Ink-jet printer

Abstract: A thermal inkjet printer [590] is operated in a spray-mode by deliberately firing ink droplets from a printhead while the meniscus of the remaining ink [104] in the printhead is settling down. Generally, the drops will not travel in a direction perpendicular to the printing surface [102]. By calibrating the printhead, one can determine how many drops are needed to be fired within the boundaries of a pixel to achieve any given optical density. Drops may be fired at rates above 50 kHz, and, depending on the ink, above 70 kHz. Ink [104] with a viscosity of 10 centi-Poise or less, and even 2 centi-Poise or less, may be used. When printing both text and non-text on the same surface [104], a digital representation of an image to be printed is analyzed and divided into non-text image fields and text fields. Each non-text image field is printed on the printing surface [102] by projecting the corresponding ink droplets in the spray-mode. Each text field is printed on the printing surface [102] in a text-mode, in which the firing rate is typically reduced to 5 - 10 kHz and the corresponding ink droplets are projected substantially perpendicular to the printing surface [102].

Description: This invention relates generally to an inkjet printer. Two ever-present and normally conflicting goals of inkjet printing is that what is printed should be as life-like or sharp as possible and also that the printing process should be as fast as possible. One way to increase both the sharpness of text and the lifelikeness of non-text images is to increase the print resolution, that is, the number of ink drops that are applied per unit printing area and are accurately placed with separation within that area. The unit printing area is typically the so-called "pixel," which is the common abbreviation for "picture element." The conventional way of controlling the number of ink dots per pixel is to improve the repeatable accuracy in the "aim" of every printhead in the inkjet printer. One problem with this is that every time an ink drop is fired from the printhead, it takes time for the meniscus of the printing fluid (the "ink") in the printhead to settle down and level out enough to fire the next drop accurately. In such conventional printheads, it is also important for the meniscus to return to a stable initial condition so that the drops that are fired will be of approximately the same size. In order to damp out and avoid resonant modes on the meniscus and thereby to try to ensure that the ink drops are ejected straight and placed accurately within a pixel, conventional printheads lower the firing velocity and the firing rate based substantially on the viscosity of the ink. Normal firing velocities are on the order of ten meters per second (m/s); and common firing rates are less than 10 kHz. US Patent No. 4,503,444 (Tacklind, 5 March 1985) describes a thermal inkjet printer that emits discrete drops whose volume varies to create a printed gray scale. In the Tacklind inkjet printer, ink droplets are fired in groups, so that each subsequent droplet in a group is fired before the previous one separates from the meniscus of the ink in the printhead. The individual droplets within each group then merge on the surface of the medium to be printed because all the droplets are designed to travel along the same direction to go to the same area. Darker pixels can be created by increasing the number of droplets fired into them. Although the Tacklind system makes it possible to use a heating element for firing the drops that are physically undersized for the particular ink being used, it still relies on an ability to fire droplets straight, that is, in a direction normal to an ejection plane of the printhead, which is typically also normal to the pixel. The maximum single-droplet emission rate for the ink used in the basic embodiment of the Tacklind system is roughly 10 kHz. This typical rate is much lower than the firing rate that one could achieve by the printhead. US-A-4,794,411 discloses that spray resulting from misdirected drops was frequently observed in thermal ink-jet heads. The problem was considered worse for printing in a "multidrop" mode, that is printing groups of drops in bursts at 50 kHz drop rates. Xerox Disclosure Journal, vol.10, no. 4 of July/August 1985 discloses a system for ejecting ink drops from a print head at different angles by having three resistors in each ink ejection chamber. JP-A-1257059 discloses a system having two heating elements one of which is operable to produce an infinate number of fine ink drops, the other of which is operable to produce a single ink drop. The present invention seeks to provide improved inkjet printing. According to an aspect of the present invention there is provided a method of printing using an inkjet printer as specified in claim 1. According to another aspect of the present invention there is provided a thermal inkjet printer as specified in claim 5. According to the invention an inkjet printer can operate at a rate much higher than is required for the meniscus of the ink to settle down, which is substantially the maximum possible rates of conventional systems such as Tacklind's. The thermal inkjet printer of the present invention can operate in a spray-mode. In this spray-mode operation, the inkjet printer does not require the meniscus of the ink to settle down before ejecting another droplet. Thus, the inkjet printer can operate at a rate much higher than conventional inkjet printers. Another benefit is that although the printer can operate