JP2011197287A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a superior image by eliminating a difference between the actual temperature of a heater and the result of detection by a temperature detection means caused by the arrangement position of the temperature detection means.SOLUTION: An image forming apparatus includes a deriving means which derives, from the result of detection by a thermistor 21, when two adjacent heating elements 20b1 and 20b2 are made to generate heat one by one; a temperature rise gradient at a position where the thermistor 21 is disposed is derived for the respective heating elements, and derives a difference between the temperature rise gradients of the heating elements 20b1 and 20b2; a storage means, in which the relation of the difference between the temperature rise gradients for the heating elements 20b1 and 20b2 and the correction value of set temperature of the heater 20 is stored in advance; and a correction means which derives the correction value of the set temperature of the heater 20, corresponding to the difference between the temperature risen gradients derived by the deriving means from the storage means and corrects the set temperature of the heater 20 by using the correction value.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer having a function of forming an image on a recording material such as a sheet.

2. Description of the Related Art Conventionally, as an image heating apparatus in an image forming apparatus, a heat roller type or a film heating type apparatus has been widely used. In particular, a method of suppressing power consumption as much as possible without supplying power to the image heating device during standby, specifically, heating a toner image on a recording material through a film between a heater having a heating element and a pressure roller. An image heating apparatus using a film heating method is known.
In such a film heating type image heating apparatus, a temperature detection element (temperature detection means) for detecting the temperature of the heater is disposed on the back surface of the heater (opposite the sliding surface with the film). This is because the toner image on the recording material is heated at an appropriate temperature without causing problems such as poor fixing and high temperature offset. The energization control unit controls power supply to the heating element so that the heater temperature detected by the temperature detection element is maintained at a predetermined substantially constant temperature.

Here, when the position of the temperature detection element in contact with the back surface of the heater swings upstream and downstream in the recording material conveyance direction, the detection result of the temperature detection element and the actual heating status of the heater differ depending on the individual case. There is a concern that
In such a case, there is a concern that the possibility of problems such as poor fixing and high temperature offset increases. In order to prevent this problem, the position of the temperature detection element on the back surface of the heater becomes severe, and the tolerance of the mounting position of the temperature detection element must be kept small. Therefore, in order to increase the productivity of the heater, it is essential to increase the accuracy of the contact position of the temperature detection element.
In order to solve this problem, Patent Document 1 proposes a method of installing two heating elements on the heater, upstream and downstream, as a method of reducing the temperature gradient in the upstream and downstream direction of the recording material conveyance of the heater. .

Japanese Patent Laid-Open No. 10-221986

However, in the case of the conventional technology as described above, there are concerns that the following problems will occur.
FIG. 11A shows a non-sheet passing surface side of the heater proposed in Patent Document 1. FIG. 60 is a heater, 60a is a ceramic substrate, 60b is a heating element, 60c is an insulating protective layer, 60d is an AC electrode for supplying power to the heating element, 61 is a thermistor, 61a is a DC energizing part for the thermistor, and 61b is a DC electrode part. 62 are thermo protectors.
FIG. 11B shows the temperature distribution in the nip when the heater shown in FIG. 11A is driven. In a heater with a single heating element, the temperature difference in the temperature detection element contact position where the temperature detection element comes in contact with the heater is about 50 ° C., whereas the heating element 60b shown in FIG. The temperature difference between the two heaters is about 20 ° C., and an improvement of about 30 ° C. is observed.
However, in recent years, there is a tendency to bring the heating element closer to the upstream and downstream ends of the heater in the recording material conveyance direction in order to secure a margin when the heater runs away, and the heat distribution in the upstream and downstream direction of the heater is larger than before. It has become. An example is shown in FIG. The difference between the maximum temperature and the minimum temperature in the nip is the difference when the heating element is closer to both ends ((2) in the figure) than the difference when the heating element is closer to the center ((1) in the figure). Is getting bigger.

Below, the case where the heat generating body has approached both ends is demonstrated in detail. FIG. 12B is a schematic cross-sectional view of the heater with the heat generating elements approaching both ends.
70 is a heater, 70a is a ceramic substrate, 71a1 and 71a2 are heating elements, 72 is a thermistor, 73 is sliding glass, and 74 is insulating glass. The substrate width of the heater is 8 mm, the heating element width is 1.5 mm, and the distance between the heating elements is 3 mm. The size of the temperature detecting element for detecting the temperature of the heater is 0.5 mm.
FIG. 13 shows the heat distribution in the nip portion N in the recording material conveyance direction when the heater shown in FIG. The temperature of the heater varies depending on the position, and the difference is about 5 ° C. at maximum within the thermistor installation range. For this reason, the detection result of the thermistor is shifted by about 5 ° C. at maximum in terms of temperature depending on the installation position of the thermistor.
Since the temperature of the heater is controlled by the detection result of the temperature detection element, if the detection result of the temperature detection element deviates, the temperature of the heater also deviates from a predetermined set temperature. If the heater temperature deviates from the set temperature, image defects such as offset and fixing failure may occur. For this reason, it is necessary to strictly manage the arrangement position of the temperature detection element, but there is a problem that the strict management leads to a decrease in productivity and an increase in cost.

  The present invention has been made in view of the circumstances as described above, and obtains a better image by eliminating the difference between the actual temperature of the heater and the detection result of the temperature detection means due to the position of the temperature detection means. With the goal.

In order to achieve the above object, the present invention provides:
An image heating apparatus that nipping and conveying a recording material on which a developer image is formed and heating it at a nip formed by a heating member having a heater and a pressure member pressed against each other,
A heating element provided in the heater so as to heat the entire width direction of the recording material sandwiched and conveyed at the nip portion, provided side by side in the conveying direction of the recording material sandwiched and conveyed at the nip portion, Two adjacent heating elements that can generate heat independently,
A temperature detecting means which is disposed between the two adjacent heating elements and detects the temperature of the heater;
In an image forming apparatus provided with an image heating apparatus having
From the detection result by the temperature detection means when the two adjacent heating elements are heated one by one, the temperature increase gradient at the position where the temperature detection means is disposed is derived for each heating element,
Deriving means for deriving a difference in temperature rise gradient between the two adjacent heating elements;
Storage means for storing in advance a relationship between a difference in temperature rise gradient between the two adjacent heating elements and a correction value of a control parameter at the time of image heating processing;
A correction means for deriving a correction value of the control parameter corresponding to the difference in temperature rise gradient derived by the deriving means from the storage means, and correcting the control parameter using the correction value;
It is characterized by providing.

