On-demand type ink-jet print head having an air flow path

An on-demand type ink-jet printer with a thin cylindrical tip end coaxially projecting into a cylindrical passageway. Ink droplet formation is controlled by a piezoelectric transducer. A stream of air is forced through an annular space formed between the tip end and passageway. The air pressures acting on a meniscus at the tip end is controlled by the position of the tip end within the passageway. A solvent for the ink saturates a porous member surrounding the end of the passageway to control drying of the ink.

This invention relates to an on-demand type ink-jet print head, and more 
particularly to an on-demand type ink-jet print head having an air flow 
path, as auxiliary means for ejecting ink droplets. 
The U.S. Pat. No. 4,106,032 entitled "APATUS FOR APPLYING LIQUID 
DROPLETS TO A SURFACE BY USING A HIGH SPEED LAMINAR AIR FLOW TO ACCELERATE 
THE SAME," issued to Miura, et al., shows an on-demand type ink-jet 
printhead having an air flow path as auxiliary means for increasing the 
velocity of a flying ink-droplet in order to obtain a clear picture. 
However, in a conventional on-demand type ink-jet print head having an air 
flow path, it is difficult to eject ink droplets which fly with a stable 
direction and velocity. 
It is, therefore, an object of this invention to provide an on-demand type 
ink-jet print head having an air flow path in which ink droplets are 
stably ejected. 
According to this invention, an on-demand ink-jet print head has an ink 
chamber; a piezoelectric element; a nozzle orifice; and an air flow path. 
The nozzle orifice has a thin wall and is provided in the air flow path.

