Precision ejection ink-jet printing apparatus

A precision ejection ink-jet printing apparatus to prevent self-ejection. Self-ejection is an ejection of coloring agent particles from an ink-ejection opening without an application of ejection voltage to an ejection electrode. The precision ejection ink-jet printing apparatus includes an ink chamber filled with a pigment type ink, an electrophoretic electrode for causing coloring agent particles in the pigment type ink to concentrate at ink ejection openings, a plurality of ejection electrodes for causing ejection of the coloring agent particles concentrated at the ink ejection openings toward a printing medium, and voltage controller for controlling a voltage to be applied to the electrophoretic electrode. The voltage controller gradually increases the applied voltage up to a predetermined target voltage. The gradual increase in voltage moderates the electrophoretic motion of the coloring agent particles, preventing self-ejection.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an ink-jet printing apparatus. 
More specifically, the invention relates to an ink-jet printing apparatus 
for controlling a coloring agent particle in a pigment type ink by 
electrophoretic effect. 
2. Description of the Related Art 
In recent years, non-impact printing methods are attracting attention 
because they generate little noise during printing. An ink-jet printing 
method is quite dominant its capability of direct printing on a printing 
medium with a simple mechanism, and of printing on plain paper. Various 
systems of ink-jet printing apparatus have been proposed. Conventionally, 
an electrostatic ink jet printing apparatus prints by applying a voltage 
between an electrode provided on the back surface of the printing medium 
and a needle shaped ejection electrode, thereby making a coloring agent of 
an ink or the like fly toward the print medium by an electrostatic force 
of the generated electric field, as disclosed in Japanese Unexamined 
Patent Publication Nos. Showa 60-228162 and Heisei 8-309993. 
FIG. 7 is a general illustration of the conventional electrostatic ink-jet 
printing apparatus. The shown conventional ink-jet printing apparatus 
includes an ink chamber 1 filled with a pigment type ink 10, an 
electrophoretic electrode 3 for causing coloring agent particles in the 
pigment type ink to move to concentrate at the ink ejection openings 2 by 
electrophoretic effect, a plurality of ejection electrodes for ejecting 
the coloring agent particle concentrated at the ink ejection openings 2, 
toward a printing medium 4, and an opposing electrode 9 arranged on a back 
surface of the printing medium 4 in opposition to the ejection electrodes 
5. 
The ink ejection openings 2 are separated per respective ejection 
electrodes 5 by flow passage walls 8 so that a convex meniscus of the 
pigment type ink 10 can be formed at the tip end of respective ejection 
electrodes. The ink chamber 1 connects to a not shown ink tank through an 
ink supply port 6 and an ink drain port 7 by a not shown tube. The pigment 
type ink 10 in the ink chamber 1 is under a back pressure and forced to 
circulate. 
FIG. 8 is a chart of a waveform of a voltage that is applied to the 
electrophoretic electrode and the ejection electrode of the conventional 
ink-jet printing apparatus. Operation of the conventional ink-jet printing 
apparatus will be discussed with reference to FIGS. 7 and 8. The shown 
ink-jet printing apparatus utilizes electrophoretic effect to orient the 
coloring agent particles in one direction by applying an electric field to 
the pigment type ink containing charged coloring agent particles. Namely, 
by applying a constant voltage VI to the electrophoretic electrode 3 to 
apply the electric field to the ink chambers 1 filled with the pigment 
type ink 10, the coloring agent particles in the pigment type ink 10 move 
toward the ink ejection openings 2 at a certain electrophoretic motion 
speed to form a convex meniscus of the pigment type ink 1 at the tip ends 
of the ejection electrodes 5. Ejection occurs by electrostatic force when 
a pulse of voltage V2 is applied to the ejection electrodes 5 for a pulse 
period T0 particles move to concentrate to the tip end portion of the 
By electrostatic force, the coloring agent particles overcome the meniscus, 
the surface tension of the pigment type ink, viscosity, and so forth, to 
fly from the tip end of the ejection openings 5 toward the opposing 
electrode 9 as fine flying particles at a timing synchronized with the 
pulse form ejection voltage and to be deposited on a printing medium 4. 
A problem encountered in the conventional ink-jet printing apparatus is a 
possibility of self-ejection causing coloring agent particles to eject 
without application of the pulse form ejection voltage on the ejection 
electrodes. Self-ejection degrades the image quality of what is printed. 
Before discussion will be given for the cause of the self-ejection, brief 
discussion will be given for mobility of the coloring agent particle, 
charge relaxation time, and the time constant of deformation of meniscus. 
