JP2018010230A - Image forming apparatus and method for controlling image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a failure in fixing in an image forming apparatus including a fixing device that performs variable control of fixing pressure.SOLUTION: An image forming apparatus comprises: a fixing device 20 that conveys a recording medium P carrying a toner image T to a nip part formed between a fixing roller 21 and a pressure roller 22 to fix the toner image T to the recording medium P; fixing pressure control means that controls fixing pressure at the nip part of the fixing device 20; and density adjustment means that adjusts an image density of an image to be formed according to the fixing pressure of the fixing device 20.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an image forming apparatus and an image forming apparatus control method.

  Various image forming apparatuses using an electrophotographic system have been devised as image forming apparatuses such as copying machines, facsimiles, and printers, and are well-known techniques. In the image forming process, an electrostatic latent image is formed on the surface of a photoconductor as an image carrier, and the electrostatic latent image on the photoconductor is developed with a toner or the like as a developer to be visualized and developed. This is established by a process in which the transferred image is transferred onto a recording medium (also referred to as paper) by a transfer device to carry the image, and an unfixed toner image on the recording medium is fixed by a fixing device using pressure, heat, or the like.

  Various types of fixing devices have been proposed. For example, the fixing device includes a fixing roller (fixing member) and a pressure roller in pressure contact with the fixing roller, and is formed by pressure contact between the pressure roller and the fixing roller. A roller fixing system is known in which a recording medium carrying an unfixed toner image is heated and fixed while being nipped and conveyed by a fixing nip portion (also referred to as a nip portion).

  Further, for example, a fixing roller disposed opposite to the pressure roller, and an endless fixing belt (fixing member) stretched between the fixing roller and the heating roller are provided. There is known a belt fixing system in which heat of a heating roller is applied to a recording medium through a fixing belt at a fixing nip portion formed by pressure contact so that an unfixed toner image is pressed and fixed on the recording medium.

  By the way, when printing thick paper such as thick paper and envelopes in the image forming apparatus, an excessive fixing pressure (also referred to as a fixing pressure at the nip portion or a pressure level) is applied to the fixing device. In some cases, curling or hot offset, which is one mode of fixing failure, may occur. Hot offset is a phenomenon that occurs when the toner layer is divided when the toner is overheated and the cohesive force of the toner falls below the adhesive force between the fixing roller and the paper.

  Conventionally, when printing thick paper, etc., printing was started by manually reducing the fixing pressure of the fixing device, etc., but wrinkles and fixing defects due to reduced work efficiency or forgetting to release it Has occurred.

  On the other hand, a technique for variably controlling the fixing pressure of the fixing device is known. For example, Patent Document 1 discloses an electrophotographic apparatus that includes a sensor that detects the image density of a document image and adjusts the fixing pressure of the fixing device based on an output signal of the sensor. Patent Document 2 discloses an image forming apparatus that adjusts a pressure contact maintaining time before depressurization based on image information in a fixing device that variably controls the fixing pressure.

  However, in a fixing device that controls the fixing pressure, the fixing performance is reduced by depressurization (decreasing the pressure level), and therefore, when the image density varies due to changes over time, environmental conditions, etc., one aspect of fixing failure There was a problem that a cold offset was generated. The cold offset is a phenomenon in which a part of the toner image is removed by the adhesive force or electrostatic attraction force with the fixing roller when the vicinity of the interface between the toner and the paper is not sufficiently melted. For this reason, it is desired that neither fixing failure of hot offset nor cold offset occurs.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that can suppress a fixing defect in an image forming apparatus including a fixing device that variably controls a fixing pressure.

  In order to achieve this object, an image forming apparatus according to the present invention transports a recording medium carrying an unfixed toner image to a nip formed between a fixing member and a pressure member, and A fixing unit that fixes an image on the recording medium; a fixing pressure control unit that controls a fixing pressure at the nip portion of the fixing unit; and an image density of an image to be formed according to the fixing pressure of the fixing unit. Density adjusting means for adjusting.

  According to the present invention, fixing failure can be suppressed in an image forming apparatus including a fixing device that variably controls the fixing pressure.

