JP4550625B2 - Fixing device - Google Patents

Description

  The present invention relates to a fixing device that performs a toner image fixing process on a sheet on which a toner image has been transferred in an image forming unit of the image forming apparatus, and particularly exhibits an excellent effect in preventing the occurrence of an offset phenomenon. The present invention relates to a fixing device.

  Conventionally, a fixing device applied to an image forming apparatus as described in Patent Document 1 is known. This fixing device performs a fixing process by heating the paper on which the toner image has been transferred in the image forming unit of the image forming apparatus. The fixing device includes a fixing roller having a heating source therein and is opposed to the fixing roller. A pressure roller and a transfer process toward the nip formed on the contact surface between the peripheral surface of the fixing roller and the peripheral surface of the pressure roller, each rotating about its axis. The sheet is fed and heated from the fixing roller, whereby the sheet is subjected to a fixing process.

  By the way, since the toner on the paper is in a molten state when heated by the fixing roller, the melted toner adheres to the peripheral surface of the fixing roller and is transferred to the next fed paper, so-called offset phenomenon. In some cases, the cleaning web is in contact with the peripheral surface of the fixing roller in order to wipe off the toner adhering to the peripheral surface of the fixing roller. The cleaning web is formed in a band shape, and the wiping surface is shifted and updated every predetermined number of sheets. However, for example, when the wiping surface of the cleaning web is just before the update, the toner moves from the cleaning web to the fixing roller, and the offset can be suppressed particularly depending on the type of paper (for example, thick paper). There was a problem that it was impossible.

Therefore, in the fixing device described in Patent Document 1, the pressing force against the circumferential surface of the fixing roller of the cleaning web is variable according to the type of paper. For example, when the paper is recycled paper, the pressing force on the recycled paper is set slightly higher than that of normal plain paper, and when the paper is thicker, the pressing force is further increased. It has been adopted.
JP 2003-76196 A

  However, even if the pressing force of the cleaning web is variable, when the wiping surface of the cleaning web is just before renewal, the wiping force is reduced because a large amount of toner is attached to the wiping surface. There is a problem that the offset phenomenon cannot be prevented by the toner adhering to the peripheral surface of the fixing roller slipping through the wiping surface.

  Further, the structure that makes the pressing force of the cleaning web variable is considered to be considerably complicated, and there is a problem that the manufacturing cost increases due to the increase in the number of parts and the number of assembly steps.

  The present invention has been made to solve such a problem, and is capable of reliably preventing an offset phenomenon caused by a web immediately before the wiping surface is updated while suppressing an increase in manufacturing cost. The purpose is to provide.

  According to the first aspect of the present invention, a fixing roller that heats the peripheral surface of a sheet rotated in a predetermined direction and having a toner image transferred thereon by an image forming unit, and the peripheral surface faces the peripheral surface of the fixing roller. A web roller for wiping the toner adhering to the peripheral surface of the fixing roller at the web side nip formed between the web and the fixing roller in a state where the side nip is formed and rotating in synchronization with the fixing roller. And a web cleaning unit configured to perform a fixing process on the sheet by conveying the sheet toward the fixing-side nip portion, and the web cleaning unit wipes the web every time a predetermined number of sheets are fixed. In the fixing device configured to update the surface, the fixing is performed in a state where a predetermined number of sheets immediately before the web is updated is passing through the fixing-side nip portion. When the length dimension of the unfixed portion of the paper that has not reached the nip portion becomes shorter than the circumferential length between the web-side nip portion and the fixing-side nip portion in the rotation direction of the fixing roller, the web is wiped off. It is characterized by being configured to update the surface.

  According to such a configuration, in a state where a predetermined number of sheets corresponding to the wiping surface immediately before the web is updated passes through the fixing side nip portion, the sheet that has not reached the fixing side nip portion is not fixed. Since the web wiping surface is updated when the length dimension of the processing portion becomes shorter than the circumferential length between the web-side nip portion and the fixing-side nip portion in the rotation direction of the fixing roller, the circumferential surface of the fixing roller The portion wiped by the dirty wiping surface immediately before the update in (that is, the portion that is not likely to be wiped (non-wiping portion)) is the peripheral surface of the pressure roller after the paper passes through the fixing side nip portion. Therefore, the sheet does not come into contact with the non-wiping portion, and therefore, the occurrence of the offset phenomenon with respect to the rear end portion of the sheet is reliably prevented.

  According to a second aspect of the present invention, in the first aspect of the present invention, the control means for controlling the update timing of the wiping surface of the web and the timing of feeding the next sheet to the fixing side nip portion, and the fixing side nip A sheet sensor for detecting a rear end of the sheet conveyed toward the section at a predetermined position of the image forming section, and the control means corresponds to the predetermined number of sheets corresponding to the web immediately before the wiping surface is updated. A wiping surface update timing discriminating unit for discriminating the update timing of the wiping surface of the web based on the detection result of the paper sensor for eye paper, and the web cleaning means based on the discrimination result of the wiping surface update timing discriminating unit And a control signal output unit that outputs a control signal for updating the wiping surface of the web.

  According to this configuration, first, the rear end of the predetermined number of sheets corresponding to the web immediately before the wiping surface is updated (hereinafter referred to as the preceding sheet) at the predetermined position of the image forming unit is detected by the sheet sensor. The detection result is input to the wiping surface update timing determination unit. Then, the wiping surface update timing determination unit to which the detection result is input determines the update timing of the wiping surface of the web and transmits the determination result to the control signal output unit. The control signal output unit outputs a control signal to update the wiping surface of the web toward the web cleaning unit based on the determination result, so that the fixing roller peripheral surface wiped by the dirty wiping surface before being updated This prevents the non-wiping portion from contacting the precursor paper, thereby preventing the offset phenomenon caused by the non-wiping portion coming into contact with the preceding paper.

  According to a third aspect of the present invention, there is provided a fixing roller that heats a peripheral surface of a sheet that is rotated in a predetermined direction and has a toner image transferred by an image forming unit, and the peripheral surface faces the peripheral surface of the fixing roller. A web roller for wiping the toner adhering to the peripheral surface of the fixing roller at the web side nip formed between the web and the fixing roller in a state where the side nip is formed and rotating in synchronization with the fixing roller. And a web cleaning unit configured to perform a fixing process on the sheet by conveying the sheet toward the fixing-side nip portion, and the web cleaning unit wipes the web every time a predetermined number of sheets are fixed. In the fixing device configured to update the surface, the fixing is performed in a state where a predetermined number of sheets immediately before the web is updated is passing through the fixing-side nip portion. When the length dimension of the unfixed portion of the paper that has not reached the nip portion becomes shorter than the circumferential length between the web-side nip portion and the fixing-side nip portion in the rotation direction of the fixing roller, the web is wiped off. In addition to updating the surface, the sheet to be subjected to the fixing process next is conveyed toward the fixing-side nip portion at a timing when the front edge of the sheet is removed from the peripheral surface of the fixing roller wiped by the wiping surface immediately before the update. It is characterized by being.

  According to such a configuration, in addition to the operation of the first aspect of the invention, the sheet to be subjected to the fixing process next is the timing at which the front end of the sheet is removed from the peripheral surface of the fixing roller wiped by the wiping surface immediately before the update. Since the sheet is conveyed toward the fixing side nip, the non-wiping portion of the fixing roller has already passed the fixing side nip when the next sheet reaches the fixing side nip. Occurrence of the offset phenomenon for the target is reliably prevented.

