JP2018010163A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent image defects from occurring in fixation processing.SOLUTION: A fixing device 25 includes: a first fixing member 41 that is provided with a release layer 41C on an outermost peripheral side and heats a surface of a paper sheet; a second fixing member 42 that forms a nip part 45 between the first fixing member 41 and the second fixing member 42 and applies pressure to the paper sheet from its rear surface; a charging device 47 that is in non-contact with the release layer 41C of the first fixing member 41 and performs discharging to the release layer 41C; and a discharge control circuit for controlling the charging device 47. The discharge control circuit performs discharging between: the charging device 47 and the release layer 41C of the first fixing member 41, with a current of a first set value, within a period from the issuance of a print start instruction to the start of secondary supply of paper sheets; and, after the lapse of the period, the charging device 47 and the release layer 41C of the first fixing member 41, with a current of a second set value which is smaller than the first set value.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a fixing device and an image forming apparatus in an electrophotographic image forming apparatus.

  An electrophotographic image forming apparatus includes a fixing device that fixes a toner image transferred onto the surface of a recording material to the recording material. The fixing device includes a first fixing member that heats the surface of the recording material to which the toner image is transferred, and a second fixing member that pressurizes the recording material from the back surface. Generally, the first fixing member is formed in a roller shape or an endless belt shape, and the second fixing member is formed in a roller shape. The first fixing member and the second fixing member are arranged so that their peripheral surfaces face each other, and the recording material is heated and heated by passing the recording material through a nip formed between the first fixing member and the second fixing member. Pressure is applied to fix the toner image on the recording material.

  In many fixing devices, a release layer formed of a fluororesin is provided on the outermost peripheral side of the first fixing member in order to improve the release property. In many cases, a release layer formed of a fluororesin is also provided on the outermost peripheral side of the second fixing member.

  In such a fixing device, when the recording material passes through the nip portion between the first fixing member and the second fixing member, a negative charge is generated on the surface of the release layer of the first fixing member due to frictional charging. Is accumulated. In particular, when positively charged toner is used, unfixed toner on the surface of the recording material becomes negative charge accumulated in the release layer of the first fixing member before the nip portion. There is a possibility that an image defect is caused by being attracted and scattered. Further, the toner attracted to the release layer of the first fixing member and adsorbed on the surface of the first fixing member may adhere to the recording material passing through the nip portion to cause an image defect.

  In this regard, Patent Document 1 below discloses a fixing device that repels toner with respect to the fixing roller by applying a bias voltage having the same polarity as the toner to the core of the fixing roller corresponding to the first fixing member. Have been described.

Japanese Patent Laid-Open No. 04-335386

  However, the fixing device described in Patent Document 1 has a configuration in which a power source is connected to the core metal of the fixing roller, and a bias voltage is directly applied to the core metal from the power source. In the case of such a contact-type configuration, the cored bar is insulated and supported in order to prevent the current flowing through the cored bar by applying a bias voltage from leaking to other parts of the image forming apparatus or the outside of the image forming apparatus. A structure needs to be provided in the fixing device. The addition of such an insulating structure leads to an increase in the size of the apparatus, and does not meet the recent request for downsizing the fixing device or the image forming apparatus.

  On the other hand, an electrode is disposed at a position away from the outer peripheral surface of the release layer of the first fixing member, and discharging is performed between the electrode and the release layer of the first fixing member, thereby removing the release layer from the toner. A configuration is conceivable in which the toner is charged to the same polarity as that of the toner to thereby prevent the toner on the recording material from being attracted to the first fixing member. Such a non-contact configuration can easily reduce the size of the fixing device or the image forming apparatus as compared with the contact configuration.

  However, since the non-contact configuration is a configuration in which the release layer of the first fixing member is charged by discharge, the first fixing member is separated from the contact-type configuration in which a voltage is directly applied. It takes time to increase the potential of the mold layer to a potential that can sufficiently suppress the toner on the recording material from being attracted to the release layer. For this reason, the start and stop of the rotation of the first fixing member and the second fixing member are synchronized with the start and stop of the discharge of the first fixing member with respect to the release layer, and from the start to the stop of the discharge. In the meantime, when discharging is performed at a constant current value, the following problems occur.

  That is, in response to a print start instruction, rotation of the first fixing member and the second fixing member is started, and at the same time, discharge of the first fixing member to the release layer is started and recording is performed almost simultaneously with this. When the conveyance of the recording material (paper) stored in the material storage unit (paper feed cassette) to the transfer unit is started, the potential of the release layer of the first fixing member is applied to the release layer on the recording material. The recording material reaches the nip portion between the first fixing member and the second fixing member before reaching a potential that can sufficiently suppress the toner from being attracted. As a result, for example, toner scattering as described above cannot be sufficiently suppressed at the front side in the conveyance direction of the recording material, and there is a possibility that an image defect may occur.

  The present invention has been made in view of, for example, the above-described problems, and an object of the present invention is to provide a fixing device and an image forming apparatus that can prevent image defects from occurring during fixing processing.

  In order to solve the above problems, a fixing device of the present invention is a fixing device that is provided in an image forming apparatus and fixes a toner image formed on the surface of a recording material to the recording material. A mold layer is provided, a nip portion is formed between the first fixing member for heating the surface of the recording material and the first fixing member, and the second fixing for pressurizing the recording material from the back surface thereof. A charging device that is in non-contact with the release layer of the first fixing member and discharges the release layer; and a control unit that controls the charging device, the control unit includes: During a predetermined period included between the time when the print start instruction is issued and the time when the recording material reaches the nip portion, the current of the first set value is supplied to the charging device by supplying the current to the charging device. The first fixing member is discharged between the release layer and the predetermined period After the lapse of time, a current having a second set value smaller than the first set value is supplied to the charging device to discharge between the charging device and the release layer of the first fixing member. It is characterized by.

