Method of cleaning a contact charger of an electrophotographic apparatus

Abstract: A method of cleaning the contact charger of an electrophotographic image-forming apparatus which includes a photoconductive body and a contact charger to which a charge voltage (V1) of a predetermined level is applied to charge the outer surface of the photoconductive body during an image-forming operation is described. First, a charge voltage (V1) of a level lower than the said predetermined level is applied to the contact charging unit. Then, positively charged toner particles adhering to the contact charging unit are transferred to the photoconductive body by shutting off the charge voltage.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to an apparatus using an electrophotographic developing process. More particularly, it relates to a method of controlling the charging of the apparatus for the purpose of cleaning toner adhered to the outer surface of a contact charger. Electrophotographic developing processes are widely used in printers and facsimile systems as well as copying machines to produce images on a printing medium in response to video signals. A laser beam printer is one common example of a printer which uses such an electrophotographic developing process. Laser beam printers make images on printing paper through a series of electrophotographic image-forming steps of charging, exposure, developing, transferring, fixing and discharging. Conventional laser beam printers employs a Scorotron technique to perform the charging operation through applying a high voltage to a thin wire to generate a discharge which is then transferred to the outer surface of the photoconductive body. However, in this Scorotron technique, the surface potential of the photoconductive body (-600V to -800V) is relatively lower than the voltage (about -3KV to -4KV) applied to the charge roller. The discharge of high voltage causes the generation of harmful materials such as ozone (O 3 ), oxides of nitrogen (NO x ), etc. The contact charging technique was developed to solve the above problem and voltages of about -1.2KV to -1.5KV, which are lower than those achieved by the Scorotron technique, are V applied to the outer surface of the photoconductive body so that its outer surface is charged, whilst generating little ozone. This contact charging technique is in wider use than the Scorotron technique. FIG. 1 is a schematic depiction of an engine mechanism of a laser beam printer which uses an electrophotographic developing process. The first step of the electrophotographic developing process is charging the photoconductive body of the laser beam printer to a predetermined polarity. That is, a conductive roller 10 of FIG. 1 is charged to a negative polarity charge voltage V1 and a photoconductive drum 12, contacting the conductive roller 10, is also charged to a negative polarity. The second step is the exposing of the parts of the photoconductive drum 12 that correspond to an image to a laser beam using a light emitting diode laser 14, so that an electrostatic latent image is formed on the photoconductive drum 12. The non-exposed parts of the photoconductive drum 12 are maintained at the same potential, and the potential of the exposed parts are altered by this exposure. The electrostatic latent image is not visible to the human eye. The third step involves changing the electrostatic latent image formed on the outer surface of photoconductive drum 12 into a visual image. An outer surface of a developing roller 16 is uniformly charged to a negative polarity by a development voltage V2. Accordingly, toner contained in a toner hopper is friction-charged to a negative polarity and moved to a developing area by developing roller 16. The amounts of toner and carrier are controlled by a doctor blade 18 and the toner is moved to the developing area. Some toner is abnormally charged and takes on a positive polarity. Some of the toner particles, moved to the developing area, are carried to the exposed parts of the photoconductive drum 12 by the ambient electric fields so that the latent image is visualized as a toner image. The fourth step of the electrophotographic developing process involves transferring the toner image formed on the photoconductive drum 12 onto printing papers. Positive charges generated by a transfer voltage V3 are transferred to printing papers S by a transfer roller 20. The attraction between these positive charges and the charged toner particles causes the toner to move away from the photoconductive drum 12 and onto the printing papers S. The toner on the printing papers S is fused to the printing papers S through a fifth, fusing step. The toner on the printing papers S is firmly fused to the printing papers S by a pressure roller 26 and a heat roller 28 so that the toner image is changed into a semipermanent image. The printing papers S is discharged to an output tray in a printing papers delivery step. A conveyor roller 22 of FIG. 1 conveys the printing papers S to a register roller 24. The register roller 24 aligns the printing papers conveyed by conveyor roller 22. The laser beam printer has sensors for monitoring the operating state of the printer, the printing papers conveyance state, etc. A first sensor S1 located in a printing paper conveyance path between the conveyor roller 22 and the register roller 24 monitors the state of the printing papers that are moved to register roller 24, and a second sensor S2, mounted in a printing paper output path, monitors the state of the printing papers output to a top output tray (not shown). The toner adhering to the photoconductive drum 12 is transferred to the printing papers S during the transferring step, and a small amount of the positively charged toner or other toner that has not yet been cleaned off may still remain on the photoconductive drum 12 in such a manner that the positively charged toner accumulates on the outer surface of the conductive roller 10. As a result, the charge voltage V1 of the conductive