Printing apparatus with focusing of toner particles

Information

  • Patent Grant
  • 6254221
  • Patent Number
    6,254,221
  • Date Filed
    Thursday, December 18, 1997
    27 years ago
  • Date Issued
    Tuesday, July 3, 2001
    23 years ago
Abstract
Various methods and apparatus are disclosed for facilitating the loading, transportation, and modulation of toner particles on a print head, as well as the transfer of toner particles onto a print medium. These methods and apparatus relate to the optimization of various elements on the print head to improve the speed and control of toner particles on the print head, as well as to the alteration of the electric field in the vicinity of the end of the print head.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The present invention relates generally to xerographic printing and, more particularly, to altering the electric field in the vicinity of a toner transfer region of a toner imaging device and a receiving surface.




2. Background of the Related Art




In modem society, among the most common and useful printing devices are printers that are used in conjunction with computers to print a variety of subject matter, such as text, graphics, and even photographic reproductions. These “computer” printers may be categorized in any number of ways. However, for the purposes of this discussion, these types of printers will be categorized, initially, as monochromatic and color printers. Monochromatic printers use a single color ink or toner, which is a form of powdered imaging material that can be charged and moved with electric fields. Most monochromatic printers are capable of producing gray and black images on a print medium, such as paper, transparencies, etc. Color printers, on the other hand, typically contain several colors of ink or toner, such as cyan, magenta, and yellow, which produce the color images, as well as black, which produces the black and gray images. As described in greater detail below, just as certain monochromatic printers have the ability to produce certain shades of gray, these color images may be produced, to some extent, in different color hues and saturations.




As far as computer printers are concerned, color printers are a relatively recent innovation. Therefore, historically, computer printers have been categorized primarily based upon the type of technology used to deliver the ink onto the paper. Such technological categories of printers have included, for instance, daisy wheel printers, ink jet printers, and laser printers. Arguably the most popular printers in today's market, for both monochromatic and color printers, are ink jet printers and laser printers. Unfortunately, each of these types of printers exhibit certain disadvantages, particularly when used as color printers.




Ink jet printers print directly onto paper. In other words, the ink is not deposited on an intermediate substrate which is then transferred from the intermediate substrate to the paper. Rather, ink jet printers use thermally generated bubbles or piezoelectric drivers to expel or “jet” ink drops onto the print-receiving medium. Advantageously, such printers are relatively inexpensive and operate satisfactorily for a variety of purposes. However, ink jet technology demonstrates very limited gray scale level writing ability at the present time. In other words, ink jet printers can only produce a few shades of gray. To provide these limited gray scale levels, ink jet printers may use diluted and full strength inks, smaller ink drops, or modulated drop sizes. In view of these limitations, ink jet printers are unlikely ever to achieve more than a few gray levels.




Toner jet printers also print directly onto paper. To provide this type of direct printing, toner jet printers typically pass toner through an array of holes that is placed in the print head very near the paper. A ring electrode is placed around each hole to control the toner that passes through each hole. This control is possible because the toner is charged prior to delivering it to the array. Accordingly, activation of an electrode essentially pulls the toner through the activated hole, and an electrode may be placed behind the print medium to pull the toner onto the paper.




The saturation of the toner on the paper may be controlled, to some extent, by the time that the particular electrode is activated. In other words, in a monochromatic printer, the electrode may be activated for a relatively short period to produce a gray image and for a relatively long period to produce a black image. Similarly, in a color printer, the electrode is activated for a relatively short period of time to produce a light colored image and for a relatively long period of time to produce a darker colored image.




Disadvantageously, the holes in the array tend to get plugged with toner, so the arrays need to be cleaned periodically. This maintenance may require the array to be removed from the printer for cleaning or replacement, or the printer may be provided with a self-cleaning mechanism that periodically produces a charge in an attempt to attract the charged toner particles away from the array. In an effort to address these concerns, the holes in the array may be made larger to help alleviate the plugging problem. However, this solution is detrimental because increasing the size of the holes increases pixel size, thereby causing the resolution of the printer to suffer.




Laser printers present another set of advantages and disadvantages. On one hand, laser printers are very reliable, require little maintenance, and are capable of printing at relatively high speeds as compared with ink jet printers. On the other hand, laser printers are more complicated and more expensive than comparable ink jet printers. Furthermore, laser printers are essentially analog devices, and it is difficult to control the analog process tightly enough to get satisfactory color control. Rather, various shades of gray or various color densities are produced by the use of “super pixels,” i.e., tight groupings of regular pixels having various different colors and/or densities to produce a given effect when viewed at a distance by the human eye.




In an effort to improve upon existing printers, electrostatic printers using traveling wave toner transport devices, sometimes called digital packet printing devices, are under development. Such devices use microscopic patterns of electrodes that are formed using semiconductor fabrication techniques to control small numbers of toner particles. Because of the precise control of the toner that these devices theoretically make possible, it is thought that these devices could produce print images having a higher resolution and much better gray scale control than existing printers. Furthermore, it is thought that these devices could provide high operating speed at a potentially lower cost. However, known traveling wave toner devices have not attained these theoretical advantages.




As discussed in detail below, the present inventors have discovered a variety of problems with currently known traveling wave toner transport devices, as well as a variety of ways to address such problems and improve traveling wave toner transport technology.




SUMMARY OF THE INVENTION




In accordance with one aspect of the present invention, there is provided a toner imaging device. A target electrode is positioned near the toner imaging device to attract toner from the toner imaging device. One or more secondary electrodes may be positioned in the vicinity of the toner imaging device and/or the vicinity of the target electrode. The electrodes alter the electric field, such as by reducing its divergence, to facilitate toner transfer.











BRIEF DESCRIPTION OF THE DRAWINGS




The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:





FIG. 1

illustrates a perspective view of a printer in accordance with the present invention;





FIG. 2

illustrates a block diagram of a print engine in accordance with the present invention;





FIG. 3

illustrates a portion of a print head having a traveling wave transport surface;





FIG. 4

illustrates a graphical depiction of forces on a particle being transported by a traveling wave;





FIG. 5

illustrates a cross-sectional view taken along line


5





5


of

FIG. 3

;





FIG. 6

illustrates a perspective view of a portion of a print head having a traveling wave transport surface;





FIG. 7

illustrates an embodiment of a print head surface having channel-defining rails that terminate before reaching the launch end of the print head;





FIG. 8

illustrates a first embodiment of a loading/modulation scheme of the print engine;





FIG. 9

illustrates a second embodiment of a loading/modulation scheme of the print engine;





FIG. 10

illustrates a third embodiment of a loading/modulation scheme of the print engine;





FIG. 11

illustrates a fourth embodiment of a loading/modulation scheme of the print engine;





FIG. 12

illustrates a multiplexing scheme for use with transfer electrodes;





FIG. 13

illustrates a page wide print head as a top view of

FIG. 8

;





FIG. 14

illustrates a scanning print head, along with other portions of a print engine that uses the scanning print head;





FIG. 15

illustrates an embodiment of the print engine for use with a color printer;





FIG. 16

illustrates a graphical depiction of the inertial motion of a toner particle in a transfer gap after leaving the end of the print head;





FIG. 17

illustrates a basic apparatus for facilitating transfer of toner from the print head onto a print medium;





FIG. 18

illustrates an electric field produced by traveling wave electrodes near the launch end of the print head;





FIG. 19

illustrates an electric field produced by traveling wave electrodes near the launch end of the print head, where the launch end includes a dielectric runway;





FIG. 20

illustrates an electric field produced by traveling wave electrodes near the launch end of the print head, where the traveling wave electrodes near the launch end are selectively controlled to shape and/or control the direction and amplitude of the electric field in the vicinity of where the toner particles leave the print head;





FIG. 21

illustrates a side view of the launch end of the print head including two transfer electrodes;





FIG. 22

illustrates an apparatus having a charge-concentrating target electrode for facilitating transfer of toner from the print head onto a print medium;





FIG. 23

illustrates an apparatus having a slit electrode for facilitating transfer of toner from the print head onto a print medium;





FIG. 24

illustrates an apparatus having planarizing electrodes for facilitating transfer of toner from the print head onto a print medium;





FIG. 25

illustrates an apparatus having focusing electrodes for facilitating transfer of toner from the print head onto a print medium;





FIG. 26

illustrates an apparatus having focusing and planarizing electrodes for facilitating transfer of toner from the print head onto a print medium;





FIG. 27

illustrates another apparatus having focusing and planarizing electrodes for facilitating transfer of toner from the print head onto a print medium;





FIG. 28

illustrates the apparatus of

FIG. 26

having planarizing electrodes with different voltages;





FIG. 29

illustrates an apparatus having focusing and planarizing electrodes for facilitating transfer of toner from the print head onto a print medium, where the print head has an altered substrate;





FIG. 30

illustrates an apparatus having focusing and planarizing electrodes for facilitating transfer of toner from the print head onto a print medium, where a dielectric member is positioned over the launch end of the print head;





FIG. 31

illustrates a vertical transfer scheme;





FIG. 32

illustrates voltage waveforms on drive electrodes; and





FIG. 33

illustrates an angled transfer scheme.











