Color ink jet drop generator using a solid acoustic cavity

The drop projection device has an array of nozzles communicating with an ink cavity fed from an associated pressurized ink reservoir and has an acoustic cavity closely associated with the ink cavity. The acoustic cavity is filled with a solid material, and may be separated from the ink cavity by a membrane that may be selected of any ink compatible material for transmitting disturbances from the solid material to the ink channel (the membrane is not required, however). A transducer is mounted to the rear of the solid filled cavity, essentially in air. The transducer is a block of piezoelectric material separated into a plurality of parallel fingers by slices made from one side of the block. The height-to-width or height-to-thickness ratios are less than 10:4. The solid acoustic cavity is filled with a material having an acoustic impedance which is substantially equal to the ink acoustic impedance. The cavity itself is defined by a material having a high acoustic impedance. The narrow, shallow ink channel across the face of the acoustic cavity is less than 2 sq. mm. in cross-section to easily expel air bubbles which form during start/stop of the ink streams. The height of the ink channel is less than 0.1 of an acoustic wavelength in ink so that the channel does not act as a separate acoustic cavity with its own standing wave pattern.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to the material disclosed in application Ser. 
No. 794,729, filed on Nov. 4, 1985, entitled "Inkjet Drop Generator" and 
application Ser. No. 794,730 filed Nov. 4, 1985, entitled "Stimulator for 
Inkjet Printer," both of which are incorporated herein by reference. 
FIELD OF THE INVENTION 
This invention relates generally to fluid drop generators and more 
particularly to the generation of a matrix of uniform fluid droplets from 
a linear array of fluid jets for use in printing apparatus such as inkjet 
printing devices and the like. 
BACKGROUND OF THE INVENTION 
Historically, printing has been done by applying ink to a specially 
configured key or carrier and mechanically impacting the key or carrier on 
a recording medium such as paper to form an impression of the carrier. 
More recently, non-impact printing devices have been developed, where 
intelligent patterns (alphanumeric characters, common graphics and the 
like) are deposited on a recording medium. Non-impact printing devices 
utilize a variety of methods of forming the intelligence patterns 
including chemically active and chemically inert processes, using either 
fluids or solids as the marking or printing medium, and requiring either 
specially treated recording media or untreated recording media. 
It has been known to print by depositing discrete droplets of printing 
fluid on a recording medium in a predetermined pattern. Previous attempts 
to achieve such a method of printing utilize a continuous stream of fluid 
which separates into droplets which are charged and electrostatically 
deflected so that they form the desired pattern on the recording medium. 
Such methods produce acceptable resolution typically only when the charge 
per unit mass is accurately controlled for each drop. This can be 
accomplished in two ways: the droplets are either given equal charge per 
unit mass and then deflected by an electrostatic field whose intensity is 
controlled by the input signal, or the droplets are given a charge per 
unit mass according to the input signal and then deflected using a 
constant electrostatic field. Existing embodiments of both of these 
methods require that the fluid droplets be substantially uniform which has 
proven difficult to achieve. Once the stream of uniform droplets has been 
attained, it is usually necessary to provide voltages in the range of 
2,000 to 10,000 volts for the electrostatic field. Such voltages are 
difficult and expensive to produce and control. Also, the process of 
charging the droplets themselves sometimes causes electrolysis of the 
printing fluid, creating corrosive bi-products which may cause electrode 
deterioration. 
In an effort to obtain droplets of uniform size, different methods have 
been applied in the prior art. First, the printing fluid is delivered to a 
nozzle at sufficient pressure to assure that a continuous jet of fluid is 
issued from the nozzle. The jet stream is separated into droplets by using 
radial oscillations of vibration induced in the nozzle itself by means of 
magnetic drivers or piezoelectric crystals. Vibrations cause regularly 
spaced varicosities in the ink stream, aiding the natural tendency of the 
stream to separate into droplets and making the ensuing droplets more 
uniform than would otherwise occur. Such devices typically provide for 
having a plurality of ink streams issuing from a row or rows of nozzles 
and require a support means for the nozzles which contains the ink channel 
and a resonant acoustic cavity such as shown in U.S. Pat. No. 3,373,437. 
