DE69619609T2 - Compressible liquid ink cartridge for coincidence printing systems - Google Patents

Description

The present invention relates to the field of computer-controlled printing devices. In particular, the field of replaceable ink cartridges for drop-on-demand liquid ink printers that require ink overprint.

Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use different actuators, different marking materials, and different receiving media. Digital printing systems currently in use include: electrophotographic laser printers, electrophotographic LED printers, dot matrix printers, thermal paper printers, film imagesetters, thermal wax printers, dye diffusion thermal transfer printers, and ink jet printers. However, electronic printing systems have not yet been able to replace mechanical printing machines to any significant extent, even though the conventional printing process requires very complex setup and is only economically viable when several thousand copies of a given page are to be printed. There is therefore a need for improved digitally controlled printing systems, for example for producing high quality color images at high speed and at low cost using plain paper.

Inkjet printing is widely used in the field of digitally controlled electronic printing systems, among other things because of its contactless, quiet operation, the use of plain paper and the absence of toner transfer and toner fixing.

Many types of inkjet printing mechanisms have been invented. These can be divided into two categories, namely continuous inkjet printing systems (CIJ) and drop-on-demand printing systems (DOD). Continuous inkjet printing systems date back to at least 1929, see US-A-1,941,001.

US-A-3,373,437 describes an arrangement of continuous inkjet nozzles, in which the ink jets to be printed are selectively charged and deflected towards the receiving medium. This technique is called binary deflection of continuous inkjet nozzles and is used by several manufacturers, including Elmjet and Scitex.

US-A-3,416,153 describes a method for achieving a variable optical density of printed dots in continuous ink jet printing using electrostatic dispersion of a charged drop stream to modulate the number of drops passing through a small orifice. This technique is used in the ink jet printers manufactured by Iris Graphics.

US-A-3,946,398 describes a drop-on-demand inkjet printer that applies a high voltage to a piezoelectric crystal, thereby bending the crystal, exerting pressure on an ink container and ejecting drops as needed. Many types of piezoelectric drop-on-demand printers have been invented that use piezoelectric crystals in bending, printing, shearing and squeezing processes. Piezoelectric drop-on-demand printers using hot melt inks (e.g. printers from Tektronix and Dataproducts) are successfully used in the commercial sector and, with image resolutions of up to 720 dpi, also for home and office printers (Seiko Epson). Piezoelectric drop-on-demand printers have the advantage that they can be used with a wide range of inks. However, piezoelectric printing mechanisms require complex high-voltage drive circuitry and piezoelectric crystal arrays, which is disadvantageous in terms of manufacturing and performance.

GB-A-2,007,162 describes an electrothermal drop-on-demand inkjet printer that applies an energy pulse to an electrothermal transducer (heating element) that is in thermal contact with the ink in a nozzle.

The heater rapidly heats the water-based ink to a high temperature, causing a small amount of ink to quickly evaporate and form a bubble. This bubble formation creates a pressure wave, which in turn causes ink drops to be ejected from small openings on the edge of the heater carrier. This technology is known as BubblejetTM (trademark of the Japanese Canon K.K.) and is used in a variety of printing systems from Canon, Xerox and other manufacturers.

US-A-4,490,728 describes an electrothermal drop ejection system which also works by forming bubbles. In this system, drops are ejected in a direction perpendicular to the plane of the heating element carrier, through nozzles formed in an aperture plate arranged above the heating element. This system is known as Thermal Ink Jet and is manufactured by Hewlett-Packard. In this document, the term Thermal Ink Jet is used for both the Hewlett-Packard system and the systems known as BubblejetrM.

The thermal ink jet printing process requires approximately 20 uJ over a period of approximately 2 us to eject one drop at a time. The power consumption in activity of 10 watts of each heating element is disadvantageous in itself and requires special inks, requires complicated driver electronics and accelerates the deterioration of the heating elements.

Other inkjet printing systems have also been described in the technical literature, but these are not currently in commercial use. For example, US-A-4,275,290 describes a system in which the simultaneous application of heat pulses and hydrostatic pressure to predetermined printhead nozzles allows ink to flow freely onto paper spaced apart by spacers that can be guided past the printhead. US-A-4,737,803, US-A-4,737,803 and US-A-4,748,458 describe inkjet recording systems in which the simultaneous application of a heat pulse and an electrostatic attraction field to ink in thermal print nozzles causes ink drops to be ejected onto a print sheet.

Each of the inkjet printing systems described above has advantages and disadvantages. There remains an undeniable need for improved inkjet technology that offers advantages in terms of, for example, cost, speed, quality, reliability, power consumption, simplicity of design and operation, durability and consumables.

These and other objects of the present invention are achieved by the characterizing features of claim 1. Preferred embodiments can be found in the subclaims.

The printing mechanism is based on a new printing principle called "Liquid lnk Fault Tolerant" (LIFT) drop-on-demand printing.

The copending applications entitled "A Liquid Ink Printing Apparatus and System" and "Coincident Drop-Selection, Drop-Separation Printing Method and System" describe new methods and devices for achieving significant improvements to overcome the prior art problems discussed above. These inventions offer important advantages, e.g. with respect to drop size and positioning accuracy, with respect to achievable printing speeds, with respect to power consumption, durability and thermal stresses during operation, as well as with respect to other printer performance characteristics and the manufacture and properties of the inks that can be used. An important object of the present invention is to extend the structures and methods described in the patent applications to further advance printing technology.

The invention is explained in more detail below using embodiments shown in the drawing.

Show it

Fig. 1(a) is a simplified schematic block diagram of an exemplary printing apparatus according to the invention.

Fig. 1(b) a section through various nozzle tips according to the invention.

Fig. 2(a) to 2(f) Simulations of the flow dynamics of tropical selection.

Fig. 3(a) is a finite element fluid dynamics simulation of an operating nozzle according to an embodiment of the invention.

Fig. 3(b) successive meniscus positions during a droplet selection and separation cycle.

Fig. 3(c) the temperatures at different points during a drop selection cycle.

Fig. 3(d) the measured surface tension plotted against temperature for different ink additives.

Fig. 3(e) shows the energy pulses applied to the nozzle heater to produce the temperature curves shown in Fig. 3(c).

Fig. 4 is a schematic block diagram of a printhead driver circuit for practicing the invention.

Fig. 5 predicted manufacturing yields for an A4 page width color printhead incorporating features of the invention, with and without error tolerance.

Fig. 6 is a general block diagram of a printing system with a print head.

Fig. 7 is a schematic view of an ink cartridge and a cartridge holder.

Fig. 8 is an exploded perspective view of a four-color ink cartridge.

Fig. 9 is an exploded view of an ink channel assembly for a four-color printhead.

Fig. 10 is an exploded perspective view of a four-color ink and paper cartridge.

In a general aspect, the invention provides a drop-on-demand printing system wherein means for selecting the drops to be printed create a positional difference between selected drops and non-selected drops, but not sufficient to cause the ink drops to overcome the ink surface tension and separate from the ink body, and wherein an alternative means is provided for causing separation of the selected drops from the ink body.

Separating the drop selectors from the drop separators significantly reduces the energy required to select the ink drops to be printed. Only the drop selectors need to be controlled with individual signals applied to each nozzle. The drop separators can be a field or a state applied to all nozzles simultaneously.

The drop selection devices can be selected from the following list, without the list being understood as limiting:

1) Electrothermal reduction of the surface tension of pressurized ink.

2) Electrothermal bubble generation, where the bubble volume is not sufficient to generate droplet ejection.

3) Piezoelectric, where the volume change is not sufficient to generate droplet ejection.

4) Electrostatic attraction with one electrode per nozzle.

The droplet separators can be selected from the following list, without the list being understood as limiting:

1) Proximity (receiving medium is close to the print head).

2) Proximity with oscillating ink pressure.

3) Electrostatic attraction

4) Magnetic attraction

The table "Drop-on-demand printing technology measuring strips" shows some desirable properties of the drop-on-demand printing technology. The table also shows some methods by which some embodiments described here or in the other reference applications achieve improvements over the prior art.

Drop-on-demand printing technology measuring strips

Measuring strip method for achieving an improvement over the state of the art

High-speed operation Affordable, page-wide printheads with more than 10,000 nozzles. Monolithic A4-page printheads can be manufactured using standard 300 mm (12 in.) silicon wafers.

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

Full-color operation Raster process colors at 800 dpi with stochastic screening

Ink versatility Low ink operating temperature and no need for bubble formation

Low power consumption Energy-saving operation because the drop selection devices do not need to eject the drop completely.

Low cost Monolithic printhead with aperture plate, low waste, few electrical connections, use of modified existing CMOS manufacturing equipment.

Low production waste. Error tolerance integrated in the print head.

High reliability Fault tolerance built into the printhead. No cavitation, kogation (deposition of ink residue on the head). Reduced thermal shocks.

Small number of electrical connections Shift register, control circuit and driver circuit can be integrated into a monolithic print head using common CMOS processes.

Use of existing VLSI manufacturing facilities CMOS compatibility. This is achievable because the heat driver energy is less than 1% of the heater driver power of thermal ink jet systems.

Electronic collationA new page compression system that achieves 100:1 compression without significant image degradation results in a compressed data set small enough to print any combination of thousands of pages stored on inexpensive magnetic disk drives in real time.

For thermal inkjet systems and piezoelectric inkjet systems, a drop velocity of about 10 m/s is preferred to ensure that the selected ink drops overcome the ink surface tension, separate from the ink body and impact the receiving medium. These systems convert electrical energy into kinetic energy only with very low efficiency. The efficiency of thermal inkjet systems is about 0.02%. This means that the driver circuits for thermal inkjet printheads must switch high currents. The driver circuits for piezoelectric inkjet heads must either switch high voltages or drive high capacitive loads. The total power consumption of full-page thermal inkjet printheads is therefore very high. An A4 full-page thermal inkjet print head for 800 dpi resolution, printing a black four-colour image in one second, consumes approximately 6 kW of electrical energy, most of which is dissipated as waste heat. The problems associated with dissipating this amount of heat preclude the manufacture of low-cost, fast and compact full-page thermal inkjet systems with high resolution.

