Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2002-08-28
2004-10-12
Hsieh, Shih-Wen (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S010000, C347S014000, C347S023000
Reexamination Certificate
active
06802589
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid-jetting apparatus including a liquid-jetting head provided with nozzle openings through which liquid particles are jetted, and a method of driving the liquid-jetting apparatus.
2. Description of the Related Art
An ink-jet recording apparatus, such as an ink-jet printer or plotter, is a representative example of a liquid-jetting apparatus. The ink-jet recording apparatus moves a recording head, i.e., a liquid-jetting head, in a scanning direction, moves a recording sheet (printable recording medium) in a feed direction, jets ink particles through the nozzle openings of the recording head as the recording head is moved in the scanning direction to print images (characters) on a recording sheet. Ink particles (liquid particles) are jetted, for example, by changing the pressure of the ink in pressure chambers communicating with the nozzle openings, respectively.
The pressure of the ink is changed by a pressure-generating element, such as a piezoelectric vibrator (piezoelectric member). The piezoelectric vibrator is transformed when a driving pulse is applied thereto to change the volume of the pressure chamber. Consequently, the pressure of the ink contained in the pressure chamber changes to jet an ink particle through the nozzle opening.
Since the ink in the nozzle openings of the recording head is exposed to the atmosphere, the solvent, such as water, of the ink evaporates gradually and the viscosity of the ink in the nozzle openings increases, which deteriorates the image quality of recorded images. When the viscosity of the ink filling the nozzle opening is excessively high, it is possible that an ink particle jetted through the nozzle opening deviates from a normal direction.
Therefore, measures are taken for the ink-jet recording apparatus to prevent increase in the viscosity of the ink filling the nozzle openings. The measures include a flushing operation for forcibly jetting the ink having an increased viscosity to a non-recording region outside a recording region or a stirring operation for stirring the ink by vibrating meniscus of the ink. The meniscus is the exposed free surface of the ink in the nozzle opening.
The conventional flushing operation uses a jetting waveform included in a driving signal which is also used for jetting an ink particle for a recording operation.
Generally, a driving signal for driving an ink-jet device to jet an ink particle for recording has a first voltage-raising part to apply a voltage to the piezoelectric member such that the pressure chamber is expanded and the pressure in the pressure chamber is reduced to a reduced pressure, a first voltage holding part to apply a voltage to the piezoelectric member such that the pressure chamber is maintained at the reduced pressure, a first voltage-reducing part to apply a voltage to the piezoelectric member such that the pressure chamber is contracted and the pressure in the pressure chamber is raised to a raised pressure, a second voltage holding part to apply a voltage to the piezoelectric member such that the pressure chamber is maintained at the raised pressure, and a second voltage-raising part to apply a voltage to the piezoelectric member such that the pressure chamber is restored to its original state.
Generally, ink particles including those for the flushing operation are jetted at a jetting speed on the order of 7 m/s, and the weight of the ink particles is, for example, 13 ng.
Usually, parts of the recording head corresponding to the nozzle openings are covered with a cap.
The inventors of the present invention acquired knowledge that it is possible that thickenings of lines or failure in forming dots occurs during printing after removing the cap if the ink has a high pigment concentration. The thickening of lines and failure in forming dots occur when bubbles are formed in the nozzle openings. The inventors of the present invention made the following analytical studies to find what forms bubbles in the nozzle opening.
The solvent, such as water, of the ink filling the nozzle opening covered with a cap and forming a meniscus evaporates and the viscosity of the ink increases. An ink particle jetted through the nozzle opening deviates from a normal direction or the nozzle opening is clogged with the ink if the viscosity of the ink is thus increased.
Increase in the viscosity of the ink starts from a peripheral part of the nozzle opening. Whereas the viscosity of the ink in a peripheral part of the meniscus started to increase in about two minutes after the nozzle opening has been covered with the cap, the viscosity of the ink in a central did not start to increase in about five minutes after the nozzle opening has been covered with the cap. Thus, the viscosity of the ink forming the meniscus increases un-uniformly in a period of two to five minutes after the nozzle opening has been covered with the cap.
An ordinary flushing operation applies pressure to the ink so that the entire meniscus is pushed out of the nozzle opening as shown in FIG.
9
(
b
). Therefore, if the viscosity of the ink in a peripheral part of the nozzle opening and that of the ink in a central part of the nozzle opening are differently increased, the meniscus is un-uniformly deformed by the flushing operation and the meniscus is liable to break. If the flushing operation is continued with the meniscus in an easily breakable state, it is highly possible that the ink adheres to the periphery of the outlet of the nozzle opening, the meniscus is broken and, eventually, bubbles are formed in the ink.
Actually, printing troubles, such as the thickening of printed lines and failure in printing dots, occur only in two to five minutes after the nozzle openings have been covered with the cap.
Therefore, when the viscosity of the ink forming the meniscus is expected to be un-uniformly increased, the flushing operation must supply energy sufficient to overcome the strength of a film of the ink having the increased viscosity to the ink filling a central part of the nozzle opening to jet an ink particle, and the flushing operation must be continued to remove gradually the ink having the increased viscosity forming the peripheral part of the meniscus.
The conventional flushing operation uses large ink particles that are jetted during a printing operation for printing large dots and are capable of exerting a large force enough to blow off the ink of the increased viscosity to achieve flushing in a short time. The term “large dot” signifies the largest dot that can be formed by the relevant ink-jet recording apparatus.
It was found that particles of some ink jetted immediately after the completion of the flushing operation are subject to so called “wet deviation” and reduces printing accuracy if large ink particles are jetted for the flushing operation. The term “wet deviation” signifies the deviation of a jetted ink particle from a normal flying direction due to the drawing effect of the ink adhered to the periphery of the outlet of the nozzle opening during the preceding flushing operation on the jetted ink particle.
The occurrence of wet deviation immediately after the flushing operation may be due to the adhesion of a mist of the ink generated during the flushing operation that jets large ink particles to the periphery of the outlet end of the nozzle opening.
An ink having a high pigment concentration and prone to thicken has a tendency to cause wet deviation immediately after the flushing operation.
Recently, inks having a high pigment concentration have been developed. The use of such an ink having a high pigment concentration will make wet deviation immediately after the flushing operation more serious.
For example, experiment showed that wet deviation occurs when the flushing operation is started after an interval of one second or longer from an ink-jetting cycle preceding the flushing operation if the flushing operation that jets large ink particles uses an ink having a high pigment concentration of ten-odd percent; that is wet devi
Chang Junhua
Matsumoto Keiji
Yonekubo Shuji
Hsieh Shih-Wen
Seiko Epson Corporation
Sughrue & Mion, PLLC
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