DE69623135T2 - Device for printing with liquid ink - Google Patents

Description

The invention relates to the field of computer-controlled printing devices, in particular to the field of drop-on-demand inkjet printing systems (DOD systems).

A wide variety of digitally controlled printing systems have been invented, and many are currently in production. These printing systems use a wide variety of actuation mechanisms, ink types and recording media. Examples of printing systems in use today include: electrographic laser printers, electrographic LED printers, dot matrix impact printers, thermal paper printers, film recorders, thermal wax printers, dye diffusion thermal transfer printers and ink jet printers. However, to date these electronic printing systems have not significantly displaced mechanical printing machines, although this traditional process requires extensive set-up and is rarely economically viable unless several thousand copies of a given page are to be printed. There is therefore a need for improved digitally controlled printing systems capable, for example, of producing high quality color images at high speed, inexpensively and using plain paper.

Today, inkjet printing is recognized as an outstanding option in the field of digitally controlled electronic printing, for example because of its non-contact operation, low noise, use of plain paper and also because there is no toner transfer and no fusing.

Many types of inkjet printing mechanisms have been developed. These can be divided into those that use a continuous ink jet (CIJ) and those that dispense ink drops on demand (DOD). CIJ inkjet printing has been known at least since 1929: US-A- 1,941,001.

US-A-3,373,437 from 1967 describes an arrangement of CIJ inkjet nozzles in which the ink drops to be applied are selectively charged and deflected towards the recording medium. This technique is known as binary deflection CIJ and is used by several manufacturers, including Elmjet and Scitex.

US-A-3,416,153 from 1966 describes a method for producing dots printed in variable optical density in the CIJ printing process, whereby the number of drops passing through a small opening is modulated by electrostatic dispersion of a charged droplet stream. This technique is used in the inkjet printers manufactured by Iris Graphics.

US-A-3,946,398 from 1970 describes a DOD inkjet printer that applies a high voltage to a piezoelectric crystal, causing the crystal to bend and thereby apply pressure to an ink supply and eject drops as needed. Many different piezoelectric drop-on-demand printers have since been invented that utilize the bending, pushing, shearing and squeezing effects of piezoelectric crystals. Piezoelectric DOD printers have achieved commercial success with hot-melt inks (e.g. Tetronix and Dataproducts printers) and at image speeds of up to 720 dpi in home and office printers (Seiko Epson). Piezoelectric DOD printers have the advantage of being able to work with a wide variety of inks. However, piezoelectric printing devices typically require complex, high-voltage drive circuits and large piezoelectric crystal arrays, and this is disadvantageous in terms of production characteristics and performance.

GB 2 007 162 of 1979 describes an electrothermal DOD inkjet printer which applies a current pulse to an electrothermal transducer (heater) in thermal contact with ink in a nozzle. The heater rapidly heats the water-based ink to a high temperature, whereupon a A small amount of ink quickly evaporates and a bubble forms. This bubble formation leads to a pressure wave, which causes the ink drops to be ejected through small openings arranged along the edge of the heating element carrier. This technology is known as the BubblejetTM process (trademark of Canon KK, Japan) and is used in numerous printing systems from Canon, Xerox and other manufacturers.

US-A-4,490,728 from 1982 describes an electrothermal droplet ejection system, which also works by forming bubbles. In this system, the drops are ejected in a direction perpendicular to the plane of the heating element carrier through nozzles that are formed in a nozzle plate positioned above the heating element. This system is known as a thermal inkjet system and is manufactured by Hewlett-Packard. In the following, the term thermal inkjet system is used both for the Hewlett-Packard system and for the systems commonly known under the name BubblejetTM.

Thermal inkjet printing typically requires about 20 uJ to eject a drop over a period of about 2 us. The 10 W active power consumption of each heater is disadvantageous in itself; it also requires special inks, complicates the driver electronics and accelerates the deterioration of the heaters.

Other ink jet printing systems have been described in the technical literature but are not currently in commercial use. For example, US-A-4,275,290 and 4,737,803 describe a system in which the simultaneous application of heat pulses and hydrostatic pressure to predetermined print head nozzles allows the ink to flow freely onto paper that is being passed straight beneath the print head and is held at a distance by spacers. US-A-4,737,830, 4,752,784 and 4,748,458 describe ink jet recording systems in which the simultaneous application of heat pulses and an electrostatic attraction field to ink in print head nozzles causes the ink drops to be ejected onto a sheet to be printed on.

The inkjet printing systems described above all have advantages and disadvantages. However, there remains a widely recognized need for an improved inkjet printing system solution that offers advantages such as cost, speed, quality, reliability, power consumption, simplicity of design and operation, durability and consumables.

It is an object of the invention to provide inkjet printing systems which offer significant advances towards the above-mentioned advantages. The invention provides a drop-on-demand printing mechanism in which the means for selecting the ink drops to be deposited create a difference in position between selected drops and non-selected drops, but this difference is insufficient for the ink drops to overcome the surface tension of the ink and detach from the bulk of the ink, and in which additional means are provided which cause the selected drops to detach from the bulk of the ink.

According to a preferred aspect of the invention, the means for creating a positional difference between selected drops and non-selected drops consists of electrothermally reducing the surface tension of ink under pressure.

According to an alternative preferred aspect of the invention, the means for generating a positional difference between selected drops and non-selected drops consists of the electrothermal generation of water vapor bubbles, wherein the water vapor bubble is insufficient to cause the selected drops to detach from the mass of ink in the nozzle.

According to a further alternative preferred aspect of the invention, the means for establishing a positional difference between selected drops and non-selected drops consist of activating a piezoelectric transducer which is in direct or indirect mechanical contact with the ink and which, when activated, causes a change in the volume of an ink container which is in communication with the ink in the printing nozzles, wherein the Volume change is not sufficient to cause the selected drops to detach from the mass of ink in the nozzle.

According to a further alternative preferred aspect of the invention, the means for generating a positional difference between selected drops and non-selected drops consists of electrostatic attraction of electrically conductive ink, wherein the electrostatic attraction is insufficient to cause detachment of the selected drops from the mass of ink in the nozzle.

According to a further alternative preferred aspect of the invention, the means for separating the selected drops from the mass of ink consists in arranging the printing medium such that the selected drops come into contact with the printing medium and unselected drops do not come into contact with the printing medium.

According to a preferred aspect of the invention, the means for detaching the selected drops from the mass of ink comprise arranging the printing medium in such a way that the selected drops come into contact with the printing medium and the speed at which the selected drops are absorbed by the printing medium and/or wet the surface is greater than the egress speed of the ink from the printing nozzle.

According to a further preferred aspect of the invention, the ink pressure oscillates at a frequency that is equal to an integer multiple of the frequency with which the drops are ejected from the nozzle.

According to an alternative preferred aspect of the invention, the means for detaching the selected drops from the mass of ink consists of electrostatically attracting electrically conductive ink towards the recording medium.

According to a preferred aspect of the invention, the electric field that produces the electrostatic attraction is applied substantially uniformly to all nozzles.

According to a preferred aspect of the invention, the different electrical force acting on selected drops and non-selected drops is essentially due to the position difference between selected drops and non-selected drops.

According to an alternative preferred embodiment of the invention, the means for detaching the selected drops from the mass of ink consist of magnetic attraction of ink containing magnetically active substances in the direction of the recording medium.

According to a preferred aspect of the invention, the magnetic field generating the magnetic attraction is applied substantially uniformly to all nozzles.

According to a preferred aspect of the invention, the different magnetic force acting on selected drops and non-selected drops is essentially due to the position difference between selected drops and non-selected drops.

In another particularly preferred embodiment of the invention, the methods and devices use an acoustic wave as a simultaneous force in drop selection.

In other preferred embodiments of the invention, the methods and devices vary the drop size of the deposited ink by varying the distance between the printheads of the invention and the printing area.

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

Show it:

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

Fig. 1(b) is a cross-sectional view of one type of nozzle tip according to the invention;

Fig. 2(a) to 2(f) fluid dynamic simulations of droplet selection;

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

Fig. 3(b) successive meniscus positions during drop selection and detachment;

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

Fig. 3(d) Surface tension versus temperature curves for different ink additives;

Fig. 3(e) shows the current pulses applied to the nozzle heating elements to produce the temperature curves shown in Fig. 3(c);

Fig. 4 is a block diagram of the print head driver circuit for carrying out the invention;

Fig. 5 planned production yields for a color print head containing features of the invention in the width of an A4 page with and without error tolerance;

Fig. 6 is a generalized block diagram of a printing system incorporating an embodiment of the invention;

Fig. 7 is a cross-sectional view of an exemplary embodiment of a print head nozzle used for the computer simulations shown in Figs. 8 to 18;

Fig. 8(a) the current sub-pulses applied to the print head during a single activation pulse of the heating element;

Fig. 8(b) the temperature at different locations of the nozzle during the drop selection process;

Fig. 9 is a graph of the meniscus position versus time during the drop selection process;

Fig. 10 is a diagram of the position and shape of the meniscus at 5 µs intervals during the drop selection process;

Fig. 11 the resting position of the ink meniscus before the drop selection process;

Fig. 12-17 the meniscus position and the thermal lines at different stages during the drop selection process;

Fig. 18 Liquid streamlines 50 us after onset of the heater pulse for drop selection;

Fig. 19(a) to 19(i) the droplet ejection cycle of a nozzle operating with thermal proximity droplet detachment;

Fig. 20(a) and 20(b) respectively an acoustic wave applied to the ink and a space-time diagram of the presence of ink along the nozzle axis for selected and non-selected drops; and

Fig. 21(a) shows a cross-section through an embodiment of the print head and the printing plate, with Figs. 21(b) to 21(d) showing cross-sections through a nozzle, adapted to different drop sizes.

In general, the invention represents a drop-on-demand printing mechanism in which the means for selecting the drops to be applied create a positional difference between selected drops and non-selected drops, but which is not sufficient for the ink drops to overcome the surface tension of the ink and detach from the bulk of the ink, and in which alternative means are provided for causing the selected drops to detach from the bulk of the ink.

By separating the drop selection means from the drop release means, the energy required to select the ink drops to be applied is significantly reduced, since now only the drop selection means have to be actuated by the signals supplied to the individual nozzles.

The drop selection means can be a field or a state that is applied to all nozzles simultaneously.

Among other things, the means for selecting drops can be selected from the following list:

1) Electrothermal reduction of the surface tension of ink under pressure.

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

3) Piezoelectric devices where the volume change is not sufficient to cause the drop ejection.

4) Electrostatic attraction with one electrode per nozzle.

The means for removing the drops can be selected from the following list, among others:

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

2) Proximity method with oscillating ink pressure.

3) Electrostatic attraction.

4) Magnetic attraction.

The table "Objectives of DOD Printing Technology" outlines some desirable features of drop-on-demand printing technology. The table also lists some methods by which some embodiments described herein or in other of my related applications enable improvements over the prior art.

