JP2014153610A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to properly adjust a condition of collecting toner from a toner carrier to a toner supply member according to the amount of fine powder toner on the toner carrier.SOLUTION: An image forming apparatus comprises: a developing apparatus having a toner carrier for supplying toner to an electrostatic latent image formed on an image carrier, and a toner supply member for forming a toner layer on the toner carrier by use of a magnetic brush; voltage application means which applies a developing bias to the toner supply member and the toner carrier; layer thickness detection means which detects a thickness of the toner layer remaining on the toner carrier; and control means which changes at least one of a potential difference ΔV of a DC voltage to be applied to the toner carrier and the toner supply member during collecting the remaining toner and an AC voltage to be applied to the toner supply member, so that the thickness of the remaining toner layer falls within a predetermined range, when the remaining toner layer thickness detected by the layer thickness detection means is beyond the predetermined range.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine thereof, and in particular, a two-component developer containing toner and a magnetic carrier is used, and only a toner is carried on a toner carrying member. The present invention relates to an image forming apparatus for developing the above electrostatic latent image.

  Conventionally, as a developing method using dry toner in an image forming apparatus using an electrophotographic process, a one-component developing method using no carrier and a non-magnetic toner using a magnetic carrier (hereinafter also simply referred to as a carrier) are used. There is known a two-component developing system that uses a two-component developer to be charged and develops an electrostatic latent image on a photosensitive member by a magnetic brush composed of toner and carrier formed on a developing roller.

  The one-component development method is suitable for high image quality because the electrostatic latent image on the image carrier is not disturbed by the magnetic brush, but toner adheres to the regulating blade, resulting in non-uniform layer formation. It sometimes caused image defects.

  In the case of color printing in which color superposition is performed, since the color toner is required to be transparent, it needs to be a non-magnetic toner. Therefore, in a full-color image forming apparatus, a two-component development system in which toner is charged and conveyed using a carrier is often employed. However, the two-component development method can maintain a stable charge amount for a long time and is suitable for extending the life of the toner, but is disadvantageous in terms of image quality due to the influence of the magnetic brush described above.

  As one of the means for solving these problems, a magnetic roller (toner supply member) is used to transfer the developer onto a developing roller (toner carrier) installed in a non-contact manner with respect to the photoreceptor (image carrier). In this process, a non-magnetic toner is transferred onto the developing roller while leaving the magnetic carrier on the magnetic roller to form a thin toner layer, and the toner is attached to the electrostatic latent image on the photosensitive member by an AC electric field. A scheme has been proposed.

  In the above developing system, among the toner on the developing roller, a coarse powder (large diameter) toner having a low charge amount and a high developability is easily selectively developed on the image carrier, and a fine powder (small diameter) toner having a high charge amount. The so-called selective development occurs in which the toner remains without being consumed. Therefore, when continuous printing (printing durability) is performed, problems such as a decrease in image density and toner adhesion to the surface of the developing roller occur.

  Therefore, as a method for preventing an increase in fine toner on the developing roller due to such selective development, for example, Patent Documents 1 and 2 disclose that the bias between the magnetic roller and the developing roller is imaged at a predetermined timing during non-image formation. A technique is disclosed in which the toner on the developing roller is collected on the magnetic roller by performing control differently from the formation. Patent Document 2 also describes that the density of a test pattern image formed at a predetermined timing is detected and whether or not toner collection from the developing roller to the magnetic roller is necessary is determined based on the detected value. .

  Further, Patent Document 3 has a sensor for detecting the thickness of the toner layer on the toner carrier, and at the time of non-image formation (non-printing) after toner removal operation (toner recovery operation) after completion of the final image. A developing method or a developing device that detects a toner layer thickness on a toner carrier by a sensor and controls the duty ratio and / or frequency of the AC voltage of the toner carrier at the next printing based on the detection result is disclosed.

Japanese Unexamined Patent Publication No. 7-72733 JP 2009-8979 A JP 2008-20640 A

  By the way, the amount of fine powder toner remaining on the developing roller is not always constant, and varies depending on the charged state of the two-component developer and the usage environment of the developing device. However, none of the above Patent Documents 1 to 3 make the toner recovery condition from the magnetic developing roller to the magnetic roller appropriate according to the amount of fine powder toner on the developing roller.

  The present invention has been made in order to solve the above-described problems. In an image forming apparatus for developing an electrostatic latent image on an image carrier by carrying only the toner on the toner carrier, the toner carrier is provided. An object of the present invention is to provide an image forming apparatus that optimizes the conditions for collecting toner from a toner carrier to a toner supply member according to the amount of fine powder toner on the body.

  In order to achieve the above object, a first configuration of the present invention includes an image carrier, a developing device, a voltage application unit, a layer thickness detection unit, and a control unit. The image forming apparatus executes a residual toner collecting operation for collecting the residual toner on the body on the toner supply member. The image carrier carries an electrostatic latent image. The developing device forms a toner layer using a magnetic brush on the toner carrier, and a toner carrier for supplying toner to the electrostatic latent image formed on the image carrier that is disposed in contact with and non-contact with the image carrier. A toner supply member. The voltage applying means applies a developing bias in which an AC voltage is superimposed on a DC voltage to the toner supply member and the toner carrier. The layer thickness detecting means detects the remaining toner layer thickness on the toner carrier. When the residual toner layer thickness detected by the layer thickness detection unit is outside the predetermined range, the control unit is configured to perform a residual toner collecting operation, and a potential difference ΔV between the DC voltages applied to the toner carrier and the toner supply member. Alternatively, at least one of the AC voltages applied to the toner supply member is changed so that the remaining toner layer thickness falls within a predetermined range.