  In this aspect of the present invention, a larger current is supplied to the charging device during a predetermined period included between the time when the print start instruction is issued and the time when the recording material reaches the nip portion than after the predetermined period has elapsed. It discharges by supplying. As a result, the current flowing between the charging device and the release layer of the first fixing member due to the discharge within the predetermined period causes the discharge between the charging device and the release layer of the first fixing member after the predetermined period. It becomes larger than the current flowing between them. Therefore, the electric potential of the release layer of the first fixing member can be quickly increased by the discharge within the predetermined period. Therefore, in response to the print start instruction, rotation of the first fixing member and the second fixing member is started, and at the same time, discharge between the charging device and the release layer of the first fixing member is started, When the conveyance of the recording material accommodated in the recording material accommodating portion to the transfer portion is started almost at the same time, before the recording material reaches the nip portion between the first fixing member and the second fixing member. In addition, the potential of the release layer of the first fixing member can be increased to a potential that can sufficiently suppress the toner on the recording material from being attracted to the release layer. Thereby, when the recording material passes through the nip portion between the first fixing member and the second fixing member, it is possible to suppress scattering of unfixed toner on the surface of the recording material, It is possible to suppress the toner from adsorbing to the release layer of the first fixing member and adhering to the recording material passing through the nip portion. Therefore, image defects can be suppressed.

  Further, in the above-described fixing device of the present invention, the control unit starts primary paper feeding to convey the recording material from the recording material storage unit to the registration roller pair from the time when the print start instruction is issued, and further By supplying the current of the first set value to the charging device within a predetermined period included until the time when the secondary feeding for conveying the recording material from the registration roller pair to the transfer unit is started. The charging device and the release layer of the first fixing member are discharged, and after the predetermined period of time, the charging device and the charging device are supplied by supplying a current of the second set value to the charging device. It is good also as a structure discharged between the mold release layers of a 1st fixing member.

  According to this aspect of the present invention, when the secondary paper feeding is started, the value of the current supplied to the charging device is switched from the first set value to the second set value, so that the recording material is in the nip portion. Before reaching the release layer, the potential of the release layer of the first fixing member is increased to a potential that can sufficiently suppress the toner on the recording material from being attracted to the release layer, and then the potential is increased. Can be stabilized.

  Further, in the above-described fixing device of the present invention, the control unit has a current of a third set value smaller than the second set value from the time when printing is finished and the image forming apparatus shifts to the standby mode. It is good also as a structure which discharges between the said charging device and the mold release layer of a said 1st fixing member by supplying to the said charging device.

  In this aspect of the present invention, during the standby mode, the discharge continues while supplying the current of the third set value to the charging device. Therefore, during the standby mode, the potential of the release layer of the first fixing member is zero. Instead, a constant potential corresponding to the current of the third set value is maintained. Therefore, when a print start instruction is issued during the standby mode, charging is performed by switching the current supplied to the charging device from the third set value to the second set value or the first set value in accordance with the print start instruction. The discharge between the apparatus and the release layer of the first fixing member is continued, and at the same time, the conveyance of the recording material accommodated in the recording material accommodation portion to the transfer portion is started. Before the recording material reaches the nip portion between the fixing member and the second fixing member, the potential of the release layer of the first fixing member is drawn to the toner on the recording material to the release layer. Can be rapidly increased to a potential that can sufficiently suppress the above. Thereby, even when a print start instruction is issued during the standby mode, it is possible to suppress image defects due to toner scattering or the like.

  In the above-described fixing device of the present invention, the charging device includes a needle-like discharge electrode and a pair of ground electrodes, and the discharge electrodes are separated from the pair of ground electrodes, respectively. Preferably, the tip of the discharge electrode is arranged between the electrodes, and the tip of the ground electrode does not protrude toward the release layer of the first fixing member.

  In the fixing device of the present invention described above, each ground electrode of the charging device is preferably connected to a ground path of the image forming apparatus via a Zener substrate.

  In the above-described fixing device of the present invention, the surface resistance of the second fixing member is set to a value smaller than the surface resistance or volume resistance of the first fixing member, and the second fixing member has a surface resistance. The surface is preferably grounded.

  In the above-described fixing device of the present invention, the volume resistance of the second fixing member may be set to a value smaller than the surface resistance or volume resistance of the first fixing member.

  In the above-described fixing device of the present invention, the first fixing member and the second fixing member are provided on the downstream side of the first fixing member and the second fixing member in the traveling direction of the recording material. The recording apparatus further includes a pair of discharge rollers that discharges the recording material traveling between the fixing members from the fixing device, and of the pair of discharge rollers, the volume resistance of the discharge roller that contacts the back surface of the recording material. The first fixing member may be set to a value smaller than the surface resistance or volume resistance.

  Further, in the above-described fixing device of the present invention, the recording material is provided on the downstream side of the first fixing member and the second fixing member in the traveling direction of the recording material, contacts the back surface of the recording material, and records the recording material. It is good also as a structure further equipped with the static elimination member which neutralizes a material.

  In order to solve the above problems, an image forming apparatus according to the present invention includes the above-described fixing device according to the present invention.