  In this way, the non-wiping portion of the peripheral surface of the fixing roller faces the peripheral surface of the pressure roller after the sheet on which the fixing process is being performed at the fixing side nip portion passes through the fixing side nip portion. At the same time, since the non-wiping portion reaches the fixing-side nip portion after the non-wiping portion has passed the fixing-side nip portion (that is, the non-wiping portion in the fixing roller is always the Therefore, there is no opportunity for the non-wiping portion to come into contact with the paper, and therefore no offset phenomenon occurs.

According to a fourth aspect of the present invention, in the third aspect of the present invention, a control means for controlling the update timing of the wiping surface of the web and the timing of feeding the next sheet to the fixing side nip portion, and the fixing side nip portion A sheet sensor for detecting a trailing edge of the sheet conveyed toward the sheet at a predetermined position of the image forming unit, and the control unit is configured to detect the predetermined number of sheets corresponding to the web immediately before the wiping surface is updated. A wiping surface update timing discriminating unit for discriminating an update timing of the wiping surface of the web based on a detection result of the paper sensor for the sheet of paper, and a paper sensor for the paper next to the predetermined number of sheets Based on the determination results of the sheet conveyance timing determination unit that determines the timing of moving the next sheet to the fixing-side nip portion based on the detection result and the wiping surface update timing determination unit. A control signal to update the wiping surface of the web is output to the web cleaning unit, and the next sheet is conveyed to the fixing side nip portion toward the image forming unit based on the determination result of the sheet conveyance timing determination unit. And a control signal output unit.

  According to this configuration, in addition to the operation of the second aspect of the invention, the trailing edge of the image forming unit of the next sheet (hereinafter referred to as the subsequent sheet) of the predetermined number of sheets is also detected by the sheet sensor, and the detection result is obtained. This is input to the paper transport timing determination unit. Then, the sheet conveyance timing determination unit to which this detection result is input determines the timing at which the subsequent sheet is directed toward the fixing side nip (that is, the timing at which the subsequent sheet is not applied to the non-wiping unit of the fixing roller). The determination result is transmitted to the control signal output unit. The control signal output unit outputs a control signal for conveying the succeeding sheet toward the fixing-side nip portion based on the determination result, so that the succeeding sheet is not wiped on the peripheral surface of the fixing roller. The portion reaches the fixing side nip portion in a state where the portion passes the fixing side nip portion. Accordingly, since the subsequent sheet is prevented from coming into contact with the non-wiping portion of the fixing roller, the offset phenomenon due to the non-wiping portion coming into contact with the subsequent sheet is prevented.

  In this way, the control unit determines the update timing of the wiping surface of the web based on the detection result of the paper sensor for the preceding paper, and the subsequent based on the detection result of the paper sensor for the subsequent paper. Control for the web cleaning unit and the image forming unit based on the determination results of the sheet conveyance timing determination unit that determines the timing of the sheet toward the fixing-side nip, and the wiping surface update timing determination unit and the sheet conveyance timing determination unit, respectively. By providing a control signal output unit that outputs a signal, the offset phenomenon is effectively prevented without adding a new member or mechanism for preventing the offset phenomenon to the fixing device.

  The invention according to claim 5 is the invention according to any one of claims 1 to 4, characterized in that pressure roller cleaning means for cleaning the peripheral surface of the pressure roller is provided. It is.

  According to such a configuration, the toner transferred from the peripheral surface of the fixing roller to the peripheral surface of the pressure roller by the peripheral surfaces abutting each other in the fixing-side nip portion in a state where the sheet does not exist in the fixing-side nip portion, Since it is removed by the pressure roller cleaning means, the back side of the paper is prevented from being contaminated by the toner present on the peripheral surface of the pressure roller.

  According to the first aspect of the present invention, the non-wiping portion of the peripheral surface of the fixing roller is the pressure roller after the sheet on which the fixing process is being performed at the fixing side nip portion passes through the fixing side nip portion. Since the non-wiping portion reaches the fixing-side nip portion after the non-wiping portion has passed the fixing-side nip portion, the non-wiping portion comes into contact with the sheet. Therefore, it is possible to prevent the offset phenomenon from occurring on any paper.

  In addition, such a workaround for the offset phenomenon is devised to use the existing components of the fixing device without adding new members to prevent the offset phenomenon. It does not increase the number of man-hours, and the increase in manufacturing cost can be suppressed accordingly.

  According to the second aspect of the present invention, the wiping surface update timing determining section for determining the update timing of the wiping surface of the web based on the detection result of the sheet sensor for the preceding sheet, and the wiping surface updating timing determining section. By providing a control signal output unit that outputs a control signal toward the web cleaning unit based on the determination result, it is possible to reliably prevent the offset phenomenon from occurring on the sheet that is actually subjected to the fixing process. .

  According to the invention described in claim 3, in addition to the effect of the invention described in claim 1, the sheet to be subjected to the fixing process next is a timing at which the front end of the sheet is removed from the peripheral surface of the fixing roller wiped by the wiping surface immediately before the update. Since the sheet is conveyed toward the fixing side nip portion, it is possible to reliably prevent the offset from occurring for the sheet to be subjected to the fixing process next.

  According to the fourth aspect of the present invention, a new member or mechanism for preventing the offset phenomenon is added to the fixing device by providing the control means with a sheet conveyance timing determination section in addition to the wiping surface update timing determination section. Therefore, it is possible to reliably prevent the occurrence of the offset phenomenon with respect to the sheet next to the sheet that has been subjected to the fixing process while suppressing an increase in manufacturing cost.

  According to the fifth aspect of the present invention, since the pressure roller cleaning means for cleaning the peripheral surface is also provided on the pressure roller side, the fixing side in the state where the sheet does not exist in the fixing side nip portion. The toner transferred from the peripheral surface of the fixing roller to the peripheral surface of the pressure roller by the peripheral surfaces abutting each other in the nip portion is removed by the pressure roller cleaning means, whereby the back surface side of the sheet is removed from the pressure roller. It is possible to prevent contamination by toner existing on the peripheral surface.

  FIG. 1 is a front sectional view for explaining an outline of an internal structure of a printer to which a fixing device according to the present invention is applied. As shown in FIG. 1, a printer (image forming apparatus) 10 includes a sheet storage / return unit 12 that stores sheets P to be subjected to printing processing, and one sheet fed from a sheet bundle stored in the sheet storage / return unit 12. An image forming unit 13 that performs an image transfer process on each sheet P and a fixing unit 14 that performs a fixing process on the sheet P subjected to the transfer process in the image forming unit 13 are provided in the apparatus main body 11. In addition, a paper discharge unit 15 for discharging the paper P on which the fixing process has been performed by the fixing unit 14 is provided at the top of the apparatus main body 11.

  The paper storage unit 12 includes a paper cassette 121 that can be inserted into and removed from the apparatus main body 11. A pickup roller 122 is provided at the upper right end of the sheet cassette 121 in FIG. 1 to feed out the sheets P one by one from the bundle of sheets loaded in the sheet cassette 121. The paper P fed out of the paper cassette 121 by driving the pickup roller 122 is supplied to the image forming unit 13 through the paper feed conveyance path 123 and the registration roller pair 124 provided at the downstream end of the paper feed conveyance path 123. It is supposed to be paper.