  According to the first configuration of the present invention, the potential difference ΔV of the DC voltage applied to the toner carrier and the toner supply member when the residual toner collecting operation is executed based on the layer thickness of the collected residual toner on the toner carrier. Alternatively, by performing control to change at least one of the AC voltages applied to the toner supply member, the recoverability of the recovered residual toner can be improved when the layer thickness of the recovered residual toner is larger than the target range. As a result, it is possible to perform printing with a stable image density even when continuously printing low-print-rate images with low toner consumption. Further, when the layer thickness of the collected residual toner is smaller than the target range, the toner on the toner carrier is prevented from being collected more than necessary, thereby causing the toner external additive to adhere to the surface of the toner carrier. Charge accumulation and fogging can be suppressed.

1 is a schematic diagram showing an internal structure of a color printer 100 as an example of an image forming apparatus of the present invention. Side sectional view of the developing device 3a mounted on the color printer 100 The figure which shows an example of the bias waveform applied to the developing roller 23 and the magnetic roller 22 1 is a block diagram showing a control path of a color printer 100 of the present invention. Timing chart showing operation during continuous printing in the color printer 100 of the present invention The flowchart which shows the execution procedure of the continuous printing in the color printer 100 of this invention The graph which shows the change of the alternating current waveform applied to the developing roller 23 when changing the duty ratio while making the area center voltage constant. In the embodiment, a graph showing the relationship between the number of printed sheets, the remaining toner layer thickness, and the potential difference ΔV when developing bias control is performed during the remaining toner collecting operation. In the embodiment, a graph showing the relationship between the number of printed sheets and the image density when developing bias control is performed during the residual toner collecting operation. In the embodiment, a graph showing the relationship between the number of printed sheets, the residual toner layer thickness, and the potential difference ΔV when the development bias control is not performed during the residual toner collecting operation. In the embodiment, a graph showing the relationship between the number of printed sheets and the image density when the developing bias control is not performed during the residual toner collecting operation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to the present invention. Here, a tandem type color printer is shown. In the main body of the color printer 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  These image forming portions Pa to Pd are provided with photosensitive drums 1a, 1b, 1c and 1d which carry visible images (toner images) of the respective colors, and are further illustrated by a driving means (not shown). 1, an intermediate transfer belt 8 that rotates in the clockwise direction is provided adjacent to each of the image forming portions Pa to Pd. The toner images formed on the photosensitive drums 1 a to 1 d are sequentially transferred onto the intermediate transfer belt 8 that moves while contacting the photosensitive drums 1 a to 1 d, and then transferred onto the transfer paper at the secondary transfer roller 9. The image is transferred onto P at a time, and further fixed on the transfer paper P in the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d in the counterclockwise direction in FIG.

  The transfer paper P to which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the color printer 100 main body, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a (seamless) belt having no seam is mainly used. A blade-shaped belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed downstream of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure device 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure device 4 to each photosensitive drum. Electrostatic latent images corresponding to image signals are formed on 1a to 1d. Each of the developing devices 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner by a replenishing device (not shown). The toner is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically attached to form a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 4. It is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, the cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d. Transcribed above. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched around a plurality of stretching rollers including an upstream conveying roller 10 and a downstream driving roller 11, and the rotation of the driving roller 11 by a driving motor (not shown) is performed. When the intermediate transfer belt 8 starts to rotate in the clockwise direction, the transfer paper P is conveyed from the registration roller pair 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and a full color image is formed. Transcribed. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed on only one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller pair 15.

  On the other hand, when images are formed on both sides of the transfer paper P, a part of the transfer paper P that has passed through the fixing unit 7 is once protruded from the discharge roller pair 15 to the outside of the apparatus. Thereafter, the transfer paper P is redistributed to the registration roller pair 12b in a state where the image surface is reversed by rotating the discharge roller pair 15 in the reverse direction to be distributed to the paper conveyance path 18 by the branching section 14. Then, after the next image formed on the intermediate transfer belt 8 is transferred by the secondary transfer roller 9 to the surface of the transfer paper P where the image is not formed, and conveyed to the fixing unit 7 to fix the toner image. The paper is discharged to the discharge tray 17 through the discharge roller pair 15.

  Further, an image density sensor 40 is disposed at a position facing the driving roller 11 with the intermediate transfer belt 8 interposed therebetween. As the image density sensor 40, an optical sensor provided with a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is used. When measuring the toner adhesion amount on the intermediate transfer belt 8, if the measurement light is irradiated from the light emitting element to each reference image formed on the intermediate transfer belt 8, the measurement light is reflected by the toner, and the belt surface. Is incident on the light receiving element as light reflected by the light.

