Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-07-11
2004-02-03
Meier, Stephen D. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S035000
Reexamination Certificate
active
06685300
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a driving device and a driving method for an ink jet printing head which executes printing of letters, figures, images, etc. on objects such as paper, by changing the volumes of its pressure-generation chambers which are filled with ink by means of actuators (piezoelectric vibrators etc.) and thereby discharging minute ink drops from its nozzles corresponding to the pressure-generation chambers.
DESCRIPTION OF THE PRIOR ART
Various types of drop-on-demand ink jet printers have been proposed as disclosed in Japanese Publication of Examined Patent Applications No.SHO-53-12138, Japanese Patent Application Laid-Open No.HEI10-193587, etc. In a so-called drop-on-demand ink jet printer, the volume of a pressure-generation chamber which is filled with ink is changed (increased/decreased) by means of an actuator such as a piezoelectric vibrator, and thereby a minute ink drop is discharged from a nozzle corresponding to the pressure-generation chamber.
FIG. 1
is a schematic cross sectional view showing the composition of an ink jet printing head. Referring to
FIG. 1
, the ink jet printing head
1
includes a plurality of pressure-generation chambers
17
, a nozzle plate
11
as a wall for the pressure-generation chambers
17
, nozzles
15
which are provided to the nozzle plate
11
corresponding to each of the pressure-generation chambers
17
for discharging ink drops
19
, a vibration plate
13
, piezoelectric actuators
14
corresponding to each of the pressure-generation chambers
17
for vibrating the vibration plate
13
and thereby changing the volumes of the pressure-generation chambers
17
so as to cause the ink drop discharge from the nozzles
15
, and the ink chambers
18
which are connected to the pressure-generation chambers
17
for supplying ink from an unshown ink tank to the pressure-generation chambers
17
via ink supply channels
16
, a wall
12
disposed between the nozzle plate
11
and the vibration plate
13
to form the ink chambers
18
.
In the ink jet printing head having the composition shown in
FIG. 1
, when the vibration plate
13
is vibrated by a piezoelectric actuator
14
, the volume of a corresponding pressure-generation chamber
17
filled with ink is changed by the vibration, and thereby an ink drop
19
is discharged from a corresponding nozzle
15
. The discharged ink drop
19
reaches the surface of an object such as paper and thereby forms an ink dot on the object. By repetition of the formation of the ink dots, letters, figures, images, etc. based on an image data are printed on the object.
The ink jet printing head
1
is required to discharge the ink drops
19
stably in order to realize precise printing on the object based on the image data. However, in actual printing by the ink jet printing head, desired printed output can not always be obtained since the ink drop discharge tends to become unstable due to various factors. One of the factors is evaporation of volatile ingredients of the ink.
The ink which is used for the ink jet printing generally includes water as the main solvent, and coloring agents (various organic dyes etc.) and surface-active agents are added to the main solvent. When such type of ink is employed, the water as the main solvent of the ink tends to evaporate from the surface of the ink at the opening of the nozzle
15
as an ink drop discharge pause period (that is, a period in which the ink drops are not discharged from the nozzle
15
) becomes longer, thereby the ink becomes viscous or fixed partially, causing the nozzle
15
to stop up.
In order to resolve the ink fixation problem and maintain constant viscosity of the ink, “ink refresh operation” is generally executed to the ink jet printing head, in which the ink jet printing head is withdrawn from the printing zone and the ink in the nozzles
15
is refreshed by means of forcible ink drop discharge, forcible ink suction by use of a pump, etc. However, in order to attain the stable ink drop discharge from the nozzles
15
and obtain high quality printed output, the above ink refresh operation has to be repeated frequently, thereby the consumption of the ink and printing cost are increased considerably and the need of handling and processing large amount of waste ink occurs.
Another method for avoiding the ink fixation problem without causing the increase of ink consumption has been disclosed in Japanese Patent Application Laid-Open No.SHO57-61576, in which a weak vibration, by which the ink drop discharge from the nozzle
15
is not caused, is applied by the piezoelectric actuator
14
to the vibration plate
13
and the pressure-generation chamber
17
also in the ink drop discharge pause periods.
FIGS. 2A through 2E
are schematic cross sectional views of a nozzle of an ink jet printing head for explaining the above method disclosed in the Japanese Patent Application Laid-Open No.SHO57-61576.
Referring to
FIG. 2A
, ink
23
which is packed in the pressure-generation chamber
17
contacts air at the opening
21
of the nozzle
15
.
Referring to
FIG. 2B
, at the interface (meniscus surface)
22
between the ink and air, evaporation of water in the ink
23
occurs and thereby a high viscosity ink layer
24
is formed near the meniscus surface
22
.
When the weak vibration, which does not cause the ink drop discharge from the nozzle
15
, is applied by the piezoelectric actuator
14
to the vibration plate
13
and the pressure-generation chamber
17
, the meniscus exhibits small vibration as shown by the arrows in
FIGS. 2C and 2D
.
By the small meniscus vibration, the high viscosity ink layer
24
diffuses into the low viscosity ink
23
as shown in
FIG. 2E
, and thereby the viscosity of the ink in the pressure-generation chamber
17
becomes uniform. The reference character “
25
” shown in
FIG. 2E
indicates the ink having the uniform viscosity.
The above method is effective and applicable as long as the ink drop discharge pause period is short, however, the method can only slow the increase of the ink viscosity since the method does not execute replacement of the ink
23
in the nozzle
15
. If the ink drop discharge pause period becomes long, fixation of the ink
23
in the nozzle
15
occurs eventually and thereby the following ink drop discharges become difficult or impossible.
In order to resolve the above problems, another ink jet printing method disclosed in Japanese Patent Application Laid-Open No.HEI9-29996 includes a “small vibration application process” in which the small vibration (which does not cause the ink drop discharge from the nozzle
15
) is applied to the pressure-generation chamber
17
in the ink drop discharge pause period and an “ink refresh process” in which the ink jet printing head
1
is withdrawn from the printing zone periodically and the ink near the openings of the nozzles
15
and ink in the pressure-generation chambers
17
are forcibly discharged. In the ink refresh process, the amplitude of a refresh driving voltage waveform which is applied to the piezoelectric actuator
14
for the forcible ink drop discharge for the ink refresh operation is set larger than that of an discharge driving voltage waveform which is employed for ordinary ink drop discharge for printing, thereby discharge of large amount of ink and replacement of ink in the pressure-generation chamber
17
are conducted so as to avoid the fixation and bodying up of the ink for the long term.
However, in the above ink jet printing method of Japanese Patent Application Laid-Open No.HEI9-29996 employing the combination of the small meniscus vibration and the ink refresh operation (forcible ink drop discharge), the ink refresh operation (ink refresh process) is executed according to a fixed refresh operation cycle. The fixed refresh operation cycle has to be set short so as to be able to cover the severest possible condition within the operation-guaranteed ranges of the ink jet printing head
1
. Therefore, also in the ink jet printing method of Japanese Patent Application Laid-Open No.HEI9-29996, i
Ishiyama Toshinori
Okuda Masakazu
Fuji 'Xerox Co., Ltd.
Mouttet Blaine
Sughrue & Mion, PLLC
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