Micro-dot ink jet recorder

An ink jet recorder in which ink droplets having different, large and small diameters are produced, only the small diameter ink droplets of the ink droplets are selectively charged and deflected to record dots on a recording medium. The property of the ink employed in the recorder is selected to meet the condition K.ltoreq.N.sup.n .times.T.sup.m (N represents the viscosity of the ink, T represents the surface tension thereof, and n, m and K are positive constants, respectively), thereby maintaining the diameter of the small diameter ink droplets at a substantially constant value in the operation temperature range of the recorder.

BACKGROUND OF THE INVENTION 
This invention generally relates to an ink jet recorder of the charging 
deflection type in which after ink droplets emerging from a nozzle are 
charged, they are deflected in a predetermined direction by the 
application of electric field so as to form recording dots on a recording 
medium. More particularly, this invention relates to an improvement of the 
micro-dot ink jet recorder in which two kinds of ink droplets of large 
diameter and small diameter are alternately emitted from a nozzle and only 
the ink droplets of small diameter are used for recording. 
In an ink jet recorder, ink droplets are emitted from a nozzle by applying 
a high frequency excitation voltage on a piezoelectric device mounted to 
the nozzle. The produced ink droplets are varied in their form by 
controlling the excitation voltage applied to the nozzle. For example, 
three forms of the ink droplets due to different excitation voltages are 
disclosed in U.S. Pat. No. 4,050,077 to Takahiro Yamada et al. assigned to 
the same assignee as this application. 
For realizing a high quality image in the ink jet recorder, it is most 
important that the ink droplets have uniform diameters. Nevertheless, in 
actual practice, small diameter ink droplets will follow larger diameter 
normal ink droplets, which will degrade the recorded image quality. 
Therefore, in the prior art, the excitation voltage was controlled so as 
to not produce such small diameter ink droplets, and so the small droplets 
have not been used for recording in the ink jet recorder. 
Yamada, who is one of the inventors of the present invention and others 
proposed to use, for recording, these small diameter ink droplets which 
conventionally have been avoided. This is because the small diameter ink 
droplets provide small dots, which permits the recording to be more 
precise and the gradation of the recorded image to be more minute. U.S. 
Pat. No. 4,050,077 mentioned above discloses a micro-dot ink jet recorder 
in which only small diameter ink droplets between the ink droplets of 
small diameter and large diameter are used for recording, i.e., only the 
small diameter ink droplets are selectively charged and deflected. 
Further, U.S. Pat. No. 4,408,211 to Yamada discloses a method for carrying 
out the charging and deflection of the small diameter ink droplet by means 
of a common electrode in a micro-dot ink jet recorder. The micro-dot ink 
jet recorder using small ink droplets is a remarkable invention in that 
the small ink droplets can be produced without reducing the diameter of 
the ink jet nozzle. Incidentally, a physical mechanism in which both large 
diameter ink droplets and small diameter ink droplets are emitted from the 
nozzle has not been clarified sufficiently as yet, but Yamada et al. 
experimentally found the condition of the excitation voltage for assuring 
the alternate production of both large and small diameter ink droplets 
with uniform diameters as shown in U.S. Pat. No. 4,050,077. 
SUMMARY OF THE INVENTION 
We have found that the micro-dot ink jet recorder gave rise to some 
problems causing color shear in printing, unstable dot diameters and 
sticking of the ink droplets onto the control electrodes thereby making it 
impossible to provide normal ink dots. As a result of this investigation, 
it was found that these problems result from the fact that the temperature 
increase inside the recorder in operation makes the diameter of the ink 
droplets even smaller than a predetermined diameter thereof; more 
specifically, when the diameter of the small ink droplets become finer 
than the predetermined value, they are undesirably deflected by the 
electric field so as to land at positions greatly displaced from the 
target positions on a recording medium, vary in their trajectory under the 
influence of the ambient air current and in an extreme case they are 
deposited on the control electrodes to produce a spark. The specifics of 
such problems will be explained in detail later. 