The material must be rigid in order to hold the nozzles fixed for accurate 
printing and implies a metallic material which implies it has a high 
acoustic impedance. For such a device to be effective, the resonant 
acoustic cavity within the support means must typically include the ink 
channel itself to be excited by either plates or pistons which in turn are 
excited by a piezoelectric transducer. 
Another approach to droplet formation utilizes printing fluid delivered to 
the nozzle under sufficient pressure to form a meniscus at the nozzle not 
high enough to produce flow through the nozzle. In this method, the fluid 
is drawn from the nozzle electrostatically in a ray-like jet which is then 
deflected electrostatically as desired. The electrostatic field which 
draws the jet of fluid from the nozzle is constant, producing a continual 
stream of printing fluid. The stream breaks into a succession of droplets 
with essentially uniform mass and charge. A time varying electrostatic 
field controlled by the input signal is then used to deflect the droplets 
as required for the formation of alphanumeric characters. The foregoing 
printing processes and mechanisms make use of a continuous flow of 
printing fluid, with the flow to be diverted to a reject basing or 
collector whenever no characters are patterns are to be printed. This may 
result in a more complicated system for hindering the flow of printing 
fluid than would otherwise be desired. 
In another type of device which is shown in U.S. Pat. No. 4,331,964, a 
piezoelectric transducer is employed to create acoustic waves in a solid 
rubber cavity. 
In the type of device such as shown in the U.S. Pat. No. 4,331,964, it is 
effectively a dual cavity system in which the ink channel is one cavity 
which in turn receives acoustical energy from another rubber filled cavity 
which is cylindrical, and is excited by a cylindrical piezoelectric. A 
membrane is specified between the two cavities. 
Ink or rubber has lower acoustic impedance than the support that forms the 
cavity (for water-based ink vs. steel, the ratio is about 1/25). As a 
result, an acoustical standing wave is set up in this cavity by the 
transducer which vibrates typically at a frequency in the range of 50-150 
KHz. In order for the breakup of jet streams to be uniform, the standing 
wave pattern must be uniform along the jet array. 
A further critical element of the design system is the piezoelectric 
transducer which is used to produce uniform vibrations into the acoustical 
cavity. If the vibrations are not uniform, the acoustical standing wave 
pattern will most likely also not be uniform. For example, U.S. Pat. No. 
2,716,708 is a device for launching ultrasonic waves. Grooves are cut into 
the piezoelectric material to form a linear array of elements. The 
elements vibrate in antiphase. Therefore, this device does not function 
properly for use as an inkjet even if the elements vibrated in phase due 
to the excessive relative width of the array. Another patent showing a 
transducer is U.S. Pat. No. 4,550,606. This patent shows an ultrasonic 
transducer array with controlled excitation pattern. Similar disclosures 
are found in U.S. Pat. Nos. 4,095,232 and 4,138,687. However, in all these 
cases, the application is directed to generating ultrasonic compressional 
waves in materials (human tissue) with scattering centers for the purpose 
of imaging the scattering centers from their echoes. Such a device as 
shown in the figures of the patents would not function in an inkjet 
device. The piezoelectrics shown therein would not generate the necessary 
amplitude signals with uniformity of amplitude of vibration because the 
device is designed to generate vibrations several wavelengths away from 
itself. Near the device, uniformity of the vibration would not be adequate 
for an inkjet printer. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a general objective of the present invention to provide an improved 
printing method and apparatus for recording with writing fluids, and an 
improved drop projection means for use with such apparatus. 
It is another objective of this invention to provide a drop projection 
means which projects droplets having a measured and reliable volume and 
mass for each droplet from a nozzle towards the printing medium in 
response to electrical signals. 
Yet another objective of this invention is to provide an improved method 
and apparatus for recording alphanumerical and graphic intelligence 
patterns on a recording medium by means of deposition of droplets of 
printing fluid on the recording medium in an economical and reliable 
manner. 
It is a further objective of this invention to provide a drop projecting 
means which projects drops from a nozzle responsive to electrical signals 
and in which the volume of the drops is controlled by the applied 
electrical signals. 
