JP2009116096A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of forming an excellent image over a long term by suppressing fogging in a developing device and suppressing a defective image such as fogging or ghost. <P>SOLUTION: The image forming apparatus includes the developing device 4 developing an electrostatic latent image formed on a photoreceptor drum 1 with developer, and a primary transfer roller 92 transferring a developer image on the photoreceptor drum 1, wherein the developer remaining on a surface of the photoreceptor drum 1 without being transferred by the primary transfer roller 92 is collected by the developing device 4. The apparatus has image density detection sensors 301 and 302 detecting the density of the image developed by the developing device 4 at a plurality of positions in the rotary shaft direction of an image carrier, compares results of detection at the plurality of positions in the rotary shaft direction of the image carrier detected by the image density detection sensors 301 and 302, and performs fogging suppressing operation based on the result of comparison. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cleanerless transfer type image forming apparatus.

  2. Description of the Related Art Conventionally, there is a cleanerless image forming apparatus that eliminates a cleaning device and removes and collects transfer residual toner on a photoconductor after the transfer process from the photoconductor by “development simultaneous cleaning” and reuses it. is there.

  The simultaneous development cleaning is a method in which the transfer residual toner present on the surface of the photosensitive member that should not be developed with toner is collected by the developing device by the fog removal bias during the developing process of the electrostatic latent image.

  On the other hand, as a charging means, in recent years, a roller charging system using a conductive roller as a contact charging member in place of a corona charger is preferably used from the viewpoint of charging stability. In the roller charging method, a conductive elastic roller (charging roller) is brought into pressure contact with a member to be charged and a voltage is applied to the member to charge the member to be charged.

  However, when the contact charging device is used in a cleanerless image forming apparatus that removes and collects transfer residual toner on the photoconductor after the transfer process, there are the following problems. That is, when the transfer residual toner on the photoconductor passes through the charging portion which is the contact nip portion of the photoconductor and the contact charging device, the transfer residual toner adheres to the contact charging device and contaminates the toner, thereby causing a charging failure. It can be a cause.

  Even if the toner has a normal charge polarity, there are a toner whose polarity is reversed by being affected by a transfer bias or a peeling discharge, and a toner whose charge amount is reduced due to charge removal. For this reason, the transfer residual toner includes a normal charge polarity, a reverse polarity (reversal toner), and a low charge amount. Among them, the reversal toner and the toner with a small charge amount are likely to adhere to the contact charging device when passing through the charging portion which is the contact nip portion between the photosensitive member and the contact charging device.

  In addition, in order to remove and collect the transfer residual toner on the photosensitive member by the developing device, the charged polarity of the transfer residual toner on the photosensitive member that is carried to the developing unit after passing through the charging unit must be a normal polarity. is necessary. Reversal toner and toner with an inappropriate charge amount cannot be removed and collected from the photoreceptor to the developing unit, and cause defective images. Therefore, it is necessary to suppress the generation as much as possible.

  In view of this, an image forming apparatus is proposed in which residual toner equalizing means is provided downstream of the transfer portion in the rotation direction of the photosensitive member, and toner charge amount control means is provided further downstream and upstream of the charging means for charging the photosensitive member. (Patent Document 1).

  In this image forming apparatus, the residual toner equalizing unit distributes and distributes the pattern-like residual toner image on the photosensitive member, which is carried from the transfer unit to the toner charge amount control unit unit, to the non-patterned surface. To do. Next, the toner charge amount control unit to which the normal polarity is applied performs the normal polarity charging process on the entire transfer residual toner, and the adhesion of the transfer residual toner to the charging unit is effectively suppressed.

  By the above means, in the cleanerless image forming apparatus, it is possible to suppress the transfer residual toner from adhering to the charging means and to stably charge the potential of the photoreceptor. Further, since the charge of the transfer residual toner becomes uniform and the potential of the photosensitive member is stabilized, the transfer residual toner is efficiently collected by the developing device, and a good image free from ghost and defective charging is obtained. be able to.

JP 2001-215798 A

  However, when image formation is repeatedly performed in a cleanerless image forming apparatus and the developer deteriorates, fog toner in the developing device increases. The fog toner includes many toners having a reverse charge polarity or a charge amount close to zero, and is easily attached to the charging device. The charging device to which the toner is attached cannot be charged to the charging potential aimed at the photoreceptor. Therefore, the potential difference (= fogging potential difference) Vback between the voltage applied to the developing device and the surface potential of the photosensitive member becomes smaller than the target. When Vback, which is the fog removal potential difference, decreases, the fog amount of the developing device further increases. As a result, the toner adheres to the charging device more and more, and the charged potential shifts, the fog of the developing device deteriorates, and the problem of a defective fog image arises.

