JP2019032451A - Fixing device and image forming apparatus - Google Patents

Abstract

To eliminate static electricity from a trace of peeling electrification occurring when a rear end of a sheet passes through a fixing nip without reducing a life of a fixing film to prevent a peeling offset.SOLUTION: A fixing device fixes an unfixed image on a recording medium and comprises: a heating member; a heat transfer member for transferring heat from the heating member to the recording medium; a pressure member that is in pressure contact with the heat transfer member to form a fixing nip for sandwiching the recording medium with the heat transfer member; and discharge means that is provided downstream in a conveyance direction of the recording medium with respect to the fixing nip and is not in contact with the heat transfer member.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a heating member for heating a recording medium and a heating device using the heating member, and more particularly to a heating type fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.

  2. Description of the Related Art Conventionally, in image forming apparatuses such as copying machines and printers that employ an electrophotographic system, many of them employ a heat fixing system as a means for performing a fixing process on paper carrying an unfixed image. Equipment. The fixing device includes a fixing film that rotates along a film guide, a heating source that is disposed in the fixing film and heats the fixing film, and a pressure roller in which a heat-resistant elastic layer is formed on an aluminum or iron cored bar. ing.

  In the fixing device, a fixing film is pressed against a pressure roller by a spring or the like, and a sheet carrying unfixed toner is passed through a fixing nip having a pressing width formed by the pressure contact, and heated and heated. By pressing, unfixed toner is fixed on the paper. For the purpose of preventing electrostatic offset, a fixing bias having the same polarity as the toner is applied to the fixing film. This is because an electrostatic repulsive force acts between the negatively charged film surface layer and the toner, which has the effect of pressing the unfixed toner against the paper.

  In such a fixing device, a sheet left in a low-humidity environment or the second side of double-sided printing occurs between the trailing edge of the sheet and the fixing film when the trailing edge of the sheet having high electrical resistance passes through the fixing nip. Peeling discharge is likely to occur due to a potential difference.

  As a result, the surface layer of the fixing film at the peeling discharge portion is locally charged with a polarity opposite to that of the toner, and a “peeling offset” is generated that attracts unfixed toner on the subsequent paper and offsets after one round of the fixing film. There is a problem of doing. In this phenomenon, the peeling discharge portion is locally charged with a strong positive charge, and therefore, the toner is offset to the film surface layer without being completely discharged by the fixing bias.

  FIG. 12 is a diagram illustrating a mechanism of peeling offset.

  As described above, in the fixing device, unfixed toner is fixed on the sheet by heating and pressurizing in the fixing nip. At this time, an electrostatic offset is prevented by applying a negative bias having the same polarity as that of the toner to the fixing film (FIG. 12A).

  On the other hand, in order to hold the unfixed toner on the paper, positive charges having a polarity opposite to that of the toner are supplied in the transfer operation upstream of the fixing device. For this reason, when the rear end of the conveyed paper passes through the fixing nip, a potential difference is generated between the surface layer of the negatively charged fixing film and the rear end of the positively charged paper, and peeling discharge occurs (FIG. 12 ( b)).

  As a result, the surface layer of the fixing film at the peeling discharge portion locally reverses the polarity and is charged with a positive charge having a polarity opposite to that of the toner. Then, the unfixed toner on the subsequent sheet comes into contact with the peeling charge trace and is electrostatically peeled off on the surface layer of the fixing film, resulting in white streaks in a solid black image or a halftone image, and further, one round of the fixing film. A “peeling offset” that appears as black streaks after offset occurs (FIGS. 12C and 12D).

  In order to solve this problem, Patent Document 1 discloses a technique of applying a bias having the same polarity as the fixing film bias to the elastic separation member brought into contact with the fixing film, or grounding.

  FIG. 13 is a schematic cross-sectional view of a fixing device related to Patent Document 1.

  According to Patent Document 1, it is possible to prevent a peeling offset by removing electricity with an elastic separation member in contact with a positive charge of a peeling charge trace on a fixing film generated at the rear end of a sheet.

