JP2010066582A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that stably performs image formation by preventing image failure resulting from a decrease in the height of the level of an agent and a temperature increase of a conveying member in a developing apparatus of a vertical stir configuration. <P>SOLUTION: The image forming apparatus includes: a developing sleeve 28 that conveys two-component developer; a first conveying screw 26 that recovers and conveys the developer after development; a second conveying screw 25 disposed above the first conveying screw 26 and used to supply the two-component developer to the developing sleeve 28; an inductance detection sensor 20 for detecting a toner concentration; and a toner concentration reference value correcting means. In the image forming apparatus, a drive source M1 for driving the developing sleeve 28 and a drive source M2 for driving the first and second conveying screws 26 and 25 are separate drive sources. The apparatus has a control section that controls the rotational speed of the first and second conveying screws 26 and 25 based on an amount of correction of a reference value of an inductance detection sensor 20, which is corrected by the toner concentration reference value correcting means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a recorded image display device, and a facsimile.

  In an image forming apparatus such as a copying machine using an electrophotographic system, a developer is attached to an electrostatic latent image formed on an image carrier such as a photosensitive drum to make a visible image. As a developing apparatus according to the prior art used for such development, one using a two-component developer composed of a toner and a carrier is known, and an example thereof is shown in FIG. FIG. 4 is a schematic sectional view of a developing device according to the prior art.

  As shown in FIG. 4, in a developing device using a two-component developer, two first and second conveying screws (conveying members) 106 and 105 that convey the two-component developer while stirring are horizontally provided. Many are arranged. The second conveying screw 105 is used to supply the developer to the developer carrier 108 (hereinafter referred to as a developing sleeve) and collect the developer after passing through the developing region. The first conveying screw 106 is used for mixing and stirring the developer collected from the developing sleeve 108 and the newly supplied developer.

  On the other hand, in recent years, in an image forming apparatus using an electrophotographic system such as a copying machine or a printer, there has been a strong demand for downsizing of the apparatus main body in order to achieve space saving. In order to arrange a plurality of developing devices, there is a strong demand for downsizing. Therefore, a vertical stirring type developing device having the configuration shown in FIG. 6 is known (for example, see Patent Document 1).

  Unlike the developing device 101d shown in FIG. 4, the developing device 101e shown in FIG. 6 is characterized in that conveying screws for stirring and mixing the two-component developer are arranged vertically. The first conveying screw 106 mixes and stirs the developer collected from the developing sleeve 108 and the newly supplied developer while conveying the developer in the stirring chamber 104 below the developing container 102. Used. The second conveying screw 105 is used to convey the developer in the developing chamber 103 above the developing container 102 and supply it to the developing sleeve 108. As described above, the vertical stirring type developing device 101e has an advantage that the space occupied in the horizontal direction can be reduced because the developing chamber 103 and the stirring chamber 104 are arranged in the vertical direction. Therefore, for example, a tandem color image forming apparatus in which a plurality of developing devices are arranged in parallel in the horizontal direction can be reduced in size.

  In the developing device 101 d shown in FIG. 4, the developer is supplied from the developing chamber 103 to the developing sleeve 108, and the developed developer is collected again in the developing chamber 103. For this reason, there is a possibility that the developer whose toner concentration is reduced by the development is supplied to the developing sleeve 108 without being sufficiently stirred. On the other hand, in the vertical agitation type developing device 101e shown in FIG. 6, the developer after the development is not collected only by supplying the developer to the developing sleeve 108 from the upper developing chamber 103, so that the toner is always used. A developer having a uniform concentration can be supplied to the developing sleeve 108. Therefore, it is possible to obtain a uniform image with no image density unevenness due to toner density unevenness and no density difference in the longitudinal direction.

JP-A-5-333691

  The vertical stirring type developing device 101 e supplies a part of the developer to the developing sleeve 108 while conveying the developer in the longitudinal direction by the second conveying screw 105. The developer once coated on the developing sleeve 108 is collected by the first conveying screw 106 after passing through the developing region. For this reason, as shown in FIG. 5, the amount of developer conveyed by the second conveying screw 105 decreases as it goes downstream in the conveying direction. Conversely, the amount of developer conveyed by the first conveying screw 106 increases as the amount of developer recovered from the developing sleeve 108 increases, so that it goes downstream in the conveying direction. Therefore, in the vertical stirring type developing device 101e, the height of the surface of the developer is inclined in the longitudinal direction of the conveying screws 105 and 106.

