WO1996032282A9 - A high speed digital fabric printer - Google Patents

Abstract

A high speed digital color fabric printing system using drop on demand printing technology. A bi-level image memory is provided to store a digital representation of the fabric pattern to be printed. The pattern to be printed can be altered by changing the contents of the bi-level page memory. The system does not require the manufacture of printing plates. Using a printing head with 126,080 active nozzles, two meter wide cloth can be printed with full color images at 400 dpi at a speed of 1 square meter per second.

Description

A HIGH SPEED DIGITAL FABRIC PRINTER

Field of the Invention

The present invention is in the field of computer controlled printing devices. In particular, the field is drop on demand systems for fabric printing.

Background of Invention

There is currently no high speed fabric printing system available in which the image to be printed can be directly controlled by a computer system. All current high speed fabric printers require the manufacture of printing plates.

Various digitally controlled printing techniques have been adapted for printing on fabric. For example, ink-jet printers have been used for low-speed fabric printing for some years. Color laser electrophotographic printers have been used for fabric printing via a paper based transfer system. Direct digital fabric printing has some potential advantages over conventional plate or drum based printers. Amongst these are the following advantages:

1) The time and cost savings of eliminating the plate-making stage

2) The ability to print small runs of a particular pattem cost effectively

3) Near-perfect color registration, as all of the required colors can be printed in a single pass

4) The ability to print non-repeating images of any length

5) The potential compact size of direct digital fabric printers 6) High image resolution

However, current direct digital fabric printers fall far short of the requirements of commercial fabric printing systems.

The principle disadvantage of current systems is printing speed. There is no currently available direct digital fabric printer which can operate at sufficient speed to be commercially viable in any but specialist applications.

Commercial fabric printing requires printing speeds of approximately one meter per second, printing on fabric that is two meters wide. Such a system can be constructed using current thermal ink-jet technology; however, many severe technical problems prevent the easy construction of such a system. One of these is power dissipation. Existing thermal inkjet printers consume approximately 20 microJoules of energy for each drop ejected. A four color process printer (cyan, magenta, yellow and black) capable of printing one square meters of fabric per second at 400 dpi using theπnal inkjet technology with an energy requirement of

20 microJoules per dot will have a power consumption of 19.8 kW when printing four color black. The dissipation of this amount of power from a small print-head presents significant techmcal difficulties. Such a system must also be built using a large number of print heads, resulting in high manufacturing costs. Reliability is also low, as existing thermal ink-jet devices are not fault tolerant

Summary ofthe Invention My concurrently filed applications entitled "Liquid Ink Printing

Apparatus and System" and "Coincident Drop-Selection, Drop Separation Printing Method and System" describe systems that afford advantages toward overcoming the above-noted problems. The system produces tiny droplets of liquid ink under the control of digital electronic impulses. Systems can be built which are fast enough for medium volume color fabric printing at high quality. The printing heads of such systems can operate in a self-cooling manner, where all of the energy required to eject a drop can be dissipated in the printed ink drops without raising the temperature of the ink above operating limits. This feature can eliminate the power dissipation problem of thermal ink-jet technology. Print heads with many thousands of nozzles can be made fault-tolerant while simultaneously reducing manufacturing costs. This reduces the production cost and increases the reliability of direct digital fabric printing systems using printing technology. The current invention is a digital color fabric printing system using such printing technology of my above-noted application. Thus, in one aspect, the present invention constitutes a digital printing system for printing on fabric material, comprising means for moving a fabric wet of uniform width along a transport path from a supply to a take up station, a digital print head assembly located along said transport path and including an integral array of print nozzles extending across the width dimension of the web transport path, ink supply means for providing fabric printing ink to the nozzles of said array, and control means for separating said print head assembly, in timed relation with the movement of said web and under the control of pattern data, to print predetermined fabric patterns.

In another aspect, the present invention constitutes a digital printing system for printing on a fabric web, including a raster image processing computer for producing digitally halftoned binary image data, digital memory means for receiving for storing said binary image data, a plurality of digital printing heads, a fabric web transport system which moves said fabric past said printing heads for printing, and an ink reservoir and ink pressure regulation system which maintains predetermined positive pressure ink flow to said heads.

B e Pescription of he Prawjngs

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 manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance.

Figure 6 shows a schematic system diagram of color fabric design and printing system in accordance with one preferred embodiment ofthe invention.

Figure 7 shows a simplified schematic diagram of a preferred print head driver system for a digital color fabric printer in accordance with the invention.

Figure 8 shows the major modules and the fabric path of a fabric printer using one preferred printer embodiment.

Figure 9(a) shows a top view of one preferred configuration of the device.

Figure 9(b) shows a side view of one preferred configuration of the device. Figure 10 shows a perspective view of one possible configuration of the device.

Petailed Pesc ption of he Preferred Embodiments

Consistency: the image quality generated is consistent, as each dot is digitally controlled. 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 altemative 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 foUowing 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 characteristics of drop on demand printing technology. The table also lists some methods by which some embodiments described herein, or in other of my related applications, provide improvements over the prior art.

DOD printing technology targets

Target Method of achieving improvement over prior art

High speed operation Practical, low cost, pagewidth printing heads with more than 10,000 nozzles. Monolithic A4 pagewidth print heads can be manufactured using standard 300 mm (12") silicon wafers

High image quality High resolution (800 dpi is sufficient for most applications), six color process to reduce image noise

Full color operation Halftoned process color at 800 dpi using stochastic screening Ink flexibility Low operating ink temperature and no requirement for bubble formation

Low power Low power operation results from drop selection means requirements not being required to fully eject drop

Low cost Monolithic print head without aperture plate, high manufacturing yield, small number of electrical connections, use of modified existing CMOS manufacturing facilities

High manufacturing Integrated fault tolerance in printing head yield

High reliability Integrated fault tolerance in printing head. Elimination of cavitation and kogation. Reduction of thermal shock.

Small number of Shift registers, control logic, and drive circuitry can be electrical connections integrated on a monolithic print head using standard CMOS processes

Use of existing VLSI CMOS compatibility. This can be achieved because the manufacturing heater drive power is less is than 1 % of Thermal Ink Jet facilities heater drive power

Electronic collation A new page compression system which can achieve 100:1 compression with insignificant image degradation, resulting in a compressed data rate low enough to allow real-time printing of any combination of thousands of pages stored on a low cost magnetic disk drive.

