GB2037235A - Electrostatic lens for an ink jet printer - Google Patents

Abstract

An electrostatic lens (10) for use in an ink jet printer having ink jet nozzles (1) charging electrodes (3) and a single pair of deflection plates (6) that divert charged droplets over a shared gutter (9) toward copy sheet (8), aligns or focuses charged droplets from all the nozzles on the sheet (8) despite misalignment of nozzles relative to a print line on the target. The lens (10) provides at least one upstream and one downstream electrode between which a voltage is applied to produce a focussing field. In Figure 5 the lens (40) a pair of upstream electrodes (42, 43) a pair of downstream electrodes (44, 45) and an intermediate electrode formed by apertured, earthed plate (41). <IMAGE>

Description

SPECIFICATION Electrostatic lens and ink jet printers using the lens This invention relates to an electrostatic lens and to an ink jet printer using such a lens. More specifically, this invention relates to an electrostatic lens for an ink jet printer for aligning or changing the trajectories of charged droplets emitted at high velocities from a nozzle.

The trajectories of charged ink droplets are difficult to align because the droplets are small, typically from 25 to 625 microns in diameter, and consequently the nozzle orifices are small and difficult to manufacture and assemble. A coarse alignment must be achieved to align the trajectories of droplets emitted by a nozzle with a charging tunnel and a pair of closely spaced deflection plates. The charging tunnel diameter is normally only about from 3 to 10 times the droplet diameter whereas considerably larger spacing separates the deflection plates. Once a coarse alignment is obtained, a vernier or fine alignment is often desired yet difficult to achieve.

The alignment difficulty is compounded in multiple jet printers, for example, in a multi-jet printer as disclosed in US Patent No. 3373437 to Sweet and Cumming, the trajectories of the multiple jets must be aligned relative to each other so as to print a straight row of droplets to match a line of print or pixel positions on a target. Heretofore, electrical techniques have been used to correct for the misalignment of one trajectory relative to another. The target is moved at a constant velocity past the print line. The electrical command to place a droplet at a given pixel position is delayed (or accelerated) a small amount to allow the target to move a distance corresponding to the misalignment of the jet trajectory for that pixel position. Alternatively, the charge on the errant droplet is increased (or decreased) to vary its deflection and hence placement on the target.Clearly, the alignment is achieved at the expense of increased complexity to the electrical control circuits for the printer.

It is an object of the invention to overcome the alignment problem of prior art, charged ink droplet systems.

According to the present invention there is provided an electrnstafic lens for changing the trajectory of a charged fluid droplet in flight including: upstream and downstream electrodes positioned adjacent the trajectory of a charged droplet including means for coupling to a voltage source for establishing a focusing electric field between the electrodes; said focusing electric field including a centreline path over which a trajectory of a charge droplet is substantially unchanged by the focusing field; and said focusing electric field being such as to change a trajectory of a charged droplet that is offset from the centreline path.

The invention also provides an ink jet printer which includes such a lens.

The lens of the invention establishes a focusing electric field along the intended trajectory of a droplet. The focusing field extends in a direction generally parallel to the trajectory of a droplet in contrast to the generally normal direction of the electric field created by conventional deflection plates. The focusing field is preferably given sym- metry at least on two sides of a droplet's trajectory thereby allowing a properly aligned droplet to traverse the focusing field without having a course correction imparted to it.

The cylindrical lens effect is achieved with four linear electrodes. At an upstream position, an electrode is placed equidistant above and below the intended droplet trajectory. At a downstream position, the remaining two electrodes are placed equidistant above and below the intended droplet trajectory. The four electrodes are substantially parallel and orthogonal to the trajectory. A potential difference coupled between the upstream and downstream fields creates two electric fields whose boundaries resemble two half cylinders abutting at a tangent plane parallel to their bases. The tangent plane defines a path over which a charged droplet is not deflected. Droplets that enter the field above or below the tangent plane are focused to a line on that plane a determinable distance downstream.The focal line is constant for droplets having substantial ly the same velocities and mass to charge ratios.

