JP6155210B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and more particularly, to a developing bias applied to a developing device that is mounted on the image forming apparatus and develops an electrostatic latent image on an image carrier into a toner image. It relates to a control method.

  In the conventional developing device, a developing bias is applied to the developing roller (toner carrier), and the toner is moved to the photosensitive drum side by a developing electric field formed between the developing roller and the photosensitive drum (image carrier). A so-called jumping development method is known.

  In the above developing apparatus, there is a problem that density unevenness due to a change in the gap (development gap) between the developing roller and the photosensitive drum (image carrier), or a density difference occurs between the edge portion of the solid image and its peripheral portion. There was a point.

  Therefore, as a developing bias, a blank pulse bias including a pulse portion that applies a DC voltage superimposed on a DC voltage and a pause portion that applies only a DC voltage by stopping the application of the AC voltage is used. A method for solving the above-described problems has been proposed.

  For example, Patent Document 1 includes a developing bias applying unit that applies an oscillating voltage obtained by superimposing an AC voltage on a DC voltage to a developing unit included in a process cartridge, and according to the usage amount (cumulative print number) of the process cartridge. It has been proposed to make corrections in a direction that promotes image forming conditions by making the period for setting the AC voltage pause time variable.

  Further, Patent Document 2 includes a magnetic roller that carries a two-component developer and supplies only the toner in the two-component developer to the developing roller, and the developing direction of the alternating voltage oscillation portion (pulse portion) of the blank pulse bias. The voltage value in the recovery direction (the direction in which the toner moves from the photosensitive drum side to the developing roller side) in a state where the voltage in the direction (the direction in which the toner moves from the developing roller side to the photosensitive drum side) is fixed. Proposed to change.

JP-A-10-39719 JP 2008-225229 A

  By the way, in the image forming apparatus, in particular, when the thickness of the recording medium is large, in order to ensure sufficient fixability and improve the image quality, the driving speed of each part of the apparatus according to the thickness and type of the recording medium to be conveyed ( There is known an image forming apparatus in which the process speed can be switched in a plurality of stages. That is, when the recording medium is plain paper, the image forming process is performed at a normal driving speed, and when the recording medium is thick paper, the image forming process is performed at a lower speed than usual.

  Here, when the driving speed of the developing device is made lower than usual, the time for the toner carried on the developing roller to pass through the portion facing the photosensitive drum (developing nip) becomes longer. As a result, the development time becomes excessive, and the density difference between the edge portion of the solid image and the other portions becomes large.

  In view of the above-described problems, the present invention provides a density unevenness due to a change in the development gap over a long period of time and a solid image edge portion regardless of the process speed when a blank pulse bias is used as the development bias applied to the development roller. An object of the present invention is to provide an image forming apparatus capable of suppressing both density differences.

  In order to achieve the above object, a first configuration of the present invention is an image forming apparatus including an image carrier, a developing device, a voltage applying device, a driving mechanism, and a control unit. An electrostatic latent image is formed on the image carrier. The developing device is disposed opposite to the image carrier, and includes a toner carrier that carries toner on the surface. By applying a developing bias to the toner carrier, the toner carried on the toner carrier is transferred to the image carrier side. The electrostatic latent image formed on the surface of the image carrier is developed into a toner image. The voltage application device applies a blank pulse bias including a pulse portion that applies an AC voltage having a rectangular waveform and a pause portion that does not apply an AC voltage as a developing bias to the toner carrier. The drive mechanism drives each part of the apparatus including the developing device. The control unit controls the drive mechanism and the voltage application device. The control unit controls the drive mechanism to change the process speed for performing the image forming process, and varies the ratio of the pulse part to the pause part in the developing bias according to the change in the process speed.

  According to the first configuration of the present invention, when a blank pulse bias is used as the developing bias, appropriate developability according to the process speed is ensured. Generation of density unevenness and density difference between the edges of a solid image can be effectively suppressed over a long period of time.

1 is a schematic cross-sectional view illustrating an overall configuration of a color printer 100 according to an embodiment of the present invention. Sectional drawing of the developing device 3a used for the color printer 100 of this invention 1 is a block diagram showing a control path of a color printer 100 of the present invention. The graph which shows an example of the waveform P of the developing bias applied to the developing roller 23 Sectional drawing which shows the other structural example of the voltage application apparatus 43 which applies a bias to the developing roller 23 and the magnetic roller 22 of the developing device 3a used for the color printer 100 of this invention. The graph which shows the density difference d1 of the peak part and flat part of a density change of a solid image in an Example. A graph showing the density change d2 during one rotation of the developing roller 23 in the example.