A further objective of this invention is to provide a printing apparatus of 
the inkjet type which is simple in construction, reliable in operation, 
capable of printing characters at high speed with low power consumption 
and having a minimal cost and weight. 
The foregoing and other objectives of this invention are achieved by a drop 
projection device in which an array of nozzles communicates with an ink 
cavity fed from an associated pressurized ink reservoir and having an 
acoustic cavity closely associated with the ink cavity. The acoustic 
cavity is filled with a solid material, and may be separated from the ink 
cavity by a membrane that may be selected of any ink compatible material 
for transmitting disturbances from the solid material to the ink channel 
(the membrane is not required, however). A transducer is mounted to the 
rear of the solid filled cavity, essentially in air. 
The transducer comprises a block of piezoelectric material separated into a 
plurality of parallel fingers by slices made from one side of the block. 
Preferably, the height-to-width or height-to-thickness ratios are less 
than 10:4. This form requires a limited number of manufacturing steps, 
thereby reducing manufacturing costs. It provides for transducer motion 
which is more uniform than for known prior art shapes. Further, the 
operating frequency can be close to the piezoelectric resonant frequency 
and still maintain the required uniformity of motion across the entire 
face of the transducer. As a result, the piezoelectric drive voltage will 
be less, which will reduce the cost of the piezoelectric drive circuitry. 
Testing shows that the variation in amplitude and phase of motion across 
the piezoelectric base is less than 10%, thereby providing a drop 
generator which will operate properly over a wide range of environmental 
temperatures and ink viscosities. 
The preferred embodiment of the present invention further includes a solid 
acoustic cavity filled with a material having an acoustic impedance which 
is substantially equal to the ink acoustic impedance. The cavity itself is 
defined by a material having a high acoustic impedance. Fewer and less 
costly parts will be required to form this acoustic material filled 
cavity. It would not be obvious to a person of skill in the art based on 
the prior art known to the inventor that acoustic waves of sufficient 
amplitude and uniformity could be generated with this type of arrangement, 
utilizing the combination of a solid acoustic cavity with a piezoelectric 
attached to the rear of the cavity and extending into air space defined by 
the framework of the inkjet generator. 
Further, the preferred form of the present invention includes a narrow, 
shallow ink channel across the face of the acoustic cavity. Preferably, 
the ink channel is less than 2 sq. mm. in cross-section to easily expel 
air bubbles which form during start/stop of the ink streams. Air bubbles 
can be expelled from larger channels, but a large cross-flow of fluid 
would be needed to force loose the bubbles; this would require larger 
pumps or pressure accumulators, or control valves and circuitry, larger 
ink inlet and outlet ports into the channel, and attendant higher costs. 
The inkjet printer is then much less compact. The larger ports would 
effectively increase the size of the resonant cavity and thus, change the 
uniformity of the standard wave pattern. In this circumstances, the 
required input energy from the piezoelectric transducer is increased. 
The height of the ink channel in the preferred embodiment is less than 0.1 
of an acoustic wavelenth in ink so that the channel does not act as a 
separate acoustic cavity with its own standing wave pattern. Theoretical 
analysis of the device for selection of the cavity dimensions to optimize 
uniformity of the standing wave pattern can be more easily accomplished 
than for a large ink cavity; therefore, the development costs for the 
inkjet printer of the present invention will be less. 
The use of a smaller ink cavity has other advantages. As unprinted ink 
recirculates in this type of inkjet printer, its more volatile components 
evaporate and it becomes more viscous. As a result, its sound speed 
changes. Changes in environment temperature can also alter ink sound speed 
if ink temperature is not controlled. Ink sound speed influences the 
standing wave pattern in the acoustic cavity filled with ink. Since for 
the present invention, the ink channel is small enough so that it does not 
support standing waves within itself, the effects of changes in ink sound 
speed are minimized. As a result, ink temperature and viscosity do not 
have to be maintained within as limited a range as with a large cavity. As 
a result, the cost and complexity of the claimed inkjet printer are less, 
and cold startup time is reduced. 
