JP6172593B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and more specifically, an image forming apparatus that transfers a toner image on a latent image carrier or an intermediate transfer member to a recording material by a transfer bias applied by a transfer unit. It is about.

  In an electrophotographic image forming apparatus, an electrostatic latent image obtained by forming optical image information on a latent image carrier such as a uniformly charged photoconductor is converted into toner from a developing device. This visible image is transferred directly onto a recording material such as transfer paper or via an intermediate transfer member such as an intermediate transfer belt, and fixed on the recording material to form an image. In many of such image forming apparatuses, a DC transfer bias is applied by a transfer unit when transferring from an image carrier such as a photosensitive member or an intermediate transfer member onto a recording material.

  In recent years, a wide variety of recording materials have been used as recording materials used in image forming apparatuses, such as paper with a high-quality leather pattern and Japanese paper-like paper. Some of such recording materials have irregularities on the surface by embossing or the like for the purpose of giving a sense of quality. When a toner image is transferred to such a recording material, the toner is less easily transferred to the concave portion on the surface of the recording material than the convex portion. Therefore, when a toner image is transferred to a recording material having a large unevenness, the toner cannot be sufficiently transferred to the recessed portion, and the image density of the recessed portion is relatively smaller than the image density of the protruding portion. It tends to be low. As a result, a shading pattern that is uneven on the surface of the recording material tends to occur in the image.

  As a method for improving the transfer failure to the surface concave portion of the recording material, a method using a transfer bias (hereinafter referred to as “superimposed transfer bias”) in which the AC component is superimposed on the DC component and the polarity changes with time is known. For example, it is proposed in Patent Documents 1 to 4. By switching the transfer mode to a DC transfer mode or a DC / AC superimposed transfer mode (hereinafter referred to as “superimpose transfer mode”) according to the type of recording material to be passed, a wide variety of recordings including a recording material having a large surface unevenness Good transferability can be obtained for each material.

  Further, it is known that the transferability to a recording material depends on the deterioration state of the toner. For example, when an image having a low image area ratio is continuously output, the toner balance in the developing device is small, and thus a large amount of toner stirred for a long time remains in the developing device. The toner subjected to stress due to such long-term agitation causes the external additive to be buried or released, thereby deteriorating the fluidity of the developer or changing the charging characteristics of the toner. As a result, the transfer rate decreases and it becomes difficult to obtain good transferability.

  As a method for improving the decrease in the transfer rate due to such toner deterioration, there is known a method of replacing toner in the developing device by replenishing new toner while forcibly consuming the deteriorated toner in the developing device. For example, it is proposed in Patent Documents 5-7.

  However, the influence of the deteriorated state of the toner on the transferability depends on whether the recording material has irregularities, in other words, whether the transfer bias used is a DC transfer bias or a superimposed transfer bias. It turns out that there is a difference. Specifically, when transferring to a recording material with irregularities as in the superimposed transfer mode using a superimposed transfer bias, the deterioration state of the toner has a large effect on the transferability, and transferability when using deteriorated toner is used. The deterioration becomes remarkable. That is, even when the toner is in a deteriorated state in which the transferability is within the allowable range in the direct current transfer mode, the transferability may exceed the allowable range in the superimposed transfer mode. This is considered to be due to the fact that when the toner is deteriorated, the toner cannot follow the time change of the bias in the superimposed transfer mode, and the toner in the transfer region cannot exhibit the target behavior.

  The present invention has been made in view of the above background, and an object of the present invention is to improve transferability when a superimposed transfer bias is used even in a state where toner deterioration in a developing device is progressing. An image forming apparatus is provided.

In order to achieve the above object, the present invention provides a latent image carrier that carries a latent image corresponding to image information on the surface, and a toner image by attaching toner to the latent image on the latent image carrier using a developer. A developing device that performs a developing process, and a transfer unit that directly transfers the toner image on the latent image carrier formed by the developing process to a recording material, or transfers the toner image to a recording material via an intermediate transfer member. When an image is formed on a recording material with small surface irregularities, a DC transfer bias consisting of only a DC component is used. When an image is formed on a recording material with large surface irregularities , an AC component is superimposed on the DC component. as used superimposed transfer bias whose polarity changes with time, a transfer bias switching means for switching the transfer bias said transferring means is applied when transferring the toner image to the recording material, the between images during continuous image forming operation The amount of toner to forcibly consume the toner in the image device, the DC transfer than in the continuous image forming operation using the bias control so it is increased during the continuous image forming operation using the superimposed transfer bias characterized by chromatic and control means for the.

  According to the present invention, it is possible to obtain an excellent effect that the transferability when the superimposed transfer bias is used can be improved even when the toner in the developing device is being deteriorated.

1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 2 is an enlarged configuration diagram illustrating an enlarged image forming unit for K in the printer. FIG. 6 is a schematic diagram illustrating a state in which a DC transfer bias and a superimposed transfer bias are switched and applied to a secondary transfer unit. FIG. 6 is a waveform diagram illustrating an example of a waveform of a secondary transfer bias including a superimposed transfer bias output from a secondary transfer bias power source of the printer. It is a block diagram showing a configuration example of a secondary transfer bias application unit. 6 is a control flowchart for executing a print job in which a superimposing transfer mode is selected. 6 is a control flowchart when executing a print job in which a DC transfer mode is selected. (A) is a graph showing the superimposed transfer bias used in the embodiment. (B) is a graph showing the superimposed transfer bias used in the first modification. It is a table | surface which shows the result of an effect confirmation test. 1 is a schematic configuration diagram illustrating an example of a direct transfer type one-drum type image forming apparatus. FIG. 2 is a schematic configuration diagram illustrating an example of a direct transfer type one-drum type image forming apparatus using a transfer belt as a transfer member. 1 is a schematic configuration diagram illustrating an example of a direct transfer type tandem image forming apparatus. 1 is a schematic configuration diagram showing an example of a direct transfer type one-drum type image forming apparatus using a transfer charger as a transfer member. FIG. 3 is a schematic configuration diagram illustrating an example of an intermediate transfer type tandem type image forming apparatus using a paper conveyance belt as a secondary transfer member.

Hereinafter, an embodiment of an electrophotographic color printer (hereinafter simply referred to as “printer”) will be described as an image forming apparatus to which the present invention is applied.
First, the basic configuration of the printer according to the present embodiment will be described.
FIG. 1 is a schematic configuration diagram showing a printer according to the present embodiment. The printer according to this embodiment includes four image forming units 1Y, 1M, 1C, and 1K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images, and a transfer. The apparatus includes a transfer unit 30, an optical writing unit 80, a fixing device 90, a paper feed cassette 100, and a registration roller pair 101.

