JP2010139817A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long-life fixing means that prevents paper from being wrapped around a fixing belt or fixing roller and is free from image quality deterioration. <P>SOLUTION: The fixing means includes at least a first roller, a second roller pressed against the first roller, and a pressing-force adjusting means for adjusting the pressing force of the second roller. The image forming apparatus includes: a stiffness detecting means for detecting the stiffness of paper; an image pattern recognizing means for recognizing a toner image transferred to paper; and a control means for controlling the pressing-force adjusting means based on signals from the stiffness detecting means and image pattern recognizing means. The pressing-force adjusting means varies the pressing force of the second roller by a signal from the control means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using a fixing unit that heat-fixes toner on a sheet.

  In Patent Document 1, the rubber hardness of the elastic layer of the fixing roller is made smaller than the rubber hardness of the elastic layer of the pressure roller, and the fixing roller is pushed by the pressure roller at the nip portion, and the nip portion exit of the fixing roller. A configuration is described in which the radius of curvature of the sheet is made small and the winding of the sheet is less likely to occur. In addition, when media separation material (peeling claw) is provided at a position away from the fixing roller and borderless printing is performed, medium type information such as the basis weight and thickness of the medium, the size of the medium, and the conveyance direction of the medium is obtained. Thus, a configuration is described in which it is determined whether or not borderless printing is possible based on this type information, and when borderless printing is determined to be possible, borderless printing is executed.

  In the fixing means disclosed in Patent Document 2, a peeling member is provided, and the tip of the peeling member is brought into contact with the belt member. The belt member corresponding to the fixing roller is brought into contact with the pressure member (roller) over which the belt member is suspended, thereby making the contact force of the peeling member weak and preventing the belt member from being damaged. The shape of the nip is such that the pressure member side is recessed to facilitate peeling.

JP 2007-78788 A JP 2004-191405 A

  In an electrophotographic image forming apparatus, an electrostatic latent image is formed on a photosensitive member by laser scanning, powder toner is adhered to the latent image by a developing unit, and the toner image is transferred to an intermediate transfer member by a transferring unit. The toner image is transferred from the intermediate transfer member to the sheet by the second transfer unit, and the toner is melted and fixed on the sheet by heat and pressure by the fixing unit.

  In the fixing unit, a nip portion that presses the backup roller against the heated fixing belt or the fixing roller to sandwich the paper is formed, and heat and pressure are applied to the toner on the paper while the paper is conveyed. Here, when the paper is thin and has low rigidity, the paper may be wound around the fixing belt or the fixing roller by the adhesion force of the melted toner.

  Therefore, in Patent Documents 1 and 2, the roller on the fixing belt side is deformed to make the nip concave, the sheet is discharged in the direction away from the fixing belt at the exit of the nip, and the radius of curvature at the exit of the nip is reduced. Thus, a configuration for preventing the winding of the paper has been proposed.

  When the pressing force of the backup roller is increased in such a configuration, the paper discharge direction is further away from the fixing belt, the radius of curvature of the nip outlet is smaller, and the paper is more difficult to wind. However, when the pressing force is increased, the strain of the elastic layer of the roller is increased, the life of the elastic layer is shortened, and the life of the fixing unit is also shortened.

  On the other hand, use the peeling claw so that the image is not formed up to the leading edge of the paper, and even if the area where the image is formed is wrapped, the leading edge of the peeling claw enters between the blank area where the image is not formed and the belt. In this configuration, the paper is peeled off to prevent the paper from being wound around the fixing belt or the fixing roller.

  Since it is desirable that the margin at the front end of the paper is as small as possible, the gap between the peeling claw and the fixing belt or the fixing roller is set as small as possible. However, there is a case where the margin is reduced due to variations in the conveyance speed of the paper, and the paper may be caught in the gap between the peeling claws. As a countermeasure, a configuration in which the peeling claw is brought into contact with the fixing belt with a weak force has been proposed. With this configuration, there is no wrapping, but the surface of the fixing belt or the fixing roller is damaged by the contact of the peeling claw, and there is a problem of image quality deterioration in which the trace appears in the image.

