EP0765231A1 - Compensation de la puissance de chauffage en fonction de la charge d'impression dans des systemes d'impression thermique - Google Patents

Compensation de la puissance de chauffage en fonction de la charge d'impression dans des systemes d'impression thermique

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Publication number
EP0765231A1
EP0765231A1 EP96912648A EP96912648A EP0765231A1 EP 0765231 A1 EP0765231 A1 EP 0765231A1 EP 96912648 A EP96912648 A EP 96912648A EP 96912648 A EP96912648 A EP 96912648A EP 0765231 A1 EP0765231 A1 EP 0765231A1
Authority
EP
European Patent Office
Prior art keywords
ink
drop
nozzles
printing
temperature
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP96912648A
Other languages
German (de)
English (en)
Inventor
Kia Silverbrook
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Eastman Kodak Co
Original Assignee
Eastman Kodak Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Eastman Kodak Co filed Critical Eastman Kodak Co
Publication of EP0765231A1 publication Critical patent/EP0765231A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/135—Nozzles
    • B41J2/14—Structure thereof only for on-demand ink jet heads
    • B41J2/14451—Structure of ink jet print heads discharging by lowering surface tension of meniscus
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/04563—Control methods or devices therefor, e.g. driver circuits, control circuits detecting head temperature; Ink temperature
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/04568—Control according to number of actuators used simultaneously
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/04—Ink jet characterised by the jet generation process generating single droplets or particles on demand
    • B41J2/045—Ink jet characterised by the jet generation process generating single droplets or particles on demand by pressure, e.g. electromechanical transducers
    • B41J2/04501—Control methods or devices therefor, e.g. driver circuits, control circuits
    • B41J2/04583—Control methods or devices therefor, e.g. driver circuits, control circuits controlling heads based on discharge by lowering the surface tension of meniscus