DESCRIPTION OF THE SPECIFIC EMBODIMENTS




1. Introduction




Turning now to the drawings, and referring initially to

FIG. 1

, a printer is illustrated and generally designated by a reference numeral


10


. The printer


10


includes a print engine


12


that is housed within a case


14


. A print medium, such as paper, is stored in an input tray


16


. Upon receiving an appropriate print command from an associated source, such as a computer (not shown), paper is fed from the input tray


16


into the print engine by a sheet feeding device. The paper, generally following the path illustrated by the dotted lines


18


, passes through the print engine


12


and into a receiving tray


20


that is located in the upper portion of the case


14


.




The print engine


12


includes toner reservoirs


22


, a toner imaging device


26


, and a fuser assembly


28


. Although the print engine


12


will be discussed in great detail below, toner from the reservoirs


22


is generally charged and loaded onto the toner imaging device


26


. The toner imaging device


26


applies the appropriate image to the paper. This image is then fixed onto the paper by the filser


28


prior to the paper being deposited in the receiving tray


20


.




It should be appreciated that

FIG. 1

illustrates a schematic depiction of one type of printer


10


. Indeed, due to the general nature of the illustrated printer


10


many elements that the printer


10


may include have not been shown, such as detailed paper transport mechanisms, human interface controls and displays, a system controller board, input/output connections, and power supplies. The printer


10


is illustrated in

FIG. 1

as a desk top printer that prints on individual sheets of paper, normally at a speed of 10 to 30 pages per minute. However, it should be understood that the print engine


12


may be used in various types of printers. Furthermore, although the print engine


12


is described below as being configured to print color images, the various aspects of the print engine


12


are also applicable to monochromatic print engines.




A generalized block diagram of the print engine


12


is illustrated in FIG.


2


. The toner storage device


30


may include the toner reservoirs


22


, mentioned above, or any other suitable toner storage device. These toner particles are delivered to a toner loading mechanism


34


which charges the toner particles and delivers the charged toner particles to the toner imaging device


26


that includes a print head


36


. As will become apparent below, a control circuit


38


may be coupled to the toner loading mechanism


34


and/or the print head


36


to control (1) the delivery of the toner particles onto the print head


36


, (2) the movement of the toner particles on the print head


36


, (3) the manner in which the toner particles are ejected onto the paper from the print head


36


, and (4) the movement of the print head


36


, if, as discussed subsequently, the print head


36


is coupled to a scanner. The control circuit


38


also may modulate the toner particles on the print head


36


. However, as will also become apparent from the following discussion, a separate modulator


40


may be used to modulate the toner particles on the print head


36


. In this case, the control circuit


38


may also be coupled to the modulator


40


to control the manner in which the modulator


40


operates on the toner particles on the print head


36


.




2. Toner Transportation On A Traveling Wave Device




Prior to discussing specific embodiments of the print engine


12


as a whole and the manner in which each of these embodiments functions, it is important to understand the manner in which the print head


36


transports toner. A portion of one embodiment of the print head


36


is illustrated in FIG.


3


. The print head


36


includes a traveling wave drive assembly that causes the toner particles


42


to move generally in the direction of the arrow


44


. In this embodiment, the traveling wave drive assembly includes a plurality of electrodes


46


that extend generally perpendicular to the direction of toner motion as illustrated by the arrow


44


. Specifically, the electrodes


46


are arranged in groups of six electrodes


46




a


,


46




b


, and


46




c


to produce a 6-phase traveling wave that causes the packets of toner particles


48


to move generally in the direction of the arrow


44


. Each electrode


46


in each group of electrodes


46




a


,


46




b


, and


46




c


is connected in order from phase one through phase six, which is illustrated in

FIG. 3

by the designations Ø1, Ø2, Ø3, Ø4, Ø5, and Ø6, respectively. As described in detail below, each of the electrodes having the same phase, e.g., each of the electrodes


01


in the groups


46




a


,


46




b


, and


46




c


, may be coupled to a common bus that provides the appropriate phase signal to the appropriate electrode


46


in each of the groups of electrodes


46




a


,


46




b


, and


46




c


.




With this general physical embodiment in mind, the basic concept of such a traveling wave device may be understood by referring additionally to

FIG. 4

for a moment. In

FIG. 4

, a charged toner particle


42


is shown diagrammatically as traveling on an electrostatic wave. As can be seen, the toner particle travels in the X direction, which is the same direction designated by the arrow


44


. Indeed, the particle


42


is moved in the X direction by electrical forces designated by the arrows generally marked F


x


. It should be noted that the toner particle


42


, in this illustration, carries a positive charge. It should also be noted that the toner particle


42


, as depicted in

FIG. 4

, always experiences a net hold down force, F


y


, in one quadrant where F


x


is positive.




It is possible for a particle to experience either positive or negative forces depending on its phase and position relative to the traveling wave. At “slow” speeds, the particle can be expected to sit at the zero-crossing at point B. There is a simple restoring force which tends to hold the particle at that position. The other zero-crossing at point D is an unstable equilibrium point. As the drive speed is increased, the particle


42


will lag behind the zero-crossing in a region where there is positive drive force. When this lag exceeds kx=x(x=λ/2), the force becomes negative, and the particle


42


slips phase and no longer moves synchronously with the traveling wave. In steady state, the magnitude of the lag is determined by a balance between the drive force and any drag present, and aerodynamic drag is usually believed to dominate.




3. Optimization Of Transport Parameters




As mentioned above, the print head


36


illustrated in

FIG. 3

is a physical device that approximates the behavior of such an ideal device. The print head


36


includes a series of electrodes


46


disposed on or near the transport surface, and these electrodes


46


produce voltages that approximate the ideal traveling wave. A dielectric layer, described below, is typically fabricated over the electrodes


46


to insulate and protect them. With this type of device in mind, there are four factors that may be used to describe any particular version of such a device: (1) the toner particle diameter; (2) the spatial wavelength of the traveling wave; (3) the electrode pitch, i.e., the number of electrodes per wavelength; and (4) the dielectric thickness.




It has been discovered that currently known traveling wave toner transport devices suffer from certain problems related to toner transportation. For instance, the devices provide jerky control of the toner motion, thus making the creation of the desired image unduly difficult. Also, the toner particles move at a rate of less than 200 millimeters per second on existing devices, while toner velocities of approximately 1 meter per second are needed to produce a printer capable of printing 20 pages per minute.




With these problems in mind, the relationship of these factors may be optimized to provide a print head


36


that produces reliable packet motion at more useful speeds. The maximum drive force is obtained for a wavelength of approximately 4.25 times the particle diameter. The wavelength range of 3.0 to 4.5 times the average particle diameter provides particularly good results, although improvements may be seen from 2.0 to less than 12.0 times the average particle diameter. The dielectric thickness is of relatively minor importance, however, because it is generally possible to increase the amplitude of the drive voltage on the electrodes


46


to compensate for the dielectric thickness. Of course, since it is usually desirable to operate at the lowest possible voltages, the optimum dielectric thickness exhibits a dielectric breakdown strength that is just sufficient to withstand the peak drive voltages. The maximum drive voltage is then limited by the onset of Paschen discharge in the air over the device. Also, a thinner dielectric layer may be used if lower speed operation is targeted.




It should be noted that it is not always best to choose a wavelength that provides maximum drive force, because the maximum drive force varies by only about 10% for a wavelength range of 2.5 to 8.8 times the diameter of the toner particles


42


. This force determines the maximum particle transport speed along the device, which is limited by air drag on the particle


42


. However, since there is typically one toner packet


48


per wavelength, the net toner throughput is inversely proportional to the wavelength. In fact, toner throughput may actually be even higher because, for longer wavelengths, toner packets


48


may consist of two or more rows of toner particles


42


. However, wavelengths where only one row of toner particles


42


fits, i.e., wavelengths less than about 7 toner particle diameters, are typically advantageous because maximum gray scale resolution can then be obtained. Thus, within a range of wavelengths where the force is varying slowly, the shortest wavelength will generally give the maximum net toner throughput. Of course, the existence of particle size distributions in real toners typically dictates that a slightly larger wavelength should be used so that the largest toner particles


42


present in significant numbers receive adequate drive force. Typically, for spherical (polymerized) toners, such as toner available from Nippon Zeon, which have relatively narrow size distributions, peak performance is achieved for wavelengths of about 3 to 4 times the average toner diameter.




In regard to the pitch of the electrodes


46


, it should be noted that the discrete electrodes


46


produce fringe fields that may produce an uneven drive force. This phenomenon is in contrast to the magnetic drive forces that occur in ac motors and stepping motors, for example, where a stable position can be set anywhere in between the poles by putting intermediate currents into the coils on adjacent poles. Pure sine wave drives of the discrete electrodes


46


where the pitch of the electrodes


46


is similar to the diameter of the toner particles


42


tend to produce a jerky drive where toner particles


42


step from electrode to electrode. This phenomenon is especially noticeable at low speeds. It is possible to smooth this motion by setting the pitch of the electrodes


46


, as measured between electrode centers, at less than the diameter of the average toner particle


42


. It is believed that a pitch of less than about half the particle diameter produces particle motion that is indistinguishable from that of an ideal traveling wave, but pitches between one half and one diameter are usable as well. For instance, if the wavelength is 3 to 4 diameters, and the pitch is about a half diameter, then at least 6 to 8 electrodes per wavelength are used to produce a smooth drive.