JP-A-9-222820

  However, a small amount of paper powder (pulp) or filler (calcium carbonate, talc, kaolin, etc.) positively charged in the transfer operation is electrostatically adhered to the surface layer of the negatively charged fixing film. . Further, when the fixing film is used for a long time, the surface layer is worn and the releasability is lowered, and the toner melted by heating at the fixing nip gradually becomes offset to the surface layer of the fixing film. Therefore, when the elastic separation member is in contact with the fixing film surface layer as in the configuration of Patent Document 1, deposits such as paper components and offset toner are formed at the contact points between the fixing film and the separation member after a long period of use. It becomes easy to accumulate. As a result, the function of removing the positive charge on the fixing film generated by the peeling discharge at the trailing edge of the paper is hindered by the deposit, and a peeling offset occurs. Furthermore, the film surface layer may be damaged by the inorganic mineral contained in the paper filler, and the life may be deteriorated.

  The present invention has been proposed in view of the prior art as described above. An object of the present invention is to provide an image forming apparatus capable of preventing a peeling offset without deteriorating the film life even when a peeling electrification mark is generated on a fixing film.

  The fixing device of the present invention is a fixing device for fixing an unfixed image on a recording medium, and includes a heating element, a heat transfer member for transferring heat from the heating element to the recording medium, and the heat transfer. A pressure member that presses against the member and forms a fixing nip that sandwiches the recording medium with the heat transfer member; and the heat transfer provided downstream of the fixing nip in the conveyance direction of the recording medium. A fixing device comprising a member and a non-contact discharge means.

  According to the present invention, the discharge means that is not in contact with the heat transfer member is provided downstream of the fixing nip in the conveyance direction of the recording medium. For this reason, even when peeling charging occurs when the trailing edge of the recording medium passes through the fixing nip, peeling offset can be prevented without deteriorating the life of the fixing film.

1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a longitudinal sectional view of a fixing device according to an embodiment of the present invention. It is a figure which shows operation | movement of embodiment of this invention. It is a block diagram which shows the structure of embodiment of this invention. It is a flowchart which shows operation | movement of embodiment of this invention. It is a block diagram which shows the structure of other embodiment of this invention. It is a figure which shows the structure of the transfer apparatus of other embodiment of this invention. It is a graph which shows the relationship between the transfer voltage and transfer detection current of other embodiment of this invention. It is a graph which shows the relationship between the bias of the separator of other embodiment of this invention, and the surface potential of a fixing film. It is a flowchart which shows operation | movement of other embodiment of this invention. It is a figure which shows the effect of embodiment of this invention. It is a figure which shows the mechanism of peeling offset. It is a cross-sectional schematic diagram of a conventional fixing device.

  Hereinafter, embodiments of the present invention will be described in detail. Note that the components and numerical values described in the following embodiments are merely examples, and are not intended to limit the scope of the present invention only to them.

<First Embodiment>
(Overall configuration of image forming apparatus)
FIG. 1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus A includes an image forming apparatus main body 100 and an image reading apparatus 200 provided on the upper part of the image forming apparatus main body 100. In the image forming apparatus main body 100, a sheet feeding unit 10, an image forming unit 20, a fixing device 30, and a sheet discharging unit are sequentially arranged from the lower side (upstream in the sheet conveying direction) to the upper side (downstream in the sheet conveying direction) in FIG. 40 is provided. Further, a paper refeed unit 50 is provided on the right side of the image forming unit 20 and the fixing device 30.

  In the paper feeding unit 10, the paper P stacked on the paper feeding cassette 11 or the manual paper tray 16 is fed to the image forming unit 20. The paper detection sensor S <b> 1 is a sensor that detects whether or not the paper P is stored in the paper feed cassette 11.

  The paper P (recording medium) stored in the paper feed cassette 11 is fed to the separation roller pair 13 as the pickup roller 12 rotates. When the paper P is being double-fed, it is separated into one sheet by a separating roller pair 13 composed of a normal rotation roller and a reverse roller, and is conveyed to a paper feed path PS1 indicated by a solid line. The paper P transported to the paper feed path PS1 is further transported by the paper feed roller pair 14, and the skew of the paper P is corrected by following the leading edge of the nip of the registration roller pair 15 that has stopped rotating. The pre-registration sensor S <b> 2 detects the timing at which the leading edge of the paper P reaches the nip of the registration roller pair 15.

  When the paper P is fed from the manual paper tray 16, the paper is separated into one sheet by the supply roller 17 and the separation pad 18 and drawn into the apparatus. Then, the paper is conveyed to the paper feed roller pair 14 by the supply roller pair 19, and the skew of the paper P is corrected by the registration roller pair 15. The sheet P whose skew has been corrected is conveyed to the image forming unit 20 by a registration roller pair 15 that rotates at a predetermined timing.