  Patent Document 1 describes a configuration for determining whether or not peeling is possible by obtaining information on a sheet. However, if it is determined that peeling is not possible, it is only displayed to the operator so that printing itself is not performed, and there is no description about a method for printing on such paper.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus provided with a long-life fixing unit that does not wind a sheet and does not deteriorate image quality.

  In order to achieve the above object, the present invention provides a latent image forming means for forming a latent image on a photoreceptor, a developing means for forming a toner image by attaching toner to the latent image, and a toner image on the photoreceptor. An intermediate transfer body to be transferred; a first transfer means for transferring a toner image on the photoconductor to the intermediate transfer body; a paper transport means for transporting paper; and a toner image on the intermediate transfer body. The present invention is intended for an image forming apparatus having a second transfer unit that transfers to a sheet and a fixing unit that melts and fixes the toner image on the sheet.

  In the first means of the present invention, the fixing means adjusts the pressing force of the second roller, the second roller pressing the first roller, and the pressing force of the second roller. Means for detecting the rigidity of the paper, an image pattern recognition means for recognizing the toner image transferred to the paper, and the pressing from the signals of the rigidity detection means and the image pattern recognition means. Control means for controlling force adjusting means, and the pressing force adjusting means adjusts the pressing force of the second roller according to a signal from the control means.

  According to a second means of the present invention, in the first means, the fixing means is a fixing belt, a heating roller that heats the fixing belt, and a pressure roller that is the first roller whose surface is an elastic body. And a tension roller and a backup roller that is the second roller pressed against the pressure roller via the fixing belt, and the surface of the pressure roller is relative to the surface of the backup roller. It is characterized by low hardness.

According to a third means of the present invention, in the second means, the fixing means has a cooling roller for lowering the surface temperature of the fixing belt, and a retracting means for allowing the cooling roller to contact and retract the fixing belt. And
When the pressing force adjusting means increases the pressing force of the backup roller according to the signal of the control means, the retracting means brings the cooling roller into contact with the fixing belt and returns the pressing force of the backup roller. The cooling roller is retracted.

  The present invention is configured as described above, and can provide an image forming apparatus provided with a long-life fixing unit that does not wind a sheet of paper around a fixing belt or a fixing roller and that does not deteriorate image quality.

  The configuration and operation of the first embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 is a schematic configuration diagram of a fixing unit and a control unit according to an embodiment of the present invention, and shows a state in which a pressing force of a backup roller 64 as a second roller is a normal value and the cooling roller 66 is retracted. .

  A fixing belt 61 for nipping and conveying the paper is stretched between a pressure roller 63, a heating roller 62, and a tension roller 65, which are first rollers. The fixing belt 61 has a polyimide layer made of polyimide having a thickness of 80 to 100 μm, and has a coat layer in which a surface of the polyimide layer is coated with a fluororesin of about 30 μm in order to improve releasability. The fixing belt 61 is provided with a silicon rubber layer of several hundreds μm between the surface coat layer and the polyimide layer in order to improve the fixability by pressing the toner along the unevenness of the paper 4 against the paper having a rough surface. May be.

  There is a heater 68 in the heating roller 62, and the temperature sensor 69 measures the surface temperature of the fixing belt 61 and is controlled to maintain a predetermined fixing temperature. The heating roller 62 is made of a metal having a good thermal conductivity such as aluminum, and the surface is coated with a fluororesin or the like to prevent contamination. The pressure roller 63 and the backup roller 64 are rotated by a drive mechanism (not shown) to convey the paper 4 in the direction of arrow a in the figure.

  The pressure roller 63 has a low hardness rubber layer 631 formed on a core metal such as aluminum, and the backup roller 64 has a high hardness rubber layer 641 formed on a core metal such as aluminum. By pressing the backup roller 64 against the pressure roller 63 via the paper 4 and the fixing belt 61, the nip shape becomes concave on the pressure roller 63 side, and the paper 4 is discharged in a direction away from the surface of the fixing belt 61. Therefore, it is difficult to wind around the fixing belt 61.

  For example, the pressure roller 63 has a diameter of 60 mm, the low hardness rubber layer 631 has a thickness of 10 mm, the rubber hardness is 10 ° (JIS A type), the backup roller 64 has a diameter of 60 mm, and the high hardness rubber layer 641 has a thickness of 10 mm. When the rubber hardness is 30 ° (JIS A type) and the width is 250 mm, the pressing force is 500 N or more, preferably 600 to 700 N.