  The four image forming units 1Y, 1M, 1C, and 1K use toners of different colors as image forming materials, but the other configurations are the same and are replaced when the lifetime is reached. Taking an image forming unit 1K for forming a K toner image as an example, as shown in FIG. 2, this includes a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, and a charge eliminating device (not shown). And a charging device 6K, a developing device 8K, and the like. These devices are held by a common holding body and integrally attached to and detached from the printer main body, so that they can be exchanged at the same time.

  The photoreceptor 2K has a drum shape having an outer diameter of about 60 [mm] in which an organic photosensitive layer is formed on the surface of a drum base, and is rotated in a clockwise direction in the drawing by a driving unit (not shown). The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. Charge like this. In this embodiment, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed. The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method of bringing a charging member such as a charging roller into contact with or close to the photosensitive member 2K, a method using a charging charger may be adopted.

  The uniformly charged surface of the photoconductor 2K is optically scanned by a laser beam emitted from the optical writing unit 80 and carries an electrostatic latent image for K. The electrostatic latent image for K is developed by a developing device 8K using K toner (not shown) to become a K toner image. Then, primary transfer is performed on an intermediate transfer belt 31 as an intermediate transfer member described later.

  The drum cleaning device 3K removes the transfer residual toner adhering to the surface of the photoreceptor 2K after the primary transfer process (primary transfer nip described later). It includes a cleaning brush roller 4K that is driven to rotate, a cleaning blade 5K that abuts the free end of the cleaning brush roller 4K in a cantilevered state, and the like. The transfer residual toner is scraped off from the surface of the photoreceptor 2K by the rotating cleaning brush roller 4K, and the transfer residual toner is scraped off from the surface of the photoreceptor 2K by the cleaning blade. The cleaning blade is in contact with the photosensitive member 2K in the counter direction in which the cantilevered support end side is directed to the downstream side in the drum rotation direction from the free end side.

  The static eliminator neutralizes the residual charge on the photoreceptor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation.

  The developing device 8K includes a developing unit 12K that includes a developing roll 9K as a developer carrying member, and a developer transport unit 13K that stirs and transports a K developer (not shown). The developer transport unit 13K includes a first transport chamber that houses the first screw member 10K and a second transport chamber that houses the second screw member 11K. Each of the screw members includes a rotary shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade projecting in a spiral manner on the peripheral surface thereof.

  The first transfer chamber that houses the first screw member 10K and the second transfer chamber that houses the second screw member 11K are partitioned by a partition wall. On the other hand, at both ends of the partition wall in the screw axis direction, communication ports for communicating both the transfer chambers are formed. The first screw member 10K is directed from the back side to the front side in the direction orthogonal to the paper surface in the drawing while stirring the K developer (not shown) held in the spiral blade in the rotation direction in accordance with the rotation drive. Transport. Since the first screw member 10K and a later-described developing roll 9K are arranged in parallel so as to face each other, the conveying direction of the K developer at this time is also a direction along the rotation axis direction of the developing roll 9K. . Then, the first screw member 10K supplies K developer along the axial direction to the surface of the developing roll 9K.

  After the K developer transported to the vicinity of the front end of the first screw member 10K in the drawing passes through the communication opening provided in the vicinity of the front end of the partition wall in the drawing and enters the second transport chamber. The second screw member 11K is held in the spiral blade. And with the rotational drive of the 2nd screw member 11K, it is conveyed toward the back | inner side from the near side in a figure, stirring in a rotation direction.

  In the second transfer chamber, a toner concentration sensor (not shown) is provided on the lower wall of the casing, and detects the K toner concentration of the K developer in the second transfer chamber. As the K toner density sensor, a sensor composed of a magnetic permeability sensor is used. Since the magnetic permeability of the K developer containing K toner and magnetic carrier has a correlation with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

  In this printer, Y, M, C, and K toner replenishing means (not shown) for individually replenishing Y, M, C, and K toners into the second storage chamber of the developing device for Y, M, C, and K, respectively. Is provided. The control unit of the printer stores Vtref for Y, M, C, and K, which are target values of output voltage values from the Y, M, C, and K toner density detection sensors, in the RAM. When the difference between the output voltage value from the Y, M, C, K toner density detection sensor and the Vtref for Y, M, C, K exceeds a predetermined value, Y, The M, C, K toner supply means is driven. As a result, Y, M, C, and K toners are replenished into the second transfer chamber in the developing device for Y, M, C, and K.

  The developing roll 9K accommodated in the developing unit 12K faces the first screw member 10K and also faces the photoreceptor 2K through an opening provided in the casing. The developing roll 9K includes a cylindrical developing sleeve made of a nonmagnetic pipe that is rotationally driven, and a magnet roller that is fixed inside the developing roll 9K so as not to rotate with the sleeve. Then, the K developer supplied from the first screw member 10K is carried on the sleeve surface by the magnetic force generated by the magnet roller, and is conveyed to the developing region facing the photoreceptor 2K as the sleeve rotates.

  A developing bias having the same polarity as the toner and larger than the electrostatic latent image of the photosensitive member 2K and smaller than the uniform charging potential of the photosensitive member 2K is applied to the developing sleeve. As a result, a developing potential for electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K. Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

  In FIG. 1, the Y, M, and C image forming units 1Y, 1M, and 1C also have Y, M, and Y on the photoreceptors 2Y, 2M, and 2C in the same manner as the K image forming unit 1K. , C toner images are formed.

  Above the image forming units 1Y, 1M, 1C, and 1K, an optical writing unit 80 serving as a latent image forming unit is disposed. The optical writing unit 80 optically scans the photoreceptors 2Y, 2M, 2C, and 2K with laser light emitted from a laser diode based on image information sent from an external device such as a personal computer. By this optical scanning, electrostatic latent images for Y, M, C, and K are formed on the photoreceptors 2Y, 2M, 2C, and 2K. Specifically, in the entire area of the uniformly charged surface of the photoreceptor 2Y, the potential of the portion irradiated with the laser light becomes an electrostatic latent image smaller than the potential of the other portion (background portion). The optical writing unit 80 irradiates the photosensitive member through a plurality of optical lenses and mirrors while polarizing the laser light L emitted from the light source in the main scanning direction by a polygon mirror rotated by a polygon motor (not shown). It is. You may employ | adopt what performs optical writing by the LED light emitted from several LED of the LED array.

  Below the image forming units 1Y, 1M, 1C, and 1K, there is disposed a transfer unit 30 as a transfer device that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. . In addition to the intermediate transfer belt 31, the transfer unit 30 includes a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, and 35K, a nip forming roller 36, a belt cleaning device. 37, a toner image detection sensor 38, and the like.

  The intermediate transfer belt 31 is stretched by a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. Then, it is moved endlessly in the same direction by the rotational force of the driving roller 32 that is driven to rotate counterclockwise in the figure by a driving means (not shown).

