Writing method for ink jet printer using electro-rheological fluid and apparatus thereof

A writing method is disclosed for an ink jet printer using an electro-rheological fluid wherein an electro-rheological fluid reservoir is provided in between static pressure tubes circuitously communicating with a larger-diameter portion and a smaller-diameter portion of a venturi tube. The pressure difference created in the venturi tube forces the injection of the electro-rheological fluid, and a writing potential is applied to the exit of the static pressure tubes from which the electro-rheological fluid is ejected so as to control the ejected amount.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for writing images 
using ink and, particularly, to a writing method for an ink jet printer 
using electro-rheological fluid and apparatus thereof which controls the 
ejection of ink by using an electrical potential for varying the viscosity 
of the fluid and a pressure difference of a venturi tube. 
2. Description of the Background Art 
Electro-rheological fluid is well-known for its electro-field 
responsiveness. The electro-rheological fluid was first disclosed in U.S. 
Pat. No. 2,417,850 by Winslow in 1943, and has been proposed in various 
forms in U.S. Pat. No. 3,047,057 by Winslow, USSR patent 1391951 by 
Lysenkov and U.S. Pat. No. 4,812,251 by Stangroom. 
Such electro-rheological fluids proposed by the aforementioned publications 
are basically made of electric-viscosity liquid containing a powdery 
additive of a minute particle diameter which, if an electric field is 
applied thereto, become varied in the viscosity. Here, viscosity has been 
known to vary proportional to the strength of the applied electric field, 
which is referred to as electric viscosity effect. The electric viscosity 
effect is that the viscosity of a fluid is varied depending on the 
strength of an applied electric field, and varies almost concurrently with 
the electric field application. Among the above-described 
electro-rheological fluids, there is one whose viscosity varies from a 
liquid state to a nearly solid state even by an electric field below 10 
KV/mm. 
Utilizing ink made with electro-rheological fluid and an appropriate 
controller, written images can be created. Technology for writing images 
using such electro-rheological ink has been disclosed (IS&T conference 91' 
11). 
A conventional head for ejecting such electro-rheological ink is 
illustrated in FIG. 1 which utilizes a nozzle sheet 1 and a pair of 
support sheets 2 and 3 that are stacked above and below the nozzle sheet 
respectively. Nozzle sheet 1 has an ink reservoir 1a for receiving ink of 
a certain quantity and a nozzle 1b for ejecting ink therefrom. Upper 
support sheet 2 has an ink supplying aperture 2a and an electrode plate 4, 
and lower support sheet 3 has another electrode plate 4'. In this 
configuration, a predetermined pressure is kept with respect to the inside 
and outside of nozzle 1b. The viscosity of the ink inside the nozzle is 
varied according to a writing potential, illustrated schematically at 
reference numeral 5, applied to a pair of electrode plates 4 and 4'. When 
the ink has a low viscosity, i.e. liquid, the ink is ejected due to the 
pressure difference between the inside and outside of the nozzle. That is, 
the viscosity of the ink inside the nozzle is varied by the strength of 
the electric field formed by the two electrodes, so that the ink is 
ejected when it has a high viscosity, i.e. nearly solid, and is not 
ejected when the ink has a low viscosity. Such a technology for ejecting 
electro-rheological ink according to the writing potential requires an 
additional means for creating the pressure difference inside and outside 
the nozzle and holding the pressure difference. This causes the apparatus 
to be complicated and expensive while impeding its miniaturization. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a writing 
method for an ink jet printer using an electro-rheological fluid which 
enables the electro-rheological fluid to jet at low pressure and low 
voltage. 
It is another object of the present invention to provide an apparatus which 
accomplishes the above writing method. 