We have carried out many experiments to solve the problems of the micro-dot 
ink jet recorder mentioned above. As a result, it was found that the small 
diameter ink droplets maintain their predetermined diameter under a 
certain condition of ink property, i.e. surface tension and viscosity even 
when the temperature is increased to the highest temperature at the 
operation state of the recorder, and therefore, the problems mentioned 
above can be solved by using an ink which meets the above condition in a 
micro-dot ink jet recorder. 
An object of this invention is to provide a micro-dot ink jet recorder in 
which small diameter ink droplets employed for recording maintain their 
diameter at a substantially constant value in the normal operating 
temperature range of the recorder. 
To attain this object, in accordance with this invention, there is provided 
a micro-dot ink jet recorder comprising nozzle means for emitting ink, 
driving means mounted on the nozzle means for providing mechanical 
displacement to the nozzle in response to an excitation voltage so as to 
alternately emit ink droplets of large diameter and small diameter from 
the nozzle, and electrode means for selectively charging only the small 
diameter ink droplets and deflecting them in a predetermined direction by 
the application of an electric field, in which the surface tension T of 
the ink and the viscosity N thereof are in the highest temperature state 
of the recorder in the following relation: N.sup.n .times.T.sup.m 
.gtoreq.K (n, m=positive constant, K=constant), and K is set to such a 
value at the diameter of the small diameter droplets is not substantially 
changed at the above highest temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a micro-dot ink jet recorder with charging electrodes and 
deflecting electrodes formed as common electrodes. Ink pressurized by an 
ink system 1 incorporating a pump is supplied to a nozzle 2 and emitted as 
an ink column 6 from a nozzle opening 3. A piezoelectric device 4 mounted 
on the nozzle 2 is excited by the voltage form a high frequency power 
supply 5 so as to vibrate the ink column 6. Thus, the ink column 6 is 
separated alternately into large diameter ink droplets 7a and small 
diameter ink droplets 7b from its tip. Control electrodes 8a and 8b are 
oppositely provided so as to cover the region where the ink column 6 is 
separated into the ink droplets 7a and 7b. These electrodes are supplied 
with recording signals (charging signals) from recording signal sources 9a 
and 9b and voltages from deflecting power sources 10a and 10b respectively 
so that the ink droplets are selectively charged in accordance with the 
recording signals and subsequently deflected by the electric field. The 
deflected ink droplets pass over a gutter 11 and reach a recording medium 
12 to form dots 13. The ink droplets not used to form the recording 
pattern travel straight without being charged and are deflected, and 
collected by the gutter 11. 
Now, the relations between excitation voltage and the shapes of the 
produced ink droplets will be explained on the basis of the experimental 
results shown in FIGS. 2 and 3. FIG. 2 shows the various ways in which the 
ink droplets are formed when the excitation voltage is changed and the 
charging states of the ink droplets thus formed (the charged droplets are 
indicated with + marks). Below FIG. 2 is a sketch of the ink column tip 
portion marked with Greek letters on the points where the ink droplets are 
separated. The ink droplets are charged at timings of the .alpha. points. 
As seen from FIG. 2, the separation sequence of mode C allows the charging 
of only the small diameter ink droplets, and so the excitation voltage in 
this case means an optimum excitation voltage. FIG. 3 shows the timing 
relations between the excitation voltage of a period T applied to the 
piezeoelectric device 4 and the charging signals applied to the electrodes 
8a and 8b, together with the states of the ink droplets formed with the 
elapse of time. As seen from FIG. 3, in mode C, the large diameter ink 
droplets and the small diameter ink droplets are separated from the ink 
column at constant time intervals. If the electrodes 8a and 8b are 
supplied with charging signal pulses with a pulse width of approx. T/2 at 
timings as shown in FIG. 3, the charging is performed when the small 
diameter ink droplets are separated from the ink column 6, but it is not 
performed when the large diameter ink droplets are separated. Therefore, 
only the small diameter ink droplets can be charged. 
The operation of charging and deflection, which are performed by the common 
electrodes 8a and 8b shown in FIG. 1, will be explained below. These 
electrodes 8a and 8b are biased by D.C. voltages 10a and 10b with opposite 
polarities, respectively so that an electric field with the electrode 8b 
at a positive potential is generated between the electrodes 8a and 8b. The 
ink is placed at a ground potential. Therefore, if the ink column 6 is 
placed at a central position between the electrodes 8a and 8b, it is also 
placed at a ground potential without being charged by the bias voltage. 