It is possible to construct discrete electrode configurations with various spatial duty cycles (ratio of electrode width to pitch). However, the dominant mechanism that produces jerky motion is simply the pitch-to-particle diameter ratio as described above and not image forces, at least for typical toner charge levels. Thus, varying the spatial duty cycle produces little effect. Therefore, it is typically advantageous to use equal width lines and spaces for ease of manufacturability.




In general, it is convenient to use an even number of electrodes per wavelength, because the drive circuitry is simpler in that half the phases are direct inverses of the other half. Thus, a six-phase drive at a wavelength of 3 to 4 times the average particle diameter is found to be a simple configuration that produces a reasonably smooth traveling wave drive and good packet throughput. Such a drive configuration gives more stable motion of the toner packets


48


at all speeds, thus minimizing the risk of packet breakup during acceleration, for example, as well as higher maximum operating speeds than a three-phase drive configuration at the same wavelength. More phases are clearly possible, at least within the resolution limits of the manufacturing technology adopted, but more phases may not gain significant practical advantage over a six-phase drive scheme.




4. Transport Device Construction and Pixel Formation




Although the manner in which the toner particles


42


are transported on the surface of the print head


36


has been discussed in great detail above, it should not be forgotten that the toner particles


42


are to form pixels. Accordingly, the print head


36


includes a plurality of rails


60


that extend along the surface of the print head


36


perpendicular to the plurality of electrodes


46


, as illustrated in

FIG. 3

, to form a ladder array


63


. In one embodiment, the rails


60


are separated by the width of a single pixel so that the packets


48


of toner particles


42


are approximately one pixel wide. For example, the rails


60


have centers placed approximately 42 microns apart to produce a 600 dot per inch (DPI) printer. Thus, if the diameter of the toner particles


42


is approximately 8 to 12 microns, 3 to 5 toner particles


42


may be placed side by side in each channel


62


to form a packet


48


. In accordance with another embodiment, the rails


60


may be spaced apart at some fraction of a pixel width to form smaller transport channels to increase the control of the toner used to form a single pixel. In keeping with the example of a 600 DPI printer, one embodiment of the print head


36


might use rails


60


on 21 micron centers to create channels


62


that are approximately one half of a pixel width wide. This sub-pixel spacing would increase the control of toner flow at the expense of a reduction in net throughput of the print head


36


. Also, if the toner particles


42


are launched in single particle packets, the mutual repulsion between toner particles in multi-particle packets would be eliminated. Thus, the toner spread during transfer across a gap caused by such repulsion also would be eliminated, causing the resolution of the pixel formed by the single particle packets to improve. Two particle packets would provide the next best resolution, and so on.




As mentioned above and as explained in detail below, the electrodes


46


are typically covered by a suitable layer of dielectric material on which the toner particles


42


move. However, the rails


60


may extend above the surface to form actual physical barriers which tend to keep the packets


48


of toner particles


42


within the respective channels. One such embodiment is illustrated in

FIG. 5

, which is a cross-sectional view taken generally along line


5





5


in FIG.


3


. In this embodiment, the rails are typically from 3 to 8 microns in height, though generally it is thought that heights between 40 and 60 percent of the diameter of the toner particles


42


is typically sufficient to restrain the packets


48


of toner particles


42


within the channel


62


. In one embodiment, the channels rails


60


illustrated in

FIG. 5

are made of a dielectric material. The rails


60


may be formed, for instance, by covering the electrodes


46


with a relatively thick layer of dielectric material and etching through portions of the dielectric material to produce the channels


62


defined between the dielectric rails


60


. Alternatively, a layer of dielectric material may be fabricated over the electrodes


46


and then covered with an appropriate mask, such as a layer of photoresist. Once windows have been masked and etched in the layer of photoresist (not shown) dielectric material may be deposited over the layer of photoresist and into the windows in any suitable manner, such as by sputtering or chemical vapor deposition to create the rails


60


. After the rails


60


have been created, the layer of photoresist may be removed in any suitable manner, such as by a piranha etch or an ash process.




In an alternate embodiment, the rails


60


may be formed of electrically conductive material, such as a suitable metal or polysilicon. In this embodiment, the rails


60


may have an appropriate voltage (e.g., 50 to 100 volts) applied to them to create divider electrodes near or on the top surface of the print head


36


. The fabrication of these divider electrodes is described along with the other features of an embodiment of the print head


36


illustrated in FIG.


6


. As can be seen in

FIG. 6

, the primary structures of the print head


36


are fabricated on a suitable substrate


70


, such as a silicon wafer. A layer of dielectric material


72


, such as silicon oxide, is formed on the substrate


70


. The layer of dielectric material


72


primarily prevents the subsequently deposited electrodes from interacting electrically with one another through the substrate


70


. Of course, if the substrate


70


is made of an insulative or dielectric material, such as glass, the dielectric layer


72


may be redundant.




The traveling wave electrodes


46


are formed over the layer of dielectric material


72


. Although the electrodes


46


may be fabricated by any suitable method, a layer of photoresist (not shown) may be applied to the surface of the dielectric layer


72


and etched to form windows where the electrodes


46


are to be formed. A layer of conductive material, such as a suitable metal or polysilicon, is then deposited over the layer of photoresist and into the windows. The layer of conductive material formed over the photoresist may be removed by a suitable etch or by chemical mechanical planarization, and the photoresist may then be removed to form the electrodes


46


.




Alternatively, a layer of conductive material may be deposited over the layer of dielectric material


72


, and a layer of photoresist (not shown) may be deposited over the layer of conductive material. The photoresist may be developed and etched to form windows which define the areas between the subsequently formed electrodes


46


. A suitable etch may be performed to remove portions of the layer of the conductive material that has been exposed through the windows, and the remaining photoresist may then be removed to leave the electrodes


46


on the surface of the dielectric layer


72


.




Once the traveling wave electrodes


46


have been formed, a layer of dielectric material


74


is deposited over the electrodes


46


. The divider electrodes


76


are then deposited over the layer of dielectric material


74


in any suitable manner, such as the methods previously described as being used to form the electrodes


46


. A layer of dielectric material


78


is then deposited over the divider electrodes


76


. The divider electrodes


76


, in this embodiment, do not protrude above the surface of the print head


36


. Rather, the divider electrodes


76


are biased to repel the charged toner particles


42


in the packets


48


and, thereby, create voltage barriers between adjacent channels


62


. It should also be appreciated that other structures that are not shown, such as bus electrodes and interconnections, may be formed during the described fabrication processes.




The repulsive fields near these divider electrodes


76


do exhibit certain apparent disadvantages, as compared to the raised dielectric rails


60


, in that they may reduce the number of particles that can move together in a packet, and they tend to exert lateral forces on some of the toner particles


42


as they leave the end


80


of the print head


36


. The reduced packet size tends to lower the maximum toner throughput of the process, and the lateral forces tend to deflect some of the toner particles from the desired straight-line trajectories. Accordingly, for these reasons, the dielectric rails


60


which extend above the surface of the print head


36


to create physical rather than electrical barriers may be advantageous.




To address one of these apparent disadvantages, the divider electrodes


76


may be terminated before they reach the launch end


80


of the print head


36


as illustrated in FIG.


6


. This early termination significantly reduces the lateral scatter of the toner particles


42


at the launch end


80


. In fact, it is believed that the lateral forces exerted on the toner particles


42


by the divider electrodes


76


are reduced by a factor of about 100 when the divider electrodes


76


are terminated before the last electrode


46


. However, if the divider electrodes


76


are terminated just before the last electrode


46


, e.g., on the next to the last electrode


46


, the toner particles


42


closest to the divider electrodes


76


tend to receive a larger forward driving force. This larger force may result in these toner particles having trajectories different than the trajectories of the interior toner particles. Therefore, it may be advantageous to terminate the divider electrodes


76


on about the third or fourth electrode


46


before the end


80


, as illustrated, to avoid this phenomenon, but the earlier the divider electrodes


76


terminate the more distance the toner particles


42


will have to spread laterally due to other factors, such as their mutual repulsion, surface defects, and fringe fields.




As yet another alternative, rails


60


having divider electrodes


76


which extend above the traveling surface of the print head


36


may be created. For instance, once the dielectric layer


78


has been formed over the divider electrodes


76


, a portion of the dielectric layer


78


between the divider electrodes


76


may be removed to create a rail


60


that extends above the surface of the print head


36


. Such rails are depicted in

FIG. 6

by the dotted lines


82


and


84


, with the understanding that the dielectric material


78


between the dotted lines


82


and


84


is removed, as discussed above, by any suitable method.