In thermal ink jet (TTJ) and piezoelectric inkjet systems, a drop velocity of 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 strike the recording medium. These systems have a very low efficiency of conversion of electrical energy into drop kinetic energy. The efficiency of ΗJ systems is approximately 0.02%). This means that the drive circuits for TJJ print heads must switch high currents. The drive circuits for piezoelectric inkjet heads must either switch high voltages, or drive highly capacitive loads. The total power consumption of pagewidth ΗJ printheads is also very high. An 800 dpi A4 full color pagewidth ΗJ print head printing a four color black image in one second would consume approximately 6 kW of electrical power, most of which is converted to waste heat. The difficulties of removal of this amount of heat precludes the production of low cost, high speed, high resolution compact pagewidth TD systems. One important feature of embodiments of the invention is a means of significantly reducing the energy required to select which ink drops are to be printed. This is achieved by separating the means for selecting ink drops from the means for ensuring that selected drops separate from the body of ink and form dots on the recording medium. 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 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

Method Advantage Limitation

1. Electrothermal Low temperature Requires ink pressure reduction of surface increase and low drop regulating mechanism. Ink tension of selection energy. Can be surface tension must reduce pressurized ink used with many ink substantially as temperature types. Simple fabrication. increases

CMOS drive circuits can be fabricated on same substrate

2. Electrothermal Medium drop selection Requires ink pressure reduction of ink energy, suitable for hot oscillation mechanism. Ink viscosity, combined melt and oil based inks. must have a large decrease with oscillating ink Simple fabrication. in viscosity as temperature pressure CMOS drive circuits can increases be fabricated on same substrate

3. Electrothermal Well known technology, High drop selection energy, bubble generation, simple fabrication, requires water based ink, with insufficient bipolar drive circuits can problems with kogation, bubble volume to be fabricated on same cavitation, thermal stress cause drop ejection substrate

4. Piezoelectric, with Many types of ink base High manufacturing cost, insufficient volume can be used incompatible with change to cause drop integrated circuit processes, ejection high drive voltage, mechamcal complexity, bulky 5. Electrostatic Simple electrode Nozzle pitch must be attraction with one fabrication relatively large. Crosstalk electrode per nozzle between adjacent electric fields. Requires high voltage drive circuits

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 pressurized ink". This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of TU), compatibility with CMOS VLSI chip fabrication, 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 reduction of ink viscosity, combined with oscillating ink pressure". This drop selection means is particularly suited for use with inks which exhibit a large reduction of viscosity with increasing temperature, but only a small reduction in surface tension. This occurs particularly with non-polar ink carriers with relatively high molecular weight This is especially applicable to hot melt and oil based inks.

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

Drop separation means

Means Advantage Limitation

1. Electrostatic Can print on rough Requires high voltage attraction surfaces, simple power supply implementation

2. AC electric field Higher field strength is Requires high voltage AC possible than electrostatic, power supply synchronized operating margins can be to drop ejection phase. increased, ink pressure Multiple drop phase reduced, and dust operation is difficult accumulation is reduced

3. Proximity Very small spot sizes can Requires print medium to be

(print head in close be achieved. Very low very close to print head proximity to, but power dissipation. High surface, not suitable for not touching, drop position accuracy rough print media, usually recording medium) requires transfer roller oi belt

4. Transfer Very small spot sizes can Not compact due to size of

Proximity (print be achieved, very low transfer roller or transfer head is in close power dissipation, high belt proximity to a accuracy, can print on transfer roller or rough paper belt

5. Proximity with Useful for hot melt inks Requires print medium to be oscillating ink using viscosity reduction very close to print head pressure drop selection method, surface, not suitable for reduces possibility of rough print media. Requires nozzle clogging, can use ink pressure oscillation pigments instead of dyes apparatus

6. Magnetic Can print on rough Requires uniform high attraction surfaces. Low power if magnetic field strength, permanent magnets are requires magnetic ink used

Other drop separation means may also be used.

The preferred drop separation means depends upon the intended use. For most applications, method 1: 'Εlectrostatic 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 various 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 incorporated by reference:

'A Liquid ink Fault Tolerant printing mechanism' (Filing no.: PN2308);

'Electrothermal drop selection in printing' (Filing no.: PN2309);

'Drop separation in printing by print media proximity' (Filing no.:

PN2310); 'Drop size adjustment in Proximity printing by varying head to media distance' (Filing no.: PN2311);

'Augmenting Proximity printing with acoustic ink waves' (Filing no.:

PN2312);

'Electrostatic drop separation in printing' (Filing no.: PN2313); 'Multiple simultaneous drop sizes in Proximity printing' (Filing no.:

PN2321);

'Self cooling operation in thermally activated print heads' (Filing no.:

PN2322); and

'Thermal Viscosity Reduction printing' (Filing no.: PN2323). A simplified schematic diagram of one preferred printing system according to the invention appears in Figure 1(a).

An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or 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 raster image processor (RJP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital

Halftoning unit 54. Halftoned bitmap image data is stored in the image memory 72. Depending upon the printer and system configuration, the image memory 72 may be a full page memory, or a band memory. Heater control circuits 71 read data from - l i ¬ the image memory 72 and apply time- varying electrical pulses to the nozzle heaters

(103 in figure 1(b)) that are part of the print head 50. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops 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. Altematively, 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 printers according to the invention, an extemal 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 extemal field 74 is a constant electric field, as the ink is easily made to be electrically 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 print medium as a means of discriminating between selected drops and unselected drops.

For small drop sizes gravitational force on the ink drop is very small; approximately 10^ of the surface tension forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented in any direction in relation to the local gravitational field. This is an important requirement for portable printers.

Figure 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process. The nozzle is etched in a substrate 101, which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates have several advantages, including:

1) High performance drive transistors and other circuitry can be fabricated in SCS;

2) Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing 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. MonoUthic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The eUmination ofthe 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 MiUer 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 appUed 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 siUcon, siUcon nitride, amorphous siticon dioxide, crystaUine siUcon 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 equiUbrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus extemal 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 soUd 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 tip 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 print medium, but would instead retum to the nozzle.

Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end of 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 drop then travels to the recording medium under the influence of the extemal 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 for each heat pulse. As the heat pulses are electricaUy controUed, drop on demand ink jet operation can be achieved.

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

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

Figure 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle. The vertical axis of the graph is temperamre, in units of 100°C. The horizontal axis of the graph is time, in units of 10 μs. The temperamre 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 temperamre 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 temperamre 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 Uttie below the boiling point of the ink for the duration of the pulse, and a rapid faU in temperamre at the end of the pulse. To achieve this, the average energy appUed to the heater is varied over the duration of 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 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency aUows finer control over the power appUed 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 temperamre coefficient of surface tension

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

Where γ-ris the surface tension at temperamre T, k is a constant, T_c is the critical temperamre of the Uquid, Λf is the molar mass of the Uquid, x is the degree of association ofthe Uquid, and p is the density of the Uquid. This equation indicates that the surface tension of most Uquids faUs to zero as the temperamre reaches the critical temperature of the Uquid. For most Uquids, 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 smaU 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 temperamre 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 sUghtly with temperamre. 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 prefeπed to achieve large operating margins, while as Uttie as lOmN/m can be used to achieve operation of the print head according to the present invention.

