JPH11194651A - Fixing device and image forming device equipped with the device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To save wastefully consumed power and to excellently keep the carrying accuracy of recording material even in the case of consecutively fixing many sheets of recording material in a film heating type fixing device. SOLUTION: A fixing film 2 is pressed to a heater 1 by a pressure roller 4, whereby a fixing nip part 6 is constituted. While the recording material P on which an unfixed toner image is carried is held and carried by the nip part 6, the toner image is thermally fixed on the recording material P by heat from the heater 1. The device is provided with thermistors 7 and 14 respectively detecting the temperature of the heater 1 and the temperature of the roller 4. The target temperature of the nip part 6 attained when the heater 1 is heated and the recording material is not introduced to the nip part 6 is changed according to detected temperature detected by the thermistor 14 abutting on the roller 4. The wastefully consumed power is saved, and a situation that the temperature of the roller 4 rises higher than necessary and the roller 4 is expanded is prevented, so that the recording material P is excellently carried even in the case of consecutively fixing the recording material P.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film heating type fixing device and an image forming apparatus provided with the fixing device.

[0002]

2. Description of the Related Art FIG. 7 is a longitudinal sectional view of a film heating type fixing device. In FIG. 7, reference numeral 1 denotes a ceramic heater (hereinafter simply referred to as a “heater”) as a heating element, 2 denotes a heat-resistant fixing film pressed to the heater 1, and 4 denotes a pressure roller that presses the fixing film 2 to the heater 1. is there. Also,
The recording material P is guided to a fixing nip portion 6 between the pressure roller 4 and the fixing film 2 by a guide member 5, heated by the heater 1, and toner is fixed on the recording material P.

The heater 1 is adhered to the heater holder 30 with a highly heat-resistant adhesive. The fixing film 2 is guided by the guide member 8 attached to the heater holder 30 and the outer peripheral surface of the heater holder 30 itself. It is configured to rotate smoothly. The rotation of the fixing film 2 and the conveyance of the recording material P are performed by the pressing roller 4.
Has been done by That is, when the driving source pressure roller 4 by (not shown) is rotated in the arrow R4 direction by the conveying force generated by the pressure roller 4 is pressed against the heater 1, the recording material P is conveyed in the arrow K _P direction The fixing film 2 rotates in the direction of arrow R2.
Further, the recording material P fixed as described above is conveyed to the paper discharge roller 10 by the guide member 9 and discharged out of the apparatus.

In the image forming apparatus having the above-described fixing device, a thermistor (first temperature detecting element) 7 bonded with a highly heat-conductive adhesive or grease is provided on the back surface of the heater 1. The fixing temperature is controlled to be a desired temperature while detecting the temperature with the thermistor 7. Therefore, the thermistor 7 is provided inside the minimum sheet passing area of the recording material P.

In general, the temperature of the heater 1 is controlled only by the temperature information of the thermistor 7. However, the temperature of the fixing nip 6 can be controlled even if the temperature of the heater 1 is constant.
Since the temperature varies with the temperature of the pressure roller 4 and the type of the recording material P, stable fixing performance cannot be obtained in all cases.

Therefore, in recent years, as shown in FIG. 7, the temperature information of the pressure roller 4 is referred to by a pressure roller thermistor (second temperature detecting element) 14 which is in contact with the outer periphery of the pressure roller 4. Meanwhile, a technique of determining the temperature of the heater 1 and keeping the temperature of the fixing nip 6 constant has been put to practical use.
In such an apparatus, the recording material P is fixed to the fixing nip 6
When heat passes through the pressure roller 4, heat is removed from the surface of the pressure roller 4 and the surface temperature decreases. By detecting the temperature decrease of the pressure roller 4 at this time by the pressure roller thermistor 14, the heater 1 Temperature adjustment is being performed. Therefore, the pressure roller thermistor 14 is also provided in the minimum sheet passing area of the recording material P similarly to the thermistor 7.

[0007]

In the above-mentioned prior art, the temperature of the fixing nip 6 is kept constant even between the recording material P being fixed and the recording material P to be subsequently fixed (hereinafter referred to as "paper interval"). Is maintained so that not only wasteful power is consumed but also the temperature of the pressure roller 4 rises more than necessary and expands, and the conveyance accuracy when a large number of recording materials P are continuously fixed is reduced. There was a problem of adverse effects.