However, if the electrodes 8a and 8b are supplied with the charging 
signals 9a and 9b with the same phase, the ink droplets are charged with 
the charging amount corresponding to the amplitude of the charging 
signals. Since they are deflected by the electric field between the 
electrodes 8a and 8b at the same time as the charging, they fly in a 
predetermined direction. In this way, if the ink droplets are generated 
with the optimum excitation voltage and the charging signal applied at the 
timings as shown in FIG. 3, only the small uniform diameter ink droplets 
can be adopted for recording. 
However, it has been confirmed by experiments that if the ink temperature 
is increased with the operation temperature of the recorder, the small and 
uniform diameter ink droplets will not be produced. FIG. 4 shows one 
example of the temperature characteristics of the diameter of the small 
ink droplet diameter in the conventional ink. The abscissa represents the 
operation temperature, of the recorder, i.e. the ink temperature while the 
ordinate represents the diameter .phi.d of the small diameter ink 
droplets. As seen from FIG. 4, when the ink temperature exceeds 25.degree. 
C., the diameter .phi.d starts to become small, and when the ink 
temperature exceeds 30.degree. C., that diameter abruptly becomes small. 
Thus, particularly when the ink temperature exceeds 30.degree. C., the 
color reproduction will greatly deteriorate because of increased 
deflection of the ink droplets, the recording dot diameter will become 
unreasonably small, the record will be confused because of unstabilized 
flying of the ink droplets, and the fine ink droplets will stick to the 
control electrodes to make the recording impossible. FIGS. 5(a) and (b) 
illustrate the time-sequential manner in which the small diameter ink 
droplets are formed at room temperature (20.degree. C.) and at a higher 
temperature (30.degree. C.), respectively. As seen from the figure, in the 
case of room temperature (FIG. 5(a)), the small diameter droplet will be 
sharply separated from the large diameter droplet, whereas in the case of 
a higher temperature (FIG. 5(b)), a part of the small diameter droplet is 
absorbed into the large diameter droplet in the separation process, 
resulting in a smaller diameter droplet than in the case of room 
temperature. 
In order to solve the above problem caused by the increase of the ink 
temperature, we have carried out the experiments as shown in FIG. 6. FIG. 
6 illustrates the states of the small diameter ink droplets formed when 
the ink temperature is changed for several kinds of ink with different 
surface tensions and viscosities, which are main ink properties changed 
with temperature. The ink viscosity is gradually increased from ink (A) 
toward ink (O), which can be performed by increasing the concentration of 
the wetting agent, e.g. polyethylene glycol or ethylene glycol, contained 
in the ink. The surface tension and viscosity of the ink as well as the 
diameter of the small diameter ink droplets were measured changing the ink 
temperature in the range of 10.degree. C.-40.degree. C. The measurement 
result is that the ink (A), for example, provides at 10.degree. C. a 
surface tension of approximately 61 dyne/cm.sup.2 and a viscosity of 1.6 
cp.g/cm.sup.3, but when the ink temperature is increased, the ink (A) 
provides a surface tension and viscosity both reduced in the direction to 
the left and bottom in the graph of FIG. 6. In the graph, the points 
indicated with circle marks .circle. are characteristic points where the 
diameter of the small diameter ink droplets is at a predetermined value to 
permit the normal dots to be recorded. The points indicated with triangle 
marks .DELTA. are characteristic points where that diameter becomes 
slightly small, but the recorded dots don't provide any problem in 
practical use. The points indicated with cross marks X are characteristic 
points where that diameter abruptly becomes small to make it impossible to 
record the normal dots. The ink (A) provides an abnormality when the ink 
temperature exceeds 30.degree. C. The cross mark points correspond to the 
temperature range exceeding 30.degree. C. in the graph of FIG. 4. When the 
ink viscosity is increased by increasing the concentration of the wetting 
agent in the ink, the ink surface tension inversely tends to be reduced. 
