AU5446298A - Microchannel marking engine - Google Patents

Description

WO 99/25557 PCT/US97/21124 1 MICROCHANNEL MARKING ENGINE BACKGROUND OF THE INVENTION The present invention concerns printing. In has particular, although the exclu sive, application to portable color printers. 5 The printing art is quite mature. Vast amounts of research and expense have been dedicated to optimizing the quality and minimizing the cost of printers, particularly those intended for the consumer market. Given the difficulty of meeting the demands of the human eye, the results of these efforts have in fact been remarkable. Still, the techniques employed to achieve these results have tended to be complicated and expensive. io Printers early employed in offices for small-computer output employed hammers and print-wire matrices. These were noisy and slow and produced low-quality output. Quality improved with the use of thermal printers, but these required special paper and tended to be slow, too. A greater quality advance accompanied the advent of laser print ers, but their mechanisms are complicated, and they remain relatively expensive despite 15is the high volumes in which they have been produced. And none of these technologies lend themselves well to color imaging. Ink-jet and ink-bubble technologies have addressed these shortcomings to a sig nificant extent. Ink-jet printers squirt charged ink at the paper, deflecting the ink elec trostatically to direct it to the desired location. This approach is simple in comparison 20 with, say, laser printers, and it lends itself to color printing, since successive jets of dif ferent-colored ink can be applied to the same locations. Ink-bubble approaches are similarly direct: they employ explosive energy to propel ink drops to the paper from an array of sources. But the ballistic nature of the ink delivery in both of these approaches tends to make the image quality quite dependent on the type of paper or other image 25 medium. SUMMARY OF THE INVENTION Despite the apparent simplicity of a ink-jet and ink-bubble approaches, I have devised a way of printing in a manner that is even simpler and lends itself to embodiment in printers that are more robust, faster, and less expensive.