  Further, since the fog toner has the charging polarity as described above and the transfer efficiency is low and a large amount of residual toner is generated, the effects of the residual toner equalizing means and the toner charge amount control means provided downstream of the transfer portion are effective. As a result, the charging potential of the photosensitive member is further shifted. As a result, the toner recovery efficiency in the developing unit is lowered, and there is a problem that a defective image is generated due to ghost after one rotation of the photoreceptor on which the image is formed.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing fogging of a developing device and forming a good image over a long period of time.

  In order to solve the above problems, a typical configuration of an image forming apparatus according to the present invention includes a developing unit that develops an electrostatic latent image formed on an image carrier with a developer, and a development on the image carrier. An image forming apparatus that collects the developer remaining on the surface of the image carrier without being transferred by the transfer unit, and developed by the developing unit. It has an image density detection sensor that detects the image density at a plurality of positions in the image carrier rotation axis direction, and compares and compares the detection results at the plurality of positions in the image carrier rotation axis direction detected by the image density detection sensor. A fog suppression operation is performed based on the result.

  According to the present invention, it is possible to suppress the fogging of the developing device and form a good image over a long period.

[First embodiment]
A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment.

(Image forming device)
As shown in FIG. 1, the image forming apparatus 100 is a color laser printer using a cleanerless system. The image forming apparatus 100 forms and outputs a full-color image on a transfer material according to image information from an external host device that is communicably connected to the image forming apparatus main body (apparatus main body). Examples of the transfer material include paper, an OHP sheet, and cloth, and the maximum sheet passing size of the image forming apparatus 100 is A3 size.

  The image forming apparatus 100 is a quadruple drum system (inline system), and has yellow (PY), magenta (PM), cyan (PC), and black (PBk) as an image forming unit as a plurality of image forming units. Each image forming unit includes a process cartridge 8. Each process cartridge 8 continuously transfers multiple toner images (developer images) onto an intermediate transfer member (intermediate transfer belt) 91 and then transfers them onto a transfer material P at a time to obtain a full color print image.

  The image forming portions PY, PM, PC, and PBk for each color have the same configuration except that the color of the developer used is different. Hereinafter, when there is no particular need to distinguish, subscripts Y, M, C, and Bk that indicate elements of each image forming unit will be omitted, and a general description will be given.

  When a four-color full-color image is formed, a color-separated image signal is generated in accordance with a signal from an external host device that is communicably connected to the image forming apparatus 100. In accordance with the image signal, toner images of the respective colors are formed in the process cartridges 8Y, 8M, 8C, and 8Bk of the image forming units PY, PM, PC, and PBk.

  In each of the process cartridges 8Y, 8M, 8C, and 8Bk, an electrophotographic photosensitive member (photosensitive drum) 1 as an image carrier is charged by a charging unit (charging roller) 2. An electrostatic latent image is formed on the photosensitive drum 1 by scanning and exposing the uniformly charged surface by the exposure unit 3. A toner image is formed on the electrostatic latent image by supplying toner as a developer by a developing device (developing means) 4.

  The toner images of the respective colors formed on the respective photosensitive drums 1 are sequentially superimposed and transferred onto an intermediate transfer belt 91 as a moving intermediate transfer member (second image carrier). The full-color toner image formed on the intermediate transfer belt 91 is transferred onto the transfer material P that has been conveyed to the secondary transfer portion where the intermediate transfer belt 91 and the secondary transfer roller 10 as the secondary transfer unit face each other. Are collectively transferred. The transfer material P onto which the toner image has been transferred is conveyed to the fixing unit 13 and fixed to the toner image, and then discharged outside the apparatus main body.

(Photosensitive drum 1, charging roller 2)
As shown in FIG. 2, the photosensitive drum 1 is a rotating drum type organic photoconductor (OPC) drum having an outer diameter of 30 mm. And it is rotationally driven centering on the rotating shaft at a process speed (circumferential speed) of 150 mm / sec. The photosensitive drum 1 has, on the surface of an aluminum cylinder (conductive drum substrate), an undercoat layer, a photocharge generation layer, and a charge transport layer (thickness: 20 μm) that in turn suppresses light interference and improves the adhesion of the upper layer. It is made by repainting.

  The charging roller 2 is a contact charger that uniformly charges the photosensitive drum 1 to a negative polarity by applying a voltage under a predetermined condition. The charging roller 2 is a roller having a diameter of 14 mm in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are sequentially laminated from the bottom around a metal core (support member) 2a. The charging roller 2 is urged in the direction of the photosensitive drum 1 by the pressing spring 2 h, is pressed with a predetermined pressing force against the surface of the photosensitive drum 1, and is driven to rotate by the photosensitive drum 1.

