HK1091441B - Apparatus for depositing droplets - Google Patents

Description

Device for depositing droplets

Technical Field

The present invention relates to depositing droplets on a substrate.

Background

Inkjet printers are one type of device that deposits droplets on a substrate. Inkjet printers typically include an ink path from an ink supply to a nozzle path. The nozzle channel terminates in a nozzle orifice from which ink drops are ejected. Ink drop ejection is controlled by pressurizing ink in an ink channel with an actuator. The actuator may be a piezoelectric deflector, a thermal bubble jet generator or an electrostatically deflected element. A typical printing assembly has an array of ink channels with corresponding nozzle holes and associated actuators. Drop ejection from each nozzle aperture can be independently controlled. In a drop-on-demand printing assembly, each actuator is actuated to selectively eject a drop at a particular pixel location of an image as the printing assembly and a printing substrate are moved relative to each other. In high performance printing assemblies, typically the nozzle openings have a diameter of 50 μm or less, such as about 25 μm, and are spaced at a pitch of 100-300 nozzles/inch with a resolution of 100-300 dpi or greater, providing drops having a drop volume of about 1-70 picoliters (pl) or less. The drop ejection frequency is typically 10KHz or greater.

U.S. patent No. 5265315 to Hoisington et al, which is incorporated herein by reference in its entirety, describes a printing assembly having a semiconductor body and a piezoelectric actuator. The body is made of silicon and etched to form ink chambers. The nozzle holes are formed by one single nozzle plate attached to the silicon body. The piezoelectric actuator has a layer of piezoelectric material that changes geometry or bends in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber disposed along the ink path. Piezoelectric inkjet printing assemblies are also described in Fishbeek et al, U.S. patent No. 4825227 and Hine, U.S. patent No. 4937598, which are also incorporated herein by reference in their entirety.

Printing accuracy is affected by many factors, including the uniformity of the size and velocity of the droplets ejected by the multiple nozzles in the assembly and at the multiple assemblies of the printer. The uniformity of ink drop size and ink drop velocity is in turn affected by factors such as the dimensional uniformity of the ink channels, acoustic interference effects, contamination of the ink flow channels, and the uniformity of actuator drive.

In many ink jet systems, ink is delivered through a supply conduit to a pumping chamber that communicates with the nozzles. Ink is periodically ejected from the nozzle by rapidly compressing the volume of the pumping chamber under the action of an electromechanical transducer, such as a piezoelectric element. The rapid compression is performed before and/or after the corresponding rapid expansion of the chamber volume. During the expansion portion of the ink drop ejection cycle, the ink pressure in the pumping chamber is greatly reduced, increasing the tendency of air dissolved in the ink within the chamber to create bubbles on the surface of the chamber. Gas bubbles are particularly prone to nucleation sites in the cavity, such as sharp corners, micro-cracks or pits, or foreign particles deposited on the cavity surface where the gas can settle. If the expansion/compression cycles occur at a sufficiently high frequency, the bubble size increases from one cycle to the next, forming rectified diffusion (rectified diffusion). The presence of bubbles in the pumping chamber prevents the application of pressure to the ink in a desired manner to eject a selected volume of ink drops from the nozzle at a selected time, resulting in a degradation of print quality over time. Rectified diffusion is more problematic for high quality inkjet systems because such systems use viscous inks that require higher pressures and frequencies to properly eject.

If the frequency of the pressure oscillations in the pumping chamber is low, the bubbles that nucleate inside the pumping chamber expand but re-dissolve before the next stroke as shown in FIG. 1. At time D, during the expansion stroke, a bubble 20 is formed. Later, at time E, during the compression stroke, the bubbles 22 are now smaller due to the pressure increase and due to the diffusion of gas from the bubbles back into the fluid in the pumping chamber. In this low frequency case, by time F, the bubbles dissolve.

If the frequency of pressure oscillations in the pumping chamber is high, the bubbles do not have time to re-dissolve during the compression cycle before being subjected to another expansion cycle. FIG. 2 shows bubbles over multiple pumping cyclesHow the radius increases cyclically as a whole. Figures 3A-3C illustrate the effect of increasing the radius of the bubble in the pumping chamber. Referring to fig. 2-3C, at time G, during the compression stroke of pumping chamber 34, printing element 30 ejects a droplet 32. Inside the pumping chamber 34, there is a meniscus 33, the bubble 36 having a radius R₃₆. Later, at time H, during the compression stroke, bubble 38 (not shown) has a radius R₃₈. This radius increases still further during the next expansion stroke. Later, at time point I during the expansion stroke, in the print chamber 34 with meniscus 42, the bubble 40 increases in size. This process continues as before, producing a radius R₄₄And a bubble 44 (not shown) having a radius R₄₆Air bubble 46 (not shown). Eventually, a large bubble volume 48 is formed in the pumping chamber. At this time, the ink droplet volume and velocity may be reduced or, in an extreme case, ejection may be completely hindered because the energy that would otherwise be used to eject a droplet is used to compress the bubble.

