A NOZZLE CLEARING PROCEDURE FOR LIQUID INK PRINTING

Abstract: A method of clearing nozzles of thermally activated drop on demand printing head which have been blocked by dried ink by applying a consecutive sequence of heater energizing pulses. The method is particularly applicable to coincident forces drop on demand printing systems. A series of consecutive pulses is applied to the heater. These pulses accumulate heat at the nozzle tip, and the ink in contact with the tip, above the boiling point of the ink. The formation of vapor bubbles at the tip can dislodge a crust of dried ink, which is then expelled with the ink drop. A single power pulse of longer than normal duration, or greater than normal energy, can be used. However, this may require substantial additional electronic circuitry to generate. The need for this circuitry is eliminated by applying an integral number (greater than one) of consecutive power pulses of the energy, duration, and time varying power profile that is used to expel drops on demand under normal operating conditions. A nozzle clearing sequence can be automatically applied to all nozzles before printing each page.

Description: A NOZZLE CLEARING PROCEDURE FOR LIQUID INK PRINTING Field of the Invention The present invention is in the field of computer controlled printing devices. In particular, the field is thermally activated drop on demand (DOD) printing systems. Background of the Invention Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, for example, being able to produce high quality color images at a high-speed and low cost, using standard paper. Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Many types of ink jet printing mechanisms have been invented. These can be categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink jet. Continuous ink jet printing dates back to at least 1929: Hansell, US Pat. No. 1,941,001. Sweet et al US Pat. No. 3,373,437, 1967, discloses an array of continuous ink jet nozzles where ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection CIJ, and is used by several manufacturers, including Elmjet and Scitex. Hertz et al US Pat. No. 3,416,153, 1966, discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics. Kyser et al US Pat. No. 3,946,398, 1970, discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 720 dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance. Endo et al GB Pat. No. 2,007,162, 1979, discloses an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in thermal contact with ink in a nozzle. The heater rapidly heats water based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble. The formation of these bubbles results in a pressure wave which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet™ (trademark of Canon K.K. of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers. Vaught et al US Pat. No. 4,490,728, 1982, discloses an electrothermal drop ejection system which also operates by bubble formation. In this system, drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater. This system is known as Thermal Ink Jet, and is manufactured by Hewlett-Packard. In this document, the term Thermal Ink Jet is used to refer to both the Hewlett- Packard system and systems commonly known as Bubblejet™. Thermal Ink Jet printing typically requires approximately 20 μJ over a period of approximately 2 μs to eject each drop. The 10 Watt active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements. Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, U.S. Patent No.4,275,290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head. U.S. Patent Nos. 4,737,803; 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a print sheet Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved ink jet printing approach, providing advantages for example, as to cost, speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables. Summary of the invention My concurrently filed applications, entitled "Liquid Ink Printing Apparatus and System" and "Coincident Drop-Selection, Drop-Separation Printing Method and System" describe new methods and apparatus that afford significant improvements toward overcoming the prior an problems discussed above. Those inventions offer important advantages, e.g., in regard to drop size and placement accuracy, as to printing speeds attainable, as to power usage, as to durability and operative thermal stresses encountered and as to other printer performance characteristics, as well as in regard to manufacturability and the characteristics of useful inks. One important purpose of the present invention is to further enhance the structures and methods described in those applications and thereby contribute to the advancement of printing technology. More particularly, the invention provides a method for clearing blocked nozzles of drop on demand printing heads the method including the step of applying sufficient energy to the heater to cause ink in a blocked nozzle to be raised above the boiling point of the ink. A preferred aspect of the invention is that the energy applied to the heater is generated by applying electric power for a predetermined duration, and the heater is electrically resistive. A further preferred aspect of the invention is that the electric power is substantially similar to the electric power which is applied when a drop is to ejected on demand during normal printing operation. A further preferred aspect of the invention is that the duration is an integral multiple, greater than one, of the duration of the electric power which is applied when a drop is to ejected on demand during normal printing operation. In another aspect, the present invention constitutes a drop on demand printing apparatus comprising a print head having a plurality of drop ejection nozzles, an ink supply and means for energizing said print head to effect drop ejections, a system for clearing ink from blocked nozzles comprising: (1) means for heating ink in said nozzles; and (2) means for energizing said heating means to cause ink in the nozzles to be raised above its boiling point Brief Description of the Drawings Figure 1 (a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention. Figure 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention. Figures 2(a) to 2(f) show fluid dynamic simulations of drop selection. Figure 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention. Figure 3(b) shows successive meniscus positions during drop selection and separation. Figure 3(c) shows the temperatures at various points during a drop selection cycle. Figure 3(d) shows measured surface tension versus temperature curves for various ink additives. Figure 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of figure 3(c) Figure 4 shows a block schematic diagram of print head drive circuitry for practice of the invention. Figure 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance. Figure 6 shows a generalized block diagram of a printing system using a print head. Figure 7(a) shows the power applied to a heater during a four pulse clearing cycle. Figure 7(b) shows the temperature histories of various points in the nozzle during a four pulse clearing cycle. Figures 8(a) to 8(h) show thermal contours during a four pulse clearing cycle. Detailed Description of Preferred Embodiments In one general aspect, the invention constitutes a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an alternative means is provided to cause separation of the selected drops from the body of ink. The separation of drop selection means from drop separation means significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles. The drop selection means may be chosen from, but is not limited to, the following list: 1) Electrothermal reduction of surface tension of pressurized ink 2) Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection 3) Piezoelectric, with insufficient volume change to cause drop ejection 4) Electrostatic attraction with one electrode per nozzle The drop separation means may be chosen from, but is not limited to, the following list: 1) Proximity (recording medium in close proximity to print head) 2) Proximity with oscillating ink pressure 3) Electrostatic attraction 4) Magnetic attraction The table "DOD printing technology targets" shows some desirable 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 In thermal ink jet (TU) and piezoelectric ink jet systems, a drop velocity of approximately 10 meters per second is preferred to ensure at 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 TU print heads must switch high currents. The drive circuits for piezoelectric ink jet 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 TU 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 pagewiddi TU 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 ti at the drop separation means can discriminate between selected and unselected drops. Drop selection means Other drop selection means may also be used. The preferred drop selection means for water based inks is method 1: "Electrothermal reduction of surface tension of pressurized ink". This drop selection means provides many advantages over otiier 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: 'Εlectrothermal 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 Other drop separation means may also be used. The preferred drop separation means depends