An important feature of embodiments of the invention is a means for substantially reducing the energy required to select which ink drops are to be printed. This is achieved by separating the means for selecting the ink drops from the means for ensuring that the selected drops separate from the ink body and form dots on the receiving medium. It is only necessary to control the drop selection means by applying individual signals to each nozzle. The drop separation means may be an array or a state applied to all nozzles simultaneously.

The table "Drop Selection Devices" shows some of the possible means for selecting drops according to the invention. The drop selection device only needs to cause a sufficient change in position of the selected drops to enable the drop selection device to distinguish between selected and non-selected drops. Drop selection devices

Other drop selection devices can also be used.

The preferred drop selection device for water-based inks is the 1st method, namely "Electrothermal reduction of the surface tension of the pressurized ink". This drop selection device offers many advantages over other systems, including energy-saving operation (approximately 1% of thermal inkjet systems), compatibility with CMOS VLSI chip manufacturing, low voltage operation (approximately 10 V), high nozzle density, low temperature operation, and a wide range of suitable ink formulations. The surface tension of the ink must decrease significantly as the temperature increases.

The preferred drop selection device for heat-melting and oil-based inks is the 2nd method, namely: "Electrothermal reduction of the ink viscosity, combined with an oscillating ink pressure". This drop selection device is particularly suitable for the use of inks which show a large reduction in their viscosity with increasing temperature, but only a small reduction in their surface tension. This occurs particularly with non-polar ink carriers with a relatively high molecular weight and is particularly true for heat-melting and oil-based inks.

The Drop Separators table shows possible methods for separating selected drops from the ink body, ensuring that the selected drops form dots on the print media. The drop separators distinguish between selected drops and unselected drops to ensure that unselected drops do not form dots on the print media. Droplet separators

Other droplet separation devices can also be used.

The preferred droplet separation device depends on the intended use. For most applications, the 1st method, "Electrothermal Attraction", or the 2nd method, "AC Field", is best. For applications where smooth coated paper or film is used and very high speed is not important, the 3rd method, "Proximity", is best. For high speed, high quality systems, the 4th method, "Transfer Proximity", can be used. The 6th method, "Magnetic Attraction", is suitable for portable printing systems where the printing medium is too rough for the proximity method and where the pressure required for electrostatic The high voltages required for drop separation are undesirable. There is no clear "best" drop separation device applicable to all circumstances. Further details of the various types of printing systems according to the invention are described in the following Australian patent application filed on 12 April 1995:

"A liquid ink fault tolerant (LIFT) printing mechanism" (reference number: PN2308);

"Electrothermal drop selection in LIFT printing" (reference number: PN2309);

"Drop separation in LIFT printing by print media proximity" (reference number: PN2310);

"Drop size adjustment in proximity LIFT printing by varying head to media distance" (reference number: PN2311);

“Augmenting proximity LIFT printing with acoustic ink waves” (reference number: PN2312);

"Electrostatic drop separation in LIFT printing" (reference number: PN2313);

"Multiple simultaneous drop sizes in proximity LIFT printing" (reference number: PN2321);

"Self cooling operation in thermally activated print heads" (reference number: PN2322); and

"Thermal viscosity reducing LIFT printing" (reference number: PN2323).

Fig. 1(a) shows a simplified schematic representation of a preferred printing system according to the invention.

The image source 52 may be raster image data from a scanner or a computer, layout data in the form of a page description language (PDL), or other forms of digital image representation. This image data is converted into a page image consisting of pixels by the image processing system 53. The image processing system 53 may be a raster image processor (RIP) in the case of PDL image data or a pixel image processing device in the case of raster image data. Halftone data generated by the image processing system 53 is rasterized. The rasterization is carried out by the digital raster unit 54. The rasterized bitmap image data is stored in the image memory 72. Depending on the printer and system configuration, the image memory 72 may be a full-page memory or a band memory. The heater control circuits 71 read data from the image memory 72 and apply time-varying electrical pulses to the nozzle heaters (see 103 in Fig. 1 (b)), which are part of the print head 50. These pulses can be applied to the corresponding nozzles at the appropriate time in such a way that the selected drops form dots on the receiving medium 51 at the positions provided by the data in the image memory 72.

The receiving medium 51 is movable relative to the print head 50 by a paper transport system 65, which is controlled by a paper transport controller 66, which in turn is controlled by a microcontroller 315. The paper transport system 65 can take on many different mechanical forms and is only shown schematically in Fig. 1 (a). In the case of full-page print heads, it is common to move the receiving medium 51 past a stationary print head 50. In the case of scanning printing systems, it is common to move the print head 50 in a raster pattern along one axis (the secondary scanning direction) and the receiving medium 51 along the orthogonal axis (the main scanning direction). The microcontroller 315 is also controllable by the ink pressure regulator 63 and the heater control circuits 71.

For printing using surface tension reduction, the ink is contained in a pressurized ink reservoir 64. In the idle state (where no ink drop is ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. A constant ink pressure can be achieved by pressurizing the ink reservoir 64 under the control of an ink pressure regulator 63. For larger printing systems, the ink pressure can be generated and controlled very precisely by locating the surface of the ink in the ink reservoir 64 a precise distance above the print head 50. This ink level can be regulated by a simple float valve (not shown).

To print with viscosity reduction, ink is pressurized in an ink reservoir 64, with the ink pressure being subjected to oscillation. The means for generating this oscillation may be a piezoelectric actuator installed in the ink channels (not shown).

When properly aligned with the drop separation devices, the selected drops form dots on the receiving medium 51, while unselected drops remain part of the ink body.

The ink is distributed to the rear of the printhead 50 via an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the printhead 50 to the front where the nozzles and actuators are located. In the thermal selection method, the ink 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 ink body and moves to the receiving medium 51. A suitable external field 74 is a constant electric field because the ink is easy to make electrically conductive. In this case, the paper guide 67 can be made of electrically conductive material and used as one electrode for generating the electric field. The other electrode can be the print head 50 itself. Another embodiment uses the proximity of the print medium as a means of distinguishing between selected drops and non-selected drops.

For small drop sizes, the gravitational force of the ink drop is very small, about 10-4 of the surface tension forces, so gravity is ignorable in most cases. This makes it possible to align the print head 50 and the receiving medium 51 in one direction in relation to the local gravitational field. This is an important requirement for portable printers.

Figure 1(b) shows a detailed enlargement of a section through a microscopic nozzle tip embodiment of the invention made using a modified CMOS process. The nozzle is etched into a carrier 101, which may be made of silicon, glass, metal, or other suitable material. When non-semiconductor carriers are used, a semiconductor material (e.g., amorphous silicon) may be deposited on the carrier and integrated driver transistors and data distribution circuitry may be formed in the surface semiconductor layer. Single crystal silicon (SCS) carriers have several advantages, including:

1) High-performance driver transistors and other circuits can be made of single crystal silicon;

2) Printheads can be manufactured in existing facilities (manufacturing facilities) using common VLSI processing systems;

3) Single crystal silicon has high mechanical strength and strength; and

4) Single crystal silicon has a high thermal conductivity.

In this example, the nozzle has a cylindrical shape with the heating element 103 forming an annulus. The nozzle tip 104 is formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS driver circuit. The nozzle tip is passivated with silicon nitride. The protruding nozzle tip controls the point of contact of the pressurized ink 100 on the printhead surface. The printhead surface is also hydrophobized to prevent ink from accidentally scattering across the printhead.

Many other nozzle configurations are possible, and nozzle embodiments of the present invention may vary in shape, size, and materials used. Monolithic nozzles, etched from the substrate on which the heating element and electronics are formed, have the advantage of not requiring a hole plate. Elimination of the hole plate has a significant cost saving effect on manufacturing and assembly. Recent methods of eliminating hole plates include the use of "vortex" actuators, such as those described in US-A-4,580,158 and US-A-5,371,527. However, the actuation of these elements is complex and the elements are difficult to manufacture. The preferred method of eliminating hole plates for printheads of the present invention is to integrate the holes into the actuator substrate.

This type of nozzle can be used with printheads that use different droplet separation techniques.

Operation with electrostatic droplet separation

As a first example, Fig. 2 shows the operation using thermal reduction of surface tension and electrostatic droplet separation.

Fig. 2 shows the results of energy transport and drop selection fluid dynamics simulations using FIDAP, a commercial fluid dynamics software package from Fluid Dynamics, Inc. of Illinois, USA. This simulation is performed on a thermal drop selection nozzle embodiment with a diameter of 8 µm and an ambient temperature of 30ºC. The heater is applied with a total energy of 276 nJ with 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity is 1.84 cPs at 30ºC. Water-based ink with a sol of 0.1% palmitic acid is used to achieve improved surface tension reduction with increasing temperature. A section through the nozzle tip from the central axis of the nozzle to a radial distance of 40 µm is shown. The heat flow in the different nozzle materials, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide and water-based ink, is simulated using the respective values for density, heat capacity and thermal conductivity of the materials. A time step of 0.1 us is used for the simulation.

Fig. 2(a) shows a resting state immediately before the heater is activated. Here, an equilibrium state is achieved with no ink coming out of the nozzle in the resting state by ensuring that the ink pressure plus external electrostatic field is not sufficient to overcome the surface tension of the ink at ambient temperature. In the resting state, the meniscus of the ink does not protrude from the printhead surface, so no significant concentration of the electrostatic field occurs at the meniscus.

