Electrostatic cleaning of electrodes in an electrographic printer

Abstract: The print head (82) of an electrographic printer has an array of electrodes (76) which co-operate with a dielectric belt (80). Periodically, printing with the printer is interrupted, (e.g. after every 100 pages of printing, or about every minute), and then substantially all of the electrodes (76) are energized to burn off contaminants from the electrodes, (e.g. for the time it takes to print four pages or about 3 seconds). A host computer (90) controls the operation of the printer components.

Description: In electrographic printers, discretely controlled ionization is employed to form the charged image on a dielectric surface. The dielectric surface then passes through a developer unit for developing a latent image thereon, and the developed electrostatic latent image is then transferred to a sheet of copy material (typically paper). Uniform ion formation and transport in the electric field is essential to guarantee the quality of the imaging. Ionization is accomplished by selectively energizing electrodes of an electrode array. During printing, only a very small proportion of the electrodes will be energized at any one time. Contamination of the electrodes occurs by the deposition of polymers and aerosols that are present in the environment, and by the deposition of compounds which result from the ionization of gas in the ion imaging process in the presence of other contaminants. Subtle degradation of the electrodes can occur due to the migration of non-precious metals through precious metal protective electrode coatings. As a result of the build-up of contaminants on the electrodes, the quality of the print can decrease remarkably. The contaminants may ultimately provide a microscopically thin insulative blockage layer on the electrode surfaces. This latter effect is particularly insidious since "Townsend Multiplication" phenomena can result in the regeneration of charge carriers that become unstable causing infrequent, but noticeable, imaging defects. Methods of cleaning the electrodes have been suggested, such as in US-A-4,638,339. Such cleaning systems typically comprise wiping or otherwise physically acting upon the electrode and/or dielectric belt surfaces to remove contaminants during a non-print portion of the cycle of operation. Such procedures are not very effective. DE-A-3436013 forms the basis for the preambles of claims 1 and 5 and discloses a method of cleaning the electrodes during periodic temporary interruption of printing. The present invention, provides a method of electrostatic cleaning of the ion electrodes of an electrographic printer which method is very effective, very easy to implement and control, and results in a minimum disturbance of the printing cycle. According to one aspect of the invention there is provided a method of cleaning a print head of an electrographic printer having an array of electrodes and electric means for selectively energizing electrodes in the electrode array whereby the array changes the charge applied to a co-operating dielectric belt of the printer, which includes the steps of: (a) periodically temporarily interrupting electrographic printing, and b) while printing is temporarily interrupted, activating substantially all of the electrodes of the electrode array by the said electric means to effect cleaning of the electrodes, characterised in that step (a) is practised by stopping application of toner from a developer unit to the dielectric belt by moving the development unit away from the dielectric surface without moving the dielectric belt. While the invention is applicable to a wide variety of electrographic printing apparatus it is particularly useful with the electrographic printing elements disclosed in US-A-4,638,339 since the relative positioning and the construction of the components there (with sufficient gap to support Townsend Multiplication) provide a very effective "burn off" of the contaminants. While the cycles of interruption and energization can vary widely depending upon the particular circumstances, typically printing is interrupted after the printing of every 100 pages, and substantially all of the electrodes are energized for a time period equal to the approximate time it takes to print about 4 pages. By the practice of such a technique, not only can a clean print head be kept clean, the print quality of a contaminated print head can be brought back to acceptability over a period of time. For example, a moderately contaminated print head can be brought back to acceptability after about 300 pages of print, whereas a badly contaminated head would reach acceptability after about 2,000 pages of printing. According to another aspect of the invention there is provided an electrographic printer comprising an array of electrodes extending adjacent a dielectric member, electric means for selectively energizing the electrodes in the electrode array to effect change in the charge level of the dielectric member during normal printing to effect electrographic printing, and means arranged to periodically temporarily interrupt electrographic printing and then to activate the electric means so as to energize substantially all of the electrodes in the electrode array for a period of time sufficient to effect cleaning of said electrodes while electrographic printing has been temporarily interrupted, characterised by the means to temporarily interrupt printing comprising means to stop application of toner from a developer unit to the dielectric belt by moving the developer unit away from the dielectric surface without moving the dielectric belt. The invention may be applied to known apparatus and methods with minimal modifications thereof. For example, when employed with an electrographic printer such as shown in US-A-4,638,339 having a host computer controlling the printer functions, including the selective energization of the electrodes, all that is necessary is a simple, straightforward change in the software which any programmer can readily implement. The software changes would typically provide for periodic temporary interruption of the printing (preferably after the count of the number of printed pages), movement of the developer away from the dielectric belt, energization of substantially all of the electrodes (pins) for a predetermined period of time and restarting the electrographic printing procedure. In order that the invention may be better understood, it will now be described by way of example with reference to the accompanying drawings, in which; FIGURE 1 is a diagrammatic view illustrating the method steps according to the invention; and FIGURE 2 is a schematic view illustrating one form of apparatus according to the invention. The basic steps of the method according to the invention for effecting electrostatic cleaning of an electrographic printer are illustrated diagrammatically in FIGURE 1. Typical electrographic printing is illustrated by box 10. The electrographic printing is periodically temporarily interrupted, as indicated by box 11. The period involved may be a predetermined printing time period, and/or a predetermined number of pages that have been printed since the last cleaning cycle. For example, the temporary printing interrupting step 11 may be practised after every 100 pages printed, or after each minute of actual printer operation. Once printing has been interrupted in step 11, the next step is the movement of the developer unit in step 12, away from the dielectric belt of the electrographic printer. The next step (13) is to energize all of the electrodes of the electrode array of the print head for a period of time sufficient to effect cleaning of the electrodes while the electrographic printing has been temporarily interrupted. Typically, this would be the time that it took to print four pages, or about three seconds. Then energization of all of the electrodes in step 13 is terminated, and printing is resumed in step 14. FIGURE 2 schematically illustrates apparatus according to the invention. The print head 82 is illustrated in its preferred form, from FIGURES 6 and 7 of US-A-4,638,339, the reference numerals in FIGURE 2 corresponding to the related reference numerals in FIGURES 6 and 7 of the US patent. The print head 82 includes an array of individual electrodes 76. The electrodes 76 are mounted on contoured belt supporting elements 84, 85 which are shaped to provide a desired spacing between the dielectric belt 80 (shown in phantom line in FIGURE 2) and the electrodes 76. Typically the spacing would be between about 0.15 mm to 0.4 mm, for a dielectric thickness of 0.25 mm and a dielectric constant of 3.0. The dielectric belt 80 comprises an electrically conductive ground plane backing element and a reinforcing layer (not shown). The bending modulus of the dielectric belt 80 is sufficiently high to preclude contact with the electrodes 76 when the dielectric member 80 is under tension, and the configuration of the elements 84, 85 provides two closely spaced support areas defined by smooth part-cylindrical surfaces which engage the dielectric belt 80, with a flattened area 83 therebetween with an unsupported region of the dielectric member above. Because of the bending modules and the flattened area 83, a predetermined spacing is maintained between the electrode array and the dielectric member 80, so that electrostatic charge deposition may reliably occur in the unsupported region of the dielectric member. A suitable connector 88 establishes electro-connection between the electrodes 76, through cables 89 and a drive circuit 90. The drive circuit 90 comprises means (in association with the control therefor) for selectively energizing the electrodes 76 to effect printing. Control for the drive circuit 90 may be provided by a host computer 91, which includes a built-in counter 92 and timer 93, although mechanical and/or circuitry elements could be used in place of a computer. The computer 91 also controls the position and operation of a developer unit 96 which applies toner to the dielectric belt 80 during printing. Apparatus according to the invention may be essentially the same as for conventional electrographic printers, the only difference being in the software used to control the computer 91 and structure for moving unit 96. The computer 91, with associated software, comprises means for periodically temporarily interrupting the electrographic printing (after printing of a complete page and before another page is started), including cessation of application of toner from unit 96 to belt 80, and means for periodically activating the circuitry 90 for energizing substantially all of the electrodes 76 in the electrode array at the same time, for a period of time sufficient to effect cleaning of the contaminates from the electrodes while the electrographic printing has been temporarily interrupted. The computer 91 also controls a mechanism mounting the developer unit for moving the developer unit 96 away from the dielectric belt 80. Again, the computer 91 would typically control energization of substantially all of the electrodes so that after each about 100 pages of printing was counted, the electrodes would be energized for a period of time equal to the approximate time it takes to print about four pages (e.g. after about one minute of actual printing all of the electrodes 76 would be energized for about three seconds). The burn off of contaminants is extremely effective in the case of the design of the print head 82 illustrated in FIGURE 2. The invention can maintain the original print head cleanliness and it is also possible to improve the quality of contaminated print heads to an acceptable level. Of course a slightly different control of the electrodes may be provided when improving print head quality, as opposed to maintaining the cleanliness of an already clean print head, and the printer interruption and electrode energization cycles may be different. For example, depending upon the particular printer design it may be desirable to energize all the electrodes for some time, only half of them for another time, and the other half for a further period of time.