As the intermediate transfer belt 31, a belt having the following characteristics is used. The thickness is about 20 [μm] to 200 [μm], preferably about 60 [μm]. The volume resistivity is about 1 × 10 ^7.5 [Ω · cm] to 1 × 10 ¹³ [Ω · cm], preferably about 1 × 10 ⁹ [Ω · cm]. This volume resistivity is measured with a Hiresta HRS probe manufactured by Mitsubishi Chemical Corporation under conditions where the applied voltage is 100 V and the measurement time is 10 seconds. The surface resistivity is 1 × 10 ¹⁰ [Ω / □] to 1 × 10 ¹² [Ω / □]. This surface resistivity is measured using a Hiresta HRS probe manufactured by Mitsubishi Chemical Corporation under conditions where the applied voltage is 500 V and the measurement time is 10 seconds. As a material of the intermediate transfer belt 31 of the present embodiment, for example, a carbon dispersed polyimide resin can be used.

  The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2Y, 2M, 2C, and 2K. As a result, primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 31 and the photoreceptors 2Y, 2M, 2C, and 2K abut are formed. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K by a transfer bias power source (not shown). As a result, a transfer electric field is formed between the Y, M, C, and K toner images on the photoreceptors 2Y, 2M, 2C, and 2K and the primary transfer rollers 35Y, 35M, 35C, and 35K. The Y toner formed on the surface of the Y photoconductor 2Y enters the Y primary transfer nip as the photoconductor 2Y rotates. Then, the image is primarily transferred from the photoreceptor 2Y to the intermediate transfer belt 31 by the action of the transfer electric field and nip pressure. The intermediate transfer belt 31 on which the Y toner image has been primarily transferred in this way then passes sequentially through the primary transfer nips for M, C, and K. Then, the M, C, and K toner images on the photoreceptors 2M, 2C, and 2K are sequentially superimposed on the Y toner image and primarily transferred. A four-color superimposed toner image is formed on the intermediate transfer belt 31 by this superimposing primary transfer.

The primary transfer rollers 35Y, 35M, 35C, and 35K are made of an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof, and have the following characteristics. doing. The external shape is 16 [mm]. The diameter of the cored bar is 10 [mm]. Further, a current that flows when a voltage of 1000 [V] is applied to the primary transfer roller core while a grounded metal roller having an outer diameter of 30 [mm] is pressed against the sponge layer with a force of 10 [N]. The resistance R of the sponge layer calculated from I based on Ohm's law (R = V / I) is about 3 × 10 ⁷ Ω. A primary transfer bias is applied to such primary transfer rollers 35Y, 35M, 35C, and 35K by constant current control. Instead of the primary transfer rollers 35Y, 35M, 35C, and 35K, a transfer charger, a transfer brush, or the like may be employed.

  The nip forming roller 36 of the transfer unit 30 is disposed outside the loop of the intermediate transfer belt 31, and the intermediate transfer belt 31 is sandwiched between the secondary transfer back roller 33 inside the loop. As a result, a secondary transfer nip where the front surface of the intermediate transfer belt 31 and the nip forming roller 36 abut is formed. While the nip forming roller 36 is grounded, a secondary transfer bias is applied to the secondary transfer back roller 33 by a secondary transfer bias power source 200. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the nip forming roller 36 to cause the negative polarity toner to electrostatically move from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done.

  Below the transfer unit 30, a paper feed cassette 100 is provided that stores a plurality of recording papers P as recording materials in a stacked state. In this paper feed cassette 100, a paper feed roller 100a is brought into contact with the top recording paper P of the paper bundle, and this recording paper P is fed into the paper feed path by being driven to rotate at a predetermined timing. Send it out. A registration roller pair 101 is disposed near the end of the paper feed path. The registration roller pair 101 stops the rotation of both rollers as soon as the recording paper P delivered from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is resumed at a timing at which the sandwiched recording paper P can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording paper P is directed to the secondary transfer nip. Send it out. The four-color superimposed toner image on the intermediate transfer belt 31 brought into intimate contact with the recording paper P at the secondary transfer nip is collectively transferred onto the recording paper P by the action of the secondary transfer electric field and nip pressure, and the recording paper Combined with the white color of P, a full color toner image is obtained. When the recording paper P having the full-color toner image formed on the surface in this way passes through the secondary transfer nip, the recording paper P is separated from the nip forming roller 36 and the intermediate transfer belt 31 by the curvature.

The secondary transfer back roller 33 is formed by laminating a resistance layer on a metal core made of stainless steel or aluminum. The resistance layer is made by dispersing conductive particles such as carbon or metal complex in polycarbonate, fluorine rubber, silicon rubber or the like, or rubber such as NBR or EPDM, rubber of NBR / ECO copolymer, polyurethane semiconductive Made of rubber. The volume resistivity is 10 ⁶ to 10 ¹² [Ω · cm], preferably 10 ⁷ to 10 ⁹ [Ω · cm]. Further, it may be a foam type having a hardness of 20 ° to 50 ° or a rubber type having a rubber hardness of 30 ° to 60 °. However, since it is in contact with the nip forming roller 36 via the intermediate transfer belt 31, a non-contact portion even with a small contact pressure. Sponge type that does not cause is desirable. This is to prevent characters and fine lines from being easily lost as the contact pressure between the intermediate transfer belt 31 and the secondary transfer back roller 33 increases.

  The volume resistivity of the secondary transfer back roller 33 was such that the electrode roller was brought into contact with the peripheral surface of the secondary transfer back roller 33 with a force of 5 [N], and 1000 V was applied to the core metal of the secondary transfer back roller 33. While the secondary transfer back roller 33 is rotated for 1 minute in this state, the volume resistivity for one rotation of the roller is sequentially measured, and the average value of each measured volume resistivity is adopted.