Fig. 2(b) shows thermal contours at 5ºC intervals 5 us after the start of the heater excitation pulse. When the heater is activated, the ink in contact with the nozzle tip heats up rapidly. The reduction in surface tension causes the heated portion of the meniscus to expand rapidly relative to the cool ink meniscus. This causes a convective heat flow that carries the heat over part of the free surface of the ink at the nozzle tip. The heat must be distributed over the ink surface and not just where the ink is in contact with the heater, because the viscous drag against the solid heater element prevents the ink in immediate contact with the heater from moving.

Fig. 2(c) shows the thermal contours at 5ºC intervals 10 µs after the start of the heater excitation pulse. The temperature increase causes a reduction in the surface tension and a disruption of the force equilibrium. Since the entire meniscus has been heated, a flow motion of the ink starts.

Fig. 2(d) shows the thermal contours at 5ºC intervals 20 us after the start of the heater excitation pulse. The ink pressure has caused a flow of the ink to a new meniscus position, with the ink protruding from the printhead. The electrostatic field is concentrated by the protruding conductive ink drop.

Fig. 2(e) shows the thermal contours at 5ºC intervals 30 us after the start of the heater excitation pulse, a time which is also 6 us after the end of the heater excitation pulse, since the heater excitation pulse lasts 24 us. The nozzle tip has cooled rapidly due to dissipation through the oxide layers and by dissipation into the flowing ink. The nozzle tip is "water cooled" by the ink, so to speak. The electrostatic attraction causes the ink drop to accelerate towards the receiving medium. If the heater excitation pulse were much shorter (in this case less than 16 us), the ink would not accelerate towards the print medium, but would instead flow back into the nozzle.

Fig. 2(f) shows the thermal contours at 5ºC intervals 26 us after the start of the heater excitation pulse. The temperature at the nozzle tip is now less than 5ºC above ambient temperature. This causes an increase in the surface tension around the nozzle tip. When the flow rate at which the ink is drawn out of the nozzle exceeds the viscosity-induced rate of ink flow through the nozzle, a constriction of the ink occurs in the nozzle tip region and the selected droplet separates from the ink body. The selected droplet then migrates to the receiving medium under the influence of the external electrostatic field. The meniscus of ink at the nozzle tip returns to its rest position ready for the next heater excitation pulse to select the next ink droplet. For each heater excitation pulse, a selected and separated ink droplet forms a dot on the receiving medium. Under electrical control of the Heater excitation pulses can be used to achieve drop-on-demand inkjet operation.

Fig. 3(a) shows successive meniscus positions during the drop selection cycle at 5 µs intervals starting with the start of the heater excitation pulse.

Fig. 3(b) shows a meniscus position versus time plot showing the motion at the center of the meniscus. The heater excitation pulse begins 10 us after the start of the simulation.

Fig. 3(c) shows the resulting temperature curve with respect to various points in the nozzle. The vertical axis of the curve represents temperature in units of 100ºC. The horizontal axis of the curve represents time in units of 10 µs. The temperature curve shown in Fig. 3(b) was calculated by the FIDAP program in time steps of 0.1 µs. The local ambient temperature is 30ºC. Temperature values at three points are shown:

A - Nozzle tip: These are the temperature values at the contact circle between the passivation layer, the ink and the air.

B - Meniscus Center: These temperature values are taken at a circle of the ink meniscus halfway between the nozzle tip and the meniscus center.

C - Carrier surface: This is a point on the printhead surface 20um from the nozzle center. The temperature increases by only a few degrees. This indicates that the active circuitry can be located very close to the nozzles without suffering any loss of performance or lifetime due to increased temperatures.

Fig. 3(e) shows the power applied to the heater. Optimum operation requires a sharp rise in temperature at the beginning of the heating pulse, a maintenance of the temperature at a value slightly below the boiling point of the ink for the duration of the pulse, and a rapid drop in temperature towards the end of the pulse. To achieve this, the average power applied to the heater during the Pulse duration is changed. In this case, the change is achieved by pulse frequency modulation of sub-pulses of 0.1 us, each of which has an energy of 4 nJ. The peak power applied to the heater is 40 mW, the average power during the duration of the heater pulse is 11.5 mW. The frequency of the sub-pulse in this case is 5 MHz. This can be changed without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control of the power applied to the heater. A sub-pulse frequency of 13.5 MHz is suitable because this frequency has the effect of minimizing radio frequency interference.

Inks with a negative temperature coefficient of surface tension

The fact that the surface tension of the ink must decrease with increasing temperature is not a significant limitation, since most pure liquids and many mixtures exhibit this property. There are no precise equations expressing the relationship between surface tension and temperature for any liquid. However, the following empirical equation, derived by Ramsay and Shields, gives satisfactory results for many liquids:

Where γT is the surface tension at a temperature T, k is a constant, Tc is the critical temperature of the liquid, M is the molar mass of the liquid, x is the degree of association of the liquid, and p is the density of the liquid. This equation shows that the surface tension of most liquids drops to zero when the temperature reaches the critical temperature of the liquid. For most liquids, the critical temperature is significantly above the boiling point at atmospheric pressure. To obtain an ink with a large change in surface tension for a small change in temperature in the range of an ejection temperature, the addition of surfactants is recommended.

The choice of surfactants is important. For example, water-based inks for thermal inkjet printers often contain isopropyl alcohol (2-propanol) to reduce surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4ºC, lower than water. As the temperature increases, the alcohol evaporates faster than the water, so the alcohol concentration decreases, causing an increase in surface tension. A surfactant such as 1-hexanol (boiling point 158ºC) can be used to achieve this effect. This agent provides a surface tension that decreases slowly with increasing temperature. However, a relatively large drop in surface tension with increasing temperature is desirable to maximize the operating range. A decrease in surface tension of 20 mN/m over a temperature range of 30°C is preferred to achieve wide operating ranges, with a value of 10 mN/m being usable to achieve the operation of the printhead according to the invention.

Inks with large negative change in surface tension with respect to temperature (-ΔYT)

Various methods can be used to achieve a large negative change in surface tension with increasing temperature. Two of these methods are:

1) The ink may contain a low concentration sol of surfactant that is solid at ambient temperature but melts at a threshold temperature. Particle sizes of less than 100 nm are desirable. Suitable melting points of surfactants for water-based inks are between 50ºC and 90ºC, preferably between 60ºC and 80ºC.

2) The ink may comprise an oil/water microemulsion having a phase inversion temperature above the maximum ambient temperature but below the boiling point of the ink. For stability reasons, the phase inversion temperature of the microemulsion is preferably 20ºC or more above the maximum non-operating temperature of the ink. A phase inversion temperature of approximately 80ºC is suitable.

Inks with surfactant sols

Inks can be formulated as a sol of small particles of a surfactant that melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids containing 14 to 30 carbon atoms, such as:

Since the melting point of sols with a small particle size is usually lower than that of the bulk material, it is advantageous to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature. A good example of this is arachidic acid.

The carboxylic acids are available in high purity and at low cost. The amount of surfactants required is very small, so the cost of adding such an additive to the ink is negligible. A mixture of carboxylic acids with slightly varying chain lengths can be used to spread the melting points over a temperature range. Such mixtures normally cost less than the pure acid.

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

Preparation of inks with sols from surfactants

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

An example of the preparation of a surfactant sol is given below:

1) Add the carboxylic acid to the clarified water under exclusion of oxygen.

2) Heat the mixture above the melting point of the carboxylic acid. The water can be brought to the boil.

3) Sonicate the mixture until the typical size of the carboxylic acid droplets is between 10 and 100 nm.

4) Allow the mixture to cool.

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

6) Add an alkali, e.g. NaOH, to ionize the carboxylic acid molecules on the surface of the particles. A pH of about 8 is suitable. This step is not mandatory, but helps to stabilize the sol.

7) Centrifuge the sol. Since the density of carboxylic acid is lower than that of water, the smaller particles will collect on the outside of the centrifuge and the larger particles will collect in the center of the centrifuge.

8) Filter the sol with a microporous filter to remove particles larger than 500 nm.

9) Add the surfactant sol to the ink. The sol is only needed in a very diluted concentration.

The ink formulation contains either dyes or pigments, bactericidal agents, agents to improve the electrical conductivity of the ink if electrostatic droplet separation is used, wetting agents and other agents as required.

Antifoaming agents are normally not required since bubble formation does not occur during the drop ejection process.

Sole made from cationic surfactants

Inks containing anionic surfactants are generally unsuitable for use with cationic dyes or pigments because the cationic dyes or pigments precipitate or coagulate with the anionic surfactant. In order to use cationic dyes or pigments, a sol of cationic surfactants is required. The alkylamine family is suitable for this purpose.

The following table shows suitable alkylamines:

The process for preparing sols from cationic surfactants is essentially the same as that for anionic surfactants, with the difference that an acid is used instead of an alkali to adjust the pH and increase the charge on the surfactant particles.

A pH of 6 using HCl is usually suitable.

Microemulsion-based inks

An alternative means of achieving a strong reduction in surface tension at a temperature threshold is to base the ink on a microemulsion. A microemulsion with a phase inversion temperature in the range of the desired ejection threshold temperature is selected. Below the phase inversion temperature, the microemulsion is an oil-in-water emulsion, above the phase inversion temperature, the microemulsion is a water-in-oil emulsion. At low temperatures, the microemulsion prefers At temperatures well below the phase inversion temperature, the surfactant prefers a strong curvature around the water. At temperatures close to the phase inversion, the microemulsion forms a continuous "sponge" of topologically connected water and oil.

There are two mechanisms by which the surface tension is reduced here. In the range of the phase inversion temperature, the surfactant prefers surfaces with very little curvature. As a result, the molecules of the surfactant migrate to the contact surface between the ink and air, which has a much weaker curvature than the oil emulsion. This reduces the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from an oil-in-water to a water-in-oil emulsion, changing the contact surface from water/air to oil/air. The oil/air contact surface has a lower surface tension.

There are many ways to produce microemulsion-based inks.

