JP2016018128A - Image forming apparatus and cancelling method - Google Patents

Abstract

An image forming apparatus capable of reducing the number of executions of pressure release of a fixing device is provided.
A control unit 200 controls whether or not the pressure release mechanism releases the pressure applied to the fixing nip according to the length of the portion of the sheet P that is sandwiched by the fixing nip of the fixing device 130. Therefore, the number of executions of pressure release of the fixing device 130 is reduced as compared with the conventional case.
[Selection] Figure 4

Description

  The present invention relates to an image forming apparatus provided with a fixing device.

  According to Patent Document 1, an image forming apparatus that weakens the pressure applied to a fixing device when a sheet jam is detected (hereinafter referred to as pressure release) has been proposed. The reason for releasing the pressure is to prevent damage to the fixing device when removing the sheet held between the fixing devices, or to facilitate the user to pull out the sheet.

JP 2010-139732 A

  The fixing device includes a heater folder storing a heater, a cylindrical fixing film disposed so as to wrap the heater folder, a pressure roller that forms a fixing nip together with the fixing film, and presses the heater folder against the pressure roller. It consists of a pressure spring and the like. The pressure release is realized by rotating the cam and pushing up the pressure release plate to push back the pressure spring and weaken the pressure applied to the heater folder by the pressure spring. When the image forming apparatus always detects a jam, if the pressure release of the fixing device is always released, the pressure release plate that pushes back the pressure spring, the cam that pushes up the pressure release plate, the gear that drives the cam, and the like are quickly worn out. This is because cams, gears, and the like are rotationally driven against the pressure applied by the pressurizing spring, so that a mechanical load is applied to each time the pressure is released. SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of reducing the number of times pressure is released from a fixing device.

The present invention is, for example,
Image forming means for forming a toner image on a sheet;
Fixing means for pressurizing and fixing an unfixed toner image formed on the sheet by the image forming means on the sheet;
Release means for releasing the pressure applied to the fixing nip of the fixing means;
Control means for controlling whether or not the release means releases the pressure applied to the fixing nip according to the length of the portion of the sheet that is sandwiched by the fixing nip of the fixing means. An image forming apparatus is provided.

  According to the present invention, it is controlled whether or not the pressure is released according to the length of the portion of the sheet that is sandwiched between the fixing nips. .

FIG. 3 is a cross-sectional view illustrating an example of an image forming apparatus. FIG. FIG. 5 is a diagram illustrating a fixing device as viewed from the downstream side to the upstream side in the sheet conveyance direction. The block diagram of a control system. FIG. 4 is a diagram illustrating a positional relationship between a fixing nip and a sheet. 7 is a flowchart showing a method for releasing the pressure applied to the fixing nip. 7 is a flowchart showing a method for releasing the pressure applied to the fixing nip.

  The image forming apparatus 100 will be described with reference to FIG. The photosensitive drum 122 is an image carrier including an organic photosensitive member or an amorphous silicon photosensitive member, and rotates clockwise at a predetermined peripheral speed (process speed). The charging roller 123 is a charging unit, and uniformly charges the peripheral surface of the photosensitive drum 122 with a predetermined polarity and a predetermined potential. The laser optical box 108 irradiates a charged surface of the photosensitive drum 122 with laser light corresponding to image information to form an electrostatic latent image. The developing roller 121 develops the electrostatic latent image using a developer (a developer material) such as toner to form a toner image. The toner image may be primarily transferred to the intermediate transfer member and then secondarily transferred to the sheet P. These members function as image forming means for forming a toner image on the sheet.

  The paper feed roller 102 feeds the sheets P from the paper feed cassette to the transport path one by one. Conveying rollers 103 and registration rollers 104 arranged in the conveying path convey the sheet P further downstream in the conveying direction. The transfer roller 106 supplies a charge having a polarity opposite to that of the toner from the back surface of the sheet P, whereby the toner image on the photosensitive drum 122 is transferred to the sheet P. The sheet P to which the unfixed toner image is transferred is separated from the photosensitive drum 122 and sent to the fixing device 130. The fixing unit 130 is a unit that holds an unfixed toner image formed on a sheet together with the sheet at a fixing nip and presses the toner image on the sheet by pressing. In the fixing device 130, the heat generated by the heater 132 is transmitted to the sheet P and the toner image through the fixing film 133, and the toner image is melted and fixed to the sheet P. The fixing film 133 is pressed against the pressure roller 134 by a pressure mechanism, and a fixing nip is formed between the fixing film 133 and the pressure roller 134, and the sheet P passes through the fixing nip. . That is, heat and pressure are applied to the sheet P and the toner image. Thereafter, the sheet P is conveyed by the FU roller 110 and the FD roller 111 and discharged to the FD tray 113. The fixing film 133 is an example of a fixing member, and the pressure roller 134 is an example of a pressure member.

