Background of the Invention Many different types of printing have been invented, a large number of which are presently in use. The known forms of printers have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.

In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.

Many different techniques of ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore,"Non-Impact Printing: Introduction and Historical Perspective", Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).

Ink Jet printers themselves come in many different types. The utilization of a continuous stream ink in ink jet printing appears to date back to at least 1929 wherein US Patent No. 1941001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing. US Patent 3596275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electro-static field

so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also US Patent No. 3373437 by Sweet et al) Piezo-electric ink jet printers are also one form of commonly utilized ink jet printing device. Piezo-electric systems are disclosed by Kyser et. al. in US Patent No.

3946398 (1970) which utilizes a diaphragm mode of operation, by Zolten in US Patent 3683212 (1970) which discloses a squeeze mode of operation of a piezo electric crystal.

Stemme in US Patent No. 3747120 (1972) discloses a bend mode of piezo-electric operation, Howkins in US Patent No.

4459601 discloses a Piezo electric push mode actuation of the ink jet stream and Fischbeck in US 4584590 discloses a sheer mode type of piezo-electric transducer element.

Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in US Patent 4490728.

Both the aforementioned references disclose ink jet printing techniques which rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media.

Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.

As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes.

These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables.

Recently, one of the present inventors has proposed an inkjet printing system which utilizes a change in surface tension effects so as to allow for the ejection of a drop from a chamber. The heaters are placed around an external aperture where a meniscus forms so that a substantial portion of the heat is transmitted to the area adjacent the meniscus so as to reduce the surface tension. European Patent Application Nos. 96116117.1,96116116.3 and 96116118.9 disclose systems which utilize this principle in the construction of printhead devices.

Unfortunately, when forming large printhead arrays of nozzles, many complex interrelated effects can result.

Further, the operation of an ink jet printing system may rely upon many subtle complex effects.

Summary of the Invention It is the object of the present invention to provide for an improved form of operation of an inkjet printhead.

In accordance with a first aspect of the present invention, there is provided a drop on demand print head system comprising: a plurality of ink ejection nozzles formed in a first wafer surface, the ink ejection nozzles being grouped into ink supply groups; each of the ink supply groups including a group ink supply channel etched through the wafer; a pressure oscillation concentration member attached to the back surface of the wafer so as to form a cavity comprising an ink supply channel interconnecting the group ink supply channel between the oscillation concentration member and the wafer surface; and a spatial movement member attached to the oscillation concentration member and adapted to impart controlled spatial movements to the oscillation concentration member, the oscillation member adapted to concentrate pressure fluctuations in the group ink supply channels in the vicinity of each group of ink ejection nozzles so as to provide for a momentary increased pressure fluctuation of ink at the ink ejection nozzles.

Preferably, the ink ejection nozzles each contain a

selection mechanism to enable or disable the ejection of drops by the nozzle on the occurrence of the momentary increased pressure fluctuation. The selection mechanism can comprise a heater for heating the vicinity of a surface layer of ink to be ejected from the ink ejection nozzle.

In one embodiment, the oscillation concentration member concentrates pressure fluctuations at a focal point in the ambient atmosphere slightly outside the ink ejection nozzle.

In a second embodiment, the oscillation concentration member concentrates pressure fluctuations at a point in the group ink supply channel slightly above the group of ink ejection nozzles.

The group ink supply channel can be formed via an anisotropic etch of the wafer terminating in a pit at the point of the ink ejection nozzles. The spatial movement can comprise imparting a substantially planar pressure wave to a first surface of the spatial movement member.

The group ink supply channels are preferably spaced apart from one another across the surface of the wafer, and the wafer preferably can include a series of extended wafer pillars between the spaced apart group ink supply channels and the pressure oscillation concentration member concentrating fluctuations in ink pressure between the wafer pillars and an adjacent surface of the pressure concentration member on a top surface of the pillar.