  The image forming unit 13 performs a transfer process on the paper P based on image information transmitted from a computer or the like (not shown), and rotates around a drum core extending in the front-rear direction (a direction perpendicular to the paper surface of FIG. 1). A charger 132, an exposure device 133, a developing device 134, a transfer device 135, and a conveyor belt 136 are arranged in a clockwise direction from a position immediately above the photosensitive drum 131 so as to follow a peripheral surface of the photosensitive drum 131 provided in a possible manner. The cleaning device 137 and the static eliminator 138 are provided.

  The photosensitive drum 131 is for forming an electrostatic latent image and a toner image along the electrostatic latent image on the peripheral surface, and an amorphous silicon layer is laminated on the peripheral surface, whereby these images are displayed. It is suitable for forming.

  The charger 132 forms a uniform charge on the peripheral surface of the photosensitive drum 131 rotating in the clockwise direction around the drum core. In the example shown in FIG. A method of applying a charge to the peripheral surface is adopted. A charging roller that applies a charge while rotating the driven surface while contacting the peripheral surface of the photosensitive drum 131 is used instead of the charger 132 as a member that applies a charge to the peripheral surface of the photosensitive drum 131. Also good.

  The exposure device 133 irradiates the peripheral surface of the rotating photosensitive drum 131 with a laser beam to which intensity is applied based on image data transmitted from an external device such as a computer, and thereby the photosensitive drum 131 is irradiated with the laser beam. An electrostatic latent image is formed on the peripheral surface of the photosensitive drum 131 by erasing the charge on the peripheral surface irradiated with the laser beam.

  The developing device 134 supplies toner to the peripheral surface of the photosensitive drum 131 to attach the toner to the portion where the electrostatic latent image is formed on the peripheral surface. An image is formed.

  The transfer unit 135 applies a positively charged toner image formed on the peripheral surface of the photosensitive drum 131 to the paper P sent to a position immediately below the photosensitive drum 131 by driving the registration roller pair 124. A negative charge having a polarity opposite to that of the toner image is applied to the paper P.

  The conveyance belt 136 conveys the paper P, which has been subjected to the transfer process by the transfer unit 135, toward the fixing unit 14, and includes a driving roller 135a provided immediately downstream of the transfer unit 135, and a fixing unit. 14 and a driven roller 135b provided on the immediately upstream side. The conveyance belt 136 circulates in synchronization with the rotation of the photosensitive drum 131, so that the paper P is conveyed toward the fixing unit 14.

  Accordingly, the sheet P that has reached the position immediately below the photosensitive drum 131 is transferred to the surface of the sheet P on which the toner image on the circumferential surface of the photosensitive drum 131 that is positively charged by the transfer unit 135 and the photosensitive drum 131 is negatively charged. Then, after the sheet P is subjected to the transfer process, the sheet P is fed toward the fixing unit 14 around the conveyor belt 136.

  The cleaning device 137 is for removing the toner remaining on the peripheral surface of the photosensitive drum 131 after the transfer process and cleaning it. The static eliminator 138 erases the charge remaining on the peripheral surface of the photosensitive drum 131. The peripheral surface of the photosensitive drum 131 from which the charge has been erased by the static eliminator 138 is transferred to the next image. It goes to the charger 132 again for the forming process.

  The fixing unit 14 performs a fixing process by heating the toner image of the paper P that has been transferred by the image forming unit 13. The fixing unit 14 is provided with a fixing device 20 which will be described in detail later, and a paper discharge conveyance path 141 is provided on the downstream side of the fixing unit 14, and the fixed paper P passes through the paper discharge conveyance path 141. Are discharged to the paper discharge unit 15.

  The paper discharge unit 15 has a paper discharge tray 151 whose upper surface is formed in an arc shape, and the paper P discharged from the paper discharge port 152 through the paper discharge conveyance path 141 is the paper discharge tray 151. Will be accepted.

  FIG. 2 is an explanatory diagram of a front sectional view of the fixing device 20. As shown in FIG. 2, the fixing device 20 is arranged in a predetermined housing 21 so that the peripheral surface of the fixing roller 30 faces the peripheral surface of the fixing roller 30 below the fixing roller 30. The pressure roller 40 and the web cleaning means 50 for cleaning the peripheral surface of the fixing roller 30 are provided internally.

  Then, the sheet P after the transfer process includes a fixing roller 30 that is driven and rotated clockwise around the roller center, and a pressure roller 40 that is driven and rotated counterclockwise around the roller center. By passing through the fixing side nip portion N <b> 1 in between, the heat from the fixing roller 30 is obtained and the fixing process is performed, and the toner transferred to the peripheral surface of the fixing roller 30 is removed by the web cleaning unit 50. It has become.

  The fixing roller 30 includes an outer cylinder 31 formed by coating a peripheral surface of a metal cylinder with a predetermined material, and a halogen lamp or the like built in the outer cylinder 31. And a heat source 32.

  The outer cylindrical body 31 is concentrically fixed to one end portion and has an unillustrated annular gear, and driving rotation of a fixing roller motor 34 provided at an appropriate location outside the housing 21 is not illustrated. This is transmitted to the annular gear 331 via a mechanism, whereby the outer cylinder 31 is driven and rotated around the cylinder center.

  The pressure roller 40 includes a cylindrical pressure roller body 41 and a pressure roller shaft 42 that is concentric with the pressure roller body 41 and penetrates the end wall of the pressure roller body 41. Yes. The pressure roller 40 is driven by the driving rotation of the fixing roller 30 by being pressed against the peripheral surface of the outer cylindrical body 31 of the fixing roller 30 by an urging force of an urging means (not shown) in the fixing-side nip portion N1. And is designed to rotate.

  In this embodiment, a cleaning roller (pressure roller cleaning means) 43 whose peripheral surface is in pressure contact with the peripheral surface of the pressure roller main body 41 is provided at a lower position in the housing 21. The cleaning roller 43 is coated with a material suitable for wiping off toner on its peripheral surface, and is rotated around the axis center following the rotation of the pressure roller body 41 around the pressure roller shaft 42. The toner attached to the peripheral surface of the pressure roller body 41 is removed by the driven rotation of the cleaning roller 43 with respect to the pressure roller body 41.

  The web cleaning means 50 includes a pressing roller 51 that comes into contact with the peripheral surface of the outer cylindrical body 31 of the fixing roller 30 via a web 58 having a peripheral surface at a position directly above the fixing roller 30 in the housing 21. A web support cylinder 52 that supports a web roll 59 around which a web 58 is wound at an upper right position of the roller 51, and a web provided at the left position of the web support cylinder 52 via the pressing roller 51. A winding cylinder 53 for winding the web 58 drawn from the roll 59 and a web winding solenoid 54 for driving and rotating the winding cylinder 53 around the cylinder center are provided.

  The pressing roller 51 includes a pressing roller shaft 511 arranged in parallel with the heat roller core in the extending direction of the fixing roller 30, and a pressing roller main body 512 concentrically fitted to the pressing roller shaft 511. Yes. The pressing roller body 512 is formed of an elastic material such as rubber, and the pressing roller shaft 511 is compressed and elastically deformed by being urged toward the fixing roller 30 by the urging force of the urging means (not shown). A web-side nip portion N2 that nips the web 58 is formed in the cut portion.

