JP2011022290A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably convey a recording medium to a transfer nip regardless of thickness of the recording medium (from thin paper to super-thick cardboard), and to suppress the occurrence of an abnormal image caused by discharge on an upstream side of the transfer nip. <P>SOLUTION: The image forming apparatus which transfers a toner image on the recording medium carried and conveyed on a transfer belt 24 has a separation roller 36 which separates the recording medium carried on the belt 24 from the belt 24 in accordance with the thickness of the recording medium on the upstream side of the transfer nip formed by a tensioning and laying roller 21 on an intermediate transfer belt 6 side, and by a transfer roller 9 on the belt 24 side across the belt 6 and the belt 24. When the thickness of the recording medium is equal to or under prescribed thickness, the recording medium is conveyed to the transfer nip while it is carried on the belt 24 without being separated therefrom by the separation roller 36. When the thickness of the recording medium exceeds the prescribed thickness, the recording medium is separated from the belt 24 by the separation roller 36 and is made to go along the peripheral surface of the intermediate transfer belt 6 on the upstream side of the transfer nip. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for transferring a toner image onto a recording medium carried by an endless belt member.

  2. Description of the Related Art Conventionally, there has been known an image forming apparatus that includes an endless belt member that conveys a recording medium, and that transfers a toner image to the recording medium at a transfer nip that includes an image carrier and a transfer roller that are opposed to each other with the belt member interposed therebetween. It has been. However, in the case of a recording medium with low rigidity, such as thin paper, the entry into the transfer nip is delayed, the leading edge of the recording medium is deformed when entering the transfer nip, or the recording medium cannot enter the transfer nip. There is a risk of jamming.

  Therefore, as disclosed in Patent Document 1, an image forming apparatus including a transfer belt that electrostatically attracts and conveys a recording medium has been proposed. In the image forming apparatus disclosed in Patent Document 1, a recording nip is electrostatically attracted and conveyed to the transfer belt described above, and a transfer nip composed of an image carrier and a transfer roller facing each other with the transfer belt interposed therebetween. The toner image is transferred to the recording medium. As a result, even if the recording medium has low rigidity such as thin paper, the recording medium is adsorbed to the transfer belt and conveyed as described above, so that the leading end is conveyed to the transfer nip without being deformed. The conveyance of the medium to the transfer nip is stabilized.

JP 2004-133419 A

  However, the above prior art has the following problems.

  In order to transfer the toner image to the recording medium, a transfer voltage is applied to the transfer roller so as to flow a constant transfer current necessary for the transfer. However, the impedance of the paper increases as the thickness of the recording medium increases. Therefore, in order to cause the same transfer current to flow through recording media having different thicknesses, the transfer voltage to be applied must be increased as the thickness increases. Here, as recording media having different thicknesses, as shown in FIG. 7A, thin paper, plain paper, thick paper, and ultra-thick paper are illustrated in order of thickness. As shown in FIG. 7B, when the same transfer current (30 to 40 μA in this case) is applied to recording media having different thicknesses during transfer, the transfer voltage applied to the transfer roller is 2.0 for thin paper. Although it was -2.5 kV, it was 5.5-6.0 kV with ultra-thick paper.

  However, when a thick recording medium 51 such as ultra-thick paper is sucked and conveyed by the transfer belt 52 and a transfer voltage is applied at the transfer nip, the applied transfer voltage is high (here, 5 kV or more). As shown in c), discharge occurs between the toner image and the paper upstream of the transfer nip. Then, the polarity (here, negative polarity) of the toner image at the location exposed by the discharge upstream of the transfer nip is reversed. Therefore, even if a transfer voltage having a polarity opposite to the normal toner polarity is applied to the transfer roller 53 at the transfer nip, the toner that has been reversed as described above is not transferred to the paper, and the phenomenon is abnormal. End up. In particular, the smaller the amount of toner per unit area on the image carrier (the intermediate transfer belt 54 in FIG. 7C), that is, the lower the density, the smaller the amount of toner charge at that location, so the polarity tends to reverse and abnormal. It was manifested as an image (see FIG. 7A).