[0008] Therefore, in recent years, as shown in FIG. 8, the temperature T _B of the fixing nip portion 6 between the sheet (E unit), lower than the target temperature T _B0 in the recording material P is passed (D unit) A method of controlling the temperature to the target temperature T _B1 has been proposed.

However, in such a method, when the pressure roller 4 is started from a sufficiently cooled state, the heat of the pressure roller 4 is removed by the first recording material P, and When the recording material P is fixed, the temperature of the fixing nip 6 is controlled to be lowered, so that the temperature of the pressure roller 4 is lowered more than necessary, and the fixing is not performed between the second and several sheets. There is a problem that curling occurs due to a sufficient temperature difference or a large temperature difference between the pressure roller 4 and the heater 1.

Accordingly, the present invention prevents wasteful power consumption, and maintains the recording material conveyance accuracy during continuous fixing satisfactorily.
It is an object of the present invention to provide a fixing device that prevents a fixing failure or curling of a recording material during continuous fixing, and an image forming apparatus including the fixing device.

[0011]

According to a first aspect of the present invention, there is provided a fixing device, comprising: Form a fixing nip between the rotating body and
A fixing device for thermally fixing the unfixed toner image on the recording material by the heat of the heating body while nipping and conveying the recording material carrying the unfixed toner image by the fixing nip portion,
Temperature detecting means for detecting a temperature of the fixing nip portion, comprising a first temperature detecting element abutting on the heating element and a second temperature detecting element abutting on the pressurizing rotating body; Temperature control means for controlling heat energy generated by the heating element according to the temperature detected by the temperature detection means, wherein the temperature control means heats the heating element and introduces the recording material into a fixing nip portion. The target temperature of the fixing nip when not performed is changed according to the temperature detected by the second temperature detecting element.

In the fixing device according to claim 2, when the temperature of the pressing rotator is equal to or lower than a predetermined temperature, the temperature control means controls the temperature when the recording material is not introduced into the fixing nip. The target temperature of the fixing nip is controlled to be equal to or lower than the target temperature when the recording material is introduced into the fixing nip.

According to a third aspect of the present invention, there is provided an image forming apparatus, wherein an image forming section for forming a toner image on a recording material, and an unfixed toner image formed on the recording material in the image forming section are thermally fixed. Item 1. The fixing device according to Item 1 or 2.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an external perspective view showing an example of a facsimile apparatus as an image forming apparatus according to the present invention, and FIG. 2 is a longitudinal sectional view showing a schematic configuration.

(Outline of Facsimile Apparatus) In FIGS. 1 and 2, reference numeral 130 denotes a flatbed scanner which reads an original B placed on an original platen glass 130a. Reference numeral 102 denotes a pressure plate which can be opened and closed by a user up and down, and presses a document B placed on a document table glass 130a. Reference numeral 108 denotes a contact image sensor that reads an image.

On the upper surface of the pressure plate 102, a document table 102b configured to be able to stack a plurality of documents S is provided. An original transport unit 106 transports the original S while separating the original S one by one, and reads the original S while transporting the original S to the sheet reading unit 107. Here, the document transport unit 106 is provided in a part of the pressure plate 102, and moves up and down as the pressure plate 102 is opened and closed. Reference numeral 109 denotes a document discharge tray on which the discharged documents S are stacked.

Reference numeral 104 denotes a recording device section comprising a laser beam printer; 112, a cassette sheet feeding section; 125, a manual sheet feeding section; 111, an image forming section; 116, a fixing section; This is a recording material discharge tray to be loaded.

An operation unit 105 includes a display unit and an input unit, a control unit (temperature control unit) 120 for controlling the facsimile apparatus, and an image information unit 121 that receives image information transmitted through a communication circuit (not shown). A receiving unit that receives and transfers image information to the control unit 120.

(Contact Image Sensor Unit) The contact image sensor 108 includes an illuminating means for illuminating the reading surface of the document, a lens through which the reflected light from the reading surface of the document passes, and a reflection light disposed immediately below the lens. And an image sensor for forming an image. Here, the lens is of a long focus type, and is set so that the focal point is on the surface of the platen glass 130a having a thickness of 3 mm.

(Flat Bed Scanner Unit) The flat bed scanner unit 130 is provided with a document table glass 130a on which the document B is placed in order to read a thick document B such as a book in a stationary state.