Definitions

  • the present invention is in the field of computer controlled printing devices.
  • the field is thermally activated drop on demand (DOD) printing systems.
  • DOD drop on demand
  • 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.
  • 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 CLI, 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 CLT prmting 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 f et printer which applies a high voltage to a piezoelectric crystal, causing the crysial 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.
  • BubblejetTM trademark of Canon K.K. of Japan
  • Thermal Ink Jet printing typically requires approximately 20 ⁇ J over a period of approximately 2 ⁇ s 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.
  • 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.
  • One important purpose of the invention is to provide means and methods to regulate the voltage of the power supply of a printing head to compensate for varying load conditions caused by differing quantities of power consuming pixel actuators being simultaneously active.
  • the invention constitutes a printing apparatus of the kind having a plurality of separately energizable pixel actuator elements and means for supplying energizing power to selected pixel elements during energization periods, an improved power control system comprising logic means for determining and signaling the number of pixel actuator elements to be energized during forthcoming energization period, and means for receiving signals from said logic predeterminedly varying the power output to said actuator elements in response to said signals thereto.
  • a preferred aspect of the invention is that the printing head pixel actuator operates on a thermal principle.
  • a further preferred aspect of the invention is that the printing head operates on the coincident forces printing approach.
  • a further preferred aspect of the invention is that the power supply voltage required is calculated before the operation of the print head, and stored as digital information in an electronic memory.
  • a further preferred aspect of the invention is that the power supply voltage determining unit consists of a counter which provides a digital representation of the number of pixel actuators currently active, the output of which forms part or all of the address of a digital electronic look-up table, the data output of which is connected to a digital to analog converter.
  • a further preferred aspect of the invention is that the counter function includes circuitry which reduces the number of digital bits required to represent the number of pixel actuators currently active.
  • a further preferred aspect of the invention is that the heater power supply voltage is determined according to the equation:
  • a further preferred aspect of the invention is that the power supply voltage is also compensated for thermal lag.
  • a further preferred aspect of the invention is that the power supply voltage is also compensated for ambient temperature.
  • a further preferred aspect of the invention is that the power supply voltage is also compensated for both thermal lag and ambient temperature.
  • a further preferred aspect of the invention is that the heater power supply voltage is determined according to the equation
  • Figure 1 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 diagram of the power supply load circuit.
  • Figures 7(a) and 7(b) show simplified block diagrams of methods of controlling head power in relation to the power function in accordance with an embodiment of the invention.
  • Figure 8 shows a detail of the On pixel counter 402.
  • 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.
  • 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:
  • the drop separation means may be chosen from, but is not limited to, the following list:
  • 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.
  • TJJ thermal ink jet
  • piezoelectric ink jet systems a drop velocity of approximately 10 meters per second is preferred to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink, and st ⁇ ke the recording medium.
  • These systems have a very low efficiency of conversion of elect ⁇ cal energy into drop kinetic energy.
  • the efficiency of ⁇ J systems is approximately 0.02%) This means that the d ⁇ ve circuits for TJJ p ⁇ nt heads must switch high currents.
  • the d ⁇ ve circuits for piezoelect ⁇ c ink jet heads must either switch high voltages, or d ⁇ ve highly capacitive loads.
  • the total power consumption of pagewidth ⁇ J p ⁇ ntheads is also very high
  • An 800 dpi A4 full color pagewidth ⁇ J p ⁇ nt head printing a four color black image in one second would consume approximately 6 kW of elect ⁇ cal power, most of which is converted to waste heat
  • the difficulties of removal of this amount of heat precludes the production of low cost, high speed, high resolution compact pagewidth 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.
  • Drop selection means shows some of the possible means for selecting drops in accordance with the invention.
  • the drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops.
  • the prefe ⁇ ed drop selection means for water based inks is met.iod 1 : ' ⁇ lectrothermal reduction of surface tension of pressurized ink". This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of T J), 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 prefe ⁇ ed drop selection means for hot melt or oil based inks is method 2: ' ⁇ lectrothermal 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 appUcable 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.
  • the prefe ⁇ ed drop separation means depends upon the intended use. For most apphcations, method 1 : “Electrostatic attraction”, or method 2: “AC electric field” are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: “Proximity” may be appropriate. For high speed, high quality systems, method 4: 'Transfer proximity” can be used. Method 6: “Magnetic attraction” is appropriate for portable printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances. Further details of various types of printing systems according to the present invention are described in the foDowing Austrahan patent specifications file- on 12 April 1995, the disclosure of which are hereby incorporated by reference:
  • LIFT Liquid ink Fault Tolerant
  • An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or other forms of digital image representation.
  • This image data is converted to a pixel-mapped page image by the image processing system 53.
  • This may be a raster image processor (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 half toned.
  • Halftoning is performed by the Digital Halftoning unit 54.