In accordance with a further aspect of the present invention, there is provided a method of ejecting drops on demand from a series of ink ejection nozzles, each of the nozzles including a selection mechanism for causing a physical change in the vicinity of the nozzle which in turn causes the ejection of ink from the nozzle, the method comprising the steps of: inducing a pressure wave fluctuation in an ink supply chamber attached to the ink ejection nozzles; concentrating the pressure wave fluctuation in the vicinity of the ink ejection nozzles; utilizing the selection mechanism to select drops to be

ejected from the ink ejection nozzles simultaneously with the pressure wave fluctuation.

The step of concentrating pressure wave fluctuations in the vicinity of the ink ejection nozzles further can comprise simultaneously dispersing pressure wave fluctuations spaced apart from the ink ejection nozzles.

Brief Description of the Drawings Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 and Fig. 2 illustrate schematically, the operational principles of the preferred embodiment ; Fig. 3 to Fig. 6 illustrate the utilization of different pressure pulses in conjunction with thermal heating; Fig. 7 to Fig. 11 illustrate computational fluid dynamic calculations of pressure variations within a modelled ink jet chamber; Fig. 12 to Fig. 15 are schematic sectional view of a portion of a print head illustrating different concentration mechanisms; Fig. 16 is a side perspective view of an element of a pagewidth printhead; Fig. 17 is an enlarged view of a portion of Fig. 16 ; Fig. 18 is a perspective view of a series of elements of Fig. 16; Fig. 19 is a side perspective, partly in section, of a portion of a printhead; Fig. 20 is an enlarged view of a portion of Fig. 19; Fig. 21 to Fig. 23 are schematic section view of edge portions of an ink ejection pit; and Fig. 24 illustrates the surface of the plastic former member utilized in the concentration of pressure pulses in accordance with the preferred embodiment.

Description of Preferred and Other Embodiments

Investigations and simulations of different printhead types especially those constructed in accordance with the principles of the aforementioned European patent specifications utilizing an oscillating pressure in the ejection of ink has increased the level of understanding of the operation of such printheads. In particular, it has been found that a particular previously unknown effect, which will be described hereinafter, occurs when the pulse which is applied to the actuator mechanism has particular characteristics. The corresponding resulting pressure experienced at an ink nozzle leads to a particularly unique phenomenon which is thought responsible for the ejection of ink. The preferred embodiment is directed at utilizing this phenomenon to provide enhanced operation of an ink jet printer.

Turning initially to Fig. 1 and Fig. 2, there will now be explained the basic operational principles of an inkjet arrangement illustrative of the teachings of the preferred embodiment. The arrangement of Fig. 1 and Fig. 2 being operational in accordance with the principles described in the aforementioned European patent applications.

In the operation of the preferred embodiment, an fluctuating ink pressure is applied to an ink supply chamber 10. The application of the oscillating pressure can be via the utilization of a piezo-electric oscillator element 12 or other such mechanism attached to a substrate 13.

Preferably, a series of ink ejection nozzles e. g. 14,15 are also provided. Each ink ejection nozzle e. g. 14 has a circular heater e. g. 16 around an exterior area of the nozzle in accordance with the teachings of the aforementioned European Patent Applications.

Normally, the oscillation in ink pressure within the nozzle chamber 10 is regulated so that the surface tension effects across each ink ejection nozzle e. g. 14,15 are sufficient to hold the ink within the nozzle chamber. When it is desired to eject a drop from a nozzle e. g. 14, the

heater 16 is pulsed so as to cause the surface tension to be reduced due to the increase in temperature of the meniscus 14. Hence, as shown in Fig. 2, during the next high pressure wave, the surface tension around the ink ejection nozzle 14 is insufficient to hold the ink within the nozzle chamber 10 and a drop 20 is ejected. In an adjacent nozzle 15, where no heat has been applied, no ink drop is ejected as, although the ink bulges 21, the surface tension characteristics are sufficient to retain the ink within the nozzle chamber 15.

Investigation of systems operational in accordance with the principles of Fig. 1 and Fig. 2 have lead to the elucidation of a number of interesting effects.

It has been found that, in order to cleanly eject a drop from a nozzle chamber in a controlled manner, it is necessary to provide for a conjunction of events to assist in drop ejection. In particular, at a certain pressure, above ambient, a drop will be ejected from the chamber. The utilization of the thermal heating of the meniscus results in a reduction in the pressure above ambient required for a drop ejection and a consequential ejection of the drop.