  Next, the pressing force adjusting means will be described. One end of the pressure spring 642 that presses the backup roller 64 toward the pressure roller 63 is supported by the pressure lever 643. The pressure lever 643 rotates around the fulcrum 646 by the rotation of the cam 645 attached to the cam drive motor 644, and changes the spring length of the pressure spring 642, so that the backup roller 64 against the pressure roller 63. Change the pressing force.

  Next, the retracting means will be described. The cooling roller 66 is a metal roller such as aluminum and is coated with a fluororesin to prevent adhesion of dirt on the surface. If air flows as a hollow, it becomes possible to cool more stably. The retracting means includes a rotating lever 665 and a lever motor 664. The cooling roller 66 is attached to a rotation lever 665 and makes contact with and retracts from the fixing belt 61 by rotation of the lever motor 664. The cooling roller 66 is composed of, for example, an aluminum roller having a diameter of 20 mm.

  The peeling claw 67 is disposed near the exit of the nip portion between the pressure roller 63 and the backup roller 64.

  The rigidity detection unit 201 detects the rigidity of the sheet 4 conveyed for image formation. The image pattern recognition unit 202 determines whether the image to be formed on the paper is a solid image of two or more colors, what ratio the image is formed in the width direction of the paper, and whether the image is formed near the leading edge of the paper. recognize. The temperature sensor 69 detects the surface temperature of the fixing belt 61. The control means 601 determines the operation of the cam drive motor 644 based on the information of the rigidity detection means 201 and the image pattern recognition means 202, and sends a signal to the motor driver A647. Further, the operation of the lever motor 664 is determined based on the information of the temperature sensor 69, and a signal is sent to the motor driver B666.

  FIG. 2 shows a state in which the pressing force of the backup roller 64 is increased and the cooling roller 66 is brought into contact with the fixing belt 61 by the fixing unit according to the embodiment of the present invention.

  The rigidity of the sheet is low by the rigidity detection means 201 (for example, when the sheet thickness is 0.1 mm or less and the conveyance direction of the sheet 4 coincides with the CD (cross direction) direction of the sheet 4) and the image pattern is recognized. The means 202 recognizes that the image formed on the paper 4 is a solid image of two or more colors and is formed wide in the width direction of the paper 4 and the image is formed near the leading edge of the paper 4. When the control unit 601 determines that the sheet 4 may be wound around the fixing belt 61, the control unit 601 sends a signal to the motor driver A647, operates the cam drive motor 644, and rotates the cam 645 to apply the signal. The pressure lever 643 is rotated to push up the pressure spring 642. As a result, the pressing force of the backup roller 64 against the pressure roller 63 increases, the concave deformation of the low-hardness rubber layer 631 increases, and the discharge direction of the paper 4 from the nip outlet is further away from the fixing belt 61. The paper 4 can be reliably peeled off. The pressing force of the backup roller 64 is set to 800 to 1300N, desirably 1000 to 1300N.

  At this time, if the temperature of the fixing belt 61 measured by the temperature sensor 69 is higher than the temperature defined by the increase in the nip width due to the increase of the pressing force and the sheet thickness assumed from the rigidity of the sheet 4, the control means 601 sends a signal to the motor driver B 666 to operate the lever motor 664 and rotate the lever 665 to bring the cooling roller 66 into contact with the fixing belt 61. The cooling roller 66 removes heat from the surface of the fixing belt 61 to lower the temperature and prevent hot offset of the toner on the paper 4. At this time, since the temperature of the paper 4 is also lowered, the decrease in rigidity of the paper 4 due to the temperature rise is reduced, and the paper 4 is reliably peeled off.

  FIG. 3 is a flowchart showing the operation of the fixing means of the embodiment of the present invention. The description in parentheses below corresponds to the notation in the figure.