US Patent 3596275 describes the prior art high velocity ink jet device for which the instant invention is especially suited. Therein, a fluid ink is forced upon a volume through a small nozzle under high pressure. The natural tendency of the resultant stream emitted from the nozzle to break up into droplets is promoted by acoustically stimulating the ink at a frequency of about 120 kilohertz. The droplets tend to form at regular intervals and at a constant size. The ink is conductive. As the droplets separate from the fluid column emitted from the nozzle, the droplets pass a charging electrode, often a closed tunnel, where charge is induced on them by a voltage coupled to the charging electrode.

The charged droplet is propelled along a trajectory towards a target that is moving at right angles to its flight. Before the droplet reaches the target it passes between parallel plates, A steady state electric field normal to the path of the droplet is created by a 2000-4000 volt potential difference coupled between the deflection plates.

The amount of charge on the droplet determines the amount of deflection imparted to it by the deflection field.

US Patent 3373437 mentioned above describes a binary ink jet system in which two deflection plates are shared by a plurality of linearly aligned nozzles.

The binary feature is that the droplet from a given nozzle either is charged and deflected toward a pixel position on the target or remains uncharged and is collected in a gutter. The charge on the droplets sent to the paper is intended to be equal.

US Patent 3877036 discloses an ink jet alignment electrode. The electrode, however, is positioned to act on the fluid column at a location prior to droplet formation.

An electrostatic lens, and an ink jet printer including the lens, in accordance with the invention, will now be described, by way of example, with refer ence to the accompanying drawings, in which: Figure 1 is a side view in cross-section of a multi-nozzle ink jet printer employing a cylindrical electrostatic lens according to the present invention.

Figure 2 is a plan view of a multi-nozzleinkjet printer of Figure 1.

Figure 3 is a view of the cylindrical electrostatic lens in Figures 1 and 2 looking upstream from the target toward the nozzles.

Figure 4 is a cross-section, elevation view of the lens along lines 4-4 in Figure 3. Also, this figure illustrates the focusing field and the focal distance for the lens.

Figure 5 is a cross-section, elevation view of another embodiment of an electrostatic lens. The lens in this figure employs an intermediate electrode between upstream and downstream electrodes. The lens employs two focusing fields and has a focal distance generally as depicted.

Herein, the ink jet system described is of the type disclosed in the above named US Patent 3596275.

Briefly, a transducer modulates or stimulates ink in a chamber or tube coupled to a nozzle. The ink is subjected to pressures of from about 1.4 to 10.7 kg.cm-2. The modulation of the ink causes a stream of discrete droplets of like velocity, mass, shape and trajectory to be emitted from the nozzle. The modulating apparatus and circuitry is not shown to simplify and thereby clarify the present discussion.

For details on that apparatus, the reader is referred to US Patent 3956275.

Figures 1 and 2 are a side view and plan view of a multiple nozzle ink jet printer. Like elements in the various figures have the same reference numbers.

The printer includes the nozzle 1 that emits a stream of droplets along a trajectory indicated by dashed line 2. The droplets are charged at charging electrode 3 as indicated by the circle 4 having the minus sign indicating a net negative charge. For the polarities given, the negatively charged droplets are deflected upwardly along the path indicated by dashed line 5 by the deflection plates 6 and 7. The deflected droplets head toward the target 8 and the uncharged, low charged, or oppositely charged droplets are collected by the gutterS. The cylindrical, electrostatic lens 10 focuses the charged droplets to a common focal line 12 on the target. The droplet 4 is diverted over the path indicated by the dashed line 13 by the lens. The dashed line 14 (actually a plane) is the centreline or axis of lens 10.Charged droplets that travel through the lens along the centreline do not have their trajectories altered.

The lens 10 can also be located upstream of the deflection plates. Specifically, lens 10 can be positioned between the charging electrode 3 and the deflection plates 6 and 7.

The printer of Figures 1 and 2 is a binary printer similar to that disclosed in US Patent 3373437 mentioned at the outset. Printing is achieved by moving the target 8 at generally right angles to the ink jet path or trajectory 2. The target is moved at a constant velocity in the upward direction in Figure 1 as indicated by arrow 15. Four drive rollers 16a, b, c and d are coupled to an appropriate drive source (not shown) to advance the target.

