GB2282569A

GB2282569A - Droplet generator.

Info

Publication number: GB2282569A
Application number: GB9420384A
Authority: GB
Inventors: Guruge Elmo Lakshma Perara
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1993-10-08
Filing date: 1994-10-10
Publication date: 1995-04-12
Also published as: WO1995010415A1; EP0673318A1; GB9320729D0; GB9420384D0

Abstract

A fluid dispenser suitable for printing includes a supply channel 25 along which a main fluid is transmitted. In the stable state of the dispenser, the main fluid is constrained to follow wall 33 due to the Coanda effect. When the pressure of a control fluid in a channel 28 is increased by application of a pulse to a heating element 29, it causes a wave front to he formed in the main fluid which is intercepted by a splitter 36 and a droplet 35 of main fluid is ejected. The device is suited to generating ink droplets in an ink jet printer, in which the droplet velocity is dependent on the flow rate of the main fluid, and the droplet size is dependent on the pulse. <IMAGE>

Description

FLUID DISPENSER This invention relates to a fluid dispenser and particularly, but not exclusively, to a dispenser for dispensing very small quantities of a fluid, such as printing ink, at a rapid rate, on demand. The device is therefore particularly suitable for use in an ink jet printing head.

Figure 1 of the accompanying drawings is a schematic cross-sectional view of a known type of fluid dispenser, as used, for example, in drop-on-demand print heads. The dispenser 1 includes a fluid supply system 2, comprising a reservoir 3 which is connected to a main chamber 4 of the dispenser via a pipe 5. A drive mechanism 6, which may be a piezoelectric driver, is fitted to one end of the chamber 4, and an outlet jet 7 is provided at the opposite end. The drive mechanism 6 acts as a reciprocating pump. On the outward stroke of the mechanism the fluid, such as printing ink, is drawn into the chamber from the reservoir, and on the forward stroke the fluid is pushed towards the jet 7, so that a drop of fluid is ejected therefrom.

The quality of such a dispenser is determined by the quantity of fluid ejected at each stroke or strokes of the drive mechanism, the velocity with which the fluid is ejected, and the rate of ejection. For a given geometry of the chamber, the pressure at which the fluid is supplied to the chamber and the operational characteristics of the drive mechanism determine all of those parameters. By increasing the supply pressure and the displacement of the drive mechanism in the forward stroke, either independently or as combined parameters, the ejection quality can be improved. However, if the supply pressure is to be increased above the pressure at the outlet of the jet (which in print heads is generally atmospheric pressure.

the fluid column cannot be contained in the chamber during the off periods of the dispenser i.e. during periods when no fluid is to be ejected from that particular jet. Fluid will therefore drip out of the jet during those periods.

Hence, the single most influential parameter in achieving high-quality ejection on demand in these known dispensers is the maximum obtainable displacement of the drive mechanism, which is clearly limited.

In another known type of ink jet print head known as a bubble-jet print head, the piezo-electric driver is replaced by an electric heating element which is located in a chamber which contains ink. Each time the heating element is energised, it generates sufficient heat to cause the ink adjacent its surface to boil. This results in the formation of a bubble, which displaces a small volume of the ink in the chamber, thereby causing a droplet of ink to be ejected from a nozzle at the exit of the chamber.

A feature common to both of the above-described types of known dispenser is the fact that the velocity of the droplets ejected from the nozzle is directly controlled by the pressure induced by the thrust of the piezo-electric transducer or by the bubble, as the case may be. This dependence of the velocity of the droplet on the control pressure limits the operational capability of the print head. Furthermore, the size of the droplets is controlled by the nozzle design, which is fixed for each particular device.

Our co-pending European Patent Application No. EP-A-0436509 describes an improved fluid dispenser. This dispenser is illustrated schematically in Figure 2 of the drawings of the present application. The dispenser comprises a substrate 8 (which may be silicon) into which is machined a main chamber 9 to which fluid is fed under pressure via an inlet 10. At the outlet end 11 of the chamber is a fluid controller 12 comprising an inlet channel 13, control chambers 14 and 15, and outlet channels 16 and 17. The outlet channel 16 leads to a dispensing outlet 18, whilst the outlet channel 17 conducts the fluid back into the chamber 9 via a port 19 and a connecting path (not shown). Alternatively, the channel 17 might be arranged to conduct the fluid back to the fluid supply (not shown).