  The image forming apparatus 100 is provided with a door 112 for removing a jammed sheet. The door sensor 101 is a sensor that detects whether the door 112 is open or closed. A plurality of sheet sensors such as a top sensor 105 and a paper discharge sensor 109 are arranged on the conveyance path, and detect arrival of the leading edge of the sheet P and passage of the trailing edge of the sheet P. For example, if the trailing edge of the sheet P does not pass even after a predetermined time has elapsed after the leading edge of the sheet P has passed, it is determined that a jam has occurred in the sheet P.

  FIG. 2a is a schematic cross-sectional view of the fixing device 130 in a pressurized state. FIG. 2B is a schematic cross-sectional view of the fixing device 130 in a pressure release state. FIG. 3 is a diagram illustrating the fixing device 130 as viewed from the downstream side (left side in FIG. 2A) to the upstream side (right side in FIG. 2A) in the conveyance direction of the sheet P. The fixing device side plate 142 is disposed on both side surfaces of the fixing device 130 and holds the components of the fixing device 130. As shown in FIG. 3, the pressure plate 138 is arranged and fixed so as to bridge between the two fixing device side plates 142. The pressure spring 135, the pressure release plate 136, the fulcrum member 141, and the pressure release cam 137 are disposed on both side surfaces of the fixing device 130. That is, there are two of each of these parts.

  The folder stay 139 is disposed so as to penetrate through the fixing film 133. Both ends of the folder stay 139 are in contact with the pressure release plate 136. The pressure springs 135 disposed at both ends of the folder stay 139 urge the folder stay 139 through the pressure release plate 136 in the direction of the arrow F1. That is, the folder stay 139 transmits the pressure applied by the pressure spring 135 to the heater folder 140 via the pressure release plate 136. Thus, the pressure spring 135, the folder stay 139, etc. constitute a pressure mechanism. The heater folder 140 is a holding member for holding the heater 132. The heater folder 140 presses the heater 132 and the fixing film 133 against the pressure roller 134 by the applied pressure transmitted from the folder stay 139. That is, the folder stay 139, the heater folder 140, and the heater 132 are support members that are in contact with part of the inner surface of the cylindrical fixing film 133. The fixing film 133 is pressed against the supporting member by a pressure spring 135 that is an elastic body, so that a part of the outer surface of the fixing film 133 contacts the pressure roller 134 that is a pressure member to form a fixing nip. ing. Thus, the urging force (elastic force) of the pressure spring 135 acts as a pressing force and acts on the fixing nip.

  As shown in FIG. 3, the roller shaft 131 of the pressure roller 134 is fixed by two fixing device side plates 142 and receives pressure applied from the heater folder 140. In addition, the pressure roller 134 rotates together with the fixing film 133 and conveys the sheet P while nipping the gear 145 fixed to the roller shaft 131 by driving means such as a motor. That is, the fixing film 133 rotates following the pressure roller 134.

  A pressure release mechanism that releases the pressure applied to the fixing nip of the fixing device 130 will be described. In order to release the pressure of the fixing device 130, driving means such as a motor rotates the pressure release cam 137 so that the flat portion of the pressure release cam 137 is positioned as shown in FIG. The rotation shaft 146 of the pressure release cam 137 is also connected to the pressure release cam 137 disposed on the opposite side surface of the fixing device 130. The two pressure release cams 137 are driven to rotate by rotating the gear 147 fixed to the rotating shaft 146 by driving means such as a motor. By rotating the pressure release cam 137, the pressure release plate 136 is lifted with the fulcrum member 141 as a fulcrum. By this operation, the pressing force of the pressure spring 135 transmitted to the folder stay 139 is reduced, and the pressing force acting on the nip portion of the fixing device 130 is weakened.

  The home sensor 143 can detect whether the fixing device 130 is in a pressure state or a pressure release state. Home sensor 143 is formed of, for example, a photo interrupter. The home sensor 143 outputs a signal corresponding to the position of the light shielding plate 144 that is rotationally driven together with the rotation shaft 146 of the pressure release cam 137. In the pressurized state, the light from the light emitting element is blocked by the light shielding plate 144 and does not reach the light receiving element, so the output of the home sensor 143 is at the first level. Since the light from the light emitting element reaches the light receiving element in the pressure release state, the output of the home sensor 143 becomes the second level. The position of the pressure release cam 137 can be controlled by controlling the rotational drive with respect to the pressure release cam 137 in accordance with the level change of the output signal of the home sensor 143.