Before the description of the embodiments of this invention, conventional 
on-demand type ink-jet print heads will be described with reference to 
FIGS. 1 and 2. 
Referring to FIG. 1, a first conventional example of an ink-jet head has a 
first nozzle 11 and a second nozzle 12 having an opening facing the first 
nozzle 11. Air flow 13 is caused to flow out of the second nozzle 12 and 
the ejection speed of an ink droplet leaving the nozzle is greatly 
increased when it is carried on this air flow. However, it was necessary 
to first drive the ink droplet into the inside 14 of the nozzle. 
In order to drive the ink droplet in this, a pulse pressure is applied to 
the ink by using an electrical mechanical conversion means such as a 
piezoelectric element. When this pulse pressure was too small for the ink 
pushed out of the first nozzle 11 to reach the inside 14 of the second 
nozzle, it was impossible to form a stable ink droplet under the influence 
of a complicated movement of air flow between the two nozzles. 
Therefore, there has been a limitation in the formation of a small volume 
ink droplet responsive to a reduced pulse pressure. Furthermore, the air 
which has passed through the passageway 15 between the two nozzles is 
abruptly accelerated in the inside of the second nozzle 12. Therefore, the 
ink meniscus 16 in the first nozzle was subjected to a force which forced 
it back toward the inside of the nozzle 11 as indicated by the arrow 17. 
As a result, air disadvantageously flowed into the ink, and even the pulse 
pressure was not able to eject the ink. In order to prevent such a state, 
when air flow was used, it was necessary to apply a fixed pressure to the 
ink so that the ink meniscus 16 can be located stably within the inside of 
the first nozzle 11. 
Referring to FIG. 2, in a second conventional example of an ink jet 
printer, a pipe for air supply 19 is attached to the outside of a 
piezoelectric element 18 incorporating an ink-jet head for blowing air 
from the end onto a recording paper. It is also possible to heighten the 
velocity of a flying ink droplet by using air flow as an auxiliary means 
after the ejection of an ink droplet. However, since the opening is larger 
than the opening of the first example of FIG. 1, it was necessary to 
supply a large amount of air in order to form a sufficiently high-speed 
air flow. As a result, a large-sized pump was required which brought about 
the problem of increasing installation cost and noise. 
In addition, as is shown in FIG. 2, when a high-speed air flow moves at the 
fore end of the head, a swirl 21 of air flow is produced in front of the 
nozzle orifice 20, to form a turbulent flow. This turbulent flow made the 
flying direction and velocity of an ink drop unstable. An ejection of an 
ink drop was difficult when the volume of an ink drop was too small. 
Therefore, in order to obtain a stable ejection of an ink drop, it was 
necessary to increase the ejection energy and to drive an ejected droplet 
to a part 22 which is away from the nozzle. It was difficult to so 
increase energy in cases where the volume of ink small was too small. 
Referring to FIGS. 3, 4, a first embodiment of this invention comprises an 
ink-jet head 104 composed of an ink chamber 100, a cylindrical 
piezoelectric element 101 on the ink chamber 100, a nozzle 102 fixed to 
one end of the ink chamber, and a supply passageway 103 fixed to the other 
end of the piezoelectric element for introducing ink from a tank outside, 
and air flow formation means 106 having an aperture or guide passageway 
105 for causing pressurized air in the vicinity of the nozzle 102 to flow 
out toward a recording paper. The air flow formation means 16 is composed 
of laminated plate members 107a, 107b and 107c. The pressurized air is 
supplied from an external pump (not shown) throgh an air inlet 108 to the 
vicinity of the nozzle. 
The wall of the orifice 110 (FIG. 4) of the nozzle 102 is made extremely 
thin and the orifice 110 is arranged to be located inside of the guide 
passageway 105 of the air flow. The pressurized air introduced into the 
vicinity of the nozzle is abruptly accelerated in the aperture or inlet 
111 of the guide passageway 105 to form an air flow directed toward the 
recording paper. Because there is an abrupt acceleration of air in the 
aperture or inlet 111 of the passageway, a large difference in pressure 
occurs due to the inertia effect in the inlet 111 of the passageway. Most 
of the pressure of the pressurized air introduced to the vicinity of the 
nozzle forms a difference in pressure in the inlet 111. The velocity of 
air flow is approximately uniform around the periphery of the orifice 110 
inside the guide passageway 105. Therefore, the generation of pressure due 
to the inertia effect can be disregarded, but the generation of pressure 
due to the viscosity effect of the air is to be recognized. However, this 
pressure due to viscosity effect is so small, as compared with the 
pressure due to inertia effect in the aperture or inlet 111 of the 
passageway, that it can effectively be disregarded. 
In the vicinity of the outlet 112 of the guide passageway 105, the section 
of the passageway is wider than it is in the vicinity of the inlet because 
the outlet has no nozzle orifice 110. The high-speed air flow passing in 
the periphery of the orifice 110 reduces the speed of the air in the 
vicinity of the outlet 112. This brings about pressure due to the inertia 
effect in the vicinity of the outlet 112. However, this inertia caused 
pressure is directed reversely to the pressure due to viscosity effect in 
the periphery of the orifice 110. 
It has been experimentally confirmed that it is possible to make the air 
pressure in the periphery of the orifice 110 approximately equal to 
atmospheric pressure by offsetting the two pressures (inertia and 
viscosity) against each other. While it is needless to say that this 
offsetting effect varies depending upon the location of the orifice 110 in 
the aperture or guide passageway 105, it was confirmed that the offsetting 
effect is obtained sufficiently by disposing the orifice within the 
section equivalent to the second and third quarters of the entire length 
of the aperture or guide passageway 105. As a result, it is possible for 
the ink meniscus inside the orifice to remain almost stably within the 
inside of the orifice without being forced further inwardly or being 
forced outwardly. 
Thus, this embodiment dispenses with the need for a pressurizing system for 
moving the ink through an ink tank as in the first conventional example. 
This embodiment enables the realization of a simple and a low-cost device. 
Further, the wall of the nozzle orifice 110 is made extremely thin as is 
shown in FIG. 4. Therefore, even when pulse pressure forces the ink 
meniscus 113 to the outside of the orifice, as is shown in the figure, it 
is possible for the air flow to pass uniformly around the periphery of the 
orifice and the ink meniscus without causing a large turbulence. As a 
result, the ink meniscus 113 may always be stably pushed out, which 
enables a much stabler ink drop formation than is possible in the second 
conventional example. 
In addition, as the ink mensicus which has been pushed out is subject to a 
force acting in the direction which pulls it out of the orifice due to 
viscosity resistance cause by the air flow in the periphery. Even when the 
ink meniscus itself is pushed ut, it has insufficient kinetic energy to 
separate itself from the orifice. Thus, it is possible for the ink to be 
ejected as a drop of ink and to be carried in the air flow. 
The wall 110 is preferably as thin as possible; however, on the other hand, 
it is also preferable to make it as thick as possible from the viewpoint 
of manufacturing technique. As a result of measurement of ink ejection 
properties which has been made while varying the wall thickness of the 
orifice, it was experimentally confirmed that, for example, when the inner 
diameter of a nozzle orifice is 50 .mu.m, an almost stable ink droplet 
ejection is possible if the outer diameter is not greaer than 
approximately 75 .mu.m. Experimentation also made it clear that the 
permissible range of outer diameter variations varies approximately in 
proportion to the variation of the inner diameter. Good ink ejection is 
possible when the ratio of the inner diameter to the outer diameter does 
not exceed 1.5. 
As described above, an ejection of an extremely minute droplet, which was 
impossible in the prior art, is enabled due to the effect of the viscosity 
resistance of the ink meniscus after being pushed out. Good half-tone 
recording is enabled simply by varying the volume of a droplet. 
Referring to FIG. 5, in a second embodiment, a porous member 114 is dispoed 
in a position opposite the nozzle 102. In a porous member 101 opposing the 
nozzle 102, and a plate member 107f on the outer wall part are formed 
bores 116 and 117 through which ink may pass before the ink droplet is 
ejected from the nozzle 102. 
The same liquid that is the prime solvent for the ink in the nozzle 102 
fills the porous member 114. This liquid evaporates from the surface 118 
of the porous member 114. The amount of evaporation varies in accordance 
with the steam pressure of the prime solvent for ink in the chamber 115. 
Evaporation stops when the solvent in member 114 reaches the saturated 
vapor pressure. Actually, as the vapor diffuses to the outside, through 
the through bores 116 and 117, evaporation from the surface 118 of the 
porous member continues slightly. The prime solvent for the ink is 
supplied due to capillary action on the surface 118 of the porous member. 
As a result, the prime solvent for the ink is stored in the container 119 
and is drawn to the surface 118 of the porous member 114 through a conduit 
120 and a connector pipe 121. In this way, the space close to the nozzle 
102 always contains vapor of the prime solvent for ink, with a high steam 
pressure which is close to the saturation value, and, thus, the ink in the 
nozzle 102 never dries. 
Furthermore, there always is some high density vapor of the prime solvent 
for ink in the space close to the nozzle, whether or not the ink-jet 
operation is being carried out. The ink drying prevention function works 
adequately at all times of operation. Thus, there is a remarkable 
heightened reliability of ink-jet recording. 
Those who are skilled in the art will readily perceive how to modify the 
invention. Therefore, the appended claims are to be construed to cover all 
equivalent structures which fall within the true scope and spirit of the 
invention.