  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 an embodiment of the present invention. Here, a tandem color printer is illustrated. 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 further rotate clockwise in FIG. An intermediate transfer belt 8 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 the transfer sheet S at a time, and further fixed onto the transfer sheet S 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 sheet S on which the toner image is transferred is accommodated in a sheet cassette 16 below the main body of the color printer 100, and is conveyed to the secondary transfer roller 9 via the sheet 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 charging devices 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, which 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 charging devices 2a to 2d, and then light is irradiated by the exposure device 4 to each photosensitive member. Electrostatic latent images corresponding to image signals are formed on the drums 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 rotates the driving roller 11 by the main motor 45 (see FIG. 3). When the intermediate transfer belt 8 starts to rotate in the clockwise direction, the transfer sheet S 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 obtained. Is transcribed. The transfer sheet S on which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer sheet S conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 to fix the toner image on the surface of the transfer sheet S, thereby forming a predetermined full color image. The transfer sheet S on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the transfer paper S, the image is directly discharged 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 sheet S, a part of the transfer sheet S 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 sheet S is distributed to the sheet conveyance path 18 by the branching section 14 by rotating the discharge roller pair 15 in the reverse direction, and is re-conveyed to the registration roller pair 12b with the image surface reversed. 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 S where the image is not formed, and conveyed to the fixing unit 7 and the toner image is fixed. The paper is discharged to the discharge tray 17 through the discharge roller pair 15.

  Further, an image density sensor 46 is disposed at a position facing the driving roller 11 with the intermediate transfer belt 8 interposed therebetween. As the image density sensor 46, 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 generally 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.

  The reflected light from the toner and the belt surface includes regular reflection light and irregular reflection light. The specularly reflected light and irregularly reflected light are separated by the polarization separation prism and then incident on separate light receiving elements. Each light receiving element photoelectrically converts the received regular reflection light and irregular reflection light and outputs an output signal to the control unit 90 (see FIG. 3). Then, the toner correction is performed for each color by detecting the toner amount from the change in the characteristics of the output signals of the regular reflection light and the irregular reflection light, and adjusting the development bias characteristic value and the like in comparison with a predetermined reference density. .

  FIG. 2 is a side sectional view showing a configuration of the developing device 3a mounted on the color printer 100 of the present invention. 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 source 43 is connected to the developing roller 23 and the magnetic roller 22 via a bias control circuit 41 (see FIG. 3). Specifically, the developing roller 23 is connected to a first power source 43a including a DC power source and an AC power source, and the magnetic roller 22 is connected to a second power source 43b including a DC power source and an AC power source. As a result, a developing bias in which an AC voltage is superimposed on a DC voltage is applied to the developing roller 23, and a supply bias in which an AC voltage is superimposed on a DC voltage is applied to the magnetic roller 22. The developing bias includes a pulse part that applies a rectangular waveform AC voltage to the DC voltage, and a pause part that pauses the application of the AC voltage. A specific waveform of the developing bias will be described later.

  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 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 image input unit 40 is a receiving unit that receives image data transmitted to the color printer 100 from a personal computer or the like. 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 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 main motor 45 is connected to the photosensitive drums 1a to 1d, the developing devices 3a to 3d, the primary transfer rollers 6a to 6d, the fixing unit 7, the secondary transfer roller 9, the driving roller 11, and the conveying rollers of each unit through a gear train. And each part of the apparatus is driven based on a control signal from the control unit 90. In the present invention, the driving speeds of the respective units including the developing devices 3a to 3d and the fixing unit 7 can be switched according to the thickness and type of the recording medium.

  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. The RAM 93 (or ROM 92) also stores the ratio of the developing bias pulse portion to the rest portion that changes according to the process speed, as will be described later. 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.

  FIG. 4 is a graph showing an example of a waveform P of the developing bias applied to the developing roller 23 in the developing devices 3a to 3d. As shown in FIG. 4, the development bias waveform P is a blank pulse bias having a pulse portion T <b> 1 that applies an AC bias having a rectangular waveform and a pause portion T <b> 2 that stops applying an AC bias.

