Electrostatic printer

Abstract: Direct electrostatic printing (DEP) is enhanced by the provision of a shutter mechanism (54) for preventing toner dislodged from a printhead structure from being deposited on copy substrates. The shutter is interposed between the printhead structure and the copy substrate during a cleaning cycle. The dislodged toner is removed from between the printhead structure and the copy substrate with a combination suction and toner collection device.

Description: This invention relates to electrostatic printing devices, and more particularly to a toner removal and recovery system for a direct electrostatic printing apparatus. Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography, wherein latent electrostatic images formed on a charge-retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper. A lesser utilized form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This form of printing differs from the xerographic form, in that the toner material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in US-A-3,689,935, with the printer incorporating a multilayered particle modulator or printhead comprising a layer of insulation material, a continuous layer of conductive material on one side of the insulation layer, and a segmented layer of conductive material on the other side of the insulation layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is electrically insulated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer, while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator, the density of the particle stream being modulated according to the the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinges upon a print-receiving medium translated relative to the particle modulator to provide line-by-line scan printing. In this device the supply of the toner to the control member is not uniformly effected, and irregularities are liable to occur in the image on the image-receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner. US-A-4,491,855 discloses a method and apparatus utilizing a controller having a plurality of openings or slits to control the passage of charged particles and to record a visible image by the charged particles impinging directly on an image-receiving member. Disclosed therein is a device for supplying the charged particles to a control electrode that has allegedly made high-speed and stable recording possible. The improvement lies in that the charged particles are supported on a support member, and an alternating electric field is applied between the support member and the control electrode, which purports to obviate the problems noted above. Thus, this device is said to supply the charged particles to the control electrode without scattering them. US-A-4,568 955 discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a recording electrode and a signal source connected thereto for propelling the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. An A.C. and a D.C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the developer found between the adjacent electrodes along electric lines of force therebetween thereby to liberate developer from the developing roller. In a modified form of this device, a toner reservoir is disposed beneath a recording electrode which has a top provided with an opening facing the recording electrode and an inclined bottom for holding a quantity of toner. In the toner reservoir are disposed a toner-carrying plate as the developer-carrying member, secured in a position such that it faces the end of the recording electrode at a predetermined distance therefrom, and a toner agitator for agitating the toner. US-A-4,647,179 discloses a toner-transport apparatus for use in forming powder images on an imaging surface. The apparatus is characterized by the provision of a traveling electrostatic wave conveyor for the toner particles for transporting them from a toner supply to an imaging surface. The conveyor comprises a linear electrode array consisting of spaced-apart electrodes to which a multiphase a.c. voltage is connected such that adjacent electrodes have phase-shifted voltages applied thereto, which cooperate to form a travelling wave. US-A-3,872,361 discloses an apparatus in which the flow of particulate material along a defined path is controlled electrodynamically by means of elongated electrodes curved concentrically to a path, as axially spaced rings or interwound helices. Each electrode is axially spaced from its neighbors by a distance about equal to its diameter and is connected with one terminal of a multi-phase alternating high voltage source. Adjacent electrodes along the path are connected with different terminals in a regular sequence, producing a wave-like, non-uniform electric field that repels electrically charged particles axially inwardly and tends to propel them along the path. US-A-3,778,678 discloses a similar device. US-A-3,801,869 discloses a booth in which electrically-charged particulate material is sprayed onto a workpiece having an opposite charge, so that the particles are electrostatically attracted to the workpiece. All of the walls that confront the workpiece are made of electrical insulation material. A grid-like arrangement of parallel, spaced-apart electrodes, insulated from each other, extends across the