It has been found that the surface of the print heads along which the toner particles travel tends to be rough, sticky, or incapable of holding a neutral charge relative to the toner particles, thus hampering the ability of the device to transport the toner particles properly. Also, the materials and methods used to fabricate the print heads may determine an upper limit on the voltage differential that may be applied between phase lines, thus limiting the force that can be used to overcome toner sticking. Accordingly, the surface of the print head


36


on which the toner particles


42


move may be optimized to enhance the speed and controllability of the toner particles


42


.




In view of the embodiments discussed above, the surface of the print head


36


on which the toner particles


42


move may be either the dielectric layer


74


or the dielectric layer


78


. However, to facilitate the following discussion of the surface characteristics, we will use as the example the dielectric layer


74


which covers the electrodes


46


. First, a discussion of the structural characteristics of the surface of the dielectric layer


74


is in order. It should be appreciated that, in the embodiments described above, the electrodes


46


protrude upwardly from the surface of the dielectric layer


72


. Typically, the electrodes


46


are approximately 0.5 to 1.0 microns in height. Therefore, when the dielectric layer


74


is applied over the top of the electrodes


46


, the surface of the dielectric layer


74


may exhibit a washboard effect. This washboard-type surface can disrupt toner motion.




To optimize transportation of the toner


42


across the surface of the dielectric layer


74


, the surface of the dielectric layer


74


should be smooth. As one possibility, the dielectric material chosen for the dielectric layer


74


should be capable of providing a smooth non-conformal coating over the raised electrodes


46


, while being thin enough to provide other advantages which will be discussed later. One particularly useful dielectric material is benzocyclobutene, which is sold by Dow Chemical Company under the tradename Cyclotene. Cyclotene may be applied over the electrodes


46


in any suitable manner, such as by spin coating or sputtering. The upper surface of the Cyclotene layer is quite flat even at thicknesses of about 0.5 microns over the electrodes


46


.




As mentioned previously, the toner particles


42


carry an electrical charge. Certain dielectric materials used to fabricate the dielectric layer


74


may exhibit a charge exchange with the toner particles


42


. Such a charge exchange causes the toner particles


42


to exhibit a tendency to stick to the dielectric surface


74


. However, Cyclotene is particularly advantageous in that it readily reaches a state of charge equilibrium with toner so that the toner remains properly charged. Thus, due to the smooth, non-conformal upper surface of the Cyclotene, in combination with its properties which limit charge exchange, the toner particles


42


tend to move smoothly over the upper surface of the dielectric layer


74


.




Although the Cyclotene does exhibit certain advantageous properties, other materials and/or techniques may also be suitable to produce a dielectric layer


74


having similar performance characteristics. For instance, dielectric materials that have rougher or more conformal surface characteristics may be used. These generally disadvantageous surface characteristics may be removed or minimized with an appropriate polishing process, such as chemical mechanical planarization. A smoother surface may also be created by a controlled etch or by a reflow process.




Of course, as discussed above, a smooth upper surface is only one advantage possessed by Cyclotene, the other advantage being its ability to limit charge exchange with the toner particles


42


. To the extent that dielectric materials other than Cyclotene also possess such a characteristic, the use of such dielectric materials may be advantageous as compared to the use of other dielectric materials which do not exhibit such a characteristic. However, even certain dielectric materials which do not limit charge exchange with the toner particles


42


may also be suitable for use as the dielectric layer


74


. If such dielectric materials are used, the performance of the dielectric layer


74


may be enhanced by precharging the surface of the dielectric layer


74


to inhibit charge exchange with the toner particles


42


by pre-establishing a state of triboelectric charge equilibrium between the surface and the toner.




Cyclotene also possesses another characteristic which makes it particularly advantageous for use as the dielectric layer


74


. Specifically, Cyclotene exhibits a irelatively high dielectric strength, sometimes referred to as dielectric breakdown, of approximately 300 volts per micron. The dielectric strength of the dielectric material


74


may be important because relatively high voltages may be applied to the traveling wave electrodes


46


to overcome the aerodynamic drag, which tends to inhibit the motion of the toner particles


42


along the channels


62


, and to enable particles to accelerate from rest to catch the traveling wave.




5. Loading and Modulation Of Toner Particles




To this point in the discussion the construction of the print head


36


and the manner in which the toner particles


42


move along it have been discussed, but the manner in which the toner particles


42


are loaded and modulated to form the desired images on a suitable print medium has not been discussed. It has been found that methods of modulating toner, i.e., controlling when and how much toner is provided by each channel, which have been disclosed to date are essentially unworkable. Accordingly, reference is now made to

FIGS. 8-11

where four alternative apparatus and methods are illustrated for providing workable loading and modulation.




Referring initially to

FIG. 8

, a first embodiment of a print engine


150


is illustrated. The end


80


of the print head


36


is positioned a suitable distance from a print medium


152


, such as a piece of paper. The print medium


152


generally travels in the direction of the arrow


154


. As can be seen, toner packets


48


are illustrated as being deposited on the print medium


152


to form a desired image. To control the print engine


150


to produce the desired image, a donor roll


156


, such as those known in the art, is positioned a suitable distance away from the surface of the print head


36


across from the loading zone of the ladder array


63


. The donor roll


156


rotates generally in the direction of curved arrow


158


and carries a plurality of toner particles


42


on its surface. The toner particles


42


are typically stored in a toner storage device


30


(

FIG. 2

) prior to being deposited onto the donor roll


156


. The toner particles


42


tend to adhere to the surface of the donor roll


156


by image forces. A doctor blade (not shown) associated with the donor roll


156


, or any other suitable mechanism, may be used to produce a relatively consistent layer of toner particles


42


on the surface of the donor roll


156


.




As alluded to previously, and as described in greater detail below, the toner particles


42


are deposited into the channels


62


of the ladder array


63


near the end of the channels that is opposite the end


80


. Because the toner particles


42


are charged, a variety of methods and mechanisms may be used to deliver toner particles near the loading end of the ladder array


63


. These methods are generally similar to ac and dc development methods used in mono-component jump-gap development systems in conventional electrophotography.




To selectively load toner to form packets in an imagewise manner, a moving pattern of electrode voltages may be created for each channel


62


. This will allow toner packets


48


to form inside the loading zone of the ladder array


63


. The loading zone may be, for example, about 1 millimeter wide, which corresponds to about 30 wavelengths times 6 phases to equal 180 “loading” electrodes per channel


62


. If this scheme were applied to a page width, e.g., 8.5 inch, print head, close to 1 million transistors and connections would be used to control packet formation in the loading zone. Due to the high voltages, e.g., 75 to 150 volts, currently used to accomplish loading and transport, this great number of high voltage transistors may be prohibitively expensive for the majority of possible commercial applications. Indeed, the number of connections used in this configuration may be cost prohibitive at any drive voltage.




The toner may be supplied by applying a combination of DC and AC voltage to a donor roll to cause the toner particles to detach from the donor roll and travel across the gap to the loading zone of the ladder array


63


. This process is similar to the jump gap development of electrostatic images on photoreceptors in some laser printers, except here the latent image moves on a traveling electrostatic wave instead of a moving photoconductor surface. It should also be noted that the width of the loading zone should typically be larger than the distance that the packets move between successive cycles of toner deposition. For a given print speed of 10 pages per minute, for example, the paper speed would be about 2 inches per second, which corresponds to about 1200 pixels per second for 600 dpi printing. If about 8 packets per pixel are used to provide maximum color density, the print head would have to deliver about 9600 packets per second.




In jump gap development, the toner particles


42


are generally transported across a gap of about 300 microns using a 2000 Hz waveform. Thus, the traveling wave transports the toner packets


48


formed in the loading zone for about 0.5 milliseconds before the next wave of toner particles


42


arrive in the loading zone. During this time the toner packets


48


advance about 5 wavelengths, which corresponds to about 150-200 microns for wavelengths of 30-40 microns. Thus, the toner packets travel only a fraction of the width of the loading zone before the next toner packets are formed if the traveling wave drive frequency is about 10 k Hz.





FIG. 9

illustrates a system


160


similar to the system


150


, so like reference numerals are used to designate similar elements to avoid confusion. Unlike the system


150


, in the system


160


, toner particles


42


are loaded in an unmodulated manner. As the toner particles


42


on the donor roll


156


move past the first phase 1 electrode at the upper end of the print head


36


, the toner particles


42


are attracted onto the print head


36


once each phase. The toner packets


48


are then transported down the print head


36


toward the end


80


in an unmodulated manner as compared with the system


150


previously described in FIG.


8


.




To provide appropriate modulation of the toner packets


48


in the system


160


, the print head


36


illustrated in the system


160


includes one or more barrier electrodes


46


B. When a barrier electrode


46


B is energized, the toner packets


48


tend to stack up behind the barrier electrode


46


B because the activation of the barrier electrode


46


B prevents the toner packets


48


from being transported down the remainder of the print head


36


. Because the barrier electrodes


46


B may be used to control the modulation of the toner packets


48


on the print head


36


, the simpler loading arrangement may be used to ensure that a given supply of toner packets


48


are being loaded onto the print head


36


. Like the system


160


, the system


162


illustrated in

FIG. 10

is loaded in an unmodulated manner. However, in contrast to the system


160


, the system


162


does not include any barrier electrodes. Rather, it should be noticed that the system


162


includes a pickup roll


164


that is positioned between the loading zone and the launch end


80


of the print head


36


. The pickup roll


164


rotates generally in the direction of the curved arrow


166


. It should be noted that the pickup roll


164


should not be placed too near the launch end


80


, because the pickup roll


164


could disturb the electric field near the launch end


80


.