  • Halftoned bitmap image data is stored in the image memory 72.
  • 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 transpo ⁇ control system 66, which in turn is controlled by a microcontroller 315.
  • the paper transport system shown in figure 1 (a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50. However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the sub-scanning direction) and the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion.
  • the microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71.
  • ink is contained in an ink reservoir 64 under pressure.
  • the ink pressure In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop.
  • a constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63.
  • 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).
  • 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).
  • the ink is distributed to the back surface of the head 50 by an ink channel device 75.
  • the ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles and actuators are situated.
  • the nozzle actuators are electrothermal heaters.
  • 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.
  • the paper guide or platen 67 can be made of electrically conductive material and used as one electrode generating the electric field.
  • the other electrode can be the head 50 itself.
  • Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops.
  • Figure 1 (b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process.
  • the nozzle is etched in a substrate 101 , which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer.
  • Single crystal silicon (SCS) substrates have several advantages, including:
  • Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment;
  • SCS has high mechanical strength and rigidity
  • SCS has a high thermal conductivity
  • the nozzle is of cylindrical form, with the heater 103 forming an annulus.
  • the nozzle tip 104 is formed from sihcon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry.
  • the nozzle tip is passivated with silicon nitride.
  • the protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface.
  • the print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head.
  • Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used.
  • Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate.
  • This type of nozzle may be used for print heads using various techniques for drop separation.
  • FIG. 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 ⁇ m, at an ambient temperature of 30°C. The total energy applied to the heater is
  • the ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30°C is 1.84 cPs.
  • the ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature.
  • a cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 ⁇ m is shown.
  • Heat flow in the various materials of the nozzle including silicon, silicon nitride, amorphous sihcon dioxide, crystalline sihcon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials.
  • Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus.
  • Figure 2(b) shows thermal contours at 5°C intervals 5 ⁇ s after the start of the heater energizing pulse.
  • the heater When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surface of the ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directly in contact with the heater from moving.
  • Figure 2(c) shows thermal contours at 5°C intervals 10 ⁇ s after the start of the heater energizing pulse.
  • the increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces.
  • the ink begins to flow.
  • Figure 2(d) shows thermal contours at 5°C intervals 20 ⁇ s after the start of the heater energizing pulse.
  • the ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head.
  • the electrostatic field becomes concentrated by the protruding conductive ink drop.
  • Figure 2(e) shows thermal contours at 5°C intervals 30 ⁇ s after the start of the heater energizing pulse, which is also 6 ⁇ s after the end of the heater pulse, as the heater pulse duration is 24 ⁇ s.
  • the nozzle tip has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink.
  • the nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter (less than 16 ⁇ s in this case) the mk would not accelerate towards the print medium, but would instead return to the nozzle.
  • Figure 2(f) shows thermal contours at 5°C intervals 26 ⁇ s after the end of the heater pulse.
  • the temperature at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around the nozzle tip.
  • the rate at which the ink is drawn from the nozzle exceeds the viscously hrnited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink.
  • the selected drop then travels to the recording medium under the influence of the external electrostatic field.
  • the meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop.
  • Figure 3(a) shows successive meniscus positions during the drop selection cycle at 5 ⁇ s intervals, starting at the beginning of the heater energizing pulse.
  • Figure 3(b) is a graph of meniscus position versus time, showing the movement of the point at the centre of the meniscus.
  • the heater pulse starts 10 ⁇ s into the simulation.
  • Figure 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle.
  • the vertical axis of the graph is temperature, in units of 100°C.
  • the horizontal axis of the graph is time, in units of 10 ⁇ s.
  • the temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 ⁇ 5 time steps.
  • the local ambient temperature is 30 degrees C. Temperature histories at three points are shown:
  • a - Nozzle tip This shows the temperature history at the circle of contact between the passivation layer, the ink, and air.
  • B - Meniscus midpoint This is at a circle on the ink meniscus midway between the nozzle tip and the center of the meniscus.
  • C - Chip surface This is at a point on the print head surface 20 ⁇ m from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures.
  • Figure 3(e) shows the power applied to the heater.
  • Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse.
  • the average energy applied to the heater is varied over the duration of the pulse. In this case, the variation is achieved by pulse frequency modulation of
  • the sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI).
  • RFID radio frequency interference
  • ⁇ r is the surface tension at temperature T
  • k is a constant
  • T c is the critical temperature of the liquid
  • M is the molar mass of the liquid
  • x is the degree of association of the liquid
  • p is the density of the liquid.
  • water based ink for thermal inkjet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying.
  • Isopropyl alcohol has a boiling point of 82.4°C, lower than that of water.
  • a surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature.
  • a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude.