To accomplish the first object, a writing method is provided for an ink jet 
printer using an electro-rheological fluid wherein an electro-rheological 
fluid reservoir is provided in the middle of static pressure tubes 
circuitously communicating with a larger-diameter portion and a 
smaller-diameter portion of a venturi tube on which a predetermined 
pressure acts. The pressure difference in the venturi tube forces the 
ejection of the electro-rheological fluid. Also, the ejected amount of 
electro-rheological fluid is controlled by applying a writing potential to 
the exit of the static pressure tubes from which the electro-rheological 
fluid is ejected. 
To accomplish the second object, a writing apparatus is provided for an ink 
jet printer using an electro-rheological fluid comprising means for 
producing pressure and a venturi tube which has a larger section at the 
entrance and a smaller section at the exit so as to produce the pressure 
difference at the entrance and exit by the pressure producing means. The 
writing apparatus further comprises static pressure tubes circuitously 
communicating with the larger-diameter and smaller-diameter of the venturi 
tube and an electro-rheological fluid reservoir installed in the middle of 
the static pressure tubes. An ink valving means is also provided for 
controlling the flow of ink at the exit of the static pressure tube. 
In the present invention, the viscosity of the electro-rheological fluid is 
varied by the ink valving means, and a pressure difference is produced in 
the venturi tube by the pressure producing means to eject the 
electro-rheological fluid. The ejection of the fluid is made possible even 
under the conditions that the ink valving means has a low voltage and the 
pressure difference of the venturi tube is low. Further, the pressure 
producing means and the ink valving means are installed inside the venturi 
tube, thereby simplifying the structure.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, reference number 18 represents a venturi tube having 
static pressure tubes 13 and 13' connecting two points A and B of 
different passage sections and filled with electro-rheological fluid 19. 
An ink valving means is provided comprising two opposing writing 
electrodes 11 and 12 to which a writing potential signal is applied. The 
writing electrodes 11 and 12 are provided on the side of static pressure 
tube 13 which is connected at point B of the smaller section. Writing 
electrodes 11 and 12 are connected to a writing signal generator 10 and a 
writing potential representing a printing signal is applied to the 
electrodes. Static pressure tube 13' has a reservoir 14 for storing 
predetermined quantities of electro-rheological fluid 19. Reservoir 14 is 
connected to a fluid supply tank (not shown) to be continuously supplied 
with the electro-rheological fluid so that the fluid in the reservoir 
keeps a specific height. Venturi tube 18 has a pressure generating means 
(not shown) for producing the pressure difference between points A and B. 
The pressure generating means forces air to flow at high speed from point 
A of the larger section to point B of the smaller section, so that 
pressure is lower at point B of the smaller section. The pressure 
generating means can be a suitable means for creating air flow, such as a 
fan. 
As illustrated in FIG. 3, writing electrodes 11 and 12 are inserted into 
static pressure tube 13 to a predetermined length. The exit of venturi 
tube 18 is positioned so that it is spaced apart from paper 17, conveyed 
by a platen 16, by a specific interval H, as shown in FIG. 2. 
The operation of the writing apparatus using electro-rheological fluid of 
the present invention will be described below. 
First, for instance, if a fan (not shown) forces air to flow from point A 
to point B at high speed, a pressure difference is produced between points 
A and B. Then, electro-rheological fluid 19 flows to static pressure tube 
13 of the lower pressure point B. At the same time, when the writing 
potential produced by writing signal generator 10 is applied to writing 
electrodes 11 and 12, the viscosity of electro-rheological fluid 19 is 
varied according to the strength of the potential thereby varying the 
shearing yield stress of electro-rheological fluid 19. Accordingly, due to 
the shearing yield stress of electro-rheological fluid 19 determined by 
the strength of the applied writing potential and the ejection force 
difference of electro-rheological fluid 19 due to the pressure difference, 
the amount of electro-rheological fluid 19 ejected onto paper 17 is 
controlled. 
