Image forming device

Abstract

To provide an image forming apparatus that can improve the ability to fix a toner image to a sheet.SOLUTION: A control unit 100 of an image forming apparatus, when receiving a print command, starts heating of a heating unit 81 so that a detection temperature detected by a temperature sensor ST reaches a predetermined target temperature; when the detection temperature is equal to or higher than a rotation start temperature lower than the target temperature, starts rotation of the heating unit 81 with the nip pressure between the heating unit 81 and a pressure unit 82 being adjusted to a first nip pressure; on condition that a nip pressure change condition is satisfied that the detection temperature is lower than the target temperature and equal to or higher than a nip pressure change temperature higher than the rotation start temperature, causes a nip pressure change mechanism to change the nip pressure from the first nip pressure to a second nip pressure larger than the first nip pressure; and on condition that a sheet supply condition is satisfied that the detection temperature is lower than the target temperature and equal to or higher than a sheet supply temperature higher than the nip pressure change temperature, causes a sheet supply unit 20 to start supply of a sheet S.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus including a fixing device that fixes a toner image onto a sheet. [Background technology]

[0002] Conventionally, there is known an image forming apparatus that includes a sheet supply unit that supplies a sheet, a heating unit that heats the sheet, a fixing unit having a pressure unit that sandwiches the sheet between the heating unit, a temperature detection unit that detects the temperature of the heating unit, and a control unit (Patent Document 1). In this technology, if the temperature of the heating unit reaches a supply start temperature before a predetermined time has elapsed from a predetermined timing after the heating unit starts heating, the control unit starts supplying the sheet after the predetermined time has elapsed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-152693 A Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, some image forming apparatuses are equipped with a nip pressure change mechanism capable of changing the nip pressure between the heating section and the pressure section. When the nip pressure between the heating section and the pressure section is increased by the nip pressure change mechanism, the heat from the heating section is easily transferred to the pressure section, so that the pressure section can be heated well by the heating section. If both the heating section and the pressure section can be sufficiently heated to a temperature suitable for fixing before the supplied sheet reaches the fixing device, the fixability of the toner image on the sheet can be improved.

[0005] SUMMARY OF THE PRESENT EMBODIMENTS In view of the above, an object of the present invention is to provide an image forming apparatus capable of improving the fixation of a toner image onto a sheet. [Means for solving the problem]

[0006] To achieve the above-mentioned objective, an image forming apparatus includes a sheet supplying unit that supplies a sheet, a processing unit that transfers a toner image onto the sheet supplied from the sheet supplying unit, and a fixing unit that fixes the toner image onto the sheet transported from the processing unit, the fixing unit having a heating unit that heats the sheet and a pressure applying unit that sandwiches the sheet between the heating unit and the pressure applying unit, a nip pressure changing mechanism that can change the nip pressure between the heating unit and the pressure applying unit to a first nip pressure and a second nip pressure that is greater than the first nip pressure, a temperature sensor for detecting the temperature of the heating unit, and a control unit. When the control unit receives a print command, it starts heating the heating unit so that the temperature detected by the temperature sensor becomes a predetermined target temperature, and when the detected temperature is equal to or higher than a rotation start temperature which is lower than the target temperature, it starts rotating the heating unit with the nip pressure set to the first nip pressure, and when a nip pressure change condition is satisfied that the detected temperature is lower than the target temperature and equal to or higher than a nip pressure change temperature which is higher than the rotation start temperature, it changes the nip pressure from the first nip pressure to the second nip pressure by the nip pressure change mechanism, and starts supplying a sheet by the sheet supply unit, when a sheet supply condition is satisfied that the detected temperature is lower than the target temperature and equal to or higher than a sheet supply temperature which is higher than the nip pressure change temperature.

[0007] According to this configuration, by increasing the nip pressure between the heating section and the pressure applying section, the heat of the heating section is easily transferred to the pressure applying section, and the pressure applying section can be heated well by the heating section. Moreover, the heating of the pressure applying section can be started before the supply of the sheet starts. As a result, both the heating section and the pressure applying section can be sufficiently heated to a temperature suitable for fixing by the time the sheet, whose supply has started thereafter, reaches the fixing device. This improves the fixability of the toner image on the sheet.

[0008] In the above-mentioned image forming apparatus, the control unit can be configured to set the sheet supply temperature to a first sheet supply temperature when the detected temperature is lower than the rotation start temperature when a print command is received, and to set the sheet supply temperature to a second sheet supply temperature lower than the first sheet supply temperature when the detected temperature is equal to or higher than the rotation start temperature when a print command is received.

[0009] If the detected temperature is equal to or higher than the rotation start temperature when a print command is received, the heating section and the pressure section are already warm to a certain extent, so that the heating section and the pressure section can be heated to a temperature suitable for fixing in a relatively short time. Therefore, in this case, by lowering the sheet supply temperature, the time from when the print command is received until the sheet supply starts can be shortened. This makes it possible to shorten the time until printing of the first sheet is completed.

[0010] In the above-described image forming apparatus, the control unit can be configured to start supplying a sheet by the sheet supply unit on the condition that both the sheet supply condition and a second sheet supply condition, that is, a sheet supply time has elapsed from a predetermined timing after the timing when rotation of the heating unit is started, are satisfied.

[0011] According to this, it is possible to wait until the sheet supply time has elapsed before starting to supply the sheet, which reduces the difference in temperature between the part of the heating unit detected by the temperature sensor and the other parts, so that the heating unit and the pressure unit can be heated evenly, and the fixability of the toner image on the sheet can be improved.

[0012] In the above-mentioned image forming apparatus, the control unit can be configured to set the sheet supply time to a first sheet supply time if the detected temperature is less than the rotation start temperature when a print command is received, and to set the sheet supply time to a second sheet supply time shorter than the first sheet supply time if the detected temperature is equal to or higher than the rotation start temperature when a print command is received.

[0013] If the detected temperature is equal to or higher than the rotation start temperature when a print command is received, the heating section and the pressure section are already warm to a certain extent, so that the heating section and the pressure section can be heated to a temperature suitable for fixing in a relatively short time. Therefore, in this case, by shortening the sheet supply time, the time from when the print command is received until the sheet supply starts can be shortened. This makes it possible to shorten the time until printing of the first sheet is completed.

[0014] In the above-mentioned image forming apparatus, the control unit can be configured to change the nip pressure from the first nip pressure to the second nip pressure by the nip pressure change mechanism when both the nip pressure change condition and a second nip pressure change condition, that is, a nip pressure change time has elapsed from the timing when rotation of the heating unit started, are satisfied.

[0015] In the image forming apparatus described above, the predetermined timing is the timing when the heating unit starts to rotate, and the nip pressure change time can be shorter than the sheet supply time.

[0016] In the image forming apparatus described above, the control unit can be configured to start supplying a sheet by the sheet supply unit when the detected temperature becomes equal to or higher than the sheet supply temperature after a third sheet supply time longer than the sheet supply time has elapsed from a predetermined timing, and when the detected temperature becomes equal to or higher than a third sheet supply temperature which is lower than the target temperature and higher than the sheet supply temperature.