  When the toner adhesion amount is large, the reflected light from the belt surface is blocked by the toner, so that the light reception amount of the light receiving element is reduced. On the other hand, when the adhesion amount of toner is small, conversely, the amount of reflected light from the belt surface increases, resulting in an increase in the amount of light received by the light receiving element. Therefore, the density of each color is corrected by detecting the density of the reference image of each color based on the output value of the received light signal based on the amount of reflected light received and adjusting the characteristic value of the developing bias in comparison with the predetermined reference density. Is done.

  FIG. 2 is a side cross-sectional view showing the configuration of the developing device 3 a mounted on the color printer 100. Here, the developing device 3a disposed in the image forming unit Pa of FIG. 1 will be described, but the configuration of the developing devices 3b to 3d disposed in the image forming units Pb to Pd is basically the same, and thus described. Is omitted.

  As shown in FIG. 2, the developing device 3a includes a developing container 20 in which a two-component developer (hereinafter also simply referred to as a developer) is accommodated. The developing container 20 is divided into a first wall and a second wall by a partition wall 20a. The first and second agitating chambers 20b and 20c are divided into agitating chambers 20b and 20c, and a toner (positively charged toner) supplied from a toner container (not shown) is mixed with a carrier and agitated and charged. The stirring screw 21a and the second stirring screw 21b are rotatably arranged.

  Then, the developer is agitated by the first agitating screw 21a and the second agitating screw 21b while being conveyed in the axial direction, and passes through the developer passages (not shown) formed at both ends of the partition wall 20a. It circulates between the second stirring chambers 20b and 20c. In the illustrated example, the developing container 20 extends obliquely upward to the left, and a magnetic roller 22 is disposed above the second stirring screw 21b in the developing container 20, and the developing is performed obliquely upward to the left of the magnetic roller 22. Rollers 23 are arranged opposite to each other. The developing roller 23 faces the photosensitive drum 1a on the opening side (left side in FIG. 3) of the developing container 20, and the magnetic roller 22 and the developing roller 23 rotate in the clockwise direction in the drawing.

  The developing container 20 is provided with a toner concentration sensor (not shown) facing the first stirring screw 21a, and a replenishing device (not shown) according to the toner concentration detected by the toner concentration sensor. Then, toner is supplied into the developing container 20 through the toner supply port 20d.

  The magnetic roller 22 includes a non-magnetic rotating sleeve 22a and a fixed magnet body 22b having a plurality of magnetic poles enclosed in the rotating sleeve 22a. In the present embodiment, the magnetic poles of the fixed magnet body 22 b have a five-pole configuration including a main pole 35, a regulation pole (head cutting pole) 36, a transport pole 37, a separation pole 38, and a pumping pole 39. The magnetic roller 22 and the developing roller 23 face each other with a predetermined gap at the facing position (opposing position).

  Further, a spike cutting blade 25 is attached to the developing container 20 along the longitudinal direction of the magnetic roller 22 (direction perpendicular to the paper surface of FIG. 2), and the spike cutting blade 25 is rotated in the direction of rotation of the magnetic roller 22 (see FIG. 2 (clockwise direction) 2, it is positioned upstream of the position where the developing roller 23 and the magnetic roller 22 face each other. A slight gap (gap) is formed between the tip of the ear cutting blade 25 and the surface of the magnetic roller 22.

  The developing roller 23 includes a nonmagnetic developing sleeve 23a and a developing roller side magnetic pole 23b fixed in the developing sleeve 23a. The developing roller side magnetic pole 23b is different in polarity from the opposing magnetic pole (main pole) 35 of the fixed magnet body 22b.

  A developing bias power supply 43 (see FIG. 5 for both) is connected to the developing roller 23 and the magnetic roller 22 via a bias control circuit 41. A DC voltage (hereinafter referred to as Vslv (DC)) and an AC voltage (hereinafter referred to as Vslv (AC)) are applied to the developing roller 23, and a DC voltage (hereinafter referred to as Vmag (DC)) is applied to the magnetic roller 22. And an AC voltage (hereinafter referred to as Vmag (AC)) is applied.

  As described above, the first stirring screw 21a and the second stirring screw 21b circulate in the developing container 20 while the developer is being stirred to charge the toner, and the second stirring screw 21b causes the developer to move to the magnetic roller 22. Be transported. Since the regulation pole 36 of the fixed magnet body 22 b faces the ear cutting blade 25, a nonmagnetic material or a magnetic body having a polarity different from that of the regulation pole 36 is used as the ear cutting blade 25. A magnetic field is generated in a direction attracting the gap with the sleeve 22a.

  Due to this magnetic field, a magnetic brush is formed between the ear cutting blade 25 and the rotating sleeve 22a. When the magnetic brush on the magnetic roller 22 is regulated by the cutting blade 25 and then moved to a position facing the developing roller 23, the magnetic field attracted by the main pole 35 of the fixed magnet body 22b and the developing roller side magnetic pole 23b. Therefore, the magnetic brush contacts the surface of the developing roller 23. Then, a toner thin layer is formed on the developing roller 23 by a potential difference ΔV between Vmag (DC) applied to the magnetic roller 22 and Vslv (DC) applied to the developing roller 23 and a magnetic field.

