JP5097602B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile using an electrophotographic process, and more particularly to an image forming apparatus capable of supplying toner to a developing device.

  In general, in a developing device using a one-component developer, by repeating image formation, a toner having a small particle size adheres to the surface of a toner carrier (developing roller) due to a reflection force due to a high charge amount. This prevents the toner particles from being frictionally charged with the toner carrier. Therefore, the toner is conveyed to the electrostatic latent image carrier (photosensitive drum) in a state where uniform charging is not performed on the toner carrier, which may cause problems such as image density reduction and image fogging. Such a phenomenon is likely to occur particularly when the document printing rate on the image is low, that is, when toner particles are likely to remain on the developing roller due to a small amount of toner flying from the developing roller to the photosensitive drum.

  Therefore, by applying an AC bias superimposed on a DC bias to the developing roller during non-image formation, a pattern with a high printing rate such as a solid image is developed to cause a large amount of toner to fly to the photosensitive drum side, An image forming apparatus capable of executing a so-called refresh mode in which toner on a developing roller is forcibly consumed has been developed. For example, Patent Document 1 discloses an image area ratio (average printing ratio) per unit driving time of a developing apparatus. There has been proposed an image forming apparatus that calculates and changes the forced consumption of toner according to the image area ratio.

  On the other hand, in an image forming apparatus that supplies (installs) toner to a developing device using a toner container, toner on the developing roller is not consumed during toner installation immediately after replacement of the toner container (hereinafter referred to as initial toner installation). For this reason, the charge amount of the toner in the developing device tends to be insufficient, and image density lowering and image fogging may occur. Another problem is that a part of the toner layer formed on the surface of the developing roller is disturbed and separated, and is developed as a solid and is confirmed as a random black spot on the image.

In order to solve such a problem, Patent Document 2 discloses that the above-described refresh mode is executed at the time of initial toner installation, thereby removing defective charging toner from the developing roller and correcting image defects such as a decrease in the density of the initial image. Techniques for preventing are disclosed.
JP 2003-76079 A JP 2004-170650 A

  However, when the method of Patent Document 2 is used, ozone is easily adsorbed on the surface of the photosensitive member by continuous application of the charging bias in the refresh process. For this reason, especially in a low-temperature and low-humidity environment, ozone adsorption becomes prominent, and the drum surface is oxidized and deteriorated, resulting in poor charging of the drum surface and variations in charging. As a result, there is a problem that image fogging and density unevenness occur in the output image immediately after the initial toner installation.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an image forming apparatus capable of effectively preventing initial image defects immediately after initial toner installation when the refresh process is executed at the time of initial toner installation. Objective.

  In order to achieve the above object, the present invention has a toner carrier that is disposed opposite to an image carrier, carries toner and supplies the image carrier, and a toner image corresponding to an electrostatic latent image on the surface of the image carrier. A developing device for forming the toner, a toner container for supplying toner to the developing device, a charging device for applying a charging bias to charge the surface of the image carrier, and a toner image formed on the image carrier directly or A transfer unit that transfers the toner onto the sheet via an intermediate transfer member; and a transfer bias application unit that selectively applies a positive or negative bias to the transfer unit. In the image forming apparatus capable of executing a refresh process for discharging toner from the toner carrier side to the image carrier side during the initial toner installation mode for replenishment, Measuring means for measuring the cumulative driving time of the image carrier from the end of the toll mode, and the sheet interval during continuous image formation until the cumulative driving time of the image carrier measured by the measuring means reaches a predetermined time And a control means for extending the time to be longer than the time between the reference sheets.

  According to the present invention, in the image forming apparatus configured as described above, the transfer bias applying unit applies a transfer reverse bias having the same polarity as the toner to the transfer unit between sheets.

  According to the present invention, in the image forming apparatus configured as described above, the control unit extends the sheet interval time to at least the time required for one rotation of the image carrier.

