WO1997011133A1

WO1997011133A1 - Ink composition based on a microemulsion

Info

Publication number: WO1997011133A1
Application number: PCT/US1996/014117
Authority: WO
Inventors: Kia Silverbrook
Original assignee: Eastman Kodak Company
Priority date: 1995-09-06
Filing date: 1996-09-04
Publication date: 1997-03-27
Also published as: AUPN522395A0

Abstract

Drop selection by surface tension reduction is a drop-on-demand printing mechanism in which ink in a liquid state is retained in a printing nozzle at a pressure greater than atmospheric pressure, but insufficient to overcome the quiescent temperature surface tension of the ink and expel the ink from the nozzle. The surface tension of the ink decreases with increasing temperature, and the nozzle includes an electrically activated means of heating the ink to a temperature less than the boiling point of the ink. The surface tension reduces to a value insufficient to retain the ink in the nozzle, whereby a drop of the ink emerges from the nozzle. An ink composition is disclosed which has the property of a large reduction of surface tension with increasing temperature. The ink is based on a microemulsion with a phase inversion temperature around the desired drop ejection temperature. At the phase inversion temperature, the microemulsion forms a bicontinuous structure between the oil and water components of the ink. At this temperature, the surfactant has a high affinity for surfaces of low curvature, and therefore migrates to the ink/air interface, lowering the surface tension.

Description

INK COMPOSITION BASED ON A MICROEMULSION

Field of the Invention

The present invention is in the field of computer controlled pπntmg devices In particular, the field is an ink composition for drop on demand (DOD) printing systems

Background of the Invention

Many different types of digitaDy controlled pπnung systems have been invented, and many types are currently in production These printing systems use a variety of actuauon mechanisms, a variety of marking mateπals, and a vaπety of recording media Examples of digital prinϋng systems in current use include laser electrophotographic pπnters, LED electrophotographic pπnteis, dot matπx impact pπnters, thermal paper pπnters; film recorders, thermal wax pπnters, dye diffusion thermal transfer pπnters, and ink jet printers However, at present, such electronic prinung systems have not significantly replaced mechanical pπntmg presses, even though this conventional method requires very expensive setup and is seldom commeicially viable unless a tew thousand copies of a particular page are to be pπnted Thus, there is a need for improved digitally controlled pπntmg systems, foi example, being able to produce high quality color images at a high¬ speed and low cost, using standard papei Inkjet printing has become recognized as a prominent contender in the digitalh controlled, electronic pπnting arena because, e g . ot its non-impact, low-noise characteπstics, its use ot plain paper and its avoidance ot toner transfers and fixing

Many types ot ink ιet pπnting mechanisms have been invented These can be categonzed as either continuous ink jet (CU) oi diop on demand

(DOD) ink ιet Continuous ink jet printing dates back to at least 1929 Hansell, US Pat No 1.941.001

Sweet et al US Pat No 3,373.437. 1967, discloses an array ot continuous ink jet nozzles where ink drops to De pπnted ate selectively charged and deflected towards the recording medium. This technique is known as binary deflection CIJ, and is used by several manufacturers, including Elmjet and Scitex.

Hertz et al US Pat. No. 3,416.153. 1966, discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics.

Kyser et al US Pat. No. 3.946,398, 1970, discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 720 dpi for home and office printers (Seiko Epson).

Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance. Endo et al GB Pat. No. 2.007, 162. 1979, discloses an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in thermal contact with ink in a nozzle. The heater rapidly heats water based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble. The formation of these bubbles results in a pressure wave which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet™ (trademark of Canon K.K. of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers.

Vaught et al US Pat. No. 4,490,728, 1982, discloses an electrothermal drop ejection system which also operates by bubble formation. In this system, drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater. This system is known as Thermal Ink Jet, and is manufactured by Hewlett-Packard. In this document, the term Thermal Ink Jet is used to refer to both the Hewlett- Packard system and systems commonly known as Bubblejet™. Thermal Ink Jet printing typically requires approximately 20 μJ over a period of approximately 2 μs to eject each drop. The 10 Watt active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements. Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, U.S. Patent No. 4.275.290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head. U.S. Patent Nos. 4.737,803; 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a pπnt sheet.

Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved ink jet printing approach, providing advantages for example, as to cost, speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables.

The printing mechanism is based on a new printing principle called "Liquid Ink Fault Tolerant" (LIFT) Drop on Demand printing.

Summary of the invention

The invention provides a microemulsion based LIFT ink.

A preferred feature is that the phase inversion temperature is between 40^CC and 90°C.

A further preferred feature is that the phase inversion temperature is between 60°C and 70°C. A preferred feature is that the surfactant is of the group poly(oxyethylene)alkylphenylene with the general formula.

HO(CH2CH2O)_mC H<;CnH2n+l

A preferred feature is that m is between 5 and 50. A further preferred feature is that m is between 8 and 12.

A preferred feature is that n is between 5 and 20. A further preferred feature is that n is between 7 and 11. An alternative form of the invention is a microemulsion based hot melt LIFT ink. A preferred feature of the alternative form of the invention is that the phase inversion temperature is at least 20°C above the melting point of the hot melt ink.

Brief Description of the Drawings

Figure 1 (a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention.

Figure 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention.

Figures 2(a) to 2(f) show fluid dynamic simulations of drop selection. Figure 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention.

Figure 3(b) shows successive meniscus positions during drop selection and separation.

Figure 3(c) shows the temperatures at various points during a drop selection cycle.

Figure 3(d) shows measured surface tension versus temperature curves for various ink additives.

Figure 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of figure 3(c) Figure 4 shows a block schematic diagram of print head drive circuitry for practice of the invention. Figure 5 shows projected manufactuπng yields for an A4 page width color pnnt head embodying features of the invenuon, with and without fault tolerance

Figure 6 shows a generalised block diagram of a printing system using a LIFT head.

Figure 7 shows a cross section of an example LIFT print head nozzle used tor computer simulations shown in figures 8 to 18

Figure 8(a) shows the power sub-pulses applied to the print head toi a single heater energizing pulse. Figure 8(b) shows the temperature at vaπous points in the nozzle duπng the drop selection process.

Figure 9 is a graph of meniscus position versus time for the drop selection piocess

Figure 10 is a plot of meniscus position and shape at 5 μs intervals dunng the drop selection process.

Figure 11 shows the quiescent position of the ink meniscus before the diop selection process.

Figures 12 to 17 show the meniscus position and thermal contours at vanous stages duπng the drop selection piocess Figure 18 shows fluid streamlines 50 μs after the beginning of the diop selection heater pulse.

Summai y ot preferred forms of the invention

Drop selection by surface tension reduction is a drop-on-demand pπnting mechanism in which ink in a liquid state is retained in a pπnting nozzle at a piessuie greater than atmospheπc pressure, but insufficient to oveicome the quiescent temperature surface tension ot the ink and expel the ink from the nozzle The surface tension ot the ink decreases with inci easing temperatuie, and the nozzle includes an electπcally activated means ot heating the ink to a temperatuie less than the boiling point ot the ink The surface tension reduces to a value insufficient to retain the ink in the nozzle, whereby a drop of the ink emerges from the nozzle.

An ink composition is disclosed which has the property of a large reduction of surface tension with increasing temperature. The ink is based on a microemulsion with a phase inversion temperature around the desired drop ejection temperature. At the phase inversion temperature, the microemulsion forms a bi- continuous structure between the oil and water components of the ink. At this temperature, the surfactant has a high affinity for surfaces of low curvature, and therefore migrates to the ink/air interface, lowering the surface tension. In one general aspect, the invention constitutes a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an alternative means is provided to cause separation of the selected drops from the body of ink.

The separation of drop selection means from drop separation means significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles.

The drop selection means may be chosen from, but is not limited to. the following list:

1 ) Electrothermal reduction of surface tension of pressurized ink

2) Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection

3) Piezoelectric, with insufficient volume change to cause drop ejection

4) Electrostatic attraction with one electrode per nozzle

The drop separation means may be chosen from, but is not limited to. the following list: 1 ) Proximity (recording medium in close proximity to print head) 2) Proximity with oscillating ink pressure 3) Electrostatic attraction

4) Magnetic attraction

The table "DOD printing technology targets" shows some desirable characteπstics of drop on demand prmtmg technology. The table also hsts some methods by which some embodiments described herein, or in other of my related applications, provide improvements over the pπor art.

DOD printing technology targets

In thermal ink jet (TIJ) and piezoelectπc ink jet systems, a drop velocity ot approximately 10 meters per second is preferred to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink. and stnke the recording medium These systems have a very low efficiency of conversion of electπcal energy into drop kinetic energy The efficiency of TIJ systems is approximately 0.02%). This means that the dπve circuits for TD pnnt heads must switch high currents The dπve circuits for piezoelectric ink jet heads must either switch high voltages, or dπve highly capacitive loads The total powei consumption ot pagewidth TIJ pπntheads is also very high An 800 dpi A4 full coloi pagewidth TIJ print head printing a four color black image in one second would consume approximately 6 kW of electrical power, most ot which is converted to waste heat The difficulties ot removal of this amount ot heat piecludes the production ot low cost, high speed, high resolution compact pagewidth TIJ systems

One important feature of embodiments of the invention is a means ot significantly reducing the energy required to select which ink drops are to be pπnted This is achieved by separating the means tor selecting ink drops trom the means toi ensunng that selected drops sepaiate tiom the bod\ ot ink and toim dots on the recording medium Only the drop selection means must be driven b\ individual signals to each nozzle The diop sepaiation means can be a field oi condition applied simultaneously to all nozzles The table "Drop selection means" shows some of the possible means for selecting drops in accordance with the invention. The drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops.

Drop selection means

Other drop selection means may also be used The preferred drop selection means for water based inks is method 1 "Electrothermal reduction of surface tension of pressuπzed ink" This drop selection means provides many advantages over other systems, including, low power operation (approximately 1 % of ΗJ), compatibility with CMOS VLSI chip fabπcation, low voltage operation (approx 10 V), high nozzle density, low temperature operation, and wide range of suitable ink formulations The ink must exhibit a reduction in surface tension with increasing temperature

The preferred drop selection means for hot melt or oil based inks is method 2 "Electrothermal reducuon of ink viscosity, combmed with oscillaung ink pressure" This drop selection means is particularly suited tor use with inks which exhibit a large reducuon of viscosity with increasing temperature, but only a small l eduction in surface tension This occurs particularly with non-polai ink carriers with relatively high molecular weight This is especially applicable to hot melt and oil based inks

The table "Diop separation means" shows some of the possible methods tor separating selected drops from the body of ink. and ensuring that the selected drops form dots on the pπnting medium The drop separation means discriminates between selected drops and unselected drops to ensure that unselected diops do not form dots on the printing medium

Other drop separation means may also be used.

The preferred drop separation means depends upon the intended use. For most applications, method 1: "Electrostatic attraction", or method 2: "AC electric field'^* are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: "Proximity" may be appropriate. For high speed, high quality systems, method 4: "Transfer proximity" can be used. Method 6: "Magnetic attraction" is appropriate for portable printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances.

