Ink jet printing head

An ink jet printing head comprising a head body made from a material having high specific stiffness and the head body includes an ink passage which is kept small to retain the high body stiffness. The ink passage comprises a narrow slot connected with internal holes in the body which lead to ink inlet and exit ports. A nozzle plate having a plurality of orifices is fixed to the front of the head body and a piezoelectric crystal is fixed to the back of the head body. The piezoelectric crystal is kept thin compared to the head thickness, typically on the order of 1/20 to 1/30 of the body thickness, so that the effect of the crystal on the resonant characteristics of the assembly is kept small. The ink jet head provides a plurality of columns or jets of ink which are excited in such a way as to break up into uniformly and equally spaced drops at a fixed distance from the nozzle plate containing the orifices which produce the jets.

BACKGROUND OF THE INVENTION 
One type of electrostatic pressure ink jet system is described in Sweet et 
al, U.S. Pat. No. 3,373,437, wherein the pressurized electrically 
conductive fluid is ejected from a plurality of orifices and broken into 
plural streams of uniform drops. As each drop breaks off from its fluid 
filament, it may be selectively charged by an associated charge electrode. 
This system operates binarily, giving a drop either a predetermined charge 
or leaving it in an uncharged condition. The drops then pass through an 
electrostatic deflection field so that the charged drops are deflected to 
a drop catcher or gutter, while the uncharged drops are undeflected and 
continue past the deflection field to impact a recording medium for 
printing. 
The charge on a drop is established in accordance with the field produced 
by the charge electrode at the instant the drops break off from the 
filament. In the apparatus shown in U.S. Pat. No. 3,739,393 to Lyons et 
al, a plurality of streams is generated by forcing the ink through a set 
of orifices in an orifice plate and the streams are stimulated to produce 
drops by vibrating the orifice plate at a point near one end and 
propagating a traveling wave along the plate to stimulate successive 
orifices which causes some difference in breakoff distance in the streams 
and also some phase difference, that is, a difference in time between 
successive stream breakoffs due to the traveling wave excitation. More 
uniform drop breakoff is achieved in the apparatus shown in U.S. Pat. No. 
3,882,508 by tapering the orifice plate along its length to compensate for 
the attenuation of the traveling wave along the orifice plate; however, 
this change does not correct the phase difference. 
It is therefore the object of this invention to provide an ink jet head of 
simplified design which produces a plurality of ink streams each producing 
uniform drop breakoff and phasing. 
SUMMARY OF THE INVENTION 
The ideal solution to achieve this objective is to design the ink jet head 
so that the first natural resonance of the head is at a frequency greater 
than the operating frequency. However, using existing engineering 
materials, it is not possible to design an ink jet head within the 
constraints of our desired dimensions and operating frequency which can 
operate in this ideal mode. 
Briefly according to the invention, the objective is achieved by keeping 
the resonant frequency of the head as high as possible by using a high 
specific stiffness material and a design which retains the advantages of 
this material so that a uniform mode shape is produced at the operating 
frequency with nodal lines parallel to the ink jet array. 
The ink jet head comprises a head body made from a material having a high 
specific stiffness and the head body includes a slot cummunicating with 
ink inlet and exit passages and extending to one face of the head body. A 
nozzle plate having a plurality of orifices is fixed to this face of the 
head body with the orifices in alignment with the ink slot so that a 
plurality of ink streams is formed when pressurized ink is introduced into 
the ink inlet passage. An electromechanical transducer having a thickness 
small with respect to the thickness of the head body is fixed to the 
opposite face of the head body so that, when the transducer is energized 
with a suitable high frequency sine wave, the ink streams are broken up 
into uniform spaced drops at a fixed distance from the nozzle plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The ink jet head according to the invention comprises a head body 10 having 
a nozzle plate 14 containing orifices 16 attached to the front of the body 
and an electromechanical transducer 18 attached to the back of the body as 
shown in FIGS. 2 and 4. The purpose of the ink jet head is to provide 
several columns or jets of fluid such as ink which is excited in such a 
way as to break up into uniformly and equally spaced drops at a fixed 
distance from the nozzle plate containing the orifices which produce the 
jets. 
The basic head body as shown in FIG. 1 is a block of material with an ink 
passage 12 formed in it. Any high specific stiffness material which is 
chemically compatible with the ink and with other materials in the head 
may be used. Stainless steel is one material that can be used and ceramic 
materials such as glass, alumina and silicon carbide may also be used. The 
specific stiffness is defined by the relation E/.rho. where E is Young's 
Modules of Elasticity and .rho. is the density of the material. The 
specific stiffness for the materials listed above varies from 
107.times.10.sup.6 inches for stainless steel to 600-800.times.10.sup.6 
inches for silicon carbide. The ink passage 12 includes a small slot 28 
extending to the face 36 of the head body to which the nozzle plate 14 is 
fixed and ink inlet opening 30 and outlet opening 31 which extend through 
the end faces of the head body to intersect with ink slot 28. The slot 28 
is kept small to retain the high body stiffness. By keeping the dimensions 
of the block small and compact, the resonant frequencies are kept high and 
resonances in the frequency range of interest, typically 100 kilohertz to 
200 kilohertz, are minimized. Although the shape of the head body is shown 
as rectangular, other shapes can be used as well, such as cylindrical with 
the faces either parallel or perpendicular to the cylindrical axis. 