Goals of DOD printing technology

Objective Procedure for achieving improvements over technology the state of the art

Very fast operation Convenient, cost-effective, page-wide printheads with more than 10,000 nozzles. Monolithic printheads the width of an A4 page can be manufactured using standard 300 mm (12") silicon boards.

High image quality High resolution (800 dpi is sufficient for most applications), six-color process for reduced image noise.

Four-color printing Raster color printing process with 800 dpi using stochastic screening.

Ink flexibility Low operating ink temperature, no blistering required.

Low power consumption Because the drop selection agents do not have to completely eject the drops, the power consumption is low.

Low cost Monolithic printhead without nozzle plate, high production speed, low number of electrical connections, use of modified, existing CMOS manufacturing equipment.

High production yield Error tolerance integrated in the print head.

High reliability Fault tolerance integrated in the print head. No cavitation and kogation. Reduction of thermal shocks.

Small number of electrical connections Shift registers, control logic and driver circuits can be integrated into a monolithic print head using standard CMOS processes.

Use of existing VLSI production equipment CMOS compatibility. This can be achieved by making the drive power of the heater less than 1% of the drive power of a thermal inkjet heater.

Electronic collation A new page compression system can produce a 10:1 image with only insignificant image degradation. compression, thereby achieving a compressed data rate low enough to permit printing of any combination of thousands of pages stored on an inexpensive magnetic disk.

For thermal inkjet (TIJ) and piezoelectric inkjet printing systems, a drop velocity of about 10 m/s is preferred to allow the selected drops to overcome the surface tension of the ink, detach from the bulk of the ink and reach the recording medium. Many systems have very low efficiency in converting electrical energy into kinetic drop energy. The efficiency of TIJ systems is about 0.02%. This means that the drive circuits of the TIJ printheads must switch high currents. The drive circuits of piezoelectric inkjet printheads must either switch high voltages or drive highly capacitive loads. The total current consumption of page-wide TIJ printheads is also very high. An A4-page-wide, four-color TIJ printhead printing at 800 dpi, producing a four-color image in one second, would consume about 6 kW of electrical power, most of which is converted into excess heat. The difficulty of dissipating this amount of heat prevents the production of low-cost, compact, high-resolution, page-wide, high-performance TIJ systems.

An important feature of embodiments of the invention is the ability to significantly reduce the energy required to select the ink drops to be deposited. This is achieved by separating the means for selecting the ink drops from the means for causing the selected drops to detach from the mass of ink and form dots on the recording medium. Only the drop selection means need be driven by the signals applied to the individual nozzles. The means for detaching the drops may consist of a field or condition applied simultaneously to all nozzles.

The table "Drop selection means" shows some of the possibilities for selecting drops according to the invention. The drop selection means only have to cause a change in the position of the selected drops in such a way that that the droplet separators can distinguish between selected and non-selected drops. Droplet selection agents

In addition, other drop selection agents can be used.

The preferred method for selecting the drops for water-based inks is method 1: "Electrothermal reduction of the surface tension of ink under pressure". This method for selecting the drops offers advantages over other systems numerous advantages, including: low power consumption (about 1% of that of TIJ systems), compatibility with CMOS VLSI chip manufacturing, low voltage operation (about 10 V), high nozzle density, low temperature operation, and the ability to use a wide variety of inks. The ink must exhibit a reduction in surface tension as the temperature increases.

The preferred means of drop selection for hot-melt or oil-based inks is method 2: "Electrothermal reduction of ink viscosity in combination with oscillating ink pressure". This drop selection means is particularly suitable for use with inks that exhibit a strong decrease in viscosity with increasing temperature but only a small reduction in surface tension. This is particularly the case with non-polar ink carriers with a relatively high molecular weight. And it is particularly true for hot-melt and oil-based inks.

The "Drop Detachment Agents" table lists some of the ways in which selected ink drops can be detached from the ink mass and it can be ensured that the selected drops form dots on the printing medium. The drop detachment agents distinguish between selected drops and non-selected drops and ensure that non-selected drops do not form dots on the printing medium. Drop Detachment Agents

It can However, other means can also be used to remove the drops.

The preferred drop release agent depends on the intended use. For most applications, Method 1: "Electrostatic Attraction" or Method 2: "Alternating Electric Field" are best. For applications that work with smooth coated paper or film and where very high speed is not important, Method 3: "Proximity Process" may also be appropriate. For high-speed quality systems, Method 4: "Proximity Transfer" can be used. Method 6: "Magnetic Attraction" is suitable for portable printing systems where the print media is too rough for proximity printing and the high voltages required for electrostatic drop release are not desirable. So there is no clear "best" means of releasing the drops under all circumstances:

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

The image source 52 can be raster image data supplied by a scanner or computer or contour image data in the form of a page description language (PDL) or other forms of digital image reproduction. The image data is converted by the image processing system 53 into a pixel map image page. In the case of PDL image data, this can be a raster image processor (RIP) or in the case of raster image data, a pixel image manipulation. The halftone data generated by the image processing unit 53 is rasterized. The rasterization is carried out by means of the digital raster unit 54. The rasterized bitmap image data is stored in the image memory 72. Depending on the configuration of the printer and the system, the image memory 72 can be a full page memory or a strip memory. Heater control circuits 71 read data from the image buffer 72 and apply time-varying electrical pulses to the nozzle heaters (103 in Fig. 1(b)) forming part of the print head 50. The pulses are applied to the correct nozzle at the correct time such that selected drops form dots at the correct position on the recording medium 51 as designated by the data in the image buffer 72.

The recording medium 51 is moved relative to the print head 50 by means of a paper transport system 65 which is electrically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. Figure 1(a) shows only a schematic representation of a paper transport system, and many different mechanical configurations are possible. For page-wide print heads, it is most convenient to transport the recording medium 51 past a stationary print head 50. However, for scanning printing systems, it is usually most convenient to transport the head 50 in a relative raster motion along one axis (the secondary scanning direction) and the print medium 51 along the perpendicular axis (the primary scanning direction). The microcontroller 315 can also control the ink pressure regulator 53 and the heater control circuits 71.

When printing with reduced surface tension, the ink is held under pressure in an ink container 64. In the idle state (when no ink drop ejected), the ink pressure is not sufficient to overcome the surface tension of the ink and eject a drop. A constant ink pressure can be achieved by applying pressure to the ink container 64 under the control of an ink pressure regulator 63. Alternatively, in larger printing systems, the ink pressure can be generated and controlled very precisely by maintaining the surface of the ink in the container 64 at a certain level above the print head 50. This ink level can be controlled by a simple float system (not shown).

In viscosity reduction printing, the ink is under pressure in an ink tank 64 and the ink pressure is caused to oscillate. This oscillation of the pressure can be caused by a piezoelectric actuator (not shown) mounted in the ink lines.

When properly arranged in conjunction with the drop release means, selected drops then form dots on the recording medium 51, while unselected drops do not release from the ink mass.

The ink is supplied to the rear surface of the printhead 50 via an ink manifold 75. The ink preferably flows through slots and/or openings etched into the silicone substrate of the printhead 50 to the front surface where the nozzles and actuators are located. In the case of thermal selection, the nozzle actuators are electrothermal heating elements.

In some types of printers according to the invention, an external field 74 is required so that the selected droplet detaches from the mass of ink and moves towards the recording medium 51. A constant electric field is suitable as the external field 74, since the ink can be made electrically conductive in a simple manner. In this case, the paper guide or printing plate 67 can be made of an electrically conductive material and used as the electrode generating the electric field. The head 50 itself can serve as the other electrode. In another embodiment, not falling within the scope of the invention, For example, the proximity of the printing medium serves as a means of distinguishing between selected drops and non-selected drops.

For small drop sizes, the force of gravity acting on the ink drop is very small, about 10-4 of the surface tension forces, so gravity can be ignored in most cases. This makes it possible to orient the print head 50 and the recording medium 51 in any direction relative to the local gravity field. This is important for portable printers.

Fig. 1(b) shows a close-up of a cross-section of a single, microscopic nozzle tip according to 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 any other suitable material. If carriers other than semiconductor material are used, a semiconductor material (e.g., amorphous silicon) may be deposited on the carrier, and the integrated driver transistors and data distribution circuits may then be formed in the semiconductive top layer. Single crystal silicon substrates (SCS) offer several advantages, including:

1) SCS can be used to manufacture high-performance driver transistors and other circuits.

2) The printheads can be manufactured in existing facilities using standard VLSI processing equipment.

3) SCS has high mechanical strength and rigidity, and

4) SCS has high thermal conductivity.

In this example, the nozzle is cylindrical, with the heating element 103 being formed as a ring. The nozzle tip 104 consists of silicon dioxide layers 102 that are applied during the manufacture of the CMOS driver circuit. The tip is passivated with silicon nitride. The protruding nozzle tip determines the point of contact of the pressurized ink 100 on the printhead surface. In addition, the printhead surface is hydrophobized to prevent uncontrolled spreading of the ink on the front of the printhead.

Many other nozzle configurations are possible and nozzle designs according to the invention can vary in shape, dimensions and materials used. Monolithic nozzles, which are etched into the carrier on which the heating element and driver electronics are formed, offer the advantage of not requiring a nozzle orifice plate. Elimination of the nozzle orifice plate results in significant cost savings in manufacturing and assembly. Recent techniques for eliminating nozzle orifice plates include the use of "whirl" actuators of the type described in US-A-4,580,158 and US-A-5,371,527. However, these are complicated to operate and difficult to manufacture. The preferred way to eliminate nozzle orifice plates in printheads according to the invention is to integrate the orifice into the actuator carrier.

This nozzle type can be used with print heads that use a variety of different droplet detachment techniques.

Working method with electrostatic droplet detachment

Fig. 2 shows the first example of the operation with thermal reduction of the surface tension and electrostatic droplet detachment.

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

Fig. 2(a) shows the resting state immediately before the heater is activated. By ensuring that the ink pressure plus the external electrostatic field is not sufficient to overcome the surface tension of the ink at the ambient temperature, an equilibrium state is established such that no ink exits the nozzle in the resting state. In the resting state, the meniscus of the ink does not protrude significantly above the printhead surface, so that the electrostatic field is not significantly concentrated at the meniscus.

In Fig. 2(b) the thermal lines are shown in 5ºC increments 5 us after the start of the heater activation pulse. When the heater is activated the ink in contact with the nozzle tip is heated rapidly. The reduction in surface tension causes the heated area of the meniscus to expand rapidly relative to the cooler ink meniscus. This creates a convective flow that rapidly transports this 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 in the area where the ink is in contact with the heater. This is because the ink in direct contact with the heater is prevented from moving by viscous friction in the fixed heater.

Fig. 2(c) shows the thermal lines in 5ºC increments 10 us after the start of the heater activation pulse. The increase in temperature causes a reduction in surface tension, thus disturbing the force balance. When the entire meniscus is heated, the ink begins to flow.

