Incremental printing of symbolic information – Ink jet – Ejector mechanism
Patent
1991-12-12
1997-01-28
Braun, Fred L.
Incremental printing of symbolic information
Ink jet
Ejector mechanism
347 22, B41J 203, B41J 2165
Patent
active
055981976
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to ink jet printers and, more particularly, to the printhead of a so-called continuous ink jet printer.
Printers of this type have a printhead with one or more nozzles connected to a supply of ink, a string of droplets being caused to flow from the nozzle or nozzles by means of an oscillator, usually a piezoelectric transducer. The row of droplets is directed towards a gutter, but selective droplets can be charged as they leave the nozzle and then deflected in an electric field in order to impinge on a substrate, individual droplets being charged appropriately in order to print at the correct position.
One particular problem with printers of this type is found with low viscosity inks which include a solvent component to enable relatively quick drying, and results from seepage of ink through the nozzle at the end of a printing run. Ink remaining in an ink passage to the nozzle can seep from the nozzle, the solvent from the ink then evaporating and leaving ink residues around the nozzle exit which can interfere with the correct direction of the ink stream when the printer is next started. Clearing such a printhead is time consuming and wasteful of operator time.
There is a need therefore to provide a system in which agglomerations of ink residues can be prevented from forming around the nozzle exit.
GB-A-2085807 discloses a continuous ink jet printer which has an external stopper to which is fixed a cleaning pad. The stopper covers the nozzle during periods of non-printing, and the nozzle is moved back and forth prior to printing so that the nozzle can be cleaned.
According to the present invention there is provided a printhead for a continuous ink jet printer, the printhead having an ink channel; a nozzle at one end of the ink channel; and, an oscillator disposed to cause ejection of ink in the channel through the nozzle in use; characterised by means provided internally of the printhead for closing off the nozzle from the channel to prevent seepage of ink therethrough during periods of non-printing.
In a first embodiment, the printhead body has a circular recess in an end face thereof, the oscillator being a circular piezoelectric transducer disposed in the recess so as to provide a short ink chamber adjacent the end face of the printhead body and being arranged to expand and contract in the direction of its axis when an excitation voltage is applied thereto. The ink channel connects with the recess for feeding ink thereto. A nozzle plate is detachably mounted on the end face of the body and has one or more nozzles. The means for closing off the nozzle or nozzles comprises a plunger carrying a closure member and sliding in a central bore. In this embodiment, the recess surrounds the central bore, being connected to it by a generally radial ink passageway. In this specification, "circular" is to be taken to include "annular".
In a second embodiment, a main channel extends substantially in alignment with the axis of the nozzle and a second channel extends to the nozzle inclined to the nozzle axis. Through this second channel, ink is arranged to pass and a piezoelectric crystal is arranged around the second channel to cause it to be squeezed when the piezoelectric vibrates, and the means for closing off the nozzle comprises a plunger carrying a closure member and sliding in the first channel to close off the inlet end of the nozzle.
Alternatively, the oscillator may be a rod of piezoelectric material which, when a modulated electrical signal is fed to it, vibrates to cause vibration of the ink in the channel and thus ejection of the ink through the nozzle at a predetermined frequency. In a third embodiment of the present invention, the means for sealing off the nozzle from the channel comprises a closure member mounted on the end of the piezoelectric rod adjacent the nozzle and the piezoelectric rod is movable into engagement with the inlet end of the nozzle so that the closure member closes the nozzle to prevent further emission of ink.
Three examples of printheads constructed i
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Braun Fred L.
Domino Printing Sciences PLC
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