  According to the present invention, in the image forming apparatus configured as described above, during the execution of the refresh process, a charging bias is applied to the charging device while repeating ON / OFF at a predetermined cycle.

  According to the first configuration of the present invention, the pre-driving time of the developing device is ensured by extending the sheet interval time until the cumulative driving time of the image carrier after the initial toner installation reaches a predetermined time, and developing It is possible to promote the charging of the toner in the apparatus and suppress the decrease in the initial density immediately after the initial toner installation.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the transfer reverse bias is applied to the transfer unit between the sheets by the transfer bias applying unit, thereby applying the transfer reverse bias. Can be made uniform so that the surface of the image carrier is uniformly charged, and variations in the surface potential of the image carrier during image output can be suppressed.

  According to the third configuration of the present invention, in the image forming apparatus having the second configuration, the transfer reverse bias is applied by extending the sheet interval time to at least the time required for one rotation of the image carrier. Since the transfer means is close to the entire outer peripheral surface of the image carrier, the charge on the surface of the image carrier can be made uniform over the entire circumference.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the charging bias is applied by repeating ON and OFF at a predetermined cycle when the refresh process is executed. As a result, since the toner on the toner carrier is ejected little by little toward the image carrier when the charging bias is OFF, the cleaning burden on the toner on the image carrier can be reduced and poor cleaning can be prevented. Further, it is possible to discharge positively charged and negatively charged toners existing on the toner carrier.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the overall configuration of the image forming apparatus of the present invention. As shown in FIG. 1, an image forming apparatus (here, a monochrome printer) 100 includes a paper feed cassette 2 that accommodates sheets stacked on the lower part of the main body. Above this paper feed cassette 2 is formed a paper transport path 4 that extends substantially horizontally from the front of the main body to the rear of the main body and further extends upward to reach the paper discharge section 3 formed on the upper surface of the main body. A pickup roller 5, a feed roller 6, an intermediate conveyance roller 7, a registration roller pair 8, an image forming unit 9, a fixing unit 10, and a discharge roller pair 11 are arranged in this order from the upstream side along the conveyance path.

  The paper feed cassette 2 is provided with a paper stacking plate 12 that is rotatably supported with respect to the paper feed cassette 2 by a rotation fulcrum 12a provided at the rear end in the paper transport direction. The sheet thus formed is pressed against the pickup roller 5. A retard roller 13 is disposed in front of the paper feed cassette 2 so as to be in pressure contact with the feed roller 6, and when a plurality of sheets are simultaneously fed by the pickup roller 5, A sheet is rolled by the feed roller 6 and the retard roller 13 so that only the uppermost sheet is conveyed.

  Then, the sheet fed by the feed roller 6 and the retard roller 13 is conveyed to the registration roller pair 8 by changing the conveyance direction to the rear of the apparatus by the intermediate conveyance roller 7, and the timing is adjusted by the registration roller pair 8. And conveyed to the image forming unit 9.

  The image forming unit 9 forms a predetermined toner image on a sheet by an electrophotographic process. The image forming unit 9 is a photosensitive drum 14 that is an image carrier that is rotatably supported in the clockwise direction in FIG. Above the charging device 15, the developing device 16, the cleaning device 17, the transfer roller 18 disposed so as to face the photosensitive drum 14 across the paper conveyance path 4, and the photosensitive drum 14. A laser scanning unit (LSU) 19 is arranged, and a toner container 20 for supplying toner to the developing device 16 as needed is arranged above the developing device 16.

  For example, the photosensitive drum 14 is formed by laminating a photosensitive layer on an aluminum drum. As the photosensitive material for forming the photosensitive layer, an amorphous silicon photoreceptor or an organic photoreceptor (OPC photoreceptor) is used. As the charging device 15, a scorotron charging device using a corona discharge charging device is used, and tungsten or stainless steel is formed inside a metal shield case having an opening formed in a bowl shape facing the photosensitive drum. A corona wire and a grid, which are discharge members composed of, etc., are provided, a predetermined voltage is applied to the corona wire to generate a corona discharge, and a voltage is applied to the grid to control the amount of electrons. As a result, the surface of the photosensitive drum 14 is uniformly charged with a predetermined polarity and potential.