  The web winding solenoid 54 includes a solenoid (not shown) that is energized by supplying current and demagnetized by interrupting current supply, and an operating piece (not shown) made of a magnetic material that operates by excitation and demagnetization of the solenoid. ing. A gear mechanism (not shown) is interposed between the operating piece and the winding cylinder 53. The forward / reverse movement of the operating piece due to the excitation / demagnetization of the solenoid is transmitted to the winding cylinder 53 via a crank mechanism, a gear mechanism, etc. (not shown), whereby the winding cylinder 53 is intermittently centered. It is designed to rotate counterclockwise.

  According to the web cleaning means 50, the winding cylinder 53 is rotated counterclockwise by driving the web winding solenoid 54 in a state where the fixing roller 30 is driven and rotated clockwise. The web 58 is intermittently pulled out sequentially from the web roll 59 mounted on the web support cylinder 52, and is sequentially wound around the winding cylinder 53 via the web-side nip portion N2.

  In this embodiment, each time 30 sheets P are fixed, the operation piece of the web take-up solenoid 54 reciprocates by one stroke, whereby the web 58 is pulled out from the web roll 59 on the average by about 1 mm. Accordingly, the contaminated portion of the web 58 is updated with a new one in the web-side nip portion N2.

  In this embodiment, every time 30 sheets P are fixed, the contaminated portion of the web 58 is updated to a new one in the web-side nip portion N2, as a result of many implementation tests. If the fixing device 20 performs a fixing process on the paper P with more than 30 sheets without updating the web-side nip portion N2 portion of the web 58, the portion of the web 58 is saturated by the toner removed from the peripheral surface of the fixing roller 30. If the fixing process is continued as it is, the toner wiped off by the web 58 and held on the web 58 moves to the peripheral surface of the fixing roller 30 conversely.

  When the toner on the web 58 moves to the peripheral surface of the fixing roller 30, a so-called offset phenomenon occurs in which the toner is transferred to the surface of the succeeding sheet P2 during the fixing process of the next sheet P (following sheet P2). Then, since the transfer failure in which the surface of the paper P is contaminated occurs, the portion of the web side nip portion N2 of the web 58 is updated with a new portion every 30 sheets of development processing in order to avoid this.

  In such a fixing device 20, on the upstream side wall (the right side wall in FIG. 2) of the casing 21, the sheet P transported by the transport belt 136 to the portion facing the fixing-side nip portion N <b> 1 is stored in the casing 21. An inlet 211 for introduction into the inside is provided, and a downstream side wall (the left side wall in FIG. 2) is used to lead out the sheet P that has been subjected to fixing processing to a portion facing the fixing side nip N1. An outlet 212 is provided.

  On the inlet 211 side in the housing 21, a guide plate 22 inclined upward is provided for guiding the paper P from the conveyance belt 136 to the fixing side nip portion N <b> 1, while on the outlet 212 side. A peeling claw member 23 having a peeling claw piece at the tip is provided. The upper peeling claw member 23 is for peeling the paper P to be wound around the circumferential surface of the fixing roller 30 by bringing the peeling claw into contact with the circumferential surface of the fixing roller 30. The lower peeling claw member 23 is for peeling the paper P to be wound around the circumferential surface of the pressure roller 40 by bringing the peeling claw into contact with the circumferential surface of the pressure roller 40.

  Further, on the downstream side of each peeling claw member 23 (left side in FIG. 2), a discharge roller pair 142 for directing the paper P after the fixing process to the discharge conveyance path 141 is provided. Between the roller pair 142 and the paper discharge conveyance path 141, the detection piece 711 rotates counterclockwise around the support shaft 712 against the biasing force of the biasing member (not shown) due to interference with the paper P. Accordingly, a limit switch type paper front end sensor 71 for detecting the front end of the fixed paper P is provided, and by the detection by the paper front end sensor 71, the number of paper P to be fixed in the fixing device 20 can be counted. It has become.

  According to the present invention, in the printer 10 having the fixing device 20 configured as described above, the fixing device 20 performs a fixing process on the last of a predetermined number of sheets P (30 sheets in the present embodiment). When applying, the following first to third measures are implemented, and even if the web 58 wiping portion (wiping surface) 581 is in the most dirty state, the fixing process is performed on the paper P that has been subjected to fixing processing at that time. This prevents the offset phenomenon from occurring.

  That is, firstly, the web 58 is moved by approximately 1 mm by driving one batch of the web winding solenoid 54 (one current supply to the solenoid of the web winding solenoid 54 and one current interruption). 58 wiping portions 581 (portions of the web 58 located at the web-side nip portion N2) are updated with a new 1 mm of the web 58. Incidentally, in FIG. 2, since the dimension of the web side nip N2 is exaggerated for convenience of illustration, the wiping part 581 of the web 58 is also exaggerated in the left-right direction.

  Next, as the second, the timing of updating the wiping portion 581 of the web 58 to a new 1 mm is being applied to a predetermined number of last sheets P (the 30th sheet P in the present embodiment). In a certain state, the peripheral surface (contaminated peripheral surface (non-wiping portion) 301) of the fixing roller 30 corresponding to the dirty wiping portion 581 is a portion on the upstream side of the fixing side nip portion N1 of the last paper P (undecided). The timing is controlled so as not to be applied to the arrival processing portion P ′).

  Finally, thirdly, the sheet P (the 31st sheet P in the present embodiment) conveyed next to the predetermined number of the last sheets P is not affected by the contaminated peripheral surface 301 of the fixing roller 30. The timing of feeding the next sheet (31st sheet P) to the fixing side nip portion N1 is controlled.

  By adopting such first to third measures, when the contaminating peripheral surface 301 of the fixing roller 30 reaches the fixing-side nip portion N1 by the rotation of the fixing roller 30, the predetermined number of the last sheets P are Since the paper P has already passed through the fixing side nip portion N1 and the next paper P has not yet reached the fixing side nip portion N1, the contaminated peripheral surface 301 has the preceding paper P and the following paper P. Between the sheet P and the sheet P (preceding sheet P1) and the subsequent sheet P (subsequent sheet P2), thereby reliably preventing the occurrence of the offset phenomenon. It can be done.

  Hereinafter, the first to third measures will be described in more detail with reference to FIGS. FIG. 3 is an explanatory diagram for explaining a positional relationship between a predetermined number of the last sheet P (preceding sheet P1) and the subsequent sheet P (following sheet P2) and the contaminating peripheral surface 301 of the fixing roller 30. It is. FIG. 4 is an explanatory diagram for explaining the state before and after the update of the wiping portion 581 of the web 58. FIG. 4A shows the state immediately before the wiping portion 581 of the web 58 is updated at the web-side nip portion N2. (B) shows the state where the contaminated peripheral surface 301 of the fixing roller 30 has reached the fixing side nip portion N1. Further, FIG. 5 is a schematic diagram linearly showing the length dimension of each part in the state of FIG.

First, as shown in FIG. 3, the most upstream portion of the web side nip portion N2 is the most upstream point N21, the most downstream portion is the most downstream point N22, and the most upstream portion of the fixing side nip portion N1 is the most upstream portion. After setting the most upstream point N11,
(A) Arc length A: Length of the circumferential surface of the fixing roller 30 reaching the most upstream point N11 in the clockwise direction starting from the most downstream point N22. (B) Arc length B: Clockwise starting from the most upstream point N21. Length of peripheral surface of fixing roller 30 reaching most upstream point N11 (c) Unfixed processing length C: Length of unfixed processing portion P ′ of sheet P being fixed at fixing-side nip portion N1 (d) Paper distance D: In the present invention, when defined as the distance between the most upstream point N11 during the fixing process and the front edge of the next paper P to be conveyed,
C <A (i)
D> B (ii)
The wiping unit 581 of the web 58 is updated so as to satisfy these conditions, and control for setting the conveyance timing of the succeeding paper P2 is performed. In this control, it is assumed that the peripheral speed of the fixing roller 30 and the transport speed of the paper P by the transport roller 136 are set to be the same.