  According to this aspect of the present invention, toner scattering can be suppressed when the recording material passes through the nip portion between the first fixing member and the second fixing member during the fixing process. It is possible to suppress the toner from adsorbing to the release layer of the first fixing member and adhering to the recording material. Therefore, image defects can be suppressed.

  According to the present invention, it is possible to suppress the occurrence of image defects in the fixing process.

1 is an explanatory diagram illustrating an image forming apparatus including a fixing device according to an exemplary embodiment of the present invention. 1 is a cross-sectional view illustrating a fixing device according to an embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a first fixing member, a second fixing member, a charging device, and the like in the fixing device according to the embodiment of the present invention. It is explanatory drawing which shows the discharge electrode of the charging device in the fixing device of embodiment of this invention. FIG. 3 is a block diagram illustrating a charging device and a discharge control circuit in the fixing device according to the embodiment of the present invention. 6 is a flowchart illustrating a discharge control process in the fixing device according to the embodiment of the present invention. 6 is a graph showing a change in surface potential of a release layer of a first fixing member when printing is started in the fixing device according to the embodiment of the present invention. 6 is a graph showing a change in surface potential of a release layer of a first fixing member when printing is started during a standby mode in the fixing device according to the embodiment of the present invention. It is explanatory drawing which shows the discharge roller, the static elimination brush, etc. in the fixing device of other embodiment of this invention.

(Image forming device)
FIG. 1 shows an image forming apparatus including a fixing device according to an embodiment of the present invention. In FIG. 1, an image forming apparatus 1 is a multifunction device capable of color printing, for example, and has a print function, a copy function, a factory function, and the like. The image forming apparatus 1 includes a housing 2. A paper feed cassette 3 is provided as a recording material storage unit at the lower portion of the housing 2, and a paper as a recording material is stored in the paper feed cassette 3. A paper discharge tray 4 is provided at the top of the housing 2, and the printed paper is discharged to the paper discharge tray 4.

  In the housing 2, a driving roller 5, a driven roller 6, and an intermediate transfer belt 7 are provided. The driving roller 5 is rotated by a driving source such as a motor, and the driven roller 6 is rotated following the driving roller 5. The intermediate transfer belt 7 is stretched between the driving roller 5 and the driven roller 6.

  A plurality of image forming units 8 are provided in the housing 2. These image forming units 8 form a color toner image on the intermediate transfer belt 7. Each image forming unit 8 includes a photosensitive drum 11, a charger 12, a developing device 13, and a primary transfer roller 14. The charger 12 charges the photosensitive drum 11. The developing device 13 attaches toner to the electrostatic latent image formed by irradiating the charged photosensitive drum 11 with laser light, and forms a toner image on the photosensitive drum 11. The primary transfer roller 14 transfers the toner image formed on the photosensitive drum 11 to the intermediate transfer belt 7. An exposure unit 15 is provided in the housing 2. In each image forming unit 8, the exposure unit 15 irradiates the photosensitive drum 11 charged by the charger 12 with laser light to form an electrostatic latent image. Further, a toner container 16 that contains toner is provided in the housing 2.

  In the housing 2, a transport path 20 is provided for transporting paper from the paper feed cassette 3 to the paper discharge tray 4. Then, along the conveyance path 20 from the upstream side toward the downstream side, the paper feed roller 21, the conveyance roller pair 22, the registration roller pair 23, the secondary transfer roller 24 as a transfer unit, and the embodiment of the present invention. A fixing device 25, a paper discharge roller pair 26, and the like are provided. The paper feed roller 21 extracts paper one by one from the paper feed cassette 3. The transport roller pair 22 transports the paper extracted from the paper feed cassette 3. The registration roller pair 23 adjusts the timing at which the sheet conveyed from the sheet feeding cassette 3 is sent to the secondary transfer roller 24. The secondary transfer roller 24 transfers the toner image transferred to the intermediate transfer belt 7 onto the surface of the paper sent out by the registration roller pair 23. The fixing device 25 performs a fixing process for fixing the toner image transferred on the surface of the paper to the paper. The paper discharge roller pair 26 discharges the paper on which the toner image is fixed to the paper discharge tray 4.

  An operation unit 31 for operating the image forming apparatus 1 is provided on the upper side of the housing 2, and a start button 32 is provided on the operation unit 31. Further, an electric circuit board 33 is provided in the housing 2, and the electric circuit board 33 has a main power supply unit 34 that is a main power supply of the image forming apparatus 1 and device control for controlling the operation of the image forming apparatus 1. A portion 35 and the like are provided. Further, the image forming apparatus 1 is provided with a grounding path 36 for grounding electrical components and electrical circuits provided in the image forming apparatus 1.

  The apparatus control unit 35 issues a print start instruction when the start button 32 of the operation unit 31 is pressed, when a facsimile is received, or when a print command is received from an external personal computer. The apparatus control unit 35 sets or switches the operation mode of the image forming apparatus 1. The operation mode includes, for example, a standby mode and a sleep mode. The standby mode is a mode in which an operation state or a state where power is supplied is maintained for a part of the configuration of the image forming apparatus 1 so that the printing operation can be started quickly. The sleep mode is a configuration that accepts an operation input or a configuration that communicates with an external device in order to reduce power consumption when a state in which no print operation is performed continues for a long time, except for a few predetermined configurations, In this mode, power supply to each component of the image forming apparatus 1 is cut off.

(Fixing device)
FIG. 2 shows the fixing device 25 according to the embodiment of the present invention. FIG. 3 shows the first fixing member 41, the second fixing member 42, the charging device 47, and the like in the fixing device 25. FIG. 4 shows the discharge electrode 48 in the charging device 47.