Reference numeral 102e denotes an original pressing plate,
The document B is supported so as to be vertically movable with respect to the document 2, and presses the document B on the document table glass 130 a by closing the pressure plate 102. Reference numeral 130b denotes a frame serving as a structure of the flatbed scanner unit 130, which supports the platen glass 130a.

Here, the pressure plate 102 is provided with a hinge (not shown) as a rotation center portion between the pressure plate 102 and the frame 130b, and the pressure plate 102 is configured to be swingable by a predetermined angle with respect to the frame 130b. User presses the platen 102
And the original B is placed on the original platen glass 130a.

The contact image sensor 108 is driven in the sub-scanning direction of the document B (direction A in the figure) by a driving means (not shown). The contact-type image sensor 108 is urged against the lower surface of the platen glass 130a, and moves in the sub-scanning direction by the above-described driving means.
Since the distance between the platen glass 8 and the platen glass 130a is always kept constant, the height of the focal position of the contact type image sensor 108 on the platen glass 130a becomes constant, and a good image without a focal position shift is obtained. be able to.

When reading the document B on the platen glass 130a, the reading is performed while moving the contact image sensor 108 in the sub-scanning direction (the direction of arrow A).

(Original Conveying Unit) The original conveying unit 106 first conveys the original S loaded on the original placing table 102b by the preliminary conveying roller 102d and the preliminary conveying pressing plate 102c.
Separation piece 106a and separation roller 106b pressed against it
And one by one, and further, a paper feed roller 106e,
The sheet is conveyed to the sheet reading unit 107 by the feeding roller 106f pressed by the sheet feeding roller 106e by the pressing spring. 107
a is a document pressing roller. When reading the original S, the contact type image sensor 108 is
The operation is performed in a state where the apparatus is stopped below 07a. Here, the original S is moved to the original platen glass 130 by the original pressing roller 107a.
The image information of the document S is read while being conveyed on the surface a. The read original S is an original discharge roller 1
The sheet is discharged to the document discharge tray 109 by the sheet discharge roller 106h and the document discharge roller 106h.

As described above, the original transporting section 106 includes the pressure plate 1
The document feeder 106 is provided on a portion of the document feeder 02 when the pressure plate 102 is opened, so that the document feeder 106 can be easily lifted when a document jam occurs.

The document table 102b is provided with a slider 102a slidable in a direction perpendicular to the direction in which the document S is conveyed (the width direction of the document S), and is placed on the document table 102b by the slider 102a. Both sides of the document S can be aligned.

(Recording Unit) The recording unit 104 is disposed at the lower part of the apparatus, and feeds the recording material P in a cassette 112a in which recording materials P of a general-purpose size are stacked and accommodated.
Then, the skew of the recording material P is corrected by the registration roller pair 124, and the recording material P is fed to the upper image forming unit 111 in synchronization with the print signal from the control unit 120.

Then, the surface of a drum type electrophotographic feeling (hereinafter referred to as "photosensitive drum") 111a as a carrier is uniformly charged by a charging member 111b, and the surface is exposed by a laser scanner 110 based on image information. Let it. As a result, an electrostatic latent image is formed on the photosensitive drum 111a, and toner as a developer of the image forming unit 111 is
The developer is transferred to an electrostatic latent image on the photosensitive drum 111 a by a developing sleeve 111 c as a developer carrier,
A toner image is formed on a. Then, the formed toner image is transferred to the recording material P by the transfer roller 111f, and the recording material P is further conveyed to the upper fixing unit 116. In the fixing unit 116, the unfixed toner image transferred onto the recording material P is fixed according to a fixing process described later, and the recording material P is discharged from the recording material discharge tray 11 by the discharge roller pair 10.
3 is sequentially discharged.

On the other hand, the recording material P stacked on the manual paper feed unit 125 is separated into one by a paper feed roller 125a and a separation pad 125b disposed at a position opposite to the recording material P. The skew of P is corrected, and the toner image is transferred by the upper image forming unit 111, fixed by the fixing unit 116, and sequentially discharged by the discharge roller 10 in the same manner as described above. .

Next, the fixing unit 116 will be described with reference to FIG. In FIG. 3, reference numeral 1 denotes a ceramic heater (hereinafter referred to as a "heater") as a heating element, 2 denotes a heat-resistant fixing film pressed against the heater 1, and 4 denotes a pressing roller for pressing the fixing film 2 against the heater 1. . Further, the recording material P is guided by a guide member 5 to a pressure contact portion (hereinafter, referred to as “fixing nip portion”) 6 between the pressure roller 4 and the fixing film 2, heated by the heater 1, and Is fixed.

