NL7915014A - METHOD AND APPARATUS FOR ELECTROSTATIC SCANNING WITH INK JETS - Google Patents

Description

ï D HO / Se / Xerox 1451

Method and device for electrostatic scanning with ink jets.

The invention relates to ink jets and more particularly ink jet recording using a continuous stream of ink delivered from an opening prior to droplet formation.

The speed and deployability achieved using non-impact printing processes have led to the development of different types of registration. One type is called ink-jet printing, and several types are known. In one type, droplets of ink are formed, whereby an ink-filled cavity is deformed to eject ink from that cavity through the opening towards a receiving medium. According to another type, a meniscus of ink is maintained in the orifice and extracted therefrom by electrostatic charge, after which the ink lands on a receiving medium. Another type consists of using magnetic ink on which magnetic field forces in addition to electrostatic field forces effect selective deflection of the ink to the desired positions on the receiving medium.

In the type of inkjet printing to which the present invention pertains, a pressurized conductive fluid is delivered through a cavity from which it escapes through an orifice in the form of a continuous flow. The ink in the cavity is disrupted, for example by means of a periodic excitation of piezoelectric crystals disposed within the cavity, thereby breaking up the continuous flow into substantially uniform droplets spaced approximately the same distance apart . The point at which the continuous flow is broken up into drops is hereinafter referred to as the point of drop formation. At the point of droplet formation, droplet charge electrodes with a potential applied thereon 7915014 -2-t induce a charge in the droplets. Selective deflection of the droplets is then obtained by passing the droplets through an electric field formed by deflection electrodes on which a voltage is applied. The electric field generated by the deflection electrodes acts on the charged droplet to selectively deflect it to a predetermined location on the receiving medium or on a trough.

In the formation of a continuous stream of droplets 10, the number of elements required for the selective deflection by deflector plates to either a trough or to one of several locations on the printing surface presents design problems.

The requirement that these elements be located near the path that the ink follows between the opening and the receiving medium, the amount of information to be processed by the electronic circuits in cases where the selective deflection by the deflection plates may either be directed to the or the receiving medium, and if the flow is directed to the receiving member, to a number of predetermined locations thereon and the space occupied by the number of elements along the path followed by the ink drops, causes design problems which affect the quality of the pressure system can affect the cost, fabrication and packing density of the nozzles within the drop generator 25. The greater the number of elements required along the path followed by the ink drops between the droplet point and the receiving medium, the greater the resulting distance between the droplet generators. to nozzles and the receiving medium and the greater the distance, the more accurate the system alignment and deflection parameters must be to hit the drop target.

U.S. Pat. No. 3,877,036 is directed to a type of inkjet printing in which the continuous stream of ink is broken up into droplets which are charged at the point of drop formation and which are selectively deflected by an electric field force to either a trough or the printing media and if the drops are Direct to the printing medium 7915014-3 at a particular location thereon. This patent is of relevance because it discloses vernier beam directional electrodes placed near the continuous flow of droplets and in place for the point of droplet formation. The potential applied to the electrodes is varied only to direct the continuous current relative to the droplet charge electrodes, and when the correct direction of the continuous current droplets is obtained, the voltage applied to the vernier electrodes is kept constant. Thus, there is no scanning of the closed continuous flow in this patent.

U.S. Pat. Nos. 1,941,001 and 3,689,936 are of interest because an electrode is placed therein near an apparently continuous ink flow. Although the former US patent uses an electrically alternating field applied to the electrodes between which the continuous ink flow moves, the object and means employed are such that the alternating field does not cause ink drop scanning behind the droplet formation point, but, in contrast, a vibration or oscillation of the continuous flow to facilitate drop formation. In the second cited U.S. patent, the electrostatic attraction between one or more electrodes and a closed continuous ink flow 25 is used to deflect the current by electrostatic attraction applied in accordance with the energy of a signal to be recorded. However, the continuous current is not scanned according to a grid pattern but instead deflected in accordance with a signal to be reproduced in order to obtain a reproduction of the signal and the information displayed by the signal to be reproduced is recorded on the deflection electrodes.