  In FIG. 2, the fixing device 25 includes a casing 40, a first fixing member 41, a second fixing member 42, a charging device 47, a Zener substrate 51, and a pair of discharge rollers 53 and 54. Although not shown, the fixing device 25 is provided with a drive mechanism for rotating the second fixing member 42. Further, the fixing device 25 is provided with a heater as a heat source for heating the first fixing member 41. The casing 40 is provided with an inlet 40A and an outlet 40B. The sheet conveyed through between the intermediate transfer belt 7 and the secondary transfer roller 24 enters the fixing device 25 from the introduction port 40A. Further, the sheet that has undergone the fixing process in the fixing device 25 is discharged from the discharge port 40B to the discharge tray 4 side.

  The first fixing member 41 is a member that heats the surface of the paper. As shown in FIG. 3, the first fixing member 41 is formed by laminating a base material layer 41A, an elastic layer 41B, and a release layer 41C from the inside to the outside. The first fixing member 41 is an endless belt formed by rolling such a film-like multilayer body into a cylindrical shape. The base material layer 41A is formed of, for example, a metal such as stainless steel or nickel or a resin having heat resistance. The elastic layer 41B is formed of an elastic material such as silicon rubber. The release layer 41 </ b> C is formed of a fluororesin excellent in release properties such as PTFE (polytetrafluoroethylene) and PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer). The diameter of the first fixing member 41 and the thickness of each layer are not limited. For example, the diameter of the first fixing member 41 is about 30 mm, and the thickness of the base material layer 41A is about 50 μm. The elastic layer 41B has a thickness of about 200 μm, and the release layer 41C has a thickness of about 20 μm. Further, the length of the first fixing member 41 is set to a value equal to or greater than, for example, the maximum sheet width dimension that can be handled by the image forming apparatus 1. A support member 43 and a pressing member 44 are provided on the inner peripheral side of the first fixing member 41. The support member 43 is a member that supports the pressing member 44 and the like, and the pressing member 44 is a member that presses a part of the first fixing member 41 toward the second fixing member 42.

  The second fixing member 42 is a member that pressurizes the sheet from its back surface. The second fixing member 42 is a roller formed by providing an elastic layer 42B on the outer peripheral side of the cylindrical core member 42A and providing a release layer 42C on the outer peripheral side of the elastic layer 42B. The core material 42A is made of a metal such as aluminum or iron. The elastic layer 42B is made of an elastic material such as silicon rubber. The release layer 42C is formed of a fluororesin having excellent release properties such as PTFE and PFA. The diameter of the second fixing member 42 and the thickness of each layer are not limited. For example, the diameter of the second fixing member 42 is about 30 mm, and the thickness of the core member 42A is about 2 mm. The thickness of the layer 42B is about 5 mm, and the thickness of the release layer 42C is about 20 μm. The length of the second fixing member 42 is the same as the length of the first fixing member 41, for example.

  Further, the surface resistance of the second fixing member 42 is set to a value smaller than the surface resistance or volume resistance of the first fixing member 41 by adding carbon or the like to the release layer 42C, for example. The surface of the second fixing member 42 is grounded.

  The second fixing member 42 is pressed against the first fixing member 41 by, for example, urging both end portions of the core member 42A toward the first fixing member 41 with a spring or the like. Thereby, as shown in FIG. 3, a nip portion 45 is formed between the first fixing member 41 and the second fixing member 42. The second fixing member 42 is rotationally driven by a driving mechanism (not shown) provided in the fixing device 25. The drive mechanism includes a motor and gears. Since the second fixing member 42 is pressed against the first fixing member 41, when the second fixing member 42 rotates, the first fixing member 41 also rotates. The sheet is heated and pressurized as it passes through the nip 45 between the rotating first fixing member 41 and the second fixing member 42. As a result, the toner image on the surface of the paper is fixed on the paper.

  The charging device 47 is a device that charges the release layer 41C of the first fixing member 41 to the same polarity as the polarity of the toner of the toner image formed on the surface of the paper by corona discharge. In this embodiment, the toner of the toner image is positively charged. Accordingly, the charging device 47 positively charges the release layer 41C of the first fixing member 41.

  The charging device 47 includes a discharge electrode 48 and a pair of ground electrodes 49. As shown in FIG. 4, the discharge electrode 48 is an elongated metal plate, and a plurality of needle-like electrode pieces 48A are provided on one edge thereof. Each ground electrode 49 is an elongated metal plate. The lengths of the discharge electrode 48 and each ground electrode 49 are set to be substantially the same as the length of the first fixing member 41, for example. Further, as shown in FIG. 3, the discharge electrode 48 is disposed between the pair of ground electrodes 49 in a state of being separated from the pair of ground electrodes 49. The discharge electrode 48 and the pair of ground electrodes 49 are arranged in parallel. The discharge electrode 48 is supported between the pair of ground electrodes 49 via the insulating member 50. By configuring the charging device 47 in this way, the release layer 41C of the first fixing member 41 can be easily and reliably charged by corona discharge.

  Further, the charging device 47 is arranged so that the extension direction of the discharge electrode 48 coincides with the direction of the rotation axis of the first fixing member 41, and the discharge electrode 48 and the first fixing member 41 are parallel to each other. ing. Further, the charging device 47 is arranged such that the discharge electrode 48 and each ground electrode 49 are not in contact with the first fixing member 41. Specifically, the charging device 47 is disposed in the vicinity of the first fixing member 41 so that the tip of each electrode piece 48A of the discharge electrode 48 approaches the outer peripheral surface of the release layer 41C of the first fixing member 41. Has been placed. The charging device 47 is insulated and supported by a casing 40 of the fixing device 25 via a support member (not shown).