1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention. 1 is a schematic diagram of a main part of an image forming apparatus. FIG. 3 is a schematic top perspective view of FIG. 2. FIG. 2 is a schematic configuration diagram illustrating an example of a fixing device. FIG. 3 is a configuration diagram illustrating an example of a pressure depressurizing unit of a fixing device. It is an example of the development output bias table according to the paper type and the fixing pressure. It is an example of a dot thinning rate table according to the paper type and the fixing pressure. It is an example of a dot thinning-out rate table according to basis weight and fixing pressure. It is explanatory drawing of adjustment of the fixing pressure according to the amount of skews. It is explanatory drawing of the deviation adjustment in the longitudinal direction of the fixing pressure according to skew amount. 6 is a flowchart illustrating an example of processing executed by the image forming apparatus.

  Hereinafter, a configuration according to the present invention will be described in detail based on the embodiment shown in FIGS.

[First Embodiment]
In the image forming apparatus (image forming apparatus 1) according to the present embodiment, an unfixed toner image (toner image) is formed in a nip portion formed between a fixing member (fixing roller 21) and a pressure member (pressure roller 22). A fixing means (fixing device 20) for conveying the recording medium (recording medium P) carrying T) and fixing the unfixed toner image on the recording medium, and a fixing pressure for controlling the fixing pressure at the nip portion of the fixing means. The image forming apparatus includes control means (control unit 30, pressure depressurization means 60) and density adjustment means (control part 30) for adjusting the image density of the image to be formed according to the fixing pressure of the fixing means. In addition, the code | symbol in embodiment and the example of application are shown in a parenthesis.

(Image forming device)
An image forming apparatus according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic configuration diagram showing a monochrome electrophotographic apparatus which is an embodiment of an image forming apparatus.

  In the image forming apparatus 1, a process cartridge 2 is disposed near the center of the apparatus, and an exposure device 4 for forming a latent image on the photosensitive member 3 disposed on the photosensitive member 3 inside the process cartridge 2 is provided. It is arranged.

  Under the process cartridge 2, a transfer roller 5 is installed, and an unfixed toner formed on the surface of the photosensitive member 3 by applying a high potential to the transfer roller 5 to create a potential difference between the photosensitive member 3 and the transfer roller 5. An image (hereinafter also referred to as a toner image) is transferred to a recording medium.

  A sheet feeding tray 6 for loading and storing recording media is disposed at the lower part of the apparatus. The recording medium fed by the sheet feeding device 7 passes between the transfer roller 5 and the photoreceptor 3 and is fixed to the fixing device. Then, the toner image is thermally fixed to the recording medium. Then, the paper is discharged onto a paper discharge tray 9 on the upper surface of the image forming apparatus 1 by a paper discharge roller 8.

(Image forming operation)
Next, an image forming operation will be described with reference to FIGS. FIG. 2 is a schematic view of the main part of the image forming apparatus 1, and FIG. 3 is a schematic top perspective view of FIG.

  The photosensitive member 3 of the process cartridge 2 is rotationally driven in the counterclockwise direction in the drawing by the driving means, and the surface of the photosensitive member 3 is uniformly charged to a predetermined polarity by the charging roller 10. The surface of the charged photoconductor 3 is irradiated with laser light from the exposure device 4, whereby a desired electrostatic latent image is formed on the surface of the photoconductor 3. Black toner 13 is applied from the developing roller 12 to the electrostatic latent image formed in this way, and visualized as a toner image.

  Further, the transfer roller 5 is driven to run in the clockwise direction in the drawing, and the toner image T is transferred from the photoreceptor 3 when the recording medium P passes through the transfer roller 5. Residual toner adhering to the surface of the photoreceptor 3 after transferring the toner image T is removed from the surface of the photoreceptor 3 by the cleaning blade 15 to prepare for the next image formation.

  At this time, the recording medium P is fed from the paper feed tray 6 and is sent toward the transfer position by the registration roller pair 14 in timing with the toner image carried on the photosensitive member 3. A transfer voltage having a polarity opposite to the toner charging polarity of the toner image on the surface of the photoreceptor 3 is applied to the transfer roller 5, whereby the toner image on the surface of the photoreceptor 3 is transferred to the recording medium P. The recording medium P to which the toner image T has been transferred is guided to the fixing device 20 by the conveying member 11 and the like, and when passing through the fixing device 20, the toner image T is fixed by heat and pressure. The recording medium P that has passed through the fixing device 20 is discharged by a paper discharge roller 8 to a paper discharge tray 9 formed on the upper surface of the apparatus main body.