Fig. 2(d) shows the thermal lines in 5ºC steps 20 us after the start of the activation pulse for the heater element. Due to the pressure of the ink, the meniscus has taken a new position and now protrudes from the print head. The electrostatic field is concentrated by the excellent conductive ink drop.

Fig. 2(e) shows the thermal lines in 5ºC increments 30 us after the start of the heater activation pulse, i.e. 6 us after the heater pulse ends, since the heater pulse duration is 24 us. The nozzle tip has cooled rapidly due to heat dissipation through the oxide layers and heat conduction into the flowing ink. In effect, the nozzle tip is "water cooled" by the ink. Electrostatic attraction causes the ink drop to begin to accelerate toward the recording medium. If the heater pulse were much shorter (in this case, less than 16 us), the ink would not accelerate toward the printing medium, but would retreat into the nozzle.

Fig. 2(f) shows the thermal lines in 5ºC increments 26 us after the heater pulse has ended. The temperature at the nozzle tip is now less than 5ºC above ambient temperature. This increases the surface tension around the nozzle tip. When the rate at which the ink is drawn out of the nozzle tip exceeds the viscosity-limited ink flow rate through the nozzle, the ink in the nozzle tip area constricts and the selected droplet detaches from the bulk of the ink. This causes the selected droplet to land on the recording medium under the influence of the external electrostatic field. The meniscus of ink at the nozzle tip then retracts to the rest position awaiting the next heat pulse to select the next ink droplet. With each heat pulse, an ink droplet is selected, detaches and forms a dot on the recording medium. Since the heat pulses are electrically controlled, drop-on-demand inkjet printing can be achieved.

In Fig. 3(a), successive meniscus positions during the drop selection cycle are shown in 5 µs steps, starting with the onset of the activation pulse for the heater element.

Fig. 3(b) shows the movement of the point in the middle of the meniscus in a graph of the meniscus position versus time. The pulse for the heating element starts 10 us after the start of the simulation.

Fig. 3(c) shows the resulting temperature/time curve at different points on the nozzle. The vertical axis shows the temperature in units of 100ºC. The horizontal axis shows the time in units of 10 us. The temperature curve shown in Fig. 3(b) was calculated using FIDAP in 0.1 us time steps. The ambient temperature is 30ºC. The temperature developments at three points are shown:

A - Nozzle tip: This curve shows the temperature development at the contact circle between passivation layer, ink and air.

B - Meniscus Center: This is a circle of the ink meniscus in the middle between the nozzle tip and the meniscus center.

C - Chip surface: This curve refers to a point on the printhead surface 20 um from the nozzle center. The temperature increases by only a few degrees. This indicates that the active circuits can be placed very close to the nozzles without degrading performance or lifetime due to high temperatures.

Fig. 3(e) shows the current applied to the heating element. For optimum function, the temperature must rise sharply at the beginning of the activation pulse for the heating element, be kept slightly below the boiling point of the ink during the pulse duration, and fall rapidly again at the end of the pulse. To do this, the average energy applied to the heating element is varied over the pulse duration. In the present case, this variation is achieved by pulse frequency modulation in partial pulses of 0.1 us with an energy of 4 nJ each. The peak current applied to the heating element is 40 mW, the average current applied to the heating element over the duration of the pulse is 11.5 mW. The partial pulse frequency in this case is 5 MHz. This can be easily changed without significantly affecting the operation of the print head. A higher partial pulse frequency enables finer control of the current applied to the heating element. A partial pulse frequency is suitable of 13.5 MHz, as this frequency is also suitable for minimizing the effect of radio frequency interference (FRI).

Inks with a negative temperature coefficient of surface tension

The requirement 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 these properties. There are no exact equations relating the surface tension to temperature for any liquid. However, for many liquids, the following empirical equation derived by Ramsay and Shields is sufficient:

where γT is the surface tension at 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 ρ is the density of the liquid. From this equation it can be seen 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 well above the boiling point under atmospheric pressure, so the addition of surfactants is recommended to obtain an ink which shows a large change in surface tension with a small change in temperature around a practical ejection temperature.

The choice of surfactant 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. The boiling point of isopropyl alcohol is 82.4ºC, which is lower than that of water. As the temperature increases, the alcohol evaporates faster than water, which reduces the alcohol concentration and increases the surface tension. To reverse this effect, a surfactant such as 1-hexanol can be used. (boiling point 158ºC) and achieve a slight decrease in surface tension with temperature. However, a relatively large decrease in surface tension with temperature is desired in order to maximize the range of functions. A decrease in surface tension of 20 mN/m over a temperature range of 30ºC is preferred in order to achieve broad ranges of functions, although 10 mN/m is sufficient for the print head to function according to the invention.

Inks with strong negative change in surface tension

To achieve a strong negative change in surface tension with increasing temperature, various methods are used, including the following two:

1) The ink may contain a low concentration surfactant sol that is solid at ambient temperatures but melts at a threshold temperature. Particle sizes below 1,000 Å are desirable. Suitable melting points for surfactants are between 50ºC and 90ºC for a water-based ink, preferably between 60ºC and 80ºC.

2) The ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) above the highest ambient temperature but below the boiling point of the ink. For stability, the PIT of the microemulsion is preferably at least 20ºC above the maximum non-operating temperature to which the ink is exposed. A PIT of about 80ºC is suitable.

Inks with surfactant sol

Inks can be prepared as a sol of small particles of a surfactant that melt in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms such as:

Since the melting point of sols with small particle size is usually slightly below that of the corresponding dry material, it is preferable to choose a carboxylic acid whose melting point is slightly above the desired droplet detachment temperature. A good example of this is arachidic acid.

These carboxylic acids are available in high purity and are inexpensive. The amount of surfactant required is very small, so the cost of adding the agent to the ink is negligible. To spread the melting points over a temperature range, a mixture of carboxylic acids with slightly different chain lengths can be used. Such mixtures usually cost less than the pure acid.

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

Production of inks with surface-active sols

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

For example, the preparation of a surfactant sol can be done as follows:

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

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

3) Ultrasonication of the mixture until the typical size of the carboxylic acid droplets is between 100 Å and 1,000 Å.

4) Allow the mixture to cool.

5) Remove the larger particles at the top of the mixture by decanting.

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

7) Centrifuge the sol. Since carboxylic acid has a lower density than water, smaller particles collect on the outside of the centrifuge and larger particles in the middle.

8) Filter the sol through a microporous filter to eliminate particles larger than 5,000 Å.

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

The prepared ink also contains either dye(s) or pigment(s), bactericidal agents, agents to improve the electrical conductivity of the ink in the case of electrostatic drop detachment, wetting agents and, if necessary, other substances.

Antifoaming agents are normally not required since no bubbles are formed during the droplet ejection.

Cationic surfactant sols

Inks made with anionic surfactant sols are generally not suitable for use with cationic dyes or pigments because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant. If cationic dyes and pigments are to be used, a cationic surfactant is required. The alkylamine family is suitable for this purpose.

The following table lists various suitable alkylamines:

The process for preparing cationic surfactant sols is essentially the same as that for anionic surfactant sols, with the exception that an acid is used instead of an alkali to adjust the pH balance and increase the charge of the surfactant particles. A pH value of 6 adjusted using HCl is suitable.

Microemulsion-based inks

Another way to achieve a strong reduction in surface tension at a certain temperature threshold is to formulate the ink on the basis of a microemulsion. A microemulsion is selected with a phase inversion temperature (PIT) in the range of the desired threshold temperature for drop ejection. Below the phase inversion temperature, the microemulsion is an oil-in-water emulsion (O/W), and above the phase inversion temperature, it is a water-in-oil emulsion (W/O). At low temperatures, the surfactant forming the microemulsion prefers a strongly curved surface around oil, and at temperatures significantly above the phase inversion temperature, it prefers a strongly curved surface around water. At temperatures close to the At the phase inversion temperature, the microemulsion forms a continuous “sponge” of topologically connected water and oil.

The surface tension is reduced by two mechanisms. Around the phase inversion temperature, the surfactant prefers surfaces with only a very slight curvature. As a result, the molecules of the surfactant migrate to the ink/air interface, whose curvature is much less than the curvature of the oil emulsion. This reduces the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from O/W to W/O, which also changes the ink/air interface from water/air to oil/air. The oil/air interface has a lower surface tension.

There are many options for producing microemulsion-based inks.

To ensure that the drops are ejected quickly, it is preferable to choose an oil with a low viscosity.

In many cases, water is a suitable polar solvent. In some cases, however, other polar solvents may be necessary. In these cases, polar solvents with a high surface tension should be chosen so that a strong reduction in surface tension can be achieved.

The surfactant can be chosen so that a phase inversion temperature is obtained in the desired range. For example, surfactants from the group of poly(oxyethylene)alkylphenyl ethers (ethoxylated alkylphenols, general formula: CnH2n+1C₄H₆(CH₂CH₂O)mOH) can be used. By increasing m, the hydrophilicity can be increased, and by increasing n, the hydrophobicity can be increased. Values for m of about 10 and for n of about 8 are suitable.

Cost-effective commercial preparations are obtained by polymerizing ethylene oxide and alkylphenols in different molar ratios, whereby the exact number of oxyethylene groups fluctuates around the chosen mean value.

These commercially available preparations are sufficient; high-purity surfactants with a specific number of oxyethylene groups are not required.

The formula for this surfactant is: C₈H₁₇C₄H₆(CH₂CH₂O)nOH (average n = 10).

Synonyms include: Octoxynol-10, PEG-10 Octylphenyl ether and PEO (10) Octylphenyl ether.

The hydrophilic/lipophilic balance (HLB) is 13.6, the melting point is 7ºC and the cloud point is 65ºC.

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

Trade name Supplier

Akyporox OP100 Chem-Y GmbH

Alkasurf OP-10 Rhone-Poulenc Surfactants and Specialties

Dehydrophen POP 10 Pulcra SA

Hyonic OP-10 Henkel Corp.

Iconol OP-10 BASF Corp.

Igepal O Rhone-Poulenc France

Macol OP-10 PPG Industries

Malorphen 810 Huls AG

Nikkol OP-10 Nikko Chem. Co. Ltd.

Renex 750 ICI Americas Inc.

Rexol 45/10 Hart Chemical Ltd.

Synperonic OP10 ICI PLC

Terric X10 ICI Australia

These materials are available in bulk at low cost (less than a dollar per lb.) and thus add less than 10 cents per liter to the prepared microemulsion-based ink with a 5% surfactant concentration.

Other suitable ethoxylated alkylphenols include:

In addition to controlling surface tension, microemulsion-based inks have other advantages:

1) Microemulsions are thermodynamically stable and do not segregate. Therefore, they can be stored for very long periods. This is especially important for office and portable printers that may only be used now and then.

2) The microemulsion spontaneously forms a certain droplet size and does not need to be stirred, centrifuged or filtered to obtain a certain droplet size range of the emulsified oil.

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

4) Flocculation of oil-miscible pigments is prevented because they are enclosed in the oil microdroplets.