According to the invention, there is provided an apparatus of the type described, in which conductive fluid droplets are formed from a continuous flow of fluid which, aided by excitations, is broken up into droplets near a charge electrode, which charges at least selected droplets and 7915014-4- according to the Invention scanning electrodes are arranged adjacent to the continuous fluid flow to periodically deflect the current.

The invention will be elucidated on the basis of the drawings of exemplary embodiments.

In the drawings shows:

Figure 1 shows a schematic of one single scanning ink jet,

Figure 2 shows a schematic representation of a system \ 10 of a number of the ink jets of Figure 1,

Figure 3 shows a schematic representation of a two-dimensional system of a number of ink jets from Figure 1

Figure 4 shows a schematic representation of a suitable alternative scanning electrode, Figure 5 shows a schematic representation of another suitable alternative scanning electrode,

Figure 6 is a schematic representation of a scanning electrode suitable for de-skewing, Figures 7A and 7B schematic representation of the timing of signals used in the invention, and

Figure 8 is a schematic representation of the electronic circuit used in the invention.

Fig. 1 shows a single scanning ink jet generated according to the invention.

The nozzle 1 of the drop generator 10 (see Figure 2) emits a continuous electrically grounded current 2 of ink, which travels through a split ring electrode 3, comprising electrodes 4 and 5. A time-changing voltage, which produces the scanning signal can be applied to either electrode 4 over lead 52 or electrode 5 over lead 54 or to each electrode sequentially, but a different signal can also be applied to each of electrodes 4 and 5 simultaneously, whereby the continuous current 2, a charge with opposite polarity to the voltage on the ring electrode 3 is induced.

The attraction between the induced charge in the continuous current 2 and the voltage with a polarity opposite to that of the ring electrode 3 is 7915014 '-5- · causes the current to be drawn to either the electrode 4 or the electrode 5 depending on the of the relative sizes of the electrodes 4 and 5, thus moving the continuous current 2 laterally 5 in a direction substantially perpendicular to the axis of the continuous current 2. Following the passage through the split ring electrode 3, the continuous current 2 moves ink by an additional earth screen 6, which is electrically grounded. The screen 6 with the grounded conductor 51 10 is used when the length of the charge electrode 7 is insufficient to shield uncharged droplets from the scanning electrode and is placed between the ring electrode 3 and the droplet charge electrode 7. When the screen is electrically charged, it prevents the scan signal from inducing charge on the droplets 8. The droplet charge electrode 7 with the lead 50 is located at the point of droplet formation, which, as mentioned above, is the point where the continuous current 2 is broken up when a periodic disturbance is exerted on the ink 20 in a manifold or cavity in the drop generator 10 which communicates with the nozzle. 1.

Due to the scanning movement applied to the continuous current 2 through the electrode 3, the continuous current 2 will make a scanning movement between positions 2A and 2B. As shown in FIG. 1 and as will be indicated in the rest of the description, the scanning movement of the continuous current 2 between positions 2A and 2B results in a lateral distribution of drops between the point of drop formation on the drop charge electrode. 7 and the printing surface 11. Drops drawn as. closed drops 9 are unloaded while the other drops, drops 8, are charged.

The drop deflecting electrode 12 is electrically coupled to an appropriate voltage source V and may be located either above or below the side distribution site of drops 8 and 9. As shown in Figure 1, the drop deflecting electrode 12 is located below the site of side 7915014. .-6 - distribution of drops 8 and 9 and since the drop deflection electrode can only act on charged drops 9, the polarity of the voltage source V must be opposite O '

are that of the charge on the charged droplets 9, where 5 is attracted by the droplet 9 in a downward trajectory resulting in a landing in the gutter 11. Moreover, it will be clear that the gutter 11 can be placed either above or below the lateral distribution of drops 8 and 9. The polarity of the voltage of the source V.

O _________ 10 who. therefore, applied to the deflecting electrode 12 must be selected in accordance with the location of the drip deflecting electrode 12 and the trough 11. As shown in Figure 1, the unloaded droplets 8 can strike the paper, while the loaded droplets 9 are deflected to the trough. This is advantageous in order to minimize the ink spatter normally generated when printing using loaded droplets on paper. to avoid. It will be understood, however, that if desired, the relationship between the trough 13 (preferably with a vacuum at the opening 53), the pressure deflecting electrode 12 and the pressure surface 11 can be arranged such that uncharged droplets normally enter the trough 13 and charged droplets are deflected towards the pressure surface 11 of a receiving medium 7.