  First, the pressing force of the backup roller 64 is set for normal sheet thickness and rigidity, and the cooling roller 66 is retracted to an initial state. The temperature control of the fixing belt 61 will not be described here. When there is a print instruction and the paper 4 is sent out from the paper cassette (YES), the image pattern recognition unit 202 sends the information on the leading margin of the paper 4 to the control unit 601 (is the blank enough)? When it is determined that the margin is sufficient (YES), the control unit 601 checks whether the pressing force has increased. If the pressing force has increased (YES), the control unit 601 sends a signal to the motor driver A647 to turn the cam drive motor 644 on. Operate, rotate the cam 645 to rotate the pressure lever 643, and return the pressure spring 642 (returning pressing force). Next, the control means 601 checks whether the cooling roller 66 is in contact with the fixing belt 61. If it is in contact (YES), it sends a signal to the motor driver B666, operates the lever motor 664, and rotates the lever 665. Printing is performed after the cooling roller 66 is separated from the fixing belt 61 (cooling roller return). If the cooling roller 66 is not in contact (NO), printing is performed as it is. If the pressing force has not increased (NO), printing is performed.

  Then, the presence or absence of a print instruction for the next page is confirmed.

  If it is determined after the print instruction that the margin is not sufficient (NO), it is checked whether the image is a solid image of two or more colors and close to the full width of the paper 4. If it is monochrome or not nearly full width, for example 50% or less of the width (NO), as described above, the pressing force and the contact of the cooling roller 66 are confirmed, and after performing the operation corresponding to each, printing is performed. Do.

  When the image is a solid image of two or more colors and is close to the full width of the sheet 4 (YES), the control unit 601 determines whether the sheet 4 has sufficient rigidity based on the information of the rigidity detection unit 201. If the paper 4 is thick and has sufficient rigidity (YES), as described above, the pressing force and the contact of the cooling roller 66 are confirmed, and the printing is performed after performing the operation corresponding to each.

  If the rigidity is insufficient (NO), the control means 601 sends a signal to the motor driver A647, operates the cam drive motor 644, rotates the cam 645, rotates the pressure lever 643, and pressurizes the pressure spring. 642 is pushed up to increase the pressing force.

  Next, based on information from the temperature sensor 69, it is determined whether the temperature of the fixing belt 61 is appropriate for the thin paper 4 having low rigidity. Up to this point, when the normal thickness of the paper 4 is printed and the temperature of the fixing belt 61 is high (high), the control unit 601 sends a signal to the motor driver B666 to operate the lever motor 664 and rotate it. The moving lever 665 is rotated to bring the cooling roller 66 into contact with the fixing belt 61 and the temperature of the fixing belt 61 is lowered, and then printing is performed.

  When the temperature of the fixing belt 61 is appropriate for the thin paper 4 having low rigidity (appropriate), printing is performed without bringing the cooling roller 66 into contact with the fixing belt 61.

  FIG. 4 is a schematic configuration diagram of the rigidity detecting means.

  The thickness detection roller 27 is supported so as to be vertically movable in the drawing, and is pressed against the transport roller 25 by a spring C26. A paper detection sensor 29 is disposed downstream of the thickness detection roller 27 and the conveyance roller 25.

The transport roller 25 is rotated in the direction of arrow e in the figure by driving means (not shown). The displacement sensor A20 measures the vertical movement of the thickness detection roller 27.
The transport roller 25 is desirably a rubber roller in order to secure a frictional driving force for transporting the paper, and desirably has a rubber hardness of 50 ° (JIS A type) or higher in order to increase the thickness detection accuracy.

  The rigidity detection lever 33 is supported so as to be rotatable about the fulcrum B36. The spring B32 pulls the rigidity detection lever 33 in the direction of arrow f in the figure and abuts against the stopper 37. The displacement sensor B30 measures the rotational displacement of the rigidity detection lever 33.

  The conveyed paper pushes up the thickness detection roller 27, and the displacement sensor A20 measures the displacement.

  Further, the rigidity detection lever 33 is rotated by the rigidity, and the output of the displacement sensor B30 is changed. The higher the paper rigidity, the larger the rotation angle of the rigidity detection lever, and the greater the change in the output of the displacement sensor B30. The outputs of the displacement sensor A20 and the displacement sensor B30 are input to the data processing means 38, and the data processing means 38 obtains the rigidity of the paper.