5) Using a microemulsion can reduce the mixing of different colors on the surface of the printing medium.

6) The viscosity of microemulsions is very low.

7) The need to add wetting agents can be reduced or eliminated altogether.

Dyes and pigments in microemulsion-based inks

Oil-in-water mixtures can contain very high oil contents - up to 40% - and still form O/W microemulsions. This allows for a high dye or pigment content.

Mixtures of dyes and pigments can also be used. For example, a microemulsion-based ink mixture containing both dye and pigments can be composed as follows:

1) 70% water

2) 5% water-soluble dye

3) 5% surfactant

4) 10% oil

5) 10% oil miscible pigment

The following table summarizes the nine basic combinations of colorants in the oil and water phases of the microemulsion that can be used:

The ninth combination without colorants is useful for printing transparent coatings, UV inks and selective gloss effects.

Since many dyes are amphiphilic, large amounts of dye can also be dissolved in the oil/water interface, since this layer has a very large surface area.

In addition, there may be several dyes or pigments in each phase and also 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 various colorants used. This raises two problems:

1) The absorption spectrum tends to become broader by averaging the absorption peaks of both colorants, and this results in a tendency toward "dirty" colors. To obtain brilliant colors, dyes and pigments must be carefully selected according to their absorption spectra, not just their human-perceivable color.

2) The color of the ink may appear different on different substrates. When a dye and pigment are used in combination, the color of the dye will tend to appear less represented in the color of the printed ink on papers with higher absorption, as the dye is absorbed by the paper while the pigment remains more "on the paper". This can be used as an advantage in some cases.

Surfactants with Krafft point in drop selection temperature range

For ionic surfactants, there is a temperature (the Krafft point) below which solubility is very low and the solution contains essentially no micelles. Above the Krafft temperature, micelle formation becomes possible and the solubility of the surfactant increases rapidly. When the critical micelle concentration (CMC) exceeds the solubility of a surfactant at a given temperature, the minimum surface tension is reached at the point of highest solubility rather than at the critical micelle concentration. Below the Krafft point, surfactants are usually much less effective.

This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures only a portion of the surfactant is in solution. When the nozzle heating element is switched on, the temperature rises and a larger portion of the surfactant goes into solution, which reduces the surface tension.

The surfactant selected should have a Krafft point that is close to the upper temperature range to which the ink is raised. This provides a maximum margin between the surfactant concentration present at ambient temperatures and the surfactant concentration in solution present at the drop selection temperature.

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

The following table summarizes some commercially available surfactants whose Krafft points are 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 cloud point in the drop selection temperature range

To produce an ink where the surface tension decreases with increasing temperature, non-ionic surfactants with polyoxyethylene (POE) chains can be used. At low temperatures, the POE chain is hydrophilic and keeps the surfactant in solution. As the temperature increases the water structure around the POE portion of the molecule is disrupted and the POE portion becomes hydrophobic. As temperatures continue to rise, the surfactant is increasingly repelled by the water and this leads to an increasing concentration of the surfactant at the air/ink interface and hence lower surface tension. The temperature at which the POE portion of a nonionic surfactant becomes hydrophilic is related to the cloud point of the surfactant. POE chains by themselves are not particularly suitable as the cloud point is generally above 100ºC.

In order to lower the cloud point of POE chains without causing strong hydrophobicity at low temperatures, polyoxypropylene (POP) can be combined with POE to form POE/POP block polymers.

Symmetric POE/POP block polymers are available in two main configurations, namely:

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

2) surfactants with POP segments at the ends of the molecules and a POE segment in the middle, such as the surfactants of the meroxapol class (generic CAS 9003-11-6).

The following table summarizes some commercially available Poloxamer and Meroxapol variants with high surface tension at room temperature in combination with a cloud point above 40º and below 100ºC:

Other poloxamer and meroxapol variants can be easily produced synthetically using known techniques. It is desirable to have as high a surface tension as possible at room temperature and a cloud point between 40ºC and 100ºC, preferably between 60ºC and 80ºC.

Suitable variants of meroxapol, for example, may be [HO(CHCHaCH₂O)x(CH₂CH₂O)y(CHCH₃CH₂O)₂HO], in which the average x and z values are about 4 and the average y value is about 15.

When salts are used to increase the electrical conductivity of the ink, the effect of the salt on the cloud point of the surfactant must also be taken into account.

The cloud point of POE-type surfactants is increased by ions that disrupt the water structure (e.g. I⊃min;) because this makes more water molecules available to form hydrogen bonds with free POE oxygen pairs. The cloud point of POE-type surfactants is increased by ions that disrupt the water structure (e.g. Cl⊃min;OH⊃min;) because fewer water molecules are available to form hydrogen bonds. Bromide ions have a relatively small effect. The composition of the ink can be tailored to a specific temperature range by changing the lengths of the POE and POP chains in a block polymer surfactant by selecting the salts added to increase electrical conductivity (e.g. Cl⁻ to Br⁻ to I⁻). The best choice of salts to increase ink conductivity is probably NaCl because of its low cost and non-toxicity. NaCl slightly lowers the cloud point of non-ionic surfactants:

Hot melt inks

The ink need not be liquid at room temperature. If the print head and ink container are heated above the melting point of the ink, solid, hot-melt inks can also be used. The hot-melt ink must be designed so that the surface tension of the molten ink decreases as the temperature decreases. For many formulations using waxes and other substances, a reduction of about 2 mN/m is typical. However, in order to achieve good operating margins in cases where the focus is on reducing surface tension rather than reducing viscosity, a reduction in surface tension of about 20 mN/m is desirable.

The temperature difference between the resting temperature and the drop selection temperature can be larger for hot melt inks than for water-based inks because water-based inks are limited by the boiling point of water.

The ink must be liquid at rest temperature. The rest temperature must be higher than the highest ambient temperature the printed page is likely to be exposed to. In addition, the rest temperature must also be as low as possible to reduce the current required to heat the print head and to obtain the largest possible range between rest temperature and drop ejection temperature. A rest temperature between 60ºC and 90ºC is generally is appropriate, although other temperatures are also possible. A range between 160ºC and 200ºC is generally suitable as a droplet ejection temperature.

The increased reduction of surface tension with increasing temperature can be achieved in different ways.

1) Addition of a dispersion of microfine particles of a surfactant having a melting point substantially above the resting temperature but substantially below the drop ejection temperature to the hot-melt ink in its liquid phase.

2) A polar/non-polar microemulsion with a phase inversion temperature that is preferably at least 20ºC above the melting points of the polar and non-polar compounds.

In order to achieve a strong reduction in surface tension with temperature, the hot-melt ink carrier should preferably have a high surface tension (above 30 mN/m) at rest temperature. This generally excludes alkanes, for example waxes. Suitable materials generally have strong intermolecular attraction, which can be achieved by multiple hydrogen bonds, such as polyols, such as hexanetrol, which has a melting point of 88ºC.

Reduction of the surface tension of various solutions

Fig. 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations with the following additives:

1) 0.1% stearic acid sol

2) 0.1% palmitic acid sol

3) 0.1% Pluoric 10R5 solution (BASF trademark)

4) 0.1% Pluoric L35 solution (BASF trademark)

5) 0.1% Pluoric L44 solution (BASF trademark)

Working method with reduction of viscosity:

The example described below is not claimed.

This example uses thermal viscosity reduction and proximity drop detachment in combination with a hot melt ink. Before the printer is operated, the solid ink in the reservoir 64 is melted. The reservoir, the ink line 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., about 100 cP). The ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is designed such that its viscosity decreases as the temperature increases. The ink pressure oscillates at a frequency that is an integer multiple of the drop ejection frequency from the nozzle. The oscillation of the ink pressure causes the ink meniscus at the nozzle tips to oscillate, but this oscillation is small because of the high viscosity of the ink. At normal operating temperatures, these oscillations are not strong enough to cause drop detachment. When the heater 103 is turned on, the ink forming the selected drop is heated so that the viscosity is reduced to a value that is preferably less than 5 cP. The reduced viscosity causes the ink meniscus to continue to move through the high pressure portion of the ink printing cycle. The recording medium 51 is located close enough to the print head 50 that the selected drops come into contact with the recording medium 51, but far enough away from it that the unselected drops do not come into contact with the recording medium 51. When the recording medium 51 is touched, a portion of the selected drop solidifies and adheres to the recording medium. As the ink pressure drops, the ink begins to withdraw back into the nozzle. The bulk of the ink separates from the ink that has adhered to the recording medium. The ink meniscus 100 at the nozzle tip begins to oscillate again at a lower amplitude. As the remaining heat is dissipated into the ink mass and the print head, the viscosity of the ink rises again to the viscosity at rest. With each heat pulse, an ink drop is selected, which detaches from the ink mass and forms a dot on the recording medium 51. Since the If heat pulses are electrically controlled, drop-on-demand inkjet printing can be achieved.

Image processing for print heads

One object of the printing systems according to the invention is to achieve a print quality corresponding to that to which people are accustomed from quality color prints printed by means of the offset process. This can be achieved by high-resolution printing at about 1,600 dpi. However, printing at 1,600 dpi is difficult and expensive. Comparable results can also be achieved by printing at 800 dpi with 2 bits per pixel for cyan and magenta and one bit per pixel for yellow and black. In the following, this color model is referred to as CC'MM'YK. If high-quality monochrome images are also to be printed, two bits per pixel for black can also be used. This color model is referred to as CC'MM'YKK'.

Applications using printheads according to the invention

The printing devices and methods of the invention are useful in a wide variety of applications including (but not limited to): color and monochrome office printing, short run digital printing, high performance digital printing, multi-color printing, spot color printing, offset insert printing, low cost printers with scanning print heads, high performance printers with page-wide print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copier machines, label printers, large format plotters, photographic duplicators, digital photo processing printers, portable printers integrated with digital instant cameras, video printing, photo CD image printing, portable "digital personal assistant" printers, wallpaper printing, indoor and outdoor sign and poster printing, and fabric printing.

Balancing printheads for environmental conditions

In drop-on-demand printing systems, the droplets should always be of a consistent and predictable drop size and position. Undesirable variations in the size and position of the ink droplets lead to differences in the optical density of the resulting print and thus to lower visible print quality. Therefore, these variations must be kept within a small ratio to the nominal volume of the ink droplet or the pixel pitch. Many environmental variables can be compensated for to reduce their impact to negligible levels. For some factors, active compensation can be achieved by changing the current applied to the nozzle heater.

In an optimal temperature profile of a print head embodiment, the active area of the nozzle tip is quickly heated to the ejection temperature, maintained at the ejection temperature during the pulse duration and then quickly cooled back to the ambient temperature.

This optimum is not achievable due to the stored heat capacities and the thermal conductivities of the various materials used to make the nozzles of the invention. However, improved performance can be achieved by deriving the current pulse curve by gradually refining the finite element simulation of the print head. The current applied to the heating element can be varied over time using various techniques, including

1) Varying the voltage applied to the heating element,

2) Modulating the width of a series of short pulses (PWM),

3) Modulating the frequency of a series of short pulses (PFM).