  First, satisfying the expression (i) means that, as shown in FIG. 5, the unfixed processing is the length dimension of the unfixed processing portion P ′ of the sheet P being fixed at the fixing-side nip portion N1. Since the length C is shorter than the arc length A which is the length dimension of the circumferential surface of the fixing roller 30 starting from the most downstream point N22 and reaching the most upstream point N11 in the clockwise direction, FIG. When the contaminated peripheral surface 301 on the peripheral surface of the fixing roller 30 reaches the fixing side nip portion N1, as shown in FIG. 4B, the downstream end of the sheet P on which the fixing process has been performed is already at the fixing side nip. Therefore, the contaminated peripheral surface 301 does not come into contact with the paper P, that is, the offset phenomenon does not occur on the paper P.

  Further, the fact that the expression (ii) is satisfied means that the inter-sheet distance D, which is the distance between the most upstream point N11 during the fixing process and the front end of the sheet P to be conveyed next, is the most upstream point. Since it is longer than the arc length B, which is the length dimension of the circumferential surface of the fixing roller 30, starting from N 21 and reaching the most upstream point N 11 in the clockwise direction, the contaminated circumference of the fixing roller 30 shown in FIG. When the portion of the surface 301 corresponding to the most upstream point N21 reaches the most upstream point N11 of the fixing side nip portion N1, as shown in FIG. 4B, the front end of the succeeding sheet P2 is still the fixing side nip portion. N1 has not been reached, and therefore, an offset phenomenon is prevented from occurring in the succeeding paper P2.

  According to FIG. 5 in which the arc length A, the arc length B, the unfixed processing length C, and the inter-sheet distance D are linearly extended and compared in the state shown in FIG. I can understand.

  Next, such timing control will be described with reference to FIG. FIG. 6 is a block diagram showing an embodiment of timing control that does not cause an offset phenomenon in the paper P by the control means 60. As shown in FIG. 6, the control means 60 includes a central processing unit (CPU) 61 as a central processing unit, a read only memory (ROM) 62 that is a read-only storage device attached to the CPU 61, and a CPU 61. And a RAM (Random Access Memory) 63 configured to be readable and writable for various types of data generated temporarily.

  The CPU 61 determines the update timing of the wiping unit 581 of the web 58 for the predetermined number of last sheets P (preceding paper P1) on which the fixing process is performed (wiping surface update timing determination unit). ) 611, a sheet conveyance timing determination unit 612 for determining the conveyance timing of the subsequent sheet P2 conveyed toward the fixing-side nip portion N1 next to the preceding sheet P1, the wiping unit update timing determination unit 611, and the sheet conveyance A control signal output unit 613 that outputs a control signal to each predetermined device based on the determination result by the timing determination unit 612 is provided.

  The wiping unit update timing discriminating unit 611 is the last one of the predetermined number of sheets P that are currently undergoing the fixing process (30 sheets in the present embodiment, and 30 sheets will be used as an example of the predetermined number). It is determined whether or not it is the first one. For this purpose, a limit switch type paper rear end sensor 72 (FIG. 4) for detecting the rear end of the paper P is provided immediately upstream of the registration roller pair 124 in the paper feed conveyance path 123. Each time the rear end of P is detected, the detection signal is input to the wiping unit update timing determination unit 611.

  Incidentally, the sheet trailing edge sensor 72 detects the detection piece after the detection piece 721 once rotates counterclockwise around the support shaft 722 against the urging force of the urging member (not shown) due to interference with the paper P. When the 721 is returned to the original by the urging force of the urging means, the rear end of the sheet P that goes to the image forming unit 13 via the registration roller pair 124 is detected.

  The wiping unit update timing discriminating unit 611 counts the detection signal from the paper trailing edge sensor 72 every time it is input. When the integrated value reaches “30”, the paper P is used for the fixing process. In this case, it is determined that the wiping portion 581 of the web 58 should be updated.

  Further, the wiping unit update timing determination unit 611 wipes the web 58 based on the detection signal from the sheet front end sensor 71 (FIG. 1) in a state where the 30th sheet P is subjected to fixing processing by the fixing device 20. The timing at which the part 581 should be updated is discriminated. This determination is made immediately after the sheet front end sensor 71 detects the front end of the sheet P and immediately after the unfixed processing length C (FIG. 4A) of the sheet P becomes shorter than the arc length A of the fixing roller 30. Is done. At this point in time, the arc length A, which is the length of the circumferential surface from the most downstream point N22 of the web side nip portion N2 in the rotation direction of the fixing roller 30 to the most upstream point N11 of the fixing side nip portion N1, is known. It can be calculated from the conveyance speed of the paper P. Note that the detection signal of the sheet front end sensor 71 is also input to the sheet conveyance timing determination unit 612.

  The wiping unit update timing discriminating unit 611 detects the web 58 when the time required for the unfixed processing length C to become shorter than the arc length A elapses after the paper front end sensor 71 detects the front end of the paper P. A command signal for immediately updating the wiping unit 581 is output to the control signal output unit 613. In order to measure the passage of time, the RAM 63 is provided with a timer 631, and the wiping unit update timing determination unit 611 determines a time point at which the wiping unit 581 should be updated based on the time measured by the timer 631. The determination by the wiping unit update timing determination unit 611 can also be performed using a signal from the sheet trailing edge sensor 72 or a rotation start signal from the registration roller pair 124.

  When the processed number of sheets P exceeds 30 sheets and becomes a new first sheet, the sheet conveyance timing determination unit 612 fixes the nip on the fixing side of the new first sheet P (the following sheet P2). The conveyance timing toward the portion N1 is determined such that the inter-paper distance D ((A) in FIG. 4) is larger than the arc length B of the fixing roller 30. This conveyance timing is determined based on the balance with the determination point of the wiping unit 581 update of the wiping unit update timing determination unit 611.

  That is, “D> B” from the arc length B that is the circumferential length from the most upstream point N21 of the web side nip N2 in the fixing roller 30 to the most upstream point N11 of the fixing side nip N1 in the rotation direction of the fixing roller 30. A satisfactory front end position (reference position X) of the succeeding paper P2 is set in advance, and the wiping unit update timing determination unit 611 of the wiping unit 581 is set even though the front end of the subsequent paper P2 has reached the reference position X. When the update is not determined (that is, when the determination result from the wiping unit update timing determination unit 611 is not input to the sheet transfer timing determination unit 612), the sheet transfer timing determination unit 612 is configured to fix the subsequent sheet P2 on the fixing side. It is determined that the conveyance toward the nip portion N1 is temporarily suspended until the determination result from the wiping unit update timing determination unit 611 is input.When a determination result is input from the wiping unit update timing determination unit 611, the sheet conveyance timing determination unit 612 determines whether to resume the sheet conveyance.

  On the other hand, if the wiping unit update timing determination unit 611 has already determined whether the wiping unit 581 has been updated when the subsequent sheet P2 reaches the reference position X, the subsequent sheet P2 is directly transferred to the fixing-side nip N1. It is determined that it is transported toward the vehicle.