entire area of every wall, parallel to a surface of the wall and in intimate juxtaposition thereto. Each electrode is connected with one terminal of an alternating high voltage source, every electrode with a terminal different from each of the electrodes laterally adjacent to it, to produce a constantly varying field that electrodynamically repels particles from the wall. While the primary purpose of the device disclosed is for powder painting, it is contended therein that it could be used for electrostatic or electrodynamic printing. The latter three devices all utilize a relatively high voltage source (e.g. 5-10 kV) operated at a relatively low frequency, e.g. 50 Hz, for generating the travelling waves. In a confined area such as a tube or between parallel plates the use of high voltages is tolerable and in the case of the '869 patent even necessary since a high voltage is required to charge the initially uncharged particles. US-A-4,743,926 discloses an electrostatic printing apparatus including a structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. Alternatively, the toner particles can be delivered to a charge-retentive surface bearing latent images. The developer or toner delivery system is adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a pair of charged toner conveyors which are supported in face-to-face relation. A bias voltage is applied across the two conveyors to cause toner of one polarity to be attracted to one of the conveyors, while toner of the opposite polarity is attracted to the other conveyor. One of the charged-toner conveyors delivers toner of the desired polarity to an apertured printhead where the toner is attracted to various apertures thereof from the conveyor. In another embodiment of the '926 patent, a single charged toner conveyor is supplied by a pair of three-phase generators which are biased by a dc source which causes toner of one polarity to travel in one direction on the electrode array, while toner of the opposite polarity travels generally in the opposite direction. In an additional embodiment disclosed in the '926 patent, a toner charging device is provided which charges uncharged toner particles to a level sufficient for movement by one or the other of the aforementioned charged toner conveyors. The toner in a device such as disclosed in the '926 patent is extracted from the "tops" of the clouds via the fringe fields that extend into the clouds from around the apertures. The efficiency of toner usage in a charged toner conveyor is currently limited by the relatively dilute toner density in the "tips" of the toner clouds that are transported thereby. US-A-4,814,796 discloses a direct electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer through apertures in the printhead onto the copy medium disposed intermediate the printhead and the conductive shoe. The structure for delivering developer or toner is adapted to deliver toner containing a minimum quantity of wrong sign and size toner. To this end, the developer delivery system includes a conventional magnetic brush which delivers toner to a donor roll structure which, in turn, delivers toner to the vicinity of apertures in the printhead structure. US-A-4,780,733 discloses a direct electrostatic printing apparatus including structure for delivering developer or toner particles to a printhead forming an integral part of the printing device. The printing device includes, in addition to an apertured printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer through apertures in the printhead onto the copy medium disposed intermediate the printhead and the conductive shoe. Toner is delivered to the printhead via a pair of oppositely charged toner conveyors. One of the conveyors is attached to the printhead and has an opening therethrough for permitting passage of the toner from between the conveyors to areas adjacent the apertures in the printhead. US-A-4,755,837 discloses a direct electrostatic printing apparatus including structure for removing wrong sign developer particles from a printhead forming an integral part of the printing device. The printing device includes, in addition to the printhead, a conductive shoe which is suitably biased during a printing cycle to assist in the electrostatic attraction of developer passing through apertures in the printhead onto the copy medium disposed intermediate the printhead and the conductive shoe. During a cleaning cycle, the printing bias is removed from the shoe and an electrical bias suitable for creating an oscillating electrostatic field which effects removal of toner from the printhead is applied to the shoe. The toner particles so removed are attracted to the copy medium in areas away from the image areas. With regard to the device described in US-A-4,568,955, it is obvious to anyone skilled in electrostatics that the toner resting in the bottom of the reservoir under the force of gravity alone must be uncharged or of very nearly neutral charge. Thus, even though some toner may be charged by friction with the agitator installed in the bottom of the reservoir, other nearby toner must acquire charge of the opposite polarity. As a result, any toner extracted from the bed by the toner-carrying plate, with its inclined end immersed in the bed of toner, must be toner having a charge which