  In the image forming unit 20, the surface of the photosensitive drum 21 (image carrier) is uniformly charged by the charging roller 22. Laser light corresponding to the image information is emitted from the laser unit 23. In the portion of the photosensitive drum 21 irradiated with the laser light, the electric charge charged by the charging roller 22 is removed, and an electrostatic latent image corresponding to the image information is formed. The electrostatic latent image formed here is visualized as a toner image when the developer is attached by the developing roller 24.

  This toner image is conveyed to the transfer nip portion N1 by the rotation of the photosensitive drum 21. In accordance with this timing, the paper P is conveyed from the registration roller pair 15 to the transfer nip portion N1. The conveyed paper P is nipped and conveyed by the transfer roller 25 (transfer member) that contacts the photosensitive drum 21 at the transfer nip portion N1, and the toner image formed on the photosensitive drum 21 is transferred by the transfer roller 25. .

  The laser light emitted from the laser unit 23 is controlled based on image data of a document read by the image reading device 200 and image data transmitted from the outside.

  Next, the paper P on which the toner image is formed is conveyed to the fixing device 30. The fixing device 30 includes a fixing film 31 and a pressure roller 32 that contacts the fixing film 31 and pressurizes the fixing film 31 with a predetermined pressure. A fixing nip N <b> 2 is formed between the fixing film 31 and the pressure roller 32. The paper P on which the toner image is formed is sent to the fixing nip portion N2, and is nipped and conveyed by the fixing film 31 and the pressure roller 32, and is heated and pressed, whereby the toner image is fixed on the paper P. . The fixing sensor S3 detects that the leading edge of the paper P has passed through the fixing nip portion N2.

  Next, the paper P on which the toner image is fixed is conveyed to the paper discharge unit 40 and discharged to the discharge tray 42 by the discharge roller pair 41.

  When images are formed on both sides of the paper P, the discharge roller pair 41 after the leading edge of the paper P on which the image is formed on the first surface passes through the fixing sensor S3 and before the trailing edge of the paper P passes through the discharge roller pair 41. Is temporarily stopped, and the discharge roller pair 41 is further rotated in the reverse direction. As a result, the paper P is reversed and conveyed to the paper refeed unit 50.

  The paper P transported to the paper refeed unit 50 is transported through a refeed path PS2 indicated by a broken line by refeed roller pairs 51 and 52, and is transported to a registration roller pair 15 by a refeed roller pair 53. . Then, after the skew is corrected by the registration roller pair 15, the toner image is formed on the second surface of the paper P by being conveyed to the transfer nip portion N1. Thereafter, the toner image is fixed on the paper P by being conveyed through the fixing nip N2 in the same manner as when the image is formed on the first side, and the paper P on which the image is formed on both sides is discharged by the discharge roller pair 41 to the discharge tray 42. Is discharged.

(Fixing device)
FIG. 2 is a longitudinal sectional view of the fixing device 30. In the following description, regarding the fixing device 30 or a member constituting the fixing device 30, the longitudinal direction is a direction orthogonal to the recording medium conveyance direction, and the short direction is the same direction as the recording medium conveyance direction. It is.

  The fixing device 30 includes a ceramic heater 311 as a heat generating member, a fixing film 31 as a flexible member, a stay 310 as a guide member, and a pressure roller 32 as a pressure member. The stay 310, the fixing film 31, the ceramic heater 311 and the pressure roller 32 are all elongated members in the longitudinal direction.

  The stay 310 is formed in a vertical cross-sectional shape using a predetermined material having heat resistance and rigidity, and a ceramic heater 311 is held on the stay 310. The fixing film 31 (heat transfer member) has heat resistance and is endless (cylindrical). The fixing film 31 transmits heat from the ceramic heater 311 to the paper P. The fixing film 31 is fitted on the stay 310. The inner peripheral length of the fixing film 31 and the outer peripheral length of the stay 310 are larger in the fixing film 31 by, for example, about 3 mm. Therefore, the fixing film 31 is externally fitted to the stay 310 with a margin in the circumference. A lubricant (not shown) is interposed between the inner peripheral surface of the fixing film 31 and the outer peripheral surface of the stay 310. This reduces the sliding resistance when the stay 310 and the inner peripheral surface of the fixing film 31 rotate in contact with each other. Each member in the fixing device will be described in detail.

