JP2012027249A - Developing device and image forming device equipped therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a developing device capable of forming a toner image of high image quality by suppressing the occurrence of leakage and an image forming device equipped therewith.SOLUTION: A developing device 122 comprises a toner carrier 43, bias application means 57 for applying a developing bias to the toner carrier 43, leakage generating means 57 for applying a leakage generation bias for generating voltage leakage between the toner carrier 43 and a predetermined image carrier 121 to the toner carrier 43, and a control part U for switching between a leakage generation mode for controlling the leakage generating means 57 to generate the voltage leakage and a developing mode for setting the developing bias based on the leakage generation bias and controlling the bias application means 57 to apply the developing bias to the toner carrier 43. The control part U comprises a state detection part 74 for obtaining state information required for setting the application time of the leakage generation bias and a setting part 75 for setting the application time according to the state information in the leakage generation mode.

Description

  The present invention relates to a developing device used for electrophotographic image formation and an image forming apparatus including the developing device.

  An image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system supplies a developer to an electrostatic latent image formed on an image carrier (for example, a photosensitive drum or a transfer belt) and supplies the developer. A developing device that forms a toner image on the image carrier by developing the electrostatic latent image is included.

  The developing device includes, as main components, a storage unit that stores a developer containing toner, a magnetic roller that receives the developer from the storage unit and carries a developer layer (so-called magnetic brush layer), and a magnetic roller. The toner is received from the magnetic brush layer on the toner carrying surface based on the potential difference between the toner carrying surface, and then the toner is supplied to the image carrier based on the potential difference between the toner and the toner image to develop the toner image. And a developing roller for applying a bias to the magnetic roller and the developing roller.

  In the developing device having the above-described configuration, when the AC bias applied to the developing roller is increased, it becomes easy to supply an amount of toner necessary for forming a toner image to the image carrier. However, if the AC bias is simply increased, the potential difference between the peak value of the AC bias and the surface potential of the image carrier increases, and a leak (air discharge) occurs between the image carrier and the developing roller. There is a fear. When the leak occurs, a so-called development leak occurs, in which random black spots (noise) are generated on the image carrier, and a toner image with good image quality is not formed. On the other hand, if the AC bias is lowered in order to avoid development leakage, it becomes difficult to supply an amount of toner necessary for forming a toner image to the image carrier, and image density unevenness occurs in the toner image. As a result, a good quality toner image is not formed.

  Therefore, the developing device needs to set the AC bias so that development leakage and image density unevenness do not occur. As a technique related to setting of such an AC bias, the one of Patent Document 1 is known. The developing device of Patent Document 1 includes a leak generating unit that applies an AC bias to the developing roller to generate a leak between the image carrier and the developing roller, and a leak detecting unit that detects the leak. Based on the AC bias generated in the above, an optimum AC bias that does not cause development leakage and image density unevenness is to be set.

Japanese Patent Laid-Open No. 2003-287942

  If the developing device is used for a long period of time or the number of printed sheets increases, toner adheres to the toner carrying surface of the developing roller or an external additive peeled off from the toner adheres to the toner carrying surface. The capacitive component changes. For this reason, when the developing device starts the developing operation, electric charges are gradually accumulated on the toner carrying surface. Charge accumulation increases the surface potential of the toner carrying surface, and the potential difference between the peak value of the AC bias applied to the developing roller and the surface potential of the image carrier becomes larger than a predetermined value. Therefore, even if a leak is intentionally generated and an optimum AC bias is set as in the developing device of Patent Document 1, the potential difference becomes larger than a predetermined value as the charge is accumulated. Leakage is likely to occur between the developing roller. As a result, a good quality toner image is not formed.

  Therefore, in view of the above circumstances, an object of the present invention is to provide a developing device capable of forming a toner image with good image quality while suppressing occurrence of a leak, and an image forming apparatus including the developing device. .

  In order to achieve the above object, a developing device according to the present invention includes a storage unit that stores a developer containing toner, a toner carrying surface that receives and carries the toner from the storage unit, and the toner carrying surface. The toner is supplied from a toner carrier to a predetermined image carrier to develop a toner image on the predetermined image carrier, and a developing bias is applied to the toner carrier to remove the toner from the toner carrier surface. Bias applying means for supplying to the predetermined image carrier, and leak generating means for applying a leak generation bias for generating a voltage leak between the toner carrier and the predetermined image carrier to the toner carrier; A leak generation mode for controlling the leak generation means to generate the voltage leak, and setting the developing bias based on the leak generation bias, Controlled to a and a control unit that performs a developing mode for applying the developing bias to the toner carrier. The control unit includes a state detection unit that acquires state information necessary for setting the application time of the leak generation bias in the leak generation mode, and a setting unit that sets the application time according to the state information.