  Next, the photosensitive drum 14 is exposed by a laser beam from a laser scanning unit (LSU) 19 based on the input image data to form an electrostatic latent image in which charging is attenuated. A toner image is formed on the surface of the photosensitive drum 14 by attaching toner to the image. For example, when an OPC photoconductor is used as the photoconductor drum and a toner image is formed with positively charged toner having an average particle size of 5.0 to 9.0 μm, the drum surface is uniformly charged to about +430 V (bright potential) by the charging device 15. The surface potential (dark potential) after exposure is 90V.

  The toner image formed on the photoconductive drum 14 is transferred by the transfer roller 18 to a sheet supplied to a transfer position formed at the nip portion between the photoconductive drum 14 and the transfer roller 18. The sheet on which the toner image has been transferred is separated from the photosensitive drum 14 and conveyed toward the fixing unit 10. The fixing unit 10 is disposed on the downstream side of the image forming unit 9 in the sheet conveying direction. The sheet on which the toner image is transferred in the image forming unit 9 is a heating roller 21 provided in the fixing unit 10 and The toner image that is sandwiched and heated by the pressure roller 22 pressed against the heating roller 21 is fixed.

  The paper that has passed through the image forming unit 9 and the fixing unit 10 is discharged to the paper discharge unit 3 by the discharge roller pair 11. On the other hand, the toner remaining on the surface of the photosensitive drum 14 is removed by the cleaning device 17. Then, the photosensitive drum 14 is charged again by the charging device 15, and image formation is performed in the same manner.

  FIG. 2 is a side sectional view of the developing device mounted on the image forming apparatus of the present invention. As shown in FIG. 2, the developing device 16 includes a casing 31a that stores a one-component developer made of magnetic toner, and a cover 31b that seals the toner stored in the casing 31a so that the toner does not leak outside. A casing 31 is provided with a first agitating and conveying screw 32, a second agitating and conveying screw 33, a developing roller 35, and a regulating blade 36.

  The inside of the casing 31a is partitioned into a first storage chamber 38 and a second storage chamber 39 by a partition plate 37 extending in the longitudinal direction. In the first storage chamber 38, a first stirring and conveying screw 32 is provided. The second storage chamber 39 is provided with a second stirring and conveying screw 33. Further, the partition plate 37 is not provided at both ends in the longitudinal direction (paper surface direction) of the casing 31a, and this portion is a passage through which the toner moves between the first storage chamber 38 and the second storage chamber 39. It has become.

  The first agitating and conveying screw 32 and the second agitating and conveying screw 33 are respectively composed of rotating shafts 32a and 33a and spiral blades 32b and 33b integrally formed on the outer peripheral surfaces of the rotating shafts 32a and 33a. It is rotatably supported in the casing 31a so as to be parallel. The first agitation and conveyance screw 32 and the second agitation and conveyance screw 33 are configured to circulate and convey the toner in the first storage chamber 38 and the second storage chamber 39 by rotating in a predetermined direction. Also, a toner supply port (see FIG. 1) through which toner is supplied from the toner container 20 (see FIG. 1) to the cover 31b so that toner can be supplied into the casing 31a according to the detection result of the toner density sensor (not shown). Not shown).

  The developing roller 35 is rotatably supported in the first storage chamber 38 so as to be substantially parallel to the first stirring and conveying screw 32 and the second stirring and conveying screw 33. A magnet roller (not shown) made of a permanent magnet having a plurality of magnetic poles is fixed inside the developing roller 35. When the developing roller 35 rotates according to the rotation of the photosensitive drum 14, the magnet roller Toner is attached (carried) to the surface of the developing roller 35 by the magnetic force to form a toner layer. Then, the toner adhered to the developing roller 35 in a predetermined developing area is caused by the surface potential of the photosensitive drum 14 and the developing bias applied to the developing roller 35 (for example, DC bias Vdc = 280 V, AC bias Vpp = 1.7 kV, frequency). 2.3 kHz) and fly to the photosensitive drum 14 and adhere to the photosensitive layer, and a toner image is formed on the surface of the photosensitive drum 14.

