JP2010002670A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device for reducing warm-up time before starting a fixing operation. <P>SOLUTION: This image forming device has a fixing device 60 provided with a heating roller 61 heated by a heat source 62, and a pressure roller 65 provided facing the heating roller, to hold a paper sheet 22 with pressure together with the heating roller, and for fixing an unfixed toner image on the paper sheet, by heating the paper sheet 22 conveyed by a nip part 69 formed between the heating roller and the pressure roller and by pressurizing the paper sheet 22 with a prescribed pressure. The image forming device is provided with a pressure variable device 67 for changing the pressure of the pressure roller, and a controller 68 for controlling an operation of the pressure variable device. The controller controls the operation of the pressure variable device, to make the pressure of the pressure roller when starting the fixing operation higher than the prescribed pressure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine having these functions in combination.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, as a fixing device for fixing an unfixed toner image on a sheet, a heating roller as a heating member heated by a heat source, and a sheet together with the heating roller provided facing the heating roller. There is known a fixing device that includes a pressure roller that sandwiches and presses the sheet, and fixes the unfixed toner image on the sheet while nipping and conveying the sheet at a nip formed between the heating roller and the pressure roller. ing.

  As an image forming apparatus provided with such a fixing device, for example, Patent Document 1 includes a pair of roller-shaped fixing members, each of which includes a first fixing roller and a second fixing roller each having a heater therein. Image forming apparatus having a fixing device that fixes a toner image on a recording sheet conveyed through the nip portion by applying heat and pressure while nipping and conveying the recording sheet at a nip portion between the fixing roller and the second fixing roller Is disclosed.

JP 2001-75414 A

  By the way, in an image forming apparatus having a fixing device for fixing an unfixed toner image on a sheet by heating and pressing the sheet nipped and conveyed by a nip portion formed between a heating roller and a pressure roller. When the power of the apparatus is turned on from the OFF state, the pressure roller is heated and the heating roller is heated to start warming up.

  FIG. 6 is a graph showing a change in temperature of the heating roller and a change in pressure applied to the pressure roller since the power was turned on in an image forming apparatus having a conventional fixing device. FIG. The temperature change of the roller is shown, and FIG. 6B shows the pressure change of the pressure roller. FIG. 6A also shows the temperature change of the pressure roller from the time of turning on the power. The temperature change of the heating roller is represented as L1, and the temperature change of the pressure roller is represented as L2. Further, in FIG. 6B, the pressure change of the pressure roller is represented as L3.

  As shown in FIG. 6A, in the image forming apparatus provided with the conventional fixing device, when the image forming apparatus is turned on, the heating roller is heated by the heat source, and the predetermined temperature is determined from the temperature T0 when the power is turned on. Is heated to a temperature T1 set in order to obtain the fixing strength. On the other hand, the pressure roller is pressed against the heating roller and heated by heat transfer from the heating roller. When the heating roller is heated to the temperature T1, the pressure roller is heated from the temperature T0 when the power is turned on to the temperature T11. Further, as shown in FIG. 6B, a pressure F1 set to obtain a predetermined fixing strength is applied to the pressure roller.

  In this way, in the image forming apparatus that performs the fixing operation in a state in which the pressing roller F1 is applied to the pressure roller to obtain the predetermined fixing strength and the heating roller is heated to the temperature T1, as shown in FIG. When a pressure F1 is applied to the pressure roller at time t1 after the power is turned on and the heating roller is heated to temperature T1 at time t2 after the power is turned on, the warm-up is completed and the fixing operation is started. Then, a fixing operation for fixing an unfixed toner image on the sheet is performed at a nip portion formed between the heating roller and the pressure roller.

  However, in the image forming apparatus, the time t1 required from when the power is turned on to when the pressure F1 is applied to the pressure roller may be relatively short, but from when the power is turned on until the heating roller is heated to the temperature T1. The time t2 required for this is long, and there is a problem that the warm-up time until the start of the fixing operation is long for the entire image forming apparatus.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above technical problem, and an object thereof is to provide an image forming apparatus capable of shortening the warm-up time until the start of the fixing operation.