  For this reason, the coating surface height tends to be low at the downstream of the second conveying screw 105 in the conveying direction (A portion in FIG. 5), so that the problem of poor coating due to insufficient supply of the developer to the developing sleeve 108 is likely to occur. is there. The occurrence of such a coating defect appears as an uneven density and appears in the image, and therefore a countermeasure is required.

  The surface height of the second conveying screw 105 downstream in the conveying direction depends on the pumping property of the developer in the pumping unit that transfers the development from the first conveying screw 106 to the second conveying screw 105. If the developer pumping performance at the pumping section is poor, the amount of developer supplied to the second transport screw 105 per unit time is reduced, so that the surface height of the second transport screw 105 downstream in the transport direction is high. It will be lower. The pumping performance of the developer varies depending on the toner concentration of the developer. In particular, the pumping performance decreases as the toner concentration decreases. As the toner concentration decreases, the developer fluidity tends to increase, and the developer with increased fluidity becomes slippery because the frictional resistance with the surface of the conveying screw decreases, and as a result, from the bottom up It is thought that it becomes difficult to pump up to.

  For these reasons, it is desirable that the toner concentration of the developer is not lowered from the initial toner concentration as much as possible from the viewpoint of drawability. However, in practice, the toner concentration generally decreases (controls to decrease) as the durability of the developer progresses. The reason for this is the accumulation of external additives on the carrier surface. As the durability progresses, the external additive detached from the toner gradually adheres to the surface of the carrier and accumulates on the surface. When the external additive accumulates on the surface of the carrier, the charge imparting ability with respect to the toner is lowered, so that the charge amount of the developer having advanced durability is lowered. However, when the charge amount of the developer is lowered, fogging and toner scattering are deteriorated. Therefore, in order to maintain the charge amount, control is often performed to lower the toner concentration. As means for controlling the toner density to be lowered in order to maintain the charge amount, there is means for developing the reference toner image and controlling the toner density so that the density becomes constant (patch detection). There is also a means for controlling the toner density according to the driving time of the developing device and the number of prints.

  When such control is performed, the toner density is controlled to gradually decrease as the durability progresses, and accordingly, the pumping-up performance of the developer also decreases. As a result, there has been a problem that the developing device in a state where the durability has progressed inevitably causes a coating defect due to insufficient supply of the developer to the developing sleeve as described above.

  A possible solution to this problem is to increase the rotational speed of the conveying screw. When the rotation speed of the conveying screw is increased, the amount of agent pumped from the first conveying screw 106 to the second conveying screw 105 within a unit time increases. For this reason, even if the developer lifting property deteriorates, the amount of the agent conveyed by the second conveying screw 105 increases. As a result, it is possible to avoid coating defects by maintaining a high surface level on the downstream side of the second conveying screw 105 in the conveying direction.

  However, if the rotational speed of the conveying screw is constantly increased, the temperature rise due to friction at the rubbing portion between the screw and the bearing tends to be prominent. As the temperature rises, the toner adhering to the end of the screw is hardened by heat and becomes an aggregate. There is a possibility that the agglomerates clog the SB gap and become white stripes, or when the agglomerates are transferred after being developed, transfer omission may occur and a whiteout image may be generated. In addition to the temperature rise, problems such as a decrease in durability of the screw end seal have also occurred.

  Accordingly, the present invention provides an image forming apparatus capable of suppressing the occurrence of image defects due to a decrease in the coating surface height and the temperature rise of the conveying member and performing stable image formation in a developing device having a vertical stirring configuration. To do.