The heater 1 is bonded to the heater holder 30 with an adhesive having high heat resistance, and guides the fixing film 2 by the guide member 8 attached to the heater holder 30 and the outer peripheral surface of the heater holder 30 itself so that the heater 1 rotates smoothly. Is configured. The rotation of the fixing film 2 and the conveyance of the recording material P are performed by the pressure roller 4. That is, when the pressure roller 4 is rotated by a driving source (not shown), the recording material P moves in the direction of arrow K _P due to the conveyance force generated by the pressure roller 4 being pressed against the surface of the heater 1 and the fixing film is moved. 2 rotates in the direction of arrow R2.

The recording material P fixed as described above is conveyed to the paper discharge roller 10 by the guide member 9 and discharged out of the apparatus.

In the image forming apparatus having the above-described fixing device, a thermistor (first temperature) adhered to the back surface of the heater 1 through a part of the hole of the heater holder 30 with a highly thermally conductive adhesive or grease. A sensing element 7 is provided, and controls the fixing temperature while detecting the temperature with the thermistor 7. Therefore, thermistor 7
Is provided inside the minimum sheet passing area of the recording material P.

As shown in FIG. 3, the thermistor 7 surrounds the periphery of the temperature detecting element 7a with a sponge 7b, and the sponge 7b is attached to a holder 7c by bonding or the like. The sponge portion 7b has a role of improving the contact between the thermistor 7 and the heater 1 by elastically pressing the thermistor 7 against the back surface of the heater 1 and increasing the temperature detection accuracy of the thermistor 7.

On the other hand, the pressure roller 4 is provided with a pressure roller thermistor 14 at the same position as the thermistor 7 in the longitudinal direction, and detects the surface temperature of the pressure roller 4. The pressure roller thermistor (second temperature detection element) 14 surrounds the periphery of the temperature detection element section 14a with a sponge section 14b. The temperature detecting element part 14a is sponge part 1 by the leaf spring 14d.
4b is pressed against the pressure roller 4 via the tape 14c. Here, the tape 14c is made of a heat-resistant material of about 60 μm and a material having good releasability so that the toner on the pressure roller 4 does not adhere to the surface.
And the like. FIG.
As shown in FIG.
1 is in contact therewith to remove toner adhering to the pressure roller 4 and to prevent toner from adhering to the surface of the pressure roller thermistor 14.

As shown in FIG. 4, connectors 20a and 20b are provided at both left and right ends of the heater 1 in the longitudinal direction, and a power controller (not shown) supplies AC 100V.
Is supplied, the heater 1 generates heat and is controlled to a predetermined temperature. However, when the recording material P reaches the fixing nip portion 6 and fixing is started, the temperature of the pressure roller 4 by the recording material P decreases. Therefore, the degree of the decrease is detected by the pressure roller thermistor 14, and the following temperature control is performed. Incidentally, reference numerals 21a and 21b in the figure are a part of the frame of the image forming apparatus main body.

Generally, as shown in FIG. 5, the temperature T _{A of the} heater 1, the temperature T _{C of} the pressure roller 4, and the fixing nip 6
Temperature T _B of constantly changing at a certain ratio, no significant change in the ratio γ and the temperature T _A of the heater 1, the temperature T _C of the pressure roller 4 changes. That is, a _{(T B -T C) / (} T A -T C) = γ = C ( constant) (1). In this embodiment, the temperature T _C of the pressure roller 4 is first detected from this content, and the temperature T _B of the fixing nip 6, which is the actual temperature of the heating unit, is fixed from this T _C (hereinafter referred to as “fixing temperature T C”). _B0 ”), it is determined how many times the temperature T _{A of the} heater 1 needs to be set.

That is, the equation (1) is modified to calculate T _A = {(T _B0 − (1−γ) T _C )} / γ (2), and the heater 1 is set to the target temperature T _A. So that
The power is changing.

