Incremental printing of symbolic information – Ink jet – Ejector mechanism
Patent
1991-08-12
1994-08-09
Grimley, A. T.
Incremental printing of symbolic information
Ink jet
Ejector mechanism
347 90, G01D 1518
Patent
active
053370716
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
In continuous ink jet printers the natural instability of at least one Jet of ink is driven by a modulating mechanism at a suitable frequency to produce a well defined train of droplets. During the break up of the jet into the discrete droplets, a secondary instability occurs when the ligaments joining the droplets finally snap. This results in a secondary set of micro-droplets, of radius less than 1.mu., which are entrained with the main droplets. In order to print, the droplets are individually and selectively charged as they pass a charging electrode assembly and are then deflected or not, depending upon whether they are charged or not, as they pass through an electrostatic field adjacent to at least one deflection electrode. Either the deflected charged droplets are used for printing and the uncharged undeflected droplets are collected in a gutter, or vice versa. It is unavoidable that many of the micro-droplets also become charged, but, because their charge to mass ratio differs from that of the main droplets, they do not follow the same trajectory as the main droplets. Indeed their deflection is greater, and their trajectories more random, than those of the main droplets, particularly as the charged micro-droplets will have the same polarity as charged main droplets and be repelled by them. If left uncontrolled the micro-droplets produce deposits at undesirable places inside the print head and these eventually grow large enough to interefere with the printing mechanism. The problem is particularly acute in systems which are now becoming preferred, in which uncharged undeflected droplets are used for printing as in that case the majority of the droplets are charged. The phenomenon is particularly significant in high resolution printers, which use fast-drying inks, and which are required to run continuously for extended periods of time.
In accordance with the present invention, in a continuous ink jet printer of the kind comprising means for producing at least one jet of ink, a modulating mechanism for causing the jet to break up into a train of main droplets, a charging electrode assembly for selectively applying an electrostatic charge to the droplets, and at least one deflection electrode for producing an electrostatic field to deflect charged ones of the droplets so that either the deflected charged droplets or the undeflected uncharged droplets are used for printing, the other main droplets being collected by a gutter; there is provided adjacent to the upstream end of the deflection electrode (s) and to the side of the train towards which the charged droplets are deflected, a subsidiary electrode portion defining a cavity which opens towards the path of the train of droplets and arranged such that air entrained by the train of droplets produces, use, a vortex in the cavity, the subsidiary electrode portion being at a potential such that any charged micro-droplets in the train are initially deflected out of the train towards the subsidiary electrode portion, whereupon they are entrained by the air flow and carried into the cavity where they are deposited. This controlled deposit of the micro-droplets in a safe area very beneficial.
In a multi-jet printer, in which there is a planar array of trains of droplets, there will be a common cavity extending parallel to the plane of the array and perpendicular to the flight paths of the trains of droplets.
The subsidiary electrode portion may form an upstream end part of the or one deflection electrode. However, in order to avoid any unnecessary increase in the droplet flight path: an adjacent deflection electrode is preferably foreshortened at its upstream end to accommodate the subsidiary electrode portion, from which it is insulated, and the subsidiary electrode portion is controlled at a different potential from the adjacent deflection electrode, so that, in spite of the cavity causing at least part of the subsidiary electrode portion to be spaced further from the droplet train path(s) than the adjacent deflection electrode: there will be s
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East Amanda H.
Janse Van Rensburg Richard W.
Dang T. A.
Elmjet Limited
Grimley A. T.
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