The pressure difference between A and B can be set according to Bernoulli's 
equation and a continuity equation, as follows: 
EQU P.sub.A -P.sub.B =.sub..gamma.a V.sub.B.sup.2 /2g(1-(A.sub.A 
/A.sub.B).sup.2)+(Z.sub.B -Z.sub.A)+h.sub.L ! 
where, 
P.sub.A and P.sub.B are pressures at A and B; 
.gamma..sub.a is the specific weight of air; 
Z.sub.A and Z.sub.B are heights at A and B with respect to a specific 
reference; 
A.sub.A and A.sub.B are sectional areas; 
h.sub.L is loss; and 
V.sub.B is air speed at B. 
Thus, the pressure difference between points A and B is determined by the 
above factors in the above equation. Here, if the pressure difference 
between A and B is set to be greater than .sub..gamma. h (see FIG. 2), 
electro-rheological fluid 19 can be ejected from static pressure tube 13. 
That is, electro-rheological fluid 19 ejected from static pressure tube 13 
is ejected at an ejecting pressure of P.sub.i =(P.sub.A 
-P.sub.B)-.sub..gamma. h. Here, since ejecting height h is not 
substantially more than 10 mm, the rising height of capillary h' is 
considered. The rising height of capillary h' may be expressed as 
h'=4.sigma.cos.theta./.sub..gamma. D. Here, .sigma. is the surface tension 
of electro-rheological fluid and D is the diameter of static pressure tube 
13. 
Meanwhile, the shearing yield stress .tau. of electro-rheological fluid 19 
is defined as .tau.=F/.pi.DL, and the ejecting pressure as P.sub.i 
=4F/.pi.D.sup.2. Here, F is the ejection force from static pressure tube 
13, D is the diameter of static pressure tube 13, and L is the length of 
writing electrodes 11 and 12. If the shearing yield stress is set to be 
greater than the ejecting pressure, that is, .tau.&gt;P.sub.i D/4L, the 
injecting of electro-rheological fluid 19 can be controlled. Here, the 
shearing yield stress of electro-rheological fluid 19 is varied according 
to the strength of the potential applied to writing electrodes 11 and 12. 
This can be expressed as follows in connection with the above equations. 
EQU .tau.&gt;D/4L .sub..gamma.a {V.sub.B.sup.2 /2g(1-(A.sub.A 
/A.sub.B).sup.2)+(Z.sub.B -Z.sub.A)+h.sub.L }-.sub..gamma. h! 
Therefore, if the shearing yield stress of electro-rheological fluid 19 
satisfies the above equation, ejecting the ink from static pressure tube 
13 can be controlled. 
FIG. 4 illustrates the relationship of the average speed at the venturi 
exit and the ejection amount at the static pressure tube with respect to a 
set passage section ratio (A.sub.A /A.sub.B) and the injection height h 
according to an embodiment of the present invention. Here, the mean 
required amount of ink to print a dot is 10.sup.-10 liters. To print five 
A4 sheets per minute at 400 dpi (dot per inch), an injection amount of 
about 5.46.times.10.sup.-11 mm.sup.3 per second is required. As shown in 
FIG. 4, in order to eject electro-rheological fluid 19 at 
5.46.times.10.sup.-11 mm.sup.3 per second, it is sufficient that the speed 
of the venturi exit, though depending upon factors in the above equations, 
is above 15 m per second. To produce the exit speed of air, a discharge 
pressure of P.sub.P =.sub..gamma.a V.sup.2 /2 g (where .sub..gamma.a is 
the specific weight of air) of a pump should be about 1.4.times.10.sup.-3 
atm. This can be sufficiently accomplished even with an ordinary fan. 
As described above, since the writing method for an ink jet printer using 
an electro-rheological fluid and apparatus thereof according to the 
present invention does not need high pressure nor high voltage, it is 
favorable to high integration and low-cost production. Further, since the 
ejecting force of ink at the ink outlet is low and, thus, allowing for low 
shearing yield stress of the electro-rheological fluid, the present 
invention is advantageous in selecting a electro-rheological fluid and 
lowering its cost. Furthermore, the ink outlet is simplified and is not 
deformed by high temperature or high pressure, thereby making the life of 
the outlet semipermanent.