WO 99/25557 PCT/US97/21124 2 In accordance with my invention, the printing surface is provided by a print head that forms a plurality of capillaries terminating in an array of respective capillary outlets on the printing surface. The capillaries contain ink that capillary action ordinarily so re tains in the capillaries that it does not mark paper brought into contact with the print sur 5 face. But the ink's dielectric constant exceeds that of air, and electrode pairs selectively apply potential differences across respective capillary outlets at which marks are to be made. As a result, electrophoresis causes ink to bulge outward from the associated capil lary outlets and into contact with the paper, leaving ink marks at the desired pixel loca tions. 10 So the printer's marking mechanism does not itself require any moving parts at all; it requires only appropriate solid-state control circuitry and the print head, which can simply be a block that forms the capillaries and provides their associated electrodes. Moreover, the ink column in a given capillary is required to move only a minute distance in order to make a mark. This means that a single mark can be made in an ex 5is tremely brief period of time, and, since all that is required to make a mark is a single capillary and its associated electrode pair, the print head can readily be provided with a large number of capillaries and associated electrodes so that many pixels-typically, a whole row's worth or more-can be printed simultaneously. So the printing speed can be made relatively high with very little cost. 20 Additionally, since the ink merely bulges from the capillary-i.e., it is not pro jected through the air-the print quality is not as sensitive to paper type as it is when, say, ink-jet printers are employed. Indeed, since the ink only bulges from the capillary and can thus the kept heated by the print head until the instant at which contact with the paper cools it, the invention lends itself to the use of hot-melt inks, which are well known 25 for their lack of sensitivity to the type of image medium on which they are used. And hot-melt inks assume solid form when the printer is not in use, so they contribute further to the printer's operational robustness. Another aspect of the invention also enables it to be embodied in particularly compact printers. The paper or other image medium is advanced past the print head by a 30 reciprocating electrostatic gripper. Electric fields generated by gripper electrodes that an advancement gripper includes draw the paper to the gripper, which then advances by an WO 99/25557 PCT/US97/21124 3 incremental distance, advancing the paper with it. The advancement gripper then re leases the paper, typically after another, retention gripper grips it to hold it in place, and returns, grips the paper again, and again advances the paper after the retention gripper releases it. The distance by which the advancement gripper advances is typically so S small-the spacing of one or two image rows-as to be visually imperceptible, and the retention gripper remains stationary. So the printer needs essentially no space to ac commodate feed-mechanism motion. BRIEF DESCRIPTION OF THE DRAWINGS The invention description below refers to the accompanying drawings, of which: 0 Fig. 1 is a perspective view, partly broken away, of a color printer that employs the present invention's teachings; Fig. 2 is a cross-sectional view of the printer's print head taken at lines 2-2 of Fig. 1; Figs. 3A-D are more-detailed views of one of the print head's capillary outlets, 15 illustrating the mechanism by which the printer marks the image medium; Fig. 4 is a cross-sectional view of one of the print-head modules that make up the print head; Fig. 5 is an isometric view of the print head, illustrating the conductor paths by which control voltages are applied to the print head's capillary outlets; 20 Figs. 6A and 6B are footprint diagrams that illustrate the cooperation of staggered capillary rows to provide a rectangular pixel layout; Figs. 7A and 7B are similar footprint diagrams illustrating the cooperation of staggered capillary rows to provide a hexagonal pixel layout; Fig. 8 is a cross-sectional view of the print head illustrating the ink-supply ap 25 proach that the printer employs; Fig. 9 is a cross-sectional view of the printer showing the paper-feed mechanism that the printer uses; Fig. 10 is an isometric view of a gripper surface illustrating the layout of its elec trode fingers; 30 Figs. 1 1A-D are timing diagrams that illustrate the paper-feed mechanism's op erating sequence; WO 99/25557 PCT/US97/21124 4 Fig. 12 is a simplified block diagram of capillary-driver circuitry for driving the capillary electrodes in a one-bit-per-pixel version of the present invention; and Fig. 13 is a simplified block diagram of capillary-driver circuitry for driving the capillary electrodes in a multi-bit-per-pixel version of the present invention. 5 DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT Fig. 1 illustrates a printer 10 that employs the present invention's teachings. In a manner that will be described in more detail below, grippers 12, 14, and 16 advance pa per 18 from a paper supply 20 past a print head 22. With the aid of a vibrating print plate 24, the print head employs the present invention's teachings to apply an image to 10 the paper by marking it with hot-melt ink from a cartridge 26. To this end, battery powered circuitry 28 receives image-data signals from a source not shown and operates the print head 22 in accordance with the image data thus received. It also operates the grippers and vibrating print plate and supplies the power to melt the hot-melt ink. Fig. 2, which is a cross-section taken at lines 2-2 of Fig. 1, partially illustrates the 15is marking mechanism by which the illustrated embodiment operates. In a manner that will be described in more detail below, ink-supply channels 30, which extend the length of the print head 22, are filled with hot-melt ink that NiCr heating elements 32 keep molten. Each of the ink-supply channels forms a row of, say, 4000 capillaries 33 at its base. The illustrated printer is a color printer. It employs the conventional ink-color 20 selection, namely, cyan, magenta, yellow, and black. Although Fig. 2 shows only a sin gle capillary row for each color, more may be provided to speed printing or for other rea sons, as will be explained below. None of these features is critical to the present inven tion. A piezoelectric actuator 34 causes the print plate 24 to reciprocate with a fre 25 quency of, say, 2 kHz through a vertical travel on the order of 75 tm between extended and retracted positions. In its extended position, the print plate's resilient core 36 urges the paper 18 into contact with the print head's bottom surface. There it is marked by hot melt ink that selectively applied electric fields have caused to bulge from selected capil laries' outlets despite the capillary action, as will now be explained by reference to 30 Figs. 3A-C.