The electromechanical transducer is attached to the back of the head body 
and the thickness of the transducer is kept thin compared to the head 
thickness. The preferred electromechanical transducer is a piezoelectric 
crystal and a suitable transducer is the lead zirconate-lead titanate 
ceramic sold under the tradename of PZT by Vernitron Piezoelectric 
Division, Bedford, Ohio. By utilizing a thin crystal, the effect of the 
crystal on the resonant characteristics of the assembly is kept small. The 
stiffness and mass of the head body are so much greater than those of the 
crystal that the resonant characteristics are essentially those of the 
head body alone. 
The ratio in percent of the first resonant frequency of the total head 
f.sub.ot and the first resonant frequency of the head body alone f.sub.oh 
is plotted in FIG. 5 versus the thickness ratio t/T for a steel head body 
and a PZT4 crystal. Similar curves can be drawn for other material 
combinations. This figure illustrates the percent reduction in the first 
resonant frequency of the head due to the presence of the crystal plate 
versus the thickness ratio of the crystal and head body. In order to keep 
the reduction within 10%, it can be seen from FIG. 5 that the thickness 
ratio should be less than 5%. Typical dimensions for an ink jet head are 
0.5 inch for the head body thickness T and 0.020 inch for the crystal 
thickness t. This corresponds to a thickness ratio t/T of 4% and this 
design produces less than a 10% reduction in the first resonant frequency 
of the head. 
As shown in FIG. 2, the head body has a nozzle plate 14 bonded to its front 
surface 36 so that the orifices 16 are in alignment with the narrow slots 
28 in the head body. The nozzle plate can be bonded to the head body by 
any suitable process which produces a uniform rigid bond line and is 
chemically inert to the ink so that the nozzle plate is forced to follow 
the vibratory motion of the head body as shown dotted in FIG. 2C. Ink 
inlet port 32 is fitted within internal hole 30 and a piezoelectric 
crystal 18 is bonded to the back surface 38 of the head body. The crystal 
18 can be bonded to the head body by any suitable process which is capable 
of producing a rigid bond that is thin with respect to the crystal 
thickness to promote the maximum transfer of energy from the crystal to 
the head body. The preferred bonding material is a suitable epoxy bonding 
material. 
A sinusoidally varying voltage from source 20 is applied to the crystal 18 
to provide the excitation to the jets 22 so that the jets break up at a 
fixed distance 24 from the nozzle plate into a series of uniformly and 
equally spaced drops 26. The drive from crystal 18 produces a vibration at 
the face of the head body as shown dotted in FIG. 2. It is important to 
the production of drop breakoff at a fixed distance 24 from the nozzle 
that the nodal points 34 of the vibration be parallel to the row of 
orifices in nozzle plate 14. The ink jet head shape and dimensions are 
chosen to operate at a particular frequency at which the head is driven so 
that the proper vibrational mode is produced as shown in FIG. 2. 
When multiple columns of jets are desired, each is provided with a separate 
slot 28 behind its orifices as shown in FIG. 3. The head body 11 has two 
ink slots 28' and ink inlet opening 30' and exit opening 31' which 
intersect with each ink slot 28'. The assembled head has a nozzle plate 15 
having two rows of orifices 17. The nozzle plate is fixed to head body 11 
so that the rows of orifices 17 are aligned with the ink slots 28'. This 
structure maintains the high stiffness of the assembly and produces the 
nodal points 34' parallel to the rows of orifices as shown in FIG. 4 so 
that, when transducer 19 is excited by a suitable sine wave voltage, 
uniform breakoff can also be obtained in each of the multiple columns of 
jets provided in this head. This structure has the advantage relative to 
other multi-column heads where a single cavity serves all of the columns. 
In these heads the nozzle plate covering this large cavity becomes a 
relatively weak diaphragm, thereby introducing complex resonant 
characteristics. 
Several other advantages of this head are not related to its resonant 
characteristics. One advantage is that the piezoelectric crystal is kept 
out of contact with the ink, thereby eliminating the need to pass crystal 
drive current through the ink and preventing chemical attack of the 
crystal, crystal electrodes or crystal bonding material by the ink. 
Another advantage is that gaskets and "O" rings are not required to seal 
the ink passages and assembly screws are eliminated. A third advantage is 
that the small ink passages permit high ink velocities through the passage 
when in a flow-through or flushing mode, thereby facilitating removal of 
air bubbles or contaminants when they affect operation, which is typically 
during the startup mode. An additional advantage is that the small 
physical size and weight of the head makes it desirable for incorporating 
it into a complete ink jet print head assembly which includes the head 
described plus charge plates, deflection plates and gutters. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in the form and details may 
be made therein without departing from the spirit and scope of the 
invention.