The nip forming roller 36 is formed by laminating a resistance layer and a surface layer made of conductive rubber on a cored bar made of stainless steel or aluminum. In this example, the outer diameter of the roller is 20 [mm], and the core metal is stainless steel having a diameter of 16 [mm]. The resistance layer is a rubber having a hardness of 40 to 60 degrees [JISA] made of a copolymer of NBR / ECO. The surface layer is made of a fluorine-containing urethane elastomer, and the thickness is desirably 8 to 24 [μm]. The reason is that the surface layer of the roller is often manufactured by a coating process. Therefore, when the surface layer thickness is 8 [μm] or less, the influence of the resistance unevenness due to the uneven coating is large, and a leak occurs at a location where the resistance is low. There is a possibility that it is not preferable. In addition, wrinkles are generated on the roller surface and the surface layer is liable to crack. On the other hand, when the thickness of the surface layer exceeds 24 [μm], the resistance increases, and when the volume resistivity is high, the voltage when a constant current is applied to the core of the secondary transfer back roller 33 may increase. is there. When the voltage exceeds the variable voltage range of the constant current power supply, the current is less than the target current, or when the voltage variable range is sufficiently high, a high voltage path from the constant current power supply to the secondary transfer back roller cored bar or two Leakage is likely to occur due to the next transfer back roller core metal becoming high voltage. Further, when the surface layer of the nip forming roller 36 is thicker than 24 [μm], the hardness becomes high, and there is a problem that the adhesion to a recording medium (paper or the like) or an intermediate transfer belt is deteriorated. The surface resistivity of the nip forming roller 36 is 1 × 10 ^6.5 [Ω / □] or more, and the volume resistivity of the surface layer of the nip forming roller 36 is 1 × 10 ¹⁰ [Ω · cm] or more, more preferably 1 × 10 ¹² [Ω · cm] or more. In this embodiment, a nip forming roller having a surface layer laminated is used, but a type in which only a resistance layer is laminated on a core metal may be used.

  The toner image detection sensor 38 is disposed outside the loop of the intermediate transfer belt 31. In the entire area of the intermediate transfer belt 31 in the circumferential direction, the intermediate transfer belt 31 is opposed to the grounded driving roller 32 with a gap of about 5 mm. The toner image detection sensor 38 is an optical sensor of one light emission and two light reception type, and detects the amount of adhesion of the toner image primarily transferred onto the intermediate transfer belt 31 by converting the received light into the amount of adhesion.

  A fixing device 90 is disposed on the right side of the secondary transfer nip in the drawing. The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source such as a halogen lamp and a pressure roller 92 that rotates while contacting with the fixing roller 91 with a predetermined pressure. The recording paper P fed into the fixing device 90 is sandwiched between the fixing nips in such a posture that the unfixed toner image carrying surface is in close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full color image is fixed. The recording paper P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus.

  When a monochrome image is formed, a support plate (not shown) that supports the primary transfer rollers 35Y, 35M, and 35C for Y, M, and C in the transfer unit 30 is moved. Then, the primary transfer rollers 35Y, 35M, 35C, and 35K are moved away from the photoreceptors 2Y, 2M, and 2C. As a result, the front surface of the intermediate transfer belt 31 is separated from the photoconductors 2Y, 2M, and 2C, and the intermediate transfer belt 31 is brought into contact with only the K photoconductor 2K. In this state, of the four image forming units 1Y, 1M, 1C, and 1K, only the K image forming unit 1K is driven to form a K toner image on the photoreceptor 2K.

  The secondary transfer bias power supply 39 has a DC power supply and an AC power supply, and can output a DC voltage superposed with an AC voltage as a secondary transfer bias. The output terminal of the secondary transfer bias power source 39 is connected to the core metal of the secondary transfer back roller 33. The potential of the core metal of the secondary transfer back roller 33 is almost the same as the output voltage value from the secondary transfer bias power source 39. Further, the core metal of the nip forming roller 36 is grounded (ground connection). Instead of grounding the core metal of the nip forming roller 36 while applying the superimposed transfer bias to the core metal of the secondary transfer back roller 33, the secondary transfer is performed while applying the superimposed transfer bias to the core metal of the nip forming roller 36. The core metal of the back roller 33 may be grounded. In this case, the polarity of the DC voltage is varied. Specifically, as shown in the figure, when a superimposed transfer bias is applied to the secondary transfer back roller 33 under the condition that negative polarity toner is used and the nip forming roller 36 is grounded, the direct current voltage is set to the toner. Using the same negative polarity, the time average potential of the superimposed transfer bias is set to the same negative polarity as that of the toner. On the other hand, when the secondary transfer back roller 33 is grounded and the superimposed transfer bias is applied to the nip forming roller 36, a DC voltage having a positive polarity opposite to that of the toner is used. The time average potential is set to a positive polarity opposite to that of the toner. Instead of applying the superimposed transfer bias to the secondary transfer back surface roller 33 or the nip forming roller 36, a DC voltage may be applied to one of the rollers and an AC voltage may be applied to the other roller.

  As the AC voltage, a sinusoidal waveform is used, but a rectangular waveform may be used. When the recording paper P is not a paper having a large surface irregularity such as rough paper but a paper having a small surface irregularity such as plain paper, a shading pattern that follows the uneven pattern does not appear. For this reason, a transfer bias composed only of a DC voltage may be applied. However, when using a paper with large surface irregularities such as rough paper, it is necessary to switch the transfer bias from only a DC voltage to a superimposed transfer bias.

  Untransferred toner that has not been transferred to the recording paper P adheres to the intermediate transfer belt 31 that has passed through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 37 in contact with the front surface of the intermediate transfer belt 31. A cleaning backup roller 34 disposed inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

FIG. 3 is a schematic diagram showing a state in which the DC transfer bias and the superimposed transfer bias are switched and applied to the secondary transfer unit.
The secondary transfer bias power supply 39 of the present embodiment includes a DC power supply 201 and an AC / DC superimposed power supply 202. In FIG. 5A, the switch 203 is switched to apply a DC transfer bias from the DC power supply 201, and in FIG. 5B, the switch 203 is switched to apply a superimposed transfer bias from the AC / DC superimposed power supply 202. In order to conceptually show the switching between the DC power supply 201 and the AC / DC superimposed power supply 202, the switch 203 is shown to be switched. However, as will be described later with reference to FIG. It may be configured.

FIG. 4 is a waveform diagram showing an example of the waveform of the secondary transfer bias composed of the superimposed transfer bias output from the secondary transfer bias power source.
As described above, the secondary transfer bias of the present embodiment is applied to the core metal of the secondary transfer back roller. When a secondary transfer bias is applied to the core of the secondary transfer back roller, a potential difference (transfer bias) is generated between the core of the secondary transfer back roller 33 and the core of the nip forming roller. . In this embodiment, the potential difference (transfer bias) is a value obtained by subtracting the potential of the core metal of the nip forming roller 36 from the potential of the core metal of the secondary transfer back surface roller 33. The time average value of the potential difference in the configuration using a negative polarity toner as in the present embodiment, the potential of the nip forming roller 36 is set to the potential of the secondary transfer back roller 33 when the polarity becomes negative. In other words, the polarity is larger on the side opposite to the charging polarity of the toner (in the present embodiment, on the plus side). Therefore, the toner is electrostatically moved from the secondary transfer back roller side to the nip forming roller side.