[0017] When the detected temperature reaches the sheet supply temperature after the third sheet supply time has elapsed from the specified timing, the gradient of the temperature rise of the fixing unit is small, so if sheet supply is started at the timing when the detected temperature reaches the sheet supply temperature, there is a possibility that the temperatures of the heating unit and the pressure unit have not yet reached a temperature suitable for fixing when the sheet reaches the fixing unit. Therefore, by waiting until the detected temperature reaches the third sheet supply temperature before starting sheet supply, the temperatures of the heating unit and the pressure unit can be raised to a temperature suitable for fixing when the sheet reaches the fixing unit. This improves the fixability of the toner image on the sheet.

[0018] In the image forming apparatus described above, the control unit is capable of executing a first printing mode in which a toner image is fixed to a sheet with a first nip pressure, and a second printing mode in which a toner image is fixed to a sheet with a second nip pressure, and when executing the first printing mode, the nip pressure is not changed from the first nip pressure, and when executing the second printing mode, the nip pressure is changed from the first nip pressure to the second nip pressure by a nip pressure change mechanism, provided that a nip pressure change condition is satisfied. Effect of the Invention

[0019] According to the present invention, the fixation of a toner image onto a sheet can be improved. [Brief description of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view showing an image forming apparatus according to an embodiment. [Diagram 2] FIG. [Diagram 3] FIG. 4 is a perspective view showing a nip pressure changing mechanism. [Figure 4] 1A is a diagram showing a nip pressure changing mechanism when the nip pressure is a strong nip pressure, and FIG. 1B is a diagram showing the periphery of the nip portion. [Diagram 5] 1A is a diagram showing a nip pressure changing mechanism when the nip pressure is a weak nip pressure, and FIG. 1B is a diagram showing the periphery of the nip portion. [Figure 6] FIG. 2 is a diagram showing the relationship between a control unit and an object controlled by the control unit. [Figure 7] 10 is a flowchart illustrating an example of an operation of a control unit to supply a sheet. [Figure 8] 10 is a flowchart illustrating an example of an operation of changing a nip pressure by a control unit. [Figure 9] 10 is a timing chart showing an example of an operation of the control unit in a case where a temperature detected by a temperature sensor when a print command is received is lower than a rotation start temperature. [Figure 10]10 is a timing chart showing an example of an operation of the control unit in a case where the temperature detected by the temperature sensor when a print command is received is equal to or higher than the rotation start temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Next, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. As shown in FIG. 1, a color printer 1 as an example of an image forming apparatus includes, within a main body housing 2, a sheet supply section 20 that supplies a sheet S, a process section 30 that transfers a toner image onto the sheet S supplied from the sheet supply section 20, a fixing device 80 that fixes the toner image onto the sheet S transported from the process section 30, a discharge section 90 that discharges the sheet S on which the image has been formed, and a control section 100.

[0022] An opening 2A is formed in the upper part of the main body housing 2, and this opening 2A is opened and closed by an upper cover 3 that is rotatably supported by the main body housing 2. The upper surface of the upper cover 3 serves as a discharge tray 4 that accumulates sheets S discharged from the main body housing 2. A plurality of LED mounting members 5 that hold LED units 40 are provided on the lower surface of the upper cover 3.

[0023] The sheet supply unit 20 is provided at the bottom of the main body housing 2, and includes a supply tray 21 that is detachable from the main body housing 2, and a supply mechanism 22 that transports the sheets S from the supply tray 21 to the process unit 30. The supply mechanism 22 includes a pickup roller 23, a separation roller 24, a separation pad 25, and a registration roller 26. In the sheet supply unit 20, the sheets S in the supply tray 21 are sent out by the pickup roller 23. Next, the sheets S are separated one by one by the separation roller 24 and the separation pad 25. Thereafter, the registration roller 26 aligns the leading ends of the sheets S, and then transports the sheets S toward the process unit 30.

[0024] The process section 30 includes four LED units 40, four process cartridges 50, and a transfer unit . The LED unit 40 is pivotably connected to the LED mounting member 5, and is appropriately positioned and supported by a positioning member provided in the main body housing 2.

[0025] The process cartridges 50 are arranged side by side in the front-rear direction between the upper cover 3 and the sheet supply unit 20. Each process cartridge 50 is configured to include a photosensitive drum 51, a charger 52, a developing roller 53, a toner storage chamber 54 that stores toner, and the like.

[0026] The transfer unit 70 is provided between the sheet supply section 20 and each process cartridge 50, and includes a drive roller 71, a driven roller 72, a conveyor belt 73, and a transfer roller 74. The drive roller 71 and the driven roller 72 are arranged in parallel and spaced apart in the front-rear direction, and an endless conveyor belt 73 is stretched between them. The outer circumferential surface of the conveyor belt 73 is in contact with the photosensitive drum 51. Four transfer rollers 74 are arranged inside the conveyor belt 73, facing each photosensitive drum 51. The transfer rollers 74 sandwich the conveyor belt 73 between themselves and the photosensitive drum 51. The sheet S is conveyed by the conveyor belt 73 and the photosensitive drum 51.

[0027] The fixing device 80 includes a heating section 81 that heats the sheet S, and a pressure section 82 that sandwiches the sheet S between the heating section 81 and the pressure section 82. The pressure section 82 presses the heating section 81. The structure of the fixing device 80 will be described in detail later.

[0028] In the process unit 30 thus configured, first, the surface of the photosensitive drum 51 is uniformly charged by the charger 52 and then exposed to light by the LED unit 40. As a result, an electrostatic latent image based on image data is formed on the photosensitive drum 51. Thereafter, toner is supplied to the electrostatic latent image by the developing roller 53, so that a toner image is carried on the photosensitive drum 51.

[0029] The sheet S supplied from the sheet supply unit 20 onto the conveyor belt 73 passes between the photosensitive drum 51 and a transfer roller 74 disposed inside the conveyor belt 73, whereby the toner image formed on the photosensitive drum 51 is transferred onto the sheet S. Then, the sheet S passes between a heating unit 81 and a pressure unit 82, whereby the toner image transferred onto the sheet S is fixed onto the sheet S.

[0030] The discharge section 90 includes a discharge-side transport path 91 and a plurality of transport rollers 92. The sheet S on which the toner image has been fixed is transported along the discharge-side transport path 91 by the transport rollers 92, discharged to the outside of the main body housing 2, and accumulated in the discharge tray 4.

[0031] 2, the fixing device 80 includes a heating unit 81 and a pressure applying unit 82. The heating unit 81 includes a heater 110 and a rotatable roller 120. The pressure applying unit 82 is a member that forms a nip portion NP between the heating unit 81 and the pressure applying unit 82. The pressure applying unit 82 includes a belt 130, a nip forming member N, a holder 140, a stay 200, a belt guide G, and a sliding sheet 150.

[0032] Heater 110 is a halogen lamp that emits light and generates heat when energized, and uses radiant heat to heat roller 120. Heater 110 is disposed so as to pass inside roller 120 along the axial direction along which the rotation axis of roller 120 extends.

[0033] Roller 120 is a cylindrical roller that is long in the axial direction, and is heated by heater 110. Roller 120 has an iron alloy tube 121 and an elastic layer 122 that covers the outer circumferential surface of tube 121. Elastic layer 122 is made of rubber such as silicone rubber. Roller 120 is rotatably supported by a side frame 83 (see FIG. 3), which will be described later, and is driven to rotate counterclockwise in FIG. 2 by input of driving force from a fixing motor M2 (see FIG. 6) provided in main body housing 2.