  A predetermined oscillating voltage (bias voltage Vdc + Vac) obtained by superimposing an AC voltage of a predetermined frequency on a DC voltage is applied to the charging roller 2 through the cored bar 2a from the power source 20 as voltage applying means. The voltage applied to the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (−500 V) and an AC voltage (sine wave, frequency: 1150 Hz) having a peak-to-peak voltage controlled to have a constant discharge current amount. It is. The peripheral surface of the rotating photosensitive drum 1 is charged to a predetermined potential (−500 V (charging potential = dark potential Vd)) by the charging roller 2. The charging portion a is a contact portion between the charging roller 2 and the photosensitive drum 1.

  The charged photosensitive drum 1 receives an image exposure L from the exposure unit 3 to form an electrostatic latent image corresponding to the image information. The exposed portion potential Vl was set to -150V. The exposure part b is an irradiation position of the image exposure L on the photosensitive drum 1. Note that voltage application means such as the power supplies 20, 21, and 22 provided in the image forming apparatus 100 are controlled by a control circuit 130 as a control means that controls the overall operation of the apparatus.

(Developer 4)
The developing device 4 is a two-component contact developing device (two-component magnetic brush developing device). The developing device 4 includes a developing container (developing device main body) 40, a developing sleeve 41, a developer regulating member (developer regulating blade) 42, a developer stirring member (stirring screw) 43 and 44, and a two-component developer (developer). 46.

  The developer 46 is a mixture of mainly resin toner particles (toner) and magnetic carrier particles (carrier) contained in the developing container 40. A negatively chargeable non-magnetic toner having an average particle diameter of 6 μm is used as the toner, and a magnetic carrier having an average particle diameter of 40 μm is used as the carrier.

  The developing sleeve 41 is a developer carrying member having a magnet roller fixedly disposed therein. The developing sleeve 41 is rotatably disposed in the developing container 40 with a part of its outer peripheral surface exposed to the outside. The developing sleeve 41 is disposed so as to face the developer regulating blade 42 with a predetermined gap, and forms a thin developer layer on the developing sleeve 41 as the developing sleeve 41 rotates. The developing sleeve 41 was disposed opposite to the photosensitive drum 1 while keeping the closest distance (S-Dgap) to the photosensitive drum 1 at 350 μm. The developing portion c is a facing portion between the photosensitive drum 1 and the developing sleeve 41.

  The developer thin layer on the developing sleeve 41 comes into contact with the surface of the photosensitive drum 1 at the developing portion c and rubs the photosensitive drum 1 appropriately. A predetermined developing bias voltage is applied to the developing sleeve 41 from a power source (not shown) as voltage applying means. The developing bias voltage applied to the developing sleeve 41 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, an oscillating voltage obtained by superimposing Vdc of −350 V, Vac of 1800 Vpp and frequency = 2300 Hz is superimposed. Therefore, since the charging potential is −500V, the fog removal potential Vback is 150V.

  Then, the toner in the developer 46 conveyed to the developing unit c is selectively attached in accordance with the electrostatic latent image formed on the photosensitive drum 1 by the electric field generated by the developing bias voltage. Is developed as a toner image. The toner adheres to the exposed bright portion on the photosensitive drum 1 and the electrostatic latent image is reversely developed. The developer thin layer on the developing sleeve 41 that has passed through the developing portion c is returned to the developer reservoir in the developing container 40 as the developing sleeve 41 continues to rotate.

  The stirring screws 43 and 44 are disposed on the bottom side in the developing container 40. The agitating screws 43 and 44 rotate in synchronization with the rotation of the developing sleeve 41, and have a function of agitating and mixing the supplied toner with a carrier to give the toner a predetermined charged charge.

  The movement of the developer by the stirring screws 43 and 44 will be described. FIG. 3 is a view of the developing device 4 as viewed from above. As shown in FIG. 3, the agitation screws 43 and 44 each convey the developer 46 in the opposite direction in the longitudinal direction. As a result, the developer 46 is supplied to the developing sleeve 41, and the developer 46 whose toner concentration (ratio of toner in the developer) has been reduced by the developing process is conveyed to the toner replenishing section, and the developer 46 is supplied to the developing container. Circulate within 40. On the upstream side wall surface of the screw 44 of the developing device 4, a toner concentration sensor 45 (see FIG. 2) that detects a change in the magnetic permeability of the developer 46 and detects the toner concentration in the developer 46 is provided. The concentration is detected.

  According to the detection result, toner is replenished from the toner replenishment unit 5 through the toner replenishment opening 47 of the developer 4 by the rotation of the screw 51 of the developer replenishment container (toner replenishment unit) 5 connected to the developer 4 as appropriate. Done. As a result, the toner concentration in the developer 46 is kept constant. Further, not only the sensor 45 but also a calculation unit (not shown) for counting the image ratio at the time of image formation is provided for toner supply, and the toner supply amount is determined according to the image ratio counted by the calculation unit. The toner is replenished.

  The replenished toner is conveyed by the agitating screw 44, mixed with the carrier, given a suitable charge, and then carried to the vicinity of the developing sleeve 41. A thin layer is formed on the developing sleeve 41 for development. The If the replenished toner cannot be sufficiently charged and is transported to the developing unit c with a low charge amount or reverse polarity, it becomes fog toner. Therefore, the replenished toner must have an appropriate charge amount to be used for development before being conveyed to the developing unit c.