  In FIG. 4, the offset voltage Voff of the superimposed transfer bias is equal to the DC component value of the superimposed transfer bias in the sine wave superimposed transfer bias as in this embodiment. The peak-to-peak voltage Vpp of the superimposed transfer bias is equal to the peak-to-peak voltage in the AC component of the superimposed transfer bias. In the printer according to this embodiment, as already described, the secondary transfer bias is applied to the core of the secondary transfer back roller 33 and the core of the nip forming roller 36 is grounded. The potential difference between both cores corresponds to the secondary transfer bias applied to the core of the secondary transfer back roller 33.

  When the polarity of the secondary transfer bias is the same negative polarity as that of the toner, the negative polarity toner is electrostatically pushed out from the secondary transfer back roller 33 side to the nip forming roller 36 side in the secondary transfer nip. As a result, the toner on the intermediate transfer belt 31 is transferred onto the recording paper P. On the other hand, when the polarity of the secondary transfer bias is a positive polarity opposite to the toner, the negative polarity toner is directed from the nip forming roller 36 side to the secondary transfer back roller 33 side in the secondary transfer nip. Pulls electrostatically. As a result, the toner transferred to the recording paper P is attracted again to the intermediate transfer belt 31 side. However, since the time average value of the secondary transfer bias (in this example, the same value as the offset voltage Voff) has a negative polarity, a negative polarity toner is formed in the secondary transfer nip from the secondary transfer back roller 33 side. The action of pushing out toward the roller 36 is relatively large. In FIG. 4, a return potential peak value Vr indicates a positive peak value having a polarity opposite to that of the toner.

  When an image is formed on a recording paper P having a large surface irregularity, such as Japanese paper or embossed paper, the toner is applied as described above by using a superimposed transfer bias as a secondary transfer bias. While reciprocating, the sheet is moved from the intermediate transfer belt 31 side to the recording paper P side and transferred onto the recording paper. Thereby, the transfer rate of the toner to a paper recessed part can be improved, and generation | occurence | production of the light / dark pattern according to the surface unevenness | corrugation can be suppressed. On the other hand, when the recording paper P with small unevenness, such as normal transfer paper, is used, sufficient transferability can be obtained by using a DC transfer bias having only a DC component as the secondary transfer bias.

  As described above, in this embodiment, as the secondary transfer bias, a DC transfer mode in which a DC transfer bias is applied to transfer an image onto the recording sheet P, and a superimposed transfer bias in which an AC is superimposed on a DC are applied. The image forming apparatus has a superimposing transfer mode for transferring an image onto the recording paper P, and can be switched between them. Then, by switching the transfer mode to the direct current transfer mode or the superimposed transfer mode according to the type of the recording paper P to be passed, good image transfer can be performed on both the paper with small unevenness and the paper with large unevenness. it can. The transfer mode may be switched automatically according to the setting of the type of recording paper P. Alternatively, the user may be able to specify the transfer mode. These settings are provided so that they can be set from the operation panel of the image forming apparatus.

FIG. 5 is a block diagram illustrating a configuration example of the secondary transfer bias applying unit.
In the example shown in the figure, the power source to which the bias is applied is switched using two relays. As shown in the figure, the DC power supply 201 applies a DC transfer bias to the secondary transfer back roller 33 via the relay 301. The AC / DC superimposed power source 202 applies a superimposed transfer bias to the secondary transfer back roller 33 via the relay 302. The connection and disconnection of the two relays 301 and 302 are controlled by the control unit 300 via the relay driving unit 204, and the DC transfer bias or the superimposed transfer bias as the secondary transfer bias is switched.

Next, the operation in the pre-refresh mode (pre-toner forced consumption control) in this embodiment will be described.
FIG. 6 is a control flowchart for executing a print job in which the superimposed transfer mode is selected.
In the pre-refresh mode, when the transfer bias switching condition (specific execution condition) that the superimposing transfer mode is selected is satisfied, the toner forced consumption control is performed before the image forming operation in the superimposing transfer mode is started. Mode. In this embodiment, even if the direct current transfer mode is selected, as will be described later (see FIG. 7), this pre-refresh mode is not executed. Since the contents of the pre-refresh mode are the same in any of the image forming units 1Y, 1M, 1C, and 1K, the K image forming unit 1K will be described below.

  In the pre-refresh mode, the optical writing unit 80 forms a predetermined electrostatic latent image for the toner consumption pattern on the photosensitive member 2K, and develops this with the developing device 8K, thereby removing the toner in the developing device 8K. Consume. In this embodiment, the toner consumption pattern (toner image) formed on the photoreceptor 2K in this way is primarily transferred to the intermediate transfer belt 31 and then collected by the belt cleaning device 37. At this time, the nip forming roller 36 is kept away from the intermediate transfer belt 31 by a contact / separation mechanism (not shown).

  In the present embodiment, when the superimposing transfer mode is selected, the advance refresh mode is not necessarily performed, and whether or not it can be performed is based on the cumulative average of the image area ratios of images formed in the past predetermined period. Determined. Specifically, the control unit 300 acquires pixel information when the optical writing unit 80 writes an electrostatic latent image on the photosensitive member 2K, and from the pixel information, a past predetermined period (of the developing device 8K). The image area ratio of the image formed on the basis of the driving time is calculated. The calculated image area ratio is sequentially stored in a storage device (image area ratio storage means) (not shown). Then, an image area ratio cumulative average is calculated from the calculated image area ratios for a plurality of periods (S1), and if the calculated image area ratio cumulative average is below a predetermined threshold A (No in S2), pre-refreshing is performed. The mode is executed (S3, S4), and if it is equal to or greater than the predetermined threshold A (Yes in S2), the pre-refresh mode is not executed.

  The amount of toner to be forcibly consumed during the pre-refresh mode may be constant, but in the present embodiment, it is determined based on the above-described average image area ratio (S3). Specifically, the smaller the cumulative image area ratio average (the greater the difference from the threshold value A), the more the toner is consumed.

  The determination as to whether or not to perform the pre-refresh mode is performed individually for each of the image forming units 1Y, 1M, 1C, and 1K. A pre-refresh mode is implemented in the forming units 1Y, 1M, 1C, 1K. At this time, the amount of toner forcibly consumed by the image forming units 1Y, 1M, 1C, and 1K is determined according to the cumulative average of the image area ratios of the image forming units 1Y, 1M, 1C, and 1K. Different for each image forming unit. Note that when any one of the conditions for executing the pre-refresh mode is satisfied, the pre-refresh mode may be executed only for the image forming unit.

  When the pre-refresh mode is executed as described above, the print job is subsequently started in the superimposed transfer mode (S5). In this embodiment, forced toner consumption control is performed as necessary even during a print job. This control is based on a non-development processing period (inter-paper handling) that exists between the development processing periods of each image during a continuous image forming operation (during a print job) in which images are continuously formed on a plurality of recording papers P. This is a control mode in which toner forcible consumption control is performed during (period). Hereinafter, this control mode is referred to as an inter-sheet refresh mode (inter-image toner forced consumption control). In the present embodiment, as shown in FIG. 7, even during the continuous image forming operation in the DC transfer mode, the paper interval refresh mode is executed if a predetermined toner forced consumption control execution condition is satisfied.