In order to obtain accurate results, a short-term simulation of the fluid dynamics with a free surface image is also required, since the convection in the ink and the flow of the ink have a significant influence on the temperature that can be achieved with a certain current curve.

By integrating appropriate digital circuits into the printhead carrier, it is possible to control the current applied to each nozzle individually. One way to achieve this is to send a large number of different digital pulse sequences across the entire printhead chip and to select the appropriate pulse sequence for the individual nozzles using multiplexing circuits.

Examples of the environmental factors that can be compensated are summarized in the Environmental Compensation table. This table also indicates which environmental factors can best be compensated globally (for the entire printhead), per chip (for each chip individually in composite multi-chip printheads) and per nozzle. Environmental Compensation

In most applications, it will not be necessary to compensate for all of these variables. Some variables will have a smaller impact and then compensation will only be necessary if very high image quality is required.

Printhead driver circuits

Fig. 4 is a schematic block diagram of the electronic operation of an example of a print head driver circuit according to the invention. This control circuit uses analog modulation of the power supply voltage applied to the print head to modulate the current applied to the heater element; there is no provision for individual control of the current applied to each nozzle. Fig. 4 is a block diagram of a system having an 800 dpi page-wide print head for color printing using the CC'MM'YK color model. The print head 50 has a total of 79,488 nozzles with 39,744 main nozzles and 39,744 redundant nozzles. The main nozzles and the redundant nozzles are divided into six colors, with each color divided into eight drive phases. Each drive phase has a shift register which converts serial data from a head controller ASIC 400 into parallel data for activating the heater driver circuits. There are a total of 96 shift registers, each of which supplies 828 nozzles with data. Each shift register consists of 828 shift register stages 217, whose outputs are logically are ANDed with the phase enable signal via a NAND gate 215. The output of the NAND gate 215 drives an inverting buffer 216 which in turn drives the drive transistor 201. The drive transistor 201 activates the electrothermal heater 200 which may be a heater 103 of the type shown in Fig. 1(b). To maintain the validity of the shifted data during the enable pulse, the shift register clock is stopped and the enable pulse is kept active by means of a clock latch 218 which is shown as a simple gate for clarity but may preferably be any glitch-free clock control circuit from a known group of such circuits. Stopping the shift register clock eliminates the need for parallel data storage in the print head, but does add some complexity to the control circuitry for the head controller ASIC 400. The data is fed to either the main nozzles or the redundant nozzles via the data router 219, depending on the state of the corresponding signal on the error status bus.

In Fig. 4, the print head is shown in a simplified form; the various means for improving production yield, such as block error tolerance, are not shown. Driver circuits for different print head designs can be easily derived from the device described here.

The digital information representing dot patterns to be printed on the recording medium is stored in page or stripe memory 1513, which may be identical to image memory 72 shown in Fig. 1(a). The data, contained in 32 bit words representing dots of one color, is read from page or stripe memory 1513 by means of addresses selected by address MUX 417 and control signals generated by memory interface 418. The addresses are generated by address generators 411 which are part of "single color circuits" 410, one of which exists for each of the six color components. The addresses are generated based on the positions of the nozzles relative to the printing medium. Since the relative position of the nozzles may vary for different printheads, the address generators 411 are preferably programmable. The address generators 411 typically generate the addresses corresponding to the position of the main nozzles. However, if there are faulty nozzles, the positions of nozzle blocks containing faults can be marked in the fault map RAM 412. The fault map RAM 412 is read as the page is printed. If the memory indicates a fault in the nozzle block, the address is changed and the address generators 411 generate the address corresponding to the position of the redundant nozzles. The data read from the page or stripe memory 1513 is buffered in the buffer 413 and converted into four consecutive bytes by the multiplexer 414. The timing of these bytes is adjusted in the FIFO 415 to the timing of the data representing other colors. The data is then buffered in buffer 430 to form the 48-bit main data bus to the printhead 50. The buffering of the data is done because the printhead may be located relatively far from the printhead controller ASIC. Data from the error map RAM 412 is also input to the FIFO 416. The timing of this data is adjusted to the data output of the FIFO 415 and the data is buffered in buffer 431 to form the error status bus.

The print head 50 is powered by the programmable power supply 320. The voltage of the power supply 320 is controlled by the DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains a two-way RAM 317. The contents of the two-way RAM 317 are programmed by the microcontroller 315. Temperature is compensated by changing the contents of the two-way RAM 317. The values are calculated by the microcontroller 315 based on the temperature sensed by the thermal sensor 300. The signal from the thermal sensor 300 is fed to the analog-to-digital converter (ADC) 311. The ADC 311 is preferably integrated into the microcontroller 315.

The head controller ASIC 400 contains control circuits for thermal inertia and print density compensation. Thermal inertia compensation requires that the head 50 power supply voltage be a time-varying voltage synchronized with the heater activation pulse. This is accomplished by programming the programmable power supply 320 to maintain this voltage.

The DAC 313 generates an analog time-varying programming voltage from the data read from the dual-port RAM 317. The data is read out according to an address generated by the counter 403. The counter 403 generates a complete address cycle during the duration of an activation pulse. This synchronization is ensured by the fact that the counter 403 is clocked by the system clock 408 and the top count pulse of the counter 403 is used to switch the activation counter 404. The value of the activation counter 404 is then decoded by the decoder 405 and buffered in the buffer 432 to generate the activation pulses for the head 50. If the number of states in the count value is less than the number of clock periods in an activation pulse, the counter 403 may include a prescaler. To precisely compensate for the thermal inertia of the heating element, sixteen voltage states are sufficient. These sixteen states can be specified using a four-bit connection between the counter 403 and the two-way RAM 317. However, these sixteen states must not be evenly spaced in time. To allow for non-linear timing of these states, the counter 403 can also include a ROM or other device that causes the counter 403 to count non-linearly. Alternatively, fewer than sixteen states can 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 ("active pixels"). The "active pixels" are counted by active pixel counter 402. An active pixel counter 402 is provided for each of the eight activation phases. The number of activation phases in a printhead according to the invention depends on the particular design. Four, eight and 16 are suitable, but the number of activation phases need not be a power of two. Active pixel counters 402 may be combinatorial logic pixel counters 420 which determine how many bits in a data nibble are active. The number is then accumulated in adder 421 and accumulator 422. A latch 423 holds the accumulated value active for the duration of the activation pulse. The multiplexer 401 selects the output of the latch 423 corresponding to the current value 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 active pixels is not necessary; the four most significant bits of the count value are sufficient.

The combination of the four bits of the thermal inertia compensation address and the four bits of the print density compensation address means that the two-way RAM 317 must have an eight-bit address. This means that the two-way RAM 317 contains 256 numbers in a two-dimensional array. The two dimensions are time (for thermal inertia compensation) and print density. A third dimension - temperature - can also be provided. Since the ambient temperature of the head changes only slowly, the microcontroller 315 has sufficient time to calculate a matrix of 256 numbers to compensate for thermal inertia and print density at the current temperature. At periodic intervals (for example, several times per second), the microcontroller senses the current print head temperature and calculates this matrix.

The clock of the print head 50 is generated by the print head clock generator 407, starting from the system clock 408, and buffered in the buffer 406. To facilitate testing of the ASIC of the print head control, JTAG test circuits 499 can also be provided.

Comparison with thermal inkjet technology

The table "Comparison between Thermal Inkjet Technology and the Present Invention" compares aspects of printing according to the present invention with thermal inkjet technology.

The direct comparison between the present invention and thermal inkjet technology is made because both technologies are drop-on-demand systems that use heat activation and liquid ink. Although these two technologies appear similar, they operate on different principles.

Thermal inkjet printers work on the following basic operating system: A heat pulse generated by an electrical resistance heater causes a bubble to suddenly form in the liquid ink. The rapid and consistent bubble formation can be achieved by overheating the ink so that enough heat is transferred to the ink before the bubble formation is complete. For water-based ink, temperatures of around 280ºC to 400ºC are required. The bubble formation creates a pressure wave that pushes a drop of ink out of the nozzle opening at high speed. The bubble then collapses, drawing ink from the reservoir to refill the nozzle. Thermal inkjet printing is very successful commercially because of the high nozzle packing density and the use of well-known integrated circuit manufacturing techniques. However, thermal inkjet technology faces major technical problems, such as precision manufacturing of the many parts, production yield, crackling noise, printing speed, drive transistor current, power loss dissipation, formation of side droplets, thermal stress, differential thermal expansion, cogitation, cavitation, directional diffusion, and difficulties in ink formulation.

The printing process according to the invention offers many of the advantages of the thermal ink jet printing process and overcomes - completely or to a significant extent - many of the problems inherent in thermal ink jet technology. Comparison between the thermal ink jet process and the invention

Yield and fault tolerance

In most cases, monolithic integrated circuits cannot be repaired if they are not fully functional during manufacture. Yield is the percentage of functional devices produced in a board production run. Yield has a direct impact on manufacturing costs. A device with a 5% yield is effectively 10 times more expensive to manufacture than an identical device with a 50% yield.

The yield is measured in three important steps:

1) Production yield

2) Yield of board sorting

3) Results of the final examination

For large molds, it is usually the board sort yield that has the greatest impact on overall yield. Page-wide color printheads according to the invention are very large compared to typical VLSI circuits. Good board sort yield is critical to cost-effective printhead manufacturing.

Fig. 5 shows a diagram of board sorting yield versus defect density for a monolithic, A4 page-wide color print head according to the invention. The head is 215 mm long and 5 mm wide. The yield 198 without defect tolerance is calculated using the Murphy method, which is widely used for predicting yield. For a defect density of one defect per cm2, the Murphy method gives a yield of less than 1%. This means that more than 99% of the heads produced would have to be rejected. This low yield is highly undesirable because it makes the manufacturing cost of the print head unacceptably high.

In the Murphy method, the effect of an uneven distribution of defects is determined by approximation. Fig. 5 also contains a yield curve 197 without defect tolerance, which explicitly depicts the concentration of defects by taking into account a defect cluster factor. The defect cluster factor is not a parameter that can be controlled in production, but rather a characteristic of the production process. The defect cluster factor for production processes can be assumed to be about 2; in this case, the yield predictions correspond quite closely to the Murphy method.

One solution to the problem of low yield is to be able to replace the faulty functional units by integrating redundant functional units into the chip, thus building in fault tolerance.

For memory chips and most die-integrated devices (WSI devices), the physical location of redundant subunits on the chip is not important. However, for printheads, the redundant subunit can be a print element or several. And these must be in a fixed spatial relationship to the page to be printed. In order to be able to print a dot in the same position as a faulty element, the redundant elements must not be offset in a direction that does not correspond to the scanning direction. On the other hand, faulty elements can be replaced by redundant elements offset in the scanning direction. To ensure that the redundant element prints the dot in the same position as the faulty element, the offset in the scanning direction can be compensated by changing the timing of the data fed to the redundant element.