With regard to the time-varying scanning signal 25 on the electrode 3, it is noted that the shape of the signal is chosen such that the lateral distribution of droplets 8 and 9 is effected. In a raster scan where a uniform distribution of droplets on the receiving medium in the printing surface 11 is desired, it is usually sufficient to use a sawtooth tension, which obtains a maximum level as the function of the square root of time, and then falls back to the initial level. The saw tooth tension will be discussed in detail with reference to Fig. 7. In case aberrations in the lateral distribution occur due to manufacturing or system defects, the appropriate portion of the scan signal can be changed to correct irregularities in the lateral droplet distribution. As shown in Fig. 1, the shape of the scan signal is chosen to obtain a substantially uniform distribution of 79 1 50 1 4 -7- of drops 8 and 9, the scan movement of the continuous stream 2 being plotted over three drip periods and all recoil drops are collected in the chute 13. When scanning drops (lateral distribution of drops between one series of maximum and minimum deflection points), which just reach the paper, the first three drops have been collected and have already disappeared from sight . The next three drops are still on a path to the paper. The last three drops are down .......

10 deflected on their way to the gutter.

The scan signal can be applied to either the electrode 4 or the electrode 5, or to both electrodes 4 and 5 to obtain a final attraction between the continuous current 2 and the split ring electrode 3.

When the scan voltage is applied to the electrode 4, a continuous current 2 is drawn from the normal path to the electrode 4; when the scan signal is applied to the electrode 5, the continuous current 2 is withdrawn from the normal path to the electrode 5? when the scan voltage is alternately applied to both the electrode 4 and the electrode 5, the continuous current 2 is withdrawn from the normal path to the electrode 4 over a maximum distance determined by the maximum voltage level on the scan signal, it moves again back through the normal path position to the electrode 5 when the scan signal is applied to the electrode 5 and oscillating continues between the maximum deflection points between the electrodes 4 and 5 alternating between the electrode 4 and 5 during the application of the scan signal. a first scan voltage 30 is applied to the electrode 4 and simultaneously a second scan voltage 30 to the electrode 5, a continuous current 2 is attracted to the electrode with the highest voltage to an extent determined partly by the difference between the voltage levels and partly by the magnitude of the charge induced in the continuous flow 2.

The geometry of the trickle charge electrode 7, the grounded screen 6 and the split ring 3 need not be limited to a circular shape, but can be of any suitable 7915014-8 shape. For example, the deflection of the continuous flow 2 can be applied to such an extent that the openings in the elements are in the form of an ellipse or even slots. Furthermore, with an array of scanning 5 beams similar to that of FIG. 1 (see FIGS. 2 and 3), it is desirable to bring the scanning electrodes, ground shields, and charged electrodes into the most compact configuration possible to match the beam site density within the array.

It will be appreciated that the scanning electrode need not be used in the form of a split ring, nor should there be an electrode on either side of the continuous current 2. That is, that only one electrode can be used on one side of the current 2 and a raster scan can be used laterally on one side of the normal path of the continuous current 2.

This is schematically indicated in Figures 4 and 5 where a single electrode is applied on one side of the current 2. In Fig. 4, the scanning electrode 14 has a flat shape 20 and can be made, for example, from a number of bars.

In Fig. 5, the scanning electrode 15 is cylindrical and may be, for example, a rod or wire.

As shown in Figure 2, a system comprising a single row of beams is similar to that of Figure 1 with the same elements arranged as in this figure. In Figures 1 and 2, the same reference numerals are used for corresponding elements. A part of the raster in the printing surface 11 is assigned to each beam. It will be clear that the assigned part in the plane 11 for each radius can be such that the assigned parts are continuous on the plane 11 or can be separated from each other by a certain distance. A two-dimensional array of rays similar to that of FIG. 1 is shown schematically in FIG. In Figure 3, the second row of beams is offset or interposed with respect to the first row. Such a shift can be applied to get different results, such as reducing the density in each row to get more space between the rays.

