Abstract:
A print command is received and a short, single, electrical precursor pulse is generated to preheat the ink components near the surface of the heating element. The precursor pulse returns precipitated ink components near the resistor to solution. For best results, the precursor pulse occurs approximately 1.50 microseconds prior to the main drive pulse even though other times, such as 1.00 microsecond, can produce adequate results. The main drive pulse then occurs to print the required information.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to inkjet printing. More particularly, the present invention relates to changing the distribution characteristics of ink exiting the print head.  
         BACKGROUND OF THE INVENTION  
         [0002]    Thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a print head surface that is in proximity to a printable medium. The nozzles are arranged in the surface of the print head such that the expulsion of a droplet of ink from a predetermined number of nozzles relative to a particular position of the print medium results in the printing of a portion of a desired character or image. Controlled repositioning of the print medium and/or printhead and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles so that the selected firing of orifices can produce a multicolored image by the inkjet printer.  
           [0003]    Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature that exceeds the boiling point of the ink solvent. The heating creates a gas-phase bubble of ink. Each nozzle is coupled to a small, unique ink firing chamber filled with ink that has an individually addressable heating element thermally coupled to the ink. This heating element is typically a resistor. As the bubble nucleates and expands, it displaces a volume of ink that is forced out of the nozzle and deposited on the print medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of the ink feed channels.  
           [0004]    The superheat temperature of the ink is the temperature at which the liquid ink undergoes a phase change from a liquid state to a gaseous state. The inks used in typical thermal inkjet printers have a superheat temperature in the range of 250° 0  C. to 300° C.  
           [0005]    After the deactivation of the heater resistor and the expulsion of ink from the firing chamber, ink flows back into the firing chamber to fill the volume vacated by the ink that was expelled. It is desirable to have the ink refill the chamber as quickly as possible to enable rapid firing of the nozzles of the print head. The faster the nozzles can fire, the faster the print speed that can be obtained.  
           [0006]    Inks used in these types of print heads must have certain desirable characteristics. For example, inks that have a high decel characteristic cannot be used. Decel is the loss of drop velocity and weight that occurs during high speed firing of the print head. Inks that have a high decel reduce the print quality because of the misdirection and low drop weight of the ink drops. This is one cause of banding in the print output as well as other quality problems.  
           [0007]    Various methods for improving the ink decel characteristic have been tried. For example, adding or removing components have shown improvements. However, changing the ink formulation in this manner compromises desirable ink properties such as ink stability and performance. Limited selections of inks, therefore, are available for inkjet use. There is a resulting unforeseen need to be able to use a larger range of inks in inkjet printers while still producing a high quality print output.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention encompasses a process for reducing a decel characteristic of ink. The ink is part of an inkjet pen device that has a print head. The print head comprises heating elements that are coupled to the ink such that the electrical heating of the heating element causes the ink&#39;s temperature to increase.  
           [0009]    When the printer receives a print command from a computer apparatus, the printer&#39;s controller generates a single, short, electrical pulse to the heating element. In the preferred embodiment, this pulse is less than 1 microsecond. In one embodiment, the pulse is in the range of 0.20- 0.60 microseconds and occurs more than 1.00 microsecond prior to the product main drive pulse. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 shows a cross-sectional view of a thermal inkjet pen incorporating the print head of the present invention.  
         [0011]    [0011]FIG. 2 shows an isometric view of an inkjet print head of the present invention.  
         [0012]    [0012]FIG. 3 shows a plot of a typical velocity versus time firing without the precursor pulses of the present invention.  
         [0013]    [0013]FIG. 4 shows a plot of a plot of velocity versus time firing with the precursor pulses in accordance with the present invention.  
         [0014]    [0014]FIG. 5 shows a flow chart of the precursor electrical pulse process of the present invention.  
         [0015]    [0015]FIG. 6 shows a block diagram of a printer system in accordance with the present invention.  
         [0016]    [0016]FIG. 7 shows an inkjet printer in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    The present invention provides a higher quality print output by reducing the decel characteristics of liquid inks. By applying a precursor pulse to the heating resistor, the ink decel can be reduced by 50- 90%.  
