Abstract:
An ink drop ejecting printhead includes a substrate having an ink drop emitting opening; a heater on the substrate surrounding the opening; and an ink supply communicating with the opening to supply ink, whose surface tension decreases inversely with its temperature, to the opening under positive pressure relative to ambient. A variable-energy electrical power supply connected to the heater, whereby application of an electrical pulse of sufficient energy to the heater will cause separation of an associated ink drop from the ink supply. A power supply control is adapted to regulate the energy of electrical pulses applied to the heater from the power supply, whereby the volumes of separated ink drops are proportional to the energy of the associated electrical pulses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to Commonly assigned U.S. Pat. No. 5,880,759, entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM filed in the name of Kia Silverbrook on Dec. 3, 1996; and to Commonly assigned U.S. Pat. No. 6,022,099, entitled INK PRINTING WITH DROP SEPARATION filed in the names of James M. Chwalek and John A. Lebens on Jan. 21, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of digitally controlled printing devices, and in particular to liquid ink drop-on-demand printheads which integrate multiple nozzles on a single substrate and in which a poised liquid meniscus on a nozzle is expanded to a pre-determined volume and is separated for printing by thermal activation. Furthermore, the volume separated drop may be controlled to permit continuous toning and grayscale toning of images. 
     BACKGROUND OF THE INVENTION 
     Ink jet printing has become a preferred technology for the printing of color images. The term “ink jet” as used herein is intended to include all drop-on-demand or continuous ink jet propulsion systems including, but not limited to, bubble jet, thermal ink jet, piezoelectric and continuous. 
     There are two types of drop-on-demand ink jet printers that dominate the market. Drop-on-demand thermal ink jet printers operate by rapidly heating a small volume of ink, causing it to vaporize and expand, thereby ejecting ink through an orifice or nozzle and causing it to land on selected areas of a recording medium. The sequenced operation of an array of such orifices moving past a recording medium writes a dot pattern of ink on the recording medium, forming text or pictorial images. The print head typically includes an ink reservoir and channels to replenish the ink to the region in which vaporization occurs. An arrangement of thermal ink jet heaters, ink channels, and nozzles is disclosed in U.S. Pat. No. 4,882,595. 
     The drop-on-demand piezoelectric printers operate by using a separate piezoelectric transducer for each nozzle generating a pressure pulse to expel the drops. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Color rendition is accomplished by adding a few (typically three) color ink reservoirs and associated nozzles and ejection means so that dots of different colors may be overlaid on an appropriate recording medium. 
     The above methods as practiced suffer from drawbacks, notable the difficulty of achieving continuous tone (grayscale) color reproduction. One method used to provide continuous tone color reproduction, namely the deposition of multiple drops with the same volume onto a single image pixel, allows only a limited number of gray levels corresponding to the number of drops deposited. The volume of ink drops has also been controlled in piezoelectric drop-on-demand printers by varying the applied energy, such as by adjusting the pulse height or pulse width of the applied electrical signal. This method tends to allow only a small volume variation and exhibits a drop velocity variation with size, making placement difficult on the recording medium. 
     Commonly assigned co-pending U.S. patent applications Ser. No. 08/750,438 entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM filed in the name of Kia Silverbrook on Dec. 3, 1996, discloses a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. The invention provides a drop-on-demand printing mechanism wherein the means of selecting drops to be printed is by thermal reduction of the surface tension of the selected drop producing a difference in position between selected drops and drops which are not selected but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an additional means is provided to cause separation of the selected drops from the body of ink. To cause separation of the drop the system requires either proximity mode, for which the recording medium must be in close proximity to the orifice in order to separate the drop from the orifice, or the use of an electric field between recording medium and orifice. A drop volume adjustment can be made, for example in proximity mode, by altering the distance between print head and recording medium which requires increased system complexity. 
     Commonly assigned co-pending U.S. patent application Ser. No. 08/787,657 entitled INK PRINTING WITH DROP SEPARATION filed in the names of James M. Chwalek and John A. Lebens on Jan. 21, 1997, discloses a printing system where by the separation of the selected drop is achieved by controlled thermal activation propelling it to the recording medium without the need for proximity or an electric field. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to improve upon the above invention by demonstrating a new mode of operation. This mode, which was not previously predicted, permits control of the separated drop volume for continuous toning and gray scale toning of images. A volume change of at least a factor of 3 can be obtained with only a small change in drop velocity. 
     It is another object of the present invention to demonstrate a new mode of operation for a drop-on-demand printhead wherein electrothermal pulses applied to an annular heater located around the rim of a nozzle controls the expansion of a poised meniscus, the separation of the drop, and the volume of the separated drop, propelling it to the recording medium. Electrothermal pulses applied to selected nozzles heat the ink in those nozzles, altering material properties of the ink, including a reduction in the surface tension of the ink and causing it to expand past its initially poised state. Heating the ink adjacent to the heater surface to a temperature greater than its boiling point results in separation of the drop. By controlling the heating, a pre-determined drop volume may be delivered to the recording medium. This pre-determined drop volume may consist of more than one drop ejected as a result of a singly-applied electrothermal pulse. 