(Transfer part)
Next, the transfer part will be described. An intermediate transfer unit 9 as a transfer unit is provided so as to face the photosensitive drums 1 of the image forming units PY, PM, PC, and PBk. The intermediate transfer unit 9 includes an intermediate transfer belt 91, a driving roller 94, a tension roller 95, and a secondary transfer counter roller 96.

  The intermediate transfer belt 91 is an endless belt as an intermediate transfer member (second image carrier). The intermediate transfer belt 91 is stretched around the rollers 94, 95, and 96 with a predetermined tension, and moves in the direction of the arrow in the figure. The material of the intermediate transfer belt 91 is preferably a resin-based or rubber belt containing a metal core, or a belt made of resin and rubber. As the primary transfer roller 92, one made of a conductive sponge was used.

  The toner image formed on the photosensitive drum 1 enters a primary transfer nip portion (transfer portion) d that is a portion where the photosensitive drum 1 and the intermediate transfer belt 91 are opposed to each other. In the transfer part d, a primary transfer roller 92 as a primary transfer unit is brought into contact with the back side of the intermediate transfer belt 91, and magenta, cyan, and black toner images are sequentially transferred from the first color (yellow). . In consideration of transfer efficiency with respect to the toner transferred to the exposed portion (exposed portion potential Vl: −150 V), a voltage of +350 V was applied to the first to fourth colors as the primary transfer bias voltage.

  The four-color full-color image formed on the intermediate transfer belt 91 is then collectively transferred to the transfer material P by the secondary transfer roller 10 as a secondary transfer unit. The transfer material P is supplied from a transfer material feeding unit (not shown), and is fed to the secondary transfer roller 10 by a feeding roller 12 as a conveying unit at a predetermined timing.

  The transfer material P onto which the toner image has been transferred is conveyed to a roller fixing device 13 as a fixing unit, where the toner image is melt-fixed on the transfer material P by heat and pressure, and is conveyed outside the machine as a color print image. . The secondary transfer residual toner remaining on the intermediate transfer belt 91 is cleaned by a cleaning blade 11a as a cleaning unit provided in the intermediate transfer belt cleaner 11, and is prepared for the next image forming process.

  Further, as shown in FIG. 2, the toner charge control means 6 and the residual toner image equalizing means 7 are brought into contact with the photosensitive drum 1 between the transfer portion d and the charging portion a in the rotation direction of the photosensitive drum 1. ing. The residual toner uniformizing means 7 and the toner charge amount control means 6 are located downstream of the transfer portion d in the rotation direction of the photosensitive drum 1 and upstream of the charging portion a, and sequentially from the upstream of the rotation direction of the photosensitive drum 1. Has been placed. The residual toner uniformizing means 7 forms a contact portion e with the photosensitive drum 1, and the toner charge amount control means 6 forms a contact portion f with the photosensitive drum 1.

  Both the toner charge amount control means 6 and the residual toner equalization means 7 have brush portions 61 and 71 made of conductive fibers. The toner charge amount control means 6 and the residual toner equalizing means 7 are fixedly supported and arranged substantially parallel to the longitudinal direction of the photosensitive drum 1 (image carrier rotation axis direction), and the brush portions 61 and 71 are photosensitive. It contacts the drum 1 surface. A predetermined voltage is applied from the power source 22 to the toner charge amount control means 6 and the residual toner equalization means 7. The brush parts 61 and 71 are formed by adding carbon or metal powder to fibers such as rayon, acrylic, and polyester to control the resistance value.

  The toner charge amount control means 6 and the residual toner equalization means 7 apply an electric field to the toner charged to zero or reverse polarity in the transfer residual toner to make the transfer residual toner an appropriate charge amount. Then, the untransferred toner that has passed through the toner charge amount control means 6 adheres to the charging roller 2 due to the mirroring force with the photosensitive drum 1 when further passing through the charging portion a located downstream in the rotation direction of the photosensitive drum 1. Pass without. Further, the charge amount of the transfer residual toner is reduced to an appropriate charge amount that can be collected by the developing device 4 due to the effect of charge removal by the AC voltage applied to the charging roller 2 at the charging portion a.

  In this way, the untransferred toner that has passed through the charging portion a is removed from the photosensitive drum 1 by the development unit 4 by simultaneous development cleaning.

(Cover)
The details of the occurrence of fogging will be described. The term “fogging” refers to toner that adheres to the white background, and the toner adhesion to the white background is suppressed by the fog removal potential Vback formed by the charging potential and the development potential.