at a much higher rate than conventional inkjet printers, it can use substantially the same hardware in the conventional printers. The printhead has a heating element, ink and an opening through which the ink is ejected from the printhead towards a printing surface. The heating element, typically a resistor, is energized by electrical power pulses. Each pulse superheats the ink in the immediate vicinity of the resistor. The superheated ink vaporizes and generates a short-lived (less than about 1 micro second) pressure pulse, which is substantially in excess of one atmosphere. This pressure pulse delivers momentum to the ink, pushing the ink away from the resistor so that a void or bubble is created in the ink. The pressure in this void quickly (in about 2 microseconds) drops to the saturation pressure of the fluid. With ambient temperature much lower than the boiling point of the fluid, this saturation pressure is much less than one atmosphere. The external atmospheric pressure eventually causes the bubble to collapse and it is the collapse process which causes the droplet to separate from the bulk of the fluid and proceed away from the opening of the printhead. In the spray-mode, the heating element is sequentially energized at a firing rate by applying to the heating element a series of pulses of power, such as current pulses. Each pulse causes a void in the ink and projects a droplet from the printhead toward the printing surface. Several droplets are typically fired into a single pixel of the printing surface; the number of droplets needed to achieve any given degree of grayness in the pixel is determined by conventional pre-calibration techniques. In the spray-mode, at least one of the droplets is deliberately projected onto the printing surface in a direction that is not perpendicular to the printing surface. The firing rate can be at or greater than 50 kHz, and may even be as high as 70 kHz or higher. The firing rate will depend in part on the viscosity of the chosen ink. The printer makes possible the use of ink with a viscosity of 10 centi-Poise or less, and even 2 centi-Poise or less. The inkjet printer according to the invention can operate in a dual-mode. In a preferred operation, a digital representation of an image to be printed is divided into non-text image fields and text fields. Each non-text image field is printed on the printing surface by projecting the corresponding ink droplets in the spray-mode. Each text field is printed on the printing surface in a text-mode, in which the firing rate is typically reduced to 5 - 10 kHz and the corresponding ink droplets are projected onto the printing surface in a direction that is substantially perpendicular to the printing surface. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figures 1A-1F illustrate the firing of a single droplet onto a printing surface using a conventional thermal inkjet printhead. Figures 2A-2D illustrate the manner in which conventional inkjet printers create pixels with different optical densities by increasing the number of well-placed. uniform ink drops within each pixel. Figures 3A-3D illustrate the manner in which multidrop inkjet printers such as Tacklind's create pixels with different optical densities. Figures 4A-4F illustrate the manner in which the inkjet printer fires ink drops onto the printing surface in the spray mode. Figures 5A-5D illustrate the way in which the inkjet printer creates pixels with different optical densities. Figure 6 is a block diagram of some of the logical electrical components of a thermal inkjet printer. Figures 1A-1F, and 4A-4F, are cross-sections through the respective printheads taken along a plane that is perpendicular to the printing surface and to a plane defined by an ejection opening of the printhead. Figure 1A illustrates the general structure of a conventional thermal inkjet printhead. The printhead 100 fires ink droplets into pixels on a printing surface 102. Printing fluid such as ink 104 is contained within a housing 106, which will typically consist of a portion of a multi-layered construction. The housing 106 thus holds a reservoir of the ink 104. A printhead 100 has an opening 107, through which drops are ejected roughly straight towards the printing surface 102. In a stable or initial state, the ink 104 will have a meniscus 108 that forms across the opening 107. In conventional printheads, the desired trajectory for the ink drops is along a line 109, which extends from a center point of the opening 107 perpendicular to the meniscus 108 when it is at rest, and also preferably perpendicular to the printing surface 102. For thermal inkjet printers, a heating element 110. which is typically a resistor, is mounted opposite the opening 107 and is controlled and driven by conventional circuitry. Conventional heating elements are commonly energized using current pulses with an amplitude on the order of one Ampere and a duration on the order of a few microseconds. Typical heating elements can superheat the ink in the immediate vicinity of the elements to a temperature much higher than the ink's boiling temperature, with a maximum cycle time on the order of 80 kHz. As is discussed both above and below, however, conventional printheads are not able to utilize the full 80 kHz potential of the heating elements; furthermore, they would be even more poorly suited to use even higher speed heating elements that may become available in the future. Figure 1B illustrates what happens when the heating element 110 is energized by a pulse. The ink immediately adjacent to