  When the amount of charge accumulated on the toner carrying surface by the developing operation of the toner image increases, the time required to generate a voltage leak becomes long even when the same leak generation bias is applied. Therefore, if the application time of the leak generation bias is fixed or set short, a voltage leak cannot be generated. However, according to the developing device of the present invention, the control unit acquires state information necessary for setting the application time of the leakage occurrence bias in the leakage generation mode, and sets the application time according to the state information. Even if the amount of stored charge on the support surface increases, voltage leakage can occur. As a result, the development bias set by the control unit based on the leak occurrence bias in the development mode is an optimum bias that does not cause voltage leak. As a result, a good quality toner image is formed.

  In a preferred embodiment of the present invention, the state detection unit acquires the number of development operations of the toner image as the state information, and the setting unit detects the leak as the number of development operations of the toner image increases. The application time of the generation bias is increased.

  As the number of toner image development operations increases, the amount of charge accumulated on the toner carrying surface increases. However, according to the above configuration, as the number of development operations increases, that is, as the amount of accumulated charge increases, the application time of the leak generation bias increases, so that a leak can be generated.

  In another preferred embodiment of the present invention, the state detection unit is a counter that counts the number of development operations, and the control unit performs the leak generation mode every predetermined number of development operations.

  According to the above configuration, since the counter is used as the state detection unit, it is easy for the control unit to set the leak occurrence mode at an optimal timing.

  An image forming apparatus according to the present invention includes an image carrier on which a toner image is formed, a developing device that supplies toner to the image carrier and develops the toner image on the image carrier, and the toner image A developing device having the above-described configuration is used as the developing device, including a transfer member to be transferred onto the sheet and a fixing unit for fixing the toner image on the sheet onto the sheet.

  According to the developing device and the image forming apparatus, a leak generation bias is reliably generated, and an optimal development bias is set based on the leak generation bias, so that a toner image with good image quality can be formed by suppressing the occurrence of the leak. Is possible.

1 schematically shows an internal structure of an image forming apparatus according to an embodiment of the present invention. It is sectional drawing which shows the internal structure of a developing device roughly. It is a schematic diagram explaining the developing operation and control configuration of the developing device. It is a figure for demonstrating generation | occurrence | production of a leak. It is a block diagram which shows the structure of the control part of a developing device. It is a figure which shows the result of the experiment conducted in order to find the optimal application time according to status information, and has shown the relationship between application time and alternating current bias for every predetermined | prescribed number of printed sheets. It is a figure which shows the result of the experiment conducted in order to find the optimal application time according to status information, and has shown the relationship between application time and alternating current bias for every predetermined | prescribed number of printed sheets. It is a figure which shows the result of the experiment conducted in order to find the optimal application time according to status information, and has shown the relationship between application time and alternating current bias for every predetermined | prescribed number of printed sheets. It is a figure which shows the result of the experiment conducted in order to find the optimal application time according to status information, and has shown the relationship between application time and alternating current bias for every predetermined | prescribed number of printed sheets. It is a figure which shows the difference of the application time between the fixed structure example which fixed this invention and application time. It is a control flow figure of a control part.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

  FIG. 1 schematically shows an internal structure of the image forming apparatus according to the present embodiment. The image forming apparatus 10 is, for example, a tandem color printer, and includes an apparatus main body 11 having a box shape. The apparatus main body 11 includes an image forming unit 12 that forms an image based on image information transmitted from an external device such as a computer, and an image formed by the image forming unit 12 and transferred to a sheet P (sheet). A fixing unit 13 that performs a fixing process and a paper storage unit 14 that stores the transfer paper P are included. A paper discharge unit 15 for discharging the paper P after the fixing process is provided on the upper portion of the apparatus main body 11.

  The image forming unit 12 forms a toner image on the paper P fed from the paper storage unit 14. In the present embodiment, the image forming unit 12 is directed from the upstream side (right side of the paper surface in FIG. 1) to the downstream side. Sequentially arranged magenta unit 12M using magenta toner (developer), cyan unit 12C using cyan toner, yellow unit 12Y using yellow toner, and black toner And a black unit 12K.

  Each unit 12M, 12C, 12Y, 12K includes a photosensitive drum 121 and a developing device 122. The photosensitive drum 121 forms an electrostatic latent image and a toner image (visible image) along the electrostatic latent image on the peripheral surface, and corresponds to the development while rotating counterclockwise in FIG. Toner is supplied from the device 122. Each developing device 122 is supplied with toner from a toner cartridge (not shown) mounted in the apparatus main body 11.