  According to the present invention, it is possible to obtain a better image by eliminating the difference between the actual temperature of the heater and the detection result of the temperature detection means due to the arrangement position of the temperature detection means.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to the present invention. 1 is a schematic diagram of a heater in Embodiment 1. FIG. FIG. 3 is a diagram for explaining the thermistor according to the first embodiment. The figure showing the positional relationship of the heater and the thermistor in Example 1. FIG. FIG. 3 is a diagram illustrating the thermistor position in the first embodiment. FIG. 3 is a diagram illustrating the thermistor position in the first embodiment. The figure for demonstrating the heater in Example 2. FIG. FIG. 6 is a diagram for explaining Example 2; The figure showing the transition of the throughput reduction of the small size paper of the main bodies A and B. FIG. 6 is a schematic view of a pressure release mechanism in Embodiment 3. The figure for demonstrating the heater of a prior art example. The figure for demonstrating the heater of a prior art example. The figure which shows the temperature distribution of the heater of a prior art example.

  DETAILED DESCRIPTION Exemplary embodiments for carrying out the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.

The overall configuration of the image forming apparatus and the image forming process in Embodiment 1 will be described below. FIG. 1A is a cross-sectional view showing an example of a schematic configuration of an image forming apparatus according to the present invention. The image forming apparatus shown in this embodiment is a laser printer using an electrophotographic process method.
The image forming apparatus shown in FIG. 1A includes a photosensitive drum 1 as an image carrier. The photosensitive drum 1 is rotationally driven at a predetermined process speed in the direction of arrow a by a main body driving motor 12 as an image carrier driving means.
Around the photosensitive drum 1, in order along the rotation direction, a charging roller 2 as a charging device, an exposure device 3, a developing device 4, a transfer roller (transfer member) 5 as a transfer device, and a cleaning blade 6 of a cleaning device. Is arranged.
A feeding cassette 7 that stores a recording material P such as paper (sheet) is disposed at the lower part of the apparatus main body. Then, in the order along the conveyance path of the recording material P, the feeding roller 15, the conveyance roller 8, the top sensor 9, the photosensitive drum 1 and the transfer roller 5, the conveyance guide 10, the fixing device 11 as an image heating device, and the discharge roller 13. The discharge tray 14 is disposed.

The operation of the image forming apparatus configured as described above will be described.
The image forming process speed of the image forming apparatus is 200 mm / sec, and the recording material is conveyed at 200 mm / sec.
The photosensitive drum 1 rotated and driven in the direction of arrow a by the main body drive motor 12 is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 2. The photosensitive drum 1 after charging is exposed based on image information by an exposure device (laser scanner) 3 of a laser optical system on the outer peripheral surface of the drum. That is, the exposure device 3 exposes the uniformly charged surface of the photosensitive drum 1 with the laser beam L that is ON / OFF controlled according to the time-series electric digital image signal of the image information. By this exposure, the exposed portion potential of the uniformly charged surface of the photosensitive drum 1 is attenuated, and an electrostatic latent image corresponding to the image information is formed on the outer peripheral surface of the photosensitive drum. The electrostatic latent image is developed by the developing device 4. The developing device 4 includes a developing roller 4a. A developing bias is applied to the developing roller 4a, and toner as a developer adheres to the electrostatic latent image on the photosensitive drum 1, thereby developing the toner image (developer image). (Visualized).

The recording material P stored in the feeding cassette 7 is fed by the feeding roller 15 and conveyed to the transfer nip T between the photosensitive drum 1 and the transfer roller 5 by the conveying roller 8. Before the recording material P is conveyed to the transfer nip T, the leading edge of the recording material P is detected by the top sensor 9 provided between the conveyance roller 8 and the transfer nip T, and the recording material P is synchronized with the toner image on the photosensitive drum 1. It is done. A transfer bias having a polarity opposite to the polarity of the toner is applied to the transfer roller 5, and the toner image on the photosensitive drum 1 is transferred onto the recording material P.

The recording material P carrying an unfixed toner image in the transfer process is conveyed along the conveyance guide 10 to the fixing device 11. Then, the recording material P carrying the unfixed toner image is nipped and conveyed by the fixing nip N in the fixing device 11 and heated and pressurized, whereby the unfixed toner image is permanently fixed on the recording material P. As established. The recording material P after the toner image is fixed is discharged onto a discharge tray 14 on the upper surface of the apparatus main body by a discharge roller 13. After the toner image is transferred, the toner remaining on the surface without being transferred to the recording material P is removed by the cleaning blade 6 of the cleaning device, and the next image formation is performed.
By repeating the above operation, image formation can be performed one after another.

Hereinafter, the fixing device 11 will be described in detail.
FIG. 1B is a cross-sectional view showing a schematic configuration of the fixing device 11 of the present embodiment. The fixing device shown in this embodiment is a film fixing type device using a cylindrical film. In the following description, the longitudinal direction refers to a direction orthogonal to (crossing) the conveyance direction (sheet passing direction, passage direction) Y of the recording material P (rotational axis direction of the pressure roller 26, width direction of the recording material P). Say.
The fixing device 11 includes a fixing rotating body (hereinafter referred to as a fixing film) 25 as a cylindrical flexible sleeve (flexible film) as a rotating body that forms a fixing nip portion N by contacting (pressing) each other. , And a roller-shaped pressure rotating body (hereinafter referred to as a pressure roller) 26. Further, a ceramic heater (hereinafter referred to as a heater) 20 for heating the unfixed toner image t on the recording material P via the fixing film 25, a temperature control means 27 for controlling the temperature of the heater 20, and a rotating body driving means. And a fixing motor 29.

The heater 20 has a resistor pattern (resistance layer, resistance heating element, heating resistor) as a heating element that generates heat when energized on a substrate having high thermal conductivity, insulation, and heat resistance such as alumina, and generates heat when energized. The toner image on the recording material is melted and fixed. The configuration of the heater will be described in detail later. The heater 20 is supported on the lower surface of the film guide 22 disposed in the hollow portion of the fixing film 25 with the resistor pattern facing upward.
The film guide 22 is a member that is formed of a heat-resistant resin and is elongated in the longitudinal direction, and guides the inner surface of the fixing film 25 by two left and right arc portions 22a and 22a. The film guide 22 not only functions as a support member that supports the heater 20 but also functions as a guide member that guides the rotation of the fixing film 25.

  The fixing film 25 has a small heat capacity, is formed of a heat resistant resin such as polyimide in a cylindrical shape, has a diameter of 24 mm, and a release layer 25a such as a fluororesin is provided on the surface. More specifically, the fixing film 25 has a base layer (about 50 μm) made of polyimide (PI) and a release layer (about 12 μm) 25a made of fluororesin (PEA, PTFE, etc.) on the surface, A primer layer having a role of connecting the two layers with the release layer is provided. The fixing film 25 composed of the base layer, the primer layer, and the release layer has a thickness of about 70 μm, and the above-described heater 20 and film guide 22 are provided in the hollow portion. The fixing film 25 is pressed against the heater 20 by the elasticity of the pressure roller 26, and the surface of the fixing film 25 that contacts the heater is in contact with the lower surface of the heater 20.