  • a surface tension decrease of 20 mN/m over a 30°C temperature range is prefe ⁇ ed to achieve large operating margins, while as little as lOmN/m can be used to achieve operation of the print head according to the present invention.
  • the ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less than 1,000 A are desirable. Suitable surfactant melting points for a water based ink are between 50°C and 90°C, and preferably between 60°C and 80°C.
  • the ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink.
  • PIT phase inversion temperature
  • 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 can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range.
  • surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as:
  • the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature.
  • a good example is Arachidic acid. These carboxylic acids are available in high purity and at low cost.
  • the amount of surfactant required is very small, so the cost of adding them to the ink is insignificant
  • a mixture of carboxylic acids with slighdy varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant.
  • the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation.
  • this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide.
  • the surfactant sol can be prepared separately at high concentration, and added to the ink in the required concentration.
  • An example process for creating the surfactant sol is as follows:
  • the ink preparation will also contain either dye(s) or pigment(s , bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required.
  • Anti-foaming agents will generally not be required, as there is no bubble formation during the drop ejection process.
  • Inks made with anionic surfactant sols are generally unsuitable for use with cationic dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the 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.
  • the method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an aj__ali is used to adjust the pH balance and increase the charge on the surfactant particles.
  • a pH of 6 using HCl is suitable.
  • a microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantly 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.
  • PIT phase inversion temperature
  • the surfactant prefers surfaces with very low curvature.
  • 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.
  • 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.
  • microemulsion based inks 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.
  • water is a suitable polar solvent.
  • different polar solvents may be required.
  • polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable.
  • the surfactant can be chosen to result in a phase inversion temperature in the desired range.
  • surfactants of the group poly(oxyethylene)alkylphenyl ether ethoxylated alkyl phenols, general formula:
  • C-H_- C H6(CH 2 CH 2 O)_OH) can be used.
  • the hydrophilicity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable.
  • Synonyms include Octoxynol-10, PEG- 10 octyl phenyl ether and POE (10) octyl phenyl ether
  • the HLB is 13.6, the melting point is 7°C, and the cloud point is 65°C.
  • ethoxylated alkyl phenols include those listed in the following table:
  • Microemulsions are thermodynamically 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.
  • 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.
  • the amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors.
  • Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets.
  • the use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium.
  • 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.
  • the ninth combination is useful for printing transparent coatings, UV ink, and selective gloss highlights.
  • 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.
  • the absorption spectrum will tend to become broader, as the absorption peaks of both colorants are averaged. This has a tendency to 'muddy' the colors. To obtain brilliant color, careful choice of dyes and pigments based on their 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 the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, while the pigment will tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances.
  • the Krafft point For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and there is a rapid increase in solubility of the surfactant If the critical micelle concentration (CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usually much less effective below the Krafft point. This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures, only a portion of the surfactant is in solution. When the nozzle heater is turned on, the temperature rises, and more of the surfactant goes into solution, decreasing the surface tension.
  • CMC critical micelle concentration
  • a surfactant should be chosen with a Krafft point which is near the top of the range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature.
  • the concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures.
  • Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature.
  • the POE chain is hydrophilic, and maintains the surfactant in solution.
  • the structured water around the POE section of the molecule is disrupted, and the POE section becomes hydrophobic.
  • the surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension.
  • the temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C
  • Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures.
  • 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(CHCH 3 CH 2 O) x (CH 2 CH 2 O) y (CHCH 3 CH 2 O)-OH] varieties where the average x and z are approximately 4, and the average y is approximately 15 may be suitable.
  • the cloud point of POE surfactants is increased by ions that disrupt water structure (such as I ' ), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs.
  • the cloud point of POE surfactants is decreased by ions that form water structure (such as Cl “ , OH " ), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively little effect
  • the ink composition can be 'tuned' for a desired temperature range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl " to Br “ to I " ) that are added to increase electrical conductivity. NaCI is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCI slightly lowers the cloud point of nonionic surfactants. Hot Melt Inks
  • the ink need not be in a liquid state at room temperature.
  • Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink.
  • the hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately
  • 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 the 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 the printed page. T he quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures.