  Incidentally, when the succeeding sheet P2 reaches the reference position X, the sheet trailing edge sensor 72 is used as a sensor for detecting the leading edge of the succeeding sheet P2, and after the sheet trailing edge sensor 72 detects the leading edge of the next sheet. Can be determined by measuring the elapsed time of time by the timer 631.

  The control signal output unit 613 immediately outputs a control signal toward the web take-up solenoid 54 when the command signal based on the determination result from the wiping unit update timing determination unit 611 is input, and also determines the sheet conveyance timing. When a command signal based on the determination result from the unit 612 is input, a control signal is output toward the registration roller pair 124 according to the type of the command signal.

  Then, the web take-up solenoid 54 that has received the control signal from the control signal output unit 613 is driven to rotate the take-up cylinder 53 by a predetermined amount, so that the wiping portion 581 of the contaminated web 58 is renewed. Will be updated. When the contamination peripheral surface 301 of the fixing roller 30 reaches the fixing-side nip portion N1 by this update, the rear end of the 30th sheet P is already out of the fixing-side nip portion N1. The phenomenon does not occur.

  Further, the registration roller pair 124 stops driving when a control signal from the paper conveyance timing determination unit 612 is input, whereby the conveyance of the paper P to the transfer unit 13 is temporarily stopped, and the wiping unit 581 is activated. The transfer process for the paper P is temporarily interrupted until there is no interference with the succeeding paper P2, so that the succeeding paper P2 waits with its front end positioned at the pair of dyst rollers 124. In this state, when the paper conveyance timing determination unit 612 determines that the conveyance of the paper P should be resumed, the control signal output unit 613 controls the resumption of driving toward the registration roller pair 124 based on the determination result. In order to output the signal, the registration roller pair 124 resumes driving, whereby the transport of the paper P that has been temporarily stopped is resumed, and after the transfer process is performed by the image forming unit 13, the registration roller pair 124 proceeds to the fixing-side nip N 1. It will be.

  When the subsequent paper P2 that has been temporarily stopped reaches the fixing side nip portion N1, the contaminated peripheral surface 301 of the fixing roller 30 is already out of the fixing side nip portion N1, and therefore the subsequent paper P2 is not contaminated by the offset phenomenon.

  If the control signal output unit 613 has already output a control signal toward the web take-up solenoid 54 when the front end of the succeeding paper P2 reaches the reference position X, the condition “D> B” is satisfied. Is already satisfied, the control signal is not output from the control signal output unit 613 toward the registration roller pair 124. Therefore, the succeeding sheet P2 remains on the fixing side without being temporarily stopped at the reference position X. It is fed toward the nip portion N1.

  Hereinafter, the control flow of the control means 60 configured as described above will be described with reference to FIG. FIG. 7 is a flowchart showing a flow of control by the control means 60. When the control starts, first, in step S1, it is determined whether or not the fixing process for the sheet P in the fixing-side nip portion N1 of the fixing device 20 is the 30th sheet. If the fixing process is the 30th sheet P (YES in step S1), the unfixed portion P ′ of the sheet P (preceding sheet P1) that has been subjected to the fixing process at the fixing side nip portion N1. Whether the unfixed processing length C, which is the length, is shorter than the arc length A, which is the length of the circumferential surface of the fixing roller 30, starting from the most downstream point N22 and reaching the most upstream point N11 in the clockwise direction, is updated. When it is determined by the timing determination unit 611 and becomes shorter (YES in step S2), a control signal is output from the control signal output unit 613 to the web take-up solenoid 54, thereby driving the web take-up solenoid 54. Thus, the wiping portion 581 of the web 58 is updated with a new portion (step S3).

  Next, the inter-sheet distance D, which is the distance between the most upstream point N11 and the front end of the succeeding sheet P2, during the fixing process for the 30th new sheet P (following sheet P2), is the most upstream point. Whether or not it is longer than the arc length B, which is the length of the circumferential surface of the fixing roller 30 reaching the most upstream point N11 in the clockwise direction starting from N21, is determined by the sheet conveyance timing determination unit 612, and if not longer (in step S4) NO), a control signal is output from the control signal output unit 613 toward the registration roller pair 124, and the registration paper pair 124 is temporarily stopped by this, and the leading edge of the succeeding paper P2 is positioned in the registration roller pair 124. At standby.

  Whether or not the image forming process is completed (specifically, the power switch of the printer 10) when the condition “D> B” is satisfied by the progress of the fixing process of the preceding sheet P1 (YES in step S4). Whether or not has been turned off) is determined, and if not completed (NO in step S6), the process returns to step S1, whereas if the process is completed (YES in step S6), the control is terminated.

  As described above in detail, the fixing device 20 according to the present invention includes the fixing roller 30 that heats the sheet P having the toner image on the surface thereof from the peripheral surface by the transfer process conveyed from the image forming unit 13 while rotating the sheet P. The pressure roller 40 that rotates synchronously with the fixing roller 30 in a state where the peripheral surface presses the peripheral surface of the fixing roller 30, and the toner attached to the peripheral surface of the fixing roller 30 is transferred to the web 58 and the fixing roller 30 by the web 58. A web-side nip portion N2 formed between the fixing roller 30 and the pressure roller 40. The fixing-side nip portion is formed between the fixing roller 30 and the pressure roller 40. By feeding the paper P toward N1, the fixing process is performed on the paper P.

  The web cleaning unit 50 is configured to update the wiping unit 581 of the web 58 for each fixing process of the predetermined number of sheets P, and the predetermined number of sheets corresponding to the wiping unit 581 immediately before the web 58 is updated. The length of the unfixed portion P ′ of the sheet P that has not reached the fixing side nip portion N1 in a state where the eye sheet P (preceding sheet P1) is passing through the fixing side nip portion N1 (unfixed processing) When the length C) becomes shorter than the circumferential length (arc length A) between the web-side nip portion N2 and the fixing-side nip portion N1 in the rotation direction of the fixing roller 30, the wiping portion 581 of the web 58 is updated. Then, the sheet P to be subjected to the fixing process is conveyed toward the fixing side nip portion N1 at a timing when the front end of the sheet P is removed from the peripheral surface of the fixing roller 30 wiped by the wiping unit 581 immediately before the update. That.

  According to this configuration, in the state where the preceding paper P1 corresponding to the wiping portion 581 of the web 58 immediately before being updated is passing through the fixing side nip portion N1, the paper P that has not reached the fixing side nip portion N1. The length dimension (unfixed processing length C) of the unfixed processing portion P ′ is determined from the circumferential length (arc length A) between the web-side nip portion N2 and the fixing-side nip portion N1 in the rotation direction of the fixing roller 30. Since the wiping portion 581 of the web 58 is updated when it becomes shorter, the contaminated peripheral surface 301 that has been wiped by the dirty wiping portion 581 immediately before the update on the peripheral surface of the fixing roller 30 is highly likely not to be wiped. The sheet P faces the peripheral surface of the pressure roller 40 after passing through the fixing side nip portion N1, and the sheet P does not come into contact with the non-wiping portion. Is part The occurrence of the offset phenomenon with respect to the fixing processing portion P 'can be reliably prevented.