is low in absolute value and/or of mixed polarity. It should also be noted that since the toner-carrying plate has a relatively coarse grid structure (less than two lines per mm), it must operate at high voltages (> 1000 volts rms) and at relatively low frequency (< 1000 Hz). In other words, from the coarse grid structure and the fact that it is alleged to extract toner from a reservoir, it is evident that this device is intended to operate much like the electric curtain of '869 which normally transports bipolar material. Another feature of the toner-carrying plate, which necessitates the handling of neutral or mixed polarity toner, is the absence of any means to aid the return of the toner to the reservoir. If the toner did possess a net charge, the pile of toner accumulated in the reservoir near the end of the toner-carrying plate would produce a strong repulsive field and prevent additional toner from escaping from the toner-carrying member. Experience with transporting charge toner via a travelling wave shows that charged toner must be assisted off the carrying plate or it will block and back up on the plate in a manner analogous to a traffic jam, and further toner transport comes to a halt. Still another feature of this device, which restricts it to the use of low charged toner or very low toner density in the transported cloud, called "smoke", is the relatively large distance (-2 mm) between the toner-carrying plate and the control aperture. Because of these features, this printer is restricted to printing at very low speeds (< 1 cm/sec ) and is incapable of printing page length (-27 cm) images without plugging the apertures. The present invention overcomes these limitations and makes it possible to repeatedly print page length images at high speeds (>2 cm/sec) for extended periods. The present invention is directed to a direct electrostatic printing (DEP) apparatus comprising a supply of charged toner disposed adjacent one side of an apertured printhead structure and an image-receiving member disposed adjacent the other side thereof. Toner particles which accumulate on the side of the printhead structure adjacent the image-receiving member are dislodged by toner particles from the supply which are caused to bombard the side of the printhead structure. The close proximity of the imaging member to the dislodged particles and the relative voltages cause the dislodged particles to be attracted to the imaging member in the interdocument area. As will be appreciated, the toner which is attracted to the imaging member is both unattractive and wasteful. Thus, in accordance with the present invention, there is provided direct electrostatic printing apparatus, comprising a supply of charged toner particles; an apertured printhead structure through which toner particles pass in image configuration; an image-receiving member disposed adjacent one side of the printhead; means for supporting the image-receiving member; means for attracting toner particles from the printhead to the image-receiving member; means for periodically causing toner particles to bombard the side of the printhead structure to dislodge toner particles which have accumulated on the side, characterised by interposing means for interposing between the printhead and the path of the image-receiving member to prevent dislodged toner from being deposited on the image-receiving member and means for removing the dislodged toner from between the printhead and the interposing means. In a further aspect of the invention there is provided a method of printing images, including the steps of supplying toner particles to an apertured printhead structure; supporting an image-receiving member to one side of the printhead structure; attracting toner particles from the printhead structure to the image-receiving member; periodically causing toner particles to bombard the side of the printhead structure facing the image-receiving member to dislodge toner particles which have accumulated on the printhead, characterised by preventing dislodged toner from being deposited on the image-receiving member by positioning interposing means between the printhead and the image-receiving member, and removing the dislodged toner from between the printhead and the interposing means. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of electrostatic printing apparatus of the present invention in its inactive position, and Figure 2 is a schematic illustration of the Fig. 1 apparatus in its operative position. The printing apparatus 10 includes a developer delivery or conveying system 12, a printhead structure 14 and a backing electrode or shoe 16. The developer delivery system 12 includes a charged toner conveyor (CTC) 18 and a magnetic brush developer supply 20. The CTC 18 comprises a base member 22 and an electrode array comprising repeating sets of electrodes 24, 26, 28 and 30 to which are connected A.C. voltage sources V₁, V₂ V₃ and V₄ which voltage sources are phase-shifted one from the other so that an electrostatic travelling wave pattern is established. The effect of the traveling wave pattern is to cause already charged toner particles 34 delivered to the conveyor via the developer supply 20 to travel along the CTC to an area opposite the printhead apertures 35 where they come under the influence of