The mobility .alpha. [(m/s)/V/m)] of the coloring agent particle is 
generally expressed by .alpha.=.epsilon..zeta./6.pi..mu., wherein 
.epsilon. is a dielectric constant of a medium, .zeta. is a zeta 
potential, .mu. is a viscosity. The mobility .alpha. is a characteristic 
value specific to the pigment type ink used and is used for deriving the 
speed of motion of the coloring agent particles as they move to 
concentrate at the ink ejection openings in the electric field generated 
by a voltage applied to the electrophoretic electrode, 
The charge relaxation time is a period required to establish a balanced 
condition of the influence of the electric field caused in the pigment 
type ink by the voltage applied to the electrophoretic electrode, for 
which a ratio of an electric conductivity .sigma. of the pigment type ink 
and the dielectric constant .epsilon., and a time constant 
.epsilon./.alpha. may provide references. The time constant 
.epsilon./.alpha. is also a characteristic value specific to the pigment 
type ink to be used similar to the mobility .alpha.. For instance, a time 
constant .epsilon./.alpha. of a pure water is about 1 .mu.s. 
The time constant of deformation of meniscus can be an indicia of the 
condition of variation of shape of the meniscus and is associated with the 
surface tension and viscosity of the pigment type ink, and the motion 
speed and degree of concentration of the coloring agent particles. Since 
the surface tension and the viscosity of the pigment type ink are 
characteristic values specific to the pigment type in question, they 
should be constant. Therefore, a primary factor in determining the time 
constant of deformation of the meniscus is the motion speed and degree of 
concentration of the coloring agent particles, which is dependent on the a 
voltage applied to the electrophoretic electrode. Accordingly, by 
controlling; the voltage applied to the electrophoretic electrode, the 
time constant of deformation of meniscus can be varied. 
The reason will be discussed hereinafter. FIG. 5 shows the influence of the 
electric field caused in the pigment type ink upon the instantaneous 
change of the voltage applied to the electrophoretic electrode to a target 
voltage V1, until establishment of balance. By applying the targeted 
voltage V1 all at once, the coloring agent particles move toward the ink 
ejection openings simultaneously at a speed which can be calculated from 
the foregoing mobility .alpha.. By simultaneous motion of the coloring 
agent particles toward the ink ejection openings, abrupt concentration of 
the coloring agent particles causes a meniscus to form. The shape of the 
meniscus is varied. Due to variation of the shape of the meniscus and 
certain external factor, the coloring agent particles can be ejected 
unwantedly. At this time, the time constant of deformation of meniscus to 
be an indicia of the condition of variation of the shape of the meniscus 
becomes smaller than a time constant .epsilon./.sigma. as shown in FIG. 6. 
SUMMARY OF THE INVENTION 
An object of the present invention is to simply prevent self-ejection from 
ejecting coloring agent particles without a pulse-form ejection voltage. 
According to one aspect of the present invention, an ink-jet printing 
apparatus comprises: 
an ink chamber filled with a pigment type ink; 
an electrophoretic electrode for causing coloring agent particles in the 
pigment type ink to concentrate at ink ejection openings; 
a plurality of ejection electrodes for causing ejection of the coloring 
agent particles concentrated to the ink ejection openings toward a 
printing medium; and 
voltage control means for controlling a voltage applied to the 
electrophoretic electrode to gradually increase the applied voltage to a 
predetermined target voltage. 
Preferably, the voltage control means elevates the voltage applied to the 
electrophoretic electrode in stepwise fashion up to the predetermined 
target voltage, with a time constant of deformation of meniscus of the 
pigment type ink smaller than a time constant of the pigment type ink 
determined by an electric conductivity of the pigment type ink and a 
dielectric constant of the pigment type ink. 
In the alternative, the voltage control means may elevate the voltage 
applied to the electrophoretic electrode in stepwise fashion up to the 
predetermined target voltage, with a time constant of deformation of 
meniscus of the pigment type ink greater than a time constant of the 
pigment type ink determined by an electric conductivity of the pigment 
type ink and a dielectric constant of the pigment type ink. 
In the further alternative, the voltage control means may elevate the 
voltage applied to the electrophoretic electrode up to the predetermined 
target voltage with a time constant substantially equal to a time constant 
of the pigment type ink.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be discussed hereinafter in detail in terms of 
the preferred embodiment of the present invention with reference to the 
accompanying drawings. In the following description, numerous specific 
details are set forth in order to provide a thorough understanding of the 
present invention. It will be obvious, however, to those skilled in the 
art that the present invention may be practiced without these specific 
details. In other instances, well-known structures are not shown in detail 
in order to avoid unnecessarily obscuring the present invention. 
FIG. 1 shows a general construction of an ink-jet printing apparatus 
according to the present invention. In the shown ink-jet printing 
apparatus, in addition to the conventional construction shown in FIG. 7, a 
voltage control means 13 for controlling a voltage applied to the 
electrophoretic electrode 3 from a power source 12 is provided. 