Here, since the ratio γ changes depending on the heat capacity and thermal conductivity of the components constituting the fixing unit 116, the position of the thermistor 7, and the like, it is necessary to change the ratio γ depending on the configuration of the fixing unit 116. Is γ = 0.6
For example, _assuming that the fixing temperature T _B0 in the fixing nip portion 6 is 160 ° C., the temperature T _C of the pressure roller 4 which is lowered by the sheet passing is calculated by the equation (2) according to Expression (2), as shown in FIG. The temperature T _A is changed.

As described above, the necessary temperature T _{A of the} heater 1 (hereinafter, referred to as “target temperature T _A0 ”) is determined by the temperature T _C of the pressure roller 4, and the heater 1 is controlled to reach the required temperature. Turn on electricity. Whether the target temperature T _A0 has been reached is determined by the thermistor 7 attached to the back of the heater 1.
Is detected and controlled by

In this embodiment, the target temperature T _A0 is
The temperature is continuously calculated and changed every 0.1 seconds according to the temperature T _C of the pressure roller 4. This sampling time is determined by the fixing unit 1
The temperature accuracy is improved by performing sampling for a shorter period of time, although it varies depending on the configuration of FIG.

[0044] Further, in this embodiment, as shown in FIG. 5, the temperature T _A of the heater 1 even reaches the target temperature T _A0, the temperature T _C of the temperature T _A and the pressure roller 4 of the heater 1 In order to prevent the curl due to the temperature difference, the control is performed such that the fixing is not determined unless the pressure roller 4 reaches a predetermined temperature (hereinafter, referred to as “fixable temperature T _C0 ”).

[0045] Incidentally, in addition to during sheet passing shown in FIG. 5 (D unit), between not the fixing nip portion 6 of the recording material P paper (E unit) Temperature Temperature T _C of the pressure roller 4 is in a predetermined When the temperature is equal to or lower than T _C1, the temperature T _B of the fixing nip 6 is controlled to be the same as the fixing temperature T _B0 . By controlling in this manner, even when the pressure roller 4 starts up from a sufficiently cooled state, the temperature T _C of the pressure roller 4 drops unnecessarily, and the temperature of the pressure roller 4 decreases from the second to several sheets. Fixing does not become insufficient, and curling due to a large temperature difference between the pressure roller 4 and the heater 1 does not occur. Further, since the temperature T _B of the fixing nip portion 6 is not controlled to be equal to or higher than the fixing temperature T _B0 , the risk of the heater 1 and the fixing film 2 being damaged due to the temperature is avoided even when the sheet interval becomes long. can do.

[0046] On the other hand, when the temperature T _C of the pressure roller 4 is equal to or higher than the predetermined temperature T _C1, the temperature T _B of the fixing nip portion 6, (hereinafter referred to as "target temperature T _B1") lower than the fixing temperature T _B0 Temperature
It is controlled to become. This control not only saves unnecessary power but also prevents the temperature T _C of the pressure roller 4 from unnecessarily rising and expanding, so that a large number of recording materials P are continuously fixed. Even if you do
There is no adverse effect on transport accuracy.

In this embodiment, the target temperature T _B1 of the fixing nip 6 is experimentally set to about 110 ° C.

Next, the operation of the facsimile apparatus will be described with reference to the flowchart shown in FIG.

The control unit 120 receives image information by a copying operation by a user or a receiving operation from a remote place (S1). The heater 1 is energized to control so that the temperature T _{A of the} heater 1 reaches the target temperature T _A0 and the temperature T _C of the pressure roller 4 reaches the fixable temperature T _C0 as shown in FIG. 5 (S2). . The recording material P is set to 1
Paper is fed (S3). The image forming unit 111 forms an unfixed toner image on the recording material P (S4). Fixing unit 1
In step S5, the unfixed toner is fixed in accordance with the above-described temperature control (S5). The recording material P is discharged out of the apparatus by the discharge roller 10 (S6). It is determined whether the image information includes two or more images (S7). If there is only one image information, the operation ends (S8).

If it is determined in S7 that the image information includes two or more sheets, it is first checked whether the next recording material P has been fed (S9). If the next recording material P is fed, controls leave so as fixing temperature T _B the temperature T _B0 of the fixing nip portion 6 returns to S4 (S10). In S9, the next recording material P
There If not fed, the temperature T _C of the pressure roller 4 is to check whether a given temperature T _C1 or higher (S11).
In S11, if T _C> T _C1, the temperature T _B of the fixing nip portion 6 and controlled to the target temperature T _B1, the flow returns to S9 (S12). In step S11, if T _C ≦ T _C1 , the temperature T _B of the fixing nip 6 is controlled to be the fixing temperature T _B0 ,
The process returns to S9 (S13).