Inks With Large -Δy_z

Several methods may be used to achieve a large negative change in surface tension with increasing temperamre. Two such methods are: 1) The ink may contain a low concentration sol of a surfactant which is soUd at ambient temperamres, but melts at a threshold temperamre. Particle sizes less than 1,000 A are desirable. Suitable surfactant melting points for a water based ink are between 50°C and 90°C, and preferably between 60°C and 80°C.

2) The ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperamre, 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 temperamre encountered by the ink. A PIT of approximately 80°C is suitable.

Inks with Surfactant Sols

Inks can be prepared as a sol of smaU particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxyUc acids with between 14 and 30 carbon atoms, such as:

Name Formula m.p. Synonym

Tetradecanoic acid CH₃(CH.)₁₂COOH 58°C Myristic acid

Hexadecanoic acid CH₃(CH₃)_uCOOH 63°C Palmitic acid

Octadecanoic acid

71°C Stearic acid

Eicosanoic acid CH₃(CH₂)₁₆COOH 77°C Arachidic acid

Docosanoic acid CH₃(CH.)₂₀COOH 80°C Behenic acid

As the melting point of sols with a smaU particle size is usually stightly less than ofthe bulk material, it is preferable to choose a carboxyUc acid with a melting point stightly above the desired drop selection temperamre. A good example is Arachidic acid.

These carboxyUc acids are available in high purity and at low cost. The amount of surfactant required is very smaU, so the cost of adding them to the ink is insignificant A mixture of carboxyUc acids with sUghtiy varying chain lengths can be used to spread the melting points over a range of temperamres. Such mixmres will typicaUy cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxyUc 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 ionisable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation.

In the case of carboxyUc acids, this can be achieved by adding an alkati 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 process for creating the surfactant sol is as foUows: 1) Add the carboxyUc acid to purified water in an oxygen free atmosphere. 2) Heat the mixmre to above the melting point of the carboxyUc acid. The water can be brought to a boil.

3) Ultrasonicate the mixture, until the typical size of the carboxyUc acid droplets is between lOOA and 1,000A.

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 carboxyUc acid molecules on the surface of the particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps stabiUze the sol.

7) Centrifuge the sol. As the density of the carboxyUc 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 wiU also contain either dye(s) or pigment(s), bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required.

Anti-foaming agents wiU generaUy not be required, as there is no bubble formation during the drop ejection process.

Cationic surfactant sols

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

Various suitable alkylamines are shown in the following table:

Name Formula Synonym

Hexadecylamine CH₃(CH₂)_UCH₂NH₂ Palmityl amine

Octadecylamine CH₃(CH₂)₁₆CH₂NH₂ Stearyl amine

Eicosylamine CH₃(CH₂)_lgCH₂NH₂ Arachidyl amine

Docosylamine CH-CCH^CrLNH, Behenyl amine

The method of preparation of cationic surfactant sols is essentiaUy similar to that of anionic surfactant sols, except that an acid instead of an alkati is used to adjust the pH balance and increase the charge on the surfactant particles. A pH of 6 using HCl is suitable.

Microemulsion Based Inks

An alternative means of achieving a large reduction in surface tension as some temperamre threshold is to base the ink on a microemulsion. A microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperamre. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the microemulsion is water in oU (W/O). At low temperamres, the surfactant forming the microemulsion prefers a high curvamre surface around oil, and at temperamres significantly above the PIT, the surfactant prefers a high curvature surface around water. At temperamres close to the PIT, the microemulsion forms a continuous 'sponge' of topologically connected water and oil.

There are two mechanisms whereby this reduces the surface tension.

Around the PIT, the surfactant prefers surfaces with very low curvamre. As a result, surfactant molecules migrate to the ink/air interface, which has a curvamre which is much less than the curvamre 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 ink/air interface changes from water/air to oil/air. The oil/air interface has a lower surface tension.

There is a wide range of possibiUties 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 group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula: C_nH_;to+-C H₆(CH->CH₂O)_mOH) can be used. The hydrophihcity ofthe surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable.

Low cost commercial preparations are the result of a polymerization of various molar ratios of ethylene oxide and alkyl phenols, and the exact number of oxyethylene groups varies around the chosen mean. These commercial preparations are adequate, and highly pure surfactants with a specific number of oxyethylene groups are not required. The formula for this surfactant is C_gHι₇C₄H₆(CH₂CH₂O)_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.

Commercial preparations of this surfactant are available under various brand names. SuppUers and brand names are Usted in the foUowing table:

Trade name Supplier

Akyporox OP100 Chem-Y GmbH

Alkasurf OP-10 Rhone-Poulenc Surfactants and Specialties

Dehydrophen POP 10 Pulcra SA

Hyonic OP- 10 Henkel Corp.

Iconol OP-10 BASF Corp.

Igepal O Rhone-Poulenc France

Macol OP-10 PPG Industries

Malorphen 810 Huls AG

Nikkol OP- 10 Nikko Chem. Co. Ltd.

Renex 750 ICI Americas Inc.

Rexol 45/10 Hart Chemical Ltd.

Synperonic OP 10 ICI PLC

Teric XlO ICI AustraUa

These are available in large volumes at low cost (less than one doUar per pound in quantity), and so contribute less than 10 cents per Uter to prepared microemulsion ink with a 5% surfactant concentration.

Other suitable ethoxylated alkyl phenols include those listed in the foUowing table: Trivial name Formula HLB Cloud point

Nonoxynol-9 CΛ,C₄H,(CH_aCH_aO^OH 13 54°C

Nonoxynol-10 C₉H„C H₆(CH₂CH₂O).₁₀OH 13.2 62°C

Nonoxynol-11 C₉H₁,C₄H₆(CH₂CH₂O)._1IOH 13.8 72°C

Nonoxynol-12 C,H„C₄H₆(CH₂CH₂O).₁₂OH 14.5 81°C

Octoxynol-9 C₈H₁₇C₄H₆(CH₂CH₂O).,OH 12.1 61°C

Octoxynol-10 C₈H₁₇C₄H₆(CH₂CH₂O)_₁₀OH 13.6 65°C

Octoxynol-12 C₈H₁₇C H₆(CH₂CH₂O).₁₂OH 14.6 88°C

Dodoxynol-10 y _dC₄H,(CH_aCH_aO)._lβOH 12.6 42°C

Dodoxynol-11 C₁₂H₂₃C₄H₆(CH₂CH₂O).₁₁OH 13.5 56°C

Dodoxynol-14 C₁₂H₂₅C H₆(CH₂CH₂O).₁₄OH 14.5 87°C

Microemulsion based inks have advantages other than surface tension control:

1 ) Microemulsions are thermodynamicaUy stable, and wiU not separate. Therefore, the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadicaUy.