In use of the dispenser, fluid is fed under pressure to the main chamber 9 via the inlet 10, the pressure being applied to the fluid by, for example, a cylinder of gas (e.g. C) ) a compressor, a pump or other suitable means. The pressurised fluid enters the inlet channel 13 of the fluid controller 12. Fluid also enters the control chambers 14 and 15 via ports 20 and 21, respectively. At the outer ends of the control chambers 14 and 15 are driver devices 22 and 23, respectively, (shown dotted) either of which can be energised, exclusively of the other, to increase the fluid pressure in the respective control chamber.The fluidic device operates in such a manner that if the driver device 22 is energised, the fluid pressure in the control channel 14 will cause the main fluid entering the inlet channel 13 to veer towards the outlet channel 17 and thence back to the chamber or to the fluid supply, as the case may be.

Even if the driver device 22 is subsequently de-energised, the fluid will continue to follow the path through the outlet channel 17, by virtue of the Coanda effect whereby the main fluid jet attaches itself to either of the walls forming the outlet channels 16 or 17. The device, therefore, acts as a fluid amplifier where a small amount of control pressure leads to a large effect on the main fluid jet.

When a drop of fluid is to be dispensed from the outlet 18, the driver device 23 is momentarily energised instead of the driver device 22, so that the fluid flow from the main chamber 9 switches over to the outlet channel 16, and fluid is dispensed from the outlet 18.

As soon as the required quantity of fluid has been dispensed, the flow is switched back to the channel 17 by re-energisation of the driver device 22, so that the fluid again circulates back to the main chamber or to the fluid supply.

Hence, a given quantity of fluid can be dispensed, at high velocity, in a very short period, merely by suitably controlling the energising electric pulses fed to the driver devices 22 and 23. As no moving parts are required in order to effect the flow switching operation, high-speed switching can be readily achieved. Furthermore, as the supply pressure is decoupled from the control pressures, high droplet ejection velocities can be achieved.

It is an object of the present invention to provide an improved fluid dispenser which requires only a single driver device.

According to the invention there is provided a fluid dispenser for dispensing a main fluid, the dispenser comprising a supply channel; fluid supply means for feeding said main fluid to the supply channel under pressure; a first fluid path along which the main fluid is fed from the supply channel; a second fluid path including a fluid dispensing outlet; a control channel containing control fluid and having a control outlet adjacent the first fluid path; and means for changing pressure in said control fluid such that a wave front is formed in the main fluid and a droplet of said main fluid is dispensed from the fluid dispensing outlet.

Preferably, the main fluid flow lollows said first fluid path due to Coanda effect except when diverted by change of pressure of the control fluid.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figures 1 and 2 are schematic cross-sectional views through respective prior fluid dispensers as described above; Figure 3 is a schematic cross-sectional view of the outlet region of a fluid dispenser in accordance with the invention; Figure 4 is an explanatory diagram illustrating the operation of the dispenser of Figure 3; and Figure 5 is a schematic cross-sectional view of part of another dispenser in accordance with the invention.

Referring to Figure 3 of the drawings, the outlet and fluid control region 24 of a fluid dispenser in accordance with the invention comprises a supply channel 25 which extends from a supply chamber (not shown) and which carries a pressurised main (or supply) fluid which is to be dispensed, on demand, from an outlet opening 26. The main fluid may be, for example, a printing ink.

A control channel 27 contains a control fluid, which may be the same as, or different from, the main fluid. The channel 27 has a control outlet 28 through which control fluid can be ejected. A thermal element 29, which may conveniently be a resistor, is disposed within the control channel. The resistor is energisable, as required, from a power supply 30 via a switching device 31, or from a pulse generator. Momentary energisation of the resistor 29 causes the generation of sufficient heat to cause the formation of a bubble in the control fluid, forcing a small quantity of the control fluid out of the control outlet 28.