In order to achieve this object, an image forming apparatus according to the present invention provides:
A heating member that is heated by a heat source; and a pressure roller that is provided to face the heating member and presses the sheet together with the heating member, and is formed between the heating member and the pressure roller. An image forming apparatus having a fixing device for fixing an unfixed toner image on a sheet by heating the sheet nipped and conveyed by the nip portion and pressurizing with a predetermined pressure.
A pressing force varying means for varying the pressing force of the pressure roller;
Control means for controlling the operation of the pressure application variable means;
With
The control unit controls the operation of the pressing force varying unit so that the pressing force of the pressure roller at the start of the fixing operation is larger than the predetermined pressing force;
It is characterized by that.

  According to the image forming apparatus according to the present invention, the paper to be nipped and conveyed by the nip formed between the heating member and the pressure roller is heated and pressurized with a predetermined pressure, whereby the indeterminate state on the paper is determined. In an image forming apparatus having a fixing device for fixing a received toner image, at the start of a fixing operation for obtaining a predetermined fixing strength by making the pressure applied by the pressure roller at the start of the fixing operation larger than the predetermined pressure The fixing temperature can be lowered, and the warm-up time until the start of the fixing operation can be shortened. In addition, since the fixing temperature at the start of the fixing operation can be lowered, the temperature difference between the heating member that presses and conveys the sheet and the pressure roller becomes small at the start of the fixing operation, and curling of the sheet can be suppressed.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, terms indicating a specific direction and position (for example, “up”, “down”, “right”, “left” and other terms including those terms) are used as necessary. These terms are used for easy understanding of the invention with reference to the drawings, and the technical scope of the present invention is not limited by the meaning of these terms.

  FIG. 1 is a schematic configuration diagram schematically showing an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 1 is an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine having a combination of these functions. Currently, various types of electrophotographic image forming apparatuses have been proposed. The illustrated image forming apparatus is a so-called tandem color image forming apparatus.

  The image forming apparatus 1 includes an endless intermediate transfer belt 2. The intermediate transfer belt 2 is wound around a pair of rollers 3 and 4 disposed on the right side and the left side of the drawing, respectively. The roller 3 on the right side of the drawing is drivingly connected to a motor (not shown) and is rotated based on the driving of the motor. Further, as the roller 3 rotates, the intermediate transfer belt 2 and the roller 4 on the left side of the drawing that supports the intermediate transfer belt 2 are rotated counterclockwise in the drawing.

  On the outer side of the intermediate transfer belt 2 supported by the roller 3 disposed on the right side in the drawing, a secondary transfer roller 5 is disposed to face the roller 3 with the intermediate transfer belt 2 interposed therebetween. The secondary transfer roller 5 is in contact with the outer peripheral surface of the intermediate transfer belt 2, and the contact portion (nip portion) forms the secondary transfer region 6.

  An intermediate transfer belt cleaning member 7 is provided outside the intermediate transfer belt 2 supported by the roller 4 disposed on the left side in the drawing. The intermediate transfer belt cleaning member 7 is pressed against the roller 4 via the intermediate transfer belt 2, and the contact portion forms a collection region 8 for collecting untransferred toner.

  Further, on the lower side of the intermediate transfer belt 2 moving from the roller 4 on the left side in the drawing to the roller 3 on the right side in the drawing, yellow (Y), magenta (M), cyan (in order from the left side to the right side in the drawing). Four image forming units 10 (10Y, 10M, 10C, and 10K) that generate toner images of colors corresponding to the four colors C) and black (K) are arranged along the intermediate transfer belt 2.

  Each image forming unit 10 has a cylindrical photoconductor 11 as an electrostatic latent image carrier. The primary transfer is arranged around the photosensitive member 11 so as to face the photosensitive member 11 via the charger 12, the developing device 13, and the intermediate transfer belt 2 in order along the rotation direction (clockwise direction in the drawing). A roller 14 and a photosensitive member cleaning member 15 are disposed. An exposure device 17 is disposed below the four photoconductors 11. The exposure device 17 irradiates each photoconductor 11 with light between the charger 12 and the developing device 13 to perform exposure. It has become.