The pressure stay 31 is a metal rigid pressure stay, and is attached to the side without the heater between the arc portions 22 a and 22 a of the film guide 22 in the fixing film 25. The pressure stay 31 is a member elongated in the longitudinal direction, and both ends thereof are supported by a device chassis (frame body) side plate (not shown) via a pressure spring. The pressure stay 31 uniformly transmits the pressure applied by the pressure spring to the heater 20 through the film guide 22, and the fixing film 25 is sandwiched between the heater 20 and the pressure roller 26 so as to be on the recording material P. A fixing nip portion N having a predetermined width necessary for heating and fixing the unfixed toner image t is formed.
The fixing film 25 is rotationally driven without wrinkles at a predetermined peripheral speed while being in close contact with and sliding on the lower surface of the heater 20 in the direction of the arrow b by the rotational driving of the pressure roller 26 at least when the unfixed toner image is fixed. Here, the predetermined peripheral speed means a peripheral speed substantially the same as the transport speed of the recording material P transported from the transfer nip T. Here, the heater 20, the fixing film 25, the film guide 22, and the pressure stay 31 constitute a heating member.

The pressure roller 26 as a pressure member has a heat-resistant elastic layer 26b having elasticity such as silicone rubber on the surface of a metal core 26a, and a release layer composed of a fluororesin or the like on the surface layer. A layer 26c is provided. More specifically, the pressure roller 26 has a diameter of 25 mm, the rubber thickness of the elastic layer 26b is 3.5 mm, and the fixing nip portion N has a width of 8 mm.
The pressure roller 26 is arranged in parallel with the fixing film 25 and is configured such that the rotation axis direction of the pressure roller 26 and the longitudinal direction of the heater 20 are parallel to each other, and both end portions of the core metal 26a are not shown. Bearings are supported rotatably on the chassis side plate.
The pressure roller 26 is rotationally driven by the fixing motor 29 in the direction of arrow c at a predetermined peripheral speed. When the pressure roller 26 is driven to rotate, a rotational force acts on the fixing film 25 by a pressure frictional force in the fixing nip N between the pressure roller 26 and the surface of the fixing film 25 that does not contact the heater. As a result, the fixing film 25 slides along the lower surface of the heater 20, and the outer periphery of the film guide 22 is driven to rotate in the direction of the arrow b.

  The temperature control means 27 includes a thermistor 21 as temperature detection means attached to the surface of the heater 20 that is not in contact with the fixing film, and an energization control circuit unit 23 including a CPU and a memory such as a ROM or a RAM. Here, the energization control circuit unit 23 corresponds to a derivation unit, a storage unit, and a correction unit. The energization control circuit unit 23 executes a position estimation method for temperature detection means (temperature detection element) in the image heating apparatus. Although detailed description of the control by the energization control circuit unit 23 will be described later, the functions of the derivation unit, storage unit, and correction unit will be described. The deriving means derives the temperature increase gradient at the position where the thermistor 21 is provided from each of the heating elements based on the detection result by the thermistor 21 when two adjacent heating elements are heated one by one. The difference between the temperature rise gradients of the two matching heating elements is derived. The storage means stores in advance a relationship between a difference in temperature rise gradient between two adjacent heating elements and a correction value of the set temperature of the heater 20 as a control parameter during fixing processing (image heating processing). It is. The correction means derives from the storage means a correction value for the set temperature of the heater 20 corresponding to the difference in temperature rise gradient derived by the derivation means, and corrects the set temperature of the heater 20 using this correction value. .

The energization control circuit unit 23 controls the triac 24 to energize the resistor pattern of the heater 20 from the AC power source 28. The heater 20 is heated by energization, and the thermistor 21 detects the heater temperature at that time. The energization control circuit unit 23 takes in the detected temperature of the heater 20 from the thermistor 21 and controls the triac 24 based on the detected temperature to control the energization amount from the AC power source 28 to the heater resistor pattern. As a result, the energization control circuit unit 23 maintains the temperature of the heater 20 at a predetermined fixing temperature (target temperature, set temperature).
In a state where the heater 20 is heated to the fixing temperature and the pressure roller 26 is rotationally driven, the recording material P is introduced into the fixing nip portion N from the leading end, whereby the recording material P is fixed to the fixing film at the fixing nip portion N. 25 and the pressure roller 26. During the conveyance process, the heater 20 heats the unfixed toner image t through the fixing film 25 to heat and press-fix the toner image t on the surface of the recording material P. After completion of the fixing process, the recording material P passes through the fixing nip portion N and is separated from the fixing film 25 and discharged.

Hereinafter, the configuration of the heater 20 will be described.
The heater 20 is formed in an elongated shape in the longitudinal direction. In this embodiment, the heater 20 is 10 mm in the recording material conveyance direction, and is formed longer than the width dimension of the recording material P in the width direction of the recording material P. FIG. 2A is a schematic sectional view of the heater.
Reference numeral 20a denotes a substrate having high thermal conductivity, insulation and heat resistance such as alumina. Reference numerals 20b1 and 20b2 denote resistor patterns (hereinafter referred to as heating elements) such as Ag / Pd, which generate heat when energized, and have a thickness of about 10 μm and a width of about 1 mm to 5 mm in the recording material conveyance direction, and are provided on the substrate 20a. . Here, the heating element 20b1 and the heating element 20b2 are arranged on the non-sheet passing surface side of the heater 20 (the side opposite to the side through which the recording material passes with respect to the heater 20) of the substrate 20a. The heating elements 20b1 and 20b2 are provided so as to heat the recording material P over the entire width direction of the recording material P sandwiched and conveyed at the fixing nip portion N, and are arranged side by side in the recording material conveyance direction. .
Reference numeral 20 c denotes a glass layer as a surface protective layer that constitutes a sliding surface with the fixing film 25. Reference numeral 20d denotes an insulating glass layer which is provided on the surface of the substrate 20a where the heating elements 20b1 and 20b2 are disposed, and serves to protect the heating elements 20b1 and 20b2 and to insulate them from the surroundings.

  FIG. 2B is a view of the heater 20 as viewed from the non-sheet passing surface side. 20e is an AC contact for driving the heater, and 20f is a conductive pattern. The heater 20 is provided with AC contacts 20e for driving at both ends, and the heating element 20b1 and the heating element 20b2 are configured to be independently drivable (heat generation is possible).