2) The microemulsion wiU form spontaneously with a particular drop size, and does not require extensive stirring, centrifuging, or filtering to ensure a particular range of 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 mixmre 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.

7) The requirement for humectants can be reduced or eliminated. Dves and pigments in microemulsion based inks

OU in water mixtures can have high oil contents - as high as 40% - and still form O/W microemulsions. This aUows a high dye or pigment loading.

Mixmres of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye and pigment is as foUows:

1) 70% water

2) 5% water soluble dye

3) 5% surfactant

4) 10% oil

5) 10% oil miscible pigment

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

Combination Colorant in water phase Colorant in oil phase

1 none oil miscible pigment

2 none oil soluble dye

3 water soluble dye none

4 water soluble dye oil miscible pigment

5 water soluble dye oil soluble dye

6 pigment dispersed in water none

7 pigment dispersed in water oil miscible pigment

8 pigment dispersed in water oU soluble dye

9 none none

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

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 wiU 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 briltiant 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 absoφtive papers, as the dye wiU be absorbed into the paper, while the pigment wiU tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances.

Surfactants with a Krafft point in the drop selection temperamre range For ionic surfactants there is a temperamre (the Krafft point) below which the solubihty is quite low, and the solution contains essentially no miceUes. Above the Krafft temperature miceUe formation becomes possible and there is a rapid increase in solubihty of the surfactant Ifthe critical miceUe concentration (CMC) exceeds the solubihty of a surfactant at a particular temperamre, then the mimmum surface tension wiU be achieved at the point of maximum solubihty, rather than at the CMC. Surfactants are usuaUy much less effective 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 temperamre rises, and more of the surfactant goes into solution, decreasing the surface tension.

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

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

Formula Krafft point

C₁₆H₃₃SO/Na⁺ 57°C

CASO/ a^* 70°C

C₁₆H₃₃SO₄ Na⁺ 45°C

Na^S Ciy^SO.-Na^* 44.9°C r-o₄s(CH₂)₁₆so₄-r 55°C

C₁₆H₃₃CH(CH₃)C₄H₆SO₃Na⁺ 60.8°C

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 faUs with increasing temperature. At low temperatures, the POE chain is hydrophiUc, and maintains the surfactant in solution. As the temperamre increases, the stractured 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 temperamre at which the POE section of a nonionic surfactant becomes hydrophiUc is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generaUy 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 temperamres.

Two main configurations of symmetrical POE POP block copolymers are avaUable. 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 (generically CAS 9003-11-6)

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 (genericaUy also CAS 9003-11-6)

Some commercially avaUable varieties of poloxamer and meroxapol with a high surface tension at room temperamre, 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 weU known techniques. Desirable characteristics are a room temperamre 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)_y(CHCH₃CH₂O)_zOH] varieties 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 electrical conductivity ofthe 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 strucmre (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 strucmre (such as Cl^", OH^"), as fewer water molecules are available to foπn hydrogen bonds. Bromide ions have relatively Uttie effect The ink composition can be 'tuned' for a desired temperamre 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 F) that are added to increase electrical conductivity. NaCl is Ukely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl stightly lowers the cloud point of nonionic surfactants.

Hot Melt Inks

The ink need not be in a Uquid state at room temperamre. SoUd 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point ofthe ink. The holt melt ink must be formulated so that the surface tension of the molten ink decreases with temperamre. A decrease of approximately 2 mN/m wiU 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 reduction in surface tension rather than a reduction in viscosity.

The temperamre difference between quiescent temperamre and drop selection temperature may be greater for a hot melt ink than for a water based ink, as water based inks are constrained by the boiling point of the water. The ink must be Uquid at the quiescent temperamre. The quiescent temperamre should be higher than the highest ambient temperamre Ukely to be encountered by the printed page. T he quiescent temperamre 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 temperamres. A qmescent temperature between 60°C and 90°C is generaUy suitable, though other temperamres may be used. A drop ejection temperamre of between 160°C and

200°C is generaUy suitable.

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

1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the d^τop 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 temperamre, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperamre. This generally excludes alkanes such as waxes. Suitable materials wiU 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 foUowing additives: 1) 0.1% sol of Stearic Acid

2) 0.1% sol of Palmitic acid

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

4) 0.1 % solution of Pluronic L35 (trade mark of BASF)

5) 0.1 % solution of Pluronic L44 (trade mark of BASF) Inks suitable for printing systems of the present invention are described in the foUowing Austratian patent specifications, the disclosure of which are hereby incoφorated by reference:

'Ink composition based on a microemulsion' (Filing no.: PN5223, filed on 6 September 1995);

'Ink composition contaimng surfactant sol' (Filing no.: PN5224, filed on 6 September 1995);

'Ink composition for DOD printers with Krafft point near the drop selection temperamre 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. Prior to operation of the printer, soUd ink is melted in the reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperamre at which the ink 100 is Uquid, but exhibits a relatively 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 ofthe ink reduces with increasing temperamre. The ink pressure oscillates at a frequency which is an integral multiple ofthe drop ejection frequency from the nozzle. The ink pressure osciUation causes osciUations of the ink meniscus at the nozzle tips, but this osciUation is smaU due to the high ink viscosity. At the normal operating temperamre, these oscillations are of insufficient amptitude to result in drop separation. When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51, but sufficiently far away that the unselected drops do not contact the recording medium 51. Upon contact with the recording medium 51, part of the selected drop freezes, and attaches to the recording medium.

As the ink pressure faUs, ink begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium. The meniscus of the ink 100 at the nozzle tip then returns to low amplimde oscillatioa 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 electricaUy controlled, drop on demand ink jet operation can be achieved.

Manufacturing of Print Heads

Manufacturing processes for monotithic 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 incoφorated by reference:

'A monotithic printing head' (Filing no.: PN2301); 'A manufacturing process for monotithic printing heads' (Filing no.: PN2302);

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

'Power requirement reduction in monoUthic printing heads' (Ftiing no.: PN2305);

'A manufacmring process for monoUthic print heads using anisotropic wet etching' (Ftiing no.: PN2306); 'Nozzle placement in monoUthic drop-on-demand print heads' (Filing no.:

PN2307);

'Heater strucmre for monoUthic print heads' (Filing no.: PN2346); 'Power supply connection for monoUthic print heads' (Filing no.: PN2347); 'External connections for Proximity print heads' (Filing no.: PN2348); and 'A self- aligned manufacturing process for monoUthic print heads' (Filing no.: PN2349); and

'CMOS process compatible fabrication of print heads' (Filing no.: PN5222, 6 September 1995). 'A manufacturing process for LIFT print heads with nozzle rim heaters'

(Ftiing no.: PN6238, 30 October 1995);

A modular LIFT print head' (Filing no.: PN6237, 30 October 1995);

'Method of increasing packing density of printing nozzles' (Ftiing no.: PN6236, 30 October 1995); and 'Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets' (Filing no.: PN6239, 30 October 1995).