  A charging device 123 is provided immediately below each photosensitive drum 121, and an exposure device 124 is provided below each charging device 123. The circumferential surface of each photosensitive drum 121 is uniformly charged by a charging device 123, and laser light corresponding to each color based on image data input from a computer or the like is charged from the exposure device 124 to the circumference of the photosensitive drum 121 after charging. The surface is irradiated. Thereby, an electrostatic latent image is formed on the peripheral surface of each photosensitive drum 121. When the developing device 122 supplies toner to the electrostatic latent image, the toner image is developed on the peripheral surface of the photosensitive drum 121.

  Above the photosensitive drum 121, a transfer belt 125 is disposed between the driving roller 125a and the driven roller 125b. The transfer belt 125 is pressed against the circumferential surface of the photosensitive drum 121 by a transfer roller 126 provided corresponding to each photosensitive drum 121, and is driven with a driving roller 125 a and a driven roller in synchronization with the photosensitive drum 121. Circulate between 125b.

  Along with the rotation of the transfer belt 125, first, primary transfer of a magenta toner image is performed on the surface of the transfer belt 125 by the photosensitive drum 121 of the magenta unit 12M. Next, the cyan toner image is transferred to the transfer position of the magenta toner image on the transfer belt 125 by the photosensitive drum 121 of the cyan unit 12C in an overcoated state. Similarly, the transfer of the yellow toner image by the yellow unit 12Y and the transfer of the black toner image by the black unit 12K are performed in the overcoating state. As a result, a color toner image is formed on the surface of the transfer belt 125.

  A drum cleaning device 127 that removes and cleans residual toner on the peripheral surface of the photosensitive drum 121 is provided at the left position of each photosensitive drum 121 in FIG. The peripheral surface of the photosensitive drum 121 cleaned by the drum cleaning device 127 goes to the charging device 123 for a new charging process. Waste toner removed from the peripheral surface of the photosensitive drum 121 by the drum cleaning device 127 is collected in a toner collection bottle (not shown) through a predetermined path.

  At the left position of the image forming unit 12 in FIG. 1, a paper conveyance path 111 extending in the vertical direction is formed. A conveyance roller pair 112 is provided at an appropriate position on the sheet conveyance path 111, and the sheet P from the sheet storage unit 14 is conveyed toward the transfer belt 125 by driving the conveyance roller pair 112. In addition, a secondary transfer roller 113 (transfer member) that is in contact with the surface of the transfer belt 125 at a position facing the driving roller 125a is provided in the sheet conveyance path 111. The secondary transfer roller 113 forms a transfer nip portion with the surface of the transfer belt 125. The sheet P is pressed and nipped by the transfer belt 125 and the secondary transfer roller 113 at the transfer nip portion, whereby the toner image on the transfer belt 125 is secondarily transferred to the sheet P.

  The sheet storage unit 14 includes a manual feed tray 141 that can be freely opened and closed on the right side wall of the apparatus main body 11 in FIG. 1, and a paper tray 142 that is removably mounted in the apparatus main body 11 below the exposure device 124. including. In the paper tray 142, a paper bundle P1 formed by stacking a plurality of papers P is stored. The paper tray 142 is configured by a box that opens upward, and the paper bundle P1 can be stored. The uppermost sheet P of the sheet bundle P1 stored in the sheet tray 142 is fed out one by one toward the sheet conveyance path 111 by driving the pickup roller 143. The paper P fed out from the paper tray 142 is conveyed toward the transfer nip portion through the paper conveyance path 111 by driving the conveyance roller pair 112.

  The fixing unit 13 performs a fixing process on the toner image on the sheet P that has been secondarily transferred. The fixing unit 13 includes a heating roller 131 having an energized heating element as a heating source therein, a fixing roller 132 disposed to face the heating roller 131, and a fixing belt 133 stretched between the fixing roller 132 and the heating roller 131. And a pressure roller 134 disposed to face the fixing roller 40 with the fixing belt 133 interposed therebetween. The sheet P supplied to the fixing unit 13 is subjected to a fixing process by obtaining heat from the fixing belt 133 while passing between the pressure roller 134 and the high-temperature fixing belt 133. The color-printed paper P for which the fixing process has been completed is discharged toward the paper discharge tray 151 of the paper discharge section 15 through the paper discharge conveyance path 114 extended from the upper part of the fixing section 13.

