Technical Field
[0001] The present invention relates to continuous ink jet printers and more particularly
to the drop generator used in continuous ink jet printers.
Background Art
[0002] In continuous ink jet printing, ink is supplied under pressure to a manifold that
distributes the ink to a plurality of orifices, typically arranged in linear array
(s). The ink is expelled from the orifices in jets which break up due to surface tension
in the ink into droplet streams. Ink jet printing is accomplished with these droplet
streams by selectively charging and deflecting some droplets from their normal trajectories.
The deflected or undeflected droplets are caught and recirculated and the others are
allowed to impinge on a printing surface.
[0003] JP 61-258762 A discloses that when a cavity block is vibrated by a piezoelectric vibrator, the resultant
sound wave is transmitted to ink contained in a liquid chamber such that an ink droplet
is jetted out through an orifice. When the wavelength of the sound wave of the vibrator
is λv and the height (L2) of the vibrator is set to L2 = nλv/2, (where n=1, 2, ...),
the vibrator is set into resonance at the frequency used. The cavity block is provided
with a row of notched slots in the direction parallel to an orifice plate so that
a transverse vibration mode is hardly generated, and vibration occurs only in the
longitudinal direction (the direction toward the orifice).
[0004] U.S. Patent No. 4,999,647 describes a drop generator design for use in long array ink jet printers. The drop
generator consists of a rectangular block of metal that contains a fluid cavity and
to which an orifice plate is bonded. The block is designed to vibrate in the first
longitudinal mode of the height direction. As the length is greater than the height,
poisson ratio induced couplings can produce non-uniform vibration at the orifice plate
face of such a rectangular block. To minimize the non-uniformities produced by the
poison ratio induced coupling, vertical slots have been cut the block from front to
back, slots perpendicular to the array direction. These slots effectively segment
the drop generator reducing the effect of the poisson ratio coupling. The slots are
closed at the top and bottom faces of the drop generator to maintain the stiffness
of the block to inhibit flexing motion down the length.
[0005] While this design works well, it has some limitations. As the flow rate of ink through
the drop generator increases, the bore of the drop generator must be increased to
avoid turbulent flow which can affect jet directionality. As the bore of the part
is increased, the resonant frequency of the longitudinal mode is reduced. The lowering
of the resonant frequency reduces the print speed of the printer. One can increase
the resonant frequency of the drop generator by reducing the height dimension of the
block. The height of the drop generator can be reduced only so far before there is
insufficient height for the inclusion of the slots.
[0006] U.S. Patent No. 4,188,635 describes a different type of resonant block for use in stimulating ink jet arrays.
This resonant block has a small slot cut into the face to which the orifice plate
is secured. The slot serves as a fluid manifold. The slot must be kept small to maintain
the stiffness of the body. A piezoelectric transducer is bonded to the drop generator
on the face opposite the orifice plate face. The piezoelectric must be thin compared
with the thickness of the block. This design has node lines on the orifice plate face
which run parallel to the array of jets. This design is not applicable to long arrays
of jets since the flow requirements of a long array necessitate a large fluid cavity
to maintain non turbulent flow. The introduction of a large fluid cavity into this
design lowers the resonant frequency significantly, so that the design is no longer
viable for use in a high speed ink jet printer.
[0007] U.S. Patent No. 4,827,285 describes another type of drop generator. This patent describes a drop generator
consisting of an orifice plate which is vibrated by means of two piezoelectric crystals
that are bonded directly to the orifice plate. A fluid manifold is bonded directly
to the orifice plate. Driving the piezoelectric crystals causes the outer edges of
the orifice plate to be displaced, inducing the orifice plate to flex. The plate flexure
causes the orifices to vibrate, stimulating the jets. This drop generator concept
is only useful where the array length is small, as longer arrays require larger fluid
cavities to handle the fluid flow. The mass of the larger fluid cavities has a negative
effect on the operating frequency range. This design is also intrinsically fragile
with the orifice plate being mounted by means of the brittle piezoelectric elements.