  The thickness of the toner layer on the developing roller 23 varies depending on the resistance of the developer and the rotational speed difference between the magnetic roller 22 and the developing roller 23, but can be controlled by ΔV. When ΔV is increased, the toner layer on the developing roller 23 is thickened, and when ΔV is decreased, the toner layer is thinned. The range of ΔV at the time of development is generally about 100V to 350V.

  FIG. 3 is a diagram illustrating an example of a bias waveform applied to the developing roller 23 and the magnetic roller 22. As shown in FIG. 3A, the developing roller 23 has a developing bias power supply 43 with a combined waveform Vslv (solid line) obtained by superimposing a rectangular wave Vslv (AC) having a peak-to-peak value of Vpp1 on Vslv (DC). Applied. Also, the magnetic roller 22 has a composite bias Vmag (broken line) in which a square wave Vmag (AC) having a peak-to-peak value of Vpp2 and a phase different from Vslv (AC) superimposed on Vmag (DC) is developed bias. Applied from the power source 43.

  Therefore, the voltage applied between the magnetic roller 22 and the developing roller 23 (hereinafter referred to as between the MSs) is a composite waveform Vmag-Vslv having Vpp (max) and Vpp (min) as shown in FIG. Become. Note that Vmag (AC) is set so that the duty ratio is larger than Vslv (AC). Actually, an AC voltage having a partially distorted shape is applied instead of a complete rectangular wave as shown in FIG.

  The toner thin layer formed on the developing roller 23 by the magnetic brush is conveyed to the opposite portion between the photosensitive drum 1 a and the developing roller 23 by the rotation of the developing roller 23. Since Vslv (DC) and Vslv (AC) are applied to the developing roller 23, the toner flies due to a potential difference with the photosensitive drum 1a, and the electrostatic latent image on the photosensitive drum 1a is developed. .

  When the rotating sleeve 22a further rotates in the clockwise direction, the magnetic brush is now separated from the surface of the developing roller 23 by a horizontal (roller circumferential direction) magnetic field generated by a different polarity separation pole 38 adjacent to the main pole 35. Toner remaining without being used for development is collected from the developing roller 23 onto the rotating sleeve 22a. When the rotating sleeve 22a further rotates, a repulsive magnetic field is applied by the peeling pole 38 of the fixed magnet body 22b and the scooping pole 39 having the same polarity, so that the toner is detached from the rotating sleeve 22a in the developing container 20. Then, after being stirred and conveyed by the second agitating screw 21b, a magnetic brush is again formed on the rotating sleeve 22a by the scooping pole 39 as a two-component developer uniformly charged with an appropriate toner concentration, It is conveyed to 25.

  As described above, the toner remaining on the developing roller 23 is collected on the magnetic roller 22 (rotating sleeve 22a) by the magnetic brush. However, since a developing bias is applied so that the toner moves from the magnetic roller 22 side to the developing roller 23 side during printing, the remaining toner on the developing roller 23 is not completely peeled off, and a part of the developing roller 23 is removed. Remain on top. The toner remaining on the developing roller 23 is mainly fine powder toner having a high charge amount, and it is difficult for the toner to fly to the photosensitive drums 1a to 1d, which may cause a decrease in image density in subsequent printing.

  In the color printer 100 of the present invention, it is possible to perform a residual toner collecting operation in which the toner thin layer remaining on the developing roller 23 is peeled off and collected on the magnetic roller 22 side. The residual toner collecting operation is executed every predetermined number of printed sheets between sheets on which image formation (printing) is not performed. Further, when executing the residual toner collecting operation, Vmag (DC) applied to the magnetic roller 22 and Vslv (applied to the developing roller 23) so that the toner moves smoothly from the developing roller 23 to the magnetic roller 22. DC) is made smaller than that during printing.

  By the way, particularly when a low printing rate image with a low toner consumption is continuously printed, the charge amount of the toner in the developing device increases, and fine powder toner with a high charge amount tends to remain on the developing roller 23. The residual toner amount on the developing roller during the residual toner collecting operation increases with time. Accordingly, in the color printer 100 of the present invention, the layer thickness of the collected residual toner on the developing roller 23 is detected, and ΔV is changed based on the detection result, thereby improving the recoverability of the residual toner from the developing roller 23. ing. A specific execution procedure of the remaining toner collecting operation will be described later.

  FIG. 4 is a block diagram illustrating an example of a control path used in the color printer 100 of the present invention. In addition, since various controls of each part of the apparatus are performed when the color printer 100 is used, the control path of the entire color printer 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 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 in accordance with an output signal from the control unit 90. In accordance with a control signal from the control circuit 41, the charging bias power source 42 is applied to the charging rollers in the charging devices 2a to 2d, the developing bias power source 43 is applied to the magnetic roller 22 and the developing roller 23 in the developing devices 3a to 3d, and the transfer bias power source 44 is supplied. Applies a predetermined bias to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, respectively.

  The in-machine temperature / humidity sensor 45 constantly detects the temperature and humidity inside the color printer 100. The detection result is transmitted to the control unit 90 via an I / F 96 described later.

  The operation unit 50 is provided with a liquid crystal display unit 51 and LEDs 52 that indicate various states, and displays the state of the color printer 100 and displays the image forming status and the number of copies to be printed. Various settings of the color printer 100 are performed from a printer driver of a personal computer.

