Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-11-09
2002-01-08
Pham, Hai C. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S017000
Reexamination Certificate
active
06336696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to acoustic ink printing and, more particularly, to methods and means for improving the visual quality of images printed from a high-speed acoustic ink printhead by shifting temperature induced visual artifacts to a high spatial frequency beyond the visual acuity of humans. Portions of the ink contained within the printhead are selectively cooled using counter-flowing heat absorbing fluids and images are generated so that adjacent pixels on a page are produced from interlaced opposite transverse hot-to-cold and cold-to-hot ink well portions of the printhead.
2. Description of Related Art
In acoustic ink printing, an array of ejectors forming a printhead is covered by pools of liquid ink. Each ejector selectively directs a beam of sound energy against a free surface of the liquid ink. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of ink are ejected from the liquid surface to impact upon a target medium, such as a sheet of paper, to complete the printing process.
Typically, the ejectors are arranged in a linear array that is aligned perpendicular to the movement of the recording medium which receives the ejected ink droplets. Alternatively, the ejectors may be arranged in an array of rows and columns, with the rows stretching across the width of the recording medium and the columns of ejectors arranged approximately perpendicular along the movement of the printhead relative to the recording medium. Often, the columns of ejectors are not arranged exactly perpendicular to the ejector rows, but at oblique angles with the rows. In other words, the ejector rows of the array are staggered.
Each ejector for an acoustic ink printer must be supplied with ink and a good ink supply system should maintain a constant flow of ink to the ejectors. A flowing ink supply system cools the ink and stabilizes the ink temperature more easily. Additionally, the flowing ink supply system keeps the ink free of various contaminants, such as paper dust which might settle upon the free surfaces of the ink, by sweeping the contaminants away. The constantly flowing ink also maintains a fresh ink supply to the free surfaces. Without the constant flow of ink, the differing evaporation rates of the constituents within inks that contain volatile components may adversely affect the uniformity of the ink composition associated with each ejector and, therefore, would also affect the uniformity of performance of the ejectors.
Ideally, each ejector when activated ejects an ink droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate the same under ideal conditions.
As can be appreciated, the specialized inks used in acoustic printers are sensitive to temperature. As a general rule, ink drop volume increases with temperature, so temperature non-uniformities can lead to unintended variations in the ink density on the receiver medium. The effect of thermally-induced ink volume variations across the face of standard acoustic ink printheads such as may be caused by the constant flow of ink through the printhead is visible to the naked eye. This results in an overall poor quality image on the transfer medium.
As a general rule of thumb, the total drop diameter non-uniformity should be held to within a target value of 5%. That target value is an approximate upper limit to achieve sufficient uniform optical density. However, a temperature difference of 1.3° C. across only four centimeters of the acoustic ink printhead can account for as much as 1.6% of the total drop diameter non-uniformity.
The present invention solves or substantially mitigates the problem of total drop diameter non-uniformity due to heat generation and thermal effects that occur in acoustic ink printheads.
SUMMARY OF THE INVENTION
The present invention uses at least two counter-flowing heat absorbing fluid flows to selectively cool portions of the ink in acoustic printheads so that the visual effects caused by thermally-induced ink volume variations are shifted to a high spatial frequency on the receiver medium and are thereby masked to the human eye.
In accordance with one aspect of the invention, a device is provided in an acoustic ink printhead for interlacing temperature induced ink drop volume artifacts to a pixel level frequency above the visual acuity of humans, preferably above about 300 dots per inch. The device includes first and second heat sinks on the printhead adapted to develop, respectively a first temperature gradients in first and second ink drops ejected from the printhead. The first temperature gradient is preferably oriented in a first direction transverse to the longitudinal path of the moving printhead. The second temperature gradient is preferably oriented in a second direction opposite the first direction of the first gradient and transverse to the longitudinal path of printhead motion. The printhead draws ink for adjacent pixels marked on the recording medium from ink wells associated with the first and second gradients in an alternating fashion so that temperature induced ink volume variation artifacts are carried or shifted to the pixel level frequency, preferably above 300 dpi. Ink droplet delivery alternates in a spatial direction transverse to the longitudinal path L across the face of the printhead between oversized drops produced from ink adjacent a first one of the heat sinks and undersized drops produced from ink adjacent a second one of the heat sinks.
In accordance with one aspect of the present invention, therefore, an apparatus is provided for cooling ink in an acoustic printhead having a plurality of rows of ink ejectors arranged on the printhead for ejecting a plurality of rows of ink drops as the printhead translates adjacent an ink drop receiving medium in alternate linear first and second translation directions. The apparatus includes a first tank containing a volume of a first thermally conductive fluid and a second tank containing a volume of a second thermally conductive fluid. A first inlet and a first outlet port on the first tank enable a transverse flow of the first thermally conductive fluid through the printhead. Similarly, a second inlet port and a second outlet port on the second tank enable a transverse flow of the second thermally conductive fluid through the printhead. The first tank has a first surface adapted to conduct thermal energy from a first portion of the ink in the printhead and the second tank includes a second surface adapted to conduct thermal energy from a second portion of the ink in the printhead. The first inlet and outlet ports are arranged on the first tank to establish a flow of the first thermally conductive fluid in a first direction transverse of the translation direction of the printhead. The second inlet and outlet ports are arranged on the second tank to establish a second flow of the second thermally conductive fluid in a second direction opposite the first direction and transverse the translation direction of the printhead.
In accordance with still another aspect of the invention, the first and second tanks are adapted to establish substantially equal and opposite thermal gradients in the ink contained within the printhead. The first tank forms a first thermal gradient in the first direction transverse the translation direction of the printhead. The second tank forms a second thermal gradient in the second direction opposite the first direction and transverse the translation direction of the printhead. Preferably, the first and second thermal gradients have substantially identical characteristics. In that way, the visual effects on the receiver medium caused by ink drop size variation occurring across lead rows of the printhead are effectively cancelled or masked by the substantially equal and opposite thermal effects that are created in the trailing ejector rows.
In accordance with yet another aspect of the invention, the fi
Ellson Richard N.
Elrod Scott
Fay Sharpe Fagan Minnich & McKee LLP
Pham Hai C.
Xerox Corporation
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