  The waveform P of the development bias is set to 50% or more in the direction in which the electrostatic latent image is developed with toner, for example, as shown in FIG. That is, in the pulse part T1 for applying the AC voltage, the voltages P11 and P12 in the direction P1 (hereinafter referred to as the first direction) P1 in which the electrostatic latent images formed in advance on the surfaces of the photosensitive drums 1a to 1d are developed. The application time T3 is equivalent to the application time T4 of the voltages P21 and P22 in the direction P2 in which the toner is pulled back to the developing roller 23 (hereinafter referred to as the second direction). Thus, when T3 is a developing bias equal to or greater than T4, even when the developing gap is widened, a large amount of toner can reach the photosensitive drum 37, so that image unevenness when the developing gap widens, In particular, image unevenness due to a decrease in the density of a halftone image can be suppressed.

  The waveform P of the developing bias has an AC voltage cycle number (pulse number) of 2 or more in the pulse portion T1, and is set to 2 cycles as shown in FIG. 4, for example. When such a developing bias is used, the voltages P11 and P12 in the first direction P1 and the voltages P21 and P22 in the second direction P2 are alternately applied twice or more. As a result, the toner adhering to the latent image portion is alternately subjected to the force in the direction in which the photoconductive drums 1a to 1d are firmly attached and the force in the direction of peeling off from the photoconductive drums 1a to 1d. The rearrangement of the toner advances, and the toner adheres uniformly to the latent image portion. Further, the toner adhering to the non-latent image portion (background portion) is reattached to the latent image portion by the above-described two types of forces acting alternately. Therefore, a more faithful toner image corresponding to the shape of the electrostatic latent image is formed.

  Further, in the waveform P of the developing bias, the AC voltage P22 immediately before the transition to the pause portion T2 of the pulse portion T1 is a voltage in the second direction P2. When such a developing bias is used, immediately after the force in the direction of pulling the toner back to the developing roller 23 is applied, a transition is made to the resting portion T2 where no AC voltage is applied. As a result, the force in the direction in which the toner moves from the photoconductive drums 1a to 1d to the developing roller 23 can be applied for a long time with such a weak force that the toner attached to the latent image portion is not peeled off. Only the unnecessary toner adhering to the non-latent image portion can be peeled off without peeling off the toner adhering to the toner image, and image fog can be suppressed. Therefore, it is possible to suppress image unevenness when the development gap is narrowed, particularly image unevenness due to peeling of a solid image.

  As described above, the developing devices 3a to 3d can effectively suppress the occurrence of image unevenness due to the variation in the developing gap by applying the blank pulse bias as described above as the developing bias.

  The development bias may be only an AC voltage or a superimposed voltage obtained by superimposing an AC voltage on a DC voltage as long as the AC voltage is as described above. The voltage value of the DC voltage is preferably lower than the absolute values of the voltages P21 and P22 in the second direction P2 of the AC voltage, and is preferably 150 V or less, for example. The peak-to-peak value of the AC voltage is preferably 500 to 2000V, and is set to 1200V or 1500V, for example. Moreover, it is preferable that the frequency of an alternating voltage is 1-6 kHz.

  As a preferred specific example of the developing bias, for example, as shown in FIG. 4, the interval (duration) between the pulse part T1 and the rest part T2 is equal, the duty ratio (T3 / (T3 + T4)) = 0.5, the maximum voltage. V1 = 1000V, the minimum voltage V2 = −200V, the voltage V3 of the pause portion T2 is 0V, and the number of cycles is two cycles.

  In the color printer 100 of the present invention, as described above, the image processing speed (process speed) is switched between three levels according to the thickness and type of the transfer sheet S being conveyed. That is, when the transfer paper S is plain paper, image formation processing is performed at a normal driving speed (full speed mode), and when the transfer paper S is thick paper, the normal 3/4 speed (intermediate speed mode) or 1 The image forming process is performed at the second speed (half speed mode). Thereby, when using thick paper, it is possible to secure a sufficient fixing time and improve the image quality.

  On the other hand, when the developing devices 3a to 3d are driven together with the fixing unit 7 using the main motor 45 as described above, when the full speed mode is switched to the intermediate speed mode or the deceleration mode, the driving speeds of the developing devices 3a to 3d ( The rotation speed of the developing roller 23) is also 3/4 or 1/2 of the full speed mode. As a result, since the time on which the toner on the developing roller 23 passes through the developing nip portion becomes long, the developing time becomes excessive, and the density difference becomes large between the edge portion of the solid image and the other portion, so-called rear end. Accumulation occurs.

