Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-04-20
2002-04-16
Gordon, Raquel Yvette (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06371599
ABSTRACT:
This application is based on application Nos. 10-116707 and 10-119439 filed in Japan, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ink jet recording apparatus based on an electrostatic actuator and a drive unit and method for an ink jet head.
2. Description of the Prior Art
Some of the ink jet heads for ink jet recording apparatuses use electrostatic actuators.
The ink jet head based on the electrostatic jet head comprises an oscillating plate provided in a pressure chamber that communicates with an ink injecting nozzle, an electrode (herein called a first electrode) provided on the oscillating plate, and a second electrode provided opposing the first electrode across a slight space, wherein the pressure of ink filled in the pressure chamber is altered by means of deforming the oscillating plate with an electrostatic force generated between the first electrode and the second electrode by applying an electric voltage across the two electrodes, thus causing the ink to be ejected from the nozzle to fly and land on a recording medium to form an image there.
Such a drive unit for the ink jet head based on the electrostatic actuator is essentially a unit for applying a voltage across the first and second electrodes, wherein the unit applies a voltage across the two electrodes to cause the oscillating plate of the electrostatic actuator to deform in a direction of expanding the pressure chamber and, when the voltage applied across the two electrodes is abruptly cut off, the oscillating plate restores to the original position of the oscillating plate thus creating a pressure rise in the pressuring chamber to cause an ink drop to be ejected from the nozzle.
A problem with such an ink jet head drive unit has been that a certain electric charge remains between the first and second electrodes after the voltage is applied and cause image fluctuations.
This is due to the fact that the amount of the electric charge remaining on the electrodes typically varies depending on whether the ink jet head is continuously operated or sporadically operated in ejecting ink drops from the ink jet head, as they cause a different amount of deformation to the oscillating plate even if the applied voltage is the same.
Moreover, even if the ink jet head is operated continuously, polarization occurs after a while in dielectric members existing in the second electrode or the oscillating plate as a result of the DC voltage applied across the electrodes for a prolonged period of time, consequently causing fluctuations in the amount of ink ejection and the ejection speed.
These problems can be more conspicuous when the oscillating plate is made of a semiconductor material.
Several techniques have been invented to prevent these problems of the residual electric charge and polarization and disclosed, for example, in Japanese Laid-Open Publication Hei 7-214770, Hei 8-72237, Hei 8-267744, and Hei 7-214780.
The technique disclosed in Japanese Laid-Open Publication Hei 7-214770 is to prevent the accumulation of the electric charge by applying a voltage for a certain period of time across the oscillating plate and an individual electrode by means of a charging circuit, and causing ink to fly by means of abruptly discharging the charge stored through a discharging circuit.
The technique disclosed in Japanese Laid-Open Publication Hei 8-72237 is to prevent the accumulation of the electric charge by shorting the high potential side and the low potential side of the power source when the head is not operating.
The technique disclosed in Japanese Laid-Open Publication Hei 8-267744 is to improve the printing quality by means of suppressing unnecessary ink release or fluctuations of ink drop diameters due to cross talk by means of constituting a system in such a way that a piezoelectric member deforms in a directions along the ink chamber when a voltage is applied by a voltage application means as the piezoelectric member is affixed only on a separating wall via an adhesive layer.
The technique disclosed in Japanese Laid-Open Publication Hei 7-214780 is to cancel the remaining polarization by applying a voltage in the forward direction between the oscillating plate and an individual electrode when the system is printing, and applying a voltage between the oscillating plate and the individual electrode in the backward direction when the system is not printing.
However, it has been learned that the residual electric charges cannot be completely erased within the operating cycle of the ink jet head by simply grounding the electrodes or shorting between the first and second electrodes during the non-operating period in consideration of the operating cycle of the ink jet head, i.e., the period from one printing operation to the next in the ink jet head recording apparatus.
This is due to the structure of the ink jet head particularly to the electrode unit that causes the oscillating plate to operate.
Since the oscillating plate of the ink jet head that uses the electrostatic actuator is manufactured by the micromachine technology based on the semiconductor device manufacturing process, the oscillating plate and the first electrode provided on the oscillating plate are both made of silicon substrates, and the surface of the first electrode is covered with an insulating film in order to prevent shorting between the first and second electrodes when they accidentally come into contact. Consequently, the oscillating plate area has electrostatic an capacity caused by barrier areas developed in the semiconductor and the insulating film, thus leaving a residual electric charge accumulated by the voltage applied during the operation. In order to eliminate this residual electric charge, it is necessary to discharge the electrostatic capacity, and the time required for this discharge depends on the time constant of the electrostatic capacity, so that the discharge requires as much time as approximately 1.014 sec if an oxide film is formed as the insulation film because of the high dielectric constant of the oxide film (the time T is calculated as T=∈
RC
, where ∈=4, R=1×10
10
, c=1×10
−12
).
In various types of ink jet type printers (not only those using electrostatic actuators), the operating cycle time is from 250 to 500 micro seconds at the most when they are printing continuously using one ink jet head, so that the operating time is much shorter than the time required for erasing the residual electric charge, consequently making it impossible to erase the residual charge completely when the system is printing continuously.
As a consequence, since the degree of the residual electrical charge varies depending on whether the time between one printing to the next is longer or shorter, the amount of deformation of the oscillating plate due to the application of the drive voltage varies, resulting in fluctuations of the amount of ink ejected and the density or dot size of each printed pixel, hence fluctuations of images. This problem has not yet been completely solved.
The phenomenon remains the same even if the first and second electrodes are short-circuited in the non-operating course as was disclosed by mentioned above, and the residual electrical charge cannot be erased completely within the non-operating time in case of a continuous printing process, so that the amount of the remaining charge on the electrodes varies with the length of the non-operating time and the image deterioration problem associated with the residual charge cannot be solved.
SUMMARY OF THE INVENTION
The first objective of the present invention is to provide an ink jet head recording apparatus that can prevent image deterioration due to the residual electrical charges on the first electrode and/or the second electrodes.
The second objective of the present invention is to provide a drive unit and method for an ink jet head that can drive the ink jet head such as the one described above.
The third objective of th
Asano Masaki
Fujii Yasuhisa
Higashino Kusunoki
Hotomi Hideo
Gordon Raquel Yvette
Minolta Co. , Ltd.
Sidley Austin Brown & Wood
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