Below, the structure of the thermistor 21 is demonstrated.
3A is a view of the thermistor 21 as seen from the surface in contact with the heater 20, and FIG. 3B is a side view of the thermistor 21. The side surface refers to a surface obtained by rotating the thermistor 21 by 90 ° about the rotation axis aa ′ shown in FIGS. 3A and 3B from the surface of FIG.
21a is a chip thermistor, 21b is a holder, 21c is ceramic paper, 21d is a dumet wire, and 21e is a heat-resistant electric wire. The thermistor 21 is composed of an oxide semiconductor using nickel, manganese, cobalt, or the like whose electric resistance value changes with temperature, and detects the temperature of the heater 20.
A chip thermistor 21a, which is an oxide semiconductor, is placed on a dedicated holder 21b via a ceramic paper 21c that acts as a cushion, and the holder 21b is pressed against the surface of the heater 20 that is not in contact with the fixing film. The temperature is detected. In addition, the change of the electrical resistance value of the chip thermistor 21a is output via the dumet wire 21d and the heat-resistant electric wire 21e.

The positional relationship between the heater 20 and the thermistor 21 will be described with reference to FIGS. FIG. 4A is a schematic sectional view of the heater 20. The thermistor 21 is provided between the heating element 20b1 and the heating element 20b2 in the recording material conveyance direction on the non-sheet passing surface side of the heater 20. The thermistor mounting position is in the range of 2.4 mm to 5.6 mm from the upstream end of the heater substrate in the recording material transport direction in consideration of tolerances of surrounding components. Hereinafter, this range is referred to as a thermistor installable range.
FIG. 4B is a view of the heater 20 as seen from the non-sheet passing surface side. The position of the thermistor 21 in the longitudinal direction of the heater is set at a position that enters the inside of the recording material having the minimum width among the recording materials that can pass the paper (paper passing area of the recording material having the minimum width among the recording materials that can pass the paper).

FIG. 5A shows the temperature distribution in the nip portion of the heater 20, more specifically, the temperature distribution on the non-sheet passing surface side of the heater 20. The horizontal axis in FIG. 5A is the position in the recording nip conveyance direction in the fixing nip N, and the vertical axis is the temperature. FIG. 5A shows a “thermistor installable range”.
Since the heater 20 is heated by the two heating elements 20b1 and 20b2, as shown in FIG. 5A, there are high temperature portions near both ends in the recording material conveyance direction, and the temperature is low in the central portion. . Due to the temperature distribution in the recording material conveyance direction, the temperature difference within the thermistor installation range is about 5 ° C. at the maximum. Therefore, even when the actual temperature of the heater 20 is the same, the detection result of the thermistor 21 is converted into a temperature depending on the installation position of the thermistor 21, and a difference of about 5 ° C. at maximum occurs.

In order to eliminate the difference in the detection results depending on the installation position, as a feature of the present embodiment, the energization control circuit unit 23 estimates (predicts, estimates) the thermistor position, and from the result, the set temperature (temperature control) of the heater 20 Temperature, control temperature). Hereinafter, a thermistor position estimation method executed by the energization control circuit unit 23 will be described.
The heater 20 has heating elements 20b1 and 20b2 that can be energized independently. Of these two heating elements, first, only one of the heating elements (here, “heating element 20b1” as an example) is energized. The target temperature at this time is 120 ° C., and the input power is 50% of the maximum power. The energization ends when the detection result of the thermistor 21 reaches 120 ° C., which is the target temperature. At that time, the detection temperature Ts1 of the thermistor 21 at the start of energization, the detection temperature Te1 at the end of energization, and the time t1 from the start of energization to the end are recorded. From these results, the temperature rise gradient (temperature rise rate) α1 at the position where the thermistor 21 is disposed when the heating element 20b1 is energized is obtained from the following formula 1.

Subsequently, after waiting until the temperature detected by the thermistor 21 is 50 ° C. or lower, only a heating element (here, “heating element 20b2”) other than the heating element energized previously is energized. The target temperature at this time is 120 ° C., and the input power is 50% of the maximum power. The energization ends when the temperature detected by the thermistor 21 reaches the target temperature of 120 ° C. At that time, the detection temperature Ts2 of the thermistor 21 at the start of energization, the detection temperature Te2 at the end of energization, and the time t2 from the start of energization to the end are recorded. From these results, the temperature rise gradient α2 at the position where the thermistor 21 is disposed when the heating element 20b2 is energized is obtained from the above equation 1.
The difference Δα (difference in temperature rise gradient) is obtained from the following equation 2 from the respective temperature rise gradients α1 and α2.

Here, it is assumed that the thermistor 21 is installed near the heating element 20b1 (2.5 mm from the upstream end of the heater). In the state where the thermistor 21 is close to the heating element 20b1, the detection result of the thermistor 21 rises faster when only the heating element 20b1 generates heat than when only the heating element 20b2 generates heat. This is because the distance between the thermistor 21 and the heating element 20b1 is shorter than the distance between the thermistor 21 and the heating element 20b2.
In addition, when the thermistor 21 is located near the heating element 20b2, the detection result of the thermistor 21 rises faster when only the heating element 20b2 generates heat. That is, when the thermistor 21 is close to the heating element 20b1, the relationship between α1 and α2 is α1> α2, and conversely, when the thermistor 21 is close to the heating element 20b2, α1 <α2. Further, the closer the thermistor 21 is to one of the heating elements, the larger the difference between α1 and α2, and the larger the value of Δα.

  Consider a case where the thermistor 21 is installed at an intermediate point (4 mm from the upstream end of the heater) between the heating element 20b1 and the heating element 20b2. Since the distance between the thermistor 21 and each of the heating elements 20b1 and 20b2 is substantially the same, the temperature rise gradients α1 and α2 of the thermistor 21 when the heating elements 20b1 and 20b2 are driven independently are substantially equal. Therefore, the value of Δα is close to zero.

FIG. 5B shows the position of the thermistor 21 and the value of Δα when the position of the thermistor 21 is intentionally shifted in the configuration of the present embodiment. Note that the “thermistor position” on the horizontal axis in FIG. 5B indicates the distance from the upstream end of the heater substrate.
From FIG. 5B, it can be seen that the value of Δα increases as the position of the thermistor 21 moves away from the intermediate point (the position of the thermistor position 4 mm) between the two heating elements 20b1 and 20b2. Therefore, it can be said that Δα is an index indicating how much the installation position of the thermistor 21 is deviated from the intermediate point between the two heating elements 20b1 and 20b2 (thermistor position 4 mm).
The aim of the installation position of the thermistor 21 is an intermediate point between the two heating elements 20b1 and 20b2, and the set temperature of the heater 20 is also set based on the examination result when the thermistor 21 is installed at the target installation position. Therefore, if the thermistor 21 is shifted from the target installation position, the detection result of the thermistor 21 is shifted from the detection result when the thermistor 21 is installed at the target installation position.