  In FIG. 7A, HT (D = 0.6), HT (D = 1.0), and solid (D = 1.6) in order from the smallest toner amount per unit area. Represents. The smaller the toner amount (thinner density) and the thicker the recording medium (higher transfer voltage), the more easily the polarity of the toner is reversed due to discharge, and the abnormal image becomes apparent. In FIG. 7C, reference numeral 55 denotes a stretching roller that stretches the intermediate transfer belt 54, and faces the transfer roller 53 with the belts 52 and 54 interposed therebetween to form a transfer nip.

  Therefore, an object of the present invention is to stably convey the recording medium to the transfer nip regardless of the thickness of the recording medium (from thin paper to ultra-thick paper), and to detect abnormal images caused by discharge upstream of the transfer nip. It is to suppress the occurrence.

  In order to solve the above problems, the present invention provides an image carrier that carries and rotates a toner image, an endless belt member that carries and rotates a recording medium, and the image carrier that sandwiches the belt member. A transfer means for forming a transfer nip; a transfer bias applying means for applying a transfer bias to the transfer means for transferring a toner image of the image carrier onto the recording medium; and the transfer nip according to the thickness of the recording medium. Separating means for separating the recording medium carried on the belt member upstream from the belt member, and when the thickness of the recording medium is equal to or less than a predetermined thickness, the recording medium is separated If the recording medium exceeds the predetermined thickness, the recording medium is separated from the belt member by the separating means. Said And characterized in that along from shooting nip the peripheral surface of the upstream image bearing member.

  According to the present invention, a recording medium having a predetermined thickness or less (a recording medium having low rigidity) can be more stably conveyed to the transfer nip by being carried by the belt member. In addition, a recording medium (a recording medium having high rigidity) exceeding a predetermined thickness is separated from the belt member upstream of the transfer nip and is placed along the image carrier, thereby recording the toner image on the image carrier upstream of the transfer nip. Toner polarity reversal due to discharge with the medium can be suppressed. Thereby, regardless of the thickness of the recording medium, the recording medium can be stably conveyed to the transfer nip, and the occurrence of abnormal images due to discharge upstream of the transfer nip can be suppressed.

1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. (A) is a figure which shows the state which is conveying the recording medium, without separating from a transfer belt, (b) is a figure which shows the state which is separating and conveying a recording medium from a transfer belt. It is a perspective view which shows the mechanism which raises / lowers a separation roller. FIG. 6 is a schematic cross-sectional view of an image forming apparatus according to a second embodiment. (A) is a figure which shows the state which is conveying the recording medium, without separating from a transfer belt, (b) is a figure which shows the state which is separating and conveying a recording medium from a transfer belt. It is a figure which shows the mechanism in which a separation nail contact | abuts or separates from a transfer belt. It is a figure explaining a technical background, (a) is a figure which shows the relationship between paper thickness, a toner amount, and an abnormal image, (b) is a figure which shows the relationship between the transfer current and transfer voltage with respect to thin paper and thick paper, (c). FIG. 4 is a diagram showing a state of toner polarity reversal due to discharge.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

[First Embodiment]
The image forming apparatus according to the first embodiment will be described with reference to FIGS. FIG. 1 is a schematic cross-sectional view of the image forming apparatus according to the first embodiment. FIG. 2 is a diagram illustrating a state of the recording medium conveyed to the transfer nip, and FIG. 2A is a diagram illustrating a state in which the recording medium is conveyed without being separated from the transfer belt. FIG. 7B is a diagram illustrating a state in which the recording medium is separated from the transfer belt and conveyed. FIG. 3 is a perspective view showing a mechanism for raising and lowering the separation roller.

  As shown in FIG. 1, the photosensitive drums (latent image carriers) 1Y, 1M, 1C, and 1k are rotationally driven in the direction of arrow A at a predetermined peripheral speed (process speed). The surface of each photosensitive drum is uniformly charged to a predetermined polarity and potential by charging devices 2Y, 2M, 2C, and 2k. The exposure devices 3Y, 3M, 3C, and 3k expose the charged surface on each photosensitive drum based on image information input from an external device such as an image scanner (not shown) or a computer. By this exposure, an electrostatic latent image corresponding to the image information is formed on each photosensitive drum.