The flow from the supply channel 25 leaves the channel via an outlet channel 32 and is normally held by the Coanda effect in contact with a wall region 33, so that it passes into a return path 34, along which it can be returned to the supply chamber. However, each time the resistor 29 is energised, the control fluid emerging from the outlet 28 causes a momentum to be transferred to the oncoming high pressure supply jet and the supply fluid moves away from the wall 33. This is insufficient in itself to emit a droplet from the supply stream.

However, the intermittent pulsing action of the transducer in the control channel forms the on-coming supply jet into a wave front on its free surface through momentum transfer, as illustrated in Figure 4 where the dotted line represents the wave front. The free surface is the surface of the main fluid which is not in contract with the convex wall 33 of the supply channel 25. A splitter plate 36 is a arranged to shave-off the wave front to deliver individual high speed droplets 35 from the outlet opening 26. When the resistor 29 is de-energised, so that the emission of control fluid from the channel outlet 28 is arrested, the flow of main fluid moves back into contact with the wall 33 and its flow back to the supply chamber via the return path 34 is resumed.

It will therefore be apparent that each time an energising pulse is applied to the resistor 29 a droplet of the main fluid is dispensed from the opening 26. The device can therefore be used in ink jet printing, and a number of the devices can be assembled side-byside to form a print head for dot matrix printing. This permits the dispensing of very closely spaced fluid droplets. A common splitter plate may be used over which a plurality of separately controlled supply jets are directed. In this embodiment, the transducer frequency and the drop ejection rate are equal. However, in other arrangement this is not necessarily the case and one may be higher than the other. No nozzle arrangement is required for the formation and dispensing of the droplets.

The thermal resistive element may be integrated into the body of the dispenser, in the control channel 27 or on or near the wall 33. Alternatively, it may be fabricated on a separate substrate, which is then located to act as a cover plate for the dispenser body.

The dispenser operation is stable, because it is biased to operate in a steady state, in which the main fluid is delivered to a return path until the transducer 29 is energised to cause ejection of a droplet. It should be noted that the dispenser of the present invention does not operate on the same principle as the known "bubble jet" print heads, in which energisation of the resistance element causes a bubble, which forces a drop of ink out of a printing nozzle.

In the above described embodiment, a bubble within the control region causes pressure, at the control channel outlet, on the main fluid path to cause momentary diversion of the main fluid to the fluid dispensing outlet and the formation of a wave front. In other embodiments, the pressure in the control fluid may be adjusted to produce a differential pressure between it and the main fluid flow without necessarily also causing ejection of control fluid into the main fluid path. The pressure difference created is sufficient to produce the required change in the direction of the main fluid flow such that it moves for an instant from its stable flow path. A diaphragm may be located between the control channel and the supply channel to retain the control fluid separate from the supply fluid.

The dispenser may advantageously be micromachined from a block of material or fabricated by electroforming, electroplating, chemical etching or moulding. It may alternatively be formed by assembling separately-fabricated modules. The dispenser may be used for depositing droplets for printing or for imaging applications, as well as other nonprinting applications where there is a requirement for dispensing precise volumes of fluids.

The dispenser of the present invention has a number of advantages over known devices. The main fluid supply pressure is independent of the control pressure. As a result, the velocity of emission of the droplet will directly depend on the supply pressure and not on the control pressure, thereby yielding drop velocities in excess of 4 m/second which are much higher than achievable with previous piezo-electric and thermal systems. The droplet size is controlled by the Coanda effect and wave front formation, and not by the dimensions of a nozzle, and the Coanda effect is controlled by the duration and magnitude of the control pulses. These features have been confirmed by experiment, where we have shown that the droplet size and shape are, as we predicted, defined by the control pressure, the geometry of the channels and the shape and position of the splitter. A dispenser in accordance with the invention may operate with a velocity and throw distance which exceeds those of previous devices. This enables printing to be effected on surfaces which are further from the print head (e.g. a few centin-tres), which is required for industrial printing applications, such as printing on cans, boxes, containers, and the like.