The stiffness of the paper may be obtained only from the thickness of the paper obtained from the value of the displacement sensor A20. However, when the sheets are double-fed, the apparent thickness is thick, but the rigidity of one sheet may be low and the sheet may be wound. If the value of the displacement sensor B30 is used, the rigidity can be reliably detected.
The paper detection sensor 29 detects the leading edge of the paper and conveys the paper at a predetermined distance therefrom.

  The predetermined distance is an amount capable of correcting the skew by pressing the leading end of the sheet against the nip portion between the registration roller A41 and the registration roller B42, and is set to the distance. A registration roller A41 whose surface is made of metal and a registration roller B42 whose surface is made of rubber are arranged. The registration roller B42 is rotated by a driving mechanism (not shown) to convey the paper in the direction of arrow g in the figure.

  After the skew correction, the registration roller A41 and the registration roller B42 convey the paper.

  FIG. 5 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention.

  In the image forming apparatus 1, each color developing unit, which is a developing unit, is arranged on a transfer belt 2, which is an intermediate transfer member, and a color image is formed on the transfer belt 2 with toner. The toner is transferred to a sheet to be conveyed, and the fixing unit 60 melts and fixes the toner with heat and pressure to form a color image.

  There are four developing units, which are a K developing unit 501 having black toner, a C developing unit 502 having cyan toner, an M developing unit 503 having magenta toner, and a Y developing unit 504 having yellow toner. The developing unit includes a toner hopper 53 that stores toner, a developing roller 52 that forms a toner layer and brings the toner into contact with the photosensitive drum 54 that is a photosensitive member, a drum cleaner 57 that cleans the photosensitive drum 54, and a photosensitive drum 54. A charging unit 56 for charging and an exposure unit 55 for writing an electrostatic latent image on the photosensitive drum 54 are included.

  The intermediate transfer belt 2 is stretched around a plurality of rollers and is conveyed by a belt driving roller 3. The belt cleaner 91 removes residual toner on the intermediate transfer belt 2. A primary transfer roller 58 serving as a transfer unit is disposed inside the intermediate transfer belt 2 so as to face the photosensitive drum 54.

  The paper conveyance path 8 passes from the paper accumulation means 41 for accumulating paper, passes between the secondary transfer roller 7 and the intermediate transfer belt 2 via the pick roller 9, the separation roller 11, and the rigidity detection means 201. To the fixing means 60. A paper path switching guide 70 for switching the paper path is provided downstream of the rigidity detection device 201, and the paper determined to be double-fed from the paper thickness and rigidity is transferred from the double-fed paper withdrawal path 71 to the double-fed paper withdrawal unit 72. You can also send it.

  When forming an image, the surface of the photosensitive drum 54 is charged by the charger 56, and light corresponding to the image is applied by the exposure device 55 to drop the potential on the photosensitive drum 54. The portion reaches the developing roller 52 by the rotation of the photosensitive drum 54, and when it comes into contact with the toner layer, the charged toner adheres to the image position.

  The toner image on the photosensitive drum 54 is transferred onto the intermediate transfer belt 2 at a portion where the primary transfer roller 58 presses the intermediate transfer belt 2.

  The toner image on the photosensitive drum 54 is transferred onto the intermediate transfer belt 2 to form a color toner image. Then, the toner image is transferred onto the sheet conveyed by the secondary transfer roller 7 by the conveyance of the intermediate transfer belt 2. The sheet on which the toner image is transferred is conveyed to the fixing unit 60 by the conveyance belt 81, and the toner is melted and fixed by heat and pressure to form a color image.

  When the control unit 601 determines from the information of the image pattern recognition unit 202 and the rigidity detection unit 201 that the pressing force needs to be increased, the pressing force is increased and the peeling is performed reliably. If it is determined in 601 that the fixing belt 61 needs to be cooled, the cooling roller 66 is brought into contact with the fixing belt 61.

  According to the present embodiment, it is possible to reliably prevent the paper from being wound around the fixing belt 61 and extend the life of the pressure roller 63 by reducing the frequency of increasing the pressing force of the backup roller 64. In addition, the temperature of the fixing belt 61 is lowered by the cooling roller 66 and the temperature is quickly adjusted to an appropriate temperature for a thin sheet, thereby enabling continuous printing without leaving a printing interval. Further, since the rigidity of the sheet decreases with an increase in temperature, if the temperature of the fixing belt 61 is decreased with the cooling roller 66, the temperature of the sheet also decreases, so that the decrease in rigidity is small and winding is less likely to occur.