Claims: A method of cleaning a print head (82) of an electrographic printer having an array of electrodes (76) and electric means (90) for selectively energizing electrodes in the electrode array, whereby the array (76) changes the charge applied to a co-operating dielectric belt of the printer, which includes the steps of (a) periodically temporarily interrupting (11) electrographic printing, and (b) while printing is temporarily interrupted, activating (13) substantially all of the electrodes (76) of the electrode array by the said electric means (90) to effect cleaning of the electrodes, characterised in that step (a) is practised by stopping application of toner from a developer unit (96) to the dielectric belt (80) by moving the developer unit (96) away from the dielectric surface without moving the dielectric belt (80). A method according to Claim 1 characterised in that steps (a) and (b) are practised without moving the print head (82). A method according to any preceding Claim characterised in that step (b) is practised for about 3 seconds for approximately every minute of printing. A method according to any of Claims 1 to 3 characterised in that step (a) is practised approximately every 100 pages printed, and step (b) is practised to energize substantially all of the electrodes for a time period equal to the approximate time it takes to print about 4 pages. An electrographic printer comprising an array of electrodes (76) adjacent a dielectric member (80), and electric means (90) for selectively energising the electrodes (76) in the electrode array to effect change in the charge level of the dielectric member (80) during printing to effect electrographic printing, and means (90-93) arranged to periodically temporarily interrupt electrographic printing, and then to activate the electric means (90) to energize substantially all of the electrodes (76) in the electrode array for a period of time sufficient to clean the electrodes, characterised by the means to temporarily interrupt printing comprise means to stop application of toner from a developer unit (96) to the dielectric belt (80) by moving the developer unit away from the dielectric surface without moving the dielectric belt. A printer according to Claim 5 characterised in that said means (90-93) comprise a controlling computer (91).

Inventors: FAGEN BEN W, FEHER PETER, KEGELMAN THOMAS B, MILLIARD NORMAN, RAYMOND MICHAEL W, RIZZA ROBERT S, FAGEN, BEN W., FEHER, PETER, KEGELMAN, THOMAS B., MILLIARD, NORMAN, RAYMOND, MICHAEL W., RIZZA, ROBERT S.
Applicants: AFK INC, AFK, INC.
Application year: 1989
Main classification: G03G15
IPC codes: G03G15, B41J2