Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-04-01
2001-08-14
Le, N. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06273559
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for projecting an electrically conducting liquid in the form of at least one continuous stimulated jet.
The invention also relates to a multi-nozzle printing device embodying this process.
A printing device conform with the invention may be used in any industrial domain related to marking, coding, addressing and industrial decoration.
STATE OF THE ART
In the current state of the art, there are two main printing technologies using stimulated continuous ink jets. These processes are the deviated continuous ink jet technique and the binary continuous ink jet technique.
According to the deviated continuous ink jet technique, pressurized electrically conducting ink is discharged through a calibrated nozzle. The ink jet thus formed is broken off at regular time intervals always at the same point in space, under the action of a periodic stimulation device. This forced fragmentation of the ink jet is usually induced by periodic vibrations of a piezoelectric crystal on the inlet side of the nozzle. Starting from this break off point, the continuous jet is transformed into a stream of identical and uniformly spaced ink drops. A first group of electrodes is located close to the break off point, the function of which is to selectively transfer a variable and predetermined quantity of electric charge to each drop in the jet. All drops in the jet then pass through a second group of electrodes, in which there is a constant electric field. Each drop is then deflected proportionally to the electric charge that has already been assigned to it, and which directs it towards a specific point on a medium to be printed. Undeflected drops are recovered in a gutter and are recycled to an ink circuit.
In ink jet printers based on this technique, a specific device is usually provided to maintain constant synchronization between instants at which the jet is broken off and instants at which drop charge signals are applied.
This technology is characterized mainly by the fact that a variable quantity of electric charge is selectively transferred to each drop in the jet, such that multiple deflection levels are created. Due to this characteristic, a single nozzle can print an entire pattern (character or graphic pattern) in segments (lines of points with a given width). The changeover from one segment to the next takes place by moving the print medium in front of the printing device, continuously and perpendicular to the segments.
Several single nozzle printing devices (usually two to four) can be grouped within the same housing, for applications requiring a slightly greater print width.
Multi-nozzle printing devices have to be used when print widths become large. Document EP-A-0 512 907 describes a multi-nozzle (eight nozzles) printing device using the deviated continuous ink jet technology. Even greater print widths can be obtained by putting several multi-nozzle printing devices together.
Stimulated continuous ink jet printing devices using the binary continuous jet technique are different from printing devices making use of the deviated continuous jet technique mainly due to the fact that only a predetermined quantity of electric charge can be transferred to each drop in the jet, on request. Therefore only one value of the drop deflection is created. Consequently, multi-nozzle printing devices are necessary to print characters or patterns, in which the center-to-center distance between the nozzles usually corresponds to the spacing between impacts on the medium to be printed. In general, drops to be used for printing (“drops to be printed” in the rest of the text) are the undeflected drops. This technique is particularly suitable for high speed printing applications such as addressing, printing of high resolution color prints, etc.
In printing devices making use of a binary continuous ink jet, some components of groups of charging and deflection electrodes can be made common to these two groups of electrodes. In all cases, electrodes dedicated to charging drops in each jet must be controlled individually, at the same frequency at which the drops are formed and at voltages of up to 350 V.
Major cost and design problems arise with the manufacturing of nozzles and electrodes for a multi-nozzle printing device operating according to the binary continuous ink jet technique, and with their positioning when a very fine pitch is necessary.
Cost problems are due to the large number of charging electrodes and the large number of high voltage electronic circuits connected to these electrodes, which result in large and complex connections.
Design problems are related to the very dense high voltage connections close to the jets, which cause undesirable crosstalk. The only way to reduce the effect of this crosstalk on the print quality is to reduce the drop usage ratio, and consequently the print speed.
In the article entitled “Binary Continuous Thermal Ink Jet Break off Length Modulation” by Donald J. DRAKE, published in the Xerox Disclosure Journal, Volume 14, No. 3, May-June 1989, a multi-nozzle binary continuous jet printing device is suggested in which the design has been modified to overcome the disadvantages mentioned above.
In accordance with the conventional binary continuous jet technology, this article proposes to use two electrode groups, each of which is formed by a flat electrode. However in this case, each electrode is common to all jets and a constant electric voltage is applied to it. The drops to be printed and the drops to be recycled are selected by individual control of the stimulation of each ink jet on the print head. Consequently, an individual stimulation device is provided for each jet.
With this layout, the connections associated with the stimulation devices are located on the inlet side of the nozzles and therefore are not close to the jets. Furthermore, the voltages carried on the connections are less than voltages required for charging the drops. Therefore the effects of crosstalk are reduced.
According to the article by Donald J. DRAKE, a low level or high level stimulation signal is applied to each of the jets on request. The point at which the jet breaks when a low level stimulation signal is applied is further from the nozzle than when a high level stimulation signal is applied to the jet.
In the first case, the jet break off point is located facing the first electrode, or the charging electrode, which is at a constant voltage V
c
. The drop that detaches at this instant then carries a charge Q
1
and is subjected to a deflection equal to an angle &dgr;
1
within the field created by the second electrode, or deflection electrode, which is kept at a constant voltage V
d
. This drop is recovered by the gutter and is recycled to the printing device ink circuit.
When the break distance is shorter because a high level stimulation signal is applied on the jet, the jet breaks at a point slightly before the charged electrode. The charge Q
2
carried by the drop is then smaller than in the previous case. The deflection &dgr;
2
induced by the deflection plane is also smaller. The drop then avoids the gutter and reaches the medium to be printed.
In this article, the difference between two jet stimulation levels is such that the distance d between jet break off points for each of the two levels is equal to the wavelength &lgr; of the stimulated jet, i.e. the stream of drops. The value &lgr; is provided by the ratio of the speed V
j
of the jet to the frequency F of the stimulation signal, &lgr;=V
j
/F.
However, there are three serious disadvantages to the operating method and design suggested in this article, which limit the extent to which this process can be applied to continuous ink jet printers.
The first disadvantage is due to the fact that the distance d between the two jet break off points is equal to the wavelength &lgr; of the stream of drops. This makes it very difficult to use the jet when long break-short break transitions occur. It is found that when a drop to be printed is followed by a drop to be recycled, the condition d
Perrin Max
Vago Stephane
Imaje S.A.
Le N.
Pearne & Gordon LLP
Vo Anh T. N.
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