  The regulating blade 36 regulates the amount of toner supplied to the photosensitive drum 14, that is, the amount of toner adhering to the developing roller 35. For example, a magnetic material such as SUS (stainless steel) is used. The regulating blade 36 is disposed so that a predetermined gap is formed between the tip of the regulating blade 36 and the developing roller 35, and a magnetic field generated in the gap between the regulating blade 36 and the developing roller 35 and the gap. The amount of toner adhering to the developing roller 35 is regulated, and a thin toner layer of several tens of microns is formed on the surface of the developing roller 35.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. The ROM 92 (or RAM 93) also stores a reference value for the cumulative driving time of the photosensitive drum 14 used for setting the sheet interval time, a reference sheet interval time, and the like after completion of the initial installation mode as will be described later. ing. The timer 95 measures the cumulative driving time of the photosensitive drum 14 after the initial toner installation is completed.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. Examples of the parts and devices controlled by the control unit 90 include, for example, the paper feeding cassette 2, the fixing unit 10, the developing device 16, the laser scanning unit 19, the toner container 20, the image input unit 40, the bias control circuit 41, and the operation unit 50. Etc.

  When the image forming apparatus 100 is a printer as shown in FIG. 1, the image input unit 40 is a receiving unit that receives image data transmitted from a personal computer or the like. When the image forming apparatus 100 is a copying machine, Scanning optical system equipped with a scanner lamp that illuminates the document during copying and a mirror that changes the optical path of the reflected light from the document, a condensing lens that focuses the reflected light from the document and forms an image It is an image reading unit composed of a CCD or the like that converts image light into an electrical signal. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power source 42, the developing bias power source 43, and the transfer bias power source 44, and operates each of these power sources according to an output signal from the control unit 90. These power sources are bias controlled. A predetermined bias is applied to the charging device 15, the developing roller 35, and the transfer roller 18 in accordance with a control signal from the circuit 41.

  The operation unit 50 is provided with a liquid crystal display unit 51, LEDs 52 indicating various states, and a numeric keypad 53, and the user operates the operation unit 50 to input instructions, thereby making various settings of the image forming apparatus 100. And execute various functions such as image formation. The liquid crystal display unit 51 displays the state of the image forming apparatus 100, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. The numeric keypad 53 is used for setting the number of copies to be printed and for inputting the other party's FAX number when the image forming apparatus 100 also has a FAX function.

  In addition, the operation unit 50 includes a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, and various settings of the image forming apparatus 100 in a default state. A reset button or the like is provided for use.

  In the image forming apparatus of the present invention, when the toner in the developing device 16 and the toner container 20 runs out, the toner container 20 is replaced, and the toner is supplied to the developing device 16 for a longer time than when toner is replenished during image formation. Perform initial toner installation mode. As described above, the toner immediately after being replenished in the initial toner installation mode is supplied to the developing roller 35 without sufficient stirring as described above. Therefore, the toner charge amount is not sufficient, the image density is lowered, and the development is performed. A part of the toner thin layer formed on the surface of the roller 35 is disturbed and separated, and the toner 35 is solidified and developed to generate random black spots on the image.

  Therefore, in the present invention, in the initial toner installation mode after the toner container 20 is replaced, a refresh process is performed in which the toner is ejected from the developing roller 35 to the photosensitive drum 14 and the toner is forcibly consumed. ing. FIG. 4 is an example of a timing chart showing ON / OFF timing of charging and developing bias in the refresh process. In the timing chart shown in FIG. 4, the application of the charging bias is started 760 seconds after the start of initial toner installation (0 sec), and the application of the developing bias is started almost simultaneously (after 761 sec). Therefore, the execution time of the refresh process is 140 seconds, the cumulative ON time of the charging bias is about 93 seconds, and the cumulative OFF time is about 47 seconds. The toner on the developing roller is discharged onto the drum surface when the charging bias is OFF.