  FIG. 4 is a block diagram of the control system. The control unit 200 is a unit that comprehensively controls the entire image forming apparatus 100, and includes, for example, a CPU, a ROM that stores a control program, a RAM that stores data, a gate element, and the like. The conveyance control unit 201 and the pressure release control unit 202 are functions realized by the CPU executing a control program. The conveyance control unit 201 determines the conveyance status of the sheet P from detection information from the top sensor 105 and the like, and controls the rotation of the conveyance motor 203 that rotationally drives the photosensitive drum 122, the pressure roller 134, and the like. The conveyance control unit 201 may detect whether or not a jam has occurred in the sheet P based on detection information received from the top sensor 105 or the paper discharge sensor 109. When the door sensor 101 detects that the door 112 is opened, the conveyance control unit 201 may stop the conveyance motor 203 and notify the pressure release control unit 202 that the door 112 has been opened. The pressure release control unit 202 controls the pressure release motor 204 that rotationally drives the pressure release cam 137 based on information indicating the conveyance status of the sheet P received from the conveyance control unit 201 and detection information such as the home sensor 143. The pressing force of the fixing device 130 is controlled. Note that the conveyance control unit 201 may determine whether the pressure release condition is satisfied based on the conveyance status of the sheet P and detection information such as the home sensor 143, and may transmit a pressure release command to the pressure release control unit 202. . Thus, the pressure release cam 137, the pressure release motor 204, and the like function as release means for releasing the pressure applied to the fixing nip of the fixing device 130. Whether the conveyance control unit 201 and the pressure release control unit 202 cause the pressure release motor 204 to release the pressure applied to the fixing nip according to the length of the portion of the sheet P that is held by the fixing nip of the fixing device 130. It functions as a control means for controlling. The storage unit 230 stores a control program, data necessary for control, and the like.

  The conveyance control unit 201 and the pressure release control unit 202 may further have a plurality of functions. The nip determination unit 211 uses a timer 212 to measure an elapsed time t1 from the time when the top sensor 105 detects the leading edge of the sheet P. Further, the nip determination unit 211 may count the elapsed time t2 from the time when the top sensor 105 detects the trailing edge of the sheet P using the timer 212. The top sensor 105 is an example of a sheet detection unit disposed on the upstream side in the conveyance direction with respect to the fixing device 130. The timer 212 functions as a time measuring unit that measures the elapsed time from when the leading edge of the sheet is detected by the top sensor 105. The measuring unit 213 is an example of a measuring unit that measures the length of the portion of the sheet that is sandwiched between the fixing nips. Here, the measuring unit 213 measures the length X1 of the front end of the sheet held by the fixing nip and the length X2 of the rear end of the sheet. The leading edge determination unit 214 determines whether or not the leading edge of the sheet P is held in the fixing nip. The trailing edge determination unit 215 determines whether the trailing edge of the sheet P is sandwiched in the fixing nip. The determination unit 220 determines the paper type of the sheet P. The threshold setting unit 221 sets thresholds th1 and th2 according to the paper type. In the first embodiment, the determination unit 220 and the threshold setting unit 221 are optional. These functions may exist in the conveyance control unit 201, may exist in the pressure release control unit 202, or may be distributed in the conveyance control unit 201 and the pressure release control unit 202. .

<Release conditions>
5A to 5E are diagrams for explaining various cases in which a part of the sheet exists in the fixing nip. Note that pu indicates the position of the upstream end of the fixing nip. p1 indicates the position of the leading edge of the sheet. Note that pd indicates the position of the downstream end of the fixing nip. p2 indicates the position of the trailing edge of the sheet. That is, the fixing nip is a section from the position pu at the upstream end to the position pd at the downstream end.

  In FIG. 5a, the central portion of the sheet P exists in the fixing nip. Therefore, the length X1 from the position pu at the upstream end of the fixing nip to the position p1 at the leading end of the sheet P exceeds the threshold th1. In such a case, drawing the sheet P requires a considerable force for the user. Therefore, it is necessary to reduce the pressure applied to the fixing device 130. The threshold th1 is a distance at which the sheet P can be easily pulled out without damaging the fixing film 133 and the pressure roller 134, and is determined in advance by experiment, simulation, or the like.

  In FIG. 5b, the position p1 of the leading edge of the sheet P is quite close to the position pu of the upstream end of the fixing nip. That is, the length X1 of the front end held by the fixing nip does not exceed the threshold th1. In such a case, the sheet P can be easily pulled out without damaging the fixing film 133 and the pressure roller 134. Therefore, it is not necessary to reduce the pressure applied to the fixing device 130.

  In FIG. 5c, the position p1 of the leading edge of the sheet P is far from the position pu at the upstream end of the fixing nip. That is, the length X1 of the front end held by the fixing nip exceeds the threshold th1. Therefore, it is difficult to easily pull out the sheet P without damaging the fixing film 133 and the pressure roller 134. Therefore, it is necessary to reduce the pressure applied to the fixing device 130.