  In addition, the operation unit 50 includes a start button for instructing the user to start image formation, a stop / clear button used for stopping image formation, and various settings of the color printer 100 when making the default settings. A reset button or the like to be used is provided.

  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 control signals to and receive input signals from the operation unit 50, such as a temporary storage unit 94 for storing image data and the like, a counter 95, and each device in the color printer 100. At least an 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 program for controlling the color printer 100, numerical values necessary for control, and the like that are not changed during use of the color printer 100. The RAM 93 stores necessary data generated during control of the color printer 100, data temporarily required for control of the color printer 100, and the like. Further, the RAM 93 (or ROM 92) collects the magnetic roller 22 and the recovery detected by the image density sensor 40 used to correct the developing bias applied to the developing roller 23 in the residual toner recovery operation on the developing roller 23 described later. A threshold value of the residual toner patch density, a bias correction table indicating the correlation between the collected residual toner patch density and the developing bias are also stored. The temporary storage unit 94 temporarily stores an image signal input from an image input unit (not shown) that receives image data transmitted from a personal computer or the like and converted into a digital signal. The counter 95 counts the accumulated number of printed sheets.

  The control unit 90 transmits a control signal from the CPU 91 to the respective units and devices in the color printer 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 the image forming units Pa to Pd, the exposure device 4, the fixing unit 7, the intermediate transfer belt 8, the secondary transfer roller 9, the image density sensor 40, and the bias control circuit 41. And the operation unit 50.

  Further, when the developing bias correction mode in the residual toner collecting operation on the developing roller 23 is set, the control unit 90 reads the collected residual toner patch density of each color formed on the intermediate transfer belt 8 by the image density sensor 40. The function of detecting the collected residual toner layer thickness on the developing roller 23 based on the output value of the image density sensor 40, and correcting the developing bias applied to the magnetic roller 22 and the developing roller 23 in the residual toner collecting operation according to the detection result It has a function to do. The developing bias applied to the magnetic roller 22 and the developing roller 23 can be adjusted by the developing bias power supply 43 and the bias control circuit 41.

  FIG. 5 is a timing chart showing an operation at the time of continuous printing in the color printer 100 of the present invention, and FIG. 6 is a flowchart showing an execution procedure of continuous printing in the color printer 100 of the present invention. A procedure for adjusting the developing bias according to the collected residual toner layer thickness in the residual toner collecting operation on the developing roller 23 along the steps of FIG. 6, referring to FIGS. 1 to 4 and 5 as necessary. Will be described.

  When a print start command is input from a personal computer or the like (step S1), a predetermined developing bias is applied to the developing devices 3a to 3d by the developing bias power source 43 and the bias control circuit 41. For example, Vslv (AC) of the developing roller 23 has a peak-to-peak value of 1500 V, a positive duty ratio of 37%, and Vslv (DC) is fixed to +50 V. Further, the Vmag (AC) of the magnetic roller 22 is fixed at a peak-to-peak value of 2500 V and a positive duty ratio of 63%, and Vmag (DC) is adapted to adjust the image density according to the density detection value of the image density sensor 40. Set to Usually, it is in the range of about +300 to 400V.

  Thereafter, electrostatic latent images are written on the photosensitive drums 1a to 1d by the exposure device 4, developed into toner images by the developing devices 3a to 3d, and then subjected to primary transfer onto the intermediate transfer belt 8 and onto the transfer paper P. After the secondary transfer, the fixing unit 7 fixes the transfer paper P.

  On the other hand, the counter 95 counts the number of printed sheets n (step S2). The control unit 90 determines whether or not the number of printed sheets n has reached the number of remaining toner collection operations to be performed (here, 4 sheets) (step S3). If the number of remaining toner collection operations has been reached (YES in step S3), then the control unit 90 determines whether or not the number n of printed sheets has reached the measured number of remaining collected toner (here, 100) (in this case, 100). Step S4). When the number of printed sheets n has not reached the collected remaining toner measured number (NO in step S4), only the remaining toner collecting operation is executed (step S5). In the remaining toner collecting operation, as shown in FIG. 5, Vmag (DC) is lowered to + 130V and ΔV is set to 80V.

  On the other hand, if the number of printed sheets n has reached the measured number of collected remaining toner (YES in step S4), first, a remaining toner collecting operation is executed (step S6). Thereafter, patch images (recovered residual toner patch images) are formed on the photosensitive drums 1 a to 1 d using the recovered residual toner on the developing roller 23. The formed residual toner patch image is transferred onto the intermediate transfer belt 8, and the patch image density C is detected by the image density sensor 40 (step S7).

  Since the collected residual toner has a high charge amount and is difficult to be developed to the photosensitive drums 1a to 1d, as shown in FIG. 5, Vslv (DC) is shifted from + 50V at the time of normal residual toner collection to + 100V. Accordingly, Vmag (DC) is also shifted from + 130V to + 180V. As a result, while the ΔV is kept at 80V, the residual toner on the developing roller 23 is reliably developed toward the photosensitive drums 1a to 1d so that the patch image density C can be accurately detected.