  In order to solve the above-described problems, a typical configuration of an image forming apparatus according to the present invention includes a developer carrying that carries a two-component developer composed of toner and a carrier on the surface and conveys it to a developing region facing the image carrier. A first conveying member that collects and conveys the two-component developer peeled off from the developer carrying member after development, and the two-component developer disposed above the first conveying member. A second conveying member for supplying the two-component developer to the developer carrying member, a toner concentration detecting means for detecting a toner concentration of the two-component developer, and the toner concentration detecting means And a toner density reference value correcting means for correcting the reference value of the image forming apparatus, wherein a driving source for driving the developer carrier and a driving source for driving the first and second conveying members are respectively provided. With another drive source And a control unit for controlling the rotation speeds of the first and second conveying members based on the correction amount of the reference value of the toner density detecting means corrected by the toner density reference value correcting means. To do.

  According to the present invention, in a developing device having a vertical stirring configuration, it is possible to suppress the occurrence of image defects due to a decrease in the level of the agent surface and the temperature rise of the conveying member, and to perform stable image formation.

[First embodiment]
A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings.

(Image forming device)
FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment. As shown in FIG. 1, the image forming apparatus includes four image forming portions P (Pa, Pb, Pc, Pd) as image forming means. Each of the image forming units Pa to Pd includes a drum-shaped electrophotographic photosensitive member that rotates in an arrow direction (counterclockwise) as an image carrier, that is, a photosensitive drum 1 (1a, 1b, 1c, and 1d). Yes. Around the photosensitive drums 1a to 1d, there are a charger 2 (2a, 2b, 2c, 2d), a laser beam scanner 3 (3a, 3b, 3c, 3d) as exposure means, and a developing device 4 (4a, 4b, 4c, 4d), transfer roller 6 (6a, 6b, 6c, 6d), and cleaning means 19 (19a, 19b, 19c, 19d). The laser beam scanner 3 is disposed above the photosensitive drum 1 in the figure. Since the image forming units Pa to Pd have the same configuration, they will be collectively referred to as the image forming unit P, and description of the image forming units Pa to Pd will be omitted.

  Next, an image forming sequence in the normal mode of the entire image forming apparatus having the above configuration will be described.

  First, the photosensitive drum 1 is uniformly charged by the charger 2. In the normal mode, the photosensitive drum 1 rotates at a process speed (circumferential speed) of 286 mm / sec in the clockwise direction indicated by the arrow.

  Next, the uniformly charged photosensitive drum 1 is subjected to scanning exposure with the laser beam modulated by the image signal by the laser beam scanner 3. The laser beam scanner 3 incorporates a semiconductor laser, and this semiconductor laser is controlled in response to an original image information signal output from an original reading apparatus having a photoelectric conversion element such as a CCD, and emits laser light.

  As a result, the surface potential of the photosensitive drum 1 charged by the charger 2 changes in the image portion, and an electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image is reversely developed by the developing device 4 to be a visible image, that is, a toner image.

  In the present embodiment, the developing device 4 uses a two-component developing system that uses a developer in which toner and a carrier are mixed as a developer.

  Further, by performing the above process for each of the image forming portions Pa to Pd, toner images of four colors of yellow, magenta, cyan, and black are formed on the photosensitive drums 1a to 1d.

  In the present embodiment, an intermediate transfer belt (intermediate transfer member) 5 is disposed below the image forming units Pa to Pd. The intermediate transfer belt 5 is suspended by rollers 51, 52, and 53 and is movable in the direction of the arrow.

  The toner image on the photosensitive drum 1 (1a to 1d) is once transferred to the intermediate transfer belt 5 as an intermediate transfer member by a transfer roller 6 (6a to 6d) as a primary transfer unit. As a result, toner images of four colors of yellow, magenta, cyan, and black are superimposed on the intermediate transfer belt 5 to form a full color image. Further, the toner remaining without being transferred onto the photosensitive drum 1 is collected by the cleaning means 19.

  The full-color image on the intermediate transfer belt 5 is taken out from the feeding cassette 12 and transferred to a recording sheet S such as paper that has advanced via a feeding roller 13 and a feeding guide 11 as a secondary transfer means. Transfer is performed by the action of the transfer roller 10. The toner remaining on the surface of the intermediate transfer belt 5 without being transferred is collected by the intermediate transfer belt cleaning means 18.