In order to replace all nozzles, a complete set of spare nozzles must be available, providing 100% redundancy. Requiring 100% redundancy would typically more than double the chip area, dramatically reducing yield before replacement with the redundant units and largely negating the benefits of fault tolerance.

In printhead designs according to the invention, the minimum physical dimensions of the printhead chip are determined by the width of the page to be printed, the sensitivity of the printhead chip, and the constraints on manufacturing the ink channels that carry the ink to the back of the chip. The minimum practical size of a full-color, page-wide printhead for printing an A4 page is approximately 215 mm x 5 mm. This size allows 100% redundancy using 1.5 µm CMOS manufacturing technology without significantly increasing the chip area. Therefore, a high level of fault tolerance can be provided without significantly reducing the primary yield.

For fault-tolerant devices, the usual yield equations are not applicable. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and taken into account in the yield equation. Fig. 5 shows the fault-tolerant sort yield 199 for a page-wide A4 color print head with different forms of fault tolerance built into the yield equation. The curve gives the predicted yield as a function of both from the defect density as well as from the defect clustering. From the yield prediction shown in Fig. 5 it can be seen that through a thoroughly implemented defect tolerance the board sorting yield can be increased from less than 1% to more than 90% under the same manufacturing conditions. This reduces the manufacturing costs by a factor of 100.

In order to improve the yield and reliability of printheads containing thousands of print nozzles, and thus make page-wide printheads feasible in practice, the integration of fault tolerance is strongly recommended. However, fault tolerance is not an essential part of the invention.

Designs of printing systems

Fig. 6 shows a diagram of a digital electronic printing system with a printhead according to the invention. A monolithic printhead 50 prints an image 60 consisting of a plurality of ink droplets onto a recording medium 51. The medium is usually paper, but can also be overhead transparencies, cloth, or numerous other substantially flat surfaces 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 is convertible into a two-dimensional pixel array. Typical image sources are image scanners, digitally stored images, images in a page description language (PDL) such as Adobe Postscript, Adobe Postscript level 2 or Hewlett-Packard PCL 5, images generated by a procedure call based raster device such as Apple QuickDraw, Apple Quickdraw GX or Microsoft GDI, or text in electronic form such as ASCII. The image data is then converted by an image processing system 53 into a two-dimensional pixel arrangement suitable for the particular printing system. The printing system may be a color or monochrome printing system and the data is typically between 1 and 32 bits per pixel depending on the image source and the specifications of the printing system. If the image source is a page description, the image processing system may be a raster image processor (RIP) or, if the image comes from a scanner, a two-dimensional image processing system.

If halftone images are desired, a screening system 54 is required. Suitable screening types are based on ordered scattered dot dither or error scattering. Modifications of these systems are suitable, generally known as stochastic screening or frequency modulation screening. The screening system commonly used for offset printing - ordered dot cluster dither - is not recommended because this technique unnecessarily loses effective image resolution. The output of the screening system is a binary monochrome or color image at the resolution of the printing system of the invention.

The binary image is processed by a data synchronization circuit 55 (which can be integrated into the head controller ASIC 400 as shown in Figure 4) which feeds the pixel data to the data shift registers 56 in the correct sequence. Forming data sequences is necessary to compensate for nozzle placement and paper movement. Once the data is loaded into the shift registers 56, it is fed in parallel to the heater driver circuits 57. The driver circuits 57 then electronically connect the appropriate heater elements 58 at the correct time to the voltage pulse generated by the pulse shaping circuit 61 and voltage regulator 62. The heater elements 58 heat the tip of the nozzles 59, thereby changing the physical properties of the ink. Ink drops 60 emerge from the nozzles in a pattern that corresponds to the digital pulses applied to the heater driver circuits. The pressure of the ink in the ink reservoir 64 is controlled by the pressure control 63. Selected ink drops 60 are released from the bulk of the ink by the selected drop release means and come into contact with the recording medium 51. During printing, the recording medium 51 is continuously moved relative to the print head 50 by the paper transport system 65. If the print head 50 extends across the entire width of the printing area of the recording medium 51, the recording medium 51 only needs to be moved in one direction and the print head 50 can be stationary. When using a smaller print head 50, a raster scanning system must be implemented. This is usually done by having the print head 50 scan along the shorter dimension of the recording medium 51 while the recording medium 51 is moved in the direction of its longer dimension.

Computer simulation of nozzle dynamics

The details of the function of the print heads according to the invention have been computer simulated to a very large extent. Fig. 8 to 18 show some results of an exemplary simulation of the operation of a preferred nozzle design with electrothermal drop selection by reducing the surface tension in combination with electrostatic drop detachment.

Computer simulation is extremely useful for determining the properties of phenomena that are difficult to observe directly. The jet function is difficult to observe experimentally for several reasons, including

1) the nozzles that can be used are microscopically small, with important phenomena occurring in the dimension range below 1 µm,

2) the time for ejection of a drop is only a few microseconds, which requires high-speed observation,

3) important phenomena take place inside opaque solid materials, which makes direct observation impossible,

4) some important parameters, such as heat flow and velocity vector fields of the fluids are difficult to observe at any scale,

5) the costs of manufacturing test nozzles are high.

Computer simulation overcomes the above problems. A leading software package for fluid dynamics simulation is FIDAP, manufactured by Fluid Dynamics International Inc. of Illinois, USA (FDI). FIDAP is a registered trademark of FDI. Although other simulation programs are commercially available, FIDAP was chosen because of its high accuracy in calculating transient fluid dynamic states, energy transport and surface tension. FIDAP version 7.51 was used.

The simulations combine aspects of energy transport and fluid dynamics. Since the nozzle used as an example has a cylindrical shape, the axisymmetric simulation was chosen. There are four deviations from the cylindrical shape. These concern the connections to the heating elements, the laminar air flow generated by the paper movement, gravity (if the print head is not aligned vertically) and the presence of adjacent nozzles in the substrate. The effects of these factors on the drop ejection are minor.

In order to achieve convergence in simulations of short-term processes on free surfaces with a variable surface tension in the micrometer range and short-term processes in the microsecond range using FIDAP 7.51, the simulation must be carried out dimensionlessly.

Only the area in the nozzle tip is simulated, since most of the phenomena relevant for droplet selection occur in this area. The simulation extends 40 µm outwards from the symmetry axis of the nozzle.

At the start of the simulation, the entire nozzle and ink are at the ambient temperature of the device, which in this case is 30ºC. During operation, the ambient temperature of the device will be slightly higher than the ambient temperature of the air, as an equilibrium temperature based on the print density will be established over the period of many drop ejections. Most of the energy from each drop selection is dissipated with the ink drop. The heat remaining in the nozzle is distributed very evenly between drop ejections due to the high thermal conductivity of silicon and convection in the ink.

Geometry of the simulated nozzle

Fig. 7 shows the geometry and dimensions of the preferred nozzle embodiment depicted in this simulation.

The nozzle is formed on a single crystal silicon substrate 2020. The substrate has an epitaxial boron-doped silicon layer 2018, which serves as an etching limit during the manufacture of the nozzle. An epitaxial silicon layer 2019 serves as an active substrate for the manufacture of CMOS driver transistors and data distribution circuits. Various layers applied during CMOS manufacture are located on this substrate. These are a thermal oxide layer 2021, a first oxide intermediate layer 2022, a first metal layer 2023, a second oxide intermediate layer 2024, a second metal layer 2025 and a passivation oxide layer 2026. The nozzles and heating elements are formed by subsequent processing of the boards. These structures include the active heater element 2027(a), an EDS shield formed from "excess" heater element material 2027(b), and a silicon nitride passivation layer 2028.

The heating element is located on a narrow "rim" etched out of the various oxide layers. This serves to reduce the "thermal mass" of the material around the heating element and prevent the ink from spreading on the surface of the printhead.

The print head is filled with electrically conductive ink 2031. An electric field is applied to the print head by means of an electrode that is in electrical contact with the ink and another electrode located behind the recording medium.

The nozzle radius is 8 um, the diagram is to scale.

Theoretical calculation basis

The theoretical basis for the fluid dynamics and energy transport calculations using the finite element method and the way in which this theoretical basis is applied in the FIDAP computer program are described in detail in the FIDAP Theory Manual 7.0 (April 1993) published by the FDI, the contents of which are hereby incorporated into this description.

Material properties

The table "Properties of the materials used for the FIDAP simulation" gives the approximate physical properties of the materials used to manufacture the inventive printhead.

The properties of the "ink" used for this simulation correspond to those of a water-based ink with a 25% pigment addition. In order to achieve a pronounced reduction in surface tension with temperature, the ink contains a suspension of fine palmitic acid particles (hexadecanoic acid particles). The surface tensions were measured at different temperatures using a surface tension meter.

The values used in the example simulation using the FIDAP program are shown in the table "Properties of the materials used for the FIDAP simulation". Most of the values were obtained by direct measurement or taken from the CRC Handbook of Chemistry and Physics, 27th edition, or the Handbook of Chemistry by Lange, 14th edition. Properties of the materials used for the FIDAP simulation

Fluid dynamic simulations

Fig. 8(a) shows the power applied to the heater. The maximum power applied to the heater is 40 mW. This power is pulse frequency modulated to obtain a desirable temporal distribution of the power delivered to the heater. The current pulses each have a duration of 0.1 us and each deliver 4 nJ of energy to the heater. To allow the meniscus to assume its rest position, the drop selection pulse is started 10 us after the start of the simulation. The total energy delivered to the heater during the drop selection pulse is 276 nJ.

Fig. 8(b) shows the temperature at different points of the nozzle during the simulation.

Point A is where the ink meniscus contacts the nozzle edge. To obtain optimal performance, it is desirable to heat this point as close as possible to the boiling point of the ink, but without exceeding the boiling point, and to maintain it at this temperature for the duration of the drop selection pulse. The "jagged" temperature curve is due to the pulse frequency modulation of the current supplied to the heating element. The "jaggedness" can be reduced by increasing the pulse frequency and correspondingly reducing the pulse energy.

Point B is located on the ink meniscus approximately halfway between the meniscus center and the nozzle tip.

Point C is located on the silicon surface 20 um from the nozzle center. This shows that the temperature rise when selecting a droplet is very small even at a short distance from the nozzle. This allows devices, such as driver transistors, to be placed very close to the nozzles.

Fig. 9 shows a position/time curve of a point in the meniscus center.

Fig. 10 shows the meniscus position and shape at different times during the drop selection pulse. The times shown are at the beginning of the drop selection pulse (10 us after the start of the simulation) and then at 5 us intervals until 60 us after the start of the heater pulse.

Fig. 11 shows the temperature lines in the nozzle immediately before the start of the drop selection pulse 9 us after the start of the simulation. The surface tension balances the combined effect of the ink pressure and the external constant electric field.