Referring to Figure 2, a plurality of nozzles 1 through 1c are representative of the multiple nozzles of a printer. For good quality image reproduction, a printer should have about 40 nozzles per cm. This means that to cover a 21.6 cm standard paper width, about 860 nozzles are deployed as illustrated in Figure 2. The packing density is reduced if the nozzles are aligned in two or more rows with one row offset one nozzle or pixel position from the other. The lens 10 is appropriateforthe multiple row arrangementof nozzles provided allowance is made for one row to be focused to a different line than the other. In addition, the offset between rows can be made large enough to accommodate a lens for each row.

In Figure 2, each nozzle 1-1c has a separate charging electrode 3-3c that charges droplets travelling the generally parallel paths 2-2c. The object is to place a droplet, when called for by a video signal, at adjacent pixel positions 18-18c on the target. The scan line of 18-18c pixels should be straight. However, any misalignment of the nozzles or any error in the amount of charge places on a droplet by the charging electrodes causes the droplet to miss the pixel location. The result is a distortion of an image constructed from a raster pattern of multiple pixel lines.

Heretofore, the alignment of the nozzles to the pixel locations has included electrical techniques. For example, should nozzle 1a tend to place its droplets slightly above pixel position 18a on the target, the video signal applied to electrode 3a is delayed, relative to nozzles 1, 1 b and 1 c, short duration to allow the target to move the amount of the offset.

Alternatively, the amount of change induced on the droplet is increased or decreased to vary the deflection and amount to correctly place a droplet at a given pixel position. The delay or magnitude change are applied to subsequent droplets.

The present invention uses lens 10 for the alignment of droplets. In Figure 2, the lens 10 is seen in plan view as shared by all the nozzles.

Referring to Figures 3 and 4, lens 10 is made up of an insulating member 20 having a rectangular tunnel or hole 21 for passage of droplets. The upstream face of the insulator 20 has rectangular electrodes 22 and 23 at the long sides of the rectangular entrance to the tunnel 21. The upstream electrodes 22 and 23 are coupled to ground potential, by way of example.

The downstream face of insulator 20 has rectangular electrodes 24 and 25 at the long sides of the rectangular exit to tunnel 21. The downstream electrodes are coupled to a high positive voltage indicated by the +A symbol. As an example, the insulator 20 is a phenolic insulator board of the type used for printed circuit boards and the electrodes 22-25 are copper strips formed by conventional evaporation and chemical etching techniques. The +A voltage is preferably about 1500 volts for a 1.52 mm thick board 20. The length of the tunnel 21 is about 1.55 mm ie the conductors are about 15 microns in thickness.

Briefly referring to Figure 1,the lens 10 establishes a field that focuses droplets to a line 12 that corresponds to the scan line of pixels 18-18c. The focusing field is better described in connection with Figure 4. The focusing electric field is represented by the dashed lines 27 and 28 emanating from the edges of the upstream and downstream electrodes 22-25 and confined substantially within the region defined by the semi-circles 27a and 28a along the length of the electrodes. The envelope of the field lines is analogous to two half-cylinders abutting at a tangent plane parallel to their bases. The abutting tangent plane is normal to the drawing and is conveniently defined by centreline 14.

The plane defined by centreline 14 is a path through the focusing field comprising fields 27 and 28 over which a charged droplet remains unaffected.

However, a droplet such as the negatively charged droplet 29 that is on a trajectory 31 offset from the centreline is focused to the focal line 12 by the focusing field. Likewise, the droplet 30 below the centreline 14 is focused to the focal line 12. All other droplets travelling trajectories lying above, below or between the paths 31 and 32 are also focused to line 12.

The focusing fields 27 and 28 extend in the direction of droplet travel from the upstream electrodes 22 and 23 to the downstream electrodes 24 and 25. At the entrance to the tunnel 21, the focusing fields include a high density flux region that has vertical force components of significant magnitude.