  On the other hand, the recording sheet S to which the toner image has been transferred is sent to a fixing device (heat roller fixing device) 16 where the image is fixed and discharged to a discharge tray 17.

  In the present embodiment, the photosensitive drum 1 that is a drum-shaped organic photosensitive member that is usually used is used as the image carrier. However, an inorganic photosensitive member such as an amorphous silicon photosensitive member can also be used. It is also possible to use a belt-like photoreceptor. The charging method, transfer method, cleaning method, and fixing method are not limited to the above methods.

(Developing device 4)
Next, the operation of the so-called vertical stirring type developing device 4 will be described with reference to FIGS. FIG. 2 is a cross-sectional view of the developing device 4 according to this embodiment. FIG. 3 is a longitudinal sectional view of the developing device 4 according to this embodiment.

  As shown in FIGS. 2 and 3, the developing device 4 includes a developing container 22 in which a two-component developer containing toner and a carrier as a developer is accommodated. Further, the developing container 22 includes a developing sleeve 28 as a developer carrying member and a regulating blade (ear cutting member) 29 that regulates the ears of the developer carried on the developing sleeve 28.

  In the present embodiment, the inside of the developing container 22 is partitioned into a developing chamber 23 and an agitating chamber 24 by a partition wall 27 having a substantially central portion extending in a direction perpendicular to the sheet surface. 23 and the stirring chamber 24.

  In the developing chamber 23 and the stirring chamber 24, a first conveying screw (first conveying member) 26 and a second conveying screw (second conveying member) 25 are arranged as developer agitating / conveying means, respectively. . The first conveying screw 26 is disposed substantially parallel to the bottom of the stirring chamber 24 along the axial direction of the developing sleeve 28, and rotates to allow the developer in the stirring chamber 24 to move in one direction along the axial direction. Transport. The second conveying screw 25 is disposed at the bottom of the developing chamber 23 substantially parallel to the first conveying screw 26 and conveys the developer in the developing chamber 23 in the opposite direction to the first conveying screw 26. . As described above, the developer is transported by the rotation of the first and second transport screws 26 and 25, so that the developer passes through the openings (that is, the communication portions) 41 and 42 at both ends of the partition wall 27. Cycled between.

  In the present embodiment, the developing sleeve 28, the first conveying screw 26, and the second conveying screw 25 are not all driven by the same drive motor (drive source). Specifically, the first and second conveying screws 26 and 25 are driven by a drive motor (second drive source) M2 that is different from the drive motor (first drive source) M1 that drives the developing sleeve 28. . With this configuration, it is possible to arbitrarily change the ratio of the rotational speeds of the first and second conveying screws 26 and 25 to the rotational speed of the developing sleeve 28.

  The first and second conveying screws 26 and 25 have a diameter of 18 mm and a pitch of 20 mm, and are rotated at a rotation speed of 390 rpm.

  There is an opening at a position corresponding to the developing region of the developing container 22 facing the photosensitive drum 1, and the developing sleeve 28 is rotatably disposed in the opening so as to be partially exposed in the photosensitive drum direction. Yes.

  Here, the diameter of the developing sleeve 28 is 20 mm, the diameter of the photosensitive drum 1 is 40 mm, and the closest region between the developing sleeve 28 and the photosensitive drum 1 is a distance of about 400 μm. Thus, the developer conveyed to the developing unit is set so as to be developed in a state where the developer is brought into contact with the photosensitive drum 1.

  The developing sleeve 28 is made of a nonmagnetic material such as aluminum or stainless steel. Inside the developing sleeve 28, a magnet roller 28m as a magnetic field means is installed in a non-rotating state. The magnet roller 28m includes a developing pole S2 disposed facing the photosensitive drum 1 in the developing unit, a magnetic pole S1 disposed facing the regulating blade 29, a magnetic pole N1 disposed between the magnetic poles S1 and S2, and a developing unit. Magnetic poles N2 and N3 are arranged to face the chamber 23 and the stirring chamber 24, respectively. The rotation speed of the developing sleeve 28 is set to 250 rpm.