  In FIG. 4, the charging bias is repeatedly applied ON and OFF in a predetermined cycle. However, the charging bias may be continuously turned OFF for a predetermined time, and the toner may be discharged collectively. For example, if the OFF time is continued for 47 seconds, the same amount of toner as in the timing chart shown in FIG. 4 can be discharged. However, when the toner on the developing roller is ejected all at once, a large amount of toner moves to the photosensitive drum side, which places a burden on the cleaning device 17 (see FIG. 1) and may cause problems such as defective cleaning. is there. Further, there are positively charged toners and negatively charged toners among the poorly charged toners on the developing roller. By repeatedly applying ON and OFF, both positively charged and negatively charged toners can be discharged. Therefore, it is preferable to apply the charging bias while repeating ON / OFF at a predetermined cycle as shown in FIG.

  Here, the refresh process is executed at the end of the initial toner installation mode, and the refresh process is also completed at the same time as the end of the initial toner installation mode. However, the refresh process is started after the end of the initial toner installation mode. Also good.

  In the present invention, after completion of the initial toner installation mode, the inter-sheet time during continuous image formation is extended beyond the reference inter-sheet time until the cumulative driving time of the photosensitive drum reaches a predetermined time. It has been found that the density unevenness due to toner and the initial density drop immediately after installation due to insufficient charging of the toner can be effectively suppressed.

  FIG. 5 is a graph showing the relationship between the cumulative driving time of the photosensitive drum after the initial toner installation (added value of the drum driving time when image output is performed) and the variation in the surface potential of the photosensitive drum. The greater the variation in drum surface potential is, the more the drum is judged to have deteriorated due to ozone. Therefore, the horizontal axis of the graph represents the cumulative driving time of the drum, and the vertical axis represents the surface potential within one page of printing (A4 size). The measurement results were plotted by taking the variation ΔV0.

  As apparent from FIG. 5, the surface potential variation ΔV0 within one page of printing gradually decreases from 27V immediately after the initial installation as the cumulative driving time of the drum becomes longer, and when the cumulative driving time reaches about 800 sec. It converged around 18V. This is because when the refresh process is executed in the initial installation mode, the drum surface is oxidized and deteriorated by ozone due to the application of the charging bias to the drum surface, and the surface potential varies greatly. This is probably because the drum surface is polished by driving and the deteriorated drum surface is restored to its original state after a predetermined time.

  FIG. 6 is a graph showing the relationship between the sheet interval immediately after the initial toner installation and the variation in the surface potential of the photosensitive drum. The measurement results are plotted with the inter-paper time on the horizontal axis of the graph and the surface potential variation ΔV0 within one printed page (A4 size) on the vertical axis.

  As shown in FIG. 5, the surface potential variation ΔV0 immediately after the initial toner installation is as large as 27V due to the deterioration of the drum surface due to the application of the charging bias during the execution of the refresh process. As can be seen from FIG. 6, when the paper interval time during continuous printing is increased from the normal 340 msec, ΔV0 also gradually decreases and extends to about 600 msec, which corresponds to the time for one rotation of the photosensitive drum. ΔV0 decreased to about 18V.

  The reason for this is considered as follows. That is, for example, as shown in FIG. 4, when the charging bias is repeatedly applied ON / OFF at a predetermined cycle in the refresh process, the charge on the drum surface is canceled by a static eliminator (not shown) before image formation. However, a charging history remains, and the surface potential variation ΔV0 when the drum surface is subsequently charged increases. On the other hand, a reverse transfer bias having the same polarity as the toner is applied to the transfer roller 18 in order to prevent toner adhesion to the transfer roller 18 (see FIG. 1) and scattering of the toner transferred onto the paper. . Since this reverse transfer bias is smaller than the charging bias applied to the drum surface and is continuously applied, the charging history of the drum surface can be reduced by increasing the paper interval time, i.e., the transfer reverse bias application time. It cancels out and the charge becomes uniform.