  Further, the tip of the discharge electrode 48, that is, the tip of each electrode piece 48 </ b> A does not protrude toward the release layer 41 </ b> C of the first fixing member 41 than the tip of each ground electrode 49. Specifically, the position P1 of the tip of each electrode piece 48A is located farther from the release layer 41C of the first fixing member 41 than the position P2 of the surface where the tips of both ground electrodes 49 are in contact. Thereby, discharge can be stabilized. The tip of each electrode piece 48A may be extended to a position P2 on the surface where the tips of both ground electrodes 49 are in contact.

  Each ground electrode 49 is connected to a ground path 36 of the image forming apparatus 1 through a Zener substrate 51 as shown in FIG. The Zener substrate 51 is attached to the casing 40 of the fixing device 25, for example. Each ground electrode 49 is connected to the ground path 36 of the image forming apparatus 1 through a circuit formed on the Zener substrate 51. The circuit formed on the Zener substrate 51 has a Zener diode 52. The Zener substrate 51 has a function of controlling the potential of the release layer 41C when discharging is performed between the charging device 47 and the release layer 41C of the first fixing member 41. The Zener potential of the Zener substrate 51 is selected so that the potential of the release layer 41C can sufficiently suppress the toner on the paper from being attracted to the release layer 41C when the discharge is performed. ing.

  On the other hand, the pair of discharge rollers 53 and 54 in the fixing device 25 is disposed in the vicinity of the discharge port 40 </ b> B of the casing 40. The sheet that has passed through the nip portion 45 between the first fixing member 41 and the second fixing member 42 is discharged between the pair of discharge rollers 53 and 54 to the outside of the casing 40, and the conveyance path 20. Are guided to advance toward the paper discharge tray 4 side.

(Control of discharge)
FIG. 5 shows the charging device 47 and the discharge control circuit 55. In FIG. 5, a discharge control circuit 55 is a circuit that performs a discharge control process for controlling discharge by the charging device 47. The discharge control circuit 55 is formed on an electric circuit board (not shown) provided in the fixing device 25. Note that the discharge control circuit 55 may be formed on the electric circuit board 33 of the image forming apparatus 1. In addition, the discharge control unit 58 in the discharge control circuit 55 is realized by causing the arithmetic processing unit included in the device control unit 35 that controls the operation of the image forming apparatus 1 to read and execute a computer program for performing the discharge control process. May be.

  The discharge control circuit 55 includes a power source 56, a switching unit 57, and a discharge control unit 58. The power source 56 is a DC constant current power source, and generates a current to be supplied to the discharge electrode 48 of the charging device 47.

  The switching unit 57 is a circuit that sets the value of the current supplied from the power source 56 to the discharge electrode 48 of the charging device 47 to any one of the first set value, the second set value, and the third set value. . The second set value is that during the fixing process, the potential of the release layer 41C of the first fixing member 41 is pulled, and the toner on the paper is attracted to the release layer 41C by the discharge of the charging device 47. Is a current value that can be maintained at a potential that can be sufficiently suppressed. The first set value is a current value larger than the second set value. The third set value is a current value smaller than the second set value.

  The discharge control unit 58 is a circuit that controls the power source 56 and the switching unit 57 of the charging device 47, and includes an arithmetic processing unit and the like. The discharge controller 58 switches the power source 56 on and off in accordance with the start of printing, the end of printing, the mode of the image forming apparatus 1, and the like. Further, the discharge control unit 58 controls the switching unit 57 according to the start of printing, the end of printing, the mode of the image forming apparatus 1, the paper feed status, and the like, and the current supplied to the discharge electrode 48 of the charging device 47 Switch values.

  In the present embodiment, as an example of a configuration in which the value of the current supplied to the discharge electrode 48 is set to be switchable, the case where the current output from the constant current power supply 56 is changed by the switching unit 57 is exemplified. Instead of this, a variable constant current power source may be used.

  FIG. 6 shows an example of discharge control processing by the discharge control circuit 55. The discharge control process shown in FIG. 6 is a discharge control process in a case where printing is performed immediately after the image forming apparatus 1 returns from the sleep mode, the mode is then shifted to the standby mode, and then the mode is shifted to the sleep mode.

  In FIG. 6, it is assumed that the image forming apparatus 1 returns from the sleep mode (step S1), and immediately after that, a print start instruction is issued (step S2). Each image forming unit 8 starts operating in response to a print start instruction. As a result, a toner image is formed on the intermediate transfer belt 7. In response to the print start instruction, the motor or the like starts driving in the fixing device 25, and the second fixing member 42 and the first fixing member 41 start rotating upon receiving this power. In the fixing device 25, the heater starts operating, and the first fixing member 41 is heated.

  Further, in response to the print start instruction, the image forming apparatus 1 starts primary paper feeding. The primary paper feed is a process of transporting paper from the paper feed cassette 3 to the registration roller pair 23. That is, the paper feed roller 21 and the transport roller pair 22 start rotating in response to a print start instruction. Thus, one of the sheets stored in the sheet feeding cassette 3 is extracted by the sheet feeding roller 21, and the extracted sheet is conveyed toward the registration roller pair 23 by the conveying roller pair 22.