  In the inter-sheet refresh mode, the optical writing unit 80 forms a predetermined electrostatic latent image for the toner consumption pattern on the photosensitive member 2K, and this is converted into an electrostatic latent image corresponding to image information existing before and after that. The development processing is continuously performed by the developing device 8K, thereby forcibly consuming the toner in the developing device 8K. The toner consumption pattern (toner image) formed on the photoreceptor 2K in this way is primarily transferred to the intermediate transfer belt 31 together with the toner image corresponding to the image information, and then collected by the belt cleaning device 37. At this time, the nip forming roller 36 is separated from the intermediate transfer belt 31 by the contact / separation mechanism immediately before the tip of the toner consumption pattern on the intermediate transfer belt 31 enters the secondary transfer nip, and the toner consumption pattern on the intermediate transfer belt 31 is detected. Immediately after the rear end passes through the secondary transfer nip, the nip forming roller 36 contacts the intermediate transfer belt 31 by the contact / separation mechanism.

  In the present embodiment, whether or not the inter-sheet refresh mode can be performed is determined based on the cumulative average of the image area ratios of images formed in the most recent plurality of periods when each image is formed during the continuous image forming operation. Specifically, the image area ratio cumulative average is calculated in the same manner as in the pre-refresh mode (S6, S12), and if the image area ratio cumulative average is below a predetermined threshold (No in S7, No in S13), The inter-paper refresh mode is executed in the next inter-paper correspondence period (S8, S9, S14, S15). In the present embodiment, the predetermined threshold is different between the DC transfer mode and the superimposing transfer mode, and the threshold B used in the DC transfer mode is set to a value smaller than the threshold C used in the superimposing transfer mode. The

  The toner amount forcibly consumed during the inter-sheet refresh mode is the cumulative average of the image area ratio calculated from the sum of the toner consumption due to image formation and the toner consumption forcibly consumed in the inter-sheet refresh mode in the DC transfer mode. If there is, the threshold B is set, and if it is the superimposed transfer mode, the threshold C is set (S8, S14). However, since the inter-paper correspondence period is a very short time, there is an upper limit on the amount of toner that can be consumed within that period. Therefore, there is a case where the target amount of toner cannot be forcibly consumed in one paper interval refresh mode. In this case, for the amount that could not be forcibly consumed, the inter-paper refresh mode is performed in the subsequent inter-paper correspondence period, and forcible consumption for the amount that could not be consumed between the paper is performed.

  As an example of each threshold described above, threshold A and threshold C are set to 5%, and threshold B is set to 3%. In addition, for example, a toner consumption pattern having an image area ratio of 56% can be used, and the color overlap of the toner consumption pattern is two colors of Y and C and M and K. However, the present invention is not limited to this. Is not to be done. Further, the calculation of the image area ratio is performed every predetermined time (when the developing device is driven), but may be a specification calculated for each printing sheet.

[Modification 1]
Next, a modified example of the superimposed transfer bias in the present embodiment (hereinafter referred to as “modified example 1”) will be described.
FIG. 8A is a graph showing the superimposed transfer bias used in the above-described embodiment, and FIG. 8B is a graph showing the superimposed transfer bias used in the first modification.
As shown in FIG. 8A, the superimposing transfer bias of the above-described embodiment has a time average value (Vave) of the superimposing transfer bias equal to the offset voltage Voff. As shown in FIG. 8B, the time average value (Vave) of the superimposed transfer bias is set closer to the transfer direction than the offset voltage Voff.

  In the first modification, the ratio of the time (return time) during which the bias value opposite to the transfer direction from the offset voltage Voff is applied is set to 10% of one cycle of the superimposed transfer bias. The ratio of the return time is preferably 4% or more and 45% or less. In addition, as shown to Fig.8 (a), the ratio of the return time of embodiment mentioned above is 50%.

  According to the first modification, the toner transfer rate to the concave portion when forming an image on the recording paper P having a large surface unevenness as compared with the case where the return time ratio is 50% as in the above-described embodiment. Is high, and the generation of a light and shade pattern can be further suppressed.

FIG. 9 is a table showing the results of an effect confirmation test conducted by the present inventors.
In this effect confirmation test, 5000 sheets of white paper are passed in advance to make the toner in the developing device deteriorate, and then a test image is continuously formed on a recording paper with large unevenness, and the transferability level of the image is checked. Evaluation was performed. The evaluation of the transferability level is performed by visual ranking in five stages, and rank 5 is the best and rank 1 is the worst. The allowable transferability level is rank 4.

Other test conditions are as follows.
Temperature and humidity environment: 23 ° C, 50%
Paper passing through: T6000 <70W> A4
Paper passing image: White (Y: 0%, C: 0%, M: 0%, K: 0%)
Evaluation paper: Rezac 66A4 (continuous amount: 130 kg)
Evaluation image: Blue full-color image (Y: 0%, C: 100%, M: 100%, K: 0%)
Evaluation item: Transferability (recessed voids)

  As shown in FIG. 9, when the DC transfer mode is selected and neither the pre-refresh mode nor the inter-paper refresh mode is executed (Test A), the transferability level is evaluated even at the initial image formation of the test image. The rank was 2 when 2000 sheets have elapsed since the start of test image formation and when 5000 sheets have elapsed since the start of test image formation.

  Also, as shown in FIG. 9, when the superposition transfer mode (return time ratio = 50%) is selected and neither the pre-refresh mode nor the inter-sheet refresh mode is executed (test B), the direct current transfer is performed. Although it was improved from rank 2 in the mode, the evaluation of the transferability level was rank 3 in all evaluation periods.

  As shown in FIG. 9, when the superposition transfer mode (return time ratio = 50%) is selected and the pre-refresh mode is executed but the inter-sheet refresh mode is not executed (test C), the test image is displayed. For a while after the initial image formation, an acceptable rank 4 was obtained due to the effect of forced consumption of deteriorated toner in the pre-refresh mode. However, as test image formation continues further, the amount of deteriorated toner in the developing device gradually increases, and when 2000 sheets have elapsed since the start of test image formation, the evaluation drops to rank 3.5, and 5000 sheets since the start of test image formation. As time passed, the rating dropped further to rank 3.

  As shown in FIG. 9, when the superposition transfer mode (return time ratio = 50%) is selected and both the pre-refresh mode and the inter-sheet refresh mode are executed (test D), In the evaluation, the acceptable rank 4 was maintained at any evaluation time.