Control of Print Heads

Means of providing page image data and controlling heater temperamre in print heads of the present invention is described in the following AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference:

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

'Heater power compensation for temperature in printing systems' (FiUng no.: PN2314);

'Heater power compensation for thermal lag in printing systems' (FiUng no.: PN2315); 'Heater power compensation for print density in printing systems ' (FiUng no.: PN2316);

'Accurate control of temperature pulses in printing heads' (Filing no.: PN2317);

'Data distribution in monoUthic print heads' (FiUng no.: PN2318); 'Page image and fault tolerance routing device for printing systems' (FiUng no.: PN2319); and 'A removable pressurized Uquid ink cartridge for printers' (FiUng no.:

PN2320).

Image Processing for Print Heads An objective of printing systems according to the invention is to attain a print quatity which is equal to that which people are accustomed to in quality color pubUcations 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 caUed CC'MM' YK. Where high quatity monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein caUed CC'MM' YKK'. Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this mvention and other printing systems are described in the foUowing AustraUan 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

Printing apparams and methods of this invention are suitable for a wide range of applications, including (but not timited to) the foUowing: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color prmting, offset press supplemental printing, low cost printers using scanning print heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimtie and copying machines, label printing, large format plotters, photographic duplication, printers for digital photographic processing, portable printers incoφorated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal Digital Assistants', wallpaper printing, indoor sign printing, biUboard printing, and fabric printing. Printing systems based on this invention are described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference:

'A high speed color office printer with a high capacity digital page image store' (FiUng no.: PN2329); 'A short run digital color printer with a high capacity digital page image store' (FiUng no.: PN2330);

'A digital color printing press using printing technology' (Filing no.: PN2331);

'A modular digital printing press' (Filing no.: PN2332); 'A high speed digital fabric printer' (FiUng no.: PN2333);

'A color photograph copying system' (Filing no.: PN2334); 'A high speed color photocopier using a printing system' (FiUng no.: PN2335);

'A portable color photocopier using printing technology' (Filing no.: PN2336); 'A photograph processing system using printing technology' (Filing no.:

PN2337);

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

PN2338); 'A PhotoCD system with integrated printer' (Filing no. : PN2293);

'A color plotter using printing technology' (Filing no.: PN2291);

'A notebook computer with integrated color printing system' (FiUng no.:

PN2292);

'A portable printer using a printing system' (FiUng no.: PN2300); 'Fax machine with on-line database interrogation and customized magazine printing' (FiUng no.: PN2299);

'Miniature portable color printer' (FiUng no.: PN2298);

'A color video printer using a printing system' (Filing no.: PN2296); and

'An integrated printer, copier, scanner, and facsimile using a printing system' (Ftiing 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 quahty. 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 appUed to the nozzle heaters.

An optimum temperamre profile for one print head embodiment involves an instantaneous raising of the active region of the nozzle tip to the ejection temperamre, maintenance of this region at the ejection temperamre for the duration ofthe pulse, and instantaneous cooling ofthe 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 print head. The power appUed to the heater can be varied in time by various techniques, including, but not limited to:

1) Varying the voltage appUed 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 free surface modeling is required, as convection in the ink, and ink flow, significantly affect the temperamre achieved with a specific power curve.

By the incoφoration of appropriate digital circuitry on the print head substrate, it is practical to individuaUy control the power appUed to each nozzle. One way to achieve this is by 'broadcasting' a variety of different digital pulse trains across the print head chip, and selecting the appropriate pulse train for each nozzle using multiplexing circuits.

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

Compensation for environmental factors

Factor Scope Sensing or user Compensation compensated control method mechanism

Ambient Global Temperature sensor Power supply voltage Temperamre moimted on print head or global PFM pattems

Power supply Global Predictive active Power supply voltage voltage flucmation nozzle count based on or global PFM pattems with number of print data active nozzles

Most appUcations wiU not require compensation for all of these variables. Some variables have a minor effect and compensation is only necessary where very high image quatity 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 apptied to the print head to achieve heater power modulation, and does not have individual control of the power appUed 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 logicaUy anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in mm 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 weU known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for a paraUel 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 manufacmring 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 pattems of dots to be printed on the recording medium is stored in the Page or Band memory 1513, 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 411, 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 411 are preferably made programmable. The

Address generators 411 normally generate the address corresponding to the position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles contaimng 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 foπn the fault status bus.

The programmable power supply 320 provides power for the head 50. The voltage of the power supply 320 is controUed 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 MicrocontroUer 315. Temperamre is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroller 315 based on temperamre sensed by a thermal sensor 300. The thermal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311. The ADC 311 is preferably incoφorated in the MicrocontroUer 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. Altematively, fewer than sixteen states may be used.

For print density compensation, the printing density is detected by counting the number of pixels to which a drop is to be printed ('on' 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 print head in accordance with the invention depend upon the specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that the number of enable phases is a power of two. The On Pixel Counters 402 can be composed of combinatorial logic pixel counters 420 which determine how many bits in a nibble of data are on. This number is then accumulated by the adder 421 and accumulator 422. A latch 423 holds the accumulated value vatid for the duration of the enable pulse. The multiplexer 401 selects the output of the latch 423 which coπesponds to the current enable phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of the dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most sigmficant four bits of this count are adequate. Combining the four bits of thermal lag compensation address and the four bits of print density compensation address 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 aπay. These two dimensions are time (for thermal lag compensation) and print density. A third dimension - temperamre - can be included. As the ambient temperamre of the head varies only slowly, the microcontroUer 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at the cuπent temperamre.

Periodically (for example, a few times a second), the microcontroUer senses the current head temperature and calculates this matrix.

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.

Comparison with thermal ink iet technology

The table "Comparison between Thermal ink jet and Present Invention" compares the aspects of printing in accordance with the present invention with thermal ink jet printing technology.

A direct comparison is made between the present invention and thermal inkjet technology because both are drop on demand systems which operate using thermal actuators and Uquid ink. Although they may appear simtiar, the two technologies operate on different principles. Thermal ink jet printers use the foUowing fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in Uquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transfeπed to the ink before bubble nucleation is complete. For water based ink, ink temperatures of approximately 280°C to 400°C are required. The bubble formation causes a pressure wave which forces a drop of ink from the aperture with high velocity. The bubble then coUapses, drawing ink from the ink reservoir to re-fiU the nozzle. Thermal ink jet printing has been highly successful commerciaUy due to the high nozzle packing density and the use of weU established integrated circuit manufacmring techmques. However, thermal inkjet printing technology faces sigmficant technical problems including multi-part precision fabrication, device yield, image resolution, 'pepper' noise, printing speed, drive transistor power, waste power dissipation, satelUte drop formation, thermal stress, differential thermal expansion, kogation, cavitation, rectified diffusion, and difficulties in ink formulation.