WO 99/25557 PCT/US97/21124 5 Fig. 3A diagrammatically illustrates a column of ink 38 held by capillary action in one of the capillaries 33. That drawing also shows two electrodes 40 and 42 disposed at opposite sides of the capillary outlet. The electrodes are embedded in a Teflon coat ing 44, which resists wetting by the ink in the capillary. Fig. 3A illustrates the situation S in which there is no difference in electrical potential between the two electrodes 40 and 42. It can be seen that capillary action prevents the ink column from effectively marking paper that has been brought into contact with the bottom head surface. To cause the ink to mark the paper, the printer of the present invention applies a voltage of, say, 200 V to electrode 40 while keeping electrode 42 at ground potential. o10 The capillary outlet is on the order of only 40 tm across, so the applied potential differ ence causes electric fields on the order of millions of volts per meter at the capillary out let. The attendant, similarly high field gradients cause the hot-melt ink, which has been chosen for its high dielectric constant, to bulge outward into the field thus formed and thereby mark the paper, as Fig. 3B illustrates. A hot-melt ink suitable for this purpose is 15 Piccotex 75LC hot-melt ink, available from Hercules Incorporated of Wilmington, Delaware. When the ink column comes into contact with the (relatively cool) paper, its tip solidifies in a matter of microseconds into a crust on the paper. As Fig. 3C illustrates, the printer then removes the electrodes' potential difference, so the (still-liquid) ink col 20 umn tends to withdraw back into the capillary. At the same time, the vibrating print plate 24 withdraws the paper into its retracted position with the help of a further gripper mechanism 46 (Fig. 2) embedded in its surface, as will be described in more detail be low. This assists in breaking the contact between the crust thus formed and the with drawing ink column. 25 Grippers 12, 14, and 16 then advance the paper 18 by a small advancement dis tance. In a color version of the invention, the spacing between capillary rows containing different-colored inks is chosen to be an integer number of advancement steps so that a paper location at which a capillary in one row has deposited ink of one color will even tually be positioned in registration with the corresponding capillary in the row that con 30 tains the next color so that ink of a different color may be deposited on top of the ink crust 47 that was deposited in the Fig. 3B operation.

WO 99/25557 PCT/US97/21124 6 Fig. 4 depicts in more detail a single head module 48, which provides a single row of ink capillaries. The module includes a body 50 of insulating material such as an Al20 3 -powder matrix in which the capillary 33 has been formed by one of the many known microfabrication techniques. A NiCr resistor 52 deposited on one side of the s head module 48 extends between the grounded electrode 42 and a similarly deposited power-supply rail 54. The ink that the head module 48 contains is in solid form when the printer is not in use, but turning the printer on applies power to the resistor 52, which thereupon heats the head module 48 and thus liquefies the hot-melt ink that it contains. Conductors 56 printed on the head module 48's opposite face connect the driven 10 electrode 40 to a printed-circuit board backplane (not shown) that leads to drive circuits in the printer's circuit module 28 (Fig. 1). Since similar conductors are provided on the corresponding face of an adjacent head module, an insulating layer 60 insulates resis tor 52 from the adjacent module's conductors. It was previously stated that a printer employing the present invention's teachings 1s may provide more than one capillary row for each color. Clearly, such an arrangement can be used to increase printing speed. But Fig. 5 illustrates a head arrangement that provides two capillary rows per color for another purpose. As Fig. 5 shows, the capillary outlets 64 that module 48 provides are staggered with respect to the adjacent module 62's capillary outlets 66. The purpose of this arrangement is to enhance the printer's spatial 20 resolution. If it proves inconvenient for a single capillary row to provide the number of capillaries per unit row length that the desired image resolution requires, one solution is to use different print-head modules to print different ones of a given row's pixels. For example, if head modules 48 and 62 contain the same ink color and their respective capil laries are staggered as shown, module 48 can deposit, say, the odd-numbered pixels in a 25 given row, and module 62 can deposit the even-numbered pixels in the same row. Fig. 6A illustrates this concept. Let us assume that a given capillary row deposits the nth row of image pixels at time t = n, where time is stated in paper-advancement pe riods. That is, the paper is advanced at times t = 1, 2, .... Rectangles 68 represent the module-48 capillaries' footprints on the paper at time t = n, while rectangles 70 represent 30 the module-62 capillaries' footprints on the paper at the same time. If it takes N paper advancement steps for the row of marks made by module 62 to draw even with the row WO 99/25557 PCT/US97/21124 7 of module-48 capillary outlets, then rectangles 72 represent the location at time t = n of the marks that the module-62 capillaries made on the paper at time t = n - N. So if mod ule 48's electrodes receive signals for a given image row's odd pixels N paper advancement steps after module 62's electrodes receive signals for that image row's even 5 pixels, the resultant resolution is twice that achievable by one module only. The resul tant pixel arrangement is illustrated in Fig. 6B, in which the module-62 capillaries' maarks are labeled A and the module-48 capillaries' marks are labeled B. Such a staggered relationship between capillary rows can also be used to achieve a different, hexagonal effect, as Figs. 7A and 7B illustrate by diagrams respectively cor 10 responding to those of Figs. 6A and 6B. To achieve the hexagonal effect, not only are the two capillary rows' footprints staggered "horizontally" (i.e., in the direction trans verse to paper advancement), but their respective sequences of row marks on the paper also staggered "vertically" (i.e., in the direction parallel to paper advancement). In such an arrangement, the spacing between adjacent capillary rows is the product of an odd in 15 teger and the distance between printed rows on the paper, and the paper-advancement mechanism advances the paper by two row spacings at a time. Consequently, a given module's capillaries print all of the pixels in a row, but only on alternate rows. In Figs. 6 and 7, footprints are depicted respectively as rectangular and hexago nal. These shapes reflect the conceptual pixels' shapes, and it may be beneficial for the 20 capillaries' cross sections also to be so shaped. But some embodiments will employ cir cular capillary cross sections for all pixel arrangements. Just as capillary action retains ink in the capillaries, some of this invention's em bodiments will also use capillary action to feed ink from the ink cartridge 26 (Fig. 1) to the ink-supply channels 30 (Fig. 2). As Fig. 8 illustrates, the cartridge 26 is snap fit into 25 a receptacle 80 formed on the print head 22. It rests on a heater pad 82, which heats the cartridge 26 and thus the ink in longitudinally extending ink reservoirs 84. These reser voirs communicate at the cartridge rear with respective tubes 86 (Fig. 1), which fit into respective print-head openings 88 that communicate with respective supply channels 30. The tubes 86 are formed in a connector 90 that additionally provides an electrical con 30 nection between the heater 82 (Fig. 8) and a power supply in the electronics module 28.