Between the loading zone and the launch end


80


of the print head


36


, one or more transfer electrodes


46


X may be positioned rather than the typical traveling wave electrodes


46


. If the toner packets


48


being transported across the transfer electrode


46


X are not needed to form the image on the print medium


152


, the transfer electrode


46


X is activated to repel the toner packet


48


so that the pickup roll


164


, which is biased to attract the toner particles


42


, captures the unwanted toner packet


48


. The unused toner is returned to a toner sump by means not shown, possibly jumping back to the donor roll


156


across a small gap. Alternatively, the modulation may also take place during transfer to the intermediate roll


172


of

FIG. 11

, and the unused toner may be returned to the sump by a means not shown.




The transfer electrodes


46


X may be individually addressable electrodes in each channel (or in each pair of one-half pixel channels, etc.) which may be energized to transfer selected toner packets


48


to the pickup roll


164


. The width of each transfer electrode


46


X is advantageously about one-third to one-half the wavelength to ensure an effective disturbance of particle motion when activated, while not impeding toner motion when not activated. The transfer electrodes


46


X may be formed as individual electrodes in each channel that are wider than the normal drive electrodes


46


, or one or more electrodes having the same width as the drive electrodes


46


may be locally connected in each channel to create a transfer electrode


46


X.




Because drive amplitudes may currently range from 100 to 300 volts peak-to-peak, relatively expensive high-voltage transistors are used in the drive circuitry. Thus, it may be advantageous to multiplex the modulation drive. One such scheme is illustrated in

FIG. 12

, where various transfer electrodes


46


X are multiplexed by at least a 2-to-1 ratio. A common drive line (not shown) may be connected to two or more transfer electrodes


46


X, where the modulation location of the transfer electrodes is staggered by 1/n wavelengths, with n being the number of pixels to be multiplexed. Since signals of the modulation line have an effect only when toner particles are over the corresponding transfer electrodes


46


X, the staggered locations allow time-division multiplexing of the drive signals for the adjacent channels. In low-cost printer design, where net throughput is sacrificed in favor of cost, it is possible to increase the practical level of multiplexing by separating the toner packets


48


further to make more space for more transfer electrodes


46


X per packet space. This can be done without increasing the drive wavelength by, for example, adding an additional full-width transfer electrode


46


X that is modulated to remove every second toner packet


48


(or to leave every nth toner packet for even more space) from the entire print width.




Another alternative system


170


is illustrated in FIG.


11


. In the previously discussed embodiments of

FIGS. 8

,


9


, and


10


, the toner packets


48


are transferred directly from the print head


36


to the print medium


152


. However, in the system


170


, a transfer roll


172


is positioned near the end


80


of the print head


36


, much like the pickup roll


164


in the system


162


. However, unlike the pickup roll


164


, the transfer roll


172


picks up all of the toner packets


48


as it rotates generally in the direction of the curved arrow


174


. These toner packets


48


have been modulated, by any appropriate means, such as by using the transfer electrodes


46


X described above, so the toner packets


48


form an image on the transfer roll


172


. The transfer roll


172


then transfers the toner packets


48


in a “conventional” manner, i.e., by contact, onto the print medium


152


, which is illustrated as being positioned an appropriate distance from the transfer roll


172


.




6. Print Head Types




The print head


36


may include several chips mounted side by side to form a page wide print head


36


P that is approximately the width of the print medium


152


. For instance,

FIG. 13

essentially illustrates a top view of the device


150


illustrated in

FIG. 8

with the donor roll


156


illustrated in phantom lines. As illustrated, a plurality of chips


100


are coupled side by side by a carrier


190


to form the print head


36


P that is approximately the width of the print medium


152


. It is thought that a page wide print head


36


P will maximize the potential throughput of the printer.




Alternatively, as illustrated in

FIG. 14

, one or more chips may be coupled to a scanning device, such as a swathing print carriage similar to those known in the art, to form a print head


36


S that scans across the print medium


152


to create the desired image. Although the use of a scanning print head


36


S may reduce potential throughput, this type of scanning print head


36


S nonetheless appears to offer various advantages as compared with the page wide print head discussed above. First, because the scanning print head


36


S has many fewer channels and, thus, uses many fewer high voltage drivers, the scanning print head


36


S is much less expensive than a comparable page wide print bead


36


P. Second, instead of “splicing” several chips, which are 1 to 5 centimeters in width, together to form a page wide print head


36


P, a single chip may be used to form the scanning print head


36


S. Third, because the relatively expensive driver circuitry may be contained elsewhere in the printer, the scanning print head


36


S may be disposable, although it may be configured to be refillable in order to minimize the printing cost per page.




The scanning print head


36


S may be positioned directly across a gap from the print medium


152


, so that toner packets


48


are transferred directly onto the print medium


152


. However, as illustrated in

FIG. 14

, the scanning print head


36


S is advantageously positioned to write images onto an intermediate drum


172


S. The intermediate drum


172


S may be sized to accept the largest image to be accommodated by the printer, and its surface may be compliant to enhance pressure transfer onto the print medium


152


.




The scanning head


36


S may write images onto the surface of the intermediate drum


172


S in a raster scan pattern. As one example, the scanning head


36


S may be held stationary during a full rotation of the drum


172


S to write an image around the circumference of the drum. The scanning head


36


S may then be incrementally moved along a path parallel to the axis of rotation of the drum


1




72


S to write the next image around the drum, and so on. Alternatively, the scanning head


36


S may move along the width of the drum


172


S as the drum remains stationary to write an image across the width of the drum


172


S. The drum


172


S may then be incrementally rotated by one or more scan widths so that the scanning head


36


S may write the next line of the image. The scanning head may write in only one direction, termed as “unidirectional,” or it may write in both directions, termed as “bidirectional.”




Although the raster scanning methods described above may be used, the scanning print head


36


S advantageously writes images onto the intermediate drum


172


S in a continuous spiral pattern that makes one revolution for each swath width. This spiral writing approach may be significantly more efficient than the typical back-and-forth scanning devices used by most ink jet printers, which spend most of their time accelerating and decelerating. indeed, the spiral writing approach may permit the scanning device to use a stepper motor having reduced torque and power requirements as compared with those used in ink jet printers. Furthermore, the scanning print head


36


S may incorporate a small toner jet arrangement, rather than a small traveling wave toner transport device, because the spiral writing technique may also provide advantages for these types of printers. As an enhancement to the spiral writing method, it may be useful to rotate the print medium or the drum slightly to align the spiral pattern with the vertical or horizontal edge of the print medium to avoid aliasing problems with horizontal or vertical lines.




Once written on the drum, the image then may be transferred to the print medium


152


with one additional revolution of the intermediate drum


172


S. The image may be subsequently fused to the print medium


152


, or, in one particularly advantageous situation, a transfix mechanism (not shown) is used, thus transferring the toner to the print medium


152


with a combination of heat and pressure to fuse the image to the print medium simultaneously.




Furthermore, it should be appreciated that a separate print head


36


is used for each color of toner. Accordingly, in a color printer that uses black, yellow, cyan, and magenta toner, four separate print heads


36


, along with the other associated mechanisms, are used. One such exemplary system


200


is illustrated in FIG.


15


. The print head


36


A transports black toner particles


42


A from the donor roll


156


A to the print medium


152


, which is moving in the direction of the arrow


154


. Similarly, the print head


36


B transports yellow toner particles


42


B from the donor roll


156


B, the print head


36


C transports cyan toner particles


42


C from the donor roll


156


C, and the print head


36


D transports magenta toner particles


42


D from the donor roll


156


D. Of course, if a color printer uses scanning print heads


36


S, two or more scanning print heads


36


S may be used simultaneously to transfer toner onto the intermediate drum


172


S to build the desired color image.




7. Electric Field At The Launch End Of The Print Head




Although we have discussed various manners in which toner particles are loaded, transported, and modulated, we have not yet discussed in detail the manner in which toner particles are transferred to the print medium. Generally speaking, as illustrated in

FIGS. 8

,


9


, and


10


, the toner packets


48


may be transferred directly from the end


80


of the print head


36


to the print medium


152


. Alternatively, as illustrated in

FIG. 11

, the toner packets


48


may be removed from the print head


36


by a transfer roll


172


and deposited onto the print medium


152


by the transfer roll


172


. Focusing on the former transfer situation, it has been determined that the inertia of the toner packets


48


, as they are transported along the print head


36


, is generally insufficient to carry the toner packet across a transfer gap of 200 microns or more because of air drag. Indeed, it has even been found that, on currently known print heads, the toner particles often fail to “jump” off the end of the channels and onto the nearby paper, and, thus, these particles merely collect at the end of the channels. Assuming that the toner particles


42


are 8 microns in diameter and that they are moving at a velocity of 1 meter per second, the curve


210


illustrated in

FIG. 16

demonstrates, through a numerical simulation, that a toner particle


42


quickly loses inertia due to air drag as it attempts to cross a 200 micron gap.