  • a quiescent temperature between 60°C and 90°C is generally suitable, though other temperatures may be used.
  • a drop ejection temperature of between 160°C and 200°C is generally suitable.
  • 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.
  • 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.
  • 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.
  • Figure 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations containing the following additives:
  • operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink is as follows.
  • solid ink Prior to operation of the printer, solid ink is melted in the reservoir 64.
  • the reservoir, ink passage to the print head, ink channels 75, and 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 the viscosity of the 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 this oscillation is small due to the high ink viscosity. At the normal operating temperature, these oscillations are of insufficient amplitude to result in drop separation.
  • the heater 103 When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle.
  • the recording medium 51 is arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51, but sufficiently far away that the unselected drops do not contact the recording medium 51.
  • part of the selected drop freezes, and attaches to the recording medium.
  • 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 the ink increases to its quiescent level as remaining heat is dissipated to the bulk ink and print head.
  • One ink drop is selected, separated and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved.
  • An objective of printing systems according to the invention is to attain a print quality which is equal to that which people are accustomed to in quality color 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 quality monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein called CC'MM'YKK'.
  • Printing apparatus and methods of this invention are suitable for a wide range of applications, including (but not limited to) the following: color and monochrome office 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 duplication, 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.
  • the variation can be minimized by appropriate head design. In other cases, the variation can compensated by active circuitry.
  • Ambient temperature Changes in ambient temperature can affect the quiescent meniscus position, and the temperature achieved by the 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 the heater pulse can be compensated by altering the power supplied to the heater.
  • Nozzle temperature It is not practical to compensate for temperature independently for each nozzle. Reliable operation of heads requires that the difference between the nozzle temperature and the 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.
  • Nozzle radius The variation in nozzle radius for nozzles supphed 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 ⁇ m using modern semiconductor manufacturing equipment.
  • 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.
  • Ink contarninants The ink must be free of contaminants larger than approximately 5 ⁇ m, which may lodge against each other and clog the nozzle. This can be achieved by placing a 5 ⁇ m absolute filter between the ink reservoir and the head.
  • Ink surface tension characteristics The most important requirement of the ink is the surface tension characteristics. The ink must be formulated so that the surface tension is high enough to retain the ink in the nozzle at ambient temperatures within the design hmits, 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 cont_minants which affect surface tension. 7) Ink drying: If the period between drop ejections from a nozzle becomes too long, then the ink at the exposed meniscus may dry out to the 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.
  • Pulse width can be accurately controlled, and may be set very close to the minimum pulse width. Higher reliability can be achieved by making the pulse width considerably longer than the minimum. For a 7 ⁇ m nozzle using water based ink as herein described, the minimum pulse width is approximately 10 ⁇ s. The nominal pulse width is set at 18 ⁇ s to give a wide operating margin. Pulse width has almost no effect on drop size.
  • 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 the '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.
  • Print media roughness This is particularly significant for proximity printing, where media roughness may be a significant fraction of the head to media distance. Protruding fibers in a paper medium may cause the ink drop to wick into the paper sooner than intended, resulting in less ink transfe ⁇ ed to the paper, and a smaller drop size. This can be compensated by using coated paper, compressing the paper fibers with rollers before printing, and/or coating or wetting the paper immediately prior to printing.
  • the performance of nozzles is sensitive to the temperature and duration of thermal pulses applied to the nozzle tip.
  • the 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 the heater, too much ink may be ejected, the ink may boil, and the energy used by the 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 the required ejection energy of the nozzle under various static and dynamic environmental circumstances.
  • An optimum temperature profile for a 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 the pulse, and instantaneous cooling of the region to the ambient temperature.
  • This optimum is not achievable due to the stored heat capacities and thermal conductivities of d e various materials used in the fabrication of the nozzles.
  • 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.
  • a transient fluid dynamic simulation with free surface modeling is required, as convection in die ink, and ink flow, significantly affect on the temperature achieved with a specific power curve. Compensation techniques
  • 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 the pulse, and instantaneous cooling of the region to the ambient temperature.
  • the power applied to the heater can be varied in time by various techniques, including, but not hmited to: 1) Varying the voltage applied to the heater
  • 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.
  • 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).
  • the enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a range of well known glitch free clock control circuits.
  • Stopping the clock of the shift register removes the requirement for a parallel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400.
  • Data is routed to either the main nozzles or the redundant nozzles by the data router 219 depending on the state of the appropriate signal of the fault status bus.