79150H

-9- 'further, to limit the number of drip impact sites in the pressure plane covered by each ray to make aerodynamic interaction less important and finally to obtain a highly interlaced image scan, thereby providing more freedom to string inter-beam boundaries. Moreover, it will be appreciated that a drop deflection electrode and trough can be used for each row of the array or if desired, a single deflection electrode and a single trough can be used for every two rows of jets for simplicity.

The deflection electrode is biased to deflect charged droplets from both rows.

It has been found that the amount of deflection of the continuous current 2 varies with the square of the applied voltage. I.e. if the scan signal is doubled in voltage, the deflection of the continuous current 2 is quadrupled. It has also been found that a scan signal frequency of about 32 KHz electrohydrodynamically breaks up the continuous current 2, causing droplet formation with a frequency which is an undesirable combination of the scan frequency and the desired droplet formation frequency. Therefore, the scan signal frequency should be kept at a frequency of about 32 KHz or less.

Various combinations of parameters can be selected for the practice of the present invention. A combination of parameters suitable for the practice of the present invention is as follows.

A drop generator disturbance of about 120 KHz, a scan signal sawtooth voltage of about 400 volts maximum, 30 and 0 volts minimum, a distance of about 3 mils between the continuous current 2 and the scanning electrode, a scanning electrode of 10 mils along the current, a charging voltage of about 20 volts on the charging electrode 7 and a voltage of about 300 volts on the drop deflecting electrode 12.

The movement of the receiving medium along the printing surface can be continuous or discontinuous. A discontinuous movement can be obtained by a stepping motor, so that the paper remains stationary for one scanning period and during the recoil of the paper. continuous flow is moved.

7915014 -10-

If the beams are properly aligned, no inclination of the printed lines occurs on the stepped medium. However, with continuous movement of the receiving medium in the printing surface, skew will occur in the printing line due to the different impact times of droplets generated during a scanning period. One method of compensating for this is the tilting of the system of rays and the drop generator in a direction opposite to the direction of the tilt in the pressure line. Another method of shifting the pressure line tilt is to use a more pigmented electrode with two or more segments as the scanning electrode. Such an electrode with, for example, four segments is indicated in Fig. 6 from the front side. The scanning electrode in FIG. 6 is similar to that in FIG. 1 except that four segments are provided instead of two. The four segments are indicated as A, B, C and D. Each segment, when biased, will draw continuous current to itself along lines a, b, c and d depending on which segment is activated. The direction a corresponds to segment A etc.

The thick dot in the center of the four directions indicates the continuous flow in the non-scan or home mode. The direction of deflection of the continuous current depends on the identity of the addressed electrode segment and the magnitude of the voltage levels applied to the addressed segments. By providing a continuous DC bias voltage to the selected electrode segments, the continuous current from the home position can be held in a direction that shifts the printed line tilt. Each beam in a beam system may similarly be biased to a selective home position from which scanning is accomplished by applying a time-varying or periodic scanning signal to selected scanning electrode segments.

Figures 7 and 8 show a timing chart for the embodiments in Figure 1 in which a single electrode 5 is used as the scanning electrode. In this case, the position 2A from the continuous current 2 represents the zero or "no off signal" position of the continuous current 2 and the position 2B represents the maximum deflection of the continuous current 2 towards the electrode. 5.

The following description follows the passage in fig. 7A from top to bottom and from left to right in fig. 8. A master clock for providing clock signals has a sufficiently high frequency f to provide the desired degree of accuracy for the system and to obtain the desired ink output. This is clear from the following. The signal form of the master clock 20 is used to clock the address counter 21 and the address counter 30. Information latches 22 and 31 are coupled with resp. cf counters 21 and 30, which are coupled again to the dead memories 23 and 32, respectively, which provide the crystal control signal and the scanning electrode signal in digital form, respectively. The digital form of the signals are transformed by digital-analog converters 24 and 33 into analog signals. These analog crystal control and scan signals are amplified in amplifiers 20 and 34, respectively.