However, because the toner particles


42


are electrically charged, an electric field may be applied across the gap


214


(see

FIG. 17

) between the end


80


and print medium


152


to help the toner particles


42


travel across the gap


214


and onto the print medium


152


. One basic method of applying an electric field across the gap


214


involves placing an electrode


212


behind the print medium


152


, as illustrated in FIG.


17


. The electrode


212


develops a charge opposite that of the toner particles


42


to attract the toner particles


42


onto the print medium


152


. Although, upon initial consideration, the electrode


212


would appear to be a clean and simple solution to transporting the toner particles


42


across the gap


214


between the end


80


and the print medium


152


, it has been determined that the toner particles


42


tend to spread vertically, i.e., in a direction normal to the transport surface of the print head


36


, as well as laterally, i.e., in a direction in the plane of the transport surface and normal to the dividers.




One of the factors responsible for such spreading is discussed in reference to FIG.


18


.

FIG. 18

illustrates the launch end


80


of a print head


36


, including the last ten electrodes


46


. As can be seen from the plurality of equipotential lines


216


, the electric field produced by the electrodes


46


near the launch end


80


of the print head


36


tends to be quite divergent, as illustrated by the curvature of the equipotential lines


216


, and, thus, contributes to variations in the launch angles of the toner particles which have various charge values and diameters. The ideal electric field at the launch end


80


and across the gap would be completely non-divergent or planar, so that it would essentially impart all toner particles with the same launch condition and initial trajectory.




Unfortunately, producing a planar electric field is quite difficult to accomplish because the electrodes


46


are responsible for producing the electric fields which transport the toner particles


42


along the surface of the print head


36


. If the electrodes


46


are terminated too far from the launch end


80


, the toner particles


42


will tend to slow and stop. However, it has been determined that a dielectric runway


220


may be formed between the last electrode


46


and the launch end


80


of the print head


36


, as illustrated in FIG.


19


. By continuing the dielectric surface of the print head


36


several microns beyond the last electrode


46


, the launch end


80


of the print head


36


is distanced from the high field gradient and high intensity electric field surrounding the last electrode


46


. Thus, the runway


220


provides a relatively “electrically smooth” area for the introducing of the toner particles


42


into the transfer field generated by the electrode


212


. In this embodiment, the launch end


80


of the print head


36


is approximately 15 to 30 microns away from the last electrode


46


. Clearly, the equipotential lines


216


near the launch end


80


of the runway


220


are much less divergent than the equipotential lines


216


near the last electrode


46


.




It should also be noted that the toner particles


42


experience an attraction to the dielectric surface along the runway


220


. This electrostatic attraction tends to hold the toner particles


42


against the runway surface until they reach the launch end


80


. This electrostatic attraction is believed to be due to a local polarization of the runway dielectric in response to the charge on the toner particles


42


. Since this attractive force is proportional to the dielectric constant of the material used to form the runway


220


, it may be desirable to use a different dielectric material for the runway


220


than for the conveyor portion of the print head


36


in order to adjust the attractive forces to an optimum level. For instance, rather than using the Cyclotene dielectric layer


78


with a dielectric constant of 2.7, it may be desirable to use a dielectric, such as polyvinyl flouride or polyimide.




However, there are methods other than providing a runway devoid of electrodes for reducing the strength and/or divergence of the electric field at the launch end


80


. In one alternative embodiment, illustrated in

FIG. 20

, the voltage waveforms applied to the last six electrodes


46


, for instance, may be monotonically reduced in amplitude. The equipotential lines


216


in

FIG. 20

depict the electric field near the launch end


80


of the print head


36


where the voltage amplitudes on the last six electrodes, i.e., one wavelength, are ramped linearly down to zero volts on the last electrode


46


. Although the equipotential lines


216


in

FIG. 20

exhibit more curvature at the launch end


80


than at the end of the runway


220


in

FIG. 19

, they still show a dramatic improvement over the curvature of the equipotential lines


216


at the launch end


80


in FIG.


18


.




Although the alteration of the electric field in the transfer region as described with reference to

FIGS. 19 and 20

may be advantageous, other methods and apparatus may be employed separately or in combination to further facilitate transfer of the toner packets


48


from the print head


36


onto the print medium


152


. For instance, one or more transfer electrodes


46


X may be used to help extract the toner packets


48


from the ac field created by the electrodes


46


of the traveling wave array. As illustrated in

FIG. 21

, the last two electrodes of the array


63


are transfer electrodes


46


X that may be controlled independently of the traveling wave electrodes


46


. The transfer electrodes


46


X are illustrated as being spaced twice as far apart as the traveling wave electrodes


46


. The larger spacing of the transfer electrodes


46


X is theoretically useful, both for better separation of traveling wave fields and transfer fields and for accelerating the toner packets


48


for higher speed launch into the transfer region. However, the transfer electrodes


46


X may be spaced at the same intervals as the traveling wave electrodes


46


, because they can still serve to pull the toner packets


48


out of the ac field and shield the transfer region from that field. It may be advantageous to bias the transfer electrodes


46


X such that they match the planarized field in the air immediately above them.




8. Focusing Of The Electric Field In The Gap




The transfer of the toner packets


48


across the gap


214


between the print head


36


and the print medium


152


cannot only be facilitated by altering the electric field in the launch region in the ways discussed above, but the transfer can also be facilitated by modifying the target electrode configuration of

FIG. 17

to include additional electrodes. These additional electrodes are arranged and energized to form an “electrostatic lense” that helps to focus and control the travel of the toner packets


48


across the gap


214


.




However, before discussing any specific embodiments, a few properties of the electric field in the launch region should be discussed. It is believed that the toner particles


42


will decelerate if the transport field wavelength is less than or equal to about three times the particle diameter. Thus, near the end


80


of the print head


36


, it may be desirable to use spatial wavelengths that are greater than about three times the diameter of the average toner particle


42


. These variable field conditions extend with significant amplitude out to approximately one half the traveling wave wavelength into the gap


214


. Thus, it may be advantageous to gain control over the variable direction of motion of toner particles


42


during that first one half wavelength of travel into a gap


214


. Once beyond that distance, dc fields should be sufficient to direct the further motion of toner particles


42


.




Given these considerations, there are certain parameters that may facilitate the design of a suitable electrostatic lense. First, the electrostatic lenses should provide an electric field in the gap


214


that is, at most or all points, of similar magnitude to the peak field experienced by toner particles


42


as they move along the traveling wave, e.g., no more than a factor of about 4. Second, near the end


80


of the print head's surface, the electric field should be nearly parallel or even slightly converging to prevent particles from moving far off axis in the initial higher-field region. Third, there should be a significant focusing or restoring field tending to bring particles back to the axis at the target point on the surface of the print medium


152


.




Various electrode structures may accomplish one or more of these goals. In the embodiment of the electrostatic lense structure


228


illustrated in

FIG. 22

, an electrode


230


, which is capable of concentrating the electric field, is placed behind the print medium


152


. The electrode


230


may resemble a knife edge or a wire, for instance. In contrast to the somewhat planar electrode


212


, it will be appreciated that the electrode


230


will provide a much more concentrated field for attracting the toner particles


42


. Although the resulting field still might contain outward components as the toner particles


42


leave the end


80


of the print head


36


, the field will also contain inward focusing components in the last portion of the gap


214


.





FIG. 23

illustrates another electrostatic lense structure


232


. The lense


232


includes a target electrode


234


, which may be similar to the field-concentrating electrode


230


or the planar electrode


212


. Regardless of the type of target electrode used, however, a slit electrode


236


is interposed between the print head


36


and the print medium


152


. The slit electrode


236


is charged opposite the target electrode


234


so that it focuses the toner particles


42


by repulsion as they pass through the slit


238


. After passing through the slit


238


in the slit electrode


236


, toner particles experience a focusing and attractive field as they continue toward the print medium


152


.




Another embodiment of an electrostatic lense


250


is illustrated in FIG.


24


. Like the lense


232


, the lense


250


includes a target electrode


252


, which may be similar to the field-concentrating electrode


230


or the planar electrode


212


. Regardless of the type of target electrode used, however, a pair of electrodes


254


and


256


may be placed near the end


80


of the print head


36


to shape the electric field near the launch site of the toner particles. These electrodes


254


and


256


may be flat or curved. The shape of the electrodes


254


and


256


and/or the voltage on them may be adjusted to achieve a parallel or slightly converging electric field near the end


80


of the print head


36


with an appropriate magnitude to prevent the above-mentioned undesirable end effects and launch behaviors. Typically, the electrodes


254


and


256


are charged opposite the target electrode


252


.




The effect of an electrostatic lense in the gap


214


can also be achieved by creating a fringe-field using a lense structure


260


located near the print medium


152


, as illustrated in FIG.