[0034] The belt 130 is a cylindrical member that is long in the axial direction of the roller 120 and has flexibility. The belt 130 forms a nip portion NP between the heating unit 81, specifically the roller 120. The belt 130 has a base material made of metal or resin, and a release layer that covers the outer peripheral surface of the base material. When the roller 120 rotates, the belt 130 rotates clockwise in FIG. 2 due to friction with the roller 120 or the sheet S. A lubricant such as grease is applied to the inner peripheral surface 131 of the belt 130. A nip forming member N, a holder 140, a stay 200, a belt guide G, and a sliding sheet 150 are arranged inside the belt 130.

[0035] The nip forming member N is a member that forms a nip portion NP by nipping the belt 130 between itself and the roller 120. The nip forming member N includes an upstream nip forming member N1 and a downstream nip forming member N2.

[0036] The upstream nip forming member N1 has an upstream pad P1 and an upstream fixed plate B1. The upstream pad P1 is made of rubber such as silicone rubber. The upstream pad P1 forms an upstream nip portion NP1 by sandwiching the belt 130 between the upstream pad P1 and the roller 120. The upstream pad P1 is fixed to the surface of the upstream fixed plate B1 on the roller 120 side. The upstream fixed plate B1 is made of metal or the like.

[0037] The downstream nip forming member N2 is disposed downstream of the upstream nip forming member N1 in the movement direction of the belt 130 in the nip portion NP. The downstream nip forming member N2 has a downstream pad P2 and a downstream fixed plate B2. The downstream pad P2 is made of rubber such as silicone rubber. The downstream pad P2 sandwiches the belt 130 between itself and the roller 120 to form the downstream nip portion NP2. The downstream pad P2 is fixed to the surface of the downstream fixed plate B2 on the roller 120 side. The downstream fixed plate B2 is made of metal or the like.

[0038] The downstream pad P2 is separated from the upstream pad P1 in the moving direction of the belt 130 in the nip portion NP. Therefore, between the upstream nip portion NP1 and the downstream nip portion NP2, there is an intermediate nip portion NP3 where the pressure from the pressurizing portion 82 does not directly act. Although the belt 130 contacts the roller 120 in this intermediate nip portion NP3, there is no member that sandwiches the belt 130 between the roller 120, so almost no pressure is applied. Therefore, the sheet S passes through the intermediate nip portion NP3 while being heated by the roller 120 and being hardly pressurized. The nip portion NP includes an upstream nip portion NP1, a downstream nip portion NP2, and an intermediate nip portion NP3.

[0039] The holder 140 is a member that holds the nip forming member N. The holder 140 is made of a heat-resistant resin or the like.

[0040] The stay 200 is a member that is located on the opposite side of the nip forming member N with respect to the holder 140 and supports the holder 140. The stay 200 includes a first stay 210 and a second stay 220 that is connected to the first stay 210. The first stay 210 is a member that supports the holder 140. The first stay 210 is made of metal or the like.

[0041] The belt guide G is a member that guides an inner circumferential surface 131 of the belt 130. The belt guide G is made of a heat-resistant resin or the like. The belt guide G has an upstream guide G1 and a downstream guide G2.

[0042] The sliding sheet 150 is a sheet-like member for reducing frictional resistance between each of the pads P1, P2 and the belt 130. The sliding sheet 150 is sandwiched between the inner circumferential surface 131 of the belt 130 and each of the pads P1, P2 at the nip portion NP. The sliding sheet 150 is made of an elastically deformable material. Note that the sliding sheet 150 may be made of any material, and may be made of, for example, a resin containing polyimide.

[0043] As shown in FIG. 3, the fixing unit 80 further includes a frame FL and a nip pressure changing mechanism 300.

[0044] The frame FL is a frame that supports the heating unit 81 and the pressure unit 82, and is made of metal or the like. The frame FL includes side frames 83 and brackets 84 that are disposed on both sides of the heating unit 81 and the pressure unit 82 in the axial direction of the roller 120, and a connection frame 85 that connects the side frames 83.

[0045] The side frame 83 is a frame that supports the heating unit 81 and the pressure unit 82. The side frame 83 has a spring engagement portion 83A that engages with one end of a first spring 320, which will be described later.

[0046] The bracket 84 is a member that supports the pressure applying unit 82 movably in a direction approaching or moving away from the heating unit 81, and is fixed to the side frame 83. In detail, the bracket 84 has a first elongated hole 84A that movably supports the end of the first stay 210 via the holder 140. The first elongated hole 84A is an elongated hole that is long in the direction in which the heating unit 81 and the pressure applying unit 82 face each other.

[0047] Nip pressure change mechanism 300 is a mechanism that changes the nip pressure of nip portion NP. More specifically, nip pressure change mechanism 300 can change the nip pressure of heating unit 81 and pressure unit 82 to a first nip pressure and a second nip pressure that is greater than the first nip pressure. Hereinafter, the first nip pressure may be referred to as a "weak nip pressure" and the second nip pressure may be referred to as a "strong nip pressure."

[0048] 3 and 4(a), nip pressure change mechanism 300 includes arm 310, first spring 320, second spring 330, and cam 340. Arm 310, first spring 320, second spring 330, and cam 340 are provided on both sides of roller 120 in the axial direction relative to heating unit 81 and pressure unit 82, respectively.

[0049] The arm 310 is a member that supports the pressure applying portion 82. The arm 310 is rotatably supported by the side frame 83. The arm 310 has an arm body 311 and a cam follower 350.

[0050] The arm body 311 has one end 311A ​​rotatably supported by the side frame 83, the other end 311B to which the first spring 320 is connected, and an engagement hole 311C that supports the pressure member 82. The engagement hole 311C is disposed between the one end 311A ​​and the other end 311B, and engages with a buffer member BF attached to the first stay 210. The arm body 311 further has a guide protrusion 312 extending toward the cam 340.

[0051] Cam follower 350 is attached so as to be movable relative to guide protrusion 312, and is capable of contacting cam 340. Cam follower 350 is made of resin or the like, and has a cylindrical portion 351 that fits into guide protrusion 312, a contact portion 352 provided at one end of cylindrical portion 351, and a flange portion 353 provided at the other end of cylindrical portion 351. Cam follower 350 is supported by guide protrusion 312 so as to be movable in the direction in which guide protrusion 312 extends.

[0052] The first spring 320 is a spring that applies a biasing force to the pressure applying portion 82. The first spring 320 is a tension coil spring made of metal or the like, and one end is connected to the spring engaging portion 83A of the side frame 83 and the other end is connected to the other end portion 311B of the arm main body 311.

[0053] The second spring 330 is a compression coil spring made of metal or the like, and is disposed between the cylindrical portion 351 and the arm main body 311 with the guide protrusion 312 inserted into the space surrounded by the compression coil spring.

[0054] Cam 340 is supported by side frame 83 so as to be rotatable between a first cam position shown in Fig. 4(a) and a second cam position shown in Fig. 5(a). Cam 340 is made of resin or the like, and has a first portion 341 and a second portion 342. First portion 341 and second portion 342 are located on the outer circumferential surface of cam 340.

[0055] As shown in FIG. 4(a), the first portion 341 is the portion closest to the cam follower 350 when the cam 340 is located at the first cam position. When the cam 340 is located at the first cam position, the first portion 341 is separated from the cam follower 350. The second portion 342 is the portion that comes into contact with the cam follower 350 when the cam 340 is located at the second cam position. In detail, the second portion 342 comes into contact with the contact portion 352 of the cam follower 350 when the cam 340 rotates approximately 270 degrees in the clockwise direction from the first cam position as shown in FIG. 5(a).