  FIG. 6 is a schematic configuration diagram of a fixing unit and a control unit according to the second embodiment of the present invention.

  The difference from the fixing unit of the first embodiment is that the fixing roller 632 is used instead of the fixing belt 61.

  The fixing roller 632 has a low-hardness rubber layer 633 formed on a metal core such as aluminum, and the surface is coated with a fluororesin having a thickness of about 30 μm in order to improve the peelability. The backup roller 64 has a high hardness rubber layer 641 formed on a metal core such as aluminum. By pressing the backup roller 64 against the fixing roller 632, the nip shape becomes concave on the fixing roller 632 side, and the paper 4 is discharged in a direction away from the surface of the fixing roller 632, so that it is difficult to wind.

  For example, the fixing roller 632 has a diameter of 60 mm, the low hardness rubber layer 632 has a thickness of 10 mm, the rubber hardness is 10 ° (Asker A type), the backup roller 64 has a diameter of 60 mm, and the high hardness rubber layer 641 has a thickness of 10 mm. When the hardness is 30 ° (Asker A type) and the width is 250 mm, the pressing force is 500 N or more, preferably 600 to 700 N. There is a heater 68 in the heating roller 62, and the temperature sensor 69 measures the surface temperature of the fixing roller 632 and is controlled to maintain a predetermined fixing temperature. The heating roller 62 is made of a metal having a good thermal conductivity such as aluminum, and the surface is coated with a fluororesin or the like to prevent contamination. The fixing roller 632 and the backup roller 64 are rotated by a driving mechanism (not shown) to convey the paper 4 in the direction of arrow a in the figure.

  The fixing roller 632 has a thick low-hardness rubber layer in order to make the nip shape concave toward the fixing roller 632 side. When a heater or the like is placed in the core of the fixing roller 632, it takes time to raise the surface of the fixing roller 632 to a temperature for fixing the toner on the paper, and the temperature of the surface is lowered by passing the paper. In addition, it takes time to return to the specified temperature, which is not suitable for high-speed continuous printing.

  As in this embodiment, the heating roller 62 is made of a metal having good thermal conductivity, and the heater 68 is disposed therein to directly heat the surface of the fixing roller 632, thereby shortening the time for increasing the surface temperature. It is possible to perform high-speed continuous printing.

  The change of the pressing force of the backup roller 64 and the operation of the cooling roller 66 are the same as in the first embodiment.

  According to this embodiment, it is possible to extend the life of the fixing roller by reducing the frequency of increasing the pressing force of the backup roller while reliably preventing the paper from being wound. In addition, the fixing temperature is lowered by the cooling roller, and the temperature is quickly adjusted to an appropriate temperature on a thin sheet, thereby enabling continuous printing without leaving a printing interval. Furthermore, since the rigidity of the paper decreases with an increase in temperature, if the surface temperature of the fixing roller 632 is lowered by the cooling roller 66, the temperature of the paper also decreases, so that the rigidity is less reduced and winding is less likely to occur. Further, since the fixing belt 61 is not used, the configuration is simplified.

  Further, in the embodiment, if the toner image of two or more colors is not formed as a wide solid image, the image is recognized by the image pattern recognition means 202 because the paper is thin and does not wind even if the rigidity is low. It is not necessary to increase the pressing force of the backup roller 64 as determined by the control means 601.

  Further, if the paper is thick and has high rigidity, even if a toner image of two or more colors is formed on almost the entire surface near the leading edge of the paper, it will not be wound. Therefore, the result of the rigidity detecting means 201 is judged by the control means 601 and backed up. The pressing force of the roller 64 may not be increased. Thus, it is possible to extend the life of the pressure roller 63 by reducing the frequency of increasing the pressing force of the backup roller 64.

  In addition, it has a cooling roller 66 for lowering the surface temperature of the fixing belt 61, and a retracting means for allowing the cooling roller 66 to come into contact with and retract from the fixing belt 61. The pressing force adjusting means is backed up by a signal from the control means 601. When the pressing force of the roller 64 is increased, the retracting unit brings the cooling roller 66 into contact, and when the pressing force of the backup roller 64 is returned, the cooling roller is separated.