  The fog toner contains a lot of toner that does not have an appropriate charge amount, such as a state where the charge amount is close to zero or a state of reverse polarity. When image formation is repeated, the developer deteriorates. The deterioration of the developer means a decrease in carrier charging ability due to adhesion of toner to the carrier surface or scraping of the carrier surface, or a reduction in toner charging ability due to release or accumulation of toner external additives.

  When the developer is deteriorated, it becomes difficult to maintain an appropriate charge amount, and the charge amount is likely to be zero, low, or reverse polarity without being charged, causing fogging. A cleanerless image forming apparatus once becomes a transfer residual toner, and a physical pressure or an electric field is applied to the transfer part or the charging part, and the deformed toner or external additives are released. The toner in a deteriorated state in which the charging ability is remarkably lowered is reused.

  The fog phenomenon is likely to occur when a large amount of toner is replenished in a short period of time. This is because the replenished toner is not sufficiently mixed by the stirring screws 43 and 44 in the developing device and is transported to the developing unit c without proper charging. If image formation is performed immediately after leaving the image forming apparatus without using the image forming apparatus for a long period of time after the last image formation, the toner charge amount decreases due to the leaving, and the image cannot be properly charged. It is used for formation and the fog is deteriorated.

  In a cleanerless image forming apparatus, fogging not only appears in an output product but also greatly affects the image forming apparatus. Since the fog toner includes many toners having a charge amount close to zero and those having a reverse polarity, the transfer efficiency is low, and the effects of the toner charge amount control means 6 and the residual toner equalizing means 7 are also normal transfer residuals. It is lower than the toner and easily adheres to the charging roller 2. The fog transfer residual toner that does not adhere to the charging roller 2 has an inappropriate charge amount, so that the recovery efficiency in the developing device 4 is low, and the toner adheres to the surface of the photosensitive drum, thereby hindering not only charging but also exposure.

  Further, when the toner adheres to the charging roller 2, the charging potential shifts in a direction in which the fog removal potential difference for suppressing the fogging decreases, and the fog toner increases more and more. As a result, the charging roller 2 is further contaminated with the transfer residual toner, and the deviation range of the charging potential is increased, and more fog toner is generated. In this case, both the charging roller 2 and the photosensitive drum 1 are contaminated with the transfer residual toner, and must be replaced. The downtime due to the replacement time of the charging roller 2 and the photosensitive drum 1 and the shortening of the replacement life are significant disadvantages for the user, such as an increase in running cost. Therefore, before the charging roller 2 is contaminated with the transfer residual toner, it is necessary to deal with it immediately.

  Further, in a state where the fog appears on the entire surface, the fog toner which has become a transfer residual toner not only lightly adheres to the surface of the charging roller 2 or the photosensitive drum, but also is repeatedly melted and discharged, so that the toner is firmly melted. It will be in a worn state. If it is lightly attached, it can be removed by applying an electric field. However, if it is firmly attached like fusion, it cannot be removed even if an electric field is applied. Therefore, in order to reliably suppress the fogging, it is necessary to quickly detect and respond to the beginning of the fogging deterioration.

  An image with a very low image density, such as a fog density, is affected by the surface property (background) of the photosensitive drum 1 on which the image is placed, and is difficult to detect with high accuracy. In particular, in a cleanerless image forming apparatus, since there is no cleaner on the photosensitive drum 1, there is residual toner on the photosensitive drum, and it is difficult to determine which timing is used as the background.

  For example, the fog toner may be attached to the photosensitive drum during image formation or immediately after completion of image formation. With a single image density sensor, the state where the fog toner is attached is detected as the background, and therefore it is impossible to accurately detect whether it is the background or the fog toner. If the background level to be detected is given a threshold value, and the background above a certain level is judged to be fogging, and if it is detected that it is fogging, the fogging level is already very bad and a fogging image is generated. End up.

  Therefore, as shown in FIG. 3, image density detection sensors 301 and 302 are provided at two locations in the longitudinal direction of the photosensitive drum 1 (in the image carrier rotation axis direction) so as to face the photosensitive drum 1. Image density detection sensors 301 and 302 detect the image density of the toner developed on the photosensitive drum. The image density detection sensors 301 and 302 have a light emitting unit and a light receiving unit. The light emission unit emits light with a predetermined light amount, and the light density is detected by detecting the reflected light amount of the ground and the reflected light amount of the image unit when there is no image. In accordance with the detection results of the image density detection sensors 301 and 302, the toner replenishment and the development electric field are controlled to keep the density of the output product constant.

  The image density detection sensor 301 is positioned upstream of the stirring screw 43 in the toner conveyance direction, and the image density detection sensor 302 is installed downstream of the stirring screw 43 in the conveyance direction. By disposing two image density detection sensors at two locations in the longitudinal direction of the photosensitive drum 1, the density difference in the longitudinal direction can be detected, and the fog can be accurately detected.