Jetting at higher frequencies is desirable because the output can be increased by making the linear velocity higher. One important limitation of the operating frequency is the resonant frequency of the ink jet. The resonant frequency is determined by the round trip travel time of the pressure wave in the pumping chamber. Thus, making the pumping chamber smaller increases the natural frequency of the ink jet and allows higher operating frequencies. Making the nozzle diameter smaller also helps to operate at higher frequencies, but this also requires smaller drop volumes. It is also possible to spray at a higher frequency by reducing the time for which the pressure is applied, but this requires a higher pressure. Generally, the sound pressure ranges from about 2 atmospheres below the ambient pressure during the expansion stroke to about 2 to 3 atmospheres above the ambient pressure during the compression stroke. The higher the injection frequency, the more problems are caused by rectified diffusion.

Disclosure of Invention

In general, in one aspect, the invention features an apparatus for depositing droplets on a substrate. The device includes: a support for the substrate; a droplet ejection assembly including a pumping chamber; a controller; and a source of static pressure for increasing the total pressure in the pumping chamber above a threshold pressure level to avoid rectified diffusion type bubble growth in the pumping chamber. The droplet ejection assembly is positioned above the support for depositing droplets on the substrate and includes, in addition to the pumping chamber, a displacement member and an orifice for ejecting the droplets. The controller provides a signal to the displacement member to eject a droplet.

In some implementations, the absolute value of the static pressure is greater than about 1.5 atmospheres.

In some implementations, the signal is provided at a frequency greater than about 8000 Hz. In other implementations, the signal is provided at a frequency greater than about 8000Hz and a static pressure having an absolute value greater than about 1.5 atmospheres.

The ejected droplets may be ink or other suitable droplet-forming material. The substrate may be paper or any other suitable substrate.

The pressure source may comprise a pressurised gas. The gas may be filtered to remove particulate matter. Moisture or vaporized solvent may be added to the gas. The gas may be air or any other suitable gas.

Another aspect of the invention features an apparatus that includes a support for a substrate; a droplet ejection assembly including a pumping chamber; a controller; a housing structure; and a source of static pressure to increase the total pressure in the pumping chamber above a threshold pressure level to avoid rectified diffusion-type bubble growth in the pumping chamber. The droplet ejection assembly is positioned above the support for depositing droplets on the substrate. The droplet ejection assembly includes, in addition to the pumping chamber, a displacement member and an orifice for ejecting the droplets. The controller provides a signal to the displacement member to eject a droplet. The housing structure and the support together form an enclosed area through which droplets are ejected onto the substrate. The housing structure also forms an entrance gap and an exit gap with the support through which the substrate travels. The inlet gap may be about 0.002 to 0.04 inches. The outlet gap may be about 0.002 to 0.04 inches.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

Drawings

FIG. 1 is a graph of ink pressure versus time for low frequency vibration;

FIG. 2 is a graph of ink pressure and bubble radius versus time for high frequency vibrations;

FIGS. 3A-3C illustrate bubble growth in an ideal printhead;

FIG. 4 is a side view of an apparatus for printing on a substrate;

FIG. 5 is a schematic side view of a printing station of the apparatus of FIG. 4;

FIG. 6 is a side view of another embodiment;

FIG. 7 is a graph of relative concentration versus applied sound field;

like reference symbols in the various drawings indicate like elements.

Detailed Description

Figure 4 shows an apparatus 50 for the continuous deposition of ink droplets onto a substrate 52, such as paper. Substrate 52 is pulled from a roll 54 located on a supply table 56 and fed to a series of droplet deposition stations 58 for depositing droplets of a plurality of different colors on substrate 52. Each droplet deposition station 58 has a droplet ejection assembly 60 above substrate 52 for depositing droplets on substrate 52. Each deposition station 58 is a substrate support structure 62 (e.g., a non-porous platen) beneath the substrate 52. After the substrate 52 exits the last deposition station 64, the substrate may proceed to a pre-trim station 66. The pre-finishing station 66 may be used to dry the substrate 52. It may also be used for uv or other radiation curing of the substrate 52. The substrate 52 then proceeds to a finishing station 68 where it is folded and cut into finished products 70. The substrate feed speed is about 0.25 to 5.0 m/s or higher. The droplet ejection assembly can eject droplets of ink. It may also eject a radiation curable material or other material that may be delivered as droplets.