upon the intended use. For most applications, method 1: "Electrostatic attraction", or method 2: "AC electric field" are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: "Proximity" may be appropriate. For high speed, high quality systems, method 4: 'Transfer proximity" can be used. Mediod 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 foDowing Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'A Liquid ink Fault Tolerant (LIFT) printing mechanism' (Filing no.: PN2308); 'Electrothermal drop selection in LIFT printing' (Filing no.: PN2309); 'Drop separation in LIFT printing by print media proximity' (Filing no.: PN2310); 'Drop size adjustment in Proximity LIFT printing by varying head to media distance' (Filing no.: PN2311); 'Augmenting Proximity LIFT printing with acoustic ink waves' (Filing no.: PN2312); 'Electrostatic drop separation in LIFT printing' (Filing no.: PN2313); 'Multiple simultaneous drop sizes in Proximity LIFT printing' (Filing no.: PN2321); 'Self cooling operation in thermally activated print heads' (Filing no.: PN2322); and 'Thermal Viscosity Reduction LIFT 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 otiier 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 (RIP) 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 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 die 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 di op. A constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown). For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown), When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51, while unselected drops remain part of the body of ink. The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to die 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 external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51. A convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive. In this case, me 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 die ink drop is very small; approximately 10 "4 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 die 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 d e 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 otiier 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 tiiis 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 d e 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 die invention may vary in shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have d e advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent metiiods for eliminating orifice plates include die use of 'vortex' actuators such as those described in Domoto et al US Pat. No. 4,580,158, 1986, assigned to Xerox, and Miller et al US Pat. No. 5,371,527, 1994 assigned to Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred method for elimination of orifice plates for print heads of die 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 Prop Separation As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is shown in figure 2. Figure 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA. This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 μm, at an ambient temperature of 30°C. The total energy applied to die heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and die 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 d e central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 μs. Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes die nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantiy from the print head surface, so the electrostatic field is not significantiy concentrated at d e 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, die ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of die meniscus to rapidly expand relative to die 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 die ink surface, and not just where me ink is in contact with die heater. This is because viscous drag against the solid heater prevents the ink direcdy 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 die equilibrium of forces. As the entire meniscus has been heated, die 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 die 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 tiiermal contours at 5°C intervals 30 μs after the start of the heater energizing pulse, which is also 6 μs after the end of die heater pulse, as the heater pulse duration is 24 μs. The nozzle tip has rapidly cooled due to conduction tiirough the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by die ink. Electrostatic attraction causes the ink drop to begin to accelerate towards die recording medium. Were d e heater pulse significantiy shorter (less than 16 μs in this case) die ink would not accelerate towards die print medium, but would instead return to the nozzle. Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end of die 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 d e nozzle tip. When die rate at which the ink is drawn from die nozzle exceeds die viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and die selected drop separates from the body of ink. The selected drop tiien travels to the recording medium under die influence of the external electrostatic field. The meniscus of the ink at die nozzle tip then returns to its quiescent position, ready for die 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 electrically controlled, 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 die meniscus. The heater pulse starts 10 μs into the simulation. Figure 3(c) shows die resultant curve of temperature with respect to time at various points in die nozzle. The vertical axis of the graph is temperature, in units of 100°C. The horizontal axis of the graph is time, in units of 10 μs. The temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 μs time steps. The local ambient temperature is 30 degrees C. Temperature histories at three points are shown: A - Nozzle tip: This shows me temperature history at the circle of contact between the passivation layer, die ink, and air. B - Meniscus midpoint: This is at a circle on the ink meniscus midway between die nozzle tip and die centre of the meniscus. C - Chip surface: This is at a point on the print head surface 20 μm from the centre of die nozzle. The temperature only rises a few degrees. This indicates tiiat active circuitry can be located very close to die nozzles without experiencing performance or lifetime degradation due to elevated temperatures. Figure 3(e) shows the power applied to die heater. Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a litde below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of die pulse. To achieve tiiis, the average energy applied to the heater is varied over the duration of die pulse. In this case, the variation is achieved by pulse frequency modulation of 0.1 μs sub-pulses, each witii an energy of 4 nJ. The peak power applied to the heater is 40 mW, and die average power over die 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 significantiy affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power applied to die 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 (RFT). Inks with a negative temperature coefficient of surface tension The requirement for the surface tension of the ink to decrease with increasing temperature is not a major restriction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids: Yτ Where γ^is die surface tension at temperature T, k is a constant, _T c is the critical temperature of the liquid, M is die molar mass of the liquid, x is the degree of association of d e liquid, and p is die density of die liquid. This equation indicates mat the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, die critical temperature is substantially above the boiling point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, die 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 die surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C, lower tiian tiiat of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse tiiis effect, and achieve a surface tension which decreases slighdy with temperature. However, a relatively large decrease in surface tension witii temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins, while as litde as lOmN/m can be used to achieve operation of the print head according to d e present invention. Inks With Larpe - . Several methods may be used to achieve a large negative change in surface tension with increasing temperature. Two such methods are: 1 ) The ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less tiian 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 temperature, but below the boiling point of die ink. For stability, the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80°C is suitable. Inks with Surfactant Sols Inks can be prepared as a sol of small particles of a surfactant which melts in die desired operating temperature range. Examples of such surfactants include carboxylic acids witii between 14 and 30 carbon atoms, such as: As e melting point of sols with a small particle size is usually slighdy less than of d e bulk material, it is preferable to choose a carboxylic acid with a melting point slighdy above the desired drop selection temperature. A good example is Arachidic acid. These carboxylic acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding diem to d e ink is insignificant A mixture of carboxylic acids witii slighdy varying chain lengths can be used to spread d e melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of die surfactant It is desirable tiiat die polar end be ionizable in water, so that the surface of die surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxylic acids, tiiis can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide. Preparation of Inks with Surfactant Sols The surfactant sol can be prepared separately at high concentration, and added to die ink in me required concentration. An example process for creating the surfactant sol is as follows: 1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil. 3) Ultrasonicate the mixture, until the typical size of the carboxylic acid droplets is between lOOA and l.OOOA. 4) Allow the mixture to cool. 5) Decant the larger particles from die top of the mixture. 