Printing in accordance with the present invention has many of the advantages of thermal inkjet printing, and completely or substantiaUy eliminates many of the inherent problems of thermal inkjet technology.

Comparison between Thermal inkjet and Present Invention

Resonance Serious problem limiting Very small effect as nozzle design and pressure waves are small repetition rate

Practical resolution Approx. 800 dpi max. Approx. 1,600 dpi max.

Self-cooling No (high energy required) Yes: printed ink carries operation away drop selection energy

Drop ejection High (approx. 10 m/sec) Low (approx. 1 m/sec ) velocity

Crosstalk Serious problem requiring Low velocities and careful acoustic design, pressures associated with which limits nozzle refiU drop ejection make crosstalk rate. very small.

Operating thermal Serious problem timiting Low: maximum temperamre stress print-head life. increase approx. 90°C at centre of heater.

Manufacturing Serious problem limiting Same as standard CMOS thermal stress print-head size. manufacturing process.

Drop selection Approx. 20 μJ Approx. 270 nJ energy

Heater pulse period Approx. 2-3 μs Approx. 15-30 μs

Average heater Approx. 8 Watts per Approx. 12 mW per heater. pulse power heater. This is more than 500 times less than Thermal Ink- Jet

Heater pulse Typically approx. 40V. Approx. 5 to 10V. voltage

Heater peak pulse Typically approx. 200 mA Approx. 4 mA per heater. current per heater. This requires This aUows the use of small bipolar or very large MOS MOS drive transistors. drive transistors.

Fault tolerance Not implemented. Not Simple implementation practical for edge shooter results in better yield and type. reliability

Constraints on ink Many constraints including Temperamre coefficient of composition kogation, nucleation, etc. surface tension or viscosity must be negative.

Ink pressure Atmospheric pressure or Approx. 1.1 atm less

Integrated drive Bipolar circuitry usuaUy CMOS, nMOS, or bipolar circuitry required due to high drive current

Differential Significant problem for Monolithic constmction thermal expansion large print heads reduces problem

Pagewidth print Major problems with yield, High yield, low cost and heads cost, precision long Ufe due to fault constmction, head life, and tolerance. Self cooling due power dissipation to low power dissipation. Yield and Fault Tolerance

In most cases, monotithic 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 manufacmring cost. A device with a yield of 5% is effectively ten times more expensive to manufacmre than an identical device with a yield of 50%.

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 Umitation 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 manufacmre 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, Muφhy's method predicts a yield less than 1 %. This means that more than 99% of heads fabricated would have to be discarded. This low yield is highly undesirable, as the print head manufacmring 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 expticitly models the clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controllable parameter in manufacmring, 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 incoφorate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units.

In memory 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 aUow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 100% redundancy would normaUy 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 ofthe head chip, and manufacmring constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for a fuU width, fuU color head for printing A4 size paper is approximately 215 mm x 5 mm. This size aUows the inclusion of 100% 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 specificaUy 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 modeUng 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 1% to more than 90% under identical manufacmring conditions. This can reduce the manufacturing cost by a factor of 100.

Fault tolerance is highly recommended to improve yield and reliabUity 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 foUowing Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference:

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

'Nozzle dupUcation 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).

Fabric printing svstems using minting technology

The present invention provides high speed digital color fabric printing system which uses drop on demand printing systems described above and in my other related applications. The printer accepts information supphed by an extemal raster image processor (RIP) in the form of a halftoned raster at 300 dots per inch. This is stored in a bi-level image memory. Many fabric printing units can be supphed with information from a single RTP, and can print simultaneously. The contents of the image memory can then be prmted using the printing head. This system has a number of advantages over conventional fabric printing presses. These include: 1 ) Fast turn- around of new designs

2) SmaU lot sizes of a particular design are practical, as the design can be changed frequently and effectively instantly.

3) Reliability: the system is fault tolerant, increasing reliabitity.

4) Perfect color registration: the four process colors are printed using a monolithic siUcon printing head. The nozzles of this head can be fabricated with a relative position tolerance of less than one micron. This eliminates the need to align four color passes, as is usuaUy required. Registration is a serious problem in conventional fabric printing systems, as the fabric tends to stretch, making multi-pass registration very difficult.

5) Consistency: the image quality generated is consistent, as each dot is digitally controUed.

Table 5, "Example product specifications," shows the specifications of one possible configuration of a high performance color fabric printing system capable of printing fabric at one meter per second.

Example product specifications

Configuration Floor standing, web fed

Fabric width 2 meters

Printer type LIFT full width printing head

Number of nozzles 126,080 active nozzles, 126,080 spare nozzles

Printing speed 1 square meter per second

Printer resolution 400 dpi

Dimensions ( XD X H) 2,400 X 4,800 X 1,600 mm

ReUabitity Fault tolerant at print head and module level

Image format DigitaUy halftoned bitmap, CMYK

Memory Capacity 64 MBytes

Connectivity 100 BaseT Ethernet, SCSI The table "LIFT head type Fabric-4-400" (Appendix A) is a summary of some characteristics of an example full color printing head system capable of printing cloth at 400 dpi at a rate of one square meter per second.

Figure 6 shows a simplified system configuration for a high speed color design and fabric printing system. Images are scanned, graphics are created, and pages are laid out using computer based color design workstations 576. These can be based on personal computers such as the Apple Macintosh and IBM and compatible personal computers, or on workstations such as those manufactured by

Sun and Hewlett-Packard. Altematively, they can be puφose built fabric design workstations. Information is communicated between these workstations using a digital commumcations local area network 577 such as Ethernet Information can also be brought into the system using wide area networks such as ISDN, or by physical media such as floppy disks, hard disks, optical disks, magnetic tape, and so forth. Color images can be scanned using a scanner 579 and incoφorated in the fabric design. Other devices, such as color printers can be connected to the network for proofing fabric designs.

When the image is completed, it is sent to the raster image processor (RTP) 551. The raster image processor converts the image information (which may be in the form of a page description language) into a raster image. This module also performs halftoning, to convert the continuous tone image data from the scanned photographs, graphics and other sources into bi-level image data. Systems providing less sophisticated fabric design capabiUties may not require a raster image processor, as the fabric design may be in raster form.