  When images are formed on both sides of the recording medium P, the recording medium P having the toner image T fixed on one side is switched back by reversing the paper discharge roller 8 and sent to the reversing unit 16 for recording. The recording medium P is fed again to the registration roller pair 14 with the front and back sides of the medium P reversed. A toner image is transferred from the photoreceptor 3 to the back surface of the re-feeded recording medium P, and the back image is fixed by the fixing device 20 and discharged to the paper discharge tray 9 to complete double-sided printing.

  In this embodiment, the case where the image forming apparatus 1 is a monochrome electrophotographic image forming apparatus is taken as an example, but it is needless to say that the image forming apparatus 1 may be a color electrophotographic image forming apparatus.

(Fixing device)
Next, the configuration of the fixing device 20 included in the image forming apparatus 1 will be described. FIG. 4 is a schematic configuration diagram illustrating an example of the fixing device 20. The fixing device 20 includes a fixing member (fixing roller 21) that heats and melts a toner image, and a pressure member (pressure roller 22) that is in contact with the outer peripheral surface of the fixing member and forms a nip portion with the fixing member. ).

  The fixing roller 21 is a thin cylindrical body that rotates in the direction of the arrow in FIG. 4, and a heater 25 as a heat source is fixed inside the cylindrical body. The fixing roller 21 is a multi-layer structure in which an elastic layer and a release layer (surface layer) are laminated in this order on a cylindrical cored bar, and a nip is formed by contacting the pressure roller 22. To do.

  The innermost cored bar portion of the fixing roller 21 is formed of an iron-based material such as SUS304, for example. A heater 25 is provided inside the cored bar portion of the fixing roller 21. As the heater 25, for example, a halogen heater can be used. Alternatively, the fixing roller 21 may be heated from the outer peripheral side of the fixing roller 21 using electromagnetic induction overheating means. In this embodiment, a halogen heater is provided inside the fixing roller 21, and the halogen heater is a rod-shaped heater extending in the axial direction of the fixing roller 21, and both end portions thereof are fixed to the side plate of the fixing device 20. Yes.

  The fixing roller 21 is heated by a heater 25 whose output is controlled by a power supply device included in the image forming apparatus 1, and heat is applied to the toner image T on the recording medium P from the surface thereof. Note that the output control of the heater 25 is performed based on a detection result by temperature detection means (thermistor, thermopile, etc.) provided so that the surface temperature of the fixing roller 21 can be detected. Further, the temperature of the fixing roller 21 (fixing temperature) can be controlled to a desired temperature by such output control of the heater 25.

  As the elastic layer of the fixing roller 21, for example, an elastic material such as fluorine rubber, silicone rubber, and foamable silicone rubber is used. In particular, when silicone rubber is used as the elastic layer, it is preferable to coat a fluorine layer on the elastic layer in order to improve the swelling resistance. Further, by providing an elastic layer on the core of the fixing roller 21, the separation property of the recording medium P after passing through the nip portion is improved and the glossiness of the output image when forming a color image is improved. .

  In addition, as the release layer of the fixing roller 21, for example, PFA (tetrafluoroethylene bar fluoroalkyl vinyl ether copolymer resin), polyimide, polyetherimide, PES (polyether sulfide), or the like can be used. By providing the release layer on the surface layer of the fixing roller 21, the releasability (peelability) with respect to the toner image T is ensured.

  The pressure roller 22 includes a cored bar portion, an elastic layer, and a release layer. For example, the pressure roller 22 may be configured similarly to the fixing roller 21. The pressure roller 22 is pressed against the fixing roller 21 by a pressure unit, and a nip portion is formed between the pressure roller 22 and the fixing roller 21.

  A guide plate 23 for guiding the conveyance of the recording medium P is disposed on the entrance side of the nip portion between the fixing roller 21 and the pressure roller 22. For example, the guide plate 23 may be fixed to the side plate of the fixing device 20. Further, a separation plate 24 is disposed at a position facing the outer peripheral surface of the fixing roller 21 and in the vicinity of the exit side of the nip portion. The separation plate 24 prevents a problem that the recording medium P after the fixing process is wound around the fixing roller 21 along with the rotation of the fixing roller 21.