  The developing sleeve 28 rotates in the direction of the arrow (counterclockwise) in FIG. 3 at the time of development and carries a two-component developer whose layer thickness is regulated by the cutting of the magnetic brush by the regulating blade 29. Then, the carried two-component developer is transported to a developing area facing the photosensitive drum 1 and supplied to the electrostatic latent image formed on the photosensitive drum 1 to develop the latent image.

  The regulating blade 29 is composed of a non-magnetic member 29a and a magnetic member 29b such as an iron material, and is disposed upstream of the photosensitive drum 1 in the developing sleeve rotation direction. The nonmagnetic member 29 a is formed of plate-like aluminum or the like extending along the longitudinal axis of the developing sleeve 28. Then, both the developer toner and the carrier pass between the tip of the regulating blade 29 and the developing sleeve 28 and are sent to the developing region.

  By adjusting the gap between the regulating blade 29 and the surface of the developing sleeve 28, the amount of the developer magnetic brush carried on the developing sleeve 28 is regulated and the amount of developer conveyed to the developing region is adjusted. Is done. In the present embodiment, the amount of developer coat per unit area on the developing sleeve 28 is regulated to 30 mg / cm 2 by the regulating blade 29. The gap between the regulating blade 29 and the developing sleeve 28 is set to 200 to 1000 μm, preferably 400 to 700 μm. In this embodiment, it is set to 600 μm.

(Toner supply control method)
A toner replenishment control method will be described. An opening (pumping portion) 41 for pumping the developer from the first transport screw 26 to the second transport screw 25 has an inductance detection sensor (toner concentration detection means) for detecting the toner concentration of the two-component developer. ) 20 is installed. The toner is replenished in the developing device by the toner replenishing means 32 disposed above the developing device 4 in accordance with the detection output of the sensor 20.

  Since the two-component developer is mainly composed of a magnetic carrier and a non-magnetic toner, if the toner concentration of the developer (the ratio of the toner particle weight to the total weight of the carrier particles and the toner particles) changes, the magnetic carrier and the non-magnetic toner The apparent permeability due to the mixing ratio changes. When this apparent permeability is detected by the sensor 20, as shown in FIG. 8, the detection output (Vsig) changes substantially linearly according to the toner concentration (T / D ratio). That is, the detection output of the sensor 20 corresponds to the toner concentration of the two-component developer present in the developing device 4.

  That is, as the toner concentration increases, the proportion of non-magnetic toner in the developer increases, so the apparent permeability of the developer decreases and the detection output decreases. On the contrary, when the toner concentration is lowered, the apparent magnetic permeability of the developer is increased, so that the detection output is increased. In this way, the toner density of the developer can be detected using the sensor 20.

  The control unit 80 compares the detected Vsig with a toner density reference value Vref recorded in advance in a non-volatile memory unit (not shown), and based on the calculation result of the difference (Vsig−Vref), the toner replenishment time. To control. Since the toner density reference value Vref is an output value corresponding to the target toner density, Vsig is controlled so as to approach this toner density reference value.

  For example, when Vsig−Vref> 0, the toner density of the developer is in a state lower than the target toner density, and therefore the necessary toner replenishment amount is determined according to the difference. Therefore, the larger the difference between Vsig and Vref, the more toner is supplied. Further, when Vsig−Vref ≦ 0, the toner concentration is higher than the target, so that the toner supply is stopped and the toner concentration is lowered by the toner consumption by the image forming operation. The toner replenishment control is performed as described above.

  The initial toner concentration of the developer used in this embodiment is 7%, and the initial toner concentration reference value of the sensor 20 is adjusted so that the output when the toner concentration is 7% is 2.5V. Yes. The present invention is not limited to such numerical values, and the initial toner density and the adjustment value of the initial toner density reference value Vref may be other numerical values.

  As described above, as the developer durability increases, the charge imparting ability of the carrier decreases. For this reason, if the sensor 20 is controlled so that the toner density is maintained at 7%, which is the initial toner density, the charge amount of the developer gradually decreases, resulting in a problem that fogging and toner scattering deteriorate. End up.