  Next, it is determined whether or not the layer thickness of the collected residual toner calculated from the detected patch image density C is within the target range. Specifically, the image density sensor 40 when the layer thickness of the collected residual toner is the lower limit value of the target range using the relationship between the detection value of the image density sensor 40 measured in advance and the layer thickness of the collected residual toner. The threshold value C1 of the image density sensor 40 when the threshold value C1 is the upper limit value is determined, and it is determined whether or not C1 ≦ C ≦ C2 is satisfied (step S8).

  If the layer thickness of the collected residual toner is not within the target range, that is, if the detected value of the patch image density C does not satisfy C1 ≦ C ≦ C2 (NO in step S8), then whether C> C2 or not Is determined (step S9). If C> C2 (YES in step S9), the layer thickness of the collected residual toner is larger than the target range, so the development bias is controlled so as to decrease ΔV (step S10). Specifically, as shown in FIG. 5, by changing Vmag (DC) from +130 V to +100 V, ΔV is reduced from 80 V to 50 V, and the recoverability of residual toner from the developing roller 23 to the magnetic roller 22 is improved. . Then, the number of printed sheets n is reset in order to determine the next collected residual toner measurement timing (step S11).

  On the other hand, when C> C2 is not satisfied (NO in step S9), the layer thickness of the collected residual toner is smaller than the target range (C <C1), so that the developing bias is controlled to increase ΔV (step S12). The number n of prints is reset (step S11). When the layer thickness of the collected residual toner is within the target range (C1 ≦ C ≦ C2) (YES in step S8), the number n of printed sheets is reset without changing ΔV during the remaining toner collecting operation (step S8). S11).

  Thereafter, it is determined whether or not printing has been completed (step S13). If printing has been completed, the process is terminated. If printing continues, the process returns to step S2, and the same processing is repeated thereafter (steps S2 to S13).

  As described above, by performing the control to change the developing bias based on the layer thickness of the collected residual toner on the developing roller 23, when the layer thickness of the collected residual toner is larger than the target range, ΔV is decreased. By changing the developing bias, it is possible to improve the recoverability of the residual toner collected on the developing roller 23. As a result, it is possible to perform printing with a stable image density even when continuously printing low-print-rate images with low toner consumption.

  In addition, if the layer thickness of the collected residual toner on the developing roller 23 is too small, the chance of new toner coming into contact with the developing roller 23 increases. As a result, toner external additives such as resin beads contained in the toner adhere to the surface of the developing roller 23. Since the adhered toner external additive increases the resistance of the developing roller 23, the apparent bias value of Vslv (DC) applied to the developing roller 23 increases (charge accumulation). When image formation is performed in this state, a so-called fog image in which toner adheres to the white solid portions of the photosensitive drums 1a to 1d is generated.

  In the above control, when the layer thickness of the residual toner is smaller than the target range, the developing bias is changed in the direction of increasing ΔV so that the toner on the developing roller 23 is not collected more than necessary.

  In the control shown in FIGS. 5 and 6, Vmag (DC) applied to the magnetic roller 22 and Vslv (DC) applied to the developing roller 23 based on the collected residual toner layer thickness on the developing roller 23. Although the potential difference ΔV is changed, the present invention is not limited to this, and parameters (Duty ratio, Vpp, frequency) of Vslv (AC) applied to the developing roller 23 instead of changing the potential difference ΔV or with the change of the potential difference ΔV. f) may be changed.

  Each parameter will be described with reference to FIG. 7. The ratio (= A / A + B) of the time (A) of the positive waveform with respect to one cycle (A + B) of the alternating waveform of Vslv (AC) is the duty ratio, and the positive potential and The negative potential difference (peak-to-peak value) is Vpp, and Vslv (DC) is set to the area center voltage at which the areas S1 and S2 are equal. Here, other parameters are changed so as to keep Vslv (DC) (= area center voltage) constant.

  When positively charged toner is used, the duty ratio of Vslv (AC) is set so that the positive side becomes larger, for example, 70% on the positive side and 30% on the negative side (A: B = 7: 3). Therefore, when Vslv (DC) is kept constant and Vpp of Vslv (AC) is increased, the negative potential (bottom value) of Vslv (AC) is greatly lowered as shown by the broken line in FIG. 7, and Vslv (AC) And the potential difference between Vmag (DC) becomes smaller. Therefore, the toner easily moves from the developing roller 23 to the magnetic roller 22, and the residual toner recoverability to the magnetic roller 22 is improved. In order to prevent the toner on the developing roller 23 from being collected more than necessary, Vpp may be reduced.

  When the duty ratio is changed, for example, if the positive side is set to 80% and the negative side is set to 20% (A: B = 8: 2), the negative waveform increases as shown by the one-dot chain line in FIG. The ratio of the portion below the voltage V1 (minus side peak voltage) is 80% of the entire waveform (peak to peak value). That is, the electric field that contributes to toner movement to the magnetic roller 22 increases in proportion to the duty ratio. Therefore, by increasing the duty ratio, fine powder toner having a high charge amount can be moved to the magnetic roller 22 side, and an effect equivalent to increasing Vpp can be obtained. In order to prevent the toner on the developing roller 23 from being collected more than necessary, the duty ratio may be reduced.