Therefore, in this embodiment, the detection result of the thermistor 21 is corrected using the Δα.
Δα is an index indicating the deviation of the installation position of the thermistor 21 from the midpoint of the two heating elements 20b1 and 20b2. That is, if Δα is large, the deviation from the intermediate point is large, and the deviation of the detection result of the thermistor 21 is also large. As a result of an increase in the deviation of the detection result of the thermistor 21, the actual temperature of the heater 20 is also greatly deviated from a predetermined set temperature.

FIG. 6 shows the result of studying the relationship between the difference ΔT between the actual temperature of the heater 20 and the set temperature ΔT (where ΔT is “ΔT = set temperature−the actual temperature of the heater 20”) and Δα in the configuration of the present embodiment. Shown in (a). When Δα is large, ΔT is also large, which indicates that the actual temperature of the heater 20 is lower than the set temperature.
Therefore, by changing the set temperature of the heater 20 in accordance with the value of Δα, the actual temperature of the heater 20 can always be kept constant.
The correction values of Δα and the set temperature of the heater 20 in the configuration of the present embodiment are as shown in Table 1 below. As shown in Table 1, the relationship between Δα and the correction value of the set temperature of the heater 20 is stored in advance in the energization control circuit unit 23.

And after calculating | requiring (DELTA) (alpha) as mentioned above, the difference by the installation position of the thermistor 21 is made into the value which added the correction value predetermined according to the result of (DELTA) (alpha) to the setting temperature of the heater 20 as a new setting temperature. Can be eliminated. In this way, the actual temperature of the heater 20 can be made constant regardless of the installation position of the thermistor 21.
FIG. 6B shows the relationship between the thermistor position and the actual temperature of the heater.
Before the correction, the actual temperature of the heater surface decreases as the thermistor moves away from the intermediate point between the two heating elements 20b1 and 20b2, that is, as Δα increases. On the other hand, after the correction, the actual temperature of the heater is substantially constant regardless of the position of the thermistor.

  As described above, according to the present embodiment, in the heater having two heating elements, each heating element is energized independently, the temperature in each case is detected by the thermistor, and the detection results thereof. Thus, the positional relationship between the heating element and the thermistor can be estimated. By correcting the set temperature of the heater based on this estimation result and optimizing the heater temperature control, the difference due to the position of the thermistor can be improved, and a good image can be obtained regardless of the position of the thermistor. It becomes possible to provide.

In addition, although the present Example demonstrated the case where two heat generating bodies were provided, it does not restrict to this. The case where three or more heating elements are provided will be described below.
When a plurality of heating elements are provided, the plurality of heating elements are respectively provided so as to heat the recording material P over the entire width direction of the recording material P that is nipped and conveyed by the fixing nip N, and in the recording material conveyance direction. They will be placed side by side. The plurality of heating elements are provided so as to be able to generate heat independently. The thermistor 21 is disposed between the heating elements provided at the front and rear ends in the recording material conveyance direction among the plurality of heating elements.
Similarly to the above, the temperature rise gradient at the position where the thermistor 21 is disposed is derived for all the heating elements from the detection result by the thermistor 21 when the plurality of heating elements are heated one by one. Further, differences in temperature rise gradients are derived for all sets of two adjacent heating elements among the plurality of heating elements.

At this time, if the thermistor 21 is positioned between two adjacent heating elements in the set in which the difference in temperature increase gradient is the minimum value, the energization control circuit unit 23 determines (estimates). Further, the energization control circuit unit 23 compares the magnitudes of the temperature rise gradients for the two heating elements to determine which of the heating elements has the larger temperature rise gradient. Here, the energization control circuit unit 23 corresponds to a determination unit.
Further, in each set of two adjacent heat generating elements among the plurality of heat generating elements, the relationship between the difference in temperature rise gradient with respect to the two adjacent heat generating elements and the correction value of the set temperature of the heater 20 is the energization control circuit unit 23. Are stored in advance as follows. That is, the relationship is such that the temperature increase gradient for one of the two adjacent heating elements is greater than the temperature increase gradient for the other and the case where the temperature increase gradient is not the other. It is stored in advance for each of two matching heating elements. Here, the case where it is not so is a case where the temperature increase gradient with respect to the other is larger than the temperature increase gradient with respect to the one. By knowing which of the two adjacent heating elements in the set where the difference in temperature rise gradient is the minimum, the temperature rise gradient for which the heating element is large, the position of the thermistor 21 is the heat generation of which of the two heating elements. It can be estimated whether it is close to the body side.
The set temperature of the heater 20 can be corrected using the relationship shown below. The relationship is that the temperature rises for two adjacent heating elements corresponding to the heating elements determined to have a large temperature rising gradient among the two adjacent heating elements in the set where the difference in temperature rising gradient is the minimum value. This is the relationship between the difference in gradient and the correction value of the set temperature of the heater 20.

Here, if three heating elements 20b1, 20b2, and 20b3 are sequentially provided as a plurality of heating elements along the recording material conveyance direction, the thermistor 21 is disposed between the heating element 20b1 and the heating element 20b3. The Rukoto. Then, temperature increase gradients α1, α2, and α3 are derived for the three heating elements 20b1, 20b2, and 20b3, respectively. Further, differences in temperature rise gradients (Δα1, Δα2) are derived for all the sets of two adjacent heating elements (a set of heating elements 20b1 and 20b2 and a set of heating elements 20b2 and 20b3), respectively.
Further, in the set of the heating element 20b1 and the heating element 20b2, the relationship between the temperature rise gradient difference Δα1 and the correction value of the set temperature of the heater 20 is stored in advance as follows. That is, the relationship is stored in advance corresponding to the case where the temperature increase gradient for one of the heat generating element 20b1 and the heat generating element 20b2 is greater than the temperature increase gradient for the other, and the case where it is not. ing. The same applies to the set of the heating element 20b2 and the heating element 20b3.
If Δα1 is smaller than Δα2 and the temperature rise gradient with respect to the heat generator 20b1 is greater than the temperature rise gradient with respect to the heat generator 20b2, the set temperature of the heater 20 is corrected using the following relationship. . The relationship between Δα1 and the correction value of the set temperature of the heater 20 corresponding to the heating element 20b1 having a large temperature rising gradient in the pair of the heating element 20b1 and the heating element 20b2 in which the difference in temperature rising gradient is Δα1. It is a relationship.