  FIG. 2 is a cross-sectional view schematically showing the internal structure of the developing device 122. The developing device 122 includes a developing container 40 that defines an internal space of the developing device 122. The developing device 122 stores a developer containing a non-magnetic toner and a magnetic carrier in the developing container 40, and can also transport the developer while stirring the developer. The magnet roller 42 disposed above the developer reservoir 41, the developing roller 43 (toner carrier) disposed opposite to the magnet roller 42 at a position obliquely above the magnet roller 42, and disposed opposite the magnet roller 42. Developer regulating blade 44.

  The developer storage section 41 is composed of two adjacent developer storage chambers 41 a and 41 b extending in the longitudinal direction of the developing device 122, and the developer storage chambers 41 a and 41 b are formed integrally with the developer container 40. The partition plates 47 partition each other in the longitudinal direction, but communicate with each other at both ends in the longitudinal direction. Further, screw feeders 45 and 46 that agitate the developer by rotation are rotatably mounted in the developer storage chambers 41a and 41b. Since the screw feeders 45 and 46 are set so that the rotation directions are opposite to each other, the developer is conveyed between the developer storage chamber 41a and the developer storage chamber 41b while being stirred. By this stirring, the toner and the carrier are mixed and the toner is charged.

  The magnet roller 42 is disposed so as to extend in the longitudinal direction of the developing device 122, and can rotate clockwise in FIG. A fixed so-called magnet roll (not shown) is arranged inside the magnet roller 42. The magnet roll has a plurality of magnetic poles, for example, a drawing pole 50, a regulation pole 51, and a main pole 52. The scooping pole 50 faces the developer storage section 41, the regulating pole 51 faces the developer regulating blade 44, and the main pole 52 faces the developing roller 43.

  The magnet roller 42 magnetically pumps the developer from the developer storage portion 41 onto the peripheral surface 48 by the magnetic force of the pumping pole 50. The developer drawn up is magnetically held as a developer layer (magnetic brush layer) on the peripheral surface 48 of the magnet roller 42, and conveyed toward the developer regulating blade 44 as the magnet roller 42 rotates. .

  The developer regulating blade 44 is located upstream of the developing roller 42 as viewed from the rotation direction of the magnet roller 42, and regulates the layer thickness of the developer layer that is magnetically attached to the peripheral surface 48 of the magnet roller 42. The developer regulating blade 44 is a plate member made of a magnetic material extending along the longitudinal direction of the magnet roller 42, and is supported by a predetermined support member 54 fixed at an appropriate position of the developing container 40. Further, the developer regulating blade 44 has a regulating surface 53 (that is, the leading end surface of the developer regulating blade 44) that forms a regulating gap G having a predetermined size with the peripheral surface 48 of the magnet roller 42.

  The developer regulating blade 44 formed of a magnetic material is magnetized by the regulating pole 51 of the magnet roller 42, and the magnetic path is formed between the regulating surface 53 of the developer regulating blade 44 and the regulating pole 51, that is, in the regulating gap G. Is formed. When the developer layer adhering on the peripheral surface 48 of the magnet roller 42 is conveyed into the regulation gap G by the rotation of the magnet roller 42 by the drawing upper pole 50, the layer thickness of the developer layer is regulated in the regulation gap G. Thus, a uniform developer layer having a predetermined thickness is formed on the peripheral surface 48.

  The developing roller 43 is disposed so as to extend in the longitudinal direction of the developing device 122 and so as to extend in parallel to the magnet roller 42, and can rotate clockwise in FIG. A gap D having a predetermined size is formed between the peripheral surface 55 of the developing roller 43 (hereinafter referred to as a toner carrying surface) and the peripheral surface 48 of the magnet roller 42. The developing roller 43 is disposed so as to face the photosensitive drum 121 through an opening formed in the developing container 40, and a gap having a predetermined dimension is also formed between the toner carrying surface 55 and the peripheral surface of the photosensitive drum 121. Is formed.

  Next, the developing operation of the developing device 122 will be described with reference to FIG. The developing device 122 is configured to perform a developing operation by applying a bias to each of the magnetic roller 42 and the developing roller 43. The bias applying unit includes a first applying unit 56, a second applying unit 57, and the like. Further included. As shown in the figure, the first application means 56 has a DC voltage source 61 and an AC voltage source 62 connected in series, and is connected to the magnet roller 42. A bias obtained by superimposing the DC bias output from the DC voltage source 61 and the AC bias output from the AC voltage source 62 is applied to the magnet roller 42. The second application unit 57 includes a DC voltage source 63 and an AC voltage source 64 connected in series, and is connected to the developing roller 43. A bias in which a DC bias output from the DC voltage source 63 and an AC bias output from the AC voltage source 64 are superimposed (hereinafter referred to as a developing bias) is applied to the developing roller 42.