Further details of vaπous types of printing systems according to the present invention are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'A Liquid ink Fault Tolerant (LIFT) pπnting mechanism' (Filing no PN2308),

'Electrothermal drop selection in LIFT printing' (Filing no PN2309). 'Drop separation in LIFT printing by print media proximity' (Filing no PN2310),

'Drop size adjustment in Proximity LIFT pnnting by varying head to media distance' (Filing no PN2311),

'Augmenting Proximity LIFT pnnting with acoustic ink waves' (Filing no PN2312), 'Electrostatic drop separation in LIFT pnnting' (Filing no PN2313),

'Multiple simultaneous drop sizes in Proximity LIFT printing' (Filing no PN2321),

'Self cooling operation in thermally activated pnnt heads' (Filing no PN2322), and 'Thermal Viscosity Reduction LIFT printing' (Filing no PN2323)

A simplified schematic diagram ot one pieteπed pπnting system accoiding to the invention appears in Figure 1 (a)

An image source 52 may be raster image data from a scanner oi computei oi outline image data in the form of a page description language (PDL) oi other forms of digital image representation This image data is converted to a pixel-mapped page image by the image processing system 53 This may be a iastei image pi ocessor (RIP) in the case of PDL image data or ma\ be pixel image manipulation in the case ot 1 aster image data Continuous tone data produced b\ the image piocessing unit 53 is halttoned Halftoning is pertoimed by the Digital Halftoning unit 54 Halttoned bitmap image data is stoied in the image memon 72 Depending upon the pπntei and system configuration the image memor> 72

be a tull page memory, oi a band memoiy Heater contiol cncuits 71 read data tiom the image memoiy 72 and applv time-varying electπcal pulses to the nozzle heateis (103 in figuie 1 (b)) that aie part ot the print head 50 These pulses aie applied at an appiopnate time, and to the appropriate nozzle so that selected diops will form spots on the recording medium 51 in the appropriate position designated by the data in the image memory 72.

The recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. The paper transport system shown in figure 1(a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50. However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the sub-scanning direction) and the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroller 315 may also control the ink pressure regulator

63 and the heater control circuits 71.

For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. A constant ink pressure can be achieved by applying pressure to the ink reservoir

64 under the control of an ink pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown).

For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown).

When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51 , while unselected drops remain part of the body of ink. The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters

In some types of pπnters according to the invention, an external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51 A convenient external field 74 is a constant electric field, as the ink is easily made to be electncally conductive In this case, the paper guide or platen 67 can be made of electrically conductive material and used as one electrode generating the electric field The other electrode can be the head 50 itself Another embodiment uses proximity of the pnnt medium as a means ot discnm ating between selected drops and unselected drops

Foi small drop sizes gravitational torce on the ink drop is very small, appioximately W^'4 of the surface tension torces. so giavity can be ignored in most cases This allows the pnnt head 50 and recording medium 51 to be oriented in any dnection in relaϋon to the local gravitauonal field This is an important lequnement tor portable pπnters

Figure 1 (b) is a detail enlargement ot a cross section of a single micioscopic nozzle tip embodiment of the invention, tabiicated using a modified CMOS piocess The nozzle is etched in a substrate 101 , which may be silicon, glass metal oi any other suitable matenal It substiates which are not semiconductoi mateπals are used, a semiconducting material (such as amorphous silicon)

be deposited on the substrate, and mtegiated drive uansistois and data distribution cncuitry may be lormed in the surface semiconducting laver Single crystal silicon (SCS) substrates have several advantages including 1 ) High peitormance dπve tiansistors and othei circuiuv can be tabiicated in SCS

2) Pnnt heads can be tabiicated in existing facilities (tabs ) using standaid VLSI pi cessing equipment,

3) SCS has high mechanical strength and rigidity, and 4) SCS has a high thermal conductivity In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is passivated with silicon nitride. The protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface. The print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head.

Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al US Pat. No. 4.580.158, 1986, assigned to Xerox, and Miller et al US Pat. No. 5,371,527, 1994 assigned to

Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred method for elimination of orifice plates for print heads of the invention is incorporation of the orifice into the actuator substrate.

This type of nozzle may be used for print heads using various techniques for drop separation.

Operation with Electrostatic Drop Separation

As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is shown in figure 2.

Figure 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois. USA. This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 μm. at an ambient temperature of 30°C. The total energy applied to the heater is 276 nJ. applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30°C is 1.84 cPs. The ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 μs.

Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus. Figure 2(b) shows thermal contours at 5°C intervals 5 μs after the start of the heater energizing pulse. When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surface of the ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directly in contact with the heater from moving.

Figure 2(c) shows thermal contours at 5°C intervals 10 μs after the start of the heater energizing pulse. The increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow.

Figure 2(d) shows thermal contours at 5°C intervals 20 μs after the start of the heater energizing pulse. The ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by the protruding conductive ink drop.

Figure 2(e) shows thermal contours at 5°C intervals 30 μs after the start of the heater energizing pulse, which is also 6 μs after the end of the heater pulse, as the heater pulse duration is 24 μs. The nozzle up has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter (less than 16 μs in this case) the ink would not accelerate towards the pnnt medium, but would instead return to the nozzle.

Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end ot the heater pulse. The temperature at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around the nozzle tip When the rate at which the ink is drawn from the nozzle exceeds the viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink. The selected diop then travels to the recording medium under the influence of the external electrostatic field. The meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop. One ink drop is selected, separated and forms a spot on the recording medium tor each heat pulse. As the heat pulses are electncally controlled, drop on demand ink jet opeiation can be achieved.

Figure 3(a) shows successive meniscus positions dunng the drop selection cycle at 5 μs intervals, starting at the beginning of the heater energizing pulse

Figure 3(b) is a graph ot meniscus position versus time, showing the movement ot the point at the centre ot the meniscus. The heater pulse starts 10 μs into the simulation

Figure 3(c) shows the resultant curve ot temperatuie with respect to time at arious points in the nozzle. The vertical axis ot the graph is temperature, in units of 100°C. The horizontal axis of the graph is time, in units of 10 μs. The temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 μs time steps. The local ambient temperature is 30 degrees C. Temperature histories at three points are shown: A - Nozzle tip: This shows the temperature history at the circle of contact between the passivation layer, the ink, and air.

B - Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the meniscus.

C - Chip surface: This is at a point on the print head surface 20 μm from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures.

Figure 3(e) shows the power applied to the heater. Optimum operation requires a shaφ rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy applied to the heater is varied over the duration ot the pulse. In this case, the variation is achieved by pulse frequency modulation of 0.1 μs sub-pulses, each with an energy of 4 nJ. The peak power applied to the heater is 40 mW, and the average power over the duration of the heater pulse is 1 1.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be vaned without significantly affecting the operation of the pnnt head. A higher sub-pulse frequency allows finer control over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI).

Inks with a negative temperature coefficient of surface tension

The requirement for the surface tension of the ink to decrease w ith increasing temperature is not a major restriction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids:

Where gris the surface tension at temperature T, k is a constant, T_c is the critical temperature of the liquid, is the molar mass of the liquid, x is the degree of association of the liquid, and r is the density of the liquid. This equation indicates that the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, the critical temperature is substantially above the boiling point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, the admixture of surfactants is recommended.

The choice of surfactant is important. For example, water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C. lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature. However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins, while as little as lOmN/m can be used to achieve operation of the print head according to the present invention. (1 mN/m equals 1 dyne/cm).

Inks with a negative temperature coefficient of surface tension

The requirement for the surface tension of the ink to decrease with increasing temperature is not a major restnction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids:

Where γ_ris the surface tension at temperature T, k is a constant, T_c is the critical temperature of the liquid, M is the molar mass of the liquid, x is the degree of association of the liquid, and p is the density of the liquid. This equation indicates that the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, the critical temperature is substantially above the boihng point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, the admixture of surfactants is recommended.

The choice of surfactant is important. For example, water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C. lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature. However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins, while as little as lOmN/m can be used to achieve operation of the print head according to the present invention. ( 1 mN/m equals 1 dyne/cm). Tnks With Large -ΔΎ.

Several methods may be used to achieve a large negaϋve change in surface tension with increasing temperature. Two such methods are:

1 ) The ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink. For stability, the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of about 60°C to about 80°C is suitable.

2) The ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less than 1,000 A are desirable. Suitable surfactant melting points for a water based ink are between about 50°C and about 100°C, and preferably between about 70°C and about 90°C.

Microemulsion Based Inks Inks with a large reducuon in surface tension with increasing temperature can be prepared by using a microemulsion as the ink carrier. Microemulsions are transparent dispersions containing two immiscible liquids with droplet sizes of 0.01 μm to 0. lμm diameter. Unlike macroemulsions and miniemulsions. which require intense agitauon to form, microemulsions are thermodynamically stable and can generally be obtained by mixing the ingredients gently Microemulsions may be water-external (O W) or oil external (W/O). oi both. To obtain a microemulsion where the air/liquid surface tension decreases with temperature, it is preferable to use a mixture which forms a microemulsion which is (O/W) at low temperatures, and (W/O) at high temperatures A microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperature Below the PIT. the microemulsion is oil in water (O/W). and above the PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvatuie. surface around oil. and at temperatures significantly above the PIT, the surfactant preters a high curvature surface around water At temperatures close to the PIT. the microemulsion forms a continuous 'sponge' of topologically connected water

T ere are two mechanisms whereby this reduces the surface tension. Around the PIT, the surfactant prefers surfaces with very low curvature. As a result, surfactant molecules migrate to the hquid/air interface, which has a curvature which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from O/W to W/O, and therefore the liquid/air interface changes from water/air to oil/air. The oil/air interface has a lower surface tension. There is a wide range of possibilities for the preparation of microemulsion based inks.

For fast drop ejection, it is preferable to chose a low viscosity oil In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be required In these cases, polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable

The surfactant can be chosen to result in a phase inversion temperature in the desired range For example, surfactants of the gioup poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula C_nH:_n+|C₄Hft(CH2CH2O)_mOH) can be used. The hydrophilicity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n Values ot m of approximately 10. and n ot approximately 8 are suitable

Low cost commercial preparations aie the lesult ot a polymenzation of vanous molar ratios of ethylene oxide and alkyl phenols, and the exact number ot oxyethylene gioups varies around the chosen mean These commercial preparations are adequate, and highly pure surfactants with a specific numbei ot oxyethylene groups are not required

The formula for this surfactant is CχHi7C H6(CH:CH:O)_nOH (average n= 10) Synonyms include Octoxynol-10, PEG- 10 octyl phenyl ether and POE (10) octyl phenyl ether. The HLB is 13.6, the melting point is 7°C, and the cloud point is 65°C.

Ink Composition Ink compositions according to the present invention generally comprise an aqueous liquid vehicle, a colorant or mixture of colorants, a surfactant sol, where the surfactant has a melting point substantially greater than the ambient temperature at the printing nozzle, but less than the activation temperature of the nozzle, and optional additional components. The ink composition contains water in the range of approximately

507c to 957c, and preferably from about 657c to about 857c, although the amount can be outside these ranges. The ink composition contains oil in the range of approximately 27c to 307o, and preferably from about 57o to about 107c although the amount can be outside these ranges. The ink composition contains surfactant (such as poly(oxyethylene)alkylphenyl ether) in the range of approximately 17c to 157c. and preferably from about 37 to about 87c, although the amount can be outside these ranges. The ink composition contains colorant in the range of approximately 0.17c to 207e, and preferably from about 17c to about 5 7c, although the amount can be outside these ranges. The ink composition contains optional additional components (such as biocides, sequestering agents, cosolvents. viscosity modifiers, humectants. and polymers) in the range of approximately 07c to about 207c. and preferably from 07c to about 57o, although the amount can be outside these ranges.

Suitable ink compositions can be prepared by any suitable process. Typically, the inks are prepared by simple mixing of the ingredients at room temperature. Vigorous mixing is not required, as microemulsions are thermodynamically stable, and so will form spontaneously under the appropriate conditions of mixture concentrations and temperature. However, some mixing is desirable to reduce the time that the microemulsion takes to form. Process Colors

Many commercial applications require process color printing, and these applications generally use four colors of ink. These colors are cyan, magenta, yellow, and black. Anionic (acid), cationic (basic) and nonionic dyes may be used. Acid dyes yield colored anions in aqueous solution and cationic dyes yield colored cations. Acid dyes typically contain carboxylic or sulphonic acid , and basic dyes usually contain quaternary nitrogen groups. The dyes may also be oil soluble, and be dissolved in the oil constituent of the microemulsion. Pigments may also be used, and these may be dispersed in either the water constituent or the oil constituent of the microemulsion.