For rapid drop ejection, an oil with low viscosity is preferred.

In many cases, water is a suitable polar solvent. However, in some cases, solvents with different polarities may be required. In these cases, polar solvents with a high surface tension should be chosen so that a large drop in surface tension can be achieved.

The surfactant can be selected in such a way that it exhibits a phase inversion in the desired temperature range. For example, surfactants from the group of poly(oxyethylene)alkylphenyl ethers (ethoxylated alkylphenols, general formula: CH2n+1C₄H₆(CH₂CH₂O)mOH) can be used. The hydrophilicity of the surfactants can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values for m of approximately 10 and for n of approximately 8 are suitable.

Low-cost commercial approaches are the result of polymerizing different molar ratios of ethylene oxide and alkylphenols, with the exact number of oxyethylene groups varying around the chosen mean. These commercial approaches are suitable, but high-purity surfactants with a specific number of oxyethylene groups are not required.

The formula for this surfactant is C₈H₁₇C₄H₆(CH₂CH₂O₂)nOH (average n = 10).

Synonyms include octoxynol-10, PEG-10 and octylphenyl ether as well as POE(10) octylphenyl ether.

The HLB is 13.6, the melting point is 7ºC, the cloud point is 65ºC.

Commercial formulations of this surfactant are available under various brand names. The following table shows the suppliers and the corresponding brand names:

Brand Name Provider

Akyporox OP100 Chem-Y GmbH

Alkasurf OP-10 Rhone-Poulenc Surfactants and Specialties

Dehydrophen POP 10 Pulcra SA

Hyonic OP-10 Handle

Iconol OP-10 BASF

Igepal O Rhone-Poulenc France

Macol OP-10 PPG Industries

Malorphen 810 Hüls AG

Nikkol OP-10 Nikko Chem. Co. Ltd.

Renex 750 ICI Americas Inc.

Rexol 45/10 Hart Chemical Ltd.

Synperonic OP10 ICI PLC

Teric X10 ICI Australia

These products are available in large quantities at low cost (less than DM 3.00 per kg) and thus contribute less than DM 0.15 per liter to the prepared microemulsion ink at a 5% concentration of surfactants.

Other suitable ethoxylated alkylphenols are listed in the following table:

Microemulsion-based inks have further advantages in terms of surface tension control:

1) Microemulsions are thermodynamically stable and do not separate. The storage time can therefore be very long. This is particularly important for office printers and portable printers that may only be used sporadically.

2) The microemulsion spontaneously forms a specific droplet size. Extensive stirring, centrifugation or filtering is not required to ensure a specific range of emulsified oil droplet sizes.

3) The amount of oil contained in the ink can be quite large, so that dyes that are soluble in oil or water or both can be used. A mixture of dyes, one soluble in water and one soluble in oil, is also possible to achieve certain colors.

4) Oil-miscible pigments do not flocculate because they are embedded in the oil microdroplets.

5) The use of a microemulsion can reduce the mixing of different colored dyes on the surface of the printing medium.

6) The viscosity of the microemulsions is very low.

7) Wetting agents can be used in smaller quantities or omitted altogether.

Dyes and picimens in microemulsion-based inks

Oil-in-water mixtures can have a high oil content, up to 40%, and still form oil-in-water microemulsions. This allows for high dye or pigment loading.

Mixtures of dyes and pigments can be used. A microemulsion-based ink mixture with dyes and pigments can, for example, be composed as follows:

1) 70% water

2) 5% water-soluble dye

3) 5% surfactants

4) 10% oil

5) 10% oil miscible pigments

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

The ninth combination without dyes can be used for printing on transparent coatings, UV inks and selective highlights.

Since many dyes are amphiphilic, large amounts of dyes are also soluble in the oil-water interface, since this layer has a very large surface area.

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

When multiple dyes or pigments are used, the absorption spectrum of the resulting ink is the weighted average of the absorption spectra of the different dyes used. This presents two problems:

1) The absorption spectrum becomes broader because the absorption peaks of both dyes are averaged. This tends to result in "washed out" colors. To obtain brilliant colors, the dyes and pigments must be carefully selected in relation to their absorption spectra and not just in relation to the color perceived by the eye.

2) The colour of the ink may vary on different substrates. When a dye and a pigment are used in combination, the dye tends to have a smaller contribution to the colour on more absorbent papers. printed color because the dye is absorbed into the paper, while the pigment tends to "sit" on the paper. This can be used as an advantage in certain circumstances.

Surfactants with a Krafft point in the drop selection temperature range

For ionic surfactants, there is a temperature (the Krafft point) below which 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 the solubility of the surfactant. If the critical micelle concentration (CMC) exceeds the solubility of a surfactant at a certain temperature, then the minimum surface tension is not reached at the critical micelle concentration but at the point of maximum solubility. Surfactants are usually much less effective below the Krafft point.

This factor can be used to achieve a greater reduction in surface tension as the temperature increases. At ambient temperatures, only a portion of the surfactant is in the solution. When the nozzle heater is turned on, the temperature increases and more surfactant enters the solution, reducing the surface tension.

A surfactant should be selected that has a Krafft point near the maximum of the temperature range to which the ink is heated. This provides a maximum limit between the concentration of the surfactant in the solution at ambient temperature and the concentration of the surfactant in the solution at the drop selection temperature.

The concentration of the surfactant should be approximately equal to the critical micelle concentration at the Krafft point. In this way, the surface tension is reduced to the maximum at elevated temperatures and to the minimum at ambient temperatures.

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

Krafft point formula

C16 H33 SO3 -Na+ 57ºC

C18 H37 SO3-Na+ 70ºC

C16 H33 SO4 -Na+ 45ºC

Na?O4 S(CH2 )16 SO4 -Na+ 44.9ºC

K?O4 S(CH2 )16 SO4 -K+ 55ºC

C16 H33 CH(CH3 )C4 H6 SO3 -Na+ 60.8ºC

Surfactants with a cloud point in the range of the drop selection temperature

Non-ionic surfactants with polyoxyethylene (POE) chains can be used to produce an ink whose surface tension drops as the temperature increases. At low temperatures, the polyoxyethylene chain is hydrophilic and keeps the surfactant in solution. As the temperature increases, the structured water in the polyoxyethylene portion of the molecule is disrupted and the polyoxyethylene portion becomes hydrophobic. The surfactant is increasingly repelled by water at higher temperatures, resulting in an increasing concentration of the surfactant at the air-ink interface, thereby reducing the surface tension. The temperature at which the polyoxyethylene portion of a non-ionic surfactant becomes hydrophilic relates to the cloud point of that surfactant. The polyoxyethylene chains themselves are not particularly suitable as the cloud point is generally above 100ºC.

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

Two main configurations of symmetric POE/POP block copolymers are available, namely:

1) Surfactants with POE segments at the ends of the molecules, and a POP segment in the center, such as the poloxamer class of surfactants (CAS 9003-11-6).

2) Surfactants with POP segments at the ends of the molecules and a POE segment in the center, such as the Meroxapol class of surfactants (also CAS 9003-11-6).

Some commercially available variants of poloxamer and meroxapol with a high surface tension at room temperature combined with a cloud point above 40ºC and below 100ºC are listed in the following table:

Further variations of poloxamer and meroxapol can be easily synthesized using known techniques. Desirable properties are a surface tension at room temperature that is as high as possible and a cloud point between 40ºC and 100ºC and preferably between 60ºC and 80ºC.

Meroxapol variants [HO(CHCH₃CH₂O)x(CH₂CH₂O)y(CHCH₃CH₂O)zOH] are suitable, in which the average value of x and z is about 4 and in which the average value of y is about 15.

If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered.

The cloud point of POE surfactants is increased by ions that disrupt the water structure (e.g. F) as this provides more water molecules available to form hydrogen bonds with POE oxygen ion pairs. The cloud point of POE surfactants is decreased by ions that form a water structure (e.g. CL 0 V) as there are fewer water molecules available to form hydrogen bonds. Bromide ions have a relatively small effect. The ink composition is "tunable" to a desired temperature range by changing the length of the POE and POP chains in a block copolymer surfactant and by changing the selection of salts (e.g. GE to B(to F)) added to increase electrical conductivity. NaCl is probably the best choice for increasing the conductivity of the ink due to its low cost and non-toxicity. NaCl slightly lowers the cloud point of the non-ionic surfactants.

Reduction of surface tension of various solutions

Fig. 3(d) shows the measured effect of temperature on the surface tension of different aqueous batches containing the following additives:

1) 0.1% sol from stearic acid

2) 0.1% sol from palmitic acid

3) 0.1% solution of Pluronic 10R5 (brand name of BASF)

4) 0.1% solution of Pluronic L35 (brand name of BASF)

5) 0.1% solution of Pluronic L44 (brand name of BASF)

Inks suitable for printing systems according to the invention are described in the following Australian patents:

"Ink composition based on a microemulsion" (Case No. PN5223, filed September 6, 1995);

"Ink composition containing surfactant sol" (Case No. PN5224, filed September 6, 1995);

"Ink composition for DOD printers with Krafft point near the drop selection temperature sol" (Case No. PN6240, filed October 30, 1995); and

"Dye and pigment in a microemulsion based ink" (Case No. PN6241, filed October 30, 1995).