  The magnetic brush layer on the peripheral surface 48 of the magnet roller 42 is transported toward the developing roller 43 as the magnet roller 42 rotates after the layer thickness is uniformly regulated by the developer regulating blade 44. A large number of magnetic brushes DB come into contact with the toner carrying surface 55 of the rotating developing roller 43. At this time, the first applying means 56 and the second applying means 57 apply a bias to the magnet roller 42 and the developing roller 43. As a result, a predetermined potential difference is generated between the peripheral surface 48 of the magnet roller 42 and the toner carrying surface 55 of the developing roller 43. Due to this potential difference, only the toner T moves from the magnetic brush DB to the toner carrying surface 55 at a position where the peripheral surface 48 and the toner carrying surface 55 are opposed (at a position where the main pole 52 (FIG. 2) and the toner carrying surface 55 are opposed). However, the carrier C of the magnetic brush DB remains on the peripheral surface 48. As a result, the toner layer TL having a predetermined thickness is carried on the toner carrying surface 55 of the developing roller 43.

  The toner layer TL on the toner carrying surface 55 is conveyed toward the peripheral surface of the photosensitive drum 121 as the developing roller 43 rotates. Since the superimposed DC voltage and AC voltage are also applied to the photosensitive drum 121, a predetermined potential difference is generated between the peripheral surface of the photosensitive drum 121 and the toner carrying surface 55 of the developing roller 43. ing. Due to this potential difference, the toner T of the toner layer TL moves to the peripheral surface of the photosensitive drum 121. As a result, the electrostatic latent image on the peripheral surface of the photosensitive drum 121 is developed, and a toner image is formed.

  Of the toner T in the toner layer TL, the residual toner RT that does not move to the photosensitive drum 121 and remains on the toner carrying surface 55 is transferred to the toner carrying surface 55 and the magnet roller 42 as the developing roller 43 rotates. When it is transported to a position facing the peripheral surface 48, it is collected by the magnetic brush DB. When the magnetic brush DB having the collected toner RT is conveyed downstream from the main pole 52 as the magnet roller 42 rotates, the magnetic brush DB is separated from the peripheral surface 48 by the magnetic force of the separation pole (not shown) of the magnet roll. It peels and returns to the developer storage part 41 (FIG. 2). That is, the residual toner RT is returned to the developer storage unit 41.

By the way, in the developing device 122 having the above-described configuration, the toner carrying surface 55 of the developing roller 43 is fixed on the toner carrying surface 55 at the initial stage of development when the toner image developing operation is started and during the development endurance when the number of toner image developing operations increases. Even when the developing bias is applied, the potential difference between the developing bias and the surface potential of the photosensitive drum 121 is different. This will be specifically described with reference to FIG. That is, when the developing operation is started and a developing bias is applied to the toner carrying surface 55 (at the initial stage of development), the AC bias peak value Vmax of the developing bias and the image portion (toner of the toner image on the photosensitive drum 121). Between the surface potential of the image bias and the surface potential V ₀ of the white background portion (portion where the toner does not adhere) of the toner image. A potential difference occurs.

  The toner moves from the toner carrying surface 55 to the photosensitive drum 121 in accordance with the image portion potential difference and the white background potential difference, and the electrostatic latent image is developed. However, when the development durability is reached, the image portion potential difference and the white background potential difference are developed. As shown by the arrows, in particular, the image portion potential difference becomes larger than a predetermined value at the initial stage of development, and voltage leakage (air discharge, hereinafter simply referred to as leakage) occurs between the toner carrying surface 55 and the image portion. It tends to occur. The fluctuation of the image portion potential difference causing the leak is due to a so-called charge-up phenomenon. Further, the DC bias Vdc of the developing bias also varies as shown by the arrow due to the charge-up phenomenon.

  For example, when the number of developments for developing a toner image increases (that is, during development endurance), the charge-up phenomenon causes residual toner to adhere to the toner carrying surface 55 of the developing roller 43 or an external additive peeled off from the toner may adhere This is because the electrical capacitance component of the toner carrying surface 55 changes. When the developing device 122 performs a developing operation in a state where the capacitance component is changed, charges are gradually accumulated on the toner carrying surface 55. Charge accumulation increases the surface potential of the toner carrying surface 55. Therefore, the image portion potential difference and the white background portion potential difference become larger than a predetermined value. Therefore, when a constant developing bias is always applied to the toner carrying surface 55, a leak occurs.