Cyan Colorants

Suitable cyan colorants include: Duasyn® Acid Blue AE-SF VP 344 (Hoechst). Duasyn® Direct Turquoise Blue FRL-SF VP 368 (Hoechst), Hostafine® Blue B2G (Pigment, Hoechst), Aized® Direct Blue 199 (Hodagaya Chemical Co.). Aized® Acid Blue 9 (Hodagaya Chemical Co.) and Water Blue 9 (Orient Chemical Industries).

Magenta Colorants

Suitable magenta colorants include: Duasyn® Brilliant Red F3B-SF VP 218 (Hoechst), Duasyn® Red 3B-SF VP 346 (Hoechst), Hostafine® Rubine F6B (Pigment. Hoechst), Aizen® Direct Red 227 (Hodagaya Chemical Co.). Aizen® Acid Red 52 (Hodagaya Chemical Co.), Aizen® Acid Red 289 (Hodagaya Chemical Co.) and Water Red 27 (Orient Chemical Industnes).

Yellow Colorants

Suitable yellow colorants include: Duasyn® Brilliant Yellow GL- SF VP 220 (Hoechst). Duasyn® Direct Yellow 6G-SF VP 216 (Hoechst). Duasyn® Acid Yellow XX-SF VP 413 (Hoechst). Hostafine® Yellow GR (Pigment. Hoechst). Hostafine® Yellow HR (Pigment. Hoechst), Aizen® Direct Yellow 86 (Hodagaya Chemical Co ), Aizen® Acid Yellow 23 (Hodagaya Chemical Co ) and Water Yellow 6C (Oπent Chemical Industries)

Black Colorants

Suitable black colorants include Duasyn® Direct Black HEF-SF VP 332 (Hoechst), Duasyn® Black RL-SF VP 228 (Hoechst) Hostafine® Black T (Pigment, Hoechst), Hostafine® Black TS (Pigment, Hoechst), Aized® Direct Blue 199 (Hodagaya Chemical Co ), Aizen® Black 3000S (Hodagaya Chemical Co ), Water Black 100-L (Orient Chemical Industπes), Water Black 187-L (Orient Chemical Industries), Water Black R-510 (Orient Chemical Industries), Bonjet® Black 814-L (Orient Chemical Industπes), Microjet-C® (Pigment Orient Chemical Industries) and FW18. Carbon Black Pigment (Degussa Corp )

Other colorants

Some pπntmg applications require colorants other than the typical cyan magenta yellow, and black of process color printing In some cases, spot coloi pπnung with specific colors is required In these cases, dyes or pigments with the lequired color may be substituted tor the abovementioned dyes after routine assessment ot the compatibility of the dye or pigment with the ink formulation pπnting mechanism, and printing substrate In some security printing and othei applications ultraviolet inks are required In these cases, dves and/or pigments ith absorption peaks in the ultraviolet part of the spectrum and with little absorption in the visible part ot the spectrum, may be used These dyes or pigments should also be assessed toi compatibility pigment with the ink formulation, pπnting mechanism, and pnnting substrate For the formulation ot magnetic inks, fine particles ot metal (such as iron) or metal oxides may be used in coniunction with dispeision techniques known to the art Diffusely reflective inks may be cieated bv the inclusion ot tine particles of oxides such as titania silica and alumina 'Metallic inks with high specular reflection, may be created by the inclusion ot tine particle size prepaiations ot metals or alloys such as aluminium, copper, chromium, and steel. These oxides and metals should also be used in conjunction with a suitable dispersant.

Other ink components

Biocides may be included in the ink to inhibit the growth of microorganisms. This is especially relevant if the ink is to be stored for long periods before use. Examples of biocides which may be included in the ink are sodium benzoate, Troysans® (Troy Chemical Corp.), Nuosept® (Huls America Inc.), Dowicides® (Dow Chemical), thiazolone compound, and Nopcocides® (Henkel Coφ.). Other known additives, such as sequestering agents, cosolvents, viscosity modifiers, humectants, and polymers may be added to the ink to improve the properties of the ink for specific applications. In all cases, the final ink composition should be tested to ensure that the additives have not adversely affected the surface tension properties desirable for ink drop ejection.

Surfactant commercial availability Commercial preparations of this poly(oxyethylene)alkylphenyl ether are available under various brand names. Suppliers and brand names are listed in the following table:

These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per liter to prepared micioemulsion ink with a 57c surfactant concentration

Other suitable ethoxylated alkyl phenols include those listed in the following table

Othei advantages ot microemulsion based inks

Micioemulsion based inks have advantages other than surface tension control

1 ) Micioemulsions aie thermodynamically stable and will not sepaiate Theietoie. the stoi age time can be veiy long This is especially significant toi office and portable pπnters, which may be used spoiadicalh

2) The micioemulsion will form sponuneously with a paiticulai diop size, and does not iequire extensive stirπng, centπtuging, or filtering to ensuie a particulai range ot emulsified oil drop sizes 3) The amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors

4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets

5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium

6) The viscosity of microemulsions is very low compared to emulsions

7) The requirement for humectants can be reduced or eliminated

Dves and pigments in microemulsion based inks

Oil in water mixtures can have high oil contents - as high as 409c - and still form O W microemulsions This allows a high dye or pigment loading

Mixtures of dyes and pigments can be used An example of a microemulsion based ink mixture with both dye and pigment is as follows

1 ) 707c water

2) 57o watei soluble dye

3) 57c surfactant

5) 107c oil miscible pigment

The following table shows the nine basic combinations ot colorants in the oil and water phases ot the microemulsion that may be used

The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highlights.

As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil- water boundary layer as this layer has a very large surface area.

It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments in each phase.

When using multiple dyes or pigments the absoφtion spectrum of the resultant ink will be the weighted average of the absoφtion spectra of the different colorants used. This presents two problems:

1 ) The absoφtion spectrum will tend to become broader, as the absoφtion peaks of both colorants are averaged. This has a tendency to 'muddy' the colors. To obtain brilliant color, careful choice of dyes and pigments based on their absoφtion spectra, not just their human-perceptible color, needs to be made. 2) The color of the ink may be different on different substrates. If a dye and a pigment are used in combination, the color of the dye will tend to have a smaller contribution to the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, while the pigment will tend to 'sit on top^" of the paper. This may be used as an advantage in some circumstances.

Inks with Surfactant Sols

Inks with a large reduction in surface tension with increasing temperature can also be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as:

As the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature. A good example is Arachidic acid.

These carboxylic acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding them to the ink is insignificant. A mixture of carboxylic acids with slightly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid.

It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant. It is desirable that the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxylic acids, this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide.

Preparation of Inks with Surfactant Sols

The surfactant sol can be prepared separately at high concentration. and added to the ink in the required concentration.

An example piocess for creating the surfactant sol is as follows:

1 ) Add the carboxylic acid to purified water in an oxygen free atmosphere.

2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil, however, a temperature of 95°C is suitable. 3) Ultrasonicate the mixture to reduce particle size.

4) Allow the mixture to cool.

5) Decant the larger particles from the top of the mixture.

6) Add an alkali such as NaOH to ionize the carboxylic acid molecules on the surface of the particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps prevent the sol from flocculating.

7) Centπtuge the sol. As the density of the carboxylic acid is lower than water, smaller particles will accumulate at the outside of the centrifuge, and larger particles in the centre.

8) Filter the sol using a microporous filter to eliminate any particles above 5000 A

9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration.

The ink preparation will also contain either dye(s) or pιgment(s). bactericidal agents, agents to enhance the electπcal conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required.

Anti-foaming agents will generally not be required, as there is no bubble tormation duπng the drop ejection process

Results tiom Initial Preparation of Carboxylic Acids Sols ot steaπc acid and palmitic acid were made using ultrasonication. The surface tension of these sols was measured using a platinum πng surface tensiometer.

0 77c u/vι wl of Palmitic Acid (m.p = 63.0°C)

0.1 % w/w sol of Stearic Acid (m.p. = 71.5°C)

The surface tension ot steanc acid was lower than expected at low temperatures. It is likely that this was due to contamination of the sample or the experimental equipment, as this problem did not occur with palmitic, arachidic oi behenic acids, which have melting points both above and below that of stearic acid

Cationic surfactant sols

Inks made with aniomc surfactant sols are generally unsuitable toi use with cationic dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant. To allow the use ot cauonic dyes and pigments, a canonic surfactant sol is required The family of alkylamines is suitable for this puφose

Various suitable alkylamines are shown in the following table

The method of preparauon of cauonic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an alkali is used to adjust the pH balance and mcrease the charge on the surfactant parucles A pH of 6 using HCl is suitable

An example process for creaung cauonic surfactant sols is as follows

1 ) Add the alkylamine to purified water in an oxygen free atmosphere

2) Heat the mixture to above the melung point of the alkylamine The water can be brought to a boil, however, a temperature of 95 °C is suitable

3) Ultrasomcate the mixture to reduce particle size 4) Allow the mixture to cool

5) Decant the larger particles from the top of the mixture

6) Add an acid such as HCl to ionize the alkylamine molecules on the surface ot the particles A pH of approximately 6 is suitable This step is not absolutely necessary, but helps prevent the sol from flocculaung 7) Centπfuge the sol As the density of the alkylamine is lower than water smaller parucles will accumulate at the outside of the centrifuge, and larger particles in the centre 8) Filter the sol using a microporous filter to eliminate any parucles abov e 5000

A 9) Add the surfactant sol to the ink preparation The sol is required only in v ery dilute concentration The ink preparation will also contain either dye(s) or pιgment(s), bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required

Surfactants with a Krafft point in the drop selection temperature ranpe. For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the solution contains essentially no micelles.

Above the Krafft temperature micelle formation becomes possible and there is a rapid increase in solubility of the surfactant. If the critical micelle concentration

(CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usually much less effecuve below the

Krafft point.

This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures, only a portion of the surfactant is in solution. When the nozzle heater is turned on, the temperature nses, and more ot the surfactant goes into solution, decreasing the surface tension

A surfactant should be chosen with a Krafft point which is near the top ot the range ot temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature

The concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatuies

The following table shows some commercially available surfactants with Krafft points in the desired range.

Surfactants with a cloud point in the drop selection temperature range

Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature.

At low temperatures, the POE chain is hydrophihc, and maintains the surfactant in solution. As the temperature increases, the structured water around the POE section ot the molecule is disrupted, and the POE section becomes hydrophobic.

The surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowenng surface tension The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point ot that surfactant. POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C

Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point of POE chains without introducing a strong hydi ophobicity at low temperatures.

Two main configurations of symmetπcal POE/POP block copolymeis are available. These are:

1 ) Surfactants with POE segments at the ends ot the molecules, and a POP segment in the centre, such as the poloxamer class ot surfactants (geneπcally CAS 9003-1 1 -6)

2) Surfactants with POP segments at the ends ot the molecules, and a POE segment in the centre, such as the meroxapol class ot surfactants (geneπcally also CAS 9003-1 1-6) Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40°C and below 100°C are shown in the following table:

Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 4()°C and HXXC. and preferably between 60°C and 80°C.

