is orthogonal to the direction along which the droplets are ejected, and is therefore particularly effective in "flushing out" dust grains and bubbles from the nozzle.

[0009] Providing a continuous stream through the ink chamber is not easy with an "end shooter" system. In the concept known from the prior art (see for example US 6,705,704) the barrier divides the ink chamber longitudinally. During use, a continuous stream of ink is maintained in the U-shaped path in the chamber: toward the nozzle on one side of the barrier, across the nozzle and away from the nozzle on the opposite side of the barrier. This arrangement has some benefits, but it is not appropriate in all cases.

[0010] WO 89/02577 discloses a linear arrangement of ink jet nozzles, supplied with ink from pressure chambers alternately arranged on opposite sides of this system, to allow close spacing of these ink jet nozzles. At the opposite end of the ink jet nozzle, each pressure chamber communicates with a low acoustic impedance chamber to reflect a pulsating vacuum from the pressure chamber back through the chamber as a positive pulsation as well as to avoid transmitting pressure pulsing to the ink supply system. In addition, the high impedance transition between the low sound impedance chamber connected to one pressure chamber and the pressure chamber for an adjacent ink stream provides a continuous stream corridor from the port of the ink supply system for one ink stream to the port of the ink supply system for the adjacent ink stream. This enables continuous circulation by ink convection when the ink jets are not used, to prevent pigmentation in the pigment ink and to transfer ink containing dispersed air from the area of ​​the ink jet nozzle to the venting passage.

[0011] The object of the present invention is to provide an improved drop deposition method and apparatus for this purpose in which the beneficial effects of a relatively high fluid stream velocity beyond the nozzle can be obtained in an end shooter configuration. [0012] Accordingly, the present invention relates in one aspect to a drop deposition device, consisting of an elongated fluid chamber for storing a fluid for drop deposition, a nozzle attached at one end to the chamber for ejecting droplets; a high impedance channel compared to the impedance of the fluid chamber connected to the chamber at said end; actuators associated with the chamber in such a way as to cause droplets to be ejected through the nozzle by generating long acoustic waves in the fluid chamber; and a fluid supply assembly adapted to supply fluid to the chamber through a high impedance channel adapted so that during operation the fluid flow through the high impedance channel to the chamber is at least equal to the maximum flow through the nozzle in the direction of drop deposition.

[0013] Preferably, the high impedance channel has an outlet directly adjacent the nozzle.

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

[0015] Preferably, the high impedance channel connects the chamber and the branched supply line that maintains a constant volume of droplet ejected.

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

[0017] In one embodiment of the invention, the impedance of the high impedance chamber is at least five, and preferably at least ten times greater than the fluid chamber impedance.

[0018] In one embodiment of the invention, the cross-sectional area of ​​the fluid chamber is at least five, and preferably at least ten times greater than for a high impedance channel.

[0019] The flow of fluid through the high impedance channel into the chamber may be at least twice, at least five times, or at least ten times greater than the maximum flow through the nozzle in the direction of ejecting droplets.

[0020] Fluid flow velocity through the high impedance channel, through the nozzle, may be at least equal to, at least twice as high, at least five times greater, or at least ten times greater than the maximum flow velocity through the nozzle in the direction of ejecting droplets.

[0021] The present invention further encompasses a method of depositing droplets from an elongate fluid chamber containing a fluid used for depositing droplets and having at one end a nozzle associated with a droplet dispensing chamber; the method comprising the steps of establishing a continuous fluid flow in the chamber for depositing droplets along the chamber, away from the nozzle, which stream reaches the chamber adjacent the nozzle through a channel having a cross-sectional area substantially smaller than that of the fluid chamber; and also generating longitudinal acoustic waves in the chamber to cause the droplet to be ejected through the nozzle, wherein the fluid flow through the high impedance channel is at least equal to the maximum flow through the nozzle in the direction of droplet ejection.

[0022] Conveniently, the stream leaving the channel is orthogonally directed relative to the direction of the drop being ejected through the nozzle.

[0023] Preferably, the fluid stream through the high impedance channel is at least two, preferably at least five times greater, and more preferably at least ten times greater than the maximum stream upstream of the nozzle in the direction of ejecting droplets.

[0024] Preferably, the flow velocity of the fluid through the high impedance channel through the nozzle is at least equal to, at least twice as high, at least five times greater or at least ten times greater than the maximum flow velocity through the nozzle in the direction of droplet ejection.