  When the frequency is increased, Vpp is high and the development time is shortened, and when the frequency is decreased, Vpp is low and the development time is long. Therefore, by increasing the frequency, the same effect as increasing the Vpp or Duty ratio can be obtained. Further, by reducing the frequency, an effect equivalent to reducing the Vpp or Duty ratio can be obtained.

  When the parameter of Vslv (AC) is changed in this way, the oscillating electric field acting between the developing roller 23 and the magnetic roller 22 can be changed, and the charge amount compared to the case where Vslv (DC) is changed. This is advantageous for the movement of high-powder fine toner.

  Further, the number of executions (execution interval) of the remaining toner collecting operation, the collected remaining toner patch image, and the density detection shown in FIG. 5 is appropriately determined according to the specifications of the developing devices 3a to 3d, the use environment, the average printing rate, and the like. Change it. For example, based on at least one of the temperature and humidity inside the color printer 100 detected by the in-machine temperature / humidity sensor 45, the execution interval of the formation of the remaining toner patch image and the density detection may be changed. Specifically, the toner charge amount is high in a low temperature and low humidity environment, and the toner charge amount is low in a high temperature and high humidity environment. The number may be changed to less than 80 (for example, 80).

  Further, based on the toner consumption per predetermined number of printed sheets, the execution interval of the formation of the remaining toner patch image and the density detection may be changed. Specifically, since the toner charge amount increases as the toner consumption amount decreases, the number of executions of the formation of the remaining toner patch image and the density detection may be changed to less than 100 sheets (for example, 80 sheets). The toner consumption amount is obtained from an integrated print rate obtained by adding up the print rates for each image calculated based on the digital signal in the temporary storage unit 94.

  As a result, the interval between the formation of the collected residual toner patch image and the density detection becomes appropriate, and the collection of the collected residual toner patch image and the density detection are frequently performed while improving the recoverability of the collected residual toner on the developing roller 23. It is also possible to suppress an increase in the waiting time of the user due to the execution.

  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 threshold values C1 and C2 of the patch image density shown in the above embodiment, the reference value of the developing bias at the time of printing and the remaining toner collecting operation, the specifications, the usage environment, the average printing rate, etc. of the developing devices 3a to 3d, etc. It may be changed appropriately according to the situation.

  In the above-described embodiment, the recovery residual toner patch image formed on the photosensitive drums 1 a to 1 d is transferred onto the intermediate transfer belt 8, and the patch image density C is detected by the image density sensor 40. Not limited to this, an image density sensor capable of detecting the density of the toner image on each of the photosensitive drums 1a to 1d is provided for each of the photosensitive drums 1a to 1d, and the recovery formed on the photosensitive drums 1a to 1d. The remaining toner patch image density may be directly detected.

  In the above-described embodiment, the intermediate transfer type color printer 100 as shown in FIG. 1 has been described as an example of the image forming apparatus of the present invention. However, the present invention is not limited to this. Development roller (toner carrier) in which a two-component developer is placed in non-contact with a photoreceptor (image carrier) using a magnetic roller (toner supply member), such as a color machine, a monochrome printer, or a direct transfer type color printer When moving up, it is applicable to various image forming apparatuses using a developing system in which only a non-magnetic toner is transferred onto the developing roller while leaving the magnetic carrier on the magnetic roller to form a thin toner layer. . Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

  Using the color printer 100 as shown in FIG. 1, the relationship between the residual toner layer thickness on the developing roller 23 and the image density was investigated. As shown in FIGS. 5 and 6, the test method includes a case where the development bias control is performed during the residual toner collection operation (the present invention) and a case where the development bias control is not performed during the residual toner collection operation (comparative example). The residual toner layer thickness and the image density (ID; image density) when a test image having a printing rate of 1% was continuously printed were measured. The test was performed in a cyan image forming portion Pa including the photosensitive drum 1a and the developing device 3a. The results are shown in FIGS.

FIG. 8 is a graph showing the relationship between the number of printed sheets, the remaining toner layer thickness, and the potential difference ΔV when developing bias control is performed during the residual toner collecting operation, and FIG. 9 shows the relationship between the number of printed sheets and the image density. It is a graph to show. As is apparent from FIGS. 8 and 9, when developing bias control is performed during the remaining toner collecting operation, the remaining toner layer thickness (displayed with a ♦ mark in FIG. 8) starts when the number of printed sheets exceeds 1,500. ) Began to increase from the initial value (0.11 mg / cm ² ), but the potential difference ΔV (represented by □ in FIG. 8) during the residual toner collection operation after 2,000 sheets was printed decreased after 50 V from the initial value 80 V. Further, by reducing the voltage to 20 V after printing 2,500 sheets, the residual toner layer thickness increases only to 0.135 mg / cm ² even after printing 3,000 sheets, and is within an appropriate range (0.10 to 0). .15 mg / cm ² ). Further, the ID value (indicated by ■ in FIG. 9) was 1.25 even after printing 3,000 sheets, and was hardly lowered from the initial value (1.3).