  Therefore, in this embodiment, as a means for maintaining the charge amount of the developer, a toner density reference value correcting means 81 based on a patch detection method for changing the toner density reference value of the sensor 20 so that the density of the reference toner image is constant is used. ing. The toner density reference value correction means 81 using the patch detection method will be described below.

  When the chargeability of the carrier decreases due to durability and the charge amount of the developer gradually decreases, in the case of the two-component development method, the density of the reference toner image (reference patch) developed with a predetermined contrast gradually increases. In order to keep the density of the reference patch substantially constant, it is necessary to keep the charge amount of the developer substantially constant. To maintain the charge amount of the developer constant with a carrier having a reduced charging capacity, the toner density must be set. Need to lower. Therefore, in this embodiment, reference patches are formed at predetermined intervals, and the control unit 80 corrects the toner density reference value Vref of the sensor 20 so that the reference patch density becomes substantially constant, thereby changing the toner density. Control is performed so that the charge amount of the developer becomes substantially constant.

  The execution interval of patch detection control in this embodiment is executed every 40 prints. When the execution timing of the patch detection control is reached, the reference patch is developed on the photosensitive drum 1 with a predetermined contrast. Thereafter, this reference patch is transferred to the intermediate transfer body 5 and its density signal (detection result) Vps is detected by an optical density sensor (image density detection means) 70 facing the intermediate transfer body. The density signal Vps is compared with the initial reference signal Vpr recorded in advance in the memory.

  When Vps−Vpr> 0, it is determined that the density of the reference patch is higher than the initial density, and the toner density reference value Vref of the detection sensor 20 is increased, that is, corrected to lower the toner density.

  When the density signal of the reference patch is lower than the initial reference signal, that is, when Vps−Vpr <0, the toner density reference value Vref of the sensor 20 is lowered. That is, correction is performed to increase the toner density.

  The correction amount of the toner density reference value is determined by the difference between the density signal of the reference patch obtained from Vps−Vpr and the initial reference signal. The larger the difference, the larger the correction amount of the toner density reference value.

  Thus, by changing the toner density based on the density signal of the reference patch, it becomes possible to keep the charge amount of the developer substantially constant, and the occurrence of fog and toner scattering that tends to become noticeable as the charge amount decreases. Can be suppressed.

  As described above, when the control is performed so as to reduce the toner concentration, the fluidity of the developer is increased. Therefore, in the vertical stirring type developing device 4, the pumping performance in the pumping unit 41 is lower than the initial level. Will end up. If the developer draw-up performance is reduced, the surface height on the downstream side of the second conveying screw 25 is lowered, which may cause a coating failure due to insufficient supply of the developer. Therefore, in this embodiment, the rotational speeds of the first and second conveying screws 26 and 25 are controlled based on the correction amount of the toner density reference value of the sensor 20 corrected by the patch detection method.

  As described above, the rotation speed of the first and second conveying screws 26 and 25 when the developer is in the initial state is set to 390 rpm. The rotation speed of the developing sleeve 28 is set to 250 rpm.

  As a result of the detection of the density of the reference patch, when the toner density reference value of the sensor 20 is corrected to be lower than the initial value, as shown in Table 1, the first and second conveying screws 26 are based on the target value of the toner density. , 25 is also corrected. For example, when the target value is corrected so that the toner density is 6.2%, the rotation speed of the first and second conveying screws 26 and 25 is changed to 507 rpm to perform image formation. At this time, the speed of the developing sleeve 28 is controlled so as to increase only the rotational speed of the first and second conveying screws 26 and 25 without changing the speed at 250 rpm in order to maintain developability. In this way, even if the toner concentration decreases and the developer pumping performance decreases, the rotation speed of the conveying screw can be increased to compensate for this, so image defects due to a decrease in the developer pumping amount Can be suppressed.

  Further, when the toner density reference value of the sensor 20 is corrected so as to be higher than the initial value, that is, when the toner density target value is corrected to 7% or more, there is no possibility that the pumping-up performance of the developer is lowered. For this reason, as shown in Table 1, the rotational speeds of the first and second conveying screws 26 and 25 are not changed from the initial speed.