(Fixing film)
In order to reduce the heat capacity and shorten the start-up time, the film thickness of the fixing film 31 is preferably 100 μm or less. As a material of the fixing film 31, a heat-resistant single layer film such as PTFE, PFA, FEP or the like, or a composite in which PTFE, PFA, FEP, etc. are coated on the outer peripheral surface of a film such as polyimide, polyamideimide, PEEK, PES, PPS, etc. A layer film can be used. In the present embodiment, a polyimide film having a film thickness of 75 μm coated with PFA or PTFA on the outer peripheral surface is used as the fixing film 31. The outer diameter of the fixing film 31 was 24.0 mm. In addition, as a countermeasure against fixing explosion, a high voltage power supply (not shown) that applies a negative bias having the same polarity as the toner is provided, and the sequence and output are controlled by the CPU.

(Stay)
The stay 310 can be made of, for example, a high heat resistant resin such as polyimide, polyamideimide, PEEK, PPS, or liquid crystal polymer, or a composite material of these resins with ceramic, metal, glass, or the like. In the present embodiment, the stay 310 is formed in a vertical cross-sectional shape by a liquid crystal polymer. Both ends of the stay 310 in the longitudinal direction are held by a pair of side plates (not shown) of the fixing device 30. A concave groove is provided along the longitudinal direction on the surface of the stay 310 on the pressure roller 32 side, and a ceramic heater 311 is held in the groove.

(Pressure roller)
The pressure roller 32 has a metal core 32a and an outermost release layer 32c provided around an elastic body layer 32b provided around the metal core 32a. The pressure roller 32 is held by a pair of side plates of the fixing device 30 in a freely rotatable state at both ends in the longitudinal direction of the cored bar 32a. In the present embodiment, the core metal 32a is made of aluminum, the elastic layer 32b is made of silicon rubber containing a microballoon, and the release layer 32c is made of a PFA tube. The outer diameter of the pressure roller 32 was 20 mm, and the thickness of the release layer was 30 μm. The pressure roller 32 disposed below the fixing film 31 and in parallel with the fixing film 31 is pressed toward the stay 310 by pressure means such as a pressure spring at both ends of the core metal. As a result, a fixing nip portion N2 having a predetermined width necessary for heating and fixing the unfixed toner image on the paper P is formed between the surface of the pressure roller 32 and the ceramic heater 311 with the fixing film 31 interposed therebetween.

(Separator)
The sheet that has exited the fixing nip N2 is separated from the fixing film 31 and guided to the discharge roller pair 41 downstream in the transport direction. Separation of the paper from the fixing film 31 is basically performed by the curvature separation. However, when the toner melted by a weak paper or high-printing paper adheres the fixing film 31 and the paper, the separation of the curvature is completed. There is no time. In this case, a separation plate 33 (conductive member) is provided in the vicinity of the fixing film 31 over the entire rotation axis direction of the fixing film 31 on the downstream side in the conveyance direction of the fixing nip. The separation plate 33 is disposed in non-contact with the fixing film 31. If the interval (hereinafter referred to as “separation plate gap”) is narrow, the separation layer 33 contacts the fixing film 31 and the surface layer is scraped. Therefore, the separation plate gap is precisely designed and is defined as 200 to 350 μm in this embodiment, and the material of the separation plate 33 is a conductive metal member such as SUS or copper. There is also provided a separation plate high-voltage power supply (not shown) for applying a negative bias having the same polarity as that of the fixing film 31, and the sequence and output are controlled by the CPU.

(Operation)
FIG. 3 is a diagram illustrating the operation of the present embodiment.

  The peeling discharge between the trailing edge of the paper P and the fixing film 31 causes dielectric breakdown due to a concentrated electric field formed at the moment when the trailing edge of the paper positively charged at the transfer nip leaves the surface layer of the fixing film 31 that is negatively charged. Occurs. At this time, the surface layer of the fixing film 31 that is uniformly charged to, for example, −1000 V by constant voltage control from the high-voltage power source of the fixing film 31 locally fluctuates toward a positive potential from about 0 to +200 V by peeling discharge (see FIG. 3 (a)).