  A paper feeding unit 21 is provided at the lower part of the image forming apparatus 1, and the paper 22 as a recording medium stacked and accommodated in the paper feeding unit 21 is rotated from the uppermost one by the rotation of the paper feeding roller 23. Each sheet is sent out to the sheet conveyance path 40. Further, a manual paper feed unit 31 is provided, and the paper 32 stacked on the paper feed unit 31 is sent out to the paper transport path 40 by the paper feed roller 33.

  The sheet conveyance path 40 includes nip portions of the conveyance roller pairs 41 and 42 corresponding to the paper feeding units 21 and 31, respectively, nip portions of the conveyance roller pair 43 and the timing roller pair 44 arranged substantially in the vertical direction, and intermediate transfer. A secondary transfer region 6 which is a nip portion between the belt 2 and the secondary transfer roller 5, a fixing device 60 disposed above the secondary transfer region 6, which will be described later, and a conveyance roller provided above the fixing device 60. The pair 45 and the discharge roller pair 46 extend through the nip portions to the discharge tray 47.

  Also, in the paper transport path 40, during duplex printing, the paper discharge roller pair 46 reversely rotates to switch back the papers 22 and 32, and the paper 22 and 32 are reversed with the front and back sides and the front end in the transport direction reversed. , 32 are fed into the secondary transfer region 6. The switchback path 50 extends from the paper discharge roller pair 46 through the nip portions of the transport roller pairs 51, 52, 53, 54 to the timing roller pair 44 of the paper transport path 40.

  Next, the operation of the image forming apparatus 1 configured as described above will be described. The image forming apparatus 1 can be switched between a monochrome mode in which a monochrome image is formed using one color toner such as black and a color mode in which a color image is formed using four color toners. Hereinafter, an example of the image forming operation in the color mode will be described. In the following, the paper 22 from the paper supply unit 21 is described as the paper, but the paper 32 from the paper supply unit 31 is the same as that of the paper 22.

  First, in each image forming unit 10, the outer peripheral surface of the photoconductor 11 that is rotationally driven at a predetermined peripheral speed is charged by the charger 12. Next, light corresponding to image information is projected from the exposure device 17 on the outer peripheral surface of the charged photoconductor 11 to form an electrostatic latent image. Subsequently, the electrostatic latent image is made visible by the developer toner supplied from the developing device 13. The toner images of the respective colors formed on the photoconductor 11 are sequentially transferred (primary transfer) from the photoconductor 11 onto the intermediate transfer belt 2 in the order of yellow, magenta, cyan, and black by the primary transfer roller 14. A toner image of each color is superimposed on the transfer belt 2.

  The four color toner images superimposed on the intermediate transfer belt 2 are conveyed to the secondary transfer region 6 as the intermediate transfer belt 2 moves. On the other hand, the paper 22 stacked on the paper supply unit 21 is conveyed to the secondary transfer area 6 in accordance with the timing, and the four color toner images are transferred from the intermediate transfer belt 2 to the paper 22 in the secondary transfer area 6. Are collectively transferred (secondary transfer). The sheet 22 onto which the four color toner images have been transferred is conveyed further downstream in the sheet conveyance path 40, and after the unfixed toner image is fixed on the sheet 22 by the fixing device 60, the conveyance roller pair 45 and the discharge roller The paper is sent to the paper discharge tray 47 via the pair 46.

Next, the configuration of the fixing device of the image forming apparatus according to the present embodiment will be described.
FIG. 2 is a schematic configuration diagram schematically showing the configuration of the fixing device of the image forming apparatus. As shown in FIG. 2, the fixing device 60 includes a heating roller 61 as a heating member that heats and melts an unfixed toner image transferred from the intermediate transfer belt 2 in the secondary transfer region 6 and fixes it to the paper 22. A pressure roller 65 that is provided opposite to the heating roller 61 and presses the sheet 22 with the heating roller 61 and pressurizes, and a variable pressure as a pressure variable means that varies the pressure of the pressure roller 65. Device 67.

  The heating roller 61 is drivingly connected to a motor (not shown), and is driven to rotate counterclockwise in the drawing based on the driving of the motor. The heating roller 61 is formed in a cylindrical shape, and a heat source 62 is provided inside the heating roller 61, and the heating roller 61 is heated by the heat source 62. As the heat source 62, various heat sources such as an infrared heater such as a halogen lamp can be used.

