Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-05-20
2002-03-19
Gordon, Raquel Yvette (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06357860
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ink jet printers of the type in which drops of ink can be charged electrically, and then deflected by an electric field, in order to control the destinations of the ink drops.
Normally, such deflection type ink jet printers are continuous jet printers, in which the ink jet runs continuously and drops not used for printing are caught by a gutter (and typically re-circulated to the ink supply). Such printers may be arranged either so that undeflected ink drops pass from the ink gun to the gutter, and drops are deflected out of the path leading to the gutter in order to be printed, or so that drops are deflected into the gutter and printing takes place with undeflected drops. In either case, the printer may be constructed to apply different levels of the deflection to different drops, so as to provide a range of printing positions.
One known type of deflection ink jet printer typically has only one ink jet nozzle, and the drops are deflected to a variety of possible printing positions. Such printers are typically used for printing information and indicia such as “sell-by” dates, code numbers, bar codes and logos onto foodstuffs and packages (e.g. yoghurt pots, eggs, milk cartons etc), manufactured articles, packaging and other articles which are conveyed past the print head on a conveyor belt or other conveying mechanism. Devices of this type are described, for example, in U.S. Pat. No. 5,481,288 (and WO-A-89/03768), U.S. Pat. No. 5,126,752 (and EP-A-0424008), U.S. Pat. No. 5,434,609 (and EP-A-0487259) and U.S. patent application Ser. No. 940667 (and EP-A-0531156), all of which are incorporated herein by reference. In another type of deflection ink jet printer, a plurality of ink jet nozzles are arranged in a row, and typically undeflected drops from each nozzle are used for printing while deflected drops are caught by the gutter (either a common gutter for all jets or a plurality of gutters). This type of printer is normally used for printing graphics.
In a normal continuous jet deflection type ink jet printer the ink leaves the nozzle in an unbroken stream of ink and breaks into drops a short distance from the nozzle. The ink jet is modulated, typically by applying a vibration to it in accordance with a modulation drive signal, in order to ensure that it breaks into drops in a controlled manner and at a desired frequency. The length of time between the moments when successive drops break from the ink jet is known as the drop period. Normally the drop period is controlled by, and can be determined from, the frequency of the modulation drive signal. The phase position of the moments when successive drops break from the ink jet will be referred to as the drop separation phase.
An electrically conductive ink is used and the voltage of the ink at the nozzle is held constant. An electrode, known as the charge electrode, is provided adjacent the path of the ink jet at the point where it breaks into drops. A voltage on the charge electrode will induce an electric charge in the part of the ink jet which is close to the electrode, and when a drop separates from the ink jet some of this charge is trapped on the drop. A deflection electrode arrangement creates an electric field which acts on the charge trapped on the drop to deflect it from the direction in which the ink jet is travelling when it leaves the nozzle.
In normal practice, different levels of deflection are applied to different drops by providing different voltages to the charge electrode for different drops, and thereby capturing different quantities of charge on different drops. As an alternative, it has been proposed (e.g. in U.S. Pat. No. 4,122,458) to provide different strengths of the electric field for different drops. Whatever aspect of the system is changed to apply different levels of deflection to different drops, the changes must be made with a correct phase relative to the drop separation phase so as to ensure that each drop is deflected correctly. Therefore it is necessary to conduct an operation, known as phasing, to discover the drop separation phase.
During phasing a special signal is applied to the charge electrode. The frequency of this special signal corresponds to the drop period and its waveform is chosen so that the quantity of charge trapped on the ink drops depends on the phase position of the special signal relative to the drop separation phase. Normally the special signal is applied at several different phase angles during a phasing operation. By monitoring the level of charge trapped on the ink drops during phasing it is possible to identify the drop separation phase. The details of the phasing operation can vary greatly. U.S. Pat. No. 5,481,288 (and WO-A-89/03768) shows one approach. U.S. Pat. No. 3,761,941 shows a different approach.
The phasing operation depends on being able to detect the level of charge captured on the ink drops. One way of doing this is to provide an electrode, known as a phase sensor electrode, downstream of the charge electrode. The phase sensor electrode is very close to the path of the drops and a brief current signal is induced in it by each charged drop as it passes. It is optionally possible also to provide another electrode (known as a time of flight sensor electrode) further along the path of the ink drops, spaced by a known distance from the phase sensor electrode, which is also placed very close to the ink path and has a current signal induced in it by charged drops passing it. By measuring the time between signals induced on these two electrodes, it is possible to measure the ink jet velocity.
FIGS. 1 and 2
show plan and side views, respectively, of the main components of an example of an ink jet printer head using a phase sensor electrode and a time of flight sensor electrode. In
FIGS. 1 and 2
, the ink jet is emitted as a continuous stream from the nozzle
1
of an ink gun, and passes through a slot in a charge electrode
3
. The continuous ink stream from the nozzle
1
breaks up into drops while it is in the slot in the charge electrode
3
. The ink is electrically conductive and the ink gun is held at a fixed potential (usually zero volts for convenience and safety). The voltage on the charge electrode
3
induces a charge in the portion of the ink jet within the slot of the charge electrode, and as ink drops separate from the ink stream, the charge is captured in the drops. The amount of charge captured in each drop is controlled by varying the voltage applied to the charge electrode
3
(e.g. in the range 0 to 255 V). In this way, the charging signal applied to the charge electrode
3
controls the extent of the subsequent deflection of the ink drops.
The drops of ink then pass over the phase sensor electrode
5
, which is used to detect the level of charge of the drops during a phasing operation as described above. The drops then pass between two deflection electrodes
7
,
9
, which are maintained at substantially different potentials (typically with a difference of 6 to 10 kV between them), so as to provide a strong electric field. This field deflects the charged ink drops, and the extent of deflection depends on the amount of charge on each drop. Drops with zero charge, or only a minimal charge, will pass through the field experiencing no deflection, or only minimal deflection, and will be caught by a gutter
11
. Drops with higher levels of charge will be deflected sufficiently to miss the gutter
11
and will therefore continue in flight until they reach the surface
13
to be printed onto, and form a dot thereon. The range of possible deflection paths for dots to be printed ranges from the minimum degree of deflection necessary to miss the gutter
11
to the maximum amount of deflection possible before the deflected dot strikes the deflection electrode
7
. The maximum and minimum deflected paths for printing are illustrated in FIG.
1
.
Drops of ink having a minimal level of charge, so that the angle of deflection is not sufficient for the drop to escape the gutter
11
, will pass over a
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Gordon Raquel Yvette
Linx Printing Technologies PLC
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