  The developing devices 4Y, 4M, 4C, and 4k contain yellow (Y), magenta (M), cyan (C), and black (k) toners, which are chromatic toners, respectively. The aforementioned electrostatic latent images are developed by these developing devices 4Y, 4M, 4C, 4k, and toner images are formed on the respective photosensitive drums 1Y, 1M, 1C, 1k. A reversal development method is used in which toner (developer) is attached to the exposed portion of the electrostatic latent image for development.

  Further, the intermediate transfer belt (image carrier) 6 is disposed so as to be in contact with each of the photosensitive drums 1Y, 1M, 1C, and 1k. The intermediate transfer belt 6 is stretched around a plurality of stretching rollers 20, 21, and 22 and rotates in the direction of arrow G at a predetermined speed (here, 250 to 300 mm / sec). Here, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 22 is a driving roller for the intermediate transfer belt 6, and the tension roller 21 is a counter roller for secondary transfer. It is.

  The toner images on the photosensitive drums 1Y, 1M, 1C, and 1k are electrostatically and primarily transferred sequentially onto the intermediate transfer belt 6 by the primary transfer units 5Y, 5M, 5C, and 5k. As a result, a full-color image in which the four unfixed toner images are superimposed on the intermediate transfer belt is obtained. On the other hand, the transfer residual toner is cleaned on the surface of each photosensitive drum 1 after the primary transfer by the cleaning devices 11Y, 11M, 11C, and 11k.

  The transfer belt 24 is an endless belt member that supports and rotates a recording medium. The transfer belt 24 is stretched around a plurality of stretching rollers 25, 26, and 27 and is rotated in the direction of arrow B at a predetermined speed (here, 250 to 300 mm / sec). The secondary transfer roller 9 is disposed at a position facing the stretching roller 21. The secondary transfer roller 9 is a transfer unit that forms a transfer nip with the stretching roller 21 on the intermediate transfer belt 6 side with the transfer belt 24 interposed therebetween.

  Recording media (hereinafter referred to as “paper”) 7 stored in a feeding cassette (not shown) are fed one by one by a feeding roller (not shown) and further conveyed in the direction of arrow B by a registration roller 8. The registration roller 8 is a transport unit that is provided upstream of a suction roller 28 described later and transports the recording medium to the transfer belt 24. The registration roller 8 temporarily stops the recording medium 7 that has been fed, and puts the recording medium 7 on the transfer belt 24 so as to synchronize with the timing at which the tip of the toner image on the intermediate transfer belt 6 reaches the transfer nip. Supply. The driving of the registration roller 8 is controlled by the registration roller drive control device 30.

  The suction roller 28 is a suction unit that electrostatically attracts the recording medium to the transfer belt 24. The suction roller 28 includes a pair of a roller disposed inside the transfer belt 24 and a roller disposed outside. The suction roller 28 applies a suction bias whose constant current is controlled by the suction bias applying means 32 to a roller disposed inside the transfer belt when the supplied recording medium 7 is nipped and conveyed. The suction bias applied by the suction bias applying means 32 is controlled by the recording medium conveyance control device 33 according to the thickness of the recording medium. As will be described in detail later, when the thickness of the recording medium is equal to or less than a predetermined thickness, the recording medium that does not need to be separated has a predetermined current (here, −15 to 15) in order to stabilize the carrying force with the transfer belt 24. −30 μA). On the other hand, when the thickness of the recording medium exceeds a predetermined thickness, the suction bias applied to the suction roller 28 is reduced (here, −5 μA) compared to the case where the thickness of the recording medium is equal to or less than the predetermined thickness. The following current is applied) to facilitate separation from the transfer belt 24.

  The separation roller (rotating member) 36 is a separating unit that separates the recording medium carried on the transfer belt 24 upstream of the transfer nip from the transfer belt 24 according to the thickness of the recording medium. The separation roller 36 is provided on the surface of the transfer belt 24 opposite to the surface that carries the recording medium, and is provided so as to be able to enter the transfer belt 24.

  As shown in FIG. 3, both ends of the separation roller 36 are supported by a separation roller support base 38, and an eccentric cam 37 is in contact with the separation roller support base 38. The separation roller 36 is moved up and down by rotating the eccentric cam 37. Here, the separation roller 36 is a φ5 metal roller, and is disposed at a position approximately 50 mm upstream from the transfer nip in the recording medium conveyance direction. ) Invade. On the other hand, when it is not pushed up, the penetration amount is set to 0 mm. The lifting operation of the separation roller 36 is controlled by the separation roller lifting drive control device 31.