  In order to remove the charge from the peeling charge, −1500 V having the same polarity as that of the fixing film 31 is applied to the conductive separation plate 33 disposed downstream of the fixing nip portion N2 in the film rotation direction. The separation plate 33 of the present embodiment is characterized in that it is disposed in non-contact with the fixing film 31. For this reason, in the surface layer of the fixing film 31 that is uniformly charged to −1000 V, the potential difference is smaller than the discharge start voltage, so no discharge from the separation plate 33 occurs, and the potential difference is greater than the discharge start voltage. Discharge occurs selectively only on the peeling charge trace (0 to +200 V) near the positive potential. As a result, the peeling offset is improved by effectively neutralizing the peeling charge trace (FIG. 3B).

  In the present embodiment, the voltage applied to the separation plate 33 is set to −1500 V. However, the discharge start voltage varies depending on environmental conditions and the like, so the applied voltage may be changed from −1500 V accordingly.

  When the bias of the separation plate 33 is constantly applied even during paper feeding or when a higher voltage than necessary is applied, the deposits of paper components accumulate on the separation plate 33 and come into contact with the fixing film 31 to cause a risk of damaging the surface layer. . This is because the paper contains fillers (calcium carbonate, talc, kaolin, etc.) in the main raw material wood pulp, and these are positively charged by passing through the transfer nip. Because it is attracted. Therefore, the sequence control for applying the bias of the separation plate 33 is performed only when the trailing edge of the sheet passes through the fixing nip portion N2.

  The trigger for bias sequence control of the separation plate 33 in the present embodiment is not particularly limited as long as it is a configuration or sensor capable of detecting the trailing edge of the paper P. For example, a clutch signal of a registration roller motor, a transmission sensor, a flag sensor, or the like that is provided in the conveyance path upstream of the fixing nip portion N2 and detects the presence or absence of paper can be used.

  Further, the discharge discharge between the trailing edge of the sheet and the fixing film 31 becomes stronger as the air and the sheet P are dried, particularly in a low humidity environment. Therefore, in this embodiment, the environment provided in the image forming apparatus main body 100 is used. Whether the operation is ON / OFF is determined based on the detection result of the temperature / humidity sensor.

For example, the absolute water content in the unit space calculated from the environmental temperature and humidity is 2.0 [g / kgD. A. In the following, the sheet resistance becomes as high as 10 ¹³ to 10 ¹⁵ Ω / cm, which causes peeling discharge. In this case, the CPU determines from the detection result of the environmental sensor, performs the sequence control of the bias of the separation plate 33 based on the rear end detection signal, and removes the charge from the peeling.

  FIG. 4 is a diagram illustrating functional blocks of the present embodiment.

  In the present embodiment, an environmental sensor 600 (moisture amount detection unit) that detects the temperature and humidity of the surrounding environment, a paper rear end detection unit 601 (passage detection unit) that detects the rear end of the conveyed paper P, and fixing. A fixing film high voltage circuit 602 (first bias applying means) for applying a high voltage to the film 31, a separation plate high voltage circuit 603 (second bias applying means) for applying a high voltage to the separation plate 33, and detection signals of sensors. And a CPU 604 that performs sequence control of the bias of the separation plate 33 is provided. The sheet trailing edge detection unit 601 is based on a detection signal of an optical sensor or a flag sensor provided in the conveyance path of the sheet P, or a transfer current detection unit according to a second embodiment to be described later. Then, it is detected that the trailing edge of the sheet P has passed through the fixing nip portion N2.

  FIG. 5 is a flowchart showing the operation of the present embodiment.

  First, after copying is started (step S1), referring to the environmental sensor 600 (step S2), the absolute water content (water content in the air) contained in the air around the image forming apparatus main body is 2.0 g / kgD. The CPU 604 detects whether or not it is greater than or equal to A (step S3). As a result, 2.0 g / kg D.D. If it is greater than or equal to A (step S3), a normal copy operation is performed (step S4), and the copy is terminated (step S8). On the other hand, 2.0 g / kg D.D. When it is detected that the humidity environment is less than A (step S3), the detection result of the sheet trailing edge detection unit 601 is referred to (step S5), and the separation plate 33 after the trailing edge of the sheet passes through the fixing nip N2. Is applied (step S6). Thereafter, if there is no subsequent sheet (step S7), the copying operation is terminated (step S8), and if there is (step S7), the detection result of the sheet trailing edge detection unit 601 is referred again (step S5). The operation is continued until the subsequent sheet runs out.