Meroxapol [HO(CHCH₁CH2θ)_x(CH2CH_:0)_v(CHCH,CH2θ)_zOH] vaneties where the average x and z are approximately 4. and the average y is approximately 15 may be suitable. If salts are used to increase the electncal conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I^"), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl^', OH^"), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively little effect. The ink composition can be 'tuned' for a desired temperature range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl^" to Br^" to I^") that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl slightly lowers the cloud point of nonionic surfactants.

Hot Melt Inks The ink need not be in a liquid state at room temperature. Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately 2 mN/m will be typical of many such preparations using waxes and other substances However, a reduction in surface tension ot approximately 20 mN/m is desnable in order to achieve good operating margins when relying on a reducuon in surface tension rather than a reduction in viscosity

The temperature difference between quiescent temperature and diop selection temperature may be greater for a hot melt ink than foi a water based ink. as water based inks are constrained by the boiling point of the water

The ink must be liquid at the quiescent temperature. The quiescent temperatuie should be higher than the highest ambient tempeiatuie likely to be encountered by the pnnted page. T he quiescent temperature should also be as low as piactical. to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop election tempeiatures. A quiescent temperature between 60°C and 90°C is generally suitable, though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generally suitable.

There are several methods of achieving an enhanced reduction in surface tension with increasing temperature.

1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the liquid phase. 2) A polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the polar and non-polar compounds.

To achieve a large reduction in surface tension with temperature, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes such as waxes. Suitable materials will generally have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88°C.

Surface tension reduction of various solutions

Figure 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations containing the following additives:

1 ) 0.19c sol of Stearic Acid

2) 0.1 c sol of Palmitic acid

3) 0.1 solution of Pluronic® 10R5 (trade mark of BASF)

4) 0.17c solution of Pluronic® L35 (trade mark of BASF) 5) 0. 1 c solution of Pluronic® L44 (trade mark of BASF)

Commercial preparations of this surfactant are available under vanous brand names. Suppliers and brand names are listed in the following table:

These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per liter to prepared microemulsion ink with a 57c surfactant concentration.

Other suitable ethoxylated alkyl phenols include those listed in the following table:

Microemulsion based inks have advantages other than surface tension control

1) Microemulsions are thermodynamically stable, and will not separate Therefore, the storage time can be very long This is especially significant for office and portable pπnters, which may be used sporadically

2) The microemulsion will form spontaneously with a particular drop size, and does not require extensive stirring, centnfuging, or filtering to ensure a particular range ot emulsified oil drop sizes

3) The amount ot oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used It is also possible to use a mixture ot dyes, one soluble in water, and the other soluble in oil, to obtain specific colors

4) Oil miscible pigments are prevented from flocculatmg, as they are trapped in the oil miciodroplets

5) The use ot a micioemulsion can reduce the mixing ot different dve colors on the surface ot the pnnt medium

6) The viscosity ot micioemulsions is vei y low

7) The requirement tor humectants can be reduced or eliminated

Dyes and pigments in microemulsion based inks

Oil in water mixtures can have high oil contents - as high as 407c and still form O/W microemulsions This allows a high dye or pigment loading

Mixtuies ot dyes and pigments can be used An example ot a mici oemulsion based ink mixture with both dye and pigment is as follows l ) 707c water

2) 57c water soluble dye

3) 57c surfactant 5) 107o oil miscible pigment

The following table shows the nine basic combinations of colorants in the oil and water phases ot the microemulsion that may be used

The ninth combination, with no colorants, is useful for pπnting transparent coatings. UV ink, and selective gloss highlights

As many dyes are amphiphilic, large quantities ot dyes can also be solubilized in the oil-water boundary layer as this layer has a very large surface area

It is also possible to have multiple dyes oi pigments in each phase and to nave a mixtuie ot dyes and pigments in each phase

When using multiple dyes or pigments the absoφtion spectium ot the lesultant ink will be the weighted average ot the absoφtion spectia of the difteient colorants used This presents two problems

1 ) The absorption spectium will tend to become broadei as the absoiption peaks ot both colorants are averaged This has a tendency to muddy' the colors To obtain bnlliant coloi, careful choice ot dves and pigments based on then absorption spectia. not just then human-perceptible coloi needs to be made

2) The color ot the ink may be different on different substrates It a dye and a pigment aie used in combination, the color ot the dye will tend to have a smaller contπbuuon to the printed ink color on more absoφtive papers, as the dye will be absorbed into the paper, while the pigment will tend to 'sit on top' ot the paper This may be used as an advantage in some circumstances

Surfactants with a Krafft point in the drop selection temperature range For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the solution contains essentially no micelles Above the Krafft temperature micelle formation becomes possible and there is a rapid increase in solubility of the surfactant If the cntical micelle concentration (CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, lather than at the CMC Surfactants are usually much less effecuve below the Kiattt point

This factor can be used to achieve an increased reduction in surface tension with increasing temperature At ambient temperatures, only a portion ot the surfactant is in solution When the nozzle heater is turned on, the temperature πses, and moie ot the surfactant goes into solution, decreasing the surface tension

A surfactant should be chosen with a Krafft point which is near the top ot the lange ot tempeiatures to which the ink is raised This gives a maximum mai m between the concentration ot surfactant in solution at ambient tempeiatures. and the concentiauon of surfactant in solution at the drop selection tempeiature

The concentiauon ot surfactant should be approximately equal to the CMC at the Kratft point In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient tempeiatures

The following table shows some commercially available surfactants with Kiattt points in the desπed range

Surfactants with a cloud point in the drop selection temperature range

At low temperatures, the POE chain is hydrophilic, and maintains the surfactant in solution. As the temperature increases, the structured water around the POE section of the molecule is disrupted, and the POE section becomes hydrophobic.

The surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant. POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C

Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures.

Two main configurations of symmetrical POE/POP block copolymers are available. These are:

1 ) Surfactants with POE segments at the ends of the molecules, and a POP segment in the centre, such as the poloxamer class of surfactants (genetically CAS 9003-1 1-6)

2) Surfactants with POP segments at the ends of the molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (generically also CAS 9003-11-6) Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud pomt above 40°C and below 100°C are shown in the following table:

Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40°C and 100°C, and preferably between 60°C and 80°C.

Meroxapol [HO(CHCH₃CH₂O)_x(CH2CH₂O),(CHCH,CH2O)_zOH] vaneties where the average x and z are approximately 4. and the average y is approximately 15 may be suitable. If salts are used to increase the electncal conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I^"), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl^", OH^"), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively little effect. The ink composition can be 'tuned' for a desired temperature range by altenng the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl^" to Br^" to I^") that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity NaCl slightly lowers the cloud point ot nonionic surfactants.

Hot Melt Inks The ink need not be in a liquid state at room temperature. Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point ot the ink. The hot melt ink must be formulated so that the surface tension ot the molten ink decreases with temperature. A decrease of approximately 2 mN/m will be typical of many such preparations using waxes and other substances However, a reduction in surface tension of approximately 20 mN/m is desirable in order to achieve good operating margins when relying on a i eduction in surface tension rather than a reduction in viscosity.

The temperature difference between quiescent temperature and drop selection temperature may be greater for a hot melt ink than tor a water based ink. as water based inks are constrained by the boiling point of the water

The ink must be liquid at the quiescent tempei ture The quiescent temperatuie should be higher than the highest ambient temperature likely to be encountered by the pπnted page. T he quiescent temperatuie should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures. A quiescent temperature between 60°C and 90°C is generally suitable, though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generally suitable.

1 ) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the liquid phase. 2) A polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the polar and non-polar compounds.

To achieve a large reduction in surface tension with temperature, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes such as waxes. Suitable materials will generally have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols. such as Hexanetetrol. which has a melting point of 88°C.

Surface tension reduction of various solutions

1 ) 0.1 c sol of Stearic Acid

2) 0.1 c sol of Palmitic acid

3) 0.17c solution of Pluronic 10R5 (trade mark of BASF)

4) 0.1 c solution of Pluronic L35 (trade mark of BASF) 5) 0.1 c solution of Pluronic L44 (trade mark of BASF)

Inks suitable for pπnting systems of the present invention are described in the following Australian patent specifications, the disclosure of which are hereby incorporated by reference:

'Ink composition based on a microemulsion' (Filing no.: PN5223. filed on 6 September 1995); Tnk composition containing surfactant sol' (Filing no ^• PN5224, filed on

6 September 1995), 'Ink composition for DOD printers with Krafft point near the drop selection temperature sol' (Filing no.: PN6240, filed on 30 October 1995); and 'Dye and pigment in a microemulsion based ink' (Filing no PN6241, filed on

30 October 1995)

Operation Using Reduction of Viscosity

As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as follows Pπor to operation of the printer, solid ink is melted in the reservoir 64 The reservoir, ink passage to the print head, ink channels 75, and pnnt head 50 are maintained at a temperature at which the ink 100 is liquid, but exhibits a lelatively high viscosity (for example, approximately 100 cP) The Ink 100 is retained in the nozzle by the surface tension of the ink The ink 100 is formulated so that the viscosity of the ink reduces with increasing temperature The ink pressuie oscillates at a frequency which is an integral multiple of the drop ejection tiequency tiom the nozzle The ink pressure oscillation causes oscillations ot the ink meniscus at the nozzle tips, but this oscillation is small due to the high ink viscosity At the normal operaung temperature, these oscillations aie of insufficient amplitude to lesult in drop separation When the heatei 103 is eneigized. the ink forming the selected drop is heated, causing a i eduction in viscosity to a value which is preferably less than 5 cP The reduced viscosity lesults in the ink meniscus moving further dunng the high pressuie part ot the ink pressuie cycle The lecording medium 51 is arranged sufficiently close to the pnnt head 50 so that the selected diops contact the recording medium 51 but sufficiently tai away that the unselected drops do not contact the iecoiding medium 51 Upon contact w ith the iecoiding medium 51 , part ot the selected diop tieezes. and attaches to the recording medium As the ink piessuie falls, ink begins to mo e back into the nozzle The body of ink separates from the ink which is tiozen onto the iecoiding medium The meniscus of the ink 100 at the nozzle tip then returns to low amplitude oscillation. The viscosity of the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head. One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved.

Manufacturing of Print Heads

Manufacturing processes for monolithic print heads in accordance with the present invention are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference:

'A monolithic LIFT printing head' (Filing no.: PN2301);

'A manufacturing process for monolithic LIFT printing heads' (Filing no.:

PN2302); 'A self-aligned heater design for LIFT print heads' (Filing no.: PN2303); 'Integrated four color LIFT print heads' (Filing no.: PN2304);

'Power requirement reduction in monolithic LIFT printing heads' (Filing no.:

PN2305); 'A manufacturing process for monolithic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monolithic drop-on-demand print heads' (Filing no.:

PN2307); 'Heater structure for monolithic LIFT print heads' (Filing no.: PN2346): 'Power supply connection for monolithic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (Filing no.: PN2348); and 'A self-aligned manufacturing process for monolithic LIFT print heads'

(Filing no.: PN2349); and 'CMOS process compatible fabrication of LIFT print heads' (Filing no.: PN5222, 6 September 1995). 'A manutactuπng process for LIFT print heads with nozzle nm heaters' (Filing no.: PN6238, 30 October 1995); 'A modular LIFT print head' (Filing no.: PN6237, 30 October 1995); 'Method of increasing packing density of pπnung nozzles' (Filing no.: PN6236, 30 October 1995); and

'Nozzle dispersion for reduced electrostatic interaction between simultaneously pnnted droplets' (Filing no.: PN6239, 30 October 1995).