  This utilizes the tendency that the fog starts to deteriorate from the upstream side in the conveying direction of the stirring screw 43. Fog tends to occur when a large amount of toner is replenished in a short period of time and the toner is not sufficiently mixed and charged. For this reason, the upstream side in the transport direction of the stirring screw 43, which has a short transport distance from the toner replenishing port 47, is transported to the developing unit c without being sufficiently mixed and charged, and thus fogging deteriorates from the upstream side. Start to do.

  Therefore, when the fog starts to deteriorate, the image density of the detection value of the upstream sensor 301 increases when the detection values of the downstream sensor 302 and the upstream sensor 301 in the transport direction are compared. Unlike a single sensor, the upstream sensor 301 and the downstream sensor 302 can detect and compare to determine whether the background is changing or the fog, and the fog level is accurately determined. can do.

  As shown in FIG. 6, the control recording unit 30 includes a CPU 31, a memory 32, a toner supply control unit 34 that controls the toner supply unit 5, and an image forming control that controls an image forming unit (charging roller 2, developing device 4, etc.). A unit 35 is provided. The control recording unit 30 exchanges control signals with the toner replenishing unit 5, the image forming unit, and a clock 33 (an apparatus capable of measuring the image formation time) which is a measuring means built in the main body.

  The operation statuses of the clock 33 and the image forming unit are recorded in the CPU 31 and the memory 32, and the leaving time from the previous image formation is calculated.

  Further, the memory 32 and the CPU 31 calculate the replenishment toner amount and determine the replenishment toner amount when image data or the like is input. The toner replenishment control unit 34 controls toner replenishment to the developing device 4 and replenishes toner according to a signal from the CPU 31. At this time, the toner supply amount for a predetermined period is calculated by recording the toner supply amount in the memory 32. Further, the image forming conditions are also controlled by the control recording unit 30.

(Fog detection, fog suppression operation)
Hereinafter, a series of flow from the fog detection timing to the fog suppression operation will be specifically described with reference to FIG.

  First, when an input signal for image formation is input (step 1 = S1), the apparatus leaving time Th from the previous image formation is read from the control recording unit 30 (S2). It is determined whether the read leaving time Th exceeds a threshold T (10 hours) (S3).

  If the threshold value T is not exceeded in S3, the amount of toner replenished during a predetermined period is read (S4), and it is determined whether the read amount of replenished toner exceeds the threshold value H (S5). The threshold value H is whether or not the replenishment amount for 10 sheets of A4 size up to the previous time is 0.5 g or more.

  If the threshold value H is not exceeded in S5, it is determined that the probability of occurrence of fogging is low, and the process proceeds to a state where image formation can be started (S10), and the image forming operation is performed.

  If the threshold value T is exceeded in S3, the charge amount of the toner in the developing device is lowered by being left, and there is a possibility of fogging. If the threshold value H is exceeded in S5, the replenishment toner is not sufficiently charged, and fogging may occur. Therefore, the fog density is detected by the image density detection sensors 301 and 302 (S6).

  To detect fog, a white background of about 15 mm is formed in the longitudinal direction (image carrier rotation axis direction) on the photosensitive drum, and the sensors 301 and 302 are used for the white background (a plurality of positions in the image carrier rotation axis direction). Detect the image density. Then, the values of the upstream sensor 301 and the downstream sensor 302 are compared to determine whether the threshold value P (density difference 0.03) is exceeded (S7).

  If the threshold value P is not exceeded in S7, it is determined that fogging has not yet occurred, and a transition is made to a state where image formation can be started (S10), and an image forming operation is performed.

  If the threshold value P is exceeded in S7, it is determined that the fog is starting to deteriorate, and a fog suppression operation is performed (S8). Specifically, as the fog suppression operation, idling of the developing device 4 is performed for 1 minute to increase the charge amount in the developing device, thereby suppressing the occurrence of fog.

  Further, feedback is made to the image forming conditions so as not to cause fogging (fogging suppression operation) (S9). Specifically, a developing electric field in which the fog removal potential difference Vback is increased by about 10 V is applied so that even if the fog toner exists in the developing device, it is difficult to develop.

  In addition, a cleaning operation by a cleaning member for cleaning the toner attached to the charging roller 2 or a cleaning operation for applying a cleaning electric field may be provided, and the cleaning operation may be performed in response to S8. If the charging roller 2 is kept in a clean state by the cleaning operation, even if fog toner adheres, the absolute amount is small and the influence on the image formation is small.

  Further, since the transfer residual toner adheres not only to the charging roller 2 but also to the toner charge amount control means 6 and the residual toner equalizing means 7, it is necessary to clean. In S8, the control for discharging the toner adhering to the toner charge amount control means 6 and the residual toner equalization means 7 is also effective for suppressing fogging.

  Further, the transfer electric field may be optimized to prevent the occurrence of transfer residual toner as much as possible. Further, not only idling of the developing device 4 but also discharging control for discharging the fog toner in the developing device, or replenishment control for reducing the toner replenishment amount after the next time by some percentage from normal, The fog can be suppressed.