6) Add an alkali such as NaOH to ionize die carboxylic acid molecules on the surface of die particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps stabilize the sol. 7) Centrifuge the sol. As the density of die carboxylic acid is lower than water, smaller particles will accumulate at the outside of d e centrifuge, and larger particles in the centre. 8) Filter the sol using a microporous filter to eliminate any particles above 5000 A. 9) Add die surfactant sol to the ink preparation. The sol is required only in very dilute concentration. The ink preparation will also contain either dye(s) or pigment(s), bactericidal agents, agents to enhance the electrical conductivity of d e ink if electrostatic drop separation is used, humectants, and other agents as required. Anti-foaming agents will generally not be required, as there is no bubble formation during die 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 die anionic 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: The method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an alkali is used to adjust die pH balance and increase the charge on the surfactant particles. A pH of 6 using HC1 is suitable. Microemulsion Based Inks An alternative means of achieving a large reduction in surface tension as some temperature threshold is to base d e ink on a microemulsion. A microemulsion is chosen with a phase inversion temperature (PIT) around die desired ejection tiireshold temperature. Below the PIT, the microemulsion is oil in water (O W), and above die PIT die microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantiy above the PIT, the surfactant prefers a high curvature surface around water. At temperatures close to the PIT, the microemulsion forms a continuous 'sponge' of topologically connected water and oil. There are two mechanisms whereby this reduces die surface tension. Around die PIT, the surfactant prefers surfaces with very low curvature. As a result, surfactant molecules migrate to the ink/air interface, which has a curvature which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from O/W to W/O, and therefore the 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 possibilities for the preparation of microemulsion based inks. For fast drop ejection, it is preferable to chose a low viscosity oil. In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be required. In tiiese cases, polar solvents witii 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(oxyedιylene)alkylphenyl ether (etiioxylated alkyl phenols, general formula: C n H2n + ιC H6(CH 2 CH 2 O) m OH) can be used. The hydrophilicity of die surfactant can be increased by increasing m, and d e 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 die result of a polymerization of various molar ratios of etiiylene oxide and alkyl phenols, and die exact number of oxyediylene groups varies around die 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 8 Hι 7 C H6(CH 2 CH 2 θ) n OH (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 die cloud point is 65°C. Commercial preparations of mis surfactant are available under various brand names. Suppliers and brand names are listed in the following table: These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per hter to prepared microemulsion ink with a 5% surfactant concentration. Otiier suitable ethoxylated alkyl phenols include tiiose listed in die following table: Microemulsion based inks have advantages other than surface tension control: 1) Microemulsions are therm odynamically stable, and will not separate. Therefore, the storage time can be very long. This is especially significant for office and portable printers, which may be used sporadically. 2) The microemulsion will 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 mixture of dyes, one soluble in water, and die other soluble in oil, to obtain specific colors. 4) Oil miscible pigments are prevented from flocculating, as they are trapped in die oil microdroplets. 5) The use of a microemulsion can reduce die mixing of different dye colors on die 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 Oil in water mixtures can have high oil contents - as high as 40% and still form O/W microemulsions. This allows a high dye or pigment loading. Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with botii dye and pigment is as follows: 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 die microemulsion that may be used. The nintii combination, witii no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highlights. As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil-water boundary layer as this layer has a very large surface area. It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments in each phase. When using multiple dyes or pigments the absorption spectrum of the resultant ink will be the weighted average of the absorption spectra of the different colorants used. This presents two problems: 1) The absorption spectrum will tend to become broader, as die absorption peaks of both colorants are averaged. This has a tendency to 'muddy' die colors. To obtain brilliant color, careful choice of dyes and pigments based on their absorption 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 die printed ink color on more absorptive papers, as die dye will be absorbed into the paper, while the pigment will tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances. Surfactants with a Krafft point in the drop selection temperature range For ionic surfactants there is a temperature (the Krafft point) below which the solubihty is quite low, and die solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and tiiere is a rapid increase in solubility of the surfactant. If the critical micelle concentration (CMC) exceeds die solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at die point of maximum solubility, rather than at the CMC. Surfactants are usually 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 die nozzle heater is turned on, die temperature rises, and more of the surfactant goes into solution, decreasing die surface tension. A surfactant should be chosen witii 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 temperatures, and die concentration of surfactant in solution at the drop selection temperature. The concentration of surfactant should be approximately equal to the CMC at die Krafft point. In this manner, the surface tension is reduced to die maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures. The following table shows some commercially available surfactants with Krafft points in the desired range. Surfactants with a cloud point in the drop selection temperature range Non-ionic surfactants using polyoxyetiiylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature. At low temperatures, the POE chain is hydrophilic, and maintains the surfactant in solution. As the temperature increases, the structured water around die POE section of die molecule is disrupted, and die POE section becomes hydrophobic. The surfactant is increasingly rejected by die water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C Polyoxypropylene (POP) can be combined with POE in POE POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures. Two main configurations of symmetrical POE POP block copolymers are available. These are: 1) Surfactants with POE segments at the ends of die 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 d e molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (generically also CAS 9003-11-6) Some commercially available varieties of poloxamer and meroxapol witii a high surface tension at room temperature, combined with a cloud point above 40°C and below 100°C are shown in the following table: Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40°C and 100°C, and preferably between 60°C and 80°C. Meroxapol [HO CHCHjCHzOMCHzCHzO^CHCHjCHzOkOH] 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 of die ink, then the effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I " ), as tiiis makes more water molecules available to form hydrogen bonds witii d e POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl " , OH " ), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively httie effect The ink composition can be 'tuned' for a desired temperature range by altering the lengdis of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl " to Br ' to I " ) that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl slightly lowers the cloud point of nonionic surfactants. Hot Melt Inks The ink need not be in a liquid state at room temperature. Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that die surface tension of the molten ink decreases witii temperature. A decrease of approximately 2 mN/m will be typical of many such preparations using waxes and otiier 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 tiian a reduction in viscosity. The temperature difference between quiescent temperature 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 d e water. The ink must be liquid at the quiescent temperature. The quiescent temperature should be higher than the highest ambient temperature likely to be encountered by die printed page. T he quiescent temperature should also be as low as practical, to reduce die power needed to heat die print head, and to provide a maximum margin between the quiescent and die drop ejection temperatures. A quiescent temperature between 60°C and 90°C is generally suitable, though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generally suitable. There are several methods of achieving an enhanced reduction in surface tension with increasing temperature. 1 ) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the liquid phase. 2) A polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the polar and non-polar compounds. To achieve a large reduction in surface tension with temperature, it is desirable tiiat the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes such as waxes. Suitable materials will generally have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88°C. Surface tension reduction of various solutions Figure 3(d) shows die measured effect of temperature on die surface tension of various aqueous preparations containing the following 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 following Australian patent specifications, die disclosure of which are hereby incorporated by reference: 'Ink composition based on a microemulsion' (Filing no.: PN5223, filed on 6 September 1995); 'Ink composition containing surfactant sol' (Filing no.: PN5224, filed on 6 September 1995); 'Ink composition for DOD printers with Krafft point near the drop selection temperature sol' (Filing no.: PN6240, filed on 30 October 1995); and 'Dye and pigment in a microemulsion based ink' (Filing no.: PN6241, filed on 30 October 1995). Operation Using Reduction of Viscosity As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination witii hot melt ink, is as follows. Prior to operation of the printer, solid ink is melted in die reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature 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 die viscosity of die ink reduces with increasing temperature. The ink pressure oscillates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle. The ink pressure oscillation causes oscillations of the ink meniscus at the nozzle tips, but tiiis oscillation is small due to die high ink viscosity. At the normal operating temperature, these oscillations are of insufficient amplitude to result in drop separation. When die heater 103 is energized, die 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 fiirther during the high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficiendy close to die print head 50 so tiiat die selected drops contact the recording medium 51, but sufficiendy far away that die 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 die ink pressure falls, 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 amplitude oscillation. The viscosity of d e ink increases to its quiescent level as remaining heat is dissipated to die bulk ink and print head. One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. Manufacturing of Print Heads Manufacturing processes for monolithic print heads in accordance with die present invention are described in die following Australian patent specifications filed on 12 April 1995, die disclosure of which are hereby incorporated by reference: 'A monolithic LIFT printing head' (Filing no.: PN2301); 'A manufacturing process for monolithic LIFT printing heads' (Filing no.: PN2302); 'A self-aligned heater design for LIFT print heads' (Filing no.: PN2303); 'Integrated four color LIFT print heads' (Filing no.: PN2304); 'Power requirement reduction in monolithic LIFT printing heads' (Filing no.: PN2305); 'A manufacturing process for monolithic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monolithic drop-on-demand print heads' (Filing no.: PN2307); 'Heater structure for monolithic LIFT print heads' (Filing no.: PN2346); 'Power supply connection for monolithic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (Filing no.: PN2348); and 'A self- aligned manufacturing process for monolithic LIFT print heads' (Filing no.: PN2349); and 'CMOS process compatible fabrication of LIFT print heads' (Filing no.: PN5222, 6 September 1995). 'A manufacturing process for LIFT print heads witii nozzle rim heaters' (Filing no.: PN6238, 30 October 1995); 'A modular LIFT print head' (Filing no.: PN6237, 30 October 1995); 'Method of increasing packing density of printing nozzles' (Filing 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 temperature in print heads of die present invention is described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'Integrated drive circuitry in LIFT print heads' (Filing no.: PN2295); 'A nozzle clearing procedure for Liquid Ink Fault Tolerant (LIFT) printing' (Filing no.: PN2294); 'Heater power compensation for temperature in LIFT printing systems' (Filing no.: PN2314); 'Heater power compensation for thermal lag in LIFT printing systems' (Filing no.: PN2315); 'Heater power compensation for print density in LIFT printing systems' (Filing no.: PN2316); ' Accurate control of temperature pulses in printing heads' (Filing no.: PN2317); 'Data distribution in monolithic LIFT print heads' (Filing no.: PN2318); 'Page image and fault tolerance routing device for LIFT printing systems' (Filing no.: PN2319); and 'A removable pressurized liquid ink cartridge for LIFT printers' (Filing no.: PN2320). Image Processing for Print Heads An objective of printing systems according to die invention is to attain a print quality which is equal to that which people are accustomed to in quality color pubhcations printed using offset printing. This can be achieved using a print resolution of approximately 1,600 dpi. However, 1,600 dpi printing is difficult and expensive to achieve. Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and black. This color model is herein called CC'MM' YK. Where high quahty monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein called CC'MM'YKK'. Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and other printing systems are described in the following Australian patent specifications filed on 12 April 1995, die disclosure of which are hereby incorporated 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 apparatus and metiiods of tiiis invention are suitable for a wide range of applications, including (but not limited to) die following: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color printing, 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, facsimile and copying machines, label printing, large format plotters, photographic duphcation, printers for digital photographic processing, portable printers incorporated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal Digital Assistants' , wallpaper printing, indoor sign printing, billboard printing, and fabric printing. Printing systems based on this invention are described in d e following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'A high speed color office printer witii a high capacity digital page image store' (Filing no.: PN2329); 'A short run digital color printer witii a high capacity digital page image store' (Filing no.: PN2330); 'A digital color printing press using LIFT printing technology' (Filing no.: PN2331); 'A modular digital printing press' (Filing no.: PN2332); 'A high speed digital fabric printer' (Filing no.: PN2333); 'A color photograph copying system' (Filing no.: PN2334); 'A high speed color photocopier using a LIFT printing system' (Filing no.: PN2335); A portable color photocopier using LIFT printing technology' (Filing no.: PN2336); 'A photograph processing system using LIFT printing technology' (Filing no.: PN2337); 'A plain paper facsimile machine using a LIFT printing system' (Filing no.: PN2338); 'A PhotoCD system with integrated printer' (Filing no.: PN2293); 'A color plotter using LIFT printing technology' (Filing no.: PN2291); 'A notebook computer with integrated LIFT color printing system' (Filing no.: PN2292); 'A portable printer using a LIFT printing system' (Filing no.: PN2300): 'Fax machine with on-line database interrogation and customized magazine printing' (Filing no.: PN2299); 'Miniature portable color printer' (Filing no.: PN2298); 'A color video printer using a LIFT printing system' (Filing no.: PN2296); and 'An integrated printer, copier, scanner, and facsimile using a LIFT printing system' (Filing no.: PN2297) Compensation of Print Heads for Environmental Conditions It is desirable that drop on demand printing systems have consistent and predictable ink drop size and position. Unwanted variation in ink drop size and position causes variations in the optical density of die resultant print, reducing die perceived print quahty. These variations should be kept to a small proportion of die nominal ink drop volume and pixel spacing respectively. There are many factors which can affect drop volume and position. In some cases, the variation can be minimized by appropriate head design. In otiier cases, the variation can compensated by active circuitry. 