These forces are represented by the vectors 34 and 35. In the centre region of the fields 27 and 28, the field and force vectors are parallel to the centreline 14 and have the same direction as the droplet for the polarities shown. These parallel forces accelerate the charged droplets shown. As a result, the charged droplets are under the influence of the focusing forces 34 and 35 longer than they are corresponding defocusing forces at the tunnel exit represented by vectors 37 and 38. When the +Apotential is coupled to the upstream electrodes 22 and 23 and the ground potential is coupled to the downstream electrodes 24 and 25, the charged droplets are decelerated as they enterthetunnel 21.In this case, the charged droplets once again are under the influence of the focusing forces for a longer time than the defocusing forces are at the exitto the lens. Similarly, a positively charged droplet will be focused by the field shown in Figure 4 by first being decelerated and then accelerated. The focusing forces always predominate over the defocusing forces regardless of the relative polarities.

Experimentation shows that the focusing forces represented by the vectors 34 and 35 are not offset by the effects of the defocusing forces represented by the vectors 37 and 38. In other words, despite what appears to be equal and opposite forces, the focusing forces 34 and 35 prevail and bend the trajectory 31 of a droplet 30 so as to intersect the centreline 14 at the focal line 12. This is because the time spent in the region of the focusing fields is greater than the time spent in the regions of the defocusing fields. Similarly, the trajectory 32 of a droplet 30 below the centreline 14, is bent by the focusing forces 34 and 35 to intersect the focus point despite the defocusing forces 37 and 38.

The symbol fin Figure 4 is representative of the focal length of the lens. For convenience it is measured from the entrance to tunnel 21 to the empirically determinable focus line 12. As mentioned earlier, the focus fvaries for a change in the focusing field potential. When +A is increased, f is decreased. Also, when the amount of charge on droplets 29 and 30 are increased, f is decreased and when the amount of charge on the droplets is decreased, f is increased.

Figure 3 shows the lens 10 looking from the target upstream toward the nozzles 1-1c. The insulator board 20 is shown with the conductive copper everywhere but along the narrow rectangular sides of the exit to tunnel 21. Since electrodes 24 and 25 (as well as electrodes 22 and 23) are coupled to the same potential, the two electrodes could be electrically coupled by copper deposited on the vertical, exposed areas of the board 20. The vertical, conductive edges should be spaced a significant distance from the end nozzles 1 and 1c so the distortion to the cylindrically shaped fields 27 and 28 are minimized.

Figure 5 illustrates another embodiment of the instant invention employing multiple, cylindrical focusing fields. The lens 40 is similar in construction to lens 10 but includes an intermediate electrode 41 between upstream electrodes 42 and 43 and downstream electrodes 44 and 45. Electrode 41 is a metal plate having a rectangular hole or tunnel 46 in it that matches the rectangular tunnels 47 and 48 in insulators 49 and 50 abutted against member 41. The intermediate electrode 41 is fabricated from 1.6 mm thick aluminium sheet and the insulators 49 and 50 from 1.52 mm phenolic board. The upstream and downstream electrodes 42-45 are on the parallel, long edges of the tunnel orifices as in the case of the electrodes 22-25 on lens 10. The height of the tunnels 46-48 is about 1.27 mm for droplets of about 25 to 250 microns in diameter.

Upstreams and downstream electrodes 42-45 are all coupled to a high voltage (represented by the symbol +A) of about +1500 volts, for example, and the intermediate electrode is grounded. Alternatively, the intermediate electrode 41 can be coupled to + 1500 volts, for example, and the upstream and downstream electrodes 42-45 to ground.