  In FIG. 5d, the trailing edge of the sheet P exists in the fixing nip. In particular, the length X2 from the position pd at the downstream end of the fixing nip to the position p2 at the rear end of the sheet P does not exceed the threshold th2. In such a case, the sheet P can be easily pulled out without damaging the fixing film 133 and the pressure roller 134. Therefore, it is not necessary to reduce the pressure applied to the fixing device 130. The threshold th2 is a distance at which the sheet P can be easily pulled out without damaging the fixing film 133 and the pressure roller 134, and is determined in advance by experiments, simulations, or the like. The threshold value th1 and the threshold value th2 may be the same value.

  In FIG. 5e, the trailing edge of the sheet P exists in the fixing nip. In particular, the length X2 from the position pd at the downstream end of the fixing nip to the position p2 at the rear end of the sheet P exceeds the threshold th2. In such a case, drawing the sheet P requires a considerable force for the user. Therefore, it is necessary to reduce the pressure applied to the fixing device 130.

<Flowchart>
Specific processing performed by the control unit 200 will be described with reference to the flowchart of FIG. In S1, the conveyance control unit 201 of the control unit 200 determines whether or not a conveyance stop condition is satisfied. The conveyance stop condition is, for example, that a jam is detected by any one of the sheet sensors, or that the door 112 is opened by the door sensor 101. The conveyance stop condition may be that an upper controller than the control unit 200 has transmitted an emergency stop command. When the conveyance stop condition is satisfied, the process proceeds to S2.

  In step S2, the conveyance control unit 201 determines whether a jam has occurred in the paper discharge unit. There are two types of jams that occur in the paper discharge unit: delay jams and stay jams. Delay jam in the paper discharge unit means that the paper discharge sensor 109 cannot detect the leading edge of the sheet P even though the elapsed time after the top sensor 105 detects the leading edge of the sheet P exceeds a predetermined time ta. Say. A staying jam at the paper discharge unit means that the paper discharge sensor 109 detects the trailing edge of the sheet P even though the elapsed time after the paper discharge sensor 109 detects the leading edge of the sheet P exceeds a predetermined time tb. Say what you can't do. If a delay jam or a stay jam has occurred in the paper discharge sensor 109, the conveyance control unit 201 cannot accurately determine the position of the sheet P. Further, since the fixing device 130 exists between the top sensor 105 and the paper discharge sensor 109, there is a high possibility that the sheet P exists in the fixing nip. Therefore, since there is a high possibility that the pressure of the fixing device 130 needs to be released, the process proceeds to S9. In step S9, the conveyance control unit 201 instructs the pressure release control unit 202 to release pressure, and the pressure release control unit 202 drives the pressure release motor 204 in accordance with this instruction, so that the pressure release is executed. 2a and 2b, the pressure release cam 137 of the pressure release mechanism is rotated by a predetermined amount, the movable end of the pressure release plate 136 is pushed up, the pressure spring 135 is compressed, and the folder stay 139 is compressed. The pressing force applied to the pressure nip is also reduced. On the other hand, if a jam does not occur in the paper discharge unit, the process proceeds to S3.

  In step S3, the conveyance control unit 201 determines whether or not the sheet P exists in the fixing nip. If the sheet P exists in the fixing nip, the process proceeds to S4. On the other hand, if the sheet P does not exist in the fixing nip, the pressure of the fixing device 130 is not released, and the processing for this flowchart is ended (actually returns to S1). Here, a method for determining whether or not the sheet P exists in the fixing nip will be exemplified. The nip determination unit 211 of the conveyance control unit 201 measures an elapsed time t1 from when the top sensor 105 detects the leading edge of the sheet P using a timer 212 which is a time measuring unit. Further, the nip determination unit 211 uses the timer 212 to measure the elapsed time t2 from the time when the top sensor 105 detects the trailing edge of the sheet P. The nip determination unit 211 determines that the sheet P exists in the fixing nip if the elapsed time t1 exceeds the predetermined time tc but the elapsed time t2 does not exceed the predetermined time td. The sheet P is conveyed at a constant speed (speed v). The distance L1 from the detection position of the top sensor 105 to the position pu at the upstream end of the fixing nip is known. Similarly, the distance L2 from the detection position of the top sensor 105 to the position pd on the downstream side of the fixing nip is also known. The predetermined time tc is the time from when the leading edge of the sheet P reaches the detection position of the top sensor 105 until it reaches the position pu at the upstream end of the fixing nip. Therefore, the equation tc = L1 / v holds. The predetermined time td is the time from when the trailing edge of the sheet P reaches the detection position of the top sensor 105 until it reaches the position pd at the downstream end of the fixing nip. Therefore, the equation td = L2 / v is established.