These data indicate that the boundary between the normal mark points 
.circle. and the triangle mark points .DELTA. and the boundary between 
the triangle mark points .DELTA. and the abnormal mark points X roughly 
draw curves as shown by a broken line and a solid line in the graph of 
FIG. 6, respectively. Namely, when the surface tension and viscosity are 
present above the broken line, the diameter of the small diameter ink 
droplets is maintained at a predetermined value, thereby permitting the 
normal dots to be recorded. When they are present between the broken line 
and the solid line, that diameter is reduced to the value of approx. 90% 
of the predetermined value, but it is possible to record the dots so that 
no problem occurs in practical use. 
Assuming that the ink viscosity is N, and the ink surface tension is T, the 
approximate equation of the curve represented by the solid line is 
EQU N.sup.n .times.T.sup.m =K 
where n, m and K are positive constants. The curve represented by this 
equation is a boundary between conditions where the small diameter ink 
droplets having a substantially constant diameter are or are not formed, 
and this boundary is referred to as a stable formation boundary of the 
small diameter ink droplets. 
In order to record the normal dots, the condition N.sup.n .times.T.sup.m 
.gtoreq.K must be satsified. Namely, the values of the viscosity and the 
surface tension of the ink must be present in the region over the solid 
line curve of FIG. 6. FIG. 6 shows that some inks provide the surface 
tension and viscosity within the stable formation boundary of the smaller 
ink droplets at 40.degree. C., which is the highest temperature of the ink 
jet recorder. One example thereof is an ink (O). 
FIG. 7 shows a characteristic of the temperature vs. the diameter of the 
smaller diameter ink droplets in the ink (O). As seen from the figure, 
that diameter is maintained constant in the temperature range of 
10.degree. C.-40.degree. C. This characteristic of the ink (O) is 
apparently different from that of the conventional ink as shown in FIG. 4. 
The conventional ink has a viscosity set at 1.7-2 (cp.g/cm.sup.3) at 
25.degree. C., containing a wetting agent of approx. 10%. The setting of 
such an extent of the viscosity is because raising the viscosity too much 
in the conventional recorder will increase the pressure loss at the nozzle 
to reduce the jetting speed of the droplets. On the other hand, the ink 
employed in the micro-dot ink jet recorder of this invention preferably 
contains a wetting agent of approx. 30-50% and a viscosity of approx. 4 
(cp.g/cm.sup.3) at 25.degree. C. These values are not limitative as long 
as the viscosity and surface tension are present in the region over the 
stable formation boundary of the small ink droplets. In the case of an 
oily ink, its viscosity can be increased by increasing the content of 
resin, e.g. acryle resin. The pressure loss at the nozzle caused by the 
viscosity increase can be compensated for by enhancing the ink 
transmission pressure in the ink system. 
In the case where the ink jet recorder emits ink at a speed of 40 m/sec 
from a nozzle having a diameter of approx. 65 .mu.m, and produces ink 
droplets by exciting the piezoelectric device at a frequency of approx. 
138 kHz, n.apprxeq.1, m.apprxeq.3 and K.apprxeq.1.7.times.10.sup.5 apply 
to the equation representative of the stable formation boundary of small 
diameter ink droplets. In the case that the nozzle diameter is increased, 
the value of K correspondingly increases, and in the case that the nozzle 
diameter is decreased. the value K correspondingly decreases. For example 
where the nozzle diameter is 70 .mu.m and 60 .mu.m, 
K.apprxeq.1.9.times.10.sup.5 and K.apprxeq.1.5.times.10.sup.5, 
respectively. In each cases, mere slight changes arise in the values of n 
and m. On the other hand, when the excitation frequency applied to the 
piezoelectric device is set to be higher or the diameter of the small 
droplet is set to increase, the emitting speed of the ink droplet should 
be made higher. However, the values of n and m change slightly. 
In FIG. 8, the chain line represents the stable formation boundary of small 
ink droplets, and the solid line and the broken line represent the states 
of the ink droplets i.e. dots when the temperature of the inventive ink 
and the conventional ink are changed from 10.degree. C. to 40.degree. C. 
in the ink jet recorder, respectively. As seen from the figure, the 
conventional ink gives rise to poor dots at 30.degree. C. while the 
inventive ink provides normal dots even at 40.degree. C.