  The pressure roller 65 is formed in a cylindrical shape, and both end portions 66 thereof are rotatably supported by support members (not shown). The supporting member is connected to a pressing force varying device 67 as a pressing force varying means for varying the pressing force of the pressure roller 65 with respect to the heating roller 61.

  The variable pressure device 67 moves the support member that supports both ends 66 of the pressure roller 65 toward the heating roller 61 or in a direction away from the heating roller 61, so that the pressing force of the pressure roller 65 against the heating roller 61 is increased. Can be changed. The pressure roller 65 is brought into pressure contact with the heating roller 61 by a pressure application varying device 67 and is rotated in the clockwise direction in the drawing as the heating roller 61 rotates.

  The fixing device 60 is controlled by a control device 68 that comprehensively controls the operation of the image forming apparatus 1. The control device 68 operates the heat source 62, rotates the heating roller 61, and drives the fixing device 60. Various controls such as operation of the variable pressure device 67 are performed. The control device 68 also controls the operation of the heat source 62 and the operation of the pressurizing variable device 67 in accordance with the input signal 68a that is turned on from the power-off state of the device. . The control device 68 is configured with, for example, a microcomputer as a main part.

  Although not shown in the fixing device 60, a heating roller temperature detection sensor that detects the temperature of the heating roller 61, a pressure roller temperature detection sensor that detects the temperature of the pressure roller 65, and a pressure applied to the heating roller 61. A pressure detection sensor for detecting the pressure of the roller 65 is provided, and the temperature of the heating roller 61 and the pressure of the pressure roller 65 are accurately controlled based on the detection value from the detection sensor. ing.

  In the image forming apparatus 1 including the fixing device 60 configured as described above, when the power of the apparatus is changed from the OFF state to the ON state and the power is turned on, the pressure roller 65 is applied by the variable pressure device 67. The heating roller 61 is heated by the heat source 62 and warm-up is started.

  In the present embodiment, when the unfixed toner image on the paper 22 is fixed at the nip portion 69 formed between the heating roller 61 and the pressure roller 65, the pressure roller 65 is used to obtain a predetermined fixing strength. A predetermined pressing force (first pressing force) F1 is applied to the heating roller 61 at a predetermined temperature (first temperature) T1, and the sheet 22 held and conveyed by the nip portion 69 is heated to a predetermined temperature. Can be heated and pressurized with a predetermined pressing force to obtain a predetermined fixing strength. At the start of the fixing operation for starting the fixing operation, the pressure roller 65 receives the first pressing force F1. A large pressing force is applied, and before the heating roller 61 is heated to the first temperature T1, the fixing operation is started in a state where the heating roller 61 is heated to a temperature lower than the first temperature T1. Note that the control device 68 controls the operation of the variable pressure device 67 so that the pressure force of the pressure roller 65 at the start of the fixing operation is larger than the first pressure force F1.

  FIG. 3 is a graph showing the relationship between the temperature of the heating roller 61 and the pressing force of the pressure roller 65 for obtaining a predetermined fixing strength. In FIG. 3, the pressing force of the pressing roller 65 against the heating roller 61 is shown. The horizontal axis represents the temperature of the heating roller 61 and the vertical axis represents the temperature. As shown in FIG. 3, the temperature of the heating roller 61 for obtaining a predetermined fixing strength and the pressing force of the pressure roller 65 show a substantially inversely proportional relationship. From this graph, the predetermined fixing strength is obtained. That is, in order to obtain the same fixing strength, if the temperature of the heating roller 61 is increased, the pressing force of the pressure roller 65 can be reduced, and if the pressing force of the pressing roller 65 is increased, the heating roller 61 is increased. It can be seen that the temperature can be lowered. As described above, the relationship between the temperature and the pressing force (pressure contact force) in fixing shows a substantially inversely proportional relationship. As a result, in the present embodiment, when the fixing operation is started, the pressing force of the pressure roller 65 is made larger than the first pressing force F1, and the heating roller 61 obtains a predetermined fixing strength at a temperature lower than the first temperature T1. Start fixing operation.