1) Ambient temperature: Changes in ambient temperature can affect the quiescent meniscus position, and die temperature achieved by die heater pulse. Changes in the quiescent meniscus position can be compensated by altering the ink pressure or the strength of the external electric or magnetic field. Changes in the temperature achieved by die heater pulse can be compensated by altering the power supphed to the heater. 2) Nozzle temperature: It is not practical to compensate for temperature independently for each nozzle. Reliable operation of heads requires that the difference between die nozzle temperature and die ambient temperature measured at the substrate is small. This can be achieved using a substrate with high thermal conductivity (such a silicon), and allowing adequate time between pulses for the waste heat to dissipate. 3) Nozzle radius: The variation in nozzle radius for nozzles supplied from a single ink reservoir should be minimized, as it its difficult to supply different electric field strengths or ink pressures on a nozzle by nozzle basis. Fortunately, the variation in nozzle radius can readily be kept below 0.5 μm using modern semiconductor manufacturing equipment. 4) Print density: Different numbers of ink drops may be ejected in each cycle. As a result the load resistance of the head may vary widely and rapidly, causing voltage fluctuations due to the finite resistance of the power supply and wiring. This can be accurately compensated by digital circuitry which determines the number of drops to be ejected in each cycle, and alters the power supply voltage to compensate for load resistance changes. 5) Ink contaminants: The ink must be free of contaminants larger than approximately 5 μm, which may lodge against each other and clog die nozzle. This can be achieved by placing a 5 μm absolute filter between the ink reservoir and the head. 6) Ink surface tension characteristics: The most important requirement of d e ink is the surface tension characteristics. The ink must be formulated so tiiat the surface tension is high enough to retain the ink in the nozzle at ambient temperatures within the design limits, and falls below the ejection threshold at temperatures achievable by the heater. Many ink formulations can meet these criteria, but care must be taken to control contaminants which affect surface tension. 7) Ink drying: If die period between drop ejections from a nozzle becomes too long, then the ink at the exposed meniscus may dry out to die extent that drop ejection is affected or prevented. This can be compensated by ejecting one or more drops from each nozzle between each printed page, and capping the printhead during idle periods. 8) Pulse widdi: Heater pulse widtii can be accurately controlled, and may be set very close to the minimum pulse widtii. Higher reliability can be achieved by making the pulse width considerably longer tiian the minimum. For a 7 μm nozzle using water based ink as herein described, the minimum pulse widtii is approximately 10 μs. The nominal pulse widtii is set at 18 μs to give a wide operating margin. Pulse width has almost no effect on drop size. 9) Clogged or defective nozzles: In many cases, clogged nozzles may be cleared by providing a rapid sequence of pulses to the heater, raising the ink above the boiling point. The vapor bubbles thus formed can dislodge d e 'crust' of dried ink. Persistent clogged nozzles may be periodically cleared using a solvent. Nozzles which are defective or permanently clogged can be automatically replaced by redundant nozzles using inbuilt fault tolerance. 10) Print media roughness: This is particularly significant for proximity printing, where media roughness may be a significant fraction of d e head to media distance. Protruding fibers in a paper medium may cause die ink drop to wick into the paper sooner than intended, resulting in less ink transferred to the paper, and a smaller drop size. This can be compensated by using coated paper, compressing die paper fibers witii rollers before printing, and/or coating or wetting the paper immediately prior to printing. Nozzle temperature control The performance of nozzles is sensitive to the temperature and duration of tiiermal pulses applied to die nozzle tip. If too httie energy is supplied to the heater, die temperature at the nozzle tip will not rise fast enough for a drop to be ejected in the allotted time, or the ejected ink drop may be smaller than required. If too much energy is supphed to d e heater, too much ink may be ejected, die ink may boil, and d e energy used by die print head will be greater than required. This energy may then exceed the limit for self-cooling operation. The amount of energy required to activate a nozzle can be determined by dynamic finite element analysis of the nozzle. This method can determine die required ejection energy of die nozzle under various static and dynamic environmental circumstances. An optimum temperature profile for a head involves an instantaneous raising of die active region of die nozzle tip to the ejection temperature, maintenance of this region at the ejection temperature for the duration of die pulse, and instantaneous cooling of the region to the ambient temperature. This optimum is not achievable due to die stored heat capacities and diermal conductivities of the various materials used in the fabrication of die nozzles. 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. To obtain accurate results, a transient fluid dynamic simidation with free surface modeling is required, as convection in the ink, and ink flow, significantly affect on the temperature achieved with a specific power curve. Compensation techniques Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power applied to die nozzle heaters. An optimum temperature profile for a print head involves an instantaneous raising of the active region of the nozzle tip to the ejection temperature, maintenance of this region at the ejection temperature for the duration of d e pulse, and instantaneous cooling of the region to the ambient temperature. This optimum is not achievable due to die stored heat capacities and thermal conductivities of the various materials used in the fabrication of the nozzles. 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 apphed to die heater can be varied in time by various techniques, including, but not limited to: 1) Varying the voltage apphed to die heater 2) Modulating die width of a series of short pulses (PWM) 3) Modulating die 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 on the temperature achieved with a specific power curve. By the incorporation of appropriate digital circuitry on the print head substrate, it is practical to individually control the power apphed 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 of the environmental factors which may be compensated for is hsted in the table "Compensation for environmental factors". This table identifies which environmental factors are best compensated globally (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle. Compensation for environmental factors Most applications will not require compensation for all of these variables. Some variables have a minor effect, and compensation is only necessary where very high image quahty is required. Print head drive circuits Figure 4 is a block schematic diagram showing electronic operation of the print head driver circuits. Figure 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using the CC'MM'YK color model. The print head 50 has a total of 79,488 nozzles, with 39,744 main nozzles and 39,744 redundant nozzles. The main and redundant nozzles are divided into six colors, and each color is divided into 8 drive phases. Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel data for enabling heater drive circuits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each shift register is composed of 828 shift register stages 217, the outputs of which are logically anded with phase enable signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as shown in figure 1(b). To maintain the shifted data valid during the enable pulse, the clock to the shift register is stopped die enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a range of well known glitch free clock control circuits. Stopping the clock of die shift register removes the requirement for a parallel data latch in the print head, but adds some complexity to die control circuits in the Head Control ASIC 400. Data is routed to eitiier the main nozzles or the redundant nozzles by die data router 219 depending on die state of the appropriate signal of the fault status bus. The print head shown in figure 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance. Drive circuits for different configurations of print head can readily be derived from d e apparatus disclosed herein. Digital information representing patterns of dots to be printed on the recording medium is stored in d e Page or Band memory 1513, which may be the same as d e Image memory 72 in figure 1(a). Data in 32 bit words representing dots of one color is read from die Page or Band memory 1513 using addresses selected by die address mux 417 and control signals generated by die 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 containing faults can be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as die page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so tiiat the Address generators 411 generate the address corresponding to d e position of the redundant nozzles. Data read from die Page or Band memory 1513 is latched by die latch 413 and converted to four sequential bytes by die multiplexer 414. Timing of these bytes is adjusted to match mat of data representing other colors by the FIFO 415. This data is then buffered by die buffer 430 to form the 48 bit main data bus to die 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 d e data output of the FIFO 415, and buffered by d e buffer 431 to form me fault status bus. The programmable power supply 320 provides power for die head 50. The voltage of die power supply 320 is controlled by die 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 die Microcontroller 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by die microcontroller 315 based on temperature 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 incorporated in die Microcontroller 315. The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time- varying voltage which is synchronized widi the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce tiiis voltage. An analog time varying programming voltage is produced by die DAC 313 based upon data read from me 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 die 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 die enable counter 404. The count from the enable counter 404 is then decoded by die decoder 405 and buffered by die buffer 432 to produce die 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 odier device which causes the counter 403 to count in a non-linear fashion. Alternatively, 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 phases in a head depend upon the specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that the number of 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 valid for the duration of die enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to die current enable phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of die dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and die most significant 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 array. These two dimensions are time (for thermal lag compensation) and print density. A third dimension - temperature - can be included. As the ambient temperature of the head varies only slowly, the microcontroller 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at the current temperature. Periodically (for example, a few times a second), d e microcontroller senses the current head temperature and calculates this matrix. The foDowing equation can be used to calculate the matrix of numbers to be stored in the dual port RAM 317: Where: Vps is the voltage specified to die programmable power supply 320; Row is the output resistance of the programmable power supply 320, including die connections to the head 50; R H is the resistance of a single heater; p is a number representing the number of heaters that are turned on in the current enable period, as provided by die multiplexer 401; n is a constant equal to the number of heaters represented by one least significant bit of p; t is time, divided into number of steps over the period of a single enable pulse; P(t) is a function defining die power input to a single heater required to achieve improved drop ejection. This function depends upon the specific geometry and materials of the nozzle, as well as various characteristics of the ink. It is best determined by comprehensive computer simulation, combined witii experimentation; TE is the temperature required for drop ejection in °C; and TA is die 'ambient' temperature of the head as measured by the temperature sensor in °C. To reduce execution time and simplify programming for me microcontroller, most or all of the factors can be pre-calculated, and simply looked up in a table stored in the microcontroller's ROM. Comparison with thermal ink iet technology The table "Comparison between Thermal ink jet and Present Invention" compares the aspects of printing in accordance with die present invention witii thermal ink jet printing technology. A direct comparison is made between the present invention and thermal ink jet technology because both are drop on demand systems which operate using thermal actuators and liquid ink. Although tiiey may appear similar, die two technologies operate on different principles. Thermal ink jet printers use the following fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in liquid ink. Rapid and consistent bubble formation can be achieved by superheating die ink, so that sufficient heat is transferred 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 die aperture with high velocity. The bubble tiien collapses, drawing ink from the ink reservoir to re-fill the nozzle. Thermal ink jet printing has been highly successful commercially due to the high nozzle packing density and die use of well established integrated circuit manufacturing techniques. However, thermal ink jet printing technology faces significant technical problems including multi-part precision fabrication, device yield, image resolution, 'pepper' noise, printing speed, drive transistor power, waste power dissipation, satellite drop formation, diermal stress, differential tiiermal expansion, kogation, cavitation, rectified diffusion, and difficulties in ink formulation. Printing in accordance with die present invention has many of the advantages of diermal ink jet printing, and completely or substantially eliminates many of the inherent problems of thermal ink jet technology. Comparison between Thermal ink jet and Present Invention Yield and Fault Tolerance In most cases, monolithic integrated circuits cannot be repaired if they are not completely functional when manufactured. The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has a direct influence on manufacturing cost. A device with a yield of 5% is effectively ten times more expensive to manufacture 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 limitation on total yield. Full pagewidth color heads in accordance with this invention are very large in comparison witii typical VLSI circuits. Good wafer sort yield is critical to the cost-effective manufacture of such heads. Figure 5 is a graph of wafer sort yield versus defect density for a monolithic full widtii color A4 head embodiment of me invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according to Murphy's method, which is a widely used yield prediction method. With a defect density of one defect per square cm, Murphy's method predicts a yield less than 1%. This means that more than 99% of heads fabricated would have to be discarded. This low yield is highly undesirable, as the print head manufacturing cost becomes unacceptably high. Murphy's method approximates the effect of an uneven distribution of defects. Figure 5 also includes a graph of non fault tolerant yield 197 which explicitly models the clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacturing process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Murphy's method. A solution to the problem of low yield is to incorporate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units. In memory chips and most Wafer Scale Integration (WSI) devices, die 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 witii redundant actuators which are displaced in die scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction. To allow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 100% redundancy would normally more than double the chip area, dramatically reducing die 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 die head chip are determined by the width of the page being printed, the fragility of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for a full widtii, full color head for printing A4 size paper is approximately 215 mm x 5 mm. This size allows the inclusion of 100% redundancy without significantiy increasing chip area, when using 1.5 μm CMOS fabrication technology. Therefore, a high level of fault tolerance can be included widiout significantly decreasing primary yield. When fault tolerance is included in a device, standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and included in die yield equation. Figure 5 shows the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100. Fault tolerance is highly recommended to improve yield and reliability of print heads containing thousands of printing nozzles, and tiiereby make pagewidth printing heads practical. However, fault tolerance is not to be taken as an essential part of the present invention. Fault tolerance in drop-on-demand printing systems is described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'Integrated fault tolerance in printing mechanisms' (Filing no.: PN2324); 'Block fault tolerance in integrated printing heads' (Filing no. : PN2325); 'Nozzle duplication for fault tolerance in integrated