The frequency of the crystal control signal f ^ is equal to the master clock frequency f divided by the value N .. As an example it can be said that f has the value of 1.2l6KHz and the value of 128 so that the crystal control signal has the frequency f ^ has 72 KHz. The frequency of the scan electrode signal f is equal to the frequency of the master clock divided by N2, where N2 is equal to times the number of drops desired from a single beam during one complete cycle of the scan electrode signal including the flyback time . As shown in Figure 1, a total of 12 drops is formed during one cycle of the scan electrode signal including the retrace time, ie nine drops during the active scan of the continuous current 35 and three during the retrace of the continuous current to the home position. In this example, N2 is equal to 12 times or equal to 1536. This provides a scanning electrode signal 7915014-12 frequency f of about 6KHz.

Ο

An internal reset signal generated by the address counter 21 is used to clock the shift register 40, and the internal reset signal generated by the address counter 30 is used to reset the shift register 40, thereby obtaining one reset pulse per cycle of the scan electrode signal. This reset pulse is indicated in FIG. 7A as the scan signal synchronizing pulse.

The shift register 40 is coupled to the information ”; Latch 41, which provides an erase signal pattern for each continuous current scan. An erasure signal, in conjunction with the foregoing description of FIG. 1, is a signal that causes droplets to load and then land in the gutter. If it is desired not to write on the paper during the backflow of the continuous flow to the home position, the erase signal is applied during that flyback of the continuous flow. It may also be desirable to obliterate or to collect droplets in the trough during active scanning of the continuous stream for purposes such as, for example, halftones. Moreover, a different erase signal pattern may be desirable for text, for example. As a result, the information latch 41 can include either hardware or software.

The function of the erasure signal pattern is to function as a synchronization signal for the scan signal and the crystal control signal (the frequency of the crystal control signal is equal to the frequency of droplet formation). The exchange signal pattern in the information block 41 is loaded in parallel in the shift register 40 upon receipt of a scan signal synchronizing pulse upon resetting the shift register 40. Upon receiving an internally formed reset pulse 21 from the address counter 21 at the clock input of the shift register 40, consequently, the erase signal is transmitted in series format from register 40. The serial format of the erase signal is sent to one input of the OR gate 44 and to the gate input of the latch 42. Information 43 correlated with parts of text or figures to be printed by the 7915014 -13 nozzle which in that regard is important during the scan, it is input into the gate of the latch 42, which is clocked by the internally formed reset signal generated by the address counter 21. Thus, information is serially shifted from the information latch 42 at the same frequency as the frequency of drop formation.

In the context of the description of Figure 1, uncharged drops may hit the receiving member at the pressure point so that information is at a logical zero level for drops, which must be printed at a logical "1" level when the drops are to be in the trough are taken care of. The information signal appears on the other input of the OR gate 44 and this gate forms an output when either information or the erase signal is at the logic level 15 "1". The output of gate 44 is amplified by amplifier 45 to an appropriate level (about 20 volts in the present example).

Three complete scans are shown in Fig. 7 t.w. scan A, B and C. During each scan 20, 12 drops are formed, indicated by the letters G. ^ to G ^ and the numbers 1 to 8. The first of the 12 drops G ^, can be used as a guard drop to reduce the aerodynamic attraction on the successive drops whereby the drops 1 to 8 can be used for printing if desired and the last three drops G ^ to G ^ are collected in the trough during the recoil of the continuous flow . Thus, any number of drops can be formed during a scan period by setting the parameters wherein the number of drops available for printing is set to 8 in this example to conveniently operate 8 bits per byte microprocessors. where the more complex minicomputers and computers can be used to print a plurality of drops per scan.

The erase signal pattern of Figure 7A corresponds to the aforementioned desire not to print the first and last three drops of the twelve drops generated during the signal scan period.

7915014 -14-

The erase signal will bring gate 44 to a logic high level, resulting in the loading of the drop formed at the point of drop formation. This drop will then be deflected by the drop deflecting electrode 12 of FIG. 1 to the trough 13. During each scan this will occur for the four drops up to. The scan information signal Δ is low only for drops 2 to 6 and consequently only those drops will be printed in the absence of an erase signal. Since the output of gate 44 is low only in the absence of both a logic high level in the information signal and a logic high level in the erase signal, the droplet charge electrode 7 during the scan A only during the formation of the drops 2 to 6 are not under pretension. During the samples B and C, correspondingly only drops 5 are printed. The three scan periods of Figure 7A will result in the print pattern of Figure 7B.