  FIG. 4 is a graph showing the temperature change of the heating roller and the pressure change of the pressure roller from the time of turning on the power in the image forming apparatus including the fixing device according to the present embodiment. Shows the temperature change of the heating roller, and FIG. 4B shows the pressure change of the pressure roller. FIG. 4A also shows the temperature change of the pressure roller from the time of turning on the power. The temperature change of the heating roller is represented as L11, and the temperature change of the pressure roller is represented as L12. Further, in FIG. 4B, the pressure change of the pressure roller is represented as L13.

  In the image forming apparatus 1 provided with the fixing device 60, when the image forming apparatus 1 is turned on, as shown in FIG. 4A, the heating roller 61 moves from the state of the temperature T0 when the power is turned on to the heat source 62. The pressure roller 65 that is heated by the pressure roller and pressed against the heating roller 61 is heated by heat transfer from the heating roller 61. As shown in FIG. 4B, pressure is applied to the pressure roller 65 by a variable pressure device 67. In this embodiment, the pressure of the pressure roller 65 at the start of the fixing operation is applied. Is set to a pressing force (second pressing force) F2 larger than the first pressing force F1 set to obtain a predetermined fixing strength.

  In this manner, the pressure of the pressure roller 65 at the start of the fixing operation is set to the second pressure F2 that is larger than the first pressure F1, so that the temperature of the heating roller 61 is higher than the first temperature T1. A predetermined fixing strength can be obtained at a low temperature (second temperature) T2. In the image forming apparatus 1, a second pressure F2 is applied to the pressure roller 65 at a time t1 from when the power is turned on. When the heating roller 61 is heated to the second temperature T2 at time t12 from the time of charging, the warm-up is completed, the fixing operation is started, and the nip formed between the heating roller 61 and the pressure roller 65 A fixing operation for fixing the unfixed toner image on the paper 22 is started in the section 69. When the heating roller 61 is heated to the temperature T2, the pressure roller 65 reaches the temperature T21 from the temperature T0 when the power is turned on by heat transfer from the heating roller 61.

  In the fixing device 60, when the unfixed toner image on the paper 22 is fixed by the nip portion 69 formed between the heating roller 61 and the pressure roller 65, the heating roller 61 is heated to the first temperature T1. And a state in which the first pressure F1 is applied to the pressure roller 65, and a state in which the heating roller 61 is heated to the second temperature T2 and a second pressure F2 is applied to the pressure roller 65. The second temperature T2 is set so that a predetermined fixing strength can be obtained, that is, the same fixing strength can be obtained.

  After the heating roller 61 is heated to the second temperature T2 and the fixing operation is started with the second pressure F2 applied to the pressure roller 65, the heating roller 61 is further heated by the heat source 62. At time t13, the heating roller 61 is heated to the first temperature T1 set to obtain a predetermined fixing strength. Then, when the heating roller 61 reaches the set first temperature T1, the operation of the variable pressure device 67 is controlled so that the pressure applied to the pressure roller 65 is returned to the first pressure F1. Thus, by making the pressure applied to the pressure roller 65 higher than the first pressure F1 only at the start of the fixing operation, the load on the heat roller 61 and the pressure roller 65 can be suppressed. It can suppress that the lifetime of the pressure roller 65 falls.

  In the present embodiment, as a pressing force variable means for changing the pressing force of the pressing roller 65, a pressing member that supports the pressing roller 65 is moved to the heating roller 61 side or away from the heating roller 61. Although the pressure variable device 68 is used, the present invention is not limited to this, and other pressure variable means may be used.

  In the image forming apparatus 1, when the second pressure F2 is applied to the pressure roller 65 at time t11, the second pressure is applied to the pressure roller 65 until the heating roller 61 is heated to the second temperature T2. Although it is held in a state where the pressing force F2 is applied, the second pressing force F2 may be applied to the pressure roller 65 at time t12 when the heating roller 61 is heated to the second temperature T2. .