The halftoned image to be printed on the fabric is stored in a bi-level image memory. In the case of a 300 dpi, 2 meter x 1 meter color image, the Bi-level image memory requires approximately 133 MBytes. This can be implemented in DRAM. However, typically, two square meters of non-repeating print pattem is not required. The amount of memory required is proportional to the area of the repeating section of the pattem to be printed. The Bi-level image memory may be a section of the main memory of the raster image processor. Once a binary image of the fabric design has been created, it can be sent to the appropriate digital color fabric printing module 599 for printing. The data is transferred by a digital data link 578. If the data must be changed quickly, this should be a high speed data link. The high speed data Unk may be FDDI, Ethernet, SCSI or other data transfer system.

Figure 7 is a schematic process diagram of a head, memory, and driver circuit of a fabric printing press 599. The computer interface 551 writes the binary data representation of the image to the bi-level image memory 505. When an image is to be printed, the bi-level image memory 505 is read in real-time. This data is then processed by the data phasing and fault tolerance system 506. This unit provides the appropriate delays to synchronize the print data with the offset positions of the nozzles of the printing head. It also provides altemate data paths for fault tolerance, to compensate for blocked nozzles, faulty nozzles or faulty circuits in the head. The printing head 50 prints the image 60 composed of a multitude of ink drops onto the fabric 598. The bi-level image processed by the data phasing and fault tolerance circuit 506 provides the pixel data in the coπect sequence to the data shift registers 56. Data sequencing is required to compensated for the nozzle arrangement and the movement of the fabric. When the data has been loaded into the shift registers, it is presented in paraUel to the heater driver circuits 57. At the coπect time, these driver circuits wtil electronically connect the coπesponding 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, reducing the attraction of the ink to the nozzle surface material. Ink drops 60 escape from the nozzles in a pattem which coπesponds to the digital impulses which have been apphed to the heater driver circuits. The ink drops 60 faU under the influence of their momentum plus gravity or another field type towards the fabric 598. The various subsystems are coordinated under the control of one or more control microcomputers 511. Figure 8 shows a simplified mechanical schematic diagram of a possible implementation ofthe invention. The drive electronics 561 provide data for the printing head 563. The head 563 prints on one side of the fabric 598 only.

The fabric 593 is supphed on a roU 591. The fabric supply roU is driven by a motor 593. The speed of the motor 593 is controUed by the control electronics 561. After printing, the printed fabric is wound onto a take-up roU 592.

The take-up roU 592 is driven by a motor 594 which is controUed by the control electronics 561. The control electronics adjusts the speeds of the motors 593 and

594 so that the fabric speed past the print head 563 is coπectly adjusted for the printing speed ofthe head. A fabric supply tensioning mechanism 595 regulates the tension of the fabric as the fabric leaves the supply roU 591. Another fabric tensioning system 596 adjusts the tension of the fabric wound onto the take-up roll 592. After printing, the fabric moves through a forced air drying region 597, which may use heated air to accelerate drying. This allows the size of the unit to be reduced.

Gravity feed ofthe ink is a convenient way to obtain a stable and accurate ink pressure at the heads. Gravity feed allows the ink to be replenished without interrupting the print cycle. The ink reservoirs 572 can contain an automatic level maintaining system, which may consist of a master reservoir 578 which is connected to a supply reservoir 579. The ink level in the reservoir 579 is regulated by a mechamsm which may be a float valve, or may be an electrical level sensor which controls an electromechanical valve. The level of ink in the reservoir 579 is adjusted such that the ink pressure caused by the difference in height between the head and the ink level is the optimum operating pressure for the head. The ink flowing to the master reservoirs 578 can be piped from a central reservoir which feeds aU of the printing modules in a print shop. In this manner, no manual filling of the ink reservoirs of the individual print modules is required.

Physical configuration There are many possible physical configurations of the invention. Figure 9(a) shows a top view of one possible configuration of the fabric printer 599. The fabric supply roU 591 and fabric takeup roU 592 are shown in this diagram. Also shown are the ink reservoirs 572.

Figure 9(b) shows a side view of one possible configuration ofthe fabric printer 599. The fabric supply roU 591 and fabric takeup roU 592 are shown in this diagram, as weU as an outline of a human figure for scale.

Figure 10 shows a perspective view of one possible configuration of the fabric printer 599. This shows the scale of the machine, with the large fabric supply roll 591 and takeup roU 592. The waUs around the takeup and supply rolls are to prevent personal injury while the machine is operating. They can be omitted to allow easier access to the rolls for replacement

Each roll can hold approximately 5,000 meters of cloth (depending upon cloth thickness) and would weigh in excess of one ton when fuUy laden. The

5,000 meters of cloth can be printed in 10,000 seconds when printed at full speed. Therefore, the cloth roU will need to be replaced approximately every three hours when the system is fuUy operational.

The foregoing describes one embodiment of the present invention.

Modifications, obvious to those skiUed in the art, can be made thereto without departing from the scope of the invention.

Appendix A

LIFT head type Fabric-4-400

This is a four color pnnt head for fabπc pnnting It produces high quality pnnted fabπc at high speed Pnnt speed is approxunately one square metre per second Resolution is 400 dpi Full color results with perfect registration can be achieved

Basic specifications Derivation

Resolution 400 dpi Specification

Pnnt head length 2,007 mm Width of pnnt area, plus 5 mm

Pnnt bead width __ 5 mm Denved from physical and layout constraints of head

Ink colors 4 CMYK

Page size — Continuous Specification

Print area width . 2,002 mm Pixels per line / Resolution

Pnnt area length 500 mm Total length of active pnnting

Page pnnting ume — . 1 9 seconds Denved from scans, lines per page and dot pnnting rate

Pages per minute . 26 ppm 60/(120% of pnnt time m seconds)

Basic IC process . 2 micron CMOS Recommendation

Bitmap memory requirement . 118 3 MBytes Bitmap memory required for one scan (cannot pause)

Pixel spacing — . 63.5 μm Reciprocal of resolution

Pixels per line — . 31,520 Acnve nozzles / Number cf colors

Lines per page — . 7,874 Scan distance times resolution

Pixels per page — . 248,188,480 Pixels per line tunes lines per page

Drops per page — . 992,753,920 Pixels per page tones simultaneous mk colors

Average data rate — . 626 MBytes se Pixels per second • in* colors/ 8 MBits

Ejection energy per drop — - Z307 nJ Energy applied to heater m finite element simulations

Energy to pnnt full black page — . 2290 J Drop ejection energy tunes drops per page

Recording medium speed . 265 cm sec ]/( resolution tunes actuation penod times phases)

Yield and cost Derivation

Number of chips per head — - 10 Recommendation

Wafer size — - 300 mm (12") Recommendation

Chips per wafer — - 36 From chip size and recommended wafer size

Pnnt bead chip area — . 100 cm² Chip width tunes length

Yield without fault tolerance — - 099% Usmg Murphy's method, defect density - 1 per cm²

Yield with fault tolerance — - 90% See fault tolerant yield calculations (D-tl/cm², CF=2)

Functional pnnt heads per month — - 32,400 Assuming 10,000 wafer sums per month

Pnnt head assembly cost — - $800 Estimate

Factory overhead per pnnt head — - $103 Based on SI 20m. cost for refurbished 1.5 μm Fab line amomsed over 5 years, plus SI 6m. PA operatmg cost

Wafer cost per pnnt head — ■ $185 Based on matenals cost of $600 per wafer

Approx. total print head cost — - $1,088 Sum of print head assembly, overhead, and wafer costs AppendixA (cont'd.)