  The guide 35 is a guide member that guides the recording medium separated from the transfer belt 24 by the separation roller 36 toward the transfer nip along the peripheral surface of the intermediate transfer belt 6.

  The recording medium electrostatically attracted to the transfer belt 24 passes through the secondary transfer nip formed by the stretching roller 21 and the transfer roller 9 as the transfer belt 24 rotates in the direction of arrow B. At that time, in order to transfer the toner image on the intermediate transfer belt 6 to a recording medium, a transfer bias controlled by a constant current having a polarity opposite to that of the toner image on the intermediate transfer belt 6 is transferred from the secondary transfer bias applying unit 44 to the transfer roller. 9 is applied. Here, a current of +30 to +40 μA is supplied, and the toner image on the intermediate transfer belt 6 is collectively transferred (secondary transfer) to the recording medium 7.

  The recording medium 7 onto which the toner image has been transferred is conveyed to the stretching roller 26 and separated from the transfer belt 24 by the separation claw 29. Next, the toner is conveyed and introduced into a fixing device (not shown), and is subjected to a heat and pressure fixing process of the toner image. On the other hand, the intermediate transfer belt 6 after transfer separation is cleaned of transfer residual toner, paper dust, and the like by a cleaning device 12.

  The image information control device 34 has exposure information and information (paper information) of a recording medium to which the embodied toner image is transferred. Based on the information obtained by the image information control device 34, the recording medium conveyance control device 33 includes a registration roller drive control device 30, an adsorption bias application unit 32, a separation roller elevating drive control device 31, and a secondary transfer bias application unit. 44 is controlled.

  Next, an operation according to the thickness of the recording medium in the image forming apparatus having the above configuration will be described.

Here, as a recording medium, JAPAN TAPPI No. A paper having a Gurley hardness specified in No. 40 of 400 to 1500 mgf and a basis weight of 50 to 300 g / m ² was used.

  Further, as the intermediate transfer belt 6, an appropriate amount of carbon black as an antistatic agent is contained in a resin such as polyimide or polycarbonate, or various rubbers, and its volume resistivity is 1E + 9 to 1E + 14 [Ω · cm], and the thickness is 0.07. What was set to -0.1 [mm] was used.

  The transfer roller 9 is made of an elastic layer of an ion conductive foamed rubber (NBR rubber) and a cored bar, has an outer diameter of 24 mm, a roller surface roughness Rz = 6.0 to 12.0 (μm), and a resistance value of N / A transfer roller of 1E + 5 to 1E + 7Ω was used with 2 kV applied at N (23 ° C., 50% RH) measurement.

  Of the pair of suction rollers 28, the suction roller disposed inside the transfer belt 24 is made of an elastic layer of an ion conductive solid rubber (NBR rubber) and a cored bar, has an outer diameter of 18 mm, and a resistance value of N / A rubber roller of 1E + 5 to 1E + 6Ω was used at 50 V applied with N (23 ° C., 50% RH) measurement. The suction roller disposed outside the transfer belt 24 is a fur brush roller, having a bristle length of 5 mm, a cored bar diameter of 8 mm, an outer diameter of 18 mm, and a resistance value of 100 V measured by N / N (23 ° C., 50% RH). A voltage of 1E + 5 to 1E + 6Ω was used. The fur brush penetrates the transfer belt 24 by 1.5 to 2 mm.

  In the following description, the basis weight is used as the information regarding the thickness of the recording medium, but the present invention is not limited to this.

First, a case where the thickness of the recording medium is a predetermined thickness (basis weight 180 g / m ² in this case) or less will be described. When a recording medium having a basis weight of 180 g / m ² or less is sent to the transfer belt 24, the separation roller 36 does not enter the transfer belt 24 and does not push up the transfer belt 24 as shown in FIG. Further, the registration roller 8 rotates so that the outer peripheral speed becomes equal to the conveying speed of the transfer belt 24. Further, a predetermined current (in this case, a current of −15 to −30 μA) flows through the suction bias controlled at a constant current by the suction bias applying means 32. As a result, the recording medium 7 is conveyed from the suction portion to the transfer nip while being electrostatically attracted to the transfer belt 24.