Control of Print Heads

Means of providing page image data and controlling heater temperature in prmt heads of the present invention is described in the following Australian patent specifications filed on 12 Apnl 1995, the disclosure of which are hereby incoφorated by reterence:

'Integrated drive circuitry in LIFT pnnt heads' (Filing no.: PN2295). 'A nozzle clearing procedure for Liquid Ink Fault Tolerant (LIFT) printing' (Filing no.: PN2294);

'Heater power compensation for temperature in LIFT printing systems'

(Filing no.: PN2314); 'Heater power compensation for thermal lag in LIFT printing systems' (Filing no.. PN2315); 'Heatei power compensation for pnnt density in LIFT printing systems'

(Filing no.: PN2316), 'Accurate control ot temperature pulses in pπnting heads' (Filing no

PN2317); 'Data distribution in monolithic LIFT print heads' (Filing no : PN2318), 'Page image and fault tolerance routing dev ice toi LIFT pπnting sy stems'

(Filing no.: PN2319); and 'A removable pressurized liquid ink cartridge toi LIFT pπnteis^' (Filing no.: PN2320). Image Processing for Print Heads

An objective of printing systems according to the invention is to attain a print quality which is equal to that which people are accustomed to in quality color publications printed using offset printing. This can be achieved using a print resolution of approximately 1,600 dpi. However, 1 ,600 dpi printing is difficult and expensive to achieve. Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and black. This color model is herein called CC'MM'YK. Where high quality monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein called CC'MM'YKK'. Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and other printing systems are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Four level ink set for bi- level color printing' (Filing no.: PN2339);

'Compression system for page images' (Filing no.: PN2340); 'Real-time expansion apparatus for compressed page images' (Filing no.:

PN2341 ); and 'High capacity compressed document image storage for digital color printers' (Filing no.: PN2342);

'Improving JPEG compression in the presence of text' (Filing no.: PN2343); 'An expansion and halftoning device for compressed page images' (Filing no.:

PN2344); and 'Improvements in image halftoning' (Filing no.: PN2345).

Applications Using Print Heads According to this Invention

Pπnting apparatus and methods of this invention are suitable for a wide range of applications, including (but not limited to) the following: color and monochrome office pnnting. short run digital printing, high speed digital printing, process color printing, spot color printing, offset press supplemental printing, low cost pπnters using scanning print heads, high speed pπnters using pagewidth pnnt heads, portable color and monochrome pπnters, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copying machines, label printing, large format plotters, photographic duplication, pπnters for digital photographic processing, portable printers incoφorated into digital 'mstant' cameras, video prmtmg, pnnting of PhotoCD images, portable pπnters for 'Personal Digital Assistants', wallpaper pnnting, indoor sign pnntmg, billboard pnnting, and fabnc pnntmg

Prmtmg systems based on this invention are descnbed in the following Australian patent specifications filed on 12 Apπl 1995, the disclosure ot which are hereby incoφorated by reference

'A high speed color office pnnter with a high capacity digital page image store' (Filing no PN2329), 'A short run digital color printer with a high capacity digital page image stoie' (Filing no PN2330), 'A digital color printing press using LIFT printing technology' (Filing no

PN2331), 'A modular digital printing press' (Filing no PN2332), 'A high speed digital fabric printer' (Filing no PN2333), 'A color photograph copying system' (Filing no PN2334), 'A high speed color photocopier using a LIFT pπnting system' (Filing no

PN2335), 'A portable color photocopier using LIFT printing technology' (Filing no

PN2336), 'A photograph processing system using LIFT printing technology' (Filing no PN2337),

'A plain paper facsimile machine using a LIFT printing system' (Filing no

PN2338). 'A PhotoCD system with integrated pπntei ' (Filing no PN2293) 'A coloi plotter using LIFT printing technology' (Filing no PN2291 ), 'A notebook computer with integrated LIFT color printing system' (Filing no PN2292), 'A portable printer using a LIFT printing system' (Filing no.: PN2300); 'Fax machine with on-line database interrogation and customized magazine printing' (Filing no.: PN2299); 'Miniature portable color printer' (Filing no.: PN2298); 'A color video printer using a LIFT printing system' (Filing no.: PN2296); and 'An integrated printer, copier, scanner, and facsimile using a LIFT printing system' (Filing no.: PN2297)

Compensation of Print Heads for Environmental Conditions It is desirable that drop on demand printing systems have consistent and predictable ink drop size and position. Unwanted variation in ink drop size and position causes variations in the optical density of the resultant print, reducing the perceived print quality. These variations should be kept to a small proportion of the nominal ink drop volume and pixel spacing respectively. Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power applied to the nozzle heaters.

An optimum temperature profile for one print head embodiment involves an instantaneous raising of the active region of the nozzle tip to the ejection temperature, maintenance of this region at the ejection temperatuie for the duration of the pulse, and instantaneous cooling of the region to the ambient temperature.

This optimum is not achievable due to the stored heat capacities and thermal conductivities of the various materials used in the fabrication of the nozzles in accordance with the invention. However, improved performance can be achieved by shaping the power pulse using curves which can be derived by iterative refinement of finite element simulation of the pnnt head. The power applied to the heater can be varied in time by vanous techniques, including, but not limited to: 1 ) Varying the voltage applied to the heater 2) Modulating the width of a series of short pulses (PWM) 3) Modulating the frequency of a series of short pulses (PFM)

To obtain accurate results, a transient fluid dynamic simulation with tiee surface modeling is required, as convection in the ink, and ink flow, significantly affect on the temperature achieved with a specific power curve

By the incoφoraϋon of appropnate digital circuitry on the print head substrate, it is practical to individually control the power applied to each nozzle One way to achieve this is by 'broadcastmg' a vanety of different digital pulse trains across the pnnt head chip, and selecting the appropnate pulse tiain foi each nozzle using multiplexing circuits

An example of the environmental factors which may be compensated for is listed in the table "Compensation for environmental factors" This table identifies which environmental factors are best compensated globally (for the entire pnnt head), per chip (for each chip in a composite multi-chip print head), and per nozzle

Compen sation for environmental factors

Most applications will not require compensation for all of these variables. Some variables have a minor effect, and compensation is only necessary where very high image quality is required.

Print head drive circuits

Figure 4 is a block schematic diagram showing electronic operation of an example head driver circuit in accordance with this invention. This control circuit uses analog modulation of the power supply voltage applied to the print head to achieve heater power modulation, and does not have individual control of the power applied to each nozzle. Figure 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using the CC'MM'YK color model. The print head 50 has a total of 79.488 nozzles, with 39.744 main nozzles and 39.744 redundant nozzles. The main and redundant nozzles are divided into six colors, and each color is divided into 8 drive phases. Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel data for enabling heater drive circuits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each shift register is composed of 828 shift register stages 217, the outputs of which are logically anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200. which may be a heater 103 as shown in figure 1(b). To maintain the shifted data valid during the enable pulse, the clock to the shift register is stopped the enable pulse is active by a clock stopper 218. which is shown as a single gate for clarity, but is preferably any of a range of well known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for a parallel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to either the main nozzles or the redundant nozzles by the data router 219 depending on the state of the appropriate signal of the fault status bus. The print head shown in figure 4 is simplified, and does not show- various means of improving manufacturing yield, such as block fault tolerance.

Drive circuits for different configurations of print head can readily be derived from the apparatus disclosed herein.

Digital information representing patterns of dots to be pπnted on the recording medium is stored in the Page or Band memory 151 . which may be the same as the Image memory 72 in figure 1(a). Data in 32 bit words representing dots of one color is read from the Page or Band memory 1513 using addresses selected by the address mux 417 and control signals generated by the Memory Interface 418. These addresses are generated by Address generators 41 1. which forms part of the 'Per color circuits' 410, for which there is one for each of the six color components. The addresses are generated based on the positions of the nozzles in relation to the print medium. As the relative position of the nozzles may be different for different print heads, the Address generators 41 1 are preferably made programmable. The Address generators 41 1 normally generate the address corresponding to the position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so that the Address generators 411 generate the address corresponding to the position of the redundant nozzles. Data read from the Page or Band memory 1513 is latched by the latch 413 and converted to four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other colors by the FIFO 415. This data is then buffered by the buffer 430 to form the 48 bit main data bus to the print head 50. The data is buffered as the print head may be located a relatively long distance from the head control ASIC. Data from the Fault Map RAM 412 also forms the input to the FIFO 416. The timing of this data is matched to the data output of the FIFO 415, and buffered by the buffer 431 to form the fault status bus.

The programmable power supply 320 provides power for the head 50. The voltage of the power supply 320 is controlled by the DAC 313. which is part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains a dual port RAM 317. The contents of the dual port RAM 317 are programmed by the Microcontroller 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroller 315 based on temperature sensed by a thermal sensor 300. The theimal sensor 300 signal connects to the Analog to Digital Converter (ADC) 31 1. The ADC 311 is preferably incoφorated in the Microcontroller 315.

The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time-varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage. An analog time varying programming voltage is produced by the DAC 313 based upon data read from the dual port RAM 317. The data is read according to an address produced by the counter 403. The counter 403 produces one complete cycle of addresses^' during the period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408. and the top count of the counter 403 is used to clock the enable counter 404. The count from the enable counter

404 is then decoded by the decoder 405 and buffered by the buffer 432 to produce the enable pulses for the head 50. The counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To allow non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used.

For pnnt density compensation, the pnnting density is detected by counting the number of pixels to which a drop is to be pπnted Con^' pixels) in each enable period. The 'on' pixels are counted by the On pixel counters 402. There is one On pixel counter 402 for each of the eight enable phases. The number of enable phases in a pnnt head in accordance with the invention depend upon the specific design Four, eight, and sixteen are convenient numbers, though there is no requπement that the number ot enable phases is a power ot two. The On Pixel Counters 402 can be composed of combinatoπal logic pixel countei s 420 which determine how many bits in a nibble of data aie on This number is then accumulated by the adder 421 and accumulatoi 422 A latch 423 holds the accumulated value valid tor the duration of the enable pulse The multiplexer 4 1 selects the output of the latch 423 which corresponds to the current enable phase. as determined by the enable counter 404. The output ot the multiplexer 401 forms part of the address ot the dual port RAM 317. An exact count ot the number ot 'on^' pixels is not necessaiy. and the most significant toui bits ot this count are adequate

Combining the four bits of theimal lag compensation address and the tour bits ot pnnt density compensation addiess means that the dual port RAM 317 has an 8 bit address. This means that the dual port RAM 317 contains 256 numbers, which are in a two dimensional array These two dimensions aie time (toi thermal lag compensation) and pnnt density A third dimension - temperature - can be included As the ambient temperature of the head vanes only slowly, the microconti oiler 315 has sufficient time to calculate a matnx of 256 numbers compensating for thermal lag and pnnt density at the current temperature Periodically (tor example, a few times a second), the microcontroller senses the current head temperature and calculates this matnx

The clock to the print head 50 is generated from the system clock 408 by the Head clock generator 407, and buffered by the buffer 406 To facilitate testing of the Head control ASIC, JTAG test circuits 499 may be included The clock to the LIFT pnnt head 50 is generated from the system clock 408 by the Head clock generator 407, and buffered by the bufter 406 To facilitate testing ot the Head contiol ASIC, JTAG test circuits 499 may be mcluded

Compaπson with thermal ink let technology The table "Compaπson between Thermal ink jet and Present

Invention' compaies the aspects ot printing in accordance with the piesent invention w ith theimal ink ]et pπnting technology

A dnect compaπson is made between the present invention and theimal ink ]et technology because both aie drop on demand systems which opeiate using theimal actuatois and liquid ink Although they may appear similai the tw o technologies operate on different principles

Theimal ink jet pπnters use the following fundamental operating pπnciple A. theimal impulse caused by electncal resistance heating lesults in the explosive toimation ot a bubble in liquid ink Rapid and consistent bubble toimation can be achieved by superheating the ink, so that sufficient heat is tiansteπed to the ink betoie bubble nucleation is complete Foi atei based ink, ink tempeiatuies ot approximately 280°C to 400°C are required The bubble toimation causes a piessuie wave which forces a diop ot ink tiom the apeituie with high elocitv The bubble men collapses, drawing ink from the ink reservoπ to le-fill the nozzle Theimal ink jet pnnting has been highly successful commercially due to the high nozzle packing density and the use ot well established integrated circuit manutactuπng techniques However, thermal ink jet prinung technology faces significant technical problems including multi-part precision fabπcation, device yield, image resolution, 'pepper' noise, printing speed, dπve transistor power, waste power dissipation, satellite drop formation, thermal stress, differenual thermal expansion, kogauon, cavitation, rectified diffusion, and difficulties in ink formulation

Pπnting in accordance with the present invention has many of the advantages ot thermal ink jet prmtmg, and completely or substantially elunmates many ot the inherent problems of thermal ink jet technology

Yield and Fault Tolerance

In most cases, monolithic integrated circuits cannot be repaired if they are not completely functional when manufactured. The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has a direct influence on manufacturing cost. A device with a yield of 57c is effectively ten times more expensive to manufacture than an identical device with a yield of 507c.