the heating element 110 is super-heated above its boiling temperature to create a pressure pulse so that a small void or "bubble" 112 forms within the printhead cavity. In this state, the direction that is perpendicular or normal to the meniscus 108 is also approximately perpendicular to the printing surface 102. This normal direction is indicated by the arrow N, and by the desired line of trajectory 109. In Figure 1C, the power to the heating element 110 has been turned off after the pulse. The atmospheric pressure forces the bubble 112 to collapse ejecting a drop 114 of ink through the opening 107. In Figure 1D, the drop 114 has separated from the remaining fluid and travels towards the printing surface 102 substantially in the normal direction N indicated in Figure 1B. Immediately upon separation of the drop 114, an empty space is left within the region of the printing fluid, so that a cross-section through the printhead would show the meniscus 108 extending from either side of the opening 107 and forming a generally concave, curved surface. Figure 1E shows the previously separated drop 114 having adhered to the printing surface 102. This figure also illustrates that, in returning to its stable, substantially planar state, the meniscus 108 will exhibit a number of resonance modes, that is, it will be uneven with uneven curvature until it has settled down to the minimum-energy shape shown in Figure 1A. During the settling time, however, there is no constant normal direction for the meniscus 108. For example, the normal direction for the approximate center of the meniscus shown in Figure 1E is indicated by the arrow N, but a drop fired in this direction would not be coincident with the previously fired drop 114. In Figure 1F, the meniscus has settled down somewhat compared with Figure 1E, but it is still not in its resting state, so that the normal direction N is still not perpendicular to the printing surface 102. In order to ensure accuracy and repeatability, therefore, known printheads must wait for the meniscus 108 to settle down to being approximately planar before the next drop is fired. Although, as Figures 1E and 1F show, the heating element may be energized to form a new bubble before the meniscus settles down completely, the conventional printhead can still not operate with the firing rate as high as the cycle time for the heating element 110. In great part, this is because of the need of conventional systems to allow the meniscus 108 to settle. Figures 2A-D illustrate the way in which conventional thermal inkjet printheads create pixels with increasing optical density. As the figures indicate, darker pixels are created simply by accurately placing a larger number of ink dots within the area of the pixel. One can see that accurate placement of the fired ink droplets is essential for ensuring the proper optical density for the pixel. As is pointed out above, however, this means that one must wait until the meniscus settles down before firing another drop. Another shortcoming of such systems is that an entire pixel can normally not be generated with a single pass of the printhead: rather, the printhead may require to make multiple passes over the pixels in order to give each its proper optical density. Figure 3 illustrates the way in which the multidrop Tacklind printhead creates pixels with different optical densities. In Figure 1C, the droplet 114 has not yet separated from the meniscus 108. In the Tacklind system, a second droplet is generated by energizing and de-energizing the heating element 110 before the first droplet has completely separated. A third droplet may also be generated before the second separates, and so on. With the droplets all travelling along the same direction towards the same area, the droplets collapse to a single dot when they reach the medium to be printed. A two-droplet drop will thereby cover more of the surface of the pixel than will a one-droplet drop. The Tacklind system changes the optical density of a pixel by increasing the number of droplets applied at the various points within the pixel. This is illustrated in Figures 3A-3D, in which each printed portion of the pixel in Figure 3B has more droplets per dot than does the pixel shown in Figure 3A, but fewer droplets per dot than in the pixel illustrated in Figure 3C. In Figure 3D, there are so many droplets per dot that they almost cover the entire pixel, that is, they provide almost the maximum optical density. When it comes to black-and-white printing, maximum optical density corresponds to the most black portion of the gray scale, whereas minimum optical density would correspond to no ink at all, that is, the most white portion of the gray scale. As before, however, the Tacklind printer still relies on accurate placement within the pixel of each drop applied to the printing surface. Most attempts to increase the printing speed of known inkjet printers involve improvements in the mechanical structure of the printer or the mechanical and chemical properties of the ink. Mechanical changes include those such as a smaller opening in the printhead (the meniscus can then settle faster and it becomes easier to fire droplets faster but still perpendicular to the printing surface), and faster printhead carriages. Improvements in inks are intended to produce inks with faster settling times or different heat absorption properties. All of these attempts are designed to increase the speed at which drops can be fired perpendicular to the printing surface. The described embodiments go against this conventional