  Thus, even when three or more heating elements are provided, the difference between the actual temperature of the heater and the detection result of the thermistor due to the position of the thermistor can be eliminated. It can be made constant regardless of the installation position of 21.

  In the present embodiment, the case where the control parameter during the fixing process is the set temperature of the heater 20 has been described. However, the present invention is not limited to this, and the fixing process using the temperature detecting unit is used to obtain a better image. Any control parameter may be used as long as it is performed. In the second embodiment described below, a case will be described in which the control parameter during the fixing process is a threshold that is set in advance and is a threshold that is used when the number of images formed per unit time is reduced. This is a form in which a thermistor is used to optimize the throughput of a small-sized recording material.

  Example 2 will be described below. In this embodiment, a case where there are two thermistors as temperature detecting means will be described. As a feature of this embodiment, since there are two thermistors, it is possible to optimize the throughput of a small-sized recording material in addition to the correction of the set temperature. Since specific examples of the constituent members of the image forming apparatus in the present embodiment are the same as those in the first embodiment, the re-explanation of the same constituent members as those in the first embodiment is omitted.

In this embodiment, a thermistor 21 (main thermistor) and a sub-thermistor 30 are provided as two thermistors. Here, the thermistor 21 corresponds to a first temperature detection element included in the temperature detection means, and the sub-thermistor 30 corresponds to a second temperature detection element included in the temperature detection means.
The positional relationship between the heater 20, the thermistor 21 and the sub-thermistor 30 in this embodiment will be described with reference to FIGS. 7 (a) and 7 (b). FIG. 7A is a view of the heater 20 as viewed from the non-sheet passing surface side. FIG. 7B is a schematic cross-sectional view of the heater 20.

The thermistor 21 transports a recording material having a size smaller than the maximum recording material capable of forming an image in the fixing nip portion N with respect to the longitudinal direction (the width direction orthogonal to the recording material transport direction of the image forming surface of the recording material). In this case, the temperature of the heater 20 in the passage region is detected. In the thermistor 21, the heater in the longitudinal direction is installed at a position that falls inside the smallest width of the recording material that can be passed.
The sub-thermistor 30 detects the temperature of the heater 20 with respect to the longitudinal direction in a passage area when the maximum size recording material is conveyed and a non-passage area when the small size recording material is conveyed. In this way, the sub-thermistor 30 is installed to detect the temperature in the vicinity of the longitudinal end of the heater. In this embodiment, the sub-thermistor 30 is 99 mm from the paper passing center (the center of the recording material in the width direction of the recording material). It is installed in the position. The installation positions of the thermistor 21 and the sub-thermistor 30 in the recording material conveyance direction are within the thermistor mounting position on the non-sheet passing surface side of the heater 20 as in the first embodiment.

  In each of the thermistor 21 and the sub-thermistor 30, the detection result correction operation based on the thermistor position is performed in the same manner as in the first embodiment to obtain Δα, that is, Δαa and Δαb, and the detection result is corrected based on the position of each thermistor. . The set temperature of the heater 20 can be corrected from the result of the thermistor 21. Furthermore, in this embodiment, it is possible to optimize the throughput of a small-sized recording material from the result of the sub-thermistor 30. Hereinafter, a method for optimizing the throughput of a small-sized recording material from the detection result of the sub-thermistor 30 will be described.

In the fixing device according to the present exemplary embodiment, when a recording material having a width smaller than the maximum sheet passing width (LTR size (width 216 mm)) is passed, the temperature excessively increases in a portion corresponding to a non-sheet passing portion. There is. This occurs because the recording material that takes heat away does not pass through the non-sheet passing portion, and heat accumulates in that portion.
FIG. 8A shows the temperature transition of the sheet passing portion and the non-sheet passing portion when an A5 size recording material is fed. As shown in FIG. 8A, the temperature of the non-sheet passing portion greatly increases as the number of sheets passing increases. Further, when the sheet is continuously passed, the temperature of the non-sheet passing portion is further increased, and there is a concern that the heat resistance temperature of each member may be exceeded.
Therefore, in order to prevent an excessive temperature rise in the non-sheet passing portion, a sub thermistor 30 is installed in the vicinity of the end portion in the longitudinal direction of the heater. When the temperature of the end portion in the longitudinal direction of the heater is detected by the sub-thermistor 30 and the temperature of the end portion reaches a predetermined temperature (threshold value) (becomes the threshold value or exceeds the threshold value), the end portion The throughput is lowered to lower the temperature. In other words, when the temperature of the edge reaches a predetermined temperature, the number of images formed per unit time is made smaller than before reaching this temperature. This suppresses the temperature rise in the non-sheet passing portion. For this reason, the throughput of the small size recording material gradually decreases.

Since the throughput of the small-sized recording material is determined by the detection result of the sub-thermistor 30, if the position of the sub-thermistor 30 is shifted, the throughput is lowered earlier than the original performance. For this reason, there is a slight difference in the throughput performance of the small size recording material depending on the individual main bodies. As an example, a change in throughput when an A5 size paper is passed through two main bodies A and B having the same configuration will be described. Here, the main body A and the main body B are different from each other only in the position condition of the sub-thermistor 30, and the other components are the same.
As shown in FIG. 8 (b), the position of the sub-thermistor 30 in the recording material conveyance direction is such that the main body A is at an intermediate point between the heating element 20b1 and the heating element 20b2 (4 mm from the heater end). It is at a position (2.5 mm from the end of the heater) close to the heating element 20b1 side.
According to Table 2 below, the throughput when the small-sized recording materials of the main bodies A and B pass is reduced by one step every time the temperature of the sub-thermistor 30 becomes equal to or higher than the threshold temperature (exceeds the threshold temperature). .

FIG. 9A shows the change in throughput when the A5 size recording material is passed through the main bodies A and B having the above specifications.
In the main body A, the throughput drops from 18 ppm (page per minute) to 12 ppm in the eleventh sheet, whereas in the main body B, the throughput drops to 12 ppm in the eighth sheet. That is, the throughput of the main body B is reduced by three sheets earlier. This is because the detected temperature of the sub-thermistor 30 is 250 ° C., which is the threshold value for the 11th sheet in the main body A, and 250 ° C. for the 8th sheet in the main body B. As a cause of the detection temperature reaching the threshold in the main body B earlier than that in the main body A, a difference in detection temperature due to the installation position of the sub-thermistor 30 can be considered.
Therefore, by detecting the thermistor position in this embodiment and correcting the detection temperature of the sub-thermistor 30, it is possible to eliminate the throughput performance difference of the small size recording material due to the displacement of the installation position of the sub-thermistor 30. .

