Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2001-03-21
2003-06-03
Vo, Anh T. N. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S047000
Reexamination Certificate
active
06572223
ABSTRACT:
This invention relates generally to the field of continuous ink jet print head design. More specifically, it relates to improving print resolution by redesigning the ink flow patterns emanating from printhead nozzles.
BACKGROUND OF THE PRIOR ART
Traditionally, digitally controlled ink jet printing capability is accomplished by one of two technologies. Typically, ink is fed through channels formed in a printhead. Each channel includes a nozzle from which ink drops are selectively ejected and deposited upon a medium.
The first technology, commonly referred to as “drop on demand” ink jet printing, provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a drop that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle.
The second technology, commonly referred to as “continuous stream” or “continuous” ink jet printing, uses a pressurized ink source which produces a continuous stream of ink drops. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink drops. The ink drops are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink drops are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When a print is desired, the ink drops are not deflected and are thereby allowed to strike a print media. Alternatively, deflected ink drops may be allowed to strike the print media, while non-deflected ink drops are collected in the ink capturing mechanism.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink drops from a filament of working fluid and deflect those ink drops. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink drops and non-printed ink drops. Printed ink drops flow along a printed ink drop path ultimately striking a print media, while non-printed ink drops flow along a non-printed ink drop path ultimately striking a catcher surface. Non-printed ink drops are recycled or disposed of through an ink removal channel formed in the catcher.
Traditionally, ink jet nozzles for both “drop on demand” and “continuous” ink jet printheads are formed in an array or row, often a linear array or row, and fixed in a single plane, the nozzles in a row being equally spaced. A row of nozzles is comprised of “end nozzles” (commonly referred to as end jets, etc.) which are nozzles at each end of the row, and “inner nozzles” positioned inside the end nozzles within the row. The ink streams and ink drops ejected from end nozzles and inner nozzles, respectively, are referred to as end streams and end drops and as inner streams and inner drops, respectively. As such, one would expect the pattern of printed ink drops
20
, printed on a recording medium
22
, to mirror the pattern of the nozzles of the linear array, as shown in
FIG. 1
a.
However, it has been observed that ink stream flow patterns of end nozzles are out-of-line or incongruent when compared to ink stream flow patterns of inner nozzles, resulting in a failure of the pattern of printed ink drops
20
, printed on a recording medium
22
, to mirror the pattern of the nozzles of the linear array. Referring to
FIG. 1
b
printed ink drops
21
, ejected from end nozzles, are printed on the recording medium at a location displaced perpendicularly relative to other printed ink drops
20
, ejected from inner nozzles. This perpendicular direction is commonly referred as a “fast scan” direction, since many commercial printers scan the printhead rapidly over a recording medium in this direction to print a pattern of drops known as an image swath. The reduction in ink drop placement accuracy degrades the printing performance of the end nozzles and of the printhead. Additionally, ink drop misplacement in the fast scan direction causes a reduction in overall image print quality.
It was theorized in the late 1970's and early 80's that this problem in print resolution stemmed from the fact that ink drops or ink streams ejected from end nozzles, positioned at an end of the nozzle array, were exposed to the ambient air, more so than ink drops or ink streams ejected from inner nozzles, positioned within the nozzle array. Ink ejected form end nozzles was thought to be subjected to aerodynamic drag, a force directed in a line along the trajectory of the ink drops but opposing their motion. This force reduced the velocity of streams of ink or ink drops ejected from end nozzles relative to the velocity of ink streams or ink drops ejected from inner nozzles. Thus, ink drops
21
ejected from end nozzles were caused to strike the print medium
22
at a later time than ink drops
20
ejected from inner nozzles. The resultant printed image of printed ink drops ejected from a linear array of nozzles was curved rather than in a straight line (see
FIG. 1
b
), as desired, thus creating image artifacts and reducing image resolution. Such aerodynamic drag could reduce resolution in all inkjet printers including drop on demand and continuous ink jet printers.
In order to improve print resolution, various efforts were directed toward compensating for the velocity reduction due to aerodynamic drag. A substantially uniform line of ink drops from all of the in-line nozzles of the multi-nozzle array, was desired, and it was reasoned the if end drops could be made to strike the recording medium at the desired location by compensation for drag, higher print resolution would result.
Methods for correcting the printed location of end drops have been disclosed in “Reducing Drop Misregistration from Differential Aerodynamic Retardation in a Linear Ink Jet Array,” IBM Technical Disclosure Bulletin, Volume 17, No. 10 by D. E. Fisher and D. L. Sipple as early as March of 1975. One correction method used control algorithms to vary the time of flight of drops from the nozzle to the recording medium and thus to cause an ink stream curvature opposite to that caused by the aerodynamic drag. A method set forth for correcting the effects of aerodynamic drag was to use a compensating velocity across the array. Alternatively, a decreased path length was found to similarly compensate.
U.S. Pat. No. 3,562,757, issued to Bischoff, corrected for drag on a drop-to-drop basis. Every other drop was guttered thereby increasing the distance between drops used for printing so that the all drops experienced some drag.
U.S. Pat. No. 3,596,275, issued to Sweet, disclosed use of an extraneous collinear stream of air with the stream of ink drops to reduce the effects aerodynamic drag. A fan, or the like, was necessary to generate the airflow.
U.S. Pat. No. 4,077,040, issued to Hendriks, reduced the effect of aerodynamic retardation or drag between streams by utilizing drop streams on the perimeter of the array which were never printed but instead continually guttered to produce a counter airflow tending to reduce retardation of drop streams emitted from the other nozzles.
U.S. Pat. No. 4,185,290, issued to Hoffman, caused each of the streams of drops ejected from end nozzles at each end of the array to have an initial velocity higher than the initial velocity of the streams of drops ejected form inner nozzles inside the end nozzles of the array, thereby compensating for the aerodynamic drag on ink streams at the end of the array. The higher initial velocity of drops e
Chwalek James M.
Delametter Christopher N.
Faisst, Jr. Charles F.
Hawkins Gilbert A.
Lee Yung-Rai R.
Eastman Kodak Company
Vo Anh T. N.
Zimmerli William R.
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