Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2002-04-09
2003-06-17
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06578954
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ink jet printing head and an ink jet printing device for printing letters, images, etc. on a medium such as paper by ejecting ink drops from nozzles, and in particular, to an ink jet printing head and an ink jet printing device capable of realizing high-frequency ink drop ejection and high-speed printing.
DESCRIPTION OF THE RELATED ART
In a drop-on-demand ink jet printer, a pressure wave (acoustic wave) is generated in a pressure generation chamber which is filled with ink by use of pressure generation means such as a piezoelectric actuator and thereby an ink drop is ejected from a nozzle that is connected with the pressure generation chamber. Such drop-on-demand ink jet printers are well known today as disclosed in Japanese Publication of Examined Patent Applications No.SH053-12138, Japanese Patent Application Laid-Open No.HEI10-193587, etc.
FIG. 1
is a cross-sectional view showing an example of a printing head of a conventional ink jet printing head. Each pressure generation chamber
121
of the printing head is connected with a nozzle
122
for ejects ink drops and an ink supply channel
124
for guiding ink from an ink tank (unshown) to the pressure generation chamber
121
via a common ink channel
123
.
To the bottom of the pressure generation chamber
121
, a vibration plate
125
is provided. In order to eject an ink drop, a piezoelectric actuator
126
which is provided outside the pressure generation chamber
121
deforms the vibration plate
125
and thereby changes the volume (capacity) of the pressure generation chamber
121
. The change of volume causes a pressure wave in the pressure generation chamber
121
and thereby part of the ink packed in the pressure generation chamber
121
is ejected from the nozzle
122
to outside as an ink drop
127
. The ink drop
127
flying from the nozzle
122
reaches a medium such as paper and thereby forms an ink dot. Letters, images, etc. are printed and recorded on the medium by repeating the ink dot formation according to specific image data.
Various types of driving waveforms are applied to the piezoelectric actuator
126
depending on the size of the ink drop
127
to be ejected from the nozzle
122
. For the ejection of large-diameter ink drops
127
for printing letters, deep-color parts, etc., a driving waveform as shown in
FIG. 2
is generally used. First, the voltage applied to the piezoelectric actuator
126
is raised (voltage increase process
111
), thereby the volume of the pressure generation chamber
121
is rapidly decreased and thereby the ink drop ejection is carried out. Thereafter, the voltage is returned to the bias voltage Vb (voltage decrease process
112
).
FIG. 3
is a schematic diagram showing the action of a meniscus in a nozzle when the ink drop ejection is carried out. The meniscus
132
which is almost flat in the beginning ((A) of
FIG. 3
) moves outward as the pressure generation chamber
121
is compressed, and thereby an ink drop
133
is ejected from the nozzle
131
((B) of FIG.
3
). Due to the decrease of ink in the nozzle
131
caused by the ink drop ejection, a concave meniscus
132
is formed in the nozzle
131
((C) of FIG.
3
). The surface of the concave meniscus
132
gradually returns to the nozzle opening due to surface tension of ink and thereafter recovers to the original state, that is, the state before the ink drop ejection ((D), (E) and (F) of FIG.
3
).
FIG. 4
is a graph showing the change of position of the meniscus when the ink drop ejection is carried out. As shown in
FIG. 4
, the meniscus
132
which withdrew widely (y=−60 &mgr;m) just after the ink drop ejection (t=0) returns to the initial position (y=0) after vibrating. In this document, the action of the meniscus returning to the initial position after the ink drop ejection will be referred to as “refill”, and the time (t
r
) necessary for the meniscus to return first to the nozzle opening surface (y=0) after the ink drop ejection will hereafter be referred to as “refill time”.
When the repeated and continuous ink drop ejection is carried out by an ink jet printing head, if an ink drop is ejected before the refill after the previous ink drop ejection is completed, the uniformity of the diameter and speed of ink drops is deteriorated and thereby the continuous ink drop ejection becomes unstable. In other words, stable ink drop ejection is impossible until a time t
r
or more elapses after the previous ink drop ejection. Therefore, the refill time t
r
is a critical characteristic value dominating the maximum ejection frequency (printing speed) of an ink jet printing head.
Besides the refill time t
r
, the number of nozzles also dominates the printing speed. As the number of nozzles increases, the number of dots that can be formed in a unit time increases and thereby the printing speed increases. Therefore, in ordinary ink jet printers of these days, a multi-nozzle printing head, having a plurality of ink drop ejection mechanisms (ejectors) which are connected together, is generally employed.
FIG. 5
is a schematic diagram showing the basic composition of a multi-nozzle ink jet printing head. An ink tank
157
is connected with a common ink channel
153
. To the common ink channel
153
, a plurality of pressure generation chambers
151
are connected via ink supply channels (unshown). By such composition, the ink drop ejection can be carried out from a plurality of ejectors at the same time and thereby the printing speed can be increased.
However, in order to realize stable ink drop ejection in such a multi-nozzle ink jet printing head, the common ink channel has to be designed properly, that is, pressure wave interference (crosstalk) etc. between the ejectors (which are connected with the common ink channel) has to be eliminated. Therefore, some methods for preventing the crosstalk between the ejectors by enlarging the acoustic capacitance of the common ink channel have been proposed so far.
For example, in an ink jet printing head disclosed in Japanese Patent Application Laid-Open No.SHO56-75863 (hereafter, referred to as “prior art #1”), the capacity of the common ink channel is set to more than twice as large as the total capacity of the pressure generation chambers (including nearby channels) and thereby the crosstalk is suppressed.
In Japanese Patent Application Laid-Open No.SHO52-49034 and Japanese Patent Application Laid-Open No.HEI9-141864, pressure damping means (air damper, pressure absorber, etc.) is provided to the common ink channel in order to realize a large acoustic capacitance even in a common ink channel of a limited capacity.
In Japanese Patent Application Laid-Open No.SHO59-26269 (hereafter, referred to as “prior art #2”), based on the number (N) of ejectors connected with the common ink channel and the impedance (Z
S
) of the ink supply channel, the impedance (Z
R
) of the common ink channel is set so as to satisfy a condition Z
R
≦Z
S
/(10N) and thereby the crosstalk is suppressed.
However, according to evaluations of experimentally manufactured multi-nozzle ink jet printing heads which have been performed by the present inventors, it became clear that the conventional ink jet printing heads explained above are not necessarily capable of guaranteeing stable ink drop ejection. The problems with the conventional ink jet printing heads will hereafter be explained referring to some concrete examples.
First, when a plurality of ejectors (which are connected together by the common ink channel) carries out the ink drop ejection simultaneously, the refill time of each ejector increases, and further, variation of refill time occurs between ejectors.
FIG. 6
is a graph showing an experimental result of the refill time of the conventional multi-nozzle ink jet printing head of FIG.
5
. In the experiment, a multi-nozzle ink jet printing head having 32 ejectors was used, and the refill time of each ejector was measured under different ejection conditions. An air damper was provided to the commo
Ishiyama Toshinori
Murakami Atsushi
Okuda Masakazu
Barlow John
Brooke Michael S.
Fuji 'Xerox Co., Ltd.
Scully Scott Murphy & Presser
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