Hereinafter, a method for correcting the detected temperature of the sub-thermistor 30 will be described.
The detection temperature correction method is the same as that in the first embodiment, and the correction is performed for both the thermistor 21 and the sub-thermistor 30. The threshold temperature correction related to the throughput of the small-sized recording material is changed by adding a predetermined correction value to the threshold temperature shown in Table 2 according to the result of ΔαSub based on the detection result of the sub-thermistor 30. The correction value of the threshold temperature in the present embodiment is as shown in Table 3 below.

FIG. 9B shows a change in throughput after the sub-thermistor 30 is corrected.
As shown in FIG. 9B, the timing at which the throughputs of the main body A and the main body B are decreased is the eleventh sheet for the main body A and the tenth sheet for the main body B, which is almost the same timing as before the correction.

As described above, according to the present embodiment, in the image forming apparatus having two thermistors, the positional relationship between the heating element and the thermistor is estimated, and the set temperature of the heater is corrected based on the estimated result. The throughput of a small size recording material can be optimized. This makes it possible to efficiently provide a good image regardless of the installation position of the thermistor.

In this embodiment, in the image forming apparatus having a plurality of thermistors, the throughput of the small-size recording material is optimized based on the detection result of the sub-thermistor. However, the present invention is not limited to this. For example, in an image forming apparatus having one thermistor, the throughput of a small-sized recording material may be optimized based on the detection result of the thermistor. In the image forming apparatus having a plurality of thermistors, the positional relationship between the heating element and the thermistor may be estimated, and a plurality of control parameters during the fixing process may be corrected based on the estimated result.
In the present embodiment, the case where there are two heating elements has been described. However, the present invention is not limited to this, and three or more heating elements may be provided as in the first embodiment.

Example 3 will be described below. In this embodiment, a fixing device provided with a pressure release mechanism capable of releasing pressure applied between a heating device and a pressure roller will be described. Since specific examples of the constituent members of the image forming apparatus in the present embodiment are the same as those in the first embodiment, the re-explanation of the same constituent members as those in the first embodiment is omitted.
The fixing device according to the present exemplary embodiment is provided with a pressure release mechanism as a configuration unique to the present exemplary embodiment. The pressure release mechanism is installed for the purpose of preventing film breakage that occurs during jam processing and improving usability (convenience) that facilitates jam processing.

Hereinafter, the pressure release mechanism will be described with reference to FIGS.
Reference numeral 32 denotes a heating device, 33 denotes a pressure roller, 34 denotes a pressure release lever, 35 denotes a pressure release sheet metal, and 36 denotes a pressure spring. FIG. 10A shows a state during printing, and pressure is applied between the heating device 32 and the pressure roller 33. FIG. 10B shows the pressure release state. In this state, pressure is not applied between the heating device 32 and the pressure roller 33 or is reduced.
Release of the applied pressure is performed by operating the applied pressure release lever 34.
In the state shown in FIG. 10A, when a force of a certain level or more is applied to the action point T1 of the pressure release lever 34, the pressure release lever 34 rotates clockwise in FIG. 10A around the rotation axis P1. Rotate. Then, the end 34b opposite to the end 34a where the application point T1 of the pressure release lever is located pushes the pressure release sheet metal 35 in the direction of the arrow in FIG. . The pushed-up pressure release sheet metal 35 rotates counterclockwise around the rotation axis P <b> 2, and at this time, the pressure spring 36 is pushed up to release the pressure force between the heating device 32 and the pressure roller 33. (See FIGS. 10A and 10B).

By the operation of the pressurizing release mechanism, the pressurizing force of the heating device 32 and the pressure roller 33 is lowered, so that the heater 20 fixed by the pressurizing force can move. If the fixing film 25 is rotated by a jamming process or the like of the fixing device in a state where the heater 20 can move, there is a concern that the heater 20 is rotated along with the fixing film 25 and the position of the heater 20 is shifted.
In such a case, the positional relationship between the heater 20 and the thermistor 21 changes, and the thermistor 21 detects the temperature on the heater 20 at a location different from the previous one. Here, the thermistor 21 is held against the heater 20 by being pressed against the heater 20, and the heater between the heating device 32 and the pressure roller 33 in the fixing nip N is released. 20 is configured to be movable.
Since the heater 20 has a heat distribution, if the place where the temperature is detected changes, the detection result also shifts. Therefore, the temperature of the heater 20 that performs temperature adjustment based on the detection result of the thermistor 21 also changes. For this reason, each time the pressure is released, the position of the thermistor 21 shifts, and the actual temperature of the heater 20 shifts accordingly, causing problems such as offset and fixing failure due to a difference in the set temperature of the heater. Concerned.

  In the case of the configuration in the present embodiment, the heater 20 can move up to about 1 mm in consideration of the range of free movement after installation. From the temperature distribution in the nip shown in FIG. 5A, it can be seen that when the temperature detection position is moved by 1 mm, a maximum deviation of about 4 ° C. occurs. In this embodiment, in order to correct this difference, correction of the detection result based on the installation position of the thermistor 21 is performed when the pressure is released and then returned to the normal state, that is, when the nip portion is formed again. It is performed every time. The correction method of the detection result based on the installation position of the thermistor 21 is the same as that of the first embodiment, and the positional relationship between the heater 20 and the thermistor 21 generated at the time of releasing the applied pressure is performed every time when the applied pressure is released from the released state. It is possible to correct the difference in the detection result due to the deviation.

  As described above, in this embodiment, in the fixing device having the pressure release mechanism, the positional relationship between the heater and the thermistor is always estimated after the pressurization release operation is completed, and the set temperature of the heater is based on the estimated result. Is configured to correct. As a result, it is possible to suppress the occurrence of problems such as offset and fixing failure due to deviations in the set temperature of the heater, and to provide a good image regardless of the position of the thermistor that fluctuates with the pressure release operation. Is possible.

In the present embodiment, the case of a fixing device having a pressure release mechanism with respect to the configuration of the first embodiment has been described. However, the present invention is not limited to this. Even in the case of a fixing device having a pressure release mechanism with respect to the configuration of the second embodiment, it may be configured as described above, and optimization of the throughput of a small-sized recording material can be performed from the pressure release state. You may comprise so that it may implement every time when it returns to a normal state. In other words, the control parameter correction at the time of the fixing process may be performed every time when the pressure is released from the released state to the normal state.
In this embodiment, the positional relationship between the heater and the thermistor is estimated when the pressure is released and then returns to the normal state. However, the heater and thermistor in the configurations of the first and second embodiments are used. The timing for estimating the positional relationship may be set as appropriate. For example, it is preferable that the positional relationship between the heater and the thermistor is estimated every time the apparatus is turned on (at the start-up operation after the power is turned on), and correction based on the estimated result is performed. In addition, the positional relationship between the heater and the thermistor is changed each time the image forming operation is performed when the apparatus starts up from the sleep state or when a predetermined time (for example, overnight (8 hours)) has elapsed. It is good that it estimates and correct | amends based on the estimated result.