Fig. 5 shows the components of a high frequency droplet deposition station 58 that can avoid significant rectified diffusion. In this device, the total pressure of the ink in the pumping chamber is raised so that the minimum total pressure reached during the expansion stroke is high enough to avoid rectified diffusion type bubble growth in the pumping chamber. This is achieved by increasing the pressure within pumping chamber 92 and in droplet ejection zone 86 by enclosing the printhead including pumping chamber 92 and ink supply 98 in housing 80 and maintaining housing 80 at a high pressure level with pressurized air supplied through slits 84 via manifold 82, as shown schematically in fig. 5. Manifold 82 is connected to the compressor using a quick connector (not shown). Droplet ejection assembly 58 is positioned above substrate 52 (e.g., paper). A source of static pressure is applied within the housing structure 80 through a manifold 82 with slits 84. The pressure applied in this manner reduces turbulence in and around enclosed region 86. Turbulence can degrade print quality because the primary drops and the smaller corresponding satellite drops can be misdirected due to the turbulent air. The substrate 52 passes through an inlet gap 88 and an outlet gap 90 located on top of the substrate support structure 62 (e.g., a non-porous platen). The platen is preferably non-porous because a porous platen may create too much drag when the substrate 52 is pulled across the platen at high pressure. The inlet gap 88 and the outlet gap 90 are about 0.002 to 0.04 inches measured above the substrate 52. If the gap is too large, the power requirements are limited; if the gap is too small, the image may become blurred or paper may be jammed. If the pressure is too low, rectified diffusion may occur, while if the pressure is too high, the structural requirements of the housing structure 80 may not be fulfilled. Preferably, the static pressure is about 1.5 to 10 atmospheres absolute (0.5 to 9 atmospheres above ambient). Droplet ejection assembly 58 includes a pumping chamber 92 with an associated ink channel 94. The ink channel 94 is connected to an ink inlet 96 connected to an ink tank 98 containing ink 100. The entire ink reservoir 98 is maintained under static pressure. This can be accomplished by a small aperture 103 in ink reservoir 98. Minor differences (e.g., 0.1-0.3 pounds per square inch) in the pumping chamber 92 due to tank height differences relative to the pumping chamber 92 can be corrected using a pump 102 (e.g., a small centrifugal blower-type pump). Water or other solvents may be added to the gas to inhibit drying of the nozzle. To slow the degradation of the ink, the gas may be air, or the gas may have a smaller oxygen content than air. Increasing the oxygen content relative to air can slow the curing of uv curable inks. In addition, the gas may be filtered, for example, using a HEPA filter, to remove particulates and excess moisture.

Fig. 6 shows another embodiment in which a rotating drum 104 below the printing substrate 52 is used in place of the stationary, curved support 62 below the housing 80 in the apparatus shown in fig. 5.

FIG. 7 is a graph of relative concentration (Ci/C0) versus applied acoustic pressure showing the relative concentration of air required to prevent bubble growth versus applied acoustic pressure for various equilibrium bubble radii and various static pressures in a 100kHz pressure field. Ci is the air concentration in the ink, and C0 is the air concentration in the ink when the air is saturated. The quantity 100(Ci/C0) represents percent saturation. If the ink is in contact with air for a long time, the Ci/C0 ratio will become 100% saturated. In many ink jet systems, the ink is degassed prior to use in order to avoid bubble problems. Degassing the ink reduces the relative concentration values, allowing operation at higher applied acoustic fields without bubble growth. Increasing the static pressure also allows operation at higher acoustic pressures without bubble growth. In the graph, P0 is the static pressure. The X-axis represents the amplitude of the sound pressure field. At a given static pressure, applied acoustic pressure field, and relative concentration of air in the ink, a bubble of a given size either grows or shrinks. Increasing the static pressure, decreasing the relative concentration of air in the ink and decreasing the amplitude of the applied oscillating pressure field can cause the bubble to move in the direction of shrinkage. As an example, a curve labeled with Rn of 5 μm to P0 of 1 atm is a curve of a bubble having an equilibrium radius (i.e., a radius in the case where no sound pressure is applied) of 5 μm and a static pressure of 1 atm. This curve shows that applying a sound field of (+/-)40000 pa does not grow bubbles even at a relative concentration of 100% (Ci/C0 ═ 1). If it is desired that such bubbles do not grow in a (+/-)100000 pa pressure field, it is necessary to reduce the relative concentration to about 27%. As another example, a curve labeled with Rn of 0.2 μm: P0 of 5 atm is a curve of a bubble having an equilibrium radius (i.e., a radius without applied sound pressure) of 0.2 μm and a static pressure of 5 atm.