  As described above, according to the image forming apparatus 1 according to the present embodiment, the sheet 22 that is nipped and conveyed by the nip portion 69 formed between the heating roller 61 and the pressure roller 65 is heated and a predetermined pressure is applied. (First Pressurizing Force) In the image forming apparatus 1 having the fixing device 60 that fixes the unfixed toner image on the paper 22 by pressurizing with F1, the pressing force of the pressure roller 65 at the start of the fixing operation is the predetermined pressure. By making the applied pressure F1 larger than this, the fixing temperature at the start of the fixing operation for obtaining a predetermined fixing strength can be lowered, and the warm-up time until the fixing operation is started can be shortened.

  Since the fixing temperature at the start of the fixing operation can be lowered, as shown in FIG. 4, the temperature difference (T2-T21) between the heating roller 61 and the pressure roller 65 at the start of the fixing operation is set to a predetermined fixing. In order to obtain the strength, the heating roller 61 is heated to the first temperature T1, and the heating at the start of the fixing operation in the case where the fixing operation is started in a state where the first pressing force F1 is applied to the pressure roller 65. The temperature difference (T1-T11) between the roller 61 and the pressure roller 65 can be reduced, and the sheet 22 nipped and conveyed by the heating roller 61 and the pressure roller 65 is curled at the start of the fixing operation. Can be suppressed.

  In the present embodiment, the heating roller 61 is heated to a temperature T2 lower than the normal temperature T1 by increasing the pressing force of the pressure roller 61 at the start of the fixing operation after turning on the power to be higher than the normal pressing force F1. However, even after the power is turned on, the pressure applied to the pressure roller 61 at the start of the fixing operation is normally set at the time of starting the fixing operation from the standby state where the fixing operation is not performed (when returning). It may be made larger than the applied pressure F1. Even in such a case, the fixing operation can be started in a state where the heating roller 61 is heated to a temperature T2 lower than the normal temperature T1, and the time until the fixing operation is started can be shortened. Further, since the fixing temperature at the start of the fixing operation can be lowered by increasing the pressing force of the pressure roller 65 to be higher than the normal pressing force F1 at the start of the fixing operation, the set temperature of the heating roller 61 in the standby state is set. Can be lowered.

  In the fixing device 60 described above, the heating roller 61 having the heat source 62 is used as the heating member heated by the heat source. However, the heating member heated by the heat source is wound around the heating roller having the heat source and the fixing roller. An endless fixing belt may be used.

  FIG. 5 is a schematic configuration diagram schematically showing a configuration of a fixing device using a fixing belt as a heating member. The fixing device 70 shown in FIG. 5 has the same configuration as the fixing device 60 except that the configuration of the heating member is different from that of the fixing device 60 shown in FIG. Description is omitted.

  The fixing device 70 is provided opposite to the fixing belt 74 as a heating member that heats and melts the unfixed toner image transferred from the intermediate transfer belt 2 in the secondary transfer region 6 and fixes it to the paper 22. A pressure roller 65 as a pressure member that sandwiches and presses the paper 22 together with the fixing belt 74 and a pressure variable device 67 as a pressure variable unit that varies the pressure of the pressure roller 65 are provided.

  The fixing belt 74 is wound around a heating roller 71 that has a heat source 72 and is rotatably provided, and a fixing roller 73 that is rotatably provided at a position separated from the heating roller 71, and the fixing roller 73 has a cylindrical shape. And is arranged to face the pressure roller 65 with the fixing belt 74 interposed therebetween.

  Also in the fixing device 70, both ends 66 of the pressure roller 65 are supported by a support member (not shown), and a pressing force variable device 67 is connected to the support member. The variable pressure device 67 moves the support member that supports both ends 66 of the pressure roller 65 toward the fixing belt 74 or in a direction away from the fixing belt 74, so that the pressure roller 65 applies pressure to the fixing belt 74. Can be changed.

  In the fixing device 70, the pressure roller 65 is drivingly connected to a motor (not shown) and is driven to rotate clockwise in the drawing based on the driving of the motor. When the pressure roller 65 is brought into pressure contact with the fixing belt 74 by the pressing force varying device 67, the fixing belt 74 is driven to rotate counterclockwise in the drawing as the pressure roller 65 rotates. Is heated by a heating roller 71 heated by a heat source 72 at the time of rotation.