  Therefore, the developing device 122 according to the present embodiment provides a control configuration that can set an optimal developing bias while suppressing the occurrence of leakage even when the amount of accumulated charge on the toner carrying surface 55 increases. . Hereinafter, the control unit U having the control configuration will be specifically described.

  As shown in FIG. 5, the control unit U includes a leak generation mode execution unit 70 that generates a leak, and a development mode execution unit 71 that performs a normal development operation. First, the leak occurrence mode execution unit 70 will be described.

  The leak generation mode execution unit 70 intends the toner carrying surface 55 to have a leak occurrence bias that generates a leak between the image portion of the toner image having a predetermined image pattern formed in advance on the photosensitive drum 121 and the toner carrying surface 55. The leak generation mode to be applied is performed. The leak occurrence mode execution unit 70 executes the leak occurrence mode periodically or when the setting of the development operation is changed (for example, when image density adjustment is performed). Leak generation mode execution unit 70 includes a leak generation instruction unit 72, a leak detection unit 73, a state detection unit 74, an application time setting unit 75, and a storage unit 76.

  The leak generation instructing unit 72 controls the second application unit 57 (leak generation unit) to apply an AC bias (that is, a leak generation bias) from the AC voltage source 64 to the toner carrying surface 55, thereby intentionally leaking. generate. The leak generation instructing unit 72 can generate a leak by changing the AC bias stepwise (for example, by 20V).

  The leak detector 73 detects the occurrence of a leak by detecting an overcurrent flowing between the photosensitive drum 121 and the toner carrying surface 55 due to the leak, and is based on the value of the overcurrent. The leak occurrence bias at which the leak occurred is calculated.

  The state detection unit 74 acquires state information necessary for setting the application time for applying the leak occurrence bias to the toner carrying surface 55. In the present embodiment, the number of toner image development operations, that is, the number of printing operations (number of printed sheets) for printing a toner image on a sheet is used as status information. The state detection unit 74 includes a counter that counts the number of printed sheets. In addition to the number of printed sheets, the status information includes a total driving time when the developing device 122 is driven, a total toner consumption consumed by the developing device 122, a total printing amount printed by the developing device 122, and the like.

  The application time setting unit 75 sets the application time of the leak occurrence bias according to the state information (number of printed sheets) acquired by the state detection unit 74.

6 to 9 are diagrams showing the results of experiments conducted to find the optimum application time according to the state information, and show the relationship between the application time and the AC bias for each predetermined number of printed sheets. In the figure, “X” indicates that a leak has been detected, and “◯” indicates that no leak has been detected. In the experiment, the AC bias was changed stepwise (in increments of 20 V) in the range of 1280 V to 1160 V, and the time (application time) required until leakage occurred for each AC bias was measured. In the experiment, 300,000 sheets were printed at a printing speed of 40 sheets / min. Other experimental conditions are shown below.
Rotational speed of the photosensitive drum 121: 210 mm / sec
Rotational speed of the developing roller 43: 1.6 times the rotational speed of the photosensitive drum 121 (same rotational direction as the photosensitive drum 121)
Magnet roller rotation speed: 1.2 times the rotation speed of the developing roller 43 (same rotation direction as the developing roller 43)
Distance between photosensitive drum 121 and toner carrying surface 55: 0.12 mm
Distance (gap D) between toner carrying surface 55 and peripheral surface 48 of the magnet roller: 0.3 mm
Surface potential of the photosensitive drum 121: +230 V (white background portion), +20 V (image portion)
Development bias: Frequency 3.85 kHz, Duty 45%, DC bias 70V,
AC bias 1160V ~ 1280V
Bias to magnet roller: Frequency 3.85 kHz, Duty 70%,
DC bias 200V, AC bias 2200V
Toner: Particle size 6.8 μm, positive chargeability Leak detection by the leak detection unit 73 was performed on the image portion.

  As shown in FIG. 6, when the voltage was applied at 1280 V during initial development when the number of printed sheets was less than 50,000, a leak was detected with an application time of 1 second or less, and no leak was detected at 1260 V or less. In addition, as shown in FIG. 7, when applied at 1260 V during development durability when the number of printed sheets is 50,000 or more, a leak was detected with an application time of 1 to 2 seconds, but with an application time of 1 second or less. No leak was detected. Further, no leak was detected at 1240 V or less.

