Abstract:
A novel structure for a bubble-jet type ink-jet printhead is provided. A substrate is covered with a nozzle plate perforated by a predetermined number of nozzle holes a predetermined distance from said nozzle plate. The structure is surrounded by walls, within which form a common ink chamber. Each nozzle hole has, on the substrate underneath, a set of resistive elements. One of the resistive elements encircles an edge of a nozzle hole while another lyes directly underneath the perforation. During operation of the printhead, the encircling elements form a doughnut-shaped bubble forming an imaginary or virtual chamber within the doughnut from the rest of the common chamber. After formation of the doughnut-shaped bubble, the resister underneath the perforation forms a big bubble which causes ink to be ejected through the nozzle hole. The structure that allows for the above is easy to manufacture, and produces high quality print.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. § 119 from my application entitled BUBBLE-JET TYPE INK-JET PRINT HEAD WITH DOUBLE HEATER filed with the Korean Industrial Property Office on Mar. 15, 2001 and there duly assigned Ser. No. 2001-13452. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink-jet printhead, and more particularly, to a bubble-jet type ink-jet printhead having an improved heater for forming bubbles. 
     2. Description of the Related Art 
     The ink ejection mechanisms of an ink-jet printer are largely categorized into two types: an electro-thermal transducer type (bubble-jet type) in which a heat source is employed to form a bubble in ink causing ink droplets to be ejected, and an electromechanical transducer type in which a piezoelectric crystal bends to change the volume of ink causing ink droplets to be expelled. 
     Meanwhile, an ink-jet printhead having this bubble-jet type ink ejector needs to meet the following conditions. First, a simplified manufacturing procedure, low manufacturing cost, and high volume production must be allowed. Second, to produce high quality color images, creation of minute satellite droplets that trail ejected main droplets must be prevented. Third, when ink is ejected from one nozzle or ink refills an ink chamber after ink ejection, cross-talk with adjacent nozzles from which no ink is ejected must be prevented. To this end, a back flow of ink in the opposite direction of a nozzle must be avoided during ink ejection. Fourth, for a high speed print, a cycle beginning with ink ejection and ending with ink refill must be as short as possible. Fifth, a nozzle and an ink channel for introducing ink into the nozzle must not be clogged by particles or solidified ink. 
     However, the above conditions tend to conflict with one another, and furthermore, the performance of an ink-jet printhead is closely associated with structures of an ink chamber, an ink channel, and a heater, the type of formation and expansion of bubbles, and the relative size of each component. 
     In efforts to overcome problems related to the above requirements, ink-jet print heads having a variety of structures have been proposed. However, ink-jet printheads having the structures proposed may satisfy some of the aforementioned requirements but do not completely provide an improved ink-jet printing approach. Accordingly, it is highly desirable to have a bubble-jet type ink-jet printhead whose fabrication process is simplified without a decrease in the ejection energy of ink. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a bubble-jet type ink-jet printhead which improves ejection energy and eliminates the need for a separate ink chamber by connecting a plurality of heaters in parallel to form bubbles at predetermined time intervals. 
     Accordingly, to achieve the above object, the present invention provides a bubble-jet type ink jet printhead having a substrate, a nozzle plate having a plurality of nozzles, the nozzle plate being separated a predetermined distance from the substrate, walls for closing the space between the substrate and the nozzle plate and then forming a common chamber between the substrate and the nozzle plate a plurality of first resistive layers formed on the substrate within the common chamber corresponding to the plurality of nozzles, each of the plurality of first resistive layers being centered around the central axis passing through the center of each nozzle a plurality of second resistive layers disposed within the plurality of first resistive layers, wherein each second resistive layer is connected in parallel to each first resistive layer to thereby form a bubble on a central axis passing through the center of each nozzle a plurality of pairs of electrically conductive layers formed on the substrate, each pair being connected to the first and second resistive layers and extending to the outside of the common chamber; and a plurality of electrode pads which are disposed at the outside of the common chamber on the substrate and electrically connected to the electrically conductive layers. 
     Preferably, the second resistive layer has resistance greater than the first resistive layer, and the second resistive layer is longer and narrower than the first resistive layer. Preferably, ink feed grooves are formed at two opposite ends of the common chamber on the substrate for supplying ink to the common chamber or an ink feed groove is formed at the center of the substrate for supplying ink to the common chamber. 
     Preferably, a boundary barrier is provided for dividing the common chamber into a plurality of regions and allowing ink to flow from one region to another by spatially connecting the plurality of regions disposed within the common chamber, wherein the boundary barrier has a height equal to the gap between the substrate and the nozzle plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIGS. 1A and 1B are cross-sections showing the structure of a bubble-jet type ink-jet printhead along with an ink ejection mechanism; 
     FIG. 2 is a partial perspective view of a bubble-jet type ink-jet printhead; 
     FIG. 3 is a partial cross-section of another bubble-jet type ink-jet printhead; 
     FIG. 4 is a partial cross-section of another bubble-jet type ink-jet printhead; 
     FIG. 5 is an exploded perspective view showing the schematic structure of an ink-jet cartridge, to which a bubble-jet type ink-jet printhead according to a first embodiment of the present invention is applied; 
     FIG. 6 is a plan view showing the structure of a bubble-jet type ink-jet printhead according to a first embodiment of the present invention; 
     FIG. 7 is a cross-section taken along line  7 - 7 ′ of FIG. 6; 
     FIG. 8A shows an electrical connection structure of a resistive layer according to a first embodiment of the present invention; 
     FIG. 8B is a graph of an electric energy on each resistive layer according to a first embodiment of the present invention; 
     FIGS. 9A-9D are schematic cross-sections showing steps of formation of bubbles and ejection of an ink droplet according to a first embodiment of the present invention; 
     FIG. 10 is a schematic plan view of the bubble-jet type ink-jet printhead according to the first embodiment of the present invention of FIG. 5; 
     FIG. 11 is a cross-section taken along line  11 - 11 ′ of FIG. 10; 
     FIG. 12 is a cross-section taken along line  12 - 12 ′ of FIG. 10; 
     FIG. 13 is a schematic plan view of a bubble-jet type ink-jet printhead according to a second embodiment of the present invention; 
     FIG. 14 is a schematic plan view of a bubble-jet type ink-jet printhead according to a third embodiment of the present invention; 
     FIG. 15 is a cross-section taken along line  15 - 15 ′ of FIG. 14; 
     FIG. 16 is schematic plan view of a bubble-jet type ink-jet printhead according to a fourth embodiment of the present invention; and 
     FIG. 17 illustrates an alternative design of the resistive heater elements that can be applied to the first through fourth embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A and 1B, a bubble-jet type ink ejection mechanism will now be described. When a current pulse is applied to a first heater  12  consisting of resistive heating elements formed in an ink channel  10  where a nozzle  11  is formed, heat generated by the first heater  12  boils ink  14  to form a bubble  15  within the ink channel  10 , which causes an ink droplet I to be ejected. 
     In FIGS. 1A and 1B, a second heater  13  is provided so as to prevent a back flow of the ink  14 . First, the second heater  13  generates heat, which causes a bubble  16  to shut off the ink channel  10  behind the first heater  12 . Then, the first heater  12  generates heat and the bubble  15  expands to cause the ink droplet I to be ejected. 
     FIG. 2 is a perspective view showing a part of an ink-jet printhead disclosed in U.S. Pat. No. 4,882,595. Referring to FIG. 2, a rectangular heater  26  is formed on a substrate  20 . A chamber  25  for providing a space for the heater  26 , and an intermediate layer  24  for forming an ink channel  27  for introducing ink into the chamber  25  are provided. A nozzle plate  21  having a nozzle  22  corresponding to the chamber  25  is disposed on the intermediate layer  24 . Ink is filled in the chamber  25  through the ink channel  27  and in the nozzle  22  connected to the chamber  25 . In the ink-jet printhead having the above structure, since the chamber  25  delimited by the intermediate layer  24  is limited by the ink channel  27  through which ink is supplied only in one direction, ink refills the chamber  25  at low speed. Thus, the ink-jet printhead has the restriction of ejection driving frequency. 
     To overcome the above problem, an ink-jet printhead having a structure shown in FIG. 3 has been proposed. Referring to FIG. 3, a round-shaped heater  36  is formed on a substrate  30 , and adjacent nozzles  32  are interconnected by a common chamber  34  instead of an independent chamber as shown in FIG.  2 . Thus, if power is applied to the round-shaped heater  36  to generate heat, a plurality of bubbles  37  are formed by the round-shaped heater  36 . In this case, the plurality of bubbles  37  form an imaginary (or virtual) ink chamber  35 . Ink I is filled in the imaginary ink chamber  35 . Then, the plurality of bubbles  37  expand and coalesce to form a larger bubble. The expansion energy of the bubbles  37  causes an ink droplet  38  to be ejected from the nozzle  32 . 
     The ink-jet printhead having the structure as described above can be improved to eliminate the need for a complicated manufacturing process caused by formation of an ink chamber in the ink-jet printhead of FIG.  2  and the reliability of products. However, the ink-jet printhead of FIG. 3 can further be improved as FIG. 3 relies entirely on ink ejection energy caused by the expansion of bubbles  37  formed around the perimeter of the imaginary (or virtual) ink chamber  35  and not on the expansion of a bubble formed within the imaginary ink chamber  35 . 
     To solve the above problem, an ink-jet printhead having a structure as shown in FIG. 4 has been proposed. Referring to FIG. 4, a hemispherical shape is formed on a substrate  40 , in which a heater  45  having a hemispherical shape is disposed. The heater  45  generates heat to grow bubbles  47  formed on a flange  46  of the heater  45  further to form a barrier and expand bubbles  48  around the hemispherical shape of the heater  45 , thereby causing an ink droplet  49  to be ejected from the nozzle  42 . Thus, the structure illustrated in FIG. 4 allows for the formation of a virtual (or imaginary) ink chamber  43  caused by doughnut shaped bubble  47  located beneath the periphery of nozzle hole  42 , but also on the driving force of bubbles  48  generated by heater  46  located within the virtual ink chamber  43 , leading to a more effective ink ejection with high ejection energy and slim possibility of forming satellite droplets after ink droplet  49  is expelled. 
     The ink-jet printhead having the structure as described above is constructed such that the ink droplet  49  is ejected by the bubbles  48  generated by the hemispherical heater  45 , thereby increasing ejection energy compared to the ink-jet printhead of FIG.  3 . However, since a hemispherical shape is formed on a substrate, the fabrication process is complicated and thus the manufacturing cost is high. What is needed is a structure that is both simple and inexpensive to manufacture but maintains all the benefits of the structure of FIG.  4 : the formation of a virtual chamber by a doughnut shaped bubble and the generation of bubbles within the virtual chamber  43  to further provide a driving force for the ejection of ink droplet  49 . 
     FIG. 5 illustrates an ink-jet printhead according to the present invention. Referring to FIG. 5, a head mount portion  301  is disposed at the upper center of a cartridge  300  for storing ink. A head  100  according to the present invention is inserted into the head mount portion  301 . The head  100  includes a substrate  102  and a nozzle plate  101 . Walls  103  having a predetermined height are arranged in parallel at regular intervals on the substrate  102 , and ink feed grooves  107  are formed at the center portions of both ends of the substrate  102  in the direction in which the walls  103  extend. The wall  103  separates the substrate  102  and the nozzle plate  101  by the predetermined height, between which a common chamber that will be described below is formed. A plurality of resistive layers  104  are disposed at the bottom of the common chamber. 
     Referring to FIGS. 6 and 7, each resistive layer  104  includes a first resistive layer  104   a  and a second layer  104   b . The first resistive layer  104   a  is centered around a central axis passing through the center of each nozzle  108  formed in the nozzle plate  101 . The second resistive layer  104   b  is connected in parallel to the inside of the first resistive layer  104   a . It is preferable that the second resistive layer  104   b  is narrower than the first resistive layer  104   a  and arranged in a long coil type. A plurality of electrically conductive layers  105  are connected to the resistive layers  104 , and the electrically conductive layers  105  extend to the outside of both walls  103 , where they are coupled to a plurality of pads  106 . 
     Turning to FIG. 5, each pad  106  on the substrate  102  contacts each terminal  201  disposed on a flexible printed circuit (FPC) board  200 . An opening  204  for penetrating the head  100  is also disposed on the FPC board  200 . Here, the pads disposed on the substrate  102  correspond one-to-one to the terminals  201  disposed on the FPC board  200 . Further, each terminal  201  on the FPC board  200  is connected to a corresponding contact terminal  203  through a wiring line  202 . When the cartridge  300  is mounted to a head transport device (not shown) of an ink-jet printer, each contact terminal  203  is in contact with each terminal (not shown) disposed in the head transport device. 
     Referring to FIG. 8A, which shows an electrical connection structure of the resistive layer  104  according to a first embodiment of the present invention, resistors R 1  and R 3  are portions of a circular or closed polygonal first resistive layer  104   a  and a resistor R 2  is the second resistive layer  104   b . Thus, voltages across the resistors R 1 , R 2  and R 3  are equal. 
     The second resistive layer  104   b  is narrower and longer than the first resistive layer  104   a . Other embodiments include having the second resistive layer made out of a material having a higher resistivity than the first resistive layer. In any case, the resistance in the second resistive layer  104   b  is larger than that in the first resistive layer  104   a . If a voltage is applied from the outside to the resistive layers  104   a  and  104   b , the power VI dissipated at the second resistive layer  104   b , which is the work performed per unit time, is less than the power VI′ dissipated at the first resistive layer  104   a , because P=VI and V=IR, therefore P=V 2 /R, and the resistance of the second resistive layer  104   b  is greater than the resistance of the first resistive layer  104   a , as shown in FIG.  8 B. 
     FIG. 8B graphically represents electric energy applied to each resistive layer  104   a  or  104   b  according to a first embodiment of the present invention. Power VI′ is delivered to the first resistive layer  104   a  and power VI is delivered to the second resistive layer  104   b . If electric energy Ev is required for each resistive layer  104   a  or  104   b  to form a big bubble, the time t 1  required for the first resistive layer  104   a  to receive Ev is shorter than the time t 2  required for the second resistive layer  104   b  to receive Ev, because power VI′ dissipated in the first resistive layer is greater than power VI dissipated in the second resistive layer  104   b , as shown in FIG.  8 B. As described above, an important feature of this invention is that the resistances of the first and second resistive layers  104   a  and  104   b  are made to be different from each other. This is intended to make the time at which a big bubble is formed at each resistive layer  104   a  or  104   b  different. 
     A process of forming bubbles and ejecting an ink droplet in the bubble-jet-type ink-jet printhead according to the first embodiment of the present invention constructed as above will now be described with reference to FIGS. 9A-9D. Firstly, a common chamber  109  is filled with ink  110  in a state in which the first and second resistive layers  104   a  and  104   b  are electrically unloaded (refer to FIG.  9 A). Next, bubbles  111  and  112  are formed by the first and second resistive layers  104   a  and  104   b , respectively, to which a DC pulse is applied. In this case, since the resistance of the first resistive layer  104   a  is less than that of the second resistive layer  104   b , a larger amount of current flows through the first resistive layer  104   a . As a result, the bubble  111  formed on the first resistive layer  104   a  is larger than the bubble  112  formed on the second resistive layer  104   b . If the bubble  111  formed on the first resistive layer  104   a  continues to grow to completely fill the space between the substrate  102  and the nozzle plate  101 , the bubble  111  forms an isolated virtual chamber  113  having a doughnut shape within the common chamber  109 . Here, since a small size of the bubble  112  is formed on the second resistive layer  104   b  as well, the bubbles  111  and  112  formed on the first and second resistive layers  104   a  and  104   b , respectively, exert expansion energy on the ink  110  thus pushing a small amount of ink droplet  114  outward the corresponding nozzle  108  (refer to FIG.  9 B). 
     As time progress, the bubbles  111  and  112  become larger, and when the bubble  112  reaches a large volume as shown in FIG. 9C, the ink droplet  114  is ejected from the nozzle  108  by the expansion of the bubbles  111  and  112 , the main ejection force being generated by the expansion of the bubble  112 . 
     After ejection of the ink droplet  114  through the nozzle  108 , the bubbles  111  and  112  shrink as shown in FIG. 9D, and the ink  110  begins to refill, which returns to the state shown in FIG.  9 A. The shrinkage of the bubbles  111  and  112  is attributed to the cooling of the first and second resistive layers  104   a  and  104   b  due to the cutoff of the DC pulse. According to the above embodiment, the virtual chamber formed by the bubble  111  spatially separates the ink  110  to be ejected through the nozzle  108 . The tail of the ink droplet ejected by the maximum growth of the bubble  112  in the virtual chamber is cut off to prevent the formation of a satellite droplet. 
     FIG. 10 is a schematic plan view of the bubble-jet type ink-jet printhead according to the first embodiment of the present invention of FIG.  5 . FIGS. 11 and 12 are schematic cross-sections taken along lines  11 - 11 ′ and  12 - 12 ′ of FIG. 10, respectively. Referring to FIGS. 10,  11 , and  12 , ink feed grooves  107  for supplying ink to be filled in the common chamber  109  are provided at either end of the substrate  102 . The opposite sides of the common chamber  109  are sealed by the wall  103  as shown in FIG.  11 . 
     Both ends of the common chamber  109  are sealed by a sealing portion (not shown) when the head ( 100  of FIG. 5) is inserted into the head mount portion ( 301  of FIG. 5) of the cartridge ( 300  of FIG. 5) for holding ink. The ink feed groove  107  is connected with the inside of the cartridge  300  for supplying ink. Thus, ink is introduced through the ink feed grooves  107  in the directions indicated by arrows shown in FIG. 12 to fill the common chamber  109 . 
     FIG. 13 is a schematic plan view of a bubble-jet type ink-jet printhead according to a second embodiment of the present invention. Here, the same reference numeral as shown in FIG. 10 represents the same element having the same function. Referring to FIG. 13, the basic configuration in this embodiment is the same as in the first embodiment. A difference is in the position at which an ink feed groove is formed. That is, an ink feed groove  113  is formed in parallel to the walls  103  in the shape of a long hole at the central portion of the substrate  102 . Both ends of the common chamber  109  are sealed by walls  114 . In this way, the ink feed groove  113  may be formed at various positions. 
     FIG. 14 is a schematic plan view of a bubble-jet type ink-jet printhead according to a third embodiment of the present invention. FIG. 15 is a schematic cross-section taken along line  15 - 15 ′ of FIG.  14 . Here, the same reference numeral as shown in FIG. 10 represents the same element having the same function. Referring to FIGS. 14 and 15, the basic configuration of an ink-jet printhead in this embodiment is the same as in the first embodiment. A plurality of square-shaped boundary barriers  116  are disposed at regular intervals between the resistive layers  104  on the substrate  102 , thereby providing a partitioned region for each resistive layer  104 . The height of the boundary barrier  116  is made equal to the gap between the substrate  102  and the nozzle plate  101 . The boundary barrier  116  is provided to prevent cross-talk between adjacent nozzles  108  due to pressure generated by bubble formation when bubbles are formed on the resistive layer  104  and to increase ink ejection efficiency at a corresponding nozzle  108  where ink ejection is attempted. 
     The structure for suppressing cross-talk as described above may be provided within a common chamber in various forms. A modified example for this structure is shown in FIG. 16, which depicts the fourth embodiment of the present invention. Referring to FIG. 16, a plurality of boundary barriers  118  formed in a rectangular shape with a predetermined length is disposed between the resistive layers  104  on the substrate  102 . The height of the boundary barrier  118  is equal to the gap between the substrate  102  and the nozzle plate  101 . 
     It can be appreciated that the first resistive layer can take on other shapes than just circular. FIG. 17 illustrates a structure of a bubble-jet type ink-jet printhead  150  having a hexagonal first resistive layer  154   a . The hexagonal first resistive layer can be employed in all four embodiments of the present invention. In addition, the first resistive layer may be any closed polygon and may be applied to all four embodiments of the present invention. 
     As described above, a bubble-jet type ink-jet printhead according to the present invention is constructed such that a big bubble is formed on each resistive layer with a predetermined time interval by connecting a plurality of resistors in parallel. Thus, this increases the ejection efficiency of ink droplet without an additional means. Furthermore, a boundary barrier is provided to prevent a back flow of ink thereby avoiding cross-talk between adjacent nozzles. In particular, ink refills the virtual chamber for each nozzle from every direction, thereby allowing for continuous high-speed ink ejection. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.