Operation using viscosity reduction

A second example, namely the operation of an embodiment using viscosity reduction and proximity drop separation in conjunction with heat-melting ink, is described below. Prior to operation of the printer, the solid ink in the ink container 64 is melted. The container, the ink channel to the printhead, the ink channels 75 and the printhead 50 are maintained at a temperature at which the ink 100 is liquid but has a relatively high viscosity (e.g., 100 cP). The ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is formulated such that the viscosity of the ink decreases with increasing temperature. The ink pressure oscillates at a frequency that is an integrated multiple of the drop ejection frequency from the nozzle. The oscillation of the ink pressure causes oscillations of the ink meniscus at the nozzle tips, but this oscillation is small due to the high viscosity of the ink. At normal operating temperature, these oscillations have an amplitude that does not result in drop separation. When the heater 103 is activated, the ink forming the selected drop heats up, causing a reduction in viscosity to a value that is preferably less than 5 cP. The reduced viscosity causes the ink meniscus to continue to move during the high pressure phase of the ink printing cycle. The receiving medium 51 is spaced sufficiently close to the printhead 50 so that the selected drops contact the receiving medium 51. However, the receiving medium 51 is spaced far enough that the unselected drops do not contact the receiving medium 51. Upon contact with the receiving medium 51, a portion of the selected drops freeze and adhere to the receiving medium. When the ink pressure drops, the ink flows back into the nozzle. The ink body separates from the ink frozen on the receiving medium. The meniscus of the ink 100 at the nozzle tip returns to a low amplitude oscillation. The viscosity of the ink increases to its resting value while the remaining heat is dissipated to the ink mass and to the printhead. An ink drop is selected, separates, and forms a dot on the receiving medium 51 for each heat pulse. If the heat pulses are controlled electrically, drop-on-demand inkjet operation is achievable.

Manufacturing of print heads

The manufacturing processes for monolithic printheads according to the present invention are described in the following Australian patent applications filed on 12 April 1995:

"A monolithic LIFT printing head" (reference number: PN2301);

"A manufacturing process for monolithic LIFT printing heads" (reference number: PN2302);

"A self-aligned heater design for LIFT print heads" (reference number: PN2303);

"Integrated four color LIFT print heads" (reference number: PN2304);

"Power requirement reduction in monolithic LIFT printing heads" (reference number: PN2305);

"A manufacturing process for monolithic LIFT printing heads using anisotropic wet etching" (reference number: PN2306);

“Nozzle placement in monolithic drop-on-demand print heads” (reference number PN2307);

"Heater structure for monolithic LIFT print heads" (reference number: PN2346);

"Power supply connection for monolithic LIFT print heads" (reference number: PN2347);

"External connections for proximity LIFT print heads" (reference number: PN2348); and

"A self-aligned manufacturing process for monolithic LIFT print heads" (reference number: PN2349); and

"CMOS process compatible fabrication of LIFT print heads" (reference number: PN5222, September 6, 1995);

"A manufacturing process for LIFT print heads with nozzle rim heaters" (reference number: PN6238, October 30, 1995);

"A modular LIFT print head" (Case number PN6237, October 30, 1995);

"Method of increasing packing density of printing nozzles" (reference number: PN6236, 30 October 1995); and

"Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets (reference number: PN6239, October 30, 1995).

Control of print heads

Means for providing page image data and controlling heater temperatures in printheads according to the present invention are described in the following Australian patent applications filed on 12 April 1995:

"Integrated drive circuitry in LIFT print heads" (reference number: PN2295);

"A nozzle clearing procedure for liquid ink fault tolerant (LIFT) printing" (reference number: PN2294);

"Heater power compensation for temperature in LIFT printing systems" (reference number: PN2314);

"Heater power compensation for thermal lag in LIFT printing systems" (reference number: PN2315);

"Heater power compensation for print density in LIFT printing systems" (reference number: PN2316);

“Accurate control of temperature pulses in printing heads” (reference number: PN2317);

"Data distribution in monolithic LIFT printing heads" (reference number: PN2318);

"Page image and fault tolerance routing device for LIFT printing systems" (reference number: PN2319); and

"A removable pressurized liquid ink cartridge for LIFT printers" (reference number: PN2320).

Image processing for print heads

One object of the printing systems according to the invention is to achieve a print quality comparable to that to which users are accustomed in publications printed by offset printing. This can be achieved using a print resolution of approximately 1,600. However, printing at 1,600 dpi is difficult and expensive. Similar results can be achieved with 800 dpi, with 2 bits per pixel for cyan and magenta and one bit per pixel for yellow and black. This color model is referred to as CC'MM'YK'. When monochrome images have to be printed in high quality, two bits per pixel for black can be used. This model is referred to here as CC'MM'YKK'. Color models, halftones, data compression and real-time enhancement systems suitable for use in systems according to the invention and in other printing systems are described in the following Australian patent applications filed on 12 April 1995:

"Four level ink set for bi-level color printing" (reference number: PN2339);

"Compression system for page images" (reference number: PN2340);

"Real-time expansion apparatus for compressed page images" (reference number: PN2341); and

"High capacity compressed document image storage for digital color printers" (reference number: PN2342);

"Improving JPEG compression in the presence of text" (case number: PN2343);

"An expansion and halftoning device for compressed page images" (reference number: PN2344); and

"Improvements in image halftoning" (case number: PN2345).

Applications for using printheads according to the invention

The printing devices and methods of the present invention are suitable for a wide range of applications including (but not limited to) color and monochrome office printing, low volume digital printing, high speed digital printing, process color printing, spot color printing, offset insert printing, low cost scanning printhead printing, high speed page printhead printing, portable color and monochrome color printers, color and monochrome copiers, color and monochrome fax machines, combined paper, fax and copier machines, label printing, large format plotters, photographic reproduction, digital photographic processing printers, portable printers integrated with digital "instant" cameras, video printing, photo CD image printing, portable "personal digital assistant" printers, wallpaper printing, indoor poster printing, display board printing, and fabric printing.

Printing systems based on this invention are described in the following Australian patent applications filed on 12 April 1995:

"A high speed color office printer with a high capacity digital page image store" (reference number: PN2329);

"A short run digital color printer with a high capacity digital page image store" (reference number: PN2330);

"A digital color printing press using LIFT printing technology" (reference number: PN2331);

"A modular digital printing press" (reference number: PN2332);

"A high speed digital fabric printer" (reference number: PN2333);

"A color photograph copying system" (reference number: PN2334);

"A high speed color photocopier using a LIFT printing system" (reference number: PN2335),

"A portable color photocopier using LIFT printing technology" (reference number: PN2336);

"A photopgraph processing system using LIFT printing technology" (reference number: PN 2337);

"A plain paper facsimile machine using a LIFT printing system" (reference number: PN2338);

"A PhotoCD system with integrated printer" (reference number: PN2393);

"A color plotter using LIFT printing technology" (reference number: PN2291);

"A notebook computer with integrated LIFT color printing system" (reference number: PN2292);

"A portable printer using a LIFT printing system" (reference number: PN2300);

"Fax machine with online database interrogation and customized magazine printing" (reference number: PN2299);

"Miniature portable color printer" (reference number: PN2298);

"A color video printer using a LIFT printing system" (reference number: PN2296),

"An integrated printer, copier, scanner, and facsimile using a LIFT printing system" (reference number: PN2297).

Composition of printheads for environmental conditions

It is desirable for drop-on-demand printing systems to have a consistent and predeterminable size and position of the ink drops. Undesirable variations in the size and position of the ink drops cause variations in the optical density and the resulting print, which degrades the perceived print quality. These variations should be limited to a small range of the nominal ink drop volume or pixel pitch.

There are many factors that can affect drop volume and drop position. In some cases, the change can be minimized by an appropriate head design. In other cases, the change can be compensated by active circuits.

1) Ambient temperature: Changes in ambient temperature can affect the meniscus rest position and the temperature achieved by the heating pulse. Changes in the meniscus rest position can be compensated by changing the ink pressure or the strength of the external electric or magnetic field. Changes in the temperature achieved by the heating pulse can be compensated by changing the heating supply current.

2) Nozzle temperature: It is not practical to compensate the temperature independently for each nozzle. Reliable operation of the heads requires that the The difference between the nozzle temperature measured at the substrate and the ambient temperature is small. This can be achieved by using a substrate with high thermal conductivity (such as silicon) and by allowing adequate time between pulses to allow the waste heat to dissipate.

3) The nozzle radius variation for nozzles fed from a single ink reservoir should be kept small, since it is difficult to provide different electric field strengths or ink pressures on a nozzle-by-nozzle basis. Fortunately, the nozzle radius variation can easily be kept below 0.5 µm with modern semiconductor manufacturing machines.

4) Print density: Different numbers of ink drops can be ejected in each cycle. The charge resistance of the head can therefore vary widely and rapidly, causing voltage fluctuations due to the power supply’s final resistance and wiring. This can be precisely compensated for by digital circuits that determine the number of drops to be ejected in each cycle and adjust the supply voltage accordingly to compensate for the charge resistance changes.

5) Ink contamination: The ink must be free of contamination that is larger than approximately 5 µm, which can accumulate and clog the nozzle. To do this, it is possible to place a filter with a mesh size of 5 µm absolute between the ink tank and the print head.

6) Ink surface tension characteristics: The most important requirement for the ink is the surface tension characteristics. The ink must be formulated to have a surface tension high enough to keep the ink in the nozzle within design limits at room temperatures and to drop below the ejection threshold at temperatures achievable by the heater. Many ink formulations can meet these criteria, but care must be taken to avoid contaminants that affect surface tension.

7) Ink drying: If the time between drop ejections from a nozzle becomes too long, the ink on the exposed meniscus can dry to the point that drop ejection is impaired or even prevented. This can be compensated for by ejecting one or more drops from each nozzle between each printed page and by covering the print head during idle times.

8) Pulse width: The heating pulse width is precisely controlled and can be set very close to the minimum pulse width. Greater reliability can be achieved by setting the pulse width significantly longer than the minimum width. For a 7 um nozzle using a water-based ink as described here, the minimum pulse width is approximately 10 um. The nominal pulse width is set to 18 us to ensure a wide operating envelope. The pulse width has virtually no effect on the droplet size.

9) Clogged or faulty nozzles: In many cases, clogged nozzles can be cleared by applying a rapid series of pulses to the heating element, which heats the ink above boiling point. The vapor bubbles that are formed can break up the "crust" of dried ink. Frequently clogged nozzles can be cleaned from time to time with a solvent. Nozzles that are defective or permanently clogged can be automatically replaced with redundant nozzles as part of built-in fault tolerance.