There are three major yield measurements:

1 ) Fab yield

2) Wafer sort yield

3) Final test yield For large die, it is typically the wafer sort yield which is the most serious limitation on total yield. Full pagewidth color heads in accordance with this invention are very large in comparison with typical VLSI circuits. Good wafer sort yield is critical to the cost-effective manufacture of such heads. Figure 5 is a graph of wafer sort yield versus defect density for a monolithic full width color A4 head embodiment of the invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according to Muφhy's method, which is a widely used yield prediction method. With a defect density of one defect per square cm, Murphy's method predicts a yield less than 1 c. This means that more than 997c of heads fabricated would have to be discarded. This low yield is highly undesirable, as the print head manufacturing cost becomes unacceptably high.

Muφhy's method approximates the effect of an uneven distribution of defects. Figure 5 also includes a graph of non fault tolerant yield 197 which explicitly models the clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacturing process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Muφhy's method. A solution to the problem of low yield is to incorporate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units.

In memoiy chips and most Wafer Scale Integration (WSI) devices, the physical location of redundant sub-units on the chip is not important. However. in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction.

To allow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 1007o redundancy would normally more than double the chip area, dramatically reducing the primary yield before substituting redundant units, and thus eliminating most of the advantages of fault tolerance.

However, with print head embodiments according to this invention, the minimum physical dimensions of the head chip are determined by the width of the page being printed, the fragility of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for a full width, full color head for printing A4 size paper is approximately 215 mm x 5 mm. This size allows the inclusion of 1007c redundancy without significantly increasing chip area, when using 1.5 μm CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without significantly decreasing primary yield.

When fault tolerance is included in a device, standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and included in the yield equation. Figure 5 shows the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 19 to more than 907c under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100.

Fault tolerance is highly recommended to improve yield and reliability of print heads containing thousands of printing nozzles, and thereby make pagewidth printing heads practical. However, fault tolerance is not to be taken as an essential part of the present invention. Fault tolerance in drop-on-demand printing systems is described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference:

'Integrated fault tolerance in printing mechanisms' (Filing no.: PN2324); 'Block fault tolerance in integrated printing heads' (Filing no.: PN2325):

'Nozzle duplication for fault tolerance in integrated printing heads' (Filing no.: PN2326);

'Detection of faulty nozzles in printing heads' (Filing no.: PN2327); and 'Fault tolerance in high volume printing presses' (Filing no.: PN2328). When fault tolerance is included in a device, standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analysed and included in the yield equation. Figure 5 shows the fault tolerant son yield 199 for a full width color A4 LIFT head which includes various forms of fault tolerance, the modelling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 17c to more than 907c under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100. The acronym LIFT contains a reference to Fault Tolerance. Fault tolerance is highly recommended to improve yield and reliability of LIFT print heads containing thousands of pnnting nozzles, and thereby make pagewidth LIFT printing heads practical. However, fault tolerance is not to be taken as an essential part of the definition of LIFT printing for the purposes of this document. Fault tolerance in drop-on-demand printing systems is described in the follo ing Australian patent specifications filed on 12 Apπl 1995. the disclosure of which are hereby incoφorated by reference:

'Integrated fault tolerance in printing mechanisms' (Filing no.: PN2324. ret^': LIFT F01 ); 'Block fault tolerance in integrated pπnting heads^' (Filing no.: PN2325. ref: LIFT F02); 'Nozzle duplication for fault tolerance in integrated printing heads' (Filing no : PN2326, ref: LIFT F03);

'Detection of faulty nozzles in printing heads' (Filing no : PN2327, ref LIFT F04); and 'Fault tolerance in high volume LIFT printing presses' (Filing no..

PN2328, ref. LIFT F05).

Printing System Embodiments

A schematic diagram of a digital electronic pnnting system using a pnnt head of this invenuon is shown in Figure 6. This shows a monolithic pnntmg head 50 printing an image 60 composed of a multitude of ink drops onto a recording medium 51. This medium will typically be paper, but can also be overhead transparency film, cloth, or many other substantially flat surfaces which will accept ink drops. The image to be pπnted is provided by an image source 52. which may be any image type which can be converted into a two dimensional arrav ot pixels. Typical image sources are image scanners, digitally stored images, images encoded in a page description language (PDL) such as Adobe Postscπpt. Adobe Postscript level 2, or Hewlett-Packard PCL 5, page images generated by a proceduie-call based rasteπzer, such as Apple QuickDiaw. Apple Quickdraw GX. or Miciosott GDI, or text in an electionic form such as ASCII. This image data is then converted by an image processing system 53 into a two dimensional airay ot pixels suitable tor the particular pπnting system This may be color oi monochrome, and the data will typically have between 1 and 32 bits pei pixel, depending upon the image source and the specifications ot the printing system The image piocessing system may be a raster image piocessoi ( RIP) it the souice image is a page description, or may be a two dimensional image piocessing s stem it the soui e image is tiom a scanner

It conunuous tone images are lequired, then a halftoning system 54 is necessary Suitable types of halftoning are based on dispeised dot oideied dithei oi enoi diffusion Variations ot these, commonly known as stochastic sci eening oi frequence modulation sci eening aie suitable The halftoning system commonlv used for offset printing - clustered dot ordered dither - is not recommended, as effective image resolution is unnecessarily wasted using this technique. The output of the halftoning system is a binary monochrome or color image at the resolution of the printing system according to the present invention. The binary image is processed by a data phasing circuit 55 (which may be incoφorated in a Head Control ASIC 400 as shown in figure 4) which provides the pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and the movement of the paper. When the data has been loaded into the shift registers 56, it is presented in parallel to the heater driver circuits 57. At the correct time, the driver circuits 57 will electronically connect the corresponding heaters 58 with the voltage pulse generated by the pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, affecting the physical characteristics of the ink. Ink drops 60 escape from the nozzles in a pattern which corresponds to the digital impulses which have been applied to the heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by the pressure regulator 63. Selected drops of ink drops 60 are separated from the body of ink by the chosen drop separation means, and contact the recording medium 51. During printing, the recording medium 51 is continually moved relative to the print head 50 by the paper transport system 65. If the print head 50 is the full width of the print region of the recording medium 51, it is only necessary to move the recording medium 51 in one direction, and the print head 50 can remain fixed. If a smaller print head 50 is used, it is necessary to implement a raster scan system. This is typically achieved by scanning the print head 50 along the short dimension of the recording medium 51, while moving the recording medium 51 along its long dimension.

Computer simulation of nozzle dynamics

Details of the operation of print heads according to this invention have been extensively simulated by computer. Figures 8 to 18 are some results . from an example simulation of a preferred nozzle embodiment s operation using electrothermal drop selection by reduction in surface tension, combined with electrostatic drop separation

Computer simulation is extremely useful in determining the characteristics of phenomena which are difficult to observe directly Nozzle operation is difficult to observe expenmentally for several reasons, including

1) Useful nozzles are microscopic, with important phenomena occurnng at dimensions less than 1 μm

2) The time scale of a drop ejection is a few microseconds, requinng very high speed observations 3) Important phenomena occur mside opaque solid mateπals, making direct observation impossible

4) Some important parameters, such as heat flow and fluid velocity vector fields aie difficult to directly observe on any scale

5) The cost of fabrication of expenmental nozzles is high Computer simulation overcomes the above problems A leading software package for fluid dynamics simulation is FIDAP, produced by Fluid Dynamics International Inc of Illinois, USA (FDI) FIDAP is a registered trademaik of FDI Other simulation programs are commercially available, but FIDAP was chosen tor its high accuracy in transient fluid dynamic, energy transport, and surface tension calculations The version ot FIDAP used is FIDAP 7 51

The simulations combine energy transport and fluid dynamic aspects Axi-symmetπc simulation is used, as the example nozzle is cvlindncal in form Theie are tour deviations from cyhndncal form These are the connections to the heater, the laminar air flow caused by paper movement gravity (it the piinthead is not veitical). and the presence ot adjacent nozzles in the substiate The effect ot these tactois on diop ejection is minor

To obtain convergence tor transient tree surface simulations with vaπable surface tension at micrometer scales with microsecond tiansients using FIDAP 7 51. it is necessary to nondimensionahze the simulation Only the region in the tip of the nozzle is simulated, as most phenomena relevant to drop selection occur in this region The simulation is from the axis of symmetry of the nozzle out to a distance of 40 μm

A the beginning of the simulation, the entire nozzle and ink is at the device ambient temperature, which in this case is 30°C During operation, the device ambient temperature will be slightly higher than the air ambient temperature, as an equilibrium temperature based on pnnting density is reached over the penod ot many drop ejections Most of the energy of each drop selection is earned away with the ink drop The remaining heat in the nozzle becomes very evenly distributed between drop ejections, due to the high thermal conductivity of silicon, and due to convection in the ink

Geometry ot the simulated nozzle

Figure 7 shows the geometry and dimensions of the a preferred nozzle embodiment modeled in this simulation The nozzle is constructed on a single crystal silicon substrate 2020

The substrate has an epitaxial boron doped silicon layer 2018 which is used as an etch stop duπng nozzle fabncation An epitaxial silicon layei 2019 provides the active substiate tor the fabrication of CMOS drive transistois and data distribution cncuits On this substrate are several layers deposited CMOS processing These are a thermal oxide layer 2021, a first interlevel oxide lavei 2022, first le el metal 2023 second interlevel oxide layer 2024, second level metal 2025, and passivation oxide layei 2026 Subsequent processing ot the aters toims the nozzles and heateis These structures include the active heater 2027(a) an ESD shield formed from 'spate' heater matenal 2027(b). and a silicon nitπde passivation la ei 2028 The heater is atop a narrow 'nm etched fiom the vanous oxide layei s This is to leduce the 'thermal mass ot the material aiound the heater, and to pi event the ink tiom spreading across the surface ot the print head

The pnnt head is filled with electπcally conductive ink 20 1 An electπc field is applied to the print head, using an electrode which is in electπcal contact with the ink and another electrode which is behind the iecording medium The nozzle radius is 8 μm, and the diagram is to scale.

Theoretical basis of calculations

The theoretical basis for fluid dynamic and energy transport calculations using the Finite Element Method, and the manner that this theoretical basis is applied to the FIDAP computer program, is described in detail in the

FIDAP 7.0 Theory Manual (April 1993) published by FDI, the disclosure of which is hereby incoφorated by reference.

Material characteristics

The table "Properties of materials used for FIDAP simulation" gives approximate physical properties of materials which may be used in the fabrication of the print head in accordance with this invention.