  Therefore, by utilizing this property, it is possible to effectively suppress the variation in the drum surface potential immediately after the initial toner installation due to the surface deterioration of the photosensitive drum. That is, by extending the sheet interval time from the reference sheet interval time until the cumulative driving time of the photosensitive drum 14 reaches a predetermined time, the transfer reverse bias application time is extended to uniformize the drum surface charge, Variations in drum surface potential during image output can be suppressed. In addition, the toner in the developing device is insufficiently charged immediately after the initial toner installation. However, since the pre-driving time of the developing device 16 becomes longer by extending the paper interval, the toner is charged immediately after the initial toner installation. It can be promoted to suppress a decrease in initial concentration.

  Note that the threshold of the cumulative driving time of the drum that continues to extend the paper interval time may be set as appropriate according to the characteristics of the photosensitive drum 14. For example, in the case of a photosensitive drum that behaves as shown in FIG. 5, the interval between sheets is extended until the cumulative drive time after the completion of the initial toner installation reaches 840 sec, as will be described later. Further, the extension time from the reference sheet interval time may be set as appropriate according to the characteristics of the photosensitive drum 14, the required initial density, and the like, but in order to make the drum surface charge uniform over the entire circumference. Needs to bring the entire outer peripheral surface of the photosensitive drum 14 close to the transfer roller 18 to which a transfer reverse bias is applied. For this reason, it is preferable to set the inter-paper interval after the extension to be at least the time required for one rotation of the photosensitive drum.

  FIG. 7 is a timing chart showing an example of the ON / OFF timing of the transfer bias during continuous printing. Here, a case will be described in which the photosensitive drum diameter is 30 mm, the drum linear velocity is 170 mm / sec, and double-sided printing is performed using positively charged toner. In FIG. 7, T1 and T2 indicate the ON timing and OFF timing of the transfer positive bias (−) on the first surface on both sides, and T1DU and T2DU indicate the ON timing and OFF timing of the transfer positive bias (−) on the second surface on both sides. T3 and T4 are the ON timing and OFF timing of the transfer positive bias for removing the reversely charged (negatively charged) toner from the transfer roller 18 after double-sided printing.

  FIG. 7A shows normal ON / OFF timings, from the end of the previous output to the start of transfer on the first side of both sides (T1), and from the end of transfer on the first side of both sides (T2) to the transfer of the second side of both sides. The time between sheets until the start (T1DU) is set to 340 msec. FIG. 7B shows the ON / OFF timing until the drum cumulative driving time reaches a predetermined time (in this case, 840 sec) after the initial toner installation is completed, and the transfer time (T1 to T2) for both sides of the first surface, The transfer time (T1DU to T2DU) on the second surface and the cleaning time (T3 to T4) of the transfer roller are not changed, and only the time between sheets is extended by 120 msec to 560 msec.

  Here, since the time required for one rotation of the photosensitive drum is 30 (mm) × 3.14 / 170 (mm) × 1000≈554 (msec), the inter-paper time after the extension is required for one rotation of the photosensitive drum. It's over time. Although the transfer bias ON / OFF timing at the time of duplex printing has been described with reference to FIG. 7, the transfer bias ON / OFF timing at the time of single-sided continuous printing is also described in exactly the same manner. Further, ON and OFF (T3, T4) of the transfer positive bias for removing the reversely charged (negatively charged) toner from the transfer roller 18 may be executed for each printing sheet or every predetermined number of sheets. You may do it.

  FIG. 8 is a flowchart illustrating a control example of the sheet interval time after the initial toner installation mode in the image forming apparatus of the present invention. With reference to FIGS. 1 to 7, the procedure for determining the paper interval after the initial toner installation mode will be described in accordance with the steps of FIG. First, when the user replaces the toner container 20 (step S1), the control unit 90 determines whether or not it is an initial toner installation mode in which toner is supplied to the developing device 16 for a longer time than when toner is supplied during image formation. Judgment is made (step S2).