  The above phenomenon is that the optimum setting of the time ratio (hereinafter referred to as the blank ratio) between the pulse part T1 and the pause part T2 differs according to the process speed in each use mode (full speed mode, intermediate speed mode or half speed mode). Is the cause.

  Therefore, in the present invention, the blank ratio of the developing bias is changed according to the process speed. Specifically, the time ratio of the pause portion T2 is increased stepwise as the process speed decreases in the order from the full speed mode to the intermediate speed mode and the half speed mode. According to the above control, an appropriate development time corresponding to the process speed is secured, so that the occurrence of rear end accumulation can be effectively suppressed.

  Further, if the time ratio of the pause portion T2 becomes too large, density unevenness that occurs in the cycle of the pause portion T2 may become noticeable. Therefore, the interval of the pause portion T2 is set to be equal to or less than the maximum ratio at which the density unevenness does not occur in the cycle of the pause portion T2 at each process speed, and appropriate suppression of rear end accumulation and density unevenness can be achieved. It is preferable to set the length.

  Note that if the blank ratio is changed by changing the interval of the pulse portion T1, the developability may be affected. Therefore, it is preferable to change the blank ratio by changing only the interval of the pause portion T2 without changing the interval of the pulse portion T1.

  FIG. 5 is a side sectional view showing another configuration example of the developing device 3a mounted on the color printer 100 of the present invention. Portions common to FIG. 2 are denoted by the same reference numerals and description thereof is omitted. As shown in FIG. 5, the voltage application device 43 is provided to be branched from the first power source 43 a connected to the developing roller 23, and between the first power source 43 a and the developing roller 23, and connected to the magnetic roller 22. A second power source 43b (dual bias control), and the first power source 43a and the second power source 43b are grounded to a common ground.

  According to this configuration, a developing bias in which an AC voltage is superimposed on a DC voltage is applied to the developing roller 23 from the first power supply 43a. On the other hand, a bias obtained by superimposing an AC voltage on a DC voltage from the second power source 43b is applied to the magnetic roller 22 as a supply bias based on a bias applied from the first power source 43a.

  As shown in FIG. 2, in the method (single bias control) in which the first power source 43 a and the second power source 43 b are separately connected to the developing roller 23 and the magnetic roller 22, respectively, between the developing roller 23 and the magnetic roller 22. The duty ratio of the bias waveform (development bias and supply bias combined waveform) applied between (MS) and the bias waveform (development bias) applied between the developing roller 23 and the photosensitive drum 1a (between DS). The duty ratio of (waveform) needs to be set to be inverted so that it becomes 100% in total, and if one duty ratio is to be set high, the other has to be set low.

  On the other hand, according to the dual bias control shown in FIG. 5, the bias waveform applied between the MSs (the combined waveform of the development bias and the supply bias) becomes equal to the bias waveform of the second power supply 43b, and the first power supply 43a Not affected by bias. Further, the bias waveform applied between the DSs can be controlled only by the bias of the first power supply 43a without being influenced by the second power supply 43b.

  That is, the bias waveform between the DS and the bias waveform between the MS can set the voltage and duty ratio of each bias independently of each other. For this reason, the duty ratio between the MSs is increased (for example, 70%) so that the formation of a thin toner layer from the magnetic roller 22 to the developing roller 23 and the toner recovery from the developing roller 23 to the magnetic roller 22 are maintained well. ) By keeping the duty ratio between DS large (for example, 50%) while maintaining, it is possible to increase the time for the toner to fly to the photosensitive drum 1a, and to improve the developability. Further, since the voltage (Vmin) in the direction in which the toner is pulled back to the developing roller 23 increases to the minus side, the reciprocating motion (toner cloud) of the toner can be increased.

  Also in the configuration shown in FIG. 5, as the process speed decreases in the order from the full speed mode to the intermediate speed mode and the half speed mode, the time ratio of the pause unit T2 is increased stepwise so that an appropriate value corresponding to the process speed is obtained. Since the development time is secured, the occurrence of rear end accumulation can be effectively suppressed. In addition, by setting the interval of the pause portion T2 to such a length that the density unevenness that occurs in the cycle of the pause portion T2 is not noticeable, it is possible to achieve both suppression of rear end accumulation and suppression of density unevenness. . The blank ratio is preferably changed by changing only the interval of the pause portion T2 without changing the interval of the pulse portion T1.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in each of the above-described embodiments, the present invention uses a two-component developer, forms a magnetic brush on the magnetic roller 22, moves only the toner from the magnetic roller 22 to the developing roller 23, and transfers the toner from the developing roller 23 to the photoconductor. Although the present invention is applied to the developing devices 3a to 3d of the non-contact developing method that causes the toner to fly to the drums 1a to 1d, the developing roller 23 is provided without using the magnetic roller 22, and the developing roller 23 is used with a one-component developer. The present invention can also be applied to a non-contact developing type developing device in which a developer layer (toner layer) formed on the photosensitive drums 1a to 1d is caused to fly.