The properties of 'ink' used in this simulation are that of a water based ink with 25% pigment loading. The ink contains a suspension of fine particles of palmitic acid (hexadecanoic acid) to achieve a pronounced reduction in surface tension with temperature. The surface tensions were measured at various temperatures using a surface tensiometer.

The values which have been used in the example simulation using the FIDAP program are shown in the table "Properties of materials used for FIDAP simulation". Most values are from direct measurement, or from the CRC Handbook of Chemistry and Physics. 72nd edition, or Lange's handbook of chemistry. 14th edition.

Properties of materials used for FIDAP simulation

Fluid dynamic simulations

Figure 8(a) shows the power applied to the heater. The maximum power applied to the heater is 40 mW. This power is pulse frequency modulated to obtain a desirable temporal distribution of power to the heater The power pulses are each of a duration of 0.1 μs. each dehveπng 4 nJ of energy to the heater. The drop selection pulse is started 10 μs into the simulation, to allow the meniscus to settle to its quiescent position The total energy delivered to the heater during the drop selection pulse is 276 nJ

Figure 8(b) shows the temperature at various points in the nozzle dunng the simulation

Point A is at the contact point of the ink meniscus and the nozzle nm. For optimal operation, it is desirable that this point be raised as close as possible to the boiling point of the ink, without exceeding the boiling point, and maintained at this temperature for the duration ot the drop selection pulse The 'spiky' temperature curve is due to the pulse frequency modulation ot the powei applied to the heater. This 'spikiness^' can be reduced by increasing the pulse frequency, and proportionally reducing the pulse energy.

Point B is a point on the ink meniscus, approximately midway between the centre ot the meniscus and the nozzle tip. Point C is a point on the surface of the silicon, 20 μm from the centre of the nozzle. This shows that the temperature nse when a drop is selected is very small a short distance away from the nozzle This allows active devices, such as drive transistors, to be placed very close to the nozzles Figure 9 shows the position versus time of a point at the centre ot the meniscus

Figure 10 shows the meniscus position and shape at vanous times dunng the drop selection pulse The tunes shown are at the start of the drop selection pulse, (10 μs mto the simulation), and at 5 μs intervals, until 60 μs attei the start ot the heater pulse

Figure 11 shows temperature contours in the nozzle just before the beginning ot the drop selection pulse, 9 μs into the simulation The surface tension balances the combined effect of the ink pressure and the external constant electnc field Figure 12 shows temperature contouis in the nozzle 5 μs aftei beginning ot the drop selection pulse. 15 μs into the simulation The reduction in surface tension at the nozzle tip causes the surface at this point to expand lapidlv cai lying the heat aiound the meniscus The ink has begun to move, as the surface tension is no longer high enough to balance the combined effect of the ink piessuie and the external constant electnc field The centie ot the meniscus begins to mov e tastei than the outside, due to viscous drag at the nozzle w alls In figures 12 to 1 ^" tempeiatute contouis are shown starting at 35°C and increasing in 5°C intervals

Figuie 13 shows temperature contours in the nozzle 10 μs attei beginning ot the drop selection pulse. 20 μs into the simulation Figure 14 sho s temperature contours in the nozzle 20 μs attei beginning ot the drop selection pulse. 30 μs into the simulation

Figuie 15 shows temperature contours in the nozzle 30 μs attei beginning ot the drop selection pulse. 40 μs into the simulation This is 6 μs attei the end ot the drop selection pulse, and the nozzle has begun to cool down Figure 16 shows temperature contours in the nozzle 40 μs after beginning of the drop selection pulse, 50 μs into the simulation If is clear from this simulation that the vast majority of the energy of the drop selection pulse is carried away with the selected drop Figure 17 shows temperature contours in the nozzle 50 μs after beginning of the drop selection pulse, 60 μs into the simulation At this time, the selected drop is beginnmg to 'neck', and the drop separation process is beginning

Figure 18 shows streamlines in the nozzle at the same time as figure 17 The approximate duration of three consecutive phases in the drop ejection cycle are

1 ) 24 μs heater energizing cycle

2) 60 μs to reach drop separation

3) 40 μs to return to the quiescent position The total ot these times is 124 μs, which results in a maximum diop repetition rate (diop tiequency) of approximately 8 Khz

Pioximitv Drop Separation

Drop separation of liquid ink by pnnt media proximity pieterably opeiates under the following conditions 1 ) The difference in meniscus positions between selected and unselected drops is greater than the surface roughness ot the print medium

2) The surface roughness of the pnnt medium is less than appioximately 307c of the ink drop diameter

3) The rate at which the volume of the ink diop inci eases due to wetting the pnnt medium surface and/oi soaking into a porous print medium is gi eater than the iate ot flow trom the ink nozzle under the applied ink piessuie

These conditions can be met over a wide iange ot nozzle iadn. ink types media and pnnt resolutions For hot melt pnnting, the molten ink drop freezes when in contact with the prmt medium, and the characteristics of ink absoφtion into the print medium are not as important.

The principle of operation ot proximity separation pnnting is shown in Figure 19(a) through Figure 19(i). In this case, the drop is selected by electrothermal transducers, which heat the ink at the nozzle tip, causing an increase in temperature at the meniscus. The increased temperature causes a reduction of surface tension below a critical surface tension, resulting in ink egress from the nozzle tip. In Figures 19(a) to 19(i) 1 is the selected drop, 10 is the nozzle from which the selected drop 1 was produced, 1 1 is a nozzle in which the heater 103 was not activated, and therefore no drop was selected, 5 is the direction of pnnt medium movement, 51 is the print medium, 100 is the body ot ink. 101 is silicon, 102 is silicon dioxide, 103 is the electrothermal actuator (also referred to as "heater"), and 109 is the pnnt head hydrophobic layer.

Figure 19(a) shows a cross section through two adjacent nozzles 10 and 1 1 in the quiescent state. The nozzles are in close proximity to the recording medium 51 which is moving relative to the nozzles in the direction 5. The cross section is at an angle ot 45 degrees to the direction ot media movement, through the plane ot the diagram. The nozzles 10 and 1 1 represent two staggered nozzles offset by one pixel width in the direction normal to the plane of the diagram All surfaces ot the nozzle have a hydrophobic surface layer 109. and the ink 100 is hydrophilic The ink is under pressure, resulting in the ink meniscus bulging Figure 19(b) shows the ink in the two nozzles shortly after an energizing pulse has been applied to the heater 103 ot nozzle 10, but not ot nozzle 1 1 The heat is conducted to the ink surface, where the resultant rise in temperatuie causes a local deciease in the surface tension ot the ink. The decrease in surface tension may be the result of the natural properties ot the ink. but is preferably enhanced by the inclusion ot an agent in the ink which causes a significant tall in surface tension at the temperature to which the ink is heated This agent may be a surfactant which is in the form ot a suspended solid particles at the quiescent temperature, but melts when the heaters are activated. When in solid form, the surfactant has little effect on surface tension. When molten, surfactant molecules rapidly migrate to the ink surface, causing a significant decrease in surface tension. In this case, the surfactant is 1-Hexadecanol, a 16 carbon alcohol with a melting point of 50°C.

Figure 19(c) shows the drop evolution a short time later. The selected drop 1 takes on a substantially cylindrical form due to a surface tension gradient from the nozzle tip to the centre of the meniscus, and due to viscous drag slowing ink movement near the walls of the nozzle. In this case, there are no external electrostatic or magnetic fields applied, and gravity is not significant on this scale.

Figure 19(d) shows the selected drop 1 at the instant that it contacts the recording medium 51. The "tilt" of the selected drop is due to the laminar air flow between the print head and the recording medium 51, caused by the movement of the recording medium. In many practical situations the recording medium will be paper, which will typically have a surface which is rough on the scale of the distance between the nozzle and the recording medium. This roughness will cause variation in the time of contact between the drop 1 and the recording medium 51. and therefore cause variations in the pπnted dot area. This variation can be minimized by using coated paper and/or passing the recording medium through compression rollers immediately pπor to printing.

Figure 19(e) shows the selected drop as it begins to "soak into" the recording medium 51.

Figure 19(f) shows the selected drop a short time later. The ink is absorbed by the recording medium at a rate approximately proportional to the gradient of the saturation. In many fibrous print media such as paper, the circle of contact between the print medium and the ink meniscus will not follow the lateral absoiption of the ink into the print medium. This is because the surface fibers do not become fully wetted. Ink flow into the print medium is highly dependent upon print medium composition. In many circumstances ink can be made to flow more quickly into the printing medium 51 by wetting the medium before printing. This may be achieved by using a series of rollers. The technology for continuously applying an even coat of liquid using rollers is well known in the offset printing industry. Most offset printing systems use damping rollers to apply a thin coating of fount solution, and inking rollers to apply a thin coating of ink, to the printing plates. Figure 19(g) shows the selected drop 1 immediately after it has separated from the body of ink 100. The ink will separate if the rate of ink flow into the porous recording medium 51 exceeds the flow rate of pressurized ink from the nozzle 10. This can be achieved for a wide range of inks, media, and nozzle radii. Non-porous media such as plastic or metal films can also be used. In this case, drop separation occurs when the rate of volume increase of a drop as it wets the non-porous medium exceeds the rate of ink flow from the nozzle 10. For some combinations of ink and non-porous media, the medium may need to be coated with an agent to promote wetting. Figure 19(h) shows the selected drop 1 after it has mostly soaked into the recording medium. Momentum of the ink returning to the nozzle carries the meniscus at the nozzle 10 past the quiescent position. The degree of this "overshoot" is very small compared to conventional thermal ink jet or piezoelectric ink jet systems. Figure 19(i) shows the nozzle 1 after the meniscus has returned to the quiescent position, and is ready for the next drop selection pulse. The selected drop 1 is shown fully absorbed into the pnnt medium 51. The rate of absorption is highly dependent upon the print medium, and the selected drop 1 may not be completely absorbed by the time a drop of a different color is printed at the same location. In some circumstances this may degrade print quality, in which case a more absorptive print medium can be used, a different ink composition can be used, a print head with greater separation between colors can be used, the pnnt medium can be heated to promote fast drying, or a combination of the above techniques can be used. Acoustic ink waves for proximity separation pnnting

Correctly applied acoustic waves in the ink ot proximity pnnting systems ot the invention can achieve several benefits

1 ) Drop growth can proceed faster when the penod ot maximum forward ink velocity caused by the acoustic wave coincides with the drop growth penod

2) The amount of ink delivered to the recording medium by a selected drop can be reduced when the drop separating time coincides with a period of reduced k pressure, as less ink will flow out of the nozzle, and the drop will separate earlier

3) The degree of variation in the amount of ink delivered to the recording medium will be reduced, as both the contact time and separating ϋme of the selected drop are influence by the acoustic wave which can be created with a highly accurate and stable tiequency and amplitude

4) The use ot pigments instead ot dyes is augmented, as the ink is constantly agitated by the acoustic waves, reducing one ot the major problems of pigments, which is pigment settling in the ink

5) Blocking ot nozzles with dned ink is reduced, as the constant motion of the ink meniscus stirs the ink in the vicinity ot the meniscus, replacing drying ink with

' tiesh" ink

Figure 20(a) show s the acoustic wave 820 applied to the ink Figuie 20(b) is a space/time diagiam showing the ink occupanc) along the nozzle axis for both selected drops 821 and diops which have not been selected 822 The graph shows ink position versus time toi a small region along the nozzle axis, ranging from a small distance inside the body of the ink 100 (at the bottom ot the graph) to a small distance within the papei 51 (at the top ot the giaph)

The two graphs aie supei imposed to allow direct comparison ot the selected diop with the unselected drop

The graph ot ink occupancy toi unselected drops 822 shows a sinusoidal oscillation of the same tiequency as the acoustic wave 820, but with a certain phase shift ΔΦ The degiee of phase shift depends upon the shape and dimensions ot the ink nozzles and ink reservons, and the fluid characteπstics ot the mk. The phase shift will approach 90° as the frequency of the acoustic wave approaches the resonant frequency of the ink in the nozzle. The phase shift is easily compensated by altering the phase of the dnve voltage to the piezoelectric or other transducer which is used to create the acoustic wave. The wave shape for the unselected drop is shown as being sinusoidal. The actual shape will have substantial harmonic distortion, and depends upon the geometry of the nozzle tip and the fluid characteristics of the ink.