Next, a case where the thickness of the recording medium exceeds a predetermined thickness (here, basis weight 180 g / m ² ) will be described. When a recording medium having a basis weight exceeding 180 g / m ² is sent to the transfer belt 24, the separation roller 36 enters the transfer belt 24 by a predetermined amount and pushes up the transfer belt 24 as shown in FIG. As a result, the recording medium 7 is separated from the transfer belt 24 by the curvature of the separating roller 36 pushed up. Further, the registration roller rotates so that the outer peripheral speed is higher than the conveying speed of the transfer belt 24 by a predetermined ratio (here, 0.5%). Thus, the separated recording medium 7 enters the transfer nip while being curved along the intermediate transfer belt 6 while being regulated by the guide 35 and curved upward. Further, the suction bias controlled at a constant current by the suction bias applying means 32 passes a current smaller than the above-described predetermined current (here, a current of −5 μA or less). That is, when the thickness of the recording medium exceeds a predetermined thickness, the suction bias applied to the suction roller 28 is made smaller than in the case where the thickness of the recording medium is equal to or less than the predetermined thickness. As a result, the separation of the recording medium 7 by the separation roller 36 can be performed more reliably and stabilized. As described above, when the thickness of the recording medium exceeds the predetermined thickness, the recording medium is separated from the transfer belt 24 by the separation roller 36, and is stably formed on the peripheral surface of the intermediate transfer belt 6 before the transfer nip. It is conveyed along.

  As described above, a recording medium having a predetermined thickness or less (a recording medium having low rigidity such as thin paper) can be more stably conveyed to the transfer nip by being carried by the transfer belt 24 and conveyed. In addition, a recording medium (a recording medium having high rigidity such as cardboard) exceeding a predetermined thickness is separated from the transfer belt 24 upstream of the transfer nip and is allowed to follow the peripheral surface of the intermediate transfer belt 6. As a result, even if the secondary transfer bias applied to flow a constant transfer current necessary for transferring the toner image is high, the discharge between the toner image on the intermediate transfer belt 6 and the recording medium upstream of the transfer nip is not caused. Can be suppressed. Therefore, toner polarity reversal due to the discharge can be suppressed. Thereby, regardless of the thickness of the recording medium, the recording medium can be stably conveyed to the transfer nip, and the occurrence of abnormal images due to discharge upstream of the transfer nip can be suppressed.

[Second Embodiment]
The image forming apparatus according to the second embodiment will be described with reference to FIGS. FIG. 4 is a schematic cross-sectional view of an image forming apparatus according to the second embodiment. FIG. 5 is a diagram illustrating a state of the recording medium conveyed to the transfer nip, and FIG. 5A is a diagram illustrating a state in which the recording medium is conveyed without being separated from the transfer belt. FIG. 7B is a diagram illustrating a state in which the recording medium is separated from the transfer belt and conveyed. FIG. 6 is a diagram showing a mechanism for bringing the separation claw into contact with or separating from the transfer belt.

  The image forming apparatus according to the present embodiment further includes a separation claw 39 that contacts or separates from the transfer belt 24 as a separation unit. Since other configurations are the same as those of the above-described embodiment, members having the same functions are denoted by the same reference numerals, and detailed description thereof is omitted here.

  Similar to the separation roller 36, the separation claw 39 is a separation unit that separates the recording medium carried on the transfer belt 24 upstream of the transfer nip from the transfer belt 24 according to the thickness of the recording medium. The separation claw 39 is a separation assisting member that assists in separation of the recording medium from the transfer belt 24 pushed up by the separation roller 36. As shown in FIG. 4, the separation claw 39 is disposed upstream of the transfer nip in the recording medium conveyance direction. Further, the separation claw 39 is provided near the separation roller 36 via the transfer belt 24 on the side of the transfer belt 24 that carries the recording medium, and is provided so as to be switchable so as to approach or move away from the transfer belt 24. It has been.