However, maintaining such a pressure above ambient is likely to result in a continual outflow of ink from a nozzle after an initial drop has been ejected. The utilization of an oscillating ink pressure was therefore thought to eliminate the continual outflow of ink from the nozzle.

However, detailed studies of ink jetting arrangements have revealed further interesting physical effects. These effects will now be discussed with reference to Fig. 3 to Fig. 6. Turning initially to Fig. 3, which shows schematically a plot of an example pressure oscillation with time. An equilibrium pressure line 22 is provided above which the surface tension will be insufficient to hold the ink within a nozzle chamber. When a heater pulse is applied during the period 23, the equilibrium pressure drops 24 as a result of the reduction in surface tension. If a pressure

pulse 25 is simultaneously applied but is of insufficient magnitude, no drop will be ejected from the nozzle.

If however, as illustrated in Fig. 4, a pressure pulse 26 is applied having a magnitude exceeding the equilibrium pressure and applied for a predetermined time, then the nozzle will eject a drop irrespective of any fluctuation 24 in the equilibrium pressure.

It has been surprisingly found, as illustrated in Fig.

5, that should a short high pressure pulse 27 be applied simultaneously whilst the drop in equilibrium pressure 24 occurs, a drop is ejected with an anomolously high velocity.

Further, it has also been found that, as illustrated in Fig.

6, when the same high pressure pulse 27 is applied simultaneously to a nozzle which has not undergone any change in equilibrium pressure, no drop is ejected even though the ink pressure has momentarily exceeded the equilibrium pressure.

Hence the utilization of a short high pressure pulse is considered highly significant in the ejection of ink from systems such as that previously described.

Detailed computational fluid flow simulations were carried out of the high pressure pulse effect and indicative results are shown in Fig. 7 to Fig. 12 with the time axis showing tens of microseconds. A fluid chamber, similar to that shown in Fig. 1 was modelled utilizing the FIDAP simulation system.

In a first example simulation, a pressure pulse or fluctuation, without a spike, as shown in Fig. 7 was applied at approximately the point 11 of Fig. 1. The particular shape of the waveform of Fig. 7 was taken as indicative of the likely pressure fluctuation for a simulated system at the meniscus. Simultaneous thermal heating of the meniscus was also applied. In Fig. 8, there is shown the corresponding velocity fluctuation near the back of the meniscus which indicates that no ejection occurred.

In a second example, as shown in Fig. 9, the same

pressure input waveform was simulated with the addition of a series of pressure spikes 28,29. In this case no thermal heating occurred. The resulting fluid velocity fluctuation near the meniscus was found to be as illustrated in Fig. 10 and again shows that no ejection occurred.

In a third example, the pressure waveform of Fig. 9 was again utilized in conjunction with thermal heating and, in this case the result was as illustrated in Fig. 11 wherein a drop was found to be ejected with a terminal velocity of approximately 2.5 m/s.

The pulse 28 (Fig. 9) represents a short period of high pressure and it is thought that the pulse 28 is a significant cause for imparting momentum to a drop as it is ejected from the nozzle. This leads to a significant refinement of the ink jetting arrangement of Fig. 1 and Fig.

2. The simulations suggest that it is desirable to concentrate an input pressure wave at or near the surface of the meniscus at substantially the same time as the heater pulse is applied so as to cause a"hyper pressure pulse" which imparts significant momentum to the ink around the meniscus resulting in a drop being ejected with a high drop velocity.

It is therefore considered desirable to provide a system which enhances the production of high pressure pulses. Turning now to Fig. 12, there is illustrated schematically, a sectional view of a first concentrating type device 30. The device 30 can include a series of piezoelectric actuators 31 which are controlled so as to impart a substantially planar wave on the surface 33 of an acoustic transmission medium 34 which can comprise a plastic injection molded member. The member 34 is glued 49 to the back surface of the wafer 40. The plastic injection molded member 34 will therefore undergo movement in accordance with the control signal sent to the piezoelectric actuator member 31. The profile of the surface of the member 34 results in a series of plane waves e. g. 35 being generated within each

fluid chamber e. g. 45. Unfortunately the anisotropically etched walls 39 of the nozzle chamber can result in reflections 52 of the planar pressure waves in an uncontrolled manner. This in turn results in the membrane eg. 48 and ink ejection nozzles e. g. 50 undergoing pressure fluctuations in an uncontrolled manner.