  Here, the flow of toner supply control will be described. FIG. 7 is a flowchart showing the flow of toner supply control. As shown in FIG. 7, when the image forming operation is started (S1), it is determined whether it is a patch detection timing (S2). When patch detection is not performed, image formation is performed as it is (S7), and the image formation operation is terminated (S8).

  When performing patch detection, patch detection is executed (S3), and Vps−Vpr> 0 or Vps−Vpr <0 is determined (S4).

  When Vps−Vpr> 0, the correction amount of Vref is determined from the difference between Vps−Vpr, and the correction is performed to reduce the toner density (S5). Then, only the rotational speeds of the first and second conveying screws 26 and 25 are increased based on the target value of the toner density (S6). Then, image formation is performed (S7), and the image formation operation is terminated (S8).

  When Vps−Vpr <0, the correction amount of Vref is determined from the difference between Vps−Vpr and corrected to increase the toner density (S9). In this case, the rotational speeds of the first and second conveying screws 26 and 25 are not changed. Then, image formation is performed (S7), and the image formation operation is terminated (S8).

  In this embodiment, the inductance detection sensor 20 is used as the toner concentration detection means. However, other toner concentration detection means, for example, an optical density sensor that detects the toner density based on the reflected light from the developer may be used. Good.

  As described above, in the image forming apparatus using the vertical stirring type developing device 4, even when the toner density is controlled to be kept constant in order to maintain the charge amount of the developer substantially constant, the developer surface It becomes possible to control the height within an appropriate range. As a result, it is possible to suppress the occurrence of fogging and toner scattering, as well as the occurrence of poor coating due to the decrease in the coating surface height.

  In addition, by appropriately controlling the speed of the conveying screw according to the toner concentration of the developer that changes depending on the durability, it is possible to suppress the occurrence of image defects due to the temperature rise of the conveying screw and to perform stable image formation. it can.

[Second Embodiment]
Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. About the part which overlaps with said 1st embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted. FIG. 9 is a flowchart showing the flow of toner replenishment control according to this embodiment.

  The image forming apparatus according to this embodiment does not detect the density of the reference patch as in the first embodiment, and corrects the toner density reference value according to the durability state of the developer.

  As described above, the charge imparting ability of the carrier decreases with the endurance, but the degree of the decrease is almost linear with the time the developer is used for image formation, that is, the accumulated rotation time (integrated drive time) of the developing sleeve. Decline. Therefore, if the accumulated rotation time of the developing sleeve 28 from the initial state of the developer is measured, the amount of decrease in the developer amount accompanying the decrease in the charge imparting ability of the carrier, and further the amount of charge of the developer is made substantially constant. Therefore, the toner density target value can be almost inferred.

  Therefore, in the present embodiment, the rotation speeds of the first and second conveying screws 26 and 25 are controlled from the toner density target value determined based on the accumulated rotation time of the developing sleeve 28 from the initial state of the developer.

  When the developing operation starts and the developing sleeve 28 rotates, the rotation time is measured and the accumulated rotation time from the initial stage is stored in a memory (not shown). The rotation time varies depending on the sheet size to be printed. Naturally, when a large sheet size is printed, the rotation time of the developing sleeve 28 becomes longer.

  The process speed of the image forming apparatus in the present embodiment is 286 mm / sec as in the first embodiment. For example, when printing one A4 size image, the rotation time of the developing sleeve 28 is 1.01 sec. Table 2 shows toner density target values based on the accumulated rotation time of the developing sleeve 28. As shown in Table 2, the toner density reference value of the sensor 20 is corrected so as to lower the target value of the toner density as the integrated rotation time of the developing sleeve 28 increases, and the first and second conveying screws 26 and 25 are corrected. Is determined. The rotational speeds of the first and second conveying screws 26 and 25 at each toner density target value are controlled based on Table 1 as in the first embodiment.

  Here, the flow of toner supply control will be described. As shown in FIG. 9, when the image forming operation is started (S21), the stacking rotation time of the developing sleeve 28 is read (S22). Then, it is determined whether or not it is necessary to change the toner density reference value Vref according to the read stacking rotation time (developer durability state) (S23).