electrostatic fringe fields emanating from the printhead 14, and ultimately under the influence of the field created by the voltage applied to the shoe 16. To enhance the interaction between the fringe fields and the toner travelling on the CTC, the distance between the CTC and the printhead should be less than three wavelengths, or 12 electrode spacings on the CTC for a four-phase CTC, and preferably less than one wavelength. A narrow CTC/printhead spacing facilitates a high delivery rate of usable toner and therefore a high printing speed By way of example, the developer comprises any suitable insulative non-magnetic toner/carrier combination having pyrogenic silica sold under the trademark 'Aerosil' contained therein in an amount approximately equal to 0.3 to 0.5% by weight, and also having zinc stearate contained therein in an amount approximately equal to 0.1 to 1.0% by weight. It should be appreciated however that the optimal amount of additives (Aerosil and zinc stearate) will vary depending on the base toner material, coating material on the CTC, and the toner supply device. The printhead structure 14 comprises a layered member including an electrical insulation base member 36 fabricated from a polyimide film having a thickness in the order of 0.025 to 0.05 mm. The base member is clad on the one side thereof with a continuous conductive layer or shield 38 of aluminum which is approximately 1 µm thick. The opposite side of the base member 36 carries a segmented conductive layer 40 thereon which is fabricated from aluminum and has a thickness similar to that of the shield 38. The total thickness of the printhead structure is in the order of 0.027 to 0.52 mm. A plurality of holes or apertures 35 (only one of which is shown), approximately 0.15 mm in diameter, are provided in the layered structure in a pattern suitable for use in recording information. The apertures form an electrode array of individually-addressable electrodes. With the shield 38 grounded, and with 0-100 volts applied to an addressable electrode, toner is propelled through the aperture associated with that electrode. The aperture extends through the base 36 and the conductive layers 38 and 40. With a negative 350 volts applied to an addressable electrode 40 via switch 39, and a dc power source 41, toner is prevented from being propelled through the aperture 35. Image intensity can be varied by adjusting the voltage on the control electrodes between 0 and minus 350 volts. Addressing the individual electrodes can be effected in any manner using electronically-addressable printing elements. The electrode or shoe 16 has an arcuate sectional shape as shown, but the present invention is not limited to such a configuration. The shoe which is positioned on the opposite side of a plain paper record medium 42 from the printhead 14 supports the record medium in an arcuate path in order to provide an extended area of contact between the medium and the shoe. The record medium 42 may comprise roll paper or cut sheets of paper fed from a supply tray (not shown). The sheets of paper are spaced from the printhead 14 a distance in the order of 0.05 to 0.75 mm as they pass thereby. As a general rule, the smaller the spacing, the higher the resolution at higher printing speeds, though at the expense of maintaining greater precision in the gap between the printhead and paper. The record medium 42 is transported in contact with the shoe 16 via edge transport roll pairs 43. During printing the shoe 16 is electrically biased to a dc potential of approximately 400 volts via a dc voltage source 44. Toner on the CTC not passed through the printhead is removed from the CTC downstream with an electrostatic pickoff device comprising a biased roll 46 and scraper blade 48. A suction pickoff device can be used in lieu of the electrostatic one. To allow for wrong-sign toner becoming agglomerated on electrode 40 of the printhead 14, a switch 50 is periodically actuated such that a dc biased AC power supply 52 is connected to the shoe 16 to effect cleaning of the printhead. The voltage from the source 52 is supplied at a frequency which causes toner travelling through the apertures 35 into the gap between the paper and the printhead to oscillate and bombard the printhead. Momentum transfer between the oscillating toner and any accumulated toner on the control electrodes 40 of the printhead causes the toner on the control electrodes to become dislodged. In order to prevent the dislodged toner from being deposited on the record medium 42, a shutter 54 is moved from its inactivated position shown in Figure 1, to its activated position shown in Figure 2. Movement of the shutter between its active and inactive positions is effected by means of a solenoid 56. In the position shown in Figure 2, the shutter 54 blocks the dislodged toner, thereby precluding contact thereof with the paper 42. A combination suction source and storage container 58 serves to draw the toner through a conduit 60 after it is dislodged from electrode 40 of the printhead structure 14. At the fusing station, a fuser assembly 62 permanently affixes the transferred toner powder images to record medium 42. Preferably, fuser assembly 62 includes a heated fuser roller 64 adapted to be pressure engaged with a back-up roller 66, with the toner