WO 99/25557 PCT/US97/21124 8 Any convenient method made be used to transport the ink from the cartridge to the ink-supply channels. Preferably, however, the conduits are formed by materials that the ink tends to wet and are so sized that capillary action alone will cause the ink to flow to the supply channels and thereby to the marking capillaries. 5 Fig. 8 also shows that the print head 24 includes a cover 92 that closes a cavity 94 in which the individual head modules are mounted. The cover 92 forms a recess 96 that communicates both with the print-head exterior and with air holes 98 formed at the print head modules' upper ends to permit air to be displaced as the capillaries' ink column ex tend and retract. The print-head modules' upper surfaces may also be provided with a o10 Teflon coating 100 to discourage ink from bleeding out the air holes. Fig. 9 illustrates the illustrated embodiment's paper-feed mechanisms. Embed ded in the upper surfaces of grippers 12, 14, and 16, as well as print pad 24, which also serves as a gripper, are gripper electrodes interdigitated in a manner that Fig. 10 illus trates. A first set of elongated electrodes 102 is connected to a positive-voltage supply 15is pad 104 and interdigitated with a second set of elongated electrodes 106 connected to a negative-voltage supply pad 108. The spacing between adjacent electrodes is on the or der of mm, so the potential difference between the two supply pads, which is on the order of 200 V when the gripper is activated, sets up substantial electric fields above the gripper. The gripper thereby draws the first paper sheet tightly to itself. But the first 20 sheet acts to shield all sheets above it, so it is only one that the gripper attracts. When an image is to be printed on a new paper sheet, actuators 112 and 114 ad vance gripper plates 12 and 14 into engagement with the bottom sheet in the paper sup ply 20. Those plates' gripper electrodes are energized and thereby draw the bottom sheet 18 to their upper surfaces. Actuators 112 and 114 then retract the gripper plates 25 and thereby pull the bottom sheet past retention lips 116 and 118. The printer then re moves power from gripper 12 but not from gripper 14, which therefore retains its hold on the paper. While gripper 14 retains its hold on the paper, piezoelectric actuator 114 ad vances gripper plate 14 and thus the paper sheet one advancement step to the right. The 30 advancement step is one pixel-row spacing in the case of the pixel organization of WO 99/25557 PCT/US97/21124 9 Figs. 6A and B. In the case of Figs. 7A and B's pixel spacing, the advancement step is two pixel rows. Gripper 12's electrodes are then powered again to hold the paper sheet in place, and gripper 14's electrodes release the paper sheet. While gripper 12 holds the paper in S place, gripper 14's actuator moves it back to the left, where it again grips the paper. Gripper 12 then releases the paper again, and gripper 14 again advances the paper sheet to the right as before. This advancing operation feeds the paper sheet into the space between the print head 22 and the print plate 24, which itself has gripper electrodes embedded in its upper 10 surface. The print plate 24's electrodes are energized in synchronism with those of grip per 12 and so timed as to cooperate with the printing process, as Figs. 11A-D illustrate. Fig. 11A represents the energization state of the gripper electrodes on the reten tion grippers, i.e., the electrodes on gripper plate 12 and print plate 34. Fig. 11 B repre sents the positions of the the print plate 24 and the activated capillaries' ink columns. 15 Those drawings show that the capillaries' ink columns and the print plate 24 assume their advanced positions, in which the ink column can mark the paper, at time time tj, while the retention grippers' electrodes are in the energized state. At time t 2 , the print plate and ink columns retreat to their retracted positions while the print plate's gripper is still en ergized and thus pulls the deposited ink crust out of contact with the still-liquid ink col 20 umn. The mark thus having been made, it is time for the advancement gripper 14 to grip the paper, and it does so at time t 3 , as is illustrated by Fig. 11 C, which represents the energization state of the advancement gripper's electrodes. The retention grippers then release the paper at time t 4 so that the advancement gripper can begin advancing the pa 25 per to the right. Fig. 1 1D, which represents the advancement gripper 14's position, shows that gripper 14 begins that advance at time ts. By time t6, the paper has been ad vanced to the point where the next marking is to take place, so the retention grippers grasp the paper again at time t7. With the the retention grippers thus holding the paper sheet in position, the advancement gripper releases the paper at time t8, and it returns to 30 the left at time t9.

