Electrostatic ink jet head

Details Electrostatic ink jet head Electrostatic ink jet head

2001-02-26

2003-01-28

Gordon, Raquel Yvette (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06511158

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrostatic ink jet head that is provided with a micro-actuator utilizing static electricity.
2. Description of the Related Art
FIG. 1
is a perspective view of a conventional ink jet head that utilizes static electricity.
FIG. 2
is a sectional view, taken along the line II—II of
FIG. 1
, showing the structure of one actuator of the ink jet head shown in FIG.
1
. In these figures, reference numeral
10
indicates an electrode substrate, reference numeral
20
indicates a liquid chamber/diaphragm substrate, and reference numeral
30
indicates a nozzle substrate. This nozzle substrate
30
is provided with a nozzle
31
, and the liquid chamber/diaphragm substrate
20
is provided with ink liquid chambers
21
that communicate with the nozzle
31
. A conductive diaphragm
22
is disposed as a part of the ink liquid chamber
21
, and also serves as a part of a common electrode. The diaphragm
22
is thin and has a low rigidity, so as to be flexible. The electrode substrate
10
has individual electrodes
11
outside the ink liquid chambers
21
that are arranged at predetermined intervals. Reference numeral
12
indicates a protection film for preventing short-circuiting between the diaphragm
22
and the individual electrode
11
. Reference numeral
13
indicates a sealing member that seals openings in which the individual electrode
11
is disposed. As shown in
FIG. 1
, the electrostatic ink jet head has a plurality of actuators, and each of the actuators discharges ink droplets.
In
FIGS. 1 and 2
, a voltage is applied between the diaphragm
22
and the individual electrode
11
. The diaphragm
22
is displaced toward the individual electrode
11
due to the static electricity. Here, the applied voltage is turned off to return the diaphragm
22
to the original location at which the diaphragm
22
was situated prior to the application of the voltage. This mechanical behavior of the diaphragm
22
with respect to the static electricity is used for discharging the ink in an electrostatic ink jet apparatus. In
FIG. 2
, the space between the substrate
20
having the diaphragm
22
and the individual electrode
11
is normally sealed by the sealing member
13
so as to ensure isolation from the outside. This space is called a “gap chamber”, and the part of the gap chamber immediately below the diaphragm
22
is referred to as a diaphragm chamber.
When a voltage is applied between the diaphragm
22
and the individual electrode
11
in the electrostatic ink jet head described above, the diaphragm
22
is displaced due to static electricity that acts between the diaphragm
22
and the individual electrode
11
. Therefore, the diaphragm
22
is made so thin as to reduce the driving voltage. As a result, the driving voltage can be low, but the rigidity of the diaphragm
22
becomes too low. The existence of air or gas in the diaphragm chamber or the gap chamber has an adverse influence on the behavior of the diaphragm
22
. When the diaphragm
22
approaches the individual electrode
11
, the diaphragm
22
is subjected to the compressive resistance of the air. As a result, the voltage at the contact point between the diaphragm
22
and the individual electrode
11
(hereinafter referred to as “contact voltage”) becomes higher in a dynamic state than in a static state.
There is another problem with the conventional electrostatic ink jet head.
FIGS. 3A and 3B
illustrate the problem of the conventional electrostatic ink jet head.
FIG. 3A
shows a displacement D of the diaphragm when the driving frequency is low, and
FIG. 3B
shows a displacement d of the diaphragm
22
when the driving frequency is high. The diaphragm
22
of the electrostatic ink jet head (Reference numeral
22
′ indicates the diaphragm
22
displaced and brought in contact with the individual electrode
11
.) needs to be dynamically vibrated at a frequency on the order of and up to 10 kHz. The diaphragm chamber is originally small in volume, and the diaphragm
22
moves within the small space. As a result, the diaphragm
22
is subjected to the compressive resistance of the air, and the air is unlikely to return into the diaphragm chamber once it moves out of the diaphragm chamber. If the driving condition (the shape of the driving voltage pulse) is the same, the amount of air moving out of the diaphragm chamber varies with the frequency of the driving voltage pulse. The higher the frequency, the larger the amount of air that cannot return to the diaphragm chamber. As a result, the diaphragm
22
moves closer to the individual electrode
11
.
FIG. 6
shows operation results of the conventional electrostatic ink jet head. As shown in
FIG. 6
, as the frequency becomes higher, the amount of air that cannot return to the diaphragm chamber becomes larger. As a result, the diaphragm
22
is vibrated at a location closer to the individual electrode, as shown in FIG.
3
B. Accordingly, the distance between the diaphragm
22
and the individual electrode
11
actually becomes shorter, and the contact voltage becomes lower. In this manner, the frequency characteristics lead to a problem when the frequency becomes high. This phenomenon is peculiar to an electrostatic actuator that drives a diaphragm by static electricity, and should be eliminated when a high-frequency driving operation is carried out.
The above problem arises only when the contact driving operation is performed, with the diaphragm being in contact with the electrodes. In a non-contact driving operation, the above problem of frequency dependence is not caused or can be neglected.
As described before, the diaphragm is subjected to the compressive resistance of the air in the gap chamber in the conventional electrostatic ink jet head. As a result, there will be a problem that the contact voltage increases. To solve this problem, there have been several suggestions. For instance, Japanese Laid-Open Patent Application No. 7-299908 discloses an electrostatic ink jet head in which a space for the air, as well as the diaphragm chamber, is formed in the gap chamber, so that the diaphragm displaced toward the electrodes is not subjected to the compressive resistance of the air. This will result in a larger gap chamber.
However, there has been no suggestion as to a method to solve the problem that arises in a high-frequency driving operation. This is because such a problem is unlikely caused in a conventional electrostatic ink jet head having the maximum driving frequency of 10 kHz, for instance.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide electrostatic ink jet heads in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an electrostatic ink jet head in which the volume of the diaphragm chamber is relative to the volume of the gap chamber, and the volume of the gap chamber except the diaphragm chamber can be smaller than that in the prior art.
Further specific objects of the present invention are: to improve the frequency dependence of the electrostatic actuator simply by setting the waveform of the driving voltage; to improve the frequency dependence of the electrostatic actuator having a certain gap configuration; to improve the frequency dependence of the electrostatic actuator by changing the structure and configuration; and to improve the frequency dependence of the electrostatic actuator both by changing the structure and configuration and by setting the waveform of the driving voltage.
The above objects of the present invention are achieved by an electrostatic ink jet head that comprises a diaphragm, and an electrode that faces the diaphragm, with a predetermined gap chamber being maintained between the electrode and the diaphragm. In this electrostatic ink jet head, a pulse voltage is applied between the electrode and the diaphragm so as to deform the diaphragm by static electricity. Ink droplets are discharged by a mechanical recovering force of the deforme

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