  In S4, the measurement unit 213 of the conveyance control unit 201 measures the length X1 of the front end of the sheet and the length X2 of the rear end of the sheet that are sandwiched by the fixing nip. X1 = t1 · v−L1 and X2 = L2−t2 · v.

  In step S5, the leading edge determination unit 214 of the conveyance control unit 201 determines whether the leading edge of the sheet P is nipped in the fixing nip. For example, if 0 <X1 = <Ln, the leading edge of the sheet P is held in the fixing nip. Here, Ln is the length of the fixing nip and is the distance from the position pu to the position pd. If the leading edge of the sheet P is sandwiched in the fixing nip, the process proceeds to S6. If the leading edge of the sheet P is not sandwiched in the fixing nip, the process proceeds to S7.

  In S6, the leading edge determination unit 214 determines whether or not the length X1 of the leading edge of the sheet P sandwiched by the fixing nip exceeds the threshold th1. The threshold th1 is 3 mm, for example. The case where the length X1 exceeds the threshold th1 is the case shown in FIG. 5c. As described above, in this case, it is difficult to remove the sheet P without damaging the fixing device 130. Therefore, the process proceeds to S7 to release the pressure. On the other hand, if the length X1 does not exceed the threshold th1, the process proceeds to S9. The case where the length X1 does not exceed the threshold th1 is a case as shown in FIG. 5B, and the sheet P can be pulled out without damaging the fixing device 130. Therefore, since it is not necessary to perform pressure release, the processing according to this flowchart is terminated.

  In step S7, the trailing edge determination unit 215 of the conveyance control unit 201 determines whether the trailing edge of the sheet P is nipped in the fixing nip. For example, if 0 <X2 = <Ln, the trailing edge of the sheet P is held in the fixing nip. If the trailing edge of the sheet P is held in the fixing nip, the process proceeds to S8. This corresponds to the case shown in FIG. 5d or 5e. On the other hand, if the trailing edge of the sheet P is not pinched in the fixing nip, the process proceeds to S9. This means that the sheet P exists in the fixing nip, but neither the leading edge nor the trailing edge exists in the fixing nip. That is, this corresponds to the case shown in FIG.

  In S8, the trailing edge determination unit 215 determines whether or not the length X2 of the trailing edge of the sheet P sandwiched by the fixing nip exceeds the threshold th2. The threshold th2 is 3 mm, for example. The case where the length X2 exceeds the threshold th2 is the case shown in FIG. 5e. As described above, in this case, it is difficult to remove the sheet P without damaging the fixing device 130. Therefore, the process proceeds to S9 to release the pressure. On the other hand, if the length X2 does not exceed the threshold th2, this process is terminated. The case where the length X2 does not exceed the threshold th2 is a case as shown in FIG. 5d, and the sheet P can be pulled out without damaging the fixing device 130. Therefore, since it is not necessary to perform pressure release, the processing according to this flowchart is terminated.

  In this embodiment, the fixing pressure is not released when the leading edge and the trailing edge of the sheet P are in the upstream 3 mm region and the downstream 3 mm range of the fixing nip. This area is an experimentally determined distance, and increases or decreases depending on the apparatus configuration, the pressure applied by the fixing device, and the like. When obtaining experimentally, it is essential that the fixing device is not damaged when the user performs jam processing and that the user can easily perform jam processing.

  By controlling as described above, in the case where the positional relationship between the fixing nip and the sheet P is as shown in FIGS. 5b and 5d, the pressure of the fixing device is not released. it can.

  Therefore, while maintaining the protection performance of the fixing device and the jam handling ability of the user, the number of times the pressure of the fixing device is released is reduced, and an inexpensive image forming apparatus is provided by using cheap parts with a short life in the pressure releasing mechanism. be able to.

<Threshold setting method according to paper type>
In the above-described embodiment, regardless of the thickness of the sheet P, when the leading end of the sheet P is in a predetermined area on the upstream side in the fixing nip and the trailing end of the sheet P in the predetermined area on the downstream side in the fixing nip. When there was, the release mechanism did not release the pressure and the pressure was maintained. As described above, the predetermined area is defined by the thresholds th1 and th2, and is a fixed value without depending on the paper type (paper thickness, basis weight, brand, etc.). However, the lengths X1 and X2 at which the user can pull out the sheet P without damaging the fixing device 130 differ depending on the paper type. For example, with thick paper and thin paper, thick paper requires more force to pull out. Therefore, the threshold values th1 and th2 for thick paper need to be smaller than the threshold values th1 and th2 for thin paper. For example, the threshold values th1 and th2 may be set to 3 mm for thick paper, and the threshold values th1 and th2 for thin paper may be set to 5 mm, respectively. For brands with a large friction coefficient on the sheet surface and brands with a small coefficient of friction, the former thresholds th1 and th2 need to be smaller than the latter thresholds th1 and th2. Therefore, in this embodiment, a step of setting the thresholds th1 and th2 according to the paper type of the sheet P is added. Thereby, the number of times of pressure release is appropriately reduced according to the paper type.