[0025] Surprisingly, the droplets can be ejected effectively by producing acoustic waves in the fluid chamber, despite the presence of the channel in the vicinity of the nozzle, providing a stream through the high speed nozzle. This is achieved by shaping the channel with high impedance compared to the impedance of the fluid chamber. By using a high impedance channel having a cross section that is small compared to the cross section of the fluid chamber, it becomes possible to place it (even at a continuous flow velocity that is equal to or slightly higher than the maximum flow velocity through the nozzle in the direction of ejecting droplets) in such a way that a high speed jet is established in the nozzle to flush out any grains of dust or bubbles.

[0026] The benefits of establishing such a flow through the channel into the chamber (instead of vice versa) are such that there is no tendency to block bubbles or dust grains in the chamber.

[0027] Preferably the stream at the outlet of the high impedance channel is directed orthogonally relative to the direction in which the droplets are ejected and orthogonal to the length of the fluid chamber. The outlet from the high impedance channel is preferably located in the immediate vicinity of the nozzle; in fact, the cross section of the nozzle inlet may extend into the high impedance channel.

[0028] This 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 printhead in ink jet technology,

Figure 2 is a longitudinal section of the printhead in the inkjet technology shown in Figure 1;

Figure 3 is a longitudinal cross-sectional view of the printhead in ink jet technology, according to one embodiment of the present invention; and Figure 4 is a longitudinal cross-sectional view of the printhead in ink jet technology according to another embodiment of the present invention.

[0029] Figure 1 shows a conventional print head in ink jet technology in which the head uses piezoelectric material to generate a longitudinal acoustic wave in the ink channels having a nozzle in an end shooter configuration. Print head 1 is equipped with in a piezoelectric actuator 2 that cooperates with the cover plate 8 to form longitudinal ink passages 3. The longitudinal walls 9 of the piezoelectric material are common to adjacent channels and can move in or out of one of these channels to change the volume of the channel. Electrodes 6

-5 are used to establish an excitatory electric field through at least part of the piezoelectric wall.

[0030] The nozzles 5 are placed in a nozzle plate 4 which is attached to the piezoelectric actuator in such a way that it closes one end of each of the ink channels 3. The manifold 7 in the cover plate makes it possible to refill the ink channels. [0031] Figure 2 shows a longitudinal section through the drop deposition device as illustrated in Figure 1.

[0032] The effect of transverse movements of each or both walls surrounding each ink channel is the production of longitudinal acoustic waves 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 ejected through the nozzle 5. Drops can be ejected in binary or grayscale mode in which a number of drops combine at the nozzle before being ejected to form droplets of varying sizes. The ink ejected through the nozzle 5 is supplemented by the channel make-up stream, represented by arrow 22, through the manifold 7 to the chamber 3.

[0033] The problem that has been observed when using this design is that the dust grains or air bubbles in the ink that are passed through the channel 3 through the channel refilling stream will be stopped at the end of the channel adjacent to the nozzle plate 4 and may cause temporary or permanent stopping of the nozzle 5. It has been found that even a relatively small bubble, if allowed to remain at the end of the channel adjacent to the nozzle plate, will block the nozzle. This is due to changes in ink pressure that accompany the ejection of droplets, which promotes the increase in bubble size.

[0034] One embodiment of this invention is shown in Figure 3, where the components that remain substantially unchanged from the system shown in Figure 2 have the same reference numerals.

[0035] In this embodiment of the present invention, an additional stream path has been established, represented by arrow 31. This stream is carried through a channel 32 which runs directly parallel to the length of the ink channel 3. The channel 32 may conveniently be located below the chamber 3, in other words, outside the plane which contains the ink channel system 3 in such a way that it does not increase the distance between adjacent channels and thus between adjacent nozzles. A stream 31 may be specific to only one ink channel 3, such a side channel 32 of the ink for each ink channel 3; alternatively, one relatively wide channel 32 may support all or some of the ink channels 3.

[0036] The high impedance channel 33 extends from channel 32 to channel 3, adjacent the nozzle plate 5. [0037] It should be noted that the position of the nozzle 5 relative to the longitudinal path of the channel 3 has been adjusted in such a way that the outlet of the high impedance channel 33 is in the immediate vicinity of the nozzle 5. Therefore, the cross-sectional area of ​​the inlet to the nozzle is seen to extend to high impedance interiors 33.

[0038] Those skilled in the art will be aware that the illustration of Figure 3 is somewhat schematic and also that there is, especially regarding the establishment of the lateral flux indicated by arrow 31, a wide range of design techniques by which such flux is

-6 possible to obtain. It is important to be aware that the channel 32 or other structure supplying the ink to the high impedance channel 33 is passive, and that means that its volume does not change when dropping.