FIG. 10 is a graph showing the relationship between the number of printed sheets, the remaining toner layer thickness, and the potential difference ΔV when developing bias control is not performed during the residual toner collecting operation, and FIG. 11 is the relationship between the number of printed sheets and the image density. It is a graph which shows. As is apparent from FIGS. 10 and 11, when the potential difference ΔV (represented by □ in FIG. 10) during the residual toner collecting operation is maintained at 80 V, the residual toner layer thickness (see FIG. 10) after printing 2,000 sheets. (Indicated by a mark) reached the upper limit (0.15 mg / cm ² ) of the proper range, and increased to 0.18 mg / cm ² after printing 3,000 sheets. Further, the ID value (indicated by ■ in FIG. 11) was 1.1 after printing 3,000 sheets, which was greatly reduced from the initial value (1.3).

  From the above results, it was confirmed that by executing the control of the present invention, it is possible to maintain a stable image density even when an image with a low printing rate is continuously printed. Here, the test was performed in the cyan image forming portion Pa, but it has been confirmed that similar results can be obtained in the magenta, yellow, and black image forming portions Pb to Pd. Further, it has been confirmed that similar results can be obtained when at least one of Vpp, Duty ratio, and frequency of Vslv (AC) applied to the developing roller 23 is changed instead of the potential difference ΔV.

  The present invention uses a two-component developer comprising a magnetic carrier and a toner, and an image provided with a developing device that develops an electrostatic latent image on an image carrier while holding only the charged toner on the toner carrier. It can be used for a forming apparatus. By utilizing the present invention, the residual toner collection condition from the toner carrier to the toner supply member is changed by changing the potential difference ΔV between the toner carrier and the toner supply member according to the amount of residual toner collected on the toner carrier. Therefore, an image forming apparatus capable of preventing a decrease in image density even when a low printing rate image with low toner consumption is continuously printed.

1a to 1d Photosensitive drum (image carrier)
2a to 2d Charging device 3a to 3d Developing device 4 Exposure device 8 Intermediate transfer belt 22 Magnetic roller (toner supply member)
23 Developing roller (toner carrier)
40 Image density sensor (layer thickness detection means)
41 Bias Control Circuit 42 Charging Bias Power Supply 43 Development Bias Power Supply 45 Internal Temperature Humidity Sensor (Internal Temperature Humidity Detection Means)
90 Control unit (control means)
94 Temporary storage unit 95 Counter 100 Color printer Pa to Pd Image forming unit

Claims (8)

An image carrier on which an electrostatic latent image is carried;
A toner carrier for supplying toner to an electrostatic latent image formed on the image carrier that is disposed in contact with the image carrier in a non-contact manner, and a toner that forms a toner layer on the toner carrier using a magnetic brush A developing device having a supply member;
Voltage application means for applying a developing bias in which an AC voltage is superimposed on a DC voltage to the toner supply member and the toner carrier,
In an image forming apparatus that performs a residual toner collecting operation of collecting the residual toner on the toner carrier on the toner supply member during non-image formation,
A layer thickness detecting means for detecting a residual toner layer thickness on the toner carrier;
If the residual toner layer thickness detected by the layer thickness detecting means is outside the predetermined range, the potential difference ΔV of the DC voltage applied to the toner carrier and the toner supply member when the residual toner collecting operation is executed, or An image forming apparatus comprising: a control unit configured to change at least one of the AC voltage applied to the toner supply member so that the remaining toner layer thickness falls within a predetermined range.   When the remaining toner layer thickness detected by the layer thickness detecting unit is larger than a predetermined range, the control unit decreases the ΔV from a reference value when executing the remaining toner collecting operation, and the remaining toner layer thickness is set to a predetermined value. The image forming apparatus according to claim 1, wherein the ΔV is increased from a reference value when the value is smaller than the range.   The control means, when the residual toner layer thickness detected by the layer thickness detection means is larger than a predetermined range, a peak-to-peak value of the AC voltage applied to the toner supply member when the residual toner collecting operation is executed, At least one of the ratio and frequency is made larger than a reference value, and when the residual toner layer thickness is smaller than a predetermined range, at least one of the peak-to-peak value, duty ratio, and frequency is made smaller than the reference value. The image forming apparatus according to claim 1.   2. The detection of the residual toner layer thickness on the toner carrier by the layer thickness detection means is executed at a predetermined timing wider than the execution interval of the residual toner collection operation during non-image formation. The image forming apparatus according to claim 3.   2. The image density sensor according to claim 1, wherein the layer thickness detecting means is an image density sensor that detects a density of a collected residual toner patch image formed on the image carrier using residual toner on the toner carrier. The image forming apparatus according to claim 4.   The image forming apparatus according to claim 5, wherein the voltage applying unit sets a direct current voltage applied to the toner carrier when forming the collected residual toner patch image to be equal to or higher than that during image formation. . In-machine temperature / humidity detection means is provided to detect the temperature and humidity inside the device.
The control unit changes an execution interval of detection of a residual toner layer thickness on the toner carrier by the layer thickness detection unit based on a detection result of the in-machine temperature / humidity detection unit. The image forming apparatus according to claim 6.   The control unit changes an execution interval of detection of a residual toner layer thickness on the toner carrier by the layer thickness detection unit based on a toner consumption amount per predetermined number of printed sheets. Item 8. The image forming apparatus according to Item 7.