  Furthermore, as shown in FIG. 8, when applied at 1240 V during development durability when the number of printed sheets is 100,000 or more, a leak was detected with an application time of 2 to 3 seconds, but with an application time of 2 seconds or less. No leak was detected. Moreover, no leak was detected at 1220 V or less. Furthermore, as shown in FIG. 9, when applied at 1220 V during development durability when the number of printed sheets is 300,000 or more, a leak was detected in an application time of 3 to 4 seconds, but in an application time of 3 seconds or less. No leak was detected. Further, no leak was detected at 1200 V or lower.

  From the results shown in FIG. 6 to FIG. 9, it was confirmed that the AC bias at which leakage occurs decreases as the number of printed sheets increases. This is because, as described with reference to FIG. 4, when the amount of accumulated charge on the toner carrying surface 55 increases as the number of printed sheets increases, the surface potential of the toner carrying surface 55 increases.

  Further, from the results shown in FIGS. 6 to 9, when the number of printed sheets increases, the accumulated charge amount on the toner carrying surface 55 increases, so that leakage occurs even when the same AC bias is applied. It was confirmed that the time required to make it longer. From this, it can be seen that if the application time is fixed or set short, the leak cannot be detected if the leak occurs beyond the fixed application time.

  Specifically, when the time required for a leak to occur when the AC bias is set to 1280 V is about 1 second (see FIG. 6), the leak is detected even if the application time is fixed at 1 second. Can do. However, when the AC bias is set to 1240 V and the time required to generate a leak is 2 to 3 seconds (see FIG. 8), the leak may be detected if the application time is fixed at 1 second. Can not.

  From the above knowledge, the application time setting unit 75 sets the application time to 2 seconds when the state detection unit 74 detects that the number of prints is less than 50,000, as shown in FIG. When it is detected that the number is 50,000 or more, the application time is set to 3 seconds, and when it is detected that the number of printed sheets is 100,000 or more, the printing time is set to 4 seconds and printing is performed. When it is detected that the number of sheets is 300,000 or more, the printing time is set to 5 seconds. On the other hand, the fixed configuration example in which the application time is fixed is set to 1 second regardless of the number of printed sheets.

  Thus, in the present embodiment, the application time setting unit 75 increases the application time as the number of printed sheets increases. As a result, even if the amount of accumulated charge on the toner carrying surface 55 increases, a leak can be reliably generated. On the other hand, since the application time is fixed to 1 second in the fixed configuration example, the leak is generated only when the actual leak generation bias is 1280V. Note that the above-described set application time of the present embodiment is appropriately changed depending on the change in the experimental conditions described above, the usage environment of the developing device 122, and the like.

  The storage unit 76 stores the application time described above as an application time table. The application time setting unit 75 refers to the application time table and sets the application time according to the number of printed sheets acquired by the state detection unit 74.

  Next, the development mode execution unit 71 will be described. The development mode execution unit 71 performs a development mode for printing a toner image on a sheet in accordance with a user instruction, and includes a development bias setting unit 77 and a bias application instruction unit 78.

  The development bias setting unit 77 sets an AC bias applied to the toner carrying surface 55 from the AC voltage source 64 of the second application unit 57 based on the leak generation bias generated by the leak generation mode execution unit 70. For example, when the leak occurrence bias is 1240V as shown in FIG. 8, the development bias setting unit 77 sets the AC bias to 1220V lower than 1240V. In this way, by setting the AC bias slightly lower than the leak occurrence bias, an amount of toner sufficient to form a toner image is supplied from the toner carrying surface 55 to the photosensitive drum 121 while suppressing the occurrence of leak. be able to.

  The bias application instruction unit 78 controls the first application unit 56 to apply a bias to the magnet roller 42 and also controls the second application unit 57 to apply the AC bias set by the development bias setting unit 77 to the toner carrying surface. 55 is applied. As a result, a predetermined potential difference is generated between the circumferential surface 48 of the magnet roller 42 and the toner carrying surface 55 and between the toner carrying surface 55 and the circumferential surface of the photosensitive drum 121. The toner image is formed according to the potential difference as described with reference to FIG.

  Hereinafter, the control flow of the leak occurrence mode execution unit 70 will be described with reference to the flowchart of FIG. First, the leak occurrence mode execution unit 70 starts the leak occurrence mode regularly or when the setting of the developing operation is changed (step S1). Next, in step S <b> 2, a leak occurrence bias (first occurrence voltage) at which a leak occurred when the leak occurrence mode was executed at the previous time is applied to the toner carrying surface 55. Here, when the first generated voltage is 1260 V, for example, the application time of the first generated voltage is 3 seconds.