printing heads' (Filing no.: PN2326); 'Detection of faulty nozzles in printing heads' (Filing no.: PN2327); and 'Fault tolerance in high volume printing presses' (Filing no.: PN2328). Printing System Embodiments A schematic diagram of a digital electronic printing system using a print head of tiiis invention is shown in Figure 6. This shows a monolithic printing head 50 printing an image 60 composed of a multitude of ink drops onto a recording medium 51. This medium will typically be paper, but can also be overhead transparency film, cloth, or many other substantially flat surfaces which will accept ink drops. The image to be printed is provided by an image source 52, which may be any image type which can be converted into a two dimensional array of pixels. Typical image sources are image scanners, digitally stored images, images encoded in a page description language (PDL) such as Adobe Postscript, Adobe Postscript level 2, or Hewlett-Packard PCL 5, page images generated by a procedure-call based rasterizer, such as Apple QuickDraw, Apple Quickdraw GX, or Microsoft GDI, or text in an electronic form such as ASCII. This image data is then converted by an image processing system 53 into a two dimensional array of pixels suitable for the particular printing system. This may be color or monochrome, and die data will typically have between 1 and 32 bits per pixel, depending upon die image source and the specifications of the printing system. The image processing system may be a raster image processor (RIP) if the source image is a page description, or may be a two dimensional image processing system if the source image is from a scanner. If continuous tone images are required, then a halftoning system 54 is necessary. Suitable types of halftoning are based on dispersed dot ordered dither or error diffusion. Variations of these, commonly known as stoctiastic screening or frequency modulation screening are suitable. The halftoning system commonly used for offset printing - clustered dot ordered dither - is not recommended, as effective image resolution is unnecessarily wasted using this technique. The output of the halftoning system is a binary monochrome or color image at the resolution of the printing system according to the present invention. The binary image is processed by a data phasing circuit 55 (which may be incorporated in a Head Control ASIC 400 as shown in figure 4) which provides die pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and the movement of the paper. When the data has been loaded into the shift registers 56, it is presented in parallel to the heater driver circuits 57. At the correct time, the driver circuits 57 will electronically connect the corresponding heaters 58 with the voltage pulse generated by die pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, affecting die physical characteristics of the ink. Ink drops 60 escape from me nozzles in a pattern which corresponds to the digital impulses which have been applied to die heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by die pressure regulator 63. Selected drops of ink drops 60 are separated from me body of ink by the chosen drop separation means, and contact the recording medium 51. During printing, the recording medium 51 is continually moved relative to the print head 50 by the paper transport system 65. If the print head 50 is the full widtii of the print region of the recording medium 51 , it is only necessary to move the recording medium 51 in one direction, and die print head 50 can remain fixed. If a smaller print head 50 is used. it is necessary to implement a raster scan system. This is typically achieved by scanning the print head 50 along the short dimension of the recording medium 51 , while moving the recording medium 51 along its long dimension. The binary image is processed by a data phasing circuit 55 (which may be incorporated in a Head Control ASIC 400 as shown in figure 4) which provides the pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and die movement of die paper. When die data has been loaded into the shift registers 56, it is presented in parallel to die heater driver circuits 57. At the correct time, the driver circuits 57 will electronically connect the corresponding heaters 58 with the voltage pulse generated by die pulse shaper circuit 61 and die voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, affecting the physical characteristics of the ink. Ink drops 60 escape from the nozzles in a pattern which corresponds to die digital impulses which have been applied to die heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by the pressure regulator 63. Selected drops of ink drops 60 are separated from the body of ink by the chosen drop separation means, and contact the recording medium 51. During printing, the recording medium 51 is continually moved relative to the print head 50 by the paper transport system 65. If the print head 50 is the full width of the print region of the recording medium 51, it is only necessary to move the recording medium 51 in one direction, and die print head 50 can remain fixed. If a smaller print head 50 is used, it is necessary to implement a raster scan system. This is typically achieved by scanning the print head 50 along die short dimension of die recording medium 51, while moving the recording medium 51 along its long dimension. Nozzle clearing cycles for print heads If a nozzle has an extended idle period, die ink in the nozzle may dry out. Under some circumstances, the ink may dry to the extent that normal operation of the nozzle is prevented. One well known method of reducing die ink drying problem is by using additives (such as humectants) in the ink. There is extensive literature (patents and industry journals) detailing the formulation of inks with controlled drying properties. This hterature can be referenced to aid formulation of inks with desirable properties. However, ink formulation may not entirely eliminate the problem. Other well known methods are capping the print head when not in use, and wiping the print head witii a flexible blade. In the nozzle clearing method disclosed herein, a series of consecutive pulses, or an extended high power pulse, is apphed to die heater. These pulses accumulate heat at the nozzle tip, raising the temperature of the nozzle tip, and d e ink in contact with the tip, above the boiling point of the ink. The formation of vapor bubbles at the tip can dislodge a crust of dried ink, which is then expelled with the ink drop. A single power pulse of longer than normal duration, or greater than normal energy, can be used. However, this may require substantial additional electronic circuitry to generate. The need for this circuitry is eliminated by applying an integral number (greater than one) of consecutive power pulses of the energy, duration, and time varying power profile mat is used to expel drops on demand under normal operating conditions. It is desirable that this feature can be provided with die minimum additional cost above die normal operating requirements of a drop on demand print head. One preferred mode of operation in accordance witii die invention uses a fixed duration heater power pulse with interleaved phases and a variable power function with respect to time within each pulse. In this situation, it is electronically inconvenient (and potentially more expensive) to provide a single pulse of extended duration. Instead, a sequence of consecutive pulses can be apphed to the heater of the blocked nozzle, without the normal idle period between pulses. In most cases, a sequence of four consecutive normal operating pulses will be adequate to raise the nozzle tip and ink meniscus above the ink boiling point, and supply enough energy to the ink to cause sufficient vapor formation to dislodge the blockage. The excess heat in the ink and nozzle tip will rapidly be removed by ink drops flowing from the nozzle tip. A nozzle clearing sequence can be automatically apphed to all nozzles before printing each page. So that the power supply and timing circuits of die printing system do not need to be modified, die nozzle clearing pulses are heater energizing pulses of the duration and time-varying voltage normally used to eject a drop. In the 7 μm nozzle used as an example herein, the pulses are of 18 μs duration, with a time varying power profile as shown in figure 3(a). The number of consecutive pulses applied can be chosen depending upon d e specific design. Four pulses is a convenient number, but two pulses may suffice in the majority of circumstances. It is convenient to choose an integral number of power pulses to eliminate the need for special timing circuits. The nozzle clearing pulse sequence is applied under the control of a microprocessor. As a contiguous series of four pulses is not part of the normal operation of the print head, digital circuitry to correctly sequence the enable pulses to the head during a nozzle clearing operation must be provided. This digital logic is extremely simple, and can be included in a data phasing and fault tolerance ASIC with die addition of less than 100 gates. Figure 7(a) shows a power function for a four pulse clearing cycle at a local ambient temperature of 30 degrees C. This power function is a contiguous sequence of four 'normal' 18 us pulses. This is not an