  As a result, the fog is improved, and the fog density is confirmed again by the image density sensors 301 and 302 (S6), and the fog density level is detected (S7). The control in S6 to S9 is performed until the fogging is below a predetermined level.

  By performing the fog suppression operation as described above, a good image can be output over a long period of time without causing fog. By suppressing the fogging, it is possible to reduce the adhesion of the transfer residual toner to the charging roller 2 and the photosensitive drum 1, so that the charging roller 2, the photosensitive drum 1, and the developing device 4 can be used until the expected life, Running costs can be reduced.

(Experiment)
An experiment was conducted in which image formation was continuously performed while outputting an image with an image ratio of 5% and occasionally an image with an image ratio of 100%.

Specifically, first, 5,000 images with an image ratio of 5% are output. Thereafter, the power source of the image forming apparatus is turned off, and the image forming apparatus is stopped without operating for 10 hours or more. When 10 hours or more have elapsed, the power is turned on again, 10 images with an image ratio of 100% are continuously output, and then an image with an image ratio of 5% is output again. At this time, the fog on the paper is confirmed in an image with an image ratio of 5% output after 100% of the image ratio. If there is no problem when the fog is confirmed, output is continued up to 5,000 images with an image ratio of 5%.

  As a result, in the conventional image forming apparatus, fogging occurs on 20,000 sheets, and it is necessary to replace the photosensitive drum 1 and the charging roller 2. In the image forming apparatus 100 of the present embodiment, it is possible to perform image formation without generating fog up to 45,000 sheets, and it has been proved that it is effective in suppressing fog.

  In addition, if an environmental sensor capable of detecting temperature and humidity is provided, and the threshold values T and H are changed depending on the temperature and humidity, fogging can be more reliably suppressed. The threshold values T, H, and P are preferably determined in consideration of the balance of the entire image forming system.

  Although the toner charge control means 6 and the residual toner image equalizing means 7 are described as brush-like shapes, the present invention is not limited to this, and may be any member such as a brush rotating body or an elastic roller body. Also good. The voltage applied to the residual toner image uniformizing means 7 is only a DC voltage, but the present invention is not limited to this. Even when a voltage obtained by superimposing an AC voltage on a DC voltage is applied for the purpose of improving the recoverability of the transfer residual toner to the residual toner image equalizing means 7, the effect of the present invention can be obtained.

Further, the image carrier may be of a direct injection charging type provided with a charge injection layer having a surface volume resistivity of 10 ⁹ to 10 ¹⁴ Ω · cm. Even when the charge injection layer is not used, for example, the same effect can be obtained when the charge transport layer is in the above resistance range. Alternatively, an amorphous silicon photoreceptor having a surface layer volume resistivity of about 10 ¹³ Ω · cm may be used.

  As the waveform of the alternating voltage component (AC component, voltage whose voltage value periodically changes) of the oscillating electric field applied to the contact charging member or the developing member, a sine wave, a rectangular wave, a triangular wave, or the like can be used as appropriate. It may be a rectangular wave formed by periodically turning on / off a DC power supply.

  Further, the exposure means as the information writing means for the charging surface of the photosensitive drum 1 as the image carrier is not only the laser scanning means of the embodiment but also a digital exposure means using a solid light emitting element array such as an LED. There may be. An analog image exposure unit using a halogen lamp or a fluorescent lamp as a document illumination light source may be used. In short, any device capable of forming an electrostatic latent image corresponding to image information may be used.

[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.

  In the first embodiment, the fog suppression control in which the fog image is suppressed in advance and no fog image is formed has been described. In the image forming apparatus 100 of the present embodiment, the fog suppression operation is performed during post-rotation after the end of image formation.

(Fog detection, fog suppression operation)
Hereinafter, a series of processes from the fog detection timing to the fog suppression operation will be described with reference to FIG. The operations of S1 to S5 are the same as those in the first embodiment, but if the thresholds T and H are exceeded in S3 and S5, it is recorded that the thresholds T and H are exceeded, and the image Formation is performed (S11).

  After the image formation in S11 is completed, the fog density is confirmed and the fog suppression operation is performed at the time of post-rotation after the completion of the image formation, or between papers in the case of continuous image formation (S6). The operations of S6 to S9 are the same as in the first embodiment.

(effect)
In the case of detecting the image density on the intermediate transfer belt, the present embodiment also compares the detection values of the plurality of sensors 301 and 302 as compared to the case of detection by a single sensor, as in the first embodiment. By calculating, it is possible to detect the start of deterioration of fog more quickly.

  In addition, an image formation with an image ratio of 5% and an image with an image ratio of 100% at a predetermined interval was occasionally performed while being output. As a result, fogging occurs with 20,000 sheets in the conventional image forming apparatus, fogging occurs with 45,000 sheets in the first embodiment, and fogging occurs up to 53,000 sheets in the present embodiment. The image could be formed without any problems.