trend and achieve much higher firing rates using existing structures and inks, as Figures 4A-4F and 5A-5d illustrate. Figures 4A-4F illustrate the "spraying" method used in the present invention for high-speed inkjet printing. Figure 4A shows the printhead when the meniscus 108 is level and a bubble 112 has been formed by pulsing the heating element 110. The printing fluid that can be used typically has a lower viscosity than that used in conventional inkjet printers, but the situation illustrated in Figure 4A is generally the same as is illustrated in Figure 1B. As Figure 4B illustrates, a first drop 410 has been ejected from the printhead; the drop travels towards the printing surface 102 in the normal direction indicated by the arrow N in Figure 4A. If one were to fire a drop before the meniscus 108 settles down, the drop would in almost all cases not follow the same path as the previous drop 410. Rather, it would travel in a direction normal to the meniscus, but since the meniscus itself is not at this point planar, the "normal" direction itself will be changing. The second drop will therefore travel at a different normal direction, which is indicated by the arrow N in Figure 4B. In other words, rather than firing drops in a constant normal direction, the printhead "sprays" drops onto the printing surface; although each drop will be ejected in a direction normal to some point of the meniscus, in general the drops will not travel in a direction that is normal to the printing surface itself. In Figure 4C, the new drop 412 is proceeding approximately in the direction N indicated in Figure 4B. The next time the bubble 112 collapses, yet another drop will be ejected in a different direction, which is normal to some point on the meniscus at the instant of ejection. This new direction is indicated as the new direction N shown in Figure 4C. In Figure 4D, this new drop 414 is proceeding in the direction shown in Figure 4C. Because of the high pulse rate of the heating element 110 according to the invention, however, in the situation shown in Figure 4D, a second firing cycle may have taken place before the drop 414 completely separates from the meniscus 108. Another droplet 416 may, as in the Tacklind System, combine on the medium as a single dot. In Figures 4E and 4F, an additional drop 418 is fired; the drop proceeds towards the printing surface 102 in a direction that is normal to the meniscus shown in Figure 4D. Figures 4A-4F illustrate several features used in the invention. First, it is not necessary to wait until the meniscus has settled before firing the next drop. This allows the use of inks with viscosity 10 centi-Poise or less, or even 2 centi-Poise or less. Prototypes have operated successfully with an ink viscosity in the range of 1 to 2 centi-Poise. The system may be used for either black-and-white or color printing, or both; the viscosity of the ink is one of the main factors in determining the maximum firing rate (and thus printing speed) for the invention. No special paper is required for using these inks. Yet another advantage of the relatively low-viscosity inks that the preferred system can use is that they allow for superimposition of printed images. Note, however, that the system can eliminate the need for multiple passes of the printhead in order to print a pixel since drops "sprayed" from the printhead will typically be distributed roughly randomly over the entire surface of the pixel to create any given, pre-calibrated optical density. Second, as Figure 4C illustrates, it is not necessary for the ejected drops to be all of the same size. Third, as Figure 4D illustrates, the system also allows for operation in a multi-drop mode, as in the Tacklind System. Indeed, the firing rate for the heating element 110 is substantially limited by the collapsing time for the bubbles that form in the low-viscosity ink. For inks with a viscosity in the range of 1-2 centi-Poise, the collapsing time for bubbles large enough to generate an inkjet is typically from about 14 to about 20 microseconds. This means that the preferred printhead can be fired with a rate at or above (indeed, well above) 50 kHz and even at a rate in the range of 70-80 kHz; this also allows the drop velocity to be in the range of 15-20 m/s. Figures 5A-5D illustrate the way in which the printhead creates pixels with different optical densities. In Figures 5A, the printhead has been operated well below its potential firing rate, that is, with a conventional firing rate that allows the meniscus to settle after one drop has been fired and before a subsequent drop is fired. As is discussed above, this rate may be as low as 10 kHz or lower, but can in general be determined using conventional calibration methods for any given ink viscosity. As Figure 5A shows, the printhead can place drops as accurately as conventional printheads simply by lowering the firing rate enough to allow the meniscus of the ink to settle between firings. In Figure 5B, the printhead has been operated with the fast firing rate that produces the sequence illustrated in Figures 4A-4F. Note that the average optical density of the pixel depicted in Figure 5B is at least roughly the same as that of the pixel depicted in Figure 5A, but that it will have been produced much faster. Figures 5C and 5D illustrate pixels with increasing optical densities that are roughly the same as the optical densities of the pixels illustrated in Figures 3C and 3D, respectively. Once again. although the optical densities can be the same, the system creates the pixels much