Fig. 12 shows temperature lines of the nozzle 5 us after the start of the drop selection pulse, 15 us after the start of the simulation. The reduction in surface tension at the nozzle tip causes surface stretching at this point, causing heat to spread rapidly around the meniscus. The ink has already started to move because the surface tension is no longer high enough to compensate for the combined effect of the ink pressure and the external constant electric field. Due to frictional resistance on the nozzle walls, the meniscus center begins to move faster than the outer meniscus regions. Figs. 12 to 17 are temperature lines starting at 35ºC and shown at 5ºC intervals.

Fig. 13 shows temperature lines in the nozzle 10 us after the start of the droplet selection pulse, 20 us after the start of the simulation.

Fig. 14 shows temperature lines in the nozzle 20 us after the start of the droplet selection pulse, 30 us after the start of the simulation.

Fig. 15 shows temperature lines in the nozzle 30 us after the start of the droplet selection pulse, 40 us after the start of the simulation. This time is 6 us after the end of the droplet selection pulse, and the nozzle has already started to cool down.

Fig. 16 shows temperature lines in the nozzle 40 us after the start of the drop selection pulse, 50 us after the start of the simulation. From this simulation it is clear that the most of the energy of the drop selection pulse is dissipated with the selected drop.

Fig. 17 shows temperature lines in the nozzle 50 us after the start of the drop selection pulse, 60 us after the start of the simulation. At this point, the selected drop begins to "pinch" and the drop detachment process begins.

Fig. 18 shows streamlines in the nozzle which correspond in time to Fig. 17.

The approximate duration of the three consecutive phases of the droplet ejection cycle is:

1) For the heating element activation cycle 24 us,

2) until the drop detaches 60 us,

3) until returning to the rest position 40 us.

Together, these times amount to 124 us, resulting in a maximum droplet repetition rate (droplet frequency) of about 8 KHz.

The proximity method of droplet detachment:

The following example is outside the scope of the claims.

The detachment of drops of liquid ink by approaching the recording medium preferably takes place under the following conditions:

1) The difference in meniscus positions between selected and non-selected drops is larger than the surface roughness of the recording medium.

2) The surface roughness of the recording medium is less than about 30% of the ink drop diameter.

3) The rate at which the volume of the ink droplet increases by wetting the surface of the recording medium and/or being absorbed by a porous recording medium is higher than the flow rate through the nozzle under the applied ink pressure.

These conditions can be achieved with many different nozzle radii, ink types, media and print resolutions.

With hot-melt inks, the molten ink drop solidifies when it comes into contact with the recording medium, and the absorption properties of the ink towards the recording medium are then not so important.

The principle of operation of droplet detachment printing using the proximity system is shown in Fig. 19(a) to Fig. 19(i). In this case, the droplet is selected by electrothermal transducers that heat the ink at the nozzle tip, thereby increasing the temperature at the meniscus. The higher temperature lowers the surface tension below a critical surface tension, causing ink to exit the nozzle tip.

In Fig. 19(a) to 19(i), the selected drop is designated by 1, the nozzle that produced the selected drop is designated by 10, a nozzle whose heater 103 was not activated so that no drop was selected is designated by 11, the direction of movement of the recording medium is designated by 5, the recording medium is designated by 51, the mass of the ink is designated by 100, silicon is designated by 101, silicon dioxide is designated by 102, the electrothermal actuator (also called "heater") is designated by 103, and the hydrophobic layer of the printhead is designated by 109.

Fig. 19(a) shows a cross section through two adjacent nozzles 10 and 11 in the resting state. The nozzles are close to the recording medium 51, which moves relative to the nozzles in the direction 5. The cross section passes through the plane of the diagram at an angle of 45º to the direction of movement of the medium. The nozzles 10 and 11 represent two nozzles offset by the width of one pixel in a direction perpendicular to the plane of the diagram. All nozzle surfaces have a hydrophobic surface layer 109, the ink 100 is hydrophilic. The ink is under pressure, causing the ink meniscus to bulge.

Fig. 19(b) shows the ink in two nozzles shortly after an activation pulse is applied to the heating element 103 of the nozzle 10, but not to the heating element of the nozzle 11. The heat is conducted to the ink surface where the resulting temperature increase causes a local reduction in the surface tension of the ink. The reduction in surface tension may result from the natural properties of the ink but is preferably enhanced by the addition to the ink of an agent which causes a significant drop in surface tension at the temperature to which the ink is heated. This may be a surfactant which is in the form of suspended solid particles at rest but which melts when the heating elements are activated. In its solid form the surfactant has little effect on surface tension. When molten the surfactant molecules migrate rapidly to the ink surface causing a substantial reduction in surface tension. In this case the surfactant is 1-hexadecanol, a 16-carbon alcohol which melts at 50ºC.

Fig. 19(c) shows the drop evolution a short time later. The selected drop 1 assumes a substantially cylindrical shape due to a surface tension gradient from the nozzle tip to the meniscus center and due to viscous friction which slows the movement of the ink near the nozzle walls. In this case, no external electrostatic or magnetic fields are applied and gravity is not important at this scale.

Fig. 19(d) shows the selected drop 1 at the moment it comes into contact with the recording medium 51. The "tilt" of the selected drop is due to the laminar air flow between the print head and the recording medium 51 caused by the movement of the recording medium. In many practical cases, the recording medium will be paper, which normally has a rough surface in proportion to the distance between the nozzle and the recording medium. This roughness causes the contact time between the drop 1 and the recording medium 51 to vary and the area of the printed dot to change. This variation can be minimized by using a coated paper and/or by passing the recording medium through print rollers immediately before printing.

Fig. 19(e) shows the selected drop at the time it begins to be "sucked up" by the recording medium 51.

Fig. 19(f) shows the selected drop a short time later. The ink is absorbed by the recording medium at a rate approximately equal to the saturation gradient. For many fibrous printing media, such as paper, the circle of contact between the recording medium and the ink meniscus does not follow the lateral absorption of the ink into the printing medium. The reason for this is that the surface fibers are not completely wetted.

The flow of ink into the printing medium is highly dependent on the composition of the recording medium. In many cases, faster absorption of ink into the recording medium 51 can be achieved by wetting the medium prior to printing. This can be done using a series of rollers. The technology of continuously applying a uniform layer of liquid using rollers is well known in offset printing. Most offset printing systems apply a thin wet layer to the printing plates using dampening rollers and a thin layer of ink using inking rollers.

Fig. 19(g) shows the selected drop 1 immediately after it has separated from the mass of ink 10. Ink separation occurs when the flow rate of ink into the porous recording medium 51 exceeds the flow rate of pressurized ink out of the nozzle 10. This can be achieved for a wide range of inks, media and nozzle radii. However, non-porous media such as plastic or metal foils can also be used. In this case, the drop separates when the rate of volume increase of the drop as it wets the non-porous medium exceeds the flow rate of ink out of the nozzle 10. For some combinations of ink and porous material, it may be necessary to coat the medium with a wetting agent.

Fig. 19(h) shows the selected drop 1 after it has been largely sucked out of the recording medium. The movement impulse that is reflected in the nozzle The receding ink carries the meniscus at the nozzle 10 beyond its resting position. The amount of this "overshoot" is very small compared to conventional thermal inkjet or piezoelectric inkjet systems.

Fig. 19(i) shows the nozzle 10 after the meniscus has returned to the rest position awaiting the next drop selection pulse. The selected drop 1 is shown fully absorbed by the recording medium 51. The absorption rate is highly dependent on the recording medium and it may happen that the selected drop 1 is not fully absorbed by the time a drop of a different color is printed at the same position. In some cases this may lead to a deterioration in print quality; in this case a more absorbent recording medium, a different ink composition, a printhead with greater separation between colors may be used, the recording medium may be heated to speed up the drying process, or several of the above techniques may be combined.

Creating sound waves when printing with drop detachment using the proximity system:

This example does not fall within the scope of the claims.

When used correctly, applying sound waves to the ink in proximity printing systems can provide several benefits:

1) If the period of the maximum forward velocity of the ink generated by the sound wave coincides with the period of droplet growth, the droplet can grow faster.

2) The amount of ink deposited on the recording medium by a selected droplet can be reduced if the droplet release time coincides with a period of reduced ink pressure, as less ink will flow out of the nozzle and the droplet will release sooner.

3) The degree of fluctuation in the amount of ink supplied to the recording medium is reduced because both the contact time and the release time of the selected droplet can be influenced by the sound wave, which can be generated with highly accurate and stable frequency and amplitude.

4) The use of pigments instead of dyes is favored because the ink is constantly agitated by the sound waves, reducing one of the major problems of pigments, namely the settling of the pigment in the ink.

5) The problem of nozzle blockage by dried ink is reduced, since the constant movement of the ink meniscus moves the ink around the meniscus and replaces drying ink with "fresh" ink.

Fig. 20(a) shows the sound wave 820 applied to the ink.

Fig. 20(b) is a space-time diagram showing the presence of ink along the nozzle axis for both selected drops 821 and unselected drops 822. The diagram illustrates the ink position as a function of time for a small region of the nozzle axis extending from a point slightly inside the bulk of the ink 100 (in the diagram below) to a point slightly inside the paper 51 (in the diagram above).

The two diagrams are superimposed to allow a direct comparison between the selected drop and the non-selected drop.

The graph of the presence of ink in non-selected drops 822 shows a sinusoidal oscillation at the frequency of the sound wave 820, but with some phase shift ΔΦ. The amount of phase shift depends on the shape and dimensions of the ink nozzles and ink containers and on the flow properties of the ink. As the sound wave approaches the resonant frequency of the ink in the nozzle, the phase shift approaches 90º. The phase shift can be easily changed by changing the phase of the drive voltage for the piezoelectric or other transducer used to generate the sound wave. The waveform for the non-selected drop is shown in the drawing as sinusoidal. The actual shape will exhibit considerable harmonic distortion and depends on the geometry of the nozzle tip and the flow characteristics of the ink.

Fig. 20(b) relates specifically to a head embodiment with eight drive phases and four ink colors (e.g., CMYK colors). Only one ink color is shown. The phases of the sound waves in the other ink colors are offset by 90º, 180º, and 270º from the phase of the sound wave 820. The eight drive phases of the drop ejection cycle extend over two periods of the sound wave 820. Each drop ejection cycle includes two drive phases per ink color. These are separated by 360º from the sound wave and do not affect the same nozzles, but rather nozzles provided alternately with it. Periods 829 and 830 are two consecutive heater drive periods of a nozzle (in this case, the nozzle with the selected drop 821). In period 830, the heating elements of the alternating nozzles of the same ink color can be activated.