  • the print head shown in figure 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance.
  • Drive circuits for different configurations of print head can readily be derived from the apparatus disclosed herein.
  • Digital information representing patterns of dots to be printed on the recording medium is stored in the Page or Band memory 1513, which may be the same as the Image memory 72 in figure 1(a).
  • Data in 32 bit words representing dots of one color is read from the Page or Band memory 1513 using addresses selected by the address mux 417 and control signals generated by the Memory Interface 418.
  • Address generators 411 which forms part of the
  • the addresses are generated based on the positions of the nozzles in relation to the print medium. As the relative position of the nozzles may be different for different print heads, the Address generators 41 1 are preferably made programmable. The Address generators 41 1 normally generate the address co ⁇ esponding to the position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in the Fault Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so that the Address generators 411 generate the address co ⁇ esponding to the position of the redundant nozzles.
  • Data read from the Page or Band memory 1513 is latched by the latch 413 and converted to four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other colors by the FIFO 415. This data is then buffered by the buffer 430 to form the 48 bit main data bus to the print head 50.
  • the data is buffered as the print head may be located a relatively long distance from the head control ASIC.
  • Data from the Fault Map RAM 412 also forms the input to the FIFO 416. The timing of this data is matched to the data output of the FIFO 415, and buffered by the buffer 431 to form the fault status bus.
  • the programmable power supply 320 provides power for the head
  • the voltage of the power supply 320 is controlled by the DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316.
  • the RAMDAC 316 contains a dual port RAM 317.
  • the contents of the dual port RAM 317 are programmed by the Microcontroller 315. Temperature is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroller 315 based on temperature sensed by a thermal sensor 300.
  • the thermal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311.
  • ADC Analog to Digital Converter
  • the ADC 311 is preferably incorporated in the Microcontroller 315.
  • the Head Control ASIC 400 contains control circuits for thermal lag compensation and print density.
  • Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time-varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage.
  • An analog time varying programming voltage is produced by the DAC 313 based upon data read from the dual port RAM 317. The data is read according to an address produced by the counter 403.
  • the counter 403 produces one complete cycle of addresses during the period of one enable pulse. This synchronization is ensured, as the counter 403 is clocked by the system clock 408, and the top count of the counter 403 is used to clock the enable counter 404.
  • the count from the enable counter 404 is then decoded by the decoder 405 and buffered by the buffer 432 to produce the enable pulses for the head 50.
  • the counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To allow non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used.
  • 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.
  • 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 the enable pulse.
  • the multiplexer 401 selects the output of the latch 423 which co ⁇ esponds to the cu ⁇ ent enable phase, as determined by the enable counter 404.
  • the output of the multiplexer 401 forms part of the address of the dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most significant four bits of this count are adequate.
  • the dual port RAM 31 7 has an 8 bit address.
  • the dual port RAM 317 contains 256 numbers, which are in a two dimensional a ⁇ ay. These two dimensions are time (for thermal lag compensation) and print density.
  • a third dimension - temperature - can be included.
  • the microcontroller 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at the cu ⁇ ent temperature. Periodically (for example, a few times a second), the microcontroller senses the cu ⁇ ent head temperature and calculates this matrix.
  • V P is the voltage specified to the programmable power supply 320
  • R OUT is the output resistance of the programmable power supply 320, including the 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 cu ⁇ ent enable period, as provided by the 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 the 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 with experimentation
  • 7_ is the temperature required for drop ejection in °C
  • T A is the 'ambient' temperature of the head as measured by the temperature sensor in °C.
  • the table "Comparison between Thermal ink jet and Present Invention” compares the aspects of printing in accordance with the present invention with thermal ink jet printing technology. A direct comparison is made between the present invention and thermal ink jet technology because both are drop on demand systems which operate using thermal actuators and liquid ink. Although they may appear similar, the 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 the ink, so that sufficient heat is transfe ⁇ ed to the ink before bubble nucleation is complete.
  • ink temperatures of approximately 280°C to 400°C are required.
  • the bubble formation causes a pressure wave which forces a drop of ink from the aperture with high velocity. The bubble then 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 the use of well established integrated circuit manufacturing techniques.
  • 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, thermal stress, differential thermal expansion, kogation, cavitation, rectified diffusion, and difficulties in ink formulation.
  • Printing in accordance with the present invention has many of the advantages of thermal inkjet printing, and completely or substantially eliminates many of the inherent problems of thermal ink jet technology.
  • yield 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%.
  • Figure 5 is a graph of wafer sort yield versus defect density for a monolithic full width color A4 head embodiment of the invention.
  • the head is 215 mm long by 5 mm wide.
  • the non fault tolerant yield 198 is calculated according to
  • 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
  • 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.