  If it is determined from the image pattern and the rigidity of the paper that the pressing force of the backup roller needs to be increased, and the pressing force increases, the nip width between the pressure roller and the backup roller increases. Since the amount of heat applied to the sheet also increases, the amount of heat becomes excessive in the case of a thin sheet, and hot offset occurs. A sheet having low rigidity has a small thickness, so that hot offset is more likely to occur due to an increase in the load of a backup roller for peeling.

  To prevent hot offset from occurring on thin paper, the fixing belt temperature should be lower than when fixing other standard thickness paper. However, until the fixing temperature drops, the paper is not passed through the fixing means. It is necessary to stop the paper, and the printing efficiency of the image forming apparatus decreases. For example, when multiple thickness sheets are mixed and put in the paper cassette, when different thickness sheets are sent from multiple paper cassettes, thick paper or standard paper When a thin sheet is conveyed later, it is necessary to stop the sheet passing.

  Therefore, by continuously reducing the temperature of the fixing belt with a cooling roller and quickly setting it to an appropriate temperature for a thin sheet, printing can be continuously performed without leaving a printing interval. Furthermore, since the rigidity of the paper decreases with an increase in temperature, if the temperature of the fixing belt is lowered with a cooling roller, the temperature of the paper also decreases, so that the rigidity is less decreased and winding is less likely to occur.

FIG. 3 is a schematic configuration diagram of a fixing unit and a control unit according to the first embodiment of the present invention. FIG. 3 is a schematic configuration diagram of a fixing unit and a control unit according to the first embodiment of the present invention. 3 is an operation flowchart according to the first embodiment of the present invention. It is a schematic block diagram of the rigidity detection means which concerns on the Example of this invention. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 5 is a schematic configuration diagram of a fixing unit and a control unit according to a second embodiment of the present invention.

Explanation of symbols

  1: image forming apparatus, 2: intermediate transfer belt, 4: paper accumulation unit, 7: transfer roller, 8: conveyance path, 33: rigidity detection lever, 35: spring C, 38: rigidity determination unit, 52: development roller, 54: photosensitive drum, 55: exposure device, 56: charger, 58: primary transfer roller, 60: fixing device, 61: fixing belt, 62: heating roller, 63: pressure roller, 64: backup roller, 65: Tension roller, 66: cooling roller, 201: rigidity detection means, 202: image pattern recognition means, 501 to 504: development unit, 601: control means.

Claims (3)

  A latent image forming unit that forms a latent image on the photosensitive member; a developing unit that attaches toner to the latent image to form a toner image; an intermediate transfer member onto which the toner image on the photosensitive member is transferred; and the photosensitive member A first transfer unit that transfers a toner image on the body to the intermediate transfer member, a paper transfer unit that transfers paper, and a second transfer unit that transfers the toner image on the intermediate transfer member to the paper. In the image forming apparatus having a fixing unit that melts and fixes the toner image on the paper, the fixing unit includes at least a first roller, a second roller pressed against the first roller, and the second roller. A pressing force adjusting unit that adjusts the pressing force of the roller, a rigidity detecting unit that detects the rigidity of the paper, an image pattern recognition unit that recognizes a toner image transferred to the paper, and the rigidity detecting unit. And image par Control means for controlling the pressing force adjusting means from the signal of the key recognition means, and the pressing force adjusting means adjusts the pressing force of the second roller according to the signal of the control means. Image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the fixing unit includes a fixing belt, a heating roller that heats the fixing belt, a pressure roller that is the first roller whose surface is an elastic body, and a tension roller. A backup roller that is the second roller pressed against the pressure roller via the fixing belt, and the surface of the pressure roller has a relatively lower hardness than the surface of the backup roller An image forming apparatus.   The image forming apparatus according to claim 2, wherein the fixing unit includes a cooling roller for lowering a surface temperature of the fixing belt, and a retracting unit that allows the cooling roller to contact and retract from the fixing belt, When the pressing force adjusting means increases the pressing force of the backup roller according to the signal of the control means, the retracting means brings the cooling roller into contact with the fixing belt and returns the pressing force of the backup roller. An image forming apparatus for retracting the cooling roller.

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