  A control unit 30 (control means, controller) illustrated in FIG. 4 is a control unit of the image forming apparatus 1, and includes, for example, a CPU as a calculation unit, a ROM or RAM as a storage unit, and an I / I as an input / output unit. It is composed of a microcomputer including an O interface.

  The control unit 30 functions as a control unit of the fixing device 20 and controls each unit of the fixing device 20 such as driving control of each roller of the fixing device 20 and control of each heater based on the detection result of each temperature sensor. The control unit of the fixing device 20 and the control unit of the image forming apparatus 1 may be configured separately.

  In addition, the control unit 30 controls a pressurizing / depressurizing unit 60 described later. The pressure depressurizing unit 60 and the control unit 30 function as a fixing pressure control unit.

  Further, as will be described later, the control unit 30 executes image processing for adjusting the image density of an image to be formed according to the fixing pressure of the fixing device 20, the fixing pressure, the type of the recording medium P, the basis weight, and the like. It functions as a density adjusting means.

  In the present embodiment, the roller fixing type fixing device has been described as an example. However, the fixing device 20 to which the configuration of the present invention can be applied is not limited to this, and other configurations such as a belt fixing method. Of course, it may be a fixing device.

(Pressure and pressure release means)
The fixing device 20 variably controls the fixing pressure (adjusts the pressure level) to prevent problems caused by excessive fixing heat such as wrinkles, curls, and hot offsets when printing thick paper such as thick paper and envelopes. Pressure depressurizing means.

  FIG. 5 is a configuration diagram illustrating an example of the pressure / decompression unit 60 of the fixing device 20. The pressure depressurizing means 60 provided in the fixing device 20 rotates around a support shaft 63 a and can be brought into contact with and separated from the roller shaft 22 j of the pressure roller 22, and one of the pressure arms 63. An elastic member 54 that is a spring spring having both ends fixed to the end portion and the side plate of the fixing device 20; a contact / separation cam 65 that rotates by the driving force of the actuator and presses the other end portion of the pressure arm 63; The filler 64 is a disk-shaped encoder that rotates together with the contact / separation cam 65, and a photo sensor 66 that detects the rotational position of the filler 64.

  The elastic force f of the elastic member 54 is rotated in the direction away from the roller shaft 22j of the pressure roller 22 around the support shaft 63a (in the direction indicated by the dotted arrow (7) in the figure). Thus, the pressing force of the contact / separation cam 65 acts on one end of the pressure arm 63 and the pressure arm 63 resists the elastic force of the elastic member 54 in the direction opposite to the rotation direction. Then, it acts on the other end of the pressure arm 63 so as to rotate (so as to rotate in the arrow direction (3) of the solid line in the figure).

  Further, the degree of pressing against the pressure arm 63 changes depending on the cam phase of the contact / separation cam 65, and the pressurization and depressurization state of the nip portion N can be switched.

  Here, the pressure roller 22 moves the pressure roller 22 toward the fixing roller 21 and presses the pressure roller 22 (pressure operation, increasing the pressure level), and moves it away from the fixing roller 21. Detachment (depressurization operation, lowering the pressurization level) is performed as follows. This will be described with reference to FIG.

(Pressurizing operation)
First, electric power is supplied to the actuator to drive the actuator, and this driving force is transmitted to the drive shaft of the contact / separation cam 65, and the contact / separation cam 65 rotates in the direction indicated by the solid arrow in the figure ( When rotated by a certain rotation angle in 1), the contact / separation cam 65 presses the end of the pressure arm 63 in a specific direction (right direction in the figure) (solid arrow direction (2) in the figure). The pressing force of the contact / separation cam 65 at this time is a force that resists larger than the elastic force f of the elastic member 54.

  When the end of the pressurizing arm 63 is pushed, the pressurizing arm 63 rotates about the support shaft 63a as a center axis (indicated by the solid arrow (3) (counterclockwise (CCW) direction)). Then, the arm portion of the pressure arm 63 comes into contact with the roller shaft 22j of the pressure roller 22 and acts to push toward the fixing roller 21 (solid arrow direction (4) in the figure). As a result, the pressure roller 22 comes into contact with the fixing roller 21 and is brought into a pressure contact state with a constant pressure, thereby forming a nip portion N (solid arrow direction (5) in the figure).

  As described above, when the fixing device 20 is driven and a fixing operation is performed, the pressure depressurizing unit 60 presses the pressure roller 22 against the fixing roller 21 with a constant pressure. This pressurized state is maintained as long as power is supplied to the actuator.