  Further, the present invention is not limited to the color printer 100 as shown in FIG. 1, and development by a non-contact development system such as a monochrome printer, a monochrome copying machine, a digital multifunction machine, a tandem or rotary color copying machine, or a facsimile machine. The present invention can be applied to various image forming apparatuses provided with the apparatus. Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

  A color printer 100 as shown in FIG. 1 equipped with the developing devices 3a to 3d shown in FIG. 2 is provided in a full speed mode, an intermediate speed mode (3/4 speed of the full speed mode), a half speed mode (1 in the full speed mode). / 2 speed) when using each mode, the edge of the solid image when the time ratio T1: T2 (blank ratio) between the pulse portion T1 and the rest portion T2 of the developing bias shown in FIG. 4 is changed. The occurrence of density unevenness due to the density difference in the area and the change in the development gap was investigated. In the test, in the cyan image forming portion Pa including the photosensitive drum 1a and the developing device 3a, the process speeds (photosensitive drum peripheral speeds) in the full speed mode, the intermediate speed mode, and the half speed mode are 180 mm / sec and 135 mm, respectively. / Sec and 90 mm / sec.

  As the conditions of the testing machine, a photosensitive drum 1a having an amorphous silicon layer having a thickness of 20 μm as a photosensitive layer was used. Further, the developing bias applied to the developing roller 23 was applied by superimposing an alternating voltage of 70V DC voltage with a peak-to-peak value (Vpp) = 1500V, a duty ratio of 0.5, and a frequency of 4700 Hz at an interval of two pulses. This is a blank pulse bias composed of a pulse part T1 and a pause part T2 to which an alternating voltage is turned off and only a DC voltage of 70V is applied.

  Then, the interval of the pause portion T2 in the full speed mode (default) is set to two pulses of the AC waveform (blank ratio 2: 2), and the blank ratio is changed from 5: 1 to 5: 5 by changing the interval of the pause portion T2. Then, the density difference at the edge portion of the solid image and the occurrence of density unevenness due to the fluctuation of the development gap were evaluated.

  As an evaluation method of the density difference at the edge portion of the solid image, a solid image with a printing rate of 100% is printed, and as shown in FIG. 6, the density difference between the peak portion and the flat portion of the density change of the solid image in the transport direction. Measure d1. Then, the case where the density difference d1 is equal to or less than a predetermined threshold that can be visually recognized is indicated by ◯, the case where the density difference d1 exceeds the threshold and is slightly conspicuous visually, Δ, and the case where the density difference d1 greatly exceeds the threshold and is conspicuous visually.

  As a method for evaluating density unevenness caused by fluctuations in the development gap, a halftone image with a printing rate of 25% is printed, and as shown in FIG. 7, the density change d2 is measured while the developing roller 23 makes one round. Then, when the density change d2 and the density unevenness are not more than a predetermined threshold that can be visually recognized, ◯, when the density change d2 exceeds the threshold and the density unevenness is slightly noticeable visually, Δ, when the density unevenness is greatly exceeded, the density unevenness is visually observed. The case where it was conspicuous was marked with x. Tables 1 to 3 show the results in the full speed mode, the intermediate speed mode, and the half speed mode, respectively.

  As is apparent from Tables 1 to 3, in the intermediate speed mode and the half speed mode, a density difference (rear end accumulation) is more likely to occur in the edge portion of the solid image than in the full speed mode, and the pause portion T2 As the time ratio was increased, the density difference (rear end accumulation) at the edge portion of the solid image was eliminated. This is a setting to suppress the developability downstream of the developing nip when the time ratio of the pause portion T2 is increased, and the toner floating between the developing roller 23 and the photosensitive drum 1a jumps to the photosensitive drum 1a side. This is because excessive development (suction phenomenon) at the rear end portion of the solid image is suppressed.