  As shown in FIG. 6, the separation claw 39 is connected to the solenoid 40 via a separation claw support base 43 and a spring 41. By switching the driving of the solenoid 40, the separation claw 39 is switched so as to approach or move away from the transfer belt 24. The separation claw 39 is provided with a roller (rotating body) 42, and the roller 42 comes into contact with the transfer belt 24 so that the position accuracy of the tip of the separation claw 39 and the transfer belt 24 is obtained.

  The switching operation for moving the separation claw 39 closer to or away from the transfer belt 24 is performed by controlling the driving of the solenoid 40 by the recording medium conveyance control device 33 described above.

  Next, an operation according to the thickness of the recording medium in the image forming apparatus having the above configuration will be described. The recording medium, the intermediate transfer belt, the transfer roller, and the suction roller are the same as those in the above-described embodiment.

First, a case where the thickness of the recording medium is a predetermined thickness (basis weight 180 g / m ² in this case) or less will be described. When a recording medium having a basis weight of 180 g / m ² or less is sent to the transfer belt 24, the separation roller 36 does not enter the transfer belt 24 and does not push up the transfer belt 24 as shown in FIG. Further, the separation claw 39 is moved away from the transfer belt 24 to a position where the recording medium is not separated from the transfer belt 24. Further, the registration roller 8 rotates so that the outer peripheral speed becomes equal to the conveying speed of the transfer belt 24. Further, a predetermined current (in this case, a current of −15 to −30 μA) flows through the suction bias controlled at a constant current by the suction bias applying means 32. As a result, the recording medium 7 is conveyed from the suction portion to the transfer nip while being electrostatically attracted to the transfer belt 24.

Next, a case where the thickness of the recording medium exceeds a predetermined thickness (here, basis weight 180 g / m ² ) will be described. When a recording medium having a basis weight exceeding 180 g / m ² is sent to the transfer belt 24, the separation roller 36 enters the transfer belt 24 by a predetermined amount and pushes up the transfer belt 24 as shown in FIG. The separation claw 39 is brought close to the transfer belt 24 in order to reliably separate the recording medium pushed up by the separation roller 36 from the transfer belt 24. As a result, the recording medium 7 is separated from the transfer belt 24 by the curvature of the separation roller 36 pushed up. Further, the recording medium pushed up by the separation roller 36 is reliably separated from the transfer belt 24 by the separation claw 39 brought close to the transfer belt 24. Further, the registration roller rotates so that the outer peripheral speed is higher than the conveying speed of the transfer belt 24 by a predetermined ratio (here, 0.5%). Thus, the separated recording medium 7 enters the transfer nip while being curved along the intermediate transfer belt 6 while being regulated by the guide 35 and curved upward. Further, the suction bias controlled at a constant current by the suction bias applying means 32 passes a current smaller than the above-described predetermined current (here, a current of −5 μA or less). That is, when the thickness of the recording medium exceeds a predetermined thickness, the suction bias applied to the suction roller 28 is made smaller than in the case where the thickness of the recording medium is equal to or less than the predetermined thickness. Thereby, the separation of the recording medium 7 by the separation roller 36 and the separation claw 39 can be performed more reliably and stably. As described above, when the thickness of the recording medium exceeds the predetermined thickness, the recording medium is separated from the transfer belt 24 by the separation roller 36, and is stably formed on the peripheral surface of the intermediate transfer belt 6 before the transfer nip. It is conveyed along.

  As described above, similarly to the above-described embodiment, a recording medium having a predetermined thickness or less (a recording medium having a low rigidity such as thin paper) is carried by the transfer belt 24 and conveyed more stably. Can be transported to. In addition, a recording medium (a recording medium having high rigidity such as cardboard) exceeding a predetermined thickness is separated from the transfer belt 24 upstream of the transfer nip and is allowed to follow the peripheral surface of the intermediate transfer belt 6. As a result, even if the secondary transfer bias applied to flow a constant transfer current necessary for transferring the toner image is high, the discharge between the toner image on the intermediate transfer belt 6 and the recording medium upstream of the transfer nip is not caused. Can be suppressed. Therefore, toner polarity reversal due to the discharge can be suppressed. Thereby, regardless of the thickness of the recording medium, the recording medium can be stably conveyed to the transfer nip, and the occurrence of abnormal images due to discharge upstream of the transfer nip can be suppressed.