  In addition, as shown in FIG. 9, when the superimposed transfer mode (return time ratio = 10%) is selected and both the pre-refresh mode and the inter-sheet refresh mode are executed (test E), the transfer level is set. Evaluation was maintained at a higher rank of 4.5 at all evaluation periods.

  In the above description, the intermediate transfer type tandem image forming apparatus that transfers the toner images on the photoreceptors 2Y, 2M, 2C, and 2K to the recording paper P via the intermediate transfer belt 31 is taken as an example. As shown in FIG. 10, the same applies to a direct transfer type one-drum type image forming apparatus that directly transfers a toner image formed on a single photosensitive member 2 onto a recording sheet P. In the example shown in FIG. 10, a transfer bias power supply 139 having a DC power supply and an AC power supply is connected to the core of the transfer roller 135 that forms a transfer nip with the photoconductor 2, and the DC is connected to the transfer nip. The transfer bias or the superimposed transfer bias is selectively applied. The configuration shown in FIG. 10 is for the case where the normal charging polarity of the toner is positive. As the transfer roller 135, a core having a foam layer or a surface coat layer formed on a core metal can be used.

  Further, the direct transfer type image forming apparatus may have a configuration in which a transfer nip is formed between the photosensitive member 2 and the transfer belt 235 as shown in FIG. In the configuration shown in FIG. 11, the transfer belt 235 is stretched between two support rollers, and the bias roller 235a and the bias brush 235b are brought into contact with or near the inner peripheral surface of the transfer belt portion forming the transfer nip. Yes. A transfer bias power supply 239 having a DC power supply and an AC power supply is connected to the bias roller 235a and the bias brush 235b, and a DC transfer bias or a superimposed transfer bias is selectively applied to the transfer nip. Yes. The configuration shown in FIG. 11 is for the case where the normal charging polarity of the toner is negative.

  In the configuration shown in FIG. 11, two members, the bias roller 235a and the bias brush 235b, are used as the bias applying member, but both of them may be roller members or both of them may be brush members. Further, the bias applying member may be composed of one member. Further, the bias applying member may be a non-contact type charger. In the configuration shown in FIG. 11, the bias applying member is disposed at a position slightly shifted from the inner peripheral surface of the transfer belt portion forming the transfer nip and shifted to the downstream side in the transport direction, but the transfer nip is formed. A bias applying member may be disposed on the inner peripheral surface of the transfer belt portion.

  Further, as shown in FIG. 12, the direct transfer type image forming apparatus directly transfers the toner images formed on the four photosensitive members 2Y, 2M, 2C, and 2K directly onto the recording paper P so as to overlap each other. A transfer type tandem image forming apparatus may be used. In the configuration shown in FIG. 12, a transfer nip is formed between each of the four photoconductors 2Y, 2M, 2C, and 2K and the transfer belt 335. In the configuration shown in FIG. 12, the bias roller 335a and the backup roller 335b are in contact with the inner peripheral surface of the transfer belt portion forming each transfer nip or in the vicinity thereof. Each bias roller 335a is connected to a transfer bias power source 339 having a DC power source and an AC power source, and is configured to selectively apply a DC transfer bias or a superimposed transfer bias to each transfer nip. In FIG. 12, only the transfer bias power supply 339 corresponding to the transfer nip of the M photoconductor 2M is shown, and the transfer bias power supply corresponding to the transfer nip of another photoconductor is not shown. Further, the configuration shown in FIG. 12 is for the case where the normal charging polarity of the toner is negative.

  Further, as shown in FIG. 14, the intermediate transfer type image forming apparatus may have a configuration in which a paper conveying belt 536 is used as a secondary transfer member. In the configuration shown in FIG. 14, a paper transport belt 536 is stretched between two support rollers 536 a and 536 b, and is formed on the inner peripheral surface of the paper transport belt portion that forms a secondary transfer nip with the intermediate transfer belt 31. A transfer bias power source 539 having a direct current power source and an alternating current power source is connected to the supporting roller 536a in contact therewith. As a result, the DC transfer bias or the superimposed transfer bias is selectively applied to the secondary transfer nip. However, as in the case of the above-described embodiment, a transfer bias power source is connected to the secondary transfer back surface roller 533 that is in contact with the inner peripheral surface of the intermediate transfer belt portion that forms the secondary transfer nip, and the secondary transfer nip is connected to the secondary transfer nip. On the other hand, a configuration may be adopted in which a DC transfer bias or a superimposed transfer bias is selectively applied.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
Development in which a latent image carrier such as the photosensitive member 2 that carries a latent image corresponding to image information on the surface, and a developing process for forming a toner image by attaching toner to the latent image on the latent image carrier using a developer The toner image on the latent image carrier formed by the apparatus 8 and the developing process is directly transferred to a recording material such as recording paper P, or is transferred to a recording material via an intermediate transfer member such as an intermediate transfer belt 31. The transfer bias applied by the transfer means when transferring the toner image to the recording material according to a transfer means such as the transfer unit 30 to be transferred and a predetermined transfer bias switching condition, a DC transfer bias consisting only of a DC component, or Implementation of predetermined toner forced consumption control in an image forming apparatus having a transfer bias switching unit such as a control unit 300 for switching to a superimposed transfer bias in which an AC component is superimposed on a DC component and the polarity changes over time When the condition is satisfied, the toner forcible consumption control unit such as the control unit 300 for performing the toner forced consumption control for forcibly consuming the toner in the developing device is included, and the predetermined toner forced consumption control execution condition is as follows: And a specific implementation condition that a transfer bias switching condition for switching the transfer bias to the superimposed transfer bias is satisfied, and the toner forced consumption control means sets the superimposed transfer bias when the specific implementation condition is satisfied. Prior to the start of the used image forming operation (print job in the superimposed transfer mode), advance toner forced consumption control such as advance refresh mode in which the toner forced consumption control is performed is performed.
According to this, before the image forming operation using the superimposed transfer bias is started, the toner in the developing device is forcibly consumed by performing advance toner forced consumption control, and the amount of deteriorated toner in the developing device is reduced. be able to. As a result, a toner image with less deteriorated toner can be formed during an image forming operation using the superimposed transfer bias, thereby reducing the influence of the deteriorated toner on the transferability when the toner image is transferred to the recording material. be able to. Specifically, as described above, excellent transferability can be obtained even when image formation is performed using a superimposed transfer bias on a recording material having large surface irregularities in a state where toner deterioration in the developing device is progressing. Can be obtained, and an image can be formed with high quality in which the generation of the light and shade pattern corresponding to the unevenness is suppressed.
In addition, according to this aspect, by performing such advance toner forced consumption control, there is a demerit that the time for starting the image forming operation using the superimposed transfer bias is delayed or the downtime occurs. However, in general, the superimposed transfer bias is used when an image is formed on a special recording material having surface irregularities as described above. In such a case, image quality tends to be emphasized. Therefore, even if such disadvantages occur, the present invention that can improve transferability is useful.