  On the other hand, in the full speed mode and the intermediate speed mode, the density unevenness due to the development gap fluctuation becomes more prominent as the time ratio of the pause portion T2 increases. Further, in the half speed mode, density unevenness due to development gap fluctuation was not recognized even when the time ratio of the resting portion T2 was changed. From this, it can be seen that as the process speed increases, the density unevenness that occurs in the period of the pause portion T2 becomes more conspicuous as the time ratio of the pause portion T2 increases.

  From the above results, in each of the full speed mode, the intermediate speed mode, and the half speed mode, the blank pulse bias used as the developing bias is optimally set (5: 1 to 5: 2 in the full speed mode, and in the intermediate speed mode). 5: 3, 5: 4 to 5: 5) in the half-speed mode, it was confirmed that both the suppression of the density difference at the edge portion of the solid image and the density unevenness due to the development gap fluctuation can be effectively suppressed. . Here, the test was performed using 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.

  The present invention is applicable to an image forming apparatus that uses a blank pulse bias as a developing bias and can change a process speed. By utilizing the present invention, it is possible to provide an image forming apparatus that can effectively suppress the occurrence of density unevenness due to a change in the development gap and the density difference between the edges of a solid image over a long period of time, regardless of changes in the process speed. Become.

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d charging device 3a to 3d developing device 4 exposure device 7 fixing unit 22 magnetic roller (developer carrier)
23 Developing roller (toner carrier)
41 Bias control circuit (voltage application device)
43 Development bias power supply (voltage application device)
43a 1st power supply 43b 2nd power supply 45 Main motor (drive mechanism)
90 Control unit (control means)
100 color printer (image forming device)

Claims (5)

An image carrier on which an electrostatic latent image is formed;
A toner carrier that is disposed opposite to the image carrier and carries toner on the surface thereof, and by applying a developing bias to the toner carrier, the toner carried on the toner carrier is transferred to the image carrier side. A developing device that develops the electrostatic latent image formed on the surface of the image carrier into a toner image;
A voltage application device that applies a blank pulse bias including a pulse part that applies an alternating voltage of a rectangular waveform and a pause part that does not apply an alternating voltage to the toner carrier as a developing bias;
A drive mechanism for driving each part of the apparatus including the developing device;
A control unit for controlling the driving mechanism and the voltage application device;
In an image forming apparatus comprising:
An amorphous silicon photosensitive layer having a thickness of 20 μm is formed on the surface of the image carrier,
The developing bias includes a pulse voltage obtained by superimposing the AC voltage having a peak-to-peak value of 1500 V, a duty ratio of 0.5, and a frequency of 4700 Hz on a DC voltage of 70 V at an interval of two pulses, and turning off the AC voltage. And the blank pulse bias including the pause unit to which only the DC voltage is applied,
The control unit controls the driving mechanism to perform image forming processing at a full speed mode of 180 mm / sec, an intermediate speed mode of 135 mm / sec, and a half speed mode of 90 mm / sec. As well as change
The ratio of the pulse part to the pause part when the process speed is set to the full speed mode is 5: 1 to 5: 2, and the ratio of the pulse part to the pause part when the intermediate speed mode is set to 5: 3. An image forming apparatus characterized in that a ratio of the pulse part to the pause part in the half speed mode is set to 5: 4 to 5: 5 . The control unit varies the ratio of the pulse part to the pause part in the developing bias by changing only the duration of the pause part without changing the duration of the pulse part. Item 2. The image forming apparatus according to Item 1. The developing device further includes a developer carrier that carries a two-component developer including a magnetic carrier and a toner, and the toner is formed using a magnetic brush formed of the two-component developer on the developer carrier. The image forming apparatus according to claim 1 , wherein a toner layer is formed on the carrier . The image forming apparatus according to claim 3, wherein the voltage application device applies a rectangular waveform AC voltage to the developer carrier or a rectangular waveform AC voltage superimposed on the DC voltage . The voltage application device is:
A first power source connected to the toner carrier and applying a bias, in which a rectangular waveform AC voltage is superimposed on a DC voltage, to the toner carrier;
A first branch is provided between the first power source and the toner carrier and is connected to the developer carrier, and applies a bias in which a rectangular waveform AC voltage is superimposed on a DC voltage to the developer carrier. Two power supplies,
With
The image forming apparatus according to claim 4, wherein a bias of the second power supply is applied to the developer carrying member so as to overlap a bias of the first power supply .