In a first modification, illustrated in Fig. 13, the profile of the surface 47 of the member 34 is shaped so as to focus the emitted wave at a point 36 outside the membrane. This ensures a more uniform pressure arrives at the membrane 48 and minimal side reflections occur.

In a second modification illustrated in Fig. 14, the focal point 36 is moved before the membrane 48. This will lead to better concentration of the pulse in the vicinity of the membrane.

Turning now to Fig. 15, there is illustrated a further modification wherein the position of surface 47 is adjusted to provide a greater depth of channel 45, with the focal point 40 being suitable moved.

By utilization of a suitably and simply molded part 34, a printhead can be constructed such that, on operation of the piezo electric oscillator, a significant concentrated pulse is experienced at the nozzles or membrane 48. Hence, a printhead element can be formed as shown in Fig. 16 in the form of a wafer 40 containing a large number of anisotropically etched pits 45 which can be anisotropically etched from the back surface of the wafer utilizing the techniques disclosed in the aforementioned European patent applications. At the bottom of each pit 46, a series of nozzles can be provided which are formed on the front surface of the wafer utilizing standard micro-electro mechanical (MEMS) processing techniques. Fig. 17 illustrates a single pit 45 in more detail showing more clearly the ink ejection nozzle 46.

As illustrated in Fig. 18, in one form of construction, a pagewidth printhead can be formed from a number of smaller

segments 40 with each segment providing for an interlocking end portion e. g. 50 (Fig. 16) which is designed to be interlocked with an adjacent segment so as to form a combined printhead.

As shown in Fig. 19 and 20, the plastic member 34 can be fixed to the wafer portion 40 so as to include a profiled surface 47 formed in accordance with the aforementioned discussion so as to provide for pressure wave concentration at the bottom of a corresponding pit e. g. 48.

Unfortunately, the utilization of the vibrating plastic member 34 is likely to still cause significant reflections in the areas 57 and 58 (Fig. 16) which may interfere with the production of high pressure pulses. This is illustrated in Fig. 21 wherein a pressure pulse 62 produced at the surface 60 of the member 34 is likely to cause significant energy to be reflected 63 to the area of the ink nozzle membrane. The arrows 64,65 illustrating the normal arrows to the plane wave in an adjacent nozzle chamber. This problem can be significantly alleviated by further shaping the surface of the member 34. Example reshapings are shown in Fig. 22 and Fig. 23. In Fig. 22 the surface 60 is profiled so as to concentrate the pulse at the surface 57. Similarly, Fig. 23 represents a second example wherein the walls of the member are formed so as to ensure substantial continual back reflection from the surface 57.

Turning finally to Fig. 24, there is illustrated a final bottom surface view of a section of the plastic member 34 which includes the profiled surface 35 for concentrating a pressure pulse in addition to a cavity 36 which is provided to mate with the ends of each pit and is provided to dampen any pressure pulse where it is not required in accordance the principles of Fig. 22 and 23.

It can therefore be seen that, through the utilization of an appropriately profiled plastic former member a plane pressure wave at the back of the plastic member can be concentrated at the bottom of the anisotropically etched

pits so as to provide for a"hyperpressure pulse"which assists in the ejection of ink from an ink ejection nozzle.

It would be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. For example, the disclosed embodiments neglect the effect of the"refractive index"of waves propagating between materials. Additionally, no account of impedance mismatch between the member materials and the fluid has been taken or of the effect of transient reflections or resonance buildup in any chamber. Further, only a single general piezo element has been assumed.

Obviously, multiple piezo elements could be used in different diving arrangements to provide for predetermined effects. Each of these effects may lead to slight modification of profiles in accordance with materials utilized. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.