  In response to the print start instruction, the discharge controller 58 turns on the power supply 56 of the charging device 47 and further controls the switching unit 57 to set the value of the current supplied to the discharge electrode 48 of the charging device 47 to the first value. Set to the set value. As a result, discharge by the current having the first set value is started between the charging device 47 and the release layer 41C of the first fixing member 41 (step S3).

  Subsequently, the discharge control unit 58 determines whether or not secondary paper feeding has been started (step S4). The secondary paper feeding is a process for transporting paper from the registration roller pair 23 to the secondary transfer roller 24. In the secondary paper feeding, the toner image formed on the intermediate transfer belt 7 is transferred to the correct position on the paper when the paper passes between the intermediate transfer belt 7 and the secondary transfer roller 24. In addition, the timing at which the sheet is fed from the registration roller pair 23 to the secondary transfer roller 24 is adjusted. For example, in the conveyance path 20, a sensor (for example, an optical sensor) (not shown) that detects passage of paper is provided between the registration roller pair 23 and the secondary transfer roller 24. The discharge controller 58 determines that the secondary paper feeding has started when the sensor detects the passage of the front edge of the paper in the traveling direction.

  When the discharge control unit 58 determines that secondary paper feeding has started (step S4: YES), the discharge control unit 58 controls the switching unit 57 to set the value of the current supplied to the discharge electrode 48 of the charging device 47. Switch from the first set value to the second set value. As a result, discharge by the current of the second set value is started between the charging device 47 and the release layer 41C of the first fixing member 41 (step S5).

  Subsequently, the discharge control unit 58 determines whether or not printing is completed (step S6). For example, the apparatus control unit 35 detects that a sheet has been discharged to the discharge tray 4 via a sensor (for example, an optical sensor), and counts the number of sheets on which printing has been performed. When the instructed number of sheets are printed and these sheets are discharged to the discharge tray 4, the apparatus control unit 35 issues a print end notification. The discharge controller 58 determines that the printing has been completed based on the print completion notification.

  When printing is completed, the image forming apparatus 1 shifts to the standby mode (step S7). During the standby mode, the rotation of the first fixing member 41 and the second fixing member 42 of the fixing device 25 and the heating of the first fixing member 41 are continued. However, the amount of heat for heating the first fixing member 41 can be kept low compared to when the printing operation is performed.

  When the discharge control unit 58 determines that printing is completed (step S6: YES), the discharge control unit 58 controls the switching unit 57 to set the current value supplied to the discharge electrode 48 of the charging device 47 to the first value. Switch from the setting value of 2 to the third setting value. As a result, the discharge by the current of the third set value is started between the charging device 47 and the release layer 41C of the first fixing member 41 (step S8).

  Subsequently, the discharge controller 58 determines whether or not a print start instruction is issued during the standby mode (step S9). When a print start instruction is issued during the standby mode (step S9: YES), the process proceeds to step S5. In this case, the discharge is performed by the current of the second set value from the time when the print start instruction is issued until the end of printing.

  On the other hand, when a predetermined time has elapsed without issuing a print start instruction during the standby mode, the image forming apparatus 1 shifts to the sleep mode. In the sleep mode, in the fixing device 25, the rotation of the first fixing member 41 and the second fixing member 42 is stopped, and the heating of the first fixing member 41 is also stopped.

  When the image forming apparatus 1 shifts to the sleep mode (step S10: YES), the discharge control unit 58 turns off the power source 56 of the charging device 47. Thereby, the discharge between the charging device 47 and the release layer 41C of the first fixing member 41 is stopped (step S11).

  As described above, in the fixing device 25, for example, when a print start instruction is issued immediately after returning from the sleep mode, the second setting value that is larger than the second set value is started from the time point until the secondary sheet feeding is started. A discharge between the charging device 47 and the release layer 41 </ b> C of the first fixing member 41 is performed by a current having a set value of 1. Thus, the potential of the release layer 41C of the first fixing member 41 is quickly increased to a potential that can sufficiently suppress the toner on the sheet from being attracted to the release layer 41C in a short time. Can do. Specifically, in response to a print start instruction, the paper extracted from the paper feed cassette 3 by the paper feed roller 21 passes through the conveyance path 20 and the first fixing member 41 and the second fixing member of the fixing device 25. Before reaching the nip portion 45 with the member 42, the potential of the release layer 41C of the first fixing member 41 is sufficiently suppressed from attracting the toner on the sheet to the release layer 41C. It can be increased to a possible potential. Thereby, it is possible to suppress the toner on the paper from being scattered due to frictional charging of the release layer 41 </ b> C of the first fixing member 41 when the paper passes through the nip portion 45. Further, it is possible to suppress the toner from adsorbing to the outer peripheral surface of the release layer 41 </ b> C due to the frictional charging and adhering the adsorbed toner to the sheet passing through the nip portion 45. Therefore, image defects can be suppressed.

  Here, FIG. 7 shows a case where a print start instruction is issued immediately after returning from the sleep mode, and in response, discharge is started between the charging device 47 and the release layer 41C of the first fixing member 41. The change of the surface potential of the mold release layer 41C is shown. In the graph of FIG. 7, the solid line indicates the change in the surface potential of the release layer 41 </ b> C when the discharge is started with the current of the first set value when a print start instruction is issued (time point ta). Show. The alternate long and two short dashes line indicates a change in the surface potential of the release layer 41 </ b> C when a discharge start instruction is issued (time point ta) and discharge is started by the current of the second set value accordingly. Va is a potential capable of sufficiently suppressing the toner on the paper from being attracted to the release layer 41C. As can be seen from the graph of FIG. 7, the release layer 41 </ b> C is started after a print start instruction is issued when the discharge is started by the current of the first set value rather than the discharge is started by the current of the second set value. The time until the potential reaches Va is short.