(Decompression operation)
First, when the contact / separation cam 65 is rotated by a constant rotation angle in the arrow direction (6) of the dotted line in the figure so that the contact / separation cam 65 is returned from the angular position in the pressurized state by the driving force from the actuator, The cam 65 is released from pressing the end of the pressure arm 63. At the same time, the end of the pressure arm 63 can be moved in the direction indicated by the dotted arrow (8) (left direction in the figure).

  When the pressing of the pressing arm 63 by the contact / separation cam 65 is released, the pressing arm 63 rotates about the support shaft 63a as a center axis by the elastic force f of the elastic member 54 (indicated by the dotted arrow direction (7) ( Clockwise (CW) direction)). Then, the arm portion of the pressure arm 63 is released from pressing the roller shaft 22j of the pressure roller 22, and moves so as to be separated from the fixing roller 21 in the direction opposite to the fixing roller 21 (indicated by the dotted arrow (9) in the figure). ). As a result, the pressure roller 22 is released from pressing the fixing roller 21 and moves in a direction away from the fixing roller 21 to eliminate the nip portion N (indicated by the dotted arrow (10) in the figure).

  As described above, when the image forming apparatus 1 is in the off-sleep state, the pressing of the pressure roller 22 to the fixing roller 21 is released by the pressure depressurizing means 60 before that (completely depressurized state). ).

  Here, by reading the rotational position of the filler 64 with the photosensor 66, the phase (cam phase) of the contact / separation cam 65 can be detected, and the pressurization level and the depressurization state of the nip portion can be detected. Thus, based on the phase position of the contact / separation cam 65 read by the photosensor 66, the actuator for driving the cam device is switched between the pressurized state and the complete depressurized state, and the pressure level is different as the pressurized state. It is possible to stop at a desired position. For example, the complete depressurization state and the pressurization state can be switched by the rotation angle of the contact / separation cam 65, and the pressurization level in the pressurization state can be adjusted to a plurality of pressurization states.

  Since the necessary fixing pressure (nip pressure) differs for each paper thickness and paper type when passing paper, it has a plurality of pressure levels.

  The fixing device 20 includes a fixing pressure detecting unit that detects a fixing pressure. For example, the fixing pressure detecting means calculates the fixing pressure based on the reading result by the photo sensor 66. Further, for example, the fixing pressure may be detected based on detecting the rotational driving torque of the fixing roller, or a strain gauge as disclosed in a reference document (Japanese Patent Laid-Open No. 2003-50518) is used. It can also be detected.

  Further, the pressurizing / depressurizing means 60 may be configured to be capable of adjusting a plurality of pressurizing levels, and is not limited to the configuration shown in FIG. As the pressurizing / depressurizing means 60, other new or known structures can be applied.

(Control when changing the fixing pressure)
A control table as shown in FIGS. 6 to 8 is stored in the storage unit of the control unit 30. FIG. 6 is an example of a development output bias table showing the development output bias corresponding to the paper type (thin paper, plain paper, thick paper, envelope) of the recording medium P and the fixing pressure [N / mm ² ]. FIG. 7 is an example of a dot thinning rate table indicating the paper type of the recording medium P and the image dot thinning rate [%] corresponding to the fixing pressure. FIG. 8 is an example of a dot thinning rate table showing the basis weight [g / m ² ] of the recording medium P and the image dot thinning rate [%] according to the fixing pressure.

  The image forming apparatus 1 refers to FIGS. 6 to 8 based on the information (paper type, basis weight, etc.) of the recording medium P included in the print job, information on the set recording medium P, and the fixing pressure. Then, an image density (development output bias, image dot thinning rate, etc.) corresponding to this is set, and printing is executed under the set conditions. In this embodiment, an example in which the image density is adjusted based on the information on the recording medium P and the fixing pressure will be described. However, any image density may be used as long as the image density is adjusted based on at least the fixing pressure. .

  In the adjustment of the image density, for example, it is possible to print at a low image density by reducing the absolute value of the development output bias or by thinning out the image dots regardless of the solid image or the halftone image.

  When the depressurization amount is large (fixing pressure is small), the fixability of the fixing device 20 is deteriorated. Therefore, when printing is performed at the same image density as when no depressurization is performed, a cold offset may occur. . Therefore, it is possible to prevent cold offset by reducing the image density as in this embodiment. Also, the yield rate can be improved.