optimum power curve for nozzle clearing. The advantage of diis curve is that it is simple to implement if the standard power profile is that show in figure 3(a). Figure 7(b) is the resultant curve of temperature with respect to time in the nozzle when the power function P(t) of Figure 7(a) is apphed. This curve is calculated using transient energy transport and fluid dynamic simulation. Temperature histories at four points are shown: A - Heater centre: This is the hottest part of the operating nozzle. The maximum temperature reached is approximately 180°C. B - Nozzle tip: This shows the temperature history at the circle of contact between the passivation layer, the ink, and air. The temperature history exhibits a rise to just under 100°C for the first power pulse. The temperature at this point for the second and subsequent power pulses reaches more than 130°C, well above die boiling point of the ink. Vapor bubbles formed at die nozzle tip should dislodge the ink 'crust' in the majority of circumstances. C - meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and die centre of the meniscus. The ink temperature rise at this point is considerably delayed when compared to the temperature history for normal operation (figure 3(b)). This is because the strong convection established by the falling surface tension at the ink meniscus is interrupted. D - Chip surface: This is at a point on the head surface 23 μm from the heater. The temperature only rises approximately five degrees. This indicates that active circuitry can be located very close to the nozzles even when nozzle clearing sequences are to be used. Nozzle fluid dynamics and eneηrv transport during clearing cvcle Various time steps of a fluid dynamic and energy transport simulation of a nozzle are shown in figures 8(a) to 8(h). These diagrams show a cross section of die nozzle tip, from the axis of symmetry out to a distance of 22.1 μm. Figure 8(a) shows the nozzle in the quiescent state, where the surface tension balances the ink pressure and external electrostatic or magnetic field. In diis diagram, 100 is die ink, 101 is silicon, 102 is silicon dioxide, 103 shows die position of die heater, 104 is the tantalum passivation layer, and 108 is the sihcon nitride passivation layer. The hydrophobic coating is applied to the exposed silicon nitride layer. The nozzle tip and ink is at die device ambient temperature, which in this case is 30°C. Figure 8(b) shows the nozzle 4 μs after the start of the first power pulse to the heater. At this time the heater power is 97 mW. Temperature contours are shown starting at 35°C (marked) and increasing in 5°C intervals Figure 8(c) shows the nozzle 19 μs after the start of the first power pulse to the heater. The heater power is 43 mW. This diagram shows that ink convection has been significantiy inhibited from the normal operation. Figure 8(d) shows die nozzle 8 μs after the start of the second power pulse to the heater. The heater power is 41 mW. The nozzle tip in contact with the ink is at approximately 120°C. As the ink modeled is a water based ink, the ink should begin to vaporize at this time. Figure 8(e) shows the nozzle 3 μs after the start of the third power pulse to the heater. The heater power is 61 mW. The nozzle tip in contact with the ink is maintained at approximately 125°C. Figure 8(f) shows the nozzle 13 μs after the start of the third power pulse to the heater. The heater power is 40 mW. Figure 8(g) shows the nozzle 5 μs after the start of the fourth power pulse to the heater. The heater power is 66 mW. Figure 8(h) shows the nozzle 7 μs after the heater is turned off. This shows rapid cooling of the substrate, with the highest temperatures (84 °C) now in the ink. The dried ink at the meniscus has been dislodged, and excess heat is being carried away witii the ink drop,. Subsequent ink drops can be ejected on demand using normal heater pulse energy and timing. The total heater energy for the four pulse sequence is 3.72 μJ. These simulations are an approximation primarily intended to ascertain the temperature profiles of the ink. The ink is modeled in the liquid phase only, so vaporization is not modeled. To approximate the effect of the dried ink on drop ejection, die affect of temperature on surface tension was eliminated between die temperatures of 40°C and 80°C in this model. Actual ink drying is a complex phenomenon which is highly dependent upon ink chemistry, and is not currently amenable to computer modeling. A key aspect of this invention is to raise the ink at the meniscus above the boiling point of the ink. It is another aspect of this invention to achieve this temperamre increase in nozzles with the minimum additional circuitry by providing an integral (and fixed) number of consecutive pulses to the heater, wherein each pulse is a standard operating pulse. The foregoing describes several preferred embodiments of die present invention. Modifications, obvious to those skilled in the art, can be made diereto without departing from the scope of die invention.

Claims: I Claim; 1. A method for clearing ink blocked nozzles of a thermal drop on demand printing head, said method including the step of applying sufficient energy to die heater of such printing head to cause ink in a blocked nozzle to be raised above the boiling point of said ink. 2. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 1 where said drop on demand printing mechanism comprises: (a) a body of ink associated with said nozzles; (b) 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; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) 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. 3. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 1 where said drop on demand printing mechanism comprises: (a) a body of ink associated with said nozzles; (b) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (c) 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, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 4. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 1 where said drop on demand printing mechanism comprises: (a) a body of ink associated witii said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (b) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (c) 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. 5. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 1 where said drop on demand printing mechanism uses a thermal drop selection or ejection step. 6. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 1 where said energy apphed to said heater is generated by applying electric power for a predetermined duration, and said heater is electrically resistive. 7. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 5 where said energy apphed to said heater is generated by applying electric power for a predetermined duration, and said heater is electrically resistive. 8. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 6 where said electric power is substantially similar to the electric power which is apphed when a drop is selected or ejected on demand during normal printing operation. 9. A method for clearing blocked nozzles of drop on demand printing heads as claimed in claim 8 where said duration is an integral multiple, greater than one, of the duration of die electric power which is apphed when a drop is selected or ejected on demand during normal printing operation. 10. In a drop on demand printing apparatus comprising a print head having a plurality of drop ejection nozzles, an ink supply and means for energizing said print head to effect drop ejections, a system for clearing ink from blocked nozzles comprising: (1) means for heating ink in said nozzles; and (2) means for energizing said heating means to cause ink in the nozzles to be raised above its boding point. 11. The invention defined in claim 10 wherein said energizing means is an electric power source and said heating means is a resistive heater element 12. The invention defined in claim 11 further comprising control means for operating said electric power source to energize said heater in a series of predetermined duration pulses. 13. The invention defined in claim 12 wherein said pulse's duration is an integral multiple greater than one of the normal drop ejection or selection operation period of said heater means. 14. The invention defined in claim 10 where said apparatus comprises: (a) 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; (b) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (c) 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. 15. The invention defined in claim 10 where said apparatus comprises: (a) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (b) 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, said drop selecting means being capable of producing said difference in miniscus position in d e absence of said drop separation means. 16. The invention defined in claim 10 where said apparatus comprises: (a) a body of ink associated witii said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (b) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (c) 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.

Inventors: SILVERBROOK KIA, SILVERBROOK, KIA
Applicants: EASTMAN KODAK CO, SILVERBROOK KIA, EASTMAN KODAK COMPANY, SILVERBROOK, KIA
Application year: 1996
Main classification: B41J2
IPC codes: B41J2