(Aspect B)
In the aspect A, the image processing apparatus further includes an image area ratio storage unit that stores an image area ratio of an image formed in a past predetermined period, and the toner forced consumption control unit is configured to display the image when the specific execution condition is satisfied. When it is determined whether or not to perform the advance toner forced consumption control according to the image area rate (image area rate cumulative average) stored in the area rate storage means, and it is determined not to perform the advance toner forced consumption control Is characterized in that the advance toner forced consumption control is not performed.
When the image area ratio of an image formed within a predetermined period in the past is low, toner deterioration in the developing device is progressing, so prior to forced toner consumption before starting the image forming operation using the superimposed transfer bias It is necessary to control. However, when the image area ratio of an image formed within a predetermined period in the past is high, the amount of deteriorated toner remaining in the developing device is small, and thus the deterioration of transferability due to the deteriorated toner is small. In this aspect B, since advance toner forced consumption control is not performed in a situation where the transferability deterioration due to deteriorated toner is small, it is possible to suppress the delay in image formation start time and the occurrence of downtime due to unnecessary advance toner forced consumption control. .

(Aspect C)
In the aspect A or B, the image area ratio storage unit that stores the image area ratio of an image formed in the past predetermined period, and the toner forced consumption control unit, when the specific implementation condition is satisfied, According to the image area ratio stored in the image area ratio storage means, a toner amount to be forcibly consumed by the prior toner forced consumption control is determined, and the preliminary toner is forcibly consumed so that the determined toner amount is forcibly consumed. It is characterized by performing forced consumption control.
If the image area ratio of an image formed in the past predetermined period is low, a lot of deteriorated toner remains in the developing device, but if the image area ratio is high, the deteriorated toner in the developing device is small. Therefore, if the amount of toner forcibly consumed by the advance toner forced consumption control is constant, the amount of toner to be forcibly consumed with respect to the deteriorated toner amount in the developing device is insufficient, and image formation using the superimposed transfer bias is performed. However, if the toner amount forcibly consumed with respect to the deteriorated toner amount in the developing device is excessive, the toner is wasted. In the aspect C, since an appropriate amount of toner corresponding to the amount of deteriorated toner remaining in the developing device can be forcibly consumed, such a problem can be reduced.

(Aspect D)
In any one of the above aspects A to C, the toner forced consumption control unit develops each image during a continuous image forming operation when a toner forced consumption control execution condition different from the specific execution condition is satisfied. The inter-image toner forced consumption control such as the inter-paper refresh mode in which the toner forced consumption control is performed during the non-development processing period that is out of the development processing period during which the latent image corresponding to the image information is developed. It is characterized by performing.
According to this, even when toner deterioration in the developing device progresses during the continuous image forming operation, it is possible to suppress deterioration in image quality due to the deteriorated toner without causing downtime.

(Aspect E)
In the aspect D, the toner forced consumption control means sets the amount of toner to be forcibly consumed by the inter-image toner forced consumption control to be higher than that during the continuous image forming operation using the DC transfer bias. Control is performed such that the number of continuous image forming operations used is increased.
Image quality deterioration due to toner deterioration occurs not only when an image is formed on a recording material with large surface irregularities using a superimposed transfer bias, but also when an image is formed on a recording material with small surface irregularities using a DC transfer bias. However, the degree of influence of the deteriorated toner on the image quality deterioration is greater in the former case than in the latter case. According to the aspect E, in the continuous image forming operation using the DC transfer bias that has little influence on the deterioration of the image quality due to the deteriorated toner, the toner amount to be forcibly consumed by the inter-image toner forced consumption control is reduced. In continuous image forming operations using superimposed transfer bias, which has a large influence on the deterioration of image quality due to deteriorated toner while suppressing wasteful consumption of toner, the amount of toner to be forcibly consumed is increased by inter-image toner forced consumption control. And sufficient image quality can be maintained.

(Aspect F)
In any one of the above aspects A to E, the superimposing transfer bias has a time average value (Vave) set to a polarity in a transfer direction in which the toner image is transferred from the latent image carrier or intermediate transfer member side to the recording material side. And is set closer to the transfer direction than the center value (Voff) of the maximum value and the minimum value.
According to this, compared to the case of using a superimposed transfer bias whose time average value Vave is the same as the offset voltage Voff, good transferability can be obtained at the time of image formation on a recording material having large surface irregularities.

DESCRIPTION OF SYMBOLS 1 Image forming unit 2 Photoconductor 3 Drum cleaning device 6 Charging device 8 Developing device 30 Transfer unit 31 Intermediate transfer belt 33,533 Secondary transfer back roller 35 Primary transfer roller 36 Nip forming roller 37 Belt cleaning device 39 Secondary transfer bias power source DESCRIPTION OF SYMBOLS 80 Optical writing unit 90 Fixing device 101 Registration roller pair 135 Transfer roller 139, 239, 339, 539 Transfer bias power source 200 Secondary transfer bias power source 201 DC power source 202 AC / DC superposition power source 203 Switch 204 Relay drive means 235, 335 Transfer belt 235a Bias roller 235b Bias brush 300 Control means 301, 302 Relay 335a Bias roller 335b Backup roller 536 Paper conveying belt

JP 2006-267486 A JP 2008-058585 A Japanese Patent Laid-Open No. 9-14681 JP-A-4-088788 JP 2008-216601 A JP 2006-47651 A JP 2007-108623 A

Claims (2)

A latent image carrier that carries a latent image according to image information on the surface;
A developing device for performing a developing process for attaching a toner to the latent image on the latent image carrier with a developer to form a toner image;
A transfer means for transferring the toner image on the latent image carrier formed by the development process directly to a recording material or transferring it to a recording material via an intermediate transfer member;
When an image is formed on a recording material with small surface irregularities, a DC transfer bias consisting of only a DC component is used. When an image is formed on a recording material with large surface irregularities , an alternating current component is superimposed on the DC component and polarity Transfer bias switching means for switching the transfer bias applied by the transfer means when transferring the toner image to the recording material so as to use a superimposed transfer bias that changes over time ,
The amount of toner that forcibly consumes the toner in the developing device between images during the continuous image forming operation is more continuously formed using the superimposed transfer bias than during the continuous image forming operation using the DC transfer bias. image forming apparatus, characterized by chromatic and control means for controlling so it is increased during operation, the. The image forming apparatus according to claim 1 .
The superimposed transfer bias has a time average value (Vave) set to a polarity in a transfer direction in which the toner image is transferred from the latent image carrier or intermediate transfer member side to the recording material side, and a central value between the maximum value and the minimum value. An image forming apparatus that is set closer to the transfer direction than (Voff).

Images