  • redundant sub-units In memory chips and most Wafer Scale Integration (WSI) devices, the physical location of redundant sub-units on the chip is not important. However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered to compensate for the displacement in the scan direction.
  • the minimum physical dimensions of the head chip are determined by the width of the page being printed, the fragility of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip.
  • the minimum practical size for a full 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 significantly increasing chip area, when using 1.5 ⁇ m CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without significantly decreasing primary yield.
  • standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and included in the yield equation.
  • Figure 5 shows the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation.
  • This graph shows projected yield as a function of both defect density and defect clustering.
  • the yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance can increase wafer sort yield from under 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 thereby make pagewidth printing heads practical. However, fault tolerance is not to be taken as an essential part of the present invention. Fault tolerance in drop-on-demand printing systems is described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference:
  • An individual print head heater can be designed to consume only a small amount of power, for example, an average of 60 mW when active.
  • a high performance printing head in accordance with my above-cited applications may have several thousand nozzles, and therefore several thousand heaters. Of these heaters, a portion may operate simultaneously.
  • the maximum number of heaters which can be energized simultaneously is equal to the total number of heaters divided by the number of enable phases in the head.
  • the table "LIFT head type A4-4-800" (Appendix A) is a summary of some characteristics of an example 800 dpi full color A4 print head.
  • This print head contains a total of 26,496 active nozzles, being 6,624 nozzles of each of cyan, magenta, yellow, and black. These operate in 8 phases, resulting in a maximum of 3,312 heaters simultaneously energized. This maximum occurs if the head is printing four-color black across the entire line. With 60 mW per heater, the power consumption is 199 watts. When the image to be printed is white, then no heaters are energized, and the power consumption is near zero.
  • a power supply system table to support such rapid and extreme load fluctuations requires careful design.
  • FIG. 6 shows a conceptual diagram which includes the output resistance (ROUT) 71 of the power supply 70.
  • the output resistance includes the resistance of any series active elements used to control the power supply voltage, the wiring from the power supply to the head, and the connections to the head.
  • the resistance of an individual heater circuit (Rg) 72 comprises the resistance of the heater, the on resistance of the drive transistor, and the resistance of the wiring connecting the drive transistor and heater to V * and V " .
  • VH will typically be in the region of 10 Volts, as higher voltages cause difficulties with breakdown of the drive transistors, and lower voltages require higher cu ⁇ ent to achieve the desired heater power. If each heater (and drive transistor) requires 60 mW and Vvisor is 10 Volts, then the cu ⁇ ent through each heater is 6 mA, and R is 1,666 Ohms. When all 3,312 heaters are on, the total cu ⁇ ent is 19.9 Amps. The output resistance ROUT will typically be approximately 0.1 Ohm in practical low cost systems. With a load cu ⁇ ent of 20 Amps, the voltage developed across R OUT is 2 Volts. Therefore, Vps must be 12 Volts in order to maintain VH at 10 Volts. When few heaters are enabled, V/_ must be approximately equal to VH. In this example, a two volt difference in V/_ is required to maintain V H constant for minimum and maximum drop ejection rates.
  • V PS in order to maintain the required heater voltage VH when differing numbers of heaters are enabled. This is achieved as shown in figure 7(a).
  • the apparatus includes an On pixel counter (402) which provides a number representing the number of heaters which are to be energized (i.e. number of pixels that are 'on') during the present enable phase.
  • the output of the counter 402 is connected to a voltage calculation process 310 which determines an appropriate power supply voltage based on the number of pixels which are 'on'.
  • the number of pixels which may be turned on in one enable phase can be between 0 and
  • On pixel counter 402 determines the voltage according to the following equation:
  • Vps is the voltage specified to the programmable power supply 320 VH is the required voltage to be supplied to the LIFT heater (and drive transistor)
  • Ro is the output resistance of the programmable power supply 320, including the connections to the head 50
  • R H is the resistance of a single heater, plus the 'on' resistance of the drive transistor p is an integer representing the number of heaters that are turned on in the cu ⁇ ent enable period, as provided by the multiplexer 401 n is a constant equal to the number of LIFT heaters represented by one least significant bit of/?. It is not necessary to calculate V S using equation 1 as the head is being used. The values of Vps can be pre-calculated and stored in an electronic memory.
  • Figure 7(b) shows a system where information containing the required values of V _ are stored in an electronic memory 318 such as a ROM. This info ⁇ nation is read from the memory 318 by a microcontroller 315, and written to a dual port RAM 317. There is an On pixel counter 402 for each of the enable phases of the LIFT head. In the example of a high speed head with sixteen phases, sixteen
  • On pixel counters are required.
  • the output of the On pixel counter 402 which co ⁇ esponds to the present enable phase is selected by the multiplexer 401 , which generates an address for the dual port RAM 317.
  • the output of the dual port RAM is connected to a Digital to Analog Converter (DAC) 313.
  • DAC Digital to Analog Converter
  • the output of the DAC controls the programmable power supply 320 which powers the head 50.
  • FIG 8 is a detail of the On pixel counter 402.
  • the On pixel counter 402 consists of a divide by 'n' counter 406 which is clocked by the pixel clock, is enabled by the pixel data, and is reset at the beginning of its corresponding phase enable period.
  • the top count output of the divide-by- n counter 406 is use- to clock a counter 407.
  • This counter produces the number p.
  • the output of the counter 407 is latched with a latch 408 which is clocked at the beginning of the co ⁇ esponding enable period. This is used to hold the pixel count/? stable during the enable period.