In the conditions above the curve, the bubbles grow with time, and in the conditions below the curve, the bubbles shrink. In all cases shown in fig. 7, an atmosphere of 0.2 μm for Rn and 5 atm for P0 is the least likely to cause bubble growth due to rectified diffusion. In this case, bubbles (Ci/C0 ═ 1) do not grow in the air-saturated ink until the applied sound field exceeds 450000 pa. By degassing the ink to a relative concentration of 0.2, an acoustic pressure field in excess of 580000 pascals can be applied without bubble growth. FIG. 7 shows that the effect of reducing the relative concentration Ci/C0 is limited. For example, for the nucleation site size Rn 1-Rn 5 μm, even if Ci/C0 is reduced to 1% (which is difficult), the maximum acoustic field that can be applied in the jet is only about 150000 pa. Conversely, by increasing the static pressure, a 4 times higher acoustic field can be applied without growing bubbles. Henry's law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas in contact with the liquid. Therefore, when the air pressure on the ink increased from 1 atmosphere to 5 atmospheres, the relative density decreased to 1/5. If 100% of the ink at 1 atm is pumped into the ink tank, which is now at 5 atm, Ci/C0 is 20%. Of course, measures are taken so that the ink entering the pumping chamber does not rebalance to 100% saturation. Rebalancing can be avoided by reducing the surface area of the jetting fluid that is in contact with air and/or jetting the fluid at a rate fast enough to prevent rebalancing.

Many embodiments have been described above. Nevertheless, various modifications may be made without departing from the spirit and scope of the invention. For example, the deposited droplets may be ink or other material. For example, the deposited droplets may be a material that is curable by ultraviolet or other radiation or other material that can be delivered as droplets. For example, the apparatus may be part of a precision dispensing system. Accordingly, other embodiments are within the scope of the following claims.

Claims (17)

1. An apparatus for depositing droplets on a substrate, the apparatus comprising:

a support for said substrate;

a droplet ejection assembly positioned above the support for depositing the droplets on the substrate on the support, the droplet ejection assembly comprising a pumping chamber, a displacement member, and an orifice for ejecting the droplets;

a controller for providing a signal to the displacement member to eject a droplet; and

a source of static pressure for increasing the total pressure in the pumping chamber above a threshold pressure level to avoid rectified diffusion type bubble growth in the pumping chamber.

2. The apparatus of claim 1, wherein the apparatus further comprises:

a housing structure forming with the support an enclosed area through which the droplets are ejected onto the substrate, the housing structure further forming with the support an inlet gap and an outlet gap through which the substrate travels.

3. The apparatus of claim 1 or 2, wherein the static pressure is greater than 1.5 atmospheres absolute.

4. The apparatus of claim 1 or 2, wherein the signal is provided at a frequency greater than 8000 Hz.

5. The apparatus of claim 1 or 2, wherein the absolute value of the static pressure is greater than 1.5 atmospheres and the frequency is greater than 8000 Hz.

6. The apparatus of claim 1 or 2, wherein the droplets comprise ink.

7. The apparatus of claim 1 or 2, wherein the substrate comprises paper.

8. The apparatus of claim 1 or 2, further comprising a continuously moving support.

9. The device of claim 1 or 2, wherein the source of static pressure comprises a pressurized gas.

10. The apparatus of claim 9, further comprising filtering the pressurized gas to remove particulate matter.

11. The apparatus of claim 9, further comprising adding moisture to the source of pressurized gas.

12. The apparatus of claim 9, further comprising adding a solvent to the source of pressurized gas.

13. The apparatus of claim 9, wherein the gas is air.

14. The apparatus of claim 9, wherein the oxygen content of the gas is less than the oxygen content of air.

15. The apparatus of claim 9, wherein the oxygen content of the gas is greater than the oxygen content of air.

16. The device of claim 2, wherein the inlet gap is 0.002-0.04 inches.

17. The device of claim 2, wherein the exit gap is 0.002-0.04 inches.