  Further, when the fixing device 25 shifts to the standby mode, the release layer 41 </ b> C between the charging device 47 and the first fixing member 41 is generated during the standby mode by a current having a third set value smaller than the second set value. Is discharged between the two. Thereby, during the standby mode, the potential of the release layer 41C of the first fixing member 41 is lower than the potential that can sufficiently suppress the toner on the sheet from being attracted to the release layer 41C (zero). Can be held at a predetermined potential. Therefore, when a print start instruction is issued during the standby mode, the set value is switched from the third set value to the second set value and the discharge is continued, so that the paper feed cassette 3 receives the print start instruction. Before the conveyed paper reaches the nip portion 45, the potential of the release layer 41C of the first fixing member 41 can be sufficiently suppressed from attracting the toner on the paper to the release layer 41C. The potential can be quickly increased to a proper potential. Thereby, it is possible to suppress the image defect due to the scattering of the toner described above.

  Here, FIG. 8 shows a change in the surface potential of the release layer 41 </ b> C of the first fixing member 41 when a print start instruction is issued during the standby mode of the image forming apparatus 1. In the graph of FIG. 8, the solid line is discharged by the current of the third set value during the standby mode, and when the print start instruction is issued during the standby mode (time point tb), the set value is changed accordingly. 3 shows a change in the surface potential of the release layer 41C when the discharge is continued by switching from the set value 3 to the second set value. The alternate long and two short dashes line indicates that the discharge is not performed during the standby mode, and when a print start instruction is issued during the standby mode (time tb), the discharge when the discharge is started with the current of the second set value accordingly. The change in the surface potential of the mold layer 41C is shown. Va indicates a potential that can sufficiently suppress the toner on the paper from being attracted to the release layer 41C. Vb represents a potential formed in the release layer 41C by discharging with the current of the third set value during the standby mode. As can be seen from the graph of FIG. 8, the release layer 41 </ b> C after the print start instruction is issued is more when the discharge is performed with the current of the third set value during the standby mode than when the discharge is not performed during the standby mode. The time until the potential reaches Va is short.

  In the fixing device 25, the surface resistance of the second fixing member 42 is set to a value smaller than the surface resistance or volume resistance of the first fixing member 41, and the surface of the second fixing member 42 is grounded. ing. Thereby, when the release layer 41C of the first fixing member 41 is charged by the charging device 47, the electric field in the direction in which the toner of the toner image on the paper is pressed against the surface of the paper is changed between the first fixing member 41 and the second fixing member 41. It is formed between the fixing member 42. Therefore, the effect of suppressing the scattering of toner on the paper when the paper passes through the nip portion 45 can be enhanced.

In the fixing device 25, the surface resistance of the second fixing member 42 is set to a value smaller than the surface resistance or volume resistance of the first fixing member 41, and the surface of the second fixing member 42 is grounded. Instead, the following configuration can be employed.
(A) The volume resistance of the second fixing member 42 is set to a value smaller than the surface resistance or volume resistance of the first fixing member 41. Then, the core material 42A of the second fixing member 42 is grounded. The volume resistance of the second fixing member 42 is the resistance between the core material 42A and the outer peripheral surface of the release layer 42C. The volume resistance of the first fixing member 41 is a resistance between the base material layer 41A and the outer peripheral surface of the release layer 41C. The volume resistance of the second fixing member 42 can be reduced by adding carbon or the like to the release layer 42C and adding carbon or metal flakes or the like to the elastic layer 42B. Further, the volume resistance value of the second fixing member 42 can be appropriately set by adjusting the amount of carbon or metal flake to be added based on the thickness of the release layer 42C and the elastic layer 42B. .
(B) Of the pair of discharge rollers 53 and 54, the volume resistance of the discharge roller 54 that contacts the back surface of the paper is set to a value smaller than the surface resistance or volume resistance of the first fixing member 41. Then, the metal core 54A of the discharge roller 54 is grounded. The volume resistance of the discharge roller 54 is a resistance between the core material 54 </ b> A of the discharge roller 54 and the outermost peripheral surface of the discharge roller 54. Further, the volume resistance of the discharge roller 54 can be changed by adding carbon or metal flakes or the like to, for example, silicon rubber forming the elastic layer 54B provided on the outer peripheral side of the core material 54A of the discharge roller 54. it can.
(C) A neutralizing member that contacts the back surface of the sheet and neutralizes the sheet is added downstream of the nip portion 45 between the first fixing member 41 and the second fixing member 42 in the sheet traveling direction. .

  The above (c) will be further described. For example, as shown in FIG. 9, in the fixing device 25, a neutralizing brush 61 as a neutralizing member is provided on the downstream side of the discharge roller 54 that contacts the back surface of the sheet. The tip of the static elimination brush 61 contacts the back surface of the paper that passes between the pair of discharge rollers 53 and 54 and is discharged from the discharge port to the outside of the fixing device 25. The static elimination brush 61 is grounded.

  In any of these configurations, when the release layer 41C of the first fixing member 41 is charged by the charging device 47, the electric field in the direction in which the toner is pressed against the sheet is changed to the first fixing member 41 and the second fixing member. It can be formed between the member 42 and the effect of suppressing the scattering of toner on the paper can be enhanced.