The circuit of FIG. 8 can be used for a number of rays such as e.g. those from fig. 2 and 3.

The only required multi-beam addition is that there is a separate information path for each drop of charge electrode 7, the information path consisting of latch 42, gate 44 and amplifier 45. For the remainder 25, the circuit shown in FIG. .8 are identical to the case where the circuit controls only a single beam.

The system according to the invention lends itself to binary dripping which solves the problem of the cost and speed of data channel electronics. The voltage required on the droplet charging electrode 7 has a relatively low value, thereby reducing the problem of an accurate print caused by the electrostatic interaction due to charged drops with a high charge while also solving the problem of ink spatter. The present invention allows the use of a shorter working distance between nozzle and pressure surface, e.g. about 1.3 cm. This reduces the problem of accurate drop placement due to errors in the nozzles. The shorter working distance also allows for a relatively small deflection of the web, also reducing the problem of accuracy due to aerodynamic interaction. The present invention allows the use for normal elements, so that there are no problems with regard to the manufacture of the deflection plate and the gutter.

In the simultaneous scanning of a plurality of beams, the present invention reduces the beam density to any practical range of about 40

up to 120 beams per 2.5 cm while the pixel density of about 300 to 500 elements per 2.5 cm is possible. The deflection is binary and the paper web straight and compact, while manufacturing and coupling problems are solved. Aerodynamic droplet displacement effects are significantly reduced while relatively simple control electronics can be used. a

It will be clear that electrostatic deflectors according to the figures such as the electrode 12 do not.

20 need be limited to the shape of a biasing electrode. Any means that can attract or repel electrostatically charged drops can be used to send charged drops to either the printing surface or a trap. Electrostatic deflectors e.g.

25 may include an electrostatically charged member charged with a polarity of charges capable of effecting the desired deflection or an element capable of causing a charged charge to be induced by the proximity of a charged droplet, thereby causing an electrostatic interaction between the organ and the drop is formed. The use of the term electrostatic deflectors means all suitable means.

It will be understood that the present invention is particularly suitable for raster output scanning and can be used as a single printer with a number of inputs eg magnetically described tapes, cassettes, plates, computer outputs, facsimile transmission etc. The present invention can also be used in 7915014 7915014

Claims (10)

1. Ink jet recording apparatus in which conductive fluid droplets are formed from a continuous fluid flow promoted by excitations, broken up in droplets near a charge electrode, which charges at least selected droplets, characterized by scanning electrodes near the continuous fluid flow to periodically deflect them. "..... 2. Device according to claim 1, characterized by electrostatic deflecting means for deflecting droplets charged by the charging electrode to a collecting means or a printing surface. 3. Device according to claim 2, characterized in that the droplets are deflected by the deflecting means in a direction substantially perpendicular to the direction in which the continuous current is periodically deflected by the scanning means. Device according to claim 1, characterized by electrical earth means for electrically shielding the conductive fluid flow at the point of droplet formation from voltages associated with the scanning electrode means to deflect the continuous current. Device as claimed in claim 1, characterized in that the scanning electrode means comprise a single electrode near the continuous current. Device according to claim 1, characterized in that the scanning electrode means comprise at least two electrodes placed on either side of the continuous fluid flow. 7. Device according to claim 1, characterized by a number of continuous currents, wherein a charging electrode is placed near each current. 8. Device as claimed in claim 7, characterized in that a scanning electrode is placed near each continuous current. 9. A method of recording information by means of ink jets, wherein conductive fluid droplets are formed from a continuous flow of fluid which breaks up 7915014 '-18- promoted by excitations into droplets near a charging electrode which charges at least selected droplets characterized by periodically deflecting the continuous flow. 10. Method according to claim 9, characterized by electrostatically deflecting charged droplets by means of deflecting means to either a collecting means or to a printing surface, wherein the charged drops are periodically deflected by the deflecting means perpendicular to the direction of the continuous flow by the scanning means. . 7915014