The objectives and advantages of the present invention will be more clearly 
understood from the following detailed description of the invention, 
wherein

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The apparatus 10 shown in FIG. 1 includes a pressurized ink reservoir 14 
which may be any suitable reservoir for the particular printing fluid 
used. The ink source 14 feeds through a tube 16 into the jet projecting 
device on the printhead 18. A valve 12 allows purging of air from the 
printhead during initial filling with ink. Ink streams 11 flow out of the 
nozzle plate 13. 
Turning to details of the inkjet apparatus, it includes a support frame 20 
supporting a material 22 which defines an acoustic cavity 24. The material 
of 24 is chosen to have an acoustic impedance approximately equal to the 
ink acoustic impedance so that sound waves generated to transducer 26 may 
be conveyed easily through the material to the ink channel itself. The 
acoustic cavity 24 has a size and shape important to the transmission of 
uniform standing wave patterns from the piezoelectric material to the ink 
channel 30 which is defined in channel face plate 31. In the present 
invention, a simple rectangular shape is preferred to be used for the 
cavity 24. Testing has demonstrated that by utilizing a rectangular 
cross-sectional acoustical cavity 24 in combination with the transducer as 
defined and a small ink channel, that acoustic standing waves of 
sufficient amplitude and uniformity can be transmitted from the 
piezoelectric transducer 26 to the ink in the channel 30. 
The piezoelectric transducer 26 itself, which is energized by the 
electronic signal generator 32 has a comb-like shape. That is, a solid 
block of piezoelectric material may be formed into the transducer 26 of 
this invention by cutting narrow slits 34a, 34b, 34c, 34d, into the 
transducer material, all from the same side. A length of backing material 
36 is left inplace to enable the transducer to hold its shape. This 
backing material is bonded to the solid acoustic material at the junction 
38. The ink channel plate 31 is provided, bonded across the forward 
opening of the acoustic cavity 24, and receives the acoustic disturbances 
which are generated by the piezoelectric transducer. The height h of the 
ink channel 30 (shown in FIG. 2) is less than 0.1 of an acoustical 
wavelength in ink, so that the channel does not act like a separate 
acoustic cavity with its own standing wave pattern. This minimizes the 
effect of changes in ink sound speed with changes in the thickness, 
temperature and viscosity of the ink. A slot 33 allows the ink to reach 
nozzle plate 13 to be ejected toward the paper 12. The reasons and 
advantages of this are discussed above in the summary of the invention. 
It should be noted that the material 22 used to define the acoustic cavity 
24 may be of steel, ceramic or any other material with a high acoustic 
impedance. The solid material 22 which fills the cavity 24 may be of 
rubber, plastic, epoxy or any other material which has an acoustic 
impedance approximately equal to that of the ink. This material may be 
either molded into the cavity or bonded to the inner surface of the cavity 
with a suitable bonding material. 
In an alternative and highly useful embodiment of this invention, a thin 
membrane 40 may be placed across the surface of the second opening; that 
is, the opening of the acoustic cavity 24. In contrast to prior work in 
this field where selection of material for the membrane was significant, 
the membrane of the present invention may be made of rubber, polyethylene 
or a thin sheet of any other material which is chemically compatible with 
the ink. The membrane need not be used at all if the material used to fill 
the acoustic cavity 24 is itself ink compatible. 
This would be provided by the use of solid acoustic materials in the 
rectangular cavity 24 such as plastic, epoxy resin, or an epoxy resin 
mixed with tiny glass hollow spheres. 
The piezoelectric transducer 26 is made of standard piezoelectric crystal 
material. Its advantages reside in all the cuts being made from a single 
side of the device, and using very thin cuts to form the slices 34a-d. 
Finite element analysis has shown that this shape is a significant 
improvement over those used in the prior art in generating waves which are 
uniform over the full surface of the transducer, and which may be 
uniformly transmitted from this surface 38 to the ink channel. The 
stimulator itself is a piezoelectric plate cut with a diamond saw to form 
the channels 34a-d into the comb-like shape. This shape, as stated above, 
vibrates uniformly in the x direction when excited by a signal generator 
32 near its first resonant frequency for the x direction. When coupled to 
an acoustic ink cavity as shown herein, the vibration of the piezoelectric 
26 generates uniform pressure fluxuations in the ink channel which in turn 
cause uniform breakoff of the inkjets. 