  Also in the fixing device 70, the operation of the fixing device 70 is controlled by a control device 68 that comprehensively controls the operation of the image forming apparatus 1, and the control device 68 operates the heat source 72, the pressure roller for the fixing device 70. Various controls such as the rotational driving of 65 and the operation of the variable pressure device 67 are performed. The control device 68 also controls the operation of the heat source 72 and the operation of the pressurizing variable device 67 in accordance with the input signal 68a that is turned on from the power-off state of the device. .

  Also in the image forming apparatus including the fixing device 70, the pressing force of the pressure roller 65 at the start of the fixing operation is set to a pressing force F2 larger than the first pressing force F1 set to obtain a predetermined fixing strength. Thus, the heating roller 61 can obtain a predetermined fixing strength at a temperature T2 lower than the first temperature T1 set to obtain a predetermined fixing strength, and the second pressure force is applied to the pressure roller 65. When F2 is applied and the heating roller 61 is heated to the second temperature T2, the warm-up is completed and the fixing operation is started, and the nip portion 79 formed between the fixing belt 74 and the pressure roller 65 is started. Then, a fixing operation for fixing the unfixed toner image on the paper 22 is started.

  In this way, an endless fixing belt 74 wound around a heating roller 71 having a heat source 72 and a fixing roller 73 is used as a heating member, and a nip formed between the fixing belt 74 and the pressure roller 65 is used. Even in the image forming apparatus having the fixing device 70 that fixes the unfixed toner image on the paper 22 by heating and pressurizing the paper 22 sandwiched and conveyed by the section 79 with a predetermined pressure, it is applied at the time of starting the fixing operation. By making the pressing force of the pressure roller 65 larger than the predetermined pressing force F1, the fixing temperature at the start of the fixing operation can be lowered, and the time until the start of the fixing operation can be shortened.

  The present invention is not limited to the illustrated embodiments, and it goes without saying that various improvements and design changes can be made without departing from the scope of the present invention. For example, although the embodiment has been described with respect to a tandem color printer, the present invention can also be applied to other image forming apparatuses such as a 4-cycle color printer.

1 is a schematic configuration diagram schematically illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram schematically illustrating a configuration of a fixing device of the image forming apparatus. 6 is a graph showing the relationship between the temperature of the heating roller 61 and the pressure applied by the pressure roller 65 to obtain a predetermined fixing strength. 6 is a graph showing a change in temperature of the heating roller and a change in pressure applied to the pressure roller since the power was turned on in the image forming apparatus including the fixing device according to the present embodiment. FIG. 2 is a schematic configuration diagram schematically showing a configuration of a fixing device using a fixing belt as a heating member. 6 is a graph showing a change in temperature of a heating roller and a change in pressure applied to a pressure roller since the power was turned on in an image forming apparatus including a conventional fixing device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 22, 32 Paper 60, 70 Fixing device 61, 71 Heating roller 62, 72 Heat source 65 Pressing roller 67 Pressurizing variable device 68 Control device 69, 79 Nip part 73 Fixing roller 74 Fixing belt

Claims (5)

A heating member that is heated by a heat source; and a pressure roller that is provided to face the heating member and presses the sheet together with the heating member, and is formed between the heating member and the pressure roller. An image forming apparatus having a fixing device for fixing an unfixed toner image on a sheet by heating the sheet nipped and conveyed by the nip portion and pressurizing with a predetermined pressure.
A pressing force varying means for varying the pressing force of the pressure roller;
Control means for controlling the operation of the pressure application variable means;
With
The control unit controls the operation of the pressing force varying unit so that the pressing force of the pressure roller at the start of the fixing operation is larger than the predetermined pressing force;
An image forming apparatus.   The image forming apparatus according to claim 1, wherein the heating member is a heating roller having a heat source.   The image forming apparatus according to claim 1, wherein the heating member is an endless fixing belt wound around a heating roller having a heat source and a fixing roller.   The image forming apparatus according to claim 1, wherein the fixing operation start time is a fixing operation start time after power is turned on.   The image forming apparatus according to claim 1, wherein the fixing operation starts when the fixing operation starts from a standby state.

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