Additionally as the piezoelectric elements are electrically driving to produce the
vibration in the direction normal to the plane of the orifice plate, they are also
made to expand in the direction parallel to the plane of the orifice plate. This expansion
of the crystals can couple into other vibrational modes of the drop generator resulting
in non-uniform stimulation down the orifice array.
[0008] U.S. Patent No. 4,245,225 describes a drop generator which places a cylindrical piezoelectric concentrically
inside an larger cylinder. The space between the cylinders serves as a plenum for
the ink. Ink is allowed to flow through holes in the wall of the outer cylinder to
the orifices from which the jets of ink are formed. The inner and outer surface of
the piezoelectric cylinder are metallized to form electrodes. The piezoelectric cylinder
can be electrically driven by means of these electrodes to expand and contract radially.
This radial mode vibration then drives a liquid cavity resonance in the space between
the inner and outer cylinders. The pressure oscillations produced by this cavity resonance
in turn cause the stimulation of the jets of ink. As mentioned in the patent the radial
expansion of the piezoelectric also produces a length change in the piezoelectric
due to the Poisson's ratio. As the length of the cylinder is increased, standing waves
can be produced down the length of the cylinder. These standing wave down the length
couple back into the radial vibration so that the radial vibration is no longer uniform
down the length of the cylinder or the ink jet array. The means to locate the inner
piezoelectric cylinder also can couple the vibration of the inner cylinder to the
outer cylinder. The resulting vibrations of the outer cylinder will tend to interfere
with the desired uniform stimulation amplitude down the ink jet array. In addition
to these problems, the need to place the piezoelectric in contact with the ink produces
problems related to shielding the electrodes and the piezoelectric from the ink. As
a result of all these problems, this design is not viable for use in high speed ink
jet printers. And although
U.S. Patent No. 4,245,227 describes a similar drop generator design, with the change that the outer cylinder
rather than the inner one is piezoelectric, it suffers from all the same problems
as the previous design.
[0009] Clearly a new means is needed for stimulating long ink jet arrays at high operating
frequencies.
Summary of the Invention
[0010] It is an object of the present invention to provide a drop generation means with
a long jet array and high operating frequency for use in a high speed ink jet printer.
[0011] In accordance with one aspect of the present invention a drop generator as defined
in claim 1 is provided for use in an ink jet printer, where the drop generator is
capable of stimulating long ink jet arrays at high operating frequencies. Specific
embodiments of the invention are defined in the dependent claims.
[0012] Other objects and advantages of the invention will be apparent from the following
description and the appended claims.
Brief Description of the Drawings
[0013]
Fig. 1 shows a cross sectional view of a hollow tube, to illustrate displaced and
undisplaced shapes of the hollow tube as it undergoes a radial bending mode;
Fig. 2 is a long rectangular block drop generator illustrating a preferred embodiment
of the present invention;
Fig. 3 illustrates the fluid cavity geometry on the interior of the drop generator;
Fig. 4 is an end wall view of a fluid channel of the drop generator of Figs. 2 and
3, designed in accordance with the present invention; and
Fig. 5 illustrates one embodiment of a design concept according to the present invention
for tuning the resonant frequency of the end sections of a drop generator to match
the central sections of the drop generator.
Detailed Description of the Invention
[0014] The present invention discloses a drop generator for use in ink jet printing, which
employs multiple lobed squashing resonances to stimulate an array of jets. The geometry
of the ends of the drop generator is configured to make the resonant frequencies of
the end sections approximately equal to that of the central portion of the drop generator.
Piezoelectric drive elements are placed to effectively drive the desired resonant
mode while suppressing undesirable resonant modes.
[0015] The basic concept behind the present invention is best understood by considering
a cross sectional view of a hollow tube 10, as illustrated in Fig. 1. In addition
to the circularly symmetric modes of the tube employed in
U.S. Patents No. 4,245,225 and
No. 4,245,227, the tube 10 also has various radial bending modes. The lowest order of such modes
has two lobes 12 where the tube bulges out, and two areas 14 where it is squeezed
in. Higher order modes have increasing numbers of lobes and higher resonant frequencies.