     According to a feature of the present invention, an ink drop ejecting printhead includes a substrate having an ink drop emitting opening; a heater on the substrate surrounding the opening; and an ink supply communicating with the opening to supply ink, whose surface tension decreases inversely with its temperature, to the opening under positive pressure relative to ambient. A variable-energy electrical power supply connected to the heater, whereby application of an electrical pulse of sufficient energy to the heater will cause separation of an associated ink drop from the ink supply. A power supply control is adapted to regulate the energy of electrical pulses applied to the heater from the power supply, whereby the volumes of separated ink drops are proportional to the energy of the associated electrical pulses. 
     According to another feature of the present invention, a process for ejecting ink from a printhead includes the steps of communicating an ink supply, whose surface tension decreases inversely with its temperature, with an ink-emitting opening to supply ink to the opening; applying positive pressure relative to ambient to the ink supply; adjustably applying pulses of heat to the ink at the opening of sufficient energy to cause separation of associated ink drops from the ink supply; and variably adjusting the applied heat pulse energy, whereby the volume of the separated ink drops are proportional to the energy of associated heat pulses. 
     The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
     FIG.  1 ( a ) shows a simplified block schematic diagram of one exemplary printing apparatus in which the present invention is useful. 
     FIG.  1 ( b ) shows a cross section of the nozzle tip in accordance with the present invention. 
     FIG.  1 ( c ) shows a top view of the nozzle tip in accordance with the present invention. 
     FIG. 2 shows a simplified block schematic diagram of the experimental setup used to test the present invention. 
     FIGS.  3 ( a ) to  3 ( e ) shows the variation in separated drop volume in accordance with the present invention. 
     FIG. 4 shows the relationship between heater pulse duration and separated drop volume in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     FIG.  1 ( a ) is a drawing of a drop-on-demand ink jet printer system utilizing the ink jet head with drop separation means. An image source  10  may be raster image data from a scanner or computer, or outline image data in the form of a page description language, or other forms of digital image representation. This image data is converted to half-toned bitmap image data by an image processing unit  12  which also stores the image data in memory. Heater control circuits  14  read data from the image memory and apply time-varying electrical pulses to the nozzle heaters that are part of a printhead  16 . These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops will form spots on a recording medium  18  in the appropriate position designated by the data in the image memory. Optimal operation refers to an operating state whereby ink drops are separated and ejected from one or more pressurized nozzle orifices by the application of electrical pulses to the heater surrounding the nozzle without the need for an external drop separation means. 
     Recording medium  18  is moved relative to printhead  16  by a recording medium transport system  20 , which is electronically controlled by a recording medium transport control system  22 , which in turn is controlled by a micro-controller  24 . A recording medium guide or platen  21  is shown directly below printhead  16 . The recording medium transport system shown in FIG.  1 ( a ) is schematic only, and many different mechanical configurations are possible. In an alternative embodiment, a transfer roller could be used in place of the recording medium transport system  20  to facilitate transfer of the ink drops to recording medium  18 . Such transfer roller technology is well known in the art. In the case of page width printheads, it is most convenient to move recording medium  18  past a stationary printhead  16 . However, in the case of scanning print systems, it is usually most convenient to move printhead  16  along one axis (the sub-scanning direction) and recording medium  18  along the orthogonal axis (the main scanning direction), in a relative raster motion. 
     Micro-controller  24  may also control an ink pressure regulator  26  and heater control circuits  14 . Ink is contained in an ink reservoir  28  under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. The ink pressure for optimal operation will depend mainly on the nozzle orifice diameter, surface properties (such as the degree of hydrophobicity) of a bore  46  and a rim  54  of the nozzle, surface tension of the ink, and power as well as temporal profile of the heater pulse. A constant ink pressure can be achieved by applying pressure to ink reservoir  28  under the control of ink pressure regulator  26 . Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in reservoir  28  an appropriate distance above printhead  16 . This ink level can be regulated by a simple float valve (not shown). The ink is distributed to the back surface of printhead  16  by an ink channel device  30 . The ink preferably flows through slots and/or holes etched through the silicon substrate of printhead  16  to the front surface, where the nozzles and heaters are situated. 
     FIG.  1 ( b ) is a detail enlargement of a cross-sectional view of a single nozzle tip of the drop-on-demand ink jet printhead  16  according to a preferred embodiment of the present invention. An ink delivery channel  40 , along with a plurality of nozzle bores  46  are etched in a substrate  42 , which is silicon in this example. In this example, delivery channel  40  and nozzle bore  46  were formed by anisotropic wet etching of silicon, using a p +  etch stop layer to form the shape of nozzle bore  46 . Ink  70  in delivery channel  40  is pressurized above atmospheric pressure, and forms a meniscus  60  which protrudes somewhat above nozzle rim  54 , at a point where the force of surface tension, which tends to hold the drop in, balances the force of the ink pressure, which tends to push the drop out. 