Second Embodiment
Next, a second embodiment of the present invention will be described. The present embodiment is basically based on the configuration of the first embodiment. In the following description, differences from the first embodiment will be described.

  In the second embodiment, the intensity of the peeling discharge is predicted from the fluctuation of the transfer current when the trailing edge of the sheet comes out of the transfer nip, and the bias intensity of the separation plate 33 is varied according to the current fluctuation threshold.

In the first embodiment, the sequence control of the separation bias is performed based on the detection results of the environmental sensor and the sheet trailing edge detection unit. However, in this embodiment, a transfer current detection unit is provided to detect the transfer current variation at the trailing edge of the sheet. The edge is detected, and the sequence and intensity of the separator bias are controlled according to the current fluctuation value.
(Block configuration)
FIG. 6 is a block diagram showing the configuration of the present embodiment.

  The image forming apparatus according to the present embodiment includes a transfer high-voltage circuit 605 (third bias applying unit) that applies a high voltage to the transfer roller 25, a transfer current detection unit 606 that detects a transfer current flowing through the transfer roller 25, and a fixing film. A fixing film high voltage circuit 602 that applies a high voltage to 31, a separation plate high voltage circuit 603 that applies a high voltage to the separation plate 33, and a bias sequence control and a high voltage control of the separation plate 33 in response to a detection signal from the transfer current detection unit 606. CPU 604 that performs the above.

(Transfer device)
FIG. 7 is a diagram illustrating a configuration of the transfer apparatus according to the present embodiment.

  The toner image developed on the photosensitive drum 21 is rotationally moved to the transfer nip portion N1 by the rotational driving of the photosensitive drum 21. In accordance with this timing, the paper P stored in the paper feed cassette 11 is transported by the pickup roller 12 and the paper feed roller pair 14 and is transported to the transfer nip portion N1 between the photosensitive drum 21 and the transfer roller 25. The

  Then, a predetermined DC voltage having a polarity opposite to that of the developer is applied to the transfer roller 25 from the transfer high-voltage circuit 605 of the high-voltage power supply unit at the timing when the sheet P is conveyed to the transfer nip portion N1, thereby applying to the photosensitive drum 21. The adhered toner images are sequentially electrostatically transferred onto the paper P. In this embodiment, since the toner has a negative polarity, a positive polarity DC voltage +2.0 kV corresponding to the opposite polarity is applied to the transfer roller 25.

  In this embodiment, a contact transfer roller system that contacts the photosensitive drum 21 is used. However, a system in which a toner image is transferred to an intermediate transfer belt and then transferred to a sheet P may be used.

  The transfer roller 25 in this embodiment is an ion conductive rubber roller in which a sponge rubber obtained by foaming a mixed rubber of NBR rubber (acrylonitrile butadiene rubber) and hydrin rubber is disposed on a metal shaft.

  As the transfer roller 25, a single layer structure is used, and the metal shaft body 25a, the conductive foam layer 25b formed on the outer periphery of the metal shaft body 25a, and the metal shaft body 25a and the conductive foam layer 25b are bonded. Mainly composed of an adhesive layer 25c.

  The configuration of the metal shaft body 25a is not limited, and a metal core made of a metal solid body, a metal cylinder body hollowed out inside, and the like are used. There is no particular limitation.

The conductive foam layer 25b is preferably set to a hardness range of 20 to 35 ° / 500 g load on Asker C (sponge hardness meter), and the electric resistance value is set to 10 ⁴ to 10 ⁸ Ω. It is preferably 10 ⁵ to 10 ⁷ Ω.

  Examples of matrix components include ethylene-propylene-diene rubber (EPDM) and acryl-nitrile-butadiene rubber (NBR), which are mixed with hydrin rubber as an ionic conductive agent.

  It is formed of a forming material in which an electronic conductive conductive agent as a resistance adjusting agent, a foaming agent, a foaming aid, a softening agent, a plasticizer, a filler, a vulcanizing agent, and a vulcanization accelerator are blended.

  Examples of the electron conductive conductive agent include carbon oxides such as carbon black, graphite, a solid solution of zinc oxide and aluminum oxide, a solid solution of tin oxide and antimony oxide, and a solid solution of indium oxide and tin oxide.

  These may be used alone or in combination of two or more. Examples of the foaming agent include dinitropentamethylenetetramine (DPT), azodicarbonamide (ADCA), 4,4-oxybisbenzenesulfonyl hydragit (OBSH), and the like. These may be used alone or in combination of two or more. It is done.