[0056] 4(a) and (b), when the cam 340 is located at the first cam position, the nip pressure between the heating unit 81 and the pressure unit 82 is strong nip pressure. Also, as shown in Figs. 5(a) and (b), when the cam 340 is located at the second cam position, the nip pressure between the heating unit 81 and the pressure unit 82 is weak nip pressure.

[0057] Nip pressure change mechanism 300 changes the nip pressure from weak nip pressure to strong nip pressure by rotating cam 340 counterclockwise from the second cam position shown in Fig. 5(a) to the first cam position shown in Fig. 4(a). Also, nip pressure change mechanism 300 changes the nip pressure from strong nip pressure to weak nip pressure by rotating cam 340 clockwise from the first cam position shown in Fig. 4(a) to the second cam position shown in Fig. 5(a).

[0058] As shown in FIG. 6, the color printer 1 further includes a developing motor M1, a fixing motor M2, a process motor M3, a main motor M4, a developing clutch C1, a switching mechanism SW, a nip pressure changing clutch C2, and a temperature sensor ST.

[0059] The developing motor M1 is a motor that mainly drives and rotates each developing roller 53, and is configured to be capable of rotating in both forward and reverse directions. In this embodiment, the direction of rotation of the developing motor M1 during printing is the forward direction. The developing motor M1 and each developing roller 53 are connected via gears and clutches (not shown). The developing motor M1 is connected to the switching mechanism SW via a developing clutch C1 and gears (not shown). The developing motor M1 is connected to a cam 340 of the nip pressure change mechanism 300 via a nip pressure change clutch C2 and gears (not shown).

[0060] The fixing motor M2 is a motor for driving and rotating the heating unit 81. More specifically, the fixing motor M2 drives and rotates the roller 120. The process motor M3 is a motor for applying a driving force to members constituting the process unit 30. In more detail, the process motor M3 drives and rotates the photosensitive drum 51 and the like. The main motor M4 is a motor for applying a driving force to rollers constituting the sheet supply unit 20. In detail, the main motor M4 drives and rotates the pickup roller 23, the separation roller 24, the registration roller 26, and the like.

[0061] The developing clutch C1 is, for example, an electromagnetic clutch. The developing clutch C1 is switchable between a first transmission state in which the driving force of the developing motor M1 is transmitted to the switching mechanism SW and a first disconnection state in which the driving force of the developing motor M1 is not transmitted to the switching mechanism SW.

[0062] The switching mechanism SW is a mechanism for switching the state of the developing roller 53 between a pressure-contact state in which the developing roller 53 is in pressure contact with the photosensitive drum 51 and a separated state in which the developing roller 53 is separated from the photosensitive drum 51. When the developing roller 53 is in the separated state and the developing motor M1 is rotating forward, the switching mechanism SW switches the state of the developing roller 53 from the separated state to the pressure-contact state when the developing clutch C1 becomes the first transmission state. Also, when the developing roller 53 is in the pressure-contact state and the developing motor M1 is rotating forward, the switching mechanism SW switches the state of the developing roller 53 from the pressure-contact state to the separated state when the developing clutch C1 becomes the first transmission state.

[0063] The nip pressure change clutch C2 is, for example, an electromagnetic clutch. The nip pressure change clutch C2 is switchable between a second transmission state in which the driving force of the developing motor M1 is transmitted to the cam 340 of the nip pressure change mechanism 300 and a second disconnection state in which the driving force of the developing motor M1 is not transmitted to the cam 340 of the nip pressure change mechanism 300.

[0064] When the cam 340 is located at the second cam position (see FIG. 5(a)) and the developing motor M1 is rotating forward, and the nip pressure change clutch C2 is in the second transmission state, the cam 340 rotates counterclockwise from the second cam position to the first cam position shown in FIG. 4(a) in the illustrated direction. When the cam 340 is located at the first cam position and the developing motor M1 is rotating backward, and the nip pressure change clutch C2 is in the second transmission state, the cam 340 rotates clockwise from the first cam position to the second cam position shown in FIG. 5(a) in the illustrated direction.

[0065] The temperature sensor ST is a sensor for detecting the temperature of the heating unit 81. For example, a thermistor can be used as the temperature sensor ST. In this embodiment, the temperature sensor ST is provided to detect the temperature of the roller 120. The temperature sensor ST may be in contact with the roller 120 or may not be in contact with the roller 120. The temperature sensor ST may be a temperature sensor included in the fixing device 80, or may be a temperature sensor provided separately from the fixing device 80.

[0066] The control unit 100 is equipped with a CPU, RAM, ROM, input / output circuits, etc., and executes control by performing various arithmetic processing based on print commands output from an external computer, information output from the temperature sensor ST, etc., and programs and data stored in the ROM, etc.

[0067] When control unit 100 receives a print command, it first starts heating unit 81, and then starts rotating heating unit 81. Thereafter, control unit 100 switches the nip pressure of heating unit 81 and pressure unit 82 from weak nip pressure to strong nip pressure using nip pressure changing mechanism 300 as necessary, and then starts supplying sheet S using sheet supply unit 20. In this embodiment, the nip pressure is weak nip pressure when control unit 100 (color printer 1) is in a standby state waiting to receive a print command.

[0068] When the control unit 100 receives a print command, it starts heating the heating unit 81 so that the detected temperature T detected by the temperature sensor ST becomes a predetermined target temperature TT. Specifically, when the control unit 100 receives a print command, it starts heating the heating unit 81 by starting the supply of electricity to the heater 110. The target temperature TT is a temperature that is set according to, for example, the thickness and conveying speed of the sheet S in order to fix the toner image transferred onto the sheet S to the sheet S.

[0069] After starting heating of heating unit 81, if detected temperature T is equal to or higher than rotation start temperature TR, control unit 100 starts rotation of heating unit 81 with the nip pressure between heating unit 81 and pressure unit 82 set to weak nip pressure. Specifically, if detected temperature T is equal to or higher than rotation start temperature TR, control unit 100 starts driving fixing motor M2 to start rotation of roller 120 of heating unit 81. Rotation start temperature TR is a temperature lower than target temperature TT.

[0070] After starting the rotation of the heating unit 81, the control unit 100 starts supplying the sheet S on the condition that the sheet supply condition that the detected temperature T is equal to or higher than the sheet supply temperature TS is satisfied. The sheet supply temperature TS is lower than the target temperature TT and higher than the nip pressure change temperature TN described later.

[0071] In this embodiment, the control unit 100 starts supplying the sheet S on the condition that both the above-mentioned sheet supply condition and a second sheet supply condition, that is, a sheet supply time ZS has elapsed from a predetermined timing after the timing when the rotation of the heating unit 81 is started, are satisfied. In this embodiment, the predetermined timing is the timing when the rotation of the heating unit 81 is started. Hereinafter, the predetermined timing may be referred to as the "rotation start timing."