10) Media roughness: This is particularly relevant to proximity printing, where media roughness can be a significant part of the head-to-media clearance. Protruding fibers in a paper media can cause the ink drop to soak into the paper faster than intended, resulting in less ink being transferred to the paper and a smaller drop. This can be compensated for by using coated paper, by compacting the paper fibers with rollers before printing, and/or by coating or wetting the paper immediately before printing.

Control of nozzle temperature

The behavior of the nozzles depends on the temperature and the duration of the thermal pulses applied to the nozzle tip.

If too little energy is applied to the heater, the temperature at the nozzle tip will not rise fast enough to eject a drop in the allotted time, or the ejected drop may be smaller than required. If too much energy is applied to the heater, too much ink may be ejected, or the ink may begin to boil, and the energy consumed by the printhead may be greater than required. This energy may then exceed the limit at which self-cooling operation is possible. The amount of energy required to activate a nozzle can be determined by performing a dynamic finite element analysis of the nozzle. This method can determine the required ejection energy of the nozzle under various static and dynamic environmental conditions.

An optimal temperature profile for a printhead includes spontaneously raising the active area of the nozzle tip to ejection temperature, maintaining this area at ejection temperature for the duration of the pulse, and spontaneously cooling the area to ambient temperature.

This optimum is not achievable due to the stored heat capacity and thermal conductivity of the various materials used to make the nozzles. However, improved performance can be achieved by shaping the energy pulse using curves that can be derived by gradually refining finite element simulations of the printhead. To achieve accurate results, a transient fluid dynamics simulation with free surface modeling is required, since convection in the ink and ink stream can significantly affect the temperature achievable with a given energy curve.

Compensation techniques

Many environmental variables can be compensated for so that their impact is reduced to a negligible level. Active compensation for some factors can be achieved by varying the power applied to the nozzle heating elements.

An optimal temperature profile for a printhead involves immediately raising the active area of the nozzle tip to the ejection temperature, holding that area at the ejection temperature for the duration of the pulse, and immediately cooling the area to ambient temperature.

However, this optimum is not achievable due to the stored heat capacities and thermal conductivities of the various materials used in the manufacture of the nozzles of the invention. However, improved performance can be achieved by shaping the power pulse using curves derived by gradually refining the finite element simulation of the printhead. The power applied to the heater can be varied over time by various techniques, including, but not limited to:

1) Modifying the voltage applied to the heating element

2) Modulating the width of a series of short pulses (PWM / Pulse Width Modulation)

3) Modulating the frequency of a series of short pulses (PFM / Pulse Frequency Modulation)

To obtain accurate results, a simulation of the transitional flow dynamics with free-surface modeling is required, since the convection in the ink and in the ink stream significantly influences the temperature achieved with a given power curve.

By incorporating a corresponding digital circuit on the print head carrier, individual control of the power applied to each nozzle can be achieved. One possibility for this is "broadcasting", i.e. collectively addressing a large number of different digital pulse paths over the printhead chip and selecting the appropriate pulse path for each nozzle using multiplexing circuits.

An example of the environmental factors that can be compensated is shown in the "Environmental Factor Compensation" table. This table shows which environmental factors can best be compensated globally (for the entire printhead), per chip (for each chip in a composite multi-chip printhead), and per nozzle. Environmental Factor Compensation

Most applications do not require compensation for all of these variables. Some variables have only a minor effect and compensation is only needed where high image quality is important.

Printhead driver circuits

Fig. 4 shows a schematic block diagram of the electronic operation of the printhead driver circuits. Fig. 4 shows a block diagram for a system with a full-page 800 dpi printhead printing four colors in the CC'MM'YK color model. The printhead 50 includes a total of 79,488 nozzles with 39,744 main nozzles and 39,744 redundant nozzles. The main nozzles and redundant nozzles are divided into six colors, with each color divided into 8 drive phases. Each drive phase has a shift register that converts the serial data from an ASIC 400 for printhead control into parallel data for activating the heater driver circuits. There are a total of 96 shift registers, each with 828 nozzles. Each shift register is comprised of 828 shift register stages 217, the outputs of which are logically linked to the phase enable signal by a NAND gate 215. The output of the NAND gate 215 drives an inverting buffer 216, which in turn drives the driver transistor 201. The driver transistor 201 activates the electrothermal heater 200, which may be a heating element 103, as shown in Fig. 1(b). To ensure that the stored data remains valid during the enable pulse, the clocking into the shift register is stopped. The enable pulse is activated by a clock stopper 218, which is shown as a single gate for clarity, but preferably is within the range of known glitch-free clock control circuits. Stopping the clocking into the shift register eliminates the need to provide a parallel data buffer in the print head, but it does add complexity to the control circuitry in the head controller ASIC 400. The data is routed by the data router 219 to either the main nozzles or the redundant nozzles, depending on the status of the signal corresponding to the error status bus.

The print head is shown in a simplified manner in Fig. 4 and does not show the various means necessary to reduce the reject rate in production, for example the block fault tolerance. The driver circuits for various Printhead configurations can be easily derived from the device described here.

The digital data representing the dot patterns to be printed on the receiving medium is stored in the page or band memory 1513, which may be identical to the image memory 72 in Fig. 1(a). The data in 32-bit words representing the dots of one color are read from the page or band memory 1513 using the addresses selected by the address multiplexer 417 and generated by the memory interface 418. These addresses are generated by the address generators 411, which in turn form part of the "per color circuits" 410, one for each of the six color components. The addresses are generated based on the positions of the nozzles with respect to the print medium. Since the relative position of the nozzles may be different for different print heads, the address generators 411 are preferably programmable. The address generators 411 normally generate the address corresponding to the position of the main nozzles. However, if defective nozzles are present, the location of the blocks containing defective nozzles can be identified in the error table read-write memory 412. The error table read-write memory 412 is read while the page is being printed. If the memory indicates an error in the nozzle block, the address is changed so that the address generators 411 generate the address corresponding to the position of the redundant nozzles. The data read from the page or band memory 1513 is latched by the latch 413 and converted into four consecutive bytes by the multiplexer 414. The timing of these bytes is adjusted to the data representing the other colors from the FIFO memory 415. This data is then buffered by buffer 430 to form the 48-bit main data bus to printhead 50. The data is buffered because the printhead may be relatively far from the head control ASIC. The data from error table random access memory 412 also forms the input to FIFO memory 416. The timing of this data is matched to the data output of FIFO memory 415 and is buffered by buffer 431 to form the error status bus.

The programmable power supply 320 supplies power to the print head 50. The voltage of the power supply 320 is controlled by the digital-to-analog converter 313, which in turn is part of a combination of a random access memory and a digital-to-analog converter (RAMDAC) 316. The contents of the dual-port RAM 317 are programmed by the microcontroller 315. The temperature is compensated by changing the contents of the dual-port RAM 317. These values are calculated by the microcontroller 315 based on the temperature determined by a thermal sensor 300. The signal from the thermal sensor 300 is applied to the analog-to-digital converter (ADC) 311. The analog-to-digital converter 311 is preferably integrated into the microcontroller 315.

The head control ASIC 400 includes control circuits for compensating for thermal shift and print density. Compensating for thermal shift requires that the power supply to the print head 50 be a rapidly changing voltage that is synchronized with the heater activation pulse. This is accomplished by programming the programmable power supply 320 to generate this voltage. The digital-to-analog converter 313 generates an analog, time-varying programming voltage based on the data read from the dual port RAM 317. The data is read according to the address generated by the counter 403. The counter 403 generates a complete address cycle during the period of an activation pulse. Synchronization is ensured by clocking the counter 403 from the system clock 408, and the upper count of the counter 403 is used to clock the activation counter 404. The count from the activation counter 404 is then decoded by the decoder 405 and buffered by the buffer 432 to generate the activation pulses for the print head 50. The counter 403 may include a frequency divider upstream if the number of states in the count is less than the number of clock pulses in an activation pulse. Sixteen states are required to compensate for the thermal delay of the heater. These sixteen states may 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 spaced linearly in time. To allow for non-linear spacing of these states in time, the counter 403 may also include a read only memory or other Means for causing the counter 403 to count in a non-linear manner. Alternatively, fewer than sixteen states may be used.

To compensate for print density, print density is determined by counting the number of pixels for which a drop is to be printed in each activation period ("on" pixels). The "on" pixels are counted by the on-pixel counters 402. There is one counter 402 for each of the eight activation phases. The number of phases in a printhead according to the invention depends on the particular design. Four, eight and sixteen phases are suitable, although the number of phases need not necessarily be divisible by two. The on-pixel counters 402 may comprise combinational logic pixel counters 420, which in turn determine how many bits in a data packet are on. This number is then accumulated by the adder 421 and the accumulator 422. A latch 423 holds the accumulated value valid for the duration of the activation pulse. The multiplexer 401 selects the output of the latch 423 corresponding to the current activation phase as determined by the activation 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 pixels "on" is not required, the most significant four bits of this count being sufficient.

The combination of the four bits of the thermal shift compensation address and the four bits of print density compensation means that the dual port RAM 317 has an 8-bit address. This requires that the dual port RAM 317 contains 256 digits in a two-dimensional array. These two dimensions are time (for thermal shift compensation) and print density. A third dimension, temperature, can also be included. Since the ambient temperature of the head changes slowly, the microcontroller 315 has enough time to calculate a matrix of 256 digits for thermal shift compensation and print density at the current temperature. The microcontroller periodically (e.g., twice a second) measures the head temperature and calculates this matrix.