faster, since it does not need the meniscus to become planar before firing the next drop. By comparing Figures 5A and 5B one can see that a slow firing rate with highly accurate drop placement (Figure 5A) in general produces more uniform drop patterns with sharper edges than does the high-speed firing method, even though the average optical densities may be the same. Such sharp edges are usually preferable for printing text, since non-uniform boundaries for ink patterns within text pixels make the letters of the text look "fuzzy." Consequently, the trend of most manufacturers of thermal inkjet printers is to attempt to fit more well-placed drops or "dots" in each unit area; this is normally expressed in terms of "dots per inch," or "dpi". Most of the highest quality conventional thermal inkjet printers now sold operate at between 300 dpi and 600 dpi for both text and non-text images. On the other hand, the well-defined drop pattern best suited for text will often make non-text images look "grainier" than necessary, since a viewer, at close distances, can often easily distinguish the individual dots (pixels) that make up the non-text image. The "random", non-uniform pattern of drops that the system can produce (see Figure 5B) is not as "crisp" as the pattern in Figure 5A, but the non-uniformity of the pattern will normally give non-text image (as opposed to text) pixels a much "smoother" appearance; indeed, the boundary between adjacent pixels will often not even be distinguishable. As a result, prototype non-text images produced using the preferred high-speed firing method at 300 dots-per-inch (dpi) were noticeably more life-like than the same images produced using conventional low-rate printing at 300 dpi. In a dual-mode operating method according to the invention the printer may be switched between a text-mode and a non-text image mode. In the text-mode, the printhead is fired at a low rate, that is, at the firing rates of conventional printheads. to place drops accurately with a uniform pattern (Figure 5A). The resolution of the printer can thereby be increased to that typical for high-quality text printing, which at present is roughly 600 dpi. If a printhead for 600 dpi is used, resolution can be decreased to 300 dpi for the non-text image mode in any known manner. Standard calibration methods (to determine optical densities as a function of activation periods) may be used for both the text and non-text image modes. The image mode is described above; the firing rate is increased to near the limit allowed by the cycle time of the heating element and the collapse rate of the bubbles. As is described above, for known inks. this will be above 50 kHz and even above 70 kHz. The firing rate of the printhead may be controlled by known hardware or software. Switching between the non-text image and text-modes can be done either manually or automatically. Manual switching can involve physically pressing a button, turning a selector, or using some other actuator that indicates to the printer or controlling processor which mode is to be used, or it can be done by entering a suitable command or parameter into the software with any standard input device such as a keyboard or mouse. Automatic switching can be accomplished in conjunction with known text-recognition software. Such software (found, for example, in document scanners) analyzes the bit or pixel pattern of a field and determines whether the field contains recognizable text features such as letters, punctuation marks, underlining, italics, and so on. This known software typically also determines the boundaries of text fields on a page. Since such software normally operates on a bitmapped, digitized (scanned), stored representation of the page, as opposed to on the page directly, it can just as well operate on a bit-mapped representation, stored in memory, of an image to be printed. When the printhead of the thermal inkjet printer is printing in a field of text, the software can set the printhead into the text-mode, in which the firing rate is reduced to a conventional rate that allows the meniscus to settle after each drop is fired (see Figure 5A). Over fields that the software interprets as non-text, however, the printhead can be switched into the spray-mode as described above, in which drops are fired faster than the time required for the meniscus to settle. This dual-mode operation will speed up printing of non-text images (and also soften the contours of the pixels) while still allowing sharp printing of text. Figure 6 is a block diagram of logical electrical components of a thermal inkjet printer 590. Some digital representations of the image to be printed on the printing surface 102 are stored as data in a memory 600. A control circuit 602 fetches printing parameters such as the firing rate, either from the memory 600, from some other conventional device, or from internal settings or calculations. Other parameters fetched are the non-text image and text data. Based on the parameters fetched, the control circuit sent control signals to a drive circuit 604, which pulses the heating element 110. Such data storage, control, and drive circuitry are well known for thermal inkjet printers. However, in the printer according to the invention, the drive circuit 604 can operate the printhead 100 in both the spray-mode to print non-text images, and in the text-mode to print text. Also, only one printhead is shown in the figures. In an actual printer, typically, there can be many more printheads, and the described system is applicable to each of them.