[0072] To explain in more detail the supply of sheet S when a print command is received, in this embodiment, if the detected temperature T is less than the rotation start temperature TR when the control unit 100 receives the print command, the control unit 100 sets the sheet supply temperature TS to a first sheet supply temperature TS1 and sets the sheet supply time ZS to a first sheet supply time ZS1. Then, the control unit 100 starts supplying sheet S on the condition that both the sheet supply condition that the detected temperature T is equal to or higher than the first sheet supply temperature TS1 and the second sheet supply condition that the first sheet supply time ZS1 has elapsed since the rotation start timing are satisfied.

[0073] Furthermore, even if both the sheet supply condition and the second sheet supply condition are satisfied, if the detected temperature T becomes equal to or higher than the first sheet supply temperature TS1 after a third sheet supply time ZS3 that is longer than the first sheet supply time ZS1 has elapsed from the rotation start timing, the control unit 100 starts supplying the sheet S when the detected temperature T becomes equal to or higher than a third sheet supply temperature TS3 that is higher than the first sheet supply temperature TS1. The third sheet supply temperature TS3 is a temperature lower than the target temperature TT, similar to the first sheet supply temperature TS1.

[0074] Furthermore, if the detected temperature T is equal to or higher than the rotation start temperature TR when the control unit 100 receives a print command, it sets the sheet supply temperature TS to a second sheet supply temperature TS2 lower than the first sheet supply temperature TS1 and sets the sheet supply time ZS to a second sheet supply time ZS2 shorter than the first sheet supply time ZS1.The control unit 100 then starts supplying the sheet S on the condition that both the sheet supply condition that the detected temperature T is equal to or higher than the second sheet supply temperature TS2 and the second sheet supply condition that the second sheet supply time ZS2 has elapsed since the rotation start timing are satisfied.

[0075] The control unit 100 can execute a first printing mode and a second printing mode. The first printing mode is a printing mode in which a toner image is fixed to the sheet S with a weak nip pressure, and the second printing mode is a printing mode in which a toner image is fixed to the sheet S with a strong nip pressure. When the control unit 100 receives a printing command in which a sheet S with a first thickness is specified by the user, the control unit 100 executes the second printing mode. Also, when the control unit 100 receives a printing command in which a sheet S with a second thickness that is thicker than the first thickness is specified by the user, the control unit 100 executes the first printing mode.

[0076] To give a specific example, the second print mode is a print mode for printing on plain paper, and the first print mode is a print mode for printing on envelopes. When the control unit 100 receives a print command specifying plain paper from the user, the control unit 100 executes the second print mode, and when the control unit 100 receives a print command specifying envelopes from the user, the control unit 100 executes the first print mode.

[0077] When the first printing mode is executed, the control unit 100 does not change the nip pressure of the heating unit 81 and the pressure unit 82 from the initial nip pressure, which is the weak nip pressure. That is, when the first printing mode is executed, the control unit 100 starts heating of the heating unit 81, starts rotation of the heating unit 81, and then starts supplying the sheet S while keeping the nip pressure at the weak nip pressure.

[0078] When the second printing mode is executed, the control unit 100 changes the nip pressure of the heating unit 81 and the pressure unit 82 from weak nip pressure to strong nip pressure. That is, when the second printing mode is executed, the control unit 100 starts heating of the heating unit 81, starts rotation of the heating unit 81, changes the nip pressure from weak nip pressure to strong nip pressure, and then starts supplying the sheet S.

[0079] Specifically, when the second printing mode is executed, the control unit 100 changes the nip pressure from weak to strong nip pressure on the condition that the detected temperature T is equal to or higher than the nip pressure change temperature TN, which is a temperature lower than the target temperature TT and higher than the rotation start temperature TR.

[0080] In this embodiment, when both the above-mentioned nip pressure change condition and a second nip pressure change condition that a nip pressure change time ZN has elapsed from the timing when the heating unit 81 starts rotating are satisfied, the control unit 100 changes the nip pressure from the weak nip pressure to the strong nip pressure. The nip pressure change time ZN is shorter than the sheet supply time ZS. More specifically, the nip pressure change time ZN is shorter than the first sheet supply time ZS1.

[0081] When printing in the second print mode is completed, control unit 100 changes the nip pressure of heating unit 81 and pressure unit 82 from strong nip pressure to weak nip pressure by nip pressure change mechanism 300. In other words, when printing in the second print mode is completed, control unit 100 returns the nip pressure from the strong nip pressure to the weak nip pressure, which is the nip pressure when in a standby state waiting for receipt of a print command.

[0082] When changing the nip pressure from strong nip pressure to weak nip pressure, first, while the developing roller 53 is in the pressure contact state and the developing motor M1 is rotating forward, the control unit 100 sets the developing clutch C1 to the first transmission state, drives the switching mechanism SW, and causes the switching mechanism SW to set the developing roller 53 to the separated state. After that, the control unit 100 sets the developing clutch C1 to the first disconnection state, and stops the switching mechanism SW.

[0083] Next, the control unit 100 stops the developing motor M1 once, and then rotates the developing motor M1 in the reverse direction. Then, the control unit 100 puts the nip pressure changing clutch C2 into the second transmission state and rotates the cam 340 from the first position to the second position, thereby changing the nip pressure from the strong nip pressure to the weak nip pressure. When the nip pressure becomes the weak nip pressure, the control unit 100 puts the nip pressure changing clutch C2 into the second disconnection state.

[0084] Next, an example of the operation of the control unit 100 will be described with reference to a flowchart. As shown in FIG. 7, when a print command is received, the control unit 100 starts energizing the heater 110, thereby starting heating of the heating unit 81 (S11).

[0085] Next, the control unit 100 determines whether the detected temperature T when the print command is received is less than the rotation start temperature TR (S21). If the detected temperature T is less than the rotation start temperature TR (S21, Yes), the control unit 100 determines whether the detected temperature T is equal to or greater than the rotation start temperature TR (S22).

[0086] When the detected temperature T becomes equal to or higher than the rotation start temperature TR (S22, Yes), the control unit 100 starts driving the fixing motor M2, thereby starting the rotation of the heating unit 81 (roller 120) (S23).

[0087] Thereafter, the control unit 100 determines whether the elapsed time Z from the start of rotation of the heating unit 81 is equal to or greater than the first sheet supply time ZS1 (S31). If the elapsed time Z is equal to or greater than the first sheet supply time ZS1 (Yes in S31), the control unit 100 determines whether the detected temperature T is equal to or greater than the first sheet supply temperature TS1 (S32).

[0088] If the detected temperature T is equal to or higher than the first sheet supply temperature TS1 (Yes in S32), the control unit 100 determines whether the elapsed time Z from when the heating unit 81 starts rotating is less than the third sheet supply time ZS3 (S33).

[0089] If the elapsed time Z is less than the third sheet supply time ZS3 (S33, Yes), the control unit 100 rotates the pickup roller 23 of the sheet supply unit 20 to supply one sheet S (S37).

[0090] In step S33, if the elapsed time Z is equal to or greater than the third sheet supply time ZS3 (No), the control unit 100 determines whether the detected temperature T is equal to or greater than the third sheet supply temperature TS3 (S34). If the detected temperature T is equal to or greater than the third sheet supply temperature TS3 (S34, Yes), the control unit 100 rotates the pickup roller 23 to supply one sheet S (S37).

[0091] In step S21, if the detected temperature T when the print command is received is equal to or higher than the rotation start temperature TR (No), the control unit 100 starts driving the fixing motor M2 to start rotating the heating unit 81 (roller 120) (S24).