25


. The lense structure


260


includes a target electrode


262


, which may be a field-concentrating electrode, for instance. The target electrode


262


is flanked by additional focusing electrodes


264


and


266


that are charged to repel and focus the toner particles


42


as they approach the target area on the print medium


152


. The lense structure


260


may exhibit a tendency to disrupt the final toner image on the print medium


152


due to the translation of the fringe field through the print medium


152


, a disruption referred to as electrostatic shearing. To accommodate for this possibility, the adhesion of toner particles


42


to the print medium


152


may be increased, for example, by the application of heat to the print medium


152


and/or to the structure


260


. This can reduce the tendency of toner to be displaced once it has reached the print medium


152


.




The combined influences of various of the above-mentioned structural features can be utilized in a toner transfer system which provides driving force, launch field planarization, electrostatic capture and confinement of charged toner particles to a target axis, and focusing toward a well-defined target. One such combined electrostatic lense


280


is illustrated in FIG.


26


. The combination of the wire target electrode


282


, the planarizing electrodes


284


and


286


, and the focusing electrodes


288


and


290


may produce a good field shape both near the end


80


of the print head


36


and near the target area of the print medium


152


.




Although a number of possible configurations may be envisioned, possibly driven by manufacturing considerations, another embodiment of a combined electrostatic lense


300


is illustrated in FIG.


27


. The lense


300


includes a wire or planar target electrode


302


flanked by a pair of focusing electrodes


304


and


306


. A planarizing structure


308


located near the launch end


80


of the print head


36


includes a dielectric member part coated on two surfaces


310


and


312


with a thin conductive layer. The conductive layer on the surfaces


310


and


312


provides planarizing and focusing elements near the end


80


of the print head


36


. Rather than using a dielectric substrate, these elements may be made entirely of metal, and the surfaces


310


and


312


might be angled or curved in such a way as to achieve the desired field shape.




Aside from the requirement that the components fit into the available space, transfer of toner from the end of the traveling-wave transport device across an air gap to the print medium places no specific requirements on the size or number of focusing elements which drive the transfer. Indeed, any arrangement that generates fields in the gap that are of sufficient magnitude and direction to drive the toner particles from the end of the transport device to a well defined location in plane of the print medium may be suitable.




There are also some considerations unique to this design, primarily because of the imposed asymmetry (distortion) of the focusing field arising from the differences in the dielectric properties of the subregions of the space between field-shaping electrodes. The silicon wafer on which the transport array is built has a thickness of about one half millimeter and a dielectric constant of about twelve. The one half millimeter or so of air above the array has a dielectric constant of about one. This imposes an asymmetry in the electric field which has the effect of making the focusing system become asymmetric, as illustrated in FIG.


26


.




This potential problem can be managed in various ways. First and simplest, the applied voltages can be made to differ on the top and bottom field-shaping electrodes, like the electrodes


284


and


286


, by an amount which compensates for the material's asymmetry, as illustrated in FIG.


28


. Second, the silicon substrate can be physically modified to cause this asymmetry to tend to vanish near the end


80


of the array


63


, as illustrated in FIG.


29


. Third, the region of space below the upper field-shaping electrode, such as the electrode


284


, and above the surface of the launch end


80


of the print head


36


can be occupied by a dielectric material


320


, as illustrated in FIG.


30


. One or more of these approaches may be used to address the concern of asymmetry.




The above discussion has been directed to the transfer of toner packets


48


across a gap


214


. However, as the size of the gap


214


decreases toward zero, many of the concerns mentioned above cease to be concerns. Once the gap


214


has been reduced to near zero, e.g., below


50


microns, the transfer field near the end


80


of the print head


36


is smooth enough to reduce concerns about the divergence of the toner paths, and the driving force across the small gap also prevents divergence of toner paths. Such a contact or near-contact transfer scheme may be suitable for monochrome applications. It may also be used in color applications, particularly if intermediate transfer surfaces are employed.




Finally, given the possibility of the toner striking the print medium


152


at a position which is above or below the nominal target location, depending upon the local positioning accuracy of the above-mentioned focusing elements, a calibration technique may be used on a pixel-by-pixel basis to compensate for misalignment of pixels from color plane to color plane as well as within a single plane. One approach to calibration involves the generation of a test image and the evaluation of the image by manual or automated means to provide information to the image-modulating electronics within the printing device for correction of pixel misalignment. Such information could be stored within a PROM inside the printer or in the printer driver software.




9. Vertical Toner Transfer




The methodologies described above have dealt primarily with the transfer of toner particles


42


across a gap to a print medium


152


where the direction of toner motion has been confined to the plane of the toner transport device. However, even with the improvements discussed above, it may still be difficult to produce an adequate image. Therefore, an alternative transfer methodology, termed herein as “vertical toner transfer,” is described below. Transferring toner particles


42


vertically from the print head


36


before the end


80


, rather than longitudinally from the end


80


, results in image information being maintained more accurately, and also allows for relaxed manufacturing tolerances on the location of the end


80


of the print head


36


.




As illustrated in

FIG. 31

, the print head


36


V includes a plurality of electrodes


46


that produce the traveling wave for transporting the toner particles


42


. A transfer electrode


46


V is located at a prescribed location on the print head


36


V, typically near the end


80


. The transfer electrode


46


V is energized to repel toner particles


42


generally vertically away from the surface of the print head


36


V to the nearby print medium


152


. The physical size of the transfer electrode


46


V should be large enough to prevent any toner particles


42


from jumping over it and returning to the surface of the print head


36


V. It is currently believed that the transfer electrode


46


V should be at least about 1.5 times the diameter of the average toner particle


42


and at least one third of the wavelength of the traveling wave drive.




However, even for a large transfer electrode


46


V, fringe fields may be generated between the transfer electrode


46


V and the nearby drive electrodes


46


. These fringe fields can produce undesirable launch conditions for toner particles


42


, depending on where the toner particles


42


are relative to the phase of the traveling wave drive as they leave the surface. Such launch conditions can have the effect of blurring the arrival position of the toner particles


42


on the print medium


152


.




To address this problem, the electric field in the transfer region should be nearly planar from the point of launch to the point of arrival. Of course, small gaps are advantageous, with gaps of 50 to 150 microns being particularly useful. However, it is possible to minimize the scattering problem caused by fringe fields by carefully matching the fields produced by the drive electrodes


46


and the transfer electrode


46


V. The goal is to cause the drive field to disappear or to be greatly diminished when the toner particles


42


reach the transfer location. One way to achieve this is to use a pulse drive, instead of a sine wave drive, with a significant asymmetry in the pulse. With a pulse drive, when the last drive electrode


46


turns off, the nearest energized electrode


46


is suddenly nearly a full wavelength back, and the resulting fringe fields at the transfer location are minimized. For example, using a six-phase drive scheme, an effective approach is to use a drive that energizes only one or two of the six electrodes


46


in each wavelength at a time, while the drive holds the remaining electrodes


46


at ground, as illustrated by the voltage waveforms for the electrodes


46


in FIG.


32


. This approach sharpens the confinement location for toner particles


42


within the wavelength, e.g., there is less phase lag variation. Therefore, a near planar dc transfer field can be established from the transfer electrode


46


V to the print medium


152


.




Although this drive scheme accomplishes the objective stated above, it does exhibit certain disadvantages. For example, this scheme may result in a possible reduction in the maximum achievable drive speed for a given device structure. Also, higher local fields are generated in the dielectric for the same drive amplitude, so an increase in the strength of the dielectric may be needed.




In terms of optimizing the transfer field, larger fields make particles traverse the gap faster and, thus, tend to reduce spreading and blurring. However, a large transfer field may also tend to pull toner particles


42


off of the surface of the print head


36


V before the particles reach the transfer electrode


46


V. Although the transport field and imaging forces tend to hold most of the toner particles


42


to the surface to resist such premature transfer, toner particles


42


that happen to slip phase and momentarily leave the surface of the print head


36


V may transfer to the print medium


152


. Accordingly, a balance should be maintained between a transfer field that is strong enough to produce reliable transfer with minimal image blurring and a transfer field that is so strong that it causes excessive premature transfer.




Because such a balance may be difficult to achieve for typical toner mass and charge distributions, a hybrid geometry may be employed. As illustrated in

FIG. 33

, toner particles


42


may be vertically transferred from near the end


80


of the print head


36


V to the print medium


152


that is angled relative to the surface of the print head


36


V. It has been found that, for both vertical transfer and end transfer, the toner particles


42


tend to leave the surface of the print head along a path that angles upwardly. This hybrid geometry takes advantage of this tendency. Because the print medium


152


is angled so that the distance between the print medium


152


and the drive electrodes


46


increases rapidly, the potential for the transfer field to remove toner particles


42


prematurely and deposit them on the print medium


152


is substantially reduced. It is believed that any suitable angle may be implemented, although having the print medium


152


angled between 30 and 60 degrees relative to the surface of the print head


36


V appears to be particularly useful.




In one particularly advantageous embodiment, the print medium


152


is angled at about 45 degrees relative to the surface of the print head


36


V, with the pivot point of the print medium


152


being about 70 to 100 microns from the location of the transfer electrode


46


V. With this configuration, the transfer distance is between about 70 and 100 microns. By applying a dc bias voltage of about 70 to 100 volts on the print medium


152


and by grounding the transfer electrode


46


V, the toner particles


42


transfer cleanly to the print medium


152


with minimum premature transfer.