WO 99/25557 PCT/US97/21124 10 The cycle begins again at time t 10 and repeats until the entire image has been written on the paper sheet. In the process, the paper sheet advances beyond the reach of the first two gripper plates 12 and 14. To continue the advancement process, a further piezoelectric actuator 120 (Fig. 9) moves advancement gripper plate 16 to the left and S right in synchronism with the left-and-right movements of advancement gripper plate 14, its gripper electrodes being energized in synchronism with that plate's. Gripper plate 16 thus cooperates with print plate 24 just as advancement gripper plate 14 cooperates with retention gripper plate 12. But a further retention gripper plate 122 may be added to take over for the print plate 24 in the last stages of the advancement process. 10 Fig. 12 is a simplified block diagram that illustrates the data flow employed to drive the print-head electrodes that Fig. 3A's electrodes 40 and 42 exemplify . In a typi cal arrangement, the electronics module 28 includes an image memory 126, which re ceives image data from the source of the image to be printed. The source will often be a personal computer or other device that can be supplied with driver software for process 15 ing the image data into the form most compatible with the hardware organization de scribed above. Alternatively, the printer can itself be provided with circuitry that per forms such processing. Between the times at which the print-head electrodes are energized, one row of image data (or, as was explained above, a subset thereof) is fetched for each capillary 20 row and supplied to a respective one of several shift registers such as shift registers 128 and 130, which are associated with respective capillary rows. Each shift register receives its share of the image data for a full row between, say, times t 2 and tl 0 of Figs. 11A-D, and an ENABLE signal gates the shift registers' contents to respective electrode rows, as gates 132 indicate, with the timing at Fig. 11 B illustrates. Of course, the Fig. 12 repre 25 sentation is merely conceptual; as was explained above, the voltages applied to the print head electrodes ordinarily are nearly two orders of magnitude greater than conventional logic levels. Additionally, Fig. 12 depicts the printer as employing single-bit pixels, whereas the present invention's teachings are readily adapted to multi-bit pixel data. The voltage 30 applied to a capillary's outlet electrodes determines the distance by which the ink column protrudes from it. That distance, in turn, determines the size of the resultant printed dot.