[0092] Thereafter, the control unit 100 determines whether the elapsed time Z from when the heating unit 81 starts rotating is equal to or greater than the second sheet supply time ZS2 (S35). If the elapsed time Z is equal to or greater than the second sheet supply time ZS2 (S35, Yes), the control unit 100 determines whether the detected temperature T is equal to or greater than the second sheet supply temperature TS2 (S36).

[0093] If the detected temperature T is equal to or higher than the second sheet supply temperature TS2 (S36, Yes), the control unit 100 rotates the pickup roller 23 to supply one sheet S (S37).

[0094] After step S37, the control unit 100 determines whether printing is completed (S41). If printing is not completed (S41, No), the control unit 100 returns to step S37 and supplies the second and subsequent sheets S at a predetermined timing. In detail, the control unit 100 supplies the next sheet S by rotating the pickup roller 23 after a predetermined time has elapsed since the previous sheet S was supplied, so that the interval between the sheets S becomes a predetermined interval.

[0095] If printing is completed in step S37 (Yes), the control unit 100 ends the process of the sheet supply flowchart shown in FIG.

[0096] 8, when a print command is received, the control unit 100 determines whether or not to execute the second print mode (S51). If the first print mode is to be executed (S51, No), the control unit 100 ends the process of the nip pressure change flowchart shown in FIG. 8 without changing the nip pressure of the heating unit 81 and the pressure unit 82 from the weak nip pressure.

[0097] In step S51, if the second print mode is to be executed (Yes), the control unit 100 determines whether the heating unit 81 is rotating (S52). If the heating unit 81 is rotating (S52, Yes), the control unit 100 determines whether the elapsed time Z from when the heating unit 81 started to rotate is equal to or longer than the nip pressure change time ZN (S53).

[0098] If the elapsed time Z is equal to or greater than the nip pressure change time ZN (S53, Yes), the control unit 100 determines whether the detected temperature T is equal to or greater than the nip pressure change temperature TN (S54).

[0099] If the detected temperature T is equal to or higher than the nip pressure change temperature TN (S54, Yes), the control unit 100 puts the nip pressure change clutch C2 into the second transmission state, rotates the cam 340 from the second position toward the first position, and changes the nip pressure of the heating unit 81 and the pressure unit 82 from weak nip pressure to strong nip pressure (S55). When the nip pressure becomes strong nip pressure, the control unit 100 puts the nip pressure change clutch C2 into the second disconnection state.

[0100] After step S55, the control unit 100 determines whether printing has been completed (S56). If printing has been completed (Yes in S56), the control unit 100 changes the nip pressure of the heating unit 81 and the pressure unit 82 from the strong nip pressure to the weak nip pressure (S57), and ends the process of the nip pressure change flowchart shown in FIG.

[0101] Next, an example of the operation of the control unit 100 will be described with reference to a timing chart. 9 shows a case where the detected temperature T when a print command is received is lower than the rotation start temperature TR. In this case, the control unit 100 starts heating the heating unit 81, and then starts rotating the heating unit 81 when the detected temperature T reaches the rotation start temperature TR (time t11).

[0102] 9, if the detected temperature T reaches the first sheet supply temperature TS1 (time t14) before the first sheet supply time ZS1 has elapsed from the rotation start timing (before time t15), the gradient of the temperature rise of the fixing device 80 (hereinafter referred to as the "temperature gradient") is steep. The temperature gradient becomes steep, for example, when the power supply voltage is high. When the temperature gradient is steep, the control unit 100 waits until the first sheet supply time ZS1 has elapsed, and starts supplying the sheet S when the first sheet supply time ZS1 has elapsed (time t15).

[0103] Furthermore, when the temperature gradient is steep, the detected temperature T reaches the nip pressure change temperature TN before the nip pressure change time ZN has elapsed from the rotation start timing (before time t12), and then the nip pressure change time ZN elapses (time t12). In this case, the control unit 100 starts switching the nip pressure when the nip pressure change time ZN, which is shorter than the first sheet supply time ZS1, has elapsed (time t12). That is, the control unit 100 starts switching the nip pressure before the supply of the sheet S starts.

[0104] 9, when the detected temperature T reaches the first sheet supply temperature TS1 (time t16) after the rotation start timing and after the first sheet supply time ZS1 has elapsed (after time t15) and before the third sheet supply time ZS3 has elapsed (before time t17), the temperature gradient is intermediate, which is smaller than a steep gradient. When the temperature gradient is intermediate, the control unit 100 starts supplying the sheet S when the detected temperature T reaches the first sheet supply temperature TS1 (time t16).

[0105] Also, when the temperature gradient is medium, the control unit 100 starts switching the nip pressure when both the nip pressure change condition that the detected temperature T is equal to or higher than the nip pressure change temperature TN and the second nip pressure change condition that the nip pressure change time ZN has elapsed from the rotation start timing are satisfied (time t12). In this case as well, the control unit 100 starts switching the nip pressure before the supply of the sheet S is started.

[0106] 9, when the detected temperature T reaches the first sheet supply temperature TS1 (at time t18) after the third sheet supply time ZS3 has elapsed from the rotation start timing (after time t17), the temperature gradient is gentle, which is smaller than the medium gradient. The temperature gradient becomes gentle, for example, when the power supply voltage is low. When the temperature gradient is gentle, the control unit 100 does not start supplying the sheet S even if the detected temperature T reaches the first sheet supply temperature TS1, but starts supplying the sheet S when the detected temperature T reaches a third sheet supply temperature TS3 (at time t19) which is higher than the first sheet supply temperature TS1.

[0107] Furthermore, when the temperature gradient is gentle, the detected temperature T reaches the nip pressure change temperature TN (time t13) after the nip pressure change time ZN has elapsed from the rotation start timing (after time t12). In this case, the control unit 100 starts switching the nip pressure when the detected temperature T reaches the nip pressure change temperature TN (time t13) which is lower than the first sheet supply temperature TS1. In this case as well, the control unit 100 starts switching the nip pressure before the supply of the sheet S starts.

[0108] 10 shows a case where the detected temperature T when the print command is received is equal to or higher than the rotation start temperature TR. In this case, the control unit 100 starts heating the heating unit 81, and starts rotating the heating unit 81 at approximately the same timing as when the heating unit 81 starts heating (time t21).

[0109] 10, when the temperature gradient is steep, the detected temperature T reaches the second sheet supply temperature TS2 (time t23) before the second sheet supply time ZS2 has elapsed from the rotation start timing (before time t24). In this case, the control unit 100 waits until the second sheet supply time ZS2 has elapsed, and starts supplying the sheet S when the second sheet supply time ZS2 has elapsed (time t24).

[0110] 10 is a medium temperature gradient, and when the two-dot chain line is a gentle temperature gradient, the detected temperature T reaches the second sheet supply temperature TS2 after the second sheet supply time ZS2 has elapsed (after time t24) from the rotation start timing and before the third sheet supply time ZS3 has elapsed (before time t27). In this case, the control unit 100 starts supplying the sheet S when the detected temperature T reaches the second sheet supply temperature TS2. Specifically, when the temperature gradient is a medium gradient, the control unit 100 starts supplying the sheet S at time t25 when the detected temperature T reaches the second sheet supply temperature TS2, and when the temperature gradient is a gentle gradient, the control unit 100 starts supplying the sheet S at time t26 when the detected temperature T reaches the second sheet supply temperature TS2.