  FIG. 7 is a flowchart showing the processing of this embodiment, and the steps common to FIG. 6 are given the same reference numerals to simplify the description. Comparing FIG. 6 and FIG. 7, the only difference is that S0 is added before S1.

  In S0, the determination unit 220 of the conveyance control unit 201 determines the paper type of the sheet P, and the threshold setting unit 221 sets the thresholds th1 and th2 according to the paper type. For example, the determination unit 220 may determine the paper type of the sheet P based on the paper type information input from the operation unit by the user or the paper type information from the paper type sensor provided in the conveyance path. Further, paper type information is received from the threshold setting unit 221 and the determination unit 220, and based on the correspondence between the paper type stored in the storage unit 230 and the thresholds th1 and th2, the thresholds th1 and th2 corresponding to the paper type information are set. decide. In the storage unit 230, for example, paper type information indicating thick paper and threshold values th1 and th2 for thick paper are stored in association with each other. Similarly, paper type information indicating thin paper and thresholds th1 and th2 for thin paper are stored in association with each other. As described above, various paper type information and the corresponding thresholds th1 and th2 are stored in the storage unit 230 in association with each other. When the thickness of the sheet P is adopted as the paper type, the determination unit 220 may determine the thickness of the sheet P, and the threshold setting unit 221 may calculate the threshold by substituting the thickness into a predetermined function. If the thickness and the threshold value are in a linear relationship, the function is a linear function. Of course, more complex functions may be employed. It is assumed that such a function is stored in the storage unit 230.

<Summary>
As described above, according to this embodiment, the control unit 200 determines whether the pressure release mechanism releases the pressure applied to the fixing nip according to the length of the portion of the sheet that is held by the fixing nip of the fixing device 130. To control. Therefore, the number of executions of pressure release of the fixing device 130 is reduced as compared with the conventional case. Since the number of times of pressure release is reduced, the service life of the fixing device 130 and the pressure release mechanism is extended as compared with the prior art. That is, it is possible to employ an inexpensive part with a short life in the pressure release mechanism, and it is possible to provide a cheaper image forming apparatus.

  Note that a measuring unit for measuring the length of the portion of the sheet P that is sandwiched between the fixing nips may be employed. Such a measuring means may be constituted by the top sensor 105 and the timer 212, for example. Accordingly, the control unit 200 may control whether or not to cause the pressure release mechanism to release the pressure applied to the fixing nip using the elapsed time as the length of the portion of the sheet P that is sandwiched by the fixing nip. . The length of the portion sandwiched between the fixing nips may be calculated from the elapsed time and the conveyance speed, or the elapsed time may be used as it is. This is because the conveyance speed is basically constant, and it is not necessary to convert the elapsed time into a length. The timer 212 may function as a time measuring unit that measures the elapsed time from when the rear end of the sheet P is detected by the top sensor 105.

  The leading edge determination unit 214 and the trailing edge determination unit 215 may be employed as a determination unit that determines whether the lengths X1 and X2 of the portion of the sheet P sandwiched by the fixing nip exceed the threshold values th1 and th2. . As described with reference to FIGS. 5B and 5D, the control unit 200 applies the pressure applied to the fixing nip to the pressure release mechanism unless the length of the portion of the sheet P sandwiched by the fixing nip exceeds the threshold. Keep it without releasing it. As described with reference to FIG. 5c and FIG. 5e, the control unit 200 applies the pressure applied to the pressure release mechanism to the pressure release mechanism when the length of the portion of the sheet P sandwiched by the fixing nip exceeds a threshold value. Let it be released. As described above, in the case where the fixing device 130 can be damaged, the pressure release mechanism releases the pressure applied to the fixing nip. In the case where the fixing device 130 is pulled out without being damaged, the pressure release mechanism applies the pressure applied to the fixing nip. Do not cancel. As a result, the number of executions of pressure release of the fixing device 130 is reduced as compared with the conventional case. The thresholds th1 and th2 are the lengths of the portions of the sheet that are sandwiched by the fixing nip, and are the lengths that allow the sheets to be pulled out without damaging the fixing device 130. The threshold values th1 and th2 may be the length of the portion of the sheet P that is sandwiched between the fixing nips, and may be a length that allows a person to easily pull out the sheet. The thresholds th1 and th2 may be the length of the portion of the sheet P that is sandwiched between the fixing nips, and may be a length that allows the sheet to be pulled out with a predetermined force or less. The predetermined force is a force that does not damage the fixing device 130 by the sheet P. These may be determined in advance at the time of factory shipment according to the material of the fixing film 133 and the pressure roller 134 of the fixing device 130, the configuration of the drive mechanism, and the spring constant of the pressure spring 135.