  Further, the subsequent sheet P2 to be subjected to the fixing process next is conveyed toward the fixing-side nip portion N1 at a timing when the front end of the succeeding sheet P2 is removed from the peripheral surface of the fixing roller 30 wiped by the wiping unit 581 immediately before the update. Therefore, when the subsequent sheet P2 reaches the fixing side nip portion N1, the non-wiping portion of the fixing roller 30 has already passed through the fixing side nip portion N1, and therefore, an offset phenomenon for the subsequent sheet P2 occurs. Occurrence can be reliably prevented.

  As described above, the contaminated peripheral surface 301 of the fixing roller 30 is the same as the peripheral surface of the pressure roller 40 after the sheet P that is actually being subjected to the fixing process in the fixing-side nip portion N1 passes through the fixing-side nip portion N1. Oppositely, the sheet P to be fed next reaches the fixing-side nip N1 after the non-wiping portion passes the fixing-side nip N1, so that the contaminated peripheral surface 301 comes into contact with the sheet P. Therefore, no offset phenomenon occurs on any paper P.

  In addition, such a workaround for the offset phenomenon is devised to use the existing components of the fixing device without adding new members to prevent the offset phenomenon. It does not increase the number of man-hours, and the increase in manufacturing cost can be suppressed accordingly.

  In this embodiment, the control means 60 for controlling the update timing of the wiping unit 581 of the web 58 and the feeding timing of the succeeding paper P2 to the fixing side nip N1 is the web immediately before the wiping unit 581 is updated. 58, a wiping unit update timing discriminating unit 611 for discriminating the update timing of the wiping unit 581 of the web 58 based on the detection result of the paper front end sensor 71 for the preceding paper P1 corresponding to 58, and the post paper for the subsequent paper P2 Based on the determination result of the sheet conveyance timing determination unit 612 that determines the timing at which the subsequent sheet P2 is directed to the fixing side nip portion N1 based on the detection result of the end sensor 72, and the web cleaning unit 50 based on the determination result of the wiping unit update timing determination unit 611. Output a control signal to update the wiping portion 581 of the web 58, And a control signal output unit 613 for outputting a control signal to convey the succeeding sheet P2 to the fixing-side nip portion N1 to the registration roller pair 124 based on the determination result of the ring determination unit 612.

  According to such a configuration, first, the leading edge of the preceding sheet P1 corresponding to the web 58 immediately before the wiping portion 581 is updated is detected by the sheet leading edge sensor 71 in the state where the leading sheet P1 is positioned at the fixing side nip N1, and the detection result. Is input to the wiping unit update timing determination unit 611. The wiping unit update timing determination unit 611 to which the detection result is input determines the update timing of the wiping unit 581 of the web 58 and transmits the determination result to the control signal output unit 613. The control signal output unit 613 outputs a control signal for updating the wiping unit 581 of the web 58 toward the web cleaning unit 50 based on the determination result, so that the dirty wiping unit 581 before updating is used. The contaminated peripheral surface 301 on the peripheral surface of the fixing roller 30 that has been wiped off does not come into contact with the precursor paper P, and this can reliably prevent the offset phenomenon that occurs when the contaminated peripheral surface 301 comes into contact with the preceding paper P. .

  Further, the trailing edge of the image forming unit 13 of the succeeding sheet P2 to be subjected to the fixing process next to the preceding sheet P1 is detected by the sheet trailing end sensor 72, and the detection result is input to the sheet transport timing determining unit 612. Then, the sheet conveyance timing discriminating unit 612 to which this detection result is input is a timing at which the subsequent sheet P2 is directed toward the fixing-side nip N1 (that is, a timing at which the subsequent sheet P2 is not applied to the non-wiping unit of the fixing roller 30). ) And the determination result is transmitted to the control signal output unit 613.

  The control signal output unit 613 outputs a control signal for conveying the succeeding sheet P2 toward the fixing side nip portion N1 toward the image forming unit 13 based on the determination result. The contamination peripheral surface 301 of the 30 peripheral surface reaches the fixing side nip portion N1 in a state of passing through the fixing side nip portion N1. Accordingly, since the subsequent sheet P2 is prevented from coming into contact with the contaminated peripheral surface 301 of the fixing roller 30, an offset phenomenon due to the contaminated peripheral surface 301 coming into contact with the subsequent sheet P2 can be reliably prevented.

  Thus, the wiping unit update timing determination unit 611 for determining the update timing of the wiping unit 581 of the web 58 based on the detection result of the sheet front end sensor 71 for the preceding sheet P, and the subsequent sheet P2 as the target. Based on the detection result of the sheet trailing edge sensor 72, a sheet conveyance timing determination unit 612 that determines the timing at which the subsequent sheet P2 is directed to the fixing-side nip N1, and a wiping unit update timing determination unit 611 and a sheet conveyance timing determination unit 612 By providing a control signal output unit 613 that outputs a control signal toward the web cleaning unit 50 and the registration roller pair 124 based on the determination result, a new member or mechanism for preventing the offset phenomenon in the fixing device is provided. Fixing process without increasing production cost without adding It is possible to prevent the offset phenomenon reliably generated.

  In addition, in this embodiment, since the cleaning roller 43 is provided as a pressure roller cleaning means for cleaning the peripheral surface of the pressure roller 40, the sheet P does not exist in the fixing-side nip portion N1. Thus, the toner transferred from the peripheral surface of the fixing roller 30 to the peripheral surface of the pressure roller 40 by the peripheral surfaces abutting each other in the fixing-side nip portion N1 is removed by the cleaning roller 43, and thereby the back surface of the paper P It is possible to prevent the side from being contaminated by the toner present on the peripheral surface of the pressure roller 40.

  The present invention is not limited to the above embodiment, and includes the following contents.

  (1) In the above embodiment, the wiping unit 581 of the web 58 is updated every time the fixing process is performed on 30 sheets of paper P. However, the present invention updates the wiping unit 581 every 30 sheets of paper. It is not limited to this, and can be appropriately set according to various conditions such as the size of the paper P and the type of the web 58.

  (2) In the above-described embodiment, the reference position X is set corresponding to the paper P of a predetermined size (for example, A4 size). The reference position X is variable according to the size of the paper P. It may be.

  (3) In the above embodiment, the web take-up solenoid 54 is adopted to take up the web 58. However, the present invention is limited to adopting the web take-up solenoid 54 for taking up the web 58. Instead of the web winding solenoid 54, for example, a stepping motor or the like capable of precisely controlling the rotation angle may be employed.

  (4) In the above embodiment, the sheet front end sensor detects the timing at which the unfixed processing portion P ′ of the preceding sheet P1 reaches the preset unfixed processing length C in order to detect the update timing of the wiping unit 581 of the web 58. However, the present invention is limited to detecting the timing at which the unfixed processing portion P ′ of the preceding sheet P1 reaches the preset unfixed processing length C by the sheet front end sensor 71. Instead of this, it may be detected by the paper trailing edge sensor 72. This can be done because the conveyance speed of the paper P is constant, so that the unfixed processing portion P ′ is preset by the time measured after the paper trailing edge sensor 72 detects the preceding paper P1. This is because the time point when the length C is reached can be detected.

  (5) In the above embodiment, when the wiping portion 581 of the web 58 is updated, the condition that the distance between the sheets D> the arc length B is satisfied because the conveyance pitch of the sequentially conveyed sheets P is too short. If it is not possible, the registration roller pair 124 is stopped until the above condition is satisfied in a state where the front end of the succeeding paper P2 is set at the reference position X. Instead of this, “D> B” in advance. Alternatively, the paper P may be transported at a transport pitch that satisfies the above. By doing so, it is not necessary to cause the paper conveyance timing determination unit 612 to determine whether or not the registration roller pair 124 is stopped, and the control by the control unit 60 can be simplified accordingly.