  When it is determined that the initial toner installation mode is set, the initial toner installation mode is started (step S3), and the refresh process is executed at the timing shown in FIG. 4 (step S4). Specifically, the charging bias and the developing bias are applied to the charging device 15 and the developing roller 35 from the charging bias power source 42 and the developing bias power source 43 by the control signal from the bias control circuit 41, and the toner does not adhere to the transfer roller 18. In this manner, a transfer reverse bias having the same polarity as that of the toner is applied from the transfer bias power supply 44.

  Then, when the charging bias is turned off, the toner flies on the photosensitive drum 14 as solid black, and the toner on the developing roller 35 is consumed. Thereafter, the cleaning is continued until the toner on the photosensitive drum 14 is completely removed by the cleaning device 17. When the cleaning is completed, a transfer positive bias is applied to the transfer roller 18, and the reverse polarity toner on the transfer roller 18 is removed. Clean and finish the refresh process.

  After the initial toner installation and refresh process is completed, the cumulative driving time of the photosensitive drum 14 is measured using the timer 95 (step S5). The control unit 90 determines whether or not the measured cumulative driving time is longer than 840 sec (step S6). If the cumulative driving time is 840 sec or less, the inter-paper time during continuous printing is set to the reference inter-paper time 340 msec. 120 msec to 560 msec (step S7). Then, the process returns to step S5 and the measurement of the cumulative driving time of the photosensitive drum 14 is continued. If the cumulative drive time exceeds 840 sec in step S6, the reference paper interval time is set to 340 msec (step S8), and the process is terminated.

  According to such control, it is possible to set an appropriate sheet interval time corresponding to the cumulative driving time of the photosensitive drum after the initial toner installation. That is, the charge applied at the time of refresh is extended by extending the time between sheets (transfer reverse bias application time) to the time required for one rotation of the photosensitive drum within a predetermined time immediately after the initial toner installation in which the drum surface is oxidized with ozone. The bias can be canceled and uniformed, and variations in drum surface potential can be suppressed. Further, by extending the preliminary drive time of the developing device immediately after the initial toner installation, in which the toner in the developing device is difficult to be charged, it is possible to suppress a decrease in the initial image density after the toner installation.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, the dimensions, materials, shapes, relative arrangements, and the like of the components described in the above embodiment are not intended to limit the scope of the present invention only to specific examples unless otherwise specified. Not too much. The transfer bias application timing and ON / OFF cycle shown in FIG. 7 are merely examples, and can be appropriately changed according to the specifications of the photosensitive drum and the developing device, the composition of the toner to be used, and the like. For example, if you want to suppress only the decrease in the initial density without considering the variation in the surface potential of the photosensitive drum, the toner after the initial toner installation is secured by extending the paper interval and securing the preliminary drive time of the developing device. What is necessary is just to promote the charging. In this case, it is not always necessary to apply the transfer reverse bias during the sheet interval, and the transfer bias may be turned off.

  Further, the present invention is not limited to the monochrome laser printer shown in FIG. 1, but an analog monochrome copying machine, a rotary or tandem color copying machine, a color printer, a copying machine such as an analog monochrome copying machine, a facsimile, etc. Of course, the present invention can be applied to various image forming apparatuses having a toner container for supplying toner to the developing device.

  The present invention comprises a developing device that has a toner carrier that is disposed opposite to an image carrier and carries toner and supplies the toner to the image carrier, and forms a toner image corresponding to the electrostatic latent image on the surface of the image carrier. A toner container for supplying toner to the developing device, a charging device for applying a charging bias to charge the surface of the image carrier, and a toner image formed on the image carrier on a sheet directly or via an intermediate transfer member A transfer means for transferring, and a transfer bias applying means for selectively applying a positive or negative bias to the transfer means; and toner carrying in the initial toner installation mode for supplying toner to the developing device after replacing the toner container In an image forming apparatus capable of executing a refresh process for discharging toner from the body side to the image carrier side, the cumulative drive time of the image carrier from the end of the initial toner installation mode is calculated. Measuring means for measuring, and control means for extending the inter-paper time during continuous image formation beyond the reference inter-paper time until the cumulative driving time of the image carrier measured by the measuring means reaches a predetermined time. It is a thing.