10. Conclusion




It should be appreciated that a system fabricated in view of the teachings set forth above may be capable of providing monochromatic or color printing at least on the order of about 10 to 30 pages per minute at a resolution meeting or exceeding 600 DPI. Indeed, due to the speed of toner delivery and the control over such toner, such a system may also be capable of printing a single pixel at any one of at least sixteen different gray levels or color densities. In regard to various color densities, it may be advantageous to use color toner that is somewhat transparent so that consecutive toner packets


48


applied to form a single pixel gradually increase the color density of the pixel.




While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.



Claims
  • 1. A print engine comprising:a toner imaging device having a toner delivery end; a target electrode being energizable to attract toner from the toner delivery end of the toner imaging device onto a surface; and at least two secondary electrodes being positioned in spaced relation to and on opposing sides of the target electrode, the at least two secondary electrodes being energizable to affect by repulsion a focusing or planarizing electric field adjacent the toner delivery end of the toner imaging device.
  • 2. The print engine, as set forth in claim 1, wherein the toner imaging device comprises a traveling wave toner transport device.
  • 3. The print engine, as set forth in claim 1, wherein the target electrode comprises a linear electrode.
  • 4. The print engine, as set forth in claim 1, wherein the target electrode is positioned across a gap from the toner delivery end of the toner imaging device.
  • 5. The print engine, as set forth in claim 1, comprising a first secondary electrode, a second secondary electrode, a third secondary electrode, and a fourth secondary electrode, the first secondary electrode and the second secondary electrode being positioned generally above the toner imaging device, and the third secondary electrode and the fourth secondary electrode being positioned generally below the toner imaging device.
  • 6. The print engine, as set forth in claim 1, wherein the surface comprises a print medium.
  • 7. The print engine, as set forth in claim 1, wherein the surface comprises an intermediate transfer surface.
  • 8. A print engine comprising:a toner imaging device, the toner imaging device being positioned across a gap from a first side of a print medium to deliver toner to the first side of the print medium; a target electrode being positioned adjacent a second side of the print medium, the target electrode being energizable to attract the toner from the toner imaging device across the gap and onto the first side of the print medium; a first focusing electrode being positioned on one side of and in spaced relation to the target electrode, the first focusing electrode being energizable to repel the toner delivered by the toner imaging device; and a second focusing electrode being positioned in spaced relation to the target electrode and on a side of said target electrode oppsite said one side, the second focusing electrode being energizable to repel the toner delivered by the toner imaging device.
  • 9. The print engine, as set forth in claim 8, wherein the toner imaging device comprises a traveling wave toner transport device.
  • 10. The print engine, as set forth in claim 8, wherein the traveling wave toner transport device comprises:a substrate; a transport surface fabricated on the substrate, the transport surface terminating in the toner delivery end, the toner delivery end being positioned across the gap from the first side of the print medium and being adapted to deliver the toner to the first side of the print medium, the substrate below the transport surface being positioned a greater distance from the print medium than the toner delivery end of the transport surface.
  • 11. The print engine, as set forth in claim 8, wherein the target electrode comprises a linear electrode.
  • 12. The print engine, as set forth in claim 8, wherein the first focusing electrode is positioned generally above the toner imaging device, and wherein the second focusing electrode is positioned generally below the toner imaging device.
  • 13. The print engine, as set forth in claim 8, further comprising:a first planarizing electrode being positioned in spaced relation to the target electrode and the toner imaging device, the first planarizing electrode being energizable to reduce divergence of an electric field in the gap; and a second planarizing electrode being positioned in spaced relation to the target electrode and the toner imaging device, the second planarizing electrode being energizable to reduce divergence of the electric field in the gap.
  • 14. The print engine, as set forth in claim 13, wherein the first planarizing electrode and the second planarizing electrode are energizable to repel the toner delivered by the toner imaging device.
  • 15. The print engine, as set forth in claim 13, wherein the first planarizing electrode is positioned generally above the toner imaging device, and wherein the second planarizing electrode is positioned generally below the toner imaging device.
  • 16. A print engine comprising:a traveling wave toner transport device, the traveling wave toner transport device having a toner delivery end being positioned across a gap from a first side of a print medium, the toner delivery end being adapted to deliver toner to the first side of the print medium; a target electrode being positioned adjacent a second side of the print medium adjacent the toner delivery end of the traveling wave toner transport device, the target electrode being energizable to attract the toner from the toner delivery end of the traveling wave toner transport device across the gap and onto the first side of the print medium; a first focusing electrode being positioned above the target electrode adjacent the second side of the print medium, the first focusing electrode being energizable to repel the toner delivered by the traveling wave toner transport device; a second focusing electrode being positioned below the target electrode adjacent the second side of the print medium, the second focusing electrode being energizable to repel the toner delivered by the traveling wave toner transport device; a first planarizing electrode being positioned above the toner delivery end of the taveling wave toner transport device, the first planarizing electrode being energizable to reduce divergence of an electric field near the toner delivery end; and a second planarizing electrode being positioned below the toner delivery end of the traveling wave toner transport device, the second planarizing electrode being energizable to reduce divergence of ah electric field near the toner delivery end.
  • 17. A printer comprising:a housing; a toner imaging device disposed within the housing, the toner imaging device having a toner delivery end; a print medium feed mechanism disposed generally within the housing, the print medium feed mechanism being operable to transport a print medium past the toner delivery end of the toner imaging device, the toner delivery end being positioned across a gap from a first side of the print medium and being adapted to deliver toner to the first side of the print medium; a target electrode being positioned adjacent a second side of the print medium adjacent the toner delivery end of the toner imaging device; a first focusing electrode being positioned on one side of and in spaced relation to the target electrode; a second focusing electrode being positioned in spaced relation to the target electrode and on a side of said target electrode opposite said one side; and a control circuit being coupled to the target electrode, the first focusing electrode, and the second focusing electrode, the control circuit energizing the first focusing electrode and the second focusing electrode to focus the toner by repulsion as the toner is transported across the gap.
  • 18. The printer, as set forth in claim 17, wherein the toner imaging device comprises a traveling wave toner transport device.
  • 19. The printer, as set forth in claim 17, wherein the target electrode comprises a linear electrode.
  • 20. The printer, as set forth in claim 17, wherein the first focusing electrode is positioned generally above the toner imaging device, and wherein the second focusing electrode is positioned generally below the toner imaging device.
  • 21. The printer, as set forth in claim 17, wherein the control circuit energizes the first focusing electrode and the second focusing electrode to focus trajectories of toner particles at the target electrode.
  • 22. The printer, as set forth in claim 17, further comprising a toner loading device disposed within the housing, the toner loading device being positioned to deliver toner to the toner imaging device.
  • 23. A printer comprising:a housing; a toner imaging device disposed within the housing, the toner imaging device having a toner delivery end; a print medium feed mechanism disposed generally within the housing, the print medium feed mechanism being operable to transport a print medium past the toner delivery end of the toner imaging device, the toner delivery end being positioned across a gap from a first side of the print medium and being adapted to deliver toner to the first side of the print medium; a target electrode being positioned adjacent a second side of the print medium adjacent the toner delivery end of the toner imaging device; a first focusing electrode being positioned above the target electrode adjacent the second side of the print medium; a second focusing electrode being positioned below the target electrode adjacent the second side of the print medium; a first planarizing electrode being positioned above the toner delivery end of the toner imaging device; a second planarizing electrode being positioned below the toner delivery end of the toner imaging device; and a control circuit being coupled to the target electrode, the first focusing electrode, the second focusing electrode, the first planarizing electrode, and the second planarizing electrode, the control circuit energizing the target electrode to attract the toner from the toner delivery end of the toner imaging device across the gap and onto the first side of the print medium, and the control circuit energizing the first focusing electrode and the second focusing electrode to focus toner as the toner is transported across the gap, and the control circuit energizing the first planarizing electrode and the second planarizing electrode to planarize the toner by repulsion as the toner is transported across the gap.
  • 24. The printer, as set forth in claim 23, wherein the toner imaging device comprises a traveling wave toner transport device.
  • 25. The printer, as set forth in claim 23, wherein the target electrode comprises a linear electrode.
  • 26. The printer, as set forth in claim 23, wherein the first planarizing electrode carries a voltage different than a voltage carried by the second planarizing electrode.
  • 27. The printer, as set forth in claim 23, further comprising a toner loading device disposed within the housing, the toner loading device being positioned to deliver toner to the toner imaging device.
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 08/993,846, filed on Dec. 18, 1997, and entitled “Optimization Of Transport Parameters For Traveling Wave Toner Transport Devices,” to U.S. patent application Ser. No. 08/993,896, filed on Dec. 18, 1997, and entitled “Toner Transport Device Having Improvements For Transferring Toner Particles,” to U.S. patent application Ser. No. 08/993,651, filed on Dec. 18, 1997, and entitled “Traveling wave and Vertical Toner Transfer,” and to U.S. patent application Ser. No. 08/993,650, filed on Dec. 18, 1997, and entitled “Scanning Print Head.”

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