Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-04-07
2001-07-03
Le, N. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06254225
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to a method of supplying power to a continuous ink jet printhead that maintains a proper directionality of a stream of droplets at the end of a printing operation.
BACKGROUND OF THE INVENTION
Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical presses, even though this conventional method requires very expensive set up and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems that are able to produce high quality color images at a high speed and low cost using standard paper.
Ink jet printing is a prominent contender in the digitally controlled electronic printing arena because, e.g., of its non-impact. low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop on demand ink jet. Continuous ink jet printing dates back to a least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
Conventional continuous ink jets utilize electrostatic charging tunnels that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a “catcher”) may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium.
A novel continuous ink jet printer is described and claimed in U.S. patent application Ser. No. 08/954,317 filed Oct. 17, 1997, now U.S. Pat. No. 6,079,821 and assigned to the Eastman Kodak Company. Such printers use asymmetric heating in lieu of electrostatic charging tunnels to deflect ink droplets toward desired locations on the recording medium. In this new device, a droplet generator formed from a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter is provided for each of the ink nozzle bores. Periodic actuation of the heater element via a train of uniform electrical power pulses creates an asymmetric application of heat to the stream of droplets to control the direction of the stream between a print direction and a non-print direction.
While such continuous ink jet printers have demonstrated many proven advantages over conventional ink jet printers utilizing electrostatic charging tunnels, the inventors have noted certain areas in which such printers may be improved. In particular, the inventors have noted that at the end of a printing operation, the next droplet or droplets directed toward the gutter may be directed toward the printing medium instead. While the cause of such droplet misdirection is not entirely understood, the applicants speculate that the principal cause is the non-instantaneous thermal response time of the heated portion of the nozzle to cool back to ambient temperature. Since the amount of the drop deflection is directly related to the temperature of the ink, and since the heated half of the ink jet nozzle does not cool instantaneously, applicants speculate that, after the end of a printing operation, the first ink droplet formed is misdirected away from the ink gutter and toward the printing medium due to the residual heat of the ink jet nozzle. Whether or not the second or third subsequent droplets are similarly misdirected is dependent upon the residual heat of the print head in the vicinity of the nozzles, the viscosity and thermal properties of the ink, and other thermal and fluid dynamic factors. Any such misdirected droplets can interfere with the objective of obtaining high image quality printing from such devices.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a continuous ink jet method of printing that maximizes print resolution by preventing the misdirection of ink droplets at the end of a printing operation.
It is another object of the present invention to provide a continuous ink jet printing method that prevents ink drop misdirection which may be used in a asymmetric heat-type printer without the need for making structural changes in such a printer.
Both of these objects are realized by the method of the invention, which generally comprises the step of applying a deflection correcting heat pulse from a second heating element that is disposed opposite to the first heating element after the first heating element generates its last operational heat pulse.
While the deflection correcting heat pulse may be of the same duration and magnitude as the operational heat pulses generated by the first heating element, the duration is preferably slightly longer in the preferred embodiment. The deflection correcting heat pulse is preferably generated at a time period that substantially corresponds to one wave length of the electrical pulse frequency, ±50%.
While the second heating element must generate at least one deflection correcting heat pulse after the first heating element has generated its last operational heat pulse, it is within the scope of the invention that the second heating element may subsequently generate a second and a third deflection correcting heat pulse.
The specific power level and frequency of the electrical pulses used to drive the first and second heating elements will vary with the particular model of printer. Typically, each of the heat generating electrical pulses may have a voltage of between 4 and 6 volts, and a current of 8 and 12 milliamps. Additionally, the period of pulse generation may be between 5 and 7 microseconds.
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Anagnostopoulos Constantine N.
Chwalek James M.
Jeanmaire David L.
Eastman Kodak Company
Le N.
Stevens Walter S.
Vo Anh T. N.
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