[0111] Furthermore, regardless of the temperature gradient, controller 100 starts switching the nip pressure when both a nip pressure change condition that detected temperature T is equal to or higher than nip pressure change temperature TN and a second nip pressure change condition that nip pressure change time ZN has elapsed from the rotation start timing are satisfied (time t22). In this case as well, controller 100 starts switching the nip pressure before the supply of sheet S starts.

[0112] According to the present embodiment described above, by increasing the nip pressure between the heating unit 81 and the pressure applying unit 82, the heat of the heating unit 81 is easily transferred to the pressure applying unit 82, and the heating unit 81 can heat the pressure applying unit 82 well. Then, by starting to switch the nip pressure and increasing the nip pressure before the supply of the sheet S is started, the heating of the pressure applying unit 82 can be started before the supply of the sheet S is started. As a result, both the heating unit 81 and the pressure applying unit 82 can be sufficiently heated to a temperature suitable for fixing by the time the sheet S, whose supply has started thereafter, reaches the fixing device 80. This can improve the fixability of the toner image on the sheet S.

[0113] Furthermore, the nip pressure of heating unit 81 and pressure unit 82 is changed by nip pressure change mechanism 300 on the condition that the nip pressure change condition, that is, the detected temperature T is equal to or higher than the nip pressure change temperature TN, is satisfied, so that the nip pressure can be changed by nip pressure change mechanism 300 after the entire fixing device 80 has warmed up to a certain degree and the lubricants in each portion have softened to a certain degree. This allows nip pressure change mechanism 300 to be driven smoothly with a smaller driving force.

[0114] Furthermore, since the supply of sheet S is started on the condition that both the sheet supply condition that the detected temperature T is equal to or higher than the sheet supply temperature TS and the second sheet supply condition that the sheet supply time ZS has elapsed from the rotation start timing are satisfied, it is possible to wait until the sheet supply time ZS has elapsed before starting the supply of sheet S. This makes it possible to reduce the difference in temperature between the part of heating unit 81 detected by temperature sensor ST and the other parts, i.e., the temperature unevenness, so that heating unit 81 and pressure unit 82 can be heated uniformly, and the fixability of the toner image on sheet S can be further improved.

[0115] Furthermore, if the detected temperature T is equal to or higher than the rotation start temperature TR when a print command is received, the heating unit 81 and the pressure unit 82 are already warmed to a certain extent, so that the heating unit 81 and the pressure unit 82 can be heated to a temperature suitable for fixing in a relatively short time. In this case, the sheet supply temperature TS is set to the second sheet supply temperature TS2, which is lower than the first sheet supply temperature TS1, thereby shortening the time from when the print command is received until the supply of the sheet S begins. This makes it possible to shorten the time until printing of the first sheet S is completed.

[0116] Similarly, if the detected temperature T is equal to or higher than the rotation start temperature TR when a print command is received, the sheet supply time ZS is set to a second sheet supply time ZS2 that is shorter than the first sheet supply time ZS1, thereby shortening the time from when the print command is received until the supply of the sheet S begins. This makes it possible to shorten the time until printing of the first sheet S is completed.

[0117] Furthermore, when the detected temperature T reaches the first sheet supply temperature TS1 after the third sheet supply time ZS3 has elapsed from the rotation start timing, the temperature gradient is small and gentle, so if the supply of the sheet S is started at the timing when the detected temperature T reaches the first sheet supply temperature TS1, there is a possibility that the temperatures of the heating section 81 and the pressure section 82 have not reached a temperature suitable for fixing when the sheet S reaches the fixing device 80. Therefore, in this case, by waiting until the detected temperature T reaches the third sheet supply temperature TS3 before starting the supply of the sheet S, it is possible to raise the temperatures of the heating section 81 and the pressure section 82 to a temperature suitable for fixing when the sheet S reaches the fixing device 80. This makes it possible to improve the fixability of the toner image on the sheet S.

[0118] Although the embodiment has been described above, the present invention is not limited to the above embodiment, and can be modified as appropriate as exemplified below.

[0119] For example, in the above embodiment, the nip pressure of the heating unit 81 and the pressure applying unit 82 is changed from weak nip pressure to strong nip pressure when both the nip pressure change condition that the detected temperature T is equal to or higher than the nip pressure change temperature TN and the second nip pressure change condition that the nip pressure change time ZN has elapsed since the rotation start timing are satisfied, but the present invention is not limited to this. For example, the nip pressure may be changed from weak nip pressure to strong nip pressure when the detected temperature T is equal to or higher than the nip pressure change temperature TN.

[0120] In the above embodiment, the supply of the sheet S is started when both the sheet supply condition that the detected temperature T is equal to or higher than the sheet supply temperature TS and the second sheet supply condition that the sheet supply time ZS has elapsed from the rotation start timing are satisfied, but the present invention is not limited to this. For example, the supply of the sheet S may be started when the detected temperature T is equal to or higher than the sheet supply temperature TS.

[0121] In the above embodiment, when the detected temperature T is less than the rotation start temperature TR when the print command is received, the sheet supply temperature TS is set to the first sheet supply temperature TS1, and when the detected temperature T is equal to or greater than the rotation start temperature TR when the print command is received, the sheet supply temperature TS is set to the second sheet supply temperature TS2, but this is not limiting. For example, the sheet supply temperature TS may be the same regardless of the detected temperature T when the print command is received.

[0122] In the above embodiment, if the detected temperature T is less than the rotation start temperature TR when the print command is received, the sheet supply time ZS is the first sheet supply time ZS1, and if the detected temperature T is equal to or greater than the rotation start temperature TR when the print command is received, the sheet supply time ZS is the second sheet supply time ZS2, but this is not limiting. For example, the sheet supply time ZS may be the same regardless of the detected temperature T when the print command is received.

[0123] In the above embodiment, two types of printing modes, the first printing mode and the second printing mode, can be executed, and the nip pressure of the heating unit 81 and the pressure applying unit 82 is changed from weak nip pressure to strong nip pressure when the second printing mode is executed, but this is not limited to the above. For example, three or more types of printing modes may be executed, and the nip pressure may be changed from weak nip pressure to strong nip pressure when at least one type of printing mode is executed. Also, only one type of printing mode may be executed. Also, the nip pressure may be changed from weak nip pressure to strong nip pressure whenever printing is executed.

[0124] In the above embodiment, the timing when the rotation of the heating unit 81 starts (rotation start timing) is exemplified as the predetermined timing, but the predetermined timing is not limited to this. The predetermined timing may be a timing after the timing when the rotation of the heating unit 81 starts. For example, the predetermined timing may be a timing when the detected temperature T reaches the nip pressure change temperature TN.

[0125] In the above embodiment, the rotation of the heating unit 81 is started by starting the drive of the fixing motor M2, but this is not limited to the above. For example, when the fixing device 80 is connected to the motor via a clutch, the rotation of the heating unit 81 may be started by putting the clutch into a transmission state. When the fixing device 80 is connected to the motor via a clutch, the motor that drives the fixing device 80 does not have to be a dedicated motor like the fixing motor M2 in the above embodiment.

[0126] In addition, the specific configuration of the nip pressure change mechanism is not limited to the configuration of the above embodiment. For example, in the above embodiment, the nip pressure change mechanism 300 is configured to be able to change the nip pressure of the heating unit 81 and the pressure unit 82 to two types of nip pressure, namely, a weak nip pressure and a strong nip pressure, but is not limited thereto, and may be configured to be able to change the nip pressure to three or more types of nip pressure.