For ease of explanation, the invention will be described in terms of the lowest order
mode, recognizing that the same principles can be applied to the higher order radial
bending modes.
[0016] The lowest order radial bending mode can be effectively driven by symmetrically placing
two piezoelectric elements 18 on the walls of the tube 10. In a preferred embodiment,
the length of the piezoelectric elements around the circumference should be less than
one quarter of the circumference of the tube to prevent the piezoelectrics from extending
across the node lines of the radial bending mode. When driven in phase at the appropriate
frequency, the symmetrically placed piezoelectrics will effectively drive this two
lobe radial bending mode. They will also not be very effective at driving other order
radial bending modes.
[0017] This lowest order radial bending mode has associated with it a whole family of resonant
modes with different profiles down the length of the tube and different resonant frequencies.
The lowest order of this family of modes maintains the same phase for the bending
pattern down the length of the tube. The second such mode will have the two ends of
the tube bending out of phase. Higher order modes will have increasing numbers of
phase shifts down the length. These modes can be thought of as compound bending modes,
where the tube wall bends both radially and axially.
[0018] For the particular application in an ink jet printer, it is desirable to utilize
the lowest order mode of one families of radial bending modes, preferably the one
with constant phase of the radial bending down the length of the tube. That is, it
is desirable for ink jet stimulation to use radial bending modes which don't have
an axial bending mode component. By employing two symmetrically placed rows of piezoelectric
elements extending down the length of the cylinder which are driven in phase, the
radial bending mode having consistent phase down the length can be driven while the
vibration of the modes with axial bending mode components can be suppressed.
[0019] While it is possible to drive the two lobe radial bending mode with a single piezoelectric,
the use of symmetrically placed piezoelectric elements 18 on the sides of the cylinder
10 provides much higher drive efficiency for driving the desired resonant mode. Furthermore,
the use of a single piezoelectric is less selective, that is, it is not as effective
at suppressing the higher order radial bending modes as the symmetric pair of piezoelectrics.
Therefore, a cylinder with a single row of piezoelectric drive elements will suffer
from more interference from unwanted radial bending modes than does one with symmetrically
placed piezoelectrics.
[0020] Ignoring, for the time, the problem with the ends of the tube 10, such as the need
to seal the ends and means to locate the drop generator, etc., which can be at least
partially addressed by using the dovetail grooves 23 for mounting, persons skilled
in the art can recognize that placing orifices at the lobes of the radial bending
mode will result in the desired displacement of the orifices to produce stimulation.
Driving the mode which maintains a constant bending phase down the length of the tube
will then produce quite uniform stimulation. Long arrays on ink jets can be stimulated
at operating frequencies of over 100 kHz using such a design concept.
[0021] The need to terminate the ends of the tube-like drop generator to contain the ink,
supply fluid ports, and provide means to locate the drop generator can affect the
stimulation uniformity of the drop generator. The present invention overcomes this
problem by designing the end of the drop generator with a cut 25 to resonate at a
similar frequency to the tube-like center of the drop generator.
[0022] While the description thus far uses a cylindrical model to explain the concepts,
it will be understood by those skilled in the art that, in practice, other shapes
can be used. Such shapes, for example, might have a square or rectangular cross section,
or even have more than four sides. Such cross sectional shapes should have a height
to width ratio close to one that is between 0.5 and 2. In general, the cross sectional
shape should be consistent with the symmetry of the desired operational radial bending
mode shape. For example, 3 sided or 6 sided cross sections might by utilized for drop
generator with the 3 lobed radial bending mode as the desired operational mode. Matching
the cross sectional shape to the desired lobe shape facilitates the placement of the
piezoelectric drive elements so that the desired mode can be driven. To effectively
drive the higher order radial bending modes while suppressing the lower order modes,
three or more rows of piezoelectrics should be used as dictated by the desired lobe
shape.
[0023] In Fig. 2, there is illustrated a preferred embodiment of the present invention,
comprising a long rectangular block 27. The drop generator 27 is a rectangular block
made of stainless steel with approximate measurements of a length of 25.4cm (10 inches),
a width of 1.68cm (0.66 inches), and a height of 1.32cm (0.52 inches). A fluid cavity
28 comprises a through hole 20 machined through the length of the block. A long, narrow
slot 22 is machined into the bottom face of the block, connecting to the through bore.