WO 99/25557 PCT/US97/21124 11 So multi-bit pixel data can specify which of a set of predetermined voltages to apply to a given capillary's electrodes. A printer that employs the present invention's teachings in a multi-bit embodiment may use an arrangement such as that which Fig. 13 illustrates. Fig. 13 shows the shift register 134 for a single capillary row. One of its 5 stages 136 may contain the data used to specify the voltage to the applied to electrode 40. Stage 136's contents may be, say, a four-bit number, which a decoder 138 uses to select among sixteen electronic switches 140 by which electrode 40 can be connected to a se lected line of an electrode-voltage bus 142. The voltages on these lines are the outputs of respective taps of a voltage divider 144 whose input is the output of a gated voltage o10 source 146. Source 146's output is a repetitive pulse whose timing Fig. 11 B depicts and whose amplitude at least equals the voltage corresponding to the digital image data's full-range value. It is thus apparent that a printer embodying the present invention's teachings can be exceedingly simple and robust mechanically. The print head is a simple manifold 15 structure that has no moving parts. Ink application is controlled by arrays of electrodes, which can be provided on a simple flex-print substrate. It is well suited to use with hot melt inks, so the method is not sensitive to the type of paper being used-and it contains no liquid ink when it is not in use. Moreover, the use of reciprocating electrostatic grip pers greatly contributes to the compactness of the resultant printer package; since their 20 travel is microscopic, a full-color printer can be made that is only slightly larger than the paper supply that it includes. The present invention thus constitutes a significant ad vance in the art.

Claims (33)

1. For marking on an image medium, an apparatus comprising: A) a print head forming an ink capillary having a capillary outlet and can be filled with ink that tends to remain in the capillary by capillary action and has a dielectric constant that exceeds the dielectric constant of air; B) a positioning mechanism that places the the image medium in proximity to the capillary outlet; and C) a capillary driver operable in response to image-data signals selectively so to impose a potential difference across the capillary outlet, in accordance with image data that the image-data signals represent, as to create an electric-field gradient that forces ink from the capillary through the capil- lary outlet into contact with an image medium placed by the positioning mechanism in proximity to the capillary outlet.

2. An apparatus as defined in claim 1 wherein the positioning mechanism includes a reciprocating mechanism that moves the image medium alternately into advanced and retracted positions, which respectively are in greater and lesser proximity to the capillary outlet.

3. An apparatus as defined in claim 2 further including a medium-advancement mechanism, which advances the image medium with respect to the capillary outlet.

4. An apparatus as defined in claim 3 wherein the medium-advancement mechanism advances the image medium only while the image medium is in the retracted position.

5. An apparatus as defined in claim 2 wherein: A) the capillary driver operates alternately in: i) a marking mode, in which it imposes the potential difference across the capillary outlet in accordance with the image data; and ii) a non-marking mode, in which it keeps potential difference im- posed across the capillary outlet low enough to avoid contact of the image medium by the ink regardless of the image data; B) the reciprocating mechanism places the image medium in the advanced position when the capillary driver operates in the marking mode; and C) the reciprocating mechanism places the image medium in the retracted position when the capillary driver operates in the non-marking mode.

6. An apparatus as defined in claim 2 wherein the reciprocating mechanism com- prises a piezoelectric actuator.

7. An apparatus as defined in claim 1 wherein: A) the print head comprises an ink manifold that forms a row of ink capillar- ies to which the first-mentioned ink capillary belongs, each of the ink capillaries in the row of ink capillaries forming a capillary outlet thereof and being adapted for filling thereof with ink that tends to remain therein by capillary action and has a dielectric constant that exceeds the dielectric constant of air; B) the positioning mechanism places the image medium in proximity to a plurality of the ink capillaries' capillary outlets; and C) the capillary driver selectively so imposes respective potential differences across the capillary outlets, in accordance with image data that the image- data signals associate with respective row positions, as to create electric- field gradients that force ink from respective capillary outlets into contact with the image medium placed by the positioning mechanism in proxim- ity to the capillary outlets.