  When such control is employed, the block diagram of the control by the control means 60 is obtained by erasing the sheet conveyance timing determination unit 612 and the registration roller pair 124 in the block diagram shown in FIG. Further, the control flowchart by the control means 60 is obtained by deleting steps S4 and S5 from the flowchart shown in FIG.

FIG. 3 is a front sectional view illustrating the outline of the internal structure of the printer to which the fixing device according to the invention is applied. FIG. 3 is an explanatory diagram of a front sectional view of the fixing device. FIG. 10 is an explanatory diagram for explaining a positional relationship between a predetermined number of last sheets (preceding sheets) and subsequent sheets (following sheets) and a contaminated peripheral surface of the fixing roller. FIG. 4 is an explanatory diagram for explaining the positional relationship between a predetermined number of last sheets (preceding sheets) and subsequent sheets (following sheets) and a contaminated peripheral surface of the fixing roller; A state immediately before the wiping portion of the web is updated in the nip portion, (B) shows a state in which the contaminating peripheral surface of the fixing roller has reached the fixing-side nip portion. It is the schematic diagram which showed linearly the length dimension of each part in the state of (c) of FIG. It is a block diagram showing an embodiment of timing control that does not cause an offset phenomenon in the paper P by the control means. It is a flowchart which shows the flow of the timing control by a control means.

Explanation of symbols

10 Printer (image forming device)
DESCRIPTION OF SYMBOLS 11 Apparatus main body 12 Paper storage part 121 Paper cassette 122 Pickup roller 123 Paper feed conveyance path 124 Registration roller pair 13 Image formation part 131 Photosensitive drum 132 Charger 133 Exposure apparatus 134 Development apparatus 135 Transfer device 136 Carrying roller 136a Drive roller 136b Driven Roller 137 Cleaning device 138 Static eliminator 14 Fixing unit 141 Discharge transport path 142 Discharge roller pair 15 Discharge unit 151 Discharge tray 20 Fixing device 21 Housing 211 Inlet 212 Exit 22 Guide plate 23 Peeling member 30 Fixing roller 301 Contamination circumference Surface (non-wiping part)
31 outer cylinder 32 heat source 34 fixing roller motor 40 pressure roller 41 pressure roller body 42 pressure roller shaft 43 cleaning roller (pressure roller cleaning means)
DESCRIPTION OF SYMBOLS 50 Web cleaning means 51 Press roller 511 Press roller axis | shaft 512 Press roller main body 52 Web support cylinder 53 Winding cylinder 54 Web winding solenoid 58 Web 581 Wiping part (wiping surface) 59 Web roll 60 Control means 61 CPU
611 Wiping unit update timing determination unit 612 Paper transport timing determination unit 613 Control signal output unit 62 ROM
63 RAM 631 Timer 71 Paper front end sensor 711 Detection piece 712 Support shaft A Circular arc length (the length of the circumferential surface of the fixing roller 30 from the most downstream point N22 to the most upstream point N11 in the clockwise direction)
B Arc length (the length of the circumferential surface of the fixing roller 30 from the most upstream point N21 to the most upstream point N11 in the clockwise direction)
C Unfixed processing length (length of unfixed processing portion P ′ of paper P being fixed at fixing side nip N1)
D Distance between sheets (distance between the most upstream point N11 during the fixing process and the front end of the sheet P to be conveyed next)
N1 Fixing side nip N2 Web side nip P Paper P1 Preceding paper P2 Subsequent paper

Claims (5)

In a state where a fixing roller that rotates in a predetermined direction and heats the paper on which the toner image is transferred in the image forming unit on its peripheral surface, and the fixing side nip portion is formed by the peripheral surface facing the peripheral surface of the fixing roller. A pressure roller that rotates in synchronization with the fixing roller; and a web cleaning unit that wipes off toner adhering to the peripheral surface of the fixing roller at a web-side nip formed between the web and the fixing roller, The paper cleaning unit is configured to perform fixing processing on the sheet by being conveyed toward the fixing side nip portion, and the web cleaning unit is configured to update the wiping surface of the web every time a predetermined number of sheets are fixed. In the fixing device
In a state where a predetermined number of sheets immediately before the web is updated is passing through the fixing side nip portion, the length dimension of the unfixed processing portion of the sheet not reaching the fixing side nip portion is fixed. A fixing device configured to renew a wiping surface of a web when it becomes shorter than a circumferential length between a web side nip portion and a fixing side nip portion in a rotation direction of a roller. Control means for controlling the timing of updating the wiping surface of the web and the timing of feeding the next sheet to the fixing side nip, and the rear end of the sheet conveyed toward the fixing nip as the image forming unit And a paper sensor for detecting at a predetermined position,
The control means determines the update timing of the wiping surface of the web based on the detection result of the paper sensor for the predetermined number of sheets corresponding to the web immediately before the wiping surface is updated. And
2. A control signal output unit that outputs a control signal to update the wiping surface of the web toward the web cleaning unit based on a determination result of the wiping surface update timing determination unit. Fixing device. In a state where a fixing roller that rotates in a predetermined direction and heats the paper on which the toner image is transferred in the image forming unit on its peripheral surface, and the fixing side nip portion is formed by the peripheral surface facing the peripheral surface of the fixing roller. A pressure roller that rotates in synchronization with the fixing roller; and a web cleaning unit that wipes off toner adhering to the peripheral surface of the fixing roller at a web-side nip formed between the web and the fixing roller, The paper cleaning unit is configured to perform fixing processing on the sheet by being conveyed toward the fixing side nip portion, and the web cleaning unit is configured to update the wiping surface of the web every time a predetermined number of sheets are fixed. In the fixing device
In a state where a predetermined number of sheets immediately before the web is updated is passing through the fixing side nip portion, the length dimension of the unfixed processing portion of the sheet not reaching the fixing side nip portion is fixed. The web wiping surface is updated when it becomes shorter than the circumferential length between the web side nip portion and the fixing side nip portion in the rotation direction of the roller, and the front end of the sheet to be subjected to the fixing process is immediately before the update. A fixing device configured to convey toward a fixing-side nip portion at a timing at which the fixing roller is wiped off from the peripheral surface of the fixing roller. Control means for controlling the update timing of the wiping surface of the web and the timing of feeding the next sheet to the fixing side nip portion, and the rear end of the sheet conveyed toward the fixing side nip portion as the image forming portion And a paper sensor for detecting at a predetermined position,
The control means determines the update timing of the wiping surface of the web based on the detection result of the paper sensor for the predetermined number of sheets corresponding to the web immediately before the wiping surface is updated. And
A sheet conveyance timing determination unit for determining a timing of moving the next sheet to the fixing side nip portion based on a detection result of the sheet sensor for the next sheet of the predetermined number of sheets;
A control signal to update the wiping surface of the web is output to the web cleaning unit based on the determination result of the wiping surface update timing determination unit, and the image forming unit is output to the image forming unit based on the determination result of the paper transport timing determination unit. The fixing device according to claim 3, further comprising a control signal output unit configured to convey the next sheet toward the fixing side nip portion.   The fixing device according to claim 1, further comprising a pressure roller cleaning unit that cleans a peripheral surface of the pressure roller.