[0039] During operation, a side ink jet 31 is established and it is at least equal and preferably greater than the maximum ink jet through the nozzles in the direction of ejecting droplets. Ink flows through channel 33 with high impedance and enters channel 3:

- in the immediate vicinity of the nozzle;

- in a direction perpendicular to the discharge direction of the ejection;

- at a relatively high speed.

[0040] For these reasons, the jet is particularly effective when flushing out dust particles from the nozzle that can block the nozzle, as well as even small bubbles that, if in one position, can grow and block the nozzle. These bubbles and dust grains then pass along the length of channel 3 and escape through the conduit 7.

[0041] The stream supplementing the channel after ejecting the drop was illustrated by arrow 22 and is dominated by the stream from the conduit adjacent to the active channel due to its lower fluid impedance than that present in channel 33. At pressure values ​​generated in channel 3 of the order of 1 or 2 atmospheres, make-up fluid can reach averaged speeds close to 0.1 ms-1.

[0042] In the case of acoustic operation, pressure waves in the fluid inside the channel are generated simultaneously with the make-up stream and at a speed of about 500 ms-1. The make-up stream only occurs when fluid is ejected according to the control of pressure waves.

[0043] The size of the side stream is selected in such a way that the time for which the channel is exposed to dust grains (or other, see above) is kept below a certain level. For the needs of basic graphic applications, it is allowed that individual nozzle defects up to 1000 pixels along may be tolerated. Graphic images for basic applications will allow defects no larger than 40 pixels. "Photo" quality images require less than 20 pixels. Printing by devices with special applications (e.g. printed circuit boards, displays, electronics, etc.) can have even more stringent requirements.

[0044] Another issue to consider regarding the size of the fluid stream is the fluid velocity at the rear end of the nozzle. Bubbles entrained during the operation of the device will migrate towards the channel and in the absence of reaction they can be blocked and significantly increase the risk of ejection errors. Depending on the fluid type and its cavitation conditions, ejection faults may accelerate. To minimize the time that dust can cause a defect in ejection, the side stream is adjusted to provide a fluid velocity that causes the fluid in the chamber to be flushed out within the time it takes to eject 1000 pixels in a single nozzle.

[0045] The velocity of the side stream will depend on the flow through the channel 33 and the relative cross-sectional areas for the channel 33 and the chamber 3.

[0046] If the flow through the channel 33 is equal to the maximum flow through the nozzle (which is greater than the time-averaged make-up stream, which level depends on the chamber's duty cycle and printing data), and also if the cross-sectional area of ​​the channel 33 is one-tenth of the surface cross-section of chamber 3, then ten times faster jet velocity through the nozzle can be expected.

[0047] Preferably, the side stream is opposed to the main make-up stream, whereby the active chamber is protected against the influence of impurities from the ink source due to the smaller size of the channel providing the side stream.

[0048] An important aspect to consider when designing the recirculation stream is the fluid underpressure that, if large, can cause unwanted cavitation. The embodiment described here requires that the side channel provide significant impedance, whereby a pressure significantly greater than atmospheric must be applied to the connected manifold to produce the necessary flow velocity in the actuation chamber. Conveniently, the opposite manifold (which must provide a negative pressure for the nozzle, which must be kept below atmospheric pressure) can be used to provide only moderate negative pressure (relative to atmospheric), thereby reducing the risk of cavitation.

[0049] The cross-sectional area of ​​the high-impedance channel 33 is substantially smaller than the cross-sectional area of ​​the channel 3. In one arrangement, the ink channels 3 are 300 pm high and 75 pm wide. High impedance channels can run through the width of chamber 3 for ink with a dimension of 75 pm, with a thickness (in the direction of longitudinal channel 3) 30 pm, with a cross-sectional area of ​​one tenth of the cross-sectional area of ​​channel 3 for ink. In variants, the high impedance channel 33 may run over less than the entire ink channel width and may extend to a greater or lesser extent in the direction of channel length 3.

[0050] The modification is shown in Figure 4. In this case, the high impedance channel takes the form of a recess 41 cut out in the nozzle plate 4. This nozzle plate can be designed to be thicker, so that it can both include this cavity and provide a nozzle of the same length as in the previously described embodiment.

[0051] Although this invention has been described with reference to the printhead, it is understood that this invention finds a much wider application for drop deposition devices. Similarly, it is understood that the high impedance channel, in communication with the chamber and at the end of the nozzle, can take many forms beyond those described herein, and the walls of the piezoelectric material are only one example of actuators associated with the chamber for ejecting droplets through the nozzle by making longitudinal acoustic waves in the fluid chamber.