(Operation)
The operation of this embodiment will be described with reference to FIGS. FIG. 8 is a diagram showing the relationship between the transfer voltage (FIG. 8A) and the transfer detection current (FIG. 8B), and FIG. 9 shows the bias of the separation plate 33 (FIG. 9A) and the fixing film 31. It is a figure which shows the relationship of surface potential (FIG.9 (b)).

  In the transfer operation of the present embodiment, constant voltage control is performed, and a constant voltage of +1000 to +2000 V is applied to the sheet passing area where the sheet P passes through the transfer nip (FIG. 8A). The transfer current detector 606 detects the fluctuation ΔI and the timing of the trailing edge from the current fluctuation at the moment when the trailing edge of the paper P passes through the transfer nip portion N1 (FIG. 8B). The detected current fluctuation is compared with the threshold value stored in the CPU 604 in advance to determine the bias intensity of the separation plate 33, and the bias sequence control of the separation plate 33 is performed according to the detected rear end timing.

  In this embodiment, two threshold values stored in advance in the CPU 604 are provided: a first threshold value with a fluctuation ΔI of the trailing edge transfer current of 15 μA and a second threshold value with 30 μA.

  For example, when the fluctuation ΔI is equal to or less than the first threshold (ΔI ≦ 15 μA), the positive charge at the rear end of the sheet due to the transfer operation is weak, and peeling discharge between the rear end of the sheet P and the fixing film 31 does not occur or is slight. The bias of the separation plate 33 is not applied. When the fluctuation ΔI exceeds the first threshold and is equal to or less than the second threshold (15 <ΔI ≦ 30 μA), a peeling discharge trace of about 0 to +100 V is generated on the surface of the fixing film (FIG. 9B). A separator plate bias of 1500 V is applied to the region. Further, when the fluctuation ΔI is large and exceeds the second threshold value (30 μA <ΔI), a peeling discharge trace of about +100 to +300 V is generated on the fixing film 31, and therefore, a separation plate bias of −2500 V is applied to the region. (FIG. 9A).

  In the transfer current detection unit 606, when a constant voltage is applied from the transfer high voltage circuit 605 to the transfer roller 25, the transfer current that fluctuates due to the load of the paper P or the like is measured over time, and the CPU determines the change ΔI to determine the threshold I do. However, the means for measuring the transfer current is not limited to the transfer current detector 606, and the current flowing when the transfer voltage is applied is measured directly or indirectly by measuring the value of the current flowing through the base layer of the photosensitive drum 21 with an ammeter. If it can be measured automatically, the configuration is not limited.

  The reason why the bias of the separation plate 33 is inadvertently applied constantly regardless of the threshold value or avoids increasing the strength constantly is to prevent the accumulation of positively charged deposits of paper components on the separation plate 33 and to operate for a long time. This is to reduce the risk of damaging the surface layer of the fixing film 31.

  Next, the operation of the present embodiment will be described based on the flowchart shown in FIG.

  First, after copying is started (step S9), the transfer current detector 606 is referred to (step S10), and the transfer current fluctuation ΔI is measured (step S11). Next, the CPU 604 determines whether or not the variation ΔI is equal to or less than 15 μA which is the first threshold (step S12). If the variation ΔI is equal to or less than the first threshold (step S12), a normal image forming operation is performed (step S13). ), The copy is terminated (step S18).

  On the other hand, if the variation ΔI exceeds the first threshold value (step S12), the CPU 604 determines whether the second threshold value is 30 μA or less (step S14). As a result, if the variation ΔI is equal to or smaller than the second threshold value (step S14), a separation plate bias of −1500 V is applied after the trailing edge of the sheet has passed through the fixing nip portion N2 (step S15).

  On the other hand, if the second threshold value is exceeded (step S14), a bias of -2500V separator 33 is applied (step S16). That is, as the transfer current fluctuation ΔI detected by the transfer current detector 606 is larger, the separator plate high voltage circuit 603 applies a larger absolute value bias to the separator plate 33 with the same polarity as the output of the fixing film high voltage circuit 602. To do.

  Thereafter, if there is no subsequent sheet (step S17), the copying operation is terminated (step S18). If there is (step S17), the detection result of the transfer current detection unit 606 is referred again (step S10). The operation continues until there is no more paper.