     In this example, the nozzle is of cylindrical form, with heater  50  forming an annulus. The heater is made of polysilicon doped at a level of about thirty ohms/square, although other resistive heater material could be used. Nozzle rim  54  is formed on top of heater  50  to provide a contact point for meniscus  60 . The width of the nozzle rim in this example is from about 0.6 μm to about 0.8 μm. Heater  50  is separated from substrate  42  by thermal and electrical insulating layers  56  to minimize heat loss to the substrate. 
     The layers in contact with the ink can be passivated with a thin film layer  64  for protection, which can also include a layer to improve wetting of the nozzle with the ink in order to improve refill time. The printhead surface can be coated with a hydrophobizing layer  68  to prevent accidental spread of the ink across the front of the printhead. The top of nozzle rim  54  may also be coated with a protective layer which could be either hydrophobic or hydrophillic. 
     FIG.  1 ( c ) is an enlargement of a top view of a single nozzle of drop-on-demand ink jet printhead  16  according to a preferred embodiment of the present invention. Nozzle rim  54  and heater annulus  50  located directly under nozzle rim  54  surrounds the periphery of nozzle bore  46 . A pair of power and ground connections  59  from the drive circuitry to heater annulus  50  are shown, and are fabricated to lie in the heater plane below the nozzle rim. 
     Heater control circuits  14  supply electrical power to the heater for a given time duration. Optimum operation provides a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature above the boiling point of the ink in an annular volume just to the ingress of the nozzle/heater interface for part of the duration of the heater pulse, and a rapid fall in temperature at the end of the heater pulse. The heater pulse controls the expansion of a poised meniscus, the separation of the drop, and the volume of the separated drop. The power and duration of the applied heater pulse that is necessary to accomplish this depends mainly on the geometry and thermal properties (such as thermal conductivity, specific heat, and density) of the materials in and around the heater including the thermal properties of the ink as well as the surface tension and viscosity of the ink. Thermal models can be used to guide the selection of geometrical parameters and materials as well as operating ranges of the ink pressure, heater pulse power and duration. It is recognized that a certain degree of experimentation may be necessary to achieve the optimal conditions for a given geometry. 
     For small drop sizes, gravitational force on the ink drop is very small; approximately 10 −4  of the surface tension forces, so gravity can be ignored in most cases. This allows printhead  16  and recording medium  18  to be oriented in any direction in relation to the local gravitational field. This is an important requirement for portable printers. 
     The ink jet head with drop separation means shown schematically in FIGS.  1 ( b ) and  1 ( c ) was fabricated as described above and experimentally tested. A schematic diagram of the experimental set up used to image drops emitted from printhead  16  is shown in FIG. 2. A CCD camera  80  connected to a computer  82  and printer  84  was used to record images of the drop at various delay times relative to the heating pulse. Printhead  16  was angled at thirty degrees from the horizontal so that the entire heater  50  could be viewed. Because of the reflective nature of the surface, a reflected image of the drop appeared together with the imaged drop. An ink reservoir and pressure control means  86 , shown as one unit, was included to poise the ink meniscus at a point below the threshold of ink release. A fast strobe  88  was used to freeze the image of the drop in motion. A heater power supply  90  was used to provide a current pulse to heater  50 . Strobe  88 , camera  80 , and heater power supply  90  may be synchronously triggered by a timing pulse generator  92 . In this way, the time delay between strobe  88  and heater power supply  90  may be set to capture the drop at various points during its formation. 
     EXPERIMENTAL RESULTS 
     A 16 μm diameter nozzle, fabricated as described above and shown schematically in FIGS.  1 ( b ) and  1 ( c ), was mounted in the test setup shown schematically in FIG.  2 . The nozzle reservoir was filled with de-ionized water and pressurized to a pressure of 13.2 kPa, below the measured critical pressure of 17.0 kPa. The nozzle contained a hydrophobizing Teflon® layer. FIG.  3 ( a ) is an image of a separated drop taken 220 μs after the start of a 70 μs duration, 115 mW electrical pulse applied to heater  50 . The application of the thermal energy to the de-ionized water in the nozzle has changed some of the physical properties of the de-ionized water, including decreasing its surface tension and viscosity. The result is a separated drop whose volume may be controlled by the electrical pulse applied to heater. Note that the image is taken at a tilt of thirty degrees from horizontal with a reflected image of the poised meniscus also appearing. For pulse durations below 65 μs separation of the de-ionized water from the nozzle did not occur. FIGS.  3 ( b )- 3 ( d ) are images of separated drops taken 220 μs after the start of 100, 130, and 160 μs duration, 115 mW electrical pulses applied to heater  50 . As can be seen from FIG. 3, the size and hence volume of the drop is increasing with the duration of the heater pulse. In FIG.  3 ( e ), two drops are separated with the application of a 200 μs duration electrical pulse. 
     In FIG. 4, the drop volume computed from the drop images of FIG. 3 is plotted against the duration of the electrical pulse applied to heater. The line through the data points is a linear least squares fit. As can be seen from FIG. 4, the drop volume is proportional to the duration of the electrical pulse applied to heater even when more than one drop is produced from a single electrical pulse. With the ability to control drop volume in such a manner, continuous toning and grayscale toning of images is possible. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.