         [0018]    [0018]FIG. 1 illustrates a preferred embodiment thermal inkjet pen of the present invention. This inkjet pen includes a main unitary body housing ( 110 ) of a suitable plastic material. The housing ( 110 ) contains a reticulated polyurethane foam material ( 112 ) for storing the ink. The foam material ( 112 ) provides the necessary capillary backpressure at the print head of the pen to prevent ink from dripping out of the pen. This pen further includes an output or print head support section ( 114 ) that includes a small output opening ( 116 ) adjacent to which is mounted a thin film thermal inkjet print head ( 118 ). The print head support section ( 114 ) has interior walls ( 120  and  122 ) that define the contour of a large diameter standpipe and an air accumulating section ( 124 ).  
         [0019]    As seen in FIG. 1, the air accumulating section ( 124 ) of the standpipe is the upper portion thereof just beneath the wire mesh filter ( 126 ). Air accumulates in this section ( 124 ) when the pen is operating in the orientation shown in FIG. 1. As a result of the wire mesh screen ( 126 ), air bubbles entering the standpipe from the print head are trapped. The screen ( 126 ) prevents air in the foam ( 112 ) from being drawn down into the standpipe.  
         [0020]    The thermal inkjet pen also includes, in the preferred embodiment, an electrical connection ( 150 ). The electrical connection ( 150 ) provides the ability for signals from the inkjet printer in which the pen is place to communicate with the inkjet pen. Also in the preferred embodiment, the electrical connection ( 150 ) is comprised of a plurality of electrical contacts that enable the printer&#39;s controller to address each of the heating elements in the print head ( 1   18 ). This heating element addressing scheme is well known in the inkjet printing art and is not discussed further.  
         [0021]    The present invention is not limited to the type of inkjet pen illustrated in FIG. 1. Alternate embodiments of the inkjet pen can include other types of inkjet pens or cartridges. The present invention encompasses any inkjet pen comprising an inkjet print head having means for heating the ink with electrical pulses.  
         [0022]    A greatly magnified isometric view of a portion of the thermal inkjet print head ( 118 ) of the present invention is illustrated in FIG. 2. Several elements of the print head have been sectioned to reveal an ink-firing chamber ( 201 ) within the inkjet print head. Many such firing chambers are typically arranged in a group around an ink supply plenum for efficient and high quality printing. Additional groups may be located in the print head to allow for individual colors to be printed from each group.  
         [0023]    Associated with each firing chamber ( 201 ) is an orifice ( 203 ) or nozzle that is relative to the firing chamber ( 201 ) so that the ink, which is rapidly heated in the firing chamber by a heater resistor ( 209 ), is forcibly expelled as a droplet from the orifice ( 203 ). Part of a second orifice ( 205 ), associated with another ink firing chamber, is also shown. In the preferred embodiment, these orifices ( 203  and  205 ) are on the order of  50  μm, or less, in diameter.  
         [0024]    The heater resistors are selected by a microprocessor and associated circuitry in the printer, discussed in association with FIG. 5, in a pattern related to the data entered to the printer. The ink is expelled from the selected orifices to create a defined character or figure on the print medium.  
         [0025]    The print medium (not shown) is held parallel to the orifice plate ( 211 ) and perpendicular to the direction of the ink droplet expelled from the orifice ( 203 ). Ink is supplied to the firing chamber ( 201 ) via an opening ( 207 ) commonly referred to in the art as an ink feed channel. This ink is supplied to the ink feed channel ( 207 ) from a much larger ink reservoir, illustrated in FIG. 1, by way of an ink plenum that is common to all firing chambers in a group.  
         [0026]    Once the ink is in the firing chamber ( 201 ), it remains there until it is rapidly heated to boiling by the heater resistor ( 209 ). In the preferred embodiment, the heater resistor ( 209 ) is a thin film resistance structure disposed on the surface of a silicon substrate ( 213 ) and connected to electronic circuitry of the printer by way of conductors disposed on the substrate ( 213 ). The heater resistor placement is typically staggered in three or more parallel lines of heater resistor with adjacent heater resistors placed non-colinearly. Print heads having increased complexity typically have some portion of the electronic circuitry constructed in integrated circuit form on the silicon substrate ( 213 ). Various layers of protection such as passivation layers and cavitation barrier layers of protection may further cover the heater resistor ( 209 ) to protect it from corrosive and abrasive characteristics of the ink. Thus, the ink-firing chamber ( 201 ) is bounded on one side by the silicon substrate ( 213 ) with its heater resistor ( 209 ) and other layers and bounded on the other side by the orifice plate ( 211 ) with its attendant orifice ( 203 ). The other sides of the firing chamber ( 201 ), as well as the ink feed channel ( 207 ), are defined by a polymer barrier layer ( 215 ).  