  In Examples 1 to 3, the fixing film type fixing device has been described. However, the present invention is not limited to this. That is, the present invention can be suitably applied to any image heating apparatus having the following heating element and temperature detecting means. The heating element is a heating element provided in the heater so as to heat the recording material across the entire width direction of the recording material nipped and conveyed at the fixing nip portion, and is a recording material to be nipped and conveyed at the fixing nip portion. A plurality of heating elements are provided side by side in the transport direction and can generate heat independently. The temperature detection means detects the temperature of the heater, and is a temperature detection means disposed between the heating elements provided at the front end and the rear end in the recording material conveyance direction among the plurality of heating elements. . Further, the image heating apparatus according to the present invention is not limited to the example in the case of functioning as the above-described fixing device, and can also be applied as an apparatus for giving gloss to a toner image fixed on a sheet. .

  DESCRIPTION OF SYMBOLS 11 Fixing device; 20 Heater; 20b1, 20b2 Heat generating body; 21 Thermistor; 22 Film guide; 23 Current supply control circuit part; 25 Fixing film; 26 Pressure roller; 27 Temperature control means;

Claims (6)

An image heating apparatus that nipping and conveying a recording material on which a developer image is formed and heating it at a nip formed by a heating member having a heater and a pressure member pressed against each other,
A heating element provided in the heater so as to heat the entire width direction of the recording material sandwiched and conveyed at the nip portion, provided side by side in the conveying direction of the recording material sandwiched and conveyed at the nip portion, Two adjacent heating elements that can generate heat independently,
A temperature detecting means which is disposed between the two adjacent heating elements and detects the temperature of the heater;
In an image forming apparatus provided with an image heating apparatus having
From the detection result by the temperature detection means when the two adjacent heating elements are heated one by one, the temperature increase gradient at the position where the temperature detection means is disposed is derived for each heating element,
Deriving means for deriving a difference in temperature rise gradient between the two adjacent heating elements;
Storage means for storing in advance a relationship between a difference in temperature rise gradient between the two adjacent heating elements and a correction value of a control parameter at the time of image heating processing;
A correction means for deriving a correction value of the control parameter corresponding to the difference in temperature rise gradient derived by the deriving means from the storage means, and correcting the control parameter using the correction value;
An image forming apparatus comprising: Three or more heating elements are provided,
The temperature detecting means is disposed between the heating elements provided at the front end and the rear end in the transport direction among the three or more heating elements,
The derivation means includes
From the detection result by the temperature detection means when the three or more heating elements are heated one by one, the temperature increase gradient at the position where the temperature detection means is disposed is derived for all the heating elements, Deriving temperature rise gradient differences for all pairs of two adjacent heating elements,
The storage means
In each group, the relationship between the difference in temperature rise gradient between two adjacent heating elements and the correction value of the control parameter is as follows:
When the temperature increase gradient for one of two adjacent heating elements is greater than the temperature increase gradient for the other, and when the temperature increase gradient for the other is greater than the temperature increase gradient for the one In correspondence with each of the above, each pair of adjacent heating elements is stored in advance,
It is determined that the two adjacent heating elements between which the temperature detecting means is located are the two adjacent heating elements in which the difference between the temperature rise gradients derived by the deriving means is a minimum value. A judgment means is further provided for comparing the magnitude of the temperature rise gradient with respect to the two heating elements to judge which temperature rise gradient with respect to which heating element is large,
The correction means includes
The temperature rise for two adjacent heating elements corresponding to the heating element determined to have a large temperature rising gradient among the two adjacent heating elements of the set having the minimum difference in temperature rising gradient by the determining means. The image forming apparatus according to claim 1, wherein the control parameter is corrected using a relationship between a gradient difference and a correction value of the control parameter. A release means for releasing the applied pressure between the heating member and the pressure member in the nip portion;
The temperature detecting means is configured to be movable with respect to the heater when the pressing force between the heating member and the pressure member in the nip portion is released by the releasing means.
After the pressing force between the heating member and the pressure member in the nip portion is released by the releasing unit, the correction unit performs correction when the nip portion is formed again. The image forming apparatus according to claim 1 or 2. The control parameter is a set temperature of the heater,
The temperature detection means passes when a recording material having a size smaller than the maximum recording material capable of image formation is conveyed in the nip portion with respect to a width direction orthogonal to the conveyance direction on the image forming surface of the recording material. Configured to sense the temperature of the heater in an area;
4. The heater according to claim 1, wherein the heater is controlled so that the temperature of the heater detected by the temperature detection unit becomes a set temperature of the heater corrected by the correction unit. 5. 2. The image forming apparatus according to item 1. The control parameter is a threshold value that is set in advance and is used when the number of images formed per unit time is reduced.
The temperature detecting means is a passing area when a recording material of a maximum size capable of image formation is transported in the width direction orthogonal to the transport direction of the image forming surface of the recording material, and the maximum size capable of image formation. Configured to detect the temperature of the heater in the non-passage area when a recording material having a size smaller than the recording material is conveyed,
4. The number of images formed per unit time is reduced when the temperature of the heater detected by the temperature detection unit is equal to or higher than a threshold value corrected by the correction unit. The image forming apparatus according to claim 1. The control parameter includes a set temperature of the heater,
The temperature detection means passes when a recording material having a size smaller than the maximum recording material capable of image formation is conveyed in the nip portion with respect to a width direction orthogonal to the conveyance direction on the image forming surface of the recording material. A first temperature detection element for detecting the temperature of the heater in the region;
The heater is controlled so that the temperature of the heater detected by the first temperature detection element becomes the set temperature of the heater corrected by the correction unit,
The control parameter includes a threshold value that is set in advance and is used to reduce the number of images formed per unit time.
The temperature detecting means is a passing area when a recording material of a maximum size capable of image formation is transported in the width direction orthogonal to the transport direction of the image forming surface of the recording material, and the maximum size capable of image formation. A second temperature detection element for detecting the temperature of the heater in the non-passage area when a recording material of a size smaller than the recording material is conveyed,
2. The number of images formed per unit time is reduced when the temperature of the heater detected by the second temperature detection element is equal to or higher than a threshold value corrected by the correction unit. 4. The image forming apparatus according to any one of items 3.

Images