The orifice array, located at the bottom of the drop generator, in the area of 24
in Fig. 3, is secured to the block and is centered over the slot 22.
[0024] Continuing with Fig. 2, rows of piezoelectric elements 18 are secured to the front
and back faces of the block. When electrically driven, these piezoelectric elements
expand and contract in the z direction. This causes the side walls of the drop generator
to flex. The driven mode corresponds to the two lobe squash mode described for the
cylinder. As the piezoelectric elements are attached down the entire length of the
drop generator, and the electrical signal is applied uniformly to all the elements,
the flexing force is applied uniformly down the length of the drop generator.
[0025] The drop generator is mounted by means of thin wall stainless steel tubes (not shown)
which are bonded into dovetailed grooves 23, as shown in Figs. 2, 3 and 4. Thin wall
tubing has been found to supply sufficient rigidity for locating stability of the
drop generator, while providing minimal vibrational coupling of the drop generator
to the support frame. The dovetail grooves 23 have been found to minimize the risk
of the mounting tubes breaking loose, while still facilitating easy removal of the
tubing at refurbishment.
[0026] The fluid manifold comprises a through bore 20 which runs the length of the block
and a long narrow fluid channel 32 connecting the through bore to the orifice plate
face of the drop generator. The narrow fluid channel stops approximately 1.3cm (1/2")
from each end. As best illustrated in Fig. 4, the end wall 22 of the fluid channel
32 is tapered to improve the fluid flow at each end of the drop generator. Ink is
supplied by fluid fittings (not shown). The fluid fittings are bonded into counter
bores 30 as shown in Figs. 3 and 4. An alignment feature 34 for locating the orifice
plate over the fluid cavity is also illustrated. Such alignment features were described
in
U.S. Patent No. 4,999,647.
[0027] In accordance with a preferred embodiment of the present invention, it is desirable
to design the ends of the drop generator to have a resonant frequency approximately
equal to that of the center of the drop generator. To better understand this concept,
consider, for instructive purposes only, the response of a narrow cross sectional
slice of the drop generator to the flexing force is considered. Initially, this response
can be examined as if this section of the drop generator were independent of the other
sections.
[0028] It is well known in the art, that the vibration response of such a section depends
on the relationship between the resonant frequency of the cross sectional slice and
the frequency of the driving force. As a result, the vibration amplitude of such a
slice will reach its peak value when the driving frequency equals the resonant frequency.
The phase of the vibration relative to the driving force also shifts as the driving
frequency is varied across the resonance. Similarly, the vibrational amplitude and
phase of any other cross sectional slice of the drop generator will depend on the
relationship between the driving frequency and the resonant frequency for that slice.
[0029] Along the length of the fluid cavity slot, the uniform cross section produces the
desired consistent resonant frequency. Near the ends of the drop generator, however,
the fluid cavity slot must terminate, to keep ink from spraying out the ends of the
drop generator. As a result, the cross section of the drop generator at each end of
the body does not match the cross section in the middle of the body. The resonant
frequency of the end section of the drop generator therefore, does not match the resonant
frequency of the central sections of the body. The filling in of the slot tends to
stiffen the cross section, raising the resonant frequency. Consequently, the vibrational
amplitude and phase of the vibration at the end sections will not match the central
sections.
[0030] It will be well understood by those skilled in the art, that in a typical drop generator,
the different cross sectional sections are not truly independent of each other. Differences
in vibration amplitude and phase are coupled from section to section. Therefore, the
different vibration amplitude and phase of the end sections are coupled into the rest
of the drop generator, affecting the vibration all along the drop generator.