When the heater is turned on 823, a differential evolution begins with respect to the presence of ink at the selected drop 821 and the non-selected drop 822. The flow of ink out of the nozzle is assisted by the ink being in a period of maximum forward velocity due to the sound wave 820. At time 824, this differential evolution is no longer reversible because the oscillating equilibrium between surface tension and oscillating ink pressure is broken. Ink continues to exit the nozzle until the ink comes into contact with the recording medium 51 at time 825. The ink wets the surface of the recording medium 51 and is absorbed by the medium, as shown by the overlap of ink and recording medium in the space/time domain 832. When the volumetric flow rate into the recording medium and/or along its surface exceeds the flow rate out of the nozzle at the moment of detachment and at position 826, the selected drop 821 detaches from the bulk of ink 100. After the moment of detachment at 821, the ink meniscus of both the ink remaining in the recording medium 51 and the bulk of ink 100 rapidly contracts. Detachment is This is aided by the fact that it occurs at the time of low ink pressure, when the ink for the selected drops flows back into the nozzle.

The ink on the side of the nozzle from the separation point 826 is rapidly drawn back into the nozzle by the surface tension of the ink. The ink meniscus performs a damped oscillation at the resonant frequency of the ink in the nozzle tip for a short time. This damped oscillation is superimposed on the oscillation produced by the sound wave. In most cases it will not be necessary or practical to match the resonant oscillation of the ink in the nozzle to the frequency of the sound wave. In the example shown in Fig. 20(b), the heater on period is 18 µs and the drop ejection cycle is 144 µs. The period of the sound wave is 72 µs, so the frequency of the sound wave is 13.8 KHz. The resonant frequency of the ink column is 25 KHz.

At time 827, when the sound wave is in the same phase as the start of activation of the heating element 823, the ink meniscus has not yet sufficiently returned to the resting oscillation state of the unselected drops 822. However, the other, alternately provided nozzles of the same ink color are ready to activate the heating element for the duration 830 at time 827. At the next time 828, when the sound wave is in the same phase as the start of activation of the heating element 823, the heating element of the same nozzle can be activated again because the meniscus has returned to the resting oscillation state with a small error margin by then.

The area 832 of ink absorbed by the recording medium 51 initially becomes thicker in the drawing and then thinner as time progresses. The actual area of ink in the recording medium only thins, slows down and stops at a certain thickness. The thinning of the ink area is because Fig. 20 is a space/time diagram of the presence of the ink along the nozzle axis and the recording medium 51 is moving relative to the nozzle axis. By the time 833 when the next selected drop from the nozzle has reached the recording medium, the edge of the previous drop has already been happens. The second ink drop flows back in the recording medium, combines with the first ink drop, and thus creates a continuous ink layer as subsequent ink drops are selected.

A simple means of generating a sound wave in the ink is to place a piezoelectric crystal so that it displaces the ink in the ink channel behind the nozzles. The piezoelectric crystal must extend the entire length of the nozzle row so that a sound wave of the same amplitude and phase is applied to all nozzles. The amplitude of the voltage applied to the piezoelectric crystal depends on the physical properties of the crystal, the nozzle dimensions, the shape, position and dimensions of the ink container, the positioning of the piezoelectric crystal relative to the ink nozzles and ink container, the flow characteristics of the ink and other factors. Because it is easy and inexpensive to try different voltages, amplitudes and phases of the drive voltage, the experimental approach is a more efficient means of deriving suitable drive waveforms than calculations.

In the example shown in Fig. 20, the sound wave has a frequency of 13.8 KHz. This is within the normal range of human hearing and can be perceived as a disturbing high-pitched tone if the wave is transmitted to a greater extent into the air and exits the printer case. The level of disturbance is subjective and will vary greatly from person to person. For example, some people are disturbed by the 15.625 KHz line frequency of NTSC and PAL televisions, while most people do not notice this sound at all. There are several remedies for the noise problem. One is to ensure that the frequency of the sound pitch is above 20 KHz, the normal maximum audible frequency. Another solution is to insulate the print head with sound-absorbing material. This material only needs to be highly absorbent at the fundamental frequency of the sound wave, since the second harmonic is above 20 KHz. Another solution is to minimize the acoustic coupling between ink and air (via the ink channel or other components) at the appropriate frequency.

Drop size adjustment for printers with proximity drop detachment

This example does not fall within the scope of the claims.

Fig. 21(a) shows a cross-section of a proximity-release printhead and a print plate for a web-transport printing system.

The print head 50 prints six colors (CC'MM'YK) in high quality full color printing using the digital halftone process. The print head is approximately 8 mm wide and 600 µm thick. The print head is located at a distance DHtoP from the recording medium 51, which moves in the direction indicated by arrow 5. The recording medium 51 presses against the platen 67. The platen 67 should have a highly polished and optically flat surface to reduce friction with the recording medium and to ensure accurate positioning over the entire printing area. Alternatively, the platen may consist of two or more rollers (not shown) to further reduce friction. A belt (not shown) may be wound around the rollers to maintain the positional accuracy of the recording medium 51. The plate 67 is attached to a piezoelectric ceramic 31 with a polarization axis 33. The piezoelectric crystal is attached to a plate 30, which in turn is mechanically fixed with respect to the print head 50 during the printing process. Electrodes 32 are applied to the piezoelectric crystal 31. A voltage is applied to the electrodes 32 to compensate for the distance DHtoP.

The ink 100 is supplied to the head via the ink channel 75. The ink channel 75 may also serve the function of holding the print head rigidly in position and correcting misalignments within the print head. Alternatively, these functions may be performed by other means. The thermal heating elements are supplied with power via the two power connectors 38 and 39. Since these connectors can be made of a conductive material which can easily be several hundred microns thick and since these connectors can extend the entire length of the print head, high currents can be supplied to the print head with little voltage drop. This is important because Page-wide color print heads can handle up to 20 A when several thousand nozzles are activated simultaneously.

A paper guide rests against the recording medium 51 under light pressure, the pressure being provided by an elastically deformable material 35 acting against a fixed block 34. The guide 36 serves two purposes: to apply the recording medium to the platen under pressure together with the paper transport roller 65 and to temporarily press down any fibers that may protrude from a recording medium, such as paper. Pressing down protruding fibers is useful to improve print quality by reducing the variation in the distance between the print head and the effective surface of the recording medium. Protruding fibers do not have as great an effect on the size of the printed dot as might be expected given the small distance between the nozzle and the closed area of the recording medium. This is because the ink drop does not penetrate the surface of small protruding fibers or is sucked along their surface at the same speed as it is sucked up from the actual surface. Therefore, the time before the drop is released and hence the total amount of ink dispensed will not vary to any great extent. Depending on the printing speed, the type of recording medium and other aspects of the printing system, the guide 36 may not be necessary or may be replaced by pinch rollers to reduce friction.

Fig. 21(b) shows a small distance DHtoP between the print head and the recording medium 51. This results in a small volume of the selected droplet 1 at the moment of contact between the selected droplet and the recording medium. The value DHtoP is achieved by applying a voltage Vnom + ΔV to the piezoelectric crystal.

Fig. 21(c) shows a nominal distance DHtop between the print head and the recording medium 51. This results in a nominal volume of the selected drop 1 at the moment of contact between the selected drop and the recording medium. The DHtoP value is achieved by applying a voltage Vnom to the piezoelectric crystal, where Vnom is the nominal voltage. Vnom can be zero or chosen so that the full required adjustment range can be covered with a single-pole adjustment voltage. ΔV can be positive or negative, depending on the crystal orientation and choice of electrodes.

Fig. 21(d) shows a relatively large distance DHtop between the print head and the recording medium 51. This results in a relatively large volume of the selected drop 1 at the moment of contact between the selected drop and the recording medium. The value DHtoP is achieved by applying a voltage Vnom to the piezoelectric crystal.

The volume of ink deposited on the recording medium is not equal to the volume of the selected drop at the moment of contact with the recording medium, since the ink continues to flow from the nozzle while the selected drop is already absorbed by the recording medium. However, the volume of ink deposited on the recording medium is approximately proportional to the volume of the selected drop at the moment of contact, within an operating range dependent on the ink, the recording medium and the nozzle characteristics.

According to an alternative embodiment of the device, a piezoelectric crystal is used to change the position of the print head with respect to a fixed printing plate. This arrangement is functionally equivalent and has no significant disadvantages compared to the device, except that it is in many cases more difficult to manufacture.

Many different arrangements of the piezoelectric crystal are possible, including those in which the crystal operates in shear mode and arrangements that use multi-layer stacks of piezoelectric crystals to reduce the required control voltage.

Various general and preferred embodiments of the invention have been described above. Modifications to the general and specific embodiments are possible for those skilled in the art without departing from the scope of the claims.

Claims (10)

1. Device for printing according to the "drop-on-demand" ink printing principle, with - a print head (50) having a plurality of droplet-emitting nozzles, - a mass of ink (100) associated with the nozzles (59) and forming a meniscus at each nozzle, - a pressure means (63) which subjects the ink mass to a pressure which is at least 2% above atmospheric pressure, at least during the drop selection and detachment phase, - a drop selection device (71, 313, 52-58) acting on the surface of the meniscus by rapidly heating the ink of the meniscus in contact with the nozzle tip to selectively address predetermined nozzles during the drop selection phase and to create a difference in meniscus position between ink in addressed and non-addressed nozzles, the force being insufficient to cause detachment of ink in addressed nozzles, marked by - a drop detachment device (74) which, using an external field, causes ink from addressed nozzles to detach from the ink mass (100) in the form of drops and to be withdrawn in the form of droplets, while ink in non-addressed nozzles can be retained. 2. Apparatus according to claim 1, characterized in that the drop selection device (71, 313, 52-58) causes ink in addressed nozzles to move to selected points, being retained by cohesive forces within the nozzles, but protruding further from the nozzles than ink in non-addressed nozzles, and that the drop detachment device draws the ink protruding further from the nozzle to the printing area. 3. Device according to claim 1, characterized in that the drop selection device causes an electrothermal reduction of the surface tension of the ink under pressure. 4. Device according to claim 1, characterized in that the drop detachment device has means for electrostatically attracting electrically conductive ink, wherein the force is not sufficient to cause the ink to detach from the ink mass in the nozzle. 5. Device according to claim 1, characterized in that the ink pressure caused by the pressure medium oscillates. 6. Device according to claim 5, characterized in that the ink pressure oscillates at a frequency that is an integer multiple of the frequency at which the drops are ejected from the nozzle. 7. Device according to claim 1, characterized in that the drop detachment device electrostatically draws electrically conductive ink to a printing area. 8. Device according to claim 1, characterized in that the drop detachment device magnetically attracts ink containing magnetically active substances to a printing area. 9. Device according to claim 1, characterized in that the pressure means comprises a device that applies an acoustic wave to the ink mass, and that the acoustic wave can be applied to both the addressed and the non-addressed nozzles. 10. Device according to claim 9, characterized in that the device generating the acoustic wave - has a piezoelectric crystal that is connected to the ink mass and, when electrically excited, causes a displacement of the ink in the ink mass, and - means for applying an oscillating voltage to the piezoelectric crystal at a frequency that is an integer multiple of the frequency at which the drops are ejected from the print head.