8. An apparatus as defined in claim 7 wherein: A) the ink manifold forms a plurality of rows of ink capillaries, each of the ink capillaries in each row of ink capillaries forming a capillary outlet thereof and being adapted for filling thereof with ink that tends to remain therein by capillary action and has a dielectric constant greater than the dielectric constant of air; B) the positioning mechanism places the image medium in proximity to capillary outlets in a plurality of the rows of ink capillaries; and C) the capillary driver selectively so imposes respective potential differences across the capillary outlets formed by a plurality of the the rows of ink capillaries, in accordance with image data that the image-data signals as- sociate with respective row positions, as to create electric-field gradients that force ink from respective capillary outlets into contact with the image medium placed by the positioning mechanism in proximity to the capil- lary outlets.

9. An apparatus as defined in claim 8 wherein: A) the apparatus further includes a medium-advancement mechanism, which advances the image medium in a medium-advancement direction with re- spect to the capillary outlets; and B) the rows of capillaries are organized into pairs of rows thereof, the capil- lary outlets in one row of each pair being offset, in the direction transverse to the medium-advancement direction, from corresponding capillary out- lets in that pair's other row.

10. An apparatus as defined in claim 7 further including an ink heater.

11. An apparatus as defined in claim 1 wherein the apparatus further includes a me- dium-advancement mechanism, which advances the image medium with respect to the capillary outlet in a medium-advancement direction and includes: A) an advancement gripper that alternately grips and releases the image me- dium; B) an advancement actuator that advances the advancement gripper in the medium-advancement direction while the advancement gripper grips the image medium and withdraws the advancement gripper in the opposite di- rection when the advancement gripper has released the image medium.

1 12. An apparatus as defined in claim 11 wherein the advancement gripper grips the image medium by generating electric fields that draw the image medium to the advance- ment gripper mechanism and releases the image medium by reducing the strength of the electric fields that the advancement gripper generates.

1 13. An apparatus as defined in claim 12 wherein: A) the apparatus further includes a retention gripper that alternately grips and releases the image medium; B) the advancement actuator advances the advancement gripper in the me- dium-advancement direction when the retention gripper has released the image medium; and C) the advancement actuator withdraws the advancement gripper in the op-

8 posite direction while the retention gripper grips the image medium.

1 14. An apparatus as defined in claim 13 wherein the retention gripper grips the image medium by generating electric fields that draw the image medium to the retention gripper mechanism and releases the image medium by reducing the strength of the electric fields that the retention gripper generates.

i

15. An apparatus as defined in claim 11 wherein the advancement actuator comprises

2 a piezoelectric actuator.

l

16. An apparatus as defined in claim 1 further including an ink heater.

1 17. For marking on an image medium, an apparatus comprising:

2 A) a print head operable to mark an image medium brought into a marking

3 position thereof; and

4 B) a medium-advancement mechanism, which advances the image medium

5 through the print head's marking position in a medium-advancement di-

6 rection and includes: i) an advancement gripper that alternately grips and releases the im- age medium; and ii) an advancement actuator that advances the advancement gripper in the medium-advancement direction while the advancement gripper grips the image medium and withdraws the advancement gripper in the opposite direction when the advancement gripper has re- leased the image medium.

18. An apparatus as defined in claim 17 wherein the advancement gripper grips the image medium by generating electric fields that draw the image medium to the advance- ment gripper mechanism and releases the image medium by reducing the strength of the electric fields that the advancement gripper generates.

19. An apparatus as defined in claim 18 wherein: A) the apparatus further includes a retention gripper that alternately grips and releases the image medium; B) the advancement actuator advances the advancement gripper in the me- dium-advancement direction when the retention gripper has released the image medium; and C) the advancement actuator withdraws the advancement gripper in the op- posite direction while the retention gripper grips the image medium.

20. An apparatus as defined in claim 19 wherein the retention gripper grips the image medium by generating electric fields that draw the image medium to the retention gripper mechanism and releases the image medium by reducing the strength of the electric fields that the retention gripper generates.