[0031] By changing the geometry of the end section of the drop generator, in accordance
with the present invention, it is possible to shift its resonant frequency to match
that of the center of the drop generator. The result is a drop generator with acceptably
uniform vibration amplitude down the length of the array. One preferred embodiment
utilizes cuts 25 on the top surface of the drop generator. These cuts, which run parallel
to the fluid cavity slot, start about even with the end of the fluid cavity slot and
extend to the ends of the drop generator as shown in Fig. 4. The cut 25 gradually
increases in depth, reaching full depth approximately where the taper of the fluid
cavity slot ends. For the embodiment described herein, therefore, the depth of the
cut is 1.32mm (0.052") and the width is 1.57mm (0.062").
[0032] While the slot shown is a preferred embodiment, it will be recognized that other
cuts or features can be employed, while still within the scope of the invention to
achieve the concept of the present invention, which is to tune the resonant frequency
of the end section of the drop generator to match that of the center section. One
such alternative embodiment, for example, is to seal the ends of the fluid cavity
slot with low modulus materials, such as a low durometer rubber, that would have minimal
effect on the resonant frequency of the ends.
[0033] The design concept of the present invention, of tuning the resonant frequency of
the drop generator end sections to match that of the center, is applicable to other
drop generator designs which do not employ radial bending modes. One such design 50,
for example, is shown in Fig. 5. The design of Fig. 5 has a height of 4.8cm (1.9"),
a width of 3.35cm (1.32"), and a thickness of 1.24cm (0.49"). The resonant mode shape
is primarily that of a rectangle in the longitudinal mode, with end sections 52.
[0034] It is well known that the velocity of sound down long thin rods is lower than that
of a bulk solid of the same material. This difference is caused by the Poisson's ratio.
In the thin rod, if a section of the rod is compressed down in the axial direction,
the radial dimension will expand as a result of the Poisson's ratio. In a rod having
a large radial dimension or other large sample, if the piece is compressed in one
dimension, the radial expansion is impeded by the radial bulk of the object. As a
result of this radial motion being impeded, the material acts stiffer in the axial
direction. The higher apparent stiffness for the larger diameter rod or the bulk sample
yields a higher effective velocity of sound than for the thin rod.
[0035] Similarly, the apparent velocity of sound is lower near the walls of an object than
in central portion of the object. In the central portion of the object, lateral motion
due to Poisson's ratio in response to a compression or dilation in one direction is
inhibited by the mass of the surrounding areas. Near the surface the lack of mass
in part of the surrounding region allows lateral motion due to Poisson's ratio to
occur in response to a compression or dilation in a direction parallel to the surface.
The difference in the ability to move laterally in these two cases produces the apparent
difference in the velocity of sound for the two regions.
[0036] By virtue of the ends of the drop generator having a different apparent velocity
of sound than the central portion, the ends of the block tend to have a slightly different
resonant frequency than the central portion. By contouring the side walls, which shifts
the resonant frequency of the end sections of the drop generator closer to that of
the center of the drop generator, the stimulation of the ink jets can be made more
uniform.
[0037] As mentioned above, the preferred embodiment of the drop generator utilizes two rows
of piezoelectric elements symmetrically placed extending down the length of the drop
generator. These are driven to flex the sides of the drop generator to excite the
radial bending mode. When driven, however, the piezoelectric elements expand also
in the length direction, parallel to the axis of the fluid cavity. If the piezoelectrics
are not appropriately sized and placed, they can excite undesirable axial bending
modes down the length of the cavity. This problem can be avoided by identifying the
wavelength for the axial bending modes which have resonant frequencies near the desired
operating frequency. The length of the piezoelectric crystals should then be greater
than ½ of the wavelength of such axial bending modes and less than one wavelength.
This will ensure that the ends of the crystal, where most of the driving force is
concentrated, will not be able to work in concert to excite such axial bending modes.
[0038] Alternatively, shear mode poled piezoelectric materials may be used to drive the
drop generator. As the shearing action of such a piezoelectric element does not induce
a length change in the piezoelectric, such piezoelectric transducers have less of
a tendency to excite axial bending modes.
Industrial Applicability and Advantages
[0039] The present invention is useful in the field of ink jet printing, and has the advantage
of providing an improved drop generator design, particularly for a long array ink
jet printer. An additional advantage of the present invention is to provide stimulation
of long ink jet arrays at high operating frequencies.