There are two focusing fields associated with lens 40 including the upstream field made up of the upper and lower cylindrical fields 55 and 56 and the downstream field made up of the upper and lower cylindrical fields 57 and 58. The centreline 60 defines the path over which the trajectory of a charged droplet is not bent. For the polarities shown, the upstream field extends in a direction opposite to the flight of the droplet, and the downfield field extends in the same direction of the flight of the droplet. The defocusing forces represented by vectors 61 and 62 at the entrance to lens 40 and vectors 67 and 68 at the exit to the lens are found not to prevent the focusing of offset charged droplets 52 and 53 at the focal line 70. The focusing forces due to the intermediate electrode 41 and represented by the vectors 63-66 are predominant because of the greater time spent in the focusing region. That is, the acceleration and deceleration of the droplets always act to favour focusing rather than defocusing. The opposing po larity of the fields of lens 40 are selected so that no net accelerating or decelerating energy is given to the droplets passing through it. In contrast, the single field lens, eg lens 10, imparts a very small amount of accelerating or decelerating energy to a charged droplet. The amount of net energy change is negligible yet, surprisingly, the focusing effect is realised.

The focal distance f is measured, for convenience, from the edge of the upstream edge of the intermediate electrode 41 to the focal line 70.

The function of lens 40 was tested by directing a stream of droplets through the lens and charging every third droplet. The uncharged droplets, by definition, are not effected by an electric field but they establish a base line for measurements. A lens was constructed like lens 40 above. About +1500 volts was coupled to the intermediate electrode 41. A ground potential was coupled to the upstream and downstream electrode 42-45. Every third droplet emitted by a nozzle 1 was charged negatively by synchronously coupling about +650 volts to a charging tunnel 3. The uncharged droplet trajectory was about 250 microns offset from the centreline of the lens. The charged droplets were focused at about 3 cm downstream from the lens.

The foregoing described lenses are novel components for ink jet applications. The focusing fields associated with lens 10 and 40 operate on charged droplets analogously to a half-cylinder, glass lens that focuses light rays entering its flat base to a line in space parallel to the base.

Other focusing field shapes including portions parallel to the droplet trajectories can be devised that are analogous to spherical and other optical lenses.

Claims (9)

1. An electrostatic lens for changing the trajectory of a charged fluid droplet in flight including: upstream and downstream electrodes positioned adjacent the trajectory of a charged droplet including means for coupling to a voltage source for establishing a focusing electric field between the electrodes; said focusing electric field including a centreline path over which a trajectory of a charged droplet is substantially unchanged by the focusing field; and said focusing electric field being such as to change a trajectory of a charged droplet that is offset from the centreline path.

2. The lens of Claim 1 including two upstream electrodes positioned on opposite sides of the droplet trajectory and having means for coupling to a first voltage and two downstream electrodes positioned on opposite sides of the droplet trajectory and having means for coupling to a second voltage.

3. The lens of Claim 1 or Claim 2 wherein said upstream and downstream electrodes have dimensions that cause a cylindrical focusing field to extend generally normal to the trajectories of a plurality of charged droplets.

4. The lens of Claim 1 wherein said electrodes have shapes to create a focusing electric field to focus a charged droplet entering the field to a focal point or line determinable by the focusing field.

5. The lens of any one of Claims 1 to 4 including an intermediate electrode positioned between the upstream and downstream electrodes and including means for coupling to a voltage source for establishing an upstream focusing electric field between the upstream and intermediate electrodes and a downstream focusing field between the intermediate and downstream electrodes; said upstream and downstream focusing fields including the centreline path over which the trajectory of a charged droplet is substantially unchanged by the two fields. and said focusing fields changing the trajectory of a charged droplet that is offset from the centreline path.

6. An electrostatic lens substantially as hereinbefore described with reference to the accompanying drawings.

7. An ink jet printer comprising a plurality of ink jet nozzles aligned for emitting continuous streams of droplets along generally parallel trajectories, charging means associated with said nozzles for charging the droplets emitted by the nozzles and an electrostatic lens according to any one of Claims 1 to 6 having means for coupling to a voltage source for establishing the focusing electric field or fields in the path of the droplets emitted by the nozzles to focus the trajectories of charged droplets to substantially a straight line near which a target to be printed is positioned and wherein the centreline path of the lens intersects said straight line.

8. The printer of Claim 7 including deflection means for establishing a deflection electric field generally normal to the trajectories of the droplets for deflecting charged droplets.

9. The printer of Claim 8 including gutter means positioned between the charging means and a target for collecting droplets not intended for a target.