The following equation can be used to calculate the matrix of numbers to be stored in the dual-port RAM 317:

where:

VPS is the voltage of the programmable power supply 320;

RouT is the output resistance of the programmable power supply 320, including the connections to the printhead 50;

RH is the resistance of a single heating element;

p is the number of heaters turned on in the power activation time as provided by the multiplexer 401;

n is a constant equal to the number of heating elements, represented by a least significant bit of p;

f is the time divided by the number of steps over the duration of a single activation pulse;

P(t) is a function defining the power applied to a single heater to achieve improved drop ejection. This function depends on the specific geometry and materials of the nozzle as well as on various characteristics of the ink. It is best determined by comparative computer simulation combined with experiments;

TE is the temperature required for droplet ejection in ºC; and

TA is the "ambient temperature" of the head in ºC as measured by the temperature sensor.

To reduce execution time and simplify programming of the microcontroller, most or all of the factors can be calculated in advance and looked up in a table stored in the microcontroller's ROM.

Comparison with thermal inkjet technology

In the table "Comparison between thermal inkjet technology and the present invention" the printing according to the invention is compared with the thermal inkjet technology.

A direct comparison is made between the present invention and thermal inkjet technology because both are drop-on-demand systems that operate using thermal elements and liquid ink. Although both systems appear to have similarities, they operate on different principles.

Thermal inkjet printers work on the following basic principle. A thermal pulse generated by an electrical resistance causes an explosive formation of a bubble in a liquid ink. Rapid and consistent bubble formation is achievable by heating the ink to a high degree so that sufficient heat is transferred to the ink before bubble nucleation is complete. For water-based inks, ink temperatures of approximately 280ºC to 400ºC are required. The bubble formation creates a pressure wave that ejects an ink droplet from the orifice at high velocity. The bubble then collapses, causing ink to be drawn from the ink reservoir to refill the nozzle. Thermal inkjet printing is very successful commercially because of the high nozzle packing density and the use of proven integrated circuit manufacturing techniques. However, thermal inkjet technology also presents significant technical problems, such as the production of multi-part precision components, production waste, image resolution, noise, print speed, control transistor performance, waste heat dissipation, satellite droplet formation, thermal stresses, thermal expansion differences, kogation, cavitation, linear diffusion and difficulties in ink formulation.

Printing according to the invention has many of the advantages of thermal inkjet printing while completely or largely eliminating the problems associated with thermal inkjet technology. Comparison between thermal inkjet technology and the present invention

Production waste and fault tolerance

In most cases, monolithic circuits are irreparable if they are defective during manufacture. The percentage of functional devices produced from a wafer run is called yield. Yield has a direct impact on manufacturing costs. A device with a 5% yield is approximately ten times more expensive to manufacture than an identical device with a 50% yield.

There are three basic methods for measuring yield:

1) Manufacturing yield

2) Wafer grade yield

3) Audit income

For large tools, it is usually the wafer grade yield that most limits the overall yield. Inventive full-page color heads are very large compared to typical VLSI circuits. A good wafer yield is important for the cost-effective production of such heads.

Fig. 5 shows a curve plotting wafer yield against defect density for an embodiment of the invention of a monolithic full-page color print head in A4 format. The head is 215 mm long and 5 mm wide. The non-defect tolerant yield 198 is calculated according to the Murphy method, which is widely used as a method for predicting yield. With a defect density of one defect per cm², the Murphy method results in a yield of less than 1%. This means that over 99% of the heads manufactured would have to be disposed of. This low yield is undesirable because the print head manufacturing costs would be unreasonably high.

The Murphy method determines the effect of an uneven distribution of defects. Fig. 5 shows a curve for a non-fault-tolerant yield 197 that explicitly models the defect grouping by including a defect grouping factor. The defect grouping factor is an uncontrollable parameter in manufacturing, but is a characteristic feature of this process. The defect grouping factor for manufacturing processes can take on a value of about 2, in which case the yield predictions closely match the Murphy method.

One solution to the problem caused by low yield is to provide fault tolerance by adding redundant functional units on the chip that can replace the faulty functional units.

In memory chips and in most wafer integration devices, the physical location of redundant subunits on the chip is not important. In printheads, however, the redundant subunit may comprise one or more print actuators. These must have a fixed spatial relationship to the page to be printed. In order to print a dot in the same position as the faulty actuator, the redundant actuators must not be arranged in the direction in which no scanning takes place. Faulty actuators, however, can be replaced by redundant actuators arranged in the scanning direction. To ensure that the If the redundant actuator prints the dot in the same position as the faulty actuator, the timing of the data transmitted to the redundant actuator can be changed to compensate for the shift in the scanning direction.

To replace all nozzles, a full set of spare nozzles must be provided, providing 100% redundancy. 100% redundancy would typically double the chip area, dramatically reducing primary yield before redundant units are replaced, and eliminating much of the benefit of fault tolerance.

However, in embodiments of the invention, the smallest physical dimensions of the printhead chip are determined by the width of the page to be printed, the sensitivity of the printhead chip, and limitations in the manufacturing of the ink channels that transport ink to the back of the chip. The smallest practical size for a full-page color printhead for A4-format printing is approximately 215 mm x 5 mm. This size allows 100% redundancy without significantly increasing the chip area when using a 1.5 um CMOS manufacturing technique. Thus, a high degree of fault tolerance is possible without significantly limiting the primary yield.

When fault tolerance is built into a device, the standard yield equations are not applicable. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and incorporated into the yield equation. Fig. 5 shows the fault-tolerant wafer sort yield 199 for a full-page color printhead for A4-size printing, which includes various forms of fault tolerance, the modeling of which has been incorporated into the yield equation. This curve shows the projected yield as a function of defect density and defect grouping. The yield projection in Fig. 5 shows that a thoroughly implemented fault tolerance can increase the wafer sort yield from less than 1% to over 90% under the same manufacturing conditions. This can reduce manufacturing costs by a factor of 100.

Fault tolerance is recommended to improve the yield and reliability of printheads with thousands of print nozzles, making printheads suitable for Full page format may be possible. However, error tolerance does not form an essential part of the present invention.

Fault-tolerant drop-on-demand printing systems are described in the following Australian patent applications filed on 12 April 1995:

"Integrated fault tolerance in printing mechanisms" (reference number: PN2324);

"Block fault tolerance in integrated printing heads" (reference number: PN2325);

"Nozzle duplication for fault tolerance in integrated printing heads" (reference number: PN2326);

"Detection of faulty nozzles in printing heads" (reference number: PN2327); and

"Fault tolerance in high volume printing presses" (reference number: PN2328).

Printing system examples

Fig. 6 shows a schematic representation of a digital, electronic printing system with a print head according to the invention. The monolithic print head 50 prints an image 60 composed of a plurality of ink drops on a receiving medium 51. This medium is typically paper, but can also be transparency film, cloth or another substantially flat surface that can receive ink drops. The image to be printed is provided by an image source 52, which can be any type of image that can be converted into a two-dimensional pixel array. 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 raster device based on procedure calls 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 a monochrome or a multi-color printing system, with the data typically comprising 1 to 32 bits per pixel, depending on the image source and 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 it may be a two-dimensional image processing system if the source image comes from a scanner.

If halftone images are required, then a screening system 54 is needed. Suitable screening methods are based on the dispersed dot ordered dither method or on error diffusion. Variants of these are also suitable, which are known as stochastic screening or frequency modulation screening. The screening system often used for offset printing - clustered dot ordered dither - is not recommended because the effective image resolution is unnecessarily lost with this technique. As an output, the screening system produces a binary single-color or multi-color image with the resolution of the printing system according to the invention.

The binary image is processed by a data phasing circuit 55 (which may be incorporated into a head control ASIC 400 as shown in Figure 4) which passes the pixel data in the correct order to the data shift registers 56. Sequencing of the data is necessary to compensate for nozzle placement and paper movement. Once the data has been loaded into the shift registers 56, it is passed in parallel to the heater driver circuits 57. At the appropriate time, the driver circuits 57 electronically connect the appropriate heaters 58 to the voltage pulse generated by the pulser circuit 61 and the voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, which affects the physical properties of the ink. The ink drops 60 emerge from the nozzles in a pattern corresponding to the digital pulses applied to the heater driver circuits. The pressure of the ink in the ink reservoir 64 is regulated by the pressure regulator 63. The selected drops of ink drops 60 are separated from the ink body by the selected separators and contact the receiving medium 51. During the printing process, the receiving medium 51 is continuously moved relative to the print head 50 by the paper transport system 65. If the print head 50 encompasses the full width of the print area of the receiving medium 51, the receiving medium 51 only needs to be moved in one direction and the print head 50 can remain in position. If a smaller print head 50 is used, a raster scanning system must be implemented. This is typically achieved by having the print head 50 move toward the short side of the receiving medium 51, while the receiving medium 51 moves toward the long side.

Ink cartridge for printing

The current invention is a replaceable ink cartridge and cartridge connection system suitable for use with full-color printheads that use inks under pressure. The ink cartridge includes a rigid container containing ink wells that include at least one flexible surface. For a monochrome ink cartridge, one ink well is sufficient. For a full-color process printhead, four ink wells are used. These wells contain ink in the colors cyan (blue-green), magenta (purple), yellow and black. The ink wells do not need to be the same size. Since more black ink is typically used than colored ink, the black ink well can be larger than the other wells. The ink wells include a means by which ink can flow from the wells to the printhead when the ink cartridge is installed in the printing device. Coincidence force printing systems required ink that was under pressure. The present invention uses a single printing system to provide identical pressure to all four ink colors. This is achieved by applying pressure to the fluid surrounding the ink vessels in the ink cartridge. Since the ink vessels comprise a flexible membrane, the pressure in the ink cartridge is transferred to the ink. A four-color ink cartridge has five fluid ports in the rigid container. There is an ink outlet port for each of the four colors and an outlet port for the pressurized fluid. Ink pressure can be built up by a pump connected to the printing fluid inlet port.