LIFT head type Fabric-4-400

Nozzle and actuation specifications Derivation

Nozzle radius— — .20 μm Specification

Number of actuation phases— _ 8 Specification

Nozzles per phase- — 15.760 From page width, resolution and colors

Active nozzles per head- — 126,080 Actuation phases times nozzles per phase

Redundant nozzles per bead- _. 126,080 Same as active nozzles for 100% redundancy

Total nozzles per head- -.252,160 Active plus redundant nozzles

Drop rate per nozzle- — 4,167 Hz 1/S healer active period times number of phases)

Heater radius- _ 20.5 μm From nozzle geometry and radius

Heater thin film resistivity— _ 2.3 μΩm For heater formed from TaAl

Heater resistance- -.2,963 Ω From heater dimensions and resistivity

Average heater pulse current- —.5.1 mA From heater power and resistance

Heater active period- — .30 μs From finite element simulations

Settling time petween pulses— — 210 μs Active period * (actuation phases-])

Clock pulses per line- -. 18,011 Assuming multiple clocks and no transfer register

Clock frequency— .- 7.5 MHz From clock pulses per line, and lines per second

Drive transistor on resistance- _ 28 n From recommended device geometry

Average head drive voltage- — 15.2 V Heater current * (heater +drive transistor resistance)

Drop selection temperature — rr. SO'C Temperature at which critical surface tension is reached

Heater peak temperature — _ 120 ^«C From finite element simulations

Ink specifications Derivation

Basic ink carrier — _™ Water Specification

Surfactant.. — 1 -Hexadecanol Suggested method of achieving temperature threshold

Ink drop volume- _ 45 pl From finite element simulations

Ink density- — 1.030 g/cm^s Black ink density at 60°C

Ink drop mass— 46.4 ng Ink drop volume times ink density

Ink specific beat capacity— _ 4.2J/KgΛ-C Ink carrier characteristic

Max. energy for self cooling— — 5,818 nI/drop Ink drop heat opacity times temperature increase

Total ink per color per page — _ 11.17 ml Drops per page per color times drop volume

Maximum ink flow rate per color— — 5.91 ml/sec Ink per color per page /page print time

Full black ink coverage — -. 44.6 ml/m² Ink drop volume x colors x drops per square metre

Ejection ink surface tension- __ 38.5 mN/m Surface tension required for ejection

Ink pressure— -. 3.9 kPa 2 x Ejection ink surface tension /nozzle radius

Ink column height— „ . 381 mm Ink column height to achieve ink pressure

Claims

I Claim: A digital printing system for printing on fabric material, compnsmg:

(a) means for moving a fabric web of uniform width along a transport path from a supply to a take up station;

(b) a digital print head assembly located along said transport path and including an integral array of print nozzles extending across the width dimension of the web transport path;

(c) ink supply means for providing fabric printing ink to the nozzles of said array; and

(d) control means for operating said print head assembly, in timed relation with the movement of said web and under the control of pattern data, to print predetermined fabric patterns.

2. The invention defined in claim 1 wherein integral array of print nozzles includes a pluraUty of web width rows of nozzles and said ink supply means suppUes a pluraUty of different color inks respectively to different nozzle rows.

3. The invention defined in claim 2 wherein said array comprises a monolithic siUcon printing head having nozzles and driver circuitry fabricated thereon.

4. The invention defined in claim 1 wherein print head assembly comprises:

( 1 ) a pluraUty of heater elements respectively associated with tips of nozzles of said array;

(2) a plurality of driver means for energizing said heater elements to reduce the surface tension or viscosity of ink on said nozzle tips;

(3) manifold means for coupling said ink supply means to said nozzle means; (4) means for maintaining ink in said manifold at a predetermined pressure above ambient;

(5) means for selectively addressing predetermined driver means to energize their associated heater element; and

(6) drop separating means for attractively separating ink from nozzles having addressed driver means without separating ink from non-addressed nozzles.

5. The invention defined in claim 4 wherein said nozzles, heater elements and driver means are fabricated on a monoUthic siUcon chip.

6. The invention defined in claim 5 wherein said print head assembly is substantially self-cooling.

7. The invention defined in claim 2 wherein said control means comprises an image formation work station, raster image processor means for producing digital halftone binary image data from said work station data and a plurality digital memory means associated respectively with each print head array for storing actuation data from said processor means to be repeatedly addressed to said nozzle arrays during pattern printing.

8. The invention defined in claim 1 wherein print head assembly comprises:

(1) a pluraUty of heater elements respectively associated with tips of nozzles of said aπay;

(2) a plurality of driver means for energizing said heater elements to reduce the surface tension or viscosity of ink on said nozzle tips;

(3) manifold means for coupling said ink supply means to said nozzle means;

(4) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (5) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and

(6) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while aUowing ink to be retained in non-selected nozzles.

9. The invention defined in claim 1 wherein print head assembly comprises:

( 1 ) a pluraUty of heater elements respectively associated with tips of nozzles of said array;

(2) a plurality of driver means for energizing said heater elements to reduce the surface tension or viscosity of ink on said nozzle tips;

(3) manifold means for coupling said ink supply means to said nozzle means;

(4) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and

(5) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while aUowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means.

10. The invention defined in claim 1 wherein print head assembly comprises:

(1) a pluraUty of heater elements Respectively associated with tips of nozzles of said array;

(2) a plurality of driver means for energizing said heater elements to reduce the surface tension or viscosity of ink on said nozzle tips; (3) manifold means for coupling said ink supply means to said nozzle means;

(4) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperamre range;

(5) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and

(6) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles.

11. A digital printing system for printing on fabric web including:

(a) a raster image processing computer for producing digitally halftoned binary image data;

(b) digital memory means for receiving storing and binary image data;

(c) a pluraUty of digital printing heads;

(d) a fabric web transport system which moves said fabric past said printing heads for printing; and

(e) an ink reservoir and ink pressure regulation system which maintains predetermined positive pressure ink flow to said heads.

12. The invention defined in claim 11 wherein said printing heads are integral and extend transversely across the path of web transport.

13. The invention defined in claim 12 wherein said ink reservoir system provides a pluratity of different color inks respectively to different printing heads.

14. The invention defined in claim 13 wherein the different printing heads recovered different image data so as to provide accurate registration of different color patterns during printing on the fabric passing said printing heads.