[0127] As an example, the nip pressure changing mechanism may be configured to change the nip pressure to a first nip pressure, a second weak nip pressure, and a second strong nip pressure. The second weak nip pressure and the second strong nip pressure are both second nip pressures that are greater than the first nip pressure, and the second strong nip pressure is greater than the second weak nip pressure. In this case, for example, when the first printing mode is executed, the nip pressure may be changed from the first nip pressure to the second weak nip pressure, and when the second printing mode is executed, the nip pressure may be changed from the first nip pressure to the second strong nip pressure.

[0128] In the above embodiment, nip pressure change mechanism 300 includes second spring 330 and cam follower 350, but is not limited thereto, and may not include second spring 330 and cam follower 350, and may have a configuration in which cam 340 can directly contact arm body 311. In the above embodiment, nip pressure change mechanism 300 moves pressure unit 82 relative to heating unit 81, but is not limited thereto, and may have a configuration in which heating unit 81 moves relative to pressure unit 82.

[0129] Furthermore, the specific configuration of the fixing unit is not limited to that of the above embodiment. For example, in the above embodiment, a halogen lamp that uses radiant heat is exemplified as heater 110 of heating unit 81, but this is not limiting. For example, the heater may be a ceramic heater or a carbon heater that uses heat generated by a resistor, or an IH heater that inductively heats the heating unit. Furthermore, the heater may be disposed outside roller 120 instead of inside roller 120.

[0130] In the above embodiment, the heating unit 81 is a so-called heating roller including the roller 120, but is not limited thereto. For example, the heating unit may be configured to include a cylindrical belt or a nip member disposed inside the belt and sandwiching the belt between the heating unit and the pressure unit. In the above embodiment, the pressure unit 82 is configured to include the belt 130 and the nip forming member N, but is not limited thereto. For example, the pressure unit may be a so-called pressure roller including a core metal and an elastic layer formed around the core metal.

[0131] In the above embodiment, the fixing device 80 is configured to include the nip pressure change mechanism 300, but the present invention is not limited to this. For example, the nip pressure change mechanism may be provided separately from the fixing device. Also, a part of the nip pressure change mechanism may be provided in the fixing device, and the remaining part may be provided separately from the fixing device.

[0132] Furthermore, the specific configuration of the sheet supply unit is not limited to that of the above embodiment. For example, the sheet supply unit may be configured to supply sheets from a so-called manual feed tray instead of a supply tray that is detachable from the main body housing 2.

[0133] The specific configuration of the process unit is not limited to that of the embodiment. For example, the process unit may be configured to include a scanner unit that exposes the surface of the photosensitive drum 51 by emitting a light beam instead of the LED unit 40. The process unit may be configured to include an intermediate transfer belt and a secondary transfer roller instead of the conveyor belt 73. The process unit may be configured to include a photosensitive belt instead of the photosensitive drum 51.

[0134] In the above embodiment, the image forming apparatus is exemplified as a color printer 1, but is not limited thereto, and may be, for example, a monochrome printer capable of forming only monochrome images. Also, the image forming apparatus is not limited to a printer, and may be, for example, a copier or a multifunction machine.

[0135] Furthermore, the elements described in the above embodiment and modified examples may be implemented in any combination. [Explanation of symbols]

[0136] 1. Color printer 20 Sheet supply section 30 Process Section 80 Fixing unit 81 Heating section 82 Pressurizing section 100 Control section 300 Nip pressure change mechanism S Seat ST Temperature Sensor

Claims

1. a sheet supply unit that supplies a sheet; a process unit that transfers a toner image onto a sheet supplied from the sheet supply unit; a fixing unit for fixing a toner image on a sheet conveyed from the process unit, the fixing unit including a heating unit for heating the sheet and a pressure unit for sandwiching the sheet between the heating unit and the sheet; a nip pressure changing mechanism capable of changing a nip pressure between the heating unit and the pressing unit between a first nip pressure and a second nip pressure greater than the first nip pressure; A temperature sensor for detecting a temperature of the heating unit; A control unit, When the control unit receives a print command, heating the heating unit so that the temperature detected by the temperature sensor reaches a predetermined target temperature; When the detected temperature is equal to or higher than a rotation start temperature that is lower than the target temperature, the rotation of the heating unit is started with the nip pressure set to the first nip pressure; changing the nip pressure from the first nip pressure to the second nip pressure by the nip pressure changing mechanism under the condition that a nip pressure change condition is satisfied, in which the detected temperature is lower than the target temperature and is equal to or higher than a nip pressure change temperature that is higher than the rotation start temperature; starting the supply of a sheet by the sheet supply unit under a condition that a sheet supply condition is satisfied in which the detected temperature is lower than the target temperature and is equal to or higher than a sheet supply temperature that is higher than the nip pressure change temperature; The control unit is A first printing mode in which a toner image is fixed to a sheet with the first nip pressure and a second printing mode in which a toner image is fixed to a sheet with the second nip pressure can be executed, When the first printing mode is executed, the nip pressure is not changed from the first nip pressure, When the second printing mode is executed, the nip pressure is changed from the first nip pressure to the second nip pressure by the nip pressure changing mechanism on condition that the nip pressure change condition is satisfied.

1. An image forming apparatus comprising:

2. The control unit is If the detected temperature is lower than the rotation start temperature when a print command is received, the sheet supply temperature is set to a first sheet supply temperature; When a print command is received and the detected temperature is equal to or higher than the rotation start temperature, the sheet supply temperature is set to a second sheet supply temperature that is lower than the first sheet supply temperature.

2. The image forming apparatus according to claim 1,

3. The control unit is The sheet supply unit starts supplying a sheet on the condition that both the sheet supply condition and a second sheet supply condition that a sheet supply time has elapsed from a predetermined timing after the timing when the rotation of the heating unit is started are satisfied.

3. The image forming apparatus according to claim 1, wherein the first and second electrodes are arranged in a first direction.

4. The control unit is If the detected temperature is lower than the rotation start temperature when a print command is received, the sheet supply time is set as a first sheet supply time; When the detected temperature is equal to or higher than the rotation start temperature when a print command is received, the sheet supply time is set to a second sheet supply time that is shorter than the first sheet supply time.

4. The image forming apparatus according to claim 3.

5. The control unit is The image forming apparatus according to claim 3 or claim 4, characterized in that when both the nip pressure change condition and a second nip pressure change condition, that is, a nip pressure change time has elapsed from the timing at which rotation of the heating section is started, are satisfied, the nip pressure is changed from the first nip pressure to the second nip pressure by the nip pressure change mechanism.

6. the predetermined timing is a timing at which the rotation of the heating unit is started, 6. The image forming apparatus according to claim 5, wherein the nip pressure change time is shorter than the sheet supply time.

7. The control unit is When the detected temperature becomes equal to or higher than the sheet supply temperature after a third sheet supply time that is longer than the sheet supply time has elapsed from the predetermined timing, when the detected temperature becomes equal to or higher than a third sheet supply temperature that is lower than the target temperature and higher than the sheet supply temperature, the sheet supply unit starts supplying a sheet.

7. The image forming apparatus according to claim 3, wherein the first and second electrodes are arranged in a first direction.

Images