FIG. 2 shows a waveform of a voltage to be applied to the electrophoretic 
electrode 3 by the voltage control means 13, together with a voltage 
waveform of the pigment type ink. Referring to FIG. 2, the voltage control 
means 13 varies the voltage applied to the electrophoretic electrode 3 in 
stepwise fashion so that a time constant of deformation of meniscus (shown 
by thick line in FIG. 2) is always smaller than a time constant 
.epsilon./.sigma. of the pigment type ink. In FIG. 2, a voltage V11 is 
initially applied to the electrophoretic electrode 3 for a period T1. 
Then, a voltage V12 is applied to the electrophoretic electrode for a 
period T2, and finally a target voltage is applied. 
FIG. 3 shows another waveform of a voltage to be applied to the 
electrophoretic electrode 3 by the voltage control means 13, together with 
a voltage waveform of the pigment type ink. Referring to FIG. 3, the 
voltage control means 13 varies the voltage applied to the electrophoretic 
electrode 3 in stepwise fashion so that a time constant of deformation of 
meniscus (shown by thick line in FIG. 2) is always greater than a time 
constant .epsilon./.sigma. of the pigment type ink. In FIG. 3, a voltage 
Vs1 is initially applied to the electrophoretic electrode 3 for a period 
T3. Then, a voltage Vs2 is applied to the electrophoretic electrode for a 
period T4, and finally a target voltage is applied. In FIGS. 2 and 3. 
V11&gt;Vs1 and V12&gt;Vs2. 
Operation upon using the waveform of the applied voltage as shown in FIG. 2 
will be discussed with making reference to FIG. 1. 
Initially, a voltage V11 is applied to the electrophoretic electrode 3 for 
a period T1 to apply an electric field to the ink chamber 1 filled with 
the pigment type ink 10. Then, the coloring agent particles in the pigment 
type ink simultaneously cause motion toward the ink ejection openings at 
an electrophoretic speed determined by the mobility .alpha. and the 
electric field generated by the voltage V11. Thus, the coloring agent 
particles initiate motion toward the ink ejection openings 2 to cause 
deformation of meniscus at an earlier time than the elapsing of the charge 
relaxation time. However, at this condition, the intensity of the electric 
field generated is not sufficient to cause flying of the coloring agent 
particles from the ink ejection openings 2. Subsequently, after a period 
corresponding to the time constant .epsilon./.sigma., the surface of the 
pigment type ink 10 reaches a balanced condition. Then, the coloring agent 
particles stop (sequence a). 
Next, after application of the voltage V11 to the electrophoretic electrode 
3, another voltage V12 is applied to the electrophoretic electrode 3 for a 
period T2. Then, the electric field generated by application of the 
voltage V12 is applied to the ink chamber 1 filled with the pigment type 
ink 10. Then, from the condition of the sequence a, the coloring agent 
particles in the pigment type ink 10 cause motion toward the ink injection 
openings at an electrophoretic speed determined by the mobility .alpha. 
and the electric field generated by application of the given voltage V12. 
Thus, the coloring agent particles simultaneously cause the motion toward 
the ink ejection opening at an electrophoretic speed determined by the 
mobility .alpha. and the electric field generated by the voltage V12. 
Thus, the coloring agent particles initiate motion toward the ink ejection 
openings 2 to cause deformation of meniscus at earlier timing than the 
elapsing of the charge relaxation time. However, at this condition, the 
intensity of the electric field generated is not sufficient to cause 
flying of the coloring agent particles from the ink ejection openings 2. 
Subsequently, after a period corresponding to the time constant 
.epsilon./.sigma., the surface of the pigment type ink 10 reaches a 
balanced condition. Then, the coloring agent particles stop (sequence b). 
After application of the voltage V12 to the electrophoretic electrode 3, 
the target voltage V1 is applied to the ink chamber 1 filled with the 
pigment type ink 10. From the condition of the sequence b, the coloring 
agent particles cause motion to the ink ejection openings 2 at the 
electrophoretic speed determined by the mobility .alpha. and the electric 
field caused by the targeted voltage V1. The coloring agent particles 
undergo simultaneous motion toward the ink ejection openings 2, again 
causing deformation of meniscus at an earlier time than the elapsing of 
the charge relaxation time. Thus, convex meniscus of the pigment type ink 
10 is formed at the tip ends of the ejection electrodes (sequence c). 
At this condition, a pulse form ejection voltage having a peak voltage V2 
and a pulse period T0 as shown in FIG. 8 is applied to the ejection 
electrodes 5, performing ejection of the coloring agent particles. Then, 
the motion energy of the coloring agent particles caused by electrostatic 
force overcomes constraint forces, such as the meniscus, surface tension 
of the pigment type ink, viscosity and so forth, to generate a fine flying 
droplet group 11 flying from the tip ends of the ejection electrodes 5, to 
be deposited on the printing medium 4, the a timing synchronous with the 
timing of application of the pulse form ejection voltage. 