  On the other hand, when the amount of depressurization is small (fixing pressure is large), the fixability of the fixing device 20 is improved, but if the fixing pressure is too large, hot offset may occur. Therefore, in this embodiment, the image density is reduced to prevent hot offset under conditions of excessive fixing pressure.

  As described above, according to the image forming apparatus according to the present embodiment, when the fixing pressure is variably controlled by the fixing device 20, the image processing on the recording medium P is performed by performing image processing such as reducing the image density. By reducing the amount of toner and avoiding hot offset by depressurizing, and by avoiding cold offset by reducing the amount of toner, it is possible to suppress both hot offset and cold offset fixing defects. Can do.

  Note that each of the tables described above may be a table in which an optimum value is set in advance according to the model of the image forming apparatus 1, the configuration of the fixing device 20, and the like. In addition, the adjustment of the image density is not limited to the adjustment of the development output bias and the image dot thinning rate, and other conditions (exposure amount, etc.) may be used in combination with these or in accordance with or in place of them. You may make it adjust.

[Second Embodiment]
Hereinafter, other embodiments of the image forming apparatus according to the present invention will be described. In addition, the description about the same point as the said embodiment is abbreviate | omitted suitably.

  It is preferable that the image forming apparatus 1 further includes a toner adhesion amount detection unit that detects the toner adhesion amount on the photosensitive member 3. In addition to the control described in the first embodiment, it is possible to control to a desired image density (adhesion amount) with higher accuracy by adopting control in consideration of the detection result by the toner adhesion amount detection means. .

  As the toner adhesion amount detecting means on the photosensitive member 3, for example, an optical detection sensor such as a transmission type photo sensor can be used.

  Depending on the detection result of the toner adhesion amount detection means, for example, when the toner adhesion amount on the photoconductor 3 is large, the development output bias is offset to the + side so that an excessive amount of toner is not developed. Can do. Note that the larger the absolute value of the development output bias, the larger the amount of toner developed on the photoreceptor.

  According to the second embodiment, the image forming condition to be adjusted by depressurization is further optimized by taking into account the toner adhesion amount (toner adhesion amount on the photoconductor) before being transferred to the recording medium P. For example, it is possible to eliminate density defects such as extremely thin and too dark.

[Third Embodiment]
It is preferable that the image forming apparatus 1 further includes a skew detection unit that detects the skew of the recording medium P.

  As the skew detection means, for example, a first irradiation unit that is installed on one end side in the width direction of the recording medium P in the conveyance path of the recording medium P, and irradiates the recording medium P with light, and the recording medium P A second irradiating unit that irradiates the recording medium P with light, and a light receiving unit that receives light from the first and second irradiating units. A configuration for detecting the skew amount can be applied based on a comparison between the amount of light received from the first irradiation unit and the amount of light received from the second irradiation unit. Also, other new or known configurations can be applied.

  FIG. 9 is a graph showing an example of adjusting the fixing pressure according to the skew amount detected by the skew detecting means when the recording medium P is plain paper. When the skew amount of the recording medium P is large, fixing wrinkles may occur depending on the fixing conditions, but the fixing pressure is adjusted according to the skew amount as shown in FIG. 9 according to the detection result of the skew detecting means. Even when the skew amount is large, fixing wrinkles can be prevented by releasing the fixing pressure.

  According to the third embodiment, by further adjusting the fixing pressure in accordance with the skew amount, it is possible to suppress fixing failure when skew occurs.

[Fourth Embodiment]
Further, in the image forming apparatus 1, when the skew of the recording medium P is large, the conveyance becomes unstable, and contact with the conveyance rib or the like causes damage to the leading end of the recording medium P or a paper jam (jam). There is a risk.

  Therefore, the fixing device 20 preferably includes a skew correction unit (fixing pressure fluctuation unit) that corrects the skew of the recording medium P.

  The skew correction means, for example, can make a deviation in the fixing pressure in the longitudinal direction of the fixing device 20 by using independent contact / separation cams on both ends in the longitudinal direction of the fixing device 20. Note that the configuration is not particularly limited as long as it is possible to make a deviation in the fixing pressure in the longitudinal direction of the fixing device 20.