  Furthermore, when the recording medium is separated from the transfer belt 24, not only the separation roller 36 but also the separation claw 39 assists the separation, so that the separation can be performed more reliably and the separation can be made more stable.

[Other Embodiments]
In the above-described embodiment, an image forming apparatus using four image forming units each including a photosensitive drum and process means (charging means, developing means, cleaning means) acting on the photosensitive drum is illustrated, but the number of use is limited. What is necessary is just to set suitably as needed.

  In the above-described embodiment, the intermediate transfer belt is exemplified as the image carrier. However, the present invention is not limited to this. The image carrier may be, for example, a photosensitive drum, a belt-shaped photosensitive member, or a drum-shaped intermediate transfer member.

  Further, in the above-described embodiment, the configuration in which the photosensitive drum and the process means (charging means, developing means, cleaning means) acting on the photosensitive drum are incorporated in the main body of the image forming apparatus is exemplified, but the present invention is not limited to this. . A process cartridge that integrally includes a photosensitive drum and process means (charging means, developing means, cleaning means) that act on the photosensitive drum may be detachable from the main body of the image forming apparatus.

  In the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine combining these functions. The same effect can be obtained by applying the present invention to any image forming apparatus that transfers a toner image onto a recording medium carried on an endless belt member.

6 ... Intermediate transfer belt 9 ... Transfer roller 24 ... Transfer belt 28 ... Adsorption roller 29 ... Separation claw 32 ... Adsorption bias application means 33 ... Recording medium conveyance control device 34 ... Image information control device 35 ... Guide 36 ... Separation roller 39 ... Separation Claw 40 ... Solenoid 42 ... Roller

Claims (6)

An image carrier that carries and rotates a toner image;
An endless belt member that rotates while carrying a recording medium;
Transfer means for forming a transfer nip with the image carrier across the belt member;
Transfer bias applying means for applying a transfer bias to the transfer means for transferring the toner image of the image carrier to a recording medium;
Separating means for separating the recording medium carried on the belt member upstream from the transfer nip according to the thickness of the recording medium from the belt member;
Have
When the thickness of the recording medium is equal to or less than a predetermined thickness, the recording medium is conveyed to the transfer nip while being carried by the belt member without being separated by the separating means,
When the thickness of the recording medium exceeds a predetermined thickness, the recording medium is separated from the belt member by the separating means and is along the peripheral surface of the image carrier upstream of the transfer nip. Image forming apparatus.   The belt member has an adsorbing means for electrostatically adsorbing the recording medium, and when the thickness of the recording medium exceeds a predetermined thickness, the thickness of the recording medium is less than the predetermined thickness. 2. The image forming apparatus according to claim 1, wherein a suction bias applied to the suction unit is reduced. Conveying means for conveying a recording medium to the belt member upstream from the adsorption means;
When the thickness of the recording medium is equal to or less than a predetermined thickness, the conveying means rotates so that the outer peripheral speed is the same as the conveying speed of the belt member,
3. The image forming according to claim 2, wherein when the thickness of the recording medium exceeds a predetermined thickness, the conveying unit rotates so that an outer peripheral speed is higher than a conveying speed of the belt member. apparatus.   4. A guide member that guides the recording medium separated from the belt member by the separating unit along the peripheral surface of the image carrier toward the transfer nip. The image forming apparatus according to any one of the above. The separating means is provided on the surface of the belt member opposite to the surface carrying the recording medium, and has a rotating member that can enter the belt member.
The rotating member does not enter the belt member when the thickness of the recording medium is equal to or less than a predetermined thickness, and the rotating member does not enter the belt member when the thickness of the recording medium exceeds a predetermined thickness. The image forming apparatus according to claim 1, wherein the recording medium is separated from the belt member by the curvature thereof. The separating means further includes a separation assisting member that is provided on the surface side of the belt member that carries the recording medium and in the vicinity of the rotating member via the belt member, and approaches or moves away from the belt member. And
The separation assisting member is kept away from the belt member when the thickness of the recording medium is equal to or less than a predetermined thickness, and close to the belt member when the thickness of the recording medium exceeds a predetermined thickness. The image forming apparatus according to claim 5, wherein the image forming apparatus assists separation of the recording medium from the belt member into which the rotating member has entered.

Images