The present invention is clearly distinguished from the Coanda effect dispenser disclosed in our above-mentioned European patent application. Thus, whereas the previous dispenser used two control channels to operate its bistable fluidic output controller, the present invention uses a monostable fluid control device which requires only a single control channel. Energisation of the control jet can be accomplished by any means capable of imparting pressure onto the control fluid and advantageously such means may be an actuator.

Such actuators may include piezoelectric transducers, magnetostrictive transducers and thermal transducers depending on the performance requirements. The transducer may be located in the control channel or could be arranged outside it. For example, the walls of the channel may include a flexible portion which is deformed by a piezoelectric transducer arranged adjacent its outer surface.

As the number of control channels is reduced from two to one, the size of the device can be reduced. This reduction in size enables the design of high resolution printheads and fluid dispensers. Furthermore, power consumption is reduced and manufacturing costs are also reduced as the construction is simplified compared to previously known dispensers.

In another embodiment, as illustrated in Figure 5, the control channel 37 is configured to include back flow restrictors 38, which maximize the effect of the transduction action by thermal transducer 39 on the supply jet for high performance, In this arrangement.

the back flow restrictors 38 have a triangular cross-section, but other geometries could also be suitable. Figure 5 also illustrates an aperture 40 for excess fluid collection and the integrated return path 41 for the supply fluid. A diaphragm 42 is also included between the control channel and the supply channel. It allows transmission of the change in pressure whilst keeping the control and supply (or main) fluids separate.

Claims

1. A fluid dispenser for dispensing a main fluid, the dispenser comprising a supply channel: fluid supply means for feeding said main fluid to the supply channel under pressure; a first fluid path along which the main fluid is fed from the supply channel; a second fluid path including a fluid dispensing outlet; a control channel containing control fluid and having a control outlet adjacent to the first fluid path; and means for changing pressure in said control fluid such that the a wave front is formed in the main fluid and droplet of said main fluid is dispensed from the fluid dispensing outlet.

2. A fluid dispenser as claimed in Claim 1, wherein the main fluid is caused to follow said first fluid path by Coanda effect except when diverted by change of pressure of the control fluid.

3. A fluid dispenser as claimed in Claim 1 or Claim 2, wherein said first fluid path is arranged to conduct said main fluid back to the fluid supply means.

4. A fluid dispenser as claimed in any preceding claim wherein the means for changing the pressure comprises an actuator located at/or near the control channel.

5. A fluid dispenser as claimed in claim 4 wherein the actuator is located in the control channel.

6. A fluid dispenser as claimed in claim 4 or 5 wherein the actuator is a transducer.

7. A fluid dispenser as claimed in claim 4, 5 or 6 wherein the actuator is a thermal element which can be energised to cause formation of a bubble in said control fluid and thereby produce a change in pressure in said control fluid.

8. A fluid dispenser as claimed in any preceding claim, comprising a substrate in which the supply channel, the control channel, the first and second fluid paths and the fluid dispensing outlet are defined as cavities.

9. A fluid dispenser as claimed in Claim 8, in which the cavities are formed by micromachining.

10 A fluid dispenser as claimed in Claim 8 or Claim 9, in which an actuator for changing the pressure in the control fluid is provided on a cover plate which is subsequently bonded to the substrate with the actuator in the control channel cavity.

11. A fluid dispenser as claimed in any preceding claim wherein a diaphragm is located between the control channel and the supply channel.

12. A fluid dispenser as claimed in any preceding claim wherein the control channel includes back flow restriction means.

13. A fluid dispenser as claimed in claim 12 wherein the back flow restriction means comprises a part of the control channel having at least one substantially triangular crosssectional configuration.

14. A fluid dispenser as claimed in any preceding claim and including splitter means arranged to intercept the wave front to form said droplet.

15. A fluid dispenser as claimed in claim 14 wherein the splitter means is an elongate sharp edge.

16. A fluid dispenser substantially as hereinbefore described with reference to Figure 3, 4 or 5 of the accompanying drawings.

17. A dispenser head comprising a plurality of fluid dispensers as claimed in any preceding claim, assembled in a stacked arrangement.

18. A dispenser head as claimed in claim 17 and including common splitter means for a plurality of main fluid flows arranged to flow adjacent one another and separately controllable.