21. An apparatus as defined in claim 17 wherein the advancement actuator comprises a piezoelectric actuator.

22. For marking on an image medium, a method comprising the steps of: A) providing a print head that forms an ink capillary having a capillary out- let; B) positioning an image medium adjacent the capillary outlet; C) filling the ink capillary with ink that tends to remain in the ink capillary by capillary action and has a dielectric constant exceeding the dielectric constant of air; and D) so imposing a potential difference across the capillary outlet as to create an electric-field gradient that forces the ink through the capillary outlet into contact with the image medium.

23. A method as defined in claim 22 wherein: A) the ink is a solid at room temperature; and B) the method further includes: i) so heating the ink as to melt it; and ii) keeping the ink molten while imposing the potential difference across the capillary outlet.

24. A method as defined in claim 22 wherein: A) the image medium is a sheet material; and B) the method further comprises the steps of: i) repeatedly employing an advancement gripper alternately to grip and release the same sheet of the image medium; ii) repeatedly advancing the advancement gripper in a medium- advancement direction while the advancement gripper grips the same sheet of the image medium and withdrawing the advance- ment gripper in the opposite direction while the sheet is released from the advancement gripper; and iii) repeatedly performing the step of imposing the potential difference across the capillary outlet between advancements of the advance- ment gripper.

25. A method as defined in claim 24 wherein the step of repeatedly employing the advancement gripper alternately to grip and release the same sheet of the image medium comprises gripping the image medium by generating electric fields that draw the image medium to the advancement gripper and releasing the image medium by reducing the strength of the generated electric fields.

26. A method as defined in claim 25 further comprising: A) repeatedly employing a retention gripper alternately to grip and release the same sheet of the image medium; and B) performing the step of repeatedly advancing the advancement gripper while the image medium is released by the retention gripper and perform- ing the step of repeatedly withdrawing the advancement gripper while the retention gripper grips the image medium.

27. A method as defined in claim 26 wherein the step of repeatedly employing the retention gripper alternately to grip and release the same sheet of the image medium comprises gripping the image medium by generating electric fields that draw the image medium to the retention gripper and releasing the image medium by reducing the strength of the electric fields that the advancement gripper generates.

28. A method as defined in claim 24 further comprising: A) repeatedly employing a retention gripper alternately to grip and release the same sheet of the image medium; and B) performing the step of repeatedly advancing the advancement gripper while the image medium is released by the retention gripper and perform- ing the step of repeatedly withdrawing the advancement gripper while the retention gripper grips the image medium.

29. For marking an image medium consisting of sheet material, a method comprising the steps of: A) providing a print head operable to mark an image medium brought into a marking position thereof; B) repeatedly employing an advancement gripper alternately to grip and re- lease the same sheet of the image medium while it is located in the print head's marking position; C) repeatedly advancing the advancement gripper in a medium-advancement direction while the advancement gripper grips the same sheet of the image medium and withdrawing the advancement gripper in the opposite direc- tion while the sheet is released from the advancement gripper; D) repeatedly operating the print head to mark the sheet between advance- ments of the advancement gripper.

30. A method as defined in claim 29 wherein the step of repeatedly employing the advancement gripper alternately to grip and release the same sheet of the image medium comprises gripping the image medium by generating electric fields that draw the image medium to the advancement gripper and releasing the image medium by reducing the strength of the electric fields that the advancement gripper generates.

31. A method as defined in claim 30 further comprising: A) repeatedly employing a retention gripper alternately to grip and release the same sheet of the image medium; and B) performing the step of repeatedly advancing the advancement gripper while the image medium is released by the retention gripper and perform- ing the step of repeatedly withdrawing the advancement gripper while the retention gripper grips the image medium.

32. A method as defined in claim 31 wherein the step of repeatedly employing the retention gripper alternately to grip and release the same sheet of the image medium comprises gripping the image medium by generating electric fields that draw the image medium to the retention gripper and releasing the image medium by reducing the strength of the electric fields generated to draw the image medium to the retention grip- per.

33. A method as defined in claim 29 further comprising: A) repeatedly employing a retention gripper alternately to grip and release the same sheet of the image medium; and B) performing the step of repeatedly advancing the advancement gripper while the image medium is released by the retention gripper and perform- ing the step of repeatedly withdrawing the advancement gripper while the retention gripper grips the image medium.