The above operation is repeated until all the received image information is recorded.

In the present embodiment, the temperature T _B of the fixing nip 6 between the sheets is changed to two levels of T _B0 and T _B1 by the temperature T _C of the pressure roller 4. And the temperature T _B of the fixing nip 6.
And a function between the temperature T _C of the pressure roller 4 (for example, T
_B = αT _C + β, where α and β are arbitrarily determined coefficients) to control the temperature between the sheets, so that more optimal temperature management can be performed.

[0053]

As described above, according to the present invention,
The target temperature of the fixing nip when the heating element is heated and the recording material is not introduced into the fixing nip is changed according to the detected temperature detected by the second temperature detecting element brought into contact with the pressure rotating body. Not only saves unnecessary power, but also prevents the temperature of the pressurizing rotator from unnecessarily rising and expanding, so that even when a large number of recording materials are continuously fixed, The transfer can be stably performed without adversely affecting the transfer accuracy.

Further, even when the pressurizing rotator is started from a sufficiently cooled state, the temperature of the pressurizing rotator drops unnecessarily, and the fixing is insufficient between the second and several sheets. It is possible to effectively prevent problems such as the occurrence of curl due to a large temperature difference between the pressure rotating body and the heating body.

Further, when the temperature of the pressure rotating body is equal to or lower than a predetermined temperature, the target temperature of the fixing nip when the recording material is not introduced into the fixing nip is set to the target temperature when the recording material is introduced. By controlling the temperature to be equal to or lower than the target temperature, it is possible to prevent the heating element and the fixing film from being damaged by heat even when the sheet interval becomes long.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the appearance of a facsimile apparatus as an example of an image forming apparatus according to the present invention.

FIG. 2 is a longitudinal sectional view showing a schematic configuration of a facsimile apparatus as an example of an image forming apparatus according to the present invention.

FIG. 3 is a longitudinal sectional view showing a schematic configuration of a fixing device according to the present invention.

FIG. 4 is a longitudinal sectional view of a fixing device along a longitudinal direction.

FIG. 5 is a diagram illustrating temperature control of the fixing device according to the first embodiment.

FIG. 6 is a flowchart showing an operation in the first embodiment.

FIG. 7 is a longitudinal sectional view showing a schematic configuration of a conventional fixing device.

FIG. 8 is a diagram illustrating temperature control of a conventional fixing device.

FIG. 9 is a diagram illustrating a relationship between a pressure roller temperature and a heater temperature.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Heating body (heater) 2 Fixing film 4 Pressure rotating body (pressure roller) 6 Fixing nip part (pressure contact part) 7 Temperature detecting means (first temperature detecting element, thermistor) 14 Temperature detecting means (second temperature) (Sensing element, pressure roller thermistor) 111 image forming unit 116 fixing device 120 temperature control means (control unit) P recording material

Claims (3)

[Claims] A recording material having an unfixed toner image, wherein a heat-resistant fixing film is pressed against a heating body by a pressure rotating body to form a fixing nip between the fixing film and the pressure rotating body. A fixing device that heat-fixes the unfixed toner image onto the recording material by the heat of the heating body while nipping and transporting the toner image by the fixing nip portion; and a first temperature detection element contacted with the heating body; Temperature detecting means for detecting a temperature of the fixing nip portion, the temperature detecting means having a second temperature detecting element in contact with the pressure rotating body, and heat energy generated by the heating element based on the temperature detected by the temperature detecting means A temperature control unit that controls the target temperature of the fixing nip when the heating element is heated and the recording material is not introduced into the fixing nip. Temperature of 2 A fixing device that changes according to a temperature detected by a detection element. 2. The fixing device according to claim 1, wherein the temperature control unit is configured to set a target temperature of the fixing nip when the recording material is not introduced into the fixing nip when the temperature of the pressing rotator is equal to or lower than a predetermined temperature. The fixing device according to claim 1, wherein the temperature is controlled to be equal to or lower than a target temperature when the recording material is introduced into the fixing nip portion. 3. The fixing device according to claim 1, wherein an image forming unit that forms a toner image on the recording material, and the unfixed toner image formed on the recording material is thermally fixed in the image forming unit. An image forming apparatus comprising: