Patent Application
20050179716
The invention relates generally to the field of digitally controlled printing devices, and more specifically to an apparatus and method for controlling temperature profiles in ejection mechanisms of these devices.
The state of the art of inkjet printing technologies is relatively well developed. A wide variety of inkjet printing devices are available for commercial purchase from consumer desktop printers that produce general documents to commercial wide format printers that produce huge photographic quality posters. Hewlett-Packard Company of Palo Alto Calif., for example, has been particularly active in the development of thermal inkjet printing devices.
A thermal inkjet printer typically comprises a transitionally reciprocating printhead that is fed by a source of ink to produce an image-wise pattern upon some type of receiver. Such printheads are comprised of an array of nozzles through which droplets of ink are ejected by the rapid heating of a volume of ink that resides in a chamber behind a given nozzle. This heating is accomplished through the use of a heater resistor that is positioned within the print head in the vicinity of the nozzle. The heater resistor is driven by an electrical pulse that creates a precise vapor bubble that expands with time to eject a droplet of ink from the nozzle. After the drop is ejected and the electrical pulse is terminated, the ink chamber refills and is ready to further eject additional droplets when the heater resistor is again energized.
The quality of an ejected droplet from a thermal inkjet printhead is dependent upon the precision of the vapor bubble that is produced by the heater resistor, and is therefore also dependent upon how evenly the heater resistor produces heat. Since it is also desirable to shape heater resistors to better control the quality of the ejected droplet, physical characteristics such as current crowding become an issue. Since electrical current will always follow the shortest path, current will crowd and produce more heat in the shorter path when there is both a shorter and a longer path for the current to flow within a particular structure.
U.S. Pat. No. 6,367,147 issued to Giere et al. teaches that multiple heater resistors that are disposed at various angles to one another require coupling devices to connect the resistors and thus turn the current from one heater resistor to another. Since these coupling devices incorporate both short and long paths in which current will flow, the coupling devices must incorporate a compensation resistor to correct the flow of current in a manner that will force the current to flow evenly within the coupling device. In Giere et al., a segmented heater resistor includes a first heater resistor segment and a second heater resistor segment. The coupling device provides serial coupling from the first resistor segment to the second resistor segment with the compensation resistance reducing current crowding within the coupling device.
Heater resistors that are connected in various series and parallel combinations are also subject to the current crowding effect, unless they provide equal paths for the flow of current. In the case where the current paths are not equal, some form of coupler must afford any change of angle of one resistor to another. In this case, the coupler will exhibit uneven heating through the current crowding effect, and compensation resistance within the coupler must be employed. The use of compensation resistors is complicated, costly and expensive. Additionally they produce a voltage drop within the coupler, causing drive voltage inefficiencies. The present invention is directed to overcoming one or more of the problems set forth above.
According to one aspect of the present invention, a heater includes a first resistor segment having an electrical resistivity and a second resistor segment. A coupling segment is positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical resistivity, wherein the ratio of the resistivity of the coupling segment to the resistivity of the first resistor segment is substantially zero.
According to another aspect of the present invention, a printhead includes a nozzle and a drop forming mechanism positioned about the nozzle. The drop forming mechanism includes a first resistor segment having an electrical resistivity, a second resistor segment, and a coupling segment positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical resistivity, wherein the ratio of the resistivity of the coupling segment to the resistivity of the first resistor segment is substantially zero.
According to another aspect of the present invention, a heater includes a first resistor segment having an electrical conductivity and a second resistor segment. A coupling segment is positioned between the first resistor segment and the second resistor segment. The coupling segment has an electrical conductivity, wherein the electrical conductivity of the coupling segment is greater than the electrical conductivity of the first resistor segment.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
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. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements.
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Higher resistance heater resistors are generally desirable for thermal inkjet applications, to minimize the voltage drops of the electrical feed lines that supply current to the inkjet heater assemblies. However, the use of higher resistances in the inkjet heater resistors that minimize these drops also tend to produce more undesirable heat in the areas that experience current crowding.
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The resistor element(s) 130 and/or the coupling segments 120 can also be constructed from polysilicon that have high and low resistivity regions. Through doping (or the addition of impurities) the resistivity of polysilicon can be varied from about 800 micro-ohms per centimeter to 80,000 micro-ohms per centimeter. This is enough, for example, to obtain a 100 to 1 ratio in resistivity. This is accomplished by doping the polysilicon lightly in a first region thus creating a region of high resistivity, and doping the polysilicon heavily in a second region thus creating a region of low resistivity. Dopants that are suitable for such purposes are elements such as Phosphorus, Boron or Arsenic. By doping the coupling segment 120 heavily and then doping the upper resistor arm 140 and lower resistor arm 150 less heavily, favorable heating profiles such as discussed above are also achieved.
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The same is true for the conductivity ratio of the coupling segment 120 as compared to the conductivity of the materials used to produce the individual heater resistor elements 130. In this sense, current crowding still exists but the conductivity of coupling segment 120, as compared to the conductivity of the resistor element 130, is so high that little or no heat is generated within coupling segment 120. Although one example embodiment discloses that the conductivity of the coupling segment 120 is in the order of at least 100 times greater than the conductivity of the materials used to produce the individual heater resistor element 130, other conductivity ratios will work depending on the specific application contemplated. Example conductivity ratios include ratios greater than 100×.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