  If it is not necessary to change Vref, the image forming operation is performed as it is (S26), the accumulated rotation time of the developing sleeve 28 is stored (S27), and the image forming operation is ended (S28).

  When it is necessary to change Vref, the correction amount of Vref is determined according to the loading rotation time (S24), and the rotation speeds of the first and second conveying screws 26 and 25 are corrected (S25). Then, an image forming operation is performed (S26), the accumulated rotation time of the developing sleeve 28 is stored (S27), and the image forming operation is terminated (S28).

  In the present embodiment, since the reference patch is not created and its density is not detected as in the first embodiment, it is not necessary to install a sensor for detecting the density of the reference patch. Costs can be reduced.

  Further, since it is necessary to temporarily interrupt the normal image forming operation in order to form the reference patch, there is a demerit that productivity is reduced by that amount. There is no loss of sex.

  As described above, in the image forming apparatus using the vertical stirring type developing device, even when the target value of the toner density is controlled based on the accumulated rotation time of the developing sleeve 28, the developer surface height is set to an appropriate range. Can be controlled within. As a result, it is possible to suppress the occurrence of fogging and toner scattering, as well as the occurrence of poor coating due to the decrease in the coating surface height.

  In addition, by appropriately controlling the speed of the conveying screw according to the toner concentration of the developer that changes depending on the durability, it is possible to suppress the occurrence of image defects due to the temperature rise of the conveying screw and to perform stable image formation. it can.

1 is a configuration diagram of an image forming apparatus according to a first embodiment. It is a cross-sectional view of the developing device according to the first embodiment. It is a longitudinal cross-sectional view of the developing device according to the first embodiment. It is a schematic explanatory drawing of the conventional developing device. It is a schematic explanatory drawing of the agent surface height in the conventional vertical stirring type developing device. It is a schematic explanatory drawing of the conventional vertical stirring type developing apparatus. 4 is a flowchart illustrating a flow of toner supply control according to the first embodiment. FIG. 4 is a diagram illustrating a relationship between a toner concentration of a developer and an inductance detection sensor output. 12 is a flowchart illustrating a flow of toner supply control according to the second embodiment.

Explanation of symbols

M1, M2 ... drive motors (first and second drive sources)
1 ... Photosensitive drum (image carrier)
4 ... developing device 20 ... inductance detection sensor 22 ... developing container 23 ... developing chamber 24 ... stirring chamber 25, 26 ... conveying screw (first and second conveying members)
28 ... Development sleeve (developer carrier)
30: Developer replenishment port 31 ... Toner hopper 32 ... Toner replenishing means 70 ... Optical density sensor (image density detecting means)
80: Control unit 81: Toner density reference value correcting means

Claims (4)

A developer carrier for carrying a two-component developer comprising a toner and a carrier on the surface and transporting the developer to a development area facing the image carrier;
A first conveying member that collects and conveys the two-component developer peeled off from the developer carrier after development; and
A second conveying member that is disposed above the first conveying member and supplies the two-component developer to the developer carrier while conveying the two-component developer;
Toner concentration detecting means for detecting the toner concentration of the two-component developer;
An image forming apparatus comprising: a toner density reference value correcting means for correcting a reference value of the toner density detecting means;
The drive source for driving the developer carrying member and the drive source for driving the first and second transport members are separate drive sources, respectively.
And a controller that controls the rotation speeds of the first and second conveying members based on a correction amount of a reference value of the toner density detecting unit corrected by the toner density reference value correcting unit. Image forming apparatus.
The control unit controls to increase the rotation speed of the first and second conveying members when correcting the reference value of the toner density detecting means in a direction to decrease the toner density. Item 2. The image forming apparatus according to Item 1.
The toner density reference value correcting means detects the density of the reference toner image developed on the image carrier by an optical image density detecting means installed in the image forming apparatus, and based on the detection result, The image forming apparatus according to claim 1, wherein the reference value of the toner density detecting unit is corrected.
  The image forming apparatus according to claim 1, wherein the toner density reference value correcting unit corrects the reference value of the toner density detecting unit based on an integrated driving time of the developer carrier.

Images