The drop size in the coincidence force printheads can be influenced by the ink pressure. To maintain a consistent ink drop size, the pressure should be controlled. This can be achieved by any state-of-the-art means, such as mechanical control, analog electronic control, or digital electronic control.

Fig. 8 shows a schematic representation of a replaceable ink cartridge 770. This ink cartridge is shown "inserted" into a receptacle 782. The cartridge is shown for a four-color printer so that there are four ink wells. The cartridge contains a teal ink well 772, a purple ink well 773, a yellow ink well 774 and a black ink well 775. Each ink well may be a completely flexible "pocket" made from a thin plastic or rubber membrane, or it may be a solid well with one or more flexible or movable surfaces. The ink wells are contained in a solid container 771. The container 771 may be made of injection molded plastic, metal or other materials. The container 771 must be completely sealed except for the four ink outlet ports 776 and the printing fluid inlet port 783.

The properties of the printing fluid surrounding the ink vessels are not of essential importance. Suitable fluids include air, water and oil. When fully flexible ink vessels are used, it is recommended that these vessels be supported by suitable means to avoid damage during handling of the ink cartridge. This can be achieved by embedding the ink vessels in a solid or semi-solid "sponge-like" material 781 through which the printing fluid can flow. For example, expanded plastic foams such as polyurethane foam are suitable. Alternatively, separate compartments for each ink color can be injection molded, as shown in Fig. 9.

The receptacle 782 includes five fluid connectors. Four fluid connectors 786 are for the ink outlet, one fluid connector 787 for the printing fluid inlet. These fluid connectors can be designed as simple metal tubes with tapered ends. When the ink cartridge is inserted into the receptacle, the ink outlet fluid connectors pierce a seal (which can be designed to be resealable) to open a path for the ink from the ink wells to the print head. The ink can flow to the print head via tubes. For a full-color process printing ink cartridge with four ink colors, four tubes are required. These are a teal ink tube 777, a purple ink tube 778, a yellow ink tube 779 and a black ink tube 780.

The inlet port 783 for the printing fluid is connected to the pump 784 through the fluid connector 787 when the ink cartridge is inserted into the receptacle 782. The pressure is regulated by the pressure regulator 785. The pressure regulator is operable by mechanical or electronic means. There are numerous means in the art for regulating the pressure of fluids.

Figure 9 shows an exploded perspective view of one possible physical configuration of a replaceable ink cartridge 770. The cartridge shown is designed for a four-color printer and therefore includes four ink wells. The cartridge contains a teal ink well 772, a purple ink well 773, a yellow ink well 774, and a black ink well 775. The movable surface of the ink well is a membrane 765 which is vacuum formed from a thin, flexible plastic or latex material. The ink wells are arranged in a rigid, injection molded plastic container which is formed in two parts, a housing 760 and a lid 761. During manufacturing, the flexible membrane is arranged in the housing of the container and filled with ink of one of the four colors. The lid 761 is then ultrasonically welded to the housing 760. The housing is sealed except for the four ink outlet ports 776 and the printing fluid inlet port 783. A channel 766 connects all four containers to a printing fluid inlet port. The ribs 763 both serve to precisely place the flexible membrane 765 and to provide contact between the container housing 760 and the container lid 761 for ultrasonic welding. The container may also have "polarizing" ribs 762 to prevent it from being inserted in the wrong direction. The four ink outlet ports 776 may have porous inserts to prevent ink from leaking out during handling. The printing fluid inlet port 783 does not require this porous insert.

This configuration can be manufactured at low cost using injection molded components. Designing the machines to produce these ink cartridges in high volume is not a problem.

Ink distribution in full-color printheads

When a full-color print head is included in a printing system, it is necessary to provide the four ink colors to the single head while avoiding color mixing. This can be achieved by using parts manufactured using common plastic injection molding techniques.

Figure 9 shows an exploded view of a mechanical diagram of one possible ink delivery configuration for a full-color printhead. The full-color printhead 740 includes four nozzle bands that print ink of the four process colors. In this diagram, the head 740 is facing down, and the back of the head is visible. The recommended method for manufacturing large monolithic heads includes a process for directly etching "ink nozzle cylinders" into the silicon substrate to form tapered cylindrical holes. In this case, the ink can be fed from the back of the head. The head also includes a band of "blue-green nozzles" 741 to print blue-green ink. Another band of "purple nozzles" 742 is for printing purple ink. Another band of "yellow nozzles" 743 is for printing yellow ink. Another band of "black nozzles" 744 is used for printing black ink.

The ink must be filtered to prevent particle contamination from entering the printhead. If particles larger than the nozzle tip diameter enter the printhead, a nozzle may become clogged. A suitable type of filter is a membrane filter with a mesh size of 10 µm absolute, which can be mounted on the back of the 740 monolithic printhead.

An ink supply manifold 750 supplies ink of the corresponding color to all nozzles. The distance between the nozzles of the different colors is approximately 1 mm, so that the ink supply manifold 750 can be manufactured using a precision injection molding process for plastic materials. A plastic inner wall of 0.5 mm thickness and a dimensional tolerance of 0.1 mm is suitable for this design and can be achieved with currently available injection molding processes. The ink supply manifold 750 has four channels for the ink flow. Channel 751 supplies the teal nozzles 741 with teal ink. Channel 752 supplies the magenta Nozzles 742 with purple ink. Channel 753 feeds yellow nozzles 743 with yellow ink. Channel 754 feeds black nozzles 744 with black ink. Ink supply manifold 750 can be manufactured with a simple, two-part, vertical pull injection mold.

An ink tube attachment 760 is provided to connect ink tubes containing ink of the appropriate color to the ink supply manifold 750. The ink tube attachment 760 has four channels for the liquid ink flow. The tube attachment 761 feeds cyan ink into the cyan ink channel 751. The tube attachment 762 feeds magenta ink into the magenta ink channel 752. The tube attachment 763 feeds yellow ink into the yellow ink channel 753. The tube attachment 764 feeds black ink into the black ink channel 754. The ink tube attachment 760 can be manufactured using a simple three-part injection mold with a horizontal pull (not shown in this diagram) and a vertical drive.

The two-piece ink channel assembly is designed to be easily manufactured using standard injection molding techniques. The purpose of the ink channel assembly is to provide a simple connection from "macroscopic" ink tubes to the "microscopic" ink nozzles.

Combined ink cartridge and paper cassette

In some cases, it may be advantageous to combine the ink cartridge with a paper cassette. This is particularly the case for printers that do not use "plain" paper in standard or A4 format. An example is photo or video printers, which may be designed to print on glossy paper that is the format of "photographs" (approximately 100 mm x 150 mm).

Fig. 10 shows an example of the physical configuration of an ink cartridge and paper cassette for printing photos. The cassette contains a plurality of paper sheets (for example, 24) and four ink tanks. The ink tanks hold enough ink of each color to print on the sheets.

This is an exploded perspective view of one possible physical configuration of a replaceable ink cartridge 770 that includes paper 768. The cartridge shown is designed for a four-color printer, so there are four ink wells. The movable well surface is a membrane 765 that is formed from a thin, flexible plastic or latex material. The ink wells are arranged in a rigid injection molded plastic container that consists of two parts, a housing 760 and a lid 761. During manufacturing, the flexible membrane is placed in the housing of the container and filled with ink of one of the four colors. The lid is ultrasonically welded to the housing 760.

Combining paper and ink supplies in a cassette or cartridge reduces the complexity of handling supplies for the user.

Some other ink cartridge configurations include:

1) A single-color ink cartridge for monochrome printing.

2) A single-colour cartridge for replacing ink in cases where the relative use of the different colours differs significantly.

3) Two-color cartridges for printing special colors, such as black text with red highlights.

4) Seven-colour cartridges for printing using the CC'MM'YKK' method.

5) For battery-operated devices, a battery can also be provided as part of the cartridge/cassette.

Many other physical configurations of this invention can be readily devised without departing from the scope of the invention.

Although the invention has been described with particular reference to preferred embodiments, the invention is not limited thereto, but may be subject to changes and modifications within the scope thereof.

Claims (4)

1. Liquid ink drop-on-demand coincidence force printing system with the following components: - a print head (50) having a plurality of droplet emitting nozzles (59); - an ink supply manifold (750) having an ink inlet opening and an ink outlet opening connected to the nozzles; - a pressure generating device (784, 785) which generates an overpressure; - an ink container hermetically sealed against the atmosphere with an ink outlet opening connected to the ink supply distributor (750) and an inlet opening for the control pressure connected to the pressure generating device (784, 785); - an ink supply inside the container; - whereby the ink in the distributor and at the droplet-emitting nozzles can be subjected to control pressure via the ink container; and - a drop selection device by which the ink in the nozzles is subjected to a selection force for moving the meniscus; marked by - a drop separation device that causes displaced meniscus ink volumes to separate as drops and move to a printing area. 2. Printing system according to claim 1, characterized in that the pressure generating device applies a pressure of at least 2% above ambient pressure to the ink in the manifold and forms a meniscus with an air/ink interface at least during the selection and separation of the drops, and that the drop selection device operates across the air/ink interface to select predetermined nozzles and create a difference in meniscus position between ink in selected and non-selected nozzles. 3. A printing system according to claim 1, characterized in that an ink body is provided associated with the nozzles, which forms an air/ink interface at each nozzle; that the drop selection device operates across the air/ink interface to select predetermined nozzles and to create a difference in meniscus position between ink in selected and non-selected nozzles; and that the drop separation device separates ink from selected nozzles as drops from the ink body while allowing ink to be retained in non-selected nozzles, the drop selection device being capable of creating the difference in meniscus position in the absence of the drop separation device. 4. A printing system according to claim 1, characterized in that the ink in the manifold exhibits a drop in surface tension of at least 10 mN/m over a temperature range of 30°C, and in that the drop selection device operates across the air/ink interface to select predetermined nozzles and create a difference in meniscus position between ink in selected and non-selected nozzles.