<Effect of the above-mentioned embodiment>
FIG. 11 is a diagram illustrating the effect of the above-described embodiment.

  In Patent Document 1, a bias having the same polarity as that of the fixing film is constantly applied to the separation blade that is in contact with the fixing film during the printing operation, and the charge on the peeling charge is removed. However, as the toner is used for a long time, the deposits of offset toner and paper components accumulate in the nip between the separation blade and the fixing film, so that the function of removing the charge of the peeling charge is hindered, and the peeling offset occurs. Moreover, the film surface layer is damaged by the inorganic minerals (talc, kaolin, etc.) of the paper component, and the durability life is deteriorated.

  On the other hand, in the above-described embodiment, the conductive separation plate 33 that is not in contact with the fixing film 31 is appropriately selected according to the detection results of the environmental sensor 600 and the paper trailing edge detection unit 601 or the transfer current detection unit 606. The bias of the separation plate 33 is applied in sequence and intensity. For this reason, it is possible to effectively neutralize the peeling charge traces regardless of long-time use. Further, since the separation plate 33 is disposed in a non-contact manner and sequence control is performed, adhesion of the paper components to the separation plate 33 can be prevented, so that the static elimination function is deteriorated and the surface layer of the fixing film 31 is damaged. Less risk.

A: Image forming device N1: Transfer nip portion N2: Fixing nip portion P ... Paper 20 ... Image forming portion 21 ... Photosensitive drum 25 ... Transfer roller 30 ... Fixing device 31 ... Fixing roller 32 ... Pressure roller 33 ... Separating plate 311 ... Ceramic heater 600 ... Environmental sensor 601 ... Paper trailing edge detector 602 ... Fixing film high-voltage circuit 603 ... Separation plate high-voltage circuit 604 ... CPU
605 ... Transfer high voltage circuit 606 ... Transfer current detector

Claims (8)

A fixing device for fixing an unfixed image on a recording medium,
A heating member;
A heat transfer member for transferring heat from the heat generating member to the recording medium;
A pressure member that presses the heat transfer member and forms a fixing nip that sandwiches the recording medium with the heat transfer member;
A fixing apparatus comprising: the heat transfer member and a non-contact discharge unit provided downstream of the fixing nip in the conveyance direction of the recording medium. A first bias applying means for applying a first bias to the heat transfer member;
The discharge means includes a conductive member provided in non-contact with the heat transfer member on the downstream side in the conveyance direction of the recording medium with respect to the fixing nip, and a first member having the same polarity as the first bias. The fixing device according to claim 1, further comprising: a second bias applying unit that applies two biases. Further comprising passage detection means for detecting that the rear end of the recording medium has passed through the fixing nip;
The second bias applying means applies a second polarity having the same polarity as the first bias to the conductive member when the passage detecting means detects that the trailing edge of the recording medium has passed through the fixing nip. The fixing device according to claim 2, wherein a bias is applied.   The passage detection unit detects that the rear end of the recording medium has passed through the fixing nip based on a detection signal of an optical sensor or a flag sensor provided in a conveyance path of the recording medium. The fixing device according to claim 3. A water content detecting means for detecting the amount of water in the air;
The second bias applying unit applies the second bias having the same polarity as the first bias to the conductive member when the moisture amount in the air is equal to or less than a predetermined value by the moisture amount detecting unit. The fixing device according to claim 2, wherein the fixing device is applied. A fixing device according to claim 3 or 4,
An image carrier for carrying an image to be transferred to the recording medium;
A transfer member that contacts the image carrier to form a transfer nip, and transfers the image on the image carrier to the recording medium as the unfixed image;
Third bias applying means for applying a bias to the transfer member;
Current detecting means for detecting a current value flowing through the transfer member;
When the recording medium passes through the transfer nip, the second bias applying means causes the second bias applying means to be applied to the conductive member when the current detecting means detects a current fluctuation value larger than a predetermined value. An image forming apparatus, wherein the second bias having the same polarity as the first bias is applied.   The second bias applying unit applies a second bias having the same polarity as the first bias and a larger absolute value to the conductive member as the current fluctuation value detected by the current detecting unit is larger. The image forming apparatus according to claim 6.   8. The passage detection unit according to claim 6, wherein the passage detection unit determines that the rear end of the recording medium has passed through the fixing nip based on a current fluctuation value detected by the current detection unit. The image forming apparatus described in 1.

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