  As described with reference to FIG. 7, the thresholds th1 and th2 may be changed according to the paper type of the sheet P. The paper type is, for example, the basis weight, thickness or brand of the sheet P. As described above, the length that can be pulled out differs between thick paper and thin paper. Therefore, by changing the threshold value according to the paper type, it is possible to further reduce the number of times the pressure is released compared to the case where the threshold value is uniformly determined assuming thick paper.

  As described with reference to S1 in FIG. 6 and FIG. 7, when the conveyance control unit 201 detects a jam of the sheet P or when the door sensor 101 detects that the door 112 is opened, it corresponds to the lengths X1 and X2. It may be determined whether to release the pressure. As described with respect to S2 and S10, the control unit 200 causes the pressure release mechanism to release the pressure applied to the fixing nip when it detects a jam that has occurred in the sheet discharge portion downstream of the fixing device 130. May be. This is because when a delay jam or a stay jam occurs in the sheet discharge unit, the position of the sheet P is unknown, and in most cases, the sheet P is held in the fixing nip. Therefore, in such a case, the pressure release mechanism may release the pressure applied to the fixing nip.

  As described with reference to FIGS. 2 a, 2 b, 3, etc., the pressure release mechanism is a pressure release mechanism that is controlled by the control unit 200 and releases the pressure applied to the pressure roller 134 of the fixing film 133 by the pressure mechanism. A cam 137, a pressure release plate 136, a pressure release motor 204, or the like may be employed. The pressure release plate 136 is an example of a plate-like member that moves the movable end of the pressure spring 135 against the elastic force of the pressure spring 135 that is an elastic body. The other end of the pressure spring 135 may be fixed to the pressure plate 138 as a fixed end. The pressure release cam 137 is an example of a cam mechanism that contacts the pressure release plate 136 and moves the pressure release plate 136. With such a simple mechanism, it is possible to apply pressure to the fixing nip or release the pressure.

  By the way, conventionally, when the sheet P is jammed or the door is opened, the pressure applied to the fixing nip can be controlled according to the length of the portion of the sheet P that is sandwiched by the fixing nip. There wasn't. Therefore, there is a demand for control means that can solve such problems. According to the embodiment described above, the control unit 200 described above is provided as such a control means.

  The pressure mechanism and the pressure release mechanism described above function as an adjusting unit that adjusts the pressure applied to the fixing nip. That is, the applied pressure changes according to the rotational position of the pressure release cam 137. Therefore, the pressure release control unit 202 can adjust the pressure applied to the pressure release cam 137 by driving the pressure release motor 204 according to the rotational position of the pressure release cam 137 detected by the home sensor 143. Become. As described above, the pressing force may be adjusted according to the lengths X1 and X2 of the portion of the sheet P that is sandwiched between the fixing nips.

  DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus, 105 ... Top sensor, 109 ... Paper discharge sensor, 130 ... Fixing device, 133 ... Fixing film, 134 ... Pressure roller, 135 ... Pressure spring, 136 ... Pressure release plate, 137 ... Pressure release cam 138 ... Pressure plate, 139 ... Folder stay

Claims (22)