Next, operation upon using the waveform of the applied voltage as shown in 
FIG. 3 will be discussed with references to FIG. 1. 
Initially, a voltage Vs1 is applied to the electrophoretic electrode 3 for 
a period T3 to apply an electric field to the ink chamber 1 filled with 
the pigment type ink 10. Then, the coloring agent particles in the pigment 
type ink simultaneously cause motion toward the ink ejection opening at an 
electrophoretic speed determined by the mobility .alpha. and the electric 
field generated by the voltage Vs1. Thus, the coloring agent particles 
initiate motion toward the ink ejection openings 2, causing a deformation 
of the meniscus at an earlier time than the elapsing of the charge 
relaxation time. However, at this condition, since the time constant of 
deformation of meniscus is greater than the time constant 
.epsilon./.sigma., the motion of the coloring agent particles stops at a 
time where a balance condition is established on the surface of the 
pigment type ink, thus stopping deformation of the shape of the meniscus 
at the midway (sequence a). 
Next, after application of the voltage Vs1 to the electrophoretic electrode 
3, another voltage Vs2 is applied to the electrophoretic electrode 3 for a 
period T3 to apply an electric field to the ink chamber 1 filled with the 
pigment type ink 10. Then, the coloring agent particles in the pigment 
type ink simultaneously cause motion toward the ink ejection openings at 
an electrophoretic speed determined by the mobility .alpha. and the 
electric field generated by the voltage Vs2. Thus, the coloring agent 
particles initiate motion toward the ink ejection openings 2 to causing a 
deformation of meniscus at an earlier time than the elapsing of the charge 
relaxation time. However, even at this condition, since the time constant 
of deformation of meniscus is greater than the time constant 
.epsilon./.sigma., the motion of the coloring agent particles stops at a 
time where a balance condition is established on the surface of the 
pigment type ink, thus stopping deformation of the shape of the meniscus 
at the midway (sequence b). 
After application of the voltage Vs2 to the electrophoretic electrode 3, 
the target voltage V1 is applied to the ink chamber 1 filled with the 
pigment type ink 10. From the condition of the sequence b, the coloring 
agent particles cause motion to the ink injection openings 2 from the 
condition of at an electrophoretic speed determined by the mobility 
.alpha. and the electric field caused by the target voltage V1. The 
coloring agent particles again undergo simultaneous motion toward the ink 
ejection openings 2. However, since the time constant of deformation of 
the meniscus is greater than the time constant .epsilon./.sigma., the 
coloring agent particle stops again at the condition where the surface of 
the pigment type ink 10 reaches a balanced condition. Thus, deformation of 
the meniscus due to concentration of the coloring agent particles again 
stops at the midway (sequence c). 
At this time, a convex meniscus of the pigment type ink 10 is formed at the 
tip ends of the ejection electrodes. At this condition, a pulse form 
ejection voltage having a peak voltage V2 and a pulse period T0 as shown 
in FIG. 8 is applied to the ejection electrodes 5 performing ejection of 
the coloring agent particles. Then, the motion energy of the coloring 
agent particles caused by the electrostatic force overcomes constraint 
forces, such as meniscus, surface tension of the pigment type ink, 
viscosity and so forth, to generate a fine flying droplet group 11 flying 
from the tip ends of the ejection electrodes 5 to be deposited on the 
printing medium 4, with a timing synchronous with the timing of 
application of the pulse form ejection voltage. As set forth above, when 
the voltage waveform applied to the electrophoretic electrode shown in 
FIG. 3 is used, variation of the shape of the meniscus is variable 
depending upon the time constant .epsilon./.sigma. of the pigment type 
ink, so that the variation of the shape of the meniscus can be predicted. 
FIG. 4 shows a further waveform of the voltage applied to the 
electrophoretic electrode 3 by the voltage control means 13. In this 
embodiment, the voltage control means controls the voltage applied to the 
electrophoretic electrode 3 so that the applied voltage is gradually 
increased during a period provided by the time constant .epsilon..sigma. 
of the pigment type ink. 
As set forth above, according to the present invention, by gradually 
causing deformation of the meniscus of the pigment type ink during a 
period set by the time constant of the pigment type ink, self-ejection of 
the coloring agent particle without application of the pulse form ejection 
voltage to the ejection electrodes is avoided. Thus, the quality of the 
image to be formed by ejection of the ink can be stabilized. 
Although the present invention has been illustrated and described with 
respect to exemplary embodiment thereof, it should be understood by those 
skilled in the art that the foregoing and various other changes, omissions 
and additions may be made therein and thereto, without departing from the 
spirit and scope of the present invention. Therefore, the present 
invention should not be understood as limited to the specific embodiment 
set out above but to include all possible embodiments which can be 
embodied within a scope encompassed and equivalents thereof with respect 
to the feature set out in the appended claims.