Method of driving ink jet recording head

Details Method of driving ink jet recording head Method of driving ink jet recording head

2001-04-19

2004-10-05

Pham, HarHai (Department: 2853)

Incremental printing of symbolic information

Ink jet

Controller

C347S011000, C347S012000, C347S068000

Reexamination Certificate

active

06799821

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for driving an ink jet recording head which method ejects fine ink droplets through a nozzle to record characters or images.
BACKGROUND ART
One of such recording heads, what is called an on-demand ink jet recording head that ejects ink droplets through a nozzle depending on printed information, is conventionally commonly known (for example, see Japanese Patent Publication No. SHO 53-12138).
FIG. 15
is a sectional view schematically showing a basic configuration of one of such on-demand ink jet recording heads which is called a Kyser type.
In this Kyser type recording head, on an ink upstream side, a pressure generating chamber
91
and a common ink chamber
92
are connected together via an ink supply hole (ink supply passage)
93
, and on an ink downstream side, the pressure generating chamber
91
and a nozzle
94
are connected together, as shown in FIG.
15
. Additionally, a bottom plate portion of the pressure generating section
91
, which is located at the bottom of
FIG. 15
, comprises a diaphragm
95
having a piezoelectric actuator
96
on its rear surface.
With this configuration, during a printing operation, the piezoelectric actuator
96
is driven depending on printed information to displace the diaphragm
95
, thereby changing the volume of the pressure generating chamber
91
rapidly to generate a pressure wave in the pressure generating section
91
. The pressure wave causes a part of an ink filled in the pressure generating chamber
91
to be injected to an exterior through the nozzle
94
and ejected as ink droplets
97
. The ejected ink droplets
98
arrived in a recording medium such as recording paper to form recording dots. Characters or images are recorded on the recording medium by repeating the formation of recording dots based on printing information.
The ink droplet ejecting operation will be further described. With this on-demand ink jet recording method or system, a single ink droplet is ejected whenever a driving voltage is applied to the piezoelectric actuator
96
. In the prior art, however, to eject a single ink droplet, a trapezoidal driving voltage waveform is generally applied to the piezoelectric actuator
96
.
The trapezoidal driving voltage waveform comprises a first voltage changing process
51
for linearly increasing a voltage V applied to the piezoelectric actuator
96
from a reference value up to a predetermined value V
1
to compress the pressure generating chamber
91
to eject the ink droplet
97
, a voltage maintaining process
52
for maintaining the applied voltage V at the predetermined value V
1
for a certain amount of time (time t
1
′), and a second voltage changing process
53
for subsequently returning the applied voltage V
1
to the reference voltage to return the compressed pressure generating chamber
91
to its original state, as shown in FIG.
16
.
Movement of the piezoelectric actuator caused by an increase or decrease in driving voltage depends on the structure or polarization of the piezoelectric actuator, so some piezoelectric actuators move in a direction opposite to the movement direction of the above-mentioned piezoelectric actuator. Since, however, the reversely operating piezoelectric actuator performs an ejection operation similar to that described above when an opposite driving voltage is applied, a piezoelectric actuator that moves in a direction that compresses the pressure generating chamber when the applied voltage increases, while moving in a direction that inflates the pressure generating chamber when the applied voltage decreases will be described in the following “BEST MODE FOR CARRYING OUT THE INVENTION” for simple explanation.
In this ink jet recording head, since a single pixel is formed when the ink droplet
97
impacts on recording paper to form a recording dot, if the recording dot has a large diameter, it appears granular to prevent high image quality from being obtained. Thus, a dot size required to obtain a smooth image that does not appear granular (high image quality) is empirically assumed to be 40 &mgr;m or less, and a dot size of 25 &mgr;m or less is considered very preferable. Evidently, the size of the ejected ink droplet
97
may be reduced in order to obtain a small dot size. The relationship between the ink droplet size and the dot size depends on a flying speed (droplet speed) of the ink droplet
97
, a physical property of the ink (e.g. viscosity or surface tension), the type of recording paper, or the like, but the dot size is normally about twice as large as the ink droplet size. Consequently, to obtain a dot size of 40 &mgr;m, the ink droplet size must be 20 &mgr;m, and to obtain a smaller size, for example, a dot size of 25 &mgr;m or less, the ink droplet size must be 12.5 &mgr;m or less.
On the other hand, it is theoretically known that if the ink droplet
97
is to be ejected through the nozzle
94
using a pressure wave, the volume q of the ejected ink droplet
97
is proportional to {circle around (1)} the opening area A
n
of the nozzle
94
, {circle around (2)} the speed (droplet speed) Vd of the ink droplet
97
, and {circle around (3)} the resonance frequency (specific cycle) Tc of the pressure wave in the pressure generating chamber
91
(acoustic fundamental vibration mode) in the as shown in Equation (1). Accordingly, to reduce the size of the ink droplet
97
, the nozzle opening diameter, the droplet speed V
d
, and the resonance frequency T
c
of the pressure wave may be correspondingly reduced.
q∝T
c
V
d
A
n
  (1)
Thus, first, the resonance frequency T
c
of the pressure wave will be discussed. The resonance frequency T
c
of the pressure wave is reduced by reducing the volume of the pressure generating chamber
91
or increasing the rigidity of walls of the pressure generating chamber while reducing the acoustic capacity of the pressure generating chamber
91
. When, however, the resonance frequency T
c
of the pressure wave is extremely reduced, for example, down to the order of several &mgr;s, a refilling operation is prevented from being operated smoothly, resulting in adverse effects on ejection efficiency, maximum driving frequency, or the like. Accordingly, the resonance frequency T
c
of the pressure wave has a minimum limit between 10 and 20 &mgr;s.
Next, the droplet speed V
d
of the ink droplet
97
will be described. The droplet speed V
d
affects the impact position accuracy of the ink droplet
97
, and a lower droplet speed reduces the impact position accuracy of the ink droplet
97
because the ink droplet
97
is affected by an air flow. Consequently, the droplet speed V
d
of the ink droplet
97
cannot be extremely reduced only to reduce the droplet size, and must after all have a fixed value or more (normally about 4 to 10 m/s) in order to obtain high image quality.
Next, the nozzle opening diameter will be described. Due to the above described reasons, it is empirically known that if the resonance frequency T
c
of the pressure wave in the pressure generating chamber
91
filled with an ink is set between about 10 and 20 &mgr;s, the droplet speed V
d
of the ink droplet
97
is set between about 4 and 10 m/s, and the piezoelectric actuator
96
is driven using the driving voltage waveform shown in
FIG. 16
, then the minimum ink droplet size obtained is equivalent to the nozzle diameter
97
. Accordingly, to obtain an ink droplet size of 20 &mgr;m, the nozzle diameter must be 20 &mgr;m, and to obtain an ink droplet size less than 20 &mgr;m, the nozzle diameter must be less than 20 &mgr;m. Forming a nozzle diameter less than 20 &mgr;m, however, makes manufacturing very difficult and increases the likelihood that the nozzle is blocked, thus significantly degrading the reliability and durability of the head. Thus, in fact, a nozzle diameter between 25 and 30 &mgr;m is presently a lower limit, so that under the above described conditions, the minimum droplet size obtained is between about 25 and 30 &mgr;m. It is expected that if the blocking problem is solved i

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