  Since the conveyance force increases on the side where the fixing pressure is high, for example, as shown in FIG. 10A, by adding a deviation of the fixing pressure in the direction in which the skew is generated, the recording medium P is fixed by the fixing device 20. The skew can be corrected. Note that, as shown in FIG. 10B, the skew amount + is a case where the sheet is skewed to the right with respect to the sheet conveyance direction.

  According to the fourth embodiment, the skew can be further corrected in the fixing device 20, the transportability of the recording medium P after the paper passing through the fixing device 20 can be stabilized, and paper damage such as ear folding can be prevented. .

[Fifth Embodiment]
FIG. 11 is a flowchart illustrating a control example when the processes described in the first to fourth embodiments are executed.

  When printing is started in the image forming apparatus 1 (S101), it is first determined based on the sheet information or the like whether or not the sheet is a thick sheet that needs to be depressurized (a pressure level is weakened) (S102). ). In the case of thick paper (S102: Yes), the pressure is released to a pressure level suitable for the thick paper under the control of the pressure depressurizing means 60 (S103).

  Next, the fixing pressure detecting means detects the fixing pressure (S104). Next, the density adjusting means refers to the control table (FIGS. 6 to 8) using the detected fixing pressure and paper information (type, basis weight, etc.) as keys, and image density (development output bias, image dot thinning rate). Etc.) is adjusted (S105).

  If the skew detection means detects that a skew has occurred (S106: Yes), the fixing pressure is adjusted, or the fixing pressure deviation in the longitudinal direction is adjusted by the skew correction means to correct it. (S107). The above processing is appropriately executed to complete the printing (S108).

  The above-described embodiment is a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Process cartridge 3 Photoconductor 4 Exposure apparatus 5 Transfer roller 6 Paper feed tray 7 Paper feed device 8 Paper discharge roller 9 Paper discharge tray 10 Charging roller 11 Conveying member 12 Developing roller 13 Black toner 14 Registration roller pair 15 Cleaning Blade 16 Reversing unit 20 Fixing device 21 Fixing roller 22 Pressure roller 22j Roller shaft 23 Guide plate 24 Separating plate 25 Heater 30 Control unit 54 Elastic member 60 Pressure depressurizing means 63 Pressure arm 63a Support shaft 64 Filler 65 Contact / separation cam 66 Photosensor P Recording medium T Toner image

JP-A-6-175533 JP 2012-88441 A

Claims (8)

Fixing means for conveying a recording medium carrying an unfixed toner image to a nip formed between a fixing member and a pressure member, and fixing the unfixed toner image on the recording medium;
Fixing pressure control means for controlling the fixing pressure at the nip portion of the fixing means;
An image forming apparatus comprising: a density adjusting unit that adjusts an image density of an image to be formed according to the fixing pressure of the fixing unit.   The image forming apparatus according to claim 1, wherein the density adjusting unit adjusts the image density according to the fixing pressure of the fixing unit and the type or basis weight of the recording medium.   The image forming apparatus according to claim 1, wherein the density adjusting unit adjusts the image density by a process of changing at least one of a development output bias and an image dot thinning rate.   The image forming apparatus according to claim 1, further comprising a fixing pressure detecting unit configured to detect the fixing pressure. A toner adhesion amount detecting means for detecting the toner adhesion amount on the photoreceptor;
5. The image forming apparatus according to claim 1, wherein the density adjusting unit adjusts the image density according to a detection result of the toner adhesion amount detecting unit. Comprising skew detecting means for detecting skew of the recording medium,
The image forming apparatus according to claim 1, wherein the fixing pressure control unit variably controls the fixing pressure based on a detection result of the skew detection unit. The fixing unit includes a fixing pressure changing unit that changes a fixing pressure in a longitudinal direction orthogonal to a conveyance direction of the recording medium,
7. The image according to claim 6, wherein the fixing pressure control means controls the fixing pressure fluctuation means based on a detection result of the skew detecting means to give a deviation to the fixing pressure in the longitudinal direction. Forming equipment. An image forming apparatus comprising a fixing unit that conveys a recording medium carrying an unfixed toner image to a nip formed between a fixing member and a pressure member and fixes the unfixed toner image on the recording medium. Control method,
Processing for controlling the fixing pressure at the nip portion of the fixing means;
And a process for adjusting an image density of an image to be formed in accordance with the fixing pressure of the fixing unit.