         [0027]    The barrier layer ( 215 ), in the preferred embodiment, is made of an organic polymer plastic that is conventionally deposited upon the substrate ( 213 ) and its various protective layers. After deposition, the layer is photolithographically defined into desired geometric shapes and etched.  
         [0028]    Polymers suitable for the purpose of forming a barrier layer ( 215 ) include products sold under the names PARAD, VACREL, and RISTON by E.I. DuPont De Nemours and Co. of Wilmington, Del. Such materials can withstand temperatures as high as 300° C. and have good adhesive properties for holding the orifice plate of the print head in position. In the preferred embodiment, the barrier layer ( 215 ) has a thickness of about 19 to 30 μm after the print head is assembled with the orifice plate ( 211 ).  
         [0029]    The orifice plate ( 211 ) is secured to the silicon substrate ( 213 ) by the barrier layer ( 215 ). In the preferred embodiment, the orifice plate ( 211 ) is constructed of nickel with a plating of palladium (other cases: gold or rhodium) to resist the corrosive effects of the ink. The diameter of an orifice ( 203 ) in the orifice plate ( 211 ), in the preferred embodiment, is in the range of 25 to 52 μm.  
         [0030]    Most types of ink experience a phenomenon referred to in the art as decel. Decel is the loss of drop velocity and weight due to high frequency firing of the ink nozzles. This phenomenon typically occurs when the firing of the nozzles is continuous over a time less than one second and where the frequency of firing is greater than 5 kHz. The decel characteristics of different colors of ink are illustrated in FIG. 3.  
         [0031]    [0031]FIG. 3 is a plot of the velocity of the ink drop, in meters per second, versus the time of firing, in seconds. The three most commonly used inks for ink pens are shown. These colors are yellow ( 301 ), magenta ( 302 ), and cyan ( 303 ). It can be seen that there is almost no decel experienced by yellow ( 301 ). Magenta ( 302 ) experiences a small amount of decel. Cyan ( 303 ) experiences the greatest amount of decel. In fact, the velocity of the cyan drops is reduced to half of the initial velocity by 0.40 seconds after printing has started.  
         [0032]    The graph of FIG. 3 assumes a 9000 Hz rate of nozzle discharge and a 50° C. ambient print head temperature. Due to the heating method for firing the ink pen nozzles, the print head nozzles typically experience a higher ambient temperature than normal room temperature. Temperatures higher than 50° C. improve decel while lower temperatures make it worse.  
         [0033]    The process and ink pen apparatus of the present invention provides a precursor electrical pulse prior to the main drive pulse. The main drive pulse is an electrical pulse of 1.0-2.0μsec. duration. The main drive pulse occurs whenever a drop of ink is required to be fired from the inkjet pen&#39;s nozzles. The main drive pulse, in the preferred embodiment, has an amplitude of less than 5.0 VDC.  
         [0034]    Alternate embodiments use different amplitudes for the main drive pulse and the precursor pulse. Depending on the technology (e.g., CMOS, TTL) used to generate the pulses and the power supply, these voltages may be different from each other and/or different from 5.0 VDC.  
         [0035]    In the preferred embodiment, the precursor pulse duration is substantially in the range of 0.20- 0.60 μsec. and has an amplitude substantially the same as the main drive pulse. Also in the preferred embodiment, the precursor pulse occurs approximately 1.50 μsec. before each main drive pulse. The precursor pulse preheats the ink near the surface of the resistor to enable ink components that were shocked out of solution by previous main drive pulses to go back into solution just long enough to enable a full velocity firing.  
         [0036]    Alternate embodiments of the present invention use other values of pulse duration that are substantially close to the preferred embodiment range may reduce ink decel an appropriate amount to improve print quality. Also, a pulse that occurs substantially close to 1.00 μsec. before the product main drive pulse may provide enough of a decel improvement for adequate print quality.  
         [0037]    [0037]FIG. 4 illustrates a plot of the ink drop velocity versus time firing using the precursor pulse process of the present invention. The plot of FIG. 4 illustrates only the color cyan since it experienced the worst decel prior to the precursor pulse. This plot shows that the decel has been greatly reduced over the prior art method of nozzle firing.  
         [0038]    [0038]FIG. 5 illustrates a flowchart of the precursor pulse process of the present invention. The process starts by receiving a print command (step  501 ) from the computer that is coupled to the printer. The print command also includes the information to be printed by the inkjet printer.  
         [0039]    The inkjet printer&#39;s controller then controls the power provided to the print head in order to generate the precursor pulse (step  502 ) at least 1.50 μsec. prior to the main drive pulse. Next, the controller provides the necessary switching of the power to the print head in order to fire the appropriate print nozzles at the proper time in order to print the information from the computer.  
         [0040]    [0040]FIG. 6 illustrates a block diagram of the inkjet printer apparatus of the present invention. The inkjet printer is controlled by a controller logic ( 601 ) that uses the precursor pulse process of the present invention. This controller, in the preferred embodiment is a microprocessor such as a MOTOROLA 6800. Other types of microprocessors or microcontrollers are used in alternate embodiments. Additional embodiments use discrete logic or programmable logic that act as a controller.  
         [0041]    The controller logic ( 601 ) is coupled to a computer device such as an APPLE G3 or other such device. In alternate embodiments, the computer device can be a personal digital assistant or any other computer-like device that has the ability to send print commands.  
         [0042]    The controller logic ( 601 ) is coupled to memory such as ROM ( 605 ) and RAM ( 606 ). The ROM ( 605 ) stores the permanent instructions to be executed by the controller logic ( 601 ) in operating the inkjet pens as well as moving the inkjet pens and print medium to their respective proper locations in order to print a document as required by the computer attached to the printer apparatus. The RAM ( 606 ) is used to store the document or other information to be printed by the printer. The RAM ( 606 ) can also be used to store temporary data used by the controller. In alternate embodiments, the RAM ( 606 ) is not required as the memory in the computer device coupled to the inkjet printer apparatus is used to store the required information.  
         [0043]    A power supply ( 602 ) provides the required voltage levels for the controller ( 601 ) as well as the rest of the components of the inkjet printer apparatus. Different embodiments require different voltage levels.  
         [0044]    The controller logic ( 601 ) provides the addressing capability to access each resistor in the print head ( 603 ). The controller logic ( 601 ) also generates the required precursor and main drive pulses of the present invention. The timing of the precursor pulses and the main drive pulses are all controlled by the controller logic ( 601 ).  
         [0045]    [0045]FIG. 7 illustrates an inkjet printer that uses the inkjet pen and electrical precursor pulse of the present invention. The printer is comprised of a print medium holder ( 701 ) in which the print medium ( 702 ) rests. In one embodiment, the print medium is paper.  
         [0046]    One or more inkjet pens ( 705 ) that use the precursor pulse in the present invention are installed in an inkjet penholder ( 704 ). The inkjet penholder slides along a track ( 703 ) that is built into the printer for purposes of allowing the inkjet pen to print across substantially the entire width of the print medium.  
         [0047]    The inkjet pen may hold a single color, such as black, or multiple colors. These colors, in the preferred embodiment, include yellow, magenta, and cyan. Alternate embodiments use other colors and other combinations of colors.  
         [0048]    In the preferred embodiment, the inkjet printer of FIG. 7 is coupled to a computer. The computer provides the information that is to be printed. This information can be transmitted to the inkjet printer over a parallel or serial bus. In the preferred embodiment, the computer is a personal computer that runs an operating system such as MACINTOSH or WINDOWS. Alternate embodiments use other types of computers including personal digital assistants or mainframe computers.  
         [0049]    The printer&#39;s controller provides the commands to the print head to activate the inkjet pen&#39;s nozzles at the appropriate time to print the information on the desired medium. The printer&#39;s controller is also responsible, according to the process illustrated in FIG. 5, for controlling the precursor pulses to the print head in order to reduce the decel characteristic of the ink.  
         [0050]    In summary, the present invention reduces the decel characteristics of ink by generating a single, electrical, precursor pulse, with a duration of 0.20- 0.60 μsec. more than 1.00μsec. prior to the main drive pulse. This reduces the decel by 50- 90%. By reducing the decel in all inks, a wider range of inks can be used that were previously excluded due to high decel characteristics.  
         [0051]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.