  Further, by performing the fog suppression operation at the time of post-rotation after completion of image formation, the user does not enter the fog suppression operation before the output product is output after the user presses the copy start button. For this reason, the time from when the user inputs an image forming signal (by pressing the copy button) until the output is output is faster than in the first embodiment. Specifically, the time from when an image formation signal is input to when an output product is output is 65.0 seconds in the first embodiment, which is significantly delayed when the fogging suppression operation is activated. On the other hand, in the present embodiment, by changing the fogging confirmation and the fogging suppression operation after the image formation, the image can be quickly formed and output in 10.5 seconds.

  By using a sensor with a short detection time, changing the threshold value, and checking the fog density for each image forming sheet or between paper sheets, the fog can be more reliably suppressed while reducing downtime as much as possible. Can do.

  It should be noted that the timing for detecting the fog density may also be performed at the time of normal image density confirmation.

[Third embodiment]
Next, a third embodiment of the image forming apparatus according to the present invention will be described. About the part which overlaps with said 1st embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted.

  In the above embodiment, the fog density of the photosensitive drum 1 is detected. In the image forming apparatus according to the present exemplary embodiment, image density detection sensors 301 and 302 are disposed on the opposing portion of the intermediate transfer belt 91 to detect the fog density of the image on the intermediate transfer belt.

  Compared to checking on the photosensitive drum, checking the image density on the intermediate transfer belt detects the fog density in a process close to output, so that the fog of the output can be detected with high accuracy.

  The fog density of all colors of YMCK can be detected only by a set of image density detection sensors 301 and 302 in which a plurality of image densities of all colors of YMCK are installed in the image carrier rotation axis direction. For this reason, compared with the case where the sensors 301 and 302 are arranged on the photosensitive drums 1 of the respective colors, the number of sensors becomes ¼, and there are advantages such as cost reduction and downsizing.

1 is a configuration diagram of an image forming apparatus according to a first embodiment. It is sectional drawing of a process cartridge. It is a block diagram of a developing device. It is a flowchart of a fog detection and a fog suppression operation. It is a schematic diagram of a control recording part and its periphery. It is a flowchart of the fog detection and the fog suppression operation according to the second embodiment.

Explanation of symbols

H ... Threshold L ... Image exposure P ... Transfer materials PBk to PY ... Image forming portion T ... Threshold a ... Charging portion b ... Exposure portion c ... Development portion d ... Transfer portion e, f ... Contact portion 1 ... Photosensitive drum 2 ... Charging Roller 2a ... Core 2b ... Lower layer 2c ... Intermediate layer 2d ... Surface layer 2h ... Pressing spring 3 ... Exposure means 4 ... Developer 5 ... Toner replenishment unit 6 ... Toner charge control means 7 ... Residual toner image equalizing means 8 ... Process Cartridge 9 ... Intermediate transfer unit 10 ... Secondary transfer roller 11 ... Intermediate transfer belt cleaner 11a ... Cleaning blade 12 ... Feeding roller 13 ... Roller fixing device 20 ... Power source 21, 22 ... Power source 30 ... Control recording unit 31 ... CPU
32 ... Memory 33 ... Clock 34 ... Toner supply control unit 35 ... Image formation control unit 40 ... Developer container 41 ... Developer sleeve 42 ... Developer regulating blades 43, 44 ... Stir screw 45 ... Sensor 46 ... Developer 47 ... Toner supply opening DESCRIPTION OF SYMBOLS 51 ... Screw 61, 71 ... Brush part 91 ... Intermediate transfer belt 92 ... Next transfer roller 94 ... Drive roller 95 ... Tension roller 96 ... Secondary transfer opposing roller 100 ... Image forming apparatus 130 ... Control circuit 301, 302 ... Image density detection Sensor

Claims (5)

Developing means for developing the electrostatic latent image formed on the image carrier with a developer;
Transfer means for transferring a developer image of the image carrier,
In the image forming apparatus in which the developer remaining on the surface of the image carrier without being transferred by the transfer unit is collected by the developing unit,
An image density detection sensor for detecting the density of the image developed by the developing means at a plurality of positions in the image carrier rotation axis direction;
An image forming apparatus that performs a fog suppression operation based on detection results of a plurality of positions in the direction of the rotation axis of the image carrier detected by the image density detection sensor.   The image forming apparatus according to claim 1, wherein whether or not to detect the image density by the image density detection sensor is determined according to a replenishment toner amount to be supplied to the developing unit.   3. A measuring means for measuring the time of image formation, and determining whether or not the image density is detected by the image density detection sensor according to a standing time from the previous image formation. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the image density detection sensor detects a density of an image formed on the image carrier.   4. The image forming apparatus according to claim 1, wherein the image density detection sensor detects a density of an image transferred from the image carrier. 5.

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