Figure 20(b) is specifically related to a head embodiment with eight dnve phases and four k colors (for example, CMYK colors). Only one ink color is shown. The phases of the acoustic waves in the other ink colors are 90°. 180°, and 270° out of phase with the phase of the acoustic wave 820. The eight dnve phases in the drop ejection cycle extend over two penods of the acoustic wave 820. There are two drive phases per ink color in one drop ejection cycle These are separated by 360° ot the acoustic wave, and do not apply to the same nozzle, but to interleaved nozzles. The periods 829 and 831 are two successive heater dnve periods of one nozzle (in this case, the nozzle with the selected drop 821. The penod 830 is the period in which the heaters of the alternate nozzles of the same ink color may be enabled

At the time that the heater is turned on 823 the ink occupancy history ot the selected drop 821 begins to diverge from the ink occupancy history ot drops which are not selected 822. Ink flow from the nozzle is aided by being at a penod ot maximum forward velocity caused by the acoustic wave 820 At the tune 824 this divergence is irreversible, as the oscillating equilibπum between surface tension and oscillating ink piessure is broken. Ink continues to emerge tiom the nozzle until the ink contact the recording medium 51 at time 825 Ink wets the surface ot the iecoiding medium 51. and is absoibed into the medium, as is sho n by the ink ov erlapping the iecoiding medium in the space-time legion 832 The selected drop 821 separates from the body of ink 100 when the rate ot volume flow into and/or along the surface ot the recording medium exceeds the rate of flow from the nozzle at the separation time and position 826. After the instant ot separation at 826 the ink meniscus rapidly contracts for both the ink which remains in the recording medium 51 and tor the body of ink 100. The separation is aided by occurring at a time of low ink pressure, when the ink for unselected drops is flowing back into the nozzle. Ink on the nozzle side of the separation point 826 rapidly moves back into the nozzle by ink surface tension. The ink meniscus undergoes a damped oscillation at the resonant frequency of the ink in the nozzle tip for a short period This damped oscillation is superimposed on the oscillation caused by the acoustic wave. In most cases, it will be neither necessary nor practical to match the resonant frequency of the ink in the nozzle with the frequency of the acoustic wave. The example shown in Figure 20(b) the heater on period is 18 μs, and the drop ejection cycle is 144 μs. The period of the acoustic wave is 72 μs. therefore the frequency ot the acoustic wave is 13 8 KHz. The resonant frequency of the ink column is 25 Khz At the time 827 that the acoustic wave is at the same phase as the start of heater energizing 823, the ink meniscus has not returned sufficiently to the quiescent oscillating state of drops which are not selected 822. However, the alternate nozzle of the same ink color are ready for heater energizing for the period 830 at the time 827 At the next time 828 the acoustic wave is at the same phase as the start ot neater energizing 823 the heater of the same nozzle can again be energized, as the meniscus has returned to the quiescent oscillating state with very minor en oi

The region ot ink 832 which has been absorbed into the lecording medium 51 is shown first growing thicker, then thinner with progressing time. The actual ink region in the recording medium only gets thinner, slows, and stops at a certain thickness. The thinning of the ink region is because Figure 20 is a space/time diagram of ink occupancy along the axis ot the nozzle, and the iecoiding medium 51 is moving relative to the nozzle axis. By the time 833 that the next selected drop tor the nozzle has reached the recording medium the edge of the pievious drop has been passed. The second ink spot flows back in the recording matenal to join with the fust ink spot, thereby providing a continuous layer of ink when subsequent drops aie selected

A simple means ot achieving an acousuc wave in the ink is by placing a piezoelectπc crystal in such a way that it displaces the ink in the ink channel behind the nozzles The piezoelectnc crystal should be the entire length ot the low ot nozzles to ensure that all nozzles receive a n acoustic wave ot the same amplitude and phase The amplitude of the voltage applied to the piezoelectnc crystal depends upon the physical characteπstics of the crystal, the dimensions ot the nozzles, the shape, location and dimensions of the ink reservoir, the placement ot the piezoelectπc crystal in relauon to the ink nozzles and ink reservoir, the fluid chaiactenstics ot the ink. and other tactors The simplicity and low cost of trying dif tenng voltages, amplitudes and phases ot dnve voltage makes expeπmentation a moie effective means ot denying the appropriate dnve waveforms than calculation In the example shown in Figure 20, the frequency ot the acousuc wave is 13 8 KHz This is within the normal audible range of humans, and may be peiceived as an annoying high pitched hiss it significant amplitudes ot the wave aie transmitted to the an and escape the printer enclosure The level ot annoyance peiceived is sub)ectιve, and highly vaπable from person to peison Foi example onlv some people aie annoyed by the 15 625 KHz line tiequency emitted by NTSC and PAL television sets while most people aie unawaie ot the sound Theie aie seveial lemedies to the problem ot sound emission One is to ensure that the acoustic wave tiequency is abov e 20KHz. the normal maximum audible tiequencv Anothei solution is to encase the print head assembly with acoustic absoiptiv e mateπal This need only absorb strongly at the fundamental tiequencv ot the acoustic wav e, as the second harmonic is above 20KHz Anothei solution is to minimize the acoustic coupling between the ink and the air (via the ink cnannel assemblv and othei components) at the appiopnate tiequencv

Diop size aduistment in proximity sepaiation printers

Figuie 21 (a) shows a cioss section ot a Pioximity sepaiation pnnt head and platen assembly tor a web fed pπnting system The pnnt head 50 pπnts six colors (CC'MM'YK) for high quality full color pπnting usmg digital halftoning. The head is approximately 8 mm wide and 600 μm thick The pnnt head is positioned a distance D_Hlop away from the iecording medium 51 which moves in a direction shown by the arrow 5 The iecording medium 51 is tensioned against a platen 67. The platen 67 should have a highly polished and optically flat surface to reduce friction with the recording medium, and to maintain positioning accuracy across the entire pnnt region The platen may alternatively be formed by two or more rollers (not shown), to reduce friction further. The rollers may be surrounded by an band (not shown) to maintain positional accuracy of the recording medium 51 The platen 67 is fixed to a piezoelectnc ceramic 31 which has an axis of polanzation 33 The piezoelectric crystal is fixed to a plate 30 which is mechanically fixed m relation to the pnnt head 50 dunng printing Electiodes 32 are applied to the piezoelectric crystal 31 To adjust the distance O_Htop <* voltage is applied to the electrodes 32 Ink 100 is supplied to the head by the ink channel assembly 75 The ink channel assembly 75 may also serve the function of holding the pnnt head πgidly in place, and ot correcting waφ in the pnnt head Alternatively, these functions may be provided by alternative means. Power to actuate the thermal heatei s is supplied by the two power connections 38 and 39 Because these connections can be manufactured trom a conductive metal which can leadily be seveial hundred micions thick, and because these connections may be the entire length ot the pnnt head, high currents can be supplied to the print head with a small voltage diop This is important, as pagewidth color print heads may consume as much as 20 Amps when seveial thousand nozzles are actuated simultaneously A papei guide lightly contacts the recording medium 51 undei piessuie piov ided by an elastically detormable mateπal 35 acting against a fixed block 34 The guide 36 has two purposes to tension the iecoiding medium against the platen in coniunction with the papei transport tollei 65, and to tempoiaπl.v flatten any fibers which may protrude trom a iecording medium such as papei It is desirable to flatten piotruding fibers to improve pnnt quality bv reducing vanations in the distance tiom the print head to the effective surface ot the iecoiding medium. Protruding fibers do not have as significant an affect on the printed dot size as may be implied by the reduced distance from the nozzle to the closed part of the recording medium. This is because the ink drop will not soak into or wick along the surface of a small protruding fibers as fast as it will soak into the bulk surface. Therefore, the time before ink drop separation, and thus the total amount of ink delivered, will not vary greatly. Depending upon the printing speed, the recording medium type, and other aspects of the printing system, the guide 36 may not be necessary, or may be replaced by tensioned rollers to reduce friction.

Figure 21 (b) shows a small distance On,_oP between the print head and the recording medium 51. This results in a small volume of the selected drop 1 at the instant ot contact between the selected drop and the recording medium. This value of D_w, _P is achieved by applying a voltage of V„_om + ΔV to the piezoelectπc crystal.

Figure 21(c) shows a nominal distance D _toP between the pnnt head and the recording medium 51. This results in a nominal volume of the selected drop 1 at the instant ot contact between the selected drop and the recording medium. This value ot D _lop is achieved by applying a voltage of V„_om to the piezoelectnc crystal wheie V„_n„, is the nominal voltage. V_πom may be zero, or may be biased so that the full lange ot required adjustment can be achieved with a unipolar adiustment v oltage. ΔV may be positive or negative, depending upon the crystal onentation and choice of electrodes

Figuie 21 (d) shows a relatively large distance D_Hk>p between the pnnt head and the recording medium 51. This results in a relatively large volume ot the selected drop 1 at the instant of contact between the selected diop and the iecoiding medium. This value of D_w,_oP is achieved by applying a voltage of V,₁₍₎„ ΔV to the piezoelectric crystal

The volume ot ink delivered to the recording medium is not equal to the volume ot the selected drop at the instant of contact with the iecording medium, as ink continues to flow trom the nozzle while the selected diop is soaking into the iecording medium. However, the volume ot ink delivered to the iecording medium will be approximately proportional to the volume ot the selected drop at the instant of contact over an operating range determined by ink, recording medium, and nozzle characteristics.

An alternative configuration of the apparatus is to use a piezoelectric crystal to alter the position of the print head in relation to a fixed platen, instead of vice versa. This arrangement is equivalent in function, with no significant disadvantage over the preferred apparatus, except that in many cases it will be more difficult to manufacture.

It is possible to derive many different arrangement of piezoelectric crystal, including arrangements where the crystal operates in shear mode, and arrangements which use multiple stacked layers of piezoelectric crystal to reduce the magnitude of the control voltage required. These variations are obvious to those skilled in the art, and are within the scope of the invention.

The foregoing describes one embodiment of the present invention. Modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention.

Claims

The Claims Defining the Invention are as follows

1 A microemulsion based inkjet ink

2 An ink composition as claimed in claim 1 with a phase inversion temperature between 40°C and 90°C

3 An ink composition as claimed in claim 1 with a phase inversion temperature between 60°C and 70°C

4 An ink composition as claimed in claim 1 wherein the surfactant is ot the group poly(oxyethylene)alkylphenylene with the general formula HO(CRCH,O)_nC₄H₆C_nH₂„„

5 An ink composition as claimed in claim 3 where m is between 5 and 50

6 An ink composition as claimed in claim 3 where m is between 8 and 12

7 An ink composition as claimed in claim 3 wheie n is between 5 and 20

8 An ink composition as claimed in claim 3 wheie n is between 7 and 1 1

9 An ink composition substantially as described herein

10 A micioemulsion based hot melt ink jet ink

1 1 An ink composition as claimed in claim 10 w ith a phase invei sion tempeiatuie which is at least 20°C above the melting point of the hot melt ink