  As a result, when executing the refresh process together with the initial toner installation mode, it is possible to set the optimal sheet interval time according to the cumulative driving time of the image carrier after the initial toner installation is completed, so that the preliminary driving time of the developing device is ensured. Thus, it is possible to provide an image forming apparatus capable of preventing a decrease in image density immediately after initial toner installation.

  Further, if a transfer reverse bias having a polarity opposite to that of the toner is applied to the transfer means between the sheets, the application time of the transfer reverse bias is also extended due to the extension of the sheet interval, so that the surface of the image carrier in the refresh process The charging history can be canceled by the reverse transfer bias to make the charging uniform, and variations in the surface potential of the image carrier during image output can be suppressed. At this time, if the time between the sheets is extended to the time required for one rotation of the image carrier, the charge can be made uniform over the entire circumference of the image carrier, so that the variation in surface potential can be more effectively suppressed. it can.

FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus of the present invention. These are sectional views of a developing device used in the image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. These are timing charts showing an example of ON / OFF timing of charging and developing bias in the refresh process. These are graphs showing the relationship between the cumulative driving time of the photosensitive drum after the initial toner installation and the variation in the surface potential of the photosensitive drum. FIG. 6 is a graph showing the relationship between the sheet interval immediately after the initial toner installation and the variation in the surface potential of the photosensitive drum. These are timing charts showing an example of the ON / OFF timing of the transfer bias during continuous printing. FIG. 5 is a flowchart illustrating an example of control of a sheet interval time after the initial toner installation mode in the image forming apparatus of the present invention.

Explanation of symbols

14 Photosensitive drum (image carrier)
DESCRIPTION OF SYMBOLS 15 Charging device 16 Developing device 17 Cleaning device 19 Transfer roller (transfer means)
20 Toner container 35 Developing roller (toner carrier)
41 Bias control circuit (transfer bias applying means)
42 Charging bias power supply 43 Development bias power supply 44 Transfer bias power supply (transfer bias applying means)
90 Control unit (control means)
91 CPU
92 ROM
93 RAM
95 Timer (measuring means)
100 Image forming apparatus

Claims (4)

A developing device that has a toner carrier that is disposed opposite to the image carrier and carries toner and supplies the toner to the image carrier, and forms a toner image corresponding to the electrostatic latent image on the surface of the image carrier;
A toner container for supplying toner to the developing device;
A charging device for applying a charging bias to charge the surface of the image carrier;
Transfer means for transferring the toner image formed on the image carrier onto a sheet directly or via an intermediate transfer member;
Transfer bias application means for selectively applying a positive or negative bias to the transfer means,
In an image forming apparatus capable of executing a refresh process for discharging toner from the toner carrier side to the image carrier side in an initial toner installation mode in which toner is supplied to the developing device after replacing the toner container.
Measuring means for measuring the cumulative driving time of the image carrier from the end of the initial toner installation mode, and continuous image formation until the cumulative driving time of the image carrier measured by the measuring means reaches a predetermined time An image forming apparatus comprising: control means for extending the sheet interval time to a reference sheet interval time.   The image forming apparatus according to claim 1, wherein the transfer bias applying unit applies a transfer reverse bias having the same polarity as the toner to the transfer unit between sheets.   The image forming apparatus according to claim 2, wherein the control unit extends a paper interval time to at least a time required for one rotation of the image carrier.   4. The image forming apparatus according to claim 1, wherein a charging bias is applied to the charging device while being repeatedly turned on and off at a predetermined period during the refresh process. 5.

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