FIG. 3 shows a disassembled or exploded perspective view of a two-cavity, 
acoustically driven ink drop generator developed in accordance with an 
alternative embodiment of this invention to provide a plurality of ink 
cavities driven by the acoustic transducer 26 of this invention. A more 
detailed view of the relative orientation of the paths followed by the ink 
drops after being expelled through the orifice plate past the deflection 
electronics is shown in FIGS. 4 and 5. 
In a preferred embodiment shown in FIG. 3, the cavity is tapered from the 
surface 17 at which the solid acoustic material is coupled to the acoustic 
source 24 toward the surface which is covered by the membrane 40 that 
acoustically couples the acoustic cavity 24 with the ink cavities 30 of 
ink cavity plate 31. The tapering is to concentrate, smooth and propagate 
through the metallic membrane 40 to a uniform pulsating effect as created 
by the acoustic source 26 through the apex of the cavity 24 into the 
relatively smaller ink cavity slots 30 of ink cavity plate 31. In this 
way, uniform breakoff of the ink drops at the relatively high frequency of 
operation is achieved. 
Ink is supplied through ink inlets 30 which communicate through opening 32 
in the impervious membrane 20 to the ends of the ink cavity slots 22. Ink 
flow-through outlets 12 are provided attached to the far ends of the ink 
cavity slots 30 so that an ink flow through the full length of the slot 
without bubbles or other disturbances in the ink flow can be achieved. As 
the ink flows through the slots 30, the pulsating output of the acoustic 
sources as concentrated through the solid filled cavity 24 is coupled 
through the plastic or metallic membrane 31 to the slots 30. As can be 
seen, the multiple ink cavity slots 30 disposed external to the acoustic 
cavity 24 and the membrane 31 are all in simultaneous acoustic 
communication with the single acoustic cavity. In this way, simultaneous 
generation of the drops from the slots through the orifice plate 13 is 
achieved, even at the relatively high frequency (about 110 KHz) of 
operation of the system. The use of multiple parallel slots also allows 
for simultaneous use of different colored inks. Several types of 
piezoelectric transducers, as in FIG. 3, may be used. The transducer is 
essentially in block form with slots 42 cut substantially across the depth 
of the block transducer. These slots have been found to reduce the bulk 
motion of the block, and enchance the piston-like motion of the transducer 
toward and away from the membrane 20 and cavity slots 22 to produce the 
desired pulsating effect on the ink filaments emanating from the orifices 
of plate 40. A low cost assembly of the entire system is achieved by using 
this structure inasmuch as the piezoelectric transducer can be bonded or 
molded into the transducer cavity 14 in alignment with the solid filled 
acoustic cavity, after the acoustic cavity has been appropriately filled 
with the desired low density propagating material. In fact, it is possible 
to eliminate the transmissive membrane entirely if the material of the 
acoustic cavity is made chemically compatible with the ink. 
It should also be noted that the ink cavity slots 30 are relatively short, 
or about 1/16 of an acoustic wave length at high frequency operation. This 
improves the uniformity of the standing wave created at the outlet nozzles 
or orifices to promote uniformity of generation of the ink drops from 
slots 30 through the orifice plate 13. It can also be seen from FIGS. 4 
and 5 that the present arrangement readily lends itself to charging of the 
drops after passage of the drops through the orifice plate 13 by 
appropriately formed electrodes 60 aligned with each opening in the 
orifice plate. After appropriate charging of the generated drops, the 
drops pass between plates 62, 64 which provide appropriate deflection of 
each drop either to reach the recording medium 66 or the collecting gutter 
68. A slight modification of the ink cavity and the placement of the 
charging electrodes 60 allows for converging of the outputs of the 
multiple ink cavities at about the same spots on the recording media as 
shown in FIG. 3. In this way high speed, or high density, or multi-color 
printing can be easily and reliably achieved. 
Other embodiments may occur to a person of skill in the art who studies the 
invention disclosure. Therefore, the scope of this invention is to be 
limited only by the following claims.