roller 10 may become lower than the ideal. The sequence for cleaning the toner remaining on the conductive roller 10 will now be described with reference to FIG. 2. FIG. 2 shows a timing chart of voltages applied to the conductive roller 10, the developing roller 16 and the transfer roller 20. "CHV" denotes a charge voltage applied to conductive roller 10, and V1 corresponds to a voltage level of CHV. "DEV" denotes a development voltage applied to the developing roller 16, and V2 corresponds to a voltage level of DEV. "THV" denotes a transfer voltage applied to the transfer roller 20, and V3 and V4 each correspond to voltage levels of THV. CHV is constantly applied to the conductive roller 10 while a main motor, a driving system, rotates. Referring to FIG. 2, in an interval T1, photoconductive drum 12 is not yet exposed to light, and an interval T2 is a printing interval. During T2, THV attains + V3, and the toner image on the photoconductive drum 12 is transferred to printing paper. CHV attains V1 during intervals T3 and T5. The positively charged toner adhering to the outer surface of the conductive roller 10 is moved to the photoconductive drum 12 during T4 where CHV is 0 volts. This is because the outer surface of the photoconductive drum 12 is charged to a negative polarity by negative voltage V1. The positively charged toner, moved to the photoconductive drum 12, is collected from the negatively charged developing roller 16. T6 of DEV applied to developing roller 16 indicates a period for which the photoconductive drum 12 rotates by ℓ1 as shown in FIG. 1. During T1, the amount of the toner used for a toner image formed on the photoconductive drum 12 from the developing roller 16 is to be minimized. A voltage of -V4 is applied to the transfer roller 20 during T1 of THV applied to the transfer roller 20 so as to return the toner, attached to the transfer roller 20, to the photoconductive drum 12. After printing period T2, if V1, applied to the conductive roller 10, drops to zero, there is little potential difference between photoconductive drum 12 and the conductive roller 10, and the amount of the positively-charged toner that is moved to the photoconductive drum 12 becomes reduced. However, it is impossible completely to clean the positively charged toner adhering to the conductive roller 10. EP 0 540 341 is an example of an apparatus employing a conductive roller for contact charging a photoconductive body. During a cleaning step for removing relatively positively charged toner particles from the conductive roller, the voltage applied to the conductive roller is set at 0V, whereby positively charged toner transfers to the photoconductive body which is at a relatively more negative voltage. It is an objective of the present invention to provide a way of enhancing the cleaning efficiency of the contact charging unit. According to the present invention, there is provided a method of cleaning a contact charging unit of an electrophotographic image-forming apparatus comprising a photoconductive body and a contact charging unit to which a charge voltage of a predetermined level is applied to charge the outer surface of the photoconductive body during an image forming operation, the method comprising: (i) shutting off the charge voltage for transferring positively charged toner particles adhering to the contact charging unit to the photoconductive body, characterised by the step of (ii) prior to step (i) applying a charge voltage of a negative level lower than the predetermined level to the contact charging unit. The method may be carried out during warm-up of the apparatus. In one preferred variation, the apparatus further comprises an operating panel, and the method further comprises detecting the input of a cleaning key from the operating panel and then applying the charge voltage of a negative level lower than said predetermined level to the contact charging unit. The method may be carried out before the main motor of the apparatus stops after completion of an image-forming operation. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described by way of example with reference to the accompanying drawings in which: FIG. 1 is a schematic view of an engine mechanism of a laser beam printer using an electrophotographic developing process; FIG. 2 is a timing diagram of voltages applied to a charge roller, a developing roller and a transfer roller in accordance with FIG. 1; FIG. 3 is a block diagram of a laser beam printer using an electrophotographic developing process; FIG. 4 is a flow diagram of a video controller 34 for controlling a charge voltage in accordance with the present invention; and FIG. 5 is a timing diagram of voltages in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 is a block diagram of a laser beam printer using an electrophotographic developing process. The laser beam printer includes a video control unit 30, a print engine unit 40, and an operation panel 38. The video control unit 30 consists of a computer interface 32, a video central processing unit (CPU) 34, and an engine interface 36. The computer interface 32 is connected between a host computer and the video CPU 34 and interfaces input/output signals. The video CPU 34 has a read only memory (ROM) which stores a control program in accordance with the present invention, and a random access memory (RAM) that temporarily stores various data produced by the host computer and OPE 38. The video CPU 44 converts input data from the computer interface 32 to corresponding image data so that it can be processed by the printer engine unit 40 according to a predetermined program, and then sends the converted image data into the printer engine unit 40. The engine interface 36 that is connected between the video CPU 44 and the printer engine unit 40 interfaces input/output (I/O) signals with the printer engine unit 40 under the control of the video CPU 34. The OPE 38 is equipped with a set of keys such a printing papers selection key by which data is fed into the