Image forming means for forming a toner image on a sheet;
A fixing unit that sandwiches the sheet on which the toner image is formed by the image forming unit in a fixing nip and presses and fixes the sheet on the sheet;
Releasing means for releasing the pressure of the fixing nip of the fixing means;
Control means for controlling whether or not the release means releases the pressure applied to the fixing nip according to the length of the portion of the sheet that is sandwiched by the fixing nip of the fixing means. Image forming apparatus.   The image forming apparatus according to claim 1, further comprising a measuring unit that measures a length of a portion of the sheet that is sandwiched between the fixing nips. The measuring means includes
A sheet detection unit disposed upstream of the fixing unit in the transport direction;
Having time measuring means for measuring the elapsed time from when the leading edge of the sheet is detected by the sheet detecting means,
The control means controls whether the release means releases the pressure applied to the fixing nip by using the elapsed time as the length of the portion of the sheet held by the fixing nip. The image forming apparatus according to claim 2. The measuring means includes
A sheet detection unit disposed upstream of the fixing unit in the transport direction;
Having time measuring means for measuring the elapsed time from when the trailing edge of the sheet is detected by the sheet detecting means,
The control means controls whether the release means releases the pressure applied to the fixing nip by using the elapsed time as the length of the portion of the sheet held by the fixing nip. The image forming apparatus according to claim 2. The control means includes
A determination unit that determines whether a length of a portion of the sheet held by the fixing nip exceeds a threshold;
If the length of the portion of the sheet that is sandwiched by the fixing nip does not exceed the threshold value, the control unit causes the release unit to maintain the pressure applied to the fixing nip without releasing the pressure, and 5. The apparatus according to claim 1, wherein if the length of a portion sandwiched between the fixing nips exceeds the threshold value, the releasing unit releases the pressure applied to the fixing nip. 6. The image forming apparatus described.   The threshold value is a length of a portion of the sheet that is sandwiched by the fixing nip, and is a length that allows the sheet to be pulled out without damaging the fixing unit. Item 6. The image forming apparatus according to Item 5.   6. The threshold value according to claim 5, wherein the threshold value is a length of a portion of the sheet that is sandwiched by the fixing nip, and is a length that allows a person to easily pull out the sheet. Image forming apparatus.   The threshold value is a length of a portion of the sheet that is sandwiched between the fixing nips, and is a length that allows the sheet to be pulled out with a predetermined force or less. The image forming apparatus described.   The image forming apparatus according to claim 5, wherein the threshold value is changed according to a paper type of the sheet.   The image forming apparatus according to claim 9, wherein the sheet type of the sheet is a basis weight, a thickness, or a brand of the sheet. It further has a detecting means for detecting a jam of the sheet,
The control unit causes the release unit to release the pressure applied to the fixing nip according to the length of the portion of the sheet that is sandwiched by the fixing nip when the detection unit detects the jam. The image forming apparatus according to claim 1.   12. The image according to claim 11, wherein the control unit causes the release unit to release the pressure applied to the fixing nip when detecting the jam generated in the sheet discharge portion on the downstream side of the fixing unit. Forming equipment. It further has a detecting means for detecting that the door is opened,
When the detecting means detects that the door is opened, the control means applies a pressing force of the fixing nip to the releasing means according to the length of the portion of the sheet that is sandwiched by the fixing nip. The image forming apparatus according to claim 1, wherein the image forming apparatus is released.   The image forming apparatus according to claim 1, wherein a portion of the sheet that is sandwiched between the fixing nips is a leading end of the sheet.   The image forming apparatus according to claim 1, wherein a portion of the sheet that is sandwiched between the fixing nips is a rear end of the sheet. The fixing means is
A pressure member;
A fixing member;
A pressure mechanism that pressurizes the fixing member against the pressure member;
The release means is
16. The image according to claim 1, further comprising a release mechanism that is controlled by the control unit and that releases pressure applied to the pressure member by the fixing member by the pressure mechanism. Forming equipment. The fixing member is a cylindrical fixing film, and a support member is pressed against a part of the inner surface of the fixing film by an elastic body so that a part of the outer surface of the fixing film contacts the pressure member. Is configured to form a fixing nip,
The image forming apparatus according to claim 16, wherein the release mechanism moves the support member against an elastic force of the elastic body. The release mechanism is
A plate-like member that moves the movable end of the elastic body against the elastic force of the elastic body;
The image forming apparatus according to claim 17, further comprising a cam mechanism that contacts the plate-like member and moves the plate-like member. A release method for releasing a pressure applied to a fixing nip of a fixing device that pressurizes an unfixed toner image formed on a sheet and fixes the toner image on the sheet.
A release method comprising: controlling whether or not to release the pressurizing force of the fixing device in accordance with the length of a portion of the sheet that is sandwiched by the fixing nip.   The length of the portion of the sheet that is sandwiched in the fixing nip is not released unless the length of the portion that is sandwiched in the fixing nip exceeds a threshold value. The release method according to claim 19, wherein if the pressure exceeds a threshold value, the pressure applied to the fixing nip is released. Image forming means for forming a toner image on a sheet;
A fixing unit that holds the sheet on which the toner image is formed by the image forming unit in a fixing nip and pressurizes and fixes the sheet on the sheet;
Control means for controlling the pressure applied to the fixing nip in accordance with the length of the portion of the sheet that is sandwiched by the fixing nip of the fixing means when a sheet jam occurs or the door is opened. An image forming apparatus comprising: Image forming means for forming a toner image on a sheet;
A fixing unit that holds the sheet on which the toner image is formed by the image forming unit in a fixing nip and pressurizes and fixes the sheet on the sheet;
Adjusting means for adjusting the pressure applied to the fixing nip of the fixing means;
Control means for controlling the adjustment means, the control means for causing the adjustment means to adjust the applied pressure according to the length of the portion of the sheet that is sandwiched between the fixing nips of the fixing means. An image forming apparatus.