  When a leak occurs at the first generated voltage (1260V) (YES in step S3), a second generated voltage that is one step lower than the first generated voltage in step S4 (1240V lower by 20V than 1260V). Is applied to the toner carrying surface 55. The application time of the second generated voltage is 3 seconds.

  If a leak occurs at the second generated voltage (1240 V) (YES in step S5), the second generated voltage is set as a new leak generating bias in step S6. The development mode execution unit 71 sets an optimal AC bias based on the new leak occurrence bias. On the other hand, if no leak has occurred in the second generated voltage (NO in step S5), in step S7, the first generated voltage (1260V) is set as the leak occurrence bias again. The development mode execution unit 71 maintains the AC bias set at the previous time.

  Returning to step S3, if no leak occurs in the first generated voltage (1260V) (in the case of NO), in step S8, the third generated voltage (1260V higher than 1260V), which is one step higher than the first generated voltage. 1280V) increased by 20V and set to the toner carrying surface 55. The application time of the third generated voltage is 3 seconds. The application time of the second generated voltage and the third generated voltage is changed depending on the number of printed sheets, but is set constant through one continuous leak occurrence detection. This is because the application time is set so that the leak can be reliably detected even if the voltage causing the leak changes. Since the control flow of FIG. 11 illustrates the case where the number of printed sheets is 50,000 or more and less than 100,000, the application time is set to 3 seconds.

  When a leak occurs at the third generated voltage (1280 V) (YES in step S9), the third generated voltage is set as a new leak occurrence bias in step S10. The development mode execution unit 71 sets an optimal AC bias based on the new leak occurrence bias. On the other hand, when no leak has occurred at the third generated voltage (NO in step S9), the first generated voltage (1260V) is again set as the leak occurrence bias in step S11. The development mode execution unit 71 maintains the AC bias set at the previous time.

  According to the developing device 122 according to the present embodiment described above, the control unit U acquires state information necessary for setting the application time of the leak occurrence bias in the leak occurrence mode, and sets the application time according to the state information. To do. Specifically, the control unit U increases the application time as the number of printed sheets increases. Therefore, even if the amount of accumulated charge on the toner carrying surface 55 increases as the number of printed sheets increases, a voltage leak can surely occur. As a result, the development bias set by the control unit U based on the leak occurrence bias in the development mode is an optimum bias that does not cause a leak. As a result, a good quality toner image is formed.

  Further, since a counter is used as the state detection unit 74, it becomes easy for the control unit U to set the leak occurrence mode at an optimal timing.

10 Image forming apparatus 42 Magnet roller 43 Developing roller (toner carrier)
55 Toner carrying surface 57 Second application means (leak generation means)
70 Leak generation mode execution unit 71 Development mode execution unit 72 Leak generation instruction unit 73 Leak detection unit 74 State detection unit 75 Application time setting unit 76 Storage unit 77 Development bias setting unit 78 Bias application instruction unit 121 Photosensitive drum 122 Developing device U Control unit

Claims (4)

A storage section for storing a developer containing toner;
A toner carrying surface for receiving and carrying the toner from the reservoir, and supplying the toner to a predetermined image carrier from the toner carrying surface to develop a toner image on the predetermined image carrier; Body,
Bias applying means for applying a developing bias to the toner carrier and supplying the toner from the toner carrying surface to the predetermined image carrier;
Leak generating means for applying a leak generating bias for generating a voltage leak between the toner carrier and the predetermined image carrier to the toner carrier;
A leak generation mode for controlling the leak generation means to generate the voltage leak, and the development bias is set based on the leak generation bias, and the bias application means is controlled to apply the development bias to the toner carrier. A control unit that performs a developing mode to be applied;
With
The control unit includes a state detection unit that acquires state information necessary for setting the application time of the leak generation bias and a setting unit that sets the application time according to the state information in the leak generation mode. . The developing device according to claim 1,
The state detection unit obtains the number of development operations of the toner image as the state information,
The developing unit is configured to increase the application time of the leak occurrence bias as the number of times of developing the toner image increases. The developing device according to claim 2,
The state detection unit is a counter that counts the number of development operations.
The control unit is a developing device that performs the leak generation mode every predetermined number of development operations. An image carrier on which a toner image is formed;
A developing device for supplying toner to the image carrier and developing the toner image on the image carrier;
A transfer member for transferring the toner image onto a sheet;
A fixing unit for fixing the toner image on the sheet onto the sheet;
With
An image forming apparatus in which the developing device according to claim 1 is used as the developing device.

Images