powder images contacting fuser roller 64. In this manner, the toner powder image is permanently affixed to substrate 42. After fusing, a chute (not shown) guides the advancing substrate 42 to a catch tray (also not shown) for removal from the printing machine by the operator. A typical width for each of the electrodes for the traveling wave grid is 0.025 to 0.10 mm. Typical spacing between the centers of the electrodes is twice the electrode width, and the spacing between adjacent electrodes is approximately the same as the electrode width. Typical operating frequency is between 1 and 10 kHz for 125 lpi grids 0.10 mm electrodes, the drive frequency for maximum transport rate being 2 kHz. A typical operating voltage is relatively low (i.e. less than the Paschen breakdown value) and is in the range of 30 to 1000 Volt depending on grid size, a typical value being approximately 500 V for a 125 lpi grid. Stated differently, the desired operating voltage is approximately equal to 100 times the spacing between centers of adjacent electrodes measured in units of 25 µm. While the electrodes may be exposed metal such as Cu or Al, it is preferred that they be covered or overcoated with a thin oxide or insulation layer. A thin coating having a thickness of about half of the electrode width will sufficiently attenuate the higher harmonic frequencies and suppress attraction to the electrode edges by polarization forces. A slightly conductive overcoating will allow for the relaxation of charge accumulation by charge exchange with the toner. To avoid excessive alteration of the toner charge as it moves about the conveyor, however, a thin coating of a material which is non-tribo active with respect to the toner is desirable. A weakly tribo-active material which maintains the desired charge level may also be utilized. A preferred overcoating layer comprises a strongly injecting active matrix material such as that disclosed in US-A-4,515,882. As disclosed therein, the layer comprises an insulating film forming continuous phase comprising charge transport molecules and finely-divided charge injection enabling particles dispersed in the continuous phase. A polyvinylfluoride film available from the E. I. duPont de Nemours and Company under the tradename 'Tedlar' has also been found to be suitable for use as the overcoat. While a single CTC has been disclosed, it will be appreciated that cooperating charged toner conveyors, such as disclosed in the above '733 patent could be utilized. Also, any toner supply, including a magnetic brush loaded donor, as described in the '796 patent, can be utilized in conjunction with the above-described toner removal and recovery system.

Claims: Direct electrostatic printing apparatus, (10) comprising: a supply (12) of charged toner particles; an apertured printhead structure (14) through which toner particles pass in image configuration; an image-receiving member (42) disposed adjacent one side of the printhead; means for supporting the image-receiving member; means for attracting toner particles from the printhead to the image-receiving member; means for periodically causing toner particles to bombard the side of the printhead structure to dislodge toner particles which have accumulated on the side, characterised by interposing means (54) for interposing between the printhead and the path of the image-receiving member to prevent dislodged toner from being deposited on the image-receiving member and means (60) for removing the dislodged toner from between the printhead and the interposing means (54). Apparatus according to claim 1, wherein the interposing means (54) comprises a shutter member. Apparatus according to claim 2, including means for moving the shutter member to its operating position intermediate the printhead and the image-receiving member simultaneously with the actuation of the means for periodically causing toner particles to bombard the side of the printhead. Apparatus according to any one of claims 1 to 3, wherein the removing means (60) comprises a suction device. A method of printing images, including the steps of: supplying toner particles to an apertured printhead structure; supporting an image-receiving member to one side of the printhead structure; attracting toner particles from the printhead structure to the image-receiving member; periodically causing toner particles to bombard the side of the printhead structure facing the image-receiving member to dislodge toner particles which have accumulated on the printhead, characterised by preventing dislodged toner from being deposited on the image-receiving member by positioning interposing means between the printhead and the image-receiving member, and removing the dislodged toner from between the printhead and the interposing means. A method according to claim 5, including the step of positioning the shutter member intermediate the printhead and the image receiving member simultaneously with the actuation of the means for periodically causing toner particles to bombard the side of the printhead structure. A method according to claim 5 or claim 6, wherein the removing step comprises the use of a suction device.

Inventors: SCHMIDLIN FRED W, SCHMIDLIN, FRED W.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: G03G17
IPC codes: B41J2, B41M1, G01D15, G03G15