  In the above-described embodiment, the charging device 47 and the first fixing device 25 are separated by the current of the first set value within the period from when the print start instruction is issued until the secondary paper feeding is started. The case where discharge is performed between the mold layers 41C is taken as an example. However, the discharge period with the current of the first set value is from when the print start instruction is issued until the sheet reaches the nip portion 45 between the first fixing member 41 and the second fixing member 42. Any period other than the period from when the print start instruction is issued until the secondary paper feeding is started may be used. .

  Further, in the above-described embodiment, when a print start instruction is issued during the standby mode, the charging device 47 is charged with the current of the second set value from the time when the print start instruction is issued until the printing is completed. However, instead of this, when a print start instruction is issued during the standby mode, the discharge is started from the time when the print start instruction is issued. The discharge is performed with the current of the first set value until the secondary paper feed is started, and the discharge is performed with the current of the second set value between the start of the secondary paper feed and the end of printing. May be performed.

  In the above-described embodiment, the case where the first fixing member 41 is an endless belt has been described as an example. However, the first fixing member 41 may be a roller.

  In the above-described embodiment, the fixing device 25 is applied to the image forming apparatus 1 having a tandem structure capable of color printing. However, the present invention is not limited thereto, and the fixing device 25 may be replaced with another method. It can also be applied to the image forming apparatus. Further, the fixing device 25 can be applied to an image forming apparatus dedicated to monochrome printing. Further, the image forming apparatus is not limited to a multifunction machine, and may be a printer, a copier, a facsimile machine, or the like.

  The present invention can be changed as appropriate without departing from the spirit or concept of the invention which can be read from the claims and the entire specification, and a fixing device and an image forming apparatus accompanying such a change are also included in the present invention. It is included in the technical idea.

1 Image forming device 3 Paper feed cassette
20 Conveyance path 21 Paper feed roller 22 Conveyance roller pair 23 Registration roller pair 24 Secondary transfer roller (transfer part)
25 Fixing Device 26 Paper Discharge Roller Pair 33 Electric Circuit Board 35 Device Control Unit 36 Ground Path 41 First Fixing Member 41A Base Material Layer 41B Elastic Layer 41C Release Layer 42 Second Fixing Member 42A Core Material 42B Elastic Layer 42C Release Mold layer 45 Nip part 47 Charging device 48 Discharge electrode 48A Electrode piece 49 Ground electrode 50 Insulating member 51 Zener substrate 53, 54 Discharge roller 55 Discharge control circuit (control part)
56 Power source 57 Switching unit 58 Discharge control unit 61 Static elimination brush (static elimination member)

Claims (10)

A fixing device provided in an image forming apparatus and fixing a toner image formed on a surface of a recording material on the recording material,
A first fixing member which is provided with a release layer on the outermost peripheral side and heats the surface of the recording material;
A second fixing member that forms a nip portion with the first fixing member and pressurizes the recording material from the back surface;
A charging device that is in non-contact with the release layer of the first fixing member and discharges the release layer;
A control unit for controlling the charging device,
The controller supplies a current of a first set value to the charging device within a predetermined period included between the time when a print start instruction is issued and the time when the recording material reaches the nip portion. To discharge between the charging device and the release layer of the first fixing member, and after the predetermined period, a current having a second set value smaller than the first set value is supplied to the charging device. A fixing device that discharges between the charging device and the release layer of the first fixing member by supplying.   The control unit starts primary paper feeding to convey the recording material from the recording material storage unit to the registration roller pair from the time when the print start instruction is issued, and further transfers the recording material from the registration roller pair to the transfer unit. During a predetermined period included between the time when the secondary paper feeding to be started is started, the current of the first set value is supplied to the charging device, whereby the charging device and the first fixing member are supplied. Discharge between the charging layer and the release layer of the first fixing member by supplying a current of the second set value to the charging device after the predetermined period has elapsed. The fixing device according to claim 1, wherein the fixing device is discharged between the two.   The control unit supplies the charging device with a current having a third set value smaller than the second set value from the time when printing is finished and the image forming apparatus shifts to the standby mode. The fixing device according to claim 1, wherein a discharge is caused between the device and a release layer of the first fixing member. The charging device includes a needle-like discharge electrode and a pair of ground electrodes,
The discharge electrode is disposed between the pair of ground electrodes in a state of being separated from the pair of ground electrodes, respectively.
4. The fixing device according to claim 1, wherein a tip of the discharge electrode does not protrude toward a release layer of the first fixing member than a tip of each ground electrode.   The fixing device according to claim 4, wherein each ground electrode of the charging device is connected to a ground path of the image forming apparatus via a Zener substrate.   The surface resistance of the second fixing member is set to a value smaller than the surface resistance or volume resistance of the first fixing member, and the surface of the second fixing member is grounded. The fixing device according to claim 1.   6. The fixing device according to claim 1, wherein the volume resistance of the second fixing member is set to a value smaller than the surface resistance or volume resistance of the first fixing member. . The recording material is provided on the downstream side of the first fixing member and the second fixing member in the traveling direction of the recording material, and travels between the first fixing member and the second fixing member. It further comprises a pair of discharge rollers for discharging the recording material from the fixing device,
The volume resistance of the discharge roller contacting the back surface of the recording material among the pair of discharge rollers is set to a value smaller than the surface resistance or volume resistance of the first fixing member. Item 6. The fixing device according to any one of Items 1 to 5.   A discharging member is further provided on the downstream side of the first fixing member and the second fixing member in the advancing direction of the recording material, and is in contact with the back surface of the recording material to discharge the recording material. The fixing device according to claim 1, wherein   An image forming apparatus comprising the fixing device according to claim 1.

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