Landscapes

  • Ink Jet (AREA)

Abstract

Procédé et appareil servant à compenser les effets de résistance affectant l'alimentation en courant des têtes d'impression thermiques 'à nombre de gouttelettes fournies à la demande'. Comme le nombre de gouttelettes produites simultanément peut varier de zéro à plusieurs milliers, les variations de la charge résistive instantanée influant sur l'alimentation en courant doivent normalement suffire pour provoquer de notables variations de la taille des gouttelettes. L'appareil compense justement ces variations d'une manière prédictive précise; il comporte en effet un compteur qui indique un nombre représentatif de celui de gouttelettes devant être éjectées pendant la période de validation en cours. La sortie du compteur est reliée à un dispositif qui détermine la tension d'alimentation requise pendant la période de validation. Ce résultat sert à son tour à piloter l'alimentation programmable du système de chauffage de la tête.
EP96912648A 1995-04-12 1996-04-09 Compensation de la puissance de chauffage en fonction de la charge d'impression dans des systemes d'impression thermique Withdrawn EP0765231A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
AUPN2316A AUPN231695A0 (en) 1995-04-12 1995-04-12 Heater power compensation for print density in lift printing systems
AUPN2316/95 1995-04-12
PCT/US1996/004885 WO1996032271A1 (fr) 1995-04-12 1996-04-09 Compensation de la puissance de chauffage en fonction de la charge d'impression dans des systemes d'impression thermique

Publications (1)

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EP0765231A1 true EP0765231A1 (fr) 1997-04-02

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EP96912648A Withdrawn EP0765231A1 (fr) 1995-04-12 1996-04-09 Compensation de la puissance de chauffage en fonction de la charge d'impression dans des systemes d'impression thermique

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EP (1) EP0765231A1 (fr)
JP (1) JPH10501489A (fr)
AU (1) AUPN231695A0 (fr)
WO (1) WO1996032271A1 (fr)

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Publication number Priority date Publication date Assignee Title
US6334660B1 (en) * 1998-10-31 2002-01-01 Hewlett-Packard Company Varying the operating energy applied to an inkjet print cartridge based upon the operating conditions
JP2001225457A (ja) * 2000-02-15 2001-08-21 Canon Inc 記録装置および記録装置の電源制御方法
CN113959350B (zh) * 2021-11-03 2023-11-28 Tcl华星光电技术有限公司 喷墨打印检测系统和检测方法

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Publication number Priority date Publication date Assignee Title
JPS5840512B2 (ja) * 1978-10-04 1983-09-06 株式会社リコー インクジェット記録装置
US4164745A (en) * 1978-05-08 1979-08-14 Northern Telecom Limited Printing by modulation of ink viscosity
JPS5579175A (en) * 1978-12-11 1980-06-14 Nec Corp Device for forming ink drop
FR2448979B1 (fr) * 1979-02-16 1986-05-23 Havas Machines Dispositif destine a deposer sur un support des gouttes d'encre
JPS61242850A (ja) * 1985-04-22 1986-10-29 Canon Inc インクジエツト記録装置
JPH0790646B2 (ja) * 1986-10-15 1995-10-04 セイコーエプソン株式会社 サ−マルプリンタの印字制御装置
DE3885238T2 (de) * 1987-11-27 1994-03-03 Canon Kk Tintenstrahlaufzeichnungsgerät.
US4910528A (en) * 1989-01-10 1990-03-20 Hewlett-Packard Company Ink jet printer thermal control system
GB8912245D0 (en) * 1989-05-26 1989-07-12 Pa Consulting Services Liquid jet recording process
US5036337A (en) * 1990-06-22 1991-07-30 Xerox Corporation Thermal ink jet printhead with droplet volume control
CA2074906C (fr) * 1991-08-01 2000-09-12 Hiromitsu Hirabayashi Appareil d'enregistrement a jet d'encre a fonction de controle de la temperature
EP0568162A1 (fr) * 1992-04-29 1993-11-03 Francotyp-Postalia GmbH Dispositif pour la commande d'une tête d'impression électrothermique
JP3384597B2 (ja) * 1992-11-30 2003-03-10 ヒューレット・パッカード・カンパニー インク転移印刷装置及び方法

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See references of WO9632271A1 *

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Publication number Publication date
WO1996032271A1 (fr) 1996-10-17
AUPN231695A0 (en) 1995-05-04
JPH10501489A (ja) 1998-02-10

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