printer, and a display that displays the information output during the printing operation. The printer engine unit 40 includes a video interface 42, an engine central processing unit (CPU) 44, an input/output (I/O) interface 46, a sensing circuit 58, an instrument driver 50, and an electrophotographic developing unit 52, and is connected to the video control unit 30. The video interface 42 links the video control unit 30 with the engine CPU 44. The engine CPU 44 has control over the instrument driver 50 and the electrophotographic developing unit 52 under the control of the video CPU 34, and prints out an image corresponding to the image data output by the video control unit 30. The engine CPU 44 finds out whether or not there is a fault in the operation of the printer engine unit 40 (such as printing paper feeding, printing paper conveyance, etc.) through the sensing circuit 48. The sensing circuit 48 controls sensors which respectively monitor the operating state of each of the components, the printing paper conveyance state, and the amount of toner and applies the outputs of the sensors to the engine CPU 44. The instrument driver 50 allows the actuation of various operating sections of the laser beam printer used for printing papers feeding, printing papers conveyance, and printing operation. The electrophotographic developing unit 52 prints on printing papers pictorial images corresponding to the image information under the control of the engine CPU 44. A process of cleaning positively charged toner adhering to the conductive roller 10 will now be described with reference to FIGS. 1, 4 and 5. This cleaning process may be performed either when a warm-up of a laser beam printer employing the electrophotographic developing process is carried out by turning on the printer, or when there is a key board input for cleaning. In addition, the printer may go into cleaning operation at any time if the main motor of the printer has not stopped after the printing operation has been completed. In the preferred embodiment of the present invention, a process of cleaning positively charged toner that is adhering to conductive roller 10 in response to a cleaning key input will now be described. Referring first to FIG. 4, video CPU 34 that is in standby mode checks, at step 60 if there is cleaning key input from OPE 38. If a command to print is input from a host computer without any key input for cleaning, the video CPU 34 sets CHV to a normal voltage level, and goes to step 64. The video CPU 34 prints at step 60 a pictorial image corresponding to the image information that is input from the host computer, and returns to step 60. When a key input for cleaning is input from the OPE 38, the video CPU 34 sets CHV at step 66, applied to conductive roller 10, to a cleaning level. Once a command to print is input from the host computer, the video CPU 34 outputs the preset CHV to perform the printing and cleaning operation at step 68, concurrently. Referring to FIG. 5, once a command to print is input from the host computer, the video CPU 34 controls the electrophotographic developing unit 52 to apply CHV, DEV and THV to the conductive roller 10, magnetic roller 16, and the transfer roller 20, respectively. The periods T1, T2, T4 and T5 and voltage levels V1 to V4 are similar to those of the above-described conventional system, but there is a difference in V1' that is applied to the conductive roller 10 after T2. CHV of V1' applied to the conductive roller during T3 has a negative value lower than V1, so as to increase the amount of the positively charged toner collected by making increasing the potential difference between the photoconductive drum 12 and the conductive roller 10 in T4 wherein CHV, being applied to the conductive roller 10, drops to zero. Once the cleaning operation is completed through the above control sequence, the video CPU 34 checks at step 70 if the main motor has stopped. If the video CPU 34 determines that the main motor goes on operating, the printing and cleaning operation is repeatedly carried out. When the main motor stops, the cleaning process is complete. As described above, the present invention may easily collect the positively charged toner by applying to the contact charging unit a voltage of negative level lower than the charge voltage, applied during the printing operation, before shutting off the charge voltage, during warm-up, prior to stopping the main motor, or in response to a key input. The present invention may be applied to any apparatus employing an electrophotographic developing process and having a contact charging unit equally as well as to a laser beam printer.

Claims: A method of cleaning a contact charging unit (10) of an electrophotographic image-forming apparatus comprising a photoconductive body (12) and a contact charging unit (10) to which a charge voltage (V 1 ) of a predetermined level is applied to charge the outer surface of the photoconductive body (12) during an image forming operation, the method comprising: i) shutting off the charge voltage for transferring positively charged toner particles adhering to the contact charging unit (10) to the photoconductive body (12), characterised by the step of ii) prior to step i) applying a charge voltage of a negative level lower than the predetermined level to the contact charging unit. A method according to claim 1 which is carried out during warm-up of the apparatus. A method according to claim 1 wherein the apparatus comprises an operating panel, the method further comprising: detecting the input of a cleaning key from the operation panel; and then applying the charge voltage of a negative level lower than the said predetermined level to the contact charging unit. A method according to claim 1 which is carried out before the main motor of the apparatus stops after completion of an image-forming operation.

Inventors: KO CHANG-KYUNG, KO, CHANG-KYUNG
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1997
Main classification: G03G15
IPC codes: G03G15, G03G21