Claims: A method of printing using an ink-jet printer (590), which includes ink (104), a heating element (110) and a drive circuit operable to send power to the heating element (110), to heat the ink (104) and to eject droplets of ink (104) towards a printing surface (102), the method including the steps of: sending power, by the drive circuit (604), in pulses, at a firing rate, to the heating element (110) to heat the ink (104), with each pulse ejecting a droplet of ink (104) towards the printing surface (102); selecting a text mode or a spray mode of printing; the drive circuit (604) sending said pulses to the heating element (110) in the selected mode; wherein in the spray mode the firing rate of the pulses is intentionally set at a frequency in which drops are fired faster then the time required for the meniscus to settle, in order to eject a plurality of droplets in directions that are not perpendicular to the printing surface (102) so as to produce a random pattern of droplets on the printing surface (102), and in the text mode the firing rate of the pulses is set at a frequency in which the meniscus settles between the firing of droplets. A method as in claim 1, wherein in the spray mode said frequency of the pulses is at least 70 kHz. A method as in claim 1 or 2, wherein the ink (104) has a viscosity of less than 10 centi-Poise. A method as in claim 1,2 or 3, including the steps of: arranging a digital representation of an image to be printed into non-text image fields and text fields; printing each non-text image field on the printing surface (102) in the spray-mode; printing each text field on the printing surface (102) in the text-mode, in which the corresponding ink droplets are projected onto the printing surface (102) in a direction substantially perpendicular to the printing surface (102). A thermal inkjet printer comprising: ink (104); a heating element (110) thermally coupled to the ink (104); a drive circuit (604) operable to send power in pulses at a firing rate, to the heating element (110) to heat the ink (104), each pulse being operative to eject a droplet of ink (104) towards a printing surface (102); wherein the drive circuit (604) can operate in both a text mode and a spray mode of printing and can send pulses to the heating element (110) in the selected mode; means for selecting said mode; in which in the spray mode the drive circuit (604) is operable to send pulses to the heating element at a firing rate intentionally set at a frequency in which droplets are fired faster than the time required for the meniscus to settle, in order to eject a plurality of droplets in directions that are not perpendicular to the printing surface (102) so as to produce a random pattern of droplets on the printing surface (102), and in which in the text mode the drive circuit (604) is operable to fire pulses at a frequency at which the meniscus settles between the firing of droplets. A thermal inkjet printer as in claim 5, wherein the driving circuit is operable in the spray mode to send pulses at a frequency of at least 70 kHz. A thermal inkjet printer as in claim 5 or 6, wherein the ink (104) has a viscosity of less than 10 centi-Poise. A thermal inkjet printer as in claim 5,6 or 7 wherein the drive circuit (604) is operable in the text-mode to send pulses to the heating element (110) such that the corresponding ink droplets are projected onto the printing surface (102) in a direction substantially perpendicular to the printing surface (102).

Inventors: MEYER JOHN D, TAUB HOWARD H, MEYER, JOHN D., TAUB, HOWARD H.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1996
Main classification: B41J2
IPC codes: B41J2, G01D15